Ion conductor for sodium ion battery and secondary battery comprising same

Abstract

An ion conductor for a sodium ion battery according to the present invention is a material represented by chemical formula 1 below. [Chemical formula 1] Na3-cZnaGabS4-cIc (here, 0.9≤a≤1.1, 0.9≤b≤1.1, and 0.05≤c≤0.2)

Description

Ionic conductor for sodium ion battery and secondary battery including same

[0001] This invention was supported by the following national research and development project.

[0002] [Project ID] 1415184637

[0003] [Assignment Number] 20012196

[0004] Ministry of Trade, Industry and Energy

[0005] [Name of Project Management (Specialist) Institution] Korea Institute of Industrial Technology Planning and Evaluation

[0006] [Research Project Name] Industrial Technology Alchemist Project

[0007] [Research Project Name] AI-Based Supercritical Materials

[0008] The present invention relates to an ion conductor for a sodium ion battery and a secondary battery including the same.

[0009] In all-solid-state batteries, sodium ion conductors are increasingly attracting attention as a potential alternative to Li ion conductors. Accordingly, research and development have been conducted to develop ion conductors applicable to sodium-based all-solid-state batteries, resulting in the discovery of several compounds that exhibit acceptable levels of ionic conductivity along with chemical, electrochemical, and environmental stability.

[0010] Among these, Na3ZnGaS4 is a relatively recently discovered material characterized by its corner-sharing T2 supertetrahedrons, in which Na ions reside in open channels formed by intertwined T2 chains. This structural feature differentiates Na3ZnGaS4 from other Na ion conductors, which have high ionic conductivity but are mostly composed of isolated tetrahedral arrays, making it inert and electrochemically stable in a wide range of solvents, including aqueous solutions.

[0011] However, the low ionic conductivity of Na3ZnGaS4 (<10 -3 mS·cm -1) is a factor limiting the application of this compound. To solve this problem, the Zn / Ga ratio included in Na3ZnGaS4 is adjusted to introduce vacancies at specific Na sites, or Na + Ca ion 2+ Methods such as partial replacement with ions have been developed, which can increase ionic conductivity by up to 100 times (Na 2.8 Zn 0.8 Ga 1.2 S4: 0.084 mS·cm -1 , Na 2.7 Ca 0.15 ZnGaS4: 0.059 mS·cm -1 ) is being performed. Despite this improvement in ionic conductivity, in order to maintain an overpotential of less than 100 mV under fast charge / discharge (C / D) conditions of all-solid-state batteries, the ionic conductivity must be at least 0.4 mS cm. -1 Therefore, further improvement of ionic conductivity is essential.

[0012] The object of the present invention is to provide an ion conductor for a sodium ion battery, which exhibits inactivity in a wide range of solvents including aqueous solutions, has electrochemical stability, and has improved ion conductivity, and a secondary battery including the same.

[0013] One aspect of the present invention is to provide an ion conductor for a sodium ion battery represented by the following chemical formula 1.

[0014] [Chemical Formula 1]

[0015] Na 3-c Zn a Ga b S 4-c I c

[0016] (Here, 0.9≤a≤1.1, 0.9≤b≤1.1, 0.05≤c≤0.2)

[0017] Another aspect of the present invention provides a secondary battery comprising: a positive electrode; a negative electrode disposed opposite the positive electrode with a predetermined gap therebetween; and an electrolyte disposed between the positive electrode and the negative electrode; wherein at least one of the positive electrode, the negative electrode, or the electrolyte comprises an ion conductor according to the chemical formula 1.

[0018] The ion conductor according to the present invention is not only inert in the atmosphere, but also has stable electrochemical properties when in contact with an electrode material and has excellent Na ion conductivity that can be applied to an all-solid-state battery.

[0019] Figure 1a is Na 3-x ZnGaS 4-x I x XRD pattern of , and Fig. 1b is a transmission electron microscope image of Na taken from the yellow square. 2.8 ZnGaS 3.8 I 0.2 is the electron diffraction pattern, Fig. 1c shows the variation of the lattice parameter according to 'x', and Fig. 1d shows the atomic arrangement showing different separations between S and Na based on ICSD 234888.

[0020] Figure 2a is Na 3-x ZnGaS 4-x I x is the Raman spectrum of Na3ZnGaS4, and Fig. 2b is the Raman spectrum of Na3ZnGaS4, Na 2.9 ZnGaS 3.9 I 0.1 and Na 2.8 ZnGaS 3.8 I 0.2 of 23 This is the Na NMR spectrum.

[0021] Figure 3a is an arrangement of Na ions showing the connection of Na clusters, and Figure 3b shows the change in Na-Na distance according to 'x' (B and B' are average values).

[0022] Figure 4a shows Na at room temperature (RT). 3-x ZnGaS 4-x I xis the EIS spectrum of Na between -20 and +60 ℃, and Fig. 4b is 3-x ZnGaS 4-x I x is the Arrhenius plot of Na, and Fig. 4c is 3-x ZnGaS 4-x I x In Fig. 4d, the change in the ionic conductivity and activation energy at 30 ℃ according to 'x', and Fig. 4e, the change in the steady-state current according to the applied voltage, and Fig. 4e, the change in the steady-state current according to the applied voltage at 30 ℃ for Na 2.9 ZnGaS 3.9 Br 0.1 EIS spectrum of (inset is Na 2.9 ZnGaS 3.9 Br 0.1 ) is the Arrhenius plot.

[0023] Figure 5 shows Na under a continuous flow of moist N2. 2.9 ZnGaS 3.9 I 0.1 And it shows the change in the concentration of H2S gas liberated from Na3PS4.

[0024] Figure 6 is 0.1 mV·s -1 In Na 2.9 ZnGaS 3.9 I 0.1 is a linear sweep voltammogram.

[0025] Figure 7 shows the Na2Sn|Na during constant current Na alloying / dealloying at various current densities. 2.9 ZnGaS 3.9 I 0.1 |This shows the voltage change of the Na2Sn symmetric cell.

[0026] Figure 8 is 0.02 mA·cm -2 Na during constant current C / D cycle 0.5 TiS2|Na 2.9 ZnGaS 3.9 I 0.1 |Na 0.5 This shows the voltage change of the TiS2 symmetric cell.

[0027] Fig. 9 is Na2Sn|Na 2.9 ZnGaS3.9 I 0.1 |This is the C / D profile according to the rate of the TiS2 cell.

[0028] Fig. 10 is Na2Sn|Na 2.9 ZnGaS 3.9 I 0.1 |These are the cycle characteristics of the TiS2 cell.

[0029] The terms and words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to best describe his or her own invention.

[0030] Hereinafter, the present invention will be described in detail based on preferred embodiments thereof with reference to the attached drawings. The embodiments described in this specification and the configurations depicted in the drawings are merely preferred embodiments of the present invention and do not fully represent the technical ideas of the present invention. Therefore, various equivalents and modified examples may exist that can replace them at the time of this application, and the scope of the present invention is not limited to the embodiments described below.

[0031]

[0032] [Embodiment 1]

[0033] The first embodiment of the present invention is an ion conductor for a sodium ion battery represented by the following chemical formula 1.

[0034] [Chemical Formula 1]

[0035] Na 3-c Zn a Ga b S 4-c I c

[0036] (Here, 0.9≤a≤1.1, 0.9≤b≤1.1, 0.05≤c≤0.2)

[0037] The above a is preferably 0.9 to 1.1, and more preferably a may be 0.95 to 1.05, 0.96 to 1.04, 0.97 to 1.03, 0.98 to 1.02, or 0.99 to 1.01.

[0038] The above b is preferably 0.9 to 1.1, and more preferably b may be 0.95 to 1.05, 0.96 to 1.04, 0.97 to 1.03, 0.98 to 1.02, or 0.99 to 1.01.

[0039] The above c is preferably 0.05 to 0.2, and more preferably c may be 0.06 to 0.2, 0.07 to 0.18, or 0.08 to 0.15.

[0040] The above ionic conductor may have a space group of I41 / acd, which is the same as the crystallographic structure of Na3ZnGaS4.

[0041] The above ionic conductor may be inert in a humid atmosphere.

[0042] The ionic conductivity of the above ionic conductor at 30°C is 0.4 mS·cm -1 Above, 0.5 mS·cm -1 Above, 0.6 mS·cm -1 Above, 0.7 mS·cm -1 Above, 0.8 mS·cm -1 Above, 0.9 mS·cm -1 Above, 1.0 mS·cm -1 It could be strange.

[0043]

[0044] [Embodiment 2]

[0045] A second embodiment of the present invention is a secondary battery including an ion conductor for a sodium ion battery.

[0046] A secondary battery according to a second embodiment comprises a positive electrode, a negative electrode disposed opposite the positive electrode with a predetermined gap therebetween, and an electrolyte disposed between the positive electrode and the negative electrode, wherein at least one of the positive electrode, the negative electrode, or the electrolyte comprises an ion conductor according to the chemical formula 1.

[0047] The above positive electrode, negative electrode and electrolyte may be applied in any possible form known in the secondary battery field, and may preferably be configured as an all-solid-state battery.

[0048]

[0049] <Example>

[0050] For the synthesis of ionic conductors, powders of Na2S, Zn, Ga2S3, S, and NaI were used as raw materials.

[0051] When the raw material was synthesized, Na 3-x ZnGaS 4-x I x The ionic conductor was synthesized by weighing the raw materials to have a composition of (x = 0.05, 0.1, 0.15, and 0.20), ball milling the prepared raw materials (15 mL stainless steel pot, 4 mm stainless steel ball) under the conditions of 40 Hz, 10 min, sealing them in a quartz tube under vacuum, heating them to 750 ℃ ​​at a heating rate of 5 ℃ / min, maintaining them for 12 hours, and slowly cooling the synthesized material to 300 ℃ at a cooling rate of 5 ℃ / h, and then leaving it at room temperature.

[0052]

[0053] <Comparative Example 1>

[0054] For comparison with the examples, NaCl was used to obtain S 2- Part of Cl - In order to obtain an ion-substituted ionic conductor, through the same process as in the above example, Na 2.9 ZnGaS 3.9 Cl 0.1 was synthesized.

[0055]

[0056] Comparative Example 2

[0057] For comparison with the examples, NaBr was used to obtain S 2- Some of the Br - In order to obtain an ion-substituted ionic conductor, through the same process as in the above example, Na 2.9 ZnGaS 3.9 Br 0.1 was synthesized.

[0058]

[0059] Crystal structure analysis

[0060] Na 3-x ZnGaS 4-x I x To confirm the crystal structure and phase purity of (x = 0.05, 0.1, 0.15, and 0.20), X-ray diffraction (XRD) analysis and transmission electron microscopy analysis were performed, and the results are shown in Fig. 1.

[0061] As shown in Fig. 1a, the synthesized Na 3-x ZnGaS 4-x I x (x = 0.05, 0.1, 0.15 and 0.20) maintained the crystallographic structure (space group I41 / acd) of Na3ZnGaS4 up to x = 0.2, and no impurities were detected.

[0062] Also, as shown in Fig. 1b, Na obtained along the

[0101] area axis 2.8 ZnGaS 3.8 I 0.2 Electron diffraction pattern of I - After doping, there was no substantial change in the crystallographic structure and no impurities appeared.

[0063] Also, the lattice dimensions are Na 3-x ZnGaS 4-x I x As 'x' increased in , the unit cell (UC) volume was found to gradually decrease, but the change was less than 0.7% (Fig. 1c).

[0064] In the Na3ZnGaS4 compound, Na ions occupy two different crystallographic sites (Na1 and Na2) to form interpenetrating T2 chains. Na conduction is known to occur primarily via the Na2-Na2 pathway, and Na conduction can be facilitated via Na2-Na2 sites with limited vacancy concentration determined by charge balance.

[0065] Figure 1d shows the atomic arrangement and interatomic distances between S and Na in the initial Na3ZnGaS4 (ICSD 234888). However, I - S at the S3 site by ion doping 2- is I - is replaced by , with a maximum of 0.36 Å(r(S 2- ) = 1.84 Å, r(I - ) = 2.2 Å) may disturb the distribution of Na ions.

[0066] Figure 2a is Na 3-x ZnGaS 4-x I x The Raman spectrum of , with an intensity of 315.9 cm -1 Normalized to the peak of 100 cm -1 and 600 cm -1 Na within the range 3-x ZnGaS 4-x I x Comparing the spectra, two prominent Stokes bands are observed in all spectra at 315.9 cm -1 and 380.5 cm -1 was observed at . According to the Raman spectrum, 380.5 cm -1 The peak intensity in decreases continuously as 'x' increases. (Zn / Ga)S4 tetrahedron is I - The polarization due to substitution and the effect on the polarization change in the asymmetric vibration (F2) gradually weakens as 'x' increases, so 380.5 cm -1The peak intensity in gradually decreases, indicating that the I anion is bound to the S site, and I - Because the amount of substitution was small, the peak position hardly changed.

[0067] Figure 2b shows the NMR spectrum of pure Na3ZnGaS4. 2.9 ZnGaS 3.9 I 0.1 and Na 2.8 ZnGaS 3.8 I 0.2 The NMR spectra of the Na3ZnGaS4 samples were compared. All spectra showed a small peak at 10 ppm, which was likely caused by surface Na compounds (e.g., NaOH) formed during sample preparation or transportation. Excluding this peak, the Na3ZnGaS4 peak was well fitted with two subpeaks at an area ratio of 1:2, which correspond to two crystallographically distinct Na ions at the 16f (Na1) and 32g sites (Na2). In comparison, the Na 2.9 ZnGaS 3.9 I 0.1 and Na 2.8 ZnGaS 3.8 I 0.2 From the peak (indicated by an asterisk), it appears that the Na2 distribution is disturbed due to the presence of Na ions (Na3) in an environment different from the existing Na2 sites. In addition, the area of ​​the new peak (Na3) was similar to that of the Na1 and Na2 peaks, regardless of the iodide content. Therefore, it appears that the Na2 and Na3 sites are approximately half-filled due to the disturbance of the Na2 distribution. This distribution of Na ions can not only increase the pore concentration of the Na2 site, but also reduce the Na2-Na2 distance for conduction. In addition, the binding of the I anion, which disturbs the Na2 stability, can significantly shorten the (Na2-Na2) internal distance.

[0068] Figure 3a shows the arrangement of Na ions showing the connection of Na clusters, and Figure 3b shows Na 3-xZnGaS 4-x I x (x = 0.05, 0.1, 0.15, and 0.20) shows the change in the Na-Na distance according to the x value. As shown in Fig. 3b, I - When doped (Na2-Na2) intra It shows a tendency to decrease rapidly to 3.41 Å (x = 0.05) and then gradually increase as 'x' increases.

[0069]

[0070] Electrochemical impedance spectroscopy (EIS) analysis

[0071] Na at room temperature 3-x ZnGaS 4-x I x Electrochemical impedance spectroscopy (EIS) analysis was performed. For EIS analysis, the synthesized sample was compressed between indium foils (50 μm thick) in a polyoxymethylene (POM) mold (13 mm in diameter) at 150°C and 350 MPa to form pellets with a thickness of approximately 500 μm.

[0072] EIS spectra were recorded by applying a sine wave with an amplitude of ±10.0 mV at a frequency ranging from 1 MHz to 0.1 Hz. To obtain temperature-dependent EIS spectra, the cell was maintained at the specified temperature for 2 h.

[0073] As shown in Fig. 4a, I - The resistance showed a significant change due to doping. The resistance, which was 12.9 kΩ in Na3ZnGaS4, decreased to 1.8 kΩ at x = 0.05 and further decreased to 54 Ω at x = 0.1. However, as 'x' increased beyond 0.1, it gradually increased to 103 Ω at x = 0.15 and 265 Ω at x = 0.2. This increase in resistance is due to the increase in (Na2-Na2) as 'x' increases. intra This may be due to a gradual increase in (see Figure 3b).

[0074] The behavior of this resistance is determined by the activation energy (E a ) is closely related to the change in Na between -20 ℃ and +60 ℃, as shown in Fig. 4b. 3-x ZnGaS 4-x I x The Arrhenius plot shows that the slope initially decreases but then increases again at x = 0.15.

[0075] Activation energy values ​​and ionic conductivity (σ) at 30°C ion ) is summarized in Fig. 4c, and the activation energy and ionic conductivity values ​​show an inverse relationship. In particular, at x = 0.1, 1.12 mS cm -1 The highest ionic conductivity is associated with the lowest activation energy of 0.22 eV.

[0076] Also, as shown in Fig. 4d, the electronic conductivity (σ e ) was at a negligible level.

[0077] Meanwhile, as confirmed in Fig. 4e, Br - Ion I - When doped with the same amount of ions, no significant improvement in ionic conductivity was observed, and Cr - Iondo Br - It showed the same results as ion.

[0078]

[0079] Moisture stability assessment

[0080] Since the surface reactivity of crystalline materials containing Na is sensitive to slight changes in chemical composition, particle shape, and crystal structure, the moisture stability of the synthesized materials was evaluated.

[0081] Moisture stability evaluation was performed by adding 50 mg of Na to humidified N2 gas (dew point +5 ℃, approximately 5500 ppm). 2.9 ZnGaS 3.9 I 0.1was passed through the chamber containing Na at a flow rate of 1.6 L / min. And Na 2.9 ZnGaS 3.9 I 0.1 The time-dependent concentration change of H2S, the product of the reaction between H2O, was monitored for 30 minutes.

[0082] Figure 5 shows that unlike Na3PS4, Na is dispersed despite containing iodide and Na ions. 2.9 ZnGaS 3.9 I 0.1 This shows that it is inert in the humid atmosphere.

[0083]

[0084] Electrochemical stability test

[0085] Na2Sn is used as the reference electrode, and Na 2.9 ZnGaS 3.9 I 0.1 The electrochemical stability of a cell using a mixture of a solid electrolyte and carbon fiber (CF) as a working electrode was investigated. The mixture was used because direct contact between the solid electrolyte and a flat current collector can often lead to an underestimation of the decomposition current.

[0086] The cell has an open circuit voltage (+0.82 V vs. Na / Na + ), 0.1m V·s -1 The charge was scanned using linear sweep voltammetry at a rate of (Fig. 6). The voltammogram showed a cutoff current of 3 μA cm -2 When set to , it exhibits an inherent stability window of 2.63 V.

[0087] The determination of the stability window depends on the choice of cutoff level and scan rate, but Na / Na + The anode limit of +0.07 V is the Na alloy / dealloy voltage (approximately +0.3 V vs. Na / Na + ) is significantly lower than that of Na. 2.9 ZnGaS 3.9 I 0.1This means that the Na2Sn cathode combination is available. Also, +2.56 V vs. Na / Na + The cathode limitation of most sulfide-based anodes is Na 2.9 ZnGaS 3.9 I 0.1 This means that it is commercially available.

[0088] Meanwhile, Na3ZnGaS4 and Na 2.9 ZnGaS 3.9 I 0.1 There was a significant difference in the voltage change during the constant current Na alloying / dealloying in the symmetrical cells (Na2Sn∥Na2Sn) between the two (Fig. 7). In both cases, the voltage response increased with increasing current density, but the magnitude of the voltage change was much larger in the case of Na2Sn∥Na3ZnGaS4∥Na2Sn. For example, in the case of Na3ZnGaS4, the voltage response was 0.1 mA cm -2 Compared to about ±1.0 V in Na 2.9 ZnGaS 3.9 I 0.1 In the case of Na3ZnGaS4, it was approximately ±0.032 V. This difference is due to the unique ionic values ​​between the two materials, but the overpotential (η) increased continuously with the cycle. However, the increase in the overpotential (η) of Na3ZnGaS4 was 0.02 mA cm -2 It was most prominent during the initial alloying / dealloying in Na, with the overpotential (η) increasing from ±0.044 V to ±0.185 V over 50 cycles (Fig. 7, lower inset panel). In comparison, Na 2.9 ZnGaS 3.9 I 0.1 In the case of , the overvoltage (η) increased only slightly from 0.003 V to 0.005 V (Fig. 7, upper insert panel). From this, according to the low anode limit confirmed in Fig. 6, Na 2.9 ZnGaS 3.9 I 0.1 It can be seen that Na2Sn has excellent compatibility.

[0089] Compatibility between a solid ion conductor and a sulfide-based anode according to an embodiment of the present invention 0.5 TiS2∥Na 0.5 A symmetric cell composed of TiS2 was used for the study. Figure 8 shows the voltage change during 50 C / D cycles. The voltage fluctuated within the range of ±100 mV, and the amplitude variation during cycling was minimal. The absence of an increasing trend in the overpotential (η) over repeated cycles indicates that Na 2.9 ZnGaS 3.9 I 0.1 This means that it can be chemically and electrochemically compatible with this TiS2 anode.

[0090]

[0091] Solid electrolyte performance evaluation

[0092] Na as a solid electrolyte for all-solid-state batteries 2.9 ZnGaS 3.9 I 0.1 The performance was evaluated when applied.

[0093] The all-solid-state battery was fabricated by pressing a multilayer stack of Na2Sn (50 mg)∥Na2.9ZnGaS3.9I0.1 (150 mg)∥cathode composite (5 mg) in a POM mold at 350 MPa for 1 minute, and the cathode composite used a mixture of TiS2 (30 wt%), solid electrolyte (65 wt%), and carbon fiber (5 wt%).

[0094] Figure 9 shows Na2Sn∥Na at various rates. 2.9 ZnGaS 3.9 I 0.1 ∥Shows the C / D profile of the TiS2 cell.

[0095] At a C-rate of 0.1C, the cell has 216 mAh·g -1 It showed a reversible capacity of , which is higher than the theoretical capacity of TiS2 (239 mAh·g -1 ) corresponds to 90% of the capacity. As the C-rate increases, the capacity decreases, but the cell still has a capacity of 118 mAh·g at 1C. -1 By passing Na2.9 ZnGaS 3.9 I 0.1 High ionic conductivity was confirmed.

[0096] Additionally, due to the electrochemical stability upon contact with the electrode material, the entire cell showed acceptable cyclability during repeated C / D cycles at 0.5 C (Fig. 10).

[0097] The discharge capacity in the first cycle is 117 mAh·g -1 However, from the second cycle onwards, it was 156m mAh·g -1 The capacity increased rapidly above. In the following cycles, the capacity increased slightly, reaching 162 mAh·g during the subsequent 25 C / D cycles. -1 reached 140 mAh·g -1 The capacity was gradually reduced to 89.7% over 100 C / D cycles. Except for one cycle, the Coulombic efficiency was also maintained above 99%.

Claims

1. An ion conductor for a sodium ion battery represented by the following chemical formula 1. [Chemical Formula 1] On 3-c Zn a Ga b S 4-c AND c (Here, 0.9≤a≤1.1, 0.9≤b≤1.1, 0.05≤c≤0.2) 2. In paragraph 1, An ionic conductor for a sodium ion battery, wherein c is 0.06 to 0.

2.

3. In paragraph 1, An ionic conductor for a sodium ion battery, wherein c is 0.07 to 0.

18.

4. In paragraph 1, An ionic conductor for a sodium ion battery, wherein c is 0.08 to 0.

15.

5. In paragraph 1, The space group of the above ionic conductor is I4. 1 / acd, an ionic conductor for sodium ion batteries.

6. In any one of paragraphs 1 to 5, The above ion conductor is an ion conductor for a sodium ion battery, which is inert in a moist atmosphere.

7. In any one of paragraphs 1 to 5, The ionic conductivity of the above ionic conductor at 30°C is 0.4 mS cm. -1 Ideal ionic conductor for sodium ion batteries.

8. In any one of paragraphs 1 to 5, The ionic conductivity of the above ionic conductor at 30°C is 1 mS cm. -1 Ideal ionic conductor for sodium ion batteries.

9. Bipolar, A cathode and a positive electrode are positioned opposite each other at a predetermined distance from each other. Containing an electrolyte disposed between the positive and negative electrodes, A secondary battery, wherein at least one of the positive electrode, negative electrode or electrolyte comprises a solid ion conductor as described in any one of claims 1 to 5.

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