Pinning Effect Enhanced Structural Stability toward a Zero-Strain Layered Cathode for Sodium-Ion Batteries.

Abstract

Layered oxides as the cathode materials of sodium-ion batteries are receiving extensive attention due to their high capacity and flexible composition. However, the layered cathode tends to be thermodynamically and electrochemically unstable during sodiation and de-sodiation. Herein, we for the first time propose the pinning effect and controllable pinning point in sodium storage layered cathodes to enhance the structural stability and achieve the optimal electrochemical performance. 0%, 2.5% and 7.3% transition-metal (TM) occupancies in Na-site as pinning points are obtained through adjusting the feeding ratio of iron in Na 0.67 Mn 0.5 Co 0.5- x Fe x O 2 system. Results show that 2.5% TM pinning is beneficial to restrain the potential slab sliding and enhance the structural stability, resulting in an ultra-low volume variation of 0.6% and maintaining the smooth two-dimensional channel for Na-ion transfer. Finally, the Na 0.67 Mn 0.5 Co 0.4 Fe 0.1 O 2 cathode with the optimal TM pinning delivers outstanding cycling performance of over 1000 cycles and superior rate capability up to 10C.