JP7615048B2 - Composite electrode and battery using same - Google Patents

Description

The present invention relates to a composite electrode and a battery using the same.

In recent years, with the development of portable devices such as personal computers and mobile phones, the demand for batteries as their power source has expanded significantly. In batteries used for such purposes, electrolytes (electrolytic solutions) using flammable organic solvents as diluting solvents have traditionally been used as a medium for moving ions. In batteries using such electrolyte solutions, problems such as electrolyte leakage, fire, and explosion may occur. In order to solve such problems, and to ensure essential safety, development of all-solid-state batteries is underway that use solid electrolytes instead of liquid electrolytes and that are composed of all other elements that are solid. Since such all-solid-state batteries use solid electrolytes, there is no risk of fire, they do not leak, and they are less likely to have problems such as deterioration of battery performance due to corrosion.

Various all-solid-state batteries have been proposed. For example, Patent Document 1 (JP 2009-193940 A) discloses that in a pressed powder all-solid-state battery of a sulfide-based solid electrolyte and lithium cobalt oxide, the surface of the lithium cobalt oxide is coated with lithium niobate to reduce the interface resistance. Reducing the interface resistance leads to improved charge/discharge characteristics. The battery disclosed in Patent Document 1 is an all-solid-state battery using a pressed powder, and the energy density of the electrode decreases if pores remain between the particles or if a conductive additive that ensures electronic conduction between active materials is added.

In contrast, all-solid-state batteries using sintered electrodes instead of compact electrodes have also been proposed. Such batteries have the advantage that the energy density is high because the sintered electrodes do not contain conductive additives. For example, Patent Document 2 (WO2019/093222A1) discloses an all-solid-state lithium battery comprising an oriented positive electrode plate that is a lithium composite oxide sintered body plate with a porosity of 10 to 50%, a negative electrode plate that contains Ti and is capable of inserting and desorbing lithium ions at 0.4 V (vs. Li/Li ⁺ ) or more, and a solid electrolyte having a melting point lower than the melting point or decomposition temperature of the oriented positive electrode plate or negative electrode plate. This document discloses various materials such as Li ₃ OCl, xLiOH.yLi ₂ SO ₄ (wherein x+y=1, 0.6≦x≦0.95) (e.g., 3LiOH.Li ₂ SO ₄ ) as solid electrolytes having such low melting points. Such solid electrolytes can be infiltrated into the gaps between the electrodes as a melt, resulting in strong interfacial contact, which is believed to result in significant improvements in battery resistance and rate performance during charging and discharging, as well as a significant improvement in battery manufacturing yields.

Incidentally, lithium batteries that have not only a solid electrolyte layer but also an electrolyte solution layer are known. For example, Patent Document 3 (JP 2017-54792 A) discloses a lithium battery that has a positive electrode layer, an anode layer made of a self-supporting foil, a solid electrolyte layer disposed between the positive electrode layer and the anode layer, and an electrolyte solution layer disposed between the solid electrolyte layer and the anode layer. It is said that an organic electrolyte solution, an ionic liquid electrolyte solution, or a mixture thereof can be used as the electrolyte that constitutes the electrolyte solution layer.

JP 2009-193940 A WO2019/093222A1 JP 2017-54792 A

As described above, an all-solid-state battery using a sintered electrode and a low-melting-point solid electrolyte can improve battery resistance and rate performance during charging and discharging, as well as improve battery manufacturing yields (see Reference 2). However, when a cell was constructed using a sintered electrode as disclosed in Reference 2 and a low-melting- _point solid electrolyte such as _3LiOH.Li2SO4 and operated as a battery, it was found that the discharge amount was lower than the theoretical capacity expected from the amount of active material.

The inventors have now discovered that by impregnating ionic liquid into the gaps of a composite electrode comprising a porous sintered plate composed of an electrode active material and a low-melting point solid electrolyte filled in the pores, it is possible to provide a composite electrode that can significantly improve the discharge capacity when incorporated into a battery.

Therefore, the object of the present invention is to provide a composite electrode that can significantly improve the discharge capacity when incorporated into a battery.

According to one aspect of the present invention,
A porous sintered plate made of an electrode active material;
A solid electrolyte having a melting point of 600° C. or less, which is filled in the holes of the porous sintered plate;
An ionic liquid impregnated in gaps between the porous sintered plate and the solid electrolyte;
A composite electrode is provided, comprising:

According to another aspect of the present invention,
The composite electrode;
A counter electrode;
a solid electrolyte layer provided between the composite electrode and a counter electrode;
A battery is provided comprising:

Composite electrode The composite electrode of the present invention comprises a porous sintered plate, a solid electrolyte, and an ionic liquid. The porous sintered plate is composed of an electrode active material. The solid electrolyte is a solid electrolyte having a melting point of 600°C or less, and is filled into the holes of the porous sintered plate. The ionic liquid is impregnated into the gaps between the porous sintered plate and the solid electrolyte. By impregnating the gaps of the composite electrode including the porous sintered plate composed of the electrode active material and the low-melting point solid electrolyte filled into the holes with the ionic liquid in this way, it is possible to provide a composite electrode that can significantly improve the discharge capacity when incorporated into a battery.

It is believed that this significant improvement in discharge capacity is due to the fact that the ionic liquid penetrates between the electrode active material (porous sintered plate) and the solid electrolyte (at the interface, etc.), thereby ensuring a stable ion conduction path at that location. In particular, in all-solid-state lithium secondary batteries, gaps are likely to form between the electrode active material and the solid electrolyte due to the expansion/contraction of the electrode active material that accompanies charging and discharging, and the ion conduction path is likely to be interrupted in these gaps. In this regard, it is believed that the presence of the ionic liquid in these gaps assists lithium ion conduction, thereby solving the above problem.

(1) Porous Sintered Plate The porous sintered plate is composed of an electrode active material. The electrode active material may be either a positive electrode active material or a negative electrode active material. That is, the positive electrode active material and/or the negative electrode active material is in the form of a sintered plate. The sintered plate does not need to contain an electronic conduction assistant or a binder, so that the energy density of the electrode can be increased. The porous sintered plate is a sintered body having a large number of holes (voids), and a solid electrolyte and an ionic liquid can be present in the holes. In addition, a protective layer for suppressing the reaction between the electrode active material and the solid electrolyte, or a layer for reducing the interface resistance may be introduced between the electrode active material and the solid electrolyte.

The proportion of the electrode active material in the total volume of the composite electrode (regardless of whether it is the positive electrode or the negative electrode) is preferably 50 to 80 volume %, more preferably 55 to 80 volume %, even more preferably 60 to 80 volume %, and particularly preferably 65 to 75 volume %. Within this range, a sufficient amount of solid electrolyte and ionic liquid can be present in the pores of the porous sintered plate while ensuring a high energy density, so that the advantages of the solid electrolyte and ionic liquid (improved battery resistance and rate performance during charging and discharging, improved battery manufacturing yield, and improved discharge capacity) can be more effectively realized.

(1a) Positive Electrode Active Material The positive electrode active material that can be used to form the porous sintered plate can be a positive electrode active material generally used in lithium secondary batteries, but preferably contains a lithium composite oxide. The lithium composite oxide is an oxide represented by Li _x MO ₂ (0.05<x<1.10, M is at least one transition metal, and M typically contains one or more of Co, Ni, Mn, and Al). The lithium composite oxide preferably has a layered rock salt structure or a spinel structure. More preferably, the positive electrode active material has a layered rock salt structure. Examples of lithium composite oxides having a layered rock salt structure include Li _x CoO ₂ (lithium cobalt oxide), Li _x NiO ₂ (lithium nickel oxide), Li _x MnO ₂ (lithium manganese oxide), Li _x NiMnO ₂ (lithium nickel manganese oxide), Li _x NiCoO ₂ (lithium nickel cobalt oxide), Li _x CoNiMnO ₂ (lithium cobalt nickel manganese oxide), Li _x CoMnO ₂ (lithium cobalt manganese oxide), Li ₂ MnO ₃ , and solid solutions of the above compounds. Particularly preferred are Li _x CoNiMnO ₂ (lithium cobalt nickel manganese oxide) and Li _x CoO ₂ (lithium cobalt oxide, typically LiCoO ₂ ). Particularly preferred lithium composite oxides having a layered rock salt structure are lithium cobalt nickel _manganese oxide (e.g., Li( _{Ni0.5Co0.2Mn0.3} ) _O2 ) or lithium cobalt oxide (typically _LiCoO2 ). On the other hand, examples _of lithium composite oxides having a spinel structure include _LiMn2O4 - _based materials and _{LiNi0.5Mn1.5O4 - based} materials.

The lithium composite oxide may contain one or more elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te, Ba, Bi, and W. In addition, LiMPO ₄ (wherein M is at least one selected from Fe, Co, Mn, and Ni) having an olivine structure can also be suitably used.

From the viewpoint of improving the energy density of the battery, the thickness of the positive electrode active material or its sintered plate is preferably 50 to 350 μm, more preferably 100 to 325 μm, even more preferably 125 to 300 μm, and particularly preferably 150 to 275 μm.

(1b) Negative electrode active material As the negative electrode active material that can constitute the porous sintered plate, an oxide-based negative electrode active material generally used in lithium secondary batteries can be preferably used. Particularly preferred negative electrode active materials include a material capable of inserting and desorbing lithium ions at 0.4 V (vs. Li/Li ⁺ ) or more, and preferably contain Ti. A negative electrode active material that satisfies such conditions is preferably an oxide containing at least Ti. Preferred examples of such negative electrode active materials include lithium titanate Li ₄ Ti ₅ O ₁₂ (hereinafter, LTO), niobium titanium composite oxide Nb ₂ TiO ₇ , and titanium oxide TiO ₂ , more preferably LTO and Nb ₂ TiO ₇ , and even more preferably LTO. Note that LTO is known to have a typical spinel structure, but other structures can also be adopted during charging and discharging. For example, in LTO, the reaction proceeds in two phases, Li ₄ Ti ₅ O ₁₂ (spinel structure) and Li ₇ Ti ₅ O ₁₂ (rock salt structure), during charging and discharging. Therefore, LTO is not limited to the spinel structure.

From the viewpoint of improving the energy density of the battery, the thickness of the negative electrode active material or its sintered plate is preferably 50 to 350 μm, more preferably 100 to 325 μm, even more preferably 125 to 300 μm, and particularly preferably 150 to 275 μm.

(2) Solid electrolyte The solid electrolyte filled in the holes of the porous sintered plate is not particularly limited as long as it has a melting point of 600 ° C or less, but preferably has a melting point of 250 to 550 ° C, more preferably 275 to 500 ° C, and even more preferably 300 to 450 ° C. By having such a melting point, the solid electrolyte can be filled in the holes of the porous sintered plate through pressure, heating, etc. The proportion of the solid electrolyte in the total volume of the composite electrode other than the electrode active material is preferably 60 to 99 volume %, more preferably 70 to 99 volume %, even more preferably 80 to 99 volume %, and particularly preferably 90 to 99 volume %.

The above _- mentioned low melting point solid electrolyte is preferably a _LiOH.Li2SO4 -based solid electrolyte. _{The LiOH.Li2SO4 -} based solid electrolyte is a composite compound of LiOH and _Li2SO4 , and has a typical composition of the general formula: _{xLiOH.yLi2SO4} (wherein x+ _y = ₁ , 0.6≦x≦0.95), and a representative example is _3LiOH.Li2SO4 (wherein x=0.75, y=0.25 in the above general formula ₎ . Preferably, the _LiOH.Li2SO4 - _based solid electrolyte is a solid _electrolyte identified as _3LiOH.Li2SO4 by X-ray diffraction. This preferred solid electrolyte contains _3LiOH.Li2SO4 as the main _phase . Whether or not 3LiOH.Li ₂ SO ₄ is contained in the solid electrolyte can be confirmed by identifying the X-ray diffraction pattern using 032-0598 of the ICDD database. Here, "3LiOH.Li ₂ SO ₄ " refers to a substance whose crystal structure can be regarded as being the same as 3LiOH.Li ₂ SO ₄ , and the crystal composition does not necessarily have to be the same as 3LiOH.Li ₂ SO _4. In other words, as long as it has a crystal structure equivalent to 3LiOH.Li ₂ SO ₄ , those whose composition is outside of LiOH:Li ₂ SO ₄ = 3:1 are also included in the solid electrolyte of the present invention. Therefore, even if the solid electrolyte contains a dopant such as boron (for example, 3LiOH.Li ₂ SO ₄ in which boron is dissolved and the X-ray diffraction peak is shifted to the higher angle side), it will be referred to as 3LiOH.Li ₂ SO ₄ in this specification as long as the crystal structure can be considered to be the same as 3LiOH.Li ₂ SO _4. Similarly, the solid electrolyte used in the present invention is also allowed to contain inevitable impurities.

Therefore, the LiOH.Li ₂ SO ₄ solid electrolyte may contain a heterogeneous phase in addition to the main phase 3LiOH.Li ₂ SO _4. The heterogeneous phase may contain a plurality of elements selected from Li, O, H, S, and B, or may be composed of only a plurality of elements selected from Li, O, H, S, and B. Examples of the heterogeneous phase include LiOH, Li ₂ SO ₄ , and/or Li ₃ BO ₃ derived from the raw material. These heterogeneous phases are considered to be unreacted raw materials remaining when forming 3LiOH.Li ₂ SO ₄ , but since they do not contribute to lithium ion conduction, it is desirable that the amount of the heterogeneous phase other than Li ₃ BO ₃ is small. However, the heterogeneous phase containing boron such as Li ₃ BO ₃ may be contained in a desired amount because it can contribute to improving the lithium ion conductivity retention rate after long-term storage at high temperatures. However, the solid electrolyte may be composed of a single phase of 3LiOH.Li ₂ SO ₄ in which boron is dissolved.

It is preferable that the LiOH.Li ₂ SO ₄ solid electrolyte (particularly 3LiOH.Li ₂ SO ₄ ) further contains boron. By further containing boron in the solid electrolyte identified as 3LiOH.Li ₂ SO ₄ , the decrease in lithium ion conductivity can be significantly suppressed even after long-term storage at high temperatures. It is presumed that boron is incorporated into one of the sites of the crystal structure of 3LiOH.Li ₂ SO ₄ , improving the stability of the crystal structure against temperature. The molar ratio (B/S) of boron B to sulfur S contained in the solid electrolyte is preferably more than 0.002 and less than 1.0, more preferably 0.003 or more and 0.9 or less, and even more preferably 0.005 or more and 0.8 or less. If B/S is within the above range, it is possible to improve the maintenance rate of lithium ion conductivity. In addition, if B/S is within the above range, the content of unreacted heterogeneous phases containing boron is low, so that the absolute value of lithium ion conductivity can be increased.

The LiOH.Li ₂ SO ₄ based solid electrolyte may be a compact of a powder obtained by pulverizing a molten solid, but is preferably a molten solid (i.e., solidified after being heated and melted).

The LiOH.Li ₂ SO ₄ solid electrolyte is melted and enters the pores in the positive electrode (positive electrode active material) and/or negative electrode (negative electrode active material) to form a part of the composite electrode, but the remaining part is preferably interposed between the positive electrode and the negative electrode as a solid electrolyte layer. The thickness of the solid electrolyte layer (excluding the part that enters the pores in the positive electrode and negative electrode) is preferably 1 to 100 μm, more preferably 3 to 50 μm, and even more preferably 5 to 40 μm, from the viewpoint of charge/discharge rate characteristics and insulating properties of the solid electrolyte.

(3) Ionic Liquid Ionic liquid is a salt that exists as a liquid over a wide temperature range (e.g., room temperature) and typically has a melting point of 100° C. or less. The ionic liquid is impregnated into the gaps between the porous sintered plate and the solid electrolyte. The ionic liquid contains an ionic liquid cation, an ionic liquid anion, and an electrolyte. Ionic liquid cations include imidazolium, pyridinium, pyrrolidinium, piperidinium, ammonium, and phosphonium cations, and examples thereof include 1-ethyl-3-methylimidazolium cation (EMI), 1-methyl-1-propylpyrrolidinium cation (MPPy), N-methyl-N-propylpyrrolidinium cation (P13), N-methyl-N-propylpiperidinium cation (PP13), N-butyl-N-methylpyrrolidinium cation (BMP), N,N-diethyl-N-methyl-N(2-methoxyethyl)ammonium cation (DEME), tetraamyl(pentyl)ammonium cation, tetraethylammonium cation, N-butyl-N-methylpyrrolidinium cation, derivatives thereof, and any combinations thereof. Examples of ionic liquid anions include bis(trifluoromethanesulfonyl)imide anion (TFSI), bis(fluorosulfonyl)imide anion (FSI), inorganic fluorine anions, and combinations thereof. Examples of the electrolyte include lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium tetrafluoroborate, and combinations thereof. As the glyme-based ionic liquid, a mixed solution of an oligoether-based solvent (G3, G4, etc.) and LiTFSI can also be used.

及びビス（フルオロスルホニル）イミドアニオン（ＦＳＩ）：

から選択される、アニオンと、
（ｉｉｉ）ビス（トリフルオロメタンスルホニル）イミドリチウム塩（ＬｉＴＦＳＩ）及びビス（フルオロスルホニル）イミドリチウム塩（ＬｉＦＳＩ）から選択されるリチウム塩である、電解質と、
を含む。特に好ましくは、アニオンがＴＦＳＩであり、かつ、リチウム塩がＬｉＴＦＳＩである。この組成のイオン液体は、固体電解質（特にＬｉＯＨ・Ｌｉ_２ＳＯ_４系固体電解質）に対して、高温（例えば１００℃あるいは１２０℃以上）での電池動作時においても、化学的に安定であり、電池出力の向上に寄与する。また、イオン液体中におけるリチウム塩の濃度は、０．１０～２．５ｍｏｌ／Ｌであるのが好ましく、より好ましくは０．５～２．５ｍｏｌ／Ｌ、さらに好ましくは１．０～２．０ｍｏｌ／Ｌである。このような範囲内であると、望ましいリチウムイオン伝導度が得られ、その結果、電池出力の向上をより効果的に実現できる。 Preferred ionic liquids are
(i) a cation; and
(ii) Bis(trifluoromethanesulfonyl)imide anion (TFSI):

and bis(fluorosulfonyl)imide anion (FSI):

and an anion selected from
(iii) an electrolyte, which is a lithium salt selected from bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) and bis(fluorosulfonyl)imide lithium salt (LiFSI);
Particularly preferably, the anion is TFSI and the lithium salt is LiTFSI. An ionic liquid of this composition is chemically stable against a solid electrolyte (particularly a LiOH.Li ₂ SO ₄ -based solid electrolyte) even during battery operation at high temperatures (for example, 100°C or 120°C or higher), and contributes to improving battery output. The concentration of the lithium salt in the ionic liquid is preferably 0.10 to 2.5 mol/L, more preferably 0.5 to 2.5 mol/L, and even more preferably 1.0 to 2.0 mol/L. Within such a range, a desirable lithium ion conductivity is obtained, and as a result, improvement in battery output can be more effectively achieved.

及び１－メチル－１－プロピルピロリジニウムカチオン（ＭＰＰｙ）：

から選択されるのが好ましく、より好ましくはＭＰＰｙである。 In the above preferred ionic liquid, the type of cation is not particularly limited, but may be 1-ethyl-3-methylimidazolium cation (EMIm):

and 1-methyl-1-propylpyrrolidinium cation (MPPy):

It is preferably selected from the following, more preferably MPPy.

(4) Operating Temperature The composite electrode according to the present invention, when incorporated into a battery as a positive electrode or a negative electrode, can operate at preferably 50 to 200° C., more preferably 60 to 180° C., even more preferably 70 to 160° C., particularly preferably 80 to 160° C., and even more particularly preferably 100 to 150° C. By being capable of operating at such high temperatures, the ionic conductivity of the solid electrolyte, ionic liquid, and electrode active material is improved, and the electrode can operate at a high output relative to room temperature.

(5) Battery The composite electrode of the present invention is used in a battery together with a solid electrolyte layer and a counter electrode. That is, according to a preferred embodiment of the present invention, a battery is provided that includes the composite electrode of the present invention, a counter electrode, and a solid electrolyte layer provided between the composite electrode and the counter electrode. When the composite electrode is a positive electrode, the counter electrode is a negative electrode. On the other hand, when the composite electrode is a negative electrode, the counter electrode is a positive electrode. The counter electrode may also be the composite electrode of the present invention.

Manufacturing method The battery having the composite electrode of the present invention can be manufactured, for example, by i) preparing a positive electrode sintered plate (with a current collector formed as necessary) and a negative electrode sintered plate (with a current collector formed as necessary), ii) sandwiching a solid electrolyte between the positive electrode sintered plate and the negative electrode sintered plate and applying pressure, heating, etc. to integrate the positive electrode, the solid electrolyte, and the negative electrode, and iii) immersing the obtained cell in an ionic liquid and allowing it to penetrate into the cell under reduced pressure (for example, vacuuming to 10 Pa). The positive electrode, the solid electrolyte, and the negative electrode may be bonded by other methods. In this case, examples of the method for forming a solid electrolyte between the positive electrode and the negative electrode include a method of placing a molded body or powder of the solid electrolyte on one electrode, a method of applying a paste of the solid electrolyte powder on the electrode by screen printing, a method of impacting and solidifying the powder of the solid electrolyte by an aerosol deposition method or the like using the electrode as a substrate, and a method of depositing the solid electrolyte powder on the electrode by electrophoresis to form a film.

The present invention will be described more specifically with reference to the following examples. In the following examples, Li _{( Ni0.5Co0.2Mn0.3} ) _O2 is abbreviated as "NCM( ₅₂₃ )" and _Li4Ti5O12 is abbreviated as _{" LTO} ".

Examples 1 to 4
(1) Preparation of solid electrolyte powder (1a) Preparation of raw material powder _Li2SO4 powder (commercial _product , purity 99% or more), LiOH powder (commercial product, purity 98 _% or more), and _Li3BO3 (commercial product, purity 99% or more) were mixed in a molar ratio of 3:1:0.05 to obtain a raw material mixed powder. These powders were handled in a glove box in an Ar atmosphere with a dew point of -50°C or less, and sufficient care was taken to prevent deterioration such as moisture absorption.

(1b) Melt synthesis The raw material mixed powder was placed in a crucible in an Ar atmosphere, and the crucible was set in an electric furnace and heat-treated at 430° C. for 2 hours to produce a melt. The melt was then cooled in the electric furnace at 100° C./h to form a solidified product.

(1c) Mortar Grinding The obtained coagulated product was pulverized in a mortar in an Ar atmosphere to obtain a solid electrolyte powder having an average particle size D50 of 5 to 50 μm.

(2) Preparation of positive electrode plate (2a) Preparation of NCM (523) molded body Commercially available (Ni _0.5 Co _0.2 Mn _0.3 ) (OH) ₂ powder (average particle size 9 μm) and Li ₂ CO ₃ powder (average particle size 2.5 μm) were mixed so that the molar ratio of Li / (Ni + Co + Mn) was 1.15, and then held at 840 ° C. for 15 hours to obtain a powder consisting of NCM (523) particles. After pulverizing this powder, it was passed through a nylon mesh, and then uniaxially pressed in a mold to produce an NCM (523) molded body. The thickness of the NCM (523) molded body was set to a value such that the thickness after sintering was 100 μm.

(2b) Preparation of NCM (523) sintered plate The NCM (523) molded body was placed on a setter and placed in a sintering sheath. The obtained laminate was heated to 920 ° C. at a heating rate of 200 ° C. / h and held for 10 hours to perform sintering. After sintering, the temperature was lowered to room temperature and the sintered body was removed from the sheath. In this way, the NCM (523) sintered plate was obtained as a positive electrode plate. An Au film (thickness 100 nm) was formed as a current collecting layer on one side of the obtained NCM (523) sintered plate by sputtering, and then laser processed into a circular shape with a diameter of 10 mm.

(3) Preparation of negative electrode plate (3a) Preparation of LTO green sheet Commercially available LTO powder (volume-based D50 particle size 0.7 μm), a dispersion medium, a binder, a plasticizer, and a dispersant were mixed. The obtained negative electrode raw material mixture was stirred under reduced pressure to adjust the viscosity, thereby preparing an LTO slurry. The slurry thus prepared was formed into a sheet on a film to form an LTO green sheet. The thickness of the LTO green sheet after drying was set to a value such that the thickness after firing was 100 μm.

(3b) Firing of LTO green sheet The obtained green sheet was cut into 25 mm squares and placed on a setter. The green sheet on the setter was placed in a firing sheath, heated at a heating rate of 200°C/h, and fired at 800°C for 5 hours. A Au film (thickness 100 nm) was formed as a current collecting layer by sputtering on the surface of the obtained LTO sintered body plate that had been in contact with the setter, and then laser processed into a circular shape with a diameter of 10 mm.

(4) Melting In a glove box in an Ar atmosphere, the solid electrolyte powder was pressed into a mold to form a pellet-shaped solid electrolyte having a diameter of 10 mm. The pellet-shaped solid electrolyte was sandwiched between positive and negative electrode plates having a diameter of 10 mm, a weight was placed on the obtained laminate, and the solid electrolyte was melted by heating at 400° C. for 45 minutes to form a cell.

(5) Image analysis The obtained cell was cross-sectionally polished by ion milling from a direction perpendicular to the stacking surface (a surface parallel to the electrode surface), and then a cross-sectional SEM image of the solid electrolyte impregnated portion in each of the composite electrodes (positive and negative electrodes) from a direction horizontal to the cell stacking surface was obtained by SEM (scanning electron microscope). The cross-sectional SEM image was an image with a magnification of 50,000 times. The obtained image was first subjected to a 100% blurring process with a 2D filter using image analysis software (Image-ProPremier manufactured by MediaCybernetics), and then subjected to binarization processing, and the proportion (volume %) of the solid electrolyte and the remainder derived from the pores in the total capacity other than the electrode active material in each of the composite electrodes (positive and negative electrodes) was calculated. The threshold value for binarization was set using Otsu's binarization as a discriminant analysis method.

Similarly, a cross-sectional SEM image of the composite electrode was obtained from a direction horizontal to the cell stacking surface (plane parallel to the electrode surface). The cross-sectional SEM image was taken at a magnification of 5000x. The obtained image was first processed with a 2D filter to achieve 100% blurring, and then binarized using image analysis software (Image-ProPremier, manufactured by MediaCybernetics), and the proportion (volume %) of the electrode active material and other materials in the total capacity of each of the composite electrodes (positive and negative electrodes) was calculated. The threshold value for binarization was set using Otsu's binarization as a discriminant analysis method.

(6) Preparation and impregnation of ionic liquid (Examples 2 to 4 only)
In Example 2, ionic liquid IL1 was obtained by dissolving commercially available LiFSI (lithium bis(fluorosulfonyl)imide) in commercially available MPPyFSI (N,N-methylpropylpyrrolidinium bis(fluorosulfonyl)imide) in a glove box under an Ar atmosphere, and adjusting the concentration of LiFSI to 1.5 M. On the other hand, in Examples 3 and 4, ionic liquid IL2 was obtained by dissolving commercially available LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) in commercially available MPPyTFSI (N,N-methylpropylpyrrolidinium bis(trifluoromethanesulfonyl)imide) in a glove box under an Ar atmosphere, and adjusting the concentration of LiTFSI to 1.5 M.

The obtained cell was immersed in ionic liquid in a glove box in an Ar atmosphere. After that, Ar was evacuated using a rotary pump until it reached 10 Pa, and the ionic liquid was impregnated into the cell. Note that in Example 1, the cell was not impregnated with ionic liquid.

(7) Charge/Discharge Test (7a) Preparation of Cell for Charge/Discharge Measurement In a glove box in an Ar atmosphere, the prepared cell was placed in a coin cell (manufactured by Hosen Co., Ltd., coin cell CR2032) and sealed to prepare a cell for charge/discharge measurement.

(7b) Charge/Discharge Evaluation The discharge capacity of the prepared charge/discharge test cell at an operating temperature of 100°C (Examples 1-3) or 120°C (Example 4) was measured in the voltage range of 2.7V-1.5V by the following procedure. After charging at a rate of 0.01C until the battery voltage reached the upper limit of the voltage range, the value when the battery was discharged at a constant current (CC) of 200μA (Examples 1-4) or 40μA (Examples 1 and 2) until the lower limit of the voltage range was taken as the discharge capacity. The discharge capacity of Example 1 (a comparative example in which the cell was not impregnated with ionic liquid) was taken as a reference value of 100, and the discharge capacity of Example 2 was calculated as a relative value.

(8) Evaluation results Image analysis showed that the proportion of solid electrolyte in the total capacity of the parts other than the electrode active material (residual pores and solid electrolyte) in the positive electrode as a composite electrode was 77%. Although it was not possible to perform microstructural observation in a state containing ionic liquid, it is presumed that all or part of the remaining 23% part not containing solid electrolyte is impregnated with ionic liquid. Similarly, image analysis showed that the proportion of electrode active material in the total capacity of the positive electrode as a composite electrode was 65%.

Image analysis also revealed that the proportion of solid electrolyte in the total volume of the parts of the negative electrode as a composite electrode other than the electrode active material (residual pores and solid electrolyte) was 94%. Although it was not possible to observe the microstructure when it contained ionic liquid, it is presumed that ionic liquid is impregnated into all or part of the remaining 6% of the part that does not contain solid electrolyte. Image analysis also revealed that the proportion of electrode active material in the total volume of the negative electrode as a composite electrode was 68%.

As a result of the charge/discharge evaluation, as shown in Table 1, the discharge capacity (relative value) of Examples 2 to 4 in which the cells were impregnated with ionic liquid was 114 to 153, relative to the discharge capacity of Example 1 (Comparative Example) in which the cell was not impregnated with ionic liquid, which was 100, and it was discovered that a high discharge capacity can be achieved by impregnating the cell with ionic liquid. This is presumably because the ionic liquid penetrates into pores not filled with solid electrolyte and into the interface between the solid electrolyte and the electrode active material, thereby supporting lithium ion conduction between the solid electrolyte and the electrode active material, and the charge/discharge results also showed that ionic liquid was contained in the electrode portion in the cells of Examples 2 to 4 compared to Example 1.

Of particular note, in Examples 3 and 4 using ionic liquid IL2 containing LiTFSI at 1.5 mol/L in MPPyTFSI, a higher discharge capacity was obtained compared to Examples 1 and 2 using ionic liquid IL1 containing LiFSI at 1.5 mol/L in MPPyFSI. That is, it was found that LiTFSI-based ionic liquids are more effective in terms of improving output than LiFSI-based ionic liquids. In particular, in Example 4 using LiTFSI-based ionic liquid, it was possible to operate at 120°C (higher than the operating temperature of 100°C in Examples 1 to 3), and a very high discharge capacity was obtained. The superiority of LiTFSI-based ionic liquids over LiFSI-based ionic liquids is an unexpected result. This is because the ionic conductivity of the LiTFSI-based ionic liquid IL2 used in Examples 3 and 4 is 3.8 mS/cm at 25° C., which is lower than the ionic conductivity of the LiFSI-based ionic liquid IL1 used in Example 2, which is 8.3 mS/cm at 25° C. It is generally understood that the LiFSI-based ionic liquid IL1, which has a high ionic conductivity, is advantageous in terms of battery performance. However, the results revealed this time are contrary to such a general understanding, and the LiTFSI-based ionic liquid IL2, which has a low ionic conductivity, provided advantageous results, which can be said to be evidence that the superiority of the LiTFSI-based ionic liquid IL2 was unexpected. This is thought to be a unique characteristic of the combination of the LiOH.Li ₂ SO ₄ -based solid electrolyte and the LiTFSI-based ionic liquid IL2.

Claims (10)

A porous sintered plate made of an electrode active material;
A solid electrolyte having a melting point of 600° C. or less, which is filled in the holes of the porous sintered plate;
An ionic liquid impregnated in gaps between the porous sintered plate and the solid electrolyte;
Equipped with
The composite electrode , wherein the solid electrolyte is identified as 3LiOH.Li2SO4 by X-ray diffraction and further _{contains boron} . The composite electrode according to claim 1, wherein the proportion of the solid electrolyte in the total volume of the composite electrode other than the electrode active material is 60 to 99 volume %. The composite electrode according to claim 1 or 2, which can operate at 50 to 200°C when incorporated into a battery as a positive or negative electrode. The composite electrode according to any one of claims 1 to 3, wherein the electrode active material has a layered rock salt structure. The composite electrode according to any one of claims 1 to 4, wherein the proportion of the electrode active material in the total volume of the composite electrode is 50 to 80 volume %. The ionic liquid is
Cation,
an anion selected from bis(trifluoromethanesulfonyl)imide anion (TFSI) and bis(fluorosulfonyl)imide anion (FSI);
an electrolyte, which is a lithium salt selected from bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) and bis(fluorosulfonyl)imide lithium salt (LiFSI);
The composite electrode according to any one of claims 1 to 5 , comprising: 7. The composite electrode of claim 6 , wherein the anion is TFSI and the lithium salt is LiTFSI. 8. The composite electrode according to claim 6 , wherein the cation is a 1-methyl-1-propylpyrrolidinium cation (MPPy). A composite electrode according to any one of claims 1 to 8 ;
A counter electrode;
a solid electrolyte layer provided between the composite electrode and a counter electrode;
Equipped with a battery. The battery according to claim 9 , wherein the counter electrode is also the composite electrode according to any one of claims 1 to 8 .