Feasibility\nof Chemically Modified Cellulose Nanofiber\nMembranes as Lithium-Ion Battery Separators

Abstract

Chemical modification of cellulose\nis beneficial to produce highly\nporous lithium-ion battery (LIB) separators, but introduction of high\ncharge density adversely affects its electrochemical stability in\na LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)/graphite\nfull cell. In this study, the influence of carboxylate functional\ngroups in 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidized cellulose\nnanofibers (TOCNs) on the electrochemical performances of the LIB\nseparator was investigated. X-ray photoelectron spectroscopy and in\noperando mass spectrometry measurements were used to elucidate the\ncause of failure of the batteries containing TOCN separators in the\npresence and absence of sodium counterions in the carboxylate groups\nand additives. For the TOCN separator with sodium carboxylate functional\ngroups, it seems that Na deposition is the dominant reason for poor\nelectrochemical stability of the cell thereof. The poor performance\nof the protonated TOCN separator, attributed to a high amount of gas\nevolution, is dramatically improved by adding 2 wt % of vinylene carbonate\n(VC) because of suppressed gas evolution. Unveiling the failure mechanism\nof the TOCN separators and successively implementing the strategies\nto improve performance, for example, removing Na, adding VC, and adjusting\ncycling rates, enable a remarkable cycling performance in the NMC/graphite\nfull cell at ≈2 C (3 mA/cm²) of a fast discharging\nrate. Despite the aforementioned efforts and compromises required,\nan increased charge density of the TOCN is beneficial to acquire a\nmechanically stronger separator. In conclusion, the manufacturing\nprocess of cellulose nanofibers needs to be carefully adjusted to\nacquire a desired separator property. To the best of our knowledge,\nit is first reported to perform operando gas evolution measurements\nto systematically investigate the electrochemical stability of nanocellulose\nas an LIB separator material. The results elucidate not only the challenges\nfor extensive applications of hygroscopic biomaterials for commercial\nLIBs but also the practical solutions to achieve high electrochemical\nstability of the materials.