LiCoO2 Electrode Research Articles

An advanced all-solid-state Li-ion battery model

A new advanced mathematical model is proposed to accurately simulate the behavior of all-solid-state Li-ion batteries. The model includes charge-transfer kinetics at both electrode/electrolyte interfaces, diffusion and migration of mobile lithium ions in the electrolyte and positive electrode. In addition, electrical double layers are considered, representing the space-charge separation phenomena at both electrode/electrolyte interfaces. The model can be used to simultaneously study the individual overpotential and impedance contributions together with concentration gradients and electric fields across the entire battery stack. Both galvanostatic discharge and impedance simulations have been experimentally validated with respect to 0.7 mAh Li/LiPON/LiCoO2 thin film, all-solid-state, batteries. The model shows good agreement with galvanostatic discharging, voltage relaxation upon current interruption, and impedance measurements. From the performed AC and DC simulations it can be concluded that the overpotential across the LiPON electrolyte is most dominant and is therefore an important rate-limiting factor. In addition, it is found that both ionic and electronic diffusion coefficients in the LiCoO2 electrode seriously influence the battery performance. The present model is generally applicable to all-solid-state batteries where combined ionic and electronic transport takes place and allows for optimizing the battery components to increase the effective energy density, which leads to a decreasing demand for materials and costs.

Water‐Based Paintable LiCoO2 Microelectrodes: A High‐Rate Li‐Ion Battery Free of Conductive and Binder Additives

AbstractA simple, low‐cost, and environmentally friendly water‐based colloidal spray‐painting process is developed to obtain mechanically stable microelectrodes of pure LiCoO2 on stainless steel disks, which display reversible high‐rate discharge capacity and good cyclability. By using a simple nanostructuring process with commercially available LiCoO2, the use of dispersant, conductive, and binder additives during the electrode preparation is completely avoided. The nanostructuring approach is based on: i) planetary ball milling and annealing of LiCoO2 microparticles and ii) LiCoO2 surface modification through dissociative H2O adsorption during contact with air‐ and water‐based colloid preparation. The coating technique involves: i) water‐based colloid preparation, ii) spray‐coating, and iii) post‐treatment (drying and calcination) processes. The effects of particle size distribution, shape, crystallinity, specific surface area, and LiCoO2 surface modification on the electrode architecture (roughness, thickness, packing density, and porosity) are studied, together with their impact on the final electrochemical performances of batteries assembled with such a LiCoO2 electrode.

Tracking Battery Swelling in Uncompressed Li-Ion Cells Via in-Operando X-Ray Radiography and Micro-Tomography

Li-ion battery swelling during cycling is known to be a major cause of battery electrode degradation and can contribute significantly to cell failure [1,2]. The main cause of battery swelling is due to the intercalation of Li into the carbon and LiCoO2 electrodes. This swelling is known to occur anisotropically due to the electric field and Li salt distributions in the electrode [3]. We have used x-ray radiography and µCT to map this swelling, in-operando under charge and discharge conditions, in the layers of an uncompressed Li-ion battery. This study gives insight into the relationship between the battery state of charge and the electrode swelling. The change in swelling due to the electrode geometry is also investigated. Li-ion pouch cells were cycled at 0.5, 1 and 2 C while undergoing x-ray radiography and tomography at the BMIT beamline of the Canadian Light Source. Synchrotron radiography allowed the tracking of the electrode swelling in two dimensions with high temporal (10 s/frame) and spatial resolution (3 µm/pixel). µCT measurements of Li-ion cells were carried out under the same battery cycling conditions and scans were taken at 0, 25, 75 and 100 % state of charge (3 µm/pixel, 225 s/scan). Reconstruction and segmentation of the collected images allowed for the measurement of each electrode volume. From these results it appears that electrode swelling depends on a number of factors, including electrode geometry, charge/discharge rate and battery age. There is also a noticeable degree of irreversibility in the swelling of the electrode, which may be a possible metric for battery degradation. The quantification of the carbon electrode swelling corresponds closely with the theoretical value for lithium intercalation to LiC6, although there is a discrepancy between the swelling value obtained for the LiCoO2 electrode and theory. These results give a valuable insight into the mechanical dynamics of Li-ion batteries under in-operando conditions and will provide validation to, and data for, battery models including stress-strain and degradation models. References Bond, T., Zhou, J., & Cutler, J. (2017). Electrode Stack Geometry Changes during Gas Evolution in Pouch-Cell-Type Lithium Ion Batteries. Journal of The Electrochemical Society, 164(1), A6158–A6162.Chen, C., Wei, Y., Zhao, Z., Zou, Y., & Luo, D. (2019). Investigation of the swelling failure of lithium-ion battery packs at low temperatures using 2D/3D X-ray computed tomography. Electrochimica Acta, 305, 65–71.Zhang, N., & Tang, H. (2012). Dissecting anode swelling in commercial lithium-ion batteries. Journal of Power Sources, 218, 52–55. Figure 1 (a) 2D x-ray radiography of a Li-ion pouch cell and (b) 3D reconstruction of a µCT of a Li-ion pouch cell section. Figure 1

Constructing multidimensional conducting networks on LiCoO2 cathode for enhanced rate performance and cycle stability

The effects of different assemblies of Super P, carbon nanotubes (CNTs) and graphene as hybrid conductive additives on the rate performance and cyclic stability of LiCoO2 (LCO) cathode material were systematically investigated. The results indicated that adding graphene, CNTs or mixture of them into the conventional Super P conductive agent was effective to reduce the overall mass ratio of the carbonaceous conductive additive in the LCO electrode while significantly improving the rate performance and cycle stability. The best electrochemical performance was achieved on the electrode with 1 wt% (G + SP) and 1 wt% CNTs. Microstructural investigations indicated that a multidimensional conducting network had been constructed within this cathode, which provided efficient electronic and ionic transportation pathways, as evidenced by the reduced transport resistance and improved Li-ion diffusion dynamics. With this composition, a high discharge capacity of 118 mA h g−1 was obtained at a current density of 10 C (1400 mA g−1), and a high capacity retention of 92.3% was maintained after 100 cycles at 1 C.

Recovery of Laminar LiCoO2 Materials from Spent Mobile Phone Batteries by High-Temperature Calcination

Sustainable recovery of laminar LiCoO2 materials from spent mobile phone batteries by high-temperature calcination was studied. Graphite powders were removed from the anode when the copper thin foil was attacked. Spent LiCoO2 and aluminum thin foil on the cathode were separated by pulverization and sieving. Spent cathodic materials were calcined at 900 °C to remove impurities and recover the laminar structure of LiCoO2. The structure and morphology of the recovered active materials were studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray. The laminar structure of the recovered LiCoO2 was found to be a favorable feature for the Li+ intercalation/deintercalation. Electrochemical properties of the recovered LiCoO2 electrode during the galvanostatic charge/discharge processes were tested by cyclic voltammetry. The recovered LiCoO2 electrode shows a reversibility and a specific capacitance of 14.0 F/g (at 50 mV/s) indicating that it exhibits favorable properties for use as pseudocapacitor. It is found comparatively that the present process is a more environment-friendly and a lower-cost recycling method, and hence more feasible for industrial applications than the other reported processes.

In Situ Observation of the Insulator-To-Metal Transition and Nonequilibrium Phase Transition for Li1-xCoO2 Films with Preferred (003) Orientation Nanorods.

It is notoriously difficult to distinguish the stoichiometric LiCoO2 (LCO) with a O3-I structure from its lithium defective O3-II phase because of their similar crystal symmetry. Interestingly, moreover, the O3-II phase shows metallic conductivity, whereas the O3-I phase is an electronic insulator. How to effectively reveal the intrinsic mechanism of the conductivity difference and nonequilibrium phase transition induced by the lithium deintercalation is of vital importance for its practical application and development. Based on the developed technology of in situ peak force tunneling atomic force microscopy (PF-TUNA) in liquids, the phase transition from O3-I to O3-II and consequent insulator-to-metal transition of LCO thin-film electrodes with preferred (003) orientation nanorods designed and prepared via magnetron sputtering were observed under an organic electrolyte for the first time in this work. Then, studying the post-mortem LCO thin-film electrode by using ex situ time-dependent XRD and conductive atomic force microscopy, we find the phase relaxation of LCO electrodes after the nonequilibrium deintercalation, further proving the differences of the electronic conductivity and work function between the O3-I and O3-II phases. Moreover, X-ray absorption spectroscopy results indicate that the oxidation of Co ions and the increasing of O 2p-Co 3d hybridization in the O3-II phase lead to electrical conductivity improvement in Li1-xCoO2. Simultaneously, it is found that the nonequilibrium deintercalation at a high charging rate can result in phase-transition hysteresis and the O3-I/O3-II coexistence at the charging end, which is explained well by an ionic blockade model with an antiphase boundary. At last, this work strongly suggests that PF-TUNA can be used to reveal the unconventional phenomena on the solid/liquid interfaces.

Stretchable fabric-based LiCoO[formula omitted], electrode for lithium ion batteries

There is a strong need for stretchable energy storage devices that can accommodate applications such as smart textiles, wearable sensors, flexible display devices, and biomedical implants. Here, we report the development and characterization of a stretchable fabric-based LiCoO2 electrode for lithium ion batteries. The performance of the fabric-based electrode is evaluated in a battery based on solid polymer nanocomposite electrolyte. The battery exhibits a capacity of 0.14 mAh cm−2 and a good cycling stability over 100 charge/discharge cycles. The mechanical properties of the stretchable fabric-based electrode are also evaluated including the tensile strength and % elongation. The electrical conductivity of the silver fabric-based LiCoO2 electrode is found to be higher than that of the rigid Al foil-based LiCoO2 electrode, and it is observed to increase during stretching. The stretchable fabric-based LiCoO2 cathode is a viable and promising candidate for the eventual integration in stretchable smart textile applications.

Degradation mechanism of LiCoO2 under float charge conditions and high temperatures

Lithium-ion batteries (LIBs) consisting of LiCoO2 and graphite electrodes exhibit a trade-off relationship between their reversible capacity and cycle/calendar life in terms of the charge cut-off voltage. That is to say, a higher charge cut-off voltage leads to a larger reversible capacity and shorter cycle life. In order to develop LIBs that satisfy both performance criteria (i.e., have a high reversible capacity as well as a long cycling life), the degradation mechanism of the LiCoO2 electrode under float charge conditions and high temperatures is investigated while focusing on the relationship between the structural deterioration of the electrode and capacity fading. Durability tests performed on graphite/LiCoO2 cells under float charge conditions (4.4 V at 60 °C) induced a drop in the open-circuit voltage as well as capacity fading in the LiCoO2 electrode along with the dissolution of a large number of cobalt ions. Acoustic emission histometry, X-ray diffraction, and transmission electron microscopy analyses of the LiCoO2 electrode after the float charge tests revealed that the degradation of the LiCoO2 electrode during the float charge tests occurred as per the following steps: (1) the HF generated by the decomposition of LiPF6 reacts with the charged LiCoO2 electrode, (2) the charged LiCoO2 electrode is disproportionated into CoO2 and Co2+ ions, and finally (3) the CoO2 having an O1 structure decomposes into cobalt oxides containing cobalt ions in a lower oxidation state, which is associated with the evolution of oxygen gas.

In-Depth Analysis of Binder Distribution within Lithium-Ion Battery Electrode Using SAICAS

Microstructure and mechanical properties of lithium-ion battery (LIB) electrodes become more important as their loading levels increase highly for acheiving high energy density. In this point of view, we should control polymeric binder materials as well as nano-sized conductive materials within LIB electrodes. However, due to the limitation of appropriate analysis tools, it remains one of the challenging issues in LIBs. Fortunately, we have started using a new analysis tool, surface and interfacial cutting analysis system (SAICAS, Daipla Witnes, Japan), to measure adhesion properties of LIB composite electrodes as a function of depths. Based on the general principle, the adhesion strength is approximately propotional to the binder content, the polymeric binder distribution within the electrodes can be unveiled with a SAICAS. In this study, we prepared two LiCoO2 electrodes dried at totally different temperatures, 130 and 230oC, and used to confirm the feasibility to figure out the polymeric binder distribution in the electrode by using SAICAS. First, the adhesion strengths of those electrodes were deliberately measured at every 10 μm depth from the electrode surface as shown in Figure. Simultaneously, not only surface and cross-sectional SEM images but also corresponding EDS atomic mapping were checked to find correlation with adhesion strength. The electrochemical properties such as DC-IR, rate capability, and cycle life at room and high temperatures were also investigated.

Electrochemical and Spectroscopic Analysis of the Ionogel-Electrode Interface.

Ionogels, pseudo-solid-state electrolytes consisting of an ionic liquid electrolyte confined in a mesoporous inorganic matrix, have attracted interest recently due to their high ionic conductivity and physicochemical stability. These traits, coupled with their inherent solution processability, make them a viable solid electrolyte for solid-state battery systems. Despite the promising properties of ionogels, there have been very few investigations of the electrode-ionogel interface. In the present study, X-ray photoelectron spectroscopy, Raman spectroscopy, and electrochemical measurements were utilized to probe the surface reactions occurring at the electrode-ionogel interface for several electrode materials. Our results indicate that the sol acidity initiates breakdown of the organic constituents of the sol and reduction of the transition metals present in the electrode materials. This chemical attack forms an organic surface layer and affects the electrode composition, both of which can impede Li+ access. By modifying the silica sol-gel reaction via a two-step acid-base catalysis, these interfacial reactions can be avoided. Results are shown for a LiCoO2 electrode in which a high Li-ion capacity and stable cycling were achieved.

High Rate Li-Ion Batteries with Cation-Disordered Cathodes

Summary Cation disordering in electrode materials for Li-ion batteries is deemed as an undesired structural feature for achieving high electrochemical activity owing to sluggish Li diffusion in the structure. Therefore, extensive efforts have been made to develop electrode materials having a well cation-ordered structure. We report a new class of cation-disordered material that can achieve reasonable Li diffusion by controlling the way of the cation distribution rather than the degree of cation disordering itself. The fully cation-disordered LiFeSO4F, activated by changes in the synthesis temperature and the Li/Fe ratio, delivers a substantially improved electrochemical performance. It exhibits a superior rate capability up to 100 C (36-s discharge) with ∼ 60 mAh/g and excellent capacity retention for 2,500 cycles at 5 C charge/20 C discharge rate. These results extend the scope of electrode materials to include cation-disordered materials and provide new opportunities for designing them for high-performance Li-ion batteries.

Improving cyclic stability of LiCoO2 charged to 4.5 V by using succinic anhydride as co-solvent

Enhancing the cutoff charge voltage of LiCoO2 can obviously increase energy density but deteriorate its cyclic stability. To address this issue, succinic anhydride (SA) is chosen as an efficient co-solvent to improve the cyclic stability of LiCoO2 electrode charged to 4.5 V (vs. Li/Li+). LiCoO2/Li half-cell with 10 wt% SA achieves 39% increased discharge capacity after 200 cycles at 0.5 C (1 C = 185 mA h g−1), compared to the one in base electrolyte between 3 and 4.5 V. Additionally, the mechanism of how SA improves the cyclic stability of LiCoO2 under high charge voltage is elucidated by electrochemical impedance spectroscopy, linear sweep voltammetry, scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. All the results consistently show that SA yields a more compact and stable passivation layer on the surface of LiCoO2 than that without SA, leading to the much higher cyclic stability for LiCoO2 electrode at high cutoff voltage.

(Invited) Understanding Interfacial Reaction of LiCoO2 Positive Electrode in Aqueous Lithium-Ion Batteries

Since the risk of catch fire using non-aqueous electrolyte solution, aqueous solution-based rechargeable lithium batteries (ARLB) have been highlighted. However, the conventional positive electrodes of lithium transition-metal oxide such as LiCoO2 (LCO) and LiNi1/3Mn1/3Co1/3O2 (NMC) have suffered from poor cyclability in aqueous medium. Representatively, the layered two-dimensional structure of LCO shows notably poor stability, possibly due to the surface degradation from water [1] and proton [2]. The understanding of interfacial reaction of LCO in the aqueous electrolyte solution is still superficial however. Here we present degradation phenomena of LCO electrode in aqueous medium using various X-ray measurement techniques, and suggest the solution to avoid such an irreversible electrochemical reaction. The aqueous solution was prepared with 0.5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and pH was controlled to ~6.8 and 10. In both cases, there was no evidence for the formation of cathode-electrolyte interphase (CEI) on LCO in contrast to the one with non-aqueous electrolyte solution. The direct contact of aqueous electrolyte solution to LCO surface results in the short-range disorder of LCO structure such as the distortion of octahedral CoO6, and irreversible Li+ desertion during 10 cycles. To improve electrochemical reversibility and structural stability of LCO, we prepared the organic protection layer that opened the Li+ mass transport route while inhibiting H2O contact from hydrophobic surface. As a result, the capacity retention was improved to ~85% during 30 cycles at pH ~ 6.8. Furthermore, we developed the way to protect LCO surface by anion engineering and in the absence of protection layer, which give insight into the inner Helmholtz plane (IHP) structure and its effect for LCO degradation in aqueous medium.

Surface Film Degradation on LiCoO2 Electrode by Hydrogen Fluoride Attack at Moderately Elevated Temperature and CuO Addition to Mitigate the Degradation

The degradation of the surface film on a LiCoO2 (LCO) electrode by hydrogen fluoride (HF) attack at moderately elevated temperature (70°C) and the effect of CuO as a HF scavenger were studied. Toward this end, a Li/LCO half-cell was “pre-cycled” to deposit a surface film on the LCO electrode and then stored at 70°C to simulate exposure to moderately elevated temperature. Then, the Li/LCO half-cell was “re-cycled” at 25°C to examine any film damage that occurred during high-temperature storage. It was found that HF attack during storage damages the surface film. Electrolyte oxidation is followed on the film-damaged LCO surface owing to a loss of passivating ability. Concomitantly, a surface film is deposited, which “repairs” the damaged surface. In parallel, the LCO electrode is lithiated (reduced) by the Li+ and electrons produced during electrolyte oxidation. The net result is self-discharge (lithiation) of the LCO electrode. This process (film damage and repair) continues until the Li+ and electron storage sites in LCO are completely filled. The HF scavenging action of CuO added into the LCO electrode effectively mitigates the self-discharge of the LCO electrode by decreasing the HF concentration, which alleviates the damage to the surface film.

Correction to Unveiling the Intrinsic Cycle Reversibility of a LiCoO2 Electrode at 4.8-V Cutoff Voltage through Subtractive Surface Modification for Lithium-Ion Batteries.

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionGet e-AlertsCorrection to Unveiling the Intrinsic Cycle Reversibility of a LiCoO2 Electrode at 4.8-V Cutoff Voltage through Subtractive Surface Modification for Lithium-Ion BatteriesWon Mo SeongWon Mo SeongMore by Won Mo Seong, Kyungho YoonKyungho YoonMore by Kyungho Yoon, Myeong Hwan LeeMyeong Hwan LeeMore by Myeong Hwan Lee, Sung-Kyun JungSung-Kyun JungMore by Sung-Kyun Jung, and Kisuk Kang*Kisuk KangMore by Kisuk Kanghttp://orcid.org/0000-0002-8696-1886Cite this: Nano Lett. 2018, 18, 12, 8071Publication Date (Web):November 12, 2018Publication History Published online12 November 2018Published inissue 12 December 2018https://doi.org/10.1021/acs.nanolett.8b04393Copyright © 2018 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1349Altmetric-Citations4LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (275 KB) Get e-AlertsGet e-Alerts Nano Letters Get e-Alerts

Unveiling the Intrinsic Cycle Reversibility of a LiCoO2 Electrode at 4.8-V Cutoff Voltage through Subtractive Surface Modification for Lithium-Ion Batteries.

The thermodynamic instability of the LiCoO2 layered structure at >0.5Li extraction has been considered an obstacle for the reversible utilization of its near theoretical capacity at high cutoff voltage (>4.6 V vs Li/Li+) in lithium-ion batteries. Many previous studies have focused on resolving this issue by surface modification of LiCoO2, which has proven to be effective in suppressing phase transformation. To determine the extent to which surface protection of LiCoO2 is effective despite its thermodynamic instability and presumably incomplete reversibility involving the O1 phase, here we verify the intrinsic reversibility of bulk LiCoO2 with extended lithium extraction by ruling out the effect of a surface. Specifically, first, we show that, contrary to conventional belief, electrochemical cycling of LiCoO2 at a cutoff voltage of 4.8 V (vs Li/Li+) results in better cycle stability and lower polarizations than those at 4.6 V. We demonstrate, using an exhaustive suite of characterization tools, that the rapid cycle degradation under high-voltage cycling is mostly caused by the formation of a surface resistive layer; however, these damaged surfaces are leached out faster than they are accumulated above a certain potential, which results in superior cyclability compared with that achieved for less oxidative 4.6-V cycling. This beneficial leaching out of the resistive surface layer serves as a "subtractive" surface modification and plays a role in enhancing the cycle stability and is distinguished from conventional "additive" surface modification such as coating. This approach allows us to decouple factors of the bulk and surface degradations that contribute to the capacity fade and leads to the finding that, in the absence of a resistive surface, the capacity retention of a LiCoO2 electrode with 4.8-V cutoff cycling can be intrinsically high, indicating that the instability of the crystalline Li xCoO2 ( x < 0.5) has a limited effect on the cycle stability. Our findings also explain why the strategy of coating foreign materials on the surface of LiCoO2 can improve the high-voltage cycling to some extent despite the expected thermodynamic instability of the highly charged phase.

Dynamic impact of LiCoO2 electrodes for Li-ion battery aging evaluation

As Li-ion batteries (LIBs) see increasing usage for electric vehicles, the effect of transportation history on battery state-of-health remains unclear. In particular, the influence of dynamic impact on battery behavior is investigated in terms of its effect on battery aging. Standard LiCoO2 electrodes were dynamically impacted using a nano-impactor setup, and the relationship between impact cycle number (N = 0, 3, 12, and 1000) and electrochemical performance is studied. Li-ion half cells constructed from impacted electrodes display decreasing first cycle Coulombic and energy efficiency with number of impacts, with electrochemical impedance spectroscopy analysis revealing an increase in the charge-transfer resistance. Accelerated capacity fading through long-term cycling is observed for N = 1000 impacts, with significant voltage hysteresis and decay occurring by cycle 50. Additional ex-situ material characterization of cycled electrodes reveals a negative Raman shift by ∼2–3 cm−1 for the A1g and Eg bands for the N = 1000 case as compared to pristine LiCoO2, correlating to retention of material stress and electrochemical degradation via capacity fading. These results connect mechanical and electrochemical analysis of LIB electrodes, elucidating the relationship between dynamic impact and cell aging.

Ultra‐Fast Cycling of Nanoscale Thin‐Film LiCoO2 Electrodes in Aqueous Electrolytes

AbstractAdditive‐free nanoscale LiCoO2 thin‐films deposited on Si substrates using DC sputtering show exceptional electrochemical performance due to the unique kinetics of the nanoscale thin‐film in aqueous environment. At extremely high scan rates and galvanostatic current densities of up to 100 mV s−1 and 200 C respectively, a capacity retention equivalent to 97 mAh g−1 (4.8 μAh cm−2, 48.3 μAh cm−2 μm−1) is obtained. A significant contribution of non‐diffusion controlled kinetics in a LiCoO2 electrode is shown.

Enhanced Electrochemical Stability of Molten Li Salt Hydrate Electrolytes by the Addition of Divalent Cations.

Water can be an attractive solvent for Li-ion battery electrolytes owing to numerous advantages such as high polarity, nonflammability, environmental benignity, and abundance, provided that its narrow electrochemical potential window can be enhanced to a similar level to that of typical nonaqueous electrolytes. In recent years, significant improvements in the electrochemical stability of aqueous electrolytes have been achieved with molten salt hydrate electrolytes containing extremely high concentrations of Li salt. In this study, we investigated the effect of divalent salt additives (magnesium and calcium bis(trifluoromethanesulfonyl)amides) in a molten salt hydrate electrolyte (21 mol kg–1 lithium bis(trifluoromethanesulfonyl)amide) on the electrochemical stability and aqueous lithium secondary battery performance. We found that the electrochemical stability was further enhanced by the addition of the divalent salt. In particular, the reductive stability was increased by more than 1 V on the Al electrode in the presence of either of the divalent cations. Surface characterization with X-ray photoelectron spectroscopy suggests that a passivation layer formed on the Al electrode consists of inorganic salts (most notably fluorides) of the divalent cations and the less-soluble solid electrolyte interphase mitigated the reductive decomposition of water effectively. The enhanced electrochemical stability in the presence of the divalent salts resulted in a more-stable charge–discharge cycling of LiCoO2 and Li4Ti5O12 electrodes.

Improved electrochemical performance of LiCoO2 electrodes for high-voltage operations by Ag thin film coating via magnetron sputtering

ABSTRACTIncreasing the operation voltage of LiCoO2 (LCO) is a direct way to enhance the energy density of the Li-ion batteries. However, at high voltages, the cycling stability degrades very fast due to the irreversible changes in the electrode structure, and formation of an unstable solid electrolyte interface layer. In this work, Ag thin film was prepared on commercial LCO cathode by using magnetron sputtering technique. Ag coated electrode enabled an improved electrochemical performance with a better cycling capability. After 100 cycles, Ag coated LCO delivers a discharge capacity of 106.3 mAh g-1 within 3 - 4.5 V at C/5, which is increased by 45 % compared to that of the uncoated LCO. Coating the electrode surface with Ag thin film also delivered an improved Coulombic efficiency, which is believed to be an indication of suppressed parasitic reactions at the electrode interface. This work may lead to new methods on surface modifications of LCO and other cathode materials to achieve high-capacity Li-ion batteries for high-voltage operations.