Storage performance of Mg2+ substituted NaMnPO4 with an olivine structure

Abstract

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.