The first and second order Raman spectra of graphite during the first lithiation and delithiation have been investigated in a typical lithium-ion battery electrolyte. In situ, real-time Raman measurements under potential control enable the probing of the graphitic negative electrode surface region during ion insertion and extraction. The experimental results reveal the staging formation of a single particle within a free standing graphitic electrode. In particular, the in situ behaviour of the double resonance 2D band during the lithiation and delithiation of graphitic carbon has not been previously reported. The 2D band was observed to shift from 2681 to 2611 cm(-1) and the band shape transformed into a single Lorentzian from 0.24 to 0.15 V vs. Li/Li(+), providing further information on the electronic structure and C-C bonding of stage 3 and 4 graphite intercalation compounds. The behaviour of the 2D band is in keeping with the Daumas-Hérold model of electrochemically derived intercalation, where the graphene layers are flexible and deform around domains of intercalating lithium ions.

Ni-rich lithium nickel manganese cobalt (NMC) oxide cathode materials promise Li-ion batteries with increased energy density and lower cost. However, higher Ni content is accompanied by accelerated degradation and thus poor cycle lifetime, with the underlying mechanisms and their relative contributions still poorly understood. Here, we combine electrochemical analysis with surface-sensitive X-ray photoelectron and absorption spectroscopies to observe the interfacial degradation occurring in LiNi 0.8 Mn 0.1 Co 0.1 O 2 –graphite full cells over hundreds of cycles between fixed cell voltages (2.5–4.2 V). Capacity losses during the first ∼200 cycles are primarily attributable to a loss of active lithium through electrolyte reduction on the graphite anode, seen as thickening of the solid-electrolyte interphase (SEI). As a result, the cathode reaches ever-higher potentials at the end of charge, and with further cycling, a regime is entered where losses in accessible NMC capacity begin to limit cycle life. This is accompanied by accelerated transition-metal reduction at the NMC surface, thickening of the cathode electrolyte interphase, decomposition of residual lithium carbonate, and increased cell impedance. Transition-metal dissolution is also detected through increased incorporation into and thickening of the SEI, with Mn found to be initially most prevalent, while the proportion of Ni increases with cycling. The observed evolution of anode and cathode surface layers improves our understanding of the interconnected nature of the degradation occurring at each electrode and the impact on capacity retention, informing efforts to achieve a longer cycle lifetime in Ni-rich NMCs.

An in situ Raman spectroelectrochemical study of Li intercalation into graphite flakes with different thicknesses ranging from 1.7 nm (3 graphene layers) to 61 nm (ca. 178 layers) is presented. The lithiation behavior of these flakes was compared to commercial microcrystalline graphite with a typical flake thickness of ∼100 nm. Li intercalation into the graphitic flakes was observed under potential control via in situ optical microscopy and Raman spectroscopy. As graphite flakes decreased in thickness, a Raman response indicative of increased tensile strain along the graphene sheet was observed during the early stages of intercalation. A progressively negative wavenumber shift of the interior and bounding modes of the split G band (E(i) and E(b)) is interpreted as a weakening of the C-C bonding. Raman spectra of Li intercalation into thin graphitic flakes are presented and discussed in the context of implications for Li ion battery applications, given that intercalation induced strain may accelerate carbon negative electrode aging and reduce long-term cycle life.

The interface between solid electrolytes and lithium metal electrodes determines the performance of an all-solid-state battery in terms of the ability to demand high power densities and prevent the formation...