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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.

Wound LiCoO 2 /graphite and Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 /graphite cells with 1M LiPF 6 EC:EMC electrolyte containing either 0, 1 or 2 wt.% vinylene carbonate were studied using the High Precision Charger at Dalhousie University, automated cell storage and AC impedance. Vinylene carbonate (VC) was found to improve the coulombic efficiency of the cells, decrease charge endpoint capacity slippage and decrease self discharge, in all cases primarily by slowing electrolyte oxidation at the positive electrode. The beneficial impacts of VC are greater in LiCoO 2 cells than in Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 cells. One percent VC is enough to derive the benefits without causing an impedance rise in the cells.

In order to enhance the performance of Li or Li-ion batteries many kinds of electrolytes have been studied and developed over the years. 1,2 These electrolytes are needed to give optimum interfacial properties between the electrolyte and the various anodes or cathodes. So far, the salts such as LiClO 4 , LiPF 6 , LiBF 4 , LiAsF 6 , and LiOSO 2 CF 3 . LiOSO 2 CF 3 have been used in these batteries. However, there are advantages and disadvantages with each salt. These salts typically do not provide a balance of desired properties such as high ionic conductivity, high chemical/thermal stability, low toxicity, and a wide potential window. For example, LiClO 4 can induce explosions under some conditions. 3,4 LiPF 6 and LiBF 4 have poor chemical/thermal stability. 5-7 LiAsF 6 is regarded as one of the best salts because it gives the highest coulombic efficiency, but it degrades into toxic products. 8-12 LiOSO 2 CF 3 , which is more stable and safer than other salts, has the disadvantage of low ionic conductivity and like LiTFSI is corrosive toward Al current collectors at high potentials. 1 Due to its good ionic conductivity, electrochemical stability and low corrosion of Al, LiPF 6 is often used as the preferred salt in Li-ion rechargeable batteries.Recently, lithium bis(trifluoromethylsulfonylimide) [LiN(CF 3 SO 2 ) 2 , (HQ-115, available from 3M Company) LiTFSI] was introduced as a salt which acts as a good plasticizer for polyethylene oxide (PEO) electrolytes. [13][14][15] For application in polymer electrolyte systems, LiTFSI-PEO complexes exhibit rather low crystallinity and a low glass transition temperature (T g ), resulting in ionic conductivity above 10 Ϫ6 S cm Ϫ1 at 25ЊC. 14 The ionic conductivity of LiTFSI in propylene carbonate (PC) solvent is also comparable to that of LiPF 6 . Furthermore, this kind of imide salt has good chemical stability and safety characteristics. 13-15 Because of the above advantages of LiTFSI, several studies have been made of LiTFSI-based electrolytes. [16][17][18][19][20][21][22] However, the practical use of this salt in liquid electrolytes has not materialized due to its severe corrosion of Al which is commonly used as a cathode current collector above 3.6 V vs. Li/Li ϩ . 23 Very recently, a new imide salt, lithium bis(perfluoroethylsulfonylimide) [LiN(SO 2 C 2 F 5 ) 2 , Li-BETI] was introduced, in which the perfluoroalkyl groups are extended from -CF 3 (TFSI anion) to -C 2 F 5 (BETI anion). 23 Li-BETI has a relatively high ionic conductivity (ca. 10 mS cm Ϫ1 ) at a concentration of 1 mol dm Ϫ3 (ϭM) in PC:1,2-dimethoxyethane (DME) (1:1), and has excellent thermal stability up to 340ЊC compared to that of LiPF 6 , which undergoes thermal decomposition between 60 and 100ЊC. The relatively high ionic conductivity of Li-BETI/PC ϩ DME electrolyte can be attributed to the remarkable delocalization of charge in this molecule (see Fig. 1). 13 Compared to LiTFSI, Li-BETI offers a further advantage of improved corrosion performance with a repassivation potential of 4.2 V vs. Li/Li ϩ . 2,...