JP2007123236A - Electrode for nonaqueous electrolyte secondary battery and nonaqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery capable of storing and releasing sodium ion. <P>SOLUTION: Slurry as an electrode mixture is produced by adding an electrode active material containing bismuth (Bi)particles, a binder and acetylene black as a conductive agent with weight ratios of 85:10:5 into a solution with a polyvinylidene fluoride as a binder dissolved in N-methyl-2-pyrrolidone and kneading them. A specific surface area of the acetylene black is, for instance, 92 m<SP>2</SP>/g with a primary particle size of around 35 nm. Next, the slurry prepared, after being coated on a copper foil of a collector, is dried in a vacuum, and rolled by a rolling mill to form an active material layer, and complete a work electrode 1 by mounting a work electrode tab to it. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

  Currently, non-aqueous electrolyte secondary batteries that use a non-aqueous electrolyte as a secondary battery with a high energy density, for example, charge and discharge by moving lithium ions between the positive electrode and the negative electrode are widely used. Yes.

In such a non-aqueous electrolyte secondary battery, a lithium transition metal composite oxide having a layered structure such as lithium nickelate (LiNiO ₂ ) or lithium cobaltate (LiCoO ₂ ) is generally used as a positive electrode, and lithium is occluded as a negative electrode. In addition, carbon materials that can be released, lithium metal, lithium alloys, and the like are used (for example, see Patent Document 1).

In addition, a non-aqueous electrolyte in which an electrolyte salt such as lithium tetrafluoroborate (LiBF ₄ ) or lithium hexafluorophosphate (LiPF ₆ ) is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate is used. ing.

On the other hand, recently, research on non-aqueous electrolyte secondary batteries using sodium ions instead of lithium ions has been started. The negative electrode of this nonaqueous electrolyte secondary battery is formed of a metal containing sodium. Sodium is abundantly contained in seawater, and the cost can be reduced by using sodium.
JP 2003-151549 A

  Since the charge / discharge reaction of the nonaqueous electrolyte secondary battery using sodium is performed by dissolution and precipitation of sodium ions, charge / discharge efficiency and charge / discharge characteristics are not good.

  Moreover, when charging and discharging are repeated, dendritic precipitates (dendrites) are likely to be generated in the nonaqueous electrolyte. Therefore, an internal short circuit may occur due to the dendrite, and it is difficult to ensure sufficient safety.

  Furthermore, in a non-aqueous electrolyte secondary battery using sodium ions, when a negative electrode containing highly practical graphite that can occlude and release lithium ions is used, sodium ions are sufficiently occluded and A high charge / discharge capacity density cannot be obtained without being released. Similarly, even when a negative electrode containing silicon is used, sodium ions are not occluded and released from the negative electrode.

  The objective of this invention is providing the electrode for nonaqueous electrolyte secondary batteries which can occlude and discharge | release sodium ion, and a nonaqueous electrolyte secondary battery using the same.

  (1) The electrode for a nonaqueous electrolyte secondary battery according to the first invention can occlude and release sodium ions and contains bismuth as a single component or a main component.

  In the electrode for a non-aqueous electrolyte secondary battery according to the first invention, sodium ions are sufficiently occluded and released by containing bismuth as a single component or main component. Further, the cost can be reduced by using sodium rich in resources and inexpensive bismuth.

  (2) The electrode for a nonaqueous electrolyte secondary battery may further include a current collector made of metal, and bismuth may be formed on the current collector. In this case, bismuth can be easily formed on the current collector.

  (3) A nonaqueous electrolyte secondary battery according to a second invention comprises a negative electrode comprising the electrode for a nonaqueous electrolyte secondary battery according to the first invention, a positive electrode, and a nonaqueous electrolyte containing sodium ions, The negative electrode includes a negative electrode mixture containing bismuth applied on the current collector, and the negative electrode mixture includes carbon as a conductive agent in an amount of 10% by weight or more based on the negative electrode mixture.

  In the non-aqueous electrolyte secondary battery according to the second invention, reversible charging / discharging can be performed by using the negative electrode comprising the non-aqueous electrolyte secondary battery electrode according to the first invention. In addition, the cost of the non-aqueous electrolyte secondary battery can be reduced.

  Further, when the negative electrode mixture contains carbon, the conductivity in the negative electrode is improved and the permeability of the nonaqueous electrolyte to the negative electrode is improved. In particular, the charge / discharge characteristics are improved when the negative electrode mixture contains 10% by weight or more of carbon with respect to the negative electrode mixture.

  (4) The carbon may include carbon black. In this case, the conductivity in the negative electrode is further improved, and the permeability of the nonaqueous electrolyte to the negative electrode is further improved.

  (5) The positive electrode may include a sodium-containing oxide. In this case, sodium ions are more likely to be occluded and released from the positive electrode as compared to electrodes containing other than oxides (sulfides, fluorides, chlorides, etc.). Thereby, charge / discharge characteristics are improved.

  (6) The non-aqueous electrolyte may contain sodium hexafluorophosphate. In this case, safety is improved.

  (7) The nonaqueous electrolyte may contain one or more selected from the group consisting of cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles and amides. Good. In this case, the cost can be reduced and the safety is improved.

  According to the present invention, the nonaqueous electrolyte secondary battery electrode contains bismuth as a single component or main component, so that sodium ions are sufficiently occluded and released from the nonaqueous electrolyte secondary battery electrode. Further, the cost can be reduced by using sodium rich in resources and inexpensive bismuth.

  Hereinafter, an electrode for a nonaqueous electrolyte secondary battery according to the present embodiment and a nonaqueous electrolyte secondary battery using the same will be described. The non-aqueous electrolyte secondary battery according to the present embodiment is generally called a sodium secondary battery because sodium ions are used.

  The nonaqueous electrolyte secondary battery according to the present embodiment includes a working electrode, a counter electrode, and a nonaqueous electrolyte.

  The various materials described below and the thicknesses and concentrations of the materials are not limited to those described below, and can be set as appropriate.

(1) Production of electrode (working electrode) for non-aqueous electrolyte secondary battery Electrode active material containing bismuth (Bi) powder in a solution obtained by dissolving polyvinylidene fluoride as a binder in N-methyl-2-pyrrolidone, A slurry as an electrode mixture is prepared by adding and kneading so that the weight ratio of the acetylene black as the binder and the conductive agent is, for example, 85: 10: 5. In the present embodiment, the acetylene black has a specific surface area of, for example, 92 m ² / g and a primary particle diameter of about 35 nm. Acetylene black is a kind of high-purity carbon black excellent in conductivity.

  Then, after apply | coating the produced slurry on the copper foil of a collector with a doctor blade method, it dries in a vacuum and forms an active material layer by rolling with a rolling roller. Then, the copper foil on which the active material layer is formed is cut into a size of 2 cm × 2 cm, for example, and a working electrode tab is attached to complete the electrode for the nonaqueous electrolyte secondary battery.

(2) Production of non-aqueous electrolyte As the non-aqueous electrolyte, an electrolyte salt dissolved in a non-aqueous solvent can be used.

  Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, and the like, which are usually used as non-aqueous solvents for batteries. Is mentioned.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate and the like, and those in which some or all of these hydrogen groups are fluorinated can be used. For example, trifluoropropylene carbonate, fluoro Examples include ethyl carbonate.

  Examples of chain carbonic acid esters include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, and the like. Some of these hydrogen groups are fluorinated. It is possible to use.

  Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5. -Trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether and the like.

  Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl. Ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1 -Dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethy Glycol dimethyl and the like.

  Nitriles include acetonitrile and the like, and amides include dimethylformamide and the like.

Examples of electrolyte salts include non-peroxides that are soluble in nonaqueous solvents such as sodium hexafluorophosphate (NaPF ₆ ), sodium tetrafluoroborate (NaBF ₄ ), NaCF ₃ SO ₃ , and NaBeTi. Use expensive ones. In addition, 1 type may be used among said electrolyte salt, and may be used in combination of 2 or more type.

In the present embodiment, as a nonaqueous electrolyte, a nonaqueous solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 50:50 is mixed with 1 mol / liter of sodium hexafluorophosphate (NaPF ₆ ) as an electrolyte salt. The one added to a concentration of 1 is used.

(3) Production of Test Cell FIG. 1 is a schematic explanatory diagram of a test cell using the electrode for a nonaqueous electrolyte secondary battery according to the present embodiment.

As shown in FIG. 1, in an inert atmosphere, a lead is attached to the working electrode 1, and a lead is attached to a counter electrode 2 made of, for example, sodium metal. Instead of the counter electrode 2 made of sodium metal, for example, sodium cobaltate represented by NaCoO ₂ , Na _x Co _y O ₂ (0 <x ≦ 1.0, and y <1.5) is represented. You may use the counter electrode 2 which contains other materials, such as sodium cobaltate, as a single component or a main component.

  Next, the separator 4 is inserted between the working electrode 1 and the counter electrode 2, and the working electrode 1, the counter electrode 2, and the reference electrode 3 made of, for example, sodium metal are disposed in the cell container 10. Then, the test cell is manufactured by injecting the nonaqueous electrolyte 5 into the cell container 10.

(4) Effect in this Embodiment As can be seen from the two-phase phase diagram of sodium and bismuth shown in FIG. 2, sodium and bismuth are alloyed. However, there was no knowledge until the present application as to whether bismuth can occlude and release sodium ions.

  In the present embodiment, by using a nonaqueous electrolyte secondary battery electrode containing bismuth as a single component or main component, sodium ions are sufficiently occluded and released from the nonaqueous electrolyte secondary battery electrode. Is done. Further, the cost can be reduced by using sodium rich in resources and inexpensive bismuth.

  In addition, by using the electrode for a nonaqueous electrolyte secondary battery as described above as the working electrode 1, it is possible to perform reversible charging / discharging and to provide an inexpensive nonaqueous electrolyte secondary battery. it can.

  Moreover, when the electrode mixture of the working electrode 1 contains carbon, the conductivity of the working electrode 1 is improved and the permeability of the nonaqueous electrolyte 5 to the working electrode 1 is improved. In particular, when the electrode mixture of the working electrode 1 contains 10% by weight or more of carbon with respect to the electrode mixture, the charge / discharge characteristics are improved. In particular, the conductivity of the working electrode 1 can be further improved by using carbon black as a kind of the carbon. This is because carbon black has a larger specific surface area than other carbons such as graphite, and therefore the proportion of carbon black in the volume of the working electrode 1 is increased.

  Here, in a nonaqueous electrolyte secondary battery using lithium ions, when carbon black is added to the working electrode (negative electrode), lithium ions are adsorbed to the carbon black at a low potential. As a result, the discharge capacity density decreases.

  For these reasons, in non-aqueous electrolyte secondary batteries that use lithium ions, carbon black cannot be added to the working electrode, or even if added, the amount of carbon black significantly reduces the discharge capacity density. The amount must not be limited.

  On the other hand, in the non-aqueous electrolyte secondary battery using sodium ions as in the present embodiment, sodium ions are not adsorbed on the carbon black in the working electrode 1 or a small amount even if adsorbed. Therefore, carbon black can be used effectively as a conductive agent for the working electrode 1.

And in this Embodiment, it is preferable that the primary particle diameter of the carbon black as a electrically conductive agent added to the working electrode 1 is about 35 nm, and it is preferable that a specific surface area is 90 m < ² > / g or more. Thus, by adding a conductive agent having a large specific surface area to the working electrode 1, the flexibility of the working electrode 1 and the permeability of the nonaqueous electrolyte 5 to the working electrode 1 are improved. This improves the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery. In addition, said softness | flexibility means what is called softness, The working electrode 1 can be processed into various shapes by this softness | flexibility.

(A) Example 1
As shown below, the charge / discharge characteristics of the electrode for the nonaqueous electrolyte secondary battery (working electrode 1) were examined using the test cell prepared based on the above embodiment.

  FIG. 3 is a graph showing the charge / discharge characteristics of the nonaqueous electrolyte secondary battery electrode of Example 1.

  In the produced test cell, discharge was performed at a constant current of 0.86 mA until the potential of the working electrode 1 with respect to the reference electrode 3 reached 0V. Then, charging and discharging characteristics were examined by charging until the potential of the working electrode 1 based on the reference electrode 3 reached 1.0 V at a constant current of 0.86 mA. The charge / discharge test was conducted up to 1 to 3 cycles. The results are shown in Table 1. Table 1 shows the value of DC resistance measured for 1 cm of the working electrode 1.

  As shown in Table 1, the initial discharge capacity density per gram of the active material of the working electrode 1 was about 361 mAh / g, and it was found that charging / discharging was performed satisfactorily. There was no significant decrease in discharge capacity density over the course of the cycle. It is also estimated that sodium ions are not adsorbed on the carbon black in the working electrode 1.

(B) Example 2
In Example 2, the weight ratio of the electrode active material containing bismuth powder and the above binder to a solution obtained by dissolving polyvinylidene fluoride as a binder in N-methyl-2-pyrrolidone is 95: 5. A test cell of a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the slurry as an electrode mixture was produced by kneading after addition.

  And the charge / discharge characteristic of the electrode for nonaqueous electrolyte secondary batteries (working electrode 1) of this example was investigated. The charge / discharge test was conducted up to 1 to 3 cycles. The results are shown in Table 1 above. Table 1 shows the value of DC resistance measured for 1 cm of the working electrode 1 of Example 2.

  As shown in Table 1, the initial charge capacity density per gram of the active material of the working electrode 1 was about 217 mAh / g, and it was found that charge and discharge were performed satisfactorily. It is also estimated that sodium ions are not adsorbed on the carbon black in the working electrode 1.

(C) Example 3
In Example 3, the weight ratio of the electrode active material containing bismuth powder, the binder, and acetylene black as the conductive agent to a solution of polyvinylidene fluoride as a binder in N-methyl-2-pyrrolidone was 90. A test cell for a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the slurry was prepared as an electrode mixture by adding and kneading so as to be 5: 5. .

  And the charge / discharge characteristic of the electrode for nonaqueous electrolyte secondary batteries (working electrode 1) of this example was investigated. The charge / discharge test was conducted up to 1 to 3 cycles. The results are shown in Table 1 above. Table 1 shows the value of the DC resistance measured for 1 cm of the working electrode 1 of Example 3.

  As shown in Table 1, the initial charge capacity density per gram of the active material of the working electrode 1 was about 280 mAh / g, and it was found that charge and discharge were performed satisfactorily. It is also estimated that sodium ions are not adsorbed on the carbon black in the working electrode 1.

(D) Summary From the above, it has been clarified that sodium ions are reversibly occluded and released from the working electrode 1. Thereby, the effectiveness of the new electrode for nonaqueous electrolyte secondary batteries which replaces the conventional electrode for nonaqueous electrolyte secondary batteries using lithium ion was able to be confirmed.

  Moreover, it turned out that it is preferable to use the working electrode 1 of Example 1 with small DC resistance from the said Table 1. That is, it has been found that a higher discharge capacity density can be obtained when the electrode mixture of the working electrode 1 contains 10% by weight or more of acetylene black with respect to the electrode mixture.

  The electrode for nonaqueous electrolyte secondary batteries and the nonaqueous electrolyte secondary battery according to the present invention can be used in various power sources such as portable power sources and automobile power sources.

It is a schematic explanatory drawing of the test cell of the electrode for nonaqueous electrolyte secondary batteries which concerns on this Embodiment. It is a two-phase phase diagram of sodium and bismuth. 3 is a graph showing charge / discharge characteristics of a nonaqueous electrolyte secondary battery electrode of Example 1. FIG.

Explanation of symbols

1 Working Electrode 2 Counter Electrode 3 Reference Electrode 4 Separator 5 Nonaqueous Electrolyte 10 Cell Container

Claims (4)

An electrode for a non-aqueous electrolyte secondary battery, which can occlude and release sodium ions and contains bismuth as a single component or a main component. A current collector made of metal;
The electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the bismuth is formed on the current collector. A negative electrode comprising the electrode for a nonaqueous electrolyte secondary battery according to claim 1, a positive electrode, and a nonaqueous electrolyte containing sodium ions,
The negative electrode includes a negative electrode mixture containing the bismuth applied on the current collector,
The non-aqueous electrolyte secondary battery, wherein the negative electrode mixture includes 10% by weight or more of carbon as a conductive agent with respect to the negative electrode mixture. The non-aqueous electrolyte secondary battery according to claim 3, wherein the carbon includes carbon black.

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