Disordered Carbon Anode for Lithium-Ion Battery I. An Interfacial Reversible Redox Action and Anomalous Topology Changes

The authors investigated mechanisms for an interfacial reversible surface redox action and a solid electrolyte interphase (SEI) formation based on LiF with surface topology changes for a disordered carbon electrode in LiPF6/propylene carbonate-based electrolyte. The electrochemical reaction mechanism that delivers a large capacity was found to occur differently in two potential regions, region A (1.4 to 0.4 V) and B (0.4 to 0.0 V). During the reversible process in region A, a lithium alkoxide (C-OLi) is formed on the carbonyl edge planes as solvated lithium complex of the C-O-Li+(solv.)n (n = 1, 2, or 3). The formation of the lithium alkoxide traps solvents on the edge surfaces preventing further intrusion into stacked graphene sheets, that in turn suppresses a exfoliation and stabilized repeated redox cycles. In region A, a Li2CO3 and a LiF are formed as an SEI film by the reduction of solvents and LiPF6 making a smooth surface. region B involves three parallel processes, viz., a lithium intercalation into graphene layers, a lithium metal cluster deposition at nanosized voids or pores, and a formation of LiF by disproportionation of the C-OLi formed at region A. Since the LiF is produced rapidly by a proposed repeated cycle, the electrode surface generates a rough anomalous topology.