Article pubs.acs.org/JPCC
Surface-Modified Lithiated H2V3O8: A Stable High Energy Density Cathode Material for Lithium-Ion Batteries with LiPF6 Electrolytes Mário Simões,*,† Yoann Mettan,‡ Simone Pokrant,† and Anke Weidenkaff§ Solid State Chemistry and Catalysis group, Empa, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland Belenos Clean Power Holding Ltd, c/o EM Microelectronic Marin SA, Rue des Sors 3, CH-2074 Marin-Epagnier, Switzerland § Materials Chemistry, Institute for Materials Science, University of Stuttgart Heisenbergstr. 3, DE-70569 Stuttgart, Germany † ‡
S Supporting Information *
ABSTRACT: A new one-pot multistep process was developed to coat and stabilize LixH2V3O8 as cathode material for Li-ion batteries with LiPF6-based electrolyte. The crystal structure of LixH2V3O8 was preserved after depositing an amorphous AlOx(OH)y coating. The sample containing higher aluminum content (3 wt % nominal) revealed a homogeneous surface coverage with 3 to 10 nm thickness, whereas for lower aluminum content no clear coverage could be detected by imaging. Spectroscopic techniques were used to confirm the presence of aluminum in all coatings. The electrochemical characterization of LixH2V3O8 cathodes coated with AlOx(OH)y containing 1.5 wt % nominal aluminum showed higher stability for lithium-ion intercalation compared to that of bare LixH2V3O8. For instance, after 200 cycles this material revealed 89% capacity retention compared to 67% for uncoated LixH2V3O8. These results enabled the preparation of cathodes working below 4.2 V with a specific energy higher that 0.5 W h kg−1 for more than 200 cycles.
1. INTRODUCTION Spinels, olivines, and layered compounds have shown interesting properties for cathodic application in lithium-ion batteries. Improving electrochemical energy storage technology, allowing the development of cheap batteries based on abundant elements and with energy density higher than that of today’s state of the art, is a key element for a successful transition from vehicles powered by internal combustion engines (ICEs) to electric drivetrains.1 Further developments in lithium-ion battery technology are needed to allow a more competitive use not only in portable applications but also in electric mobility and in power systems as utility backup. Vanadium is a cheap, widely available, and light element possessing a high oxidation number. These characteristics make it a desirable candidate for intercalation materials in lithium-ion batteries. Vanadium oxides, such as V2O5 are already available in high quantities and low price because of mature synthesis in industrial scale. Vanadium pentoxide, with its layered structure, is one of the most studied vanadate for cathodic applications.2−7 Understanding the evolution of the crystal structure in function of the intercalated Li+ content and its correlation to the electrochemical performance and stability has raised great attention. Close to three lithium equivalents can be inserted into the V2O5 crystal starting from α-V2O5 forming ω-Li3V2O5 when the battery is first discharged down to 1.6 V. Afterward, less than two lithium equivalents can be reversibly intercalated. An irreversible structural change to the ω-V2O5 phase is responsible for this property and restricted the broad © 2014 American Chemical Society
dissemination of this compound as a cathode. Doping techniques were applied to increase electronic conductivity and lithium percolation to stabilize the V2O5 under cycling conditions.8,9 Xerogels and aerogels of hydrated V2O5 are also established because of the ability of these materials to reversibly intercalate high amounts of lithium, allowing the preparation of electrodes with high specific charge.10−12 Intercalation materials such as Li x V 3 O 8 13−16 and H2V3O817−20 were also proposed. The protonated compound H2V3O8 possesses remarkable characteristics for a cathode material. It can be synthesized by hydrothermal reactions, leading to particles with high aspect ratio. Typically, needles with submicrometer diameter (usually in the range of 100 nm) and 10 to 100 μm in length allow for fast lithium diffusion inside the structure during intercalation−deintercalation processes. The morphology allows also good interaction with carbon additives for increased electronic conductivity even without the need of using polymeric binders when preparing electrodes because of high surface area and the formation of a stable network. The electrochemical properties of H2V3O8 are remarkable as up to four lithium equivalents (ca. 400 A h kg−1) can be intercalated in the structure between 4.2 and 1.5 V versuss Li/Li+20,21 with a mean potential close to 2.7 V. This leads to a specific energy density above 1 kW h kg−1, a value Received: March 13, 2014 Revised: May 28, 2014 Published: June 13, 2014 14169
dx.doi.org/10.1021/jp502546w | J. Phys. Chem. C 2014, 118, 14169−14176
The Journal of Physical Chemistry C
Article
electron microscopy−EDX (SEM/EDX), He-ion microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis−differential scanning calorimetry (TGA-DSC), and electrochemical characterization). The objective of this work was to increase the capacity retention of the lithium intercalation under charge and discharge cycling of H2V3O8 as lithium-ion battery cathode by means of a surface coating with aluminum oxyhydroxide.
considerably higher than that of current state of the art cathode materials.22,23 The electrochemical properties of H2V3O8 are competitive with 5 V compounds in terms of energy density, without electrolyte window stability issues. The reversible Li+ intercalation reaction in H2V3O8 occurs below 4.5 V and allows the use of standard electrolytes based on LiPF6/carbonates.24 Crystal water plays a vital role in stabilizing the H2V3O8 structure, but even traces of water weakly bonded to the compound can greatly influence the overall stability of batteries running liquid electrolytes based on carbonates and LiPF6 salt. Lithium-ion batteries with high energy density can be prepared with H2V3O8 cathodes, but stability issues on long-term cycling have to be solved when such electrolytes are used. Published data normally show a few dozen cycles with fast capacity fading.25 Prado-Gonjal et al.20 recently showed that a discharge down to 1 V versus Li/Li+ would allow the intercalation of up to 5 lithium equivalents. The authors were able to detect V3+ species in the host material. Under the described conditions, vanadium dissolution can be expected, but only a few parts per million of vanadium were found in the electrolyte. Wu et al.26 revealed that traces of water present in the battery electroactive core can lead to HF formation. HF is reacting with V2O5 cathodes through a self-catalyzed process, both at open circuit and under potential. The formation of vanadium oxyfluorides could be responsible for the severe capacity fading under charge−discharge cycles. The use of nonfluorinated salts such as LiClO4 allow for better stability in conjunction with vanadium oxide H2V 3O 8 cathodes,18,27 but its use in commercial batteries is still under debate.28,29 In contrast to the majority of lithium-ion battery cathodes available, H2V3O8 shows low thermal stability. It is known that structural water present in this compound (also written as V3O7·H2O) is irreversibly released when heated above 200 °C.2,18 It is generally accepted that the anhydrous material is obtained at a temperature close to 350 °C. This property limits the choice of possible compounds to coat its surface. Several metal oxides like MgO, Al2O3, SiO2, TiO2, ZnO, SnO2, ZrO2, glasses, and phosphates, for example, have been extensively studied as surface coatings for lithium-ion cathodes. It has been reported that these coatings prevent the direct contact of the intercalation compound with the electrolytic solution, suppress undesirable phase transitions, and improve structural stability. As a result, side reactions and heat generation during cycling can be decreased.30 However, because of either processing limitations related to solvent and pH compatibility or thermal stability of H2V3O8, the common wet and solid-state chemistry methods used to decorate the surface of cathodic materials with the referred metal oxides are not suited for H2V3O8. With the aim of limiting side reactions between the surface of H2V3O8 and the electrolyte, a low-temperature water-based process was developed to deposit a coating on the surface of lithiated H2V3O8. To our knowledge, this is the first report of a successful coating for H2V3O8 fibers. The deposition of AlOx(OH)y species was performed by a one-pot multistep reaction starting with phase pure H2V3O8, followed by a chemical lithiation in which the concentration of V(IV) in the structure increases with the lithiation extent. The excess of V(IV) induced by the chemical lithiation allows for the redoxassisted aluminum oxyhydroxide deposition. Electroactive materials (EAM) with nominal aluminum concentrations ranging from 0.5 to 3 wt % were prepared and characterized by several techniques (transmission electron microscopy− energy-dispersive X-ray spectrometry (TEM/EDX), scanning
2. EXPERIMENTAL SECTION 2.1. Hydrothermal Synthesis of H2V3O8 and Preparation of Coated LixH2V3O8. The host compound for lithium intercalation used in the present work was H2V3O8. This material was synthesized by a hydrothermal reaction following a procedure described elsewhere.21,31 The synthesis reaction proceeded as follows: 3 g of VOSO4·5H2O was dissolved in 50 mL of deionized water. Afterward, 2 mL of 25 wt % ammonia (NH4OH) were added to the previous suspension. A dense precipitate formed immediately. The precipitate was then filtered, and the wet solid product was collected. This compound was transferred into a Teflon vessel of an autoclave and dispersed in 1000 mL of distilled water. After 1 mL of 12 M HCl was added, the autoclave was sealed and the suspension treated hydrothermally for 48 h at 220 °C. At the end of the reaction, the green solid was filtered, washed with water and isopropanol, and dried overnight at 80 °C in air. The reaction used to lithiate the fibers with lithiated carbohydrates is also described elsewhere.21 The chemical lithiation of the H2V3O8 fibers was performed using lithium ascorbate as a reducing and complexing agent in aqueous media at room temperature. The desired amount of H2V3O8 was first dispersed in distilled water and then pH neutralized with LiOH· H2O (Alfa Aesar monohydrate 56.5% min.). At the same time, a solution containing 30 mg of ascorbic acid (Sigma-Aldrich, Lascorbic acid 99%) dissolved in 15 mL of Milli-Q water (18.2 MΩ cm) and 15 mg of LiOH·H2O was prepared. This lithium ascorbate solution was subsequently added to the suspension containing the H2V3O8 fibers. After a few minutes under stirring, the pH decreases from 10 to a value close to 7 and the lithiation reaction is then completed. The suspended fibers show now a blue color, indicating an increase of the vanadium(IV) concentration in the material. Considering the charge balance in the compound, this fact is an indication that Li+ was incorporated in the H2V3O8 structure. The desired amount of Al(NO3)3·9H2O was added to this suspension causing the pH to drop to a value between 4.5 and 6 depending on the aluminum salt concentration. The suspension was then heated to 80 °C, and a few drops of a 0.1 M ammonia solution were added slowly. The pH increased to a value between 7 and 8. After 2 h under stirring, the pH of the suspension reached a stable value between 6 and 7. The suspension was then filtered and washed once with water and a polar organic solvent. The filtered compound was finally dried at 80 °C before being stabilized for 1 h at 180 °C under air. Scheme 1 briefly illustrates the coating reaction pathway. Scheme 1. Reaction Pathway for the AlOx(OH)y Deposition
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dx.doi.org/10.1021/jp502546w | J. Phys. Chem. C 2014, 118, 14169−14176
The Journal of Physical Chemistry C
Article
microscopy, XRD, XPS, TGA-DTA, and electrochemical characterization. 3.1.1. Physicochemical Characterizations. The morphology of the surface modifications was evaluated by several imaging techniques, with and without chemical analysis. He-ion microscopy technique was employed to obtain high-resolution images of the surface of pristine and modified LixH2V3O8 fibers. Images registered by such technique on uncoated and 1.5Al_OxHy and 3Al_OxHy fibers are presented in Figure 1.
The coated samples were labeled regarding the nominal aluminum wt % concentrations: for example, the sample containing 1.5 nominal wt % of aluminum is labeled as 1.5Al_OxHy. 2.2. TEM, SEM, EDX, He-Ion Microscopy, XRD, XPS, and TGA Characterizations. The samples were characterized by transmission electron microscopy using a Philips CM30 microscope (300 kV) equipped with a LaB6 filament and a JEOL 2200 FS TEM/STEM microscope (200 kV) equipped with a Schottky field emission electron source, an in-column energy filter, and energy dispersive X-ray spectroscopy detector. The morphology of the fibers was further evaluated by scanning electron microscopy using a FEI Nova NanoSEM 230 with EDX detector and by He-ion microscopy allowing highresolution surface characterization. The crystal structure was evaluated by X-ray diffraction. The powder diffraction patterns were obtained using a PANanalytical X′Pert PRO system equipped with a copper tube, a Johansson monochromator (Cu Kα1 radiation, 1.5406 Å) and an X′Celerator linear detector operating in Bragg−Brentano geometry (θ/2θ). The diffraction patterns were recorded between 5° and 80° (2θ). X-ray photoelectron spectroscopy spectra were acquired using a PHI Quantum 2000 spectrometer with monochromatic Al Kα X-ray radiation (1486.6 eV). The spectra were recorded at room temperature applying a photoelectron takeoff angle of 45° with respect to the surface plane, and all spectra were calibrated to the C 1s line at 284.6 eV. The TGA experiments were performed with a NETZSCH STA 409 CD thermobalance. An alumina crucible was used and heated at 7.5 °C min−1 from 40 to 650 °C in synthetic air with a flow rate of 50 mL min−1. 2.3. Electrochemical Measurements. The pristine LixH2V3O8 was characterized electrochemically in lithium-ion half cells. The cathode electrodes were prepared by mixing 20 mg of pristine and coated LixH2V3O8 electroactive material with 10 mg of carbon black (Super P Li from TIMCAL) in 5 mL of tetrahydrofuran (THF from Alfa Aesar 99%). This suspension was then dispersed ultrasonically for a few minutes until a dark homogeneous suspension was obtained. A desired volume was transferred to a mortar and gently stirred with a pestle at room temperature. The THF evaporation leads to a very dark paste which was then transferred to a titanium current collector (geometric surface close to 1.3 cm2). The electrode was allowed to dry in air before being heat-treated at 180 °C for 10 min. Electrodes containing close to 4 mg of EAM could be prepared by such a method. The batteries were assembled in an argonfilled glovebox (
