Research On High Energy Lithium-ion Power Batteries For Electric Vehicles: Electrical Performance And Thermal Safety

With the increase of energy density,thermal safety accidents in lithium ion power battery systems characterized by thermal runaway have occurred frequently,affecting the competitiveness of the electric vehicle market.Lithium ion power battery composed of high nickel ternary cathode material and silicon-carbon cathode material is the most promising next generation battery system.As two typical layered high nickel ternary cathode materials,LiNi_0.8Co_0.15Al_0.05O₂（NCA）and LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）are expected to be commercialized in lithium ion power batteries.Therefore,the electrical performance and thermal safety of NCM811|SiC and NCA|SiC pouch cells were studied in this thesis.Firstly,the electrical performance of the NCM811|SiC and NCA|SiC pouch cells was studied to analyze the discharge capacity and cycle stability,including the constant current cycle test under the voltage window of 3～4.2 V and the cycle performance test under different charge-discharge ratios（0.2 C,0.5 C,1 C,2 C,3 C,4 C）.In the constant current cycle performance test,the initial capacities of NCM811|SiC pouch cells and NCA|SiC pouch cells were 1000.3 m Ah and 1002.7 m Ah,respectively.After 200 cycles of charging and discharging,the discharge capacities of NCM811|SiC pouch cells and NCA|SiC pouch cells were 857.2 m Ah and 891.5 m Ah,respectively,with corresponding capacity retention rates of 85.7%and 88.9%.In the rate performance test,the discharge capacity of NCA|SiC pouch cells were higher than that of NCM811|SiC pouch cells under various rate charging and discharging cycles.It was found that the cycle stability and rate performance of NCA|SiC pouch cells are better than that of NCM811|SiC pouch cells.Secondly,the thermal safety of two kinds of high nickel lithium ion power batteries was tested and analyzed at the monomer level.The NCM811|SiC and NCA|SiC pouch cells had been subjected to an accelerating rate calorimeter（ARC）test and lateral heating test.In the ARC test,the T₁ and T₂ of NCA|SiC pouch cells were higher than that of NCM811|SiC pouch cells,and the T₃ of NCA|SiC pouch cells and max{d T/dt}were also higher.In the lateral heating test,under the same heating conditions,the time of thermal runaway of NCA|SiC pouch cells were about 60 s later than that of NCM811|SiC pouch cells,but the maximum temperature of NCA|SiC pouch cells were about 100℃higher than that of NCM811|SiC pouch cells.The study found that compared with NCM811|SiC pouch cells,NCA|SiC pouch cells are relatively less prone to thermal runaway.If thermal runaway occurs,NCA|SiC pouch cells will be more severe.Finally,the thermal safety of two kinds of high nickel lithium ion power batteries was tested and analyzed at the material level to explore the underlying reasons for the monomer safety performance of NCM811|SiC pouch cells and NCA|SiC pouch cells.In the test,the reaction system inside the lithium ion battery was divided into a single system（single cathode,single anode,and single electrolyte）,a binary system（cathode+anode,cathode+electrolyte,and anode+electrolyte）and a full battery system（cathode+anode+electrolyte）.The DSC-TG-MS testing was conducted on materials from different systems to analyze the heat generation,weight loss,and gas production of each system.It was found that the thermal stability of NCA is higher than that of NCM811.However,when the cathode material was mixed with the anode electrode and electrolyte,thermal safety of NCA system was worse than that of NCM811 system.This was because the oxygen release temperature of NCA is higher than that of NCM811,and it reacts more seriously with the silicon-carbon anode electrode at a higher temperature.This finding can explain the deterioration of the thermal stability of NCA|SiC pouch cells after thermal runaway.