Solid electrolyte and lithium ion battery

Abstract

Provided is a halide-based solid electrolyte which exhibits high lithium ion conductivity at room temperature. This solid electrolyte contains Li, Mα, Mβ, and Cl, wherein Mα is at least one element that is selected from the group consisting of Zn, Mg, Ca, Sr, and Ba, and Mβ is at least one element that is selected from the group consisting of Al, Ga, Bi, Er, Ge, and Zr. This solid electrolyte has a crystal structure that belongs to an orthorhombic crystal of the space group Pnma.

Description

Solid electrolyte and lithium-ion batteries

[0001] The present disclosure relates to solid electrolytes and lithium-ion batteries.

[0002] In recent years, there has been a strong demand for smaller size and improved reliability (safety) for batteries that serve as power sources for electronic devices. Therefore, all-solid-state batteries, which use solid electrolytes instead of liquid electrolytes, have attracted attention. One typical type of solid electrolyte is a sulfide-based solid electrolyte. However, because sulfide-based solid electrolytes can generate toxic hydrogen sulfide, solid electrolytes that do not belong to the sulfide-based solid electrolyte category are desired from a safety perspective.

[0003] Halides have been proposed as sulfur-free solid electrolytes. For example, in Non-Patent Document 1 (Ryoji Kanno et al., Chemistry Letters, 1989, pp. 223-226), Li ２ ZnCl ４ has been reported as a lithium ion conductive solid electrolyte. ２－ａ M1 １－ａ M2 ａ X ４ (wherein M1 is at least one element selected from the group consisting of Mg and Zn, M2 is at least one element selected from the group consisting of Al, Ga, Y, In, and Bi, and X is at least one element selected from the group consisting of F, Cl, Br, and I, and 0<a<1) and has a spinel structure (Fd-3m structure), and discloses that the solid electrolyte material has high lithium ion conductivity. ２ ZnCl ４ It has been disclosed that the conductivity of the material can be increased by doping it with an element.

[0004] WO2020 / 194897

[0005] Ryoji Kanno et al., "Inonic Conductivity and Structure of Double Chloride Li2ZnCl4 in the LiCl-ZnCl2 System", Chemistry Letters, 1989, pp. 223-226

[0006] In Non-Patent Document 1, Li ２ ZnCl ４ The conductivity of the Fd-3m and Pnma polymorphs has been reported, and it has been shown that the Pnma structure, which is a high-temperature phase, has higher conductivity. However, the ionic conductivity of the Pnma structure at room temperature is 10 －１０ In addition, in Patent Document 1, as described above, Li with an Fd-3m structure is ２ ZnCl ４ Although it has been proposed to increase the conductivity by element doping, the ionic conductivity at room temperature is only 10 －５ S / cm, and further improvement in conductivity is required for practical use.

[0007] The present inventors have now discovered that Li, M α , M β In the solid electrolyte containing M and Cl, α and M β They have found that by employing a specific metal element as the cation, and giving it a crystal structure belonging to a rectangular crystal of the space group Pnma, it is possible to provide a halide-based solid electrolyte that exhibits high lithium ion conductivity at room temperature.

[0008] Therefore, an object of the present invention is to provide a halide-based solid electrolyte that exhibits high lithium ion conductivity at room temperature.

[0009] According to the present disclosure, the following aspects are provided: [Aspect 1] Li, M α , M β and a solid electrolyte containing Cl, α is at least one element selected from the group consisting of Zn, Mg, Ca, Sr, and Ba, and M βis at least one element selected from the group consisting of Al, Ga, Bi, Er, Ge, and Zr, and the solid electrolyte has a crystal structure belonging to an orthorhombic crystal of the space group Pnma. [Aspect 2] M α The solid electrolyte according to aspect 1, wherein M contains Zn. β The solid electrolyte according to aspect 1 or 2, wherein the solid electrolyte comprises Ga. [Aspect 4] A solid electrolyte represented by the following composition formula: Li ２－ａ (M α １－ｂ M β ｂ ) ｚ＋ ｃ Cl ２－ａ＋ｚ×ｃ (wherein z is M α and M β wherein the average ionic valence is -2<a<2, 0<b<1, and 0<c<2). [Aspect 5] The solid electrolyte according to any one of Aspects 1 to 4, which exhibits a peak in the range of 17.7°≦2θ≦18.9° in an X-ray diffraction pattern. [Aspect 6] Li ２ ZnCl ４ A solid electrolyte according to any one of Aspects 1 to 5, wherein Li is deficient with respect to a stoichiometric ratio of Li represented by the following composition formula: A lithium ion battery comprising the solid electrolyte according to any one of Aspects 1 to 6.

[0010] FIG. 1 is a schematic cross-sectional view showing an example of an all-solid-state battery according to the present invention.

[0011] Solid Electrolyte The solid electrolyte according to the present invention is α , M β , and Cl. α is at least one element selected from the group consisting of Zn, Mg, Ca, Sr, and Ba. β is at least one element selected from the group consisting of Al, Ga, Bi, Er, Ge, and Zr. The solid electrolyte has a crystal structure belonging to an orthorhombic system of the space group Pnma. α , M β In the solid electrolyte containing M and Cl, α and Mβ By employing a specific metal element as the cation, and giving the cation a crystal structure belonging to an orthorhombic crystal of the space group Pnma, it is possible to provide a halide-based solid electrolyte that exhibits high lithium ion conductivity at room temperature.

[0012] As mentioned above, various halide-based solid electrolytes that do not contain sulfur have been reported (see, for example, Non-Patent Document 1 and Patent Document 1). However, the ionic conductivity of conventional halide-based solid electrolytes at room temperature (hereinafter referred to as room temperature conductivity) is not sufficiently high, and further improvement in conductivity has been desired for practical use. This problem is successfully solved by the present invention. That is, the present inventors have found that Li ２ ZnCl ４ Focusing on the basic composition, Li ２ ZnCl ４ In order to improve the conductivity of ２ ZnCl ４ We have prepared materials by adding various cations (M) to Li and then annealed them to freeze the Pnma structure, which is a high-temperature phase, at room temperature. As a result of examining various combinations using various cations (M), we found that adding Ga as M and annealing it produced Li with a Pnma structure doped with M. ２－ｘ Zn １－ｘ M ｘ Cl ４ can be obtained, and 10 －４ It was found that the material exhibited a high room temperature conductivity of 1000 S / cm. The reason why the conductivity improved by adding Ga is not necessarily clear, but it is thought that the substitution of divalent Zn with trivalent Ga resulted in a loss of Li to maintain electrical neutrality, and the generation of vacancies at the Li site contributed to the improvement of conductivity. Therefore, it is thought that a similar effect can be achieved with cations having a higher valence than Zn, as with Ga, and it is thought that high room temperature conductivity can be achieved even when Al, Ga, Bi, Er, Ge, and / or Zr is added as M instead of or together with Ga. Furthermore, if Mg, Ca, Sr, or Ba has the same valence as divalent Zn, Li ２ ZnCl ４ Since it is expected that the Zn site can be substituted without changing the crystal structure of Li ２ ZnCl４ Instead of Li ２ MgCl ４ , Li ２ CaCl ４ , Li ２ SrCl ４ , or Li ２ BaCl ４ It is expected that even for the basic composition, high room temperature conductivity can be exhibited by doping with a cation (M). ｘ MCl ４ By combining these cation species and controlling the Li ratio, it is possible to provide a solid electrolyte that exhibits high lithium ion conductivity at room temperature.

[0013] As described above, the solid electrolyte of the present invention is α , M β and Cl and has a crystal structure belonging to an orthorhombic crystal of the space group Pnma. ２－ａ (M α １－ｂ M β ｂ ) ｚ＋ ｃ Cl ２－ａ＋ｚ×ｃ (wherein z is M α and M β wherein the average ionic valence is -2<a<2, 0<b<1, and 0<c<2).

[0014] M α is an element that becomes a divalent cation. α is at least one element selected from the group consisting of Zn, Mg, Ca, Sr, and Ba, and preferably contains Zn.

[0015] M β is an element that becomes a trivalent or tetravalent cation. β is at least one element selected from the group consisting of Al, Ga, Bi, Er, Ge, and Zr, and preferably contains Ga. βThe coefficient b multiplied by satisfies 0<b<1, preferably 0.01≦b≦0.5, more preferably 0.05≦b≦0.3, and further preferably 0.1≦b≦0.2.

[0016] The coefficient c is 0<c<2, preferably 0.5≦c≦1.5, more preferably 0.7≦c≦1.3, and even more preferably 0.9≦c≦1.1, and typically c=1.

[0017] The coefficient a is −2<a<2, preferably 0≦a≦1, more preferably 0.05≦a≦0.6, and further preferably 0.05≦a≦0.4. α , M β The solid electrolyte of the present invention can take any value within the above range to balance the charge with Li and Cl. ２ ZnCl ４ It is preferable that Li is deficient with respect to the stoichiometric ratio of Li represented by the composition formula: Li deficiency has the advantage that vacancies are generated at the Li site, improving the lithium ion conductivity. ２ ZnCl ４ However, this composition formula is merely an example composition formula used as a guide for defining the Li deficiency, and therefore does not contradict the composition of the solid electrolyte of the present invention.

[0018] z is M α and M β is the average ionic valence of M α The value obtained by multiplying the valence (i.e., 2) by the coefficient (1-b), and M β The valence (i.e., 3 or 4) is multiplied by a coefficient b.

[0019] When determining whether an unknown solid electrolyte has the above composition, chemical analysis is performed on the unknown solid electrolyte to determine whether the constituent elements are Li, M, α , M β In addition, in order to confirm whether an unknown solid electrolyte has the above composition formula, for example, Li, M, or Cl may be used. α and M βcan be quantified using an ICP-emission spectrometer. Cl can be quantified using an ion chromatograph.

[0020] The solid electrolyte of the present invention typically exhibits a peak in the range of 17.7°≦2θ≦18.9° in an X-ray diffraction (XRD) pattern, and more typically exhibits a peak in the range of 18.0°≦2θ≦18.6°. The peak in the above range is due to a crystalline structure belonging to an orthorhombic crystal of the space group Pnma.

[0021] The solid electrolyte of the present invention has a molecular weight of 1×10 at room temperature (e.g., 25° C.). －８ Preferably, the lithium ion conductivity is 1×10 S / cm or more, more preferably 1×10 －６ S / cm or more, more preferably 1×10 －４ The higher the lithium ion conductivity, the more desirable it is. Therefore, the upper limit is not limited, but it is typically 1×10 ―２ S / cm or less, more typically 1×10 －３ S / cm or less.

[0022] The solid electrolyte of the present invention is lithium chloride (LiCl), M α chloride, M β The solid electrolyte precursor powder can be produced by weighing, mixing, and milling raw material powders of the chlorides of the above in a predetermined molar ratio, and then annealing the resulting solid electrolyte precursor powder in an inert gas (argon, etc.) atmosphere. The annealing process converts the precursor powder into a solid electrolyte having a crystal structure belonging to an orthorhombic crystal of the Pnma space group. The annealing temperature is not particularly limited as long as it can provide the Pnma crystal structure, but is preferably 200 to 350°C, more preferably 250 to 300°C. The annealing time at the above temperature is preferably 0.5 to 12 hours, more preferably 1 to 4 hours.

[0023] According to a preferred embodiment of the present invention, there is provided a lithium-ion battery containing the solid electrolyte of the present invention. This lithium-ion battery is preferably a lithium-ion secondary battery, more preferably an all-solid-state lithium-ion secondary battery. Figure 1 shows a schematic diagram of an example of a lithium-ion battery 10 according to the present invention.

[0024] The positive electrode 12 includes a positive electrode active material. The positive electrode active material preferably includes a lithium composite oxide. Examples of lithium composite oxides include lithium nickel manganese oxide (LNMO) (typically LiNi ０．５ Mn １．５ O ４ ), lithium nickel cobalt manganese oxide (NCM) (typically Li(Ni,Co,Mn)O ２ ), lithium cobalt oxide (LCO) (typically LiCoO ２ ), lithium nickel cobalt aluminate (NCA) (typically Li(Ni,Co,Al)O ２ ) and lithium iron phosphate (LFP) (typically LiFePO ４ ), and combinations thereof. NCM, LCO, and NCA have a layered rock salt structure. LNMO has a spinel structure. LFP has an olivine structure. A lithium composite oxide having a layered rock salt structure, such as NCM, is preferred. The positive electrode 12 preferably further contains a solid electrolyte and / or an electron-conducting additive (carbon black, etc.) in addition to the positive electrode active material. The positive electrode 12 in this embodiment is formed by integrating these materials by pressure or heat. Therefore, as shown in FIG. 1, the positive electrode 12 is typically in the form of a positive electrode layer. When the positive electrode 12 contains a solid electrolyte, it is preferable to use the solid electrolyte according to the present invention described above as such a solid electrolyte.

[0025] The negative electrode 14 is 0.1 V (vs. Li / Li ＋ ) or more, and includes a negative electrode active material capable of inserting and desorbing lithium ions. Examples of the negative electrode active material include lithium titanate (LTO) (typically Li ４ Ti ５ O １２ ), SiO, Si or Si alloy, and TiO ２The negative electrode active material is preferably LTO, SiO, or TiO because of its non-flammability. ２ In terms of cycleability, LTO is particularly preferred. Although LTO is typically known to have a spinel structure, it can also have other structures during charge and discharge. For example, LTO can have Li ４ Ti ５ O １２ (spinel structure) and Li ７ Ti ５ O １２ The reaction proceeds in the coexistence of two phases, a solid electrolyte (rock salt structure) and a solid electrolyte (rock salt structure). Therefore, LTO is not limited to a spinel structure. From the viewpoint of improving ionic conductivity, it is preferable that the negative electrode 14 further contains a solid electrolyte in addition to the negative electrode active material. The negative electrode 14 may further contain an electron conduction aid (carbon black, etc.). The negative electrode 14 in this embodiment is formed by integrating these materials by pressure or heat. Therefore, as shown in FIG. 1, the negative electrode 14 is typically in the form of a negative electrode layer. When the negative electrode 14 contains a solid electrolyte, it is preferable to use the solid electrolyte according to the present invention described above.

[0026] The electrolyte layer 16 is a layer containing an electrolyte and disposed between the positive electrode 12 and the negative electrode 14. Typically, the electrolyte layer 16 contains a solid electrolyte, and is preferably made of a solid electrolyte. In this case, the electrolyte layer 16 can be said to also function as a separator. The solid electrolyte contained in the electrolyte layer 16 is preferably the solid electrolyte according to the present invention described above.

[0027] At least one of the positive electrode 12, the negative electrode 14, and the electrolyte layer 16 contains a solid electrolyte according to the present invention. The solid electrolyte exhibits high ionic conductivity (e.g., lithium ion conductivity). Furthermore, this solid electrolyte is non-flammable and chemically stable, and does not generate hydrogen sulfide gas, thereby realizing an inherently safe lithium-ion battery 10. However, it is not necessary for all of the positive electrode 12, the negative electrode 14, and the electrolyte layer 16 to contain the solid electrolyte; at least one of them may contain the solid electrolyte.

[0028] The solid electrolyte according to the present invention may be mixed with other substances and used as an electrolyte material. In this case, the solid electrolyte is preferably the component with the largest mass ratio among the components contained in the electrolyte material, i.e., the main component. The mass ratio of the main component in the electrolyte material is preferably 50 wt % or more, more preferably 60 wt % or more, and even more preferably 70 wt % or more.

[0029] As described above, in the lithium ion battery 10 of the present invention, sulfide-free materials can be used for the positive electrode 12, the negative electrode 14, and the electrolyte layer 16. In other words, it is preferable that no part of the positive electrode 12, the negative electrode 14, or the electrolyte layer 16 contains sulfide. This makes it possible to provide an intrinsically safe lithium ion battery 10 that does not generate toxic gases such as hydrogen sulfide.

[0030] The lithium-ion battery 10 preferably further includes a positive electrode current collector 18 and a negative electrode current collector 20. The positive electrode current collector 18 is preferably provided on the surface of the positive electrode 12 opposite the electrolyte layer 16, and the negative electrode current collector 20 is preferably provided on the surface of the negative electrode 14 opposite the electrolyte layer 16. Examples of materials that can be used to form the positive electrode current collector 18 and the negative electrode current collector 20 include aluminum (Al), copper (Cu), titanium (Ti), nickel (Ni), stainless steel (SUS), carbon, platinum (Pt), platinum (Pt) / palladium (Pd), gold (Au), silver (Ag), and ITO (indium-tin oxide).

[0031] The container 22 is not particularly limited as long as it can accommodate a single lithium-ion battery 10 or a stack of multiple lithium-ion batteries 10 stacked in series or parallel. In particular, if the lithium-ion battery 10 is an all-solid-state battery, there is no concern about electrolyte leakage, so a relatively simple container shape can be adopted for the container 22. For example, a chip shape for mounting on an electronic circuit or a laminate cell shape (e.g., a multi-layer product of aluminum (Al) / polypropylene (PP)) for thin and wide space applications can be adopted.

[0032] The present invention will be explained in more detail by the following examples, but the present invention is not limited to the following examples.

[0033] Example 1 (1) Preparation of electrolyte In an argon atmosphere having a dew point of −60° C. or less, LiCl, ZnCl ２ , and GaCl ３ Each raw material powder was mixed with LiCl:ZnCl ２ :GaCl ３ The raw material powders were weighed to a molar ratio of 1.9:0.9:0.1, and then pulverized and mixed in a mortar. The resulting mixed powder was placed in a zirconia pot and milled for 20 hours at 400 rpm using a planetary ball mill to obtain a solid electrolyte precursor powder. The resulting precursor powder was placed in an alumina crucible and annealed for 1 hour at 250°C in an argon atmosphere to obtain a solid electrolyte powder.

[0034] (2) Conductivity Measurement The solid electrolyte powder was placed in a mold consisting of a resin sleeve and upper and lower stainless steel punches, and uniaxially pressed at 250 MPa. Impedance measurements were performed at room temperature with leads connected to the upper and lower punches, and the lithium ion conductivity was calculated from the measurement results. The results are shown in Table 1.

[0035] (3) XRD An XRD pattern of the solid electrolyte powder was obtained using an X-ray diffractometer under the following measurement conditions: X-ray: CuKα radiation, voltage: 30 kV, current: 10 mA, and measurement range (2θ): 10° to 70°. This measurement was carried out in an argon atmosphere with a dew point of −60° C. or lower. In the obtained XRD pattern, a peak was observed in the 2θ range of 17.7° to 18.9°, and as a result of peak analysis, the electrolyte of this example was assigned to a crystalline structure of the space group Pnma.

[0036] Example 2 (Comparative) In the electrolyte preparation of Example 1, LiCl and ZnCl ２ Each raw material powder was mixed with LiCl:ZnCl ２ The electrolyte was prepared and evaluated in the same manner as in Example 1, except that the amounts were weighed so as to give a molar ratio of 2:1.

[0037] Example 3 (Comparative) An electrolyte was prepared and evaluated in the same manner as in Example 1, except that the annealing treatment was not carried out.

[0038] Results Table 1 shows the compositions and measurement results of the solid electrolytes prepared in Examples 1 to 3. ２－ａ (M α １－ｂ M β ｂ ) ｚ＋ ｃ Cl ２－ａ＋ｚ×ｃ (wherein z is M α and M β In order to confirm whether or not the following conditions are satisfied (a is the average ionic valence of 1, -2<a<2, 0<b<1, and 0<c<2), the composition formula and each coefficient applied to the above general formula are shown in Table 2.

[0039]

[0040]

Claims

1. Li, M α , M β and a solid electrolyte containing Cl, α is at least one element selected from the group consisting of Zn, Mg, Ca, Sr, and Ba, and M β is at least one element selected from the group consisting of Al, Ga, Bi, Er, Ge, and Zr, and the solid electrolyte has a crystal structure belonging to an orthorhombic crystal of space group Pnma.

2. M α The solid electrolyte of claim 1 , wherein 3. M β The solid electrolyte according to claim 1 or 2, wherein contains Ga.

4. The following composition formula: Li ２－ａ (M α １－ｂ M β ｂ ) ｚ＋ ｃ Cl ２－ａ＋ｚ×ｃ (wherein z is M α and M β wherein a is an average ionic valence of -2<a<2, 0<b<1, and 0<c<2.

5. The solid electrolyte according to claim 1 or 2, which exhibits a peak in the range of 17.7°≦2θ≦18.9° in an X-ray diffraction pattern.

6. Li ２ ZnCl ４ 3. The solid electrolyte according to claim 1, wherein Li is deficient with respect to the stoichiometric ratio of Li represented by the composition formula:

7. A lithium ion battery comprising the solid electrolyte according to claim 1 or 2.

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