Manipulating the microstructure of Na2TiV(PO4)3 for aqueous Na ion storage

Polyanionic materials are among the most promising candidates as electrochemically active materials in sodium-ion secondary batteries, yet designing structures facilitating ion diffusion and electron transfer for electrodes that operate on multi-electron redox reactions proves both intriguing and challenging. In this study, we examine the electrochemical properties of mixed-polyanion Na2VTi(PO4)(3)/NaVP2O7, pure Na2VTi(PO4)(3), and disordered Na2VTi(PO4)(3) in an aqueous electrolyte. The former two polyanionic phosphates achieve an ultrahigh capacity (195.68 mAh g(-1) at 0.2 A g(-1)) through three-sodium storage, whereas the disordered phosphate stores charge via a capacitive mechanism. Experimental tests and first-principle calculations demonstrate that enhancing the occupancy of asymmetric Na in the lattice establishes a new pathway for Na+ ions transport and narrows the band gap for electron transfer. Our work gives an insight into the local chemical environment of polyanionic phosphates composed of complex microstructures and provides a feasible strategy for obtaining electrode materials with exceptional performance.