Why are lithium iron phosphate batteries safer than ternary batteries?

As the new energy vehicle and energy storage industries are developing rapidly, the safety of lithium batteries has become the focus of the industry. As mainstream battery types, lithium iron phosphate batteries and ternary batteries are favored by the industry for their excellent safety. This article will analyze in depth the reasons why lithium iron phosphate batteries are safer than ternary batteries from multiple aspects such as material system, thermal stability, charge and discharge characteristics, and overcharge and overdischarge tolerance.

Material system

1.Differences in thermal stability of cathode materials

Lithium iron phosphate (LiFePO₄) adopts olivine crystal structure. With strong chemical bonds between Fe-O and P-O, it is stable at high temperature and not easy to decompose and release oxygen. However, ternary materials (NCM, NCA) have layered structures. The higher the nickel content, the worse the stability. They begin to decompose at 200-250℃. The released oxygen easily triggers oxidation reaction when it comes into contact with the electrolyte, leading to thermal runaway.

Comparison of chemical activity of materials

The chemical activity of lithium iron phosphate materials is relatively low. During normal charging and discharging, the electrochemical reaction of lithium iron phosphate is relatively stable, and there are fewer side reactions between the electrode material and the electrolyte. This means that during the long-term use of the battery, there will not be excessive gas and heat accumulation due to the occurrence of a large number of side reactions.

Due to its high nickel content, ternary materials have strong chemical activity. During the charging and discharging process, especially under extreme conditions such as high temperature and overcharging, complex side reactions are prone to occur between ternary materials and electrolytes, such as decomposition of electrolytes and dissolution of transition metals. These side reactions will not only cause battery capacity decay, but also generate a large amount of heat and gas, increase the internal pressure of the battery, and seriously threaten battery safety.

Differences in thermal stability affect safety limits

1.Different thermal runaway temperatures

The thermal runaway starting temperature of lithium iron phosphate battery reaches 500℃-600℃, which reserves sufficient reaction time for the battery management system (BMS) to start protection in extreme conditions such as high temperature and short circuit. In contrast, the thermal runaway temperature of ternary lithium battery is only 200℃-300℃, which is very easy to trigger thermal runaway in the face of abnormal working conditions. It is difficult for BMS to intervene in time, and the fire spreads rapidly.

2.Severity of thermal runaway reaction

Lithium iron phosphate batteries have high thermal stability and decompose slowly during thermal runaway, releasing little heat and gas, effectively suppressing the spread of thermal runaway. Ternary lithium batteries, on the other hand, release a large amount of heat and oxygen instantly due to rapid material decomposition, igniting the electrolyte and causing a violent combustion, resulting in a sudden increase in the temperature of the battery pack. Thermal runaway causes a chain reaction in the module, leading to serious safety accidents.

Charge and discharge characteristics affect safety performance

1.Differences in overcharge tolerance

The charging and discharging voltage platform of lithium iron phosphate batteries is stable, the voltage rises slowly when overcharged, and the material structure is stable, lattice collapse is not likely to occur, greatly reducing the risk of overcharging.

When the ternary lithium battery is overcharged, the positive electrode material undergoes an irreversible phase change, releasing oxygen, and the electrolyte decomposes to produce flammable gas, causing a sharp increase in internal pressure, which is very likely to cause bulging, fire, or even explosion.

2.Cyclic stability comparison

The structure of lithium iron phosphate batteries changes little during long-term cycles, and the expansion and contraction of electrodes are low, which can maintain structural integrity and reduce the risk of internal short circuits.

Due to the layered structure characteristics of ternary lithium batteries, the electrodes expand and contract significantly during charging and discharging. As the number of cycles increases, the material is prone to powdering and falling off, piercing the diaphragm and causing a short circuit, affecting safety and life.

Summary: In summary, lithium iron phosphate batteries are significantly better than ternary batteries in terms of safety due to their unique material system, excellent thermal stability, good charge and discharge characteristics, and safety performance in practical applications. With the continuous advancement of technology, lithium iron phosphate batteries are expected to play their safety and reliability advantages in more fields and promote the sustainable and healthy development of the new energy industry.