Why Lithium Iron Phosphate Batteries Are Safer Than Ternary Lithium Batteries

Ternary lithium batteries and lithium iron phosphate batteries exhibit distinct characteristics, with the key differences lying in “energy density” and “safety.” Ternary lithium batteries offer higher energy density, yet their safety is often a concern. In contrast, lithium iron phosphate batteries, though lower in energy density, are generally considered safer. For instance, taking the commonly used 18650 cell (with a diameter of 18 mm and height of 65 mm) as an example, the maximum capacity of a ternary lithium battery can reach 3500mAh, while a lithium iron phosphate battery of the same size typically achieves only around 2000mAh.

So, how safe are lithium iron phosphate batteries?  

A lithium iron phosphate battery is a type of lithium-ion battery that uses lithium iron phosphate as the cathode material. Common cathode materials for lithium-ion batteries include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, ternary materials, and lithium iron phosphate. The P-O bond in the crystalline structure of lithium iron phosphate is highly stable and resistant to decomposition. Even under high temperatures or overcharging conditions, it does not break down or generate heat like lithium cobalt oxide, nor does it form strongly oxidizing substances. This inherent stability contributes to its superior safety.

Advantages of lithium iron phosphate batteries:  

- Long service life: They can achieve over 4,000 cycles at 80% depth of discharge (DOD), translating to a lifespan of 10 to 15 years under normal conditions.  

- High safety: These batteries undergo rigorous safety testing and are unlikely to explode, even in traffic accidents.  

- Fast charging: With a dedicated charger, a full charge can be achieved in 40 minutes at 1.5C.  

- High-temperature resistance: They can withstand temperatures of 350–500°C.  

- High capacity, no memory effect, and environmental friendliness: They are non-toxic, pollution-free, and made from abundant, low-cost raw materials.

Are ternary lithium batteries safe?

1. Ternary lithium batteries are generally considered safe cathode materials for lithium-ion batteries. They contain no heavy metals harmful to humans, and the oxygen in their olivine structure is not easily released, enhancing material stability.  

2. The production process of ternary lithium batteries is similar to that of other lithium-ion batteries, involving steps such as mixing, coating, rolling, electrode formation, and winding. Due to the relatively poor conductivity of ternary lithium materials, particles are typically made smaller, resulting in more uniform internal arrangement and a balanced voltage platform. This promotes operational stability.  

3. During charging and discharging, the oxidative capability of iron ions is weak, and no oxygen is released, reducing the likelihood of redox reactions with the electrolyte. This ensures a safer charging and discharging process. Even under high-rate discharge or overcharge conditions, severe redox reactions are unlikely to occur. Additionally, the volume change during lithium intercalation and deintercalation offsets the expansion of the carbon anode, helping maintain structural stability and minimizing the risk of battery rupture.

However, ternary lithium batteries still face safety challenges. In accidents involving external impact, the battery separator may be damaged, leading to a short circuit. The heat generated during a short circuit can cause thermal runaway, rapidly increasing temperatures to over 300°C.  

The thermal stability of ternary lithium batteries is relatively poor. At temperatures below 300°C, oxygen molecules may be released, which can react with combustible electrolytes and carbon materials inside the battery. This accelerates cathode decomposition and may lead to combustion within a very short time. In comparison, lithium iron phosphate batteries can withstand temperatures of 700–800°C without releasing oxygen molecules, making them significantly safer.

Which is safer: lithium iron phosphate or ternary lithium battery?

The difference in safety primarily stems from the cathode material. Both materials decompose at certain temperatures, but ternary lithium decomposes at around 200°C, releasing oxygen and causing intense chemical reactions that can rapidly ignite the electrolyte. Lithium iron phosphate, on the other hand, decomposes at 700–800°C without releasing oxygen, resulting in less intense combustion. In short, ternary lithium materials are more prone to catching fire than lithium iron phosphate materials. However, it is important to note that this refers specifically to the materials themselves.

The overall safety of a power system depends on more than just the cathode material. Lithium-ion batteries require a Battery Management System (BMS) to provide critical protections such as overcharge prevention, over-discharge prevention, temperature control, and over-current protection. In case of an abnormality, the BMS can immediately cut off the current. Therefore, it is inaccurate to simply label ternary lithium batteries as unsafe. While ternary lithium materials are more prone to thermal decomposition, this does not mean ternary lithium batteries are inherently unsafe. Similarly, the stability of lithium iron phosphate materials does not automatically guarantee safety.

A more critical consideration is how to prevent a failing cell from affecting others. Due to manufacturing limitations, it is impossible to maintain 100% consistency across all cells.

In summary, compared to ternary lithium batteries, lithium iron phosphate batteries are less prone to explosion or combustion under various extreme conditions such as overcharging, short circuits, compression, impact, disassembly, or high-temperature exposure.