Studies of the effects of electrolyte additives on the performance of lithium-ion batteries
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Electrolyte additives are an effective way to improve the lifetime and performance of Li-ion cells. Electrolyte additives can modify the solid electrolyte interphase layers on the positive and/or negative electrodes, resulting in improved Li-ion cells. However, the function of additives and why they work are poorly understood. Through a series of high precision cycling and storage experiments, combined with measurements of gas evolution and cell impedance, the effectiveness of electrolyte additives and additive combinations have been investigated. This combination of experimental methods proved to be useful and was used to probe the performance of additives and understand their role in Li-ion cells in a timely fashion. Cells containing a combination of additives resulted in better performance than those containing a single additive, demonstrating the distinctive contributions of each additive to improvements to cell performance and lifetime. Li[Ni1-x-yMnxCoy]O2/graphite pouch cells containing the additive prop-1-ene-1,3-sultone (PES) had equivalent performance to cells containing the “famous” additive vinylene carbonate, except that prop-1-ene-1,3-sultone nearly eliminated all gas production during cycling and storage at elevated temperature. Cells containing PES combined with methylene methane disulfonate (MMDS) and tris(trimethylsilyl) phosphite (TTSPi) exhibited superior performance during cycling and storage experiments. In general, the impedance of Li[Ni1-x-yMnxCoy]O2/graphite cells varied strongly with voltage above 4.3 V, and was basically reversible during a single cycle, but increased slowly and continuously during dozens of charge-discharge cycles to 4.4 V or 4.5 V. Additive blends of PES combined with MMDS and TTSPi were very effective at reducing this impedance growth. Further work will address why this is the case.