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% after cycling 1000 times at 5 C, which was far superior to that (only 47%) 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 Na₄MnV(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% and 10%, 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% and 20%, respectively. 

Inhibition mechanism of the negative Jahn-Teller effects and performance/cost ratios comparison of phosphate cathode materials for NIBs (Image by IPE)