LMO CATHODE COMPOSITION

Abstract

A cathode composition for a lithium-ion cell or battery of the general formula: Li.sub.1+xMn.sub.1−xO.sub.2, wherein the composition is in the form of a single phase having a rock salt crystal structure such that an x-ray diffraction pattern of the composition has an absence of peaks below a 20 value of 35; and the value of x is greater than 0, and equal to or less than 0.3. The compound is also formulated into a positive electrode, or cathode, for use in an electrochemical cell.

Claims

1. A cathode composition for a lithium-ion battery of the general formula:
Li.sub.1+xMn.sub.1-xO.sub.2 wherein the composition is 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 Kα radiation source has an absence of peaks below a 20 value of 35; and the value of x is greater than 0, and equal to or less than 0.3.

2. The cathode composition according to claim 1, wherein the x-ray diffraction pattern of the composition has an absence of a peak at a 2θ value of 18.

3. The cathode composition according to claim 1, wherein the single phase crystal structure is absent of any spinel or layered structures.

4. The cathode composition according to claim 4, wherein the single phase crystal structure does not exhibit either a R3(bar)m and/or a C2/m space group.

5. The cathode composition according to claim 1, wherein the value of x is equal to or greater than 0.1.

6. The cathode composition according to claim 1, wherein the value of x is equal to or greater than 0.17.

7. The cathode composition according to claim 1, wherein the value of x is equal to or greater than 0.2.

8. The cathode composition according to claim 1, wherein the value of x is equal to or greater than 0.2 and equal to or less than 0.3.

9. The cathode compound according to claim 1, wherein the value of x is equal to or greater than 0.1 and equal to or less than 0.2.

10. The cathode composition according to claim 1, wherein the value of x is equal to 0.2.

11. The cathode composition according to claim 1, wherein the single phase crystal structure exhibits the Fm3(bar)m space group.

12. The cathode composition according to claim 1, wherein the composition is expressed as the general formula:
(a)LiMnO.sub.2(1-a)Li.sub.2MnO3 wherein two precursors are provided in proportions defined by a, and a has a value in the range greater than 0, and less than 1; and the precursors are mixed by a ball milling process.

13. The composition according to claim 12, wherein the value of a is equal or greater than 0.15 and equal to or less than 0.7.

14. The composition according to claim 12, wherein the value of a is equal or greater than 0.15 and equal to or less than 0.4.

15. The composition according to claim 12, wherein the composition is 0.4LiMnO.sub.2.0.6Li.sub.2MnO.sub.3.

16. An electrode comprising the cathode composition according to claim 1.

17. The electrode according to claim 16, wherein the electrode comprises electroactive additives and/or a binder.

18. The electrode according to claim 17, wherein the electroactive additive is selected from at least one of carbon or carbon black.

19. The electrode according to claim 17, wherein the polymeric binder is selected from at least one of PVDF, PTFE, NaCMC or NaAlginate.

20. An electrochemical cell comprising a cathode according to claim 16, an electrolyte, and an anode.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0016] In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying Figures, in which:

[0017] FIG. 1 shows powder X-ray Diffraction patterns of various Li—Mn—O compounds that exist, with their space groups and lattice parameters;

[0018] FIG. 2 shows powder X-ray Diffraction patterns of the synthesised cathode compositions Li.sub.1.2Mn.sub.0.8O.sub.2 (or 0.4LiMnO.sub.2.0.6Li.sub.2MnO.sub.3) compound in Example 1;

[0019] FIGS. 3a and b shows first cycle galvanostatic load curves for the first cycle of Li.sub.1.2Mn.sub.0.8O.sub.2 and Li.sub.1.1Mn.sub.0.9O.sub.2 at 45° C. at C/10 respectively, and FIGS. 3c and d show evolution of the charge and discharge capacity as a function of cycle for Li.sub.1.2Mn.sub.0.8O.sub.2 and Li.sub.1.1Mn.sub.0.9O.sub.2 respectively;

[0020] FIG. 4 shows preparation of Li.sub.1.2Mn.sub.0.8O.sub.2 with a conventional planetary ball mill at 400 rpm with ZrO.sub.2 balls (upper line) compared to the high energy ball mill with 700 rpm and WC balls (lower line);

[0021] FIG. 5 shows preparation of Li.sub.1.2Mn.sub.0.8O.sub.2 with starting materials Li.sub.2MnO.sub.3, Li.sub.2O.sub.2 and Mn.sub.2O.sub.3 (upper line) compared to starting materials Li.sub.2MnO.sub.3 and LiMnO.sub.2 (lower line);

[0022] FIG. 6 shows Electrochemical data of Li.sub.1.2Mn.sub.0.8O.sub.2 prepared from Li.sub.2MnO.sub.3, Li.sub.2O.sub.2 and Mn.sub.2O.sub.3 with C/10 charge, and C/5 discharge; and

[0023] FIG. 7 shows X-ray diffraction patterns of the precursors (lower line), ball milled for 40 min (second to lower line), ball milled for 5 h (second to higher line) and ball milled for 10 h (higher line).

DETAILED DESCRIPTION

[0024] The present invention will now be illustrated with reference to the following examples.

Example 1—Synthesis of the Lithium Rich Manganese Oxide Cathode Compositions

[0025] Material comprising LiMnO.sub.2 and Li.sub.2MnO.sub.3 precursors were mixed in different molar proportions in accordance with Table 1 using WC jars and balls. All materials were handled at all times under inert atmosphere (in an Argon filled glovebox) and never exposed to ambient atmosphere, ie protected against moisture and oxygen at all times. A planetary ball milling (Fritsch Planetary Micro Mill PULVERISETTE 7 premium line which can deliver energy which are approximately 150% above that which can be achieved through conventional milling) was employed and the milling was performed at a speed rate of 700 rpm for 10 minutes, following 30 minutes break. Phase purity was assessed after repeating this milling and resting cycle for at least 30 times, i.e. for a total milling time of at least 5 hours. However, it is possible that less milling times are necessary to achieve phase purity. The phase transformation is assessed by X-ray diffraction. If the phase transformation is not complete, the same program is repeated, and so on.

TABLE-US-00001 LiMnO.sub.2 Li.sub.2MnO.sub.3 Resulting Composition Stoichiometry 0.7 0.3 0.7LiMnO.sub.2•0.3Li.sub.2MnO.sub.3 Li.sub.1.1Mn.sub.0.9O.sub.3 0.6 0.4 0.6LiMnO.sub.2•0.4Li.sub.2MnO.sub.3 Li.sub.1.13Mn.sub.0.87O.sub.2 0.5 0.5 0.5LiMnO.sub.2•0.5Li.sub.2MnO.sub.3 Li.sub.1.17Mn.sub.0.83O.sub.2 0.4 0.6 0.4LiMnO.sub.2•0.6Li.sub.2MnO.sub.3 Li.sub.1.2Mn.sub.0.8O.sub.2 0.3 0.7 0.3LiMnO.sub.2•0.7Li.sub.2MnO.sub.3 Li.sub.1.23Mn.sub.0.77O.sub.2 0.2 0.8 0.2LiMnO.sub.2•0.8Li.sub.2MnO.sub.3 Li.sub.1.27Mn.sub.0.73O.sub.2 0.15 0.85 0.15LiMnO.sub.2•0.85Li.sub.2MnO.sub.3 Li.sub.1.28Mn.sub.0.72O.sub.2

[0026] Alternate starting materials can be used here including but not limited to Mn.sub.2O.sub.3, MnO.sub.2, Li.sub.2O, Li.sub.2O.sub.2, Mn.sub.2O.sub.4, LiMn.sub.2O.sub.4. An addition route for the preparation of Li.sub.1.2Mn.sub.0.8O.sub.2 was tried with Li.sub.2O, Mn2O3 and MnO2 which resulted in the same phase as shown in FIG. 2.

[0027] Alternatively a conventional planetary ball mill was used, the Retsch PM 100 mill. Here both mills were used to prepare Li.sub.1.2Mn.sub.0.8O.sub.2 at a milling speed of 400 rpm with ZrO.sub.2 balls. Here it can be clearly understood that the lower density of the milling media and the rotation rate will result in significantly lower energy collisions. The conditions employed by both mills were successful in obtaining the disordered rock salt phase (FIG. 4) with a composition of Li.sub.1.2Mn.sub.0.8O.sub.2 indicating that preparation at high energies is not the only route to cathode composition production.

[0028] Alternatively, mechanofusion or conventional Physical Vapour Deposition techniques can be considered to prepare these cathode compositions.

Example 2—Structural Analysis and Characterisation of the Lithium Rich Manganese Oxide Cathode Compositions

[0029] The materials according to Example 1 were examined with Powder X-Ray Diffraction (PXRD) which was carried out utilising a Panalytical Aeris benchtop XRD with a Cu Kα Radiation. The range of measurement was 10-90° 2 theta.

[0030] FIG. 2 shows a representative Powder X-ray Diffraction pattern of the synthesised compositions. These are characteristic of a cation disordered rock salt structure. All of the patterns appear to show the broad major peaks consistent with a face centred cubic lattice with the Fm3(bar)m space group, as shown in FIG. 2. It should be noted that broad peaks are displayed due to the ball milling process which without wishing to be bound by theory results in a cathode composition with a small crystallite size. There is no evidence for the presence of the layered precursors which had a layered LiMnO.sub.2 or Li.sub.2MnO.sub.3 structures with the space group R3(bar)m or C2/m, which would indicate that the ball-milling synthesis has converted the material of the precursors into the cathode composition of the present inventions. No presence of extra peaks due to impurities was observed. The example XRD in FIG. 2 shown for Li.sub.1.2Mn.sub.0.8O.sub.2 was indicative of all samples with a slight shifting of the peak positions in accordance with changes to the bulk lattice parameters as a result of the changing composition. It should also be noted that no crystalline phases for other common Li.sub.pMn.sub.fO.sub.q (database patterns shown in FIG. 1 for comparison) can be seen in the final product. FIG. 7 also shows how the reaction proceeds as the initial precursors are slowly removed on continual milling as the disordered rock salt phase becomes the dominant feature.

Example 3—Electrochemical Analysis of the Lithium Rich Manganese Oxide Cathode Compositions

[0031] The cathode compositions according to Example 1 were characterised electrochemically through galvanostatic cycling performed with a BioLogic BCS series potentiostats. All the samples were assembled as powdered cathodes into Swagelok type cells with a metallic lithium counter/reference electrode and cycled between 2 and 4.8 V vs. Li.sup.+/Li at a current rate of C/10 as defined by a capacity of 300 mAh/g. The electrolyte employed was LP40 (a 1M solution of LiPF.sub.6 in 1:1 w/w ratio of EC; DEC).

[0032] FIGS. 3a-d shows the potential curves during the charge and subsequent discharge of the first cycle for compositions 0.7LiMnO.sub.2.0.3Li.sub.2MnO.sub.3 and 0.4LiMnO.sub.2.0.6Li.sub.2MnO.sub.3 according to Example 1. Both samples present high discharge capacities above 200 mAh/g at C/10. The values of the first discharge capacities for all materials which have been prepared are detailed in the Table below.

[0033] The 0.4LiMnO.sub.2.0.6Li.sub.2MnO.sub.3 cathode composition exhibits a sloping region at the beginning of charge, until ca. 4 V vs Li+/Li and a high potential plateau centred at around 4.2 V vs Li+/Li that appears to be irreversible on the first discharge. This general feature could be considered consistent for all the prepared materials with Li>1.1 per formula unit with the length of the plateau correlating to the amount of lithium in the material: the more lithium is present the longer the plateau.

[0034] The 0.7LiMnO.sub.2.0.3Li.sub.2MnO.sub.3 composition exhibits a different first charge. A long sloping region is observed up to the high potential cut-off of 4.8 V vs Li+/Li. No potential plateau is observed, less irreversibility on discharge is seen and therefore a higher first cycle coulombic efficiency.

TABLE-US-00002 First charge First discharge Coulombic capacity capacity efficiency at Phase purity (mAh/g) (mAh/g) the first cycle 0.7LiMnO2•0.3Li2MnO3 No crystalline phase 230 209 90.8% detectable except rock-salt structure (Fm3(bar)m) 0.6LiMnO2•0.4Li2MnO3 No crystalline phase 279.5 249.6 89.3% detectable except rock-salt structure (Fm3(bar)m) 0.5LiMnO2•0.5Li2MnO3 No crystalline phase 281 250 88.9% detectable except rock-salt structure (Fm3(bar)m) 0.4LiMnO2•0.6Li2MnO3 No crystalline phase 315.9 245.3 77.7% detectable except rock-salt structure (Fm3(bar)m) 0.3LiMnO2•0.7Li2MnO3 No crystalline phase 317.8 264 83.1% detectable except rock-salt structure (Fm3(bar)m) 0.2LiMnO2•0.8Li2MnO3 No crystalline phase 346.2 239.6 69.2 detectable except rock-salt structure (Fm3(bar)m) 0.15LiMnO2•0.85Li2MnO3 No crystalline phase 376.9 242.9 64.4% detectable except rock-salt structure (Fm3(bar)m)