Why lithium iron phosphate batteries more safe than ternary lithium batteries?

[Pete_UZ]

Ternary lithium battery (e.g. NCM, NCA) and lithium iron phosphate battery have different characteristics, and the main contradiction focuses on “energy density” and “safety”. Ternary lithium batteries have higher energy density, but their safety is often questioned. Although the energy density of lithium iron phosphate battery is small, it is said to be safer.

   

1. Is lithium iron phosphate battery safe?

Lithium iron phosphate battery refers to a lithium-ion battery using lithium iron phosphate as the cathode material. The cathode materials of lithium ion batteries mainly include lithium cobalt oxide, lithium manganate, lithium nickel oxide, ternary materials, lithium iron phosphate, etc. The P-O bond in lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperature or overcharge, it will not collapse and heat like lithium cobalt oxide or form strong oxidizing substances, so it has good safety.
 
2. Advantages of lithium iron phosphate battery

Lithium iron phosphate battery has long service life and cycle life of more than 4000 times(in the condition of 80%DOD). Under the same conditions, the lithium iron phosphate battery can be used for 10 to 15 years.

Safe to use. Lithium iron phosphate battery has passed strict safety test and will not explode even in traffic accidents.

Fast charging. Using a special charger, the battery can be fully charged after charging at 1.5C for 40 minutes.

Lithium iron phosphate battery is resistant to high temperature, and the hot air value of lithium iron phosphate battery can reach 350 to 500 ℃.

Lithium iron phosphate battery has large capacity.

Lithium iron phosphate battery has no memory effect.

Lithium iron phosphate battery is green, non-toxic, pollution-free, with a wide range of raw materials and low price.
 
3. Is ternary lithium battery safe?

3.1 Ternary lithium battery is a safe cathode material for lithium ion battery at present. It does not contain any heavy metal elements harmful to human body. Oxygen in its olivine structure is difficult to precipitate, which improves the stability of the material.

3.2 The production process of ternary lithium battery is roughly the same as that of other lithium battery varieties, and its core processes include: batching, coating, rolling, film making and winding. In the batching process, the conductivity of ternary lithium material is relatively poor. Therefore, the particles are generally made smaller. The objective effect of this is that the internal arrangement is more uniform, which promotes it to form a balanced voltage platform, and can maintain the stability of the battery during operation.

3.3 Charge and discharge are two basic working states of lithium battery. When the ternary lithium battery is charged and discharged, because the oxidation capacity of iron ion is not strong and oxygen will not be released, it is naturally difficult to have redox reaction with electrolyte, which makes the charging and discharging process of ternary lithium battery in a safe environment. Moreover, it is difficult for ternary lithium batteries to undergo severe redox reaction in the process of high rate discharge or even overcharge and discharge. At the same time, after lithium is de intercalated, the lattice change will reduce the cell volume, which just offsets the increased volume of carbon negative electrode in the reaction. Therefore, during charge and discharge, the ternary lithium battery can maintain the stability of physical structure and eliminate the hidden danger of battery burst caused by volume increase.

Ternary lithium batteries are unreliable in terms of safety. In a car accident, the impact of external force will damage the battery diaphragm, resulting in a short circuit, and the heat generated during the short circuit will cause the battery heat to get out of control and quickly raise the temperature to more than 300 ℃.

The thermal stability of ternary lithium battery is poor. Oxygen molecules will be decomposed when it is less than 300 ℃. When it meets the combustible electrolyte and carbon materials in the battery, the generated heat will further aggravate the decomposition of the positive electrode and deflagration will occur in a very short time. In contrast, another widely used lithium iron phosphate battery can adhere to 700-800 ℃, and will not decompose oxygen molecules, so its safety is much higher.
 
4. Who is safer, lithium iron phosphate battery or ternary lithium battery?

The so-called “safety” difference is mainly in the cathode material. Both materials will decompose when reaching a certain temperature, and the ternary lithium material will decompose at about 200 degrees. Moreover, the chemical reaction of ternary lithium material is more intense, which will release oxygen molecules, and the electrolyte will burn rapidly under the action of high temperature. Lithium iron phosphate will decompose at 700-800 degrees, will not release oxygen molecules like ternary lithium materials, and the combustion is not so intense. To put it simply, ternary lithium materials are easier to catch fire than lithium iron phosphate materials. Note that we are only referring to “materials”.

The final decision is the safety of the whole power system. Due to the characteristics of lithium battery, BMS battery management system must be provided. The main functions include overcharge protection), over discharge protection, over temperature protection, over-current protection and other functions. Once an accident occurs, the current can be cut off immediately. Therefore, it is not appropriate to simply think that ternary lithium battery is unsafe. Ternary lithium materials are easy to pyrolyze, but it does not mean that ternary lithium batteries are unsafe; Lithium iron phosphate material is not easy to pyrolyze, but it does not mean that lithium iron phosphate battery is safe.

In fact, we should pay more attention to how to prevent failed battery cells from harming other cells. Limited by the process level, it is impossible to maintain the consistency of all battery cells at 100%.

The above is the safety comparison between lithium iron phosphate battery and ternary lithium battery.