Microstructure of Silicon Anodes in Solid-State Batteries -- From Crystalline to Amorphous

TL;DR

This study uses cryo-TEM to analyze how silicon microstructure evolves during cycling in solid-state batteries, revealing microstructural changes from crystalline to amorphous states and emphasizing the importance of initial material choice.

Contribution

It provides detailed insights into the microstructural evolution of silicon anodes during cycling, highlighting the transition from crystalline to amorphous phases and implications for electrode stability.

Findings

Post-lithiation, silicon shows a mix of crystalline and amorphous phases.

Delithiation results in predominantly amorphous microstructure.

A stable microstructure forms after several cycles, affecting electrode design.

Abstract

Silicon offers great promise as a potential anode active material and the optimum alternative to lithium metal in all-solid-state lithium-ion batteries. However, its practical application is limited by severe volume expansion (~300%) during lithiation, leading to cracking upon delithiation. In this study, we investigated the microstructural evolution of microcrystalline silicon electrodes in a solid-electrolyte-free environment using cryogenic scanning transmission electron microscopy (STEM) during electrochemical cycling. A controlled workflow prevents ambient exposure, and cryo-TEM ensures structural integrity. After the first lithiation, the electrode shows a heterogeneous mix of crystalline Li15Si4, various amorphous LixSi phases, and residual crystalline silicon. After delithiation, the structure becomes predominantly amorphous with thread-like features and minimal remaining crystallinity. By the 10th delithiation, the microstructure is more uniform, with thread-like regions mainly at grain boundaries. Our results reveal that starting from a crystalline phase, a stationary microstructure emerges in bulk silicon only after several cycles. Thus, to have a more controlled behavior of the electrode and minimize cracking, the starting material should be carefully chosen along with an optimized electrode architecture to help stabilize the microstructure throughout cycling.