Site-selective Mg-doping regulated charge storage in NaFe2PO4(SO4)2 for high energy sodium-ion batteries

The absence of compatible cathodes with higher specific capacity and energy density hampers the full-scale commercial adaptation of sodium-ion batteries (NIB). Engineering NASICON cathodes with Fe redox centre and mixed polyanions is promising to overcome the bottlenecks. Our study uses a chemo-mechanical route to synthesise NaFe2−xMgxPO4(SO4)2 (NFMPS), where the dopant, Mg, is strategically positioned at the Fe sites. To our knowledge, such a chemo-mechanical synthesis of metal-phosphosulphate has not been attempted so far, and our work also presents Mg2+ doping at the Fe site, in particular, for a NASICON-type NaFe2PO4(SO4)2 for the first time. With varying dopant concentrations, NFMPS is optimised to show a remarkable reversible capacity of around 111 mAh g−1 at C/20 with a corresponding energy density of 324 Wh kg−1. Even after 100 cycles at a C/5 current rate, the material retains 86.45 % of its initial capacity. An in-depth analysis of sodium-ion storage in NFMPS was conducted using electrochemical investigation, ex-situ characterisation methods and DFT calculations, where the presence of mixed polyanion and the dopant seem to enhance reversible sodium-ion (de)intercalation synergistically. DFT calculations indicate that the presence of Mg2+ can affect the localised electronic state of NFMPS and reduce the energy band gap of the material as evidenced from the electrical conductivity measurements for NFMPS. Ex-situ XRD studies at various (de)sodiation states showed that Mg-doping helps in retaining the material's structural integrity and providing larger lattice sites for enhanced sodium-ion diffusion (ranging from 10−11 to 10−12 cm2 s−1). Higher working voltages, better sodium-ion transport, and capacity retention make NFMPS a promising candidate as a sodium-ion battery cathode.