JP4693371B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a nonaqueous electrolyte secondary battery including a negative electrode, a positive electrode, and a nonaqueous electrolyte.

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

As such a nonaqueous electrolyte secondary battery, a lithium transition metal composite oxide such as LiCoO _{2 is} generally used for the positive electrode, and a lithium metal, a lithium alloy, or a carbon material capable of inserting and extracting lithium is used for the negative electrode. As a water electrolyte, a solution obtained by dissolving an electrolyte salt made of a lithium salt such as LiBF ₄ or LiPF ₆ in an organic solvent such as ethylene carbonate or diethyl carbonate is used.

  In recent years, such non-aqueous electrolyte secondary batteries have come to be used as power sources for various portable devices, and non-aqueous electrolyte secondary batteries with higher energy density are desired.

However, in the conventional non-aqueous electrolyte secondary battery, the lithium transition metal composite oxide such as LiCoO ₂ used for the positive electrode is large in weight and has a small number of reaction electrons, so that the capacity per unit weight is sufficiently increased. It was difficult.

  For this reason, it is indispensable to develop a positive electrode material capable of obtaining a high energy density with a high capacity. In recent years, studies have been conducted using sulfur alone as a positive electrode material. Sulfur alone has a large theoretical capacity of 1675 mAh / g, and is one of promising positive electrode materials for next-generation secondary batteries.

  In a positive electrode made of simple sulfur (S) (hereinafter referred to as a sulfur positive electrode), polysulfide generated during discharge is dissolved in the electrolytic solution. When such a dissolution-precipitation type sulfur positive electrode is used, there is a problem that the discharge product reacts at the counter electrode (negative electrode) and self-discharge occurs, and the charge / discharge capacity is limited to the solubility of the electrolytic solution (for example, patent document) 1).

  Moreover, when using the carbonate type electrolyte solution currently used for the lithium ion battery as an electrolyte solution of a sulfur positive electrode, since polysulfide does not melt | dissolve, charging / discharging cannot be performed (for example, refer patent document 2).

  For this reason, an ether solvent having a low flash point is used as the electrolyte solution for the sulfur positive electrode. In this case, there is a concern about safety.

  Therefore, J.H. L. Wang et al. Reported that charging and discharging are possible even in carbonate-based electrolytes by mixing sulfur alone and activated carbon and performing heat treatment or mechanical milling with a ball mill (for example, see Non-Patent Document 1). .

If the positive electrode obtained by the above method is used, the energy density of charging / discharging is lower than that of a normal sulfur positive electrode, but it is possible to charge / discharge single sulfur in a carbonate-based electrolyte.
JP 2003-123840 A JP 2003-242964 A JLWang et al., Electrochem Commun. 4 (2002) 499-502.

  However, since sulfur alone does not contain lithium, a carbon material or silicon material that does not contain lithium cannot be used as the negative electrode. In the negative electrode containing lithium currently developed, sufficient safety, life, energy density, etc. are not shown.

  The objective of this invention is providing the nonaqueous electrolyte secondary battery which can perform reversible charging / discharging and a high energy density is obtained.

The non-aqueous electrolyte secondary battery according to the present invention is capable of inserting and extracting lithium, a positive electrode containing a mixture of lithium sulfide and a conductive agent in a metastable amorphous state obtained by mechanical milling treatment of lithium sulfide and a conductive agent, and lithium A negative electrode containing a non-aqueous material and a non-aqueous electrolyte.

  In the non-aqueous electrolyte secondary battery according to the present invention, the positive electrode includes an active material composed of lithium sulfide in an amorphous state and a conductive agent, so that charging and discharging are performed with almost no sulfur dissolved in the non-aqueous electrolyte. And a high energy density can be obtained.

The positive electrode is obtained by subjecting lithium sulfide and a conductive agent to mechanical milling .

  Thereby, the adhesion between the lithium sulfide and the conductive agent is improved, and the conductivity of the entire positive electrode is improved. Thereby, a reversible charging / discharging reaction is performed and a high energy density is obtained.

The lithium sulfide may contain Li ₂ S. In this case, since Li ₂ S has a large lithium content, a higher energy density can be obtained.

  The conductive agent may include a carbon material. In this case, the conductivity of the whole positive electrode is further improved at low cost.

  The material capable of inserting and extracting lithium may include a silicon material or a carbon material.

  In this case, when the positive electrode contains lithium sulfide, a silicon material or a carbon material that does not contain lithium can be used. Thereby, safety is increased.

  According to the present invention, since the positive electrode includes an active material and a conductive agent made of amorphous lithium sulfide, it is possible to charge and discharge without almost dissolving sulfur in the non-aqueous electrolyte, High energy density can be obtained.

  Hereinafter, the nonaqueous electrolyte secondary battery according to the present embodiment will be described.

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

  The positive electrode includes an active material made of lithium sulfide, a conductive agent, and a binder.

The positive electrode active material contains at least one of Li ₂ S, Li ₂ S ₂ , Li ₂ S ₄ , Li ₂ S _{8 and the} like as lithium sulfide. In this case, it is preferable to use Li ₂ S having a large lithium content.

  Lithium sulfide has almost no conductivity, and reversibility of charge / discharge is extremely low when a solvent having low solubility in lithium sulfide is used as the electrolyte. Therefore, the influence which a electrically conductive agent has on the electrode characteristic of a positive electrode is large.

  Reversible charge / discharge characteristics cannot be obtained simply by mixing lithium sulfide and a conductive agent using a mortar or the like.

  Therefore, in the present embodiment, mechanical milling is performed on lithium sulfide and the conductive agent. Here, mechanical milling means that a mechanical reaction is easily caused by applying mechanical energy to a substance using a planetary ball mill or the like to obtain a metastable amorphous material that cannot be obtained by heat treatment. It is a process that can.

  In this embodiment, a metastable amorphous mixture of lithium sulfide and a conductive agent is obtained by mechanical milling treatment of lithium sulfide and a conductive agent. The chemical reaction caused by the mechanical energy input in this way is called a mechanochemical effect.

  By performing a mechanical milling treatment of lithium sulfide and a conductive agent, a positive electrode active material containing lithium and capable of being charged / discharged with almost no sulfur dissolved in a non-aqueous electrolyte solvent can be produced. Thereby, a negative electrode containing no lithium can be used.

  As the conductive agent, for example, a conductive carbon material, nitride, carbide, or the like can be used. Details of the carbon material, nitride, and carbide will be described later.

  As the binder, at least one selected from polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber, carboxymethyl cellulose, or the like is used. Can do.

  When the amount of the binder added is large, the ratio of the active material contained in the positive electrode becomes small, so that a high capacity cannot be obtained. Therefore, the addition amount of the binder is in the range of 0.1 to 30% by weight of the whole positive electrode, preferably in the range of 0.1 to 20% by weight, more preferably in the range of 0.1 to 10% by weight. To do.

  Examples of the conductive carbon material include ketjen black, acetylene black, and graphite.

  Examples of the conductive nitride include titanium nitride (TiN), tantalum nitride (TaN), niobium nitride (NbN), and tungsten nitride (WN).

  Examples of the conductive carbide include titanium carbide (TiC), tantalum carbide (TaC), niobium carbide (NbC), and tungsten carbide (WC).

  In addition, when the addition amount of the conductive agent is small, the conductivity of the positive electrode cannot be sufficiently improved. On the other hand, when the addition amount is too large, the ratio of the active material in the positive electrode is reduced to obtain a high capacity. Disappear. Therefore, the addition amount of the conductive agent is in the range of 0.1 to 30% by weight of the whole positive electrode, preferably in the range of 0.1 to 20% by weight, and more preferably in the range of 0.1 to 10% by weight. .

  In addition, as the current collector of the positive electrode, foamed aluminum, foamed nickel, or the like can be used in order to increase conductivity.

  As the negative electrode, for example, a carbon material such as graphite capable of inserting and extracting lithium, lithium metal, a lithium alloy, or the like is used.

  In order to obtain a non-aqueous electrolyte secondary battery having a high energy density, it is desirable to use silicon having a large capacity as the negative electrode. In particular, as proposed in JP 2001-266851 A and JP 2002-83594 A (or WO 01/029912), a silicon negative electrode using a roughened foil as a current collector, and a silicon negative electrode having a columnar structure Alternatively, it is preferable to use a silicon negative electrode having copper (Cu) diffused therein, or a silicon negative electrode having at least one of these characteristics.

  Examples of the nonaqueous solvent for the nonaqueous electrolyte in the present embodiment include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, and nitriles that are usually used as nonaqueous solvents for batteries. Or amides etc. are mentioned, At least 1 sort (s) selected from these can be used.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and the like, and trifluoropropylene carbonate or fluoroethyl carbonate in which a part or all of these hydrogen groups are fluorinated.

  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, and some or all of these hydrogens are fluorinated. Also mentioned.

  Examples of esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate or 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-cineole or crown ether.

  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, Pentylphenyl ether, methoxytoluene, benzylethyl 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 or the La ethylene glycol dimethyl and the like.

  Examples of nitriles include acetonitrile, and examples of amides include dimethylformamide.

As an example of the lithium salt added to the nonaqueous solvent of the nonaqueous electrolyte, a lithium salt generally used as an electrolyte in a conventional nonaqueous electrolyte secondary battery can be used. For example, LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiAsF ₆ , and at least one selected from lithium difluoro (oxalato) borate may be used. Or two or more of them may be used in combination.

  According to the nonaqueous electrolyte secondary battery according to the present embodiment, the mechanical milling process is performed on lithium sulfide and the conductive agent, so that the adhesion between the lithium sulfide and the conductive agent is improved, and the entire positive electrode is electrically conductive. Is improved. Thereby, a reversible charging / discharging reaction is performed and a high energy density is obtained.

  Hereinafter, the nonaqueous electrolyte secondary battery of an Example is demonstrated. In addition, the nonaqueous electrolyte secondary battery in this invention is not limited to what was shown in the following Example, It can implement by changing suitably in the range which does not change the summary.

Example 1
The positive electrode of Example 1 was produced as follows. After mixing lithium sulfide (Li ₂ S), which is an active material, in a proportion of 60% by weight of the entire positive electrode and ketjen black, which is a conductive agent, in a proportion of 30% by weight of the entire positive electrode, mechanical milling is performed with argon (using a planetary ball mill). Ar) Performed under atmosphere. The number of rotations was 300 rpm, and the processing time was 10 hours. Thereafter, polytetrafluoroethylene as a binder was mixed at a ratio of 10% by weight, and then press-molded to obtain an electrode. Further, the electrode was dried at 50 ° C. under vacuum to produce a positive electrode.

  Lithium metal cut into a predetermined size was used for the negative electrode. Moreover, lithium metal was cut into a predetermined size to prepare a reference electrode.

As a non-aqueous electrolyte, electrolyte salt of lithium hexafluorophosphate (LiPF ₆ ) is adjusted to a concentration of 1.0 mol / l in an electrolytic solution in which ethylene carbonate and diethylene carbonate are mixed at a volume ratio of 30:70. What was added to was used.

  As shown in FIG. 1, in an inert atmosphere, the positive electrode 1 is used as a working electrode, the lithium metal is used for a negative electrode 2 as a counter electrode, and a reference electrode 3. The test cell of Example 1 was produced by injecting the nonaqueous electrolyte 5. A separator 4 is interposed between the positive electrode 1 and the negative electrode 2.

In the test cell of Example 1, after charging at a charging current 0.5 mA / cm ² until the charge cutoff potential 3.0V (vs.Li/Li ^+), the discharge cutoff potential at a discharge current 0.5 mA / cm ² The battery was discharged to 1.5 V (vs. Li ^/ Li ⁺ ), and the charge / discharge characteristics were examined. The measurement results are shown in FIG.

(Comparative Example 1)
The positive electrode of Comparative Example 1 was produced as follows. Percentage of sulfur alone (S) as an active material of 60% by weight of the whole positive electrode, ketjen black as a conductive agent of 30% by weight of the whole positive electrode, and polytetrafluoroethylene as a binder of 10% by weight of the whole positive electrode Then, press molding was performed. Then, it was dried at 50 ° C. under vacuum.

Then, after preparing the to test cell in the same manner as in Example 1, except that the final charging potential was 4.2V (vs.Li/Li ^+), was measured charge-discharge characteristics in the same manner as in Example 1 .

In the test cell of Comparative Example 2, after charging at a charging current 0.5 mA / cm ² until the charge cutoff potential 4.2V (vs.Li/Li ^+), the discharge cutoff potential at a discharge current 0.5 mA / cm ² The battery was discharged to 1.5 V (vs. Li ^/ Li ⁺ ), and the charge / discharge characteristics were examined. The measurement results are shown in FIG.

(Comparative Example 2)
The positive electrode of Comparative Example 2 was produced as follows. Lithium sulfide as an active material was mixed at 60% by weight of the whole positive electrode, ketjen black as a conductive agent was mixed at 30% by weight of the whole positive electrode, and polytetrafluoroethylene as a binder was mixed at a ratio of 10% by weight of the whole positive electrode. Thereafter, a test cell was produced in the same manner as in Comparative Example 2 above. Thereafter, the charge / discharge characteristics were measured in the same manner as in Comparative Example 1 except that the discharge end potential was set to 1.0 V (vs. Li ^/ Li ⁺ ). The measurement results are shown in FIG.

(Evaluation 1)
Initial discharge capacity density and average discharge potential of nonaqueous electrolyte secondary battery of Example 1, potential difference of charge / discharge of nonaqueous electrolyte secondary battery of Comparative Example 1, and initial charge capacity of nonaqueous electrolyte secondary battery of Comparative Example 2 The density is shown in Table 1.

In Example 1, by charging to 3.0 V (vs. Li ^/ Li ⁺ ), the initial discharge capacity density per gram of active material is 218 mAh / g, and the initial charge capacity density per gram of active material. Was 290 mAh / g, and the average discharge potential was 1.79 V (vs. Li / Li ⁺ ). The charge / discharge capacity density is obtained by dividing the flowing current by the weight of the active material.

In Comparative Example 1, the discharge reaction occurred in the vicinity of 2.4 V (vs. Li ^/ Li ⁺ ), and the charge reaction occurred in the vicinity of 4.0 V (vs. Li ^/ Li ⁺ )). As a result, the charge / discharge potential difference was about 1.6 V, and the energy loss increased.

  In Comparative Example 2, the charge capacity density at the initial stage was only about 15 mAh / g, and the battery could hardly be charged. It was found that the charge / discharge reaction at the electrode was not performed well only by mixing lithium sulfide and ketjen black.

  From the above results, it was found that reversible charging / discharging can be performed without causing energy loss by performing mechanical milling treatment of lithium sulfide and ketjen black.

(Example 2)
A test cell was produced using a positive electrode produced in the same manner as in Example 1 and a negative electrode made of carbon. A reference electrode was not used.

The charge / discharge characteristics were measured in the same manner as in Example 1 except that the charge end potential was 3.0 V (vs. Li ^/ Li ⁺ ) and the discharge end potential was 0.5 V (vs. Li ^/ Li ⁺ ). It was. The measurement results are shown in FIGS.

(Evaluation 2)
In Example 2, the discharge capacity density per gram of active material in the initial stage was 218 mAh / g, and the average discharge potential was 1.55 V (vs. Li / Li ⁺ ). Further, the charge / discharge efficiency after 10 cycles was about 80%.

  Here, the charge / discharge efficiency (%) indicates a ratio between the discharge capacity density (mAh / g) and the charge capacity density (mAh / g) in each cycle.

  From the above results, it was found that the positive electrode produced by mechanical milling treatment of lithium sulfide and ketjen black can be charged and discharged satisfactorily even when a negative electrode not containing lithium is used.

(Example 3)
In Example 3, acetylene black was used instead of ketjen black as a conductive agent, and a test cell was prepared in the same manner as in Example 1 except that. The charge / discharge characteristics were measured in the same manner as in Example 1.

Example 4
In Example 4, a test cell was produced in the same manner as in Example 1 except that graphite was used instead of ketjen black as the conductive agent. The charge / discharge characteristics were measured in the same manner as in Example 1.

(Evaluation 3)
In Example 3, the discharge capacity density per gram of active material in the initial stage was 110 mAh / g, and the average discharge potential was 1.86 V (vs. Li / Li ⁺ ).

In Example 4, the discharge capacity density per gram of active material in the initial stage was 101 mAh / g, and the average discharge potential was 1.88 V (vs. Li / Li ⁺ ).

  Table 2 shows the measurement results of the initial discharge capacity density and average discharge potential in Examples 1, 3, and 4.

The specific surface areas of ketjen black, acetylene black and graphite are about 1000 m ² / g, 100 m ² / g and 10 m ² / g, respectively. It was found that the larger the specific surface area of the conductive agent, the higher the discharge capacity density in the initial stage, but the lower the average discharge potential.

(Example 5)
In Example 5, a positive electrode was produced as follows. After mixing lithium sulfide as an active material in a proportion of 60% by weight of the whole positive electrode and ketjen black as a conductive agent in a proportion of 30% by weight of the whole positive electrode, mechanical milling treatment was performed in an argon (Ar) atmosphere using a planetary ball mill. went. The number of rotations was 300 rpm, and the processing time was 10 hours.

  And the measurement by the X-ray-diffraction apparatus (XRD) was performed about the produced positive electrode. The measurement results are shown in FIG.

(Comparative Example 3)
In Comparative Example 3, a positive electrode was produced as follows. Measurement was performed with an X-ray diffractometer on a positive electrode prepared by mixing lithium sulfide as an active material in a proportion of 60% by weight of the whole positive electrode and ketjen black as a conductive agent in a proportion of 30% by weight of the whole positive electrode. The measurement results are shown in FIG. 7 similarly to the measurement results of Example 5.

(Evaluation 4)
As shown in FIG. 7, the lithium sulfide peak of the positive electrode obtained by the mechanical milling process appeared at a position corresponding to the lithium sulfide peak of the positive electrode obtained by mixing.

  The peak intensity of lithium sulfide of the positive electrode obtained by mechanical milling was reduced to 1/3 of the peak intensity of lithium sulfide of the positive electrode obtained by mixing. As a result, the mechanical milling process changes the lithium sulfide into an amorphous structure, or the lithium sulfide reacts with the conductive agent ketjen black to form a new amorphous material. Can be considered.

  The nonaqueous electrolyte secondary battery according to the present invention can be used as various power sources such as a portable power source and an automobile power source.

It is the schematic of the test cell produced in Example 1,2,3,4,5 and Comparative Example 1,2,3. FIG. 3 is a diagram showing charge / discharge characteristics of a test cell of Example 1. It is the figure which showed the charging / discharging characteristic of the test cell of the comparative example 1. It is the figure which showed the charging / discharging characteristic of the test cell of the comparative example 2. It is the figure which showed the charging / discharging characteristic of the test cell of Example 2. FIG. It is the figure which showed the charging / discharging cycle characteristic of the test cell of Example 2. It is the figure which showed the measurement result by the X-ray-diffraction apparatus of the positive electrode of Example 5 and Comparative Example 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Reference electrode 4 Separator 5 Nonaqueous electrolyte 10 Cell container

Claims (4)

A positive electrode comprising a mixture of lithium sulfide and a conductive agent in a metastable amorphous state obtained by mechanical milling of lithium sulfide and a conductive agent;
A negative electrode containing a material capable of inserting and extracting lithium;
A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte. The non-aqueous electrolyte secondary battery according to claim 1 , wherein the lithium sulfide contains Li ₂ S. The non-aqueous electrolyte secondary battery according to claim 1 , wherein the conductive agent includes a carbon material. The non-aqueous electrolyte secondary battery according to claim 1 , wherein the material capable of inserting and extracting lithium includes a silicon material or a carbon material.

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