Study Of The Properties And Working Mechanism Of Ti⁴⁺/Cr³⁺ Doped Li₃V₂?PO₄?₃ Cathode Materials For Lithium Ion Batteries

Lithium vanadium phosphate,as a positive electrode material for lithium ion batteries with high operating voltage and energy density,excellent structural stability and low temperature performance,has been the focus of research.But the low intrinsic electronic conductivity and ionic conductivity greatly limit its development.Ion doping as a method that can effectively improve the performance of materials is often used to improve the electrochemical performance of materials.At present,the research is mainly focused on improving the electrochemical performance of lithium vanadium phosphate by ion doping,and the mechanism of ion-doping is rarely reported.In this paper,Ti⁴⁺ and Cr³⁺ ions near V in the periodic table are selected as doping ions.The selection of doping ions mainly considers two factors: 1.Effects of unequal and equivalent ion doping on charge balance and ion transport mechanism;2.The influence of d orbital electrons on the magnetic and electrical properties of the material.Ti⁴⁺?d⁰?,which is high valence and electron-free in the d orbit,is selected for doping,and its effect on the lattice structure and electrochemical performance of lithium vanadium phosphate is studied.Lithium vanadium phosphate materials with different doping ratios are synthesized by sol-gel method,and then the prepared materials are characterized by XRD,NMR,SEM and electrochemical performance.The XRD results show that the structure and lattice parameters of the materials have not changed dramatically.The NMR 7Li and 31 P spectra of the materials are tested,and the local environment changes of Li and P in the materials under different doping amounts are explored,and the local lithium ion exchange rate accelerates.The results of the electrochemical performance test show that the material with a doping ratio of 4% exhibits the most excellent electrochemical performance.The specific capacity of LT0.08VP/C at 10 C has a specific capacity of 110 m Ah/g and the capacity retention rate is 99.3% after 100 cycles.Combining the test results of solid-state nuclear magnetic resonance and the electrochemical performance tests,a structural model of lithium vacancies is proposed,and the mechanism of Ti⁴⁺ doping modification is explained.The effect of Cr³⁺?d³? doping equivalent to V³⁺ and with 3 electrons in the d orbital on the performance and structure of lithium vanadium phosphate is explored.Similarly,using citric acid as the reducing agent and carbon source,samples with different doping amounts are prepared by the sol-gel method,and a series of structural and morphological properties of the materials are characterized.The XRD diffraction pattern and refined results show that with the increase of the doping ratio,the lattice parameters of the sample are gradually decreasing,indicating that Cr³⁺ has entered the lattice.Solid NMR 7Li and 31 P spectra are tested on materials with different doping ratios.Three new lithium signals are observed from the 7Li spectra,indicating that Cr³⁺ doping introduced three new lithium sites.The electrochemical performance of the material is tested,and it is found that a slope of solid solution-like properties appears in the charge-discharge curve around 3.7V.Combining the results of electrochemical performance test and NMR,the effect of Cr³⁺ doping on the local structure of the material is analyzed.