On the stability of Li intercalated fine-grained graphitic material

In order to study the performance, cycling stability and safety of electrochemical storage devices, various characterisation methods are applied. Magic angle spinning (MAS) NMR enables ex situ studies of electrochemical reactions with high resolution. In the present work, we follow the changes that occur in the MAS NMR spectra of lithiated graphitic materials after storage. The measuring time has been significantly shortened by using fine-grained graphitic material with 65% C-13-isotope enrichment. Our results show that for C-13 graphitic electrode material with a low lithiation degree, visible changes in the C-13 NMR spectra are observed already in few hours, whereas the samples with a higher lithiation degree demonstrated greater stability even after several days. We analyzed the deintercalation behavior in these materials using Li-7 MAS NMR and oxidation processes using in situ X-ray photoelectron spectroscopy. During storage Li ions migrate to the other interlayers (redistribution), large pores and to the surface, where interaction with air takes place. Effects of charge transfer from Li to graphite across four graphitic layers guarantees that C-13 NMR spectra are less affected by these effects. The key finding is that special care has to be paid to the ex situ characterisation of electrochemically prepared graphitized materials. (C) 2020 Elsevier Ltd. All rights reserved.