JP2015204256A - Method for producing coated positive electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a coated positive electrode active material that achieves both improvement in electron conductivity and improvement in cycle characteristics.SOLUTION: A method for producing a coated positive electrode active material includes: a Ti oxide layer formation step of forming a Ti oxide layer with a layer number within a range of 10-200 by an atomic layer deposition method, on a surface of a positive electrode active material that contains Li and Mn and is an oxide having a spinel structure; and a firing step of firing the Ti oxide layer at a temperature within a range of 600-900°C.

Description

  The present invention relates to a method for producing a coated positive electrode active material that achieves both improved electronic conductivity and improved cycle characteristics.

  With the rapid spread of information-related equipment and communication equipment such as personal computers, video cameras, and mobile phones in recent years, development of batteries that are used as power sources has been regarded as important. Also in the automobile industry and the like, development of high-power and high-capacity batteries for electric vehicles or hybrid vehicles is being promoted. Currently, lithium batteries are attracting attention among various batteries from the viewpoint of high energy density.

In the field of such lithium batteries, conventionally, attempts have been made to improve the performance of lithium batteries by paying attention to the electrode active material interface. For example, Patent Document 1 describes that LiNi _0.5 Mn _1.5 O ₄ is coated with TiO ₂ by a CVD method or a PVD method. Patent Document 2 also describes that LiNi _0.5 Mn _1.5 O ₄ is coated with TiO ₂ .

  By the way, an atomic layer deposition (ALD) method is known as a film forming method of a vapor phase process. The ALD method is a method of depositing an atomic layer contained in a material to form a film, and for example, Patent Document 3 discloses a technique for depositing an electrode material using the ALD method. Patent Document 4 describes that an electrode material is deposited by the ALD method.

JP 2012-048771 A JP 2013-149433 A JP 2013-143375 A Special table 2012-517717 gazette

For example, LiNi _0.5 Mn _1.5 O ₄ has low electron conductivity. On the other hand, it is assumed that the electron conductivity is improved by introducing oxygen deficiency by firing. However, by introducing oxygen vacancies, trivalent Mn is produced from tetravalent Mn, and trivalent Mn is more likely to elute than tetravalent Mn.

  This invention is made | formed in view of the said situation, and it aims at providing the manufacturing method of the covering positive electrode active material which made compatible the improvement of electronic conductivity, and the improvement of cycling characteristics.

  In order to solve the above-described problem, in the present invention, a range of 10 to 200 layers is formed by atomic layer deposition on the surface of a positive electrode active material that is an oxide containing Li and Mn and having a spinel structure. A coated positive electrode active material comprising: a Ti oxide layer forming step for forming a Ti oxide layer therein; and a firing step for firing the Ti oxide layer within a range of 600 ° C. to 900 ° C. A manufacturing method is provided.

  According to the present invention, it is possible to obtain a coated positive electrode active material in which both improvement in electronic conductivity and improvement in cycle characteristics are achieved by forming a Ti oxide layer by ALD and then firing.

  In this invention, there exists an effect that the covering positive electrode active material which made the improvement of electronic conductivity and the improvement of cycling characteristics compatible can be obtained.

It is a schematic sectional drawing which shows an example of the manufacturing method of the covering positive electrode active material of this invention. It is a schematic sectional drawing explaining the ALD method in this invention. It is a result of the cell resistance measurement with respect to the battery for evaluation obtained by the Example and the comparative example. It is the result of the cycle characteristic evaluation with respect to the battery for evaluation obtained by the Example and the comparative example. It is a result of the XPS measurement with respect to the covering positive electrode active material produced in the Example and the comparative example. It is the result of the cell resistance measurement with respect to the battery for evaluation obtained by the reference example.

  Hereinafter, the manufacturing method of the covering positive electrode active material of this invention is demonstrated in detail.

  FIG. 1 is a schematic cross-sectional view showing an example of a method for producing a coated positive electrode active material of the present invention. In FIG. 1, first, a positive electrode active material 1 containing Li and Mn and having an spinel structure is prepared (FIG. 1 (a)). Next, a Ti oxide layer 2 in the range of 10 to 200 layers is formed on the surface of the positive electrode active material 1 by ALD (FIG. 1B). Next, the Ti oxide layer 2 is baked in the range of 600 ° C. to 900 ° C. to obtain the coated positive electrode active material 10 (FIG. 1C).

  According to the present invention, it is possible to obtain a coated positive electrode active material in which both improvement in electronic conductivity and improvement in cycle characteristics are achieved by forming a Ti oxide layer by ALD and then firing. As described above, by introducing oxygen vacancies by firing, the electron conductivity can be improved, but there is a problem that the cycle characteristics are deteriorated. On the other hand, in the present invention, by forming a Ti oxide layer, it is possible to distribute a Ti element having a high oxygen binding property on the active material surface. Therefore, introduction of oxygen vacancies and solid solution of Ti in the positive electrode active material occur during firing, and a coated positive electrode active material that achieves both improved electronic conductivity and improved cycle characteristics can be obtained. Moreover, by introducing oxygen vacancies by firing, the electron conductivity can be improved, but the Li ion conductivity can be lowered due to the decrease in crystallinity. On the other hand, it is considered that doping with Ti element can suppress a decrease in crystallinity and also can suppress a decrease in Li ion conductivity.

In addition, the ALD method has an advantage that a uniform and thin film (Ti oxide layer) can be formed as compared with other vapor deposition methods. Here, the characteristics of the ALD method, the PLD method, and the CVD method are compared. As shown in FIG. 2A, the ALD method can form a uniform and thin film. On the other hand, as shown in FIG. 2B, the PLD method cannot form a uniform film due to its mechanism. Similarly, as shown in FIG. 2C, the CVD method cannot form a uniform film due to its mechanism. Furthermore, the thickness uniformity is low. On the other hand, in the present invention, a uniform and thin film can be formed by using the ALD method.
Hereinafter, the manufacturing method of the covering positive electrode active material of this invention is demonstrated for every structure.

1. Ti oxide layer forming step The Ti oxide layer forming step in the present invention comprises 10 layers to the surface of the positive electrode active material which is an oxide containing Li and Mn and having a spinel structure by an atomic layer deposition method. This is a step of forming a Ti oxide layer within the range of 200 layers.

(1) Positive electrode active material The positive electrode active material in the present invention is an oxide containing Li and Mn and having a spinel structure. Mn contained in the positive electrode active material is usually tetravalent. Moreover, the positive electrode active material in this invention may contain only Li and Mn as a cation, and may contain another element. Examples of other elements include at least one of Ni, Zr, Nb, V, Y, and W, and among these, Ni and Zr are preferable. Moreover, in this invention, it is preferable that a positive electrode active material contains Li, Mn, and Ni as a cation. In this case, the positive electrode active material may further contain at least one of Zr, Nb, V, Y, and W. In particular, in the present invention, the positive electrode active material is preferably LiNi _x Mn _2-x O ₄ . x is usually 0 <x <2 and preferably 0.5 ≦ x ≦ 1.

  The positive electrode active material in the present invention preferably has a high average potential. Specifically, it is preferably 4.5 V or more, more preferably 4.55 V or more, and further preferably 4.6 V or more with respect to the Li metal potential. The average potential of the positive electrode active material can be determined by, for example, cyclic voltammetry measurement, 3V-5V constant voltage charge / discharge measurement (0.1 mV / sec).

Although the shape of a positive electrode active material is not specifically limited, For example, a particulate form can be mentioned. The average particle diameter (D ₅₀ ) of the particles is preferably in the range of 0.1 μm to ₅₀ μm, for example.

(2) Ti oxide layer In this invention, the Ti oxide layer in the range of 10 layers-200 layers is formed in the surface of the said positive electrode active material by ALD method.

  In the ALD method, first, a first compound (Ti source) is introduced onto the surface of the positive electrode active material, adsorbed on the surface of the positive electrode active material, and then the excess first compound is removed by purging. In this way, a layer composed of the first compound is formed. Subsequently, the second compound (O source) is introduced into the surface of the layer composed of the first compound, reacted with the layer composed of the first compound, and then removed by purging excess second compound. To do. In this way, an atomic layer in which two kinds of precursors are reacted is formed on the surface of the positive electrode active material. Note that this atomic layer corresponds to one Ti oxide layer.

The types of precursors in the present invention may be two types or three or more types. Examples of the Ti source used as the precursor include TiCl ₄ , Ti [N (CH ₃ ) ₂ ] ₄ , Ti [N (C ₂ H ₅ ) ₂ ] ₄ , Ti [N (C ₂ H ₅ ) CH ₃ ]. ₄ etc. can be mentioned. Among these, TiCl ₄ and Ti [N (CH ₃ ) ₂ ] ₄ are preferable. The Ti source usually contains tetravalent Ti. Examples of the O source used as the precursor include water and ozone.

  In the present invention, the formation of an atomic layer (one Ti oxide layer) is repeated. The number of cycles is usually 10 times or more, and preferably 20 times or more. On the other hand, the number of cycles is usually 200 times or less and preferably 150 times or less.

Moreover, although the reaction tank temperature in ALD method is not specifically limited, For example, it is preferable to exist in the range of 200 to 300 degreeC. Further, after the precursor is introduced, excess precursor is removed by purging. Examples of the purge gas include N ₂ gas and Ar gas.

The Ti oxide layer in the present invention may contain only Ti element and O element, and may further contain other elements. In the former case, “only” does not include residues derived from raw materials. Further, the thickness of the Ti oxide layer is, for example, in the range of 0.5 nm to 20 nm. Further, the coverage of the Ti oxide layer is, for example, 70% or more, more preferably 80% or more, and further preferably 90% or more. The coverage can be determined by, for example, X-ray photoelectron spectroscopy (XPS). The Ti oxide layer in the present invention is preferably amorphous. In the conventional CVD method or PLD method, a crystalline TiO ₂ layer is formed. However, in the case of the ALD method, a crystalline TiO ₂ layer is usually not formed, and an amorphous Ti oxide layer (TiO _x ) Is formed.

2. Firing step The firing step in the present invention is a step of firing the Ti oxide layer within a range of 600 ° C to 900 ° C.

  A calcination temperature is 600 degreeC or more, for example, it is more preferable that it is 650 degreeC or more, and 700 degreeC or more is more preferable. If the firing temperature is too low, Ti may not be dissolved in the positive electrode active material. The firing temperature is, for example, 900 ° C. or less, preferably 850 ° C. or less, and more preferably 800 ° C. or less. This is because if the firing temperature is too high, an undesired crystal phase (heterophase) may be formed. The firing time is, for example, in the range of 1 minute to 10 hours, and preferably in the range of 1 hour to 5 hours. The firing atmosphere is not particularly limited, and examples thereof include an inert gas atmosphere and an air atmosphere.

  In the present invention, the Ti element contained in the Ti oxide layer is usually dissolved in the positive electrode active material by firing. It can be confirmed by X-ray photoelectron spectroscopy (XPS) measurement that Ti element contained in the Ti oxide layer is dissolved in the positive electrode active material. In particular, in the present invention, it is preferable that no Ti (2p) peak is confirmed. This is because the cell resistance can be lowered.

In the present invention, oxygen deficiency usually occurs on the surface of the positive electrode active material by firing. The occurrence of oxygen deficiency on the surface of the positive electrode active material can be confirmed by surface atomic state analysis by TEM-EELS. The amount of oxygen deficiency is not particularly limited, but for example, it is preferably in the range of 1% to 10%. When the positive electrode active material before firing is LiNi _0.5 Mn _1.5 O ₄ and the positive electrode active material after firing is LiNi _0.5 Mn _1.5 O _4-y , y / 4.

3. Coated positive electrode active material The coated positive electrode active material obtained by the present invention is preferably used in a lithium battery. A lithium battery usually has a positive electrode active material layer, a negative electrode active material layer, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer.

  The positive electrode active material layer is a layer containing a coated positive electrode active material, and may further contain at least one of a conductive material and a binder as necessary. Examples of the conductive material include carbon materials such as acetylene black, ketjen black, and carbon fiber. Examples of the binder include fluorine-containing binders such as PTFE and PVDF. Moreover, it is preferable that the thickness of a positive electrode active material layer exists in the range of 0.1 micrometer-1000 micrometers, for example.

  The negative electrode active material layer is a layer containing at least a negative electrode active material, and may further contain at least one of a conductive material and a binder as necessary. Examples of the negative electrode active material include a metal active material and a carbon active material. Examples of the metal active material include Li alloy, In, Al, Si, and Sn. On the other hand, examples of the carbon active material include graphite such as mesocarbon microbeads (MCMB) and highly oriented graphite (HOPG), and amorphous carbon such as hard carbon and soft carbon. The conductive and binder materials are the same as described above. The thickness of the negative electrode active material layer is preferably in the range of 0.1 μm to 1000 μm, for example.

The form of the electrolyte layer is not particularly limited, and examples thereof include a liquid electrolyte layer, a solid electrolyte layer, and a gel electrolyte layer. The liquid electrolyte layer is usually a layer using a non-aqueous electrolyte. The type of non-aqueous electrolyte varies depending on the type of battery. For example, the non-aqueous electrolyte of a lithium battery usually contains a lithium salt and a non-aqueous solvent. Examples of the lithium salt include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiClO _4, and LiAsF ₆ ; and LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiC An organic lithium salt such as (CF ₃ SO ₂ ) ₃ can be used. Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), butylene carbonate (BC), γ-butyrolactone, sulfolane, Examples include acetonitrile, 1,2-dimethoxymethane, 1,3-dimethoxypropane, diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and any mixture thereof.

  Examples of the solid electrolyte material include a sulfide solid electrolyte material, an oxide solid electrolyte material, a nitride solid electrolyte material, and a halide solid electrolyte material. The gel electrolyte layer can be obtained, for example, by adding a polymer to the non-aqueous electrolyte and gelling. Although the thickness of an electrolyte layer is not specifically limited, For example, it is preferable to exist in the range of 0.1 micrometer-1000 micrometers, and it is more preferable to exist in the range of 0.1 micrometer-300 micrometers.

  The lithium battery usually has a positive electrode current collector that collects current from the positive electrode active material layer and a negative electrode current collector that collects current from the negative electrode active material layer. Further, the lithium battery may be a primary battery or a secondary battery, but among them, a secondary battery is preferable. This is because it can be repeatedly charged and discharged and is useful, for example, as a vehicle-mounted battery.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Example]
(Preparation of coated positive electrode active material)
A positive electrode active material (LiNi _0.5 Mn _1.5 O ₄ ) having a spinel structure was prepared, and a precursor was introduced into the surface of the positive electrode active material by using the ALD method to form one Ti oxide layer. . Note that TiCl ₄ and H ₂ O were used as the precursor. The temperature for gasifying the precursor was 20 ° C., and the temperature of the reaction vessel was 300 ° C. Further, the carrier gas N ₂ gas was used. This operation was repeated 10 cycles, 100 cycles or 200 cycles to obtain a Ti oxide layer. Thereafter, each sample was fired at 600 ° C., 800 ° C., and 900 ° C. for 3 hours in an air atmosphere. This obtained the covering positive electrode active material.

(Preparation of positive electrode active material layer)
First, acetylene black (AB) was mixed with the obtained coated positive electrode active material, and a polyvinylidene fluoride (PVDF) binder dissolved in n-methylpyrrolidone (NMP) was added to prepare a slurry. The mixing ratio (weight ratio (wt%)) of the coated positive electrode active material, AB, and PVDF was set to coated positive electrode active material: AB: PVDF = 85: 10: 5. Next, the obtained slurry was applied to the surface of an Al foil (thickness: 15 μm) as a positive electrode current collector using a doctor blade method, and dried in air at about 80 ° C. to remove NMP. . Thereafter, the obtained Al foil was vacuum-dried at 130 ° C. for 10 hours, and subsequently pressed to bond the Al foil and the positive electrode active material layer to produce a positive electrode. In addition, the area of the top part (circular shape) of the obtained positive electrode active material layer was 1.77 cm ² (diameter 1.5 cm).

(Preparation of negative electrode active material layer)
A negative electrode active material layer using graphite as a negative electrode active material was prepared.

(Production of electrolyte material)
Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio EC: EMC = 3: 7. Next, lithium hexafluorophosphate (LiPF ₆ ) having a concentration of 1 mol / dm ³ was dissolved as a supporting salt in the obtained mixed solvent to obtain an electrolyte material.

(Production of evaluation battery)
A CR2032-type bipolar coin cell (evaluation battery) was obtained using the above member.

[Comparative example]
An evaluation battery was obtained in the same manner as in the example except that the Ti oxide layer was not formed by the ALD method. Further, an evaluation battery was obtained in the same manner as in the example except that the firing was not performed after the formation of the Ti oxide layer.

[Evaluation]
(Charge / discharge test)
A charge / discharge test was performed using the evaluation batteries obtained in the examples and comparative examples. A charging / discharging test was performed with the process of detaching Li ⁺ from the test electrode (positive electrode) as “charging” and the process of inserting Li ⁺ into the test electrode (positive electrode) as “discharging”. For the measurement, a charge / discharge test apparatus (HJ-1001 SM8A manufactured by Hokuto Denko) was used. First, charge / discharge at the first cycle was performed under conditions of a current value of 0.2 mA / cm ² , 3.5 V to 4.9 V, and 25 ° C. The 1C rate was calculated from the obtained capacity. The cell resistance measurement was calculated from the applied voltage after 10 seconds of 5C discharge after adjusting to SOC 60%. As for the cycle characteristics, charging / discharging was performed at 60 ° C. and 2 C.

  The result of the cell resistance measurement is shown in FIG. As shown in FIG. 3, when the number of Ti oxide layers was the same, the cell resistance was reduced by firing at 600 ° C. to 900 ° C. as compared with the case where no firing was performed. In particular, when fired at 800 ° C., the cell resistance decreased by about 25% compared to the case without firing. Moreover, when not firing, the cell resistance increased as the number of Ti oxide layers increased. On the other hand, when firing at 600 ° C. and 800 ° C., the cell resistance was reduced even when the number of Ti oxide layers was increased. In particular, when the baking was performed at 800 ° C. and the number of Ti oxide layers was 10 to 100, the cell resistance was greatly reduced. On the other hand, when firing at 900 ° C., the cell resistance increased as the number of Ti oxide layers increased. The cause is considered to be an increase in particle diameter due to high-temperature firing and the appearance of a heterogeneous phase.

  Moreover, the result of a cycle characteristic is shown in FIG. As shown in FIG. 4, in the sample in which 10 or 100 Ti oxide layers were formed and then fired at 800 ° C., an increase in initial capacity and an improvement in cycle characteristics were confirmed.

(XPS measurement)
X-ray photoelectron spectroscopy (XPS) measurement was performed on the coated positive electrode active materials prepared in Examples and Comparative Examples. The result is shown in FIG. As shown in FIG. 5A, when 100 Ti oxide layers were formed, the peak of Ti (2p) was confirmed, but the peak disappeared in the sample fired at 800 ° C. thereafter. From this, it is considered that the Ti element contained in the Ti oxide layer was dissolved in the active material. On the other hand, in the sample in which 200 Ti oxide layers were formed and then fired at 800 ° C., a peak of Ti (2p) was slightly confirmed around 458 eV as shown in FIG. As shown in FIG. 3 described above, even when firing at the same 800 ° C., when 200 Ti oxide layers are formed, the cell resistance is higher than when 100 Ti oxide layers are formed. it was high. This suggests that cell resistance increases when Ti element remains on the surface of the positive electrode active material.

[Reference example]
A Ti oxide layer was formed using a PLD (pulse laser deposition) method and a CVD (chemical vapor deposition) method instead of the ALD method. The thickness of the Ti oxide layer was about 1 nm. This corresponds to the thickness of the ten Ti oxide layers in the above-described embodiment. Then, it baked at 800 degreeC, the battery for evaluation was produced like the Example, and cell resistance was measured. The result is shown in FIG. As shown in FIG. 6, the cell resistance was significantly lower in the ALD method than in the PLD method and the CVD method. This suggests that the active material particles could not be uniformly coated by the PLD method and the CVD method, but the active material particles could be uniformly coated by the ALD method.

DESCRIPTION OF SYMBOLS 1 ... Positive electrode active material 2 ... Ti oxide layer 10 ... Coated positive electrode active material

Claims (1)

Ti oxide layer that forms a Ti oxide layer in the range of 10 to 200 layers on the surface of a positive electrode active material containing Li and Mn and having a spinel structure by an atomic layer deposition method Forming process;
A firing step of firing the Ti oxide layer within a range of 600 ° C. to 900 ° C .;
A method for producing a coated positive electrode active material, comprising:

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