Unravelling composition and mass-loading effects in monolithic non-stoichiometric silicon nitride anodes for lithium-ion batteries

Abstract

Silicon is a promising alternative to the conventional graphite anodes due to its high theoretical capacity and favorable lithiation potential for lithium-ion batteries (LIBs) with liquid as well as solid-state electrolytes. However, lithiation-induced extreme volume change causes severe mechanochemical deformation and continuous formation of solid-electrolyte interphase leads to cell failure. One of the strategies to mitigate this problem is alloying silicon with a suitable element that can alter the surface electrochemistry and/or lithiation pathways, and acts as mechanical buffer. Nonetheless, these benefits come with a compromise on the specific capacity, which strongly influences the mass loading of the electrodes, highlighting the need to deconvolute the intertwined influence of composition and mass loading when designing high performance electrodes. In this work, we systematically studied the influence of composition and mass loading in monolithic amorphous silicon and non-stoichiometric silicon nitride (SiN_x) electrodes on their electrochemical performance as LIB anodes. The incorporation of nitrogen in the electrode matrix clearly improves the electrochemical stability at the expense of reduced specific capacity, while higher mass loading accelerates capacity fading, most critically in amorphous silicon electrodes. Postmortem analysis reveals that such capacity fading in the electrodes with higher mass loading can be related to delamination due to evolved tensile stress during the charge–discharge cycle. Yet, nitrogen-rich SiN_x monolithic electrodes accommodate strain more effectively. These findings demonstrate that while pristine Si delivers high specific capacity and long-term stability in thin films, thicker (>1 µm) monolithic electrodes benefit from higher nitrogen content in SiN_x, which provides more stable cycling and sustained capacity.