KR102528259B1 - A novel phase-convertible magnesium solid-state electrolyte and preparation method thereof - Google Patents

Abstract

The present invention relates to (a) a magnesium salt; and (b) succinonitrile.
According to the present invention, a new phase-change magnesium solid electrolyte that has very high conductivity of magnesium ions in the solid phase, excellent chemical and thermal stability, and can be widely applied to all types of electrodes regardless of shape, and an all-solid-state battery including the same can provide.

Description

A novel phase-convertible magnesium solid-state electrolyte and preparation method thereof}

The present invention relates to a novel phase-change magnesium solid electrolyte and a manufacturing method thereof, and more particularly, to a magnesium all-solid-state battery with excellent stability by preparing a new concept phase-change magnesium solid electrolyte using a reversible phase change of succinonitrile. It relates to a technology that can be applied as a high-speed ion conductor of

The reversible and alternating intercalation of lithium ions from the two electrodes through the liquid electrolyte layer is a key function utilized in modern battery technology. However, with current battery technology already reaching the maximum achievable energy density, the demand for higher energy density, safer and cheaper batteries continues to grow, driving the development of next-generation technologies using new and innovative materials. As a promising candidate, magnesium (Mg) batteries are attracting great attention due to their attractive properties such as high volumetric capacity (3,832 mAh cm ⁻³ ) and relatively low reduction potential (−2.37 V vs standard hydrogen electrode). Considering that volumetric capacity rather than gravimetric capacity is a key criterion for stationary energy storage systems (ESS), the large capacity of Mg batteries is a great advantage for next-generation batteries. Divalent Mg ²⁺ ions can transfer two electrons per single cation, potentially providing higher energy densities compared to conventional lithium-ion batteries. In addition, non-toxic Mg is relatively inexpensive and abundant in nature, making it more attractive in the age of lithium-ion batteries.

However, practical application of Mg-based batteries is difficult because metallic Mg tends to form an ionic insulating layer on its surface when in contact with an electrolyte. Originally, many efforts have been devoted to developing Mg-compatible electrolytes. For example, Grignard reagent-based electrolytes composed of organic magnesium and organic aluminum chloride compounds have been widely studied because the reactive chloride compounds are effective in removing contaminants and preventing the formation of passivating layers. However, since battery packages or cell components are vulnerable to these highly reactive electrolytes, important safety issues such as leakage, volatilization and electrolyte explosion remain serious unresolved issues. Considering that the main target of Mg batteries is large-scale battery systems (e.g., ESS), a high level of safety must be ensured first to prevent serious explosion accidents.

In addition, solid electrolytes (SE), which undoubtedly have high chemical and thermal stability, will be a promising option to provide a much improved level of safety. Therefore, the main research direction for next-generation batteries is the development of solid electrolytes with ion conductivity similar to that of current liquid electrolytes. Lithium-ion conductive solid electrolytes have already caught up with the conductivity of liquid electrolytes, but the development of Mg ²⁺ ion conductors has been delayed mainly due to the slow diffusion of Mg ²⁺ ions by strong electrostatic bonding with the host material (Fig. 1). see a through c). For example, multivalent ionic conducting hosts that use conventional cation hopping for ion diffusion have been difficult to achieve practically acceptable conductivity at room temperature. In addition, unlike liquid electrolytes, which always form conformal contact even on rough solid surfaces, high interfacial resistance due to unavoidable poor contact between hard solid particles is an important problem in next-generation batteries using solid electrolytes. Therefore, in realizing an efficient all-solid-state Mg battery with a high-conductivity solid electrolyte, it is necessary to design a new ion conduction host with a fast diffusion channel based on a unique conduction mechanism and solve the difficult contact resistance between solid particles. there is.

Therefore, the inventors of the present invention developed a new concept of phase-convertible magnesium solid-state electrolyte (PCE) using the reversible phase transition of succinonitrile (N≡C-CH ₂ -CH ₂ -C≡N, SN). was produced, and the present invention was completed with attention to the fact that it could be applied as a high-speed ion conductor of a magnesium all-solid-state battery with excellent stability.

Patent Document 1. Patent Publication No. 10-2019-0048265 Patent Document 2. Patent Registration No. 10-1946266

Non-Patent Document 1. Aurbach, D. et al., Nature 407, 724-727 (2000) Non-Patent Document 2. Manthiram, A. et al., Nat. Rev. Mater. 2, 1-16 (2017)

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is that the conductivity of magnesium ions in the solid phase is very high, chemical and thermal stability is excellent, and it can be widely applied to all types of electrodes regardless of shape. A possible new phase change is to provide a magnesium solid electrolyte.

In addition, another object of the present invention is to provide a method for preparing a new phase-change magnesium solid electrolyte by using a liquid-solid phase transition and using a safe and simple process that does not require a solvent or a pressurization process.

The present invention for achieving the object as described above, (a) a magnesium salt; and (b) succinonitrile.

The magnesium salt may be magnesium bis(trifluoromethanesulfonimide).

The molar percentage of the magnesium salt may be 1 to 5 mol%.

The phase change magnesium solid electrolyte may further include an iodine additive.

In addition, the present invention provides an all-solid-state battery including the phase-change magnesium solid electrolyte.

In addition, the present invention comprises the steps of (I) adding a magnesium salt to molten succinonitrile to form a mixture; and (II) stirring the mixture at 60° C. or higher.

In step (I), an iodine additive may be further added.

According to the present invention, it is possible to provide a new phase-change magnesium solid electrolyte that has very high conductivity of magnesium ions in the solid phase, excellent chemical and thermal stability, and can be widely applied to all types of electrodes regardless of shape.

In particular, when the iodine additive is additionally included in the phase change magnesium solid electrolyte according to the present invention, the conductivity of magnesium ions is further improved by greatly reducing the interfacial resistance on the surface of the magnesium metal, so it is expected to be applied as an electrolyte for an all-solid-state battery.

1 is a conceptual diagram comparing the characteristics of a phase change magnesium solid electrolyte (PCE) according to the present invention and a conventional liquid electrolyte and an inorganic solid electrolyte.
Figure 2 is data analyzing the characteristics of phase change magnesium solid electrolytes (PCE) prepared using different concentrations of Mg (TFSI) ₂ salts from Examples of the present invention [(a) FT-IR spectrum, (b) real Image, (c) differential scanning calorimetry (DSC) graph, (d) X-ray diffraction (XRD) pattern graph of 0 mol% and 1 mol% SN electrolytes, (e) temperature-dependent phase transition of 1 mol% SN electrolyte image].
3 is data investigating magnesium ion conduction characteristics of a phase-change magnesium solid electrolyte (PCE, 1 mol% SN) according to an embodiment of the present invention [(a) Nyquist at room temperature using a SUS│PCE│SUS symmetrical cell ( Nyquist) plot (inset shows an equivalent circuit), (b) temperature dependence of PCE ionic conductivity (Arrhenius plot), (c) comparison of the properties of previously known magnesium solid electrolytes and PCE, (d) three succinonitrile molecules Shape and binding energy of magnesium ion with (e) mean square displacement curve, (f) theoretically calculated temperature dependence of ionic conductivity (Arrhenius plot, inset shows the calculated temperature dependent diffusion coefficient of magnesium ion in SN) ].
4 is a process of impregnating a Mo ₆ S ₈ electrode with a phase change magnesium solid electrolyte (PCE) according to an embodiment of the present invention and data analyzing the morphology of the Mo ₆ S ₈ electrode [(a) monolithic (monolithic) ) Schematic diagram of the manufacture of PCE, (b) real image showing the wetting characteristics of PCE on the Mo ₆ S ₈ electrode before and after the heating process, (c) scan of the cross section of the Mo ₆ S ₈ electrode without 1 mol% SN electrolyte (d) Scanning electron microscope (SEM) image of a cross section of a Mo ₆ S ₈ electrode with 1 mol% SN electrolyte, (e) Energy dispersion of the white dotted box area in (d) X-ray spectroscopy (EDS) analysis and corresponding EDS basic mapping results].
Figure 5 is data showing the electrochemical characteristics of the Mo ₆ S ₈ electrode having a phase change magnesium solid electrolyte (PCE) according to an embodiment of the present invention [(a) Mo ₆ S ₈ │PCE│SUS precision constructed by asymmetric cells Comparison of Nyquist plots for Mo ₆ S ₈ electrodes in contact and poor contact conditions (closed circle: initial, open circle: after 24 hours, insets show schematic images for poor and fine contact), (b) Mg Nyquist plots for 1 mol% SN electrolyte with iodine additive (1 mol%+I ₂ ) and 1 mol% SN electrolyte without iodine additive (1 mol%) at room temperature using a cell constructed by │PCE│Mo , (c) Magnified Nyquist plot in the gray dotted circle region in (b), (d) Mg│PCE+I ₂ │Mo ₆ S ₈ voltage profile of Mg stripping and deposition in the cell].

Hereinafter, a novel phase-change magnesium solid electrolyte and a manufacturing method thereof according to the present invention will be described in detail with examples and accompanying drawings.

First, in the present invention (a) a magnesium salt; and (b) succinonitrile.

Specifically, the present invention relates to a phase-convertible electrolyte (PCE) of a new concept using the reversible phase transition of succinonitrile (SN), wherein succinonitrile has a degree of rotational freedom and It is known as a plastic crystal composed of weakly functioning molecules. The high polarity and high conformational disorder of succinonitrile crystals dissolve Mg ²⁺ salts and facilitate ion transport in the solid state. In addition, due to the reversible solid-liquid transition above room temperature, the monolithic structure of PCE can form conformal contact with a solid electrode like a conventional liquid electrolyte through a simple heating and cooling process. Monolithic PCE is expected to simultaneously solve the current issues of low ionic conductivity and poor contact. In addition, the high thermal and electrochemical stability of succinonitrile can ensure a high level of safety for future all-solid-state Mg-ion batteries.

Therefore, crystalline succinonitrile capable of phase change was selected for the Mg ion-conducting host because of its monolithic structure and high-speed cation transport through 3-dimensional ion diffusion. As shown in Figure 1d and e, the succinonitrile molecule in the plastic crystalline phase (i.e. solid phase) at room temperature has two conformations of the trans and gauche types and rotation of the central CC single bond (i.e. between the trans and gauche types). mutual conversion), and can induce diffusion of cations dissolved in the succinonitrile matrix. In particular, inherent properties such as weak interactions between succinonitrile molecules and low rotational energy barriers mainly contribute to the dynamic motion of dissolved cations. More importantly, the monolithic nature of succinonitrile-based electrolytes is important for providing high ionic conductivity as they can provide a homogeneous and favorable environment for cation diffusion. So far, succinonitrile-based electrolytes have been studied only for monovalent ions (especially lithium ions), but succinonitrile is also advantageous for ion conduction and has unique properties for all types of cations, including multivalent cations such as Mg ²⁺ . It is promising as a much more challenging polyvalent ion conductor because it can potentially be applied to In this regard, if the high polarity of succinonitrile can dissolve polyvalent Mg salts and exhibit high Mg ²⁺ ionic conductivity, the feasibility of monolithic succinonitrile-based Mg ²⁺ ion electrolyte through phase transition is sufficient. do.

In addition, magnesium bis(trifluoromethanesulfonimide) can be preferably used as the magnesium salt (a), but is not limited thereto.

In addition, the molar percentage of the magnesium salt may be 1 to 5 mol%, where the molar percentage of the magnesium salt is 1 mol% means '1 mol% Mg salt of succinonitrile', and a solid electrolyte at room temperature Considering the fact that can be obtained, the mole percentage of the magnesium salt is most preferably 1 mol%.

In addition, the phase-change magnesium solid electrolyte may further include an iodine (I ₂ ) additive, which greatly improves the conductivity of magnesium ions by greatly reducing the interfacial resistance on the surface of the magnesium metal, so that it can be applied as an electrolyte for an all-solid-state battery. It is expected.

Therefore, the present invention can provide an all-solid-state battery including the above-described phase-change magnesium solid electrolyte.

Further, in the present invention, (I) adding a magnesium salt to molten succinonitrile to form a mixture; and (II) stirring the mixture at 60° C. or higher.

A conventional solution-based method for preparing a solid electrolyte mainly disperses or dissolves solid particles into a solution phase of a solid electrolyte using a dissolving agent such as N-methylformamide (NMF), hydrazine, and hexane. However, most of these solvents are toxic, and practical applications are limited because an additional solvent drying step must be used. In addition, a pelletizing or pressing process is also required to densely fill the empty space between the particles.

However, the method for producing a phase-change magnesium solid electrolyte according to the present invention utilizes the liquid-solid phase transition of a phase-change electrolyte (PCE) that does not require a solvent or a pressurization process, thereby producing a phase-change magnesium solid electrolyte in a simple and safe process. there is. In addition, due to the unique properties of molten PCE, it is easy to deform and chemically stable, and has an effect that can be widely applied to all types of electrodes regardless of shape.

In the step (I), an iodine additive may be further added, and the phase-change magnesium solid electrolyte to which the iodine (I ₂ ) additive is added significantly improves the conductivity of magnesium ions by greatly reducing the interfacial resistance at the magnesium metal surface. contribute

Hereinafter, specific examples and test examples are described together with various data of the accompanying drawings.

[Example] Preparation of phase change magnesium-ion electrolyte (PCE)

Magnesium(II) bis(trifluoromethanesulfonimide) salt [Mg(TFSI) _2, Solvionic, 99.5%] and succinonitrile (Sigma-Aldrich, 99%) were dried prior to use. The magnesium(II) bis(trifluoromethanesulfonimide) salt was added to molten succinonitrile (SN) to form a mixture. Then, the mixture was stirred at 60° C. or higher overnight to prepare PCE of various concentrations [molar percentage of Mg(TFSI) ₂ 1 mol% (1 mol% SN), 3 mol% (3 mol% SN), 5 mol% (5 mol% SN)]. In addition, PCE to which an iodine (I ₂ ) additive (Sigma-Aldrich, ≥99.8%) was optionally added at a molar concentration of 0.1 M was prepared in the same manner.

On the other hand, Mo ₆ S ₈ electrodes were prepared to evaluate electrochemical properties, and Mo ₆ S ₈ , carbon and polyvinylidene fluoride (80:10:10% by weight) were mixed with an excess of 1-methyl-2-p. Molybdenum foil was coated by forming into a slurry under Rolidone (NMP). After drying the coating solution overnight, it was placed in a vacuum oven at 80°C. Mo ₆ S ₈ electrodes were then punched and assembled into 12 mm disks.

[Test Example] Instrument analysis and characteristic evaluation method

Powder X-ray diffraction patterns were obtained using Dmax2500/PC with CuK _α radiation at a scan rate of 3° mm ⁻¹ (2 θ ). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on SDT-Q600 to evaluate the thermal properties of PCE under air and N ₂ atmospheres (heating rate 10° C. min ^-1 ). A Fourier transform infrared spectrum was measured using a Nicolet iS10 in the range of 4,000 to 400 cm ^-1 . Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) were obtained using a Regulus 8230. X-ray photoelectron spectroscopy (XPS) was performed using Nexsa. Electrochemical impedance spectroscopy was performed using VMP3 in the frequency range of 50 mHz to 1 MHz and applied amplitude of 10 mV, and was measured by cyclic voltammetry at a sweep rate of 0.1 mV s ^-1 . Constant current cycle measurement was evaluated under a current density of 0.01 mA cm ^-2 (1 hour per cycle) using WBCS 3000.

In addition, the electronic structure and energetics of the molecule were modeled with the DMol3 program using the density functional theory. The exchange and correlation energies were treated by a generalized gradient approximation, and the Perdew-Burke-Ernzerhof function was used to describe the exchange-correlation interaction between electrons. A global trajectory cutoff of 5.0 ㅕ was selected and used for all calculations. A high-quality set double-numeric polarized basis was adopted, and the binding energy was calculated by the following equation:

In the above formula,

,

및

은 각각 SN과 상호 작용하는 Mg²⁺의 기저 상태, Mg²⁺의 흡착종 및 SN 분자의 총 전자에너지이다.

,

and

are the total electron energies of the ground state of Mg ²⁺ interacting with SN, the adsorbed species of Mg ²⁺ and the SN molecule, respectively.

All molecular modeling procedures including geometrical optimization were implemented using PACKMOL. The runs for predicting the mobility of ions in the SN were performed using the Large-Scale Atomic/Molecular Massively Parallel Simulation program provided by Sandia National Research Laboratories. implemented. All intermolecular and intramolecular interactions were accounted for using the polymer coherent force field parameterized in the ab-inito calculations. The PCFF force field used to describe the interaction between Mg ²⁺ and SN does not differentiate between the van der Waals potential and Coulomb interaction parameters of Mg ²⁺ and N atoms in the gauche and trans conformations. Nevertheless, the PCFF force field can explain the morphological changes of the gauche-trans transition. Therefore, the MD simulation part focuses on the morphological changes of SN and Mg ²⁺ ion mobility. The MSD of Mg ²⁺ ions at time t in the NPT process is given by:

는 시간 t에서 i 번째 원자의 현재 위치를 나타낸다. In the above formula,

denotes the current position of the ith atom at time t.

를 통해 계산되었으며, 여기서 σ는 등방성 이온 전도도이고, N은 cm³ 당 양이온 수이고, e는 전자 전하이고, k _b 는 볼츠만 상수이고, D는 음이온의 확산계수이다. The change of the MSD curve with temperature can be used to estimate the relative diffusivity of a molecular system undergoing a temperature change. The ionic conductivity of Mg ions is determined by the Nernst-Einstein relation

where σ is the isotropic ionic conductivity, N is the number of cations per cm ³ , e is the electronic charge, k _b is the Boltzmann constant, and D is the anion diffusion coefficient.

[Results of device analysis and characteristic evaluation]

Figure 2 shows data analyzing the characteristics of the phase change magnesium solid electrolyte (PCE) prepared using different concentrations of Mg (TFSI) ₂ salt from the examples of the present invention [(a) FT-IR spectrum, (b) real Image, (c) differential scanning calorimetry (DSC) graph, (d) X-ray diffraction (XRD) pattern graph of 0 mol% and 1 mol% SN electrolytes, (e) temperature-dependent phase transition of 1 mol% SN electrolyte image] was shown.

First, the chemical properties of each PCE according to the different salt molar percentages of the solid phase were investigated using Fourier Transform Infrared Spectroscopy (FT-IR) (see Fig. 2a). As indicated by the notation of the 1 mol% (1 mol% Mg salt of SN) sample, signals related to the stretching oscillations of the TFSI anion were identified after addition of the Mg ²⁺ salt. The peaks at 1061, 1142 and 1354 cm ⁻¹ coincided with SNS vibrations, symmetric and asymmetric stretching vibrations of SO ² , respectively. After adding the salt, the characteristic peak of pure SN (0 mol%) was unchanged, indicating that Mg(TFSI) ₂ can be stably dissolved in SN without side reactions. Even after adding 5 mol% of the salt, no significant change in the spectrum was observed, indicating high compatibility of the Mg dopant in the SN.

It is interesting to note that the physical behavior of PCE varies greatly with salt concentration. All SN electrolytes appear transparent at an elevated temperature of 70 °C regardless of the salt concentration, but show concentration-dependent characteristics after solidification (i.e., cooling to room temperature) (see Fig. 2b). Both 0 mol% (pure SN) and 1 mol% SN are completely solid at room temperature, while 3 mol% SN consists primarily of a solid phase but is mixed with some liquids. In the case of 5 mol% SN, it hardly converts to the solid phase and maintains the original liquid state. This temperature-dependent phase transition behavior is because Mg salt and SN form a eutectic system, and the T _m of SN decreases when Mg salt is added.

Differential scanning calorimetry (DSC) was performed to carefully compare the T _m of each SN (see Fig. 2 c). As the concentration of Mg salt increased from 0 to 5 mol%, the T _m of each SN was It decreased gradually from 59 °C to 37 °C. In addition, all PCEs have an endothermic peak around -32 ℃ corresponding to the plastic crystal transition temperature ( T _pc ), and since dissolved salts affect the activation barrier of the gauche-trans transition, T _pc increases when the salt concentration increases. showed a gradual negative shift. In short, molten SN can stably dissolve the Mg(TFSI) ₂ salt, and the concentration of the salt has a great influence on the thermal behavior of PCE. To obtain a solid-state electrolyte at room temperature, 1 mol% SN is the most preferred choice.

In addition, the crystal structures of pure SN (0 mol% SN) and PCE (1 mol% SN) in the solid phase were investigated using X-ray diffraction (XRD) (see Fig. 2d). 0 mol% SN exhibited typical properties of a plastic crystalline phase. The two dominant peaks at 19° and 28° corresponded to the (110) and (200) planes of the cubic structure, respectively, and the wide hump region from about 20° to 30° in the XRD pattern indicates the highly rotated plastic crystal phase. indicate a disability. 1 mol% SN showed a consistent XRD pattern with the plastic crystal phase of 0 mol% SN, and no significant change was observed. It can be seen that the Mg ²⁺ salt was stably dissolved in the molten SN and the plastic crystal properties of the SN host were preserved even after dissolving the Mg ²⁺ salt. However, 1 mol% SN has a relatively intense peak at 28° compared to pure SN, which is consistent with previous results showing salt-induced local ordering of SN by the interaction between salt and SN molecules. 1 mol% SN can be easily liquefied and solidified by simple heating to 60 °C and cooling to room temperature (see Fig. 2e), respectively, and is in close contact with a solid electrode using a transformable liquid PCE at moderately elevated temperatures. can form

Additionally, the thermal and electrochemical stability of SN was investigated to confirm its applicability as a solid electrolyte in Mg ion batteries. In contrast to highly flammable organic solvents (tetrahydrofuran, THF), SN showed rather low flammability and was thermally stable above 100 °C in air. In particular, no significant oxidation current was observed up to 6 V in cyclic voltammetry (CV) tests, which proves the rationality of using SN as a safe host for Mg batteries.

3 shows the data of examining the magnesium ion conduction characteristics of the phase-change magnesium solid electrolyte (PCE, 1 mol% SN) according to an embodiment of the present invention [(a) Nyquist at room temperature using a SUS│PCE│SUS symmetrical cell ( Nyquist) plot (inset shows an equivalent circuit), (b) temperature dependence of PCE ionic conductivity (Arrhenius plot), (c) comparison of the properties of previously known magnesium solid electrolytes and PCE, (d) three succinonitrile molecules Shape and binding energy of magnesium ion with (e) mean square displacement curve, (f) theoretically calculated temperature dependence of ionic conductivity (Arrhenius plot, inset shows the calculated temperature dependent diffusion coefficient of magnesium ion in SN) ] was shown.

In order to evaluate the ionic conductivity of PCE, first, a blocking symmetric cell was prepared and the impedance was measured using electrochemical impedance spectroscopy (EIS) (see a and b in FIG. 3). A Nyquist plot of PCE (1 mol% SN) shows a typical EIS profile of the blocking electrode, and the ionic conductivity was measured as 2.808 x 10 ^-5 S cm ^-1 at room temperature (see Fig. 3a). Conductivities higher than 10 ⁻⁵ S cm ⁻¹ at room temperature for multivalent ions have rarely been achieved. Based on the ionic conductivity with temperature (see b in FIG. 3), the activation energy ( E _a ) for ion conduction in the solid-state PCE can be derived. In the Arrhenius plot, E _a was calculated to be 0.674 eV, which is significantly lower than previously known values, confirming that SN acts as a promising ionoconductive host for multivalent Mg ²⁺ ions. It should be noted that such a high ionic conductivity was achieved at room temperature without a high-temperature sintering process. The ionic conductivity was measured by wetting the molten electrolyte on a glass microfiber filter (GF/F) and cooling it at room temperature (see the inset of FIG. 3B). In particular, the PCE according to the present invention showed almost the highest conductivity even at the lowest heat treatment temperature. PCE exhibits better ion conductivity than conventional solid electrolytes for Mg ²⁺ conduction such as borohydride-based electrolytes, liquid crystal-based electrolytes, and phosphate-based solid electrolytes (see FIG. 3 c). Phosphate-based solid electrolytes (R5, R6 and R7 in FIG. 3c) can exhibit significantly high ionic conductivity (> 10 ^-5 S cm ^-1 ) at room temperature, but the high temperature sintering (> 750 °C, The red bars in Fig. 3 c) limit the practical application of the electrolyte. In addition to the high ionic conductivity of PCE, the voltage window is wide enough to ensure high electrochemical stability (blue bar in Fig. 3c). The unique properties such as high ionic conductivity, simple fabrication process, wide electrochemical window and high chemical stability are the great advantages of PCE used in advanced Mg ²⁺ ion conducting solid electrolytes.

In addition, in order to fundamentally understand how Mg ²⁺ ions dissolve in SN and coordinate with SN molecules, ' ab initio ' calculations were performed by density functional theory (DFT). The arrangement of Mg ²⁺ ions and SN molecules and binding energy between them were calculated (see d in FIG. 3). Monovalent lithium ions showed a binding energy of -3.33 eV, but divalent Mg ²⁺ ions interacted more strongly with SNs due to strong Coulomb interactions. The binding energy is greatly affected by the number of trans configurations. The binding energy of Mg ²⁺ with three gauche configurations is -15.93 eV, but it gradually increases as the gauche ligand converts to the trans configuration. The binding energy with three trans corresponds to -17.95 eV, suggesting that the binding with trans is more favorable than gauche for Mg ²⁺ ion (see d in FIG. 3). In other words, interaction with Mg ²⁺ ions can promote the configurational transition of gauche to trans due to the reduced activation energy for the transition. In addition, the configurational transitions affected by Mg ²⁺ ions can also be visually provided using simulations of configurational transitions of gauche molecules adjacent to Mg ²⁺ ions, where the easy rotation of SN molecules is beneficial for conducting positive ions. Considering , this result fundamentally supports that the SN host enables high-speed ionic conduction even in the solid phase.

In addition, the fluid motion of Mg ²⁺ ions in the SN host was further investigated based on molecular dynamic simulations at various molecular ratios (99 to 1) between Mg(TFSI) ₂ and SN. A periodic boundary condition with an amorphous cell was applied, and the cell became the lowest energy by energy minimization. The cell was then equilibrated in an isothermal-isobaric (NPT) ensemble with repeated 1,000 ps at various temperatures ranging from 250 K to 350 K at 25 K intervals. 3e shows the mean square displacement (MSD) of Mg ²⁺ ions, and the kinetic ability (MSD) of Mg ²⁺ ions was calculated by inductive calculation. Based on the calculation results, the ionic conductivity was estimated to be 1.17 x 10 ^-4 S cm ^-1 at room temperature (see f in Fig. 3). The calculated coefficient roughly follows the Arrhenius behavior from which the activation energy for ionic conduction can be calculated. The obtained activation energy of 0.487 eV indicates that the Mg ²⁺ ion strongly interacts with the SN molecule in the trans form (see the inset of f in FIG. 3 ). The theoretical values of both the ionic conductivity and the activation energy barrier show slight deviations from the experimental values, which can be attributed to the weaker interaction between N and Mg ²⁺ in the MD simulations than in the DFT calculations. However, the above calculation result can be regarded as a basic basis for how Mg ²⁺ ions rapidly diffuse in the SN host.

4 shows a process of impregnating a Mo ₆ S ₈ electrode with a phase change magnesium solid electrolyte (PCE) according to an embodiment of the present invention and data analyzing the morphology of the Mo ₆ S ₈ electrode [(a) monolithic (monolithic) ) Schematic diagram of the manufacture of PCE, (b) real images showing the wetting characteristics of PCE on the Mo ₆ S ₈ electrode before and after the heating process, (c) scan of the cross section of the Mo ₆ S ₈ electrode without 1 mol% SN electrolyte Electron microscope (SEM) image, (d) Scanning electron microscope (SEM) image of a cross-section of Mo ₆ S ₈ electrode with 1 mol% SN electrolyte, (e) Energy dispersive X in the white dotted box area in (d) -ray spectroscopy (EDS) analysis and the corresponding basic mapping result of EDS] are shown.

In addition to the high conductivity of the bulk PCE, inducing conformal contact between the solid electrolyte and the active material, in particular, is essential to achieve solid-state batteries with high-speed performance. Since SN has a relatively low T _m at about 60 °C, mild heating conditions above T _m can easily liquefy PCE. After cooling to room temperature, it spontaneously undergoes a phase transformation back to the plastic crystalline solid phase. The simple liquid-solid phase transition of PCE is an important technical idea of the present invention because the liquid-phase coating can form perfect conformal contact with the electrode. As schematically shown in Fig. 4a, PCE melted by mild heating above T _m directly fell onto the electrode, and liquid PCE naturally penetrated into the electrode. The wetting process allows the PCE to completely occupy the empty space of the electrode and form conformal contact with the electrode particle. Finally, natural cooling to room temperature induces solidification of the PCE while maintaining intimate contact with the electrode. This approach has significant advantages over previous results dealing with solution processes of solid electrolytes to induce favorable interfacial contacts.

To further confirm the close contact formation between the PCE and the electrode through the liquid-solid phase transition of the PCE, the wettability of the PCE on the Mo ₆ S ₈ cathode having numerous internal pores was tested (see Fig. 4b). Immediately after the molten PCE was dropped onto the Mo ₆ S ₈ electrode, spherical droplets were formed. When heated above T _m , the PCE droplets easily penetrated the porous electrode by filling the voids between the inner Mo ₆ S ₈ particles. Due to its high fluidity, the molten PCE can easily occupy the voids and cover the electrodes (see Fig. 4b). Interfacial characteristics between PCE and Mo ₆ S ₈ electrodes were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis (see c to e in FIG. 4 ). The pure Mo ₆ S ₈ electrode (see Fig. 4c) contained visible voids between the cathode particles, but the void space was greatly reduced in the case of the Mo ₆ S ₈ electrode wetted by PCE (see Fig. 4 d ). . As a result of EDS elemental mapping, it was confirmed that the PCE successfully occupied the inside of the electrode and uniformly covered the electrode particles (see Fig. 4e). In addition, the chemical stability of PCE for Mo ₆ S ₈ was tested using XRD. The characteristic peak of Mo ₆ S ₈ did not change after contact with PCE, and only other detectable peaks were associated with SN molecules, indicating that PCE is compatible with Mo ₆ S ₈ .

5 shows the electrochemical characteristics of the Mo ₆ S ₈ electrode having a phase-change magnesium solid electrolyte (PCE) according to an embodiment of the present invention [(a) Mo ₆ S ₈ │PCE│Precision constructed by an asymmetric cell Comparison of Nyquist plots for Mo ₆ S ₈ electrodes in contact and poor contact conditions (closed circle: initial, open circle: after 24 hours, insets show schematic images for poor and fine contact), (b) Mg Nyquist plots for 1 mol% SN electrolyte with iodine additive (1 mol%+I ₂ ) and 1 mol% SN electrolyte without iodine additive (1 mol%) at room temperature using a cell constructed by │PCE│Mo , (c) Nyquist plot enlarged in the gray dotted circle region of (b), (d) voltage profile of Mg stripping and deposition in Mg│PCE+I ₂ │Mo ₆ S ₈ cell].

The effect of liquid-treated PCE on the electrochemical properties of the interface was investigated using time-dependent EIS (see Fig. 5a). Two different cell arrangements were prepared, a “bad contact” cell was assembled by placing Mo ₆ S ₈ electrodes directly on pre-solidified PCE, which is pelletized PCE. In contrast, "precise contact" cells were prepared by forcibly pre-immersing an electrode in a sufficient amount of liquid PCE, which was then solidified while maintaining precise contact with the electrode material. Interestingly, the initial resistances of the good contact (solid blue line, 1.59 kΩ) and poor contact (solid red line, 1.03 kΩ) were similar. However, they changed significantly after being left at room temperature for 24 hours. The interfacial resistance of the defective contact greatly increased to about 789% (red dotted line, 9.16 kΩ), but the interface resistance of the precise contact decreased by about 26% (blue dotted line, 1.17 kΩ). The differential arrangement of the two cells can explain the different behavior. Precision contact progressed to “more precise contact” by further wetting the electrodes while standing. However, simply placing the electrode on the pre-solidified PCE did not improve the contact and lost the original contact. These results show how important the formation of intimate contacts is in achieving high-speed solid electrolytes.

In addition, the chemical compatibility of PCE to Mg metal was evaluated using a non-blocking asymmetric cell (see FIG. 5 b and c). Despite the high oxidation stability of SN, PCE showed high interfacial resistance with Mg metal, which may have been affected by the formation of an ion insulating layer on the Mg metal surface. To identify the factors causing the insulating layer, Mg metal was immersed in SN with different salt concentrations of 0 mol% and 1 mol%, and the resulting surface was analyzed using X-ray photoelectron spectroscopy (XPS). bar. Mg metal immersed in 0 mol% SN (pure SN) showed undetectable signals except for the Mg peak, whereas samples stored in 1 mol% SN (i.e. PCE) showed distinct peaks of elements S and F. . It is well known that the only source of elements S and F is TFSI ^- anions, and Mg metal tends to form a passivation layer on the surface through reaction with TFSI ^- anions. TFSI ^- has been generally used as a counter anion based on its high ionic conductivity, but its incompatibility with Mg metal must be addressed.

In addition, in order to mitigate undesirable side reactions, when an iodine additive effective in reducing the interfacial resistance is used by forming a Mg ²⁺ ion conductive MgI ₂ layer on the surface of the Mg metal, the actually high interfacial resistance is significantly reduced with the help of the iodine additive. (see b and c in FIG. 5). In addition, actual deposition of Mg metal on the Mo substrate was observed in the cell configuration of Mg metal|PCE+I ₂ |Mo, indicating that the Mg ²⁺ ion channels in PCE are active. To prove the reversible Mg ²⁺ ion transport through the PCE layer, galvanostatic cycling measurements were performed with the Mg|PCE|Mo ₆ S ₈ cell (see Fig. 5d). A large overpotential for the intercalation and deintercalation of Mg ²⁺ ions in Mo ₆ S ₈ was observed in the reference electrolyte (1 mol% SN without iodine), which gradually increased upon cycling. It is shown that the large overpotential originates from the formation of an ionic insulating layer caused by the decomposition of TFSI anions in metallic Mg. This result, in contrast to the electrolyte containing the iodine additive, significantly reduced the overpotential and was suppressed even after repeated cycling. This is consistent with the EIS results (see b and c in Fig. 5), indicating that the iodine additive can help reduce the large resistivity on the Mg metal surface. As a result, PCE can provide excellent ion transport capability for divalent Mg ²⁺ ions, and the unique phase transition characteristics of monolithic hosts are highly desirable for inducing precision contact in all-solid-state batteries.

As described above, according to the present invention, a new phase-change magnesium solid electrolyte has very high conductivity of magnesium ions in the solid phase, excellent chemical and thermal stability, and can be widely applied to all types of electrodes regardless of shape. can provide

In particular, when an iodine additive is additionally included in the phase-change magnesium solid electrolyte, the conductivity of magnesium ions is further improved by significantly reducing the interfacial resistance on the surface of the magnesium metal, so it is expected to be applied as an electrolyte for an all-solid-state battery.

Claims (7)

(a) a magnesium salt;
(b) succinonitrile; and
(c) an iodine additive; a phase change magnesium solid electrolyte comprising a. According to claim 1,
The magnesium salt is a phase change magnesium solid electrolyte, characterized in that magnesium bis (trifluoromethanesulfonimide). According to claim 2,
Phase change magnesium solid electrolyte, characterized in that the molar percentage of the magnesium salt is 1 to 5 mol%. delete An all-solid-state battery comprising the phase-change magnesium solid electrolyte according to any one of claims 1 to 3. (I) adding a magnesium salt to molten succinonitrile to form a mixture; and
(II) stirring the mixture at 60 ° C. or higher; A method for producing a phase-change magnesium solid electrolyte comprising,
A method for producing a phase change magnesium solid electrolyte, characterized in that in step (I) further adding an iodine additive. delete

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