JP2015035324A - Negative electrode mixture for nonaqueous secondary battery and nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide negative electrode active material capable of realizing a nonaqueous secondary battery having an excellent capacity maintenance rate.SOLUTION: A negative electrode mixture for a nonaqueous secondary battery comprises lithium-containing layer shape chrome oxide LiCrOor natrium-containing layer shape chrome oxide NaCrOas negative electrode active material.

Description

  The present invention relates to a negative electrode mixture for a non-aqueous secondary battery including lithium or sodium-containing layered chromium oxide as a negative electrode active material, and a non-aqueous secondary battery including a negative electrode layer including a layer made of the negative electrode mixture.

In recent years, various studies have been conducted on electrode active materials for lithium secondary batteries having high voltage and high energy density in order to further improve battery performance. For example, lithium-containing layered metal oxides such as LiFeO ₂ and LiCoO ₂ have been widely studied as positive electrode materials, but there are few reports on their use as negative electrode active materials. Patent Document 1 discloses a negative electrode active material for a secondary battery that charges and discharges by a conversion reaction. For example, a negative electrode active material (for example, LiFeO ₂₎ obtained by adding lithium to a metal compound such as Fe ₂ O _3. Etc.) to improve the initial charge and discharge efficiency by performing a conversion reaction. However, lithium-containing layered metal oxides such as LiFeO ₂ and LiCoO ₂ described in Patent Document 1 obtain a large capacity by inserting and detaching Li up to 0.01 V with respect to the Li standard. However, because of the large change in morphology due to the conversion reaction and the volume change due to expansion and contraction, repeated charge / discharge cycles will cause cracks in the negative electrode and break the electron conduction path, resulting in poor charge / discharge cycle characteristics and capacity. There is a problem that the maintenance rate is low.

JP 2012-028264 A

  Therefore, an object of the present invention is to provide a negative electrode active material capable of providing a non-aqueous secondary battery having an excellent capacity retention rate.

As a result of intensive studies to solve the above problems, the present inventor has found that lithium-containing layered chromium oxide LiCrO ₂ or sodium-containing layered chromium oxide NaCrO ₂ is based on Li standards when used as a negative electrode active material. On the other hand, even if Li was inserted up to 0.01 V, no conversion reaction was caused, and it was found that the initial form could be maintained even after repeated charge and discharge, and the present invention was completed. Since the negative electrode active material of the present invention does not cause a conversion reaction, even if the charge / discharge cycle is repeated, a decrease in charge / discharge cycle characteristics can be suppressed.

According to the present invention, there is provided a negative electrode mixture for a non-aqueous secondary battery containing lithium-containing layered chromium oxide LiCrO ₂ or sodium-containing layered chromium oxide NaCrO ₂ as a negative electrode active material.

  Furthermore, according to the present invention, the negative electrode layer includes a positive electrode layer, a negative electrode layer, a separator disposed between the positive electrode layer and the negative electrode layer, and the nonaqueous electrolyte. A non-aqueous secondary battery having a layer made of an agent is provided.

  The non-aqueous secondary battery obtained by using the negative electrode active material of the present invention has an excellent capacity maintenance rate even after repeated charge and discharge.

1 (a), (b) and (c) show XRD spectra of LiCrO ₂ , NaCrO ₂ and LiCoO ₂ before and after 50 charge / discharge cycles, respectively.

In the present invention, lithium-containing layered chromium oxide LiCrO ₂ (hereinafter abbreviated as “LiCrO ₂ ”) and sodium-containing layered chromium oxide NaCrO ₂ (hereinafter abbreviated as “NaCrO ₂ ”) are produced by a known method. can do. LiCrO ₂ and NaCrO ₂ are, for example, chromium glycol (III) and lithium nitrate or sodium nitrate at a temperature of room temperature to 150 ° C. in a glycolic acid aqueous solution adjusted to a low pH of 2 or less at a molar ratio of 1: 1. While stirring, the solvent is evaporated to produce a gel, and the obtained gel is calcined at a temperature of 300 to 500 ° C. for 3 to 12 hours under an inert atmosphere (for example, argon or nitrogen gas). Can be obtained by performing the main baking at a temperature of 650 to 1000 ° C. for 3 to 24 hours in an inert atmosphere. The calcination time may be a time that allows removal of nitric acid. Alternatively, commercially available LiCrO ₂ and NaCrO ₂ can be used.

In addition to LiCrO ₂ or NaCrO ₂ , the negative electrode mixture of the present invention may be an optional component known in the art as a constituent component of the negative electrode mixture for non-aqueous secondary batteries, for example, a conductive agent. (Eg, graphite, carbon black (acetylene black, ketjen black, etc.), carbon nanotube, carbon nanofiber, fullerene, etc.), binder (eg, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, etc.) (PTFE) and other fluorine-based binders, and styrene-butadiene rubber (SBR) and rubber-based binders such as butadiene rubber (BR), such as carboxymethyl cellulose, polyamic acid, polyimide, polyamideimide, polyacrylic acid, lithium polyacrylate, Polyacrylic acid sodium One or more of the beam) and the like. The proportion of each component in the total solid content in the negative electrode mixture is such that LiCrO ₂ or NaCrO ₂ (negative electrode active material) is 60% by mass to 98.5% by mass, the binder is 1% by mass to 20% by mass, and the conductivity. The agent is preferably 0.5% by mass or more and 30% by mass or less. A negative electrode layer can be obtained by forming the negative electrode mixture layer of the present invention on one surface of a negative electrode current collector.

When the ratio of LiCrO ₂ or NaCrO ₂ as the negative electrode active material is less than 60% by mass, it may be difficult to obtain a sufficient discharge capacity, and when it exceeds 98.5% by mass, the ratio of the binder is decreased. Therefore, the adhesion to the current collector is reduced, the negative electrode active material may be easily detached, and the output may not be obtained due to the decrease in conductivity. When the ratio of the binder is less than 1% by mass, the binding property is reduced, and thus the negative electrode active material and the carbon material as the conductive agent may be easily detached from the current collector. Since the ratio of the carbon material as the active material and the conductive agent is lowered, there is a possibility that the battery performance is lowered. If the proportion of the conductive agent is less than 0.5% by mass, it may be difficult to obtain sufficient conductivity, and if it exceeds 30% by mass, the proportion of the negative electrode active material that greatly contributes to battery performance decreases. In some cases, the discharge capacity may be reduced.

The negative electrode layer containing the negative electrode active material of the present invention can be obtained using a method known as a method for obtaining a negative electrode layer in the technical field, and is not particularly limited. For example, a coating method (metal mask printing method) , Electrostatic coating method, dip coating method, spray coating method, roll coating method, doctor blade method, gravure coating method, screen method, etc.), and compacting method (crimping method, pellet method, etc.). . For example, a paste containing LiCrO ₂ or NaCrO ₂ , an optional component such as a conductive agent and a binder, and a solvent is prepared, and the obtained paste is applied onto a foil-like negative electrode current collector, dried, and pressed. Then, a negative electrode mixture layer is formed on the negative electrode current collector, or a negative mixture containing LiCrO ₂ or NaCrO ₂ and optional components such as a conductive agent and a binder is press-molded to form a preform. Then, the negative electrode layer can be obtained by applying a current collector material to one surface of the obtained preform by an application method such as vapor deposition, sputtering, or lamination. The negative electrode layer thus obtained can be combined with other constituent members of the battery, for example, a positive electrode layer, a separator, and a nonaqueous electrolyte to constitute a nonaqueous secondary battery. The negative electrode current collector can be, for example, a rolled metal foil, an electrolytic metal foil, or a metal mesh, but is limited to a metal that does not cause an alloying reaction with Li at a Li reference potential of 3.0 V or less.

  The positive electrode layer can be obtained by forming a positive electrode material layer on at least one surface of the positive electrode current collector. The positive electrode material is a positive electrode active material (for example, a lithium-containing active material such as a composite oxide containing lithium and a transition metal, a lithium sulfide, an intercalation compound containing lithium, and a lithium phosphate compound, as well as desorption and insertion of Li And an optional component known in the art as a constituent of the positive electrode material for a non-aqueous secondary battery, for example, a conductive agent (for example, graphite, carbon black ( Acetylene black, ketjen black, etc.), carbon nanotubes, carbon nanofibers, fullerenes, etc.), binders (eg, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), etc.) Binders and rubber-based bars such as styrene butadiene rubber (SBR) and butadiene rubber (BR) Nda, carboxymethylcellulose, polyacrylic acid, polyimide, polyamic acid, one or more of a polyamide-imide) and the like. The positive electrode current collector is made of a metal material such as aluminum, nickel, titanium, or stainless steel. The positive electrode layer can be obtained according to the method described above as a method for obtaining the negative electrode layer, and is not particularly limited. For example, a coating method (metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll) (Coating method, doctor blade method, gravure coating method, screen method, etc.) and compacting method (pressure bonding method, pellet method, etc.). A paste containing a positive electrode active material, an optional component such as a conductive agent and a binder, and a solvent is prepared, and the obtained paste is applied onto a foil-like positive electrode current collector (eg, copper, gold) and then dried. Press to form a positive electrode material layer on the positive electrode current collector, or press form a positive electrode material containing a positive electrode active material, a conductive agent, a binder and other optional components to form a preform. The positive electrode layer can be obtained by applying a film-like or foil-like current collector material to one surface of the obtained preform by an application method such as vapor deposition, sputtering, or lamination.

  A separator will not be specifically limited if it is a porous film | membrane which can function as a separator in the said technical field. Examples of the material constituting the separator include an organic material such as a porous membrane made of polyolefin such as polypropylene (PP) and polyethylene (PE), and an inorganic material such as a porous membrane made of ceramic. The porous film may be composed of one or more organic or inorganic materials, and may be a composite film composed of an organic material and an inorganic material. There are no particular restrictions on the number or material of the membranes that make up the composite membrane. Particularly, a porous film made of polyolefin having a multilayer structure, for example, a three-layer structure of PP / PE / PP is preferably used.

In the present invention, the electrolyte is lithium ion conductive, and examples of the electrolyte include a lithium salt electrolyte, a polymer electrolyte, and a solid electrolyte that are dissolved in a non-aqueous solvent to constitute an electrolytic solution. The non-aqueous solvent constituting the electrolytic solution is preferably an aprotic polar organic solvent. For example, a high-dielectric constant, high-viscosity cyclic carbonate compound such as ethylene carbonate (EC) or propylene carbonate (PC), and dimethyl carbonate ( Low viscosity chain carbonate compounds such as DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) are preferred. Examples of lithium salt electrolytes include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), Bis (pentafluoroethanesulfonyl) imide lithium (LiN (C ₂ F ₅ SO ₂ ) ₂ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), bis (trifluoromethanesulfonyl) imide lithium (LiN (CF ₃ SO ₂ ) ₂ ), lithium tris (trifluoromethanesulfonyl) methide (LiC (CF ₃ SO ₂ ) ₃ ), lithium chloride (LiCl), lithium bromide (LiBr) and the like. The concentration of the lithium salt in the electrolytic solution is preferably 0.3 to 5M, more preferably 1 to 3M, still more preferably 1 to 2.5M, and particularly preferably 0.8 to 1.5M. It is. When the lithium salt concentration exceeds 3M, the viscosity of the electrolytic solution becomes excessively high, and the discharge capacity at a high rate and the discharge capacity at a low temperature are reduced. On the other hand, when the concentration of the lithium salt is less than 1M, the supply cannot follow the consumption of lithium ions, and the discharge capacity at a high rate and the discharge capacity at a low temperature are reduced.

  The nonaqueous secondary battery obtained by using the negative electrode mixture of the present invention may have a positive electrode current collector, a positive electrode terminal connected to the negative electrode current collector, a negative electrode terminal, and the like in addition to the above-described constituent members. it can. The structure of the negative electrode layer and the positive electrode layer is such that a negative electrode layer and a positive electrode layer are opposed to each other through a separator and an electrolyte, and a laminated structure in which flat positive electrodes and negative electrodes are alternately stacked, or a strip-shaped positive electrode and negative electrode The type and shape of these constituent members that may have any structure, which are formed by being rolled up and rolled up, are appropriately selected according to the use of the nonaqueous secondary battery. be able to. Examples of the shape of the non-aqueous secondary battery include, but are not limited to, a coin shape, a cylindrical shape, and a square shape.

  The present invention will be described in more detail with reference to the following examples and comparative examples, but the scope of the present invention is not limited by these examples.

<Example 1>
(I) Synthesis of LiCrO ₂ negative electrode active material In a solvent obtained by dissolving 16 g of glycolic acid in 160 ml of ultrapure water whose pH was controlled to 2 or less with nitric acid, 12.0045 g (0.0300 mol) was obtained. ) And 2.0685 g of lithium nitrate (0.0300 mol) were added (molar ratio of chromium (III) nitrate to lithium nitrate 1: 1). While the obtained solution was stirred at 80 ° C., the solvent was evaporated to form a gel-like precipitate. The gel-like precipitate was collected and pre-baked at 350 ° C. for 24 hours under an inert atmosphere (N ₂ gas). Next, main baking was performed at 950 ° C. for 48 hours under an inert atmosphere to obtain LiCrO ₂ . It was.
(Ii) Production of negative electrode layer (working electrode) LiCrO ₂ obtained as described above, polyvinylidene fluoride (manufactured by Kureha Co., Ltd.) and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) in a mass ratio of 85: 10: 5 And dispersed in N-methyl-2-pyrrolidone (Wako Pure Chemical Industries, Ltd.) to obtain a negative electrode mixture slurry. Next, the obtained slurry was applied onto a rolled copper foil (thickness: 15 μm) as a negative electrode current collector by a doctor blade method, punched into a circle having a diameter of 16 mm, dried and rolled to obtain a negative electrode layer.
(Iii) Production of positive electrode layer (counter electrode) Foil-like lithium metal (Honjo Metal Co., Ltd., thickness 25 μm) was smoothed by a roller, and then punched into a circle having a diameter of 19 mm to obtain a counter electrode.
(Iv) Preparation of Electrolytic Solution In a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 4: 3, a lithium salt LiPF ₆ as an electrolyte has a molar concentration of 1M. An electrolytic solution was obtained by dissolving in
(V) Separator A porous film made of polypropylene (PP) and having a thickness of 25 μm was used as a separator.
(Vi) Current collector A rolled copper foil having a thickness of 15 μm was used as a current collector.
(Vii) Production of Battery A 2032 type coin battery was produced by incorporating the above battery constituent members into a 2032 type (diameter 20 mm, thickness 3.2 mm) coin cell.

<Example 2>
Sodium nitrate instead of lithium nitrate (2.5497g, 0.0300 mole) were synthesized NaCrO ₂ using, except using the NaCrO ₂ as the negative electrode active material, produce a battery in the same manner as in Example 1 did.

<Comparative Example 1>
A battery was fabricated in the same manner as in Example 1 except that LiCoO ₂ was used as the negative electrode active material. LiCoO ₂ was obtained by a sol-gel method using lithium nitrate and cobalt nitrate.

Evaluation method <X-ray diffraction (XRD) method>
(1) XRD measurement before charge / discharge The negative electrode active materials used in Examples 1 and 2 and Comparative Example 1 (that is, LiCrO ₂ , NaCrO _2, and LiCoO ₂ , respectively), and used for the production of the negative electrode layer The powdered negative electrode active material before the analysis was analyzed by the XRD method to obtain an XRD spectrum. The X-ray diffractometer used for the analysis was an X-ray diffractometer Ultima IV manufactured by Rigaku Corporation. The 2θ measurement range was 10 ° to 50 °, the scan speed was 10 ° per minute, and the X-ray used The wavelength was CuΚα rays, and software PDXL manufactured by Rigaku Corporation was used for identification of diffraction peaks.
(2) XRD measurement after charge / discharge The coin batteries of Examples 1 and 2 and Comparative Example 1 after being subjected to 50 charge / discharge cycles in the following “electrochemical property evaluation” were disassembled in an argon box, and dimethyl After washing with carbonate (DMC), the negative electrode mixture layer is peeled off, a sample is taken and placed in an air non-exposed cell that shuts off the atmosphere, and the negative electrode mixture layer is placed under the above-mentioned “(1) Using the same measuring apparatus and conditions as described in “XRD measurement before charge and discharge”, analysis was performed by the XRD method to obtain an XRD spectrum. For identification of diffraction peaks, software PDXL manufactured by Rigaku Corporation was used.
(3) Evaluation result by XRD measurement The negative electrode active materials of Examples 1 and 2 and Comparative Example 1 before charging and discharging and after being subjected to 50 charging / discharging cycles (that is, LiCrO ₂ , NaCrO ₂ and LiCoO ₂ , respectively) XRD spectra of are shown in FIGS. When the XRD spectrum before charging / discharging is compared with the XRD spectrum after charging / discharging cycle, the peak corresponding to the sharp peak existing in the XRD spectrum before charging / discharging also exists in the XRD spectrum after charging / discharging cycle. When it is determined that no conversion reaction has occurred during the discharge cycle, and a peak corresponding to a sharp peak in the XRD spectrum before charge / discharge is not observed in the XRD spectrum after the charge / discharge cycle, conversion is performed during the charge / discharge cycle. It was determined that a reaction occurred. From the XRD spectra of FIGS. 1A to 1C, in Examples 1 and 2, a peak corresponding to a sharp peak existing in the XRD spectrum before charge / discharge is present in the XRD spectrum after 50 charge / discharge cycles. Therefore, it was determined that no conversion reaction occurred during the charge / discharge cycle. On the other hand, in Comparative Example 1, a peak corresponding to a sharp peak existing in the XRD spectrum before charge / discharge was not recognized in the XRD spectrum after 50 charge / discharge cycles, and a broad peak appeared. About the comparative example 1, it is thought that the conversion reaction advanced during the charging / discharging cycle until the structure irreversibly changed from crystalline to amorphous.

<Electrochemical characteristics evaluation>
(1) Evaluation method:
The coin battery produced in each example was charged and discharged at a temperature of 25 ° C., a cutoff potential of 0.01 to 1.5 V (vs. Li / Li ⁺ ), and 0.1 C (1 C is a current value that can be fully charged in 1 hour) The cycle was repeated 50 times, the discharge capacity of the first charge / discharge cycle was determined as the initial discharge capacity, and the discharge capacity after 50 charge / discharge cycles was determined. According to the following formula (1), the discharge capacity after 50 charge / discharge cycles with respect to the initial discharge capacity was determined as the capacity retention rate (%).
Capacity retention rate (%) = 100 x (discharge capacity after 50 cycles) / (initial discharge capacity) ... Formula (1)
(2) Results of electrochemical property evaluation The initial discharge capacities and capacity retention rates of Examples 1 and 2 and Comparative Example 1 are shown in Table 1 below.

  The initial discharge capacities of Examples 1 and 2 and Comparative Example 1 were the same value, but the capacity maintenance rate of Example 1 was 90.2%, and the capacity maintenance rate of Example 2 was 89.3%. The capacity retention rate of Comparative Example 1 was 12.3%. It can be seen that Examples 1 and 2 showed a considerably higher capacity retention rate than Comparative Example 1.

Claims (2)

A negative electrode mixture for a non-aqueous secondary battery comprising lithium-containing layered chromium oxide LiCrO ₂ or sodium-containing layered chromium oxide NaCrO ₂ as a negative electrode active material.   2. The negative electrode mixture according to claim 1, comprising: a positive electrode layer; a negative electrode layer; a separator disposed between the positive electrode layer and the negative electrode layer; and a nonaqueous electrolyte, wherein the negative electrode layer is disposed on the negative electrode current collector. A non-aqueous secondary battery having a layer comprising:

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