Polymer electrolytes with high Li + -ion conductivity provide a route toward improved safety and performance of Li + -ion batteries. However, most polymer electrolytes suffer from low ionic conduction and an even lower Li + -ion contribution to the conductivity (the transport number, t + ), with the anion typically transporting over 80% of the charge. Here, we show that subtle and potentially undetected associations within a polymer electrolyte can entrain both the anion and the cation. When removed, the conductivity performance of the electrolyte can be improved by almost 2 orders of magnitude. Importantly, while some of this improvement can be attributed to a decreased glass transition temperature, T g , the removal of the amide functional group reduces interactions between the polymer and the Li + cations, doubling the Li + t + to 0.43, as measured using pulsed-field-gradient NMR. This work highlights the importance of strategic synthetic design and emphasizes the dual role of T g and ion binding for the development of polymer electrolytes with increased total ionic conductivity and the Li + ion contribution to it.

The necessity of well-tuned reactivity for successful controlled polymer synthesis often comes with the price of limited monomer substrate scope. We demonstrate here the on-demand interconversion between living radical and cationic polymerization using two orthogonal stimuli and a dual responsive single catalyst. The dual photo- and electrochemical reactivity of 10-phenylphenothiazine catalyst provides control of the polymer’s molar mass and composition by orthogonally activating the common dormant species toward two distinct chemical routes. This enables the synthesis of copolymer chains that consist of radically and cationically polymerized segments where the length of each block is controlled by the duration of the stimulus exposure. By alternating the application of photochemical and electrochemical stimuli, the on-demand incorporation of acrylates and vinyl ethers is achieved without compromising the end-group fidelity or dispersity of the formed polymer. The results provide a proof-of-concept for the ability to substantially extend substrate scope for block copolymer synthesis under mild, metal-free conditions through the use of a single, dual reactive catalyst.

Polymer electrolytes (PEs) offer a promising avenue toward safer, more mechanically robust and high power density lithium-ion batteries. In PEs, conduction is achieved through the dissolution and subsequent transport of the lithium cation and organic anion, yet only lithium transport provides useful current between the two electrodes and must be maximized. As such, PEs are rationally designed to include solvation groups that only moderately interact with the Li + cations to enable high ionic conductivity (σ) and a high Li + transference number (t + ). Herein, we report a polysiloxane-based PE grafted with cyano-containing side chains that exhibits a total ionic conductivity of 6.9 × 10 −4 S/cm and a Li + transference number of 0.48 at 90 °C, demonstrating significant performance improvements compared to the typical poly(ethylene oxide) (PEO) benchmark. Wide-angle X-ray scattering data indicate that there is no ion aggregation in these systems up to a salt loading of r = [Li + ]/[CN] = 0.3. The high ion dissolution ability of the present PE is attributed to its high dielectric permittivity and the modest Li + -side-chain interaction due to the introduction of the polar cyano group, as probed by electrochemical impedance spectroscopy and infrared/Raman spectroscopy, respectively. Moreover, the side-chain length is critical to ion transport and cation selectivity. With a short alkyl chain length, the polymer matrix effectively solvates salt ions and offers good cation selectivity, while the ion mobility is limited by the chain rigidity; with a longer chain length, the polymer segmental motion increases, while the salt dissolution ability drops and the polymer is less cation-selective. These results demonstrate the vast potential of nonpolar flexible polymers grafted with polar side chains as host materials for PEs.

Solid-state polymer electrolytes offer a safer alternative to traditional lithium-ion batteries based on organic electrolytes. The focus is here on imidazole functionalized polymer electrolytes, where the imidazole ligand promotes salt dissolution, while its functionalization allows to tune the dynamic interactions between the cations in solution and the imidazole ligand tethered to the polymer backbone. Although careful choice of polymer backbone and imidazole linker functionality have resulted in polymer electrolytes with increased total ionic conductivities and a boost in the Li + ion contribution, improvements in performance through modifications of the imidazole heterocycle remain underexplored. In this work, we systematically investigate poly(methylsiloxane) (PMS) polymers functionalized with a series of halogen-substituted imidazoles and show that Li + ion conduction can be tuned by electron-deficient heterocycle ligand. When the number of halogen substituents increases, the Li + ion mobility also increases as measured by pulsed-field-gradient NMR and NMR relaxometry. Although beneficial for Li + transport, electron-deficient imidazole ligands result in clustering, as indicated by wide-angle X-ray scattering, and poor salt dissolution, which in turn impedes the overall ionic transport. This work highlights the importance of synthetic design and the necessity for high salt solvation and weak cation-polymer binding to obtain high Li + transport numbers.

Polymers that are elastic while supporting charge transport are desirable for flexible and soft electronics. Many polymers with bulky and conjugated redox-active pendant units have high glass transition temperatures (T g) in their neutral form that will not lead to elasticity at room temperature. Their behavior in charged form in the solid state without an electrolyte has not been extensively studied. Here, the design strategy of polymeric ionic liquid where two weakly interacting ionic groups are used to maintain a low T g is shown to lead to flexible redox active polymers. The use of a flexible ethylene backbone and redox-active phenothiazine (PTZ)-based pendant group resulted in polymers with relatively low T g that are electrically conductive. PTZ that was N-substituted with 2-(2-ethoxyethoxy)­ethoxy)­ethyl was found to promote solubility of the polymer and lower the T g of the neutral polymer by ∼150 °C relative to that of the T g of a variant without the N-substituent. Doping with trifluoromethanesulfonimide leads to an electrically conductive polymer without significantly increasing the T g. Physical characterization by UV–vis–NIR spectroscopy, electron spin resonance spectroscopy, and impedance spectroscopy verified that the molecular design leads to an efficient charge hopping between the PTZ groups.