The invention relates to new lithium insertion electrode materials based on orthosilicates derivatives, electrochemical generators and variable optical transmission devices having an electrode containing these materials.

Electrode materials are mainly derivatives of transition metals chalcogenides, especially oxides having a lamellar structure like Li_xCoO₂ or spinels like Li_2xMn₂O₄. These materials usually operate via a lithium insertion mechanism with less than one lithium and electron exchanged per transition metal in the unit formula (0 ≤ x ≤ 0.6). Besides, these materials suffer from facile loss of the transition metal which is leached into the electrolyte. Cobalt is prohibitively expensive for large scale application.

Materials with the olivine structure LiFePO₄ (triphylite) and the quasi-isomorphous delithiated material noted as "□FePO₄" where "□" represents the lithium site vacancies, have the advantage of an operating voltage of 3.5 V vs. Li⁺/ Li°, i.e. in the stability window of both liquid and polymer electrolytes with a flat discharge (lithium intercalation) plateau. The materials arc however limited by relatively both slow Li⁺ diffusion kinetics and low electronic conductivity due to the high electronegativity of the phosphate anion. The absence of non-stoichiometry or mutual miscibility for both phases (LiFePO₄ and □FePO₄) is an advantage in terms of the shape of the discharge curve but limits the kinetics of phase transformation which may lead ultimately at high current densities to fragmentation of the material and loss of contact with the carbon grains acting as current collector. The phosphate of this type offers the possibility of exchanging only one lithium per unit formula.

Conversely, it is known that the compounds with either the Li₂SO₄, Li₃PO₄, Li₄SiO₄ and their solid solutions possess relatively high diffusion coefficients for lithium ions, but have no redox activity.

In the present invention, orthosilicates containing the SiO₄^4- tetranion as the building block are used as redox materials by incorporation of at least one transition elements with at least two valency levels. Lithium ingress or egress from the structure to compensate for the valency change of the redox couples during electrode operation and keep overall electroneutrality.

The general formula of the materials of the invention is:

        Li_xM_m-tT_t [SiO₄]

wherein:

• M represents Mn²⁺ or Fe²⁺ and mixtures thereof;
• T represents a metal in the +3 oxidation state, chosen among: Al³⁺, Ti³⁺, Cr³⁺, Fe³⁺, Mn³⁺, Ga³⁺, Zn³⁺, V³⁺; all M, T, being elements residing in octahedral or tetrahedral sites.

The stoichiometric coefficient t, is positive and comprised between zero (inclusive) and 1 (inclusive). Other conditions arc: 0≤x≤2; 1 ≤ m ≤2 ; and x+2m = 4.

Silicon is the second most abundant element in the earth crust, and silicates structures are chemically very robust, and thus electrochemically stable. The preferred transition metals besides iron and manganese are chosen among: titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, niobium and tantalum. The compounds containing a large fraction of iron and/or manganese are preferred due to the abundance and lack of toxicity of these elements. In addition, metals with fixed valency or redox states whose potential lies outside the domain of operation of the electrode can be substituted partially for the transition elements to improve the diffusion characteristics of the lithium ions and/or increase the number of available sites, help control grain size and allow the existence of continuous solid solution between the oxidized and reduced form of the electrode material. These elements are preferably chosen among lithium, magnesium, calcium, zinc aluminum, gallium and tin. For instance, these substitutions allows for the coexistence of iron and/or manganese in two different oxidation states in the same phase, or introduce specific interactions with other elements having redox levels close to those of Fe and Mn (eg.: Fe²⁺/Ti⁴⁺ ⇔ Fe³⁺/Ti³⁺, Mn²⁺/V⁵⁺ ⇔ Mn³⁺/V⁴⁺ etc.) both of which are favorable to electronic conductivity, while disorder on the anionic site provides preferential diffusion sites for Li⁺.

Another aspect of the invention relates to the possibility to substitute the silicon sites by various elements. The orthosilicate anion being isosteric with the sulfate, phosphate, germanate and vanadate anions, the corresponding elements can easily replace silicon in this type of structure, as does boron and aluminum, offering a wide choice of materials with complete control of the charge density on the anionic sites. Similarly, the propensity of orthosilicates structures to accommodate large fractions of either vacancies or interstitial ions is very favorable to lithium ion diffusion while maintaining the integrity of the structure during lithium intercalation-desintercalation processes.

The materials according to the invention have preferably the orthorombic structures determined by X-ray and electron diffraction for lithium phosphate (a and g forms), both the low and high temperature form, where the metal ions occupy partially either the seven unoccupied tetrahedral sites left or the "stuffed olivine" stucture with a partial occupancy of the four octahedral sites (ASTM N° 25-1020 and 15-760 ) or that of lithium orthosilicate Li₄SiO₄. Capacities well in excess of 160 mAh/g can be obtained, which are superior than for the conventional electrode materials. Another advantage of the materials of the present invention is to prevent the leaching of transition metals into the electrolyte due to the high charge of the orthosilicate anion SiO₄^4- which binds selectively the multiply charged cations, including the transition elements.

The compounds of the invention are especially useful, when used alone or in admixture with other redox-active compounds, as a component of stable electrode materials in electrochemical cells, and primary or secondary batteries in which the negative electrode is a source of lithium ions. Typical examples include, in a non limiting way, metallic lithium, lithium alloys, nanodispersions of such alloys in a lithium oxide matrix, such as those obtained by reduction of SnO or SnO₂, a lithium intercalation compound in carbon, especially graphite or carbonaceous matters resulting from the pyrolysis of organic derivatives, like petroleum coke or polyparaphenylene, lithium-titanium spinels of general formula Li_1+yTi_2-x/4O₄ (0≤x≤1, 0≤y≤1, independently) and its solid solutions with other spinels, or a lithium-transition metal mixed nitride like Li_3-xCo_xN (0 ≤ x ≤ 1, ). These materials can be used alone or in admixtures.

Other applications are in supercapacitors or as counter-electrode for controlled reflection-transmission windows ("smart windows") due to the low optical absorption of the compounds in different oxidation states.

The compounds of the present invention are easily prepared by the techniques of inorganic chemistry and ceramics known to the man skilled in the art. In general, oxides, salts or derivatives of the elements to be incorporated in the final composition of the material are mixed in a predetermined ratio and heated at a preset temperature to ensure reaction. Preferably, the metallic salts are those derived from anions that easily decompose at moderate temperatures to leave the oxides. These include the organic salts like acetates, oxalates and in general carboxylic acid derivatives, betadiketonates, alkoxides or mixtures thereof. Preferred inorganic salts similarly are chosen among hydroxides, nitrates, carbonates, sulfates. The so-called sol-gel techniques are especially appreciated as they allow to incorporate the elements in a highly dispersed form which results in homogeneous powders of controllable grain size at relatively low temperature. The silicon alkoxides are especially useful for this purpose as they hydrolyze into silica precursors at room temperature and the silica gel thus formed have a high surface area and adsorbs large quantities of metal salt leading to homogeneous materials upon heat treatment. A wide choice of silicon, titanium, germanium, aluminum derivatives alkoxides are commercially available Variations of this technique are the coprecipitation of elements form aqueous or non-aqueous media and the Pechini process where polyester-polyacid is formed in solution and chelates the ion from solution this step being followed by drying and pyrolysis. The control of the oxidation level of the elements, especially transition elements can be achieved by working in a controlled environment. Low partial pressures of oxygen can be maintained with vacuum, an inert gas or reducing mixtures like hydrogen or carbon monoxide/carbon dioxide mixture. Well defined oxygen activities can thus be obtained from known thermodynamical data. Conversely, air or oxygen bring the transition element to high valence states.

The redox state of the material can also be controlled by chemical extraction or insertion of lithium from the structure, thus mimicking the electrode operation. In a non limiting way, known oxidants able to extract lithium can be chosen among bromine, chlorine, iodine, the nitrosonium or nitronium salts of stable anions like NO⁺ BF₄^- or NO₂⁺PF₆^-. Reducing agents, in a non limiting way can be similarly chosen among: the iodide ion, LiBH₄, the radical-anion derivatives of lithium with condensed aromatic molecules (naphtalene, anthracene, benzophenone), organometallic reagents like butyl lithium, lithium thiolates. The desired degree of lithium content of the compounds of the invention can also be obtained electrochemically.

The following examples are provided to illustrate the present invention and shall by no means by construed as limited its scope.

To 90 ml of water and 10 ml of methanol are added 20.4 g of lithium acetate dihydrate Li(C₂H₃O₂)•2(H₂O), 24,5 g of manganese acetate tetrahydrate (Mn(C₂H₃O₂)₂•4(H₂O)) and 14.8 ml of tetramethoxysilane. The mixture is milled with zirconia pellets in a heavy walled plastic container on a roller mill for 60 hours. The thick slurry is poured onto a PTFE evaporating dish and dried at 90°C in an oven with air circulation. The resulting mass is powdered in a mortar and the organic residues are removed by heat treatment at 350°C in air. The resulting brownish-green powder is transferred to an alumina boat and the compound is treated at 800°C in a electrically heated furnace under a reducing gas stream containing 8% hydrogen diluted in argon. 13 g of Li₂MnSiO₄ are obtained (81% yield).

71.8 g of ferrous oxide FeO and 89.9 of lithium metasilicate Li₂SiO₃ are ball-milled with high carbon steel ball for 4 hours. The resulting mixture is transferred into a quartz ampoule in which some iron turnings are added. The ampoule is evacuated under secondary vacuum while kept at 200°C and sealed, The reaction

        FeO + Li₂SiO₃ ⇒ Li₂FeSiO₄

is complete at 800°C in 4 hours. After cooling, the ampoule is opened under argon and the iron turnings are removed magnetically.

Iron protoxide FeO (71.8 g) and lithium metasilicate Li₂SiO₃ (90 g) are ball-milled with high carbon steel ball for 4 hours. The resulting mixture is transferred into a quartz ampoule in which some iron turnings are added. The ampoule is evacuated under secondary vacuum while kept at 200°C and sealed. The reaction

        FeO + Li₂SiO₃⇒ Li₂FeSiO₄

is complete at 800°C in 4 hours.

21 6 g of iron oxalate dihydrate, 1.79 g manganese oxalate dihydrate and 5.17 g of lithium carbonate are milled similarly as in example 1 with 13.4 g of polydiethoxysiloxane (Gelest Inc, Tullytouwn PA, USA catalog N° PSI-023) in 100 ml of 96 % ethyl alcohol. The resulting slurry is evaporated and calcined at 500°C under air. The cake is pulverized in a ball mill for 20 minutes and the resulting powder is treated at 1050°C for 10 hours in an tubular furnace under an equivolumic mixture of CO₂ and CO. The resulting compound has the formula Li_1.4Fe_1.2Mn_0.1O₄.

To 50 ml of methanol are added 19.4 g of lithium acetate dihydrate Li(C₂H₃O₂) • 2(H₂O), 19.6 g of manganese acetate tetrahydrate Mn(C₂H₃O₂)₂•4(H₂O), 14.8 ml of tetramethoxysilane and 2.75 ml of titanium tetraisopropoxide diluted in 20 ml of isopropanol. The mixture is milled with zirconia pellets in a heavy walled plastic container on a roller mill for 60 hours. The slurry is poured onto a PTFE evaporating dish and dried at 80°C in a air circulation oven. The resulting mass is powdered in a mortar and the organic residues are removed by heat treatment at 350°C in air. The resulting powder is transferred to an alumina boat and the compound is treated as in example 1 at 800°C under a reducing gas stream containing 8 % hydrogen diluted in argon. The compound obtained corresponds to the formula Li_1.9Mn_0.8 Ti_0.1SiO₄ where Ti is in the +3 state and manganese is divalent,

3 g of the compound of example 2, Li₂FeSiO₄ are suspended in 20 ml of acetonitrile and treated with 1.7 g of bromine under stirring at room temperature. After two hours, the suspension is filtered and washed with acetonitrile leaving a solid having the composition Li₁FeSiO₄, This corresponds to an electrode capacity of 172 mAh/g. Similarly, 7 g of the material of example 4 are treated with 3 g of bromine in 50 ml of acetonitrile. After filtration, the material Li_0.1Fe_1.2Mn_0.1O₄ is obtained. It corresponds to an electrode capacity of 260 mAh/g.

5 g of the compound of example 1, Li₂MnSiO₄ are suspended in 40 ml of nitromethane and treated with 12 g nitronium hexafluorophosphate NO₂PF₆ under stirring at room temperature. After two hours, the suspension is filtered and washed with nitromethane leaving a brownish-purple solid having the composition MnSiO₄. This corresponds to an electrode capacity of 345 mAh/g

8 g of the compound of example 1, Li₂MnSiO₄ and 3.36 g of lithium iron phosphate LiFePO₄ are ball-milled with steel balls under an inert atmosphere until the grain size does not exceed 4 microns. The mixture is introduced in a closed-end nickel tube which is evacuated and welded. The tube is treated at 750°C for 10 hours and allowed to cool to room temperature. The solid solution obtained has the formula Li_1.7Mn_0.7 Fe₀.₃Si₀.₇P₀.₃O₄.

A solid-state battery using a 10 mm lithium anode laminated on a 10 mm nickel foil, a 25 mm polymer electrolyte and a 45 mm positive electrode is constructed with a total active surface of 4 cm². The electrolyte is a crosslinked condensation polymer prepared according to J. Electrochem. Soc. 141-7, 1915 (1994) and Journal of Power Sources, 54-1, 40 (1995) with lithium trifluoromethane sulfonimide Li(CF₃SO₂)₂N as a solute corresponding to a 16:1 ratio of oxygens from polymer ether groups to Li⁺. The positive electrode is obtained from a slurry of 53% by volume of PEO, molecular weight 200 000, 40 % by volume of the material of example 4 and delithiated according to example 6, and 7% by volume of Ketjenblack® in acetonitrile, by coating technique on a 12 mm aluminium foil. 1.2 lithium per formula unit can be cycled between 3.5 and 2.4 volts.

A gel type rocking-chair ("lithium ion") battery is constructed using a 40 mm anode made from graphite flakes (4 mm) with an ethylene-propylene-diene binder (5%) by volume, and laminated on a 10 mm copper foil, a 45 mm electrolyte containing a vinylidene fluoride-hexafluoropropylene copolymer (Solvay, Belgium), 30 ww%, fumed silica (5 ww%) with gelled with a liquid elecrolyte consisting of 1.2 molar lithium hexafluorophate in equivolumic ethylene carbonate-ethylmethyl carbonate (65 ww%) mixed solvent The positive electrode consists in a blend of the electode material of example 5 (80 ww%), Ketjenblack® (8 ww%) and the vinylidene fluoride-hexafluoropropylene copolymer used in the electrolyte (12 ww%). The battery is housed in a flat metal-plastic laminate container allowing for the passage of current leads. At 25°C, the battery operation allows the exchange of 1.3 lithium per manganese in the positive electrode in the voltage range 2 - 4.3 volts.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention and including such departures from the present description as come within known or customary practice within the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.