WO2020194823A1 - 全固体二次電池 - Google Patents
全固体二次電池 Download PDFInfo
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- WO2020194823A1 WO2020194823A1 PCT/JP2019/041711 JP2019041711W WO2020194823A1 WO 2020194823 A1 WO2020194823 A1 WO 2020194823A1 JP 2019041711 W JP2019041711 W JP 2019041711W WO 2020194823 A1 WO2020194823 A1 WO 2020194823A1
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- solid electrolyte
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- secondary battery
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
- C01D15/00—Lithium compounds
- C01D15/06—Sulfates; Sulfites
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
- H01M10/0561—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of inorganic materials only
- H01M10/0562—Solid materials
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2002/00—Crystal-structural characteristics
- C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
- C01P2002/72—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by d-values or two theta-values, e.g. as X-ray diagram
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2002/00—Crystal-structural characteristics
- C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
- C01P2002/74—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by peak-intensities or a ratio thereof only
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M2300/00—Electrolytes
- H01M2300/0017—Non-aqueous electrolytes
- H01M2300/0065—Solid electrolytes
- H01M2300/0068—Solid electrolytes inorganic
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- the present invention relates to an all-solid-state secondary battery.
- Non-Patent Document 1 proposes to use a solidified body obtained by homogeneously melting Li 2 SO 4 and Li OH and then quenching it as a solid electrolyte. In particular, it is said that this solid electrolyte can be used for devices that operate at low temperatures.
- Non-Patent Document 1 cannot be said to have sufficiently high lithium ion conductivity at room temperature. Further, the solid electrolyte of Non-Patent Document 1 has a small temperature dependence of conductivity, and the effect of increasing conductivity due to temperature increase cannot be expected. That is, this solid electrolyte cannot be said to be a material having sufficient lithium ion conductivity from room temperature to high temperature. In response to these problems, the present inventors have obtained the finding that the solid electrolyte represented by 3LiOH ⁇ Li 2 SO 4 exhibits high lithium ion conductivity at 25 ° C. However, it has been found that the material having the above composition alone has another problem that the lithium ion conductivity tends to decrease when the material is held at a high temperature for a long time.
- the present inventors have stated that by further adding boron to the solid electrolyte identified as 3LiOH / Li 2 SO 4 , the decrease in lithium ion conductivity can be significantly suppressed even after being held at a high temperature for a long time. I got the knowledge of.
- an object of the present invention is to provide an all-solid-state secondary battery provided with a 3LiOH ⁇ Li 2 SO 4- based solid electrolyte capable of significantly suppressing a decrease in lithium ion conductivity even after being held at a high temperature for a long time. There is.
- an all-solid-state secondary battery using a solid electrolyte which is a solid electrolyte identified as 3LiOH ⁇ Li 2 SO 4 by X-ray diffraction, wherein the solid electrolyte further contains boron. Will be done.
- the solid electrolyte used in the all-solid secondary battery of the present invention is a solid electrolyte identified as 3LiOH ⁇ Li 2 SO 4 by X-ray diffraction. And this solid electrolyte further contains boron. 3LiOH ⁇ Li 2 SO 4 by causing further contains boron in solid electrolyte identified as can significantly suppress a decrease in lithium ion conductivity even after holding at a high temperature for a long time. That is, as described above, the present inventors have obtained the finding that the solid electrolyte represented by 3LiOH ⁇ Li 2 SO 4 exhibits high lithium ion conductivity at 25 ° C.
- the above composition alone has another problem that the lithium ion conductivity tends to decrease when the lithium ion conductivity is held at a high temperature for a long time.
- it is possible to solve the above problem further be contained boron solid electrolyte identified as 3LiOH ⁇ Li 2 SO 4.
- the mechanism by which the maintenance of ionic conductivity can be improved by the inclusion of boron is not clear, but according to X-ray diffraction measurements, the inclusion of boron shifts the diffraction peak of 3LiOH / Li 2 SO 4 slightly to the higher angle side. since that boron is incorporated into one of the sites of the crystal structure of 3LiOH ⁇ Li 2 SO 4, it is presumed that to improve the stability against the temperature of the crystal structure.
- the solid electrolyte used in the present invention is preferably used for a power storage element such as a lithium ion secondary battery and a capacitor, and particularly preferably used for a lithium ion secondary battery.
- the lithium ion secondary battery may be an all-solid-state battery (for example, an all-solid-state lithium-ion secondary battery).
- the lithium ion secondary battery may be a liquid battery (for example, a lithium air battery) in which a solid electrolyte is used as a separator and an electrolytic solution is provided between the separator and the counter electrode.
- the solid electrolyte used in the present invention is a solid electrolyte identified as 3LiOH ⁇ Li 2 SO 4 by X-ray diffraction. That is, the solid electrolyte contains 3LiOH ⁇ Li 2 SO 4 as the main phase. Whether or not the solid electrolyte contains 3 LiOH / Li 2 SO 4 can be confirmed by identifying the X-ray diffraction pattern using 032-0598 of the ICDD database.
- “3LiOH / Li 2 SO 4 " refers to a crystal structure that can be regarded as the same as that of 3LiOH / Li 2 SO 4, and the crystal composition does not necessarily have to be the same as that of 3LiOH / Li 2 SO 4 .
- the solid electrolyte is a main phase in addition 3LiOH ⁇ Li 2 SO 4, may be included heterophase.
- the heterogeneous phase may contain a plurality of elements selected from Li, O, H, S and B, or may consist only of a plurality of elements selected from Li, O, H, S and B. It may be.
- Examples of the heterogeneous phase include LiOH, Li 2 SO 4 and / or Li 3 BO 3 derived from the raw material. Regarding these heterogeneous phases, it is considered that unreacted raw materials remained when forming 3 LiOH / Li 2 SO 4 , but since they do not contribute to lithium ion conduction, the smaller the amount, the better, except for Li 3 BO 3. desirable.
- a heterogeneous phase containing boron such as Li 3 BO 3
- the solid electrolyte may be composed of a single phase of 3LiOH / Li 2 SO 4 in which boron is dissolved.
- the solid electrolyte used in the present invention further contains boron.
- 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, still more preferably. It is 0.005 or more and 0.8 or less.
- the absolute value of the lithium ion conductivity may be lowered, but if the B / S is within the above range, the content of the unreacted heterogeneous phase containing boron is low, so that the lithium ion conductivity is low.
- the absolute value of the degree can be increased.
- the lower limit is not particularly limited, but is typically 0.08 ° or more, and more typically 0.1 °. That is all.
- the solid electrolyte used in the present invention may be a green compact obtained by crushing a melt-coagulated product, but a melt-solidified product (that is, one solidified after heating and melting) is preferable.
- the solid electrolyte used in the present invention is produced through a step of forming a solid body by melting and cooling a raw material containing LiOH, Li 2 SO 4 and Li 3 BO 3. can do.
- the raw material used in this case has a composition represented by xLiOH, Li 2 SO 4 , yLi 3 BO 3 (in the formula, 2.0 ⁇ x ⁇ 4, 0.002 ⁇ y ⁇ 1) for ionic conductivity. It is preferable from the viewpoint, but it is not limited to this as long as the desired characteristics can be obtained (for example, 1.0 ⁇ x ⁇ 4 may be obtained).
- a solidified body is formed by cooling a melt of a raw material containing LiOH, Li 2 SO 4 and Li 3 BO 3 (preferably a raw material having the above composition), and (b) It can be carried out by crushing the solidified body to obtain a solid electrolyte powder, and (c) forming the solid electrolyte powder, or by forming the solid electrolyte powder by melting the solid electrolyte powder again and then cooling and solidifying the solid electrolyte powder.
- the cooling of the melt in the above (a) may be either rapid cooling or slow cooling (for example, furnace cooling).
- the pulverization method in (b) above can be carried out by putting a boulder such as zirconia balls and a solidified body of a solid electrolyte into a container and pulverizing according to a known method and conditions.
- the molding in the step (c) can be performed by various methods such as pressing (for example, die pressing and rubber pressing), and is preferably a die pressing.
- the temperature lowering rate at the time of cooling after the solid electrolyte powder is melted again in the step (c) is preferably 10 to 1000 ° C./h, more preferably 10 to 100 ° C./h.
- the solid electrolyte used in the present invention is preferably used for the all-solid-state secondary battery. That is, according to a preferred embodiment of the present invention, an all-solid secondary battery using the above-mentioned solid electrolyte is provided.
- This all-solid-state secondary battery includes a solid electrolyte according to the present invention between the positive electrode and the negative electrode. Then, at least a part or all of the solid electrolyte constitutes a lithium ion conductive material layer.
- the positive electrode a positive electrode generally used for a lithium secondary battery can be used, but it is preferable that the positive electrode contains a lithium composite oxide.
- the lithium composite oxide is Li x MO 2 (0.05 ⁇ x ⁇ 1.10, M is at least one transition metal, and M is typically 1 of Co, Ni, Mn and Al. It is an oxide represented by (including seeds and above).
- the lithium composite oxide preferably has a layered rock salt structure or a spinel-type structure. Examples of lithium composite oxides having a layered rock salt structure include Li x CoO 2 (lithium cobaltate), Li x NiO 2 (lithium nickelate), Li x MnO 2 (lithium manganate), and Li x NimnO 2 (nickel).
- Lithium manganate Li x NiCoO 2 (lithium nickel cobaltate), LixCoNiMnO 2 (cobalt, nickel manganate lithium), Li x ComnO 2 (cobalt lithium manganate), Li 2 MnO 3 , and the above compounds.
- Examples include a solid solution of and. Particularly preferred are Li x CoNiMnO 2 (lithium cobalt oxide manganate) and Li x CoO 2 (lithium cobalt oxide, typically LiCoO 2 ).
- Examples of the lithium composite oxide having a spinel structure include LiMn 2 O 4 series materials, LiNi 0.5 Mn 1.5 O 4 series materials, and the like.
- Lithium composite oxides include 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 may contain one or more elements selected from. Further, LiMPO 4 having an olivine structure (M is at least one selected from Fe, Co, Mn and Ni in the formula) and the like can also be preferably used.
- the positive electrode may be in the form of a mixture of a positive electrode active material, an electron conductive auxiliary agent, a lithium ion conductive material, a binder and the like, which is generally called a mixture electrode, or a sintered plate obtained by sintering a positive electrode raw material powder. It may be in the form of.
- the sintered plate may be a dense body or a porous body, and a solid electrolyte may be contained in the pores of the porous body. Further, a protective layer for suppressing the reaction between the positive electrode and the solid electrolyte and a layer for reducing the interfacial resistance may be introduced between the positive electrode active material and the solid electrolyte.
- a negative electrode generally used for lithium secondary batteries can be used.
- general negative electrode materials include carbon-based materials, metals or semimetals such as Li, In, Al, Sn, Sb, Bi, and Si, or alloys containing any of these.
- an oxide-based negative electrode may be used.
- a particularly preferable negative electrode contains a material capable of inserting and removing lithium ions at 0.4 V (vs. Li / Li + ) or higher, and preferably contains Ti.
- the negative electrode active material satisfying such conditions is preferably an oxide containing at least Ti.
- Preferred examples of such a negative electrode active material include lithium titanate Li 4 Ti 5 O 12 (hereinafter, LTO), niobium-titanium composite oxide Nb 2 TIO 7 , and titanium oxide TiO 2 , and more preferably LTO and Nb. 2 TiO 7 , more preferably LTO.
- LTO is typically known to have a spinel-type structure, other structures may be adopted during charging / discharging. For example, LTO reacts in a two-phase coexistence of 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.
- the negative electrode may be in the form of a mixture of a negative electrode active material, an electron conductive auxiliary agent, a lithium ion conductive material, a binder and the like, which is generally called a mixture electrode, or a sintered plate obtained by sintering a negative electrode raw material powder. It may be in the form of.
- the sintered plate may be a dense body or a porous body, and a solid electrolyte may be contained in the pores of the porous body. Further, a protective layer for suppressing the reaction between the negative electrode and the solid electrolyte and a layer for reducing the interfacial resistance may be introduced between the negative electrode active material and the solid electrolyte.
- a positive electrode having a current collector and a negative electrode having a current collector are prepared, and ii) a solid electrolyte is sandwiched between the positive electrode and the negative electrode to pressurize the battery.
- a solid electrolyte is sandwiched between the positive electrode and the negative electrode to pressurize the battery. This can be done by integrating the positive electrode, the solid electrolyte, and the negative electrode by heating or the like.
- the positive electrode, the solid electrolyte, and the negative electrode may be bonded by other methods.
- a method of placing a molded body or powder of the solid electrolyte on one of the electrodes, and a screen printing of a paste of the solid electrolyte powder on the electrode examples thereof include a method of colliding and solidifying the solid electrolyte powder 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 an electrophoresis method to form a film.
- Examples 1-17 Preparation of raw material powder Li 2 SO 4 powder (commercially available, purity 99% or more), LiOH powder (commercially available, purity 98% or more), and Li 3 BO 3 (commercially available product, purity 99% or more) are shown.
- the raw material mixed powder was obtained by mixing so as to have the molar ratio shown in 1. These powders were handled in a glove box in an Ar atmosphere with a dew point of ⁇ 50 ° C. or lower, and sufficient care was taken not to cause deterioration such as moisture absorption.
- a pellet-shaped solid electrolyte having a diameter of 10 mm was formed by pressing a solid electrolyte powder with a die at a pressure of 250 MPa in a glove box in a molten Ar atmosphere.
- a pellet-shaped solid electrolyte is sandwiched between two stainless steel (SUS) electrodes having a diameter of 10 mm and a thickness of 0.5 mm, a weight of 15 g is placed on the obtained laminate, and the mixture is heated at 400 ° C. for 45 minutes. By doing so, the solid electrolyte was melted. Then, the melt was cooled at 100 ° C./h to form a solidified body.
- SUS stainless steel
- the solid electrolyte in the ion conductivity measurement cell was measured lithium ion conductivity C 1 in the same manner as described above.
- the conductivity retention rate after holding it at 150 ° C. for 100 hours was asked.
- Quantitative analysis of boron and sulfur was performed on the solid electrolytes obtained in each example. Quantitative analysis was performed on each of boron and sulfur by ICP emission spectroscopy (ICP-AES) and a calibration curve method. Each analytical value of boron and sulfur was converted into the number of moles and calculated as B / S.
- ICP-AES ICP emission spectroscopy
- Results Table 1 summarizes the production conditions and evaluation results of the solid electrolytes of Examples 1 to 17.
- the weight loss is 1% or less in the steps of melting the raw material mixed powder containing LiOH, Li 2 SO 4 and Li 3 BO 3 to synthesize a solid electrolyte, and the step of remelting the solid electrolyte powder. It is presumed that the composition of Li, O, H, S and B constituting the solid electrolyte is almost unchanged from the composition at the time of preparation.
- boron is assumed that a solid solution in the backbone of 3LiOH ⁇ Li 2 SO 4 crystalline phase. It was found to contain a solid electrolyte identified as 3LiOH ⁇ Li 2 SO 4 in agreement with 032-0598 in the ICDD database except for the high angle shift.
- Examples 1 to 4, 6 to 10 and 12 to 17 synthesized by adding Li 3 BO 3 the B / S was a value larger than 0 in the chemical analysis, and the solid electrolyte contained boron. I understood.
- the ionic conductivity retention rate is as small as 75% or less, and as in Example 10, the B / S is 0.002 or more, so that the ionic conductivity retention rate is as large as 80% or more. It turned out to be.
- the conductivity of Examples 1 and 3 after holding at 150 ° C. for 100 hours was compared, it was found that the conductivity was low in Example 1. It is presumed that this is because the content of unreacted heterogeneous phase was high because the amount of Li 3 BO 3 added was large, and it was found that the B / S indicating the amount of boron added was preferably less than 1.0.
- Example 14 has higher ionic conductivity than Example 17. Focusing on the peak intensity ratio by X-ray diffraction (I Li2SO4 / I LHS), since the value is larger in Example 17, it is presumed that Li 2 SO 4 is left as a heterogeneous phase, which inhibits the ionic conductivity It is presumed that it was done. Therefore, Li 2 SO 4 is the case is detected as heterophase, the peak intensity ratio (I Li2SO4 / I LHS) is considered to preferably less than 1.1.
- An all-solid-state secondary battery was prepared using the solid electrolytes of Examples 3 and 5, and the resistance increase rate after holding at 150 ° C. for 100 hours was confirmed.
- a positive electrode a dense sintered plate of lithium cobalt oxide having a current collector layer formed on one side was prepared, and as a negative electrode, a dense sintered plate of lithium titanate having a current collector layer formed on one side was prepared.
- a powder press body of a solid electrolyte was sandwiched between these positive electrode plates and negative electrode plates, and cells were formed while pressurizing. The obtained cell was allowed to stand at 150 ° C., AC impedance was measured, and the resistance R 0 immediately after the temperature was raised to 150 ° C.
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Abstract
Description
本発明の全固体二次電池に用いる固体電解質は、X線回折により3LiOH・Li2SO4と同定される固体電解質である。そして、この固体電解質はホウ素をさらに含む。3LiOH・Li2SO4と同定される固体電解質にホウ素をさらに含有させることで、高温で長時間保持した後においてもリチウムイオン伝導度の低下を有意に抑制することができる。すなわち、前述したとおり、本発明者らは、3LiOH・Li2SO4で表される固体電解質が25℃において高いリチウムイオン伝導度を呈するとの知見を得ている。しかしながら、上記組成のみでは高温で長時間保持した場合にリチウムイオン伝導度が低下しやすいとの別の問題があることが分かってきた。この点、3LiOH・Li2SO4と同定される固体電解質にホウ素をさらに含有させることで上記問題を解決することができる。ホウ素の含有によりイオン伝導度維持度を向上できるメカニズムは定かではないが、X線回折測定によると、ホウ素を含有させることにより、3LiOH・Li2SO4の回折ピークがわずかに高角側にシフトしていることから、ホウ素は3LiOH・Li2SO4の結晶構造のサイトのいずれかに取り込まれ、結晶構造の温度に対する安定性を向上させているものと推察される。
本発明の好ましい態様によれば、本発明に用いる固体電解質は、LiOH、Li2SO4及びLi3BO3を含む原料を溶融して冷却することによって凝固体を形成する工程を経て製造することができる。この場合に用いる原料はxLiOH・Li2SO4・yLi3BO3(式中、2.0≦x≦4、0.002≦y≦1)で表される組成を有するのがイオン伝導度の観点から好ましいが、所望の特性が得られるかぎりこれに限定されない(例えば1.0≦x≦4であってもよい)。例えば、固体電解質の製造は、(a)LiOH、Li2SO4及びLi3BO3を含む原料(好ましくは上記組成の原料)の溶融物を冷却することによって凝固体を形成し、(b)凝固体を粉砕することによって固体電解質粉末とし、(c)固体電解質粉末を成形すること又は固体電解質粉末を再度溶融後冷却して固化することによって固体電解質を形成することにより行うことができる。上記(a)における溶融物の冷却は急冷又は徐冷(例えば炉冷)のいずれでもよい。上記(b)における粉砕の方法は、公知の手法及び条件に従い、容器にジルコニアボール等の玉石と固体電解質の凝固体を投入して粉砕することにより行うことができる。上記(c)工程における成形は、プレス(例えば金型プレス、ラバープレス)等の様々な手法により行うことができ、好ましくは金型プレスである。上記(c)工程における固体電解質粉末の再度の溶融後の冷却時の降温速度は10~1000℃/hであるのが好ましく、より好ましくは10~100℃/hである。
前述のとおり、本発明に用いる固体電解質は全固体二次電池に用いられるのが好ましい。すなわち、本発明の好ましい態様によれば上記固体電解質を用いた全固体二次電池が提供される。この全固体二次電池は、正極と負極との間に本発明による固体電解質を備える。そして、固体電解質の少なくとも一部又は全部がリチウムイオン伝導材料層を構成する。
(1)原料粉末の準備
Li2SO4粉末(市販品、純度99%以上)、LiOH粉末(市販品、純度98%以上)、及びLi3BO3(市販品、純度99%以上)を表1に示されるモル比となるように混合して原料混合粉末を得た。これらの粉末は、露点-50℃以下のAr雰囲気中のグローブボックス中で取り扱い、吸湿等の変質が起こらないように十分に注意した。
Ar雰囲気中で原料混合粉末を高純度アルミナ製のるつぼに投入し、このるつぼを電気炉にセットし、430℃で2時間熱処理を行い溶融物を作製した。引き続き、電気炉内にて100℃/hで溶融物を冷却して凝固物を形成した。
得られた凝固物をAr雰囲気中にて乳鉢で粉砕することによって、平均粒径D50が5~50μmの固体電解質粉末を得た。
Ar雰囲気中のグローブボックス内で、固体電解質粉末を250MPaの圧力で金型プレスすることによって、直径10mmのペレット状の固体電解質を形成した。直径10mm、厚さ0.5mmの2枚のステンレス鋼(SUS)電極の間にペレット状の固体電解質を挟み、得られた積層物の上に15gの重しを載せ、400℃で45分加熱することにより固体電解質を溶融させた。その後、100℃/hで溶融物を冷却して凝固体を形成した。
得られた凝固体(固体電解質)に対して以下の評価を行った。
固体電解質をX線回折装置(XRD、X線源:CuKα線)で分析することによりX線回折パターンを得た。なお、金属Si粉を内部標準として添加して2θ位置を合わせた。得られたX線回折パターンとICDDデータベースの032-0598とを対比することによって、3LiOH・Li2SO4結晶相の同定を行い、3LiOH・Li2SO4の有無を判定した。また、上記得られたXRDプロファイルに基づき、3LiOH・Li2SO4と同定される2θ=18.4°付近のピークの半値幅を算出した。さらに、3LiOH・Li2SO4と同定される2θ=18.4°付近のピーク強度ILHSに対する、LiOHと同定される2θ=20.5°付近のピーク強度ILiOHの比(ILiOH/ILHS)を算出した。同様に、3LiOH・Li2SO4と同定される2θ=18.4°付近のピーク強度ILHSに対する、Li2SO4と同定される2θ=22.2°付近のピーク強度ILi2SO4の比(ILi2SO4/ILHS)を算出した。結果は表1に示されるとおりであった。
固体電解質のリチウムイオン伝導度を一般的な交流インピーダンス測定を用いて以下のようにして測定した。まず、Ar雰囲気中において、固体電解質を2枚のステンレス鋼(SUS)電極の間に挟み、セル(宝泉株式会社製、コインセルCR2032)に入れて密閉し、イオン伝導度測定用セルを作製した。このイオン伝導度測定用セルを150℃の恒温乾燥器に入れ、交流インピーダンス測定装置(BioLogic社製、VMP3)を用いて交流インピーダンス法によりコンダクタンス(1/r)を測定した。測定した値とリチウムイオン伝導度σ=L/r(1/A)の式に基づき、初期リチウムイオン伝導度C0を算出した。
各例で得られた固体電解質についてホウ素と硫黄の定量分析を行った。ホウ素及び硫黄の各々についてICP発光分光分析法(ICP-AES)にて、検量線法で定量分析を行った。ホウ素及び硫黄の各分析値をモル数に換算し、B/Sとして算出した。
例1~17の固体電解質の作製条件及び評価結果を表1にまとめて示す。例1~17において、LiOH、Li2SO4及びLi3BO3を含む原料混合粉末を溶融して固体電解質を合成する工程や、固体電解質粉末を再度溶融する工程において、重量減は1%以下と非常に小さいものであり、固体電解質を構成するLi、O、H、S及びBの組成は調合時の組成からほとんど変化していないものと推測される。
例3と例5の固体電解質を用いて全固体二次電池を作製し、150℃100時間保持後の抵抗増加率を確認した。正極として、片側の面に集電層を形成したコバルト酸リチウムの緻密焼結板を用意し、負極として、片側の面に集電層を形成したチタン酸リチウムの緻密焼結板を用意した。これらの正極板及び負極板で固体電解質の粉末プレス体を挟み込み、加圧しながらセル化した。得られたセルを150℃で静置し、交流インピーダンス測定を行い、固体電解質部分の抵抗として、150℃昇温直後の抵抗R0と、150℃100時間保持後の抵抗をR1とを測定した。得られた抵抗値から抵抗増加率R1/R0を算出したところ、例3(ホウ素を添加した実施例)では1.0であったのに対し、例5(ホウ素を添加しなかった比較例)では1.3となり、3LiOH・Li2SO4にホウ素を添加した電解質を用いた全固体二次電池では抵抗の増加が小さいことが分かった。このことから、例5のセルでは150℃で100時間経過後に固体電解質部分の抵抗増加に起因して充放電容量が低下するが、例3のセルでは150℃で100時間経過しても固体電解質部分の抵抗増加に起因する容量低下がなく充放電できることが分かった。
Claims (7)
- X線回折により3LiOH・Li2SO4と同定される固体電解質であって、前記固体電解質がホウ素をさらに含む、固体電解質を用いた全固体二次電池。
- 前記固体電解質中に含まれる硫黄Sに対する、前記ホウ素Bのモル比である、B/Sが0.002超1.0未満である、請求項1に記載の全固体二次電池。
- 前記固体電解質は、CuKαを線源としたX線回折パターンにおける、3LiOH・Li2SO4と同定される2θ=18.4°付近のピークの半値幅が0.500°以下である、請求項1又は2に記載の全固体二次電池。
- 前記固体電解質は、CuKαを線源としたX線回折パターンにおける、3LiOH・Li2SO4と同定される2θ=18.4°付近のピーク強度ILHSに対する、LiOHと同定される2θ=20.5°付近のピーク強度ILiOHの比である、ILiOH/ILHSが0.234未満である、請求項1~3のいずれか一項に記載の全固体二次電池。
- 前記固体電解質は、CuKαを線源としたX線回折パターンにおける、3LiOH・Li2SO4と同定される2θ=18.4°付近のピーク強度ILHSに対する、Li2SO4と同定される2θ=22.2°付近のピーク強度ILi2SO4の比である、ILi2SO4/ILHSが1.10未満である、請求項1~4のいずれか一項に記載の全固体二次電池。
- 前記固体電解質が溶融凝固体である、請求項1~5のいずれか一項に記載の全固体二次電池。
- 正極と負極との間に前記固体電解質を備える、請求項1~6のいずれか一項に記載の全固体二次電池。
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