Lithium secondary battery with easy estimating state of charge

Abstract

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same. The positive electrode contains a first positive active material comprising an iron phosphate compound and a second positive active material comprising a lithium composite metal oxide together in a positive electrode composite layer having a multilayer structure. Since this allows for a large voltage deviation depending on the state of charge (SOC) of the secondary battery, there is an advantage in that the state of charge (SOC) can be easily estimated and / or measured with high reliability when applied to the secondary battery. Furthermore, the positive electrode for the lithium secondary battery contains a small amount of lithium composite metal oxide in the first positive electrode composite layer adjacent to the positive electrode current collector among the plurality of positive electrode composite layers. This allows heat generated by the lithium composite metal oxide to be easily released to the outside, thereby further enhancing the safety of the secondary battery.

Description

Technology Field

[0001] The present invention relates to a lithium secondary battery that can easily determine the charge state of the battery. Background Technology

[0003] Recently, secondary batteries are being widely applied not only to small devices such as portable electronic devices, but also to medium and large devices such as battery packs or power storage devices for hybrid or electric vehicles.

[0004] A secondary battery comprises an electrode assembly including a positive electrode, a negative electrode, and a separator, and an electrolyte, and includes a multilayer outer casing that protects them as a body, and such a secondary battery can be used in the form of a battery module equipped with multiple cells.

[0005] Since high energy density is required to apply such secondary batteries to medium and large-scale devices, LiNi with a Ni content of 60% or more a Co b Mn c High capacity is achieved by using a layered lithium nickel metal oxide (O2, 0.6≤a≤0.9, a+b+c=1) as the cathode active material.

[0006] However, the degradation of battery performance caused by the decrease in structural stability due to the increase in nickel content in lithium nickel metal oxides, particularly the severe deterioration of battery performance and reduced thermal stability in high-temperature environments, is a problem that is hindering commercialization.

[0007] Meanwhile, lithium iron phosphate (LiFePO4) having an olivin structure is a promising active material with excellent lifespan characteristics and superior safety features, including overcharging and over-discharging, because it has the best structural stability. In particular, lithium iron phosphate (LiFePO4) has excellent high-temperature stability due to the strong bonding strength of PO4, and because it contains iron, which is resource-abundant and inexpensive, it is cheaper than the aforementioned LiCoO2, LiNiO2, or LiMn2O4, and because it has low toxicity, it has less impact on the environment.

[0008] However, as shown in Figure 1, lithium iron phosphate (LiFePO4) does not show a large change in voltage during charging and discharging and remains constant, making it difficult to estimate the state of charge (SOC) of the secondary battery. Consequently, there is a problem in that power management for devices using the secondary battery is not easy.

[0009] Therefore, there is a need to develop a technology that includes lithium iron phosphate (LiFePO4) as the cathode active material, which not only provides high battery safety but also facilitates the estimation of the battery's charge state. Prior art literature

[0011] Republic of Korea Published Patent No. 10-2020-0024980 Republic of Korea Published Patent No. 10-2014-0017470 The problem to be solved

[0012] Accordingly, the objective of the present invention is to provide a lithium secondary battery that includes lithium iron phosphate (LiFePO4) as a positive electrode active material, which has high safety and allows for easy estimation of the battery's charge state. means of solving the problem

[0014] In order to solve the aforementioned problem,

[0015] In one embodiment of the present invention,

[0016] n (where n≥2) positive composite layers are positioned on the positive current collector,

[0017] The first positive composite layer in contact with the surface of the positive current collector comprises a first positive active material comprising an iron phosphate compound represented by the following chemical formula 1 and a second positive active material comprising a lithium complex metal oxide represented by the following chemical formula 2, and

[0018] n-1 anode composite layers disposed on the first anode composite layer provide a positive electrode for a lithium secondary battery comprising a first positive active material comprising an iron phosphate compound represented by Chemical Formula 1:

[0019] [Chemical Formula 1]

[0020] LiFe a M 1 1-a XO4

[0021] [Chemical Formula 2]

[0022] Li x [Ni y Co z Mn w M 2 v ]O2

[0023] In the above Chemical Formulas 1 and 2,

[0024] M 1 It is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and

[0025] X is one or more selected from the group consisting of P, Si, S, As, and Sb, and

[0026] a is 0≤a≤0.5, and

[0027] 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

[0028] x, y, z, w, and v are respectively 1.0≤x≤1.30, 0.1≤y<1, 0≤z≤1, 0≤w≤1, and 0≤v≤0.1, where y+z+w+v=1.

[0030] At this time, the concentration of the second positive active material may tend to decrease as the first positive composite layer proceeds from the surface in contact with the positive current collector to the other surface.

[0031] In addition, the second positive active material may be included in an amount of less than 10 weight% relative to the weight of the total positive composite layer.

[0032] In addition, the total thickness of the anode composite layer may be 50㎛ to 300㎛, and the thickness of the first anode composite layer among the anode composite layers may be 10% to 60% of the total thickness of the anode composite layer.

[0033] Furthermore, when applied to a secondary battery, the above-mentioned positive electrode may have a voltage change of 5mV to 60mV per 1% of SOC in the range of 30~70% SOC; and / or may have a voltage change of 0.1mV to 60mV per 1% of SOC in the range of 65~95% SOC.

[0035] In addition, in one embodiment of the present invention,

[0036] The present invention provides an electrode assembly for a lithium secondary battery comprising a positive electrode according to the invention described above; a negative electrode; and a separator interposed between the positive electrode and the negative electrode.

[0037] Here, the cathode comprises a cathode composite layer on a cathode current collector, and the cathode composite layer may comprise one or more carbon-based cathode active materials selected from natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotube, fullerene, activated carbon, acetylene black, and Ketjen black.

[0039] Furthermore, in one embodiment of the present invention,

[0040] Electrode assembly according to the present invention as described above;

[0041] A battery case into which the above electrode assembly is inserted; and

[0042] A lithium secondary battery comprising an electrolyte composition injected into a battery case together with an electrode assembly is provided.

[0043] Here, the above lithium secondary battery is not particularly limited in type, but specifically, it may be a prismatic secondary battery. Effects of the invention

[0045] The cathode for a lithium secondary battery according to the present invention contains a first cathode active material comprising an iron phosphate compound and a second cathode active material comprising a lithium composite metal oxide together in a cathode composite layer having a multilayer structure, thereby enabling a large voltage deviation according to the state of charge (SOC) of the secondary battery, and thus has the advantage of easily estimating and / or measuring the state of charge (SOC) with high reliability when using the secondary battery.

[0046] In addition, the positive electrode for the lithium secondary battery contains a small amount of lithium composite metal oxide in the first positive electrode composite layer adjacent to the positive electrode current collector among the plurality of positive electrode composite layers, thereby allowing heat generated by the lithium composite metal oxide to be easily released to the outside, thus having the effect of further improving the safety of the secondary battery. Brief explanation of the drawing

[0048] Figure 1 is a graph showing the voltage change according to the state of charge (SOC) of a lithium secondary battery (anode active material: graphite) containing lithium iron phosphate (LiFePO4) as the positive electrode active material. FIG. 2 is a cross-sectional view showing the structure of a positive electrode for a lithium secondary battery according to the present invention. Specific details for implementing the invention

[0049] The present invention is capable of various modifications and may have various embodiments, and specific embodiments are to be described in detail in the detailed description.

[0050] However, this is not intended to limit the invention to specific embodiments, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0051] In the present invention, terms such as "comprising" or "having" are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not excluding in advance the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0052] Furthermore, in the present invention, when a part such as a layer, film, region, or plate is described as being "on" another part, this includes not only cases where it is "immediately above" the other part, but also cases where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "under" another part, this includes not only cases where it is "immediately below" the other part, but also cases where there is another part in between. Additionally, in the present application, being "placed on" may include cases where it is placed on the lower part as well as on the upper part.

[0054] The present invention will be described in more detail below.

[0056] cathode for lithium secondary batteries

[0057] In one embodiment of the present invention,

[0058] n (where n≥2) positive composite layers are positioned on the positive current collector,

[0059] The first positive composite layer in contact with the surface of the positive current collector comprises a first positive active material comprising an iron phosphate compound represented by the following chemical formula 1 and a second positive active material comprising a lithium complex metal oxide represented by the following chemical formula 2, and

[0060] n-1 anode composite layers disposed on the first anode composite layer provide a positive electrode for a lithium secondary battery comprising a first positive active material comprising an iron phosphate compound represented by Chemical Formula 1:

[0061] [Chemical Formula 1]

[0062] LiFe a M 1 1-a XO4

[0063] [Chemical Formula 2]

[0064] Li x [Ni y Co z Mn w M 2 v ]O2

[0065] In the above Chemical Formulas 1 and 2,

[0066] M 1 It is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and

[0067] X is one or more selected from the group consisting of P, Si, S, As, and Sb, and

[0068] a is 0≤a≤0.5, and

[0069] 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

[0070] x, y, z, w, and v are respectively 1.0≤x≤1.30, 0.1≤y<1, 0≤z≤1, 0≤w≤1, and 0≤v≤0.1, where y+z+w+v=1.

[0072] The positive electrode for a lithium secondary battery according to the present invention comprises a positive electrode current collector and a positive electrode composite layer having a multilayer structure in which two or more individual composite layers are stacked on the positive electrode current collector.

[0073] Specifically, the anode composite layer has a structure in which n (where n≥2) individual anode composite layers are stacked on an anode current collector. At this time, the anode composite layer stacked on the surface in contact with the anode current collector is the first anode composite layer, and the second anode composite layer to the nth anode composite layer are sequentially stacked on the first anode composite layer so that n individual anode composite layers are located on the anode current collector.

[0074] The number of layers is not particularly limited as long as the anode composite layer has a structure of two or more layers, but specifically, it may be 2 to 10 layers; 2 to 8 layers; 2 to 6 layers; or 2 to 4 layers. By controlling the number of layers of the anode composite layer to the above range, the present invention can improve the energy density of the electrode while preventing a decrease in the manufacturing efficiency of the anode, and at the same time, effectively release heat generated during the charging and discharging of the battery to the outside.

[0075] In addition, the above-mentioned positive composite layer is manufactured by applying, drying, and pressurizing a slurry containing a positive active material capable of reversibly intercalating and deintercalating lithium ions during charging and discharging of the battery, wherein the positive active material may be included in each layer in different types and / or amounts.

[0076] Specifically, the anode according to the present invention comprises a first anode composite layer in contact with the surface of an anode current collector, comprising a first anode active material comprising an iron phosphate compound represented by the following chemical formula 1 and a second anode active material comprising a lithium complex metal oxide represented by the following chemical formula 2, and n-1 anode composite layers disposed on the first anode composite layer, comprising a first anode active material comprising an iron phosphate compound represented by chemical formula 1:

[0077] [Chemical Formula 1]

[0078] LiFe a M 1 1-a XO4

[0079] [Chemical Formula 2]

[0080] Li x [Ni y Co z Mn w M 2 v ]O2

[0081] In the above Chemical Formulas 1 and 2,

[0082] M 1 It is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and

[0083] X is one or more selected from the group consisting of P, Si, S, As, and Sb, and

[0084] a is 0≤a≤0.5, and

[0085] 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

[0086] x, y, z, w, and v are respectively 1.0≤x≤1.30, 0.1≤y<1, 0≤z≤1, 0≤w≤1, and 0≤v≤0.1, where y+z+w+v=1.

[0088] The iron phosphate compound represented by Chemical Formula 1 above is a promising active material that has excellent lifespan characteristics and superior safety features, including overcharging and over-discharging, because it has the best structural stability due to having an olivin structure. In particular, the iron phosphate compound has excellent high-temperature stability due to the strong bonding strength of PO4, and is cheaper than the aforementioned LiCoO2, LiNiO2, or LiMn2O4 because it contains iron, which is resource-abundant and inexpensive, and has low toxicity, thus having less impact on the environment. On the other hand, since the voltage of the iron phosphate compound does not change significantly and remains constant during charging and discharging, it is difficult to determine the state of charge (SOC) of the secondary battery, which presents a problem in that power management for devices using the secondary battery is not easy.

[0089] Accordingly, the positive electrode according to the present invention has a configuration in which an iron phosphate compound represented by Formula 1, having an olivine crystal structure with excellent stability throughout the positive electrode composite layer, is included as a first positive electrode active material, and a second positive electrode active material including a lithium composite metal oxide represented by Formula 2 is included in the first positive electrode composite layer that is in contact with the positive electrode current collector. This allows the state of charge (SOC) of the battery to be easily measured with high reliability while increasing the stability of the lithium secondary battery, and at the same time, by including a small amount of lithium composite metal oxide in the first positive electrode composite layer adjacent to the positive electrode current collector among the plurality of positive electrode composite layers, heat generated by the lithium composite metal oxide can be easily released to the outside, thereby having the effect of further improving the safety of the secondary battery.

[0090] Here, the iron phosphate compound represented by the above chemical formula 1 is a lithium phosphate containing iron, and in some cases, another transition metal (M 1 It may have a doped form. For example, the iron phosphate compound is LiFePO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.5 Mn 0.5It may include PO4, etc.

[0091] In addition, the lithium complex metal oxide represented by the above chemical formula 2 is a metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn) together with lithium, and in some cases, other transition metals (M 2 It may have a form doped with ). In a specific example, more specifically, the lithium composite metal oxide is Li(Ni 0.6 Co 0.2 Mn 0.2 )O2, Li(Ni 0.7 Co 0.15 Mn 0.15 )O2, Li(Ni 0.8 Co 0.1 Mn 0.1 )O2, Li(Ni 0.9 Co 0.05 Mn 0.05 )O2, Li(Ni 0.6 Co 0.2 Mn 0.1 Zr 0.1 )O2, Li(Ni 0.6 Co 0.2 Mn 0.15 Zr 0.05 )O2 and Li(Ni 0.7 Co 0.1 Mn 0.1 Zr 0.1 It may include one or more types selected from the group consisting of )O2.

[0092] In addition, the first cathode active material may have an average particle size of 0.5 to 5 μm, and specifically, may have an average particle size of 0.5 to 1.0 μm; 0.8 to 1.2 μm; 1.0 to 2.0 μm; 1.5 to 3.0 μm; or 2.0 to 3.0 μm.

[0093] In addition, the first positive active material is uniformly dispersed throughout n positive composite layers, and as the position of the positive composite layers changes from the first positive composite layer in contact with the positive current collector to the nth positive composite layer furthest from the positive current collector, the average particle size of the first positive active material contained in each positive composite layer may tend to decrease.

[0094] Specifically, the first positive active material included in the first positive composite layer may have an average particle size of 1.2 to 1.9 μm, and the first positive active material included in the nth positive composite layer (wherein≥2) may have an average particle size of 0.5 to 0.9 μm.

[0095] As an example, the first positive active material included in the first positive composite layer may have an average particle size of 1.2 to 1.9 μm, and the first positive active material included in the second positive composite layer may have an average particle size of 0.5 to 0.9 μm.

[0096] As another example, the first positive active material included in the first positive composite layer may have an average particle size of 1.5 to 1.9 μm, the first positive active material included in the second positive composite layer may have an average particle size of 1.1 to 1.4 μm, and the first positive active material included in the third positive composite layer may have an average particle size of 0.5 to 0.9 μm.

[0097] The present invention can increase the ion mobility of the anode by controlling the average particle size of the first anode active material contained in the individual anode composite layer to decrease as it progresses from an adjacent position to the most separated position in the anode current collector, and at the same time, can improve the adhesion between the anode current collector and the anode composite layer.

[0098] In addition, the second positive active material comprising a lithium composite metal oxide represented by the above chemical formula 2 may be contained only in the first positive composite layer, and may be contained in an amount of less than 10 weight% with respect to the total weight of the positive composite layer, specifically in an amount of 0.1 to 9.9 weight%; 0.5 to 8.0 weight%; 0.5 to 6.0 weight%; 0.1 to 5.0 weight%; 0.1 to 3.0 weight%; 1.0 to 3.0 weight%; 2.5 to 5.0 weight%; 4.0 to 8.0 weight%; or 6.0 to 9.9 weight% with respect to the total weight of the positive composite layer.

[0099] The present invention can prevent the reliability of SOC estimation from decreasing due to insufficient voltage change of the anode according to the battery charge state caused by a negligible content by controlling the content of the second anode active material contained in the first anode composite layer to the above range relative to the total weight of the anode composite layer, and can prevent severe heat generation in the anode during charging and discharging due to an excessive amount of the second anode active material.

[0100] In addition, the second positive active material has a structure dispersed within the first positive composite layer, and may exhibit a tendency for the concentration of the second positive active material to change in the thickness direction of the first positive composite layer. Specifically, the concentration of the second positive active material may show a tendency to decrease as the first positive composite layer progresses from the surface in contact with the positive current collector to the surface in contact with the second positive composite layer formed in the first positive composite layer.

[0101] The above-mentioned second positive active material can be mixed with the first positive active material, which has a small voltage change according to the state of charge (SOC) in the first positive composite layer, to perform the function of increasing the voltage deviation according to the state of charge (SOC). In this case, the second positive active material has a configuration in which its concentration increases as it is closer to the positive current collector within the first positive composite layer, thereby not only allowing the voltage deviation according to the state of charge (SOC) to be expressed with higher reliability, but also having the advantage of easily controlling heat generation at the positive electrode by facilitating the transfer of heat generated by the second positive active material during the charging and discharging of the battery to the positive current collector.

[0102] The positive electrode for a lithium secondary battery according to the present invention has the above-described configuration, which not only provides high safety of the battery but also enables large voltage changes depending on the charge state, thereby having the advantage of easy state of charge (SOC) estimation.

[0103] Specifically, when applied to a secondary battery, the anode may exhibit a voltage change of 5mV to 60mV per 1% of SOC at 25℃ and in the range of 30 to 70% SOC, and specifically, may exhibit a voltage change of 10mV to 50mV; 20mV to 50mV; 30mV to 50mV; 35mV to 45mV; 40mV to 48mV; or 42mV to 45mV.

[0104] As an example, when the above-mentioned positive electrode is applied to a secondary battery containing graphite as a negative electrode active material, it can exhibit a voltage change of 43.1 mV to 44.0 mV per 1% of SOC in the range of 30 to 70% SOC at 25°C.

[0105] In addition, when applied to a secondary battery, the above-mentioned positive electrode may exhibit a voltage change of 0.1mV to 60mV per 1% of SOC at 25℃ and in the range of 65~95% SOC, and specifically, may exhibit a voltage change of 0.1mV to 40mV; 0.1mV to 20mV; 0.1mV to 10mV; 1mV to 10mV; 2.5mV to 10mV; or 3.0mV to 7.0mV.

[0106] As an example, when the above-mentioned positive electrode is applied to a secondary battery containing graphite as a negative electrode active material, it can exhibit a voltage change of 2.5 mV to 9 mV per 1% of SOC in the range of 25°C and SOC 65~95%.

[0107] When the positive electrode according to the present invention is applied to a secondary battery, it has a large voltage deviation as described above for each section of the state of charge (SOC), thereby allowing the state of charge of the battery to be easily estimated and / or measured with high reliability, thus providing the advantage of convenient power management for devices using secondary batteries, and the amount of voltage change may vary depending on the type of negative electrode active material.

[0108] Meanwhile, the positive electrode for a lithium secondary battery according to the present invention may further include a conductive material, a binder, other additives, etc. in the positive electrode composite layer as needed.

[0109] In this case, the first and second cathode active materials contained in each cathode composite layer may be included in an amount of 85 parts by weight or more based on the weight of each cathode composite layer, and specifically, may be included in an amount of 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0110] In addition, the conductive material is used to improve the electrical performance of the anode, and while commonly used in the industry may be applied, specifically, it may include one or more selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Denka black, Ketjen black, Super-P, Channel black, furnace black, lamp black, thermal black, graphene, and carbon nanotubes.

[0111] In addition, the conductive material may be included in an amount of 0.1 to 5 parts by weight based on the weight of each anode composite layer, and specifically, may be included in an amount of 0.1 to 4 parts by weight; 2 to 4 parts by weight; 1.5 to 5 parts by weight; 1 to 3 parts by weight; 0.1 to 2 parts by weight; or 0.1 to 1 part by weight.

[0112] In addition, the binder serves to bind the cathode active material, cathode additive, and conductive material together, and any binder having this function can be used without particular limitation. Specifically, the binder may include one or more resins selected from the group consisting of polyvinylidenefluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinylidenefluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof. As one example, the binder may include polyvinylidenefluoride.

[0113] In addition, the binder may be included in an amount of 1 to 10 parts by weight based on the weight of each anode composite layer, specifically 2 to 8 parts by weight; or 1 to 5 parts by weight of conductive material.

[0114] In addition, the total thickness of the anode composite layer is not particularly limited, but specifically may be 50㎛ to 300㎛, and more specifically may be 100㎛ to 200㎛; 80㎛ to 150㎛; 120㎛ to 170㎛; 150㎛ to 300㎛; 200㎛ to 300㎛; or 150㎛ to 190㎛.

[0115] In addition, among the individual anode composite layers constituting the anode composite layer, the first anode composite layer in contact with the anode current collector may have its thickness adjusted within a certain range. Specifically, the thickness of the first anode composite layer may be 10% to 60% of the total thickness of the anode composite layer, and more specifically, it may be 10% to 40%; 30% to 50%; 10% to 20%; or 40% to 60% of the total thickness of the anode composite layer.

[0116] The present invention can not only prevent the reduction of the energy density of the electrode by controlling the total thickness and individual thicknesses of the anode composite layer to the above range, but also achieve high adhesion between the anode current collector and the anode composite layer.

[0117] Furthermore, the positive current collector provided in the above positive electrode may be one that has high conductivity without causing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, etc. may be used, and in the case of aluminum or stainless steel, one that has been surface-treated with carbon, nickel, titanium, silver, etc. may be used.

[0118] In addition, the average thickness of the above current collector can be appropriately applied in the range of 5 to 500 μm, taking into account the conductivity and total thickness of the anode being manufactured.

[0120] Electrode assembly for lithium secondary battery

[0121] In addition, in one embodiment of the present invention,

[0122] The present invention provides an electrode assembly for a lithium secondary battery comprising a positive electrode according to the invention described above; a negative electrode; and a separator interposed between the positive electrode and the negative electrode.

[0124] The electrode assembly for a lithium secondary battery according to the present invention is equipped with the positive electrode of the present invention described above, and thus can achieve a large voltage deviation according to the state of charge (SOC) of the secondary battery. Therefore, when using the secondary battery, the state of charge (SOC) can be easily estimated and / or measured with high reliability, and heat generated by the lithium composite metal oxide can be easily released to the outside, thereby providing the advantage of further improving the safety of the secondary battery.

[0125] Here, since the above-mentioned anode has the same configuration as the anode for a lithium secondary battery of the present invention described above, a description of the detailed configuration is omitted.

[0126] In addition, the above-mentioned cathode has a cathode composite layer manufactured by applying, drying, and pressing a cathode active material onto a cathode current collector in the same manner as the anode, and may optionally further include a conductive material, a binder, other electrolyte additives, etc. as needed.

[0127] At this time, the above-mentioned cathode active material may be one commonly used in the industry, but specifically, it may include one or more carbon-based cathode active materials among natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotube, fullerene, activated carbon, acetylene black, and Ketjen black.

[0128] In addition, the above-mentioned cathode composite layer may include a binder to enable adhesion with the cathode current collector, while also allowing the cathode active material, conductive material, and other additives to be bonded together. Examples of such binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used. The above binder may be included in an amount of 1 to 10 parts by weight based on the weight of the cathode composite layer, specifically 2 to 8 parts by weight; or may be included in an amount of 1 to 5 parts by weight of the conductive material.

[0129] In addition, the cathode composite layer may have an average thickness of 100㎛ to 200㎛, and specifically, may have an average thickness of 100㎛ to 180㎛, 100㎛ to 150㎛, 120㎛ to 200㎛, 140㎛ to 200㎛, or 140㎛ to 160㎛.

[0130] In addition, the above-mentioned negative electrode may include a negative current collector having high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, calcined carbon, etc., may be used as the negative current collector, and in the case of copper or stainless steel, a surface-treated one such as carbon, nickel, titanium, silver, etc. may be used. Furthermore, similar to the positive current collector, the negative current collector may form fine irregularities on its surface to strengthen the bonding force with the negative active material, and various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics are possible. Moreover, the average thickness of the negative current collector may be appropriately applied in the range of 3 to 500 μm, taking into consideration the conductivity and total thickness of the manufactured negative electrode.

[0132] Furthermore, the separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The separator is not particularly limited as long as it is commonly used in the industry, but specifically, a sheet or nonwoven fabric made of chemically resistant and hydrophobic polypropylene; glass fiber; or polyethylene may be used, and in some cases, a composite separator in which inorganic particles / organic particles are coated by an organic binder polymer on a porous polymer substrate such as the sheet or nonwoven fabric may be used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as the separator. In addition, the pore diameter of the separator may be an average of 0.01 to 10 μm, and the thickness may be an average of 5 to 300 μm.

[0134] lithium secondary battery

[0135] Furthermore, in one embodiment of the present invention,

[0136] Electrode assembly according to the present invention as described above;

[0137] A battery case into which the above electrode assembly is inserted; and

[0138] A lithium secondary battery comprising an electrolyte composition injected into a battery case together with an electrode assembly is provided.

[0140] A lithium secondary battery according to the present invention comprises an electrode assembly including the positive electrode of the present invention described above, and the electrode assembly may have a structure that is inserted into a battery case together with an electrolyte composition.

[0141] At this time, since the above electrode assembly has the same configuration as the electrode assembly of the present invention described above, a description of the detailed configuration is omitted.

[0142] In addition, the above electrolyte composition may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used when manufacturing lithium secondary batteries, but is not limited to these.

[0143] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0144] The above-mentioned organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. For example, the above-mentioned organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; and aromatic hydrocarbon-based solvents such as benzene and fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having C2 to C20 structures and may include a double bond, a directional ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.In this case, using a mixture of cyclic carbonate and chain carbonate in a volume ratio of about 1:1 to 9 can result in excellent performance of the electrolyte.

[0145] In addition, the above lithium salt may be used without special restrictions as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the above 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, etc.

[0146] In addition, the concentration of the lithium salt can be used within the range of 0.1M to 2.0M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0147] In addition to the above electrolyte components, the above electrolyte may further include one or more additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate; or pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be included in an amount of 0.1% to 5% by weight based on the total weight of the electrolyte.

[0148] As described above, a lithium secondary battery comprising the cathode active material composition according to the present invention or a cathode manufactured using the same stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in fields such as portable devices like mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0149] Furthermore, the lithium secondary battery according to the present invention is not limited in its external shape depending on the application of the battery, and its form may be adopted by a case commonly used in the industry. For example, the lithium secondary battery may be a battery comprising a cylindrical or prismatic battery case using a can, or a pouch or coin-type battery case.

[0150] As an example, the lithium secondary battery may be a prismatic secondary battery comprising a prismatic can as a battery case.

[0152] The present invention will be explained in more detail below through examples and experimental examples.

[0153] However, the following examples and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples.

[0155] Examples 1–6 and Comparative Examples 1–3. Preparation of cathodes for lithium secondary batteries

[0156] N-methylpyrrolidone solvent is injected into a homo mixer, and to form the first to third anode composite layers, LiFePO4 (hereinafter 'LFP') as the first anode active material and LiNi as the second anode active material 0.8 Co 0.1 Mn 0.1O2 (hereinafter referred to as 'NCM', average particle size: about 2 μm), carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were each added. Then, the mixture was mixed at 3,000 rpm for 60 minutes to prepare a slurry for forming a first anode composite layer, a slurry for forming a second anode composite layer, and a slurry for forming a third anode composite layer, respectively.

[0157] At this time, the slurry prepared to form each anode composite layer was prepared to include 48.5 parts by weight of anode active material, 1 part by weight of conductive material, and 0.5 parts by weight of binder based on solid content, and the content ratio (unit: parts by weight) of the first anode active material and the second anode active material included in each slurry and the average particle size (unit: μm) of the first anode active material were controlled as shown in Table 1.

[0158] An aluminum foil (average thickness: 12 μm) was prepared as an anode current collector, and the slurry for forming the first to third anode composite layers prepared earlier was sequentially cast onto the prepared aluminum foil, dried in a vacuum oven at 130°C, and then rolled to manufacture an anode. At this time, the total thickness of the rolled anode composite layer was 150 μm, and the thickness of the individual anode composite layer was 50 μm for a 3-layer structure and 75 μm for a 2-layer structure.

[0159] composite layer structure 1st anode composite layer Second anode composite layer Third anode composite layer NCM content LFP content LFP particle size LFP content LFP particle size LFP content LFP particle size Example 1 2nd floor 5 45 1.5 50 1.2 - - Example 2 3rd floor 1.5 48.5 1.5 50 1.2 50 0.8 Example 3 3rd floor 7.5 42.5 1.5 50 1.2 50 0.8 Example 4 3rd floor 13.5 36.5 1.5 50 1.2 50 0.8 Example 5 3rd floor 22.5 27.5 1.5 50 1.2 50 0.8 Example 6 3rd floor 7.5 42.5 1.2 50 1.2 50 1.2 Comparative Example 1 1st floor 100 - - - - - - Comparative Example 2 1st floor - 100 1.5 - - - - Comparative Example 3 1st floor 5 95 1.5 - - - -

[0161] Examples 7–12 and Comparative Examples 4–6. Preparation of electrode assemblies and lithium secondary batteries

[0162] A negative electrode active material was prepared by mixing natural graphite and artificial graphite in a weight ratio of 1:1. A negative electrode slurry was formed by mixing 97 parts by weight of the negative electrode active material and 3 parts by weight of styrene-butadiene rubber (SBR) with water, and then casting it onto a copper foil serving as a negative electrode current collector. The copper foil on which the negative electrode slurry was cast was dried in a vacuum oven at 130°C and rolled to manufacture a negative electrode. At this time, the thickness of the negative electrode composite layer was 130 μm.

[0163] An electrode assembly was fabricated by placing the manufactured cathode and the anode prepared in the previous examples and comparative examples, respectively, as shown in Table 2 below, facing each other, and interposing a separator made of 18 μm polypropylene between them. Each manufactured electrode assembly was inserted into a prismatic battery case, an electrolyte composition was injected into the battery case, and the case was sealed to manufacture a prismatic lithium secondary battery. At this time, as the electrolyte composition, a solution was used in which lithium hexafluorophosphate (LiPF6, 1.0 M) and vinyl carbonate (VC, 2 wt%) were mixed into a mixture of ethylene carbonate (EC):dimethyl carbonate (DMC):diethyl carbonate (DEC) = 1:1:1 (volume ratio).

[0164] Type of anode used Example 7 The anode prepared in Example 1 Example 8 The anode prepared in Example 2 Example 9 The anode prepared in Example 3 Example 10 The anode prepared in Example 4 Example 11 The anode prepared in Example 5 Example 12 The anode prepared in Example 6 Comparative Example 4 The anode prepared in Comparative Example 1 Comparative Example 5 The anode prepared in Comparative Example 2 Comparative Example 6 The anode prepared in Comparative Example 3

[0166] Experimental Example.

[0167] To evaluate the effect of the cathode for a lithium secondary battery according to the present invention, the following experiment was performed.

[0169] a) Evaluation of adhesion between the anode current collector and the anode composite layer

[0170] The surface of the anode composite layer of the anode prepared in the examples and comparative examples was attached to a slide glass using double-sided tape. Double-sided tape was also attached to the surface of the anode current collector, and a portion of it (less than 10 mm from the end of the adhesive surface) was peeled off and attached to a 25 mm x 100 mm PET film in the longitudinal direction. Then, the slide glass was inserted into the lower holder of the UTM equipment (LLOYD Instrument LF Plus), and the portion of the PET film attached to the anode current collector was inserted into the upper holder of the UTM equipment. A force was applied at 300 mm / min at 25°C at a 180° angle to measure the force required to peel off the anode current collector and the anode composite layer (adhesion force between the anode current collector and the anode composite layer). The results are shown in Table 3 below.

[0172] b) Evaluation of voltage deviation by SOC range

[0173] In the examples and comparative examples, the lithium secondary batteries manufactured were each subjected to three charge-discharge cycles, and the voltage according to the SOC was measured to calculate the voltage deviation occurring when the SOC changes by 1% for each section. At this time, the charge-discharge was performed three times at a rate of 0.1C with a cutoff potential of 2.8V and a maximum charge voltage of 3.6V. In addition, the voltage deviation was calculated for a first section including an SOC of 30~55%; a second section including an SOC of 30~70%; and a third section including an SOC of 65~95%, and the results are shown in Table 3 below.

[0175] c) Evaluation of battery heat generation during charging and discharging

[0176] Overcharging was performed on the secondary batteries fabricated in the examples and comparative examples, and the surface and internal temperatures of the overcharged secondary batteries were measured. Specifically, a thermal sensor was installed on the inside of the case of each target secondary battery, and after charging to 4.2V, the charged battery was overcharged at a constant current of 1A until it reached 10V. Then, the constant voltage of 10V was maintained for 6 hours. After 6 hours had elapsed, the internal temperature of the battery was measured using the thermal sensor installed on each secondary battery, and the surface of each secondary battery was photographed with a thermal imaging camera to measure the temperature of the secondary battery three times. The average value was calculated and measured as the surface temperature of the battery during overcharging. The measured results are shown in Table 3 below.

[0177] Adhesion between the entire house and the composite layer [gf / cm] Voltage change [mV] for a 1% change in SOC Battery temperature during overcharging Section 1 Section 2 Section 3 interior surface Example 7 55 12.3 43.5 5.1 45±1℃ 36±1℃ Example 8 49 11.8 43.1 2.9 41±1℃ 33±1℃ Example 9 57 12.4 43.5 5.1 42±1℃ 33±1℃ Example 10 57 13.3 43.9 7.9 45±1℃ 36±1℃ Example 11 55 14.2 44.3 8.5 54±1℃ 41±1℃ Example 12 56 12.3 43.5 5.0 44±1℃ 35±1℃ Comparative Example 4 50 28.2 52.1 58.3 63±1℃ 54±1℃ Comparative Example 5 22 11.6 43.0 2.3 41±1℃ 32±1℃ Comparative Example 6 47 12.2 43.5 5.0 47±1℃ 38±1℃

[0179] As shown in Table 3, it can be seen that the cathode for a lithium secondary battery according to the present invention has excellent battery safety and can easily estimate and / or measure the state of charge (SOC).

[0180] It can be seen that the positive electrode of the embodiment according to the present invention has a high adhesion strength between the positive electrode current collector and the positive electrode composite layer of 49 gf / mm or higher, and that the internal and external temperatures of the battery are low, at less than 55°C and 41°C, respectively, when overcharged. This implies that the safety of the secondary battery containing the positive electrode is high. In addition, it was confirmed that the positive electrodes of the embodiment increase the voltage change amount per 1% of SOC in the second section, which includes a state of charge (SOC) of 50%, to 43.1 mV or higher. In particular, in the case of a battery containing lithium iron phosphate (LiFePO4) as the positive electrode active material, it was confirmed that the voltage change amount per 1% of SOC in the first and third sections, where the voltage difference according to the change in state of charge (SOC) is minimal, shows a large voltage deviation of 11.8 mV or higher and 2.9 mV or higher, respectively. This means that the state of charge (SOC) of the battery can be easily estimated with high reliability.

[0182] From these results, it can be seen that the anode according to the present invention can achieve a large voltage deviation according to the state of charge (SOC) of the secondary battery, so the state of charge (SOC) can be easily estimated and / or measured with high reliability when applied to the secondary battery, and the safety of the secondary battery is further improved because heat generated by the lithium composite metal oxide can be easily released to the outside.

[0184] Although the present invention has been described above with reference to preferred embodiments, those skilled in the art or those with ordinary knowledge in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and technical scope of the invention as described in the claims set forth below.

[0185] Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification, but should be determined by the claims. Explanation of the symbols

[0187] 1: Cathode for lithium secondary batteries 10: Positive current collector 20: Multilayer structured anode composite layer 21: Individual anode composite layer 21a: First anode composite layer 21b: n-th anode composite layer

Claims

Claim 1 A positive electrode for a lithium secondary battery, wherein n (wherein≥2) positive electrode composite layers are positioned on a positive electrode current collector, the first positive electrode composite layer in contact with the surface of the positive electrode current collector comprises a first positive electrode active material comprising an iron phosphate compound represented by the following Chemical Formula 1 and a second positive electrode active material comprising a lithium complex metal oxide represented by the following Chemical Formula 2, and n-1 positive electrode composite layers disposed on the first positive electrode composite layer comprise a first positive electrode active material comprising an iron phosphate compound represented by Chemical Formula 1, and the concentration of the second positive electrode active material tends to decrease as the first positive electrode composite layer proceeds from the surface in contact with the positive electrode current collector to the other surface: [Chemical Formula 1] LiFe a M 1 1-a XO4[Chemical Formula 2]Li x [Ni y Co z Mn w M 2 v ]O2 In the above Chemical Formulas 1 and 2, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Co, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, X is P, a is 0≤a≤0.5, 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 x, y, z, w, and v are each 1.0≤x≤1.30, 0.1≤y<1, 0≤z, 0≤w, 0≤v≤0.1, and y+z+w+v=1. Claim 2 delete Claim 3 A lithium secondary battery cathode according to claim 1, wherein the second cathode active material is included in an amount of less than 10 weight% with respect to the weight of the total cathode composite layer. Claim 4 A positive electrode for a lithium secondary battery according to claim 1, wherein the total thickness of the positive electrode composite layer is 50㎛ to 300㎛. Claim 5 A positive electrode for a lithium secondary battery according to claim 1, wherein the thickness of the first positive electrode composite layer is 10% to 60% of the total thickness of the positive electrode composite layer. Claim 6 In claim 1, the positive electrode is a positive electrode for a lithium secondary battery having a voltage change amount of 5mV to 60mV per 1% of SOC in the range of 30~70% SOC when applied to a secondary battery. Claim 7 In claim 1, the positive electrode is a positive electrode for a lithium secondary battery having a voltage change amount of 0.1mV to 60mV per 1% of SOC in the range of 65~95% SOC when applied to a secondary battery. Claim 8 An electrode assembly for a lithium secondary battery comprising a positive electrode according to claim 1; a negative electrode; and a separator interposed between the positive electrode and the negative electrode. Claim 9 An electrode assembly for a lithium secondary battery according to claim 8, wherein the cathode comprises a cathode composite layer on a cathode current collector, and the cathode composite layer comprises one or more carbon-based cathode active materials selected from natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotube, fullerene, activated carbon, and acetylene black. Claim 10 A lithium secondary battery comprising: an electrode assembly according to claim 8; a battery case into which the electrode assembly is inserted; and an electrolyte composition injected into the battery case together with the electrode assembly. Claim 11 A lithium secondary battery characterized in that, in claim 10, the lithium secondary battery is a prismatic secondary battery.

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