WO2019043402A1

WO2019043402A1 - Process for producing particulate carbon-coated lithium iron phosphate, carbon coated lithium iron phosphate and its uses

Info

Publication number: WO2019043402A1
Application number: PCT/GB2018/052468
Authority: WO
Inventors: Mark Copley; Maria Elena RIVAS-VELAZCO; Enrico Petrucco
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2017-09-04
Filing date: 2018-08-31
Publication date: 2019-03-07
Also published as: CN111132929A; GB201714100D0; EP3678989A1

Abstract

A process for producing particulate carbon-coated lithium iron phosphate comprising a milling step and a calcination step. The process uses polyvinyl butyral having a having a molecular weight distribution such that at least 75% of the polyvinyl butyral has a molecular weight in the range from 30000 to 90000. The process enables the formation of lithium iron phosphate with improved electrochemical properties.

Description

PROCESS FOR PRODUCING PARTICULATE CARBON-COATED LITHIUM IRON PHOSPHATE, CARBON COATED LITHIUM IRON PHOSPHATE AND ITS USES

Field of the Invention

The present invention relates to lithium transition metal phosphate materials, their preparation and use as a cathode material in secondary lithium ion batteries.

Background of the Invention

Lithium metal phosphates with olivine structures have emerged as promising cathode materials in secondary lithium ion batteries. Advantages of lithium metal phosphates compared with other lithium compounds include the fact that they are relatively benign environmentally, and have excellent safety properties during battery handling and operation.

Melting processes, hydrothermal processes and solid-state processes are the most common synthesis routes for the preparation of lithium metal phosphates.

Relatively poor electrochemical performance of lithium metal phosphates has been attributed to their poor electronic conductivity, and their performance has been significantly improved by coating the particles with electronically conductive carbon. There remains a need for lithium metal phosphates which can be made by simple, cost effective and scalable processes, employ low cost precursors, and exhibit advantageous electrochemical properties such as increased capacity.

Summary of the Invention

The present inventors have found that the electrochemical performance carbon-coated lithium iron phosphate can be improved by controlling the properties of the carbon-containing precursor used in its preparation. In particular, the present inventors have found that it is particularly advantageous to use a polyvinyl butyral with particular properties. As the skilled person will be aware, polyvinyl butyrals (PVBs) are typically copolymers with the Formula I below:

Formula I

As the skilled person will understand, the copolymer typically includes (e.g. consists of) vinyl alcohol residues (z), vinyl butyral residues (x) and optionally vinyl acetate residues (y).

These residues are typically distributed throughout the copolymer (i.e. it is not typically a block copolymer). The values of x, y and z in Formula I can be controlled to control the properties of the PVB. Typically, the weight % of vinyl butyral residues (the residue of bracket x) is referred to as the butyryl content. Typically, the weight % of vinyl alcohol residues (the residue of bracket z) is referred to as the hydroxyl content. Typically, the weight % of vinyl acetate residues (the residue of bracket y) is referred to as the acetyl content. The acetyl content may be the remainder after the hydroxyl content and butyryl content has been accounted for. Note that acetyl residues need not be present (i.e. the value of y may be zero). (The weight % of residues recited herein is intended to include the polymer backbone shown in Formula I.)

PVBs may be formed by reaction of a copolymer of polyvinyl alcohol and polyvinyl acetate with butyraldehyde, or by reaction of polyvinyl alcohol with butyraldehyde. The ratio of vinyl alcohol to vinyl acetate in the original copolymer, and the amount of butyraldehyde reacted with the copolymer, controls the butyryl, hydroxyl and acetyl content of the PVB.

The butyryl, hydroxyl and acetyl content of the PVB, together with its molecular weight, affect its viscosity. As demonstrated in the Examples below, use of PVBs in which the viscosity, butyryl content, hydroxyl content and/or molecular weight are controlled to particular levels as the carbon source in the production of carbon-coated lithium iron phosphate leads to materials which exhibit improved electrochemical performance, e.g. improved capacity. In particular, without wishing to be bound by theory, the present inventors believe that an intermediate molecular weight in combination with a relatively high butyryl content leads to improved interaction with the iron and/or lithium containing precursors used to make the carbon coated lithium iron phosphate. Accordingly, in a first preferred aspect the present invention provides a process for producing particulate carbon-coated lithium iron phosphate, the process comprising:

a milling step in which lithium-containing precursor, iron-containing precursor and carbon-containing precursor are combined and subjected to milling; and

a calcination step in which the product of the milling step is calcined to provide carbon coated particulate lithium iron phosphate,

wherein the carbon-containing precursor is polyvinyl butyral having a molecular weight distribution such that at least 75% of the polyvinyl butyral has a molecular weight in the range from 30000 to 90000. The polyvinyl butyral may have a butyryl content of at least 70 wt%. The polyvinyl butyral may have a hydroxyl content of 30 wt% or less. The polyvinyl butyral may have a viscosity in the range from 50 to 350 cP when measured as a 10 wt% solution in isopropyl alcohol at a shear rate of 100 1/s. In a second preferred aspect present invention provides a process for producing particulate carbon-coated lithium iron phosphate, the process comprising:

wherein the carbon-containing precursor is polyvinyl butyral having a butyryl content of less than 84 wt% and a hydroxyl content of at least 16 wt%. The polyvinyl butyral may have a viscosity in the range from 50 to 350 cP when measured as a 10 wt% solution in isopropyl alcohol at a shear rate of 100 1/s. The polyvinyl butyral may have a molecular weight in the range from 30000 to 90000.

In a third preferred aspect present invention provides a process for producing particulate carbon-coated lithium iron phosphate, the process comprising:

wherein the carbon-containing precursor is polyvinyl butyral having a viscosity in the range from 50 to 350 cP when measured as a 10 wt% solution in isopropyl alcohol at a shear rate of 100 1/s. The polyvinyl butyral may have a butyryl content of at least 70 wt%. The polyvinyl butyral may have a hydroxyl content of 30 wt% or less. The polyvinyl butyral may have a molecular weight in the range from 30000 to 90000. In a fourth preferred aspect, the present invention provides particulate carbon-coated lithium iron phosphate obtained or obtainable by a process described herein. In a further preferred aspect, the present invention provides use of carbon-coated lithium iron phosphate of the present invention for the preparation of a cathode of a secondary lithium ion battery. In a further preferred aspect, the present invention provides a cathode which comprises carbon-coated lithium iron phosphate of the present invention. In a further preferred aspect, the present invention provides a secondary lithium ion battery, comprising a cathode which comprises carbon-coated lithium iron phosphate of the present invention. The battery typically further comprises an anode and an electrolyte.

Brief Description of the Drawings

Figure 1 shows the results of electrochemical testing of the lithium iron phosphate samples prepared in Example 1 and Comparative Examples 1 and 2.

Detailed Description

Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

The present invention provides a process for making particulate carbon-coated lithium iron phosphate, using polyvinyl butyral as a carbon-containing precursor. The PVB may have a molecular weight in the range from 30,000 to 90,000, e.g. from 40,000 to 80,000. Typically, the molecular weight distribution is such that at least 70%, at least 75%, at least 80%, at least 90% at least 95% or at least 99% (e.g. by number) of the PVB molecules have a molecular weight in the recited range.

The PVB may have a butyryl content in the range from 70 wt% to 95 wt%. The butyryl content may be at least 70 wt%, at least 75 wt%, or at least 78 wt%. The butyryl content may be 98 wt% or less, 95 wt% or less, 90wt% or less, 86 wt% or less, 85 wt% or less, 84 wt% or less, or 83 wt% or less. The present inventors believe that a relatively high butyryl content is advantageous, particularly in combination with a relatively low molecular weight, since this results in carbon coated lithium iron phosphate with improved electrochemical properties. The hydroxyl content of the PVB may be in the range from 5 wt% to 30 wt%. The hydroxyl content may be at least at least 2 wt%, at least 5 wt%, at least 8 wt%, at least 10 wt%, at least 14 wt%, at least 15 wt%, at least 16 wt% or at least 17 wt%. It may be 30 wt% or less, 25 wt% or less or 22 wt% or less. The present inventors believe that a relatively low hydroxyl content is advantageous, particularly in combination with a relatively low molecular weight, since this results in carbon coated lithium iron phosphate with improved

electrochemical properties.

As discussed above, the butyryl content is the wt% of butyryl residues in the PVB polymer and the hydroxyl content is the wt% of hydroxyl residues in the PVB polymer. The PVB may optionally include acetyl residues, and the content of the acetyl residues may be the balance of the content of the PVB. In other words, the sum of the acetyl content, the butyryl content and the hydroxyl content may be 100 wt%. Alternatively, where acetyl is not present, the sum of the butyryl content and the hydroxyl content may be 100 wt%.

The PVB may have a viscosity in the range from 50 to 350 cP when measured as a 10 wt% solution in isopropyl alcohol at a shear rate of 100 1/s. It may have a viscosity of at least 50 cP, at least 70 cP, at least 90 cP or at least 100 cP. It may have a viscosity of 350 cP or less, 300 cP or less, 250 cP or less 200 cP or less or 150cP or less. The present inventors have found that when the viscosity of the PVB is too high or too low, the capacity of the resulting carbon-coated lithium iron phosphate is reduced.

The particulate carbon-coated lithium iron phosphate of the present invention typically has the formula Li_xFe_yP04, in which x is 0.8-1.2 and y is 0.8-1.2, and in which up to 10 atom % (e.g. up to 5 atom %) of the Fe may be replaced with a dopant metal, up to 10 atom % (e.g. up to 5 atom %) of the phosphate may be replaced with S0₄ and/or Si0₄, and up to 10 atom % of the Li may be replaced with Na and/or K. The lithium iron phosphate may have the formula LiFeP0₄, in which up to 10 atom % (e.g. up to 5 atom %) of the Fe may be replaced with a dopant metal and up to 10 atom % (e.g. up to 5 atom %) of the phosphate may be replaced with S0₄ and/or Si0₄, and up to 10 atom % of the Li may be replaced with Na and/or K. The lithium iron phosphate may have the formula Li_xFe_yP0₄, in which x is 0.8-1.2 and y is 0.8-1.2. The lithium iron phosphate may have the formula LiFeP0₄. The dopant metal may be one or more selected from Mn, Co, Ni, Al, Mg, Sn, Pb, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, Zr, Cd. The dopant metal may be one or more selected from Mn, Al, Ti and Zr. It may be preferred that the lithium iron phosphate is undoped. Where the lithium iron phosphate is doped, typically dopant-containing precursor is added in the milling step.

The lithium iron phosphate may have a crystallite size of 90nm or less, e.g. 85nm or less, when determined by Rietveld analysis of XRD data. The crystallite size may be at least 60nm or at least 70nm. Smaller observed crystallite sizes can indicate a shorter lithium diffusion distance within the particles, thereby enhancing electrochemical performance.

The carbon-coated lithium iron phosphate is typically prepared by a process comprising a milling step and a calcination step. The milling step may be a dry milling step, or may be a wet milling step, e.g. carried out in the presence of a liquid, such as water or an organic solvent. Suitable organic solvents include isopropyl alcohol, glycol ether, acetone and ethanol. The milling step may be a high energy milling step. The term "high energy milling" is a term well understood by those skilled in the art, to distinguish from milling or grinding treatments where lower amounts of energy are delivered. For example, high energy milling may be understood to relate to milling treatments in which at least 100 kWh of energy is delivered during the milling treatment, per kilogram of solids being milled. For example, at least 150 kWh, or at least 200 kWh may be delivered per kilogram of solid being milled. There is no particular upper limit on the energy, but it may be less than 500 kWh, less than 400 kWh, or less than 350 kWh per kilogram of solids being milled. Energy in the range from 250 kWh/kg to 300 kWh/kg may be typical. The milling energy is typically sufficient to cause mechanochemical reaction of the solids being milled. In the milling step lithium-containing precursor, iron-containing precursor and

carbon-containing precursor are combined and subjected to milling. If phosphorus is not provided as part of one of the iron- or lithium- containing precursors added in the milling step, a separate phosphorous-containing precursor (e.g. phosphate-containing precursor) is typically added. The nature of the lithium- and carbon- containing precursors is not particularly limited in the present invention.

Suitable lithium-containing precursors include lithium carbonate (L12CO3), lithium hydrogen phosphate (LJ2HPO4) and lithium hydroxide (LiOH). L12CO3 may be preferred. Suitable iron-containing precursors include iron phosphate (FePCU) and iron oxalate. The iron phosphate may be hydrated (e.g. FePCU.xHaO) or may be dehydrated. FePCU may be preferred. Typically, the iron-containing precursor and the lithium precursor (and optionally phosphorous-containing precursor and / or dopant-containing precursor) are combined in suitable proportions to give the desired stoichiometry to the lithium iron phosphate product.

The amount of PVB added is not particularly limited in the present invention. For example, the amount of carbon precursor may be selected to give a carbon content of 1 to 5 wt% in the carbon-coated lithium iron phosphate, e.g. 1 to 3 wt%. The amount of carbon precursor added in the milling step may be in the range from 3 to 15 wt%, e.g. 3 to 7 wt%.

In the calcination step, the product of the milling step is typically calcined under an inert atmosphere to provide the particulate carbon-coated lithium iron phosphate. The calcination step performs two functions. Firstly, it results in pyrolysis or carbonisation of the carbon precursor to form a conductive carbon coating on the lithium iron phosphate particles.

Secondly, it results in crystallisation and the formation of the lithium iron phosphate into the desired olivine structure. Typically, the calcination is carried out in an inert atmosphere, for example in an inert gas such as argon or nitrogen. It may alternatively be carried out in a reducing atmosphere. It is typically carried out at a temperature in the range from 550°C to 800°C, e.g. from 600°C to 750°C, or from 600°C or 650°C to 700°C. 680°C is particularly suitable. Typically, the calcination is carried out for a period of 3 to 24h. The calcination time depends on the scale of manufacture (i.e. where larger quantities are prepared, longer calcination times may be preferred. At a commercial scale, 8 to 15 hours may be suitable, for example. The process of the present invention may further comprise the step of forming an electrode (typically a cathode) comprising the carbon-coated lithium iron phosphate. Typically, this is carried out by forming a slurry of the particulate carbon-coated lithium iron phosphate, applying the slurry to the surface of a current collector (e.g. an aluminium current collector), and optionally processing (e.g. calendaring) to increase the density of the electrode. The slurry may comprise one or more of a solvent, a binder, additional carbon material and further additives.

Typically, the electrode of the present invention will have an electrode density of at least 2.3 g/cm³. It may have an electrode density of 2.8 g/cm³ or less, or 2.65 g/cm³ or less. The electrode density is the electrode density (mass/volume) of the electrode, not including the current collector the electrode is formed on. It therefore includes contributions from the active material, any additives, and additional carbon material, and any binder used. The lithium iron phosphate may be capable of being formed into an electrode having an electrode density as defined above when formed into an electrode, e.g. by the electrode formation method of the Examples.

The process of the present invention may further comprise constructing a battery or electrochemical cell including the electrode comprising the carbon-coated lithium iron phosphate. The battery or cell typically further comprises an anode and an electrolyte. The battery or cell may typically be a secondary (rechargeable) lithium ion battery.

The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention, and are not intended to limit its scope. Examples

Experiments were conducted to determine the effect of polyvinyl butyral (PVB) properties on the carbon-coated LiFePCU obtained.

L12CO3, and hydrated FeP0₄ were mixed in the desired proportions to obtain stoichiometric LiFeP0₄, along with PVB as carbon source (at 4.5wt%). The precursors were subjected to roller ball milling for 24 hours, using 10mm YSZ media. The samples were then calcined in argon at 680°C for 5 hours, to form olivine lithium iron phosphate coated with conductive carbon. Three different PVBs were tested. Their properties are shown in Table 1 below. PVBs with the properties listed below are readily available from companies including Kurarat Europe GmbH, Sigma Aldrich, Eastman Chemical and Sekisui Japan.

Table 1

The viscosity was determined in 10 wt% solutions in I PA at a shear rate of 100 1/s.

Table 2 below shows which PVBs were used to prepare the lithium iron phosphate of the Examples and the Comparative Examples.

Table 2

Electrochemical Analysis

The obtained lithium iron phosphate was formed into electrodes, using an electrode coating formulation. The electrode coating formulation had a solids content of approximately 40% by weight. The solids portion consisted of 90wt% of active material from the Examples, 5wt% carbon black (C65 from Imerys™), 5 wt% binder (Solef 5130™ (polyvinylidene fluoride, 10wt% binder in n-methyl pyrrolidone). The coating formulations were used to cast electrodes on a 20μηι aluminium foil using a vacuum coater, to provide an electrode loading of 5 mg/cm² (the electrode loading refers to the mass of active material per area of electrode). The coated electrodes were calendared to provide an electrode density of 2.0 g/cm³. The electrodes were then dried for 12 hours at 120°C. Electrochemical coin cells (2032 button cell from Hohsen™) were formed. The electrolyte was LP30 from Solvonic™, which is 1 M LiPF6 in 1 : 1 by weight mixture of dimethyl carbonate and ethylene carbonate. The anode was 0.75mm thickness lithium, and the separator was a glass microfiber filter (Whatman™ GF/F). The pressure used to crimp the coin cell was 750 psi.

The electrochemical performance of the samples was measured using a voltage window of 4.0V to 2.0V. The result of the electrochemical analysis is shown in Figure 1.

The results show that at all charge rates tested, the lithium iron phosphate of Example 1 , prepared using PVB A, has a higher capacity then the lithium iron phosphate of the

Comparative Examples. XRD Analysis

The lithium iron phosphate particles were subjected to XRD and the crystallite

determined by Rietveld analysis. The results are shown in Table 3 below.

Table 3

Smaller observed crystallite sizes can indicate a shorter lithium diffusion distance within the particles, thereby enhancing electrochemical performance.

Claims

1. Process for producing particulate carbon-coated lithium iron phosphate, the process comprising:

- a milling step in which lithium-containing precursor, iron-containing precursor and carbon-containing precursor are combined and subjected to milling; and

wherein the carbon-containing precursor is polyvinyl butyral having a molecular weight distribution such that at least 75% of the polyvinyl butyral has a molecular weight in the range from 30000 to 90000.

2. Process according to claim 1 wherein the polyvinyl butyral has a butyryl content of at least 70 wt%, and optionally 86 wt% or less.

3. Process according to any one of the preceding claims wherein the polyvinyl butyral has a hydroxyl content of 30 wt% or less, and optionally 14 wt% or more

4. Process according to any one of the preceding claims wherein the polyvinyl butyral has a viscosity in the range from 50 to 350 cP when measured as a 10 wt% solution in isopropyl alcohol at a shear rate of 100 1/s.

5. Process according to any one of the preceding claims wherein the iron-containing precursor is iron phosphate.

6. Process according to any one of the preceding claims wherein the lithium-containing precursor is lithium carbonate.

7. Process according to any one of the preceding claims wherein the milling step is a wet high energy milling step.

8. Process according to any one of the preceding claims, in which the lithium iron phosphate has the formula Li_xFe_yP04, in which x is 0.8-1.2 and y is 0.8-1.2, and in which up to 10 atom % of the Fe may be replaced with a dopant metal, up to 10 atom % of the phosphate may be replaced with S0₄ and/or Si0₄ and up to 10 atom % of the Li may be replaced with Na and/or K.

9. Process according to any one of the preceding claims, further comprising forming an electrode comprising the carbon-coated lithium iron phosphate.

10. Process according to claim 9, further comprising constructing a battery including the electrode.

1 1. Use of a polyvinyl butyral having a a molecular weight distribution such that at least 75% of the polyvinyl butyral has a molecular weight in the range from 30000 to 90000 and a butyryl content of at least 70 wt% as a carbon-containing precursor in the manufacture of lithium iron phosphate.

12. Particulate carbon-coated lithium iron phosphate obtained or obtainable by a process according to any one of claims 1 to 10.

13. Use of particulate carbon-coated lithium iron phosphate according to claim 12 in the preparation of an electrode for a secondary lithium ion battery.

14. An electrode for a secondary lithium ion battery comprising particulate carbon-coated lithium iron phosphate according to claim 12.

15. A secondary lithium ion battery comprising an electrode according to claim 14.