Correlation of the irreversible lithium capacity with the active surface area of modified carbons

Negative electrodes of lithium–ion batteries are generally based on graphite. Higher storage capacities can be obtained with disordered carbons, however they demonstrate a noticeable hysteresis and irreversibility, which can preclude a practical application. In this paper, the main parameters which may affect the irreversible capacity are analyzed and we show that the irreversible lithium consumption which occurs at the negative carbon electrode during the first charge (Cirr) is proportional to the active surface area (ASA). Composites with a reduced ASA have been obtained after coating a hard carbon or milled graphite samples with a thin film of pyrolytic carbon. Deactivating the surface by pyrolytic carbon deposition allows the irreversible capacity to be noticeably reduced, being lower than in graphite, while the reversible capacity is 50% higher than in graphite. The electrochemical properties of this new C/C composite are investigated by galvanostatic cycling at various current densities and by impedance spectroscopy. The main effect of the dense carbon coating is to hinder the diffusion of solvated lithium ions to the active sites of the carbon host during the first discharge, giving rise to a moderate development of the Solid Electrolyte Interphase (SEI).