The Effect of VC Content in Electrolyte on Lithium Iron Phosphate

Vinylene Carbonate (VC) has excellent adaptability to both high and low temperatures. Adding VC to the electrolyte can help form a more stable SEI layer on the surface of the anode in lithium batteries. Therefore, the content of VC has a significant impact on the performance of lithium iron phosphate (LiFePO₄) batteries. This paper specifically studies the effect of VC content on the electrochemical performance of LiFePO₄ batteries under low-temperature conditions. LiFePO₄ batteries are widely used in new energy vehicles and energy storage systems due to their high safety, long cycle life, environmental friendliness, and relatively low cost. However, they face issues such as discharge capacity decay and reduced energy retention at low temperatures (especially below -20°C), which limits their use in cold regions. To address this issue, functional additives like VC are often added to the electrolyte to improve low-temperature ionic conductivity, suppress side reactions, and stabilize the electrode interface, thereby enhancing the battery's low-temperature performance.

In this study, four kinds of LiFePO₄ battery electrolytes with different VC contents (3.0%, 3.2%, 3.5%, 3.8%) were prepared and assembled into pouch cells. The cathode used LiFePO₄, and the anode used artificial graphite. All batteries were prepared under the same process conditions and underwent formation and capacity dispersion. Electrochemical tests were conducted on each group of batteries at -30°C to systematically compare key performance indicators such as low-temperature discharge capacity, direct current resistance (DCR), energy efficiency, and cycle life.

1. Low-Temperature Discharge Capacity

By testing the CV curves of the battery under low-temperature conditions and calculating the battery capacity from the integrated area of the CV curves.

When the VC content is 3.5%, the battery shows the highest discharge capacity at -30°C (5.6 Ah), significantly better than other groups (3.0%, 3.2%, and 3.8% correspond to 4.6 Ah, 5.0 Ah, and 5.0 Ah, respectively). An appropriate amount of VC can lower the freezing point of the electrolyte, increase ion mobility, and suppress solvent crystallization, thereby improving low-temperature discharge performance.

2. Direct Current Resistance (DCR)

DCR tests of the battery show that when the VC content is 3.5%, the DCR is lowest (0.76 mΩ), indicating lower interfacial resistance and higher lithium-ion transport efficiency. Excessive VC content (e.g., 3.8%) leads to an increase in DCR, suggesting that too much VC may increase interfacial reaction resistance.

3. Low-Temperature Energy Efficiency

When the VC content is 3.5%, the battery exhibits the highest energy efficiency at -30°C (82.0%), surpassing other groups (72.5%~79.0%). This is attributed to VC promoting the formation of a stable SEI film, reducing polarization losses, and enhancing energy conversion efficiency.

4. Cycle Life

After 300 cycles, the battery with 3.5% VC content shows the highest capacity retention (97.5%), outperforming other groups (90.0%~96.1%). VC can dynamically repair SEI film defects during cycling, inhibit electrolyte decomposition and gas generation, thereby extending the battery's cycle life at low temperatures.

ACEY-BA3040-20 battery cycle test equipment  is used to test the lifespan, reliability, capacity and other parameters of the battery pack through cyclic charge and discharge test.

• VC preferentially reduces on the graphite anode surface before the solvent molecules during the initial charging and discharging process, forming a dense SEI film rich in inorganic components such as Li₂CO₃. This film has a relatively high lithium-ion diffusion coefficient.

• An appropriate amount of VC can optimize the solvation structure of the electrolyte, reduce viscosity, and improve ionic conductivity at low temperatures.

• Excessive VC may lead to an overly thick SEI film or increased impedance, which is actually detrimental to performance improvement.

Although the article conducted experiments comparing different amounts of VC and performed certain tests, identifying the optimal VC content in this experiment, it did not include a control group without VC. Therefore, it cannot be concluded that adding VC necessarily improves the battery's performance.