A cathode composition

Abstract

A cathode composition for a battery has the general formula: Li1+aMn1-bMg1-cO2; wherein the values of a, b and c are greater than 0. A method of making the cathode composition is also disclosed, wherein a lithium manganese nickel oxide precursor is subjected to high energy ball milling, the precursor having oxidation states that are equal to the oxidation states of the respective elements in the final cathode composition. The composition preferably has a single phase rock salt structure, wherein there are no peaks below a 2ϴ value of 35 in the XRD pattern. Preferably, the manganese component in the composition comprises a plurality of oxidation states, in particular a mixture of Mn(III) and Mn(IV). A cathode comprising the cathode composition and an electrochemical cell comprising the cathode are also claimed.

Description

24 09 24 TECHNICAL FIELD 5 The present invention relates to a cathode composition. More specifically, the present invention relates to a cathode composition for a battery. The present invention further relates to a method of making the cathode composition for a battery. The present invention further relates to a cathode comprising the cathode composition and an electrochemical cell comprising the cathode. 10 BACKGROUND Lithium-rich cathode compositions for batteries are widely known. Typically, lithium-rich cathodes require a layered structure, which can lead to problems with defects, trapped 15 lithium ions and collapse of the layers during cycling of the battery. In addition, lithium-rich cathode compositions are usually prepared via high temperature synthesis, which is energy-intensive, expensive and can result in difficulties in controlling the reactions that take place to form the composition. More recently, lithium-rich disordered rock salt (DRS) structures have been investigated as cathode materials. However, it is currently 20 a challenge to improve the cycling stability and battery performance in lithium-rich DRS structures with known compositions and methods. The present invention has been devised to mitigate or overcome at least some of the above-mentioned problems. 25 SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention, there is provided a cathode composition for a battery of the general formula: Lii+aMni-bMgi-cO2; wherein the values 30 of a, b and c are greater than 0, and wherein the values of b and c are less than 1. 24 09 24 The present invention thereby provides stable and electrochemically-active magnesiumbased cathode compositions for batteries. In these compositions, the cathode is based only on manganese and magnesium in the DRS structure, whereby magnesium acts as a stabilising structural agent. The present inventors have shown for the first time that 5 such compositions increase cycling stability and battery performance, and not only can the above cathode compositions be produced, but also that they can be produced in a straightforward manner and cycled. The compositions themselves are cheaper to produce and more practical than any 10 previously reported cathode composition. In addition, the present inventors have demonstrated electrochemical activity in these compositions for the first time. Advantageously, magnesium acts as a stabilising structural agent in the DRS structure, which enables the stability of the cathode to be significantly improved in comparison to known lithium-based cathode compositions. 15 The composition may be in the form of a single phase having a rock salt crystal structure such that an x-ray diffraction pattern of the composition using a Cu Ka radiation source has an absence of peaks below a 20 value of 35. 20 Conventional ordered or layered lithium and manganese-rich compositions (Lh+xTmi.xCk where Tm is predominately Mn) have lithium ions sitting in both the alkali and transition metal sites. In addition to the above, x-ray diffraction patterns of these conventional compositions will have a peak at a 20 value of 18. However, in the present invention the x-ray diffraction pattern of the cathode composition has an absence of a peak at a 20 25 value of 18. In other words, the single phase crystal structure of the present invention is absent of any spinel or layered structures, and is considered purely as a single phase rock salt crystal structure. The single phase crystal structure does not exhibit either a R3(bar)m and / or a C2 / m space group. 24 09 24 In particular, the compositions are provided as a single phase rock salt crystal structure (i.e. face centred cubic lattice with the Fm3(bar)m space group), which may be a disordered rock salt crystal structure. 5 The manganese component of the composition may comprise a plurality of oxidation states. The manganese component of the composition may comprise Mn(lll) and Mn(IV), and the composition may have the general formula: 10                                 Lii+aMn(lll)i-biMn(IV)i-b2Mgi-cO2 Where 1 - b = (1 - b1) + (1 - b2). The cathode composition may have the formula: 15                     LU x y 2qMn(III) yMn(IV)2 2x qMgy+qO2 3~3~2~~        2      3—3—3 In some cases, q may be 0 such that the cathode composition has the formula: Lu x         yMn(IV)2 2xMgyO2 3 3 2                2            3 3 20 The value of x may be greater than approximately 0.1 and less than approximately 0.8. The value of x may be greater than approximately 0.4 and less than approximately 0.7. 25 The value of y may be less than approximately 0.3. The value of y may be greater than approximately 0.05 and less than approximately 0.15. 30 The composition may be Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502. The composition may be Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02. 24 09 24 In accordance with a second aspect of the present invention, there is provided a method 5 of making the cathode composition according to the first aspect, the method comprising: providing a lithium manganese magnesium oxide precursor; high-energy milling the precursor with a plurality of milling balls at a milling speed for a milling time period to form the cathode composition; wherein the precursor has oxidation states that are equal to the oxidation states of the respective elements in the cathode composition. 10 Contrary to known high temperature synthesis methods, the present method of making the cathode compositions does not require high temperatures. In addition, the present method provides more control over the oxidation states of the transition metals in the compositions, since the precursors can be selected according to the required oxidation 15 states and applying the method to the precursors results in a cathode composition based on the oxidation states of the precursors without any risk of oxidation during the milling process. The milling speed may be at least approximately 400 rpm. 20 The milling speed may be between approximately 400 rpm and approximately 1000 rpm. Preferably, the milling speed is between approximately 400 rpm and approximately 700 rpm. 25 The milling speed may be approximately 700 rpm. The milling time period may be between approximately 10 hours and approximately 180 hours. 30 24 09 24 The milling time period may be between approximately 40 hours and approximately 100 hours. The milling time period may be between approximately 40 hours and approximately 80 5 hours. The milling time period may be between approximately 40 hours and approximately 60 hours. 10 The milling time period may comprise intermittent periods of 30 minute milling and 15 minute resting. The ratio of precursor powder to milling balls by weight may be between 1:4 and 1:20. 15 The lithium manganese magnesium oxide precursor may comprise a mixture of the following components: lithium manganese (III) oxide, LiMnO2, and / or manganese (III) oxide, Mn2O3 lithium manganese (IV) oxide, Li2MnO3 and / or manganese (IV) oxide, MnO2, 20          magnesium oxide, MgO, and optionally lithium oxide, Li2O. The components of the precursor may be provided in stochiometric amounts according to the final cathode composition. In particular, the components of the precursor may be 25 provided in amounts such that the relative proportions of the different cation species in the precursor is the same as the relative proportions of the different cation species in the final cathode composition. The lithium manganese magnesium oxide precursor may comprise lithium oxide, Li2O, 30 manganese (III) oxide, Mn2O3, manganese (IV) oxide, MnO2, and magnesium oxide, MgO. In this case, the components may be provided in the following molar proportions: 24 09 24 Lithium oxide, U2O:----- 3   6   4 5 Manganese oxide, Mn2O3: Manganese oxide, MnO2:   --y Magnesium oxide, MgO:    y The lithium manganese magnesium oxide precursor may comprise lithium oxide, Li2O, 10 lithium manganese oxide, LiMnO2, manganese (IV) oxide, MnO2, and magnesium oxide, MgO. In this case, the components may be provided in the following molar proportions: 15 Lithium oxide, U2O: -- — 3     3 Lithium manganese oxide, LiMnO2: x Manganese oxide, MnO2:   --y Magnesium oxide, MgO:    y 20 The lithium manganese magnesium oxide precursor may comprise lithium manganese oxide, LiMnO2, lithium manganese oxide, Li2MnO3, and magnesium oxide, MgO. In this case, the components may be provided in the following molar proportions: 25 Lithium manganese oxide, LiMnO2: x-^ 2 2x lithium manganese oxide, U2MnOs: --y Magnesium oxide, MgO: y 24 09 24 The lithium manganese magnesium oxide precursor may comprise lithium oxide, Li2O, lithium manganese oxide, Li2MnO3, manganese (IV) oxide, MnO2, and magnesium oxide, MgO. 5 In this case, the components may be provided in the following molar proportions: Lithium oxide, Li2O: ---2   4 2 2x Lithium manganese oxide, Li2MnO3: Manganese oxide, Mn2Os: 10 Magnesium oxide, MgO: y The plurality of milling balls may comprise tungsten carbide. The high-energy milling may be performed within a ball mill jar comprising tungsten carbide. 15 The plurality of milling balls may comprise zirconium oxide. The high-energy milling may be performed within a ball mill jar comprising zirconium oxide. Each of the plurality of milling balls may have a diameter of approximately 0.5 mm to approximately 1 cm. 20 In accordance with a third aspect of the present invention, there is provided a cathode comprising the cathode composition according to the first aspect. In accordance with a fourth aspect of the present invention, there is provided an 25 electrochemical cell comprising a cathode according to the third aspect, an electrolyte and an anode. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the 30 claims and / or in the following description and drawings, and in particular the individual 24 09 24 features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to 5 amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS 10 One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows X-ray diffraction patterns of the cathode compositions Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502 and Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02 in Examples 15    1 and 2, respectively; Figure 2 shows voltage as a function of capacity (C / 30 cycling at room temperature) for the cathode compositions Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502 and Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02 in Examples 1 and 2, respectively; and 20 Figure 3 shows voltage as a function of capacity (C / 10 cycling at room temperature) for the cathode compositions Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502 and Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02 in Examples 1 and 2, respectively. 25 DETAILED DESCRIPTION The present invention relates to a cathode composition for a battery of the general formula: Lin-aMn1.bMg1.cO2; wherein the values of a, b and c are greater than 0. 24 09 24 The composition has a disordered rock salt structure, i.e. it comprises regions of rock salt having different cations, wherein the regions are disordered. In this way the rock salt effectively forms a composite rock salt material. 5 The inventors have determined that the cathode composition can be made using a ball milling technique. Starting materials are selected according to the desired cathode material, as will be explained in more detail below. The starting materials are then subjected to ball milling for a milling time period. The ball milling process typically takes place at room temperature, or at low temperatures such that the oxidation state of the 10 metal cations is not altered during the process. Milling parameters can be selected as appropriate. After the ball milling process is complete, the resulting material is the disordered rock salt cathode composition. The ball milling process advantageously allows the oxidation state of the metals to be 15 maintained into the final cathode composition. In this way, the oxidation state, and hence the final cathode composition, can be carefully controlled. The manganese component of the composition may comprise a plurality of oxidation states, such as Mn(lll) (i.e. a 3+ oxidation state) and Mn(IV) (i.e. a 4+ oxidation state). 20 In this case, the formula of the composition is given by: Lii+aMn(lll)i-biMn(IV)i-b2Mgi-cO2 where 1 - b = (1 - b1) + (1 - b2). 25 The parameters a, b and c may be inter-related. In particular, the cathode composition may have the formula: LU x yMn^III) yMn^IV^ 2xMgyO2 3 3 2              2           3 3 30 In this case: 24 09 24 1   X   y a =----- 3    3    2 b1 = 1+^-% 1 , 2x b2 = - + — 3    3 c = y 5 The cathode composition may also have the formula: LU x y 2qMn(III) yMn(IV)2 2x qMgy+q02 3323     2    333 10 In the above composition, which comprises both Mn(lll) and Mn(IV), the starting material is a lithium manganese magnesium oxide. The lithium manganese magnesium oxide precursor may comprise a mixture of powdered oxide materials. The materials include oxides of lithium, manganese 3+, 15 manganese 4+ and magnesium, i.e. all the cation species that must be present in the final cathode material. In particular the precursor may comprise the following components: 20          lithium manganese (III) oxide, LiMnO2, and / or manganese (III) oxide, Mn2O3, lithium manganese (IV) oxide, Li2MnO3 and / or manganese (IV) oxide, MnO2, magnesium oxide, MgO, and optionally lithium oxide, U2O. 25 The proportions of the starting materials are chosen to provide stochiometric amounts of the cations that correspond to the stochiometric amounts in the final material. In particular, the components of the precursor may be provided in amounts such that the relative proportions of the different cation species in the precursor is the same as the relative proportions of the different cation species in the final cathode composition. For example, where the lithium manganese magnesium oxide precursor comprises lithium oxide, U2O, manganese (III) oxide, Mn2O3, manganese (IV) oxide, MnO2, and magnesium oxide, MgO, the components may be provided in the following molar 5 proportions: Lithium oxide, U2O:----- 3   6   4 Manganese oxide, M^Os: 2 2x Manganese oxide, MnO2:   --y 10 Magnesium oxide, MgO:    y 24 09 24 Where the lithium manganese magnesium oxide precursor comprises lithium oxide, U2O, lithium manganese oxide, LiMnO2, manganese (IV) oxide, MnO2, and magnesium oxide, MgO, the components may be provided in the following molar proportions: Lithium oxide, U2O: -- — 3     3 Lithium manganese oxide, LiMnO2: x 2 2x Manganese oxide, MnO2:   3-y Magnesium oxide, MgO:    y Where the lithium manganese magnesium oxide precursor comprises lithium manganese oxide, LiMnO2, lithium manganese oxide, Li2MnOa, and magnesium oxide, MgO, the components may be provided in the following molar proportions: 25 Lithium manganese oxide, LiMnO2: x-^ 2 2x lithium manganese oxide, U2MnOa: --y Magnesium oxide, MgO: y 24 09 24 Where the lithium manganese magnesium oxide precursor may comprise lithium oxide, Li2O, lithium manganese oxide, Li2MnO3, manganese (IV) oxide, MnO2, and magnesium oxide, MgO, the components may be provided in the following molar proportions: 5 Lithium oxide, Li2O: --- 2   4 2 2x Lithium manganese oxide, Li2MnO3: --y Manganese oxide, Mn2Os: Magnesium oxide, MgO: y 10 The ball milling process parameters may be any suitable parameters. In particular: The ball milling time period is a time that is sufficient to produce the cathode composition, which may be for example at least 10 hours. 15 The material of the balls and / or the container may be any suitable material such as for example tungsten carbide or zirconium oxide. A ratio of balls to precursor may be any suitable ratio to provide sufficient milling to produce the cathode composition. For example the ratio of precursor to milling balls by 20 weight may be between 1:4 and 1:20, though other suitable ratios may be used. The milling speed may be any suitable milling speed that is sufficient to produce the cathode composition, which may be for example at least approximately 400 rpm. 25 The composition can be incorporated into a cathode in any suitable form. Aspects of the present invention include a method of making the cathode composition for a battery, a cathode comprising the cathode composition, and an electrochemical cell comprising the cathode. The present invention will now be illustrated with reference to the following examples. 24 09 24 EXAMPLE 1 5 Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502 a = 0.075 b = 0.125; b1 =0.325; b3 = 0.8 c = 0.95 10 A lithium manganese magnesium oxide (LiMnMgO2) precursor was prepared. The precursor comprised 1.347g (6.74 wt%) U2O, 14.279g (71.4 wt%) LiMnO2, 3.918g (19.59 wt%) MnO2, and 0.45g (2.25 wt%) MgO. The precursor was provided in a 20 ml ball mill jar made of tungsten carbide. 80 tungsten carbide milling balls each having a diameter of 5 mm were also provided in the ball mill jar. High-energy milling was used 15 to mill the precursor at a milling speed of 700 rpm for a milling time period of at least 10 hours to form a cathode composition of the formula: Lil.075Mn(lll)o.675Mn(IV)o.2Mg(ll)o.0502. This results in formation of 20g of Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502 powder inside the 20 ball mill jar. The composition has a disordered rock salt crystal structure. The oxidation states of the elements in the precursor are as follows: Element in Precursor Oxidation State Li in U2O +1 Li in LiMnO2 +1 Mn in LiMnO2 +3 Mn in MnO2 +4 Mg in MgO +2 24 09 24 The oxidation states of Li, Mn and Mg in the cathode composition Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502are equivalent to the oxidation states of the elements in the precursor as provided in the above table. 5 The X-ray diffraction pattern 102 of the cathode composition Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502 is shown in Figure 1, demonstrating the crystalline structure of the composition. The XRD pattern is characteristic of a cation disordered rock salt structure. The pattern appears to show the broad major peaks consistent with a face centred cubic lattice with the Fm3(bar)m space group, as shown in Figure 1. 10 There is no evidence for the presence of the layered precursors such as LiMnO2. Figures 2 and 3 show voltage as a function of capacity using C / 30 cycling at room temperature and C / 10 cycling at room temperature, respectively, for the cathode composition Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502. This demonstrates that the 15 composition can be cycled effectively. EXAMPLE 2 20 Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02 a = 0.05 b = 0.15; b1 = 0.35; b2 = 0.8 c = 0.90 25 A lithium manganese magnesium oxide (LiMnMgO2) precursor was prepared. The precursor comprised 1.352g (6.76 wt%) Li2O, 13.802g (69.01 wt%) LiMnC>2, 3.933g (19.67 wt%) MnO2, and 0.912g (4.56 wt%) MgO. The precursor was provided in a 20 ml ball mill jar made of tungsten carbide. 80 tungsten carbide milling balls each having a diameter of 5 mm were also provided in the ball mill jar. High-energy milling was used 30 to mill the precursor at a milling speed of 700 rpm for a milling time period of at least 10 hours to form a cathode composition of the formula: Lii.osMn(lIl)o.65Mn(IV)o.2Mg(ll)o.i02. This results in formation of 20g of Lii.osMn(l I l)o.6sMn(l V)o.2Mg(l I)o.i02 powder inside the ball mill jar. The composition has a disordered rock salt crystal structure. The oxidation states of the elements in the precursor are as follows: Element in Precursor Oxidation State Li in U2O +1 Li in LiMnO2 +1 Mn in LiMnO2 +3 Mn in MnO2 +4 Mg in MgO +2 24 09 24 The oxidation states of Li, Mn and Mg in the cathode composition Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02are equivalent to the oxidation states of the elements in the precursor as provided in the above table. 10 The X-ray diffraction pattern 104 of the cathode composition Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02 is shown in Figure 1, demonstrating the crystalline structure of the composition. The XRD pattern is characteristic of a cation disordered rock salt structure. The pattern appears to show the broad major peaks consistent with a face centred cubic lattice with the Fm3(bar)m space group, as shown in Figure 1. 15 There is no evidence for the presence of the layered precursors such as LiMnO2. Figures 2 and 3 show voltage as a function of capacity using C / 30 cycling at room temperature and C / 10 cycling at room temperature, respectively, for the cathode composition Lii.osMn(lll)o.6sMn(IV)o.2Mg(lI)o.i02. This demonstrates that the composition 20 can be cycled effectively. Many modifications may be made to the specific embodiments described above without departing from the scope of the invention as defined in the accompanying claims. Features of one embodiment may also be used in other embodiments, either as an 25 addition to such embodiment or as a replacement thereof.

Claims

24 09 241. A cathode composition for a battery of the general formula:5                                       Lii+aMni-bMgi-cO2wherein the values of a, b and c are greater than 0, and wherein the values of b and c are less than 1.10   2. The cathode composition according to claim 1, wherein the composition has theform of a single phase having a rock salt crystal structure that has an x-ray diffraction pattern when using a Cu Ka radiation source having an absence of peaks below a 20 value of 35.15   3. The cathode composition according to claim 1 or claim 2, wherein themanganese component of the composition comprises a plurality of different oxidation states.

4. The cathode composition according to claim 3, wherein the manganese 20 component of the composition comprises Mn(lll) and Mn(IV), and the composition has the general formula:Lii+aMn(lll)i-biMn(IV)i-b2Mgi-cO2Where 1 - b = (1 - b1) + (1 - b2).25   5. The cathode composition according to claim 4, wherein the cathode compositionhas the formula:LU x y 2qMn(III) yMn(IV)2 2x qMgy+q02 3323     2    3336. The cathode composition according to claim 5, wherein the value of x is greater 30 than 0.1 and less than 0.8.24 09 247.     The cathode composition according to claim 5 or claim 6, wherein the value of xis greater than 0.4 and less than 0.7.5   8. The cathode composition according to any of claims 5 to 7, wherein the value ofy is less than 0.3.

9. The cathode composition according to any of claims 5 to 8, wherein the value of y is greater than 0.05 and less than 0.15.1010. The cathode composition according to any of claims 5 to 9, wherein the composition is Lii.o75Mn(lll)o.675Mn(IV)o.2Mg(ll)o.o502.

11. The cathode composition according to any of claims 5 to 9, wherein the15 composition is Lii.o5Mn(lll)o.65Mn(IV)o.2Mg(ll)o.i02.

12. A method of making the cathode composition of any of claims 1 to 11, the method comprising:providing a lithium manganese magnesium oxide precursor;20           high-energy milling the precursor with a plurality of milling balls at a milling speedfor a milling time period to form the cathode composition;wherein the precursor has oxidation states that are equal to the oxidation states of the respective elements in the cathode composition.25   13.    The method according to claim 12, wherein the milling speed is at least 400 rpm.

14. The method according to claim 12 or claim 13, wherein the milling speed isbetween 400 rpm and 1000 rpm.30   15. The method according to any one of claims 12 to 14, wherein the milling speedis between 400 rpm and 700 rpm.24 09 2416. The method according to any of claims 12 to 15, wherein the milling time period is between 10 hours and 180 hours.

517. The method according to any of claims 12 to 16, wherein the milling time period is between 40 hours and 100 hours.

18. The method according to any of claims 12 to 17, wherein the milling time period 10 is between 40 hours and 80 hours.

19. The method according to any of claims 12 to 18, wherein the milling time period is between 40 hours and 60 hours.15   20. The method according to any of claims 12 to 19, wherein the ratio of precursorpowder to milling balls by weight is between 1:4 and 1:20.

21. The method according to any of claims 12 to 20, wherein the lithium manganese magnesium oxide precursor comprises a mixture of the following components:20          lithium manganese (III) oxide, LiMnO2, and / or manganese (III) oxide, Mn2O3,lithium manganese (IV) oxide, Li2MnO3 and / or manganese (IV) oxide, MnO2, magnesium oxide, MgO, and optionally, lithium oxide, Li2O.25   22. The method according to any of claims 12 to 20, wherein the lithium manganesemagnesium oxide precursor comprises lithium oxide, Li2O, manganese oxide, Mn2O3, manganese oxide, MnO2, and magnesium oxide, MgO, in the following molar proportions:30 Lithium oxide, Li2O: 3   6   4Manganese oxide, M^Os:2 2xManganese oxide, MnO2:   --yMagnesium oxide, MgO:    y24 09 24523. The method according to any of claims 12 to 22, wherein the plurality of milling balls comprises tungsten carbide and / or zirconium oxide, and wherein the high-energy milling is performed within a ball mill jar being made of a material comprising tungsten carbide and / or zirconium oxide.1024. A cathode comprising the cathode composition according to any of claims 1 to 11.

25. An electrochemical cell comprising a cathode according to claim 24, an 15 electrolyte and an anode.

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