Indium-MOF as Multifunctional Promoter to Remove Ionic Conductivity and Electrochemical Stability Constraints on Fluoropolymer Electrolytes for All-Solid-State Lithium Metal Battery

Abstract

Fluoropolymers promise all-solid-state lithium metal batteries (ASLMBs) but suffer from two critical challenges. The first is the trade-off between ionic conductivity (σ) and lithium anode reactions, closely related to high-content residual solvents. The second, usually consciously overlooked, is the fluoropolymer's inherent instability against alkaline lithium anodes. Here, we propose indium-based metal–organic frameworks (In-MOFs) as a multifunctional promoter to simultaneously address these two challenges, using poly(vinylidene fluoride–hexafluoropropylene) (PVH) as the typical fluoropolymer. In-MOF plays a trio: (1) adsorbing and converting free residual solvents into bonded states to prevent their side reactions with lithium anodes while retaining their advantages on Li+ transport; (2) forming inorganic-rich solid electrolyte interphase layers to prevent PVH from reacting with lithium anodes and promote uniform lithium deposition without dendrite growth; (3) reducing PVH crystallinity and promoting Li-salt dissociation. Therefore, the resulting PVH/In-MOF (PVH-IM) showcases excellent electrochemical stability against lithium anodes, delivering a 5550 h cycling at 0.2 mA cm−2 with a remarkable cumulative lithium deposition capacity of 1110 mAh cm−2. It also exhibits an ultrahigh σ of 1.23 × 10−3 S cm−1 at 25 °C. Moreover, all-solid-state LiFePO4|PVH-IM|Li full cells show outstanding rate capability and cyclability (80.0% capacity retention after 280 cycles at 0.5C), demonstrating high potential for practical ASLMBs.

1. Introduction

The energy density of traditional Li-ion batteries has approached their theoretical limits, and using liquid electrolytes has raised widespread safety concerns [1–4]. All-solid-state Li metal batteries (ASLMBs) are considered the "holy grail" of next-generation electrochemical energy-storage technologies due to their theoretically high energy density and intrinsic safety [5–7]. Solid-state electrolytes are one of the most fundamental components that determine the electrochemical performance of ASLMBs, requiring high ionic conductivity (σ), interfacial contact, electrochemical stability, high strength, etc. [8–10]. Among various candidates, solid-state polymer electrolytes (SPEs) stand out because they exhibit excellent flexibility enabling decent interfacial compatibility with electrodes, good processability promising large-scale production, and tailored physical/chemical properties endowing multifunctionality [11, 12]. In particular, fluoropolymers provoke great attention because they have high dielectric constants (εr = 8–12) that significantly enhance the dissolution and dissociation of Li salts, resulting in higher σ compared to other SPEs [13–15]. In addition, fluoropolymers show excellent elasticity, mechanical strength (~50 MPa), and thermal stability (Td: 400 °C) [16, 17]. With these merits, fluoropolymers are promising for developing high-performance SPEs [18, 19]. However, fluoropolymers suffer from two critical problems, hindering their implementation in next-generation ASLMBs.

The first problem is the compromise between the σ and Li anode side reactions, which strongly correlate with residual solvents [20, 21]. Generally, solution casting is the most common approach for preparing fluoropolymer SPEs, which involves polar solvents such as N,N-dimethylformamide (DMF) and N-methylpyrrolidone (NMP) [22, 23]. These solvents have high polarity and boiling points, making them difficult to completely remove upon drying. As a result, fluoropolymer SPEs usually exhibit high residual solvent content of 10–17 wt% [24–26]. On the one hand, the residual solvents act as plasticizers and form complexes with Li+ (e.g., Li(DMF)x+), thereby accelerating Li+ migrations and enhancing σ to 10−4 S cm−1 [27–30]. On the other hand, the residual solvents can react with lithium anodes, leading to side reactions and degradation of the electrolyte–electrode interface. This trade-off between ionic conductivity and electrochemical stability is a major challenge for fluoropolymer-based SPEs.