Twenty seven LiCoO 2 /graphite wound prismatic cells containing a variety of electrolyte additives as well as high or low surface area LiCoO 2 were studied during high temperature storage using an automated storage system. The same cells had been previously studied using high precision coulometry. Cells were initially cycled to measure the capacity, charged and then stored for one month at either 40 or 60 • C, then cycled again to measure the reversible and irreversible capacity loss. The process was then repeated. During storage, the open circuit potential was automatically measured every 6 hours. The mechanisms responsible for the voltage drop which occurred during storage and the capacity loss after storage were analysed using a Li inventory model. The voltage drop during storage is caused primarily by parasitic reactions (electrolyte oxidation, transition metal dissolution, etc.) that insert Li into the positive electrode, because the potential of the Li x C 6 electrode is virtually constant on the stage-2/stage-1 plateau even if its Li content changes due to solid electrolyte interface (SEI) growth. The experimental results show that the combination of the electrolyte additive, vinylene carbonate, and low surface area LiCoO 2 minimizes the voltage drop and capacity loss during storage presumably by reducing the amount of electrolyte oxidation occurring at the positive electrode. The same cells had charge endpoint capacity slippages that were closest to 0.00%/cycle during cycling tests monitored with high precision coulometry. Storage experiments, in concert with precision coulometry, allow a clear picture of the effect of additives to be determined.Lithium-ion batteries are now being used in electrified vehicles. The cycle and calendar life requirements in vehicular applications are far more demanding than in computer and phone applications. Therefore it is utmost importance to understand cell degradation mechanisms and to use new electrode materials, electrolytes and electrolyte additives to minimize degradation.Capacity loss in Li-ion batteries occurs during storage and cycling. 1-5 There are many possible undesired or parasitic processes, such as dissolution of transition metals from charged positive electrodes, corrosion of current collectors, electrolyte oxidation at the positive electrode, electrolyte reduction at the negative electrode leading to SEI growth, etc. that lead to capacity loss. Capacity retention and storage life of Li-ion cells are critically dependent on the stability of the passivation layers that form on both electrodes. Control of the electrode/electrolyte interfaces is therefore key to obtain Li-ion cells with long lifetimes.It was suggested by Broussely et al. 2 that lithium consumption at the negative electrode affected cell capacity during storage at high temperature. They also concluded that electrolyte oxidation at the positive electrode resulted in additional losses during storage at high voltage.Electrolyte additives, such as vinylene carbonate (VC), are known to improve cycl...

Wound LiCoO 2 /graphite cells with 1 M LiPF 6 EC:EMC electrolyte containing 1 wt%, 2 wt% vinylene carbonate (VC), 0.3 wt% trimethoxyboroxine (TMOBX) and 2 wt% VC + 0.3 wt% TMOBX were subjected to extended storage studies. After storage, the electrodes were studied using the symmetric cell and electrochemical impedance spectroscopy (EIS) approach described by previous workers. This approach allows the impact of an additive on the impedance of the negative and the positive electrode to be distinguished. Compared to the control cells, adding 1 wt% VC reduced the positive electrode impedance and only slightly affected the negative electrode impedance. Adding 2 wt% VC reduced the positive electrode impedance and greatly increased the negative electrode impedance. An addition of 0.3 wt% TMOBX greatly decreased the positive electrode impedance and slightly increased the negative electrode impedance. Compared to the cells with 2% VC only, adding 2% VC + 0.3% TMOBX decreased the positive electrode impedance without affecting the negative electrode impedance leading to a significant reduction in full cell impedance. These results help explain why the combination of VC and TMOBX additives can be effective in LiCoO 2 /graphite cells designed for long life time.Lithium-ion batteries have high gravimetric and volumetric energy densities which make them suitable for portable electronics and electric vehicle applications. However, parasitic reactions between the electrolyte and the electrochemically active material limit their lifetime, especially at elevated temperatures. Electrolyte additives are generally used in commercial batteries to improve capacity retention and calendar life. [1][2][3] Although it is very apparent that electrolyte additives play an important role, the details of how they work are poorly understood. The most-studied additive, vinylene carbonate (VC) has been shown to change the chemistry of the passivation film on the graphite electrode. 4-7 It is not clear whether this changed film is actually a better film, because recent experiment by Xiong 8 show that only at 60 • C are the parasitic reactions with electrolyte reduced in rate in the presence of VC: at lower temperatures, the reactions are accelerated. Burns et al. and Sinha et al.,9,10 have shown that VC strongly reduces the rate of reactions between the electrolyte and the charged positive electrode, and it seems that the major impact of VC may be at the positive electrode.Burns et al. studied electrolyte additives in wound prismatic cells using high precision coulometry and electrochemical impedance spectroscopy (EIS). 11 These methods show how additives affect the cycling performance, coulombic efficiency, charge and discharge end-point capacity slippage rates and potential drop during storage. As a motivation for the work in this paper, Figure 1 reviews some of the earlier work by Burns et al., 11 where cells were first tested for 600 hours at 40 • C on the high precision charger, then impedance spectra were collected and then cells were cycled f...

LiCoO 2 /graphite and LiCoO 2 /Li 4 Ti 5 O 12 wound prismatic cells were examined with and without electrolyte additives using the high precision charger at Dalhousie University. The additives tested were vinylene carbonate, trimethoxyboroxine, and lithium ͑bis͒ trifluoromethanesulfonimide. The voltage curves, charge and discharge end point positions, fade, and coulombic efficiency were compared to gain an understanding of the effects of the electrolyte additives on the cells. Long term cycling data ͑capacity loss over 750 cycles͒ was compared with predicted lifetime measurements based on high precision coulometry. Design of experiments was used in order to help interpret the results from the 20 groups of cells tested.Lithium-ion batteries ͑LIBs͒ used in electrified vehicles and for grid energy applications require larger capacities, longer cycle life, and longer calendar life than previously required in portable electronics ͑e.g., laptops and cell phone͒ applications. Electrolyte additives have been extensively studied and are used to improve the lifetime of LIBs. [1][2][3][4][5][6][7][8] One of the most commonly used electrolyte additives is vinylene carbonate ͑VC͒. 2-5 Aurbach et al. 2 studied the impact of VC and found that its reduction at a graphite negative electrode takes place before the reduction of ethylene carbonate, which forms a flexible and cohesive polymeric surface species that acts as a more stable solid electrolyte interface ͑SEI͒. The authors also believed that this type of unique surface reaction may be occurring on the positive electrode, stabilizing its SEI as well. Ota et al. 3 examined the improved SEI formed on graphite with the addition of VC. Another paper from the same group showed the beneficial impact of VC at higher temperature and attributed it to increased Li + ion mobility and stated that the addition of VC has a large impact on the negative electrode but could benefit the positive electrode as well. 4 Other less studied additives include trimethoxyboroxine ͑TMOBX͒ and LiN͑CF 3 SO 2 ͒ 2 ͑called HQ-115 here͒. TMOBX is made of a ͑BO͒ 3 ring with methyl groups attached to the boron atoms. Mao et al. 6 showed that the presence of ͑BO͒ 3 rings dissolved in electrolyte reduced the capacity loss during cycling of 18,650-size cells ͑LiCoO 2 /graphite and LiMn 2 O 4 /graphite͒ and that the additive with the methyl groups attached to the ring proved more effective. Small concentrations ͑Ͻ 1%͒ were added to see this benefit. 6 HQ-115 is a Li-salt ideal for organic electrolyte-based lithium batteries. HQ-115 has better thermal stability than LiPF 6 resulting from the strong covalent bonding nature of the negative ion. 7,8 It is believed by the authors that HQ-115 is used as an electrolyte additive in prismatic and pouch-type Li-ion cells to limit gas generation during operation.A major problem for researchers is the inability to conveniently determine the cycle life and calendar life of lithium-ion batteries under actual duty cycles that will be used in the field. For example, a pure elect...

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.

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