The contrast between wet and dry areas
- Dry area: that is, the area where the electrolyte is insufficiently wetted; the lithium ion transmission path in this area is blocked, and the internal resistance will be higher than other areas, resulting in local overheating.
- Wet area: the electrolyte-rich area; the side reactions between excess electrolyte and active substances will intensify, causing the battery to produce gas (such as CO2, CH4) and expand, which will cause the shell to deform, etc.
- Experimental data show that when the electrolyte surface density difference in a region exceeds 10%, the capacity retention rate will drop by 15% after the battery is cycled 300 times. When the electrolyte is evenly distributed, the cycle drop is only 5% to 8%.
Chain reaction of insufficient electrolyte wetting
- The diaphragm micropores that are not completely soaked in electrolyte will lead to uneven lithium ion transmission rate, resulting in “ion channel blockage”.
- Inadequate wetting can lead to local current density differences within the battery of up to 3 times, thereby accelerating the growth of lithium dendrites.
- Insufficient infiltration of the negative electrode electrolyte will lead to obstruction of lithium insertion and even serious lithium precipitation, resulting in a decrease in battery capacity and deterioration of electrical performance.
“Wetting hysteresis” effect at the edge of the electrode
- Due to the high structural stress at the edge of the core, the electrolyte penetration rate is 20%~30% slower than that in the central area, forming an annular liquid-lean zone.
- The formation of a lean zone will cause the electrical performance in this area to deteriorate and gradually expand to the normal area, and the battery performance will decay sharply in the later stage.
The hidden threat of trapped bubbles
- Insufficient electrolyte infiltration will cause bubbles to remain in some areas inside the battery. The bubbles will occupy the pore space, resulting in a decrease in the effective liquid injection volume, affecting the final battery liquid retention volume.
- Residual bubbles will also expand at high temperatures and cause interface delamination.
Dual effects of viscosity and temperature
- The viscosity of the electrolyte and the temperature during injection are also important performance factors.
- The viscosity of the electrolyte changes significantly with temperature (for example, the viscosity of a certain electrolyte is 1.5 mPa·s at 25°C, but rises to 8 mPa·s at -10°C). Low-temperature injection is prone to wall adhesion and agglomeration.