Lithium secondary battery with easy state estimation

Abstract

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same, and the positive electrode contains both a first positive electrode active material including an iron phosphate compound and a second positive electrode active material including a lithium composite metal oxide in a multi-layered positive electrode composite layer, so that a large voltage deviation can be realized according to the state of charge (SOC) of the secondary battery. Therefore, there is an advantage that the state of charge (SOC) when the secondary battery is used can be easily estimated and / or measured with high reliability. 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 multiple positive electrode composite layers, so that heat generated by the lithium composite metal oxide can be easily released to the outside, and thus the safety of the secondary battery is further improved.

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery that allows easy determination of the state of charge of the battery.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0045864, filed on April 13, 2022, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference. [Background technology]

[0003] BACKGROUND ART In recent years, secondary batteries have been widely used not only in small devices such as portable electronic devices but also in medium to large devices such as battery packs for hybrid cars and electric cars, and power storage devices.

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

[0005] To apply such secondary batteries to medium- to large-scale equipment, high energy density is required. Therefore, LiNi with a Ni content of 60% or more is used. a Co b Mn c O2 (0.6≦a≦0.9, a+b+c=1), a layered lithium nickel metal oxide is used as the positive electrode active material, achieving high capacity.

[0006] However, the increased nickel content in lithium nickel metal oxides leads to a decrease in structural stability, which in turn leads to a serious deterioration in battery performance, particularly in high-temperature environments, and a decrease in thermal stability, which has hindered their commercialization.

[0007] Meanwhile, lithium iron phosphate (LiFePO4), which has an olivine structure, is a promising active material with the advantages of excellent structural stability, excellent life characteristics, and excellent safety in all aspects, including overcharge and overdischarge. In particular, lithium iron phosphate (LiFePO4) has excellent high-temperature stability due to the strong binding strength of PO4, and is cheaper than the aforementioned LiCoO2, LiNiO2, or LiMn2O4 because it contains iron, which is a resource that is abundant and inexpensive. It also has low toxicity, which reduces the impact on the environment.

[0008] However, as shown in Figure 1, the voltage of lithium iron phosphate (LiFePO4) does not change significantly during charging and discharging, and remains constant. This makes it difficult to estimate the state of charge (SOC) of the secondary battery, which makes it difficult to manage the power supply of devices that use secondary batteries.

[0009] Therefore, there is a need to develop a technology that contains lithium iron phosphate (LiFePO4) as the positive electrode active material, which not only ensures high battery safety but also makes it easy to estimate the battery's state of charge. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Korean Patent Publication No. 10-2020-0024980 [Patent Document 2] Korean Patent Publication No. 10-2014-0017470 Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, an object of the present invention is to provide a lithium secondary battery that contains lithium iron phosphate (LiFePO4) as a positive electrode active material, has high battery safety, and allows easy estimation of the state of charge of the battery. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems, in one embodiment, the present invention provides a positive electrode for a lithium secondary battery, in which n (n≧2) positive electrode composite layers are located on a positive electrode current collector, a first positive electrode composite layer in contact with the surface of the positive electrode current collector includes a first positive electrode active material including an iron phosphate compound represented by Chemical Formula 1 below, and a second positive electrode active material including a lithium composite metal oxide represented by Chemical Formula 2 below, and n-1 positive electrode composite layers disposed on the first positive electrode composite layer include first positive electrode active materials including the iron phosphate compound represented by Chemical Formula 1:

[0013] [Chemical formula 1] Life a M 1 1-a XO4

[0014] [Chemical formula 2] Li x [Ni y Co z Mn w M 2 v ]O2

[0015] In the above chemical formula 1 and chemical formula 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 one or more elements selected from the group consisting of P, Si, S, As, and Sb; 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 1.0≦x≦1.30, 0.1≦y<1, 0≦z≦1, 0≦w≦1, 0≦v≦0.1, respectively, and y+z+w+v=1.

[0016] In this case, the first positive electrode mixture layer may have a tendency for the concentration of the second positive electrode active material to decrease from the surface in contact with the positive electrode current collector to the other surface.

[0017] The second positive electrode active material may be included in an amount of less than 10 wt % based on the weight of the entire positive electrode mixture layer.

[0018] The positive electrode mixture layers may have a total thickness of 50 μm to 300 μm, and the thickness of the first positive electrode mixture layer among the positive electrode mixture layers may be 10% to 60% of the total thickness of the positive electrode mixture layers.

[0019] Furthermore, when the positive electrode is applied to a secondary battery, it may have a voltage change of 5 mV to 60 mV per 1% SOC in the range of 30 to 70% SOC, and / or a voltage change of 0.1 mV to 60 mV per 1% SOC in the range of 65 to 95% SOC.

[0020] In one embodiment, the present invention provides an electrode assembly for a lithium secondary battery, including the above-described positive electrode according to the present invention, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0021] Here, the negative electrode includes a negative electrode composite layer on a negative electrode current collector, and the negative electrode composite layer may include one or more carbon-based negative electrode active materials selected from natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotubes, fullerene, activated carbon, acetylene black, and ketjen black.

[0022] Furthermore, in one embodiment, the present invention provides a lithium secondary battery including the above-described electrode assembly according to the present invention, a battery case into which the electrode assembly is inserted, and an electrolyte composition injected into the battery case together with the electrode assembly.

[0023] Here, the type of the lithium secondary battery is not particularly limited, but specifically, it can be a prismatic secondary battery. [Effects of the Invention]

[0024] The positive electrode for a lithium secondary battery according to the present invention includes a multi-layered positive electrode composite layer containing both a first positive electrode active material including an iron phosphate compound and a second positive electrode active material including a lithium composite metal oxide, and can realize a large voltage deviation according to the state of charge (SOC) of the secondary battery. This has the advantage that the state of charge (SOC) of the secondary battery during use can be easily estimated and / or measured with high reliability.

[0025] Furthermore, in the positive electrode for the lithium secondary battery, by containing a small amount of lithium composite metal oxide in the first positive electrode composite layer, which is 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, which has the effect of further improving the safety of the secondary battery. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a graph showing the change in voltage according to the state of charge (SOC) of a lithium secondary battery (negative electrode active material: graphite) containing lithium iron phosphate (LiFePO4) as a positive electrode active material. [Figure 2] 1 is a cross-sectional view showing the structure of a positive electrode for a lithium secondary battery according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] Because the present invention is susceptible to various modifications and can have various embodiments, specific embodiments are described in detail in the detailed description.

[0028] However, this is not intended to limit the invention to any particular embodiment, but should be understood to include all modifications, equivalents, or alternatives falling within the spirit and scope of the invention.

[0029] In the present invention, the terms "comprise" and "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the presence or possibility of addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0030] Furthermore, in the present invention, when a layer, film, region, plate, or other portion is described as being "on" another portion, this includes not only the case where it is "directly on" the other portion, but also the case where there is another portion therebetween. Conversely, when a layer, film, region, plate, or other portion is described as being "under" the other portion, this includes not only the case where it is "directly under" the other portion, but also the case where there is another portion therebetween. Furthermore, in this application, being "located on" can include not only the case where it is located at the top, but also the case where it is located at the bottom.

[0031] The present invention will now be described in more detail.

[0032] <Positive electrodes for lithium secondary batteries> In one embodiment, the present invention provides a positive electrode for a lithium secondary battery, in which n (n≧2) positive electrode composite layers are located on a positive electrode current collector, a first positive electrode composite layer in contact with the surface of the positive electrode current collector includes a first positive electrode active material including an iron phosphate compound represented by Chemical Formula 1 below, and a second positive electrode active material including a lithium composite metal oxide represented by Chemical Formula 2 below, and n−1 positive electrode composite layers disposed on the first positive electrode composite layer include first positive electrode active materials including the iron phosphate compound represented by Chemical Formula 1:

[0033] [Chemical formula 1] Life a M 1 1-a XO4

[0034] [Chemical formula 2] Li x [Ni y Co z Mn w M 2v ]O2

[0035] In the above chemical formula 1 and chemical formula 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 one or more elements selected from the group consisting of P, Si, S, As, and Sb; 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 1.0≦x≦1.30, 0.1≦y<1, 0≦z≦1, 0≦w≦1, 0≦v≦0.1, respectively, and y+z+w+v=1.

[0036] The positive electrode for a lithium secondary battery according to the present invention includes a positive electrode current collector and a positive electrode composite layer having a multilayer structure in which two or more individual composite layers are laminated on the positive electrode current collector.

[0037] Specifically, the positive electrode composite layer has a structure in which n (n≧2) individual positive electrode composite layers are stacked on a positive electrode current collector. In this case, the positive electrode composite layer stacked on the surface adjacent to the positive electrode current collector is a first positive electrode composite layer, and a second positive electrode composite layer or an nth positive electrode composite layer is sequentially stacked on the first positive electrode composite layer, resulting in n individual positive electrode composite layers being located on the positive electrode current collector.

[0038] The positive electrode composite layer is not particularly limited in the number of layers as long as it has a structure of two or more layers, and specifically may have 2 to 10 layers, 2 to 8 layers, 2 to 6 layers, or 2 to 4 layers. By adjusting the number of stacked positive electrode composite layers within the above range, the present invention can improve the energy density of the electrode while preventing a decrease in the manufacturing efficiency of the positive electrode, and at the same time, can effectively release heat generated during charging and discharging of the battery to the outside.

[0039] The positive electrode composite layer is manufactured by applying, drying, and pressing a slurry containing a positive electrode active material that is capable of reversibly intercalating and deintercalating lithium ions during charging and discharging of the battery, and the positive electrode active material may be contained in different types and / or amounts in each layer.

[0040] Specifically, in the positive electrode according to the present invention, a first positive electrode composite layer in contact with the surface of a positive electrode current collector includes a first positive electrode active material including an iron phosphate compound represented by Chemical Formula 1 below, and a second positive electrode active material including a lithium composite metal oxide represented by Chemical Formula 2 below, and n-1 positive electrode composite layers disposed on the first positive electrode composite layer include the first positive electrode active material including the iron phosphate compound represented by Chemical Formula 1:

[0041] [Chemical formula 1] Life a M 1 1-a XO4

[0042] [Chemical formula 2] Li x [Ni y Co z Mn w M 2 v ]O2

[0043] In the above chemical formula 1 and chemical formula 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 one or more elements selected from the group consisting of P, Si, S, As, and Sb; 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 1.0≦x≦1.30, 0.1≦y<1, 0≦z≦1, 0≦w≦1, 0≦v≦0.1, respectively, and y+z+w+v=1.

[0044] The iron phosphate compound represented by Chemical Formula 1 has an olivine structure and excellent structural stability, making it a promising active material with excellent lifespan characteristics and excellent safety in all aspects, including overcharge and overdischarge. In particular, the iron phosphate compound has excellent high-temperature stability due to the strong binding strength of PO4. It also contains iron, which is abundant and inexpensive, making it less expensive than the aforementioned LiCoO2, LiNiO2, or LiMn2O4. It also has low toxicity and therefore less impact on the environment. However, iron phosphate compounds have a problem in that the voltage change during charge and discharge is constant and not large, making it difficult to estimate the state of charge (SOC) of secondary batteries, which makes it difficult to manage the power supply of devices that use secondary batteries.

[0045] Therefore, the positive electrode according to the present invention has a configuration in which the positive electrode composite layer contains, as a first positive electrode active material, an iron phosphate compound represented by Chemical Formula 1 having an olivine crystal structure, which has excellent stability overall, and the first positive electrode composite layer that is in contact with the positive electrode current collector further contains a second positive electrode active material that includes a lithium composite metal oxide represented by Chemical Formula 2. This improves the stability of the lithium secondary battery and makes it possible to easily measure the state of charge (SOC) of the battery with high reliability. At the same time, by including a small amount of lithium composite metal oxide in the first positive electrode composite layer that is adjacent to the positive electrode current collector among the multiple positive electrode composite layers, heat generated by the lithium composite metal oxide can be easily released to the outside, thereby further improving the safety of the secondary battery.

[0046] Here, the iron phosphate compound represented by the above formula 1 is a lithium phosphate oxide containing iron, and in some cases, other transition metals (M 1 ) may have a doped form. For example, the iron phosphate compound may be LiFePO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.5 Mn 0.5 It may contain PO4 etc.

[0047] The lithium composite 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 ) may be doped. In a specific example, more specifically, the lithium composite metal oxide may have a doped form of 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 )O2.

[0048] The first positive electrode active material may have an average particle size of 0.5 to 5 μm, specifically 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.

[0049] In addition, the first positive electrode active material is uniformly dispersed throughout the n positive electrode composite layers, and the average particle size of the first positive electrode active material contained in each positive electrode composite layer may tend to decrease as the position of the positive electrode composite layer changes from the first positive electrode composite layer in contact with the positive electrode current collector to the nth positive electrode composite layer farthest from the positive electrode current collector.

[0050] Specifically, the first positive electrode active material contained in the first positive electrode composite layer may have an average particle size of 1.2 to 1.9 μm, and the first positive electrode active material contained in the nth positive electrode composite layer (where n≧2) may have an average particle size of 0.5 to 0.9 μm.

[0051] As one example, the first positive electrode active material contained in the first positive electrode mixture layer may have an average particle size of 1.2 to 1.9 μm, and the first positive electrode active material contained in the second positive electrode mixture layer may have an average particle size of 0.5 to 0.9 μm.

[0052] As another example, the first positive electrode active material contained in the first positive electrode composite layer may have an average particle size of 1.5 to 1.9 μm, the first positive electrode active material contained in the second positive electrode composite layer may have an average particle size of 1.1 to 1.4 μm, and the first positive electrode active material contained in the third positive electrode composite layer may have an average particle size of 0.5 to 0.9 μm.

[0053] The present invention adjusts the average particle size of the first positive electrode active material contained in the individual positive electrode composite layers to decrease from the position adjacent to the positive electrode current collector to the position furthest away, thereby increasing the ion mobility of the positive electrode and simultaneously improving the adhesion between the positive electrode current collector and the positive electrode composite layer.

[0054] Furthermore, the second positive electrode active material containing the lithium composite metal oxide represented by Chemical Formula 2 above may be contained only in the first positive electrode composite layer, and may be contained in an amount of less than 10 wt % relative to the weight of the entire positive electrode composite layer. Specifically, the second positive electrode active material may be contained in an amount of 0.1 to 9.9 wt %, 0.5 to 8.0 wt %, 0.5 to 6.0 wt %, 0.1 to 5.0 wt %, 0.1 to 3.0 wt %, 1.0 to 3.0 wt %, 2.5 to 5.0 wt %, 4.0 to 8.0 wt %, or 6.0 to 9.9 wt % relative to the weight of the entire positive electrode composite layer.

[0055] By controlling the content of the second positive electrode active material contained in the first positive electrode composite layer within the above range relative to the weight of the entire positive electrode composite layer, the present invention can prevent a decrease in the reliability of SOC estimation due to an insufficient positive electrode voltage change depending on the state of charge of the battery caused by a small content, and can prevent significant heat generation in the positive electrode during charge and discharge due to an excessive amount of the second positive electrode active material.

[0056] The second positive electrode active material may be dispersed within the first positive electrode composite layer, and the concentration of the second positive electrode active material may vary in the thickness direction of the first positive electrode composite layer. Specifically, the first positive electrode composite layer may have a decreasing concentration of the second positive electrode active material from a surface in contact with the positive electrode current collector to a surface in contact with the second positive electrode composite layer formed on the first positive electrode composite layer. Here, "decreasing concentration" may refer to a decrease in the content or content ratio of the second positive electrode active material relative to the total weight of the positive electrode composite layer.

[0057] The second positive electrode active material is mixed with the first positive electrode active material, which has a small voltage change according to the state of charge (SOC), in the first positive electrode composite layer, and functions to increase the voltage deviation according to the state of charge (SOC). In this case, the second positive electrode active material has a configuration in which its concentration increases the closer to the positive electrode current collector within the first positive electrode composite layer. This not only enables more reliable voltage deviation according to the state of charge (SOC), but also has the advantage of easily transferring heat generated by the second positive electrode active material to the positive electrode current collector during charging and discharging of the battery, thereby easily controlling heat generation in the positive electrode.

[0058] The positive electrode for a lithium secondary battery according to the present invention has the above-described configuration, which not only ensures high battery safety but also simplifies estimation of the state of charge (SOC) of the battery since it can realize a large voltage change depending on the state of charge.

[0059] Specifically, when the positive electrode is applied to a secondary battery, it can exhibit a voltage change of 5 mV to 60 mV per 1% SOC at 25°C in the range of 30 to 70% SOC, specifically, a voltage change of 10 mV to 50 mV, 20 mV to 50 mV, 30 mV to 50 mV, 35 mV to 45 mV, 40 mV to 48 mV, or 42 mV to 45 mV.

[0060] As an example, when the 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% SOC at 25° C. in the range of 30 to 70% SOC.

[0061] Furthermore, when the positive electrode is applied to a secondary battery, it can exhibit a voltage change of 0.1 mV to 60 mV per 1% SOC at 25°C in the range of 65 to 95% SOC, specifically, a voltage change of 0.1 mV to 40 mV, 0.1 mV to 20 mV, 0.1 mV to 10 mV, 1 mV to 10 mV, 2.5 mV to 10 mV, or 3.0 mV to 7.0 mV.

[0062] As an example, when the 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% SOC at 25° C. in the range of SOC 65 to 95%.

[0063] When applied to a secondary battery, the positive electrode according to the present invention has such a large voltage deviation for each state of charge (SOC) range, thereby enabling the state of charge of the battery to be easily estimated and / or measured with high reliability, which is advantageous in that it facilitates power management of devices using the secondary battery. The amount of voltage change may vary depending on the type of negative electrode active material.

[0064] Meanwhile, the positive electrode for a lithium secondary battery according to the present invention may further contain a conductive material, a binder, other additives, and the like in the positive electrode mixture layer, if necessary.

[0065] In this case, the first positive electrode active material and the second positive electrode active material contained in each positive electrode composite layer may be contained in an amount of 85 parts by weight or more, specifically 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more, based on the weight of each positive electrode composite layer.

[0066] The conductive material is used to improve the electrical performance of the positive electrode and may be any conductive material commonly used in the art. Specifically, the conductive material may include at least one 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, summer black, graphene, and carbon nanotubes.

[0067] The conductive material may be contained in an amount of 0.1 to 5 parts by weight, specifically 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, based on the weight of each positive electrode composite layer.

[0068] The binder functions to bind the positive electrode active material, the positive electrode additive, and the conductive material together, and any material having this function may be used without particular limitation. Specifically, the binder may include one or more resins selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof. For example, the binder may include polyvinylidene fluoride.

[0069] The binder may be contained in an amount of 1 to 10 parts by weight, specifically 2 to 8 parts by weight, or the conductive material may be contained in an amount of 1 to 5 parts by weight, based on the weight of each positive electrode mixture layer.

[0070] Furthermore, the total thickness of the positive electrode composite layer is not particularly limited, but may specifically be 50 μm to 300 μm, more specifically 100 μm to 200 μm, 80 μm to 150 μm, 120 μm to 170 μm, 150 μm to 300 μm, 200 μm to 300 μm, or 150 μm to 190 μm.

[0071] Furthermore, among the individual positive electrode composite layers constituting the positive electrode composite layer, the first positive electrode composite layer in contact with the positive electrode current collector may have a thickness adjusted to a certain range. Specifically, the thickness of the first positive electrode composite layer may be 10% to 60% of the total thickness of the positive electrode composite layer, more specifically, 10% to 40%, 30% to 50%, 10% to 20%, or 40% to 60% of the total thickness of the positive electrode composite layer.

[0072] By adjusting the total thickness and individual thicknesses of the positive electrode composite layers within the above ranges, the present invention can not only prevent a reduction in the energy density of the electrode, but also realize high adhesive strength between the positive electrode current collector and the positive electrode composite layer.

[0073] Furthermore, the positive electrode current collector provided in the positive electrode may be made of a material that has high conductivity without inducing chemical changes in the battery, such as stainless steel, aluminum, nickel, titanium, or calcined carbon. In the case of aluminum or stainless steel, it may be surface-treated with carbon, nickel, titanium, silver, or the like.

[0074] The average thickness of the current collector is preferably 5 to 500 μm, taking into consideration the conductivity and total thickness of the positive electrode to be produced.

[0075] <Electrode assembly for lithium secondary battery> In one embodiment, the present invention provides an electrode assembly for a lithium secondary battery, including the above-described positive electrode according to the present invention, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0076] The electrode assembly for a lithium secondary battery according to the present invention includes the above-described positive electrode of the present invention and can realize a large voltage deviation according to the state of charge (SOC) of the secondary battery. Therefore, the state of charge (SOC) of the secondary battery during use can be easily estimated and / or measured with high reliability. Furthermore, heat generated by the lithium composite metal oxide can be easily released to the outside, which has the advantage of further improving the safety of the secondary battery.

[0077] Here, the positive electrode has the same structure as the positive electrode for the lithium secondary battery of the present invention described above, and therefore, detailed description of the structure will be omitted.

[0078] In addition, the negative electrode includes a negative electrode composite layer prepared by applying a negative electrode active material to a negative electrode current collector, drying, and pressing the negative electrode active material in the same manner as the positive electrode, and may optionally further include a conductive material, a binder, other electrolyte additives, and the like, as needed.

[0079] In this case, the negative electrode active material may be one commonly used in the art, and specifically, may include one or more carbon-based negative electrode active materials selected from natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotubes, fullerene, activated carbon, acetylene black, and ketjen black.

[0080] The negative electrode mixture layer may also contain a binder to bond the negative electrode active material, conductive material, and other additives while providing adhesion to the negative electrode current collector. Examples of such binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used alone or in combination. The binder may be present in an amount of 1 to 10 parts by weight, specifically 2 to 8 parts by weight, or 1 to 5 parts by weight of the conductive material, based on the weight of the negative electrode mixture layer.

[0081] The negative electrode composite layer may have an average thickness of 100 μm to 200 μm, specifically 100 μm to 180 μm, 100 μm to 150 μm, 120 μm to 200 μm, 140 μm to 200 μm, or 140 μm to 160 μm.

[0082] The negative electrode may also include a negative electrode current collector that has high conductivity without inducing chemical changes in the battery. For example, the negative electrode current collector may be made of copper, stainless steel, nickel, titanium, calcined carbon, or the like. In the case of copper or stainless steel, it may also be surface-treated with carbon, nickel, titanium, silver, or the like. The negative electrode current collector, like the positive electrode current collector, may have a finely textured surface to strengthen its bonding with the negative electrode active material, and may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric. The average thickness of the negative electrode current collector may be preferably 3 to 500 μm, taking into account the conductivity and total thickness of the negative electrode to be manufactured.

[0083] The separator is a thin insulating membrane interposed between the positive and negative electrodes, exhibiting high ion permeability and mechanical strength. The separator may be any commonly used material in the art, including sheets or nonwoven fabrics made of chemically resistant and hydrophobic materials such as polypropylene, glass fiber, or polyethylene. In some cases, composite separators may be used, in which inorganic particles or organic particles are coated with an organic binder polymer on a porous polymer substrate such as a sheet or nonwoven fabric. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also function as the separator. The separator may have an average pore diameter of 0.01 to 10 μm and an average thickness of 5 to 300 μm.

[0084] <Lithium secondary battery> Furthermore, in one embodiment, the present invention provides a lithium secondary battery including the above-described electrode assembly according to the present invention, a battery case into which the electrode assembly is inserted, and an electrolyte composition injected into the battery case together with the electrode assembly.

[0085] The lithium secondary battery according to the present invention includes an electrode assembly including the above-described positive electrode of the present invention, and the electrode assembly may have a structure in which it is inserted into a battery case together with an electrolyte composition.

[0086] At this time, the electrode assembly has the same structure as the electrode assembly of the present invention described above, and therefore, a detailed description of the structure will be omitted.

[0087] In addition, examples of the electrolyte composition 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 producing lithium secondary batteries, but are not limited to these.

[0088] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0089] The organic solvent may be any solvent capable of serving as a medium through which ions involved in the electrochemical reaction of the battery can move. For example, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone, ether solvents such as dibutyl ether and tetrahydrofuran, ketone solvents such as cyclohexanone, aromatic hydrocarbon solvents such as benzene and fluorobenzene, dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can enhance the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, the cyclic carbonate and the chain carbonate are mixed in a volume ratio of approximately 1:1 to 9 to achieve excellent electrolyte performance.

[0090] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries, without particular limitation. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCFSO, LiC4F9SO, LiN(C2F5SO3), LiN(C2F5SO2), LiN(CF3SO2), LiCl, LiI, or LiB(C2O4)2.

[0091] The lithium salt can be used at a concentration in the range of 0.1 M to 2.0 M. When the lithium salt concentration is within the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0092] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. In this case, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0093] As described above, the lithium secondary battery including the positive electrode according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0094] Furthermore, the lithium secondary battery according to the present invention is not limited in shape depending on the intended use of the battery, and may be shaped according to cases commonly used in the art. For example, the lithium secondary battery may be a battery including a cylindrical or prismatic battery case using a can, a pouch-type battery case, or a coin-type battery case.

[0095] As one example, the lithium secondary battery may be a prismatic secondary battery including a prismatic can as a battery case.

[0096] The present invention will be described in more detail below with reference to examples and experimental examples.

[0097] 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 and experimental examples.

[0098] <Examples 1 to 6 and Comparative Examples 1 to 3. Production of Positive Electrodes for Lithium Secondary Batteries> N-methylpyrrolidone solvent was poured into a homomixer, and LiFePO4 (hereinafter referred to as "LFP") was added as the first positive electrode active material and LiNi was added as the second positive electrode active material to form the first to third positive electrode composite layers. 0.8 Co 0.1 Mn 0.1 O2 (hereinafter referred to as "NCM", average particle size: approximately 2 μm), carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were added. The mixture was then mixed at 3,000 rpm for 60 minutes to prepare a first positive electrode composite layer slurry, a second positive electrode composite layer slurry, and a third positive electrode composite layer slurry.

[0099] At this time, the slurry prepared for forming each positive electrode composite layer was prepared to contain 48.5 parts by weight of the positive electrode active material, 1 part by weight of the conductive material, and 0.5 parts by weight of the binder based on the solid content, and the content ratio (unit: parts by weight) of the first positive electrode active material and the second positive electrode active material contained in each slurry and the average particle size (unit: μm) of the first positive electrode active material were adjusted as shown in Table 1.

[0100] An aluminum thin plate (average thickness: 12 μm) was prepared as a positive electrode current collector, and the previously prepared first to third positive electrode composite layer slurries were sequentially cast onto the prepared aluminum thin plate, dried in a vacuum oven at 130°C, and rolled to prepare a positive electrode. The total thickness of the rolled positive electrode composite layer was 150 μm, and the thickness of the individual positive electrode composite layer was 50 μm for the three-layer structure and 75 μm for the two-layer structure.

[0101] [Table 1]

[0102] <Examples 7 to 12 and Comparative Examples 4 to 6. Manufacture of electrode assemblies and lithium secondary batteries> A negative electrode active material was prepared by mixing natural graphite and artificial graphite in a 1:1 weight ratio. 97 parts by weight of the negative electrode active material and 3 parts by weight of styrene-butadiene rubber (SBR) were mixed with water to form a negative electrode slurry, which was then cast onto a copper sheet as a negative electrode current collector. The copper sheet onto which the negative electrode slurry was cast was dried in a vacuum oven at 130°C and rolled to produce a negative electrode. The thickness of the negative electrode composite layer was 130 μm.

[0103] The fabricated negative electrode was placed opposite a positive electrode prepared in each of the examples and comparative examples, as shown in Table 2 below, with an 18 μm polypropylene separator interposed between them to fabricate an electrode assembly. Each fabricated electrode assembly was inserted into a prismatic battery case, and an electrolyte composition was injected into the battery case, after which the case was sealed to fabricate a prismatic lithium secondary battery. The electrolyte composition used was a solution prepared by mixing lithium hexafluorophosphate (LiPF6, 1.0 M) and vinyl carbonate (VC, 2 wt %) in a mixture of ethylene carbonate (EC): dimethyl carbonate (DMC): diethyl carbonate (DEC) = 1:1:1 (volume ratio).

[0104] [Table 2]

[0105] <Experimental Example> In order to evaluate the effect of the positive electrode for a lithium secondary battery according to the present invention, the following experiment was carried out.

[0106] a) Evaluation of the adhesive strength between the positive electrode current collector and the positive electrode composite layer The surface of the positive electrode composite layer of the positive electrodes prepared in the Examples and Comparative Examples was attached to a glass slide using double-sided tape. Double-sided tape was also attached to the surface of the positive electrode 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 so that it was connected longitudinally. The glass slide was then placed in the lower holder of a UTM device (LLOYD Instrument LF Plus), and the portion of the PET film attached to the positive electrode current collector was placed in the upper holder of the UTM device. A force of 300 mm / min at 25°C and a 180° angle was applied to measure the force required to peel the positive electrode current collector from the positive electrode composite layer (adhesion strength between the positive electrode current collector and the positive electrode composite layer). The results are shown in Table 3 below.

[0107] b) Evaluation of voltage deviation by SOC section The lithium secondary batteries manufactured in each of the examples and comparative examples were each charged and discharged three times, and the voltages corresponding to the SOC were measured. The voltage deviations generated when the SOC changed by 1% were calculated for each section. The charge and discharge cycles were carried out three times at a 0.1C rate with a cutoff potential of 2.8V and a maximum charge voltage of 3.6V. The voltage deviations were 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.

[0108] C) Evaluation of battery heat generation during charging and discharging The secondary batteries fabricated in the examples and comparative examples were overcharged, and the surface and internal temperatures of the overcharged secondary batteries were measured. Specifically, a thermal sensor was attached to the inside of each secondary battery case, and the battery was charged to 4.2 V. The charged battery was then overcharged to 10 V at a constant current of 1 A. The constant voltage of 10 V was then maintained for six hours. After six hours, the internal temperature of each battery was measured using the thermal sensor attached to the 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 used as the surface temperature of the battery during overcharge. The measurement results are shown in Table 3 below.

[0109] [Table 3]

[0110] As shown in Table 3, it can be seen that the positive electrode for a lithium secondary battery according to the present invention is excellent in battery safety, and the state of charge (SOC) can be easily estimated and / or measured.

[0111] The positive electrodes of the examples according to the present invention exhibited a high adhesive strength between the positive electrode current collector and the positive electrode composite layer of 49 gf / mm or more, and the internal and external temperatures of the battery during overcharge were low, below 55°C and below 41°C, respectively. This indicates that secondary batteries including the positive electrodes are highly safe. Furthermore, it was confirmed that the positive electrodes of the examples exhibited a voltage change per 1% SOC of 43.1 mV or more in the second section including a 50% SOC. In particular, in the case of a battery containing lithium iron phosphate (LiFePO4) as the positive electrode active material, the voltage change per 1% SOC was confirmed to be 11.8 mV or more and 2.9 mV or more, respectively, in the first and third sections, where voltage differences due to changes in SOC are slight. This indicates that the battery's SOC can be easily estimated with high reliability.

[0112] These results show that the positive electrode according to the present invention can realize a large voltage deviation depending on the state of charge (SOC) of the secondary battery, and therefore the state of charge (SOC) when used in a secondary battery can be easily estimated and / or measured with high reliability, and the heat generated by the lithium composite metal oxide can be easily released to the outside, thereby further improving the safety of the secondary battery.

[0113] Although the present invention has been described above with reference to preferred embodiments, it will be understood by those skilled in the art or those with ordinary knowledge in the art that various modifications and changes can be made to the present invention without departing from the spirit and technical scope of the present invention as set forth in the claims below.

[0114] Therefore, the technical scope of the present invention should not be limited to the content described in the Summary of the Invention of the specification, but should be defined by the claims. [Explanation of symbols]

[0115] 1: Positive electrode for lithium secondary batteries 10: Positive electrode current collector 20: Multilayered positive electrode composite layer 21: Individual positive electrode composite layer 21a: First positive electrode composite layer 21b: nth positive electrode composite layer

Claims

1. n (n≧2) positive electrode composite layers are located on a positive electrode current collector, The first positive electrode composite layer in contact with the surface of the positive electrode current collector includes a first positive electrode active material including an iron phosphate compound represented by the following Chemical Formula 1, and a second positive electrode active material including a lithium composite metal oxide represented by the following Chemical Formula 2: The (n-1)th positive electrode composite layer disposed on the first positive electrode composite layer includes a first positive electrode active material including an iron phosphate compound represented by Chemical Formula 1 below: [Chemical formula 1] LiFe ａ M １ １－ａ XO ４ [Chemical formula 2] Li ｘ [Ni ｙ Co ｚ Mn ｗ M ２ ｖ ]O ２ In the Chemical Formula 1 and the Chemical Formula 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, 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; A positive electrode for a lithium secondary battery, wherein x, y, z, w, and v are 1.0≦x≦1.30, 0.1≦y<1, 0≦z, 0≦w, 0≦v≦0.1, respectively, and y+z+w+v=1.

2. A positive electrode for a lithium secondary battery as described in claim 1, wherein the first positive electrode composite layer has a tendency for the concentration of the second positive electrode active material to decrease as it progresses from the surface in contact with the positive electrode current collector to the other surface.

3. The positive electrode for a lithium secondary battery according to claim 1 , wherein the second positive electrode active material is contained in an amount of less than 10 wt % based on the weight of the entire positive electrode mixture layer.

4. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the total thickness of the positive electrode mixture layer is 50 μm to 300 μm.

5. A positive electrode for a lithium secondary battery as described in 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 layers.

6. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode has a voltage change of 5 mV to 60 mV per 1% SOC in the range of 30% to 70% SOC when used in a secondary battery.

7. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode has a voltage change of 0.1 mV to 60 mV per 1% SOC in the range of 65% to 95% SOC when used in a secondary battery.

8. An electrode assembly for a lithium secondary battery, comprising the positive electrode according to any one of claims 1 to 7, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

9. the negative electrode includes a negative electrode mixture layer on a negative electrode current collector, 9. The electrode assembly for a lithium secondary battery of claim 8, wherein the negative electrode mixture layer comprises at least one carbon-based negative electrode active material selected from the group consisting of natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotubes, fullerene, and activated carbon.

10. The electrode assembly according to claim 8 . a battery case into which the electrode assembly is inserted; and A lithium secondary battery comprising an electrolyte composition injected into a battery case together with an electrode assembly.

11. The lithium secondary battery according to claim 10, wherein the lithium secondary battery is a prismatic secondary battery.

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