Analysis of the cause of lithium battery dark spot

 The failure of graphite anode materials mainly occurs on the surface of graphite. Lithium battery dark spot is generated by the electrochemical reaction of the graphite surface with the electrolyte to form the solid electrolyte interface phase (SEI).

Due to the interaction of a series of complex chemical and physical mechanisms inside the battery, certain failure phenomena often occur. This paper mainly analyzes the cause of the lithium battery dark spot phenomenon generated on the anode surface of lithium iron phosphate batteries.

Lithium battery dark spot in the anode

In the 100% SOC state, after high temperature storage, recharging and discharge tests found that the battery capacity decayed to a certain extent and the scattering difference became larger.

There are differences in battery charge retention capacity. The dissection of the battery found that the anode surface of the battery with this winding structure produced uneven lithium battery dark spots, and the degree of lithium battery dark spot in each pole group was different.

And the dark spot at the end is more obvious at the cathode, which has precipitated on the surface of the pole piece, resulting in an increase in the thickness of the anode. It seriously reduces the performance of lithium-ion batteries and inflence the lithium ion battery applications.

Lithium battery dark spot composition

The position of the dark spot of the lithium battery at the end of the electrode piece and the composition of the normal electrode piece were analyzed, and the composition of the normal anode was 100% C.

In addition to 50% to 96% C elements (mainly graphite material), the dark spot position of a lithium battery also contains 4% to 44% O elements (mainly electrolytes), 1% to 3% F/P elements (mainly electrolyte salt LiPF6), and 2% Na elements that are measured at individual points.

In this way, the surface of the lithium battery dark spot in the anode the is no longer dense. The graphite expands and the interlaminar structure is destroyed. There are many cracks caused by the splitting of graphite particles and abnormal holes caused by the failure of interpowder bonding.

The stripping of graphite continuously consumes electrolyte and active lithium, resulting in reduced capacity dispersion and cycling performance. As the surface graphite expands and peels off, the falling anode powder easily overlaps with the cathode, triggering the battery to self-discharge.

The formation of lithium battery dark spot

Anode carbon materials have complex structures and a wide variety, and their conductive behavior varies depending on the electrolyte system. The chemical reaction of the electrolyte has an important impact on the battery capacity and charge-discharge characteristics.

Therefore, the failure analysis of the anode surface should begin with the structural characteristics of the carbon anode material, the composition of the electrolyte system, and the degree of matching of the interface reaction between the anode and the electrolyte.

Anode material structural properties

There are many kinds of carbon materials, such as lithium-ion battery anodes, and they have different structural characteristics with different raw materials and manufacturing processes, so the performance of different types of carbon materials, such as lithium-ion battery anodes, varies greatly.

Theoretically, the better the layered structure of carbon anode materials, the more conducive to the insertion and removal of lithium ions. In general, we use graphitization to reflect the structural perfection of carbon materials. Studies have shown that carbon materials with high graphitization and hybrid carbon atoms can produce excellent SEI films and large lithium storage spaces.

Heat treatment at different temperatures can change the microstructure and graphitization degree of carbon materials, and with the increase of heat treatment temperature, the lithium insertion structure gradually increases, and the graphitization degree also gradually increases.

It shows that there is little or no lithium intercalation in graphite in this area, which verifies that the interlayer peeling of graphite particles in the dark spot area of a lithium battery is serious.

Electrolyte system composition

A solid electrolyte interface (SEI) film is formed by decomposition of the electrolyte on the graphite surface. Its chemical composition and properties depend on the composition and properties of the anode material and electrolyte, which have an important impact on the performance and capacity of the battery.

This protective film ensures that the electrolyte does not undergo reduction and decomposition under the lithium insertion potential, so that lithium ions undergo reversible intercalation in the graphite material.

Lithium salt anion PF6 is the most fundamental cause of the difference in behavior between PC and EC interfaces. The EC-based system forms a stable LiF-rich SEI film.
The PC-based system results in a low content of LiF, a decomposition product.

Studies have shown that low LiF content is the root cause of the inability of PC-based electrolyte decomposition products to form dense and stable SEI films. The electrolyte and active lithium are consumed in film formation and solvent intercalation, and their macroscopic morphology is manifested as dark spots in lithium batteries.

Effect of Na content on the anode

CMC is called a weakly acidic cation exchange resin, and when a small amount of water or weak acid is present, in a small current state, cmc can undergo a cation exchange reaction. ionization occurs to form Na ions.

The Na element is more likely to come from cmc, and insufficient internal moisture control of the battery is the main reason for the dark slag of lithium batteries produced by the anode sheet.

Effect of moisture to produce dark spot

The water contained in the organic electrolyte reacts with the organic solvent to form the corresponding alcohol. Secondly, the organic electrolyte containing water will react with the anode during the first charge and discharge of the battery.

On the one hand, it will consume the limited Li+ in the battery, so that the irreversible capacity of the battery will increase.

On the other hand, the large amount of reaction product gas will also lead to an increase in the internal pressure of the battery, which may eventually cause the anode graphite to peel off and the structure to collapse.

The presence of water will also cause LiPF6 hydrolysis to produce HF, which can further consume the limited Li+ in the battery and accelerate electrolyte deterioration. At the same time, it can lead to an increase in the viscosity of the organic electrolyte and a decrease in conductivity.

Lithium battery dark spot worsen conditions

During the storage process, the lithium-ion battery anode is at a low potential for a long time, which will cause the electrolyte to continuously decompose and consume active Li on the surface of the graphite anode. The cause of lithium dark spot formation has a great effect on battery maintenance, and it can maintain the battery by preventing this situation.

The main factor in reversible capacity loss during storage. Factors such as the SOC and ambient temperature of the battery will affect the rate of decay of the battery’s reversible capacity during storage.

• The electrochemical polarization of anodes is more pronounced.
• Anode pole pieces are brittle and less elastic.

It will affect the stability of the electrode conductive network, which is not conducive to the conduction of Li+, and the dark spot phenomenon will deteriorate.

Conclusion

Anode graphitization degree is low, and graphite layer spacing is small, affecting graphite without lithium insertion or less lithium insertion. The electrolyte affects the intercalation of active lithium between layers of graphite particles.

The moisture residue inside the battery leads to an intensification of the graphite intercalation reaction. At the same time, the water residue leads to the decomposition of electrolyte gas production, resulting in peeling of the anode graphite.

The combined effect of these conditions is the main reason for lithium battery dark spot, and low-temperature storage or low-temperature (0°C and below) charging and discharging will lead to difficulty in lithium intercalation and aggravation of side reactions, and the phenomenon of lithium battery dark spot will worsen.