Ion conducting glass-ceramics, method for manufacturing same and all-solid-state secondary battery including same

Abstract

An ion conducting glass-ceramics represented by the general formula (I): Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b, wherein M and N are different and selected from P, Si, Ge, B, Al and Ga; x, y, a and b are integers indicating the stoichiometric ratio depending on the species of M and N; and the content of Na.sub.2S is more than 60 mol % and less than 80 mol %.

Claims

1. An ion conducting glass-ceramics represented by a general formula (I): Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b, wherein the Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b is Na.sub.2SP.sub.2S.sub.5SiS.sub.2; and a content of Na.sub.2S is more than 60 mol % and less than 80 mol %.

2. The ion conducting glass-ceramics according to claim 1, wherein the Na.sub.2SP.sub.2S.sub.5SiS.sub.2 contains 0.1 to 10 mol % of SiS.sub.2.

3. The ion conducting glass-ceramics according to claim 1, which does not have a glass transition temperature of a corresponding glass having the same general formula as the ion conducting glass ceramics.

4. The ion conducting glass-ceramics according to claim 1, wherein the ion conducting glass-ceramics comprises crystalline portions dispersed in amorphous glass components.

5. The ion conducting glass-ceramics according to claim 4, wherein the crystalline portions have a cubic Na.sub.3PS.sub.4 crystal structure.

6. An ion conducting glass-ceramics represented by a general formula (I): Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b, wherein M and N are different and selected from P, Si, Ge, B, Al and Ga; x, y, a and b are integers indicating a stoichiometric ratio depending on the species of M and N; and a content of Na.sub.2S is more than 60 mol % and less than 80 mol %, wherein the ion conducting glass-ceramics comprises crystalline portions dispersed in amorphous glass components, and wherein the crystalline portions have a cubic Na.sub.3PS.sub.4 crystal structure.

7. The ion conducting glass-ceramics according to claim 6, wherein the Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b is Na.sub.2SP.sub.2S.sub.5SiS.sub.2, Na.sub.2SP.sub.2S.sub.5GeS.sub.2, Na.sub.2SP.sub.2S.sub.5B.sub.2S.sub.3, Na.sub.2SP.sub.2S.sub.5Al.sub.2S.sub.3, Na.sub.2SP.sub.2S.sub.5Ga.sub.2S.sub.3, Na.sub.2SSiS.sub.2GeS.sub.2, Na.sub.2SSiS.sub.2B.sub.2S.sub.3, Na.sub.2SSiS.sub.2Al.sub.2S.sub.3, Na.sub.2SSiS.sub.2Ga.sub.2S.sub.3, Na.sub.2SGeS.sub.2B.sub.2S.sub.3, Na.sub.2SGeS.sub.2Al.sub.2S.sub.3, Na.sub.2SGeS.sub.2Ga.sub.2S.sub.3, Na.sub.2SB.sub.2S.sub.3Al.sub.2S.sub.3, Na.sub.2SB.sub.2S.sub.3Ga.sub.2S.sub.3 or Na.sub.2SAl.sub.2S.sub.3Ga.sub.2S.sub.3.

8. A method for manufacturing the ion conducting glass-ceramics according to claim 6, comprising the steps of: subjecting a starting material mixture containing Na.sub.2S, M.sub.xS.sub.y and N.sub.aS.sub.b at a predetermined proportion that provides the general formula (I): Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b to mechanical milling treatment to obtain a glass; and subjecting the glass to heat treatment at a temperature at or above the crystallization temperature thereof to convert the glass to the ion conducting glass-ceramics.

9. The method for manufacturing the ion conducting glass-ceramics according to claim 8, wherein the mechanical milling treatment is carried out with a planetary ball mill under conditions of 50 to 600 rpm, 0.1 to 50 hours and 1 to 100 kWh/kg of the starting material mixture.

10. An all-solid-state secondary battery comprising at least a positive electrode, a negative electrode and a solid electrolyte layer between the positive electrode and the negative electrode, wherein the solid electrolyte layer comprises the ion conducting glass-ceramics according to claim 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the conductivity at room temperature of the glass-ceramics of Example 1;

(2) FIG. 2 is a graph showing the temperature dependency of the conductivity for the glass-ceramics of Example 2;

(3) FIG. 3 shows XRD patterns for the glass-ceramics of Example 3;

(4) FIG. 4 shows Raman spectra for the glass-ceramics of Example 3; and

(5) FIG. 5 shows a cyclic voltammogram for the glass-ceramics of Example 4.

MODE FOR CARRYING OUT THE INVENTION

(6) (Ion Conducting Glass-Ceramics)

(7) The glass-ceramics may have crystalline portions dispersed in amorphous glass components. The proportion of the crystalline portions is preferably, relative to the whole glass-ceramics, 50% by weight or more and more preferably 80% by weight or more. The upper limit of the proportion is 100% by weight. The proportion of the crystalline portions can be determined on solid-state NMR.

(8) The glass-ceramics is preferably the one that does not have the glass transition temperature of the corresponding glass.

(9) The ion conducting glass-ceramics of the present invention is represented by the general formula (I): Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b, wherein M and N are different and selected from P, Si, Ge, B, Al and Ga; x, y, a and b are integers indicating the stoichiometric ratio depending on the species of M and N; and the content of Na.sub.2S is more than 60 mol % and less than 80 mol %. Specifically, Na.sub.2SP.sub.2S.sub.5SiS.sub.2, Na.sub.2SP.sub.2S.sub.5GeS.sub.2, Na.sub.2SP.sub.2S.sub.5B.sub.2S.sub.3, Na.sub.2SP.sub.2S.sub.5Al.sub.2S.sub.3, Na.sub.2SP.sub.2S.sub.5Ga.sub.2S.sub.3, Na.sub.2SSiS.sub.2GeS.sub.2, Na.sub.2SSiS.sub.2B.sub.2S.sub.3, Na.sub.2SSiS.sub.2Al.sub.2S.sub.3, Na.sub.2SSiS.sub.2Ga.sub.2S.sub.3, Na.sub.2SGeS.sub.2B.sub.2S.sub.3, Na.sub.2SGeS.sub.2Al.sub.2S.sub.3, Na.sub.2SGeS.sub.2Ga.sub.2S.sub.3, Na.sub.2SB.sub.2S.sub.3Al.sub.2S.sub.3, Na.sub.2SB.sub.2S.sub.3Ga.sub.2S.sub.3, Na.sub.2SAl.sub.2S.sub.3Ga.sub.2S.sub.3 and the like may be mentioned. Among these, Na.sub.2SP.sub.2S.sub.5SiS.sub.2 is particularly preferred. The glass-ceramics may further contain an additional ion conductive material such as NaI and Na.sub.3PO.sub.4.

(10) The present inventors believe that the ion conducting material of the present invention contains 3 components and therefore can provide glass and glass-ceramics containing a higher amount of Na ions than the material containing 2 components such as Na.sub.2SP.sub.2S.sub.5.

(11) Furthermore, the Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b contains more than 60 mol % and less than 80 mol % of Na.sub.2S. When the amount of Na.sub.2S is within this range, the material can have improved ion conductivity compared to the corresponding glass. The content of Na.sub.2S is more preferably more than 65 mol % and less than 80 mol % and further preferably 67 to 78 mol %.

(12) When M.sub.xS.sub.y represents P.sub.2S.sub.5, the content of N.sub.aS.sub.b is preferably in the range such that the crystal structure of Na.sub.2SP.sub.2S.sub.5 can be maintained. In the case of Na.sub.2SP.sub.2S.sub.5SiS.sub.2, the proportion of SiS.sub.2 in Na.sub.2SP.sub.2S.sub.5SiS.sub.2 is preferably in the range of 0.1 to 10 mol %. When the proportion is in this range, Na.sub.2SP.sub.2S.sub.5SiS.sub.2 can have significantly improved conductivity compared to that of Na.sub.2SP.sub.2S.sub.5, resulting in provision of an all-solid-state secondary battery having an improved charge and discharge efficiency. The proportion of SiS.sub.2 is more preferably in the range of 4 to 8 mol %.

(13) (Method for Manufacturing Ion Conducting Glass-Ceramics)

(14) The method for manufacturing the ion conducting glass-ceramics includes the steps of:

(15) (i) subjecting a starting material mixture containing Na.sub.2S, M.sub.xS.sub.y and N.sub.aS.sub.b at a predetermined proportion that provides the general formula (I): Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b to mechanical milling treatment to obtain a glass; and

(16) (ii) subjecting the glass to heat treatment at a temperature at or above the crystallization temperature thereof to convert the glass to the ion conducting glass-ceramics.

(17) (1) Step (i)

(18) The mechanical milling treatment in the step (i) is not particularly limited as to the treatment instrument and treatment conditions as far as the starting materials are sufficiently mixed and allowed to react.

(19) The treatment instrument used may usually be a ball mill. Ball mills are preferred because they can provide high mechanical energy. Among ball mills, a planetary ball mill is preferred because it can efficiently generate high impact energy due to rotation of a pot as well as revolution of a stage.

(20) The treatment conditions may be appropriately selected according to the treatment instrument used. When a ball mill is used for example, the starting materials may be further uniformly mixed and allowed to react when the rotation speed is increased and/or the treatment period is extended. The term and/or in the context of A and/or B means A, B or A and B. Specifically, when a planetary ball mill is used, the conditions may be the rotation speed of 50 to 600 rpm, the treatment period of 0.1 to 50 hours and 1 to 100 kWh/kg of the starting material mixture. More preferred treatment conditions may be the rotation speed of 200 to 500 rpm, the treatment period of 1 to 20 hours and 6 to 50 kWh/kg of the starting material mixture.

(21) (2) Step (ii)

(22) The glass obtained in the step (i) is subjected to heat treatment to convert the glass to the ion conducting glass-ceramics. The heat treatment is carried out at a temperature at or above the crystallization temperature of the glass.

(23) The glass transition temperature (T.sub.g) varies depending on the proportion between Na.sub.2S, M.sub.xS.sub.y and N.sub.aS.sub.b. For example, Na.sub.2SP.sub.2S.sub.5 has the glass transition temperature in the range of 180 to 200 C. and the first crystallization temperature (T.sub.c) in the range of 190 to 240 C. The upper limit of the temperature during heat treatment is not particularly limited and generally is the first crystallization temperature+100 C.

(24) The heat treatment period is the period during which the glass may be converted to the ion conducting glass-ceramics. The heat treatment temperature is high with a shortened heat treatment period and is low with a lengthened heat treatment period. The heat treatment period is generally in the range of 0.1 to 10 hours.

(25) (Application of Ion Conducting Glass-Ceramics)

(26) The ion conducting glass-ceramics may have any applications for which ion conductivity is required. For example, solid electrolyte layers for all-solid-state secondary batteries and all-solid-state capacitors, conductive layers for sensors and the like may be mentioned. Among these, the ion conducting glass-ceramics is preferably used as a solid electrolyte layer for all-solid-state secondary batteries.

(27) The all-solid-state secondary battery generally includes, but is not particularly limited to, at least a positive electrode, a negative electrode and a solid electrolyte layer between the positive electrode and the negative electrode.

(28) (1) Solid Electrolyte Layer

(29) The solid electrolyte layer contains the ion conducting glass-ceramics (Na.sub.2S-M.sub.xS.sub.yN.sub.aS.sub.b). The solid electrolyte layer may further contain, in addition to the ion conducting glass-ceramics, an electrolyte (e.g., NaI or Na.sub.3PO.sub.4) that is usually contained in all-solid-state secondary batteries. The solid electrolyte layer preferably contains the ion conducting glass-ceramics at a proportion of 80% by weight or more and more preferably the ion conducting glass-ceramics accounts for the whole amount of the solid electrolyte layer. The solid electrolyte layer preferably has a thickness of 1 to 1000 m and more preferably 1 to 200 m. The solid electrolyte layer may be obtained, for example, in the form of pellets by pressing the starting materials thereof.

(30) (2) Positive Electrode

(31) The positive electrode is not particularly limited. The positive electrode may contain only a positive electrode active material or may further contain a binder, a conductive agent, an electrolyte and the like mixed with the positive electrode active material.

(32) The positive electrode active material may include various transition metal compounds such as Na.sub.0.44MnO.sub.2, NaNi.sub.0.5Mn.sub.0.5O.sub.2, FeS, TiS.sub.2, NaCoO.sub.2, NaFeO.sub.2, NaCrO.sub.2, Na.sub.3V.sub.2(PO.sub.4).sub.3 and NaMn.sub.2O.sub.4, sulfur, sodium sulfide, sodium polysulfide and the like.

(33) The binder may include, for example, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyvinyl acetate, poly(methyl methacrylate), polyethylene and the like.

(34) The conductive agent may include natural graphite, artificial graphite, acetylene black, ketjen black, Denka black, carbon black, vapor grown carbon fibers (VGCFs) and the like.

(35) The electrolyte may include those used for solid electrolyte layers.

(36) The positive electrode can be obtained, for example, in the form of pellets by mixing the positive electrode active material and an optional binder, conductive agent, electrolyte and the like and pressing the obtained mixture.

(37) The positive electrode may be formed on a current collector such as aluminum or copper.

(38) (3) Negative Electrode

(39) The negative electrode is not particularly limited. The negative electrode may contain only a negative electrode active material or may further contain a binder, a conductive agent, an electrolyte and the like mixed with the negative electrode active material.

(40) The negative electrode active material may include metals such as Na, In and Sn, Na alloys, graphite, hard carbon and various transition metal oxides such as Li.sub.4/3Ti.sub.5/3O.sub.4, Na.sub.3V.sub.2(PO.sub.4).sub.3 and SnO.

(41) The binder, conductive agent and electrolyte may be those mentioned above in the section of the positive electrode.

(42) The negative electrode can be obtained, for example, in the form of pellets by mixing the negative electrode active material and an optional binder, conductive agent, electrolyte and the like and pressing the obtained mixture. The negative electrode active material that is a metal sheet (foil) of a metal or an alloy thereof can be used as it is.

(43) The negative electrode may be formed on a current collector such as aluminum or copper.

(44) (4) Method for Producing All-Solid-State Secondary Battery

(45) The all-solid-state secondary battery can be obtained, for example, by stacking and pressing the positive electrode, the electrolyte layer and the negative electrode.

EXAMPLES

(46) The present invention is hereinafter further specifically illustrated by way of Examples which do not limit the present invention.

Example 1

(47) Step (i): Mechanical Milling Treatment

(48) Na.sub.2S (Nagao Co., Ltd; purity: 99.1%), P.sub.2S.sub.5 (Aldrich; purity: 99.9%) and SiS.sub.2 (Fruuchi Chemical Corporation; purity: 99.999%) were weighed at the mole percentages so that (100x)Na.sub.3PS.sub.4.xNa.sub.4SiS.sub.4 with x=0, 2, 4, 5 and 6 was obtained and respectively charged in a planetary ball mill. The materials were then subjected to mechanical milling treatment to obtain mixtures respectively having the compositions of Na.sub.3PS.sub.4 (75Na.sub.2S.25P.sub.2S.sub.5), 98Na.sub.3PS.sub.4.2Na.sub.4SiS.sub.4, 96Na.sub.3PS.sub.4.4Na.sub.4SiS.sub.4, 95Na.sub.3PS.sub.4.5Na.sub.4SiS.sub.4 and 94Na.sub.3PS.sub.4.6Na.sub.4SiS.sub.4.

(49) The planetary ball mill used was Pulverisette P-7 from Fritsch and the mill included a pot of 45 ml containing 500 balls each having a diameter of 4 mm with the pot and balls being made of ZrO.sub.2. The mechanical milling treatment was carried out at a rotation speed of 510 rpm at room temperature for 20 hours in a dry nitrogen-filled glove box.

(50) The method as described above follows the descriptions in Experimental in Akitoshi Hayashi et al., Journal of Non-Crystalline Solids 356 (2010) 2670-2673.

(51) The five different samples (80 mg each) after the mechanical milling treatment as described above were pressed (pressure: 370 MPa) to obtain pellets each having a diameter of 10 mm and a thickness of about 1 mm. It was confirmed by DTA that the obtained pellets partly contained cubic Na.sub.3PS.sub.4 due to the reaction by milling and also contained glass components.

(52) Step (ii): Heat Treatment

(53) The five different pellets containing glasses as above were heated from room temperature (25 C.) towards 220 C. that is at or above the crystallization temperature in order to convert the glasses to glass-ceramics. After reaching to 220 C., the pellets of glass-ceramics were cooled towards room temperature.

(54) The results of conductivity measurements of pellets of glass-ceramics at room temperature are shown in FIG. 1. As shown in FIG. 1, it is found that the pellets of the Na.sub.3PS.sub.4 glass-ceramics had the conductivity of 210.sup.4 Scm.sup.1 while all pellets of glass-ceramics containing the SiS.sub.2 components had an increased conductivity. It is also found that the glass-ceramics of 94Na.sub.3PS.sub.4.6Na.sub.4SiS.sub.4 had the conductivity (7.410.sup.4 Scm.sup.1) that was about 3.7 times higher than that of the Na.sub.3PS.sub.4 glass-ceramics.

Example 2

(55) The change in the conductivity of the pellets of the Na.sub.3PS.sub.4 glass-ceramics and of the 95Na.sub.3PS.sub.4.5Na.sub.4SiS.sub.4 glass-ceramics as a function of the temperature of the pellets is shown in FIG. 2. In FIG. 2, the filled circles represent the measurement result for the pellets of the 95Na.sub.3PS.sub.4.5Na.sub.4SiS.sub.4 glass-ceramics and the open circles represent the measurement result for the Na.sub.3PS.sub.4 glass-ceramics. The glass-ceramics were treated at 270 C.

(56) FIG. 2 indicates the following.

(57) It is found that the pellets of the 95Na.sub.3PS.sub.4.5Na.sub.4SiS.sub.4 glass-ceramics have a higher conductivity than the pellets of the Na.sub.3PS.sub.4 glass-ceramics at any temperature in the range of 25 to 160 C.

Example 3

(58) In the same manner as Example 1, pellets of Na.sub.3PS.sub.4 (75Na.sub.2S.25P.sub.2S.sub.5), 95Na.sub.3PS.sub.4.5Na.sub.4SiS.sub.4, 90Na.sub.3PS.sub.4.10Na.sub.4SiS.sub.4, 75Na.sub.3PS.sub.4.25Na.sub.4SiS.sub.4, 33Na.sub.3PS.sub.4.67Na.sub.4SiS.sub.4 and Na.sub.4SiS.sub.4 (67Na.sub.2S.33SiS.sub.2) glass-ceramics were obtained. Due to the variation in the crystallization temperature according to the compositions, the samples were subjected to heat treatment at a temperature selected between 220 to 360 C. according to the crystallization temperature.

(59) The XRD patterns for the pellets of the obtained glass-ceramics are shown in FIG. 3. From FIG. 3, it is found that the pellets of 95Na.sub.3PS.sub.4.5Na.sub.4SiS.sub.4 and 90Na.sub.3PS.sub.4.10Na.sub.4SiS.sub.4 glass-ceramics have the same cubic Na.sub.3PS.sub.4 crystal structure as Na.sub.3PS.sub.4.

(60) The Raman spectra for the pellets of Na.sub.3PS.sub.4 (75Na.sub.2S.25P.sub.2S.sub.5), 90Na.sub.3PS.sub.4.10Na.sub.4SiS.sub.4, 75Na.sub.3PS.sub.4.25Na.sub.4SiS.sub.4, 33Na.sub.3PS.sub.4.67Na.sub.4SiS.sub.4 and Na.sub.4SiS.sub.4 (67Na.sub.2S.33SiS.sub.2) glass-ceramics are shown in FIG. 4. From FIG. 4, it is found that the glass-ceramics contain discrete anions of PS.sub.4.sup.3- and SiS.sub.4.sup.4- without bridging sulfurs and have local structures depending on the loaded compositions of (100x)Na.sub.3PS.sub.4.xNa.sub.4SiS.sub.4. With the pellets of the 90Na.sub.3PS.sub.4.10Na.sub.4SiS.sub.4 glass-ceramics, the peak of PS.sub.4.sup.3- derived from Na.sub.3PS.sub.4 was mainly observed, which result does not conflict with the experimental result from XRD showing that cubic Na.sub.3PS.sub.4 was present.

Example 4

(61) FIG. 5 shows a cyclic voltammogram which was obtained by cyclic voltammetry with a working electrode of stainless steel, an electrolyte of pellets (thickness: about 1 mm) of the 90Na.sub.3PS.sub.4.10Na.sub.4SiS.sub.4 glass-ceramics and a counter electrode of metal sodium on stainless steel. The sweeping condition was 5 mV/sec at room temperature.

(62) It is found from FIG. 5 that with the glass-ceramics, reduction/oxidation current corresponding to dissolution and deposition of Na was repeatedly observed at around 0 V. In addition, because sweeping towards the oxidation side up to +5 V did not result in observation of high oxidation current due to decomposition of the electrolyte or the like, it is found that the glass-ceramics is electrochemically stable up to +5 V.