DE102011106326B3 - Process for the preparation of nanoparticulate lithium transition metal phosphates; nanoparticulate lithium transition metal phosphate and cathode with it - Google Patents

Abstract

The present invention relates to a process for preparing a nanoparticulate lithium transition metal phosphate LiMPO4 starting from carbon-coated lithium transition metal phosphate, in which the carbon layer is subsequently removed.

Description

The present invention relates to a process for the preparation of nanoparticulate lithium transition metal phosphates, a lithium transition metal phosphate obtainable by the process according to the invention, a cathode containing as active material the lithium transition metal phosphate obtainable by the process according to the invention and a secondary lithium ion battery containing such a cathode.

Doped and undoped mixed lithium metal oxides, in particular lithium transition metal phosphates, have meanwhile acquired great importance as electrode materials in so-called lithium-ion batteries. For example, lithium ion batteries, also called secondary lithium ion batteries, are considered to be promising battery models for battery-powered motor vehicles. Lithium ion batteries are also used, for example, in power tools, computers and mobile phones. In these batteries, in particular the electrodes and the electrolytes consist of lithium-containing materials.

Since the publications by Goodenough et al. (J. Electrochem.Soc., 144, 1188-1194, 1997), lithium iron phosphate (LiFePO ₄ ) in particular has been of great interest for use as a cathode material in rechargeable secondary lithium ion batteries. Lithium iron phosphate offers higher safety properties in the delithiated state than conventional spinel or layer oxide based lithium compounds, such as lithium manganese oxide, lithium cobalt oxide and lithium nickel oxide, as needed in particular for the future use of batteries in electric cars, electrically powered tools in the future.

The pure lithium iron phosphate material was prepared by the so-called "carbon coating" (Ravet et al., Meeting of Electrochemical Society, Honolulu, 17-31 October 1999, EP 1 049 182 B1 ), whereby an increased capacity of the carbon-coated material is obtained at room temperature (about 160 mAh / g).

A disadvantage of lithium iron phosphate is, in particular, the presence of the redox couple Fe ²⁺ / Fe ³⁺ , which has a significantly lower redox potential than Li / Li ⁺ (3.45 V versus Li / Li ⁺ ) than, for example, the redox couple Co ³⁺ / Co ⁴⁺ in ^FIG LiCoO ₂ (3.9 V vs. Li / Li ⁺ ).

In particular, lithium manganese phosphate LiMnPO ₄ is of interest in view of its Li / Li ⁺ higher Mn ²⁺ / Mn ³⁺ redox couple (4.1 volts). LiMnPO ₄ was also reported by Goodenough et al. in the US 5,910,382 A described.

Lithium cobalt phosphate, LiCoPO ₄ , was found in the EP 571 858 A1 Mentioned as a potential cathode material in secondary lithium ion batteries.

In general, lithium transition metal phosphates are considered to be safe electrode materials as opposed to, for example, lithium manganese oxide or lithium cobalt oxide. In contrast to the above oxides, which are also referred to as so-called layer oxides, they are stable in the charged state even at higher temperatures and do not react under oxygen and heat emission.

However, lithium transition metal phosphates are subject to certain crystal chemical limitations in their electrochemical behavior compared to the purely oxide cathode materials lithium manganese oxide, lithium cobalt oxide.

Electrochemical delithiation / lithiation (charge / discharge) involves lithium diffusion along one-dimensional transport paths through the lattice. Structural dislocations that cause these channels to block greatly affect the electrochemical behavior of the materials. The electronic conductivity is via the so-called "Polaronenhopping" and is limited by the small number of charge carriers. Even in disorder-free crystals, the ionic and electronic conductivity of lithium transition metal phosphates is generally low.

Various approaches have been proposed to improve conductivity. One approach is, for example, introducing conductive components into the finished electrode formulation, another is the use of the carbon-coated compounds already described above. Another variant is the so-called nanoscaling of the active materials, which in particular intervenes in the microstructural design of the crystal particles.

In particular, the property of so-called "nanoscale", i. H. a low primary crystallite size for lithium transition metal phosphates is an essential prerequisite for optimizing their capacity and their charge / discharge characteristics. Smaller crystallites have short diffusion and conduction paths and are therefore better electrochemical loadable. In particular, for lithium iron phosphate and lithium manganese phosphate is known that the nanoscale is a necessary prerequisite for a good electrochemical utilization of the transition metal.

Conductive additives can, as already mentioned, be in highly dispersed mechanical distribution, as a coating on the particle surfaces or as particles of a composite of LiFePO ₄ and conductive component. In addition to carbon as a conductive additive, copper and silver have also been proposed in finely divided form. The coating of LiMPO ₄ particles is usually carried out by the reaction of decomposable (organic) carbon precursor compounds on lithium transition metal phosphates LiMPO ₄ ( EP 1 049 182 B1 ).

The combination of conductivity improvement by addition of conductive substances such as carbon or carbon precursor compounds and crystallite size leads to so-called nanocomposite systems, in which in particular the limitation of crystallite growth by the resulting decomposition products of the carbon precursor compounds is important, which act as so-called sintering inhibitors.

It is also possible to improve the electronic properties of LiMPO ₄ compounds, as already described by Goodenough US 6,514,640 B1 proposed to partially substitute, that is to dope, the transition metals M by foreign ions.

In general, the physical requirements of a high degree of order and a small crystallite size are placed on primary crystallites. From a process engineering point of view, however, it is difficult to influence both factors at the same time, since the factors influencing both parameters are in opposite directions.

Small crystallites are usually obtained at low temperatures, however, the required reaction times in the calcination process are very long, that is, one obtains a low time yield.

In addition, full conversion of the starting compounds can not always be guaranteed, especially in solid state processes. In addition, defects can remain in the formed crystals, which negatively affect the product quality in terms of their electronic properties.

On the other hand, if you calcine at higher temperatures, the reaction will be faster, and at the same time, defects will heal better. However, the primary crystallites also grow more strongly, which has a negative effect on the electronic properties of the materials.

In principle, large crystals can indeed be reduced in a subsequent step by grinding processes, but these processes are complex and associated with a high energy input, which usually usually leads to renewed formation of defects. The annealing of the defects that are formed by the milling processes carried out after the synthesis of the compound can in turn only take place in a subsequent annealing process.

It was therefore an object to provide a novel process in which nanocrystalline nanoparticulate lithium transition metal phosphates can be obtained without the primary crystallites have defects and thus have a high electronic activity.

This object is achieved by a process in which, starting from a carbon-coated lithium transition metal phosphate as starting material, a nanoparticulate carbon-free lithium transition metal phosphate LiMPO _{4 is} obtained in which the carbon layer of the starting material is completely or partially removed.

Since the starting material, the carbon-coated lithium transition metal phosphate has a very small (primary) crystallite size, since the carbon acts as a sintering inhibitor in a first process step for the preparation of the starting material, which will be described below, the carbonaceous starting material in the process according to the invention at relatively low temperatures are annealed in air, wherein the existing carbon layer is completely or even partially removed by oxidation.

In principle, it is not important for the performance of the process according to the invention how the carbon-coated starting material was prepared. It can be either solid-state, wet-chemical or hydrothermal or by sol-gel synthesis or flame pyrolysis prepared lithium transition metal phosphate, which is either already obtained as a carbon-coated material ( EP 1 049 182 B1 ), or is subsequently coated in a second process step ( WO 2005/051840 A1 ).

It is possible in developments of the invention to carry out the preparation of the starting compound and removal of the carbon layer in a single process step, or perform in a single reactor by z. B. the carbon-coated lithium transition metal phosphate is solid-state chemically prepared and then annealed in air, wherein the carbon layer is removed again.

Temperatures of 300 ° C to 500 ° C are usually selected as temperatures for the annealing step. Surprisingly, no change in the crystallite size of the starting material takes place at the selected lower temperatures according to the invention. Here runs the oxidation of the Carbon selective. Depending on the annealing time, depending on the temperature, residual carbon contents which vary between 0.5 to 5% by weight can be adjusted.

The object of the invention is further achieved by a nanoparticulate lithium transition metal phosphate obtainable by the process according to the invention.

Surprisingly, the lithium transition metal phosphate obtained by the process according to the invention has a primary crystallite size of <100 nm, so that the lithium transition metal phosphate according to the invention has excellent electronic properties when used as active material in cathodes of secondary lithium ion batteries. In developments of the invention, the primary crystallite sizes are <90 nm or <85 nm.

In embodiments of the invention, the lithium transition metal phosphate obtained according to the invention has a monomodal particle size distribution, which is particularly suitable for use as an electrode material, since the material is very homogeneous

The transition metal in the lithium transition metal phosphate obtainable according to the invention is selected according to the invention from at least one transition metal from the group consisting of cobalt, iron, manganese, nickel. In preferred embodiments of the present invention, the transition metal is cobalt or manganese. The lithium transition metal phosphate is thus lithium cobalt phosphate LiCoPO ₄ or lithium manganese phosphate LiMnPO ₄ .

In the context of this invention, the term "a lithium transition metal phosphate" or "a lithium transition metal phosphate LiMPO ₄ " as used herein means that the lithium transition metal phosphate may be doped or undoped.

Under a doped lithium transition metal phosphate is a compound of the formula LiM is _'y M _x PO understood _4, wherein M = Fe, Co, Ni or Mn, in particular Co or Mn, M' in the present of M different and at least one metal cation from the group consisting of Co, Ni, Mn, Fe, Nb, Ti, Ru, Zr, B, Zn, Mg, Ca, Cu, Cr or combinations thereof, but preferably Co, Ni, Mn, Fe, Ti, B, Mg, Zn , Ca and Nb, in other embodiments of the invention Mg, Nb, or Zn, x is a number <1 and> 0.001 and y is a number> 0.001 and <0.99. Typical preferred compounds are, for. LiNb _y Fe _x PO ₄ , LiMg _y Fe _x PO ₄ LiB _y Fe _x PO ₄ LiMn _y Fe _x PO ₄ , LiMg _y Co _x PO ₄ , LiCo _y Fe _x PO ₄ , LiMn _z Co _y Fe _x PO ₄ , LiMn _y Co _x PO ₄ , LiMg _y Fe _x Mn _1-xy PO ₄ , LiZn _y Fe _x Mn _1-x- yPO 4, LiMg _y Co _x Mn _1-xy PO ₄ , LiZn _y Co _x Mn _1-xy PO ₄ , LiZn _y Co _x PO ₄ with 0.001 ≤ x, y, z ≤ 1).

The doped or undoped lithium transition metal phosphate most preferably has either an ordered or modified olivine structure, as stated above.

Structurally, lithium transition metal phosphates in ordered olivine structure can be described in the rhombic space group Pnma (No. 62 of the International Tables), where the crystallographic arrangement of the rhombic unit cell is chosen such that the a axis is the longest axis and the c axis the shortest axis the unit cell Prima is such that the mirror plane m of the olivine structure is perpendicular to the b-axis. Then, the lithium ions of the lithium metal phosphate in Olivinstruktur arrange parallel to the crystal axis [010] or perpendicular to the crystal surface {010}, which is thus the preferred direction for the one-dimensional lithium ion conduction.

The term "modified olivine structure" means that there is a modification either to the anionic (eg, phosphate by vanadate) and / or cationic sites in the crystal lattice, with substitution by aliovalent or like charge carriers, to provide better diffusion of the lithium ions and to allow improved electronic conductivity.

Surprisingly, it has been found that inventive carbon-free undoped and doped lithium cobalt phosphate or lithium manganese phosphate is not inferior when used as the active material in cathodes of secondary lithium ion batteries conventional carbon-containing corresponding derivatives. Surprisingly, these materials do not require any additive addition and differ from the analogous iron compounds.

More than 95% of the theoretical Co ^II -Co ^III transition was achieved for cobalt in LiCoPO ₄ .

The present invention further relates to an electrode for a lithium secondary lithium ion battery, which contains a lithium transition metal phosphate according to the invention, in particular non-doped or doped lithium cobalt phosphate LiCoPO ₄ or non-doped or doped lithium manganese phosphate LiMnPO ₄ as the active material.

The electrode according to the invention further contains a binder. As binders it is possible to use any binder known per se to the person skilled in the art, such as, for example, polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyvinylidene difluoride-hexafluoropropylene copolymers (PVDF-HFP), Ethylene-propylene-diene terpolymers (EPDM), tetrafluoroethylene-hexafluoropropylene copolymers, polyethylene oxides (PEO), polyacrylonitriles (PAN), polymethyl methacrylates (PMMA), carboxymethylcelluloses (CMC), their derivatives, and mixtures thereof.

In further embodiments of the present invention, the electrode further contains a lithium metal-oxygen compound selected from doped or undoped lithium metal oxides, lithium metal phosphates (other than those of the invention), lithium metal vanadates, and mixtures from that. It is of course also possible that two, three or even more, different lithium metal oxygen compounds are included. It will be understood by those skilled in the art that, of course, only lithium metal oxygen compounds that have the same functionality (that is, act as a cathode active material) may be included in such an electrode formulation.

The lithium metal oxygen compound is preferably selected from doped or undoped lithium manganese oxide, lithium cobalt oxide, lithium iron manganese phosphate, lithium manganese phosphate. The second lithium-metal-oxygen compound is particularly advantageous in particular cathode formulations and is typically present in an amount of about 3-50% based on the lithium transition metal phosphate according to the invention.

The invention is explained below with reference to examples and figures, without these being to be understood as limiting.

1 shows the variation of the carbon content of LiCoPO ₄ according to the invention as a function of the temperature in the process according to the invention.

2 shows the charge / discharge of inventive carbon-free LiCoPO ₄ in the first cycle.

3 shows the specific capacity of various lithium cobalt phosphates obtained according to the invention.

4 shows SEM images of carbon-free lithium cobalt phosphate, obtainable by a direct solid state process of the prior art ( 4a ) and the method according to the invention ( 4b ).

embodiments

1. Preparation of electrodes

For electrode preparation, active material (60% by weight) was mixed with AB100 (20% by weight) as conductive additive and also Hostaflon (20% by weight) as binder. The individual components were mortared in agate mortar.

From the resulting electrode mass, about 50-52 mg samples were placed in an aluminum mesh pocket and pressed at a pressure of 10 tons for 10 seconds.

The pressed electrode was dried at 130 ° C under vacuum over a period of 8-12 h and then measured in test cells with excess electrolyte against lithium as a reference electrode and counter electrode.

The electrolyte used was a mixture of ethylene carbonate / dimethyl carbonate (EC: DMC 1: 1 wt% with 1 M LiPF ₆ as conductive salt (LP1-U33 / U34 UBE, LP30-17 Merck).

The electrochemical characterization was carried out by galvanostatic measurement in a potential range of 3.0 V-5.3 V at a charge / discharge rate of C / 20.

2. Preparation of the starting compounds

The preparation of the starting compounds, ie the carbon-coated LiCoPO ₄ and LiMnPO _{4 was} carried out either by solid-state synthesis ( JP H09-134724 A . EP 571858 A1 . EP 1049182 B1 ), wet-chemical way ( EP 1379468 B1 ) or by hydrothermal route ( WO 2006/097324 A2 ).

The carbon coating was done either in situ according to the EP 1049182 B1 or after isolation of the non-carbon coated intermediate (e.g., as in U.S. Pat WO 2006/097324 A2 and the EP 1379468 B1 ).

3). Preparation of inventive lithium transition metal phosphates

3.1 Preparation of LiCoPO ₄ and LiMnPO ₄

The carbon-coated LiCoPO ₄ and LiMnPO ₄ (carbon content about 5% by weight) obtained as in section 2 were then tempered for 20 hours in air at 400 ° C. This gave a carbon-free LiCoPO ₄ . The result was the same regardless of the preparation (ie, wet chemical, solid-state chemical or hydrothermal) of the LiCoPO ₄ or LiMnPO ₄ originally used. Also, the nature of the application of the carbon coating, ie directly on carbon precursor in mixture with the starting materials of the lithium transition metal phosphates, or on the already synthesized lithium transition metal phosphate showed no differences.

How out 1 can be seen, depending on the temperature of the residual carbon content of the product obtained by the process according to the invention LiCoPO _{4 can be} adjusted. At a temperature of 300 ° C, no carbon is removed, at temperatures around 400 ° C, a carbon-free product is obtained. The same result was obtained for LiMnPO ₄ . Again, the synthetic nature of the lithium transition metal phosphate had no effect on the result.

2 shows the charge / discharge of an electrode with carbon-free LiCoPO ₄ according to the invention (starting material obtained according to the EP 1379468B1 ) as active material without further additive addition in the first cycle measured against Li / Li ⁺ . The LiCoPO _{4 had} a primary crystallite size of 80 nm. The discharge reached a value of 155 mAh / g. Purest LiCoPO ₄ has a specific capacity of 167 mAh / g. However, the measured sample had a Li ₃ PO ₄ minor phase of 3 to 4% by weight. Therefore, the practical specific capacity of the measured sample is 163 mAh / g. Thus, 155 mAh / g represent about 95% of the expected specific capacity of the prepared cathode material LiCoPO ₄ of 163 mAh / g.

3 shows the influence of the crystallite size on the specific capacity LiCoPO ₄ obtained according to the invention and carbon-coated LiCoPO ₄ as active material in an electrode. It can be seen from this that the smaller crystallite size of 63 nm of the LiCoPO ₄ obtained according to the invention, even without carbon coating in the electrode formulation, shows better results than carbon-coated material with a somewhat larger crystallite size of 83 nm.

The materials with crystallite sizes of 304 and 391 nm show inferior results and prove the influence of crystallite size on the electronic properties of these materials. The crystallite sizes were determined as primary crystallite sizes from X-ray diffraction measurements on a Bruker AXS instrument (Software TOPAS 4.2).

4 shows in 4a Comparison of particle morphology of carbon-free LiCoPO ₄ obtainable by a solid state process of the prior art ( EP 571 858 A1 ); and in 4b of carbon-free LiCoPO ₄ obtainable by the process according to the invention. The product obtained according to the invention has a significantly lower primary crystallite size than the product prepared by the process of the prior art.

Claims (14)

Process for the preparation of a nanoparticulate lithium transition metal phosphate LiMPO ₄ starting from carbon-coated lithium transition metal phosphate, in which the carbon layer is subsequently completely or partially removed. The method of claim 1, wherein the transition metal M in LiMPO _{4 is} selected from at least one selected from the group consisting of Co, Fe, Mn, Ni. The method of claim 1 or 2, wherein the carbon layer is removed by oxidation. The process of claim 3, wherein the removal is accomplished by heating the carbon-coated lithium transition metal phosphate at a temperature in the range of 300 to 500 ° C. The method of claim 4, wherein the heating is carried out for a period of 2 to 16 hours. A method according to any one of the preceding claims, wherein the carbon-coated lithium transition metal phosphate has a total carbon content of 0.5 to 5% by weight. Nanoparticulate lithium transition metal phosphate obtainable by the process according to one of the preceding claims 1 to 6. Lithium transition metal phosphate according to claim 7 having a primary crystallite size <100 nm. Lithium transition metal phosphate according to claim 8 having a monomodal particle size distribution. The lithium transition metal phosphate of any one of preceding claims 7 to 9, wherein the transition metal is at least one of Co, Fe, Mn, Ni. The lithium transition metal phosphate of claim 10 having the formula LiCoPO ₄ or LiMnPO ₄ . The lithium transition metal phosphate according to any of the preceding claims 7 to 11, which is doped with further foreign ions selected from the group Co, Ni, Mn, Fe, Nb, Ti, Ru, Zr, B, Zn, Mg, Ca, Cu, Cr or combinations thereof. Cathode for a secondary lithium ion battery, containing as active material a lithium transition metal phosphate according to one of claims 7 to 12. The cathode of claim 13, wherein the lithium transition metal phosphate is doped or undoped LiCoPO ₄ .

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