Negative electrode active material and negative electrode and lithium secondary battery containing the same

Abstract

To provide a negative electrode active material for lithium secondary batteries with good lifespan characteristics. [Solution] A secondary particle assembled from multiple primary silicon particles, amorphous carbon filling the spaces between the primary silicon particles and covering the surface of the primary silicon particles and the surface of the secondary particle, and a 0.5 μm particle present inside the amorphous carbon or independently of the secondary particle. 2 A negative electrode active material is provided which contains fine silicon particles having the following cross-sectional area, wherein the proportion of the fine silicon particles is 6.5 area% or less (excluding 0 area%).

Description

Technical Field

[0001] The present invention relates to a negative electrode active material, a negative electrode containing the same, and a lithium secondary battery, and more particularly, to a silicon-based negative electrode active material, a negative electrode containing the same, and a lithium secondary battery.

Background Art

[0002] Lithium ion batteries (LIBs) have a high energy density and are easy to design, so they are adopted and used as the main power supply for mobile electronic devices. In the future, their application scope to electric vehicles, power storage devices for renewable energy, etc. is further expanding.

[0003] In order to apply to new application fields, research on LIB materials having characteristics such as higher energy density and longer life is continuously required.

[0004] In particular, in the case of negative electrode materials, research has been carried out on various substances such as carbon, silicon, tin, germanium, etc.

[0005] Among these, silicon-based negative electrode materials have a very high energy density compared to the currently commercialized graphite negative electrode materials and have attracted much attention.

[0006] However, silicon-based negative electrode materials have fatal drawbacks such as the formation of an unstable SEI layer due to the side reaction between the silicon surface and the electrolyte, resulting in a decrease in electrochemical characteristics, or the pulverization of electrode materials due to internal stress caused by the rapid volume expansion during charge and discharge.

[0007] In order to solve this problem, a plan to nanonize the silicon-based active material or to composite it by secondary particle formation of silicon and carbon has been studied, but the problems due to the continuous increase of SEI and the collapse of the conductive network have not been completely solved at present.

Summary of the Invention

Problems to be Solved by the Invention

[0008] One of the various objects of the present invention is to provide a negative electrode active material for a lithium secondary battery having good life characteristics.

Means for Solving the Problems

[0009] According to one aspect, secondary particles assembled from a plurality of silicon primary particles, amorphous carbon filled between the silicon primary particles to cover the surface of the silicon primary particles, and covering the surface of the secondary particles, and present inside the amorphous carbon or existing independently of the secondary particles, microparticle silicon having a cross-sectional area of 0.5 μm 2 A negative electrode active material containing microparticle silicon having the following cross-sectional area, wherein the proportion of the microparticle silicon is 6.5 area% or less (excluding 0 area%), is provided.

[0010] In one embodiment, the proportion of the microparticle silicon may be 3.0 area% or less.

[0011] In one embodiment, the silicon primary particles may be flaky nanosilicon sections.

[0012] In one embodiment, the silicon primary particles may be at least one selected from the group consisting of SiOx (x = 0), SiOx (0 <x ≤ 0.5), SiOx (0.5 <x <2), SiC, and Si alloys.

[0013] In one embodiment, in the volume distribution measured using a laser diffraction particle size analyzer for the silicon primary particles, when the cumulative volume distribution percentage reaches 50%, the corresponding particle diameter is d v 50, and the d v 50 may be 80 to 120 nm.

[0014] In one embodiment, in the volume distribution measured for the secondary particles using a laser diffraction particle size analyzer, the particle size corresponding to the point when the cumulative volume distribution percentage reaches 50% is defined as D. v If it is 50, then D v 50 may be 3 to 15 μm.

[0015] In one embodiment, the amorphous carbon may be formed from at least one selected from the group consisting of sucrose, phenol resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin or vinyl chloride resin, coal-based pitch, petroleum-based pitch, polyvinyl chloride, mesoface pitch, tar, block copolymer, polyol, and low molecular weight heavy oil.

[0016] In one embodiment, the negative electrode active material may further contain crystalline carbon.

[0017] In one embodiment, the crystalline carbon may be at least one selected from the group consisting of natural graphite, artificial graphite, expanded graphite, graphene, carbon black, and fullerene.

[0018] In one embodiment, the crystalline carbon may be arranged on the surface of the secondary particles.

[0019] In one embodiment, the negative electrode active material may contain 5 to 70% silicon relative to the total weight of the negative electrode active material.

[0020] In another embodiment, a negative electrode containing the negative electrode active material is provided.

[0021] In one embodiment, after 100 charge-discharge cycles, when the negative electrode is heated from 25°C to 200°C at a rate of 5°C / min using a differential scanning calorimeter (DSC), the cumulative heat from 80°C to 200°C may be 180 J / g or less.

[0022] In yet another embodiment, a lithium secondary battery is provided, comprising a negative electrode, a positive electrode, and an electrolyte, wherein the negative electrode comprises the negative electrode active material. [Effects of the Invention]

[0023] The negative electrode active material of the present invention can significantly improve the lifespan characteristics of lithium secondary batteries by controlling the proportion of silicon nanoparticles to a certain level or lower.

[0024] The effects of the present invention are not limited to those described herein, but should be understood to include all effects that can be inferred from the detailed description herein or the configurations described in the claims. [Brief explanation of the drawing]

[0025] [Figure 1] This graph shows the measurement results of the capacity retention rate of lithium secondary batteries after 1 to 500 cycles for each embodiment. [Figure 2] This graph shows the measurement results of the capacity retention rate of lithium secondary batteries after 1 to 500 cycles for each embodiment. [Modes for carrying out the invention]

[0026] One aspect of the present invention will be described below based on specific examples. However, the provisions described herein may be embodied in various different forms and are therefore not limited to the embodiments described herein.

[0027] Where the specification as a whole states that a part "includes" a certain component, this means, unless otherwise stated, that it may further include other components rather than excluding them.

[0028] Where a range of numerical values ​​is given herein, unless otherwise specified, the value shall have the precision of significant figures provided in accordance with the standard rules of chemistry for significant figures. For example, 10 includes the range of 5.0 to 14.9, and the figure 10.0 includes the range of 9.50 to 10.49.

[0029] negative electrode active material As mentioned earlier, silicon-based active materials have a fatal flaw: the rapid volume expansion that occurs during charging and discharging causes internal stress that leads to the pulverization of the electrode material. To overcome this, research is being intensively pursued in the direction of nano-sizing silicon-based active materials.

[0030] However, our research has shown that during the nano-processing of silicon-based active materials, fine silicon particles are inevitably formed independently of the silicon composite. These fine silicon particles continuously form an SEI (Solid Electrolyte Interphase) layer during charging and discharging, contributing to a decrease in the battery's lifespan. Specifically, as charging and discharging progress, the formation of SEI due to side reactions between the fine silicon particles and the electrolyte becomes more severe. As a result, the volume expansion of the negative electrode active material promotes particle cracking, allowing the electrolyte to penetrate into the cracks and causing continuous degradation of the negative electrode active material. Furthermore, our further research has shown that reducing the proportion of such fine silicon particles to below a certain level delays the occurrence of side reactions and significantly improves the lifespan of the negative electrode active material.

[0031] In other words, one aspect of the present invention provides a secondary particle assembled from a plurality of silicon primary particles, amorphous carbon filling the spaces between the silicon primary particles and covering the surface of the silicon primary particles and the surface of the secondary particle, and a 0.5 μm particle present inside the amorphous carbon or independently of the secondary particle.2 The present invention relates to a negative electrode active material containing fine silicon particles having the following cross-sectional area, characterized in that the proportion of the fine silicon particles is 6.5 area% or less (excluding 0 area%). The proportion of the fine silicon particles may preferably be 5.0 area% or less, more preferably 4.5 area% or less, even more preferably 4.0 area% or less, even more preferably 3.5 area% or less, and most preferably 3.0 area% or less.

[0032] The silicon nanoparticles form very small silicon clusters, separate from the silicon primary particles that constitute the secondary particles. Such clusters may exist outside the secondary particles or inside the amorphous carbon that constitutes the matrix of the secondary particles, or inside the crystalline carbon if the negative electrode active material further contains crystalline carbon. In other words, the silicon nanoparticles may be completely embedded in the matrix of the secondary particles, partially embedded, or not embedded at all.

[0033] The aforementioned silicon nanoparticles cannot be quantified using a laser diffraction particle size analyzer, but their area ratio can be measured by the following method.

[0034] (1) A mixture is obtained by mixing a 1.0-2.0 wt% aqueous solution of carboxymethyl cellulose (CMC) with the negative electrode active material in a weight ratio of 97:3.

[0035] (2) The mixture is applied to a copper thin film and dried to form a negative electrode active material layer with a thickness of 50 to 60 μm.

[0036] (3) An ion beam milling apparatus (IB-19510CP, JEOL) is used to secure a cross-section of the negative electrode active material layer.

[0037] (4) A cross-sectional image of the negative electrode active material layer at a magnification of 2,000 times is taken using a field emission scanning electron microscope (JSM-7001F, manufactured by JEOL Ltd.).

[0038] (5) A histogram is extracted from the taken cross-sectional image using image analysis software (ImageJ, manufactured by National Institute of Health). After deriving the median value of the brightness at two maximum values as the threshold, the ratio (S1 / S0×100) of the total area (S1) of fine particle silicon having the following cross-sectional area to the total area (S0) of silicon particles is calculated, and this is taken as the area ratio of fine particle silicon (unit: area %). 2 The silicon primary particles may be at least one selected from the group consisting of SiOx (x = 0), SiOx (0 < x ≤ 0.5), SiOx (0.5 < x < 2), SiC, and Si alloys. The Si alloy may be, for example, a Si-Z alloy (where Z is at least one element selected from the group consisting of alkali metals, alkaline earth metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof and does not contain Si), but is not limited thereto.

[0039] The silicon primary particles may be scaly nano-silicon slices, but are not limited thereto. When the silicon primary particles have a scaly structure, the spheroidization degree may be 0.5 or less, preferably 0.1 to 0.5, and more preferably 0.1 to 0.3. When the spheroidization degree satisfies the above range, the expansion rate of the electrode plate during negative electrode manufacturing can be reduced and the battery life can be improved. Here, the spheroidization degree means the ratio of the major axis to the minor axis in the cross-section of the silicon primary particles, and may mean, for example, "aspect ratio" or "length / thickness ratio".

[0040]

[0041] ​In the volume distribution measured using a laser diffraction particle size analyzer (e.g., Mastersizer 3000) for the silicon primary particles, when the cumulative volume distribution percentage reaches 50%, the particle diameter corresponding thereto is d v 50. Then, the d v 50 may be 80 to 120 nm. When the value of the d v 50 is too large, volume expansion becomes excessive, which may be disadvantageous for the life characteristics. Conversely, when the value is too small, the capacity and initial efficiency may decrease.

[0042] The crystal grain size of the silicon primary particles may be 10 nm or more, preferably 13 nm or more, and more preferably 15 nm or more. When the crystal grain size of the silicon primary particles is less than 10 nm, it may be disadvantageous in realizing the capacity of the battery. On the other hand, the larger the crystal grain size of the silicon primary particles, the more advantageous it is in realizing the capacity of the battery. Therefore, in the present invention, the upper limit is not particularly limited, but considering the life stability of the battery, it can be set to 30 nm. The crystal grain size can be determined by the following formula after analyzing the crystal structure by scanning a sample powder pellet to be measured for Cu-Kα rays with an XRD (X’pert, Malvern Panalytical) device.

[0043]

Equation

[0044] In the volume distribution measured using a laser diffraction particle size analyzer (e.g., Mastersizer 3000) for the secondary particles assembled from a plurality of the silicon primary particles, when the cumulative volume distribution percentage reaches 50%, the particle diameter corresponding thereto is D v 50. Then, the D v 50 may be 3 to 15 μm. The D vIf the value of 50 is too large, electrode expansion due to volume expansion of the negative electrode active material can become severe, potentially leading to problems with lifespan and stability. Conversely, if the value is too small, the reduced dispersibility can make the electrode manufacturing process less efficient.

[0045] Amorphous carbon fills the spaces between the silicon primary particles, coating the surfaces of the silicon primary particles and the secondary particles. The amorphous carbon interposes itself between multiple silicon primary particles, physically linking them together. During the charge-discharge process, it blocks contact between silicon and the electrolyte, reducing the generation of by-reactants and delaying the degradation of the negative electrode active material. It also more effectively buffers the volume expansion of silicon nanoparticles, acts as an electron transfer pathway between the silicon nanoparticles and the electrolyte, improving conductivity and reducing electrode resistance. The amorphous carbon may be derived from at least one selected from the group consisting of sucrose, acrylic resin, phenol resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin or vinyl chloride resin, coal-based pitch, petroleum-based pitch, polyvinyl chloride, mesoface pitch, tar, block copolymer, polyol, and low molecular weight heavy oil, and is preferably derived from at least one selected from the group consisting of coal-based pitch, petroleum-based pitch and acrylic resin, but is not limited thereto.

[0046] The softening point of the amorphous carbon is preferably 250°C or lower, more preferably 220°C or lower, even more preferably 200°C or lower, and most preferably 185°C or lower. If the softening point of the amorphous carbon does not meet the above range, the formation of excessive silicon fine particles may be detrimental to the lifespan characteristics.

[0047] The negative electrode active material according to one aspect of the present invention may further contain crystalline carbon. The crystalline carbon can compensate for insufficient electrical conductivity and also act as a buffer against volume expansion.

[0048] The crystalline carbon may be at least one selected from the group consisting of natural graphite, artificial graphite, expanded graphite, graphene, carbon black, and fullerene, and is preferably natural graphite, but is not limited thereto. The purity of the graphite may be high-purity grade with a fixed carbon content of 99% by weight or more, more preferably 99.95% by weight or more, but is not limited thereto.

[0049] Natural graphite is graphite that occurs naturally, and includes flake graphite, high crystalline graphite, and amorphous (microcrystalline or cryptocrystalline) graphite.

[0050] Artificial graphite is graphite that has been synthesized artificially, produced by heating amorphous carbon at high temperatures, and includes primary or electrographite, secondary graphite, and graphite fiber.

[0051] Expanded graphite may be produced by intercalating chemicals such as acids or alkalis between layers of graphite and then heating it to expand the vertical layers of its molecular structure.

[0052] Graphene consists of a single layer or multiple layers of graphite.

[0053] Carbon black is a crystalline substance with less regularity than graphite, and it can be converted into graphite by heating it at approximately 3,000°C for a long period of time.

[0054] Fullerenes are carbon mixtures containing at least 3% by weight of fullerenes, which are polyhedral bundle compounds consisting of 60 or more carbon atoms.

[0055] The crystalline carbon may be arranged on the surface of the secondary particles or may exist independently of the secondary particles. When the crystalline carbon is arranged on the surface of the secondary particles, it effectively prevents contact between the silicon nanoparticles and the electrolyte, contributing to the stable electrochemical reversibility of the silicon nanoparticles, and can also contribute to improving the lifetime characteristics of the negative electrode active material based on the stable electrochemical stability with the electrolyte at the outermost layer.

[0056] A negative electrode active material according to one aspect of the present invention may contain 5 to 70% silicon relative to the total weight of the negative electrode active material. When the silicon content satisfies the above range, it may be advantageous in terms of the initial efficiency, life characteristics, and output characteristics of the secondary battery.

[0057] The negative electrode active material according to one aspect of the present invention may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as a preferred example, it may be manufactured by a method comprising the steps of (a) manufacturing a mixture in which silicon-containing particles are dispersed in an organic solvent, (b) grinding the mixture and then spray-drying it to manufacture a silicon precursor, (c) compounding the silicon precursor with amorphous carbon and then compounding it to manufacture composite particles, and (d) heat-treating the composite particles.

[0058] <Step (a)> A mixture of micrometer-sized silicon particles and an organic solvent is produced, in which silicon-containing particles are dispersed in the organic solvent.

[0059] The organic solvent may be an alcohol that volatilizes easily without oxidizing the silicon particles, and may be, for example, at least one selected from the group consisting of methanol, ethanol, propanol, butanol, and propylene glycol, and preferably isopropyl alcohol, but is not limited thereto.

[0060] The weight ratio of the silicon-containing particles to the organic solvent may be 2:95 to 30:70, preferably 5:95 to 25:75, and more preferably 8:92 to 20:80, but is not limited thereto. When the weight ratio of the silicon-containing particles to the organic solvent satisfies the above range, there is an advantage in that the milling efficiency can be maximized.

[0061] <Step (b)> The mixture obtained in step (a) is pulverized and then spray-dried to produce a silicon precursor.

[0062] The grinding step may be carried out by at least one physical method selected from the group consisting of milling, stirring, mixing, and compression, which are high-energy processes.

[0063] The grinding step may be carried out by at least one method selected from the group consisting of bead milling, ball milling, mechanofusion milling, shaker milling, planetary milling, attritor milling, and disk milling.

[0064] The balls that can be used for the aforementioned ball milling may be, for example, zirconia balls, and there are no restrictions on the type of ball. The size of the ball may be, for example, about 0.3 to 10 mm, but is not limited to this.

[0065] The aforementioned planetary ball mill can efficiently mix and grind a mixture by a mixing method in which the ball rotates and revolves without contact with the composition.

[0066] In the spray drying step, the solid content of the slurry is preferably 8% or more and less than 15%, more preferably 9% or more and 14%, and even more preferably 10% or more and 13%. If the solid content of the slurry is too low, it may negatively affect the lifespan characteristics due to the formation of excessive silicon particles, and conversely, if the content is too high, it may become difficult to produce the precursor.

[0067] <Step (c)> Amorphous carbon and crystalline carbon are added to the silicon precursor obtained in step (c), and then composite particles are produced by compounding.

[0068] The compounding step may be carried out by a melt mixing method.

[0069] The reaction time for the compounding step may be 1 minute to 12 hours, preferably 5 minutes to 6 hours, and more preferably 10 minutes to 3 hours. If the compounding time is too short, the effect of improving the structural stability of the negative electrode active material may be insufficient, and conversely, if it is too long, the capacity and lifespan of the negative electrode active material may decrease.

[0070] The reaction temperature of the compounding step may be 40 to 300°C, preferably 60 to 280°C, and more preferably 100 to 250°C.

[0071] The reaction atmosphere in the compounding step may be air or an inert gas atmosphere, and preferably an inert gas atmosphere, but is not limited thereto.

[0072] <Step (d)> The composite particles obtained in step (c) are heat-treated to produce the negative electrode active material.

[0073] The heat treatment temperature in the heat treatment step may be 700 to 1100°C, preferably 800 to 1050°C, and more preferably 900 to 1000°C.

[0074] The heat treatment time in the heat treatment step may be 1 to 10 hours, and preferably 3 to 7 hours.

[0075] The heat treatment step may be carried out in an inert gas atmosphere containing nitrogen, hydrogen, argon, or a mixture thereof, or under vacuum.

[0076] negative electrode Another aspect of the present invention provides a negative electrode comprising the negative electrode active material.

[0077] The negative electrode, after 100 charge-discharge cycles, may have a cumulative heat value of 180 J / g or less when heated from 25°C to 200°C at a rate of 5°C / min using a differential scanning calorimeter (DSC), preferably 170 J / g or less, more preferably 160 J / g or less, even more preferably 150 J / g or less, even more preferably 140 J / g or less, and most preferably 130 J / g or less.

[0078] The negative electrode may be manufactured, for example, by mixing the negative electrode active material, a binder, and a selectively conductive agent in a solvent to produce a negative electrode active material composition, and then molding it into a certain shape, or by coating it onto a current collector such as copper foil.

[0079] The negative electrode may further include, in addition to the negative electrode active material described above, a negative electrode active material commonly used as a negative electrode active material for lithium secondary batteries in the art. Commonly used negative electrode active materials may include, for example, at least one selected from the group consisting of lithium metal, metals alloyable with lithium, transition metal oxides, non-transition metal oxides, and carbon-based materials.

[0080] The metals capable of alloying with lithium may be, for example, Si, Sn, Al, Ge, Pb, Bi, Sb, Si-Y alloy (where Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element or a combination thereof, and does not contain Si), Sn-Y alloy (where Y is an alkali metal, alkaline earth metal, Group 13 to Group 16 element, transition metal, rare earth element or a combination thereof, and does not contain Sn). and so on, but are not limited thereto. The element Y may be, for example, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po or a combination thereof, but is not limited thereto.

[0081] The transition metal oxide may be, for example, lithium titanate, vanadium oxide, lithium vanadate and the like, but is not limited thereto.

[0082] The non-transition metal oxide may be, for example, SnO2, SiOx (0 < x ≤ 2) and the like, but is not limited thereto.

[0083] The carbon-based material may be, for example, crystalline carbon, amorphous carbon or a mixture thereof, but is not limited thereto. The crystalline carbon may be graphite such as amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, and the amorphous carbon may be soft carbon (low-temperature calcined carbon) or hard carbon, mesophase pitch carbide, calcined coke and the like, but is not limited thereto.

[0084] When the aforementioned negative electrode active material is used in combination with a carbon-based material, the oxidation reaction of the silicon-based active material is suppressed, an SEI film is effectively formed to create a stable coating, and electrical conductivity is improved, thereby further enhancing the charge-discharge characteristics of lithium.

[0085] Conventional negative electrode active material may be used by mixing and blending it with the aforementioned negative electrode active material, or by coating it on the surface of the aforementioned negative electrode active material, or in any other combination form.

[0086] The binder used in the negative electrode active material composition may be added in an amount of 1 to 50 parts by weight based on 100 parts by weight of the negative electrode active material, as a component that assists in the bonding of the negative electrode active material with conductive agents and other materials, and with the current collector. For example, it may be added in an amount of 1 to 30 parts by weight, 1 to 20 parts by weight, or 1 to 15 parts by weight based on 100 parts by weight of the negative electrode active material.

[0087] Examples of such binders include, but are not limited to, polyvinylidene fluoride, polyvinylidene chloride, polybenzimidazole, polyimide, polyvinyl acetate, polyacrylonitrile, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polystyrene, polymethyl methacrylate, polyaniline, acrylonitrile butadiene styrene, phenolic resins, epoxy resins, polyethylene terephthalate, polytetrafluoroethylene, polyphenylene sulfide, polyamide-imide, polyetherimide, polyethylene sulfone, polyamide, polyacetal, polyphenylene oxide, polybutylene terephthalate, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0088] The negative electrode may further selectively contain a conductive agent to provide a conductive path to the negative electrode active material and further improve electrical conductivity.

[0089] Any conductive agent commonly used in lithium batteries can be used as the conductive agent. Examples include carbon-based materials such as carbon black, acetylene black, Ketjen black, and carbon fibers (e.g., vapor-grown carbon fibers); metal-based materials such as metal powders or metal fibers of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or conductive materials containing mixtures thereof. The content of the conductive agent may be adjusted as appropriate. For example, the weight ratio of the negative electrode active material to the conductive agent may be in the range of 99:1 to 90:10.

[0090] The solvent may be N-methylpyrrolidone (NMP), acetone, water, etc. The solvent content is 1 to 10 parts by weight based on 100 parts by weight of the negative electrode active material. When the solvent content is within the above range, the process of forming the active material layer is easy.

[0091] The current collector is generally manufactured with a thickness of 3 to 500 μm. The current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy may be used.

[0092] Furthermore, by forming fine irregularities on the surface of the current collector, the bonding force of the negative electrode active material can be strengthened, and it may be used in various forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0093] The negative electrode plate can be manufactured by directly coating the manufactured negative electrode active material composition onto a current collector, or by casting it onto another support, peeling the negative electrode active material film from the support, and laminating it onto a copper foil current collector. The negative electrode is not limited to the forms listed above, and may be in forms other than those listed above.

[0094] The negative electrode active material composition may be used not only for manufacturing electrodes for lithium secondary batteries, but also for manufacturing printable batteries by being printed on a flexible electrode substrate.

[0095] Lithium-ion rechargeable battery A further aspect of the present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, and an electrolyte, wherein the negative electrode comprises a negative electrode active material.

[0096] The lithium secondary battery may have a capacity retention rate of 60% or more after 500 cycles in a voltage range of 2.75V to 4.2V, preferably 65% ​​or more, more preferably 70% or more, even more preferably 75% or more, and most preferably 80% or more.

[0097] The positive electrode may be manufactured from a positive electrode active material composition which is a mixture of a positive electrode active material, a conductive agent, a binder, and a solvent.

[0098] As the positive electrode active material, any lithium-containing metal oxide commonly used in the art may be used.

[0099] For example, Li a A 1-b B b D2 (wherein the above formula, 0.90 ≤ a ≤ 1.8 and 0 ≤ b ≤ 0.5), Li a E 1-b B b O 2-c D c (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05), LiE 2-b B b O 4-c D c (In the above formula, 0≦b≦0.5 and 0≦c≦0.05), Li a Ni 1-b-c Co b Bc D α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2), Li a Ni 1-b-c Co b B c O 2-α F α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2), Li a Ni 1-b-c Co b B c O 2-α F α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2), Li a Ni 1-b-c Mn b B c D α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2), Li a Ni 1-b-c Mn b B C O 2-α F α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2), Li a Ni 1-b-c Mn b B C O 2-α F α (In the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2), Li a Ni b E c G d O2 (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0.001 ≤ d ≤ 0.1), Li a Ni b Co c Mn d G e O2 (In the above formula, 0.90≦a≦1.8, 0≦b≦0, 9, 0≦c≦0.5, 0≦d≦0.5, 0.001≦e≦0.1), Li a NiGb O2 (wherein the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1), Li a CoG b O2 (wherein the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1), Li a MnG b O2 (wherein the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1), Li a Mn2G b O4 (where 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1 in the above formula), QO2, QS2, LiQS2, V2O5, LiV2O5, LiIO2, LiNiVO4, Li (3-f) J2(PO4)3(0≦f≦2), Li (3-f) A compound represented by either the chemical formula Fe2(PO4)3 (0≦f≦2) or LiFePO4 may be used.

[0100] In the above chemical formula, A is Ni, Co, Mn, or a combination thereof; B is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements, or a combination thereof; D is O, F, S, P, or a combination thereof; E is Co, Mn, or a combination thereof; F is F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; Q is Ti, Mo, Mn, or a combination thereof; I is Cr, V, Fe, Sc, Y, or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu, or a combination thereof.

[0101] Of course, a compound having a coating layer on its surface may be used, or a mixture of the compound and a compound having a coating layer may be used. This coating layer may contain coating element compounds of the coating element oxide, coating element hydroxide, coating element oxyhydroxyl, coating element oxycarbonate, or coating element hydroxycarbonate. The compounds forming these coating layers may be amorphous or crystalline. The coating elements included in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof. The coating layer formation step may use any coating method (e.g., spray coating, immersion method, etc.) that does not adversely affect the physical properties of the positive electrode active material by using such elements in the compound, and since this is well understood by those engaged in the field, a detailed explanation will be omitted.

[0102] For example, LiNiO2, LiCoO2, LiMn x O 2x (x=1, 2), LiNi 1-x Mn x O2(0 <x<1)、LiNi 1-x-y Co x Mn y O2 (0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5), LiFeO2, V2O5, TiS, MoS, etc. may also be used.

[0103] In the positive electrode active material composition, the conductive agent, binder, and solvent may be the same as those used in the negative electrode active material composition described above. In some cases, a plasticizer may be further added to the positive electrode active material composition and the negative electrode active material composition to form voids inside the electrode plate. The content of the positive electrode active material, conductive agent, binder, and solvent is at levels commonly used in lithium batteries.

[0104] The prepared positive electrode active material composition can be directly coated onto a positive electrode current collector and dried to produce a positive electrode plate. Alternatively, the positive electrode active material composition can be cast onto another support, and the resulting film, obtained by peeling it off the support, can be laminated onto the positive electrode current collector to produce a positive electrode plate.

[0105] The positive electrode current collector has a thickness of 3 to 500 μm and is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., may be used. The positive electrode current collector can also have fine irregularities formed on its surface to enhance the adhesion to the negative electrode active material, and can take various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0106] The positive and negative electrodes may be separated by a separator, and any separator commonly used in lithium batteries can be used. Particularly preferred is one that exhibits low resistance to ion movement of the electrolyte and excellent moisture-absorbing capacity for the electrolyte. For example, materials selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or combinations thereof, may be in the form of a nonwoven or woven fabric. The separator may have a void diameter of 0.01 to 10 μm and a thickness of generally 5 to 300 μm.

[0107] Lithium salt-containing non-aqueous electrolytes consist of a non-aqueous electrolyte and lithium. Non-aqueous electrolytes include non-aqueous electrolyte solutions, solid electrolytes, and inorganic solid electrolytes.

[0108] As the non-aqueous electrolyte, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0109] As the organic solid electrolyte, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionic dissociation groups may be used.

[0110] As the inorganic solid electrolyte, for example, lithium nitrides, halides, sulfates such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, and Li3PO4-Li2S-SiS2 may be used.

[0111] The lithium salt can be any of those commonly used in lithium batteries, and is a substance that dissolves easily in the non-aqueous electrolyte, such as LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10At least one of the following substances may be used: LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, lithium chloroborate, lithium lower aliphatic carboxylate, lithium 4-phenylborate, imide, etc.

[0112] Lithium secondary batteries can be classified into lithium-ion batteries, lithium-ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used. They can also be classified into cylindrical, prismatic, coin-type, pouch-type, etc., depending on their shape, and into bulk type and thin-film type depending on their size.

[0113] Since the manufacturing methods for these batteries are widely known in this field, a detailed explanation will be omitted.

[0114] The examples of this specification will be described in more detail below. However, the experimental results described below are representative of those found in the aforementioned examples, and the scope and content of this specification cannot be narrowed or limited by these examples. The effects of some of the specific examples of this specification not explicitly shown below will be described in detail in the relevant sections.

[0115] Comparative Example 1 MGS (metal grade silicon) and isopropyl alcohol are added to the bead mill in a weight ratio of 1:10, d v After grinding to 50-109 nm, use a spray dryer to dry D v 50 7.5 μm silicon precursors were prepared. The solid content of the slurry used for spray drying was set to 9%. The silicon precursor, petroleum-based pitch, and graphite were loaded into a custom-made coating machine in a ratio of 5:4:1 based on weight after carbonization was completed, and the coating process was carried out for 20 minutes. The petroleum-based pitch used had a softening point of 275°C. The coated particles were heat-treated at 930°C to obtain a negative electrode active material, which was then classified using a 400-mesh sieve.

[0116] Example 1 The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that a coating process was carried out using petroleum-based pitch with a softening point of 210°C during the manufacturing of the negative electrode active material.

[0117] Example 2 The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that a coating process was carried out using petroleum-based pitch with a softening point of 180°C during the manufacturing of the negative electrode active material.

[0118] Example 3 The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that a coating process was carried out using petroleum-based pitch with a softening point of 150°C during the manufacturing of the negative electrode active material.

[0119] Example 4 The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that the solid content of the slurry used for spray drying during the manufacturing of the negative electrode active material was set to 11%.

[0120] Example 5 The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that the solid content of the slurry used for spray drying during the manufacturing of the negative electrode active material was set to 13%.

[0121] Comparative Example 2 The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that the solid content of the slurry used for spray drying during the manufacturing of the negative electrode active material was set to 15%.

[0122] Example 6 The negative electrode active material was manufactured in the same manner as in Example 2, except that the solid content of the slurry used for spray drying during the manufacturing of the negative electrode active material was set to 13%.

[0123] Example 7 The negative electrode active material was produced in the same manner as in Example 3, except that the solid content of the slurry used for spray drying during the production of the negative electrode active material was 13%.

[0124] Experimental Example 1: Measurement of Area Ratio of Silicon Microparticles Each of the negative electrode active materials obtained in Examples 1 to 7 and Comparative Examples 1 and 2 was mixed with an aqueous solution of carboxymethyl cellulose (CMC) at 1.0 to 2.0 wt% in a weight ratio of 3:97 to obtain a mixture.

[0125] The mixture was applied onto a copper thin film and dried to form a negative electrode active material layer with a thickness of 50 to 60 μm.

[0126] A cross-section of the negative electrode active material layer was secured using an ion beam milling apparatus (IB-19510CP, JEOL).

[0127] A cross-sectional image of the negative electrode active material layer at a magnification of 2,000 times was taken using a field emission scanning electron microscope (JSM-7001F, manufactured by JEOL Ltd.).

[0128] From the taken cross-sectional image, a histogram was extracted using image analysis software (ImageJ, manufactured by National Institute of Health). After deriving the median value of the brightness at two maximum values as the threshold, the ratio (S1 / S0×100) of the total area (S1) of fine particle silicon having the following cross-sectional area to the total area (S0) of silicon particles was calculated, and this was defined as the area ratio of fine particle silicon (unit: area%). The results are shown in Table 1 below. 2 The ratio (S1 / S0×100) of the total area (S1) of fine particle silicon having the following cross-sectional area to the total area (S0) of silicon particles was calculated, and this was defined as the area ratio of fine particle silicon (unit: area%). The results are shown in Table 1 below.

[0129]

Table 1

[0130] Experimental Example 2: Measurement of Cumulative Calorific Value The negative electrode active materials obtained in Examples 1 to 7 and Comparative Examples 1 and 2 were uniformly mixed with a conductive material (Super P) and a binder (SBR-CMC) in a weight ratio of 93:3:4 (active material:conductive material:binder) to produce a negative electrode slurry. The prepared negative electrode slurry was coated onto a 20 μm thick copper foil current collector, dried at 120°C for 30 minutes, and then rolled (pressed) to produce a negative electrode.

[0131] A CR2032-type coin-shaped half-cell was fabricated using metallic lithium as the counter electrode, a PE separation membrane as the separation membrane, and a mixed solvent containing EC (ethylene carbonate), DEC (diethyl carbonate), and DMC (dimethyl carbonate) in a volume ratio of 3:5:2, with 1.0 M LiPF6 dissolved in it, as the electrolyte.

[0132] A coin-type half-cell was charged with a constant current at a rate of 1.0C until the voltage reached 4.2V (vs.Li), and then charged with a constant voltage until the current was reduced to 0.05C while maintaining the 4.2V voltage. After the fully charged coin cell was left to rest for 10 minutes, the discharge cycle was repeated 100 times, in which the cell was discharged with a constant current of 1.0C until the voltage reached 2.7V (vs.Li).

[0133] After drying and washing the aforementioned coin-shaped half-cells in a dry room environment, they were destroyed to extract the negative electrode active material. The extracted negative electrode active material powder was then subjected to differential scanning calorimeter (DSC) analysis. The heat generation values ​​in Table 2 below represent the cumulative heat from 80°C to 200°C when the temperature is increased from 20°C to 200°C at a rate of 5°C / min.

[0134] [Table 2]

[0135] When the negative electrode active material and electrolyte come into direct contact, an SEI layer is inevitably formed. Generally, when the SEI layer is densely formed, it acts as a coating layer, preventing the formation of further SEI layers. However, in the case of negative electrode active materials with large volume changes, such as silicon, the volume changes during charging and discharging cause cracks in the SEI layer. The electrolyte then penetrates through these gaps, repeatedly forming new SEI layers and accelerating electrode degradation. Referring to Table 2, it can be seen that Comparative Example 1 shows the highest cumulative heat value among the experimental group. This means that a large amount of SEI is continuously formed. Examples 1 to 7 show a decrease in cumulative heat value compared to Comparative Example 1, and in particular, Examples 2, 3, 6, and 7 show a significant decrease in cumulative heat value to 2 / 3 of that of Comparative Example 1.

[0136] Experiment Example 3: Battery Performance Evaluation (1) Manufacturing of negative electrode pieces A slurry of the negative electrode active material was prepared by mixing 15.0% by weight of the prepared negative electrode active material, 80.0% by weight of natural graphite, 2.0% by weight of carbon fiber conductive material, 1.5% by weight of carboxymethylcellulose, and 1.5% by weight of styrene-butadiene rubber binder in solution. The prepared slurry was coated onto a copper foil current collector and dried, and then the negative electrode was manufactured by rolling to a mixture density of 1.55 g / cc.

[0137] (2) Manufacturing of positive electrode pieces A cathode active material slurry was prepared by mixing 96.0% by weight of NCM cathode active material, 2.0% by weight of acetylene black conductive agent, and 2.0% by weight of polyvinylidene fluoride (PVdF) binder in an N-methylpyrrolidone (NMP) solvent. This slurry was then coated onto an Al thin film current collector, dried, and then rolled to produce a cathode with a composite density of 3.2 g / cc.

[0138] (3) Cell manufacturing The characteristics of the manufactured negative electrode active material were evaluated using a 30mm x 40mm 6-stack pouch cell. The negative electrode was 13.00 cm². 2 The positive electrode is 12.30 cm. 2Six sheets of material were cut and coated, with the coated surfaces facing each other, and stacked in pairs to form cells. Each electrode was separated by a separator membrane. At this time, the N / P ratio, which is the volume ratio per unit area of ​​the negative electrode to the positive electrode, was set to 1.1. 5.0 g / Ah of electrolyte, consisting of a mixed solvent of ethylene carbonate and diethyl carbonate (EC:DEC=3:7) in which 1.0 M LiPF6 was dissolved, with 5% FEC added, was added to the completed dry cell to form cells.

[0139] (4) Capacity retention rate measurement The prepared cells were charged twice at 0.1C at 25°C with a current of 0.1C until the voltage reached 4.2V, and then charged at a constant voltage until the current was reduced to 0.05C while maintaining the 4.2V voltage. After the charged cells were left to rest for 10 minutes, they were discharged at a constant current of 0.1C until the voltage reached 2.75V (performed twice, initial formation).

[0140] Subsequently, the cell was charged with a constant current at 25°C at a rate of 1.0C until the voltage reached 4.2V, and then charged with a constant voltage until the current dropped to 0.05C while maintaining 4.2V. After the fully charged coin cell was allowed to rest for 10 minutes, the cycle of discharging with a constant current of 1.0C until the voltage reached 2.75V was repeated (cycles 1 to 500).

[0141] The capacity retention rate characteristics were calculated using the following formula (where n is an integer from 1 to 500).

[0142]

number

[0143] Figures 1 and 2 are graphs showing the measured capacity retention rate of lithium secondary batteries after 1 to 500 cycles for each embodiment, and Table 3 below shows the measured capacity retention rate after 500 cycles.

[0144] [Table 3]

[0145] Table 3 shows that Examples 1-7 had a lower content of fine silicon particles compared to Comparative Example 1, and that the capacity retention rate after 500 cycles was relatively high. In particular, Examples 2, 3, 6, and 7, in which the content of fine silicon particles was kept below 3.0% by weight, showed a capacity retention rate of 80% or more after 500 cycles, confirming a significant improvement in lifespan characteristics.

[0146] The descriptions herein provided herein are illustrative, and a person with ordinary skill in the art to which one aspect of this specification belongs will understand that it is possible to easily modify it into other specific forms without altering the technical idea or essential features described herein. Therefore, the embodiments described herein should be understood in all respects as illustrative and not limiting. For example, each component described in a single form may be implemented in a distributed manner, and similarly, components described as distributed may be implemented in a combined manner.

[0147] The scope of this specification is defined by the claims set forth below, and all modifications or alterations derived from the meaning and scope of the claims and the concept of equivalents thereof should be construed as being included within the scope of this specification.

Claims

1. A secondary particle assembled from multiple primary silicon particles, amorphous carbon filling the spaces between the primary silicon particles and covering the surface of the primary silicon particles and the surface of the secondary particle, and a 0.5 μm particle present inside the amorphous carbon or independently of the secondary particle. ２ A negative electrode active material comprising fine silicon particles having the following cross-sectional area, A negative electrode active material in which the proportion of the aforementioned silicon nanoparticles is 6.5 area percent or less (excluding 0 area percent).

2. The negative electrode active material according to claim 1, wherein the proportion of the fine silicon particles is 3.0 area percent or less (excluding 0 area percent).

3. The negative electrode active material according to claim 1, wherein the silicon primary particles are flake-shaped nanosilicon slices.

4. The negative electrode active material according to claim 1, wherein the silicon primary particles are at least one selected from the group consisting of SiOx (x=0), SiOx (0 < x ≤ 0.5), SiOx (0.5 < x < 2), SiC, and Si alloys.

5. In the volume distribution measured for the aforementioned silicon primary particles using a laser diffraction particle size analyzer, the particle size corresponding to the point when the cumulative volume distribution percentage reaches 50% is defined as d. ｖ If it is 50, then the above d ｖ The negative electrode active material according to claim 1, wherein 50 is 80 to 120 nm.

6. In the volume distribution measured for the aforementioned secondary particles using a laser diffraction particle size analyzer, the particle size corresponding to the point where the cumulative volume distribution percentage reaches 50% is defined as D. ｖ If it is 50, then D ｖ The negative electrode active material according to claim 1, wherein 50 is 3 to 15 μm.

7. The anode active material according to claim 1, wherein the amorphous carbon is formed from at least one selected from the group consisting of sucrose, phenol resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin or vinyl chloride resin, coal-based pitch, petroleum-based pitch, polyvinyl chloride, mesoface pitch, tar, block copolymer, polyol, and low molecular weight heavy oil.

8. The negative electrode active material according to claim 1, further comprising crystalline carbon.

9. The negative electrode active material according to claim 8, wherein the crystalline carbon is at least one selected from the group consisting of natural graphite, artificial graphite, expanded graphite, graphene, carbon black, and fullerene.

10. The negative electrode active material according to claim 1, comprising 5 to 70% silicon relative to the total weight of the negative electrode active material.

11. A negative electrode comprising the negative electrode active material according to any one of claims 1 to 10.

12. The negative electrode according to claim 11, wherein, after 100 charge-discharge cycles, when the temperature is raised from 25°C to 200°C at a rate of 5°C / min using a differential scanning calorimeter (DSC), the cumulative heat from 80°C to 200°C is 180 J / g or less.

13. A lithium secondary battery comprising a negative electrode, a positive electrode, and an electrolyte, A lithium secondary battery comprising the negative electrode active material described in any one of claims 1 to 10.

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