In situ7Li nuclear magnetic resonance observation of the electrochemical intercalation of lithium in graphite: second cycle analysis

Graphite is used in the lithium-ion batteries as a negative electrode. We use continuous, in situ, operando 7Li nuclear magnetic resonance (NMR) to show, in real time, the progressive intercalation and de-intercalation of lithium in graphite when a battery is charged and discharged. We obtain all the Li-graphite intercalation compound stages through an electrochemical path. We explain the overvoltages by transient entropic and Peltier effects. The sample is a plastic cell, NMR compatible, made of commercial graphite, commercial electrolyte and lithium metal foil. We analyze the NMR characteristics of the Li-GIC stages: line shift, quadrupolar frequencies, line width (Li diffusion), line intensity and area as a function of x = Li/C6. This allows us to estimate the lithium quantities in each stage at each step. Two facts differ from the theoretical stage n formation: for the C/20 cycling rate, we find an hysteresis in the filling/emptying of the dilute (LiC9n) stages, and we find another NMR line synchronous with LiC6. The lithium metal line also provides quantitative information on the lithium deposited as dendrites when x diminishes, in de-intercalation. This paper presents experimental NMR data over two cycles, and is an extension of the first cycle analysis published earlier.