Porous Organic Cage-Based Quasi-Solid-State Electrolyte with Cavity-Induced Anion-Trapping Effect for Long-Life Lithium Metal Batteries

Abstract

Porous organic cages (POCs) with permanent porosity and excellent host–guest property hold great potentials in regulating ion transport behavior, yet their feasibility as solid-state electrolytes has never been testified in a practical battery. Herein, we design and fabricate a quasi-solid-state electrolyte (QSSE) based on a POC to enable the stable operation of Li-metal batteries (LMBs). Benefiting from the ordered channels and cavity-induced anion-trapping effect of POC, the resulting POC-based QSSE exhibits a high Li+ transference number of 0.67 and a high ionic conductivity of 1.25 × 10−4 S cm−1 with a low activation energy of 0.17 eV. These allow for homogeneous Li deposition and highly reversible Li plating/stripping for over 2000 h. As a proof of concept, the LMB assembled with POC-based QSSE demonstrates extremely stable cycling performance with 85% capacity retention after 1000 cycles. Therefore, our work demonstrates the practical applicability of POC as SSEs for LMBs and could be extended to other energy-storage systems, such as Na and K batteries.

1. Introduction

With the rapid development of portable electronic devices, electric vehicles, and energy storage grids, the demand for high energy density batteries is ever-increasing [1–3]. Lithium metal has been considered as the “Holy Grail” anode material for the next-generation batteries due to its lowest electrode potential (−3.04 V vs. standard hydrogen electrode), small mass density (0.53 g cm−3), and highest theoretical specific capacity (3860 mAh g−1) [4–7]. However, traditional liquid electrolytes (LEs) with flammable and volatile properties are prone to form unstable solid electrolyte interphase (SEI) during cycling due to their high reactivity with lithium metal, resulting in uneven deposition of lithium and severe dendrite growth that can penetrate the separator to cause serious safety issues [8, 9].

Solid-state electrolytes (SSEs) are expected to become an ideal candidate for LMBs due to their nonflammable nature, low reactivity with lithium metal, and high mechanical strength that can suppress dendrite growth [10–13]. Although significant progress has been made in the SSEs field, their wide application is still limited by the poor interfacial contact between electrodes and electrolytes, as well as low ionic conductivity [14, 15], alternatively, with the introduction of a small amount of liquid to SSEs, namely quasi-solid-state electrolytes (QSSEs), whom not only maintain a solid-state phase to avoid leakage but also improve the interfacial contact between electrodes and electrolytes [16, 17]. Specifically, the liquid can act as a plasticizer to dissolve lithium salts and improve the ionic conductivity [18]. Recently, the mechanistic study on Li+ transportation also suggested that solvent-assisted Li+ hopping is the main transport pathway in QSSEs [19].

Porous organic cages (POCs), as an emerging class of crystalline molecular-based materials with permanent porosity [20], have found wide applications such as molecular recognition [21], separation [22–24], gas adsorption [25], and detection [26], with benefit from their porosity and excellent host–guest properties [27]. Different from other crystalline porous materials [28, 29], due to the discrete cage structure at the molecular level, the POCs can both dissolve in specific solvents and periodically stack to form crystalline organic solids, which would facilitate the solution-processing operation [30]. Meanwhile, POCs possess the following advantages in the solid state that bestows them great potential as SSEs: (1) crystalline nanoparticles with high surface areas are beneficial for improving the interfacial contact with electrodes; (2) the uniform porosity across the three-dimensional (3D) framework would facilitate the high-throughput transport with a homogeneous Li+ flux, leading to the uniform deposition of Li+ [31–36].