Anode active material for lithium secondary batteries

Abstract

Disclosed are an anode active material for lithium secondary batteries, the anode active material comprising: a core part including a carbon-silicon complex and having a cavity therein; and a coated layer which is formed on the surface of the core part and includes a phosphor-based alloy.

Claims

1. An anode active material for a lithium secondary battery, comprising. a core including a carbon-silicon composite and having a cavity formed inside; and a coating layer formed on a surface of the core and including a phosphorus (P)-based alloy wherein the coating layer entirely covers the surface of the core wherein the carbon-silicon composite comprises carbon-silicon bonds.

2. The anode active material for the lithium secondary battery according to claim 1, wherein the coating layer has a plurality of domains which are protruded from the coating layer surface.

3. The anode active material for the lithium secondary battery according to claim 1, wherein the phosphorus-based alloy is an alloy of phosphorous and a metal selected from the group consisting of Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Pd, Ag, In, Sn, and W.

4. The anode active material for the lithium secondary battery according to claim 1, wherein the carbon-silicon composite further includes oxygen.

5. The anode active material for the lithium secondary battery according to claim 1, wherein a density of the coating layer is in a range of 1.2 to 5.8 g cm.sup.−3, and a thickness of the coating layer including the domains is equal to or less than 0.1 μm.

6. The anode active material for the lithium secondary battery according to claim 1, wherein a specific surface area of the core is equal to or less than 50 m.sup.2/g.

7. The anode active material for the lithium secondary battery according to claim 1, wherein an average particle diameter of the core is in a range of 0.5 to 100 μm.

8. An anode for a lithium secondary battery, the anode comprising: a current collector; and an anode active material layer formed on at least one surface of the current collector and including an anode active material and binder, wherein the anode active material is an anode active material defined in claim 1.

9. An anode for a lithium secondary battery according to claim 8, wherein the binder is polyimide.

10. A lithium secondary battery comprising: a cathode; an anode; and a separator interposed between the cathode and the anode, wherein the anode is an anode defined in claim 8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings illustrate a preferred embodiment of the present disclosure and together with the foregoing disclosure, serve to provide further understanding of the technical spirit of the present disclosure. However, the present disclosure is not construed as being limited to the drawings.

(2) FIG. 1 is a diagram illustrating an apparatus for manufacturing an anode active material for a lithium secondary battery according to an exemplary embodiment.

(3) FIG. 2 is a diagram illustrating a process of manufacturing an anode active material for a lithium secondary battery according to an exemplary embodiment.

(4) FIG. 3 is a flowchart schematically illustrating a procedure of Embodiment example 1.

(5) FIG. 4a is a diagram schematically illustrating a section of the carbon-silicon composite according an embodiment of the present invention and FIG. 4b is a photographic image of an active material obtained in Embodiment example 1.

(6) FIG. 5 illustrates a volume expansion result of electrodes manufactured in Embodiment example 1 and Comparative example 1 during charge/discharge cycles.

(7) FIG. 6 illustrates a cycle characteristics result of electrodes manufactured in Embodiment example 1 and Comparative example 1.

BEST MODE FOR EMBODIMENT OF THE INVENTION

(8) Hereinafter, the present disclosure will be described in detail. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation.

(9) FIG. 1 illustrates an embodiment of an apparatus for manufacturing an anode active material for a lithium secondary battery according to the present disclosure. Also, FIG. 2 illustrates an embodiment of a process of manufacturing an anode active material for a lithium secondary battery according to the present disclosure. However, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the disclosure, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the disclosure.

(10) An anode active material for a lithium secondary battery according to the present disclosure comprises a carbon-silicon composite including a core wherein a cavity is formed therein, and a coating layer formed on a surface of the core and including a phosphorus (P)-based alloy (See FIGS. 4a and 4b).

(11) The anode active material functions to absorb lithium ions in a cathode active material during charging and then generates electric energy along the concentration gradient of lithium ions during the deintercalation of the absorbed lithium ions. If Si having high charging/discharging capacity is used as anode active material, Si forms an alloy with lithium ions during the absorption of lithium ions to cause volume expansion. To solve the volume expansion problem of Si, the present disclosure introduced a coating layer including a phosphorus-based alloy on a surface of a carbon-silicon composite having a cavity formed therein. The structure of the carbon-silicon composite will be explained herein below, referring to FIG. 4a.

(12) The carbon-silicon composite (400) is prepared by carbonizing a silicon polymer particle in a reducing atmosphere, and includes not only a carbon-silicon bond but also a carbon-silicon oxide bond containing oxygen. The use of the carbon-silicon composite contributes to alleviation of volume expansion of Si.

(13) Also, such a carbon-silicon composite (400) has a cavity (410) inside the core, and due to this cavity, a surface area of the carbon-silicon composite is increased, and when the volume of Si expands, the cavity may act as a buffer. The cavity is located at the approximate center in the core of the carbon-silicon composite. Therefore, the center of the carbon-silicon composite is empty. The cavity occupies from 20 to 80 volume % of the carbon-silicon composite. Such cavity relieves the stress generated from the volumetric expansion.

(14) The carbon-silicon composite (400) includes the core (410) having the cavity formed therein and the coating layer which includes the phosphorus (P)-based alloy and covers the core. The phosphorus (P)-based alloy is an alloy containing phosphorus, and includes alloys of phosphorus and metals such as Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Pd, Ag, In, Sn, and W. The phosphorus-based alloy is coated on the surface of the carbon-silicon composite to reduce degradation of the active material caused by the volume expansion.

(15) The coating layer (430) include the layer (431) entirely covering the core. Further, the coating layer (430) may further include a plurality of domains (432) protruded from the layer (431) like embossed shape. The domains (432) are formed of and filled with the same material as the layer (431), both of which includes the phosphorus (P)-based alloy, and are continuously connected to the layer (431). Besides, the domains (432) may be formed at a distance from each other and may in total occupy from 40 to 60% of surface area of the layer (431). The domains (432) may have a hemisphere shape or a similar shape with the hemisphere. Therefore, the domains (432) may have a circular top-view and the height (thickness) and/or diameter of each of the domains (432) can be different. In the present disclosure, the word ‘circle’ or ‘circular’ means any circle-like or circle-similar shape including oval and distorted circle. The domains may be formed all over the layer covering the core surface. Also, an average diameter of the domains or a thickness of the coating layer including the domains is preferably equal to or less than 0.1 μm, and because the thickness greater than 0.1 μm hinders diffusion of the lithium ion through the domains, the excessively thick coating layer is not preferred.

(16) Particularly, a density of the phosphorus-based alloy of the present disclosure is preferably in a range of 1.2 to 5.8 g cm.sup.−3.

(17) A specific surface area of the core of the carbon-silicon composite is preferably equal to or less than 10 m.sup.2/g. When the specific surface area of the core is greater than 10 m.sup.2/g, initial efficiency of the anode may reduce. In the present disclosure, a lower bound of the specific surface area of the core is not specially limited. A preferred lower bound may be 5 m.sup.2/g, however this is just an example and is not limited thereto.

(18) Also, the core may have a particle diameter in a range of 0.5 μm to 100 μm, preferably, 0.5 μm to 40 μm. When the average particle diameter of the core is less than 0.5 μm, initial efficiency of the anode may reduce due to fine powder of the core, and when the average particle diameter is greater than 100 μm, procedural efficiency of the anode slurry coating may reduce and scratches on an electrode may increase.

(19) The anode active material of the present disclosure may be manufactured by the following method.

(20) First, silicon polymer particles with the metal catalyst supported on the surface are prepared by treating silicon polymer particles with a metal chloride solution.

(21) The silicon polymer represents a polymer containing at least one hetero atom such as silicon (Si) within a polymer repeat unit. The silicon polymer used in the present disclosure is not limited to a specific type, but may use particles of silicone resin, for example, polysiloxane, polysilane, and polycarbosilane. More preferably, an emulsion type in which silicon polymer particles in latex form are uniformly dispersed in a dispersion medium may be used.

(22) Also, the metal chloride solution may be one that metal chloride selected from PdCl.sub.2, SnCl.sub.2, and CuCl.sub.2 is dissolved in an aqueous solution of nitrohydrochloric acid or hydrochloric acid, or in a polar organic solvent of dimethylformamide (DMF), hexamethylphosphoramide (HMPA), dimethylacetamide (DMA) or dimethylsulfoxide (DMSO).

(23) The silicon polymer particles are treated with the metal chloride solution, so that a metal from metal chloride is supported on the surface of the silicon polymer particles. The metal may act as a catalyst in a subsequent reaction process.

(24) Subsequently, the silicon polymer particles with the catalyst supported on the surface are dispersed in a plating bath filled with a plating solution containing a metal ion for a phosphorus-based alloy and phosphoric acid.

(25) Referring to FIG. 1, the plating bath has a stirrer at the bottom thereof to uniformly disperse, in the plating solution, the silicon polymer particles with the catalyst supported on the surface. As the plating solution, various types of plating solutions may be used, and a detailed description is as follows. In the case of an acidic electroless nickel plating solution, a mixture of a phosphorus-based alloy metal ion and sodium hypophosphite (NaH.sub.2PO.sub.2.H.sub.2O) in an aqueous solution of lactic acid, propionic acid (CH.sub.3CH.sub.2COOH), sodium acetate (CH.sub.3CO.sub.2Na), sodium succinate (CH.sub.3CH.sub.2COOH), malic acid or sodium citrate may be used, and in the case of an alkaline electroless nickel plating solution, a mixture of a phosphorus-based alloy metal ion and sodium hypophosphite (NaH.sub.2PO.sub.2.H.sub.2O) in an aqueous solution of ammonium chloride (NH.sub.4Cl) or sodium pyrophosphate may be used.

(26) As the metal ion used in the phosphorus-based alloy metal ion, Ni.sup.2+, Cu.sup.2+, Ti.sup.4+, Al.sup.3+, Cr.sup.2+, Cr.sup.3+, Cr.sup.6+, Mn.sup.2+, Mn.sup.3+, Mn.sup.4+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+, Zn.sup.2+, Ga.sup.3+, Ge.sup.4+, As.sup.4+, Pd.sup.2+, Ag.sup.+, In.sup.2+, In.sup.3+, Sn.sup.2+, and W.sup.6+ may be used, singularly or in combination.

(27) Subsequently, the bath is heated to form phosphorus (P)-based alloy surface-coated silicon polymer particles.

(28) The plating bath of FIG. 1 may use a hot plate to perform heating. Heating is performed while stirring the plating solution favorably by operating the stirrer. In this instance, a preferred temperature is in a range of 40 to 100° C. Through heating, phosphoric acid reacts with the phosphorus-based alloy ion, in the presence of the catalyst, on the surface of the silicon polymer particles to form phosphorus-based alloy surface-coated silicon polymer particles. Preferably, the phosphorus-based alloy may be coated while forming domains.

(29) Finally, the phosphorus (P)-based alloy surface-coated silicon polymer particles are carbonized by heat treatment in a reducing atmosphere.

(30) The reducing atmosphere is not particularly limited, but a reducing atmosphere such as H.sub.2 or N.sub.2 may be used. By carbonizing the phosphorus-based alloy surface-coated silicon polymer particles through heat treatment in a range of about 800 to 1200° C., oxygen or hydrogen inside is discharged outwards to form a cavity. In such a way, the anode active material of the present disclosure in which the phosphorus-based alloy coating layer is provided on the surface of the carbon-silicon composite having the cavity may be manufactured.

(31) In FIG. 2, a process of forming the phosphorus-based alloy on the surface of the silicon polymer particles with the metal catalyst supported on the surface and a process of forming the anode active material having the cavity formed therein by heat treatment are illustrated. The phosphorus-based alloy may be easily formed due to the metal catalyst supported on the surface of the silicon polymer particles, and the internal cavity may be formed by high heat treatment in the reducing atmosphere.

(32) The anode active material of the present disclosure manufactured as described in the foregoing may be used to manufacture an anode by a manufacturing method generally used in the art. Also, a cathode according to the present disclosure may be manufactured by a method generally used in the art. For example, the electrode may be manufactured by mixing an electrode active material of the present disclosure with a binder, a solvent, if necessary, a conductive material and a dispersant, agitating the mixture to prepare a slurry, applying the slurry to a current collector, and compressing the result.

(33) As the binder, various types of binder polymers may be used, for example, polyimide, polyvinylidene fluoride-co-hexafluoro propylene (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, and the like. In case that polyimide is used as binder for the anode active material, performance deterioration of battery which is resulted from the contraction-expansion of carbon-silicon during the battery use can be inhibited since polyimide can physically strongly bind the carbon-silicon composite.

(34) As the cathode active material, lithium-containing transition metal oxide may be preferably used, for example, any one selected from the group consisting of Li.sub.xCoO.sub.2(0.5<x<1.3), Li.sub.xNiO.sub.2(0.5<x<1.3), Li.sub.xMnO.sub.2(0.5<x<1.3), Li.sub.xMn.sub.2O.sub.4(0.5<x<1.3), Li.sub.x(Ni.sub.aCo.sub.bMn.sub.c)O.sub.2(0.5<x<1.3, 0<a<1, 0<b<1, 0<c<1, a+b+c=1), Li.sub.xNi.sub.1-yCo.sub.yO.sub.2(0.5<x<1.3, 0<y<1), Li.sub.xCo.sub.1-yMn.sub.yO.sub.2(0.5<x<1.3, 0≦y<1), Li.sub.xNi.sub.1-yMn.sub.yO.sub.2(0.5<x<1.3, O≦y<1), Li.sub.x(Ni.sub.aCo.sub.bMn.sub.c)O.sub.4(0.5<x<1.3, 0<a<2, 0<b<2, 0<c<2, a+b+c=2), Li.sub.xMn.sub.2-zNi.sub.zO.sub.4(0.5<x<1.3, 0<z<2), Li.sub.xMn.sub.2-zCo.sub.zO.sub.4(0.5<x<1.3, 0<z<2), Li.sub.xCoPO.sub.4(0.5<x<1.3) and Li.sub.xFePO.sub.4(0.5<x<1.3) or mixtures thereof, and the lithium-containing transition metal oxide may be coated with a metal such as aluminum (Al) or metal oxide. Also, besides the lithium-containing transition metal oxide, sulfide, selenide, and halide may be used.

(35) When the electrode is manufactured, a lithium secondary battery generally used in the art, in which a separator interposed between the cathode and the anode and an electrolyte solution are included, may be manufactured using the electrode.

(36) In the electrolyte solution used in the present disclosure, a lithium salt included as an electrolyte may use, without limitation, those generally used in an electrolyte solution for a lithium secondary battery, and for example, an anion of the lithium salt may be any one selected from the group consisting of F.sup.−, Cl.sup.−, Br.sup.−, I.sup.−, NO.sub.3.sup.−, N(CN).sub.2.sup.−, BF.sub.4.sup.−, ClO.sub.4.sup.−, PF.sub.6.sup.−, (CF.sub.3).sub.2PF.sub.4.sup.−, (CF.sub.3).sub.3PF.sub.3.sup.−, (CF.sub.3).sub.4PF.sub.2.sup.−, (CF.sub.3).sub.5PF.sup.−, (CF.sub.3).sub.6P.sup.−, CF.sub.3SO.sub.3.sup.−, CF.sub.3CF.sub.2SO.sub.3.sup.−, (CF.sub.3SO.sub.2).sub.2N.sup.−, (FSO.sub.2).sub.2N.sup.−, CF.sub.3CF.sub.2(CF.sub.3).sub.2CO.sup.−, (CF.sub.3SO.sub.2).sub.2CH.sup.−, (SF.sub.5).sub.3C.sup.−, (CF.sub.3SO.sub.2).sub.3C.sup.−, CF.sub.3(CF.sub.2).sub.7SO.sub.3.sup.−, CF.sub.3CO.sub.2.sup.−, CH.sub.3CO.sub.2.sup.−, SCN.sup.−, and (CF.sub.3CF.sub.2SO.sub.2).sub.2N.sup.−.

(37) In the electrolyte solution used in the present disclosure, an organic solvent included in the electrolyte solution may include, without limitation, those generally used in an electrolyte solution for a lithium secondary battery, as a representative example, any one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethylcarbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma butyrolactone, propylene sulfate and tetrahydrofuran, or mixtures thereof. Particularly, among the carbonate-based organic solvents, cyclic carbonate such as ethylene carbonate and propylene carbonate corresponds to an organic solvent having a high viscosity, and is preferred to use because it dissociates a lithium salt in an electrolyte favorably due to its high dielectric constant, and in this instance, such cyclic carbonate is more preferred to use because it contributes to form an electrolyte solution having high electrical conductivity when mixed with linear carbonate having a low viscosity and a low dielectric constant such as dimethyl carbonate and diethyl carbonate at a proper ratio.

(38) Optionally, the electrolyte solution stored according to the present disclosure may further include an additive, such as an overcharge inhibitor, used in a general electrolyte solution.

(39) Also, as the separator, a general porous polymer film employed as a separator in the art may be used, for example, a porous polymer film made from a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, arranged singularly or in a stack, or a general porous non-woven fabric, for example, a non-woven fabric made from a glass fiber having a high melting point, a polyethyleneterephthalate fiber, and the like, however the present disclosure is not limited thereto.

(40) A battery casing used in the present disclosure may employ those generally used in the art, and is not limited to a specific outer shape based on usage of the battery, and the battery casing may have, for example, a cylindrical shape using a can, a prismatic shape, a pouch shape, a coin shape, and the like.

MODE FOR EMBODIMENT OF THE INVENTION

(41) Hereinafter, a detailed description is provided through an embodiment example and a comparative example to describe embodiments and effects of the present disclosure more specifically. However, it should be understood that embodiments of the present disclosure may be modified in various forms and the scope of the invention is not limited to the following embodiment. The embodiments of the present disclosure are provided to describe the present disclosure to those skilled in the art more completely.

EMBODIMENT EXAMPLE

Embodiment Example 1

Manufacture of an Anode Active Material with an Electroless Plating Layer and a Battery Comprising the Same

(42) An electroless plating layer on silicon polymer was manufactured in accordance with the flowchart of FIG. 3.

(43) 5 ml of a suspension solution containing 48 wt % of solids including polycarbosilane was put in 100 ml of an aqueous catalyst solution containing 0.1 g of PdCl.sub.2 and 50 μl of nitrohydrochloric acid, followed by impregnation at about 5° C. for 24 hours, so that the Pd catalyst was adsorbed on the surface of the silicon polymer particles, and suction filtration and washing was conducted on the catalyst-adsorbed solution to collect powder in cake form. The collected powder was put in an electroless NiP plating solution including 15 g/L of Ni(PH.sub.2O.sub.2).sub.2.6H.sub.2O, 12.0 g/L of H.sub.3BO.sub.3, 2.5 g/L of CH.sub.3COONa, and 1.3 g/L of (NH.sub.3).sub.2SO.sub.4, and after reaction at 30° C. for about 5 minutes, a precipitation reaction of Ni was stopped by lowering the temperature with ice water. Subsequently, the resulting NiP-plated silicon polymer was collected through suction filtration, and freeze-dried to completely remove water.

(44) Powder obtained through freeze-drying was heat-treated at temperature of 1000° C. in an inert atmosphere of N.sub.2 or Ar or a reducing atmosphere in which H.sub.2 is present in part. In this instance, through reduction and carbonization of the silicon polymer, a silicon-carbon composite was formed.

(45) In FIG. 4, a photographic image of an active material obtained through electroless plating and heat treatment of a silicon polymer particulate is shown. As seen in FIG. 4, a spheric particle having a diameter of about 0.6 μm was obtained.

(46) An NMP (N-methyl pyrrolidone)-based organic slurry was prepared based on a composition including the active material, VGCF as a conductive material, and a polyimide-based binder as a binder at 95:1:4, and was applied to a Cu current collector, followed by drying and pressing processes, to manufacture an electrode. A coin cell was manufactured using the manufactured electrode, a metal lithium as an opposite electrode, and an EC+EMC organic solvent containing 1M LiPF.sub.6 as an electrolyte.

Comparative Example 1

Manufacture of an Anode Active Material without an Electroless Plating Layer and a Battery Comprising the Same

(47) An anode active material and a coin cell were manufactured by the same method as Embodiment example 1 except an electroless plating layer was not formed on silicon polymer.

EVALUATION EXPERIMENT

(48) In FIG. 5, an average of volume expansion of an electrode during three (3) charge/discharge cycles is shown. To show the extent of volume expansion of the electrode, a thickness of the electrode at charge on each cycle was measured relative to an initial thickness of the electrode. The electrode of Embodiment example 1 using the active material with NiP electroless plating and heat treatment exhibited less volume expansion on each cycle than the electrode of Comparative example 1 using the active material without electroless plate coating, and thus, was found to be effective in suppressing the volume expansion.

(49) In FIG. 6, the cycle characteristics result of the electrodes manufactured in Embodiment example 1 and Comparative example 1 are shown. It was found that the electrode of Comparative example 1 without plating exhibited the deterioration in cycle characteristics during 10 cycles, while the NiP-plated electrode of Embodiment example 1 suppressed an extent of cycle characteristic deterioration.