Abstract:

LiFePO₄ batteries are widely used in the field of energy storage because of their safety. The test object was a soft-pack LiFePO₄ LFP battery with a rated capacity of 21 Ah that was float-charged at high voltages of 4.05 V, 4.25 V, 4.50 V, and 5.0 V for 24 h at 25 ℃. The high-temperature thermal runaway and battery material thermal stability were investigated. The results show that bulging occurs at voltages of 4.25 V, 4.50 V, and 5.0 V, and the bulging increases as the voltage increases. At the 5.0 V battery rupture, the anode active material was dissolved, the copper current collector was exposed, and a substantial amount of lithium was deposited simultaneously. In the high-temperature thermal runaway test after float charging at 4.05 V, 4.25 V, and 4.50 V, the battery's rupture temperature decreased as the voltage increased. The thermal runaway-triggering temperature increased from 249.86 ℃ to 278.65 ℃, and the early rupture-released energy raised the thermal runaway trigger temperature; however, the system was unsafe. The maximum temperature of the thermal runaway increased from 484.67 ℃ to 516.08 ℃, the maximum temperature rise rate also increased significantly, and the time of thermal runaway to the maximum temperature decreased. The battery's thermal stability deteriorated after high-voltage float-charging, and the thermal runaway became more severe. The separator begins to undergo a phase transition at 120.63 ℃ and begins to decompose at 367.06 ℃. However, the positive and negative electrodes did not decompose and had good thermal stability. Therefore, the float voltage must be precisely controlled to make the battery safe and stable to use.

Key words: LiFePO₄ battery, high voltage float charge, thermal runaway, material thermal stability, safety