LI-RICH TRANSITION METAL OXIDES MATERIAL

Abstract

Li-rich transition metal oxides material, useful as cathode for Li-ion batteries and having general formula Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13-x-yAl.sub.yO.sub.2; where: 0.01≤x≤0.1; 0.01≤y≤0.1; and 0.03≤x+y<0.13.

Claims

1. Li-rich transition metal oxides material having general formula
Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2; where: 0.01≤x≤0.1; 0.01≤y≤0.1; and 0.03≤x+y<0.13.

2. The Li-rich transition metal oxides material according to claim 1, wherein 0.03≤x≤0.08.

3. The Li-rich transition metal oxides material according to claim 1, wherein 0.03≤y≤0.05.

4. The Li-rich transition metal oxides material according to claim 1, wherein the Li-rich transition metal oxides material has a general formula selected from the group consisting of Li.sub.1.23Mn.sub.0.54Ni.sub.0.13Co.sub.0.07Al.sub.0.03O.sub.2, Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.04Al.sub.0.03O.sub.2, Li.sub.1.28Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.03O.sub.2 and Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.05O.sub.2.

5. A cathode for Li-ion batteries, comprising a Li-rich transition metal oxides material according to claim 1.

6. A sol-gel process for preparing a Li-rich transition metal oxides material according to claim 1; said sol-gel process being characterized by comprising the following steps: a) mixing metal precursors with a Li excess in a solvent; b) adding a gelling agent to obtain a gel as product keeping a pH>7; c) drying a resulting solution at a temperature lower than 100° C. until a viscous mass is obtained; d) thermally treating the obtained product at a temperature between 300 and 500° C. for at least an hour; and e) annealing the resulting product of step (d) at a temperature between 800 and 1000° C.

7. The process according to claim 6, wherein the solvent is water.

8. The process according to claim 6, wherein the metal precursors are selected from the group consisting of acetates, nitrates, sulfates, carbonates, and oxides.

9. The process according to claim 6, wherein the gelling agent is selected from the group consisting of oxalic acid ascorbic acid and citric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Hereinafter there is a description of some embodiments of the invention, by mere way of explanatory and non-limiting examples, with reference to the accompanying drawings, wherein:

[0028] FIG. 1 shows XRD Patterns of Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 as function of content of cobalt. Hexagonal LiMO.sub.2 and monoclinic Li.sub.2MnO.sub.3 have been reported as reference.

[0029] FIG. 2 shows four HR-SEM images of Li.sub.1.23Mn.sub.0.54Ni.sub.0.13Co.sub.0.07Al.sub.0.03O.sub.2 at different magnifications. a) ×10.000; b) ×25.000; c) ×50.000; d) ×100.000.

[0030] FIG. 3 shows four EDX maps of Mn, Ni, Co and Al of Li.sub.1.23Mn.sub.0.54Ni.sub.0.13Co.sub.0.07Al.sub.0.03O.sub.2.

[0031] FIG. 4 shows a first charge/discharge voltage profile at C-rate=C/10 of Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 where x=0 and y varies.

[0032] FIG. 5 shows specific capacity vs cycle number plot obtained in lithium cell (rate C/10) of materials of FIG. 4.

[0033] FIG. 6 shows a) voltage profile and b) specific capacity vs cycle number plot obtained in lithium cell (rate C/10) for three Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 materials.

[0034] FIG. 7 shows a) voltage profile and b) specific capacity vs cycle number plot obtained in lithium cell (rate C/10) for Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.05O.sub.2.

[0035] FIG. 8 shows rate capability tests of three Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 materials.

[0036] FIG. 9 shows specific capacity vs cycle number obtained in lithium cell under galvanostatic cycling (rate 1C) of four Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 materials.

BEST MODE FOR CARRYING OUT THE INVENTION

[0037] Materials according to the general formula Li.sub.2.2+xMn.sub.0.54Ni.sub.0.23Co.sub.0.13−x−yAl.sub.yO.sub.2 were prepared by the synthesis process below:

[0038] Stoichiometric amounts of LiCH.sub.3COO.Math.2H.sub.2O (lithium acetate dihydrate, Sigma-Aldrich), Mn(CH.sub.3COO).sub.2.4H.sub.2O (manganese (II) acetate tetrahydrate, Sigma-Aldrich), Ni(CH.sub.3Co.sub.0).sub.2.4H.sub.2O (Nickel (II) acetate tetrahydrate, Sigma-Aldrich), Co(CH.sub.3Co.sub.0).sub.2.Math.4H.sub.2O (Cobalt (II) acetate tetrahydrate, Sigma-Aldrich) and (H.sub.0).sub.2Al(CH.sub.3Co.sub.0) (Aluminum acetate dibasic, Sigma-Aldrich) were dissolved in ultrapure water. 5 wt. % excess of lithium acetate was included in the synthesis in order to compensate the lithium loss during the high heating process.

[0039] An aqueous solution of C.sub.2H.sub.2O.sub.4 (oxalic acid, Sigma-Aldrich) around 0.38 M, acting as chelating agent, was added to the metal-acetate solution, in order to have chelating agent/metals molar ratio of 1.5/1 and left under stirring.

[0040] The pH in the mixture was kept at 8 by the addition of ammonia solution (NH.sub.4OH 32%, Sigma-Aldrich) dropwise.

[0041] Then, the solution was dried slowly by heating at 80° C. and continuously stirred until a viscous mass was obtained. The obtained gel was finally completely dried at 200° C. under vacuum for 18 hours.

[0042] To obtain the final product, two thermal treatments have been carried out in a muffle furnace in air. The resulted powder, finely milled, was pre-heated in the furnace at 450° C. for 2 hours (temperature ramp: 10° C./min). The product was recovered from furnace, milled by mortar and pestle and re-heated 12 hours at 900° C. with a temperature ramp of 10° C./min and setting a cooling rate of 0.25° C./min.

[0043] Finally, the stoichiometry of the synthesized compounds has been verified by elemental analysis using ICP-OES (Inductively coupled plasma-optical emission spectrometry).

[0044] The above described materials can be prepared by other techniques different from sol-gel, e.g. co-precipitation, solid-state molten salts and hydrothermal synthesis.

[0045] Qualitative Analysis—XRD

[0046] FIG. 1 shows the XRD patterns to compare the crystal structure of the material before and after the Al- and Li-co-doping. XRD patterns were collected using a Malvern PANalytical Empyrean with Cu Kα radiation source in the range of 2Θ degree of 10°-90° with a step of 0.026°. We can observe all the three compounds resulted crystalline and the diffraction lines appear sharp and intense. No differences can be highlighted; in fact, for all the three compounds, the main phase consists of the hexagonal phase (S.G.: R-3m) of LiMO.sub.2. Additional peaks are also visible between 2Θ of 20°-30°, corresponding to the monoclinic phase with space group C2/m, due to LiMn.sub.6 cation ordering in the transition metal layers of Li.sub.2MnO.sub.3.

[0047] Morphology

[0048] The morphology and homogeneity of all materials have been evaluated by HRSEM. FIG. 2 reports the micrographs obtained for Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 where x=0.03 and y=0.03. From images, it is possible to observe that the sample consists of agglomerates of sub-micron particles with spherical shape.

[0049] From EDX maps in FIG. 3, all elements resulted uniformly distributed and there is no presence of segregation of other phase.

[0050] From a structural point of view, the materials of the present invention can be described as modified Li-rich layered metal oxides, wherein the content of Cobalt has been highly reduced by co-doping with lithium and aluminum.

[0051] Similar to their lead compound (Li.sub.1.2Ni.sub.0.13Co.sub.0.13Mn.sub.0.54O.sub.2), Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 maintain the layered structure composed by rhombohedral Li(TM)O.sub.2 phase (TM=transition metal) belonging to space group R-3m and the monoclinic Li.sub.2MnO.sub.3 phase belonging to space group C2/m. This layered structure allows intercalation and de-intercalation of Lithium ions during the charge and discharge process. In particular, it was found that the simultaneous replacement of Co.sup.2+/.sup.3+ mixed centers in the layered structure with balanced amounts of Al.sup.3+ and Li.sup.+ positively affects the electronic disorder and stabilizes the performance compared to Co-rich phases.

[0052] Electrochemical Tests

[0053] Battery Assembly

[0054] Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 with respectively x=0-0.08, y=0.03-0.05, were used for lab tests as electrode in lithium cell. The electrode formulation consists of the active material, a conductive additive (i.e. carbon black) and polyvinylidine fluoride (PVDF) as binder in a weight ratio of 80:10:10. The powders were initially mixed and then 1-methyl-2-pyrrolidinone (NMP) was added dropwise to form a slurry. The obtained slurry was casted onto an Aluminum current collector by the use of a doctor blade and dried at 110° C. overnight. The electrode was assembled in a coin cell 2032 facing with a metallic lithium foil. A Whatman GF/D embedded with LP30 electrolyte was used as separator of the two electrodes. The compounds were tested with different electrode formulations (i.e. carbon black, carbon nanotubes and graphene) as well as at different current density.

[0055] Effect of Aluminum (FIGS. 4-5)

[0056] Here, the comparison between Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 as function of content of Aluminum (i.e. x=0 and y=0, 0.03, 0.06, 0.09) was reported. The cells were tested through galvanostatic discharge/recharge cycles using a current density of 37.7 mA/g (C/10) in a potential range of 2-4.8V. The specific capacity decreased when the content of aluminum increased.

[0057] However, when y=0.03 the best electrochemical performances were obtained with the small quantity of Aluminum, i.e. y=0.03. The specific discharge capacity in the first cycle was over 240 mAh/g with a capacity retention of 67% after 200 cycles.

[0058] Li Effect (FIG. 6)

[0059] The effect of content of lithium have been evaluated changing x=0, 0.03, 0.06, 0.08 in Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.10−xAl.sub.0.03O.sub.2.

[0060] FIG. 6 reports the electrochemical behaviour obtained in lithium cell under galvanostatic condition in the voltage range of 2-4.8V and current density of 37.7 mA/g.

[0061] Despite overlithiated samples present lower capacity values, they were able to achieve more than 200 mAh/g for prolonged cycling.

[0062] Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.05O.sub.2 (FIG. 7)

[0063] Here the electrochemical performances of Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2, where x=0.06 and y=0.05 are reported. The cells were tested through galvanostatic discharge/recharge cycling using a current density of 37.7 mA/g (C/10) in a potential range of 2-4.8V and the results are shown in FIG. 7. Similar to the other overlithiated samples, Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.05O.sub.2 specific capacity remained around 200 mAh/g and it remained stable upon cycling.

[0064] Rate Capability (FIG. 8)

[0065] The effect of overlithiation was evaluated in lithium cells also in terms of rate capability. The tests were performed changing the current density from 37.7 mA/g (C/10) up to 754 mA/g (2C). The rate capability tests of Li.sub.1.2+xMn.sub.0.54Ni.sub.0.13Co.sub.0.13−x−yAl.sub.yO.sub.2 with x=0.03 and 0.06 and y=0.03-0.05 are here reported. As evident in the figure, all the samples were able to sustain high current, achieving capacity around 120 mAh/g at 2C. Furthermore, when the current decreased again to the initial value, capacity rise to the initial values of more than 200 mAh/g.

[0066] Cycling Test (FIG. 9)

[0067] Prolonged cycling tests of overlithiated Li.sub.1.23Mn.sub.0.54Ni.sub.0.13Co.sub.0.07Al.sub.0.03O.sub.2, Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.04Al.sub.0.03O.sub.2, Li.sub.1.28Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.03O.sub.2 and Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.05O.sub.2 are here reported. The cells were tested through galvanostatic discharge/recharge cycling using a current density of 377 mA/g (1C) in a potential range of 2-4.8V and the results are reported in FIG. 9.

[0068] The discharge capacity achieved are 162.9, 157, 143.2 and 152.5 mAh/g for the first cycle and are 87, 92.9, 89.8 and 85.4 after 500 cycles, respectively for Li.sub.1.23Mn.sub.0.54Ni.sub.0.13Co.sub.0.07Al.sub.0.03O.sub.2, Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.04Al.sub.0.03O.sub.2, Li.sub.1.28Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.03O.sub.2 and Li.sub.1.26Mn.sub.0.54Ni.sub.0.13Co.sub.0.02Al.sub.0.05O.sub.2 corresponding to a capacity retention 53%, 59%, 63% and 56%, respectively.

[0069] The inventors of the present inventions have surprisingly found that increasing the amount of lithium and aluminum simultaneously, it is possible to drastically decrease the amount of cobalt and maintain, in the same time, high performance in terms of specific capacity, cycling stability and rate capability. This effect can be immediately evident from a comparison between data of FIG. 4 and data of FIGS. 6 and 7. In fact, FIG. 4 shows that when only the amount of aluminum is increased (no Li), the performance in terms of specific capacity progressively get worse by decreasing the amount of cobalt. Differently, FIGS. 6 and 7 show that it is possible to decrease the amount of Cobalt guaranteeing, in the same time, high performances in terms of specific capacity, cycling stability and rate capability if the amounts of Lithium and Aluminum have been increased.

[0070] As can be clearly evident to a skilled man of the art, the materials of the invention result very promising as cathode for lithium-ion batteries. Indeed, the above materials of the invention, even if comprise low amount of Cobalt, are able to exchange approximately 200 mAh/g in lithium cell for 200 cycles using a current density of ≈40 mA/g (C/10) in a potential range of 2-4.8V. The discharge capacity is about 120 mAh/g at 1C (≈400 mA/g) with a capacity retention of 70% after 500 cycles.

[0071] To sum up, the material of the invention shows a reduced amount of cobalt respect to other existing Li-rich materials and, in the same time, guarantees high performances in terms of specific capacity, cycling stability and rate capability.