The Impact of Electrolyte Additives Determined Using Isothermal Microcalorimetry

Isothermal microcalorimetry is a quick and simple method of determining the effect an additive or additive combination has on the parasitic reactions occurring as a function of state of charge. It can easily discriminate exactly where in the voltage range the additive is providing a benefit with just a single cycle. As a demonstrative example, the effect of varying concentrations of vinylene carbonate (VC) on a LiCoO2/graphite cell is examined. Machine-made pouch cells were used such that the cells were nominally identical except for the concentration of VC. The measured heat flow for the different cells is then identical except for the heat flow that results from parasitic reactions. It is shown that the presence of VC reduces parasitic reactions above 3.9 V, and continues to reduce these reactions with increasing state of charge. The heat flow during open circuit conditions at the top of charge also shows that the presence of VC dramatically reduces the heat due to parasitic reactions. 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 such as the consumption of active lithium in forming and repairing the solid electrolyte interphase (SEI), electrolyte oxidation, transition metal dissolution, electrode damage, etc. 1 However, it is not very well understood how these additives are functioning and exactly where in the charge-discharge cycle they prove advantageous. One such example is with the popular additive, vinylene carbonate (VC). Several studies 2‐7 report that the increase in full cell lifetime is caused by VC being reduced on the surface of graphite, which forms a more stable SEI. More recent studies 8‐13 have challenged this thought, reporting that the benefit of VC arises primarily from limitingelectrolyteoxidationandpossibletransitionmetaldissolution reactionsatthepositiveelectrode.Therefore,itisofdistinctinterestto 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. Recently the technique of isothermal microcalorimetry has been combinedwithelectrochemicalmeasurements,whichhasbeenusedto examine the thermal behavior of several lithium-ion chemistries. 14‐21 More recently, Krause et al. 22 showed how to use this technique to separate the various contributions to the thermal power and isolate parasitic energy. In this paper, this technique is used to qualitatively 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. As a demonstrative example, the effect of varying concentrations of VC on a LiCoO2/graphite cell is examined. Experimental General.— Machine-made 225 mAh LiCoO2 (LCO)/graphite pouch cells 0.4 cm thick × 2 cm wide × 3 cm high (obtained from Pred Materials Co.) were provided dry. The pouches were then filled with 0.75 g of 1M LiPF6 in 3:7 ethylene carbonate (EC):ethylmethyl carbonate (EMC) (Novolyte Technologies, now BASF) with various amounts of VC (Novolyte Technologies, now BASF) additive (0%, 0.5%, 2%, and 4% by weight) and then vacuum sealed. The electrodes