Battery and EV manufacturers carry out extensive long-term tests to estimate the lifetime of the battery and base warranty durations on those tests. The long duration of these tests slows progress in the research and development required to improve the lifetime of Li-ion batteries. This paper shows that accurate measurements of coulombic efficiency (CE) and impedance spectra of Li-ion batteries, that take a few weeks to acquire, can be used to rank the resulting lifetime of Li-ion cells. Adding one or more electrolyte additives to Li-ion batteries that act synergistically can dramatically improve the CE and long-term tests show corresponding ten-fold improvements in lifetime.In a recent review of battery technologies for electric vehicle applications, Bruce et al. 1 argue that technologies with greater energy densities than those achievable by Li-ion batteries are required in order to reach driving ranges greater than 200 km. Bruce et al. picked the Nissan Leaf as their canonical example which has an approximate 160 km range. The view that Li-ion batteries cannot lead to electric vehicles with widely acceptable driving ranges is also held by others. 2,3 Recently, the Tesla Model S, a Li-ion battery powered electric vehicle, was named Motor Trend car of the year for 2013. 4 The Tesla S has a driving range of 425 km when equipped with an 85 kWhr Li-ion battery. The Tesla S demonstrates that Li-ion batteries can power EV's for distances over 200 km in an elegant design. However, the vehicle is expensive and the 85 kWh battery must contribute at least $25,000 to the price assuming the $300/kWh USDOE target for EV battery costs. 5 No one would argue that increased energy density would not be an advantage, but the real issues with Li-ion batteries are cost and lifetime, not energy density, as far as automotive applications are concerned.Recently, a class action lawsuit was brought against Nissan by Nissan Leaf owners alleging that the Li-ion batteries in the Leaf can lose as much as 27% of their energy storage capacity within one year of use. 6 The Nissan Leaf uses a different Li-ion battery technology than the Tesla Model S. Nevertheless, EV manufacturers and users are worried about battery lifetime since the cost of a replacement battery is large.Testing the lifetime of a Li-ion battery under realistic conditions for an EV application takes years. Research and development to improve the lifetime of Li-ion batteries cannot have iterative cycles of several years before the outcome of experiments are known. We recently proposed the use of high precision measurements of the coulombic efficiency (CE) of Li-ion cells as a rapid way to screen and rank new electrode materials, electrolytes, and electrolyte additives for their impact on Li-ion cell lifetime. 7-10 In this paper it is demonstrated that short-term CE measurements coupled with initial impedance measurements can serve as a good predictor for cell lifetime and also dramatically demonstrate the beneficial impact of multiple electrolyte additives on lifetime. Figure...

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...

LiCoO2/graphite and Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells and Li[Ni1-x-yMnxCoy]O2/graphite 18650 cells were made with varying concentrations of vinylene carbonate (VC) and studied using high precision coulometry, extended cycling as well as electrochemical impedance spectroscopy (EIS). As expected, adding increased concentrations of VC (up to 6 wt%) to the control electrolyte resulted in improved coulombic efficiency, decreased charge endpoint slippage and longer cycle life. However, high concentrations of VC led to larger charge transfer resistance, especially at the graphite negative electrode. Understanding how varying amounts of VC impact cell lifetime and impedance allows for optimized electrolyte formulations to be found for different applications that may balance lifetime and power demands.

The effects of cyclic sulfate additives including ethylene sulfate (or 1,3,2-Dioxathiolane-2,2-dioxide (DTD)), trimethylene sulfate (or 1,3,2-Dioxathiane 2,2-dioxide (TMS)) and propylene sulfate (or 4-methyl-1,3,2-dioxathiolane-2,2-dioxide (PLS)) on Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 /graphite pouch cells were studied. These additives have the same SO 4 unit bonded to a different hydrocarbon ring so that the impact of changes to the ring on the electrochemical behavior can be studied. The additives were studied in pouch cells singly and in combination with 2% vinylene carbonate (VC) using high precision coulometry, AC impedance, long term storage and using a gas evolution apparatus. DTD and TMS show significant promise as electrolyte additives while PLS appears less useful. When added alone, DTD and TMS decrease cell impedance compared to control cells, improve coulombic efficiency and reduce voltage drop during storage. However, both lead to significant gas generation, comparable to control cells, during formation. DTD in combination with 2% VC gives high coulombic efficiency (CE), stable impedance during cycling and manageable amounts of gas during formation. TMS in combination with VC produces virtually no gas during formation at 40 • C. In addition, TMS in combination with VC shows high CE and impedance that is reduced during early cycling. PLS in combination with VC yields cells with relatively poor CE and impedance growth during cycling.