Lithium-ion batteries are being used in an increasing number of applications that are demanding higher energy densities and longer lifetimes. The use of electrolyte additives is a common method that has shown to extend calendar and cycle life, and reduce parasitic reactions that occur between the electrolyte and electrode materials. However, it is not very well understood how these additives are functioning and exactly where in the charge-discharge cycle they prove advantageous. Therefore, it is of interest to be able to determine the voltage-dependent advantage of a particular additive or additive combination, which can aid in the understanding of how these additives are extending lifetimes of lithium-ion batteries. Recently the technique of isothermal microcalorimetry has been combined with electrochemical measurements, which has been used to examine the thermal behavior of several lithium-ion chemistries1"8. More recently, Krause et al.9 showed how to use this technique to separate the various contributions to the thermal power and isolate parasitic energy. Here, this technique is used to qualitatively and quantitatively compare the heat flow between cells that only vary in concentration of additive. In this case, with all other sources being identical, the measured difference in heat flow arises from differences in parasitic heat. This is done as a function of state of charge, providing a simple and quick method of determining exactly where and to what extent the additive is reducing parasitic reactions occurring between the electrolyte and electrode materials. As a demonstrative example, the effect of varying concentrations of vinylene carbonate (VC) on a LiCoO2/graphite cell is examined, where VC is a widely used electrolyte additive that has been shown to extend cell lifetimes10.
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