A cathode composition

Abstract

A cathode material for a battery, wherein the cathode material has the anti-fluorite crystal structure and belongs to the family represented by the formula Li(6-x-y)Co(1-x-y)FexAlyO4, wherein x and y

Description

TECHNICAL FIELD This invention relates to a cathode material for a battery, and to a method of making a cathode for a battery. BACKGROUND Many small-scale electrified devices, such as mobile phones, tablets and smartwatches as well as larger machines, such as automobiles, rely on batteries. Across all electrified devices, a common goal is to produce higher capacity rechargeable (secondary) batteries, for example to support greater processing power in a device or to increase the range of an electric vehicle. There is also an increased focus on sustainability of batteries across their lifecycle. This takes many guises, for example improving the electrochemical stability of batteries over numerous charging cycles, improving the lifetime of the battery or assessing of the materials from which the batteries are made, with a view to using materials that are less energy intensive to extract from their ores, or are more abundant, or are less toxic to the environment. Current high-performance secondary batteries are typically referred to in general as ‘lithium ion batteries’. The cathode materials used to make lithium ion batteries are generally lithiated transition metal oxide compounds such as NMC (LiNixMnyCozO2). These are composed of metals such as cobalt and nickel, which are costly and are becoming scarce, especially in view of projected future battery demands. They are also toxic in nature and pose environmental hazard risks. There is a therefore a desire to look to cheaper and less toxic metals for alternative cathode materials to provide a greener solution for future energy storage applications, that can match or exceed current performance. As is well-known, batteries generate electricity through the use of chemical reactions occurring at the electrodes. In secondary batteries, the reactions are reversible redox reactions. During discharge of the battery a species is oxidised at the anode, which generates lithium ions (Li+) and electrons (e_), with another species, reduced at the cathode in incorporating the generated lithium ions into the cathode material. During charging of the battery, the two reactions at each electrode are reversed. In an NMC cathode, for example, the species reduced during discharge are nickel, manganese and cobalt, which are transition metals. The specific capacity of current lithium ion batteries is limited by the number of lithium ions available to reversibly intercalate into the structure of the cathode and by the higher atomic weight of the transition metal cations, since the specific capacity of a battery is normalised by mass. A recent development in cathode materials for secondary batteries has been the discovery of cathodic oxygen redox. This differs from the current redox of transition metals at the cathode as it relies on anionic redox of oxygen ligands at the cathode, as opposed to cationic redox when transition metals form the redox centre (oxygen-based ions are typically negatively charged, while transition metal ions are positively charged). Of materials proposed to take advantage of cathodic oxygen redox, Li2O has highest possible theoretical specific capacity as it contains no heavy transition metals and therefore relies only on oxygen redox. The redox reaction is given by: 2U2O <=> U2O2 + 2Li+ + 2e_ The potential of this reaction is approximately 3.0 V against the Li+ / Li redox occurring at the anode. However, despite the high theoretical specific capacity of Li2O-based cathodes, both U2O and U2O2 show low electrochemical activities and poor electrical conductivity and so needs a catalyst. Another issue is that Li2O-based cathodes release oxygen gas (O2) due to metastability of the delithiated U2O and so their electrochemical performance is further adversely impacted by their structural instability. One way of improving performance of Li2O-based cathodes is to improve the ionic and electrical conductivities of the cathode. One promising way of achieving this has been through the partial substitution of lithium with transition metals to create lithium-rich defect anti-fluorite compounds. Two such compounds are LieCoO4 and Li5FeO4. If four unit cells of U2O, effectively a single cell of LisO4, are considered, replacing two Li+ ions with one Co2+ ion and one lithium vacancy creates LieCoO4, while replacing three Li+ ions with one Fe3+ ion and two lithium vacancies creates LisFeCU Figure 1a shows a super cell of U2O, with the lithium ions labelled 10 and the oxygen ions labelled 20. The super cell adopts the anti-fluorite structure, with lithium ions 10 occupying the tetrahedral interstices created by a face-centred cubic lattice of oxygen ions 20. LieCoCUand LisFeCU offer enhanced ionic and electrical conductivities compared to Li2O, due to the vacancies present in the structure. Both LisFeO4 and LieCoO4 are theorised to have high specific capacities as a result of the high lithium content, as well as the effect of the oxygen redox in addition to the redox of the respective transition metal ions in the two structures (Co2+ in LieCoO4 and Fe3+ in LisFeO4). For example, the specific capacity of LisFeO4 has been theorised to be over 860 mAh / g, while the theoretical specific capacity of LieCoO4 is over 970 mAh / g. However, Li6CoO4 and LisFeO4 cathodes still face numerous challenges arising from poor electrochemical performance, arising mainly from structural instability and loss of oxygen from the lattice under repeated cycling. It is against this background that the invention has been devised. SUMMARY OF THE INVENTION In a first aspect, the invention provides a cathode material for a battery. The cathode material has the anti-fluorite crystal structure and belongs to the family represented by the formula U(6-x-y)CO(i-x-y)FexAlyO4. The values of x and y are greater than zero and less than one, and the sum of x and y is less than one. The value of x may be greater than or equal to 0.2 and may be less than or equal to 0.4. Preferably, the value of x may be greater than or equal to 0.25 and less than or equal to 0.4. The value of y may be greater than or equal to 0.1 and may be less than or equal to 0.25. In certain preferred embodiments, the value of x is equal to the value of y. The cathode material may have a crystal structure belonging to the space group P42 / nmc. In a second aspect, the invention provides a method of making a cathode for a battery. The method comprises: providing a precursor powder mixture comprising Li2O, LiFeO2, Co(OH)2 and AI2O3 powders in a stoichiometric ratio; ball-milling the precursor powder mixture; heating the precursor powder mixture to a heated temperature between approximately 650°C and approximately 750°C; holding the precursor powder mixture at the heated temperature for a time period of at least 10 hours; and cooling the precursor powder mixture to room temperature at a cooling rate of between 1°C per minute and 10°C per minute. The stoichiometric ratio is such that the cathode material has the formula Li6-x-yCoi.x.yFexA 1 / )4, wherein x and y are greater than zero and less than one and the sum of x and y is less than one. The heated temperature may be approximately 700°C. The time period may be at least 15 hours. The cooling rate may be at least 5°C per minute. The value of x may be greater than or equal to 0.2 and may be less than or equal to 0.4. Preferably, the value of x may be greater than or equal to 0.25 and less than or equal to 0.4. The value of y may be greater than or equal to 0.1 and may be less than or equal to 0.25. In certain preferred embodiments, the value of x is equal to the value of y. In a third aspect, the invention provides a cathode comprising the cathode material discussed above, or a cathode material made by the method described above. In a fourth aspect, the invention provides a cell comprising the cathode discussed above in combination with an anode and an electrode. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the following drawings, in which: Figure 1a shows a super-cell of U2O; Figure 1b shows a super-cell of Li(6.x-»Co(i.x.y)FexAlyO4; Figure 1c shows a super cell of Li(6-x-y)Co(i-x-y)FexAlyO4, including co-ordination tetrahedra of oxygen ions around the metal ions; Figure 2 shows the theoretical specific capacity of Li(6-x-y)CO(i-x.y)FexAlyO4 as the proportions of cobalt, iron and aluminium in the composition change; Figure 3a shows the lattice parameters of Li(6-x-y)Co(i-x-y)FexAlyO4 at various values of x and y, Figure 3b shows the unit-cell volume of Li(6-x-y)CO(i-x-y)FexAlyO4 at various values of x and y; Figure 4 shows X-ray diffraction patterns of various members of the Li(6-x-y)Co(i-x-y)FexAlyO4 family; Figures 5a and 5b show Rietveld refinements of the X-ray diffraction patterns of Li5.7Co0.7Fe0.15AI0.15O4 and Li5.5Co0.5Fe0.25AI0.25O4; Figure 6 shows a charge-discharge curve for Li5.6Co0.6Fe0.3AI0.1O4 in the first and second cycles between 3.5 V and 1.5 V; Figure 7 shows a charge discharge curve for Li5.5Co0.5Fe0.4AI0.1O4 with different values of the charging cut-off voltage over different cycles; Figure 8 shows a comparison between the charge-discharge curves for Li5.6Co0.6Fe0.3AI0.1O4 and U6COO4 when the Li5.6Co0.6Fe0.3AI0.1O4 cathode is charged to a charging cut-off voltage of 4.2 V; and Figure 9 shows a charge discharge curve for Li5.6Co0.6Fe0.2AI0.2O4 between 3.3 V and 2.9 V. DETAILED DESCRIPTION The present invention relates to a cathode material with the general formula Li(6-x^Co(i.x-y)FexAlyO4 having the anti-fluorite crystal structure, wherein the values of x and y are greater than zero and less than or equal to one. The material has a space group of PAilnmc. The inventors have found that the issues described above in relation to LieCoO4and LisFeO4 cathode materials can be at least partially mitigated by the partial substitution of iron by other metal ions, in particular by iron and aluminium. In this case, the resulting family of materials has the general formula Li(6-x-y)CO(i-x-y)FexAlyO4. Depending on the values of x and y, a different proportion of the cobalt ions are replaced by iron and aluminium ions, respectively. Figure 1b shows the crystal structure resulting from these substitutions, with the cobalt / iron / aluminium sites labelled 30. Figure 1c shows the same crystal structure as Figure 1b, albeit with the cobalt / iron / aluminium ions in different locations, but also shows the co-ordination of the oxygen ions around the respective cations, with lithium tetrahedra labelled 40, and cobalt / iron / aluminium tetrahedra labelled 50. Figure 2 shows the theoretical specific capacity (i.e., the maximum extractable capacity) of cathodes belonging to the Li(6-x-y>CO(i-x-y)FexAlyO4 family. The value on the x-axis of %(Co) represents the proportion of cobalt ions substituted into the structure in comparison to the iron and aluminium ions, with different lines representing different values ofy, representing the proportion of aluminium ions substituted into the structure. In other words, the values on the x-axis of Figure 2 represent the proportion of the number of cobalt ions compared to the combined number of cobalt, iron and aluminium ions. Travelling along any one of the lines in Figure 2 therefore results in an increase in cobalt content and a decrease in iron content. All lines in Figure 2 represent a fixed aluminium content and value of yas denoted by the labels. As can be seen, increasing the proportion of cobalt ions compared to iron ions in the structure increases the theoretical capacity of the cathode material. Equally, increasing the proportion of aluminium ions compared to both the proportion of iron and cobalt ions increases the theoretical capacity of the cathode material. This is due to the lower atomic weight of aluminium and the fact that specific capacity is normalized by mass. The inventors have found that the Li(6-x-y)Co(i.x.y)FexAlyO4 family of materials therefore enjoys largely the same benefits of LisFeCU and LieCoCXi cathodes in that the high lithium content and simultaneous cationic and anionic redox ensures a high specific capacity, while significantly reducing the cobalt content compared to Li6CoO4 cathodes. By replacing the cobalt with iron and aluminium, the cathode material makes greater use of abundant, cheap, non-toxic materials. The addition of the iron and aluminium also help to stabilise the structure of the cathode material under charge cycling. Both iron and aluminium help to ‘pin’ the crystal structure in place and prevent the loss of oxygen that causes degradation of the cathode. Aluminium is especially helpful in this regard as it is a redox-inactive element and so does not undergo any change in oxidation state. While the addition of aluminium increases the theoretical specific capacity because of its lower weight, it does at the same time decrease the number of active species to take part in the redox processes, having substituted into the structure in place of redox-active iron or cobalt, and also decreases the electrical conductivity of the crystal structure. In particular, the reduced number of redox-active species eventually reduces the practical specific capacity of the cathode material as the value of x increases, as lithium intercalation and deintercalation requires a redox process to occur. Increasing the value of x too high therefore effectively reduces the number of lithium ions that can intercalate and deintercalate into and from the cathode. Therefore, there is a balance to be struck between a number of factors (including theoretical and practical specific capacity, electrical conductivity and number of redoxactive species) in order to achieve optimal performance with reasonable and practical levels of structural integrity. A range of materials within the Li(6-x-»CO(i-x-y)FexAlyO4 family, with different values of x and y have been synthesized. The cathode materials can be synthesised by typical powder processing routes. In one method, precursor powders are initially taken in stoichiometric amounts in a ball-mill jar and then ball-milled and eventually heated in an argon atmosphere to react and obtain the final powder of Li(6-x-»CO(i-x-y)FexAlyO4 compositions. The ball-milling may occur at between 200 rpm and 400 rpm, preferably 200 rpm, using stainless steel balls in stainless steel jars for between four and eight periods of 30 minutes, with a 15 minute break in between each period to allow the powder to cool down. It will be appreciated that any ball-milling process, or similar process, may be used in order to prepare a suitably fine powder for consolidation, and to prepare a homogeneous mixture of the precursor powders. For the manufacture of Li(6-x-y)CO(i.x-y)FexAlyO4 compositions, a mixture of Li2O, LiFeO2, Co(OH)2 and AI2O2 powders are used, with the exact stoichiometric ratio of the four powders depending on the values of x and y of the intended final composition. To synthesise the cathode material, the ball-milled precursor powder mixture can be heated to between 650°C and 750°C, preferably to 700°C for at least 10 hours, preferably 15 hours, followed by cooling at a cooling rate of between 1°C per minute and 10°C per minute, preferably at least 5 °C per minute until the sample reaches room temperature. Figure 3a shows the variation in cell constants as the values of x and y change, while Figure 3b shows the change in the cell volume. It can be seen that the lattice parameters of the unit cell tend to increase with increasing cobalt content, while the lattice parameters of the unit cell tend to decrease with increasing aluminium content. This is unsurprising as the ionic radius of the Al3+ ion is smaller than that of the Fe3+ ion, which is in turn smaller than the ionic radius of the Co2+ ion. The trends seen also indicate that the substituents dope successfully into the structure to form a full solid solution. Examples Various samples in the Li(6-x-j4CO(i-x-^FexAlyO4 family were prepared. The samples were prepared using the following method: • Mixing U2O, LiFeO2, Co(OH)2 and AI2O3 powders in appropriate stochiometric ratios. • Ball-milling the mixture at 200 rpm using stainless steel balls in stainless steel jars for four periods of 30 minutes, with a 15 minute break in between each period to allow the powder to cool down. • Heating to 700°C for 15 hours; • Cooling to room temperature at a cooling rate of at least 5 °C per minute. Figure 4 shows X-ray diffraction (XRD) patterns for samples having the following compositions: Li5.4Co0.4Fe0.4AI0.2O4, U5.6Co0.6Fe0.2AI0.2O4, Li5.5Co0.5Fe0.4AI0.1O4, Li5.5Co0.5Fe0.25AI0.25O4, Li5.8Co0.3Fe0.1AI0.1O4, and LieCoO4 (i.e., x = 0.4, y = 0.2; x = 0,2, y = 0.2; x = 0.4, y= 0.1; x = 0.25, y = 0.25; x = 0.1, y = 0.1; and x = 0, y = 0). Figures 5a and 5b show Rietveld refinements of the XRD patterns for Li5.7Co0.7Fe0.15AI0.15O4 (x = 0.15, y= 0.15) and Li5. 5Co0.5Fe0.25AI0.25O4 (x = 0.25, y = 0.25), respectively, in which a suitable crystal structure was determined and refined against the observed XRD pattern. A satisfactory fit is seen in both Figures 5a and 5b, and all peaks could be successfully indexed, showing that the synthesised phase is pure in both cases. The space group of all the family members investigated was determined to be PA^nmc, except when the value of either x or y was equal to one (i.e., for Li5FeO4 and Li5AIO4). The electrochemical performance of various members of the Li(6-x-»C0(i-x-y>FexAI>O4 family was investigated and is shown in Figures 6 to 9. Figure 6 shows the cyclic performance of Li5.6Co0.6Fe0.3AI0.1O4 (x = 0.3, y = 0.1) in the first cycle (dashed line) and the second cycle (solid line). Upon cycling between 3.5 V and 1.5 V, a discharge capacity of over 250 mAh / g was observed. In the first charging cycle, a charging plateau was observed at approximately 3.28 V, while a discharge plateau was observed at approximately 2.42 V. The charging plateau becomes sloppy in the second cycle, remaining the same in subsequent cycling. The first cycle can therefore be viewed as an activation cycle. Figure 7 shows the impact of different charging cut-off voltages on the capacity of Li5.5Co0.5Fe0.4AI0.1O4 (x = 0.4, y = 0.1). Once the first cycle (solid line, flat charging profile and low capacity discharge) is complete, the cathode undergoes three further cycles, with gradually increasing charging cut-off voltages, from approximately 3.3 V (solid line), to approximately 3.4 V (dashed line), to approximately 3.55 V (dotted line). The respective discharge capacities for these cycles were observed to be approximately 210 mAh / g, approximately 235 V and just over 250 mAh / g when limiting the discharge voltage to 1.5 V. In Figure 8, the impact of charging to a higher charging cut-off voltage can be seen. With a charging cut-off voltage of 4.2 V, a capacity of only 100 mAh / g was observed for Li5.6Co0.6Fe0.3AI0.1O4 (x = 0.3, y = 0.1) when limiting the discharge voltage to 1.5 V. However, for comparison, the curve for LieCoO4 (dotted line) is also shown, which displays a still lower discharge capacity at 1.5 V despite a higher charging cut-off voltage. Finally, the inventors have established that in compositions where x = y, the cathode materials in the U(6-x-y)CO(i-x-y)FexAlyO4 family show highly reversible cycling with very low hysteresis when cycled over a small voltage range. Figure 9 shows charge and discharge cycling between 2.9 V and 3.3 V for Li5.6Co0.6Fe0.2AI0.2O4 (x = y = 0.2). While the discharge capacity is only approximately 30 mAh / g, this property may be useful for low-load applications where a high number of charge cycles is envisioned. It will be appreciated that various modifications may be made to the invention as described herein without departing for the scope of protection defined by the appended claims.

Claims

1. A cathode material for a battery, wherein the cathode material has the anti-fluorite crystal structure and belongs to the family represented by the formula Li(6-x-y)Co(i-x-y)FexAlyO4, wherein x and y are greater than zero and less than one, and the sum of x and y is less than one.

2. The cathode material of Claim 1, wherein x is greater than or equal to 0.2 and less than or equal to 0.4.

3. The cathode material of Claim 2, wherein x is greater than or equal to 0.25 and less than or equal to 0.4.

4. The material of any preceding claim, wherein y is greater than or equal to 0.1 and less than or equal to 0.25.

5. The cathode material of any preceding claim, wherein x is equal to y.

6. The cathode material of any of claims 1 to 5, having a space group of PA-Jnmc.

7. A method of making a cathode for a battery, the method comprising:providing a precursor powder mixture comprising Li2O, LiFeO2, Co(OH)2 and AI2O3 powders in a stoichiometric ratio;ball-milling the precursor powder mixture;heating the precursor powder mixture to a heated temperature between approximately 650°C and approximately 750°C;holding the precursor powder mixture at the heated temperature for a time period of at least 10 hours;cooling the precursor powder mixture to room temperature at a cooling rate of between 1°C per minute and 10°C per minute;wherein the stoichiometric ratio is such that the cathode material has the formula Li6-x- / 3oi.x.yFexAlyO4, wherein x and y are greater than zero and less than one and the sum of x and y is less than one.

8. The method of Claim 7, wherein the heated temperature is approximately 700°C.

9. The method of Claim 7 or Claim 8, wherein the time period is at least 15 hours.10 The method of any of Claims 7 to 9, wherein the cooling rate is at least 5°C perminute.

11. The method of any of Claims 7 to 10, wherein x is greater than or equal to 0.2 and less than or equal to 0.4.

12. The method of Claim 11, wherein x is greater than or equal to 0.25 and less than or equal to 0.4.

13. The method of any of Claims 7 to 12, wherein y is greater than or equal to 0.1 and less than or equal to 0.25.

14. The method of Claim 13, wherein x is equal to y15. A cathode comprising the cathode material of any of Claims 1 to 6, or a cathode material made by the method of any of Claims 7 to 14.

16. A cell comprising the cathode of Claim 15 in combination with an anode and an electrode.

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