Understanding the Electrochemical Performance of Hybrid Na(2-x)LixFeP2O7 (x= 0, 0.6) Cathode Materials

Abstract

The need for green energy is increasing every year, and renewable energy and efficient storage require large-scale lithium-ion batteries (LIBs). The use of LIBs has resulted in declining lithium (Li) reserves, which effectively increases the cost of current LIBs. Na ion batteries (NIBs) have been proposed as an alternative to the LIBs as it displays similar properties to them. However, Na ions (Na+), possessing higher ionic radii, and lower energy density than Li ion (Li+), the NIBs have limited commercialization potential. One proposed solution is the development of hybrid cathodes, where the cathode has both Li+ and Na+ in the structure and can be active in both Li and Na cells. The current work describes the development of hybrid cathode material in which the substitution of Li+ for Na+ in the configuration of Na(2-x)LixFeP2O7 (x=0,0,6) is considered to form Na1.4Li0.6FeP2O7 cathodes. The solid-state technique was used for the synthesis of Na(2-x)LixFeP2O7 (x=0.6) cathode. Structural analysis indicates the formation of phase pure crystalline materials with non-uniform and sub-micron particles. It is also noticed that the original triclinic structure of Na2FeP2O7 is preserved with the substitution of Li+. It is also revealed that Na1.4Li0.6FeP2O7 shows improved thermal stability up to 550°C when compared to Na2FeP2O7. The electrochemical performance of Na2FeP2O7 and Na1.4Li0.6FeP2O7 is investigated using various electrochemical characterizations. Both Li and Na half cells show electrochemical activity for Na1.4Li0.6FeP2O7 with decent cyclability. However, Na1.4Li0.6FeP2O7, suffers from lower electrochemical efficiency when compared to Na2FeP2O7. This could mainly be due to the lattice distortion upon substituting larger Na+ ions with smaller Li+ ions. Nevertheless, Na1.4Li0.6FeP2O7 hybrid cathode material can be used as a reference to synthesize new cathode materials with improved efficiency.