Structure and Surface Chemistry Optimization of Graphite Electrodes for High-Voltage Dual-Intercalation Batteries
Authors: Dyatkin Boris , Read Jeffrey A. 
Published: February 2020

Existing standalone grid-level power systems rely on inefficient operation of diesel generators or expensive traditional battery technologies. Dual-intercalation batteries, which rely on high-voltage ion insertion into both cathodes and anodes, offer a promising electrical energy storage alternative. The respective 372 mAh/g and 90 mAh/g capacitances for cation and anion intercalation offer grid-level charge storage densities, implementation of affordable and available materials, and simpler disposal procedures for optimized and reliable energy storage systems[1]. Despite several expected advantages, the 5.2 V intercalation process currently exhibits low first-cycle efficiencies and limited charge/discharge rates. Possible causes can be likely attributed to a poor solid-electrolyte interface (SEI) and the large anion diameters, increasing inefficiencies from long intercalation pathways. Furthermore, theoretical predictions limit cathode stage 1 intercalation to one anion per 24 carbons (compared to one cation per 6 carbons for stage 1 anode intercalation), inherently limiting the system’s energy density. To maximize capacitance and long-term cyclabilities, the carbon structure and surface chemistry of both the cathode and anode materials must be optimized for the specific electrolyte and be appropriately matched with respective properties of the opposite electrode material.

To accomplish this goal, we subjected spherical, core-shell, and stacked graphite structures to 1100 – 1800 °C vacuum annealing, impact milling, 570 °C air oxidation, C2F6 plasma discharge, and high-temperature treatments using melamine and H2. These procedures defunctionalized surfaces [2], altered defects in the sp2 lattice, or selectively grafted C=O, C-F, C-Nx, and C-H groups, respectively. Following electrochemical cycling between 4.0 and 5.2 V in 2.0 m LiPF6 in fluoroethylene carbonate (FEC), ethylmethycarbonate (EMC) electrolytes and a tris(hexafluoro-iso-propyl)phosphate (HFiP) additive, we observed different effects of specific surface chemistries on anode and cathode capacities and first-cycle efficiencies. In particular, we observe positive influences of hydrogenated anode surfaces and improved performances for oxidized cathodes. Similar modifications of spherical and layered graphite electrodes yielded asymmetric efficiencies and long-duration cyclabilities, indicating divergent influences of functionalities on specific intercalated ions. Matching the appropriate surface groups for anode and cathode dual-graphite batteries, we observe improvements in coulombic efficiencies and long-term cyclabilities. Our findings offer fundamental insights into the dual-intercalation process and provide pathways for optimization of these all-carbon, environmentally friendly energy storage systems [3] for grid-level applications.

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2. Dyatkin, B.; Gogotsi, Y., Effects of Structural Disorder and Surface Chemistry on Electric Conductivity and Capacitance of Porous Carbon Electrodes. Faraday Discussions
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3. Dyatkin, B.; Presser, V.; Heon, M.; Lukatskaya, M. R.; Beidaghi, M.; Gogotsi, Y., Development of a Green Supercapacitor Composed Entirely of Environmentally Friendly Materials. ChemSusChem
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6 (12), 2269–2280.

Figure 1