Cathode for lithium secondary battery and secondary battery comprising the same

Abstract

A cathode for a lithium secondary battery, comprising: a cathode current collector; and a cathode active material layer formed on the cathode current collector and comprising overlithiated oxide particles containing nickel and manganese, wherein the molar ratio of lithium to the total metal elements is greater than 1. The manganese content in a surface region of the overlithiated oxide particles is greater than the manganese content in a central region thereof. The nickel content in the central region of the overlithiated oxide particles is greater than the nickel content in their surface region.

Description

[Technical area]

[0001] The present disclosure relates to a cathode for a lithium secondary battery and a secondary battery including the same, and more particularly to a cathode for a lithium secondary battery having an uneven concentration distribution (ie, a concentration gradient) and a lithium secondary battery including the same. [State of the art]

[0002] Secondary batteries are batteries that can be repeatedly charged and discharged. They are often used as a power source for portable electronic devices such as mobile phones and laptops.

[0003] A lithium secondary battery has a high operating voltage and high energy density per unit weight, making it advantageous in terms of charging speed and lightweight construction. In this regard, the lithium secondary battery has been actively developed and applied in various industrial fields.

[0004] The lithium secondary battery can store electrical energy through a difference in chemical potential when lithium ions are intercalated and deintercalated between a cathode and an anode. Accordingly, the lithium secondary battery can use materials that can reversibly intercalate and deintercalate lithium ions as both the cathode and anode active materials.

[0005] As active cathode material, for example, lithium metal oxide particles with a layered crystal structure of ABO2 (e.g. lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), lithium nickel cobalt manganese oxide (NCM), lithium nickel aluminum manganese oxide (NCA), etc.) are used.

[0006] Because lithium secondary batteries are used in electric vehicles, lithium metal oxide particles with higher capacities than these lithium metal oxide particles are being researched and developed. For example, Korean Patent Publication No. 10-1369951 discloses the use of overlithiated oxide (OLO) particles to improve the capacity of lithium secondary batteries.

[0007] However, as the lithium content in the active material increases, the amount of lithium impurities remaining on the surface of the active material may increase, and the cathode density may decrease. Furthermore, cycling stability during repeated charge and discharge, as well as high-temperature stability, may deteriorate when a high voltage is applied to drive the overlithiated oxide. [Summary of the Invention][Problems to be Solved by the Invention]

[0008] An object of the present disclosure is to provide a cathode for a lithium secondary battery having high capacity and improved operational stability.

[0009] Another object of the present disclosure is to provide a lithium secondary battery with high capacity and improved stability. [Means of solving the problems]

[0010] A cathode for a lithium secondary battery comprises: a cathode current collector; and a cathode active material layer formed on the cathode current collector and comprising overlithiated oxide particles containing nickel and manganese, wherein the molar ratio of lithium to the total metal elements is greater than 1. The manganese content in a surface region of the overlithiated oxide particles is greater than the manganese content in a central region thereof. The nickel content in the central region of the overlithiated oxide particles is greater than the nickel content in their surface region.

[0011] In some embodiments, the ratio of manganese content in the surface region to manganese content in the central region may be in a range of 105% to 200%.

[0012] In some embodiments, the ratio of manganese content in the surface region to manganese content in the central region may be in a range of 105% to 150%.

[0013] In some embodiments, the ratio of the nickel content in the surface region to the nickel content in the middle region may be in a range of 50% to 95%.

[0014] In some embodiments, the ratio of the nickel content in the surface region to the nickel content in the middle region may be in a range of 60% to 95%.

[0015] In some embodiments, the central region and the surface region of the overlithiated oxide particles may have different lithium contents.

[0016] In some embodiments, the central region may have a relatively lower lithium content and the surface region may have a relatively higher lithium content.

[0017] In some embodiments, the overlithiated oxide particles may have a concentration gradient between the central region and the surface region.

[0018] In some embodiments, the overlithiated oxide particles may have a chemical structure represented by Formula 1 below: Li a [M x Ni y Mn z ]O b [Formula 1]

[0019] In Formula 1, M includes at least one of Co, Na, Ca, Y, Hf, Ta, B, Si, Ba, Ra, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi, and x, y, z, a and b satisfy 0≤x≤0.9, 0 <y≤0,9, 0,1≤z≤0,9, 1,8≤a+x+y+z≤2,2, 1,05≤a / (x+y+z)≤1,95, 1,8≤b≤2,2.

[0020] In some embodiments, the mole fraction of manganese to the total elements excluding lithium and oxygen in the overlithiated oxide particles may be 0.5 to 0.75.

[0021] In some embodiments, the central region may be a region extending radially outward from the center of the overlithiated oxide particle within a range of 0.5 µm, and the surface region may be a region extending inward from the outermost surface of the overlithiated oxide particles toward the center within a depth or thickness of 0.5 µm.

[0022] In some embodiments, the manganese and nickel content may be determined based on integrated peak intensity values ​​of manganese and nickel obtained by a line-scan EDS analysis performed along a straight line passing through the center of the cross-section of the overlithiated oxide particle exposed to the cross-section of the active cathode material layer, from one end to the other end of the particle.

[0023] In some embodiments, the overlithiated oxide particles may comprise at least one Li2MnO3 domain and one domain derived from the Li2MnO3 domain.

[0024] In some embodiments, the domain derived from the Li2MnO3 domain may comprise at least one domain selected from the group consisting of MnO2, Mn2O4, LiMnO2, LiMn2O4 and Li2Mn2O4.

[0025] A lithium secondary battery includes: the cathode for a lithium secondary battery according to the above-described embodiments; and an anode disposed opposite to the cathode. [Beneficial effects]

[0026] The cathode for a lithium secondary battery according to embodiments of the present disclosure may comprise a lithium metal oxide comprising an overlithiated oxide in a surface region and having a relatively higher manganese content in the surface region. The operating voltage of the lithium secondary battery may be increased by the lithium-rich composition, and the thermal stability of the lithium secondary battery may be improved by the increased manganese composition in the surface region.

[0027] In some embodiments, the lithium metal oxide may have a relatively lower nickel content in the surface region. Accordingly, the high-Ni composition in the mid-region may realize high-capacity properties while further improving operational stability in the surface region.

[0028] By designing the concentrations of the transition metal elements described above, a high-voltage and high-capacity secondary battery can be realized while improving high-temperature stability and cycle life. [Brief description of the characters] Fig. 1 is a schematic cross-sectional view showing a cathode for a lithium secondary battery according to exemplary embodiments. Fig. 2 is a diagram showing the concentration distribution of the cathode active material according to exemplary embodiments. Fig. 3 and Fig. 4 are schematic plan and cross-sectional views showing a lithium secondary battery according to exemplary embodiments. Fig. 5 and Fig.6 are scanning electron microscopy (SEM) cross-sectional images of active cathode material layers, including active cathode materials, prepared according to Examples 1 and 2. Fig. 7 to 10 are scanning electron microscopy (SEM) cross-sectional images of cathode active material layers including cathode active materials prepared according to Comparative Examples. [Mode for carrying out the invention]

[0029] According to exemplary embodiments of the present disclosure, a cathode for a lithium secondary battery is provided, comprising a cathode active material with an increased lithium content and a variation in metal concentration. Furthermore, a lithium secondary battery comprising the cathode is also provided.

[0030] Below, a cathode for a lithium secondary battery and a lithium secondary battery including the same according to exemplary embodiments of the present disclosure will be described in more detail with reference to the figures. However, the figures and embodiments are merely an example, and the present disclosure is not limited to the figures and embodiments described as examples. <Kathode für Lithium-Sekundärbatterie>

[0031] Fig. 1 is a schematic cross-sectional view showing a cathode for a lithium secondary battery according to exemplary embodiments.

[0032] As in Fig. 1, a cathode 100 for a lithium secondary battery may include a cathode current collector 105 and a cathode active material layer 110 formed on the cathode current collector 105.

[0033] For example, the active cathode material layer 110 may be formed on one or both surfaces of the cathode current collector 105.

[0034] The cathode current collector 105 may comprise, for example, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, preferably aluminum or an aluminum alloy.

[0035] The active cathode material layer 110 may comprise an active cathode material capable of reversibly intercalating and deintercalating lithium ions. The active cathode material may, for example, comprise lithium metal oxide particles.

[0036] The active cathode material may comprise lithium metal oxide particles. In exemplary embodiments, the lithium metal oxide particles may be in the form of overlithiated oxide particles.

[0037] According to exemplary embodiments, the overlithiated oxide particles may contain nickel and manganese, wherein the molar ratio of lithium to the total metal elements comprised in the overlithiated oxide particles may be greater than 1.

[0038] The active cathode material may comprise a plurality of overlithiated oxide particles. In some embodiments, the content of the overlithiated oxide particles may be 50 wt% or more, based on the total weight of the active cathode material, preferably 60 wt% or more, 70 wt% or more, 80 wt% or more, or 90 wt% or more.

[0039] In one embodiment, the active cathode material may consist essentially of the overlithiated oxide particles.

[0040] In one embodiment, the overlithiated oxide particle may comprise a Li2MnO3 domain (C2 / m space group) and a Li a M b O c-domain (R3m space group , where M is at least one of Co, Na, Ca, Y, Hf, Ta, B, Si, Ba, Ra, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi and a, b and c satisfy 1.8≤a+b≤2.2, 0.9≤a / b<1.05, and 1.9≤c≤2.1).

[0041] In some embodiments, in the overlithiated oxide particles, the molar ratio of the Li2MnO3 domain and the LiMO2 domain can be represented as w:1-w, where w can be 0.05 to 0.7 or 0.1 to 0.7.

[0042] The overlithiated oxide particles can be produced, for example, by a co-precipitation process.

[0043] For example, metal salts, a chelating agent (e.g., aqueous ammonia, ammonium carbonate, etc.), and a coprecipitant (e.g., sodium hydroxide, sodium carbonate, etc.) can be mixed and subjected to a coprecipitation reaction to produce metal hydroxide particles. The molar ratio between the metal salts can be adjusted, for example, according to the chemical formula of the desired overlithiated oxide particles.

[0044] For example, the metal hydroxide particles and the lithium source may be mixed and calcined such that the molar ratio between the number of moles of the lithium source and the number of moles of the metal hydroxide particles is in the range of 1.05 to 1.95, 1.1 to 1.95, 1.15 to 1.95, or 1.2 to 1.95, thereby producing the overlithiated oxide particles.

[0045] In one embodiment, the lithium source may comprise lithium hydroxide or lithium carbonate. In some embodiments, the lithium source may comprise lithium hydroxide.

[0046] In exemplary embodiments, the metal hydroxide particles as the nickel-manganese precursor and lithium hydroxide as the lithium precursor can be mixed to form a precursor mixture. The precursor mixture can then be subjected to a first and a second heat treatment to form overlithiated oxide particles.

[0047] In some embodiments, the first heat treatment may be performed at a temperature in the range of 200 to 300°C. The second heat treatment may be performed at a temperature in the range of 800 to 900°C.

[0048] As described above, the first heat treatment can be performed at a relatively low temperature to effect selective lithium intercalation into the surface region of the particles. Accordingly, the lithium concentration in the surface region can be increased. Subsequently, the second heat treatment can be performed at a high temperature to stabilize the resulting concentration gradient, yielding overlithiated oxide particles.

[0049] For example, the overlithiated oxide particles can be oxidized by applying a voltage of 4.4 V (vs Li / Li + ) or higher (e.g., 4.4 V to 4.8 V) to the overlithiated oxide particles (see Scheme 1-1 below). Alternatively, the overlithiated oxide particles can be activated by charging and discharging a lithium secondary battery comprising the overlithiated oxide particles at a voltage of 4.4 V (vs Li / Li +) or higher (see scheme 1-1 and 1-2 below).

[0050] In one embodiment, the activated particles may comprise a domain derived from a Li2MnO3 domain in the overlithiated oxide particles.

[0051] In some embodiments, the domain derived from the Li2MnO3 domain may comprise at least one of MnO2, Mn2O4, LiMnO2, LiMn2O4, and Li2Mn2O4.

[0052] For example, at least a portion of the Li2MnO3 in the overlithiated oxide particles can be converted into MnO2 and LiMnO2 by activation, as shown in Schemes 1-1 and 1-2 below. The MnO2 and LiMnO2 can reversibly intercalate and deintercalate lithium ions, as shown in Scheme 2. Accordingly, the overlithiated oxide particles can exhibit increased capacity. (charge) Li2MnO3→ MnO2+ 2Li+ + 1 / 2O2+ 2e - [Scheme 1-1] (Discharge) MnO2+ Li + + e -→ LiMnO2 [Scheme 1-2] (Charge) LiMnO2→ MnO2+ Li + + e - [Scheme 2] (Discharge) MnO2+ Li + + e - → LiMnO2

[0053] In some embodiments, the LiMnO2 in the activated particles may further react and be converted into Mn2O4, LiMn2O4 or Li2Mn2O4.

[0054] In some embodiments, the activated particles may a M b O c -domains and Li2MnO3 domains and / or domains derived from the Li2MnO3 domains.

[0055] In exemplary embodiments, the overlithiated oxide particles and / or the activated particles may be represented by Formula 1 below. Li a [M x Ni y Mn z ]O b [Formula 1]

[0056] In formula 1, M may comprise at least one of Co, Na, Ca, Y, Hf, Ta, B, Si, Ba, Ra, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi.

[0057] Where x, y, z, a and b 0≤x≤0.9, 0 <y≤0,9, 0,1≤z≤0,9, 1,8≤a+x+y+z≤2,2, 1,05≤a / (x+y+z)≤1,95, 1,8≤b≤2,2 erfüllen können.

[0058] In some embodiments, x may be in a range of 0 <x≤0,9, 0,05≤x≤0,9, 0,1≤x≤0,9, 0<x≤0,8, 0,05≤x≤0,8 oder 0,1≤x≤0,8 liegen.

[0059] In some embodiments, y may be in a range of 0 <y≤0,9, 0,05≤y≤0,9, 0,1≤y≤0,9, 0<y≤0,8, 0,05≤y≤0,8 oder 0,1≤y≤0,8 liegen.

[0060] In some embodiments, x, y, and z may satisfy 1.1≤a / (x+y+z)≤1.95, 1.15≤a / (x+y+z)≤1.95, 1.2≤a / (x+y+z)≤1.95, or 1.3≤a / (x+y+z)≤1.95.

[0061] In some embodiments, the mole fraction of manganese relative to the total elements excluding lithium and oxygen in the overlithiated oxide particles may be 0.5 to 0.75. For example, x, y, and z may satisfy 0.5≤z / (x+y+z)≤0.75.

[0062] In some embodiments, x, y, and z may satisfy 0.25≤(x+y) / (x+y+z)≤0.5.

[0063] In some embodiments, the mole fraction of cobalt relative to the total elements excluding lithium and oxygen in the overlithiated oxide particles may be 0 to 0.02. In one embodiment, the overlithiated oxide particles may not contain cobalt.

[0064] In some embodiments, b may be in a range of 1.9≤b≤2.1 or 1.95≤b≤2.05.

[0065] According to embodiments of the present disclosure, the central region and the surface region of the overlithiated oxide particles may have different manganese contents. The central region may have a relatively lower manganese content, while the surface region may have a relatively higher manganese content.

[0066] According to embodiments of the present disclosure, the ratio of the manganese content in the surface region to the manganese content in the central region, expressed as a percentage, may be in the range of 105% to 200%.

[0067] The term “content” as used herein may refer to the concentration or molar ratio of the metal element to the total metal elements in the chemical structure of the lithium metal oxide particles or the overlithiated oxide particle.

[0068] In some embodiments, the ratio of the manganese content in the surface region to the manganese content in the center region, expressed as a percentage, may be in a range of 105% to 150%. Preferably, the ratio of the manganese content may be in a range of 105% to 140%, 107% to 140%, 107% to 138%, or 107% to 136%. Within the above range, high-temperature stability can be effectively improved without deteriorating the rate characteristics of the lithium secondary battery.

[0069] As described above, increasing the manganese content in the surface region can improve the chemical stability of the active cathode material in the surface region.

[0070] For example, the Li2MnO3 domain in the surface region can be easily concentrated due to the increased manganese content. Accordingly, the activation of the Li2MnO3 region in the surface region can occur rapidly. Therefore, the activation that causes instability in the active cathode material can be preferentially and quickly completed, thereby improving the chemical stability and cycle life of the active cathode material.

[0071] Furthermore, the local concentration of the thermally stable Li2MnO3 domain in the surface region can be increased, thereby improving the thermal stability of the cathode and the secondary battery.

[0072] The central region may refer to a region extending outward from the center of the lithium metal oxide particles or the overlithiated oxide particles to a predetermined radial distance. The surface region may refer to a region extending inward from the outermost surface of the lithium metal oxide particles or the overlithiated oxide particles toward the center to a predetermined depth or thickness.

[0073] For example, the central region may be a region extending radially outward from the center within a range of 0.5 µm. The surface region may be a region extending from the outermost surface inward toward the center with a depth or thickness of 0.5 µm.

[0074] In some embodiments, the central region and the surface region of the overlithiated oxide particle may have different nickel contents. The central region may have a relatively higher nickel content, while the surface region may have a relatively lower nickel content.

[0075] According to embodiments of the present disclosure, the ratio of the nickel content in the surface region to the nickel content in the middle region may be in the range of 50% to 95%.

[0076] In some embodiments, the nickel content in the surface region to the nickel content in the center region, expressed as a percentage, may be in a range from 60% to 95%. Preferably, the ratio of the nickel content may be in a range from 60% to 90%, 60% to 89%, or 60% to 88%.

[0077] As described above, increasing the nickel content in the mid-region can improve the chemical stability of the active cathode material in the surface region. Accordingly, nickel can increase capacity in the mid-region, and the increased manganese content in the surface region can facilitate early activation, thus contributing to particle stabilization and thermal stability.

[0078] In one embodiment, the central region and the surface region of the overlithiated oxide particle may have different lithium contents. The central region may have a relatively lower lithium content, and the surface region may have a relatively higher lithium content.

[0079] Increasing the lithium content in the surface region, in combination with the increased manganese content in the surface region, can further facilitate the activation of the Li2MnO3 domain in the surface region. Accordingly, the activation rate can be increased, thereby more effectively promoting the early stabilization of the active cathode material.

[0080] Fig. 2 is a diagram showing the concentration distribution of the cathode active material according to exemplary embodiments.

[0081] As in Fig. 2, the metal elements of the lithium metal oxide particles or the overlithiated oxide particles may have a concentration gradient.

[0082] In some embodiments, lithium may have a concentration gradient that increases from the center of the particle to the surface region. Manganese may have a concentration gradient that increases from the center of the particle to the surface region. Nickel may have a concentration gradient that decreases from the center of the particle to the surface region.

[0083] As in Fig.As shown in Figure 2, the concentration of lithium relative to nickel, manganese, and lithium may be highest in both the central region and the surface region, and the concentration of manganese may be higher than the concentration of nickel. In some embodiments, the concentration difference between manganese and nickel in the central region may be lower than the concentration difference between manganese and nickel in the surface region. In some embodiments, the concentration difference between lithium and manganese in the surface region may be lower than the concentration difference between manganese and nickel in the surface region.

[0084] Fig. Figure 2 shows the concentration gradient schematically to facilitate description, and the concentration changes or concentration gradient profiles of the metal elements are not necessarily related to the Fig. 2 shown.

[0085] In some embodiments, the lithium metal oxide particle or the overlithiated oxide particle may have a secondary particle shape. The overlithiated oxide particle may have a shape in which a plurality of primary particles are aggregated together. For example, 30, 40, 50, 80, or 100 or more primary particles may be aggregated together in one overlithiated oxide particle.

[0086] In some embodiments, the overlithiated oxide particles (the secondary particles) may have an average particle diameter D50 of 2 µm to 9 µm. The average particle diameter D50 may refer to a particle diameter at the 50% point of the volume particle size distribution and may be measured using a laser diffraction method.

[0087] In some embodiments, the overlithiated oxide particles may have a specific surface area (BET) of 2.1 m 2 / g or more. Preferably, the overlithiated oxide particles have a specific surface area (BET) of 2.2 m 2 / g or more, 2.4 m 2 / g or more, or 2.5 m 2 / g or more. Preferably, the overlithiated oxide particles have a specific surface area (BET) of 2.6 m 2 / g or more, e.g. 3.0 m 2 / g or less.

[0088] Within the above-mentioned specific surface area range, the initial activation of the overlithiated oxide particles can be facilitated, further promoting the formation of stable Li2MnO3 domains on the surface region.

[0089] In some embodiments, M in Formula 1 may be included as a dopant in the overlithiated oxide particles. For example, at least one element of Co, Na, Ca, Y, Hf, Ta, B, Si, Ba, Ra, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi may be included as a dopant in the chemical or crystalline structure of the overlithiated oxide particles having a chemical structure of Li-Ni-Mn-O according to Formula 1.

[0090] In some embodiments, a coating may be formed on the overlithiated oxide particles. The coating may contain B, Al, W, Zr, Ti, Mg, and / or Co as a coating element.

[0091] The concentration or gradient of the metal element in the lithium metal oxide particles or the overlithiated oxide particles described above can be analyzed by energy-dispersive X-ray spectroscopy (EDS). For example, the concentration or gradient of the metal element can be measured or calculated using line-scan EDS analysis.

[0092] For example, line-scan EDS analysis can be performed along a straight line passing through the center of the particle on the cross-section of the particle from one end to the other end of the particle.

[0093] The metal element content in the surface region of the particle can be measured based on the integrated peak intensity values ​​of the metal elements in the surface region of the particle. The metal element content in the center region of the particle can be measured based on the integrated peak value of the metal element in the center region of the particle.

[0094] The manganese and nickel content can be measured, for example, by line-scan EDS analysis using the cross-section of the lithium metal oxide particles or the overlithiated oxide particles exposed to the cross-section of the active cathode material layer 110.

[0095] In one embodiment, when the manganese content is measured by line-scan EDS analysis by randomly selecting three lithium metal oxide particles exposed to the cross-section of the cathode active material layer 110, at least one of the particles may satisfy the manganese content ratio described above.

[0096] In one embodiment, when the manganese content is measured by line-scan EDS analysis by randomly selecting three lithium metal oxide particles exposed to the cross-section of the cathode active material layer 110, two or three of the particles may satisfy the manganese content ratio described above.

[0097] The active cathode material layer 110 may further comprise a binder and a conductive material.

[0098] In one embodiment, the binder may comprise an organic binder such as polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, or polymethyl methacrylate, etc., or an aqueous binder such as styrene-butadiene rubber (SBR). In some embodiments, the binder may be used in conjunction with a thickener such as carboxymethylcellulose (CMC).

[0099] The conductive material may include carbon-based conductive materials such as graphite, carbon nanotubes (CNT), carbon black, and graphene, etc.; metal-based conductive materials, including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3, etc.

[0100] In one embodiment, the active material loading of the active cathode material layer 110 may be 5 to 28 mg / cm 2 , 7 to 25 mg / cm 2 , 8 to 20 mg / cm 2 or 9 to 15 mg / cm 2 be.

[0101] In one embodiment, the density of the active cathode material layer 110 may be 2.5 to 3.8 g / cc, 2.6 to 3.7 g / cc, or 2.7 to 3.6 g / cc. <Lithium-Sekundärbatterie>

[0102] Fig. 3 and Fig. 4 are schematic plan and cross-sectional views showing a lithium secondary battery according to exemplary embodiments. Fig. 4 is a cross-sectional view on line II' of Fig. 3 in the thickness direction of the battery.

[0103] In Fig. 4, a separation membrane 140 is illustrated as being separated and spaced in the thickness direction for simplicity, but the separation membrane 140 may be continuously extended and wound.

[0104] As in Fig. 3 and Fig. 4, the lithium secondary battery may include an electrode assembly 150 and a casing 160 in which the electrode assembly 150 is housed.

[0105] The electrode assembly 150 may include the cathode 100 and an anode 130 disposed opposite the cathode 100.

[0106] In one embodiment, the electrode assembly 150 may include a plurality of cathodes and a plurality of anodes arranged alternately and repeatedly.

[0107] In one embodiment, the electrode assembly 150 may further include the separation membrane 140 disposed between the cathode 100 and the anode 130.

[0108] The lithium secondary battery may include the cathode 100 described above according to the embodiments of the present disclosure. As described above, the cathode 100 may include the cathode current collector 105 and the cathode active material layer 110. For example, the cathode active material layer 110 may be formed on one surface or on both surfaces (top and bottom) of the cathode current collector 105.

[0109] The active cathode material layer 110 may comprise the active cathode material described above according to embodiments of the present disclosure. The active cathode material may comprise the lithium metal oxide particles described above, and the lithium metal oxide particles may comprise overlithiated oxide particles.

[0110] When overlithiated oxide particles are used as the active cathode material, a voltage drop may occur during the operation of the lithium secondary battery. As described above, by changing the manganese concentration, the activation of the active cathode material at the surface region can be completed quickly, thereby promoting the stabilization of the active cathode material at an early stage of the lithium secondary battery's operation.

[0111] Therefore, a lithium secondary battery with improved high-temperature stability and cycle life can be achieved even under high-voltage charging and discharging processes.

[0112] The anode 130 may include an anode current collector 125 and an anode active material layer 120. The anode active material layer 120 may, for example, be formed on one or both surfaces of the anode current collector 125.

[0113] The anode active material layer 120 may, for example, comprise an anode active material, the binder, and the conductive material.

[0114] The anode current collector 125 may comprise, for example, gold, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, preferably copper or a copper alloy.

[0115] In one embodiment, the active anode material may comprise a material capable of intercalating and deintercalating lithium ions. The active anode material may comprise, for example, a lithium alloy, a carbon-based active material, a silicon-based active material, etc.

[0116] The lithium alloy may, for example, further comprise aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, and the like.

[0117] The carbon-based active material may include, for example, crystalline carbon, amorphous carbon, carbon composites, carbon fibers, and the like.

[0118] The amorphous carbon may include, for example, hard coal, coke, mesocarbon microbeads, mesophase pitch-based carbon fibers, and the like.

[0119] The crystalline carbon may include, for example, natural graphite, artificial graphite, graphite coke, graphite MCMB, graphite MPCF and the like.

[0120] The silicon-based active material can be, for example, Si, SiO (x) (0 <x<2), Si / C, SiO / C, Si-Metall und dergleichen umfassen.

[0121] In some embodiments, the anode 130 may have a larger area than that of the cathode 100.

[0122] In one embodiment, the cathode current collector 105 may include a cathode tab 106 protruding from a side of the cathode current collector 105.

[0123] The cathode tab 106 may, for example, be formed integrally with the cathode current collector 105 or connected to the cathode current collector 105 by welding, etc. The cathode current collector 105 and the cathode lead 107 may be electrically connected to each other via the cathode tab 106.

[0124] In one embodiment, the anode current collector 125 may include an anode tab 126 extending from one side of the anode current collector 125.

[0125] The anode tab 126 may, for example, be formed integrally with the anode current collector 125 or electrically connected to the anode current collector 125 by welding, etc. The anode current collector 125 and the anode lead 127 may be electrically connected to each other via the anode tab 126.

[0126] The separation membrane 140 may, for example, comprise a porous polymer film made of a polyolefin polymer such as polyethylene, polypropylene, ethylene-butene copolymer, ethylene-hexene copolymer, or ethylene-methacrylate copolymer. The separation membrane 140 may, for example, comprise a nonwoven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, or the like.

[0127] For example, the electrode assembly 150 and the electrolyte may be housed together in the pocket 160 to form a lithium secondary battery.

[0128] In one embodiment, the electrolyte may comprise a lithium salt and an organic solvent.

[0129] In one embodiment, the lithium salt may be Li + X - For example, X - at least one of F - , Cl - , Br, I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C-, CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN -and (CF3CF2SO2)2N - be.

[0130] In one embodiment, the organic solvent may include a carbonate solvent such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc.; an ester solvent such as methyl propionate, ethyl propionate, ethyl acetate, propyl acetate, butyl acetate, butyrolactone, caprolactone, valerolactone, etc.; an ether solvent such as dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), tetrahydrofuran (THF), etc.; an alcohol solvent such as ethyl alcohol, isopropyl alcohol, etc.; a ketone solvent such as cyclohexanone, etc.; an aprotic solvent such as an amide solvent (e.g., dimethylformamide), a dioxolane solvent (e.g., 1,3-dioxolane), a sulfolane solvent, a nitrile solvent, etc.

[0131] In exemplary embodiments, the upper limit of the operating voltage of the lithium secondary battery may be 4.5 V or less, based on the oxidation-reduction potential of lithium (i.e., a redox potential) (vs Li / Li + ). For example, the lithium secondary battery can operate in a voltage range of 4.5 V (vs Li / Li + ) or less.

[0132] The "upper limit of operating voltage" refers to the upper voltage reached during actual operation of the lithium secondary battery (i.e., during real-life use) and can be distinguished from the activation voltage during an activation process in a lithium secondary battery manufacturing process. Accordingly, the voltage drop and energy reduction of the lithium secondary battery can be suppressed.

[0133] In one embodiment, the lower limit of the operating voltage of the lithium secondary battery may be 1.8 V or more, 1.9 V or more, or 2.0 V or more, based on the oxidation-reduction potential of lithium (vs Li / Li + ). In some embodiments, the lower limit of the operating voltage may be 1.8 V to 2.2 V.

[0134] In some embodiments, the operating voltage range (i.e., the operating voltage range) of the lithium secondary battery may be 2 V (vs Li / Li + ) to 4.5 V (vs Li / Li + ).

[0135] Preferred examples and comparative examples of the present disclosure are described below. However, the following examples are merely exemplary and should not be construed as limiting the scope of the present disclosure. Example 1(1) Preparation of overlithiated oxide particles

[0136] Distilled water, deoxygenated from dissolved oxygen, was added to a sealed reactor, and NiSO4·6H2O and MnSO4·H2O were added in a predetermined molar ratio as nickel and manganese precursors, respectively (a first metal source mixture).

[0137] NaOH (precipitant) and NH4OH (chelating agent) were additionally introduced into the reactor to maintain the pH value in the range of 10 to 12 and to create a N2 atmosphere.

[0138] While the co-precipitation reaction was being conducted, a second metal source mixture was added in which the molar ratio of the nickel precursor was reduced and the molar ratio of the manganese precursor was increased compared to the initial precursors. The co-precipitation reaction was continued to produce metal hydroxide particles with a concentration gradient (Mn-rich in the surface region, Ni-rich in the middle region). The total duration of the co-precipitation reaction was 60 hours.

[0139] The metal hydroxide particles were washed and dehydrated using a filter press. The dehydrated metal hydroxide particles were dried at 120 °C for more than one day and classified to obtain nickel-manganese precursor powder.

[0140] The nickel-manganese precursor powder, the metal hydroxide particles, and lithium hydroxide (as lithium precursor) were placed in a calcination furnace to form a precursor mixture, and a heat treatment was carried out.

[0141] Specifically, lithium hydroxide was weighed and mixed so that the molar ratio of lithium to total metal in the precursor mixture was 1.2 or more. The calcination furnace was heated to 250 °C at a heating rate of 2 °C / min, and the first heat treatment was performed while maintaining the temperature at 250 °C for 3 hours.

[0142] The temperature of the calcination furnace was then increased to 850 °C at a rate of 2 °C / min, and the second heat treatment was carried out while maintaining the temperature at 850 °C for 8 hours. During both the first and second heat treatments, oxygen was continuously supplied to the calcination furnace at a flow rate of 10 ml / min.

[0143] The heat-treated product was naturally cooled to room temperature, then pulverized and classified to produce overlithiated oxide particles.

[0144] Analysis of the overlithiated oxide particles by ICP (normalization of the number of oxygen atoms to 2) confirmed that particles with a composition of Li 1,11 Ni 0,34 Mn (0,55) O2 has been formed. (2) Manufacture of a preliminary lithium secondary battery (hemispherical cell)

[0145] The overlithiated oxide particles, carbon black, and PVDF were mixed in a mass ratio of 92:5:3 and then dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a cathode slurry.

[0146] The cathode slurry was applied to an aluminum foil, then dried and pressed to produce a cathode with a layer of cathode active material formed thereon. During the cathode preparation, the loading amount of the cathode active material layer was set to 11 mg / cm 2 and the density of the cathode active material layer was set to 2.8 g / cc. Lithium metal was used as the counter electrode (anode).

[0147] The cathode and the anode were laminated by notching into circular shapes with a diameter of Φ14 and Φ16, respectively, and a separation membrane (PE, thickness: 13 μm) notched into Φ19 was inserted between the cathode and the anode to form an electrode assembly.

[0148] The electrode assembly was inserted into a button cell case (2016 standard), and an electrolyte was injected into the button cell case to prepare a preliminary lithium secondary battery.

[0149] The electrolyte used here was prepared by dissolving a 1M LiPF6 solution in a solvent mixture of EC / EMC (30 / 70, v / v). (3) Production of lithium secondary batteries (activation stage of preliminary lithium secondary battery)

[0150] The preliminary lithium secondary battery was subjected to CC / CV charge (0.1C constant current, CC section CUT-OFF condition: 4.6V, CV section CUT-OFF condition: 0.05C) and CC discharge (0.1C constant current, 2.0V CUT-OFF) at 25°C.

[0151] The charge and discharge cycles were repeated twice to activate the overlithiated oxide particles. Example 2

[0152] Overlithiated oxide particles (Li 1,11 Ni0,34 Mn (0,55) O2) and a lithium secondary battery were prepared in the same manner as in Example 1, except that the timing of introduction of the second metal source mixture was changed to form a steeper concentration gradient. Comparison example 1

[0153] Overlithiated oxide particles and a lithium secondary battery were prepared in the same manner as in Example 1, except that the molar ratios of nickel precursor, manganese precursor, and lithium precursor were changed so that the composition of the overlithiated oxide particles was Li 1,15 Ni 0,30 Mn (0,55) O2, and that a single metal source mixture was used during the co-precipitation reaction, thus preventing the formation of a concentration gradient of nickel and manganese. Comparison example 2

[0154] Overlithiated oxide particles and a lithium secondary battery were prepared in the same manner as in Example 1, except that the molar ratios of the nickel precursor, the manganese precursor, and the lithium precursor were changed so that the composition of the overlithiated oxide particles was Li 1,09 Ni 0,39 Mn (0,52) O2, and that a single metal source mixture was used during the co-precipitation reaction, thus preventing the formation of a concentration gradient of nickel and manganese. Comparison example 3

[0155] Overlithiated oxide particles and a lithium secondary battery were prepared in the same manner as in Example 1, except that the molar ratios of the nickel precursor, the manganese precursor, and the lithium precursor were changed so that the composition of the overlithiated oxide particles was Li 1,20 Ni 0,22 Mn (0,58)O2, and that a single metal source mixture was used during the co-precipitation reaction, thus preventing the formation of a concentration gradient of nickel and manganese. Comparison example 4

[0156] Overlithiated oxide particles and a lithium secondary battery were prepared in the same manner as in Example 1, except that the molar ratios of the nickel precursor, the manganese precursor, and the lithium precursor were changed so that the composition of the overlithiated oxide particles was Li 1,13 Ni 0,29 Mn (0,58) O2, and that a single metal source mixture was used during the co-precipitation reaction, thus preventing the formation of a concentration gradient of nickel and manganese.

[0157] The average particle diameter D50 and the specific surface area (BET) of the overlithiated oxide particles of the examples and the comparative examples were measured and are described together in Table 1 below.

[0158] The specific surface area was calculated as surface area per unit mass by measuring the physical adsorption of gas molecules on the solid surface according to ISO 9277 using a 3-Flex Adsorption Analyzer System. [TABLE 1] Average overall composition of the overlithiated oxide particles Molar ratio of lithium to all metal elements (Li / Me) D50 (µm) BET (m 2 / g) Example 1 Li 1,11 Ni 0,34 Mr (0,55) O2 1,25 7,1 2,66 Example 2 Li 1,11 Ni 0,34 Mr (0,55) O2 1,25 7,0 2,62 Comparison example 1 Li 1,15 Ni 0,30 Mr (0,55) O2 1,35 6,6 2,46 Comparison example 2 Li 1,09 Ni 0,39 Mr (0,52) O2 1,2 3,9 1,5 Comparison example 3 Li 1,20 Ni 0,22 Mr (0,58) O2 1,5 4,0 1,5 Comparison example 4 Li 1,13 Ni 0,29 Mr (0,58) O2 1,3 5,9 5,0 Experimental example(1) Analysis of the concentrations of manganese and nickel

[0159] Three cathode-active material particles were randomly sampled from the SEM cross-sections of the cathode-active material layers prepared according to the examples and comparative examples. The manganese and nickel contents in the central and surface regions of each of the three selected particles were measured using line-scan EDS instruments.

[0160] Specifically, the levels were measured by line scanning from one end to the other end of each particle along a straight line passing through the center of each particle.

[0161] The integrated peak intensity values ​​of nickel and manganese were measured in a region extending radially inward from one end toward the center within a depth of 0.5 µm (a first surface region) and in a region extending radially inward from the other end toward the center within a depth of 0.5 µm (a second surface region). The manganese and nickel contents in the surface region were determined by summing the integrated values ​​from the first surface region and the second surface region.

[0162] In addition, the contents of manganese and nickel in the center region were determined based on the integrated peak intensity values ​​of manganese and nickel from a point located 0.5 µm toward one end to a point located 0.5 µm toward the other end from the center of the particle.

[0163] The conditions for line-scan EDS analysis described above are as follows. i) Name of the device: Apreo 2S (FEI) ii) Acceleration voltage: 10 kV iii) FlatQuad detector, working distance 11 to 12 mm

[0164] Fig. 5 and Fig. 6 are scanning electron microscopy (SEM) cross-sectional images of active cathode material layers including active cathode materials prepared according to Examples 1 and 2, respectively. Fig. 7, Fig. 8, Fig. 9 and Fig. 10 are scanning electron microscopy (SEM) cross-sectional images of cathode active material layers including cathode active materials prepared according to Comparative Examples 1, 2, 3, and 4.

[0165] Fig. Figures 5 to 10 show the number of particles on which the line-scan EDS described above was performed, along with the line-scan directions given for each particle. (2) Thermal stability analysis (DSC analysis)

[0166] Using a DSC 3 thermal analysis system (product of METTLER TOLEDO), the overlithiated oxide particles of the examples and comparative examples were analyzed by differential scanning calorimetry (DSC) to measure the onset temperature at which the exothermic peak occurs.

[0167] Specifically, the lithium secondary batteries of the examples and comparative examples were charged with CC / CV (1C constant current, 4.6V and 0.05C cut-off), and then the exothermic peak onset temperature of the cathode active material layer was measured by DSC analysis. (3) Evaluation of high-temperature storage properties at 60 °C

[0168] The lithium secondary batteries of the examples and comparative examples were charged at room temperature CC-CV (1C constant current, 4.6V) and then stored in air at 60°C for 16 weeks using a sealed thermostatic device. After high-temperature storage, the batteries were discharged at a CC rate of 0.5C (2.7V cutoff), and then the discharge capacity was measured (discharge capacity after high-temperature storage). The capacity retention rate was calculated by expressing the discharge capacity after high-temperature storage as a percentage of the capacity measured before high-temperature storage (initial capacity).

[0169] The analysis results according to Example 1 are shown in Table 2. [TABLE 2] Example 1 Line-scan EDS Measuring point 1 Measuring point #2 Measuring point #3 Temperature of the exothermic DSC peak onset Maintains storage capacity at high temperatures (60 °C) Length of the major axis (L) (µm) 11,1 10,8 6,1 First surface area (0 ~ 0.5 µm) Integrated Mn peak intensity 1,613 1,544 1,524 252 °C 92 % Second surface area [(L-0.5 µm) ~ L] Integrated Mn peak intensity 2,278 1,788 1,835 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 3,891 3,332 3,359 Mid-range[(Center-0.5 µm) - (Center+0.5 µm)] Integrated Mn peak intensity 2,857 3,106 3,092 Ratio of Mn content between surface area and middle area (%) 136 % 107 % 109 % First surface area (0 - 0.5 µm) Integrated Ni peak intensity 1,037 0,524 1,047 Second surface area [(L-0.5 µm) ~ L] Integrated Ni peak intensity 0,862 0,561 0,834 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 1,899 1,085 1,881 Mid-range[(Center-0.5 µm) - (Center+0.5 µm)] Integrated Ni peak intensity 2,659 1,823 2,136 Ratio of Ni content between surface area and middle area (%) 71 % 60 % 88 %

[0170] The analysis results according to Example 2 are shown in Table 3. [TABLE 3] Example 2 Line-scan EDS Measuring point 1 Measuring point #2 Measuring point #3 Temperature of the exothermic DSC peak onset Maintains storage capacity at high temperatures (60 °C) Length of the major axis (L) (µm) 8,3 7,4 7,9 First surface area (0 ~ 0.5 µm) Integrated Mn peak intensity 2,113 2,323 2,278 258 °C 91 % Second surface area [(L-0.5 µm) ~ L] Integrated Mn peak intensity 2,756 2,751 2,633 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 4,869 5,074 4,911 Mid-range[(Center-0.5 µm) - (Center+0.5 µm)] Integrated Mn peak intensity 2,531 2,703 2,585 Ratio of Mn content between surface area and middle area (%) 192 % 188 % 190 % First surface area (0 ~ 0.5 µm) Integrated Ni peak intensity 0,714 0,516 0,854 Second surface area [(L-0.5 µm) ~ L] Integrated Ni peak intensity 0,883 0,546 0,672 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 1,597 1,062 1,526 Mid-range[(Center-0.5 µm) ~(Center+0.5 µm)] Integrated Ni peak intensity 3,123 1,831 2,834 Ratio of Ni content between surface area and middle area (%) 51 % 58 % 54 %

[0171] The metal content and DSC analysis results according to Comparative Example 1 are shown in Table 4 below.

[211] [TABLE 4] Comparison example 1 Line-scan EDS Measuring point 1 Measuring point #2 Measuring point #3 Temperature of the exothermic DSC peak onset Maintains storage capacity at high temperatures (60 °C) Length of the major axis (L) (µm) 7,0 7,2 7,9 First surface area (0 ~ 0.5 µm) Integrated Mn peak intensity 1,298 1,026 0,898 242 °C 47 % Second surface area [(L-0.5 µm) ∼ L] Integrated Mn peak intensity 1,208 1,177 1,013 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 2,506 2,203 1,911 Mid-range[(Center-0.5 µm) ~(Center+0.5 µm)] Integrated Mn peak intensity 2,210 2,344 2,331 Ratio of Mn content between surface area and middle area (%) 113 % 94 % 82 % First surface area (0 - 0.5 µm) Integrated Ni peak intensity 0,359 0,351 0,478 Second surface area [(L-0.5 µm) ∼ L] Integrated Ni peak intensity 0,342 0,360 0,325 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 0,701 0,711 0,803 Mid-range[(Center-0.5 µm) ~(Center+0.5 µm)] Integrated Ni peak intensity 0,221 0,332 0,927 Ratio of Ni content between surface area and middle area (%) 317 % 214 % 87 %

[0172] The metal content and DSC analysis results according to Comparative Example 2 are shown in Table 5 below. [TABLE 5] Comparison example 2 Line-scan EDS Measuring point 1 Measuring point #2 Measuring point #3 Temperature of the exothermic DSC peak onset Maintains storage capacity at high temperatures (60 °C) Length of the major axis (L) (µm) 3 2,6 3,4 First surface area (0 - 0.5 µm) Integrated Mn peak intensity 2,446 2,167 2,080 244 °C 61 % Second surface area [(L-0.5 µm) ∼ L] Integrated Mn peak intensity 2,198 2,192 2,404 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 4,644 4,359 4,484 Mid-range[(Center-0.5 µm) ~(Center+0.5 µm)] Integrated Mn peak intensity 5,051 4,184 5,040 Ratio of Mn content between surface area and middle area (%) 92 % 104 % 89 % First surface area (0 ~ 0.5 µm) Integrated Ni peak intensity 1,405 1,116 1,302 Second surface area [(L-0.5 µm) ~ L] Integrated Ni peak intensity 1,190 1,616 1,573 Surface area (0.5 µm First surface 2,595 2,732 2,875 Depth from both ends) area +SecondSurfaceArea Mid-range[(Center-0.5 µm) - (Center+0.5 µm)] Integrated Ni peak intensity 2,323 2,716 1,703 Ratio of Ni content between surface area and middle area (%) 112 % 101 % 169 %

[0173] The metal content and DSC analysis results according to Comparative Example 3 are shown in Table 6 below. [TABLE 6] Comparison example 3 Line-scan EDS Measuring point 1 Measuring point #2 Measuring point #3 Temperature of the exothermic DSC peak onset Maintains storage capacity at high temperatures (60 °C) Length of the major axis (L) (µm) 2,93 2,3 3,0 First surface area (0 ~ 0.5 µm) Integrated Mn peak intensity 2,668 2,592 2,701 239 °C 35 % Second surface area [(L-0.5 µm) ~ L] Integrated Mn peak intensity 2,875 2,677 2,856 Surface area (0.5 µm First surface 5,543 5,269 5,557 Depth from both ends) area +SecondSurfaceArea Mid-range[(Center-0.5 µm) - (Center+0.5 µm)] Integrated Mn peak intensity 5,950 5,688 5,591 Ratio of Mn content between surface area and middle area (%) 93 % 93 % 99 % First surface area (0 - 0.5 µm) Integrated Ni peak intensity 1,036 0,989 1,020 Second surface area [(L-0.5 µm) ∼ L] Integrated Ni peak intensity 1,002 0,883 0,808 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 2,038 1,872 1,828 Mid-range[(Center-0.5 µm) ~(Center+0.5 µm)] Integrated Ni peak intensity 1,526 1,933 1,535 Ratio of Ni content between surface area and middle area (%) 134 % 97 % 119 %

[0174] The results of metal content and DSC analysis according to Comparative Example 4 are shown in Table 7.

[220] [TABLE 7] Comparison example 4 Line-scan EDS Measuring point 1 Measuring point #2 Measuring point #3 Temperature of the exothermic DSC peak onset Maintains storage capacity at high temperatures (60 °C) Length of the major axis (L) (µm) 7,7 8,35 5,6 First surface area (0 ~ 0.5 µm) Integrated Mn peak intensity 1,930 1,850 1,819 247 °C 52 % Second surface area [(L-0.5 µm) ∼ L] Integrated Mn peak intensity 1,716 2,372 1,833 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 3,646 4,222 3,652 Mid-range[(Center-0.5 µm) - (Center+0.5 µm)] Integrated Mn peak intensity 3,965 4,110 3,804 Ratio of Mn content between surface area and middle area (%) 92 % 103 % 96 % First surface area (0 ~ 0.5 µm) Integrated Ni peak intensity 0,912 0,736 0,646 Second surface area [(L-0.5 µm) ∼ L] Integrated Ni peak intensity 0,745 0,943 0,747 Surface area (0.5 µm depth from both ends) First surface area + Second surface area 1,657 1,679 1,393 Mid-range[(Center-0.5 µm) ~(Center+0.5 µm)] Integrated Ni peak intensity 0,891 1,306 1,418 Ratio of Ni content between surface area and middle area (%) 186 % 129 % 98 %

[0175] Referring to Tables 2 to 7, improved thermal stability after high-voltage charging of the lithium secondary batteries was achieved using the overlithiated oxide particles of the examples in which the concentration gradients of manganese and nickel were formed as described above. Furthermore, a significantly increased capacity retention rate was achieved after high-voltage charging and high-temperature storage compared to the comparative examples.

[0176] Since the manganese content in the surface increased rapidly in Example 2 compared to Example 1, the thermal stability was relatively improved. However, the rate characteristics of the secondary battery may deteriorate due to the high manganese content in the surface.

Claims

[1] A cathode for a lithium secondary battery, comprising: a cathode current collector; and an active cathode material layer formed on the cathode current collector and comprising overlithiated oxide particles containing nickel and manganese, wherein the molar ratio of lithium to the total metal elements is greater than 1, wherein the manganese content in a surface region of the overlithiated oxide particles is greater than the manganese content in a central region thereof, and the nickel content in the central region of the overlithiated oxide particles is greater than the nickel content in their surface region. [2] A cathode for a lithium secondary battery according to claim 1, wherein the ratio of the manganese content in the surface region to the manganese content in the central region is in a range of 105% to 200%. [3] A cathode for a lithium secondary battery according to claim 1, wherein the ratio of the manganese content in the surface region to the manganese content in the central region is in a range of 105% to 150%. [4] A cathode for a lithium secondary battery according to claim 1, wherein the ratio of the nickel content in the surface region to the nickel content in the central region is in a range of 50% to 95%. [5] A cathode for a lithium secondary battery according to claim 1, wherein the ratio of the nickel content in the surface region to the nickel content in the central region is in a range of 60% to 95%. [6] A cathode for a lithium secondary battery according to claim 1, wherein the central region and the surface region of the overlithiated oxide particles have different lithium contents. [7] A cathode for a lithium secondary battery according to claim 6, wherein the central region has a relatively lower lithium content and the surface region has a relatively higher lithium content. [8] A cathode for a lithium secondary battery according to claim 1, wherein the overlithiated oxide particles have a concentration gradient between the central region and the surface region. [9] A cathode for a lithium secondary battery according to claim 1, wherein the overlithiated oxide particles have a chemical structure represented by the following formula 1: Li a [M x Ni y Mn z ]O b [Formula 1] (in formula 1, M includes at least one of Co, Na, Ca, Y, Hf, Ta, B, Si, Ba, Ra, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi, and x, y, z, a and b satisfy 0≤x≤0.9, 0 <y≤0,9, 0,1≤z≤0,9, 1,8≤a+x+y+z≤2,2, 1,05≤a (x+y+z)≤1,95, 1,8≤b≤2,2).[10] A cathode for a lithium secondary battery according to claim 9, wherein the molar fraction of manganese to the total elements other than lithium and oxygen in the overlithiated oxide particles is 0.5 to 0.

75. [11] The cathode for a lithium secondary battery according to claim 1, wherein the central region is a region extending radially outward from the center of the overlithiated oxide particle within a range of 0.5 µm, and the surface region is a region extending inward from the outermost surface of the overlithiated oxide particle toward the center within a depth or thickness of 0.5 µm. [12] A cathode for a lithium secondary battery according to claim 1, wherein the contents of manganese and nickel are determined based on integrated peak intensity values ​​of manganese and nickel obtained by a line-scan EDS analysis performed along a straight line passing through the center of the cross section of the overlithiated oxide particle exposed to the cross section of the cathode active material layer from one end to the other end of the particle. [13] A cathode for a lithium secondary battery according to claim 1, wherein the overlithiated oxide particles comprise at least one Li2MnO3 domain and one domain derived from the Li2MnO3 domain. [14] The cathode for a lithium secondary battery according to claim 13, wherein the domain derived from the Li2MnO3 domain comprises at least one domain selected from the group consisting of MnO2, Mn2O4, LiMnO2, LiMn2O4 and Li2Mn2O4. [15] A lithium secondary battery comprising: the cathode for a lithium secondary battery according to claim 1; and an anode arranged opposite to the cathode.

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