Effects of electrolyte-volume-to-electrode-area ratio on redox behaviors of graphite anodes for lithium-ion batteries

This study reports on the crucial effects of the ratio of electrolyte volume to electrode area on redox reactions of graphite anodes for lithium-ion batteries (LIBs). It is found that a pyrolytic graphite edge plane electrode (PGE) exhibits reversible Li+ transport behavior in LiClO4 solution, but shows a suppressed redox reaction in LiPF6 electrolyte. The reversible Li+ transport in LiClO4 is hampered by the presence of trace HF, while the depressed activity in LiPF6 is improved by adding an HF scavenger, which points to the suppressive effects of HF on the redox behavior of PGE. In addition, the redox activity of PGE in LiPF6 solution is either enhanced by decreasing electrolyte volume or diminished by decreasing surface area. It is also revealed that graphite composite electrodes of practical LIBs are also subject to the influence of HF if an excessive amount of electrolyte is employed. These observations are attributed to the HF-induced formation of a passive LiF layer, which is facilitated at high ratio of electrolyte volume to electrode area. Our results clearly demonstrate that HF impurity significantly suppresses the redox activity of graphite anodes, and that the adverse HF influence becomes predominant at a high electrolyte-to-electrode ratio.