Cause and analysis of battery aging

To reduce gas emissions and global warming, batteries have become a versatile and efficient option. However, the limited cycle life of lithium batteries limits long-term use. Battery aging is defined as a decrease in system performance, life, and reliability, and aging batteries can cause their capacity to decay and power to decay.

A typical lithium-ion battery consists of electrode active materials, binders, conductive agents, separators, current collectors, and electrolytes, and the interaction between these components plays a key role.

Therefore, the aging of these components accelerates the entire battery aging. In this article, the side reactions that occur in different parts of the battery will be analyzed in detail, describe the aging characteristics of the battery and reveal the corresponding battery aging mechanism.

The battery aging mechanism of the anode

● Formation and steady growth of the SEI

The main battery aging factor on graphite anodes is the change in solid electrolyte interphase (SEI) on the surface of the anode. SEI membranes are formed near the electrode surface to protect the anode from possible corrosion and prevent electrolyte decay during the first charge.

However, SEI is volatile. Thus, SEI growth continues to occur, leading to the depletion of active li and electrolyte breakdown. The consumption of active li leads to an increase in electrode impedance, battery capacity and power decay, and accelerated battery aging.

But SEI is relatively stable over the course of growth, and capacity loss is not significant in the short term, allowing for long-term use of lithium battery. The conditions under which lithium batteries are used, including high voltage, high SOC, and high temperature, can exacerbate these processes.

● Lithium deposition and formation of lithium dendrites

• Lithium ion concentration

Once the lithium ion concentration exceeds the saturation limit of the electrolyte, lithium metal deposition occurs on the surface of the graphite anode. In addition to causing capacity loss, the formation of lithium dendrites can also lead to short circuits and transient battery failure.

• High current

The high current means that the number of lithium increases, thus accumulating on the surface of the anode.

● Low temperature and high SOC

In this case, the likelihood of dendrite formation increases. The deposition of li on graphite is partially reversible. If the active lithium is completely coated with SEI film and loses the electrical connection to graphite, the active lithium will become dead lithium.

Therefore, some lithium dendrites are isolated and widely distributed, resulting in dendritic growth of lithium dendrites and diffusion of dead lithium. This process can lead to internal short circuits posing a threat to battery safety and have bad ifluence to battery maintenance.

Changes in the anode structure

The particle size of graphite materials has a great influence on the anode performance. Small particle materials can shorten the diffusion path between graphite materials, which is conducive to high-speed charging/discharging.

However, small particle materials have a larger specific surface area, which will consume more li+ at high temperatures, resulting in an irreversible increase in the anode’s capacity.

The volume change of the active material is also one of the causes of contact loss. In addition, the volume change of the active material has a negative effect on the porosity of the electrode. High-voltage solid-density graphite electrodes have low porosity, which can delay battery aging.

● Changes in the structure of the active material

During battery performance degradation, battery aging usually occurs inside the active material, which is manifested in the destruction of the ordered graphite structure.

Because of the new interface between lithium ions and disordered particles that react with electrolytes, ions are more difficult to embed in graphite. As a result, structural changes in the active material lead to a decrease in reversible capacity.

The battery aging mechanism of the cathode

● Dissolution of active material

Battery aging caused by the dissolution of active materials is mainly present in the cathode. For corroded cathodes, the crystal structure is destroyed, resulting in a reduced storage location for lithium, capacity decay, and battery aging.

For cells with low potential, deposits form on the surface of the anode, and catalytic SEI thickening reactions occur, resulting in the loss of lithium and increased internal resistance of the anode.

● Formation and growth of CEI

The loss of Li+ in the cathode is mainly attributed to cathode electrolyte interphase (CEI), which is similar to SEI membranes. CEI membranes consist of compounds produced by side reactions between cathodes and electrolytes.

Running the battery at high SOC will accelerate electrolyte breakdown and produce more HF to corrode the cathode, resulting in more CEI components and battery aging. The electrolyte breaks down easily at high temperatures, which will accelerate the production of CEI films.

Changes in the cathode structure

The attenuation caused by the dissolution of the active materials is mainly found in the  cathode, especially at high state of charge and low potentials. Changes in the volume of lithium ions in the anode material have a greater impact on cathode performance.

In addition, this deintercalation process causes phase transitions that distort the lattice structure. Some phase transitions are reversible, such as LiFeO4 and FePO4 coexisting during cycling. But there are also irreversible phase transitions that can cause the lattice structure to collapse, leading to battery aging.

● The battery aging mechanism of electrolytes

The electrolyte determines the cycle stability, capacity, safety, and operating conditions of lithium batteries. Traditional electrolytes are usually composed of LiPF6 and other organic carbonates.

However, LiPF6 instability can easily lead to battery aging. The reaction products generate SEI films on the anode surface and CEI films on the cathode surface.

While a stable and dense surface film can effectively reduce the reaction between the electrolyte and the electrode, continuous rupture and regeneration will continue to consume Li+, leading to battery aging.

● The battery aging mechanism of the separator

The main causes of separator of battery aging are the growth of lithium dendrites, electrolyte corrosion, blockage of separator channels, excessive circulation and structural degradation caused by high temperature or cycle times. In addition, further dissolution of transition metal ions or decomposed active substances can diffuse through the separator and block pores.

● Battery aging mechanism of current collection

Aluminum foil and copper foil are the most widely used materials for current collectors for cathodes and anodes, respectively. The performance of aluminum current collectors depends largely on the composition of the electrolyte.

In the LiPF6 electrolyte, a small amount of water can inhibit corrosion of aluminum current collectors. However, with the production of water, the oxidation decomposition products of the electrolyte undergo an electrochemical reaction on the surface of the aluminum foil, which accelerates the corrosion of the aluminum foil.

Conclusion

In order to improve the system performance, life, and reliability of a battery, analyzing battery aging may cause its capacity to decay and power decay.

Taking lithium batteries as an example, we will introduce electrode active materials, binders, conductive agents, separators, current collectors, and electrolytes. We know that the aging of these components will accelerate the battery aging.