Air degradation and rehealing of high-voltage Na0.7Ni0.35Sn0.65O2 cathode for sodium ion batteries

Abstract

Layered transition metal oxides (NaxTMO2, TM = transition metal) are the most promising cathode candidate for sodium-ion batteries (SIBs) due to their two-dimensional ion diffusion channel that enables easy ion intercalation/deintercalation, superior specific capacity, environmental benignity, and feasibility for mass production. Among the reported NaxTMO2, Na0.7Ni0.35Sn0.65O2 has the highest working plateau and an average voltage as high as 3.7 V and thus has great potential as a high-energy cathode candidate for SIBs. However, its air sensitivity upon exposure and storage, which would greatly affect the cost and feasibility of its practical application, remains unclear. Here, the degradation mechanism of Na0.7Ni0.35Sn0.65O2 upon air exposure was investigated by ex-situ X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that Na0.7Ni0.35Sn0.65O2 suffers from severe air sensitivity and completely transforms to its hydrated phase within 4 h under 25°C and 80% relative humidity. Further, the deteriorated materials were repaired by reheating under different conditions. In-situ variable temperature XRD, infrared, Raman, and thermogravimetry analyses revealed that the recovery of the electrochemical properties and crystal structure of Na0.7Ni0.35Sn0.65O2 depends on the reinsertion of sodium ions into the lattice; these sodium ions have been extracted by H2O upon air exposure. This work provides new insights into the air degradation of electrode materials for SIBs and a deep understanding of their reheating recovery strategy.