Investigating Parasitic Reactions in Anode-Free Li Metal Cells with Isothermal Microcalorimetry

Abstract

Anode-free Li metal cells are one of the most appealing energy storage technologies beyond Li-ion batteries due to their superior theoretical specific and volumetric energy densities. However, long cycle life in an anode-free cell remains elusive due to difficulties reversibly plating and stripping metallic lithium. Isothermal microcalorimetry was used to study parasitic reactions in anode-free Li metal cells for electrolytes containing different Li salts. A new cycling protocol was used to measure the parasitic heat flow both on freshly plated Li surfaces and anode surfaces after stripping lithium. Existing methods were used to measure parasitic reactions occurring at high voltage. In both low- and high-voltage measurements, electrolytes containing LiDFOB had the highest parasitic heat flow compared to an electrolyte with LiPF6 salt. In contrast to previous studies of parasitic reactions in Li-ion batteries using isothermal microcalorimetry, the LiDFOB-containing electrolytes gave the longest lifetime despite having higher parasitic heat flow. This observation was attributed to decomposition of the LiDFOB salt, and subsequent formation of a favorable SEI layer that greatly improves plating and stripping efficiency. In-situ detection of parasitic heat flow with isothermal microcalorimetry techniques will be valuable for future studies of electrolyte design in anode-free Li metal cells.