Synthesis And Electrochemical Performance Of LiVOPO₄ Cathode Materials For Lithium-ion Battery

The theoretical capacity of LiVOPO₄ cathode material is up to 166 mAh·g^-1,which is close to the theoretical capacity(170 mAh·g^-1)of LiFePO₄.However,the voltage platform of LiVOPO₄ is much higher than LiFePO₄（3.6 V）,which is 3.9 V.It is a cathode material that can provide high voltage,large capacity,and is expected to replace LiFePO₄.LiVOPO₄ has two crystal phases,α-LiVOPO₄ andβ-LiVOPO₄.Due to the poor ion conductivity of the two crystalline phases,the application in real life is limited.According to the defect of the material itself,coating the surface of LiVOPO₄ with excellent electrochemical performance carbon material,and greatly improved the ionic conductivity of LiVOPO₄.At the same time,the material of the mixed crystal phase is also synthesized,so that the structural stability of the material enhanced obviously.This experiment contains three parts.The triclinicα-LiVOPO₄/C was successfully synthesized by a two-step method.Through the discovery of different carbon-coated materials（graphite,activated carbon,acetylene black,conductive carbon black）,the use of acetylene black as carbon coating material has the greatest effect on the enhancement of the electronic conductivity ofα-LiVOPO₄.Under the Ar atmosphere protection of high temperature tube furnace,when the content of acetylene black mass is 10%,the initial discharge capacity of theα-LiVOPO₄/C sample sintered at 600℃for 10 h can reach at 111.7 mAh·g^-1.After 50 cycles of charging and discharging performance test,the capacity retention rate is 95.1%,which is 106.3 mAh·g^-1.With the same mass fraction of graphite,activated carbon and conductive carbon black as carbon coated,the first cycle discharge capacity at 0.05 C and the capacity retention after 50 cycles were 87.3 mAh·g^-1,94.4%;82.6 mAh·g^-1,94.3%;92.9 mAh·g^-1,93.8%,respectively.The electrochemical performance ofα-LiVOPO₄/C prepared using acetylene black as the carbon coating material is the most excellent.For the preparation and electrochemical performance of orthorhombicβ-LiVOPO₄/C cathode materials,the acetylene black with larger specific surface area and lower resistivity was used as carbon coating material,and theβ-LiVOPO₄/C cathode material was successfully obtained by two step method.The effects of synthetic sintering temperature,sintering time and different mass percentages of acetylene black on the electrochemical performance ofβ-LiVOPO₄/C were explored.After sintering at 600°C for 10 h,the material with a 10%mass fraction of acetylene black has the most excellent electrochemical performance.At a current rate of 0.05 C,the material has a discharge capacity of 141.4 mAh·g^-1 in the first cycle.After 50cycles of charge and discharge performance tests,the capacity is 136.3 mAh·g^-11 and the capacity retention remains as high as 96.3%.The average capacity loss per cycle is only 0.07%.In order to enhance the structural stability of LiVOPO₄ to improve its rate performance,theα_x/β_1-x-LiVOPO₄（x=0.1,0.3,0.5,0.7,0.9）composites were successfully synthesized by both usingδ-VOPO₄ andβ-VOPO₄ as reaction materials with a two-step method.Among them,the mixed crystalline phase materialα_0.5/β_0.5-LiVOPO₄/C showed the most excellent rate performance.The initial discharge capacities were 81.1 mAh·g^-11 and 69.8 mAh·g^-11 at 2 C and 5 C,respectively,while the pure phaseα-LiVOPO₄ andβ-LiVOPO₄ materials were only 56.8 mAh·g^-1,36.9 mAh·g^-11 and 30.6 mAh·g^-1,18.6 mAh·g^-1.This also shows that the electroperformance of LiVOPO₄ can be significantly enhanced by the compounding of different crystal phase materials.