Lithium Diffusion in Graphitic Carbon

Graphitic carbon is currently considered the state-of-the-artmaterial for the negative electrode in lithium ion cells, mainly due to its high reversibility and low operating potential. However, carbon anodes exhibit mediocre charge/ discharge rate performance, which contributes to severe transport-induced surface structural damage upon prolonged cycling and limits the lifetime of the cell. Lithium bulk diffusion in graphitic carbon is not yet completely understood, partly due to the complexity of measuring bulk transport properties in finite-sized nonisotropic particles. To solve this problem for graphite, we use the DevanathanStachurski electrochemical methodology combined with ab initio computations to deconvolute and quantify the mechanism of lithium ion diffusion in highly oriented pyrolytic graphite (HOPG). The results reveal inherent high lithium ion diffusivity in the direction parallel to the graphene plane (∼10-10 cm s), as compared to sluggish lithium ion transport alonggrainboundaries (∼10 cm s), indicating the possibility of rational design of carbonaceous materials and composite electrodes with very high rate capability. SECTION Energy Conversion and Storage W hile commercial lithium ion batteries can consist of multiple cathode chemistries, a vast majority of them use graphitic carbon as the negativeelectrode material because of its low cost, low operating potential, high capacity, high reversibility, and remarkable structural and interfacial stability. The use of graphitic electrodes as ion-intercalation negative-electrode hosts for rechargeable electrochemical power sources was suggested first by Rudorff and Hofmann in 1938, and many scientists have subsequently investigated them. Lithium diffusion in graphitic carbon is not yet completely understood due to a lack of reliable theoretical and experimental methods. Impedance spectroscopy, potentiostatic intermittent titration technique (PITT), and standard electrochemical methods have been used to gauge the diffusion coefficient in different types of graphitized carbons in composite electrodes and to determine their overall rate performance in lithium ion systems. However, the electrochemical response of a composite electrode consists of multiple components and, in principle, fails to resolve the highly inherent anisotropic nature of lithium diffusion in graphitic carbon. Thus, the basic electrochemical properties of graphite becomes convoluted with the parameters of mass and charge transfer involving particle contact resistances, surface films, and side reactions. Moreover, the analysis of the experimental data is extremely complicated, and consequently, the lithium ion transport rates reported for various types of composite graphite electrode architectures vary in the literature from 10 to 10 cm s. In this Letter, we present a combination of electrochemical measurements of lithium ion permeation and first-principles calculations to clarify and quantify lithium ion diffusion in HOPG. The objective of this work is to (i) determine the diffusion paths and lithium ion transport parameters in graphite and (ii) provide rational guidelines for design and synthesis of high-rate graphitic materials. In order to directly measure Li diffusion in graphite, a HOPG foil was used as a membrane in a DevanathanStachurski-type two-compartment cell. The HOPG used was made of single-crystal graphitic cuboids (i.e., graphene domains) with an angular spread of the c-axes of the crystallites of less than 1 . The graphene basal planes are exposed at the surface of the HOPG foil, whereas the plane edges are at the foil perimeters. The HOPG membrane served as a common working electrode for both compartments “A” and “B” (see Figures 1 and2), whichwere filledwith 1.2MLiPF6 inEC/ EMC (1:1) electrolyte and equipped with two sets of metallic lithium reference and counter electrodes. Further details of the materials and cell setup are described in the Supporting Received Date: February 10, 2010 Accepted Date: March 16, 2010