Degradation mechanism of tin phosphide as Na-ion battery negative electrode

The degradation mechanism of an Sn_4P_3 electrode as Na-ion battery anode was investigated by using a transmission electron microscopic observation. At the first desodiation, we confirmed that Sn nanoparticles with 6 nm in size were dispersed in an amorphous-like P matrix.Compared to this, we observed aggregated Sn particles with sizes exceeding 50 nm after the drastic capacity fading. The capacity fading mechanism was for the first time confirmed to be Sn aggregation. To improve the capacity decay, we carried out the two kinds of chargeàdischarge cycling tests under the reduced volume changes of Sn particles and P matrix by limiting desodiation reactions of Nae Sn and Na3P, respectively. The Sn_4P_3 electrode exhibited an excellent cyclability with the discharge capacity of 500 mA hg^(-1) for 420 cycles under the limited desodiation, whereas the capacity decay was accelerated under the limited sodiation. The results suggest that the Sn aggregation can be improved by the reduced volume change of the P matrix, and that it is very effective for improving anode performance of Sn_4P_3 electrode.