Effects of hexamethylenetetramine as an electrolyte additive on elevated temperature performance of graphite/LiMn₂O₄ batteries

Spinel LiMn₂O₄-based lithium-ion batteries are widely applied in electric two-wheeler and low-speed vehicles due to their low cost and low toxicity. Nevertheless, the Mn deposition at the anode originated from Mn dissolution of LiMn₂O₄ causes a poor elevated temperature cycle life of graphite/LiMn₂O₄ batteries. Herein, a graphite/LiFePO₄ cell with specific Mn²⁺ added in the electrolyte is provided as an efficient evaluation method to research Mn²⁺ deposition. Besides, an effective strategy by adding a ligand hexamethylenetetramine (HMTA) as an electrolyte additive is proposed to prevent Mn²⁺ depositing. HMTA effectively “locks in” Mn²⁺ through strong coordination, as confirmed by Raman spectroscopy and density functional theory (DFT) calculations. Moreover, the CV and XPS results indicate that the adsorption of HMTA is able to significantly inhibit side reactions at the anode/electrolyte interface. By adopting this strategy, the capacity retention of graphite/LiMn₂O₄ cell is enhanced from 68.9% to 79.0% at 0.5 C after 100 cycles under 60 °C when adding 0.5% HMTA additive into the blank electrolyte. This work provides a new insight into revealing the mechanism of Mn²⁺ regulation by introducing HMTA and improving the electrochemical performance of graphite/LiMn₂O₄ batteries at elevated temperature.