Special Synergy between Electrolyte Additives and Positive Electrode Surface Coating to Enhance the Performance of Li[Ni0.6Mn0.2Co0.2]O2/Graphite Cells

Li[Ni0.6Mn0.2Co0.2]O2 (NMC622) is one of the most attractive positive electrode materials for Li-ion cells. Ultra high precision coulometry, impedance spectroscopy, gas evolution and long-term cycling were used to explore the combined impact of several electrolyte additives and an Al2O3 surface coating on NMC622/graphite cells tested to 4.4 V at 40◦C. An excellent correlation between the short-term coulombic efficiency measurements and the long-term cycling results was noted. An electrolyte containing 1 M LiPF6 in ethylene carbonate: ethyl methyl carbonate (3:7 by weight) with additives 2 wt% prop-1-ene-1,3 sultone (PES) + 1 wt% methylene methane disulfonate (MMDS) + 1 wt% tris (trimethylsilyl) phosphite (TTSPi) (this electrolyte is called PES 211) was found to give the best performance in both coated and uncoated NMC622/graphite cells. The surface coating was found to improve coulombic efficiency, reduce impedance and improve capacity retention in all cases studied. However, uncoated NMC622/graphite cells with PES211 were found to perform better than coated NMC622/graphite cells which used any other electrolyte. The special synergy between Al2O3-coated NMC622 and PES211 needs to be understood so that even better coatings and electrolyte combinations can be developed. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0171613jes] All rights reserved.

Li-ion batteries are the best power sources for applications ranging from portable electronics to electric vehicles. 1,2 Properties such as high energy density, high power, low cost and longer life-time (calendar life) are often desired for such applications. Increasing the upper cutoff voltage in Li[Ni 1-x-y Mn y Co z ]O 2 "NMC" Li-ion cells results in increased energy density. 3 However, increasing the upper cutoff voltage increases undesirable side reactions 4 (parasitic) at the electrodeelectrolyte interface and shortens the lifetime of the cell. 5 While high energy density is important for many applications, lifetime is also important.
Electrochemical oxidation of electrolytes occurring at the positive electrode at high voltage is a primary cause of cell failure. Hence methods to hinder these parasitic reactions should be developed. Coating the surface of the positive electrode and using electrolyte additives are two popular ways to suppress the parasitic reactions and thus improve the cell lifetime. 6,7 Zheng et al. claimed that an AlF 3 coating on Li and Mn rich positive electrode materials could reduce electrolyte oxidation at high voltage by suppressing thick solid electrolyte interface (SEI) formation. 8 Recently, Mohanty et al. showed a significant suppression of impedance by using an Al 2 O 3 coating on NMC or NCA (LiNi 0.85 Co 0.1 Al 0.05 O 2 ) materials which improved cell performance. 9 Wise et al. studied the impact of an Al 2 O 3 coating on NMC442 and reported that an Al 2 O 3 coating could mitigate side reactions between the positive electrode and the electrolyte at high voltage (>4.4 V). 10 As an example of work of the Umicore authors of this paper, Figure 1 shows a comparison between the charge-discharge capacity versus cycle number of Al 2 O 3 -coated NMC622/graphite and uncoated NMC622/graphite pouch type cells built at Umicore. The cells in Figure 1 used the same electrolyte which was supplied by PanaxEtec. They were tested at 1 C charge with a constant voltage hold till C/20 followed by a 1 C discharge between 3.0 V and 4.35 V at 25 • C. The cells had a capacity of about 700 mAh with an initial typical gravimetric energy density of 180 Wh/kg @ 4.35 V. Figure 1 shows that: 1) Uncoated NMC622 -compared to Al 2 O 3 -coated NMC622 -performs quite poorly in a standard electrolyte and; 2) Under identical conditions Al 2 O 3 -coated NMC performs quite well, still retaining >85% of 1 C capacity after 1000 cycles.
Hence Al-based materials, especially Al 2 O 3 , appear to be a good choice for surface coatings on positive electrode materials. In this study, the impact of an Al 2 O 3 coating on NMC622 on the cell performance has been studied in comparison with uncoated NMC622.
Using additive blends along with positive electrode surface coatings should improve electrochemical properties. Xia et al. showed that NMC442/graphite cells with a 3 wt% LaPO 4 -coating on the surface of the NMC442 resulted in reduced electrolyte oxidation and better capacity retention, when cycled to 4.5 V when certain electrolytes were used. 14 They also showed that for those NMC442/graphite cells, the benefits of additives were greater than those of the LaPO 4 surface coating. However, Song et al. showed that a 3 wt% coating of LaPO 4 on Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 (NMC532) was highly effective at improving charge-discharge capacity retention in the absence of additives. 15 Apparently, the electrochemical properties depend on the nature of the coating, the presence or absence of additives and also on different experimental conditions. Nelson et al. reported the importance of impedance growth caused by electrolyte oxidation. 3 They studied the LaPO 4 -coated NMC442/graphite cells using simultaneous cycling and electrochemical impedance spectroscopy (EIS) experiments in the presence of control electrolyte + 2 wt% prop-1-ene-1,3-sultone (PES) + 2 wt% 1,3,2-dioxathiolane-2,2-dioxide (DTD) + 2 wt% tris-(trimethyl-silyl) phosphite (TTSPi), called PES222. They found severe impedance growth in coated cells when aggressively cycled above 4.4 V. In their studies, the behavior of coated and uncoated cells differed with changes in the upper cutoff voltages. Hence the cumulative effect of a particular combination of a positive electrode material, a surface coating and an additive blend at a particular upper cutoff voltage is unique and can only be determined from experimental observations. This article explores the effectiveness of an Al 2 O 3 coating in presence of selected electrolyte additives in NMC622/graphite cells.
To prepare uncoated NMC622, lithium carbonate and a mixed Ni-Mn-Co oxyhydroxide were homogeneously blended in a vertical single-shaft mixer by a dry powder mixing process. The blend ratio was targeted to obtain Li 1.01 (Ni 0.6 Mn 0.2 Co 0.2 ) 0.99 O 2 , which was verified by inductively coupled plasma (ICP) mass spectroscopy. The mixture was then sintered in a tunnel furnace in dry air at 900 • C for approximately 10 h. After sintering, the sample was milled in a grinding machine to a mean particle size of around 12 micrometers (μm). Al 2 O 3 -coated NMC622 was prepared from the uncoated NMC622 according to the procedures outlined in PCT patent application PCT/IB16/050257. Uncoated and coated NMC622/graphite pouch cells of 402035 size with respective capacities of 200 and 220 mAh were used in this study. The cells with NMC622 positive electrode material that was coated with 0.2 wt% of Al 2 O 3 are referred to as coated-NMC 622 cells. Both types of cells were balanced for 4.45 V operation. The uncoated and coated positive electrodes comprised of 92:4:4% by weight of active material, PVDF binder and carbon black, respectively. The negative electrodes were made from artificial graphite, carbon black and CMC/SBR binder in a 96:2:2 weight ratio. The dry pouch cells were assembled in a dry room at Umicore, vacuum sealed without any electrolyte and then shipped to our laboratory in Canada for electrolyte filling and testing.
Electrolyte filling.-The pouch cells were opened and dried at 80 • C under vacuum for about 14 h to remove residual water before filling with electrolyte. The cells were then taken into an argon-filled glove box for electrolyte filling and subsequent vacuum sealing. The argon used in the glove box was ultra high purity grade (Praxair UHP grade 99.999% purity) that was further treated by the glove box purification system to reduce water and oxygen levels below 1 ppm. 0.75 mL (0.90 g) of the electrolyte was poured into the pouch cells and they were vacuum-sealed at −90 kPa (gauge pressure) using a compact vacuum sealer (MSK-115A, MTI Corp.). After the electrolyte filling and sealing, the volume of the cell was measured using Archimedes' principle as explained in the upcoming section.
Formation steps.-Formation step 1: The cells were held at 1.5 V at 40. • C for 24 h to ensure sufficient wetting. Then, they were charged to 3.5 V at C/20 at 40 • C. After formation step 1, the volume of the cell was measured again. The difference in the volumes measured before and after formation gives the volume of any gas generated. Then, the cells were transferred into the glove box, opened to release any gas generated and vacuum sealed again.
Formation step 2: In this step, the sealed cells were charged again from 3.5 V at C/20 to 4.4 V, held at 4.4 V for 1 h and then discharged to 3.8 V. After reaching 3.8 V, the volume of the cells were measured again, the cells were transferred into the glove box, degassed and vacuum sealed again. After the two degassing processes, electrochemical impedance spectroscopy (EIS) testing was conducted at 10 • C with the cells at 3.8 V.
EIS measurement.-Impedance spectra were measured on the pouch cells before and after cycling at 3.8 V at a temperature of 10. ± 0.1 • C using Biologic VMP-3 system. Alternating current (AC) impedance spectra were collected with ten points per decade from 100 kHz to 10 mHz with a signal amplitude of 10 mV.

Ultra high precision charger (UHPC) experiment.-Selected
cells were cycled using the ultra high precision charger (UHPC) at Dalhousie University 16 between 2.8 and 4.4 V at 40. ± 0.1 • C using currents corresponding to C/20 for 16 cycles.
Cycle-store test.- Figure 2 shows the protocol used for cycle-store testing. Initially, the pouch cells were kept in temperature boxes at 40. ± 0.1 • C. The cells were first cycled between 4.4 V and 2.8 V twice at a rate of C/20 and then at C/5 for the subsequent cycles up to a desired number of cycles. During the C/5 rate cycling, the cells were left at open circuit for 24 h at the top of every charge (4.4 V), while their potentials were recorded.
Gas measurement.-The gas volume (Ex-situ) of the cells were measured after formation steps and cycling 17 with Archimedes' principle. First, the sealed pouch cells were suspended under a Shimadzu balance (AUW200D) using a fine wire hook. The cells were immersed in a beaker of de-ionized "nanopure" water (18.2 M ) maintained at 20. ± 1 • C. The weight of the cell was measured while immersed before and after gas generation. The difference in the weights ( m) of the cell before and after gas generation is equivalent to the volume change ( v) caused by change in the buoyant force. The weight Results and Discussion  Figure 3 shows that the specific capacity increased as the upper cutoff potential increased reaching a maximum of ∼250 mAh/g for the first charge to 4.6 V. The reversible capacity was 225 mAh/g for the cells tested to 4.6 V. The difference in measured specific capacities between coated and uncoated NMC622 cells were negligible, as expected based on the small weight fraction (0.2% of Al 2 O 3 ) in the coating. Figure 4 shows the data collected from the UHPC experiments (2.8-4.4 V at C/20 and 40 • C) on coated NMC622/graphite pouch cells with the selected electrolyte additive blends. In Figure 4, the legends corresponding to black dots, blue squares, red triangles, green crosses and magenta triangles represent the electrolyte additives -control, 2% VC, 2% PES, PES211 and 1% PBF respectively. Figure 4a shows the coulombic efficiency (CE) versus cycle number for the cells with the studied additives. Cells with the additive combinations showed better CE than those of the cells with control electrolyte. Cells with the additive PES211 exhibited the best CE (0.9986 at the 16 th cycle) suggesting that cells with PES211 would have the longest lifetime. A CE of 0.9986 at cycle 16 under these test conditions is the best ever measured in this laboratory on any NMC/graphite cell. Figure 4b shows the discharge capacity versus cycle number for the first 16 cycles. Cells with PES211 showed the best capacity retention. Figure 4c shows the charge end point capacity versus cycle number for the first 16 cycles. For cells with all the studied additives, the charge end point capacity increased with increasing cycle number indicating electrolyte oxidation. The charge end point capacity slippage in cells with the additive blends was less than that of cells with control electrolyte suggesting that additive blends help mitigate electrolyte oxidation at the positive electrode interface. Cells with the additive blend PES211 exhibited the lowest increase in charge end point capacity. Figure 4d shows the voltage difference between the average charge and discharge voltage ( V) versus cycle number for the first 16 cycles. V decreased during the first 5 cycles and became almost constant after that for all cells. Figure S2 shows   Figure 5 shows the CE versus cycle number in a format that allows easy comparison between coated and uncoated NMC622 cells with the same electrolyte additives. Figure 5 shows that the CE of coated NMC622 cells were always better that of the corresponding uncoated cells. However for cells with PES211, the difference in the CE between coated and uncoated cells is very small suggesting that the use of PES211 outweighs the advantage brought by the Al 2 O 3 coating. In fact, the uncoated NMC622 cells with PES211 have a higher CE than the coated cells with any of the other electrolytes. Figure 6 shows a summary of UHPC data presented in Figures 4 and S2. Figure 6a shows 5 sets of comparative histograms of coulombic inefficiency (CIE), which is equal to 1-CE, of the cells corresponding to the studied additive blends. The CIE reported in Figure 6a was the average of the CIE of the last 5 cycles. In all the 5 sets of histograms, which correspond to the 5 selected additives, the CIE of the coated cells (red) are better than that of the uncoated cells (blue) indicating the positive role played by the Al 2 O 3 coating in reducing the parasitic reactions. Upon comparing the effect of the additive blends, PES-containing additive blends outperformed all others, especially PES211. Figure 6b shows the charge end point capacity slippage data corresponding to all the additive blends. The charge end point capacity slippage per cycle (mAh/cycle) was determined by calculating the slope of the best fit line to the last five points (11-15 cycles) of the charge end point capacity versus cycle number graph (Figures 4c and  S2c). Once again, all the cells with coated NMC 622, irrespective of the additive blends, had smaller (better) charge end point capacity slippage than those of the uncoated cells. In the presence of the elec-trolyte additive combinations, the charge end point capacity slippage per cycle was lower than that of the control electrolyte. Cells with PES211 exhibited the least charge end point capacity slippage per cycle among all the additives. Figure 6c shows the increase in V per cycle corresponding to cells with all the additive blends. V per cycle was determined by calculating the slope of the best fit line to the final five points (11-15 cycles) of V vs. cycle number graphs (Figures 4d and S2d). Figure  6c shows that the increase in V (impedance growth) for both coated and uncoated cells is very small during the UHPC cycling irrespective of the nature of the additive blends. The distinction between the increase in V between cells with coated and uncoated NMC622 is negligible. Figure 6d shows the discharge capacity loss per cycle (mAh/cycle), which was determined by calculating the slope of a best fit line to the final five points (11-15 cycles) of the discharge capacity vs. cycle number graphs in Figures 4b and S2b. The discharge capacity loss per cycle for coated cells tested with control, 2% VC and 1% PBF was clearly lower (better) than that of uncoated cells. However, when PES-based additives were used, the discharge capacity loss per cycle was almost same for coated and uncoated cells and the capacity loss per cycle was smaller than for cells with the other additive blends. Figures 7a and 7b show the impedance spectra of the coated NMC622 cells after formation and after UHPC experiments (16 cycles) respectively. The diameter of the semicircle represents the sum of the charge-transfer resistances, R ct , of the positive and negative electrodes. After the formation step, the impedance of the coated cells tested with any additive blend was higher than that of the control electrolyte. However, after the UHPC experiments, PES-based additives helped lower the impedance compared to that measured after formation whereas other additives did not affect the impedance   much. Figures 7c and 7d show the impedance spectra of the cells with uncoated NMC622 after formation and after UHPC experiments respectively. The impedance data of all the tested uncoated cells after formation (Figure 7c) almost mirror the results for coated cells (Figure 7a): control electrolyte exhibited the least impedance. On the other hand, the impedance after UHPC cycling decreased significantly in the case of cells with PES-containing additives. By contrast, the impedance of cells with other additive blends as well as the control electrolyte increased and the impedance spectra developed a large low-frequency lobe. Previous work by Nelson et al. 3 and Abarbanel et al. 18 shows that this low frequency lobe is due to impedance increase at the positive electrode/electrolyte interface. Irrespective of the presence of the Al 2 O 3 coating, the impedance control achievable from PES-containing additives after UHPC cycling is far superior to other additive blends. Figure S3 shows a summary of the impedance data in Figure 7 for the coated and uncoated NMC622 cells after formation and after UHPC cycling. In Figure S3, the length of the bars is equal to the corresponding diameter of the semicircles shown in Figure 7. Figure  S3 shows that the coating is very important for impedance control in control, 2% VC and 1% PBF electrolytes, while the coating does not impact the impedance of cells containing 2% PES or PES211 strongly at all. Figures 8a, 8b and 8c show a summary of gas volume data measured after formation step 1, formation step 2 and after UHPC cycling, respectively, for both coated and uncoated NMC622/graphite cells. The intial volume of the cells was about 2.5 mL. Figure 8a shows that the volumes of gas evolved from the cells containing either 2% VC or PES-containing additives were lower than those of control electrolyte or 1% PBF after formation step 1. The gas evolved in formation step 1 is primarily ethylene, associated with the formation of the passive film on the graphite. In the case of control and 1%PBF the passivation film is dominated by the reduction products of EC so the amount of ethylene produced is large. When VC or PES are present, very few gaseous reduction products are created as the major reduction products from these additives are not gaseous. 19 Figure 8b shows that the gas evolution after formation step 2 is very small except in the cells having 2% VC. This gas is primarily CO 2 and is created at the positive electrode. 20 Figure 8c shows that none of cells exhibited any significant gas evolution after UHPC cycling. Overall, Figure 8 suggests that the evolved gas volume is almost same for both coated and uncoated NMC622/graphite cells after each step. Figure 9 shows the discharge capacity of the NMC622/graphite cells measured between 2.8 and 4.4 V for about 150 cycles using the cycle-store protocol (See Figure 2). Figures 9a, 9b, 9c, 9d and 9e correspond to the cells containing control electrolyte (black), 2% VC (blue), 2% PES (red), PES211 (green) and 1% PBF (magenta) respectively. In each of the panels in Figure 9, the closed and open circles correspond to the coated and uncoated NMC622/graphite cells respectively. In all cases, the cells with coated NMC622 had better capacity retention than the uncoated ones indicating the advantage of the Al 2 O 3 coating. Comparing the effect of additive blends, those cells containing either control electrolyte or 2% VC exhibited poor capacity retention with cells showing <70% capacity before 70 cycles. On the other hand, the cells with PES-containing additives and 1% PBF exhibited much better capacity retention especially in cells with coated NMC622. In particular, the capacity retention of cells with both PES211 and coated NMC622 was the best (>80% even after 160 cycles). Figure 10 shows a comparison between the coulombic efficiency results in Figure 5 and the long-term cycling results in Figure 9. Figure  10a shows the CE measured at cycle 16 from Figure 5 and Figure 10b shows the time (in months) for the cell capacity to reach 90% of the initial capacity during the long-term cycling protocol applied in Figure 9. The close correlation between Figures 10a and 10b is obvious which points to the value of coulombic efficiency as a predictor of cell lifetime as has been proposed before. 5 In addition, Figure 10 shows that uncoated NMC622/graphite cells with PES211 are virtually as good or better than coated NMC622/graphite cells with any other electrolyte. This really shows the effectiveness of the PES211 blend. Figure 11 shows a summary of the voltage drop (V drop ) of the tested cells during the storage portion of the cycle-store testing between 2.8 and 4.4 V at 40 • C. Figure 2 shows the typical voltage relaxation during the storage periods. Parasitic reactions at the positive electrode such as electrolyte oxidation are responsible for the long time (>1 h) voltage relaxation while direct current (DC) cell resistance is the source of the short time (<15 min.) voltage relaxation. Figure 11 does not distinguish between these two contributions. Different legends in Figure 11 have been used to indicate V drop measured after a certain number of cycles: 1, 15, 35, 70, 90 and 130. V drop is generally stable for all cells over the first 35 cycles except for cells with control electrolyte. Cells with coated NMC622 that use PES211 have the smallest and most stable V drop of all cells. Cells with uncoated NMC622 that use PES211 have the next smallest and next most stable V drop of all cells. These results are even more indicative of the stability of the PES211 electrolyte system in combination with coated NMC622. Realize that those cells have been exposed to higher potentials than the others cells (smaller V drop ) during the storage periods which would accelerate electrolyte oxidation during the storage period if the additive system had not been effective. Figure 12 shows the impedance spectra collected after cycle-store testing between 2.8 V and 4.4 V at 40 • C. Figures 12a and Figure  12b show the impedance spectra of the cells with coated and uncoated NMC622 respectively. The impedance of the coated NMC622 cells were much smaller than the corresponding cells with uncoated NMC622 implying that the Al 2 O 3 surface coating was effective in  Figure 5 for the coated (red bars) and uncoated (blue bars) NMC622/graphite cells; b) number of months of testing for the cells to reach 90% of initial capacity during the charge-store testing described by Figure 9 using the protocol shown in Figure  2. The electrolytes labeled A, B and C are 2% PES, PES211 and 1% PBF, respectively. The cells tested in Figures 9 and 10b are duplicate cells of those tested in Figure 5.  uncoated/graphite cells. The spectra were collected after the number of charge discharge cycles displayed in Figure 9. The spectra were collected at 10 • C with the cells at 3.8 V.
controlling the impedance. The impedance of the uncoated cells was quite small when tested with PES-containing additives especially PES 211. A change in the scale of the graph is required to observe the changes in the cells with 2% PES and PES211. To this end, Figure 13 shows the impedance spectra collected after formation, after UHPC cycling and after cycle-store testing for the cells tested with the PES211 additive. Both uncoated and coated NMC622/graphite cells show relatively large impedance after formation, which decreases after the UHPC cycling and cycle-store test. The results in Figures 12  and 13 indicate that PES211 could control impedance growth even in the absence of an Al 2 O 3 coating. However, the development of the low frequency lobe in the impedance spectrum of the uncoated NMC622/graphite cells is indicative of developing problems with the positive electrode/electrolyte interface. Ma et al. showed that the impedance of NMC442/graphite cells with PES211 decreased slightly during 500 cycles. 12 This result is in good agreement with the results found here for NMC622/graphite cells tested with PES211. Clearly impedance growth can be impacted by selective surface coating or additives or the combination of both. Figure 14 shows comparative histograms for the gas volume and the impedance measured after cycle-store testing. Figure 14a shows that coated NMC622 cells with 2% PES, PES211 and 1% PBF produced more gas than cells with the other additives (control and 2% VC). Figure 14a shows that NMC622/graphite cells with PES211 produced the most gas, however, these cells were tested for longer periods of time and were exposed to a higher potential during the storage periods (see discussion surrounding Figure 11) than control cells and cells with 2% VC. Coated NMC622 cells with PES211 produced less gas after 120 cycles than uncoated NMC622 cells with PES211 after only 70 cycles. This again, points to the value of the coating when Figure 13. Impedance spectra collected after "cycle-store" testing between 2.8 and 4.4 V at 40 • C for NMC622/graphite cells using PES211 additive for (a) coated NMC622/graphite cells and (b) uncoated NMC622/graphite cells. PES211 is used. However, cells with 1% PBF showed the opposite trend, pointing to the complexities of the coating/additive interaction which will, frankly speaking, take years to understand. Figure 14b shows a summary of the impedance data collected after cycle-store test, which were introduced in Figure 12. Figure 14b once again reiterates that PES-containing additives can suppress impedance growth much better than the other additives.

Conclusions
The electrochemical performances from continuous cycling tests such as discharge capacity, coulombic efficiency, voltage drop and impedance growth were systematically investigated on Al 2 O 3 -coated and uncoated NMC622/graphite pouch cells using five different electrolytes: control, 2% VC, 2% PES, 2% PES + 1% MMDS + 1% TTSPi (PES 211) and 1% PBF. In general, electrochemical properties such as coulombic efficiency, charge end point capacity slippage, voltage drop during storage and impedance growth were better in Al 2 O 3coated NMC622/graphite cells than in uncoated NMC622/graphite cells demonstrating the important role of surface coating on NMC622. Cells with PES-containing additives, especially PES211, exhibited superior performance to cells with the other additive blends. Overall, the combination of Al 2 O 3 -coating and PES211 showed the best electrochemical properties, such as much better coulombic efficiency, better capacity retention and the best impedance control. Researchers should explore other surface coating materials and electrolyte additive combinations for different NMC grades to improve the life-time of Li-ion batteries. It is beyond the scope of this paper to report the mechanisms of the synergistic action of PES211 and Al 2 O 3 -coated NMC622. It may take years to elucidate those mechanisms. The purpose of this paper is to report these exciting results and get other researchers motivated to help understand these mechanisms.