Origin of H2 Evolution in LIBs: H2O Reduction vs. Electrolyte Oxidation

Gassing in lithium-ion batteries (LIBs) is a serious challenge, especially at high voltage and elevated temperature. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) and a two-compartment cell with a newly developed aluminum edge-seal to elucidate the origin of H2 evolution in LIBs. We demonstrate that the new sealing is entirely impermeable for gaseous and liquid species, thus allowing us to measure the true H2 evolution from H2O reduction at a graphite electrode, without interference from the lithium counter-electrode. We further report that graphite//NMC full-cells without any diffusion barrier between anode and cathode show enhanced H2 generation, especially for high charging potentials and at elevated temperature. We propose that the diffusion of protic electrolyte oxidation species (R-H+) from the cathode to the anode and their subsequent reduction is the origin of enhanced H2 gassing. To prove this hypothesis, methanesulfonic acid is added to the electrolyte as a chemical source of protons. At the negative graphite electrode, all H+ can be quantitatively reduced to H2. By the use of the electrolyte additives vinylene carbonate (VC) and lithium bis(oxalato) borate (LiBOB), less H2 evolution is observed, since the reduction of both H2O and R-H+ is hindered by a more effective SEI on graphite. Finally, we demonstrate that the Al-sealed diffusion barrier between anode and cathode can stop the diffusion of oxidation products to the anode and therefore essentially eliminates the generation of H2 caused by high cathode potentials.