Origin of Excess Irreversible Capacity in Lithium-Ion Batteries Based on Carbon Nanostructures

Recent reports on high capacity lithium ion batteries based on carbon nanostructures aroused expectations of realizing high energy density devices. We have studied the performances of a wide variety of carbon nanostructures with surface areas from a few up to 1400 m(2)/g as anode materials in Li-ion batteries by using three different experimental setups aiming to clarify the origin of high capacities. The obtained charge values consumed in the initial intercalation/deintercalation cycles of Li ions for high surface area nanostructures indeed correspond to capacities that exceed the theoretical limit for pristine graphite (372 Al/kg; as LiC6) up to a factor of six. Yet, typically these large excess capacity values were irreversibly diminished during further charge/discharge cycling. Density functional theory (DFT) calculations reveal a decisive role of edge carbon atoms in high surface nanostructures as active sites that contribute not only to an initial high capacity, but to the formation of a solid-electrolyte interphase and thereby to the irreversible capacity loss (ICL). These results question the feasibility of stable large excess Li capacity values in studied carbon nanostructures, yet suggest the design of nanostructures for reducing the ICL. (C) 2015 The Electrochemical Society. All rights reserved.