KR20230065075A - Electrode active material having pores, preparation method thereof, and electrode and secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a pore-containing electrode active material, a method for preparing the same, and an electrode and a secondary battery including the same.

Description

Electrode active material having pores, preparation method thereof, and electrode and secondary battery comprising the same}

The present invention relates to a pore-containing electrode active material, a method for preparing the same, and an electrode and a secondary battery including the same.

With the trend of miniaturization and thinning of portable devices such as mobile phones, notebook computers, and camcorders, demand for general secondary batteries or lithium secondary batteries used as energy sources for these devices is increasing, and at the same time, high capacity of these secondary batteries is required.

In a typical lithium secondary battery that is currently being commercialized, a lithium-cobalt-based metal oxide is used as a cathode active material and carbon is used as an anode active material. The lithium-cobalt-based metal oxide is relatively easy to synthesize and has excellent stability and cycle characteristics, but has limitations in application to high-capacity battery technologies.

Meanwhile, an all-solid-state battery refers to a battery in which a liquid electrolyte is replaced with a solid electrolyte among battery components including a positive electrode, an electrolyte, and a negative electrode. All-solid-state secondary batteries are attracting attention as next-generation secondary batteries because they do not have the risk of battery explosion or fire compared to liquid electrolytes, simplify the manufacturing process, and have high energy density. That is, the all-solid-state battery has the advantage of higher safety compared to conventional lithium batteries using flammable organic solvents.

However, since all components of the solid-state secondary battery, such as the positive and negative electrodes as well as the electrolyte, are in a solid state, the resistance between the electrodes is greater during the movement of ions than the liquid electrolyte, and the contacted portion is detached due to the deterioration. Due to problems, etc., the binding force between the electrolyte and the electrode is weakened, and the conductivity tends to deteriorate. In particular, in the solid electrolyte manufactured for the development of the existing all-solid-state battery, when stacking with the electrode, the binding force between each part is not large, so the resistance at the boundary appears quite large after fabrication of the battery, so the performance of the mixed solid electrolyte is 100 It showed a tendency not to exert %. In addition, the performance of the battery rapidly deteriorates due to the resistance generated at the boundary between the current collector corresponding to the electrode of the battery and the electrolyte. Even if the electronic conductivity of the current collector is high, if the electronic conductivity is low at the interface between the electrolyte and the electrode active material, the overall conductivity capacity may be lowered. There is a problem in that the resistance increases due to the limited contact area and the conductivity decreases. Accordingly, the performance of an all-solid-state battery is influenced by the ionic conductivity of the solid electrolyte and the interface characteristics between the electrolyte and the electrode active material.

The present invention has an empty space in the center and uses microparticles coated with a lithium diffusion layer on the outer surface as well as the inner surface as a cathode active material to achieve an improved contact with a solid electrolyte even if the particles are broken by charging and discharging. An object of the present invention is to provide a high-performance positive electrode active material and a lithium secondary battery including the same, for example, an all-solid-state secondary battery.

According to one embodiment of the present invention, a layer structure having an average diameter of 6 to 10 μm containing 70 to 98 mol% of nickel and optionally 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively. An electrode active material is provided, which is a particle having an empty space having a diameter of 1 to 3 μm in the center and coated with a lithium diffusion layer between primary particles and on the surface of the secondary particles.

According to one embodiment of the present invention, the lithium diffusion layer is LiNbO ₃ , Li _0.5 La _0.5 TiO ₃ (lithium lanthanum titanate; LLTO), Li ₄ Ti ₅ O ₁₂ , Li ₂ ZrO ₃ , Li ₃ PO ₄ , LiTa ₂ O ₈ , or a combination thereof, an electrode active material is provided.

According to one embodiment of the present invention, the electrode active material is a positive electrode active material for a lithium secondary battery, an electrode active material is provided.

According to one embodiment of the present invention, the electrode active material is an electrode active material that is a positive electrode active material for an all solid state battery.

According to one embodiment of the present invention, the internal density is low and the tap density is low, including nickel at 70 to 98 mol%, and optionally cobalt and manganese at 0 to 15 mol% and 0 to 15 mol%, respectively. is 1.4 to 1.9 g/cm ³ , the precursor size is 4 to 6 μm after growth for 1 hour at the beginning of synthesis, and the growth time is 10 hours or more and 25 hours or less by treating the precursor at a temperature of 700 to 800 ° C. to obtain a specific surface area of 0.8 to 6 μm 1.5 m ² /g, a first step of synthesizing a cathode active material intermediate having pores therein; and a second step of coating the lithium diffusion layer by mixing lithium and a raw material capable of forming a lithium diffusion layer with the cathode active material intermediate and treating it at 400° C. to 800° C. There is provided.

According to one embodiment of the present invention, a layered structure having an average diameter of 6 to 10 μm containing 70 to 98 mol% of nickel and, optionally, 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively. A secondary battery electrode containing an electrode active material, a conductive material, and a binder is provided, which is a particle having an empty space with a diameter of 1 to 3 μm in the center and coated with a lithium diffusion layer from the surface to the inner surface of the particle.

According to one embodiment of the present invention, a secondary battery electrode is provided in which the electrode is bound on a current collector.

According to one embodiment of the present invention, a layered structure having an average diameter of 6 to 10 μm containing 70 to 98 mol% of nickel and, optionally, 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively. A positive electrode formed by including a positive electrode active material, which is a particle having an empty space with a diameter of 1 to 3 μm in the center and coated with a lithium diffusion layer from the surface to the inner surface of the particle, a negative electrode positioned facing the positive electrode, and the positive electrode An all-solid-state secondary battery including one or more unit cells including a solid electrolyte positioned between cathodes is provided.

According to one embodiment of the present invention, a secondary battery is provided in which one or more unit cells are connected in series.

According to one embodiment of the present invention, there is provided a secondary battery that maintains capacity of 85% or more based on the initial capacity during charging and discharging at 25° C. 100 times.

According to an embodiment of the present invention, microparticles having an empty space in the center and coated with a lithium diffusion layer on the outer surface as well as the inner surface are used as the cathode active material, thereby preventing contact with the solid electrolyte even if the particles are broken by charging and discharging. This can be achieved sufficiently, so that it can be used for all-solid-state secondary batteries with excellent stability and high energy density.

1 is a schematic diagram showing the configuration of a positive electrode active material composition according to an embodiment of the present invention.
2 is a diagram showing capacity characteristics by rate of an all-solid-state battery configured including a cathode active material according to an embodiment of the present invention.
3 is a graph showing the lifespan at 25° C. of an all-solid-state battery including a cathode active material according to an embodiment of the present invention.

Since the present invention may have various changes and various forms, specific embodiments are illustrated in the drawings and described in detail in the text. However, it should be understood that this is not intended to limit the present invention to the specific disclosed form, and includes all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

In this application, the terms "include" or "have" are intended to designate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, but one or more other features It should be understood that it does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

The first aspect of the present invention has a layered structure containing 70 to 98 mol% of nickel and optionally 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively, and having an average diameter of 6 to 10 μm. , Provided is an electrode active material comprising an empty space having a diameter of 1 to 3 μm in the center and coated with a lithium diffusion layer between primary particles and on the surface of the secondary particles.

The secondary particle is a polycrystalline particle commonly called an active material particle, and a single crystal constituting the secondary particle may be referred to as a primary particle. At this time, the primary particles in the form of a single crystal are particles having a size of about 100 to 500 nm, and these primary particles may gather to form secondary particles that are active material particles.

According to an embodiment of the present invention, microparticles having an empty space in the center and coated with a lithium diffusion layer on the outer surface as well as the inner surface are used as the cathode active material, so that even though particles having a size of 6 μm or more are used, a solid electrolyte and excellent It exhibits improved output characteristics by showing a contact rate, and not only has excellent life characteristics through the internal lithium diffusion layer, but also has high rate charging characteristics, so that its performance can be greatly improved when applied to an all-solid-state battery.

Although the secondary particles formed as described above have a relatively large size, they are coated with a lithium diffusion layer on the outer surface as well as the inner surface, so that even if the particles are destroyed during charging and discharging, they can maintain contact with the electrolyte, thereby exhibiting improved output characteristics. there is

For example, it is preferable that the lithium diffusion layer is uniformly coated on the surface, and a material that has little side reaction with a solid electrolyte, has electrical conductivity, and has an excellent lithium diffusion coefficient can be selected as the material. For example, the lithium diffusion layer material is LiNbO ₃ , Li _0.5 La _0.5 TiO ₃ (lithium lanthanum titanate; LLTO), Li ₄ Ti ₅ O ₁₂ , Li ₂ ZrO ₃ , Li ₃ PO ₄ , LiTa ₂ O ₈ , or a combination thereof. It may be, but is not limited thereto.

For example, the electrode active material of the present invention may be a cathode active material for a lithium secondary battery.

The term "secondary battery" of the present invention has previously been called an accumulator, and has a name of secondary cell, storage battery, or rechargeable battery in English, and converts external electrical energy into a form of chemical energy and stores it. A device that generates electricity when needed. It is also called a rechargeable battery because it can be reused many times by charging after discharging. Examples of secondary batteries include lead acid batteries, nickel-cadmium batteries (NiCd), nickel-metal hydride batteries (Ni-MH), lithium ion batteries (Li-ion), and lithium ion polymer batteries (Li-ion polymer).

As used herein, the term "lithium secondary battery" refers to a battery in which lithium ions existing in an ionic state move from the positive electrode to the negative electrode during discharging and from the negative electrode to the positive electrode during charging to generate electricity. The performance of a lithium secondary battery depends on the ability of a positive electrode material to activate lithium ions and the presence of sufficient space in the negative electrode material to insert lithium ions. In particular, since lithium is included in the cathode active material, the cathode active material substantially influences the performance of the lithium secondary battery.

At this time, the cathode active material is generally composed of a transition metal oxide, because a change in oxidation number to satisfy a charge neutral state is essential during lithium deintercalation. Characteristics required of a cathode active material include high operating voltage, small polarization during charging and discharging, high capacity and efficiency, life characteristics, and stability with an electrolyte solution.

For example, the electrode active material may be a cathode active material for an all-solid-state battery, but is not limited thereto.

As described above, even though the electrode active material of the present invention has a relatively large particle size, the lithium diffusion layer is coated from the outer surface to the inner surface, which is advantageous in contact with the solid electrolyte, and even if the particles are broken during charging and discharging, contact with the electrolyte is prevented. Since it can show improved output characteristics by maintaining it, it can be used as a positive electrode active material in an all-solid-state battery with excellent stability and high energy density.

As used herein, the term "all solid state battery" refers to a battery including a solid state electrolyte, unlike conventional lithium batteries composed of a positive electrode, a negative electrode, a separator, and a liquid electrolyte. Therefore, in the all-solid-state battery, the solid electrolyte can replace the function of the separator and thus can be eliminated. In the case of conventional lithium batteries, there is a risk of damage to the battery, such as expansion of the battery due to environmental changes such as temperature or leakage due to external shock, because a liquid electrolyte is used, so additional parts or devices are required to secure stability. In contrast, all-solid-state batteries use a solid electrolyte, so they are structurally solid and stable, and are safer because they can maintain their shape even if the electrolyte is damaged. In addition, all-solid-state batteries, as described above, have no risk of explosion or fire, reduce safety-related parts, and fill the space with more active materials that can increase battery capacity, so they have higher energy density than conventional lithium batteries. High-capacity batteries required for electric vehicles can be realized.

The second aspect of the present invention contains 70 to 98 mol% of nickel and optionally 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively, and has a low internal density and a tap density of 1.4 to 1.9 g/cm ³ , the precursor size is 4 to 6 μm after growth for 1 hour at the beginning of synthesis, and the growth time is 10 hours or more and 25 hours or less, and the specific surface area is 0.8 to 1.5 m ² /g, a first step of synthesizing a cathode active material intermediate having pores therein; And a second step of mixing lithium and a raw material capable of forming a lithium diffusion layer with the cathode active material intermediate and treating it at 400 ° C to 800 ° C to coat the lithium diffusion layer. It provides a method for producing an electrode active material.

In this case, the raw material capable of forming the lithium diffusion layer may include elements such as niobium, phosphorus, titanium, zirconium, and tantalum, but is not limited thereto. The raw materials easily combine with lithium even at a temperature of 800° C. or lower, are present on the surface of primary particles without being doped into the active material crystal, and can exhibit better lithium diffusion characteristics than the active material when forming the material.

For example, through the second step, a lithium diffusion layer uniformly coated on the surface of the particle and between the particles may be formed, but is not limited thereto.

Specifically, when forming the lithium diffusion layer through the second step, in order for the lithium diffusion layer to be well coated inside, the pores need to be open toward the inside of the particles during the heat treatment in the first step, and the size of the coating material is small. It may be more preferable, but is not limited thereto. Furthermore, as additionally described above, the coating raw material can be well diffused into the interior along the grain boundary by mixing and treating lithium together. The degree of opening of these pores may appear as a specific surface.

For example, low internal density and internal porosity of the precursor can be achieved by controlling process conditions for preparing the precursor. Specifically, the initial growth rate may be increased and the later growth may be slowed to impart internal pores, but is not limited thereto.

For example, since the lithium diffusion layer is formed along the boundary of the internal primary particles through the heat treatment of the second step, it is preferable that the pores are opened inside the particles during the heat treatment of the first step in order to more smoothly coat the lithium diffusion layer. The smaller the size of is, the more advantageous it is, and in addition to this, coating to the inside can be made more smoothly during heat treatment by mixing lithium.

The third aspect of the present invention has a layered structure containing 70 to 98 mol% of nickel and optionally 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively, and having an average diameter of 6 to 10 μm. , Provided is a secondary battery electrode containing an electrode active material, a conductive material, and a binder, which is a particle having an empty space having a diameter of 1 to 3 μm in the center and coated with a lithium diffusion layer from the surface to the inner surface of the particle.

For example, the electrode may be provided in a state bound to a current collector, but is not limited thereto. Specifically, the current collector may be a foil made of aluminum, nickel, or a combination thereof, but is not limited thereto.

For example, the conductive material may be acetylene black or carbon black, but is not limited thereto. In this case, the content of the conductive material used in the electrode may be 0.5 to 10% by weight in the case of the positive electrode and 10% by weight or less in the case of the negative electrode, but is not limited thereto.

For example, the binder is polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyacrylonitrile, nitrile rubber, polybutadiene, polystyrene, styrene butadiene rubber, polysulfide rubber, butyl rubber, hydrogenated styrene butadiene rubber, nitrocellulose , And one or more species may be selected from the group consisting of carboxymethyl cellulose, but is not limited thereto. In this case, the binder may be included in an amount of 0.1 to 15% by weight, but is not limited thereto.

The fourth aspect of the present invention has a layered structure containing 70 to 98 mol% of nickel and optionally 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively, and having an average diameter of 6 to 10 μm. , A positive electrode formed by including a positive electrode active material, which is a particle having an empty space having a diameter of 1 to 3 μm in the center and coated with a lithium diffusion layer from the surface to the inner surface of the particle, a negative electrode positioned facing the positive electrode, and between the positive electrode and the negative electrode Provided is an all-solid-state secondary battery including one or more unit cells containing a solid electrolyte located in the.

For example, the secondary battery of the present invention may be formed by connecting a plurality of unit cells in series, but is not limited thereto.

For example, the secondary battery of the present invention may be a battery with reduced capacity degradation and extended lifespan, maintaining capacity of 85% or more based on the initial capacity when charging and discharging is repeated 100 times at 25°C.

For example, the electrode active material according to the present invention can exhibit excellent effects as a positive electrode active material in a lithium secondary battery.

For example, an anode included in the secondary battery of the present invention may include an anode active material. Examples of the negative electrode active material include lithium adsorption materials such as lithium metal, lithium alloy, carbon, petroleum coke, activated carbon, graphite, or various other carbons. Not limited. In addition, the negative electrode active material may be provided in a form bound to a negative electrode current collector, for example, a foil made of copper, gold, nickel, copper alloy, or a combination thereof, but is not limited thereto.

For example, the solid electrolyte may include an organic electrolyte including a dry polymer electrolyte; inorganic electrolytes including sulfide-based, oxide-based, and phosphoric acid-based electrolytes; A composite electrolyte containing a nanoparticle filler and a polymer; or a combination thereof. An example of a sulfide-based electrolyte is Li ₁₀ GeP ₂ S ₁₂ (LGPS), an example of a garnet-type oxide-based electrolyte is Li ₇ La ₃ Zr ₂ O ₁₂ (Lithium lanthanum zirconium oxide; LLZO), and a phosphoric acid-based electrolyte Examples include, but are not limited to, Li _1.4 Al _0.4 Ti _1.6 (PO ₄ ) ₃ (LATP).

Hereinafter, the present invention will be described in more detail by examples and experimental examples. However, the following examples and experimental examples are only for exemplifying the present invention, and the scope of the present invention is not limited only to these.

Experimental example

It has a composition of 83 mol% Ni, 12 mol% Co, and 5 mol% Mn, a tap density of 1.8 g / cm ³ , and the growth rate of the precursor was adjusted to grow 5 μm per hour during synthesis, and grown for a total of 20 hours, After using a 7 μm-sized precursor with a low internal density, mixing a lithium raw material at a molar ratio of 1.02 to the transition metal, heat treatment at ⁷⁶⁰ ° C. Particles were synthesized. Particles containing pores were synthesized by heat treatment at a low temperature. Then, after dissolving ammonium niobate oxalate hydrate (C ₄ H ₄ NNbO ₉ H ₂ O) and lithium hydroxide hydrate (LiOH H ₂ O) in distilled water, they were dissolved in 1 mol% ratio relative to the transition metal. It was mixed with the active material particles, that is, the secondary particles. Thereafter, heat treatment was performed at 780° C. to prepare a cathode active material coated with a lithium diffusion layer between and on the surface of the single crystal primary particles constituting the secondary particles. At this time, the formed cathode active material was made of particles having a total diameter of 7 μm including pores having a size of 2.5 μm therein. An all-solid-state cell was constructed using the cathode active material synthesized as described above and the solid electrolyte having an average diameter of 1 μm.

Comparative Example 1

After mixing a precursor having the same composition as the experimental example, a tap density of 2.0 g/cm ³ , a growth rate per hour of 3 μm, and a size of 7 μm, and a lithium raw material at a molar ratio of 1.02 to the transition metal in the precursor, 800 ° C. Heat treatment was performed to synthesize 7 μm-sized particles having no internal pores and a specific surface area of 0.5 m ² /g. Subsequently, in the same manner as in the experimental example, lithium and Nb raw materials were wet-mixed and heat-treated at 400° C. to prepare a positive electrode active material coated with LiNbO ₃ on the surface. An all-solid-state cell was constructed using the cathode active material synthesized as described above and the solid electrolyte having an average diameter of 1 μm.

Comparative Example 2

Active material particles having a size of 3 μm were synthesized using a 3 μm-sized precursor having no internal pores having the same composition as the experimental example, and coated with LiNbO ₃ in the same manner as in Comparative Example 1 to prepare a positive electrode active material. An all-solid-state cell was constructed using the cathode active material synthesized as described above and the solid electrolyte having an average diameter of 1 μm.

Experimental Example 3

For the cells of the Experimental Example and Comparative Examples 1 and 2, the capacity characteristics by rate were measured in FIG. 2, and the high-temperature lifespan was measured and shown in FIG. 3. Furthermore, the CC retention rate and the time required for 90% capacity charging were measured at a 2C charging rate, which is a rapid charging environment, and are shown in Table 1 below.

Specifically, after mixing the sulfide-based solid electrolyte and Li ₆ PS ₅ Cl particles at a mass ratio of 7:3 with respect to the active material, layering the sulfide solid electrolyte layer and the lithium/indium alloy layer with a size of 5 μm, and then applying a pressure of 300 MPa was added to fabricate an all-solid-state cell. The charge/discharge for evaluation of the discharge rate disclosed in FIG. 2 was performed at 45° C. in the voltage range of 2.0 to 4.25 V, and the life test disclosed in FIG. cutoff) was performed under charging and discharging conditions of 0.2C.

As shown in FIG. 2, in terms of capacity characteristics by rate, the cell of Experimental Example exhibited high initial capacity and excellent output characteristics compared to the cells of Comparative Examples 1 and 2. On the other hand, the cell of Comparative Example 1 having a positive electrode active material of the same size and composition without pores inside had a significantly reduced high-rate characteristic, and the cell of Comparative Example 2 having a small-sized positive electrode active material had contact with a solid electrolyte. This was not smooth, so the initial capacity was greatly reduced.

In addition, as shown in FIG. 3 , the cell of Experimental Example exhibited a remarkably superior lifespan retention rate compared to the cells of Comparative Examples 1 and 2.

As shown in Table 1, the cell of the experimental example showed an excellent CC retention rate in a rapid charging environment, and the charging time was also shortened.

CC (%) Charging time to 90% capacity (minutes) Experimental example 81.5 35 Comparative Example 1 32.3 58 Comparative Example 2 77.3 41

*2C charge, 0.05C cutoff, 45℃

Although the above has been described with reference to preferred embodiments of the present invention, those skilled in the art or those having ordinary knowledge in the art do not deviate from the spirit and technical scope of the present invention described in the claims to be described later. It will be understood that the present invention can be variously modified and changed within the scope not specified.

Therefore, the technical scope of the present invention is not limited to the contents described in the detailed description of the specification, but should be defined by the claims.

Claims (10)

It has a layered structure containing 70 to 98 mol% of nickel and, optionally, 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively, with an average diameter of 6 to 10 μm and a center of 1 to 3 μm. An electrode active material comprising an empty space of a diameter and coated with a lithium diffusion layer between primary particles and on the surface of secondary particles.
According to claim 1,
The lithium diffusion layer is made of LiNbO ₃ , Li _0.5 La _0.5 TiO ₃ (lithium lanthanum titanate; LLTO), Li ₄ Ti ₅ O ₁₂ , Li ₂ ZrO ₃ , Li ₃ PO ₄ , LiTa ₂ O ₈ or a combination thereof. , electrode active material.
According to claim 1,
The electrode active material is a positive electrode active material for a lithium secondary battery, an electrode active material.
According to claim 1,
The electrode active material is an electrode active material that is an all solid state battery positive electrode active material.
70 to 98 mol% nickel, optionally cobalt and manganese in 0 to 15 mol% and 0 to 15 mol%, respectively, low internal density, tap density 1.4 to 1.4 1.9 g/cm ³ , the precursor size is 4 to 6 μm after 1 hour of initial growth, and the growth time is 10 hours or more and 25 hours or less, and the specific surface area is 0.8 to 1.5 m ² by treating the precursor at a temperature of 700 to 800 ° C. ^/ g, a first step of synthesizing a cathode active material intermediate having pores therein; and
A method for producing an electrode active material comprising a second step of mixing lithium and a raw material capable of forming a lithium diffusion layer with the cathode active material intermediate and treating it at 400 ° C to 800 ° C to coat the lithium diffusion layer.
It has a layered structure containing 70 to 98 mol% of nickel and, optionally, 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively, with an average diameter of 6 to 10 μm and a center of 1 to 3 μm. A secondary battery electrode containing an electrode active material, a conductive material, and a binder, which is a particle including an empty space in diameter and coated with a lithium diffusion layer from the surface to the inner surface of the particle.
According to claim 6,
The electrode is a secondary battery electrode that is bound on a current collector.
It has a layered structure containing 70 to 98 mol% of nickel and, optionally, 0 to 15 mol% and 0 to 15 mol% of cobalt and manganese, respectively, with an average diameter of 6 to 10 μm and a center of 1 to 3 μm. A positive electrode formed by including a positive electrode active material, which is a particle having an empty space of a diameter and coated with a lithium diffusion layer from the surface to the inner surface of the particle, a negative electrode facing the positive electrode, and a solid electrolyte disposed between the positive electrode and the negative electrode. An all-solid-state secondary battery comprising one or more unit cells that
According to claim 8,
A secondary battery in which one or more unit cells are connected in series.
According to claim 8,
A secondary battery that maintains a capacity of 85% or more based on the initial capacity during charging and discharging by repeating 100 times at 25 ° C.

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