FR3139132A1 - Niobium titanium mixed oxide active material substituted with tungsten - Google Patents

Abstract

Active material intended for the manufacture of electrodes, the active material comprising a monoclinic mixed oxide of substituted niobium titanium, capable of allowing the insertion and extraction of Li+ ions, the active material having the following crude formula (I): Ti(1+x)Nb(2-2x)WxO7 (I) in which x is chosen in the range from 0.05 to 0.2. Figure for the abstract: [fig. 3]

Description

Niobium titanium mixed oxide active material substituted with tungsten

The present invention relates to the field of active materials intended to form an electrode for lithium accumulators. In particular, the invention relates to an active material formed of particles of mixed oxide of niobium and titanium, part of the niobium of which is substituted by tungsten and titanium. According to a second aspect, the invention proposes a process for manufacturing such an active material. According to other aspects, the invention proposes an electrode formed from said active material and an electrochemical generator, in particular of the battery type, which comprises a negative electrode in said active material.

Lithium batteries are increasingly used as stand-alone energy sources, particularly in portable equipment, where they have replaced nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) batteries and now also for electric mobility. This development is explained by the continuous improvement in the performance of lithium accumulators associated with a drastic reduction in their production costs, thus giving them mass and volume energy densities significantly higher than those offered by the Ni-Cd and Ni-MH. While the first Li-ion accumulators had an energy density of around 85 Wh/kg, more than 200 Wh/kg can now be obtained (energy density relative to the mass of the complete Li-ion cell). For comparison, Ni-MH accumulators peak at 100-110 Wh/kg and Ni-Cd accumulators have an energy density of around 50-70 Wh/kg, this, associated with lower costs , explains that lithium batteries are now the best-selling. New generations of more efficient lithium batteries are being developed for ever more diversified applications (hybrid or all-electric automobiles, photovoltaic cell energy storage, etc.). In order to meet ever-increasing energy and sometimes power demands (per unit of mass and/or volume), new, even more efficient Li-ion battery electrode materials are essential.

The active electrode compounds used in commercial accumulators are, for the positive electrode, lamellar compounds such as LiCoO ₂ , LiNiO ₂ and mixed Li(Ni, Co, Mn, Al)O ₂ or compounds with spinel structure of composition close to LiMn ₂ O ₄ . The negative electrode is generally carbon (graphite, coke, etc.) or possibly spinel Li ₄ Ti ₅ O ₁₂ or a metal forming an alloy with lithium (Sn, Si, etc.). The theoretical and practical specific capacities of the cited negative electrode compounds are approximately 370 mAh/g for graphite and 170 mAh/g for titanium oxide, respectively.

Despite its low capacity, compared to graphite, the Li ₄ Ti ₅ O ₁₂ compound finds its place on the market thanks to its high work potential, around 1.6V vs Li+/Li, which makes it very safe and thanks to very good cyclability at high speed, which makes it the negative electrode material of choice for power applications.

Many studies have been carried out to find a compound with the same advantages as Li ₄ Ti ₅ O ₁₂ in terms of both power and safety and having a higher specific capacity. This is how niobium oxides and mixed Ti-Nb oxides were considered, niobium having a working potential close to that of titanium and making it possible to exchange 2 electrons per metal atom (Nb ⁵⁺ / Nb ^{3 +} ).

The most interesting compounds are the oxides TiNb ₂ O ₇ and Ti ₂ Nb ₁₀ O ₂₉ . They have very high theoretical capacities (388mAh/g and 396mAh/g, respectively) compared to Li ₄ Ti ₅ O ₁₂ (175mAh/g) and have a working potential close to that of Li ₄ Ti ₅ O ₁₂ , which allows them to maintain the security advantages of the latter. They are therefore very interesting candidates with a view to replacing it for applications requiring more energy.

However, the TiNb ₂ O ₇ material, more attractive in terms of cost due to the higher Ti/Nb ratio than in the Ti ₂ Nb ₁₀ O ₂₉ material, presents limitations in terms of power performance and cyclability.

The present invention aims to remedy the drawbacks mentioned above. To this end, the present invention proposes an active material intended for the manufacture of an electrode, the active material comprising a monoclinic mixed oxide of substituted niobium titanium, capable of allowing the insertion and extraction of Li+ ions, the active material presenting the following crude formula (I):

Ti _(1+x) Nb _(2-2x) W _x O ₇ (I)

in which the x value is chosen in the range from 0.05 to 0.2.

This active material requires the substitution of part of the niobium in the mixed oxide of TiNb ₂ O ₇ with tungsten and titanium while preserving the initial crystal structure. As we will see later in the , only the volume of the mesh increases with the tungsten content. The niobium content decreases at the same time as the tungsten content increases as well as the titanium content to maintain the initial stoichiometry.

Thus optimized, this new active material offers stabilization of cycling performance over time while maintaining a high work potential and limiting performance losses during slow initial cycling.

According to one provision, the active material consists of a single monoclinic mixed oxide of substituted niobium titanium of the following crude formula (I):

Ti _(1+x) Nb _(2-2x) W _x O ₇ (I)

in which the x value is chosen in the range from 0.05 to 0.2.

According to one possibility, the value x is chosen in the range from 0.10 to 0.2, in particular in the range from 0.12 to 0.18, and for example x = 0.15.

In the active material according to the invention, titanium has an oxidation degree +IV and niobium has an oxidation degree +V. The oxidation states of the metals in Ti _(1+x) Nb _(2-2x) W _x O ₇ are identical to those of the unsubstituted compound TiNb ₂ O ₇ . This is advantageous because a reduction in the degrees of oxidation, from Ti4+ to Ti3+ or from Nb5+ to Nb4+, would de facto limit the number of electrons available during the first lithiation (reduction of metals) and therefore reduce the capacity of the material.

Concretely, the active material comprises particles having an average diameter Di greater than 100 nm and less than or equal to 0.5 mm. These average diameter Di values meet the density/compactness needs of the active material to provide a satisfactory energy density. The particles having a slightly elongated overall spherical shape and whose surface has contours comprising a certain irregularity, the average diameter Di of the particles corresponds to the average value of the three dimensions measured by laser particle size analysis.

According to one provision, the active material consists solely of said particles.

In particular, the particles are divided into three populations of average diameters Di, a first population having an average diameter D1 with 0.1 µm < D1 < 0.8 µm, a second population having an average diameter D2 with 1 µm < D2 < 10 µm, and a third population having an average diameter D3 with 10 µm < D3 < 0.5 mm.

According to a second aspect, the invention proposes a process for manufacturing the active material as previously described, which comprises a solid-state synthesis,

Concretely, the precursor reagents are TiO₂, of No.₂O₅and WO_3.

According to one characteristic, the precursor reagents are used in stoichiometric proportions.

According to one provision, the process comprises the steps of:

• a) grinding the precursor reagents in powder form in a planetary ball mill so as to obtain a homogeneous powdery mixture,
• b) calcination by application of a heat treatment at a temperature between 900°C and 1200°C so as to obtain the active material.

According to one arrangement, the process comprises after step b) carrying out a step c) of low energy grinding of the active material so as to reduce any agglomerates and obtain a homogeneous powder having particles of an average diameter Di greater than 100 nm and less than or equal to 0.5 mm.

One possibility is that low energy grinding is manual grinding.

Concretely, step a) of grinding the precursor reagents is carried out at a speed of approximately 400 rpm, it includes in particular an alternation of grinding sequences and rest sequences.

According to another aspect, the invention proposes an electrode comprising the active material as previously described.

According to yet another aspect, the invention proposes an electrochemical generator, in particular of the battery type, which comprises a positive electrode and a negative electrode comprising the active material as previously described and a non-aqueous electrolyte comprising lithium.

Other characteristics and advantages will appear on reading the detailed description below, of a non-limiting example of implementation, made with reference to the appended figures in which:

represents a laser particle size analysis of different compositions of active materials according to one embodiment of the invention.

represents an X-ray diffraction (XRD) pattern of the different compositions of active materials.

represents a graph illustrating the evolution of the charged capacity as a function of the number of cycles of button cells obtained from the different compositions of active materials.

With reference to the crude formula (I) illustrated below, the present invention proposes an active material intended for the manufacture of electrodes for lithium accumulators based on a mixed niobium titanium oxide substituted by tungsten:

Ti _(1+x) Nb _(2-2x) W _x O ₇ (I)

in which the x value is chosen in the range from 0.05 to 0.2.

This active material is obtained by solid-state synthesis from a mixture of stoichiometric proportions of powders of precursor reagents comprising TiO₂, of No.₂O₅and WO₃(refer to Table 1)_.TiO₂is for example obtained from the company Huntsmann (TiO₂anatase purity 99.0%), the Nb₂O₅is obtained from the company Sigma Aldrich (purity 99.9%) and the WO₃is obtained from the company Sigma Aldrich (purity 99.9%). The initial oxidation states of Ti +IV and Niobium +V are preserved during the reaction.

Formula (I) Mass of precursors (g) X TiO ₂ No. ₂ O ₅ WO ₃ TiNb ₂ O ₇ 0 1,473 4,940 - Ti _1.05 Nb _1.9 W _0.05 O ₇ 0.05 1,689 5,080 0.237 Ti _1.1 Nb _1.8 W _0.1 O ₇ 0.1 1,769 4,794 0.470 Ti _1.15 Nb _1.7 W _0.15 O ₇ 0.15 1,827 4,489 0.694 Ti _1.2 Nb _1.6 W _0.2 O ₇ 0.2 1,893 4,196 0.918

Table 1 : Masses of precursor reagents used for the synthesis of Ti _(1+x) Nb _(2-2x) W _x O ₇

The process for manufacturing the active material comprises a step a) of grinding the precursor reagents present in powder form in a planetary ball mill (PM 100 CM, Retsch – 32 g of agate balls in a bowl also made of agate for a mass of precursor reagents of approximately 7g) with a speed of 400 rpm for 8 hours. Grinding is not carried out continuously but by repetition of grinding sequence and rest sequence lasting 5 min each.

Once the perfectly homogeneous powdery mixture has been obtained, it is placed in three alumina crucibles (diameter of 3 cm each) then put in a muffle furnace (Carbolite, Model CWF 1200) until reaching a temperature of approximately 1100°C for 4 p.m. for example (step b). At the end of the heat treatment, the active material is obtained in particulate form.

The recovered active material is manually ground in an agate mortar. Five minutes of grinding are enough to obtain a homogeneous powder in the form of particles having an average diameter Di greater than 100 nm and less than or equal to 0.5 mm. As shown on the illustrating the laser particle size distribution diagram (left column), the average diameter Di is divided into three populations similar to those of the mixed niobium titanium oxide devoid of tungsten (device used MALVERN MASTERSIZER). These three populations include a first population having an average diameter D1 with 0.4 micrometer < D1 < 0.8 micrometer, a second population having an average diameter D2 with 1 micrometer < D2 < 10 micrometers, and a third population having an average diameter D3 with 10 micrometers < D3 < 0.5 mm.

Also shown in (right column), the particles of active material subjected to ultrasound treatment lead to the same particle size distribution, which shows that the particles obtained at the end of step c) are devoid of agglomerate.

Illustrated at , an X-ray diffraction analysis of the particles obtained at the end of step c) makes it possible to verify that the crystal structure of Ti ₂ Nb ₅ O ₇ is identical to that of the active material Ti _(1+x) Nb _{( 2-2x)} W _x O ₇ for all substitution rates in W (x from 0.05 to 0.2).

Electrodes and button cells were designed using the traditional method in order to observe the properties obtained by the active material as a function of different values of x and Ti ₂ Nb ₅ O ₇ .

The active material Ti _(1+x) Nb _(2-2x) W _x O ₇ (with x between 0.05 and 0.2) and a carbon additive (Carbon Black SUPER C65 from TIMCAL) are mixed and ground manually in an agate mortar in cyclohexane (Purity >99.5%, Merck ENSURE) for 5 minutes.

After complete evaporation of the solvent, a solution of polyvinylidene fluoride (PVDF, Solef 5130, Solvay) at 8% by weight in N-methyl-2-pyrrolidone (NMP, Purity >99%, Merck) is added to the ground mixture. until reaching a mass formulation of 80% active material, 10% electronic conductor and 10% binder for a dry extract of 30%. Table 2 illustrates the proportions of the reagents used.

Precursor of ink Typical mass (mg) Ti _(1+x) Nb _(2-2x) W _x O ₇ 200 Carbon black Super C65 25 8% PVDF in NMP 310

Table 2 : Typical masses for the preparation of Ti _(1+x) Nb _(2-2x) W _x O ₇ inks

The ink obtained is then coated on an aluminum strip, using a doctor blade type with a thickness of 100 µm. After 24 hours of drying at 55°C, electrodes with a diameter of 14 mm are cut and passed under a 10-ton press.

The electrodes are then assembled in a glove box to form CR2032 type button cells. The counter electrode is Lithium metal, one of the separators is made of polypropylene felt (Viledon, Freudenberg) and the other separator is made of polypropylene (CG2500, Celgard). The electrolyte consists of a mixture of ethylene carbonate (EC) / propylene carbonate (PC) / dimethyl carbonate (DMC) (1:1:3 vol) with lithium hexafluorophosphate (LiPF6) (1M ) (LP100, UBE Industries).

The performances of the button batteries obtained are gathered at the which illustrates, in the left part, the evolution of the Lithiation capacity of the compounds Ti _(1+x) Nb _(2-2x) W _x O ₇ with x = 0; 0.05; 0.1; 0.15 and 0.2 depending on the number of cycles and at different speeds (respectively cycling at C/10, C, 2C, 3C, 5C and 10C). The right part of the illustrates the evolution of the Lithiation capacity of the same compounds with cycling at the C/10 regime from the ^31st cycle.

As can be observed, the worst performances are obtained when the electrode is TiNb ₂ O ₇ for which the loss is quickly very significant. Cycling stability and performance losses during cycles are better with the substituted active material Ti _(1+x) Nb _(2-2x) W _x O ₇ . Also illustrated in Table 3 which reports the degradation values at C/10 for each of the active materials, the best results are obtained with x = 0.1 and x = 0.15. The performance losses vary from one to two after 100 cycles between an active material in which x = 0 (loss of approximately 30%) and an active material in which x = 0.1 and 0.15 (loss of 16.7 and 14, respectively. 1%). The improvement in lithiation capabilities is also best for x = 0.1 and 0.15.

Performance degradation every 10 cycles at C/10 (%) W(x) Cycle No. 60 Cycle No. 70 Cycle No. 80 Cycle No. 90 Cycle No. 100 0 20.0 24.7 26.3 28.5 30.3 0.05 13.4 15.9 17.2 19.6 22.3 0.10 10.8 14.0 14.0 15.7 16.7 0.15 7.9 8.4 11.8 12.0 14.1 0.20 10.1 12.2 15.7 17.4 18.5

Table 3 : Loss of performance compared to initial capacity at C/10 (cycle X. Vs cycle 31)

Thus, the present invention proposes an active material consisting of a mixed oxide of Ti _(1+x) Nb _(2-2x) W _x O ₇ intended for the manufacture of an electrode for Li-ion accumulators. The use of this active material makes it possible to achieve greater cycling stability and to limit the loss of performance observed with x = 0. As visible on the and Table 3, the optimum is obtained for tungsten substitution with a value of x= 0.15. The process for manufacturing the material includes solid-state synthesis, leading to particles without agglomerate having a diameter greater than 100 nm, which makes it possible to meet the density/compactness requirements necessary to achieve energy densities satisfactory for the intended objective. The active materials proposed by the invention are thus adapted to the needs for high power (rapid charge/rapid discharge, associated with very good cyclability), and maintaining an energy density at a high level (>100Wh/kg).

It goes without saying that the invention is not limited to the alternative embodiments described above by way of example but that it includes all the technical equivalents and variants of the means described as well as their combinations.

Claims (10)

Active material intended for the manufacture of electrodes, the active material comprising
a monoclinic mixed oxide of substituted niobium titanium, capable of allowing the insertion and extraction of Li+ ions, the active material having the following crude formula (I):
Ti _(1+x) Nb _(2-2x) W _x O ₇ (I)
in which x is chosen in the range from 0.05 to 0.2. Active material according to claim 1, wherein x is selected from the range of 0.10 to 0.2. Active material according to claim 1 or 2, which comprises particles having an average diameter Di greater than 100 nm and less than or equal to 0.5 mm. Process for manufacturing the active material according to one of claims 1 to 3, which comprises solid-state synthesis. Manufacturing process according to claim 4, in which the precursor reagents are TiO₂, of No.₂O₅and WO_3. Manufacturing process according to claim 4 or 5, in which the precursor reagents are used in stoichiometric proportions. Manufacturing process according to one of claims 4 to 6, the process comprising the steps of:

• a) grinding the precursor reagents in powder form in a planetary ball mill so as to obtain a homogeneous powdery mixture,
• b) calcination by application of a heat treatment at a temperature between 900°C and 1200°C so as to obtain the active material.

Process for manufacturing the active material according to claim 7, which comprises, after step b), carrying out a step c) of low-energy grinding of the active material so as to reduce any agglomerates and obtain a homogeneous powder having particles with an average diameter Di greater than 100 nm and less than or equal to 0.5 mm. Electrode comprising the active material according to one of claims 1 to 3. Electrochemical generator, in particular of the battery type, which comprises a positive electrode and a negative electrode comprising the active material according to one of claims 1 to 3 and a non-aqueous electrolyte comprising lithium.