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Anode-free lithium metal cells store 60% more energy per volume than conventional lithium-ion cells. Such high energy density can increase the range of electric vehicles by approximately 280 km or even enable electrified urban aviation. However, these cells tend to experience rapid capacity loss and short cycle life. Furthermore, safety issues concerning metallic lithium often remain unaddressed in the literature. Recently, we demonstrated long-lifetime anode-free cells using a dual-salt carbonate electrolyte. Here we characterize the degradation of anode-free cells with this lean (2.6 g Ah−1) liquid electrolyte. We observe deterioration of the pristine lithium morphology using scanning electron microscopy and X-ray tomography, and diagnose the cause as electrolyte degradation and depletion using nuclear magnetic resonance spectroscopy and ultrasonic transmission mapping. For the safety characterization tests, we measure the cell temperature during nail penetration. Finally, we use the insights gained in this work to develop an optimized electrolyte, extending the lifetime of anode-free cells to 200 cycles. Anode-free batteries have emerged as a promising storage means to offer high energy density but still suffer from long-term reversibility. The authors analyse the cell failure mechanisms and present an optimized electrolyte to extend the lifetime of anode-free pouch cells.

Positive electrode materials remain a limiting factor for the energy density of lithium-ion batteries (LIBs). Improving the structural stability of these materials over a wider potential window presents an opportune path to higher energy density LIBs. Herein, operando neutron diffraction is used to elucidate the relationship between the structural evolution and electrochemical behavior for a series of Li-ion pouch cells containing Li(NixMnyCoz)O2 (x + y + z = 1) electrode chemistries. The structural stability of these electrodes during charge and discharge cycling across a wide potential window is found to be influenced by the ratio of transition-metal atoms in the material. Of the electrodes investigated in this study, the Li(Ni0.4Mn0.4Co0.2)O2 composition exhibits the smallest magnitude of structural expansion and contraction during cycling while also providing favorable structural stability at high voltage. Greater structural change was observed in electrodes with a higher Ni content, while decreasing ...

Part II of this 2-part series examines the impact of competitive graphite materials on NMC811 pouch cell performance using Ultra-High Precision Coulometry (UHPC), isothermal microcalorimetry, and in-situ stack growth. A simple lifetime projection of the best NMC811/graphite cells as a function of operating temperature is made. We show that graphite choice greatly impacts fractional fade, while fractional charge endpoint capacity slippage was largely unchanged due to identical cathodes. We show that an increase in graphite 1st cycle efficiency due to limited redox-active sites—favourable for minimizing Li inventory loss—is concomitant with an increase in negative electrode charge transfer resistance. Further, we demonstrate that cells with competitive artificial graphites (AG) have a lower parasitic heat flow (∼0.060 mW A−1 h−1 at 40 oC) compared to cells with natural graphites (NG), and that the cells with the AGs had minimal stack thickness change with cycling. Finally, we model SEI growth for NMC811 cells limited to 4.06 V with the square-root time model, and project that the best NMC811/graphite cells here can have decades-long lifetimes at 20 –30 °C when Li plating is avoided. Such cells are excellent candidates for grid storage applications where energy density is less important compared to long lifetime.

Unwanted lithium plating on the graphite anode of lithium ion batteries can reduce the cycle life and safety of lithium ion batteries. Increased charging rates, lower temperatures, thicker electrodes, lower Li-ion diffusion constant and larger graphite particles all increase the propensity for unwanted lithium plating. In this work, a variety of electrolyte additives and electrolytes, which extend lifetime during low rate cycling, were used in Li[Ni1/3Mn1/3Co1/3]O2/graphite (NMC111/graphite) pouch cells subjected to high rate charging at 20°C. It was found that additives and electrolytes which increased the negative electrode area-specific resistance, Rnegative, decreased the onset current, Iu, for unwanted lithium plating. Here, the processes of ion desolvation, electron and ion transport through the solid electrolyte interphase and contact resistance are lumped into the Rnegative. Under conditions where Rnegative is the dominant factor determining when unwanted Li plating occurs, the onset current for lithium plating could be well predicted by the expression: Iu = 0.080 V x S/Rnegative, where S is the geometric electrode surface area. Rnegative is easily determined using negative electrode coin-type symmetric cells. This simple rule-of-thumb relation will help guide researchers seeking to select electrolyte additives that simultaneously increase lifetime and also allow fast charging.

Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells with an ethylene carbonate-containing or a fluorinated electrolyte were used to prepare charged electrodes for studies using "pouch bags". Sealed pouch bags containing either lithiated graphite or delithiated NMC442 electrodes taken from pouch cells, and also "sister" pouch cells, were subjected to 500 h storage at elevated temperature. The electrodes recovered from the pouch bags and pouch cells after storage were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy while the gases generated were quantified using gas chromatography. The fluorinated electrolyte suppressed impedance growth of the positive electrode during storage but caused a large initial negative electrode impedance compared to the carbonate electrolyte. The solid electrolyte interface (SEI) formed by the fluorinated electrolyte at the graphite electrode hinders the consumption of CO2 generated at the delithiated NMC442 electrode, leading to more CO2 in pouch cells with fluorinated electrolyte than in cells with carbonate electrolyte. Hydrogen gas was only observed in pouch cells after storage and not in pouch bags which contained either a single negative electrode plus electrolyte or a single positive electrode plus electrolyte, suggesting the H2 results from a species created at one electrode which reacts at the other in a pouch cell.

Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells demonstrate superb performance at high voltage when ethylene carbonate (EC)-free electrolytes, using a solvent mixture that is >95% ethyl methyl carbonate (EMC) and between 2 and 5% of an "enabler", are used. The "enablers", required to passivate graphite during formation, can be vinylene carbonate (VC), methylene-ethylene carbonate (MEC), fluoroethylene carbonate (FEC) or difluoro ethylene carbonate (DiFEC), among others. In order to optimize the amount of "enabler" added to EMC, gas chromatography coupled with mass spectrometry (GC-MS) was used to track the consumption of "enabler" during the formation step. Storage tests, electrochemical impedance spectroscopy (EIS), ultrahigh precision coulometry (UHPC), long-term cycling, differential voltage analysis and isothermal microcalorimetry were used to determine the optimum amount of enabler to add to the cells. It was found that the graphite negative electrode cannot be fully passivated when the amount of "enabler" is too low resulting in gas production and capacity fade. Using excess "enabler" can cause large impedance and gas production in most cases. The choice of "enabler" also impacts cell performance. A solvent blend of 5% FEC with 95% EMC (by weight) provides the best combination of properties in NMC442/graphite cells operated to 4.4 V. It is our opinion that the experiments and their interpretation presented here represent a primer for the design of EC-free electrolytes.

Some of the problems of current electrolytes for high voltage Li-ion cells originate from ethylene carbonate (EC) which is thought to be an essential electrolyte component for Li-ion cells. Ethylene carbonate-free electrolytes containing 1 M LiPF 6 in ethylmethyl carbonate (EMC) with small loadings of vinylene carbonate, fluoroethylene carbonate, or (4R,5S)-4,5-Difluoro-1,3-dioxolan-2-one acting as “enablers” were developed. These electrolytes used in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch type Li-ion cells tested at 4.2 V and 4.5 V yielded excellent charge-discharge cycling and storage properties. The results for cells containing linear alkyl carbonate electrolytes with no EC were compared to those of cells with EC-containing electrolytes incorporating additives proven to enhance cyclability of cells. The combination of EMC with appropriate amounts of these enablers yields cells with better performance than cells with EC-containing electrolytes incorporating additives tested to 4.5 V. Further optimizing these linear alkyl carbonate electrolytes with appropriate co-additives may represent a viable path to the successful commercial utilization of NMC/graphite Li-ion cells operated to 4.5 V and above.

The effectiveness of electrolyte additives in fluorinated electrolytes containing 1 M LiPF 6 /fluoroethylene carbonate:bis (2,2,2-trifluoroethyl) carbonate (1:1 w:w) were studied in high voltage Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch cells tested to 4.5 V. The results showed that fluorinated electrolytes containing prop-1-ene-1,3-sultone alone or in combination with other additives exhibited significant improvements in terms of coulombic efficiency and charge endpoint capacity slippage during UHPC cycling, voltage drop during storage, as well as capacity retention during long-term cycling compared with state-of-the-art ethylene carbonate-based (ethylene carbonate: ethylmethyl carbonate 3:7) or sulfolane-based electrolytes (sulfolane: ethylmethyl carbonate 3:7) with some promising additive blends. These results indicate that fluorinated electrolytes offer an interesting alternative for high voltage Li-ion batteries.

The consumption of the additive prop-1-ene-1,3-sultone (PES) in Li[Ni 0.33 Mn 0.33 Co 0.33 ]O 2 /graphite (NMC(111)/graphite) pouch cells during the first cycle and after hold periods at different cell potentials was measured using gas chromatography coupled with mass spectrometry (GC-MS). GC-MS measurements of the electrolyte of NMC(111)/graphite pouch cells during the first charge showed that the amount of PES consumed, and the cell impedance, increased with increasing starting concentration up to a starting concentration of 5 wt% and then seemed to slow down at higher starting concentration. Additive consumption measurements after holding periods showed that PES is consumed in measurable amounts at the positive electrode, suggesting that PES oxidizes at potentials as low as 4.27 V vs. Li/Li + on the NMC(111) surface. Electrochemical impedance spectroscopy on symmetric cells as well as X-ray photoelectron spectroscopy of electrodes salvaged from NMC(111)/graphite cells after hold periods at various cell potentials strongly support the existence of a positive-negative electrode interaction.

The effect of LiPF 6 concentration in the 1 M–2.5 M range was studied in Li[Ni 0.4 Mn 0.4 Co 0.2 O] 2 (NMC(442))/graphite and LaPO 4 coated-NMC(442)/graphite pouch cells cycled to high voltage. Electrochemical impedance spectroscopy on symmetric cells revealed that the dramatic impedance growth observed in NMC(442)/graphite cells cycled to high voltage comes from the interface impedance of the positive electrode. The use of high LiPF 6 concentrations in the 2–2.5 M range dramatically slowed down the impedance growth of both coated and uncoated NMC(442)/graphite cells containing certain electrolyte additive blends and cycled to high voltage. However no beneficial effect was observed in control cells containing no electrolyte additive. X-ray photoelectron spectra of cycled electrodes of coated-NMC(442)/graphite cells showed that LiPF 6 concentration greatly affected the composition of the solid electrolyte interphase of both the positive and negative electrodes of cells containing additives.

The application of detailed in-situ neutron diffraction studies on lithium ion batteries has been limited in part due to the requirement of expensive deuterated carbonate-based electrolyte. This work presents an in-situ neutron diffraction study of the structural evolution of the Li x (Ni 0.4 Mn 0.4 Co 0.2 )O 2 (NMC442) positive electrode material using a recently-developed low-cost deuterated ethyl acetate-based electrolyte. Rietveld analysis show that the NMC442 c lattice parameter gradually increases until x = 0.47 (4.03 V) and then decreases during the first charge. The decreasing trend of the c lattice parameter with time during the hold at 4.7 V and 4.9 V agrees very well with the change of current. Overall the structural changes appear highly reversible when 4.7 V is used as an upper cutoff voltage, even following a 10 h hold at 4.7 V. However, the electrode/electrolyte changes dramatically when charged and held at 4.9 V. There is a significant drop in background attributed to electrolyte decomposition and an unexpected increase in the a lattice parameter is noted after the 4.9 V hold. Therefore, the electrolyte system used is both beneficial for in-situ neutron diffraction studies and battery performance until 4.7 V, but appears to degrade in combination with the electrode at 4.9 V. By comparison to Li(Ni 0.8 Mn 0.1 Co 0.1 )O 2 (NMC811), the contraction of the c lattice with increasing voltage and decreasing lithium content of the NMC442 is less rapid. The transition metal composition significantly affects the c lattice contraction above 4.0 V.

The impact of the electrolyte additives methyl phenyl carbonate, ethyl phenyl carbonate, and diphenyl carbonate was evaluated in Li[Ni 0.33 Mn 0.33 Co 0.33 ]O 2 /graphite pouch cells with or without 2% vinylene carbonate. Experiments included high precision coulometry, automated storage, electrochemical impedance spectroscopy on symmetric cells and gas chromatography coupled with mass spectrometry. Gas chromatography/mass spectrometry analysis, electrochemical studies during the first charge and impedance spectroscopy on symmetric cells indicated that phenyl carbonates act as solid electrolyte interphase modifiers rather than formers. High precision coulometry showed that cells containing 1–4 wt% methyl phenyl carbonate, ethyl phenyl carbonate or diphenyl carbonate had similar coulombic efficiencies and charge-endpoint capacity slippage as cells filled with 2 wt% vinylene carbonate. Impedance spectroscopy showed that cells containing phenyl carbonates have substantially lower impedance than cells filled with 2 wt% vinylene carbonate and produced minimal volumes of gas during cell use. Results presented in the report show that phenyl carbonates are competitive additives for 4.2 V class cells and should lead to good cycle life, low polarization and low gas evolution during normal use. Phenyl carbonates can also be used as gas-producing safety agents (to trip pressure activated disconnects) in combination with vinylene carbonate in cylindrical or prismatic cells without adverse effects.

A low-cost deuterated electrolyte suitable for in situ neutron diffraction measurements of normal and high voltage Li-ion battery electrodes is reported here. Li[Ni 0.4 Mn 0.4 Co 0.2 ]O 2 /graphite (NMC(442)/graphite) pouch cells filled with 1:0.1:2 (molar ratio) of lithium bis(fluorosulfonyl) imide (LiFSi):LiPF 6 : ethyl acetate (EA) and LiFSi:LiPF 6 :deuterated EA (d8-EA) electrolytes were successfully cycled between 2.8 V and 4.7 V at 40°C for 250 h without significant capacity loss, polarization growth, or gas production. The signal-to-noise ratio of neutron powder diffraction patterns taken on NMC(442) powder with a conventional deuterated organic carbonate-based electrolyte and filled with LiFSi:LiPF 6 :d8-EA electrolyte were virtually identical. Out of all the solvents widely available in deuterated form tested in highly-concentrated systems, EA was the only one providing a good balance between cost and charge-discharge capacity retention to 4.7 V. The use of such an electrolyte blend would half the cost of deuterated solvents needed for in situ neutron diffraction measurements of Li-ion batteries compared to conventional deuterated carbonate-based electrolytes.

The use of highly concentrated lithium bis(fluorosulfonyl)imide (LiFSi) electrolyte in ethyl acetate (EA) is reported here. Li[Ni 0.33 Mn 0.33 Co 0.33 ]O 2 /graphite (NMC(111)/graphite) and Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 /graphite (NMC(442)/graphite) pouch cells filled with a 3:32:65 (molar ratio) of LiPF 6 :LiFSi:EA electrolyte, were successfully cycled between 2.8–4.2 V and 2.8–4.4 V, respectively at 40 °C over a period of several weeks without dramatic capacity loss. Cells filled with the highly concentrated LiFSi:EA electrolyte demonstrated surprisingly low impedance compared to a conventional electrolyte composed of organic carbonates after 400–800 h of cycling at 40 °C. LiFSi was the most effective salt in reducing the amount of ethyl acetate consumed at the negative electrode during the first charge compared to LiPF 6 and lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) at equivalent molar ratios. The work reported here shows that highly unconventional solvents can be used in full Li-ion cells without any cyclic carbonates at all, provided high concentrations of the appropriate electrolyte salt are used.

The role of two homologous cyclic sulfate electrolyte additives, trimethylene sulfate (or 1,3,2-dioxathiane-2,2-dioxide, TMS) and ethylene sulfate (or 1,3,2-dioxathiolane-2,2-dioxide, DTD), used either alone or in combination with vinylene carbonate (VC) on the lifetime of LiNi1/3Mn1/3Co1/3O2(NMC)/graphite pouch cells was studied by correlating data from gas chromatography/mass spectroscopy (GC–MS), dQ/dV analysis, ultrahigh precision coulometry, storage experiments, and X-ray photoelectron spectroscopy. For VC alone, more stable and protective SEI films were observed at the surface of both electrodes due to the formation of a polymer of VC, which results in higher capacity retention. For TMS, similar chemical SEI compositions were found compared to the TMS-free electrolytes. When VC was added to TMS, longer cell lifetime is attributed to VC. For DTD, a cell lifetime that competes with VC was explained by a preferential reduction potential and a much higher fraction of organic compounds in the SEI films. ...

The effects of three well-known electrolyte additives, used singly or in combination, on LiCoO2/graphite pouch cells has been investigated using the ultra high precision charger (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS) and long term cycling Vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and lithium bis(oxalato) borate (LiBOB) were chosen for study. The results show that combinations of electrolyte additives that act synergistically can be more effective than a single electrolyte additive. However, simply using 2% VC yielded cells very competitive in coulombic efficiency (CE), charge endpoint capacity slippage and charge transfer resistance (Rct). For cells with 1% LiBOB and VC (1, 2, 4 or 6%), adding VC above 2% does not increase the CE, but increases the electrode charge transfer impedances. Rct for cells containing 1% LiBOB and VEC (0.5, 1 or 4%) decreased after long term cycling (1800 h), compared to that tested after the UHPC cycling (500 h) indicating that VEC might be useful for the design of power cells. However, the opposite behaviour (increasing Rct with cycle number) was observed for the control cells or cells containing LiBOB and/or VC.

The effectiveness of well-known electrolyte additives singly or in combination on LiCoO 2 /graphite pouch cells has been systematically investigated and compared using the ultra high precision charger (UHPC) at Dalhousie University and electrochemical impedance spectroscopy (EIS). UHPC studies are believed to identify the best electrolyte additives singly or in combination within a short time period (several weeks). Three parameters: 1) the coulombic efficiency (CE); 2) the charge endpoint capacity slippage (slippage) and 3) the charge transfer resistance ( R ct ), of LiCoO 2 /graphite pouch cells with different electrolyte additives singly or in combination were measured and the results for over 55 additive sets are compared. The experimental results suggest that a combination of electrolyte additives can be more effective than a single electrolyte additive. However, of all the additive sets tested, simply using 2 wt.% vinylene carbonate yielded cells very competitive in CE, slippage and R ct . It is hoped that this comprehensive report can be used as a guide and reference for the study of other electrolyte additives singly or in combination.

The effect of various electrolyte additives and additive combinations added to a 1 M LiPF6 EC:EMC electrolyte on the positive and negative electrodes surface of 1 year old wound LiCoO2/graphite cells and Li[Ni0.4Mn0.4Co0.2])O2/graphite cells was studied using electrochemical impedance spectroscopy (EIS) on symmetric cells. The additives tested were: vinylene carbonate (VC), trimethoxyboroxine (TMOBX), fluoroethylene carbonate (FEC), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and H2O alone or in combination. In general, compared to control electrolyte, the additives tested reduced the impedance of the positive electrode and increased the impedance of the negative electrode with the exception of LiTFSI in Li[Ni0.4Mn0.4Co0.2]O2/graphite wound cells. Higher charge voltage led to higher positive electrode impedance, with the exception of 2%VC + 2% FEC, and 2% LiTFSI. In some cases, some additives when mixed with another controlled the formation of the SEI at one electrode, and shared the formation of the SEI at one electrode when mixed with a different additive.

The effects of electrolyte additives singly or in combination on Li[Ni1/3Mn1/3Co1/3]O2 (NMC)/graphite pouch cells have been systematically investigated and compared using the ultra high precision charger (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS), an automated storage system, gas evolution measurements and selected long-term cycling experiments. The results of testing Li[Ni1/3Mn1/3Co1/3]O2 (NMC)/graphite pouch cells with different electrolyte additives singly or in combination were measured and the results for over 110 additive sets are compared. A "Figure of Merit" approach is used to rank the effectiveness of the additives and their combinations. The combination of vinylene carbonate (VC) and/or prop-1-ene-1,3 sultone (PES), a sulfur containing additive, such as methylene methane disulfonate (MMDS), as well as either tris(-trimethly-silyl)-phosphate (TTSP) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) as additives in the electrolyte can give cells with extremely high coulombic efficiency, excellent storage properties, low impedance and superior long term cycling at 55°C. Additive mixtures such as 2% PES + 1% MMDS + 1% TTSPi are especially excellent in all respects. It is hoped that this comprehensive report sets a benchmark for future studies by others and can be used as a guide and reference for the comparison of other electrolyte additives singly or in combination.

The effectiveness of two electrolyte additives, 1,3-propane sultone (PS) and methylene methanedisulfonate (MMDS) either singly or in combination with vinylene carbonate (VC) has been studied. Li(Ni1/3Mn1/3Co1/3)O2 (NMC)/Graphite pouch cells containing these additives were studied using ultra high precision coulometry, automated storage experiments and electrochemical impedance spectroscopy. The two additives have the same SO3 functional group but it seems PS is more widely used by battery manufacturers. When used alone, both additives decrease the charge transfer impedance during cycling while MMDS shows better coulombic efficiency and lower charge end point capacity slippage rate than PS. Compared with 2% VC, MMDS in combination with 2% VC significantly improves the coulombic efficiency and the charge slippage rate, reduces the charge transfer impedance during cycling and decreases the voltage drop during storage while the addition of PS to VC-containing electrolyte does not show this added benefit. Compared to PS, MMDS seems to be a more beneficial additive for improving cell performance.

Isothermal microcalorimetry is a quick and simple method of determining the effect an additive or additive combination has on the parasitic reactions occurring as a function of state of charge. It can easily discriminate exactly where in the voltage range the additive is providing a benefit with just a single cycle. As a demonstrative example, the effect of varying concentrations of vinylene carbonate (VC) on a LiCoO2/graphite cell is examined. Machine-made pouch cells were used such that the cells were nominally identical except for the concentration of VC. The measured heat flow for the different cells is then identical except for the heat flow that results from parasitic reactions. It is shown that the presence of VC reduces parasitic reactions above 3.9 V, and continues to reduce these reactions with increasing state of charge. The heat flow during open circuit conditions at the top of charge also shows that the presence of VC dramatically reduces the heat due to parasitic reactions. Lithium-ion batteries are being used in an increasing number of applications that are demanding higher energy densities and longer lifetimes. The use of electrolyte additives is a common method that has shown to extend calendar and cycle life and reduce parasitic reactions such as the consumption of active lithium in forming and repairing the solid electrolyte interphase (SEI), electrolyte oxidation, transition metal dissolution, electrode damage, etc. 1 However, it is not very well understood how these additives are functioning and exactly where in the charge-discharge cycle they prove advantageous. One such example is with the popular additive, vinylene carbonate (VC). Several studies 2‐7 report that the increase in full cell lifetime is caused by VC being reduced on the surface of graphite, which forms a more stable SEI. More recent studies 8‐13 have challenged this thought, reporting that the benefit of VC arises primarily from limitingelectrolyteoxidationandpossibletransitionmetaldissolution reactionsatthepositiveelectrode.Therefore,itisofdistinctinterestto be able to determine the voltage-dependent advantage of a particular additive or additive combination, which can aid in the understanding of how these additives are extending lifetimes. Recently the technique of isothermal microcalorimetry has been combinedwithelectrochemicalmeasurements,whichhasbeenusedto examine the thermal behavior of several lithium-ion chemistries. 14‐21 More recently, Krause et al. 22 showed how to use this technique to separate the various contributions to the thermal power and isolate parasitic energy. In this paper, this technique is used to qualitatively compare the heat flow between cells that only vary in concentration of additive. In this case, with all other sources being identical, the measured difference in heat flow arises from differences in parasitic heat. This is done as a function of state of charge, providing a simple and quick method of determining exactly where and to what extent the additive is reducing parasitic reactions. As a demonstrative example, the effect of varying concentrations of VC on a LiCoO2/graphite cell is examined. Experimental General.— Machine-made 225 mAh LiCoO2 (LCO)/graphite pouch cells 0.4 cm thick × 2 cm wide × 3 cm high (obtained from Pred Materials Co.) were provided dry. The pouches were then filled with 0.75 g of 1M LiPF6 in 3:7 ethylene carbonate (EC):ethylmethyl carbonate (EMC) (Novolyte Technologies, now BASF) with various amounts of VC (Novolyte Technologies, now BASF) additive (0%, 0.5%, 2%, and 4% by weight) and then vacuum sealed. The electrodes

The next generation of lithium ion cells for electric vehicles (EVs) requires longer lifetime and higher energy density. The use of electrolyte additives is one of the most effective ways of improving cell performance1–3. In this work, Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells demonstrate superb performance at high voltage when ethylene carbonate (EC)-free electrolytes, using a solvent mixture that is > 95% ethyl methyl carbonate (EMC) and between 2 and 5% of an “enabler”, are used. The “enablers”, required to passivate graphite during formation, can be vinylene carbonate (VC), methylene-ethylene carbonate (MEC), fluoroethylene carbonate (FEC) or difluoro ethylene carbonate (DiFEC) among others. In order to optimize the amount of “enabler” added to EMC, gas chromatography coupled with mass spectrometry (GC-MS) was used to track the consumption of “enabler” during the formation step. Storage tests, electrochemical impedance spectroscopy (EIS), ultrahigh precision coulometry (UHPC), long-term cycling, differential voltage analysis and isothermal microcalorimetry were used to determine the optimum amount of enabler to add to the cells. It was found that the graphite negative electrode cannot be fully passivated when the amount of “enabler” is too low resulting in gas production and capacity fade. Using excess “enabler” can cause large impedance and gas production in most cases. The choice of “enabler” also impacts cell performance. Figures 1a and 1b show the amount of enabler left and the percentage of EMC trans-esterification as a function of the initial enabler content after the first cycle (formation) of cells at 40°C with a current of C/20 up to 4.4 V. The dashed blue line in Figure 1a is a line of slope 1. Comparing the data for cells with FEC in Figure 1a to the dashed blue line shows that only 2% FEC was consumed during formation even if the initial FEC content was 3, 4 or 5%. By contrast when MEC is used, the amount of MEC consumed increases with the initial MEC content, since only about 0.5% MEC is left when the initial MEC content was 5%. Figure 1b shows the percentage of EMC that underwent trans-esterification producing dimethyl carbonate (DMC) and diethyl carbonate (DEC) 4, 5. The presence of lithium alkoxides caused by the reduction of linear alkyl carbonates results causes these trans-esterification reactions. Therefore, when trans-esterification of EMC occurs it is a signal that EMC was reduced at the graphite negative electrode during formation. References: 1. M. Nie, J. Xia, L. Ma, and J. R. Dahn, J. Electrochem. Soc., 162, A2066–A2074 (2015). 2. D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, and U. Heider, Electrochimica Acta, 47, 1423–1439 (2002). 3. K. Xu, Chem. Rev., 114, 11503–11618 (2014). 4. J.-Y. Eom, I.-H. Jung, and J.-H. Lee, J. Power Sources, 196, 9810–9814 (2011). 5. K. Kumai, H. Miyashiro, Y. Kobayashi, K. Takei, and R. Ishikawa, J. Power Sources, 81–82, 715–719 (1999). Figure 1

Many electrolyte additives have been shown to promote longer lifetime lithium ion cells by forming protective films on the electrodes [1]. However, the additives used usually lead to higher negative electrode resistance which can increase the likelihood of unwanted lithium plating on the graphite negative electrode especially when charging at high rates and/or low temperatures [2,3]. In this work, effects of different electrolyte additives on unwanted lithium plating were studied in Li[Ni1/3Co1/3Mn1/3O2(NMC111)/graphite pouch cells. The relation between the negative electrode impedance and the onset current for unwanted lithium plating was also studied. Experiments were performed using charge-discharge cycling with different charge rates between 2.8 V - 4.1 V at 20°C. Unwanted lithium plating was detected by the onset of rapid capacity loss during cycling and post-mortem analysis of the negative electrode. Figure 1a shows that rapid capacity loss above the blue dashed line is caused by unwanted lithium plating and cells with the addition of 2% VC, PES211 or 2% TAP demonstrate obvious lithium plating at lower C-rates. Figure 1b shows that the negative electrode impedance depends strongly on the choice of electrolyte additives and the temperature. The negative electrode impedance increases with the addition of 2% VC, PES211 or 2% TAP, suggesting a negative correlation between the negative electrode impedance and the onset current for unwanted lithium plating. Figure 1c shows the calculated current for unwanted lithium plating (Iu) by the expression: Iu = 0.080 V x S/Rnegative, where 0.080 V is the overpotential needed for lithium plating, S is the geometric electrode surface area in a full cell and Rnegative is the area specific negative electrode resistance obtained from negative/negative symmetric cells shown in Figure 1b. Figure 1c shows that the prediction of this simple rule-of-thumb relation agrees well with the trends observed in Figure 1a under conditions where Rnegative is the dominant factor for anode polarization. This simple rule of thumb can be used by those searching for electrolyte additives that simultaneously lead to enhanced life time and the ability to charge at high rate. References: 1. K. Xu, Chem. Rev., 114, 11503–11618 (2014). 2. J.-P. Jones, M. C. Smart, F. C. Krause, B. V. Ratnakumar, and E. J. Brandon, ECS Trans., 75, 1–11 (2017). 3. Q. Q. Liu, R. Petibon, C. Y. Du, and J. R. Dahn, J. Electrochem. Soc., 164, A1173-A1183 (2017). Figure 1a) The capacity loss measured from the C/20 cycles before and after high rate cycling (350 hours of high rate cycling at the C-rates indicated) at 20°C for cells with the baseline electrolyte (1M LiPF6 EC:EMC(3:7)), 2% vinylene carbonate (VC), a ternary additive blend of 2% propene sultone (PES), 1% ethylene sulfate (DTD) and 1% tris(-trimethyl-silyl)-phosphite (TTSPi) (called PES211) and 2% triallyl phosphate (TAP); b) the area-specific Nyquist plots of negative electrode symmetric coin cell impedance divided by two at 20°C and 10°C; c) the measured onset current for unwanted lithium plating for cells with different electrolytes during cycling at 20°C and 10°C as well as the calculated result plotted versus the negative electrode area specific resistance, Rnegative from symmetric cells. Figure 1

Ethylene carbonate is used in virtually every lithium-ion cell. In efforts to produce high voltage NMC/graphite cells, electrolytes without ethylene carbonate are being explored. The use of electrolytes based on fluoroethylene carbonate (FEC), difluoroethylene carbonate (di-FEC), bis(2,2,2-trifluoroethyl) carbonate (TFEC) and other solvents are explored in this presentation. Studies using ultra high precision coulometry, impedance spectroscopy, isothermal battery microcalorimetry, in-situ gas evolution, storage testing, XPS and long-term charge-discharge testing show the benefits and drawbacks of such electrolytes. Although progress is being made, there remains substantial work to do to produce high energy denisty NMC/graphite cells which can operate for years to 4.5 V. This work was done by Jian Xia, Remi Petibon, Steven Glazier, Kathlyne Nelson, Dan Abarbanel, Deijun Xiong, Leah Ellis and Alex Louli. The authors thank Xiaodong Cao of HSC Corporation for kindly providing many of the solvents used in the work.

Introduction The current state of the art electrolytes for LiBs are based on alkyl carbonate mixtures containing ethylene carbonate (EC). However, this base electrolyte limits the temperature window as well as the potential window of operation of LiBs. Designing new EC-free solvents is not always straightforward since most solvents that have low freezing point or high oxidation stability do not provide a good passivation at the graphite electrode without the presence of EC. Over the past few years, we have designed new electrolyte systems that can operate over a wide temperature window or over a wide potential window. This talk will present two main routes for the design of new EC-free electrolyte systems. This will hopefully help the scientific community in designing new electrolyte systems that will meet the needs of advanced LiBs. Discussion Designing new electrolytes compatible with graphite-based negative electrodes can be achieved using two main routes. The first route consists of using small amounts of passivating additives such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC) in combination with solvents having good properties such as low melting point or high oxidation potential. The amount of these passivating additives must be small since they often impart high impedance or high gas evolution during high voltage and/or high temperature cycling. Using this method, three new electrolyte systems were developed. These consist of Ester:VC blends [1], Sulfolane:linear carbonate:VC blends [2], as well as FEC:ditrifluoroethylcarbonate (TFEC) blends. Figure 1a shows that electrolyte blends consisting of 1M LiPF6 methyl propanoate:VC have similar capacity retention to 1M LiPF6 EC:EMC (3:7) + 2% VC electrolyte in a full cell configuration (220 mAh Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cell) when cycled at 40°C between 2.8 V and 4.2 V. This is surprising since electrolytes containing esters undergo heavy reduction at the graphite surface without the formation of a passivating layer [1]. Figure 1b also shows that this ester-based electrolyte provides superior low-temperature rate performance to EC-based electrolytes. This shows that using small amounts of passivating additives can allow the use of atypical solvents such as esters, without the use of EC. Similarly, Figure 2 shows that sulfolane-based electrolytes and fluorinated solvent-based electrolytes provide superior capacity retention to 220 mAh Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells operated to high voltage (4.5 V) and relatively high temperature (40°C). Once again, the use of VC or FEC allows the use of solvents with high oxidation potential such as sulfolane or fluorinated carbonates. The second route for the design of EC-free electrolytes is the use of the peculiar properties of highly concentrated (3 – 5M) electrolytes. Yamada et al. [3] and Jeong et al. [6] showed that high salt concentration allows for a large array of solvents to be stable against lithiated graphite. Following the same method an ester-based and additive-free electrolyte, kinetically stable against lithiated graphite and high potential, (4.78 V) was developed [7,8]. Figure 3a shows that electrolytes composed of EA:LiFSi:LiPF6 (1:0.5:0.05, molar ratio) provide better capacity retention in Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells than EC-based electrolyte containing VC when cycled up to 4.4 V. Figure 3b also shows that these highly concentrated electrolytes provide reasonable capacity retention in Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells cycled up to 4.7 V and 40°C. Conclusion Following two simple design principles, a large array of new electrolyte systems can be developed. These two principles consist of the use of small amounts of passivating additives such as FEC or VC, or the peculiar effect of high salt concentration. These two simple principles allow the use of atypical solvents that are normally not used due to their instability against lithiated graphite. The design of new EC-free electrolyte systems can play a crucial role in developing advanced Li-ion batteries. Acknowledgments The authors would like to thank 3M Canada and NSERC for the funding of this work. The authors thank Dr. Jing Li of BASF for providing some of the solvents, salts and additives used in his work. The authors also thank Xiodong Cao of HSC Corporation for providing LiFSI. Remi Petibon thanks NSERC and the Walter C. Sumner Foundation for Scholarship support. References [1] R. Petibon et al., Electrochimica Acta 154 (2015) 227–234. [2] J. Xia et al., J. Electrochem. Soc. 162 (2015) A1424–A1431. [3] Y. Yamada et al., J. Am. Chem. Soc. 136 (2014) 5039–5046. [6] S.-K. Jeong et al., Electrochem. Solid-State Lett. 6 (2003) A13–A15. [7] R. Petibon et al., Electrochimica Acta 154 (2015) 287–293. [8] R. Petibon et al., J.R. Dahn, Electrochimica Acta 174 (2015) 417. Figure 1

Introduction The Si/electrolyte interface is increasingly recognized as being a key factor in the successful implementation of Si-based materials in Li-ion batteries. Si-based negative electrode materials undergo massive volume changes during cycling. Pure Si expands by approximately 280% while 3M’s active/inactive Si alloys expand on the order of 135%. The expansion and contraction of Si has important consequences on the surface stability of these materials. This work will focus on the combined impact of material design and electrolyte choices on cell performance. Results and Discussions The 3M Si alloy is compared to pure Si in EC/EMC 3/7 + 10% FEC. Cross section SEM images are taken after 100 cycles in a half cell. While the 3M alloy shows a discernible SEI on distinct particles, the pure Si has dramatically expanded in size and completely lost its particle morphology. The impact of electrolyte choice on the surface morphology of the 3M Si alloy is then explored across a range of electrolytes compositions using cross section SEM images after 100 cycles. Further testing is performed in 200 mAh pouch cells (Lifun), with a LiCoO2 positive electrode, a Si-alloy/graphite negative electrode, and approximately 3 mAh/cm2 of reversible capacity. In order to understand the impact of electrolyte composition as well as failure mechanisms, cells were studied by long term cycling, ultrahigh precision cycling, post-cycling gas chromatography/mass spectroscopy (GC/MS) of the electrolyte, in-situ volume measurements, and electrochemical impedance spectroscopy. Fluorethylene carbonate (FEC) has long been recognized as an important electrolyte component for Si-containing full cells. GC/MS studies are used to study the consumption of FEC with cycle number as well as changes in electrolyte composition. FEC is found to be preferentially consumed relative to other electrolyte solvents and its depletion is found to generally cause sudden failure. A hierarchy of reactivity of electrolyte solvents is established. An electrolyte additive in the new product introduction process at 3M will be presented. This additive is found to help delay sudden failure as well as suppress FEC gassing. Conclusion When implementing well-designed Si-based materials in commercially-relevant full cells, the electrolyte is a key parameter to be optimized. In this work, electrolyte optimizations lead to delayed sudden failure, suppressed gassing and reduced impedance growth. These findings suggest considerable gains are still attainable through further electrolyte optimizations, which will enable deeper penetration of Si-based materials into the marketplace.