Study On Porous Structure Synthesis And Negative Immersion Modification Of Ni-rich Cathode Materials NCM811 For Lithium-ion Battery

As the extensive research of the lithium ion battery,the lithium-ion battery cathode materials become more and more significant in the technical evolution.In order to meet the high demand of lithium-ion battery,Due to the high theoretical capacity,low cost and non-toxic character of Ni-rich layered cathode materials(LiNi_1-x-yCo_xTM_yO₂,TM=Mn or Al),they have been considered as one of the most efficient cathode materials and have been received a wide attention,.The high content of Ni（Ni≥0.6）makes Ni-rich layered cathode materials possess a reversible capacity which is more than 200 mAh/g,but at the same time,it also brings a series of problems and challenges such as capacity fading.With the increase of Ni content,the structure stability of cathode materials was seriously affected,the side reaction on the surface of cathode materials and the formation of intergranular cracks would lead to drastic capacity fading.Thus,this paper conduct a number of researches about LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials,and the main work and results are shown below:（1）From the aspect of structure design,using spray-drying method rebuilt the structure of the sample which was made by nickel acetate,cobalt acetate,manganese acetate and lithium acetate.The 3D porous LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials with low cation-mixing and 6 wt%sodium alginate（functioned as blender）was successfully fabricated under the conditions of 775℃.（2）Using ethyl orthosilicate as the coating materials,the high-Ni cathode materials SiO₂@NCM811 with uniform and complete SiO₂ coating layer was prepared by hydrolytic polycondensation method:firstly,by comparing 2 different coating orders,we chose the suitable method and successfully fabricated a uniform SiO₂ coating layer on NCM811 cathode materials which can completely cover the surface of the cathode materials;and then we studied the morphology and electrochemical properties of the SiO₂@NCM811 whose raw materials have different TEOS-cathode ratios.The result of this study shows thatwhen TEOS-cathode materials ratios is 2 wt%,there is a uniform and complete SiO₂ coating layer on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials and its thickness is～38 nm,and the corresponding SiO₂@NCM811 sample perform a superb cycling performance with 80.22%capacity retention after 100 cycles at 1 C（25℃）and also have a good initial charge and discharge capacity which is 175.63 mAh/g.（3）LiMn₂O₄@NCM811 was prepared by negative pressure immersion method:prepared LiMn₂O₄@NCM811 by negative pressure immersion method with different concentrations of precursors and test their electrochemical properties.The results shows that the LiMn₂O₄coating layer not only can’t remit the chemical instability on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials and failed to enhance the cycling performance of LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials,but also introduce superfluous Liand Mn,which aggravate the cation-mixing level,and lead to inferior electrochemical performance;but from another aspect,the comparison of negative pressure immersion method sample and normal immersion method sample reveals that negative pressure immersion method can aggravate above situation,which can indirectly prove the idea of negative pressure immersion method,that is through negative pressure immersion treatment,the precursor solution can successfully permeate into the intergranular voids and reach into the cathode materials.（4）Based on the results of hydrolytic polycondensation experiment and negative pressure immersion experiment,combined them as a new modified method which named hydrolytic polycondensation assisted negative pressure immersion method.By this method a 3D-SiO₂ framework is formed in the intergranular voids and at grain boundaries（functioning as the buffer zone and transfer-bridge）and meanwhile the SiO₂ protective layer is completely and homogeneously coated on the surfaces of the pristine particles.Under the synergistic effect of both 3D-SiO₂framework and SiO₂ protective layer,the modified Ni-Rich cathode materials not only effectively relieves the chemical instability on the surface of LiNi_0.8Co_0.1Mn_0.1O₂cathode materials,but also inhabits the generation and extension of intergranular cracking inside the LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials.The as-modified LiNi_0.8Co_0.1Mn_0.1O₂ exhibits enhanced cycling performance with 92.4%capacity retention after 100 cycles at 1 C,while the capacity retention values for the pristine particles and normal coating treatment particles are only 55.4%and 82.6%,respectively.Moreover,the thermal stability（60°C）is distinctly enhanced and the rate performance is significantly improved at high rates（2,3 and 5 C）.