Irreversible Capacity Loss of Li-Ion Batteries Cycled at Low Temperature Due to an Untypical Layer Hindering Li Diffusion into Graphite Electrode

This paper deals with the occurrence of a graphite irreversible degradation mechanism in commercial Graphite (C) / lithium Nickel Manganese Cobalt oxide (NMC) lithium-ion batteries, challenging metallic lithium deposition as the major aging mechanism at low temperature cycling. In this study, commercial 16 Ah C/NMC Li-ion cells were aged during cycling at 5°C at a rate of 1C between 2.7 V and 4.2 V (namely between 0 and 100% of state of charge (SOC), respectively), with significant performance fading after 50 cycles only, while up to 4000 cycles can be performed at 45°C with the same commercial cells. The monitoring of the potential of each electrode during cycling has been performed through the successful introduction of lithium metal as reference electrode into the commercial cell. This technique demonstrated that it was more and more difficult to extract lithium ions from graphite particles to intercalate into the positive electrode as the number of cycles increased. Graphite electrodes remained unexpectedly lithiated after cells were dismantled in discharged state. A part of exchangeable lithium detected being trapped into the negative electrode as graphite intercalation compounds was observed with X-Ray Diffraction (XRD). Lithium-7 Nuclear Magnetic Resonance ( 7 Li NMR) performed on graphite electrode led to the distinction between lithium intercalated into graphite, oxidized lithium in the Solid Electrolyte Interphase (SEI) and metallic lithium present in low amounts. Coupling Focused Ion Beam (FIB) / Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Photoelectron Spectrometry (XPS) techniques demonstrated the presence of an untypical layer composed of electrolyte degradation products, hindering graphite electrode pores, particularly concentrated in the regions corresponding to interparticle cavities where lithium was found enriched and trapped.