KR102140211B1 - a method for preparing cathode active material, a cathode active material prepared thereby, and a lithium secondary battery including the same - Google Patents

Abstract

The present invention relates to a lithium secondary battery comprising a positive electrode active material for a lithium secondary battery, a method for manufacturing the same, and produced thereby. The present invention is a step of preparing a mixture of a precursor and a lithium compound of a composite metal oxide, the first heat treatment of the mixture for 2 to 5 hours at 670 ~ 750 ℃, the first heat-treated mixture at 935 ~ 950 ℃ 6-8 It characterized in that it comprises the step of performing a second heat treatment for a period of time, and a third heat treatment of cooling the second heat-treated mixture to 100°C. According to the present invention, a positive electrode active material having a small amount of LiOH can be produced on the surface, and by using this, a lithium secondary battery having improved capacity and life characteristics can be realized.

Description

A method for preparing a cathode active material, a cathode active material prepared thereby, and a lithium secondary battery including the same}

The present invention relates to a method for manufacturing a positive electrode active material, a positive electrode active material produced thereby, and a lithium secondary battery comprising the same. More specifically, the present invention relates to a positive electrode active material having a reduced amount of LiOH present on a surface and a method for manufacturing the same, and a lithium secondary battery having improved capacity and life characteristics by using the positive electrode active material.

Lithium secondary batteries have high voltage and stable charging and discharging characteristics, so they have advantages of higher output density and energy density than conventional nickel-hydrogen batteries, and are used as power sources for many portable electronic devices. LiCoO ₂ , a layered material that is widely used as a positive electrode active material for lithium secondary batteries, is actively developing alternative materials due to the limitations of cobalt's intrinsic reserves, high price, and human health. The layered system LiNi _x Co _y Mn _z O ₂ (0.2<x,y<0.8, 0<z<0.5), which is a material in which Mn and Mn are substituted at a certain ratio, is being developed as an alternative.

LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is a material in which Ni, Co, and Mn are inserted at the same ratio. Co increases structural stability in the process of lithium insertion and desorption, Ni increases capacity, Mn serves to lower the price of the active material.

The layered active material containing lithium reacts with carbon dioxide (CO ₂ ) and moisture in air to form lithium carbonate (Li ₂ CO ₃ ) and lithium hydroxide (LiOH). Especially when nickel is contained, this phenomenon is more severe. This acts as a cause of the loss of lithium present inside the active material, which in turn may cause a decrease in the capacity of the battery. In addition, lithium hydroxide (LiOH) formed on the surface may react with hydrofluoric acid (HF), which may be present in the electrolyte.

Journal of power source, 196 (2011), p.5102-5108

The present invention is to provide a method for manufacturing a cathode active material having a small amount of lithium hydroxide (LiOH) present on the surface and a cathode active material produced thereby.

In addition, the present invention is to provide a lithium secondary battery having improved capacity and life characteristics by using a positive electrode active material having a small amount of lithium hydroxide (LiOH) present on the surface.

One aspect of the present invention, as a positive electrode active material for a lithium secondary battery containing a layered structure lithium composite metal oxide, TOF-SIMS analysis results, in the case of negative polarity, the ratio of the strength of LiOH ^- to the sum of the strength of all lithium compounds is 40 % To 44%. In addition, in the case of positive polarity, the ratio of the intensity of LiOH ⁺ to the sum of the intensity of all lithium compounds may be 0.02% to 0.05%.

Another aspect of the present invention, as a method for preparing the positive electrode active material of the front side, the step of preparing a mixture of a precursor of a complex metal oxide, and a lithium compound, the mixture is subjected to primary heat treatment for 2-5 hours at 670 ~ 750 ℃ step; It may be a method of manufacturing a positive electrode active material comprising the step of secondary heat treatment of the primary heat-treated mixture for 6-8 hours at 935~950°C, and tertiary heat treatment of cooling the secondary heat-treated mixture to 100°C. .

Another aspect of the present invention may be a lithium secondary battery comprising the positive electrode active material.

According to the present invention, a positive electrode active material having a small amount of lithium hydroxide (LiOH) present on the surface can be manufactured, and a lithium secondary battery having improved capacity and life characteristics can be realized using the same.

1 is a graph showing the charge and discharge test results for a lithium secondary battery prepared according to Examples and Comparative Examples.

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

One aspect of the present invention, as a positive electrode active material for a lithium secondary battery comprising a layered structure lithium composite metal oxide, TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectroscopy) analysis results, in the case of negative polarity, the strength of the entire lithium compound It may be a positive electrode active material having a ratio of the strength of LiOH ^- to the ratio of 40% to 44%. In addition, in the case of positive polarity, the ratio of the intensity of LiOH ⁺ to the sum of the intensity of all lithium compounds may be 0.02% to 0.05%.

When the amount of LiOH present on the surface of the positive electrode active material containing the lithium composite metal oxide is greater than the above range, the effect of improving the capacity and life characteristics of the battery does not appear. It is expected that the smaller the amount of LiOH is, the better the capacity and life characteristics improvement effect is, but due to the hygroscopicity of the nickel-based oxide, the formation of LiOH cannot be completely suppressed, which makes it difficult to realize a value smaller than the lower limit of the above range. have. When the amount of LiOH is within the above range, capacity and life characteristics of the lithium secondary battery may be improved.

According to the negative polarity measurement type of TOF-SIMS, the surface of the lithium composite metal oxide is Li ^{-, LiH -, LiO -,} LiOH - is detected by the four kinds of the lithium compound present. The ratio of the strength of LiOH ^- to the sum of the strengths of all lithium compounds is a value obtained by dividing the intensity value of LiOH ^- by the sum of all four types of lithium compounds. In other words, it may mean a ratio (mass ratio) of LiOH among all lithium compounds present on the surface.

According to the positive polarity measurement method of TOF-SIMS, it is detected that three lithium compounds of Li ⁺ , LiH ⁺ and LiOH ⁺ are present on the surface of the lithium composite metal oxide. The ratio of the intensity of LiOH ⁺ is obtained by dividing the intensity value of LiOH ⁺ a sum of the intensity of the three kinds of the lithium compound of the value.

The lithium composite metal oxide may be represented by a composition formula of LiM _x O ₂ (M is one or more selected from the group consisting of Co, Ni, Mn, Fe, Al, V, and Ti, and x is 0.05 or more and 1.10 or less). Specifically, LiCoO ₂ , LiNiO ₂ , LiNi _y Co _1-y O ₂ (0<y<1), LiMn ₂ O ₄ , LiNi _x Co _y Mn _1-xy O ₂ (0.325<x<0.341, 0.325 <y<0.341) or a combination of these.

Another aspect of the present invention, the step of preparing a mixture of a precursor of a complex metal oxide, and a lithium compound, the first heat treatment of the mixture for 2 to 5 hours at 670 ~ 750 ℃, and the first heat-treated mixture 935 ~ It may be a method of manufacturing a positive electrode active material comprising the step of secondary heat treatment for 6-8 hours at 950 ℃. This aspect is characterized in that it undergoes a two-step heat treatment process as described above, through which it is possible to reduce the amount of LiOH present on the surface of the positive electrode active material. In addition, this aspect may further include a step of tertiary heat treatment after the secondary heat treatment, the secondary heat-treated mixture is cooled to 100°C.

First, a mixture of a precursor of a complex metal oxide and a lithium compound may be prepared.

The precursor of the composite metal oxide contains metal and serves as a source of the composite metal. The precursor is preferably a hydroxide form rather than a carbonate form. When the precursor is in the form of a carbonate, there is an advantage that the specific surface area of the synthesized positive electrode active material is large, but a large number of pores may be formed inside the positive electrode active material particles, so that the capacity per volume of the battery may be reduced. As such a precursor, LiM _x (OH) ₂ (M is one or more selected from the group consisting of Co, Ni, Mn, Fe, Al, V and Ti, and x is 0.05 or more and 1.10 or less) Hydroxide can be used.

The precursor of the composite metal oxide can be prepared as follows. First, considering the precursor of the composite metal oxide to be synthesized, the salt of each transition metal can be dissolved in water at a constant molar ratio to prepare a composite metal solution. At this time, one of nickel sulfate, nickel nitrate, and nickel carbonate may be used as the nickel salt, and one or more of cobalt sulfate, cobalt nitrate, and cobalt carbonate may be used as the cobalt salt, and manganese as the manganese salt One or more of sulfate, manganese nitrate, and manganese carbonates can be used. For example, nickel, cobalt, and manganese sulfate may be weighed at a constant molar ratio, and then added to water to prepare a composite metal solution. Next, a precursor of a complex metal oxide in the form of hydroxide can be synthesized by introducing a base such as NaOH, NH ₄ OH, or KOH into the complex metal solution to precipitate. It is preferable that the pH of the reaction solution is 10-12. Next, SO adsorbed on the surface of the precursor of the hydroxide form of a complex metal oxide of the precipitate washed several times with distilled water ^{_{4 2-, NH 4 +, NO}} 3 - by removing, Na ⁺ and K ^+, etc. of high purity The precursor of the composite metal oxide in the form of hydroxide can be obtained, and dried in an oven at 150° C. for 24 hours or more to obtain a composite metal hydroxide that is a precursor of the composite metal oxide.

The lithium compound functions as a lithium source, and any material capable of supplying lithium in a subsequent heat treatment process can be used. The lithium compound is not limited thereto, and one or more selected from the group consisting of Li ₂ CO ₃ , LiOH, LiNO ₃ , and LiCH ₃ COO may be used.

Next, the mixture may be subjected to primary heat treatment at 670 to 750°C for 2 to 5 hours. During the first heat treatment process, the precursor of the composite metal oxide and the lithium compound react to volatilize and remove water and carbon dioxide, and the lithium and the composite metal react to participate in the crystal structure, resulting in the layered structure of the lithium composite metal oxide. Can be formed.

When the primary heat treatment temperature is lower than 670°C, the formation of a lithium composite metal oxide having a layered structure may be delayed or not formed, and if the primary heat treatment temperature is higher than 750°C, crystal growth may be excessive during a subsequent secondary heat treatment process. Rather, battery performance may deteriorate.

If the primary heat treatment time is shorter than 2 hours, the time for lithium to participate in the crystal structure of the composite metal oxide is insufficient, so that the formation of the lithium composite metal oxide crystal structure may not be complete, and if it is longer than 5 hours, the formation of the oxide of the lithium composite metal After this is completed, heat treatment is unnecessarily unnecessary, which is uneconomical.

Next, the first heat-treated mixture may be subjected to a second heat treatment at 935 to 950°C for 6 to 10 hours. Through the secondary heat treatment process, the crystallinity of the lithium composite metal oxide formed in the primary heat treatment process may be further increased. When the lithium ion participates in the formation of the lithium composite metal oxide and the amount of lithium remaining on the surface is small, the proportion of LiOH formed by the combination of lithium and moisture decreases. The secondary heat treatment process is preferably performed at 935 to 950°C. When the secondary heat treatment temperature is higher than 935°C, as the ratio, lithium ions mostly participate in the formation of the lithium composite metal oxide and the amount of lithium remaining on the surface decreases, so the proportion of surface by-product LiOH formed by the combination of lithium and moisture decreases. Capacity and life can be improved. If the secondary heat treatment temperature is higher than 950°C, lithium may volatilize and disappear, and thus capacity and life may be reduced.

If the secondary heat treatment time is shorter than 6 hours, the LiOH reduction effect is almost non-existent, and if it is longer than 8 hours, LiOH is no longer reduced, which may be uneconomical in terms of energy consumption.

Next, a third heat treatment may be performed to cool the second heat-treated mixture to 100°C.

Primary, secondary and tertiary heat treatments can be carried out continuously. By performing the continuous heat treatment, there is no room for other processes or conditions to intervene between the heat treatment process and the heat treatment process, thereby enabling more precise heat treatment control. For such continuous heat treatment, it is preferable to use RHK (Roller Hearth Kiln). When using RHK, there is an advantage that air supply and exhaust conditions can be controlled.

Another aspect of the present invention may be a lithium secondary battery including the positive electrode active material.

As a positive electrode and a negative electrode of a lithium secondary battery, a known active material may be used. A positive electrode and a negative electrode may be prepared by mixing an active material, a binder, and a conductive agent with a solvent to prepare a slurry, and applying the slurry to a current collector such as aluminum, followed by drying and pressing. As the positive electrode active material, the positive electrode active material may be used. As the negative electrode active material, natural graphite, artificial graphite, carbon fiber, coke, carbon black, activated carbon, lithium metal or lithium alloy may be used. The negative electrode active material, the binder and the conductive agent may be mixed with a solvent to form a slurry, and then applied to the negative electrode current collector, followed by drying and pressing to prepare a negative electrode. The binder serves to fix the current collector by binding an active material and a conductive agent, and lithium ions such as polyvinylidene fluoride, polypropylene, carboxymethylcellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, and polyvinyl alcohol Those commonly used in secondary batteries can be used. As the conductive agent, artificial graphite, natural graphite, acetylene black, ketjen black, channel black, lamp black, thermal black, conductive fibers such as carbon fiber or metal fiber, conductive metal oxides such as titanium oxide, metal powders such as aluminum and nickel, etc. Can be used.

The lithium secondary battery may also include a separator, which is a porous membrane between the positive electrode and the negative electrode to prevent electrical short circuit between the two electrodes and function as a channel for ion transfer. As the separator, a single olefin or a complex of olefins such as polyethylene (PE) and polypropylene (PP), polyamide (PA), polyacrylonitrile (PAN), polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene Glycol diacrylate (PEGA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyvinyl chloride (PVC), and the like can be used.

The electrolyte may include a non-aqueous organic solvent and a lithium salt, and as the non-aqueous organic solvent, one or more selected from the group consisting of carbonate-based, ester-based, ether-based and ketone-based organic solvents may be used. Without being limited thereto, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), dimethyl sulfoxide, aceto Nitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethylcarbonate (EMC), butyrolactone, gamma-butyrolactone (GBL), valerolactone, Caprolactone, fluoroethylene carbonate (FEC), methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, pentyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate or halogen derivatives thereof Or may be used by mixing two or more. In addition, as the lithium salt, a material generally used in lithium secondary batteries may be used, but is not limited thereto, LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiAlO ₄ , LiAlCl ₄ and LiSO ₃ CF ₃ . Preferably, a 1.3M LiPF6 EC/DMC/EC = 5:3:2 solution can be used as the electrolyte.

Hereinafter, the present invention will be described in more detail through Examples and Comparative Examples. However, the present invention is not limited to this.

Example 1

Ni _1/3 Co _1/3 Mn _1/3 (OH) ₂ powder (purchased from STM) and Li ₂ CO ₃ powder (purchased from Chemmetal), metal (Ni _1/3 Co _1/3 Mn _1/3 ) and lithium (Li) were mixed in a molar equivalent ratio of 1:1.05, and 5 kg of the mixture was charged into a saggar. The saggar filled with the mixture was continuously heat-treated using 25 stages of RHK (Roller Hearth Kiln). Specifically, the temperature was raised to 700°C at a rate of 2.92°C/min, and then maintained for 2.28 hours. The temperature was raised to 935°C over 1.5 hours and then maintained for 6.84 hours. It was then cooled to 100° C. over 4.94 hours. After heat treatment, it was crushed with a roll crusher (1.5mm intervals) and classified to obtain a positive electrode active material.

As the positive electrode, a positive electrode prepared by using the positive electrode active material is used, and a lithium metal is used as the negative electrode, and ethylene carbonate (Ethylene Carbonate), ethylmethyl carbonate (EthylMethyl Carbonate), and dimethyl carbonate (Dimethyl Carbonate) are used as the electrolyte. After mixing at a volume ratio of 20:20:60, an electrolyte solution having a lithium salt concentration of 1.3M in which lithium salt LiPF ₆ was dissolved was used. As a separator, a coin cell was manufactured using a porous polyethylene membrane.

Example 2

A positive electrode active material and a coin cell were prepared according to the same method as in Example 1, except that the secondary heat treatment temperature was 940°C.

Comparative example

A positive electrode active material and a coin cell were manufactured according to the same method as in Example 1, except that the second heat treatment temperature was 930°C.

Composition evaluation of positive electrode active material (ICP analysis)

ICP analysis was performed on the positive electrode active materials of Examples and Comparative Examples to confirm the composition of the synthesized positive electrode active material, and the results are shown in Table 1. As an ICP analysis instrument, an ACTIVA m model from HORIBA scientific was used.

Li/(Ni+Co+Mn) Main element content (mol%) Ni Co Mn
Comparative example

1.061
33.5
34.1
32.4
Example 1

1.058
33.4
33.9
32.7
Example 2

1.053
33.6
34.1
32.3

Referring to Table 1, the composition of the positive electrode active material according to Examples and Comparative Examples were all similar.

Crystallinity evaluation of positive electrode active material (XRD analysis)

The positive electrode active material according to Examples and Comparative Examples was subjected to XRD analysis to measure the half width (FWHM) at the (101) crystal plane. The XRD analysis equipment of BRUKER AXS was used, and analysis was performed under the conditions of 2θ=10 to 80°, 0.01°/step, Cu-Kα (1.5418Å, 40kV/30mA). According to the results of the analysis, the half width was 0.165° in the case of the comparative example, 0.161° in the case of Example 1, and 0.156° in the case of Example 2. From these results, it can be seen that the crystallinity of the positive electrode active material according to Example 2 is the best.

Evaluation of the amount of LiOH on the surface of the positive electrode active material (TOF-SIMS analysis)

A cathode active material prepared according to Examples and Comparative Examples was subjected to a Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) analysis (ION TOF, TOF SIMS 5), and the results are shown in Table 2. There is four kinds of the lithium compound of detection, in the case of Negative polarity measurements on the surface of the lithium-metal composite oxide ^{Li -, LiH -, LiO -} -, LiOH (LiOH - intensity ratio of) = (LiOH ^- intensity value) / ( It was calculated as the sum of the intensity ^{values) - Li -, LiH -,} LiO - and LiOH. In the case of positive polarity measurement, three lithium compounds of Li ⁺ , LiH ⁺ and LiOH ⁺ are detected, (intensity ratio of LiOH ⁺ )=(intensity value of LiOH ⁺ )/(Li ⁺ , LiH ⁺ and LiOH ⁺ intensity (Sum of values).

Battery capacity characteristics

For the coin cells according to Examples and Comparative Examples, 0.2C charging and 0.1C discharging were performed at room temperature with a charge/discharger (Toyo, Toscat3100), followed by 0.2C charging and discharging. Table 2 shows the discharge capacity measured in the second cycle of the charge/discharge test.

Battery life characteristics

The test for charging and discharging to 1C after charging and discharging 0.2C at room temperature with a charge/discharger was repeated for the coin cells according to Examples and Comparative Examples. 1C charge/discharge test criteria The life characteristics of the battery were evaluated by the ratio of the discharge capacity after the 35 charge/discharge test to the discharge capacity after the 1 charge/discharge test, and the results are shown in Table 2 and FIG. 1.

calcination
Temperature Crystallinity
(Half width) TOF-SIMS intensity ratio Capacity (mAh/g)
(0.2C discharge) Life characteristics
(35 doses/1 dose) LiOH ^- LiOH ⁺
Comparative example

930℃
0.165°
51.5%
0.05%
154.7
89.6%
Example 1

935℃
0.161°
43.8%
0.03%
156.8
94.5%
Example 2

940℃
0.156°
42.7%
0.03%
155.0
94.5%

Referring to Table 2, the LiOH ^- intensity ratio shows a smaller value than Comparative Examples in Examples 1 and 2, and the LiOH ⁺ intensity ratio indicates similar values in both Examples and Comparative Examples.

In addition, the capacity of the battery according to Example 2 shows the best value. Therefore, it can be seen that when the positive electrode active material is calcined at 935°C or higher, the capacity characteristics are improved.

In addition, the life characteristics of Examples show values superior to those of Comparative Examples. From these results, it can be seen that the calcination at 935°C or higher improves the life characteristics.

Referring to FIG. 1, Examples 1 and 2 show similar life characteristics, but Comparative Examples have a lower discharge capacity than Examples 1 and 2 and a tendency to rapidly decrease the discharge capacity after 35 cycles.

Summarizing the above results, according to the present invention, it is possible to manufacture a positive electrode active material with improved crystallinity and a reduced amount of LiOH on the surface, and a lithium secondary battery manufactured using the positive electrode active material has capacity characteristics and life characteristics It can be confirmed that this is improved.

The terms used in the present invention are intended to describe specific embodiments, and are not intended to limit the present invention. Singular expressions should be considered to include plural meanings, unless the context is clear. The terms "include" or "have" mean that there are features, numbers, steps, actions, elements, or combinations thereof described in the specification, and are not intended to exclude them. The present invention is not limited by the above-described embodiments, but is intended to be limited by the appended claims. Accordingly, various forms of substitution, modification, and modification will be possible by those skilled in the art without departing from the technical spirit of the present invention as set forth in the claims, and this also belongs to the scope of the present invention. something to do.

Claims (10)

A positive electrode active material for a lithium secondary battery containing a layered-structure lithium composite metal oxide. As a result of TOF-SIMS analysis, in the case of negative polarity, a positive electrode active material having a ratio of strength of LiOH ^- to 40% to 44% of the sum of the strengths of all lithium compounds . According to claim 1, TOF-SIMS analysis, positive polarity in the case of positive polarity, the ratio of the strength of LiOH ⁺ compared to the sum of the strengths of all lithium compounds is 0.02% to 0.05% of the positive electrode active material. According to claim 1, The lithium composite metal oxide, LiM _x O ₂ (M is Co, Ni, Mn, Fe, Al, V, and one or more selected from the group consisting of Ti, x is 0.05 or more and 1.10 or less. ) The positive electrode active material for a lithium secondary battery represented by the composition formula. According to claim 1, wherein the lithium composite metal oxide is LiCoO ₂ , LiNiO ₂ , LiNi _y Co _1-y O ₂ (0<y<1), LiMn ₂ O ₄ and LiNi _x Co _y Mn _1-xy O ₂ ( A cathode active material for a lithium secondary battery comprising one or more selected from the group consisting of 0.325<x<0.341, 0.325<y<0.341). A method for manufacturing the positive electrode active material of claim 1,
Preparing a mixture of a precursor of a complex metal oxide and a lithium compound;
Primary heat treatment of the mixture at 670 ~ 750 ℃ for 2 ~ 5 hours; And
Method for producing a positive electrode active material comprising the step of secondary heat treatment for 6 to 8 hours at 935 ~ 950 ℃ higher than the primary heat treatment mixture of the primary heat treatment temperature. According to claim 5, After the second heat treatment step, the method of manufacturing a positive electrode active material comprising the step of cooling the mixture to the second heat treatment to 100 ℃. The method of claim 5, wherein the first and second heat treatments are continuously performed. The complex metal oxide precursor is M _x (OH) ₂ (M is one or more selected from the group consisting of Co, Ni, Mn, Fe, Al, V and Ti, and x is 0.05 or more. Method of producing a positive electrode active material comprising a hydroxide represented by 1.10 or less). The method according to claim 5, wherein the lithium compound comprises at least one selected from the group consisting of Li ₂ CO ₃ , LiOH, LiNO ₃ , and LiCH ₃ COO. A lithium secondary battery comprising the positive electrode active material of claim 1.

Images