Origin of the Superior Electrochemical Performance of Amorphous-Phase Conversion-Reaction-Based Electrode Materials for Na-Ion Batteries: Formation of a Bicontinuous Metal Network.

The realization of conversion type electrode materials in Na-ion batteries (NIBs) has been hindered due to the nucleation property of the active material. During the sodiation, the transition metal (TM) cations reduce to the metallic state, and the respective anions react with the sodium ions. As a result, the metal particles are surrounded by the matrix of the insulating sodium compound, resulting in loss of electrical contact among the TM particles. Here, an amorphous molybdenum sulfide (a-MoSx) electrode is made highly reversible by suppressing TM particle growth via elongating the cation diffusion pathway. Because of the long distance among Mo atoms in a-MoSx, the growth of Mo nuclei is limited. This leads to more frequent nucleation and formation of smaller particles (3-5 nm in diameter). Since the smaller particles have a larger surface area than the bigger particles, the electrical contacts among Mo particles are clearly retained. The a-MoSx anode for NIBs demonstrates a high capacity and excellent cycling retention. This work establishes that the amorphous structure enhances the reversibility and cycling stability of conversion-reaction-based electrodes by elongating the diffusion pathway of the metal ions.