Storage performance of Mg 2+ substituted NaMnPO 4 with an olivine structure.

Sodium manganese phospho-olivine, NaMnPO ₄ , is considered to be a higher-voltage alternative to the presently used iron-based electrode material, NaFePO ₄ , for sodium ion batteries. Irrespective of this advantage, the electrochemical performance of NaMnPO ₄ is still far from what is desired. Herein we provide the first report on the storage performance of NaMnPO ₄ having a structure modified by Mg ²⁺ substitution. The Mg-substituted phospho-olivines are prepared on the basis of ionic exchange reactions involving the participation of Mg-substituted KMnPO ₄ ·H ₂ O 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 Li ₄ Ti ₅ O ₁₂ 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 LiPF ₆ -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.
Competing Interests: There are no conflicts to declare.
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