Charge–Discharge Properties of a Sn₄P₃Negative Electrode in Ionic Liquid Electrolyte for Na-Ion Batteries

We evaluated the charge–discharge performance of a Sn 4 P 3 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 Sn 4 P 3 electrode to reveal the reaction mechanism. It was suggested that Na 15 Sn 4 and Na 3 P 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 Sn 4 P 3 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 Sn 4 P 3 negative electrode due to its superior electrochemical stability.