Single-Ion Conductive Polymer-Based Composite Electrolytes for High-Performance Solid-State Lithium Metal Batteries.

Flexible composite polymer electrolytes (CPEs) with inorganic electrolyte fillers dispersed in polymer electrolytes integrate the merits of the polymer and inorganic electrolytes and have attracted much attention in recent years. In order to increase the electrochemical performance, especially the low lithium (Li)-ion transference number in traditional dual-ion Li salt-containing CPEs, single-ion conductive CPEs are synthesized with a single-ion polymer conductor (SIPC) as the matrix and Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) particles as the active fillers. The single-ion conductive CPEs show a high Li-ion transference number (up to 0.96), high room-temperature (RT) ionic conductivity (>1.0 × 10-4 S cm-1 ), wide electrochemical stability window (>5.0 V, vs Li/Li+ ), and excellent long-term cycling stability with Li metal at RT (3200 h). Based on the SIPC-LLZTO CPE, the solid-state lithium metal batteries with LiFePO4 - and LiCoO2 -based cathodes deliver average discharge capacities of 159 mAh g-1 for 600 cycles and 119 mAh g-1 for 200 cycles at RT, respectively. This study sheds light on the design of high-performance CPEs for next-generation solid-state lithium metal batteries.

An initiator-free and solvent-free in-situ self-catalyzed crosslinked polymer electrolyte for all-solid-state lithium-metal batteries.

Linear polymer (e.g. polyethylene oxide, PEO) based electrolytes have been widely studied due to their flexibility and relatively good contact against electrodes. However, the linear polymers are prone to crystallization at room temperature and melting at moderate temperature, restricting their application in lithium metal batteries. To address these problems, a self-catalyzed crosslinked polymer electrolyte (CPE) was designed and prepared by the reaction of poly (ethylene glycol diglycidyl ether) (PEGDGE) and polyoxypropylenediamine (PPO) with only the bistrifluoromethanesulfonimide lithium salt (LiTFSI) added and with no any initiators. LiTFSI catalyzed the reaction by reducing the activation energy to form a crosslinked network structure, which was identified by calculation, NMR and FTIR. The as-prepared CPE has high resilience and a low glass transition temperature (Tg = -60 °C). Meanwhile, the solvent-free in-situ polymerization technique has been adopted in the assembly of the CPE with electrodes to decrease the interfacial impedance greatly and improve the ionic conductivity to 2.05 × 10-5 S cm-1 and 2.55 × 10-4 S cm-1 at room temperature and 75 °C, respectively. As a result, the in-situ LiFeO4/CPE/Li battery exhibits outstanding thermal and electrochemical stability at 75 °C. Our work has proposed an initiator-free and solvent-free in-situ self-catalyzed strategy of preparing high performance crosslinked solid polymer electrolytes.

A high performance composite separator with robust environmental stability for dendrite-free lithium metal batteries.

The garnet ceramic Li6.4La3Zr1.4Ta0.6O12 (LLZTO) modified separators have been proposed to overcome the poor thermal stability and wettability of commercial polyolefin separators. However, the side reaction of LLZTO in the air leads to deterioration of environmental stability of composite separators (PP-LLZTO), which will limit the electrochemical performance of batteries. Herein, the LLZTO with the polydopamine (PDA) coating (LLZTO@PDA) was prepared by solution oxidation, and then applied it to a commercial polyolefin separator to achieve a composite separator (PP-LLZTO@PDA). LLZTO@PDA is stable in the air, and no Li2CO3 can be observed on the surface even after 90 days in the air. Besides, LLZTO@PDA coating endows the PP-LLZTO@PDA separator with the tensile strength (up to 103 MPa), good wettability (contact angle 0°) and high ionic conductivity (0.93 mS cm-1). Consequently, the Li/PP-LLZTO@PDA/Li symmetric cell cycles stably for 600 h without significant dendrites generation, and the assembled Li//LFP cells with PP-LLZTO@PDA-D30 separators deliver a high capacity retention of 91.8% after 200 cycles at 0.1C. This research provides a practical strategy for constructing composite separators with excellent environmental stability and high electrochemical properties.

Effects of Molecular Weight on the Electrochemical Properties of Poly(vinylidene difluoride)-Based Polymer Electrolytes.

Polymer-based electrolytes have attracted ever-increasing attention for solid-state batteries due to their excellent flexibility and processability. Among them, poly(vinylidene difluoride) (PVDF)-based electrolytes with high ionic conductivity, wide electrochemical stability window, and good mechanical properties show great potential and have been widely investigated by using different Li salts, solvents, and inorganic fillers. Here, we report the influence of the molecular weight of PVDF itself on the electrochemical properties of the electrolytes by using two kinds of common PVDF polymers, i.e., PVDF 761 and 5130. Our results demonstrate that the electrolyte with a larger molecular weight (PVDF 5130) has a denser structure and lower crystallinity, and thus much better electrochemical performance, than one with a smaller molecular weight (PVDF 761). With PVDF 5130, the LiFePO4-based solid-state cells present a steady cycling performance with a capacity retention of 85% after 1000 cycles at 1 C and 30 °C. The cycle life of the LiCoO2-based solid-state cells is also extended by using PVDF 5130.

Self-Enhancing Gel Polymer Electrolyte by In Situ Construction for Enabling Safe Lithium Metal Battery.

Lithium metal battery (LMB) possessing a high theoretical capacity is a promising candidate of advanced energy storage devices. However, its safety and stability are challenged by lithium dendrites and the leakage of liquid electrolyte. Here, a self-enhancing gel polymer electrolyte (GPE) is created by in situ polymerizing 1,3-dioxolane (DOL) in the nanofibrous skeleton for enabling safe LMB. The nanofiber membrane possesses a better affinity with poly-DOL (PDOL) than commercial separator for constructing homogeneous GPE with enhanced ion conductivity. Furthermore, polydopamine is introduced on nanofiber membrane to form hydrogen bonding with PDOL and bis((trifluoromethyl)sulfonyl)imide anion, dramatically improving the mechanical strength, ionic conductivity, and transference number of GPE. Besides, molecular dynamic simulation is used to reveal the intrinsic factors of high ionic conductivity and reinforcing effect in the meantime. Consequently, the LiFePO4 //Li batteries using self-enhancing GPE show extraordinary cyclic stability over 800 cycles under high current density of 2 C, with a capacity decay of 0.021% per cycle, effectively suppressing the growth of lithium dendrites. This ingenious strategy is expected to manufacture advanced performance and high safety LMBs and compatible with the current battery production.

Emerging trends in anion storage materials for the capacitive and hybrid energy storage and beyond.

Electrochemical capacitors charge and discharge more rapidly than batteries over longer cycles, but their practical applications remain limited due to their significantly lower energy densities. Pseudocapacitors and hybrid capacitors have been developed to extend Ragone plots to higher energy density values, but they are also limited by the insufficient breadth of options for electrode materials, which require materials that store alkali metal cations such as Li+ and Na+. Herein, we report a comprehensive and systematic review of emerging anion storage materials for performance- and functionality-oriented applications in electrochemical and battery-capacitor hybrid devices. The operating principles and types of dual-ion and whole-anion storage in electrochemical and hybrid capacitors are addressed along with the classification, thermodynamic and kinetic aspects, and associated interfaces of anion storage materials in various aqueous and non-aqueous electrolytes. The charge storage mechanism, structure-property correlation, and electrochemical features of anion storage materials are comprehensively discussed. The recent progress in emerging anion storage materials is also discussed, focusing on high-performance applications, such as dual-ion- and whole-anion-storing electrochemical capacitors in a symmetric or hybrid manner, and functional applications including micro- and flexible capacitors, desalination, and salinity cells. Finally, we present our perspective on the current impediments and future directions in this field.

Layered Double Hydroxide Quantum Dots for Use in a Bifunctional Separator of Lithium-Sulfur Batteries.

Functional separators, which are chemically modified and coated with nanostructured materials, are considered an effective and economical approach to suppressing the shuttle effect of lithium polysulfide (LiPS) and promoting the conversion kinetics of sulfur cathodes. Herein, we report cobalt-aluminum-layered double hydroxide quantum dots (LDH-QDs) deposited with nitrogen-doped graphene (NG) as a bifunctional separator for lithium-sulfur batteries (LSBs). The mesoporous LDH-QDs/NG hybrids possess abundant active sites of Co2+ and hydroxide groups, which result in capturing LiPSs through strong chemical interactions and accelerating the redox kinetics of the conversion reaction, as confirmed through X-ray photoelectron spectroscopy, adsorption tests, Li2S nucleation tests, and electrokinetic analyses of the LiPS conversion. The resulting LDH-QDs/NG hybrid-coated polypropylene (LDH-QDs/NG/PP) separator, with an average thickness of ∼17 µm, has a high ionic conductivity of 2.67 mS cm-1. Consequently, the LSB cells with the LDH-QDs/NG/PP separator can deliver a high discharge capacity of 1227.48 mAh g-1 at 0.1C along with a low capacity decay rate of 0.041% per cycle over 1200 cycles at 1.0C.

Structural Engineering of Ultrathin ReS2 on Hierarchically Architectured Graphene for Enhanced Oxygen Reduction.

Herein, binary heteronanosheets made of ultrathin ReS2 nanosheets and reduced graphene oxide (RGO) with either a two-dimensional (2D) "sheet-on-sheet" architecture (2D ReS2/RGO) or a three-dimensional hierarchical structure (3D ReS2/RGO) are constructed through rational structure-engineering strategies. In the resultant 3D ReS2/RGO heteronanosheets, the ultrathin ReS2 nanosheets are bridged on the RGO surface through Re-O bonds in a vertically oriented manner, which endows the heteronanosheets with open frameworks and a hierarchical porous structure. In sharp contrast to the 2D ReS2/RGO, the 3D ReS2/RGO heteronanosheets are featured with abundant active sites and channels for efficient electrolyte ions transport. This, coupled with the strong affinity toward oxygen-containing intermediates intrinsically associated with the binary ReS2/RGO structure, imparts excellent oxygen reduction performance to the 3D ReS2/RGO heteronanosheets for potential applications in fuel cells and metal-air batteries.

All-in-One MoS2 Nanosheets Tailored by Porous Nitrogen-Doped Graphene for Fast and Highly Reversible Sodium Storage.

Though being a promising anode material for sodium-ion batteries (SIBs), MoS2 with high theoretical capacity shows poor rate capability and rapid capacity decay, especially involving the conversion of MoS2 to Mo metal and Na2S. Here, we report all-in-one MoS2 nanosheets tailored by porous nitrogen-doped graphene (N-RGO) for the first time to achieve superior structural stability and high cycling reversibility of MoS2 in SIBs. The all-in-one MoS2 nanosheets possess desirable structural characteristics by admirably rolling up all good qualities into one, including vertical alignment, an ultrathin layer, vacancy defects, and expanded layer spacing. Thus, the all-in-one MoS2@N-RGO composite anode exhibits an improvement in the charge transport kinetics and availability of active materials in SIBs, resulting in outstanding cycling and rate performance. More importantly, the restricted growth of all-in-one MoS2 by the porous N-RGO via a strong coupling effect dramatically improves the cycling reversibility of conversion reaction. Consequently, the all-in-one MoS2@N-RGO composite anode demonstrates excellent reversible capacity, outstanding rate capability, and superior cycling stability. This study strongly suggests that the all-in-one MoS2@N-RGO has great potential for practical application in high-performance SIBs.

Flexible PANI/SWCNT thermoelectric films with ultrahigh electrical conductivity.

The effects of polyaniline (PANI) with different polymerization times on the film-forming and thermoelectric properties as well as on the performance of SWCNTs/PANI composites were systematically investigated in this study. It was found that the film-forming and flexibility of PANI films improved with the increase in polymerization time. We showed that a super high conductivity of ∼4000 S cm-1 can be achieved for the SWCNTs/PANI composite film, which is the highest value for the SWCNTs/PANI system at present. Both the electrical conductivity and power factor increase by an order of magnitude than that of pure PANI films and far exceed the theoretical value of the mixture model. These results suggest that the sufficiently continuous and ordered regions on the interlayer between the filler and matrix are key to improve the electrical conductivity of composites. Finally, the maximum PF reaches 100 µW m-1 K-2 at 410 K for the 0.6CNT/PANI5h. Furthermore, it is found that the composite films have excellent environmental and structural stability. Our results can deepen the understanding of organic-inorganic thermoelectric composite systems and facilitate the practical application of flexible and wearable thermoelectric materials.

Bio-inspired spider-web-like membranes with a hierarchical structure for high performance lithium/sodium ion battery electrodes: the case of 3D freestanding and binder-free bismuth/CNF anodes.

High gravimetric energy density and volumetric energy density energy storage devices are highly desirable due to the rapid development of electric vehicles, and portable and wearable electronic equipment. Electrospinning is a promising technology for preparing freestanding electrodes with high gravimetric and volumetric energy density. However, the energy density of the traditional electrospun electrodes is restricted by the low mass loading of active materials (e.g. 20%-30 wt%). Herein, a biomimetic strategy inspired by the phenomenon of the sticky spider web is demonstrated as a high performance anode, which simultaneously improves the gravimetric and volumetric energy density. Freestanding carbon nanofiber (CNF) membranes containing over 50 wt% of bismuth were prepared by electrospinning and subsequent thermal treatment. Membranes consisting of CNF network structures bonded tightly with active Bi cluster materials, resulting in excellent mechanical protection and a fast charge transport path, which are difficult to achieve simultaneously. The composite membrane delivers high reversible capacity (483 mA h g-1 at 100 mA g-1 after 200 cycles) and high rate performance (242 mA h g-1 at 1 A g-1) as a lithium-ion battery anode. For use as a sodium ion battery, the composite membrane also shows a high reversible specific capacity of 346 mA h g-1 and outstanding cycling performance (186 mA h g-1 at 50 mA g-1 after 100 cycles). This work offers a simple, low cost and eco-friendly method for fabricating free-standing and binder-free composite electrodes with high loading used in LIBs and SIBs.