Ti-Substituted O3-NaNi0.5Mn0.3Ti0.2O2 Material with a Disordered Transition Metal Layer and a Stable Structure.

Abstract

O3-type NaNi_0.5Mn_0.5O₂ (NNM) is very competitive for sodium-ion batteries (SIBs) due to its high capacity and easy production. Nevertheless, the intricate phase transitions during the charging-discharging significantly impede its practical application. This paper proposes a strategy for successfully synthesizing NaNi_0.5Mn_0.3Ti_0.2O₂ (NNMT) by combining coprecipitation and a high-temperature solid-state method. This method introduces Ti elements while retaining the electrochemically active Ni²⁺ content, thus, the NNMT has a high initial specific capacity of 151.4 mAh g^-1 at 1 C. It is demonstrated that introducing Ti⁴⁺ leads to the transition metal layers becoming disordered by ex situ XRD, thus mitigating the irreversible phase transition of the material. In addition, Ti⁴⁺ does not have an outer electron, which can reduce electron delocalization in the transition metal layer and improve the material's cyclic stability. The NNMT possesses a capacity retention rate of 60.66% after 150 cycles, much higher than the initial NNM's 18.96%. It also exhibits an excellent discharge capacity of 86.8 mAh g^-1 at 5 C. In conclusion, the cycling and rate performance of the Ti-substituted NNMT are greatly improved without capacity loss, which offers innovative concepts for the modification means of the SIBs layered oxide cathode materials.