Enhanced Regional Electric Potential Difference of Graphdiyne Through Asymmetric Substitution Strategy Boosts Li+ Migration in Composite Polymer Solid-State Electrolyte

Abstract

Low ionic conductivity is a major obstacle for polymer solid-state electrolytes. In response to this issue, a design concept of enhanced regional electric potential difference (EREPD) is proposed to modulate the interaction of nanofillers with other components in the composite polymer solid-state electrolytes (CPSEs). While ensuring the periodic structure of the graphdiyne (GDY) backbone, methoxy-substituted GDY (OGDY) is prepared by an asymmetric substitution strategy, which increases the electric potential differences within each repeating unit of GDY. The staggered distributed electron-rich regions and electron-deficient regions on the two-dimensional plane of OGDY increase the free Li+ concentration through Lewis acid–base pair interaction. The adjacent ERRs and EDRs form uniformly distributed EREPDs, creating a continuous potential gradient that synergistically facilitates the efficient migration of Li+. Impressively, the OGDY/poly(ethylene oxide) (PEO) exhibits a high ionic conductivity (1.1 × 10−3 S cm−1) and ion mobility number (0.71). In addition, the accelerated Li+ migration promotes the formation of uniform and dense SEI layers and inhibits the growth of lithium dendrites. As a proof of concept, Li||Li symmetric cell and Li||LiFePO4 full cell and pouch cell assembled with OGDY/PEO exhibit good performance, highlighting the effectiveness of our EREPD design strategy for improving CPSEs performance.

1. Introduction

Graphdiyne (GDY) is a revolutionary two-dimensional (2D) carbon allotrope with remarkable structural characteristics [1, 2]. The uniform distribution of sp C and sp2 C in GDY results in an inhomogeneous charge distribution across the 2D carbon plane, generating abundant active sites [3–7]. Consequently, GDY exhibits excellent affinity for a wide range of substances, including metal atoms, ions, nanoclusters, polymers, and organic molecules [8–10]. This fundamental property underpins the outstanding performance of GDY-based materials in various application fields [11–18]. Furthermore, the bottom-up synthesis strategy enables precise and systematic functional group substitution at specific sites of GDY. That can facilitate the further modulation of its local electronic environment and optimization of charge distribution [19–22]. These features endow GDY-based materials with superior performance advantages in complex multicomponent applications [23, 24].

Polymer solid electrolytes (PSEs) are complex multicomponent systems typically composed of the polymer matrix (e.g., poly(ethylene oxide), PEO), lithium salts (e.g., lithium bis(trifluoromethane sulfonyl)imide, LiTFSI), and functional additives [25–29]. To enable practical applications of polymer solid electrolytes, several critical challenges must be addressed: (1) low ionic conductivity (σ), particularly at low temperatures [30, 31]; (2) high interfacial impedance [32, 33]; (3) poor electrochemical stability [34, 35]; and (4) lithium dendrite growth [36, 37]. Among these limitations, the low ionic conductivity remains the most significant drawback of polymer solid electrolytes when compared to their inorganic counterparts [38]. The incorporation of 2D layered materials into PSEs to form composite polymer solid-state electrolytes (CPSEs) has been demonstrated as an effective strategy to enhance the σ, even in the case of low additions [39–42]. Generally, 2D nanofillers with high aspect ratios can effectively reduce the crystallinity of polymers and improve the motility of polymer chain segments, thus improving the migration of Li+ in the polymer chain segment [43–45]. With the deepening research of CPSEs, it has been discovered that precisely regulating the interactions among the polymer, lithium salt, and nanofillers can further improve the σ of CPSEs [46, 47]. Ideal 2D nanofillers typically need to possess the following characteristics: (1) strong affinity with an