Engineering Hierarchical Structure and Surface of Na4mnv(Po4)3 for Ultrafast Sodium Storage by a Scalable Ball Milling Approach

Abstract

Polyanion-type Na4MnV(PO4)3  (NMVP) is attracting increasing attention as a cathode material for Na-ion battery (NIB) due to its high redox potential and crystal structure stability, while it is still suffering from poor intrinsic conductivity. Herein, to address such issue, a unique hierarchical bayberry-like NMVP@NC material, in which the ultra-small primary nanoparticles are embedded in N-doped carbon conductive network, is fabricated by a scalable ball milling approach.  The optimum NMVP@NC electrode exhibits a high reversible capacity of 103.5 mAh g -1  at 0.5 C and maintains a desirable capacity of 82.4 mAh g -1  even at an ultrahigh rate of 100 C.  After 1000 cycles at 5 C, a capacity retention ratio of 94.4% is achieved. The underlying electrochemical Na storage mechanism verified by the kinetic analysis and operando X-ray diffraction characterization reveals that the superior electrochemical performance should be attributed to the hierarchical structure with a facile ion/electron diffusion pathway and excellent interfacial compatibility. Moreover, a solid-solution and biphasic reaction mechanism for Na storage is confirmed in NMVP.  Further demonstration of commercial soft carbon//NMVP@NC full cell manifests the good potential of as-prepared material for practical applications.  Therefore, the NMVP@NC with rational architecture design is a considerable competitive cathode for the commercialization of NIB.

Keywords: Sodium-ion batteries, Na4MnV(PO4)3 cathode, hierarchical structure, Surface engineering, Na storage mechanism