A Manganese Phosphate Cathode for Long‐Life Aqueous Energy Storage

Abstract Mn‐based materials for aqueous energy storage are reaching the capacity ceiling due to the limited Mn 4+ /Mn 3+ redox. The disproportionation of Mn 3+ often occurs, forming soluble Mn 2+ and thus leading to severer capacity decays. Here, an amorphous manganese phosphate material [AMP, Na 1.8 Mn 4 O 1.4 (PO 4 ) 3 ] is fabricated using an electrochemical method for the first time. Benefitting from the open framework and the insoluble nature of Mn 2+ in AMP, the Mn 3+ /Mn 2+ and Mn 4+ /Mn 3+ redox couples can participate in the charge storage processes. The AMP electrode shows a high capacity of 253.4 mAh g −1 (912.4 F g −1 or 912.4 C g −1 ) at the current density 1 A g −1 and good rate capability. Experimental results indicate AMP experiences a mixed charge storage mechanism (i.e., cation intercalation and conversion reactions) in Na 2 SO 4 electrolyte. Besides, electrolyte engineering can effectively prevent the decomposition of AMP during cycling test, achieving capacity retention of 97% in 5000 cycles. Importantly, AMP can accommodate different cations (e.g., Mg 2+ , Ca 2+ , etc.), exhibiting great potential for aqueous energy storage.