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Understanding the Behavior of LiCoO Cathodes at Extended Potentials in Ionic Liquid - Alkyl Carbonate Hybrid Electrolytes Sowmiya Theivaprakasam, Jimmy Wu, James C. Pramudita, Neeraj Sharma, Douglas R. Macfarlane, and Sagar Mitra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05591 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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The Journal of Physical Chemistry

Understanding the Behavior of LiCoO2 Cathodes at Extended Potentials in Ionic liquid - Alkyl carbonate Hybrid Electrolytes Sowmiya Theivaprakasam a,b,c , Jimmy Wu d, James C. Pramudita d, Neeraj Sharma d, Douglas R. MacFarlane b*, Sagar Mitra c*. a IITB-Monash

bARC

Research Academy, Powai, Mumbai, India 400076.

Centre of Excellence for Electromaterials Science, School of Chemistry, Monash

University, VIC 3800, Australia. c Department

of Energy Science and Engineering, Indian Institute of Technology Bombay

(IITB), Mumbai 400076, India. d School

of Chemistry, The University of New South Wales, Sydney NSW 2052, Australia.

ACS Paragon Plus Environment


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ABSTRACT

The current electrolyte compositions makes it is hard to achieve a high energy density lithiumion battery based on LiCoO2 chemistry due to the destabilization of LiCoO 2 crystal structure beyond 4.2 V vs. Li/Li+ leading to oxygen evolution and electrolyte decomposition. Therefore, electrolyte developments may hold promise for improved performance, for example if some of the advantageous properties of ionic liquids can be introduced into a carbonate electrolyte system. Here, we report the use of a hybrid electrolyte (HE) system with a LiCoO 2 cathode and have observed an excellent electrochemical performance when cycled to 4.4 V vs. Li/Li+. This extended potential range produces higher capacity via greater ion insertion/extraction and better structural stability. A discharge capacity of 161 mAh/g (0.7 lithium extraction) was observed in the HE compared to 128 mAh/g in conventional electrolyte, after 60 cycles. The charge-discharge studies at extended potentials also indicate better capacity retention in the HE compared to the conventional electrolyte (LP30). Investigations to confirm the origin of such behavior establish that surface film formation is protecting or delaying the phase transition for LiCoO2 at extended potentials. In-situ XRD studies suggest that the electrolyte combination helps to delay the potential for monoclinic phase formation in LiCoO 2 and ex-situ XRD studies suggest less structural degradation takes place in the HE than the conventional electrolyte at the end of 60 cycles. Therefore, we believe that the future tailoring of the HE will provide a significant step towards high energy density lithium batteries.


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INTRODUCTION Lithium batteries have revolutionized energy storage in recent decades 1. The current need is to make these batteries more dense in energy so as to power electric vehicles and other large scale energy storage requirements2,3. To meet this high energy density goal, current research is focused on high potential cathodes, high capacity anodes and electrolytes with wide electrochemical potential window 4–6. Apart from the recently reported high potential cathodes, lithium cobalt oxide (LiCoO2) is still the most commercially successful cathode in lithium ion batteries 2,3,7. However, the thermal instability of delithiated LiCoO2 can lead to thermal run-away which is a serious concern to the safety of these batteries 7–10. Ionic liquids have been proposed as safe electrolytes, to avoid issues that arise due to the flammability of carbonate electrolytes 11–18. Several studies have been carried out with ionic liquids, however the specific capacities of the systems, as well as capacity retention, were not convincing compared to the conventional electrolytes 12,19–22. Studies with ionic liquids based on the fluoro sulfonyl imide (FSI) anion presented better rate capability than the conventional electrolytes with LiCoO2 cathodes23,24. However, the thermal stability of FSI anions is lower than that of tri fluoro sulfonyl imide (TFSI) anions25 and typically TFSI anions show better electrochemical stability26. Hence an ionic liquid composed of TFSI anions was used in our study. In this work, we have investigated the hypothesis that some of the desirable features of ionic liquid electrolytes could be introduced via a hybrid electrolyte consisting of a mixture of ionic liquid and traditional electrolytes with LiCoO2 cathodes13,27–30. The hybrid electrolyte with 40% percent of ionic liquid in the mixture was considered as optimum composition in order to have the best compromise between thermal stability and viscosity 13,27–31. Though the theoretical capacity of LiCoO 2 is 274 mAh/g, the practical capacity is limited to 140 mAh/g with a controlled potential cut off of 4.2V vs. Li/Li+; elevating the potential



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beyond 4.2V gives rise to structural instabilities and undesired interfacial reactions

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16,23,32–34.

This has prevented the use of LiCoO2 as a high potential cathode. Measures such as coating the cathode surface with ceramics, oxides or with electrolyte additives35–38 have been successful in extending the potential limit. Nonetheless, the safety and thermal runaway issues remain. In this work we have tackled both the voltage / capacity challenge and the thermal instability of LiCoO2 cathodes by adding ionic liquid to the conventional alkyl carbonate electrolytes. The studies were carried out initially in the standard potential window of 4.2V vs. Li/Li + to investigate the performance of hybrid electrolytes in LiCoO 2 cathodes. Further studies were carried out in the extended potential range of 4.4V vs. Li/Li+, a move towards extracting more lithium from the system31. The potential cut-off in the present study is restricted to 4.4V vs. Li/Li+, as a safe limit to avoid serious structural distortion in LiCoO 2 cathodes as shown previously 36,39,40 and as investigated here via in-situ synchrotron X-ray diffraction (XRD) studies. METHODS Electrode preparation The electrode slurry was prepared by mixing LiCoO2 cathode (BATSOL, India) with carbon black (Super C-65, Timcal, Switzerland) and Polyvinylidene fluoride-hexafluoropropylene (Sigma Aldrich) binder in the ratio of 90:5:5 with N-Methyl-pyrrolidinone (Thermo Fisher Scientific) as a solvent. The well-mixed slurry is then coated on an aluminium current collector (MTI corp) using the doctor blade technique. The coated electrode foils were dried in air oven for 24 hours. The dried electrode foil was then punched into small circular discs for cell fabrication.


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Electrochemical characterization The electrochemical studies were carried out in Swagelok two electrode cells with LiCoO 2 as working electrode and lithium metal (Alfa Aesar) as counter and reference electrode. The cells were tested with conventional carbonate electrolyte 1 M LiPF 6 : Ethylene carbonate : Dimethyl carbonate (LP30, Merck) and a hybrid electrolyte (HE) which contains 40% ionic liquid (N-methyl-N-propyl-pyrrolidinium bis(trifluromethanesulfonyl)imide) added to the conventional electrolyte mixture of 1M LiPF6 in ethylene carbonate and diethyl carbonate (1:1) (Solvionic). The cells were tested with a glass fibre separator (GF/D Whatman). Neat N-methyl-N-propyl-pyrrolidinium bis (trifluromethanesulfonyl) imide, ([C3mpyr][TFSI], Solvoionic, France) was also used for some reference studies. The cells were made in an argon glove box (Lab star, MBraun) with water and oxygen content of 0.5ppm and 1ppm respectively. The charge discharge studies were carried out galvanostatically at the C/5 rate at 20 oC with an Arbin battery tester (BT-2000 model, Arbin Instruments). Surface characterization The surface characterization of the LiCoO2 electrodes was carried out by opening the cells inside the glove box after cycling and washing with diethyl carbonate. High resolution transmission electron microscopy (HR-TEM, JEOL-2100F) and X-ray photoelectron spectroscopy (XPS, AXIS Ultra) were carried out to provide a better understanding of the electrode surface. The XPS curves were fitted using the processing software package Casa XPS. Thermogravimetric analysis (TGA, NETZSCH STA 449F3) was carried out on delithiated LiCoO2 electrodes in a nitrogen gas environment at a heating rate of 5oC / minute.



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Structural characterization Half-coin cells with 3 mm diameter holes in the casing and 5 mm diameter holes in the stainless spacer were used for the construction of coin cells for the in situ synchrotron XRD measurements. The coin cells contains Li metal, glass fibre separator with hybrid electrolyte. Cells were made 3 days before the in situ synchrotron XRD experiments. Further details regarding coin cell construction and beamline setup can be found in our previous publications41,42. In situ synchrotron XRD data were collected on the Powder Diffraction beamline43 at the Australian Synchrotron at a X-ray wavelength  = 0. 0.708735(2) Å, determined using the NIST 660a LaB6 standard reference material. Each data set was collected for 3 minutes acquisitions (with detector position movement) on the coin cell in transmission geometry throughout the charge/discharge cycles and the period between datasets was 28 minutes. This is an in situ experiment rather than an in operando experiment where data is continuously collected. Structural analysis were carried out using the software package GSAS and EXPGUI44,45. RESULTS AND DISCUSSION Cycling studies, 3.0 - 4.2 V potential window The galvanostatic charge-discharge studies were carried out initially in the potential window of 3 - 4.2 V vs. Li/Li+. The first cycle capacity in LP30 and HE electrolyte based cells were 146 and 148 mAh/g, respectively. In the second cycle, the charging capacity of LP30 and HE were 135 and 139 mAh/g respectively, whereas the discharge capacities were 134 and 138 mAh/g, respectively, indicating rapid approach to efficient cycling in both cases. At the end of 60 cycles, the discharge capacities of the LP30 and HE electrolyte were 131 and 133


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mAh/g, respectively, as shown in Figure S1(a) & S1 (b) (Supplementary information). The plots of capacity and coulombic efficiency versus cycle number are shown in the Figure 1 respectively. It was observed that the HE showed better cyclic efficiency, as shown in Figure 1b, suggesting that there is a degradation in the LP30 case, as observed by other researchers 46, which is negligible in HE.

Figure 1. Specific capacity at potential cut-off 4.2 V vs. Li/Li+ of (a) Li /LP30/ LiCoO2, (b) Li /HE/ LiCoO2 and (c) Comparison of coulombic efficiency in LP30 and HE. TEM analysis of the charged electrodes were carried out as shown in Figure S1 (c) & S1 (d). The analysis indicates the formation of a thin surface film on the electrode, which may help in preventing the undesirable side reactions that occur on direct contact with the electrolyte in a highly oxidative environment. Such film formation was also observed in our previous studies13 with LiFePO4 cathodes. Other researchers12,47,48 have also reported that ionic liquid assisted film formation on the cathodes improves cyclic stability. We investigated whether the surface films might also enhance the thermal stability of delithiated LiCoO2. In order to evaluate the thermal behaviour, differential thermal analysis were carried out on the delithiated LiCoO2 cathodes as shown in the Figure S2 (a) (samples were removed directly from the cell, placed into the TGA instrument and intentionally



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allowed to retain traces of electrolyte). LiCoO2 in neat ionic liquidwas also included as a control experiment. In Figure S2 (a), the peak at 133oC in LP30 is in accordance with the literature38 and is understood to occur due to the volatile organic solvents in the electrolyte, whereas the corresponding peak occurs at 140oC in the HE system, indicating the lower volatility of the carbonate components in the mixture, as expected from Henry’s law. The second feature that starts near 230oC can be attributed to the initiation of thermal decomposition of LiPF6 in the system. This is also supported by the absence of such a feature in the pure [C3mpyr] [TFSI] / Li0.5CoO2 sample and a shift to higher temperatures for the HE. These results suggest that the cathode material is slightly more stable in the HE. Cycling studies in the 3.0 - 4.4V potential window The charge-discharge studies were carried out galvanostatically at C/5 rate in a potential window of 3 - 4.4V vs. Li/Li+, as shown in the Figure 2. In the first cycle the discharge capacity of the LP30 cell was 164 mAh/g, whereas the discharge capacity of the HE cell was 172 mAh/g. At the end of 60 cycles, the discharge capacity of the LP30 and HE cycled cells were 128 and 161 mAh/g, respectively. The higher capacity for the HE suggests a larger proportion of lithium is being reversibly cycled from the electrodes. The capacity fade at the end of 10th and 60th cycles in LP30 were 5% and 22%, whereas in HE it was only 1% and 6% respectively. Further, rate performance of these hybrid electrolytes were tested at various C rates upto 1C. It was found that better capacity of around 124 mAh/g were attained in these hybrid electrolytes at high C rate such as 1C as shown in Figure 2e. Further understanding of improved performance of hybrid electrolytes was obtained with the help of surface studies. TEM analysis (Figure 3) revealed a similar thin surface film around the cathode as seen in the standard potential window, as discussed earlier.


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Figure 2. Comparison of LP30 and HE cells in the potential window (3-4.4V vs. Li/Li+) after a) 1st cycle, b) 60 th cycle, (c) cycle number vs. specific capacity of Li /LP30/ LiCoO 2 and Li /HE/ LiCoO2, (d) Comparison of coulombic efficiency of LP30 and HE cells for 60 cycles, (e) Rate performance of hybrid electrolytes at various C rates.



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Figure 3. TEM images of the electrodes after the first cycle in (a) Li /LP30/ LiCoO2 and (b) Li /HE/ LiCoO2 cells in the potential cut off 4.4V vs. Li/Li+. Characterisation of the surface film was carried out using XPS analysis, as shown in Figure 4 & 5, on the electrodes cycled with LP30 and HE electrolytes, as well as the fresh electrode. The XPS survey spectrum of the electrodes is illustrated in Figure S3. Table 1 shows the atomic percentage of elements present on the surface of the electrodes. It is clear that both cycled electrodes feature electrolyte decomposition products on the surface, but closer observation reveals there is more carbon, oxygen and phosphorous on the electrode cycled in LP30, whereas in HE, trace quantities of sulphur are seen, but with smaller quantities of other electrolyte components. This supports our hypothesis that the ionic liquid assists film formation, as sulphur can originate only from the anion in the ionic liquid. The XPS spectra of carbon and oxygen for the LP30 and HE cycled cells are shown in Figure 4. The C1s peaks of fresh LiCoO2 cathodes were found at 284.3 eV, 285.7 eV and 290.1 eV


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corresponding to C-C bonds from added carbon additive in the electrode and C-H bonds and C-F species from the PVDF-HFP binder 49–51. Table 1. Atomic percentage of the elements on the delithiated LiCoO 2 electrode surface Li1-

C 1s

O 1s

F 1s

Co 2p

Li 1s

P 2p

S 2p

S. No

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x

1.

LP30

40.33

12.88

16.34

0.32

28.53

1.59

--

2.

HE

34.25

6.56

7.16

0.60

51.07

---

0.36

CoO2

In the cycled electrodes, the above mentioned peaks were found with variable intensities and in addition a peak at 286.2 eV represents the C-O peak. In the C1s core peak of cycled electrodes, the intensity of C-H peak in the HE cycled electrode is comparatively higher than that of the LP30 cycled electrode which may be due to the presence ionic liquid component on the electrode surface. This in turn suggests an active role for the ionic liquid in the formation of surface film on the electrode surface. The O1s spectra of the fresh electrode includes two peaks; one at 529.45eV represents the cobalt-oxygen bond and the other at 530.9 eV represents the trace carbonate Li2CO3 species in the fresh cathode 49,52. The O1s spectra of the cycled electrodes exhibit additional peaks at 532 eV and 533 eV due to LiOH species and other oxidation products of the electrolytes on the electrode surface 49,53. The XPS spectrum of phosphorous and fluorine are shown in Figure 5. The P 2p spectra of the cycled electrodes consists of two peaks; one at 133.7eV represents the phosphate species arising from oxidation products of the electrolytes at the surface and the other at 136.4 eV



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Figure 4. C 1s and O 1s XPS spectra of the fresh and delithiated LiCoO 2 electrodes cycled in LP30 and HE. represents residual LiPF6 salt on the electrode surface51. These P 2p peaks suggests that the presence of LiPF6 and its decomposition products is lower on the HE cycled electrode.Earlier works 51,54,55 also suggests that a smaller degree of electrolyte decomposition in turn gives better electrode performance, which supports the betterment observed in the performance of HE. The F 1s spectra of the fresh electrode consists of a peak at 687 eV which represents the fluorine component in the binder whereas the cycled electrodes have two peaks, one at 684.4 eV which corresponds to the LiF component from the electrolyte, whereas the peak at 687.3eV stems from the binder as well as from trace amounts of LiPF 6 salt in the electrolyte 40,47.


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Figure 5. P 2p and F 1s XPS spectrum of the fresh and delithiated LiCoO 2 electrodes cycled in LP30 and HE. The S 2p spectra (supplementary information, Figure 4S) on the HE cycled electrode consists of a peak at 169.4eV representing the SO 2F species on the electrode36,47. The oxidised sulphur species on the electrode surface also further indicates the role of ionic liquid in the surface film. In short, the film observed in this work is a composite film with carbon, fluorine, phosphorous and sulphur species. The film formation on the surface of cathodes (LiCoO2) was reported in addition to this work by several researchers12,35,56,57 which claims that the film formed in first cycle will play a clear role in preventing further surface reactions



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of the electrolyte with the cathode. The effectiveness of such films can also be visualised through better capacity retention of the cathode in the subsequent cycles. Differential thermal analysis carried out on delithiated LiCoO 2 cathodes exhibited similar trends as reported in the earlier standard potential window section (Figure 6). It was obvious that the weight losses slightly increased in the electrodes cycled in the extended potential window as extracting lithium beyond 0.5 makes it more thermally vulnerable 38.

Figure 6. Differential thermal analysis (DTA) of delithiated LiCoO2 cathodes charged in LP30, HE and neat [C3mpyr] [TFSI] electrolytes. XRD studies were carried out on the delithiated electrodes at the end of first charge and after 60 cycles, as shown in Figure 7a. As reported by Chung et al23, the capacity fading is predominantly due to surface reactions on the cathode, but structural changes in the bulk do have some impact58. Literature reports that structural changes in LiCoO2 are notably seen in


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the (003) peak35,59–61. It was found here that after first charging cycle, the (003) peak moved to lower angles compared to the fresh electrodes, due to the de-lithiation process28 with the HE showing a lower 2 value than the conventional electrolyte. This immediately suggests more lithium extraction with the HE.

Figure 7. (a) XRD data of fresh LiCoO2, after 1st charging 61st charging in LP30 and HE. (b) Lattice and volume evolution of the LiCoO2 during charging as determined from Rietveld analysis of in situ synchrotron XRD data, boxed regions indicate patterns shown in Fig. 7c and 7d. (c,d) Stacked XRD data of selected 2θ regions corresponding to higher potential regions, (c) new reflections are observed at about 4.45V and (d) monoclinic splitting of the 104 reflection is illustrated at about 4.73V.



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Similarly, after the 61 st charging, the (003) peak of the LP30 cycled cell shows a drop in intensity, and is found at higher 2 values than that of the HE cycled electrode. This indicates better structural stability in the HE cycled electrodes and possibly more lithium extraction 23,60.

To shed further light on the structural evolution, in situ synchrotron XRD data were collected. The Rietveld refined fit for the electrode in the coin cell for the first XRD pattern is shown in Figure S5 (b) and the lattice evolution using a single R-3m phase structural model with the electrochemical charge curve shown in Figure 7b. Stacked individual patterns at selected potentials are shown in Figure 7c and 7d. On charge, the cell was held at 4.2V for 30 minutes followed by charging to 4.5V and holding at 4.5V for 60 minutes followed by further charging. The lattice evolution shows that the c lattice parameter and volume increase while the a lattice parameter decreases until 4.3V and then the parameters stabilize showing minimal change in the ramp from 4.3V to 4.45V. During the 4.5V hold the opposite trend is observed with the c lattice parameter and volume decreasing and the a lattice parameter increasing. This behavior to 4.3V is consistent with the literature, with the removal of Li ions leading to a repulsion of the CoO2- layers expanding the c lattice parameter or the stacking axis and the a lattice parameter decreasing in part due to the oxidation of Co3+ to Co4+. Early work with LiCoO2 being close to equilibrium suggested that the expansion of the c or stacking axis stabilizes around 4.12V which corresponds to 0.5 Li extracted from LiCoO2 58, additionally a monoclinic splitting was observed for the 104 reflection between 4.05-4.2V, which is also observed around 4.2V in related studies under different electrochemical conditions59. Although slightly different electrochemical conditions are used in our study, the monoclinic splitting of the 104 reflection is only observed at 4.73V with some evidence of multiple phases present at 4.57V with the 003 reflection, see Figure S5 (c). Interestingly, the R-3m layered structure is preserved to at least 4.3V, see Figure S5 (d) for a Rietveld refined


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fit, with no evidence of peak splitting or secondary phases. Minor additional reflections appear at 4.45V as seen in Figure 7d. In any case, at least to 4.3V the R-3m structure is preserved with the use of the HE, which suggests that the combination of the surface layer and HE may play a role in stabilizing this structure to lower lithium contents (allowing a larger amount of reversible Li insertion/extraction while maintaining the R-3m structure). This in turn allows the electrode to function at higher potentials with reasonable capacity retention. In addition, the surface film may be amorphous, as there is no significant peak contribution of the surface film in the ex-situ XRD taken after cycles (1 st and 60th cycles) and in the in-situ synchrotron studies of the cathodes cycled with this hybrid electrolyte. The exact mechanism of the role of the HE in potentially stabilizing the R-3m structure is intriguing and unknown at this time; speculatively solvent or a component of intercalation could be occurring or the interface layer is stabilizing the outer layers of the Li xCoO2 particle which in turn stabilizes the core. These hypotheses all require further work to understand the mechanism at play. Conclusions In this work, we have demonstrated the use of hybrid electrolytes with LiCoO 2 cathodes in the standard potential window, as well as the elevated cut-off potential of 4.4V vs. Li/Li+. The presence of ionic liquid in the electrolyte plays a key role in improving capacity retention and thermal stability with delithiated (Li1-xCoO2) cathodes especially close to x ~ 0.5 where it appears that the phase transitions (monoclinic and other phases) do not appear until higher potentials. With the help of surface studies, we show that the film formation on the cathode surface involves the ionic liquid species, forming a thinner, more stable film that reduces further electrolyte decomposition at elevated potentials (4.4V). We illustrate that the electrode maintains the R-3m to at least 4.3V and no new phases are formed. In short, without



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the use of any ceramic surface coatings or metal dopants in the cathodes, we report that the HE provides improved performance and safety with LiCoO2 cathodes. ASSOCIATED CONTENT Supporting Information. Separate file is attached for supporting information. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ABBREVIATIONS LiCoO2, Lithium cobalt oxide; Li1-xCoO2, delithiated lithium cobalt oxide; HE, Hybrid Electrolyte; LP30, Conventional battery electrolyte; Li2CO3, Lithium carbonate; PVDF, Polyvinylidene fluoride; LiFePO4, Lithium iron phosphate; XRD, X-ray diffraction; TEM, Transmission electron microscopy, TGA, Thermogravimetric analysis, XPS, X-ray Photoelectron spectroscopy. SUPPORTING INFORMATION The supporting information includes the performance of LiCoO 2 cathodes in the standard potential window, the Rietveld refinement of delithiated LiCoO 2 at extended potentials,


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performance of LiCoO2 cathodes at 40oC and the plot of ionic conductivity of hybrid electrolytes in comparison with the conventional electrolyte. ACKNOWLEDGMENT The authors extend their gratitude to SAIF IIT Bombay for their assistance in HR-TEM analysis, CeNSE, IISc Banglore for the XPS analysis, Prof. V.S Raja for the TGA facility and Mr. Ajay Krishnan for his assistance in TGA measurements. DRM is grateful to the Australian Research Council for his Australian Laureate Fellowship. NS would like to thank the Australian Research Council for providing support through the DECRA (DE160100237) and DP (DP170100269) programs. Part of this research was undertaken on the powder diffraction beamline at the Australian Synchrotron, Victoria, Australia. This work was carried out by the financial support from IITB Monash research Academy, NCPRE-IIT Bombay and Reliance Industries Limited, India. REFERENCES (1)

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Hybrid Ionic liquid for higher safety of LCO-based cathode 450x238mm (96 x 96 DPI)


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Understanding the Behavior of LiCoO2 Cathodes ... - ACS Publications

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