Performance And Simulation Of Lithium Ion Power Batteries

Lithium-ion power batteries have a good application prospect in the field of electric vehicles. The performance (such as specific energy, specific power) of batteries determines the key indicators of electric vehicles. From electrochemical principle, on the one hand the performance of cell depends on the characteristics of electrode material, electrolyte, separator et al. On the other hand, it also depends on the structural characteristics of cell. The porous electrode, composed by active material, is the place where electrode reaction occurs. Its influence is especial important. In order to clarify the influence mechanism of electrode structure on lithium ion batteries' performance, on the one hand the influence of different electrode structure (porosity) and different conductive agent on cell's performance was studied, on the other hand, mathematical model was established and the discharge behavior of lithium ion power batteries at large current was simulated. The unit step of electrochemical process inside cell was analyzed. The key step which determines lithium-ion power batteries' discharge process was analyzed.8Ah high power lithium ion batteries using spinel LiMn2O4 as cathode material and mesophase microspheres as anode material were prepared. Batteries' rate discharge performance at room temperature, cycle performance and discharge performance at low temperature were studied when cathode porosity (ε) is 0.32 and 0.244 respectively. The result shows that whenεis 0.32, cell's rate discharge performance and cycle performance excels that of cell which cathode porosity (ε) is 0.244. The ratio of discharge capacity at 25C to that at 1C rate is 91.8% and 18.1% irrespectively. When the retention of discharge capacity declines to 70%, the cycles of cell whichεis 0.32 is 1700, while that is 1060 whenεis 0.244. The difference of discharge capacity of cell with different cathode porosity at low temperature is very little. The ratio of discharge capacity at -30℃to that room temperature is 91.8% and 89.9% respectively.There exits great difference between batteriees' discharge curves at low temperature. The cell whichεis 0.32 shows good discharge voltage plateau. During discharging at low temperature, the discharge voltage of cell whichεis 0.244 is lower and the voltage recovery phenomena occur.The influence of conductive agents, plated-shape graphite (KS-6), fiber (VGCF) and conductive carbon black (Super-P), on electrochemical performance of spinel LiMn2O4 was studied. The results show that the difference of charge-discharge specific capacity of LiMn2O4 is very little when different conductive agent was used, but cycle performance differs from each other greatly. When KS-6, VGCF, SP or SP+VGCF is used as conductive agent, the capacity retention of LiMn2O4after 100 cycles at 1C rate is 58.3%, 77.6%,82.2% and 88.3% respectively. The cycle performance of LiMn2O4 can be heightened by adding surfactant to improve the dispersion of conductive agent.When KS-6, VGCF,SP or SP+VGCF is used as conductive agent, capacity retention of LiMn2O4 after 100 cycles at 1C rate is improved to 86.3%,86.8%,88.3% and 94.5% respectively.In addition, the rate performance of LiMn2O4 is improved also. The ration of discharge capacity at 2C rate to that at 1C rate is improved from 64.2% to 70.4%. The difference of rate discharge performance of 300mAh lithium ion batteries using spinel LiMn2O4 as cathode material and synthetic graphite as anode material, KS-6 and SP was used as conductive agent respectively, was studied furthermore.The cell using SP as conductive agent has a good rate discharge performance. The ratio of discharge capacity at 15C rate to that at 1C rate is 84.3%. While that of cell using KS-6 as conductive agent is only 21.8%.The mathematical model of lithium ion batteries is established based on dynamic equations of each unit step of electrochemical process inside batteries. The discharge process of 8Ah C-LiMn2O4 high power lithium ion batteries at 1C,5C,10C,15C,20C, 25C rate was simulated respectively. The effects of Bruggeman coefficient, conductivity of electrode solid matrix, and electrode reaction rate constant on cell's discharge behavior were analyzed. The modeling results can coincide with experimental results well by changing Bruggeman coefficient. The effect of conductivity of electrode solid matrix on batteries'discharge behavior is little. Electrochemical reaction rate constant of cathode materials affects cell's discharge voltage to a certain extent. These results illuminate that the transport of Li+in electrolyte phas is the key factor affecting lithium ion batteries' discharge behavior. The distribution of electrolyte concentration inside cell and Li-concentration inside LiM2O4 particles when discharged at 25C rate were analyzed. The results show that in any rejoin of porous cathode where near current collector, the lithium salt is depleted as discharge time going on. The distribution of Li- concentration inside LiMn2O4 particles is uneven at the end of discharge. These results demonstrate that the transportation of lithium salt in electrolyte phase and the diffusion of Li- in LiMn2O4 particle are the limiting steps of electrochemical reaction process when discharged at large current. The discharge process at different C rate (1C,5C,10C,15C) of 300mAh lithium ion batteries, using KS-6 or SP as cathode conductive agent respectively, were simulated also. The results show that modeling result could coincide with experimental result well using different Bruggeman coefficient for different conductive agent. This demonstrates that the structural characteristics of porous cathode vary when KS-6 or SP used as conductive agent. When SP is used as conductive agent, the transport of Li+in electrolyte phase inside cell is superior to that using KS-6. Electrode structure is the key factor which affects cell's discharge behavior.According to the simulation results of C-LiMn2O4 battery,7.5Ah High power lithium ion batteries using LiFePO4 as cathode material and mesophase microspheres as anode material were prepared by adoption of thin electrode. At room temperature, the discharge capacity at 26C rate is up to 92.3% of that at 1C rate. The discharge curves at 1C,6C, 10C, 20C,26C rate were simulated. The transport of Li- in LiFePO4 particles was modeled by phase-change diffusivity. The other electrochemical processes were modeled using the same method for modeling C-LiMn2O4 batteries. The results show that just like the cell using LiMn2O4 as cathode material, the transport of Li- in electrolyte phase determines cell's discharge behavior at large current which uses LiFePO4 as cathode material. The distribution of electrolyte concentration inside cell when discharged at 26C rate was analyzed. The result shows that in contrast to cell using LiMn2O4 cathode, the electrolyte inside porous cathode is not depleted. The distribution of Li- concentration at the surface of active particles across the thickness of electrodes is analyzed. The result shows that the diffusion of Li- in LiFePO4 particles results in gradual descending of cell's voltage at later stage when discharged at large current.