Sodium storage mechanism and electrochemical performance of layered GeP as anode for sodium ion batteries

Layered germanium phosphide, which combines the advantages of both germanium and phosphorus, is believed to be a potential anode for sodium ion battery. Here, the sodium storage mechanism and electrochemical performance of layered germanium phosphide have been deeply investigated by advanced in-situ transmission electron microscopy technique combining half-cell testing. Dynamic reaction process reveals that individual layered germanium phosphide nanoflake undergoes total area expansion of 248% without any detectable fracture or cracking in sodiation. In contrast, germanium phosphide experiences multi-step reactions, i.e. intercalation and alloying, accompanied by sequentially forming Na_xGeP (0 < x < 1/3), layered NaGe₃P₃, and amorphous NaGe and Na_yP (0 < y ≤ 3). Moreover, the germanium phosphide electrode delivers a high first discharge/charge capacity of 1124/996 mA h g^-1 within voltage window of 0.005-2.5 V. By narrowing voltage window in 0.15-1.5 V, the electrode displays good cycle stability with the capacity retention of 330 mA h g^-1 after 100 cycles. Our study provides useful fundamental understanding and practical strategy for the application of germanium phosphide as anode in sodium ion batteries.