FIELD OF THE INVENTION

The present invention relates to lithium -metal oxide nanoparticles having a general formula of xLi ₂ MnO3-(1 -x)LiNiyCo _z Mni _-y-z O2, and their preparation method, and also to their use as cathode materials for lithium ion batteries.

BACKGROUND OF THE INVENTION

Lithium-metal oxide compound of the general formula L1MO2, where M is a trivalent transition metal such as Co, Ni or/and Mn, is of interest as cathode material for lithium-ion battery. The best well-known cathode material is LiCoO ₂ , which is however relatively expensive compared to the isostructual nickel and manganese-based compounds. Efforts have therefore been made to develop less costly cathode materials, for example, by partially substituting the cobalt ions within L1C0O2 by nickel or manganese.

US 6,680, 143B2 describes a lithium metal oxide positive electrode having a general formula xLiMO ₂ (1 -x)Li ₂ MO ₃ (0<x<1 ) where M is one or more ions with an average trivalent oxidation state with at least ion being Mn or Ni, and M' is one or more ions with an average tetravalent oxidation state. The lithium metal oxide however has a relatively poor crystal structure. The lithium-metal oxide compound has the following problems: (1 ) a low coulombic efficiency of the first cycle ascribable to large irreversible capacity loss; (2) a poor rate capability caused by kinetic problems; (3) a rapid capacity fading during cycles.

Therefore, there remains a need for a lithium-metal oxide compound for use as cathode material for a lithium ion battery, which overcomes the above-mentioned shortcomings, while having high energy density, excellent thermal stability and low cost.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided lithium-metal oxide nanoparticles having a general formula xLi ₂ MnO3-(1 -x)LiNi _y Co _z Mn _-y-z O2 where 0<x<1 , 0<y<1 , 0<z<1 , wherein the nanoparticles have a primary particle size ranging from 50nm to 500nm.

According to a further aspect of the invention, there is provided a molten-salt method of preparing the lithium-metal oxide nanoparticles, comprising reacting a mixture comprising transition metal (TM) compounds of manganese (Mn), nickel (Ni) and cobalt (Co), and a lithium compound in a molten salt (MS), wherein the respective transition metal compounds are , , ,

is selected from the group consisting of lithium oxides and lithium salts.

According to a further aspect of the invention, there is provided a cathode material for a lithium ion battery comprising the lithium-metal oxide nanoparticles.

According to a further aspect of the invention, there is provided a lithium ion battery comprising the lithium-metal oxide nanoparticles as cathode materials.

The nanoparticles having the general formula of xLi ₂ MnO3-(1 -x)LiNi _y Co _z Mn _-y-z O2 according to the invention show significant improvement in specific capacity, rate capability and coulombic efficiency at the first cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the invention, taken in conjunction with the accompanying drawings, in which,

Fig. 1 is a graph showing X-ray diffraction pattern of Li1.2Mno.54Nio.13Coo.13O2 prepared according to Example 2.

Fig. 2 is a graph showing X-ray diffraction pattern of Li1.2Mno.54Nio.13Coo.13O2 prepared according to Comparative Example 1 .

Fig. 3 is SEM image of Li1.2Mno.54Nio.13Coo.13O2 prepared according to Example 2.

Fig. 4 is SEM image of Li1.2Mno.54Nio.13Coo.13O2 prepared according to Comparative Example 1 .

Fig. 5 shows the typical voltage vs. capacities curve of Li1.2Mno.54Nio.13Coo.13O2 prepared according to Example 2.

Fig. 6 shows the rate performance of Li1.2Mno.54Nio.13Coo.13O2 prepared according to

Example 2 at various rates.

DETAILED DESCRIPTION OF THE PRERRED EMBODIMENTS

According to the invention, the lithium-metal oxide nanoparticles have a general formula: xLi ₂ MnO3-(1 -x)LiNi _y Co _z Mni _-y-z O2, in which 0<x<1 , 0<y<1 , and 0<z<1 . In a preferred embodiment, . , . , _K .

embodiment, z is from 0.1 to 0.5.

The nanoparticles according to the invention have a primary particle size ranging from 50nm to 500nm, preferably from 50nm to 200nm.

The nanoparticles can be prepared by a molten-salt method according to the invention, comprising reacting a mixture including transition metal compounds of manganese (Mn), nickel (Ni) and cobalt (Co), and a lithium compound in a molten salt.

As precursors of the transition metals, the transition metal compounds are selected from the group consisting of transition metal oxides and transition metal salts. Exemplary transition metal salts include, but are not limited to, carbonate, nitrate, acetate, oxalate, hydrate and sulfate.

As lithium precursor, the lithium compound is selected from the group consisting of lithium oxides and lithium salts. Examplary lithium salts include, but are not limited to, lithium carbonate, lithium hydroxide, lithium nitrate and lithium sulfate.

The molten-salt method is based on the use of a salt with a low melting point as a reaction medium. The salt is not particularly restricted. Examples of the salt include, but are not limited to, lithium carbonate, lithium hydroxide, lithium nitrate, lithium chloride, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, barium chloride, sodium nitrate, and potassium nitrate. Preferably, potassium chloride, lithium nitrate, and lithium chloride are used as the salt.

The reaction can be carried out by mixing stoichiometric amounts of the transition metal compounds and the lithium compound, grinding the mixed compounds with an excess of the salt as described above used as a reaction medium, and heating the mixture at a temperature of from 600°C to 1000°C, under an oxygen-containing atmosphere, for example, under a flow of air or oxygen gas, for a period of from 1 h to 48h.

It has been found that the amount of the molten salt used as a reaction medium in the method may influence the formation of the nanoparticles. As the amount of the molten salt increases, the average particle size becomes small. In the method according to the invention, the salt can be suitably used in a molar ratio to the total transition metals (MS/TM) of from 2 to 32.

The obtained product is then cooled, for example by liquid nitrogen quenching. After cooling, the product is washed to remove residual molten-salt, and then dried. . - , , crystallinity can be obtained. The obtained nanoparticles show improved specific capacity, rate capability, and coulombic efficienty at the first cycle, as illustrated in Figs. 5 and 6. The nanoparticles can be advantageously used as cathode materials for lithium-ion batteries.

The following examples further illustrate the preparation of the nanoparticles by the molten-salt method according to the invention, and the characteristics of the prepared nanoparticles used as cathode material for lithium-ion battery. The examples are given by way of illustration only, and are not intended to limit the invention in any manner.

Example 1 : Preparation of xLi ₂ MnO ₃ -(1 -x)LiNi _y Co _z Mni _-y-z O ₂ powder (x=0.5; y=1/3; z=1/3)

A stoichiometric amount of lithium nitrate, Ni, Mn and Co oxides were thoroughly mixed. The mixed precursors were ground with a large excess of lithium nitrate salt (LiNO ₃ ) of which the molar ratio for the total transition metals (MS/TM) was 4. The mixture of LiNO ₃ salt and the precursors was put in an alumina crucible and heated at 950°C in air for 6 h. The powder having a particle size of 400-500nm was obtained after liquid nitrogen quenching.

Example 2: Preparation of xLi ₂ MnO ₃ -(1 -x)LiNi _y Co _z Mni _-y-z O ₂ powder (x=0.5; y=1/3; z=1/3)

The powder precursors were synthesized using a stoichiometric amount of LiOH, Ni, Mn and Co oxides which were thoroughly mixed. The mixed precursors were ground with potassium chloride salt (KCI) of which the molar ratio for the total transition metals (MS/TM) was 32. The mixture was put in an alumina crucible and heated at 800°C in air for 12 h. The powder having a particle size of 100-200nm was obtained after liquid nitrogen quenching.

Example 3: Preparation of xLi ₂ MnO ₃ -(1 -x)LiNi _y Co _z Mn-i _-y-z O ₂ powder (x=0.3; y=1/3; z=1/3)

A stoichiometric amount of lithium chloride, Ni, Mn and Co oxides were thoroughly mixed. The mixed precursors were ground with a large excess of lithium chloride salt (LiCI) of which the molar ratio for the total transition metals (MS/TM) was 4. The mixture of LiCI salt and the precursors was put in an alumina crucible and heated at 650°C in air for 12 h. The powder having a particle size of 50-200nm was obtained after cooling to room temperature.

Comparative Example 1 : Preparation of xLi ₂ MnO ₃ -(1 -x)LiNi _y Co _z Mn-i _-y-z O ₂ powder (x=0.5; y=1/3; z=1/3) by a solid-state reaction process

The powder precursors were synthesized using a stoichiometric amount of Li ₂ CO ₃ , Ni, Mn and Co oxides which were thoroughly mixed. The mixture was put in an alumina crucible and . , ». _l after liquid nitrogen quenching.

The chemical composition and crystalline phase of the powders prepared according to Example 2 and according to Comparative Example 1 were determined by X-ray diffraction (XRD) measurement. The XRD pattern of the powder prepared according to Example 2, as shown in Fig. 1 , indicates an essentially single-phase product. In contrast, the XRD pattern of the powder prepared according to Comparative Example 1 , as shown in Fig. 2, indicates a mixed-phase product. From the XRD pattern as shown in Fig. 1 , the chemical composition of the powder prepared according to Example 2 was indexed to be a-NaFeO ₂ type structure, space group R3m.

The particle morphology of the powders was observed by a scanning electron microscopy (SEM). Fig. 3 shows the SEM image of the powder prepared according to Example 2. From the SEM image, it can be seen that discrete nanoparticles having a particle size of 100-200nm were obtained by the molten-salt method according to the invention. In contrast, Fig. 4 shows the SEM image of the powder prepared according to Comparative Example 1 . From the SEM image, severe agglomeration was observed, and the powder having a particle size of from 400nm to 1 μιτι was obtained by the solid-state reaction process.

Cell assembling and electrochemical tests:

The cathode consisted of the following active materials: xLi ₂ MnO3-(1 -x)LiNiyCo _z Mni _-y-z O2 powder prepared according to Example 2, carbon black and polyvinylidene fluoride (PVDF) (with a weight ratio of 80-94 : 10-3 : 10-3). Then, a solvent, N-methyl-2-pyrrolidone (NMP), was added to these active materials, forming a slurry. The slurry was then uniformly coated on an aluminum foil, dried at 100 °C under vacuum for 10 h, pressed and cut into 12 mm cathode discs. Coin cells (CR2016) were assembled using metallic Li as the counter electrode, Celgard 2400 as the separator, and 1 mol ¹ LiPF ₆ as the electrolyte, in an Ar-filled glove box. The cycling performances of the cells were evaluated by using a Land CT2001A battery tester between 2.0V and 4.8V versus Li/Li ⁺ . The test results are shown in Figs. 5 and 6.

As shown in Figs. 5 and 6, the nanoparticles according to the invention deliver a capacity of about 300 mAh g ^"1 at room temperature at the current density of 20 mA/g with 89% coulombic efficiency at the first cycle. When discharged at 6C, the Li1.2Mno.54Nio.13Coo.13O2 nanoparticles show a capacity of 200 mAh g ^"1 .