Safety is one of the most concerned issues when using power-type lithium-ion batteries in electric vehicles. There are many influencing factors, including positive and negative materials, separators, electrolytes, battery design and power management systems, and a series of issues. The current safety tests and evaluations of lithium-ion batteries are done by sampling the finished batteries in various safety tests under different abuse conditions, and the excellent safety performance of lithium iron phosphate materials and lithium iron phosphate batteries are also tested under these conditions Out. A more important factor related to the safety of lithium-ion batteries is the possibility of short circuits and the higher probability of short circuits due to the inherent reasons of the material and the battery. The lithium secondary battery with metallic lithium as the negative electrode was abandoned because of the safety problem that lithium dendrites would pierce the separator and cause an internal short circuit during the charging and discharging process.
It is generally believed that lithium-ion batteries are safe under normal use. This can also be seen from the use of nickel-based compounds considered by the industry to be the worst in the industry as positive electrode materials by Toyota. Although the thermal stability and structural stability of lithium iron phosphate materials are the highest among all current cathode materials in terms of thermodynamics, and they have also been verified in actual safety performance tests, from the material and the possibility of internal short-circuits in the battery In terms of probability, it may be the most insecure.
First of all, in terms of material preparation, the solid-phase sintering reaction of lithium iron phosphate is a complex multiphase reaction (although some synthesis techniques claim to be a liquid-phase synthesis process, it ultimately requires a high-temperature solid-phase sintering process). There are solid phase phosphates, iron oxides and lithium salts, plus carbon precursors and reducing gas phases. In order to ensure that the iron element in lithium iron phosphate is positive divalent, the sintering reaction must be carried out in a reducing atmosphere, and a strong reducing atmosphere reduces the ferric ions to positive divalent iron ions. The possibility of further reduction of iron ions to trace elemental iron. Elemental iron can cause micro-short circuits in batteries, which is the most taboo substance in batteries. This is also one of the main reasons why Japan did not apply lithium iron phosphate to power lithium-ion batteries. In addition, a notable feature of the solid-phase reaction is the slowness and incompleteness of the reaction, which makes the possibility of trace Fe2O3 in the lithium iron phosphate. The American Argonne Laboratory attributed the poor high-temperature cycle of lithium iron phosphate to Fe2O3. Dissolution during charge and discharge cycles and precipitation of elemental iron on the negative electrode. In addition, in order to improve the performance of lithium iron phosphate, its particles must be nanosized. A significant feature of nanomaterials is low structural stability and thermal stability, and high chemical activity, which to some extent also increases the probability 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 through chemical analysis or energy spectrum analysis.
From the aspect of the preparation of lithium iron phosphate batteries, due to the small lithium iron phosphate nanoparticles, the specific surface area is relatively high, and due to the carbon coating process, the activated carbon with high specific surface area has a strong effect on the moisture in the air and other gases. Adsorption results in poor electrode processing performance, and poor adhesion of the binder to its nanoparticles. No matter in the battery preparation process or in the battery charging and discharging cycle and storage, the nanoparticles are easy to detach from the electrode, causing the internal micro short circuit of the battery.
As far as we know, lithium iron phosphate batteries have a high short-circuit rate no matter in the manufacturing process of the battery manufacturer or in the process of consumer use. Battery manufacturers often start with the battery preparation process to find the problem, and often do not recognize the problem of short-circuit caused by the inherent reasons of the lithium iron phosphate material. The American A123's 18650 lithium iron phosphate battery caught fire and exploded on an electric car several years ago, when the car was driving on the highway. Later investigations believed that the screws of the wiring were not tightened, which caused the battery to catch fire and explode due to overheating. However, we believe that the possibility of fire and explosion caused by a short circuit within the battery is greater. It is doubtful that the heat generated by the failure of tightening of the external partial screws will cause such a serious fire and explosion phenomenon of the 18650 lithium battery.
