(A) study on cobalt-substituted $Na_4Mn_3($PO_4$)_2($P_2O_7$)$ as a cathode material for Na-ion batteries

Abstract

Rechargeable Na-ion batteries (NIBs) are widely recognized as one of the promising alternative energy storage systems (ESS) for Li-ion technology due to its abundance and low cost. Polyanion type materials with a 3-D framework structure have received much attention due to their high operating voltage and structural stability. Among the various polyanion type materials, mixed-anion phosphates ($Na_4M_3($PO_4$)_2($P_2O_7$)$, (M= Co, Fe, Mn etc.)) is known to exhibit low volume change and high energy density even among polyanion compounds. However, these phosphates synthesized by the solid-state process have limitations such as poor kinetics due to sluggish redox process and poor electronic conductivity. Manganese based materials are attractive cheap and abundant materials with high redox potential but Mn compounds suffer from poor electrochemical activity and low electric conductivity. In this study, cobalt substituted Mn-based mixed-anion phosphates ($Na_4Mn_{3-x}Co_x($PO_4$)_2($P_2O_7$)$) is prepared by solution combustion synthesis (SCS) method and the electrochemical properties have been reported. The structural and the phase purity were confirmed by powder X-ray diffraction method. The electrochemical performance was evaluated using cyclic voltammetry and galvanostatic charge-discharge cycling methods. The cells made with cobalt substituted mixed-anion phosphates exhibit better electrochemical kinetics and improved electrochemical properties. The effect of substitution will be discussed in detail with the physicochemical and electrochemical characterization.