Electro-Responsive Aggregation and Dissolution of Cationic Polymer Using Reversible Redox Reaction of Electron Mediator.

Stimuli-responsive aggregation of polymer chains in water has found a variety of applications in polymer science, biology, and chemical engineering. To date, the majority of the phase transitions between the aggregated and dissolved forms has been observed by changing the solution temperature, and an active and precise control on the phase transition with a high time resolution has been challenging. Herein, a reversible phase transition of poly(allylamine-co-allylurea) (PAU) in an aqueous electrolyte is achieved by electrochemical redox cycling of hexacyanoferrate(II/III) ([Fe(CN)6 ]4-/3- ) ion pair. The aggregation and dissolution cycle can be completed in a high-resolution time frame of as short as 5 s. The strong electrostatic interaction between the protonated primary amino group of PAU and the tetravalent [Fe(CN)6 ]4- anion induces the aggregation, while the oxidation to the trivalent [Fe(CN)6 ]3- anion reduces the attractive force, and the polymer chain redissolves in solution. The ureido group of PAU helps the chain-folding process through the formation of inter/intrachain hydrogen-bonding networks, resulting in the sharp phase transition. By using [Fe(CN)6 ]4-/3- as the electron mediator, the electrochemical control on the large transparency change of polymer aqueous solution is realized for the first time.

Advancement of Electrochemical Thermoelectric Conversion with Molecular Technology.

Thermocells are a thermoelectric conversion technology that utilizes the shift in an electrochemical equilibrium arising from a temperature difference. This technology has a long history; however, its low conversion efficiency impedes its practical usage. Recently, an increasing number of reports have shown drastic improvements in thermoelectric conversion efficiency, and thermocells could arguably represent an alternative to solid thermoelectric devices. In this Minireview, we regard thermocells as molecular systems consisting of successive molecular processes responding to a temperature change to achieve energy generation. Various molecular technologies have been applied to thermocells in recent years, and could stimulate diverse research fields, including supramolecular chemistry, physical chemistry, electrochemistry, and solid-state ionics. These research approaches will also provide novel methods for achieving a sustainable society in the future.

Designing polymer coatings for lithium metal protection.

Protection of lithium metal has been one of the great challenges to realize a long-life, high-energy-density battery. Polymer coatings on lithium metal surface have been proven to be an effective protection method in terms of improved morphology, higher coulombic efficiency, and a longer cycle life. However, there is a variety of design principles of polymer coatings proposed by the research community, and the influence of polymer swelling in liquid electrolytes remains poorly understood. Herein we use crosslinking density and solvent-polymer interaction to quantitatively explain the mechanical property and the ion-transport property of polymer coatings when swollen in liquid electrolytes. Low crosslinking density is beneficial for reducing the rigidity and enhancing the viscosity of the polymer. Ion conductivity increases with the swelling ratio, and activation energy of lithium-ion transport increases in a polar polymer with strong ion-polymer coupling. We propose that polymer coatings must be combined with the emerging electrolytes with unconventional solvent compositions to realize a practical high-performance lithium metal battery. This study can provide design guidelines for polymer coatings through the optimized interactions with upcoming high-performance electrolytes.

Supramolecular Thermo-Electrochemical Cells: Enhanced Thermoelectric Performance by Host-Guest Complexation and Salt-Induced Crystallization.

Thermo-electrochemical cells have potential to generate thermoelectric voltage 1 order higher than that given by semiconductor materials. To overcome the current issues in thermoelectric energy conversion, it is of paramount importance to grow and fulfill the full potential of thermo-electrochemical cells. Here we report a rational supramolecular methodology that yielded the highest Seebeck coefficient of ca. 2.0 mV K(-1) around ambient temperatures. This is based on the encapsulation of triiodide ions in α-cyclodextrin, whose equilibrium is shifted to the complexation at lower temperatures, whereas it is inverted at elevated temperatures. This temperature-dependent host-guest interaction provides a concentration gradient of redox ion pairs between two electrodes, leading to the eminent performance of the thermo-electrochemical cells. The figure of merit for this system, zT reached a high value of 5 × 10(-3). The introduction of host-guest chemistry to thermoelectric cells thus provides a new perspective in thermoelectric energy conversion.

Direct Conversion of Phase-Transition Entropy into Electrochemical Thermopower and the Peltier Effect.

A thermocell generates thermopower from a temperature difference (ΔT) between two electrodes. The converse process of thermocells is an electrochemical Peltier effect, which creates a ΔT on the electrodes by applying an external current. The Seebeck coefficient (Se ) of the electrochemical system is proportional to the entropy change of the redox reaction; therefore, a redox system having a significant entropy change is expected to increase the Se . In this study, a thermoresponsive polymer having a redox-active moiety, poly(N-isopropyl acrylamide-co-N-(2-acrylamide ethyl)-N'-n-propylviologen) (PNV), is used as the redox species of a thermocell. PNV2+ dication undergoes the coil-globule phase transition upon the reduction to PNV+ cation radical, and a large entropy change is introduced because water molecules are freed from the polymer chains. The Se of PNV thermocell drastically increased to +2.1 mV K-1 at the lower critical solution temperature (LCST) of PNV. The entropy change calculated from the increment of Se agrees with the value evaluated by differential scanning calorimetry. Moreover, the electrochemical Peltier effect is observed when the device temperature is increased above the LCST. This study shows that the large entropy change associated with the coil-globule phase transition can be used in electrochemical thermal management and refrigeration technologies.

Exploring the local solvation structure of redox molecules in a mixed solvent for increasing the Seebeck coefficient of thermocells.

A thermocell is an emerging alternative to thermoelectric devices and exhibits a high Seebeck coefficient (Se) due to the large change of solvation entropy associated with redox reactions. Here, the Se of p-chloranil radicals/dianions (CA˙-/2-) in acetonitrile was drastically increased from -1.3 to -2.6 mV K-1 by the addition of ethanol, and the increment surpassed the estimation of the classical Born model with continuum solvent media. UV-vis spectroscopy and electrochemical measurements at various mixing ratios of acetonitrile to ethanol revealed that the strong hydrogen bonding between ethanol and oxygen atoms of CA2- forms a 4 : 1 solvent-ion pair, while the ethanol molecules binding to CA2- dissociate upon its oxidation to CA˙-. The local solvation structures of CA2- are in good agreement with density functional theory. This order-disorder transition of the local solvation structure around the CA˙-/2- ions produces a large entropy change and results in a large Se value. The tailored solvation structure of redox ions by hydrogen bonding is a versatile method applicable to a variety of redox pairs and solvents, contributing to the development of electrolyte engineering for thermocells.

Design of a robust and strong-acid MOF platform for selective ammonium recovery and proton conductivity.

Metal-organic frameworks (MOFs) are potential candidates for the platform of the solid acid; however, no MOF has been reported that has both aqueous ammonium stability and a strong acid site. This manuscript reports a highly stable MOF with a cation exchange site synthesized by the reaction between zirconium and mellitic acid under a high concentration of ammonium cations (NH4+). Single-crystal XRD analysis of the MOF revealed the presence of four free carboxyl groups of the mellitic acid ligand, and the high first association constant (pKa1) of one of the carboxyl groups acts as a monovalent ion-exchanging site. NH4+ in the MOF can be reversibly exchanged with proton (H+), sodium (Na+), and potassium (K+) cations in an aqueous solution. Moreover, the uniform nanospace of the MOF provides the acid site for selective NH4+ recovery from the aqueous mixture of NH4+ and Na+, which could solve the global nitrogen cycle problem. The solid acid nature of the MOF also results in the proton conductivity reaching 1.34 × 10-3 S cm-1 at 55 °C by ion exchange from NH4+ to H+.

Molecular Design of Organic Ionic Plastic Crystals Consisting of Tetracyanoborate with Ultralow Phase Transition Temperature.

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]-) and various organic cations. In particular, the OIPC composed of [TCB]- and spiro-(1,1')-bipyrrolidinium ([spiropyr]+) cations can transform into its plastic phase at ultralow temperature (Tp = -55 °C) while maintaining a high melting point (Tm = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high Tm was attributed to the relatively small interionic distance between [spiropyr]+ and [TCB]-. At the same time, the low Tp was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input.

Quantification of the ion transport mechanism in protective polymer coatings on lithium metal anodes.

Protective Polymer Coatings (PPCs) have been proposed to protect lithium metal anodes in rechargeable batteries to stabilize the Li/electrolyte interface and to extend the cycle life by reducing parasitic reactions and improving the lithium deposition morphology. However, the ion transport mechanism in PPCs remains unclear. Specifically, the degree of polymer swelling in the electrolyte and the influence of polymer/solvent/ion interactions are never quantified. Here we use poly(acrylonitrile-co-butadiene) (PAN-PBD) with controlled cross-link densities to quantify how the swelling ratio of the PPC affects conductivity, Li+ ion selectivity, activation energy, and rheological properties. The large difference in polarities between PAN (polar) and PBD (non-polar) segments allows the comparison of PPC properties when swollen in carbonate (high polarity) and ether (low polarity) electrolytes, which are the two most common classes of electrolytes. We find that a low swelling ratio of the PPC increases the transference number of Li+ ions while decreasing the conductivity. The activation energy only increases when the PPC is swollen in the carbonate electrolyte because of the strong ion-dipole interaction in the PAN phase, which is absent in the non-polar PBD phase. Theoretical models using Hansen solubility parameters and a percolation model have been shown to be effective in predicting the swelling behavior of PPCs in organic solvents and to estimate the conductivity. The trade-off between conductivity and the transference number is the primary challenge for PPCs. Our study provides general guidelines for PPC design, which favors the use of non-polar polymers with low polarity organic electrolytes.

Tailoring Electrolyte Solvation for Li Metal Batteries Cycled at Ultra-Low Temperature.

Lithium metal batteries (LMBs) hold the promise to pushing cell level energy densities beyond 300 Wh kg-1 while operating at ultra-low temperatures (< -30°C). Batteries capable of both charging and discharging at these temperature extremes are highly desirable due to their inherent reduction of external warming requirements. Here we demonstrate that the local solvation structure of the electrolyte defines the charge-transfer behavior at ultra-low temperature, which is crucial for achieving high Li metal coulombic efficiency (CE) and avoiding dendritic growth. These insights were applied to Li metal full cells, where a high-loading 3.5 mAh cm-2 sulfurized polyacrylonitrile (SPAN) cathode was paired with a one-fold excess Li metal anode. The cell retained 84 % and 76 % of its room temperature capacity when cycled at -40 and -60 °C, respectively, which presented stable performance over 50 cycles. This work provides design criteria for ultra-low temperature LMB electrolytes, and represents a defining step for the performance of low-temperature batteries.

High-throughput 16S rDNA sequencing of the pulmonary microbiome of rats with allergic asthma.

A decrease in microbial infection in adolescents is implicated with an increase in the incidence of asthma and allergic diseases in adulthood, indicating that the microbiome plays a critical role in asthma. However, the microbial composition of the lower respiratory tract remains unclear, hindering the further exploration of the pathogenesis of asthma. This study aims to explore the microbial distribution and composition in the lungs of normal rats and rats with allergic asthma via 16S rDNA sequencing. The DNA of the pulmonary microbiome was extracted from the left lungs collected from normal control group (NC), saline control group (SC), and allergic asthma group (AA) under aseptic conditions. After the 16s rDNA V4-V5 region was amplified, the products were sequenced using Illumina high-throughput technology and subjected to operational taxonomic unit (OTU) cluster and taxonomy analysis. The OTU values of AA increased significantly compared with those of NC and SC. Microbiome structure analysis showed that the dominant phylum of the pulmonary microbiome changed from Proteobacteria in NC to Firmicutes in AA. Linear discriminant analysis indicated that the key microbiomes involved in the three groups varied. Numerous microbiomes stably settled in the lungs of the rats in NC and AA. The structure and diversity of the pulmonary microbiome in AA differed from those in NC.

Draining Over Blocking: Nano-Composite Janus Separators for Mitigating Internal Shorting of Lithium Batteries.

Catastrophic battery failure due to internal short is extremely difficult to detect and mitigate. In order to enable the next-generation lithium-metal batteries, a "fail safe" mechanism for internal short is highly desirable. Here, a novel separator design and approach is introduced to mitigate the effects of an internal short circuit by limiting the self-discharge current to prevent cell temperature rise. A nano-composite Janus separator-with a fully electronically insulating side contacting the anode and a partially electronically conductive (PEC) coating with tunable conductivity contacting the cathode-is implemented to intercept dendrites, control internal short circuit resistance, and slowly drain cell capacity. Galvanostatic cycling experiments demonstrate Li-metal batteries with the Janus separator perform normally before shorting, which then results in a gradual increase of internal self-discharge over >25 cycles due to PEC-mitigated shorting. This is contrasted by a sudden voltage drop and complete failure seen with a single layer separator. Potentiostatic charging abuse tests of Li-metal pouch cells result in dendrites completely penetrating the single-layer separator causing high short circuit current and large cell temperature increase; conversely, negligible current and temperature rise occurs with the Janus separator where post mortem electron microscopy shows the PEC layer successfully intercepts dendrites.

Nonpassivated Silicon Anode Surface.

A stable solid electrolyte interphase (SEI) has been proven to be a key enabler to most advanced battery chemistries, where the reactivity between the electrolyte and the anode operating beyond the electrolyte stability limits must be kinetically suppressed by such SEIs. The graphite anode used in state-of-the-art Li-ion batteries presents the most representative SEI example. Because of similar operation potentials between graphite and silicon (Si), a similar passivation mechanism has been thought to apply on the Si anode when using the same carbonate-based electrolytes. In this work, we found that the chemical formation process of a proto-SEI on Si is closely entangled with incessant SEI decomposition, detachment, and reparation, which lead to continuous lithium consumption. Using a special galvanostatic protocol designed to observe the SEI formation prior to Si lithiation, we were able to deconvolute the electrochemical formation of such dynamic SEI from the morphology and mechanical complexities of Si and showed that a pristine Si anode could not be fully passivated in carbonate-based electrolytes.

Hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin-I5 - complex in aqueous I-/I3 - thermocells and enhancement in the Seebeck coefficient.

A large Seebeck coefficient (S e) of 1.9 mV K-1 was recorded for the I-/I3 - thermocell by utilizing the host-guest complexation of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin (Me18-α-CD) with the oxidized iodide species. The thermocell measurement and UV-vis spectroscopy unveiled the formation of an Me18-α-CD-pentaiodide (I5 -) complex, which is in remarkable contrast to the triiodide complex α-CD-I3 - previously reported. Although the precipitation of the α-CD-I3 - complex in the presence of an electrolyte such as potassium chloride is a problem in thermocells, this issue was solved by using Me18-α-CD as a host compound. The absence of precipitation in the Me18-α-CD and I-/I3 - system containing potassium chloride not only improved the S e of the I-/I3 - thermocell, but also significantly enhanced the temporal stability of its power output. This is the first observation that I5 - species is formed in aqueous solution in a thermocell. Furthermore, the solution equilibrium of the redox couples was controlled by tuning the chemical structure of the host compounds. Thus, the integration of host-guest chemistry with redox couples extends the application of thermocells.

A Scalable Synthesis Pathway to Nanoporous Metal Structures.

A variety of nanoporous transition metals, Fe, Co, Au, Cu, and others, have been readily formed by a scalable, room-temperature synthesis process. Metal halide compounds are reacted with organolithium reductants in a nonpolar solvent to form metal/lithium halide nanocomposites. The lithium halide is then dissolved out of the nanocomposite with a common organic solvent, leaving behind a continuous, three-dimensional network of metal filaments that form a nanoporous structure. This approach is applicable to both noble metals (Cu, Au, Ag) and less-noble transition metals (Co, Fe, Ni). The microstructures of these nanoporous transition metals are tunable, as controlling the formation of the metal structure in the nanocomposite dictates the final metal structure. Microscopy studies and nitrogen adsorption analysis show these materials form pores ranging from 2 to 50 nm with specific surface areas from 1.0 m2/g to 160 m2/g. Our analysis also shows that pore size, pore volume, and filament size of the nanoporous metal networks depend on the mobility of target metal and the amount of lithium halide produced by the conversion reaction. Further, it has been demonstrated that hybrid nanoporous structures of two or more metals could be synthesized by performing the same process on mixtures of precursor compounds. Metals (e.g., Co and Cu) have been found to stabilize each other in nanoporous forms, resulting in smaller pore sizes and higher surface areas than each element in their pure forms. This scalable and versatile synthesis pathway greatly expands our access to additional compositions and microstructures of nanoporous metals.

Suppressing Lithium Dendrite Growth with a Single-Component Coating.

A single-component coating was formed on lithium (Li) metal in a lithium iodide/organic carbonate [dimethyl carbonate (DMC) and ethylene carbonate (EC)] electrolyte. LiI chemically reacts with DMC to form lithium methyl carbonate (LMC), which precipitates and forms the chemically homogeneous coating layer on the Li surface. This coating layer is shown to enable dendrite-free Li cycling in a symmetric Li∥Li cell even at a current density of 3 mA cm-2. Adding EC to DMC modulates the formation of LMC, resulting in a stable coating layer that is essential for long-term Li cycling stability. Furthermore, the coating can enable dendrite-free cycling after being transferred to common LiPF6/carbonate electrolytes, which are compatible with metal oxide cathodes.