JP2011146359A - Alkali metal sulfur secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve charge/discharge cycle characteristics of an alkali metal sulfur secondary battery. <P>SOLUTION: The alkali metal sulfur secondary battery includes: a positive electrode containing a porous carbon material doped with an nitrogen atom, and sulfur; a negative electrode which can store and emit an alkali metal ion; and a nonaqueous electrolyte interposed between the positive electrode and the negative electrode. The nonaqueous electrolyte is made by dissolving lithium salt in an etheric organic solvent containing at least two or more oxygen in a molecule, and a ratio r of total number of oxygen atoms in the etheric organic solvent with respect to the number of lithium cations in the lithium salt satisfies 4≤r≤5. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an alkali metal sulfur secondary battery.

  Conventionally, a lithium-sulfur secondary battery using a simple sulfur as a positive electrode active material is known. The theoretical capacity density of lithium is about 3862 mAh / g, and the theoretical capacity density of sulfur is about 1675 mAh / g. Therefore, by using sulfur as the positive electrode active material and lithium as the negative electrode active material, the energy density is very high. A secondary battery can be provided.

In a normal lithium-sulfur secondary battery, sulfide ions (S ²⁻ ) in the electrolyte are oxidized to polysulfide ions (S _y ²⁻ ) during the charging reaction process in the positive electrode. The majority of the resulting polysulfide ions (S _y ^2-), by being further oxidized to precipitate as sulfur on the surface of the positive electrode. As shown in the following formula (1), the sulfur deposited on the positive electrode reacts with the polysulfide ions (S _(y-1) ²⁻ ) remaining in the vicinity of the positive electrode, thereby again producing polysulfide ions (S _y ^2- ) Elution into the electrolyte solution. The polysulfide ions eluted in the electrolyte react with the negative electrode and are reduced, whereby a discharge phenomenon (self-discharge phenomenon) occurs in the negative electrode. As a result, the charge / discharge cycle characteristics deteriorate.
S _(y-1) ^2- + S-> S _y ^2- ... Formula (1)

  For this reason, several techniques for suppressing the self-discharge phenomenon have been reported. For example, in the lithium-sulfur secondary battery described in Patent Document 1, a positive electrode containing sulfur coated with a polymer electrolyte, lithium as a negative electrode, a polymer electrolyte layer between the positive electrode and the separator or between the negative electrode and the separator, Proposals have been made in which the lithium polysulfide produced at the positive electrode during the battery reaction is diffused to the negative electrode to suppress self-discharge and to improve the charge / discharge cycle characteristics.

JP 2003-242964 A

  However, in the lithium-sulfur secondary battery described in Patent Document 1, the lithium polysulfide is suppressed from diffusing to the negative electrode using a polymer electrolyte, but this is not sufficient, and charge / discharge cycle characteristics (capacity) It has been desired to further suppress a decrease in the maintenance rate and charge / discharge efficiency.

  This invention is made | formed in view of such a subject, and it aims at providing the alkali metal sulfur secondary battery which can improve charging / discharging cycling characteristics more.

  In order to achieve the above-described object, the present inventors include a porous carbon material doped with nitrogen atoms in a positive electrode in a lithium-sulfur secondary battery, and an ether-based organic material for the number of lithium cations in a lithium salt. When a nonaqueous electrolytic solution in which the ratio r of the total number of oxygen atoms in the solvent satisfies 4 ≦ r ≦ 5 was used, the charge / discharge cycle characteristics were found to be improved as compared with the conventional one, and the present invention was completed.

That is, the alkali metal sulfur secondary battery of the present invention is
A positive electrode comprising a porous carbon material doped with nitrogen atoms and sulfur;
A negative electrode capable of occluding and releasing alkali metal ions;
The ether system with respect to the number of alkali metal cations of the alkali metal salt, wherein an alkali metal salt is dissolved in an ether organic solvent that is interposed between the positive electrode and the negative electrode and contains at least two oxygen atoms in the molecule. A non-aqueous electrolyte in which the ratio r of the total number of oxygen atoms in the organic solvent satisfies 4 ≦ r ≦ 5;
It is equipped with.

  In the alkali metal sulfur secondary battery of the present invention, the charge / discharge cycle characteristics are improved as compared with the prior art. The reason why such an effect is obtained is not clear, but is presumed as follows. That is, in the non-aqueous electrolyte, ether-based organic solvent molecules are coordinated (solvated) to the lithium cation. It is known that solvation to a lithium cation has a four-coordinate structure, and an unshared electron pair of oxygen contained in an ether-based organic solvent molecule is involved in the solvation. If the ratio r described above satisfies 4 ≦ r ≦ 5, it is considered that oxygen in almost all ether-based organic solvent molecules present in the non-aqueous electrolyte is coordinated to the lithium cation. This is a kind of bound state, and there are almost no unbound ether-based organic solvent molecules coordinated to the lithium cation, and the polysulfide ions generated during the discharge reaction are less likely to elute. It is done. In addition, the positive electrode contains a carbon-doped carbon doped with nitrogen, and the surface polarization formed on the carbon surface of the carbon-based porous material improves the adsorptivity of polysulfide ions generated during the discharge reaction. It is done. These effects suppress the elution of polysulfide ions generated during charging / discharging into the electrolyte, which is a problem in alkali metal sulfur batteries, and improve the capacity maintenance characteristics during charging / discharging cycles, and also improve the polysulfide ions. It is thought that the shuttle effect (a phenomenon in which the charge capacity becomes larger than the discharge capacity) due to elution is suppressed, and the charge / discharge efficiency is improved.

It is explanatory drawing of the evaluation cell.

  The alkali metal sulfur secondary battery of the present invention includes a positive electrode containing a porous carbon material doped with nitrogen atoms and sulfur, a negative electrode capable of occluding and releasing alkali metal ions, and a positive electrode and a negative electrode. The alkali metal salt is dissolved in an ether organic solvent containing at least two oxygen atoms in the molecule, and the ratio r of the total number of oxygen atoms in the ether organic solvent to the number of alkali metal cations in the alkali metal salt is 4 ≦ a non-aqueous electrolyte satisfying r ≦ 5. Examples of the alkali metal include lithium, sodium, and potassium, among which lithium having a high theoretical capacity density is preferable.

  In the alkali metal sulfur battery of the present invention, the negative electrode is capable of occluding and releasing alkali metal ions. Examples of the case where the alkali metal is lithium include, for example, metal lithium and lithium alloy, metal oxide, metal sulfide, and a carbonaceous material capable of occluding and releasing lithium ions. Examples of the lithium alloy include alloys of lithium with aluminum, silicon, tin, magnesium, indium, calcium, and the like. Examples of the metal oxide include tin oxide, silicon oxide, lithium titanium oxide, niobium oxide, and tungsten oxide. Examples of the metal sulfide include tin sulfide and titanium sulfide. Examples of the carbonaceous material capable of inserting and extracting lithium ions include graphite, coke, mesophase pitch-based carbon fiber, spherical carbon, and resin-fired carbon.

  In the alkali metal sulfur secondary battery of the present invention, the positive electrode is not particularly limited as long as it contains sulfur as a positive electrode active material and contains a porous carbon material doped with nitrogen atoms in the positive electrode material. For this positive electrode, for example, a positive electrode active material, a conductive material, and a binder are mixed, and an appropriate solvent is added to form a paste-like positive electrode material, which is applied to the surface of the current collector and dried. You may compress and form in order to raise an electrode density. Here, the conductive material is not particularly limited as long as it is a conductive material. For example, carbon blacks such as ketjen black, acetylene black, channel black, furnace black, lamp black, and thermal black may be used. Natural graphite such as flake graphite, artificial graphite, graphite such as expanded graphite, conductive fibers such as carbon fiber and metal fiber, or metal powder such as copper, silver, nickel, and aluminum Alternatively, an organic conductive material such as a polyphenylene derivative may be used. These may be used alone or in combination. Although it does not specifically limit as a binder, A thermoplastic resin, a thermosetting resin, etc. are mentioned. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin) , Polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrif Examples include olefin copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, and ethylene-acrylic acid copolymer. . These materials may be used alone or in combination. As the current collector, a metal plate such as stainless steel, aluminum, copper, nickel, or a metal mesh can be used.

In the alkali metal sulfur secondary battery of the present invention, the porous carbon material contained in the positive electrode is doped with nitrogen. If it carries out like this, polysulfide ion will become easy to be adsorb | sucked by a porous carbon material, and it can improve charging / discharging cycling characteristics. This porous carbon material is preferably doped with nitrogen in an atomic ratio N / C between nitrogen atoms and carbon atoms of 0.05 or more, and N / C is more preferably 0.3 or less. N / C is more preferably 0.15 or more and 0.25 or less. If N / C is 0.05 or more, higher charge / discharge cycle characteristics can be obtained, and if N / C is 0.3 or less, it is easy to produce a porous carbon material having a high specific surface area. Further, the porous carbon material doped with nitrogen preferably has a BET specific surface area calculated from nitrogen adsorption measurement at a liquid nitrogen temperature (77 K) of 20 m ² / g or more, and 300 m ² / g or more. Is more preferable, and it is more preferable that it is 1200 m < ² > / g or less. If the BET specific surface area is 300 m ² / g or more, the discharge capacity can be further increased, and if the BET specific surface area is 1200 m ² / g or less, it is easy to produce a porous carbon material. The porous carbon material doped with nitrogen is preferably contained in the positive electrode in an amount of 20% by weight or more, more preferably 40% by weight or more. Considering the amount of sulfur contained in the positive electrode, it is desirable not to exceed 50% by weight. In addition, the porous carbon material doped with nitrogen is preferably prepared using a nitrogen-containing heterocyclic compound having a carboxy group or a hydroxy group as a raw material. In this way, a porous carbon material doped with nitrogen can be produced relatively easily.

In the alkali metal sulfur secondary battery of the present invention, the non-aqueous electrolyte solution interposed between the positive electrode and the negative electrode has an alkali metal salt dissolved in an ether-based organic solvent containing at least two oxygen atoms in the molecule, The ratio r of the total number of oxygen atoms of the ether-based organic solvent to the number of alkali metal cations of the alkali metal salt satisfies 4 ≦ r ≦ 5. As the electrolyte, for example, a known supporting salt such as LiPF ₆ , LiClO ₄ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N can be used. The ether-based organic solvent is not particularly limited as long as it contains at least two or more oxygen atoms in the molecule. For example, 1,2-dimethoxyethane (monoglyme), diglyme, tetraglyme, etc. Use cyclic ethers such as chain ethers, 1,3-dioxolane, 4-methyl-1,3-dioxolane, crown ethers (for example, 12-crown-4, 15-crown-5, etc.) or a mixed solvent thereof. be able to. When the ratio r is less than 4, the initial discharge capacity decreases, which is not preferable. On the other hand, when the ratio r exceeds 5, the capacity retention rate after the charge / discharge cycle is decreased, which is not preferable.

  The alkali metal sulfur secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it is a composition that can withstand the use range of the alkali metal sulfur secondary battery. A microporous film is mentioned. These may be used alone or in combination.

  The shape of the alkali metal sulfur secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing used for an electric vehicle etc.

  The nitrogen-doped porous carbon material used in the present invention may be produced, for example, by the following method. That is, a fired product is produced by firing a mixture of a nitrogen-containing heterocyclic compound having a carboxy group or a hydroxy group and an alkaline earth metal ion in an inert atmosphere, and the alkaline earth metal ion in the fired product is produced. You may make it obtain a porous carbon material by wash | cleaning a baked product with the washing | cleaning liquid which can melt | dissolve the component derived from this, and removing this component.

  Here, the nitrogen-containing heterocyclic compound may have only one carboxy group or hydroxy group, or may have two or more. When it has two or more, it may have only one of a carboxy group and a hydroxy group, or may have both. In addition, a carboxy group is more preferable than a hydroxy group. The nitrogen-containing heterocyclic compound includes a heterocyclic compound containing one nitrogen such as pyrrole and pyridine; a heterocyclic compound containing two nitrogen such as pyrazole, imidazole, pyrazine, pyrimidine and pyridazine. A heterocyclic compound containing three nitrogen atoms such as 1,2,3-triazine, 1,2,4-triazine, 1,3,5-triazine, etc., among which pyridine is preferable. That is, preferred as the nitrogen-containing heterocyclic compound is pyridine having a carboxy group, for example, pyridine-3-carboxylic acid (nicotinic acid), pyridine-2-carboxylic acid, pyridine-4-carboxylic acid, pyridine-2. , 3-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, pyridine 2,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid, pyridine-3,4-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, Examples thereof include pyridine-2,4,5-tricarboxylic acid. Examples of alkaline earth metal ions include magnesium ions, calcium ions, strontium ions, barium ions and the like. In order to obtain a mixture of a nitrogen-containing heterocyclic compound and an alkaline earth metal ion, for example, the two may be obtained by mixing them in an aqueous solution and then evaporating and drying the water. As for the usage-amount of both, only the stoichiometric amount based on the neutralization reaction formula may be used, and you may use so that one may become excess with respect to the other. The mixture thus obtained is fired under an inert atmosphere, and examples of the inert atmosphere include a nitrogen atmosphere and an argon atmosphere. The firing temperature is, for example, 400 to 1000 ° C., although it depends on the mixture to be fired.

In the step of removing the component derived from alkaline earth metal ions, the cleaning liquid is not particularly limited as long as the component derived from alkaline earth metal ions can be dissolved. For example, alkaline earth metal ions are calcium ions. In this case, it is preferable to use water or an acidic aqueous solution. By performing such cleaning, a portion where a component derived from alkaline earth metal ions in the fired product was present becomes a cavity, and thus a porous carbon material is obtained. The porous carbon material thus obtained has a specific surface area of 20 m ² / g or more, or 300 m ² / g or more, and the ratio of nitrogen atoms to carbon atoms (N / C) is about 0.02 to 0.3. Often becomes. Incidentally, the specific surface area of the fired product before washing is 10 m ² / g or less. In consideration of such cleaning efficiency (that is, removal efficiency of components derived from alkaline earth metal), it is preferable to use a cleaning solution having high solubility of such components. In this respect, it is preferable that the alkaline earth metal ion is calcium ion because it has higher solubility in water or acidic aqueous solution than magnesium ion or barium ion.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the lithium sulfur secondary battery as an alkali metal sulfur secondary battery of this invention concretely is demonstrated.

[General procedure]
(Synthesis of nitrogen-doped porous carbon material)
A neutralized salt of nicotinic acid (pyridine-3-carboxylic acid: Tokyo Kasei) and calcium hydroxide powder (Wako Pure Chemical Industries) was prepared by the evaporation to dryness method. A reagent corresponding to the neutralization equivalent was dispersed in water, and then heated in a hot water bath at 80 ° C. to obtain a transparent solution. The transparent solution was evaporated to dryness to obtain a neutralized salt of calcium nicotinate. This neutralized salt was carbonized by heating it in an inert atmosphere (in a nitrogen stream) in a quartz reaction tube. The flow rate of nitrogen was 1 L / min, and the carbonization temperature was 500 ° C. After reaching the carbonization temperature, the temperature was held for 3 hours. The sample after carbonization was washed with water, and an excessive amount of acetic acid (Wako Pure Chemical Industries, Ltd.) was added thereto to dissolve the calcium salt produced along with the carbonization. After filtering off, washing with water and drying were performed, and the mixture was ground in an agate mortar to obtain a porous carbon material doped with nitrogen atoms. When this porous carbon material was used for nitrogen adsorption measurement at liquid nitrogen temperature, the BET specific surface area was 407 m ² / g. Further, N / C (amount ratio of nitrogen atom to carbon atom) of this porous carbon material was calculated to be 0.22 from elemental analysis. In addition, the elemental analysis of the porous carbon material was determined by a combustion method using a fully automatic elemental analyzer (manufactured by Elemental Co., VarioEL). From the above results, it was shown that the synthesized porous carbon material is a nitrogen-doped porous carbon material having a large specific surface area and having a large amount of nitrogen atoms doped in the carbon skeleton. .

(Preparation of positive electrode)
50 parts by weight of sulfur, 43 parts by weight of the obtained porous carbon material, 2 parts by weight of a conductive additive (Ketjen Black: Lion ECP600), and 5 parts by weight of a binder (polytetrafluoroethylene powder) In a proportion of parts, ethanol was used as a dispersion medium and kneaded in an agate mortar to form a sheet-like positive electrode.

(Preparation of non-aqueous electrolyte)
As the non-aqueous electrolyte, electrolytes A to H shown in Table 1 were prepared. Each of the electrolytes A to H includes a mixed solvent in which 1,3-dioxolane and dimethoxyethane are mixed at a molar ratio of 1: 1, and lithium bis (trifluoromethylsulfone) imide (LiN (CF ₃ SO ₂ ) ₂ ) and ₂ ) were mixed so as to have a molar ratio shown in Table 1. The average molecular weight of the mixed solvent was 82. This average molecular weight was determined as the sum of 74 (= 1,3-dioxolane molecular weight) × 0.5 and 90 (= dimethoxyethane molecular weight) × 0.5. In addition, a value obtained by converting the molar ratio into a ratio r of the total number of oxygen atoms contained in the mixed solvent with respect to the number of lithium cations and the value of the molar concentration of the lithium salt are also shown.

(Production of evaluation cell)
An evaluation cell 10 shown in FIG. 1 was produced. FIG. 1 is an explanatory diagram of the evaluation cell 10, wherein the upper stage is a cross-sectional view before the evaluation cell 10 is assembled, and the lower stage is a cross-sectional view after the evaluation cell 10 is assembled. In assembling the evaluation cell 10, first, the negative electrode 16, the separator 18, and the positive electrode 20 (composite material) described above are formed in the cavity 14 provided at the center of the upper surface of the stainless steel cylindrical base 12 having a thread groove on the outer peripheral surface. 5 mg) as a weight. Here, a lithium metal foil having a diameter of 16 mm and a thickness of 0.4 mm was used as the negative electrode 16, and a polyethylene separator (Tonen Chemical Co., Ltd., microporous polyethylene film) was used as the separator 18. After injecting the non-aqueous electrolyte into the cavity 14, a polypropylene insulating ring 29 is inserted, and then a stainless steel cylinder 24 fixed in a liquid-tight manner in the hole of the polypropylene ring 22 is disposed on the positive electrode 20. A stainless cup-shaped lid 26 was screwed into the cylindrical base 12. Further, an insulating resin ring 27 made of PTFE is disposed on the cylinder 24, and a pressure bolt 25 having a through hole 25a in a screw groove carved in an inner peripheral surface of an opening 26a provided at the center of the upper surface of the lid 26. The negative electrode 16, the separator 18, and the positive electrode 20 were pressed and adhered. All the evaluation cells 10 were assembled in an argon glove box. In this way, the evaluation cell 10 was assembled. In addition, since the diameter of the opening 26a provided in the upper surface center of the lid | cover 26 is larger than the diameter of the cylinder 24, the lid | cover 26 and the cylinder 24 are a non-contact state. In addition, since the packing 28 is disposed around the cavity 14, the electrolyte injected into the cavity 14 does not leak to the outside. In this evaluation cell 10, the lid 26, the pressure bolt 25, and the cylindrical base 12 are integrated with the negative electrode 16, so that the whole becomes the negative electrode side, and the column 24 is integrated with the positive electrode 20 and insulated from the negative electrode 16. Therefore, it becomes the positive electrode side. The evaluation cell 10 assembled in this way was used to evaluate charge / discharge characteristics.

[Example 1]
Using the electrolytic solution A as the non-aqueous electrolytic solution, the evaluation cell 10 was produced according to the general procedure described above.

[Example 2]
Using the electrolytic solution B as the nonaqueous electrolytic solution, the evaluation cell 10 was produced according to the general procedure described above.

[Example 3]
Using the electrolytic solution G as the nonaqueous electrolytic solution, an evaluation cell 10 was produced according to the general procedure described above.

[Comparative Example 1]
Using the electrolytic solution C as the nonaqueous electrolytic solution, the evaluation cell 10 was produced according to the general procedure described above.

[Comparative Example 2]
Using the electrolytic solution D as the non-aqueous electrolytic solution, the evaluation cell 10 was produced according to the general procedure described above.

[Comparative Example 3]
Using the electrolytic solution E as the non-aqueous electrolytic solution, the evaluation cell 10 was produced according to the general procedure described above.

[Comparative Example 4]
The evaluation cell 10 was produced according to the general procedure described above except that the electrolytic solution B was used as the nonaqueous electrolytic solution and the positive electrode was produced. The positive electrode was produced as follows. That is, 50 parts by weight of sulfur, 45 parts by weight of ketjen black, and 5 parts by weight of binder (polytetrafluoroethylene powder) were kneaded in an agate mortar using ethanol as a dispersion medium, and a sheet-like positive electrode Was made.

[Comparative Example 5]
An evaluation cell 10 was produced in the same procedure as in Comparative Example 4 except that the electrolytic solution D was used as the nonaqueous electrolytic solution.

[Comparative Example 6]
Using the electrolytic solution F as the non-aqueous electrolytic solution, the evaluation cell 10 was produced according to the general procedure described above.

[Comparative Example 7]
Using the electrolytic solution H as the non-aqueous electrolytic solution, the evaluation cell 10 was produced according to the general procedure described above.

(Charge / discharge test)
The charge and discharge characteristics of the evaluation cells of Examples 1 to 3 and Comparative Examples 1 to 7 were evaluated by a constant current method using HJ1001SM8A manufactured by Hokuto Denko. The charge / discharge conditions were a discharge current of 100 mA / g (per weight of the positive electrode mixture), and discharge was performed at a constant current until the voltage between the terminals of the evaluation cell became 1.5V. Immediately after the terminal voltage reached the final discharge voltage of 1.5 V, charging was performed with a charging current of 100 mA / g (per weight of the positive electrode mixture) until the terminal voltage reached 2.8 V. This charge / discharge cycle was repeated 50 times. From the discharge capacity (mAh / g) of the first cycle and the discharge capacity (mAh / g) of the nth cycle, the capacity retention rate (%) defined by the following equation was calculated. Further, the charge / discharge efficiency (%) defined by the following equation was calculated from the discharge capacity (mAh / g) of the nth cycle and the charge capacity (mAh / g) of the (n-1) th cycle.
Capacity retention rate (%) = discharge capacity of nth cycle / discharge capacity of 1st cycle x 100
Charge / discharge efficiency (%) = discharge capacity of the nth cycle / charge capacity of the (n-1) th cycle x 100

(Experimental result)
The experimental results of Examples 1 to 3 and Comparative Examples 1 to 7 are shown in Table 2. In Examples 1 to 3 using the electrolytic solutions A, B, and G, 80% or more of the initial (first cycle) discharge capacity was maintained even after the 50th cycle charge / discharge. This is because the ratio r of the electrolytes A, B, and G is 4 to 5, and the amount of ether organic solvent not coordinated to the lithium cation is small, so that polysulfide ions generated by the discharge reaction are eluted. This is thought to be due to suppression. In addition, it is thought that elution of polysulfide ions was further suppressed by adsorbing slightly dissolved polysulfide ions to the carbon porous material by doping nitrogen into the skeleton of the carbon porous material. .

  On the other hand, in comparison with Examples 1 to 3, the amount of the ether organic solvent is large, and Comparative Example 1 using the electrolytic solution C having a ratio r of 6 and comparison using the electrolytic solution H having a ratio r of 5.4 In Example 7, a decrease in capacity retention rate was observed. This is considered to be because the elution of polysulfide ions was suppressed by nitrogen doping, but the solubility of polysulfide ions was increased by increasing the amount of the ether organic solvent. In Comparative Example 2 in which the amount of the ether organic solvent was larger and the electrolytic solution D having a ratio r of 16 was used, the lithium salt concentration was about 1 mol / L. Decrease was observed.

  Conversely, Comparative Example 3 using an electrolytic solution E having a ratio r of 3 and a comparative example using an electrolytic solution F having a ratio r of 3.6 as compared with Examples 1 to 3 having a smaller amount of ether organic solvent. In No. 6, the initial discharge capacity itself was significantly reduced. This is because the ionic conductivity decreases as the lithium salt concentration of the electrolyte solution increases, and the capacity rapidly decreases due to the difficulty in following the charge / discharge reaction in charge / discharge operations with a high current amount. it is conceivable that.

  Comparative Example 4 is the same as Example 2 in that the electrolytic solution B is used, but the carbon material used for the positive electrode is an undoped nitrogen material. In this case, although the initial discharge capacity is large and the cycle capacity maintenance rate is relatively good, the capacity maintenance rate and discharge after repeated cycles are compared to Example 2 using a carbon material doped with nitrogen as the positive electrode. Both capacities tended to be inferior.

  In Comparative Example 5, as the carbon material used for the positive electrode, an undoped nitrogen material and an electrolytic solution D having a ratio r of 16 were used. In this case, the capacity retention rate had already decreased to 48% after the 20th cycle charge / discharge, and the effect of capacity decrease due to elution of polysulfide ions was clearly seen. This is also a result suggesting the usefulness of nitrogen doping to carbon.

  In each of the examples and comparative examples, the charge / discharge efficiency in the 10th cycle was good in Examples 1 to 3 and Comparative Examples 4 and 7 using the electrolytes A, B, and G. . This shows that the increase in the apparent charge capacity due to the shuttle effect due to the elution of polysulfide ions was suppressed, and in Examples 1 to 3 in which nitrogen doping was performed, the suppression of the shuttle effect was more excellent. It is suggested that

  From the above results, the charge / discharge cycle characteristics in the lithium-sulfur secondary battery are greatly improved by the synergistic effect of the optimization of the numerical range of the ratio r of the non-aqueous electrolyte and the nitrogen doping of the carbon material used for the positive electrode. I found out that

In the general procedure described above (synthesis of nitrogen-doped porous carbon material), when the carbonization temperature is 600 ° C., the BET specific surface area is 344 m ² / g, and N / C by elemental analysis is 0.15. A porous carbon material was obtained. When the carbonization temperature was 800 ° C., a nitrogen-doped porous carbon material having a BET specific surface area of 386 m ² / g and N / C of 0.06 by elemental analysis was obtained. When these carbon materials were used and an evaluation cell was prepared using the electrolyte D according to the general procedure described above, the capacity retention ratios of the 10th cycle were 84% and 70%, respectively. When the electrolytic solution D is changed to the electrolytic solutions A, B, and G for these evaluation cells, it is easily predicted that the capacity retention rate is further improved as in Examples 1 to 3.

  10 Evaluation Cell, 12 Cylindrical Base, 14 Cavity, 16 Negative Electrode, 18 Separator, 20 Positive Electrode, 22 Ring, 24 Column, 25 Pressure Bolt, 25a Through Hole, 26 Lid, 26a Opening, 27 Insulating Resin Ring, 28 Packing, 29 Insulation ring.

Claims (3)

A positive electrode comprising a porous carbon material doped with nitrogen atoms and sulfur;
A negative electrode capable of occluding and releasing alkali metal ions;
The ether system with respect to the number of alkali metal cations of the alkali metal salt, wherein an alkali metal salt is dissolved in an ether organic solvent that is interposed between the positive electrode and the negative electrode and contains at least two oxygen atoms in the molecule. A non-aqueous electrolyte in which the ratio r of the total number of oxygen atoms in the organic solvent satisfies 4 ≦ r ≦ 5;
Alkali metal sulfur secondary battery.   2. The alkali metal sulfur secondary battery according to claim 1, wherein the positive electrode includes the porous carbon material having a nitrogen atom to carbon atom ratio N / C of 0.05 or more and 0.3 or less. The alkali metal sulfur secondary battery according to claim 1, wherein the porous carbon material has a specific surface area of 300 m ² / g or more and 1200 m ² / g or less.

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