A Composite of Hierarchical Porous MOFs and Halloysite Nanotube as Single-ion Conducting Electrolyte Toward High Performance Solid-State Lithium Ionic Battery

Metal-organic frameworks (MOFs), as a promising rechargeable electrochemical energy storage material has emerged in the fields of solid-state lithium battery. However, low ionic conductivity and high interfacial impedance still severely hamper the application of MOFs-based solid-state electrolytes (SSE). Herein, a novel hierarchical porous H-ZIF-8 solid-state electrolyte (SSE) material has been harvested through the in-situ growth of zinc nitrate hydroxide nanosheets, expressing excellent ion conductivity of 1.04×10^-3 S cm^-1 and Li⁺ transference number of 0.71. Moreover, the morphology and structure of H-ZIF-8 was further optimized to obtain a composite H-ZIF-8/HNT by decorating halloysite nanotubes (HNT). Notably, functionalized H-ZIF-8/HNT as an electrolyte presents obvious enhancement on electrochemical properties: higher ionic conductivity of 7.74×10^-3 S cm^-1 , better single-ion transmittability (t_Li ⁺ = 0.84), good interfacial compatibility as well as excellent rate performance. More importantly, the Li/LiFePO₄ battery equipped with H-ZIF-8/HNT SSE owns efficient lithium dendrite suppression and up to 84% capacity retention (104.16 mA h g^-1 ) after 200 times galvanostatic charge/discharge cycles. This work enriches the solid-state lithium-ion composite electrolyte materials, opening an entirely new way for enhancing electrochemical performance. This article is protected by copyright. All rights reserved.

Single-Ion Conducting Polymeric Protective Interlayer for Stable Solid Lithium-Metal Batteries

With many reported attempts on fabricating single-ion conducting polymer electrolytes, they still suffer from low ionic conductivity, narrow voltage window, and high cost. Herein, we report an unprecedented approach on improving the cationic transport number (t_Li⁺) of the polymer electrolyte, i.e., single-ion conducting polymeric protective interlayer (SIPPI), which is designed between the conventional polymer electrolyte (PVEC) and Li-metal electrode. Satisfied ionic conductivity (1 mS cm^-1, 30 °C), high t_Li⁺ (0.79), and wide-area voltage stability are realized by coupling the SIPPI with the PVEC electrolyte. Benefiting from this unique design, the Li symmetrical cell with the SIPPI shows stable cycling over 6000 h at 3 mA cm^-2, and the full cell with the SIPPI exhibits stable cycling performance with a capacity retention of 86% over 1000 cycles at 1 C and 25 °C. This incorporated SIPPI on the Li anode presents an alternative strategy for enabling high-energy density, long cycling lifetime, and safe and cost-effective solid-state batteries.

Unraveling the Role of Neutral Units for Single-Ion Conducting Polymer Electrolytes

With the cationic transference number close to unity, single-ion conducting polymer electrolytes (SICPEs) are recognized as an advanced electrolyte system with improved energy efficiency for battery application. The relatively low ionic conductivity for most of the SICPEs in comparison with liquid electrolytes remains the major "bottleneck" for their practical applications. Polyethylene oxide (PEO) has been recognized as a benchmark for solid polymer electrolytes due to its high salt solubility and reasonable ionic conductivity. PEO has two advantages: (i) the polar ether groups coordinate well with lithium ions (Li⁺) providing good dissociation from anions, and (ii) the low T_g provides fast segmental dynamics at ambient temperature and assists rapid charge transport. These properties lead to active use of PEO as neutral plasticizing units in SICPEs. Herein, we present a detailed comparison of new SICPEs copolymerized with PEO units vs SICPEs copolymerized with other types of neutral units possessing either flexible or polar structures. The presented analysis revealed that the polarity of side chains has a limited influence on ion dissociation for copolymer-type SICPEs. The Li⁺-ion dissociation seems to be controlled by the charge delocalization on the polymerized anion. With good miscibility between plasticizing neutral units and ionic conductive units, the ambient ionic conductivity of synthesized SICPEs is still mainly controlled by the T_g of the copolymer. This work sheds light on the dominating role of PEO in SICPE systems and provides helpful guidance for designing polymer electrolytes with new functionalities and structures. Furthermore, based on the presented results, we propose that designing polyanions with a highly delocalized charge may be another promising route for achieving sufficient lithium ionic conductivity in solvent-free SICPEs.

Remarkable Conductivity of a Self-Healing Single-Ion Conducting Polymer Electrolyte, Poly(ethylene-co-acrylic lithium (fluoro sulfonyl)imide), for All-Solid-State Li-Ion Batteries

Single-ion conducting polymer electrolyte (SICPE) is a safer alternative to the conventional high-performance liquid electrolyte for Li-ion batteries. The performance of SICPEs-based Li-ion batteries is limited due to the low Li⁺ conductivities of SICPEs at room temperature. Herein, we demonstrated the synthesis of a novel SICPE, poly(ethylene-co-acrylic lithium (fluoro sulfonyl)imide) (PEALiFSI), with acrylic (fluoro sulfonyl)imide anion (AFSI). The solvent- and plasticizer-free PEALiFSI electrolyte, which was assembled at 90 °C under pressure, exhibited self-healing properties with remarkably high Li⁺ conductivity (5.84 × 10^-4 S cm^-1 at 25 °C). This is mainly due to the self-healing behavior of this electrolyte, which induced to increase the proportion of the amorphous phase. Additionally, the weak interaction of Li⁺ with the resonance-stabilized AFSI anion is also responsible for high Li⁺ conductivity. This self-healed SICPE showed high Li⁺ transference number (ca. 0.91), flame and heat retardancy, and good thermal stability, which concurrently delivered ca. 88.25% (150 mAh g^-1 at 0.1C) of the theoretical capacitance of LiFePO₄ cathode material at 25 °C with the full-cell configuration of LiFePO₄/PEALiFSI/graphite. Furthermore, the self-healed PEALiFSI-based all-solid-state Li battery showed high electrochemical cycling stability with the capacity retention of 95% after 500 charge-discharge cycles.

Bioinspired Redox-Active Catechol-Bearing Polymers as Ultrarobust Organic Cathodes for Lithium Storage

Redox-active catechols are bioinspired precursors for ortho-quinones that are characterized by higher discharge potentials than para-quinones, the latter being extensively used as organic cathode materials for lithium ion batteries (LIBs). Here, this study demonstrates that the rational molecular design of copolymers bearing catechol- and Li⁺ ion-conducting anionic pendants endow redox-active polymers (RAPs) with ultrarobust electrochemical energy storage features when combined to carbon nanotubes as a flexible, binder-, and metal current collector-free buckypaper electrode. The importance of the structure and functionality of the RAPs on the battery performances in LIBs is discussed. The structure-optimized RAPs can store high-capacities of 360 mA h g^-1 at 5C and 320 mA h g^-1 at 30C in LIBs. The high ion and electron mobilities within the buckypaper also enable to register 96 mA h g^-1 (24% capacity retention) at an extreme C-rate of 600C (6 s for total discharge). Moreover, excellent cyclability is noted with a capacity retention of 98% over 3400 cycles at 30C. The high capacity, superior active-material utilization, ultralong cyclability, and excellent rate performances of RAPs-based electrode clearly rival most of the state-of-the-art Li⁺ ion organic cathodes, and opens up new horizons for large-scalable fabrication of electrode materials for ultrarobust Li storage.