The Application of Isothermal Microcalorimetry to the Study of Parasitic Reactions in Lithium Ion Batteries
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In an ongoing effort to advance the understanding of how electrolyte composition impacts the performance and lifetime of lithium ion cells, this work introduces the use of isothermal microcalorimetry as a tool to measure the heat flow associated with the parasitic reactions that cause cell degradation. This technique is rapid, in situ, in operando, and the results are a direct function of voltage. The majority of this work focuses on the development of the method to extract and understand the parasitic heat flow as a function of both voltage and time. This was achieved by measuring the heat flow for cells undergoing narrow voltage range cycling at various currents, then fitting the measured total heat flow to an empirical model to isolate each component of the heat flow as a function of state of charge. Particular emphasis was placed on the heat flow resulting from parasitic reactions and as such, this work presents the first direct measurements on the voltage-dependent impact of the electrolyte composition on the parasitic reactions occurring in lithium ion cells. The parasitic heat flow was determined for LCO/graphite and several types of NMC/graphite pouch cells containing a variety of electrolyte compositions. Cells containing electrolyte additives that result in increased cell lifetimes were found to have decreased parasitic heat flows, and therefore decreased reaction rates, especially at high voltages. The determination of the time dependence of the parasitic heat flow highlighted profound changes in the reaction mechanisms as a function of voltage, including the observation of an increasing parasitic heat flow as a function of time for LCO/graphite cells containing 2%VC + 1%TTSPi + 1%MMDS above 4.3 V. Experiments on NMC442/graphite cells showed distinct differences in the voltage behaviour of the parasitic heat flow for cells containing either carbonate or fluorinated carbonate solvents, and showed that fluorinated solvents are only advantageous above 4.4 V. The results presented in this work have provided valuable insight on the complex roles of the electrode and electrolyte compositions on the parasitic reactions in lithium ion cells, as well as how those roles depend on both voltage and time.