Effect Of Heat Treatment Process On Structure And Properties Of Cathode Materials

Cathode material is one of the key factors which decide the performances of lithium-ion battery.Most of the cathode materials are transition metal oxides.Due to the polyvalent characteristics of transition metal ions,the ion valence of the cathode materials are closely related to the heat treatment process in the synthesis process.Therefore,the study on the effects of heat treatment process on the structure and properties of transition metal oxide cathode materials has important guiding significance for the material preparation.In this thesis,we have studied the effects of heat treatment process on the structures and properties of the lithium-rich layered oxide Li[Li_0.2Ni_0.13Co_0.13Mn_0.54]O₂ and vanadium phosphate cathode materials.First of all,the lithium-rich layer cathode material Li[Li_0.2Ni_0.13Co_0.13Mn_0.54]O₂ was prepared by sol-gel method,and we studied the influence of heat treatment temperature and atmosphere on the structures and electrochemical properties of the materials.When the precursor is heated directly in the temperature range of 600-900?in the air,the electrochemical active phase is formed when the temperature reaches to 600?.With the increase of the heat treatment temperature,the discharge specific capacity of the electrode material increases gradually.When the heat treatment temperature reaches up to 900?,the specific capacity of the material can reach to 280 mAh/g during discharge process.The electrochemical activity phase of the lithium-rich layer cathode material could also crystallize in the two-step process,which is calcined with argon atmosphere and then treated in the air.When calcined in argon atmosphere 700?for 4 hours and then followed by a heat treatment at 900?for in the air,the specific capacity of the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ material is60mAh/g during discharge process at the current density of 400mA/g,and discharge plateau can remain.After 200 cycles,the material,which calcined in argon atmosphere 700?for 4hours and followed by a heat treatment at 900?for in the air,remained an initial discharge capacity of 72%.These results show that two-step heat treatment is beneficial to improve the electrochemical cycle performance,and to shorten the calcination time of the material.Secondly,the influence of heat treatment process on the structure and electrochemical properties of the positive electrode material of Li-V-P-O system is studied.We Using sol-gel method to prepare the precursors of both vanadium phosphate lithium LiVOPO₄ and vanadium phosphate Li₃V₂?PO₄?₃ by using sol-gel method.The precursor of Li₃V₂?PO₄?₃ are heated by 300-500?in the air,producing?-LiVOPO₄ electrochemical activity phase.When temperature is above 600??-LiVOPO₄ phase disappeared.In contranst,when Li₃V₂?PO₄?₃precursors is heated at 750?in argon atmosphere,the expected Li₃V₂?PO₄?₃ electrochemical activity phase is produced,then Li₃V₂?PO₄?₃ phase disappeared by a followed heat treatment at 500?in the air,forming part of the?-LiVOPO₄ crystalline phases.As a result,it is difficult to obtain positive electrode material with a low oxidation state(V³⁺)vanadium Li-V-P-O system by a heat treatment in the air,while it is difficult to obtain vanadium cathode material with the high oxidation state(V⁴⁺)by a heat treatment in argon atmosphere.Finally,for the silicon-doped modification of LiVOPO₄,Si doping does not change the structure of the LiVOPO₄ lithium material,and a small amount of Si doping can improve the rate performance of LiVOPO₄ cathode material.When the molar ratio of Si incorporation is10%,the discharge specific capacity of the composite is 110mAh/g,and the specific discharge capacity can still be maintained at 25mAh/g at a current density of 200mA/g.Further,the lithium-rich layer cathode material Li[Li_0.2Ni_0.13Co_0.13Mn_0.54]O₂ was modified with this material,and the results showed that the layered structure of the material is not changed by coating of LiVOPO₄.When the coating amount is 4%,the coulomb efficiency of the composite material is improved during initial cycle,reaching 79.08%,and the discharge specific capacity is 250 mAh/g.