Revealing structure and dynamics in host–guest supramolecular crystalline polymer electrolytes by solid-state NMR: Applications to β-CD-polyether/Li+ crystal

Ion-Exchange-Induced Phase Transition Enables an Intrinsically Air Stable Hydrogarnet Electrolyte for Solid-State Lithium Batteries

Inferior air stability is a primary barrier for large-scale applications of garnet electrolytes in energy storage systems. Herein, a deeply hydrated hydrogarnet electrolyte generated by a simple ion-exchange-induced phase transition from conventional garnet, realizing a record-long air stability of more than two years when exposed to ambient air is proposed. Benefited from the elimination of air-sensitive lithium ions at 96 h/48e sites and unobstructed lithium conduction path along tetragonal sites (12a) and vacancies (12b), the hydrogarnet electrolyte exhibits intrinsic air stability and comparable ion conductivity to that of traditional garnet. Moreover, the unique properties of hydrogarnet pave the way for a brand-new aqueous route to prepare lithium metal stable composite electrolyte on a large-scale, with high ionic conductivity (8.04 × 10-4 S cm-1), wide electrochemical windows (4.95 V), and a high lithium transference number (0.43). When applied in solid-state lithium batteries (SSLBs), the batteries present impressive capacity and cycle life (164 mAh g-1 with capacity retention of 89.6% after 180 cycles at 1.0C under 50 °C). This work not only designs a new sort of hydrogarnet electrolyte, which is stable to both air and lithium metal but also provides an eco-friendly and large-scale fabrication route for SSLBs.

Porous Cyclodextrin Polymer Enables Dendrite-Free and Ultra-Long Life Solid-State Zn-I2 Batteries

Zn-I2 batteries have attracted attention due to their low cost, safety, and environmental friendliness. However, their performance is still limited by the irreversible growth of Zn dendrites, hydrogen evolution reactions, corrosion, and shuttle effect of polyiodide. In this work, we have prepared a new porous polymer (CD-Si) by nucleophilic reaction of β-cyclodextrin with SiCl4 , and CD-Si is applied to the solid polymer electrolyte (denoted PEO/PVDF/CD-Si) to solve above-mentioned problems. Through the anchoring of the CD-Si, a conductive network with dual transmission channels was successfully constructed. Due to the non-covalent anchoring effect, the ionic conductivity of the solid polymer electrolytes (SPE) can reach 1.64×10-3  S cm-1 at 25 °C. The assembled symmetrical batteries can achieve highly reversible dendrite-free galvanizing/stripping (stable cycling for 7500 h at 5 mA cm-2 and 1200 h at 20 mA cm-2 ). The solid-state Zn-I2 battery shows an ultra-long life of over 35,000 cycles at 2 A g-1 . Molecular dynamics simulations are performed to elucidate the working mechanism of CD-Si in the polymer matrix. This work provides a novel strategy towards solid electrolytes for Zn-I2 batteries.

A Bio-Inspired Methylation Approach to Salt-Concentrated Hydrogel Electrolytes for Long-Life Rechargeable Batteries

Hydrogel electrolytes hold great promise in developing flexible and safe batteries, but the presence of free solvent water makes battery chemistries constrained by H2 evolution and electrode dissolution. Although maximizing salt concentration is recognized as an effective strategy to reduce water activity, the protic polymer matrices in classical hydrogels are occupied with hydrogen-bonding and barely involved in the salt dissolution, which sets limitations on realizing stable salt-concentrated environments before polymer-salt phase separation occurs. Inspired by the role of protein methylation in regulating intracellular phase separation, here we transform the "inert" protic polymer skeletons into aprotic ones through methylation modification to weaken the hydrogen-bonding, which releases free hydrogen bond acceptors as Lewis base sites to participate in cation solvation and thus assist salt dissolution. An unconventionally salt-concentrated hydrogel electrolyte reaching a salt fraction up to 44 mol % while retaining a high Na+ /H2 O molar ratio of 1.0 is achieved without phase separation. Almost all water molecules are confined in the solvation shell of Na+ with depressed activity and mobility, which addresses water-induced parasitic reactions that limit the practical rechargeability of aqueous sodium-ion batteries. The assembled Na3 V2 (PO4 )3 //NaTi2 (PO4 )3 cell maintains 82.8 % capacity after 580 cycles, which is the longest cycle life reported to date.

Probing distribution and dynamics of lithium ions in supermolecule β-CD-PEO/Li+ solid polymer electrolytes via solid-state NMR

In this work, the distribution and dynamics of Li⁺ ions in β-CD-PEO/Li⁺ (β-CD, β-cyclodextrin; PEO, polyethylene-oxides) crystalline polymer electrolytes were investigated by solid-state NMR to enlighten the ionic conduction mechanism. Specifically, ⁷Li-⁶Li REDOR NMR and variable-contact-time ¹H-⁶Li CP/MAS NMR were adopted for the study. The results demonstrate that Li⁺ ions coordinated by polymer chains have relatively compact spatial density and fast dynamics, which facilitate the improvement of the electrochemical properties. Additionally, the variation of the distribution and dynamics of the Li⁺ ions and the ionic conduction mechanism were studied and discussed by altering the amount of the Li⁺ ions. This work deepens our understanding of the distribution and dynamics of Li⁺ ions in β-CD-PEO/Li⁺ crystals and demonstrates possible future applications of solid-state NMR on the study of the polymer electrolytes.

Direct observation of geometric and sliding ferroelectricity in an amphidynamic crystal

Sliding ferroelectricity is a recently observed polarity existing in two-dimensional materials. However, due to the weak polarization and poor electrical insulation in these materials, existing experimental evidences are indirect and mostly based on nanoscale transport properties or piezoresponse force microscopy. We report the direct observation of sliding ferroelectricity, using a high-quality amphidynamic single crystal (15-crown-5)Cd₃Cl₆, which possesses a large bandgap and so allows direct measurement of polarization-electric field hysteresis. This coordination polymer is a van der Waals material, which is composed of inorganic stators and organic rotators as determined by X-ray diffraction and NMR characterization. From density functional theory calculations, we find that after freezing the rotators, an electric dipole is generated in each layer driven by the geometric mechanism, while a comparable ferroelectric polarization originates from the interlayer sliding. The net polarization of these two components can be directly measured and manipulated. Our finding provides insight into low-dimensional ferroelectrics, especially control of the synchronous dynamics of rotating molecules and sliding layers in solids.

Biobased thermosensitive polyrotaxanes constructed by polymerization of cyclodextrin-triterpenoid inclusion complexes

Three biobased thermosensitive polyrotaxanes with alternating multiblock structures have been constructed through polymerization of inclusion complexes in a convenient tandem way.

Review of Recent Nuclear Magnetic Resonance Studies of Ion Transport in Polymer Electrolytes

Current and future demands for increasing the energy density of batteries without sacrificing safety has led to intensive worldwide research on all solid state Li-based batteries. Given the physical limitations on inorganic ceramic or glassy solid electrolytes, development of polymer electrolytes continues to be a high priority. This brief review covers several recent alternative approaches to polymer electrolytes based solely on poly(ethylene oxide) (PEO) and the use of nuclear magnetic resonance (NMR) to elucidate structure and ion transport properties in these materials.

Natural-Product-Tailored Polyurethane: Size-Dictated Construction of Polypseudorotaxanes with Cyclodextrin-Triterpenoid Pairs

Cyclodextrin (CD)-based polyrotaxanes (PRs) and polypseudorotaxanes (PPRs) have attracted considerable attention due to their unique topological structures and functions. However, limited by the simple chemical structures and the single functionalization of guest polymer units like poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG), to date the construction of CD-based PRs and PPRs with precisely controllable supramacromolecular structures is fairly rare. In this work, two kinds of molecular necklace-like PPRs with CD-triterpenoid pairs were prepared via the size-dictated construction, where the threaded guest polymer was a natural product-tailored polyurethane (PU-PEG-GA) with the alternating structure of triterpenoid and PEG segments via a simple step-growth polymerization. Taking advantage of the differentiation in host-guest interactions between β/γ-CD and triterpenoid pairs, β-CD simultaneously located on both PEG segments and triterpenoid units in PU-PEG-GA, while γ-CD selectively recognized triterpenoid units. Consequently, the assembly morphology of PU-PEG-GA was adjusted hierarchically from micelles to worms and vesicles upon addition of β-CD, whereas they gradually collapsed to disappear in the presence of γ-CD. Our biocompatible PPRs with precisely controllable supramacromolecular structures may lead to the exploration on understanding and simulating macromolecular recognition using natural products.

Supramolecular Self-Assembly of Methylated Rotaxanes for Solid Polymer Electrolyte Application

Li⁺-conducting solid polymer electrolytes (SPEs) obtained from supramolecular self-assembly of trimethylated cyclodextrin (TMCD), poly(ethylene oxide) (PEO), and lithium salt are investigated for application in lithium-metal batteries (LMBs) and lithium-ion batteries (LIBs). The considered electrolytes comprise nanochannels for fast lithium-ion transport formed by CD threaded on PEO chains. It is demonstrated that tailored modification of CD beneficially influences the structure and transport properties of solid polymer electrolytes, thereby enabling their application in LMBs. Molecular dynamics (MD) simulation and experimental data reveal that modification of CDs shifts the steady state between lithium ions inside and outside the channels, in this way improving the achievable ionic conductivity. Notably, the designed SPEs facilitated galvanostatic cycling in LMBs at fast charging and discharging rates for more than 200 cycles and high Coulombic efficiency.

Low-Molecular-Weight Supramolecular Ionogel Based on Host-Guest Interaction

Supramolecular ionogels were prepared by self-assembly of small molecules through host-guest interaction between β-cyclodextrin (β-CD) and a room-temperature ionic liquid (IL) 3-(1-methyl-3-imidazolio)propanesulfonate-lithium bis(trifluoromethanesulfonyl)imide (MIPS-LiTFSI) which contains zwitterion MIPS. ¹⁹F NMR and 2D ROESY ¹H NMR have been used to prove that only TFSI^- is involved in the complexation. ¹H NMR, FT-IR, and comparative tests indicated that the electrostatic interaction between imidazole cation and TFSI^- anion and intermolecular hydrogen bonding between three compounds also contribute to the formation of supramolecular ionogel. Ionogels with different gel-sol phase transition temperatures can be obtained by adjusting the molar ratio between β-CD and MIPS-LiTFSI. In addition, the supramolecular ionogels composed of "channel type" structural β-CD have been constructed. The ionogel with high conductivity and low activation energy open a door to new fields for special applications.