Investigation of K modified P2 Na _0.7 Mn _0.8 Mg _0.2 O ₂ as a cathode material for sodium-ion batteries

Sodium-ion batteries (NIBs) are emerging as a potentially cheaper alternative to lithium-ion batteries (LIBs) due to the larger abundance of sodium and in some cases the similar intercalation chemistry to LIBs. Here we report the solid state synthesized K-modified P2 Na _0.7 Mn _0.8 Mg _0.2 O ₂ which adopts hexagonal P6 ₃ /mmc symmetry. The second charge/discharge capacity for the as-prepared material is 115/111 mA h g ^-1 between 1.5-4.2 V at a current density of 15 mA g ^-1 , which reduces to 61/60 mA h g ^-1 after 100 cycles. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS) analysis shows a heterogeneous distribution of K and solid state ²³ Na NMR illustrates that the presence of K perturbs the local environment of Na within the P2 Na _0.7 Mn _0.8 Mg _0.2 O ₂ crystal structure. Larger scale X-ray absorption near-edge structure (XANES) data on the K L-edge also illustrate that K is present on the surface of electrodes in preference to the bulk. In situ synchrotron X-ray diffraction (XRD) data illustrates that the P2 structural motif is preserved, featuring a solid solution reaction for most of charge-discharge except at the charged and discharged states where multiple phases are present. The K-modified sample of P2 Na _0.7 Mn _0.8 Mg _0.2 O ₂ is compared with the K-free samples in terms of both structural evolution and electrochemical performance.