In the field of lithium batteries, "lithium deposition" is a term that appears very frequently. Whether it's fast charging, battery life, or low-temperature performance, many issues can ultimately be traced back to lithium evolution.

For battery researchers, lithium deposition is almost a necessary concern; For ordinary users, it is one of the important factors affecting battery life.

1. What is lithium deposition

During normal charging, lithium ions will detach from the positive electrode, migrate through the electrolyte to the negative electrode, and embed into negative electrode materials such as graphite.

Ideally, all lithium ions should enter the negative electrode structure.

But when the charging conditions are not suitable, some lithium ions do not have time to embed into the negative electrode, and will directly obtain electrons on the surface of the negative electrode, forming metallic lithium deposition.

This process is called lithium evolution.

Simply put:

The lithium that was supposed to 'enter the negative electrode' ended up 'piling up on the surface of the negative electrode'.

2. Why does lithium deposition occur

Lithium deposition is essentially due to the inability of negative electrode lithium insertion speed to keep up with charging speed.

When the speed at which lithium ions reach the surface of the negative electrode is greater than the speed at which they enter the interior of the negative electrode, lithium will deposit on the surface.

The common causes are mainly as follows.

Fast charging

The larger the charging current, the more lithium ions reach the negative electrode surface per unit time.

If the negative electrode cannot absorb in time, it is easy for lithium to precipitate.

This is also one of the problems that fast charging technology always needs to solve.

Low temperature charging

After the temperature decreases, the diffusion rate of lithium ions significantly decreases.

At this point, even if the charging current remains constant, the lithium insertion ability of the negative electrode will decrease, thereby increasing the risk of lithium deposition.

Therefore, many new energy vehicles will limit their fast charging power in winter.

High SOC state

As the battery approaches full charge, the available space in the negative electrode decreases.

When continuing to charge, lithium ions are more likely to deposit on the surface.

Therefore, lithium deposition often occurs in the later stage of charging.

3. What problems will lithium deposition bring

Firstly, there is capacity decay.

The metallic lithium formed by sedimentation may not necessarily be able to participate in subsequent cycles again.

As time goes by, the available lithium gradually decreases, and the capacity will decrease.

Secondly, there is an increase in internal resistance.

Lithium deposition can disrupt the original interface state, induce more side reactions, and continuously thicken the SEI film.

More serious is the security risk.

If the deposited lithium continues to grow, it may form needle like structures and even pierce the membrane, causing internal short circuits.

4. How to reduce the risk of lithium deposition

The common methods currently include:

Control charging rate

Optimize the structure of negative electrode materials

Improve electrolyte system

Improve low-temperature kinetic performance

Adopting smarter charging strategies

Essentially, it is to enable lithium ions to enter the negative electrode more evenly and quickly.

5. Conclusion

Lithium deposition is not an independent fault, but the result of internal dynamic imbalance in the battery.

It will affect capacity, lifespan, and safety, and is also a challenge that must be faced in the development of fast charging technology.

In a sense, understanding lithium evolution means understanding why lithium batteries age and why some batteries can achieve fast charging while still maintaining a longer lifespan.