Further identification to the SEI film on Ag electrode in lithium batteries by surface enhanced Raman scattering (SERS)

The surface enhanced Raman scattering (SERS) spectrum of the solid electrolyte interphase (SEI) film on the Ag electrode discharged to 0.0 V in lithium battery was measured by normal Raman spectrometer at different excited wavelength. Compared with the Raman spectra of pure LiOH·H₂O and Li₂CO₃ reagents, all the SERS bands of the SEI film formed on the surface of the Ag electrode can be assigned to Li₂CO₃ and LiOH·H₂O, which are the main stable components of the SEI film in the presence of trace water. LiF may be another possible stable component, however, it cannot be detected due to its inactivity in Raman spectrum.     

Electrochemical behaviors of a Li3N modified Li metal electrode in secondary lithium batteries

A lithium conductive Li₃N film is successfully prepared on Li metal surface by the direct reaction between Li and N₂ gas at room temperature. X-ray diffraction (XRD), Auger electron spectroscopy (AES), cyclic voltammetry (CV), scanning electron microscopy (SEM), AC impedance, cathodic polarization and galvanostatic charge/discharge cycling tests are applied to characterize the film. The experimental results show that the Li₃N protective film is tight and dense with high stability in the electrolyte. Its thickness is more than 159.4 nm and much bigger than that of a native SEI film formed on the lithium surface as received. An exchange current as low as 3.244 × 10⁻⁷ A demonstrates the formation of a complete SEI film at the electrode|electrolyte interface with Li₃N modification. The SEI film is very effective in preventing the corrosion of the Li electrode in liquid electrolyte, leading to a decreased Li|electrolyte interface resistance and an average short distance of 3.16 × 10⁻³ cm for Li ion diffusion from electrolyte to Li surface. The Li cycling efficiency depends on N₂ exposing time and is obviously enhanced by the Li₃N (1 h) modification. After cycling, a dense and homogeneous Li layer deposits on the Li₃N (1 h) modified Li surface, instead of a loose and inhomogeneous layer on the Li surface as received.     

Reconsideration of SEI stability: reversible lithium intercalation into graphite electrodes in trans-2,3-butylene carbonate

Lithium ion batteries with graphitic carbon anodes and LiCoO₂ cathodes are cycled reversibly in electrolytes based on trans-2,3-butylene carbonate (t-BC), even in the absence of ethylene carbonate. While the poor interfacial film (the solid electrolyte interface (SEI)) on the lithium electrode can be readily explained in terms of previous models of its stability, this highly reversible behavior of graphite is hard to account for. To explain this profound difference in the SEI stability of the two electrodes, we have taken into account the influence that the nature of the electrode (lithium metal versus graphite) and the type of the reaction site (basal plane versus edge sites) exert on the solvent reduction pathways.     

Properties of the lithium and graphite-lithium anodes in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide

The ternary [Li⁺]_0.09[MePrPyr⁺]_0.41[NTf₂⁻]_0.50 room temperature ionic liquid was obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂) in liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([MePrPyr⁺][NTf₂⁻]), and studied as an electrolyte for lithium-ion batteries. The graphite-lithium (C₆Li) anode, working together with vinylene carbonate as an additive showed ca. 90% of its initial discharge capacity after 50 cycles. The addition of vinylene carbonate to the neat ionic liquid results in the formation of the protective coating (SEI) on both the lithium and graphite anodes. The SEI formation increases the rate of the charge transfer reaction as well as protects the anode from chemical passivation (corrosion). The graphite-lithium (C₆Li) anode shows good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate as an additive to the ionic liquid.     

Thickness estimation of interface films formed on Li1−xCoO2 electrodes by hard X-ray photoelectron spectroscopy

Solid electrolyte interface (SEI) films formed on Li_1−xCoO₂ electrodes were observed with hard X-ray photoelectron spectroscopy (HX-PES). This paper particularly focuses on film thickness estimation using HX-PES with theoretical calculation. The validity of the calculation was proven by experiments using model SEI films. The native film formed on a LiCoO₂ composite electrode was estimated to be LiF with its thickness of 5 nm. Formation of Co (II) species on top of LiCoO₂ was also indicated. Storage of the electrode at 60 °C brought about considerable film growth (30-40 nm) with carbonate compounds formation. SEI film changes during charging of the LiCoO₂ electrode were also examined. The main component in the film was deduced to be LiF or a kind of fluorite, with its thickness decreased during charging. The SEI formation mechanisms are also elucidated.     

Ageing of V2O5 thin films induced by Li intercalation multi-cycling

Cyclic voltammetry, XPS, RBS and AFM have been combined to study the ageing mechanism of Li intercalation in V₂O₅ thin films prepared by thermal oxidation of vanadium metal. Multi-cycling tests were performed in 1 M LiClO₄-PC in the potential range E ∈ [3.8, 2.8 V] versus Li/Li⁺, corresponding to the α-to-δ phase transition. XPS and AFM were performed using direct anaerobic and anhydrous transfer. Capacity fading remains inferior to 20% during ∼2500 cycles. XPS shows slight modifications of the oxide composition with a V⁴⁺ concentration increasing from ∼5% prior to cycling to ∼16–27% after cycling, due to Li trapped in the oxide film and to the loss of V₂O₅ active material. The presence of lithium carbonate and lithium-alkyl carbonate species evidences the formation of the so-called SEI layer. AFM evidences the loss of crystalline material by grain exfoliation from the outer V₂O₅ layer of the oxide film. By further exfoliation, the inner VO₂ layer of the oxide film is reached and pits are formed, occupying ∼9–13% of the surface. This de-cohesion at grain boundaries is attributed to the strain generated by repeated lattice distortions. After 3300 cycles, the disappearance of lithium carbonates, whereas Li-alkyl carbonates and/or Li-alkoxides remain on the surface, indicates the dissolution and/or conversion of the SEI layer. After 4500 cycles, the oxide film became very labile and could be stripped away by rinsing to reveal the vanadium metal substrate.     

Surface film formation on a carbonaceous electrode: Influence of the binder chemistry

We have investigated the possible effect of carboxymethylcellulose (CMC) in the SEI film formation at the surface of a graphite composite electrode of LiCoO₂/graphite cells. The electrode/electrolyte interface was analyzed by XPS at different potentials of the first electrochemical cycle, and after simple contact of the electrode with the electrolyte. We could evidence a specific reactivity of CMC towards the electrolyte (LiPF₆ in a mixture of carbonate solvents), resulting in the formation of new species that contribute to the surface film composition. This result shows that the chemical reactivity of CMC towards the electrolyte takes part in the formation of the surface film, and contributes to the good properties of CMC as binder.     

Effect of fluoroethylene carbonate additive on interfacial properties of silicon thin-film electrode

A silicon thin-film electrode (thickness = 200 nm) is prepared by E-beam evaporation and deposition on copper foil. The electrochemical performance of a lithium/silicon thin-film cell is investigated in ethylene carbonate/diethyl carbonate/1.3 M LiPF₆ with and without 3 wt.% fluoroethylene carbonate (FEC). The addition of FEC remarkably improves discharge capacity retention and coulombic efficiency. The surface morphology and chemical composition of the solid electrolyte interphase (SEI) formed on the surface of the silicon thin-film electrode after cycling are studied through scanning electron microscopy and X-ray photoelectron spectroscopy analysis. A smoother and more stable SEI layer structure is generated by the introduction of the FEC additive to the electrolyte.     

Electrochemical properties of Li2ZrO3-coated silicon/graphite/carbon composite as anode material for lithium ion batteries

Silicon/graphite/disordered carbon (Si/G/DC) is coated by Li₂ZrO₃ using Zr(NO₃)₄·5H₂O and CH₃COOLi·2H₂O as coating reagents. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize Li₂ZrO₃-coated Si/G/DC composite. The Li₂ZrO₃-coated Si/G/DC composite exhibits a high reversible capacity with no capacity fading from 2nd to 70th cycle, indicating its excellent cycleability when used as anode materials for lithium ion batteries. A compact and stable solid-electrolyte interphase (SEI) layer is formed on the surface of Li₂ZrO₃-coated Si/G/DC electrode. Analysis of electrochemical impedance spectra (EIS) shows that the resistance of the coated material exhibits less variation during cycling, which indicates the integrity of electrode structure is kept during cycling. XPS shows that F and P elements do not appear in the SEI layers of Li₂ZrO₃-coated Si/G/DC electrode, while they have a relatively high content in SEI layers of Si/G/DC electrode. The improvement of Li₂ZrO₃-coated Si/G/DC is attributed to the decrease of lithium insertion depth and the formation of stable SEI film.     

Electrochemical intercalation of cationic and anionic species from a lithium perchlorate–propylene carbonate system—a rocking-chair type of dual-intercalation system

The reductive and oxidative intercalation of ionic species of lithium perchlorate (LiClO₄) in propylene carbonate (PC) medium are carried out to develop a dual-intercalation battery system. Cyclic voltammetry (CV), potentiostatic transients (i-t), galvanostatic charging, thermogravimetry (TG) and differential thermal analysis (DTA) are performed to establish the intercalation behaviour of both lithium and perchlorate ionic species. A polypropylene graphite composite electrode material containing 20 wt.% polypropylene as a binder is found to be a suitable host material for dual intercalation studies. The intercalation/de-intercalation efficiency (IDE) increases with increasing sweep rate and reaches up to 90% for Li⁺ and 65% for ClO₄⁻ ions at a sweep rate of 40 mV s⁻¹. The formation of a passive film decreases the IDE during the first intercalation/de-intercalation cycle. The open-circuit potential for a battery assembly involving these two electrodes is in the range 3.8 to 4.0 V.     

Properties of the graphite-lithium anode in N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide as an electrolyte

The ternary [Li⁺][MPPip⁺][NTf₂⁻] ionic liquid, obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂) in liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MPPipNTf₂), was used as an electrolyte, and stable at the lithium or graphite-lithium anodes. The graphite-lithium (C₆Li) anode showed good cyclability and Coulombic efficiency in the presence of a molecular additive (10 wt.% of vinylene carbonate, VC) to the ionic liquid. The electrode showed ca. 90% of its initial discharge capacity after 100 cycles. The addition of ethylene carbonate (EC) does not improve the cyclability of the anode to the same degree as that observed in the case of vinylene carbonate.     

Effects of surface lithiated TiO2 nanorods on room-temperature properties of polymer solid electrolytes

Polymer solid electrolyte with high ionic conductivity at room-temperature is most likely to be widely used in solid-state lithium batteries. In this work, the novel surface lithiated TiO₂ nanorods were firstly used as ionic conductor in polymer solid electrolyte. The surface lithiated TiO₂ nanorods-filled polypropylene carbonate polymer composite solid electrolyte (CSE) has an uniform composite structure with a thickness of about 60 μm. The ionic conductivity at room-temperature is 1.21 × 10⁻⁴ S cm⁻¹ and the electrochemical stability window is up to 4.6 V (vs Li⁺/Li). The assembled NCM622/CSE/Li solid-state battery shows a stable cycle performance with a retention capacity of 120 mAh g⁻¹ after 200 cycles at the current density of 0.3 C and a high coulomb efficiency of 99%. Compared with TiO₂ particles, this novel surface lithiated TiO₂ nanorods can provide more continuous ion transport channels and more Lewis acid-base reactive sites, provide a novel way to enhance the lithium ion transport in polymer solid electrolyte.     

Studies on the enhancement of solid electrolyte interphase formation on graphitized anodes in LiX-carbonate based electrolytes using Lewis acid additives for lithium-ion batteries

The new electrolyte systems utilizing one type of Lewis acids, the boron based anion receptors (BBARs) with LiF, Li₂O, or Li₂O₂ in carbonate solutions have been developed and reported by us. These systems open up a new approach in developing non-aqueous electrolytes with higher operating voltage and less moisture sensitivity for lithium-ion batteries. However, the formation of a stable solid electrolyte interphase (SEI) layer on the graphitized anodes is a serious problem needs to be solved for these new electrolyte systems, especially when propylene carbonate (PC) is used as a co-solvent. Using lithium bis(oxalato)borate (LiBOB) as an additives, the SEI layer formation on mesophase carbon microbeads (MCMB) anode is significantly enhanced in these new electrolytes containing boron-based anion receptors, such as tris(pentafluorophenyl) borane, and lithium salt such as LiF, or lithium oxides such as Li₂O or Li₂O₂ in PC and dimethyl carbonate (DMC) solvents. The cells using these electrolytes and MCMB anodes cycled very well and the PC co-intercalation was suppressed. Fourier transform infrared spectroscopy (FTIR) studies show that one of the electrochemical decomposition products of LiBOB, lithium carbonate (Li₂CO₃), plays a quite important role in the stablizing SEI layer formation.     

Gamma-crotonlatone as an electrolyte additive for improving the cyclability of MCMB electrode

In this paper, gamma-crotonlatone (GCL) was tested as an additive to electrolyte solutions of 1 M LiPF₆ EC:DMC (1:1, vol) for lithium-ion batteries. Smaller volume amount in the order of 10⁻⁶ GCL improved the cyclability of MCMB electrode, and decreased the impedance of MCMB/Li cell. The results of cyclic voltammetry show that the GCL has higher reduction decomposition potential at about 2.1 V. The surface morphologies and chemistry of the solid electrolyte interphase (SEI) film formed on MCMB electrodes cycled in 1 M LiPF₆ EC:DMC with and without GCL were studied by SEM, FTIR and XPS analyses, and the results show that a uniform, stable and low-resistive SEI film was formed on the surface of MCMB electrode, which cause the excellent cyclability of electrode.     

Cycling performance and thermal stability of lithium polymer cells assembled with ionic liquid-containing gel polymer electrolytes

Gel polymer electrolytes containing 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and a small amount of additive (vinylene carbonate, fluoroethylene carbonate, and ethylene carbonate) are prepared, and their electrochemical properties are investigated. The cathodic limit of the gel polymer electrolytes can be extended to 0 V vs. Li by the formation of a protective solid electrolyte interphase on the electrode surface. Using these gel polymer electrolytes, lithium metal polymer cells composed of a lithium anode and a LiNi_1/3Co_1/3Mn_1/3O₂ cathode are assembled, and their cycling performances are evaluated at room temperature. The cells show good cycling performance, comparable to that of a cell assembled with gel polymer electrolyte containing standard liquid electrolyte (1.0 M LiPF₆ in ethylene carbonate/diethylene carbonate). Flammability tests and differential scanning calorimetry studies show that the presence of the ionic liquid in the gel polymer electrolyte considerably improves the safety and thermal stability of the cells.     

4-Bromobenzyl isocyanate versus benzyl isocyanate—New film-forming electrolyte additives and overcharge protection additives for lithium ion batteries

Electrochemical properties and working mechanisms of benzyl isocyanate compounds as polymerizable electrolyte additives for overcharge protection of lithium ion batteries have been studied by cyclic voltammetry, charge–discharge cycling, overcharge tests, accelerating rate calorimetry (ARC) and in situ Fourier transform infrared spectroscopy (FTIRS). The overcharge and FTIRS data clearly reveal that 4-bromobenzyl isocyanate (Br-BIC) can electrochemically polymerize at 5.5 V (versus Li/Li⁺) to form an overcharge-inhibiting (probably insulating) film on the cathode surface. In addition, is found the Br-BIC does slightly improve the charge/discharge performance of a lithium ion battery. Furthermore, Br-BIC and benzyl isocyanate show beneficial solid electrolyte interphase (SEI) formation behaviour on graphite in propylene carbonate based electrolyte solutions.     

Investigation of positive electrodes after cycle testing of high-power Li-ion battery cells: I. An approach to the power fading mechanism using XANES

Cylindrical lithium-ion (Li-ion) cells with a nickel-cobalt oxide (LiNi_0.73Co_0.17Al_0.10O₂) positive electrode and a non-graphitizable carbon (hard carbon) negative electrode were degraded using cycle tests. The degraded cells were disassembled and examined; most attention was paid to the positive electrodes in order to clarify the origin of the power fade of the cells. X-ray absorption near-edge structure (XANES) analysis demonstrated that the crystal structure of the electrode at the surface changed from rhombohedral to cubic symmetry. Furthermore, a film of lithium carbonate (Li₂CO₃) covered the surface of the positive electrode after the cycle tests. Using a combination of X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and glow discharge optical emission spectrometry (GD-OES) measurements, a schematic model of the changes occurring in the surface structure of the positive electrode during the cycle tests was constructed. The appearance of both an electrochemically inactive cubic phase and lithium carbonate films at the surface of the positive electrode are important factors giving rise to power fade of the positive electrode.     

Solid electrolyte interphase (SEI) electrodes Part VI: Calcium-Ca(AlCl4)2-sulfuryl chloride system

The electrochemistry of the Ca/Ca(AlCl₄)₂-sulfuryl chloride (SC) system and its capability of functioning as a primary cell were studied. The OCV of this cell is about 3.4 V and it exhibits a rather flat discharge curve of 3.1 - 2.9 V at current densities of 0.4 - 2.0 mA cm^?2. Cell capacity is 35 – 45 mA h cm^?2 (for 1.1 mm thick porous carbon cathodes). Cell failure results from anodic polarization. Cells were found to resist charging and reversal abuses and no indication of calcium deposition during reversal was found. It was found that the calcium anode is covered by a thin film (most likely CaCl₂) which serves as a SEI. Its thickness on fresh electrodes is about 15 – 30 Å and it increases with time. The rate determining step for the calcium dissolution process is the ionic migration through the SEI. The ionic resistivity of the SEI is about 4 × 10⁹ Ω cm.     

Study of poly(acrylonitrile-methyl methacrylate) as binder for graphite anode and LiMn2O4 cathode of Li-ion batteries

We evaluated poly(acrylonitrile-methyl methacrylate) (AMMA, AN/MMA=94:6) as a binder for the graphite anode and the LiMn₂O₄ cathode of Li-ion batteries by studying the cycling performance of lithium half-cells. The results showed that, using AMMA binder, both graphite and LiMn₂O₄ could be cycled well in 1 m LiPF₆ 3:3:4 (weight) PC/EC/EMC electrolyte with less capacity fading. AMMA is chemically more stable than poly(vinylidene fluoride) (PVDF) against the lithiated graphite. More importantly, AMMA can help graphite to form a stable solid electrolyte interface (SEI) film. An impedance study showed that the SEI film formed with AMMA is more stable than the one formed with PVDF. Therefore, self-delithiation of the lithiated graphite can be reduced by use of AMMA instead of PVDF, which improves the storage performance of Li-ion batteries.     

Effect of electrolyte transport properties and variations in the morphological parameters on the variation of side reaction rate across the anode electrode and the aging of lithium ion batteries

In this work, for the first time, we model the variation of solid electrolyte interface (SEI) across the depth of anode electrode of lithium ion battery. It is anticipated that due to higher thickness of SEI layer at the electrode side connected to the separator, a more critical condition prevails there. The present work also investigates the effects of variations in the morphological parameters including porosity, interfacial surface area and active particle radius across anode electrode on the uniformity of side reaction. Moreover, the sensitivity of the side reaction uniformity to electrolyte parameters, such as diffusion and ionic conductivity, is studied. Results show that the ionic conductivity has a major role on the uniformity, and could reduce critical conditions in the part of electrode next to the separator. Moreover, simulation results show that increasing ionic conductivity could significantly prolong the lifetime of the battery. An increase in electrolyte diffusion improves side reaction uniformity. Results also show that positive gradients of morphological parameters across anode electrode, when parameters are changed independently, have considerable effects on uniformity of side reaction. This could be a criterion in choosing new morphologies for the part of anode electrode connected to separator.