| The LiNixCoyMnzO2(x+y+z=1,NCM)cathode is considered to be one of the most promising materials for power batteries.For the traditional poly-crystalline NCM(PC-NCM)materials,the secondary micron-sized spherical structure inevitably occurs the intergranular cracks,unavoidable parasitic reactions and Li+/Ni2+cation mixing during the charge-discharge processes.These issues could easily cause serious degradation of battery performance,especially at the harsh condition(elevated temperature and high cut-off voltage,etc.).Benefiting from the favourably structural stability and few grain boundaries,single-crystal ternary oxide cathode(SC-NCM)exhibits a better capacity retention during long-term cycling,becoming a research hotspot.However,its application is limited by the difficulties in the synthesis of SC-NCM materials and the sluggish Li+diffusion kinetics of the micron-sized primary particles.To address these problems,we have investigated the synthesis process of single crystal ternary materials,in-situ doping modification,electrolyte optimization and surface coating modification to improve the rate performance and electrochemical stability at high voltage.The main research contents and results are as follows:(1)Based on the stoichiometric ratio of the SC-NCM,it is found that the SC-NCM613 material has the better cost efficiency.Thus,a low-cost single-crystal LiNi0.6Co0.1Mn0.3O2(SC-NCM613)cathode material was successfully prepared by the coprecipitation and high temperature calcination method.The electrochemical performance,structural stability and thermal stability of the SC-NCM613 and PC-NCM613(poly-crystalline LiNi0.6Co0.1Nn0.3O2)cathodes were comprehensively investigated and assessed.In particular,the cycled life of SC-NCM613‖AG full cell at 2.8-4.2 V/1 C/45℃ increased from 270 times to 900 times charge-discharge process,where the capacity retention is retained as high as 73.9%.It is proved that SC-NCM613 cathode material has excellent cycling stability at high temperature.(2)To address the problems of low initial capacity and poor rate performance for SC-NCM613 cathode materials,we have proposed the Zr/Ti trace co-doping SC-NCM613 materials(Z/T@SC-NCM-0.2,Zr/Ti doping content was 0.2 at%).Benefiting from the homogeneous distribution of Zr/Ti,the electrochemical reaction kinetics of Z/T@SC-NCM-0.2 is significantly improved and also reduced the risk of microcracks and irreversible phase transformation during long-term cycles.Compared with undoped SC-NCM613 cathode,the capacity retention of Z/T@SC-NCM-0.2 cathode were increased from 68.5%to 88%after 150 cycles at 2.75-4.6 V/0.3 C/25℃,where the initial discharge capacity is almost the same(about 181.2-185 mAh g-1(3)To solve the serious interfacial side reaction caused by the continuous oxidation decomposition of carbonate electrolyte under high cut-off voltage(≥ 4.5 V),Tri(2-cyanoethyl)borate(TCEB)molecule was selected as electrolyte additive to improve the electrochemical properties of the SC-NCM613‖Li battery through in situ film-forming strategy.Electrochemical results showed that TCEB derived CEI film containing rich B\N could effectively reduce the decomposition of the carbonate electrolyte and improved the electrochemical stability of the SC-NCM613‖Li battery.As a result,the capacity retention of the battery using the 2-TCEB electrolyte is increased from 69.9%to 76%after 300 cycles at 2.75-4.5 V/0.5 C/25℃.Even at 2.75-4.7 V/1 C/25℃,the capacity retention of the assembled SC-NCM613‖Li battery increased from 51%to 80%after 100 cycles.(4)The Se@SC-NCM613 composite cathode materials were prepared by the melt-diffusion method.The electrochemical performance of the Se@SC-NCM613‖Li battery at high charging cut-off voltage(4.7 V or higher)is further enhanced through constructing the selenophilic surface.The initial discharge capacity values of uncoated SC-NCM613 and Se@SC-NCM613-1%cathodes are almost the same(174.3 mAh g-1and 175.6 mAh g-1at 2.75-4.7 V/1 C/25℃.After 200 cycles,the discharging capacity of the SC-NCM613 and Se@SC-NCM613-1%cathodes were 19.8 mAh g-1 and 120.8 mAh g-1 respectively,showing the better cyclic stability of Se@SC-NCM613-1%cathode.There are 87 figures,20 tables and 225 references in this thesis... |
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