WO2010051747A1 - Method of preparing cathode active material and method of forming lithium ion secondary battery - Google Patents

Abstract

A method of preparing a cathode active material having a mixed crystal is provided. The method comprises the following steps: providing a first crystalline substance having one or more members with general formulas Li_xxM_yy(XO₄)_zz, LiMXO₅, LiMXO₆ and LiMX₂O₇; providing a second crystalline substance having one or more members with general formulas LiD_cO₂, Li_iNi_1-d-eCo_dMn_eO₂, LiNi_1-f-gCo_fAl_gO₂, Li_xNi_1-yCoO₂ and Li_mMn_2-nE_nO_j and sintering the two crystalline substances to provide the mixed crystal. According to the cathode active material prepared by the method, the electrical property is superior, which enhances the duration and performance of a lithium secondary battery. Further, a method of forming a lithium ion secondary battery comprising the same is provided.

Description

METHOD OF PREPARING CATHODE ACTIVE MATERIAL AND MEHTOD OF FORMING LITHIUM ION

SECONDARY BATTERY

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Chinese Patent Application No. 200810173688.8, filed on November 7, 2008, which is hereby incorporated by reference in its entirety.

This application claims priority to Chinese Patent Application No. 200810173689.2, filed on November 7, 2008, which is hereby incorporated by reference in its entirety.

This application claims priority to Chinese Patent Application No. 200810189234.X, filed on December 26, 2008, which is hereby incorporated by reference in its entirety. This application claims priority to Chinese Patent Application No.

200810189237.3, filed on December 26, 2008, which is hereby incorporated by reference in its entirety.

This application claims priority to Chinese Patent Application No. 200810189239.2, filed on December 26, 2008, which is hereby incorporated by reference in its entirety.

This application claims priority to U.S. Patent Application No. 12/316,173, filed on December 9, 2008, which is hereby incorporated by reference in its entirety.

This application claims priority to U.S. Patent Application No. 12/316,234, filed on December 9, 2008, which is hereby incorporated by reference in its entirety. FIELD OF THE INVENTION

The present invention relates to material for a rechargeable battery, more specifically, to a method of preparing composite active compound having a mixed crystalline structure that can be used as a cathode material for lithium secondary batteries and a method of forming a lithium secondary battery having a cathode made therefrom.

BACKGROUND OF THE RELATED ART

Lithium-inserted compounds are used as cathode materials for secondary batteries to improve the battery performance in voltage, specific capacity, battery duration and self-discharge. These batteries are widely used in various electronic devices and electronic vehicles as clean energy resources.

LiFePO₄ as a cathode material for secondary battery has the advantage of non-toxicity, non-pollution, excellent safety, applicability at high temperature, and abundant raw material resources. However, LiFePO₄ has its problems as a cathode material. Compared with other cathode materials, LiFePO₄ has much lower conductivity and electrical density. Presently, to solve the problem by doping compound such as LiCoO₂ to obtain a new cathode material to enhance the electrical properties thereof, but the electrical conductivity of such cathode material is still very low, it is usually only about 10^~6 S/cm, and the battery prepared by using such cathode material has poor specific capacity and cycle performance.

SUMMARY OF THE INVENTION

In viewing thereof, the present invention is aimed to solve at least one of the problems in the art. The present invention needs to provide a method of preparing a cathode active material, which has a novel crystal structure that may enhance electrical properties of the battery significantly. Further, the present invention needs to provide a method of forming a lithium ion secondary battery having a cathode made therefrom. According to an embodiment of the invention, a method of preparing a cathode active material having a mixed crystal is provided. The method comprises the following steps: providing a first crystalline substance having one or more members with general formulas Li_xxM_y>(XO₄)_zz, LiMXO₅, LiMXO₆ and LiMX₂O₇, in which: 0<xx/zz<l and 0<yy/zz<l .l ; M may be an element selected from a group consisting Na, Mn, Fe, Co, Ni, Ti, V, Y, Mg, Ca, Nb and Zn; X may be an element selected from a group consisting of P, S, As, Mo and W; providing a second crystalline substance having one or more members with general formulas LiD _CO₂, Li₁Ni_I-^_eCO_dMn₆O₂, LiNi]-^_gCO_fAI_gO₂, Li_xNii_-yCoO₂ and Li_mMn_2-nE_nO,, in which D may be an element selected from a group consisting B, Mg, Al, Ti, Cr, Fe, Cu, Zn, Ga, Y, La and V; 0<c≤3, 0.9<i<1.2, 0<d≤0.5, 0<e≤0.3, 0<f≤0.5, 0<g≤0.3, 0.9<x≤l . l and O≤y≤l; E may be an element selected from a group consisting boron, magnesium, aluminum, gallium and the transition metals except Mn; 0.9≤m≤l . l, O≤n≤l and l<j<6; and sintering the two crystalline substances to provide the mixed crystal.

According to another embodiment of the invention, a method of preparing a cathode active material having a mixed crystal is provided. The method thereof may comprise the following steps: providing a first material having a combination of lithium, iron and phosphorous sources; providing a second material having one or more members with general formulas LiD₀O₂, Li₁Ni_I-Ci_-CCO_dMn₆O₂, LiNi_I-^_gCO_fAI_gO₂, Li_xNii_-yCoO₂ and Li_mMn_2-nE_nO,, in which: D may be an element selected from a group consisting of B, Mg, Al, Ti, Cr, Fe, Cu, Zn, Ga, Y, La and V; 0<c<3, 0.9<i<1.2, 0<d<0.5, 0<e<0.3, 0<f<0.5, 0<g≤0.3, 0.9<x≤l . l and O≤y≤l ; E may be an element selected from a group consisting of boron, magnesium, aluminum, gallium and the transition metals except for Mn; 0.9≤m≤l .l, O≤n≤l and l<j<6; and sintering the two materials to provide the mixed crystal.

According to still another embodiment of the invention, a method of providing a lithium secondary battery is provided. The method may comprise the following steps: providing electrodes having a cathode, an anode and a divider film which are wounded or stacked, the cathode comprising the cathode active material prepared by the method as described above; and providing a battery shell and electrolyte, the electrodes and electrolyte being sealed within the battery shell.

The present invention, for the first time, successfully prepares a lithium metal intercalation compound with a mixed crystal. With the mixed crystalline structure, the novel cathode material prepared by the method disclosed in the present invention significantly improves electrical properties of lithium batteries.

Other variations, embodiments and features of the present invention will become evident from the following detailed description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages of the invention will become apparent and more readily appreciated from the following descriptions taken in conjunction with the drawings in which: Fig. 1 shows an XRD pattern of a composite compound according to Example l ;

Fig. 2 shows an XRD pattern of a composite compound according to Example 2;

Fig. 3 shows an XRD pattern of a composite compound according to Example 3;

Fig. 4 shows an XRD pattern of a composite compound according to Example 5; and

Fig. 5 shows an XRD pattern of a composite compound according to Example 6.

DETAILED DESCRIPTION OF THE INVENTION

It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive.

Generally, a mixed crystal can be referred to as a solid solution. It is a crystal containing a second constituent, which fits into and is distributed in the lattice of the host crystal. One exemplary illustration of the existing solution may be found in, for example, IUPAC Compendium of Chemical Terminology 2nd

Edition (1997) . Mixed crystals have been used in semiconductors for enhancing light output in light emitting diodes (LEDs). They have also been used to produce sodium-based electrolyte for galvanic elements. The present invention, for the first time, discloses that a mixed crystal has been successfully prepared for lithium metal intercalation compounds. It is also disclosed for the first time that a mixed crystalline structure has been used as a cathode material for lithium secondary batteries. The novel cathode material disclosed in the present invention has significantly better electrical properties than traditional cathode materials. The description thereof will be described in detail with reference to accompanying figures.

A cathode active material may be provided having a mixed crystal structure. The mixed crystal structure may have a first crystalline substance having one or more members with the general formulas Li_xxM_yy(XO₄)_zz, LiMXO₅, LiMXO₆ and LiMX₂O₇, in which:

0<xx/zz<l and 0<yy/zz<l .1 ;

M may be an element selected from a group consisting of Na, Mn, Fe, Co, Ni, Ti, V, Y, Mg, Ca, Nb and Zn; and

X may be an element selected from a group consisting of P, S, As, Mo and W. The mixed crystal structure may further include a second crystalline substance having one or more members with general formulas LiD₀O₂, Li₁Ni_I-Ci_-6CO_dMn₆O₂,

Li_xNii_-yCoO₂ and Li_mMn_2-nE_nO,, in which:

D may be an element selected from a group consisting of B, Mg, Al, Ti, Cr, Fe, Cu, Zn, Ga, Y, La and V; 0<c<3, 0.9<i<1.2, 0<d<0.5, 0≤e<0.3, 0<f<0.5, 0≤g<0.3, 0.9<x≤l .l and O≤y≤l ; E may be an element selected from a group consisting of boron, magnesium, aluminum, gallium and the transition metals except Mn; and

0.9<m≤l .l, O≤n≤l and Kj<6. And the cathode active material may have electrical conductivity of about

0.001 to 10 S/cm at about 25 ⁰C.

The mixed crystal structure may be formed by sintering two or more compounds, the intermediary mixture having oxygen vacancies or metallic crystalline structures. The two or more compounds do not exhibit any major chemical reactions when mixed together. However, upon sintering, a large number of crystalline defects may be formed, thus altering the electronic states of the compounds and creating a large number of oxygen vacancies. These oxygen vacancies provide the needed carriers, thus greatly enhancing the electrical conductivity of the mixed crystal. Accordingly, the cathode active material can achieve electrical conductivity of about 0.01 to 2S/cm at about 25 ⁰C measured by Siemens per centimeter, which is greater than traditional lithium iron phosphate cathode active materials. In this embodiment, the first crystalline substance and the second crystalline substance have a molar ratio of about 1 to 0.01-0.05, taking only the lithium components in the material into consideration. The first crystalline substance may have a mixed crystalline structure with the general formula Li_xxM_yy(XO₄)_zz including one or more members selected from the group consisting of LiFePO₄, LiMnPO₄ and LiCoPO₄. hi other embodiments, single-crystalline structures including Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃ and Li₂Na V₂(PO₄)₃ may be incorporated. For the general formula LiMXO₅, the first crystalline substance may be LiTiPO₅. For the general formula LiMXO₆, the first crystalline substance may include LiVMoO₆ and LiVWO₆. For the general formula LiMX₂O₇, the first crystalline substance may include LiVP₂O₇ and LiFeAs₂O₇. In the mixed crystalline structure with the general formula Li_xxM_yy(Xθ₄)_zz, M may include element Fe and one or more members selected form the group consisting of Mn, Co, Ni, Ti, Y, Mg, Ca and Zn, and the amount of Fe is from 90 % to 100 % by molar; then the first crystalline substance can include one or more members selected from LiFePO₄, Li_{0 99}Yo _OiFePO₄ and LiR₁Fe_I-1PO₄, in which 0<i<0.1, R is an element selected from a group consisting of Mn, Co, Ni, Ti, Mg, Ca and Zn.

In the general formula Li_1nMn_2-11E₁₁O_j, E may be an element selected from a group consisting of boron, magnesium, aluminum, gallium and the transition metals except for Mn, the transition metals may be elements comprising titanium, chromium, iron, cobalt, nickel, copper, zinc and yttrium. The second crystalline substance may include one or more members selected from the group consisting of LiCoO₂, LiNiO₂, LiMn₂O₄, LiVO₂, Li_{1 03}Ni_{0 8}Co_{0 1}Mn_{0 1}O₂, LiNi_{0 8}Co_{0 15}Al_{0 05}O₂ and LiMnBO₃. The mixed crystal structure can further include a carbon, in which the carbon may be about 1-5 % of the mixed crystal structure by weight. The carbon can further enhance the electrical conductivity of the mixed crystal.

A method of preparing a cathode active material for lithium secondary batteries is provided, which may comprises the following steps: Firstly, a first material having one or more members with general formulas

Li_xxM_yy(Xθ₄)_zz, LiMXO₅, LiMXO₆ and LiMX₂O₇ may be provided in which: 0<xx/zz<l and 0<yy/zz<l .l; M may be an element selected from a group consisting of Na, Mn, Fe, Co, Ni, Ti, V, Y, Mg, Ca, Nb and Zn; X may be an element selected from a group consisting of P, S, As, Mo and W; Secondly, a second material having one or more members with general formulas LiD₀O₂, Li₁Ni_I-Cj_-6CO_dMn₆O₂, LiNii._f-gCθ_fAl_gO₂, Li_xNii_-yCoO₂ and Li_1nMn_2-11E_nO_j may be provided in which: D may be an element selected from a group consisting of B, Mg, Al, Ti, Cr, Fe, Cu, Zn, Ga, Y, La and V; 0<c<3, 0.9<i<1.2, 0<d<0.5, 0≤e<0.3, 0<f<0.5, 0≤g<0.3, 0.9<x≤l .l and O≤y≤l; E may be an element selected from a group consisting of boron, magnesium, aluminum, gallium and the transition metals except for Mn; 0.9<m<l .l, O≤n≤l and l<j<6.

And thirdly, the two materials are sintered to provide a mixed crystal.

The first material and the second material may have a molar ratio of about 1 to 0.01-0.05, taking only the lithium components in the material into consideration.

In some embodiments, the first material may include one or more members selected from the group consisting of LiFePO₄, LiMnPO₄, LiCoPO₄, Li₃Fe₂(PO₄)_S, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃, Li₂NaV₂(PO₄)₃, Li_{0 99}Yo OiFePO₄, LiR₁Fe_1-1PO₄, LiTiPO₅, LiVMoO₆, LiVWO₆, LiVP₂O₇ and LiFeAs₂O₇ in which 0<i<0.1, R is one or more members selected from a group consisting of Mn, Co, Ni, Ti, Mg, Ca and Zn. In other embodiments, the first material may include one or more members selected from LiFePO₄, Li_{0 99} Yo ₀₁FePO₄ and LiR₁Fe_1-1PO₄.

In the general formula Li_1nMn_2-11E_nO_j, E may be an element selected from a group consisting of boron, magnesium, aluminum, gallium and the transition metals except Mn, the transition metals may be elements titanium, chromium, iron, cobalt, nickel, copper, zinc and yttrium. The second material may include one or more members selected from the group consisting of LiCoO₂, LiNiO₂, LiMn₂O₄, LiVO₂, Li_{1 03}Ni_{0 8}Co₀ iMno ^₂, LiNi_{0 8}Co_{0 15}A1_{O 05}O₂ and LiMnBO₃.

In some embodiments, the method may further comprise sintering a carbon additive into the two crystalline substances, the carbon additive capable of providing the mixed crystal with about 1-5 % of carbon by weight. The carbon additive includes one or more members selected from the group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin, polyethylene glycol, and other organic and inorganic sources. However, it should be noted that the examples are for illustration purpose rather than for limitation. A person skilled in the art can use equivalents thereof to achieve the same as described herein.

And a heating rate of the sintering step ranges from 5 to 20 ⁰C per minute, a sintering temperature ranging from 500 to 800 ⁰C, and a sintering time ranging from 5 to 32 hours. The sintering atmosphere is chose according the selected materials. For example, when the first or second material is easily oxidized, the sintering atmosphere may be inert atmosphere or reduction atmosphere; and when the first or second material is not easily oxidized, the sintering atmosphere may be any atmosphere.

In another embodiment, a method of preparing a cathode active material having a mixed crystal is provided. In the present invention, the cathode active material may be used for lithium secondary batteries. However, a person normally skilled in the art can apply the method for preparing a cathode active material to other rechargeable battery, thus, the illustration and description here are for illustrative purpose rather than limitation. The method may comprise the following steps: Providing a first material having a combination of lithium, iron and phosphorous sources; Providing a second material having one or more members with the general formulas LiD₀O₂, Li₁Ni_I-Ci_-6CO_dMn₆O₂, LiNi]-^_gCO_fAI_gO₂, Li_xNi]_-yCoO₂ and Li_1nMn_2-11E_nO_j, in which D may be an element selected from a group consisting B, Mg, Al, Ti, Cr, Fe, Cu, Zn, Ga, Y, La and V; 0<c<3, 0.9<i<1.2, 0<d≤0.5, 0<e≤0.3, 0≤f<0.5, 0<g≤0.3, 0.9<x≤l .l and O≤y≤l ; E may be an element selected from a group consisting boron, magnesium, aluminum, gallium and the transition metals except Mn; 0.9<m<l .l, O≤n≤l and l<j<6; and Sintering the two materials to provide a mixed crystal.

The lithium, iron and phosphorous (Li : Fe : P) sources may have a molar ratio of about 0.95-1.1 : 1 : 0.95-1.1, and the first material and the second material may have a molar ratio of about 1 to 0.01-0.05, taking only the lithium components and phosphorous source in the material into consideration. In some embodiments, the lithium source may include one or more members selected from the group consisting of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate; the iron source may include one or more members selected from the group consisting of ferrous oxalate, ferrous carbonate, iron acetate, iron oxide, iron phosphate, iron pyrophosphate and iron nitrate. And the phosphate source may include one or more members selected from the group consisting of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, iron phosphate, phosphoric acid and lithium dihydrogen phosphate. In the formula Li_1nMn_2-11E_nO_j, E may be an element selected from a group consisting boron, magnesium, aluminum, gallium and the transition metals except Mn, the transition metals may be elements of titanium, chromium, iron, cobalt, nickel, copper, zinc and yttrium. The second material may include one or more members selected from the group consisting of LiCoO₂, LiNiO₂, LiMn₂O₄, LiVO₂, Li₁ 0₃Ni_{0 8}Cθo iMn₀ ]O₂, LiNi_{0 8}Co_{0 15}A1_{O 05}O₂ and LiMnBO₃.

In some embodiments, the method may further comprise sintering the two crystalline substances by adding carbon additive which may provide the mixed crystal with about 1-5 % of carbon by weight. The carbon additive may include one or more members selected from a group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin, polyethylene glycol, and other organic and/or inorganic sources.

And the sintering step has a heating rate ranging from 5 to 20 ⁰C per minute, a sintering temperature ranging from 500 to 800 ⁰C, and a sintering time ranging from 5 to 32 hours. The sintering atmosphere is chosen according to the different materials, for example, when the first or second material is easily oxidized, the sintering atmosphere may be an inert atmosphere or a reducing one. And when the first or second material may not be easily oxidized, the sintering atmosphere may adopt any atmosphere known in the art.

According to another embodiment of the invention, a lithium ion secondary battery may be provided, the lithium ion secondary battery having a battery shell, electrodes and electrolyte, the electrodes and electrolyte being sealed within the battery shell, the electrodes having wounded or stacked cathode, anode and divider film, the cathode further including the cathode active materials described above. The cathode may include cathode components such as the cathode active materials described above with adhesives. The adhesives can be hydrophobic or hydrophilic binding additives without any specific binder ratio restrictions. In one instance, the hydrophilic to hydrophobic adhesive binder can have weight ratios of about 0.3 : 1 to about 1 : 1. The adhesive can be solid, aqueous or as an emulsion. The concentration can be adjusted accordingly based on methods of preparing the cathode, anode and the slurry viscosity and coating. In one example, the hydrophilic adhesive solution has a concentration of about 0.5 to 4 weight percent while the hydrophobic latex binder has a concentration of about 10 to 80 weight percent.

Hydrophobic adhesives can include PTFE, styrene butadiene rubber, or mixtures thereof. Hydrophilic adhesives can include HPMC, CMC, hydroxyethyl cellulose, polyvinyl alcohol, or mixtures thereof. The binder content can be about 0.01 to 8 % by weight of the total cathode active material. In addition, conductive agents may be incorporated or added in the cathode active material, the conductive agents include, but without limitation, graphite, carbon fiber, carbon black, metal powders and fibers as well as any suitable material understood by one skilled in the art. The conductive agent can be about 0.1 to 20 % by weight of the total cathode active material. The method of preparing the cathode include using solvents to dissolve the cathode active material and mixing with adhesives and conductive agents to form a cathode slurry. The cathode slurry can be applied onto cathode collectors, dried, rolled or compressed, and sliced into pieces to produce the cathode. In one example, the slurry can be dried at about 100 to 150 ⁰C for about 2 to 10 hours. The cathode collectors include aluminum foil, copper foil, nickel-plated steel or punched stainless steel. The types of solvent to use include N-methyl pyrrolidone (NMP), dimethylformamide (DMF), diethyl formamide (DEF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), water, alcohol and mixtures thereof. The amount of solvent to use can be adjusted accordingly to provide the proper slurry coating and viscosity. In one instance, the amount of solvent can be about 40 to 90 % by weight of the cathode active material. The method of preparing the cathode and types of solvents, adhesives, conductive agents and cathode collectors can also incorporate other techniques understood by one skilled in the art. As discussed above, the lithium secondary battery includes a battery shell, electrodes and electrolyte, the electrodes and electrolyte capable of being sealed within the battery shell. The electrodes may include wounded or stacked cathode, anode and divider film with the cathode utilizing the cathode active material of the presently disclosed embodiments. The divider film can be situated between the cathode and anode for preventing electrical shortcuts and for maintaining the electrolytic solution. In one instance, the divider film can include any membrane including, but without limitation, micro-porous membrane polyolefin, polyethylene fibers, ultra-fine glass fibers and fiber paper. The anode can incorporate any anode active materials and known methods of forming such materials as known in the arts. The anode active material can be provided in slurry form and coated onto anode collectors similar to the cathode collectors above. Additionally, the anode active material may include carbon additives such as non-carbon graphite, graphite, and polymers having undergone high-temperature carbon oxidation. The carbon additive can also include pyrolytic coal, coke, organic polymer sintered materials and activated carbons. The organic polymer sintered materials include phenolic resin, epoxy resin, and carbonized products obtained by sintering.

Adhesives can utilize traditional adhesives for lithium secondary batteries including polyvinyl alcohol, PTFE, carboxymethyl cellulose (CMC), hydroxymethyl cellulose (HMC), and styrene butadiene rubber (SBR). The adhesive binder can be about 0.5 to 8 weight percent of the total anode active material. The anode active material can further include conductive agents, the conductive agent capable of increasing electrical conductivity and reducing internal resistance of the battery. The conductive agent may include, but without limitation to, carbon black, nickel powder and copper powder etc. Other conductive agents known by one skilled in the art may also be utilized and can be about 0.1 to 12 weight percent of the anode active material.

The method of preparing the anode may include: using solvents to dissolve the anode active material and mixing with adhesives and conductive agents to form anode slurry. The anode slurry can be applied onto the anode collectors similar to that of the cathode slurry described above, dried, rolled or compressed, and sliced into pieces to produce the anode. In one example, the slurry can be dried at about 100 to 150 ⁰C for about 2 to 10 hours. The types of solvent for dissolving the anode active material include N-methyl pyrrolidone (NMP), dimethylformamide (DMF), diethyl formamide (DEF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), water, alcohol and mixtures thereof. The amount and concentration of solvents to use can be adjusted accordingly to provide the proper slurry coating and viscosity. Like the cathode slurry, the amount of anode slurry applied to the anode collector can be about 40 to 90 weight percent of the anode active material.

The electrolyte for the lithium secondary battery can be a non-aqueous electrolyte, which can be formed by dissolving lithium salt in a non-aqueous solvent. The lithium salt electrolyte can include one or more members selected from lithium hexafluorophosphate (LiPF₆), lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄), lithium hexafluoroarsenate (LiAsF₆), lithium hexafluorosilicate (LiSiF₆), lithium tetraphenylborate (LiB(C₆H₅)₄), lithium chloride (LiCl), lithium bromide (LiBr), lithium aluminum tetrachloride (LiAlCl₄), LiC(So₂CF₃)₃, LiCH₃SO₃, and LiN(SO₂CF₃)₂. The non-aqueous solvent can be chain ester and ester ring mixed solution, the chain ester being one or more members of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), dimethylpropyl carbonate (DPC) and other fluoride or sulfur-containing unsaturated key chain organic esters, with the ester ring being one or more members of ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), gamma-butyrolactone (γ-BL), sodium fluoride and other lactone-containing or unsaturated organic ester rings. In one instance, the lithium salt electrolyte has a concentration of about 0.1 to 2 mole per liter.

The presently disclosed lithium secondary batteries can be provided by processes known by one skilled in the art. The preparation method may include winding or stacking cathode, anode and divider films into the battery core, and placing the battery core into the battery shell, adding the electrolyte, and sealing the battery accordingly. The winding, stacking and sealing of the batteries can utilize traditional techniques as understood by one skilled in the art. Furthermore, other known steps of manufacturing the lithium secondary battery can be incorporated.

The following will describe various embodiments of mixed-crystal cathode active materials according to the presently disclosed invention.

EXAMPLE 1

Mix LiFePO₄ and LiNiO₂ by a molar ratio of 1 : 0.02 with starch as a source of carbon which may provide 5 % carbon by weight in the final product. The

LiFePO₄ may be prepared by mixing lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate in a Li : Fe : P molar ratio of 1 : 1 : 1 , which may be added to the lithium nickel oxide at an ammonium dihydrogen phosphate to

LiNiO₂ molar ratio of 1 : 0.02 (taking into account the phosphorous and lithium components in the mixture). Alternatively, the LiFePO₄ may be prepared by a third party and added to the LiNiO₂ in the manner discussed above. In one instance, the LiFePO₄ may be produced by heating lithium, iron and phosphorous sources at about 400 to 800 ⁰C for 5 to 32 hours.

Ball-grind the mixture for 10 hours, remove and dry the mixture at 80 ⁰C. Heat the resulting powder in a nitrogen or argon atmosphere at a heating rate of 10 °C per minute to 600 ⁰C, continue sintering the product for 20 hours to provide a LiFePO₄ / LiNiO₂ / C mixed crystal cathode active material.

With a Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material as shown in Fig. 1. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiFePO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiFePO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiFePO₄ / LiNiO₂ / C in a mixed crystal form. EXAMPLE 2

Mix LiFePO₄ and LiCoO₂ in a molar ratio of 1 : 0.04 with acetylene black as a source of carbon which may provide 2% carbon by weight in the final product. The LiFePO₄ may be prepared by mixing lithium oxalate, iron oxide and diammonium hydrogen phosphate in a Li : Fe : P molar ratio of 0.95 : 1 : 1, which may be added to the lithium cobalt oxide at a diammonium hydrogen phosphate to LiCoO₂ molar ratio of 1 : 0.04 (taking into account the phosphorous and lithium components in the mixture). Alternatively, the LiFePO₄ may be prepared by a third party and added to the LiCoO₂ in the manner discussed above. In one instance, the LiFePO₄ may be produced by heating lithium, iron and phosphorous sources at about 400 to 800 ⁰C for 5 to 32 hours.

Ball-grind the mixture for 10 hours, remove and dry the mixture at 80 ⁰C. Heat the resulting powder in a nitrogen or argon atmosphere at a heating rate of 5 ⁰C per minute to 500 ⁰C, continue sintering the product for 30 hours to provide a LiFePO₄ / LiCoO₂ / C mixed crystal cathode active material. Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material as shown in Fig. 2. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiFePO₄ and LiCoO₂, there is no new peak or feature, thus indicating that the LiFePO₄ and LiCoO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiFePO₄ / LiCoO₂ / C in a mixed crystal form.

EXAMPLE 3

Mix LiFePO₄, LiMn₂O₄ and LiVO₂ in a molar ratio of 1 : 0.03 : 0.01 with carbon black as a source of carbon which may provide 0 % carbon by weight in the final product. The LiFePO₄ may be prepared by mixing lithium hydroxide, ferrous carbonate and phosphoric acid in a Li : Fe : P molar ratio of 1.05: 1 : 1.05, which may be added to the LiMn₂O₄ and LiVO₂ at a phosphoric acid to LiMn₂O₄ and LiVO₂ molar ratio of 1 : 0.03 : 0.01 (taking into account the phosphorous components in the mixture). Alternatively, the LiFePO₄ may be prepared by a third party and added to the LiCoO₂ in the manner discussed above. In one instance, the LiFePO₄ may be produced by heating lithium, iron and phosphorous sources at about 400 to 800 ⁰C for 5 to 32 hours.

Ball-grind the mixture for 10 hours, remove and dry at 80 ⁰C. Heat the resulting powder in a nitrogen or argon atmosphere at a heating rate of 20 ⁰C per minute to 800 ⁰C, continue sintering the product for 8 hours to provide a LiFePO₄ / LiMn₂O₄ / LiVO₂ mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material as shown in Fig. 3. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiFePO₄, LiMn₂O₄ and LiVO₂, there is no new peak or feature, thus indicating that the LiFePO₄, LiMn₂O₄ and LiVO₂ exist in three phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiFePO₄ / LiMn₂O₄ / LiVO₂ in a mixed crystal form. EXAMPLE 4

Mix LiOH, Ni(OH)₂, Co₂O₃ and Al₂O₃ in a molar ratio of 1 : 0.8 : 0.075 : 0.025, ball-grind the mixture for 5 hours, heat in an oxygen atmosphere at a heating rate of 7 ⁰C per minute to 800 ⁰C, continue sintering the product for 15 hours to provide a mixed crystal cathode active material. Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. In comparison with the standard XRD pattern of LiNio sCoo ₁5AI005O₂, the composite mixed crystal is determined to be

The remaining steps are the same as those used in Example Al, with the difference being that the LiNi₀ sCoo ₁₅AIo _0SO₂ substitutes the LiNiO₂ to provide cathode active material having a mixed crystal of LiFePO₄ / LiNi_{0 8}Co_{0 15}Al_{0 05}O₂ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiFePO₄ and LiNi_{0 8}Co_{0 15}Al_{0 05}O₂, there is no new peak or feature, thus indicating that the LiFePO₄ and LiNi_{0 8}Co_{0 15}Al_{0 05}O₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiFePO₄ / LiNi_{0 8}Co_{0 15}Al_{0 O5}O₂ / C in a mixed crystal form.

EXAMPLE 5

Mix LiOH, Ni(OH)₂, Co₂O₃ and MnO₂ in a molar ratio of 1.03 : 0.8 : 0.05 : 0.1, ball-grind the mixture for 5 hours, heat in an oxygen atmosphere at a heating rate of 7 ⁰C per minute to 800 ⁰C, continue sintering the product for 15 hours to provide the cathode active material having a mixed crystal cathode.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. In comparison with the standard XRD pattern of Li_{1 03}Ni_{0 8}Co_{0 1}Mn_{0 1}O₂, the composite mixed crystal is determined to be Li_{1 03}Ni_{0 8}Co_{0 1}Mn₀ ]O₂. The remaining steps are the same as those used in Example Al, with the difference being that the Li_{1 03}Ni_{0 8}Co₀ iMn_{0 1}O₂ substitutes the LiNiO₂ to provide the cathode active material having a mixed crystal of LiFePO₄ / Li_{1 03}Ni_{0 8}Co_{0 1}Mn_{0 1}O₂ 1 C. Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material as shown in Fig. 4. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiFePO₄ and Li_{1 03}Ni_{0 8}Co_{0 1}Mn₀ ]O₂, there is no new peak or feature, thus indicating that the LiFePO₄ and Li_{1 03}Ni₀ sCo_{0 1}Mn_{0 1}O₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiFePO₄ / Li] ₀₃Ni₀ sCo₀ iMn_{0 1}O₂ / C in a mixed crystal form.

EXAMPLE 6 In example 6, the steps are similar to those used in Example 1, with the only difference being that the LiMnBOs substitutes the LiNiO₂ to provide the cathode active material having a mixed crystal Of LiFePO₄ / LiMnBO₃ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material as shown in Fig. 5. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiFePO₄ and LiMnBO₃, there is no new peak or feature, thus indicating that the LiFePO₄ and LiMnBO₃ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiFePO₄ / LiMnBO₃ / C in a mixed crystal form. EXAMPLE 7

According to the disclosed method of "A Method of Preparing Lithium Battery Cathode Active Material Li_{0 99}Yo _OiFePO₄" (JOURNAL OF FUNCTIONAL MATERIALS, VOLUME 36, ISSUE 5 (2005)) to prepare Li_{0 99}Yo _OiFePO₄.

The steps are similar to those used in Example 1, with the only difference being that the Li_{0 99}Y_{0 O}iFePO₄ substitutes the LiFePO₄ to provide the cathode active material having a mixed crystal Of Li_{0 99}Y_{0 01}FePO₄ / LiNiO₂ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to Li_{0 99}Y_{0 01}FePO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the Lio ₉₉Yo _OiFePO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having Lio ₉₉ Yo ₀₁FePO₄ / LiNiO₂ / C in a mixed crystal form. EXAMPLE 8

According to a disclosed method of "The Preparation and Performance of Lithium Battery Cathode Active Material LiNi_{0 1}Fe_{0 9}PO₄" (THE CHINESE JOURNAL OF NONFERROUS METALS, VOLUME 16, ISSUE 4 (Apr. 2006)) to prepare LiTi_{0 05}Fe_{0 95}PO₄. Mix Li₂CO₃, FeC₂O₄-2H₂O, NH₄H₂PO₄ and TiO₂ in a molar ratio to the stoichiometry of LiTi_{0 05}Fe_{0 95}PO₄. Ball-grind the mixture with ethanol for 5 hours, remove and dry the mixture at room temperature. And then heat the mixture in an argon atmosphere at 320 ⁰C for 7 hours; continue sintering the product for 24 hours at 700 ⁰C to provide the LiTi_{0 05}Fe_{0 95}PO₄.

The steps are similar to those used in Example 1, with the only difference being that the LiTi_{0 05}Fe_{0 95}PO₄ substitutes the LiFePO₄ to provide the cathode active material having a mixed crystal Of LiTi_{0 05}Fe_{0 95}PO₄ / LiNiO₂ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiTi_{0 05}Fe_{0 95}PO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiTi_{0 05}Fe_{0 95}PO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiTi_{0 05}Fe_{0 95}PO₄ / LiNiO₂ / C in a mixed crystal form. EXAMPLE 9 According to a disclosed method of "The Preparation and Performance of

Lithium Battery Cathode Active Material LiNi_{0 1}Fe_{0 9}PO₄" (THE CHINESE JOURNAL OF NONFERROUS METALS, VOLUME 16, ISSUE 4 (Apr. 2006)) to prepare LiNi_{0 1} Fe_{0 9}PO₄. Mix Li₂CO₃, FeC₂O₄-2H₂O, NH₄H₂PO₄ and Ni(CH₃COO)₂-4H₂O in a molar ratio to the stoichiometry of LiNi_{0 I}Fe_{0 9}PO₄. Ball-grind the mixture with ethanol for 5 hours, remove and dry the mixture at room temperature. And then heat the mixture in an argon atmosphere at 320 ⁰C for 7 hours; continue sintering the product for 24 hours at 700 ⁰C to provide

The steps are similar to those used in Example 1, with the only difference being that the LiNio iFeo ₉PO₄ substitutes the LiFePO₄ to provide the cathode active material with a mixed crystal Of LiNi_{0 1}Fe₀9PO₄ / LiNiO₂ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiNi_{0 1}Fe_{0 9}PO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiNi_{0 1}Fe_{0 9}PO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiNi_{0 1}Fe_{0 9}PO₄ / LiNiO₂ / C in a mixed crystal form. EXAMPLE 10

According to a disclosed method of "The Preparation and Performance of Lithium Battery Cathode Active Material LiNi_{0 1}Fe_{0 9}PO₄" (THE CHINESE JOURNAL OF NONFERROUS METALS, VOLUME 16, ISSUE 4 (Apr. 2006)) to prepare LiCo_{0 O}iFe_o 99PO₄. Mix Li₂CO₃, FeC₂O₄-2H₂O, NH₄H₂PO₄ and CoO in a molar ratio to the stoichiometry Of LiCo_{0 O}iFe_{0 99}PO₄. Ball-grind the mixture with ethanol for 5 hours, remove and dry the mixture at room temperature. And then heat in an argon atmosphere at 320 ⁰C for 7 hours; continue sintering the product for 24 hours at 700 ⁰C to provide LiCo_{0 O}iFe_{o 99}PO₄.

The steps are similar to those used in Example 1, with the difference being that the LiCo_{0 01}Fe_{0 99}PO₄ substitutes the LiFePO₄ to provide a LiCo_{0 O}iFe_{o 99}PO₄ / LiNiO₂ / C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiCo_{0 O}iFe_{o 99}PO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiCoo oiFe_{0 99}PO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiCoo oiFeo ₉₉PO₄ / LiNiO₂ / C in a mixed crystal form. EXAMPLE I l

According to the disclosed method of "The Preparation and Performance of Lithium Battery Cathode Active Material LiNi_{0 I}Fe_{0 9}PO₄" (THE CHINESE JOURNAL OF NONFERROUS METALS, VOLUME 16, ISSUE 4 (Apr. 2006)) to prepare LiMn_{0 02}Fe_{0 98}PO₄. Mix Li₂CO₃, FeC₂O₄-2H₂O, NH₄H₂PO₄ and MnCO₃ in a molar ratio to the stoichiometry of LiMn_{0 02}Fe_{0 98}PO₄. Ball-grind the mixture with ethanol for 5 hours, remove and dry the mixture at room temperature. And then heat the mixture in an argon atmosphere at 320 ⁰C for 7 hours; continue sintering the product for 24 hours at 700 ⁰C to provide LiMn_{0 02}Fe_{0 98}PO₄.

The steps are similar to those used in Example 1, with the only difference being that the LiMn_{0 02}Fe_{0 98}PO₄ substitutes the LiFePO₄ to provide the cathode active material having a mixed crystal Of LiMn_{0 02}Fe_{0 98}PO₄ / LiNiO₂ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiMn_{0 02}Fe_{0 98}PO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiMn_{0 02}Fe_{0 98}PO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiMn_{0 02}Fe_{0 98}PO₄ / LiNiO₂ / C in a mixed crystal form. EXAMPLE 12 According to a disclosed method of "The Preparation and Performance of

Lithium Battery Cathode Active Material LiNi_{0 1}Fe_{0 9}PO₄" (THE CHINESE JOURNAL OF NONFERROUS METALS, VOLUME 16, ISSUE 4 (Apr. 2006)) to prepare LiMg_{0 03}Fe_{0 97}PO₄. Mix Li₂CO₃, FeC₂O₄-2H₂O, NH₄H₂PO₄ and MgO in a molar ratio to the stoichiometry Of LiMg_{0 03}Fe_{0 97}PO₄. Ball-grind the mixture with ethanol for 5 hours, remove and dry the mixture at room temperature. And then heat in an argon atmosphere at 320 ⁰C for 7 hours; continue sintering the product for 24 hours at 700 ⁰C to provide LiMgo o₃Feo ₉₇PO₄.

The steps are similar to those used in Example 1, with the only difference being that the LiMg_{0 03}Fe_{0 97}PO₄ substitutes the LiFePO₄ to provide the cathode active material having a mixed crystal of LiNϊo iFeo 9PO₄ / LiNiO₂ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiMg_{0 03}Fe_{0 97}PO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiMg_{0 03}Fe_{0 97}PO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiMg_{0 03}Fe_{0 97}PO₄ / LiNiO₂ / C in a mixed crystal form.

EXAMPLE 13 According to a disclosed method of "The Preparation and Performance of

Lithium Battery Cathode Active Material LiNi_{0 1}Fe_{0 9}PO₄" (THE CHINESE JOURNAL OF NONFERROUS METALS, VOLUME 16, ISSUE 4 (Apr. 2006)) to prepare LiCa_{0 05}Fe_{0 95}PO₄. Mix Li₂CO₃, FeC₂O₄-2H₂O, NH₄H₂PO₄ and CaO in a molar ratio to the stoichiometry of LiCa_{0 05}Fe_{0 95}PO₄. Ball-grind the mixture with ethanol for 5 hours, remove and dry the mixture at room temperature. And then heat the mixture in an argon atmosphere at 320 ⁰C for 7 hours; continue sintering the product for 24 hours at 700 ⁰C to provide LiCa_{0 05}Fe_{0 95}PO₄.

The steps are similar to those used in Example 1, with the difference being that the LiCa_{0 05}Fe_{0 95}PO₄ substitutes the LiFePO₄ to provide the cathode active material having a mixed crystal of LiCa_{0 05}Fe_{0 95}PO₄ / LiNiO₂ / C.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiCa_{0 05}Fe_{0 95}PO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiCa₀₀₅Fe_{0 95}PO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiCa_{0 05}Fe_{0 95}PO₄ / LiNiO₂ / C in a mixed crystal form.

EXAMPLE 14 According to a disclosed method of "The Preparation and Performance of

Lithium Battery Cathode Active Material LiNi_{0 1}Fe_{0 9}PO₄" (THE CHINESE JOURNAL OF NONFERROUS METALS, VOLUME 16, ISSUE 4 (Apr. 2006)) to prepare LiZn_{0 07}Fe_{0 93}PO₄. Mix Li₂CO₃, FeC₂O₄-2H₂O, NH₄H₂PO₄ and ZnCO₃ in a molar ratio to the stoichiometry of LiZn_{0 07}Fe_{0 93}PO₄. Ball-grind the mixture with ethanol for 5 hours, remove and dry the mixture at room temperature. And then heat the mixture in an argon atmosphere at 320 ⁰C for 7 hours; continue sintering the product for 24 hours at 700 ⁰C to provide LiZn_{0 07}Fe_{0 93}PO₄.

The steps are similar to those used in Example 1, with the difference being that the LiZn_{0 07}Fe_{0 93}PO₄ substitutes the LiFePO₄ to provide a LiZn_{0 07}Fe_{0 93}PO₄ / LiNiO₂ / C mixed crystal cathode active material.

Using the Rigaku D/MAX-2200/PC, an XRD pattern is carried out on the cathode active material. Viewing diffraction peaks of the sintered product, except for peaks corresponding to LiZn_{0 07}Fe_{0 93}PO₄ and LiNiO₂, there is no new peak or feature, thus indicating that the LiZn₀₀₇Fe_{0 93}PO₄ and LiNiO₂ exist in two phases and no new compound is created. Accordingly, this demonstrates that the process described above prepares a cathode active material having LiZn_{0 07}Fe_{0 93}PO₄ / LiNiO₂ / C in a mixed crystal form.

COMPARATIVE EXAMPLE Rl

Mix LiFePO₄ and LiCoO₂ in a molar ratio of 1 : 0.04 with acetylene black as a source of carbon which may provide 2 % carbon by weight in the final product. Ball-grind the mixture for 10 hours, remove and dry the mixture at 80 ⁰C to provide a combined composition OfLiFePO₄, LiCoO₂ and carbon cathode active material.

COMPARATIVE EXAMPLE R2 Mix LiFePO_4, LiMn₂O₄ and LiVO₂ in a molar ratio of 1 : 0.03 : 0.01. Ball-grind the mixture for 10 hours, remove and dry the mixture at 80 ⁰C to provide a combined composition OfLiFePO₄, LiMn₂O₄ and LiVO₂ cathode active material.

CONDUCTIVITY OF EXAMPLES 1-14 AND COMPARATIVE EXAMPLES Rl -R2

At 25 ⁰C, separately take each cathode active materials of Examples 1-14 and Comparative examples R1-R2, apply 30 MPa of pressure to provide a cylinder. Measure the height (V), diameter (d) and resistance (R) of each cylinder. Use the following formula to calculate the electrical conductivity (σ) for each sample:

Electrical conductivity σ = 4 x 1 1 (π R x d²)

The electrical conductivities of Examples 1-14 and Comparative examples R1-R2 are shown in Table 1.

Table 1. Electrical conductivities of samples at 25 ⁰C. From Table 1 , it may be determined that the cathode active materials of the present embodiments can achieve electrical conductivity up to 1.8 S / cm measured by Siemens per centimeter. Additionally, the cathode active material of Comparative example Rl, provided by simple mixing, achieves electrical conductivity of 1.5 x 10^~6 S / cm while the cathode active material of Example 2, having similar composition to that of Comparative example Rl but provided by the present disclosed method, achieves electrical conductivity of 0.24 S / cm, the latter being 160,000 times more electrically conductive than the former one. Likewise, the cathode active material of Comparative example R2, provided by simple mixing, achieves electrical conductivity of 2.4 x 10^"5 S / cm while the cathode active material of Example 3, having similar composition to that of Comparative example R2 but provided by the present disclosed method, achieves electrical conductivity of 0.6 S / cm, the latter being 25,000 times more electrically conductive than R2. TESTINGS OF EXAMPLES 1-14 AND COMPARATIVE EXAMPLES

R1-R2

(1) Battery preparation

(a) Cathode active material

Separately combine 90 grams of each of the composite cathode material from Examples 1-14 and Comparative examples R1-R2 with 5 grams of polyvinylidene fluoride (PVDF) binder and 5 grams of acetylene black to 50 grams of

N-methylpyrrolidone (NMP). Place the materials in a vacuum mixer to mix into uniform slurry. Apply a coating of about 20 microns thick on both sides of an aluminum foil, dry at 150⁰C, roll and crop to a size of 540 x 43.5 mm² to provide about 5.2 grams of cathode active material.

(b) Anode active material Combine 90 grams of natural graphite with 5 grams of poly vinylidene fluoride (PVDF) binder and 5 grams of conductive carbon black to 100 grams of N-methylpyrrolidone (NMP). Place the materials in a vacuum mixer to mix into uniform slurry. Apply a coating of about 12 microns thick to both sides of a copper foil, dry at 90⁰C, roll and crop to a size of 500 x 44 mm² to provide about 3.8 grams of anode active material. (c) Battery assembly

Separately wind each of the cathode and anode active materials with polypropylene film into a lithium secondary battery core, followed by dissolving one mole of LiPF₆ in a mixture of non-aqueous electrolyte solvent EC/EMC/DEC to provide a ratio of 1 : 1 : 1, inject and seal the electrolyte having a capacity of 3.8 g/Ali into the battery to provide separate lithium secondary batteries Al -Al 4 (Examples 1-14) and AC1-AC2 (Comparative examples R1-R2) for testing.

PERFORMANCE TESTINGS OF BATTERIES A1-A14 and AC1-AC2 Separately place each of batteries Al -Al 4 and AC1-AC2 on the testing cabinet. At 25 ⁰C, charge each battery at a current of 0.5 C with a voltage limit of 3.8 V and set the battery aside for 20 minutes. Using a current of 0.5 C, discharge the battery from 3.8 V to 2.5 V and record the discharge capacity as the battery's initial discharge capacity. Use the following equation to calculate the battery's specific discharge capacity. The test results for batteries A1-A14 and AC1-AC2 are shown in Table 2.

Specific discharge capacity = Initial discharge capacity (milliampere hour) / weight of cathode active material (grams)

Repeat the process described above: charge the battery, set it aside, and discharge each battery for 500 cycles. Record the battery's discharge capacity and use the following equation to calculate the battery's ability to maintain discharge capacity after 500 cycles. The higher the maintenance rate, the better the performance of the battery in maintaining its discharge capacity. The test results for batteries Al -Al 4 and AC1-AC2 are shown in Table 2. Capacity maintenance rate = (Discharge capacity after n^th cycle / initial discharge capacity ) x 100 %

TABLE 2. Electrical testing results for batteries A1-A14 and AC1-AC2.

From Table 2, it may be observed that the cathode active materials according to Examples 1-14 of the presently disclosed invention are able to achieve better electrical performance than Comparative examples R1-R2. Specifically, the cathode active materials of batteries A1-A14 are able to achieve specific discharge capacity of at least 121 mAh/g at 0.5 C and maintain greater than 95 % discharge capacity after 500 cycles.

Additionally, cathode active material of Comparative example Rl, provided by simple mixing, achieved specific discharge capacity of 108 mAh/g and maintained 88.21 % discharge capacity after 500 cycles while cathode active material of Example 2, having similar composition to that of Comparative example Rl but provided by the presently disclosed method, achieves specific discharge capacity of 124 mAh/g and maintains 95.90 % discharge capacity after 500 cycles. Likewise, cathode active material of Comparative example R2, provided by simple mixing, achieves specific discharge capacity of 112 mAh/g and maintains 90.09 % discharge capacity after 500 cycles while cathode active material of Example 3, having similar composition to that of Comparative example R2 but provided by the presently disclosed method, achieves specific discharge capacity of 126 mAh/g and maintains 96.67 % discharge capacity after 500 cycles. Accordingly, the cathode active materials for lithium secondary batteries and methods of preparing the same according to the presently disclosed embodiments can provide superior electrical performance, e.g., higher electrical conductivity, discharge capacity and discharge capacity maintenance or retention rate.

Although the invention has been described in detail with reference to several embodiments, additional variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method of preparing a cathode active material having a mixed crystal, comprising the following steps: providing a first crystalline substance having one or more members with general formulas Li_xxM_yy(X0₄)_zz, LiMXO₅, LiMXO₆ and LiMX₂O₇, wherein: 0<xx/zz<l and 0<yy/zz<l .1 ;

M is an element selected from a group consisting of Na, Mn, Fe, Co, Ni, Ti, V, Y, Mg, Ca, Nb and Zn; X is an element selected from a group consisting of P, S, As, Mo and W; providing a second crystalline substance having one or more members with general formulas LiD_cO₂, Li₁Ni] .^_eCO_dMn₆O₂, LiNii._f-gCθ_fAl_gO₂, Li_xMi _-yCoO₂ and Li_mMn_2-nE_nO_j, wherein:

D is an element selected from a group consisting of B, Mg, Al, Ti, Cr, Fe, Cu, Zn, Ga, Y, La and V;

0<c<3, 0.9<i<1.2, 0<d≤0.5, 0<e≤0.3, 0≤f<0.5, 0<g<0.3, 0.9<x≤l .l and O≤y≤l ; E is an element selected from a group consisting of boron, magnesium, aluminum, gallium and the transition metals except Mn;

0.9<m<l .l, O≤n≤l and Kj<6; and sintering the two crystalline substances to provide the mixed crystal.

2. The method according to claim 1, wherein the first crystalline substance and the second crystalline substance have a molar ratio of about 1 : 0.01-0.05 measured only by lithium components contained therein.

3. The method according to claim 1 , wherein M includes element Fe and one or more members selected from a group consisting of Mn, Co, Ni, Ti, Y, Mg, Ca and Zn, and wherein the amount of Fe is from 90 % to 100 % by molar.

4. The method according to claim 1, wherein the first crystalline substance includes one or more members selected from a group consisting of LiFePO₄, LiMnPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃, LiTi₂(PO₄)₃, Li₃V₂(PO₄)₃, Li₂NaV₂(PO₄)₃, Li_{0 99}Yo OiFePO₄, LiR₁FeI_-1PO₄, LiTiPO₅, LiVMoO₆, LiVWO₆, LiVP₂O₇ and LiFeAs₂O₇, and wherein 0<i<0.1, R is one or more members selected from an element group consisting of Mn, Co, Ni, Ti, Mg, Ca and Zn; and the second crystalline substance includes one or more members selected from an element group consisting of LiCoO₂, LiNiO₂, LiMn₂O₄, LiVO₂, Li_{1 03}Ni₀ sCo₀ iMn_{0 1}O₂, LiNi₀ δCoo ₁5Al₀05O₂ and LiMnBO₃.

5. The method according to claim 4, wherein the first crystalline substance includes one or more members selected from an element group consisting of LiFePO₄, Li_{O 99}Yo _OiFePO₄ and LiR₁Fe_I-1PO₄ wherein 0<i<0.1, R is one or more members selected from an element group consisting of Mn, Co, Ni, Ti, Mg, Ca and Zn.

6. The method according to claim 1, further comprising the step of: sintering the two crystalline substances by adding carbon additive which provides the mixed crystal with about 1-5 % carbon by weight.

7. The method according to claim 4, wherein the carbon additive includes one or more members selected from an element group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol.

8. The method according to claim 1, wherein the sintering step has a heating rate ranging from 5 to 20 ⁰C per minute, a sintering temperature ranging from 500 to 800 ⁰C, and a sintering time ranging from 5 to 32 hours.

9. A method of preparing a cathode active material having a mixed crystal comprising the following steps: providing a first material having a combination of lithium, iron and phosphorous sources; providing a second material having one or more members with general formulas LiD₀O₂, Li₁Ni_I-Cj_-6CO_dMn₆O₂, LiNii._f-gCθ_fAl_gO₂, Li_xNii_-yCoO₂ and Li_1nMn_2-11E_nO_j, wherein: D is an element selected from a group consisting of B, Mg, Al, Ti, Cr, Fe, Cu, Zn, Ga, Y, La and V;

0<c≤3, 0.9<i<1.2, 0<d≤0.5, 0<e≤0.3, 0≤f<0.5, 0<g≤0.3, 0.9<x≤l .l and O≤y≤l ;

E is an element selected from a group consisting of boron, magnesium, aluminum, gallium and the transition metals except for Mn;

0.9<m≤l .l, O≤n≤l and Kj<6; and sintering the two materials to provide the mixed crystal.

10. The method according to claim 9, wherein the lithium, iron and phosphorous (Li : Fe : P) sources have a molar ratio of about 0.95-1.1 : 1 : 0.95-1.1, and wherein the first material and the second material have a molar ratio of about 1 to 0.01-0.05, measured only by the lithium components and phosphorous source contained therein.

11.The method according to claim 9, wherein the second material includes one or more members selected from a group consisting of LiCoO₂, LiNiO₂, LiMn₂O₄, LiVO₂, Li_{1 03}Ni_{0 δ}Coo iMn_{0 1}O₂, LiNi₀ sCo_{0 15}Al_{0 05}O₂ and LiMnBO₃.

12. The method according to claim 9, wherein the lithium source includes one or more members selected from a group consisting of lithium carbonate, lithium hydroxide, lithium oxalate and lithium acetate; the iron source includes one or more members selected from a group consisting of ferrous oxalate, ferrous carbonate, iron acetate, iron oxide, iron phosphate, iron pyrophosphate and iron nitrate; and the phosphate source includes one or more members selected from a group consisting of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, iron phosphate, phosphoric acid and lithium dihydrogen phosphate.

13. The method according to claim 9, wherein the first material further includes a carbon source.

14. The method according to claim 13, wherein the carbon source provides the mixed crystal with about 1-5 % of carbon by weight, and wherein the carbon source includes one or more members selected from a group consisting of carbon black, acetylene black, graphite, glucose, sucrose, citric acid, starch, dextrin and polyethylene glycol.

15. The method according to claim 9, wherein the sintering step has a heating rate ranging from 5 to 20 ⁰C per minute, a sintering temperature ranging from 500 to 800 ⁰C, and a sintering time ranging from 5 to 32 hours.

16. A method of providing a lithium secondary battery, comprising the following steps: providing electrodes having a cathode, an anode and a divider film which are wounded or stacked, the cathode comprising the cathode active material prepared by the method according to any one of claims 1-15; and providing a battery shell and electrolyte, the electrodes and electrolyte being sealed within the battery shell.