Reversible Intercalation of Bis(trifluoromethanesulfonyl)imide Anions from an Ionic Liquid Electrolyte into Graphite for High Performance Dual-Ion Cells

Abstract

Dual-graphite cells have been proposed as electrochemical energy storage systems using graphite as both, the anode and cathode, whereas the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode during charge. On discharge, cations and anions are released back into the electrolyte. In this contribution, we present highly promising results for "dual-ion cells" based on intercalation of bis(trifluoromethanesulfonyl)imide anions into a graphite cathode from an ionic liquid-based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr 14 TFSI). As the compatibility of this ionic liquid with graphitic anodes is relatively poor, metallic lithium and lithium titanate (Li 4 Ti 5 O 12 ) are used as anode. As both cations and anions participate in the charge/discharge reaction and other anode materials than graphite are possible, we propose the name "dual-ion cells" for these systems. The cell performance was studied in terms of cut-off voltage, temperature, cycling stability, self-discharge and rate performance. Depending on the cut-off voltage and temperature, coulombic efficiencies of more than 99 % and specific discharge capacities exceeding 100 mAh g −1 (based on graphite cathode weight) were achieved. Furthermore, this system provides an excellent cycling stability and capacity retention above 99 % after 500 cycles, outperforming reported organic solvent-based dual-graphite or dual-ion cells. Graphite is a redox-amphoteric intercalation host and therefore cations and anions can be electrochemically intercalated at different potentials yielding so-called donor-type or acceptor-type graphite intercalation compounds (GICs).1,2 Currently, the predominantly used donor-type GIC is LiC x . The LiC x /C x redox couple is the major active compound for state-of-the-art negative electrodes in lithium ion batteries. [3][4][5][6][7] Compared to the limited number of cationic intercalation guests, there is a broad spectrum of different anions capable to form acceptor-type GICs. Examples are hexa-or tetrafluoride guest species, e. g. PF 6− , AsF In 1938, Rüdorff and Hofmann developed the first ion transfer or rocking chair cell based on the shuttling of HSO 4 − anions between two graphite electrodes ( Figure 1a).14 This cell can be considered as the ancestor of the well-known lithium-ion cell, where lithium cations are transferred between two insertion electrodes during the charge/discharge process (Figure 1b). 3 In the 1990s, a rechargeable electrochemical energy storage system, using graphite as positive and negative electrode material in combination with a non-aqueous electrolyte has been introduced by and Carlin et al. 27,28 Carlin et al. investigated the reductive and oxidative intercalation of different cations and anions from ionic liquid-based electrolytes (without any additional lithium salt), such as 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI + PF 6 − ). This system, a so-called dual-graphite cell, was bas...

Synthesis of Spherical Graphite Particles and Their Application as Cathode Material in Dual-Ion Cells

“…and the operation conditions (i.e. upper cut-off potential, operation temperature) (3,8). So far, we found a maximum specific discharge capacity of 115 mAh g -1 for the intercalation of TFSI -((CF 3 SO 2 ) 2 N -) anions into graphite at a temperature of 60 °C (11), whereas the intercalation capacity for lithium into graphitic carbon is usually very close to the theoretical value of 372 mAh g -1 (14).…”

“…By intercalation of cations, like lithium ions, donortype graphite intercalation compounds (GICs) are formed, while the intercalation of anions yields acceptor-type GICs (3).…”

Pre-Lithiation Strategies for Rechargeable Energy Storage Technologies: Concepts, Promises and Challenges

“…LICs, are a special form of "hybrid capacitors," i.e., a combination of a supercapacitor electrode and a battery electrode in one device, i.e., both storage mechanisms (Li ion insertion/extraction and adsorption/desorption) occur simultaneously during charge/discharge ( Figure 15b). As like in dual ion batteries, which are based on a similar working mechanism [150][151][152][153], not only the electrode capacities but also the number of anions and cations in the electrolyte, thus the amount of electrolyte is decisive for the available energy density. Amatucci et al [154,155] reported in 2001 the use of nanostructured lithium titanate (LTO) as negative electrode in combination with an activated carbon positive electrode.…”

Intercalation manners of perchlorate anion into graphite electrode from organic solutions

“…However, situation is changing as people begin to be aware of their outstanding merits as follows. First, many anions can intercalate into graphite reversibly [2][3][4][5][6], even though their sizes are much larger than Li + ; Second, a relatively high specific capacity of more than 110 mAh g À1 can be delivered [4,7,8]; Third, the potential for the intercalation process ranges from 4.5 to 5.5 V vs. Li/Li + , which is still a heavy task for most lithium-storage positive electrode materials at present [9][10][11]. Apart from the above, the beneficial properties also include low cost, high electronic conductivity, abundant sources et al In short, anion-intercalated graphite compounds are qualified to pave a way for some new types of electric storage devices.…”

“…Recently, much attention has been focused on ionic liquids which allow reliable operation in highly oxidizing circumstance [1,2,5,[8][9][10][12][13][14][15][16]. However, the drawbacks such as relatively poor compatibility with graphitic electrodes and high viscosity at room temperature may become problems in practical applications [10].…”