Non-aqueous electrolyte secondary battery and method for manufacturing the same

Abstract

The present invention provides a non-aqueous electrolyte secondary battery with improved cycle characteristics and a method for manufacturing the same. [Solution] A non-aqueous electrolyte secondary battery comprising an electrode assembly including a positive electrode, a negative electrode, and a separation membrane disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the positive electrode comprises a first positive electrode active material and a second positive electrode active material, the second positive electrode active material comprising a solid solution of lithium manganese oxide and lithium transition metal oxide having an oxidation potential of 4.4V or higher on a lithium basis, and is at least partially electrochemically deactivated by a charging reaction, and the negative electrode comprises a Si-based negative electrode active material.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a non-aqueous electrolyte secondary battery and a method for producing the same. [Background technology]

[0002] As technological advancements in mobile devices progress, the demand for rechargeable batteries as an energy source is rapidly increasing. Among these rechargeable batteries, lithium-ion batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used. Currently, research is being actively conducted to increase the capacity of such lithium-ion batteries using silicon materials, which have a large theoretical capacity (Patent Document 1).

[0003] However, when increasing the capacity of such lithium-ion secondary batteries, the cycle characteristics (also known as lifespan characteristics) during charging and discharging may not be sufficiently obtained, and further improvement of these characteristics is desired. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-152250 [Overview of the project] [Problems that the invention aims to solve]

[0005] The problem that this invention aims to solve is to provide a non-aqueous electrolyte secondary battery with improved cycle characteristics and a method for manufacturing the same. [Means for solving the problem]

[0006] The inventor conceived that the cycle characteristics could be improved by pre-doping the negative electrode active material with lithium ions to complement the decrease in charge-discharge efficiency that occurs during use. As a result of intensive studies, the inventor found that the cycle characteristics can be improved by adding a solid solution of lithium mangano-oxide and a lithium transition metal oxide used as a positive electrode active material for pre-doping to the positive electrode and performing the first charge at a high voltage.

[0007] The present invention may include the following aspects. [1] A non-aqueous electrolyte secondary battery including an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the positive electrode includes a first positive electrode active material and a second positive electrode active material, the second positive electrode active material includes a solid solution of a lithium mangano-oxide and a lithium transition metal oxide having an oxidation potential of 4.4 V or more based on lithium, and is at least partially electrochemically inactivated by a charge reaction, and the negative electrode includes a Si-based negative electrode active material. [2] The non-aqueous electrolyte secondary battery according to [1], wherein the lithium mangano-oxide is Li2MnO3. [3] The non-aqueous electrolyte secondary battery according to [1] or [2], wherein the lithium transition metal oxide is represented by LiMeO2 (Me is one or more elements selected from the group consisting of Co, Ni, and Mn). [4] The second positive electrode active material is Li 1+a (Ni x Co y Mn z ) 1-a O2 (where 0 < a < 0.3, 0 ≤ x < 1, 0 ≤ y < 1, 0 < z ≤ 1, and x + y + z = 1), and is composed of a lithium-excess transition metal oxide, and is the non-aqueous electrolyte secondary battery according to any one of [1] to [3]. [5] The non-aqueous electrolyte secondary battery according to any one of [1] to [4], wherein the first positive electrode active material is a nickel-containing lithium transition metal oxide, and the content of nickel is 50 mol% or more based on the total amount of transition metals. [6] The non-aqueous electrolyte secondary battery according to any one of [1] to [5], wherein the molar ratio of Mn to the total transition metals in the second positive electrode active material is higher than the molar ratio of Mn to the total transition metals in the first positive electrode active material. [7] The non-aqueous electrolyte secondary battery according to any one of [1] to [6], wherein the second positive electrode active material is contained in an amount of 0.1% by mass to 50% by mass based on the total mass of the first positive electrode active material and the second positive electrode active material. [8] The non-aqueous electrolyte secondary battery according to any one of [1] to [7], wherein the Si-based negative electrode active material is pre-doped with lithium ions derived from the second positive electrode active material. [9] The non-aqueous electrolyte secondary battery according to any one of [1] to [8], wherein the second positive electrode active material does not have an oxidation potential causing an irreversible oxidation reaction in a range of less than 4.4V.

[10] The non-aqueous electrolyte secondary battery according to any one of [1] to [9], wherein the slope on the low-temperature side of the most low-temperature exothermic peak in the DSC curve obtained by differential scanning calorimetry (DSC) with a heating rate of 10 °C / min when heating from room temperature in the positive electrode active material contained in the positive electrode is 1.5 W / °C·g or less.

[11] The non-aqueous electrolyte secondary battery according to any one of [1] to

[10] , wherein the slope on the low-temperature side of the most low-temperature exothermic peak in the DSC curve obtained by differential scanning calorimetry (DSC) with a heating rate of 10 °C / min when heating from room temperature in the positive electrode active material contained in the positive electrode is 70% or less with respect to the slope on the low-temperature side of the maximum peak of the first positive electrode active material alone.

[12] A hybrid electric vehicle (HEV) or a plug-in hybrid electric vehicle (PHEV) including the non-aqueous electrolyte secondary battery according to any one of [1] to

[11] .

[13] A method for manufacturing a non-aqueous electrolyte secondary battery, comprising an electrode assembly including a positive electrode, a negative electrode, and a separation membrane disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the positive electrode comprises a first positive electrode active material and a second positive electrode active material, the second positive electrode active material comprises a solid solution of lithium manganese oxide and lithium transition metal oxide having an oxidation potential of 4.4V or higher on a lithium basis, and is at least partially electrochemically inactivated by a charging reaction, the negative electrode comprises a Si-based negative electrode active material, and the method comprises an initial charging step of charging the non-aqueous electrolyte secondary battery at least once with a voltage of 4.4V or higher on a lithium basis, wherein lithium ions are pre-doped from the second positive electrode active material into the Si-based negative electrode active material by the initial charging step.

[14] The method for manufacturing a non-aqueous electrolyte secondary battery according to

[13] , wherein the charging voltage in the initial charging step is higher than the charging voltage for the second cycle and subsequent cycles. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery with improved cycle characteristics and a method for manufacturing the same. [Brief explanation of the drawing]

[0009] [Figure 1] The X-ray diffraction (XRD) patterns of the positive electrodes for Example 3, Comparative Example 1, and Comparative Example 2 are shown. [Figure 2] An example of a peak obtained by differential scanning calorimetry (DSC) is shown. [Figure 3] The slopes of the peaks obtained by differential scanning calorimetry (DSC) of the positive electrode active materials for Examples 1-3, Comparative Example 1, and Comparative Example 2 are shown. [Figure 4] The heat generation amounts obtained by differential scanning calorimetry (DSC) of the positive electrode active materials for Examples 1-3, Comparative Example 1, and Comparative Example 2 are shown. [Modes for carrying out the invention]

[0010] The following describes an embodiment of a non-aqueous electrolyte secondary battery. Note that the following embodiment represents one aspect of the present invention and is not limiting; it can be modified at will within the scope of the technical idea of ​​the present invention. Furthermore, the configurations and features of the embodiments can be combined in any way.

[0011] [Nonaqueous electrolyte secondary battery] The non-aqueous electrolyte secondary battery according to this embodiment includes an electrode assembly comprising a positive electrode, a negative electrode, and a separation membrane disposed between the positive and negative electrodes, and a non-aqueous electrolyte. The positive electrode comprises a first positive electrode active material and a second positive electrode active material, the second positive electrode active material comprising a solid solution of lithium manganese oxide and lithium transition metal oxide having an oxidation potential of 4.4V or higher relative to lithium, and is at least partially electrochemically deactivated by the charging reaction during the first charge, and the negative electrode comprises a Si-based negative electrode active material.

[0012] A specific example of such a secondary battery is the lithium-ion secondary battery, which has advantages such as high energy density, discharge voltage, and output stability. The following explanation will mainly use lithium-ion secondary batteries as an example, but the present invention is not limited to lithium-ion secondary batteries and can be applied to various non-aqueous electrolyte secondary batteries.

[0013] [Positive electrode] In the lithium-ion secondary battery according to this embodiment, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on one or both surfaces of the positive electrode current collector. The positive electrode active material layer may be formed on the entire surface of the positive electrode current collector or on only a portion of it.

[0014] (Positive electrode current collector) The positive electrode current collector used in the positive electrode is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., can be used as the positive electrode current collector.

[0015] The positive electrode current collector may have a thickness of 3 μm to 500 μm. Fine irregularities can be formed on the surface of the positive electrode current collector to enhance adhesion to the positive electrode active material. The positive electrode current collector can take various forms, such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0016] (Cathode active material layer) The positive electrode active material layer can be formed, for example, by applying a positive electrode active material slurry, in which a mixture of positive electrode active material, conductive material, and binder is dissolved and dispersed in a solvent, to a positive electrode current collector, followed by drying and rolling. The mixture may further contain dispersants, fillers, and other optional additives as needed. For example, the positive electrode active material may be present in an amount of 80% to 99% by mass based on the total mass of the positive electrode active material layer.

[0017] (Cathode active material) In the lithium-ion secondary battery according to this embodiment, the positive electrode active material includes a first positive electrode active material and a second positive electrode active material. The positive electrode active material layer may further include a positive electrode active material other than the first positive electrode active material and the second positive electrode active material.

[0018] (First positive electrode active material) As the first positive electrode active material, compounds that allow for reversible insertion (intercalation) and deintercalation (deintercalation) of lithium can be used. Specific examples include lithium transition metal oxides containing lithium and one or more metals such as cobalt, manganese, nickel, copper, vanadium, and aluminum. More specifically, such lithium transition metal oxides include lithium-manganese oxides (e.g., LiMnO2, LiMnO3, LiMn2O3, LiMn2O4, etc.); lithium-cobalt oxides (e.g., LiCoO2, etc.); lithium-nickel oxides (e.g., LiNiO2, etc.); lithium-copper oxides (e.g., Li2CuO2, etc.); lithium-vanadium oxides (e.g., LiV3O8, etc.); lithium-nickel-manganese oxides (e.g., LiNi 1-z Mn z O2(0 <z<1)、LiMn2-z Ni z O4 (0 < z < 2), etc.); lithium-nickel-cobalt-based oxides (e.g., LiNi 1-y Co y O2 (0 < y < 1), etc.); lithium-manganese-cobalt-based oxides (e.g., LiCo 1-z Mn z O2 (0 < z < 1), LiMn 2-y Co y O4 (0 < y < 2), etc.); lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni x Co y Mn z )O2 (0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1), Li(Ni x Co y Mn z )O4 (0 < x < 2, 0 < y < 2, 0 < z < 2, x + y + z = 2), etc.); lithium-nickel-cobalt-metal (M) oxides (e.g., Li(Ni x Co y Mn z M w )O2 (M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, 0 < x < 1, 0 < y < 1, 0 < z < 1, 0 < w < 1, x + y + z + w = 1), etc.); compounds in which the transition metal elements in these compounds are partially substituted with one or two or more other metal elements, etc. The first positive electrode active material layer can contain any one or two or more of these compounds. However, it is not limited to only these.

[0019] The oxidation potential of the first positive electrode active material can be 4.2 V or less. The first positive electrode active material enables reversible insertion and desorption of lithium at 4.2 V or less. Therefore, the first positive electrode active material contributes to the charge-discharge reaction at 4.2 V or less and has a large reversible capacity.

[0020] The first positive electrode active material preferably contains a nickel-containing lithium transition metal oxide, and more preferably a lithium transition metal oxide with a high nickel content, in terms of improving the capacity characteristics of the battery. Here, "high nickel content" means containing 50 mol% or more nickel based on the total amount of transition metal. As described above, high-nickel lithium transition metal oxides containing 50 mol% or more nickel make it possible to achieve both high capacity and improved electrode resistance characteristics of lithium-ion secondary batteries. For example, the first positive electrode active material may contain a lithium transition metal oxide containing 60 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more nickel based on the total amount of transition metal. More specifically, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, Li(Ni 0.8 Mn 0.1 Co 0.1 Lithium nickel cobalt manganese ternary cathode active materials such as O2 are preferred.

[0021] The amount of the first positive electrode active material contained in the positive electrode active material layer may be, for example, 30% by mass, 40% or more by mass, 50% or more by mass, 60% or more by mass, 70% or more by mass, 99.9% or less by mass, 90% or less by mass, or 80% or less, based on the total amount of positive electrode active material. If the content of the first positive electrode active material is within the above range, it is possible to achieve excellent capacity characteristics.

[0022] (Second positive electrode active material) The second positive electrode active material contains at least one type of lithium metal composite oxide having an oxidation potential of 4.4 V or more based on lithium and being at least partially electrochemically inactivated by a charging reaction. Specifically, the second positive electrode active material contains a solid solution of lithium manganese oxide and a lithium transition metal oxide. Electrochemical inactivation means that as a result of structural changes or property changes caused by a charging reaction, lithium ions do not return even during discharging and do not directly contribute to the charge-discharge reaction. Such a solid solution is preferable in that it has high thermal stability and low initial charge-discharge efficiency at high voltage. Examples of lithium manganese oxide include Li2MnO3, LiMnO2, and LiMn2O4. The lithium transition metal oxide can be represented by LiMeO2 (Me is one or more elements selected from Co, Ni, and Mn). Examples of the lithium transition metal oxide include the same ones as those described for the first positive electrode active material. In particular, Li 1+a (Ni x Co y Mn z ) 1-a O2 (where 0 < a < 0.3, 0 ≤ x < 1, 0 ≤ y < 1, 0 < z ≤ 1, and x + y + z = 1) is preferable. The molar ratio of Mn to the total amount of transition metals in the lithium-excess transition metal oxide is preferably 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more.

[0023] Li2MnO3 is monoclinic. During overcharging, oxygen is released rather than oxide ions being oxidized. As a result, a heterogeneous composition is formed that maintains the original monoclinic system and a MnO2-like structure from which lithium ions and oxygen have escaped. During discharging, lithium ions are inserted into the MnO2-like structure, but the accommodation capacity is almost inactivated, so most of the lithium ions pre-doped in the negative electrode active material do not return to the second positive electrode active material. Therefore, it is preferable to use a material with a large initial irreversible capacity and low charge-discharge efficiency as the second positive electrode active material.

[0024] Lithium manganese oxide contains a large amount of Mn, resulting in high resistance. Therefore, when used as an active material, measures such as reducing resistance or decreasing particle size are necessary for its full utilization. Using the solid solution described above is preferable compared to using only lithium manganese oxide such as Li2MnO3, as it results in higher electronic conductivity and can be effectively utilized as an active material. This is presumed to be because, compared to Mn alone, the inclusion of Co, Ni, etc., facilitates hybridization with oxygen 2p orbitals (electron correlation between O2p and transition metal 3d), mixed valence effects, and mitigates cation vacancies that tend to occur with Mn alone, making lithium ions more electrochemically mobile from an electronic conductivity standpoint.

[0025] The above-mentioned second positive electrode active material can pre-dope the negative electrode active material with excess lithium ions during the initial high-voltage charge. Excess lithium ions refer to lithium ions that, in addition to the capacity derived from the first positive electrode active material, are released from the second positive electrode active material and inserted into the negative electrode active material by charging at a voltage higher than the oxidation potential of the second positive electrode active material. Therefore, the decrease in charge-discharge efficiency due to repeated charge-discharge cycles can be compensated for by the excess lithium ions pre-doped into the negative electrode active material, thereby improving cycle characteristics.

[0026] It is also preferable that the second positive electrode active material described above does not have a large irreversible oxidation potential in the range of less than 4.4V. If the second positive electrode active material does not have a large irreversible capacitance in the range of less than 4.4V, a decrease in charge / discharge efficiency due to the second positive electrode active material will not occur. Therefore, it is possible to prevent a decrease in cycle characteristics due to the addition of the second positive electrode active material within the operating voltage range.

[0027] The total amount of the second positive electrode active material contained in the positive electrode active material layer is, for example, 0.1% to 70% by mass, 0.1% to 60% by mass, 0.1% to 50% by mass, 1% to 40% by mass, or 10% to 30% by mass, relative to the total amount of positive electrode active material. If the amount of the second positive electrode active material with a sufficiently large irreversible capacity is 0.1% by mass or more, improvements in the battery's cycle characteristics and electrode resistance characteristics can be expected. If it is 50% by mass or less, sufficient energy density can be obtained.

[0028] The mass ratio of the first positive electrode active material to the second positive electrode active material is, for example, within the range of 99.9:0.1 to 30:70, 90:10 to 50:50, or 80:20 to 60:40. Excellent cycle characteristics and safety can be achieved when the mass ratio of the first positive electrode active material to the second positive electrode active material is within the above range.

[0029] The molar ratio of Mn to the total amount of transition metal in the second positive electrode active material is preferably higher than the molar ratio of Mn to the total amount of transition metal in the first positive electrode active material. The molar ratio of Mn to the total amount of transition metal in the second positive electrode active material may be 0.2 or higher, 0.3 or higher, or 0.4 or higher. According to the above, the second positive electrode active material has the effect of improving the thermal stability of the positive electrode active material by containing a large amount of manganese, which has high thermal stability. In particular, it is presumed that the solid solution containing Li2MnO3 changes to a composition containing a MnO2-like structure as oxygen and lithium are released during the first charge. Such a structure can suppress the phenomenon of oxygen being released from the crystal lattice in the event of an abnormality such as an internal short circuit in the battery, thereby improving safety.

[0030] (Thermal stability evaluation) The thermal stability of the positive electrode active material can be evaluated by differential scanning calorimetry (DSC). The second positive electrode active material has the effect of improving the thermal stability of the positive electrode active material by containing a large amount of manganese, which has high thermal stability. Thermal stability can be evaluated by the amount of heat generated by DSC, specifically the amount of heat generated in the 150°C to 300°C range, where the exothermic reaction begins. By appropriately selecting the second positive electrode active material, it is possible to reduce the amount of heat generated in the 150°C to 300°C range compared to the first positive electrode active material, thereby increasing thermal stability. Therefore, by mixing the first and second positive electrode active materials, the amount of heat generated is reduced compared to the first positive electrode active material alone, thus improving the safety of lithium-ion secondary batteries.

[0031] In a differential scanning calorimetry (DSC) curve representing the amount of heat per unit temperature obtained by heating from room temperature at a heating rate of 10°C / min, the low-temperature slope of the lowest-temperature exothermic peak of the positive electrode active material, which is a mixture of the first and second positive electrode active materials, is smaller than the low-temperature slope of the maximum exothermic peak of the first and second positive electrode active materials individually. Here, the slope of the exothermic peak refers to the slope of the straight line (W / (°C·g)) connecting the point at 80% of the peak height and the point at 40% of the peak height for the rising portion of the peak. The slope of the exothermic peak is 1.7 W / °C·g or less, 1.6 W / °C·g or less, 1.5 W / °C·g or less, 1.4 W / °C·g or less, 1.3 W / °C·g or less, 1.2 W / °C·g or less, or 1.1 W / °C·g or less.

[0032] In a DSC curve representing the amount of heat per unit temperature obtained by differential scanning calorimetry (DSC) performed from room temperature at a heating rate of 10°C / min, the low-temperature slope of the exothermic peak at the lowest temperature of the positive electrode active material, which is a mixture of the first positive electrode active material and the second positive electrode active material, is 80% or less, 75% or less, 70% or less, or 65% or less compared to the low-temperature slope of the maximum peak of the first positive electrode active material alone.

[0033] Compared to using the first and second positive electrode active materials individually, mixing them reduces rapid heat generation in a short time and improves thermal stability. Therefore, using a mixture of the first and second positive electrode active materials provides stability against thermal runaway in the event of a battery malfunction.

[0034] (Conductive material) The conductive material is not particularly limited as long as it is an electrically conductive material that does not induce chemical changes. Examples of conductive materials include carbon-based materials such as artificial graphite, natural graphite, carbon black, acetylene black, Ketjen black, Denka black, thermal black, channel black, furnace black, lamp black, carbon nanotubes, and carbon fibers; metal powders and metal fibers such as aluminum, tin, bismuth, silicon, antimony, nickel, copper, titanium, vanadium, chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten, silver, gold, lanthanum, ruthenium, platinum, and iridium; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyaniline, polythiophene, polyacetylene, polypyrrole, and polyphenylene derivatives. One or more of these may be used, but are not limited to these.

[0035] The conductive material content may be 0.1% to 30% by mass, based on the total mass of the positive electrode active material layer. Preferably, the conductive material content is 0.5% to 15% by mass, and more preferably 0.5% to 5% by mass. When the conductive material content satisfies the above range, sufficient conductivity can be provided, and since the amount of positive electrode active material is not reduced, it is advantageous in that battery capacity can be secured.

[0036] (binder) Binders are added as components that promote bonding between the active material and the conductive material, or between the active material and the current collector. Any common binder used in the relevant art can be used, and the type is not particularly limited. Examples of binders include polyvinylidene fluoride (PVdF), polyvinyl alcohol (PVA), polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), polyacrylic acid, acrylamide, polyimide, fluororubber, and various copolymers thereof. One or more of these may be used, but are not limited to these.

[0037] The binder content may be 0.1% by mass or more and 30% by mass or less, based on the total mass of the positive electrode active material layer. Preferably, the binder content is 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 10% by mass or less. When the binder content satisfies the above range, sufficient adhesive force can be provided within the electrodes while preventing a decrease in the capacity characteristics of the battery.

[0038] (solvent) The solvent used in the cathode active material slurry is not particularly limited as long as it is generally used in the manufacture of cathodes. Examples of solvents include amine solvents such as N,N-dimethylaminopropylamine, diethylenetriamine, and N,N-dimethylformamide (DMF); ether solvents such as tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; amide solvents such as dimethylacetamide and N-methyl-2-pyrrolidone (NMP); and dimethyl sulfoxide (DMSO). One or more of these may be used, but are not limited to these.

[0039] The amount of solvent used should be sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, while also having a viscosity that allows for excellent thickness uniformity when applied to the positive electrode current collector, taking into account the slurry coating thickness and production yield.

[0040] [Manufacturing method for positive electrode] A conductive agent, binder, etc., are added to the positive electrode active material obtained above. At this time, other additives such as dispersants and thickeners may be added as needed. By dispersing these in a solvent, a positive electrode active material slurry is obtained. That is, the positive electrode active material slurry contains the positive electrode active material obtained above, a conductive agent, a binder, and a solvent.

[0041] A positive electrode can be manufactured by applying a slurry of positive electrode active material to a positive electrode current collector, followed by drying and rolling, thereby forming a layer of positive electrode active material on the positive electrode current collector.

[0042] Alternatively, for example, the positive electrode may be manufactured by casting the above-mentioned positive electrode active material slurry onto another support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector. Furthermore, the positive electrode active material layer may be formed on the positive electrode current collector using any other method.

[0043] [Negative electrode] In the lithium-ion secondary battery according to this embodiment, the negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on one or both surfaces of the negative electrode current collector. The negative electrode active material layer may be formed on the entire surface of the negative electrode current collector or on only a portion of it.

[0044] (Negative electrode current collector) The negative electrode current collector used in the negative electrode is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys may be used as negative electrode current collectors.

[0045] The negative electrode current collector may have a thickness of 3 μm or more and 500 μm or less. Fine irregularities can also be formed on the surface of the negative electrode current collector to enhance the adhesive force with the negative electrode active material. The negative electrode current collector can have various forms, such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc.

[0046] (Negative electrode active material layer) The negative electrode active material layer may contain a negative electrode active material, a binder, a conductive material, and an optional additive. For example, the negative electrode active material layer can be formed by applying a negative electrode active material slurry in which a mixture of a negative electrode active material, a binder, and a conductive material is dissolved or dispersed in a solvent onto the negative electrode current collector, followed by drying and rolling, or by casting the above negative electrode active material slurry onto another support and then laminating the film obtained by peeling it from the support onto the negative electrode current collector. The above mixture may further contain a dispersant, a filler, and other optional additives as required.

[0047] (Negative electrode active material) The negative electrode active material contains a Si-based material. The Si-based material is a material containing silicon as the main component. Examples of the Si-based material include, for example, silicon-based materials such as silicon powder, amorphous silicon, silicon nanofibers, and silicon nanowires; silicon alloys, silicon oxides SiO x (0 < x ≤ 2), silicon compounds such as silicon oxides doped with an alkali metal or an alkaline earth metal (such as lithium or magnesium), composites of silicon-based materials and carbonaceous materials such as Si-C, etc. One or a mixture of two or more of these can be used, but it is not limited to these. Note that the negative electrode active material layer may further contain a negative electrode active material different from the silicon-based material. In this specification, all substances contributing to the charge-discharge reaction in the negative electrode are referred to as "negative electrode active materials".

[0048] The negative electrode active material may be contained in an amount of 70% to 100% by mass, preferably 80% to 99% by mass, based on the total mass of the negative electrode active material layer. When the content of the negative electrode active material satisfies the above range, excellent energy density, electrode adhesion, and electrical conductivity can be achieved.

[0049] The negative electrode active material is included in an amount that can accept lithium ions equivalent to the first positive electrode active material plus the excess capacity provided by the second positive electrode active material.

[0050] The negative electrode active material is also included in an amount exceeding the number of moles required for the charge-discharge reaction with the total amounts of the first positive electrode active material and the second positive electrode active material. Preferably, it is included in an amount greater than 100%, 120% or less, greater than 100%, 110% or less, greater than 100%, and 105% or less of the number of moles required for the charge-discharge reaction.

[0051] (Binder and conductive material) The types and contents of binders and conductive materials used in the negative electrode active material slurry may be the same as those described for the positive electrode.

[0052] (Thickening agent) The negative electrode active material slurry used when coating the negative electrode current collector with the negative electrode active material may contain a thickening agent. Specifically, the thickening agent may be a cellulosic compound such as carboxymethylcellulose (CMC). The thickening agent may be included in an amount of, for example, 0.5% to 10% by mass based on the total mass of the negative electrode active material layer.

[0053] (solvent) The solvent used in the negative electrode active material slurry is not particularly limited as long as it is generally used in the manufacture of negative electrodes. Examples of solvents include N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and water, and one or more of these may be used, but are not limited to these.

[0054] [Method for manufacturing a negative electrode] A negative electrode active material slurry can be obtained by dissolving or dispersing the negative electrode active material in a solvent, along with a binder, conductive material, thickener, etc., as needed. After applying the negative electrode active material slurry to a negative electrode current collector, a negative electrode can be manufactured by drying and rolling, forming a negative electrode active material layer on the negative electrode current collector.

[0055] Alternatively, the negative electrode may be manufactured by casting the above-mentioned negative electrode active material slurry onto another support, peeling it off the support, and then laminating the resulting film onto the negative electrode current collector. Furthermore, the negative electrode active material layer may be formed on the negative electrode current collector using any other method.

[0056] [Separation membrane] In the lithium-ion secondary battery according to the embodiment, the separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is a material that is normally used as a separation membrane in lithium-ion secondary batteries. In particular, it is preferable that the electrolyte has low resistance to ion movement and excellent moisture-retaining capacity. For example, a porous polymer film made from a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, can be used as the separation membrane. In addition, ordinary porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, a separation membrane coated with a ceramic component or polymer material to ensure heat resistance or mechanical strength may be used.

[0057] [Non-aqueous electrolytes] In the non-aqueous electrolyte secondary battery according to this embodiment, the non-aqueous electrolyte includes, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, and solid electrolytes that can be used in the manufacture of secondary batteries.

[0058] Non-aqueous electrolytes may contain organic solvents and lithium salts, and may also contain additives as needed. Hereinafter, liquid electrolytes will also be referred to as "electrolytes."

[0059] (organic solvent) Organic solvents can be used without particular restrictions, as long as they can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Examples of organic solvents include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitrile solvents such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group from C2 to C20, and may include a double-bonded aromatic ring or ether bond); amide solvents such as dimethylformamide; dioxolane solvents such as 1,3-dioxolane; and sulfolane solvents. One or more of these can be used, but are not limited to these. In particular, carbonate-based solvents are preferred, and more preferably, a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can enhance the charge / discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.) is preferred. In this case, when the cyclic carbonate and linear carbonate are mixed in a volume ratio of 1:1 to 1:9, excellent electrolyte performance can be observed.

[0060] (Lithium salt) Lithium salts can be used without particular limitations as long as they are compounds capable of supplying lithium ions for use in lithium-ion secondary batteries. Examples of lithium salts include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, and one or more of these may be used, but are not limited to these. The lithium salt may be included in the electrolyte, for example, at a concentration of 0.1 mol / L to 2 mol / L. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0061] (Additives) Additives can be used as needed to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity. Examples of additives include vinylene carbonate (VC), haloalkylene carbonate compounds such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride. One or more of these may be used, but are not limited to these. The additive may be included in the total mass of the electrolyte in an amount of 0.1% to 15% by mass.

[0062] [Manufacturing method and usage method of non-aqueous electrolyte secondary batteries] A method for manufacturing and using a lithium-ion secondary battery according to this embodiment includes the following steps. (1) Steps for manufacturing a lithium-ion secondary battery (2) Step of performing the initial charge and discharge of the lithium-ion secondary battery. (3) Steps to use the lithium-ion secondary battery after the initial charge and discharge.

[0063] (1) Steps to manufacture a battery The lithium-ion secondary battery according to the embodiment can be manufactured by interposing a separation membrane and an electrolyte between the negative electrode and the positive electrode manufactured as described above. More specifically, an electrode assembly can be formed by placing a separator between the negative electrode and the positive electrode, and the electrode assembly can be placed in a battery case such as a cylindrical battery case or a rectangular battery case, after which an electrolyte can be injected to manufacture the battery. Alternatively, the electrode assemblies can be stacked, impregnated with an electrolyte, and the resulting product can be placed in a battery case and sealed to manufacture the battery. In addition, the battery may be manufactured by any known method.

[0064] (2) Step of performing the initial charge and discharge of the battery The manufacturing method for a lithium-ion secondary battery according to the embodiment includes an initial charging step in which the lithium-ion secondary battery manufactured as described above is charged at least once with a voltage of 4.4V or higher relative to lithium. The charging voltage in the initial charging step may be higher than or equal to the oxidation potential of the second positive electrode active material. By performing the initial charging with a voltage higher than or equal to the oxidation potential of the second positive electrode active material, lithium ions in the second positive electrode active material can be pre-doped into the negative electrode active material. The voltage is higher than the voltage at the time of basic design, preferably 4.4V or higher, 4.5V or higher, 4.6V or higher, or 4.7V or higher relative to lithium, and may be, for example, 4.8V. The charging rate during the initial charging may be, for example, in the range of 0.05C to 1.0C. A step to remove gas generated after the initial charging may also be performed.

[0065] After the initial charge, the battery is completely discharged. The discharge rate may be, for example, in the range of 0.05C to 1.0C. Lithium-ion secondary batteries may be shipped after undergoing such an initial charge-discharge process at least once. The initial charge-discharge process can be repeated 1 to 3 times.

[0066] (3) Steps to use the lithium-ion secondary battery after the initial charge and discharge. In actual use, the State of Charge (SOC) is defined by the upper and lower voltage range within which the nominal capacity can be obtained. Charging during use is performed at a voltage above the main oxidation potential of the first positive electrode active material and below the oxidation potential of the second positive electrode active material. Charging and discharging during use can be performed within ranges such as upper charge limits of 4.3V, 4.25V, 4.2V, 4.1V, 4.0V and lower discharge limits of 2.5V, 2.6V, 2.7V, 2.8V, 2.9V, 3.0V. The charge / discharge rate can be within the range of 0.1C to 1.0C.

[0067] During use, by performing charging and discharging within the above voltage range, the second positive electrode active material does not participate in charging and discharging. During the initial charge, the main redox reaction takes place between the lithium-ion-predoped negative electrode active material and the first positive electrode active material, enabling a highly reversible charge-discharge reaction. By performing a stable charge-discharge reaction, it is expected that the increase in internal resistance will be reduced even with repeated use.

[0068] [effect] Conventionally, pre-doping, which involves inserting lithium ions into the negative electrode active material beforehand, has been performed to compensate for the irreversible capacity of the negative electrode active material. While this method is effective in that it can compensate for the irreversible capacity of the negative electrode alone and improve charge-discharge efficiency, it required pre-treatment before assembling the cell. Such pre-doping of the negative electrode before cell assembly was difficult to handle because it used metallic lithium. There were also problems such as the manufacturing process becoming more complex.

[0069] In this embodiment, the lithium-ion secondary battery does not pre-dope the negative electrode active material with lithium ions before cell assembly, but rather performs pre-doping during the initial charging process after battery assembly by charging with a high voltage. Therefore, the electrodes are easier to handle before cell assembly, and the manufacturing process is simplified.

[0070] Furthermore, the battery according to the present invention provides stability against thermal runaway in abnormal situations, while simultaneously enabling control of excess charge and improving capacity retention by adjusting the usable SOC range. In addition, by combining this design and process, upper and lower voltage limits (ΔSOC setting) can be arbitrarily set according to usage conditions such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs), allowing for effective utilization of excess capacity during the initial charge and reducing capacity degradation during the durability cycle, thereby enabling the construction of a highly durable battery system. [Examples]

[0071] Examples and comparative examples are described below, but the present invention is not limited thereto. Furthermore, the considerations described below are merely illustrative inferences to aid in understanding the invention and do not limit the present invention in any way.

[0072] [Example 1] (Manufacturing of positive electrodes) The first cathode active material is LiNi with an average particle size of approximately 11 μm. 0.8 Co 0.1 Mn 0.1 O2(NCM) powder and Li2MnO3 and LiNi with an average particle size of approximately 5 μm as the second positive electrode active material. 0.326 Co 0.116 Mn 0.558 A cathode active material slurry was prepared by weighing 100 parts by mass of a positive electrode active material prepared by blending a powder of Li-rich NCM (Li:Ni:Mn:Co = 1.14:0.28:0.48:0.10 molar ratio), which is a solid solution with O2, in a mass ratio of 9:1, 2 parts by mass of PVdF as a binder, and 1.5 parts by mass of carbon black as a conductive material. These were mixed in N-methyl-2-pyrrolidone as a solvent to prepare a cathode active material slurry. The obtained cathode active material slurry was applied to aluminum foil, which served as the cathode current collector, dried at 130°C for 1 hour in a dry environment, and then rolled to form the cathode.

[0073] [Example 2] A cathode active material slurry was prepared in the same manner as in Example 1, except that the mass ratio of the first cathode active material to the second cathode active material was set to 7:3, and a cathode was prepared in the same manner.

[0074] [Example 3] A cathode active material slurry was prepared in the same manner as in Example 1, except that the mass ratio of the first cathode active material to the second cathode active material was set to 5:5, and a cathode was prepared in the same manner.

[0075] [Comparative Example 1] Except for not adding the second positive electrode active material, a positive electrode active material slurry was prepared in the same manner as in Example 1, and a positive electrode was prepared in the same manner.

[0076] [Comparative Example 2] A cathode active material slurry was prepared in the same manner as in Example 1, except that the first cathode active material was not added, and a cathode was prepared in the same manner.

[0077] [Evaluation Example 1: X-ray Diffraction (XRD) Pattern Measurement] Coin cells were manufactured using lithium metal as the counter electrode for the positive electrodes obtained in Example 3, Comparative Example 1, and Comparative Example 2. The coin cells of Example 3, Comparative Example 1, and Comparative Example 2 were initially charged to 4.65V at a charge rate of 0.2C and then discharged to 2.5V. After that, the coin cells were disassembled, the positive electrodes were removed and cleaned with dimethyl carbonate (DMC). After cleaning, the positive electrode active material layer, excluding the current collector, was scraped off, and XRD measurements were performed using Cuα as the radiation source.

[0078] After charging and discharging, the positive electrodes of Comparative Example 1 and Comparative Example 2 showed peaks at different positions around 19°. Furthermore, two peaks were observed in the positive electrode of Example 3 after charging and discharging, both attributable to the mixed first and second positive electrode active materials.

[0079] [Evaluation Example 2: Measurement of excess charge amount during initial charging] Coin cells were manufactured using lithium metal as the counter electrode for the positive electrodes obtained in Examples 1-3 and Comparative Examples 1-2. For the coin cells of Examples 1-3 and Comparative Example 2, an initial charge was performed to 4.65V at a charge rate of 0.2C, and then discharged to 2.5V. For the second cycle, charging and discharging were performed under conditions of a charge termination voltage of 4.25V and a discharge termination voltage of 2.5V (0.2C rate). Since the coin cell of Comparative Example 1 does not contain a second positive electrode active material that operates in the high potential region, charging and discharging were performed under conditions of a charge termination voltage of 4.25V and a discharge termination voltage of 2.5V (0.2C rate) for both the initial charge and the second cycle.

[0080] Here, the difference between the initial charge capacity and the initial discharge capacity is defined as the initial irreversible capacity, and the sum of the initial irreversible capacity and the charge capacity of the second cycle is defined as "2 nd The "true charge amount" is defined as follows: "2 nd "True charge" represents the capacity after two charging cycles from a completely discharged state before charging and discharging. [Mathematics 1] 2 nd True charge capacity (mhA / g) = Initial irreversible capacity (mAh / g) + 2nd cycle charge capacity (mhA / g)

[0081] Also, the initial charge capacity is 2 nd The ratio of pre-doping amount (β) is defined as follows: β represents the ratio of how much excess capacity can be charged by the initial charge, given that the charge capacity of the second cycle is 1. [Math 2] β = (Initial charge capacity) / (2 nd True charge amount)-1

[0082] The results of Evaluation Example 1, along with the manufacturing conditions described above, are summarized in the table below.

[0083] [Table 1]

[0084] Referring to Table 1, it was confirmed that by adding a second positive electrode active material and performing high-voltage charging (4.65V) during the initial charge, a surplus charge capacity corresponding to β was observed. This surplus charge capacity can be controlled by adjusting the amount of the second positive electrode active material.

[0085] [Evaluation Example 3: Evaluation of Battery Life Characteristics] (Manufacturing of negative electrodes) As the negative electrode active material, silicon monoxide (SiO) powder with an average particle size of approximately 8 μm was prepared. As the conductive material, an aqueous dispersion of carbon black (CB) and single-walled carbon nanotubes (SWCNTs) (solid content 0.4%) was prepared. Styrene-butadiene rubber (SBR) was prepared as a binder, and carboxymethylcellulose (CMC) as a thickener, in a mass ratio of 88.8:3.3:0.25:3.65:4.0. First, the negative electrode active material, CB, and CMC were mixed, and the SWCNT dispersion and water were added and kneaded. Finally, SBR was added and mixed to produce a negative electrode active material slurry. The obtained negative electrode active material slurry was uniformly coated onto copper foil and vacuum dried at 110°C for 10 hours to serve as the negative electrode.

[0086] (Manufacturing of lithium-ion rechargeable batteries) An electrode assembly was manufactured by laminating the negative electrode manufactured above with the positive electrodes of Examples 1-3 and Comparative Example 1, interposed with a polyethylene separation membrane (thickness: approximately 20 μm). After placing the electrode assembly inside the battery case, an electrolyte was injected into the case to manufacture a lithium-ion secondary battery. The electrolyte was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7, dissolving 1 M lithium salt LiPF6, and adding 1.0 mass% vinylene carbonate (VC).

[0087] (Charge / Discharge Test) For each example and comparative example, batteries using the respective positive electrodes were initially charged and discharged under conditions of 0.1C and 4.6V (discharge termination voltage 2.5V). From the second cycle onward, charging and discharging were performed for up to 300 cycles under conditions of a charging termination voltage of 4.2V and a discharge termination voltage of 2.5V. During the initial charge, the amounts of the positive and negative electrodes were adjusted so that the negative electrode was charged to approximately 90% of its design capacity. The cycle test involved two charge-discharge cycles at 25°C, followed by 300 charge-discharge cycles at 45°C under conditions of 0.5C. The discharge capacity after 300 cycles relative to the discharge capacity of the first cycle at 45°C was defined as the capacity retention rate (%).

[0088] Furthermore, DCR measurements were performed at SOC 50% before and after the cycle test. DCR was calculated by dividing the voltage drop before and after energizing at 2.5C for 10 seconds by the current. The results of Evaluation Example 2 are summarized in Table 2. In Table 2, the DCR increase rate represents the percentage increase (%) of the DCR after 300 cycles compared to the DCR after the first cycle at 45°C.

[0089] [Table 2]

[0090] In the cells of Examples 1 to 3, it was confirmed that the capacity retention rate improved compared to Comparative Example 1, which did not contain the second positive electrode active material, by adding the second positive electrode active material and adjusting the amount of excess charge to the negative electrode. Furthermore, the increase in DCR after 300 cycles was suppressed. This is thought to be because the excess lithium was pre-doped into the negative electrode by adding the second positive electrode active material and performing high-voltage charging on the first cycle, and then charging was performed under the condition of 4.2V, which causes oxidation and reduction of the first positive electrode active material, from the second cycle onward, thereby suppressing the difference in SOC between the positive and negative electrodes.

[0091] [Evaluation Example 4: Thermal Stability of Cathode Active Material] Next, differential scanning calorimetry (DSC) was performed on the positive electrode active materials of Examples 1-3 and Comparative Examples 1-2 as follows. A coin cell was prepared using each positive electrode in the same manner as in Evaluation Example 2, and the initial charge and discharge was performed, stopping at the second charging cycle. After that, the coin cell was disassembled, the positive electrode was removed and washed with dimethyl carbonate (DMC). After washing, the positive electrode active material layer, excluding the current collector, was scraped off and dried, and 4 mg was weighed. The positive electrode active material layer was sealed in a DSC pan with 6 μL of a solution in which LiPF6 was dissolved at a concentration of 1 M in a solvent mixed with ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7, and the measurement was performed by raising the temperature from room temperature at a heating rate of 10°C / min. As a result of the measurement, graphs like those shown in Figure 2 were obtained for each of Examples 1-3 and Comparative Examples 1-2. From these graphs, the slope representing the heat generation per unit time and the amount of heat generation (area) from 150°C to 300°C were calculated. Figure 3 is a graph comparing the slopes of Examples 1-3 and Comparative Examples 1-2. Figure 4 is a graph comparing the heat generation of Examples 1-3 and Comparative Examples 1-2.

[0092] Referring to Figure 4, by comparing the heat output of the positive electrode active materials in Examples 1-3 and Comparative Examples 1-2, it was confirmed that the heat output decreased as the amount of the second positive electrode active material increased. In other words, it was confirmed that the second positive electrode active material had a low heat output and high thermal stability at 150°C to 300°C. Furthermore, Examples 1-3, in which the first and second positive electrode active materials were mixed, showed a lower heat output than Comparative Example 1, which used only the first positive electrode active material.

[0093] Furthermore, for the left side of the coldest peak (i.e., the rising portion of the peak) among the DSC peaks observed after heating from room temperature, the slope (W / (°C·g)) of the straight line connecting the point at 80% of the peak top height and the point at 40% of the peak top height was calculated. Figure 3 shows the slopes for Examples 1-3 and Comparative Examples 1-2.

[0094] Comparative Examples 1 (first positive electrode active material only) and 2 (second positive electrode active material only) showed relatively sharp slopes in the DSC peaks, whereas Examples 1-3, which used both the first and second positive electrode active materials, showed gentler slopes. This slope indicates the heat flow per unit temperature, but since this test was conducted at a heating rate of 10°C / min, it also indicates the heat flow per unit time, showing the likelihood of rapid heat generation in a short period of time. Compared to Comparative Examples 1 and 2, which used only one type of positive electrode active material, Examples 1-3, which mixed the first and second positive electrode active materials, showed a reduced slope and improved thermal stability.

Claims

1. An electrode assembly comprising a positive electrode, a negative electrode, and a separator membrane positioned between the positive and negative electrodes, A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte, The positive electrode includes a first positive electrode active material and a second positive electrode active material. The second positive electrode active material contains a solid solution of lithium manganese oxide and lithium transition metal oxide having an oxidation potential of 4.4 V or higher relative to lithium, and is at least partially electrochemically deactivated by a charging reaction. The aforementioned negative electrode includes a Si-based negative electrode active material. Nonaqueous electrolyte secondary battery.

2. The lithium manganese oxide is Li ２ MnO ３ The non-aqueous electrolyte secondary battery according to claim 1.

3. The lithium transition metal oxide is LiMeO ２ A non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein Me is represented as one or more elements selected from the group consisting of Co, Ni, and Mn.

4. The second positive electrode active material is Li １＋ａ (Ni ｘ Co ｙ Mn ｚ ) １－ａ O ２ (where 0 < a < 0.3, 0 ≤ x < 1, 0 ≤ y < 1, 0 < z ≤ 1, x + y + z = 1), and is composed of a lithium-excess transition metal oxide, the non-aqueous electrolyte secondary battery according to claim 1 or 2.

5. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the first positive electrode active material is a nickel-containing lithium transition metal oxide, and the nickel content is 50 mol% or more of the total amount of transition metals.

6. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the molar ratio of Mn to the total transition metal in the second positive electrode active material is higher than the molar ratio of Mn to the total transition metal in the first positive electrode active material.

7. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the second positive electrode active material is contained in an amount of 0.1% to 50% by mass based on the total mass of the first positive electrode active material and the second positive electrode active material.

8. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the Si-based negative electrode active material is pre-doped with lithium ions derived from the second positive electrode active material.

9. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the second positive electrode active material does not have an oxidation potential in the range of less than 4.4 V that causes an irreversible oxidation reaction.

10. The positive electrode active material contained in the positive electrode has a slope on the low-temperature side of the lowest temperature exothermic peak in the DSC curve obtained by differential scanning calorimetry (DSC) performed at a heating rate of 10°C / min from room temperature, which is 1.5 W / °C·g or less, according to claim 1 or 2, for the non-aqueous electrolyte secondary battery.

11. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode active material contained in the positive electrode has a slope on the low-temperature side of the lowest temperature exothermic peak in a DSC curve obtained by differential scanning calorimetry (DSC) performed at a heating rate of 10°C / min from room temperature, and the slope on the low-temperature side of the highest temperature peak of the first positive electrode active material alone is 70% or less.

12. A hybrid electric vehicle (HEV) or plug-in hybrid electric vehicle (PHEV) comprising a non-aqueous electrolyte secondary battery according to claim 1 or 2.

13. An electrode assembly comprising a positive electrode, a negative electrode, and a separator membrane positioned between the positive and negative electrodes, A method for manufacturing a non-aqueous electrolyte secondary battery, comprising a non-aqueous electrolyte, The positive electrode includes a first positive electrode active material and a second positive electrode active material. The second positive electrode active material comprises a solid solution of lithium manganese oxide and lithium transition metal oxide having an oxidation potential of 4.4 V or higher relative to lithium, and is at least partially electrochemically deactivated by a charging reaction. The aforementioned negative electrode includes a Si-based negative electrode active material. The non-aqueous electrolyte secondary battery includes an initial charging step of charging it at least once with a voltage of 4.4V or higher based on lithium, wherein lithium ions are pre-doped into the Si-based negative electrode active material from the second positive electrode active material during the initial charging step. A method for manufacturing a non-aqueous electrolyte secondary battery.

14. The method for manufacturing a non-aqueous electrolyte secondary battery according to claim 13, wherein the charging voltage in the initial charging step is higher than the charging voltage in the second and subsequent cycles.

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