Room temperature flexible solid electrolytes and their preparation methods

Abstract

This invention provides a room-temperature flexible solid electrolyte having the following chemical formula: LiAlP 0.4a O a+x Cl 4‑2x ; 0.8≤a≤1.6, 0.01≤x≤0.2. This invention also provides a method for preparing the room-temperature flexible solid electrolyte of this invention. The solid electrolyte of this invention is flexible at room temperature and also possesses excellent ionic conductivity.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage. Specifically, this invention relates to a room-temperature flexible solid electrolyte and its preparation method. Background Technology

[0002] Given the rapid development of lithium-ion batteries, liquid electrolytes are not the best choice in terms of safety. Using new inorganic amorphous solid electrolytes to improve safety performance and energy density is currently a more promising approach.

[0003] Lithium-ion solid electrolytes can currently be classified into the following categories: oxides, sulfides, halides, organic polymers, and inorganic-organic composite solid electrolytes.

[0004] In terms of flexibility, organic polymers and inorganic-organic composite solid electrolytes have a natural advantage, with their organic segments giving them good processability. Pure polymer solid electrolytes, such as PEO (polyethylene oxide), are crystalline at room temperature, but their low ionic conductivity is a drawback, and they require higher operating temperatures to maintain good ionic conductivity.

[0005] Common oxide and sulfide solid electrolytes cannot meet the requirements, lacking both flexibility and light transmittance. Due to the lack of freely rotating chain segments in halide electrolytes, only a very small number of solid electrolytes can achieve flexibility, but these electrolytes cannot be stored at room temperature and will harden upon cooling.

[0006] Therefore, there is an urgent need for a solid electrolyte that is flexible at room temperature and also has excellent ionic conductivity. Summary of the Invention

[0007] The purpose of this invention is to provide a solid electrolyte that is flexible at room temperature and possesses excellent ionic conductivity. This solid electrolyte is amorphous in structure. Macroscopically, this solid electrolyte material maintains a polymeric, flexible state at room temperature.

[0008] Another object of the present invention is to provide a method for preparing the solid electrolyte of the present invention.

[0009] The above-mentioned objective of the present invention is achieved through the following technical solution.

[0010] In the context of this invention, the term "room temperature" refers to 25°C to 35°C.

[0011] In a first aspect, the present invention provides a room-temperature flexible solid electrolyte having the following chemical formula: LiAlP 0.4a O a+x Cl 4-2x ;in,

[0012] 0.8≤a≤1.6, 0.01≤x≤0.2.

[0013] The inventors of this application unexpectedly discovered that the addition of P2O5 powder caused lithium tetrachloroaluminate (LiAlCl4) crystals to form an amorphous structure, achieving polymer-like flexibility, while simultaneously lowering the glass transition temperature T of the original material. g However, this effect cannot be achieved by using Sb₂O₃ or other oxides for oxygen substitution. To avoid being limited by theory, it is possible that the addition of P₂O₅ powder introduces oxygen bridges, connecting the originally isolated aluminum atoms, causing the sodium tetrachloroaluminate crystals to form segments and thus an amorphous structure, achieving polymer-like flexibility while lowering the glass transition temperature. This invention achieves this by reducing the material's own T... g This enables room temperature flexibility.

[0014] The inventors of this application also unexpectedly discovered that the addition of X powder did not further alter the structure of the substrate. Instead, while maintaining flexibility, it unexpectedly increased ionic conductivity, allowing the battery to operate at room temperature. The desire not to be limited by theory may be because the addition of X powder did not further alter the substrate structure, but rather evaporated during the preparation process.

[0015] Preferably, in the room-temperature flexible solid electrolyte of the present invention, 1≤a≤1.3, 0.03≤x≤0.2.

[0016] Preferably, in the room-temperature flexible solid electrolyte of the present invention, the ionic conductivity of the solid electrolyte is 10. -6 -2×10 -3 S / cm.

[0017] Preferably, in the room-temperature flexible solid electrolyte of the present invention, the ionic conductivity of the solid electrolyte is 10. -4 -2×10 -3 S / cm.

[0018] Secondly, the present invention provides a method for preparing the room-temperature flexible solid electrolyte of the present invention, comprising the following steps:

[0019] (1) After uniformly mixing LiCl and AlCl3 powders, heat to 200-500℃ and keep warm for 1-24 hours to obtain the precursor LiAlCl4;

[0020] (2) The precursor LiAlCl4 is ground into powder and then mixed evenly with P2O5 powder. The mixture is heated to 200-400℃ and kept at that temperature for 1-5 hours to obtain the solid electrolyte LiAlP. 0.4a Oa Cl4;

[0021] (3) The solid electrolyte LiAlP 0.4a O a Cl4 was ground into powder and mixed evenly with X powder. The mixture was then heated to 200-300℃ and held at that temperature for 1-5 hours to obtain the room-temperature flexible solid electrolyte LiAlP. 0.4a O a+x Cl 4-2x ;

[0022] in,

[0023] X is SeO2 and / or TeO2;

[0024] 0.8≤a≤1.6, 0.01≤x≤0.2.

[0025] Preferably, in the method described in this invention, in step (1), LiCl and AlCl3 powders are uniformly mixed and heated to 200-300℃ and kept at that temperature for 3-6 hours to obtain the precursor LiAlCl4.

[0026] Thirdly, the present invention provides a method for preparing the room-temperature flexible solid electrolyte of the present invention, comprising the following steps:

[0027] (1) After uniformly mixing LiCl and AlCl3 powders, heat to 200-500℃ and keep warm for 1-24 hours to obtain the precursor LiAlCl4;

[0028] (2) The precursor LiAlCl4 is ground into powder and then mixed evenly with P2O5 powder and X powder. The mixture is heated to 200-300℃ and kept at that temperature for 1-5 hours to obtain a room temperature flexible solid electrolyte LiAlP. 0.4a O a+x Cl 4-2x ;

[0029] in,

[0030] X is SeO2 and / or TeO2;

[0031] 0.8≤a≤1.6, 0.01≤x≤0.2.

[0032] The room-temperature flexible lithium-ion solid electrolyte LiAlP of this invention 0.4a O a+x Cl 4-2x It can be applied to lithium-ion all-solid-state batteries.

[0033] The present invention has the following beneficial effects:

[0034] (1) The solid electrolyte of the present invention exhibits excellent machinability (i.e., flexibility) at room temperature, thereby avoiding serious interface problems. The lower melting temperature solves the cathode interface problem of lithium-ion superconducting oxide ceramic sheets, resulting in good interface compatibility. Compared to polymer PEO (polyethylene oxide) electrolytes, the solid electrolyte of the present invention can achieve an ionic conductivity of 10 at room temperature. -4 The S / cm ratio ensures operation at room temperature. Furthermore, compared to sulfide amorphous solid electrolytes, the solid electrolyte of this invention exhibits excellent oxidation resistance and will not be oxidized and decomposed by the positive electrode under high voltage, demonstrating resistance to high oxidation potentials. In addition, the solid electrolyte of this invention is compatible with sulfide solid electrolytes.

[0035] (2) The solid electrolyte of the present invention has a simple preparation process and does not require a complicated preparation method like inorganic-organic composite solid electrolytes. Attached Figure Description

[0036] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0037] Figure 1 The solid electrolyte LiAlP prepared in Example 3 is shown. 0.5 O 1.33 Cl 3.84 Electrochemical performance when assembled into a solid-state battery;

[0038] Figure 2 The XRD patterns of the solid electrolytes prepared in Examples 1-5 are shown.

[0039] Figure 3 The solid electrolyte LiAlP prepared in Example 3 is shown. 0.5 O 1.33 Cl 3.84 Impedance diagram;

[0040] Figure 4 The solid electrolyte LiAlP prepared in Example 3 is shown. 0.5 O 1.33 Cl 3.84 A diagram showing the viscosity at room temperature. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0042] The following embodiments and comparative examples of the present invention underwent some performance tests:

[0043] Ionic conductivity testing: First, the material is placed in a mold and pressed into an amorphous solid electrolyte sheet under certain pressure. Then, the test is performed using an electrochemical workstation. The test temperature is 35℃.

[0044] LSV test: First, the material is placed in a mold and pressed into an amorphous solid electrolyte sheet under a certain pressure. Then, sodium metal and carbon black positive electrodes (mixed with solid electrolyte) are placed on both sides of the amorphous solid electrolyte sheet, and the test is carried out through an electrochemical workstation.

[0045] Battery cycle testing: First, the material is rolled into an amorphous solid electrolyte film. Then, a transparent amorphous solid electrolyte film is placed on an LLZTO ceramic sheet, and a positive electrode is placed on the film. The film is melted and tightly bonded to the LLZTO and the positive electrode through heat treatment. Lithium metal is then placed on the other side of the LLZTO ceramic sheet. Finally, the LLZTO is placed in a battery casing and sealed.

[0046] Alternatively, the material is first placed in a mold and pressed into an amorphous solid electrolyte sheet under certain pressure. An LPSC solid electrolyte is added to the negative electrode side, a lithium-indium alloy is placed on the LSPC side, and a positive electrode sheet is added to the positive electrode side. All three are then pressed together in the mold and sealed. Cyclic discharge tests are then conducted using Wuhan Landian Battery testing equipment at 0.1C and a voltage of 2.5-3.6V.

[0047] Example 1

[0048] LiAlP 0.44 O 1.18 Cl 3.84 Preparation

[0049] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with P2O5 and SeO2 powders in a 1:0.22:0.04 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.44 O 1.18 Cl 3.84 .

[0050] The solid electrolyte of this embodiment has the following properties as measured:

[0051] Ionic conductivity at room temperature (35℃): 7×10 -4 S / cm;

[0052] Oxidation resistance potential: 4V;

[0053] Reduction potential: 0.5V;

[0054] Melting point: 150℃;

[0055] At room temperature, macroscopically: glassy, ​​flexible.

[0056] Example 2

[0057] LiAlP 0.48 O 1.28 Cl 3.84 Preparation

[0058] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with P2O5 and SeO2 powders in a 1:0.24:0.04 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.48 O 1.28 Cl 3.84 After measurement, the amorphous solid electrolyte of this embodiment has the following properties:

[0059] Ionic conductivity at room temperature (35℃): 1×10 -3 S / cm;

[0060] Oxidation resistance potential: 4.1V;

[0061] Reduction potential: 0.6V;

[0062] Melting point: 100℃;

[0063] At room temperature, macroscopically: glassy, ​​flexible.

[0064] Example 3

[0065] LiAlP 0.5 O 1.33 Cl 3.84 Preparation

[0066] NaCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with P2O5 and SeO2 powders in a 1:0.25:0.04 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.5 O 1.33Cl 3.84 After measurement, the amorphous solid electrolyte of this embodiment has the following properties:

[0067] Ionic conductivity at room temperature (35℃): 1.1 × 10⁻⁶ -3 S / cm;

[0068] Oxidation resistance potential: 4.3V;

[0069] Reduction potential: 1V;

[0070] Melting point: 70℃;

[0071] At room temperature, macroscopically: glassy, ​​flexible.

[0072] Figure 1 The solid electrolyte LiAlP prepared in Example 3 is shown. 0.5 O 1.33 Cl 3.84 In assembling all-solid-state batteries Li-In / / LPSC / / LiAlP 0.5 O 1.33 Cl 3.84 / / Electrochemical performance of NCM622 at 30℃. Figure 1 The battery charge-discharge curves at 30°C and 0.1C rate are shown. Figure 1 The results show that the solid electrolyte prepared in this embodiment has good charge and discharge capacity and curves, and no short circuits or open circuits were observed. This indicates that the positive electrode has good compatibility with it, which can ensure good battery operation.

[0073] Example 4

[0074] LiAlP 0.48 O 1.3 Cl 3.8 Preparation

[0075] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained precursor LiAlCl4 was ground into powder and then uniformly mixed with P2O5 and SeO2 powders in a 1:0.24:0.05 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.48 O 1.3 Cl 3.8 After measurement, the amorphous solid electrolyte of this embodiment has the following properties:

[0076] Ionic conductivity at room temperature (35℃): 1.5 × 10⁻⁶ -3 S / cm;

[0077] Oxidation resistance potential: 3.9V;

[0078] Reduction potential: 1V;

[0079] Melting point: 80℃;

[0080] At room temperature, macroscopically: glassy, ​​flexible.

[0081] Example 5

[0082] LiAlP 0.48 O 1.4 Cl 3.6 Preparation

[0083] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio and then placed in a muffle furnace. The mixture was heated to 200℃ and held for 3 hours to obtain a homogeneous precursor, LiAlCl4. The obtained LiAlCl4 precursor was then ground into powder and uniformly mixed with P2O5, TeO2, and SeO2 powders in a molar ratio of 1:0.24:0.05:0.05. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.48 O 1.4 Cl 3.6 .

[0084] Measurements showed that the amorphous solid electrolyte of this embodiment possesses the following properties:

[0085] Ionic conductivity at room temperature (35℃): 0.7 × 10⁻⁶ -3 S / cm;

[0086] Oxidation resistance potential: 4V;

[0087] Reduction potential: 1V

[0088] Melting point: 90℃;

[0089] At room temperature, macroscopically: glassy, ​​flexible.

[0090] Comparative Example 1

[0091] LiAlP 0.48 O 1.2 Preparation of Cl4

[0092] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with P2O5 powder in a 1:0.24 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.48 O 1.2 Cl4.

[0093] After measurement, the amorphous solid electrolyte in this comparative example exhibits the following properties:

[0094] Ionic conductivity at room temperature (35℃): 1×10 -4 S / cm;

[0095] Oxidation resistance potential: 4.1V;

[0096] Reduction potential: 1.5V;

[0097] Melting point: 70℃;

[0098] Macroscopically at room temperature: flexible.

[0099] The comparison between this comparative example and Example 2 shows that the ionic conductivity of the solid electrolyte decreases significantly when SeO2 is lacking. Therefore, normal operation of the battery at room temperature cannot be guaranteed.

[0100] Comparative Example 2

[0101] LiAlP 0.24 O 0.6 Preparation of Cl4

[0102] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with P2O5 powder in a 1:0.12 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.24 O 0.6 Cl4.

[0103] After measurement, the amorphous solid electrolyte in this comparative example exhibits the following properties:

[0104] Ionic conductivity at room temperature (35℃): 1×10 -6 S / cm;

[0105] Oxidation resistance potential: 4V;

[0106] Reduction potential: 1.7V;

[0107] Melting point: 250℃;

[0108] At room temperature, macroscopically: there is no flexibility.

[0109] This comparative example shows that when SeO2 is lacking and P2O5 is insufficient, the ionic conductivity of the solid electrolyte decreases significantly and it lacks flexibility at room temperature.

[0110] Comparative Example 3

[0111] LiAlP 0.48 O 1.44 Cl 3.54 Preparation

[0112] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio and then placed in a muffle furnace. The mixture was heated to 200℃ and held for 3 hours to obtain a homogeneous precursor, LiAlCl4. The obtained LiAlCl4 precursor was then ground into powder and uniformly mixed with P2O5 and SeO2 powders in a 1:0.24:0.12 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.48 O 1.44 Cl 3.54。 After measurement, the amorphous solid electrolyte in this comparative example exhibits the following properties:

[0113] Ionic conductivity at room temperature (35℃): 7×10 -4 S / cm;

[0114] Oxidation resistance potential: 4.15V;

[0115] Reduction potential: 1.5V;

[0116] Melting point: 250℃;

[0117] At room temperature, macroscopically: loss of flexibility.

[0118] This comparative example shows that when SeO2 is in excess, the solid electrolyte does not have flexibility at room temperature.

[0119] Comparative Example 4

[0120] LiAlP 0.48 O 1.44 Cl 3.54 Preparation

[0121] LiCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with P2O5 and TeO2 powders in a 1:0.24:0.12 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.48 O 1.44 Cl 3.54。

[0122] After measurement, the amorphous solid electrolyte in this comparative example exhibits the following properties:

[0123] Ionic conductivity at room temperature (35℃): 0.5 × 10⁻⁶ -3 S / cm;

[0124] Oxidation resistance potential: 4V;

[0125] Reduction potential: 1.5V;

[0126] Melting point: 250℃;

[0127] Macroscopically at room temperature: loss of flexibility

[0128] This comparative example shows that when TeO2 is in excess, the solid electrolyte does not have flexibility at room temperature.

[0129] Comparative Example 5

[0130] LiAlP 0.72 O 1.88 Cl 3.84 Preparation

[0131] NaCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with P2O5 and SeO2 powders in a 1:0.36:0.04 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlP. 0.72 O 1.88 Cl 3.84 .

[0132] After measurement, the amorphous solid electrolyte in this comparative example exhibits the following properties:

[0133] Ionic conductivity at room temperature (35℃): 0.3 × 10⁻⁶ -4 S / cm;

[0134] Oxidation resistance potential: 3.8V;

[0135] Reduction potential: 1V;

[0136] Melting point: 200℃;

[0137] Macroscopically at room temperature: loss of room temperature flexibility.

[0138] This comparative example shows that when P2O5 is in excess, the solid electrolyte does not have flexibility at room temperature.

[0139] Comparative Example 6

[0140] LiAlO 1.88 Cl 2.04 Preparation

[0141] NaCl and AlCl3 powders were uniformly mixed and ground in a 1:1 molar ratio, then placed in a muffle furnace and heated to 200℃ for 3 hours to obtain a homogeneous precursor LiAlCl4. The obtained LiAlCl4 precursor was ground into powder and then uniformly mixed with Sb2O3 and SeO2 powders in a 1:0.6:0.04 molar ratio. The mixture was heated to 250℃ and held for 2 hours. After ICP analysis, the elemental content was measured to obtain a room-temperature flexible lithium-ion solid electrolyte, LiAlO3. 1.88 Cl 2.04 .

[0142] After measurement, the amorphous solid electrolyte in this comparative example exhibits the following properties:

[0143] Ionic conductivity at room temperature (35℃): 0.3 × 10⁻⁶ -4 S / cm;

[0144] Oxidation resistance potential: 3.8V;

[0145] Reduction potential: 1V;

[0146] Melting point: 200℃;

[0147] Macroscopically at room temperature: loss of room temperature flexibility.

Claims

1. A room-temperature flexible solid electrolyte having the following chemical formula: LiAlP 0.4a O a+x Cl 4-2x ;in, 0.8≤a≤1.6, 0.01≤x≤0.

2.

2. The room-temperature flexible solid electrolyte according to claim 1, wherein, 1≤a≤1.3, 0.03≤x≤0.

2.

3. The room-temperature flexible solid electrolyte according to claim 1, wherein, The ionic conductivity of the solid electrolyte is 10. -6 -2×10 -3 S / cm.

4. The room-temperature flexible solid electrolyte according to claim 3, wherein, The ionic conductivity of the solid electrolyte is 10. -4 -2×10 -3 S / cm.

5. A method for preparing a room-temperature flexible solid electrolyte according to any one of claims 1-4, comprising the following steps: (1) After uniformly mixing LiCl and AlCl3 powders, heat to 200-500℃ and keep warm for 1-24 hours to obtain the precursor LiAlCl4; (2) The precursor LiAlCl4 is ground into powder and then mixed evenly with P2O5 powder. The mixture is heated to 200-400℃ and kept at that temperature for 1-5 hours to obtain the solid electrolyte LiAlP. 0.4a O a Cl4; (3) The solid electrolyte LiAlP 0.4a O a Cl4 was ground into powder and mixed evenly with X powder. The mixture was then heated to 200-300℃ and held at that temperature for 1-5 hours to obtain the room-temperature flexible solid electrolyte LiAlP. 0.4a O a+x Cl 4-2x ; in, X is SeO2 and / or TeO2; 0.8≤a≤1.6, 0.01≤x≤0.

2.

6. The method according to claim 5, wherein, In step (1), LiCl and AlCl3 powders are uniformly mixed and heated to 200-300℃ and kept at that temperature for 3-6 hours to obtain the precursor LiAlCl4.

7. A method for preparing a room-temperature flexible solid electrolyte according to any one of claims 1-4, comprising the following steps: (1) After uniformly mixing LiCl and AlCl3 powders, heat to 200-500℃ and keep warm for 1-24 hours to obtain the precursor LiAlCl4; (2) The precursor LiAlCl4 is ground into powder and then mixed evenly with P2O5 powder and X powder. The mixture is heated to 200-300℃ and kept at that temperature for 1-5 hours to obtain a room temperature flexible solid electrolyte LiAlP. 0.4a O a+x Cl 4-2x ; in, X is SeO2 and / or TeO2; 0.8≤a≤1.6, 0.01≤x≤0.2.

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