A Manganese Phosphate Cathode for Long-Life Aqueous Energy Storage

Mn-based materials for aqueous energy storage are reaching the capacity ceiling due to the limited Mn4+/Mn3+ redox. The disproportionation of Mn3+ often occurs, forming soluble Mn2+ and thus leading to severer capacity decays. Here, an amorphous manganese phosphate material [AMP, Na1.8Mn4O1.4(PO4)(3)] is fabricated using an electrochemical method for the first time. Benefitting from the open framework and the insoluble nature of Mn2+ in AMP, the Mn3+/Mn2+ and Mn4+/Mn3+ 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 Na2SO4 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., Mg2+, Ca2+, etc.), exhibiting great potential for aqueous energy storage.