Lithium‐7 Nuclear Magnetic Resonance Investigation of Lithium Insertion in Hard Carbon

Abstract

Lithium battery anodes based on disordered, hard carbon were prepared by low-temperature (1000°C) pyrolysis of cotton cloth. Samples were lithiated in an electrochemical cell. The Li insertion (potential vs. capacity) curve exhibits two different regions: a sloping one, from 1.1 to 0.1 V (vs. Li/Li) denoted as the high-voltage region (HVR), and a plateau between 0.1 and zero V, denoted as the low-voltage plateau (LVP). Lithium-7 high-resolution (magic angle spinning) nuclear magnetic resonance measurements in fully lithiated samples reveal three main features: a broad line at ca. 50 ppm, a relatively sharp line at 17 ppm, with a shoulder at about zero ppm (all shifts relative to aqueous LiC1). The 50 ppm component is attributed to Li intercalated between turbostratically disordered graphene planes and is associated with the LVP part of the potential curve. The 17 ppm signal arises from a Li site which resides in amorphous hydrogen-containing regions of the carbon and is correlated with the HVR part of the curve. The zero ppm component is attributed to the irreversible portion of the Li (up to -20% of the total) which constitutes the solid electrolyte interphase on the surface of the carbon grains formed by electrochemical reduction of the electrolyte. These spectral assignments were verified by running other samples which were electrochemically delithiated to varying degrees. InfroductionLithiated carbons play a key role in rechargeable lithium battery technology.1-4 Much current activity concerns the effect of carbon structure and morphology on reversible lithium capacity, in order to better understand the factors leading to improvement in overall cell capacity as well as cycle life. Commercial lithium ion batteries employ either highly graphitic or disordered carbon as an anode host material. Considerable attention is being devoted to increasing the capacity of various disordered carbons. Excess Li capacity has been correlated with both disorder5-7 and the presence of hydrogen8'° in the carbon. Recent studies suggest covalent bonding of Li to "edge" carbons found in voids and other termination sites.'7"2 The subject of the current study is a particular form of carbon consisting of relatively small ordered regions dispersed in a highly disordered matrix containing residual hydrogen. This material, hereafter referred to as hard carbon, cannot be pyrolyzed to graphite at high temperature.Nuclear magnetic resonance (NMR) is a powerful analyt-

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