Engineering High-Energy, High-Rate NASICON Cathodes via Cu Doping and Microwave-Induced Grain Refinement for Sodium-Ion Batteries

Abstract

In this work, we designed a series of precursor materials denoted as NMTP-Cux (x=0, 0.03, 0.05, 0.07, 0.1, 0.12, these materials were firstly treated by microwave irradiation. Owing to the rapid volumetric heating characteristics of microwaves, the synthesized samples exhibit a dual structural advantage. Particularly, Cu-introduced NMTP-Cu0.1 [Na2.74Mn0.94Cu0.10Ti1.01(PO4)3], derived from Na2.10Mn1.07Ti1.01(PO4)3 (NMTP), demonstrates more uniform fine grains compared with pristine NMTP. This can be explained by lattice rearrangement that is facilitated by the rapid thermal condition. Furthermore, owing to the electronic structure modulation of the Cu and the structural stabilization of the Cu-O covalent bond, the microstructure effectively suppresses the long-range cooperative Jahn-Teller distortion induced by Mn3+ through several non-equilibrium defects, such as cation vacancies, thereby significantly enhancing the structural stability of the material during the in-situ reduction of Cu2+ to Cu+ in the material synthesis. NMTP-Cu0.1 shows a three-step redox reaction involving the Mn2+/3+, Ti3+/4+, and Mn3+/4+ couples, delivering a high capacity (167.25mAh/g at 0.1 C), a considerable medium discharge voltage (2.5V), an excellent energy density (481.89 Wh/kg), and a decent cyclability.