Block Copolymer Electrolytes with Double Primitive Cubic Structures: Enhancing Solid-State Lithium Conduction via Lithium Salt Localization.

Abstract

We present a strategy for enhancing Li⁺ conduction in block copolymer electrolytes by introducing trace amounts of Li salts into polystyrene-<i>b</i>-poly(ethylene oxide) (PS-<i>b</i>-PEO), wherein Li⁺ ions preferentially coordinate with the -OH end groups of the PEO chains, resulting in the formation of double primitive cubic (<i>Im</i>3̅<i>m</i>) structures. Compared with TFSI^- anions in Li salts, smaller anions (PF₆^- and BF₄^-) could facilitate ion localization more effectively, expanding the salt concentration range for developing stable <i>Im</i>3̅<i>m</i> structures. The <i>Im</i>3̅<i>m</i> structures formed in PS-<i>b</i>-PEOs doped with LiBF₄ at <i>r</i> = 0.013-0.02 (<i>r</i> ≡ [Li⁺]/[EO]) exhibited ionic conductivities several times higher than those doped at the conventional level (e.g., <i>r</i> = 0.06). The corresponding morphology factors were more than eight times higher than those of the lamellar-forming electrolytes. Notably, the activation energy value for Li⁺ conduction in PS-<i>b</i>-PEO with one Li⁺ ion per entire PEO chain was only 0.012 eV (by Vogel-Fulcher-Tammann), indicating that Li⁺ transport was less dependent on polymer relaxation. Furthermore, modifying the PEO chain ends with three -PO₃H₂ groups further strengthened the Li⁺-mediated end-end interactions and significantly extended the salt concentration range to form <i>Im</i>3̅<i>m</i> structures. In contrast, increasing the number of -OH end groups (such as diols and triols) had minimal effect on stabilizing the network morphologies.