Inhomogeneity and relaxation phenomena in the graphite anode of a lithium-ion battery probed by in situ neutron diffraction

In this study, lithium gradients forming in the graphite anode of a commercial 18650-type lithium-ion battery during discharge and the associated relaxation processes after discharge were monitored by neutron diffraction. The experiments reveal the coexistence of several Li1-xC6 phases with different lithium contents during discharge, which can be explained by the formation of an inhomogeneity or lithium gradient in the graphite anode. The observed inhomogeneity is more pronounced at higher discharging rates, but at low temperatures it appears at a rate as low as C/10. After discharge, the inhomogeneity gradually disappears and the coexisting phases diminish in favor of one or several Li1-xC6 phases with close to mean lithium content. At room temperature these relaxation processes take 20-40 min with the main changes in the first 10 min. In contrast, at −20 °C changes are still observed after 11 h. The observed phenomena can be explained by a faster delithiation of the graphite particles than the equilibration of the resulting lithium gradient by lithium diffusion in the solid phase during discharge.