Analysis of the Interphase on Carbon Black Formed in High Voltage Batteries

Abstract

Carbon black (CB) additives commonly used to increase the electrical conductivity of electrodes in Li-ion batteries are generally believed to be electrochemically inert additives in cathodes. Decomposition of electrolyte in the surface region of CB in Li-ion cells at high voltages up to 4.9 V is here studied using electrochemical measurements as well as structural and surface characterizations. LiPF 6 and LiClO 4 dissolved in ethylene carbonate:diethylene carbonate (1:1) were used as the electrolyte to study irreversible charge capacity of CB cathodes when cycled between 4.9 V and 2.5 V. Synchrotron-based soft X-ray photoelectron spectroscopy (SOXPES) results revealed spontaneous partial decomposition of the electrolytes on the CB electrode, without applying external current or voltage. Depth profile analysis of the electrolyte/cathode interphase indicated that the concentration of decomposed species is highest at the outermost surface of the CB. It is concluded that carboxylate and carbonate bonds (originating from solvent decomposition) and LiF (when LiPF 6 was used) take part in the formation of the decomposed species. Electrochemical impedance spectroscopy measurements and transmission electron microscopy results, however, did not show formation of a dense surface layer on CB particles. The growth of earth's population with concomitant increase in energy consumption require development of renewable energy conversion technologies coupled with advanced energy storage systems like lithium batteries.1,2 In order to increase the power density in Li-ion batteries, much research is focused on developing cathode materials that can operate at high voltages (above 4.5 V vs. Li/Li + ) with a high capacity, high cycling stability, and good rate capability.3-5 However, at high voltages, all the components of positive electrodes including the Al current collector, polymer binders, conductive additives, and other possible additives have an increased risk of degradation. In addition, one of the main issues with high voltage batteries is the instability of common aprotic electrolytes at voltage above 4.5 V. 6,7 The stability of the electrolyte/cathode interphase is related to the chemistry of electrolyte solvents and salts and also to the chemistry of the components of the cathode.Carbon black (CB) additives are one of the main constituents of cathodes, added to increase the electrical percolation and thus the electronic conductivity. 8,9 Though the weight percentage of CB in commercial batteries is generally very small, it composes a rather large part of the internal surface area of a cathode due to its small particle size (≈50 nm), low density, and high surface area. CBs are generally thought of being an electrochemically inert additive in cathodes, but few studies have investigated the role of CBs at high voltages and have indicated that CBs exhibit irreversible electrochemical reactions resulting in appreciable irreversible charge capacities. [10][11][12][13][14][15][16][17][18] This charge capacity is attributed to oxid...

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