I. Operating Temperature of Lithium Iron Phosphate Batteries
Currently, the most commonly used lithium battery packs are lithium iron phosphate (LFP) battery packs. This is because lithium batteries are known to have relatively poor stability and a lower safety factor. LFP batteries, however, improve upon the cathode material of existing lithium-ion batteries, resulting in a higher safety factor. Common cathode materials for LFP battery packs include lithium cobalt oxide, lithium manganese oxide, and lithium nickel oxide, with lithium cobalt oxide being the most prevalent. Although LFP battery packs are also a type of lithium-ion battery, their cathode material demonstrates that LFP batteries currently offer higher stability among lithium-ion batteries. Furthermore, their cost is not particularly high, leading to their increasing use in large-scale industrial applications and mobile base stations.
However, lithium iron phosphate batteries have a relatively low tap density. This characteristic is precisely why lithium iron phosphate batteries are effective in power tools. However, when used in mobile phones, their drawbacks become apparent, the most obvious being insufficient capacity. During operation, lithium iron phosphate battery packs inevitably generate heat. Generally, the operating environment for lithium iron phosphate battery packs is between -50℃ and +80℃. However, in reality, lithium iron phosphate battery packs generate heat during and after operation. Therefore, it is best to keep the temperature of the lithium iron phosphate battery pack below +50℃. However, the biggest drawback of lithium iron phosphate battery packs is their low tolerance to low temperatures; simply put, lithium iron phosphate battery packs are not resistant to low temperatures, so avoid operating them outside of this temperature range.
II. Causes of Lithium Iron Phosphate Battery Explosions and Fires
Lithium iron phosphate batteries generally do not explode or catch fire. Under normal use, lithium iron phosphate batteries have a high level of safety; however, there are exceptions, and dangerous situations can still occur in extreme circumstances. This is greatly influenced by the materials chosen, the formulation, the manufacturing process, and the subsequent use of the batteries.
Although lithium iron phosphate materials have the highest thermal stability and structural stability among all cathode materials from a thermodynamic perspective, and this has been verified in actual safety performance tests, they may be the least safe in terms of the possibility and probability of short circuits occurring in the material and the battery itself.
Firstly, from the perspective of material preparation, the solid-state sintering reaction of lithium iron phosphate is a complex multiphase reaction involving solid phosphate, iron oxide, lithium salt, an added carbon precursor, and a reducing gas phase. To ensure that the iron in lithium iron phosphate is in the +2 valence state, the sintering reaction must be carried out in a reducing atmosphere. However, a strong reducing atmosphere, while reducing ferric ions to +2 valence ions, may further reduce the +2 valence ions to trace amounts of elemental iron.
Elemental iron can cause micro-short circuits in batteries, making it one of the most detrimental substances in batteries. This is one of the main reasons why Japan has not used lithium iron phosphate (LFP) in power lithium-ion batteries. Furthermore, a significant characteristic of solid-state reactions is their slowness and incompleteness, which makes it possible for trace amounts of Fe₂O₃ to exist in LFP. Argonne National Laboratory in the United States attributes the poor high-temperature cycling performance of LFP to the dissolution of Fe₂O₃ during charge-discharge cycles and the precipitation of elemental iron on the negative electrode. In addition, to improve the performance of LFP, its particles must be nanoscaled. A significant characteristic of nanomaterials is their low structural and thermal stability, but high chemical activity, which to some extent increases the probability of iron dissolution in LFP, especially under high-temperature cycling and storage conditions. Experimental results also show that the presence of iron can be detected on the negative electrode through chemical analysis or energy dispersive spectroscopy.
From the perspective of lithium iron phosphate battery manufacturing, the small size and high specific surface area of lithium iron phosphate nanoparticles, coupled with the strong adsorption of moisture and other gases from the air by the high specific surface area activated carbon due to the carbon coating process, result in poor electrode processing performance and weak adhesion of the binder to the nanoparticles. Whether during battery manufacturing, charge-discharge cycles, or storage, the nanoparticles are prone to detaching from the electrodes, causing internal micro-short circuits in the battery.
