Mn-Rich Phosphate Cathodes for Na-Ion Batteries with Superior Rate Performance

Abstract

A Mn-based NASICON-type Na4VMn(PO4)3 cathode is considered to be one of the most promising substitutions for Na3V2(PO4)3 due to the huge abundance and appropriate redox potential from Mn. However, the current Na4VMn(PO4)3/C cathode still delivers a limited electrochemical performance due to the sluggish kinetics and negative structural degradation caused by the Mn in the structure. Herein, a selective replacement of vanadium rather than manganese in the Na4VMn(PO4)3 system was developed to fully utilize the manganese element and enhance the structural stability. Both experimental and calculation results affirmed that the Al-substituted Na4V0.8Al0.2Mn(PO4)3 cathode shows favorable Na+ kinetics and structure stability. The resulting Na4V0.8Al0.2Mn(PO4)3 reveals a discharge capacity of ∼84 mA h g–1 at 40 C and renders a capacity retention of 92% after cycling 1000 times at 5 C. Inspired by the availability of Al dopants, we also demonstrated the Al-doped Mn-richer Na4.2V0.6Al0.2Mn1.2(PO4)3 to be a viable candidate for Mn-rich phosphate cathodes.