From Computer Simulation to Synthesis of Highly Lithium-Ion Conductive Block Copolymers with Bicontinuous Morphology

Single-ion conducting polyelectrolytes transport only lithium ions, thereby increasing the lithium transference number and reducing concentration polarization in batteries. While this improves the electrochemical performance and safety, it remains challenging to balance ionic conductivity and mechanical strength (viscoelastic properties). A promising solution can be ionic block copolymers (BCPs), in which one charged block enables ion conduction while the neutral block with a high glass transition temperature provides mechanical reinforcement. Ionic conductivity in these BCPs is strongly dependent on their microphase-separated morphology. Although bicontinuous structures were believed to offer optimal ion conduction, synthesizing such morphologies is difficult, because they form in a narrow phase-space window requiring precise control of block volume fractions, molecular weights, and interaction parameters. To address this, we combined theoretical and experimental approaches to study a series of poly[(LiMn-r-PEGMm)-b-PhEtMk] block copolymers, where the ion-conducting block comprises lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethanesulfonyl)imide (LiM) and poly(ethylene glycol)methyl ether methacrylate (PEGM), and the neutral block is poly(2-phenylethyl methacrylate) (PhEtMk). The length of the LiMn-r-PEGMm block was fixed at n = 20 and m = 80, while k was varied from 70 to 140. Dissipative particle dynamics simulations, coupled with a 3D scattering-diagram analysis, revealed stability regions for hexagonally packed cylinders, lamellae, and bicontinuous phases, predicting that bicontinuous phases emerge at k = 110÷120. To validate this, a series of poly[(LiMn-r-PEGMm)-b-PhEtMk] block copolymers with k ranging from 79 to 174 was synthesized. In keeping with predictions, morphologies consistent with the existence of a (disordered) bicontinuous structure were observed via AFM at k = 96 and 110. At the same time, while the storage modulus (G′) increased with rising k, the ionic conductivity decreased monotonically. The polymers with (disordered) bicontinuous structures demonstrated an optimal balance of viscoelastic properties and ionic conductivity, highlighting their potential, despite the absence of a significant conductivity enhancement in this series of BCPs.