Abstract:

To fundamentally enhance the safety of lithium-ion batteries, phosphorus-based flame retardants were added to the electrolyte, and the effects of different valence states of phosphorus-based flame retardants on safety performance and electrochemical performance were studied. The impact of phosphorus-based flame retardants on the thermal stability of the electrolyte was compared using self-extinguishing time and differential scanning calorimetry. The influence of phosphorus-based flame retardants on the basic properties of the electrolyte was analyzed through linear sweep voltammetry. Further, cyclic voltammetry tests, cycling performance tests, rate capability tests, and electrochemical impedance spectroscopy were conducted to explore the effects of varying volume ratios of phosphorus-based flame retardants on the electrochemical performance of LiFePO₄|Li half-cells. The results show that electrolytes containing 5% trimethyl phosphate (TMP) and trimethyl phosphite (TMPi) exhibit significantly reduced self-extinguishing times, with the former also expanding the electrochemical window of the electrolyte. The LiFePO₄|Li half-cell with 5% TMP demonstrates a smaller potential difference between the oxidation and reduction peaks, reduced battery polarization, and maintains a higher discharge capacity after 300 cycles, with a capacity retention rate of 99.6%. In contrast, the addition of 5% TMPi leads to a decline in discharge capacity. Methyl phosphate flame retardants exhibit weaker molecular activity and less adverse effects compared to methyl phosphite flame retardants. Under the premise of not compromising electrochemical performance, adding 5% trimethyl phosphate is more effective in improving the safety of lithium-ion batteries.

Key words: phosphorus-based flame retardants, lithium-ion batteries, electrochemical properties, self-extinguishing time (SET), frontal orbital energy