Origin of H 2 Evolution in LIBs: H 2 O 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 H 2 evolution in LIBs. We demonstrate that the new sealing is entirely impermeable for gaseous and liquid species, thus allowing us to measure the true H 2 evolution from H 2 O 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 H 2 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 H 2 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 H 2 . By the use of the electrolyte additives vinylene carbonate (VC) and lithium bis(oxalato) borate (LiBOB), less H 2 evolution is observed, since the reduction of both H 2 O 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 H 2 caused by high cathode potentials.