Research On Fluoride Coating Of Li[Ni_xCo_yMn_1-x-y]O₂ And LiMn₂O₄ Cathode Materials For Lithium-ion Batteries

Li[Ni_xCo_yMn_1-x-y]O₂ and LiMn₂O₄, with environmental beneficial, low cost and good safety, are considered to be promising cathode materials for Li-ion batteries. While charged to 4.6V, LiNi_1/3Co_1/3Mn_1/3O₂ and Li[Ni_0.4Co_0.2Mn_0.4]O₂ show higher capacity of 180～200 mAh·g^-1, but poorer cycling performance due to electrolyte decomposition. Application of LiMn₂O₄ is also limited by its poor cycleability at elevated temperatures. That is attributed to the manganese dissolution caused by corrosion of HF contained in the electrolytes. In the thesis, fluoride coating on surface of LiNi_1/3Co_1/3Mn_1/3O₂, Li[Ni_0.4Co_0.2Mn_0.4]O₂ and LiMn₂O₄ is investigated to improve their cycleability by reducing the decomposition of electrolyte and HF corrosion.AlF₃ and LiF coating on surface of Li[Ni_0.4Co_0.2Mn_0.4]O₂, SrF₂ coating on surface of Li[Ni_1/3Co_1/3Mn_1/3]O₂ is carried out by precipitation method, respectively. The fluoride amounts to 0.5mol%, 1.0mol%, 2.0mol%, 4.0mol% of base materials, respectively. XRD and SEM analysis show that uncrystaline AlF₃ and LiF, nano-crystalline SrF₂ is uniformly coated on particle surface of base materials, respectively. The initial capacity decreases slightly with increasing the fluoride amount due to enhancement of cell impedance, but the cycling stability is improved obviously. EIS analysis indicates that the fluoride coating such as AlF₃, LiF and SrF₂ can effectively reduce the interaction between electrolyte and electrode materials and increase of electrochemical impedance (especially charge-transfer impedance), leading to remarkable improvement of cycleability. In view of capacity, rate capability and cycling performance, 2.0mol% AlF₃ and LiF coated Li[Ni_0.4Co_0.2Mn_0.4]O₂, 2.0mol% SrF coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ exhibits the best electrochemical performance, showing capacity of 161.5, 171, 165.7mAh·g^-1 at 0.2C, and capacity retention of 90.7%, 82.3%, 90% after 30cycles over 2.5～4.6V respectively. SrF₂ and AlF₃ are suitable fluoride coating agent for improving the cycleability of Li[Ni_xCo_yMn_1-x-y]O₂ at higher cut-off voltage.AlF₃, LiF and SrF₂ coating on surface of LiMn₂O₄ is also carried out by precipitation method, respectively. The fluoride amounts to 0.5mol%, 1.0mol%, 2.0mol%, 4.0mol% of base materials, respectively. XRD and SEM analysis show that uncrystaline AlF₃ and LiF, nano-crystalline SrF₂ is uniformly coated on particle surface of base materials, respectively. EIS analysis indicates that AlF₃ coating can reduce electrochemical impedance, improving the rate capability of LiMn₂O₄. However, the initial capacity decreases slightly with increasing LiF and SrF₂ amount due to enhancement of cell impedance. EIS analysis also indicates that the fluoride coating such as AlF₃, LiF and SrF₂ can effectively reduce the interaction between electrolyte and electrode materials and increase of electrochemical impedance (especially charge-transfer impedance), leading to remarkable improvement of the cycling stability of LiMn₂O₄ at elevated temperatures of 55℃. In view of capacity, rate capability and cycling performance, 4.0mol% AlF₃ and LiF coated LiMn₂O₄, 2.0mol% SrF₂ coated LiMn₂O₄ exhibits the best electrochemical performance, showing capacity of 113.4, 113.7, 108.0mAh·g^-1 at 0.2C, and capacity retention of 97%, 89%, 95% after 20cycles at 55℃respectively. SrF₂ and LiF are suitable fluoride coating agent for improving the cycleability of LiMn₂O₄ at higher temperatures.