Feasibility of chemically modified cellulose nanofiber membrane as lithium ion battery separators.
Chemical modification of cellulose is beneficial to produce highly porous lithium ion battery (LIB) separators, but introduction of high charge density adversely affects its electrochemical stability in a LiNi1/3Mn1/3Co1/3O2(NMC)/graphite full cell. In this study, the influence of carboxylate functional groups in TOCN on the electrochemical performances of the lithium ion battery separator was investigated. X ray photoelectron spectroscopy (XPS) and in operando mass spectrometry measurements were used to elucidate the cause of failure of the batteries containing the TOCN separators, in the presence and absence of sodium counterions in the carboxylate groups and additives. For the TOCN separator with sodium carboxylate functional groups, it seems that Na deposition is the dominant reason for poor electrochemical stability of the cell thereof. The poor performance of the protonated TOCN separator, attributed to a high amount of gas evolution, is dramatically improved by adding 2 wt% of vinylene carbonate (VC) due to suppressed gas evolution. Unveiling the failure mechanism of the TOCN separators and successively implementing the strategies to improve performance, e.g. removing Na, adding VC, adjusting cycling rates, enable a remarkable cycling performance in the NMC/graphite full cell at 2C (3 mA/cm2) of fast discharging rate. Despite of the aforementioned efforts and compromises required, increased charge density of the TOCN is beneficial to acquire a mechanically stronger separator. In conclusion, the manufacturing process of cellulose nanofibers needs to be carefully adjusted to acquire a desired separator property. To the best of our knowledge, it is first report to perform operando gas evolution measurements to systematically investigate the electrochemical stability of nanocellulose as a LIB separator material. The results elucidate not only the challenges for extensive applications of hygroscopic biomaterials for commercial LIB, but also the practical solutions to achieve high electrochemical stability of the materials.
ACS applied materials interfaces