In another exemplary embodiment, the solid solution may be represented by Formula 3-1 below.

Li_aM1_bW_cO_d—Li_wNi_1-(x+y+z)Co_xM2_yM3_zO₂ [Formula 3-1]

(Here,

M1 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd and Nd,

M2 is at least one selected from Mn and Al,

M3 is at least one selected from Mn, Al, Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ta, Fe, Cr, Sn, Hf, Ce, W, Nd and Gd,

0≤a≤6, 0≤b≤2, 0≤c≤2, 4≤d≤8, 0.5≤w≤1.5, 0.01≤x≤0.50, 0.01≤y≤0.20, and 0.001≤z≤0.20)

Here, M3 may be W or include W. That is, when M3 is a single element, M3 is W, but when M3 includes multiple elements, M3 may include W and an element other than W. When M3 is W or includes W, W and (if present) an element other than W may have concentration gradients decreasing toward the center of the positive electrode active material from the surface of the positive electrode active material.

The presence or absence of the solid solution(s) represented by Formula 3 and/or Formula 3-1 in the positive electrode active material may be verified by XPS analysis, Raman shift analysis and/or XRD analysis.

As a result of the XRD analysis, when there is a lithium composite oxide in the form of a solid solution represented by Formula 3 and/or Formula 3-1, the solid solution may simultaneously have both diffraction peak characteristics of the first composite oxide and the second composite oxide.

For example, in the case of the solid solution represented by Formula 3, when the solid solution is formed of a second composite oxide in which M1 is Ni and M2 is W and a first composite oxide in which M3 is Al and M4 is W, the solid solution may be represented by Formula Li_aNi_bW_cO_d—Li_wNi_1-(x+y+z)CO_xAl_yW_zO₂, and here, the solid solution may have both of a diffraction peak characteristic specific for Li_aNi_bW_cO_dand a diffraction peak characteristic specific for Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂.

In addition, in the case of the solid solution represented by Formula 3-1, when a solid solution is formed of a second composite oxide in which M1 is Ni and a first composite oxide in which M2 is Al and M3 is W, the solid solution may be represented by Formula Li_aNi_bW_cO_d—Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂, and here, the solid solution may have both of a diffraction peak characteristic specific for Li_aNi_bW_cO_dand a diffraction peak characteristic specific for Li_wNi_1-(x+y+z)CO_xAl_yW_zO₂.

In this case, through X-ray diffraction (XRD) analysis using a CuKα ray for the lithium composite oxide, the lithium composite oxide(s) in the form of the solid solution(s) represented by Formula 3 and/or Formula 3-1 may have peaks corresponding to the (001) crystal face and the (003) crystal face in a X-ray diffraction spectrum range of 2θ=15° to 200, and a peak corresponding to the (104) crystal face in a range of 2θ=44° to 500. In addition, the lithium composite oxide(s) in the form of a solid solution(s) represented by Formula 3 and/or Formula 3-1 may have a peak corresponding to the (20-2) crystal face in the range of 2θ=42° to 44°.

In addition, through XPS analysis for the lithium composite oxide, in the case of W4f and O1s peaks, the lithium composite oxide in the form of the solid solution represented by Formula 3-1 may have both of a peak characteristic specific for Li_aNi_bW_cO_dand a peak characteristic specific for Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂.

Specifically, in the XPS analysis for the lithium composite oxide, a peak corresponding to W4f_7/2may be present at 35.5 eV or less, and preferably in a range of 34.0 to 35.0 eV.

In addition, in the XPS analysis for the lithium composite oxide, a peak corresponding to W4f_7/2may be present at 37.5 eV or less, and preferably in a range of 36.0 to 37.0 eV.

The W4f_7/2peak for W⁶⁺ of the oxide having a Li_aNi_bW_cO_dcomposition may be present in a range of 34.0 to 35.0 eV, and the W4f_5/2peak may be present in the range of 36.0 to 37.0 eV. On the other hand, it can be confirmed that the W4f_7/2peak for O²⁻ of the oxide having a Li_aW_cO_dor W_cO_dcomposition is present in a range exceeding 35.0 eV, and the W4f_5/2peak may be present in a range exceeding 37.0 eV.

Therefore, in the XPS analysis for the lithium composite oxide included in a positive electrode active material according to an exemplary embodiment of the present invention, the W4f_7/2peak is present in a range of 34.0 to 35.0 eV, and the W4f_5/2peak is present in a range of 36.0 to 37.0 eV, and thus the lithium composite oxide exhibits a unique peak of Li_aNi_bW_cO_d.

In addition, in the XPS analysis for the lithium composite oxide, a peak corresponding to O1s may be present in a range of 528.0 to 530.0 eV and a range of 530.0 to 531.0 eV.

The peak for O-M³⁺ binding of an oxide having a Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂composition is shown in a range of 528.0 to 529.0 eV. While the peak for O²⁻ of an oxide having a Li_aNi_bW_cO_dcomposition is present in a range of 530.0 to 531.0 eV, the peak for O²⁻ of oxides having Li_aW_cO_dand W_cO_dcompositions are also shown in a range of 530.0 to 531.0 eV, and therefore it may be difficult to distinguish these oxides.

In XPS analysis, the lithium composite oxide included in the positive electrode active material according to an exemplary embodiment of the present invention may have both of a peak corresponding to O-M³⁺ binding of an oxide having a Li_wNi_1(x+y+z)Co_xAl_yW_zO₂composition and a peak of O²⁻ of an oxide having a Li_aNi_bW_cO_dcomposition. The peak shown in a range of 530.0 to 531.0 eV may be caused by an alloy oxide to be described below.

As described above, the positive electrode active material(s) including the solid solutions(s) represented by Formula 3 and/or Formula 3-1 may be improved in lifespan characteristics, efficiency characteristics and electrochemical characteristics such as resistance, compared to a conventional positive electrode active material, by improving lithium ion conductivity in the active material, particularly, at the surface of the active material.

In addition, in another exemplary embodiment, the positive electrode active material may include a coating layer covering at least a part of the primary particle and/or a secondary particle formed by aggregating the primary particles.

For example, the coating layer may be present to cover at least a part of an exposed surface of the core-shell particle, which is the primary particle. Particularly, the coating layer may be present to cover at least a part of an exposed surface of the core-shell particle, which is the primary particle, present at the outermost region of the secondary particle.

The coating layer present as described above may contribute to improvement in physical and electrochemical characteristics of the positive electrode active material.

Here, in the core-shell particle, which is the primary particle, present at the outermost region of the secondary particle, when the shell layer covers a part of the surface of the core, the coating layer may cover at least a part of the surface of the core which is not covered by the shell layer, or may be present to cover at least a part of the shell layer.

In addition, the coating layer may be defined as a region in which an alloy oxide represented by Formula 4 below is present.

Li_eM5_fO_g [Formula 4]

(Here,

M5 is at least one selected from Mn, Al, Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, W, Nd and Gd,

0≤e≤6, 0<f≤6, and 0<g≤10)

The alloy oxide may be physically and/or chemically linked to the first composite oxide. In addition, the alloy oxide may be physically and/or chemically linked to the second composite oxide. In addition, the alloy oxide may be physically and/or chemically linked to a solid solution formed of the first composite oxide and the second composite oxide.

As described above, as the positive electrode active material according to the exemplary embodiment includes the above-described alloy oxide, the structural stability may increase, and when the positive electrode active material is used in a lithium secondary battery, the high-temperature storage stability and lifespan characteristics of the positive electrode active material may be improved. In addition, the alloy oxide may serve as a diffusion path for lithium ions in the positive electrode active material, and thus affect the improvement of the efficiency characteristics of the lithium secondary battery.

In addition, in some cases, the alloy oxide may be present on at least a part of the interface between the core-shell particles and the surface of the secondary particle, and even in the inner pore formed in the secondary particle.

As shown in Formula 4, the alloy oxide is an oxide in which lithium is complexed with a metal element represented by M5, and may be, for example, Li(W)O, Li(Zr)O, Li(Ti)O, Li(B)O, WO_x, ZrO_x, or TiO_x. However, although the above examples are merely provided for convenience of understanding, the alloy oxide defined in the present invention is not limited to the above examples. Therefore, in experimental examples to be described below, for convenience, experimental results for some examples of a representative alloy oxide that can be represented by Formula 4 will be described.

In another exemplary embodiment, the alloy oxide may be an oxide prepared by complexing lithium and at least two metal elements represented by M5, or may further include an oxide prepared by complexing lithium and at least two metal elements represented by M5. The alloy oxide prepared by complexing lithium and at least two metal elements represented by M5 may be, for example, Li(W/Ti)O, Li(W/Zr)O, Li(W/Ti/Zr)O or Li(W/Ti/B)O, but the present invention is not necessarily limited thereto.

In addition, in this case, the lithium tungsten oxide may have a lattice structure that is doped with at least one type of metal element. That is, the alloy oxide may have a lattice structure formed of a lithium tungsten oxide (e.g., Li₂WO₄, Li₂W₂O₇, Li₄WO₅, Li₆WO₆or Li₆W₂O₉), in which a part of the tungsten is substituted with a metal element. Here, as a metal element substituting for tungsten in the lattice structure, at least one selected from Mn, Al, Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, Nd and Gd may be used.

In one exemplary embodiment, the alloy oxide may have a concentration gradient which decreases from the surface toward the center of the secondary particle. Accordingly, the concentration of the alloy oxide may be reduced from the outermost surface toward the center of the secondary particle.

As described above, as the alloy oxide has a concentration gradient which decreases from the surface toward the center of the secondary particle, residual lithium present at the surface of the positive electrode active material is effectively reduced, thereby preventing a side reaction by the unreacted residual lithium beforehand, and also preventing crystallinity in the inside of the surface of the positive electrode active material from being lowered by the alloy oxide. In addition, the collapse of the entire structure of the positive electrode active material during an electrochemical reaction may be prevented by the alloy oxide.

In another exemplary embodiment, the alloy oxide may include a first alloy oxide represented by Formula 5 below and a second alloy oxide represented by Formula 6 below.

Li_hW_iO_j [Formula 5]

(Here, 0≤i≤6, 0<j≤6 and 0<k≤10)

Li_kM6_lO_m [Formula 6]

(Here,

M6 is at least one selected from Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, Nd and Gd,

0≤k≤6, 0≤l≤6, and 0<m≤10)

Here, the first alloy oxide and the second alloy oxide may have a concentration gradient which decreases from the surface toward the center of the secondary particle. Accordingly, the concentrations of the first alloy oxide and the second alloy oxide may decrease from the outermost surface toward the center of the secondary particle.

Here, it is preferable that the decrease rate of the second alloy oxide be higher than that of the first alloy oxide, and as the decrease rate of the second alloy oxide is higher than that of the first alloy oxide, the structural stability of the positive electrode active material including an alloy oxide may be maintained. In addition, it can positively improve the efficiency characteristics of a lithium secondary battery by forming an efficient diffusion path for lithium ions in the positive electrode active material.

In addition, the alloy oxide may include a third alloy oxide represented by Formula 7 below.

Li_nW_l-oM7_oO_p [Formula 7]

(Here,

M7 is at least one selected from Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ta, Fe, Cr, Sn, Hf, Ce, Nd and Gd,

0≤n≤6, 0≤o≤0.50, and 0<p≤6)

Here, the third alloy oxide may have a lattice structure in which a part of the tungsten therein is substituted with M7.

As described above, a positive electrode including a positive electrode active material according to one of various exemplary embodiments of the present invention may further improve high-temperature storage stability and lifespan characteristics when used as a positive electrode of a lithium secondary battery.

Lithium Secondary Battery

According to still another aspect of the present invention, a positive electrode including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector may be provided. Here, the positive electrode active material layer may include a positive electrode active material according to any one of various exemplary embodiments of the present invention. Therefore, since the positive electrode active material is the same as described above, for convenience, detailed description will be omitted, and hereinafter, other components that have not been described will be described.

The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in a battery and has conductivity, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium or silver may be used. In addition, the positive electrode current collector may conventionally have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector, thereby increasing the adhesive strength of a positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, foam, a non-woven fabric, etc.

The positive electrode active material layer may include a conductive material as well as the positive electrode active material, and selectively a binder when needed.

Here, the positive electrode active material may be included at 80 to 99 wt %, and specifically, 85 to 98.5 wt % with respect to the total weight of the positive electrode active material layer. When the positive electrode active material is included in the above content range, excellent capacity characteristics may be exhibited, but the present invention is not limited thereto.

The conductive material is used to impart conductivity to an electrode, and is not particularly limited as long as it has electron conductivity without causing a chemical change in a battery. A specific example of the conductive material may be graphite such as natural graphite or artificial graphite; a carbon-based material such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black or a carbon fiber; a metal powder or metal fiber consisting of copper, nickel, aluminum, or silver; a conductive whisker consisting of zinc oxide or potassium titanate; a conductive metal oxide such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and one or a mixture of two or more thereof may be used. The conductive material may be included at 0.1 to 15 wt % with respect to the total weight of the positive electrode active material layer.

The binder serves to improve attachment between particles of the positive electrode active material and the adhesive strength between the positive electrode active material and a current collector. A specific example of the binder may be polyvinylidene fluoride (PVDF), a vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene polymer (EPDM), a sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, or various copolymers thereof, and one or a mixture of two or more thereof may be used. The binder may be included at 0.1 to 15 wt % with respect to the total weight of the positive electrode active material layer.

The positive electrode may be manufactured according to a conventional method of manufacturing a positive electrode, except that the above-described positive electrode active material is used. Specifically, the positive electrode may be manufactured by applying a composition for forming a positive electrode active material layer prepared by dissolving or dispersing the positive electrode active material, and selectively, a binder and a conductive material in a solvent on a positive electrode current collector, and drying and rolling the resulting product.

The solvent may be a solvent generally used in the art, and may be dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone or water, and one or a mixture of two or more thereof may be used. In consideration of the coating thickness and production yield of a slurry, the solvent is used at a sufficient amount for dissolving or dispersing the positive electrode active material, the conductive material and the binder, and then having a viscosity that can exhibit an excellent thickness uniformity when applied to prepare a positive electrode.

In addition, in another exemplary embodiment, the positive electrode may be prepared by casting the composition for forming a positive electrode active material layer on a separate support, and laminating a film obtained by delamination from the support on a positive electrode current collector.

Moreover, according to still another aspect of the present invention, an electrochemical device including the above-described positive electrode may be provided. The electrochemical device may be, specifically, a battery, a capacitor, and more specifically, a lithium secondary battery.

The lithium secondary battery may specifically include a positive electrode, a negative electrode disposed opposite to the positive electrode, and a separator and an electrolyte, which are interposed between the positive electrode and the negative electrode. Here, since the positive electrode is the same as described above, for convenience, detailed description for the positive electrode will be omitted, and other components which have not been described below will be described in detail.

The lithium secondary battery may further include a battery case accommodating an electrode assembly of the positive electrode, the negative electrode and the separator, and selectively, a sealing member for sealing the battery case.

The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in a battery, and may be, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper or stainless steel whose surface is treated with carbon, nickel, titanium or silver, or an aluminum-cadmium alloy. In addition, the negative electrode current collector may generally have a thickness of 3 to 500 μm, and like the positive electrode current collector, fine irregularities may be formed on the current collector surface, thereby enhancing the binding strength of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, foam, a non-woven fabric, etc.

The negative electrode active material layer selectively includes a binder and a conductive material, as well as the negative electrode active material.

As the negative electrode active material, a compound enabling the reversible intercalation and deintercalation of lithium may be used. A specific example of the negative electrode active material may be a carbonaceous material such as artificial graphite, natural graphite, graphitized carbon fiber or amorphous carbon; a metallic compound capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, a Si alloy, a Sn alloy or an Al alloy; a metal oxide capable of doping and dedoping lithium such as SiO_β (0<β<2), SnO₂, vanadium oxide, or lithium vanadium oxide; or a composite including the metallic compound and the carbonaceous material such as a Si—C composite or a Sn—C composite, and any one or a mixture of two or more thereof may be used. In addition, as the negative electrode active material, a metal lithium thin film may be used. In addition, as a carbon material, both low-crystalline carbon and high-crystalline carbon may be used. Representative examples of the low-crystalline carbon include soft carbon and hard carbon, and representative examples of the high-crystalline carbon include amorphous, sheet-type, flake-type, spherical or fiber-type natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

The negative electrode active material may be included at 80 to 99 wt % with respect to the total weight of the negative electrode active material layer.

The binder is a component aiding bonding between a conductive material, an active material and a current collector, and may be generally added at 0.1 to 10 wt % with respect to the total weight of the negative electrode active material layer. Examples of the binder may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber, nitrile-butadiene rubber, fluorine rubber, and various copolymers thereof.

The conductive material is a component for further improving conductivity of the negative electrode active material, and may be added at 10 wt % or less, and preferably, 5 wt % or less with respect to the total weight of the negative electrode active material layer. The conductive material is not particularly limited as long as it does not cause a chemical change in the battery, and has conductivity, and may be, for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal black; a conductive fiber such as a carbon fiber or a metal fiber; a metal powder such as fluorinated carbon, aluminum, or nickel powder; a conductive whisker such as zinc oxide or potassium titanate; a conductive metal oxide such as titanium oxide; or a conductive material such as a polyphenylene derivative.

In an exemplary embodiment, the negative electrode active material layer maybe prepared by applying a composition for forming the negative electrode active material layer, which is prepared by dissolving or dispersing a negative electrode active material, and selectively a binder and a conductive material in a solvent, on the negative electrode current collector, and drying the resulting product, or may be prepared by casting a composition for forming the negative electrode active material layer on a separate support and laminating a film obtained by delamination from the support on the negative electrode current collector.

Meanwhile, in the lithium secondary battery, a separator is not particularly limited as long as it is generally used in a lithium secondary battery to separate a negative electrode from a positive electrode and provide a diffusion path for lithium ions, and particularly, the separator has a low resistance to ion mobility of an electrolyte and an excellent electrolyte solution impregnation ability. Specifically, a porous polymer film, for example, a porous polymer film prepared of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer and an ethylene/methacrylate copolymer, or a stacked structure including two or more layers thereof may be used. In addition, a conventional porous non-woven fabric, for example, a non-woven fabric formed of a high melting point glass fiber or a polyethylene terephthalate fiber may be used. In addition, a coated separator including a ceramic component or a polymer material may be used to ensure thermal resistance or mechanical strength, and may be selectively used in a single- or multi-layered structure.

In addition, the electrolyte used in the present invention may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte, which can be used in production of a lithium secondary battery, but the present invention is not limited thereto.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent is not particularly limited as long as it can serve as a medium enabling the transfer of ions involved in an electrochemical reaction of a battery. Specifically, the organic solvent may be an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; an aromatic hydrocarbon-based solvent such as benzene or fluorobenzene; a carbonate-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (PC); an alcohol-based solvent such as ethyl alcohol or isopropyl alcohol; a nitrile-based solvent such as R—CN (R is a linear, branched or cyclic C2 to C20 hydrocarbon group, and may include a double bonded aromatic ring or an ether bond); an amide-based solvent such as dimethylformamide; a dioxolane-based solvent such as 1,3-dioxolane; or a sulfolane-based solvent. Among these, a carbonate-based solvent is preferably used, and a mixture of a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having high ion conductivity and high permittivity to increase the charge/discharge performance of a battery and a low-viscosity linear carbonate-based compound (for example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferably used. In this case, by using a mixture of a cyclic carbonate and a chain-type carbonate in a volume ratio of approximately 1:1 to 1:9, the electrolyte solution may exhibit excellent performance.

The lithium salt is not particularly limited as long as it is a compound capable of providing a lithium ion used in a lithium secondary battery. Specifically, the lithium salt may be LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆, LiAlO₄, LiAlCl₄, LiCF₃SO₃, LiC₄F₉SO₃, LiN(C₂F₅SO₃)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)₂, LiCl, LiI, or LiB(C₂O₄)₂. The concentration of the lithium salt is preferably in the range of 0.1 to 2.0M. When the concentration of the lithium salt is included in the above-mentioned range, the electrolyte has suitable conductivity and viscosity and thus can exhibit excellent electrolytic performance. Therefore, lithium ions can effectively migrate.

To enhance lifespan characteristics of the battery, inhibit a decrease in battery capacity, and enhance discharge capacity of the battery, the electrolyte may further include one or more types of additives, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric triamide, a nitrobenzene derivative, sulfur, a quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, an ammonium salt, pyrrole, 2-methoxy ethanol or aluminum trichloride, in addition to the components of the electrolyte. Here, the additive(s) may be included at 0.1 to 5 wt % with respect to the total weight of the electrolyte.

Since the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, excellent output characteristics and lifespan characteristics, it is useful in portable devices such as a mobile phone, a notebook computer and a digital camera and an electric automobile field such as a hybrid electric vehicle (HEV).

The outer shape of the lithium secondary battery according to the present invention is not particularly limited, but may be a cylindrical, prismatic, pouch or coin type using a can. In addition, the lithium secondary battery may be used in a battery cell that is not only used as a power source of a small device, but also preferably used as a unit battery for a medium-to-large battery module including a plurality of battery cells.

According to yet another exemplary embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and/or a battery pack including the same may be provided.

The battery module or the battery pack may be used as a power source of any one or more medium-to-large devices including a power tool; an electric motor vehicle such as an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); and a power storage system.

Hereinafter, the present invention will be described in further detail with reference to examples. However, these examples are merely provided to explain the present invention, and it will not be interpreted that the scope of the present invention is limited by the examples below.

Experimental Example 1

(1) Preparation of Positive Electrode Active Material

Example 1

A spherical Ni_0.92Co_0.08(OH)₂hydroxide precursor was synthesized by a co-precipitation method. In a 90 L reactor, 25 wt % NaOH and 30 wt % NH₄OH were input into an aqueous 1.5M sulfuric acid solution of a transition metal complex in which NiSO₄·7H₂O and CoSO₄·7H₂O were mixed in a molar ratio of 92:8. The pH in the reactor was maintained at 11.5, the temperature of the reactor was maintained at 60° C., and an inert gas N₂was added to the reactor to prevent the prepared precursor from being oxidized. After the completion of synthesis and stirring, washing and dehydration were performed using filter press (F/P) equipment, thereby obtaining a Ni_0.92Co_0.08(OH)₂hydroxide precursor.

The hydroxide precursor was mixed with LiOH as a Li-containing raw material and Al₂O₃as an Al-containing raw material using a mixer, a calcination furnace was heated at a rate of 1° C. per minute while maintaining a O₂atmosphere, and maintained at a thermal treatment temperature of 650 □ for 10 hours, followed by natural cooling. The obtained positive electrode active material was mixed with a W-containing raw material (WO₃) using a mixer. In the same calcination furnace, the temperature was increased 2 □ per minute while maintaining the O₂atmosphere, and the furnace was maintained at a thermal treatment temperature of 600 □ for 5 hours, followed by natural cooling. Subsequently, each of thermal treatment and cooling were carried out once under the same conditions as above. The positive electrode active material prepared in Example 1 was confirmed to have composition formula Li_1.0Ni_0.902Co_0.079Al_0.014W_0.005O₂.

Example 2

A positive electrode active material was prepared by the same method as described in Example 1, except that a W-containing raw material (WO₃) and a cobalt source material (Co(OH)₂) were mixed such that the composition formula of the positive electrode active material obtained in Example 2 was Li_1.04Ni_0.897Co_0.084Al_0.014W_0.005O₂.

Example 3

A positive electrode active material was prepared by the same method as described in Example 1, except that a W-containing raw material (WO₃) and a magnesium source material (Mg(OH)₂) were mixed such that the composition formula of the positive electrode active material obtained in Example 3 was Li_1.04Ni_0.897Co_0.079Al_0.014W_0.005Mg_0.005O₂.

Example 4

A positive electrode active material was prepared by the same method as described in Example 1, except that a W-containing raw material (WO₃) and a manganese source material (Mn(OH)₂) were mixed such that the composition formula of the positive electrode active material obtained in Example 4 was Li_1.04Ni_0.897Co_0.079Al_0.014W_0.005Mn_0.005O₂.

Example 5

A positive electrode active material was prepared by the same method as described in Example 1, except that a W-containing raw material (WO₃) and a titanium source material (TiO₂) were mixed such that the composition formula of the positive electrode active material obtained in Example 5 was Li_1.04Ni_0.901Co_0.79Al_0.014W_0.005Ti_0.001O₂.

Example 6

A positive electrode active material was prepared by the same method as described in Example 1, except that a W-containing raw material (WO₃) and a zirconium source material (ZrO₂) were mixed such that the composition formula of the positive electrode active material obtained in Example 6 was Li_1.04Ni_0.904Co_0.079Al_0.014W_0.005Zr_0.001O₂.

Example 7

A positive electrode active material was prepared by the same method as described in Example 1, except that a W-containing raw material (WO₃), a Ti-containing source material (TiO₂) and a zirconium-containing material were mixed such that the composition formula of the positive electrode active material obtained in Example 7 was Li_1.04Ni_0.901Co_0.078Al_0.014W_0.005Ti_0.001Zr_0.001O₂.

Comparative Example 1

A positive electrode active material was prepared by the same method as described in Example 1, except that a W-containing source material (WO₃) was not mixed such that the composition formula of the positive electrode active material prepared in Comparative Example 1 was Li_1.04Ni_0.907Co_0.079Al_0.014O₂.

Comparative Example 2

Li₄NiWO₆was synthesized using a solid phase synthesis method. The lithium composite oxide Li₄NiWO₆was prepared by mixing Ni(OH)₂, WO₃and LiOH—H₂O with a mixer, and thermally treating the resulting mixture at 1000 □. A positive electrode active material was prepared by mixing 0.5 mol % of Li₄NiWO₆with the positive electrode active material prepared in Comparative Example 1.

Comparative Example 3

A positive electrode active material was prepared by the same method as described in Example 1, except that the positive electrode active material Li_1.04Ni_0.97Co_0.079Al_0.014O₂prepared in Comparative Example 1 was mixed with 0.5 mol % each of Li₂WO₄and Li₂W₂O₇synthesized by a solid phase synthesis method, but not thermally treated.

Comparative Example 4

A positive electrode active material was prepared by the same method as described in Example 1, except that the positive electrode active material Li_1.04Ni_0.97Co_0.079Al_0.014O₂prepared in Comparative Example 1 was mixed with 0.5 mol % of Li₄NiWO₆synthesized by a solid phase synthesis method, but not thermally treated.

TABLE 1

Classification

W content in
Dopant content in
Dopant content

positive electrode
solid solution
in alloy oxide

active material
(mol %)
(mol %)

Composition
(mol %)
Co
Mg
Mn
Ti
Zr

Example 1
Li_1.04Ni_0.902Co_0.079Al_0.014W_0.005O₂
0.5
—
—
—
—
—

Example 2
Li_1.03Ni_0.897Co_0.084Al_0.014W_0.005O₂
0.5
0.5
—
—
—
—

Example 3
Li_1.03Ni_0.897Co_0.079Al_0.014W_0.005Mg_0.005O₂
0.5
—
0.5
—
—
—

Example 4
Li_1.03Ni_0.897Co_0.079Al_0.014W_0.005Mn_0.005O₂
0.5
—
—
0.5
—
—

Example 5
Li_1.03Ni_0.901Co_0.079Al_0.014W_0.005Ti_0.001O₂
0.5
—
—
—
0.1
—

Example 6
Li_1.03Ni_0.901Co_0.079Al_0.014W_0.005Zr_0.001O₂
0.5
—
—
—
—
0.1

Example 7
Li_1.03Ni_0.901Co_0.078Al_0.014W_0.005Ti_0.001Zr_0.001O₂
0.5
—
—
—
0.1
0.1

Comparative
Li_1.03Ni_0.907Co_0.079Al_0.0145O₂
—
—
—
—
—
—

Example 1

Comparative
Li_1.03Ni_0.907Co_0.079Al_0.0145O₂+ WO₃
0.5
—
—
—
—
—

Example 2

Comparative
Li_1.03Ni_0.907Co_0.079Al_0.0145O₂+ Li₂WO₄/Li₂W₂O₇
0.5

Example 3

Comparative
Li_1.03Ni_0.907Co_0.079Al_0.0145O₂+ Li₄NiWO₆
0.5

Example 4

(2) XPS Analysis of Positive Electrode Active Material

XPS analysis was performed on the positive electrode active materials prepared according to the Examples and Comparative Examples to verify whether a solid solution is present in the positive electrode active material. The XPS analysis was performed using Quantum 2000 (Physical Electronics. Inc.) (Acceleration voltage: 0.5 to 15 keV, 300 W, Energy resolution: approximately 1.0 eV, Minimum analysis area: 10 micro, Sputter rate: 0.1 nm/min).

FIG. 2 is a graph showing the result of the XPS analysis for the positive electrode active material prepared according to Example 1.

Referring to FIG. 2, it can be confirmed that the W4f_7/2peak for O²⁻ of an oxide having a Li_aNi_bW_cO_dcomposition is present in a range of 34.0 to 35.0 eV, and the W4f_5/2peak is present in the range of 36.0 to 37.0 eV. On the other hand, it can be confirmed that the W4f_7/2peak for W⁶⁺ of an oxide having a Li_aW_cO_dor W_cO_dcomposition is present in a range exceeding 35.5 eV, and the W4f_5/2peak is present in a range exceeding 37.5 eV.

In the case of the W4f spectrum of the positive electrode active material prepared according to Example 1, it can be confirmed that the W4f_7/2peak is present in the range from 34.0 to 35.5 eV, and the W4f_5/2peak is present in the range from 36.0 to 37.5 eV. That is, the positive electrode active material prepared according to Example 1 exhibits a unique peak of Li_aNi_bW_cO_d.

Accordingly, as shown in FIG. 2, by confirming the presence of the unique peak corresponding to Li_aNi_bW_cO_dfrom the W4f results of the positive electrode active materials having NCA compositions, it can be seen that the positive electrode active material prepared according to Example 1 includes a composite formed of an oxide having a Li_aNi_bW_cO_dcomposition and an oxide having a Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂composition in a solid solution.

Table 2 below shows the binding energy sections in which respective peaks corresponding to W4f_7/2and W4f_5/2of the positive electrode active materials prepared according to the Examples and Comparative Examples were present.

TABLE 2

Classification

W4f_7/2(eV)

W4f_5/2(eV)

Range

34.0~
35.0~
36.0~
37.0~

35.0 eV
36.0 eV
37.0 eV
38.0 eV

Example 1
◯
◯
◯
◯

Example 2
◯
◯
◯
◯

Example 3
◯
◯
◯
◯

Example 4
◯
◯
◯
◯

Example 5
◯
◯
◯
◯

Example 6
◯
◯
◯
◯

Example 7
◯
◯
◯
◯

Comparative
X
X
X
X

Example 1

Comparative
X
◯
X
◯

Example 2

Comparative
X
◯
X
◯

Example 3

Comparative
◯
X
◯
X

Example 4

Referring to the results of Table 2, in the cases of Examples 1 to 7, compared to Comparative Example 4, it can be confirmed that, although an oxide having a Li_aNi_bW_cO_dcomposition is not separately added, a W4f peak corresponding to an oxide having a Li_aNi_bW_cO_dcomposition is observed.

(3) XRD Analysis for Positive Electrode Active Material

XRD analysis for positive electrode active materials prepared according to the Examples and Comparative Examples was performed to verify whether Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂and Li_aNi_bW_cO_dphases are present in a positive electrode active material. The XRD analysis was performed using a Bruker D8 Advance diffractometer with Cu Kα radiation (1.540598 □).

FIGS. 3 to 5 are graphs showing the XRD analysis results for the positive electrode active material prepared according to Example 1.

Referring to FIG. 3, it can be confirmed that a compound included in a positive electrode active material exhibits both of a diffraction peak characteristic specific for Li_aNi_bW_cO_dand a diffraction peak characteristic specific for Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂.

More specifically, referring to FIGS. 4 and 5, it can be confirmed that there is a diffraction peak corresponding to the (003) crystal face of Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂at 2θ of approximately 18.8°. In addition, it can be confirmed that there is a diffraction peak corresponding to the (001) crystal face of Li₄NiWO₆at 2θ of approximately 18.3°. That is, a compound included in a positive electrode active material exhibits both of a diffraction peak characteristic specific for Li_aNi_bW_cO_dand a diffraction peak characteristic specific for Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂at 2θ of 18° to 20°.

In a region included in the positive electrode active material, in which 2θ is 20° to 30°, specific diffraction peaks are shown at positions of the (020) crystal face, the (110) crystal face, the (11-1) crystal face and the (021) crystal face, indicating diffraction peaks corresponding to Li_aNi_bW_cO_d.

In addition, referring to FIGS. 3 and 5, it can be confirmed that a compound included in a positive electrode active material exhibits a diffraction peak corresponding to the (104) crystal face of Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂at 2θ of approximately 44.5°, and a diffraction peak corresponding to the (20-2) crystal face of Li₄NiWO₆at 2θ of approximately 43.2°.

Therefore, putting the results of FIGS. 3 to 6 together, it can be confirmed that a compound included in a positive electrode active material exhibits a diffraction peak characteristic specific for Li_aNi_bW_cO_dand a diffraction peak characteristic specific for Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂, indicating a solid solution represented by Formula Li_aNi_bW_cO_d—Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂.

Moreover, while all of the positive electrode active materials prepared according to Examples 2 to 7 exhibit diffraction peak patterns specific for a solid solution and a lithium alloy oxide, in the case of the positive electrode active material prepared according Comparative Example 1, only a diffraction peak pattern corresponding to a Li_wNi_1-(x+y+z)Co_xAl_yO₂composition was detected.

On the other hand, through the XRD analysis, in the cases of the positive electrode active materials prepared according to Comparative Example 2 and Comparative Example 3, no diffraction peak specific for Li_aNi_bW_cO_dwas shown, but in the case of the positive electrode active material prepared according to Comparative Example 4, a diffraction peak pattern similar to the positive electrode active material prepared according to Example 1 was shown.

In the case of the positive electrode active material prepared according to Comparative Example 4, since the oxide having a Li_aNi_bW_cO_dcomposition and an oxide having a Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂composition are present while being simply mixed, diffraction peaks respectively resulting from Li_aNi_bW_cO_dand Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂are simultaneously detected, but it does not mean that Li_aNi_bW_cO_dand Li_wNi_1-(x+y+z)Co_xAl_yW_zO₂, which are components constituting the positive electrode active material prepared according to Comparative Example 4, form a solid solution.

Additionally, by performing XRD analysis for the positive electrode active materials prepared according to Examples 5 to 7, it was confirmed whether an alloy oxide is present in a positive electrode active material. The XRD analysis was performed using a Bruker D8 Advance diffractometer with CuKα radiation (1.540598 □).

FIGS. 6 to 8 are graphs showing the results of XRD analysis for the positive electrode active materials prepared according to Examples 5 to 7.

Referring to FIG. 6, a specific peak corresponding to Li₄WO₅, which is an alloy oxide, in the positive electrode active material prepared according to Example 5 was confirmed, and particularly, while the (220) peak was not shifted, the (111) and (200) peaks were specifically shifted toward a low angle, and thus it was confirmed that there is a different type of alloy oxide in which a part of the tungsten in the lattice structure forming Li₄WO₅is substituted with Ti.

In addition, referring to FIG. 7, a characteristic peak corresponding to an alloy oxide Li₄WO₅of the positive electrode active material prepared according to Example 6 was confirmed, and particularly, as (111), (200) and (220) peaks are specifically shifted toward a low angle, it was confirmed that there are different types of alloy oxides in which a part of the tungsten in a lattice structure formed by Li₄WO₅is substituted with Zr.

Referring to FIG. 8, it can be confirmed that the positive electrode active material prepared according to Example 7 has an NCA composition, and there are peaks corresponding to Li₄WO₅, Li₆WO₆, LiTiO₂, Li₄TiO₄and Li₄ZrO₄in the positive electrode active material.

(5) Measurement of Unreacted Lithium of Positive Electrode Active Material

The measurement of unreacted lithium in each of the positive electrode active materials prepared according to the Examples and Comparative Examples was performed with an amount of 0.1M HCl used until pH 4 by pH titration. First, 5 g of each positive electrode active material was added to 100 mL of DIW, stirred for 15 minutes, and then filtered. 0.1 M HCl was added to 50 mL of the filtered solution, and an amount of HCl consumption according to pH change was measured to determine Q1 and Q2, and thereby, unreacted LiOH and Li₂CO₃were calculated.

M1=23.95 (LiOH Molecular weight)

M2=73.89 (Li₂CO₃Molecular weight)

SPL Size=(Sample weight×Solution Weight)/Water Weight

LiOH(wt %)=[(Q1−Q2)×C×M1×100]/(SPL Size×1000)

Li₂CO₃(wt %)=[2×Q2×C×M2/2×100]/(SPL Size×1000)

The measurement results of unreacted lithium obtained by the above equations are shown in Table 3 below.

TABLE 3

Classification

LiOH (ppm)
Li₂CO₃(ppm)