Visualization of regulated nucleation and growth of lithium sulfides for high energy lithium sulfur batteries†
The formation of lithium sulfides as discharge products imparts high specific energy density to lithium sulfur batteries (LSBs), however, the involvement of soluble intermediates in the battery reaction makes it challenging to achieve it reversibly for extended cycles. The precise understanding of phase transitions from the soluble intermediates to solid discharge products would aid in fundamentally resolving practical issues involving the intermediates, and thus allow the realization of long-lived high-energy-density LSBs. Herein, we utilize liquid in situ transmission-electron-microscopy (TEM) to probe detailed liquid–solid reaction processes. It reveals that the surface nature of the host materials of the polysulfides critically influences the growth mechanism of lithium sulfides. It is elucidated that polar hosts induce instantaneous nucleation of lithium sulfides, followed by diffusion-controlled-to-reaction-limited growth kinetics and a crystalline-to-amorphous phase transition. Moreover, it is verified that polysulfides are better immobilized in polar hosts, whereas polysulfide diffusion through nonpolar hosts is evidently observed, leading to the eventual degradation of cells. Based on these findings, an optimal host structure for sulfur is proposed, where the dual walls of polar (inner)/nonpolar (outer) spheres confine the polysulfides. The new cathode exhibits remarkable electrochemical performance, retaining a capacity of 4.3 mA h cm−2 over 400 cycles at a low electrolyte/sulfur ratio of 6.8 ml g−1, which rivals state-of-the-art LSBs. This work contributes the first liquid in situ TEM study of liquid–solid phase evolution for high energy electrode materials.