The (electro)chemical reactions at the interface between the oxide electrode and the organic electrolyte play an important role in limiting the capacity retention and the cycle life of Li-ion batteries. Understanding these reactions and the governing physical properties is key for improving battery performance and safety. Here we use a combination of computational and experimental techniques to understand the (electro)chemical reactions leading to the formation of the electrode-electrolyte interface (EEI) on the positive electrode in Li-ion batteries. We used DFT calculations to compute the energetics of the initial stages of ethylene carbonate (EC) decomposition on the surface of transition metal layered oxides (LixMO2; M = Mn, Fe, Co and Ni; x = 1, 0.5, 0). We found that EC dissociation, resulting in a proton transfer to the oxide surface and the reduction of surface transition metals, is energetically more favorable than electrophilic attack, nucleophilic attack and EC dissociation leaving behind an oxygen vacancy in the oxide. The dissociation reaction becomes more favorable as the Fermi level is lowered into the oxide O(2p) states, in particular by changing the transition metal from left to right in the periodic table and by decreasing the Li content. These findings were compared with the EEI components identified by infrared spectroscopy, Raman spectroscopy and X-ray photoemission spectroscopy on binder-free and carbon-free LiCoO2, LiNi1/3Mn1/3Co1/3O2 (NMC) and Ni-rich NMC electrodes. The experimental data confirm the enhanced reactivity towards the electrolyte for Ni-rich materials and for electrodes charged at high potential, i.e. highly delithiated. A reaction mechanism is proposed to explain the trends in reactivity of solvent molecules and salt ions at the oxide surfaces.

Reactivity descriptor for the oxide-electrolyte interface on positive electrodes in Li-ion batteries

256th National Meeting (2018)

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