JP2013084499A - Sulfide solid battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sulfide solid battery system capable of preventing the battery resistance from increasing.SOLUTION: The sulfide solid battery system includes a battery unit including a positive electrode layer containing a positive electrode active material, a negative electrode layer containing a negative electrode active material, and an electrolyte layer between the positive electrode layer and the negative electrode layer; voltage measuring means for measuring the voltage of the battery unit; and discharge control means for controlling the discharge so as to prevent the voltage of the battery unit measured by the voltage measuring means from falling to or below -0.5 V.

Description

  The present invention relates to a sulfide solid state battery system using a solid sulfide as an electrolyte.

  A lithium ion secondary battery has the characteristics that it has a higher energy density than other secondary batteries and can operate at a high voltage. For this reason, it is used as a secondary battery that can be easily reduced in size and weight in information equipment such as a mobile phone, and in recent years, there is an increasing demand for large motive power such as for electric vehicles and hybrid vehicles.

  A lithium ion secondary battery includes a positive electrode layer and a negative electrode layer, and an electrolyte layer disposed therebetween. Examples of the electrolyte used for the electrolyte layer include non-aqueous liquid and solid substances. Are known. When a liquid electrolyte (hereinafter referred to as “electrolytic solution”) is used, the electrolytic solution easily penetrates into the positive electrode layer and the negative electrode layer. Therefore, an interface between the active material contained in the positive electrode layer or the negative electrode layer and the electrolytic solution is easily formed, and the performance is easily improved. However, since the widely used electrolyte is flammable, it is necessary to mount a system for ensuring safety. On the other hand, when a solid electrolyte that is flame retardant (hereinafter referred to as “solid electrolyte”) is used, the above system can be simplified. Therefore, development of a lithium ion secondary battery (hereinafter referred to as “solid battery”) in a form provided with a layer containing a solid electrolyte (hereinafter referred to as “solid electrolyte layer”) is in progress.

  As a technology relating to such a lithium ion secondary battery, for example, Patent Document 1 includes an assembled battery in which a plurality of lithium secondary battery cells are connected, and the lithium secondary battery electrolyte is an inorganic solid electrolyte. A battery module that does not include overdischarge protection means for preventing overdischarge of the battery is disclosed. In Patent Document 1, since an inorganic solid electrolyte is used, charging and discharging can be performed normally by charging again after overdischarge or inversion, so that a circuit for preventing overdischarge is unnecessary. It is described.

JP 2010-225581 A

  The technique disclosed in Patent Document 1 has a problem in that when the voltage is discharged to a predetermined value or less, the subsequent battery resistance increases and the battery output decreases.

  Then, this invention makes it a subject to provide the sulfide solid battery system which can prevent the increase in battery resistance.

  As a result of intensive studies, the present inventor has found that battery resistance increases when a battery using a sulfide solid electrolyte is discharged until the battery voltage becomes −0.5 V or less. Therefore, it is considered that an increase in battery resistance can be prevented by controlling the discharge so that the battery voltage does not become −0.5 V or less. The present invention has been completed based on this finding.

In order to solve the above problems, the present invention takes the following means. That is,
The present invention relates to a battery unit having a positive electrode layer containing a positive electrode active material, a negative electrode layer containing a negative electrode active material, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer, and a voltage of the battery unit. A sulfide solid state battery system comprising: voltage detection means for detecting; and discharge control means for controlling discharge so that the voltage of the battery unit detected by the voltage detection means does not become −0.5 V or less.

  Since the present invention has a discharge control means for controlling the discharge so that the battery voltage does not become −0.5 V or less, a high resistance layer that prevents ionic conduction is formed at the interface between the positive electrode active material and the solid electrolyte. hard. By suppressing the formation of the high resistance layer, it is possible to prevent an increase in battery resistance. Further, by discharging until the state of charge (SOC) of the battery is lower than 0%, it is possible to recover the output characteristics of the battery once lowered.

  In the present invention, the positive electrode active material is preferably coated with an ion conductive oxide. Since the positive electrode active material is coated with the ion conductive oxide, it is difficult to form a high resistance layer that hinders ion conduction, and thus it is easy to prevent an increase in battery resistance.

  Moreover, in the said invention, it is preferable to provide a sulfide solid electrolyte in at least one part of a positive electrode layer and an electrolyte layer. Even in such a form, it is possible to prevent an increase in battery resistance.

  ADVANTAGE OF THE INVENTION According to this invention, the sulfide solid state battery system which can prevent the increase in battery resistance can be provided.

2 is a cross-sectional view illustrating a battery unit 1. FIG. 1 is a diagram illustrating a sulfide solid state battery system 10. FIG. It is a figure explaining DC resistance and reaction resistance. It is a figure which shows the relationship between resistance change rate and a discharge stop voltage. It is a figure which shows the relationship between the battery voltage obtained by the comparative example 2, and discharge time.

  The present invention will be described below with reference to the drawings. In addition, the form shown below is an illustration of this invention and this invention is not limited to the form shown below.

  FIG. 1 is a cross-sectional view illustrating the form of a battery unit 1 provided in a sulfide solid state battery system 10 of the present invention. In FIG. 1, description of the exterior body is omitted. As shown in FIG. 1, the battery unit 1 includes a positive electrode current collector 1a, a positive electrode layer 1b containing a positive electrode active material connected to the positive electrode current collector 1a, a negative electrode current collector 1e, and the negative electrode The negative electrode layer 1d containing the negative electrode active material connected to the collector 1e, and the solid electrolyte layer 1c disposed between the positive electrode layer 1b and the negative electrode layer 1d are included. In the battery unit 1, the positive electrode layer 1b and the solid electrolyte layer 1c contain a sulfide solid electrolyte.

  FIG. 2 is a schematic diagram showing an example of the sulfide solid state battery system 10 of the present invention (hereinafter sometimes referred to as “battery system 10”). The battery system 10 shown in FIG. 2 includes a battery unit 1, voltage detection means 2 that detects the voltage of the battery unit 1, and discharge control that controls discharge so that the voltage of the battery unit 1 does not fall below −0.5V. Part 3. In the battery system 10, the voltage detection unit 2 detects the battery voltage of the battery unit 1, and sends information related to the detected battery voltage to the discharge control unit 3. The discharge control unit 3 controls the discharge of the battery unit 1 so that the discharge of the battery unit 1 is terminated before the battery voltage of the battery unit 1 reaches −0.5V.

As will be described later, when a sulfide solid state battery using a solid electrolyte mainly composed of Li ₂ S and P ₂ S ₅ for a positive electrode layer or a solid electrolyte layer is discharged until the battery voltage becomes −0.5 V or less, Resistance increases. The detailed mechanism by which the resistance increases is unknown at this time, but the present inventor predicts as follows.
From experiments using a three-electrode cell, it has been found that when the battery voltage decreases, the potential of the positive electrode decreases and the potential of the negative electrode increases. Therefore, the potential of the positive electrode decreases during discharge. When the potential of the positive electrode falls below a predetermined potential, the solid electrolyte undergoes a reduction reaction due to the action of the positive electrode active material (metal oxide), and at this time, conduction of lithium ions to the interface between the positive electrode active material and the solid electrolyte. It is expected that this is because a high resistance layer that hinders the formation is formed.

Based on the above knowledge, in the present invention, the discharge of the battery unit 1 is terminated before the battery voltage of the battery unit 1 reaches −0.5V. Therefore, according to the battery system 10, an increase in resistance can be prevented. Note that, “having Li ₂ S and P ₂ S ₅ as main components” specifically does not include a liquid ion conductor, and the molar ratio of Li ₂ S and P in the components constituting the solid electrolyte. _{As long} as the total of the molar ratios of ₂ S ₅ is 50% or more, lithium compounds other than Li ₂ S may be included, and the ratio of Li ₂ S and P ₂ S ₅ is appropriately changed. That is possible.

  On the other hand, although active (intentional) overdischarge is considered effective for recovering a battery whose output characteristics have deteriorated, it is discharged until the battery voltage becomes −0.5 V or less as described above. Then the resistance increases. Therefore, it is effective to terminate the overdischarge for restoring the output characteristics before the battery voltage reaches −0.5V. In the present invention, even when overdischarge for recovering the output characteristics is performed, the discharge of the battery unit 1 is terminated before the battery voltage of the battery unit 1 reaches −0.5 V. Thus, it is possible to recover the output characteristics while preventing an increase in resistance.

  In the present invention, a known metal that can be used as a current collector of a lithium ion secondary battery can be used for the positive electrode current collector 1a and the negative electrode current collector 1e. As such a metal, a metal containing one or more elements selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, and In. Materials can be exemplified.

Moreover, as a positive electrode active material contained in the positive electrode layer 1b, the well-known active material which can be contained in the positive electrode layer of a lithium ion secondary battery can be used suitably. Examples of such positive electrode active materials include layered active materials such as LiCoO ₂ , LiNiO ₂ , LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiVO ₂ , LiCrO ₂ , LiFePO ₄ , LiCoPO ₄ , LiMnPO ₄ . olivine active material or the like _4, be exemplified _{LiMn 2 O 4, Li (Ni} 0.25 Mn 0.75) 2 O 4, LiCoMnO 4, Li 2 NiMn 3 O 8 spinel active materials such as it can.

  The shape of the positive electrode active material can be, for example, particulate or thin film. The average particle size (D50) of the positive electrode active material is, for example, preferably from 1 nm to 100 μm, and more preferably from 10 nm to 30 μm. Further, the content of the positive electrode active material in the positive electrode layer 1b is not particularly limited, but is preferably 40% or more and 99% or less in mass%, for example.

From the viewpoint of making it easy to prevent an increase in battery resistance by making it difficult to form a high resistance layer at the interface between the positive electrode active material and the solid electrolyte, in the present invention, the positive electrode active material is an ion conductive oxide. It is preferably coated. Examples of the lithium ion conductive oxide that coats the positive electrode active material include a general formula Li _x AO _y (where A is B, C, Al, Si, P, S, Ti, Zr, Nb, Mo, Ta, and the like). Or W and x and y are positive numbers.) Specifically, Li ₃ BO ₃ , LiBO ₂ , Li ₂ CO ₃ , LiAlO ₂ , Li ₄ SiO ₄ , Li ₂ SiO ₃ , Li ₃ PO ₄ , Li ₂ SO ₄ , Li ₂ TiO ₃ , Li ₄ Ti ₅ Examples include O ₁₂ , Li ₂ Ti ₂ O ₅ , Li ₂ ZrO ₃ , LiNbO ₃ , Li ₂ MoO ₄ , Li ₂ WO _{4 and the} like. The lithium ion conductive oxide may be a complex oxide. As the composite oxide covering the positive electrode active material, any combination of the above lithium ion conductive oxides can be employed. For example, Li ₄ SiO ₄ —Li ₃ BO ₃ , Li ₄ SiO ₄ —Li ₃ PO ₄ etc. can be mentioned. Moreover, the ion conductive oxide should just coat | cover at least one part of a positive electrode active material, and may coat | cover the positive electrode active material whole surface. In addition, the thickness of the ion conductive oxide covering the positive electrode active material is, for example, preferably from 0.1 nm to 100 nm, and more preferably from 1 nm to 20 nm. The thickness of the ion conductive oxide can be measured using, for example, a transmission electron microscope (TEM).

The positive electrode layer 1b can appropriately contain a known solid electrolyte that can be contained in the positive electrode layer of the lithium ion secondary battery. Examples of such a solid electrolyte include sulfide solid electrolytes such as Li ₃ PS ₄ and Li ₂ S—P ₂ S ₅ prepared by mixing Li ₂ S and P ₂ S ₅ . As a solid electrolyte, the case of using a sulfide solid electrolyte was produced by using a raw material composition containing Li ₂ S and _P 2 _{S 5,} the proportion of Li ₂ S to the total of Li ₂ S and _P 2 _{S 5} is particularly limited However, it is preferably 70 mol% or more and 80 mol% or less, more preferably 72 mol% or more and 78 mol% or less, and still more preferably 74 mol% or more and 76 mol% or less. This is because a sulfide solid electrolyte material having an ortho composition or a composition in the vicinity thereof can be obtained, and a sulfide solid electrolyte material having high chemical stability can be obtained. Here, the ortho generally refers to an oxo acid obtained by hydrating the same oxide and having the highest degree of hydration. In the positive electrode layer 1b, Li ₂ S is the most sulfide. The added crystal composition is called the ortho composition. In the Li ₂ S—P ₂ S ₅ system, Li ₃ PS ₄ corresponds to the ortho composition. In the case of a Li ₂ S—P ₂ S ₅ based sulfide solid electrolyte, the ratio of Li ₂ S and P ₂ S ₅ to obtain the ortho composition is Li ₂ S: P ₂ S ₅ = 75: 25 on a molar basis. is there. The positive electrode layer 1b may contain a lithium compound (solid electrolyte) other than Li ₂ S and P ₂ S ₅ . Examples of such a lithium compound include Li ₂ O, LiI, LiBr, and the like.

In the present invention, it is preferable that the sulfide solid electrolyte does not substantially contain bridging sulfur. This is because a sulfide solid electrolyte material having high chemical stability can be obtained. “Bridged sulfur” refers to bridged sulfur in a compound prepared through a process of reacting Li ₂ S with at least one sulfide selected from the group consisting of P, Si, Ge, Al, and B. For example, Li ₂ S and _P 2 _{S 5} is crosslinked sulfur _{S 3} PS-PS 3 structure formed by the reaction corresponds. Such bridging sulfur easily reacts with water and easily generates hydrogen sulfide. Furthermore, “substantially free of bridging sulfur” can be confirmed by measurement of a Raman spectrum. For example, in the case of a Li ₂ S—P ₂ S ₅ -based sulfide solid electrolyte, the peak of the S ₃ P—S—PS ₃ structure usually appears at 402 cm ⁻¹ . Therefore, it is preferable that this peak is not detected. Moreover, the peak of PS ₄ ³⁻ structure usually appears at 417 cm ⁻¹ . In the positive electrode layer 1b, the intensity _{I 402} at 402 cm ^-1 is preferably smaller than the intensity _{I 417} at 417 cm ^-1. More specifically, the strength I ₄₀₂ is preferably 70% or less, more preferably 50% or less, and even more preferably 35% or less with respect to the strength I ₄₁₇ .

  In the present invention, the form of the solid electrolyte is not particularly limited, and may be amorphous or glass ceramics in addition to crystalline. Moreover, the shape of a solid electrolyte can be made into a particulate form, a thin film form, etc., for example. The average particle diameter (D50) of the solid electrolyte can be, for example, from 1 nm to 100 μm, and preferably from 10 nm to 30 μm.

  In addition, the positive electrode layer 1b may contain a binder for binding the positive electrode active material and the solid electrolyte and a conductive material for improving conductivity. Examples of the binder that can be contained in the positive electrode layer 1b include styrene butadiene rubber (SBR). Examples of the conductive material that can be contained in the positive electrode layer 1b include vapor grown carbon fiber and acetylene. In addition to carbon materials such as black and ketjen black, metal materials that can withstand the environment when the solid battery is used can be exemplified. In addition, when the positive electrode layer 1b is manufactured through a process of applying the slurry-like composition prepared by dispersing the positive electrode active material or the like in a liquid to the positive electrode current collector 1a, the liquid in which the positive electrode active material or the like is dispersed is used. , Heptane and the like, and a nonpolar solvent can be preferably used. Moreover, the thickness of the positive electrode layer 1b is, for example, preferably from 0.1 μm to 1 mm, and more preferably from 1 μm to 100 μm. Moreover, in order to make it easy to improve the performance of the battery unit 1, the positive electrode layer 1b is preferably manufactured through a pressing process. In the present invention, the pressure when pressing the positive electrode layer can be about 100 MPa.

Moreover, as a negative electrode active material contained in the negative electrode layer 1d, a known negative electrode active material capable of occluding and releasing metal ions can be appropriately used. Examples of such a negative electrode active material include a carbon active material, an oxide active material, and a metal active material. The carbon active material is not particularly limited as long as it contains carbon, and examples thereof include mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of the oxide active material include Nb ₂ O ₅ , Li ₄ Ti ₅ O ₁₂ , and SiO. Examples of the metal active material include In, Al, Si, and Sn. Further, a lithium-containing metal active material may be used as the negative electrode active material. The lithium-containing metal active material is not particularly limited as long as it is an active material containing at least Li, and may be Li metal or Li alloy. Examples of the Li alloy include an alloy containing Li and at least one of In, Al, Si, and Sn.

  The shape of the negative electrode active material can be, for example, particulate or thin film. The average particle diameter (D50) of the negative electrode active material is, for example, preferably from 1 nm to 100 μm, and more preferably from 10 nm to 30 μm. Further, the content of the negative electrode active material in the negative electrode layer 1d is not particularly limited, but is preferably, for example, 40% or more and 99% or less in mass%.

  Furthermore, the negative electrode layer 1d can contain the solid electrolyte that can be contained in the positive electrode layer 1b. In addition, the negative electrode layer 1d may contain a binder for binding the negative electrode active material and the solid electrolyte, and a conductive material for improving conductivity. Examples of the binder and conductive material that can be contained in the negative electrode layer 1d include the binder and conductive material that can be contained in the positive electrode layer 1b. Further, when the negative electrode layer 1d is manufactured through a process of applying a slurry-like composition prepared by dispersing the negative electrode active material or the like in a liquid to the negative electrode current collector 1e, the liquid for dispersing the negative electrode active material or the like , Heptane and the like, and a nonpolar solvent can be preferably used. In addition, the thickness of the negative electrode layer 1d is, for example, preferably from 0.1 μm to 1 mm, and more preferably from 1 μm to 100 μm. Moreover, in order to make it easy to improve the performance of the battery unit 1, the negative electrode layer 1d is preferably manufactured through a pressing process. In the present invention, the pressure when pressing the negative electrode layer is preferably 200 MPa or more, more preferably about 400 MPa. In the present invention, the mass ratio of the positive electrode layer 1b and the negative electrode layer 1d is not particularly limited. However, from the viewpoint of sufficiently accepting ions moving between the positive electrode layer 1b and the negative electrode layer 1d, the negative electrode layer 1d It is preferable to increase the capacity of the positive electrode layer 1b.

  Moreover, as a solid electrolyte contained in the solid electrolyte layer 1c, a known solid electrolyte that can be used in a solid battery can be appropriately used. Examples of such a solid electrolyte include the solid electrolyte that can be contained in the positive electrode layer 1b. In addition, the solid electrolyte layer 1c can contain a binder for binding the solid electrolytes from the viewpoint of developing plasticity. Examples of such a binder include styrene butadiene rubber (SBR). However, in order to easily increase the output, the solid electrolyte layer 1c is formed on the solid electrolyte layer 1c from the viewpoint of preventing excessive aggregation of the solid electrolyte and forming a solid electrolyte layer 1c having a uniformly dispersed solid electrolyte. The binder to be contained is preferably 5% by mass or less. When the solid electrolyte layer 1c is manufactured through a process of applying the slurry-like composition prepared by dispersing the solid electrolyte or the like in the liquid to the positive electrode layer 1b or the negative electrode layer 1d, the liquid for dispersing the solid electrolyte or the like Examples thereof include heptane and the like, and a nonpolar solvent can be preferably used. The content of the solid electrolyte material in the solid electrolyte layer 1c is mass%, for example, preferably 60% or more, more preferably 70% or more, and particularly preferably 80% or more. The thickness of the solid electrolyte layer 1c varies greatly depending on the configuration of the battery. For example, the thickness is preferably 0.1 μm or more and 1 mm or less, and more preferably 1 μm or more and 100 μm or less.

  In this invention, the battery part 1 is used in the state accommodated in the exterior body. Examples of such an exterior body include a resin laminate film, a film obtained by vapor-depositing a metal on a resin laminate film, a metal case, and the like. Moreover, the battery part 1 can be produced by a method similar to a known method for producing a solid battery.

  In the present invention, the voltage detection means 2 is not particularly limited as long as the voltage of the battery unit 1 can be detected, and a known voltmeter or the like can be used as appropriate. Moreover, the discharge control part 3 will not be specifically limited if the discharge of the battery part 1 can be terminated before the battery voltage of the battery part 1 reaches -0.5V, for example, discharge It can be set as the form which has a well-known switch part which can switch continuation / stop of. In the present invention, the battery voltage immediately before the discharge of the battery unit 1 is stopped is not particularly limited as long as it is a voltage that can prevent an increase in battery resistance. However, as will be described later, in the discharge curve, about −0. Since an inflection point exists in the vicinity of 4V, for example, it is preferable to set it to -0.3V or more.

  In the above description related to the present invention, the battery system 10 having one battery unit 1 has been exemplified, but the number of battery units in the sulfide solid state battery system of the present invention is not limited to this, and the embodiment has a plurality of battery units. It is also possible to do. When the sulfide solid state battery system of the present invention has a plurality of battery parts, each battery part may be connected in series, may be connected in parallel, or a combination thereof.

  In the said description regarding this invention, although the form whose battery of this invention is a lithium ion secondary battery was illustrated, this invention is not limited to the said form. The battery of the present invention can also be configured such that ions other than lithium ions move between the positive electrode layer and the negative electrode layer. Examples of such ions include sodium ions and potassium ions. In the case where ions other than lithium ions move, the positive electrode active material, the solid electrolyte, and the negative electrode active material may be appropriately selected according to the moving ions.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

1. Production of battery part <Synthesis of sulfide solid electrolyte>
As starting materials, lithium sulfide (Li ₂ S, manufactured by Nippon Chemical Industry Co., Ltd.) and diphosphorus pentasulfide (P ₂ S ₅ , manufactured by Aldrich) were used. In a glove box under an Ar atmosphere (dew point −70 ° C.), 0.7656 g of Li ₂ S and 1.2344 g of P ₂ S ₅ were weighed and mixed in an agate mortar for 5 minutes. Then, 2 g of the obtained mixture was put into a planetary ball mill container (45 cc, made of ZrO ₂ ), 4 g of dehydrated heptane (moisture content of 30 ppm or less) was added, and 53 g of ZrO ₂ balls (φ = 5 mm) were added. The container was completely sealed (Ar atmosphere). This container was attached to a planetary ball mill (P7 manufactured by Fritsch), and mechanical milling was performed at a base plate rotation speed of 500 rpm for 40 hours. Thereafter, the obtained sample was dried so as to remove heptane on a hot plate to obtain a sulfide solid electrolyte.

<Preparation of positive electrode mixture>
224.9 mg of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (positive electrode active material, manufactured by Nichia Corporation), VGCF (vapor growth carbon fiber (“VGCF” is registered by Showa Denko KK) Trademarks.), Conductive material (made by Showa Denko KK) 3.0 mg, the above sulfide solid electrolyte 75.1 mg was weighed and mixed to obtain a positive electrode mixture. The surface of the positive electrode active material was previously coated with LiNbO ₃ by the same method as disclosed in JP 2010-73539 A.

<Preparation of negative electrode mixture>
9.06 mg of graphite (negative electrode active material, manufactured by Mitsubishi Chemical Corporation) and 8.24 mg of the above sulfide solid electrolyte were weighed and mixed to obtain a negative electrode mixture.

<Production of sulfide solid state battery>
A solid electrolyte layer was formed by placing 65 mg of the above sulfide solid electrolyte material in a 1 cm ² mold and pressing it at a pressure of 100 MPa. On the one surface side of the obtained solid electrolyte layer, 20.5 mg of the positive electrode mixture was placed and pressed at a pressure of 100 MPa to form a positive electrode layer. Next, 17.3 mg of the negative electrode mixture was placed on the other surface side of the solid electrolyte layer, and pressed at a pressure of 400 MPa to obtain a power generation element. SUS304 (positive electrode current collector, negative electrode current collector) was arranged on both surfaces of the obtained power generation element to obtain a battery part.

2. Measurement 2.1. Conditioning The obtained battery part was subjected to one cycle charge / discharge (constant current charge, constant current discharge) under the following conditions.
Charge / discharge current: 10 hour rate charge / discharge (charge / discharge current: 0.304 mA)
Charge stop voltage: 4.55V
Discharge stop voltage: 3.0V

2.2. Battery Resistance Evaluation Before Discharge Test After the above conditioning was completed, a constant current / constant voltage charge (end current: 0.015 mA) was performed with a charge stop voltage of 4.1 V, and immediately after that, the battery resistance was measured by an AC impedance method.

2.3. Discharge test After the battery resistance evaluation before the discharge test is completed, the battery is discharged to 2.5 V, and is charged with a constant current and a constant voltage as a charge stop voltage of 3.7 V (end current: 0.015 mA). Discharge was performed.
Discharge current: 1/8 hour rate charge / discharge (charge / discharge current: 24.32 mA)
Discharge stop voltage: 1 V (Example 1), 0 V (Example 2),
-0.5V (Comparative Example 1), -1V (Comparative Example 2)

2.4. Battery resistance evaluation after discharge test After the above discharge test was completed, a constant current and constant voltage charge (end current: 0.015 mA) was performed at a charge stop voltage of 4.1 V, and immediately after that, the battery resistance was measured by an AC impedance method. In the battery resistance measurement by the AC impedance method, a Cole-Cole plot as shown in FIG. 3 is obtained. As shown in FIG. 3, in this Cole-Cole plot, the point where the arc intersects the Z ′ axis is defined as DC resistance, and the size of the arc is defined as reaction resistance.

3. Results The battery resistance evaluation results are shown in Table 1 and FIG. The vertical axis in FIG. 4 is the rate of resistance change (ratio to the resistance measured before the discharge test), and the horizontal axis is the discharge stop voltage [V].

  As shown in Table 1 and FIG. 4, Example 1 and Example 2 were discharged so as not to be −0.5 V or lower, and Comparative Example 1 and Comparative Example 2 were discharged until −0.5 V. The direct current resistance was the same, and the reaction resistances of Example 1 and Example 2 were also the same, but the reaction resistances of Comparative Example 1 and Comparative Example 2 were larger than those of Example 1 and Example 2. It was.

  In FIG. 5, the discharge curve of the comparative example 2 is shown. The vertical axis of FIG. 5 is battery voltage [V], and the horizontal axis is time [s]. As shown in FIG. 5, since the inflection point is confirmed at about −0.4 V, this point is considered to be a threshold value.

  From the above, according to Example 1 and Example 2 (examples of the present invention) in which the discharge was controlled so that the voltage did not become −0.5 V or less, Comparative Example 1 in which discharging was performed until the voltage became −0.5 V or less As compared with Comparative Example 2, an increase in battery resistance could be prevented.

DESCRIPTION OF SYMBOLS 1 ... Battery part 1a ... Positive electrode collector 1b ... Positive electrode layer 1c ... Solid electrolyte layer 1d ... Negative electrode layer 1e ... Negative electrode collector 2 ... Voltage detection means 3 ... Discharge control means 10 ... Sulfide solid battery system

Claims (3)

A battery unit having a positive electrode layer containing a positive electrode active material and a negative electrode layer containing a negative electrode active material, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer;
Voltage detecting means for detecting the voltage of the battery unit;
A sulfide solid state battery system comprising: discharge control means for controlling discharge so that the voltage of the battery unit detected by the voltage detection means does not become −0.5 V or less. The sulfide solid state battery system according to claim 1, wherein the positive electrode active material is coated with an ion conductive oxide. The sulfide solid state battery system according to claim 1 or 2, wherein a sulfide solid electrolyte is provided in at least a part of the positive electrode layer and the electrolyte layer.

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