Storage performance of Mg²⁺ substituted NaMnPO₄ with an olivine structure

Sodium manganese phospho-olivine, NaMnPO4, is considered to be a higher-voltage alternative to the presently used iron-based electrode material, NaFePO4, for sodium ion batteries. Irrespective of this advantage, the electrochemical performance of NaMnPO4 is still far from what is desired. Herein we provide the first report on the storage performance of NaMnPO4 having a structure modified by Mg2+ substitution. The Mg-substituted phospho-olivines are prepared on the basis of ionic exchange reactions involving the participation of Mg-substituted KMnPO4·H2O dittmarites as structural template. Furthermore, the phosphate particles were covered with a thin layer (up to 5 nm) of activated carbon through ball-milling. The storage performance of phospho-olivines is analyzed in sodium and lithium half-ion cells, as well as in full-ion cells versus bio-mass derived activated carbon and spinel Li4Ti5O12 as anodes. The compatibility of phospho-olivines with electrolytes is assessed by utilization of several types of lithium and sodium carbonate-based solutions. In sodium half-cell, the Mg-substituted phosphate displays a multi-phase mechanism of Na+ intercalation in case when NaTFSI-based electrolyte is used. In lithium half-cell, the high specific capacity and rate capability is achieved for phospho-olivine cycled in LiPF6-based electrolyte. This is a consequence of the occurrence of dual Li+,Na+ intercalation, which encompass nano-sized domains. The utilization of the Mg-substituted phospho-olivine in the full ion cell is demonstrated.