JP3616694B2 - LiMn2O4 thin film electrode and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a LiMn ₂ O ₄ thin film electrode and a method for producing the same, and particularly to a LiMn ₂ O ₄ thin film electrode that can be advantageously used for a positive electrode for a lithium ion secondary battery.
[0002]
[Prior art]
The lithium ion secondary battery is a battery having a particularly high voltage and high energy density among secondary batteries. As a power source for electronic devices that tend to be smaller, lighter, and portable electronic devices, Has attracted attention.
[0003]
Ceramics such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ are known as positive electrode materials for this lithium ion secondary battery. Among these, LiCoO ₂ is particularly suitable for cycle stability and ease of synthesis. In this respect, it has been practically used. However, since Co, which is a raw material, is expensive, in the future, it is desired to use LiMn ₂ O ₄ made of a manganese compound, which has abundant resources and is advantageous in terms of price, as a positive electrode material.
[0004]
On the other hand, positive electrode materials made of these ceramics generally have a low electronic conductivity and poor adhesion to other materials. Therefore, when used as a positive electrode for a lithium ion secondary battery, the positive electrode material is bonded to carbon powder as a conductive agent. It was necessary to sinter after adhering the organic binder as the adhering agent at a predetermined ratio in a total ratio of about 10 wt% (20 vol%) and then pressing and adhering to a positive electrode substrate such as an Al plate or a SUS plate.
Therefore, since the positive electrode obtained in this way uses additives other than the positive electrode material, there is a problem in that the volume energy density is lowered or the adhesion between the positive electrode substrate and the positive electrode material becomes insufficient. there were.
[0005]
[Problems to be solved by the invention]
The purpose of the present invention is to eliminate the above-mentioned problems of the prior art, in particular, to establish a manufacturing technique of an electrode excellent in adhesion at the interface between the positive electrode substrate and the positive electrode material LiMn ₂ O ₄ , This is to further improve battery characteristics.
[0006]
[Means for Solving the Problems]
Japanese Patent Laid-Open No. 5-82137 proposes a method of strongly bonding a lanthanum transition metal perovskite oxide to the surface of an oxide solid electrolyte. The inventors paid attention to the technique described in this publication, which has been proposed earlier, and conducted earnest research to achieve the above object.
[0007]
As a result, the inventors applied a raw material solution obtained by adding a predetermined solvent to a mixed aqueous solution composed of a Li source and a Mn source on the positive electrode substrate, so that crystalline spinel type LiMn ₂ O ₄ was formed on the surface. The present inventors have found that a thin film can be formed with good adhesion and have arrived at the present invention.
[0008]
That is, the LiMn ₂ O ₄ thin film electrode according to the present invention is a thin film electrode formed by forming a thin film of crystalline spinel type LiMn ₂ O ₄ on the positive electrode substrate, and in particular, this crystalline spinel type LiMn ₂ O ₄ The positive electrode substrate of a solution obtained by adding a single or a mixed solution of two or more selected from diethylene glycol, glycerol and formaldehyde to an aqueous solution of a water-soluble lithium salt and manganese nitrate (Mn (NO ₃ ) ₂ ) It is characterized in that it is obtained by coating on top , drying, baking and baking.
[0009]
The manufacturing method of LiMn ₂ O ₄ thin film electrode according to the present invention, the water-soluble lithium salt and manganese nitrate (Mn _{(NO 3) 2)} aqueous solution of diethylene glycol, a solvent for either alone selected from glycerol and formaldehyde or 2 A solution obtained by adding the above mixed solvent is allowed to act on the positive electrode substrate to form a crystalline spinel type LiMn ₂ O ₄ thin film on the surface thereof.
[0010]
In addition, the method for producing a LiMn ₂ 0 ₄ thin film electrode according to the present invention includes a water-soluble lithium salt and an aqueous solution of manganese nitrate (Mn (NO ₃ ) ₂ ) on a positive electrode substrate, any one selected from diethylene glycol, glycerol, and formaldehyde. A single solvent or a solution obtained by adding and mixing two or more mixed solvents is applied, then dried and baked in a temperature range of 200 to 400 ° C , and then in a temperature range of 600 to 800 ° C. A method for producing a LiMn ₂ 0 ₄ thin film electrode, characterized by repeating the firing step.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Unlike the prior art in which LiMn ₂ O ₄ crystal powder is bound to a positive electrode using an organic binder or the like, the present invention is a raw material solution obtained by adding a predetermined solvent to a mixed aqueous solution composed of a Li source and a Mn source It is characterized in that a thin film crystal of LiMn ₂ O ₄ is produced directly on the substrate by causing the film to act on the positive electrode substrate to form a thin film electrode. In particular, the present invention uses a single solvent selected from diethylene glycol, glycerol and formaldehyde or a mixed solvent composed of two or more as the predetermined solvent, so that the LiMn ₂ O ₄ thin film can be formed by baking at a low temperature of about 300 ° C. It is characterized in that the crystal layer can be synthesized.
[0012]
According to the present invention, a LiMn ₂ O ₄ thin film having a uniform film structure having no grain boundary can be formed on the positive electrode substrate with good adhesion, and the obtained LiMn ₂ O ₄ thin film electrode has battery characteristics. Excellent performance at. That is, in the present invention, since the positive electrode material is formed on the positive electrode substrate as a thin film having excellent adhesion to the substrate without using additives other than the positive electrode material such as a conductive agent and a binder, the volume energy density It is considered that the battery characteristics at high load are improved. That is, the capacity drop at a high load (high rate charge / discharge) is reduced.
[0013]
Here, the LiMn ₂ O ₄ thin film electrode in the present invention uses a Pt plate, an Al plate, a SUS plate or the like as a positive electrode substrate, but is not limited thereto, and may be a metal that hardly reacts with LiMn ₂ O ₄ during firing. In addition, a conductive oxide such as LaMnO ₃ can be used.
[0014]
In the present invention, an electrode is obtained by forming a LiMn ₂ O ₄ thin film on the surface of the positive electrode substrate. For this purpose, a raw material solution obtained by adding any one solvent selected from diethylene glycol, glycerol and formaldehyde or a mixed solvent composed of two or more to a mixed aqueous solution composed of a Li source and a Mn source is used. The reason for this is to increase the viscosity of the raw material solution by adding an organic solvent and to improve the coatability of the raw material solution to the substrate. In addition, as a water-soluble lithium salt which is a Li source, lithium nitrate, lithium sulfate, lithium chloride, or the like can be used, but lithium nitrate (LiNO ₃ ) is preferably used. Further, the Mn source is not particularly limited, but manganese nitrate (Mn (NO ₃ ) ₂ ) is preferably used.
[0015]
In the present invention, first, the raw material solution is applied to the surface of the positive electrode substrate. In this case, the positive electrode substrate is immersed in this raw material solution, or this raw material solution is applied or sprayed onto the positive electrode substrate using a brush, or in other ways, the solution remains on the surface of the positive electrode substrate. Put it in a state.
[0016]
Next, after drying the positive electrode substrate coated with the raw material solution, baking is preferably performed in a temperature range of 200 to 400 ° C. for a predetermined time. As a result, a uniform LiMn ₂ O ₄ thin film can be formed with good adhesion on the surface of the positive electrode by baking at a relatively low temperature. That is, when the temperature is deviated from, the strong LiMn ₂ O ₄ thin film cannot be easily bonded. The heating time is 1 minute at 300 ° C.
[0017]
A LiMn ₂ O ₄ thin film layer having a certain thickness can be obtained by repeatedly performing the above-described series of operations of coating, drying, and baking.
[0018]
And further, by firing at this more preferably a temperature range of 600 to 800 ° C., the flow advances firing with crystal growth of the LiMn ₂ O _4, the adhesion at the interface between the positive electrode and the LiMn ₂ O ₄ thin film An excellent electrode can be obtained. In this case, when deviating from the above temperature range, firing is not sufficient at low temperatures, whereas LiMn ₂ O ₄ is decomposed at high temperatures, so that a thin film electrode having good characteristics cannot be obtained.
[0019]
Regarding the LiMn ₂ O ₄ thin film electrode thus produced, the properties of the LiMn ₂ O ₄ thin film are determined by X-ray diffraction, the adhesion to the substrate is observed by observing a bent fracture surface, and the structure is a scanning electron microscope ( This can be confirmed by observation using SEM.
[0020]
【Example】
A LiMn ₂ O ₄ thin film electrode was produced according to the process shown in FIG.
(1) 0.1 mol of LiNO _3: 7.04 g and 0.2mol of _{Mn (NO 3) 2 · 6H} 2 O: 57.80g of a mixed aqueous solution prepared by dissolving in pure water 100 ml, diethylene glycol was added 500 ml, A raw material solution was prepared. Here, a raw material solution in which diethylene glycol was not added to the mixed aqueous solution with a pure water amount of 600 ml was prepared as a comparative example. Glycerol or formaldehyde can be used in place of diethylene glycol.
(2) The raw material solution prepared in (1) above was applied to a 20 mm × 20 mm × 0.2 mmt Pt plate (positive electrode substrate) by brush coating, then dried at 110 ° C. for 30 minutes, and then at 1 ° C. at 300 ° C. Baking for a minute. Then, after repeating a series of operations of coating, drying and baking five times, a thin film of crystalline spinel type LiMn ₂ O ₄ is formed on the positive electrode substrate by baking at 650 ° C. in the atmosphere for 5 hours. LiMn ₂ O ₄ thin film electrodes were prepared. As the positive electrode substrate, an Al plate or a SUS plate can be used instead of the Pt plate.
[0021]
▲ 1 ▼ for LiMn ₂ O ₄ thin film electrodes identified Preparation of LiMn ₂ O ₄ thin film of crystalline phase, X-rays diffraction using a Rigaku Denki Co., Ltd. RINT-2000 Identification of LiMn ₂ O ₄ thin film crystal phase (XRD ). As a result, the XRD pattern after baking at 300 ° C. is shown in FIG. 2, and the XRD pattern after baking at 650 ° C. is shown in FIG.
[0022]
As is clear from the XRD pattern after baking shown in FIG. 2, in the example of the present invention to which diethylene glycol was added, a broad peak of LiMn ₂ O ₄ was recognized in addition to the Pt peak of the positive electrode substrate. On the other hand, only the peak of Pt was observed in the comparative example in which diethylene glycol was not added. That is, in the example of the present invention, the crystal phase of the LiMn ₂ O ₄ thin film was synthesized by baking at a low temperature of 300 ° C., but it was confirmed that LiMn ₂ O ₄ was not synthesized in the comparative example. This is presumably because diethylene glycol has the ability to suppress separation and precipitation of cations (Li ⁺ , Mn ²⁺ ).
[0023]
As apparent from the XRD pattern after firing shown in FIG. 3, after firing, the peak of LiMn ₂ O ₄ was observed for both XRD patterns of the present invention example and the comparative example, and although it was slightly as a different phase, Mn _{A 2} O ₃ peak was also observed.
[0024]
▲ 2 ▼ for LiMn ₂ O ₄ thin film electrode prepared by sintering in LiMn ₂ O ₄ micro structure observation 650 ° C. of the thin film, scanning electron a LiMn ₂ O ₄ micro structure observation of a thin film using a JEOL JSM-6400 This was performed by observation with a microscope (SEM). As a result, a photograph obtained by SEM observation of the fracture surface of the electrode obtained in the inventive example to which diethylene glycol was added is shown in FIG. 4, and a photograph obtained by SEM observation of the fracture surface of the electrode obtained in the comparative example to which diethylene glycol is not added is shown in FIG. Shown in The observed fracture surface was formed by bending and breaking the substrate. Therefore, in this SEM observation, the adhesion between the LiMn ₂ O ₄ thin film having a thickness of about 1.0 μm and the substrate can be confirmed.
[0025]
As is apparent from the SEM photograph shown in FIG. 4, the LiMn ₂ O ₄ thin film electrode obtained in the present invention example to which diethylene glycol was added had good adhesion between the LiMn ₂ O ₄ thin film and the Pt substrate, and the thin film was In the surface layer, particles of about 0.2 μm were densely covered, and there was almost no grain boundary inside, forming a uniform film. On the other hand, as is clear from the SEM photograph shown in FIG. 5, the LiMn ₂ O ₄ thin film electrode obtained in the comparative example to which no diethylene glycol was added had poor adhesion between the LiMn ₂ O ₄ thin film and the Pt substrate. In the thin film, the entire film was formed of fine particles of about 0.1 μm, and the grain boundaries were clearly recognized. This is considered to be because, in the example of the present invention to which diethylene glycol was added, the crystal phase of the LiMn ₂ O ₄ thin film was already synthesized in the low-temperature baking process, so that sintering progressed with grain growth during firing.
[0026]
▲ 3 ▼ for LiMn ₂ O ₄ thin film electrode prepared by sintering the battery characterization 650 ° C. of LiMn ₂ O ₄ thin film electrodes, and the battery was characterization in a glove box filled with dry argon. In this evaluation, the prepared LiMn ₂ O ₄ thin film electrode was used as the positive electrode, the Li foil was used as the negative electrode and the reference electrode, and a solution obtained by dissolving LiClO ₄ at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate and diethyl carbonate was electrolyzed. The test was carried out using a tripolar glass cell as shown in FIG. The test conditions were a charge / discharge rate of 0.5C and 5C, and a voltage range of 3.0 to 4.4V. As a result, FIG. 7 shows the discharge capacity cycle characteristics of the battery when the charge / discharge rate was 5C. Moreover, regarding the battery according to the present invention, FIG. 8 shows the discharge capacity cycle characteristics of the battery when the charge / discharge rate is changed, and FIG. 9 shows the charge / discharge curve at 10 cycles.
[0027]
As is clear from the graph shown in FIG. 7, the battery using the LiMn ₂ O ₄ thin film electrode according to the present invention prepared by adding diethylene glycol shows no deterioration in characteristics until about 700 cycles, whereas diethylene glycol is added. In the battery using the LiMn ₂ O ₄ thin film electrode according to the prior art produced without the linearity, linear characteristic deterioration was observed. This is considered to be related to the crystal stability (integrity) of LiMn ₂ O ₄ itself, the film structure and the adhesion to the substrate.
In either case, the initial capacity was 110 mAh / g corresponding to about 75% of the theoretical capacity of LiMn ₂ O ₄ : 148 mAh / g, but as apparent from the XRD pattern shown in FIG. In these samples, since Mn ₂ O ₃ is recognized as a different phase, it is considered that nearly 100% of LiMn ₂ O ₄ present in the thin film contributes to the battery reaction.
[0028]
As apparent from the graph shown in FIG. 8, when the charge / discharge rate is changed from 5C to 0.5C for the battery using the LiMn ₂ O ₄ thin film electrode according to the present invention, the initial capacity is 117 mAh / g, which is about 6% larger. However, there was almost no change in the discharge capacity cycle characteristics, and it was found that the charge / discharge rate dependence was small in this electrode. This is considered to be due to the fact that LiMn ₂ O ₄ , which is a positive electrode material, is thin and dense and has good adhesion to the substrate.
[0029]
As is clear from the graph shown in FIG. 9, regarding the battery using the LiMn ₂ O ₄ thin film electrode according to the present invention, a two-stage plateau (flat portion) peculiar to LiMn ₂ O ₄ is clearly confirmed in the 4V plateau region. it can. A battery using a LiMn ₂ O ₄ positive electrode is generally said to have poor battery cycle characteristics when such a two-stage plateau (flat part) appears clearly. However, the battery using the LiMn ₂ O ₄ thin film electrode according to the present invention prepared by adding diethylene glycol has good battery cycle characteristics as described above. This is presumably because the LiMn ₂ O ₄ thin film according to the present invention has no crystal defects in the LiMn ₂ O ₄ crystal and the crystal structure is very stable.
[0030]
【The invention's effect】
As described above, according to the present invention, an aqueous solution containing a specific solvent such as diethylene glycol in which a Li source and a Mn source are mixed is applied onto the positive electrode substrate, dried, and then fired. A LiMn ₂ O ₄ thin film having a uniform film structure without grain boundaries can be formed on the surface with good adhesion.
Therefore, the LiMn ₂ O ₄ thin film electrode according to the present invention has a large initial capacity, good battery cycle characteristics, small charge / discharge rate dependency, and excellent performance in battery characteristics.
[Brief description of the drawings]
FIG. 1 is a flowchart showing a manufacturing process of a LiMn ₂ O ₄ thin film electrode.
FIG. 2 is a diagram showing an X-ray diffraction pattern after baking at 300 ° C.
FIG. 3 is a diagram showing an X-ray diffraction pattern after firing at 650 ° C. FIG.
FIG. 4 is an SEM photograph showing the crystal structure of LiMn ₂ O ₄ at the fracture surface of the electrode according to the present invention.
FIG. 5 is an SEM photograph showing the crystal structure of LiMn ₂ O ₄ at the fracture surface of the electrode according to the prior art.
FIG. 6 is a diagram showing a triode glass cell used as a test cell for evaluating battery characteristics in Examples.
FIG. 7 is a graph showing the discharge capacity cycle characteristics of the battery when the charge / discharge rate is 5C.
FIG. 8 is a graph showing the discharge capacity cycle characteristics of the battery according to the present invention when the charge / discharge rate is changed.
FIG. 9 is a graph showing a charge / discharge curve in 10 cycles for the battery according to the present invention.

Claims (2)

A thin film electrode on the positive electrode substrate obtained by forming a thin film of crystalline spinel LiMn ₂ 0 _4, the thin film of crystalline spinel LiMn ₂ 0 ₄ has a water-soluble lithium salt and manganese nitrate (Mn (NO ₃ ) The solution obtained by adding any one solvent selected from diethylene glycol, glycerone, and formaldehyde to the aqueous solution of ₂ ) or a mixed solvent composed of two or more was obtained by coating on a positive electrode substrate , drying, baking, and baking. LiMn ₂ 0 ₄ thin film electrode characterized by being a thing. Obtained by adding and mixing an aqueous solution of water-soluble lithium salt and manganese nitrate (Mn (NO ₃ ) ₂ ) with a single solvent selected from diethylene glycol, glycerol and formaldehyde or a mixed solvent of two or more on a positive electrode substrate. A method for producing a LiMn ₂ 0 ₄ thin film electrode, comprising: applying a solution obtained, followed by drying, baking in a temperature range of 200 to 400 ° C. , and then baking in a temperature range of 600 to 800 ° C. .

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