High-energy Layered Cathode Materials: Interface Modifications And Lithium Storage Performances

To meet the demand of developing lithium ion batteries with high energy density,high power density and long cycle life,layer-structured cathodes have been considered to be one of the most attractive candidates.Theoretically,both LiCoO₂ and LiNi_0.8Co_0.15Al_0.05O₂ cathodes own large theoretical capacities(-280 mAh-g^-1),and the discharge capacities of these cathodes are more close to their theoretical capacities at a higher cut-off voltage?? 4.5?.However,the cathodes easily induces a series of side reactions at high cut-off voltages,which aggravate irreversible phase transition and leads to the fast capacities degradation.Thereby,the applications of the high-energy layered cathodes in power batteries were seriously blocked.As reported previously,stable interface structure can efficiently suppress irreversible phase transition of layer cathodes at high work potential.In this paper,from the aspect of the interface instability variation,we report the effects of Al₂O₃ coating and phosphate coating on the stabilization of material interface at high charging voltage.Besides,the interface structure and the element composition of cathode materials were measured by different techniques.This thesis mainly explores the following three aspects:1.Preparing Al-O coating LiCoO₂ cathode materials in organic solution.In this work,we designed a novel wet method to synthesis the Al-O coating LiCoO₂ materials.Besides,the components of surface coating were studied by controlling sintering temperature.Al₂O₃ single coating film on LiCoO₂ surface were found at 150 °C,and turned into LiAlO₂/LiCo_1-xAl_xO₂ composite layer through a subsequent heating treatment.The electrochemical performances of the coating materials obtained at different temperatures were significant improved at high voltage,especially the the coating cathode acquired at 550 °C.At a current density of 1 C between 3.0 V and 4.5 V,LiAlO₂/LiCo_1-xAl_xO₂ composite layer coating displayed the discharge capacities of 130 mAh·g^-1 with the capacity retention of 73%after 500 cycles at 25 °C.At 55 °C,the 1C corresponding 100-cycles capacity retentions delivered by the double layer coating are 147.2 mAh·g^-1?72%?.Due to the formation of the film solid solution LiCo_1-xAl_xO₂ layer,the LiCo_1-xAl_xO₂ solid solution had the same structure as pure LiCoO₂ and LiCo_1-xAl_xO₂ compounds provided easy pathways for Li+ migration.2.The structures and electrochemical properties of LiNi_0.8Co_0.15Al_0.05O₂?NCA?materials during storage in air.In this study,the surface species on the cathode material of LiNi_0.8Co_0.15Al_0.05O₂ were investigated from the influence of humidity,exposure temperature and time on NCA materials.The results show that the longer exposure time of the NCA often accompany by larger amounts of Li₂CO₃ on the surface of the NCA cathode,along with worse cation mixing and poorer electrochemical performance.Based on the voltage-testing results of the cathodes obtained after 7 days exposure,it can be concluded that the sample obtained at 4.5 V?0.1 C?for 3 cycles and then at 4.3 V?3 C?for 100 cycles delivers the capacity of 92 mAh·g^-1 with retention of 72.6%.The sample activated at 4.3 V?0.1 C?for 3 cycles and then at 4.3 V?3 C?for 100 cycles exhibits a discharge capacity of 70 mAh·g^-1 with 68.9%capacity retention.3.The synthesis and high-voltage stability of phosphate-coated LiNi_0.8Co_0.15Al_0.05O₂?NCA?cathode materials in organic solution.Herein,the phosphate have been successfully coated on the surface of NCA microspheres and the effect of ultrathin coating on the electrochemical performances of NCA as high-voltage cathode materials have been discovered.Compared with the pristine material,the phosphate-coated NCA presents outing cycling performance at high voltage.The modified sample demonstrates discharge capacity of 107.5 mAh·g^-1 at 5 C,and retains 68.8%after 300 cycles from 3.0 V to 4.5 V.