The safety issue is one of the most important concerns when power lithium-ion batteries are used in electric vehicles, which affects a number of factors, including positive and negative electrode materials, diaphragms, electrolytes and battery design and power management systems and a series of other issues. The current lithium-ion battery safety tests and assessments are done on a sample of the finished battery in different states of abuse for a variety of safety tests, and lithium iron phosphate materials and lithium iron phosphate battery's excellent safety performance is also tested under these conditions. The more important factor related to the safety of lithium-ion batteries is the possibility of short circuit and the higher probability of short circuit because of the material and the intrinsic reasons of the battery. The lithium secondary battery with lithium metal as the negative electrode was abandoned because of the safety problem of internal short circuit caused by the generation of lithium dendrites during the charging and discharging process that can puncture the diaphragm.
It is generally believed that lithium-ion batteries are safe under normal use, which can be seen from the use of the industry's worst safety nickel compounds as cathode materials from Toyota Japan. Although lithium iron phosphate materials are thermodynamically the most thermally stable and structurally stable of all cathode materials available, and have been verified in actual safety performance tests, it is again probably the least safe in terms of materials and the likelihood and chances of short circuiting within the battery.
First, in terms of material preparation, lithium iron phosphate solid phase sintering reaction is a complex multiphase reaction (although some synthesis technologies claim to be liquid phase synthesis process, but ultimately require high temperature solid phase sintering process), there are solid phase phosphate, iron oxides and lithium salts, plus carbon precursors and reducing gas phase. In order to ensure that the iron in lithium iron phosphate is positive divalent, the sintering reaction must be carried out in a reducing atmosphere, and the stronger reducing atmosphere in the reduction of trivalent iron ions to positive divalent iron ions, there is the possibility of further reduction of positive divalent iron ions to trace amounts of singlet iron. Monatomic iron can cause micro-short circuits in batteries, which is the most taboo substance in batteries, and this is one of the main reasons why lithium iron phosphate is not used in power lithium-ion batteries in Japan. In addition, a significant feature of the solid-phase reaction is the slowness and incompleteness of the reaction, which makes it possible to have traces of Fe2O3 in lithium iron phosphate, and Argonne Laboratories in the United States attributed the defect of poor high-temperature cyclability of lithium iron phosphate to the dissolution of Fe2O3 during the charge-discharge cycle and the precipitation of singlet iron on the negative electrode. In addition, in order to improve the performance of lithium iron phosphate, its particles must be nanosized. A distinctive feature of nanomaterials is their lower structural and thermal stability and higher chemical activity, which to some extent also increases the chance of iron dissolution in lithium iron phosphate, especially under high temperature cycling and storage conditions. The experimental results also show that the presence of iron is tested on the negative electrode by chemical analysis or energy spectrum analysis.
From the aspect of lithium iron phosphate battery preparation, due to the small nanoscale particles of lithium iron phosphate, high specific surface area, and due to the use of carbon coating process, the high specific surface area of activated carbon has a strong adsorption effect on moisture and other gases in the air, resulting in poor processing performance of the electrode and poor adhesion of the binder to its nanoparticles. Whether in the battery preparation process or during the charge/discharge cycle and storage of the battery, the nanoparticles are easily detached from the electrodes, causing internal micro-short circuit of the battery.
As far as we know, lithium iron phosphate batteries have a high short-circuit rate, both in the manufacturing process of battery manufacturers and in the process of consumer use. Battery manufacturers tend to find the problem from the battery preparation process, and often do not recognize the problem of short circuit due to the causes inherent in lithium iron phosphate materials. The United States A123 18650 lithium iron phosphate batteries a few years ago on the electric car fire explosion, when the car was driving on the highway. The subsequent investigation concluded that the wiring screws were not tightened, resulting in overheating that caused the battery to catch fire and explode. However, we believe that due to the internal short circuit of the battery is more likely to cause a fire and explosion. It is doubtful that the heat generated by the external partial screw not tightened will cause such a serious fire and explosion of 18650 type lithium battery.
