Pre-lithiation method of negative electrode for secondary battery

Abstract

A method of pre-lithiating an electrode for a secondary battery, the method including: a first step of bringing a lithium metal into direct contact with an electrode in an electrolyte and applying pressure to the electrode to prepare a pre-lithiated electrode; and a second step of removing the lithium metal and then applying pressure to the pre-lithiated electrode to perform a stabilization process. The electrode for the secondary battery after going through the pre-lithiation can relieve volume change of the electrode and reduce contact loss of the electrode.

Claims

1. A method of pre-lithiating an electrode for a secondary battery, the method comprising: a first step of bringing a lithium metal into direct contact with an electrode in an electrolyte and applying pressure to the electrode and the lithium metal for 0.5 to 3 hours to prepare a pre-lithiated electrode; and a second step of removing the lithium metal and then applying pressure to the pre-lithiated electrode for 2 to 6 hours for perform a stabilization process; wherein the pressure applied in the first step is in a range of 20 to 50 kgf and the pressure applied in the second step is in a range of 20 to 50 kgf.

2. The method of claim 1, wherein the first step comprises placing the lithium metal on the electrode and tightening a jig to apply pressure to the lithium metal and electrode.

3. The method of claim 1, wherein the applying pressure of the second step comprises tightening the pre-lithiated electrode on a jig to apply pressure.

4. The method of claim 1, wherein the electrode is a negative electrode.

5. The method of claim 1, wherein the electrode comprises silicon oxide.

6. The method of claim 1, further comprising wetting the electrode by immersing the electrode in an electrolyte before the first step.

7. The method of claim 6, wherein the electrode is wetted with the electrolyte for 1 to 48 hours.

8. The method of claim 1, wherein the second step is performed in an electrolyte.

9. A method of preparing an electrode for a secondary battery, the method comprising: mixing an electrode active material, a conductive material and a binder to prepare a slurry; coating the slurry on an electrode current collector; pressing the coated electrode current collector with a roller and punching the electrode current collector to prepare unit electrodes and drying the unit electrodes; and pre-lithiating the dried unit electrodes according to the pre-lithiation method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The FIGURE is a schematic diagram showing a pre-lithiation step (first step) and a stabilization step (second step) according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) Hereinafter, the present invention will be described in detail. The present invention is not limited by the following examples and experimental examples. The embodiments according to the present invention may be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

(3) The pre-lithiation of the electrode for a secondary battery according to the present invention includes a first step of directly pre-lithiating by allowing a lithium metal to directly contact an electrode in an electrolyte solution and applying pressure thereto; and a second step of separating the lithium metal and then performing a stabilization process by applying pressure to the pre-lithiated electrode.

(4) Referring to the FIGURE, in the first step of the pre-lithiation of the present invention, lithium metal is directly contacted to the electrode in the electrolyte solution and pressure is applied thereto. Lithium metal is placed on the electrode, and a constant force is applied to the electrode at the top and bottom to allow the electrode to directly contact the lithium metal to thereby perform pre-lithiation. The direct contact between the electrode and the lithium metal allows the lithium to enter the space between the electrodes similarly to the initial irreversible state, and the irreversible state is less likely to occur in the subsequent charge by the space, thereby allowing the initial irreversible level to be controlled.

(5) The electrolytic solution for carrying out the pre-lithiation process of the present invention is not limited in its kind as long as the lithium salt is dissolved in a solvent and promotes side reactions to be experienced when the electrode is initially charged. Examples of the lithium salt include one or more selected from the group consisting of LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4, LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3, LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4, CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li, (CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, and lithium 4-phenylborate, and examples of the solvent include one or a mixture of two or more selected from the group consisting of propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butylolactone, and 1,2-dimethoxyethane.

(6) In the first step, the lithium metal preferably covers an entire area of the electrode, and a method of applying a force to the lithium metal is not limited. The lithium metal is placed on the electrode and the jig is tightened, so that the pressure can be applied toward the center from above and below the contact surface of the electrode and the lithium metal. It is also possible to apply a pressure to the contact surface between the electrode and the lithium metal by using a press machine or the like.

(7) The pressure applied for the pre-lithiation of the first step may be 20 to 50 kgf, preferably 25 to 45 kgf, most preferably 30 to 40 kgf. In the case of less than 20 kgf, the pre-lithiation does not occur sufficiently, and the initial irreversible reaction occurs largely, and the volume expansion is not suppressed during the whole lithiation process, which makes the contact between the active materials undesirable. Further, when it exceeds 50 kgf, the lithiation may proceed excessively and the electrode capacity may be reduced, and the excessive pressure may be applied to the electrode, so that the active material damage may occur.

(8) Further, in the pre-lithiation in the first step, the time for applying the pressure to the electrode and the lithium metal may be 0.1 to 10 hours, more preferably 0.3 to 5 hours, and most preferably 0.5 to 3 hours.

(9) In the present invention, the electrode may be wetted by immersing the electrode in the electrolytic solution before performing the pre-lithiation in the first step. Due to the wetting, the volume of the electrode expands to a certain extent, and the distance between the electrode active material particles increases appropriately, so that the electrolyte penetrates deeply between the particles. Therefore, the contact area between the electrolyte and the electrode is increased, so that lithium ions can penetrate into the electrode more easily by direct contact.

(10) The wetting is preferably 1 hour to 48 hours, more preferably 3 hours to 30 hours, and most preferably 5 hours to 20 hours. In case of wetting less than 1 hour, the negative electrode may not expand sufficiently and electrolyte may not penetrate deeply into the negative electrode sufficiently, so that the pre-lithiation due to direct contact at a later step may not occur sufficiently. When wetting is performed for more than 48 hours, the electrode is exposed to the electrolyte for too long, resulting in poor shape stability and deterioration of the adhesive strength of the electrode, which reduces the cell balance.

(11) In the second step of the present invention, the lithium metal deposited on the electrode is removed, and the negative electrode is stabilized by applying pressure to the pre-lithiated negative electrode. Even if the electrode is subjected to the pre-lithiation process, there may be a change in the volume of the electrode as the lithium ion moves through the diffusion in the electrode. Such a change in volume increases the irreversible capacity of the electrode, and thus in the present invention, a stabilization step of applying pressure to the electrode is performed in order to further mitigate the volume change of the electrode.

(12) The pressure applied to the electrode during the stabilization process is preferably 20 to 50 kgf, more preferably 25 to 45 kgf, and most preferably 30 to 40 kgf. If it is less than 20 kgf, the pre-lithiation does not sufficiently take place, and the initial irreversible state significantly occurs which is undesirable. If it exceeds 50 kgf, the pre-lithiation proceeds excessively, which may cause a decrease in the electrode capacity. The method of applying the pressure to the pre-lithiated electrode may be performed by using a press machine or by a method of fastening the pre-lithiated electrode to the jig.

(13) The stabilization process may be performed in an electrolytic solution. Further, the time for applying the pressure to the electrode in the stabilization step is preferably 0.5 to 10 hours, more preferably 1 to 8 hours, and most preferably 2 to 6 hours. If the time for applying the pressure in the stabilization step is less than 0.5 hour, it is difficult to achieve the object of the present invention, and if it exceeds 10 hours, the shape stability of the electrode may be deteriorated.

(14) The pre-lithiation method of the present invention can be applied to a negative electrode for a secondary battery. In particular, in order to increase the capacity density of a negative electrode using a carbon material such as graphite, it is preferable to apply the pre-lithiation method of the present invention to the negative electrode using silicon and oxides thereof as its main material.

(15) The silicon oxide has a theoretical capacity density of 4200 mAh/g, which is much higher than the theoretical capacity density of carbon material of 372 mAh/g. Therefore, it can be suitably used as a negative electrode for a secondary battery, but since its shape stability is low, the initial irreversible capacity is large and there is a risk of reduction of the electrode capacity or a collapse of the cell balance, and thus a pre-lithiation process as in the present invention is necessary.

(16) The present invention is also characterized in that it provides a method of manufacturing an electrode including a process of pre-lithiation by the above-described method. A method of manufacturing an electrode according to the present invention includes: mixing an electrode active material, a conductive material and a binder to prepare a slurry; coating the mixed slurry on an electrode current collector; pressing the coated electrode current collector with a roller and punching toe electrode current collector to generate unit electrodes and drying the unit electrodes; and pre-lithiating the dried unit electrodes according to the pre-lithiation method.

(17) Except for the pre-lithiation step, it is the same as the conventional method for manufacturing a negative electrode for a secondary battery. Therefore, the step of pre-lithiation can be easily added to the existing process, and since the added process itself is simple, the cost increase is not significant, which is an advantage. In addition, the conventional physicochemical method has a disadvantage of fire and explosion because it is carried out at a high temperature, and the electrochemical method is difficult to homogenize and the cost is greatly increased. However, the lithiation method according to the present invention is advantageous in that the cost increase is relatively small and the lithiation can be performed relatively safely and uniformly, and the pre-lithiated electrode is stabilized to thereby minimize the volume change of the electrode. Therefore, the effect of reducing the initial irreversible capacity as compared with the electrode by the conventional direct contact type lithiation method and the capacity retention rate is further improved.

(18) The present invention also provides a secondary battery including a negative electrode manufactured by the above-described method.

(19) The secondary battery according to the present invention includes an electrode assembly in which two electrodes of different polarities are stacked in a state separated from each other by a separator. The electrode assembly includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a separator.

(20) Specifically, the positive electrode, for example, may be prepared by applying a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode current collector, followed by drying. If necessary, a filler may be further added to the mixture.

(21) The positive electrode active material according to the present invention may be mixed with a compound, which uses a lithium intercalation material as its main component, such as a layered compound such as lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2), or a compound substituted with one or more transition metals; lithium manganese oxides such as Li.sub.11xMn.sub.2-xO.sub.4(herein, x is between 0 and 33), LiMnO.sub.3, LiMn.sub.2O.sub.3, and LiMnO.sub.2; lithium copper oxide (Li.sub.2CuO.sub.2); vanadium oxides such as LiV.sub.3O.sub.8, LiFe.sub.3O.sub.4, V.sub.2O.sub.5, and Cu.sub.2V.sub.2O.sub.7; lithium nickel oxide expressed by LiNi.sub.1-xM.sub.xO.sub.2 (herein, M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x=0.01 to 0.3); lithium manganese composite oxide expressed by LiMn.sub.2-xM.sub.xO.sub.2 (herein, M=Co, Ni, Fe, Cr, Zn or Ta, x 32 0.01 to 0.1) or Li.sub.2Mn.sub.3MO.sub.8 (herein, M=Fe, Co, Ni, Cu or Zn); LiMn.sub.2O.sub.4 in which a part of Li in the formula is substituted with an alkaline earth metal ion; disulfide compound; and Fe.sub.2(MoO.sub.4).sub.3 or a compound oxide formed by combination thereof.

(22) The positive electrode collector generally has a thickness of 3 to 500 micrometers. The positive electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon or aluminum or stainless steel of which the surface has been treated with carbon, nickel, titanium, silver, or the like. The current collector may have fine irregularities on the surface thereof to increase the adhesion of the positive electrode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

(23) The conductive material is usually added in an amount of 1 to 50% by weight based on the total weight of the mixture including the positive electrode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum and nickel powder; conductive whiskey such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives and the like.

(24) The binder is added in an amount of 1 to 50% by weight, on the basis of the total weight of the mixture containing the positive electrode active material, as a component that assists in bonding between the active material and the conductive material and bonding to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

(25) The filler is optionally used as a component for inhibiting expansion of a positive electrode, and is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers.

(26) Further, the negative electrode may be formed by coating a negative electrode material on a negative electrode collector and drying the negative electrode active material. The negative electrode may further include the above-described components.

(27) The negative electrode current collector is generally made to a thickness of 3 to 500 micrometers. The negative electrode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel of which the surface has been treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like. In addition, like the positive electrode current collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

(28) As the negative electrode active material of the present invention, a material capable of reversibly adsorbing/discharging lithium ions such as silicon (Si) and tin may be used. As long as such a material is used, it is possible to exhibit the effects of the present invention in any of composite, alloy, compound, solid solution and composite negative electrode active material including silicon-containing material and tin-containing material. As the silicon-containing material, Si, SiO.sub.x(0.5<x<2.0) or an alloy, a compound or a solid solution which is generated by substituting part of Si or Si contained in SiOx with at least one element selected from the group consisting of B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, N, and Sn may be used.

(29) These materials may constitute a negative electrode active material alone or may constitute a negative electrode active material by a plurality of kinds of materials. Examples of constituting the negative electrode active material by the plural kinds of materials include a compound containing Si, oxygen and nitrogen, a composite of a plurality of compounds containing Si and oxygen and having different composition ratios of Si and oxygen, and the like. Among these, SiOx (0.5<; x<2.0) is preferable because the discharge capacity density is large and the expansion ratio at the time of filling is smaller than that of Si.

(30) It is possible to use a commonly known polyolefin separator or a composite separator in which an organic and inorganic composite layer is formed on the olefin based material, as a separator for insulating the electrodes between the positive electrode and the negative electrode, and the present invention is not limited to these examples.

(31) The electrolyte injected into the secondary battery is a lithium salt-containing non-aqueous electrolyte, which is composed of a nonaqueous electrolyte and lithium. As the non-aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte and the like are used.

(32) Examples of the non-aqueous electrolyte include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylenecarbonate, dimethyl carbonate, diethyl carbonate, gamma-Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate, etc.

(33) Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, a polymer including an ionic dissociation group, and the like.

(34) Examples of the inorganic solid electrolyte include nitrides, halides, and sulfates of Li such as Li.sub.3N, LiI, Li.sub.5NI.sub.2, Li.sub.3N—LiI—LiOH, LiSiO.sub.4, LiSiO.sub.4—LiI—LiOH, Li.sub.2SiS.sub.3, Li.sub.4SiO.sub.4, Li.sub.4SiO.sub.4—LiI—LiOH, and Li.sub.3PO.sub.4—Li.sub.2S—SiS.sub.2.

(35) The lithium salt is a substance that is soluble in the non-aqueous electrolyte. The examples of the lithium salt include LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4, LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3, LiCF.sub.3CO.sub.2, LiASF.sub.6, LiSbF.sub.6, LiAlC.sub.14, CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li, (CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium 4-phenylborate, imide and the like.

(36) For the purpose of improving charge/discharge characteristics, flame retardancy, etc., pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. may be added to the non-aqueous electrolyte. In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high temperature storage characteristics.

(37) Hereinafter, the present invention will be described in more detail through examples below. However, the following Examples and Experimental Examples are provided for illustrating the present invention, and the scope of the present invention is not limited by these Examples and Experimental Examples.

Example 1

(38) <Preparation of Negative Electrode>

(39) 92 wt % of negative electrode active material (graphite: SiO=7:3), 3 wt % of Denka Black (conductive material), 3.5 wt % of SBR (binder) and 1.5 wt % of CMC (thickener) were added to water as the negative electrode active material, to thereby prepare a negative electrode mixture slurry.

(40) The negative electrode mixture slurry was coated on one side of the copper collector, dried and rolled, and then punched to a predetermined size to produce a negative electrode.

(41) <High Pressure, Direct Contact Pre-Lithiation>

(42) As shown in the FIGURE, the negative electrode prepared above was wetted for 10 hours in an electrolyte (an electrolyte which is generated by dissolving 1M lithium hexafluorophosphate (LiPF.sub.6) in a solvent that is generated by mixing ethylene carbonate (EC) with ethyl methyl carbonate (DEC) by the volume ratio of 50:50), then the lithium metal is disposed on the negative electrode in a state that there is the electrolyte, and a jig is tightened to be pressed by a force of 30 kgf. After 30 minutes of lithiation, the lithium metal is removed.

(43) <High Pressure Stabilization>

(44) As shown in the FIGURE, the above-prepared pre-lithiated negative electrode is tightened to the jig again and pressed for 2 hours under a force of 30 kgf to allow the stabilization process to proceed.

(45) <Preparation of Lithium Secondary Battery>

(46) A polyolefin separator was interposed between the negative electrode and LiCoO.sub.2 as counter electrode, and an electrolyte which is generated by dissolving 1M LiPF.sub.6 in a solvent generated by mixing ethylene carbonate (EC) and ethyl methyl carbonate (DEC) by a volume ratio of 50:50, to thereby prepare a coin-type full-cell.

Example 2

(47) Secondary battery was produced under the same conditions as in Example 1, except that the pressure applied in the lithiation and stabilization step was changed to 20 kgf.

Example 3

(48) A secondary battery was manufactured under the same conditions as in Example 1, except that the electrode was pressed for 3 hours in the high-voltage stabilization step.

Example 4

(49) A secondary battery was manufactured under the same conditions as in Example 1 except that a negative electrode and a lithium metal were pressed for 1 hour at the high-voltage direct contact lithiation step.

Example 5

(50) The secondary battery was manufactured under the same conditions as in Example 1, except that the wetting time of the electrode was changed to 15 hours at the high voltage direct contact lithiation step.

Comparative Examples 1 to 4

(51) A secondary battery was manufactured under the same conditions as in Example 1, except that the pressing force at the time of lithiation, the wetting time of the electrode, and the pressing force at the time of stabilization were changed as shown in Table 1.

(52) 1. Measurement of Electrode Thickness

(53) The thickness of the negative electrode prepared above was measured using a height measuring device and the result about the increase of the thickness compared to the initial time is shown in Table 1.

(54) 2. Measurement of Capacity Retention Rate

(55) The coin-type full-cell thus prepared was subjected to a charge/discharge reversibility test using an electrochemical charge/discharge device, and the capacity retention rate is shown in Table 1. During charging, the battery was charged at a current density of 0.1 C-rate up to a voltage of 4.2 V (vs. Li/Li+), and discharged to a voltage of 2.5 V at the same current density during discharging.

(56) TABLE-US-00001 TABLE 1 Increase rate Wetting of electrode 100 cycle time of Press force at Pre-lithiation Press force at stabilization thickness after capacity electrode pre-lithiation time stabilization time pre-lithiation retention rate (hours) (kgf) (hours) (kgf) (hours) (%) (%) Example 1 10 30 0.5 30 2 31 88 Example 2 10 20 0.5 20 2 33 86 Example 3 10 30 0.5 30 3 29 90 Example 4 10 30 1 30 2 37 85 Example 5 15 30 0.5 30 2 32 88 Comparative 10 30 0.5 0 2 43 77 example 1 Comparative 10 1 0.5 0 2 51 68 example 2 Comparative 10 1 0.5 30 2 47 75 example 3 Comparative 0 30 0.5 30 2 39 73 example 4

(57) In the case of the embodiment in which the compression was strong during the pre-lithiation and stabilization, the thickness change after the pre-lithiation was smaller than that in the comparative example. The negative electrode was used to make a full cell and a cycle test was performed, and as a result of the test, the capacity retention rate up to 100 cycles was also measured higher than the comparative example.

(58) Generally, lithiation and stabilization causes lithium ions to enter the negative electrode, causing the volume of the active material to expand, causing the contact between the particles to drop, which causes the electrical contact to drop. However, as in the case of the embodiment of the present invention, when strong pressure is applied during the pre-lithiation and stabilization, the contact loss between particles due to the volume change is not greatly reduced. Therefore, it is seen that when the battery is manufactured using such an electrode and subjected to a cycle test, the capacity retention rate is increased.

(59) While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the present invention is not limited to the disclosed exemplary embodiments, and various changes and modifications may be made without departing from the technical idea of the present invention and the scope of the appended claims.

DESCRIPTION OF SYMBOLS

(60) 10: electrolyte

(61) 20: jig

(62) 30: lithium metal

(63) 40: electrode

(64) 50: pre-lithiated electrode