Forged by charge: polaron-induced matrix formation in silicon nitride conversion-type anodes for lithium-ion batteries

The quest for high-capacity anode materials is vital in developing future lithium-ion battery technologies. While silicon-based anodes offer high theoretical capacity, their commercial realization is hindered by instability associated with large volume changes. Amorphous silicon nitride (a-Si₃N₄) has emerged as a promising alternative, acting as a conversion-type anode where lithium incorporation drives the formation of a structurally robust matrix and active phases. Here, we demonstrate that charge trapping, driven by polaron and bipolaron formation, governs the structural transformation of a-Si₃N₄ during the initial lithiation. These charge-induced modifications lead to the formation of a Li-Si-N matrix that stabilizes the anode framework. Matrix generation is accompanied by the development of Si-rich regions, serving as precursors for the active phase. We identify a progression from electronically active polarons to inactive bipolaron states, establishing a direct link between charge localization and matrix formation. These insights recast charge trapping from a passive consequence to a functional design parameter for optimizing conversion-type anodes.