Ex situ TEM observations of the SnB2O4 glass electrode in all-solid-state lithium-ion batteries after charge–discharge process

Abstract

As an environmentally friendly Pb-free material, SnO–B2O3 glass is considered a next-generation negative-electrode active material for all-solid-state lithium-ion batteries. Using SnB2O4 (50SnO·50B2O3) as a negative-electrode active material, the all-solid-state cells exhibit a high initial discharge capacity of approximately 950 mAh g−1. In this study, the microstructures of the SnB2O4 glassy electrode composites are investigated to clarify the charge–discharge behavior and charge–discharge mechanisms of the SnB2O4 glassy electrode composites using transmission electron microscopy (TEM). Here, ex situ TEM observations capture Sn and Li–Sn alloy nanocrystallites in the amorphous matrix. Li+ is incorporated into Sn, and Li–Sn alloy nanocrystallites are formed after initial lithiation. Further, Li+ is extracted from Li–Sn alloys to form Sn nanocrystallites after initial delithiation. In particular, Sn is lithiated and delithiated during the first cycle. However, as the cycle number increases, the capacity deteriorates. Ex situ TEM observations further reveal that Li–Sn alloys remain unchanged even after delithiation, and the crystallite size increases significantly. It is thus found that the capacity deterioration is attributed to the irreversible Li insertion/extraction between Sn and Li–Sn alloys.