Single Crystalline LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Materials:Coating Modifications And Battery Performances Under High-temperature And High-voltage

Recently,layered lithiated transition metal oxides,such as LiCoO₂（LCO）,Li[Ni_xCo_yMn_z]O₂（NCM）and Li[Ni_0.85-yCo_0.15Al_y]O₂（NCA）,have become the most widely used cathode materials for lithium-ion batteries.Among various NCM ternary materials,LiNi_0.5Co_0.2Mn_0.3O₂（523）gradually gained a large share of the cathode market for power batteries due to its high capacity,excellent cycling stability,low cost and mature production process.Theoretically,the 523 cathode material has a specific capacity of nearly 280 mAh g^-1,and the capacity released will be closer to the theoretical capacity as the cut-off voltage increases.However,at high cut-off voltage operation（≥4.4 V）,a series of side reactions occur easily on the surface of the particles,which induces the continuous growth of SEI layer at the interface and aggravates the irreversible phase transition of the material,leading to the rapid degradation of the capacity.Because of the surface instability,the practical 523 cathodes can only provide a reversible capacity less than 180 mAh g^-1 under a cut-off voltage below4.5 V（vs.Li⁺/Li）.Moreover,523 materials with spherical morphology and nano/micro hierarchical structures are mostly reported and commercialized,which always cause the low tap density of the cathode and the low energy density of the final batteries.To solve these problems,we firstly fabricated the 523 cathode materials with a micro-sized single-crystal structure and Al-doped surface.Later,we explored LiBO₂-LiAlO₂ and LiV₂O₄ coatings on the pristine 523single-crystals for enhanced electrochemical activity and at high charging voltages and high temperatures.This thesis mainly explores the following three aspects:1.Electrochemical performance of surface Al-doped 523 single crystal cathode at different cut-off voltages.Surface Al-doped 523 single crystal with a diameter of 2-4μm was synthesized by using precursor Ni-Co-Mn and combined with futher Al³⁺coating/annealing process.Al contents in the surface and the bulk structures of product particles were about 1.05%and 0.02%analyzed by EDX and Inductively Coupled Plasma（ICP）,respectively.Coin cell testing of the as-prepared 523cathode were taken under different voltage ranges（3.0-4.3 V,3.0-4.4 V,and 3.0-4.5 V）,with corresponding 100 cycles capacity retentions of91.2%,74.0%,67.8%at 25°C,and 43.3%,40.3%,31.6%at 55°C.2.LiV₂O₄ nano-layer modified 523 single crystal cathode with much improved battery performance under 4.5 V.The influence of the coating amount on electrochemical properties of materials were investigated.The cycling performance of different cathodes were measured with a rate of 1C between 3.0 V and 4.5 V.After 100 cycles,the capacity retentions of0.2 wt.%,0.5 wt.%and 1 wt.%LiV₂O₄ are 62.2%,79.7%and 51.6%,respectively,while that of the pristine material is only 30.0%.EIS and CV results suggest that 523 cathode with 0.5 wt.%LiV₂O₄ coating display the optimal performance,which is attributed to this coating would supply a cathode surface with the best electrochemical activity and stability.3.Study on the electrochemical properties at high operating voltage and high temperature of 523 single-crystal cathode by LiBO₂-LiAlO₂double coating layers.The primary coating of B and Al content on 523single crystal surface was achieved by using lithium salt residues on the pristine 523 surface,and then transformed into LiBO₂-LiAlO₂ double layers via a further annealing operation.The samples are evaluated by coin cells testing under 3C in the voltage range of 3.0-4.5 V at 25°C.After 300 cycles,the pristine sample reveals a capacity retention of only1.9%,which much lower than that of the LiBO₂-coated（55.7%,）and the LiBO₂-LiAlO₂ dual-coated samples（73.4%）.After 500 cycles at 1 C,3.0-4.5 V and 55°C,the pristine,the LiBO₂-coated and the LiBO₂-LiAlO₂dual-coated samples showed different capacity retention of 0,26.9%and66.3%,respectively.The results showed that the battery properties of the coated 523 single-crystal cathodes were significantly improved at high temperature and high voltage,and the LiBO₂-LiAlO₂ dual-coated material exhibited the superior electrochemical properties.