Particle Structure Modulation And Application Performance Research For Ternary Cathode Materials

Ternary cathode material(Li Ni_1-x-yCo_xMn_yO₂)combines the synergistic advantages of Ni,Co and Mn transition metal elements,and is considered as one of the most promising cathode materials for lithium ion batteries（LIBs）in the market.In this thesis,Li Ni_1/3Co_1/3Mn_1/3O₂（NCM111）and Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）materials were systematically studied.The effects of the particle structure,such as the secondary particles,quasi single crystals and single crystals,on the performance of the cathode materials are specifically investigated.Moreover,the thermal stability and cycling life of the full pouch cells using the as prepared NCM111 and NCM811 as cathode material were evaluated,to obtain the effective approach to improve the performance of the ternary cathode materials.The main contents are as follows:As to the cathode materials for the high rate start-stop battery,NCM111 cathode materials with secondary particles,quasi single crystals and single crystals were synthesized and analyzed.The results show that quasi single crystal NCM111 with high capacity and good cycling performance could be obtained with the adjusting the mole ratio of Li and transition metals（Li/TM ratio）of 1.10 and sintering temperature of 960℃.The rate capability of the material was significantly improved by the surface coating of Ti O₂.Full cell testing shows that quasi single crystal NCM111 exhibited balanced electrochemical properties,including high power density of 7432 W/kg in the hybrid pulse power characterization,high rate performance of discharge capacity retention of 94.2%at 30 C rate,good storage stability of capacity retention of over 91.0%after being stored at 55℃for 30 days,and excellent cycle life of capacity retention of 87.4%and 83.3%after 6000 cycles at room temperature and 2000 cycles at high temperature during charge-discharge voltage range of 3.0～4.1 V,respectively.Compared with quasi single crystal NCM111,single crystal NCM111 presented remarkable power density loss of 400 W/kg,and secondary particle NCM111 showed much poor cycle life of 4500 cycles at room temperature.Therefore,quasi single crystal NCM111 exhibits excellent comprehensive properties and could be a suitable cathode candidate for start-stop battery system.For the high energy density power battery system application,the improvement and effect mechanism of Zr-Al element co-doping and solid electrolyte coating on the structural and surface stability of NCM811material was investigated.Under the optimized conditions at sintering temperature of 770℃,Li/TM ratio of 1.04 and Zr-Al element co-doping,the as-prepared NCM811 possessed stable structure,uniform particle size and high capacity of 210.9 m Ah/g.At the same time,the H2-H3 phase transition could be also significantly inhibited during the charge and discharge processes.The surface coating research demonstrates that the interface impedance was greatly reduced and the transfer rate of Li⁺was improved after a content of 2000 ppm solid state electrolyte(LATP,Li_1.3Al_0.3Ti_1.7（PO₄）₃)coating.Pouch cells were fabricated to evaluate the electrochemical performance during voltage range of 3.0～4.2 V,LATP coated NCM811 exhibited discharge capacity of 190.0 m Ah/g at 0.5C,energy density of 270 Wh/kg.It can be cycled for 2500 and 1500 cycles at room temperature and 45℃,respectively.Meanwhile,the cracks and pulverization of the secondary particles after cycling were inhibited obviously,which can meet the demand for high power battery system.Single crystal NCM811 was further studied to suppress the particle cracking during cycling process,as well as improve thermal stability,and meet the higher cut-off voltage demand of power battery.The results indicate that increasing sintering temperature and Li/TM ratio,prolonging sintering time can effectively promote the formation of single crystal materials.As the size of the single crystal increased,the capacity and rate capability decreased,while its structure stability and cycling stability were enhanced.The optimized results show that single crystal NCM811 with relative high capacity and good cycling stability can be synthesized at Li/TM ratio of 1.05 with the sintering temperature of 910℃and sintering time of 12 h.Full cells were fabricated and tested between the voltage range of 3.0～4.35 V,the high temperature storage testing results show that the capacity retention and recovery value of single crystal sample were as high as 90.3%and 94.0%,while for the secondary particle sample,the values were 72.3%and 78.6%,respectively.The cycle capacity retention of single crystal sample was 80.2%after 2200 cycles,while secondary particle sample was only 80.0%after 1200 cycles at room temperature.At the same time,the particle structure stability of single crystal sample was significantly improved,and the fragmentation situation was also greatly reduced after long time cycling.Therefore,single crystal NCM811 shows great potential for high voltage power battery application.The effect of additive elements on the growth of single crystal was further studied.It is found that the Sr element could play effective role for accelerating the single crystal formation of NCM811.At Sr doping amount of 500 ppm,single crystal NCM811 was synthesized at lower sintering temperature of 890℃.XRD investigation suggested that the cation mixing of Sr doped sample decreased from 2.24%to 2.02%,and the structural stability was significantly improved.EIS impedance analysis indicated that Sr additive could reduce the impedance increase and side reactions during cycling.First-principles calculation demonstrated that Sr doping can effectively suppress oxygen vacancy,and the DSC testing suggested that the exothermic temperature was increased about 30℃after Sr doping compared with the pristine NCM811,indicating the enhanced thermal stability after Sr doping.Full cell testing results showed that the cycling stability of single crystal doped with Sr was greatly enhanced,with capacity retention of 87.4%after 2300 cycles at room temperature,and86.6%after 1200 cycles at high temperature.The particle structure remained well and there was no cracking presented after cycling.The large-scale production further confirms that the Sr doping strategy is stable and reliable,the production capability reaches 3.5 tons/day and the as-prepared product shows good consistency.Therefore,the modified samples are the promising materials for the high voltage and high energy density power battery systems.