An operando X-ray diffraction study of chloroaluminateanion-graphite intercalation in aluminum batteriesChun-Jern Pana,b,1, Chunze Yuana,1, Guanzhou Zhua,1, Qian Zhangc,1, Chen-Jui Huangb, Meng-Chang Lind,Michael Angella, Bing-Joe Hwangb,e,2, Payam Kaghazchic,2, and Hongjie Daia,2

aDepartment of Chemistry, Stanford University, Stanford, CA 94305; bDepartment of Chemical Engineering, National Taiwan University of Scienceand Technology, Taipei 10607, Taiwan; cPhysikalische und Theoretische Chemie, Freie Universität Berlin, D-14195 Berlin, Germany; dCollege of ElectricalEngineering and Automation, Shandong University of Science and Technology, 266590 Qingdao, People’s Republic of China; and eNational SynchrotronRadiation Research Center, 30076 Hsinchu, Taiwan

Contributed by Hongjie Dai, April 24, 2018 (sent for review February 28, 2018; reviewed by She-huang Wu and Yongyao Xia)

We investigated rechargeable aluminum (Al) batteries composedof an Al negative electrode, a graphite positive electrode, and anionic liquid (IL) electrolyte at temperatures down to −40 °C. Thereversible battery discharge capacity at low temperatures couldbe superior to that at room temperature. In situ/operandoelectrochemical and synchrotron X-ray diffraction experimentscombined with theoretical modeling revealed stable AlCl4

−/graphiteintercalation up to stage 3 at low temperatures, whereas interca-lation was reversible up to stage 4 at room temperature (RT). Thehigher-degree anion/graphite intercalation at low temperaturesaffords rechargeable Al battery with higher discharge voltage(up to 2.5 V, a record for Al battery) and energy density. A remark-able cycle life of >20,000 cycles at a rate of 6C (10 minutes chargetime) was achievable for Al battery operating at low temperatures,corresponding to a >50-year battery life if charged/dischargedonce daily.

aluminum batteries | ionic liquids | operando X-ray diffraction |low temperature | energy storage

Developing new energy storage systems with high energy andpower density, high safety, and long cycle life at low costs is

important to applications ranging from personal electronics toelectrical vehicles and grid storage of renewable energies (1–3).Among various battery technologies, the aluminum-ion batteryhas gained attention due to the high abundance, low cost, andhigh safety of aluminum (Al) (4–6). We recently showed a re-chargeable aluminum-ion battery (AIB) system based on reversiblereduction/oxidation of aluminum at the Al negative electrodeand reversible intercalation/deintercalation of chloroaluminateanions at the graphite positive electrode in several electrolytesincluding a 1-ethyl-3-methylimidazolium-AlCl3 (EMIC-AlCl3)ionic liquid (IL) electrolyte containing chloroaluminate AlCl4

−

and Al2Cl7− anions (7–10). The Al battery exhibited promising

characteristics including a specific capacity of ∼66–90 mA h/gdepending on the graphite used, a Coulombic efficiency (CE) upto 99%, and charge/discharge rate up to 5,000 mA/g (or 75C) forover 7,000 cycles at room temperature.Various ionic liquids are known to exhibit melting points well

below 0 °C (11–14), which could be utilized to facilitate batteryoperations in cold conditions. Here we investigated the prop-erties of aluminum-ion battery at low temperatures with anacidic 1-ethyl-3-methylimidazolium chloride (EMIC)-AlCl3ionic liquid electrolyte. We found that the reversible dischargecapacity of chloroaluminate anion AlCl4

−/graphite intercala-tion at low temperatures (e.g., approximately −10 °C) could behigher than that at room temperature, and a remarkable cyclelife of >20,000 cycles could be achieved for low-temperature Albattery operation. The structural evolution of graphite positiveelectrode during AlCl4

− anion intercalation/deintercalationthrough charging/discharging cycling was investigated by in situ/operando synchrotron X-ray diffraction (XRD) measurements.Well-defined staging behavior was observed for chloroaluminate

anion–graphite intercalation (Fig. 1A). At low temperatures, stage 3intercalation was found stable to afford a higher reversible dis-charge capacity than at room temperature at which intercalationwas stable/reversible up to stage 4. Theoretical modeling wasperformed to rationalize the anion/graphite staging behaviorobserved experimentally.Aluminum battery pouch cells were fabricated for this work

containing an Al metal foil as anode, graphite powder pastedonto a carbon fiber paper with polymer binder as cathode, a glassfiber filter paper as separator, and an ionic liquid electrolyteformed by mixing EMIC and aluminum chloride (see SI Ap-pendix for details). The galvanostatic charge–discharge data foran Al–graphite cell containing an IL electrolyte made of AlCl3/EMIC mixture with a molar ratio of 1.7 (AlCl3: EMIC = 1.7:1.0)operating under a charge–discharge rate of 50 mA/g (see SIAppendix for experimental details) is shown in Fig. 1B. Thedischarge (charge) curve of the cell recorded at room tempera-ture (∼23 °C) exhibited two characteristic plateaus around 1.7 V(1.9 V) and 2.1 V (2.2 V), respectively (Fig. 1B, dashed lines; thelower voltage plateau exhibited fine features containing severalsmall plateaus). On the other hand, charge–discharge curves ofthe same cell recorded at −10 °C showed an additional plateau ata higher discharge (charge) voltage of ∼2.5 V (∼2.6 V) (Fig. 1B,solid lines). The additional discharge plateau at 2.5 V afforded alow-temperature Al battery with a higher operating voltage and ahigher cathodic capacity of ∼100 mA h/g than the ∼80-mA h/gcapacity at room temperature. Upon further cooling, the battery

Significance

Operando X-ray diffraction experiments are performed to in-vestigate chloroaluminate anion intercalation into a graphitepositive electrode during electrochemical charging and dis-charging of rechargeable aluminum batteries in situ. The re-sults revealed surprisingly ordered anion intercalation stagingbehavior in graphite despite the large anion size and the re-markably stable graphite structure upon repeated intercalationand deintercalation by the large anions. The basic under-standing will guide the design and development of new bat-tery materials and new batteries.

Author contributions: C.-J.P., C.Y., B.-J.H., and H.D. designed research; C.-J.P., C.Y., G.Z.,and C.-J.H. performed research; Q.Z. and P.K. contributed new reagents/analytic tools;C.-J.P., C.Y., G.Z., Q.Z., M.-C.L., M.A., B.-J.H., P.K., and H.D. analyzed data; and C.-J.P.,B.-J.H., P.K., and H.D. wrote the paper.

Reviewers: S.-h.W., Ming Chi University of Technology; and Y.X., Fudan University.

The authors declare no conflict of interest.

Published under the PNAS license.1C.-J.P., C.Y., G.Z., and Q.Z. contributed equally to this work.2To whom correspondence may be addressed. Email: bjh@mail.ntust.edu.tw, payamk@zedat.fu-berlin.de, or hdai1@stanford.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1803576115/-/DCSupplemental.

Published online May 14, 2018.

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still functioned even down to −40 °C (SI Appendix, Figs. S1and S2).When the battery was charged at room temperature to higher

voltages, the flat charging plateau at ∼2.5 V (Fig. 1B, dashedline) gave no corresponding plateau at a similar voltage in thedischarge curve. Charging at this voltage level led to an irreversibleprocess without stable charge/energy storage. This corroboratedwith low Coulombic efficiency ∼73% with a discharge capacityfixed at ∼80 mA h/g regardless of the degree of charging. At lowtemperatures below approximately −10 °C, charging at the ∼2.5 Vplateau allowed for reversible release of charge/energy, giving aclear, corresponding discharge plateau at ∼2.5 V (Fig. 1B, solidline), suggesting that the charged state of AlCl4

−/graphite inter-calation compound was stabilized at low temperatures and couldbe reversibly discharged to release the stored charge and energy.Cyclic voltammetry of the Al negative electrode in the same

AlCl3/EMIC = 1.7 electrolyte showed redox of Al/Al3+ at roomtemperature and low temperatures down to −40 °C (SI Appendix,Fig. S1), as the melting temperature of this IL was down to ap-proximately −50 °C (11). The positive electrodes showed a groupof graphite oxidation/reduction peaks (corresponding to the low-voltage plateau regime in Fig. 1B) in the 1.6–2.0 V (vs. Al/Al3+)range and a pair of sharp peaks at ∼2.2 V (corresponding to the∼2.2 V plateau in the charge–discharge curves in Fig. 1B, dashedline) at both room temperature (Fig. 1C) and −10 °C (Fig. 1D).At room temperature, upon scanning the voltages above ∼2.4 V,a sharp rise in oxidative current was observed but without acorresponding reduction peak in the reverse scan, consistent witha dissipative charging process (as seen in Fig. 1B, dashed lines)above this potential without stable charge/energy storage forreversible discharge. In strong contrast, at −10 °C, a new pair ofwell-defined oxidation/reduction peaks emerged at ∼2.5 V, sug-gesting reversible charging/discharging at this potential, consistentwith additional reversible plateaus in the galvanostatic charge–

discharge data recorded at −10 °C (Fig. 1B, solid line). Theseresults revealed the existence of a kinetically metastable stateof the chloroaluminate/graphite intercalation complex at a highvoltage of ∼2.5–2.6 V vs. Al/Al3+ before irreversible reactions athigher potentials.We investigated chloroaluminate anion–graphite intercalation

by in situ/operando synchrotron XRD measurements of the graph-ite cathode in an AIB cell undergoing galvanostatic charging at−10 °C at a constant 50 mA/g current density (Fig. 2A). At eachcharging voltage along the charging curve (Fig. 2B) an XRDspectrum of the graphite flakes in the cathode was recorded(Fig. 2 A, C, and D). Three main features were observed in thegraphite structure through the charging process. First, the dif-fraction peak at 2θ = 26.5° for pristine graphite split into twopeaks in the range of 22°–23.5° and 27°–28° and kept movingfarther apart in the charging voltage range of ∼1.8 V to 2.6 V(Fig. 2D). Second, a XRD peak at small angle of ∼2.8° emergedand shifted to higher angles up to ∼4.9° upon continuous charging,suggesting long-range ordering/periodicity in the intercalatedgraphite (Fig. 2C). Third, in the final stage of charging along the∼2.6 V plateau, a new XRD peak emerged at 16.3°, and the peakwas not observed at lower charging voltages spanning the 1.8-and 2.2 V plateaus (Fig. 2A). These operando XRD data sug-gested well-defined staging and long-range order existed in theanion–graphite intercalation structures.A notable feature was that at voltages along a charging plateau

(e.g., the ∼2.25 V plateau), each of the XRD peaks (i.e., the low-angle peak and the two near ∼26°) evolved by gradually loweringits intensity at its peak position, whereas a new peak next to itemerged and gradually gained intensity as charging proceededalong the plateau (Fig. 2C, small-angle peak, and Fig. 2D, leftpeak). The original peak diminished, whereas the new onebecame dominant to complete the shifting once reaching ahigher charging plateau (e.g., at ∼2.6 V in Fig. 2 C and D). The

Fig. 1. Extra battery charges at low temperatures. (A) Schematic of Al battery graphite positive electrode depicting a working mechanism of AlCl4− anions

that intercalate/deintercalate into graphite under charging and discharging, respectively. (B) Charge–discharge curve of an aluminum–graphite battery in anAlCl3/EMIC = 1.7 ionic liquid electrolyte recorded at room temperature and −10 °C under a current density of 50 mA/g. (C) Cyclic voltammetry data of thegraphite electrode of the battery recorded at room temperature. Scan rate is 1 mV/s. (D) Cyclic voltammetry data of the graphite electrode recorded at −10 °C.Scan rate is 1 mV/s.

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in situ XRD data suggested electrochemical driven structuralevolutions from stage n to stage n − 1 (n = integer; a stagingnumber corresponding to that every n graphene layers is in-tercalated by a layer of ions) of the anion–graphite intercalationcompound.To elucidate staging, we performed theoretical modeling of

various stages (n = 3–6) complexes of AlCl4−–graphite interca-

lation compounds by density functional theory (DFT) calcula-tions of the atomic structures (Fig. 3 A and B) and XRD patterns(Fig. 3C) for pristine and anion-intercalated graphite (see SIAppendix for modeling details). Calculations revealed that thegallery height of an anion intercalation layer in graphite was∼9.49–9.54 Å. The calculated periodic repeating structure anddiffraction 2θ peaks for stage n = 3–6 (SI Appendix, Fig. S4) werecompared with experimental results. Best fitting suggested thatat −10 °C, AlCl4

− intercalation into graphite during chargingalong the ∼1.8, 2.2, and 2.6 V voltage plateaus reached stagesn = 5, 4, and 3, respectively.The calculated XRD peak for pristine graphite (26.55°) was in

accord with experiments, confirming a reliable description for long-range van der Waals interaction (dispersion forces) using the D3

correction (SI Appendix, Table S1). The XRD peak positions calcu-lated using atomic coordinates of the simulated structures of the in-tercalated graphite in various stages mostly agreed with experiments(SI Appendix, Table S2) including the appearance of a diffractionpeak at ∼16.3° for the stage 3 anion–graphite intercalation (Fig.3C) seen on the 2.6 V charging plateau in experiments (Fig. 2A).Aside from simulation result, the XRD data could be un-

derstood within the Bragg’s law of diffraction,

2dn × sin  θ=mλ, [1]

where λ is the X-ray wavelength of 1.54059 Å, and dn is theperiodicity along the c axis for stage n compound,

dn = galley  height+Xn−1

i=1di, [2]

where di ∼ 3.32–3.40 Å is the interlayer spacing between sequen-tial nonintercalated graphene layers. In Eq. 1, m = integer is theorder of the diffraction pattern. For stage n = 3 with a galleryheight of 9.54 Å and a periodicity of d3 ∼ 16.29 Å from modeling,the order m = 1–5 diffraction peaks calculated from the Bragg’s

Fig. 2. In situ operando synchrotron X-ray diffraction and battery charging. (A) In situ XRD data of the graphite electrode in an aluminum–graphite batteryunder charging to various voltages at −10 °C under 50 mA/g current. The voltages were in the range of 1.858–2.622 V from the bottom to the top curves. TheXRD curves were displaced vertically for clarity. Same color adjacent curves were recorded along a voltage plateau (indicated at the right with graphitestaging number that is also labeled in the electrochemical charging curve in B). (B) The aluminum–graphite battery charging curve recorded simultaneouslywith the XRD measurement with the corresponding graphite intercalation stages labeled along the curve. In situ XRD data in the (C) small-angle and (D)larger-angle regimes showing details of the XRD peak evolutions of graphite during anion intercalation to different stages. The right-hand side labels arecharging voltages (in volts).

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law were 2θ = 5.42°, 10.85°, 16.31°, 21.81°, and 27.35°, respec-tively. Experimentally, we observed the corresponding m = 1, 3,4, and 5 peaks at 5.0°, 16.3°, 21.95°, and 27.60°, but the m =2 peak was not observed likely due to weak intensity. The newXRD peak of 16.3° emerged on the ∼2.6 V charging plateau wasthe third-order diffraction peak of the stage n = 3 anion–graphiteintercalation compound with a c axis periodicity of 16.29 Å.We observed that the small-angle (2.8°–3.8°) diffraction

peaks below ∼2.124 V during charging (Fig. 2A) in the earlystage (stage 5) intercalation. Once in stage 4 and <4 interca-lation stage, most of the diffraction peaks became sharp (Fig.2). This result suggested that chloroaluminate anion intercala-tion early on was more disordered than stage 4 and stage 3.That is, anion intercalation evolved into highly ordered, coor-dinated stages as more and more anions were inserted intographite layers. Also, one can observe peaks broadening intotwo shoulders before evolving into a sharp peak during thetransition between two stages (such as the peak in Fig. 2D atV = 2.474 V). In this case, one stage was not fully diminishedwhile the new stage was arising.We note several discrepancies between experimental and

simulated XRD data for AlCl4− intercalated graphite. The sim-

ulated peak at the small-angle position for the intercalation stagen = 3 sample was off by ∼10% from the experimentally measuredposition, and the intensity was low (Fig. 3C, zoom-in view). Therelative intensities of XRD peaks in experiments and simulations

did not show good correlation. Also, the simulated XRD patternsshowed small peaks between Bragg reflections from thegraphene sheets that were due to scattering from AlCl4

− an-ions but not observed in experiments. These discrepancieswere likely due to the inaccurate scattering parameters for Aland Cl ions used in our simulation and because the simplisticstaging model might not represent the overall structure ofintercalated graphite flakes uniformly over the large flakesizes on the order of ∼100 microns.Analysis of the in situ/operando XRD data recorded at −10 °C

for the Al battery graphite positive electrode suggested that thehigh-voltage ∼2.6 V charging plateau corresponded to a higher-energy AlCl4

− anion–graphite intercalation compound in the n =3 stage that was stable and reversible for charge–discharge at lowT but not stable at RT due to kinetic instability. At room tem-perature, up to stage n = 4 was stable, and stage 3 was kineticallyunstable, i.e., became irreversible due to thermal activationacross a small energy barrier to overwhelming irreversible sidereactions seen at >2.6 V. The irreversible reactions above ∼2.5 Vat RT (∼2.6 V at low T) with a rapid rise in oxidative current(Fig. 1 C and D) likely involved AlCl4

−. The reaction did notgenerate chlorine gas detected by mass spectroscopy in thebattery cell even up to 3 V. Based on X-ray spectroscopy thatprobed surface vs. bulk carbon atoms of graphite in an Al battery,the irreversible reaction was attributed to graphite chlorination

Fig. 3. Theoretical calculations of AlCl4− anion–graphite intercalation. (A) Side view of DFT calculated structures of stage 3–6 intercalation compounds. See SI

Appendix for details of calculations. (B) Top view of AlCl4− anions intercalated in graphite (we modeled four anions per 4 × 4 unit cell to give a theoretical

capacity of ∼100 mA h/g for stage 3, as a fit to experimentally measured capacity; SI Appendix, Table S2). (C) Calculated XRD patterns of intercalated graphitein stages 3–6 of AlCl4

− graphite intercalation compounds. (Inset) A zoom-in of the small-angle peak at 5.4° with a weak intensity. The calculated XRD patternfor pristine graphite is also presented for comparison.

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to form C–Cl bonding on the surface, defects, and edges ofgraphite but not in the bulk of the graphite flakes (8).We observed excellent performance of the Al batteries at low

temperatures (SI Appendix, Figs. S2 and S3). The battery in theAlCl3/EMIC = 1.7 electrolyte still functioned at down to −30 °Cand −40 °C, with ∼70 and ∼30% of the stage 3 capacity(∼100 mA h/g at −10 °C), respectively, due to increased viscosityand reduced ionic conductivity of the electrolyte at low tem-peratures near its freezing point (11). Coulombic efficiencies atlow temperatures were generally very high due to reduced sidereaction currents near the cutoff voltages over at RT (near ∼2.6 V;Fig. 1C vs. Fig. 1D). The battery operating at −20 °C (SI Ap-pendix, Fig. S2) showed Coulombic efficiency ∼100% and ca-pacity of ∼80 mA h/g at 100 mA/g rate (∼1.2 C). The battery alsoshowed excellent rate capability even at−20 °C, retaining∼65mA h/gcapacity at 700 mA/g current (∼10C, 6 min charging time; Fig. 4A).To our knowledge no other battery exhibited a comparable rate ca-pability at this temperature. Cycling at a low rate (∼1.2 C, 100 mA/g),the battery showed an excellent stability over 1,200 cycles (Fig. 4A). Inaddition, we tested an Al battery in an AlCl3/EMIC = 1.5 electrolyterunning at 4 °C (Fig. 4B). The battery exhibited excellent rate per-formance with a capacity of ∼80 mA h/g at 300 mA/g and ∼75 mA h/gat 500 mA/g rate and a CE higher than 99%. Charge–discharge cy-cling at a high rate of 500 mA/g (6C, ∼10 min charging/discharging

time) showed a remarkable stability with only slight capacity decayafter >20,000 cycles. This corresponded to battery life of >50 y if onecharge–discharge cycle daily. The graphite flakes in the positiveelectrode showed amazing structural integrity/reversibility even afterintercalation/deintercalation by chloroaluminate anions over>20,000 times.In summary, we investigated the electrochemical performance

of Al/graphite rechargeable batteries at low temperature. Thereversible discharge capacity of the battery at low temperatureswas superior to that at room temperature due to higher stagingstability and reversibility of chloroaluminate anion/graphite in-tercalation at low temperatures. In situ/operando electrochemicaland synchrotron X-ray diffraction experiments combined withtheoretical modeling revealed reversible stage 3 AlCl4

−/graphiteintercalation at low temperatures, whereas at room temperature,intercalation was stable up to stage 4. A remarkable cycle life andhigh rate performance were achievable for Al battery operating atlow temperatures down to −30 °C and −40 °C.

Materials and MethodsDetails of the materials, chemicals, and methods used in this study, includingpreparation of ionic liquids, fabrication of graphite electrodes, electrochemicalmeasurements for aluminum-ion batteries, preparation of battery, and facilitiesfor performing in situ operando X-ray diffraction measurements, simulation,and modeling, are provided in SI Appendix, SI Materials and Methods.

Fig. 4. Al batteries with up to 20,000 cycles at low temperatures. (A) Cycle life stability test of an aluminum–graphite battery (by measuring dischargecapacity represented by blue symbols vs. cycle number at various current densities indicated; right axis is Coulombic efficiency for red symbols) at −20 °C usingan AlCl3/1.0 EMIC = 1.7 ionic liquid electrolyte. Voltage range is 1.0–2.55 V. (B) Cycle life stability test of an aluminum–graphite battery (by measuring dis-charge capacity represented by blue symbols vs. cycle number at various current densities indicated; right axis is Coulombic efficiency for red symbols) at 4 °Cusing a AlCl3/EMIC = 1.5 ionic liquid electrolyte, Voltage range is 1.0–2.4 V.

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ACKNOWLEDGMENTS. This work was partially supported by US Depart-ment of Energy DE-SC0016165, the Global Networking Talent 3.0 plan[National Taiwan University of Science and Technology (NTUST)104DI005] from the Ministry of Education of Taiwan, the Wallenberg

Foundation Postdoctoral Scholarship Program on Environment and Energy“The Stig Hagström stipend” [The Marcus and Amalia Wallenberg founda-tion (MAW) 2013.0076], and the Taishan Scholar Project of Shandong Prov-ince of China (tsqn20161025).

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