The reasons for battery deformation and expansion after cycling
Globally renowned lithium polymer battery company-JXBT
After cycling, the reasons for battery deformation and expansion:
The pole piece expansion is mainly divided into physical and chemical expansion;
1. Physical expansion:
Reason analysis of winding cell:
After being grating or cycling several weeks, the medium-and-small size winding lithium polymer battery deforms in the thickness way. After the battery is charged and discharged, due to the deformation of the material (mainly the pole piece and the isolation film), the inner layer of the cell is too small at the corner, and the stress concentration is generated along the width of the cell, which in turn causes the cell pole piece to twist and the cell to bend and deform;
2. Chemical expansion:
The chemical reason is manifested as the expansion of the pole piece;
Lithium-ion battery cell thickness is increased while charging, which is mainly due to the expansion of the negative electrode. The positive electrode expansion rate is only 2~4%, and the negative electrode is usually composed of graphite, binder, and conductive carbon. The expansion rate of the graphite material itself reaches to 10%. The main factors affecting the change of the graphite negative electrode expansion rate include: SEI film formation , State of charge (SOC) and other factors.
(a). SEI film formation: During the first charge and discharge of lithium ion batteries, the electrolyte undergoes a reduction reaction at the graphite particles at the solid-liquid interface, forming a passivation layer (SEI film) covering the surface of the electrode material. The generation of SEI film makes the anode thickness Significant increase, and due to the SEI film, the thickness of the cell increased 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 occur the dissolution of SEI and the dynamic process of new SEI production. For example, flake graphite has a greater expansion rate than spherical graphite.
(b). During the cycle of the charged state cell, the volume expansion of the graphite anode and the cell SOC have a good periodic function relationship, that is, as lithium ions are continuously embedded in the graphite (the cell SOC increases), the volume gradually expands When lithium ions are released from the graphite anode, the cell SOC gradually decreases, and the corresponding graphite anode volume gradually decreases.
(c). (3)From the perspective of the compaction density of the material, the compaction density has a greater impact on the graphite anode. During the cold pressing of the pole piece, a large compressive stress is generated in the graphite anode film layer. This stress is baked at a high temperature in the subsequent pole piece It is difficult to completely release the process. When the battery is charged and discharged cyclically, due to the interplay of lithium ion insertion and extraction, electrolyte swelling of the adhesive, and other factors, the membrane stress is released during the cycle and the expansion rate increases. On the other hand, the compaction density determines the size of the pore volume of the anode membrane layer. The pore volume in the membrane layer is large, which can effectively absorb the expanded volume of the pole piece. The pore volume is small. When the pole piece expands, there is not enough space to absorb the expansion. The generated volume, at this time, can only expand to the outside of the membrane layer, which is expressed as the volume expansion of the anode sheet.
(d). Other factors: the adhesive strength of the adhesive (adhesive strength of the interface between the adhesive, graphite particles, conductive carbon and current collector), charge and discharge rate, the swelling ability of the adhesive and the electrolyte, the shape of the graphite particles Its bulk density and the increase in volume of the pole piece caused by the failure of the adhesive during the cycle have a certain effect on the anode expansion.
Calculation of expansion rate:
(Battery thickness after expansion-Battery thickness before expansion)/Battery thickness before expansion * 100%
The expansion coefficient is the physical characteristics of the battery, and the cycle life is the electrochemical characteristic. There is no direct relationship between the two. The cycle life refers to the number of times the battery can be used repeatedly.
The test requirements for the cycle: 1C charge and discharge current, 300 cycles, the battery not leak, no fire, no explosion, and the battery capacity ≥80%, which shows qualified.
Considering the final product, if the battery expansion does not affect the use of the product (for example, the battery expansion does not cause the housing of the product to expand/deform the case), and after the cycle meets various specifications, the battery can be used normally;
