Article pubs.acs.org/JPCC
In-Situ Studies on the Electrochemical Intercalation of Hexafluorophosphate Anion in Graphite with Selective Cointercalation of Solvent Jeffrey A. Read* U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783-1138, United States S Supporting Information *
ABSTRACT: Electrochemical cells utilizing graphite intercalation compounds at both electrodes have been proposed as an energy storage technology where the electrolyte salt is split and stored in the electrodes on charge and reformed on discharge. The anion intercalation compounds of graphite proposed as cathodes in these systems have been studied in electrolytes that are resistant to oxidation at 5 V but that are incompatible with graphite anodes. Recent work has demonstrated that electrolytes based on monofluoroethylene carbonate (FEC) and ethylmethyl carbonate (EMC) have superior oxidative stability on graphite cathodes over previously studied electrolytes and form a stable solid electrolyte interphase (SEI) on graphite anodes that allow for full dual-graphite cells to be evaluated for energy storage applications. There is still a limited understanding as to structure of the anion intercalate formed in these electrolyte systems and the effect of solvent cointercalation on cathode performance. This effort was undertaken using a number of in situ techniques to better characterize the fully intercalated composition as well as to investigate the process of solvent cointercalation. It was shown that a series of stages based on the C24PF6 composition are formed until, upon reaching full charge, the structure approaches a C20PF6 stage I composition with PF6− anion in close contact with the graphite layers and 0.7 molecules of cointercalated solvent. For the first time, we have shown that solvent molecules move with anion during the intercalation/deintercalation process while analysis of fully intercalated crystals demonstrated that there is an unusually strong preference for EMC over FEC to cointercalate in this anion intercalation compound.
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INTRODUCTION Electrochemical cells that utilize a graphite intercalation compound (GIC) as cathode have been demonstrated by a number of researchers.1−9 Anions that can be electrochemically intercalated into graphite include among others ClO4−, BF4−, PF6−, AlCl4−, CF3SO3−, and (CF3SO2)2N−, as well as the fluoride− anion acceptor complexes.1−6,9 The dual-carbon cell with nonaqueous electrolyte was first proposed as a rechargeable battery in patents by McCullough et al.10−13 In a dual-carbon cell, the lithium salt is electrolyzed on charge with the Li+ being stored in the anode and the anion intercalating into the cathode. The salt is depleted from the electrolyte on charge and replenished upon discharge with the energy capable of being stored in practical cells depending on the concentration and molecular weight of the electrolyte salt.14 Anion intercalation into graphite occurs at potentials near 5 V, and therefore, the oxidation of both electrolyte solvent and anion is of major concern.5 The electrolyte solvents that have been investigated previously include carbonates such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), as well as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), ethyl methyl sulfone (EMS), and the ionic liquid N-butyl-Nmethylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI). These solvents show varying degrees of stability This article not subject to U.S. Copyright. Published 2015 by the American Chemical Society
at high potential, with EMS giving the best overall results in terms of capacity and efficiency.5,9 The oxidative stability of both the electrolyte solvent and anion have been cited as possible reasons for cycling efficiencies of less than 100% and the poor storage performance of anion intercalated graphite.9 The electrolyte solvents PC, DMSO, DMF, EMS, and Pyr14TFSI are poor solvent choices for intercalation of lithium ions into graphite anodes as they cointercalate causing exfoliation and destruction of the graphite anode lattice, although recent work suggests that SEI forming additives may be used to stabilize the graphite anode toward the ionic liquid electrolyte based on Pyr14TFSI.15 In order to make a dual-graphite energy storage system feasible, an electrolyte system capable of high efficiencies on both the anode and cathode needed to be found. Recent work has demonstrated the higher oxidative stability of FEC based electrolytes on both 5 V spinel16 and on graphite cathodes.1 These results are supported by DFT calculations17 showing that higher oxidation potential is imparted to EC by fluorine substitution. Our recent work1 has demonstrated that electrolytes based on monofluoroethylene carbonate (FEC) and ethylmethyl carbonate (EMC) have superior oxidative Received: November 18, 2014 Revised: April 1, 2015 Published: April 1, 2015 8438
DOI: 10.1021/jp5115465 J. Phys. Chem. C 2015, 119, 8438−8446
The Journal of Physical Chemistry C
Article
peeling off the adhered graphite layers until the desired thickness was achieved. For the in-situ XRD and dilatometry measurements, the initial crystal thicknesses were measured with a micrometer in six places. Electrolyte solvents FEC (Daikin) and EMC (BASF) were tested by Karl Fischer and determined to be
