Field of the Invention

The present invention relates to cathode materials for lithium ion accumulators based on vanadium oxide compounds that exhibit a layered structure, and are stabilized by means of titanium od aluminum. The compounds of this invention are active electrode materials based on NaCI type structure, and are represented by the general chemical formula Li _x VMO ₄ , wherein typically x=2, and M stands for a non-transition metal or a mixture of non-transition metals having an oxidation state of +3 or +4.

The present invention pertains to the field of chemistry, specifically to the field of electrical energy storage on the base of reversible redox reactions. It relates to the utilization of the compounds based on lithium vanadium oxide stabilized, by means of titanium or aluminum, in lithium ion accumulators exhibiting activity in the potential window of 1 V to 4.6V with respect to the potential of metallic lithium.

Relevant Prior Art

Cathode materials for lithium ion accumulators are typically layered oxides of transition metals represented by the chemical formula LiM0 ₂ (M=Co, Ni, Mn). Their applicability is limited either by the amount of the deinserted (deintercalated) lithium in the case of LiCoO ₂ , or by a rather difficult synthesis in the case of LiNi0 ₂ , or by the electrochemical instability of the layered oxide in the case of LiMnO ₂ . Recently, more and more research has been directed to the discovery of novel cathode materials based on the polyanionic structure XO ^n" , wherein X=S, P, As, Mo or W, for example LiFeP0 ₄ , and to materials having a spinel structure (LiMn ₂ O ₄ ) with the sole aim to enhance the energy density of lithium ion accumulators. The spinel structure also enables the use of mixtures of various cations having an oxidation state of +3 and +4. In the literature have been known cases of synthesizing a LiVTi0 compound having a spinel structure, which means that the oxide structure is a three-dimensional (3D) network that is different from layered compounds wherein the oxides are in the form of a two-dimensional (2D) network; presented in the article J. Barker, M.Y. Saidi, J.L Swoyer (2003), Electrochemical insertion properties of lithium vanadium titanate, LiVTi04: Solid State Ionics, volume 167, pages 413-418 and employed as active material in lithium ion accumulators. In the literature has also been known a group of cathode materials for lithium ion accumulators having a layered structure Li ₂ MTiO ₄ (M= Fe, Mn, Ni, Co). Their synthesis and electrochemical characterization are described in the following articles: L Sebastian, J. Gopalakrishnan, Li ₂ MTi0 ₄ (M = Mn, Fe, Co, Ni): New cation-disordered rocksalt oxides exhibiting oxidative deintercalation of lithium, Synthesis of an ordered Li ₂ NiTi0 ₄ . JOURNAL OF SOLID STATE CHEMISTRY 172 (1): 171-177 APR 2003; and in M. Kuzma, R. Dominko, A. Meden, D. Makovec, M. Bele, J. Jamnik, M. Gaberscek. Electrochemical activity of Li ₂ FeTi0 ₄ and Li ₂ MnTi0 ₄ as potential active materials for Li ion batteries: a comparison with Li ₂ NiTi0 ₄ . JOURNAL OF POWER SOURCES, 2009, 189, 81-88). No examples of the utilization of pure lithium vanadium oxides having a layered structure as active cathode material for lithium ion accumulators have been known as yet in the literature.

Technical problem

One of the goals of providing modern materials for lithium ion accumulators, is to achieve the highest possible energy density of the said materials, namely over 150 mAh/g. These new materials ought to be inexpensive, widely available and environmentally innocuous. The compounds based on vanadium, titanium and aluminum satisfy the conditions of availability and environmental acceptance. A higher energy density is, however, attained only by providing a higher specific capacity. The cathode materials that are at present utilized in lithium ion accumulators exhibit a specific capacity of up to 140 mAh/g; several known laboratory materials exhibit a practical capacity of 150-180 mAh/g. Recently mentioned materials are for example Li ₂ rmTiO ₄ and L.i ₂ 7777SiO ₄ , wherein Tm= Mn, Fe, Ni, Co; their theoretical capacity is over 300 mAh/g. Their applicability is, however, limited owing to an exacting synthesis, poor resistance in the air, and a very low specific conductivity.

A solution to this problem is represented by active electrode materials for lithium ion accumulators exhibiting a layered structure, denominated as NaCI type structure, of the chemical formula Li _x VMO ₄ , wherein x = 0.1 - 3 and M stands for a non-transitional metal or a mixture of non-transition metals having an oxidation state of +3 or +4. Large deposits of vanadium ensure its availability, and lithiated vanadium oxide is environmentally acceptable. The structure forming with a non-transition metal of an oxidation state of +3 or +4 a layered structure, which may host more than one lithium for each unit of the chemical formula, combines all characteristics of a potential cathode material for lithium ion accumulators. This invention suggests the following two hitherto unknown active electrode materials based on lithiated vanadium oxide, Li ₂ VTi0 ₄ and Li ₂ VAIO ₄ that exhibit an excellent electrochemical activity in lithium ion accumulators. According to this invention are manufactured electrode materials from initial precursors TiO ₂ , V ₂ 05, AI2O3 and LiOH, which are dissolved or dispersed in an aqueous or organic medium in the presence of organic acids and/or dispergators.

Description of the Solution of the Problem

This invention is represented by the following drawings and is described by the illustrative embodiments:

FIG. 1 : a) is an X-ray powder diffractogram of the material based on Li ₂ VTi0 ₄ manufactured according to Example A. The powder diffractogram shows in the material the predominance of the crystalline compound Li ₂ VTi0 ₄ that crystallizes in the spatial group Fm3m, and b) is an X-ray powder diffractogram of the material on the base of Li ₂ VAIO ₄ manufactured according to Example C. The powder diffractogram shows in the material the predominance of the crystalline compound Li ₂ VAI0 ₄ that crystallizes in the spatial group R-3m.

FIG. 2: is a plot of charge/discharge curves during the first 24 cycles for the material based on Li ₂ VTi0 in the potential window of 2V - 4.2V with respect to metallic lithium that was manufactured according to Example A. The current density amounted to 15.2 mA/g of the active material.

FIG. 3: is a plot of charge/discharge curves during the first 9 cycles for the material based on Li ₂ VTi0 ₄ in the potential window of 1V - 4.4V with respect to metallic lithium that was manufactured according to Example A. The current density amounted to 15.2 mA/g of the active material.

FIG. 4: is a plot of charge/discharge curves during the first 12 cycles for the material based on Li ₂ VAIO ₄ in the potential window of 2V - 4.1V with respect to metallic lithium that was manufactured according to Example C. The current density amounted to 15.2 mA/g of the active material.

FIG. 5: depicts the reversible capacity of Li ₂ VTi0 ₄ in the potential window of 1V - 4.4V (squares), the reversible capacity of Li ₂ VTiO ₄ in the potential window of 2V - 4.2V (circles), and the reversible capacity of Li ₂ VAI0 ₄ in the potential window of 2V - 4.1V (triangles) dependent on the performance cycle.

The essential novelty of the proposed cathode materials in comparison with the hitherto known active materials for lithium ion accumulators resides in the utilization of compounds based on lithiated vanadium oxides having a layered structure based on the NaCI type structure and represented by the chemical formula Li _x VM0 ₄ , wherein x = 0.1 - 3 and M stands for a non-transition metal or a mixture of non-transition metals having an oxidation state of +3 or +4. According to the invention the non-transition metal may be titanium or aluminum. The new active materials for lithium ion accumulators can be represented by the following chemical formulas: l_i _x VTi0 ₄ or Li _x VAI0 ₄ , wherein the meaning of x may be within the range 1 to 3, preferably x=2. The proposed new cathode materials exhibit a considerable electroactivity (up to 2 times higher in comparison with the hitherto employed materials in lithium ion accumulators); "considerable electroactivity" means that such materials offer a capacity at least 150 mAh/g or more. The cathode materials according to this invention may be prepared from the initial precursors T1O ₂ , V2O5, AI ₂ O ₃ and LiOH by dissolving or dispersing in an aqueous or in an organic medium, in the presence of organic acids and/or dispergators. The starting materials are in the form of powders having a particle size of 5 nm to 0.5 mm. The concentration of the precursors varies within the range 10g - 100g particles per liter of the liquid in which the particles are dispersed or dissolved. The dispersing and dissolving is performed with the aid of stirrers such as magnetic stirrers, anchor stirrers, propeller stirrers, and turbine stirrers; on the other hand, the dispersing is performed by means of ultrasound sonification, and shaking. The liquid is typically a mixture of organic solvents and water, wherein the mutual ratio of organic solvents and water depends on the employed precursors, organic acids as well as dispergators. The organic acids employed in the preparation of the dispersion are typically chosen from the group of organic acids having a low number of C atoms and optionally several carboxy groups per molecule, such as citric acid, oxalic acid, and oleic acid. The concentration of organic acids in the obtained dispersion is up to 500 g per one liter of the dispersion. The employed dispergators are typically chosen from the group of nonionic dispergators, such as Brij56, Tween 80, and Pluronic 123. The concentration of the dispergators in the obtained dispersion is within the range 0.001 mol/L to 10 mol/L. The obtained dispersions are either dried at a temperature of 25°C to 100°C or lyophilized under supercritical conditions and disintegrated with the aid of a crushing or grinding apparatus or a mill, and subjected to thermal treatment at a temperature over 500°C in an inert atmosphere (nitrogen, argon, xenon, helium) or in a reductive atmosphere (mixture of the said gases with hydrogen). The heat-up time is 1°C/min to 20°C/min; the oven dwell time is 1 minute to 100 hours. After completion of thermal treatment the fabricated cathode materials are stored in an inert atmosphere (in an argon-filled chamber containing less than 10 ppm of water), or in a vacuum package. The proposed cathode materials may be utilized for electrical energy storage purposes in lithium ion accumulators as materials for the construction of positive electrodes. The fabrication of the electrodes requires in addition to synthesized active particles also an appropriate binder and an electronic conductor. As typical binders may be used polytetrafluoroethylene, polyvinylidene fluoride, polyimide, ethylene propylene diene terpolymer, cellulose, gelatin, and similar polymers. The typically employed electronic conductors carbon black, graphite, metal particles or electron conductive polymers. The binder portion in the final electrode formulation is 2 % by weight to 40 % by weight, and the electronic conductor portion is from 2 % by weight to 20 % by weight. The electrode thickness is 10 micrometers to 500 micrometers; the coating density is 2mg/cm ² to 50mg/cm ² . The fabricated positive electrode may be employed as active electrode in lithium batteries or in lithium accumulators. Synthesis of the active cathode material

The two X-ray powder diffractograms as depicted in FIG 1 represent crystal structures of layered active electrode materials of general chemical formula Li _x VM0 ₄ intended for lithium ion accumulators. The diffractograms are indexed by Miller indices representing planes wherein the X-ray diffraction condition is fulfilled for the said structures. There exist several synthesis routes for the manufacture of layered lithiated oxides based on vanadium and titanium or aluminum, respectively. There ought to be provided small particles ensuring short lithium diffusion paths in the structure, and the presence of an electronic conductor (preferably amorphous carbon) warranting a good wiring of the cathode material..

Description of electrode manufacturing:

An electrode composite based on active electrode materials for lithium ion accumulators having a general chemical formula Li _x VM0 ₄ is utilized for the manufacture of positive electrodes. The electrodes are manufactured by homogenizing the active material by means of electron conductive carbon black and a binder, whereupon the homogenous suspension is applied onto the surface of an aluminum current carrier. A typical active material coating over an aluminum current carrier is 1-5 mg/cm ² .

Description of the Solution with Working Examples: Example A:

The process for the manufacture of Li ₂ VTi0 ₄ as active electrode material for the cathode of lithium ion accumulators is performed with organic acids as follows: a) starting from titanium dioxide, lithium hydroxide and vanadium pentoxide;

b) preparing a colloidal solution of 0.002 - 0.02 moles of titanium dioxide in 9.7 - 97 ml_ of MiliQ water. All is dispersed together in an ultrasonic bath for about 2 hours. The obtained colloidal solution of titanium dioxide is supplemented with 0.004 - 0.04 moles of lithium hydroxide, and dispersed for another hour to dissolve the lithium hydroxide. c) Separately are dissolved 0.0 - 0.12 moles of oxalic acid and 0.0 - 0.12 moles of citric acid in 9.5 - 95 ml_ of MiliQ water at 60°C to which is added 0.01 mol of vanadium pentoxide, and stirred overnight.

d) The solutions (b) and (c) are cooled down to room temperature and mixed together. The mixture is dried in a rotary evaporator at 60°C until a viscous liquid is obtained that is further dried in a drying oven at 60°C overnight to obtain a powder, or dried in vacuum.

e) The obtained powder is crushed in an achate mortar for 10 minutes. Thereafter, it is placed in the oven and heated under an argon atmosphere following a chosen temperature regime: the rates of heating and cooling are 1 - 20°C/min, the maximum temperature is 950°C. The baking time is in the range 1 minute to 100 hours. Immediately after baking the electrodes are transferred into a dry chamber containing less than 10 ppm of water and oxygen.

f) The obtained material is subjected to X-ray powder analysis to evaluate whether the diffractogram corresponds to the requested structure.

g) The obtained material is used for the manufacture of a cathode appropriate for electrochemical tests.

The molar ratio of Li:V:Ti ions in the final compound is 2±X:1±X:1+X.

Example B:

The process for the manufacture of Li ₂ VTi0 ₄ as active electrode material for a cathode in lithium ion accumulators by means of nonionic dispergators is performed as follows: a) starting from titanium dioxide, lithium hydroxide and vanadium pentoxide;

b) preparing a colloidal solution of 0.002 - 0.02 moles of titanium dioxide in 9.7 - 97 ml_ of MiliQ water. All together is dispersed in an ultrasonic bath for about 2 hours. The obtained colloidal solution of titanium dioxide is supplemented with 0.004 - 0.04 moles of lithium hydroxide and dispersed for another hour, in order to dissolve the lithium hydroxide. Thereafter is added 50 ml_ to 500 mL of the organic solvent containing the dissolved nonionic dispergator, and all is homogenized together in an ultrasonic bath. c) Separately are dissolved 0.0 - 0.12 moles of oxalic acid and 0.0 - 0.12 moles of citric acid in 9.5 - 95 mL of MiliQ water at 60°C to which is added 0.01 mole of vanadium pentoxide, and stirred overnight. Then is added 50 mL to 500 mL of the organic solvent containing the dissolved nonionic dispergator with a concentration of 0.001 mol/L to 10 mol/L, and all is homogenized together in an ultrasonic bath for 1 hour.

d) The above solutions (b) and (c) are cooled down to room temperature and mixed together. The mixture is dried in a rotary evaporator at 60°C until a viscous liquid is obtained that is further dried in a drying oven at 60°C overnight to obtain a powder, or dried in vacuum.

e) The obtained powder is crushed in an achate mortar for 10 minutes.

Thereafter, it is placed in the oven and heated under an argon atmosphere following a chosen temperature regime: the rates of heating and cooling are 1 - 20°C/min, the maximum temperature is 950°C. The baking time is in the range 1 minute to 100 hours. Immediately after baking the material is transferred into a dry chamber containing less than 10 ppm of water and oxygen.

f) The obtained material is subjected to X-ray powder analysis to evaluate whether the diffractogramme corresponds to the requested structure.

g) The obtained material is used for the manufacture of a cathode appropriate for electrochemical testing.

The molar ratio of Li:V:Ti ions in the final compound is 2±X:1±X:1+X.

Example C:

The process for the manufacture of Li ₂ VAI0 ₄ as active electrode material for a cathode in lithium ion accumulators by means of organic acids is performed as follows: a) starting from aluminum oxide (AI ₂ O ₃ ), lithium hydroxide and vanadium pentoxide (V ₂ 0 ₅ );

b) preparing a solution of 0.0 - 0.12 moles of oxalic acid and 0.0 - 0.12 moles of citric acid in 10 - 100 mL of MiliQ water at 60°C to which is added 0.01 mol of vanadium pentoxide, and stirred overnight. c) Separately are dissolved 0.004 - 0.04 moles of lithium hydroxide in 10 - 100 mL of MiliQ water.

d) The solution obtained in (b) is supplemented by 0.002 - 0.02 moles of aluminum oxide and all is dispersed together in an ultrasonic bath for about 2 hours. e) To the obtained colloidal solution of aluminum oxide obtained in (d) is added the solution of lithium hydroxide obtained in (c).

f) The obtained suspension is dried in a rotary evaporator at 60°C until a powder is obtained.

g) The obtained powder is crushed in an achate mortar for 10 minutes. Thereafter, it is placed in the oven and heated under an argon atmosphere following a chosen temperature regime: the rates of heating and cooling are 1 - 20°C/min, the maximum temperature is 950°C. The baking time is in the range 1 minute to 100 hours. Immediately after baking the material is transferred into a dry chamber containing less than 10 ppm of water and oxygen.

h) The obtained material is subjected to X-ray powder analysis to evaluate whether the diffractogram corresponds to the requested structure.

g) The obtained material is used in the manufacture of a cathode appropriate for electrochemical tests.

The molar ratio of Li:V:AI ions in the final compound is 2±X:1±X:1±X. Example D:

Assembling the accumulator

The operating electrode having a surface area of 2 cm ² coated with 3-5 mg of the electrode composition based on the active electrode material for lithium ion accumulators exhibiting a layered structure, which is based on the NaCI type structure and is represented by the general chemical formula Li _x VM0 ₄ , is dipped into an aprotic electrolyte (typically 1 M solution of LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate). The selected negative electrode is made of metallic lithium, graphite or similar anode active materials for lithium ion accumulators. The electrodes are assembled into an active electrochemical cell, and separated by a typical lithium battery separator permeable to lithium ions. The electrochemical cell is installed into a prefabricated accumulator housing in a dry chamber (argon atmosphere with water content < 1ppm (1mg/L)). The accumulator thus obtained is subjected to capacity and reversibility tests.

Accumulator testing

The current leads of the fabricated accumulator are clamped to the cable of the instrument that enables the application of varying values of constant electrical current, and a simultaneous measurement of the accumulator's time-dependent voltage change. The voltage range of the operating accumulator is limited under oxidation within 4 V with respect to metallic lithium to 4.6 V with respect to metallic lithium, and under reduction within 1 V with respect to metallic lithium to 2 V with respect to metallic lithium. The capacitance of the accumulator is calculated from the electric charge flow normalized to the mass of the electrode composite.