Lithium-ion rechargeable battery

Abstract

The lithium secondary battery according to the present invention includes a positive electrode including a positive electrode active material layer, and a negative electrode including a negative electrode active material layer, the negative electrode active material layer including a negative electrode active material, the negative electrode active material being made of silicon, and a half cell including the positive electrode and a lithium metal electrode is manufactured. Then, the half cell is discharged at a constant current of 0.1 C, and a capacity-voltage graph is measured. An inflection point appears in a voltage range of 3.3 V to 3.7 V, and the capacity of the positive electrode at the inflection point is 3% to 25% of the capacity of the positive electrode at the end-of-charge voltage.

Description

[Technical Field]

[0001] This application claims priority under Korean Patent Application No. 10-2022-0014470 dated February 3, 2022, and all content disclosed in the documents of the said Korean Patent Application is incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery comprising a silicon negative electrode active material. [Background technology]

[0003] As technological development and demand for electric vehicles and energy storage systems (ESS) increase, the demand for batteries as an energy source is rapidly rising, and consequently, various research is being conducted on batteries that can meet diverse needs. In particular, research is actively being conducted on lithium-ion secondary batteries that have high energy density and excellent lifespan and cycle characteristics as power sources for such devices.

[0004] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, an electrolyte, an organic solvent, and the like. An active material layer containing a positive electrode active material or a negative electrode active material can be formed on the current collector of the positive and negative electrodes, respectively. Generally, lithium-containing metal oxides such as lithium iron phosphate and lithium nickel oxide are used as the positive electrode active material, while lithium-free carbon-based active materials and silicon-based active materials are used as the negative electrode active material.

[0005] In particular, among the negative electrode active materials, silicon-based active materials have a capacity approximately 10 times higher than that of carbon-based active materials. Therefore, there is an advantage that a high energy density can be achieved even with a thin electrode. However, silicon-based active materials are accompanied by rapid volume expansion / contraction due to changes in the crystal structure caused by the insertion / desorption of lithium during charge and discharge of the battery. As a result, the electrical contact between particles is lost, and there is a problem that the life characteristics of the lithium secondary battery deteriorate. In particular, in the high potential region of the negative electrode (region above 0.6 V), the pulverization phenomenon of silicon particles is accelerated due to the volume change accompanying the rapid potential change, which is the main cause of battery life degradation.

[0006] In order to solve the above problems, in a lithium secondary battery using a lithium nickel-based oxide as a positive electrode active material and pure silicon (pure Si 100%) as a negative electrode active material, a method has been proposed to set the cut-off voltage to 3.3 V or higher so as not to use the high potential region of the negative electrode accompanied by a rapid volume change of silicon. However, as described above, when the cut-off voltage is set high, there is a problem that the cell energy density becomes low and it is difficult to realize a high-capacity battery.

[0007] Therefore, there is a situation where the development of a lithium secondary battery that minimizes the decrease in cell energy density and improves the life characteristics is required. Summary of the Invention Problems to be Solved by the Invention

[0008] The present invention is for solving the above problems, and by using a positive electrode having a specific discharge behavior, it aims to minimize the decrease in the energy density of a cell applying a silicon negative electrode active material and to realize excellent life characteristics. Means for Solving the Problems

[0009] According to one embodiment of the present invention, a lithium secondary battery is provided which includes a positive electrode comprising a positive electrode comprising a positive electrode active material layer and a negative electrode comprising a negative electrode active material layer, wherein the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material being silicon, and after manufacturing a half cell comprising the positive electrode and a lithium metal electrode, a capacity-voltage graph measured while discharging the half cell at a constant current of 0.1C shows an inflection point in the region where the voltage is 3.3V to 3.7V, and the capacity of the positive electrode at the inflection point is 3% to 25% of the capacity of the positive electrode at the charge termination voltage. [Effects of the Invention]

[0010] The lithium secondary battery according to the present invention uses silicon as the negative electrode active material and a positive electrode whose discharge behavior satisfies specific conditions, thereby avoiding the high-potential region (above 0.6V) of the negative electrode where a rapid change in silicon volume occurs, even when a relatively low cutoff voltage is applied. As a result, the problem of degradation of the battery's lifespan characteristics caused by excessive silicon volume change can be effectively resolved, enabling discharge to relatively low voltages and achieving a high cell energy density.

[0011] Furthermore, when lithium iron phosphate and lithium nickel oxide, which have a theoretical efficiency close to 100%, are used as the positive electrode active material for the lithium secondary battery according to the present invention, the discharge region of lithium iron phosphate can be used during battery discharge as needed, thereby minimizing the loss of cell energy density.

[0012] Furthermore, the lithium secondary battery according to the present invention may contain 100 parts by weight of lithium nickel oxide and 5 to 20 parts by weight of lithium iron phosphate as the positive electrode active material. When the weight ratio of lithium nickel oxide to lithium iron phosphate satisfies the above range, a positive electrode with desired discharge characteristics can be formed, and the lithium nickel oxide, which has a relatively high reaction potential, discharges before the lithium iron phosphate, and most of the lithium nickel oxide can be discharged near the cutoff voltage. As a result, the present invention can make maximum use of the relatively high capacity of lithium nickel oxide compared to lithium iron phosphate, and has advantages in terms of material costs. [Brief explanation of the drawing]

[0013] [Figure 1] The graphs and tables shown illustrate the discharge potential of the negative electrode, the cell energy density, and the battery life characteristics of a lithium secondary battery using lithium nickel oxide as the positive electrode active material and pure silicon (100% pure Si) as the negative electrode active material, measured while varying the cutoff voltage. [Figure 2] This is a graph of the discharge curves of half-cells manufactured using the negative electrode and positive electrode manufactured in Example 1, respectively. [Figure 3] These are the discharge curve graphs of the half-cells manufactured using the negative electrode and positive electrode produced in Example 2, respectively. [Figure 4] These are the discharge curve graphs of half-cells manufactured using the negative electrode and positive electrode manufactured in Comparative Example 1, respectively. [Figure 5] These are the discharge curve graphs of half-cells manufactured using the negative and positive electrodes produced in Comparative Example 2, respectively. [Modes for carrying out the invention]

[0014] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and can be realized in a variety of different forms, provided that these embodiments are provided to complete the disclosure of the present invention and to fully inform a person ordinary skill in the art to which the invention pertains, of the scope of the invention, and the present invention is defined by the claims. Throughout the specification, the same reference numerals refer to the same components.

[0015] Unless otherwise defined, all terms used herein (including technical and scientific terms) may be used in a sense that can be commonly understood by a person of ordinary skill in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless explicitly defined otherwise.

[0016] The terms used herein are for illustrative purposes only and do not limit the invention. In this specification, the singular form includes the plural form unless otherwise specified. The terms “includes,” “has,” and “equips” as used in this specification do not preclude the presence or addition of one or more other components in addition to the components mentioned.

[0017] In this specification, when a part is said to include a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0018] In this specification, "A and / or B" means A, or B, or A and B.

[0019] In this specification, "%" means weight percent unless otherwise explicitly indicated.

[0020] In this specification, D 50This refers to the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 This can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can obtain highly reproducible and high-resolution results.

[0021] In this specification, "specific surface area" is measured by the BET method, and specifically, it can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-mino II manufactured by BEL Japan.

[0022] In this specification, the cut-off voltage or discharge termination voltage (hereinafter referred to as "cut-off voltage") of a lithium secondary battery means the voltage of the lithium secondary battery at the point where the discharge of the lithium secondary battery ends.

[0023] The present invention will be described in more detail below.

[0024] The lithium secondary battery according to the present invention includes a positive electrode containing a positive electrode active material layer and a negative electrode containing a negative electrode active material layer, wherein the negative electrode active material layer contains a negative electrode active material, and the negative electrode active material is made of silicon. After manufacturing a half cell containing the positive electrode and a lithium metal electrode, an inflection point appears in a capacity-voltage graph measured while discharging the half cell with a constant current of 0.1C, in which the voltage is 3.3V to 3.7V, and the capacity of the positive electrode at the inflection point is 3% to 25% of the capacity of the positive electrode at the charging termination voltage.

[0025] When silicon is used as the negative electrode active material, rapid volume expansion and contraction occur during battery charging and discharging due to changes in the crystal structure caused by the insertion and removal of lithium. This leads to a loss of electrical contact between particles, resulting in a decrease in the lifespan characteristics of lithium secondary batteries. In particular, the volume expansion of silicon is greatly affected by the potential of the negative electrode during charging and discharging.

[0026] Figure 1 shows graphs and tables illustrating the discharge potential of the negative electrode, cell energy density, and battery life characteristics measured while varying the cutoff voltage in a lithium secondary battery using lithium nickel oxide as the positive electrode active material and pure silicon (pure Si 100%) as the negative electrode active material. According to Figure 1, in the case of a lithium secondary battery using a silicon negative electrode, even a small increase in the discharge potential of the negative electrode at the cutoff voltage to a level of 0.05V to 0.1V significantly reduces the battery life characteristics by approximately 100 cycles or more. This is because when the discharge potential of the negative electrode increases during the discharge process, rapid expansion of the silicon occurs. Furthermore, Figure 1 shows that when the cutoff voltage of a lithium secondary battery using a silicon negative electrode increases, the discharge potential of the negative electrode decreases, improving the battery life characteristics, but the cell energy density decreases sharply. Thus, in lithium secondary batteries using silicon negative electrode active materials, cell energy density and life characteristics are inversely related (trade-off), making it difficult to achieve excellent performance in both areas.

[0027] As a result of diligent research to solve these problems, the inventors have discovered that when a positive electrode with specific discharge behavior is applied as the positive electrode of a lithium secondary battery using silicon negative electrode active material, it is possible to avoid using the high-potential region of the negative electrode where rapid expansion of silicon occurs, even when a relatively low cutoff voltage is applied. This minimizes the decrease in cell energy density and enables the realization of excellent life characteristics, thus completing the present invention.

[0028] Specifically, the lithium secondary battery of the present invention includes a positive electrode containing a positive electrode active material layer and a negative electrode containing a negative electrode active material layer, wherein the negative electrode active material layer contains a negative electrode active material, and the negative electrode active material is made of silicon. After manufacturing a half cell containing the positive electrode and a lithium metal electrode, an inflection point appears in a capacity-voltage graph measured while discharging the half cell at a constant current of 0.1C, where the voltage is in the region of 3.3V to 3.7V, and the capacity of the positive electrode at the inflection point is 3% to 25% of the capacity of the positive electrode at the charging termination voltage.

[0029] More specifically, the lithium secondary battery according to the present invention includes a negative electrode whose negative electrode active material is silicon, a positive electrode having the discharge behavior described above, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0030] The components of the lithium secondary battery of the present invention will be described in more detail below.

[0031] <Negative electrode> A negative electrode according to one embodiment of the present invention may include a negative electrode active material layer. Specifically, the negative electrode according to the present invention may include a negative electrode current collector and the negative electrode active material layer located on at least one surface of the negative electrode current collector.

[0032] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with surface treatments such as carbon, nickel, titanium, or silver, and aluminum-cadmium alloys can be used.

[0033] The negative electrode current collector can typically have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on its surface to strengthen the bonding force of the negative electrode active material. For example, it can be used in various forms such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0034] The negative electrode active material layer is located on at least one surface of the negative electrode current collector and may contain negative electrode active material. Furthermore, in addition to the negative electrode active material, the negative electrode active material layer may optionally further contain negative electrode conductive material and negative electrode binder.

[0035] The following provides a detailed explanation of each component included in the negative electrode active material layer.

[0036] The negative electrode active material is designed to combine with lithium ions that move from the positive electrode during battery charging, thereby triggering an electrochemical reaction.

[0037] The negative electrode active material according to the present invention can be silicon (Si). Since silicon has superior capacity compared to other silicon-based particles such as SiOx or SiC, when silicon is used alone as the negative electrode active material, a lithium secondary battery with high energy density can be realized.

[0038] However, as mentioned above, when silicon particles are used as the negative electrode active material, a rapid volume change occurs in the high-potential region where the negative electrode potential exceeds 0.6V during charging and discharging, which degrades the battery's lifespan characteristics. To suppress silicon expansion, the cutoff voltage can be increased to avoid using the high-potential region of the negative electrode, but in this case, the entire capacity of the positive electrode cannot be used, resulting in a problem of reduced cell energy density.

[0039] To solve these problems, the lithium secondary battery according to the present invention, as described later, applies a positive electrode having a specific discharge behavior, thereby avoiding the high-potential region (above 0.6V region) of the negative electrode where a rapid change in silicon volume occurs, even when a relatively low cutoff voltage is applied. This minimizes the change in silicon volume and increases the usable capacity of the positive electrode during discharge, thereby minimizing the decrease in cell energy density.

[0040] Average particle size D of silicon 50can be 5 μm to 20 μm, specifically 5 μm to 18 μm, and more specifically 5 μm to 15 μm. The average particle size D of the silicon 50 When it satisfies the above numerical range, during charge and discharge, it is easy to control the volume expansion, which is advantageous in terms of ensuring the capacity retention rate of the battery.

[0041] Also, when the average particle size of the silicon particles is too small, the specific surface area of the particles increases excessively, and the viscosity of the negative electrode slurry rises excessively. Therefore, the dispersion of the particles constituting the negative electrode slurry is not smooth. Further, when the size of the silicon-based active material is too small, the contact area between the silicon particles and the conductive material in the negative electrode slurry decreases due to the composite composed of the conductive material and the binder, so the possibility of the conductive network being broken increases and the capacity retention rate decreases.

[0042] On the other hand, when the average particle size of the silicon particles is too large, the surface of the negative electrode becomes non-smooth, and as a result, non-uniformity of the current density occurs during charge and discharge. Further, when the silicon particles are excessively large, the phase stability of the negative electrode slurry becomes unstable, so the processability decreases. As a result, the capacity retention rate of the battery decreases.

[0043] The BET specific surface area of the silicon is 0.1 m 2 / g to 5 m 2 / g, specifically 0.2 m 2 / g to 4 m 2 / g, and more specifically 0.3 m 2 / g to 3.5 m 2 / g, and when the BET specific surface area of the silicon satisfies the above numerical range, side reactions with the electrolyte can be prevented, and the initial efficiency and life characteristics of the battery can be improved.

[0044] The silicon can exist in crystalline or amorphous form, and preferably is not porous. The pure silicon particles can be spherical or fragment-shaped particles, but are not limited thereto, and can also have a fibrous structure or exist in the form of a silicon-containing film or coating.

[0045] The silicon can be contained in the negative electrode active material layer in an amount of 60% to 99% by weight, specifically 65% ​​to 98% by weight, and more specifically 70% to 95% by weight. When the content of the negative electrode active material satisfies the above range, the battery capacity of the negative electrode can be improved by maintaining the content of the negative electrode conductive material and the negative electrode binder at a favorable level and ensuring a sufficient negative electrode energy density.

[0046] The negative electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations in the battery it is constructed from, as long as it does not cause chemical changes and possesses electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these materials alone or a mixture of two or more materials can be used.

[0047] The negative electrode conductive material can be contained within the negative electrode active material layer in an amount of 1% to 15% by weight, specifically 2% to 13% by weight, and more specifically 5% to 12% by weight. When the content of the negative electrode active material satisfies the above range, the electrical conductivity of the negative electrode can be improved by ensuring a negative electrode conductive network.

[0048] The negative electrode binder plays a role in improving the adhesion between particles of the negative electrode active material and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more can be used.

[0049] The negative electrode binder can be contained in the negative electrode active material layer at a concentration of 1% to 15% by weight, specifically 2% to 13% by weight, and more specifically 5% to 12% by weight. When the content of the negative electrode active material satisfies the above range, the negative electrode active material particles can bind smoothly, minimizing the problem of volume expansion of the negative electrode active material, and the negative electrode active material can be easily adhered to the negative electrode current collector.

[0050] The discharge potential of the negative electrode at the cutoff voltage of the lithium secondary battery according to the present invention can be 0.6V or less, specifically 0.1V to 0.5V, and more specifically 0.1V to 0.4V. Here, the discharge potential of the negative electrode is the discharge potential of the negative electrode during the first charge-discharge cycle of the lithium secondary battery. When the value of the discharge potential of the negative electrode at the cutoff voltage satisfies the above range, the battery life characteristics can be improved by avoiding the high-potential region where a rapid change in silicon volume occurs during charging and discharging. Here, the value of the discharge potential of the negative electrode or the negative electrode discharge curve graph can be measured after manufacturing a half-cell using the negative electrode and lithium metal electrode applied to the lithium secondary battery of the present invention, while discharging the half-cell with a constant current.

[0051] <Positive electrode> The positive electrode of the lithium secondary battery according to the present invention exhibits a discharge behavior in which, after manufacturing a half cell including the positive electrode and a lithium metal electrode, an inflection point appears in a capacity-voltage graph measured while discharging the half cell with a constant current of 0.1C, in the region where the voltage is 3.3V to 3.7V, specifically 3.3V to 3.6V, and more specifically 3.3V to 3.5V, and the capacity of the positive electrode at the inflection point appears at a level of 3% to 25% of the capacity of the positive electrode at the charging termination voltage, specifically 4% to 23%, and more specifically 4% to 20%.

[0052] Here, an inflection point refers to the point in the discharge curve graph of the half-cell where the second derivative of the potential value with respect to the capacitance value is zero. The existence of such an inflection point indicates that the voltage value changes rapidly in that voltage range. The positive electrode of the present invention exhibits a rapid voltage decrease in the 3.3V to 3.7V range during discharge. This avoids the use of high-potential regions where silicon expansion is caused even at relatively low cutoff voltages, thereby increasing the usable capacitance range of the positive electrode and minimizing the decrease in cell energy density.

[0053] On the other hand, if the positive electrode capacity at the inflection point exceeds 25% of the positive electrode capacity at the charging termination voltage, when the lithium secondary battery is discharged to the inflection point, a large amount of the positive electrode capacity remains unused, resulting in a problem of reduced cell energy density. Also, if the positive electrode capacity at the inflection point is less than 3% of the positive electrode capacity at the charging termination voltage, when the lithium secondary battery is discharged to the inflection point, the high-potential region of the negative electrode (above 0.6V) where a rapid change in silicon volume occurs is used, resulting in a problem of reduced battery life characteristics.

[0054] On the other hand, the positive electrode may include a positive electrode active material layer. Specifically, the positive electrode according to the present invention may include a positive electrode current collector and the positive electrode active material layer located on at least one surface of the positive electrode current collector.

[0055] The positive electrode current collector is not particularly limited, as long as it does not cause chemical changes in the battery and is conductive. For example, the current collector can be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc.

[0056] The positive electrode current collector can have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance its adhesion to the positive electrode active material layer. For example, it can be used in various forms such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0057] The positive electrode active material layer is located on at least one surface of the positive electrode current collector and may contain positive electrode active material. Furthermore, in addition to positive electrode active material, the positive electrode active material layer may optionally further contain conductive material, binder, and dispersant.

[0058] The following provides a detailed explanation of each component included in the positive electrode active material layer.

[0059] The positive electrode active material layer according to the present invention may contain two positive electrode active materials having different reaction potentials. For example, the positive electrode active material layer may contain lithium iron phosphate and lithium nickel oxide as positive electrode active materials.

[0060] The lithium nickel oxide can be a compound represented by the following chemical formula 1.

[0061] [Chemical formula 1] Li x [Ni y Co z M 1 w M 2 v ]O 2-p A p

[0062] In the above chemical formula 1, M 1is Mn, Al, or a combination thereof, and M 2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and A can be one or more elements selected from the group consisting of F, Cl, Br, I, At, and S.

[0063] Also, in Chemical Formula 1, 1.0 ≤ x ≤ 1.30, 0.3 ≤ y < 1, 0 < z ≤ 0.6, 0 < w ≤ 0.6, 0 ≤ v ≤ 0.2, 0 ≤ p ≤ 0.2, specifically, 1.0 ≤ x ≤ 1.20, 0.3 ≤ y < 0.95, 0 < z ≤ 0.5, 0 < w ≤ 0.5, 0 ≤ v ≤ 0.05, 0 ≤ p ≤ 0.05 can be satisfied. For example, the lithium nickel-based oxide is Li[Ni 0.6 Co 0.2 Mn 0.2 O2, Li[Ni 0.8 Co 0.1 Mn 0.1 O2, Li[Ni 0.5 Co 0.2 Mn 0.3 O2, but is not limited thereto.

[0064] The lithium nickel-based oxide can be contained in the positive electrode active material layer at 82% to 94% by weight, specifically 82% to 92% by weight, and more specifically 82% to 89% by weight. When the content of the lithium nickel-based oxide in the positive electrode active material layer satisfies the above range, the battery capacity of the positive electrode can be ensured by securing sufficient positive electrode energy density.

[0065] The average particle size D of the lithium nickel-based oxide 50 can be 4 μm to 20 μm, specifically 5 μm to 20 μm, and more specifically 5 μm to 15 μm. When the average particle size D of the lithium nickel-based oxide satisfies the above range, when mixed with lithium iron phosphate described later, a high compression density can be realized, and the effect of improving the cell energy density can be obtained. 50

[0066] The BET specific surface area of ​​lithium nickel oxide is 0.1 m². 2 / g~2.0m 2 / g, specifically 0.3m 2 / g~1.5m 2 / g, more specifically 0.5m 2 / g~1.5m 2 It can be / g. When the BET specific surface area of ​​the lithium nickel oxide satisfies the above range, the side reactions with the electrolyte are reduced, thereby minimizing the problem of degradation of the battery's lifespan performance.

[0067] On the other hand, the lithium iron phosphate can be a compound represented by the following chemical formula 2.

[0068] [Chemical formula 2] Li 1+a Fe 1-b M b (PO 4-c )X c

[0069] In the above chemical formula 2, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, and X may include one or more elements selected from the group consisting of F, S, and N. In the above chemical formula 2, a, b, and c can each be -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.1, specifically -0.3 ≤ a ≤ 0.3, 0 ≤ b ≤ 0.3, and 0 ≤ c ≤ 0.05. For example, the lithium iron phosphate may be LiFePO4 with an olivine crystal structure.

[0070] Lithium iron phosphate can be present in the positive electrode active material layer at a concentration of 4% to 16% by weight, specifically 6% to 16% by weight, and more specifically 9% to 16% by weight.

[0071] Preferably, the lithium iron phosphate can be included in an amount of 5 to 20 parts by weight, specifically 7 to 20 parts by weight, and more specifically 10 to 20 parts by weight, per 100 parts by weight of lithium nickel oxide.

[0072] When the weight ratio of lithium iron phosphate to lithium nickel oxide satisfies the aforementioned range, a positive electrode exhibiting the discharge behavior described above can be manufactured. Furthermore, if the lithium iron phosphate content is too low, the negative electrode potential at the cutoff voltage increases, reducing the effect of improving lifespan characteristics. When the cutoff voltage is increased to improve lifespan characteristics, the usable capacity of the positive electrode decreases, and the cell energy density decreases. On the other hand, if the lithium iron phosphate content is too high, the cell volume increases due to the increase in the amount of positive electrode active material, and the cell energy density decreases.

[0073] Average particle size D of lithium iron phosphate 50 The average particle size D of lithium iron phosphate can be 3 μm to 10 μm, specifically 3 μm to 9 μm, and more specifically 4 μm to 9 μm. 50 When the above range is satisfied, aggregation of particles between lithium iron phosphate is prevented, and the average particle sizes of lithium iron phosphate and lithium nickel oxide are similar, allowing for a uniform mixture of lithium iron phosphate and lithium nickel oxide. As a result, a positive electrode can be formed in which the positive electrode active material is uniformly distributed.

[0074] Average particle size D of lithium nickel oxides 50 The average particle size D of lithium iron phosphate relative to this 50 The ratio can be 0.5 to 1.5, more specifically 0.5 to 1.2, and more specifically 0.5 to 1.0. Average particle size D of lithium nickel oxide 50 The average particle size D of lithium iron phosphate relative to this 50 When the ratio satisfies the aforementioned range, the average particle sizes of lithium iron phosphate and lithium nickel oxide are similar, allowing for a uniform mixture of lithium iron phosphate and lithium nickel oxide, thereby preventing localized non-uniformity of the positive electrode energy density.

[0075] The BET specific surface area of ​​lithium iron phosphate is 5 m². 2 / g~20m 2 / g, specifically 7m 2 / g~18m 2 / g, more specifically 8m 2 / g~16m 2 It can be / g. When the BET specific surface area of ​​lithium iron phosphate satisfies the above range, aggregation of lithium iron phosphate can be effectively suppressed even when the content of the positive electrode dispersant is relatively low.

[0076] On the other hand, when a combination of lithium nickel oxide and lithium iron phosphate is used as the positive electrode active material, during discharge, the lithium nickel oxide discharges before the lithium iron phosphate, and almost all of the lithium nickel oxide can be discharged near the cutoff voltage. As a result, the present invention can maximize the relatively high capacity of lithium nickel oxide compared to lithium iron phosphate, and is advantageous in terms of material costs.

[0077] Furthermore, since the lithium secondary battery according to the present invention contains lithium iron phosphate, a positive electrode active material with a theoretical efficiency close to 100%, the discharge region of lithium iron phosphate can be utilized during battery discharge as needed. Therefore, the energy density loss caused by adding lithium iron phosphate to lithium nickel-based oxides can be minimized.

[0078] The positive electrode active material can be contained in the positive electrode active material layer at a concentration of 85% to 99% by weight, specifically 90% to 99% by weight, and more specifically 93% to 99% by weight. When the content of the positive electrode active material satisfies the above range, the battery capacity of the positive electrode can be improved by ensuring sufficient positive electrode energy density.

[0079] The positive electrode conductive material is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. Examples of materials that can be used include graphite; carbon black such as carbon black, acetylene black, Ketjenblack, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Specific examples of commercially available positive electrode conductive materials include acetylene black-based materials (from companies such as Chevron Chemical Company, Denka Singapore Private Limited, and Gulf Oil Company), Ketjenblack, EC-based materials (from Armak Company), Vulcan XC-72 (from Cabot Company), and Super P (from Timcal). Preferably, the positive electrode conductive material can be a carbon nanotube.

[0080] The positive electrode conductive material can be contained in the positive electrode active material layer in an amount of 0.1% to 5.0% by weight, more specifically 0.1% to 3.0% by weight, and more specifically 0.3% to 2.0% by weight. When the content of the positive electrode conductive material in the positive electrode active material layer satisfies the above range, the electrical conductivity of the positive electrode can be improved by ensuring a positive electrode conductive network.

[0081] The positive electrode binder plays a role in bonding the positive electrode active material to conductive materials and to the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, or various copolymers thereof. One of these can be used alone or in mixtures of two or more.

[0082] The positive electrode binder can be contained within the positive electrode active material layer in an amount of 0.5% to 5.0% by weight, specifically 1.0% to 4.0% by weight, and more specifically 1.0% to 3.5% by weight. When the content of the positive electrode binder satisfies the above range, the contact area between the positive electrode binder and the positive electrode active material is increased, ensuring excellent positive electrode adhesion.

[0083] The positive electrode dispersant suppresses the phenomenon of lithium iron phosphate excessively agglomerating within the positive electrode slurry composition, enabling lithium iron phosphate to be effectively dispersed in the manufactured positive electrode active material layer.

[0084] The positive electrode dispersant may include a hydrogenated nitrile copolymer; specifically, the positive electrode dispersant may be a hydrogenated nitrile copolymer.

[0085] Specifically, the hydrogenated nitrile copolymer may be a copolymer containing structural units derived from α,β-unsaturated nitrile and structural units derived from hydrogenated conjugated diene, or a copolymer containing structural units derived from α,β-unsaturated nitrile, structural units derived from conjugated diene, and structural units derived from hydrogenated conjugated diene. As the α,β-unsaturated nitrile monomer, for example, acrylonitrile or methacrylonitrile can be used, and one or more of these can be used individually. As the conjugated diene monomer, for example, a conjugated diene monomer having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, or 2,3-methylbutadiene, can be used, and one or more of these can be used individually.

[0086] More specifically, the hydrogenated nitrile copolymer can be hydrogenated nitrile butadiene rubber (H-NBR).

[0087] The positive electrode dispersant can be contained in the positive electrode active material layer in an amount of 1.5% by weight or less, specifically 0.1% to 1.0% by weight, and more specifically 0.2% to 0.8% by weight. When the content of the positive electrode dispersant satisfies the above range, aggregation of the positive electrode conductive material in the positive electrode active material layer can be suppressed, and the positive electrode conductive network can be improved.

[0088] On the other hand, the positive electrode can be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material described above. Specifically, the positive electrode can be manufactured by first producing a positive electrode slurry composition containing the positive electrode active material, positive electrode conductive material, positive electrode binder, and / or positive electrode dispersant, then applying the positive electrode slurry composition onto a positive electrode current collector, and finally drying and rolling it.

[0089] Alternatively, the positive electrode can also be manufactured by casting the positive electrode slurry composition onto another support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0090] <Separator> A lithium secondary battery according to one embodiment of the present invention may include a separator between the negative electrode and the positive electrode.

[0091] The separator can be used without particular limitations as long as it is a separator that is normally used in lithium secondary batteries, and is particularly preferred if it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. Ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, the separator can be a porous thin film having a pore size of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0092] <Electrolytes> A lithium secondary battery according to one embodiment of the present invention may contain a non-aqueous electrolyte.

[0093] The non-aqueous electrolyte may include, but is not particularly limited to, organic solvents and lithium salts commonly used in the art.

[0094] The aforementioned organic solvent can be used without particular limitations, as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvents that can be used include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; and carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0095] Among these, carbonate-based solvents are preferred, and more preferably are mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of batteries, and linear carbonate compounds with low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate).

[0096] The lithium salt can be used without particular limitations as long as it is a compound that can provide lithium ions for use in lithium secondary batteries. Specifically, 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, or LiB(C2O4)2. The lithium salt is preferably contained in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

[0097] On the other hand, the non-aqueous electrolyte according to the present invention may further contain additives, although these are not essential, in order to further improve the physical properties of the secondary battery.

[0098] Examples of such additives include at least one selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, nitrile compounds, sultone compounds, sulfate compounds, phosphate compounds, borate compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0099] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinylethylene carbonate (VEC).

[0100] The halogen-substituted carbonate compound can be, for example, fluoroethylene carbonate (FEC).

[0101] The nitrile compound can be, for example, succinonitrile, adiponitrile, hexanetricyanide, or 1,4-dicyano-2-butene.

[0102] The sultone compound can be, for example, 1,3-propanesultone or 1,3-propenesultone.

[0103] The sulfate compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0104] The phosphate compound can be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalate) phosphate, lithium difluorophosphate, tetramethyltrimethylsilyl phosphate, trimethylsilyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, and tris(trifluoroethyl) phosphate.

[0105] The borate-based compound can be, for example, tetraphenylborate or lithium oxalyl difluoroborate (LiODFB).

[0106] The benzene-based compound may be, for example, fluorobenzene, the amine-based compound may be triethanolamine or ethylenediamine, and the silane-based compound may be tetravinylsilane.

[0107] The lithium salt compound can be one or more compounds selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bisoxalate borate (LiB(C2O4)2) and LiBF4), as a compound different from the lithium salt contained in the non-aqueous electrolyte.

[0108] On the other hand, the aforementioned additives can be used individually, or two or more can be used in combination.

[0109] The total amount of the additive can be 1% to 20% by weight, preferably 1% to 15% by weight, relative to the total weight of the electrolyte. When the additive is within this range, it can stably form a film on the electrode, suppress ignition during overcharging, and prevent side reactions from occurring or the additive from remaining or precipitating during the initial activation process of the secondary battery.

[0110] The lithium secondary battery of the present invention can be manufactured by forming an electrode assembly by placing a separator between the positive electrode and the negative electrode, placing the electrode assembly in a cylindrical or rectangular battery case, and then injecting an electrolyte. Alternatively, the electrode assemblies can be stacked, impregnated with an electrolyte, and the resulting product can be placed in a battery case and sealed.

[0111] The aforementioned battery case may be one of those commonly used in this field, and its external shape is not limited by the battery's application, and can be, for example, cylindrical, rectangular, pouch-shaped, or coin-shaped.

[0112] Preferably, the cell energy density of the lithium secondary battery according to the present invention can be 650 Wh / L or more, specifically 650 Wh / L to 800 Wh / L, and more specifically 670 Wh / L to 800 Wh / L. When the cell energy density of the lithium secondary battery satisfies the above range, it is easy to realize a high-capacity battery.

[0113] A lithium secondary battery according to one embodiment of the present invention can be used as a battery cell for powering small devices, and can also be preferably used as a unit battery in medium- and large-sized battery modules containing a large number of battery cells. Preferred examples of such medium- and large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems (ESS). In particular, it can be usefully used in areas where high output is required, such as hybrid electric vehicles (HEVs) and batteries for storing new renewable energy.

[0114] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and do not limit the scope of the present invention to these examples alone.

[0115] Examples and Comparative Examples Example 1 (1) Manufacturing of the negative electrode Silicon particles (average particle size D 50 =10μm, BET specific surface area=3m 2 A negative electrode slurry was prepared by mixing carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a weight ratio of 80:10:1:9, and adding distilled water. The solid content of the negative electrode slurry was 50% by weight.

[0116] On one surface of a 20 μm thick copper (Cu) metal thin film, the negative electrode slurry was applied at a rate of 8.5 mAh / cm². 2 After coating with the specified loading amount, the material was vacuum-dried. Subsequently, the dried negative electrode slurry was rolled (roll-pressed), dried in a vacuum oven at 130°C for 12 hours, and then punched out to produce the negative electrode.

[0117] (2) Manufacturing of the positive electrode Average particle size D 50Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2100 parts by weight, and average particle size D 50 A positive electrode active material was prepared by mixing parts by weight of LiFePO415 with a particle size of 7 μm.

[0118] The prepared cathode active material, carbon nanotube (CNT) conductive material, polyvinylidene fluoride (PVdF) binder, and hydrogenated nitrile butadiene rubber (H-NBR) dispersant were added to N-methylpyrrolidone (NMP) solvent in a weight ratio of 97.0:1.0:1.5:0.5 and stirred to produce a cathode slurry. The solid content of the cathode slurry was 70% by weight.

[0119] The positive electrode slurry is applied to one surface of a 20 μm thick aluminum thin film at a rate of 4.5 mAh / cm². 2 After coating with the specified loading amount, the material was vacuum-dried at 130°C for 10 hours. The dried cathode slurry was rolled (roll press), dried in a vacuum oven at 130°C for 12 hours, and then punched out to produce the cathode.

[0120] (3) Manufacturing of lithium secondary batteries As described above, the negative and positive electrodes and porous polyethylene separators were assembled using a stacking method to produce an electrode assembly. After placing the electrode assembly in a battery case, an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio) and lithium hexafluorophosphate (LiPF 61 moles)) was injected to produce a lithium secondary battery.

[0121] Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 5 parts by weight of LiFePO4 were mixed in during the production of the positive electrode active material.

[0122] Comparative Example 1 A lithium secondary battery was manufactured using the same method as in Example 1, except that LiFePO4 was not used in the production of the positive electrode active material.

[0123] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 30 parts by weight of LiFePO4 were mixed in during the production of the positive electrode active material.

[0124] [Table 1]

[0125] Experimental Example 1 - Evaluation of Discharge Behavior 1) Evaluation of negative electrode discharge behavior Using the negative electrodes manufactured in Examples 1-2 and Comparative Examples 1-2, and a lithium (Li) metal electrode cut into a circular shape, an electrode assembly was manufactured by interposing a porous polyethylene separator between the lithium metal electrode and the negative electrode. After placing the electrode assembly in a battery case, an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio) and lithium hexafluorophosphate (LiPF 61 moles)) was injected to manufacture a negative electrode half-cell.

[0126] The negative electrode half-cell manufactured as described above was charged to 0.005V with a constant current (CC) of 0.1C, and then the capacitance-voltage change was measured while discharging to 1.0V with a constant current (CC) of 0.1C, to obtain a discharge curve graph of the negative electrode.

[0127] 2) Evaluation of positive electrode discharge behavior Except for using the positive electrodes manufactured in Examples 1-2 and Comparative Examples 1-2, and a lithium (Li) metal electrode cut into a circular shape, the positive electrode half-cell was manufactured using the same method as the manufacturing method for the half-cell used for measuring the discharge curve of the negative electrode.

[0128] The positive electrode half-cell manufactured as described above was charged to 4.3V with a constant current (CC) of 0.1C, and then the capacitance-voltage change was measured while discharging to 2.5V with a constant current (CC) of 0.1C, to obtain a discharge curve graph of the positive electrode.

[0129] The discharge curve graphs of the positive electrode half-cell and negative electrode half-cell measured as described above are shown in Figures 2 to 5. Specifically, Figure 2 is the discharge curve graph of the half-cell manufactured using the negative electrode and positive electrode manufactured in Example 1, Figure 3 is the discharge curve graph of the half-cell manufactured using the negative electrode and positive electrode manufactured in Example 2, Figure 4 is the discharge curve graph of the half-cell manufactured using the negative electrode and positive electrode manufactured in Comparative Example 1, and Figure 5 is the discharge curve graph of the half-cell manufactured using the negative electrode and positive electrode manufactured in Comparative Example 2.

[0130] As illustrated in Figures 2, 3, and 5, an inflection point appears in the voltage range of 3.3 to 3.7 V on the positive electrode discharge curve graphs of Examples 1 and 2 and Comparative Example 2, in which lithium iron phosphate and lithium nickel oxide, which have different reaction potentials, were used together as positive electrode active materials.

[0131] On the other hand, as shown in Figure 4, no inflection points appear on the discharge curve graph of the positive electrode in Comparative Example 1, which uses lithium nickel oxide alone as the positive electrode active material.

[0132] Furthermore, referring to Figures 2 and 3, it can be confirmed that in the case of the positive electrode of Example 1, the positive electrode capacity at the inflection point (0.5 mAh) is approximately 11% of the positive electrode capacity at the charging termination voltage (4.5 mAh), and in the case of the positive electrode of Example 2, the positive electrode capacity at the inflection point (0.2 mAh) is approximately 4.7% of the positive electrode capacity at the charging termination voltage (4.25 mAh).

[0133] In contrast, Figure 5 shows that the positive electrode capacitance of Comparative Example 2 at the inflection point (1.5 mAh) is approximately 27% of the positive electrode capacitance at the charging termination voltage (5.5 mAh).

[0134] On the other hand, as can be seen from Figures 2 to 5, when setting the cutoff voltage to achieve the same positive electrode discharge capacity, the batteries of Examples 1 and 2 do not use the high-potential region of the negative electrode, while the battery of Comparative Example 1 does use the high-potential region of the negative electrode. To explain this in more detail, in Examples 1 and 2, the negative electrode potential is 0.4V or less at the cutoff voltage set so that the positive electrode discharge capacity is 4mAh, whereas in Comparative Example 1, the negative electrode potential appears high at 0.8V or more at the cutoff voltage. When the negative electrode potential appears high at the cutoff voltage as in Comparative Example 1, the negative electrode potential becomes high during the discharge process, causing rapid expansion of the silicon, which significantly reduces the lifespan characteristics. On the other hand, in Comparative Example 2, as shown in Figure 5, the negative electrode potential at the cutoff voltage is low at 0.4V or less, and silicon expansion can be suppressed, but the cutoff voltage (positive electrode potential - negative electrode potential) is very high at 3.3V, and a significant decrease in cell energy density is expected.

[0135] Experimental Example 2 - Evaluation of Energy Density of Lithium-ion Secondary Batteries After fully charging each lithium secondary battery manufactured in Examples 1 and 2 and Comparative Example 2, the cell energy density was measured while discharging them to a set cutoff voltage. Here, the cutoff voltage was set as the potential difference between the positive and negative electrodes at the inflection point on the positive electrode discharge curve graph of each battery, and the cell energy density was calculated by dividing the energy generated until the end of discharge of the lithium secondary battery (voltage (V) × capacity (Ah)) by the volume (L) of the lithium secondary battery.

[0136] [Table 2]

[0137] Table 2 above confirms that the cell energy density of the lithium secondary batteries produced in Examples 1 and 2 is significantly better than that of the lithium secondary battery produced in Comparative Example 2.

Claims

1. It includes a positive electrode containing a positive electrode active material layer and a negative electrode containing a negative electrode active material layer, The aforementioned negative electrode active material layer contains a negative electrode active material, and the negative electrode active material is made of silicon. After manufacturing the half-cell containing the positive electrode and lithium metal electrode, a capacitance-voltage graph (capacity on the horizontal axis, voltage on the vertical axis) measured while discharging the half-cell with a constant current of 0.1C shows an inflection point in the region where the voltage is 3.3V to 3.7V. This inflection point represents the point where the slope becomes steeper during discharge, passes through a maximum slope, and then becomes gentler. The capacitance of the positive electrode at the inflection point is 3% to 25% of the capacitance of the positive electrode at the charging termination voltage. The positive electrode active material layer comprises a positive electrode active material, the positive electrode active material consisting of lithium nickel manganese oxide and lithium iron phosphate. The lithium iron phosphate is contained in an amount of 5 to 15 parts by weight per 100 parts by weight of the lithium nickel manganese oxide. The average particle size D50 of lithium iron phosphate is 3 μm to 10 μm. The ratio of the average particle size D 50 of lithium iron phosphate to the average particle size D 50 of lithium nickel manganese oxide is 0.5 to 1.

5. The aforementioned lithium nickel manganese oxide is a compound represented by the following chemical formula 1, [Chemical formula 1] Lix[Ni y Co z M 1 w M 2 v ] O 2-p A p A lithium secondary battery in which, in the above chemical formula 1, M1 is Mn, M2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, A is one or more elements selected from the group consisting of F, Cl, Br, I, At, and S, and 1.0 ≤ x ≤ 1.30, 0.3 ≤ y < 1, 0 < z ≤ 0.6, 0 < w ≤ 0.6, 0 ≤ v ≤ 0.2, 0 ≤ p ≤ 0.2, and y > z, y > w, y > v.

2. The lithium secondary battery according to claim 1, wherein when the cutoff voltage of the lithium secondary battery is 3.0V, the discharge potential value of the negative electrode is 0.6V or less.

3. The average particle size D of the silicon ５０ The lithium secondary battery according to claim 1, wherein the particle size is 5 μm to 20 μm.

4. The BET specific surface area of ​​the aforementioned silicon is 0.1 m². ２ / g to 5m ２ A lithium secondary battery according to claim 1, wherein the value is / g.

5. The average particle size D of the lithium nickel manganese oxide ５０ The lithium secondary battery according to claim 1, wherein the particle size is 4 μm to 20 μm.

6. The lithium iron phosphate is a compound represented by the following chemical formula 2, [Chemical formula 2] Li １＋ａ Fe １－ｂ M ｂ (PO ４－ｃ )X ｃ The lithium secondary battery according to claim 1, wherein in the chemical formula 2, M comprises one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X comprises one or more elements selected from the group consisting of F, S, and N, and a, b, and c are -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.1, respectively.

7. The lithium secondary battery according to claim 1, wherein the energy density of the lithium secondary battery is 650 Wh / L or more.

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