WO2015138019A1

WO2015138019A1 - Negative electrode active material for energy storage devices and method for making the same

Info

Publication number: WO2015138019A1
Application number: PCT/US2014/069535
Authority: WO
Inventors: Bing Tan; Zhendong Hu; Guanghui He; Yong Che
Original assignee: Imra America,Inc.
Priority date: 2014-03-12
Filing date: 2014-12-10
Publication date: 2015-09-17

Abstract

The described embodiments provide an energy storage device that includes a positive electrode including an active material that can store and release ions, a negative electrode including a V, Nb co-doped TiO₂(B), and a non-aqueous electrolyte including lithium ions. At least one embodiment provides a negative electrode active material including V, Nb co-doped TiO₂(B). At least one embodiment provides a wet-chemistry process to prepare V, Nb co-doped TiO₂(B).

Description

NEGATIVE ELECTRODE ACTIVE MATERIAL FOR ENERGY STORAGE DEVICES

AND METHOD FOR MAKING THE SAME

BACKGROUND

1. Field of the Invention

[0001] The present invention relates to lithium-based energy storage devices generally, and, in particular, to negative electrode active materials for lithium-based energy storage devices.

2. Description of the Related Art

[0002] Ti0₂(B) is reported as an alternative negative electrode active material to compete with Li₄Ti₅0₁₂ for lithium ion batteries or capacitors. Compared to Li₄Ti₅0₁₂, Ti0₂(B) has much higher theoretical specific capacity (335 mAh/g for Ti0₂(B) as compared to 175 mAh/g for Li₄Ti₅0₁₂). It is, however, challenging to prepare Ti0₂(B) from a simple process and the prepared Ti0₂(B) generally lacks good thermal stability. For example, Ti0₂(B) prepared from a solvothermal process has low thermal stability at above 400 °C (Zheng Liu, Yuri G. Andreev, A. Robert Armstrong, Sergio Brutti, Yu Ren, Peter G. Bruce, "Nanostructured Ti0₂(B): the effect of size and shape on anode properties for Li-ion batteries", Progress in Natural Science: Materials International, 2013 (23), 235). As disclosed in U.S. Patent Application No. 14/178,428, entitled 'Negative electrode active material for energy storage devices and method of making the same', we showed an improved thermal stability of Ti0₂(B) by doping with Nb. With the incorporation of Nb, the doped Ti0₂(B) can maintain its Ti0₂(B) crystal structure at temperature even above 400 °C. Although Nb-doped Ti0₂ (B) is promising relative to Li Ti₅0₁₂, the cycling stability of a Nb-doped Ti0₂(B), however, is not particularly beneficial. For example, for Nb-doped Ti0₂ (B) (molar ratio of Nb/Ti =1/3), the capacity retention was about 59% after 1000 cycles. For many applications, it is necessary to further improve the cycling stability of Nb-doped Ti0₂(B).

SUMMARY

[0003] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0004] At least one embodiment provides an energy storage device that includes a positive electrode including an active material that can store and release ions, a negative electrode having a vanadium and niobium co-doped Ti0₂(B) (i.e., V, Nb co-doped Ti0₂(B)), and a non-aqueous electrolyte including lithium ions.

[0005] At least one embodiment provides a negative electrode active material based on V, Nb co-doped Ti0₂(B). The molar ratio of Nb/Ti in the co-doped Ti0₂(B) may be ranged from 1/19 to 2/1. The molar ratio of V/Nb in the co-doped Ti0₂(B) may be ranged from 1/100 to 2/1.

[0006] At least one embodiment provides a wet-chemistry process for the preparation of V,

Nb co-doped Ti0₂(B). In one process, V, Nb co-doped Ti0₂(B) may be prepared from a solvothermal process, where at least one titanium compound, one niobium compound, and one vanadium compound are converted into oxides at a mild temperature (e.g., 120 °C to 200 °C ) in the presence of water and an organic compound. In another process, Nb-doped Ti0₂(B) is prepared first from a solvothermal process in the presence of water and an organic compound. The prepared Nb-doped Ti0₂(B) is then impregnated with a vanadium compound and heated at > 400 °C to obtain V, Nb co-doped Ti0₂(B).

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0007] Other aspects, features, and advantages of described embodiments will become more fully apparent from the following detailed description, the appended claims, and the

accompanying drawings in which like reference numerals identify similar or identical elements.

[0008] FIG. 1 is a sectional view schematically showing an energy storage device in accordance with exemplary embodiments.

[0009] FIG. 2 is a sectional view schematically showing a structure of a portion of the energy storage device of FIG. 1 having a V, Nb co-doped Ti0₂(B) negative electrode in accordance with exemplary embodiments.

[0010] FIG. 3 shows X-ray diffraction (XRD) patterns for a Nb-doped Ti0₂ and a V, Nb co-doped Ti0₂(B) prepared from a solvothermal process.

[001 1 ] FIG. 4 shows transmission electron microscopy (TEM) images for the Nb-doped Ti0₂ and the V, Nb co-doped Ti0₂(B) prepared from the solvothermal process.

[0012] FIG. 5 are representative graphs showing constant current charge/discharge cycle properties of the Nb-doped Ti0₂ and the V, Nb co-doped Ti0₂(B) prepared from the

solvothermal process. FIG. 5 (a) is specific capacity as a function of cycle number, and FIG.

5(b) illustrates capacity retention percentage as a function of cycle number.

[0013] FIG. 6 shows X-ray diffraction patterns for a Nb-doped Ti0₂ and a V, Nb co-doped

Ti0₂(B) with V/Nb molar ratio of 0.5 prepared from a impregnation process.

[0014] FIG. 7 shows transmission electron microscopy images for the Nb-doped Ti0₂(B) and the V, Nb co-doped Ti0₂(B) with V/Nb molar ratio of 0.5 prepared from the impregnation process.

[0015] FIG. 8 shows X-ray diffraction patterns for a Nb-doped Ti0₂(B) and V, Nb co-doped Ti0₂(B) with V/Nb molar ratio of 0.15, 0.3 and 0.5, respectively, prepared from the impregnation process.

[0016] FIGs. 9 are representative graphs showing constant current charge/discharge cycle properties of the Nb-doped Ti0₂ and the V, Nb co-doped Ti0₂(B) with V/Nb molar ratio of 0.15 and 0.5, respectively, prepared from the impregnation process. FIG. 9 (a) is specific capacity as a function of cycle number, and FIG. 9(b) is capacity retention percentage as a function of cycle number.

DETAILED DESCRIPTION

[0017] Described embodiments relate to V, Nb co-doped Ti0₂(B).

[0018] Hereinafter, exemplary embodiments are described with reference to the drawing figures.

Energy Storage Devices

[0019] Energy storage devices include lithium ion capacitors and lithium ion batteries.

[0020] Referring to FIG. 1, exemplary energy storage device 100 includes negative electrode

102, negative lead tab 104, positive electrode 106, positive lead tab 108, electrolyte 110, separator

112, safety vent 114, positive electrode cap 116, positive temperature coefficient (PTC) device

118, gasket 120, insulators 122 and 124, and battery housing 126. Although the energy storage device is illustrated as cylindrical structure, any other shape, such as prismatic, aluminum pouch, or coin type may be used. Numeral 200 represents a minimum functional unit including a layer of negative electrode, a layer of positive electrode and a layer of separator between the negative and positive electrodes. Energy storage device 100 is formed by stacking or winding together several minimum functional units 200 to obtain the desired voltage/current characteristic of completed energy storage device 100. A sectional view of a minimum functional unit is illustrated in FIG. 2 in accordance with exemplary embodiments.

[0021] Referring to FIG. 2, minimum functional unit 200 of the energy storage device in FIG. 1 is shown in more detail. Unit 200 includes negative electrode 202, positive electrode 206, electrolyte 210, and separator 212. Electrolyte 210 is included in separator 212 and contains a non-aqueous lithium compound, such as LiPF₆, LiBF₄, LiC10₄, and LiBOB (lithium

bis(oxalato)borate) .

[0022] Negative electrode 202 may be formed by applying negative electrode material 203 onto one or both surfaces of current collector 201 or made only of negative electrode active material 203.

[0023] Current collector 201 is an electric conductive substrate made of stainless steel, copper, nickel, aluminum, titanium, graphite, carbon black, carbon nanotubes, graphene, conductive polymer, or the like. Aluminum is preferred because of its good electric conductivity, good chemical stability, low cost, and lightweight. Current collector 201 may be a sheet, plate, foil, mesh, expanded mesh, felt, or foam.

[0024] Negative electrode material 203 may comprise negative electrode active material, electric conductive additive, and polymer binder. Negative electrode active material may be a material capable of reversibly containing lithium ions. Electric conductive additive such as carbon black is to improve the electric conductivity of the layer of electrode material to facilitate the electrons transport to and from the current collector to the particles of the negative electrode active material. Polymer binder may bind the particles of electrode active material and carbon black together to ensure the good electric contacts among all particles and between the current collector and the electrode material layer. Both the electric conductive additive and the polymer binder generally are not electrochemically-active during the cycling, so they are not electrode active materials in the electrode material 203. According to the described embodiments, negative electrode material 203 includes V- and Nb- co-doped Ti0₂(B) as the electrode active material. The V- and Nb co-doped Ti0₂(B) is described in detail below in the section titled Negative Electrode Active Materials.

[0025] Positive electrode 206 includes current collector 205 and positive electrode material 207. Current collector 205 is preferably made from aluminum even if other electric conductive substances can be used. Positive electrode 206 is formed by applying positive electrode material 207 onto one surface or both surfaces (not shown) of current collector 205 or is made only of positive electrode material 207. Here, positive electrode material 207 may include a positive electrode active material, an electric conductive additive such as carbon black, and a polymer binder. The positive electrode active material may be any existing or prospective positive electrode active material known in the art, such as a carbonaceous material with high specific surface area, a metal oxide that can be inserted and extracted with lithium ions including LiFeP0₄ and LiMn₂0₄, metal fluoride, sulfur, and air catalyst.

Positive Electrode Active Materials

[0026] Referring to FIG. 2, as described above, positive electrode material 207 includes positive electrode active material, electric conductive additive, and polymer binder. Positive electrode active material may be a material capable of reversibly containing ions. Electric conductive additive and polymer binder may improve the electric conductivity of the electrode material layer and they are generally not electrochemically-active during cycling. [0027] The positive electrode active material may be a carbonaceous material with a high specific surface area that stores ions through an adsorption/de-sorption process. The specific surface area is preferred to be greater than 100 m²/g, preferably between 1000 m²/g and 3500 m²/g. The positive electrode active material includes activated carbon, carbon nanotubes, graphene, carbon black, carbon nanoparticles, and carbon nanocrystals.

[0028] The positive electrode active material may be a material that can store/release lithium ions through an intercalation/de-intercalation process, which may be selected from, but not limited to the existing positive electrode materials for lithium ion batteries. The positive electrode active material may be selected from LiFeP0₄, LiMn₂0₄, LiMn0₂, LiNi0₂, LiCo0₂, LiMn_{0 5}Nio.₅02, LiCo_{1 3}Ni_1/3Mn₁/30₂, xLi₂Mn0₃ (l-x)LiM0₂ (0 < x < 1; M: Mn, Co, Ni), LiNi_1.5Mno.₅0₄, LiV₃0₈, and LiVP0₄F. It may also include a non-lithiated material comprising FeP0₄, V₂0₅, and Mn0₂.

[0029] The positive electrode active material may include sulfur, which stores lithium by forming lithium sulfur species. A carbon-sulfur composite is generally used to ensure good electric conductivity of electrode film.

[0030] The positive electrode active material may include at least one air catalyst that can catalyze either the reduction process of oxygen, or the oxidation process of lithium oxide, or the both.

[0031] The positive electrode active material may include a metal fluoride that interacts with lithium ions through a conversion reaction.

Separator

[0032] Referring to FIG. 2, as described above, separator 212 includes a porous membrane that electrically separates the negative electrode from the positive electrode, while permitting ions to flow across the separator. The separator may be selected from cellulous paper, nonwoven fibers (e.g., nylon, cotton, polyesters, glass), polymer films (e.g., polyethylene (PE), polypropylene (PP), poly(tetrafluoroethylene) (PTFE), Polyvinylidene fluoride (PVDF), and poly( vinyl chloride) (PVC)), and naturally occurring substances (e.g., rubber, asbestos, wood, and sand).

Electrolyte

[0033] Referring to FIG. 2, electrolyte 210 may be a lithium-ion compound solution which is combined with other organic components. The lithium-ion compound solution include lithium hexafluoro phosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), lithium perchlorate (LiC10₄), and lithium bis(oxalato)borate (LiBOB), but is not limited thereto. In one exemplary embodiment, electrolyte 210 includes an organic solvent and a lithium ionic compound. The organic solvent dissolves the lithium ionic compound forming the lithium-ion compound solution. Examples of suitable organic solvents may be hexane, tetrahydrofuran (THF), propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), acetonitrile (ACN), but is not limited thereto.

Negative Electrode Active Materials

[0034] Referring to FIG. 2, as described above, negative electrode material 203 includes an electrode active material, which is vanadium and niobium co-doped Ti0₂ having a Ti0₂(B) crystal structure (i.e., V, Nb co-doped Ti0₂(B)).

[0035] V, Nb co-doped Ti0₂(B) was prepared by two routes. In a first route, i.e., a solvothermal process, a vanadium compound, a niobium compound, and a titanium compound were dissolved in an organic solvent such as ethylene glycol. The mixture was then stirred at room temperature or at other temperature (e.g., 80 °C) to promote the ligand exchange reactions between the metal compound and the organic solvent. After a few hours (e.g., 2 hours), water or aqueous ammonia was introduced to the mixture and the mixture was then heated to form crystallized particles. The heating temperature could be any temperature from 100 °C to 185 °C. With a heating temperature as 185 °C, the solution was under refluxing. The solution was kept at the relatively high temperature for a few hours (e.g., 4 hours) before cooling down with the formation of dispersion. Particles in the dispersion were collected by a filtering process and the filtered cake was dried at 100 °C to 250 °C in air overnight. The dried powder was finally heated at a temperature > 400 °C (e.g., 550 °C) for a few hours (e.g., 2 hours) to remove any organic impurities. Because of the solubility of vanadium in acidic ethylene glycol, part of the vanadium was dissolved in the solution after the solvothermal process and the dissolved vanadium was not incorporated into Nb-doped Ti0₂.

[0036] FIG. 3 shows the X-ray diffraction (XRD) patterns for a sample doped with V by the first route and the corresponding sample without V. The molar ratio of V/Nb in the starting precursors was 0.39/1. Since part of the vanadium was dissolved in the acidic solution, the final molar ratio of V/Nb in the V, Nb co-doped Ti0₂ was less than 0.39. XRD patterns in FIG. 3 show that both samples mainly had Ti0₂(B) crystal structure. A small amount of anatase might be present. TEM images (FIG. 4) of the two samples show that the Nb-doped Ti0₂(B) sample comprised of plate-like or sheet-like nanoparticles with the particle sizes in the range of a few nanometers to tens of nanometers. The V, Nb co-doped sample also comprised of plate-like or sheet-like nanoparticles with the particle sizes in the range of a few nanometers to tens of nanometers. With the doping of V, the particle morphology and the average particle size did not change significantly. Electrochemical evaluation (FIG. 5) of the two samples shows that the cycling stability of the V, Nb co-doped Ti0₂(B) was much better than the Nb-doped Ti0₂(B). For example, as shown in FIG. 5(b), a capacity retention about 53% of the 2^nd discharge capacity was obtained for Nb-doped Ti02(B) after 1000 cycles, while the value became 78% for the V, Nb co-doped Ti0₂(B). In another example, as shown in FIG. 5(a), a specific capacity about 85 mAh/g was obtained for the Nb-doped Ti0₂(B) after 1000 cycles, while a specific capacity about 170 mAh/g was obtained for the V, Nb co-doped Ti0₂(B). These examples show that the incorporation of vanadium into Nb-doped Ti0₂(B) by the solvothermal process can significantly improve cycling stability of the Nb-doped Ti0₂(B) .

[0037] A second route, i.e., an impregnation process, could be used to avoid the solubility issue of vanadium in acidic ethylene glycol. In this route, Nb-doped Ti0₂ was prepared by a solvothermal process, for example, the process disclosed in U.S. Patent Application No.

14/178,428. The dried Nb-doped Ti0₂ was then impregnated with a vanadium compound solution. The vanadium compound solution may include, but is not limited to ammonium vanadate, VC1₃, VOCl₃, VC1₅, and V₂0₅. The vanadium impregnated Nb-doped Ti0₂ was then dried and heated at a temperature > 400 °C (e.g., 550 °C) to form V, Nb co-doped Ti0₂(B). In principle, any vanadium compound that later can decompose into vanadium oxide can be used in this process. FIG. 6 shows XRD patterns of the Nb-doped Ti0₂(B) and the V, Nb co-doped Ti0₂(B) in which vanadium is incorporated by the impregnation process. The XRD patterns confirm that both samples have Ti0₂(B) crystal structure. Representative transmission electron microscopy (TEM) images for Nb-doped Ti0₂(B) before and after the incorporation of vanadium are shown in FIG. 7. The TEM images show that after the incorporation of vanadium, the particle morphology changed from plate-like or sheet-like particles to faceted particles with particle sizes in the range of tens of nanometers. The incorporation of vanadium into the Nb-doped Ti0₂(B) by the impregnation process could change the morphology and particle size of the Nb-doped Ti0₂(B) .

[0038] A series of samples with varied amount of vanadium were prepared from the impregnation process. FIG. 8 shows XRD patterns of the series of samples with varied amount of vanadium (V/Nb molar ratio ranged from 0.15/1 to 0.5/1). With the incorporation of small amounts of vanadium (V/Nb molar ratio was about 0.15/1), the sample maintained its Ti0₂(B) crystal structure ( the second XRD pattern from the bottom in FIG. 8). With an increase of vanadium (V/Nb molar ratio was about 0.30/1), the sample still had Ti0₂(B) crystal structure (the third XRD pattern from the bottom in FIG. 8). With a further increase of vanadium (V/Nb molar ratio was about 0.5/1), the sample showed the presence of an impurity phase that is not from either Ti0₂(B) or anatase (the top XRD pattern in FIG. 8) and the relative intensities of the characteristic peaks from Ti0₂(B) structure at the positions about 14.2 degree (2Θ) and 28.8 degree (2Θ) became almost negligible as compared to the intensity of the peak at about 25.2 degree (2Θ), suggesting that too much vanadium may not be good for the formation of Ti0₂(B) structure. The XRD pattern of V, Nb co-doped Ti0₂(B) may be affected by its synthetic conditions. Even with the same compositions, the obtained crystal structure may be slightly different from batch to batch. But as a general trend, too much vanadium will help convert the Ti0₂(B) structure into an anatase structure. Electrochemical performance of vanadium doped and un-doped NbTi₃O_x is shown in FIG. 9. FIG. 9(b) shows capacity retention percentage as a function of cycle number of the vanadium doped and un-doped NbTi₃O_x samples. Without vanadium, the Nb-doped Ti0₂(B) (i.e.,NbTi₃O_x) showed capacity retention about 59% of the 2^nd discharge capacity after 1000 cycles. With incorporation of small amounts of vanadium (V/Nb molar ratio was about 0.15/1), the capacity retention was about 67% after 1000 cycles (i.e. Vo_.15NbTi₃O_x). The capacity retention for V_0.5NbTi₃O_x was about 81% of the 2^nd discharge capacity after 1000 cycles. The comparison shows that the incorporation of vanadium by the impregnation process could improve the cycling stability of Nb-doped Ti0₂(B). FIG. 9(a) shows specific capacity as a function of cycle number of the vanadium doped and un-doped NbTi₃O_x samples. Without vanadium, the Nb-doped Ti0₂(B) (i.e.,NbTi₃0_x) showed a specific capacity about 110 mAh/g after 1000 cycles. With incorporation of small amount of vanadium (V/Nb molar ratio was about 0.15/1), the Nb-doped Ti0₂(B) showed a specific capacity about 137 mAh/g after 1000 cycles. With a further increased amount of vanadium (V/Nb molar ratio was about 0.5/1), the Nb-doped Ti0₂(B) showed a specific capacity about 120 mAh/g after 1000 cycles, which shows that too much vanadium may reduce the specific capacity of the V, Nb co-doped Ti0₂(B).

[0039] The performance of Nb-doped Ti0₂(B) was sensitive to its synthetic conditions. It is rational that the cycling stability might be slightly different for the samples synthesized from different batches. It is, however, a general observation that the cycling stability has been improved with the incorporation of vanadium into a Nb-doped Ti0₂(B) material.

[0040] The amount of vanadium incorporated into a Nb-doped could be varied. Without Nb, a V-doped Ti0₂ tends to have an anatase crystal structure. To maintain its Ti0₂(B) structure, the V/Nb molar ratio in a V, Nb co-doped Ti0₂(B) could not be too large. In general, a molar ratio of 1/1 of V/Nb might be the limit to maintain its Ti0₂(B) crystal structure. It might be possible that the molar ratio can reach 2/1 if the preparation conditions are optimized. There is no limitation in the minimum amount of incorporated vanadium. A molar ratio of V/Nb could be selected as small as 1/100.

[0041] In at least one embodiment, the negative active electrode material includes a V, Nb co-doped Ti0₂ having Ti0₂(B) crystal structure.

[0042] In at least one embodiment, the V, Nb co-doped Ti0₂(B) has a composition of vanadium, niobium, and titanium oxide. The molar ratio between Nb and Ti could be varied from 1/19 to 2/1. It is preferred to be from about 1/6 to about 1/1. The molar ratio between V and Nb could be varied from 1/100 to 2/1. It is preferred to be from about 1/10 to about 1/1. [0043] In at least one embodiment, the V, Nb co-doped Ti0₂(B) includes a carbonaceous material. The carbonaceous material generally has an average aggregate size or particle size in the range of micrometers. An aggregate refers to a particle composed of at least two smaller particles. Examples of suitable carbonaceous materials includes graphite, hard carbon, soft carbon, amorphous carbon coated graphite, amorphous carbon coated hard carbon, carbon black, carbon nanofibers, carbon nanotubes, graphene, carbon nanoparticles, carbon onion , crystalline carbon, semi-crystalline carbon, amorphous carbon and the like, but are not limited thereto.

[0044] In at least one embodiment, V, Nb co-doped Ti0₂(B) includes at least one more element selected from, but not limited to calcium, magnesium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, tungsten, aluminum, gallium, tin, antimony, bismuth, bromine, and fluorine. The incorporation of this element may further improve the electrochemical performance of the V, Nb-doped Ti0₂ (B).

SYNTHESIS PROCESS

[0045] In this invention, we synthesized V, Nb co-doped Ti0₂(B) particles either from a solvothermal process with a post-treatment heating step or from a combination of a solvothermal process and an impregnation process.

[0046] In a typical solvothermal process, TiCl , NbCl₅, and VOCl₃ are dissolved in ethylene glycol to form a clear solution. At least part of the CI^" ions in TiCU, NbCl₅, and VOCl₃ are replaced by ethylene glycol during the dissolution process. The solution may be subject to heat at a mild temperature (e.g., 60 °C to 190 °C) to improve the ligand-exchange reactions between the metal chloride and ethylene glycol. The heated solution is then cooled down. Water or aqueous ammonia is then added into the solution under stirring to induce the hydrolysis and condensation reactions of the titanium, niobium, and vanadium species. A carbonaceous material may be dispersed in the solution before or after the addition of water or aqueous ammonia. The obtained solution is then refluxed at about 185 °C for 4 hours in open air. Formed precipitates are collected by filtering the dispersion with a regular filtering paper. The collected precipitates are dried at about 110 °C and then heated at 550 °C in air for 2 hours.

[0047] In a typical combinational process, Nb doped Ti0₂(B) was prepared from a solvothermal process as being disclosed in this or a previous invention. After being collected from the solution, the as-made Nb-doped Ti0₂ is then mixed with a vanadium source (i.e., impregnation process). The mixture is then dried and heated at > 400 °C to form V, Nb co-doped Ti0₂(B).

[0048] For the solvothermal process, other titanium sources may also be used besides TiCl₄ as long as they can be dissolved in ethylene glycol or can form a complex with glycolic acid. The titanium source may be selected from, but not limited to titanium alkoxide (e.g., titanium ethoxide, titanium isopropoxide, and titanium butoxide), titanium acetylacetonate, titanium

bis(acetylacetonate)dichloride, and titanium glycolate.

[0049] The niobium source may be selected from, but not limited to niobium chloride, niobium alkoxide (e.g., niobium ethoxide, niobium isopropoxide, and niobium butoxide), niobium acetylacetonate, niobium bis(acetylacetonate)dichloride, and niobium glycolate.

[0050] The vanadium source may be selected from, but not limited to vanadium (V) oxychloride, vanadium (V) pentachloride, alkoxides of vanadium (e.g, vanadium(V)

oxytriisopropoxide), and vanadium glycolate.

[0051] The solvent is preferred to be glycol ethylene unless the titanium source is titanium glycolate. Water can be used as the solvent if the titanium source is a pre-formed titanium glycolate.

[0052] When titanium chloride is used as a titanium source, its concentration in ethylene glycol is preferably not too high, otherwise a significant amount of water will be needed for the hydrolysis and condensation reactions, which will dilute the concentration of ethylene glycol resulting in a decrease in boiling temperature of the mixture. A low boiling temperature (e.g., 100 °C) may not be high enough for the formation of crystallized Ti0₂(B) from the solvothermal process.

[0053] After the formation of titanium-ethylene glycol complex, water is needed to induce the hydrolysis and condensation reactions so that Ti0₂ can be produced. The introduction of water can be realized by adding water or aqueous ammonia. An aqueous alkaline solution may be used, but is not preferred because the alkaline metal ions will need to be removed after the solvothermal process as a contamination source.

[0054] The reaction is preferred to be carried out at a relatively high temperature (e.g., about 140 °C and above) to form crystallized particles. The crystallization may possibly occur at a lower reaction temperature such as 100 °C to 140 °C if the reaction conditions are optimized. With ethylene glycol as a solvent, the reaction can be carried out either in an open air or in a sealed container. With water as the pure solvent, the reaction is preferred to be carried out in a sealed container so that the reaction pressure will be higher than the atmosphere pressure and the formed oxide can be better crystallized.

[0055] The Ti0₂ particles in the colloidal solution can be collected by filtration. With the use of water for the hydrolysis and condensation reactions, the formed colloidal particles may be too small to be collected by using a regular filtering paper. In this case, a material with micro-sized aggregate or particles can be introduced into the reaction solution, so that the Ti0₂ colloidal particles can be formed or adsorbed on these micro-sized aggregates/particles, which can be filtered with a regular filtering paper. The Ti0₂ particles may also be collected by other processes including centrifuging, spray-drying, and freeze-drying.

[0056] The collected particles have organic impurities as being revealed by Fourier

Transform Infrared (FTIR) spectrum. A post-treatment heating process is necessary to remove the organic impurities. The heating temperature for the V, Nb co-doped Ti0₂(B) is preferred to be > 400 °C, more preferably between 400 °C and 650 °C.

[0057] If the impregnation process is used, the vanadium source may be selected from any compound containing vanadium. The vanadium source can be selected from, but is not limited to vanadium, ammonium vanadate, vanadium oxide, vanadium chloride, vanadium oxychloride, vanadium (V) pentachloride, alkoxides of vanadium (e.g, vanadium(V) oxytriisopropoxide), and vanadium glycolate.

[0058] In at least one embodiment, the titanium source is selected from, but not limited to titanium chloride, titanium alkoxide (e.g., titanium ethoxide, titanium isopropoxide, and titanium butoxide), titanium acetylacetonate, titanium bis(acetylacetonate)dichloride, and titanium glycolate.

[0059] In at least one embodiment, the niobium source is selected from, but not limited to niobium chloride, niobium alkoxide (e.g., niobium ethoxide, niobium isopropoxide, and niobium butoxide), niobium acetylacetonate, niobium bis(acetylacetonate)dichloride, and niobium glycolate.

[0060] In at least one embodiment, the vanadium source for the solvothermal process may be selected from, but not limited to vanadium (V) oxychloride, vanadium (V) pentachloride, alkoxides of vanadium (e.g, vanadium(V) oxytriisopropoxide), and vanadium glycolate.

[0061] In at least one embodiment, the molar ratio of niobium/titanium is in the range of about 1/19 to about 2/1. Higher molar ratio can be used, but not preferred considering the high cost of niobium.

[0062] In at least one embodiment, the solvent is ethylene glycol.

[0063] In at least one embodiment, the solvent is water when the titanium compound is titanium gylcoate.

[0064] In at least one embodiment, water is introduced to induce the hydrolysis and condensation reactions.

[0065] In at least one embodiment, aqueous ammonia is introduced to induce the hydrolysis and condensation reactions.

[0066] In at least one embodiment, the reaction solution is heated at a temperature about 140 °C and above.

[0067] In at least one embodiment, the reaction solution is refluxed. The refluxing time may be in the range from tens of minutes (e.g., 30 minutes) to tens of hours.

[0068] In at least one embodiment, the generated particles are collected by filtration. Other collection techniques including centrifuging and spray-drying may also be used, but they are expected to be more costly than the filtration technique.

[0069] In at least one embodiment, the collected as-made particles are dried and then heated at a temperature> 400 °C in air for a few hours (e.g., 2 hours). A heating temperature ranged from about 400 °C and about 650 °C is preferred considering the completely removal of organic impurities and the cost of heating. The heating process may also be carried out in vacuum or in an inert environment including argon.

[0070] The described embodiments of the present invention are further illustrated by the following Examples.

EXAMPLE 1 V, Nb co-doped Ti0₂(B) prepared from a solvothermal process

[0071] In a typical synthetic process for V, Nb co-doped Ti0₂, about 2.12 ml TiCl₄, 1.74 grams of NbCl₅, and 0.23 ml VOCl₃ were dissolved in 50 ml ethylene glycol. 0.11 grams of acetylene black (AB) was dispersed in the solution. 5.4 g aqueous ammonia (about 28-30 wt%) was then added into the above dispersion under stirring. The obtained solution was refluxed (e.g., about 185 °C) in open air for 1 day. Particles were collected by vacuum filtration with a regular filter paper (particle retention: 8 to 12 μηι). The as-made particles were dried at 110 °C overnight and then heated at 450 °C in air for 2 hours. The heated sample was finally heated at 550 °C for 4 hours in 5% H₂/Ar.

[0072] The XRD pattern for the V, Nb co-doped Ti0₂(B) was collected from a Rigaku Miniflex X-ray diffractometer operated at 30 kV and 10 mA with Cu Ka radiation (wavelength of about 1.5418 A). TEM images for the Nb-doped Ti0₂ (B) (NbTi₃O_x-l) and the corresponding V_aNbTi₃0_x-l (0< a < 0.39) were collected from a JEOL 3011 transmission electron microscope.

[0073] The electrochemical performance for the V, Nb co-doped Ti0₂(B) was collected from a coin cell with a lithium disk as the negative electrode and the co-doped Ti0₂(B) as the positive electrode active material. The composition of the positive electrode material was 80 wt% active material, 7.5 wt% BP2000 (carbon black, purchased from CABOT Corporation in Japan), 4 wt% of sodium carboxymethyl cellulose (i.e., cmc, purchased from Sigma-Aldrich Corporation in U.S.A.), and 3.5 wt% styrene butadiene rubber (i.e., SBR, purchased from ZEON in Japan). Electrochemical data were collected from Arbin BT-2000 battery testing instrument (purchased from Arbin Instruments in U.S.A.).

EXAMPLE 2

V, Nb co-doped Ti0₂(B) prepared from an impregnation process [0074] In this example, 2 ml TiCl₄ and 1.64 g NbCl₅ were dissolved in 50 ml ethylene glycol. 5.4 g aqueous ammonia (about 28 wt%) was added into the solution under stirring. The solution was then refluxed at about 185 °C in open air for about 4 hours. Particles were collected by vacuum filtration. The as-made particles were dried at 120 °C overnight and then 200 °C for about 45 minutes. The dried powder was then impregnated with various amount of VOCI3, which was dissolved in acidic water. The impregnated powder was dried and finally heated at 550 °C in air for two hours. Two samples were prepared with the V/Nb molar ratio about 0/1 and 0.5/1. The two samples are named as NbTi₃O_x-2and V_0. NbTi₃O_x-2, respectively.

[0075] Representative XRD patterns and electron transmission microscopy (TEM) images from a Nb-doped Ti0₂(B) and a V, Nb co-doped Ti0₂(B) are shown in FIG. 6 and FIG. 7, respectively.

EXAMPLE 3

V, Nb co-doped Ti0₂(B) prepared from an impregnation process

[0076] In a typical process, 8 ml TiCl₄ and 6.56 g NbCl₅ were dissolved in 200 ml ethylene glycol. 21.6 g aqueous ammonia (about 28 wt%) was added into the solution under stirring. 0.44 g acetylene black (AB) was then dispersed in the solution. The black dispersion was then refluxed at about 185 °C in open air for 4 hours. Particles were collected by vacuum filtration with a regular filter paper (particle retention: 8 to 12 μπι). The as-made particles were dried at 200 °C overnight. The dried powder was then impregnated with various amount of VC1₃, which was dissolved in water. The impregnated powder was dried and finally heated at 550 °C in air for four and half hours. Five samples were prepared with the V/Nb molar ratio about 0/1, 0.15/1, 0.3/1, 0.5/1, and 1/1. These samples are named as NbTi₃O_x-3, νο_.15ΝΜ¾Ο_χ-3, Vo_.3NbTi₃O_x-3, Vo_.5NbTi₃O_x-3, and VNbTi₃O_x-3, respectively.

[0077] XRD patterns (FIG. 8) were collected from a Rigaku X-ray diffractometer operated at 40 kV and 100 mA with Cu Ka radiation (wavelength of about 1.5418 A). Constant current charge/discharge curves (FIG. 9) were collected for coin cells with the co-doped Ti0₂ as the positive electrode active material and a lithium disk as the negative electrode. The positive electrode material includes 85 wt% of active material, 8 wt% of Super P (carbon black, purchased from TIMCAL in Switzerland), 4 wt% of sodium carboxymethyl cellulose (i.e., cmc, purchased from Sigma- Aldrich Corporation in U.S.A.), and 3 wt% styrene butadiene rubber (i.e., SBR, purchased from ZEON in Japan). The electrolyte was 1 M LiPF₆ in propylene carbonate (i.e., PC, purchased from BASF in U.S.A.). The test was performed at room temperature (e.g., about 21 °C).

[0078] In summary, the authors have shown that the incorporation of vanadium into

Nb-doped Ti0₂(B) has improved its cycling stability. A Nb-doped Ti0₂(B) with Nb/Ti molar ratio of 1/3 is used as an example. The doping effect of vanadium, however, is not limited to the 1/3 molar ratio of Nb/Ti. The improvement in cycling stability with the incorporation of vanadium is expected for Nb-doped Ti0₂(B) with other molar ratios of Nb/Ti.

[0079] In general, the invention has been described by way of several embodiments and examples.

[0080] An energy storage device comprises positive electrode including an active material that stores and releases ions, a negative electrode including V, Nb co-doped Ti0₂(B), and a non-aqueous electrolyte containing lithium ions.

[0081] The energy storage device could be either lithium ion battery or lithium ion capacitor. Lithium ion battery is an energy storage device including a positive active electrode material that stores/releases electrons through faradic reactions. Lithium ion capacitor is an energy storage device including a positive active electrode material that stores/releases electrons through an ion adsorption/desorption process.

[0082] The positive electrode active material may be a carbonaceous material with a high specific surface area that stores ions through an adsorption/de-sorption process. The specific surface area is preferred to be greater than 100 m²/g, preferably between 1000 m²/g and 3500 m²/g. The positive electrode active material includes activated carbon, carbon nanotubes, graphene, carbon black, carbon nanoparticles, and carbon nanocrystals.

[0083] The positive electrode active material may be a material that can store/release lithium ions through an intercalation/de-intercalation process, which may be selected from, but not limited to the existing positive electrode materials for lithium ion batteries. The positive electrode active material may be selected from LiFeP0₄, LiMn₂0₄, LiMn0₂, LiNi0₂, LiCo0₂, LiMno.₅Nio.5Ch, LiCoi/3Nii 3Mhi 30₂, xLi₂Mn0₃ (l-x)LiM0₂ (0 < x < 1; M: Mn, Co, Ni),LiNi₁.₅Mno.₅0₄, LiV₃0₈, and LiVP0₄F. It may also include a non-lithiated material comprising FeP0₄, V₂0₅, and Mn0₂

[0084] The positive electrode active material may include sulfur, which stores lithium by forming lithium sulfur species. A carbon-sulfur composite is generally used to ensure good electric conductivity of electrode film.

[0085] The positive electrode active material may include at least one air catalyst that can catalyze either the reduction process of oxygen, or the oxidation process of lithium oxide, or the both.

[0086] The positive electrode active material may include a metal fluoride that interacts with lithium ions through a conversion reaction.

[0087] The negative electrode active material in the lithium ion battery or lithium ion capacitor may include an inorganic material with Ti0₂(B) crystal structure with the compositions of titanium oxide, niobium oxide, and vanadium oxide.

[0088] The vanadium and niobium incorporated Ti0₂(B) may have at least one characteristic XRD peaks at about 28.8 degree (2Θ) or about 43.4 degree (2Θ).

[0089] The vanadium and niobium incorporated Ti0₂(B) may include niobium and titanium with a molar ratio of Nb/Ti ranged from about 1/19 to about 2/1.

[0090] The vanadium and niobium incorporated Ti0₂(B) may include vanadium and niobium with a molar ratio of V/Nb ranged from about 1/100 to about 2/1.

[0091] The vanadium and niobium incorporated Ti0₂(B) may include a carbonaceous material selected from activated carbon, carbon black, carbon nanotubes, carbon nanofibers, graphite, graphene, carbon nanocrystals, carbon nanoparticles, carbon onions, crystalline carbon, semi-crystalline carbon, and amorphous carbon.

[0092] The vanadium and niobium incorporated Ti0₂(B) may include an element selected calcium, magnesium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, tungsten, aluminum, gallium, tin, antimony, bismuth, bromine, and fluorine.

[0093] The vanadium and niobium incorporated Ti0₂(B) may have a particle morphology as plate-like or sheet-like.

[0094] The vanadium and niobium incorporated Ti0₂(B) may have an average particle size in the range of from 1 to 1000 nm, preferably from 5 to 50 nm.

[0095] The vanadium and niobium incorporated Ti0₂(B) may have a particle morphology as faceted particles.

[0096] The vanadium and niobium incorporated Ti0₂(B) may have an average particle size in the range of from 1 to 1000 nm, preferably from 20 to 100 nm. [0097] An impregnation process for making V, Nb co-doped Ti0₂(B). The process may include the steps of: (1) dissolving at least one titanium compound and one niobium compound in ethylene glycol to form a clear solution, (2) adding water or aqueous ammonia into the solution, (3) heating the solution, (4) collecting the colloidal particles, (5) mixed a vanadium compound with the collected particles, and (6) applying a thermal treatment at a temperature > 400 °C to obtain the vanadium mixed particles.

[0098] The titanium compound may be selected from titanium chloride, titanium ethoxide, titanium isopropoxide, titanium butoxide, titanium acetylacetonate, titanium

bis(acetylacetonate)dichloride, or titanium glycolate.

[0099] The niobium compound may be selected from niobium chloride, niobium ethoxide, niobium isopropoxide, and niobium butoxide, niobium acetylacetonate, niobium

bis(acetylacetonate)dichloride, or niobium glycolate.

[00100] The vanadium compound can be any compound that contains vanadium. The vanadium compound can be selected, but not limited to vanadium, ammonium vanadate, vanadium oxide, vanadium chloride, vanadium oxychloride, vanadium (V) pentachloride, alkoxides of vanadium (e.g, vanadium(V) oxytriisopropoxide), and vanadium glycolate.

[00101 ] The reaction solution may be heated at a mild temperature > 100 °C, preferably above 140 °C.

[00102] The heating temperature for the vanadium incorporated particles may be > 400 °C in air, preferably from 400 °C to 650 °C.

[00103] The heating process may also be carried out in an inert gas or reduced gas including N₂, Ar, and H₂.

[00104] The obtained product may have an average particle size in the range of 1 nm to 1000 nm, preferably from 20 nm to 100 nm.

[00105] The obtained product may include faceted particles.

[00106] A solvothermal process for making V, Nb co-doped Ti0₂(B). The process includes the steps of dissolving at least one titanium compound, one niobium compound, and one vanadium compound in ethylene glycol; adding water or aqueous ammonia into the solution; heating the solution; collecting the colloidal particles; and applying a thermal treatment at a temperature > 400 °C to the vanadium mixed particles.

[00107] The titanium compound may be selected from titanium chloride, titanium ethoxide, titanium isopropoxide, titanium butoxide, titanium acetylacetonate, titanium

bis(acetylacetonate)dichloride, or titanium glycolate.

[00108] The niobium compound may be selected from niobium chloride, niobium ethoxide, niobium isopropoxide, and niobium butoxide, niobium acetylacetonate, niobium

bis(acetylacetonate)dichloride, or niobium glycolate.

[00109] The vanadium compound may be selected from, but not limited to vanadium (V) oxychloride, vanadium (V) pentachloride, alkoxides of vanadium (e.g, vanadium(V)

oxytriisopropoxide), and vanadium glycolate.

[00110] The reaction solution may be heated at a mild temperature > 100 °C, preferably above 140 °C.

[00111] The heating temperature for the vanadium incorporated particles may be > 400 °C in air, preferably from 400 °C to 650 °C.

[00112] The heating process may also be carried out in an inert gas or reduced gas including N₂, Ar, and H₂.

[00113] The obtained product may have an average particle size in the range of 1 nm to 1000 nm, preferably from 5 nm to 50 nm.

[00114] The obtained product may include plate-like or sheet-like particles.

[00115] A carbonaceous material may be added into the solution with the dissolved titanium compound in the impregnation process or the solvothermal process.

[00116] It is understood that the described embodiments are not mutually exclusive, and elements, components, materials, or steps described in connection with one exemplary embodiment may be combined with, or eliminated from, other embodiments in suitable ways to accomplish desired design objectives.

[00117] Reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term "implementation."

[00118] As used in this application, the word "exemplary" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word exemplary is intended to present concepts in a concrete fashion.

[00119] Additionally, the term "or" is intended to mean an inclusive "or" rather than an exclusive "or". That is, unless specified otherwise, or clear from context, "X employs A or B" is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then "X employs A or B" is satisfied under any of the foregoing instances. In addition, the articles "a" and "an" as used in this application and the appended claims should generally be construed to mean "one or more" unless specified otherwise or clear from context to be directed to a singular form.

[00120] Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word "about" or "approximately" preceded the value of the value or range.

[00121] The use of figure numbers or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.

[00122] It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments.

[00123] Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

[00124] No claim element herein is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase "means for" or "step for." [00125] It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of described embodiments may be made by those skilled in the art without departing from the scope as expressed in the following claims.

Claims

CLAIMS We claim:

1. An energy storage device, comprising:

a positive electrode including an active material that stores and releases ions; a negative electrode including V, Nb co-doped Ti0₂(B); and

a non-aqueous electrolyte containing lithium ions.

2. The energy storage device of claim 1, wherein said device comprises lithium ion battery and lithium ion capacitor.

3. The energy storage device of claim 1, wherein said negative electrode comprises an inorganic material having Ti0₂(B) crystal structure which shows at least one characteristic peak at about 14.2 degree (Θ), 28.8 degree (2Θ) or about 43.4 degree (2Θ) in an X-ray diffraction spectrum obtained by using Cu Ka radiation.

4. The negative electrode of claim 3, wherein said inorganic material comprises vanadium oxide, niobium oxide, and titanium oxide.

5. The inorganic material of claim 4, wherein said material comprises niobium and titanium with a molar ratio of Nb/Ti ranged from about 1/19 to about 2/1.

6. The inorganic material of claim 4, wherein said material comprises vanadium and titanium with a molar ratio of V/Nb ranged from about 1/100 to about 2/1.

7. The energy storage device of claim 1, wherein said V, Nb co-doped Ti0₂(B) comprises a carbonaceous material selected from a group comprising activated carbon, carbon black, carbon nanotubes, carbon nanofibers, graphite, graphene, carbon nanocrystals, carbon nanoparticles, carbon onions, crystalline carbon, semi-crystalline carbon, and amorphous carbon.

8. The energy storage device of claim 1, wherein said V, Nb co-doped Ti0₂(B) comprises an element selected from a group comprising calcium, magnesium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, tungsten, aluminum, gallium, tin, antimony, and bismuth.

9. A negative electrode active material for energy storage devices, comprising V, Nb co-doped Ti0₂(B).

10. The negative electrode active material of claim 9, wherein said material comprises crystalline Ti0₂(B) having at least one characteristic peak at about 14.2 degree, 28.8 degree (2Θ) or about 43.4 degree (2Θ) in a x-ray diffraction spectrum obtained by using Cu Ka radiation.

11. The negative electrode active material of claim 9, wherein said material comprises V, Nb co-doped Ti0₂(B) with a molar ratio of Nb/Ti ranged from about 1/19 to about 2/1 and a molar ratio of V/Nb ranged from about 1/100 to about 2/1 .

12. The negative electrode active material of claim 9, wherein said material comprises a carbonaceous material selected from a group comprising activated carbon, carbon black, carbon nanotubes, carbon nanofibers, graphite, graphene, carbon nanocrystals, carbon nanoparticles, carbon onions, crystalline carbon, semi-crystalline carbon, and amorphous carbon.

13. The negative electrode active material of claim 9, wherein said V, Nb co-doped Ti0₂(B) comprises an element selected from a group comprising calcium, magnesium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, tungsten, aluminum, gallium, tin, antimony, and bismuth.

14. The negative electrode active material of claim 9, wherein said V, Nb co-doped Ti0₂(B) has a particle size in the range from 1 nm to 1000 nm.

15. The negative electrode active material of claim 9, wherein said V, Nb co-doped Ti0₂(B) has a particle size in the range from 1 nm to 100 nm.

16. A method for making V, Nb co-doped Ti0₂(B) the method comprising the steps of:

dissolving at least one titanium compound and one niobium compound in an

organic solvent to form a clear solution;

adding water or aqueous ammonia into the solution;

heating the solution;

collecting and drying the colloidal particles; mixed a vanadium compound with the dried particles;

applying a thermal treatment at a temperature > 400 °C to the vanadium mixed particles.

17. The method for making V, Nb co-doped Ti0₂(B) of claim 16, wherein the organic solvent is ethylene glycol.

18. The method for making V, Nb co-doped Ti0₂(B) of claim 16, wherein said titanium compound is selected from a group comprising titanium chloride, titanium ethoxide, titanium

isopropoxide, titanium butoxide, titanium acetylacetonate, titanium

bis(acetylacetonate)dichloride, and titanium glycolate.

19. The method for making V, Nb co-doped Ti0₂(B) of claim 16, wherein said niobium compound is selected from a group comprising niobium chloride, niobium ethoxide, niobium

isopropoxide, and niobium butoxide, niobium acetylacetonate, niobium

bis(acetylacetonate)dichloride, and niobium glycolate.

20. The method for making V, Nb co-doped Ti0₂(B) of claim 16, wherein said vanadium

compound is selected from a group comprising vanadium, ammonium vanadate, vanadium oxide, vanadium chloride, vanadium oxychloride, vanadium (V) pentachloride, alkoxides of vanadium (e.g, vanadium(V) oxytriisopropoxide), and vanadium glycolate.

21. The method for making V, Nb co-doped Ti0₂(B) of claim 16, wherein said method comprises a step of dispersing a carbonaceous material into the solution before collecting the particles.

22. The method for making V, Nb co-doped Ti0₂(B) of claim 16, wherein said co-doped Ti0₂(B) has an average particle size in the range of 1 ran to 1000 nm.

23. The method for making V, Nb co-doped Ti0₂(B) of claim 16, wherein said dried particles are heated at a temperature > 400 °C in air, preferably from 400 °C to 650 °C.

24. A method for making V, Nb co-doped Ti0₂(B), the method comprising the steps of:

dissolving at least one titanium compound, one niobium compound, and one

vanadium compound in an organic solvent;

adding water or aqueous ammonia into the solution;

heating the solution;

collecting and drying the colloidal particles;

25. The method for making V, Nb co-doped Ti0₂(B) of claim 24, wherein the organic solvent is ethylene glycol.

26. The method for making V, Nb co-doped Ti0₂(B) of claim 24, wherein said vanadium

compound is selected from a group comprising vanadium (V) oxychloride, vanadium (V) pentachloride, alkoxides of vanadium (e.g, vanadium(V) oxytriisopropoxide), and vanadium glycolate.

27. An electrode, wherein electrode material comprises V, Nb co-doped Ti0₂ as an electrode active material.

28. The electrode of claim 27 further comprising a current collector.

29. The electrode of claim 27, wherein the current collector is selected from a group comprising aluminum, stainless steel, copper, nickel, titanium, graphite, carbon black, carbon nanotubes, graphene, and conductive polymer.

30. The electrode of claim 27, wherein the electrode material further comprises an electrically conductive additive which improves electrical conductivity of the electrode material.

31. The electrode of claim 27, wherein the electrode material further comprises a polymer binder.

32. The electrode of claim 27, wherein the electrode material further comprises a carbonaceous material selected from a group comprising graphite, hard carbon, soft carbon, amorphous carbon coated graphite, amorphous carbon coated hard carbon, carbon black, carbon nanofibers, carbon nanotubes, graphene, carbon nanoparticles, carbon onion , crystalline carbon, semi-crystalline carbon, and amorphous carbon.

33. The electrode of claim 27, wherein the V, Nb co-doped Ti0₂ has Ti0₂ crystal structure which shows at least one characteristic peak at about 28.8 degree (2Θ) or about 43.4 degree (2Θ) in a x-ray diffraction spectrum obtained by using Cu Ka radiation.

34. The electrode of claim 27, wherein the molar ratio of Nb/Ti in the co-doped Ti0₂(B) is ranged from 1/19 to 2/1.

35. The electrode of claim 27, the molar ratio of V/Nb in the co-doped Ti0₂(B) is ranged from 1/100 to 2/1.

36. The electrode of claim 27, wherein the electrode material further comprises an element

selected from a group comprising calcium, magnesium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, tungsten, aluminum, gallium, tin, antimony, and bismuth.

37. The electrode of claim 27, wherein the electrode is used as a negative electrode in a lithium ion battery or in a lithium ion capacitor.

38. The electrode of claim 27, wherein the V, Nb co-doped Ti0₂(B) is synthesized from a

solvothermal process, in which at least one titanium compound, one niobium compound, and one vanadium compound are converted into oxides at a temperature between 120 °C to 200 °C in the presence of water and an organic compound.

39. The electrode of claim 27, wherein the V, Nb co-doped Ti0₂(B) is synthesized by synthesizing a Nb-doped Ti0₂(B) first from a solvothermal process and then the synthesized Nb-doped Ti0₂(B) is impregnated with a vanadium compound and heated at temperature above 400 °C to obtain V, Nb co-doped Ti0₂(B).