Mn-rich phosphate cathodes with better electrochemical performance for Na-ion batteries

A research group led by Prof. Zhao Junmei from the Institute of Process Engineering (IPE) of the Chinese Academy of Sciences developed a novel vanadium-poor and manganese-rich phosphate cathode with better electrochemical performance for Na-ion batteries.

The study was published in ACS Energy Letters on Dec. 3.

This work addresses the crucial issues on how to effectively suppress the Jahn-Teller distortions of Mn³⁺, meanwhile minimizing the cost of raw materials and enhancing the electrochemical performance in the Mn-rich phosphate cathodes.

The so called Jahn-Teller distortion means a geometric structure deformation of MnO6 octahedron, resulted from the shrinkage or elongation of the O-Mn-O bond when Mn is in the high-spin Mn³⁺ oxidation state. Such a structural distortion is usually accompanied by the serious structure degradation and sluggish kinetics, finally leading to the inferior electrochemical performance.

Mn-based phosphate cathodes are promising due to low cost and high voltage, particularly the Mn-rich NASICON structure. Thus, it's significant to effectively suppress the undesired Mn³⁺ Jahan-Teller effect in the Mn-rich phosphate cathodes.

In this study, the researchers designed the Al³⁺ as a stabilizer for selective replacement of V instead of Mn to tune the crystal structure of Mn-based Na₄VMn(PO₄)₃ system. DFT calculation affirmed that the structural distortions and the dissolution of Mn ions were effectively suppressed due to enhanced ionic-covalent character after Al substitution.

The resulting Na₄V_0.8Al_0.2Mn(PO₄)₃ achieved a capacity retention over 92 percent after cycling 1000 times at 5 C, which was far superior to that (only 47 percent) of Na₄VMn(PO₄)₃ cathode.

Benefiting from the synergistic effects among V, Al and Mn multi-elements, the Na₄V_0.8Al_0.2Mn(PO₄)₃ exhibited enhanced ion diffusion ability and electronic conductivity. As a result, the Al incorporated Na₄V_0.8Al_0.2Mn(PO₄)₃ cathode could deliver a superior rate capacity of 84 mA h g^-1 at 40 C, even with the loading as high as 5.5 mg/cm², significantly better than 62 mA h g^-1 for Na₄VMn(PO₄)₃.

Furthermore, a Mn-richer Na_4.2V_0.6Al_0.2Mn_1.2(PO₄)₃ was also proposed and developed as a viable candidate for Mn-richer phosphates cathodes.

Compared with Na₃V₂(PO₄)₃ and Na₄VMn(PO₄)₃, the raw materials costs of Al incorporated Na₄V_0.8Al_0.2Mn(PO₄)₃ cathode were reduced by nearly 44 percent and 10 percent, respectively. More impressively, those values for the Al substituted Mn-richer Na_4.2V_0.6Al_0.2Mn_1.2(PO₄)₃ could reach 50 percent and 20 percent, respectively.