Preparation,Structural Design And Modification Of Cathode Material NCM811

The high nickel ternary cathode material Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）has the advantages of high specific capacity,low cost and environmental friendliness,and has a broad application prospect in the field of energy storage.However,the high nickel property of NCM811 also leads to the problems of severe Li⁺/Ni²⁺mixing,low electronic and ionic conductivity,and poor thermal stability.In response,this thesis conducts research on the optimization of the preparation process and the modification of the material in terms of material synthesis,crystal structure design,elemental doping,in situ coating and composite electron-conducting agents.The details are as follows:1.NCM811 was prepared by co-precipitation-high-temperature solid-phase method,and the optimal final firing condition was optimized to hold the material at 780°C for 10 h.The initial discharge specific capacity of the material was 191.4 m Ah g^-1 at 1 C（3～4.3 V）,with a capacity retention rate of 79.1%after 200 cycles.2.To further improve the performance of NCM811,the cathode structure was designed with a gradual concentration gradient,and the Zr element was doped in a gradient to obtain a simultaneous gradient design with both Ni and Zr elements showing decreasing concentration from inside to outside,which not only takes into account the advantages of"high Ni inside and low Ni outside"structure stabilization,but also gives full play to the Zr element.This not only takes into account the advantages of"high nickel inside and low nickel outside"structure stabilization,but also gives full play to the role of Zr elements in stabilizing crystal structure and improving rate performance.The optimal Zr doping amount is 1 wt%,and the initial discharge capacity of the cathode at 1 C（3～4.3 V）is 193.1 m Ah g^-1,and the capacity retention rate after 200 cycles is 89.9%,which is significantly higher than that of the undoped blank sample(191.2 m Ah g^-1and 82.43%.When the cycling multiplier was raised to 10 C,its initial discharge specific capacity reached 147.1 m Ah g^-1 and still had a capacity of 129.03 m Ah g^-1 after 200 cycles（capacity retention of 87.7%）.3.To improve the interfacial stability of NCM811 electrode,1,3-dioxolan（DOL）was introduced into the basic electrolyte as a film-forming additive,and the electrode was modified by in situ polymerization of DOL on the cathode surface to produce poly-DOL（PDOL）film to achieve the effects of blocking the direct contact between electrolyte and electrode,inhibiting interfacial side reactions,and reducing crystal Li⁺/Ni²⁺mixing and fragmentation.The effect of PDOL was achieved.The optimal DOL addition amount was 15 wt%,and a conductive polymer electrolyte film of suitable thickness（6 nm）was formed on the cathode surface under this condition.The initial discharge specific capacity of the cathode was 186m Ah g^-1 at 1 C（3～4.3 V）,and the capacity retention rate was 85.6%after200 cycles,which was much higher than that of the blank sample（14.8%）.improved.In addition,the PDOL coating can still achieve effective protection of the cathode under high voltage（3～4.5 V）,and the capacity retention rate can reach 76.5%after 200 cycles（1 C）.4.The physical characteristics of various types of carbon nanotubes（SW-CNT,MW-CNT,OH-CNT,NH4-CNT）of carbon black（CB）,vapor grown carbon fiber（VGCF）,and graphene（GP）were compared from the viewpoint of improving the electronic conductivity of the cells,respectively.The results showed that VGCF had the highest conductivity,followed by MW-CNT and the lowest by GP.CB was used as a conductive agent for NCM811 in combination with other conductive substances,respectively.The optimized composite of CB and MW-CNT was most favorable for the enhancement of the electronic conductivity of NCM811,which was attributed to the ability of the granular CB to be uniformly dispersed in the MW-CNT mesh nanotubes,forming a good linear conductive network.