Role of Electron-Deficient Imidazoles in Ion Transport and Conductivity in Solid-State Polymer Electrolytes

Poly(siloxane)-Derived Ionosilicone Elastomers Reveal the Role of Interfacial Polymer Dynamics in Ionic Double-Layer Rectification

Poly(siloxane ionic liquid)s (PSILs) have highly flexible siloxane backbones, affording them low glass transition temperatures and therefore high solvent-free ionic conductivity at ambient temperature, offering promise for ion-mediated electronic devices. Here, cross-linked, highly conductive (>4 × 10-3 mS/cm) cationic and anionic PSILs (termed ionosilicones) were prepared. The backbone of these ionosilicone networks could be tuned by copolymerization with acrylate monomers to create ionosilicone-acrylate hybrid networks with intermediate properties. When two oppositely charged networks are brought into contact, an ionic double layer (IDL) consisting of fixed cations and anions is formed, and the heterojunction exhibits diode-like nonlinear conductance and ionic current rectification. Interestingly, we observe a trade-off between IDL polarization speed and rectification performance with increased ionosilicone content. We show that the more rapid interfacial polymer dynamics induced by increasing temperature switches the diode "on" in a similar manner as applying a forward DC bias voltage. To explain this multimodal switching behavior, we posit the formation of an interfacial complex with distinctly slower dynamics than the bulk, low-Tg ionoelastomers, limiting ion motion at low temperatures and under reverse bias. These findings provide insight into the key role of backbone flexibility of IDL-based device performance and shine new light on interfacial polymer dynamics as an important design criterion in bipolar ionotronic devices.

Converting a Metal-Coordinating Polymer to a Polymerized Ionic Liquid Improves Li+ Transport

Solid polymer electrolytes (SPEs) with mechanical strength and reduced flammability may also enable next-generation Li+ batteries with higher energy densities. However, conventional SPEs have fundamental limitations in terms of Li+ conductivity. While an imidazole functionalized polymer (PMS-Im) has been previously shown to have ionic conductivity related to the imidazole-Li+ coordination, herein we demonstrate that quaternization of this polymer to form an analogous imidazolium functionalized polymer (PMS-Im+) more efficiently solvates lithium salts and plasticizes the polymer. In addition, inverse Haven ratios as high as 10 indicate positively correlated Li+ transport, possibly due to percolation of nanochannels that significantly improve battery-relevant conductivity. From these combined effects, Li+ conductivity in PMS-Im+ (2.1 × 10-5 S/cm) is over an order of magnitude greater than in PMS-Im at 90 °C (1.6 × 10-6 S/cm).

Charge-Delocalized Triptycene-Based Ionic Porous Organic Polymers as Quasi-Solid-State Electrolytes for Lithium Metal Batteries

Ideal solid electrolytes for lithium (Li) metal batteries should conduct Li+ rapidly with low activation energy, exhibit a high Li+ transference number, form a stable interface with the Li anode, and be electrochemically stable. However, the lack of solid electrolytes that meet all of these criteria has remained a considerable bottleneck in the advancement of lithium metal batteries. In this study, we present a design strategy combining all of those requirements in a balanced manner to realize quasi-solid-state electrolyte-enabled Li metal batteries (LMBs). We prepared Li+-coordinated triptycene-based ionic porous organic polymers (Li+@iPOPs). The Li+@iPOPs with imidazolates and phenoxides exhibited a high conductivity of 4.38 mS cm-1 at room temperature, a low activation energy of 0.627 eV, a high Li+ transference number of 0.95, a stable electrochemical window of up to 4.4 V, excellent compatibility with Li metal electrodes, and high stability during Li deposition/stripping cycles. The high performance is attributed to charge delocalization in the backbone, mimicking the concept of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which facilitates the diffusion of coordinated Li+ through the porous space of the triptycene-based iPOPs. In addition, Li metal batteries assembled using Li+@Trp-Im-O-POPs as quasi-solid-state electrolytes and a LiFePO4 cathode showed an initial capacity of 114 mAh g-1 and 86.7% retention up to 200 cycles.