KR102306441B1 - Positive active material for rechargeable lithium battery, method of preparing the same, and rechargeable lithium battery including the same - Google Patents

Abstract

The positive electrode active material is present on the surface of the lithium metal compound and the lithium metal compound, the positive electrode active material for a lithium secondary battery comprising a conductive surface layer comprising a metal salt of an acid that is relatively stronger than phosphate and phosphoric acid, a method for manufacturing the same, and lithium containing the same A secondary battery is provided.

Description

A cathode active material for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery comprising the same

A cathode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same.

A lithium secondary battery is composed of a positive electrode and a negative electrode including an active material capable of inserting and deintercalating lithium ions, and is used by charging an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode. At this time, electric energy is produced by oxidation and reduction reactions when lithium ions are inserted and desorbed from the positive and negative electrodes.

As a cathode active material of a lithium secondary battery, an oxide made of lithium and a transition metal capable of intercalation and deintercalation is mainly used. As a cathode active material, LiCoO ₂ is most widely used. Recently, a substitute material, such as as a high-capacity and high-output and stable even required, performance and three-component system of LiCoO _2, up bingye as the use of a lithium secondary battery expansion by industries such as power tools, automobile by the mobile information electronic device Research on development is actively underway.

In particular, lithium nickel-based oxide having a high nickel content can have a high capacity and exhibit excellent electrochemical properties. However, due to the presence of a lot of residual lithium on the surface, cycle characteristics are deteriorated and stability is low, making it impossible to use for a long period of time.

One embodiment is to provide a positive electrode active material for a lithium secondary battery that exhibits a high capacity, has excellent lifespan characteristics, and has excellent stability due to a small amount of gas generation.

Another embodiment is to provide a method of manufacturing the positive active material.

Another embodiment is to provide a lithium secondary battery including the positive active material.

The positive active material for a lithium secondary battery according to an embodiment is present on a lithium metal compound and the lithium metal compound, and includes an ion conductive surface layer including a first metal salt and a second metal salt, the first metal salt and the second The metal salts are different from each other, and the first metal salt includes a metal salt of phosphoric acid, and the second metal salt includes a metal salt of an acid that is relatively stronger than the phosphoric acid.

A method of manufacturing a cathode active material for a lithium secondary battery according to another embodiment includes reacting a lithium metal compound surface with a solution containing phosphoric acid and a relatively strong acid than the phosphoric acid; and after the reaction, heat-treating the lithium metal compound to form an ion conductive surface layer comprising a first metal salt and a second metal salt on the surface, wherein the first metal salt and the second metal salt are different from each other and , The first metal salt includes a metal salt of phosphoric acid, and the second metal salt includes a metal salt of an acid that is a relatively stronger acid than the phosphoric acid.

A lithium secondary battery according to another embodiment includes a positive electrode including the above-described positive electrode active material.

It is possible to provide a positive electrode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same, which exhibits high capacity by reducing residual lithium, has excellent lifespan characteristics, and has excellent stability due to a small amount of gas generation.

1 is a diagram schematically showing the structure of a lithium secondary battery according to an embodiment.
FIG. 2 is a graph comparing the amount of gas generated according to high temperature standing at 80° C. in a state in which the cathode active material-containing cells according to Examples 1 and 2, Comparative Examples 1 and 4 were charged at 4.3V.
3 is a graph comparing the lifespan characteristics of the positive electrode active materials according to Examples 1, 2, Comparative Example 1, Comparative Example 4, Comparative Example 5, and Comparative Example 6 at a high temperature (45° C.).

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, in describing the present description, descriptions of already known functions or configurations will be omitted in order to clarify the gist of the present description.

A positive active material for a lithium secondary battery according to an embodiment will be described.

A lithium secondary battery according to an embodiment is present on a lithium metal compound and a lithium metal compound, and includes an ion conductive surface layer including a first metal salt and a second metal salt, wherein the first metal salt and the second metal salt are different from each other and the first metal salt includes a metal salt of phosphoric acid, and the second metal salt includes a metal salt of an acid that is a relatively stronger acid than the phosphoric acid. In the cathode active material for a secondary battery according to an exemplary embodiment, since surface residual lithium is reduced, lifespan characteristics are improved, and side reactions caused by residual lithium are suppressed, thereby preventing deterioration of battery performance.

The lithium metal compound may be a lithium nickel-based metal oxide, and may be represented by Formula 1 below.

[Formula 1]

Li _a Ni _b X _c Y _d O ₂

In Formula 1,

X and Y are each independently at least one selected from Co, Mn, Al, Mg, Ti, B, Ca, Sr, Ba, V, Cr, Fe, Cu and Zr, 0.95≤a=1.05, 0.4≤ b=1.00, 0≦c=0.6, 0≦d=0.6, and b+c+d=1. Specifically, 0.5<b=0.99, more specifically 0.6≤b=0.99, and more specifically 0.7≤b=0.99. When the lithium nickel-based oxide represented by Formula 1, specifically, the lithium nickel-based oxide having a relatively high ratio of b value indicating the nickel content exceeds 0.5, is used as a positive electrode active material, it has a high capacity as well as a ratio It is possible to implement a lithium secondary battery having excellent electrochemical properties such as characteristics. More specifically, the lithium metal oxide may be represented by the following formula (2).

[Formula 2]

Li _a (Ni _1-xyz Co _x Mn _y M _z )O ₂

In Formula 1, M is boron (B), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe) ), copper (Cu), zirconium (Zr), and an element selected from the group consisting of aluminum (Al), 0.95≤a≤1.05, x≤(1-xyz), y≤(1-xyz), z≤( 1-xyz), 0<x<1, 0≤y<1, 0≤z<1. More specifically, 0.95≤a≤1.05, 0<x≤1/3, for example, 0.05≤x≤1/3, 0≤y≤0.5, for example, 0.05≤y≤1/3, 0≤z ≤0.05, 1/3≤(1-xyz)≤0.95. The lithium metal oxide may be, for example, LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , or LiNi _0.85 Co _0.1 Al _0.05 O ₂ .

The positive active material for a lithium secondary battery according to an embodiment includes an ion conductive surface layer formed by performing a surface treatment process of a lithium metal compound. The ion conductive surface layer includes a first metal salt, such as a metal salt of phosphoric acid. By reducing residual lithium through the surface treatment process, the amount of gas generated can be reduced, and the lifespan characteristics can be improved. In the surface treatment process, basic residual lithium, etc. existing on the surface of the lithium metal compound may react with phosphoric acid to form a metal salt of phosphoric acid. The metal salt of phosphoric acid is a metal salt produced by reacting phosphoric acid with lithium or other metal ions, Li ₃ PO ₄ , Li ₂ HPO ₄ , LiH ₂ PO ₄ , LiMPO ₄ (M = Fe, Co, Mn, at least one selected from Ni, V, La, Ti and Al), or a combination thereof. The phosphoric acid may be, for example, H ₃ PO ₄ .

The ion conductive surface layer includes, in addition to the metal salt of phosphoric acid, a second metal salt, for example, a metal salt of an acid that is a relatively stronger acid than the phosphoric acid. In other words, since it includes a metal salt of phosphoric acid and a metal salt of an acid stronger than phosphoric acid, at least two types of metal salts of different acids (a first metal salt and a second metal salt) are included. The metal salt of phosphoric acid (the first metal salt) and the metal salt of an acid stronger than phosphoric acid (the second metal salt) are gas in a converted form of _{lithium hydroxide (LiOH) and lithium carbonate (Li 2} CO _{3 ), which are residual lithium present on the surface of the positive electrode active material.} By reducing the amount of generation and having excellent electrical conductivity, the capacity of a lithium secondary battery manufactured using the same may not be impaired.

As the acid stronger than phosphoric acid, an acid having a pKa of less than 2.15 may be used. For example, oxalic acid (OA) [pKa 1.25], phosphorous acid (HP) [pKa 1.1], fluoroacetic acid (TFA) [pKa 0.23] or It may be a combination of these. More specifically, the strong acid may be oxalic acid (OA) or trifluoroacetic acid (TFA). Accordingly, the surface layer may include a metal salt of phosphoric acid and a metal salt of oxalic acid, or a metal salt of phosphoric acid and a metal salt of trifluoroacetic acid. At this time, the metal oxalate salt is Li ₂ C ₂ O ₄ , LiHC ₂ O ₄ , (LiM) ₂ (C ₂ O ₄ ) ₃ (M = Fe, Co, Mn, Ni, V, La, Ti, or Al), or a combination thereof. The metal salt of trifluoroacetic acid may be CF ₃ COOLi, LiF, or a combination thereof. This phenomenon LiOH content reduced residue of the lithium component is effective or Li ₂ CO ₃ content reduced, the effect is negligible or even increase the Li ₂ CO ₃ content was observed when only phosphoric acid used for surface modification. In the case of including phosphoric acid and a metal salt of an acid stronger than phosphoric acid as in the present embodiment, it is effective to reduce the content of _{Li 2} CO _{3 as well as the residual lithium component LiOH.} In addition, oxalic acid or trifluoroacetic acid can be used in a lithium secondary battery to maintain stability without generating a side reaction such as corrosion of the battery.

The effect of suppressing side reactions such as battery corrosion and maintaining stability may vary depending on the type and content of the strong acid. Specifically, when the strong acid is oxalic acid, the phosphoric acid and oxalic acid may each independently be included in 0.3 mol% and 0.1 mol% based on 1 mol of the lithium metal compound, and when the strong acid is trifluoroacetic acid, the phosphoric acid and trifluoroacetic acid may each independently be included in 0.2 mol% and 0.3 mol% based on 1 mol of the lithium metal compound. For example, when the strong acid is oxalic acid, the phosphoric acid and oxalic acid may be included in a molar ratio of 3:1 with respect to 1 mole of the lithium metal compound. When the strong acid is trifluoroacetic acid, the phosphoric acid and trifluoroacetic acid are It may be included in a molar ratio of 2:3 with respect to 1 mol of the lithium metal compound.

Hereinafter, a method of manufacturing a cathode active material for a lithium secondary battery according to an exemplary embodiment will be described. The method of manufacturing a cathode active material for a lithium secondary battery according to the present embodiment includes neutralizing the surface of a lithium metal compound with a solution containing phosphoric acid and a relatively strong acid than the phosphoric acid; and forming an ion conductive surface layer including a first metal salt and a second metal salt on the surface by heat-treating the lithium metal compound after the neutralization reaction. In this case, the first metal salt and the second metal salt are different from each other, the first metal salt includes a phosphate, and the second metal salt includes a metal salt of an acid that is relatively stronger than the phosphoric acid.

For example, the method of manufacturing a positive electrode active material for a lithium secondary battery according to the present embodiment may further include removing the solvent and moisture of the solution containing the phosphoric acid and a solution containing a relatively strong acid than the phosphoric acid after the formation of the ion conductive surface layer. .

First, a solution for surface modification for surface modification of a lithium metal compound is prepared. The surface modification solution includes phosphoric acid and a relatively strong acid than the phosphoric acid. The phosphoric acid can remove LiOH, a strong basic material, and further includes an acid that is a strong acid than the phosphoric acid, so that the weak basic material Li ₂ CO _{3 is} also effectively removed. can do. At this time, the phosphoric acid and the strong acid exist in a dissolved state in a solvent.

The phosphoric acid and the strong acid are each independently added so as not to exceed 1 mol% based on 1 mol of the lithium metal compound. As the addition amount increases, the reversible capacity per mass tends to gradually decrease, and a rapid capacity decrease occurs in the region exceeding the added amount. Specifically, the phosphoric acid may be included in an amount of 0.1 mol% to 1.0 mol% based on 1 mol of the lithium metal compound, and the strong acid may be included in an amount of 0.05 mol% to 0.5 mol% based on 1 mol of the lithium metal compound. More specifically, the addition ratio with phosphoric acid showing an optimum point may vary depending on the type of the strong acid. For example, if the strong acid is oxalic acid, the phosphoric acid and oxalic acid are included in a molar ratio of 3:1, and the strong acid is trifluoro In the case of acetic acid, the phosphoric acid and trifluoroacetic acid may be included in a molar ratio of 2:3. The solvent is an organic solvent capable of dissolving phosphoric acid and a strong acid together, for example, anhydrous ethanol (CH ₃ CH ₂ OH, an alcohol). ), isopropyl alcohol (C ₃ H ₈ O), ethylene glycol (C ₂ H ₆ O ₂ ) and methylpyrrolidone (N-methyl-2-pyrrolidone) may be at least one.

The phosphoric acid and the strong acid may each independently be included in an amount of 1 wt% to 40 wt%, for example 20 wt% to 30 wt%, based on the total weight of the solution for surface modification. Considering the solubility and fairness of the phosphoric acid and the strong acid, the phosphoric acid and the strong acid must be included in the above ranges, respectively, to be effective in reducing residual lithium.

Then, the surface of the provided lithium metal compound is subjected to a neutralization reaction with the solution for surface modification. The neutralization reaction may be performed by adding a solution for surface modification to the lithium metal compound and stirring. After the neutralization reaction proceeds, a conductive surface layer is formed on the surface of the lithium metal compound by a first metal salt, such as a metal salt of phosphoric acid and a second metal salt, such as a metal salt of a strong acid, produced by reacting the residual lithium compound with phosphoric acid and a strong acid. That is, the metal salt of phosphoric acid (the first metal salt) and the metal salt of the strong acid (the second metal salt) included in the conductive surface layer are products of a neutralization reaction other than water. The conductive surface layer may include a metal salt of phosphoric acid (a first metal salt) and a metal salt of an acid stronger than phosphoric acid (a second metal salt). The conductive surface layer may be formed on a part or all of the surface of the lithium metal compound.

Then, the lithium metal compound is heat-treated to form an ion conductive surface layer. The heat treatment may be performed at 300° C. to 500° C., for example, at 300° C. to 400° C. for 1 hour to 10 hours, for example, 3 hours to 10 hours. In this case, the temperature increase rate may be 0.5 °C/min to 3.0 °C/min, for example, 1.0 °C/min to 2.0 °C/min. Under the above conditions, the solvent and moisture of the surface modification solution may be effectively removed without affecting the performance of the positive electrode active material, and an ion conductive surface layer may be formed. In the present invention, a general mixer can be used by using a minimum amount of solvent, and an efficient process can be implemented by using only a minimum amount of solvent sufficient to volatilize the solvent by heat generated during the mixing process.

The metal salt of phosphoric acid and the metal salt of a strong acid may be produced by any one of Schemes 1 to 3 below.

The metal salt of phosphoric acid may be Li ₃ PO ₄ , Li ₂ HPO ₄ , LiH ₂ PO ₄ , LiMPO ₄ (M = Fe, Co, Mn, Ni, V, La, Ti, or Al), or a combination thereof. .

For example, when the strong acid is oxalic acid, the metal salt is Li ₂ C ₂ O ₄ , LiHC ₂ O ₄ and (LiM) ₂ (C ₂ O ₄ ) ₃ (M = Fe, Co, Mn, Ni, V, La, Ti, and at least one selected from Al), the strong acid may be trifluoroacetic acid, and the metal salt _{may be at least one selected from CF 3} COOLi, LiF, and combinations thereof.

The following Reaction Scheme 1 is a reaction formula in which the surface modification solution _{removes LiOH and Li 2} CO ₃ , the following Reaction Scheme 2 is a reaction formula in the case of using oxalic acid as a strong acid, and the following Reaction Scheme 3 uses trifluoroacetic acid as a strong acid This is the reaction equation in the case

That is, the following Schemes 1 to 3 are _{a series of reaction schemes in which the residual lithium compound (LiOH, Li 2} CO _3, etc.) on the surface of the prepared lithium metal compound is neutralized with phosphoric acid and strong acid to form an ion conductive surface layer.

[Scheme 1]

H ₃ PO ₄ + 3LiOH → Li ₃ PO ₄ + 3H ₂ O

2H ₃ PO ₄ + 3Li ₂ CO ₃ → 2Li ₃ PO ₄ + 3CO ₂ + 3H ₂ O

[Scheme 2]

C ₂ H ₂ O ₄ + 2LiOH → Li ₂ C ₂ O ₄ + 2H ₂ O

C ₂ H ₂ O ₄ + Li ₂ CO ₃ → Li ₂ C ₂ O ₄ + CO ₂ + H ₂ O

[Scheme 3]

CF ₃ COOH + LiOH → CF ₃ COOLi + H ₂ O

2CF ₃ COOH + Li ₂ CO ₃ → 2CF ₃ COOLi + H ₂ O + CO ₂

CF ₃ COOH + LiOH → CF ₃ COOLi + H ₂ O

2CF ₃ COOH + Li ₂ CO ₃ → 6LiF + H ₂ O + 7CO ₂

The content ranges and content ratios of the phosphoric acid and the strong acid are the same as described above.

1 is a schematic diagram showing a lithium secondary battery according to an embodiment. Referring to FIG. 1 , a lithium secondary battery 100 according to an embodiment includes a positive electrode 114 , a negative electrode 112 opposite to the positive electrode 114 , and a separator disposed between the positive electrode 114 and the negative electrode 112 . 113, and an electrode assembly including an electrolyte (not shown) impregnated with the positive electrode 114, the negative electrode 112, and the separator 113, and the battery container 120 and the battery container containing the electrode assembly and a sealing member 140 sealing the 120 .

The positive electrode 114 includes a current collector and a positive electrode active material layer formed on the current collector. The positive active material layer includes a positive active material, a binder, and optionally a conductive material.

Al may be used as the current collector, but is not limited thereto.

The positive active material is the same as described above.

The binder serves to well adhere the positive active material particles to each other and also to adhere the positive active material to the current collector. Specific examples of the binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like, but is not limited thereto.

The conductive material is used to impart conductivity to the electrode, and any electronically conductive material can be used as long as it does not cause chemical change. Examples include natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, copper. , nickel, aluminum, metal powders such as silver, metal fibers, etc., may be used in combination with one or more conductive materials such as polyphenylene derivatives.

The negative electrode 112 includes a current collector and a negative active material layer formed on the current collector.

The current collector may use Cu, but is not limited thereto.

The anode active material layer includes an anode active material, a binder, and optionally a conductive material.

The negative active material includes a material capable of reversibly intercalating/deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and dedoping lithium, or a transition metal oxide.

The material capable of reversibly intercalating/deintercalating the lithium ions is a carbon material, and any carbon-based negative active material generally used in lithium secondary batteries may be used, and representative examples thereof include crystalline carbon, Amorphous carbon or these may be used together. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake, spherical or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon (low-temperature calcined carbon). or hard carbon, mesophase pitch carbide, and calcined coke.

The lithium metal alloy includes lithium and a group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn. An alloy of a metal selected from may be used.

Examples of the material capable of doping and dedoping lithium include Si, SiO _x (0 < x < 2), Si-C composite, and Si-Q alloy (wherein Q is an alkali metal, alkaline earth metal, group 13 to group 16 element) , transition metal, rare earth element, or a combination thereof, not Si), Sn, SnO ₂ , Sn-C composite, Sn-R (wherein R is an alkali metal, alkaline earth metal, group 13 to 16 element, transition metal, A rare earth element or a combination thereof, but not Sn), and at least one of these and SiO ₂ may be mixed and used. Specific elements of Q and R include Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, or a combination thereof.

Examples of the transition metal oxide include vanadium oxide and lithium vanadium oxide.

The binder well adheres the negative active material particles to each other and also serves to attach the negative active material to the current collector, representative examples of which include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, and carboxylation. polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene -Butadiene rubber, epoxy resin, nylon, etc., but are not limited thereto.

The conductive material is used to impart conductivity to the electrode, and any electronically conductive material can be used as long as it does not cause chemical change. Examples include natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc. A conductive material including a conductive polymer such as a carbon-based material, a metal powder such as copper, nickel, aluminum, or silver, or a metal-based material such as a metal fiber, a polyphenylene derivative, or a mixture thereof may be used.

The negative electrode 112 and the positive electrode 114 are prepared by mixing an active material, a conductive material, and a binder in a solvent to prepare a slurry, and then applying the slurry to each current collector. As the solvent, an organic solvent such as N-methylpyrrolidone may be used, and an aqueous solvent such as water may be used depending on the type of binder, but is not limited thereto. Since such an electrode manufacturing method is widely known in the art, a detailed description thereof will be omitted herein.

The electrolyte includes an organic solvent and a lithium salt.

The organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move. The organic solvent may be selected from carbonate-based, ester-based, ether-based, ketone-based, alcohol-based and aprotic solvents.

Examples of the carbonate-based solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate ( ethylpropyl carbonate (EPC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. may be used.

In particular, when the chain carbonate compound and the cyclic carbonate compound are mixed and used, the dielectric constant can be increased and the solvent can be prepared with a low viscosity. In this case, the cyclic carbonate compound and the chain carbonate compound may be mixed and used in a volume ratio of about 1:1 to 1:9.

In addition, the ester solvent is, for example, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, ν-butyrolactone, decanolide (decanolide), valerolactone, me Valonolactone (mevalonolactone), caprolactone (caprolactone) and the like may be used. As the ether solvent, for example, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. may be used, and as the ketone-based solvent, cyclohexanone, etc. may be used. have. In addition, ethyl alcohol, isopropyl alcohol, etc. may be used as the alcohol-based solvent.

The organic solvents may be used alone or in combination of one or more, and when one or more of the organic solvents are mixed and used, the mixing ratio may be appropriately adjusted according to the desired battery performance.

The electrolyte may further include an additive such as an overcharge inhibitor such as ethylene carbonate or pyrocarbonate.

The lithium salt is dissolved in an organic solvent, serves as a source of lithium ions in the battery, enables basic lithium secondary battery operation, and promotes movement of lithium ions between the positive electrode and the negative electrode.

Specific examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN(SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiN(C _x F _2x+1 SO ₂ )(C _y F _2y+1 SO ₂ ), where x and y are natural numbers, LiCl, LiI, LiB(C ₂ O ₄ ) ₂ (lithium bis(oxalato) ) borate; LiBOB), or a combination thereof.

The concentration of the lithium salt is preferably used within the range of about 0.1M to about 2.0M. When the concentration of the lithium salt is included in the above range, since the electrolyte has an appropriate conductivity and viscosity, excellent electrolyte performance may be exhibited, and lithium ions may move effectively.

The separator 113 separates the negative electrode 112 and the positive electrode 114 and provides a passage for lithium ions to move, and any one commonly used in a lithium battery may be used. That is, one having low resistance to ion movement of the electrolyte and excellent moisture content of the electrolyte may be used. For example, it may be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in the form of a nonwoven fabric or a woven fabric. For example, a polyolefin-based polymer separator such as polyethylene or polypropylene is mainly used for lithium ion batteries, and a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and optionally single-layer or multi-layer structure can be used.

Hereinafter, specific embodiments of the present invention are presented. However, the examples described below are only for specifically illustrating or explaining the present invention, and the present invention is not limited thereto. In addition, since the content not described here can be sufficiently technically inferred by a person skilled in the art, the description thereof will be omitted.

Example 1

Based on 1 mol of the lithium metal compound LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , 0.3 mol % of phosphoric acid (H ₃ PO ₄ ) and 0.1 mol % of oxalic acid (OA) were quantified.

The quantified phosphoric acid _{was added to 1.226 g of absolute ethanol (CH 3} CH ₂ OH) to prepare a mixed solution. After that, the mixed solution was added to 100 g of the positive electrode active material. Then, mixing and neutralization were continuously performed through a stirrer for about 1 minute.

Thereafter, the positive electrode active material was calcined at a low temperature at 350° C. for 6 hours at a temperature increase rate of 1.5° C./min, and then cooled naturally to prepare a surface-modified positive electrode active material.

Example 2

0.3 mol % of phosphoric acid (H ₃ PO ₄ ) in Example 1 Instead, a positive active material was prepared in the same manner as in Example 1, except that 0.5 mol % of phosphoric acid (H ₃ PO _{4 ) was used.}

Example 3

0.3 mol % of phosphoric acid (H ₃ PO ₄ ) in Example 1 Instead, a cathode active material was prepared in the same manner as in Example 1, except that 0.7 mol % of phosphoric acid (H ₃ PO _{4 ) was used.}

Example 4

0.3 mol % of phosphoric acid (H ₃ PO ₄ ) in Example 1 Instead, 0.2 mol % of phosphoric acid (H ₃ PO ₄ ) was used and 0.3 mol % of trifluoro acetic acid (TFA) was used instead of 0.1 mol % of oxalic acid, and then the cathode active material was prepared in the same manner as in Example 1. manufactured.

Example 5

A cathode active material was prepared in the same manner as in Example 1, except that LiNi _0.7 Co _0.15 Mn _0.15 O ₂ was used instead of the lithium metal compound LiNi _0.6 Co _0.2 Mn _0.2 O _{2 in Example 1.}

Comparative Example 1

LiNi _0.6 Co _0.2 Mn _0.2 O ₂ was used as a positive electrode active material.

Comparative Example 2

In the same manner as in Example 1, 0.3 mol % of phosphoric acid (H ₃ PO ₄ ) and 0.1 mol % of oxalic acid were added, mixed and neutralized, and the firing process was omitted to prepare a cathode active material.

Comparative Example 3

In the same manner as in Example 1, 0.7 mol % of phosphoric acid (H ₃ PO ₄ ) was added, mixed and neutralized, and the firing process was omitted to prepare a cathode active material.

Comparative Example 4

In Example 1 of oxalic acid A positive active material was prepared in the same manner as in Example 1, except that 0.5 mol % of phosphoric acid (H ₃ PO _{4 ) was used without addition.}

Comparative Example 5

0.3 mol % of phosphoric acid (H ₃ PO ₄ ) in Example 1 A positive active material was prepared in the same manner as in Example 1, except that, without addition, only 0.3 mol % of trifluoroacetic acid was used.

Comparative Example 6

0.3 mol % of phosphoric acid (H ₃ PO ₄ ) in Example 1 A positive active material was prepared in the same manner as in Example 1, except that, without addition, only 0.5 mol % of trifluoroacetic acid was used.

Comparative Example 7

0.3 mol % of phosphoric acid (H ₃ PO ₄ ) in Example 1 A positive active material was prepared in the same manner as in Example 1, except that only 0.1 mol % of oxalic acid was used without addition.

Comparative Example 8

In Comparative Example 1, instead _{of LiNi 0.6} Co _0.2 Mn _0.2 O _{2 LiNi 0.7} Co _0.15 Mn _0.15 O ₂ was used as the positive electrode active material.

Evaluation 1: Analysis of Residual Lithium Content

Residual lithium content is measured using the Warder method, which is an acid-base neutralization titration of mixed alkali. After measuring 10 g of active material and stirring with 100 g of pure water for 30 minutes, a filtrate is generated through a filtration filter. Neutralization titration is carried out while adding hydrochloric acid (HCl) with a concentration of 0.1N to the filtrate through a titrator, and at the first inflection point (pH 7~9 range) and the second inflection point (pH 3~6 range) on the neutralization titration curve. Residual lithium content is calculated by converting the amount of hydrochloric acid (HCl) input.

Table 1 below shows the change in the residual lithium content and the electrochemical characteristics according to whether the surface is treated, the content, and whether or not to be fired.

surface modification solution firing temperature
(℃) residual lithium electrochemical properties phosphoric acid
(mol%) strong acid
(mol%) Li ₂ CO ₃
(wt%) LiOH
(wt%) Total Li
(ppm) 0.1C capacity
(mAh/g) Coulomb Efficiency
(%) Example 1 0.3 OA
0.1 350 0.211 0.311 1259 185.4 93.8 Example 2 0.5 OA0.1 350 0.243 0.259 1209 185.0 93.6 Example 3 0.7 OA 0.1 350 0.234 0.248 1160 183.7 93.4 Example 4 0.2 TFA 0.3 350 0.084 0.289 996 185.1 93.5 Comparative Example 1 0 0 350 0.234 0.396 1588 184.8 94.1 Comparative Example 2 0.3 0A0.1 - 0.231 0.309 1331 181.4 92.5 Comparative Example 3 0.7 0A0.1 - 0.273 0.262 1274 178.7 92.0 Comparative Example 4 0.5 0 350 0.275 0.249 1241 183.4 93.1 Comparative Example 5 0 TFA0.3 350 0.11 0.303 1086 185.6 93.3 Comparative Example 6 0 TFA0.5 350 0.079 0.279 958 183.8 92.9 Comparative Example 7 0 OA0.1 350 0.275 0.294 1370 184.0 93.8

Referring to Table 1 above, when the surface of a lithium metal compound is modified using phosphoric acid and a strong acid (oxalic acid (OA) or trifluoroacetic acid (TFA)) together, lithium compounds such as _{Li 2} CO _{3 and LiOH and all} It can be confirmed that the concentration of lithium has been reduced, and at this time, due to the alternating effect of the two acids, it shows an excellent effect of reducing residual lithium even with a small amount of addition compared to the case of using one type of acid, while at the same time reversible capacity, efficiency, and high temperature life (45 ℃) was also confirmed to be improved.

In addition, when not firing, it is effective to reduce residual lithium, but a problem in that electrochemical performance, particularly, reversible capacity, is deteriorated was observed, and it was confirmed that the firing process was absolutely necessary.

Table 2 below shows the content of residual lithium according to the type of the positive electrode active material and whether or not the surface is treated.

lithium metal compound surface modification solution residual lithium phosphoric acid
(mol%) strong acid
(mol%) Li ₂ CO ₃
(wt%) LiOH
(wt%) Total Li
(ppm) Example 2 LiNi _0.6 Co _0.2 Mn _0.2 O ₂ 0.5 OA
0.1 0.243 0.259 1209 Comparative Example 1 - - 0.234 0.396 1588 Example 5 LiNi _0.7 Co _0.15 Mn _0.15 O ₂ 0.5 OA
0.1 0.188 0.636 2197 Comparative Example 8 - - 0.419 0.755 2976

According to Table 2, in the case of the lithium metal compound corresponding to the lithium nickel-based metal oxide, it can be confirmed that the effect of reducing the total lithium and lithium compounds such as _{Li 2} CO _{3 and LiOH according to whether or not the surface modification is more significant.}

Evaluation 2: Gas generation

In addition, FIG. 2 is a graph comparing gas generation according to high temperature at 80° C. in a state of being charged at 4.3V, from which the gas generation amount of Examples 1 and 2, which are positive electrode active materials surface-modified by phosphoric acid and strong acid, is compared before reforming. It can be confirmed that 50% or more was reduced compared to Example 1. In addition, in the case of Comparative Example 4 using phosphoric acid alone, _{it can be confirmed that the reduction of Li 2} CO ₃ is small and the gas generation reduction effect is relatively small.

Evaluation 3: Lifetime Characteristics

Furthermore, FIG. 3 is a graph comparing the lifespan characteristics of the positive electrode active material at high temperature (45° C.), from which the life characteristics of Examples 1 and 2, which are positive electrode active materials surface-modified by phosphoric acid and strong acid, are compared with Comparative Example 1 before modification It can be seen that the surface was improved compared to Comparative Example 4, which was surface-modified with only one kind of phosphoric acid (H ₃ PO _{4 ).} In addition, in the case of Comparative Examples 5 and 6 using TFA alone, it was confirmed that the life characteristics were poor.

Evaluation 4: Analysis of the conductive surface layer present on the surface of the positive electrode active material

The surface of the positive active material according to Example 2 and Comparative Example 1 was analyzed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (Time of Surface Flight Secondary Ion Mass Spectrometry). , the results are shown in Table 3 and Table 4 below.

(Unit: atomic %) element C O Co Ni Mn Li P S Example 2 19.3 33.6 2.2 5.5 4.6 16.9 1.6 1.3 Comparative Example 1 10.1 36.1 1.3 2.3 3.3 21.1 0.1 1.2

From Table 3, it can be seen that the surface modification layer including the _{Li 3} PO ₄ compound is formed on the surface of the active material.

(Unit: atomic %) P PO PO ₂ PO ₃

PO_x

PO _x Example 2 0.14 0.26 2.80 2.15 5.35 Comparative Example 1 0.03 0.02 0.04 0.26 0.35

From Table 4, it can be seen that the surface modification layer including various types of phosphoric oxide is formed on the surface of the positive electrode active material.

In the foregoing, specific embodiments of the present invention have been described and shown, but it is common knowledge in the art that the present invention is not limited to the described embodiments, and that various modifications and variations can be made without departing from the spirit and scope of the present invention. It is self-evident to those who have Accordingly, such modifications or variations should not be individually understood from the technical spirit or point of view of the present invention, and modified embodiments should be said to belong to the claims of the present invention.

100: lithium secondary battery
112: cathode
113: separator
114: positive electrode
120: battery container
140: sealing member

Claims (9)

lithium metal compounds; and
It is present on the lithium metal compound and comprises an ion conductive surface layer comprising a first metal salt and a second metal salt,
The first metal salt and the second metal salt are different from each other,
The first metal salt includes a metal salt of phosphoric acid,
The second metal salt includes a metal salt of an acid that is a relatively stronger acid than the phosphoric acid,
The strong acid is at least one selected from oxalic acid, phosphorous acid, trifluoroacetic acid, and combinations thereof, provided that when the strong acid is oxalic acid, the phosphoric acid and oxalic acid are 3:1 to 7:1 with respect to 1 mole of the lithium metal compound It is included in a molar ratio, and when the strong acid is trifluoroacetic acid, the phosphoric acid and trifluoroacetic acid are included in a molar ratio of 2:3 with respect to 1 mole of the lithium metal compound. The method of claim 1, wherein the metal salt of phosphoric acid is Li ₃ PO ₄ , Li ₂ HPO ₄ , LiH ₂ PO ₄ , LiMPO ₄ (M = Fe, Co, Mn, Ni, V, La, Ti and at least one selected from Al ), or a positive active material for a lithium secondary battery, which is a combination thereof. delete delete According to claim 1, wherein the lithium metal compound is a cathode active material for a lithium secondary battery represented by the following formula (1).
[Formula 1]
Li _a Ni _b X _c Y _d O ₂
In Formula 1,
X and Y are each independently at least one selected from Co, Mn, Al, Mg, Ti, B, Ca, Sr, Ba, V, Cr, Fe, Cu and Zr, 0.95≤a=1.05, 0.4≤ b=1.00, 0≦c=0.6, 0≦d=0.6, and b+c+d=1. reacting the lithium metal compound surface with a solution containing phosphoric acid and a relatively strong acid than the phosphoric acid; and
After the reaction, heat-treating the lithium metal compound to form an ion conductive surface layer comprising a first metal salt and a second metal salt on the surface,
The first metal salt and the second metal salt are different from each other,
The first metal salt includes a metal salt of phosphoric acid,
The second metal salt includes a metal salt of an acid that is a relatively stronger acid than the phosphoric acid,
The strong acid is at least one selected from oxalic acid, phosphorous acid, trifluoroacetic acid, and combinations thereof, provided that when the strong acid is oxalic acid, the phosphoric acid and oxalic acid are 3:1 to 7:1 with respect to 1 mole of the lithium metal compound It is included in a molar ratio, and when the strong acid is trifluoroacetic acid, the phosphoric acid and trifluoroacetic acid are included in a molar ratio of 2:3 with respect to 1 mol of the lithium metal compound,
A method of manufacturing a cathode active material for a lithium secondary battery. The method of claim 6 , wherein the strong acid is oxalic acid. The method of claim 6 , wherein the strong acid is trifluoroacetic acid. A lithium secondary battery comprising a positive electrode comprising the positive active material of any one of claims 1, 2 and 5 or the positive active material prepared according to the method of any one of claims 6 to 8.

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