Quantification of the Carbon-Coating Effect on the Interfacial Behavior of Graphite Single Particles.

The effect of carbon coating on the interfacial charge transfer resistance of natural graphite (NG) was investigated by a single-particle measurement. The microscale carbon-coated natural graphite (NG@C) particles were synthesized by the simple wet-chemical mixing method using a phenolic resin as the carbon source. The electrochemical test results of NG@C using the conventional composite electrodes demonstrated desirable rate capability, cycle stability, and enhanced kinetic property. Moreover, the improvements in the composite electrodes were confirmed with the electrochemical parameters (i.e., charge transfer resistance, exchange current density, and solid phase diffusion coefficient) analyzed by a single-particle measurement. The surface carbon coating on the NG particles reduced the interfacial charge transfer resistance (Rct) and increased the exchange current density (i0). The Rct decreased from 81-101 (NG) to 49-67 Ω cm2 (NG@C), while i0 increased from 0.25-0.32 (NG) to 0.38-0.52 mA cm-2 (NG@C) after the coating process. The results suggested both electrochemically and quantitatively that the outer uniformly coated surface carbon layer on the graphite particles can improve the solid-liquid interface and other kinetic parameters, therefore enhancing the rate capabilities to obtain the high-power anode materials.

Dual additive of lithium titanate and sulfurized pyrolyzed polyacrylonitrile in sulfur cathode for high rate performance in lithium-sulfur battery.

Lithium-Sulfur (Li-S) batteries have attracted much attention as next-generation batteries due to their high theoretical energy density. However, lithium polysulfide generated during the discharge loses intimate electrical contact with the carbon matrix due to its high solubility in the electrolyte, causing a high charge transfer resistance and slow redox kinetics for the discharge reactions, resulting in a low rate capability. A cathode additive having a strong chemical adsorbing site toward the polysulfide can effectively inhibit their dissolution. We now report a dual additive of lithium titanium oxide (LTO) and sulfurized polyacrylonitrile (SPAN). LTO provides a rapid charge transfer and a fast Li+ ion transfer in the cathode. On the other hand, SPAN helps to enhance the polysulfide adsorption capability. This dual additive system synergistically supplies the cathode with a strong polysulfide adsorption capability and fast redox kinetics. As a result, the dual additive exhibits high discharge capacities of 1430 mA h g-1 at 0.1C and 1200 mA h g-1 at 0.5C at the high-sulfur-loading cathode of 5.0 mg cm-2. Our findings demonstrated the manufacturing of the cathode with a strong polysulfide adsorption capability and a fast redox reaction which could then effectively improve the rate performance of the Li-S batteries.

Effect of a layer-by-layer assembled ultra-thin film on the solid electrolyte and Li interface.

Advanced all-solid-state batteries are considered as the most preferable power source for the next generation devices. Such batteries demand consumption of electrode materials with high energy and power density. One of the excellent solutions is the utilization of Li metal as anode which provides opportunity to fulfill such requirements. Yet, obstacles such as interfacial impedance and reactivity of Li metal with promising solid electrolytes prevent the consumption of the Li anode. Despite its outstanding stability under ambient conditions, high ionic conductivity and facile synthesis methods, NASICON-type Li1.3Al0.3Ti1.7(PO4)3 also suffers from the above mentioned problems. In this work, these critical issues were resolved by applying an artificial protective interlayer. Herein, the layer-by-layer polymer assembly approach of the ultra-thin interlayer of (PAA/PEO)30 on either side of solid electrolyte pellets simultaneously is presented. The introduction of the protective layer prevented a formation of mixed conduction interphase and effectively decreased the interfacial impedance. A symmetric cell with Li metal electrodes performed over 600 hours at 0.1 mA cm-2. Furthermore, an all-solid-state Li metal battery, assembled with the modified LATP solid electrolyte and LiFePO4 cathode, demonstrated an excellent electrochemical performance with an initial discharge capacity of 115 mA h g-1 and a capacity retention of 93% over 20 cycles with a coloumbic efficiency of almost 100%. The LATP with the (PAA/PEO)30 coating exhibited electrochemical stability up to 5 V.

Degradation Mechanism of All-Solid-State Li-Metal Batteries Studied by Electrochemical Impedance Spectroscopy.

Solid-state Li-metal batteries have the potential to achieve both high safety and high energy densities. Among various solid-state fast-ion conductors, the garnet-type Li7La3Zr2O12 (LLZO) is one of the few that are stable to Li metal. However, the large interfacial resistance between LLZO and cathode materials severely limits the practical application of LLZO. Here a LiCoO2 (LCO) film was deposited onto an Al-doped LLZO substrate at room temperature by aerosol deposition, and a low interfacial resistance was achieved. The LCO particles were precoated by Li3BO3 (LBO), which melted to join the LCO particles to the LLZO substrate at heating. All-solid-state Li/LLZO/LBO-LCO cells could deliver an initial discharge capacity of 128 mAh g-1 at 0.2 C and 60 °C and demonstrated relatively high capacity retention of 87% after 30 cycles. The cell degradation mechanism was studied by electrochemical impedance spectroscopy (EIS) and was found to be mainly related to the increase of the interfacial resistance between LBO and LCO. In-situ SEM analysis verified the hypothesis that the increase of the interfacial resistance was caused primarily by interfacial cracking upon cycling. This study demonstrated the capability of EIS as a powerful nondestructive in-situ technique to investigate the failure mechanisms of all-solid-state batteries.

Ionic liquid-containing cathodes empowering ceramic solid electrolytes.

Although ceramic solid electrolytes, such as Li7La3Zr2O12 (LLZO), are promising candidates to replace conventional liquid electrolytes for developing safe and high-energy-density solid-state Li-metal batteries, the large interfacial resistance between cathodes and ceramic solid electrolytes severely limits their practical application. Here we developed an ionic liquid (IL)-containing while nonfluidic quasi-solid-state LiCoO2 (LCO) composite cathode, which can maintain good contact with an Al-doped LLZO (Al-LLZO) ceramic electrolyte. Accordingly the interfacial resistance between LCO and Al-LLZO was significantly decreased. Quasi-solid-state LCO/Al-LLZO/Li cells demonstrated relatively high capacity retention of about 80% after 100 cycles at 60°C. The capacity decay was mainly because of the instability of the IL. Nevertheless, the IL-containing LCO cathode enabled the use of Al-LLZO as a solid electrolyte in a simple and practical way. Identifying a suitable IL is critical for the development of quasi-solid-state Li-metal batteries with a ceramic solid electrolyte.

Characterization of the Depth of Discharge-Dependent Charge Transfer Resistance of a Single LiFePO4 Particle.

The discharged state affects the charge transfer resistance of lithium-ion secondary batteries (LIBs), which is referred to as the depth of discharge (DOD). To understand the intrinsic charge/discharge property of LIBs, the DOD-dependent charge transfer resistance at the solid-liquid interface is required. However, in a general composite electrode, the conductive additive and organic polymeric binder are unevenly distributed, resulting in a complicated electron conduction/ion conduction path. As a result, estimating the DOD-dependent rate-determining factor of LIBs is difficult. In contrast, in micro/nanoscale electrochemical measurements, the primary or secondary particle is evaluated without using a conductive additive and providing an ideal mass transport condition. To control the DOD state of a single LiFePO4 active material and evaluate the DOD-dependent charge transfer kinetic parameters, we use scanning electrochemical cell microscopy (SECCM), which uses a micropipette to form an electrochemical cell on a sample surface. The difference in charge transfer resistance at the solid-liquid interface depending on the DOD state and electrolyte solution could be confirmed using SECCM.

Low-Refractive-Index Deep-Ultraviolet Transparent Poly(fluoroalkyl-co-methylsilsesquioxane) Resins Synthesized by Cosolvent-Free Hydrolytic Polycondensation of Organotrimethoxysilanes.

Cosolvent-free (solventless) hydrolytic polycondensation of fluoroalkyltrimethoxysilanes of linear fluoroalkyl groups of the form R = CnF2n+1C2H4 (n = 1, 4, and 8) and methyltrimethoxysilane followed by thermal curing yielded dense polymeric silsesquioxane (SQ) resins with low refractive indices and deep-ultraviolet transparency with an ultraviolet absorption edge at ∼210 nm. The refractive index at 589 nm was adjustable at ∼1.35-1.39, and the lowest value was ∼1.354 for the stiff resin and ∼1.347 for the soft resin of poly(R-co-Me-SQ) prepared at n = 8. The refractive indices of these resins were consistent with the linear combinations of molar refractivities of constituent functional groups, and there were no free-volume anomalies.

Structure Design of Long-Life Spinel-Oxide Cathode Materials for Magnesium Rechargeable Batteries.

Development of metal-anode rechargeable batteries is a challenging issue. Especially, magnesium rechargeable batteries are promising in that Mg metal can be free from dendrite formation upon charging. However, in case of oxide cathode materials, inserted magnesium tends to form MgO-like rocksalt clusters in a parent phase even with another structure, which causes poor cyclability. Here, a design concept of high-performance cathode materials is shown, based on: i) selecting an element to destabilize the rocksalt-type structure and ii) utilizing the defect-spinel-type structure both to avoid the spinel-to-rocksalt reaction and to secure the migration path of Mg cations. This theoretical and experimental work substantiates that a defect-spinel-type ZnMnO3 meets the above criteria and shows excellent cycle performance exceeding 100 cycles upon Mg insertion/extraction with high potential (≈2.5 V vs Mg2+ /Mg) and capacity (≈100 mAh g-1 ). Thus, this work would provide a design guideline of cathode materials for various multivalent rechargeable batteries.

Phenylphosphonate surface functionalisation of MgMn2O4 with 3D open-channel nanostructures for composite slurry-coated cathodes of rechargeable magnesium batteries operated at room temperature.

Spinel-type MgMn2O4, prepared by a propylene-oxide-driven sol-gel method, has a high surface area and structured bimodal macro- and mesopores, and exhibits good electrochemical properties as a cathode active material for rechargeable magnesium batteries. However, because of its hydrophilicity and significant water adsorption properties, macroscopic aggregates are formed in composite slurry-coated cathodes when 1-methyl-2-pyrrolidone (NMP) is used as a non-aqueous solvent. Functionalising the surface with phenylphosphonate groups was found to be an easy and effective technique to render the structured MgMn2O4 hydrophobic and suppress aggregate formation in NMP-based slurries. This surface functionalisation also reduced side reactions during charging, while maintaining the discharge capacity, and significantly improved the coulombic efficiency. Uniform slurry-coated cathodes with active material fractions as high as 93 wt% can be produced on Al foils by this technique employing carbon nanotubes as an electrically conductive support. A coin-type full cell consisting of this slurry-coated cathode and a magnesium alloy anode delivered an initial discharge capacity of ∼100 mA h g-1 at 25 °C.

Determining Factor on the Polarization Behavior of Magnesium Deposition for Magnesium Battery Anode.

To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/triglyme, magnesium borohydride (Mg(BH4)2)/tetrahydrofuran (THF), and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg(TFSA)2/triglyme and Mg(BH4)2/THF, while the reactions do not occur in Mg(TFSA)2/2-MeTHF. Raman spectroscopy shows that the [TFSA]- in the Mg(TFSA)2/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]- in Mg(TFSA)2/2-MeTHF and [BH4]- in Mg(BH4)2/THF coordinate to the magnesium ions. In operando SXAS measurements, the intermediate, such as the Mg+ ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA]-, strongly bound to the magnesium ion in the Mg(TFSA)2/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg(BH4)2/THF electrolyte, because the [BH4]- coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH4]-. Conversely, because [TFSA]- is weakly bound to the magnesium ion in Mg(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.

Twinning by Merohedry and Thermal Expansion of Zeolitic Clathrasil Deca-dodecasil 3R.

Rhombohedral crystal particles of zeolitic clathrasil deca-dodecasil 3R (DDR), hydrothermally synthesized from a mixture consisting of fumed silica, water, and 1-adamantanamine, were characterized by single-crystal and powder X-ray diffractometry as a function of temperature and pole figure analysis. The crystallite was bounded by six equivalent {101̅1} faces and exhibited twin-free appearance, whereas the structure was resolved with the binary twin by merohedry, defined by the twin point group 3̅2'/m'1, consisting of two twin domains with nearly equal volume fractions. This twinning modifies the positions of O atoms in the Si-O-Si framework while preserving the positions of Si atoms that define the topology of polyhedral cages. This type of twinning therefore does not disrupt the microporous channels via the 8-membered rings of the 19-hedral cages and little disturbs the adsorption and permeation of gas molecules in DDR. The cell volume of DDR increased monotonically with an increase in temperature up to ∼673 K accompanied by an elongation perpendicular to the [0001] axis and a shrinkage along the [0001] axis. Above ∼673 K, the cell volume decreased with temperature. These positive and negative volume expansion coefficients observed in this study were roughly one-half and one-third of the values currently available.

Cosolvent-free synthesis and characterisation of poly(phenyl-co-n-alkylsilsesquioxane) and poly(phenyl-co-vinylsilsesquioxane) glasses with low melting temperatures.

Thermoplastic poly(phenylsilsesquioxane) [poly(Ph-SQ)] and its copolymers with R-SQ units of linear aliphatic R groups [poly(Ph-co-R-SQ) (R = Me, Et, Pr, and Vi)] were synthesised by cosolvent-free hydrolytic polycondensation from acid-catalysed water-organotrimethoxysilane binary systems. These compounds became transparent glasses with coefficents of linear thermal expansion of ∼1 × 10-4 K-1 and Vickers hardnesses of 50-110 MPa when thermally treated at or above 100 °C. Poly(Ph-SQ) formed a fragile melt with kinetic fragility (F1/2 ⪆ 0.8) as high as that of molecular glasses. The low melting temperature and high fragility probably arise from weak attractive π interactions between phenyl groups that readily dissociate on heating, disordered Si-O frameworks, and low average molar mass (∼103 g mol-1). Proton NMR measurements confirmed other weak attractive CH/π interactions between phenyl and R groups in poly(Ph-co-R-SQ). Poly(Ph-SQ) and poly(Ph-co-R-SQ) glasses were hydrophilic and strongly bonded to a glass plate because of the presence of residual SiOH groups, while thermosetting by polycondensation was insignificant at or below 140 °C. The poly(Ph-SQ) glass was brittle, and macroscopic cracks were formed when melted on a glass plate as a result of a thermal expansion coefficient mismatch. Such crack formation was suppressed by incorporating Et-SQ, Pr-SQ, or Vi-SQ units and enhancing structural relaxation.

Ceramic-Based Flexible Sheet Electrolyte for Li Batteries.

The increasing demand for high-energy-density batteries stimulated the revival of research interest in Li-metal batteries. The garnet-type ceramic Li7La3Zr2O12 (LLZO) is one of the few solid-state fast-ion conductors that are stable against Li metal. However, the densification of LLZO powders usually requires high sintering temperatures (e.g., 1200 °C), which likely result in Li loss and various side reactions. From an engineering point of view, high-temperature sintering of thin LLZO electrolytes (brittle) at a large scale is difficult. Moreover, the high interfacial resistance between the solid LLZO electrolytes and electrodes is a notorious problem. Here, we report a practical synthesis of a flexible composite Al-doped LLZO (Al-LLZO) sheet electrolyte (75 µm in thickness), which can be mass-produced at room temperature. This ceramic-based flexible sheet electrolyte enables Li-metal batteries to operate at both 60 and 30 °C, demonstrating its potential application for developing practical Li-metal batteries.

Magnesium Storage Performance and Mechanism of 2D-Ultrathin Nanosheet-Assembled Spinel MgIn2 S4 Cathode for High-Temperature Mg Batteries.

Magnesium batteries have the potential to be a next generation battery with large capability and high safety, owing to the high abundance, great volumetric energy density, and reversible dendrite-free capability of Mg anodes. However, the lack of a stable high-voltage electrolyte, and the sluggish Mg-ion diffusion in lattices and through interfaces limit the practical uses of Mg batteries. Herein, a spinel MgIn2 S4 microflower-like material assembled by 2D-ultrathin (≈5.0 nm) nanosheets is reported and first used as a cathode material for high-temperature Mg batteries with an ionic liquid electrolyte. The nonflammable ionic liquid electrolyte ensure the safety under high temperatures. As prepared MgIn2 S4 exhibits wide-temperature-range adaptability (50-150 °C), ultrahigh capacity (≈500 mAh g-1 under 1.2 V vs Mg/Mg2+ ), fast Mg2+ diffusibility (≈2.0 × 10-8 cm2 s-1 ), and excellent cyclability (without capacity decay after 450 cycles). These excellent electrochemical properties are due to the fast kinetics of magnesium by the 2D nanosheets spinel structure and safe high-temperature operation environment. From ex situ X-ray diffraction and transmission electron microscopy measurements, a conversion reaction of the Mg2+ storage mechanism is found. The excellent performance and superior security make it promising in high-temperature batteries for practical applications.

Long-Term Stable Lithium Metal Anode in Highly Concentrated Sulfolane-Based Electrolytes with Ultrafine Porous Polyimide Separator.

Highly concentrated solutions composed of lithium bis(fluorosulfonyl)imide (LiFSI) and sulfolane (SL) are promising liquid electrolytes for lithium metal batteries because of their high anodic stability, low flammability, and high compatibility with lithium metal anodes. However, it is still challenging to obtain the stable lithium metal anodes in the concentrated electrolytes due to their poor wettability to the conventional polyolefin separators. Here, we report that the highly concentrated 1:2.5 LiFSI/SL electrolyte coupled with a three-dimensionally ordered macroporous polyimide (3DOM PI) separator enables the stable lithium plating/stripping cycling with an average Coulombic efficiency of ca. 98% for over 400 cycles at 1.0 mA cm-2. The 3DOM PI separator shows good electrolyte wettability and large electrolyte uptake due to its high porosity and polar constituent of the imide structure, allowing superior cycling performance in the highly concentrated solution, compared with the polyolefin separators. Electrochemical and spectroscopic analyses reveal that the superior cycling stability in the concentrated electrolyte is attributed to the formation of highly stable and Li+ ion conductive solid electrolyte interphase (SEI) layer derived from FSI- anions, which reduces the side reactions of SL with lithium metal, prevents the growth of lithium dendrites, and suppresses the increase in cell impedance over long-term cycling. Our findings demonstrate that polar and porous separators could effectively improve the affinity to the concentrated electrolytes and allow the formation of the anion-derived SEI layer by increasing the salt concentration of the electrolytes, achieving the long-term stable lithium metal anode.

Modifications in coordination structure of Mg[TFSA]2-based supporting salts for high-voltage magnesium rechargeable batteries.

To achieve a sustainable-energy society in the future, next-generation highly efficient energy storage technologies, particularly those based on multivalent metal negative electrodes, are urgently required to be developed. Magnesium rechargeable batteries (MRBs) are promising options owing to the many advantageous chemical and electrochemical properties of magnesium. However, the substantially low working voltage of sulfur-based positive electrodes may hinder MRBs in becoming alternatives to current Li-ion batteries. We proposed halide-free noncorrosive ionic liquid-based electrolytes incorporating Mg[TFSA]2 for high-voltage MRB applications. Upon the complexation of Mg[TFSA]2 with tetraglyme (G4) and strict control of the liquid states, the electrolytes achieved excellent anodic stability up to 4.1 V vs. Mg2+/Mg even at 100 °C. The modest electrochemical activities for magnesium deposition/dissolution in the [Mg(G4)][TFSA]2/ionic liquid electrolyte can be improved by certain modifications to the coordination state of [TFSA]-. Dialkyl sulfone was found to be effective in changing the coordination state of [TFSA]- from associated to isolated (free). This coordination change successfully promoted magnesium deposition/dissolution reactions, particularly in the coexistence of ether ligand. By contrast, the coordination of Mg2+ by strongly donating agents such as dimethyl sulfoxide and alkylimidazole led to the complexes inactive electrochemically, suggesting that interaction between Mg2+ and coordination agents predominates the fundamental electrochemical activity. We also demonstrated that an enhancement in the electrochemical activity of electrolytes contributed to improvements in the cycling ability of magnesium batteries with 2.5 V-class MgMn2O4 positive electrodes.

Phosphoric Acid Diethylmethylammonium Trifluoromethanesulfonate-Based Electrolytes for Nonhumidified Intermediate Temperature Fuel Cells.

The present study reports a new series of electrolytes for nonhumidified intermediate temperature fuel cells (IT-FCs). This series of new mixed electrolytes, composed of phosphoric acid (PA) and diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), was designed as nonhumidified IT-FC electrolytes. The mixed electrolytes show a higher thermal stability than pure PA, which is dehydrated at ITs. The thermal stability of the mixed electrolytes could be explained by the interaction between the triflate group in [dema][TfO] and PA, as indicated by Fourier transform infrared and proton nuclear magnetic resonance (1H NMR) spectroscopies. On the other hand, the ionic conductivity and proton transference number of the mixed electrolytes were similar to those of the pure PA. However, the oxygen reduction reaction (ORR) activity of a platinum catalyst is significantly enhanced in the mixed electrolytes, which was due to the several orders of magnitude increase in oxygen solubility by the addition of [dema][TfO] to PA. Specifically, for the equimolar fraction mixed electrolyte, the diffusion coefficient and the solubility of oxygen were ca. 1.47 × 10-5 cm2 s-1 and ca. 1.28 mmol dm-3 at 150 °C, respectively. The addition of [dema][TfO] to PA could significantly enhance the ORR activity. Therefore, the PA_[dema][TfO] mixed electrolyte can be one of the solutions to develop nonhumidified intermediate FC electrolytes.

Computational investigation of the Mg-ion conductivity and phase stability of MgZr4(PO4)6.

Solid electrolyte materials exhibiting high Mg-ion conductivity are required to develop Mg-ion batteries. In this study, we focused on a Mg-ion-conducting solid phosphate based electrolyte, MgZr4(PO4)6 (MZP), and evaluated the ionic conductivity of NASICON-type and ß-iron sulfate-type MgZr4(PO4)6 structures via density functional theory calculations. The calculations suggest that the migration energy of Mg is 0.63 eV for the NASICON-type structure and 0.71 eV for the ß-iron sulfate-type one, and the NASICON-type structure has higher ion conductivity. Although the NASICON-type MZP structure has not been experimentally realised, there is only an energy difference of 14 meV per atom with respect to that of the ß-iron sulfate-type structure. Therefore, in order to develop a synthesis method for the NASICON-type structure, we investigated pressure- and temperature-dependent variations in the free energy of formation using density functional perturbation theory calculations. The results suggest that the formation of the NASICON-type structure is disfavoured under the 0-2000 K and 0-20 GPa conditions.

Sol-gel-derived transparent silica-(Gd,Pr)PO4 glass-ceramic narrow-band UVB phosphors.

Silica-based monolithic transparent glass-ceramics containing (Gd,Pr)PO4 orthophosphate nanocrystals, prepared by a cosolvent-free sol-gel method, efficiently emit narrow-band ultraviolet B (UVB) photoluminescence (PL) at ∼313 nm from the 6P7/2 → 8S7/2 transition of Gd3+ ions upon excitation into the 4f-5d transition of Pr3+ ions. The formation of (Gd,Pr)PO4 nanocrystals as small as ∼5-10 nm facilitates energy transfer between rare-earth (RE) ions while avoiding optical loss by Rayleigh scattering. Unlike conventional phosphors, the aggregation of Gd3+ ions causes no concentration quenching, and the incorporation of inert RE ions to block energy transfer, such as La3+ and Y3+ ions, is unnecessary. A glass-ceramic with a Pr3+ ion fraction of 0.02 exhibited the maximum internal and external PL quantum efficiencies of ∼0.98 and ∼0.91, respectively, under excitation at 220 nm.

Poly(n-alkylsilsesquioxane) liquids prepared by cosolvent-free hydrolytic polycondensation of n-alkyltrialkoxysilanes: effects of liquid-liquid phase separation during aging and alkyl chain length on structure and viscosity.

The effect of the alkyl chain length and alkoxy groups on the viscosity and related properties of polysilsesquioxanes (PSQs) prepared by cosolvent-free hydrolytic polycondensation from n-alkyltrialkoxysilane-water binary systems via aging was investigated. n-Alkyltrialkoxysilanes with ethyl, n-propyl, and n-butyl groups gave PSQ liquids, whereas those with methyl groups yielded gels. The viscosity of the PSQ liquids remained stable over a month at room temperature despite the presence of many SiOH groups, and decreased with an increase in the alkyl chain length. The aging step was crucial for obtaining PSQ liquids with low viscosities, and both n-alkyltrimethoxysilane and n-alkyltriethoxysilane with the same alkyl group produced PSQ liquids with comparable viscosities. However, during aging, liquid-liquid phase separation occurred only in the solutions derived from alkyltrimethoxysilanes. These observations confirmed that liquid-liquid phase separation is not essential for the preparation of PSQ liquids. The temperature dependence of the viscosity indicated that the PSQ liquids are fragile.