Spinel LiNi0.5Mn1.5O4 (LNMO) is an attractive next-generation cathode material for Li-ion batteries because of its reversible specific charge at high operating potentials. However, the cycling efficiency of Li-ion cells with LNMO-based cathodes is limited by the poor anodic stability of the most commonly employed alkyl carbonate electrolytes. The electrolyte/electrode stability is investigated by in situ gas analysis techniques, including cell pressure measurements and online electrochemical mass spectrometry (OEMS), to monitor the decomposition of ethylene carbonate (EC) and dimethyl carbonate (DMC) electrolytes on LNMO electrodes. Increasing the DMC content, exchanging the LiPF6 salt for LiClO4, and elevating the cell temperature, all result in higher gas evolution rates. The major volatile side reaction products are H2, CO, CO2, and POF3 (only with LiPF6 salt), which display unique gas evolution profiles depending on electrode potential and electrolyte composition. The significantly higher gas evolution rates for the DMC-rich electrolyte are attributed to an electrolyte solution-mediated decomposition cycle, which is facilitated by the enhanced mass transport induced by the lower viscosity of DMC. Differences in reactivity of the Ni cationic redox state on the LNMO surface toward electrolyte decomposition are indicated.

In order to follow the structural changes correlated to the evolution of the lithium content in high voltage battery systems (based on a disordered LiNi 0.5 Mn 1.5 O 4 (d-LNMO) and a graphite electrode), we developed a new cylindrical cell suitable for operando neutron diffraction measurements. The cell, containing two grams of electroactive materials, is able to cycle at a fast rate (1C) with reliable electrochemical performance. The operando neutron diffraction measurements revealed the evolution of the lattice parameters of both the d-LNMO and graphite phases, notably showing the transitions between graphite lithiation stages. Furthermore, as a result of Rietveld refinements, the lithium consumption could be attributed mainly to the formation of a solid electrolyte interphase (SEI) layer on the graphite surface. This approach provides important insights helping to optimize the loading of the electroactive materials in batteries, especially for high voltage systems in which side reactions and lithium consumption can occur during cycling.

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An optimized cylindrical cell (LiNi0.5Mn1.5O4versus graphite) for operando neutron diffraction investigation during the first cycle and long-term cycling (100 cycles).