Solid ionic conductor, solid electrolyte, positive electrode material and sodium ion solid secondary battery

Abstract

To provide a lithium compound for a solid electrolyte that excels in ion conductivity, charging / discharging stability and thermodynamic stability.SOLUTION: A solid ion conductor comprises a compound crystal whose formula consists of at least one selected from the group consisting of Na5-2xAl1-xVxS4, Na5-2xAl1-xTaxS4 and Na5-2xIn1-xSbxS4, 0.375≤x≤0.625.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solid ion conductor, a solid electrolyte, a positive electrode material, and a sodium ion solid secondary battery. [Background technology]

[0002] Secondary batteries, which can be used repeatedly by recharging, are used in a wide range of applications from personal to industrial use, including hybrid and electric vehicles, mobile devices such as smartphones, power sources for PCs, and solar power generation storage, and the market for these batteries is expanding rapidly.

[0003] Currently, the most common secondary batteries are lithium-ion secondary batteries and nickel-metal hydride secondary batteries. The former uses relatively rare lithium as its main material, raising concerns about raw material supply issues as demand expands, and also raising concerns about cost. The latter has problems with relatively low charging energy density, memory effect, and limited reusability.

[0004] Given this situation, sodium-ion secondary batteries are attracting attention as the next generation of batteries, as they are abundant in resources, have few problems with raw material supply, are expected to be low cost, and can incorporate the technical knowledge gained from lithium-ion secondary batteries.

[0005] In this case, it is desirable to use a solid electrolyte, which can avoid problems such as handling and leakage, and how to deal with them. The solid electrolyte for sodium ion secondary batteries is required to have high ionic conductivity, excellent charge / discharge characteristics, and high thermal stability (see Non-Patent Document 1).

[0006] Under these circumstances, various studies have been conducted on solid electrolytes for sodium ion secondary batteries. For example, Non-Patent Document 2 can be cited as an example of such efforts, in which Na 3-x Sb 1-x W xS4 (0 < x < 0.18) is under consideration. Also, in Non-Patent Document 3, Na 5-x Al 1-x Si x S4 (0 < x < 1) is under consideration. Note that in those cases, the thermodynamic stability has not been examined.

Prior Art Documents

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0008] The problem of the present invention is to provide a solid ion conductor compound suitable for a solid electrolyte for a secondary battery having high ionic conductivity and excellent thermodynamic stability. The solid electrolyte of the current sodium ion secondary battery has an ionic conductivity at room temperature of 10 -7 ~10 -3 S / cm level, which is not sufficient. Furthermore, it is to provide a solid electrolyte for a sodium ion secondary battery and a sodium ion solid secondary battery having excellent ionic conductivity and charge stability using the compound. [Means for solving the problem]

[0009] The configuration of the present invention is shown below. (Configuration 1) The formula is Na 5-2x Al 1-x V x S4, Na 5-2x Al 1-x Ta x S4 and Na 5-2x In 1-x Sb x S 4、 0.375≦x≦0.625). (Configuration 2) The composition formula is Na4Al 0.5 V 0.5 2. The solid ionic conductor according to claim 1, comprising a crystal of a compound comprising S4. (Configuration 3) The composition formula is Na4Al 0.5 Ta 0.5 2. The solid ionic conductor according to claim 1, comprising a crystal of a compound comprising S4. (Configuration 4) The formula is Na4In 0.5 Sb 0.5 2. The solid ionic conductor according to claim 1, comprising a crystal of a compound comprising S4. (Configuration 5) A solid electrolyte comprising the solid ionic conductor according to any one of Aspects 1 to 4. (Configuration 6) A positive electrode material comprising the solid ionic conductor according to any one of Aspects 1 to 4. (Configuration 7) A sodium ion secondary battery comprising the solid ionic conductor according to any one of Aspects 1 to 4. [Effects of the Invention]

[0010] The present invention provides a solid ion conductor compound that has high ionic conductivity and excellent thermodynamic stability and is suitable for use as a solid electrolyte for a secondary battery. Furthermore, the compound is used to provide a solid electrolyte for sodium ion secondary batteries and a solid sodium ion secondary battery that are excellent in ionic conductivity and charging stability. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a structural diagram showing the structure of a first compound of the present invention. [Figure 2] FIG. 2 is a structural diagram showing the structure of a second compound of the present invention. [Figure 3] FIG. 1 is a structural diagram showing the structure of a third compound of the present invention. [Figure 4] FIG. 1 is a characteristic diagram showing the time dependence of the time ensemble mean squared displacement (MSD). [Figure 5] FIG. 1 is a characteristic diagram showing the temperature dependence (Allerius plot) of ionic conductivity σ. [Figure 6] 1 is a characteristic diagram showing the dependence of the ionic conductivity σ of Na5-2xAl1-xVxS4 on the composition ratio x, where (a) is the case where the temperature T is 500K, and (b) is the case where it is 300K. [Figure 7] 1 is a characteristic diagram showing the dependence of the ionic conductivity σ of Na5-2xAl1-xTaxS4 on the composition ratio x, where (a) is the case where the temperature T is 500K, and (b) is the case where it is 300K. [Figure 8] 10 is a characteristic diagram showing the dependence of the ionic conductivity σ of Na5-2xIn1-xSbxS4 on the composition ratio x, where (a) is the case where the temperature T is 500K, and (b) is the case where it is 300K. [Figure 9] 1 is a cross-sectional structural view of a main part showing a schematic configuration of a solid secondary battery of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in detail below. The detailed description of the present invention will be based on representative aspects, embodiments, and examples, but these are merely examples, and the present invention is not limited to such aspects, embodiments, and examples. It should be noted that "A to B" indicates A or greater and B or less.

[0013] (Embodiment 1) In the first embodiment, an outline of a material search method will be described.

[0014] The first compound Na discovered in this invention 5-2x Al 1-x V x The basic structure of S4 is a supercell of more than 100 atoms taken from the Na5AlS4 structure, and further modified in composition. The M site is assumed to be Al (aluminum)-V (vanadium), with the ratio x varying between 0.375, 0.5, and 0.625. Here, Al-V means that it contains both Al and V. The structure is shown in Figure 1, with x=0.5 as an example. In the figure, Na is represented as 11, Al as 12, V as 13, and S as 14.

[0015] The second compound Na discovered in this invention 5-2x Al 1-x Ta x The basic structure of S4 is a supercell of more than 100 atoms taken from the Na5AlS4 structure, and further modified in composition. The M site is assumed to be Al-Ta (tantalum), with the ratio x varying between 0.375, 0.5, and 0.625. This structure is shown in Figure 2, using x=0.5 as an example. In the figure, Na is represented as 21, Al as 22, Ta as 23, and S as 24.

[0016] The third compound Na discovered in this invention 5-2x In 1-x Sb x The basic structure of S4 is a supercell of more than 100 atoms taken from the Na5InS4 structure, with further compositional modifications. The M site is assumed to be In (indium)-Sb (antimony), with the ratio x varied between three levels: 0.375, 0.5, and 0.625. The structure is shown in Figure 3, with x = 0.5 as an example. In the figure, Na is represented as 31, In as 32, Sb as 33, and S as 34.

[0017] Next, the decomposition energy is calculated based on first-principles calculations, and once it is confirmed that the decomposition energy is 0.1 eV or less, the next step is carried out. Here, the standard of 0.1 eV for the decomposition energy is an empirical value derived from experience in material synthesis. In other words, it is based on the experience that material synthesis is difficult when the decomposition energy exceeds 0.1 eV.

[0018] Thereafter, the Na ionic conductivity was evaluated at 500 K and room temperature (300 K). The calculation method will be described in the Examples section. As a result, as will be described in detail in the Examples section, the first to third compounds, Na 5-2x Al 1-x V x S4, Na 5-2x Al 1-x Ta x S4, and Na 5-2x In 1-x Sb x S4 (x=0.375, 0.5, 0.625), all at 500K, 10 ‐2 S cm -1 Above, 10 at 300K ‐3 S cm -1 It was confirmed that this was the case.

[0019] In addition, the band gap was evaluated as an index of charging stability. As described in the Examples, the band gap is 5-2x Al 1-x V x S4, Na 5-2x Al 1-x Ta x S4, and Na 5-2x In 1-x Sb x It was confirmed that all of the S4 (x=0.375, 0.5, 0.625) were above 1.5 eV.

[0020] Here, high sodium ion conductivity contributes to a reduction in internal resistance and an improvement in current characteristics, and a high band gap improves charge / discharge stability. Therefore, the solid ionic conductor having the extracted Na solid electrolyte composition (formula) is suitable for use as a solid electrolyte for sodium secondary batteries, having excellent ionic conductivity and stability, including during charging and discharging.

[0021] (Embodiment 2) In the second embodiment, a sodium ion solid secondary battery (Na ion solid secondary battery) 101 will be described with reference to FIG. The Na ion solid state secondary battery 101 mainly comprises a negative electrode consisting of a negative electrode active material layer 2 and a collector electrode (negative electrode) 1, a positive electrode consisting of a positive electrode active material layer 4 and a collector electrode (positive electrode) 5, and a solid electrolyte 3. In the present invention, the solid electrolyte 3 uses a solid ion conductor having the composition of embodiment 1. Here, examples of the negative electrode active material layer 2 include Na metal, and examples of the positive electrode active material layer 4 include NaVS2. Note that NaVS2 is disclosed in Non-Patent Document 5.

[0022] By using the solid ion conductor having the composition according to the first embodiment as a solid electrolyte, it becomes possible to provide a Na-ion solid state secondary battery that is excellent in ionic conductivity and stability including charge and discharge, and also has the advantages of being a solid state battery, such as ease of handling, safety, and no leakage. In particular, the first compound Na4Al 0.5 V 0.5 S4, the second compound Na4Al 0.5 Ta 0.5 S4, or the third compound Na4In 0.5 Sb 0.5 When S4 is used, it becomes possible to provide a Na-ion solid state secondary battery with excellent ionic conductivity, low internal resistance, and high output current, and it also has strong stability in charge and discharge characteristics. Na 5-2x Al 1-x V x S4 can be obtained by, for example, mixing Na2S, Al2S3, V, and S, thoroughly crushing and mixing them, and then heating them in a vacuum sealed container. S can be added in excess to create an oxidizing atmosphere.

[0023] It should be noted that the present invention is not limited to the above-described embodiments, which are merely examples for explaining the present invention, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention. [Example]

[0024] The present invention will be described in more detail below using examples.

[0025] Example 1 In Example 1, the following simulation was performed to search for suitable materials from three perspectives: Na ion conductivity, decomposition energy, and band gap energy. More specifically, in Example 1, density functional theory was established and inorganic crystal structure data was prepared, followed by thermodynamic calculations under an ion exchange scheme to determine the ground state phase, and the ion conductivity, decomposition energy, and band gap were calculated.

[0026] 1. Density Functional Theory (DFT) Setup The projector augmented wave (PAW) formalism of the ion-electron interaction is used, and the generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerhof (PBE) is used as a function of the exchange-correlation energy. For Na, In, V, and Ta pseudopotentials, the semicore state was used as the valence state, and standard pseudopotentials were used for other atom types. Here, the kinetic energy cutoff was set to 520 eV. The mesh was allocated using a Monkhorst-Pack grid scheme with at least 1000 k-points. All calculations were performed under spin-polarized conditions, and the optimization convergence was set to less than 10 μeV / atom for energy and less than 0.1 eV / nm for residual force.

[0027] 2. Inorganic crystal structure data As for inorganic crystal structure data, Non-Patent Document 3 is referred to for the structure of Na5AlS4, and Non-Patent Document 4 is referred to for the structure of Na5InS4.

[0028] 3. Ion Substitution Scheme The ion substitution scheme was simulated based on the relationship between crystal symmetry groups and subgroups at special site concentration ratios (x = 0.325, 0.5, 0.675).

[0029] 4.Thermodynamic calculation (4-1) Based on thermodynamics, the Gibbs free energy (G) at a specific temperature (T) and pressure (P) was calculated using the following equation: G(T,P,NNa,NM,NS) = H(T,P,NNa,NM,NS) + PV(T,P,NNa,NM,NS) - TS(T,P,NNa,NM,NS). where H is enthalpy, V is volume, and S is entropy. The PV and TS terms are assumed to be negligible and are set to zero for the condensed phase at 0 K. (4-2) For the predicted compounds and their competing phases, the ground state phases were determined using a convex hull approach. (4-3) Regarding chemical potentials, the reference chemical potentials used based on the standard state of the elements were as follows: (μNa0) from Na metal, (μS0) from S solid, (μAl0) from Al metal, (μV0) from V metal, (μTa0) from Ta metal, (μIn0) from In metal, and (μSb0) from Sb metal.

[0030] 5. Calculation of Ionic Conductivity The ionic conductivity was first estimated from molecular dynamics (MD) calculations at temperatures T = 500, 600, 700, 800, and 900 K. Here, a supercell model with at least 100 atoms was used for Na orbital sampling by MD. In detail, the MD step size was set to 1 fs, and equilibration for more than 5 ps (5000 MD steps) was performed under NVT ensemble conditions before 100 ps (100,000 MD steps) MD trajectory sampling under NVT ensemble conditions. The Na diffusivity at temperature T was determined from the slope of the time ensemble mean-squared displacement (MSD) plot as follows: MSD=<[rv(t+τ)-rv(t)] 2 > where rv(t) is the position of the atom at time t, and τ is the delay between the two positions rv(t+τ) and rv(t). An example of this calculation is shown in Figure 4, where the first compound Na 5-2x Al 1-x V x Calculations were performed for S4 using X as a parameter.

[0031] The diffusion coefficient (D) of Na was calculated based on the Einstein-Smoluchowski equation, i.e., using the following formula (1): where d is the lattice dimensionality in the diffusion process.

[0032]

number

[0033] The ionic conductivity (σ) at each temperature T was calculated based on the Nernst-Einstein relationship, i.e., σ = (ze²D) / (kT), where z is the charge carrier density, e is the elementary charge, and k is the Boltzmann constant. An Arrhenius plot was then created, with Log(σ) on the vertical axis and the reciprocal of temperature (1 / T) on the horizontal axis. The ionic conductivity at the target temperature T = 300K was calculated by extrapolating the plot to T = 300K. 5-2x Al 1-x V x The Allelius plot of S4 is shown in Figure 5.

[0034] The decomposition energy was calculated by comparing the enthalpy of the predicted compound with that of other materials made up of the elements contained in that compound. For this calculation, we used the Phase Diagram App provided by MaterialsProject.org, which is considered the world standard algorithm for decomposition energy.

[0035] The band gap was calculated using the Heyd-Scruseria-Enzerhof (HSE) hybrid functional. In conventional DFT calculations, there is a tendency for the localization of electrons to be underestimated, and the band gap to also be underestimated. However, in this example, by incorporating the HSE method into the electron exchange functional, the picture of electron localization was more accurately estimated, and the underestimation of the band gap was prevented.

[0036] The resulting first to third solid ionic conductors, Na 5-2x Al 1-x V x S4, Na 5-2x Al 1-x Ta x S4, and Na 5-2x In 1-x Sb x The decomposition energy, ionic conductivity, and band gap for S4 (x=0.375, 0.5, 0.625) are listed in Table 1.

[0037] [Table 1]

[0038] As shown in Table 1, Na 5-2x Al 1-x V x S4, Na 5-2x Al 1-x Ta x S4, and Na 5-2x In 1-x Sb xIn S4 (x = 0.375, 0.5, 0.625), the decomposition energy is 0.1 eV / atom or less, and the ionic conductivity is high (10 ‐2 S cm -1 That's it, 10 ‐3 S cm -1 It was confirmed that the band gap was 1.5 eV or more. In particular, number 2 in Table 1 (Na4Al 0.5 V 0.5 S4), 5(Na4Al 0.5 Ta 0.5 S4), and 8(Na4In 0.5 Sb 0.5 S4) has an ionic conductivity of 10 at a temperature of T = 500 K. ‐1 S cm -1 Above, the ionic conductivity at T = 300K is 10 ‐2 S cm -1 Thus, for all three compositions, when X=0.5, the internal resistance is low and the charge / discharge stability remains at a high level. The dependence of ionic conductivity on the composition ratio X is shown in Figures 6 to 8. 5-2x Al 1-x V x S4, Fig. 7 is Na 5-2x Al 1-x Ta x S4, and Figure 8 shows Na 5-2x In 1-x Sb x The data are for S4, where (a) shows the case where the temperature T is 500 K, and (b) shows the case where it is 300 K. In all data, the ionic conductivity reaches a maximum when the composition ratio X is 0.5. [Industrial Applicability]

[0039] The present invention makes it possible to provide a solid ionic conductor with high sodium ion conductivity. Because of its high ionic conductivity, this ionic conductor improves the electrical characteristics of secondary batteries and is preferably used as a solid electrolyte or a positive electrode for sodium ion secondary batteries. Furthermore, this ionic conductor is solid and easy to handle, preventing problems such as leakage. In a smart society where hybrid and electric vehicles, high-performance homes, smartphones and other mobile devices are used, there is a strong demand for secondary batteries that can handle large currents and are easy to handle. Therefore, it is believed that the present invention will contribute to the development of industry and society, including for personal use. [Explanation of symbols]

[0040] 1:Collector electrode (negative electrode) 2: Negative electrode active material layer 3: Solid electrolyte 4: Positive electrode active material layer 5:Collector electrode (positive electrode) 11:Na 12:Al (M site) 13: V (M site) 14:S 21:Na 22:Al (M site) 23: Ta (M site) 24:S 31:Na 32:In (M site) 33:Sb (M site) 34:S 101: Sodium-ion secondary battery

Claims

1. The composition formula is Na ５－２ｘ Al １－ｘ V ｘ S ４ , Na ５－２ｘ Al １－ｘ Ta ｘ S ４ and Na ５－２ｘ In １－ｘ Sb ｘ S ４、 0.375≦x≦0.625).

2. The composition formula is Na ４ Al ０．５ V ０．５ S ４ The solid ionic conductor according to claim 1, which is composed of a crystal of a compound comprising:

3. The composition formula is Na ４ Al ０．５ Ta ０．５ S ４ The solid ionic conductor according to claim 1, which is composed of a crystal of a compound comprising:

4. The composition formula is Na ４ In ０．５ Sb ０．５ S ４ The solid ionic conductor according to claim 1, which is composed of a crystal of a compound comprising:

5. A solid electrolyte comprising the solid ionic conductor according to any one of claims 1 to 4.

6. A positive electrode material comprising the solid ionic conductor according to any one of claims 1 to 4.

7. A sodium ion secondary battery comprising the solid ionic conductor according to any one of claims 1 to 4.

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