Negative electrode material for lithium-ion battery

Abstract

To improve the cycle characteristics of a lithium-ion battery using a conversion-type negative electrode material.SOLUTION: A negative electrode material for a lithium ion battery is a mixture of a powder of a convertible oxide composed of iron oxide and / or silicon oxide and causing a conversion reaction with Li ions, and a powder of an oxygen ion conductor.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a negative electrode material for a lithium ion battery.

Background Art

[0002] The capacity of a lithium ion battery is determined by the amount of lithium ions that the electrode material can occlude. As this negative electrode material for a lithium ion battery, for example, as described in Patent Document 1, it is known to use an iron oxide. This iron oxide, unlike the conventionally mainstream graphite, occludes and releases Li ions by a conversion reaction. For example, in the case of magnetite, it can occlude 5 times as many Li ions as graphite.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, conversion-type oxides such as magnetite have large volume expansion and contraction when occluding and releasing Li ions. Therefore, when charge and discharge are repeated, there is a problem that the conversion-type oxide rapidly deteriorates and the charge capacity decreases.

[0005] Therefore, an object of the present invention is to improve the cycle characteristics of a lithium ion battery using a conversion-type negative electrode material.

Means for Solving the Problems

[0006] In order to solve the above problems, the present invention adds oxygen ion conductor powder to conversion-type oxide powder that becomes a negative electrode material of a lithium ion battery.

[0007] That is, the negative electrode material for a lithium-ion battery disclosed herein contains a powder of a conversion oxide composed of an iron oxide and / or a silicon oxide that undergoes a conversion reaction with Li ions, and a powder of an oxygen ion conductor, The conversion oxide is composed of magnetite, and the oxygen ion conductor is composed of samarium-doped ceria and / or yttria-stabilized zirconia. and is characterized by this. In addition, the negative electrode material for a lithium ion battery disclosed herein contains a mixture of a powder of a conversion oxide made of an iron oxide and / or a silicon oxide that undergoes a conversion reaction with Li ions and a powder of an oxygen ion conductor. The volume-based average particle diameter D50 of the oxygen ion conductor is 1 nm or more and 100 nm or less, and the volume-based average particle diameter D50 of the conversion oxide is 10 nm or more and 1000 nm or less, and is characterized by being magnetite particles.

[0008] According to this, since the oxygen ion conductor serves as a buffer material for the volume expansion and contraction of the conversion oxide, the deterioration of this conversion oxide is suppressed. Therefore, the cycle characteristics of the battery are improved. In addition, since the oxygen ion conductor acts as a catalyst for the lithium ion insertion and extraction reactions in the conversion oxide, the rate characteristics of the battery are improved. That is, since the conversion oxide stores lithium ions by the binding of Li to the oxygen of the oxide, the insertion and extraction of lithium ions involve the movement of oxygen ions. The oxygen ion conductor promotes the movement of these oxygen ions. Thus, the reaction resistance of lithium ions is reduced and the rate characteristics are improved.

[0009] As the iron oxide that becomes the above conversion oxide, α-hematite, γ-maghemite, or magnetite is preferable, and magnetite can be particularly preferably employed. An iron oxide in a mixed phase with magnetite and α-hematite or the like may also be used. From the viewpoint of improving the conversion reactivity, it is preferable that the volume-based average particle diameter D50 of such an iron oxide is about 10 nm or more and 1000 nm or less.

[0010] As the silicon oxide that becomes the above conversion oxide, those having an Si phase and an SiO2 phase are preferable, and for example, a silicon oxide represented by SiOn (0.3 < n < 1.6) is preferable. From the viewpoint of improving the conversion reactivity, it is preferable that the volume-based average particle diameter D50 of the silicon oxide is 3 μm or more and 10 μm or less.

[0011] As the oxygen ion conductor, it is preferably composed of SDC (samarium-doped ceria) and / or YSZ (yttria-stabilized zirconia), and particularly preferably SDC. The volume-based average particle diameter D50 of this oxygen ion conductor is preferably about 1 nm or more and 100 nm or less from the viewpoints of improving the buffering property of the volume expansion and contraction and improving the conversion reactivity.

[0012] In addition, the ratio of the oxygen ion conductor powder in the total amount of the conversion oxide powder and the oxygen ion conductor powder is preferably 3% by mass or more and 30% by mass or less. When the ratio is less than 3% by mass, a remarkable effect of improving the buffering property of the volume expansion and contraction or improving the conversion reactivity cannot be obtained. Since the oxygen ion conductor powder itself does not occlude Li ions, when the ratio increases, it becomes disadvantageous for improving the output of the battery. Therefore, the ratio is preferably 30% by mass or less.

Advantages of the Invention

[0013] According to the present invention, since the negative electrode material contains a mixture of a conversion oxide powder composed of iron oxide and / or silicon oxide and causing a conversion reaction with Li ions and an oxygen ion conductor powder, it is advantageous for extending the life and improving the output of the lithium ion battery.

Brief Description of the Drawings

[0014]

Figure 1

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Mode for Carrying Out the Invention

[0015] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit the present invention, its applications, or its uses.

[0016] <Lithium ion battery (secondary battery)> [Negative electrode] The lithium ion battery according to this embodiment has a negative electrode made of a negative electrode material obtained by mixing a conversion oxide powder composed of iron oxide and / or silicon oxide and capable of causing a conversion reaction with Li ions, and an oxygen ion conductor powder. This negative electrode material constitutes the negative electrode active material. The negative electrode material is mixed with a conductive assistant and a binder and applied to the surface of the current collector.

[0017] Preferred current collectors include copper foil. As the binder, styrene-butadiene rubber (SBR), a combination of the styrene-butadiene rubber with carboxymethyl cellulose as a thickener (SBR-CMC), PVdF, an imide-based binder, or a polyacrylic acid-based binder can be preferably adopted.

[0018] [Positive electrode] The positive electrode of the above lithium ion battery is obtained by mixing a positive electrode active material, a conductive assistant, and (a binder and a conductive assistant), and applying the mixture to a current collector. Preferred current collectors include aluminum foil.

[0019] Preferred positive electrode active materials include composite metal oxides of lithium containing one or more selected from the group consisting of cobalt, manganese, and nickel, lithium phosphate compounds, and lithium silicate compounds. In particular, it is preferable to adopt lithium phosphate. These positive electrode active materials can be used alone or in combination of two or more.

[0020] As the binder, polyvinylidene fluoride (PVdF) can preferably be adopted. As the conductive assistant, carbon black, acetylene black, carbon nanofiber (CNF), etc. can be adopted.

[0021] [Non-aqueous electrolyte] The non-aqueous electrolyte is formed by dissolving a lithium salt (supporting electrolyte) in a non-aqueous solvent, and additives are added as required. There is no particular limitation on the type of non-aqueous solvent, and examples include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), chain carbonates such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), cyclic carboxylic acid esters such as γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. The non-aqueous solvent may be used alone or in combination of two or more.

[0022] Preferred lithium salts include LiPF6, LiPO2F2, LiBF4, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2, etc. The lithium salt can be used alone or in combination of two or more.

[0023] As additives, there are a wettability-improving solvent and an SEI-forming solvent that improve the wettability of the electrolyte with respect to the separator. Examples of the wettability-improving solvent include dibutyl carbonate (DBC), methyl butyl carbonate (MBC), ethyl butyl carbonate (EBC), and the like. The addition amount of the wettability-improving solvent is preferably about 3% by mass or more and 10% by mass or less of the above non-aqueous solvent. Examples of the SEI-forming solvent include vinylene carbonate (VC), methyl vinylene carbonate (MVC), ethyl vinylene carbonate (EVC), fluorovinylene carbonate (FVC), vinyl ethylene carbonate (VEC), ethynyl ethylene carbonate (EEC), ethylene sulfite (ES), fluoroethylene carbonate (FEC), and the like. These SEI-forming solvents can be used alone or in combination of two or more. The addition amount of the SEI-forming solvent is preferably about 0.5% by mass or more and 5% by mass or less of the above non-aqueous solvent.

[0024] [Separator] There are no particular restrictions on the separator, but a single-layer or laminated microporous film of polyolefin such as polypropylene or polyethylene, a woven fabric, a non-woven fabric, or the like can be employed.

[0025] <Examples and Comparative Examples of Anode Materials> - Example 1 - Powder of magnetite Fe3O4 (D50 = 500 nm) as a conversion-type oxide and powder of SDC (Sm 0.2 Ce 0.8 O 2-δ )(D50 = 50 nm) as an oxygen ion conductor were mixed by a solid-phase method to obtain an anode material with an SDC concentration of 5% by mass (hereinafter referred to as "5% SDC").

[0026] - Example 2 - An anode material with an SDC concentration of 10% by mass (hereinafter referred to as "10% SDC") was obtained in the same manner as in Example 1.

[0027] - Example 3 - The negative electrode material with an SDC concentration of 20 mass% (hereinafter referred to as "20% SDC") was obtained in the same manner as in Example 1.

[0028] - Example 4 - The negative electrode material with an SDC concentration of 30 mass% (hereinafter referred to as "30% SDC") was obtained in the same manner as in Example 1.

[0029] - Example 5 - Powder (D50 = 50 nm) of YSZ (Y2O3 = 3 mol%) was adopted as the oxygen ion conductor, and the negative electrode material with a YSZ concentration of 20 mass% (hereinafter referred to as "20% YSZ") was obtained in the same manner as in Example 1.

[0030] - Comparative Example 1 - A negative electrode material of magnetite Fe3O4 powder alone without adding an oxygen ion conductor was prepared.

[0031] [Evaluation of cycle characteristics] The evaluation of cycle characteristics was carried out using a 2032-type coin cell. The mixing ratio of the negative electrode binder was 80 wt% of the negative electrode material, 5 wt% of the conductive aid, and 15 wt% of the imide-based binder. These three were mixed to prepare a binder slurry, which was applied to a copper foil as a current collector and then dried at 80°C. Next, in order to imidize the binder, a vacuum oven was used and dried at 180°C. The obtained negative electrode was placed in a glove box, and in the box, a positive electrode (counter electrode) composed of a negative electrode and Li metal was opposed through a porous separator, and a half cell was obtained by enclosing it together with an electrolytic solution in a coin cell container. The electrolytic solution was prepared by adding LiPF6 (1M) to a non-aqueous solvent obtained by mixing EC and DEC at a volume ratio of 3:7.

[0032] (Cycle characteristics) Using each negative electrode material of Examples 1 - 5 and Comparative Example 1, the above half cell was fabricated, and a charge-discharge cycle test was carried out at a potential range of 0.01 V to 3.0 V and a current density of 200 mA / g. The test temperature was 20°C. The transition of the discharge capacity is shown in Fig. 1.

[0033] Looking at Comparative Example 1 (without an oxygen ion conductor), the discharge capacity starts to decrease significantly from around the third cycle and further decreases from around the 45th cycle. In contrast, in Examples 1 - 5 where an oxygen ion conductor was added, the decrease in discharge capacity is small.

[0034] The discharge capacity retention rate at 60 cycles is the highest for Example 1 with 5% SDC. As the proportion of SDC increases, the discharge capacity retention rate decreases, but Example 4 with 30% SDC is still higher than Comparative Example 1 (Fe3O4) without an oxygen ion conductor. Example 5 (Fe3O4 + 10% YSZ) using YSZ as the oxygen ion conductor also has a higher discharge capacity retention rate than Comparative Example 1.

[0035] Next, the results of charge - discharge cycle tests at a temperature of - 10°C for Example 1 (5% SDC), Example 3 (20% SDC), and Comparative Example 1 (without an oxygen ion conductor) are shown in Figure 2. Even at a low temperature of - 10°C, in the cases where SDC is added, the capacity retention rate is higher than that of the comparative example.

[0036] From the above, it can be said that the addition of an oxygen ion conductor (SDC or YSZ) to the conversion oxide (Fe3O4) can improve the cycle characteristics not only at room temperature but also at low temperatures.

[0037] (Rate characteristics) For Example 1 (5% SDC), Example 2 (10% SDC), and Comparative Example 1 (without an oxygen ion conductor), a charge - discharge cycle test was conducted where the current density was changed from 200 mA / g to 500 mA / g → 1000 mA / g → 2000 mA / g every 5 cycles and finally returned to 200 mA / g. The results are shown in Figure 3.

[0038] The reduction rate of the discharge capacity when the current density changes from 200 mA / g to 2000 mA / g is about 60% in Comparative Example 1 (without an oxygen ion conductor). In contrast, for Example 1 (5% SDC) and Example 2 (10% SDC), the reduction rate is about 30 - 40%. Also, the reduction rate of Example 2 (10% SDC) is smaller than that of Example 1 (5% SDC).

[0039] According to the test results of the above cycle characteristics and rate characteristics (the tendency of the change in cycle characteristics and rate characteristics with respect to the change in the addition amount of SDC), it is recognized that even when the SDC addition amount is about 3%, it is effective in improving the battery. Also, it can be said that when the SDC addition amount is 5% or more and 10% or less, it is particularly effective in improving the battery.

[0040] (Impedance measurement) For Example 5 (SDC 30%) and Comparative Example 1 (without an oxygen ion conductor), using an impedance measuring device, the AC impedance measurement was performed while changing the temperature under the conditions of a measurement frequency range of 0.01 Hz to 100,000 Hz for the charged state (3V). The measurement temperatures were 10°C, 20°C, 30°C, and 40°C.

[0041] Figure 4 shows the Cole - Cole plot (complex impedance diagram) of Comparative Example 1, and Figure 5 shows the Cole - Cole plot of Example 5.

[0042] In both Comparative Example 1 (Figure 4) and Example 5 (Figure 5), the arc drawn by the Cole - Cole plot becomes larger as the temperature decreases. However, in Example 5 (Figure 5), compared with Comparative Example 1 (Figure 4), the arc is smaller, and the degree of increase in the arc with the decrease in temperature is also smaller. It is recognized that the oxygen ion conductor (SDC) acts as a catalyst for the lithium ion insertion / extraction reaction in the conversion oxide (Fe3O4), and the reaction resistance has become smaller.

[0043] <Case where the conversion oxide is a silicon oxide> - Example 6 - As the conversion oxide, silicon oxide SiO having an Si phase and an SiO2 phase was adopted, and this SiO powder (D50 = 5 μm) and SDC (Sm 0.2 Ce 0.8 O 2-δ ) powder (D50 = 50 nm) were mixed by a solid - phase method to obtain a negative electrode material (SiO + 5% SDC) with an SDC concentration of 5 mass%.

[0044] - Comparative Example 2 - An anode material (SiO) of SiO powder alone without adding an oxygen ion conductor was prepared as described above.

[0045] [Evaluation of cycle characteristics] The evaluation of the cycle characteristics was carried out by creating a half-cell with the same materials and manufacturing conditions as in the previous Examples 1-5 and Comparative Example 1, and performing a charge-discharge cycle test in a potential range of 0.01 V to 3.0 V, a current density of 200 mA / g, and a temperature of 20°C. The transition of the discharge capacity is shown in Figure 6.

[0046] In Example 6 (SiO + 5% SDC), unlike the Comparative Example (SiO), there is almost no decrease in the discharge capacity with an increase in the number of cycles. It can be seen that the addition of the oxygen ion conductor SDC to SiO is effective in improving the cycle characteristics.

Explanation of symbols

[0047] None

Claims

1. It contains a mixture of a powder of a conversion oxide composed of an iron oxide and / or a silicon oxide that undergoes a conversion reaction with Li ions, and a powder of an oxygen ion conductor, The conversion oxide is composed of magnetite, The oxygen ion conductor is composed of samarium-doped ceria and / or yttria-stabilized zirconia, and a negative electrode material for a lithium ion battery is characterized by this.

2. It contains a mixture of a powder of a conversion oxide composed of an iron oxide and / or a silicon oxide that undergoes a conversion reaction with Li ions, and a powder of an oxygen ion conductor, The volume-based average particle diameter D50 of the oxygen ion conductor is 1 nm or more and 100 nm or less, A negative electrode material for a lithium ion battery is characterized by being magnetite particles in which the volume-based average particle diameter D50 of the conversion oxide is 10 nm or more and 1000 nm or less.

3. In Claim 1 or Claim 2, A negative electrode material for a lithium ion battery is characterized in that the proportion of the powder of the oxygen ion conductor in the total amount of the powder of the conversion oxide and the powder of the oxygen ion conductor is 3% by mass or more and 30% by mass or less.

4. In Claim 1, A negative electrode material for a lithium ion battery is characterized in that the volume-based average particle diameter D50 of the oxygen ion conductor is 1 nm or more and 100 nm or less.

5. In Claim 4, A negative electrode material for a lithium ion battery is characterized by being magnetite particles in which the volume-based average particle diameter D50 of the conversion oxide is 10 nm or more and 1000 nm or less.

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