Solid-State Lithium and Li-Ion Batteries with Silica-Gel Solid Nanocomposite Electrolytes

Towards advanced lithium metal solid-state batteries: Durable and safe multilayer pouch cell enabled by a nanocomposite solid electrolyte

An extension of reliable operation temperature for an electrical energy storage device is required by specialized application, such as oil industry, military, space exploration and electric vehicles.1-2 Supercapacitors, which stores charges electrostatically, are more suitable for low temperature applications, since the operation of batteries below -30 oC is limited by its faradic charge storage mechanism. Ionic liquids are attractive electrolytes for supercapacitors, because their good (electro-)chemical stability provides safety, large voltage window and large energy density. However, supercapacitors using ionic liquid electrolytes, suffer a substantial drop of capacitance at low temperatures. This is because a large fraction of small pores is not accessible by electrolyte ions, especially for ionic liquid with low ionic mobility and large ion size.3 To enhance the performance of a supercapacitor with ionic liquid electrolyte at low temperature, studies have been focusing on facilitating ion accessibility to the electrode porous structure. Mitigating the Coulomb ion pairing (anion-cation attraction), such as adding organic solvent and binary mixture of ionic liquid or eutectic electrolyte, has been used to establish sufficient fluidity and improve ions mobility of electrolytes at low temperatures.4 Combining two different species of cations and only one anion mixtures (or vice versa) can lower the melting point of ionic mixture to -80 oC.5 In addition, adjusting the morphology of the electrode material is another way to enhance the ion transportation dynamically. For example, with larger inter-tube distance and more open porous structure, carbon nanotube showed a much better rate performance than that of the onion-like carbon at the temperature of -50 oC. Similarly accessible pore structures ae required for operation of organic electrolytes at low temperatures. With a fast development of electrode materials for supercapacitor, the choice of material extends from activated carbons, carbide-derived carbons, graphene, carbon onions and nanotubes, to many pseudocapacitive materials (carbides, nitrides, oxides, etc.), and the morphology of the material also varies in dimensions and confinement. Many unique electrolyte ion behaviors have been found or are waiting to be found in various electrode material with specific physical and chemical properties. Hence, a proper design of electrode material in conjunction with an electrolyte formulation will offer many opportunities for enhancing the supercapacitor performance at low temperatures. Reference Lin, X.; Salari, M.; Arava, L. M. R.; Ajayan, P. M.; Grinstaff, M. W., High temperature electrical energy storage: advances, challenges, and frontiers. Chemical Society Reviews 2016, 45 (21), 5848-5887.Huang, P.; Pech, D.; Lin, R.; Mcdonough, J. K.; Brunet, M.; Taberna, P.-L.; Gogotsi, Y.; Simon, P., On-chip micro-supercapacitors for operation in a wide temperature range. Electrochemistry Communications 2013, 36, 53-56.Abbas, Q.; Béguin, F., High voltage AC/AC electrochemical capacitor operating at low temperature in salt aqueous electrolyte. Journal of Power Sources 2016, 318, 235-241.Tsai, W.-Y.; Lin, R.; Murali, S.; Zhang, L. L.; McDonough, J. K.; Ruoff, R. S.; Taberna, P.-L.; Gogotsi, Y.; Simon, P., Outstanding performance of activated graphene based supercapacitors in ionic liquid electrolyte from− 50 to 80 C. Nano Energy 2013, 2 (3), 403-411.Lin, R.; Taberna, P.-L.; Fantini, S.; Presser, V.; Pérez, C. R.; Malbosc, F.; Rupesinghe, N. L.; Teo, K. B.; Gogotsi, Y.; Simon, P., Capacitive energy storage from− 50 to 100 C using an ionic liquid electrolyte. The Journal of Physical Chemistry Letters 2011, 2 (19), 2396-2401.

Solid‐state lithium batteries (SSLBs) with composite solid electrolytes (CSEs) offer enhanced energy density and high safety. However, their performance is hindered by large thickness and limited Li⁺ conductivity of CSEs, and large electrode/electrolyte interface resistance. This study develops an 18 µm‐thick CSE using a polyethylene scaffold, which incorporates garnet‐type Li6.25La3Zr2Ga0.25O12 (LLZGO) oxide and an ionic liquid (IL) additive in a poly(vinylidene fluoride‐co‐hexafluoropropylene)/polypropylene carbonate matrix, achieving a high ionic conductivity of 8.6 × 10−4 S cm−1 at 30 °C. The IL increases Li+ conduction of the CSE and reduces the interfacial resistance. The constructed LiNi0.8Co0.1Mn0.1O2 (NCM‐811)||CSE||Li cell shows remarkable charge–discharge performance. This study also integrates LLZGO and N‐propyl‐N‐methylpyrrolidinium bis(trifluorosulfonyl)imide (PMP‐TFSI) IL into a thick NCM‐811 cathode. The synergy between LLZGO and PMP‐TFSI within the cathode is examined. A high‐mass‐loading cathode with ≈20 mg cm−2 NCM‐811 is developed. The resulting composite cathode||CSE||Li cell achieves an areal capacity of ≈4 mAh cm−2. With the proposed scaffold‐supported CSE and thick NCM/LLZGO/IL composite cathode, the projected energy density of the resulting anode‐free pouch cell is ≈420 Wh kg−1. This study demonstrates a scalable and effective strategy for fabricating an oxide‐based SSLB with a high energy density and enhanced Li+ transport properties.

Carbonaceous allotropes modified ionic liquid electrolytes for efficient quasi-solid-state dye-sensitized solar cells

While known for a higher power density than batteries, electrochemical capacitors are limited by their energy density for some energy storage applications. Since the energy density of the device is proportional to the square of operating potential window, an effective way to increase energy density of the device is by increasing the window. Ionic liquid electrolytes are beneficial for this reason since they can theoretically operate at up to 6 V, though experimentally, the value is between 3-4 V, depending on the properties of electrode materials. By employing a novel electrochemical technique to study symmetric supercapacitors with carbon electrodes and ionic liquid electrolytes, we have found one of the possible reasons for the observed smaller operating potential window of ionic liquids in practical applications. We observed that even for a symmetric device, the different properties of the cation and anion results in an asymmetric performance of the two electrodes. However, a solution to this asymmetry is found by mixing two ionic liquids with the same cation, which ultimately results in a higher operating potential window of the device and therefore a higher energy density.1 Onion-like carbon (OLC) was used as the electrode material due to its large (>500 m2/g) outer surface available for ion adsorption and double layer formation, and no internal porosity that could restrict the movement of ions.2 However, electrode materials can also be made from high surface area carbons that contain a network of pores (e.g., activated or carbide-derived carbon) in which ions are adsorbed during charging. Though they boast a large operating potential window, ionic liquids are known to contain large and bulky ions. This can make it difficult to use an ionic liquid on a porous carbon with a range of pore sizes, some being smaller than the ion size, even though the specific surface area of the electrode material measured by gas adsorption is high and therefore the energy storage capability is more impressive. It has also been shown that the capacitance of porous electrodes is maximized when the ion size and pore size are equal.3 In this case, the ion is small enough to fit inside the pores while still large enough to take advantage of the surface area within the pore walls. While materials with a large outer surface allow a more accessible surface for adsorbing large ions, their capacitance is limited by their specific surface area. By designing an electrolyte based on mixed ionic liquids, we can match the ions in the mixture to the multiple pore sizes of the electrode material. Recently, it was found that the capacitance of porous carbon could be increased with the presence of multiple ions in the electrolyte mixture. Imidazolium-based ionic liquids are chosen based on their variable alkyl chain length which alters the cation size and bis(trifluoromethylsulfonyl)- imide (TFSI) is chosen as the anion for all ionic liquids. Different porous carbons with varying pore size distributions are used to illustrate the effect of ionic liquid mixture electrolytes. Finally, it has been shown previously that ionic liquid mixture electrolytes can be used to increase the operating temperature window of a supercapacitor to -50 °C – 100 °C.4 Ionic liquid mixtures have therefore been shown to improve the potential window, the capacitive performance, and the temperature window of supercapacitors, becoming an important area of study in the field of energy storage. 1. Van Aken, K. L., Beidaghi, M. & Gogotsi, Y. Formulation of Ionic-Liquid Electrolyte to Expand the Voltage Window of Supercapacitors. Angew. Chemie 127, 4888–4891 (2015). 2. McDonough, J. K. et al. Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes. Carbon N. Y. 50, 3298–3309 (2012). 3. Lin, R. et al. Solvent effect on the ion adsorption from ionic liquid electrolyte into sub-nanometer carbon pores. Electrochim. Acta 54, 7025–7032 (2009). 4. Lin, R. et al. Capacitive Energy Storage from -50 to 100 C Using an Ionic Liquid Electrolyte. J. Phys. Chem. Lett. 2, 2396–2401 (2011). Figure 1

Li-ion batteries are used widely for portable energy storages because of large capacity. Generally, the carbonate-based organic solvents are used as the electrolytes. Most of these organic solvents decompose during the first charge, resulting in formation of the solid electrolyte interphase (SEI) film, which plays an important role for Li-ion batteries on the active materials. Although there are many reports about the SEI film, its nature and function have not been clarified yet. We have already reported that lithium phosphorous oxynitride (LiPON) film acts as an artificial SEI film in 1-butyl-1-methylpyrrolidinium (BMP+) and 1-ethyl-3-methylimidazolium (EMI+) bis(trifluoromethylsulfonyl)amide (TFSA–) ionic liquids containing LiTFSA, and LiTFSA-glyme solvate ionic liquid. [1,2] Ionic liquids are expected to be the alternative electrolytes for Li-ion batteries, since ionic liquids have low flammability and low volatility, which are expected to improve safety of the batteries. However, the SEI film formed by the decomposition of ionic liquids is not preferable for deposition and dissolution of lithium. In this study, deposition and dissolution of lithium through a Li2CO3 thin film prepared by RF magnetron sputtering were investigated in BMPTFSA containing LiTFSA. The Li2CO3 thin film was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractrometer, and X-ray photoelectron spectrometer (XPS). All the electrochemical measurements were performed using an airtight three-electrode cell. The Ni electrode with or without the Li2CO3 thin film was used as a working electrode. Li foil was used as a reference and counter electrode. After electrochemical measurements, the electrode was washed with dimethyl carbonate (DMC) and characterized by SEM and XPS. SEM observation and XPS measurement showed that the Ni substrate was covered with the Li2CO3 thin film completely. The Li2CO3 thin film on the substrate was X-ray amorphous. However, the crystalline domains were observed sparsely by TEM observation. The deposition and dissolution of lithium were possible through the Li2CO3 thin film in BMPTFSA containing 1 M LiTFSA. Although the cathodic current attributed to the decomposition of the electrolytes was observed on the Ni electrode, it was suppressed on the Ni electrode with the Li2CO3 thin film. These results indicate that the Li2CO3 thin film has the possibility to act as an artificial SEI film. The depth analysis by XPS revealed that lithium deposited between the Li2CO3 thin film and Ni electrode. From AC impedance measurement, the ionic conductivity of the Li2CO3 thin film was estimated to be 1.3 × 10−8 S cm−1.

Lithium-air battery (LAB) is one candidate of energy sources beyond conventional lithium ion batteries for overcoming future energy need without worsening environmental issues. Presently, LABs have attracted interest due to its high energy density, while maintaining a long operational life. Although, significant progress has been made in developing electrochemical performance of these batteries, many technical challenges still remain unsolved. For example, the electrolytes used in rechargeable LABs have inferior conductivity, high viscosity, high volatility and flammability, decomposition on Li, and overpotential during charging [1]. To overcome these limitations, it is crucial to replace the conventional electrolyte with an ionic liquid (IL) because of its non-volatility, non-flammability, high ion density and chemical stability [2], [3]. However, there are a few issues that need to be solved when using ILs in LABs; they have high viscosity, low oxygen solubility, and low cyclability. Therefore, synthesis of an appropriate IL electrolyte is necessary due to improve the overall performance of LABs.In order to synthesis of ILs, bis(trifluoromethanesulfonyl)amide (TFSA) anion was used for its anionic similarity with Li salts. Five different types of cations such as ammonium, phosphonium, piperidinium, sulfonium, and pirolidinium were used. Subsequently, Lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) was completely dissolved at a concentration of 1.0 mol dm−3 in these ILs, and then stirred for 8 h at 60 ºC. The viscosities of prepared IL electrolytes were measured by viscometer. Then, Ketjen black (KB1600) and polyvinylidene difluoride (weight ratio = 9:1, respectively) were mixed in N-methylpyrrolidone solution. After stirring at RT for 5 min, the slurry was coated on the carbon paper followed by drying in a vacuum oven at 80 ºC for 12 h to obtain the cathode. The electrode area was set to 2.0 cm2 in which carbon loading was adjusted to 0.5 mg cm−2. Approximately 50 µl of IL electrolyte drop was deposited on the surface of as-prepared cathode (2.0 cm2 area), and subsequently anode was placed onto the center. The coin cells were assembled inside an Ar filled glove box in which moisture content was less than 1.00 ppm. To observe micro structures on the electrode surface, FE-SEM was used. The electrochemical performances were evaluated in coin cells using mainly cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests.Fig. 1 shows the result of viscosity of IL electrolytes. The viscosities of these IL electrolytes are different, and depend strongly on temperatures. The viscosity decreased with increasing temperature. It can be attributed to the reduced forces of cohesion among surfactant molecules. The viscosities of the IL electrolytes at specific temperature decrease in the order: piperidinium> pirolidinium> ammonium> phosophonium> sulfonium. The result of CV analysis has confirmed similar pattern of chemical reactions in which composition and subsequent decomposition of Li2O2 occurred. However, the current density obtained for the case of sulfonium IL electrolyte is higher than other electrolytes. GCD tests were performed to determine the cycle performance and capacity. The LAB which used low viscosity IL electrolyte, especially sulfonium IL electrolyte showed better cycling performance and capacity than that of high viscosity IL electrolytes. We investigated the temperature effects on sulfonium IL electrolyte to determine optimum performance characteristics. In this regard, the Li metal was immersed in the sulfonium IL electrolyte for 1 week at 25 ºC, 50 ºC, and 75 ºC. Subsequently, the SEM observation was performed to evaluate the morphologies of the Li metal surface. No significant change is observed in the Li metal surface after immersion when compared with the pristine lithium. This result indicates that the Li metal is highly stable in the sulfonium IL electrolyte. Then GCD measurements were performed in dry air at 25 ºC, 50 ºC, and 75 ºC. The discharge/charge capacities of LAB were found to increase with increase in temperature, and a lower overvoltage was obtained at high temperature. However, an initial discharge/charge cycle was retained up to 2nd cycles. To further examine the cycle performance, the measurement atmosphere has been changed to O2 instead of dry air. Fig. 2 shows the discharge/charge curves obtain at 75 ºC under O2 and dry air. As shown in figure, the initial cycle performance is retained up to 10th cycles, and a lower overvoltage is obtained in O2 as compared with dry air. These results indicate a notable improvement in cycle performance. In summary, the sulfonium IL electrolyte has exhibited very high performance in electrochemical tests, even worked at a high temperature under O2 atmosphere. Therefore, the proposed electrolyte design concept has great potential for not only LAB, but also next-generation batteries for various applications. Figure 1

Electrolytes with a high lithium (Li) transport number, whether liquid or polymeric, are desired for use in both lithium-ion and lithium metal rechargeable battery technologies. Liquid electrolytes pose safety problems, while polymer electrolytes suffer from poor ionic transport. Ionic liquid (IL) electrolytes were proposed as an alternative solution for their electrochemical stability and low flammability. Nevertheless, Li-salt-containing IL electrolytes still suffer from sluggish Li transport owing to the complicated Li solvation structure and dynamics. One promising route to increase Li transport number and simplify Li solvation structure and dynamics in IL electrolytes is limiting the anion mobility while having a Li salt in liquid form. In this paper, a series of macro Li salts that respond to ILs behavior were synthesized via an original and simple synthetic approach combining Michael addition and epoxidation reactions. Different backbone platforms such as polyhedral oligomeric silsesquioxane (POSS), tetraethylenepentaamine (TEPA), branched poly(ethylenimine) (BPEI), and Jeffamine® ED were used for grafting both Li sulfonyl(trifluoromethylsulfonyl) imide derivative and short poly(ethylene glycol) (PEG) chains. Benefiting from the presence of grafted PEG chains of 550 g mol–1, these Li salt electrolytes are under liquid form at room temperature and possess IL properties such as low Tg (<0 °C), low vapor pressure and volatility, and high thermal stability (decomposition temperature >300 °C). Among different IL electrolytes investigated, those based on TEPA or BPEI exhibit a good conductivity approaching 10–4 S cm–1 at 40 °C, combining a high Li+ transport number above 0.7, which is very promising for battery application.

Lithium-ion capacitor (LICs), a novel electrochemical energy storage class combined the advantages of lithium-ion batteries and supercapacitor, are attractive for high energy densities, high power densities, and long cycle life. A typical LIC is made up of a Li-ion battery (LIB) anode, an electrochemical double-layer capacitor (EDLC) cathode, and the common Li-ion electrolyte. Graphene-based nanomaterials have been extensively studied as the electrodes for Li-ion or charge storage in LIBs and EDLCs, respectively. Recently, we have developed reduced graphene oxide (rGO) in composite with MnOx with reversible lithium storage capacity of 900 mA/h. Meanwhile, we have obtained charge storage capacitance up to 150F/g using rGO electrode with an ionic liquid (IL) electrolyte. In this work, we will report our studies on the conducting characteristics of the IL/Li-ion hybrid electrolyte and their compositional impacts on both Li-ion and charge storage in the graphene-based nanocomposites towards high-performance LICs operable in a broad temperature range. The novel electrolyte series are a mixture of representative IL, e.g. EmimBF4, C8minBF4, C8mimPF6, and the common Li-ion electrolyte, e.g. 1M LiPF6-EC-DEC or 1M LiBF4-EC-DEC, with various preset volume ratios. The ionic conductivities of the hybrid electrolyte series were determined at temperatures from 25oC to 80oC, from which conduction activation energies were calculated. Further, the influences of the hybrid electrolyte composition on the ionic conductivity, activation energy, stability window, Li-ion and charge storage in the graphene-based nanocomposites were systematically studied. Results showed that ionic conductivities of the hybrid electrolytes increased in general with the addition of Li-ion electrolyte and the activation energies of the ionic conductance increased with the addition of IL. It is interesting to observe a secondary maxima point at around 1:1 volume ratio (IL: Li-ion electrolyte) and higher ionic conductivities of such hybrid electrolyte than that of the neat Li-ion electrolyte above 65oC. The hybrid electrolyte remains relatively stable up to 6V starting from the second cycle after the occurrence of the irreversible reactions corresponding to IL partial decomposition and SEI formation in the first charge/discharge cycle. The novel LIC consisting of the hybrid Li-conducting/IL electrolyte, rGO-MnOx anode, rGO cathode, is anticipated to have a high deliverable energy densities both gravimetrically and volumetricallyand operable in a broad temperature range.

The success of commercializing sodium-ion batteries is inhibited by the implementation of suitable electrolytes that have economic feasibility and stable electrochemical performance. Organic electrolytes such as Na[ClO4]- or Na[PF6]-including organic solvents, EC, PC, DMC, and DEC, are most widely used to investigate the performance of sodium-ion batteries. These electrolytes are widely accepted because of the analogous studies in LIBs, but their flammability can cause reliability issues. Moreover, the SEI layer composed of sodium cations is known to be inhomogeneous and fragile, giving a higher solubility than that based on lithium cations.(1) In this regard, ionic liquid electrolytes are a drawing huge attention to constructing safe electrochemical devices owing to their unique properties such as low volatility and low flammability. Furthermore, recent studies report ionic liquids show similar electrochemical performance at room temperature to organic electrolytes and better interfacial properties.(2-4) Despite this bright improvement, the cost of ionic liquids drags down the advantage of economical sodium-ion batteries.In this study, ionic liquid and organic liquid electrolytes are mixed to adjust the balance of cost, performance, and safety. The [C3C1pyrr][FSA] (C3C1pyrr+ = N-methyl-N-propylpyrrolidinium) ionic liquid and PC organic solvent are used as mother electrolytes with Na[FSA] (FSA = bis(fluorosulfonyl)amide) and Na[ClO4] salts. The physical and electrochemical properties of the various mixing ratio of those mixture electrolytes are investigated for sodium-ion batteries. These mixture electrolytes improve not only flammability but also show better rate and cycling performance. The detailed electrochemical properties are evaluated by comparing reference electrolytes to clarify the dual salts effect and dual solvents (ionic liquids and organic solvent) effect. This performance will be discussed based on half cell tests using a Na2FeP2O7 positive electrode and full cell tests using a hard carbon negative electrode and a NaCrO2 positive electrode.

A lithium ion capacitor (LIC) is an electric energy storage device that combines an electric double-layer capacitor (EDLC) with a lithium ion battery (LIB). Although LIC features an excellent power density like EDLC, the energy density of LIC is lower than that of LIB. Therefore, improvement of energy density is required for LIC. In the present study, to achieve high energy density of LIC, porous 3-dimensinal (3-D) current collector is applied to LIC electrodes. This 3-D current collector can increase the packing density of active material. It is, however, expected that diffusion of ionic carriers is limited when using conventional organic electrolytes because electrodes based on the 3-D current collector should be massive. As a result, LIC using the 3-D current collector may deliver limited power. Here bis(fluorosulfonyl)imide (FSI)-based ionic liquids (ILs) that have high carrier density would be useful as an electrolyte for LIC to maintain power. The purpose of this study is applying an FSI-based IL electrolyte to LICs with porous 3-D current collector. We assembled a three-electrode cell. A positive electrode using aluminum 3-D current collector was composed of activated carbon, acetylene black (AB) and polyvinylidene di-fruoride (PVdF). A negative electrode using copper 3-D current collector was composed of hard carbon, AB and PVdF. A lithium foil was used as a counter electrode and reference electrode. We used LiFSI/EMImFSI as an IL electrolyte. The cell was charged and discharged for 3000 cycles at 1.0 C-rate in a voltage range of 2.0 – 3.8 V. We also evaluated rate performance of the LIC cells by rapid charging and discharging test up to 30 C-rate. Potential profiles of positive and negative electrodes were observed during charge and discharge at the 2nd, 1000th and 3000th cycles. It turns out that the LIC cell containing the IL electrolyte can be charged and discharged reversibly as well as stably. Even though a test was long-term cycling such as 3000 cycles, the capacity has not significantly decreased. We compared rate performance among the IL electrolyte and two organic electrolytes. Although ILs have high viscosity, it is shown that the FSI-based IL electrolyte has high rate performance comparable to that of a LiPF6-based organic electrolyte. This may be ascribed to high carrier density of the IL. These results suggest that applying the IL electrolyte to a LIC electrode with the porous 3-D current collector is promising to keep or enhance energy and power capability. We will also report the effect of FSI-based electrolytes including IL systems and solvent-based ones on the charge-dicharge performance of EDLCs.

All-solid-state batteries (ASSBs) using inorganic solid electrolytes are expected to achieve high safety and high energy density as alternatives to conventional lithium-ion batteries using liquid electrolytes. However, in many cases, rate capability of ASSBs is poor compared with that of the conventional liquid system. This is because the contact interface between solid active materials and solid electrolyte is insufficient, resulting in high resistance for electrochemical properties. The solid electrode-electrolyte interface in ASSBs is related to the morphological structure such as porosity and mixed state. It has been confirmed that pressurization reduces the grain boundaries of solid electrolytes and improves conductivity1). From this improvement, it is generally known that applying high pressure when assembling ASSBs improves the contact area of the solid-solid interface, including electrode and electrolyte interface. However, the effect of pressure on particle-particle contact and battery performance is not understood enough. In this study, we investigated the effect of pressure-induced three-dimensional structure on the electrochemical properties of ASSBs by using X-ray computed tomography (CT). ASSBs were prepared at various stack pressures using a composite cathode consisting of oxide active materials LiNi1/3Co1/3Mn1/3O2 (NCM), sulfide solid electrolyte Li10GeP2S12 (LGPS), and acetylene black. Electrochemical properties were investigated in ASSBs of NCM composite cathode|LGPS solid-electrolyte|In-Li anode. The charge–discharge measurements were performed using various stack pressure, which indicates that the capacity increases as the stack pressure increases. To analyze the factors behind this result in terms of the internal structure of the ASSBs, we performed X-ray CT with synchrotron radiation at 20 keV using a homemade operando measurement cell that electrode diameter is 1 mm. This measurement system allows us to analyze the three-dimensional structure of composite cathode during monitoring applied pressure, electrochemical measurements, and X-ray CT measurements. From the X-ray CT data, we succeeded in quantifying the changes in the porosity of the composite cathode and the contact state between the active material and electrolytes by changing the pressure. Quantitative analysis revealed that the porosity and contact state correlate with the charge–discharge properties. Additionally, CT analysis revealed that the voids are preferentially presented near active materials and these voids spread the horizontal direction (not pressurization direction), resulting in different porosities of composite cathode and solid electrolyte layer. Since it was confirmed that voids remained in the vicinity of the active material even when the pressurization was continued, controlling these voids decreases the voids of the entire cathode layer. Pressure dependencies on AC impedance during X-ray CT measurements showed that the apparent conductivity of ASSBs increases linearly as the confinement stack pressure increases. This result is caused by suggesting that the decrease in porosity and tortuosity in cathode layer, which is detected by CT analysis. Additionally, date of tortuosity shows over-pressurization may bypass Lithium conduction path. The improved internal structure such as contact state due to pressurization enhances the apparent conductivity and charged-discharge characteristics of ASSBs. With the latest synchrotron radiation measurement technology and image processing technology, high-precision analysis has become possible.1) Jean-Marie Doux, Yangyuchen Yang, Darren H. S. Tan, Han Nguyen, Erik A. Wu, a Xuefeng Wang, Abhik Banerjeea and Ying Shirley Meng, J. Mater. Chem. A, 8, 5049, (2020).

Sodium-ion batteries (SIBs) are emerging as a promising alternative to lithium-ion batteries, primarily due to their plentiful raw materials and cost-effectiveness. However, the use of traditional organic liquid electrolytes in sodium battery applications presents significant safety risks, prompting the investigation of solid electrolytes as a more viable solution. Despite their advantages, single solid electrolytes encounter challenges, including low conductivity of sodium ions at room temperature and incompatibility with electrode materials. To overcome these limitations, the researchers develop composite polymer solid electrolytes (CPSEs), which merge the strengths of high ionic conductivity of inorganic solid electrolytes and the flexibility of polymer solid electrolytes. CPSEs are usually composed of inorganic materials dispersed in the polymer matrix. The final performance of CPSEs can be further improved by optimizing the particle size, relative content, and form of inorganic fillers. CPSEs show great advantages in improving ionic conductivity and interface compatibility, making them an important direction for future solid-state sodium battery research. Therefore, this paper summarizes recent advancements in composite solid electrolytes, discusses the impact of their preparation processes on performance, and outlines potential future developments in sodium-ion solid-state batteries.

Electrochemical reduction of carbon dioxide (CO 2 ) to CO is a promising strategy. However, achieving high Faradaic efficiency with high current density using ionic liquid (IL) electrolyte remains a challenge. In this study, the new IL N ‐octyltrimethyl ammonium 1,2,4‐triazole ([N 1118 ][TRIZ]) shows outstanding performance for electrochemical reduction of CO 2 to CO on the commercial Ag electrode, and the current density can be up to 50.8 mAcm −2 with a Faradaic efficiency of 90.6 %. The current density of CO is much higher than those reported in the IL electrolyte. In addition, the density functional theory (DFT) calculation further proved that [N 1118 ][TRIZ] interact with CO 2 to form [N 1118 ] + [TRIZ‐CO 2 ] − complex which played a key role in reducing the activation energy of CO 2 . The formation of [N 1118 ] + [TRIZ‐CO 2 ] − complex was verified by Fourier Transform Infrared (FTIR) spectroscopy. According to the molecular orbital theory, the electrons obtained from IL was filled in the anti‐bonding orbit ( π *) of the CO 2 , resulting in reducing the C=O bond energy. This work provides a new strategy to design novel ILs for high efficiency electrochemical reduction of CO 2 to CO.

To improve the thermal stability of lithium- and sodium-ion batteries, the room-temperature molten salts /1-ethyl-3-methyl imidazolium tetrafluoroborate and were used as ionic liquid (IL) electrolytes instead of flammable carbonate-type organic electrolyte solvents. To avoid cathodic decomposition of the IL electrolytes, a symmetric cell configuration with Na Superionic CONductor (NASICON)-type (where A is Li or Na) as both cathode and anode was tried in a coin-type cell (type 2320). As a result, both the polyanionic-based (LVP) as well as (NVP) symmetric cells using organic electrolytes were found to operate as secondary batteries and exhibited satisfactory electrochemical performances. The substitution of the organic electrolytes by the appropriate IL electrolytes in both cases resulted in the reduction in the first discharge capacities. However, the IL-based cells revealed better cyclability and a more stable behavior at elevated temperatures. The obtained electrochemical behavior of the symmetric cells was confirmed by the complex impedance measurements at 25 and . In addition, the thermal stability of LVP and NVP with the IL electrolytes was also examined.