JP7168373B2 - Positive electrode active material for lithium-ion batteries - Google Patents

Description

The present application discloses positive electrode active materials and the like used in lithium ion batteries.

As disclosed in Patent Documents 1 to 3, lithium cobalt oxide having a layered rock salt crystal phase, lithium manganate having a spinel crystal phase, and the like are widely used as positive electrode active materials used in lithium ion batteries. . On the other hand, in recent years, lithium cobaltate having a spinel-type crystal phase as disclosed in Non-Patent Document 1 has been developed, and is expected as a new positive electrode active material for lithium ion batteries.

JP 2011-001256 A JP-A-2002-289175 JP 2016-143539 A

Eungje Lee et al., ACS Appl. Mater. Interfaces 2016, 8, 27720-27729

According to the findings of the present inventors, lithium cobalt oxide having a spinel-type crystal phase disclosed in Non-Patent Document 1 has a small change in lattice constant due to the insertion and extraction of lithium ions. It is considered that when applied as a positive electrode active material, the change in volume of the positive electrode during charging and discharging can be reduced. However, when lithium cobalt oxide having a spinel crystal phase is used as a positive electrode active material to form a lithium ion battery, the initial coulombic efficiency of the battery may decrease.

As one means for solving the above problems, the present application has a spinel-type crystal phase containing lithium, cobalt, aluminum, and oxygen, and LiCo _x Al _y O _2±δ (0.85≦x<1, Disclosed is a cathode active material for a lithium ion battery, having a composition represented by 0<y≦0.15, 0.85<x+y≦1.15).

According to the new findings of the present inventors, by doping a specific amount of aluminum in the spinel-type lithium cobaltate like the positive electrode active material of the present disclosure, when applied to a lithium ion battery, the battery performance is improved. The initial coulombic efficiency is greatly increased.

1 is a schematic diagram for explaining the configuration of a lithium ion battery 10; FIG. 1 is a schematic diagram for explaining the configuration of a lithium ion battery system 100; FIG. 4 is a diagram for explaining an example of a control flow in the lithium ion battery system 100; FIG. FIG. 2 is a diagram showing X-ray diffraction peaks of positive electrode active materials according to Examples 1 to 5 and Comparative Examples 1 to 5; 1 is a diagram showing SEM images of positive electrode active materials according to Example 1 and Comparative Example 1. FIG. 1 is a diagram showing first charge-discharge curves (4.45V-2.5V) of lithium ion batteries using positive electrode active materials according to Example 1 and Comparative Example 1. FIG.

1. Positive electrode active material The positive electrode active material of the present disclosure is a positive electrode active material used in lithium ion batteries, has a spinel crystal phase containing lithium, cobalt, aluminum, and oxygen, and is LiCo _{x Aly} O _2±δ It is characterized by having a composition represented by (0.85≤x<1, 0<y≤0.15, 0.85<x+y≤1.15).

1.1. Crystal Phase The positive electrode active material of the present disclosure has a spinel-type crystal phase containing lithium, cobalt, aluminum, and oxygen. “Having a spinel-type crystal phase” means that at least a diffraction peak derived from a spinel-type crystal phase is confirmed in X-ray diffraction. For example, the positive electrode active material of the present disclosure has 2θ = 19.8 ± 0.4 °, 37.3 ± 0.4 °, 39.0 ± 0.4 ° in X-ray diffraction measurement using CuKα as a radiation source. , 45.3±0.4°, 49.7±0.4°, 60.1±0.4°, 66.1±0.4° and 69.5±0.4° A diffraction peak derived from the crystal phase is preferably confirmed. It is believed that the spinel-type lithium cobalt oxide and the positive electrode active material of the present disclosure have different crystal lattice constants in the spinel-type crystal phase. That is, after confirming the composition of the positive electrode active material by X-ray diffraction and elemental analysis, by confirming the crystal lattice constant of the spinel-type crystal phase by X-ray diffraction, "lithium, cobalt, aluminum, and oxygen It is thought that it is possible to confirm the presence or absence of a spinel-type crystal phase containing and.

In the positive electrode active material of the present disclosure, the spinel crystal phase contains lithium, cobalt, aluminum, and oxygen. In other words, it can be said that the positive electrode active material of the present disclosure is obtained by replacing some elements of spinel-type lithium cobaltate with aluminum. It is believed that this stabilizes the spinel crystal phase.

The positive electrode active material of the present disclosure has the specific spinel crystal phase described above. On the other hand, the positive electrode active material of the present disclosure may contain crystal phases other than the spinel crystal phase as long as the above problems can be solved. For example, when synthesizing a composite oxide containing lithium and cobalt, a thermally stable layered rock salt crystal phase may be formed together with the spinel crystal phase. can exert the effect of In this regard, the positive electrode active material of the present disclosure may contain a layered rock salt crystal phase in addition to the spinel crystal phase. Preferably, in the positive electrode active material of the present disclosure, only diffraction peaks derived from the spinel crystal phase are confirmed in X-ray diffraction measurement.

1.2. Composition The positive electrode active material of the present disclosure has a composition represented by LiCo _x Aly O _2±δ (0.85≦x<1, 0< _y ≦0.15, 0.85<x+y≦1.15). have. According to the findings of the present inventors, the initial coulombic efficiency of the battery is significantly increased when the aluminum substitution amount (doping amount) in the spinel-type lithium cobalt oxide is within the specific range indicated by the above composition formula.

From the viewpoint of further increasing coulombic efficiency, x in the above composition formula is more preferably 0.85 ≤ x ≤ 0.975, still more preferably 0.85 ≤ x ≤ 0.95, and particularly preferably 0.85≦x≦0.93. y is more preferably 0.025≤y≤0.15, still more preferably 0.05≤y≤0.15, and particularly preferably 0.07≤y≤0.15.

In the positive electrode active material of the present disclosure, the total molar ratio of Co and Al to Li is preferably 1 (x + y = 1), but even if Li is somewhat excessive, or Li is somewhat insufficient, Even if it is, it is possible to generate and maintain the spinel type crystal phase, and the desired effect can be exhibited. In this respect, as shown in the above composition formula, the molar ratio of Co and Al to Li should be more than 0.85 and 1.15 or less (0.85<x+y≦1.15). The lower limit is preferably 0.9 or more, more preferably 0.95 or more, and the upper limit is preferably 1.1 or less, more preferably 1.05 or less.

In the positive electrode active material of the present disclosure, the molar ratio of O to Li (O/Li) is preferably 2 from the stoichiometric ratio of spinel-type lithium cobalt oxide, but the stoichiometric ratio as a spinel-type crystal phase Even if the amount of oxygen exceeds the ratio or the amount of oxygen is partially deficient, the spinel crystal phase can be generated and maintained, and the desired effect can be exhibited. In this regard, it is preferable that the molar ratio of O to Li (O/Li) is, for example, 1.8 or more and 2.2 or less. Alternatively, δ in the above composition formula is preferably 0.2 or less.

1.3. Shape The shape and size of the positive electrode active material of the present disclosure are not particularly limited as long as they are applicable to the positive electrode of a lithium ion battery. It is preferably particulate.

1.4. Effect The cathode active material of the present disclosure is doped with a specific amount of aluminum in the spinel-type lithium cobaltate, resulting in a higher initial coulombic efficiency of the battery when applied to a lithium-ion battery compared to the case without aluminum. increases significantly. This is probably because the spinel-type crystal phase was stabilized by aluminum.

2. Method for producing positive electrode active material The positive electrode active material of the present disclosure includes, for example, a first step of mixing a lithium source, a cobalt source, and an aluminum source to obtain a mixture, and heating the mixture to form a spinel crystal phase. It can be produced through a second step of obtaining a composite oxide having.

2.1. First Step In the first step, a lithium source, a cobalt source and an aluminum source are mixed to obtain a mixture. Lithium sources include lithium compounds and metallic lithium. Lithium compounds include lithium carbonate, lithium oxide, lithium hydroxide, lithium acetate, and the like. Lithium carbonate is preferred for the solid phase method. Lithium acetate is preferred when the liquid phase method (evaporation to dryness method) is used. Cobalt sources include cobalt compounds and metallic cobalt. Cobalt compounds include cobalt carbonate, cobalt oxide, cobalt hydroxide, and cobalt acetate. In the solid phase method, cobalt oxide is preferred, and Co ₃ O ₄ is more preferred. Cobalt acetate is preferred for the liquid phase method (evaporation to dryness method). Aluminum sources include aluminum compounds and metallic aluminum. Aluminum compounds include aluminum carbonate, aluminum oxide, aluminum hydroxide, aluminum acetate and the like. Aluminum oxide is preferred when the solid phase method is employed. Aluminum acetate is preferred for the liquid phase method (evaporation to dryness method). Incidentally, the above various compounds may be hydrates.

The molar ratio of lithium, cobalt, and aluminum in the mixture may be any ratio that satisfies the composition of the positive electrode active material of the present disclosure.

The method of mixing the lithium source and the cobalt source is not particularly limited, and various methods such as dry mixing without using a solvent and wet mixing with a solvent can be employed. In the first step, the raw materials may be dissolved to form a mixture (mixed solution) of a solution, or powders may be mixed to form a powder mixture. Mixing may be performed manually using a mortar or the like, or may be performed mechanically using a ball mill or the like.

In particular, in the first step, the raw materials are dissolved in a solvent by a liquid phase method (evaporation to dryness method) to obtain a mixed solution, and then the mixed solution is evaporated to dryness to obtain a solid precursor. is preferred. Solvents used in this case include protic polar solvents such as water and alcohols. The precursor obtained after evaporation to dryness is in a state in which lithium, cobalt, and aluminum are uniformly mixed at the atomic level, and is in the form of fine particles with a large specific surface area. By heating and baking such a precursor in the second step described later, a spinel crystal phase can be generated in a short time.

2.2. Second Step In the second step, the mixture obtained in the first step is heated to obtain a composite oxide having a spinel crystal phase. Generally, in the composite oxide of lithium and cobalt, the layered rock salt crystal phase is more stable against heat than the spinel crystal phase, so if the heating temperature in the second step is too high, the spinel crystal A layered rock salt type crystal phase is generated rather than a phase. That is, in order to obtain the desired spinel crystal phase in the above mixture, it is preferable that the heating temperature in the second step is low and the heating time is long. In particular, according to the findings of the present inventors, a desired spinel crystal phase can be easily obtained by setting the heating temperature in the second step to 250° C. or higher and 600° C. or lower. The lower limit of the heating temperature is more preferably 280°C, more preferably 300°C or higher, and the upper limit is more preferably 550°C or lower, still more preferably 510°C or lower. The heating time in the second step may be adjusted according to the heating temperature. For example, in the solid-phase method, the crystallinity of the spinel-type crystal phase can be enhanced by heating for one week or longer. On the other hand, when the liquid phase method (evaporation to dryness method) is used, the crystallinity of the spinel crystal phase can be enhanced even if the heating time is 120 hours or less. The heating atmosphere in the second step may be any atmosphere capable of forming a composite oxide. For example, an air atmosphere, an oxygen atmosphere, or the like can be used.

3. Lithium Ion Battery The technology of the present disclosure also has an aspect as a lithium ion battery. FIG. 1 shows an example of the configuration of the lithium ion battery of the present disclosure. The lithium ion battery 10 shown in FIG. 1 includes a positive electrode 1, a negative electrode 2, and an electrolyte 3 disposed between the positive electrode 1 and the negative electrode 2, wherein the positive electrode 1 includes the positive electrode active material of the present disclosure. and

3.1. positive electrode 1
The positive electrode 1 may have the same configuration as the conventional one, except that it includes the positive electrode active material of the present disclosure. For example, the positive electrode 1 includes a positive electrode current collector 1a and a positive electrode active material layer 1b containing the positive electrode active material of the present disclosure. The positive electrode current collector 1a may be made of, for example, various metals. The positive electrode active material layer 1b may optionally contain a binder and a conductive aid in addition to the positive electrode active material. In addition to the positive electrode active material of the present disclosure, the positive electrode active material layer 1b may contain a positive electrode active material other than the positive electrode active material of the present disclosure within a range that can solve the above problems. Examples thereof include lithium metal composite oxides having a layered rock salt crystal phase and lithium metal phosphate compounds having an olivine crystal phase. The positive electrode active material of the present disclosure has a small expansion/shrinkage rate of the active material during charging and discharging, and is particularly advantageous in a solid battery in which interfacial contact between particles is important. In other words, the lithium ion battery of the present disclosure is preferably an all solid state battery. When an all-solid battery is employed as the lithium ion battery, it is preferable that the positive electrode active material layer 1b contains a solid electrolyte. As the solid electrolyte, an inorganic solid electrolyte such as an oxide solid electrolyte or a sulfide solid electrolyte is preferable, and a sulfide solid electrolyte is more preferable. As the sulfide solid electrolyte, for example, a solid electrolyte containing Li, P and S as constituent elements can be used. Specifically, Li ₂ SP ₂ S ₅ , Li ₂ S—SiS ₂ , LiI—Li ₂ S—SiS ₂ , LiI—Si ₂ SP ₂ S ₅ , LiI—LiBr—Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ S ₅ , LiI-Li ₂ O-Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ SP ₂ S ₅ -GeS ₂ and the like. Among these, a sulfide solid electrolyte containing Li ₂ SP ₂ S ₅ is particularly preferred. Solid electrolytes may be used singly or in combination of two or more. When a sulfide solid electrolyte is included in the positive electrode 1, the surface of the positive electrode active material is coated with a lithium niobate layer or the like from the viewpoint of suppressing the formation of a high resistance layer at the interface between the positive electrode active material and the sulfide solid electrolyte. Layers may be provided. Since the configuration other than the positive electrode active material is self-evident from common technical knowledge, further explanation is omitted.

3.2. negative electrode 2
The negative electrode 2 can employ a known negative electrode for lithium ion batteries. For example, the negative electrode 2 includes a negative electrode current collector 2a and a negative electrode active material layer 2b containing a negative electrode active material. The negative electrode current collector 2a may be made of, for example, various metals. As the negative electrode active material, a material having a lithium ion charge/discharge potential that is more base than the positive electrode active material of the present disclosure may be adopted. The negative electrode active material layer 2b may optionally contain a binder and a conductive aid in addition to the negative electrode active material. Moreover, when a solid battery is employed as the lithium ion battery, the negative electrode active material layer 2b preferably contains the solid electrolyte described above. Since the structure of the negative electrode is self-evident from common technical knowledge, further explanation is omitted.

3.3. electrolyte layer 3
The electrolyte layer 3 is for conducting lithium ions between the positive electrode 1 and the negative electrode 2 . Either an electrolytic solution or a solid electrolyte may be employed in the electrolyte layer 3 . When an electrolytic solution is employed, a separator may be arranged between the positive electrode and the negative electrode and impregnated with the electrolytic solution. On the other hand, when using a solid electrolyte, a solid electrolyte layer may be arranged between the positive electrode and the negative electrode. The solid electrolyte layer contains the solid electrolyte described above and optionally a binder. As described above, the positive electrode active material of the present disclosure has a small expansion/contraction rate of the active material during charging and discharging, and is particularly advantageous in a solid battery in which interfacial contact between particles is important. In this respect, the electrolyte layer 3 is preferably a solid electrolyte layer containing an inorganic solid electrolyte such as an oxide solid electrolyte or a sulfide solid electrolyte, and more preferably a layer containing a sulfide solid electrolyte. Since the structure of the electrolyte layer 3 is self-evident from common technical knowledge, further explanation is omitted.

3.4. Other Configurations The lithium-ion battery 10 only needs to include the positive electrode 1, the negative electrode 2, and the electrolyte layer 3 described above. Since these configurations are self-evident from common technical knowledge, further explanation is omitted.

3.5. Effect The lithium ion battery of the present disclosure employs the positive electrode active material of the present disclosure in the positive electrode, has a high initial coulombic efficiency, and has a small volume change of the positive electrode during charging and discharging. The lithium ion battery of the present disclosure is suitably used not only as a primary battery but also as a secondary battery.

4. Lithium Ion Battery System The positive electrode active material of the present disclosure has a spinel-type crystal phase that is more stable than conventional positive electrode active materials, and can function, for example, as a high-voltage active material. In this regard, when charging and discharging a lithium ion battery including the positive electrode active material of the present disclosure, the charging and discharging of the lithium ion battery is controlled by the charge and discharge control unit, and the discharge start voltage and the charge cutoff voltage are set to a high voltage. It is preferable to

FIG. 2 schematically shows a configuration example of the lithium ion battery system 100. As shown in FIG. 3 shows an example of a control flow in the lithium ion battery system 100. As shown in FIG. As shown in FIGS. 2 and 3, the lithium ion battery system 100 includes a lithium ion battery 10 including the positive electrode active material of the present disclosure, and a charge/discharge control unit 20 that controls charging and discharging of the lithium ion battery 10. In addition, the charge/discharge control unit 20 sets the discharge start potential or charge cutoff potential of the positive electrode of the lithium ion battery 10 to 4.0 V (vs. Li ⁺ /Li) or more, preferably 4.2 V (vs. Li ⁺ /Li) or more, more preferably 4.3 V (vs. Li ⁺ /Li) or more.

The charge/discharge control unit 20 may be any unit that can control charge and discharge of the lithium ion battery 10 as described above. For example, when charging the lithium ion battery 10 using a power supply, the potential of the positive electrode of the lithium ion battery 1 is sequentially measured, and if the measured potential of the positive electrode is less than a predetermined voltage, charging is continued, and the measured positive electrode is equal to or higher than a predetermined voltage, the supply of electricity from the power source is stopped to stop charging.

The same applies to the discharge start potential. That is, when the first discharge is performed after charging the lithium ion battery 10, the potential of the positive electrode is measured before the first discharge, and if the measured potential of the positive electrode is less than a predetermined voltage, lithium ion The lithium ion battery 10 is charged without discharging the battery 10, and when the potential of the positive electrode reaches a predetermined voltage or higher due to the charging of the lithium ion battery 10, the first discharge is performed.

When the charging and discharging of the lithium ion battery 10 is controlled by the charge/discharge control unit 20, the upper limit of the discharge start potential or the charge cutoff potential of the lithium ion battery 10 is not particularly limited. Even at a high potential, the effect is small. Rather, there is concern about deterioration and decomposition of battery materials. In this respect, the charge/discharge control unit 20 preferably sets the discharge start potential or charge cutoff potential of the positive electrode of the lithium ion battery 10 to 5.3 V (vs. Li ⁺ /Li) or less. More preferably, it is 5.1 V (vs. Li ⁺ /Li) or less, and still more preferably 5.0 V (vs. Li ⁺ /Li) or less.

1. Synthesis of positive electrode active material (spinel-type composite oxide) (Example 1)
Lithium acetate as a lithium source, cobalt acetate as a cobalt source, and aluminum acetate as an aluminum source were dissolved in deionized water as a protic polar solvent to obtain a mixed solution. The resulting mixed solution was heated to 250° C. on a hot plate while stirring with a stirrer, and evaporated to dryness to obtain a solid precursor. The obtained precursor was sintered at 400° C. for 2 hours in an air atmosphere to obtain a positive electrode active material (LiCo _0.9 Al _0.1 O _2±δ ) according to Example 1.

(Example 2)
A positive electrode active material (LiCo _0.9 Cr _0.1 O _2±δ ) according to Example 2 was obtained in the same manner as in Example 1, except that the sintering temperature of the precursor was 450°C.

(Example 3)
A positive electrode active material (LiCo _0.9 Cr _0.1 O _2±δ ) according to Example 3 was obtained in the same manner as in Example 1, except that the sintering temperature of the precursor was 500°C.

(Example 4)
A positive electrode active material according to Example 4 (LiCo _0.85 Al _0.15 O _2±δ ).

(Example 5)
A positive electrode active material (LiCo _0.95 Al _0.05 O _2±δ ).

(Comparative example 1)
A cathode active material (LiCoO _2±δ ) according to Comparative Example 1 was obtained in the same manner as in Example 1, except that the raw material composition ratio was Li:Co:Al=1:1:0.

(Comparative example 2)
A positive electrode according to Comparative Example 2 was prepared in the same manner as in Example 1 except that nickel acetate was used as the nickel source instead of the aluminum source and the raw material composition was Li: Co: Ni = 1: 0.9: 0.1. An active material (LiCo _0.9 Ni _0.1 O _2±δ ) was obtained.

(Comparative Example 3)
A positive electrode according to Comparative Example 3 was prepared in the same manner as in Example 1, except that chromium acetate was used as the chromium source instead of the aluminum source, and the raw material composition was Li:Co:Cr=1:0.9:0.1. An active material (LiCo _0.9 Cr _0.1 O _2±δ ) was obtained.

2. Confirmation of Crystal Phase X-ray diffraction measurement using CuKα as a radiation source was performed on the positive electrode active materials according to Examples 1 to 5 and Comparative Examples 1 to 4, and diffraction peaks were confirmed. FIG. 4 shows the results of X-ray diffraction measurement. As is clear from the results shown in FIG. 4, diffraction peaks derived from spinel-type crystal phases were confirmed in all of Examples 1-5 and Comparative Examples 1-4.

3. SEM Observation The morphology of the positive electrode active materials according to Example 1 and Comparative Example 1 was observed by SEM. The results are shown in FIG. As shown in FIG. 5, Example 1 is considered to have larger particles and higher crystallinity than Comparative Example 1.

4. Preparation of electrode The positive electrode active material, conductive aid and binder obtained were weighed so that the mass ratio of positive electrode active material: conductive aid: binder = 85: 10: 5, and wet-mixed with NMP. A slurry was obtained. The resulting slurry was applied onto an aluminum foil and dried at 120° C. overnight to obtain a positive electrode.

5. Preparation of lithium ion battery Using F-substituted carbonate-based electrolyte for the above positive electrode, negative electrode (lithium foil), and electrolyte solution, a separator is placed between the positive electrode and the negative electrode, and the electrolyte is enclosed in a coin battery. A lithium ion battery (CR2032 coin cell) was obtained for evaluation.

6. Charge/Discharge Test A charge/discharge test was performed on the prepared lithium ion battery under the following conditions to confirm the coulombic efficiency in the first charge/discharge cycle after charging at 4.45V.
CC charging: current 0.1C, termination condition 4.45V
CC discharge: current 0.1C, termination condition 2.5V

The results are shown in Table 1 below. For reference, FIG. 6 shows charge-discharge curves for Example 1 and Comparative Example 1. As shown in FIG.

As is clear from the results shown in Table 1, Examples 1-5 significantly increased the initial coulombic efficiency as compared with Comparative Examples 1-3. As shown in FIGS. 4 and 5, it is believed that the aluminum doping enhanced the crystallinity of the spinel-type crystal phase and stabilized the spinel-type crystal phase. In addition, as shown in FIG. 6, in Example 1, the average voltage was higher than that in Comparative Example 1 by about 0.13V. This is probably because the doping with aluminum enhanced the crystallinity of the spinel crystal phase and increased the electron conductivity.

From the above results, it can be concluded that replacing some of the elements of spinel-type lithium cobalt oxide with a specific amount of aluminum can exhibit superior performance as a positive electrode active material for lithium-ion batteries, rather than spinel-type lithium cobalt oxide. Do you get it.

Although Examples 1 to 5 in which the amount of aluminum substitution is 0.05 to 0.15 are shown in the above Examples, the positive electrode active material of the present disclosure is not limited to this form. As described above, the technique of the present disclosure has found the effectiveness of substituting aluminum for some elements of spinel-type lithium cobaltate, and even if the amount of aluminum substitution is less than 0.05 , it is considered that the desired effect can be exhibited as compared with the case where the amount of aluminum substitution is 0.

In the above examples, the molar ratio of lithium to other metals was adjusted to 1, but the molar ratio of other metals to lithium is limited to this as long as a spinel crystal phase can be obtained. is not. According to the findings of the present inventors, sufficient effects can be exhibited if the molar ratio of the other metal to Li is more than 0.85 and 1.15 or less.

INDUSTRIAL APPLICABILITY A lithium ion battery using the positive electrode active material according to the present invention can be widely used, for example, from a small power source for portable equipment to a large power source for vehicles.

1 positive electrode 2 negative electrode 3 electrolyte layer 10 lithium ion battery

Claims (1)

comprising a positive electrode, a negative electrode, and an electrolyte disposed between the positive electrode and the negative electrode;
The positive electrode as a positive electrode active material,
Having a spinel crystal phase containing lithium, cobalt, aluminum and oxygen,
A positive electrode having a composition represented by LiCo _x Al _y O _2±δ (0.85≦x<1, 0<y≦0.15, 0.85<x+y≦1.15 , δ≦0.2 ) having only an active material,
Lithium-ion battery.

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