Positive electrode, lithium secondary battery comprising same, and method for manufacturing lithium secondary battery

Abstract

The present invention relates to a positive electrode comprising: a current collector; a positive electrode active material layer disposed on the current collector and containing a positive electrode active material; and a transfer functional layer provided on the positive electrode active material layer and including a composite represented by Formula 1, in which gas production can be reduced by preventing a cracking phenomenon of the positive electrode active material, and excellent capacity characteristics and lifespan characteristics can be achieved by improving over-lithiation efficiency of the positive electrode. [Formula 1] MaOb (wherein M includes at least one element selected from the group consisting of iron (Fe), manganese (Mn), cobalt (Co), titanium (Ti), vanadium (V), zirconium (Zr), and nickel (Ni), and a satisfies 1.0 < = a < = 3.0, and b satisfies 1.0 < = b < = 4.0.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2022-0182380, filed on Dec. 22, 2022, the disclosure of which is incorporated herein by reference. Technical field

[0003] The present invention relates to an over-lithiated positive electrode, a lithium secondary battery including the positive electrode, and a method for manufacturing the lithium secondary battery. Background art

[0004] Due to the rapid increase in the use of fossil fuels, the demand for using alternative energy or clean energy has increased, and as part of this trend, power generation and power storage using electrochemical reactions are the most active research fields.

[0005] Currently, typical examples of electrochemical devices using electrochemical energy can be secondary batteries, and their fields of use have a tendency to expand further. In recent years, with the development of technology and the increasing demand for portable devices such as laptop computers, mobile phones, and cameras, the demand for secondary batteries as an energy source has increased significantly, and among these secondary batteries, lithium secondary batteries with high energy density (i.e., high capacity) have been intensively studied, commercialized, and widely used.

[0006] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material in which lithium ions released from the positive electrode are intercalated and deintercalated, and silicon-based active material particles with a large discharge capacity can be used as the negative electrode active material. The silicon-based active material particles can correspond to silicon (Si) or SiO x (0 < x < 2). The silicon-based active material particles have the advantages of a large theoretical capacity and low price. However, since the silicon-based active material particles have an excessive volume change during battery operation, the disadvantage is that the battery life rapidly decreases as the battery cycles.

[0007] Therefore, in order to minimize the volume change of the silicon-based active material particles, there is a method of using only a part of the total capacity of the silicon-based active material particles. For this purpose, a so-called prelithiation method is used, in which lithium ions are pre-intercalated into the negative electrode containing the silicon-based active material particles. Specifically, if lithium ions are intercalated into the negative electrode by a method such as transferring lithium metal to the negative electrode, when the lithium ions react at the irreversible part of the negative electrode, the total capacity of the negative electrode can be reduced to the reversible capacity level. Therefore, since the amount of lithium ions intercalated during battery operation can be appropriately reduced to the level required for battery operation, the volume change of the silicon-based active material particles can be minimized.

[0008] However, during the pre-lithiation process, which involves depositing lithium metal on the surface of the negative electrode, excessive heat is generated due to the alloying reaction between lithium and silicon, and the potential for fire increases due to the reaction between lithium and moisture. Furthermore, during the slotting and stamping of the negative electrode, the potential for fire increases further due to the increased reaction area between lithium and the silicon-based active material. Furthermore, there is the serious safety concern of fire ignition caused by pre-lithiated silicon-based active material particles.

[0009] Therefore, a new technology is needed that can suppress the possibility of excessive heat generation and fire while improving the battery life by pre-intercalating lithium ions into the negative electrode before the battery is operated. Summary of the Invention

[0010] [Technical Issues]

[0011] One aspect of the present invention provides a positive electrode and a method for preparing the same, which improves the safety of the battery preparation process while controlling the available area of the negative electrode, and at the same time, improves the cracking phenomenon of the positive electrode active material through overlithiation of the positive electrode.

[0012] Another aspect of the present invention provides a high-capacity lithium secondary battery and a method for preparing the same, which suppresses the cracking phenomenon of positive electrode active material particles to reduce gas generation, improves life characteristics due to controlling the available area of the negative electrode, and can achieve the unique characteristics of the negative electrode active material without adverse effects by incorporating the positive electrode.

[0013] [Technical solution]

[0014] In order to solve the above-mentioned task, according to one aspect of the present invention, a positive electrode is provided, which includes: a current collector; a positive electrode active material layer disposed on the current collector and containing a positive electrode active material; and a transfer function layer disposed on the positive electrode active material layer and containing a composite represented by Formula 1.

[0015] [Formula 1]

[0016] M a O b

[0017] Wherein, M includes at least one selected from the group consisting of iron (Fe), manganese (Mn), cobalt (Co), titanium (Ti), vanadium (V), zirconium (Zr) and nickel (Ni), and a satisfies 1.0≤a≤3.0, and b satisfies 1.0≤b≤4.0.

[0018] In order to solve the above-mentioned task, according to another aspect of the present invention, a lithium secondary battery is provided, which comprises a positive electrode, a negative electrode and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode comprises: a current collector; a positive electrode active material layer disposed on the current collector and comprising a positive electrode active material; and a transfer function layer disposed on the positive electrode active material layer and comprising a composite represented by Formula 1 and lithium. In the dQ / dV curve obtained during the initial charge at a rate of 0.1C, the peak intensity ratio (R H / L ) is less than 0.8.

[0019] [Equation 1]

[0020] R H / L =I L / I H

[0021] Among them, R H / L is the peak intensity ratio, I L is the intensity of the peak appearing at a voltage of 2.30 V ± 0.05 V, and I H It is the intensity of the peak appearing at a voltage of 2.50 V±0.05 V.

[0022] In order to solve the above-mentioned task, according to another aspect of the present invention, a method for preparing a lithium secondary battery is provided, which comprises the following steps: preparing an electrode assembly comprising a positive electrode, a negative electrode and a separator arranged between the positive electrode and the negative electrode; and performing an activation process by placing the electrode assembly in a battery shell and injecting an electrolyte solution, wherein the positive electrode is prepared by comprising the following steps: step P1, placing an initial positive electrode active material layer on a current collector, and placing a transfer functional layer on the initial positive electrode active material layer; step P2, placing a transfer laminate comprising a base film and a lithium metal layer arranged on the base film on the transfer functional layer to form a positive electrode structure in which the lithium metal layer and the transfer functional layer are in contact with each other; step P3, rolling the positive electrode structure; and step P4, after rolling, preparing a positive electrode by removing the base film from the transfer laminate, wherein, after rolling in step P3, the transfer functional layer contains a complex of formula 1 and lithium.

[0023] [Beneficial Effects]

[0024] The advantage of the positive electrode of the present invention is that, since lithium ions migrate to the negative electrode through over-lithiation by a specific lithium metal transfer method during the activation process, the available area of the negative electrode is reduced, thereby increasing the life of the battery, and reducing the process risks caused by pre-lithiation of the negative electrode, and it can be expected that the gas production can be reduced by suppressing the cracking phenomenon of the positive electrode active material particles caused by the transfer functional layer during the lithiation process.

[0025] Furthermore, the secondary battery of the present invention can increase its capacity by incorporating this positive electrode, and can be expected to improve its lifespan by reducing gas generation. In particular, when using a silicon-based negative electrode active material, due to overlithiation of lithium, only a controlled area can be used without causing lithium loss in the positive electrode, thereby having the advantage of minimizing volume changes and improving lifespan characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram illustrating step P1 in a positive electrode preparation method according to one embodiment of the present invention.

[0027] Figure 2 Schematic diagram illustrating step P1 of using a transferred laminate including a polymer layer in a method for producing a positive electrode according to one embodiment of the present invention.

[0028] Figure 3 Schematic diagram illustrating step P2 in the positive electrode preparation method according to one embodiment of the present invention.

[0029] Figure 4 Schematic diagram illustrating step P3 in the positive electrode preparation method according to one embodiment of the present invention.

[0030] Figure 5 This is a scanning electron microscope (SEM) image taken from a cross section of the positive electrode of Example 1-3 according to one embodiment of the present invention.

[0031] Figure 6 This is a scanning electron microscope (SEM) image taken from a cross section of the positive electrode of Comparative Example 1-2. DETAILED DESCRIPTION

[0032] Hereinafter, the present invention will be described in more detail for clearer understanding of the present invention.

[0033] It will be understood that the words or terms used in the specification and claims should not be interpreted as having the meanings defined in commonly used dictionaries, and it will be further understood that, based on the principle that the inventor can appropriately define the meanings of words or terms to best interpret the present invention, the words or terms should be interpreted as having meanings consistent with their meanings in the context of the relevant technical field and the technical concept of the present invention.

[0034] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to limit the present invention.In the specification, unless otherwise stated, a term in the singular may include a plural form.

[0035] It should also be understood that when used in this specification, the terms "include", "comprising" or "having" specify the presence of stated features, quantities, steps, elements or a combination thereof, but do not exclude the presence or addition of one or more other features, quantities, steps, elements or a combination thereof.

[0036] In this manual, D 50 It can be defined as the particle size at which the cumulative volume in the particle size distribution curve is 50%. For example, D can be measured by using laser diffraction. 50 Laser diffraction can typically measure particle sizes from the submicron level to several millimeters, and can obtain highly reproducible and high-resolution results.

[0037] positive electrode

[0038] The positive electrode of the present invention includes: a current collector; a positive electrode active material layer provided on the current collector and including a positive electrode active material; and a transfer function layer provided on the positive electrode active material layer and including a composite represented by the following Formula 1.

[0039] [Formula 1]

[0040] M a O b

[0041] Wherein, M includes at least one selected from the group consisting of iron (Fe), manganese (Mn), cobalt (Co), titanium (Ti), vanadium (V), zirconium (Zr) and nickel (Ni), and a satisfies 1.0≤a≤3.0, and b satisfies 1.0≤b≤4.0.

[0042] According to one embodiment of the present invention, the positive electrode includes a positive electrode active material layer. The positive electrode active material layer itself can constitute the positive electrode, but the positive electrode active material layer can be provided on a positive electrode current collector, and the positive electrode active material layer can be provided on one surface or both surfaces of the positive electrode current collector.

[0043] According to one embodiment of the present invention, the positive electrode current collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode current collector may typically have a thickness of 3 μm to 500 μm, and microscopic irregularities may be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector may be in various shapes, such as a film, a sheet, a foil, a mesh, a porous body, a foam, a non-woven fabric, etc.

[0044] According to one embodiment of the present invention, the positive electrode active material is a material capable of inducing an electrochemical reaction, wherein the positive electrode active material may be a lithium transition metal oxide. For example, the positive electrode active material may include at least one selected from the group consisting of: a layered compound, such as lithium cobalt oxide or lithium nickel oxide substituted with at least one transition metal; a lithium manganese oxide substituted with at least one transition metal; a lithium transition metal oxide composed of Li[Ni 1-y M 1 y ]O2(where M 1 A lithium nickel composite oxide comprising at least one selected from cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), chromium (Cr), zinc (Zn) and gallium (Ga), and represented by 0.01≤y≤0.7); 1+z [Ni b Mn c Co 1-(b+c+d) M 2 d ]O (2-e) A e (Among them, M 2 A lithium nickel ternary composite oxide comprising at least one selected from the group consisting of Al, Mg, Cr, Ti, silicon (Si) and yttrium (Y), A comprising at least one selected from the group consisting of fluorine (F), phosphorus (P) and chlorine (Cl), and -0.5≤z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2, 0≤e≤0.2, and b+c+d<1); and a lithium nickel ternary composite oxide comprising Li 1+x [M 3 1-q M 4 q ]PO 4-r X r (Among them, M 3 including at least one selected from the group consisting of Fe, Mn, Co and Ni, 4 An olivine-based lithium metal phosphate comprising at least one selected from the group consisting of Al, Mg and Ti, X comprising at least one selected from the group consisting of F, sulfur (S) and nitrogen (N), and -0.5≤x≤0.5, 0≤q≤0.5, and 0≤r≤0.1).

[0045] Specifically, the positive electrode active material may be composed of lithium nickel-based composite oxides, lithium nickel ternary composite oxides, olivine-based lithium metal phosphates, or combinations thereof, which may be combined to configure the positive electrode active material layer as one layer or separate layers.

[0046] According to one embodiment of the present invention, more specifically, the positive electrode active material may include a compound of Formula A below.

[0047] [Formula A]

[0048] Li 1+x [Ni a Co b Mn c M 1 (1-a-b-c) O (2-d) A d

[0049] In Formula A,

[0050] M 1 may be at least one selected from the group consisting of Al, Mg, Cr, Ti, Si, and Y, and may specifically be Al.

[0051] A is at least one selected from the group consisting of F, P, and Cl, and may specifically be F.

[0052] x may satisfy -0.5 ≤ x ≤ 0.5, specifically -0.3 ≤ x ≤ 0.3.

[0053] a may satisfy 0.6 ≤ a < 1, specifically 0.7 ≤ a ≤ 0.9.

[0054] b may satisfy 0.03 ≤ b ≤ 0.1, specifically 0.05 ≤ b ≤ 0.1.

[0055] c may satisfy 0.03 ≤ c ≤ 0.1, specifically 0.05 ≤ c ≤ 0.1.

[0056] d may satisfy 0 ≤ d ≤ 0.1, specifically 0 ≤ d ≤ 0.05.

[0057] a, b, and c satisfy 0 < a + b + c ≤ 1, specifically a + b + c = 1.

[0058] The compound of Formula A may be in the form of particles.

[0059] The compound of Formula A may be in the form of secondary particles formed by combining multiple primary particles. Specifically, the compound of Formula 1 may be in the form of secondary particles formed by combining more than 10 primary particles. Therefore, there is an effect that lithium can be uniformly inserted into and extracted from the positive electrode active material.

[0060] The D of the compound of Formula A 50 may be 5 μm to 15 μm, particularly 7 μm to 12 μm, more particularly 9 μm to l0 μm. D 50 may be the D of the secondary particles 50 . Since the positive electrode slurry is easily dispersed when the above range is satisfied, the positive electrode active material layer can be uniformly coated.

[0061] In the positive electrode active material layer, the positive electrode active material may be contained in an amount of 90 to 99 wt %, specifically 92 to 98 wt %, and more specifically 95 to 98 wt %.

[0062] According to one embodiment of the present invention, the positive electrode active material layer may further include a positive electrode binder. The positive electrode binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of positive electrode binders may be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or its various copolymers, and any one thereof or a mixture of two or more thereof may be used.

[0063] In the positive electrode active material layer, the positive electrode binder may be included in an amount of 0.5 wt % to 5.0 wt %, specifically 1.0 wt % to 2.5 wt %, and more specifically 1.0 wt % to 2.0 wt %.

[0064] According to one embodiment of the present invention, the positive electrode active material layer may further include a positive electrode conductive agent. The positive electrode conductive agent is used to provide conductivity to the electrode, wherein any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity without causing adverse chemical changes in the battery. Specific examples of positive electrode conductive agents may be: graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black and carbon fiber; powder or fiber of a metal (such as copper, nickel, aluminum and silver); conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one thereof or a mixture of two or more thereof may be used.

[0065] In the positive electrode active material layer, the positive electrode conductor may be contained in an amount of 0.5 wt % to 30.0 wt %, specifically 0.5 wt % to 10.0 wt %, and more specifically 1.0 wt % to 4.0 wt %.

[0066] According to one embodiment of the present invention, a positive electrode includes a transfer function layer on a positive electrode active material layer, and the transfer function layer includes a composite represented by the following Formula 1.

[0067] [Formula 1]

[0068] M a Ob

[0069] Among them, M includes at least one selected from the group consisting of Fe, Mn, Co, Ti, V, Zr, and Ni, and a satisfies 1.0 ≤ a ≤ 3.0, and b satisfies 1.0 ≤ b ≤ 4.0.

[0070] In the case of the over-lithiation reaction of the transfer functional layer, lithium chemically binds to the complex of Formula 1, such that a part thereof decomposes into a metal and a lithium oxide and exists as shown in the following Reaction Formula 1, and the remaining part maintains the form of Formula 1, but can exist as a lithium-containing complex represented by the following Formula 1a by reacting with lithium.

[0071] [Formula 1a]

[0072] Li k M a O b ...

[0073] Among them, M includes at least one selected from the group consisting of Fe, Mn, Co, Ti, V, Zr, and Ni, and k satisfies 0 < k < 6, a satisfies 1.0 ≤ a ≤ 3.0, and b satisfies 1.0 ≤ b ≤ 4.0.

[0074] [Reaction Formula 1]

[0075] M a O ... b + kLi -> xM + (k / 2)Li2O

[0076] Among them, M includes at least one selected from the group consisting of Fe, Mn, Co, Ti, V, Zr, and Ni, k satisfies 0 < k < 6, and a satisfies 1.0 ≤ a ≤ 3.0, and b satisfies 1.0 ≤ b ≤ 4.0.

[0077] According to an embodiment of the present invention, the reaction occurring in the transfer functional layer through over-lithiation can be a reaction that converts the transfer functional layer into a form of a lithium-containing complex (such as Formula 1a), or causes the transfer functional layer to decompose by reacting with lithium. Finally, as shown in Reaction Formula 1, the over-lithiated transfer functional layer can be a metal (such as the metal M of Formula 1), such as Fe, Mn, and Ni, Li2O can be formed as a lithium oxide, and a lithium-containing complex can be formed. It is expected that these forms of materials have the following effects: compensating for the irreversible capacity by the ionization of lithium from the lithium oxide and the lithium-containing complex, and lithium migrates to the negative electrode due to charging during the activation process, especially reducing the volume expansion by restricting the available area of the silicon negative electrode active material.

[0078] When lithium migrates due to the formation of a transfer functional layer on the positive electrode active material layer and the provision of a transfer stack containing a lithium metal layer on the transfer functional layer, overlithiation occurs, wherein the cracking of the active material particles of the positive electrode active material layer due to the diffusion of lithium can be suppressed by the presence of the transfer functional layer.

[0079] Specifically, lithium migration occurs preferentially in the transfer functional layer, with the lithium migrating to the upper portion of the positive electrode active material layer. In this case, since the lithium passes through the transfer functional layer, the diffusion of lithium in the positive electrode active material layer is reduced. As a result, diffusion into the positive electrode active material layer does not occur, or diffusion occurs only in a portion of the upper portion, thereby clearly preventing particle breakage. Specifically, since the transfer functional layer can contain lithium and simultaneously act as a buffer layer for lithium diffusion, it can protect the positive electrode active material layer, which plays a primary role during operation, while also functioning to exhibit an overlithiation effect.

[0080] According to one embodiment of the present invention, the lithium of the transfer functional layer migrates to the negative electrode earlier and can be used to compensate for the irreversible capacity because it has a stronger ionization tendency than the lithium of the positive electrode active material layer during the activation process. Due to the excess lithium, the battery can operate in a state where the lithium of the positive electrode active material layer (expressed as capacity) is not lost, and most of the lithium of the negative electrode can stop its migration in the positive electrode active material layer during the charging process because the lithium intercalation energy of the composite in the transfer functional layer is high and the migration distance to the transfer functional layer is long. That is, between the positive electrode active material layer and the transfer functional layer, the tendency to combine with lithium in the positive electrode active material layer may dominate, so that during the cycle, the diffusion of lithium through the positive electrode active material layer to the transfer functional layer does not occur. Therefore, due to the presence of the transfer functional layer, lithium loss hardly occurs during the cycle, so it can play a role in improving capacity characteristics and life characteristics.

[0081] According to one embodiment of the present invention, the transfer function layer includes a composite represented by the following Formula 1.

[0082] [Formula 1]

[0083] M a O b

[0084] Here, M includes at least one selected from the group consisting of Fe, Mn, Co, Ti, V, Zr and Ni, and a satisfies 1.0≤a≤3.0, and b satisfies 1.0≤b≤4.0.

[0085] The composite of Formula 1 may be in the form of a precursor of a commonly used positive electrode active material, thereby having the property of being able to easily accept lithium, and therefore, it can be transferred by using a transfer laminate to minimize the migration of lithium to the positive electrode active material layer, thereby easily capturing lithium. The lithium-containing composite of Formula 1a formed by bonding the composite of Formula 1 to lithium due to this series of operations can be controlled by the roller pressure after transfer, the activation process conditions, or the thickness of the transfer functional layer, and the use of the transfer laminate.

[0086] More specifically, at least one selected from the group consisting of Fe2O3, Fe3O4, Mn2O3, MnO2, Co3O4, and NiO may be used as the composite, and the composite may be in the form of a precursor of the positive electrode active material.

[0087] According to one embodiment of the present invention, as described above, due to the reaction with lithium in the transfer functional layer during the overlithiation process, the complex decomposes or bonds with lithium, wherein, when the activation process is performed, lithium is removed from the transfer functional layer, and thus the transfer functional layer contains the complex represented by Formula 1 and may also contain the metal M in the decomposition product, wherein, in this case, the metal M may be selected from M in Formula 1, and they may be of the same type. In addition, some lithium byproducts generated due to the overlithiation reaction may remain in the transfer functional layer, and thus, the transfer functional layer may also contain lithium, but its content may be trace amount.

[0088] According to one embodiment of the present invention, the thickness of the transfer functional layer may be 15 μm to 40 μm. Preferably, the thickness of the transfer functional layer may be 17 μm or greater, 18 μm or greater, 19 μm or greater, or 20 μm or greater, and may be 35 μm or less, 30 μm or less, 28 μm or less, or 25 μm or less. When the thickness of the transfer functional layer is within the above range, the degree of mitigation and suppression of cracking of the active material particles can be appropriately adjusted according to the diffusion distance of lithium, and a transfer functional layer can be formed that does not impair the resistance performance of the positive electrode.

[0089] According to one embodiment of the present invention, the transfer function layer includes a composite and may further include a conductive agent. Based on the total weight of the composite and the conductive agent, the content of the conductive agent may be 2.10 wt % to 3.00 wt %, and the amount of the conductive agent may preferably be 2.12 wt % or more, 2.15 wt % or more, 2.20 wt % or more, 2.25 wt % or more, and 2.30 wt % or more, and may be 2.90 wt % or less, 2.85 wt % or less, 2.80 wt % or less, 2.75 wt % or less, 2.70 wt % or less, 2.65 wt % or less, and 2.60 wt % or less. If the conductive agent is mixed with the lithium-containing composite within the above range, the diffusion of lithium can be promoted, the transfer efficiency of the lithium metal layer can be improved, and the problem of battery performance degradation and the problem of the coating process can be solved.

[0090] Furthermore, the transfer function layer may further include a binder, and any binder material generally used in the process of forming a positive electrode active material layer may be equally used as the binder.

[0091] According to one embodiment of the present invention, the positive electrode may further include a lithium metal layer disposed on the transfer function layer. The lithium metal layer serves to provide lithium ions to the transfer function layer and the positive electrode active material layer. The lithium metal layer comprises solid-phase lithium metal and may specifically be formed of solid-phase lithium metal. The lithium metal layer can be solid-phase diffused into the positive electrode active material layer through transfer and rolling. Thus, after activation, the lithium metal layer may be absent or may be present in an extremely thin state.

[0092] According to one embodiment of the present invention, the positive electrode may further include a polymer layer disposed on the positive electrode active material layer. The polymer layer may effectively peel the lithium metal layer from the transfer stack and allow the lithium metal layer to be easily transferred to the positive electrode active material layer during the preparation of the positive electrode. That is, the polymer layer may be separated from the transfer stack together with the lithium metal layer to be disposed on the positive electrode active material layer. The polymer layer may be in contact with the positive electrode active material layer, or the lithium metal layer may be present between the polymer layer and the positive electrode active material layer.

[0093] The polymer layer may be at least one selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methyl methacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate. Therefore, in a secondary battery comprising the positive electrode, since the polymer layer can be dissolved in the electrolyte solution contained in the secondary battery, the resistance of the battery can be prevented from increasing. Specifically, the polymer layer may include PMMA, in which case the above-mentioned effect can be further improved.

[0094] According to one embodiment of the present invention, in the positive electrode, the porosity of the positive electrode active material layer may be 10% to 40%, specifically 15% to 35%, and more specifically 25% to 30%. In this case, no additional thickness change occurs during the rolling process.

[0095] In one embodiment of the positive electrode of the present invention, a lithium metal layer is deposited on the positive electrode using a transfer method. The lithium in the lithium metal layer is then embedded in the positive electrode active material layer through solid-phase diffusion caused by rolling. During battery operation, the lithium can migrate to the negative electrode, reducing the available area of the negative electrode.

[0096] Therefore, due to the over-lithiation of lithium in the positive electrode, the available range of negative electrode capacity can be reduced without losing lithium in the positive electrode active material, thereby suppressing excessive volume changes of silicon-based active materials and improving battery life characteristics. In addition, since the capacity of the positive electrode active material can be used over the entire range, there are advantages such as increased battery efficiency and maximized energy density.

[0097] The positive electrode of one embodiment of the present invention is characterized in that, instead of pre-lithiation by contacting a lithium metal layer with a negative electrode in a conventional manner, the lithium metal layer is transferred to the positive electrode and rolled, and then the lithium ions embedded in the positive electrode are transferred to the negative electrode during the activation process of the battery.

[0098] That is, since the negative electrode is not in contact with the lithium metal layer and lithium ions are not directly embedded in the negative electrode from the lithium metal layer, the phenomenon of excessive heat generation due to the alloy reaction of lithium and silicon at the negative electrode can be avoided, and the possibility of fire caused by the reaction between lithium and moisture can be significantly reduced. In addition, since lithium ions are not embedded in the negative electrode during the process of slotting and stamping the negative electrode (because pre-lithiation has not yet been carried out), the possibility of fire during the slotting and stamping process can be significantly reduced.

[0099] Positive electrode preparation method

[0100] According to another embodiment of the present invention, a method for preparing a positive electrode includes:

[0101] Step P1, disposing an initial positive electrode active material layer on a current collector, and disposing a transfer functional layer on the initial positive electrode active material layer; Step P2, disposing a transfer laminate comprising a base film and a lithium metal layer disposed on the base film on the transfer functional layer to form a positive electrode structure in which the lithium metal layer and the transfer functional layer are in contact with each other; Step P3, rolling the positive electrode structure; and Step P4, after rolling, preparing a positive electrode by removing the base film from the transfer laminate, wherein, after rolling in Step P3, the transfer functional layer comprises a complex of Formula 1 and lithium.

[0102] [Formula 1]

[0103] M a O b

[0104] Here, M includes at least one selected from the group consisting of Fe, Mn, Co, Ti, V, Zr and Ni, and a satisfies 1.0≤a≤3.0, and b satisfies 1.0≤b≤4.0.

[0105] According to one embodiment of the present invention, step P1 of the positive electrode preparation method is a step of forming an initial positive electrode active material layer by applying an initial positive electrode active material on a current collector and then providing a transfer functional layer on the initial positive electrode active material layer.

[0106] According to one embodiment of the present invention, step P2 of the positive electrode preparation method is a step of providing a transfer stack comprising a base film and a lithium metal layer on a transfer functional layer provided on an initial positive electrode active material layer to form a positive electrode structure in which the lithium metal layer and the transfer functional layer are in contact with each other.

[0107] Reference Figure 1 , the transfer stack 300 may include a base film 310 and a lithium metal layer 320 disposed on the base film 310. The base film 310 may be used without limitation as long as it is a material that can withstand high temperature conditions occurring during the deposition of the lithium metal layer 320 on the base film 310. Specifically, the base film may include at least one of polyethylene terephthalate (PET), polyimide (PI), poly(methyl methacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate.

[0108] According to one embodiment of the present invention, a lithium metal layer may be provided on the base film. The lithium metal layer serves to supply lithium ions to the initial positive electrode active material layer. The lithium metal layer includes solid-phase lithium metal and may be specifically formed of solid-phase lithium metal.

[0109] According to one embodiment of the present invention, the thickness of the lithium metal layer in the transferred laminate may be 1 μm to 10 μm, particularly 3 μm to 9 μm, and more particularly 4 μm to 6.5 μm. When the above range is met, the degree of cracking of the positive electrode active material particles on the positive electrode surface can be reduced, thereby suppressing the reduction in the initial capacity of the battery.

[0110] According to one embodiment of the present invention, the loading capacity of the lithium metal layer (unit: mAh / cm 2 ) can be the loading capacity of the initial positive electrode active material layer (unit: mAh / cm 2), specifically 12% to 35%, and more specifically 20% to 30%. When the above range is met, since the generation of by-products is small and lithium can be easily intercalated into the positive electrode active material, the target lithium intercalation capacity can be easily achieved.

[0111] In step P2, refer to Figure 1 Initial positive electrode active material layers 120 and 120' are disposed on the positive electrode current collector 110, and transfer function layers 130 and 130' may be disposed on the initial positive electrode active material layers 120 and 120'. A transfer stack 300 may be disposed on the transfer function layers 130 and 130' to form a positive electrode structure 400 in which the lithium metal layer 320 and the transfer function layers 130 and 130' are in contact with each other.

[0112] Here, since the positive electrode current collector 110 has been described in the above description of the positive electrode, its description will be omitted.

[0113] Reference Figure 2 The transfer stack 300 may further include a polymer layer 330. The polymer layer 330 may be disposed between the base film 310 and the lithium metal layer 320. The polymer layer may effectively peel the lithium metal layer from the transfer stack and allow the lithium metal layer to be easily transferred to the positive electrode active material layer during the preparation of the positive electrode. That is, the polymer layer may be separated from the transfer stack together with the lithium metal layer to be disposed on the positive electrode active material layer. The polymer layer may be in contact with the positive electrode active material layer, or the lithium metal layer may be present between the polymer layer and the positive electrode active material layer.

[0114] Here, since the polymer layer has been described above in the description of the positive electrode, description thereof will be omitted.

[0115] The thickness of the polymer layer may be 0.1 μm to 10 μm, particularly 0.5 μm to 5 μm, and more particularly 1 μm to 2.5 μm. When the above range is met, the lithium metal layer can be easily transferred to the positive electrode active material layer, and the reverse transfer phenomenon of the positive electrode active material layer being transferred to the transfer stack can be prevented.

[0116] According to one embodiment of the present invention, step P3 of the positive electrode preparation method is a step of rolling the positive electrode structure.

[0117] Reference Figure 3, in step P3, the prepared positive electrode structure 400 may be rolled. The rolling may be performed by a rolling method. Specifically, by two rollers R spaced apart from each other at a predetermined interval in the vertical direction, pressure may be applied to the positive electrode structure 400 passing through the interval in the vertical direction, and the pressure may be a linear pressure. Through the rolling process, at least a portion of the lithium ions of the lithium metal layer contained in the transfer stack may be embedded in the transfer functional layer, and in some cases, embedded in the initial positive electrode active material layer. In this process, the initial positive electrode active material layer may become a positive electrode active material layer, and the lithium-containing complex in the transfer functional layer may satisfy Formula 1. In Figure 3 and Figure 4 In the embodiment, it is explained that a lithium metal layer is included in the positive electrode, but the lithium metal layer may not exist as a separate layer when the entire lithium metal layer is embedded in the transfer function layer or in the transfer function layer and the initial positive electrode active material layer during rolling.

[0118] The pressure applied to the positive electrode structure during the roll pressing process may be 10 kgf / cm to 90 kgf / cm, specifically 15 kgf / cm to 80 kgf / cm, and more specifically 20 kgf / cm to 40 kgf / cm.

[0119] When the rolling pressure is applied within the above range, the lithium metal layer can be efficiently transferred, and lithium can be intercalated into the positive electrode at the desired level. An appropriate cracking ratio can also be achieved. As a result, the effects of improving lifespan and achieving capacity characteristics can be achieved simultaneously.

[0120] According to one embodiment of the present invention, step P4 of the method for preparing a positive electrode is a step of preparing a positive electrode by removing the base film from the transferred laminate after rolling.

[0121] Reference Figure 4 In step P4, the positive electrode can be prepared by removing the base film 310 from the transferred laminate after rolling in step P3. In the case where the polymer layer 330 is provided between the base film 310 and the lithium metal layer 320, the base film 310 can be more easily removed through the polymer layer 330.

[0122] In addition, according to one embodiment of the present invention, the method for preparing a positive electrode may further include step P5: allowing the initial positive electrode to stand (place) for 1 minute to 600 minutes, specifically 1 minute to 30 minutes. Step P5 may be performed after step P3. Specifically, step P5 may be performed in at least one of "between step P3 and step P4" and "immediately after step P4". Since the reaction heat generated by the reaction between the lithium metal layer and the initial positive electrode active material layer in step P3 can be effectively released through step P5, the lithium is uniformly embedded in the positive electrode, thereby reducing the generation of by-products.

[0123] In addition, since the prepared positive electrode has been described previously, its description will be omitted.

[0124] lithium secondary batteries

[0125] According to another embodiment of the present invention, a lithium secondary battery includes: a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode includes: a current collector; a positive electrode active material layer disposed on the current collector and including a positive electrode active material; and a transfer function layer disposed on the positive electrode active material layer and including a composite represented by Formula 1 and lithium, and in a dQ / dV curve obtained during initial charge at a rate of 0.1C, a peak intensity ratio (R H / L ) is 0.75 or less, wherein the lithium secondary battery may be a lithium secondary battery before an activation process.

[0126] [Equation 1]

[0127] R H / L =I L / I H

[0128] Among them, R H / L is the peak intensity ratio, I L is the intensity of the peak appearing near the potential difference of 2.30 V ± 0.05 V, and I H It is the intensity of the peak that appears near the potential difference of 2.50 V ± 0.05 V.

[0129] Since the positive electrode has been described previously, its description will be omitted.

[0130] In the dQ / dV curve obtained during the initial charge at a rate of 0.1C, the peak intensity ratio (R H / L ) may be 0.75 or less, preferably 0.70 or less, 0.65 or less, 0.60 or less, and 0.57 or less. Specifically, when the transfer function layer is not applied, as the potential difference increases, the charge change amount that occurs according to the voltage change amount exhibits irregular behavior, and the positive electrode active material particles crack. However, when the transfer function layer is applied, as the potential difference increases, the charge change amount that occurs according to the voltage change amount is constant. Therefore, it can be understood that the cracking phenomenon of the positive electrode active material is suppressed and there is almost no damage.

[0131] That is, the fact that the ratio of the peak intensity at low potential to the peak intensity at high potential is lower than 0.75 may indicate that even for an overlithiated cathode, the cathode active material particles can remain intact due to the presence of the transfer functional layer, and therefore the dQ / dV curve is also stable.

[0132] In addition, according to an embodiment of the present invention, a lithium secondary battery includes: a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The positive electrode includes: a current collector; a positive electrode active material layer disposed on the current collector and including a positive electrode active material; and a transfer functional layer disposed on the positive electrode active material layer and including a composite represented by Formula 1. The lithium secondary battery may be a lithium secondary battery after an activation process. Since the positive electrode has been described in detail previously, its description will be omitted.

[0133] According to an embodiment of the present invention, the negative electrode includes a negative electrode active material layer, the negative electrode active material layer may include a negative electrode active material, and a silicon-based negative electrode active material, a carbon-based negative electrode active material, or a combination thereof may be used as the negative electrode active material.

[0134] According to an embodiment of the present invention, the negative electrode active material may include a silicon-based negative electrode active material. The silicon-based negative electrode active material may include at least one of Si and SiO x (0 < x < 2).

[0135] Si is silicon particles, which may be silicon particles (particles formed of silicon), i.e., so-called pure silicon. The silicon particles can effectively improve the capacity of the negative electrode. SiO x (0 < x < 2) may be in a form including Si and SiO2, and Si may form a phase. That is, x corresponds to the ratio of the number of oxygen (O) contained in SiO x (0 < x < 2) to Si. In the case where the silicon-based composite particles include SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0136] According to an embodiment of the present invention,a silicon-based negative electrode active material has a problem of extremely poor life characteristics due to extreme volume changes during battery operation. However, in the case of being combined with the positive electrode of the present invention, since excess lithium ions rather than the lithium that determines the capacity in the positive electrode active material transfer from the over-lithiated positive electrode to the silicon-based negative electrode active material during activation to react with the silicon-based negative electrode active material in advance, an irreversible phase is formed to reduce the available area. Therefore, the volume change during battery operation is not serious, and thus the life can be significantly improved, and the high-capacity characteristic, which is an inherent characteristic of the silicon-based negative electrode active material, can be fully realized.

[0137] According to an embodiment of the present invention, the negative electrode active material may further include a carbon-based negative electrode active material. The carbon-based negative electrode active material may include at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesophase carbon microspheres.

[0138] According to one embodiment of the present invention, the negative electrode active material layer may further include a negative electrode binder. The negative electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylate, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials in which hydrogen is replaced by lithium (Li), sodium (Na) or calcium (Ca), or various copolymers thereof.

[0139] According to one embodiment of the present invention, the negative electrode active material layer may further include a negative electrode conductive agent. The negative electrode conductive agent is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and for example, conductive materials such as the following may be used: graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal carbon black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbon powder; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0140] According to one embodiment of the present invention, the secondary battery includes a diaphragm. The diaphragm separates the negative electrode from the positive electrode and provides a migration path for lithium ions, wherein any diaphragm can be used as the diaphragm without particular limitation, as long as it is commonly used in secondary batteries, and in particular, a diaphragm having a high water retention capacity for the electrolyte and a low impedance to the transfer of electrolyte ions is preferred. Specifically, a porous polymer film can be used, for example, a porous polymer film prepared by a polyolefin polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer), or a laminated structure having two or more layers thereof can be used. In addition, conventional porous non-woven fabrics can be used, for example, a non-woven fabric formed by high melting point glass fiber or polyethylene terephthalate fiber. In addition, a coated diaphragm comprising a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a diaphragm with a single layer or multilayer structure can be optionally used.

[0141] According to one embodiment of the present invention, the secondary battery may further include an electrolyte. The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that can be used to prepare a lithium secondary battery, but the present invention is not limited thereto.

[0142] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt. Examples of non-aqueous organic solvents may include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0143] Especially, ethylene carbonate and propylene carbonate as the cyclic carbonate in the carbonate organic solvent dissociate the lithium salt in the electrolyte solution well owing to the high dielectric constant as the high viscosity organic solvent, therefore can preferably use cyclic carbonate.Due to when cyclic carbonate is mixed with low viscosity, the linear carbonate of low dielectric constant (for example dimethyl carbonate and diethyl carbonate) with appropriate ratio, can prepare the electrolyte solution with high conductivity, therefore more preferably use cyclic carbonate.

[0144] A lithium salt may be used as the metal salt, and the lithium salt is a material that is easily soluble in a non-aqueous electrolyte solution, wherein, for example, at least one selected from the group consisting of the following may be used as an anion of the lithium salt: F - 、Cl - , I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 -CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - .

[0145] In addition to the above-mentioned electrolyte components, at least one additive may be included in the electrolyte, such as a halogenated alkylene carbonate compound (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme dimethyl ether, phosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride, so as to improve the life characteristics of the battery, inhibit the reduction of the battery capacity and improve the discharge capacity of the battery.

[0146] According to one embodiment of the present invention, a method for preparing the above-mentioned lithium secondary battery is provided, wherein the preparation method includes the following steps: preparing an electrode assembly including a positive electrode, a negative electrode and a separator arranged between the positive electrode and the negative electrode; and performing an activation process by placing the electrode assembly in a battery shell and injecting an electrolyte solution.

[0147] Specifically, since the preparation method of the positive electrode is the same as described above, a description thereof will be omitted.

[0148] Regarding the lithium secondary battery of one embodiment of the present invention, the R H / L The value may be a value derived from a curve obtained during initial charge and discharge in performing an activation process.

[0149] As described above, since the secondary battery including the cathode of the present invention stably exhibits excellent capacity and capacity retention, the secondary battery is suitable for portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0150] Therefore, according to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the battery module are provided.

[0151] The battery module or battery pack can be used as a power source for at least one of the following medium to large devices: power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0152] Example

[0153] Hereinafter, preferred embodiments will be provided to better understand the present invention. It will be apparent to those skilled in the art that these examples are intended only to illustrate the present invention, and that various modifications and variations may be made within the scope and technical spirit of the present invention. These modifications and variations fall within the scope of the claims herein contained.

[0154] Electrode preparation

[0155] Example 1-1

[0156] A PET film (base film), a polymer layer (2.5 μm thick) formed of PMMA provided on the PET film, and a lithium metal layer (6.2 μm thick, with a loading of 1.26 mAh / cm) provided on the polymer layer and formed of solid-phase lithium metal were prepared. 2 ) of the transfer laminate.

[0157] Will Li[Ni 0.86 Co 0.05 Mn 0.08 Al 0.01 ]O2 is used as the positive electrode active material. The positive electrode active material is a plurality of (more than 10) Li[Ni 0.86 Co 0.05 Mn 0.08 Al 0.01 ]O2 primary particles are combined with each other to form secondary particles, and the average particle size of the secondary particles is D 50 A positive electrode was prepared comprising the following materials: an initial positive electrode active material layer comprising a positive electrode active material, PVdF as a positive electrode binder, and carbon nanotubes as a positive electrode conductor in a ratio of 98:1:1, and an aluminum foil (thickness: 12 μm) as a positive electrode current collector. The loading capacity of the initial positive electrode active material layer was 4.5 mAh / cm 2 , with a thickness of 140μm.

[0158] In addition, the average particle size (D 50 ) of Fe2O3 with a thickness of 0.5 μm, 2.86 wt% of carbon black and 2.6 wt% of polyvinylidene fluoride (PVDF) is applied to the initial positive electrode active material layer and dried to form a 30 μm thick transfer functional layer, and then the transfer stack is arranged on the transfer functional layer on the initial positive electrode active material layer so that the lithium metal layer is in contact with the transfer functional layer.

[0159] The positive electrode with the transferred laminate was then rolled using a roller pressing method and then allowed to stand for 24 hours. The pressure during the rolling process was 20 kgf / cm. As a result, the lithium ions in the lithium metal layer were intercalated into the initial positive electrode active material layer to form a positive electrode active material layer. The base film was then removed, and the rolled positive electrode was allowed to stand for 10 minutes to prepare a positive electrode comprising a positive electrode current collector, a positive electrode active material layer, a transfer functional layer, and a polymer layer.

[0160] Example 1-2

[0161] A positive electrode was prepared in the same manner as in Example 1-1, except that the thickness of the transfer function layer was 25 μm.

[0162] Examples 1-3

[0163] A positive electrode was prepared in the same manner as in Example 1-1, except that the thickness of the transfer function layer was 20 μm.

[0164] Examples 1-4

[0165] The positive electrode was prepared in the same manner as in Example 1-1, except that the average particle size (D 50 ) of 0.5 μm replaces the Fe 2 O 3 contained in the transfer function layer, and the thickness of the transfer function layer is 20 μm.

[0166] Examples 1-5

[0167] The positive electrode was prepared in the same manner as in Example 1-1, except that the average particle size (D 50 ) of Co 3 O 4 with a thickness of 0.5 μm replaces Fe 2 O 3 contained in the transfer function layer, and the thickness of the transfer function layer is 20 μm.

[0168] Examples 1-6

[0169] The positive electrode was prepared in the same manner as in Example 1-1, except that the thickness of the lithium metal layer was 3.0 μm and the loading capacity was 0.62 mAh / cm 2 .

[0170] Comparative Example 1-1

[0171] A positive electrode was prepared in the same manner as in Example 1-1, except that overlithiation was not performed, and the transfer stack and the transfer functional layer were not used.

[0172] Comparative Example 1-2

[0173] A positive electrode was prepared in the same manner as in Example 1-1, except that the transfer function layer was not used.

[0174] Comparative Examples 1-3

[0175] Will Li[Ni 0.86 Co 0.05 Mn 0.08 Al 0.01 ]O2 is used as the positive electrode active material. The positive electrode active material is in the form of secondary particles formed by multiple (more than 10) primary particles bonded to each other, and the average particle size of the secondary particles is D 50 A positive electrode was prepared comprising the following materials: an initial positive electrode active material layer comprising a positive electrode active material, PVdF as a positive electrode binder, and carbon nanotubes as a positive electrode conductor in a ratio of 98:1:1, and an aluminum foil (thickness: 12 μm) as a positive electrode current collector. The loading capacity of the initial positive electrode active material layer was 4.5 mAh / cm 2 , with a thickness of 140μm.

[0176] A positive electrode and a lithium metal counter electrode facing the positive electrode were prepared, a polyethylene separator was set between the positive electrode and the lithium metal counter electrode, and an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3 / 7 (volume ratio), lithium hexafluorophosphate (LiPF61mol)) was injected to prepare a pre-lithiation battery cell.

[0177] An electrochemical charger and discharger was connected to the pre-lithiation cell, and the positive electrode was overlithiated by electrochemically charging the pre-lithiation cell at 0.1 C to 120% of the positive electrode charge capacity.

[0178] Experimental Example 1: Electrode Cross-Section Evaluation

[0179] For the positive electrodes and negative electrodes prepared in the above-described Examples and Comparative Examples, the presence or absence of cracks in the active material particles was measured by the following method.

[0180] One day after each positive electrode was prepared, a cross-section of the positive electrode was analyzed. A reference region, 20 μm in depth and 200 μm in the in-plane direction from the surface of the active material layer, was used as the reference region. The cracked region represented the broken particles, while the particle region represented the unbroken particles. Scanning electron microscope (SEM) images were obtained at a magnification of 2500 from electrode cross-sections cut by ion milling.

[0181] In the SEM cross-sectional image, the length of the damaged active material particles was measured, and in each embodiment and comparative example, the case where the damaged active material particles were observed at a depth of more than 50% of the thickness of the lithium metal layer for transfer was evaluated as C, the case where damaged active material particles were observed but the damaged active material particles were observed only within a depth of less than 50% was evaluated as B, and the case where no damaged active material particles were observed was evaluated as C.

[0182] [Table 1]

[0183] Is there any cracking (A / B / C) Example 1-1 A Example 1-2 A Examples 1-3 A Examples 1-4 A Examples 1-5 A Examples 1-6 A Comparative Example 1-1 A Comparative Example 1-2 C Comparative Examples 1-3 A

[0184] As shown in Table 1, in the case of transferring lithium to the positive electrode including the transfer function layer, no cracking of the active material was observed, but for the positive electrode without the transfer function layer, severe cracking was observed in the active material layer. Specifically, Figure 5 This is an SEM image taken from the cross section of the positive electrode of Example 1-3, where the solid line on the top is the transfer function layer and the dotted line is the depth to which lithium has been transferred. It can be confirmed that the positive electrode active material is not damaged due to the presence of the transfer function layer on the top. In addition, Figure 6 This is an SEM image taken from a cross section of the positive electrode of Comparative Example 1-2. In this case, it can be confirmed that the active material above the positive electrode active material layer is severely damaged. This confirms that the use of a transfer functional layer can contribute to improved battery durability and performance by enabling stable lithium transfer to compensate for irreversible capacity without damaging the positive electrode active material layer.

[0185] Preparation of lithium secondary batteries

[0186] Example 2-1

[0187] The average particle size D 50 Silicon particles with a diameter of 5 μm were used as the negative electrode active material. A negative electrode was prepared comprising the following materials: an initial negative electrode active material layer comprising the negative electrode active material, carboxymethyl cellulose (CMC) as a negative electrode binder, and carbon nanotubes as a negative electrode conductive agent in a weight ratio of 80:10:10. The loading capacity of the initial negative electrode active material layer was 10 mAh / cm 2 , with a thickness of 75μm.

[0188] The positive electrode, negative electrode and porous polyethylene separator of Example 1-1 were assembled using a winding method, and an initial lithium secondary battery was prepared by injecting an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3 / 7 (volume ratio) and lithium hexafluorophosphate (LiPF6 1 mol)) into the assembled battery.

[0189] The initial lithium ion secondary battery was charged to 4.2 V at a C rate of 0.1 C and then discharged to 2.5 V for an activation process.

[0190] Examples 2-2 to 2-6

[0191] Lithium secondary batteries were prepared in the same manner as in Example 2-1, except that the positive electrodes of Examples 1-2 to 1-6 were respectively used instead of the positive electrode of Example 1-1.

[0192] Comparative Examples 2-1 to 2-3

[0193] Lithium secondary batteries were prepared in the same manner as in Example 2-1, except that the positive electrodes of Comparative Examples 1-1 to 1-3 were respectively used instead of the positive electrode of Example 1-1.

[0194] Experimental Example 2: Evaluation of initial charge and discharge characteristics

[0195] The initial charge capacity and discharge capacity of the lithium secondary batteries of Examples 2-1 to 2-6 and Comparative Examples 2-1 to 2-3 were evaluated under the following conditions, and R was obtained by the following Equation 1 using the dQ / dV curve during the initial charge process. H / L .

[0196] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cutoff) 4.2V

[0197] Discharge condition: CC (constant current) condition 2.5V

[0198] [Equation 1]

[0199] R H / L =I L / I H

[0200] Among them, R H / L is the peak intensity ratio, I L is the intensity of the peak appearing at a voltage of 2.30 V ± 0.05 V, and I H It is the intensity of the peak appearing at a voltage of 2.50 V±0.05 V.

[0201] Furthermore, the ratio of the difference between the discharge capacity of the subject example or comparative example and the discharge capacity of Comparative Example 2-1 is expressed as a percentage based on Comparative Example 2-1 (the positive electrode was not overlithiated).

[0202] [Table 2]

[0203]

[0204] Referring to Table 2, for the examples, it was confirmed that the peak RH / L The value was 0.75 or less, but in Comparative Examples 2-2 and 2-3, since overlithiation did not occur when a transfer functional layer was used as the transfer method, the peak value was greater than 0.75. In addition, the initial charge capacity of the batteries of Examples 2-1 to 2-6 increased significantly due to overlithiation, confirming that most of the transferred lithium was charged into the negative electrode, allowing overlithiation to proceed normally.

[0205] In addition, in Comparative Example 2-2, the initial charge capacity increased, but due to the absence of the transfer function layer, the active material layer was broken, resulting in a significant decrease in discharge capacity. In Comparative Example 2-3, because lithium was injected into the positive electrode using an electrochemical method rather than a transfer method, the initial charge capacity increased, but the discharge capacity was confirmed to decrease rapidly. This is probably because as the electrochemical method progresses, lithium penetrates deeper into the thickness direction of the positive electrode active material layer. As a result, the amount of unabsorbed lithium increases, resulting in a large amount of lithium byproducts generated throughout the active material layer, and thus increased lithium loss.

[0206] [Explanation of Reference Numerals]

[0207] 110: Positive electrode current collector

[0208] 120, 120': initial positive electrode active material layer

[0209] 130, 130': transfer function layer

[0210] 300: Transfer stack

[0211] 310: basement membrane

[0212] 320: Lithium metal layer

[0213] 330: polymer layer

[0214] 400: Positive electrode structure

[0215] R: Roller

Claims

1. A positive electrode comprising: current collector; a positive electrode active material layer disposed on the current collector and containing a positive electrode active material; and A transfer function layer is provided on the positive electrode active material layer and includes a composite represented by Formula 1: [Formula 1] M a O b in, M includes at least one selected from the group consisting of iron (Fe), manganese (Mn), cobalt (Co), titanium (Ti), vanadium (V), zirconium (Zr) and nickel (Ni), and a satisfies 1.0≤a≤3.0, and b satisfies 1.0≤b≤4.

0.

2. The positive electrode according to claim 1, wherein The composite represented by Formula 1 is at least one selected from the group consisting of Fe2O3, Fe3O4, Mn2O3, MnO2, Co3O4 and NiO.

3. The positive electrode according to claim 1, wherein The transfer function layer further comprises at least one selected from the group consisting of metal M, lithium metal and Li2O; and The metal M is at least one selected from the group consisting of Fe, Mn, Co, Ti, V, Zr, and Ni.

4. The positive electrode according to claim 1, wherein The transfer function layer has a thickness of 15 μm to 40 μm.

5. The positive electrode according to claim 1, wherein The transfer function layer further comprises a conductive agent.

6. The positive electrode according to claim 5, wherein The conductive agent may be included in an amount of 2.10 wt % to 3.00 wt % based on the total weight of the composite and the conductive agent.

7. The positive electrode according to claim 1, wherein The positive electrode active material is at least one selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel composite oxide, lithium nickel ternary composite oxide, lithium manganese composite oxide and lithium transition metal phosphate. 8 . The positive electrode according to claim 1 , further comprising a lithium metal layer disposed on the transfer function layer.

9. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, in, The positive electrode comprises: current collector; a positive electrode active material layer disposed on the current collector and comprising a positive electrode active material; and A transfer function layer is provided on the positive electrode active material layer and includes the composite represented by Formula 1 and lithium, and In the dQ / dV curve obtained during the initial charge at a rate of 0.1C, the peak intensity ratio R is calculated by the following equation 1: H / L Below 0.75: [Equation 1] R H / L =I L / I H Among them, R H / L is the peak intensity ratio, I L is the intensity of the peak appearing at a voltage of 2.30 V ± 0.05 V, and I H It is the intensity of the peak appearing at a voltage of 2.50 V±0.05 V.

10. The lithium secondary battery according to claim 9, wherein The negative electrode includes a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material, Wherein, the negative electrode active material comprises a silicon-based negative electrode active material.

11. A method for preparing a lithium secondary battery, the method comprising the following steps: preparing an electrode assembly comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; as well as The electrode assembly is placed in a battery case and injected with an electrolyte solution to perform an activation process. The positive electrode is prepared by the following steps: Step P1, disposing an initial positive electrode active material layer on a current collector, and disposing a transfer function layer on the initial positive electrode active material layer; Step P2, disposing a transfer stack comprising a base film and a lithium metal layer disposed on the base film on the transfer functional layer to form a positive electrode structure in which the lithium metal layer and the transfer functional layer are in contact with each other; Step P3, rolling the positive electrode structure; and Step P4, after rolling, preparing a positive electrode by removing the base film from the transferred laminate, Wherein, after the rolling in step P3, the transfer function layer comprises a complex of Formula 1 and lithium: [Formula 1] M a O b Wherein, M includes at least one selected from the group consisting of iron (Fe), manganese (Mn), cobalt (Co), titanium (Ti), vanadium (V), zirconium (Zr) and nickel (Ni), and a satisfies 1.0≤a≤3.0, and b satisfies 1.0≤b≤4.

0.

12. The method of claim 11, wherein: The rolling in step P3 is performed by a roll-to-roll method.

13. The method of claim 11, wherein: The thickness of the lithium metal layer is 1 μm to 10 μm.

14. The method of claim 11, wherein: The loading amount of the lithium metal layer is 4% to 40% of the loading amount of the initial positive electrode active material layer.

15. The method of claim 11, wherein: The transfer laminate further includes a polymer layer, and the polymer layer is disposed between the base film and the lithium metal layer.

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