Prop-1-ene-1,3-sultone Research Articles

(Invited) Investigations into the Chemical Role of Additives in Li-Ion Cells

INTRODUCTION For over a decade, the most common electrolyte solution in commercially available lithium-ion cells has remained LiPF6 dissolved in some blend of organic carbonate solvents.1 Rather than change the salt or the solvent, many industrial production lines have adopted the use of electrolyte additives to improve cycling performance, extend calendar lifetime, decrease detrimental gas formation and improve lithium-ion cell safety. The practical advantage of this move to electrolyte additives is that performance improvements can be achieved with minimal changes to existing supply chains for electrolyte salts and solvents. However, the optimization of lithium-ion cells for various applications (automotive, grid storage, etc.) would be greatly enhanced by a more detailed understanding of the cell chemistry. In particular, it is desirable to characterize the chemical and electrochemical reactions that occur during solid-electrolyte interphase (SEI) formation for the various additives in use. This presentation will demonstrate how our group has used computational and experimental methods, together, to study SEI formation for two additives, prop-1-ene-1,3-sultone (PES)2 and pyridine boron trifluoride (PBF).3 EXPERIMENTAL Density functional theory (DFT) calculations were performed with the Gaussian 09 (G09.D01) software package using the B3LYP and M06-2X hybrid functionals. The IEFPCM-UFF solvation model and its parameterization will be discussed.4 Several experimental methods will be discussed, including coulometry, in situ volumetric measurements using the Measuring Archimedes’ Gas Evolution (MAGE) instrument, gas-chromatography coupled with mass spectrometry (GC-MS) and thermal conductivity detection (GC-TCD), X-ray photoelectron spectroscopy (XPS), and isothermal microcalorimetry. Experimental details have been described previously.5–9 RESULTS AND DISCUSSION During the initial formation cycle (i.e., the first charge step), PES forms a passive SEI at the negative electrode surface via a two-electron electrochemical reduction, which produces Li2PES (Figure 1). The decomposition of this compound and its various reactions with the solvent (EC and EMC) and with other PES molecules will be discussed. These reactions are spontaneous and result in the formation of Li2SO3 and organic sulfate species (RSO3Li) at the anode. This is a good match to the S2ppeaks observed in the XPS spectrum of the anode after formation. The predicted gas-phase products, including several hydrocarbons at the anode, are also consistent with those observed by GC-MS. PBF similarly forms a passive SEI at the graphite surface by electrochemical reduction. The reduced species, LiPBF, forms a bipyridine boron trifluoride adduct, which is accompanied by the reduction of the solvent component, ethylene carbonate (EC). This reaction produces lithium ethyl carbonate, a soluble lithium semicarbonate. This reaction pathway does not produce an appreciable amount of any gas-phase species, as demonstrated by MAGE, GC-MS, and GC-TCD results. The predicted PBF-derived dimer is consistent with the C1s and N1s peaks observed in the XPS spectrum of the anode surface after formation. In summary, carefully developed theoretical methods coupled with experimental data reveal several spontaneous pathways for the reductive decomposition of two additives, PES and PBF. It is hoped that these results will prove useful for developing new and improved electrolyte additives. Moreover, these results provide new insight into the role of the solvent molecules during SEI formation that may have significance for research into new solvents and solvent blends. REFERENCES 1. K. Xu, Chem. Rev., 114, 11503–11618 (2014). 2. B. Li, M. Xu, T. Li, W. Li, and S. Hu, Electrochem. Commun., 17, 92–95 (2012). 3. M. Nie, J. Xia, and J. R. Dahn, J. Electrochem. Soc., 162, A1186–A1195 (2015). 4. D. S. Hall, J. Self, and J. R. Dahn, J. Phys. Chem. C, 119, 22322–22330 (2015). 5. C. P. Aiken et al., J. Electrochem. Soc., 161, A1548–A1554 (2014). 6. V. L. Chevrier et al., J. Electrochem. Soc., 161, A783–A791 (2014). 7. L. Madec et al., J. Phys. Chem. C, 118, 29608–29622 (2014). 8. R. Petibon, L. M. Rotermund, and J. R. Dahn, J. Power Sources, 287, 184–195 (2015). 9. J. Self, D. S. Hall, L. Madec, and J. R. Dahn, J. Power Sources, 298, 369–378 (2015). Figure 1

Study of the consumption of the additive prop-1-ene-1,3-sultone in Li[Ni0.33Mn0.33Co0.33]O2/graphite pouch cells and evidence of positive-negative electrode interaction

The consumption of the additive prop-1-ene-1,3-sultone (PES) in Li[Ni0.33Mn0.33Co0.33]O2/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.

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 Effects of a Ternary Electrolyte Additive System on the Electrode/Electrolyte Interfaces in High Voltage Li-Ion Cells

The effects of a ternary electrolyte additive system on Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells balanced for 4.7 V was thoroughly investigated. The additive system consisted of 2% prop-1-ene-1,3-sultone (PES) + 1% methylene methane disulfonate + 1% tris-(trimethylsilyl)-phosphite (TTSPi) which is denoted “PES211”. “PES211” significantly improved the capacity retention, impedance and gas evolution of NMC442/graphite pouch cells cycled at constant current up to 4.4 V or 4.5 V. X-ray photoelectron spectroscopy results suggest more stable and passivating SEI films at both graphite and NMC442 surfaces due to the preferential electrochemical reaction of PES and MMDS as well as the preferential chemical reaction of TTSPi at the graphite surface. “PES211” modifies the reactivity of the LiPF6 salt at the graphite surface and hinders its degradation at high potential. However, during extended exposure to 4.5 V, less passivating SEI films were observed leading to extensive electrolyte degradation, capacity loss, impedance increase and gas evolution even for NMC442/graphite pouch cells containing “PES211”.

A systematic study of some promising electrolyte additives in Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]/graphite and Li[Ni0.6Mn0.2Co0.2]/graphite pouch cells

Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite pouch cells were examined with and without electrolyte additives using the ultra high precision charger at Dalhousie University, electrochemical impedance spectroscopy, gas evolution measurements and “cycle-store” tests. The electrolyte additives tested were vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), pyridine-boron trifluoride (PBF), 2% PES + 1% methylene methanedisulfonate (MMDS) + 1% tris(trimethylsilyl) phosphite (TTSPi) and 0.5% pyrazine di-boron trifluoride (PRZ) + 1% MMDS. The charge end-point capacity slippage, capacity fade, coulombic efficiency, impedance change during cycling, gas evolution and voltage drop during “cycle-store” testing were compared to gain an understanding of the effects of these promising electrolyte additives or additive combinations on the different types of pouch cells. It is hoped that this report can be used as a guide or reference for the wise choice of electrolyte additives in Li[Ni1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2]O2/graphite and Li[Ni0.6Mn0.2Co0.2O2]/graphite pouch cells and also to show the shortcomings of particular positive electrode compositions.

A Study of Gas Evolution at Elevated Temperature Using Pouch Cells and Pouch Bags

Introduction A lithium-ion pouch cell cycled at elevated temperature may experience volume expansion due to gas production leading to rapid capacity fade. This gas production may be caused by impurities (LiOH and Li2CO3) on the surfaces of electrodes, 1 the instability of LiPF6,2 interactions between the electrolyte and the negative electrode or interactions between the electrolyte and the positive electrode. 3, 4 Recently, Xia et al. found that prop-1-ene-1,3-sultone (PES) suppresses gassing especially for NMC/graphite cells cycled at elevated temperature.5 Therefore, studies were made using pouch cells and pouch bags to determine the main factor which causes gas production and understand why the addition of PES can suppress gassing at elevated temperature. Experimental The LiNi1/3Mn1/3Co1/3O2/graphite [NMC (111)] pouch cells (220 mAh) were filled with 0.9 g of 1M LiPF6 in EC:EMC (3:7 v/v) (BASF, 99.99%) with 2% PES. After electrolyte filling, they were placed in a temperature box at 40.0°C at 1.5 V for 24 h. After they were charged to 3.8 V at C/20, they were transferred to a glove box for degassing. After degassing, they were charged to either 4.2 or 4.4 V, then discharged to 2.8 V and charged back to the same cutoff voltage until the current dropped to C/2000. Two pouch cells at each cutoff voltage, either 4.2 V or 4.4 V, were moved to a 60°C temperature box for storage. Their voltage and gas volume were measured using a voltmeter and ex-situ gas equipment, respectively. The other cells were transferred to an argon-filled glove box and dissembled there. The delithiated NMC (111) electrodes collected from the full cells were inserted into different pouch bags (made of the same material as pouch cells) with 0.7 g of control electrolyte with 2% PES. In order to study the effect of the LiPF6 on gas evolution at elevated temperature, some of the delithiated NMC (111) electrodes collected from the full cells were first washed with DMC three times and then inserted into different pouch bags with 0.7 g of EC/EMC with 2% PES. After these pouch bags were vacuum sealed, they were transferred to the same 60oC temperature box for storage. The volume changes of all pouch bags were measured using the ex-situgas equipment. Results and discussion Figure 1 shows that the pouch cells and pouch bags containing 2% PES continuously produce gas during storage at 60°C. The rate of gas production for these pouch cells and pouch bags during the 500 h storage period is slower than that for the corresponding pouch cells and pouch bags without 2% PES shown in Figure 1a. This suggests that the addition of PES can slow down electrolyte oxidation at the delithiated NMC (111) electrode maybe due to a better SEI formed at the electrode. However, there is still a large amount of gas generated in the pouch bags even with the addition of PES, which is much larger than that in the corresponding pouch cells. This suggests that the addition of PES does not significantly suppress some gaseous products which can be consumed at the lithiated graphite electrode but suppresses some gaseous products which cannot be consumed at the lithiated graphite electrode.

Studies of the Effect of High Voltage on the Impedance and Cycling Performance of Li[Ni0.4Mn0.4Co0.2]O2/Graphite Lithium-Ion Pouch Cells

Introduction Electrolyte additives are the most effective way to improve the calendar life and cycling performance of lithium-ion batteries. Vinylene carbonate (VC) is perhaps the most famous and widely used additive and has been shown to improve cycle and calendar life of Li-ion cells [1]. VC is less effective, however, when used in cells cycling to potentials above 4.2 V [2] or at elevated temperatures [3]. Sulfur-containing additives have recently been investigated by several research groups in the hopes of overcoming the temperature sensitivity of VC and extending the usable voltage range of Li-ion cells [4-6]. LiCoO2/graphite cells with upper cutoff potentials of 4.4 V are now common in the marketplace, but Li[Ni0.4Mn0.4Co0.2]O2 (NMC442)/graphite cells that can be charged to 4.4 V are not, since NMC/graphite cells normally do not function well when charged to high potential. One possible factor leading to severe capacity fade in NMC cells, particularly when charged to high potential, is large impedance growth. It is therefore important and valuable to measure cell impedance as a function of cycle number, time, and voltage. Experimental NMC442/graphite pouch cells containing various electrolyte additives, either singly or in combination, were studied using cycling experiments up to 4.4 and 4.5 V coupled with simultaneous electrochemical impedance spectroscopy (EIS) measurements using a frequency response analyzer (FRA). The impacts of adding prop-1-ene-1,3-sultone (PES), VC, triallyl phosphate (TAP), methylene methane disulfonate (MMDS), ethylene sulfate (DTD) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) to 1M LiPF6 ethylene carbonate:ethyl methyl carbonate (EC:EMC) electrolyte were studied. The additive combination 2% PES + 1% MMDS + 1% TTSPi, named PES211, is highlighted. Results Cells underwent either continuous charge-discharge cycling or charge-hold-discharge cycling in which the cells were held at either 4.4 or 4.5 V for 20 hours. The FRA measured cell impedance every 0.1 V of one cycle during regular intervals. Figure 1 shows a comparison of the discharge capacity (top panels) and Rct, a combination of the charge transfer resistances from both the positive and negative electrodes as well as resistance due to the motion of ions through the SEI layers at both the positive and negative electrodes, as a function of voltage (bottom panels) for NMC442/graphite cells containing PES211. Figure 1a shows the discharge capacity as a function of cycle number for cells containing PES211 undergoing continuous cycling and cycling with a 24 hr hold at 4.4 V. Despite the excellent capacity retention of the PES211–containing cell during continuous cycling, the hold at high voltage leads to severe discharge capacity fade. This effect is also seen in Figure 1b, which shows the discharge capacity as a function of time. Figure 1c shows Rct as a function of voltage for the PES211–containing cell undergoing charge-hold-discharge cycling. This cell exhibited severe impedance growth over 50 cycles (~1700 hours). Interestingly, the impedance was reversible over one cycle, while the impedance was irreversible over several cycles. Figure 1d shows Rct as a function of voltage for the PES211–containing cell undergoing continuous cycling. This cell had very low impedance at all voltages and all 285 cycles (~3000 hours). Figure 1 dramatically demonstrates that holding NMC442/graphite cells at high potentials for extended periods (as might be expected in real-life charging situations in portable electronics, for example) leads to severe impedance growth issues, at least with PES211 electrolyte, which might be overcome through the use of better additive combinations. The importance of performing experiments representative of “real-life” Li-ion cell use is demonstrated. Practical applications that use Li-on batteries often leave the cells at high voltages for extended periods of time after charging. Therefore, studying the effect of extended periods of time at high voltage on the performance and impedance growth of Li-ion cells is imperative.

Investigating the Fate of an Electrolyte Additive: A Combined Theoretical and Experimental Study of Prop-1-Ene-1,3-Sultone (PES) in Li-Ion Cells

INTRODUCTION One way to improve the cycling performance and stability of Li-ion cells is the use of electrolyte additives. In recent years, prop-1-ene-1,3-sultone (PES) has shown great promise for improving cell lifetime and decreasing gas formation.1–5Voltage cycling experiments and surface analysis studies have provided important clues for understanding the fate of PES in cells. However, the details of this additive’s mechanism of action remains unknown. This presentation will discuss computational chemistry methods, including the accurate representation of solvation for ethylene carbonate (EC)/ethylmethyl carbonate (EMC) mixtures, and the application of these methods to gain insight into the role and ultimate fate of PES in Li-ion cells. EXPERIMENTAL Calculations were performed with the Gaussian 09 (G09.D01) software package using the B3LYP/6-311++G(d,p) method. The IEFPCM-UFF solvation model and its parameterization will be discussed. A cylindrical, stainless steel capacitance cell, based on the design of Greer and Jacobs, was used for dielectric constant measurements.6 Machine-made 220 mAh graphite/Li[Ni1/3Mn1/3Co1/3]O2 (NMC) pouch cells were filled with 3:7 EC/EMC, 1 M LiPF6, and 0 – 2 % PES, and galvanostatically cycled.4 Select cells were disassembled in an argon-filled glove box for XPS surface analysis as described by Madec et al.7 Volumetric and GC-MS analysis of gas formation was performed as described by Self et al.5 RESULTS AND DISCUSSION Computational chemistry can be used to determine standard electrode potentials, free energies of reactions and transition state energies. It is, however, imperative that solvation is properly modeled to obtain meaningful results. The polarizable continuum model (PCM) is a simple yet robust approach that requires only the dielectric constant (static permittivity) of the reaction medium. Therefore, dielectric constants of EC/EMC solvent blends were measured at various compositions and temperatures. It was found that measured values do not exactly match those predicted by a simple linear combination of EC and EMC. The reactions of PES at the electrodes were then investigated. PES reduction has a calculated reduction potential of 1.0 V vs. Li/Li+, which closely matches experimental dQ/dV plots.4 The subsequent reduction is predicted to occur very rapidly and results in the reactive Li2PES compound shown in Figure 1. The decomposition of this compound and its various reactions with the solvent (EC and EMC) and with other PES molecules will be discussed. These reactions are spontaneous and result in the formation of Li2SO3 and organic sulfate species (RSO3Li) at the anode. This is a good match to the S 2p peaks observed in the XPS spectrum of the anode after formation. The predicted gas-phase products, including several hydrocarbons at the anode and the formation of O=C=S at the cathode, are also consistent with those observed by GC-MS. In summary, carefully developed theoretical methods coupled with experimental data reveal several spontaneous pathways for the reductive decomposition of PES. It is hoped that these results will also prove useful for developing new and improved electrolyte additives. REFERENCES 1. B. Li et al., J. Mater. Chem. A, 1, 12954–12961 (2013). 2. B. Li et al., Electrochimica Acta, 105, 1–6 (2013). 3. B. Li, M. Xu, T. Li, W. Li, and S. Hu, Electrochem. Commun., 17, 92–95 (2012). 4. J. Xia et al., J. Electrochem. Soc., 161, A1634–A1641 (2014). 5. J. Self, C. P. Aiken, R. Petibon, and J. R. Dahn, J. Electrochem. Soc., 162, A796–A802 (2015). 6. D. T. Jacobs and S. C. Greer, Rev. Sci. Instrum., 51, 994–995 (1980). 7. L. Madec et al., J. Phys. Chem. C, 118, 29608–29622 (2014). 8. K. Xu, Chem. Rev., 114, 11503–11618 (2014). Figure 1 – The reduced Li2PES compound predicted to form at the anode during cell formation. Figure 1

Understanding the Role of Prop-1-ene-1,3-Sultone and Vinylene Carbonate in LiNi1/3Mn1/3Co1/3O2/Graphite Pouch Cells: Electrochemical, GC-MS and XPS Analysis

The role of prop-1-ene-1,3-sultone (PES) used alone or in combination with vinylene carbonate (VC) in LiNi1/3Mn1/3Co1/3O2(NMC)/graphite pouch cells was studied by correlating data from differential capacity (dQ/dV) analysis, gas chromatography/mass spectroscopy (GC−MS), ultrahigh precision coulometry, storage experiments and X-ray photoelectron spectroscopy. VC formed more stable and protective SEI films at both graphite and NMC surfaces due to the formation of a polymer of VC. For PES-containing electrolytes, the preferential reaction of PES led to the formation of SEI films with higher oxygen content at both graphite and NMC surfaces due to the additional oxygen contribution of sulfite species and substantially less LiF compared to control and VC electrolytes. PES also led to thicker SEI films at the NMC surface. When VC was combined with PES, features of the SEI films from both VC and PES were observed. The SEI film features for VC and PES used alone or in combination can explain the improved electrochemical performance as well as the lower production of gas observed with these additives compared to control electrolyte.

Ternary Electrolyte Additive Mixtures for Li-Ion Cells that Promote Long Lifetime and Less Reactivity with Charged Electrodes at Elevated Temperatures

Li[Ni1/3Mn1/3Co1/3]O2 (NMC111)/graphite and Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells demonstrate superb performance when ternary blends of electrolyte additives are added to the cells. These ternary blends contain one of vinylene carbonate (VC) or prop-1-ene-1,3-sultone (PES) plus a sulfur containing molecule and tris(trimethylsilyl) phosphite (TTSPi). Here, the excellent charge-discharge capacity retention of cells containing these ternary blends of additives is demonstrated at both 45°C and 55°C by comparison to cells with only VC or PES additives alone. The impact of a number of sulfur- and phosphorous-containing electrolyte additives on the reactivity of charged electrode materials with electrolyte at elevated temperatures was studied using accelerating rate calorimetry (ARC). The electrolyte additives, methylene methanedisulfonate (MMDS) and TTSPi, dramatically reduce the reactivity between lithiated (charged) graphite and electrolyte. The ternary electrolyte blends of VC + MMDS + TTSPi and PES + MMDS + TTSPi show no reactivity between charged graphite and electrolyte to above 200°C. By contrast, electrolytes with no additives or with PES alone show significant reactivity with lithiated graphite below 200°C. NMC/graphite cells with VC + MMDS + TTSPi or PES + MMDS + TTPSi additives will demonstrate long life and excellent safety.

Studies of the Effect of High Voltage on the Impedance and Cycling Performance of Li[Ni0.4Mn0.4Co0.2]O2/Graphite Lithium-Ion Pouch Cells

Li[Ni0.4Mn0.4Co0.2]O2 (NMC442)/graphite pouch cells containing various electrolyte additives, either singly or in combination, were studied using cycling experiments up to 4.4 and 4.5 V coupled with simultaneous electrochemical impedance spectroscopy (EIS) measurements. The impedance of most cells increased dramatically at 4.4 and 4.5 V, but was nearly reversible over one cycle. However, during continued cycling, the impedance of all cells slowly increased at all potentials. Electrolyte additives were found to dramatically affect this behavior. The impacts of adding prop-1-ene-1,3-sultone (PES), vinylene carbonate (VC), triallyl phosphate (TAP), methylene methane disulfonate (MMDS), ethylene sulfate (DTD) and/or tris(-trimethyl-silyl)-phosphite (TTSPi) to 1M LiPF6 ethylene carbonate:ethyl methyl carbonate (EC:EMC) electrolyte were studied. PES-containing cells had dramatically lower impedance and better capacity retention than VC and TAP-containing cells during both 4.4 and 4.5 V experiments. When MMDS, DTD and/or TTSPi were added in combination with PES, the performance was improved further. Finally, continuous charge-discharge cycling was compared to cycling with a 24-hour hold applied at the top of charge at 4.4 V. The high voltage hold led to severe impedance growth which could be partially overcome through the use of optimal additive combinations.

A Comparative Study of Pyridine-Boron Trifluoride, Pyrazine-(BF3)2 and Triazine-(BF3)3 as Electrolyte Additives for Lithium-Ion Cells

The novel electrolyte additives pyridine boron trifluoride, pyrazine di-boron trifluoride and triazine tri-boron trifluoride were compared in Li[Ni1/3Mn1/3Co1/3]O2/graphite and Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells. This series of additives allowed the Lewis base:BF3 ratio to be systematically varied. The results were compared to baseline experiments on cells with well-known additives such as vinylene carbonate (VC) or prop-1-ene-1,3-sultone (PES). Increasing the BF3 content on the additive can reduce the impedance of cells during charge-discharge cycling. However the coulombic efficiency, charge endpoint capacity slippage and long-term cycling behavior of these pouch cells was not improved by increasing the content of BF3 in the additives.

Survey of Gas Expansion in Li-Ion NMC Pouch Cells

This preliminary study investigates initial gas formation in Li[Ni0.4Mn0.4Co0.2]O2 (NMC442) pouch cells with three different electrolytes: 3:7 ethylene carbonate : ethyl methyl carbonate (EC:EMC) w/ 1 M LiPF6 as the control, control + 2% prop-1-ene-1,3-sultone (PES) and control + 2% vinylene carbonate (VC). In situ volume measurements reveal three main features of gas evolution, an initial gas step, gas absorption, and a second gas step at higher voltage. Gas chromatography-mass spectrometry is employed to identify the gaseous compounds. This work illustrates the strong dependence of volume change due to gas evolution on additives, charging rate and temperature.

Sulfolane-Based Electrolyte for High Voltage Li(Ni0.42Mn0.42Co0.16)O2 (NMC442)/Graphite Pouch Cells

Ethylene carbonate (EC)-free electrolyte composed of sulfolane (SL), ethylmethyl carbonate (EMC) and vinylene carbonate (VC) was studied in Li(Ni0.42Mn0.42Co0.16)O2 (NMC442)/graphite pouch cells using in situ measurements of gas evolution, ultra high precision coulometry (UHPC), automated storage experiments, long-term cycling and electrochemical impedance spectroscopy (EIS). Although cells using 1 M LiPF6 in SL:EMC 3:7 (w:w) do not function at all, the addition of only 1% VC allows the cells to operate normally. In situ gas measurements show that NMC442/Graphite pouch cells containing SL:EMC:VC electrolyte produce less gas during formation than cells containing control (1 M LiPF6 EC:EMC 3:7 (w:w)) electrolyte or 2% VC in control electrolyte. Cycling and storage experiments show that cells containing SL:EMC:VC electrolytes can provide better performance than control electrolyte without additives and similar performance to state-of-the-art electrolyte with the ternary additive blend 2% prop-1-ene-1,3-sultone (PES) +1% methylene methanedisulfonate (MMDS) +1% tris(trimethylsilyl) phosphite (TTSPi) (“PES-211”) in EC:EMC 3:7 (w:w). The SL:EMC:VC system needs to be further explored with additional additives and from a safety perspective.

Improving cyclic stability of lithium nickel manganese oxide cathode for high voltage lithium ion battery by modifying electrode/electrolyte interface with electrolyte additive

We report a new approach to improve the cyclic stability of lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode, in which the cathode/electrolyte interface is modified by using prop-1-ene-1, 3-sultone (PES) as an electrolyte additive. The interfacial properties of LiNi0.5Mn1.5O4 cathode in PES-containing electrolyte have been investigated by scanning electron spectroscopy (SEM), transmission electron microscopy (TEM), thermal gravimetry (TG), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), cyclic voltammometry (CV), chronoamperometry (CA), and constant current charge/discharge test. It is found that the application of PES improves significantly the cyclic stability of LiNi0.5Mn1.5O4. After 400 cycles at 1C rate (1C=147mAg−1), the capacity retention of LiNi0.5Mn1.5O4 is 90% for the cell using 1.0 wt% PES, while only 49% for the cell without the additive. The characterizations from SEM, TEM, TG, XRD, and XPS confirm that the LiNi0.5Mn1.5O4/electrolyte interface is modified and a protective solid electrolyte interface film is formed on LiNi0.5Mn1.5O4 particles, which prevents LiNi0.5Mn1.5O4 from destruction and suppresses the electrolyte decomposition.

Studies of the Effect of Varying Prop-1-Ene-1,3-Sultone (PES) Concentration on Li-Ion Batteries

Introduction Electrolyte additives are the most effective way to improve the calendar life and cycling performance of lithium-ion batteries. Vinylene carbonate (VC) is perhaps the most well-known additive for Li-ion batteries and has been shown to be useful during the solid electrolyte interphase (SEI) formation [1], which can result in longer calendar life and reduced irreversible capacity (IRC). However, it has been shown that the performance of cells containing VC decreases at high temperatures and high voltages. Sulfur-containing additives have been studied in hopes to overcome the temperature sensitivity of VC. Prop-1-ene-1,3-sultone (PES) was found to have smaller IRC than VC-containing cells and was suggested to be an SEI forming additive [2]. In this study, we show that PES is a useful additive for Li-ion pouch cells and suggest that PES, when used in combination with other additives, may be more effective than VC at achieving long lifetimes and high cycling performance. Experimental Li[Ni1/3Mn1/3Co1/3]O2 (NMC)/graphite pouch cells made with 1M LiPF6in EC:EMC 3:7 (by weight) and varying amounts of PES were studied using ultra high precision coulometry and storage experiments. In addition, gas evolution measurements employing Archimedes principle and electrochemical impedance spectroscopy (EIS) measurements were performed. The PES-containing cells were compared to the same control electrolyte containing 2% VC. Results Figure 1 shows a preliminary set of results from both storage and cycling experiments of PES-containing cells compared with control electrolyte and 2% VC-containing cells. The dotted line in Figures 1a, d, and e indicates the value for 2% VC. Figure 1a shows the gas evolution during formation at 40°C and Figures 1b and c show gas evolution during storage at 60°C and 40°C, respectively. Cells containing 2, 4 and 6% PES show smaller gas during formation than that of 2% VC and all concentrations of PES studied show significantly smaller gas during 40°C and 60°C storage than VC-containing cells. The most prominent advantage of PES compared to VC is the extremely low gas evolution during high temperature storage. Figure 1d shows the voltage drop during storage from 4.2 V at 40°C for 500 hours. Figure 1e shows the average coloumbic inefficiency (CIE) during the last three cycles of cycling between 2.8 V and 4.2 V at C/20 and 40°C. Through the voltage drop and CIE, it is clear that 2% PES appears to be as good as 2% VC. This work suggests that PES is a viable alternative to VC and is superior during high temperature storage. PES may also prove to be beneficial in combination with other additives.

A Comparative Study of Sulphur-Containing Electrolyte Additives on the Reactivity Between Electrolytes and Charged Electrodes Using Accelerating Rate Calorimetry (ARC)

Introduction Recently, electrolyte additives have attracted much attention because they can help improve the lifetime of lithium-ion batteries1,2. Among them, sulphur-containing electrolyte additives are very promising. For example, 1,3,2-dioxathiolane-2,2-dioxide (DTD)1 and prop-1-ene-1,3-sultone (PES)2 can help increase coulombic efficiency (CE) while 1,3,2-dioxathiane 2,2-dioxide (TMS)1 can limit gas production during formation. However, electrolyte additives may have an effect on Li-ion battery safety because these additives modify the interfaces between the electrode materials and the electrolyte. In this work, the reactivity of lithiated graphite or delithiated Li(Ni1/3Mn1/3Co1/3)O2 (NMC) with control electrolytes (1M LiPF6 ethylene carbonate (EC):ethyl methyl carbonate (EMC) 3:7 wt% ratio) or electrolytes containing several sulphur-containing additives was studied, respectively, using accelerating rate calorimetry (ARC). Experimental The ARC sample preparation process was similar to that reported before3. 2325 coin type pellet cells were made using control electrolyte and charged to 4.2 V for the NMC electrodes or discharged to 0.0 V for the graphite electrodes using the protocol described in Reference 3. The ratios between charged electrode materials and electrolyte were 94 mg:30 mg and 140 mg:140 mg for NMC electrodes and graphite electrodes, respectively. The single-point BET surface areas of the graphite and NMC powders were measured with a Micromeritics Flowsorb 2300 instrument. Results and Discussion Table 1 shows the specific surface area results for the NMC and graphite materials used in this experiment.Figure 1 shows the molecular structures of the electrolyte additives were used in this experiment.Figure 2 shows the self-heating rate (SHR) versus temperature for the reaction of lithiated graphite or delithiated NMC with different electrolyte additives. Although there is a short-lived exothermic observation at 50°C, 5% DTD decreases the SHR for lithiated graphite. TMS causes a small exothermic peak at around 75°C but the SHR is very small. Both 5% DTD and 5% TMS help eliminate the exothermic peak at around 100°C resulting from the decomposition of metastable solid electrolyte interface (SEI)4. Furthermore, 2% PES does not dramatically increase the reactivity of the delithiated NMC with electrolyte from the starting temperature to around 250°C and PES helps decrease the SHR compared with the control electrolyte after 250°C. Further experiments showing the impact of these additives in combination with other additives like VC will be reported.In summary, some sulphur-containing electrolyte additives which show good electrochemical performance, such as DTD, TMS etc., should not compromise the safety of lithium-ion batteries.

Study of the Impact of Electrolyte Additives on High Voltage LiNi0.4Mn0.4Co0.2O2/Graphite Cells by Electrochemical Impedance Spectroscopy

Introduction Materials such as LiNi0.4Mn0.4Co0.2O2 have been proposed as good candidates for high energy density cells. Even though these types of materials show reversible cycling up to 4.7 V, the stability of typical carbonate based electrolytes towards such high voltage proves to be a problem. Electrolyte additives can improve capacity retention at high voltage, however very little is known about how these additives work. The effect of three additive combinations on the cycling of LiNi0.4Mn0.4Co0.2O2/graphite cells cycled to different upper voltage cut-offs has been studied using electrochemical impedance spectroscopy (EIS). Experimental Dry pouch LiNi0.4Mn0.4Co0.2O2/graphite cells balanced for 4.7 V with a nominal capacity of 240 mAh were obtained from a reputable cell manufacturer. The pouch cells were filled with ~0.9 g of 1M LiPF6 EC:EMC (3:7) based electrolyte with different additive combinations. The additive combinations tested were 2% vinylene carbonate (VC), 2% prop-1-ene-1,3-sultone (PES), and 2% VC + 1% Methylene Methanedisulfonate (MMDS) + 1% additive A. Cells were vacuum sealed in an argon-filled glove box. The cells then underwent a formation protocol during which they were opened and re-vacuum sealed at 3.5 V and 4.5 V to remove any gas generated during the first charge. Cells were then moved to a charger to be cycled at 40°C to various upper potential cut-offs. After a period of 400 – 500 h of cycling, the impedance of the cells was measured at 3.8 V and at 10°C. Results and discussion Figure 1 shows the discharge capacity as a function of time for cells containing various additive combinations and cycled to various upper voltage cut-offs. Figure 1 shows that all cells exhibit capacity loss when cycled to potentials above 4.4V. Cells containing 2% VC + 1% MMDS + 1% A have encouraging cycling performance at both 4.4 and 4.5 V. Figure 1 also shows that cells containing any of the additive combinations show substantial capacity fade when cycled at to 4.5 V and above.Figure 2 shows the Nyquist representation of the impedance of LiNi0.4Mn0.4Co0.2O2/graphite cells cycled for 400 - 500 h to different upper voltage cut-offs. Figure 2 shows that the additives have a dramatic impact on the impedance of cells cycled to high voltage. Figure 2 shows that the additive combination, 2% VC + 1% MMDS + 1% A, yields a very small impedance compared to other more conventional additives such as 2% VC. Figure 2 also shows that the shape of the EIS spectrum of cells containing any additive combination dramatically changes when increasing the higher voltage cut-off. Other measurements (not shown) showed that the EIS spectra of cells cycled to upper cut-offs of 4.0 V to 4.3 V are very similar. This seems to indicate the activation of various surface reactions when increasing the upper voltage to 4.4 V and above. These surface reactions seem to be dramatically hindered when the electrolyte additive ,2% VC + 1% MMDS + 1% A is used Conclusion The effect of different additive combinations on the cycling performance of LiNi0.4Mn0.4Co0.2O2/graphite cells cycled to various upper voltage cut-offs and up to a maximum of 4.7 V has been evaluated. Results showed that 2% VC which is a very useful additive in 4.2 V cells yields very poor cycling performance for high voltage applications. Other additives combinations such as 2% VC + 1% MMDS + 1% A yield dramatically improved performance. This study also showed that the impedance spectra of cells cycled to 4.4 V and above dramatically change as the upper cutoff potential is increased..Reference1. J. C. Burns et al., J. Electrochem. Soc., 160, A1668–A1674 (2013).

Comparative Study on Prop-1-ene-1,3-sultone and Vinylene Carbonate as Electrolyte Additives for Li(Ni1/3Mn13Co1/3)O2/Graphite Pouch Cells

The effectiveness of Prop-1-ene-1,3-sultone (PES) and Vinylene carbonate (VC) as electrolyte additives in Li(Ni1/3Mn1/3Co1/3)O2 (NMC)/graphite pouch cells was studied using in situ measurements of gas evolution, ultra high precision coulometry (UHPC), automated storage experiments and electrochemical impedance spectroscopy (EIS). Gas measurements show that cells containing PES produce less gas than cells containing 2% VC during formation and much less gas during elevated temperature storage (60°C) at 4.2 V. UHPC cycling and storage results show that cells containing 2% PES have higher coulombic efficiency, lower charge end point capacity slippage and smaller voltage drop during storage than cells containing 2% VC. The combination of VC and PES shows even better cycling performance than the single additives, however, at the expense of much higher impedance. EIS studies of positive and negative electrode symmetric cells show that the introduction of PES or VC to control electrolyte affects the individual electrode impedances in a similar way: both increase the negative electrode impedance and decrease the positive electrode impedance. Based on these experiments we suggest that PES is a very useful additive for NMC/graphite Li-ion pouch cells that may encounter high temperatures during storage and cycling.

Improving the High Voltage Cycling of Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/Graphite Pouch Cells Using Electrolyte Additives

Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells with 1 M LiPF6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 by weight) electrolyte containing 2 wt% vinylene carbonate (VC), 2 wt% prop-1-ene-1,3 sultone (PES), or 2 wt% PES + 1 wt% tris(trimethylsilyl) phosphite (TTSPi) + 1 wt% methylene methanedisulfonate (MMDS) (“PES-211”) were subjected to cycling tests at various upper cut-off voltages from 4.2 to 4.7 V. After every 10 cycles, the pouch cells were charged or discharged to 3.8 V and evaluated using electrochemical impedance spectroscopy (EIS). This approach allows the impact of electrolyte additives on the impedance of cells cycled to different cut-off voltages to be studied systematically. The addition of 2% PES or “PES-211” proved to be beneficial in suppressing impedance growth when cells are cycled up to and above 4.4 V. Long-term cycling tests between 3.0 V and 4.4 V at 45°C showed that “PES-211” had the best capacity retention (∼85%) and the least impedance growth after 500 cycles. The results suggest the use of “PES-211” could lead to longer lived and higher power NMC/graphite Li-ion cells with better tolerance to high voltage (4.4 V) which will improve energy density.