Sodiation of hard carbon : how separating enthalpy and entropy contributions can find transitions hidden in the voltage profile

Mercer, Michael and Affleck, Sam and Gavilan-Arriazu, Edgardo Maximiliano and Aragon Zulke, Alana and Maughan, Phil and Trivedi, Shivam and Fichtner, Maximilan and Reddy Munnangi, Anji and Leiva, Ezequiel and Hoster, Harry (2022) Sodiation of hard carbon : how separating enthalpy and entropy contributions can find transitions hidden in the voltage profile. ChemPhysChem, 23 (5): e202100748. ISSN 1439-4235

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Abstract

Sodium-ion batteries (NIBs) utilise cheaper materials than lithium-ion batteries (LIBs), and can thus be used in larger scale applications. The preferred anode material is hard carbon, because sodium cannot be inserted into graphite. We apply experimental entropy profiling (EP), where the cell temperature is changed under open circuit conditions. EP has been used to characterise LIBs; here, we demonstrate the first application of EP to any NIB material. The voltage versus sodiation fraction curves (voltage profiles) of hard carbon lack clear features, consisting only of a slope and a plateau, making it difficult to clarify the structural features of hard carbon that could optimise cell performance. We find additional features through EP that are masked in the voltage profiles. We fit lattice gas models of hard carbon sodiation to experimental EP and system enthalpy, obtaining: 1. a theoretical maximum capacity, 2. interlayer versus pore filled sodium with state of charge.