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.