Mechanistic insights into capacity discrepancies of conversion-type transition-metal compounds in wide-temperature-range lithium-ion batteries†

Conversion-type transition-metal compounds (C-TMCs) are widely used as lithium-ion battery (LIB) anodes due to their high theoretical capacity. However, a significant discrepancy in lithium storage capacity is observed across a wide range of temperatures, and a comprehensive understanding of the underlying mechanism remains elusive. Herein, we propose a methodology to clarify the capacity discrepancy mechanisms by choosing the Fe_1−xS anode as a representation. Specifically, we demonstrate lithium storage in three stages of Fe_1−xS across a wide temperature range, involving insertion, conversion, and space charge. Furthermore, we reveal that the capacity discrepancy mechanisms of Fe_1−xS across a wide temperature range are due to the differences in the amount of spin-polarized electrons that are injected into Fe, which induces the storage of different amounts of lithium ions into Li₂S during the space charge lithium storage by in situ magnetometry as a dominant technology. Higher operational temperatures of the batteries benefit from more storage of ions and electrons in Li₂S and Fe, respectively. Our work clarifies the importance of space charge in the improvement of the capacity of C-TMCs in a wide temperature range, which can guide the development of high-capacity anodes that can be used in wide-temperature-range LIBs.