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.

Active lithium in the negative electrode of a Li-ion cell reacts with electrolyte to form an ever-thickening solid electrolyte interphase. The rate of this reaction can be monitored as a function of temperature, time and electrode potential using storage and symmetric cell studies. Using the soft carbon, petroleum coke, as a model negative electrode material, experiments measuring the open circuit voltage (OCV) change with time of Li/coke cells were made to measure the rate of loss of active lithium. The capacity loss with cycle number or time of coke/coke symmetric cells was also used to measure the rate of Li loss. The results on over 100 test cells show that: 1) the reaction rate decreases by about a factor of 2-4 as the electrode potential increases from 0.005 to 1.0 V; 2) the reaction rate increases approximately 3-10 fold between 30 and 60 • C depending on the electrode potential; 3) The reaction rates are within a factor of two, and may be the same, for electrodes at OCV or undergoing cycling; 4) the reaction rate is larger when vinylene carbonate (VC) is present in the electrolyte at 30 • C for all potentials and times studied and 5) the reaction rate is about two times smaller in the presence of VC at 60 • C for potentials above 0.4 V. A significant number of further experiments are required to develop accurate theoretical models of the reactivity of intercalated lithium with electrolyte as a function of time, temperature and potential.

Lithium-ion cells for electronics and automotive applications have an excellent safety record. However, safety-related events can sometimes occur during routine testing of prototype designs, especially in the case of designs using new electrode materials, new separators and/or new electrolytes. High precision measurements of coulombic efficiency have recently been shown to have great value in predictions of the impact of electrolyte additives on cell lifetime. In order to apply those methods to prototype automotive cells, special compact temperature-controlled boxes were required that could maintain the temperature to ± 0.05 • C precision and ensure the safety of the laboratory. These versatile temperature-controlled boxes were designed to accommodate cells as small as coin cells and as large as 40 Ah automotive pouch cells. The ability of the design to prevent cell-to-cell fire propagation and to channel smoke and flame away from the rest of the laboratory was experimentally verified. It is hoped that the information presented here will be of value to those designing precision testing facilities for large Li-ion cells. The summer 2012 issue of Electrochemical Society Interface is devoted to Li-ion battery safety. Doughty and Roth 1 state that the failure rate, leading to a safety incident, of Li-ion rechargeable battery cells in the field is very small, less than 1 in 10 million. Li-ion cells that are in use in the field have all been subjected to a large number of mandatory tests to demonstrate that they are safe under normal conditions of use and also under some conditions of electrical and mechanical abuse.2 Prototype Li-ion batteries in the R + D stage, that may be hand assembled, with higher energy density, new designs, new electrode materials and/or new electrolytes may be less safe than commercial cells and hence must be handled appropriately in the testing phase in the laboratory.Our research group has recently been applying precision measurements of the coulombic efficiency of Li-ion batteries to the study of cell lifetime and the efficacy of electrolyte additives.3-5 High precision coulometry measurements made over a period of a few weeks can be used to rank electrolyte additives and their combinations for effectiveness in prolonging cell lifetime. Applying such methods to automotive-scale Li-ion batteries is expected to yield similar advantages in cell lifetime predictions.Smith et al. 3 published requirements for testing equipment to perform high precision coulometry on Li-ion cells. These experiments are generally made at low rates, C/10 and slower, but need to be made at a very stable temperature (preferably stable to better than ±0.1• C) for measurements of extremely high precision. Therefore, there is a need for temperature-controlled boxes designed for high precision cycling of automotive cells that can maintain strict temperature stability while ensuring laboratory safety.One of the authors recently visited or contacted many makers of large Li-ion cells (>20 Ah) to learn about the chambers use...