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

We present a strategy for enhancing Li + conduction in block copolymer electrolytes by introducing trace amounts of Li salts into polystyrene- b -poly(ethylene oxide) (PS- b -PEO), wherein Li + ions preferentially coordinate with the −OH end groups of the PEO chains, resulting in the formation of double primitive cubic ( Im 3̅ m ) structures. Compared with TFSI – anions in Li salts, smaller anions (PF 6 – and BF 4 – ) could facilitate ion localization more effectively, expanding the salt concentration range for developing stable Im 3̅ m structures. The Im 3̅ m structures formed in PS- b -PEOs doped with LiBF 4 at r = 0.013–0.02 ( r ≡ [Li + ]/[EO]) exhibited ionic conductivities several times higher than those doped at the conventional level (e.g., r = 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- b -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 −PO 3 H 2 groups further strengthened the Li + -mediated end–end interactions and significantly extended the salt concentration range to form Im 3̅ m structures. In contrast, increasing the number of –OH end groups (such as diols and triols) had minimal effect on stabilizing the network morphologies.