Lithium secondary battery and method for manufacturing same

Abstract

The present invention relates to a lithium secondary battery comprising: a negative electrode comprising a silicon-based negative electrode active material; a positive electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte, in which the silicon-based negative electrode active material contains 30 parts by volume or less of crystalline silicon based on a total of 100 parts by volume of the active material, and breakage of the silicon-based negative electrode active material particles is minimized, thereby suppressing side reactions with an electrolyte solution and electrode deterioration, and thus improving lifespan characteristics and resistance characteristics.

Description

TECHNICAL FIELD

[0001] Cross-reference to related applications

[0002] This application claims the priority benefit of Korean Patent Application No. 10-2022-0183612, filed on December 23, 2022, the entire disclosure of which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present invention relates to a lithium secondary battery and a method for manufacturing the lithium secondary battery including an activation process. BACKGROUND ART

[0004] With the increase in the development of technologies and demands for mobile devices, the demand for secondary batteries as an energy source is rapidly increasing. Among secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low self-discharge rate have been commercialized and widely used.

[0005] Generally, a lithium secondary battery is manufactured by the following method. A composition for forming an electrode active material layer containing an electrode active material is coated on an electrode current collector and then dried to prepare an electrode (positive electrode, negative electrode). A separator is inserted between the positive electrode and the negative electrode to form an electrode assembly, and the electrode assembly is inserted into a battery case and sealed. In addition, in order to determine whether a secondary battery is defective and to ensure the performance of the secondary battery, especially the stability of its life, an activation process must be performed before the product is shipped. The activation process repeatedly charges and discharges to activate the battery and remove gas.

[0006] In the case of a battery using silicon as a negative electrode active material, when lithium is embedded in the silicon active material, especially during initial charge and discharge, due to volume expansion, stress is generated inside and outside the active material particles, resulting in particle deformation and rupture. As a result, there are problems of generating active materials that do not participate in the electrochemical reaction, leading to capacity loss, and side reactions occurring due to exposure to the electrolyte.

[0007] However, when a stable form of amorphous lithium silicate (a-Li-Si) is formed in the silicon active material, lithium can be easily embedded and de-embedded, and thus in actual charge and discharge cycles, a single cell can be driven without deterioration. For this reason, in the activation process, a process of converting crystalline silicon (c-Si) into amorphous lithium silicate (a-Li-Si) by charge and discharge is necessary.

[0008] On the other hand, during the activation process, if charge and discharge are performed at a low rate for stable activation, the activation time is too long, thereby reducing productivity. If activation is performed rapidly to improve productivity, lithium cannot diffuse to the entire active material layer, but causes deterioration of the region near the surface, thereby reducing the life characteristics.

[0009] Accordingly, the present invention aims to provide a method for maximizing the conversion of the crystalline (c-Si) portion of silicon into amorphous lithium silicate (a-Li-Si) through stable and rapid activation, thereby minimizing the deformation and cracking of active material particles and maximizing the life characteristics accordingly. Summary of the Invention

[0010] Technical Problem

[0011] An object of the present invention is to provide a lithium secondary battery and a method for manufacturing the same, the lithium secondary battery being capable of preventing cracking of silicon active material particles used as a negative electrode active material, thereby suppressing deterioration of the electrode, reducing side reactions with the electrolyte and thus reducing the amount of gas generated, and improving resistance characteristics and life characteristics.

[0012] Technical Solution

[0013] To solve the above problems, according to an embodiment of the present invention, there is provided a lithium secondary battery including: a negative electrode including a silicon-based negative electrode active material, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the silicon-based negative electrode active material includes 30 parts by volume or less of crystalline silicon based on 100 parts by volume of the negative electrode active material.

[0014] To solve the above problems, according to another embodiment of the present invention, there is provided a method for manufacturing a lithium secondary battery, including the following steps: (S1) preparing an inactive lithium secondary battery including: a negative electrode including a silicon-based negative electrode active material, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte; and (S2) activating the inactive lithium secondary battery, wherein the activation in step (S2) includes a low-rate charging range in which charging is performed at a rate of 0.01C to 0.07C to a charging end voltage of 4.0V or more.

[0015] Advantageous Effects

[0016] The lithium secondary battery of the present invention can prevent damage to the silicon-based negative electrode active material by controlling the initial rate at a low rate and can effectively convert the crystalline state into the amorphous state; and by using an activation method of gradually increasing the rate, the efficiency of the activation process can be improved, while preventing cracking of the negative electrode active material particles, thereby suppressing side reactions with the electrolyte and deterioration of the electrode, and thus improving resistance and life characteristics. Description of the Drawings

[0017] Figure 1 A diagram showing stress changes outside the negative electrode active material particles during initial charging of the examples and comparative examples.

[0018] Figure 2It is an image that shows the amorphous phase, crystalline phase, and mixed phase in various colors by photographing the cross-section of the negative electrode in Example 2 after activation, and the Raman spectroscopy results measured by Raman spectroscopy. Detailed Description of the Invention

[0019] Hereinafter, the present invention will be described in more detail.

[0020] The terms or words used in the description and claims of the present application should not be construed as limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts consistent with the technical gist of the present invention based on the principle that the inventor can appropriately define the concept of the term to best describe his invention.

[0021] Lithium secondary battery

[0022] According to an embodiment of the present invention, the lithium secondary battery is an activated lithium secondary battery, comprising: a negative electrode containing a silicon-based negative electrode active material, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the silicon-based negative electrode active material is characterized in that, based on 100 parts by volume of the negative electrode active material, it has 30 parts by volume or less of crystalline silicon.

[0023] The silicon-based negative electrode active material is pure silicon, which is characterized by containing silicon particles. Generally, pure silicon theoretically has excellent performance, but it is not easily applied due to problems such as volume expansion. However, when the activation process according to the embodiment of the present invention is applied, the performance can be effectively realized. That is, through the activation according to the present invention, the amorphous silicon, crystalline silicon, and semi-amorphous silicon in the silicon-based negative electrode active material particles are controlled to exist within a specific range, thereby preventing the silicon particles from cracking. As a result, side reactions with the electrolyte and electrode deterioration can be suppressed, thereby providing a lithium secondary battery with improved life characteristics.

[0024] Specifically, according to an embodiment of the present invention, the silicon-based negative electrode active material contains 5 to 30 parts by volume of crystalline silicon; 30 to 85 parts by volume of amorphous silicon; and 10 to 65 parts by volume of semi-amorphous silicon, wherein, on the Raman spectrum obtained using a Raman spectrometer, the semi-amorphous silicon has a peak at a Raman shift value greater than 467 cm -1 and less than 515 cm -1 of the Raman shift value.

[0025] In addition, the crystalline silicon may have a peak at a Raman shift value of about 467 cm -1 and the amorphous silicon may have a peak at a Raman shift value of about 515 cm -1has a peak at the Raman shift value. As described above, the silicon-based negative electrode active material has a crystalline silicon content of 5 to 30 parts by volume and thus has a weak peak, thereby controlling the crystalline state inside the silicon so that the lithium insertion and extraction reactions can proceed stably, thereby suppressing volume expansion and improving the life characteristics.

[0026] Specifically, the content of the crystalline silicon may be 5 parts by volume or more, 10 parts by volume or more, preferably 12 parts by volume or more, and 27 parts by volume or less, 25 parts by volume or less, or 23 parts by volume or less. The content of the amorphous silicon may be 35 parts by volume or more, 40 parts by volume or more, 45 parts by volume or more, or 47 parts by volume or more, and 80 parts by volume or less, 75 parts by volume or less, 70 parts by volume or less, 65 parts by volume or less, or 60 parts by volume or less. The content of the semi-amorphous silicon may be 15 parts by volume or more, 20 parts by volume or more, 25 parts by volume or more, 30 parts by volume or more, or 32 parts by volume or more, and 60 parts by volume or less, 55 parts by volume or less, 50 parts by volume or less, 45 parts by volume or less, or 40 parts by volume or less.

[0027] According to an embodiment of the present invention, the silicon-based negative electrode active material may include semi-amorphous silicon, wherein the semi-amorphous silicon may have a peak at a Raman shift value greater than 467 cm -1 and less than 515 cm -1 on a Raman spectrum obtained using a Raman spectrometer. Here, the semi-amorphous silicon may refer to silicon having a phase that shows a peak detected by Raman spectroscopy when the Raman shift is within the above range, and may have two or more peaks. The semi-amorphous silicon may be a mixed phase of crystalline silicon and amorphous silicon, and may exhibit the characteristics of crystalline silicon and amorphous silicon at the same time. That is, it may be silicon having a phase that is in an incompletely converted state when the crystalline silicon is converted to amorphous silicon during the crystalline phase control of the silicon-based negative electrode active material. This phase is a phase that can suppress volume expansion during charge and discharge and helps to improve the life characteristics of the lithium secondary battery.

[0028] According to an embodiment of the present invention, the lithium secondary battery may include one or more unit electrodes having a positive electrode and a negative electrode, wherein a separator is interposed between the unit electrodes to form an electrode assembly, and the electrode assembly may be accommodated in a battery case. The secondary battery may be a cylindrical, square, or pouch-shaped secondary battery.

[0029] According to an embodiment of the present invention, the positive electrode and the negative electrode may be manufactured by coating an active material layer-forming composition containing an electrode active material on a current collector and then drying the composition.

[0030] The composition for forming a positive electrode active material layer contains a positive electrode active material, where the positive electrode active material can be a material capable of causing an electrochemical reaction, such as a lithium transition metal oxide. For example, it can be a layered compound containing two or more transition metals, such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), etc. substituted by one or more transition metals; lithium manganese oxide substituted by one or more transition metals; lithium nickel-based oxides represented by the formula LiNi 1-y M y O2 (where M is Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga, and contains at least one of the above elements, 0.01 ≤ y ≤ 0.7); lithium nickel cobalt manganese composite oxides represented by Li 1+z Ni b Mn c Co 1-(b+c+d) M d O (2-e) A e (where -0.5 ≤ z ≤ 0.5, 0.1 ≤ b ≤ 0.8, 0.1 ≤ c ≤ 0.8, 0 ≤ d ≤ 0.2, 0 ≤ e ≤ 0.2, b + c + d < 1; M = Al, Mg, Cr, Ti, Si or Y; A = F, P or Cl), such as Li 1+z Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, Li 1+z Ni 0.4 Mn 0.4 Co 0.2 O2; olivine-type lithium metal phosphates represented by the formula Li 1+x M 1-y M' y PO 4-z X z (where M = transition metal, preferably Fe, Mn, Co or Ni; M' = Al, Mg or Ti; X = F, S or N; -0.5 ≤ x ≤ +0.5, 0 ≤ y ≤ 0.5 and 0 ≤ z ≤ 0.1), etc., but not limited thereto. Based on the total weight of the positive electrode active material layer, the content of the positive electrode active material can be 80% by weight to 99% by weight.

[0031] The composition for forming a negative electrode active material layer contains a negative electrode active material, where the negative electrode active material is a silicon-based negative electrode active material and can be the above-mentioned pure silicon particles. Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 80% by weight to 99% by weight.

[0032] According to an embodiment of the present invention, the composition for forming the positive electrode active material layer and the composition for forming the negative electrode active material layer may optionally contain a binder, a conductive material, a filler, etc. in addition to the active materials contained therein. Among them, the binder, conductive material, and filler applied to the positive electrode and the negative electrode may be the same or different from each other.

[0033] The binder is a component that aids in the adhesion between the conductive material, the active material, and the current collector. Based on the total weight of the active material layer, the addition amount of the binder is generally 0.1% by weight to 10% by weight. Examples of the binder include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile rubber, fluororubber, and various copolymers thereof.

[0034] The conductive material is a component that further improves the conductivity of the active material. Based on the total weight of the active material layer, the addition amount of the conductive material may be 10% by weight or less, specifically 5% by weight or less. The conductive material is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, it can be: graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black; conductive fibers, such as carbon fibers and metal fibers; carbon fluoride; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; conductive materials, such as polyphenylene derivatives.

[0035] The current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. For example, it can be: copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum cadmium alloy, etc. In addition, the current collector usually has a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion of the active material. For example, it can be used in various forms, such as film, sheet, foil, net, porous body, foam, non-woven fabric, etc.

[0036] On the other hand, according to an embodiment of the present invention, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a channel for lithium ion movement. Any separator can be used without particular limitation as long as it is generally used as a separator in a lithium secondary battery. In particular, a separator having low resistance to the migration of electrolyte ions and excellent electrolyte moisture retention ability is preferred. Specifically, a porous polymer membrane can be used, such as a porous polymer membrane made of a polyolefin-based polymer (e.g., ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer), or having a laminated structure with two or more layers. In addition, conventional porous non-woven fabrics can also be used, such as non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc. In addition, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can also be used, and a single-layer or multi-layer structure can be optionally used.

[0037] In addition, according to an embodiment of the present invention, the electrolyte can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the preparation of a lithium secondary battery, but is not limited thereto.

[0038] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0039] As the organic solvent, any solvent can be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent can be an ester-based solvent, such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether-based solvent, such as dibutyl ether or tetrahydrofuran; a ketone-based solvent, such as cyclohexanone; an aromatic hydrocarbon-based solvent, such as benzene or fluorobenzene; a carbonate-based solvent, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (PC); an alcohol-based solvent, such as ethanol or isopropanol; nitriles, such as R-CN (R is a linear, branched, or cyclic C2-C20 hydrocarbon group and may contain a double bond, an aromatic ring, or an ether bond); amides, such as dimethylformamide; dioxolanes, such as 1,3-dioxolane; or sulfolanes. Among them, carbonate-based solvents are preferred, and more preferably a mixture of a cyclic carbonate having high ionic conductivity and high dielectric constant (e.g., ethylene carbonate, propylene carbonate, etc.) and a low-viscosity linear carbonate-based compound (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.) that can improve the charge and discharge performance of the battery. In this case, excellent electrolyte performance can be obtained by mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to about 1:9.

[0040] The lithium salt can be any compound without particular limitation as long as it can provide lithium ions for use in a lithium secondary battery. Specifically, the anion of the lithium salt can be selected from the group consisting of F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - and at least one selected from the group consisting thereof, and the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The lithium salt is preferably used in a concentration range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that excellent electrolyte performance can be exhibited and lithium ions can move effectively.

[0041] To improve the life characteristics of the battery, suppress the reduction of the battery capacity, and improve the discharge capacity of the battery, in addition to containing the constituent components of the above electrolyte, the electrolyte may further contain one or more additives, for example, a halogenated alkylene carbonate-based compound (such as difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-ethylene glycol dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidinone, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, based on the total weight of the electrolyte, the content of the additive can be 0.1% by weight to 5% by weight.

[0042] Manufacturing method of lithium secondary battery

[0043] According to one embodiment of the present invention, a method for manufacturing a lithium secondary battery includes the following steps: (S1) preparing an inactive lithium secondary battery, wherein the inactive lithium secondary battery comprises: a negative electrode comprising a silicon-based negative electrode active material, a positive electrode, a separator inserted between the positive electrode and the negative electrode, and an electrolyte; and (S2) activating the inactive lithium secondary battery, wherein the activation of step (S2) includes a low-rate charging interval, wherein the battery is charged at a rate of 0.01C to 0.07C to a charge termination voltage of 4.0V or above.

[0044] Hereinafter, a method for manufacturing a lithium secondary battery according to an embodiment of the present invention will be described in detail.

[0045] Step S1: Preparation of inactive lithium secondary battery

[0046] According to one embodiment of the present invention, step S1 is a step of providing a lithium secondary battery by injecting an electrolyte into an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, that is, a step of assembling a lithium secondary battery before activation.

[0047] Since the positive electrode, the negative electrode, the separator, and the electrolyte have been described above, a detailed description thereof will be omitted.

[0048] Step S2: Activation step of inactive lithium secondary battery

[0049] According to one embodiment of the present invention, step S2 is a step of activating the assembled inactive lithium secondary battery by charging and discharging, wherein the activation of step (S2) includes a low-rate charging interval, wherein the battery is charged at a rate of 0.01C to 0.07C to a charging termination voltage of 4.0V or above.

[0050] Although the silicon-based negative electrode active material can show excellent performance due to its high capacity characteristics, there are many problems in commercialization due to the problem of volume expansion. Specifically, in the case of silicon-based negative electrode active materials, especially in the case of negative electrode active materials containing pure silicon particles, the activation process is inevitably accompanied by the process of converting crystalline silicon into amorphous silicon. However, if the conversion to a stable amorphous phase is not performed, the volume change between the insertion and extraction of lithium may be drastic, and the resulting internal stress may cause particle rupture. This particle rupture phenomenon may cause side reactions with the electrolyte, and may not participate in the reaction due to electrical short circuits, thereby causing the problem of reduced capacity.

[0051] Accordingly, the present invention provides an activation method that can effectively convert crystalline silicon into amorphous silicon and convert the portion that has not been converted into amorphous silicon into semi-amorphous silicon to further minimize crystalline silicon, thereby exhibiting the effects of reducing cracking of negative electrode active material particles and improving life characteristics.

[0052] To achieve the above effects, the activation step according to an embodiment of the present invention is characterized in that the initial charging rate is controlled at a low rate of 0.01C to 0.07C. The initial rate may preferably be 0.01C or more, 0.02C or more, 0.03C or more, 0.04C or more, and 0.07C or less, or 0.05C or less. When the first charging is performed at a rate of 0.1C or more, stress is applied to the active material particles when lithium is intercalated into the silicon-based negative electrode active material, and this stress may exceed the limit value that the particles can withstand, thereby causing cracks. In other words, the first charging process is the process in which lithium is first intercalated into the negative electrode active material. In this process, it is important to reduce the rate so that the particles do not break and to induce the conversion into amorphous silicon by reacting with lithium sequentially from the surface of the negative electrode active material layer. This can be achieved by controlling the rate at 0.01C to 0.07C during the first charging.

[0053] According to an embodiment of the present invention, the activation of step (S2) may include: a low-rate charging interval in which charging is performed at a rate of 0.01C to 0.07C to a charging termination voltage of 4.0V or more; and a high-rate discharging interval in which discharging is performed at a rate higher than the low-rate charging rate to an interruption voltage of 2.7V to 3.3V.

[0054] During the first charging, charging is performed at a low rate so that lithium and silicon gradually react from the surface of the negative electrode active material layer, inducing the conversion to the amorphous phase. Thereafter, subsequent charge and discharge can be performed at a relatively increased rate. In this case, the rate may be 3 to 10 times the low-rate charging rate, preferably 4 to 8 times the low-rate charging rate. More specifically, the high-rate discharging rate may be in the range of 0.2C to 0.5C. When high-rate discharging is performed after low-rate charging within this range, by the low-rate charging, the active material particles on the surface of the negative electrode active material layer are converted into a state in which lithium can be easily intercalated and deintercalated, so that the activation process can be performed faster without deforming the particles, thereby improving the efficiency of the process.

[0055] In addition, the discharge cut-off voltage in the high-rate discharge range may be 2.7 V to 3.3 V, which may be higher than the discharge cut-off voltage commonly used for activation. That is, when after the initial low-rate charge range, in the high-rate discharge range the discharge terminates near the cut-off voltage without being fully discharged to the discharge cut-off voltage and then charge and discharge are performed, the shrinkage of the silicon that swells during the charge process is minimized, and thus the generation of stress is minimized, thereby preventing the rupture of particles.

[0056] According to one embodiment of the present invention, the activation in step (S2) may include: a low-rate charge range in which charging is performed at a rate of 0.01 C to 0.07 C to a charge cut-off voltage of 4.0 V or more; a high-rate discharge range in which discharging is performed at a rate higher than the low-rate charge rate to a cut-off voltage of 2.7 V to 3.3 V; and a charge-discharge range in which charging is performed at a rate higher than the high-rate discharge rate to the charge cut-off voltage and then discharging is performed to a discharge cut-off voltage of 2.8 V or less.

[0057] After performing the low-rate charge range and the high-rate discharge range, a charge-discharge range in which charge and discharge are performed over the entire available voltage range may be performed. The rate of the charge-discharge range may be higher than the high-rate discharge rate. Since the crystalline Si of the active material near the surface of the negative electrode active material layer is converted to amorphous Li-Si by the initial low-rate charge, even if high-rate charge and discharge are subsequently performed, the space where lithium can diffuse from the surface to the inside more quickly has been formed, and thus, the reaction of the internal active material can be easily induced, thereby improving the efficiency of the activation process. If the rate in the charge-discharge range is lower than the high-rate discharge rate, the reaction at the surface may be dominant, and thus the thickness of the SEI film on the negative electrode surface may increase, resulting in problems such as a decrease in capacity characteristics or ionic conductivity. Therefore, it is desirable that the rate of the charge-discharge range be higher than the rate of the high-rate discharge range.

[0058] In addition, according to one embodiment of the present invention, in the charge-discharge range, charging and discharging may be performed while gradually increasing the rate within a range of higher than the high-rate discharge rate, preferably in the range of 0.3 C to 1.0 C. In this case, for each of charging and discharging, the method of gradually increasing the rate may be applied by changing the rate once or by changing the rate two or more times. If such a method of gradually increasing the rate is applied, an increase in the efficiency of the activation process can be expected. The charging and discharging in the charge-discharge range may be repeated for one or more cycles. The activation may be completed by performing it only once, but it may be performed two or more times according to the internal state of the negative electrode active material.

[0059] According to an embodiment of the present invention, the charging cut-off voltage in the activation step may be 4.0 V to 4.8 V, and the discharging cut-off voltage may be 2.2 V to 2.8 V. These voltages may vary according to the type of the positive electrode active material applied. The charging cut-off voltage and the discharging cut-off voltage are generally wider than the driving voltage range of the battery applied, and the capacity of the battery can be maximized by activating in a wider voltage range.

[0060] Examples

[0061] The present invention will be described in detail by way of examples below. However, the embodiments of the present invention can be modified in various different forms, and the scope of the present invention should not be construed as being limited to the embodiments described in detail below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0062] Preparation Example

[0063] LiNi as the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2, carbon nanotubes as the conductive material, and PVdF as the binder were mixed in a weight ratio of 92.8:0.8:1.4 in an N-methyl-2-pyrrolidone (NMP) solvent to prepare a positive electrode paste. The paste was coated on one side of an aluminum current collector, dried and then calendered to manufacture a positive electrode.

[0064] Pure silicon with a particle size of about 5 μm (crystallite size of about 200 nm) as the negative electrode active material, Super C65 as the conductive material, SBR as the binder, and CMC as the thickener were mixed in a weight ratio of 95.6:1.5:2.3:1.1 in an N-methyl-2-pyrrolidone (NMP) solvent to prepare a negative electrode paste. The negative electrode paste was coated on one side of a copper current collector, dried and then calendered to prepare a negative electrode.

[0065] A separator was inserted between the positive electrode and the negative electrode to prepare an electrode assembly, which was placed in a battery case. Then an electrolyte was injected to prepare an inactive lithium secondary battery.

[0066] Here, as the electrolyte, an electrolyte in which 1 M of LiPF6 was dissolved in an organic solvent mixture of ethylene carbonate:ethyl methyl carbonate:diethyl carbonate in a volume ratio of 3:3:4 was used.

[0067] Example 1

[0068] The inactive lithium secondary battery is activated as follows: The secondary battery prepared in the preparation example is charged at a rate of 0.01C to 4.2V, and then discharged at a rate of 0.20C to 3V; subsequently, the lithium secondary battery is charged again at a rate of 0.20C to 4.2V and then discharged to 2.5V.

[0069] Example 2

[0070] The inactive lithium secondary battery is activated as follows: The secondary battery prepared in the preparation example is charged at a rate of 0.05C to 4.2V, and then discharged at a rate of 0.33C to 3V; subsequently, the lithium secondary battery is charged again at a rate of 0.33C to 4.2V and then discharged to 2.5V.

[0071] Example 3

[0072] The inactive lithium secondary battery is activated as follows: The secondary battery prepared in the preparation example is charged at a rate of 0.07C to 4.2V, and then discharged at a rate of 0.5C to 3V; subsequently, the lithium secondary battery is charged again at a rate of 0.5C to 4.2V and then discharged to 2.5V.

[0073] Example 4

[0074] The inactive lithium secondary battery is activated as follows: The secondary battery prepared in the preparation example is charged at a rate of 0.05C to 4.2V, and then discharged at a rate of 0.33C to 3V; subsequently, the lithium secondary battery is charged again at a gradually changing rate from 0.33C to 0.5C to 4.2V and then discharged at a rate of 0.5C to 2.5V.

[0075] Example 5

[0076] The inactive lithium secondary battery is activated as follows: The secondary battery prepared in the preparation example is charged at a rate of 0.05C to 4.2V, and then discharged at a rate of 0.33C to 3V; subsequently, the lithium secondary battery is charged again at a rate of 0.4C to 4.2V and then discharged to 2.5V.

[0077] Comparative Example 1

[0078] The inactive lithium secondary battery is activated as follows: The secondary battery prepared in the preparation example is charged at a rate of 0.33C to 4.2V, and then discharged at a rate of 0.33C to 3V; subsequently, the lithium secondary battery is charged again at a rate of 0.33C to 4.2V and then discharged to 2.5V.

[0079] Comparative Examples 2 to 6

[0080] In Comparative Examples 2 to 6, the non-active lithium secondary battery was activated in the same manner as in Comparative Example 1, except that the rates were 0.1C, 0.2C, 0.5C, 1C, and 6C, respectively.

[0081] Experimental Example 1: Variation of stress inside silicon particles with charge rate

[0082] For Example 1 and Comparative Examples 1 to 6, during charging, the stress change of silicon particles as the negative electrode active material with the initial rate at each SOC was measured. For the measurement, the CAD three-dimensional shape of Si particles with polycrystals was designed as a random polycrystalline shape using the Voronoi diagram technique. Referring to previously reported papers, constants such as the lithium diffusion coefficient inside silicon were applied, and using Comsol Multiphysics software, the stress change of silicon particles (negative electrode active material) with the initial rate at each SOC was calculated using the designed shape. The results are as Figure 1 shown.

[0083] Referring to Figure 1 , it can be seen that with the change of the initial rate during activation, the stress change inside the silicon particles is drastic, and when the stress exceeds about 2 GPa, the particles rupture. Thus, it can be confirmed that in the negative electrodes of Comparative Examples 1 to 6 other than Example 1, a part of the active material particles ruptured.

[0084] Experimental Example 2: Cross-sectional analysis of the activated negative electrode

[0085] The completed-activated batteries of the examples and comparative examples were disassembled and cleaned in DMC solvent for 1 hour to remove organic substances, such as the SEI layer of the negative electrode. Then the electrodes were sampled and cut using an ion milling device (IM-4000, manufactured by Hitachi, Ltd.) so that the vertical cross-section of the electrode could be observed. Subsequently, using a Raman spectrometer (NTEGRA SPECTRA, manufactured by NT_MDT), a Raman spectrum of this region was obtained using a laser with a resolution of 500 nm × 500 nm on the cut negative electrode cross-section. In summary, regarding the respective contents of amorphous Si (467 cm -1 ), crystalline Si (515 cm -1 ), and semi-amorphous (greater than 467 cm -1 and less than 515 cm -1 ), the results of Example 2 are shown in Figure 2 , and the results of other examples and comparative examples are shown in Table 1 below.

[0086]

[0087] Referring to Figure 2, it is possible to confirm that the part marked in red is amorphous silicon, the part marked in blue is crystalline silicon, and the part marked in green is semi-amorphous silicon. In the case of the negative electrode active material of Example 2, the amorphous silicon accounts for 46.3% of the total volume, and the crystalline silicon is in a relatively small amount of 18.3%. If the crystal content is high, it means a high possibility of early deterioration of the electrode. When the amorphous silicon and crystalline silicon are converted to the same level as in Example 2, excellent effects as a negative electrode material can be exhibited. In addition, referring to Table 1, it can be confirmed that in the cases of Examples 1 to 5, the ratio of the crystalline phase is mostly a small value of 25% or less, the ratio of the semi-amorphous phase is about 31% or more, and the ratio of the amorphous phase is also 45% or more. In the comparative examples, the ratio of the crystalline phase is significantly higher, and the ratios of the amorphous phase and semi-amorphous phase are low. As described above, the possibility of early deterioration of the electrode is quite high.

[0088] Experimental Example 3: Evaluation of the life characteristics of lithium secondary batteries

[0089] The charge and discharge of each lithium secondary battery of Examples 1 to 5 and Comparative Examples 1 to 6 were carried out, and the life characteristics of the secondary battery were evaluated using the following method. The results are shown in Table 1 below.

[0090] * Life characteristics (capacity retention rate): Charge and discharge were carried out at 1.0C from the 1st cycle to the 200th cycle.

[0091] Charging conditions: CC (constant current) / CV (constant voltage) (4.2V / 0.05C current cut-off)

[0092] Discharging conditions: CC (constant current) condition 3V

[0093] The capacity retention rate was calculated as follows:

[0094] Capacity retention rate (%) = (discharge capacity at the 200th cycle / discharge capacity at the 1st cycle) × 100

[0095] When evaluating the life under the above methods and conditions, the resistance was calculated using the voltage and current during discharge ((V0 - V1) / I, where V0 is the initial discharge voltage, V1 is the voltage after 60 seconds of discharge, and I is the applied current). The resistance increase rate was calculated using the resistance after cycling compared with the initial resistance by the following formula. The results are shown in Table 2 below.

[0096] Resistance increase rate (%) = [(resistance after 200 discharges / initial resistance) - 1] × 100

[0097]

[0098] Referring to Table 2, it can be seen that, compared with Comparative Examples 1 to 6 in which the initial magnification increases, Examples 1 to 5 according to the present invention have better life characteristics and resistance characteristics. It can be said that this is the effect caused by the fact that, compared with Examples 1 to 5, the ratio of the crystalline phase in silicon is high and the ratios of the amorphous phase and the semi-amorphous phase are low in the case of Comparative Examples 1 to 6.

Claims

1. A lithium secondary battery, comprising: a negative electrode containing a silicon-based negative electrode active material, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, Among them, wherein the silicon-based negative electrode active material contains silicon particles, and based on 100 parts by volume of the negative electrode active material, it contains 30 parts by volume or less of crystalline silicon.

2. The lithium secondary battery according to claim 1, wherein the silicon-based negative electrode active material contains 5 to 30 parts by volume of crystalline silicon; 30 to 85 parts by volume of amorphous silicon; and 10 to 65 parts by volume of semi-amorphous silicon, Among them, In the Raman spectrum obtained by using a Raman spectrometer, the semi-amorphous silicon has a wavelength greater than 467 cm -1 and less than 515cm -1 There is a peak at the Raman shift value of .

3. The lithium secondary battery according to claim 1, wherein the silicon-based negative electrode active material contains 5 to 25 parts by volume of crystalline silicon; 40 to 70 parts by volume of amorphous silicon; and 25 to 55 parts by volume of semi-amorphous silicon, Among them, On the Raman spectrum obtained using a Raman spectrometer, the semi-amorphous silicon has a peak at a Raman shift value greater than 467 cm -1 and less than 515 cm -1 .

4. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is activated.

5. The lithium secondary battery according to claim 4, wherein the activation includes a process of charging at a rate of 0.01C to 0.07C to a charging cut-off voltage of 4.0V or more.

6. A method for a lithium secondary battery according to claim 1, wherein the positive electrode contains a positive electrode active material, Among them, and 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-based composite oxide, lithium manganese-based composite oxide, and lithium transition metal phosphate.

7. A method for manufacturing a lithium secondary battery, comprising the following steps: (S1) Prepare an inactive lithium secondary battery, the inactive lithium secondary battery comprising: a negative electrode containing a silicon-based negative electrode active material, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte; and (S2) Activate the inactive lithium secondary battery, wherein the activation in step (S2) includes a low-rate charging range, wherein charging is performed at a rate of 0.01C to 0.07C to a charging cut-off voltage of 4.0V or more.

8. The method for manufacturing a lithium secondary battery according to claim 7, wherein the activation in step (S2) includes: a low-rate charging range, wherein charging is performed at a rate of 0.01C to 0.07C to a charging cut-off voltage of 4.0V or more; and a high-rate discharging range, wherein discharging is performed at a rate higher than the low-rate charging rate to an interruption voltage of 2.7V to 3.3V.

9. The method for manufacturing a lithium secondary battery according to claim 7, wherein the activation in step (S2) includes: a low-rate charging range, wherein charging is performed at a rate of 0.01C to 0.07C to a charging cut-off voltage of 4.0V or more; a high-rate discharging range, wherein discharging is performed at a rate higher than the low-rate charging rate to an interruption voltage of 2.7V to 3.3V; and a charge-discharge range, wherein charging is performed at a rate higher than the high-rate discharging rate to a charging cut-off voltage, and then discharging is performed to a discharging cut-off voltage of 2.8V or less.

10. The method for manufacturing a lithium secondary battery according to claim 9, wherein in the charge-discharge range, within the range of a rate higher than the high-rate discharging rate, charging and discharging are performed while gradually increasing the rate.

11. The manufacturing method of the lithium secondary battery according to claim 9, wherein in the charge-discharge interval, charging and discharging are performed while gradually increasing the rate within the range of 0.3C to 1.0C.

12. The manufacturing method of the lithium secondary battery according to claim 7, wherein the low-rate charging rate is from 0.02C to 0.05C.

13. The manufacturing method of the lithium secondary battery according to claim 9, wherein the high-rate discharge rate is 3 to 10 times the low-rate charging rate.

14. The manufacturing method of the lithium secondary battery according to claim 9, wherein the high-rate discharge rate is from 0.2C to 0.5C.

15. The manufacturing method of the lithium secondary battery according to claim 7, wherein the charge termination voltage is from 4.0V to 4.8V, and the discharge termination voltage is from 2.2V to 2.8V.

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