Cathode composition

Abstract

A material suitable for use as a cathode composition, comprising a lith9ium manganese oxide of the general formula Li1-xMn1-xO2 wherein x is within the range of 0.04 to 0.13. The lithium manganese ox

Description

BACKGROUND The performance and cost of lithium ion batteries primarily relies on the composition of the positive electrode (cathode). Currently available lithium-based cathode compositions are graded mainly on their energy density, electrochemical performance and the price of raw materials required to formulate the composition. Manganese would be an ideal soletransition metal centre for a lithium-based cathode composition, as the abundance of manganese in the Earth's crust is far greater than cobalt and nickel. Although much higher capacities and energy densities can currently be achieved with compositions comprising nickel, cobalt and aluminium, the cost of these metals is far greater. In addition, these nickel, cobalt and aluminium-based composition still suffer with problems of the voltage profile evolving on cycling, costly (because of the inclusion of cobalt and nickel) and show significant stability issues, such as gas loss during cycling. SUMMARY The invention resides in a lithium manganese oxide in a cathode composition, wherein the lithium manganese oxide has a cubic crystal structure, and the unit cell length of the cubic crystal structure is at least about 4.10 A. In a first aspect, provided herein is a cathode composition for a lithium-ion battery, the cathode composition comprising a lithium manganese oxide of the general formula: Lii+xMni-x02 wherein x is in the range of from 0.04 to 0.13; the lithium manganese oxide has a cubic crystal structure; and the unit cell length of the cubic crystal structure is at least 4.10 A. The inventors have found that a lithium manganese oxide having a cubic crystal structure and a unit cell length of at least about 4.10 A can provide an excellent balance in electrochemical performance when used in a cathode of an electrochemical cell. Specifically, such an electrochemical cell may exhibit an excellent balance in capacity and capacity retention, and especially good capacity retention after 50 charge-discharge cycles. The term “cubic crystal structure” refers to a crystal structure where the unit cell is in the shape of a cube, such that all length dimensions of the unit cell are equal to each other and separated by an angle of 90°. Examples of a cubic crystal structure include simple cubic, body-centred cubic (bcc) and face-centred cubic (fee). In some embodiments, the cubic crystal structure may be a simple cubic crystal structure, a body-centred cubic crystal structure and a face-centred cubic crystal structure. In preferred embodiments, the cubic crystal structure may be a face-centred cubic crystal structure. In some embodiments, the face-centred cubic crystal structure may be a rock-salt structure. A rock-salt structure is a face-centred cubic array of anions with cations in the octahedral sites of the structure. Each cation is surrounded by six anions and each anion is octahedrally coordinated by cations. The present invention relates to a lithium manganese oxide, and so the cubic crystal structure may be a face-centred cubic crystal structure, which is a disordered rock-salt crystal structure. The rock-salt structure may be disordered by virtue of two different types of cations present in the structure. The disordered rock-salt crystal structure may be a face-centred cubic crystal structure, such as a crystal structure with the Fm3(bar)m space group. Thus, in some embodiments, the lithium manganese oxide of the present invention may have a disordered rock-salt structure in which lithium and manganese ions occupy the octahedral holes of the structure and oxide ions occupy the cubic array. The term “unit cell” refers to the smallest repeating unit having the full symmetry of the crystal structure, from which the entire lattice can be built up by repetition in three dimensions. It follows that a cubic crystal structure may be characterised by its unit cell length, which is the length of one straight side of a unit cell of the cubic crystal structure. In the lithium manganese oxide for use in the cathode composition of the present invention, the unit cell length of the cubic crystal structure is at least about 4.10 A. Unexpectedly, a unit cell length at this range provides an excellent balance in capacity and capacity retention, allowing for excellent electrochemical performance in an electrochemical cell. In some embodiments, the unit cell length of the cubic crystal structure may be at least about 4.11 A. In some embodiments, the unit cell length of the cubic crystal structure may be at most about 4.16 A, such as at most about 4.15 A, such as at most about 4.14 A. Any of the foregoing may be combined to form a suitable range of the unit cell length. In some embodiments, the unit cell length of the cubic crystal structure may be in the range of from 4.10 to 4.16 A, such as from 4.11 to 4.16 A, such as from 4.11 to 4.15 A, such as from 4.11 to 4.14 A, such as from 4.11 to 4.13 A. In some embodiments, the unit cell length of the cubic crystal structure may be in the range of from 4.11 to 4.13 A. The unit cell length may be measured by X-ray diffraction. The unit cell length may be modelled through Rietveld refinement according to standard methods. In some embodiments, the lithium manganese oxide may exhibit an X-ray diffraction pattern having significant peaks at 29 = 37.5±0.2°, 43.9±0.2°, and 63.6±0.2° by X-ray diffraction pattern using Cu-Ka radiation. In some embodiments, the lithium manganese oxide may not exhibit an X-ray diffraction pattern having significant peaks at 29 = 37.7±0.2°, 44.2±9.2°, 64.1±9.2°, 77.2±9.2°, and 81.3±0.2° by X-ray diffraction pattern using Cu-Ka radiation. The lithium manganese oxide has the general formula: Lii+xMni-x02, wherein x is in the range of from 9.94 to 9.13. In some embodiments, x may be at least about 9.94, such as at least about 9.95, such as at least about 9.97. In some embodiments, x may be at least about 9.98, such as at least about 9.99. In some embodiments, x is may be most about 0.13, such as at most about 0.12, such as at most about 0.11. Any of the foregoing may be combined to form a suitable range of x. For example, in some embodiments, x may be in the range of from 0.07 to 0.13, such as from 0.08 to 0.12, such as from 0.09 to 0.11. Examples of the lithium manganese oxide include: Li1.04Mn0.96O2, Li1.05Mn0.95O2, Li1.oeMno.94O2, Li1.07Mn0.93O2, Li1.08Mn0.92O2, Li1.09Mn0.91O2, Li1.1Mno.9O2, Li1.nMno.89O2, Li1.12Mno.88O2 and Li1.13Mno.87O2. As shown in the Examples of the present invention below, the cathode composition may comprise a lithium manganese oxide of the formula selected from Li1.04Mn0.9eO2, Li1.07Mn0.93O2, Li1.1Mno.9O2, or Li1.13Mno.87O2. In particular, x may be equal to about 0.1, such as 0.1, such as 0.10. This particular lithium manganese oxide is thus Li1.1Mno.9O2. This oxide may have an excellent balance in capacity and capacity retention over a number of charge-discharge cycles. The invention also relates to a general aspect of a lithium manganese oxide of the general formula: Lii+xMni-x02 wherein x is in the range of from 0.04 to 0.13; the lithium manganese oxide has a cubic crystal structure; and the unit cell length of the cubic crystal structure is at least about 4.10 A. The lithium manganese oxide of this general aspect may have some or all of the optional features described elsewhere herein for the lithium manganese oxide of the cathode composition of the first aspect. In a second aspect, provided herein is an electrode comprising the cathode composition of the first aspect, an electroactive additive and / or a binder. The cathode composition of the first aspect may be present, based on the total mass of the electrode, in an amount of at least about 60 wt%, such as at least about 70 wt%, such as at least about 80 wt%, such as at least about 90 wt%. The cathode composition of the first aspect may be present, based on the total mass of the electrode, in an amount of at most about 98 wt%, such as at most about 96 wt%, such as at most about 94 wt%, such as at most about 92 wt%. Any of the foregoing may be combined to form a suitable range of the amount of the cathode composition. For example, in some embodiments, the cathode composition of the first aspect may be present, based on the total mass of the electrode, in an amount from 60 to 98 wt%, such as from 70 to 96 wt%, such as from 80 to 94 wt%, such as from 90 to 92 wt%. In the electrode, the electroactive additive may be for improving the electrical conductivity of the electrode. Examples of electroactive additives include carbon, for example, Super P and Carbon black. The electroactive additives in the electrode may be present, based on the total mass of the electrode, in an amount of at least about 2 wt%, such as at least about 4 wt%, such as at least about 6 wt%. The electroactive additives in the electrode may be present, based on the total mass of the electrode, in an amount of at most about 12 wt%, such as at most about 10 wt%, such as at most about 8 wt%. Any of the foregoing may be combined to form a suitable range of the amount of the electroactive additives. For example, in some embodiments, the electroactive additives in the electrode may be present, based on the total mass of the electrode, in an amount from 2 to 12 wt%, such as from 4 to 10 wt%, such as from 6 to 8 wt%. In the electrode, the binder is for adhesively holding together the cathode composition and the electroactive additives. In some embodiments, the binder is a polymeric binder. Examples of binders include PVDF, PTFE, NaCMC and NaAlginate. The binder in the electrode may be present, based on the total mass of the electrode, in an amount of at least about 2 wt%, such as at least about 4 wt%, such as at least about 6 wt%. The binder in the electrode may be present, based on the total mass of the electrode, in an amount of at most about 12 wt%, such as at most about 10 wt%, such as at most about 8 wt%. Any of the foregoing may be combined to form a suitable range of the amount of the binder. For example, in some embodiments, the binder in the electrode may be present, based on the total mass of the electrode, in an amount from 2 to 12 wt%, such as from 4 to 10 wt%, such as from 6 to 8 wt%. The overall electrochemical performance of the composite cathode can be improved by the introduction of electroactive additives, and the structural properties of the resulting composite cathode can also be improved by adding material that improves cohesion of the cathode composition and adhesion of the material to particular substrates. In a third aspect, provided herein is an electrochemical cell comprising an electrode of the second aspect, an electrolyte and an anode. In the electrochemical cell, the electrolyte may be for providing an ion transport medium between the cathode and the anode. The electrolyte may take the form of a liquid or solid. In some embodiments, the electrolyte may comprise a solvent comprising one or more carbonate compounds. In some embodiments, the electrolyte may comprise a solvent comprising one or more cyclic or linear carbonate compounds. In some embodiments, the solvent may comprise one or more cyclic carbonate compounds. In some embodiments, the solvent may comprise one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. In some embodiments, the solvent may comprise a blend of at least two different compounds, for example at least three or at least four different compounds. In some embodiments the solvent may comprise a blend of at least two different organic carbonate compounds, for example at least three or at least four different organic carbonate compounds. In some embodiments, the electrolyte may comprise an alkali metal salt. The alkali metal of the alkali metal salt may be any suitable alkali metal (Group I of the periodic table). The alkali metal salt may be a lithium, sodium, or potassium salt. The anion of the alkali metal salt may be any suitable anion. Typical anions are known to the skilled person and may be chosen based on the kind of alkali metal. In some embodiments, when the alkali metal is lithium, the anion of the salt may comprise a halogen such as fluorine. Examples of the anion include BFT, CIOT, PFe’, TFST, FST, OTf, DFOB' and TDT. In some embodiments, the one or more electrolyte components may comprise a lithium salt. In some embodiments, the electrolyte may comprise a mixture of at least two different lithium salts. Examples of suitable lithium salts include LiPFe, LiC104, LiBF4, LiTFSI, LiFSI, LiOTf, LiDFOB and LiTDI. One or more kinds of alkali metal salt may be used in accordance with the present invention. Typically, but not exclusively, when more than one kind of alkali metal salt is used, they may share a common alkali metal. In the electrochemical cell, the anode may be for facilitating oxidation of a redox reaction. In some embodiments, the anode may comprise carbon. In some embodiments, the electrochemical cell may be a lithium-ion battery cell. In a fourth aspect, provided herein is a method of preparing a lithium manganese oxide, wherein the method comprises: heating at least one lithium source and at least one manganese source to obtain a layered oxide compound comprising a mixture of lithium and manganese, wherein the lithium and the manganese are in separate layers; and milling the layered oxide compound to obtain a lithium manganese oxide of the general formula: Lii+xMni-xO2 wherein x is in the range of from 0.04 to 0.13; the lithium manganese oxide has a cubic crystal structure; and the unit cell length of the cubic crystal structure is at least about 4.10 A. The present inventors have found that the combination of heating lithium and manganese sources to produce a layered oxide compound and milling the layered oxide compound can produce the lithium manganese oxide of the first aspect. In some embodiments, the lithium manganese oxide may have some or all of the optional features described elsewhere herein for the method of the fourth aspect. In some embodiments, the method of preparing a lithium manganese oxide further involves preparing a cathode composition comprising the lithium manganese oxide. There is also provided a cathode composition / lithium manganese oxide obtained or obtainable from the method of the fourth aspect of the invention. The layered oxide compound comprises a mixture of lithium and manganese, and is produced by heating together at least one lithium source and at least one manganese source, as precursors. It is believed that the layered oxide compound, when formed from the heating step of the method, allows for an intimate mixing of all the lithium and manganese sources. This can provide a route to improved electrochemical performance at reduced milling times, thereby making production at a larger scale more effective. In some embodiments, the lithium source may be lithium hydroxide or lithium carbonate, or a combination thereof. In some embodiments, the lithium source may not be lithium oxide. The heating step makes use of cheaper, more accessible lithium sources. In some embodiments, the manganese source may be manganese(III) oxide or manganese(IV) dioxide, or a combination thereof. In some embodiments, the lithium source may be lithium hydroxide or lithium carbonate, or a combination thereof, and the manganese source is manganese(III) oxide or manganese(IV) dioxide, or a combination thereof. Heating the lithium source and the manganese source produces a layered oxide compound. In some embodiments, the layered oxide compound may comprise Mn(III). In some embodiments, the layered oxide compound may comprise Mn(IV). In some embodiments, the layered oxide compound may be LiMn(III)O2. In some embodiments, the layered oxide compound may be Li2Mn(IV)O3. In some embodiments, the layered oxide may be a combination of LiMn(III)O2 and Li2Mn(IV)O3. In some embodiments, the heating temperature may be at least about 300 °C, such as at least about 350 °C, such as at least about 400 °C, such as at least about 450 °C, such as at least about 500 °C, such as at least about 550 °C. In some embodiments, the heating temperature may be at most about 1000 °C, such as at most about 950 °C, such as at least about 900 °C, such as at least about 800 °C, such as at least about 800 °C, such as at least about 750 °C, such as at most about 700 °C. Any of the foregoing may be combined to form a suitable range of the heating temperature. For example, in some embodiments, the heating temperature is in the range of from 300 °C to 1000 °C, such as from 350 °C to 950 °C, such as from 400 °C to 900 °C, such as from 450 °C to 850 °C, such as from 500 °C to 800 °C, such as from 500 to 750 °C, such as from 550 to 750 °C, such as from 550 °C to 700 °C. In some embodiments, the heating time may be at least about 2 hours, such as at least about 3 hours, such as at least about 4 hours, such as at least about 5 hours, such as at least about 6 hours, such as at least about 7 hours, such as at least about 8 hours, such as at least about 9 hours, such as at least about 10 hours, such as at least about 11 hours, such as at least about 12 hours. In some embodiments, the heating time may be at most about 24 hours, such as at most about 23 hours, such as at most about 22 hours, such as at most about 21 hours, such as at most about 20 hours, such as at most about 19 hours, such as at most about 18 hours. Any of the foregoing may be combined to form a suitable range of the heating time. For example, in some embodiments, the heating time is in the range of from 2 hours to 24 hours, such as from 3 hours to 24 hours, such as from 3 hours to 24 hours, such as from 4 hours to 24 hours, such as from 5 hours to 24 hours, such as from 6 hours to 24 hours, such as from 7 hours to 24 hours, such as from 8 hours to 24 hours, such as from 9 hours to 24 hours, such as from 10 hours to 24 hours, such as from 11 hours to 24 hours, such as from 12 hours to 24 hours. In some embodiments, the heating may be performed under air. In some embodiments, the heating may be performed under an atmosphere which comprises at least about 20% oxygen by volume. In some embodiments, the heating may be performed under an inert atmosphere. The inert atmosphere may comprise nitrogen, carbon dioxide or a noble gas. In some embodiments, the inert atmosphere may comprise a noble gas. In some embodiments, the noble gas may be argon. The present inventors have conducted extensive tests into the milling time and milling speed to optimise the electrochemical performance, specifically the capacity and capacity retention. The method involves milling a layered oxide compound under various conditions, such as speed and time, to produce a lithium manganese oxide having a unit cell length of at least about 4.10 A in accordance with the first aspect. Milling is performed to prepare the lithium manganese oxide having a cubic crystal structure and a unit cell length of at least about 4.10 A. In some embodiments, the milling step may be performed by ball milling. In some embodiments, the milling step may be performed by planetary ball milling. Ball milling may involve the rotation of a cylindrical container filled with ceramic or metal balls and the layered oxide compound obtained from the heating step. As the container rotates, the balls may crush the material into progressively finer particles. The material may undergo a mechanochemical reaction which causes a phase change from a phase adopted by the precursors to a disordered rock-salt phase. For example, the material may undergo a mechanochemical reaction which causes a phase change from a layered phase to a disordered rock-salt phase. The intensity of the milling may be characterised by the milling speed, which is expressed as revolutions per minute (rpm). In some embodiments, the milling time may be at least about 1 hour, such as at least about 2 hours, such as at least about 3 hours, such as at least about 4 hours, such as at least about 5 hours, such as at least about 6 hours, such as at least about 7 hours. In some embodiments, the milling time is at least about 7.5 hours. In some embodiments, the milling time may be at most 5 about 0 hours, such as at most about 49 hours, such as at most about 48 hours, such as at most about 47 hours, such as at most about 46 hours, such as at most about 45 hours, such as at most about 44 hours, such as at most about 43 hours, such as at most about 42 hours, such as at most about 41 hours, such as at most about 40 hours, such as at most about 39 hours, such as at most about 38 hours, such as at most about 37 hours, such as at most about 36 hours, such as at most about 35 hours, such as at most about 34 hours, such as at most about 33 hours, such as at most about 32 hours, such as at most about 31 hours, such as at most about 30 hours, such as at most about 29 hours, such as at most about 28 hours, such as at most about 27 hours, such as at most about 26 hours, such as at most about 25 hours, such as at most about 24 hours, such as at most about 23 hours, such as at most about 22 hours, such as at most about 21 hours, such as at most about 20 hours. Any of the foregoing can be combined to form a suitable range of the milling time. For example, in some embodiments, the milling time may be in the range of from 1 hours to 50 hours, such as from 2 hours to 45 hours, such as from 3 hours to 40 hours, such as from 4 hours to 35 hours, such as from 5 hours to 30 hours, such as from 6 hours to 25 hours, such as from 7 hours to 20 hours, such as from 7.5 hours to 20 hours. In some embodiments, the milling speed may be at least about 300 rpm, such as at least about 350 rpm, such as at least about 400 rpm, such as at least about 450 rpm, such as at least about 500 rpm, such as at least about 550 rpm, such as at least about 600 rpm, such as at least about 650 rpm, such as at least about 700 rpm. In some embodiments, the milling speed may be at most about 1500 rpm, such as at most about 1400 rpm, such as at most about 1300 rpm, such as at most about 1200 rpm, such as at most about 1100 rpm, such as at most about 1000 rpm. Any of the foregoing may be combined to form a suitable range of the milling speed. For example, in some embodiments, the milling speed may be in the range of from 300 to 1500 rpm, such as from 300 to 1000 rpm, such as from 400 to 1000 rpm, such as from 500 to 1000 rpm, such as from 600 to 1000 rpm. In some embodiments, the milling speed may be from 700 to 1000 rpm. When ball milling is performed, a suitable powder:ball ratio may be selected to produce the lithium manganese oxide. The powder:ball ratio is the mass ratio between the mass of a powder of the precursors used as starting materials and the mass of balls used for ball milling. Specifically, the powder:ball ratio may be the mass ratio between the mass of a powder of the layered oxide compound and the mass of balls used for ball milling. In some embodiments, the powder:ball ratio may be at least about 1:1, such as at least about 1:2, such as at least about 1:4, such as at least about 1:5, such as at least about 1:8, such as at least about 1:10, such as at least about 1:20, such as at least about 1:100. In some embodiments, the powder:ball ratio may be at most about 1:100, such as at most about 1:20, such as at most about 1:10, such as at most about 1:8, such as at most about 1:5, such as at most about 1:4, such as at most about 1:2, such as at most about 1:1. Any of the foregoing may be combined to form a suitable In some embodiments, the powder:ball ratio may be in the range of from 1:100 to 1:1, such as from 1:20 to 1:2, such as from 1:10 to 1:5, such as from 1:8 to 1:4. These, and other aspects and embodiments of the invention, are described in further detail herein. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows (a) the capacity over 50 charge-discharge cycles for Li1.1Mno.9O2 synthesised via different precursors and conditions, and; (b) the capacity retention over 50 chargedischarge cycles for Li1.1Mno.9O2 synthesised via different precursors and conditions. Figure 2 shows X-ray diffraction patterns of (a) starting mixture of Li2CO3, MmOs and Mn02 as precursors; (b) post-heat treatment at 700 °C of Li2CO3, MmOs and Mn02 showing LiMnO2 and Li2MnO3 phases. Figure 3 shows X-ray diffraction patterns of (a) starting mixture of LiOH, MmOs and Mn02 as precursors; (b) post-heat treatment at 700 °C of LiOH, Mn2O3 and Mn02 showing LiMnO2 and Li2MnO3 phases. Figure 4 shows SEM images of (a) Li2CO3, Mn2O3 and Mn02 post-heat treatment at 700 °C showing mixture of LiMnO2 and Li2MnO3 phases; (b) LiMnO2 and Li2MnO3 phases synthesised separately and mixed showing separate LiMnO2 and Li2MnO3 phases. Scale bars are 10 pm. Figure 5 shows X-ray diffraction pattern ‘time-slices’ for Li1.1Mno.9O2 milled at 700 rpm at milling times of 5 h, 10 h, 15 h and 20 h. Figure 6 shows electrochemical testing at 30 °C at C / 10 in half cells vs Li and cycled between 2 to 4.8 V for Li1.04Mn0.9eO2, Li1.07Mn0.93O2, Li1.1Mno.9O2, Li1.13Mno.87O2, Li1.17Mno.83O2 and Li1.2Mno.8O2 milled at 700 rpm, with (a) showing the comparison of the 1st cycle capacity and (b) the capacity retention of 50 cycles. Figure 7 shows electrochemical testing at C / 10 at 30 °C in half cells for Li1.04Mn0.9eO2, Li1.1Mno.9O2, Li1.13Mno.87O2 and Li1.2Mno.8O2 milled at 1000 rpm, with (a) showing the comparison of the 1st cycle and (b) the capacity retention of 50 cycles. DETAILED DESCRIPTION The present invention will now be described by way of examples, which are intended to be illustrative and not limiting on the present invention. Ball milling was performed in a Fritsch(RTM) Pulverisette(RTM) 7. XRD patterns were measured with an Aeris(RTM) Benchtop X-ray diffractometer equipped with a PIXcel(RTM) detector in reflection geometry using CuKa radiation, with an x-ray wavelength of 1.54056A, operating at 40 kV and 40 mA. The measurement range was 30-75° 20. SEM and EDX was performed on a Phenom XL G2 Desktop SEM from Thermofisher Scientific(RTM). For electrochemical testing, the prepared compositions were mixed inside a glovebox with a carbon conductive additive and PTFE as binder in a weight ratio of 80:10:10. The chargedischarge curve was measured from a standard CR 2032 coin type cell assembled in an argon filled glove box (H2O content <Ippm). The charge-discharge cycle tests were performed at a current density of 30 mA / g cycled between 0.5 and 3 V vs. Li / Li+ on a MACCOR. A 1 M LiPFe solution obtained by dissolving in a mixture of EC and DMC in a volume ratio of 1:1 was used as electrolyte. Electrodes were prepared by combining the composition comprising lithium and titanium, acetylene black and PTFE in a weight ratio of 8:1:1 into a pellet. Metallic lithium was used as the counter electrode. The charge-discharge cycle tests were performed at 30°C (303 K) unless stated otherwise, and ambient pressure (101 kPa). Example 1 - Precursor Heating Stoichiometric amounts of Li2CO3 / Mn2O3 / MnO2 or LiOH / Mn2O3 / MnO2 (see Equations 1 and 2 below) as lithium sources and manganese sources were required for the final disordered rock-salt (DRS) phase are mixed effectively and heated to remove carbonate or hydroxide species. 0.55Li2CO3 + 0.35Mn2O3 + 0.2MnO2 Lii.iMn(III)o.7Mn(IV)o.202 + 0.55 CO2 (1) or l.lLiOH + 0.35Mn2O3 + 0.2MnO2 Lii.iMn(III)o.7Mn(IV)o.202 + 0.55 CO2 (2) The resultant material is a mix of layered LiMn(III)02 and Li2Mn(IV)O3 phases, which is then ball milled. The addition of this heating step prior to milling significantly reduces milling time to produce materials with profound electrochemical behaviour. The performance of these materials is superior to those observed using Li2O with no heat treatment, or the use of LiMnO2 and Li2MnO3 synthesised separately prior to ball milling, and may be attributed due to the intimate mixing improved by the heating step. The results are shown in Table 1. The present inventors have shown that this family materials can be rapidly synthesised by an improved ball milling methodology, via the addition of heating all precursors together for the required final DRS stoichiometry prior to ball milling. This results in a much more stable material during charge-discharge cycling compared to when starting with oxide precursors (Figure 1). Table 1 - Effect ofprecursor type on the properties ofLi1.1Mno.9O2 Precursors Heat Milling time (h) Unit cell length (A) Capacity (mAh / g) Capacity retention (10 cycles) Capacity retention (50 cycles) Li2O MmOs MnO2 No 20 4.115 245 98.7% 82.9% LiMnO2 Li2MnO2 No 15 4.129 279 93.0% 74.4% Li2COs Mn2O3 MnO2 Yes 10 4.155 236 104.0% 93.1% Example 2 - LiiCOs / M112O3 / MnOi Precursor Heating and Subsequent Milling Heating of the Li2CO3 / Mn2O3 / MnO2 (Figure 2a) precursors to a temperature of 700 °C shows removal of the carbonate species (Figure 2b). Ball milling experiments were performed using ZrO2 mill jars with ZrO2 5 mm balls and the following milling conditions: (1) 700 rpm with 1:8 powder:ball ratio (5 g of powder to 40 g of balls); (2) 400 rpm with 1:8 powder:ball ratio (5 g of powder to 40 g of balls); and (3) 700 rpm with 1:4 powder:ball ratio (10 g of powder to 40 g of balls). All three conditions (1) to (3) showed a reduction in over half the milling time in comparison to the Li2O / Mn2O3 / MnO2 analogue with no heating. With condition (1), a pure disordered rock-salt (DRS) phase formed after just 5 h of milling, compared to 20 h. With condition (2), the DRS phase formed at 40 h in comparison to 130 h. With condition (3), the DRS phase formed at 30 h in comparison to 70 h. A larger unit cell was calculated for the resulting heated precursor DRS compared to its Li2O / Mn2O3 / MnO2 precursor analogue. The electrochemical performance of these materials initially shows a reduction in the first discharge capacities; however, a profound improvement is seen in the capacity retention. The DRS, with the additional heat treatment prior to ball milling, show less capacity fade over cycle lifetime. For condition (1), the capacity retention improved from 96% to 104% for 10 cycles. For condition (2), the capacity retention improved from 72% to 96% for 10 cycles. For condition (3), the capacity retention improved from 88% to 102% for 10 cycles. The results are shown in Table 2. These results can also be reproduced for the use of Li OH (with Mn2O3 / MnO2) in the heating step. Figures 3a and 3b shows no LiOH remains after heating to a temperature of 700 °C. Ball milling at 700 rpm with a 1:8 powder:ball ratio shows a similar reduced milling time when heating with Li2CO3. Example 3 - LiMnOi / LiiMnOs Precursor Heating and Subsequent Milling With condition (1) resulting in the best electrochemical performance, it was then used to synthesise Li1.1Mno.9O2 from LiMnO2 and Li2MnO3, as alternative precursors, for comparison. Both LiMnO? and Li2MnO3 were synthesised separately and mixed together prior to ball milling, and the results were compared against the heated Li2CO3 / Mn2O3 / MnO2 precursors. Two forms of synthesising each lithium manganese oxide were investigated: (a) LiMnO2 synthesised by solid-state: 850 °C with a 5 °C ramp / min, 12 h dwell, under argon; (b) LiMnO2 purchased commercially; (c) Li2MnO3 synthesised by solid-state: 700 °C with a 5 °C / min ramp, 12 h dwell, under air; and (d) Li2MnO3 synthesised by sol-gel synthesis: dried at 400 °C, 4 h and then calcined in air at 900 °C with a 1 °C / min ramp for 12 h. Material (a) + (c) were mixed and ball milled together. Materials (b) + (d) were mixed and ball milled together. Both resultant materials ((a) + (c), and (b) + (d)) required a longer milling time of 15 h to form a pure DRS phase than the heated Li2CO3 / Mn2O3 / MnO2 precursor. The final DRS phase for (a) + (c) and (b) + (d) reported similar cubic unit cells of 4.129 A and 4.132, respectively. Both reported a higher 1st discharge capacities compared to when using the heated Li2CO3 / Mn2O3 / MnO2 precursor. However, due to poor capacity retention, the capacity recorded was much lower after 50 cycles. The results are shown in Table 2. SEM images show the morphology of the LiMnO2 / Li2MnO3 heated layered oxide material (Figure 4a), whereby there are multiple crystal phases are observed within the same particle, in comparison to LiMnO2 and Li2MnO3 synthesised separately and then mixed (Figure 4b). Figure 4a shows particles sharing more similar morphology with a fine granulometry of the primary particles, which indicates more intimate mixing. Figure 4b shows that the particles have significantly more varied sizes. Table 2 - Effect of precursor type and milling conditions on the properties ofLi1.1Mno.9O2 Precursors Heat Powder :ball ratio Milling Unit cell length (A) Capacity (mAh / g) Capacity retention (10 cycles) Speed (rpm) Time (h) Li2O MmOs MnO2 No 1:8 700 15 4.115 252 96% 1:8 400 130 4.070 320 72% 1:4 700 70 4.100 297 88% Li2CO3 Mn2O3 MnO2 Yes 1:8 700 10 4.155 236 104% 1:8 400 50 4.120 262 96% 1:4 700 30 4.152 253 102% Li2O Mn2O3 MnO2 Yes 1:8 700 10 4.156 226 109% LiMnO2 Li2MnO3 solid-state No 1:8 700 15 4.129 279 93.1% LiMnO2 Li2MnO3 sol-gel No 1:8 700 15 4.132 252 84.0% Example 4 - Li1.1Mno.9O2 obtained from different Milling Speeds Different compositions of Li1.1Mno.9O2, according to the invention, were synthesised by 5 milling at milling speeds of 400, 700 and 1000 rpm from stoichiometric amounts of Li2O, Mn20s and Mn02. All materials were synthesised via milling from the respective stoichiometric mixture of Li2O / Mn2O3 / MnO2 for each composition. The milling jars were loaded and sampled within an inert argon environment. Milling was performed with ZrO2 jars and 5 mm media in a Fritsch P7 Planetary ball mill for 75 min followed by a 15 min 10 break for x cycles depending on the milling speed used. A Lii+xMi-x02 DRS (disordered rock-salt) records a cubic unit cell where a=b=c. The unit cell is expected to vary with changes in composition due to changes in ionic radii between Li+ (60 pm), Mn3+ (63 pm) and Mn4+ (54 pm). With increasing Li, and concurrently a Mn4+ 15 increase and a Mn3+ decrease, the unit cell length is expected to decrease. However, on varying the milling speed it is possible to observe a change in the unit cell for the same composition. Thus, the milling speed affects the electrochemical performance. The results are shown in Table 3. XRD ‘time-slices’ for Li1.1Mno.9O2 milled at 700 rpm are shown in Figure 5. Table 3 - Effect of milling speed on the properties ofLi1.1Mno.9O2 Milling Speed (rpm) Milling time (h) Unit cell length (A) Capacity (mAh / g) Capacity retention (10 cycles) Capacity retention (50 cycles) 400 130 4.070 319 71.2% - 700 20 4.115 245 98.7% 82.9% 1000 5 4.134 244 94.0% 74.0% Example 5 - Li1.1Mno.9O2 obtained from Milling Speed of 700 rpm Li1.04Mn0.96O2, Li1.07Mn0.93O2, Li1.1Mno.9O2 and Li1.13Mno.87O2, according to the invention, and Li1.17Mno.83O2 and Li1.2Mno.8O2, not according to the invention, were synthesised by milling at a milling speed of 700 rpm from stoichiometric amounts of Li2O, Mn2O3 and Mn02. Sampling and re-homogenisation were performed every 5 h. Unless otherwise specified, other milling conditions generally follow the conditions used in Example 3. Table 4 summarises the results from XRD and electrochemical testing. Milling was performed for 20-25 h, until little to no precursor diffraction peaks remained. The materials milled at 700 rpm show improved crystallinity compared to the analogous synthesised at 400 rpm. Performing Rietveld refinement on the final material shows clear unit cell changes with different compositions. The experimental unit cell length correlates to the ionic radii of Li+, Mn3+ and Mn4+ components; Li1.04Mn0.96O2 with the most amount of Mn3+ (64 pm) has the largest calculated unit cell. Li1.2Mno.8O2 with the most amount of Li+ (60 pm) and Mn4+ (54 pm) has the smallest calculated unit cell. Electrochemical testing shows improved Coulombic efficiency in comparison to the 400 rpm milled DRS materials. A strong correlation is observed, for a given composition, between the unit cell and electrochemical performance whereby larger unit cells generally exhibit a lower discharge capacity but a higher capacity retention (4.148 A, 207 mAh / g, 93%), while smaller unit cells generally exhibit a higher 1st discharge capacity with lower capacity retention (4.092 A, 297 mAh / g, 61%). This assessment of the unit cell from the XRD analysis allows us to better understand and thus predict the electrochemical performance of the DRS synthesised via milling. Considering both 1st discharge capacity and capacity retention with the unit cell size we can determine the best composition at each milling condition. At a milling speed of 700 rpm, the best cathode composition is Li1.1Mno.9O2 as it provides the best balance between the electrochemical factors (Figures 6a and 6b). It has also been found that a higher content of Li+ in the LMO composition generally results in more efficient Li+ diffusion due to more favourable Li transport pathways available. Table 4 - Properties of LMO obtainedfrom a milling speed of 700 rpm Composition Milling time (h) Unit cell length (A) Crystallite Size (A) Capacity (mAh / g) Capacity retention Capacity retention (5) (10) (20) (50) Li1.04Mn0.9eO2 20 4.148 39.6 207 112 0 / / 0 114 0 / / 0 108 0 / / 0 93% Li1.07Mn0.93O2 20 4.130 46.8 220 106 0 / / 0 105 0 / / 0 99% 86% Li1.1Mno.9O2 20 4.115 50.7 245 102 0 / / 0 99% 93% 83% Li1.13Mno.87O2 25 4.107 47.9 270 95% 89% 82% 74% Li1.17Mno.83O2 25 4.101 46.9 292 87% 78% 70.1 0 / / 0 63% Li1.2Mno.sO2 25 4.092 44.8 297 89% 80% 72% 61% Example 6 - Li1.1Mno.9O2 obtained from Milling Speed of 1000 rpm Li1.04Mn0.9eO2, Lii.iMno.902 andLi1.13Mno.87O2, according to the invention, and Li1.2Mno.8O2, not according to the invention, were synthesised by milling at a milling speed of 1000 rpm from stoichiometric amounts of Li2O, MmCh and Mn02. Sampling was performed after just 5 h. Unless otherwise specified, other milling conditions generally follow the conditions used in Example 3. For Li1.1Mno.9O2, Li1.13Mno.87O2 and Li1.2Mno.8O2, an additional 2.5 h of milling was required to remove precursor diffraction peaks in the XRD. The materials milled at 1,000 rpm show improved crystallinity compared to the analogous synthesised at 400 and 700 rpm. Table 5 summarises the results from XRD and electrochemical testing. As with the compositions prepared at 700 rpm, increasing Li+ and Mn4+, and thus deceasing Mn3+, results in an overall decrease in unit cell. The electrochemistry follows a similar profile to that of the 700 rpm milled materials. The unit cell size has a strong correlation to the electrochemistry, whereby larger unit cells generally exhibit a lower discharge capacity but a higher capacity retention (4.155 A, 189 mAh / g, 106%), while smaller unit cells generally exhibit a higher 1st discharge capacity with lower capacity retention (4.117 A, 290 mAh / g, 52%). At a milling speed of 1,000 rpm, the best cathode composition is Li1.1Mno.9O2 obtained after 7.5 h of milling as it provides the best balance between the electrochemical factors (Figures 7a and 7b). Table 5 - Properties ofLMO obtainedfrom a milling speed of1000 rpm at different milling times Composition Milling time (h) Unit cell length (A) Crystallite Size (A) Capacity (mAh / g) Capacity retention Capacity retention (5) (10) (20) (50) Li1.04Mn0.9eO2 5 4.155 47.9 189 116 0 / / 0 120 0 / / 0 117 0 / / 0 106 0 / / 0 Li1.1Mno.9O2 7.5 4.147 54.0 230 102 0 / / 0 97% 90% 74% Li1.13Mno.87O2 7.5 4.138 57.3 246 96% 89% 80% 68% Li1.2Mno.sO2 7.5 4.117 57.8 290 83% 73% 65% 52%

Claims

1. A cathode composition for a lithium-ion battery, the cathode composition comprising a lithium manganese oxide of the general formula:Lii+xMni-xO2wherein x is in the range of from 0.04 to 0.13;the lithium manganese oxide has a cubic crystal structure; andthe unit cell length of the cubic crystal structure is at least 4.10 A.

2. The cathode composition of claim 1, wherein the unit cell length is at most 4.16 A, such as at most 4.13 A.

3. The cathode composition of claim 1 or claim 2, wherein the unit cell length is at least 4.11 A.

4. The cathode composition of any one of claims 1 to 3, wherein x is in the range of from 0.07 to 0.13.

6. The cathode composition of any one of claims 1 to 5, wherein the cubic crystal structure is a face-centred cubic crystal structure.

7. The cathode composition of any one of claims 1 to 6, wherein the cubic crystal structure is a disordered rock-salt crystal structure.

8. The cathode composition of any one of claims 1 to 7, wherein the lithium manganese oxide is of the formula selected from:Li1.04Mn0.9eO2,Li1.07Mn0.93O2,Li1.1Mno.9O2, orLi1.13Mno.87O2,9. An electrode comprising the cathode composition of any one of claims 1 to 8, an electroactive additive and / or a binder.

10. An electrochemical cell comprising an electrode of claim 9, an electrolyte and an anode.

11. A method of preparing a lithium manganese oxide, wherein the method comprises: heating at least one lithium source and at least one manganese source to obtain a layered oxide compound comprising a mixture of lithium and manganese, wherein the lithium and the manganese are in separate layers; andmilling the layered oxide compound to obtain a lithium manganese oxide of the general formula:Lii+xMni-x02wherein x is in the range of from 0.04 to 0.13;the lithium manganese oxide has a cubic crystal structure; andthe unit cell length of the cubic crystal structure is at least 4.10 A.

12. The method of claim 11, wherein the heating temperature is from 300 °C to 1,000 °C.

13. The method of claim 11 or claim 12, wherein the heating time is from 2 hours to 24hours.

14. The method of any one of claims 11 to 13, wherein the milling time is from 1 hours to 50 hours.

15. The method of any one of claims 11 to 14, wherein the milling speed is from 300 to 1000 rpm.

16. The method of any one of claims 11 to 15, wherein the lithium source is lithium hydroxide or lithium carbonate.

17. The method of any one of claims 11 to 16, wherein the manganese source is manganese(III) oxide and / or manganese(IV) dioxide.

18. The method of any one of claims 11 to 17, wherein the layered oxide compound is 5 LiMn(III)02 or Li2Mn(IV)O3, or a combination thereof.

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