TW201307197A - Method for producing nanoparticulate lithium transition metal phosphates - Google Patents

Abstract

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

Description

Method for preparing nanoparticle lithium transition metal phosphate

The present invention relates to a method for preparing a nanoparticle lithium transition metal phosphate, a lithium transition metal phosphate obtainable by the method of the present invention, a cathode containing a lithium transition metal phosphate obtainable by the method of the present invention as an active material, and the like A secondary lithium ion battery for the cathode.

Mixed and undoped mixed lithium metal oxides, especially lithium transition metal phosphates, have recently become extremely important electrode materials in so-called lithium ion batteries. For example, a lithium-ion accumulator, also known as a secondary lithium-ion battery, is considered a battery model that is expected to be used in battery-powered vehicles. Lithium-ion batteries are also used, for example, in power tools, computers, and mobile phones. In these batteries, especially the electrodes and electrolytes are made of a lithium-containing material.

Since the publication of Goodenough et al. (J. Electrochem. Soc., 144, 1188-1194, 1997), lithium iron phosphate (LiFePO ₄ ) has been used especially as a cathode material in rechargeable secondary lithium ion batteries. Caused great concern. Compared with conventional lithium compounds based on spinel or layered oxides such as lithium manganese oxide, lithium cobalt oxide and lithium nickel oxide, lithium iron phosphate is, for example, especially in the future in electric cars, electric tools, etc. The higher the safety is achieved in the delithiation state required for the use of the battery.

Pure lithium iron phosphate material has been improved by the so-called "carbon coating" (Ravet et al, Meeting of Electrochemical Society, Honolulu, October 17-31, 1999, EP 1 084 182 B1), at room temperature The capacitance of the carbon coating material is increased (about 160 mAH/g).

The defect of lithium iron phosphate is especially the presence of redox pair Fe ²⁺ /Fe ³⁺ . Compared with Li/Li ⁺ , the oxidation-reduction potential of Fe ²⁺ /Fe ³⁺ (3.45 V, relative to Li/Li ⁺ ) It is significantly lower than the redox potential (3.9 V, relative to Li/Li ⁺ ) of the redox pair Co ³⁺ /Co ^{4+ in} LiCoO ₂ , for example.

In particular, lithium manganese phosphate LiMnPO ₄ has attracted attention because its Mn ²⁺ /Mn ³⁺ redox pair (4.1 volts) is higher than Li/Li ⁺ . LiMnPO ₄ has also been described by Goodenough et al. in U.S. Patent 5,910,382.

Lithium cobalt phosphate LiCoPO ₄ is mentioned in EP 571 858 A1 as a potential cathode material for secondary lithium ion batteries.

Generally, lithium transition metal phosphates are considered to be safe electrode materials, unlike, for example, lithium manganese oxide or lithium cobalt oxide. Unlike the so-called layered oxide oxides described above, lithium transition metal phosphates are also stable at higher temperatures and do not release oxygen and heat during charging.

However, lithium transition metal phosphates are susceptible to certain crystallization chemical constraints with respect to their electrochemical behavior compared to pure oxide cathode materials such as lithium manganese oxide, lithium cobalt oxide, and the like.

During electrochemical delithiation/lithiation (charge/discharge), lithium diffusion proceeds along the unidirectional transport path via the crystal lattice. The disordered structure that causes the clogging of these channels severely impairs the electrochemical behavior of the materials. Electronic conduction is performed via so-called "polaron hopping" and is limited by a small number of charge carriers. The ion and electron conductivity of lithium transition metal phosphates are generally small in crystals without disorder.

It has previously been proposed to improve conductivity by different methods. One method is for example to introduce a conductive component into the final electrode formulation, and another method has used a carbon coating compound as described above. Another variant is the so-called Nai of active materials. Nanoscaling, which inter alia interferes with the microstructure design of crystalline particles.

For lithium transition metal phosphates, especially the nature of the so-called "nanoscale", that is, the small primary crystallite size is a necessary prerequisite for optimizing its capacitance and its charge/discharge characteristics. Smaller crystallites have short diffusion and routing paths and are therefore better electrochemically charged. Especially for lithium iron phosphate and lithium manganese phosphate, the nanometer scale is known to be a necessary prerequisite for better electrochemical utilization of the transition metal.

As previously mentioned, the conductive additive can be present as a coating on the surface of the particles or as a composite of LiFePO ₄ and a composite of conductive components, both in a highly dispersed mechanical distribution. In addition to the carbon as the main additive, copper and silver in a finely distributed form have also been proposed. The LiMPO ₄ particles are usually coated by converting a decomposable (organic) carbon precursor compound on the lithium transition metal phosphate LiMPO ₄ (EP 1 049 182 B1).

The combination of improving conductivity and crystallite size by adding a conductive substance such as a carbon or carbon precursor compound produces a so-called nanocomposite system in which a decomposition product which is a so-called sintering inhibitor produced by a carbon precursor compound is particularly limited. Crystal growth is important.

As proposed by Goodenough, for example in US 6,514,640, in order to improve the electronic properties of the LiMPO ₄ compound, it is likewise possible to partially replace the heterogeneous ion, i.e. to dope the transition metal M.

Generally, the physical requirements for primary crystallites to be met are high order and small crystallite size. However, from the perspective of method engineering, since the influence factors of the two parameters have opposite effects, it is difficult to affect both factors at the same time.

Small crystallites are usually obtained at low temperatures, but the necessary conversion time during the calcination process is very long, that is, a low time benefit is obtained.

Furthermore, complete conversion of the starting compound cannot always be guaranteed, especially when using solid state methods. In addition, the formed crystals retain defects that have a negative impact on the quality of the product in terms of electronic properties.

On the other hand, if calcination is carried out at a higher temperature, the conversion occurs faster and the defects are better repaired. However, primary crystallites also grow more violently, which has a negative impact on the electronic properties of these materials.

In principle, large crystals can actually be reduced by a grinding process in the latter step, but these processes are expensive and involve high energy consumption, which often leads to re-formation defects. Repairing defects formed by the grinding process after synthesizing the compound can be performed again only during subsequent tempering.

Accordingly, it is an object of the present invention to provide a novel process for obtaining nanocrystalline nanoparticle lithium transition metal phosphate wherein the primary crystallites are free of defects and thus have high electron activity.

This object is achieved by a method in which a carbon-coated lithium transition metal phosphate as a starting material is obtained, and the nanoparticle is obtained by completely or partially removing the carbon layer of the starting material. Carbon lithium transition metal phosphate LiMPO ₄ .

Since the (primary) crystallite size of the starting material carbon-coated lithium transition metal phosphate has been extremely small, when carbon acts as a sintering inhibitor in the first method step for preparing a starting material as described further below, The carbon-containing starting material in the process of the invention can be tempered in air at relatively low temperatures, wherein the existing carbon layer is completely or only partially removed by oxidation.

In principle, how to prepare a carbon coating starting material is not critical to the practice of the process of the invention. Lithium transition metal phosphates can be prepared by solid state chemical methods, wet chemical methods and hydrothermal methods or by means of sol-gel synthesis or flame spray pyrolysis, which have been obtained in the form of carbon coating materials (EP 1 049 182 B1) Or, subsequently coated in a second method step (WO 2005/051840).

In an evolution of the invention, it is possible to prepare the starting compound in a single process step or in a single reactor and to remove the carbon layer, for example by solid state chemistry to prepare a carbon coated lithium transition metal phosphate and subsequently in air. It is tempered, where the carbon layer is removed again.

Conventionally, a temperature of 300 ° C to 500 ° C is selected as the temperature of the tempering step. At the lower temperatures selected in accordance with the present invention, unexpectedly, the crystallite size of the starting material did not change. At this time, carbon oxidation is selectively performed. Depending on the tempering time, the residual carbon content fluctuating between 0.5 wt% and 5 wt% may even be set depending on the temperature.

The object of the present invention can be further achieved by a nanoparticle lithium transition metal phosphate obtainable by the process of the present invention.

Surprisingly, the primary crystallite size of the lithium transition metal phosphate obtained by the method of the present invention is <100 nm, and thus the lithium transition metal phosphate of the present invention is excellent when used as an active material in the cathode of a secondary lithium ion battery. Electronic nature. In the evolution of the invention, the primary crystallite size is <90 nm or <85 nm.

In a specific embodiment of the invention, the lithium transition metal phosphate obtained according to the invention has a monomodal particle-size distribution, which makes it particularly useful as an electrode material because It is homogeneous.

According to the present invention, the transition metal in the lithium transition metal phosphate obtainable according to the present invention is at least one transition metal selected from the group consisting of cobalt, iron, manganese, and nickel. In a preferred evolution of the 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 .

The term "lithium transition metal phosphate" or "lithium transition metal phosphate LiMPO ₄ " as used above means within the scope of the present invention that the lithium transition metal phosphate may be either doped or undoped.

By doped lithium transition metal phosphate is meant herein a compound of the formula LiM' _y M _x PO ₄ wherein M = Fe, Co, Ni or Mn, especially Co or Mn, M' is different from M and is indicated At least one metal cation of a group consisting of free Co, Ni, Mn, Fe, Nb, Ti, Ru, Zr, B, Zn, Mg, Ca, Cu, Cr or a combination thereof, but preferably represents Co, Ni, Mn, Fe, Ti, B, Mg, Zn, Ca, and Nb, in other evolutions of the invention, are 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 example, 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-xy PO ₄ , LiMg _y Co _x Mn _1-xy PO ₄ , LiZn _y Co _x Mn _1-xy PO ₄ , LxZn _y Co _x PO ₄ , of which 0.001 x, y, z 1.

As mentioned previously, it is quite preferred for the doped or undoped lithium transition metal phosphate to have an ordered or modified olivine structure.

Lithium transition metal phosphates in an ordered olivine structure can be described structurally in the orthorhombic space group Pnma (No. 62 of the International Tables), where the orthorhombic unit can be selected here. The crystallographic index of the unit cell is such that the a-axis is the longest axis of the unit cell Pnma and the c-axis is the shortest axis, and thus the mirror surface m of the olivine structure is perpendicular to the b-axis. The lithium ions of the lithium metal phosphate are thus arranged parallel to the crystal axis [010] or perpendicular to the crystal plane {010} to form an olivine structure, which is thus also a preferred direction for one-dimensional lithium ion conduction.

The term "modified olivine structure" means that the modification occurs in the crystal lattice at an anion (for example, by replacing the phosphate with vanadate) and/or a cation site, wherein the substitution occurs via an isovalent or identical charge carrier. In order to better diffuse lithium ions and improve electronic conductivity.

Surprisingly, it has been found that the undoped and doped carbon-free lithium cobalt phosphate or lithium manganese phosphate of the present invention is not inferior to the conventional carbon-containing corresponding derivative when used as an active material in the cathode of a secondary lithium ion battery. . Surprisingly, these materials do not require additional conductors and are therefore different from similar iron compounds.

Cobalt in LiCoPO ₄ achieves over 95% of the theoretical Co ^II -Co ^III transition.

The invention further relates to an electrode for a secondary lithium ion battery comprising the lithium transition metal phosphate of the invention, in particular undoped or doped lithium cobalt cobalt LiCoPO ₄ or undoped or The doped lithium manganese phosphate LiMnPO _{4 is} used as an active material.

The electrode of the present invention further contains a binder. Any adhesive known to the person skilled in the art can be used as a binder, such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride hexafluoropropyl Ethylene copolymer (PVDF-HFP), ethylene-propylene-diene terpolymer (EPDM), tetrafluoroethylene hexafluoropropylene copolymer, polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethacrylic acid Propylene oxime ester (PMMA), carboxymethyl cellulose (CMC), and derivatives and mixtures thereof.

In other embodiments of the invention, the electrode further comprises a lithium-metal-oxygen compound selected from the group consisting of doped or undoped lithium metal oxides, lithium metal phosphates (which are different from the phosphoric acids of the invention) Salt), lithium metal vanadate and mixtures thereof. Of course, it is also possible to contain two, three or even more different lithium-metal-oxygen compounds. Of course, it is self-evident that those skilled in the art will be aware that lithium-metal-oxygen compounds having only the same functionality (and thus acting as anode materials) may be included in such electrode formulations.

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

The invention is described in more detail below with reference to the embodiments and drawings, however, these embodiments and drawings are not to be considered as limiting.

Figure 1 shows the change in the carbon content of LiCoPO ₄ of the present invention as a function of temperature in the process of the present invention.

Figure 2 shows the charge/discharge curve of the carbon-free LiCoPO _{4 of the} present invention in the first cycle.

Figure 3 shows the specific capacitance of different lithium cobalt phosphates obtained in accordance with the present invention.

Figure 4 shows a direct solid state process according to the state of the art (Figure 4a) And an SEM micrograph of the carbon-free lithium cobalt phosphate obtained by the method of the invention (Fig. 4b).

1. Preparation of electrodes

To prepare the electrode, AB100 (20 wt%) as a conductive additive and Hostaflon (20 wt%) as a binder were added to the active material (60 wt%). Study individual components in an agate mortar.

Approximately 50-52 mg of the electrode block thus obtained was introduced into an aluminum mesh bag and pressurized with 10 t pressure for 10 seconds.

The extruded electrode was dried in vacuum at 130 ° C for 8-12 hours and then measured in the test chamber using the remaining electrolyte relative to lithium as the reference electrode and the opposite electrode.

As a electrolyte, a mixture of ethyl carbonate/dimethyl carbonate (EC: DMC 1:1 wt%) and 1 M LiPF ₆ as a supporting electrolyte (LP1-U33/U34 UBE, LP30-17 Merck) was used.

Electrochemical characterization was carried out at a charge/discharge rate of C/20 over a potential range of 3.0 V-5.3 V by means of a constant current measurement.

2. Preparation of starting compounds

The starting compounds, ie the carbon-coated LiCoPO ₄ and LiMnPO ₄ , are formed by a solid synthesis route (JP H09-134724, EP 571858 A1, EP 1049182 B1), by a wet chemical route (EP 1379468 B1) or by Prepared by the hydrothermal pathway (WO 2006/097324).

The carbon coating is carried out on the spot according to EP 1049182 B1 or after separation of the carbon-coated intermediate product (for example in W02006/097324 and EP) 1379468 B1).

3. Preparation of the lithium transition metal phosphate of the present invention 3.1 Preparation of LiCoPO ₄ and LiMnPO ₄

Next, the carbon coatings LiCoPO ₄ and LiMnPO ₄ (carbon content of about 5 wt%) obtained in the second part were tempered in air at 400 ° C for 20 hours. Obtain carbon-free LiCoPO ₄ . This result is also independent of the preparation of LiCoPO ₄ or LiMnPO ₄ (ie wet chemical, solid chemical or hydrothermal) which was originally used. Again, the difference is not seen on the type of carbon coating applied (i.e., directly via a mixture of a carbon precursor and a lithium transition metal phosphate starting material or by a synthesized lithium transition metal phosphate).

As can be seen from Figure 1, the residual carbon content of the LiCoPO ₄ product obtained by means of the process of the invention can be set according to temperature. At a temperature of 300 ° C, no carbon was removed, and at a temperature of about 400 ° C, a carbon-free product was obtained. The same result was obtained for LiMnPO ₄ . Also here, the type of synthesis of the lithium transition metal phosphate does not affect the results.

Figure 2 shows the charge/discharge curve of the electrode in the first cycle using the carbon-free LiCoPO ₄ (starting material obtained according to EP 1379468 B1) of the present invention as an active material without additional additional conductors, relative to the amount of Li/Li ^* Measured. The primary crystallite size of the LiCoPO ₄ is 80 nm. At the time of discharge, a value of 155 mAh/g was obtained. At its purest, LiCoPO _{4 has} a specific capacitance of 167 mAh/g. However, the measured Li ₃ PO ₄ second phase of the sample was 3 to 4 wt%. Therefore, the actual specific capacitance of the sample measured was 163 mAh/g. Thus, 155 mAh/g represents about 95% of the expected specific capacitance of the cathode material LiCoPO ₄ of 163 mAh/g.

Figure 3 shows the effect of crystallite size on the specific capacitance of LiCoPO ₄ and carbon coated LiCoPO ₄ obtained according to the present invention as an active material in an electrode. It can be seen that the smaller crystallite size of 63 nm of LiCoPO ₄ obtained according to the present invention also shows a carbon coating having a slightly larger crystallite size of 83 nm in the absence of a carbon coating in the electrode formulation. The material has better results.

Materials having a crystallite size of 304 or 391 nm exhibit poor results and exhibit the effect of crystallite size on the electronic properties of the materials. The crystallite size was determined by radiation measurement to be the primary crystallite size using a Bruker-AXS apparatus (soft body TOPAS 4.2).

Figure 4 shows LiCoPO particle morphology without carbon obtainable according by solid state of the art of (EP 571 858 A1) in FIG. 4A ₄ of the particle morphology of the carbon-free may be obtained according to the method of the present invention LiCoPO ₄ of FIG. 4b Comparison. The primary crystallite size of the product obtained using the present invention is significantly smaller than that produced by the process according to the state of the art.

Claims (15)

A method of preparing a nanoparticle lithium transition metal phosphate LiMPO ₄ , the method starting with a carbon coated lithium transition metal phosphate, wherein 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 of the group consisting of Co, Fe, Mn, and Ni. The method of claim 1 or 2, wherein the carbon layer is removed by oxidation. The method of claim 3, wherein the removing is performed by heating the carbon-coated lithium transition metal phosphate at a temperature in the range of 300 ° C to 500 ° C. The method of claim 4, wherein the heating is carried out during 2 to 16 hours. The method of any of the preceding claims, wherein the carbon-coated lithium transition metal phosphate has a total carbon content of from 0.5 to 5 wt%. A nanoparticle lithium transition metal phosphate obtainable by the method according to any one of the preceding claims 1 to 6. For example, the lithium transition metal phosphate of claim 7 has a primary crystallite size of <100 nm. A lithium transition metal phosphate as claimed in claim 8 which has a unimodal particle size distribution. The lithium transition metal phosphate according to any one of the preceding claims, wherein the transition metal is at least one of the group consisting of Co, Fe, Mn, and Ni. A lithium transition metal phosphate according to claim 10, which has the formula LiCoPO ₄ or LiMnPO ₄ . The lithium transition metal phosphate according to any one of the preceding claims, which is further characterized in that it is doped with other selected from the group consisting of Co, Ni, Mn, Fe, Nb, Ti, Ru, Zr, B, Zn, Mg. Heteropoly ions of a group of Ca, Cu, Cr or a combination thereof. A cathode for a secondary lithium ion battery comprising the lithium transition metal phosphate according to any one of claims 7 to 12 as an active material. The cathode of claim 13, wherein the lithium transition metal phosphate is doped or undoped LiCoPO ₄ . A secondary lithium ion battery comprising a cathode according to item 13 or item 14 of the patent application.