WO2010094988A1

WO2010094988A1 - Method of producing an amorphous lithium metal (ii) phosphate aerogel via non-aqueous sol-gel processing

Info

Publication number: WO2010094988A1
Application number: PCT/IB2009/000307
Authority: WO
Inventors: Jun Yoshida; Norio Sato; Nicola Huesing; Michael Stark; Jürgen HOLZBOCK
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2009-02-20
Filing date: 2009-02-20
Publication date: 2010-08-26

Abstract

The invention relates to a method of producing an amorphous lithium metal (II) phosphate aerogel, the method comprising: - dissolving (100) a lithium salt and a metal salt in a first organic solvent, - dissolving a phosphor source in the first organic solvent, - mixing (102) the dissolved salts and the dissolved phosphor source for receipt of a gel, - supercritical (106) drying of the gel for receipt of the lithium metal (II) phosphate aerogel.

Description

D e s c r i p t i o n

Method of producing an amorphous lithium metal (II) phosphate aerogel via nonaqueous sol-gel processing

Technical field

The invention relates to a method of producing an amorphous lithium metal (II) phosphate aerogel and an amorphous lithium metal (II) phosphate aerogel, wherein the porosity of the aerogel is higher than 80%.

Background and related art

Batteries belong to the most important power sources which are used in different areas of operation. Almost any electrical consumer can be equipped with batteries in order to use electrical energy which results from discharging of the battery through an electrochemical redox reaction.

Lithium is a widespread negative electrode material for batteries. This Is due to the fact that lithium has the most negative standard potential of all elements which allows realizing obtaining high battery cell voltages. Also, using lithium theoretically extremely high battery capacities are accomplishable. Indeed, since many years suitable electrode materials are developed for uptaking and releasing of lithium ions in combination with respective electrolyte materials in order to achieve such high theoretical energy densities of lithium batteries in practice. One electrode material which can be used to realize such high discharge voltages while maintaining a high capacity are lithium phosphor compounds in the form of olivines, as for example LiMPO₄, wherein M is a metal like iron, manganese, cobalt etc. Phospho-olivines such as LiMPO₄ (M = transition metal) are attractive candidates for cathode materials in lithium ion batteries, because of their excellent cyclabiiity, thermal stability, low cost and environmental benefits. However, these materials suffer from a low electronic and ionic conductivity of LiMPO₄ leading typically to poor rate capabilities. Immense technical efforts have therefore been devoted to counteract this problem, one approach being the synthesis of well dispersed and small particles to shorten the diffusion path length of lithium ions.

For example, J. Electrochβm. Soc, Vol. 144, No. 4, April 1997, p. 1188 - 1194 discloses that phospho-olivine is a positive electrode material suitable for rechargeable lithium batteries.

Similarly, J. Electrochem. Soc, Vol. 148, No. 8, A960 ■ A967, 2001 deals with the usage of olivine type lithium compounds as a possible cathode material for lithium batteries.

US 5,910,382 discloses the usage of transition metal compounds with an ordered olivine or rhombohedral Nasicon structure as electrode material for rechargeable alkali ion batteries.

R. Dominko, M. BeIe, M. Gaberscek, M. Remskar, D. Hanzel, J. M. Goupil, S.

Pejovnik, J. Jamnik, "Porous olivine composites synthesized by sol-gel technique", J. Power Sources, 2006, 153, 274-280 does disclose porous LiMPO₄/C composites (where M stands for Fe and/or Mn) with micro-sized particles which were synthesized by a sol-gel technique. The document discusses porosity in terms of qualitative results obtained from TEM micrographs and in terms of quantitative results obtained from N₂ adsorption. Porous particles are described as an inverse picture of nano particulate arrangements, where the pores serve as channels for lithium supply and the distance between the pores determines the material kinetics.

Optimized, well crystallized phosphates have also been synthesized by a sol-gel approach also termed as "chimie douce reaction¹¹ and low sintering temperatures as disclosed in WO2007/093856. Drying of the resulting gel body is performed at low temperatures in ambient pressures, resulting in a material that the authors describe as an aerogel. This aerogel is converted to crystalline LiMnPO₄ by sintering in a second step. No further evidence on the aerogel structure is given in the disclosure. Aerogels are highly porous solid materials with extremely low densities (bulk densi- ties 0.004 - 0.500 g/cm³), large, open pores, and high specific surface areas. So far, aerogels have been differently defined: 1. All materials prepared from wet gels by supercritical drying were called aerogels, irrespective of their structural properties. However, with the development of new drying techniques this definition is no longer appropriate or 2. Materials in which the typical pore structure and network are largely maintained when the pore liquid of a gel is replaced by air are called aerogels. Typically drying by simply heating a highly porous gel does not result In an aerogel. [Aerogels, N. Hϋsing, U. Schubert, Uflmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release, Wiley VCH, Weinheim 2006].

US, 4,622,310 discloses the preparation of inorganic phosphate aerogels and the method of preparing such inorganic phosphate aerogels which are characterized by high surface areas and high pore volumes.

Summary of the invention

The present invention provides a method of producing an amorphous lithium metal (II) phosphate aerogel, the method comprising dissolving a lithium salt and a metal salt in a first organic solvent, dissolving a phosphor source in the first organic solvent, mixing the dissolved salts and the dissolved phosphor source to receive a monolithic gel and supercritical drying of the gel for receipt of the lithium metal (II) phosphate aerogel.

The method according to the invention has the advantage, that amorphous lithium metal (II) phosphates can be produced with an extremely high surface area. The morphology of the lithium metal (II) phosphate amorphous nanoparticles forming a threedimensional network can further be easily controlled. The preparation of such kind of lithium metal (II) phosphate material which is built- up from amorphous nanoparticles forming a nanoporous network with a high surface area is an important step for production of highly efficient cathode materials for lithium ion batteries. By coating the lithium metal (II) phosphate material with carbon and starting a crystallization process, carbon coated crystal lithium metal (II) phosphate nanoparticles can be obtained which have a high electronic and ionic conductivity which permits the application of the resulting material as negative electrode material in batteries with extremely high charge and discharge rates. Thus, by means of the method of producing an amorphous lithium metal (II) phosphate aerogel, a material can be obtained which allows for high performance applications in lithium ion batteries.

In accordance with an embodiment of the invention, the method further comprises adding acetic acid to the dissolved salts. Acetic acid acts as a compatibilizing agent, is needed for pH-adjustment and can act as chelating species to control the chemical reactivity of the salts. In principle other carboxylic acids are also suited.

In accordance with a further embodiment of the invention, the method further comprises adding hydrazine hydrate to the dissolved salts. Hydrazine hydrate acts as a reducing agent to avoid oxidation of Mn(II) to Mn(III) or other species.

In accordance with an embodiment of the invention, the method further comprises exchanging the first organic solvent with a second organic solvent. Exchanging the first organic solvent with a second organic solvent has the advantage, that during the supercritical drying process of the gel an unwanted partial drying of the wet gel can be efficiently avoided. This is an important aspect in order to prevent shrinkage of the gel volume before or during supercritical drying.

In accordance with a further embodiment of the invention, the supercritical drying Is performed in the presence of the second organic solvent. In accordance with an embodiment of the invention, the first solvent comprises DMSO. In accordance with a further embodiment of the invention, the second solvent comprises methanol.

Using DMSO as the first solvent has the advantage, that lithium and metal salts can be easily dissolved in the solvent. The combination of DMSO and methanol has the advantage, that a highly efficient solvent exchange within a short period of time is achievable which is an important aspect for industrial scale processes where processing speed is an important aspect.

In accordance with an embodiment of the invention, the lithium salt comprises lithium acetate.

In accordance with an embodiment of the invention, the phosphor source comprises a phosphate and/or a phosphoric acid component.

In accordance with an embodiment of the invention, the amorphous lithium metal (II) phosphate aerogel contains material with the composition Li_xMyPO₄ with M=Ti, V, W, Cr, Mn, Fe, Co, Ni, Cu, Mg, Ca, Sr, Pb, Cd, Ba, Be and/or contains material of the composition Li_xFei_-y Ti_y PO₄ and/or Li_xFei._y Mn_y PO₄ with 0<y<1.

In another aspect, the invention relates to a method of producing a battery electrode material, wherein the method comprises the method of producing an amorphous lithium metal (II) phosphate aerogel according to the invention, the method further comprising mixing the aerogel with the carbon source for receipt of amorphous carbon coated nanoparticles and subsequent heat treating of said nanoparticles for formation of carbon coated crystalline lithium metal (II) phosphate nanoparticles.

As already mentioned above, such kinds of carbon coated crystalline lithium metal (II) phosphate nanoparticles have the advantage, that due to the nanometer sized dimensions of the particles highly efficient lithium ion batteries can be realized by using said carbon coated crystalline lithium metal (II) phosphate nanoparticles as negative battery electrode material.

In accordance with a further embodiment of the invention, the mixing is performed by milling, preferably ball milling.

In accordance with an embodiment of the invention, the carbon source comprises graphite and/or carbon black and/or acetylene black.

In another aspect, the invention relates to an amorphous lithium metal (II) phosphate aerogel, wherein the porosity of the aerogel is higher than 80%.

In accordance with an embodiment of the invention, the lithium metal (II) phosphate aerogel is a monolithic aerogel.

Brief description of the drawings

In the following preferred embodiments of the invention will be described in greater detail by way of example only making reference to the drawings in which:

Figure 1 is a flow chart illustrating a method of producing an amorphous lithium metal (II) phosphate aerogel and a battery electrode material,

Figure 2 shows XRD measurement results performed on a heat treated and crystallized LiMnPO₄ aerogel,

Figure 3 shows XRD results of various in-situ heat treated LiMnPO₄ aerogels at different heating temperatures,

Figure 4 shows the nitrogen sorption data for an amorphous LiMnPO₄ aerogel. Figure 5 shows a TEM image of an amorphous LiMnPO₄ aerogel.

Detailed description

Fig. 1 is a flow chart illustrating the method of producing an amorphous lithium metal (II) phosphate aerogel and a battery electrode material according to the invention. The method starts in step 100 in which a lithium salt and a metal salt are dissolved in a first organic solvent. For example, for this purpose manganese acetate and lithium acetate are dissolved in DMSO. Further, not shown in fig. 1 , a few drops of acetic acid and of hydrazine hydrate are added with steering which reduces Mn(III) and results in an almost colorless solution.

In step 102, o-phosphoric acid is dissolved in DMSO and added to the salts dissolved in step 100. This results in the formation of a gel. In step 104, this gel is 'aged' at elevated temperatures which may result in a small shrinkage of the monolithic gel body. After a short curing time at elevated temperature in which DMSO is removed from the gel to some extent, DMSO is exchanged to methanol for various hours.

This is followed by step 106 which comprises supercritical drying of the gel. This can be for example performed by means of an autoclave which for this purpose is preferably filled with methanol to avoid a partial drying of the gel body. The autoclave is kept preferably at room temperature. The monolithic gel body is placed in the autoclave in the extraction thimble to avoid fragmenting of the sample. The specimen boat is loaded into the pressure chamber, the door of the autoclave is closed and then the fill valve on the pressure vessel is carefully opened. The autoclave is filled with liquid CO₂ and the pressure increases. This is followed by a substitution of the methanol mother liquid with liquid carbon dioxide. This flushing process is performed until the solvents are completely exchanged. After complete exchange, the actual supercritical drying process is performed. This results in a pinkish, monolithic gel body with a bulk density of typically 0.04 gram per cm³. The gel is very sensitive towards compression. The surface area measured by BET analysis is higher than 400 m² per gram.

Steps 108 and 110 are only necessary if the aerogel resulting from the supercritical drying step 106 shall be used as a battery electrode material. For this purposed, in step 108 the aerogel is mixed for example by ball milling with a carbon source for receipt of amorphous carbon coated nanoparticles. After this carbon coating was performed in step 108, a crystallization process in step 110 is performed by heat treatment of the carbon coated amorphous particles. For example, by using as precursors lithium acetate and a manganese acetate solution, the resulting carbon coated crystals have a stoichiometric composition of LiMnPO₄. The carbon coated crystals are nanoparticles which compared to conventional LiMnPO₄ preparation techniques have an extremely small diameter. The particle size is typically much less than 10nm.

The carbon coating has here two advantages: first, due to the carbon coating of the LiMnPO₄ nanoparticles, the material is made electrically conductive which is an important aspect for application as electrode material in lithium ion batteries. Second, due to the carbon coating during the heat treatment process for the formation of the crystalline LiMnPO₄ particles, no sintering between LiMnPO₄ particles occurs. In other words, the particles will not form larger crystallites during the heat treatment process such that extremely regular, nanometer sized LiMnPO₄ particles can be obtained.

Fig. 2 shows an XRD (X-ray diffraction) pattern of a heat treated LJMnPO₄ aerogel. It can be seen, that all visible peaks in the diffraction diagram can be identified belonging to pure LiMnPO₄. Obviously, no impurities are present. In other words, the produced aerogel consists of pure LiMnPO₄. The measurements in fig. 2 were performed after heat treatment at 450^°C which results in a crystalline, shrunken sample. In fig. 3, various in-situ temperature dependent XRD measurements on an aerogel which was produced according to the method illustrated in fig. 1 are shown. It can be seen, that at low temperatures almost no diffraction peaks are visible which is an indication for an amorphous aerogel. By gradually increasing the temperature, it can be seen that crystallization sets in above 300⁰C. Above 600⁰C, LiMnPO₄ seems to decompose into different crystalline phases.

Fig. 4 shows the gas ad- and desorption isotherms for an aerogel measured with nitrogen at 77K according to this invention. The adsorbed volume of the gas (N₂) is depicted versus the relative pressure p/p°. Prior to the determination of the isotherm, all physisorbed material has been removed from the aerogel sample by vaccum heating at 100⁰C. The adsorptive (N₂) is automatically and continuously admitted to the aerogel sample, thus providing a measure (volumetric determination) of the adsorption under quasi-equilibrium conditions. The porous nature of the material is clearly seen and the sorption is characterized by a Type IV isotherm with a H1 hysteresis indicating mesoporosity. The specific surface area was evaluated by applying the Brunauer-Emmett-Teller theory in a pressure range from 0.05-0.5 p/p° via a 5-point analysis. It is very high with above 400 m²g^'1 and the total pore volume (at the highest value for p/p° close to 1) already indicates the high porosity of the sample. In addition, the rather low bulk density of the monolith with 0.143 gem^"3 confirms the high porosity of above 90% (the crystal density would be 3.34 gem^'3.

Fig. 5 is a TEM (transmission electron microscopy) image of an aerogel according to the invention. The TEM analysis shows a rather featureless image which clearly indicates that the aerogel sample is amorphous. No particles larger than 10nm are found and the porous nature of the structure is visible.

Claims

C l a i m s

1. A method of producing an amorphous lithium metal (II) phosphate aerogel, the method comprising:

- dissolving (100) a lithium salt and a metal salt in a first organic solvent,

- dissolving a phosphor source in the first organic solvent,

- mixing (102) the dissolved salts and the dissolved phosphor source for receipt of a wet gel, - drying of the gel via extraction with supercritical solvents for receipt of the lithium metal (Ii) phosphate aerogel.

2. The method of claim 1 , further comprising adding acetic acid to the dissolved salts.

3. The method of claim 1 or 2, further comprising adding hydrazine hydrate to the dissolved salts.

4. The method according to any of the previous claims, further comprising ex- changing the first organic solvent with a second organic solvent.

5. The method according to any of the previous claims, wherein the supercritical drying (106) is performed in the presence of the second organic solvent.

6. The method according to any of the previous claims, wherein the first solvent comprises DMSO.

7. The method according to any of the previous claims 4 to 7, wherein the second solvent comprises methanol.

8. The method according to any of the previous claims, wherein the lithium salt comprises lithium acetate.

9. The method according to any of the previous claims, wherein the phosphor source comprises a phosphate and/or a phosphoric acid component.

10. The method according to any of the previous claims, wherein the amorphous lithium metal (II) phosphate aerogel contains material with the composition Li_xMyPO₄ with M= Ti, V, W, Cr₁ Mn, Fe, Co, Ni₁ Cu, Mg, Ca₁ Sr, Pb, Cd, Ba, Be, and/or contains material of the composition U_xFei._yTi_yPO₄ and/or Li_xFei- yMn_yPO₄ with 0<y<1 .

11. A method of producing a battery electrode material, the method comprising the steps according to any of the previous claims 1 to 10, the method further comprising mixing (108) the aerogel with a carbon source for receipt of amorphous carbon coated nanoparticles and subsequent heat treating (110) of said nanoparticles for formation of carbon coated crystalline lithium metal

(II) phosphate nano particles.

12. The method of claim 11 , wherein the mixing is performed by milling.

13. The method of claim 11 or 12 wherein the carbon source comprises graphite and/or carbon black and/or acetylene black.

14.An amorphous lithium metal (II) phosphate aerogel, wherein the porosity of the aerogel is higher than 80%.

15.The amorphous lithium metal (II) phosphate aerogel of claim 14, wherein the aerogel is a monolithic aerogel.