High-nickel Ternary Material Preparation And Performance Research For Lithium-ion Batteries

To meet the requirements of new energy vehicle in actual operating environmental conditions,lifetime and long range,high nickel ternary material is widely preferred by industries for its high weight to volume ratio and environmental friendliness.However,the polycrystalline high nickel has problems,such as poor thermal stability and capacity attenuation caused by particle crack in the later stage of long cycle.This paper applies a variety of means to improve the overall performance of high nickel polycrystalline materials,including material coating and modification,as well as mixing of high nickel polycrystalline and monocrystalline materials.At the same time,the multi beam laser deflection method is used to monitor the stress changes of cathode during charge/discharge online and quantitatively.Based on the Pseudo Two-Dimensional（P2D）lithium-ion battery model,the parallel thermoelectric coupling model of high-capacity（over 100Ah）square battery is established,which provides theoretical guidance for the design of lithium-ion battery.Zr/B-NCM materials modified by zirconium boron are synthesized by coprecipitation and high temperature solid state method.XRD results show that the crystal structure is not changed after modification,and the（003）peak deviates to low angle,together with the increase of c lattice parameters,illustrating that part of Zr⁴⁺has entered into lattice,thus increasing layer spacing and Li ion diffusion coefficient.Part of B³⁺enters into oxygen atoms in layer structure transition metal,leading to the decrease of average valence state of transition metal and increase of synchysis of Li⁺/Ni²⁺.SEM and TEM results illustrate that the modified material is polycrystalline material consisting of primary particles.Due to the suppression from coating layer to Li⁺diffusion and primary particle refinement of Zr/B-NCM from steric-hinerance effect,rate performance is increased,with 2.0C/0.1C discharge rate as 85.2%and capacity retention as 83.8%at 250 cycles.Obvious coating layer is observed at the surface of the material,thus the solid/liquid interphase is stabilized and cycle performance is improved.According to XPS results,Zr and B characteristic peak is observed,which is consistent with the results of EDS,illustrating that part of Zr O₂enters into crystal lattice and others stay on the surface of material in the form as Zr O₂and Li₂Zr O₃respectively.After modification of B₂O₃,the Ni²⁺on the surface rises from 15.8%to 24.4%,more than the NCM without modification at 13.9%,and this is the important reason for the improved cycle life.To further enhance the compaction density and cycle performance of Zr/B-NCM cathode,monocrystalline with high mechanical strength,high cycle performance but low rate performance is mixed under various proportion.Cathode electrodes produced under four different proportions are first systematically studied in surface tension,wettability,porosity,compaction density,conductivity,solid state diffusion coefficient,rate performance and cycle performance.Afterwards,the best proportion is evaluated in the field of rheological behavior and yield stress of cathode slurry in industry application.The connection among electrode extension strength,surface resistance and compaction density is tested under 3 m·min^-1 leaner velocity,and the connection between cathode electrode linearly load and compaction density is simulated by using Heckel equation.Test results of prepared 11 Ah pouch full battery show that,the NCM-M7/3（polycrystalline:monocrystalline=7:3）has the best Li⁺diffusion coefficient as 9.88×10^-9 cm²·s^-1 and discharge capacity retention rate of0.5C/0.1C as 87.4%.The positive electrode prepared by mixed materials can obtain highest compaction density and best electronic conductivity,and the pouch capacity retention is 85.3%after 700 cycles.The XRD results of cycled electrode show that the（018）/（110）peak is more obvious and the half-peak is more narrow,illustrating that the NCM-M7/3 has higher structure stability.According to SEM results,the surface of NCM-M7/3 is more clear,the stacking between particles is more compact and the crack of polycrystalline is less,despite that there are coverage of reactant on both NCM-M7/3 and NCM-SP（pure polycrystalline material）cathode electrode.To evaluate the expansion behavior of both electrodes in the period of cycle,pouch battery with gasbag is placed in fixture with force sensor,and after 18 cycles,the expansion force of NCM-M7/3 and NCM-SP is 12.94%and 19.19%respectively.And the NCM-M7/3 has lower internal resistance,better high temperature storage performance,higher overcharge performance and better heat abuse safety performance.To deeply analyze the stress behavior of two cathode electrodes,wafer curvature method and Stoney equation are used with Li tablet as reference electrode and auxiliary electrode,and cathode as research electrode,to quantitatively evaluate the relationship among stress,potential and SOC during lithiation/delithiation.The results show that,under room temperature and 1/30C,nominal stress increases with SOC from 0-20%and 70-100%SOC range during first lithiation.However,the nominal stress decreases with the increase of SOC in the range of 20-70%SOC,due to change of crystal form.During full delithiation,the nominal stress of NCM-SP is15.07MPa,and that of NCM-M7/3 is only 11.60MPa（23%down）,which theoretically support the electrode structure maintenance,polycrystalline crack decrease and NCM-M7/3 cycle performance improvement.Under 0%SOC,the nominal stress does not go back to the original point because of the change of active material microstructure and electrode hole structure.To further increase the cycle performance of the material under higher temperature and to faster design primary battery,simulation,evaluation and optimization is made via control equation and physical structure on the basis of P2D model.Meanwhile,a parallel thermoelectric coupling model suitable for 155Ah battery is established.The model combines Maxwell-Cattaneo-Vernotte theory and Marcus-Hush-Chidsey dynamics to simulate Li⁺transmission inertia and electron migration behavior of the solid phase in the electrode,and test cycle performance and actual temperature distribution of the battery.The model is discharged at 1.0C rate at25℃,the relative error of the simulated temperature at the center of the tab is reduced from 5.61%to 4.15%.The new model can be used as a fast and accurate tool to optimize the design of lithium-ion battery and to develop battery thermal management system（BTMS）,and is under practical use in companies.