JP2016006798A - Sulfide-based solid electrolyte for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sulfide-based solid electrolyte for a lithium ion secondary battery, capable of reducing deterioration in battery performance during charging/discharging.SOLUTION: The solid electrolyte for a lithium ion secondary battery is synthesized from at least LiS and one or more sulfides selected from PS, PS, SiS, GeS, BS, and AlS, has an elemental sulfur component content of 1 wt.% or less, and contains an organic solvent.

Description

  The present invention relates to a lithium ion conductive sulfide-based solid electrolyte and a method for producing the same.

  Conventionally, most electrolytes exhibiting high lithium ion conductivity at room temperature are liquids, and many of the commercially available lithium ion secondary batteries use organic electrolytes. In a lithium ion secondary battery using an organic electrolyte, there is a risk of electrolyte leakage and ignition, and an all-solid battery with higher safety is desired. However, the ionic conductivity of solid electrolytes is generally low and is difficult to put into practical use.

As a solid electrolyte exhibiting high ion conductivity, a lithium ion conductive ceramic based on Li ₃ N is known. The ionic conductivity of this solid electrolyte is 10 ⁻³ Scm ⁻¹ at room temperature. However, since the decomposition voltage is low, a battery that operates at 3 V or more cannot be constructed.
Patent Document 1 discloses a sulfide-based solid electrolyte having 10 ⁻⁴ Scm ⁻¹ ion conductivity. Patent Document 2 discloses a solid electrolyte synthesized from Li ₂ S and P ₂ S ₅ and having ion conductivity of 10 ⁻⁴ Scm ⁻¹ units.
Further, Patent Document 3 discloses a sulfide-based crystallized glass obtained by synthesizing Li ₂ S and P ₂ S ₅ in a ratio of 68 to 74 mol%: 26 to 32 mol%, and has an ion conduction of 10 ⁻³ Scm ⁻¹ unit. Realize the sex.

As a method for producing a sulfide-based solid electrolyte, there are generally known a melting method in which heating and melting are performed to near the melting point of Li ₂ S and quenching, and a mechanical milling method (MM method) which is a mechanical mixing process. Sulfide-based solid electrolytes are highly reactive and react with moisture in the air to generate H ₂ S. Therefore, in the melting method, the raw material is melted in a dry environment in which moisture is sufficiently controlled, and then rapidly cooled in an inert atmosphere or the like. In the MM method, a raw material is charged into a container in a moisture-controlled space and mechanically mixed.
JP-A-4-202024 JP 2002-109955 A JP 2005-228570 A

  An object of this invention is to provide the sulfide type solid electrolyte for lithium ion secondary batteries which can reduce the deterioration of the battery performance at the time of charging / discharging.

The present inventors have found that a single sulfur component in a solid electrolyte obtained can be efficiently removed by washing a reaction product obtained by reacting a starting material with an organic solvent, thereby completing the present invention.
As a method of removing elemental sulfur from the sulfide-based solid electrolyte, there is a method of vacuum heating at a boiling point of 444 ° C. or more of elemental sulfur. However, in this case, for example, in the P ₂ S ₅ system solid electrolyte, heating is performed at a temperature of the boiling point of P ₂ S ₅ of 290 ° C. or more, which causes a significant decrease in battery performance. In addition, even in the SiS ₂ system, the generation of elemental sulfur due to the decomposition reaction of some sulfides occurred, which was not always an effective means.
According to the present invention, the following sulfide-based solid electrolyte for lithium ion secondary batteries is provided.
1. It is synthesized from at least Li ₂ S and one or more sulfides selected from P ₂ S ₃ , P ₂ S ₅ , SiS ₂ , GeS ₂ , B ₂ S ₃ and Al ₂ S _3. A solid electrolyte for a lithium ion secondary battery having a weight% or less.
2. At least Li ₂ S and one or more sulfides selected from P ₂ S ₃ , P ₂ S ₅ , SiS ₂ , GeS ₂ , B ₂ S ₃ and Al ₂ S ₃ are used as starting materials, and the starting materials are The manufacturing method of the sulfide type solid electrolyte for lithium ion secondary batteries which has the process of wash | cleaning the reaction material obtained by making it react with an organic solvent.
3. The manufacturing method of the sulfide type solid electrolyte for lithium ion secondary batteries of 2 which implements process (a), (b), and (c) shown below in this order.
(A) Step of reacting the starting material (b) Step of adding an organic solvent to the reaction and washing (c) Step of removing the added organic solvent The method for producing a sulfide-based solid electrolyte for a lithium ion secondary battery according to 2 or 3, wherein the organic solvent is a hydrocarbon-based organic solvent.
5. Lithium ion secondary containing a sulfide-based solid electrolyte for lithium ion secondary battery as described in 1 above, a complex oxide containing Li element, and one or more metal elements selected from Co, Ni, Mn and Fe Positive electrode composite for batteries.
6). An all-solid-state lithium ion secondary battery comprising a sulfide-based solid electrolyte having a content of elemental sulfur of 1% by weight or less.

ADVANTAGE OF THE INVENTION According to this invention, the sulfide type solid electrolyte for lithium ion secondary batteries which can reduce the deterioration of the battery performance at the time of charging / discharging can be provided.

The sulfide-based solid electrolyte for a lithium ion secondary battery of the present invention is obtained by reacting at least a starting material containing the following (1) and (2), and the elemental sulfur component is 1% by weight or less.
(1) Li ₂ S
(2) one or more sulfides selected from P ₂ S ₃ , P ₂ S ₅ , SiS ₂ , GeS ₂ , B ₂ S ₃ and Al ₂ S ₃

  In the present invention, the residual amount of the single sulfur component of the solid electrolyte is 1% by weight or less. If the amount is 1% by weight or less, a battery having stable performance can be produced when the solid electrolyte is used in the battery. If it exceeds 1% by weight, a reaction due to sulfur occurs during charging and discharging, which may cause a reduction in battery capacity and output. The residual amount of the elemental sulfur component is preferably 0.5% by weight or less, and more preferably 0.1% by weight or less.

  The starting material is not particularly limited, and commercially available products can be used.

The sulfide-based solid electrolyte for a lithium ion secondary battery of the present invention is obtained by, for example, reacting a reaction product obtained by reacting at least a starting material containing the above (1) and (2) with an organic solvent during the production of the solid electrolyte. What is necessary is just to implement the process to wash | clean.
By washing the sulfide-based solid electrolyte that is a reactant with an organic solvent, the elemental sulfur component that is an impurity of the solid electrolyte that is the final product can be reduced.

  The step of washing with an organic solvent targets a reaction product (solid electrolyte) obtained by reacting starting materials. Furthermore, you may wash | clean after the heating process which makes the solid electrolyte mentioned later glass-ceramic. The cleaning step may be performed once, but may be performed a plurality of times after each step. In the step of reacting the starting materials, there is a possibility that a new elemental sulfur component may be newly generated. Therefore, it is preferable to wash the reactant.

  In addition, the removal by a heating etc. can be used for the removal of a simple sulfur component. Specifically, it is possible to evaporate and remove the sulfur component by vacuum heating. However, the boiling point of elemental sulfur is as high as 444 ° C., and there is a high possibility of affecting the crystal structure of the synthesized solid electrolyte. Therefore, the method of extracting and filtering a single element sulfur component with an organic solvent as in the present invention is preferred.

Hereinafter, an embodiment of the production method of the present invention will be described in detail.
In the present embodiment, the following steps (a), (b), and (c) are performed in this order.
(A) Step of reacting starting materials (b) Step of adding an organic solvent to the reaction product obtained in step (a) and washing (c) Step of removing the added organic solvent

In the step (a), the method for reacting the starting material is not particularly limited, and a method known in this technical field can be employed. For example, a melting method in which the starting material is heated and melted to near the melting point of Li ₂ S and rapidly cooled, or a mechanical milling method (MM method) in which the starting material is mechanically mixed can be employed. The starting material is treated by these methods to obtain a reaction product. Hereinafter, a case where the MM method is adopted will be described as an example.

Various types of grinding methods can be used for the mechanical milling treatment. In particular, it is preferable to use a planetary ball mill. The planetary ball mill can efficiently generate very high impact energy by rotating the platform while the pot rotates.
The amount of Li ₂ S charged at the time of mechanical milling is preferably 30 to 95 mol%, more preferably 40 to 85 mol%, particularly 50 to 75 mol%, based on the total amount of starting materials. preferable.

The rotation speed and rotation time of the mechanical milling treatment are not particularly limited, but the higher the rotation speed, the faster the glassy electrolyte production rate, and the longer the rotation time, the higher the conversion rate of the raw material into the glassy electrolyte.
For example, when a planetary ball mill is used, the rotational speed may be set to several tens to several hundreds of revolutions / minute, and the treatment may be performed for 0.5 hours to 100 hours.

In the step (b), an organic solvent is added to the reaction product obtained in the above step (a) and washed.
As the organic solvent, those capable of extracting the elemental sulfur component which is an impurity can be used. For example, toluene, xylene, hexane, carbon disulfide and the like can be suitably used. In particular, hydrocarbon organic solvents such as toluene, xylene and hexane are preferred.

  The amount of the organic solvent to be used is not particularly limited as long as it can sufficiently extract a single sulfur component from the reaction product. Although it may be adjusted as appropriate in consideration of the properties of the reactants, it is preferable that the reactants are in a slurry state by the addition of a solvent. Usually, the addition amount of the raw material (total amount) with respect to 1 liter of solvent is about 0.1 to 1 kg. Preferably it is 0.3-1.0Kg, Most preferably, it is 0.5-0.8Kg.

The washing method is not particularly limited as long as the mixture of the reactant and the organic solvent can be sufficiently stirred and mixed. For example, use of an autoclave or a universal mixing stirrer can be mentioned.
In the case of the MM method, washing can be performed by adding an organic solvent to the container after the reaction and further continuing the rotation treatment.

  The time for the washing treatment is not particularly limited as long as the single sulfur component can be sufficiently extracted from the reaction product. Although it may be adjusted as appropriate in consideration of the properties of the reactants, it is usually preferably about 0.5 to 10 hours.

  In the step (c), the organic solvent added after the washing is removed, and the solid electrolyte is recovered. The removal of the organic solvent can be carried out, for example, by filtration. By removing the solvent, a sulfide-based solid electrolyte that is a sulfide glass is obtained.

In the present invention, the sulfide-based solid electrolyte obtained by the step (a) or (c) is further heat-treated at 200 ° C. or more and 400 ° C. or less, more preferably 270 ° C. to 320 ° C. The ionic conductivity of the solid electrolyte can be improved. This is because the reactant becomes sulfide crystallized glass (glass ceramic).
The heat treatment time is preferably 1 to 5 hours, and particularly preferably 1.5 to 3 hours.
If the heat treatment temperature is a high temperature of 400 ° C. or higher, there is a high possibility that Li ₂ P ₂ S ₆ inferior in lithium ion conductivity is generated, which is not desirable.
This heat treatment may be performed before the cleaning step (b), or may be performed after the step (c).

The solid electrolyte for a lithium ion secondary battery of the present invention can be used as an electrolyte layer of an all solid lithium ion secondary battery. Moreover, this solid electrolyte, composite oxide, etc. can be mixed, and it can also be used as a positive electrode compound material for lithium ion secondary batteries.
As the composite oxide, an element containing Li element and one or more metal elements selected from Co, Ni, Mn, and Fe can be used. For example, lithium cobaltate LiCoO ₂ composed of Li element and Co, LiNi _0.8 Co _0.2 O ₂ composed of Li element, Co, and Ni are preferable.

Incidentally, the solid electrolyte or positive electrode material for lithium ion secondary battery of the present invention, LiFePO 4 as _necessary, silicic acid compounds, may be added vitrified material such as phosphoric acid-based compound.

The all solid lithium secondary battery of the present invention includes a sulfide-based solid electrolyte in which the content of elemental sulfur is 1% by weight or less. That is, the sulfide solid electrolyte of the present invention is used for an electrolyte layer and a positive electrode mixture.
For example, an all-solid lithium secondary battery is formed by forming a layer made of the solid electrolyte of the present invention between the positive electrode and the negative electrode.

As the positive electrode material, in addition to the positive electrode mixture of the present invention, those used as a positive electrode active material in the battery field can be used. For example, in the sulfide system, titanium sulfide (TiS ₂ ), molybdenum sulfide (MoS ₂ ), iron sulfide (FeS, FeS ₂ ), copper sulfide (CuS), nickel sulfide (Ni ₃ S ₂ ), and the like can be used. Preferably, TiS ₂ can be used.
In the oxide system, bismuth oxide (Bi ₂ O ₃ ), bismuth lead acid (Bi ₂ Pb ₂ O ₅ ), copper oxide (CuO), vanadium oxide (V ₆ O ₁₃ ), lithium cobalt oxide (LiCoO ₂ ) Lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and the like can be used. It is also possible to use a mixture of these. Preferably, lithium cobaltate can be used.
In addition to the above, niobium selenide (NbSe ₃ ) can be used.

As a negative electrode material, what is used as a negative electrode active material in the battery field | area can be used. For example, carbon materials, specifically artificial graphite, graphite carbon fiber, resin-fired carbon, pyrolytic vapor-grown carbon, coke, mesocarbon microbeads (MCMB), furfuryl alcohol resin-fired carbon, polyacene, pitch-based carbon Examples thereof include fibers, vapor-grown carbon fibers, natural graphite, and non-graphitizable carbon. Or it may be a mixture thereof. Preferably, it is artificial graphite.
An alloy combined with a metal itself such as metallic lithium, metallic indium, metallic aluminum, metallic silicon, or another element or compound can be used as the negative electrode material.

Production Example 1
(1) Production of Lithium Sulfide (Li ₂ S) Lithium sulfide was produced according to the method of the first aspect (two-step method) in JP-A-7-330312. Specifically, N-methyl-2-pyrrolidone (NMP) 3326.4 g (33.6 mol) and lithium hydroxide 287.4 g (12 mol) were charged into a 10 liter autoclave equipped with a stirring blade, and 300 rpm, 130 The temperature was raised to ° C. After the temperature rise, hydrogen sulfide was blown into the liquid at a supply rate of 3 liters / minute for 2 hours. Subsequently, this reaction solution was heated under a nitrogen stream (200 cc / min), and the reacted lithium hydrosulfide was dehydrosulfurized to obtain lithium sulfide. As the temperature increased, water produced as a by-product due to the reaction between hydrogen sulfide and lithium hydroxide started to evaporate, but this water was condensed by the condenser and extracted out of the system. While water was distilled out of the system, the temperature of the reaction solution rose, but when the temperature reached 180 ° C., the temperature increase was stopped and the temperature was kept constant. The reaction was completed after the dehydrosulfurization reaction of lithium hydrosulfide (about 80 minutes) to obtain lithium sulfide.

(2) Purification of lithium sulfide After decanting NMP in the 500 mL slurry reaction solution (NMP-lithium sulfide slurry) obtained in (1) above, 100 mL of dehydrated NMP was added and stirred at 105 ° C. for about 1 hour. NMP was decanted at that temperature. Further, 100 mL of NMP was added, stirred at 105 ° C. for about 1 hour, NMP was decanted at that temperature, and the same operation was repeated a total of 4 times. After completion of the decantation, lithium sulfide was dried at 230 ° C. (temperature higher than the boiling point of NMP) under a nitrogen stream for 3 hours under normal pressure. The impurity content in the obtained lithium sulfide was measured.
Incidentally, lithium sulfite _(Li 2 _SO 3), the content of each sulfur oxide lithium sulfate _(Li 2 SO ₄₎ and lithium thiosulfate _{(Li 2 S 2 O 3)} , and N- methylamino acid lithium (NMAB) Was quantified by ion chromatography. As a result, the total content of sulfur oxides was 0.13% by mass, and LMAB was 0.07% by mass. Li ₂ S thus purified was used in the following examples.

The solid electrolyte produced in Examples and the like was evaluated by the following method.
(1) Ionic conductivity Sulfide-based solid electrolyte powder was filled in a tablet molding machine, and a pressure of 4 to 6 MPa was applied to obtain a molded body. Furthermore, a composite material in which carbon and electrolyte glass ceramic are mixed at a weight ratio of 1: 1 as an electrode is placed on both sides of the molded body, and pressure is again applied by a tablet molding machine, so that a molded body for measuring conductivity (diameter of about 10 mm and a thickness of about 1 mm). The molded body was subjected to ionic conductivity measurement by AC impedance measurement. The conductivity value was a value at 25 ° C.

(2) Evaluation of battery performance The sulfide-based solid electrolyte produced in each example and lithium composite oxide (LiCoO ₂ ) were mixed at a weight ratio of 8: 2 to 6: 4 to prepare a positive electrode mixture.
Either an indium foil or a composite produced from carbon and a sulfide-based solid electrolyte was used as the negative electrode.
A sulfide-based solid electrolyte, the above-mentioned positive electrode mixture, Ti mesh and Ti foil were laminated in this order on the negative electrode, assembled and compressed at 10 to 30 MPa to form a battery, and a charge / discharge cycle curve was obtained and evaluated.
The charge / discharge evaluation was performed by setting the cut-off voltage to a lower limit of 1.5 V and an upper limit of 3.7 V, and the ratio of the discharge capacity to the charge capacity after charging and the discharge output that is an integrated value thereof.

(3) Analysis of residual elemental sulfur component in sulfide-based solid electrolyte 1 g of the target solid electrolyte powder is placed in a 30 ml sample bottle, and 20 ml of organic solvent (dehydrated toluene) is added. Time). The solid electrolyte was allowed to settle down and the solid electrolyte was sufficiently settled. The supernatant was taken out with a syringe, passed through a Millipore filter, and the solid electrolyte was completely removed to obtain an organic solvent supernatant. The single sulfur component in the supernatant was quantified with a gas chromatograph to determine the amount of residual single sulfur component.

Example 1
16.27 g of Li ₂ S and 33.73 g of P ₂ S ₅ (manufactured by Aldrich) manufactured according to the above manufacturing example were put in a 500 ml alumina container containing 175 10 mmφ alumina balls and sealed. The above weighing and sealing operations were all carried out in a glove box, and all the instruments used were water removed beforehand by a dryer.
This sealed alumina container was mechanically milled at a rotational speed of 290 rpm for 36 hours with a planetary ball mill (PM400 manufactured by Lecce) at room temperature to obtain a reaction product (white yellow glass powder).
As a result of X-ray diffraction measurement (CuKα: λ = 1.5418４) of this powder, a peak of Li ₂ S as a raw material was not observed, and a halo pattern due to the reaction product was observed.

After the mechanical milling process, 136 ml of toluene was added to the alumina container. Then, the reaction product was washed by ball milling for 20 minutes.
After washing, the resulting slurry of the reaction product was filtered and dried to obtain a sulfide-based solid electrolyte.
This sulfide-based solid electrolyte was sealed in a SUS tube in a glove box (under an Ar atmosphere) and subjected to a heat treatment at 300 ° C. for 2 hours to obtain a sulfide-based solid electrolyte that was converted into a glass ceramic. In the X-ray diffraction measurement of this glass-ceramic solid electrolyte powder, peaks were observed at 2θ = 17.8, 18.2, 19.8, 21.8, 23.8, 25.9, 29.5, 30.0 deg. Was observed.
Further, the ionic conductivity of the glass-ceramic solid electrolyte was 1.3 × 10 ⁻³ S / cm.
The amount of simple sulfur in the solid electrolyte was 0.023% by weight both before and after the glass ceramic treatment.
A battery was formed using a sulfide-based solid electrolyte made into a glass ceramic, and evaluated according to battery performance evaluation. As a result, the charge / discharge capacity ratio was 0.85. Further, the discharge capacity at a 0.2 C rate was 136 mAh / g, and the discharge output was 0.504 Wh / g.

Example 2
The same treatment as in Example 1 except that SiS ₂ (manufactured by Alfa Aesar) was used instead of P ₂ S ₅ , and the amount of preparation at the time of slurry preparation was changed to Li ₂ S 21.39 g and SiS ₂ 28.61 g. The solid electrolyte was obtained.
The amount of simple sulfur in the solid electrolyte was 0.066% by weight.
The ionic conductivity of this solid electrolyte was 9.3 × 10 ⁻⁴ S / cm.
A battery was formed using this sulfide-based solid electrolyte and evaluated according to the battery performance evaluation. As a result, the charge / discharge capacity ratio was 0.82. Further, the discharge capacity at a 0.2 C rate was 124 mAh / g, and the discharge output was 0.441 Wh / g.

Example 3
Example 1 except that Al ₂ S ₃ (manufactured by Mitsuwa Corp.) was used instead of P ₂ S ₅ , and the amount charged during slurry preparation was changed to 15.89 g of Li ₂ S and 28.61 g of Al ₂ S _3. The solid electrolyte was obtained in the same manner as in Example 1.
The residual amount of simple sulfur in the solid electrolyte was 0.37% by weight.

Comparative Example 1
After the mechanical milling treatment, a sulfide-based solid electrolyte was produced in the same manner as in Example 1 except that the cleaning using toluene was not performed.
The residual amount of simple sulfur in the solid electrolyte was 2.53% by weight both before and after the glass ceramicization treatment.
Further, the ionic conductivity of the glass-ceramic solid electrolyte was 1.0 × 10 ⁻³ S / cm.
A battery was formed using a sulfide-based solid electrolyte made into a glass ceramic, and evaluated according to battery performance evaluation. As a result, the charge / discharge capacity ratio was 0.68. The discharge capacity at a 0.2 C rate was 103 mAh / g, and the discharge output was 0.359 Wh / g.

The solid electrolyte or positive electrode mixture for a lithium ion secondary battery of the present invention can be suitably used for an all solid lithium secondary battery.
The all-solid-state lithium secondary battery of the present invention is a lithium secondary battery used in portable information terminals, portable electronic devices, household small-sized power storage devices, motor-driven motorcycles, electric vehicles, hybrid electric vehicles, and the like. Can be used.

Claims (10)

At least Li ₂ S;
Synthesized from _{P 2 S 3, P 2 S} 5, SiS 2, GeS 2, B 2 S 3 and _Al 2 _{S 3} 1 or more sulfides selected from elemental sulfur components Ri der than 1 wt% And a solid electrolyte for a lithium ion secondary battery containing an organic solvent . The solid electrolyte for a lithium ion secondary battery according to claim 1, wherein the solid electrolyte is a powder. The solid electrolyte for lithium ion secondary batteries according to claim 1 or 2, wherein the organic solvent is a hydrocarbon organic solvent. The solid electrolyte for lithium ion secondary batteries according to any one of claims 1 to 3, wherein the organic solvent contains at least one of toluene, xylene, and hexane. The solid electrolyte for a lithium ion secondary battery according to any one of claims 1 to 4 , wherein the elemental sulfur component is 0.1 wt% or less. At least Li ₂ are synthesized from S and _P 2 _{S 5,} the lithium ion secondary battery for a solid electrolyte according to any one of claims 1 to 5. The charge of Li ₂ S is 30～95Mol% the combined total of the starting materials, lithium ion secondary battery for a solid electrolyte according to any one of claims 1-6. At least Li ₂ S and
P ₂ S ₃ , P ₂ S ₅ , SiS ₂ , GeS ₂ , B ₂ S ₃ And Al ₂ S ₃ A solid electrolyte powder for a lithium ion secondary battery synthesized from one or more sulfides selected from the group consisting of 1% by weight or less of a single sulfur component. The solid electrolyte powder for a lithium ion secondary battery according to claim 8, wherein the elemental sulfur component is 0.1 wt% or less. Lithium-ion secondary battery for a solid electrolyte according to any one of claims 1 to 7, or, all-solid-state lithium-ion secondary battery including a solid electrolyte powder for a lithium ion secondary battery according to claim 8 or 9.