WO1999059218A1 - Primary or secondary electrochemical generator - Google Patents

Primary or secondary electrochemical generator Download PDF

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Publication number
WO1999059218A1
WO1999059218A1 PCT/EP1999/003261 EP9903261W WO9959218A1 WO 1999059218 A1 WO1999059218 A1 WO 1999059218A1 EP 9903261 W EP9903261 W EP 9903261W WO 9959218 A1 WO9959218 A1 WO 9959218A1
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Prior art keywords
electrode
mesoporous
electrochemical generator
generator according
electrolyte
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PCT/EP1999/003261
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French (fr)
Inventor
Michael GRÄTZEL
François R. SUGNAUX
Nicholas Pappas
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Ecole Polytechnique Federale De Lausanne (Epfl) Sri
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Priority to EP99932452A priority Critical patent/EP1086506A1/en
Publication of WO1999059218A1 publication Critical patent/WO1999059218A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0565—Polymeric materials, e.g. gel-type or solid-type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1391—Processes of manufacture of electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1397—Processes of manufacture of electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/581—Chalcogenides or intercalation compounds thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/661—Metal or alloys, e.g. alloy coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/663—Selection of materials containing carbon or carbonaceous materials as conductive part, e.g. graphite, carbon fibres
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M6/00—Primary cells; Manufacture thereof
    • H01M6/14—Cells with non-aqueous electrolyte
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • H01M50/491—Porosity
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the invention relates to a high power density primary or secondary generator and, more specifically a generator in which at least one electrode is composed of a solid material characterized by a mesoscopic morphology.
  • An electrolyte is present in the mesoporous network of said electrode forming a bicontinuous junction of very large surface area with the electroactive solid.
  • the invention also relates to an electrode of this type having a large ion exchange capacity and high electric power density that makes it particularly suited for application as a cathode or anode in a primary or secondary electrochemical generator, such as a lithium ion battery.
  • the invention also relates to processes for obtaining an electrode of this type involving sol gel precipitation and subsequent baking of the electrically active solid material. It discloses methods to produce the specific mesoporous morphology of the electrode, required for optimal performance of the electrochemical generator. More specifically, it discloses the use of surfactant assemblies as templates to induce the desired electrode texture during the formation of the electro-active solid.
  • all its constituent elements, in particular the electrode materials, the current collector, as well as the separator and the composition of the electrolyte have been scrutinized to allow for the judicious selection of the optimal combination of said constituents.
  • the physical configuration of the generator elements notably the effect of electrode and spacer thickness and porosity as well as their conformation has been examined.
  • European patent application EP 0 709 906 A1 discloses a positive electrode composed of a sintered mass of lithium compound oxide, the mean particle size of the electrically active powder being 33 Dm. The particles are pressed into pellets of 1.5 mm size by applying high pressure with simultaneous baking at 350 to 700 °C. They report an improvement in the resistivity when baking at the elevated temperature presumably due to a degree of sintering within the active mass, thus improving somewhat the connectivity in the active material.
  • U.S. Pat. No. 5,604,057 discloses a cathode comprising amorphous microporous, sub-micron-size, lithium intercalateable manganese oxide having an internal surface area greater than about 100 m2/g.
  • Electrodes are fabricated by mixing the oxide with a binder, containing optionally a conducting polymer and heating the composite material at a temperature up to 400 °C.
  • the temperature is limited to this value to prevent crystallization of manganese oxide.
  • the high surface area and the amorphous nature of the active material structure proposed seem to increase the initial capacity of fabricated electrodes but the connectivity is hindered. Interior connectivity of the particles is poor and therefore requires a binder and/or a conducting binder within the electrode fabrication mixture.
  • Another drawback in electrode construction with the amorphous material is that the exposure to temperatures can alter its structure by crystallization, limiting the reported benefits arising from its amorphous structure.
  • 5,211 ,933 and 5,674,644 disclose a method for the low temperature preparation of the spinel LiMn2 ⁇ 4 and layered UC0O2 phases prepared at temperatures less than 400 °C using acetate precursors.
  • the LiMn2 ⁇ 4 powder obtained comprises grains or crystallites ranging in size between 0.3 Dm and 1 Dm. Pellets are pressed containing this powder and about 10% carbon black and used as positive electrodes in lithium ion batteries.
  • the particle sizes claimed are large to suit high rate discharge electrodes and connectivity within the active material particles is not ensured by special bonding, the conductivity enhanced by mixing in carbon powder to the active powder.
  • U.S. Pat. No. 5,700,442 discloses insertion compounds based on manganese oxide usable as positive electrode active material in a lithium battery, prepared by reacting D-Mn ⁇ 2 powder with a lithium compound at 150 to 500 °C for an adequate time to convert these solid precursors to a spinel type.
  • the specific surface of the resulting powder is below 7 nr ⁇ 2/g.
  • the large particle sizes, as deduced from the low specific surface area claimed here are not suited for high rate discharge electrodes.
  • European patent application EP 0 814 524 A1 discloses a spinel-type lithium manganese complex oxide for a cathode active material of a lithium ion secondary battery.
  • the average particle diameter is between 1 and 5 Dm and the specific surface area between 2 and 10 rr ⁇ 2/g.
  • the large particle sizes, thus the low specific surface areas claimed here, are not optimum for high rate discharge electrodes.
  • the invention provides and electrochemical generator in which at least one electrode consists of a mesoscopic, bicontinuous structure, composed of an interconnected solid material and of an interconnected network of mesopores.
  • the electrically active solid is employed as a host for accommodating ions by an insertion process. At the same time, it serves to sustain the flow of electric current during charging and discharging of the battery.
  • Said electrically active solid is in contact with an interconnected porous space filled with electrolyte, the latter serving for ionic transport.
  • Said electrode is characterized by the presence of an extremely large interface between the solid and the electrolyte, comprised between 10 and 3000 m 2 /g electrode material, permitting rapid exchange of ions between the solid and liquid phase.
  • the architecture of the solid phase is designed to overcome the impediment of ionic diffusion in the electrolyte encountered with conventional high surface area electrodes.
  • the specific three dimensional structure of the electrode disclosed by the invention ascertains interconnectivity and mechanical stability of the solid phase providing ease of access of the electrolyte to the entire pore space. Furthermore, it renders possible the conduction of electric current within the solid even in the absence of conductive binders which are mixed with the electrically active material in conventional batteries to enhance electronic conductivity of the electrode.
  • the electrically active material in the form of such a mesoscopic morphology is obtained, for example, by employing surfactant assemblies exerting a templating effect during the formation of the solid from water-soluble precursor compounds or by sol-gel synthesis of a xerogel and subsequent sintering under appropriate conditions.
  • Electrodes of lithium manganate, LiMn2 ⁇ 4 serving as cathode in the electrochemical generator are prepared by casting an aqueous mixture of the manganate precursor by the doctor blading technique or by screen printing or dip coating of the substrate.
  • the gel precursor is precipitated starting from homogeneous solutions of maganous diacetate or manganous diacetylacetonate at 0.13 M and LiOH at 1.3 M.
  • the [Li]/[Mn 3+ ] ratio equals to 10 for each precipitation reaction.
  • the reaction temperature is fixed at 110°C and the pH of the reactants is increased to basic values (-12-13) using the concentrated LiOH solution.
  • the manganous salt solution and the LiOH solution are separately dissolved, before mixing them in a batch reactor with agitation.
  • the reaction time is 2 hours after which the hydrolytic reaction between LiOH and the manganate salt is terminated by rapidly cooling the solution to less than 10°C in an ice bath.
  • the gel it is applied in concentrated form to the substrate. Its consistency is controlled by the proportion of water in the mixture and the effectiveness of ambient drying of the cast layer. Each layer provides 0-3 microns of the dry precursor in porous form. This process is repeated to build up layers of many microns. After deposition of the precursor, the film is heated at 700-750 °C in air with a temperature gradient of 5-10 °C/min. and few minutes rest at peak temperature.
  • Figure 2 present the morphology of the electrode obtained by scanning electron microscopy.
  • An electrode of this type having in its composition an electroactive material in the form of a mesoscopic morphology provides a high energy storage capacity and high power density and may be used in both primary and secondary electrochemical generators.
  • the electrolyte used in such a generator is preferably one containing protons or lithium ions. Other alkali or alkaline earth metals may also be used.
  • the electrolyte contains lithium ions brought into the form of one of its salts such as tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, trifluoromethane sulfonate, bis (trifluorosulfonyl) imide, tris ( trifluorosulfonyl) methide, trifluoromethanesulfonate, trifluoroacetate, tertachloroaluminate or perfluorobutane sulfonate.
  • salts such as tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, trifluoromethane sulfonate, bis (trifluorosulfonyl) imide, tris ( trifluorosulfonyl) methide, trifluoromethanesulfonate
  • the solvent of the electrolyte is an aprotic solvent or a liquid salt, such as ethlyene carbonate, propylene carbonate, dimethylcarbonate diethylcarbonate, dioxolane, butyrolactone, methoxypropionitrile, methoxy- ethoxy propionitrile, methoxy-diethoxypropionitrile, methoxyacetonitrile, tetrafluoro-propanol or mixtures of these solvents.
  • aprotic solvent or a liquid salt such as ethlyene carbonate, propylene carbonate, dimethylcarbonate diethylcarbonate, dioxolane, butyrolactone, methoxypropionitrile, methoxy- ethoxy propionitrile, methoxy-diethoxypropionitrile, methoxyacetonitrile, tetrafluoro-propanol or mixtures of these solvents.
  • a molten salt as a solvent for the lithium ion containing salt, such as methyl-ethyl-imidazolium trifluoromethansulfonate or methy-ethyl-imidazolium bis (trifluorosulfonyl) imide and corresponding di- methyl-ethyl-imidazolium salts with the above anions.
  • the material used for at least one electrode in form of a mesoporous layer may be an electrically active material, but according to the preferred embodiment, this material is chosen so as to form an insertion compound with alkali or alkaline earth metals, thereby providing a secondary electrochemical generator.
  • An electrically active material of this type will be chosen for example from the oxides or chalcogenides of transition metals or their lithiated or partially lithiated forms, such as Ti ⁇ 2, Nb2 ⁇ 5, WO3, M0O3, Mn ⁇ 2, Li y Mn2 ⁇ 4, Hf ⁇ 2, TiS2, WS2, TiSe2, LiyNi ⁇ 2,Li y CoO2, Li y (NiCo)O2, or Sn y O2.
  • an electrode which may be the cathode or the anode, depending on the electrical activity of the electrode used as the counterelectrode, is composed of mesoporous titanium dioxide in the form of anatase or in the form of a mixture of rutile and anatase. containing more than 50% anatase.
  • the titanium dioxide in mesoporous form is prone to form an intercalation compound LiyTi ⁇ 2 in which the intercalation coefficient has a high value between 0.6 and 0.8.
  • the electrochemical generator of the invention is a secondary (rechargeable) generator of the "rocking chair" type, the principles of which was descibed for the first time by M. Armand (Materials for Advanced Batteries, D.W. Murph et al. ed. Plenum press N.Y. 1980, p145).
  • the rocking chair configuration employs Ti ⁇ 2 in the anatase structure as anode material, while LiyMn2 ⁇ 4 in amorphous or crystalline form is used for the cathode.
  • the LiyMn2 ⁇ 4 (y D 2 ) material disclosed by the invention is also present in the form of a mesoporous morphology, yielding cathodes of high capacity, i.e. close to the theoretical maximum of 280 Ah kg -1 .
  • the overall capacity of such a Ti ⁇ 2 LiyMn2 ⁇ 4 rocking chair battery reaches up to 140 Ah kg -' ' , the average cell voltage being 2 volts and the power density based on the material densities 2000 to 3000 W/kg.
  • power densities of 0.5 to 1.5 mW cm ⁇ 2 Dm-1 are observed.
  • the mesoporous electrode structure of the said cell configuration and the battery device as a whole also provide for improved local heat dissipation or exchange from the solid during high rate discharge, thus protecting potentially sensitive (active) materials of the battery from degradation, as exposure to extreme temperatures is minimized.
  • O The mechanical strength is enhanced by controlled sintering process, as the interconnected loops and/or arches are created having a typical aspect ratio of ca. 4, complementing the contribution to the reduction of the system entropy resulting from the natural self-organizing by the templating effect, leading to a structural toughness and robustness of the mesoporous network that can therefore sustain the capillary forces, i.e.
  • the preparation according to the preferred embodiment of the invention results in a crystalline phase that enhances the Li + insertion kinetics as long crystalline rows are formed at high temperatures and at short times.
  • the sintered metal oxide layer density (and pore or future anhydrous electrolyte mass vs. oxide mass ratio) can be controlled by solvent (e.g. water) dilution of the oxide precursor and the aggregation state (oxide mass vs. water) of the precursor solution applied.
  • solvent e.g. water
  • the full connectivity of the mesoporous space combined with the low tortuosity enables a lower porosity (higher active mass to void (for electrolyte) ratio) to be more effective with respect to the electrolyte function i.e. Li + diffusivity.
  • the two proposed mineral electrodes instead of carbon or lithium metal offer safety in a battery device as the risk of explosion is virtually eliminated in the former case.
  • the battery concept as of the preferred embodiment of the invention described above, embodies the safety aspect by considering only non-water sensitive and low toxicity materials for electrode fabrication and for the electrolytes as well. Carbon electrode surfaces are very reactive and especially so at high current rates, and require overcharge protection. Also nickel-cobalt oxides are prone to dangerous reactions as well. The manganese oxides are safe in all respects and no charge protection is necessary.
  • FIG.1 shows a side view of an electrochemical generator of the invention with a partial sector showing the arrangement of the internal layers; (1 ,2) active material mesoporous electrode layers, (3) the porous separator layer, (4,5) the current collectors.
  • FIG.2 shows SEM images of the said mesoporous lithium manganate sintered layer under time and conditions set according to a preferred embodiment of the invention.
  • FIG.3 shows cyclic voltammograms of 1.5 micron layers of the said lithium manganate as a function of treatment temperature.
  • FIG.4 shows the comparison under shallow cycling of the various types of electrodes under identical potential scan and electrolyte conditions, in order to demonstrate the improved kinetics in the charge/discharge performance of said mesoporous lithium manganate material as a positive electrode.
  • FIG.5 shows the electrolyte function at steady-state current of the thin layer battery as perceived by the simulation model describing the electrolyte transport behavior of a completely mesoporous cell, by means of the concentration profiles of the Li + and the coupling anion species within a multilayer design.
  • FIG.6 shows examples of electrolytes and their electrochemical properties used in the cell described above.
  • FIG.7 shows examples of several differently sized Ti ⁇ 2 LiyMn2 ⁇ 4 accumulators with respect to current and voltage discharge behavior. All cells utilize pure cellulose paper (30-40 microns) as the separator.
  • a secondary electrochemical generator of the "rocking chair” type has Ti ⁇ 2/LiyMn2 ⁇ 4 negative and positive electrodes respectively and of the cell configuration depicted in Figure 1.
  • the mesoporous layers are deposited on conducting tin oxide coated glass sheets by the methods previously described.
  • Devices display 20C (or C/0.05) discharge rates under short circuit.
  • the above cell in 30 crr ⁇ 2 form battery and a thin layer configuration comprising 10 micron mesoporous layers of the active materials, according to a preferred embodiment, with a separator of pure cellulose paper of 30-40 microns, utilizing electrolyte comprised of methoxypropionitrile and lithium bis-(trifluorosulfonyl) imide, is observed to sustain a 10C (or C/0.1) discharge rate and maintain a cell voltage of 1.5V, resulting in power densities above 1500 W/kg and energy densities above 210 Wh/kg.
  • the porosity of a typical sintered layer of the nanosized oxide of an average 8Dm layer estimated by mass to surface measurements reaches as high as 30- 70%.
  • Very porous appear to be the layers in the case of 1.5 micron sintered films treated at 400 and 700 °C, the behavior of which is seen in Figure 3.
  • the two insertion levels for the spinel in the 4V range reported in the literature (W. Liu et al., J. Electrochem. Soc, Vol.143, No.
  • 700°C material appears to be less resistive and increased electrochemical reversibility and the capacity much higher as compared to the 400°C treatment due to the effective interconnectivity by sintering and material formation at the high temperature, as seen in Figure 2.
  • the Selectipur particles consist of large hollow spheres (30 micron) the walls of which appear on SEM to have primary grains of 100- 200 microns, extremely tight merged packing; the other appears as a fragmented form of the Selectipur.
  • the pre-formed powders are made into electrodes by mixing the powder with PVA (MW 100000) as binder and graphite particles (Lonza KS-10 or carbon nanotubes) as the conducting matrix. The weight percentage of the constituents are chosen maximum 3% PVA and 10% graphite.
  • the pastes are cast on CTO substrates ambient dried and then heated in an air furnace for 15min at 200 °C. As electrolyte a 1M LiCIO4 in propylene carbonate is the standard in all electrode cycling experiments to enable objective comparison.
  • FIG. 4 Depicted in Figure 4 is the comparison between the Merck powders and a 8 micron mesoporous layer under shallow cycling of the various types of electrodes under identical potential scan and electrolyte conditions, in order to demonstrate the improved kinetics in the discharge performance of said mesoporous lithium manganate material as a positive electrode.
  • Cyclic voltammetric information elucidates the kinetic aspects of the lithium insertion/extraction in the oxides of the above thin film electrodes.
  • the Selectipur and 5.2 micron particle electrodes display higher irreversibility than the electrode from the mesoporous material treated at 700 °C. as evidenced by the higher induced polarization and discharge peak potential shift toward more negative in the case of the commercial materials.
  • the mesoporous layer shows a clear advantage over the even higher capacity (larger active mass) Selectipur electrode that contains conducting carbon.
  • Selectipur 30-40Dm electrode the 8Dm mesoporous layer.
  • a 1.75 to 2 mA/cm2 delithiation current at 1.5V vs. AgCI and 7.5 mA/cm2, in both cases respectively.
  • a 1.25 mA/cm2 current was measured in the first case and 5 mA/cm 2 in the second.
  • a 3 micron LiMn2 ⁇ 4 electrode in PC 1 M Li + , at short circuit gave a 1.4 mA/cm2/micron on charge and discharge.
  • the corresponding value for the delithiation of Ti ⁇ 2 was 0.4 mA/cm2/micron.
  • the above Selectipur 30-40Dm electrode gave 0.055 mA/cm2/micron.
  • Cyclability was the normally expected for the temperature treatment applied and described in the literature. In the glove box, 10% capacity loss was observed within the first 50 cycles at 5 mV/s scan rate and between 1.4 - 2.8V as voltage limits.
  • EXAMPLE 5 Thin layer cell ionic transport conditions at steady-state are modeled for the mesoporous electrode intercalation cell.
  • the cell configuration consists of two 1 millimeter lithium intercalation mesoporous hosts separated by a 20 micron inert and insulating (meso-)porous spacer, one of the active material layers acting as the insertion and the other as the de-insertion (lithium extraction) electrode.
  • the initial electrolyte concentration in monovalent lithium salt is 1M and the free stream diffusion coefficient for Li + in the electrolyte corresponds to the case of propylene carbonate (3 10 " 6 cm 2 /s) of approximately 3 cP.
  • the sought for properties are generally low viscosity, low volatility, solubility of the lithium salt electrolytes and electrochemical stability/chemical compatibility with the electrode materials. Synthesized are room temperature ionic liquids which have very attractive features in the above respects.
  • the electrochemical stability limits fulfill the requirements for 4V cathode materials, as seen from the examples shown in Figure 6a. The potentials are expressed versus iodide/tri- iodide which is +0.15V vs. AgCI. Di-methyl-ethyl-imidazolium imide offers the best cathodic stability advantages.
  • Methoxy-ethoxy-propionitrile is also synthesized having viscosity 2.7cP, 1.1 cP being for methoxypropionitrile and 5cP for methoxy- diethoxy propionitrile.
  • the boiling points are respectively 240C, 165°C and at 10mmHg 152 O C.
  • a secondary electrochemical generator of the "rocking chair” type has Ti ⁇ 2 LiyMn2 ⁇ 4 negative and positive electrodes respectively as described in EXAMPLE 1 and of the cell configuration depicted in Figure 1 , however, with the separator layer (3) consisting of a mesoporous zirconia layer of 8 micron thickness and 60% porous, prepared as described by P.Bonhote et al., J.Phys.Chem.B. (1998), 102, 1498-1507, displays performance characteristics matching those disclosed in EXAMPLE 1.
  • This mesoporous separator offers advantages for electrolyte penetration in the phase of electrolyte filling of the said electrochemical generator and effective retention of the electrolyte within the mesopores and thus within the generator as a whole, combined with effective displacement of gas from the space of the said generator on electrolyte filling, as well as during operation, that is the effective accommodation of volumetric changes on charge/discharge cycling, allowing for electrolyte movement and gas expulsion from the generator layers(1 ,2,3) by forced capillary filling of the pores by the liquid electrolyte.

Abstract

A high power density and high capacity primary or secondary electrochemical generator in which at least one electrode (1,2) is composed of an electrically active solid material, said electrode having a mesoporous texture forming a bicontinuous junction of large specific surface area with the electrolyte. The specific morphology of the electro-active material permits high rates of ion insertion in the solid while allowing for rapid ion transport in electrolyte present in the porous space of the electrode. Specific methods for preparation of said electrode are disclosed, in particular the control of the electrode morphology by use of surfactant assemblies such as surfactant micelles exerting a templating effect during the chemical synthesis of said electro-active material.

Description

PRIMARY OR SECONDARY ELECTROCHEMICAL GENERATOR
FIELD OF THE INVENTION
The invention relates to a high power density primary or secondary generator and, more specifically a generator in which at least one electrode is composed of a solid material characterized by a mesoscopic morphology. An electrolyte is present in the mesoporous network of said electrode forming a bicontinuous junction of very large surface area with the electroactive solid.
The invention also relates to an electrode of this type having a large ion exchange capacity and high electric power density that makes it particularly suited for application as a cathode or anode in a primary or secondary electrochemical generator, such as a lithium ion battery.
The invention also relates to processes for obtaining an electrode of this type involving sol gel precipitation and subsequent baking of the electrically active solid material. It discloses methods to produce the specific mesoporous morphology of the electrode, required for optimal performance of the electrochemical generator. More specifically, it discloses the use of surfactant assemblies as templates to induce the desired electrode texture during the formation of the electro-active solid. To enhance the performance of the electrochemical generator, in particular with respect to its energy density and power density, all its constituent elements, in particular the electrode materials, the current collector, as well as the separator and the composition of the electrolyte have been scrutinized to allow for the judicious selection of the optimal combination of said constituents. The physical configuration of the generator elements, notably the effect of electrode and spacer thickness and porosity as well as their conformation has been examined.
DESCRIPTION OF PRIOR ART
European patent application EP 0 709 906 A1 discloses a positive electrode composed of a sintered mass of lithium compound oxide, the mean particle size of the electrically active powder being 33 Dm. The particles are pressed into pellets of 1.5 mm size by applying high pressure with simultaneous baking at 350 to 700 °C. They report an improvement in the resistivity when baking at the elevated temperature presumably due to a degree of sintering within the active mass, thus improving somewhat the connectivity in the active material. U.S. Pat. No. 5,604,057 discloses a cathode comprising amorphous microporous, sub-micron-size, lithium intercalateable manganese oxide having an internal surface area greater than about 100 m2/g. Electrodes are fabricated by mixing the oxide with a binder, containing optionally a conducting polymer and heating the composite material at a temperature up to 400 °C. The temperature is limited to this value to prevent crystallization of manganese oxide. The high surface area and the amorphous nature of the active material structure proposed seem to increase the initial capacity of fabricated electrodes but the connectivity is hindered. Interior connectivity of the particles is poor and therefore requires a binder and/or a conducting binder within the electrode fabrication mixture. Another drawback in electrode construction with the amorphous material is that the exposure to temperatures can alter its structure by crystallization, limiting the reported benefits arising from its amorphous structure. U.S. Pat.No. 5,211 ,933 and 5,674,644 disclose a method for the low temperature preparation of the spinel LiMn2θ4 and layered UC0O2 phases prepared at temperatures less than 400 °C using acetate precursors. The LiMn2θ4 powder obtained comprises grains or crystallites ranging in size between 0.3 Dm and 1 Dm. Pellets are pressed containing this powder and about 10% carbon black and used as positive electrodes in lithium ion batteries. The particle sizes claimed are large to suit high rate discharge electrodes and connectivity within the active material particles is not ensured by special bonding, the conductivity enhanced by mixing in carbon powder to the active powder.
U.S. Pat. No. 5,700,442 discloses insertion compounds based on manganese oxide usable as positive electrode active material in a lithium battery, prepared by reacting D-Mnθ2 powder with a lithium compound at 150 to 500 °C for an adequate time to convert these solid precursors to a spinel type. The specific surface of the resulting powder is below 7 nrι2/g. The large particle sizes, as deduced from the low specific surface area claimed here are not suited for high rate discharge electrodes.
European patent application EP 0 814 524 A1 discloses a spinel-type lithium manganese complex oxide for a cathode active material of a lithium ion secondary battery. The average particle diameter is between 1 and 5 Dm and the specific surface area between 2 and 10 rrι2/g. The large particle sizes, thus the low specific surface areas claimed here, are not optimum for high rate discharge electrodes.
BRIEF SUMMARY OF THE INVENTION
As compared to these known features of the prior art, the invention provides and electrochemical generator in which at least one electrode consists of a mesoscopic, bicontinuous structure, composed of an interconnected solid material and of an interconnected network of mesopores. The electrically active solid is employed as a host for accommodating ions by an insertion process. At the same time, it serves to sustain the flow of electric current during charging and discharging of the battery. Said electrically active solid is in contact with an interconnected porous space filled with electrolyte, the latter serving for ionic transport. Said electrode is characterized by the presence of an extremely large interface between the solid and the electrolyte, comprised between 10 and 3000 m2/g electrode material, permitting rapid exchange of ions between the solid and liquid phase. The architecture of the solid phase is designed to overcome the impediment of ionic diffusion in the electrolyte encountered with conventional high surface area electrodes. The specific three dimensional structure of the electrode disclosed by the invention ascertains interconnectivity and mechanical stability of the solid phase providing ease of access of the electrolyte to the entire pore space. Furthermore, it renders possible the conduction of electric current within the solid even in the absence of conductive binders which are mixed with the electrically active material in conventional batteries to enhance electronic conductivity of the electrode. The electrically active material in the form of such a mesoscopic morphology is obtained, for example, by employing surfactant assemblies exerting a templating effect during the formation of the solid from water-soluble precursor compounds or by sol-gel synthesis of a xerogel and subsequent sintering under appropriate conditions.
BRIEF DESCRIPTION OF THE INVENTION
Electrodes of lithium manganate, LiMn2θ4 serving as cathode in the electrochemical generator are prepared by casting an aqueous mixture of the manganate precursor by the doctor blading technique or by screen printing or dip coating of the substrate. The gel precursor is precipitated starting from homogeneous solutions of maganous diacetate or manganous diacetylacetonate at 0.13 M and LiOH at 1.3 M. The [Li]/[Mn3+] ratio equals to 10 for each precipitation reaction. The reaction temperature is fixed at 110°C and the pH of the reactants is increased to basic values (-12-13) using the concentrated LiOH solution. In each experiment, the manganous salt solution and the LiOH solution are separately dissolved, before mixing them in a batch reactor with agitation. The reaction time is 2 hours after which the hydrolytic reaction between LiOH and the manganate salt is terminated by rapidly cooling the solution to less than 10°C in an ice bath.
The gel it is applied in concentrated form to the substrate. Its consistency is controlled by the proportion of water in the mixture and the effectiveness of ambient drying of the cast layer. Each layer provides 0-3 microns of the dry precursor in porous form. This process is repeated to build up layers of many microns. After deposition of the precursor, the film is heated at 700-750 °C in air with a temperature gradient of 5-10 °C/min. and few minutes rest at peak temperature. Figure 2 present the morphology of the electrode obtained by scanning electron microscopy.
An electrode of this type having in its composition an electroactive material in the form of a mesoscopic morphology provides a high energy storage capacity and high power density and may be used in both primary and secondary electrochemical generators. The electrolyte used in such a generator is preferably one containing protons or lithium ions. Other alkali or alkaline earth metals may also be used. According to a preferred embodiment, the electrolyte contains lithium ions brought into the form of one of its salts such as tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, trifluoromethane sulfonate, bis (trifluorosulfonyl) imide, tris ( trifluorosulfonyl) methide, trifluoromethanesulfonate, trifluoroacetate, tertachloroaluminate or perfluorobutane sulfonate. According to the preferred embodiment, the solvent of the electrolyte is an aprotic solvent or a liquid salt, such as ethlyene carbonate, propylene carbonate, dimethylcarbonate diethylcarbonate, dioxolane, butyrolactone, methoxypropionitrile, methoxy- ethoxy propionitrile, methoxy-diethoxypropionitrile, methoxyacetonitrile, tetrafluoro-propanol or mixtures of these solvents. Another preferred embodiment of the invention uses a molten salt as a solvent for the lithium ion containing salt, such as methyl-ethyl-imidazolium trifluoromethansulfonate or methy-ethyl-imidazolium bis (trifluorosulfonyl) imide and corresponding di- methyl-ethyl-imidazolium salts with the above anions. The material used for at least one electrode in form of a mesoporous layer may be an electrically active material, but according to the preferred embodiment, this material is chosen so as to form an insertion compound with alkali or alkaline earth metals, thereby providing a secondary electrochemical generator. An electrically active material of this type will be chosen for example from the oxides or chalcogenides of transition metals or their lithiated or partially lithiated forms, such as Tiθ2, Nb2θ5, WO3, M0O3, Mnθ2, LiyMn2θ4, Hfθ2, TiS2, WS2, TiSe2, LiyNiθ2,LiyCoO2, Liy(NiCo)O2, or SnyO2. According to the preferred embodiment of the invention, an electrode, which may be the cathode or the anode, depending on the electrical activity of the electrode used as the counterelectrode, is composed of mesoporous titanium dioxide in the form of anatase or in the form of a mixture of rutile and anatase. containing more than 50% anatase. In the presence of lithium ions, the titanium dioxide in mesoporous form is prone to form an intercalation compound LiyTiθ2 in which the intercalation coefficient has a high value between 0.6 and 0.8. For a cell in which x = 0.8 and where the counterelectrode is made of lithium metal, the theoretical energy density is 400 Wh kg-1 assuming a mean value of the cell voltage of 1.5 V. This high capacity is associated with a high value for the specific power , the later reaching values of 2.3 kW/kg. Results of this kind can not be obtained with the technologies of the prior art, such as described by W.J. Macklin et al. (Solid state Ionics 53 -56 (1992) 694-700) in which both the specific power and capacity of a conventional Tiθ2 electrode is significantly lower.
When an electrically active compound also capable of inserting lithium ions is used for the other electrode, the electrochemical generator of the invention is a secondary (rechargeable) generator of the "rocking chair" type, the principles of which was descibed for the first time by M. Armand (Materials for Advanced Batteries, D.W. Murph et al. ed. Plenum press N.Y. 1980, p145). According to a preferred embodiment of the invention, the rocking chair configuration employs Tiθ2 in the anatase structure as anode material, while LiyMn2θ4 in amorphous or crystalline form is used for the cathode. The LiyMn2θ4 (y D 2 ) material disclosed by the invention is also present in the form of a mesoporous morphology, yielding cathodes of high capacity, i.e. close to the theoretical maximum of 280 Ah kg-1. The overall capacity of such a Tiθ2 LiyMn2θ4 rocking chair battery reaches up to 140 Ah kg-'' , the average cell voltage being 2 volts and the power density based on the material densities 2000 to 3000 W/kg. For such a Tiθ2/LiyMn2θ4 rocking chair thin layer cell configuration power densities of 0.5 to 1.5 mW cm~2 Dm-1 are observed.
The mesoporous electrode structure of the said cell configuration and the battery device as a whole also provide for improved local heat dissipation or exchange from the solid during high rate discharge, thus protecting potentially sensitive (active) materials of the battery from degradation, as exposure to extreme temperatures is minimized. OThe mechanical strength is enhanced by controlled sintering process, as the interconnected loops and/or arches are created having a typical aspect ratio of ca. 4, complementing the contribution to the reduction of the system entropy resulting from the natural self-organizing by the templating effect, leading to a structural toughness and robustness of the mesoporous network that can therefore sustain the capillary forces, i.e. not breaking under the intense local pressure when the pores are filled with electrolyte, and most importantly, able to support the strain and mechanical stresses resulting from the volumetric expansion/contraction resulting from the Li+ insertion/extraction process during the rocking chair battery operation. The later effect induces improved structural integrity on cell cycling, which produces enhanced cyclability as capacity fading due to active material disintegration and discontinuity (decreased electrical connectivity and conductivity) is minimized. Consequently, better solid phase connectivity is ensured with the above mentioned smaller sized "particles" or "grains" or described elemental structural units comprising the mesoporous electrode. The preparation according to the preferred embodiment of the invention, results in a crystalline phase that enhances the Li+ insertion kinetics as long crystalline rows are formed at high temperatures and at short times. The sintered metal oxide layer density (and pore or future anhydrous electrolyte mass vs. oxide mass ratio) can be controlled by solvent (e.g. water) dilution of the oxide precursor and the aggregation state (oxide mass vs. water) of the precursor solution applied. The full connectivity of the mesoporous space combined with the low tortuosity enables a lower porosity (higher active mass to void (for electrolyte) ratio) to be more effective with respect to the electrolyte function i.e. Li+ diffusivity. The two proposed mineral electrodes instead of carbon or lithium metal offer safety in a battery device as the risk of explosion is virtually eliminated in the former case. The battery concept, as of the preferred embodiment of the invention described above, embodies the safety aspect by considering only non-water sensitive and low toxicity materials for electrode fabrication and for the electrolytes as well. Carbon electrode surfaces are very reactive and especially so at high current rates, and require overcharge protection. Also nickel-cobalt oxides are prone to dangerous reactions as well. The manganese oxides are safe in all respects and no charge protection is necessary.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the invention are further set out in the following examples, given by way of non-limiting example with reference to the appended drawings in which:
FIG.1 shows a side view of an electrochemical generator of the invention with a partial sector showing the arrangement of the internal layers; (1 ,2) active material mesoporous electrode layers, (3) the porous separator layer, (4,5) the current collectors.
FIG.2 shows SEM images of the said mesoporous lithium manganate sintered layer under time and conditions set according to a preferred embodiment of the invention.
FIG.3 shows cyclic voltammograms of 1.5 micron layers of the said lithium manganate as a function of treatment temperature.
FIG.4 shows the comparison under shallow cycling of the various types of electrodes under identical potential scan and electrolyte conditions, in order to demonstrate the improved kinetics in the charge/discharge performance of said mesoporous lithium manganate material as a positive electrode. FIG.5 shows the electrolyte function at steady-state current of the thin layer battery as perceived by the simulation model describing the electrolyte transport behavior of a completely mesoporous cell, by means of the concentration profiles of the Li+ and the coupling anion species within a multilayer design.
FIG.6 shows examples of electrolytes and their electrochemical properties used in the cell described above.
FIG.7 shows examples of several differently sized Tiθ2 LiyMn2θ4 accumulators with respect to current and voltage discharge behavior. All cells utilize pure cellulose paper (30-40 microns) as the separator.
DETAILED DESCRIPTION OF THE INVENTION
EXAMPLE 1 According to a first embodiment, a secondary electrochemical generator of the "rocking chair" type has Tiθ2/LiyMn2θ4 negative and positive electrodes respectively and of the cell configuration depicted in Figure 1. The mesoporous layers are deposited on conducting tin oxide coated glass sheets by the methods previously described. Devices display 20C (or C/0.05) discharge rates under short circuit. The above cell in 30 crrι2 form battery and a thin layer configuration comprising 10 micron mesoporous layers of the active materials, according to a preferred embodiment, with a separator of pure cellulose paper of 30-40 microns, utilizing electrolyte comprised of methoxypropionitrile and lithium bis-(trifluorosulfonyl) imide, is observed to sustain a 10C (or C/0.1) discharge rate and maintain a cell voltage of 1.5V, resulting in power densities above 1500 W/kg and energy densities above 210 Wh/kg.
EXAMPLE 2
The porosity of a typical sintered layer of the nanosized oxide of an average 8Dm layer estimated by mass to surface measurements reaches as high as 30- 70%. Very porous appear to be the layers in the case of 1.5 micron sintered films treated at 400 and 700 °C, the behavior of which is seen in Figure 3. The two insertion levels for the spinel in the 4V range reported in the literature (W. Liu et al., J. Electrochem. Soc, Vol.143, No. 3, (1996), 879-884) for 200nm layers at 0.05mV/s, appear very distinct for our 1.5 micron thickness recorded at a sweep rate over two orders of magnitude higher, demonstrating the higher reversibility in the intercalation kinetics in comparison to the earlier materials, attainable by decreasing the particle size to the nanometer range and simultaneously ensuring effective particle connectivity and mesoporosity. The
700°C material appears to be less resistive and increased electrochemical reversibility and the capacity much higher as compared to the 400°C treatment due to the effective interconnectivity by sintering and material formation at the high temperature, as seen in Figure 2.
EXAMPLE 3
Sintering the primary particles in order to obtain a bicontinuous porous network and thus a high connectivity should therefore be determinant to performance. The idea toward improving the manganates by imparting mesoscopic morphology to a continuous phase i.e. by retaining the connectivity as well as decreasing the primary particle to the nanometer range, significant improvements to the intercalation properties are expected. Making intimate electrical contact within the active mass regardless of size and shape of the oxide, results in enhanced utilization of the oxide and is therefore very desirable. For comparison electrodes are made from the commercial spinel LiMn2θ4 materials, namely the Selectipur and 5.2 micron particle powders, both supplied by Merk. The Selectipur particles consist of large hollow spheres (30 micron) the walls of which appear on SEM to have primary grains of 100- 200 microns, extremely tight merged packing; the other appears as a fragmented form of the Selectipur. The pre-formed powders are made into electrodes by mixing the powder with PVA (MW 100000) as binder and graphite particles (Lonza KS-10 or carbon nanotubes) as the conducting matrix. The weight percentage of the constituents are chosen maximum 3% PVA and 10% graphite. The pastes are cast on CTO substrates ambient dried and then heated in an air furnace for 15min at 200 °C. As electrolyte a 1M LiCIO4 in propylene carbonate is the standard in all electrode cycling experiments to enable objective comparison. Depicted in Figure 4 is the comparison between the Merck powders and a 8 micron mesoporous layer under shallow cycling of the various types of electrodes under identical potential scan and electrolyte conditions, in order to demonstrate the improved kinetics in the discharge performance of said mesoporous lithium manganate material as a positive electrode. Cyclic voltammetric information elucidates the kinetic aspects of the lithium insertion/extraction in the oxides of the above thin film electrodes. The Selectipur and 5.2 micron particle electrodes display higher irreversibility than the electrode from the mesoporous material treated at 700 °C. as evidenced by the higher induced polarization and discharge peak potential shift toward more negative in the case of the commercial materials. The mesoporous layer shows a clear advantage over the even higher capacity (larger active mass) Selectipur electrode that contains conducting carbon.
EXAMPLE 4
The preferential kinetics are also exemplified by the comparison of the
Selectipur 30-40Dm electrode the 8Dm mesoporous layer. In the same voltammetric set up and under a potential scan of 100mV/s a 1.75 to 2 mA/cm2 delithiation current at 1.5V vs. AgCI and 7.5 mA/cm2, in both cases respectively. At 20mV/s a 1.25 mA/cm2 current was measured in the first case and 5 mA/cm2 in the second. As another example a 3 micron LiMn2θ4 electrode in PC, 1 M Li+, at short circuit gave a 1.4 mA/cm2/micron on charge and discharge. The corresponding value for the delithiation of Tiθ2 was 0.4 mA/cm2/micron. The above Selectipur 30-40Dm electrode gave 0.055 mA/cm2/micron.
Cyclability was the normally expected for the temperature treatment applied and described in the literature. In the glove box, 10% capacity loss was observed within the first 50 cycles at 5 mV/s scan rate and between 1.4 - 2.8V as voltage limits.
EXAMPLE 5 Thin layer cell ionic transport conditions at steady-state are modeled for the mesoporous electrode intercalation cell. The cell configuration consists of two 1 millimeter lithium intercalation mesoporous hosts separated by a 20 micron inert and insulating (meso-)porous spacer, one of the active material layers acting as the insertion and the other as the de-insertion (lithium extraction) electrode. The initial electrolyte concentration in monovalent lithium salt is 1M and the free stream diffusion coefficient for Li+ in the electrolyte corresponds to the case of propylene carbonate (3 10"6 cm2/s) of approximately 3 cP.
In the depicted results of the model the concentration profile of the Li+ (also the overlapping curve for the anion) appears in Figure 5a and the potential drop across the cell in Figure 5b when approx. 6 mA/cm2 are passing through the cell, the minimum current limiting the supply of Li+ to one side of the intercalating electrode. The conductivity of the host solid is not considered here. The electrode porosities are 50% and 90% for the separator layer. Anything smaller than 1000 micron (1mm) would increase the limiting currents. It is noted that the absence of supporting electrolyte in this case aids the lithium ion diffusion or transport but at a cost of potential in the form of IR drop across the cell (here just under 0.2 V, as depicted in the Figure 5b. This calculation reveals that for electrodes of 10 microns in the above configuration, currents as high as 500mA/cm2 are possible as far as electrolyte performance is concerned. In addition, the Tiθ2/cellulose paper/LiyMn2O4 cells, with active mesoporous layers of about 10 microns, could deliver more than 5mA/cm2 at short circuit, with molten salts containing 1-2 M lithium - bis (trifluorosulfonyl) imide that has a much higher viscosity (>40 cP) than in the propylene carbonate case, and thus a lower Li+ diffusion coefficient.
EXAMPLE 6
From the viewpoint of electrolyte investigation and characterization the sought for properties are generally low viscosity, low volatility, solubility of the lithium salt electrolytes and electrochemical stability/chemical compatibility with the electrode materials. Synthesized are room temperature ionic liquids which have very attractive features in the above respects. The electrochemical stability limits fulfill the requirements for 4V cathode materials, as seen from the examples shown in Figure 6a. The potentials are expressed versus iodide/tri- iodide which is +0.15V vs. AgCI. Di-methyl-ethyl-imidazolium imide offers the best cathodic stability advantages.
Solubility of more than 2M lithium-bis (trifluorosulfonyl) imide can be reached in methoxy-diethoxy propionitrile and in methoxypropionitrile. The electrochemical stability of this newly synthesized organic solvent as evidenced by the electrochemical window is given in Figure 6b. The corresponding stability range for methoxypropionitrile is depicted by the bar in this figure, and the operation limits of the lithium hosts Tiθ2 and lithium manganate are positioned against this scale (ferrocene potential is +0.4V vs. AgCI) in Figure 6c. These molecular structures are designed to exploit the strong solvating properties of the ether groups toward Li+, combined with the low viscosity for
Li+ diffusion and the relatively high boiling points of these compounds to decrease their volatility. Methoxy-ethoxy-propionitrile is also synthesized having viscosity 2.7cP, 1.1 cP being for methoxypropionitrile and 5cP for methoxy- diethoxy propionitrile. The boiling points are respectively 240C, 165°C and at 10mmHg 152OC.
EXAMPLE 7
According to a second embodiment, a secondary electrochemical generator of the "rocking chair" type has Tiθ2 LiyMn2θ4 negative and positive electrodes respectively as described in EXAMPLE 1 and of the cell configuration depicted in Figure 1 , however, with the separator layer (3) consisting of a mesoporous zirconia layer of 8 micron thickness and 60% porous, prepared as described by P.Bonhote et al., J.Phys.Chem.B. (1998), 102, 1498-1507, displays performance characteristics matching those disclosed in EXAMPLE 1. This mesoporous separator offers advantages for electrolyte penetration in the phase of electrolyte filling of the said electrochemical generator and effective retention of the electrolyte within the mesopores and thus within the generator as a whole, combined with effective displacement of gas from the space of the said generator on electrolyte filling, as well as during operation, that is the effective accommodation of volumetric changes on charge/discharge cycling, allowing for electrolyte movement and gas expulsion from the generator layers(1 ,2,3) by forced capillary filling of the pores by the liquid electrolyte.

Claims

CLAIMS What we claim is:
1. A high capacity and high power density primary or secondary electrochemical generator having two electrodes supporting different electroactive materials, said electrodes being connected together by an electrolyte, characterized in that the electroactive material used in the composition of at least one electrode includes an oxide or chalcogenide of transition metals or their lithiated or partially lithiated forms selected from Tiθ2, Nb2θ5, WO3, V2O5, M0O3, Mnθ2, LixMn2θ4, Hfθ2, T1S2,
WS2, TiSe2, LixNiO2, LixCoO2, Lix(NiCo)O2, Fe2θ3, Fe3θ4, RuOx, FexS2, RuxS2, M0S2, WS2, lrxO2, CexO2, LixNayMnOzln (n<1), lnxO3, Taχθ5, SnMχOy, where M is one of glass forming metallic elements or a mixture of these elements, or SnxO2 in mesoporous form having a pore size of 0.001 to 10 micron and a specific surface area between 2 and
2000 rrι2/g.
2. An electrochemical generator according to claim 1 characterized in that the mesoporous electrode is characterized by a three dimensional bicontinuous structure consisting of an interconnected solid phase material used for reversible ion intercalation and for electronic transport which is in contact with an interconnected porous space filled with electrolyte the latter serving for ionic transport, said electrode being characterized by a morphology of the solid phase that is designed to overcome impediment of ionic diffusion in the electrolyte encountered with conventional high surface area electrodes and a three dimensional structure ascertaining interconnectivity, mechanical stability of the solid phase as well as access of the electrolyte to the entire pore space.
3. An electrochemical generator according to claim 1 or 2 characterized in that the mesoporous electrically active material is prepared by precipitating a precursor compound of said material in aqueous solution via a sol-gel method followed by deposition of the precursor on a conductive support and sintering at a temperature between 300 and 800 °C.
4. An electrochemical generator according to any of claims 1 to 3 characterized in that the mesoporous electrically active material is prepared by chemical reaction between precursor compounds in aqueous solution in the presence of surfactant micelles, the said surfactant micelles acting as templates to produce the desired mesoporous morphology, particularly in the form of ordered hexagonal arrays.
5. An electrochemical generator according to any of claims 1 to 4 characterized in that the electrically active material is composed of elongated interconnected members, that are connected to other elongated members in at least two points of an aspect ratio of at least 4 and of which the small dimension of the member is smaller than 300 nm.
6. An electrochemical generator according to any of claims 1 to 5 characterized in that the electrically active material is prepared by inclusion of solvents to the particle or precursor in order to exert control over the texture and the morphology of the said material and of the porosity of the electrode, which can be controlled from 70% to 25% by changing the ratio of oxide precursor grains versus solvent.
7. An electrochemical generator according to any of claims 1 to 6 characterized in that the mesoporous electrically active material is comprised of mesoporous 5-20 micron beads or rods which are electrically connected together by compressing in the form of pellets of films a mixture composed of said beads or rods, of carbon powder or conducting polymer or other matrix and of a bonding material contained in solvent, then drying said mixture on a conducting support and/or baking at temperature and long enough time for binder or conductive matrix conversion to desired state.
8. An electrochemical generator according to any of claims 1 to 8 characterized in that the electrolyte contains alkali or alkaline earth metals in cationic form.
9. An electrochemical generator according to any of claims 1 to 5 or 7 to 8 and claim 6 characterized in that the alkali metal is lithium on the form of one of its salts chosen from tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, trifluoromethane sulfonate, bis-(trifluorosulfonyl) imide, tris-(trifluorosulfonyl)methide, trifluoro- methanesulfonate, trifluoroacetate, tetrachloroaluminate or perfluorobutane sulfonate.
10. An electrochemical generator according to any of claims 1 to 9, characterized in that the electrolyte includes and aprotic solvent selected from ethlyene carbonate, propylene carbonate, dimethylcarbonate, diethylcarbonate, dioxolane, butyrolactone, methoxypropionitrile, methoxy-ethoxy propionitrile, methoxy-diethoxypropionitrile, methoxyacetonitrile, tetrafluoro-propanol or mixtures of these solvents.
11. An electrochemical generator according to any of claims 1 to 10 characterized in that the electrolyte includes a molten salt as a solvent for the lithium ion containing salt, such as methyl-ethyl-imidazolium trifluoromethansulfonate, methy-ethyl-imidazolium bis (trifluorosulfonyl) imide or alkylguanidinium bis (trifluorosulfonyl) imide.
12. An electrochemical generator according to any of claims 1 to 11 , characterized in that the electrically active material of one electrode or of each of the two electrodes, is capable of forming an intercalation compound with the alkali or alkaline earth metal.
13. An electrochemical generator according to any of claims 1 to 12, characterized in that the electrically active material of one electrode is composed of Tiθ2 in anatase form having a mesoporous structure.
4. An electrochemical generator according to any of claims 1 to 13, characterized in that the negative electrode includes mesoporous Tiθ2 and the composition of the positive electrode includes LiyMn2θ4 (y <2) in amorphous or crystalline form, the electrolyte being composed of a 1 M solution of lithium bis-(trifluorosulfonyl) imide in methoxypropionitrile as a solvent.
15. An electrochemical generator according to any of claims 1 to 14, characterized in that the mesoporous electrically active material comprised of mesoporous 5-20 micron beads or rods form an electrode by templated ordering, manipulative stacking or arrangement of the said 5-20 micron secondary mesoporous particles in a manner that controls particle orientation, the secondary porosity and the said particle electrical contact by means of an additive conducting matrix or by treatment
(chemical, temperature, external energy field such as ultrasonic, electromagnetic) of the conducting additive or its precursor or of the template itself in order to form the conductive phase.
16. An electrochemical generator according to any of claims 1 to 12 or 14 to 15, and claim 13, characterized in that separator (3) is a porous or mesoporous, high porosity insulating material such as ceramic (zirconia, alumina) or glass or polypropylene, the above in the forms of continuous layers or particle spacers and the current collector-substrate (4,5) is taken from options such as carbon, graphite paper, stainless steel, titanium or aluminium alloy, DSA, or consist of the conductive matrix incorporated for conductivity enhancement. Also, characterized in that a distributed within the active mass or dendritic conductive collector of the above is used.
PCT/EP1999/003261 1998-05-12 1999-05-08 Primary or secondary electrochemical generator WO1999059218A1 (en)

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