Mass Spectrometric Study on Volatilization Behavior of Electrolyte Solvents of Lithium-Ion Batteries

For the distillation separation of organic solvents from lithium-ion battery (LIB) cells and their recycling, the volatilization behavior characteristics of lithium hexafluorophosphate (LiPF6) with several types of alkyl carbonates in open cells were studied using mass spectrometry from 296 to 773 K. Similar volatilization behaviors were observed for three different dialkyl carbonates, i.e., dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and diethyl carbonate (DEC). When the electrolytic solution from single dialkyl carbonate with LiPF6 was heated, solution mass loss occurred in three steps. In the first step near the boiling points of the dialkyl carbonates, it is expected that dialkyl carbonates (with trace POF3 for DMC and EMC) can be recovered. The residues after heating to 773 K contained LiF and Li3PO4. For electrolytic solution from mixed alkyl carbonates with LiPF6, dialkyl carbonates with higher volatilities could be recovered near their boiling points (with trace POF3 for EMC + ethylene carbonate (EC)). EC with lower volatilities than the dialkyl carbonates could be recovered at higher temperatures near the EC boiling point with trace PF5 and POF3. The residues after heating to 773 K contained LiF and Li3PO4. With moisture, POF3 generation, which must accompany hydrofluoric-acid generation, was observed from a lower temperature than that without moisture. Moisture also affected the chemical form of the residues. Lithium salts, such as Li4P2O7, LiPO3, LiF, or Li3PO4 were observed. The presence of water should be avoided when recovered electrolytic solution from LIBs is separated and recycled by distillation. Oxygen in air does not affect the volatilization behavior of LIB electrolytic solutions. These insights will be of practical importance to consider electrolytic solution recovery from LIB cells and its recycling by distillation without incineration to avoid damage by hydrofluoric-acid generation.