Study On The Synthesis And Modification Of Lithium-rich Manganese-based Cathode Materials

Lithium-rich manganese-based cathode material Li_1.2Ni_0.13Co_0.13Mn_0.54O₂（LRNCM）for lithium-ion batteries has attracted much attention owing to its high specific capacity,eco-friendliness,low cost etc.However,the material has some shortcomings,such as low initial coulombic efficiency,low discharge medium voltage,cycle capacity attenuation,and poor rate performance,and its commercial application still has a long way to go.Therefore,the synthesis and modification of LRNCM were explored and optimized in this paper.The main research contents and results are as follows:（1）Mn_0.66Ni_0.17Co_0.17CO₃ precursor was synthesized by the controlled crystallization method in a carbonate system,and the effects of different coprecipitation processing parameters on the morphology,particle distribution,and composition of the precursor were investigated.The LRNCM cathode material was prepared by lithiation and solid-state reaction using the precursor,which was prepared under optimized conditions（the reaction temperature:50°C,stirring rate:600 rpm,p H:8.5,and reaction time:18 h）.The impact of the thermal treatment on the electrochemical properties of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ were discussed.Under the optimized experimental conditions（850℃,12 h）,the lithium-rich manganese-based cathode material with good electrochemical performance(the discharge specific capacity remained at 188.7 m Ah g^-1after 100 cycles at 0.2 C)was obtained.（2）The effects of Al₂O₃ coating by the liquid-phase method on the performance of the LRNCM were investigated.The experimental results show that a surface coating modification of 0.5%–1%Al₂O₃ can improve the cycle stability of lithium-rich manganese-based materials.The optimal modification effect can be achieved by controlling the Al₂O₃coating amount of 1%Al₂O₃,with a capacity reaching 177.7 m Ah g^-1 after 100cycles at a 0.5 C rate.（3）To improve the electronic conductivity of the lithium-rich manganese-based cathode material,the polyaniline,which is a good conductive electroactive polymer,was selected to modify the surface of the LRNCM by liquid-phase method.The physical and chemical properties of the materials coated with polyaniline were systematically studied.The results showed that the coating layer was amorphous,and the optimal coating amount was 1%.The impedance value of the coated material decreased.The discharge specific capacity of the Li-rich manganese-based cathode material coated with 1 wt.%polyaniline was about 37 m Ah g^-1higher than that of the uncoated material at 5 C current density.After 100charge–discharge cycles at 0.5 C rate,LRNCM@1%PANI has a high discharge capacity of 188.8 m Ah g^-1.（4）Surface coating modification of lithium-ion conductor on lithium-rich manganese-based materials was studied.Li_6.4La₃Al_0.2Zr₂O₁₂（LLAZO）ionic conductor material with a cubic garnet structure was successfully synthesized by the sol–gel method.Accordingly,LRNCM@LLAZO composite lithium-rich manganese-based cathode material with good electrochemical performance was designed and synthesized.The effects of LLAZO coating on the morphology,surface structure,and electrochemical properties of lithium-rich manganese-based cathode materials as well as the modification mechanism were systematically studied.The experimental results show that the LLAZO modification can promote the migration of lithium ions at the interface and stabilize the cycle performance of the LRNCM@LLAZO cathode material under high voltage conditions.At 0.5 C,LRNCM@2%LLAZO has a good charge–discharge performance after 100 cycles with a discharge capacity of 175.5m Ah g^-1.（5）Li_1.4Al_0.4Ti_1.6（PO₄）₃（LATP）ionic conductor was synthesized by the sol–gel method,and the LRNCM@LATP composite with a three-dimensional lithium-ion diffusion channel on the surface was designed and synthesized by the in situ method.The synergistic effect of the LATP coating with Al,Ti element doping,and diffusion of PO₄^3-,was realized by a high-temperature heat treatment,and the XRD patterns of LATP synthesized at different sintering temperatures were characterized.The effects of LATP with different coating contents（1 wt.%,3 wt.%,5 wt.%）on the morphology,surface structure,and electrochemical properties of LRNCM materials were investigated,and the mechanism of performance improvement was analyzed.The results show that LATP can be synthesized by the sol–gel method under a sintering temperature of 600°C–800°C and sintering time of 6 h.It is determined that the Li_1.4Al_0.4Ti_1.6（PO₄）₃ coating can hinder the side reactions between the electrolyte and active material,which improves the structural stability and cycle rate performance of the material.LRNCM@1%LATP has a sound discharge specific capacity of 198 m Ah g^-1 at 0.2 C after 100 cycles,and the discharge capacity retention is 81%.（6）The hybrid modification of Li_1.4Al_0.4Ti_1.6（PO₄）₃（LATP）and polyaniline（PANI）was designed and prepared.The surface modification of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode material was carried out to study the effect of double coating on the material（LRNCM@LATP@PANI）.The hybrid modified layer reduces the interface side reaction and combines the high-ionic conductivity of Li_1.4Al_0.4Ti_1.6（PO₄）₃ with the electronic conductivity of polyaniline,which effectively improves the electrons and Li-ions transfer.LRNCM@3%LATP@1%PANI exhibits excellent electrochemical performance,and the capacity retention can reach 79%(189 m Ah g^-1)after 200 cycles at 0.2 C,25℃,2.0–4.8 V.There are 89 figures,20 tables and 332 references included in the paper.