Patent Application
20150024269
The present invention relates to electrodes that contain an active material comprising a sulfate group, and to the use of such electrodes, for example in sodium ion battery applications. The invention also relates to certain novel materials and to the use of these materials, for example as an electrode material.
Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing power in a compact system by accumulating energy in the chemical bonds of the cathode, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and migrate towards the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
From the prior art, it is known, for example from patent application WO2011078197, to prepare sodium-ion batteries containing an electrode active material comprising a mixture of a transition metal sodium phosphate and a powder of a composite metal oxide. Similarly, EP2239805 discloses electrodes comprising sodium mixed transition metal oxides, and U.S. Pat. No. 6,872,492 teaches a sodium-ion battery comprising an electrode active material comprising AaMb(XY4)cZd where A is sodium, M is one or more metals comprising one metal capable of undergoing oxidation to a higher valance state, XY4 is a phosphate or similar group and z is OH or halogen. Further prior art includes EP0743692 which teaches positive electrode materials of the formula: LixFe2(SO4)3 where 0<x<2, and U.S. Pat. No. 5,908,716 which discloses compounds of the formula: AxMy(SO4)z, and specifically discloses compounds where x=3 and z=3, and where x=1 and z=2.
In a first aspect, the present invention aims to provide a cost effective electrode that contains an active material that is straightforward to manufacture and easy to handle and store. A further object of the present invention is to provide an electrode that has a high initial charge capacity and which is capable of being recharged multiple times without significant loss in charge capacity.
Therefore, the present invention provides an electrode containing an active material of the formula:
AaMb(SO4)cXx
In one embodiment of the above formula, one or more of a, b, c and x are integers i.e. whole numbers. An embodiment where a=2 is preferred. In an alternative embodiment one or more of a, b, c and x are non-integers, i,e, fractions.
In another preferred embodiment, the present invention is directed to an electrode that contains an active material of the formula:
AaMb(SO4)cXx
In particular, the present invention provides an electrode as described above in which the active material comprises one or more transition metals and/or non-transition metals and/or metalloids (M) selected from titanium, vanadium, niobium, tantalum, hafnium, chromium, molybdenum, tungsten, manganese, iron, osmium, cobalt, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, aluminium, scandium, yttrium, zirconium, technetium, rhenium, ruthenium, rhodium, iridium, mercury, gallium, indium, tin, lead, bismuth and selenium, magnesium, calcium, beryllium, strontium, barium, boron, silicon, germanium, arsenic, antimony and tellurium;
The moiety X preferably comprises one or more atoms selected from fluorine, chlorine, bromine, iodine and hydroxide.
In a preferred electrode of the present invention, M comprises one or more elements selected from nickel, cobalt, manganese and iron.
In a preferred electrode of the present invention the molar ratio of a:b:c is 1:1:1.5. Examples of suitable compounds include those with the general formula: A2M2(SO4)3
In another preferred electrode of the present invention the molar ratio of a:b:c is 1:0.5:1. Examples of suitable compounds include those with the general formula: A2M(SO-4)2.
Other preferred materials of the present invention include: Na2M(SO4)2 and Na2M2(SO4)3 such as Na2Fe(SO4)2, Na2Fe2(SO4)3, Na2Mn(SO4)2 and Na2Mn2(SO4)3.
Electrodes according to the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices. Advantageously, the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof.
The active materials of the present invention may be prepared using any known and/or convenient method. For example, using a solution reaction using an aqueous or other solvent. However ideally, the starting materials are intimately admixed in particulate form. This can be achieved using various methods, for example by finely grinding the materials separately using a pestle and mortar or a ball mill, and then mixing them together, or the materials can be admixed whilst they are being finely ground. The grinding and admixing is of sufficient duration to produce a uniformly intermixed and finely ground powder. A solvent such as acetone or another material which is easily removed, for example a low boiling liquid, can be used to assist the grinding/admixing process and this is preferably removed prior to a heating step. Other known techniques such as high energy ball milling and microwave activation may also be used to help prepare the starting materials, for example to increase their reactivity.
Some reactions, notably some solution reactions, proceed at room temperature. However typically the active materials are made by heating the precursor materials, for example in a furnace. This is especially useful when the reaction is a solid state reaction process, i.e. a reaction in which all of the reactants are in solid form and are substantially free of any reaction medium such as a solvent. Where a solvent or other low boiling liquid is used to assist the mixing of the reactants, as described above, it is substantially removed prior to the heating step.
The heating step typically involves heating the reaction mixture either at a single temperature, or over a range of temperatures, for example up to at least 25° C., preferably up to at least 50° C., further preferably up to at least 150° C., and yet further preferably up to 600° C., although for some reactants a single or a range of reaction temperatures up to 1200° C. may be needed. Conveniently the reaction is performed under atmospheric pressure and under a non-oxidising atmosphere, for example nitrogen, argon or another inert gas, or under vacuum. Depending on the target material and the precursors used, the reaction may also be performed in a sealed reaction vessel. Advantageously, the reaction temperature is maintained for between 0.5 and 12 hours, although the exact time will depend on the reactivity of the starting materials. A dwell time of 8 hours has been found to be sufficient for many reactions.
The conversion of a sodium-ion rich material to a lithium-ion rich material may be effected using an ion exchange process.
Typical ways to achieve Na to Li ion exchange include:
In a further aspect, the present invention provides an energy storage device comprising an electrode as described above, for use as one or more of the following: a sodium ion and/or lithium ion and/or potassium ion cell; a sodium metal and/or lithium metal and/or potassium metal ion cell; a non-aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell; and an aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell. Specifically, the energy storage device may be a battery.
The present invention will now be described with reference to the following drawings in which:
Active materials used in the present invention are prepared on a laboratory scale using the following generic method:
Generic Synthesis Method:
The required amounts of the precursor materials are intimately mixed together. The resulting mixture is then heated in a tube furnace or a chamber furnace using either a flowing inert atmosphere (e.g. argon or nitrogen) or an ambient air atmosphere, at a furnace temperature of at least 50° C. until reaction product forms. When cool, the reaction product is removed from the furnace and ground into a powder.
Using the above method, active materials used in the present invention were prepared in Examples 1 to 7 summarised below in Table 1.
Product Analysis Using XRD
All of the product materials were analysed by X-ray diffraction techniques using a Siemens D5000 powder diffractometer to confirm that the desired target materials had been prepared and to establish the phase purity of the product material and to determine the types of impurities present. From this information it is possible to determine the unit cell lattice parameters.
Electrochemical Results
The target materials were tested in lithium or sodium metal anode test electrochemical cells to determine their specific capacity and also to establish whether they have the potential to undergo charge and discharge cycles. These test electrochemical cells containing the active material are constructed as follows:
Generic Procedure to Make a Lithium Metal Test Electrochemical Cell
The positive electrode is prepared by solvent-casting a slurry of the active material, conductive carbon, binder and solvent. The conductive carbon used is Super P (Timcal). PVdF co-polymer (e.g. Kynar Flex 2801, Elf Atochem Inc.) is used as the binder, and acetone is employed as the solvent. The slurry is then cast onto glass and a free-standing electrode film is formed as the solvent evaporates. The electrode is then dried further at about 80° C. The electrode film contains the following components, expressed in percent by weight: 80% active material, 8% Super P carbon, and 12% Kynar 2801 binder. Optionally, an aluminium current collector may be used to contact the positive electrode. Metallic lithium on a copper current collector may be employed as the negative electrode. The electrolyte comprises one of the following: (i) a 1 M solution of LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) in a weight ratio of 1:1; (ii) a 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) in a weight ratio of 1:1; or (iii) a 1 M solution of LiPF6 in propylene carbonate (PC) A glass fibre separator (Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes.
Generic Procedure to Make a Sodium Metal Test Electrochemical Cell
The positive electrode is prepared by solvent-casting a slurry of the active material, conductive carbon, binder and solvent. The conductive carbon used is Super P (Timcal). PVdF co-polymer (e.g. Kynar Flex 2801, Elf Atochem Inc.) is used as the binder, and acetone is employed as the solvent. The slurry is then cast onto glass and a free-standing electrode film is formed as the solvent evaporates. The electrode is then dried further at about 80° C. The electrode film contains the following components, expressed in percent by weight: 80% active material, 8% Super P carbon, and 12% Kynar 2801 binder. Optionally, an aluminium current collector may be used to contact the positive electrode. Metallic sodium on a stainless steel or copper current collector may be employed as the negative electrode. The electrolyte comprises a 0.5 M solution of NaClO4 in propylene carbonate (PC). A glass fibre separator (Whatman, GF/A) or a porous polypropylene separator wetted by the electrolyte is interposed between the positive and negative electrodes.
Cell Testing
The cells are tested in one of two ways:
Referring to
It is expected from thermodynamic considerations that the sodium extracted from the Na2Fe(SO4)2 active material during the initial charging process, enters the electrolyte, and would then be displacement ‘plated’ onto the lithium metal anode (i.e. releasing more lithium into the electrolyte). Therefore, during the subsequent discharging of the cell, it is assumed that a mix of lithium and sodium ions are re-inserted into the active material. The re-insertion process corresponds to 40 mAh/g, indicating the reversibility of the ion extraction-insertion processes. The generally symmetrical nature of the charge-discharge curves further indicates the excellent reversibility of the system.
Subsequent cycling of the cell indicates the reversibility of the ion extraction-insertion reactions. As the upper voltage limit is increased it is possible to access more and more of the sodium ions within the Na2Fe(SO4)2 structure. Within the voltage range 3.0 to 4.6 V vs. Li, a reversible discharge specific capacity of 72 mAh/g is achieved, as illustrated in
The generally symmetrical nature of the charge-discharge curves indicates the excellent reversibility of the system. The level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is extremely small, indicating the excellent kinetics of the extraction-insertion reactions. This is an important property that is useful for producing a high rate active material. The symmetrical nature of the differential capacity profile further indicates the reversibility of the ion insertion/extraction reactions.
The generally symmetrical nature of the charge-discharge curves indicates the excellent reversibility of the system. The level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is extremely small, indicating the excellent kinetics of the extraction-insertion reactions. This is an important property that is useful for producing a high rate active material. The symmetrical nature of the differential capacity profile further indicates the reversibility of the ion insertion/extraction reactions.
The generally symmetrical nature of the charge-discharge curves indicates the excellent reversibility of the system. The level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is extremely small, indicating the excellent kinetics of the extraction-insertion reactions. This is an important property that is useful for producing a high rate active material. The symmetrical nature of the differential capacity profile further indicates the reversibility of the ion insertion/extraction reactions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1201717.4 | Feb 2012 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2013/050198 | 1/30/2013 | WO | 00 |
