A study on electrolyte interactions with graphite anodes exhibiting structures with various amounts of rhombohedral phase

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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...

Section: First Cycle "Activation" For Anion Intercalation and Influenmentioning

confidence: 99%

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

Placke

Fromm

Lux

et al. 2012

307

345

“…1,2 For such use, a higher degree of safety, a longer cycle life, higher voltage and capacity will be indispensable in the future. An important key of the stability and durability of the LIB is the solid electrolyte interphase (SEI) formed at the negative electrode-electrolyte interface.…”

mentioning

confidence: 99%

Near-Shore Aggregation Mechanism of Electrolyte Decomposition Products to Explain Solid Electrolyte Interphase Formation

Ushirogata

Sodeyama

Futera

et al. 2015

To get insight of the formation mechanism of solid electrolyte interphase (SEI) film in Lithium-ion battery (LIB), we examine a probable scenario, referred to as "surface growth mechanism," for electrolyte involving ethylene carbonate (EC) solvent and vinylene carbonate (VC) additive by using density functional theory (DFT). We first extracted stable SEI film components (SFCs) for the EC/VC electrolyte and constructed probable SFC aggregates via DFT molecular dynamics. We then examined their solubility in the EC solution, their adhesion to a model graphite electrode, and the electronic properties. The results showed that the SFC aggregates are characterized by "unstable adhesion" to the graphite surface and "high electronic insulation" against the EC solution. These characteristics preclude explaining SEI growth up to a typical thickness of several tens of nanometers based on the surface growth mechanism. With the present results, we propose "near-shore aggregation" mechanism, where the SFCs formed at the electrode surface desorb into the near-shore region and form aggregates. The SFC aggregates coalesce and come into contact with the electrode to complete the SEI formation. The present model provides a novel perspective for the long-standing problem of SEI formation. Lithium-ion batteries (LIBs) have attracted considerable attention for use in larger power sources like electric vehicles and energy storage system because of their high energy densities.1,2 For such use, a higher degree of safety, a longer cycle life, higher voltage and capacity will be indispensable in the future. An important key of the stability and durability of the LIB is the solid electrolyte interphase (SEI) formed at the negative electrode-electrolyte interface.3,4 It is generally accepted that molecules in the electrolyte solution reductively decompose to form various SEI film components (SFCs), such as organic oligomers (e.g., (CH 2 OCO 2 Li) 2 , and ROCO 2 Li) and inorganic moieties (e.g., Li 2 CO 3 , LiF) at the first charging, 5 and that the SFCs precipitate on the electrode surface to form a stable SEI film with a thickness of several tens of nanometers. 6 The SEI hinders electron transport from the electrode to the electrolyte solution, preventing further electrolyte decomposition, while allowing Li + ion transport. This property decreases the irreversible capacity and improves the safety of LIBs. Additives to the electrolyte solution also have a large impact on SEI performance. Even a small amount of additive up to a few wt% significantly improves the irreversible capacity and cyclability.7 In general, the additive molecules modify the SFCs and lead to different properties of the SEI film. Despite the important roles of the SEI in LIB operation, the microscopic formation processes are still unclear because of the difficulty in operando observations of chemical reactions at the electrodeelectrolyte interfaces.The detailed formation processes of SEI have been typically assumed to involve "surface growth mechanism", mainly in the firs...

“…[24][25][26] Also graphite exhibits different SEI layer compositions on plane and edge sites. 18,27,28 In this study, the mechanism of SEI formation on a model binder-and conductive additive-free Si(100) wafer electrode is studied as a function of the electrode potential and electrolyte composition. …”

mentioning

confidence: 99%

The Mechanism of SEI Formation on a Single Crystal Si(100) Electrode

Vogl

Lux

Crumlin

et al. 2015

A fundamental study of interfacial phenomena on a Si(100) single crystal electrode in organic carbonate-based electrolytes was carried out. The SEI formation on the Si(100) single crystal electrode was investigated as a function of the electrolyte composition, electrode potential and Li x Si lithiation degree. Fourier transform infrared spectroscopy (FTIR) and X-ray photon spectroscopy (XPS) studies of the SEI layer during early stages of SEI formation indicate a strong dependence of the SEI composition on the electrolyte composition. However, the influence of the electrolyte composition becomes negligible at low potentials, when lithium alloys with Si and forms amorphous Li Currently, graphite is the most common negative electrode material in commercial lithium ion batteries for portable electronics. [1][2][3] However, lithium ion batteries for large-scale transportation applications require new inexpensive electrode materials with higher specific capacity. For decades, silicon has been considered as a promising negative electrode material, mainly because of its high specific capacity of ca. 3500 mAh/g 4 and its abundance in the earth crust. [4][5][6] The key problems that prevent its widespread use are large, up to ∼300 % volumetric changes during lithiation/delithiation processes and interfacial instability of lithium silicides in organic solvent based electrolytes. 6-12The resultant poor electrochemical cycling performance and large irreversible capacities during formation and operation of the Si negative electrode contribute to rapid degradation and failure of the battery. 6,7,13 The solid electrolyte interphase (SEI) layer, which forms at the electrode/electrolyte interface during the initial charge/discharge cycles, is the key component that determines the long-term stability and cycling behavior of carbonaceous and intermetallic negative Li-ion battery electrodes.14-16 Electrolyte reduction and SEI layer formation on a Si electrode usually take place at potentials below 1.8 V vs. Li/Li + and accompany the formation of Li-Si phases, the so-called "Si-Li alloying" process at E <0.4 V vs. Li/Li + . 17 The exact mechanism of the SEI formation processes on Si, the SEI composition and the effect on the Si electrode electrochemical cycling performance is not well understood. 18 On the other hand, SEI-forming electrolyte additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are known to alter the composition and properties of the SEI on Si and improve the electrode electrochemical performance. [19][20][21] Interestingly, recent model studies on Sn single crystal electrodes in organic carbonate electrolytes revealed a strong correlation between the crystal surface orientation and the SEI composition.22,23 A similar study of the composition of the SEI on a silicon monocrystal electrode showed strong effects of different SEI formation protocols, presence/absence of intrinsic SiO 2 , electrolyte composition and impurities e.g., HF. [24][25][26] Also graphite exhibits different SEI layer compos...