Understanding the electrochemical properties of A2MSiO4 (A = Li and Na; M = Fe, Mn, Co and Ni) and the Na doping effect on Li2MSiO4 from first-principles calculations

To explore the feasibility of regarding silicate materials as sustainable cathode materials for rechargeable Na ion batteries, the voltage plateaus, cycling stabilities, electrical conductivities and ionic conductivities of Li2MSiO4 and Na2MSiO4 (M = Fe, Mn, Co and Ni) are investigated by first principles calculations. The calculated electrochemical performance of silicate materials gives reasonable explanations for the poor capacity retention of Li2MnSiO4 as well as the reason why Li2FeSiO4 and Li2CoSiO4 exchange only one Li ion per formula unit. In comparison with Na2MSiO4, Li2MSiO4 presents higher voltage and better cycling stability. However, Na2MSiO4 displays higher electrical and ionic conductivities. Moreover, Na2NiSiO4 also presents significant potential for application as a good cathode material for Na ion batteries, as it can deliver high voltage and reversibly exchange 1.5 Li ions per formula unit. Furthermore, to make full use of the advantages of Li2MSiO4 and Na2MSiO4, a Na doped Li1.5Na0.5MSiO4 system is explored as well. The results suggest that Na doping can improve the electronic and ionic conductivities of Li2MSiO4 materials and simultaneously maintain the voltage and cycling stability. Therefore, Na ion doping should be an effective methodology to improve the performance of Li2MSiO4 cathode materials.