The current lithium-ion battery safety test and evaluation is to conduct various safety tests on the finished battery under various abuse conditions, and to test the excellent safety performance of the lithium iron phosphate material and the lithium iron phosphate battery under these conditions. A more important factor associated with the safety of lithium ion batteries is the possibility of short circuits due to the inherent causes of materials and batteries, and the higher probability of short circuits. On the other hand, lithium secondary batteries using lithium metal as a negative electrode have been abandoned due to the safety problem of internal short circuit caused by the occurrence of lithium dendrites during charging and discharging.
It is widely believed that lithium-ion batteries are safe under normal conditions of use, and it can be seen from Toyota Japan that the most unsafe nickel-based compounds in the industry are used. Although lithium iron phosphate materials are thermodynamic, their thermal stability and structural stability are among the highest of all current cathode materials and have been verified in actual safety performance testing, but the possibility of material and battery shorts is inherent. And by chance, it may be the least secure.
First, in terms of material preparation, the solid phase sintering reaction of lithium iron phosphate is a complex heterogeneous reaction (although some synthetic techniques claim to be liquid phase synthesis processes, the process of high temperature solid phase sintering is ultimately required). There are solid phase phosphates, iron oxides and lithium salts, plus a carbon precursor and a reducing gas phase. 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 the atmosphere is strongly reduced in the process of reducing iron ions to positive divalent iron ions, where there will be a positive ferrous iron. The possibility of further reduction of ions into trace element iron. Elemental iron causes the battery to be short-circuited, which is the most taboo substance in the battery. This is one of the main reasons why Japan does not use lithium iron phosphate in power lithium-ion batteries.
In addition, an important feature of the solid phase reaction is the slow and incomplete reaction, which makes it possible to trace Fe2O3 in lithium iron phosphate. The Argonne laboratory in the United States attributed the defect of 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, it is necessary to nanoparticle the particles. An important feature of nanomaterials is their low structure and thermal stability and high chemical activity, which also increases the possibility of iron dissolution in iron ferric phosphate to some extent, especially under high temperature cycling and storage conditions. The experimental results also show that the presence of iron is tested by chemical analysis or energy spectrum analysis on the negative electrode.
From the viewpoint of preparing a lithium iron phosphate battery, since the lithium iron phosphate nanosized particles are small, the specific surface area is high, and the high specific surface area activated carbon has a strong gas, such as moisture in the air, a carbon coating process. Adsorption results in poor electrode processing performance and poor adhesion of the binder to its nanoparticles. The nanoparticles are easily separated from the electrodes during battery preparation or during charge and discharge cycles and storage of the cells, resulting in internal micro-shorts of the cells.
To the best of our knowledge, lithium iron phosphate batteries have a high short circuit rate during both the battery manufacturer’s manufacturing process and the consumer’s use. Battery manufacturers often look for problems starting from the battery preparation process, and short-circuit problems caused by the inherent causes of lithium iron phosphate materials are often not recognized. A few years ago, when the car was driving on the highway, the US A123 18650 lithium iron phosphate battery exploded on the electric car. Subsequent investigations concluded that the screws of the wiring were not tightened, causing the battery to explode due to overheating. However, we believe that the possibility of a fire explosion due to an internal short circuit in the battery is greater. The heat generated by the external screws not tightening will cause severe fire and explosion of the 18650 lithium battery.