An apparatus was built to make accurate and precise in situ measurements of the volumes of gas evolved in Li-ion pouch cells during operation. With a thin film load cell accurately measuring the weight of a cell submerged in a fluid, the volume of a pouch cell can be precisely monitored using Archimedes' Principle. Examples showing the utility and sensitivity of the device have been selected from measurements made during the formation cycle (very first charge and discharge) of Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite (NMC) Li-ion pouch cells. Gas production occurs at the very beginning of the formation cycle but quickly stops for cells containing a variety of electrolytes. The volume of the pouch cell then decreases with time. The testing of cells with various electrolyte additives indicated that the common additive, vinylene carbonate, is very effective at reducing the amount of gas formed during formation, but the best results among the additives reported here were obtained by using a combination of 2% vinylene carbonate and 2% prop-1-ene 1,3-sultone. The additives vinyl ethylene carbonate and ethylene sulfite were found to delay the onset of gas production during formation.

The effects of electrolyte additives singly or in combination on Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (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[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (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.

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Vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are compared as electrolyte additives for LiCoO 2 /graphite pouch cells using the ultra high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS) and long term cycling. Both VC and FEC are useful additives that improve couloumbic efficiency (CE), reduce charge end point capacity slippage, improve long-term cycling and reduce self-discharge during storage compared to cells with control electrolyte. Increasing the concentration of VC over 2% causes a dramatic increase in charge transfer resistance at the negative electrode surface, while the same effect is not observed for FEC. Therefore larger concentrations of FEC can be added to the electrolyte without this problem. However, when 4 or 6% FEC is used, greater gas generation during extended cycling at 40 • C is detected. When only a single additive of VC or FEC is used in these LCO/graphite pouch cells tested at 40 • C, a concentration of between 2% and 4% VC appears to be optimum as that provides high CE, low charge end point capacity slippage, a small increase in charge-discharge polarization with cycling and a small self-discharge during storage. The VC content would be optimized between these limits to trade off lifetime for rate capability.

The effect of mixtures of lithium hexafluorophosphate (LiPF 6 ) and lithium bis(fluorosulfonyl)imide (LiFSI) with different molar ratios in the electrolyte of Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite pouch cells was studied using the ultra-high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS) and gas evolution measurements. Ethylene carbonate: ethyl methyl carbonate (EC:EMC, 3:7 wt.% ratio) solvent was used as the base solvent in these studies. Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite pouch cells containing both LiPF 6 and LiFSI (with a total salt content of 1 M) with or without 2% VC showed smaller or similar self-discharge, lower charge transfer resistance (R ct ), and smaller amounts of gas evolution during formation and during storage at high temperature, compared to cells containing only 1 M LiPF 6 with and without 2% VC, respectively. For cells without 2% VC, cells with 0.3 M LiPF 6 + 0.7 M LiFSI showed the smallest self-discharge and the lowest R ct after 40 • C storage. For cells with 2% VC, cells with 0.5 M LiPF 6 + 0.5 M LiFSI + 2% VC showed the lowest voltage drop and the lowest R ct after 40 • C storage. The UPHC cycling data showed that cells containing LiPF 6 :LiFSI mixtures with 2% VC showed similar coulombic efficiency (CE), and similar charge end-point capacity slippage compared to cells with 1 M LiPF 6 + 2% VC. The combination of LiFSI and LiPF 6 in electrolytes that contain 2% VC can bring benefits of improved storage properties and reduced gas evolution at high temperature while maintaining all other properties of the cells in experiments limited to 4.2 V. However, preliminary experiments at voltages up to 4.45 V suggest that LiFSI may lead to increased transition metal dissolution (Ni, Mn and Co) compared to lithium bistrifluoromethane sulfonyl imide (LiTSFI), another salt additive used to improve storage properties and reduce gassing. The use of electrolyte additives is one of the most economical and effective ways to improve the performance of Li-ion cells.1,2 Vinylene carbonate (VC) has been shown to be an effective electrolyte additive for the improvement of cell performance.3,4 Burns et al. 5 showed that LiCoO 2 /graphite cells with 2% VC demonstrated improved life-time with a higher coulombic efficiency (CE) and lower charge and discharge end-point capacity slippage rates, compared to cells without VC. Lithium hexafluorophosphate (LiPF 6 ) is the most commonly used salt in Li-ion cells, due to balance of properties, such as high dissociation constant, high conductivity and good electrochemical stability against Al corrosion.1,2,6,7Recent studies [8][9][10] have shown that lithium bis(fluorosulfonyl)imide (LiFSI) can be thermally stable up to 200• C. However, Al current collector corrosion was observed in Li-ion cells with LiFSI-based electrolytes.10-14 The Al corrosion problem for LiFSI-based electrolytes can be readily solved by the addition of LiPF 6 , which can be decomposed to form a protective film of AlF 3 . ...