KR20240037783A - Cathode for lithium secondary battery and lithium secondary battery including the same - Google Patents

Abstract

A positive electrode for a lithium secondary battery according to exemplary embodiments includes a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector and including lithium excess oxide particles. The electrochemically active surface area of the anode may be from 0.5 m ² /g to 2.5 m ² /g.

Description

Anode for lithium secondary battery and lithium secondary battery including same {CATHODE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME}

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery containing the same. More specifically, it relates to a positive electrode for a lithium secondary battery containing lithium excess oxide particles and a lithium secondary battery containing the same.

Secondary batteries are batteries that can be repeatedly charged and discharged, and are widely used as a power source for portable electronic devices such as mobile phones and laptop PCs.

Lithium secondary batteries are being actively developed and applied because they have high operating voltage and energy density per unit weight, and are advantageous in charging speed and weight reduction.

A lithium secondary battery can store electrical energy by the difference in chemical potential when lithium ions are inserted and desorbed from the positive and negative electrodes. Accordingly, lithium secondary batteries can use materials capable of reversible insertion and detachment of lithium ions as positive and negative active materials.

For example, as the positive electrode active material, lithium metal oxide particles having a layered crystal structure of ABO ₂ (e.g., lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂₎ , lithium nickel-cobalt-manganese oxide (NCM), lithium nickel-aluminum-manganese oxide (NCA), etc.) are used. The lithium metal oxide particles can realize a reversible capacity of about 100 mAh/g to 230 mAh/g.

Meanwhile, as lithium secondary batteries are applied to electric vehicles (EVs), lithium metal oxide particles with higher capacity than the lithium metal oxide particles are being researched and developed. For example, Korean Patent Publication No. 10-1369951 uses lithium over lithiated oxide particles to improve the capacity of lithium secondary batteries.

The lithium excess oxide particles are known to have a structure in which lithium is inserted into the transition metal site of the lithium metal oxide particles having an ABO ₂ layered crystal structure. Accordingly, the lithium excess oxide particles may be expressed as Li _a M _b O ₂ (M is a transition metal such as Mn, Ni, Co, etc., 1.8≤a+b≤2.2 and a/b≥1.05).

According to the results of XRD analysis of the lithium excess oxide particles, the lithium excess oxide particles have a Li ₂ MnO ₃ domain (C 2 /m space group) and a LiMO ₂ domain (R 3 m space group, M is Mn, Ni, Co, etc.) It can be included. Accordingly, the lithium excess oxide particles may be expressed as xLi ₂ MnO ₃ · (1-x)LiMO ₂ (0.05<x<1).

The lithium excess oxide particles may exhibit a reversible capacity of 250 mAh/g or more through electrochemical reaction of Li ₂ MnO ₃ .

For example, Li ₂ MnO ₃ is electrochemically inactive, but can be activated through an activation process (for example, charging and discharging a lithium secondary battery using lithium excess oxide particles at 4.4 V (vs Li/Li ⁺ ) or higher). It can be converted to LiMnO ₂ as shown in Scheme 1. The LiMnO ₂ can reversibly insert and desorb lithium ions as shown in Scheme 2 below. Accordingly, the lithium excess oxide particles may exhibit high reversible capacity according to the LiMO ₂ domain and the Li ₂ MnO ₃ domain.

[Scheme 1]

(Charge) Li ₂ MnO ₃ → MnO ₂ + 2Li ⁺ + 1/2O ₂ + 2e ^-

(Discharge) MnO ₂ + Li ⁺ + e ^- → LiMnO ₂

[Scheme 2]

(Charge) LiMnO ₂ → MnO ₂ + Li ⁺ + e ^-

(Discharge) MnO ₂ + Li ⁺ + e ^- → LiMnO ₂

However, the lithium excess oxide particles are inferior in operational reliability (eg, life characteristics, etc.).

Korean Patent Publication No. 10-1369951

One object of the present invention is to provide a positive electrode for a lithium secondary battery with improved capacity and operational reliability.

One object of the present invention is to provide a lithium secondary battery with improved capacity and operational reliability.

A positive electrode for a lithium secondary battery according to exemplary embodiments includes a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector and including a positive electrode active material containing lithium excess oxide particles represented by the following Chemical Formula 1. The electrochemically active surface area of the positive electrode expressed by Equation 1 below may be 0.5 m ² /g to 2.5 m ² /g.

[Formula 1]

Li _a [M _x Ni _y Mn _z ]O _b

In Formula 1, M is at least one of Co, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi, 0≤x≤ 0.9, 0≤y≤0.9, x+y>0, 0.1≤z≤0.9, 1.8≤a+x+y+z≤2.2, 1.05≤a/(x+y+z)≤1.95, 1.8≤b≤ It could be 2.2.

[Equation 1]

Electrochemically active surface area (m ² /g) = C(F)/[W(g)×{0.2(F/m ² )}]

In Equation 1, C is the capacitance of one anode measured by analyzing a 2-electrode symmetrical cell manufactured using two of the above anodes by cyclic voltammetry, and W is the capacitance of one anode in the 2-electrode symmetrical cell. This is the weight of the positive electrode active material included.

In one embodiment, the positive electrode may have an electrochemically active surface area of 1.1 m ² /g to 1.8 m ² /g.

In one embodiment, the XRD peak intensity ratio of the anode expressed by Equation 2 below may be 4% or more.

[Equation 2]

XRD peak intensity ratio (%) = 100 × I (110) / [I (110) + I (003)]

In Equation 2, I (110) is the maximum height of the (110) plane peak in the XRD spectrum analyzing the positive electrode active material layer, and I (003) is the maximum height of the (003) plane peak in the XRD spectrum.

In one embodiment, the content of the lithium excess oxide particles in the total weight of the positive electrode active material layer may be 80% by weight or more.

In one embodiment, the density of the positive electrode active material layer may be 2.5 g/cc to 3.8 g/cc.

In one embodiment, the mole fraction of manganese to all elements except lithium and oxygen in the lithium excess oxide particles may be 0.5 to 0.75.

In one embodiment, the mole fraction of cobalt to all elements except lithium and oxygen in the lithium excess oxide particles may be 0 to 0.02.

In one embodiment, the lithium excess oxide particles have the form of secondary particles in which a plurality of primary particles are aggregated, and the plurality of primary particles may include plate-shaped particles having an aspect ratio of 3 to 9.

In one embodiment, in a scanning electron microscope (SEM) image measuring a cross section of the lithium excess oxide particle, the ratio of the total area of the plate-shaped particles to the area of the lithium excess oxide particle may be 0.8 or more.

In one embodiment, the lithium excess oxide particles may include lithium excess oxide particles that satisfy Equation 3.

[Equation 3]

P _h /P _f ＜0.2

In Equation 3, P _f is the total area of pores present in the lithium excess oxide particles in a scanning electron microscope (SEM) image measuring the cross section of the lithium excess oxide particles. P _h is the total area of pores present in an area of 0.5R thickness from the surface of the lithium excess oxide particle, when the radius of the lithium excess oxide particle is defined as R.

In one embodiment, the lithium excess oxide particle may include a Li ₂ MnO ₃ domain and/or a domain derived from the Li ₂ MnO ₃ domain.

In one embodiment, the domain derived from the Li ₂ MnO ₃ domain may include at least one of MnO ₂ , Mn ₂ O ₄ , LiMnO ₂ , LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ .

In one embodiment, the anode may satisfy Equation 4.

[Equation 4]

(Rct2/Rct1)×100≤220%

In Equation 4, when a half-cell was manufactured using the positive electrode and the lithium counter electrode, Rct1 was charged and discharged at 1C by repeating the half-cell 100 times at 45°C and in a voltage range of 2V to 4.3V, and then electrochemically charged. This is the charge transfer resistance value of the positive electrode measured according to the impedance analysis method. Rct2 is the charge transfer resistance value of the positive electrode measured according to electrochemical impedance analysis after charging and discharging 1C by repeating 1C charging and 1C discharging of the half cell 100 times at 45°C and a voltage range of 2V to 4.5V.

A lithium secondary battery according to exemplary embodiments includes the positive electrode; And it may include a cathode opposing the anode.

In one embodiment, the upper operating limit voltage of the lithium secondary battery may be 4.5 V or less compared to the redox potential of lithium.

The positive electrode for a lithium secondary battery according to exemplary embodiments of the present invention may exhibit high capacity by including lithium excess oxide particles. Additionally, it is possible to exhibit improved lifespan characteristics by satisfying Equation 1, which will be described later.

A positive electrode for a lithium secondary battery according to some embodiments may include lithium excess oxide particles having a predetermined morphology and may have improved output characteristics.

Lithium secondary batteries according to example embodiments may include the positive electrode and have improved lifespan characteristics and output characteristics.

1 is a schematic cross-sectional view of a positive electrode for a lithium secondary battery according to example embodiments.
2 and 3 are schematic plan views and cross-sectional views, respectively, showing lithium secondary batteries according to example embodiments.
Figures 4, 6, 8, and 10 are images of the surfaces of lithium excess oxide particles of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, respectively, measured using a scanning electron microscope (SEM).
Figures 5, 7, 9, and 11 are images of cross-sections of lithium excess oxide particles of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, respectively, measured using a scanning electron microscope (SEM).

According to exemplary embodiments of the present invention, a positive electrode for a lithium secondary battery including lithium over-lithiated oxide particles (OLO) and having improved capacity, lifespan characteristics, output characteristics, etc. is provided.

Additionally, a lithium secondary battery including the positive electrode is provided.

Hereinafter, a positive electrode for a lithium secondary battery and a lithium secondary battery according to exemplary embodiments of the present invention will be described in more detail with reference to the drawings. However, the drawings and examples are illustrative, and the present invention is not limited by the drawings and examples.

Anode for lithium secondary battery

1 is a schematic cross-sectional view showing a positive electrode for a lithium secondary battery according to example embodiments.

Referring to FIG. 1, the positive electrode 100 for a lithium secondary battery may include a positive electrode current collector 105 and a positive electrode active material layer 110 formed on the positive electrode current collector 105.

For example, the positive electrode active material layer 110 may be formed on one side or both sides of the positive electrode current collector 105.

For example, the positive electrode current collector 105 may include stainless steel, nickel, aluminum, titanium, copper, or alloys thereof.

The positive electrode active material layer 110 may include a positive electrode active material capable of reversible insertion and desorption of lithium ions. For example, the positive electrode active material may include lithium metal oxide particles.

According to exemplary embodiments, the positive electrode active material may include lithium excess oxide particles.

In one embodiment, the lithium excess oxide particles include Li ₂ MnO ₃ domain (C2/m space group) and Li _a M _b O _c domain (R3m space group, M is Ni Mn, Co, Mg, V, Ti, At least one of Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, 1.8≤a+b≤2.2, 0.9≤a/b<1.05 and 1.9 ≤c≤2.1).

In some embodiments, in the lithium excess oxide particles, the molar ratio of the Li ₂ MnO ₃ domain and the LiMO ₂ domain may be expressed as w:1-w, and w may be 0.05 to 0.7, or 0.1 to 0.7. there is.

For example, the lithium excess oxide particles may be prepared according to a coprecipitation method.

For example, metal hydroxide particles can be prepared by mixing various types of metal salts, chelating agents (e.g., aqueous ammonia, ammonium carbonate, etc.) and co-precipitation agents (e.g., sodium hydroxide, sodium carbonate, etc.) and performing a co-precipitation reaction. You can. For example, the molar ratio between the various metal salts can be adjusted according to the chemical formula of the intended lithium excess oxide particle.

For example, the metal hydroxide particles and the lithium source are mixed and calcined so that the ratio of the number of moles of the lithium source to the number of moles of the metal hydroxide particles is 1.05 to 1.95, 1.1 to 1.95, 1.15 to 1.95, or 1.2 to 1.95 to obtain the excess lithium. Oxide particles can be manufactured.

In one embodiment, the lithium source may include lithium hydroxide or lithium carbonate. In some embodiments, the lithium source may include lithium hydroxide.

In one embodiment, the calcination may be performed at 400 to 1100°C. For example, the firing may be performed for 2 to 16 hours.

In some embodiments, the firing may include primary firing performed at 200 to 300°C and secondary firing performed at 700 to 1100°C.

For example, the lithium excess oxide particles can be activated by applying a voltage of 4.4V (vs Li/Li ⁺ ) or more (for example, 4.4V to 4.8V) to the lithium excess oxide particles (Scheme 1 below) 1). Alternatively, the lithium secondary battery containing the lithium excess oxide particles may be charged and discharged at a voltage of 4.4V (vs Li/Li ⁺ ) or more to activate the lithium excess oxide particles (Schemes 1-1 and 1- below) 2).

In one embodiment, the activated particle may include a domain derived from the Li ₂ MnO ₃ domain of the lithium excess oxide particle.

In some embodiments, the domain derived from the Li ₂ MnO ₃ domain may include at least one of MnO ₂ , Mn ₂ O ₄ , LiMnO ₂ , LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ .

For example, Li ₂ MnO ₃ of at least some of the lithium excess oxide particles may be converted into MnO ₂ and LiMnO ₂ by the activation as shown in Scheme 1-1 and Scheme 1-2 below. The MnO ₂ and LiMnO ₂ can reversibly insert and extract lithium ions as shown in Scheme 2 below. Accordingly, the lithium excess oxide particles can exhibit high capacity.

[Reaction Scheme 1-1]

(Charge) Li ₂ MnO ₃ → MnO ₂ + 2Li+ + 1/2O ₂ + 2e ^-

[Reaction Scheme 1-2]

(Discharge) MnO ₂ + Li ⁺ + e ^- → LiMnO ₂

[Scheme 2]

(Charge) LiMnO ₂ → MnO ₂ + Li ⁺ + e ^-

(Discharge) MnO ₂ + Li ⁺ + e ^- → LiMnO ₂

In some embodiments, LiMnO ₂ among the activated particles may be further reacted and converted into Mn ₂ O ₄ , LiMn ₂ O ₄ or Li ₂ Mn ₂ O ₄ .

In some embodiments, the activated particle may include the Li _a M _b O _c domain, and a Li ₂ MnO ₃ domain and/or a domain derived from the Li ₂ MnO ₃ domain.

In exemplary embodiments, the lithium excess oxide particles and/or the activated particles may be represented by Formula 1 below.

[Formula 1]

Li _a [M _x Ni _y Mn _z ]O _b

In Formula 1, M may be at least one of Co, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi.

0≤x≤0.9, 0≤y≤0.9 (however, x+y>0), 0.1≤z≤0.9, 1.8≤a+x+y+z≤2.2, 1.05≤a/(x+y+z) It may be ≤1.95, 1.8≤b≤2.2.

In some embodiments, 0<x≤0.9, 0.05≤x≤0.9, 0.1≤x≤0.9, 0<x≤0.8, 0.05≤x≤0.8, or 0.1≤x≤0.8.

In some embodiments, 0<y≤0.9, 0.05≤y≤0.9, 0.1≤y≤0.9, 0<y≤0.8, 0.05≤y≤0.8, or 0.1≤y≤0.8.

In some embodiments, 1.1≤a/(x+y+z)≤1.95, 1.15≤a/(x+y+z)≤1.95, 1.2≤a/(x+y+z)≤1.95, or 1.3 It may be ≤a/(x+y+z)≤1.95.

In some embodiments, the mole fraction of manganese to all elements except lithium and oxygen in the lithium excess oxide particles may be 0.5 to 0.75. For example, 0.5≤z/(x+y+z)≤0.75.

In some embodiments, 0.25≤(x+y)/(x+y+z)≤0.5.

In some embodiments, the mole fraction of cobalt to all elements except lithium and oxygen in the lithium excess oxide particles may be 0 to 0.02. In some embodiments, the lithium excess oxide particles may not contain cobalt.

In some embodiments, 1.9≤b≤2.1, or 1.95≤b≤2.05.

For example, the composition of the lithium excess oxide particles and the activated particles can be confirmed by inductively coupled plasma (ICP). For example, the lithium excess oxide particles and/or the activated particles are analyzed by ICP, and the number of oxygen atoms is normalized to 1.8 to 2.2 (preferably, 2), thereby producing the lithium excess oxide particles and/or the activated particles. Alternatively, the chemical formula of the activated particle can be obtained.

In one embodiment, the positive active material layer 110 may further include a binder and a conductive material.

In one embodiment, the binder is an organic binder such as polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, and polymethyl methacrylate; It may include an aqueous binder such as styrene-butadiene rubber (SBR). In some embodiments, the binder may be used with a thickening agent such as carboxymethyl cellulose (CMC).

In one embodiment, the conductive material is a carbon-based conductive material such as graphite, carbon nanotube (CNT), carbon black, or graphene; It may include metal-based conductive materials such as perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO ₃ , and LaSrMnO ₃ .

In one embodiment, the content of the lithium excess oxide particles in the total weight of the positive electrode active material layer 110 may be 80% by weight or more, 85% by weight or more, or 90% by weight or more. In some embodiments, the content of the lithium excess oxide particles in the total weight of the positive electrode active material layer 110 may be 98% by weight or less or 95% by weight or less.

In one embodiment, the loading amount of the positive electrode active material layer 110 may be 5 to 28 mg/cm ² , 7 to 25 mg/cm ² , 8 to 20 mg/cm ² , or 9 to 15 mg/cm ² there is.

In one embodiment, the density of the positive active material layer 110 may be 2.5 to 3.8 g/cc, 2.6 to 3.7 g/cc, or 2.7 to 3.6 g/cc.

Meanwhile, a lithium secondary battery using the lithium excess oxide particles may have inferior lifespan characteristics due to the unique electrochemical reaction of the Li ₂ MnO ₃ domain among the lithium excess oxide particles.

Additionally, a lithium secondary battery using the lithium excess oxide particles may have a relatively higher voltage operating section than a lithium secondary battery using general lithium metal oxide particles. At high voltage, side reactions between the lithium excess oxide particles and the electrolyte may intensify, and accordingly, the lifespan characteristics of the lithium secondary battery may deteriorate.

In exemplary embodiments, the electrochemical active surface area of the anode 100 represented by Equation 1 below may be 0.5 to 2.5 m ² /g. Accordingly, the lifespan characteristics of the lithium secondary battery can be improved.

[Equation 1]

Electrochemically active surface area (m ² /g) = C(F)/[W(g)×{0.2(F/m ² )}]

In Equation 1, C is one (1) anode measured by analyzing a 2-electrode symmetry cell manufactured using two anodes (100) using cyclic voltammetry. This is the electrochemical double layer capacitance (EDLC) value. W is the weight of the positive electrode active material contained in one positive electrode in the two-electrode symmetrical cell.

For example, the electrochemically active surface area of the anode 100 can be measured according to Evaluation Example 2, which will be described later.

Unlike specific surface area (BET), the electrochemically active surface area can substantially capture information related to electrochemical reactions at and near the electrode surface. Accordingly, by controlling the electrochemically active surface area, it is possible to effectively prevent deterioration of the lifespan characteristics of the lithium secondary battery due to the lithium excess oxide particles.

For example, if the electrochemically active surface area of the positive electrode 100 is less than 0.5 m ² /g, the output characteristics of the lithium secondary battery may be excessively reduced. Additionally, if the electrochemically active surface area of the positive electrode 100 exceeds 2.5 m ² /g, the lifespan characteristics of the lithium secondary battery may be inferior.

In one embodiment, the electrochemically active surface area of the anode 100 is 0.5 to 2.4 m ² /g, 0.6 to 2.3 m ² /g, 0.7 to 2.2 m ² /g, 0.8 to 2.1 m ² /g, 0.9. It may be from 2 m ² /g or from 1 to 1.9 m ² /g or from 1.1 to 1.8 m ² /g. Within the above range, the lifespan characteristics of a lithium secondary battery can be further improved.

In one embodiment, the XRD peak intensity ratio of the anode 100 represented by Equation 2 below may be 4% or more, or 5% or more. In some embodiments, the XRD peak intensity ratio may be 10% or less, 9% or less, 8% or less, or 7% or less.

[Equation 2]

XRD peak intensity ratio (%) = 100 × I (110) / [I (110) + I (003)]

In Equation 2, I (110) is the maximum height of the (110) plane peak in the XRD spectrum analyzing the positive electrode active material layer, and I (003) is the maximum height of the (003) plane peak.

If the positive electrode 100 satisfies the XRD peak intensity ratio within the above range, both the lifespan characteristics and output performance of the lithium secondary battery can be improved.

In one embodiment, the lithium excess oxide particles (and the activated particles) may have the form of secondary particles in which a plurality of primary particles are aggregated.

In some embodiments, the plurality of primary particles may include plate-shaped particles having an aspect ratio of 2.5 to 12, 3 to 9, or 4 to 7. For example, the aspect ratio may be defined as the length of the long side (or long axis) of the plate-shaped particle divided by the thickness. For example, the aspect ratio can be measured from a scanning electron microscope (SEM) image of the lithium excess oxide particle. For example, if the electrochemically active surface area of the positive electrode decreases, the output characteristics of the lithium secondary battery may decrease. However, by adjusting the morphology of the primary particles to the plate shape, a decrease in output performance can be prevented.

In some embodiments, the particle diameter D ₅₀ of the lithium excess oxide particles (the secondary particles) may be 2 ㎛ to 9 ㎛. The particle size D ₅₀ may be the particle size at 50% of the volume particle size distribution, and can be measured using a laser diffraction method.

In some embodiments, the sphericity of the lithium excess oxide particles (the secondary particles) may be 0.7 or more, 0.8 or more, or 0.9 or more.

In some embodiments, in a scanning electron microscope (SEM) image measuring the surface of the lithium excess oxide particle, the length of the long side of the plate-shaped particle is 0.1 to 3 ㎛, 0.3 to 2.5 ㎛, 0.5 to 2 ㎛, 0.75 to 0.75 ㎛. It may be 1.5 μm, or 0.8 to 1.2 μm. Additionally, the thickness of the plate-shaped particles may be 100 to 200 nm. Within the above range, the output performance of the lithium secondary battery can be further improved.

In some embodiments, in a scanning electron microscope (SEM) image measuring a cross-section of the lithium excess oxide particle, the total area of the plate-shaped particles relative to the area of the lithium excess oxide particle (excluding the pore portion) is calculated. The ratio may be at least 0.6, at least 0.7, at least 0.8, or at least 0.9.

In one embodiment, the lithium excess oxide particles may include lithium excess oxide particles that satisfy Equation 3 below. Accordingly, the lifespan characteristics of the lithium secondary battery can be further improved.

[Equation 3]

P _h /P _f ＜0.2

In Equation 3, P _f is the total area of pores present in the lithium excess oxide particles in a scanning electron microscope (SEM) image measuring the cross section of the lithium excess oxide particles. P _h is the total area of pores present in an area of 0.5R thickness from the surface of the lithium excess oxide particle, when the radius of the lithium excess oxide particle is defined as R.

In some embodiments, P _h /P _f may be less than or equal to 0.15, less than or equal to 0.1, or less than or equal to 0.05. In some embodiments, P _h /P _f may be greater than or equal to 0.01 or greater than or equal to 0.02.

In one embodiment, the anode 100 may satisfy Equation 4 below.

[Equation 4]

(Rct2/Rct1)×100≤220%

In Equation 4, when a half-cell was manufactured using the positive electrode and the lithium counter electrode, Rct1 was charged and discharged at 1C by repeating the half-cell 100 times at 45°C and in a voltage range of 2V to 4.3V, and then electrochemically charged. This is the charge transfer resistance value of the positive electrode measured according to the impedance analysis method. Rct2 is the charge transfer resistance value of the positive electrode measured according to electrochemical impedance analysis after charging and discharging 1C by repeating 1C charging and 1C discharging of the half cell 100 times at 45°C and a voltage range of 2V to 4.5V.

In some embodiments, (Rct2/Rct1)×100 may be less than or equal to 200%, less than or equal to 180%, less than or equal to 150%, or less than or equal to 130%.

lithium secondary battery

Figure 2 is a schematic plan view showing a lithium secondary battery according to example embodiments. Figure 3 is a schematic cross-sectional view of a lithium secondary battery taken along line II' of Figure 2.

Referring to FIGS. 2 and 3 , the lithium secondary battery may include an electrode assembly 150 and a case 160 that accommodates the electrode assembly 150 .

The electrode assembly 150 may include an anode 100 and a cathode 130 opposing the anode 100. In one embodiment, the electrode assembly 150 may further include a separator 140 interposed between the anode 100 and the cathode 130.

In one embodiment, the positive electrode 100 may be the positive electrode for a lithium secondary battery described above.

For example, the negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120 on the negative electrode current collector 125. For example, the negative electrode active material layer 120 may be formed on one or both sides of the negative electrode current collector 125.

For example, the negative electrode active material layer 120 may include the negative electrode active material, the binder, and the conductive material.

For example, the negative electrode current collector 125 may include gold, stainless steel, nickel, aluminum, titanium, copper, or alloys thereof.

In one embodiment, the negative electrode active material may be a material capable of inserting and desorbing lithium ions. For example, the negative electrode active material may include lithium alloy, carbon-based active material, silicon-based active material, etc.

For example, the lithium alloy may include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, etc.

For example, the carbon-based active material may include crystalline carbon, amorphous carbon, carbon composite, carbon fiber, etc.

For example, the amorphous carbon may include hard carbon, coke, mesocarbon microbeads, mesophase pitch-based carbon fiber, etc.

For example, the crystalline carbon may include natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

For example, the silicon-based active material may include Si, SiO _x (0<x<2), Si/C, SiO/C, Si-Metal, etc.

In some embodiments, the area of the cathode 130 may be larger than the area of the anode 100.

In one embodiment, the positive electrode current collector 105 may include a positive electrode tab 106 protruding from one side of the positive electrode current collector 105.

For example, the positive electrode tab 106 may be integrated with the positive electrode current collector 105 or may be electrically connected to the positive electrode current collector 105 by welding or the like. The positive electrode current collector 105 and the positive electrode lead 107 may be electrically connected through the positive electrode tab 106.

In one embodiment, the negative electrode current collector 125 may include a negative electrode tab 126 protruding on one side of the negative electrode current collector 125.

For example, the negative electrode tab 126 may be integrated with the negative electrode current collector 125 or may be electrically connected to the negative electrode current collector 125 by welding or the like. The negative electrode current collector 125 and the negative electrode lead 127 may be electrically connected through the negative electrode tab 126.

For example, the separator 140 may include a porous polymer film made of polyolefin-based polymers such as polyethylene, polypropylene, ethylene-butene copolymer, ethylene-hexene copolymer, and ethylene-methacrylate copolymer. For example, the separator 140 may include a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc.

For example, the electrode assembly 150 and the electrolyte solution may be accommodated together in the pouch case 160 to form a lithium secondary battery.

In one embodiment, the electrolyte solution may include lithium salt and an organic solvent.

In one embodiment, the lithium salt may include ^{Li + For example , ​ ​} _​ ^​ _​ ^​ _​ ^​ _​ ^​ _​ ^​ _{​ ​ ​} ^​ , (CF ₃ ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , CF ₃ SO ₃ ^- , CF ₃ CF ₂ SO ₃ ^- , ( CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- _, CF ₃ CF ₂ (CF ₃ ) ₂ CO ^- , (CF ₃ SO ₂ ) ₂ CH ^- , (SF ₅ ) ₃ C ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ (CF ₂ ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , It may be any one of SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

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

In one embodiment, the upper operating limit voltage of the lithium secondary battery may be 4.8V or less, 4.7V or less, or 4.6V or less compared to the lithium redox potential (vs Li/Li ⁺ ). The “operating upper limit voltage” refers to the upper limit voltage during actual operation (i.e., actual use) of a lithium secondary battery, and can be distinguished from the activation voltage in an activation process during the manufacturing process of a lithium secondary battery.

In some embodiments, the upper operating limit voltage of the lithium secondary battery may be 4.5V or less compared to the lithium redox potential (vs Li/Li ⁺ ). In some embodiments, the upper operating voltage limit may be 4.3V to 4.5V. Within the above range, the lifespan characteristics of a lithium secondary battery can be further improved.

In one embodiment, the lower operating limit voltage of the lithium secondary battery may be 1.8V or more, 1.9V or more, or 2.0V or more compared to the lithium redox potential (vs Li/Li ⁺ ). In some embodiments, the lower operating voltage limit may be 1.8V to 2.2V.

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

Preferred examples and comparative examples of the present invention are described below. However, the following example is only a preferred example of the present invention and the present invention is not limited to the following example.

Example 1

(1) Preparation of lithium excess oxide particles

Distilled water from which dissolved oxygen was removed was added to the closed reactor, and NiSO ₄ ·6H ₂ O and MnSO ₄ ·H ₂ O were added at a molar ratio of 41:59.

NaOH (precipitant) and NH ₄ OH (chelating agent) were additionally added to the reactor, and coprecipitation reaction was performed for 60 hours to prepare metal hydroxide particles.

The metal hydroxide particles were dried at 100°C for 12 hours.

The metal hydroxide particles and lithium hydroxide were added to a dry mixer to prepare a mixture.

The mixing ratio of the metal hydroxide particles and lithium hydroxide was adjusted so that the manufactured lithium metal oxide particles satisfied the composition according to the ICP analysis below.

The mixture was placed in a kiln, the temperature of the kiln was raised to 250°C at 2°C/min, and primary firing was performed while maintaining the temperature at 250°C for 3 hours.

After the first firing, the temperature of the furnace was raised to 850°C at a rate of 2°C/min, and the second firing was performed by maintaining the temperature at 850°C for 8 hours.

Oxygen gas was continuously passed through the furnace at a rate of 10 mL/min during the first and second calcinations.

After completion of firing, the fired product was naturally cooled to room temperature, pulverized and classified to prepare lithium excess oxide particles.

The lithium excess oxide particles were analyzed by ICP (the number of oxygen atoms was normalized to 2) and were confirmed to be Li _1.09 Ni _0.38 Mn _0.54 O ₂ .

(2) Manufacturing of spare lithium secondary battery (half coin cell)

The lithium excess oxide particles, carbon black, and PVDF were dispersed in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 92:5:3 to prepare a positive electrode slurry.

The positive electrode slurry was applied on an aluminum foil, dried, and rolled to prepare a positive electrode with a positive electrode active material layer. When manufacturing the positive electrode, the loading amount of the positive electrode active material layer was adjusted to 11 mg/cm ² and the density of the positive active material layer was adjusted to 2.8 g/cc. Lithium metal was used as the counter electrode (cathode).

The anode and the cathode were notched and laminated in a circular shape having diameters of Φ14 and Φ16, respectively, and an electrode assembly was formed by interposing a separator (PE, thickness 13 ㎛) notched at Φ19 between the anode and the cathode.

The electrode assembly was stored in a coin cell case (2016 standard), and an electrolyte was added to the coin cell case to manufacture a preliminary lithium secondary battery.

As the electrolyte solution, 1M LiPF ₆ dissolved in an EC/EMC (30:70 v/v) mixed solvent was used.

(3) Manufacturing of lithium secondary battery (activation step of preliminary lithium secondary battery)

The spare lithium secondary battery was subjected to CC/CV charging (0.1C constant current, CC section CUT-OFF conditions: 4.6V, CV section CUT-OFF conditions: 0.05C) and CC discharge (0.1C constant current, 2.0V CUT-OFF conditions) at 25°C. OFF).

The charging and discharging were repeated twice to activate the lithium excess oxide particles.

Example 2

Lithium excess oxide particles (Li _1.19 Ni _0.22 Mn _0.59 O ₂ ) were prepared by adjusting the molar ratio of NiSO ₄ ·6H ₂ O and MnSO ₄ ·H ₂ O to 27.5:72.5.

Other than that, a lithium secondary battery was manufactured in the same manner as in Example 1.

Comparative Example 1

Lithium carbonate was used as the lithium source.

Lithium excess oxide particles were manufactured by performing only the second calcination for 11 hours without performing the first calcination.

Other than that, a lithium secondary battery was manufactured in the same manner as in Example 1.

Comparative Example 2

The molar ratio of NiSO ₄ ·6H ₂ O and MnSO ₄ ·H ₂ O was adjusted to 38.1:61.9, and lithium carbonate was used as a lithium source.

Instead of performing the first firing, only the second firing was performed at 1000°C for 11 hours to produce lithium excess oxide particles (Li _1.11 Ni _0.34 Mn _0.55 O ₂ ).

Other than that, a lithium secondary battery was manufactured in the same manner as in Example 1.

Evaluation example 1: Morphology measurement

(1) Morphology of primary particles

The surface and cross section of the lithium excess oxide particles of Examples and Comparative Examples were observed using a scanning electron microscope (SEM).

Figures 4, 6, 8, and 10 are SEM images measuring the surfaces of lithium excess oxide particles of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, respectively. Figures 5, 7, 9, and 11 are SEM images measuring the cross sections of lithium excess oxide particles of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, respectively.

Referring to Figures 4 and 6, the lithium excess oxide particles of Examples 1 and 2 have the form of secondary particles in which plate-shaped primary particles are aggregated. Referring to Figures 8 and 10, the lithium excess oxide particles of Comparative Examples 1 and 2 have the form of secondary particles in which round polygonal primary particles are aggregated.

In the SEM surface image, three primary particles among one lithium excess oxide particle were randomly selected, the aspect ratios of the selected primary particles were measured, and the average value of the measured aspect ratios was calculated. When measuring the aspect ratio, for the plate-shaped primary particles, the long side length/thickness was measured, and for the round polygonal primary particles, the long axis length/short axis length were measured.

(2) Evaluation of density of primary particles

In the SEM cross-sectional image, it was confirmed whether lithium excess oxide particles satisfying Equation 3 below were observed.

If particles satisfying Equation 3 below are observed, they are marked as ○, and if they are not observed, they are marked as ×.

[Equation 3]

P _h /P _f ＜0.2

In Equation 3, P _f is the total area of pores present in the lithium excess oxide particle in the SEM cross-sectional image of the lithium excess oxide particle (e.g., a cross section through the center of the particle). P _h is the total area of pores present in a region with a thickness of 0.5R from the surface of the lithium excess oxide particle, when the radius of the lithium excess oxide particle is defined as R.

The radius R was calculated as the average value of the major and minor radii of the lithium excess oxide particles in the SEM cross-sectional image.

Evaluation Example 2: Measurement of electrochemical active surface area of anode

1) Manufacturing of a 2-electrode symmetry cell

Two lithium secondary batteries of Example 1 were prepared. The two lithium secondary batteries were disassembled to obtain two positive electrodes.

An electrode assembly was formed by interposing a separator (PP, thickness 14 ㎛, Celgard, PP1410) notched at Φ19 between the two anodes.

The electrode assembly was stored in a coin cell case (2016 standard), and an electrolyte was added to the coin cell case to produce a symmetrical cell.

As the electrolyte solution, 1M LiPF ₆ solution (solvent: EC/EMC/DEC=25/45/30 v/v%) was used.

2) Measurement of electrochemical double layer capacitance of anode

The symmetrical cell was analyzed using cyclic voltammetry to measure the electrochemical double layer capacitance (EDLC; hereinafter abbreviated as capacitance) of the anode.

The specific measurement method is as follows.

i) The symmetric cell was connected to a potential variable device (VMP-3, Biologic).

A voltage such that the potential reciprocates (initial potential → upper limit potential → initial potential → lower limit potential → initial potential) in a voltage section (i.e., non-Faraday voltage section; -0.10V to +0.10V) where Faraday current does not flow in the symmetric cell. was applied to measure the cyclic voltammogram (CV curve). When measuring the CV curve, the voltage scan rate was adjusted to 10 mV/s.

ii) The response peak current was obtained from the CV curve.

iii) i) and ii) were further performed by changing the voltage scanning rate to 20 mV/s, 30 mV/s, 40 mV/s, and 50 mV/s.

iv) A linear graph was shown with the voltage scanning speed set to the x-axis and the response peak current according to the voltage scanning speed set to the y-axis. Slope values were obtained from the linear graph (Linear regression analysis). The slope value is the total capacitance C _tot of the two anodes in the symmetrical cell.

v) In a symmetrical cell, C = 2C _tot (C is the capacitance of one anode), so C _tot was substituted into the above equation to calculate the capacitance C of one (i.e. one) anode.

3) Calculation of the electrochemically active surface area of the anode

Based on the capacitance value of the platinum (Pt) electrode (i.e., 0.2 F/m ² ), the electrochemical active surface area (EASA) of the anode was calculated according to Equation 1 below.

[Equation 1]

Electrochemically active surface area (m ² /g) = C(F)/[W(g)×{0.2(F/m ² )}]

In Equation 1, C is the capacitance value of one positive electrode calculated in 2) above, and W is the total weight of the positive electrode active material contained in one positive electrode of the symmetric cell.

In the same manner as described above, the active surface areas of the positive electrodes of Example 2, Comparative Example 1, and Comparative Example 2 were measured.

Evaluation Example 3: XRD (X-ray diffraction) analysis

The lithium secondary batteries of Examples and Comparative Examples were dismantled to obtain a positive electrode, and the positive electrode active material layer of the positive electrode was analyzed by XRD. The XRD spectrum was analyzed to calculate the XRD peak intensity ratio expressed by Equation 2 below.

XRD analysis equipment and conditions are listed in Table 1 below.

XRD(X-Ray Diffractometer) EMPYREAN Maker PANalytical Anode material Cu K-Alpha1 wavelength 1.540598 Å Generator voltage 45 kV Tube current 40mA Scan Range 10~ ^120o Scan Step Size ^0.006565o Divergence slit 1/4 ^o Antiscatter slit ^1o

[Equation 2]

XRD peak intensity ratio (%) = 100 × I (110) / [I (110) + I (003)]

In Equation 2, I(110) is the maximum height of the (110) plane peak in the XRD spectrum, and I(003) is the maximum height of the (003) plane peak in the XRD spectrum.

Evaluation Example 4: Evaluation of surface resistance (charge transfer resistance, Rct) of anode

Two lithium secondary batteries of Example 1 were prepared (hereinafter abbreviated as first battery and second battery, respectively).

At 45°C, the first battery was charged at 1C and discharged at 1C in a voltage range of 2.0V to 4.3V. The charging and discharging were repeated 100 times.

The electrochemical impedance of the first battery was measured with a potential variable (Biologic, VMP-3), and the surface resistance Rct1 of the positive electrode was confirmed.

At 45°C, the second battery was charged at 1C and discharged at 1C in a voltage range of 2.0V to 4.5V. The charging and discharging were repeated 100 times.

By measuring the electrochemical impedance of the second battery, the surface resistance Rct2 of the positive electrode was confirmed.

According to Equation 4 below, the Rct ratio (RR, % ₎ was calculated.

[Equation 4]

RR(%)=(Rct2/Rct1)×100

In the same manner as described above, the surface resistance of the anodes of Example 2, Comparative Example 1, and Comparative Example 2 was measured, and the RR value in Equation 4 was calculated.

Evaluation Example 5: Evaluation of life characteristics (45°C)

The lithium secondary batteries of Examples and Comparative Examples were CC/CV charged (1C constant current, 4.5V and 0.05C CUT-OFF) and CC discharged (1C constant current, 2.0V CUT-OFF).

The charging and discharging were repeated 100 times, and the second discharge capacity D ₂ and the 100th discharge capacity D ₁₀₀ were measured.

Capacity retention rate was calculated according to the formula below.

Capacity maintenance rate (%) = D ₁₀₀ /D ₂ × 100 (%)

EASA
( ^m2 /g) primary particle
aspect ratio Equation 3
of particles
respect XRD
peak rate
(%) RR
(%) 45℃,
100 Cycles
Capacity maintenance rate (%) Example 1 1.41 6 ○ 5.05 106 77.3 Example 2 1.56 6 ○ 6.29 122 77.0 Comparative Example 1 3.16 2 × 3.77 475 27.3 Comparative example 2 2.54 2 × 2.23 238 23.7

Referring to Table 2, the lithium secondary batteries of Examples showed improved lifespan characteristics compared to the lithium secondary batteries of Comparative Examples.

In addition, the lithium secondary batteries of the examples showed lower surface resistance (charge transfer resistance) of the positive electrode even at high voltage compared to the lithium secondary batteries of the comparative examples.

100: positive electrode 105: positive electrode current collector
106: positive tab 107: positive lead
110: positive electrode active material layer 120: negative electrode active material layer
125: negative electrode current collector 126: negative electrode tab
127: cathode lead 130: cathode
140: Separator 150: Electrode assembly
160: case

Claims (15)

positive electrode current collector; and
A positive electrode active material layer is formed on the positive electrode current collector and includes a positive electrode active material containing lithium excess oxide particles represented by Chemical Formula 1,
A positive electrode for a lithium secondary battery having an electrochemically active surface area of 0.5 m ² /g to 2.5 m ² /g, represented by Equation 1:
[Formula 1]
Li _a [M _x Ni _y Mn _z ]O _b
(In Formula 1, M is at least one of Co, Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi, 0≤x ≤0.9, 0≤y≤0.9, x+y>0, 0.1≤z≤0.9, 1.8≤a+x+y+z≤2.2, 1.05≤a/(x+y+z)≤1.95 and 1.8≤b ≤2.2),
[Equation 1]
Electrochemically active surface area (m ² /g) = C(F)/[W(g)×{0.2(F/m ² )}]
(In Equation 1, C is the capacitance of one anode measured by analyzing a 2-electrode symmetrical cell manufactured using two of the above anodes by cyclic voltammetry, and W is the capacitance of one anode in the 2-electrode symmetrical cell. is the weight of the positive electrode active material contained in).
The positive electrode for a lithium secondary battery according to claim 1, having the electrochemically active surface area of 1.1 m ² /g to 1.8 m ² /g.
The positive electrode for a lithium secondary battery according to claim 1, wherein the XRD peak intensity ratio expressed by Equation 2 is 4% or more:
[Equation 2]
XRD peak intensity ratio (%) = 100 × I (110) / [I (110) + I (003)]
(In Equation 2, I (110) is the maximum height of the (110) plane peak in the XRD spectrum analyzing the positive electrode active material layer, and I (003) is the maximum height of the (003) plane peak in the XRD spectrum.)
The positive electrode for a lithium secondary battery according to claim 1, wherein the content of the lithium excess oxide particles in the total weight of the positive electrode active material layer is 80% by weight or more.
The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode active material layer has a density of 2.5 g/cc to 3.8 g/cc.
The positive electrode for a lithium secondary battery according to claim 1, wherein the mole fraction of manganese to all elements excluding lithium and oxygen among the lithium excess oxide particles is 0.5 to 0.75.
The positive electrode for a lithium secondary battery according to claim 1, wherein the mole fraction of cobalt relative to all elements excluding lithium and oxygen among the lithium excess oxide particles is 0 to 0.02.
The method according to claim 1, wherein the lithium excess oxide particles have the form of secondary particles in which a plurality of primary particles are aggregated,
A positive electrode for a lithium secondary battery, wherein the plurality of primary particles include plate-shaped particles having an aspect ratio of 3 to 9.
The positive electrode for a lithium secondary battery according to claim 8, wherein, in a scanning electron microscope (SEM) image measuring a cross section of the lithium excess oxide particle, the ratio of the total area of the plate-shaped particles to the area of the lithium excess oxide particle is 0.8 or more.
The positive electrode for a lithium secondary battery according to claim 8, wherein the lithium excess oxide particles include lithium excess oxide particles satisfying Equation 3:
[Equation 3]
P _h /P _f ＜0.2
(In Equation 3, P _f is the total area of pores present in the lithium excess oxide particles in a scanning electron microscope (SEM) image measuring the cross section of the lithium excess oxide particles,
P _h is the total area of pores present in a region with a thickness of 0.5R from the surface of the lithium excess oxide particle, when the radius of the lithium excess oxide particle is defined as R).
The positive electrode for a lithium secondary battery according to claim 1, wherein the lithium excess oxide particles include at least one of a Li ₂ MnO ₃ domain and a domain derived from the Li ₂ MnO ₃ domain.
The method of claim 11, wherein the domain derived from the Li ₂ MnO ₃ domain includes at least one selected from the group consisting of MnO ₂ , Mn ₂ O ₄ , LiMnO ₂ , LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ . Anode for secondary batteries.
The cathode for a lithium secondary battery according to claim 1, which satisfies Equation 4:
[Equation 4]
(Rct2/Rct1)×100≤220%
(In Equation 4, when a half cell is manufactured using the positive electrode and the lithium counter electrode, Rct1 charges and discharges 1C by repeating 100 times the half cell at 45°C and a voltage range of 2V to 4.3V, and then electrically It is the charge transfer resistance value of the positive electrode measured according to the chemical impedance analysis method,
Rct2 is the charge transfer resistance value of the positive electrode measured according to electrochemical impedance analysis after charging and discharging 1C by repeating 1C charging and 1C discharging of the half cell 100 times at 45°C and a voltage range of 2V to 4.5V).
Anode according to claim 1; and
A lithium secondary battery comprising a negative electrode opposing the positive electrode.
The lithium secondary battery according to claim 14, wherein the upper operating voltage is 4.5 V or less compared to the redox potential of lithium.