Study On Electrode Materials And Full Cell Electrode Technology Of High Specific Energy Lithium Ion Batteries

It is demonstrated that material the preparation,modification and electrode fabrication of both anode and cathode materials play the critical role in achieving high specific energy lithium-ion batteries,which will have a great impact on the batteries’s commercialization and pratical application in portanle electronics,new energy electric vehicles and large-scale energy storage systems.The work of this thesis is to study the electrode active materials that can be applied to high energy density lithium-ion batteries.The synthesis,characterization and performance validation of the silicon-carbon cpmposite anode materials with a core-shell structure and the LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）cathode materials are carried out,and the results show that these materials have high specific capacities,good low temperature operation properties and outstanding cycling performances..In order to optimize the peforamnce of NCM811 electrodes,the effects of contents of the precursor s in fabricating NCM811 material on its property and the corresponding battery performance are investigated.The main achievements of this thesis work are as follows:（1）A silicon-carbon composite anode material with a core-shell structure was synthesized through a method of mechanical ball milling followed by a step of high-temperature heat treatment.Studies have shown that as the silicon content increases,the first charge/discharge specific capacities of the silicon-carbon anode increases correspondingly,but the Coulomb efficiency decreases.Lithium-ion batteries with such a higher silicon content anode show the expanded battery volume during cycling,the quickly increased thickness,weaken and instable electrode structure,and the quickly decayed capacity quickly.However,if the silicon-carbon electrode with the higher silicon content is designed to be thinner,the Li⁺transmission distance is shortened,which is conducive to Li⁺ion conduction,so that the corresponding lithium ion battery has lower resistance and higher capacity.Studies have also shown that silicon-carbon electrodes containing 9.5%silicon have the best electrochemical performance:the first specific capacity reaches 450.9 m Ah·g^-1,the Coulomb efficiency is>90%,and the capacity retention rate is still above 84.5%after 300 cycles.（2）The Ni_0.8Co_0.1Mn_0.1（OH）₂ nickel-cobalt-manganese ternary precursor was prepared by the co-precipitation method.The precursor is mixed with LiOH in proportion and then sintered twice to prepare the NCM811 nickel-cobalt-manganese ternary cathode material.The surface of NCM811 is coated with Al₂O₃,Zr O₂ and LBO by wet chemical method.Studies have shown that these cladding thicknesses are about 20-200 nm,and do not change the lattice structure of the bulk NCM811 material.The first charge/discharge specific capacities of NCM811 coated with Al₂O₃ and Zr O₂ are slightly reduced,but the Coulomb efficiencies are improved.The capacity and Coulomb efficiency of NCM811 coated with LBO are the best.The specific energy of the first charge/discharge reaches 230.0 m Ah·g^-1 and 204.3 m Ah·g^-1,respectively,and the Coulomb efficiency reaches 88.83%.The water contents of the uncoated,Al₂O₃,Zr O₂,and LBO coated NCM811 electrodes after storage at 10%RH are increased by 455.1ppm,274.5ppm,312.0ppm,and 255.1 ppm,respectively,and the corresponding capacities after 50 weeks of corresponding battery cycling are reduced to 99.50%,99.77%,99.84%and 99.91%.In addition,the direct current resistances are also increased by 19.6 mΩ,14.0 mΩ,13.6 mΩ,and 12.2 mΩ,respectively.The results show that the first charge-discharge specific capacities,coulomb efficiencies,electrode stability,cycle characteristics,and resistance stability of the NCM811 coated with LBO have been significantly improved,and the electrochemical performances are also optimal.（3）The influence of LBO content on the electrochemical performance of NCM811 electrode was further optimized,and its mechanism was explored.Studies have shown that with increasing the amount of LBO coating materials,the thickness of the LBO coating layer increases in sequence,but does not change the lattice structure of the NCM811 bulk material.The first discharge specific capacities of NCM811 with different coatings range from 192.2 to 193.1 m Ah·g^-1,and the differences are not significant.The NCM811 materials with 0.1%and 0.15%coatings have high Coulomb efficiencies,reaching 90.06% and 90.19%,respectively.As the amount of LBO coating increases,the direct current resistance increases.Under 50%state of charge,the direct current resistance of the corresponding lithium-ion batteries with NCM811 with coating amounts of 0.05%,0.1%,0.15%,and 0.2%ppm are 12.76 mΩ,13.33 mΩ,13.53 mΩ,and 14.74 mΩ,respectively.After 50 cycles,the battery capacities are reduced to 99.38%,99.64%,99.69%and 99.67%,the resistance values are increased by 24.4 mΩ,21.4 mΩ,21.6 mΩand 22.9 mΩ,respectively.The results show that the NCM811 material with 0.1%LBO coating（the thickness of the cladding layer is about 20-100 nm）has a relatively uniform cladding layer,by which the first charge-discharge specific capacity,coulomb efficiency,resistance stability,cycling and other properties of such coated NCM811-based battery can be optimized.（4）The effects of the precursor material components and their contents in the synthesis of NCM811 materials on their electrochemical and safety performance when used in lithium-ion batteries are studied.The precursor components mainly include Super-P and PVDF.They have a significant impact on electrode resistance,peel strength,battery rate capacity,discharge at room temperature,discharge at low temperature,and cycling reflected by the following points:（1）At the Super-P addition ratios of 1.0%,1.5%,2.0%,2.5%,and 3.0%,as the amount of Super-P increases,the NCM811 electrode sheet resistance decreases,and the battery internal resistance also decreases.The discharge rate characteristics are improved and the cycleability is also improved,but too much Super-P will reduce the battery capacity.The result shows that the performance of 2%added amount is the best;（2）In the cases of 1.8%,2.1%,and 2.4%of PVDF added amounts,with increasing PVDF amount,the electrode sheet resistance,the electrode peel strength,the electrode adhesion,and the strength,and the battery temperature during normal temperature discharge are all increased,but the voltage curve decreases during low temperature discharge,and the battery cycle performance decreases with increasing PVDF viscosity.Optimization experiments show that 1.8%of PVDF can give the best performance of NCM811 electrode and its lithium-ion battery.（5）The effects of electrode technology on the electrochemical and safety performance of NCM811 electrode and its lithium-ion battery are investigated.It is mainly reflected in the following three points.（1）When the compacted density of NCM811 electrode is 3.4 g·cm^-3,3.45 g·cm^-3 and 3.50 g·cm^-3,respectively,with increasing the compacted density,the electrode surface state tends to deteriorate,and more material particles are crush.Although the difference between the charge and discharge curves is not obvious,the charge and discharge power and resistance are both decreases,and the cycle performance also decreases with increasing the compaction density.The results show that the performance of NCM811 electrode with compacted density of 3.45 g·cm^-3 is more balanced;（2）the effects of cold pressing at normal temperature and hot pressing at high temperature on the performance of NCM811 electrode and its battery are studied.Through the studies of rate,low temperature,cycling and overcharge,it is found that the hot pressing at 120℃ is beneficial to improve the performance of lithium ion batteries;（3）the NCM811 electrodes are stored for 72 hours under 1%,5%,and 10%RH humidity conditions,respectively,to study the ambient humidity effect by assembling NCM811-based lithium-ion batteries.Studies have shown that with the increase of ambient humidity,the moisture value of the NCM811 electrode increases significantly,resulting in a negative effect on the electrochemical performance and short-circuit performance of lithium-ion batteries.It is concluded that under 1%ambient humidity,the performance of the NCM811 electrode and its lithium-ion battery can be optimized.