Preparation And Performance Of Polyanionic Cathode Materials For Sodium-Ion Batteries

The use of lithium-ion batteries（LIBs）has achieved great success in large-scale electrochemical energy storage systems and solid-state LIBs using lithium metal as the anode have been well developed.However,the dramatic increase in demand/cost and the limited reserves of the two most important metal elements for LIBs（Li,Co,Ni,etc.）have raised concerns about their future development.As a major representative battery system for post Li-ion batteries,sodium-ion batteries（SIBs）are highly advantageous for development in the field of large-scale energy storage and conversion,such as smart grids and low-speed electric vehicles,due to their abundant resources and low costs.The cathode material largely determines the cost and energy density of the battery,so the development of high-performance SIB materials is crucial for SIBs.NASICON（sodium-ion fast conductor）structured polyanionic SIB materials have a three-dimensional open framework,and this special structural skeleton（3D）is able to induce fast sodium ion transport and has therefore received a lot of attention from researchers,but its low electrical conductivity limits its application prospects.In this thesis,to address the intrinsic electronic conductivity of NASICON-type Na₃V₂（PO₄）₃（NVP）and Na₃V₂（PO₄）₂F₃（NVPF）,chlorine/magnesium co-doped Na₃V_1.95Mg_0.05（PO₄）_2.9Cl_0.1（NVMPL）and zinc-doped Na₃V_1.95Zn_0.05（PO₄）₂F₃（NVPFZ0.05）,and their electrochemical properties were investigated as positive electrode materials.Two specific aspects were covered as follows.1.To address the low intrinsic electronic conductivity of NVP,a new NVMPL material was prepared by introducing heterovalent ions(Cl^-and Mg²⁺)into NVP by sol-gel method.The results of Density Functional Theory（DFT）calculations showed that the conductivity of NVMPL after doping was higher than that of undoped NVP,and the exact substitution position of Cl^-in the PO₄^3-group was determined to be the O site.The reaction kinetics tests revealed that the NVMPL co-doped with heterovalent ions had a larger sodium ion diffusion coefficient and significantly broadened sodium ion transport channels.The electrochemical performance tests revealed that the NVMPL-based half cell showed a specific discharge capacity of 120m A h g^-1 at 0.1 C and 65 m A h g^-1 at 30 C.After 3000 cycles at 30 C,there was a capacity retention of 79.4%and a Coulombic efficiency of around 100%,reflecting good cycling stability.Full cells assembled with NVMPL and hard carbon pretreated by sodium also exhibited excellent rate performance.In addition,dynamic polyanionic backbones have been demonstrated as an effective cathode for zinc ion batteries.2.To address the problem of low intrinsic electronic conductivity of NVPF,a new NVPFZ0.05 material was prepared by introducing the cation Zn²⁺（electrochemically inactive）into NVPF through high-temperature solid-phase synthesis.Analysis combined with DFT calculations showed that the doped NVPFZ0.05 had higher electronic conductivity.XRD tests indicated that the Zn²⁺doping increased the lattice volume of the NVPF material,which contributes to the fast sodium ion transport.The carbon nanotubes（CNTs）was also used to composite with NVPFZ0.05 to increase the electrical conductivity.The results showed that the CNTs reinforced NVPF0.05 reached an initial discharge capacity of 127.5 m A h g^-1 at 0.1C and still delivered a capacity of 105 m A h g^-1 at 20 C.After 1000 cycles at 1 C,a specific discharge capacity of 102.3 m A h g^-1 was maintained,corresponding to a capacity retention rate of 82%,with a Coulombic efficiency close to 100%.These resultes indicates that the CNTs further enhance the electronic conductivity of the material,effectively stabilize the structure of NVPF material at electrode level and improve the rate performance and cycling stability of the material.