Cathode material

Abstract

A process for producing a lithium-manganese-rich layered oxide cathode material or a lithium-manganese-rich layered oxide cathode material precursor includes co-precipitating a dissolved Li compound and a dissolved Mn salt selected from the group consisting of Mn(CH.sub.3COO).sub.2, Mn(NO.sub.3).sub.2, MnSO.sub.4, and mixtures thereof, from an aqueous solution, in the presence of a precipitator which reacts at least with the dissolved Mn salt to form a carbonate, thereby providing a precipitate which includes MnCO.sub.3 and a lithium compound as a lithium-manganese-rich layered oxide cathode material precursor. The invention extends to a lithium-manganese-rich layered oxide cathode material or a lithium-manganese-rich layered oxide cathode material precursor, to an electrochemical cell, and to methods of making and operating an electrochemical cell.

Claims

1. A process for producing a lithium-manganese-rich layered oxide cathode material or a lithium-manganese-rich layered oxide cathode material precursor, the process comprising: co-precipitating a dissolved Li compound and a dissolved Mn salt selected from a group consisting of Mn(CH.sub.3COO).sub.2, Mn(NO.sub.3).sub.2, MnSO.sub.4, and mixtures thereof, from an aqueous solution, in the presence of a precipitator which reacts at least with the dissolved Mn salt to form a carbonate, thereby providing a precipitate which includes MnCO.sub.3 and a lithium compound as a lithium-manganese-rich layered oxide cathode material precursor, wherein the dissolved Li compound is LiOH, and wherein the process comprises: dissolving the LiOH and the precipitator together in water to form a first solution, and adding a second solution with a dissolved Mn salt selected from a group consisting of Mn(CH.sub.3COO).sub.2, Mn(NO.sub.3).sub.2, MnSO.sub.4, and mixtures thereof, and with at least one further dissolved salt, selected from a further associated group consisting of an acetate, a nitrate, a sulfate, and mixtures thereof, of a metal M selected from a further group consisting of Ni, Co, Fe, Al, Mg, Ti, and two or more of these, to the first solution to form said aqueous solution and to effect co-precipitation.

2. The process according to claim 1, wherein the process includes calcining the precipitate to convert any carbonate material to oxide material, thereby providing a calcined material; and annealing the calcined material to provide a lithium-manganese-rich layered oxide cathode material.

3. The process according to claim 2, wherein the precipitator is selected from a group consisting of urea, (NH.sub.4).sub.2CO.sub.3, NH.sub.4HCO.sub.3, and mixtures thereof.

4. The process according to claim 1, wherein the precipitator is selected from a group consisting of urea, (NH.sub.4).sub.2CO.sub.3, NH.sub.4HCO.sub.3, and mixtures thereof.

5. The process according to claim 1, wherein the aqueous solution, during co-precipitation of the dissolved Li compound and the dissolved Mn salt, has a pH greater than 7.

6. The process according to claim 1, wherein the lithium-manganese-rich layered oxide cathode material is Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2.

7. The process according to claim 1, wherein the aqueous solution, during co-precipitation of the dissolved Li compound and the dissolved Mn salt, has a pH greater than 9.

8. The process according to claim 1, wherein the aqueous solution, during co-precipitation of the dissolved Li compound and the dissolved Mn salt, has a pH between 9.5 and 10.5.

Description

(1) The invention is now described in more detail with reference to the following example and the accompanying diagrammatic drawings in which

(2) FIG. 1 shows powder XRD patterns of L.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2 cathode materials prepared in accordance with the process of the invention from four aqueous solutions each with a different pH;

(3) FIG. 2 shows first charge/discharge voltage profiles of Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2 cathode materials prepared in accordance with the process of the invention from four aqueous solutions each with a different pH, charged between 2.0 V and 4.8 V at a rate of 0.1 C (20 mA/g current density);

(4) FIG. 3 shows graphs of rate performances of Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2 cathode materials prepared in accordance with the process of the invention from four aqueous solutions each with a different pH, discharged at 20 mA/g, 50 mA/g and 100 mA/g in the voltage range 2.0 V to 4.8 V for 10 cycles and further cycling performance at 50 mA/g for 50 cycles; and

(5) FIG. 4 shows graphs of cycling efficiency over 50 cycles for Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2 cathode materials prepared in accordance with the process of the invention from four aqueous solutions each with a different pH, discharged at 50 mA/g.

EXAMPLE

(6) A facile one-pot co-precipitation synthesis method or process in accordance with the invention was employed to produce the lithium-manganese-rich metal oxide cathode material Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2. Stoichiometric ratios of Li:Mn:Ni were kept fixed for the desired Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2 layered material, while urea as a precipitator was varied from 1.0 to 1.8 times the required stoichiometric ratio, resulting in four batches of Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2 produced from aqueous solutions that varied in pH, respectively of pH 9.0 (1.0×the stoichiometric urea ratio), pH 9.5 (1.2×the stoichiometric urea ratio), pH 10.0 (1.6×the stoichiometric urea ratio) and pH 10.5 (1.8×the stoichiometric urea ratio). The synthesis procedure involved dissolving appropriate stoichiometric amounts of manganese acetate tetrahydrate [Mn(CH.sub.3COO).sub.2.4H.sub.2O] and nickel acetate tetrahydrate [Ni(CH.sub.3COO).sub.2.4H.sub.2O] in distilled water, in a first beaker. Lithium hydroxide [LiOH.H.sub.2O] and urea [CO(NH.sub.2).sub.2] were dissolved, separately from the Mn(CH.sub.3COO).sub.2.4H.sub.2O and Ni(CH.sub.3COO).sub.2.4H.sub.2O, in deionized water in a second beaker to form a base solution.

(7) The solutions in both beakers were stirred at 1000 rpm at 70° C. until completely dissolved (dissolved in 10 minutes). The metal ions acetate solution in the first beaker was then introduced dropwise to the base solution in the second beaker while stirring at 1000 rpm at 70° C. The aqueous solution in the second beaker changed in colour gradually to dark brown as a suspension with a suspended precipitate formed. The suspension was then left stirring at 70° C. for a period of time, whereafter the water was evaporated at 100° C. A fine brown/reddish powder was obtained, which was calcined in a first heating step at about 600° C. for 2 hrs (with a temperature increase rate of 5° C. min.sup.−1) in an air flowing furnace to burn off all the acetates/organics. A 2.sup.nd heating step, i.e. an annealing step, was done at a higher temperature of about 900° C., for a period of about 12 hours, to promote the formation of the final desired crystalline phase.

(8) Physicochemical information was attained using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), the Brunauer-Emmett-Teller (BET) method and a Maccor battery tester (for electrochemistry information). The physical properties of the four produced materials did not differ much according to XRD, SEM and BET (see FIG. 1, which shows the XRD patterns for the four materials). The four lithium-manganese-rich metal oxides produced were similar to those reported in literature. As seen in FIG. 1, the XRD patterns for the four Li.sub.1.2Mn.sub.0.6Ni.sub.0.2O.sub.2 oxides are all similar and are indexed to the layered structure that was reported in literature. The material is in each instance a mixture of a cubic and a monoclinic crystal structure. Rietveld refinement shows that their compositions are 90.1%, 93.4%, 94.2% and 94.3% monoclinic for material produced from aqueous solutions having a pH 9, a pH 10.5, a pH 9.5 and a pH 10.0 respectively as shown in FIG. 1.

(9) However, the electrochemistry of these four materials differed significantly.

(10) The four cathode materials were built into coin-cells and displayed exceptional electrochemical performance in delivering more than 200 mA h g.sup.−1 at a constant current density of 20 mA/g in the voltage range of 2.0 V-4.8 V.

(11) Cathode material obtained by co-precipitation from an aqueous solution with a pH of 10.0 delivered an initial high discharge capacity of 266 mA h g.sup.−1 at 20 mA/g current density and maintained a discharge capacity of more than 220 mA h g.sup.−1 at 50 mA/g after 50 cycles, as shown in FIG. 3.

(12) All four cathode materials showed good stability at starting high potentials of 4.8 V. These materials also showed high discharge capacities at potentials >4.5V at 20 mA/g. Upon the first discharge cycle, all four cathode materials suffered capacity loss as expected for Li.sub.2MnO.sub.3 based composites and was similar to the results reported by other groups studying these types of composite materials. The large capacity loss during the initial cycle was reported by others to be associated with the complete loss of Li.sub.2O during activation of the Li.sub.2MnO.sub.3 component. All electrodes exhibited a sloping voltage profile below 4.4 V, followed by a relatively long plateau around 4.5 V during the first charge process. The sloping voltage profile can be attributed to the oxidation of Ni.sup.2+ to Ni.sup.4+ ions in the LiMn.sub.0.5Ni.sub.0.5O.sub.2 component and the 4.5 V plateau voltage profile arises from the simultaneous irreversible removal of Li.sup.+ ions and oxygen (Li.sub.2O).

(13) The process of the invention produced materials with good performance; the results showed that the cathode material made from an aqueous solution wih a pH of 10.0 had the highest initial charge and discharge capacity of 373 mA h g.sup.−1 and 266 mA h g.sup.−1 respectively as shown in FIG. 2.

(14) FIG. 3 shows that the cells made with the various cathode materials all achieved more than 200 mA h g.sup.−1. The cells also showed good capacity retention properties with about 95% capacity retention at 20 mA/g after 30 cycles (See FIG. 4). From FIG. 3 it is evident that capacity decreases steadily with increasing current densities. Materials produced from an aqueous solution with a pH of 9.5 or a pH of 10.0 further delivered >200 mA h g.sup.−1 capacity even when charged with 100 mA/g current density and the cathode materials delivered stable capacity even at higher current densities.

(15) All four homogenous lithium-manganese-rich metal oxide cathode materials produced displayed steady cycling at high potentials and good rate performance, suggesting that they are promising candidates for use in high capacity lithium-ion battery applications.

(16) The process of the invention, which can be described as a facile one-pot co-precipitation synthesis method, can be used for producing a lithium-manganese-rich layered oxide cathode material or a lithium-manganese-rich layered oxide cathode material precursor. Advantageously, the process is fast and uses an inexpensive precipitator such as urea, which can be used in various ratios to produce cathode materials with different electrochemical properties. In at least one embodiment of the invention, as illustrated, the need for filtration and washing is avoided, which can potentially save water.