Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery and a method for producing the same, which includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode includes a lithium iron phosphate-based active material having an amorphous-content index (AI) defined by formula (1) of 0.28 or less, preferably 0.20 to 0.28, and more preferably 0.20 to 0.27.

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery containing a lithium iron phosphate compound as a positive electrode active material. [Background technology]

[0002] Generally, lithium secondary batteries are manufactured by forming an electrode assembly by interposing a separator between a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material, inserting the electrode assembly into a battery case, injecting a non-aqueous electrolyte, which serves as a medium for transferring lithium ions, and sealing the battery case. The non-aqueous electrolyte is generally composed of a lithium salt and an organic solvent capable of dissolving the lithium salt.

[0003] Positive electrode active materials used in lithium secondary batteries include lithium cobalt oxides, lithium manganese oxides, lithium iron phosphate compounds, lithium nickel cobalt manganese oxides, and lithium nickel cobalt aluminum oxides. Among these, lithium iron phosphate compounds are widely used as positive electrode active materials for lithium secondary batteries because of their excellent thermal stability, long life characteristics, safety, and low cost. However, lithium iron phosphate compounds have the drawback of lower energy density and reduced capacity characteristics compared to other positive electrode active materials.

[0004] In addition, lithium iron phosphate compounds have different stoichiometry and impurity content depending on the initial synthesis state and storage state. This causes deviations in the initial charge capacity and life characteristics when applied to batteries, resulting in problems such as reduced quality uniformity.

[0005] Furthermore, the amorphous phase content in lithium iron phosphate is not well controlled during the manufacturing process and is therefore unpredictable. For example, even lithium iron phosphate manufactured by the same manufacturer using the same process in the same reactor may have different amorphous phase contents. Therefore, the properties and performance of lithium iron phosphate are inconsistent, making it difficult to use in the production process of electrodes and batteries, where reproducibility and uniform quality are essential. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention is intended to solve these problems, and provides a lithium secondary battery with excellent capacity characteristics by using, as a positive electrode active material, a lithium iron phosphate active material whose AI index, which indicates the content ratio of the amorphous phase in the lithium iron phosphate active material, falls within a specific range.

[0007] The present invention also provides a method for manufacturing a lithium secondary battery, which can ensure uniform electrochemical characteristics and quality of the lithium secondary battery by measuring the amorphous content index of a lithium iron phosphate-based active material using a specific method and selecting and applying a lithium iron phosphate-based active material having the amorphous content index within a specific range. [Means for solving the problem]

[0008] According to one embodiment, the present invention provides a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode includes a lithium iron phosphate-based active material having an amorphous-content index (AI) defined by the following formula (1) of 0.28 or less, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, and even more preferably 0.22 to 0.25:

[0009]

number

[0010] In the formula (1),

[0011]

number

[0012] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0013]

number

[0014] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample.

[0015] In this case, the lithium iron phosphate active material may include a lithium iron phosphate compound represented by the following [Chemical Formula 1].

[0016] [Chemical formula 1] Li 1-a [Fe 1-x M x ] 1-y PO 4-b A b

[0017] In the above Chemical Formula 1, M is at least one selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn; A is at least one selected from the group consisting of S, Se, F, Cl, and I; and <a<0.5、0≦x<1、-0.5<y<0.5、0≦b≦0.1である。

[0018] The lithium iron phosphate compound may have a molar ratio of Li to Fe and M (Li / (Fe+M)) of 1.0 to 1.1, preferably 1.05 to 1.09, and more preferably 1.06 to 1.085. The lithium iron phosphate compound may have a molar ratio of P to Fe and M (P / (Fe+M)) of 1.01 to 1.04, and preferably 1.02 to 1.04. The lithium iron phosphate-based compound may further include a conductive coating layer.

[0019] According to another embodiment, the present invention provides a method for manufacturing a lithium secondary battery, the method including the steps of: preparing a sample by mixing a lithium iron phosphate active material and MgO in a weight ratio of 70:30; performing X-ray diffraction analysis on the sample to measure an AI value defined by the following formula (1); selecting a lithium iron phosphate active material whose AI value satisfies a predetermined range as a positive electrode active material; manufacturing a positive electrode including the selected positive electrode active material; manufacturing an electrode assembly including the positive electrode, a separator, and a negative electrode; and housing the electrode assembly in a battery case and then injecting an electrolyte.

[0020]

number

[0021] In the formula (1),

[0022]

number

[0023] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0024]

number

[0025] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample.

[0026] On the other hand, the preset range may be 0.28 or less, preferably 0.20 to 0.28, more preferably 0.2 to 0.27, and even more preferably 0.22 to 0.25. [Effects of the Invention]

[0027] The present invention uses a lithium iron phosphate active material having an amorphous content index AI defined by formula (1) that satisfies a specific range as a positive electrode active material, thereby controlling the heterogeneity of the lithium iron phosphate active material and improving the capacity characteristics of LFP batteries.

[0028] In addition, the method for manufacturing a lithium secondary battery of the present invention is characterized by measuring the amorphous content index (AI) of a lithium iron phosphate active material using a specific method, selecting a lithium iron phosphate compound whose amorphous content index satisfies a specific range, and applying the selected compound as a cathode active material. Meanwhile, the amorphous content index (AI) of the present invention is measured by comparing the peak intensities (areas) of the graph obtained by X-ray diffraction analysis of a sample prepared by mixing a lithium iron phosphate active material and MgO in a weight ratio of 70:30. Unlike the conventional spike method used to analyze the amorphous content in a crystalline structure, this method exhibits minimal variation depending on the sample or number of runs, providing excellent reproducibility and discriminability. Therefore, by using the amorphous content index (AI) defined in the present invention, secondary batteries of uniform and excellent quality can be manufactured without the cumbersome process of fabricating cells and directly measuring their performance. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is a graph showing the initial charge capacity of a lithium secondary battery according to the amorphous content index AI. DETAILED DESCRIPTION OF THE INVENTION

[0030] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0031] In the present invention, "lithium iron phosphate" refers to LiFePO4. Meanwhile, "lithium iron phosphate-based compound" refers to a compound containing lithium, iron, and phosphoric acid, and may contain various other elements and / or dopants, as in Chemical Formula 1 above. "Lithium iron phosphate-based active material" refers to a final positive electrode active material that includes lithium iron phosphate and / or a lithium iron phosphate-based compound, and may further contain additional compositional or structural components, such as a coating layer.

[0032] In the present invention, "primary particles" refers to particle units that do not appear to have grain boundaries when observed under a scanning electron microscope at a magnification of 5,000 to 20,000 times. "Average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle sizes of primary particles observed in a scanning electron microscope image.

[0033] In the present invention, the "average particle size D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the positive electrode active material powder. 50 can be measured using the laser diffraction method. For example, the positive electrode active material powder is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000). After irradiating the dispersion with ultrasonic waves of approximately 28 kHz at an output of 60 W, a graph of the volume cumulative particle size distribution is obtained, and the particle size corresponding to 50% of the volume cumulative amount is determined.

[0034] The present invention will be specifically described below. The amorphous content of a positive electrode active material affects the energy density and / or life characteristics of a lithium secondary battery. Furthermore, the amorphous content is difficult to predict, even for materials with the same chemical composition manufactured using the same method. Therefore, when designing a lithium secondary battery cell, it is important to accurately understand the amorphous content of the positive electrode active material and predict its electrochemical performance.

[0035] Conventionally, the amorphous content of lithium iron phosphate-based active materials has been measured using the spike method, in which an internal standard is added to the lithium iron phosphate-based active material, followed by X-ray diffraction analysis and Rietveld refinement to estimate the amorphous content. However, with this spike method, the calculated values ​​vary depending on the structural model and fitting parameters, and there is also significant variation within the sample, making it difficult to obtain reliable values. Therefore, in the past, quality control required the direct fabrication of lithium secondary battery cells and subsequent testing of their electrochemical properties, which was a cumbersome process.

[0036] The present inventors conducted numerous experiments to develop a secondary battery (hereinafter referred to as "LFP cell") using a lithium iron phosphate-based active material with excellent capacity characteristics and quality uniformity. As a result, they developed a new amorphous content index (AI) that can represent the amorphous content of a lithium iron phosphate-based active material and discovered that it can be used to manufacture LFP cells with excellent initial capacity characteristics, leading to the completion of the present invention. The amorphous content index (AI) is a value different from the crystalline content in the lithium iron phosphate. Rather, it is a parameter characteristic of the final LFP active material that is affected by additional characteristics such as the presence and / or content of a coating layer on the LFP and the presence and / or content of impurities, as well as the active material structure. The amorphous content index AI can be defined by the following formula (1).

[0037]

number

[0038] In the formula (1),

[0039]

number

[0040] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0041]

number

[0042] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample. The MgO is a substance with nearly 100% crystallinity.

[0043] The amorphous content W in the sample amorphrous can be calculated as follows: W amorphrous =100-W crystal =100-(W MgO +W crystal、LFP )=100-(W MgO +W MgO (W crystal、LFP / W MgO )

[0044] In the above formula, W crystal is the crystalline content in the sample, W crystal、LFP is the crystalline content in the lithium iron phosphate compound, W MgO is the content of MgO. Meanwhile, MgO is 100% crystalline and its weight is fixed at 30% by weight. According to the above formula, W amorphrous is W crystal、LFP / W MgO The smaller the amorphous content W in the sample, the greater the amorphrousis W crystal、LFP / W MgO Since MgO is 100% crystalline, the amorphous content in the sample corresponds to the amorphous content of the lithium iron phosphate compound.

[0045] On the other hand, when the proportion of crystalline material in the lithium iron phosphate active material increases, the sum of the peak areas showing crystalline lithium iron phosphate in the X-ray diffraction analysis graph, i.e.

[0046]

number

[0047] increases, and W MgO As increases,

[0048]

number

[0049] will increase. Therefore, W crystal、LFP / W MgO is inversely proportional to AI. As mentioned above, W amorphrous is W crystal、LFP / W MgO Since it has a negative correlation with AI, it has a positive correlation with AI.

[0050] As described above, the amorphous content in a lithium iron phosphate active material has a positive correlation with the AI ​​expressed by the formula (1), and therefore the content of the amorphous phase in a lithium iron phosphate active material can be relatively compared using the AI ​​as an index.

[0051] Meanwhile, in the AI ​​calculation, the present invention adds up only the peak areas of the XRD peaks of lithium iron phosphate that appear in the range of 2θ=15° to 36°, excluding peaks in the region that overlap with MgO peaks, and uses only the area of ​​the (200) peak (2θ=43°), which appears most strongly among the MgO peaks, thereby minimizing errors due to peak overlap.

[0052] Furthermore, according to the inventors' research, when a sample was prepared by mixing 100% crystalline MgO at a content of 30 wt% as an internal standard, the deviation of the result values ​​was found to be the smallest and the reliability was excellent. In this case, the MgO used was, for example, 99.95% pure MgO manufactured by ABCR, which was grounded, heat-treated at 1200°C, and then grounded again. However, the present invention is not limited to this, and any MgO with a crystallinity close to 100% can be used.

[0053] Research by the present inventors has revealed that when a lithium iron phosphate compound having an AI value of 0.28 or less, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, and even more preferably 0.22 to 0.25, is used as a positive electrode active material, the initial capacity characteristics of an LFP cell are significantly improved. The present invention will be specifically described below.

[0054] <Lithium secondary battery> First, the lithium secondary battery according to the present invention will be described. The lithium secondary battery of the present invention includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the positive electrode includes a lithium iron phosphate-based active material having an amorphous-content index (AI) defined by the following formula (1) of 0.28 or less:

[0055]

number

[0056] In the formula (1),

[0057]

number

[0058] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0059]

number

[0060] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample.

[0061] (1) Positive electrode The positive electrode according to the present invention comprises, as a positive electrode active material, a lithium iron phosphate active material having an amorphous-content index (AI) defined by the following formula (1) of 0.28 or less, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, even more preferably 0.22 to 0.27, and most preferably 0.22 to 0.25. Specifically, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the lithium iron phosphate active material.

[0062]

number

[0063] In the formula (1),

[0064]

number

[0065] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0066]

number

[0067] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample.

[0068] As mentioned above, the AI ​​value corresponds to the amorphous content in a lithium iron phosphate active material. In other words, a high AI value indicates a high amorphous content in the lithium iron phosphate active material, while a low AI value indicates a low amorphous content in the lithium iron phosphate active material. On the other hand, since the amorphous phase cannot contribute to the battery reaction, an increase in the amorphous phase content in the lithium iron phosphate active material reduces the capacity characteristics of the battery. When the AI ​​value is as low as 0.28 or less, the amorphous content in the lithium iron phosphate active material is low, enabling excellent capacity characteristics to be achieved.

[0069] The AI ​​value is affected not only by the ratio of crystalline to amorphous matter within the lithium iron phosphate particles, but also by the additional structure and amorphous content in the LFP-based active material, such as the content of the conductive carbon layer formed on the surface of the lithium iron phosphate particles and the content of impurities formed or introduced during the manufacturing process. That is, if a conductive carbon layer is formed on the surface of the lithium iron phosphate particles, the content of the conductive carbon layer can also affect the AI ​​value. Specifically, an increase in the content of the conductive carbon layer increases the AI ​​value, while a decrease in the content of the conductive carbon layer decreases the AI ​​value. When the AI ​​value of a lithium iron phosphate-based active material is less than 0.2, the content of the conductive carbon layer is low. In this case, the lithium iron phosphate-based active material may have insufficient conductivity, resulting in reduced electrochemical performance.

[0070] Meanwhile, the lithium iron phosphate active material may include, for example, a lithium iron phosphate compound represented by the following [Chemical Formula 1].

[0071] [Chemical formula 1] Li 1-a [Fe 1-x M x ] 1-y PO 4-b A b

[0072] In Chemical Formula 1, M may be any one or more selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn, and A may be any one or more selected from the group consisting of S, Se, F, Cl, and I.

[0073] Furthermore, the a may be in the range of −0.5≦a≦0.5, preferably −0.3≦a≦0.3, and more preferably −0.1≦a≦0.1. Furthermore, the x may be 0≦x<1, preferably 0≦x≦0.8, and more preferably 0≦x≦0.7.

[0074] The y may be in the range of -0.5≦y≦0.5, preferably -0.3≦y≦0.3, and more preferably -0.1≦y≦0.1. Furthermore, the b may be 0≦b≦0.1, preferably 0≦b≦0.08, and more preferably 0≦b≦0.05.

[0075] In particular, taking into consideration the electrical conductivity and the effect of improving rate characteristics and capacity characteristics due to the electrical conductivity, the LFP may more specifically be LiFePO4, Li(Fe,Mn)PO4, Li(Fe,Co)PO4, Li(Fe,Ni)PO4, or a mixture thereof, and even more specifically may be LiFePO4.

[0076] Meanwhile, the lithium iron phosphate compound may have a molar ratio of Li to Fe and M (Li / (Fe+M)), i.e., (1-a) / (1-y) in Chemical Formula 1, of 1.0 to 1.1, preferably 1.05 to 1.09, and more preferably 1.06 to 1.085. If the Li / Fe ratio is outside the above range, lithium mobility may decrease, which may result in a decrease in initial capacity.

[0077] Furthermore, the lithium iron phosphate compound may have a molar ratio of P to Fe and M (P / (Fe+M)), i.e., 1 / (1-y) in Chemical Formula 1, of 1.01 to 1.04, preferably 1.02 to 1.04, and more preferably 1.025 to 1.035. If the P / (Fe+M) ratio is too small, the polyanion PO4 in the lattice structure may be insufficient, whereas if the P / (Fe+M) ratio is too large, the Fe and M sites may become over-populated, resulting in a Li-excess state, which may degrade the capacity characteristics.

[0078] Meanwhile, the contents (moles) of Li, Fe, and P in the lithium iron phosphate compound are values ​​measured by ICP analysis. The ICP analysis method can be performed as follows.

[0079] First, approximately 10 mg of lithium iron phosphate active material was dispensed into a vial and its weight was accurately measured. Next, 2 ml of hydrochloric acid and 1 ml of hydrogen peroxide were added to the vial and dissolved at 100°C for 3 hours. Next, 50 g of ultrapure water was added to the vial, and 0.5 ml of 1000 μg / ml scandium (internal standard) was accurately added to prepare a sample solution. The sample solution was filtered through a 0.45 μm PVDF filter, and the concentrations of Li, Fe, and P were measured using an ICP-OES system (Perkin Elmer, AVIO500). If necessary, additional dilution may be performed to ensure that the measured concentrations of the sample solution fall within the calibration range of each component.

[0080] The shape of the particles of the lithium iron phosphate compound is not particularly limited, but may be spherical in consideration of the tap density. The lithium iron phosphate compound may be composed of a single primary particle or a secondary particle formed by agglomeration of a plurality of primary particles. In this case, the primary particle may be uniform or non-uniform. In the present invention, the term "primary particle" refers to a primary structure of a single particle, and the term "secondary particle" refers to an agglomerate formed by agglomeration of primary particles through physical or chemical bonding between the primary particles, i.e., a secondary structure.

[0081] Meanwhile, the lithium iron phosphate-based active material may further include a carbon-based coating layer on the lithium iron phosphate-based compound. Lithium iron phosphate-based compounds are structurally very stable but have the disadvantage of relatively low electrical conductivity. Therefore, it is preferable to improve electrical conductivity and resistance by coating the surface of the lithium iron phosphate-based compound with highly conductive carbon.

[0082] The lithium iron phosphate compound has an average particle size (D 50 ) may be 1 μm to 20 μm, preferably 2 μm to 20 μm, and more preferably 2 μm to 15 μm. If the average particle size of the lithium iron phosphate compound is less than 1 μm, the dispersion may be reduced due to aggregation between particles during the production of the positive electrode, which may result in a deterioration in the characteristics of the positive electrode. In addition, the average particle size (D 50 If the particle size exceeds 20 μm, the mechanical strength and specific surface area may decrease, the porosity between the lithium iron phosphate compound particles may become too large, resulting in a decrease in tap density, or sedimentation may occur during the preparation of the positive electrode slurry.

[0083] On the other hand, when the lithium iron phosphate compound is a secondary particle, the primary particle has an average particle diameter (D 50 ) above, the average particle size may be 100 nm to 2 μm, preferably 100 nm to 1 μm. If the average particle size of the primary particles is less than 100 nm, dispersibility may decrease due to aggregation between particles, and if the average particle size exceeds 2 μm, the packing density may decrease, resulting in a decrease in the capacitance characteristics of the electrode.

[0084] Meanwhile, the lithium iron phosphate-based compound may further include a conductive coating layer on its surface. The conductive coating layer is intended to improve the conductivity of the lithium iron phosphate-based compound and may include one or a mixture of two or more materials selected from the group consisting of carbon-based materials, metals, and conductive polymers. In particular, when a conductive coating layer of a carbon-based material is included, the conductivity can be effectively improved without significantly increasing the weight of the lithium iron phosphate-based compound.

[0085] The conductive coating layer may be formed by a conventional coating layer formation method and may be contained in an amount of 1 wt % to 7 wt %, more specifically 1 wt % to 5 wt %, based on the total weight of the lithium iron phosphate-based compound. If the content of the conductive coating layer is excessively high, exceeding 7 wt %, the battery performance may be degraded due to a relative decrease in the LFP content, while if it is less than 1 wt %, the effect of improving conductivity by forming the conductive layer may be negligible.

[0086] The lithium iron phosphate active material may be included in an amount of 85 wt % to 98 wt %, preferably 90 wt % to 98 wt %, and more preferably 94 wt % to 98 wt %, based on the total weight of the positive electrode active material layer. When the content of the lithium iron phosphate active material satisfies the above range, excellent energy density can be achieved.

[0087] The lithium iron phosphate-based compound may be prepared by a method for preparing a lithium iron phosphate-based compound known in the art, such as a solid-phase synthesis method, a sol-gel method, a hydrothermal synthesis method, or a spray pyrolysis method, and the method is not particularly limited.

[0088] For example, a lithium iron phosphate compound may be synthesized by mixing precursor materials of the lithium iron phosphate compound (e.g., an iron precursor, a phosphate precursor, and a lithium precursor) in a solid state and then calcining the mixture (solid-phase synthesis method).

[0089] Alternatively, a precursor material of a lithium iron phosphate-based compound (e.g., an iron precursor, a phosphate precursor, or a lithium precursor) may be dissolved in a solvent, and then an additive such as an acid / base or a chelating agent may be added to cause a sol-gel reaction to form a gel, which may then be calcined to produce a lithium iron phosphate-based compound (sol-gel method).

[0090] Alternatively, a precursor material of a lithium iron phosphate compound (e.g., an iron precursor, a phosphate precursor, or a lithium precursor) may be dissolved in water and then reacted under high temperature and high pressure to produce the lithium iron phosphate compound (hydrothermal synthesis method).

[0091] Alternatively, a solution containing a precursor material of a lithium iron phosphate compound (e.g., an iron precursor, a phosphate precursor, or a lithium precursor) is sprayed as droplets, and then the solvent is evaporated and heated to produce the lithium iron phosphate compound (spray pyrolysis method).

[0092] On the other hand, as the iron precursor, for example, iron oxalate (FeC2O4·2H2O), iron acetate (Fe(CH3COO)2), FeSO4, FeCO3, FeO, etc. may be used, and as the phosphoric acid precursor, for example, ammonium phosphate, ammonium dihydrogen phosphate, lithium phosphate, iron phosphate, phosphoric acid, phosphoric acid oxide (PO5, PO 10 ), diammonium hydrogen phosphate, etc. may be used, and examples of the lithium precursor that may be used include, but are not limited to, lithium chloride, lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, lithium sulfate, lithium oxide, and lithium aluminum oxide.

[0093] Meanwhile, the lithium iron phosphate-based compound prepared by the above method may be mixed with a carbon source and then heat-treated or reacted with a hydrocarbon gas to prepare a lithium iron phosphate-based active material including a conductive coating layer.

[0094] 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, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like may be used. The positive electrode current collector may typically have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0095] Meanwhile, the positive electrode active material layer may further contain a conductive material, a binder, a dispersant, and the like in addition to the lithium iron phosphate-based active material. In this case, the conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electronic conductivity without causing chemical changes in the battery that is constructed. Specific examples include graphite such as natural graphite and artificial graphite; carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotube, and other carbon-based materials; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives, and the like, and these may be used alone or in combination of two or more.

[0096] The conductive material may be contained in an amount of 0.4 to 10 wt %, preferably 0.4 to 7 wt %, and more preferably 0.4 to 5 wt %, based on the total weight of the positive electrode active material layer. When the content of the conductive material satisfies the above range, good conductivity and capacity of the positive electrode can be achieved.

[0097] The binder also serves to improve adhesion between particles of the positive electrode active material and between the positive electrode active material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination.

[0098] The binder may be contained in an amount of 1 wt % to 5 wt %, preferably 1.5 wt % to 5 wt %, more preferably 1.5 wt % to 4 wt %, and even more preferably 2 wt % to 4 wt %, based on the total weight of the positive electrode active material layer. When the binder content satisfies the above range, the adhesive strength between the current collector and the positive electrode active material layer is high, and a separate layer (e.g., a primer layer) for improving the adhesive strength is not required. When the positive electrode loading is high (e.g., 400 mg / 25 cm), 2 Even in the above cases, the positive electrode adhesive strength is maintained well, and excellent capacity characteristics and life characteristics can be achieved.

[0099] The dispersant is used to improve the dispersibility of the lithium iron phosphate-based active material, conductive material, etc., and may be, for example, hydrogenated nitrile-butadiene rubber (H-NBR), but is not limited thereto. Various dispersants that can improve the dispersibility of the positive electrode slurry may be used. The dispersant may be contained in an amount of 2 wt % or less, preferably 0.1 wt % to 2 wt %, and more preferably 0.1 wt % to 1 wt %, based on the total weight of the positive electrode active material layer. If the content of the dispersant is too low, the dispersion improvement effect will be negligible, and if the content is too high, it may adversely affect battery performance.

[0100] On the other hand, the positive electrode according to the present invention has a loading of 350 mg / 25 cm 2 ~2000mg / 25cm 2 , preferably 400 mg / 25 cm 2 ~1700mg / 25cm 2 , more preferably 450 mg / 25 cm 2 ~1000mg / 25cm 2 When the positive electrode loading amount satisfies the above range, a higher capacity characteristic can be achieved compared to a conventional LFP battery. In this case, the positive electrode loading amount is defined as a positive electrode 25 cm 2 This means the weight of the lithium iron phosphate active material contained in the area.

[0101] The positive electrode may have a porosity of 25% to 60%, preferably 28% to 55%, more preferably 28% to 40%, even more preferably 28% to 35%, and even more preferably 25% to 30%. When the porosity of the positive electrode is within the above range, both the energy density and the electrolyte impregnation property can be maintained at a good level. In the case of LFP batteries, the particle size of the lithium iron phosphate active material, which is the positive electrode active material, is small, resulting in a small pore size within the positive electrode, which reduces the electrolyte impregnation property. Therefore, it is preferable to form the positive electrode with a higher porosity than batteries using other positive electrode active materials. However, the higher the porosity of the positive electrode, the lower the energy density, which is a problem. Therefore, it is necessary to appropriately adjust the porosity of the positive electrode so that both the energy density and the electrolyte impregnation property can be maintained at a good level.

[0102] (2) Negative electrode In the present invention, the negative electrode may be a negative electrode commonly used in the art, and may include, for example, a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0103] The negative electrode may be manufactured, for example, by applying a negative electrode slurry containing a negative electrode active material, and optionally a binder and a conductive material, onto a negative electrode current collector, drying the slurry to form a negative electrode active material layer, and then rolling the layer; alternatively, the negative electrode slurry may be cast onto a separate support, and then peeled off from the support to obtain a film, which may be laminated onto the negative electrode current collector.

[0104] 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 surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. The negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and, like the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0105] The negative electrode active material layer contains, together with the negative electrode active material, a binder and a conductive material selectively. As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides capable of doping and undoping lithium such as SiOx (0 < x < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composites containing the metallic compound and the carbonaceous material such as Si-C composites or Sn-C composites, and mixtures of any one or two or more of these may be used. Further, a thin film of metallic lithium may be used as the negative electrode active material. Also, as the carbon material, either low-crystalline carbon or high-crystalline carbon may be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of high-crystalline carbon are amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes. Also, the binder and the conductive material may be the same as those described above.

[0106] (3)Separator In the present invention, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in lithium secondary batteries can be used without particular limitation. It is particularly preferred that the separator exhibits low resistance to ion migration and excellent electrolyte impregnation capacity. Specifically, porous polymer films, such as those made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminate structures of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymeric material may be used, and may be selectively used in a single-layer or multi-layer structure.

[0107] (4) Electrolyte Examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0108] For example, the electrolyte may include an organic solvent and a lithium salt. The organic solvent is not particularly limited as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specific examples of the organic solvent include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); and alcohol-based solvents such as ethyl alcohol and isopropyl alcohol. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0109] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may 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 concentration of the lithium salt is preferably in the range of 0.1M to 2.0M. When the lithium salt concentration is within this range, the electrolyte has suitable conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0110] In addition to the electrolyte components, the electrolyte may further contain additives to improve the battery's lifespan, suppress the decrease in battery capacity, and improve the battery's discharge capacity. The additives may be various electrolyte additives used in lithium secondary batteries, including, but not limited to, halocarbonate compounds such as fluoroethylene carbonate; nitrile compounds such as succinonitrile; sulfone compounds such as 1,3-propane sultone and 1,3-propene sultone; carbonate compounds such as vinylene carbonate; or combinations thereof. In this case, the additive may be present in an amount of 0.1 wt % to 10 wt %, preferably 0.1 wt % to 5 wt %, based on the total weight of the electrolyte.

[0111] The lithium secondary battery of the present invention as described above has a charge capacity superior to conventional ones. Specifically, when the lithium secondary battery of the present invention is charged to 3.7 V at 0.1 C based on the theoretical capacity (170 mAh / g) of lithium iron phosphate, the initial charge capacity may be 93% to 100%, preferably 93% to 98%, and more preferably 94% to 97% of the theoretical capacity.

[0112] Specifically, the lithium secondary battery according to the present invention may have an initial charge capacity of 158 mAh / g to 170 mAh / g, preferably 158 mAh / g to 167 mAh / g, and more preferably 159 mAh / g to 165 mAh / g when charged to 3.7 V at 0.1 C based on the theoretical capacity (170 mAh / g) of lithium iron phosphate.

[0113] <Method of manufacturing lithium secondary batteries> Next, a method for producing a lithium secondary battery according to the present invention will be described. The method for manufacturing a lithium secondary battery according to the present invention includes the steps of: (1) preparing a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30; (2) performing X-ray diffraction analysis on the sample to measure the AI ​​value expressed by the following formula (1); (3) selecting a lithium iron phosphate active material whose AI value satisfies a predetermined range as a positive electrode active material; (4) manufacturing a positive electrode including the selected positive electrode active material; (5) manufacturing an electrode assembly including the positive electrode, a separator, and a negative electrode; and (6) housing the electrode assembly in a battery case and then injecting an electrolyte.

[0114]

number

[0115] In the formula (1),

[0116]

number

[0117] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0118]

number

[0119] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample.

[0120] (1) Sample preparation step First, a sample is prepared by mixing a lithium iron phosphate active material and MgO in a weight ratio of 70:30. When the sample has this composition, the deviation of the measured values ​​during AI measurement is minimal, demonstrating excellent reproducibility.

[0121] The sample for XRD analysis is a mixture of an internal standard and a lithium iron phosphate active material, which is the target material for measurement. To measure the amorphous content in a lithium iron phosphate active material, a 100% crystalline material must be used as the internal standard.

[0122] According to the research of the present inventors, it was found that the deviation of the measurement values ​​was smallest and the reliability was excellent when MgO was used as the internal standard and the internal standard was mixed at 30% by weight of the total weight of the sample.

[0123] (2) AI measurement step The sample prepared as described above is subjected to X-ray diffraction analysis. The X-ray diffraction analysis was performed using a Bruker D8 Endeavor instrument. Specifically, depending on the amount of sample to be measured, powder was placed in the central groove of the Bruker D8 Endeavor instrument's general powder holder or small-quantity powder holder. A glass slide was used to ensure the sample height was aligned with the holder edge and the sample surface was uniform. The fixed divergence slit was adjusted to 0.3 to match the sample size, and the 2θ region was measured every 0.016 degrees for 0.5 seconds. The holder was rotated at 15 RPM during measurement to compensate for any non-uniformity that may occur during sampling.

[0124] Next, the area of ​​the peak appearing at 2θ=43°±0.5° in the X-ray diffraction graph, i.e.

[0125]

number

[0126] , and the combined area of ​​the peaks appearing in the range of 2θ = 15° to 36°, i.e.

[0127]

number

[0128] is calculated and substituted into equation (1) to calculate the AI ​​value.

[0129]

number

[0130] In the formula (1),

[0131]

number

[0132] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0133]

number

[0134] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample.

[0135] (3) Selecting the positive electrode active material Next, a lithium iron phosphate-based active material whose measured AI value satisfies a predetermined range is selected as the positive electrode active material. In this case, the predetermined range may be appropriately selected taking into consideration the electrochemical performance of the LFP cell to be ultimately manufactured, and may be, for example, 0.28 or less, preferably 0.20 to 0.28, more preferably 0.20 to 0.27, and even more preferably 0.22 to 0.25. When a lithium iron phosphate-based compound whose AI value satisfies the above range is used as the positive electrode active material, the initial capacity characteristics of the LFP cell are excellent.

[0136] (3) Positive electrode manufacturing steps Next, a positive electrode containing the selected positive electrode active material is manufactured. In this case, the positive electrode may be manufactured by a common method known in the art, except that a lithium iron phosphate-based active material having an AI value satisfying a predetermined range is used as the positive electrode active material. For example, the positive electrode may be manufactured by mixing a positive electrode active material, a binder, and a conductive material to prepare a positive electrode slurry, applying the positive electrode slurry to a positive electrode current collector, drying the positive electrode slurry to form a positive electrode active material layer, and then rolling the positive electrode. Meanwhile, the specific types and specifications of the positive electrode active material, binder, and conductive material are the same as those described above, and therefore a detailed description thereof will be omitted.

[0137] (4) Manufacturing steps of the electrode assembly Next, an electrode assembly including the positive electrode, separator, and negative electrode manufactured as described above is manufactured. The specific types and specifications of the negative electrode and separator are the same as those described above, so detailed description thereof will be omitted.

[0138] The electrode assembly may be manufactured by sequentially stacking a positive electrode, a separator, and a negative electrode. The form of the electrode assembly is not particularly limited and may be a common electrode assembly well known in the field of lithium secondary batteries, such as a wound type, a stacked type, and / or a stack and folding type electrode assembly.

[0139] (5) Secondary battery manufacturing steps Next, the electrode assembly is placed in a battery case, and an electrolyte is injected into the battery case to manufacture a lithium secondary battery.

[0140] In this case, the battery case is not particularly limited, and any common battery case well known in the field of lithium secondary batteries, such as a cylindrical, square, or pouch-shaped battery case, can be used without any limitation.

[0141] Meanwhile, the specific type and specifications of the electrolyte are the same as those described above, and the electrolyte may be injected by a general electrolyte injection method well known in the field of lithium secondary batteries.

[0142] (Example) The present invention will be described in more detail below with reference to specific examples. Experimental Example 1: Measurement of the amorphous content index AI of lithium iron phosphate compounds Four types of lithium iron phosphate active materials, A to D, manufactured in different production lots from the same manufacturer were prepared. The lithium iron phosphate active materials were LiFePO4 particles coated with an amorphous carbon layer. Each of the lithium iron phosphate active materials A to D was mixed with MgO in a weight ratio of 70:30 to prepare samples. The samples were subjected to X-ray diffraction analysis to obtain XRD graphs, and the AI ​​values ​​of the following equation (1) were calculated using the obtained XRD graphs.

[0143]

number

[0144] In the formula (1),

[0145]

number

[0146] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, and

[0147]

number

[0148] is the total area of ​​the peaks appearing in the range of 2θ=15° to 36° in the X-ray diffraction analysis graph of the sample.

[0149] Meanwhile, the XRD graph was measured using a D8 Endeavor manufactured by Bruker, and the measurement conditions were as follows. Light source: Cu tube(Cu K alpha 1, alpha 2), Wavelength=1.5406(alpha 1), 1.5444(alpha 2) X-ray Generator power: 40kV, 40mA Divergence slit: 0.5° 2θ=10~90 Step: 0.015° Time / step: 1 sec Total measurement time: 90 min The measurement results are shown in Table 1 below.

[0150] [Table 1]

[0151] Example 1 A positive electrode slurry was prepared by mixing 95 parts by weight of Sample B as a positive electrode active material, 2 parts by weight of carbon black as a conductive material, and 3 parts by weight of PVDF as a binder in an N-methylpyrrolidone solvent. The positive electrode slurry was applied to an aluminum current collector with a thickness of 15 μm, dried, and then rolled to a loading amount of 500 mg / 25 cm. 2 A positive electrode with a porosity of 29% was produced.

[0152] Meanwhile, 95 parts by weight of artificial graphite as a negative electrode active material, 3 parts by weight of SBR and 1 part by weight of CMC as binders, and 1 part by weight of carbon black as a conductive material were added to distilled water to prepare a negative electrode slurry. The negative electrode slurry was applied to a copper current collector with a thickness of 8 μm, dried, and then rolled to a loading amount of 240 mg / 25 cm. 2 A negative electrode with a porosity of 29% was produced.

[0153] The cathode and anode prepared above were stacked together with a polyethylene separator to prepare an electrode assembly, which was then placed in a battery case. An electrolyte solution prepared by dissolving 1M LiPF6 in a solvent containing a 1:1:1 mixture of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate was then injected to prepare a lithium secondary battery.

[0154] Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that Sample C was used instead of Sample B as the positive electrode active material.

[0155] Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that Sample D was used instead of Sample B as the positive electrode active material.

[0156] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that Sample A was used instead of Sample B as the positive electrode active material.

[0157] Experimental Example 2 The lithium secondary batteries prepared in Examples 1 to 3 and Comparative Example 1 were charged to 3.7 V at 0.1 C based on the theoretical capacity (170 mAh / g) of lithium iron phosphate, and the initial charge capacity was measured. The measurement results are shown in FIG. 1 and Table 2 below.

[0158] [Table 2]

[0159] As shown in FIG. 1 and Table 2, the lithium secondary batteries of Examples 1 to 3, which used lithium iron phosphate active materials B to D with AI values ​​of 0.28 or less, exhibited superior capacity characteristics compared to the lithium secondary battery of Comparative Example 1, which had an AI value of more than 0.28. Specifically, it was found that Examples 1 to 3 had a capacity percentage relative to the theoretical capacity that was 2% or more higher than Comparative Example 1. In the field of batteries, such an improvement is considered to be extremely significant. For example, a 2% difference in the capacity of the positive electrode active materials used in batteries included in electric vehicles (EVs) can have the effect of increasing the EV's driving range by 10 miles or more.

[0160] Experimental Example 3 The lithium iron phosphate active materials A and B of Experimental Example 1 were mixed with ZnO in a weight ratio of 70:30 to prepare Sample 1. X-ray diffraction analysis was performed on Sample 1, and the results showed that the active materials were LiFePO4 (Space group: Pnma, 62) and ZnO (Space group: P6 3mc Using a complete structure model for the ZnO (186), Rietveld refinement was performed on the phases present in the samples in the 10 to 120° range to measure the amorphous content in the lithium iron phosphate active material. The analysis was performed three times for each sample, and the amorphous content in the lithium iron phosphate compound was calculated as the relative ratio of the ZnO content to the lithium iron phosphate active material content, with the ZnO content fixed at 30 wt%.

[0161] Lithium iron phosphate active materials A and B from Experimental Example 1 were mixed with MgO in a weight ratio of 70:30 to prepare Sample 2. Sample 2 was subjected to X-ray diffraction analysis, and then the phases present in the sample in the 10° to 120° range were subjected to Rietveld refinement using a complete structure model for LiFePO4 (Space group: Pnma, 62) and MgO (Space group: Fm-3m, No, 225) to measure the amorphous content of the lithium iron phosphate compound. The analysis was performed three times for each sample, and the amorphous content of the lithium iron phosphate compound was calculated as the relative content of the lithium iron phosphate compound to the MgO content, which was fixed at 30 wt%. The AI ​​value of formula (1) was measured using an X-ray diffraction graph obtained by performing X-ray diffraction analysis on Sample 2. The measurement results are shown in Table 3 below.

[0162] [Table 3]

[0163] As shown in Table 3 above, the AI ​​of the present invention exhibits very small deviations depending on the number of measurements and exhibits excellent reproducibility, whereas when the amorphous content is measured by the Rietveld method, the deviations depending on the number of measurements are very large, and it is clear that reliable values ​​cannot be obtained.

Claims

1. A lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, The positive electrode includes a lithium iron phosphate-based active material having an amorphous-content index (AI) of 0.28 or less, as defined by the following formula (1): [Equation 1] In the formula (1), [Equation 2] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, [Equation 3] is the sum of the peak areas appearing in the range of 2θ = 15° to 36° in the X-ray diffraction analysis graph of the sample.

2. 2. The lithium secondary battery according to claim 1, wherein the positive electrode comprises a lithium iron phosphate-based active material having an amorphous content index AI of 0.20 to 0.

27.

3. The lithium iron phosphate active material includes a lithium iron phosphate compound represented by the following [Chemical Formula 1]: [Chemical formula 1] Li １－ａ [Fe １－ｘ M ｘ ] １－ｙ 2O ４－ｂ A ｂ 2. The lithium secondary battery of claim 1, wherein, in Chemical Formula 1, M is at least one selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn; A is at least one selected from the group consisting of S, Se, F, Cl, and I; and −0.5<a<0.5, 0≦x<1, −0.5<y<0.5, and 0≦b≦0.

1.

4. 4. The lithium secondary battery according to claim 3, wherein the lithium iron phosphate compound has a molar ratio of Li to Fe and M (Li / (Fe+M)) of 1.0 to 1.

1.

5. 4. The lithium secondary battery according to claim 3, wherein the lithium iron phosphate compound has a molar ratio of P to Fe and M (P / (Fe+M)) of 1.01 to 1.

04.

6. The lithium secondary battery according to claim 1 , wherein the lithium iron phosphate-based active material further comprises a conductive coating layer.

7. The positive electrode is 350 mg / 25 cm ２ ~2000mg / 25cm ２ 2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery has a loading of

8. 2. The lithium secondary battery according to claim 1, wherein the positive electrode has a porosity of 25% to 60%.

9. 2. The lithium secondary battery according to claim 1, wherein when the lithium secondary battery is charged to 3.7 V at 0.1 C, the charge capacity of the lithium secondary battery is 93% to 100% of the theoretical capacity.

10. preparing a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30; a step of measuring an AI value represented by the following formula (1) by subjecting the sample to X-ray diffraction analysis; selecting a lithium iron phosphate-based active material having an AI value that satisfies a predetermined range as a positive electrode active material; fabricating a positive electrode comprising the selected positive electrode active material; manufacturing an electrode assembly including the positive electrode, a separator, and a negative electrode; Injecting an electrolyte into a battery case after housing the electrode assembly in the battery case; Including, [Equation 4] In the formula (1), [Equation 5] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, [Equation 6] is the sum of the peak areas appearing in the range of 2θ = 15° to 36° in an X-ray diffraction analysis graph of the sample.

11. The method for producing a lithium secondary battery according to claim 10 , wherein the predetermined range is 0.28 or less.

12. 11. The method for producing a lithium secondary battery according to claim 10, wherein the predetermined range is 0.20 to 0.

27.

13. The lithium iron phosphate active material is LiFePO 4 containing a conductive coating layer. ４ 4. The lithium secondary battery according to claim 3, wherein

14. The lithium iron phosphate active material has an amorphous content index (AI) of 0.28 or less, as defined by the following formula (1): [Equation 7] In the formula (1), [Equation 8] is the peak area appearing at 2θ=43°±0.5° in an X-ray diffraction analysis graph of a sample in which a lithium iron phosphate active material and MgO are mixed in a weight ratio of 70:30, [Equation 9] is the sum of the peak areas appearing in the range of 2θ = 15° to 36° in the X-ray diffraction analysis graph of the sample.

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