Study On Safety Of Lithium-ion Batteries

With the large scale applications, safety study on lithium-ion batteries has been much concerned recently. Due to the high energy density and organic flammable electrolyte system, lithium-ion batteries may undergo thermal runaway under certain conditions, leading to unsafe events e.g. ignition, explosion when they are abused or misused. In particular, for battery packs used in large power system, the one of single battery, in fact, is in the abused state if the pack is seriously unbalanced, as a result, the non-safety of this single battery may cause the unsafe event of the whole pack.In this dissertation, the study on safety of lithium-ion batteries recent years was briefly summarized based on the thermal process occurred in the battery due to the abuse or misuse and a chart of energy triggering process was given. Then our investigation results of the tolerance to the various abuse disturbances for tested single ALBs (aluminum laminated batteries) with the system of LiCoO₂/MPCF at different conditions (charge state, cycling and operating etc) were followed.The tolerant abilities to abuse for over two thousand commercial ALBs after cycling were investigated. In the cycling range of this experiment, there were obvious effects of cycling on safety such as electrical and thermal tolerance, however, hardly on mechanical safety. For overcharge, short-circuit and oven tests, there was a certain cycle number range respectively, below which the cycled battery was safe and over which the cycled battery was unsafe. In conjunction with results from XRD, SEM and IR tests, which showed that IR increased along with the cycle due to the decrease of lithium intercalation and reduction of particle sizes for LiCoO₂, and also thickening of SEI film during cycling, and the formation of Lithium and Lithium compound at the surface of anode at the end of cycling, the triggering factors of thermal runaway in cycled batteries for different situations were discussed. In addition, we observed that the increasing rate of temperature at beginning during short-circuit test was slower for unsafe batteries than safe ones and an explanation was given.The storage under fully charging state at elevated temperatures deteriorated the abilityof the anti-thermal disturbance. After stored at 70-90°C for 4h, the battery was safe, however, it became unsafe for overcharge of 3C12V for stored at 90°C. Correspondingly, the charging-discharging performance became severe. Those phenomena were attributed to the IR increase with an abruptness at 80 °C due to thickening and compactness of SEI film by an enhanced interaction between intercalated anode and electrolyte at elevated temperatures.The thermal tolerance of cathode materials, including LiCoC>2, coated-LiNiCh, LiMn2O4 and mixture material of LiMn2O4 coated-LiNiCh (1:1, wt%), was investigated and compared. The battery with LiMn2O4 showed the best thermal tolerance with the maximal thermo-safety temperature of 165°C. Battery with the mixture material had excellent performance e.g. the maximal thermo-safety temperature of 150°C and passing 5C18V over-charge test, it also had similar performances e.g. capacity and cycles compared with LiCoC>2 batteries.By comparison of separators from different manufacturers, the polyolefin separators played a very important and direct role on the safety in the temperature range of its shut-down and rupture. The effect of the pressing ratio of cathode rolls on performance and safety demonstrated that an optimized ratio was needed.A unit cell model for battery was suggested to investigate the temperature distribution inside battery during discharging. It revealed that the current density and increasing rate of temperature of cathode active material and current collector were higher at the position near cathode electrode tabs compared with other places in the battery. This result consisted with observation by infrared image apparatus.