A study on electrolyte interactions with graphite anodes exhibiting structures with various amounts of rhombohedral phase (original) (raw)
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Electrochimica Acta, 2010
The first electrochemical lithium insertion was characterized for several graphite materials with high degree of crystallinity, different particle size distributions and surface morphologies in an ethylene carbonate (EC)/propylene carbonate (PC) electrolyte. For coarser graphite materials and graphites with a low superficial defect concentration, an irreversible process was observed which correlated with the electrochemical exfoliation of graphite. Different natural and synthetic graphites with similar particle size distribution and active surface area showed differences in the passivation behavior during the first electrochemical reduction. The fraction of graphite particles exfoliating during the first galvanostatic lithium insertion linearly increased with length of the irreversible plateau, which concomitantly moved to more positive potentials. This behavior can be rationalized when considering, besides the surface structure, local overpotentials for the solid electrolyte interphase formation process, and especially the overpotential distribution through the graphite electrode. These overpotentials cause a distribution of the local current density attributed to the passivation process. Optimizing the particle contacts in the electrode by applying mechanical pressure or by selecting the proper binder decreased the overpotentials and suppressed the graphite exfoliation in the EC/PC electrolyte. Therefore, both graphite surface structure and electrode engineering aspects have to be considered for successful passivation against exfoliation.
Nano letters, 2017
Li-ion batteries (LIB) have been successfully commercialized after the identification of ethylene-carbonate (EC)-containing electrolyte that can form a stable solid electrolyte interphase (SEI) on carbon anode surface to passivate further side reactions but still enable the transportation of the Li(+) cation. These electrolytes are still utilized, with only minor changes, after three decades. However, the long-term cycling of LIB leads to continuous consumption of electrolyte and growth of SEI layer on the electrode surface, which limits the battery's life and performance. Herein, a new anode protection mechanism is reported in which, upon changing of the cell potential, the electrolyte components at the electrode-electrolyte interface reorganize reversibly to form a transient protective surface layers on the anode. This layer will disappear after the applied potential is removed so that no permanent SEI layer is required to protect the carbon anode. This phenomenon minimizes th...
Electrochimica Acta, 2002
In situ AFM observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed before and after cyclic voltammetry in 1 mol dm (3 LiClO 4 dissolved in ethylene carbonate (EC), EC'/diethyl carbonate (DEC), and EC'/dimethyl carbonate (DMC) to clarify the effects of co-solvents in EC-based solutions on surface film formation on graphite negative electrodes in lithium-ion cells. In each solution, surface film formation involved the following two different processes: (i) intercalation of solvated lithium ions and their decomposition beneath the surface; and (ii) direct decomposition of solvent molecules on the basal plane to form a precipitate layer. The most remarkable difference among these solvent systems was that solvent co-intercalation took place more extensively in EC'/DEC than in EC'/DMC or EC. Raman analysis of ion Á/solvent interactions revealed that a lithium ion is solvated by three EC molecules and one DEC molecule in EC'/DEC, whereas it is solvated exclusively by EC in EC'/DMC and in EC, which suggested that the presence of linear alkyl carbonates in the solvation shell of lithium ion enhance the degree of solvent co-intercalation that occurs in the initial stage of the surface film formation. #
Characterisation of the SEI formed on natural graphite in PC-based electrolytes
Electrochimica Acta, 2004
The origin of the different Li + intercalation behaviour of raw and jet-milled natural graphite has been investigated. Jet-milled graphite is found to cycle reversibly in equal solvent mixture of propylene carbonate (PC) and ethylene carbonate (EC), whereas raw graphite does not. Using both Al Ka and synchrotron radiation (SR) Photoelectron Spectroscopy, new insight is obtained into the formation of the solid electrolyte interphase (SEI) on the two different graphite materials during electrochemical cycling in 1 M LiPF 6 in either PC:EC (1:1) or in PC with 5% vinylene carbonate (VC) as additive. Solvent reduction products are found at the surface of both raw and jet-milled graphite cycled in PC:EC (1:1), but differed in composition. The addition of VC reduces primarily the quantities of salt reaction products (LiF and Li x PF y compounds) and produces a mainly organic SEI layer. Electron diffraction from the edges for raw and jet-milled graphite particles shows a more disordered surface structure in the jet-milled particles than in the raw graphite. The more disordered surface structure can serve as a physical barrier hindering PC co-intercalation and facilitating the formation of a stable SEI layer.
ACS Applied Materials & Interfaces, 2021
Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li + /Li 0 and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode−electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anionintercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF 6 , LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g −1 were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF 6-based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g −1. The poor performance in LiPF 6-containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in Li x PF y , PO x F y , Li x PO y F z , and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI-and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, ex situ Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF 6-based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.
2014
Density functional theory (DFT) was used to investigate the effect of electrolyte additives such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl ethylene sulfite (VES), and ethylene sulfite (ES) in propylene carbonate (PC)-based Li-ion battery electrolytes on SEI formation at graphitic anodes. The higher desolvation energy of PC limits Li + intercalation into graphite compared to solvated Li + in EC. Li + (PC) 3 clusters are found to be unstable with graphite intercalation compounds and become structurally deformed, preventing decomposition mechanisms and associated SEI formation in favor of co-intercalation that leads to exfoliation. DFT calculations demonstrate that the reduction decomposition of PC and electrolyte additives is such that the first electron reduction energies scale as ES > VES > VEC >PC. The second electron reduction follows ES > VES > VEC > VC > PC. The reactivity of the additives under consideration follows ES > VES > VEC > VC. The data demonstrate the supportive role of certain additives, particularly sulfites, in PC-based electrolytes for SEI film formation and stable cycling at graphitic carbon-based Li-ion battery anodes without exfoliation or degradation of the anode structure.
Investigating the solid electrolyte interphase using binder-free graphite electrodes
Journal of Power Sources, 2008
Binder-free (BF) electrodes simplify interpretation of solid electrolyte interphase (SEI) data obtained from studies of graphite surfaces. In this work, we prepared BF-graphite electrodes by electrophoretic deposition (EPD), and the SEI layers formed on the electrode in lithium cells containing LiPF 6-and LiF 2 BC 2 O 4-bearing electrolytes were examined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that the dominant SEI species were lithium alkyl carbonates (ROCO 2 Li) and lithium alkoxides (ROLi); Li 2 CO 3 was conspicuously absent. Trigonal borate oligomers are most likely present in the SEI of graphite samples cycled in LiF 2 BC 2 O 4 electrolyte, while lithium fluorophosphates are present on graphite samples cycled in LiPF 6 electrolyte. The SEI layer coverage was greater on graphite samples cycled in LiF 2 BC 2 O 4 electrolyte than in the LiPF 6 electrolyte. Our results demonstrate that BF-graphite electrodes prepared by EPD are suitable for the study of SEI layer formed in various electrolyte systems.
Electrochimica Acta, 2019
In this study, the interplay between the electrochemical behavior, structural modifications and volumetric changes of graphite-based negative electrodes for sodium ion batteries is evaluated. The electrolyte solvents chosen for this study are ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TriGDME), and tetraethylene glycol dimethyl ether (TEGDME) while the salt is sodium trifluoromethanesulfonate (NaOTf). The volume changes undergone upon co-(de)intercalation of the [Na-solvent] + complexes are systematically investigated by means of in situ electrochemical dilatometry and correlated to the structural modification of the graphite crystalline lattice as detected by in situ X-ray diffraction. The expected staging mechanism upon co-intercalation, leading to the formation of stage-I graphite intercalation compound, is observed with all electrolytes. However, the various solvents play a substantial role on the reversibility of such structural changes in the first cycle. The effect of temperature is also investigated showing that the most stable electrochemical performance is observed with the TEGDME at room temperature, but with TriGDME at higher temperatures. Nonetheless, post-mortem scanning electron microscopy suggests that the surface layer forming after the first sodiation may dissolve in the following de-sodiation.
Journal of The Electrochemical Society, 2001
This work relates to a rigorous study of the surface chemistry ͑Fourier transform infrared, X-ray photoelectron spectroscopy͒, crystal structure ͑X-ray diffraction͒, galvanostatic, cyclic voltammetric, and impedance behavior of lithiated carbon electrodes in commonly used liquid electrolyte solutions. Two different types of disordered carbons and graphite, as a reference system, were explored in a single study. All three types of carbons develop a similar surface chemistry in alkyl carbonate solutions, which are dominated by reduction of solvent molecules and anions from the electrolyte. The differences in the crystal structure of these carbons lead to pronounced differences in the mechanisms of Li insertion into them. Whereas Li-ion intercalation into graphite is a staged process, Li-ion insertion into the disordered carbons occurs in the form of adsorption on both sides of the elementary graphene flakes and on their edges. The electroanalytical behavior of the disordered carbons was found to correlate well with their unique structure described in terms of the butterfly model. Both types of the disordered carbons reveal exceptionally good cyclability in coin-type cells ͑vs. Li counter electrodes͒, with only moderate capacity fading. Highly resolved plots of the chemical diffusion coefficient of Li-ions, D vs. potential E, for the disordered carbon electrodes were obtained. Surprisingly, a maximum in D appears on these plots at intermediate levels of Li-ion insertion corresponding to ca. 0.4-0.5 V ͑vs. Li/Li ϩ ͒. We propose that these maxima may originate from a combination of two effects, ͑i͒ repulsive interactions between the inserted species, and ͑ii͒ pronounced heterogeneity of Li insertion sites in terms of carbon-Li interactions and Li-ion mobility.