Gas Evolution in LiNi_0.5Mn_1.5O₄/Graphite Cells Studied In Operando by a Combination of Differential Electrochemical Mass Spectrometry, Neutron Imaging, and Pressure Measurements

The cycling performance and in operando gas analysis of LiNi_0.5Mn_1.5O₄ (LNMO)/graphite cells with reasonably high loading, containing a “standard” carbonate-based electrolyte is reported. The gas evolution over the first couple of cycles was thoroughly investigated via differential electrochemical mass spectrometry (DEMS), neutron imaging and pressure measurements. The main oxidation and reduction products were identified as CO₂, H₂ and C₂H₄. In different sets of experiments graphite was substituted with delithiated LiFePO₄ (LFP) and LNMO with LFP to distinguish between processes occurring at either anode or cathode and gain mechanistic insights. Both C₂H₄ and H₂ were found to be mainly formed at the anode side, while CO₂ is generated at the cathode. The results from DEMS analysis further suggest that the Ni redox couples play a profound role in the evolution of CO₂ at the LNMO/electrolyte interface. Lastly, it is shown that the cycling stability and capacity retention of LNMO/graphite cells can be considerably improved by a simple cell formation procedure.