Electronic Supplementary Information Spherical Nanoporous LiCoPO
Nanoporous LiNi1/3Co1/3Mn1/3O2 as an ultra-fast charge cathode material for aqueous rechargeable...
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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 9209--9211 9209 Cite this: Chem. Commun., 2013, 49, 9209 Nanoporous LiNi 1/3 Co 1/3 Mn 1/3 O 2 as an ultra-fast charge cathode material for aqueous rechargeable lithium batteries† Faxing Wang, Shiying Xiao, Zheng Chang, Yaqiong Yang and Yuping Wu* Nanoporous LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NP-NCM) was synthesized by using vapour grown carbon fibers (VGCFs) as templates. It exhibits excellent electrochemical performance as a cathode material for aqueous rechargeable lithium batteries, especially with ultra-fast charge capability, and 72 mA h g 1 can be achieved at a charge time of 20 s. Lithium ion batteries are now dominating the market of portable electronic devices and are also competitive power sources for electric vehicles (EVs). 1 Currently, it takes from 6 to 8 h to fully recharge EVs using a Level II-240 V charging station. 2a This long charging time is not convenient when traveling long distance. To overcome this issue, Level III fast-charging power levels have been defined and implemented. At present, although few EV manufacturers allow for fast charging due to high energy inputs and high temperatures, some fast charging techniques based on various chemistries have been reported. 2 For example, a multistage fast charging technique for high power LiFePO 4 batteries was proposed and the results showed that a fully discharged battery could be recharged within 6 min to 40% of its normal capacity, which meets the goal of USABC (U.S. Advanced Battery Consortium) for fast charging, 40% of the normal capacity of the battery within 15 min. 2a At the same time, to achieve the fast charging of electrode materials, designing superior architectures with high surface area and special nano-structure is an effective route. 3 For example, nano-LiCoO 2 with a size of 17 nm exhibited 75 mA h g 1 at a rate of 100 C (charge in 36 s). 3a Then high rate performance of nano-sized LiFePO 4 up to 400 C was reported by adding 65% of conductive agent in the electrode. 3b Recently, carbon-coated LiMn 2 O 4 retained 47% of the initial capacity at 300 C. 3c However, the batteries based on the above materials encounter the risk of safety accidents because of the flammability of the organic electrolytes and the improper use such as overcharging or short-circuit. Therefore, better energy storage devices are urgently needed. Aqueous rechargeable lithium batteries (ARLBs) were firstly introduced in 1994 and have received wide interest since 2007 due to their low cost and good safety by avoiding the use of flammable organic electrolytes. 4 More importantly, the ionic conductivity of aqueous electrolytes is high, about two orders of magnitude higher than that of organic electrolytes, 5 which enables achievement of high rate capability easily. For example, a bicontinuous NiOOH electrode delivered 75% of its 1 C capacity when the discharge rate increased to 1017 C. 5b In our previous work, it was found that LiMn 2 O 4 nanotubes with a preferred orientation of (400) planes can reach a charge capacity of 59.3 mA h g 1 at a charge rate of 600 C as cathode materials for ARLBs. 4f However, before ARLBs can be really applied as the main power sources of EVs, their energy density needs some further improvement. 4e, g Layered LiNi 1/3 Co 1/3 Mn 1/3 O 2 has attracted much attention as a possible replacement for commercial LiCoO 2 , because of its higher energy density, cheaper price and less toxicity. 6 Herein, we demonstrated a facile strategy to synthesize nanoporous LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NP-NCM) by using VGCFs as templates, and it exhibits superior electrochemical performance especially the ultra-fast charge behavior. Scheme 1 illustrates the preparation process of NP-NCM. At the initial stage of calcination, the LiNi 1/3 Co 1/3 Mn 1/3 O 2 precursor spontaneously assembles on the surface of the modified VGCFs driven by minimizing the surface energy (Fig. 1a). During the calcination process, VGCFs, citric acid and CH 3 COO are burnt away. At the same time, the precursor particles are agglomerated and finally form large secondary particles. The pores were produced as a result of release of CO 2 or H 2 O gas from the gel and VGCFs. For comparison, LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM) was prepared via the same method without using VGCFs. From the SEM micrograph (Fig. 1b), it can be seen that NP-NCM exhibits quasi-cylindrical or spherical agglomerates composed of a lot of nanoparticles as primary particles. A further New Energy and Materials Laboratory (NEML), Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China. E-mail: [email protected]; Fax: +86-21-55664223 † Electronic supplementary information (ESI) available: Preparation, character- ization, electrochemical measurements, CV curves and some galvanostatic charge and discharge curves of the NCM and NP-NCM electrodes at different scan rates. See DOI: 10.1039/c3cc44360d Received 10th June 2013, Accepted 3rd July 2013 DOI: 10.1039/c3cc44360d www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 10 July 2013. Downloaded by University of Sydney on 12/09/2013 17:57:50. View Article Online View Journal | View Issue
Transcript of Nanoporous LiNi1/3Co1/3Mn1/3O2 as an ultra-fast charge cathode material for aqueous rechargeable...
This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 9209--9211 9209
Cite this: Chem. Commun.,2013,49, 9209
Nanoporous LiNi1/3Co1/3Mn1/3O2 as an ultra-fastcharge cathode material for aqueous rechargeablelithium batteries†
Faxing Wang, Shiying Xiao, Zheng Chang, Yaqiong Yang and Yuping Wu*
Nanoporous LiNi1/3Co1/3Mn1/3O2 (NP-NCM) was synthesized by
using vapour grown carbon fibers (VGCFs) as templates. It exhibits
excellent electrochemical performance as a cathode material for
aqueous rechargeable lithium batteries, especially with ultra-fast
charge capability, and 72 mA h g�1 can be achieved at a charge
time of 20 s.
Lithium ion batteries are now dominating the market of portableelectronic devices and are also competitive power sources forelectric vehicles (EVs).1 Currently, it takes from 6 to 8 h to fullyrecharge EVs using a Level II-240 V charging station.2a This longcharging time is not convenient when traveling long distance.To overcome this issue, Level III fast-charging power levelshave been defined and implemented. At present, although fewEV manufacturers allow for fast charging due to high energyinputs and high temperatures, some fast charging techniquesbased on various chemistries have been reported.2 For example,a multistage fast charging technique for high power LiFePO4
batteries was proposed and the results showed that a fullydischarged battery could be recharged within 6 min to 40% ofits normal capacity, which meets the goal of USABC (U.S. AdvancedBattery Consortium) for fast charging, 40% of the normal capacityof the battery within 15 min.2a
At the same time, to achieve the fast charging of electrodematerials, designing superior architectures with high surface areaand special nano-structure is an effective route.3 For example,nano-LiCoO2 with a size of 17 nm exhibited 75 mA h g�1 at arate of 100 C (charge in 36 s).3a Then high rate performance ofnano-sized LiFePO4 up to 400 C was reported by adding 65% ofconductive agent in the electrode.3b Recently, carbon-coatedLiMn2O4 retained 47% of the initial capacity at 300 C.3c
However, the batteries based on the above materials encounterthe risk of safety accidents because of the flammability of theorganic electrolytes and the improper use such as overchargingor short-circuit. Therefore, better energy storage devices areurgently needed. Aqueous rechargeable lithium batteries(ARLBs) were firstly introduced in 1994 and have received wideinterest since 2007 due to their low cost and good safety byavoiding the use of flammable organic electrolytes.4 Moreimportantly, the ionic conductivity of aqueous electrolytes ishigh, about two orders of magnitude higher than that oforganic electrolytes,5 which enables achievement of high ratecapability easily. For example, a bicontinuous NiOOH electrodedelivered 75% of its 1 C capacity when the discharge rateincreased to 1017 C.5b In our previous work, it was found thatLiMn2O4 nanotubes with a preferred orientation of (400) planescan reach a charge capacity of 59.3 mA h g�1 at a charge rate of600 C as cathode materials for ARLBs.4f
However, before ARLBs can be really applied as the mainpower sources of EVs, their energy density needs some furtherimprovement.4e,g Layered LiNi1/3Co1/3Mn1/3O2 has attractedmuch attention as a possible replacement for commercialLiCoO2, because of its higher energy density, cheaper priceand less toxicity.6 Herein, we demonstrated a facile strategy tosynthesize nanoporous LiNi1/3Co1/3Mn1/3O2 (NP-NCM) by usingVGCFs as templates, and it exhibits superior electrochemicalperformance especially the ultra-fast charge behavior.
Scheme 1 illustrates the preparation process of NP-NCM.At the initial stage of calcination, the LiNi1/3Co1/3Mn1/3O2
precursor spontaneously assembles on the surface of the modifiedVGCFs driven by minimizing the surface energy (Fig. 1a). Duringthe calcination process, VGCFs, citric acid and CH3COO� are burntaway. At the same time, the precursor particles are agglomeratedand finally form large secondary particles. The pores were producedas a result of release of CO2 or H2O gas from the gel and VGCFs. Forcomparison, LiNi1/3Co1/3Mn1/3O2 (NCM) was prepared via the samemethod without using VGCFs.
From the SEM micrograph (Fig. 1b), it can be seen thatNP-NCM exhibits quasi-cylindrical or spherical agglomeratescomposed of a lot of nanoparticles as primary particles. A further
New Energy and Materials Laboratory (NEML), Department of Chemistry &
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Fudan University, Shanghai 200433, China. E-mail: [email protected];
Fax: +86-21-55664223
† Electronic supplementary information (ESI) available: Preparation, character-ization, electrochemical measurements, CV curves and some galvanostatic chargeand discharge curves of the NCM and NP-NCM electrodes at different scan rates.See DOI: 10.1039/c3cc44360d
Received 10th June 2013,Accepted 3rd July 2013
DOI: 10.1039/c3cc44360d
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9210 Chem. Commun., 2013, 49, 9209--9211 This journal is c The Royal Society of Chemistry 2013
examination by TEM reveals that the size of primary particlesranges from 50 to 100 nm and the diameter of pores in NP-NCMis in the range of 5 to 10 nm. The XRD patterns of NCM andNP-NCM are shown in Fig. 1c, which are consistent with asingle phase of a-NaFeO2 structure with the space group R%3m(JCPDS 82-1495).6a The ratios of the intensities for (003) and(104) peaks in the XRD pattern are found to be 1.83 and 1.86 forNCM and NP-NCM, respectively, which indicate no evidentcation mixing.6b Moreover, the splitting of (006)/(102) and(108)/(110) peaks in XRD patterns is regarded as an indicator ofwell-ordered layered structure.6d Nitrogen adsorption–desorptionisotherm and the Barrett–Joyner–Halenda (BJH) curve for NP-NCMare shown in Fig. 1d. The isotherm exhibits the type IV character-istics with a distinct hysteresis loop in the P/P0 range of 0.40–1.0,which suggests the presence of nanopores.6c The pore sizedistribution shows that the size of the pores is in the range of4–8 nm, which is consistent with the TEM measurement. Incontrast, no quasi-cylindrical or spherical agglomerates areobserved in the SEM micrographs (Fig. S1b of ESI†) of theoriginal NCM, demonstrating that the VGCFs are essential toachieve NP-NCM. It is well known that nano-sized cathodematerials have some drawbacks such as low volumetric energy
density.3c,7 Namely, they cannot be packed as densely onthe current collector as micro-sized materials, which meanselectrodes made of nano-sized materials have a low tap density.For practical applications, tap density of an active material isimportant to decide the weight capacity of a battery. Therefore, itwas reported that the best way to improve both rate capabilityand tap density would be to use micro-sized particles that consistof primary nanoparticles.3c,7 The NP-NCM agglomerates arecomposed of a lot of primary nanoparticles. The tap densitiesof NP-NCM and NCM are 3.2 and 2.1 g cm�3, respectively.Moreover, the nanopores can make electrolytes easily permeateinto the inside of the cathode material.
The CV curves of the electrodes (Fig. S2a and b in ESI†) in0.5 mol l�1 Li2SO4 aqueous electrolyte show that both of themexhibit one couple of oxidation–reduction at 0.70 and 0.61 Vat a scan rate of 1 mV s�1, respectively, corresponding to thede-intercalation–intercalation of lithium ions.8 When the scanrate increases, both the intensities and the peak separation ofthe redox peaks increase. It is noted that the oxidation peakof NCM is incomplete when the scan rate is higher than10 mV s�1. In contrast, the peaks of the NP-NCM electrode stillretain the well-defined shape even when the scan rate increasesto 20 mV s�1, which indicate that it can be charged anddischarged at larger current density.
The charge–discharge curves at different rates in 0.5 mol l�1
Li2SO4 aqueous electrolyte are shown in Fig. 2a. NP-NCMexhibits an initial discharge capacity of 155 mA h g�1 at 1.5 C(1 C is 160 mA g�1 here), which is higher than those ofthe recently reported LiCoO2,4b,8a LiFePO4,8b LiMn2O4,4d,g,8c
LiNiPO4,8d and LiMn0.6Fe0.4PO48e in aqueous electrolytes. Even
at a charge and discharge rate of 45 C, it can still deliver areversible capacity of 108 mA h g�1, nearly 70% of its capacity at1.5 C. However, the NCM electrode only delivers 67 mA h g�1 atthe same rate (Fig. S2c in ESI†). As for the NP-NCM cathode,these improved properties are further confirmed in the ratecapability test from 2 to 50 C (Fig. 2b). The discharge capacitiesof NP-NCM are much higher than those of NCM at the samehigh current rate. Moreover, after 50 cycles, as the charge anddischarge current densities are recovered from 50 C to 2 C, thedischarge capacity of NCM shows more than 50% capacityfading, but the NP-NCM electrode almost completely recoversthe capacity, which is much better than those of the PPy@LiNi1/3-Co1/3Mn1/3O2 composite,9a spherical LiNi1/3Co1/3Mn1/3O2
9b andother LiNi1/3Co1/3Mn1/3O2
10 in aqueous electrolytes. It also exhibits
Scheme 1 Preparation process of NP-NCM using VGCFs as templates.
Fig. 1 (a) TEM micrograph of the NCM@VGCFs precursor (450 1C, 3 h), (b) SEMand TEM (inset) micrographs of NP-NCM, (c) XRD patterns of NP-NCM and NCM,(d) nitrogen adsorption–desorption isotherms (inset) and the Barrett–Joyner–Halenda (BJH) curve of NP-NCM.
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excellent rate capability and good cycling behavior (Fig. 2c).With an initial discharge capacity of 92 mA h g�1, 92% of theinitial capacity is maintained after 50 cycles when charged at 80 C(the charge time is about 45 s as shown in Fig. S2d in ESI†) anddischarged at 3 C. When it is charged at 180 C and dischargedat 3 C, the discharge capacity could reach 72 mA h g�1 and thecharge time needed is only 20 s. After 50 cycles, the dischargecapacity of 65 mA h g�1 could be obtained and the charge timeis 15 s (Fig. 2d), which is superior to those of the previouslyreported LiNi1/3Co1/3Mn1/3O2 nanoparticles in the aqueouselectrolyte (45 mA h g�1 at 140 C and 33 mA h g�1 at 180 C),6e
or other nanostructured NMC.6,9 The improvement in the ratecapability and the cycling life of NP-NCM is attributed to itsunique structure. Firstly, the nanosized particles can shorten thediffusion length of Li+ ions. Secondly, the nanopores not onlyprovide the conducting pathway for the Li+ ions into the cathode(providing less charge transfer resistance, Fig. S2e, ESI†), but alsoact as a buffer for volume change or strain during cycling.
In summary, nanoporous LiNi1/3Co1/3Mn1/3O2 micrometersize agglomerates consisting of nanoparticles have beensuccessfully synthesized by using VGCFs as templates. Theyexhibit excellent electrochemical performance as cathode materialsfor ARLBs. Their rate capability is superior to those of the reportednanostructured LiNi1/3Co1/3Mn1/3O2 in the organic or aqueouselectrolytes. When charged at 180 C and discharged at 3 C in0.5 M Li2SO4 aqueous electrolyte, their discharge capacity couldreach 72 mA h g�1 and the charge time needed is only 20 s.Their cycling behavior is also good. The two main reasons are:
the LiNi1/3Co1/3Mn1/3O2 primary nanoparticles can decrease theLi-ion migration length and the nanopores can act as a buffer forvolume change or strain during cycling. This provides a newdirection to explore superior cathode materials such as LiCoO2
and LiMn2O4 for ARLBs with high rate capability. Of course,the micro-sized agglomerates are expected to guarantee highvolumetric and weight energy density.
The authors acknowledge the financial support from MOSTPrograms (2010DFA61770), STCSM (12JC1401200) and NSFC(21073046).
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Fig. 2 (a) The galvanostatic charge and discharge curves of NP-NCM, (b) ratecapability of NCM and NP-NCM, (c) and (d) ultra-fast charge capability ofNP-NCM. All electrochemical data were calculated based on the mass of thecathode and measured in 0.5 mol l�1 Li2SO4 aqueous electrolyte by using thesaturated calomel electrode (SCE) as the reference electrode. Activated carbonwas used as the counter electrode for the cycling test and the Ni-mesh was usedfor charge and discharge curves.
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