KR20190130911A - Positive active material for rechargeable lithium battery, method of preparing the same, and rechargeable lithium battery including the same - Google Patents

Abstract

The present invention provides a positive electrode active material for a rechargeable lithium battery comprising a lithium metal compound and an ion conductive surface layer which is on the lithium metal compound and includes phosphates and metal salts of acids that are relatively stronger than phosphoric acid, a preparation method thereof and a rechargeable lithium battery including the same.

Description

A positive electrode active material for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same TECHNICAL FIELD

The present invention relates to a cathode active material for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same.

The lithium secondary battery includes a positive electrode and a negative electrode including an active material capable of inserting and detaching lithium ions, and is used by filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode. At this time, electrical energy is produced by oxidation and reduction reactions when lithium ions are inserted and removed from the anode and cathode.

As a cathode active material of a lithium secondary battery, an oxide composed of lithium and a transition metal that can intercalate and deintercalate is mainly used. LiCoO ₂ is the most widely used positive electrode active material. Recently, as the use of lithium secondary batteries has been expanded from portable information electronic devices to industries such as power tools and automobiles, high capacity, high output, and stability have been further demanded. Accordingly, performance improvement of LiCoO ₂ and replacement of alternative materials such as three-component or olivine-based Research on development is actively underway.

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

One embodiment is to provide a positive electrode active material for a lithium secondary battery exhibiting high capacity, excellent lifespan characteristics, low gas generation and excellent stability.

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

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

A cathode active material for a rechargeable lithium battery according to one 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, wherein the first metal salt and the second metal salt are present. The metal salts are different from each other, the first metal salt comprises a metal salt of phosphoric acid, and the second metal salt comprises a metal salt of acid that is a relatively stronger acid than the phosphoric acid.

According to another embodiment, a method of manufacturing a cathode active material for a rechargeable lithium battery includes reacting a surface of a lithium metal compound with a solution including phosphoric acid and a stronger 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. The first metal salt includes a metal salt of phosphoric acid, and the second metal salt includes a metal salt of acid that is relatively stronger than the phosphoric acid.

According to another embodiment, a lithium secondary battery includes a cathode including the cathode active material described above.

It is possible to provide a cathode active material for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same, which exhibits high capacity due to reduced residual lithium, excellent life characteristics, and low amount of gas generation.

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

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

It will be described with respect to the positive electrode active material for a lithium secondary battery according to one embodiment.

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. The first metal salt includes a metal salt of phosphoric acid, and the second metal salt includes a metal salt of acid that is relatively stronger than the phosphoric acid. In the positive electrode active material for a secondary battery according to an embodiment, surface residual lithium may be reduced to improve life characteristics, and side reactions due to residual lithium may be suppressed, thereby reducing battery performance.

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

[Formula 1]

Li _a Ni _b X _c Y _d O ₂

In Chemical 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, and 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. In the case of using a lithium nickel-based oxide represented by Chemical Formula 1, specifically, a lithium nickel-based oxide having a relatively high ratio of b value indicating nickel content of more than 0.5, as well as having a high capacity, rate A lithium secondary battery having excellent electrochemical characteristics such as characteristics can be realized. 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 aluminum (Al) and are elements selected from the group consisting of 0.95≤a≤1.05, x≤ (1-xyz), y≤ (1-xyz), z≤ ( 1-xyz), 0 <x <1, 0 ≦ y <1, and 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 cathode active material for a rechargeable lithium battery according to one embodiment includes an ion conductive surface layer formed by performing a surface treatment process of a lithium metal compound. The ion conductive surface layer comprises a first metal salt, such as a metal salt of phosphoric acid. The surface treatment process may reduce residual lithium to reduce gas generation and improve life characteristics. In the surface treatment process, phosphoric acid may react with basic residual lithium, which is present on the surface of the lithium metal compound, to form a metal salt of phosphoric acid. The metal salt of phosphoric acid is a phosphoric acid reacted with lithium or other metal ions to form a metal salt, 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 used, 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 acid that is a relatively stronger acid than the phosphoric acid. In other words, metal salts of phosphoric acid and metal salts of acids that are stronger than phosphoric acid include at least two kinds of metal salts of different acids (first metal salt and second metal salt). The metal salt of phosphoric acid (first metal salt) and the metal salt of acid stronger than phosphoric acid (second metal salt) are gas in the form of converted lithium hydroxide (LiOH) and lithium carbonate (Li ₂ CO ₃ ), which are residual lithium present on the surface of the positive electrode active material. It is possible to reduce the amount of generation and excellent electrical conductivity, so that the capacity of the lithium secondary battery manufactured using the same may not be impaired.

An acid having a stronger acid than the phosphoric acid may use an acid having a pKa of less than 2.15. For example, oxalic acid (OA) [pKa 1.25], phosphate-based phosphorous acid (HP) [pKa 1.1], fluorine-based trifluoroacetic acid (TFA) [pKa 0.23] or Combinations thereof. More specifically, the strong acid may be oxalic acid (Oxalic acid, OA) or trifluoroacetic acid (Trifluoroacetic acid, TFA). Thus, the surface layer may include a metal salt of phosphoric acid and a metal salt of oxalic acid, or may include 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 combinations thereof. The metal salt of trifluoroacetic acid may be CF ₃ COOLi, LiF or a combination thereof. When only phosphoric acid is used for surface modification, it is effective to reduce LiOH content in residual lithium components, but it is ineffective for reducing Li ₂ CO ₃ content or rather increases Li ₂ CO ₃ content. When the metal salt of phosphoric acid and acid acid stronger than phosphoric acid is included as in the present embodiment, it is effective in reducing Li ₂ CO ₃ content as well as LiOH, which is a residual lithium component. In addition, oxalic acid or trifluoroacetic acid can be used in a lithium secondary battery to maintain stability without generating side reactions such as corrosion of the battery.

Effects 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 include 0.3 mol% and 0.1 mol% with respect to 1 mol of the lithium metal compound, and when the strong acid is trifluoroacetic acid, the phosphoric acid And trifluoroacetic acid may be each independently included in 0.2 mol% and 0.3 mol% with respect to 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 to 1 mol of the lithium metal compound, and 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 rechargeable lithium battery according to one embodiment is described. Method for producing a cathode active material for a lithium secondary battery according to the present embodiment comprises the steps of neutralizing the surface of the lithium metal compound with a solution containing phosphoric acid and a stronger acid than the phosphoric acid; And after the neutralization reaction, heat treating the lithium metal compound to form an ion conductive surface layer including a first metal salt and a second metal salt on the surface. 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 a relatively stronger acid than the phosphoric acid.

For example, the method of manufacturing a cathode active material for a lithium secondary battery according to the present embodiment may further include removing the solvent and water from a solution containing a phosphoric acid and a strong acid relatively stronger than the phosphoric acid after the ion conductive surface layer is formed. .

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 stronger acid than the phosphoric acid. The phosphoric acid may remove LiOH, which is a strong base material, and further includes a weak base material, Li ₂ CO ₃ , which is more acidic than the phosphoric acid. can do. At this time, the phosphoric acid and strong acid are present in a dissolved state in a solvent.

The amount of phosphoric acid and strong acid added may not independently exceed 1 mol% based on 1 mol of the lithium metal compound. As the amount added increases, the reversible capacity per mass tends to decrease gradually, and in the region exceeding the amount added, a sharp decrease in capacity occurs. In detail, 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 the phosphoric acid representing the optimum point may vary according to the type of the strong acid. For example, when 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 triplet. If 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). ), Isopropyl alcohol (C ₃ H ₈ O), ethylene glycol (C ₂ H ₆ O ₂ ) and methylpyrrolidone (N-methyl-2-pyrrolidone).

The phosphoric acid and the strong acid may each independently include 1 wt% to 40 wt%, such as 20 wt% to 30 wt%, based on the total weight of the solution for surface modification. In consideration of solubility and fairness of the phosphoric acid and the strong acid, the phosphoric acid and the strong acid should be included in the above ranges, respectively, in order to reduce residual lithium.

 Subsequently, the surface of the provided lithium metal compound is neutralized with the solution for surface modification. The process of carrying out the neutralization reaction may be performed by adding and stirring a solution for surface modification to the lithium metal compound. After the neutralization reaction proceeds, a conductive surface layer is formed on the surface of the lithium metal compound by a first metal salt generated by reacting the residual lithium compound with phosphoric acid and a strong acid, such as a metal salt of phosphoric acid and a second metal salt such as a metal salt of strong acid. That is, the metal salt of phosphoric acid (first metal salt) and the strong acid metal salt (second metal salt) contained 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 (first metal salt) and a metal salt of acid that is stronger than phosphoric acid (second metal salt). The conductive surface layer may be formed on some or all of the surface of the lithium metal compound.

Subsequently, the lithium metal compound is heat treated to form an ion conductive surface layer. The heat treatment may be carried out 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. At this time, the temperature increase rate may be 0.5 ℃ / min to 3.0 ℃ / min, for example, 1.0 ℃ / min to 2.0 ℃ / min. Under the above conditions, the solvent and water of the surface modification solution can be effectively removed without affecting the performance of the positive electrode active material, and an ion conductive surface layer can be formed. In the case of the present invention, it is possible to use a general mixer by using a minimum amount of solvent and to implement an efficient process by using only a minimum amount of solvent such that the solvent may be volatilized by the heat generated in the mixing process.

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

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, At least one selected from Ti, and Al), wherein the strong acid is trifluoroacetic acid, and the metal salt may be at least one selected from CF ₃ COOLi, LiF, and a combination thereof.

Scheme 1 below is a reaction for removing the LiOH and Li ₂ CO ₃ solution for the surface modification, Scheme 2 is a reaction in the case of using oxalic acid as a strong acid, Scheme 3 is using a trichloroacetic acid as a strong acid It is a reaction scheme of the case.

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

Scheme 1

H ₃ PO ₄ + ₃ LiOH → Li ₃ PO ₄ + 3 H 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 + ₂ LiOH → 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 range, content ratio and the like of the phosphoric acid and the strong acid are as described above.

1 is a schematic view showing a rechargeable lithium battery according to one embodiment. Referring to FIG. 1, a lithium secondary battery 100 according to an embodiment includes a positive electrode 114, a negative electrode 112 facing the positive electrode 114, and a separator disposed between the positive electrode 114 and the negative electrode 112. An electrode assembly (113) and an electrolyte assembly (not shown) that impregnates the positive electrode 114, the negative electrode 112, and the separator 113, the battery container 120 containing the electrode assembly, and the battery container. And a sealing member 140 for sealing 120.

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

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

The cathode active material is as described above.

The binder adheres the positive electrode active material particles to each other well, and also serves to adhere the positive electrode 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 are not limited thereto.

The conductive material is used to impart conductivity to the electrode, and any electronic conductive material can be used without causing chemical change, and examples thereof include natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, and copper. , Metal powders such as nickel, aluminum, silver, metal fibers and the like can be used, and one or more conductive materials such as polyphenylene derivatives can be mixed and used.

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

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

The negative electrode active material layer includes a negative electrode active material, a binder, and optionally a conductive material.

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

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

Examples of the alloy of the lithium metal include lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn. Alloys of metals selected from can be used.

Examples of materials capable of doping and undoping lithium include Si, SiO _x (0 <x <2), Si-C composites, and Si-Q alloys (wherein Q is an alkali metal, an alkaline earth metal, and a group 13 to 16 element). , A transition metal, a rare earth element or a combination thereof, not Si), Sn, SnO ₂ , Sn-C composite, Sn-R (wherein R is an alkali metal, an alkaline earth metal, an element of Group 13-16, a transition metal, A rare earth element or a combination thereof, 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 is mentioned.

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

The binder adheres well to the negative electrode active material particles, and also adheres the negative electrode active material to the current collector, and typical examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, and carboxylation. Polyvinylchloride, polyvinylfluoride, ethylene oxide containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene Butadiene rubber, epoxy resin, nylon, etc., but is not limited thereto.

The conductive material is used to impart conductivity to the electrode, and may be used as long as it is an electronic conductive material without causing chemical change, and examples thereof include natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber. A conductive material containing a conductive powder such as a metal powder such as carbon-based material copper, nickel, aluminum, silver, or a polyphenylene derivative of a metal-based material such as metal fiber, or a mixture thereof can 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, or 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 well known in the art, detailed description thereof will be omitted.

The electrolyte solution contains 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, ester, ether, ketone, alcohol and aprotic solvents.

Examples of the carbonate solvents include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), and ethylpropyl carbonate ( Ethylpropyl carbonate (EPC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and the like may be used.

In particular, when a mixture of the chain carbonate compound and the cyclic carbonate compound is used, the dielectric constant may be increased, and the solvent may be prepared with a low viscosity solvent. 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 may be, for example, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, ν-butyrolactone, decanolide, valerolactone, and merol. Valonolactone, 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. As the ketone solvent, cyclohexanone may be used. have. In addition, ethyl alcohol, isopropyl alcohol, etc. may be used as the alcohol solvent.

The organic solvent may be used singly or in mixture of one or more, and the mixing ratio in the case of mixing one or more may be appropriately adjusted according to the desired battery performance.

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

The lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions in the battery to enable the operation of a basic lithium secondary battery and to promote the 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, it can exhibit excellent electrolyte performance, and lithium ions can move effectively.

The separator 113 separates the cathode 112 and the anode 114 and provides a movement path for lithium ions, and any separator can be used as long as it is commonly used in a lithium battery. In other words, those having low resistance to ion migration of the electrolyte and excellent electrolyte-wetting ability can be used. For example, it is selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in a nonwoven or woven form. For example, a polyolefin-based polymer separator such as polyethylene or polypropylene is mainly used for a lithium ion battery, and a coated separator including a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength. Can be used as a structure.

The following presents specific embodiments of the present invention. However, the examples described below are merely for illustrating or explaining the present invention in detail, and thus the present invention is not limited thereto. In addition, the description is not described herein, so those skilled in the art that can be sufficiently technically inferred, the description thereof will be omitted.

Example 1

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

The quantified phosphoric acid was added to 1.226 g of anhydrous ethanol (CH ₃ CH ₂ OH) to form a mixed solution. Thereafter, the mixed solution was added to 100 g of the positive electrode active material. Then, the mixing and neutralization reaction was continued for about 1 minute through a stirrer.

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

Example 2

0.3 mol% phosphoric acid (H ₃ PO ₄ ) in Example 1 A positive electrode 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 instead.

Example 3

0.3 mol% phosphoric acid (H ₃ PO ₄ ) in Example 1 A positive electrode 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 instead.

Example 4

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

Example 5

A positive electrode active material was manufactured 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 the 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 (Oxalic acid) were added to mix and neutralize, 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% phosphoric acid (H ₃ PO ₄ ) was added to the mixture and the neutralization reaction, and the firing process was omitted to prepare a cathode active material.

Comparative Example 4

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

Comparative Example 5

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

Comparative Example 6

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

Comparative Example 7

0.3 mol% phosphoric acid (H ₃ PO ₄ ) in Example 1 A positive electrode 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: Residual Lithium Content Analysis

Residual lithium content is measured using the Wader method, which is an acid group neutralization titration method of mixed alkalis. 10 g of the active material was quantified and stirred with 100 g of pure water for 30 minutes to produce a filtrate through a filtration filter. Neutralization titration was performed by adding 0.1 N hydrochloric acid (HCl) to the filtrate through a titrator, and at the first inflection point (pH 7-9 region) and the second inflection point (pH 3-6 region) on the neutralization titration curve. The residual lithium content is calculated by converting the amount of hydrochloric acid (HCl).

Table 1 shows the residual lithium content change and electrochemical characteristics according to the surface treatment and content, whether or not firing.

Surface modification solution Firing temperature
(℃) Residual lithium Electrochemical properties Phosphoric Acid
(mol%) Strong mountain
(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, when the surface of the lithium metal compound is modified using phosphoric acid and strong acid (oxalic acid (OA) or trifluoroacetic acid (TFA)), lithium compounds such as Li ₂ CO ₃ and LiOH and the whole It can be seen that the concentration of lithium is reduced, and due to the alternating effect of the two acids, it shows excellent residual lithium reduction effect even with a small amount of addition compared to the case of using one acid, and at the same time reversible capacity, efficiency and high temperature life (45 ° C) was also found to be improved.

Moreover, when baking was not performed, it was effective for reducing residual lithium, but the problem that the electrochemical performance, especially the reversible capacity | capacitance decreases, was observed, and also confirmed that the baking process is necessary.

Table 2 shows the residual lithium content depending on 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 mountain
(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 seen that the reduction effect of the lithium compound and the total lithium, such as Li ₂ CO ₃ and LiOH according to the surface modification is more significant.

Evaluation 2: Gas Generation

In addition, Figure 2 is a graph comparing the gas generation amount according to the high temperature standing at 80 ℃ in a state filled with 4.3V, from which the gas generation amount of Examples 1 and 2, which is a cathode active material surface-modified by phosphoric acid and strong acid compared before It can be confirmed that more than 50% reduction compared to Example 1. In addition, in the case of Comparative Example 4 using phosphoric acid alone, the reduction of Li ₂ CO ₃ is small, it can be confirmed that the gas generation reduction effect is relatively small.

Evaluation 3: Life Characteristics

In addition, Figure 3 is a graph comparing the life characteristics of the positive electrode active material at a high temperature (45 ℃), from which the life characteristics of Examples 1 and 2, the positive electrode active material surface-modified by phosphoric acid and strong acid before the modification and It can be seen that compared with Comparative Example 4 surface-modified only with one phosphoric acid (H ₃ PO ₄ ). In addition, in 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 electrode active material according to Example 2 and Comparative Example 1 was analyzed by X-ray photoelectron spectroscopy (XPS) and 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 confirmed that the surface modification layer containing a 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 containing various types of phosphorus oxide is formed on the surface of the positive electrode active material.

While specific embodiments of the invention have been described and illustrated above, it is to be understood that the invention is not limited to the described embodiments, and that various modifications and changes can be made without departing from the spirit and scope of the invention. It is obvious to those who have. Therefore, such modifications or variations are not to be understood individually from the technical spirit or viewpoint of the present invention, and the modified embodiments shall belong to the claims of the present invention.

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

Claims (9)

Lithium metal compounds; And
Is present on the lithium metal compound, including 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 comprises a metal salt of phosphoric acid,
The second metal salt is a positive electrode active material for a lithium secondary battery comprising a metal salt of an acid that is relatively stronger than the phosphoric acid. In claim 1, wherein the metal salt of phosphoric acid is at least one selected from Li ₃ PO ₄ , Li ₂ HPO ₄ , LiH ₂ PO ₄ , LiMPO ₄ (M = Fe, Co, Mn, Ni, V, La, Ti and Al) ), Or a combination thereof, a cathode active material for a lithium secondary battery. The positive active material of claim 1, wherein the strong acid is an acid having a pKa of less than 2.15. The cathode active material of claim 1, wherein the strong acid is at least one selected from oxalic acid, phosphorous acid, trifluoroacetic acid, and a combination thereof. The cathode active material of claim 1, wherein the lithium metal compound is represented by Formula 1 below.
[Formula 1]
Li _a Ni _b X _c Y _d O ₂
In Chemical 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, and 0.95≤a = 1.05, 0.4≤ b = 1.00, 0 ≦ c = 0.6, 0 ≦ d = 0.6, and b + c + d = 1. Reacting a lithium metal compound surface with a solution comprising phosphoric acid and a stronger acid relative to 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 comprises a metal salt of phosphoric acid,
The second metal salt comprises a metal salt of an acid that is relatively stronger than the phosphoric acid,
The manufacturing method of the positive electrode active material for lithium secondary batteries. 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 including the positive electrode active material according to any one of claims 1 to 5 or the positive electrode active material prepared according to any one of claims 6 to 8.

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