PROGRESS REPORT Beyond ...xrm.phys.northwestern.edu/research/pdf_papers/2010/...©2010 WILEY-VCH...

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Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions

By Jordi Cabana , Laure Monconduit , Dominique Larcher , and M. Rosa Palacín *

espite the imminent commercial introduction of Li-ion batteries in electric rive vehicles and their proposed use as enablers of smart grids based on enewable energy technologies, an intensive quest for new electrode mate-ials that bring about improvements in energy density, cycle life, cost, and afety is still underway. This Progress Report highlights the recent develop-ents and the future prospects of the use of phases that react through con-

ersion reactions as both positive and negative electrode materials in Li-ion atteries. By moving beyond classical intercalation reactions, a variety of low ost compounds with gravimetric specifi c capacities that are two-to-fi ve times arger than those attained with currently used materials, such as graphite nd LiCoO 2 , can be achieved. Nonetheless, several factors currently handicap he applicability of electrode materials entailing conversion reactions. These actors, together with the scientifi c breakthroughs that are necessary to fully ssess the practicality of this concept, are reviewed in this report.

1. Introduction

Current concern about limited energy resources, coupled to the need to decrease greenhouse gas emissions, has brought about the need to consider renewable energies at a large scale together with the widespread use of hybrid and electric vehi-cles. However, due to the intermittent and/or diffuse nature of these renewable sources, effi cient storage and mobile systems

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[∗] Dr. M. R. Palacín Institut de Ciència de Materials de Barcelona (CSIC)Campus UAB, E-08193 Bellaterra, Catalonia (Spain)and ALISTORE-ERI European Research Institute E-mail: [email protected] Dr. J. Cabana Environmental Energy Technologies DivisionLawrence Berkeley National LaboratoryBerkeley, CA 94720 (USA) Dr. L. Monconduit Institut Charles Gerhardt-CNRS Université Montpellier IIPlace Eugène Bataillon, 34095 Montpellier (France)and ALISTORE-ERI European Research Institute. Dr. D. Larcher Laboratoire de Réactivité et Chimie des SolidesUniversité de Picardie Jules VerneCNRS UMR6007, 33 rue Saint Leu 80039 Amiens (France)and ALISTORE-ERI European Research Institute.

DOI: 10.1002/adma.201000717

is a must. Among the various energy con-version/storage systems proposed over the two last centuries, electrochemical storage and more specifi cally batteries seem to be very well positioned to satisfy these needs, but research to meet the application requirements is still an imperious need. [ 1 ]

Lithium-ion batteries were fi rst com-mercialized in 1990 [ 2 ] as a natural result of the extensive knowledge in intercala-tion chemistry accumulated by inorganic and solid state chemists in the 1970s. [ 3 , 4 ] The fi rst generation of such batteries allowed storing more than twice the energy compared to nickel or lead batteries of the same size and mass. It consisted of LiCoO 2 and carbon at the positive and negative electrode, respectively, the redox operation of both versus lithium being based on intercalation reactions. Thus, one

can consider that intercalation chemistry deserves some of the honours granted to the lithium-ion technology in the develop-ment of portable electronics. However, both existing and new emerging applications demand even better performance in terms of energy density, power, safety, price and environmental impact. [ 5 ] As a consequence, mature as the technology may seem at fi rst sight, the quest for improved materials had never been so intense.

In these almost 20 years of life of the lithium-ion battery, we have witnessed continuous progress in intercalation mate-rials, [ 6 , 7 ] and alternatives to LiCoO 2 , such as LiNi 1 − y − z Mn y Co z O 2 , LiFePO 4 and Li 4 Ti 5 O 12 , have reached the market at different levels, bringing about incremental improvements in perform-ance. Nonetheless, all these materials have intrinsic limita-tions in terms of capacity, which are derived from their redox mechanism of operation and structural aspects. Indeed, the reversible intercalation of lithium ions, which does not induce major structural changes, is mostly limited by the changes the crystal structure is able to withstand, and to the intrinsic lim-ited redox activity (i.e., number of exchanged electrons) of the transition metals. There is general consensus that such limita-tion handicaps the device in terms of energy density, so that breakthroughs in performance will only come from the devel-opment of novel concepts in materials research.

Metals and semimetals that can electrochemically form alloys with lithium have been investigated as high capacity electrode materials for several years now. [ 8 ] The capacities that can be

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Jordi Cabana is a research scientist at Lawrence Berkeley National Laboratory (USA). Prior to joining LBNL in 2008, he completed his Ph.D. in Materials Science at the Institut de Ciència de Materials de Barcelona (Spain) in 2004, and worked in Prof. Clare P. Grey’s group at SUNY-Stony Brook (USA) as a postdoctoral associate.

His research aims at improving the power and energy density of lithium-ion battery electrodes, both by exploring new compounds and optimized microstructures, and through the study of the mechanisms that govern their performance.

M. Rosa Palacín has been a staff researcher at the Solid State Chemistry Department of the Institut de Ciència de Materials de Barcelona (Spain) since 1999. After receiving her B.Sc. (1991, Hons) and her Ph.D. (1995, Hons) in Chemistry from the Universitat Autònoma de Barcelona, she worked as a postdoc in Prof. Jean-Marie

Tarascon’s group (LRCS, France). She is actively involved in the ALISTORE-ERI. Her research is focused on elec-trode materials for both lithium- and non-lithium-based batteries, placing emphasis on tailoring structure and microstructure to maximise performance.

achieved from these alloying reactions can reach extremely high values, both by weight and by volume (e.g., 8365 mAh cm − 3 and 3590 mAh g − 1 for silicon compared to 975 mAh cm − 3 and 372 mAh g − 1 for graphite). However, the practical utilization of these reactions has been severely handicapped by the huge volume changes associated to the (de)alloying process, which result in the introduction of large strains in the particles of active mate-rial and, in general, in the composite electrode. Upon cycling, such changes lead to a progressive decohesion, particle shuf-fl ing, and, subsequently, to a capacity loss. [ 9 , 10 ] The strategies used to circumvent these issues have recently been reviewed; [ 11 ] they always entail limiting the effects of these volume changes, mainly through the modifi cation of the content ratios of active material, conductive additive and polymeric binder in the elec-trode formulation, which, nonetheless, typically reduce the fi nal electrode capacities (including all components). However, the efforts in this direction proved to be very successful, and batteries containing composite negative electrodes based on Sn became a commercial reality. [ 12 ]

The advent of the 21 st century brought interest onto a new reactivity concept with the reversible electrochemical reaction of lithium with transition metal oxides, [ 13 ] according to what is conventionally referred to as “conversion reaction,” generalized as follows:

MaXb + (b · n) Li ↔ aM + bLinX (1) where M = transition metal, X = anion, and n = formal oxida-tion state of X.

Conversions reactions had already been reported for some oxides and sulfi des, [ 14–18 ] and even different degrees of revers-ibility had been observed, for instance, in high temperature cells using molten salts as electrolytes. However, they were con-sidered detrimental for the correct operation of Li-ion batteries due to their supposed irreversibility at room temperature, as opposed to the excellent results obtained with intercalation elec-trodes. As a consequence, many transition metal compounds that do not have any vacant sites in the structure were disre-garded because of the impossibility of intercalating lithium. It is not until it was proved that several oxides can deliver stable gravimetric capacities as much as three times that of carbon that these phases started to be considered as promising alterna-tives in rechargeable batteries. Since then, the reports of revers-ible conversion reactions in binary M–X compounds with X = O, N, F, S, P, and even H have boomed and the “conversion reaction” concept has now become a strategy that is the focus of a large number of peer-reviewed articles every year.

The key to the reversibility of the conversion reaction seems to lie in the formation, upon complete reduction of the metal, of nanoparticles that, owing to the large amount of interfa-cial surface, are very active toward the decomposition of the matrix of the lithium binary compound (Li n X) in which they are embedded when a reverse polarization is applied. The nanometric character of the metal particles has shown to be maintained even after several reduction-oxidation cycles. [ 19 ] The footprint of the reduction process is a representative voltage plateau with length typically equivalent to the amount of electrons required to fully reduce the compound (Figure 1 ). It is worth emphasizing that in some cases, such as X = P,

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the redox centres are not exclusively located on the transition metal, but electron transfer occurs also into bands that have a strong anion contribution. Obviously, this phenomenon will be directly correlated with the covalence of the M–X bond. As will be shown in this overview, the actual potential at which conver-sion occurs has been shown to depend on both the transition metal and the anionic species ( Table 1 ), so that, in principle, the reaction potential can easily be tuned to the application requirements. [ 20 ]

This Progress Report aims at casting light on the several issues that keep compounds that electrochemically react with lithium through a conversion reaction far from commercial application. Among those, the most relevant are i) the strong structural re-organization that takes place to accommodate the chemical changes induces large volume changes that, as in the case of alloys, result in particle decohesion and unsat-isfactory cycling performance, ii) an unacceptable, in terms of round-trip energy density loss, large voltage hysteresis that is observed between the discharge and charge steps, iii) a virtually

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Figure 1 . Typical voltage vs . composition profi le of the fi rst two and half cycles for an electrode containing a material that reacts through a conversion reaction, measured against a Li counter-electrode. The different processes that occur during this complex reaction are indicated at the appropriate voltages (see text for details). The light grey, dark grey and black balls depict X, Li and M, respectively. The scheme of the interfacial storage mechanism is reproduced with permission. [ 38 ] Copyright 2008, Elsevier.

ubiquitous large Coulombic ineffi ciency observed in the fi rst cycle. Hence, despite the promise of this concept and the progress that has been made since it was fi rst proposed, several fundamental obstacles still lie in the way to making these mate-rials a viable alternative. Our aim in this article is to provide an overview of this scientifi cally appealing fi eld. The perform-ance of the different compounds tested up to date and how it can be improved using materials engineering strategies will be comprehensively described fi rst. Thereafter, a discussion of the fundamental and complex topics that stand behind the simple “conversion reaction” picture will be discussed, with the hopes of identifying directions of research necessary to bring this family of phases closer to commercial implementation.

For the most part, the results presented here correspond to tests performed in cells using lithium metal as the nega-tive electrode and the phase of interest as positive electrode, a typical procedure to evaluate performance in the Li-ion bat-tery community. Therefore, the reduction/conversion reaction will always occur on battery discharge, while the reverse will be induced on charge. Nonetheless, some results will also be mentioned for full Li-ion cells with commercial positive elec-trodes such as LiCoO 2 or LiMn 2 O 4 . Following the convention in the lithium battery fi eld, the potential values in the text will always be referred to the Li + /Li ° redox couple, E ° (vs . H + /H 2 ) = − 3.04 V. [ 21 ] The thermodynamic values of conversion potentials that will be employed throughout the text are taken from calcu-lations in the literature [ 22 , 23 ] and correspond exclusively to the phases in their bulk (as opposed to nanoscale) and crystallized state.

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2. Characteristic Performance of Compounds Undergoing Conversion Reactions

In this section, a discussion of the perform-ance of the different binary transition metal compounds that have been shown to revers-ibly react with lithium in a battery through a conversion reaction will be done. Although related conversion reactions have been shown in the fi rst reduction for compounds with main element metals and semi-metals, the very large capacities observed are also due to the formation of Li x A (A = In, Si, Ge, Sn, Pb, Sb, Bi) [ 24–32 ] alloy phases, which result in additional issues vis-à-vis their per-formance. [ 11 ] Here, we will thus concentrate in compounds of transition metals that do not alloy with lithium. Apart from identifying the phases that have already been reported to show interesting properties, separated into categories based on the corresponding anion, the strategies used to obtain good perform-ance upon cycling will be described.

2.1. Transition Metal Oxides

Transition metal oxides are, by far, the family of compounds that react through conver-

sion reactions deserving the most attention. As a consequence, most of the discussion in the later sections will be based on data obtained for these compounds. Conversion reactions have not been observed for metals in groups 4 and 5, and, therefore, these oxides will not be discussed here.

2.1.1. Chromium

Cr 2 O 3 has been the object of several studies to evaluate its applicability as a negative electrode mainly because of its very high theoretical capacity (1058 mAh g − 1 , Figure 2 ) obtained at the lowest voltage (ca. 0.2 V, Table 1 ) among the transition metal oxides for which performance has been reported to date. In practice, values of capacity that easily exceed the theoretical can be obtained in the fi rst charge, and full reduction of chro-mium has experimentally been observed. [ 33–37 ] Unfortunately, the Coulombic effi ciency in this fi rst cycle is low, the best 1 st charge values available being well below 900 mAh g − 1 . Some authors have provided evidence that this limitation could be, at least in part, stemmed in the irreversible conversion of Cr 2 O 3 , with CrO being the phase recovered instead after a fi rst charge to 3 V. [ 36 ]

The cycling performance of powder-based electrodes is gen-erally poor unless a carbon coating is applied, [ 33 , 35 ] which comes with a toll in terms of volumetric specifi c capacity. Another approach that has been effective in improving the cycling behaviour of Cr 2 O 3 is the use of thin fi lms, [ 34 ] thanks to the good and extensive contact between the active particles and the conductive metal substrate. However, the strong limitations

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Table 1. Experimental values of potential for the plateaus associated with conversion reactions in binary transition metal compounds, M a X b . Rel-evant references are provided. See text for details.

X = O X = S X = N X = P X = F

Phase E conv [V] [a] Phase E conv [V] Phase E conv [V] Phase E conv [V] Phase E conv [V]

M = Ti TiF 3 0.95 [258]

M = V VF 3 0.4 [258]

M = Cr Cr 2 O 3 0.2 [33] CrS 0.85 [192] CrN 0.2 [225] CrF 3 1.8[c] [259]

M = Mn MnO 2 0.4 [38] MnS 0.7 [165] MnP 4 0.2[d] [231]

Mn 2 O 3 0.3 [42]

MnO 0.2 [46]

M = Fe Fe 2 O 3 0.8 [304] FeS 2 1.5 [173] FeP 2 0.3 [241] FeF 3 2.0 [c] [260]

Fe 3 O 4 0.8 [67] FeS 1.3 [178] Fe 3 N 0.7 [224] FeP 0.1 [241]

FeO 0.75 [13]

M = Co Co 3 O 4 1.1 [87] CoS 2 1.65–1.3 [b] [187] CoN 0.8 [221] CoP 3 0.3 [244] CoF 2 2.2 [c] [261]

CoO 0.8 [87] Co 0.92 S 1.4 [192] Co 3 N 1.0 [224]

Co 9 S 8 1.1 [189]

M = Ni NiO 0.6 [13] NiS 2 1.6 [194] Ni 3 N 0.6 [223] NiP 3 0.7 [250] NiF 2 1.9 [c] [261]

NiS 1.5 [196] NiP 2 0.5-0.3 [b] [233]

Ni 3 S 2 1.4 [199] Ni 3 P Slope [249]

M = Cu CuO 1.4 [125] CuS 2.0-1.7 [b] [209] Cu 3 N [229] CuP 2 0.7 [251] CuF 2 3.0 [c] [270]

Cu 2 O 1.4 [129] Cu 2 S 1.7 [208] Cu 3 P 0.8 [252]

M = Mo MoO 3 0.45 [148] MoS 2 0.6 [214]

MoO 2 Slope [151]

M = W WS 2 0.8−0.6 [b] [218]

M = Ru RuO 2 0.9 [155]

[a] “Slope” indicates the absence of distinct voltage plateau in the electrochemical profi le. [b] Two plateaus were observed in the electrochemical profi le. [c] Data collected for nanocomposites with carbon at 70 ° C. [d] Conversion of Li 7 MnP 4 to Li 3 P and Mn.

in practical thickness of the fi lm (performance worsening was observed for fi lms thicker than 175 nm) result again in a severe volumetric specifi c capacity penalty, despite the absence of carbon additives in the electrode. Further enhancements were sought after by applying concepts that have long been known in the fi eld of metallurgy; [ 34 ] a layer of a chromium-based oxide (generally, mixtures of Cr 2 O 3 and Mn x Fe y Cr 3− x − y O 4 ) [ 36 ] was formed on the surface of stainless steel disks by oxidation at high temperature using O 2 impurities (ca. 5 ppm) present in a fl ow of N 2 /H 2 gas (90:10 v/v). The result of this treatment is an integrated interface between the active material and the current collector; the thickness of the oxide layer can now be up to 500 nm due to a suitable porosity that allows for good impregnation by the electrolyte while keeping electrical con-tact. The surface area was successfully tuned by employing a suitable chemical pre-treatment of the steel using acidic solu-tions. Outstanding capacity retention was observed when several hundreds of cycles were carried out using such integrated elec-trodes, even for the thicker ones. In the absence of knowledge on the active oxide content, the specifi c capacity values, which were shown to increase with the thickness of the layer, were given in mAh cm − 2 , and, hence, cannot be directly compared to those observed in other confi gurations. However, following calculations using a simple 1D model, it is suggested that, by


optimizing the preparation procedure in order to attain suitable porosity, electrodes could be designed that outperform conven-tional carbon electrodes in terms of gravimetric capacity.

2.1.2. Manganese

Even though, in theory, the full reduction of MnO 2 to man-ganese metal holds the promise of the highest capacity (1233 mAh g − 1 , Figure 2 ) among the binary oxides studied here, the feasibility of a conversion reaction in manganese oxides has not attracted much attention, especially when compared to chromium, cobalt or copper, likely because manganese is among the most diffi cult 1 st row transition metals to reduce into a metallic state. [ 21 ]

The data available on the electrochemical activity of γ -MnO 2 show that large capacities, well above 1000 mAh g − 1 and, in some cases, close to 2000 mAh g − 1 , can be obtained upon the fi rst reduction by lithium. [ 39 , 40 ] Further, the long voltage plateau observed around 0.4 V, following a sloping region commonly expected from the lithium insertion in MnO 2 , [ 41 ] is very similar to the typical signatures of conver-sion reactions. Similar electrochemical signatures, with fi rst reduction capacities that can be higher than 1000 mAh g − 1 , have been reported for other manganese oxides, such as


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Figure 2 . Theoretical (black bars), fi rst discharge (dark grey), and charge (light grey) specifi c gravimetric capacities of different compounds that react with lithium through a conversion reaction. The experimental capac-ities are taken from a series of reports for each compound. The “error” bars are provided as an indication of the dispersion of values observed in the bibliography and, thus, have no statistical meaning. Data for com-pounds with no bar have been taken from a single literature source.

Mn 2 O 3 , [ 42 , 43 ] Mn 3 O 4 , [ 43–45 ] and MnO. [ 46–48 ] In general, rather low Coulombic effi ciencies in the fi rst cycle and rapid capacity decays with cycling were reported, the best results showing capacities of 600 mAh g − 1 after 100 cycles for a γ -MnO 2 fi lm deposited on nickel metal [ 39 ] and after 150 cycles for MnO powder. [ 48 ] Notwithstanding these disappointing results, the scarcity of systematic efforts to optimize the performance of electrodes using manganese oxides by combining materials synthesis, processing and engineering indicate that improve-ments could be achieved.

Despite the hints provided by these electrochemical signa-tures, little evidence on the reaction mechanism can be found in the literature. Nonetheless, it is interesting to note that con-fl icting conclusions have been reached by different authors. On one hand, both X-ray diffraction (XRD) patterns acquired after the reduction of MnO in a molten salt cell at 400 ° C [ 14 ] and Li 1s and Mn 2p X-ray photoelectron spectroscopy (XPS) results on γ -MnO 2 electrodes [ 39 ] have shown to be consistent with the presence of Mn and Li 2 O. In addition, electrochemical reduc-tion of Mn 2 + to Mn has been observed for Mn 0.6 Mo 0.8 V 1.2 O 6 by X-ray absorption spectroscopy (XAS). [ 49 ] On the other hand, analysis of electron energy loss spectroscopy (EELS) data on β -MnO 2 electrodes, coupled with electron diffraction (ED), suggest that manganese is indeed not completely reduced and that LiMn 3 O 4 is formed upon discharge down to 0 V. [ 50 , 51 ] Since the formation of such oxide cannot explain the huge capacity of 1600 mAh g − 1 observed for this process, an alternative mechanism of interfacial storage, which is overviewed below, is speculated. Considering that long voltage plateaus are observed for both γ - and β -MnO 2 (albeit separated by 100 mV), it is not likely that the difference in manganese dioxide polytype will have a fundamental effect. Further work is needed to clarify this reaction mechanism.

2.1.3. Iron

The fi rst observations of conversion reactions in iron oxide elec-trodes date back to the 1980s. XRD patterns of samples recov-ered from high temperature cells (400 ° C) using molten salt electrolytes already revealed that the decomposition of Fe 2 O 3 to yield Fe and Li 2 O was somewhat reversible. [ 14 , 15 ] The large capacity resulting from reducing Fe 2 O 3 was later found to be partially reversible also at room temperature. [ 52 , 53 ] This observa-tion prompted several authors to use the product of the reduc-tion of Fe 2 O 3 as the negative electrode in a Li-ion battery against compounds such as V 2 O 5 or TiS 2 , with fair results. [ 52–55 ] Despite the vast majority of the studies being devoted to the electro-chemical activity of α -Fe 2 O 3 , γ -Fe 2 O 3 was found to behave much the same way. [ 52 ]

The very large capacities associated with the conversion of Fe 2 O 3 (1007 mAh g − 1 , Figure 2 ), coupled with its low toxicity and cost, certainly make it an attractive candidate. As a result, many papers can be found today in which the electrochemical activity is evaluated for samples prepared by numerous synthetic methods, both in powder [ 56–64 ] and fi lm [ 65 , 66 ] form, and using liquid and solid [ 67 ] electrolyte setups. The very large capacities (above 900 mAh g − 1 ) sustained for even up to 50–100 cycles that have been reported to date by several groups [ 58,59 , 61 ] hold the promise of the applicability of this compound.

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Fe 3 O 4 was also the object of research in the fi rst reports of conversion reactions in metal oxides. [ 15 ] Although its behav-iour received much less attention than Fe 2 O 3 , the remarkable performance, both in terms of cycle life (900 mAh g − 1 after 50 cycles) and rate capability, observed for nano-architectured elec-trodes made of Fe 3 O 4 deposited onto nanostructured copper current collectors [ 68 ] constitutes proof that this compound should also be taken into consideration. Other studies suggest that nano-engineered powder composite electrodes with Fe 3 O 4 and carbon or metallic iron could also be of interest, [ 69–77 ] espe-cially in terms of rate capability, although efforts to prove long term retention upon cycling are still needed.

Finally, a reversible conversion reaction has also been dem-onstrated for FeO, albeit with a gradual loss upon cycling. [ 13 ]

2.1.4. Cobalt

Cobalt oxides (CoO and Co 3 O 4 ) are among the most explored transition metal oxides for their application as negative elec-trodes in Li-ion batteries. As in the case of Fe 2 O 3 and Fe 3 O 4 , the conversion reaction to form metallic Co and Li 2 O had already been observed in earlier studies, [ 78 , 79 ] but it was not until the report of the high degree of cycleability of the conversion reac-tion in 2000 [ 13 ] that intensive efforts to evaluate the perform-ance of these phases were triggered.

Several authors have now shown that CoO can effi ciently be re-formed after one or more full discharge-charge cycles between 0 and 3 V, albeit in a nanometric state whatever the microstructure of the initial samples. [ 13 , 80–83 ] Outstanding capacities of 800 mAh g − 1 or more after 100 cycles have been reported by several groups for samples prepared with different particle sizes and shapes. [ 13 , 84–86 ] Particularly signifi cant is the observation that this phase can be cycled up to 250 times at high rates with very good capacity retention and values (400–600 mAh g − 1 ). [ 19 ] However, as more cycles are performed, the experimental evidence suggests that an increasing amount of capacity is not associated to a reversible conversion reac-tion, but to a side process that involves the decomposition of the electrolyte. As a matter of fact, the magnetic susceptibility of samples recovered after long cycling are the same, and consistent with the sole presence of cobalt metal, in both the charged (oxidized) and discharged (reduced) state. [ 87 ] These side reactions with the electrolyte are commonly concomitant to the electrochemical reduction process and will be further discussed in the forthcoming section.

Despite the initial interest in CoO, Co 3 O 4 has recently attracted far more attention because of its higher theoretical capacity (715 vs. 890 mAh g − 1 , Figure 2 ), with materials chem-ists exploiting the full potential of the advanced synthetic tools available nowadays. The performance has now been evalu-ated for Co 3 O 4 prepared in powder form using a breadth of synthesis methods such as decomposition of precursors, [ 88–96 ] solvothermal, [ 97 , 98 ] precipitation, [ 99 , 100 ] growth within hard templates, [ 101 , 102 ] inverse microemulsions, [ 101 , 103 ] high energy mechanical milling (HEMM), [ 101 ] spray pyrolysis, [ 104 ] electros-pinning, [ 105 ] citrate-gel [ 106 ] and even through bio-assisted textu-ration using genetically modifi ed viruses as templates, [ 107 ] or in thin fi lm form made by pulsed laser deposition (PLD), [ 108 ] radio frequency (RF) sputtering, [ 109 ] electrodeposition [ 110 , 111 ]


and precursor decomposition. [ 112 ] Although excellent capaci-ties in excess of 800 mAh g − 1 have been observed after sev-eral cycles in some cases, [ 88 , 95 , 100 , 102 , 107 ] and the retention was shown to improve when the compound is used as part of a composite with carbon, [ 94 , 104 , 113 ] the majority of the manu-scripts available in the literature report values that are similar to those already highlighted for CoO, i.e. around or below 800 mAh g − 1 . The reason for this similarity most probably lies in the partial irreversibility of the conversion of Co 3 O 4 . Depending on the crystallite size and surface area of Co 3 O 4 , and the applied current, the electrochemical reaction with the fi rst 1 or 2 moles of lithium can proceed through insertion to form Li x Co 3 O 4 (small domains and/or low current densities) or through the conversion to Li 2 O and CoO (large domains and/or high current densities), [ 89 , 99 ] which, upon further reduction, indistinctively decompose into Co and Li 2 O. [ 102 , 108 , 112 , 114 ] How-ever, a unique path is generally observed upon oxidation of the nanometric composite, generally appearing to lead to the par-tial oxidation of cobalt to form CoO, which becomes the active material upon further cycling, instead of Co 3 O 4 . [ 99 , 102 , 112 , 114,115 ] In light of these observations, and in order to avoid unde-sired battery ineffi ciencies, one may be tempted to conclude that future efforts directed to improve the performance of CoO may be more practically sound. However, the recent observations of the growth of Co 3 O 4 upon cycling CoO/Li 2 O composite thin fi lms, supported by subsequent increase of capacity, [ 85 ] indicate that full re-conversion of the cobalt spinel might be achieved by carefully identifying the mechanisms of this ineffi ciency.

It is worth noting that the promising performance of nega-tive electrodes based on these two cobalt oxides has warranted their evaluation in Li-ion cells with a commercially viable posi-tive electrode. [ 116 ]

2.1.5. Nickel

Some reports are available in the literature of the reversible electrochemical conversion of NiO into Ni and Li 2 O. [ 117 , 118 ] Close-to-theoretical capacity (718 mAh g − 1 ) values have been observed after several tens of cycles for samples in the form of powder, [ 119 , 120 ] and of thin fi lms on both fl at [ 117,118 , 121 ] and mesh [ 120 , 122–124 ] substrates. In the case of the latter, the reticular design of the electrode leads to outstanding high rate perform-ance, with capacities of 700 mAh g − 1 reported after 20 cycles at rates as high as 10 A g − 1 . [ 122 ]

2.1.6. Copper

CuO was already well known to the lithium battery community in the 1980s as an attractive electrode for primary cells, [ 125 ] and several studies are available in the literature of the optimiza-tion of the performance of these devices. [ 126–128 ] Both CuO and Cu 2 O have been proved to transform into Cu nanoparticles and Li 2 O when reduced by lithium, [ 129–132 ] and the latter is generally observed as an intermediate both upon discharge and charge of a battery containing the former. [ 131 , 133,134 ] As a matter of fact, certain cycling ineffi ciencies have been observed for CuO, which result in the presence of Cu 2 O after a full discharge-charge cycle. [ 131 ] Such ineffi ciencies could be at the origin


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of the capacity after one cycle being, in some cases, closer to that of Cu 2 O (375 mAh g − 1 ) than to CuO (674 mAh g − 1 , Figure 2 ). [ 130 , 135–137 ] Similar to what has been described for cobalt oxides, capacities that are close to theoretical have indeed been obtained by several authors after several tens of cycles using Cu 2 O instead. [ 13 , 130 , 138–141 ] Nonetheless, it must be pointed out that cycling capacities around 600 mAh g − 1 have been reported for CuO electrodes in a handful of cases (albeit often using thin fi lm electrodes), [ 133,134 , 142–146 ] thereby implying that full utiliza-tion is possible. Understanding the origin of the ineffi ciencies coupled with the careful design of the electrodes can certainly be a successful strategy to achieve the oxidation of copper to its higher state. For instance, in situ formation of CuO has been reported upon cycling of Li 2 O/Cu 2 O composite fi lms. [ 147 ]

2.1.7. Metals from the 2 nd Transition Metal Row

Although most of literature deals with low formula weight fi rst row transition metal oxides, it is worth noting that a compound such as MoO 3 has a theoretical capacity value (1117 mAh g − 1 , Figure 2 ) that is only surpassed by MnO 2 . Indeed, the few reports available in the literature [ 148–151 ] indicate that such capac-ities can be obtained upon the fi rst discharge of MoO 3 , and that values of more than 1000 mAh g − 1 can be sustained upon sev-eral discharge/charge cycles. Another 2 nd row transition metal oxide that has deserved some attention is MoO 2 . Capacities well above its theoretical value (838 mAh g − 1 ) have been reported in the fi rst discharge, [ 152 ] and stability around 700 mAh g − 1 has been reported for several cycles using carefully designed electrodes showing mesoporosity. [ 153 , 154 ] Another strategy to boost the performance of MoO 2 consists in a fi rst thermally activated cycle during which the reduction-oxidation is per-formed at 120 ° C; such activated electrodes show capacities of 800 mAh g − 1 after 30 cycles. [ 155 ] Reduction to metallic Mo and re-oxidation to MoO 2 has been proposed based on XPS and Raman spectroscopy data. [ 155 ]

RuO 2 has a theoretical capacity of 806 mAh g − 1 , which has warranted its study as lithium battery electrode. Unfortunately, despite the very large fi rst discharge and charge capacities, drastic fading after the fi rst three cycles was found. [ 156 ] Since the only attempts reported so far were done using an non-optimized electrode mixture with no carbon, opportunities for improvement could be envisaged by tailoring the sample micro-structure or the electrode mixture composition.

2.2. Transition Metal Sulfi des

Although transition metal sulfi des were initially used as lithium primary cell materials, [ 157 , 158 ] layered phases such as TiS 2 were long considered as candidates as positive electrode materials for rechargeable lithium batteries [ 4 ] and high temperature cells based on iron sulfi de electrodes were considered as a possible energy storage option for the electric vehicle in the 1980’s. [ 18 ] On the other hand, the technical feasibility of the electrochem-ical reaction of alkali metals with sulfur is well-known; high-temperature sodium-sulfur batteries [ 159 ] are currently in use for certain stationary storage applications, mainly in Japan. In turn, a battery based on the lithium/elemental sulfur redox

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couples has a theoretical specifi c capacity of 1675 mAh g − 1 of active material and a theoretical specifi c energy 2500 Wh kg − 1 , assuming complete reaction to form Li 2 S, which triggered efforts to utilize it both in non-aqueous [ 160 , 161 ] and polymer [ 162 ] batteries. Unfortunately, what may in practice look like a simple reaction is complicated by the formation of a variety of lithium polysulfi de intermediates, which are partially soluble in the electrolyte, reducing its conductivity, and can eventually shuttle to the negative electrode where they react with lithium and pas-sivate it, all this, among other reasons, [ 163 ] resulting in severe ineffi ciencies. [ 164 , 165 ] Nonetheless, in view of the reactivity of sulfur with lithium, it is not surprising that several transition metal sulfi des have been reported to react following a conver-sion reaction to produce Li 2 S and metal nanoparticles. As described herein, these sulfi de share characteristics and issues as electrodes with elemental sulfur.

2.2.1. Manganese

The studies on manganese sulfi de as an electrode in a Li battery are rare in the literature. A convenient hydrothermal synthetic route has been successfully developed to prepare stable rock-salt-type structure α -MnS submicrocrystals, [ 166 ] which show ini-tial discharge capacities of 900–1300 mAh g − 1 depending on the synthesis temperature, that decrease to 400–600 mAh g − 1 after 20 cycles. Although the authors proposed a conversion reaction mechanism, no proof of it was provided.

2.2.2. Iron

Both FeS 2 and FeS are well known within the lithium battery fi eld. As already stated, they were the object of very intensive research in the 1980’s and early 1990’s because of their attrac-tive performance as positive electrodes in high temperature (375–500 ° C) batteries with molten salt electrolytes to be used to power electric vehicles. [ 17 , 18 ] The performance in lithium cells at lower temperatures, using composite polymer (CPE, 120–135 ° C), [ 167 , 168 ] polymer (90 ° C), [ 169 ] hybrid and polymer gel (HPE and GPE, 70 ° C) [ 170 ] or liquid electrolytes (room temperature) [ 171 ] was also evaluated. As a result of these efforts, the reduction of these sulfi des with lithium to form Li 2 S and Fe has thoroughly been characterized. In the case of FeS 2 , this con-version is preceded by an intermediate step of insertion form Li 2 + x Fe 1−x S 2 phases, [ 167 , 171–175 ] and its reversibility was found to be very dependent on the temperature; [ 171 , 175 ] re-formation of FeS 2 was reported to occur only in molten salt cells. Otherwise, mixtures of pyrrhotite (FeS y , 1 ≤ y ≤ 1.25) and elemental sulfur are obtained upon charge. The poor cycleability of sulfur (vide supra) is ultimately thought to severely handicap the capacity retention of FeS 2 electrodes. In practice, the limited revers-ibility at close-to-room temperature fi nally led to a progressive fading in the interest of iron sulfi des as electrodes in secondary lithium batteries in favour of other emerging technologies. Nonetheless, the Li/FeS 2 couple still made it to the market; it is nowadays commercialized in primary AA cells or used in reserve batteries, [ 176 ] mainly for defence uses.

The recent surge in the quest for phases that can react through conversion reactions has brought certain renewed interest in the room temperature behaviour of iron sulfi des

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as electrode materials. High initial capacities above 500 and 600 mAh g − 1 for FeS and FeS 2 , respectively, but poor revers-ibility were observed for both FeS 2 and FeS in liquid electro-lyte cells. [ 177–180 ] In contrast, notable cycling performance was reported for all-solid state cells in which the sulfi des are cycled against LiCoO 2 using glass-type electrolytes, [ 181 , 182 ] suggesting that the interaction between the organic solvents used in liquid cells and the highly reactive Li 2 S/Fe composites may be a major issue. The notably better performance also reported when polymer electrolytes are used instead, albeit at higher tempera-ture, [ 167 , 183 ] also hints at the critical impact of electrolyte choice in the cycling of iron sulfi de electrodes.

2.2.3. Cobalt

CoS 2 was once viewed as a potential alternative to FeS 2 or FeS in high temperature batteries for ground transportation [ 184 ] and a material of choice in thermally activated lithium bat-teries. [ 185–187 ] Its performance in room temperature, liquid electrolyte rechargeable batteries has only recently been evalu-ated. [ 188 , 189 ] Poor reversibility has been observed, even with considerable amounts of carbon ( > 25%) in the electrode mix-ture; the capacities obtained after a few tens of cycles (around 400 mAh g − 1 ) are well below the theoretical value (870 mAh g − 1 ). Nonetheless, such capacities would still be comparable to those of the graphite electrode (theoretical: 372 mAh g − 1 ).

Another phase in the Co-S phase diagram that has deserved some attention is Co 9 S 8 . When made using a traditional solid state method based on annealing of cobalt and sulfur in evac-uated tubes, this compound is electrochemically active and undergoes a conversion reaction to Co and Li 2 S, but it suffers severe and swift capacity losses upon cycling. [ 190 , 191 ] In contrast, a noticeable improvement in the cycling behaviour has been found when this phase is synthesized in an amorphous state by co-precipitation; the resulting capacities after 20 cycles are in excess of 500 mAh g − 1 , very close to the theoretical value (545 mAh g − 1 ). [ 191 ] Even larger capacities are observed for thin fi lms of Co 9 S 8 made by electrodeposition, [ 192 ] but only when impractically thin electrodes are employed.

Finally, an electrochemical conversion reaction with lithium has been reported for Co 0.92 S made from elemental cobalt and sulfur at high temperature. [ 193 ] Preliminary cycling experiments show that, as in the case of Co 9 S 8 , thus made samples have poor cycling properties, with rapid decay within the fi rst 10 cycles.

2.2.4. Nickel

While they are also considered as potential candidates in thermally activated batteries [ 187 ] and were at one point evalu-ated for use in primary batteries, [ 194 ] NiS 2 and NiS have not attracted as much attention as FeS 2 or CoS 2 for these applications. In general, the cycling properties of NiS 2 , [ 195 ] NiS, [ 190 , 196–199 ] and Ni 3 S 2 , [ 200–202 ] using a variety of electrolyte confi gurations (lithium salts in carbonate-based solvents, ionic liquids, polymer-based or inorganic glasses) have proved to be defi cient. The losses observed are, at least partly, due to the inef-fi ciency of the re-conversion reaction from Ni and Li 2 S, [ 195 ] as it has been shown for NiS by the presence of Ni 3 S 2 at the end


of the fi rst charge, [ 190 , 199 ] and, once again, to sulfur solubility in the electrolyte. [ 203 ] The most remarkable cycle life results have been reported for NiS in batteries operating at 80 ° C using PEO-based polymer electrolytes, which yield a promising 540 mAh g − 1 (close to the theoretical value of 591 mAh g − 1 ) after 100 cycles. [ 203 ] This observation suggests that the choice of electrolyte may be as critical in view of the performance optimization of metal sulfi de electrodes as it has already been proposed to be for lithium/sulfur batteries. [ 204–206 ] Another complementary path toward better electrochemical properties could be the use of nanoarchitectured electrodes. This concept has been proved for Ni 3 S 2 grown on Ni foam by means of a hydrothermal process; outstanding capacities of 400 mAh g − 1 , only about 50 mAh g − 1 lower than the theoretical value, have been reported after 20 cycles at high rates. [ 207 ]

2.2.5. Copper

CuS is also one of the several metal sulfi des attractive as posi-tive electrodes in lithium primary batteries. [ 158 , 208 ] Its overall conversion reaction to Cu and Li 2 S proceeds through an inter-mediate step that entails the formation of Cu 2−x S and Li 2 S and has been shown to be rather reversible. [ 190 , 209,10 ] The perform-ance of this sulfi de in a rechargeable battery with liquid elec-trolytes has been evaluated by a handful of authors [ 190 , 196 , 210,211 ] ; despite the reversibility of the reaction in the fi rst cycle, severe capacity losses are observed very early on in the life of the bat-tery due to irreversible sulfur dissolution in the electrolyte upon cycling. [ 210 ] On the contrary, sustained capacities of 350 mAh g − 1 are obtained after 60 cycles at 90 ° C if the electrolyte is a com-posite of a PEO-based polymer blended with SiO 2 fi ller. Mix-tures of CuS and S have also been used with glass ceramic electrolytes, yielding capacities of almost 700 mAh g − 1 (defi ned against the total weight of copper and sulfur in the electrode) after 20 cycles. [ 212 ] These observations are in line with the trend defi ned by the reported performance of iron and nickel sulfi des in the previous sections, providing a rather clear hint that liquid electrolytes, at least those conventionally used in Li-ion batteries, may not be a suitable choice for adequate cycling.

The electrochemical conversion of Cu 2− x S ( x = 0 and 0.24) to Li 2 S and copper has also been shown to be somewhat revers-ible. [ 209 ] As in the case of the other sulfi des reviewed so far, Cu 2 S behaves poorly, the initial capacity dropping to negligible values in less than 20 cycles. [ 213 ]

2.2.6. Molybdenum and Tungsten

The lithium intercalation chemistry of MoS 2 and WS 2 was extensively studied for their use as positive electrodes in the fi rst lithium batteries. [ 214 ] Interestingly, a result of these efforts was the discovery, back in the late 1980’s, of electrochemical conversion when more than one mole Li reacted per mole of metal sulfi de. [ 215 ] Capacities were reported to be very reversible in the fi rst charge, but no efforts to evaluate extended cycling were made until recently. A few reports of MoS 2 made following hydrothermal methods [ 216–218 ] and WS 2 prepared by reducing WS 3 with H 2 [ 219 ] or through a reaction involving rheological phases [ 220 ] have shown interesting results. Very large initial dis-charge capacities, well above the theoretical value and consistent


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with conversion reactions, are generally observed, with some variability in terms of fading upon cycling; capacities that range from 300 to 800 mAh g − 1 have been reported after 40–50 cycles. Preparation of nanocomposites with carbon have been shown to result in an improvement in capacity retention. [ 221 ]

2.3. Transition Metal Nitrides

Conversion reactions have been reported to take place for tran-sition metal nitrides when prepared as crystallized (e.g., VN, CoN, Co 3 N, Fe 3 N, Mn 4 N) or amorphous (e.g., Ni 3 N) thin fi lms a few hundred nanometres thick. [ 222–228 ] Cyclic voltammetry revealed the existence of diverse redox peaks that evolve upon cycling. Specifi c capacities of 400–500 mAh g − 1 , close to the the-oretical values for the conversion reaction, have been reported for the phases with N/M < 1 after a fi rst discharge to 0.01 V. Although un-reacted Li 3 N and metal are still observed upon charge, capacities around 350–400 mAh g − 1 are mostly main-tained upon cycling with small fl uctuations. Much larger capac-ities, in excess of 1000 mAh g − 1 in some cases, are observed for nitrides with N/M = 1, in agreement with the larger amount of nitrogen present in the structure. CrN and VN exhibit the largest capacities after the fi rst reduction (1800 mAh g − 1 and 1500 mAh g − 1 , respectively, well above the theoretical value, ca . 1200 mAh g − 1 ). Despite the absence of detectable amounts of un-reacted metal seen upon charge, the overall effi ciency is limited. Nonetheless, 1200 mAh g − 1 are obtained after the second discharge, a value that remains relatively stable upon cycling in the case of CrN. [ 226–228 ] Unfortunately, the nature of the irreversible redox processes that lead to this loss is not yet clarifi ed. Films composed of CoN exhibit fi rst discharge capaci-ties of 950 mAh g − 1 that are again only partially reversible, but 650 mAh g − 1 are still observed after 50 cycles at very high rates. Cyclic voltammetry experiments show the presence of diverse peaks that have been associated to the possible formation of ter-nary Li-M-N phases, [ 222 , 229 ] but no defi nitive experimental evi-dence has been provided so far.

The reactivity in lithium cells of electrodes using Cu 3 N powder has also been reported to entail a conversion reaction with formation of Li 3 N and Cu. [ 230 ] An uptake of 5.1 mol of Li is observed during the fi rst reduction to 0 V, equivalent to a capacity of 675 mAh g − 1 , of which only 3.2 can be removed (420 mAh g − 1 ) upon subsequent oxidation to 3 V. Good capacity retention is observed at moderately high rates. However, upon prolonged cycling additional redox features are also observed that seem to be due to the formation of copper oxides, probably associated to electrolyte degradation. This observation high-lights the complexity of the reactions taking place in nitride-based electrodes.

2.4. Transition Metal Phosphides

The reactivity of transition metal phosphides with lithium through conversion reactions to yield metallic particles and Li 3 P has only recently been discovered. This reactivity is nested on the redox character of the phosphorus centres and leads to mechanisms of reaction involving staggering uptakes of lithium

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(for instance, 7 mole Li per mole of MnP 4 ) [ 231 , 232 ] before the reduced metal starts precipitating out. Density function theory (DFT) calculations have revealed that the very covalent nature of the M-P bonds yields electronic structures around the Fermi level (i.e., the electronic states involved in the redox activity of the compounds) in which bands with a strong P(3 s ,3 p ) char-acter lie at high energy. [ 232–235 ] As a matter of fact, the attractive redox activity of phosphorus has led to efforts to investigate the performance of a Li-P battery. [ 236 ]

While no conversion has been observed for phosphides con-taining early transition metals such as Ti or V, [ 237–239 ] formation of Li 3 P and nanosized metallic particles has been found upon reduction of compounds with other fi rst row transition metals, such as Mn, Fe, Co, Ni or Cu. As reviewed hereafter, many phosphorus-rich phases have been shown to electrochemically react with lithium through conversion reactions. The very high specifi c capacities that result undoubtedly turn transition metal phosphides into a family of compounds with attractive electro-chemical properties. Taking into account the limited amount of reports on the performance of phosphides in lithium cells as opposed to oxides or sulfi des, such properties call for further efforts to evaluate the real potential of these phases.

2.4.1. Manganese

A conversion reaction to Li 3 P and Mn has been found to occur as the last step in the reduction of MnP 4 in a lithium battery, resulting in a huge specifi c capacity of more than 1400 mAh g − 1 . [ 232 ] Unfortunately, the reversibility of this process is poor, and half of the capacity is already lost during the fi rst cycle.

2.4.2. Iron

The reactivity of FeP y with y = 0.33, 0.5, 1, 2, 4 towards lithium has very recently been investigated. [ 240 ] While the iron rich phases ( y = 0.33, 0.5) did not show any redox activity, [ 241 ] direct conversion reaction to Li 3 P and Fe nanoparticles upon the fi rst discharge was demonstrated for FeP, yielding a capacity of 720 mAh g − 1 . [ 242 , 243 ] Even higher capacities in excess of 1300 mAh g − 1 have been reported for FeP 2 . [ 242 , 244 ] Finally, although large capacities of more than 1000 mAh g − 1 can be achieved during the fi rst reduction of FeP 4 , [ 240 , 242 ] preliminary characterization results suggest that this compound only reacts through an intercalation mechanism, and no conversion occurs. [ 240 ] Extended reversibility remains to be proved in all cases.

2.4.3. Cobalt

CoP 3 was the fi rst transition metal phosphide studied as an elec-trode in a lithium battery. [ 245 , 246 ] The redox conversion of this compound can lead to capacities of more than 1800 mAh g − 1 in the fi rst discharge, which were found to fade rapidly to ca . 400 mAh g − 1 . The performance of CoP has also been tested, with similar results; while the initial capacity is more than 700 mAh g − 1 , only 400 mAh g − 1 are recovered after 10 cycles. [ 247 ]

2.4.4. Nickel

As in the case of iron, several nickel phosphides, NiP y , with y = 0.33, 0.5, 2, 3, have been evaluated. The phases with y ≤ 1

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were initially found to be electrochemically inactive in powder form. [ 248 , 249 ] However, very recent results indicate that such activity rather depends on the design of the electrode; Ni 3 P electrodeposited onto Ni foam delivers 400 mAh g − 1 after 20 cycles. [ 250 ] For NiP 3 , 1475 mAh g − 1 were obtained upon the fi rst discharge, of which ca. 80% is recovered upon charge, [ 251 ] but a very fast decay within the fi rst ten cycles is found unless the voltage window is restricted, thus limiting the resulting capacity.

The case of NiP 2 can be considered paradigmatic of the direc-tions that need to be taken if transition metal phosphides are to be considered alternatives as negative electrode materials. Both the cubic and the monoclinic polymorphs convert to Li 3 P and Ni, with a intermediate step of insertion for the latter, [ 235 ] upon dis-charge to 0 V, producing a total of 1000–1300 mAh g − 1 . [ 234 ] Fair reversibility is observed upon the fi rst charge, and the capacity in powder electrodes decays noticeably within the fi rst 10 cycles, even at low rates. In an attempt to solve this issue, monoclinic NiP 2 was grown on Ni foam, which also acts as a current col-lector, through a vapour-phase procedure. [ 234 ] The strategy is rather successful; not only was the retention ameliorated, but these integrated electrodes showed remarkably enhanced rate capabilities.

2.4.5. Copper

CuP 2 can be completely reduced by lithium, yielding an initial capacity of 1325 mAh g − 1 , that fades to approx. 400 mAh g − 1 after 15 cycles. [ 252 ] No optimization efforts have been made so far with this phase. In contrast, Cu 3 P has been the object of somewhat more intensive scrutiny, and both powder [ 253–255 ] and fi lm [ 256 , 257 ] forms have now been tested. Different degrees of capacity decay upon cycling are observed for the former, the best results being reported for samples made by HEMM (ca . 300 mAh g − 1 , 80% of the theoretical capacity, after 30 cycles). [ 255 ] As for NiP 2 , both the cycle life and the rate capability can be improved signifi cantly when employing nanostructured integrated electrodes, as demonstrated for Cu 3 P nanorods grown by vapour-phase reaction onto copper foil. [ 257 ] While further improvements are certainly needed to meet the minimum standards of cycle life, the innovative work with NiP 2 and Cu 3 P shows that a lot can still be done to optimize the electrochemical performance of this family of compounds.

2.5. Transition Metal Fluorides

Transition metal fl uorides constitute a special group within the family of compounds that react with lithium through conver-sion reactions. The very high ionicity of the M-F bond results in reduction potentials to LiF and metal nanoparticles that can be around or even above 2 V (Table 1 ), which is in stark contrast with the potentials below 1.5 V observed for oxides, sulfi des, nitrides, and phosphides. This characteristic turns fl uorides into alternatives for the positive electrode with noticeably higher specifi c capacity than intercalation-based candidates, such as LiCoO 2 (280 mAh g − 1 , theoretical; 140 mAh g − 1 , practical) or LiFePO 4 (170 mAh g − 1 , theoretical).


2.5.1 Titanium, Vanadium, Chromium, Manganese

The feasibility of the insertion of lithium into the ReO 3 -type structure of TiF 3 and VF 3 to yield Li x MF 3 at 2.5 V and 2.2 V, respectively, with reversible capacity of 80 mAh g − 1 , was reported in the late 1990’s. [ 258 ] Complete reduction of these phases was not attempted until recently; [ 259 ] a plateau was observed at ca . 1 V for TiF 3 and 0.5 V for VF 3 , followed by a sloping voltage region, for a total of more than 900 mAh g − 1 . The existence of a revers-ible conversion reaction was proved using Raman spectroscopy data and, although the products at the end of discharge are amor-phous, transmission electron microscopy (TEM) patterns showed the reformation of crystalline TiF 3 upon oxidation to 3.5 V. Despite a low Coulombic effi ciency during the fi rst cycle, good capacity retention is observed, with values close to 600 mAh g − 1 and 400 mAh g − 1 after 15 cycles for Ti and V, respectively.

CrF 3 /C nanocomposites have been prepared by HEMM of CrF 2 and CF. When tested at 70 ° C, a close-to-theoretical spe-cifi c capacity of 682 mAh g − 1 , of which only 440 mAh g − 1 are reversible, was found. [ 260 ] Additionally, activity has also been reported for MnF 2 and MnF 3 when reduced down to 0.02 V, with values of 761 and 1120 mAh g − 1 , respectively, upon the fi rst reduction, but no cycling data has been provided so far. [ 23 ]

2.5.2 Iron

FeF 3 , which also crystallizes in a ReO 3 structure, is, by far, the fl uoride phase having attracted the most attention. Early studies reported the reversible insertion of 0.5 mole lithium in the structure at 3.4 V, corresponding to capacities of 140 mAh g − 1 that are only partially reversible. [ 258 ] Later on, a novel approach entailing the fabrication of C/FeF 3 nanocomposites (also con-taining some FeF 2 ) aimed at minimizing the length of the ion diffusion path (nanoscale fl uorides) while ensuring both good electronic conductivity and inter-particle contact (carbon matrix) was developed. [ 261 ] Better reversibility of the insertion process was thus achieved. Upon further reduction to 1.5 V, a plateau consistent with a conversion reaction is observed at 2 V, leading to enhanced capacities of 773 mAh g − 1 at 70 ° C. The formation of LiF and Fe was confi rmed by XRD and EELS. [ 262 , 263 ] Iron fl uo-rides have also been tested in thin fi lm form. [ 264 , 265 ] In this case, even in absence of any conducting additive, very high capacities of 1000–1500 mAh g − 1 , depending on the deposition conditions, were reported upon the fi rst reduction, yet only approximately half of it is maintained upon subsequent cycling.

FeF 3 and FeF 2 are representative examples of transition metal fl uorides that yield very high specifi c capacities at attrac-tive voltages for application as positive electrodes. One major drawback would be that conventional negative electrodes such as graphite or silicon/tin-based alloys require the positive elec-trode to act as the lithium reservoir. To address this issue, the use of LiF-Fe nanocomposites with various stoichiometries has been proposed as the starting material. It is expected that methods of preparation with control at the nanoscale, such as HEMM [ 266 ] or RF sputtering [ 267 , 268 ] will result in microstruc-tures that mimic those generated in a lithium battery, thereby leading to reverse conversion reactions and equally high capaci-ties as those obtained with FeF 3 or FeF 2 . In practice, such


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nanocomposites exhibit promising electrochemical perform-ance, yet with somewhat smaller capacity values.

2.5.3 Cobalt, Nickel, Copper

Copper and nickel fl uorides were investigated in the 1960s as positive electrode materials for primary lithium cells. [ 269 , 270 ] The performance of CuF 2 in a lithium cell has recently been reinvestigated. Two plateaus at ca. 2 V and 1.5 V and very poor reversibility were observed [ 23 ] unless CuF 2 embedded in either carbon or metal oxide mixed conducting matrices is used. Such nanocomposites enable close-to-theoretical capacities, most of which is delivered at high voltages (close to 3 V). [ 271 ] Although these fi ndings are of certain interest in view of application in primary cells, reversibility of the reactions is not discussed. The reduction reaction has been recently studied by solid state NMR and XRD [ 272 ] and found to simply consist of direct formation of Cu and LiF without intermediate phases.

Reversible conversion reactions were also reported for NiF 2 , with Li 2 NiF 4 seemingly appearing as an intermediate after partial reduction to 1 V. [ 273 ] However, disparity is found in the potential values at which it takes place: close to 2 V in NiF 2 /C nanocomposites made by HEMM (albeit cycled at 70 ° C), [ 262 ] ca . 1.5 V for electrodes prepared by conventional hand-mixing of the components, [ 23 ] and around 0.7 V in thin fi lms obtained by PLD (both at room temperature). [ 273 ] Such disparity is most probably due to the poor electronic conductivity of these very ionic compounds, which is compensated by the formation of nanocomposites with carbon. Despite the over-potential observed in the thin fi lms, interesting capacities close to 500 mAh g − 1 were reported after 35 cycles. In contrast, poor cycling was reported for the hand-mixed electrodes.

Finally, formation of LiF and Co has been revealed by XRD and XPS in thin fi lms of amorphous CoF 2 , leading to very high initial capacities of 600 mAh g − 1 . [ 274 ] Again, the potential of the conversion process seems to be very dependent on the electrode preparation conditions, with 2 V reported at 70C for nanocom-posite electrodes [ 262 ] and ca . 1 V for the thin fi lms. [ 274 ] Unfor-tunately, the only data available up to date show severe fading upon cycling to yield less than 200 mAh g − 1 after 8 cycles.

2.6. Other Phases

2.6.1. Other Simple and Polyanionic Systems

Following the promising results obtained with the simple ani-onic systems outlined so far, the focus on compounds that, a priori, can electrochemically react with lithium through conver-sion reactions has very recently been expanded to hydrides [ 275 ] oxyfl uorides [ 276 ] and oxysalts such as oxyhydroxides, [ 277 ] oxalates and carbonates. [ 47 , 278,279 ] The conversion was proved by XRD to be fully reversible for MgH 2 , and leads to huge capacities of more than 1400 mAh g − 1 , although the observed retention is poor unless capacity-constrained cycling is performed. [ 275 ] The formation of metal particles and LiH was also demonstrated for TiH 2 , NaH, LaNi 4.25 Mn 0.75 H 5 , and Mg 2 NiH 3.7 , but no data upon extended cycling was provided. One feature that makes these phases particularly exciting is the very low polarization

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(ca . 0.25 V) observed in the electrochemical profi les, which is the lowest reported to date for reversible conversion reactions (vide infra).

FeO x F 2− x /C nanocomposites (0 ≤ x ≤ 1) fully convert to Fe, LiF and an oxide phase containing both Li and Fe at ca . 2 V. [ 276 ] Their cycling stability (120 mAh g − 1 after 110 cycles) is greatly improved with respect to that of FeF 2 nanocomposites although at the expense of a lower initial discharge capacity. If the cut-off voltage is decreased from 1.5 V to 0.8 V, the Li-Fe-O phase further converts to Li 2 O and Fe. Preliminary evidence points at the re-formation of a FeO x F 2− x phase upon oxidation from these lower voltages.

The reduction of FeOOH, [ 277 ] CoC 2 O 4 [ 278 ] FeC 2 O 4 , MnCO 3 , [ 47 ] and Cd 1/3 Co 1/3 Zn 1/3 CO 3 , [ 279 ] by lithium results in very high capacities, well in excess of the theoretical value associated to a conversion reaction. In some cases, more than 1/3 of that capacity is lost after the fi rst charge, but capacity values of more than 500 mAh g − 1 are still maintained after 50 cycles, making these compounds interesting. TEM results suggest that the oxy-hydroxide polyanion decomposes into LiOH, Li 2 O, and Fe nano-particles and that only Fe 2 O 3 is formed upon charge, [ 277 ] but the carbonate structure may indeed be stable upon cycling. [ 279 ] In turn, preliminary infrared (IR) measurements on the oxalates at different points of charge and discharge indicate that the oxalate ion backbone may be maintained, thereby adding evi-dence to the formation and consumption of Li 2 C 2 O 4 . [ 278 ] How-ever, the occurrence at similar wave numbers of signals due to species resulting from electrolyte decomposition, such as alkyl-carbonates, which have actually been found at these voltages for other phases, [ 280–282 ] calls for further research to confi rm this point.

Finally, a few studies have been devoted to the reactivity of borates such as MBO 3 (M = V and Fe [ 283–285 ] ), M 3 BO 6 (M = Fe [ 283 , 286 ] and Cr [ 287 ] ), and M 3 B 2 O 6 (M = Co, Ni, and Cu) [ 288 ] as negative electrodes. Although the initial discharge capacities are close to 1000 mAh g − 1 , their response upon cycling is rather poor, with the best values (350 mAh g − 1 after 35 cycles) being reported for Fe 3 BO 6 . [ 283 ] In this case, the integrity of the borate unit upon complete reduction has been proved in the case of Cu 3 B 2 O 6 , [ 288 ] but formation of Li 3 BO 3 was shown to be concom-itant to a decomposition into Li 2 O and metallic nanoparticles, which are the only active components in further cycling.

2.6.2. Mixed Transition Metal Compounds

Several oxide systems containing two transition metals and crystallizing in a spinel structure have been studied to date, namely, MMn 2 O 4 (M = Co [ 43 ] ), MFe 2 O 4 (M = Co, [ 282 , 289,290 ] Ni, [ 290 , 291 ] Cu [ 292 ] ), and MCo 2 O 4 (M = Mn, [ 43 ] Fe, [ 282 , 293 ] Ni, [ 294 , 295 ] Cu [ 296 ] ), which show an overall performance that is similar to that of simple oxides. It is interesting to note that in all the cases reported so far the signature of their reduction is, sys-tematically, a single plateau instead of the two that could be expected from the different potentials at which the individual oxides react, suggesting that the occurrence of mixed states at the Fermi level results in simultaneous reduction to the metallic state. The actual potential of this process is domi-nated by the transition metal content; a smooth progression with x is observed in the values reported for Co 3− x M x O 4 (M =

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Mn [ 43 ] and Fe [ 282 ] ) systems, indicating that the voltages of the electrode, and, in turn, the energy density of the device, can be tuned. Upon oxidation, the mixed metal oxides are never recov-ered, the electrode mixture being always formed by the corre-sponding simple oxides. [ 282 , 291 , 293 , 296 ]

Another approach that is worth mentioning here is the inclu-sion of alkali earth elements, such as Ca, in the structure of a transition metal oxide, such as Co or Fe, to produce an inactive matrix of CaO upon the fi rst discharge. Certain improvements in the cycling properties of Ca 3 Co 4 O 9 [ 297 ] and Ca 3 Co 3 FeO 9 [ 298 ] with respect to Co 3 O 4 have been shown. Enhanced performance has also been observed for ZnM 2 O 4 (M = Co [ 299 ] and Mn [ 300 ] ) for which additional capacity is obtained from the formation of alloys between Li and Zn. Although further research is needed to confi rm that these metals are driving the improvements, these cases exemplify the richness of the strategies available to produce electrode materials that react through conversion reac-tions with desirable properties.

3. The Road Toward High Capacities upon Extended Cycling: Importance of the Conversion Mechanism

Despite the promising properties shown by the different com-pounds discussed in the previous section, their application in commercial devices is strongly handicapped by several very crit-ical ineffi ciencies, which, as we shall discuss, are deeply rooted in the reaction mechanisms. Overcoming these obstacles is an absolute requirement if this class of materials is ever to become viable, and such goal cannot be achieved without a full under-standing of their origins. Thus, the aim in this section is to decompose the multiple complexities of the conversion reac-tion and its implications, from the fi rst discharge to the failure mechanisms.

3.1. Conversion vs. Intercalation and Overpotential, Δ E conv

The electrochemical mechanism for the transformation of the pristine M a X b phase into Li n X ( n = 1–3, X = H, N, O, F, S, P) and metallic particles can entail the intermediate forma-tion of ternary Li–M–X phases before such conversion takes place (Figure 1 ). The vast majority of the phases that contain transition metals in high oxidation states and possess a some-what marked ionic character (i.e., oxides and fl uorides) elec-trochemically react by inserting lithium, thereby reducing the metal to a lower (oftentimes, its lowest) stable cationic state. Indeed, some of the compounds described in the previous section, such as MnO 2 , [ 41 ] MoO 3 , [ 301 ] MoO 2 , [ 302 ] RuO 2 , [ 303 ] or MoS 2 [ 4 ] were postulated as alternative electrode materials as far as 30 years ago precisely because of their ability to insert around 1 mole of lithium per formula unit. In turn, the redox activity of the phosphorus centres, due to the covalence of metallic phosphides, can result in extensive lithium insertion before conversion to Li 3 P and the metal is observed. [ 231,232 , 235 ] Further, copper displacement by lithium to form Li 2 CuP has been observed in Cu 3 P. [ 253,254 , 304 ] In other cases, such as CuO, [ 131 ] Cr 2 O 3 , [ 36 ] Co 3 O 4 , [ 89 ] and CuS, [ 190 ] an initial and partial


decomposition to lower binaries and Li n X has also been reported. In the case of the last two, competition between lithium insertion and partial conversion is reported. The intermediate processes can be kinetically (e.g., in Co 3 O 4 [ 89 ] or CuS, [ 190 ] NiP 3 [ 251 ] or FeP [ 242 ] ) or thermodynamically-controlled (e.g., in Fe 2 O 3 [ 305 ] ), with particle size either having an indirect effect on the current density for the former or a direct ener-getic effect for the latter.

The existence of intermediate processes indicates that direct conversion to metallic particles from some binary compounds is not energetically favoured. The fi rst step of conversion, and, to some extent, the preceding intercalation, [ 88 , 131 , 272 , 305 ] has a crucial role in the nanostructural formatting of the electrode (Figure 1 ). DFT calculations on FeF 3 suggest that the forma-tion of very small metallic nanoparticles creates a penalty in cohesive energy that makes it more favourable to completely reduce iron to the + 2 state through the formation of LiFeF 3 before Fe starts precipitating out, [ 306 ] a hypothesis that could apply to other compounds that undergo insertion prior to con-version. Quite astonishingly, despite the disintegration to form metallic particles of 1–10 nm in size dispersed a matrix of Li n X, preservation of the initial particle shape was reported in some cases. [ 13 , 131 ]

The energy penalty associated with the formation of nanoparticles is also identifi ed to be, at least in part, at the origin of the large deviations in potential ( Δ E conv ) for the conversion reactions with respect to the theoretical values that are systematically observed in Figure 3 . Note that only representative compounds for which direct conversion to metallic particles has been reported and thermodynamic data were found [ 22 , 23 ] are shown for simplicity of the qualitative analysis. First, it is interesting to note that sulfi des system-atically show higher experimental reduction potentials than oxides (Table 1 ) mainly because of the rather larger Δ E conv observed for oxides. For the same anion, there seems to be a trend, which would need to be confi rmed through system-atic studies, toward higher experimental reduction potentials when increasing Z for the metals. Interestingly, the com-pounds containing copper show the lowest values of Δ E conv , which hints at the importance of the enhanced diffusion of copper ions in the solid state. [ 307 ]

Experimental deviations from the theoretical potentials are still observed when cycling is performed at higher tempera-tures. Recent results on Cr 2 O 3 -based electrodes indicate that the voltage of the conversion plateau does not get substan-tially closer to the thermodynamic value when the temperature is increased up to 100 ° C, as would be expected if the limita-tion was stemmed only in kinetics within the electrode com-posite. [ 308 ] Galvanostatic intermittent titration technique (GITT) measurements performed for several binary oxides at 400 ° C using molten salt electrolytes reveal that the potentials at which the conversion reactions occur are still far from the thermody-namic value. [ 14,15 , 78 ] Moreover, fabrication of nanostructured conversion electrodes results in enhanced electrode kinetics and the achievement of large capacities at high cycling rates, but minor changes in Δ E conv . [ 68 , 112 , 122 ] The cristallinity of the mate-rial, [ 309 ] the surface energy [ 268 ] or the diffusion of cations and/or anions within the framework of the binary compounds surely have a crucial role in the deviations found. It is clear that to


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Figure 3 . Potential values at which conversion reactions have experimentally been observed to occur for different compounds during the fi rst discharge in a lithium battery (light grey bar) and deviation from the thermodynamic value calculated for the bulk ( Δ E conv , dark grey bar). To simplify the analysis, only compounds that were reported to undergo direct conversion, with no intermediate insertion steps, were used. The experimental values are taken from data obtained in galvanostatic mode at low to moderate current densities. In view of the drastic variability of conversion voltage values reported for metal fl uorides, only the data for nanocomposites with carbon were used.

Figure 4 . Scheme of a voltage vs. capacity profi le that highlights the contribution (excess/default) of the variation in surface energy onto the overall energy involved in an electrochemical reaction process, through a shift in voltage. This variation in surface free energy ( Δ G surf = Δ ( S · γ )) is taking place when either the quantity of surface ( S ) (nano vs . micro) or the chemical nature of the interface ( γ ) varies. During the fi rst conversion of a binary transition metal compound, both effects are simultaneously present.

achieve full understanding of all these parameters, even deeper knowledge of the conversion reaction pathways than currently available is required.

It is worth noting that, in contrast to kinetic factors (i.e., dif-fusion), the impact of surface energy (i.e,. thermodynamics) has seldom been invoked to account for the large experimental potential deviations from the theoretical values. Apart from the energy penalty associated with the formation of metal nano-particles (vide supra), the result of the electrochemical conver-sion of M a X b with Li is the formation of Li n X/M nanocompos-ites with a large contact interface between two components that have very different bonding nature (metallic vs. ionic). The large interfacial area created during conversion reactions, together with the nature of the involved phases, subsequently results in considerable surface/interfacial energy, which needs to be provided to the system during the reaction and would result in the deviations from the theoretical potential (Figure 4 ). As metals can exhibit surface energy as high as 2 J m − 2 , [ 310 ] the Li n X/M interfacial energy can easily be believed to be much higher. Further, the sole formation of metallic domains a few nanometres in size leads to calculated equilibrium overpoten-tials that can largely exceed 100 mV. Preliminary, yet still scarce reports, [ 311 ] are directed at addressing this surface effect on the voltage value.

3.2. Voltage Hysteresis

It can readily be noticed that the large conversion plateau expected for a symmetric de-conversion upon oxidation of Li n X/M (i.e., charge in a lithium battery) is generally absent or much less defi ned upon charge of the lithium cell, and is

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhwileyonlinelibrary.com

replaced by a mostly sloping curve (Figure 1 and Figure 5 ). The remarkable increase in surface area undergone by the active mate-rial in the discharge process could explain this transition. In fact, the increase in sur-face sites, [ 291 , 312 ] which will react at slightly different energies compared to bulk ones, has been shown to result in similar effects (i.e. widening of the reaction site energy range) for intercalation compounds. [ 313–315 ] The large surface area induced during the initial conversion is maintained throughout this process of de-conversion through the generation of nanoparticles of the newly formed transition metal compound (Figure 1 ). Another contribution to the drastic changes in voltage between charge and discharge in the fi rst cycle is likely to have an origin in the amorphous char-acter sometimes reported for the Li n X/M nanocomposite at the end of reduction; dif-ferences in free energy, and, therefore, in equilibrium reaction potential, are expected between crystalline and amorphous mate-rials, as expressed in [ 309 ]

�Gj ≡ �f G◦j (amorphous) − �f G◦j (crystalline) (2)

The changes in electrochemical profi le upon oxidation charge) result in a subsequent difference in the deviation

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Figure 5 . a) Voltage vs . composition and capacity profi les for the fi rst three cycles of a TiF 3 /Li cell. The theoretical potential (emf) is indicated. Δ E conv/de-conv ( η d/c in the fi gure) values at different points of the curve are provided. Reproduced with permission. [ 23 ] Copyright 2004, The Elec-trochemical Society. b) Voltage composition profi les for a representative binary fl uoride, oxide, sulfi de, and phosphide. The values of Δ E hys at the second cycle are indicated. Reproduced with permission. [ 317 ] Copyright 2008, Wiley-VCH.

from the theoretical bulk potential between the fi rst conver-sion ( Δ E conv ) and de-conversion ( Δ E de-conv ) steps (Figure 5 a). This intriguing observation clearly indicates that the factors that dominate these deviations are different in both processes. The existence of Δ E conv and Δ E de-conv induces an overall voltage hysteresis ( Δ E hys = Δ E de-conv + Δ E conv ) that, from the application point of view, produces a huge round-trip ineffi ciency. Elec-tronic conductivity measurements show that this magnitude is relatively high during the operation of NiO and Fe 2 O 3 elec-trodes, [ 318 ] suggesting that conventionally assigned sources of ineffi ciency are not behind the large Δ E hys . On the contrary, the DFT calculations on the FeF 3 system mentioned above point at a fundamental difference in reaction path upon conversion and de-conversion that could be a more prominent factor. [ 306 ] In the particular case of this iron fl uoride, these paths would

© 2010 WILEY-VCH Verlag GmbAdv. Mater. 2010, 22, E170–E192

e controlled by the very slow diffusion of iron within the fl uo-ide lattice and entail the formation of Li x [Fe 3 + 1− x Fe 2 + x ]F 3 before ull reduction to LiF and Fe, and of Li 3−3 x Fe x F 3 upon oxida-ion, before re-formation of FeF 3 . The formation of lithium-ontaining iron fl uorides upon both discharge and charge has ecently been confi rmed experimentally. [ 319 ] These fundamen-ally different reactions have different equilibrium potentials, he corresponding theoretical profi le being in excellent agree-

ent with experiments. Despite only being demonstrated for ne particular system, these fi ndings legitimately challenge ur understanding of the general mechanisms of conversion/e-conversion reactions. Considering also that the large cut-off otentials necessary to obtain a reasonable amount of capacity pon charge (typically 3 V) can be even larger than the ther-odynamic value for the dissolution/oxidation of the metals, as

efi ned by their standard potentials (e.g., Fe (s) to Fe 2 + (aq) at 2.6 V s. Li + /Li ° ), [ 21 ] renewed focus on the study of these mechanisms s certainly required if we are to understand the origins of the oltage hysteresis.

The second discharge profi le is closer to that of the fi rst harge than to the fi rst discharge (Figure 1 and Figure 5 ). espite the similarities, a signifi cant, though generally smaller,

oltage hysteresis remains and stays somewhat constant upon urther cycling. The fact that the ratio of surface-to-bulk sites nd the crystalline/amorphous character of the composite does ot signifi cantly change beyond the fi rst cycle compellingly einforces the idea that other factors that have not been fully evealed yet are behind the hysteresis. Dependence of the fi rst harge-second discharge hysteresis on the anionic species has een reported (Figure 5 b). [ 275 , 317 ] Given that the lowest values re observed for phosphides and hydrides, a correlation between ovalence and voltage hysteresis certainly seems to exist. Ascer-aining whether this correlation is coincidental and what exact ole covalence has on the reaction mechanism is an issue that lso remains to be addressed. Differences in this hysteresis ave also been shown for different transition metal oxides, [ 48 ] ut their origin has not been analyzed so far.

At this point, it is worthy of remark that the existence of a oltage hysteresis, and the consequently huge round-trip ineffi -iencies, when conversion reactions are performed in a lithium attery are, without a doubt, the most serious drawback toward pplication. Without fully understanding its origins, and, hereby, designing strategies to minimize it or use it to our dvantage, any electrode based on these reactions will never ross the barrier between laboratory and society as the associ-ted energy losses would make a battery unviable. The lack of fforts in this direction found in the literature can thus be con-idered rather surprising, and they should certainly be encour-ged in order for this particular fi eld to evolve.

.3. Extra Charge Storage at Low Potentials

nalysis of the capacity values reported upon the fi rst dis-harge below 1 V for compounds described in section 2 and in igure 2 readily reveals a trend toward higher capacity values ith respect to those predicted by reaction (1). The signature of

uch additional capacity is a sloping curve that follows the con-ersion plateau (Figure 1 ). Two different phenomena, namely,

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electrolyte decomposition [ 19 , 320 ] and interfacial storage, [ 156 ] have been proposed to account for the charge associated with this step (Figure 1 ). The nature of the former has been ana-lyzed extensively, using an array of tools such as IR, NMR, and XPS spectroscopy and mass spectrometry. [ 83 , 280 , 316 , 321–324 ] Battery discharge below 1 V was found to lead to a thin solid layer surrounded by a gel-like polymeric one that can be several nanometres thick, [ 131 ] which coat the particles, possibly contrib-uting to the preservation of their integrity. The inner layer was mainly found to be composed mostly of LiF, Li 2 CO 3 , and some alkylcarbonates, similar to the solid electrolyte interphase (SEI) layer formed on carbon. [ 325 ] In turn, the polymeric gel appears to contain polyethylene oxide (PEO) oligomers whose forma-tion is initiated by the reduction of dimethyl carbonate (DMC), used as an electrolyte solvent, to lithium alkoxides and alkylcar-bonates, and mediated by the presence of the other solvents, such as ethylene carbonate (EC) and propylene carbonate (PC), as shown in Figure 6 . [ 316 ] Apart from the charge consumed for the reduction of the solvent, this polymeric layer is also believed to enable additional lithium storage on its surface in a capacitive way, [ 320 ] thereby contributing to the observed extra capacities. An enhancement of the electrolyte decomposition can be induced by using nanoparticulate electrodes, which may lead to some of it even happening simultaneous to the conver-sion reaction, i.e., during the voltage plateaus. [ 88 , 312 ] Although it has typically been studied for materials that react at low volt-ages, side reactions involving the electrolyte have also been observed following the conversion of binary metal fl uorides, hinting at the possible catalytic effect of the generated metal nanoparticles. [ 326 , 327 ]

The second mechanism that is used to rationalize this additional capacity is based on a two-phase capacitive behaviour of the Li n X/M interface that allows for th

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