Dual-carbon decorated Na3Mn2(P2O7)(PO4) nanocomposite via freeze drying: A zero-strain cathode material for sodium ion batteries

Sodium ion batteries (SIBs) are regarded as one of the most effective and economical solutions for large-scale energy storage, and thus have motivated widespread exploration on desirable electrode materials. Manganese-based phosphates are expected to be potential cathodes for SIBs due to their stable framework, low cost, and high operating voltage. However, low electrochemical activity and severe Mn dissolution lead to unsatisfying specific capacity and poor cycle life, which greatly limit their practical applications. Herein, a dual-carbon (amorphous carbon and reduced graphene oxide) decoration strategy is proposed and applied to Na3Mn2(P2O7)(PO4) (NMPP) via freeze drying technique in order to tackle the common issues of manganese phosphate cathodes. By minimizing particle aggregation and constructing conductive network, the dual-carbon functionalized NMPP nanocomposite has overcome its intrinsic sluggishness, and delivers a reversible capacity of ∼101 mAh g−1 with a high Mn2+/Mn3+ potential of ∼3.6 V vs. Na+/Na and a favorable energy density of ∼350 Wh kg−1. Besides, the NMPP||Na battery also manifests favorable cycle performance (73% retention after 500 cycles) and decent rate ability (32 mAh g−1 at 5C). Finally, the desodiation/sodiation mechanism is investigated, which demonstrates that NMPP is based on solid-solution reaction accomplished by Na1 and Na3 extraction/intercalation with a tiny volume change of 0.87%.