Charge–Discharge Properties of a Sn4P3 Negative Electrode in Ionic Liquid Electrolyte for Na-Ion Batteries

We evaluated the charge–discharge performance of a Sn4P3 negative electrode in an ionic liquid electrolyte comprised of N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) and NaFSA. We also conducted cyclic voltammetry and transmission electron microscopy for the Sn4P3 electrode to reveal the reaction mechanism. It was suggested that Na15Sn4 and Na3P are formed via phase separation in the first sodiation and that elemental Sn and elemental P formed by following a desodiation reaction with Na ions in the subsequent cycles. The Sn4P3 electrode exhibited a high Coulombic efficiency of 99.1% at the fourth cycle and an excellent cycling performance with a high reversible capacity of 750 mA h g–1 even at the 200th cycle. We demonstrated that there are two important factors to improve the performance: (i) higher volume fraction of Sn than P and (ii) uniform dispersion of Sn nanoparticles in a P matrix. The ionic liquid electrolyte showed good applicability to the Sn4P3 negative electrode due to i...