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 Li6-xCo1-xFexO4, wherein x is greater than z

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 high 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 Li6-xCoi-xFexO4, wherein x is greater than zero and less than one. The space group of the material is P42 / nmc. The value of x may be greater than or equal to 0.4 and may be less than or equal to 0.7. Preferably, the value of x may be greater than or equal to 0.4 and less than or equal to 0.6: for example, the value of x may be approximately 0.5. 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, and Co(OH)2 powders in a stoichiometric ratio; ball-milling the precursor powder mixture; arranging the precursor in an argon atmosphere; heating the precursor powder mixture in the argon atmosphere to a heated temperature between approximately 650°C and approximately 900°C; holding the precursor powder mixture at the heated temperature in the argon atmosphere for a time period of at least 10 hours; and cooling the precursor powder mixture to room temperature. The stoichiometric ratio is such that the cathode material has the formula Li6-xCoi-xFexO4, wherein x is greater than zero and less than one. The heated temperature may be between approximately 650°C and 750°C, and may preferably be approximately 700°C. The time period may be at least 15 hours. The cooling rate may be between 1 °C per minute and 10 °C per minute. The value of x may be greater than or equal to 0.4 and may be less than or equal to 0.7. Preferably, the value of x may be greater than or equal to 0.4 and less than or equal to 0.6: for example, the value of x may be approximately 0.5. In a third aspect, the invention provides a cathode comprising the cathode material discussed above, or a cathode material made by the method discussed above. An active material of the cathode may comprise no more than trace amounts of any materials other than the cathode material. 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)FexO4; Figure 1c shows a super cell of Li(6.X)CO(i.X)FexO4, including co-ordination tetrahedra of oxygen ions around the metal ions; Figure 2 shows the theoretical specific capacity of L^Co^Fe / ^ as the value of x changes; Figure 3 shows X-ray diffraction patterns of various members of the Li(6-^Co(1.^FexO4 family; Figures 4a and 4b show Rietveld refinements of the X-ray diffraction patterns of Li5.7Co0.7Fe0.3O4 and Li5.6Co0.6Fe0.4O4; Figure 5a shows the cell volume for various members of the Li(6-X)CO(i.X)FexO4 family, from UeCoO4 (x = 0) to Li5.4Coo.4Feo.6 (x = 0.6); Figure 5b shows the a lattice parameter for various members of the Li(6-X)CO(i-x)FexO4 family, from Li6CoO4 (x = 0) to Li5.4Coo.4Feo.6 (x = 0.6); Figure 5c shows the c lattice parameter for various members of the Li(6-X)CO(i.X)FexO4 family, from Li6CoO4 (x = 0) to Li5.4Coo.4Feo.6 (x = 0.6); Figure 6 shows the charge-discharge curve for Li5.6Co0.6Fe0.4O4 (solid line) and LieCoO4 (dashed line) in the first cycle between 1.0 V and 4.5 V; and Figure 7 shows the charge-discharge curve for Li5.6Co0.6Fe0.4O4 in the first (dashed line) and second (solid line) cycles between 1.5 V and 3.4 V. DETAILED DESCRIPTION The present invention relates to a cathode material with the general formula Li(6-X)CO(i.X)FexO4 having the anti-fluorite crystal structure, wherein the value of x is greater than zero and less than or equal to one. The material crystallises in the P42 / nmc space group. 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 high 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 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 U2O-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 addressing the poor 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. One such compound is Li6CoO4. 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. 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. LieCoO4 offers an enhanced ionic and electrical conductivity compared to Li2O, due to the vacancies present in the structure. A high specific capacity for LieCoCU has been theorised as a result of the high lithium content, as well as the effect of the oxygen redox in addition to the redox of the transition metal Co2+ ion. For example, the theoretical specific capacity of LieCoCU is over 970 mAh / g. However, LieCoO4 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, caused by incomplete reconstruction of the coordination of oxygen ions around the iron ions upon repeated cycles of reduction and oxidation. The inventors have found that these issues can be at least partially mitigated by the partial substitution of cobalt by other transition metal ions, such as iron. In the case of partial substitution by iron, the resulting family of materials has the general formula Li(6-x)CO(i-X)FexO4. Depending on the value of x, a different proportion of the cobalt ions are replaced by iron ions. The unit cell shown in Figure 1b shows the crystal structure resulting from this substitution, with the cobalt / iron sites labelled 30. Figure 1c shows the same crystal structure as Figure 1b, albeit with the iron / cobalt ions in different locations, but also shows the co-ordination tetrahedra of the oxygen ions 20 around the lithium ions 10 and the iron / cobalt ions 30. Figure 2 shows the theoretical specific capacity of cathodes belonging to the belonging to the Li(6-x)CO(i-X)FexO4 family. As can be seen, the theoretical specific capacity slightly decreases as the proportion of iron ions increases relative to the proportion of cobalt ions. This is due to the decreasing amount of lithium for charge balancing purposes as Fe3+ ions replace Co2+ ions. The Li(6-x)Co(i.x)FexO4 family of materials therefore enjoys largely the same benefits of LieCoO4 cathodes in that the high lithium content and simultaneous cationic and anionic redox ensures a high specific capacity. However, the addition of iron helps stabilise the crystal structure over repeated charge and discharge cycles. Furthermore, the addition of iron reduces the cobalt content of the cathode material, which is beneficial from an environmental perspective, as well as being cheaper. Both iron and cobalt are redoxactive species and so addition of iron does not reduce the practical specific capacity of the cathode at higher values of x. Therefore, there is a balance to be struck between a number of factors (including specific capacity, material cost and electrochemical stability) in order to achieve optimal performance with reasonable and practical levels of structural integrity. A range of materials within this family, with different values of x 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)FexO4 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 homogenous mixture of the precursor powders. For the manufacture of Li(6-X)CO(i.x)FexO4 compositions, a mixture of U2O, LiFeO2 and Co(OH)2 powders are used, with the exact stoichiometric ratio of the three powders depending on the value of x of the final composition. To synthesise the cathode material, the ball-milled precursor powder mixture can be heated to between 600°C and 900°C, preferably between 600°C and 750 C, more preferably to 700°C for at least 10 hours, preferably for at least 15 hours, although preferably for less than 24 hours, followed by slow cooling at a rate of between 1°C per minute and 10°C per minute until the sample reaches room temperature. The precursor powder mixture is specifically heated and cooled in an argon atmosphere, which the inventors have found results in a desirable phase purity. Examples A variety of Li(6.X)Co(i.X)FexO4 samples were prepared with different values of x. The samples were prepared using the following method: • Mixing U2O, LiFeO2 and Co(OH)2 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 at a rate of 5°C per minute; • Slow cooling to room temperature at a rate of 4°C per minute. Figure 3 shows X-ray diffraction (XRD) patterns for samples having the following compositions: U6CoO4, Li5.7Co0.7Fe0.3O4, Li5.6Co0.5Fe0.4O4, Li5.5Co0.5Fe0.5O4, Li5.4Co0.4Fe0.6O4 and LisFeO4 (i.e. x = 0, x = 0.3, x = 0.4, x = 0.5, x = 0.6 and x = 1).. It can be seen that the XRD pattern for x = 1 differs from that of the other compositions tested, where x <0.6. It has been determined that LisFeO4 crystallises in the Pbca space group, while all the tested cobalt-substituted compositions crystallise in the more symmetric P42 / nmc space group. The inventors believe that the cobalt-substituted compositions have the P42 / nmc space group because the cobalt ions have a bigger ionic radius, and because of the increased lithium compared to the LisFeO4 composition. The purity of the synthesised phases was assessed by carrying out Rietveld refinements on the XRD patterns of the as-synthesized samples. Representative analyses are shown in Figures 4a and 4b, for samples of Li5.7Co0.7Fe0.3O4 (x = 0.3) and Li5.5Co0.5Fe0.4O4 (x = 0.4), in which suitable crystal structures were determined and refined against the respective XRD patterns shown in Figure 3. A satisfactory fit is seen between the refinement and the measured XRD pattern in both cases, and all the peaks in the both XRD patterns could be indexed, indicating that both synthesised phases are pure (i.e. containing no more than trace amounts of other materials). From the Rietveld refinements carried out, the cell volumes and lattice parameters were calculated for the determined crystal structures for x = 0, x = 0.3, x = 0.35, x = 0.45, x = 0.5 and x = 0.6. Figure 5a shows the cell volume against the value of x, while Figures 5b and 5c show the a and c lattice parameters plotted against the value of x, respectively. It should be noted that, the P42 / nmc space group falls under the tetragonal crystal system and so the a and b lattice parameters are equal. As would be expected, as the value of x increases from zero (i.e., as the iron content increases), the cell volume and lattice parameters all decrease, due to the smaller ionic radius of the Fe3+ ions being incorporated into the structure compared to the Co2+ ions being replaced. The electrochemical performance of U5.5Co0.6Fe0.4O4 (x = 0.4) was investigated and is shown in Figures 6 and 7. Figure 6 shows that, upon cycling between 1.0 V and 4.5 V in the first charge, a specific capacity in excess of 800 mAh / g was observed. The charging curve shows two plateaus. The first plateau occurs at approximately 3.22 V; this has been attributed primarily to cationic redox of Fe3+ / Fe4+ and Co2+ / Co4+, although anionic redox also occurs. This plateau is seen at approximately this voltage for all values of x. The second plateau occurs at between 3.6 V to 4.0 V; this has been attributed primarily to anionic redox of O27Oi:2~"h where n <1. The voltage at which this second plateau occurs at depends on the value of x, with an increasing value of x (higher iron content) causing the second plateau to appear at a higher voltage. The second plateau is also seen to be sloppy, especially in comparison to the first plateau. The discharge curve for the first cycle of Li5.6Co0.6Fe0.4O4 (x = 0.4) also shows two plateaus. The first plateau shows a small contribution at 2.77 V, with the main contribution coming from a significant plateau at 1.38 V, accounting for both the cationic and anionic reactions. The voltage at which the discharge plateaus appear is dependent on the value of x, increasing as the iron content (i.e., the value of x) increases. Figure 6 also shows in dashed line the charge / discharge curve of UeCoO4 (x = 0) as a reference. Figure 7 shows the first two charge / discharge cycles for Li5.6Co0.6Fe0.4O4 (x = 0.4), when cycling between 1.5 V and 3.4 V. In the first cycle, shown in dashed line, a charging plateau is again seen at 3.22 V, which remains constant over all values of x. In discharge, a plateau is seen at 2.37 V. In the second cycle, shown in solid line, the charging plateau becomes sloppy, remaining this way in subsequent cycling. The first charge / discharge cycle can therefore be viewed as an activation cycle. However, a specific capacity of over 220 mAh / g can be seen in discharge when cycling between 1.5 V and 3.4 V. Of the cathode materials investigated by the inventors, the preferred compositional range extends between x = 0.4 and x = 0.7. It has been found that this range of values of x offers the best balance between structural stability, electrochemical performance and a reduction in the amount of cobalt used for the cathode material. It has been found that increasing the iron content beyond x = 0.7 results in worsened structural stability under cycling. 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 Li6-xCoi-xFexO4, wherein x is greater than zero and less than one, and wherein the space group of the material is P42 / nmc.

2. The cathode material of Claim 1, wherein x is greater than or equal to 0.4 and less than or equal to 0.7.

3. The cathode material of Claim 1, wherein x is greater than or equal to 0.4 and less than or equal to 0.6.

4. The cathode material of Claim 3, wherein x is approximately 0.5.

5. A method of making a cathode for a battery, the method comprising: providing a precursor powder mixture comprising U2O, LiFeC>2, and Co(OH)2 powders in a stoichiometric ratio;ball-milling the precursor powder mixture;arranging the precursor in an argon atmosphere;heating the precursor powder mixture in the argon atmosphere to a heated temperature between approximately 650°C and approximately 900°C;holding the precursor powder mixture at the heated temperature in the argon atmosphere for a time period of at least 10 hours;cooling the precursor powder mixture to room temperature;wherein the stoichiometric ratio is such that the cathode material has the formula Li6-xCo1-xFexO4, wherein x is greater than zero and less than one.

6. The method of Claim 5, wherein the heated temperature is between approximately 650°C and 750°C, and is preferably approximately 700°C.

7. The method of Claim 5 or Claim 6, wherein the time period is at least 15 hours.

8. The method of any of Claims 5 to 7, wherein the cooling rate is between 1 °C perminute and 10 °C per minute.

9. The method of any of Claims 5 to 8, wherein x is greater than or equal to 0.4 and less than or equal to 0.7.

10. The method of Claim 9, wherein x is greater than or equal to 0.4 and less than or 5 equal to 0.6.

11. A cathode comprising the cathode material of any of Claims 1 to 4, or a cathode material made by the method of any of Claims 5 to 10.10 12. The cathode of Claim 11, wherein an active material of the cathode comprises nomore than trace amounts of any materials other than the cathode material of any of Claims 1 to 4 or made by the method of any of Claims 5 to 10.

13. A cell comprising the cathode of Claim 12 in combination with an anode and an15 electrode.13

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