The most complete summary of the reasons for the swelling of soft-pack lithium-ion batteries

by:CTECHi     2021-07-21

There are many reasons for the swelling of soft-packed lithium-ion batteries. Based on experimental research and development experience, the author divides the causes of lithium battery swelling into three categories. One is the increase in thickness caused by the expansion of the battery pole pieces during the cycle; the other is the swelling caused by the oxidation and decomposition of electrolyte to produce gas. The third is the swelling caused by process defects such as inadequate battery encapsulation and introduction of moisture and corner damage. In different battery systems, the leading factors for the change of battery thickness are different. For example, in the lithium titanate anode system battery, the main factor of swelling is air swelling; in the graphite anode system, the thickness of the pole piece and the gas production have the same effect on the swelling of the battery. Play a promotional role.

1. Variation of electrode pole piece thickness

Discussion on influencing factors and mechanism of graphite anode expansion

The increase in the thickness of the cell during the charging process of lithium-ion batteries is mainly attributed to the expansion of the negative electrode. The expansion rate of the positive electrode is only 2~4%. The negative electrode is usually composed of graphite, binder, and conductive carbon. The expansion rate of itself reaches ~10%, and the main influencing factors that cause the change of the expansion rate of the graphite anode include: SEI film formation, state of charge (SOC), process parameters and other influencing factors.

(1) SEI film is formed during the first charge and discharge of lithium-ion batteries, the electrolyte undergoes a reduction reaction on the graphite particles at the solid-liquid interface, forming a layer covering the electrode material The passivation layer (SEI film) on the surface, the generation of the SEI film significantly increases the thickness of the anode, and due to the generation of the SEI film, the thickness of the cell increases by about 4%. From the perspective of the long-term cycle process, according to the physical structure and specific surface of different graphite, the cycle process will cause the dissolution of SEI and the dynamic process of new SEI production. For example, flake graphite has a larger expansion rate than spherical graphite.

(2) During the cycle of the charged state battery, the volume expansion of the graphite anode has a good periodic function relationship with the SOC of the battery, that is, with the lithium ion The volume of the continuous insertion in the graphite (increasing the SOC of the cell) gradually expands. When the lithium ions are extracted from the graphite anode, the SOC of the cell gradually decreases, and the volume of the corresponding graphite anode gradually shrinks.

(3) Process parameters From the aspect of process parameters, the compaction density has a greater impact on the graphite anode. During the cold pressing of the pole piece, the graphite anode film has a greater impact. The compressive stress of the pole piece is difficult to be completely released in the subsequent high-temperature baking of the pole piece. When the battery is charged and discharged in cycles, due to the combined effect of multiple factors such as the insertion and extraction of lithium ions and the swelling of the adhesive by the electrolyte, the diaphragm stress is released during the cycle, and the expansion rate increases. On the other hand, the compaction density determines the void capacity of the anode membrane. The pore capacity in the membrane is large, which can effectively absorb the expanded volume of the pole piece. The void capacity is small. When the pole piece expands, there is not enough space to absorb the expansion. At this time, the generated volume can only expand to the outside of the membrane layer, which is manifested as the volume expansion of the anode sheet.

(4) Other factors: The bonding strength of the adhesive (the bonding strength of the bonding agent, graphite particles, conductive carbon and the interface between the current collector), charge and discharge The magnification, the swelling property of the adhesive and electrolyte, the shape and packing density of graphite particles, and the increase in the volume of the pole piece caused by the failure of the adhesive during the cycle process, etc., all have a certain degree of influence on the expansion of the anode.

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