Lithium insertion into disordered carbons prepared from organic polymers

Abstract

Part I. Electrochemical lithium insertion into polyacrylonitrile (PAN)-based disordered carbons was studied using the techniques of discharge/charge tests, cyclic voltammetry, and $^{7}Li$ NMR spectroscopy. The PAN-based carbons were prepared by vacuum pyrolysis of polyacrylonitrile (PAN) at 500, 800, and 1000℃. They showed charge capacities between 254 and 380 mAh/g in the first cycle. $^{7}Li$ NMR spectra showed two kinds of lithium insertion sites in the PAN-based carbons: a reversible site where lithium is removed in the subsequent charge process and an irreversible site where lithium remains intact. The NMR results suggest that lithium in fully Li-inserted PAN-based carbons has an ionic character, and reversible site lithium resides between negatively charged carbon layers. Part II. Disordered carbon samples were prepared from four organic polymers: poly[(Z)-1-methoxy-4-phenyl-1-buten-3-yne] [poly(MPBEY)], poly(1,4-diphenyl-1-buten-3-yne) [poly(DPBEY)], poly[5-(2-pyridyl)-2,4-pentadiyn-1-ol] [poly(PyPDO)], and poly(2,4-hexadiyn-1,6-diol) (PHDO). Electrochemical lithium insertion into these disordered carbons was studied with discharge/charge tests, cyclic voltammetry, and $^{7}Li$ nuclear magnetic resonance (NMR) spectroscopy. In the potential range of 0.0 to 2.5 V vs. Li/$Li^+$, all carbons showed discharge/charge curves with a hysteresis effect unlike typical curves for standard lithium insertion/removal processes in pyrolyzed carbons. This hysteresis may be caused by the lithium-oxygen bonding of the organolithium complexes. $^7Li$ NMR spectra showed two kinds of lithium insertion sites in all carbons: a reversible site from which lithium could be removed in the subsequent charge process and an irreversible site where lithium remains intact. The NMR results suggest that the reversible-site lithium has ionic nature in all of the fully Li-inserted carbons.