Revealing a conversion-alloying reaction mechanism behind high capacity and rate capability of SnS/N-doped graphene anode by in situ TEM

Developing an electrode material with improved ionic transport dynamics in a battery has been the focus of research. Here, we report a facile one-step hydrothermal synthesis method to prepare anode material of ultra-small SnS nanocrystals (NCs) anchored on N-doped graphene nanosheets (SnS/N-G), which is expected to significantly the dynamics of lithium transport, enabling an exceptional capacity of 1120.3 mAh g⁻¹ at 0.1 A g⁻¹ after 130 cycles and superior rate capabilities of 446.3 and 340.7 mAh g⁻¹ at 2 and 3 A g⁻¹, respectively. Furthermore, the lithiation/delithiation behaviors of SnS/N-G anode were observed in real time using in situ transmission electron microscopy to reveal the corresponding kinetics. By tracking the full lithiation procedure, in situ electron diffraction and high-resolution TEM imaging found that the original SnS phase was firstly transformed to Sn phase by conversion reaction and then to Li₂₂Sn₅ phase by alloying reaction. Notably, a stable and reversible phase transformation was established between Li₂₂Sn₅ and Sn phases during subsequent charge-discharge cycles. In the meantime, the volume expansion-induced pulverization of SnS NCs was evidently alleviated by graphene matrix that not only provided a two-dimensional support to buffer the volume change, but also improved the ion migration kinetics, as corroborated by superior rate capability.