First-principles Study On Vanadium Based Phosphate Electrode Materials For Sodium Ion Batteries

Polyanionic-type electrode materials with three-dimensional（3D）open channel and stable skeleton structures have been widely used in rechargeable batteries because of their excellent cycle stability,safety and fast ion transport.Meanwhile,vanadium-based compounds not only have multi redox potentials,but also have relatively high theoretical capacity due to the multi valence characteristics of vanadium.Therefore,vanadium-based phosphate materials are promising to be safe cathode and anode materials with stable structure and excellent electrochemical performance.In this dissertation,vanadium-based phosphate Na₃V₂（PO₄）₃ cathode and Na V₃（PO₄）₃ anode materials for sodium ion batteries（SIBs）were investigated by combining first-principles calculations and experimental tests.The main research contents and results are summarized as follows:1.Na⁺deintercalation/intercalation behavior of NASICON-type Na₃V₂（PO₄）₃cathode for SIBs was clarified.Results indicate that Na⁺in Na_xV₂（PO₄）₃（1≤x≤3）prefer to stay at M1（empty 6b sites）rather than at M2（empty 18e sites）.There are three possible Na⁺transport pathways during Na⁺intercalation into Na V₂（PO₄）₃,and the energy barrier（0.23 eV）of cooperative-transport mechanism in Na2-Na1-M2（Na1:Na⁺in M1,Na2:Na⁺in M2）model is the lowest.However,the energy barrier（0.49 eV）of three possible Na⁺transport pathways during Na⁺deintercalation from Na₃V₂（PO₄）₃ are almost the same.2.Boron substituted partial phosphorus was used to improve the electrochemical performance of Na₃V₂（PO₄）₃ cathode materials for SIBs.B doping does not change the crystal framework of parent material in the range of 0≤x≤1/3,but causes the local structure distortion with the tiny shrink of crystal lattice.After doping,the band gap is narrowed due to the occurrence of new boundary states and thus the rate performance is improved significantly.B doping into Na₃V₂（PO₄）₃ leads to a wider Na⁺diffusion pathway which reduces the energy barrier and increases the diffusion coefficient.3.The electrochemical mechanisms of Na₃V₂（PO₄）₃ used as cathode material for lithium ion（LIBs）and zinc ion batteries（ZIBs）were systematically investigated.Formation energy calculations indicate that Li⁺can directly insert into Na₃V₂（PO₄）₃cathode through the ion exchange with Na⁺,but Zn²⁺cannot,which conform to the experimental results.Meanwhile,in the mixed electrolyte containing Na⁺and Zn²⁺,Na⁺preferentially insert into Na V₂（PO₄）₃ which is obtained by electrochemical Na⁺deintercaltion from Na₃V₂（PO₄）₃ cathode,and the co-intercalation of Na⁺and Zn²⁺shows a two phase reaction process.CI-NEB calculation results show that the lowest energy pathway for Zn²⁺intercalation into NaV₂（PO₄）₃ is Zn-M2 interlayer transport accompanied with the shock of Na⁺between M1 and M2 sites.For Li⁺intercalation into Na V₂（PO₄）₃,although the model of Li-M2 interlayer transport also exhibits the lowest energy barrier,the oscillation of adjacent Na⁺near M1 occurs to Li⁺transport in Li_1/6Na V₂（PO₄）₃.4.The electrochemical properties,sodium storage mechanism and microscopic evolution process of geometrical and electronic structures during the sodiation process of NaV₃（PO₄）₃ as a new anode material for SIBs were investigated.Na V₃（PO₄）₃ exhibits excellent cycling stability,high reversible capacity（140 mAh/g）and fast-ion transport characteristics when used as anode material for SIBs.The cooperative-transport mechanism dominates the Na diffusion in Na_1.125V₃（PO₄）₃ and the activation barrier for Na⁺transport is 0.30 eV.