SYNTHESIS OF NANOSTRUCTURED LITHIUM ZIRCONIUM PHOSPHATE

Abstract

The invention relates to a process for producing lithium zirconium phosphate by means of flame spray pyrolysis using as precursors lithium and zirconium carboxylates containing 5 to 20 carbon atoms, an organic phosphate, and a solvent containing less than 10% by weight water. Lithium zirconium phosphate obtainable by this process can be used in lithium ion batteries.

Claims

1. Lithium zirconium phosphate of a general formula Li.sub.aZr.sub.bM.sub.c(PO.sub.4).sub.d, wherein M is at least one metal different from Li and Zr, 0.5 ≤ a ≤ 5.0, 0.5 ≤ b ≤ 5.0, 0 ≤ c ≤ 5, 1 ≤ d ≤ 5 characterized in that the lithium zirconium phosphate is in the form of aggregated primary particles, has a BET surface area of 5 m.sup.2/g -100 m.sup.2/g, a numerical mean particle diameter of d.sub.50 = 0.03 .Math.m -2 .Math.m, as determined by static light scattering (SLS), and a tamped density of 20 g/L - 200 g/L.

2. Process for producing lithium zirconium phosphate according to claim 1, by means of flame spray pyrolysis, characterized in that at least one solution of metal precursors, comprising a lithium carboxylate and a zirconium carboxylate, wherein each of these metal carboxylates contains 5 to 20 carbon atoms, an organic phosphate, a solvent containing less than 10% by weight water is subjected to flame spay pyrolysis.

3. Process according to claim 2, characterized in that the spray flame pyrolysis comprises the following steps: a) the solution of metal precursors is atomized to afford an aerosol by means of an atomizer gas, b) the aerosol is brought to reaction in the reaction space of the reactor with a flame obtained by ignition of a mixture of fuel gas and an oxygen-containing gas to obtain a reaction stream, c) the reaction stream is cooled and d) the solid lithium zirconium phosphate is subsequently removed from the reaction stream.

4. Process according to claim 2, characterized in that the lithium and zirconium carboxylates are, independently of each other, carboxylates selected from the group consisting of linear, branched or cyclic pentanoate (C5), hexanoate (C6), heptanoate (C7), octanoate (C8), nonanoate (C9), decanoate (D10), undecanoate (C11), dodecanoate (C12), tridecanoate (C13), tetradecanoate (C14), pentadecanoate (C15), hexadecanoate (C16), heprtadecanoate (C17), octadecanoate (C18), nonadecanoate (C19), icosanoate (C20) of lithium and/or zirconium, and the mixtures thereof.

5. Process according to claim 2, characterized in that the organic phosphate is selected from esters of phosphonic acid (H.sub.3PO.sub.3), orthophosphoric acid (H.sub.3PO.sub.4), methaphosphoric acid (HPO.sub.3), pyrophosphoric acid (H.sub.4P.sub.2O.sub.7), polyphosphoric acids, and mixtures thereof.

6. Process according to claim 2, characterized in that the organic phosphate is selected from alkyl ester, aryl ester, mixed alkyl/aryl esters, and mixtures thereof.

7. Process according to claim 2, characterized in that the organic phosphate is an alkyl ester having alkyl groups with 1 to 10 carbon atoms.

8. Process according to claim 2, characterized in that the solvent is selected from the group consisting of linear or cyclic, saturated or unsaturated, aliphatic or aromatic hydrocarbons, esters of carboxylic acids, ethers, alcohols, carboxylic acids, and the mixtures thereof.

9. Process according to claim 2, characterized in that the solution of metal precursors comprises a chelating agent selected from the group consisting of diamines and 1,3-dicarbonyl compounds.

10. Process according to claim 2, further comprising thermal treatment of the lithium zirconium phosphate, produced by means of flame spray pyrolysis, at a temperature of 600° C. - 1300° C.

11. Process according to claim 10, further comprising milling of the thermally treated lithium zirconium phosphate.

12. A method comprising incorporating the lithium zirconium phosphate according to claim 1 as a component of a solid-state electrolyte, as an additive in liquid, or gel electrolyte or as a constituent of an electrode of a lithium ion battery.

13. Electrode for a lithium ion battery comprising lithium zirconium phosphate according to claim 1.

14. Electrolyte for a lithium ion battery comprising lithium zirconium phosphate according to claim 1.

15. Lithium ion battery comprising lithium zirconium phosphate according to claim 1.

16. Lithium ion battery according to claim 15, comprising a liquid or gel electrolyte.

17. Lithium ion battery according to claim 15, wherein the battery is a solid-state battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0094] FIG. 1 is a TEM image of lithium zirconium phosphate particles prepared as described in comparative example 1.

[0095] FIG. 2 is a TEM image of lithium zirconium phosphate particles prepared as described in comparative example 2.

[0096] FIG. 3 is a TEM image of lithium zirconium phosphate particles prepared as described in example 1.

[0097] FIG. 4 shows XRD patterns of lithium zirconium phosphates prepared as described in examples 1-2 and comparative examples 1-2.

[0098] FIG. 5 shows SEM images of NMC dry coated with fumed lithium zirconium phosphate particles (a - backscattered electrons (BSE) image, b - EDX mapping of Zr, c - high resolution SEM image).

[0099] FIG. 6 shows the rate characteristics and the cycling performance of uncoated NMC and NMC dry coated with fumed lithium zirconium phosphate particles in lithium ion batteries with liquid electrolyte.

[0100] FIG. 7 shows the charge and discharge curves at the initial cycle with the discharge current of 0.066 mA (0.035C) in sulfide based all-solid-state batteries with the solid electrolyte of Li.sub.6PS.sub.5Cl.

[0101] FIG. 8 shows the charge and discharge curves at the 4.sup.th cycle with the discharge current of 1.4 mA (0.75C) in sulfide based all-solid-state batteries with the solid electrolyte of Li.sub.6PS.sub.5Cl.

EXAMPLES

Comparative Example 1

[0102] 6,34 Kilogram of an aqueous solution containing 157.8 g of LiNO.sub.3 (metal content: 4 wt%), 1733 g of Zr(NO.sub.3).sub.4 (metal content: 6 wt%) was prepared. 1190 Grams of H.sub.3PO.sub.4 (85 wt% in water) were added dropwise to the solution of metal salts under intense stirring resulting in formation of a dispersion containing white precipitate. The composition of the thus obtained compound corresponded to the formula LiZr.sub.2(PO4).sub.3.

[0103] An aerosol of 1.5 kg/h of this dispersion and 15 Nm.sup.3/h of air was formed via a two-component nozzle and sprayed into a tubular reaction with a burning flame. The burning gases of the flame consisted of 14.3 Nm.sup.3/h of hydrogen and 30 Nm.sup.3/h of air. Additionally, 25 Nm.sup.3/h of secondary air was used. After the reactor the reaction gases were cooled down and filtered.

[0104] The particle properties are shown in Table 1, the TEM image of the particles is shown in FIG. 1 and the XRD analysis (FIG. 4) showed, that the major phase of the product was the cubic zirconium phosphate.

Comparative Example 2

[0105] 6,34 Kilogram of an ethanolic solution containing 157.8 g of LiNO.sub.3 (metal content: 4 wt%), 1733 g of Zr(NO.sub.3).sub.4 (metal content: 6 wt%) was prepared. 1190 Gram of H.sub.3PO.sub.4 (85 wt% in water) was added dropwise to the solution of metal salts under intense stirring resulting in formation of a dispersion containing white precipitate. The composition of the thus obtained compound corresponded to the formula LiZr.sub.2(PO4).sub.3.

[0106] An aerosol of 1.5 kg/h of this dispersion and 15 Nm.sup.3/h of air was formed via a two-component nozzle and sprayed into a tubular reaction with a burning flame. The burning gases of the flame consisted of 8.4 Nm.sup.3/h of hydrogen and 30 Nm.sup.3/h of air. Additionally, 25 Nm.sup.3/h of secondary air was used. After the reactor the reaction gases were cooled down and filtered.

[0107] The particle properties are shown in Table 1, the TEM image of the particles is shown in FIG. 2 and the XRD analysis (FIG. 4) showed, that the major phase of the product was the cubic zirconium phosphate.

Comparative Example 3

[0108] 61.4 Gram of a solution containing 2.9 g of a commercial solution (Borchers® Deca Lithium2), containing 2 wt% lithium in the form of lithium neodecanoate dissolved in naphtha, 50 g of a commercial solution (Octa Solingen® Zirconium 12), containing 11.86 wt% Zr in the form of zirconium ethyl hexanoate dissolved in white spirit was prepared by mixing. 8.6 Grams of H.sub.3PO.sub.4 (85 wt% in water) were added dropwise to the solution of metal salts under intense stirring. A phase separation was observed, making it impossible to spray the solution homogeneously. No spray experiment was performed

Example 1

[0109] 23.75 Kilogram of a solution containing 3370 g of a commercial solution (Borchers® Deca Lithium2), containing 2 wt% lithium in the form of lithium neodecanoate dissolved in naphtha, 15 kg of a commercial solution (Octa Solingen® Zirconium 12), containing 11.86 wt% Zr in the form of zirconium ethyl hexanoate dissolved in white spirit and 5384 g of a commercial solution (Alfa Aesar), containing 16.83 wt % phosphorous in the form of triethyl phosphate were mixed, resulting in a clear solution. This solution corresponding to a composition of LiZr.sub.2(PO.sub.4).sub.3.

[0110] An aerosol of 1.5 kg/h of this dispersion and 15 Nm.sup.3/h of air was formed via a two-component nozzle and sprayed into a tubular reaction with a burning flame. The burning gases of the flame consisted of 8.5 Nm.sup.3/h of hydrogen and 30 Nm.sup.3/h of air. Additionally, 25 Nm.sup.3/h of secondary air was used. After the reactor the reaction gases were cooled down and filtered.

[0111] The particle properties are shown in table 1, the TEM image of the particles are shown in FIG. 3 and the XRD analysis (FIG. 4) showed, that the major phase of the product was the rhombohedral lithium zirconium phosphate.

Example 2

[0112] 6,18 Kilogram of a solution containing 1056 g of a commercial solution (Borchers® Deca Lithium2), containing 2 wt% Lithium in the form of lithium neodecanoate dissolved in naphtha, 2453 g of a commercial solution (Octa Solingen® Zirconium 12), containing 18.01 wt% Zr in the form of zirconium ethyl hexanoate dissolved in white spirit and 1408 g of a commercial solution (Alfa Aesar), containing 16.83 wt % phosphorous in the form of triethyl phosphate were mixed, resulting in a clear solution. A further solution, containing 60.8 g of Ca(NO.sub.3).sub.2 × 4H.sub.2O, 1200 g of ethanol and 1200 g of ethyl hexanoic acid was added under constant stirring until a clear solution was obtained. This solution corresponded to a composition of Li.sub.1.2Ca.sub.0.1Zr.sub.1.9(PO.sub.4).sub.3.

[0113] An aerosol of 1.5 kg/h of this dispersion and 15 Nm.sup.3/h of air was formed via a two-component nozzle and sprayed into a tubular reaction with a burning flame. The burning gases of the flame consisted of 8.7 Nm.sup.3/h of hydrogen and 30 Nm.sup.3/h of air. Additionally, 25 Nm.sup.3/h of secondary air was used. After the reactor the reaction gases were cooled down and filtered.

[0114] The particle properties are shown in table 1 and the XRD analysis (FIG. 4) showed, that the major phase of the product was the rhombohedral lithium zirconium phosphate.

[0115] Particle properties of the obtained products summarized in Table 1 show that only using of the inventive combination of metal carboxylates and ethyl phosphate in a solution containing no water lead to lithium zirconium phosphates with a relatively high BET surface area (17-44 m.sup.2/g vs. 4.3-4.7 m.sup.2/g for comparative examples), small particle size (76-130 nm vs. ca. 4 .Math.m for comparative examples) and low tamped density (ca. 50 g/L vs. ca 300 g/L for comparative examples).

TABLE-US-00001 Lithium zirconium phosphate properties Example BET [m.sup.2/g] d.sub.10 [.Math.m] d.sub.50 [.Math.m] d.sub.90 [.Math.m] tamped density [g/L] Comparative Example 1 4.3 0.98 4.00 16.0 303 Comparative Example 2 4.7 1.08 3.94 57.4 285 Example 1 44 0.058 0.076 1.60 52 Example 2 17 0.067 0.130 3.94 50

Analysis of Lithium Zirconium Phosphate (LZP) Coated Mixed Lithium Transition Metal Oxides by SEM-EDX

[0116] The NMC powder was mixed with the respective amount (1.0 wt%) of the fumed LZP powder from Example 1 in a high intensity laboratory mixer (SOMAKON mixer MP-GL with 0.5 L mixing unit) at first for 1 min at 500 rpm to homogeneously mix the two powders. Afterwards the mixing intensity was increased to 2000 rpm for 5 min to achieve the dry coating of the NMC particles by LZP powder.

[0117] LZP coating layer thickness on NMC particles was about 15-200 nm.

[0118] FIG. 5 shows SEM-images of NMC dry coated with LZP (FIGS. 5 a,b,c: fumed LZP of the example 1). Comparison of the back-scatter electron image (a) and the EDX-mapping of Zr (b) of NMC dry coated with fumed LZP reveals a complete and homogeneous coverage of all cathode particles with fumed LZP. No larger LZP agglomerates were detected, showing that a good dispersion of nanostructured fumed LZP particles was achieved. Additionally, no free unattached LZP particles next to the cathode particles were found, indicating the strong adhesion between the coating and the substrate. The high-resolution SEM image (c) shows a homogeneous distribution of fumed LZP with a high degree of surface coverage of the CAM.

Electrochemical Tests of Lithium Ion Batteries with Liquid Electrolytes

[0119] Electrodes for electrochemical measurements were prepared by blending 90 wt% NMC with 5 wt% PVDF (Solef PVDF 5130) as a binder and 5 wt% SUPER PLi (TIMCAL) as a conductive additive under inert gas atmosphere. N-Methyl-2-pyrrolidone (NMP) was used as a solvent. The slurry was casted on aluminum foil and dried for 20 min at 120° C. on a heating plate in air. Afterwards, the electrode sheet was dried in a vacuum furnace at 120° C. for 2 h. Circular electrodes with a diameter of 12 mm were punched out, calendered with a pressure of 90 psi, and dried again in a vacuum furnace at 120° C. for 12 h to remove any residual water and NMP. For the cycling tests the cells were assembled as CR2032 type coin cells (MTI Corporation) in an argon-filled glovebox (GLOVEBOX SYSTEMTECHNIK GmbH). Lithium metal (ROCKWOOD LITHIUM GmbH) is used as the anode material. Celgard 2500 was used as the separator. 25 .Math.L of a 1 M solution of LiPF.sub.6 in ethylene carbonate and ethyl methyl carbonate (50:50 wt/wt; SIGMA-ALDRICH) was used as electrolyte. The cells were locked with a crimper (MTI).

[0120] For electrochemical evaluations galvanostatic cycling was performed between 3.0 and 4.3 V. For the calculation of the capacities and the specific currents, only the mass of the active material was considered. For the coin half-cells during cycling, the C-rate was increased every four cycles, starting from 0.1/0.1 (Charge/Discharge) to 0.3/0.3, 0.5/0.5, 1.0/1.0, 1.0/2.0 and 1.0/4.0 C. Afterward, the cells were cycled at 0.5/0.5 C for long term stability test.

[0121] FIG. 6 shows the influence of LZP coating layers on the cycling performance. The performance of NMC dry coated with fumed LZP of the example 1 is compared with that of the uncoated NMC as a reference. It can be readily recognized from the shown graphs that the dry coated fumed LZP coating improves the stability and cycle life of NMC significantly. The NMC dry coated with fumed LZP shows a higher discharge capacity over all cycles, the rate test at the beginning and the long-term cycling test. It also shows higher initial specific discharge capacities than the uncoated NMC at 0.1 C.

Electrochemical Tests of Lithium Ion Batteries with Sulfide Based All-Solid-State Batteries

[0122] A powdered composite electrode is prepared by mixing the active material NCM (AM), conductive carbon additive carbon nanofibers (CNF), and solid electrolyte (Li.sub.6PS.sub.5Cl, SE) in the mass ratio of 63:2:35 for 30 min in an agate mortar. To fabricate a free-standing and flexible dry film, the above prepared powder electrode was mixed with 0.3 wt% of polytetrafluoroethylene (PTFE, emulsion polymerized fine powder; particle size 300-700 .Math.m; softening point 320-330° C.; molecular weight 10.sup.7-10.sup.8 g/mol) in a mortar at 100° C. After 1 min of mixing and shearing, a single flake was formed. The flake was placed on a hot plate and rolled out to the desired thickness (≈100 .Math.m). Each sample was prepared at least twice to confirm the reproducibility of the process. To prepare a free-standing electrolyte film, the solid electrolyte was mixed with 0.15 wt% of PTFE and treated in the same way as the cathode film.

[0123] A test cell was prepared for measuring the basic characteristics of the cathode composite such as the charge/discharge potential profiles, rated discharge capability.

[0124] The cell was prepared by using a die with a diameter of 13 mm. The test cell comprises of the stainless-steel outer casing with the Teflon insulator. For a typical cell, the electrolyte Li.sub.6PS.sub.5Cl powder was uniformly spread inside the die by a micro-spatula. Next, the powder was once temporally compressed and compacted into a pellet. The cathode composite powder was homogeneously distributed across the compacted electrolyte surface in the die. Then the cathode layer was compressed. On the opposite side of the cell stack, a Lithium-Indium alloy anode was placed and compressed. All the cell components were again compressed together and completely pelletized by using a hydraulic press (4 tons for 30 s were applied): 300 MPa.

[0125] After compression, the cell stack was placed inside the outer steel casing, were a screw maintains the electric contact in the cell. The screw was fastened at 3.0 Nm using a preset torque.

[0126] All above mentioned processes were carried out in an Ar filled glove box (<0.1 ppm H.sub.2O and O.sub.2).

[0127] The cycle and rate performances of the cell were measured by a battery tester CTS-Lab (BaSyTec, Germany). The standard rate performance test plan consists of 3 different discharge currents ranging from 0.066 mA (0.035 C) to 1.4 mA (0.75 C) while the charging rate is kept constant at 0.14 mA (0.075 C) including a CV step. The standard cut-off voltages for the cycle test were set at 3.63 V and 1.93 V for charge and discharge, respectively.

[0128] FIG. 7 shows the influence of LZP coating layers on the rate characteristics at the discharge current of 0.066 mA (0.035 C) at the 1.sup.st cycle with different coating amount of 0.5, 1 and 2 wt% LZP. The discharge capacities of NMC dry coated with fumed LZP of the example 1 at 0.066 mA are compared with that of the uncoated NMC as a reference. It can be readily recognized from the shown graphs that no significant improvement was observed at the initial discharge capacity at low discharge rate like 0.035 C in sulfide based all-solid-state batteries.

[0129] FIG. 8 shows the influence of LZP coating layers on the rate characteristics at the discharge current of 1.4 mA (0.75 C) at the 4.sup.th cycle with different coating amount of 0.5, 1 and 2 wt% LZP. The discharge capacities of NMC dry coated with fumed LZP of the example 1 at 0.75 C are compared with that of the uncoated NMC as a reference. It can be readily recognized from the shown graphs that the dry coated fumed LZP coating improves the discharge rate characteristics of NMC significantly. The NMC dry coated with fumed LZP shows a higher discharge capacity at high discharge rate like 0.75C in sulfide based all-solid-state batteries.