Designing Advanced Electrolytes for High-Safety and Long-Lifetime Sodium-Ion Batteries via Anion-Cation Interaction Modulation.

Safety hazards caused by flammable electrolytes have been major obstacles to the practical application of sodium-ion batteries (SIBs). The adoption of nonflammable all-phosphate electrolytes can effectively improve the safety of SIBs; however, traditional low-concentration phosphate electrolytes are not compatible with carbon-based anodes. Herein, we report an anion-cation interaction modulation strategy to design low-concentration phosphate electrolytes with superior physicochemical properties. Tris(2,2,2-trifluoroethyl) phosphate (TFEP) is introduced as a cosolvent to regulate the ion-solvent-coordinated (ISC) structure through enhancing the anion-cation interactions, forming the stable anion-induced ISC (AI-ISC) structure, even at a low salt concentration (1.22 M). Through spectroscopy analyses and theoretical calculations, we reveal the underlying mechanism responsible for the stabilization of these electrolytes. Impressively, both the hard carbon (HC) anode and Na4Fe2.91(PO4)2(P2O7) (NFPP) cathode work well with the developed electrolytes. The designed phosphate electrolyte enables Ah-level HC//NFPP pouch cells with an average Coulombic efficiency (CE) of over 99.9% and a capacity retention of 84.5% after 2000 cycles. In addition, the pouch cells can operate in a wide temperature range (-20 to 60 °C) and successfully pass rigorous safety testing. This work provides new insight into the design of the electrochemically compatibility electrolyte for high-safety and long-lifetime SIBs.

Distinctly different solvation behaviors of poly(N,N-diethylacrylamide) gels in water/acetone and water/DMSO mixtures.

The solvation behaviors and intermolecular interactions of a poly(N,N-diethylacrylamide) (PDEA) gel network in water/DMSO and water/acetone mixtures have been investigated by variable-temperature high-resolution 1H MAS NMR. Unlike decreasing volume phase transition temperature (VPTT) of the typical thermosensitive poly(N-isopropylacrylamide) (PNIPAM) gel induced by both acetone and DMSO in a water-rich region, distinct phase transition behaviors are revealed for the PDEA gel. That is, acetone is found to increase the VPTT of PDEA directly in the water-rich region while DMSO is also found to increase the VPTT of PDEA at a very low concentration but then decrease the VPTT as the concentration further increases. The above distinctly different VPTT shifts of PDEA are attributed to the different polymer-cosolvent interactions in water/acetone and water/DMSO systems. DMSO molecules with a strong water affinity are preferentially excluded by the PDEA gel network, and can promote the volume phase transition by favoring the collapse of the PDEA gel network, while acetone molecules preferentially adsorbed on the polymer interact with PDEA via non-specific van der Waals interaction, which favors the swollen state of the PDEA gel.

Dynamic mechanism of halide salts on the phase transition of protein models, poly(N-isopropylacrylamide) and poly(N,N-diethylacrylamide).

The effects of salts on protein systems are not yet fully understood. We investigated the ionic dynamics of three halide salts (NaI, NaBr, and NaCl) with two protein models, namely poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEA), using multinuclear NMR, dispersion corrected density functional theory (DFT-D) calculations and dynamic light scattering (DLS) methods. The variation in ionic line-widths and chemical shifts induced by the polymers clearly illustrates that anions rather than cations interact directly with the polymers. From the variable temperature measurements of the NMR transverse relaxation rates of anions, which characterize the polymer-anion interaction intensities, the evolution behaviors of Cl-/Br-/I- during phase transitions are similar in each polymer system but differ between the two polymer systems. The NMR transverse relaxation rates of anions change synchronously with the phase transition of PNIPAM upon heating, but they drop rapidly and vanish about 3-4.5 °C before the phase transition of PDEA. By combining the DFT-D and DLS data, the relaxation results imply that anions escape from the interacting sites with PDEA prior to full polymer dehydration or collapse, which can be attributed to the lack of anion-NH interactions. The different dynamic evolutions of the anions in the PNIPAM and PDEA systems give us an important clue for understanding the micro-mechanism of protein folding in a complex salt aqueous solvent.

Inhomogeneous-collapse driven micelle-vesicle transition of amphiphilic block copolymers.

Understanding the morphological transition dynamics related to the hydrophilic-hydrophobic interface has been a challenge due to the lack of an effective evaluation method. Herein, nuclear magnetic resonance spectroscopy was employed to study the morphological transition related chain collapse of poly(N,N'-diethylaminoethylmethacrylate)-b-poly(N-isopropylacrylamide) (PDEAEMA133-b-PNIPA322) and poly(N,N'-dimethylaminoethylmethacrylate)-b-poly(N-isopropylacrylamide) (PDMAEMA95-b-PNIPA228) and was proved to be a powerful technique in morphological transition mechanism studies once combined with dynamic light scattering and transmission electron microscopy. Unlike the cooperative coil collapse of two blocks in the PDMAEMA95-b-PNIPA228 alkaline solution upon heating which induces the assembly of a nanostructure (∼200 nm) with a hydrophobic core containing both collapsed PDMAEMA and PNIPA segments and a hydrophilic surface part consisting of un-shrunk PDMAEMA and PNIPA segments, PDEAEMA133-b-PNIPA322 with a low-temperature core-shell micelle structure showed a micelle-vesicle transition due to temperature-induced inhomogeneous-collapse of PNIPA. The PNIPA segments in the shell sequentially collapse outside (starting at the core-shell interface), accompanied by a gradual decrease in micelle size. Above the critical temperature, the residual hydrophilic PNIPA segments become too short to stabilize the micelle structure, the micelles then transform into vesicles of a slightly larger size, instead of micelle aggregation and precipitation as normally expected.

Preferential adsorption of the additive is not a prerequisite for cononsolvency in water-rich mixtures.

Cononsolvency of poly(N-isopropylacrylamide) (PNIPAM) gels in binary mixed solvents (water-acetone and water-DMSO) has been comparatively investigated by 1H HR-MAS NMR spectroscopy. The results demonstrate that, although the addition of both acetone and DMSO gives rise to cononsolvency behavior, PNIPAM preferentially interacts with acetone rather than DMSO in a water-rich regime, regardless of whether the temperature is above or below the volume phase transition temperature (VPTT). It suggests that the preferential adsorption of the additive cannot be deemed as a prerequisite for the cononsolvency in water-rich mixtures. The underlying molecular mechanism of cononsolvency involves a delicate balance between polymer-solvent and solvent-solvent interactions. Moreover, a new NOE-based NMR approach has been proposed to study the preferential adsorption in this work, which can be extensively adopted to study other relevant processes, including protein hydration, ligand binding, enzyme catalysis, etc.

Preparation and characterization of curdlan with unique single-helical conformation and its assembly with Congo Red.

Elucidating the structure-activity relationship of curdlan is hampered by a lack of characterization with unique specific conformations (i.e., single- or triple-helix). In this study, single-helical curdlan is generated in dilute NaOH solutions at 35-50 °C, and characterized with NMR, SAXS, and GPC. The conformational transition from coil to single-helix and the intramolecular hydrogen bond interaction are explored using NMR. It is found that the two aforementioned types of curdlan interact with Congo Red in very different ways. Single-helical curdlan can encapsulate Congo Red to form a stable, supramolecular dye assembly, which is demonstrated by the shortest distance between the H3 of curdlan and the phenyl groups of Congo Red, and also the same self-diffusion coefficients of Congo Red and curdlan. In contrast, random-coil curdlan interacts weakly with Congo Red and cannot enwrap it. This study offers insight into the specific structure-activity relationship of beta-(1,3)-glucans.

Inverse solubility of chitin/chitosan in aqueous alkali solvents at low temperature.

The low-temperature dissolving mechanism of chitin/chitosan in the alkali (LiOH, NaOH and KOH) aqueous solvents has not been well established yet. As revealed by our XRD and NMR methods, the prepared deacetylated chitins can be categorized as chitin (DA = 0.94-0.74), chitosan I (DA = 0.53-0.25) and chitosan II (DA < 0.25). Aqueous alkali exhibits fully different dissolving power in the above three regions, i.e., KOH > NaOH >> LiOH for chitin, KOH ≈ LiOH ≈ NaOH for chitosan I, and inverse LiOH >> KOH > NaOH for chitosan II. While in the two-alkali mixed solvent, NaOH or KOH can destroy the interaction of LiOH with D9 (chitosan II region) in the order of NaOH >> KOH, but LiOH cannot destroy the interaction of KOH with raw chitin. The varied solubility of chitin/chitosan in alkali solvent is suggested to be from the cation's preferential interaction rather than OH- ion and low temperature.

Preparation, Structure and Properties of Acid Aqueous Solution Plasticized Thermoplastic Chitosan.

This work provides a simple method for the preparation of thermoplastic chitosan using the most common dilute inorganic and organic acids in aqueous solutions, namely hydrochloric acid (HCl) and acetic acid (HAc). The melting plasticization behavior of chitosan under different concentrations and types of acid solution was investigated. By means of infrared spectra (IR), scanning electron microscope (SEM), X-ray diffraction (XRD), and other characterization methods, as well as a mechanical property test, it was found that as the acid solution concentration increased, the protonation effect was stronger and the plasticization performance showed a better trend. The structure and performance of the modified chitosan were optimal when the concentration of HCl was around 8 wt %. In addition, it was found that HCl had a better effect on the plasticization of chitosan than HAc, which was because the protonation ability of HCl was stronger than that of HAc. Unlike the casting method, the structure and properties of chitosan sheets prepared by thermoplastic processing were directly affected by protonation, however not by the interaction of anionic-cationic electrostatic attractions between the -NH3+ groups of chitosan chains and the carboxyl groups of acetic acids or the chloridoid groups of hydrochloric acid.