Effect of Ni substitution on structural stability, micromorphology, and electrochemical performance of Li2MnSiO4/C cathode materials

Abstract

The chief drawback of Li₂MnSiO₄ cathode materials is structural instability deriving from the transformation of [MnO₄] tetrahedra to [MnO₆] octahedra during oxidation from Mn(II) to Mn(III) and the Jahn–Teller effect especially from Mn(III) to Mn(IV). Based on the theory that Ni²⁺ could keep the [NiO₄] ligand unchanged and stable during oxidation/reduction, [NiO₄] tetrahedra may behave as a regional structural framework to support the total crystal lattice and alleviate structural distortion partially. In this paper we synthesize nano-Li₂Mn_1−xNi_xSiO₄/C samples with a small amount of Ni as the dopant via a hydrothermal route. Firstly, we discuss the existential state of Ni through XPS spectra, and demonstrate that both Ni²⁺ and metallic Ni coexist, especially Ni²⁺ could improve structural stability. The influence of Ni doping on morphology, structural stability, and electrochemical performance is discussed through Raman spectroscopy and electrochemical tests below. It is confirmed that there exists an optimal Ni content under which the optimal performance is displayed. In our work, the optimum Ni content is 5.0%, and the maximal discharge capacity achieved is 235 mA h g⁻¹ for the 5.0% Ni doped sample in the first cycle.