Ignition-suppressing lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery having an ignition suppression structure and a method for manufacturing the lithium secondary battery. More specifically, the present invention provides a method for suppressing ignition of a lithium secondary battery by positioning a high-capacity section including a high-capacity unit cell having a high energy density above and below a low-capacity section including a low-capacity unit cell having a relatively low energy density.

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery having an ignition suppression structure. More specifically, the present invention is characterized by providing a lithium secondary battery having a structure that can suppress ignition by adjusting the stacking position of unit cells with different energy densities.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0142601, dated October 25, 2021, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference. [Background technology]

[0003] A lithium secondary battery typically comprises a battery cell including a positive electrode, a negative electrode, an electrolyte, and a separator. A pouch cell, which is one type of battery cell, has a configuration in which an electrode assembly, in which a plurality of unit cells each including a positive electrode, a negative electrode, and a separator are stacked, is housed in a pouch-shaped battery case together with an electrolyte.

[0004] While the electrolyte and separator are important materials that affect the battery's performance, such as output and safety, it is the electrode active materials of the positive and negative electrodes that directly determine the battery's energy density. The positive electrode active material, in particular, has the greatest impact on the battery's energy density. While it is possible to increase the battery's energy density by reducing the thickness of the current collector, which supports the active materials, or the separator, this is merely an indirect method.

[0005] In current battery systems, lithium ions are inherently contained in the cathode. When a battery is charged, the lithium in the cathode is stored in a reduced state in the anode, made of materials such as graphite, while the cathode is oxidized, losing one electron for each lithium atom lost. Failure to buffer this change would result in destructive changes such as decomposition or phase transition of the cathode material, making repeated charging and discharging impossible. Therefore, elements that can buffer the redox process without destructive behavior are required, and transition metal elements with flexible oxidation states are included as chemical components of the cathode. Furthermore, a crystalline structure that allows reversible movement of lithium ions must be maintained, so a framework element that maintains the structure must also be included. Consequently, cathode materials for lithium secondary batteries are composed of three axes: lithium, which determines the charge capacity by moving between the cathode and anode, a transition metal that enables reversible redox reactions, and an oxide that acts as a framework to maintain the crystalline structure.

[0006] Generally, as the nickel content increases, the energy density in the normal voltage range increases, but the battery may become weaker in terms of stability.

[0007] Because the battery case acts as a thermal insulator, heat generated in the electrode assembly inside the battery case may not be able to be released to the outside, which may result in the lithium secondary battery igniting due to overheating. Specifically, as the heat generated in each unit cell is accumulated, the temperature inside the battery cell increases exponentially, causing the inside of the battery cell to overheat.

[0008] In recent years, there has been a trend toward increasing the energy density of lithium secondary batteries, and therefore, a method capable of suppressing the above-mentioned overheating phenomenon caused by the superposition of heat generated in the unit cells is desired. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Publication No. 10-2019-0024709 Summary of the Invention [Problem to be solved by the invention]

[0010] SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery having a structure capable of eliminating the overheating phenomenon caused by the superposition of heat generated in a plurality of unit cells.

[0011] Another object of the present invention is to provide a lithium secondary battery that has excellent thermal stability while maintaining a certain level of energy density. [Means for solving the problem]

[0012] In order to solve the above problems, the present invention provides a lithium secondary battery with an ignition suppression structure, comprising: an electrode assembly including a plurality of unit cells; and a battery case accommodating the electrode assembly, wherein the electrode assembly includes a low-capacity portion in the middle thereof and high-capacity portions located above and below the low-capacity portion, the low-capacity portion being stacked with low-capacity unit cells, and the high-capacity portion being stacked with high-capacity unit cells.

[0013] Furthermore, the low capacity portion may have a lower energy density than the high capacity portion.

[0014] The unit cell may include a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode.

[0015] Here, the positive electrode may contain either a nickel compound containing nickel or a non-nickel compound containing no nickel.

[0016] Specifically, the low capacity portion may be composed of low capacity unit cells containing a nickel compound in a positive electrode, and the nickel content of the nickel compound contained in the low capacity unit cells may be less than 60 mol % based on 100 mol % of the transition metal contained in the nickel compound. The high capacity portion may be composed of high capacity unit cells containing a nickel compound in a positive electrode, and the nickel content of the nickel compound contained in the high capacity unit cells may be 60 mol % or more based on 100 mol % of the transition metal contained in the nickel compound.

[0017] As another example, the low-capacity portion may be composed of low-capacity unit cells containing a non-nickel compound as a positive electrode active material, and the high-capacity portion may be composed of high-capacity unit cells containing a nickel compound as a positive electrode active material.

[0018] Specifically, the nickel content of the nickel compound contained in the high-capacity unit cell may be 60 mol % or more based on 100 mol % of the transition metal contained in the nickel compound.

[0019] Alternatively, the nickel content of the nickel compound contained in the high-capacity unit cell may be 60 mol % or less.

[0020] Specifically, the nickel compound may include an oxide of the following Chemical Formula 1:

[0021] [Chemical formula 1] Li x [Ni y Co z Mn w A v ]O2

[0022] A 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 <y<1、0<z≦0.6、0<w≦0.6、0≦v≦0.2である。

[0023] As an example, the nickel compound may be LiNiO2, LiNi 0.5 Co 0.5 O2, LiNi 0.6 Co 0.4 O2, LiNi 1 / 3 Co 1 / 3 Al 1 / 3 O2, LiNi 1 / 3 CO 1 / 3 Mn 1 / 3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2, and LiNi 0.7 CO 0.1 Mn 0.1 Al 0.1 The compound may be composed of one or more compounds selected from the group consisting of O2.

[0024] Specifically, the non-nickel compound may be any one of an oxide of the following Chemical Formula 2 and an oxide of the following Chemical Formula 3.

[0025] [Chemical formula 2] Li p CO 1-q D q O4

[0026] [Chemical formula 3] Li a Fe 1-b E b O4

[0027] In the above Chemical Formulas 2 and 3, x, y, z, w, and v are each such that 1.0 ≤ x ≤ 1.30, 0 ≤ y < 1, 0 < z ≤ 0.6, 0 < w ≤ 0.6, 0 ≤ v ≤ 0.2; D 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; p and q are each such that 5 ≤ p ≤ 7, 0 ≤ q ≤ 0.2; E is one or more selected from P, Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; and a and b are each such that 4 ≤ a ≤ 6, 0 ≤ b ≤ 0.5.

[0028] As an example, the non-nickel compound may be composed of one or more compounds selected from the group consisting of LiFePO4, Li6CoO4, Li6Co 0.5 Zn 0.5 O4, Li6Co 0.7 Zn 0.3 O4, Li2FeSiO4, Li5FeO4, and Li6FeO4.

[0029] Also, the ratio of the number of low-capacity unit cells constituting the low-capacity portion may be 20 to 40% based on the number of all unit cells included in the electrode assembly.

Advantages of the Invention

[0030] According to the present invention, even if heat is partially generated at a high temperature in the electrode assembly inside the battery cell, it has the effect of suppressing overheating and ignition phenomena of the lithium secondary battery.

Brief Description of the Drawings

[0031] [Figure 1] It shows the configuration of a general unit cell. [Figure 2] It shows a lithium secondary battery including a general electrode assembly and a battery case. [Figure 3] It shows the configuration of the electrode assembly of the present invention. [Figure 4]1 shows the structures of electrode assemblies manufactured in Examples and Comparative Examples. [Figure 5] 1 is a graph showing the measurement results of the initial capacity of secondary batteries manufactured in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0032] Hereinafter, detailed configurations of the present invention will be described in detail with reference to the accompanying drawings and various embodiments. The embodiments described below are shown as examples to facilitate understanding of the present invention, and the accompanying drawings are not drawn to actual scale, and the dimensions of some components may be exaggerated to facilitate understanding of the invention.

[0033] Because the present invention can be modified in various ways and can take various forms, specific embodiments are shown by way of example in the drawings and described in detail herein, but it is not intended to limit the invention to the particular forms disclosed, and it should be understood that the invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

[0034] The battery cell 1000 generally includes an electrode assembly in which a plurality of unit cells are stacked.

[0035] FIG. 1 shows the configuration of a unit cell 110' included in a typical battery cell.

[0036] Referring to FIG. 1, the unit cell 110′ is a stack of a positive electrode 111′, a negative electrode 112′, and a separator 113′ interposed between the positive electrode 111′ and the negative electrode 112′.

[0037] FIG. 2 shows an exaggerated view of the structure of a typical pouch-type battery cell 1000 among the battery cells 1000.

[0038] Referring to FIG. 2, the battery cell 1000 includes an electrode assembly 100′ in which a plurality of unit cells are stacked, a battery case 200′ that encases and houses the electrode assembly 100′, and an electrolyte 120′ that is poured and contained inside the battery case 200′.

[0039] A battery cell 1000 including a positive electrode 111' active material with high energy density may generate high-temperature heat internally. Due to its sealed structure, the inside of the battery cell 1000 may become excessively overheated, and in the worst case, the battery cell 1000 may catch fire. When high-temperature heat is generated in the positive electrode 111' active material included in each of the plurality of unit cells, the heat generated in each layer may be superimposed, and the inside of the battery case 200' may become an environment with a temperature higher than the ignition point that the battery cell 1000 can tolerate.

[0040] The lithium secondary battery of the present invention includes a battery cell 1000 including an electrode assembly 100 including a plurality of unit cells and a battery case 200 ′ that houses the electrode assembly 100 .

[0041] The unit cell includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a separator interposed between the positive electrode and the negative electrode. Although not shown, the positive electrode and the negative electrode are coated on a positive electrode current collector and a negative electrode current collector, respectively.

[0042] The positive electrode current collector and the negative electrode current collector of the present invention may be made of any material used in the technical field of lithium secondary batteries. The configuration of the current collector is not relevant to the features of the present invention, so for ease of understanding, the description will be omitted.

[0043] The positive and negative electrodes may further include a conductive material to improve electrical conductivity, and may further include a binder that helps the positive and negative electrode active materials to adhere to the current collectors.

[0044] The negative electrode active material of the present invention may be any common negative electrode active material used in the technical field of lithium secondary batteries, and is not particularly limited thereto. For example, the negative electrode active material may include graphite.

[0045] The separator of the present invention may be any common separator used in the technical field of lithium secondary batteries, and is not particularly limited thereto. The separator may be made of any insulating material having pores through which lithium ions can pass. For example, the separator may be made of polyethylene (PE) or polypropylene (PP).

[0046] The positive electrode active material of the present invention includes lithium, an oxide that acts as a framework to maintain the crystalline structure, and a transition metal that enables reversible oxidation-reduction reactions. Specifically, the transition metal includes one selected from the group consisting of nickel, cobalt, aluminum, manganese, iron, zinc, magnesium, copper, cerium, lanthanum, tungsten, vanadium, chromium, titanium, zirconium, indium, tantalum, yttrium, strontium, gallium, scandium, gadolinium, samarium, calcium, niobium, and molybdenum, and combinations thereof. More specifically, the positive electrode active material of the present invention may include any one or more of the oxides represented by the following Chemical Formula 1, Chemical Formula 2, and Chemical Formula 3:

[0047] [Chemical formula 1] Li x [Ni y Co z Mn w A v ]O2

[0048] [Chemical formula 2] Li p CO 1-q D q O4

[0049] [Chemical formula 3] Li a Fe 1-b E b O4

[0050] In the above Chemical Formulas 1 to 3, A 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; x, y, z, w, and v are 1.0≦x≦1.30, 0 <y<1、0<z≦0.6、0<w≦0.6、0≦v≦0.2であり、 D 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; p and q are 5≦p≦7 and 0≦q≦0.2, respectively; E is one or more selected from P, Al, Mg, Ni, CO, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; a and b are 4≦a≦6 and 0≦b≦0.5, respectively.

[0051] The lithium nickel cobalt oxide represented by the above chemical formula 1 is a composite metal oxide containing lithium and nickel, and is represented by LiNiO2, LiNi 0.5 Co 0.5 O2, LiNi 0.6 Co 0.4 O2, LiNi 1 / 3 Co 1 / 3 Al 1 / 3 O2, LiMnO2, LiNi 1 / 3 CO 1 / 3 Mn 1 / 3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.8 Co 0.3 Mn 0.2 O2 and LiNi 0.7 CO 0.1 Mn 0.1 Al 0.1 The compound may comprise one or more compounds selected from the group consisting of O2.

[0052] The oxides shown in the above chemical formula 2 are Li6CoO4, Li6CO 0.5 Zn 0.5 O4, Li6Co 0.7 Zn 0.3 O4, and the oxides represented by Chemical Formula 3 may include LiFePO4, Li2FeSiO4, Li5FeO4, Li6FeO4, and the like.

[0053] The battery cell 1000 of the present invention includes an electrode assembly 100 including a plurality of unit cells and a battery case 200 ′ that houses the electrode assembly 100 .

[0054] The battery cell 1000 of the present invention further includes an electrolyte in the battery case 200′. The electrolyte acts as a mediator that allows lithium cations to pass through the separator and exchange between the positive electrode active material and the negative electrode active material. The electrolyte may be in a solid or liquid form, and any common electrolyte used in the technical field of lithium secondary batteries will suffice. For example, in the case of a liquid electrolyte, the electrolyte may include a lithium salt that serves as a path for lithium cations to travel, a solvent for dissolving the lithium salt, and additives that are added to improve the characteristics of the battery cell 1000, such as its lifespan and output.

[0055] The electrode assembly 100 of the present invention includes a low-capacity section L including stacked low-capacity unit cells 110l and a high-capacity section H including stacked high-capacity unit cells 110h.

[0056] FIG. 3 shows a simplified configuration of an electrode assembly 100 included in a battery cell 1000 of the present invention.

[0057] Referring to FIG. 3, the electrode assembly 100 is formed by stacking a plurality of unit cells, each of which has a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode as a basic unit.

[0058] Specifically, a positive electrode, a separator, and a negative electrode are sequentially stacked to form the electrode assembly 100. An insulating film (or separator) to prevent short circuits may be further included between the unit cells, or a bipolar current collector having a positive electrode on one side and a negative electrode on the other side may be further included. Insulating films (or separators) may be further included at the bottom and top of the electrode assembly 100 to protect the active material and prevent direct contact between the battery case 200′ and the active material.

[0059] The unit cell of the present invention can be divided into a low-capacity unit cell 110l and a high-capacity unit cell 110h. Specifically, the unit cell is composed of a low-capacity unit cell 110l and a high-capacity unit cell 110h having a higher energy density than the low-capacity unit cell 110l.

[0060] The energy density of the unit cell is affected by the energy density of the positive electrode active material contained in the unit cell, i.e., the positive electrode active material contained in the low-capacity unit cell 110l has a lower energy density than the positive electrode active material contained in the high-capacity unit cell 110h.

[0061] The electrode assembly 100 of the present invention includes a low-capacity section L including stacked low-capacity unit cells 110l and a high-capacity section H including stacked high-capacity unit cells 110h.

[0062] The electrode assembly 100 of the present invention includes a low capacitance portion L in the middle and high capacitance portions H located above and below the low capacitance portion L.

[0063] The low-capacity section L and the high-capacity section H may each be composed of one or more unit cells. Most preferably, the low-capacity section L is composed of low-capacity unit cells 110l, and the high-capacity section H is composed of high-capacity unit cells 110h. However, this is not limiting, and even if the low-capacity section L includes a high-capacity unit cell 110h, it is sufficient that the energy density of the entire unit cells constituting the low-capacity section L is lower than the energy density of the entire unit cells constituting the high-capacity section H. Conversely, even if the high-capacity section H includes a low-capacity unit cell 110l, it is sufficient that the energy density of the entire unit cells constituting the high-capacity section H is higher than the energy density of the entire unit cells constituting the low-capacity section L.

[0064] Comparing the energy density of each region of the electrode assembly 100, the energy density of the low-capacity section L is preferably lower than the energy density of the high-capacity section H. To facilitate understanding, FIG. 4 shows the most preferable case in which the high-capacity section H of the electrode assembly 100 includes only high-capacity unit cells 110h, and the low-capacity section L includes only low-capacity unit cells 110l.

[0065] Generally, when a lithium secondary battery is charged and discharged or overcharged, the temperature inside the electrode assembly 100 rises. The temperature rise is caused by chemical reactions that occur in each unit cell. That is, when the battery is charged and discharged, chemical reactions between the positive and negative electrodes cause the generation and movement of electrons and lithium cations, generating heat as a by-product of the reaction.

[0066] Due to the unique structure of the battery cell 1000 in which the electrode assembly 100 is sealed in the battery case 200', there is a large temperature difference between the inside and outside of the battery cell 1000. Generally, the inside of the battery cell 1000 can be 40 to 80 degrees higher than the outside. In particular, the inside of the battery cell 1000 where the electrode assembly 100 is located can become so hot that the battery cell 1000 may catch fire in extreme situations such as overcharging.

[0067] Since the electrode assembly 100 includes a plurality of stacked unit cells that generate heat, the heat generated in each unit cell may be superimposed, and this superimposition of heat may cause the inside of the battery cell 1000 to be in a very high temperature environment. In particular, the heat superimposition becomes more severe toward the middle layer of the electrode assembly 100. Therefore, a method for alleviating the high temperature generated by the superimposition of heat is needed.

[0068] Generally, the energy density of a positive electrode active material significantly affects the amount of heat generated. In the present invention, the total energy density of the high-capacity unit cells 110h included in the high-capacity section H is greater than the total energy density of the low-capacity unit cells 110l included in the low-capacity section L, resulting in a greater temperature rise in the high-capacity section H. The energy density of the positive electrode active material is related to the heat resistance of the positive electrode active material. That is, a positive electrode active material with a higher energy density exhibits heat-vulnerable properties, while a positive electrode active material with a lower energy density exhibits heat-resistant properties. A positive electrode active material with heat-resistant properties is less sensitive to heat than a positive electrode active material with heat-vulnerable properties, and therefore, a unit cell including the heat-resistant positive electrode active material exhibits a relatively small temperature rise even under extreme conditions such as overcharging.

[0069] Consequently, the temperature rise of the low-capacity section L including the low-capacity unit cell 110l in the electrode assembly 100 is smaller than the temperature rise of the high-capacity section H including the high-capacity unit cell 110h.

[0070] A key feature of the present invention is that the high-capacity portions H are positioned above and below the low-capacity portions L, thereby mitigating heat buildup within the electrode assembly 100. That is, the high-capacity portions H, which have a high energy density, are positioned above and below the low-capacity portions L, which have a relatively low energy density, thereby allowing the low-capacity portions L to somewhat mitigate high-temperature heat generated in and built up in the high-capacity portions H. Preferably, one electrode assembly 100 includes one low-capacity portion L and two high-capacity portions H. In this case, it is preferable to design the number of high-capacity unit cells 110h so that the energy densities of the high-capacity portions H are the same. That is, the more the energy densities of the two high-capacity portions H are the same or similar, the more improved the temperature mitigation effect of the present invention.

[0071] The ratio of the number of low-capacity unit cells 110l constituting the low-capacity portion L of the present invention is preferably 20 to 40% based on the total number of unit cells included in the electrode assembly 100. In other words, the low-capacity portion L occupies 20 to 40% of the total electrode assembly 100. If the low-capacity unit cells 110l account for less than 20%, it is difficult to achieve the effect of mitigating heat generated by the high-capacity unit cells 110h inside the battery cell 1000. Also, if the low-capacity unit cells 110l account for more than 40%, the temperature mitigation effect can be achieved, but sufficient battery performance cannot be expected.

[0072] The positive electrode active material of the present invention can be divided into nickel compounds containing nickel and non-nickel compounds not containing nickel.

[0073] The low-capacity unit cell 110l and the high-capacity unit cell 110h of the present invention are distinguished from each other by the energy density of the positive electrode active material, as described above. Factors that affect the energy density of the positive electrode active material include the inclusion or non-inclusion of nickel and the nickel content.

[0074] That is, the positive electrode active material contained in the low capacity unit cell 1101 of the present invention preferably includes a nickel compound having a nickel content of less than 60 mol% based on 100 mol% of transition metals, or includes a non-nickel compound.

[0075] (First embodiment) The first embodiment relates to a case where the positive electrode active material of a low-capacity unit cell contains a nickel compound, more precisely, a case where the transition metal contained in the positive electrode active material of the present invention contains a nickel compound.

[0076] The nickel compound includes any one of nickel-cobalt-manganese (NCM), nickel-cobalt-aluminum (NCA), and nickel-cobalt-manganese-aluminum (NCMA).

[0077] The energy density of the positive electrode active material increases in proportion to the nickel content in the nickel compound. That is, when the nickel content in the nickel compound is high, the energy density of the unit cell increases, the heat resistance is relatively low, and the temperature rise is large. Conversely, when the nickel content in the nickel compound is low, the energy density of the unit cell decreases, the heat resistance is relatively high, and the temperature rise is small. As a result, the low-capacity portion L of the electrode assembly of the present invention has a relatively lower energy density, a higher heat resistance, and a smaller temperature rise than the high-capacity portion H.

[0078] The transition metal contained in the positive electrode active material of the low-capacity unit cell preferably contains less than 60 mol% nickel based on 100 mol% of the transition metal. If the nickel content exceeds 60 mol%, it may be difficult to fully expect the temperature mitigation effect.

[0079] In contrast to the low-capacity unit cell, the transition metal contained in the positive electrode active material of the high-capacity unit cell contains 60 mol% or more of nickel based on 100 mol% of the transition metal. If the nickel content of the positive electrode active material contained in the high-capacity unit cell is less than 60 mol%, it may be difficult to expect sufficient battery performance.

[0080] As a result, when heat is generated in the electrode assembly included in the battery cell 1000 due to charging and discharging, the low-capacity portion L can reduce the temperature inside the battery cell 1000. In addition, an effect of preventing the battery cell 1000 from catching fire in extreme situations such as overcharging can be obtained.

[0081] (Second embodiment) The second embodiment relates to a case where the positive electrode active material of the low-capacity unit cell includes a non-nickel compound. In this case, the positive electrode active material of the high-capacity unit cell included in the high-capacity section H includes a nickel compound.

[0082] The non-nickel compound includes any one of lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium iron phosphate (LFP) and lithium titanium oxide (LTO).

[0083] When the positive electrode active material included in the low-capacity unit cell includes a non-nickel compound, the energy density may be lower than that of a positive electrode active material including a nickel compound, and the positive electrode active material has high heat resistance and a small temperature rise.

[0084] When the positive electrode active material of the low capacity unit cell included in the low capacity portion L of the present invention contains the non-nickel compound, the low capacity portion L has a lower energy density than the high capacity portion H, and the temperature rise is smaller.

[0085] As a result, when heat is generated in the electrode assembly included in the battery cell 1000 due to charging and discharging, the low-capacity portion L can reduce the temperature inside the battery cell 1000. In addition, an effect of preventing the battery cell 1000 from catching fire in extreme situations such as overcharging can be obtained.

[0086] The present invention has been described in more detail above with reference to the drawings and embodiments, etc. However, the configurations described in the drawings or embodiments, etc. in this specification are merely one embodiment of the present invention and do not represent all of the technical ideas of the present invention, so it should be understood that there may be various equivalents and modifications that can replace them at the time of filing this application.

[0087] The present invention will be described in more detail below with reference to specific examples. However, these examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0088] <Examples 1 to 2 and Comparative Examples 1 to 4> As shown in Figure 4, six electrode assemblies each having 10 stacked unit cells were fabricated, and then the electrode assemblies were sealed in a pouch-type battery case to fabricate battery cells for Examples 1, 2, and Comparative Examples 1 to 4. The separator, electrolyte, anode, and current collector were all made of the same materials, with only the type of cathode active material and the number of low-capacity unit cells being different, as shown in Table 1 below. The positions of the unit cells stacked in each electrode assembly were designed as shown in Figures 4(a) to 4(f). However, the content of the cathode active material in the cathode of each unit cell was the same.

[0089] [Table 1] <Experimental Example>

[0090] In order to evaluate the performance of the secondary battery according to the present invention, the following experiment was carried out.

[0091] B) 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 2 below.

[0092] B) Initial capacity comparison Each fabricated battery was charged at 45°C in 1 / 3C constant current (CC) mode until the voltage reached 4.2V. It was then discharged at 0.33C constant current (CC) mode until the voltage reached 2.5V, and then further discharged in constant voltage (CV) mode until the current value decreased to 0.05% of the initial current value, and the initial discharge capacity was measured. The results are shown in Table 2 and Figure 5 below.

[0093] At this time, in order to objectively compare the initial capacity results of the above Examples and Comparative Examples, the initial capacity results for a secondary battery (symbol: reference) assembled using only NCM622 as the positive electrode active material are also shown in FIG. 5.

[0094] [Table 2]

[0095] As shown in Table 2 above, it can be seen that the secondary battery according to the present invention can minimize heat generation during charging and discharging.

[0096] Specifically, referring to the results of Table 2 and FIG. 5, Comparative Example 1 showed a lower internal cell temperature than Example 1, but the initial capacity of the secondary battery was very low. Comparative Example 2 showed an initial capacity of 60.26 Ah, which is almost the same as the initial capacity of Example 1, but the internal cell temperature was measured relatively high. Comparative Example 3 showed a slightly higher initial capacity than Example 1, but the internal cell temperature was measured somewhat higher. Comparative Example 4 showed a lower internal cell temperature than Example 1, but the initial capacity was the lowest, indicating somewhat inferior secondary battery performance. The initial capacity can be considered to be the result most significantly affected by the energy density of the secondary battery. Therefore, it can be seen that secondary batteries having the features of the present invention have excellent thermal stability without a significant decrease in energy density. [Explanation of symbols]

[0097] 1000: Battery cells 100, 100': Electrode assembly 200': Battery case 110, 110': unit cell 110h: High capacity unit cell 110L: Low volume unit cell 111, 111': Positive electrode 112, 112': Negative electrode 111c, 112c: Conductive material 113, 113': Separation membrane 120, 120': Electrolyte 200: Battery case H: High capacity part L: Low capacity part

Claims

1. an electrode assembly including a plurality of unit cells; a battery case that houses the electrode assembly, the electrode assembly includes a low-capacity portion in a middle portion and high-capacity portions located above and below the low-capacity portion, the low-capacity portion is composed of a low-capacity unit cell containing a non-nickel compound as a positive electrode active material, the high-capacity portion is composed of a high-capacity unit cell containing a nickel compound as a positive electrode active material, The non-nickel compound is any one of an oxide represented by the following Chemical Formula 2 and an oxide represented by the following Chemical Formula 3: [Chemical formula 2] Li p Co １－ｑ D ｑ O ４ [Chemical formula 3] Li ａ Fe １－ｂ E ｂ O ４ In Chemical Formula 2 and Chemical Formula 3, D 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; p and q are 5≦p≦7 and 0≦q≦0.2, respectively; E is one or more selected from P, Al, Mg, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; a and b are 4≦a≦6 and 0≦b≦0.5, respectively.

2. 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the low-capacity portion has a lower energy density than the high-capacity portion.

3. 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the unit cell includes a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode.

4. 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the nickel content of the nickel compound contained in the high-capacity unit cell is 60 mol % or more based on 100 mol % of the transition metal contained in the nickel compound.

5. 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the nickel content of the nickel compound contained in the high-capacity unit cell is 60 mol % or less based on 100 mol %.

6. 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the nickel compound comprises an oxide represented by the following Chemical Formula 1: [Chemical formula 1] Li ｘ [Ni ｙ Co ｚ Mn ｗ A ｖ ]O ２ A 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; x, y, z, w, and v are in the ranges of 1.0≦x≦1.30, 0<y<1, 0<z≦0.6, 0<w≦0.6, and 0≦v≦0.2, respectively.

7. The nickel compound is LiNiO ２ , LiNi ０．５ Co ０．５ O ２ , LiNi ０．６ Co ０．４ O ２ , LiNi １／３ Co １／３ Al １／３ O ２ , LiNi １／３ Co １／３ Mn １ / ３ O ２ , LiNi ０.８ Co ０.１ Mn ０.１ O ２ , LiNi ０.５ Co ０.３ Mn ０.２ O ２ , and LiNi ０.７ Co ０.１ Mn ０.１ Al ０.１ O ２ 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the lithium secondary battery is composed of one or more compounds selected from the group consisting of:

8. The non-nickel compound is Li ６ CoO ４ , Li ５ FeO ４ and Li ６ FeO ４ 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the lithium secondary battery is composed of one or more compounds selected from the group consisting of:

9. 2. The lithium secondary battery with an ignition suppression structure according to claim 1, wherein the low-capacity unit cells constituting the low-capacity portion account for 20 to 40% of the total number of unit cells included in the electrode assembly.

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