Reasons For Lithium Evolution Of Lithium-ion Batteries Is An Extremely Common Abnormal Phenomenon In The Lithium Battery Industry.

Different lithium evolution states often correspond to different reasons. Analyzing the reasons according to the lithium evolution state can improve the product yield.

From a general perspective, the reasons for lithium evolution in lithium-ion batteries are divided into five categories: lithium evolution caused by insufficient negative electrode margin; lithium evolution caused by charging mechanism; lithium evolution caused by abnormal lithium insertion path; lithium evolution caused by abnormal main material; Lithium deposition at a fixed location caused by special reasons. The following is an explanation of the specific reasons for the lithium evolution for the above five categories of reasons.

1. Lithium evolution of lithium ion batteries caused by insufficient negative electrode margin

After lithium ions are released from the positive electrode during charging, they must have a destination. Generally speaking, the fate is to be embedded in the negative electrode, but when the negative electrode is insufficient and the negative electrode can insert lithium ions less than the positive electrode deintercalated lithium ions, lithium ions can only be precipitated on the surface of the negative electrode. Insufficient excess negative electrode can be regarded as the most common cause of lithium precipitation.

According to the position of insufficient negative electrode excess, it can be subdivided into the following three groups of lithium analysis conditions:

(1) Lithium evolution with insufficient excess of conventional anode

When the negative electrode is excessively insufficient, there is not enough space for the lithium ions to be inserted from the positive electrode to the negative electrode. Therefore, only metallic lithium can be formed and precipitated on the surface of the negative electrode. Because the degree of insufficient excess of the negative electrode is generally uniform, and the lithium ions extracted from the positive electrode also uniformly come to the negative electrode, the lithium evolution caused by the insufficient negative electrode is also a uniform layer. The severity of lithium evolution is related to the insufficient negative electrode excess. The degree is closely related, the higher the degree of excess and deficiency, the more serious the lithium precipitation.

(2) Lithium analysis on yin and yang

When a battery cell is coated with heavy coating on one side of the positive electrode or lightly coated on one side of the negative electrode, it will cause the lithium on both sides of the negative electrode of the battery cell to deposit lithium on the other side, which is commonly known as the Yin and Yang side. The interface of the anode and cathode cell on the lithium-evolving side is exactly the same as that of the negative electrode with excess or insufficient lithium, while the other side is golden yellow (for graphite anode).

(3) The positive electrode head is coated without thinning out lithium

If the positive electrode head is not thinned during coating, the dressing at the positive electrode head position may be thicker, so that there will be excessive and insufficient corresponding to the negative electrode head, resulting in a strip of the negative head. Analyze lithium.

2. Lithium-ejection of lithium-ion batteries improperly manufactured by chargers

Since the evolution of lithium occurs during the charging stage, the change in the charging mechanism must also be one of the reasons for the evolution of lithium. The following is a list of several cases of lithium evolution due to the charging mechanism:

(1) Lithium precipitation by low-temperature charging

The reason for lithium precipitation during low-temperature charging is that the lithium insertion resistance of the negative electrode at low temperatures is significantly greater than that of the positive electrode. Although lithium ions can be removed from the positive electrode relatively quickly at low temperatures, they cannot be inserted into the negative electrode in time, thus causing precipitation. lithium.

(2) Lithium analysis by high rate charging

If charging at room temperature blindly increases the charging rate, the negative electrode will also cause lithium precipitation due to the inability to quickly complete lithium insertion. Under the conventional capacity type design, the maximum charge rate that the battery cell can withstand is about 1C~1.5C. If the product needs to further increase the charge current during use, then a special design of the pole piece and electrolyte is required. Otherwise, the greater the charge rate, the more serious the lithium evolution will be.

(3) Lithium overcharge

When the charging voltage or charging capacity of the battery greatly exceeds the design value, more excess lithium ions will be extracted from the positive electrode. Since the negative electrode is designed, there is no space for these excess lithium ions. Lithium is inevitable. During overcharge, the deintercalation of lithium ions from the positive electrode is uniform and does not vary with the position of the pole piece, so the lithium evolution caused by overcharge is also a uniform layer.

3. Lithium evolution of lithium-ion batteries caused by abnormal lithium insertion paths

When charging a lithium-ion battery, lithium ions are extracted from the positive electrode and then inserted into the negative electrode through the electrolyte. However, if the interface between the positive and negative electrodes is not in good contact, it will cause lithium ions to precipitate on the surface of the negative electrode. Details are as follows:

(1) Diaphragm wrinkles to analyze lithium

When the separator is wrinkled due to its own quality, after the lithium ions at the corresponding position are deintercalated from the positive electrode, they cannot be uniformly inserted into the negative electrode. As a result, the negative electrode at the corresponding position will either become brown with insufficient lithium intercalation, or it will become Stripe-like lithium with the same direction of crimping.

(2) Deformation of the battery cell to analyze lithium

When the thickness of the battery cell is large, it is easy to deform. When the deformation is serious, the pole piece corresponding to the deformed position of the battery cell may be in poor contact, resulting in the strip-shaped lithium-insertion poor area in the figure above, which will occasionally be accompanied by analysis. lithium.

(3) Conventional formation and no hot and cold pressing of lithium before formation

If the thickness of the cell is relatively large, even if the conventional formation is carried out without hot and cold pressing after liquid injection, the interface will not be too problematic. However, for some thin cells with a thickness of less than 3mm, if there is no clamping during formation, and the hot and cold pressing or fixture baking are forgotten before formation, the interface will be more miserable.

Since the contact between thin batteries is difficult to close at the interface, if no pressure is applied to the surface before and during the formation, the formation gas cannot be completely discharged and the interface contact will be affected, resulting in insufficient point-like lithium insertion and point-like lithium precipitation.

4) The fixture is formed into lithium without pressure

Since the fixture formation is often accompanied by high current and high charge SOC, the gas production rate during formation is faster, and the battery interface after formation will have obvious golden yellow, and the position corresponding to the insufficient lithium insertion will look more obvious. Whether it is a thin cell that is not hot or cold pressed before formation, or a cell that should have been formed by the fixture but not pressurized, as long as the problem is found before degassing, then the small current discharge with fixture and the formation can be performed again. The interface has been significantly improved.

(5) Lithium analysis summary:

When the lithium insertion path is abnormal, the most obvious interface abnormality of the battery cell is the brown insufficient lithium insertion area, followed by the slight lithium evolution at the corresponding position. Due to the different formation processes and materials, the phenomenon of lithium precipitation caused by poor interface contact during formation may be different from the above illustration.

4. Lithium evolution of lithium ion battery caused by abnormal main material

During the charging process, the destination of lithium ions is to penetrate the SEI film and finally be embedded in the negative electrode. If there is a problem with the SEI film or the negative electrode, causing the lithium ions to be unable to be inserted normally, then the result can only be lithium evolution.

(1) Lithium is crushed by the negative electrode

When the compaction of the negative electrode sheet exceeds its limit, lithium ions will be deposited on the surface of the negative electrode when the negative electrode structure is crushed or there is not enough space for insertion. The lithium evolution caused by the crushing of the negative electrode is not repairable like the poor chemical contact, and it has a fatal impact on the capacity and cycle of the battery.

(2) Lithium evolution caused by less electrolyte

When the battery is injected with a small amount of liquid or the aging time after injection is short, the electrolyte will not be able to completely infiltrate the negative electrode, and the position where the infiltration is not fully infiltrated will form a small black spot with no lithium embedded as shown in the figure above. There may be slight lithium evolution around the black spot.

(3) Lithium evolution with mismatched electrolyte

The principle of lithium evolution caused by this reason is not fully understood by Wenwu. It is guessed that the electrolyte and the negative electrode are not matched, which causes the SEI film to be too thick or uneven, and then hinders the insertion of lithium ions; or the electrolyte cannot Fully infiltrates into the negative electrode, causing difficulty in lithium ion insertion.

(4) Lithium evolution caused by direct separation without being formed

If the battery cell is directly charged with a separate volume without forming a small current, the SEI film cannot be effectively formed, which will affect the insertion of lithium ions into the negative electrode and cause lithium evolution during the charging process. The corresponding lithium analysis picture is like a spot as shown in the figure above.

(5) The water content exceeds the standard to analyze lithium

A small amount of water contributes to the formation of the SEI film, but when the water content exceeds the standard, it will react with the lithium salt in the electrolyte and destroy the SEI film composition, thereby affecting the insertion of lithium ions into the negative electrode and forming the irregularities in the above picture Brown area, sometimes the brown area will also occur lithium evolution.