Stabilising Ni-rich cathode interfaces with Li-ion batteries

The cathode-electrolyte interphase (CEI) in Li-ion batteries plays a key role in suppressing undesired side reactions while facilitating Li-ion transport.  Ni-rich layered cathode materials offer improved energy densities, but their high interfacial reactivities can negatively impact the cycle life and rate performance. 

In the paper 'Role of salt concentration in stablising charged Ni-rich cathode interfaces in Li-ion batteries', published in Chemistry of Materials and authored by this department, The Faraday Institution, University of Cambridge, Diamond Light Source and the Research Centre at Harwell, the role of electrolyte salt concentration was investigated.  Focus was given to LiPF₆ (0.5-5m) in altering the interfacial reactivity of charged LiN_0.8Mn_0.1Co_0.1O₂ ((NMC811) cathodes in standard carbonate-based electrolytes (EC/EMC vol %/vol % 3.7).   Extended potential holds of NMC811/Li₄Ti₅O₁₂ (LTO) cells revealed that the parasitic electrolyte oxidation currents observed were strongly dependent on the electrolyte salt concentration.

X-ray photoelectron and absorption spectroscopy (XPS/XAS) revealed that a thicker Li_xPO_yF_z-/LiF-rich CEI was formed in the higher concentration electrolytes, which suppressed reactions with solvent molecules - this resulted in a thinner, or less-dense, reduced surface layer (RSL) with lower charge transfer resistance and lower oxidation currents at high potentials.  The thicker CEI also limited access of acidic species to the RSL suppressing transition-metal dissolution into the electrolyte, as confirmed by nuclear magnetic resonance (NMR) spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES). 

The above provides insight into the main degradation processes occurring at Ni-rich cathode interfaces in contact with carbonate-based electrolytes and how electrolyte formulation can help to mitigate them.