Flexible Solid-state Batteries Research Articles

ZIF-8-derived N-doped porous carbon wrapped in porous carbon films as an air cathode for flexible solid-state Zn-air batteries

Metal-organic frameworks are a new type of catalyst precursor with high specific surface area and controllable composition, which can be modified by post-treatment and are suitable for use as cathode catalysts for Zn-air batteries (ZABs). Here, a self-doped nitrogen nanocatalyst (N-PC@CF) with a double-layered porous structure is rationally designed for flexible solid-state ZABs. The outer porous carbon shell of the N-PC@CF is highly hydrophilic and O2 permeable, while the layered porous structure exposes sufficient active sites to shorten the mass transfer distance, which would promote electrocatalytic performance and increase flexibility efficiently. The obtained N-PC@CF has an onset potential of 0.926 V and a half-wave potential of 0.843 V in the oxygen reduction reaction test, which is equivalent to commercial Pt/C. Most importantly, the maximum power density of the assembled ZAB is 134.7 mW cm−2 and it exhibits a specific capacity of 776.8 mA h g−1 at 10 mA cm−2, which is better than the 99.9 mW cm−2 of the Pt/C-based battery. An obvious improvement in the constant current discharge–charge cycle durability of the ZAB is found when compared with Pt/C. The specific capacities of ZAB with N-PC@CF as the air cathode at 5, 10 and 15 mA cm−2 are 842.7, 776.8 and 715.0 mAh g−1 (calculated by the mass of zinc consumed), respectively, corresponding to high energy densities of 1089.7, 977.3 and 842.2 Wh kg−1. A flexible solid-state battery is assembled with excellent flexibility and stability, even if the battery is folded into a large angle (160°). This work provides a new strategy for the design and synthesis of metal-free air cathodes.

Antifreezing Zwitterionic-Based Hydrogel Electrolyte for Aqueous Zn Ion Batteries

Hydrogel electrolytes have attracted great attention because the properties of integrated conductivity and mechanical flexibility endow them with potential applications in flexible solid-state batteries. However, hydrogels are easy to freeze when the temperature drops below zero, resulting in the reduction of ionic conductivity and mechanical strength of hydrogels. Herein, an antifreezing polymer hydrogel electrolyte was developed by introducing a zwitterionic group and salt into the double-network polymer. The polymer hydrogel electrolyte possessed a perfect ionic conductivity of 10.38 mS cm–1 at −20 °C and could remain unfrozen until −50 °C. A flexible solid-state Zn//LiFePO4 battery assembled with the prepared hydrogel electrolyte exhibited a discharge capacity of 126.1 and 75.1 mAh g–1 at 20 and −20 °C at 1 C, respectively. In addition, it showed quite perfect cycling performance and maintained a capacity retention of 72.6% after 1000 cycles with a favorable Coulombic efficiency (∼100%) at −20 °C at 1 C. The hydrogel electrolyte with perfect antifreezing performance is supposed to broaden the range of working temperature and practical applications of the zinc ion battery.

Enabling high-energy flexible solid-state lithium ion batteries at room temperature

Flexible solid-state batteries (FSSBs) are indispensable energy storage devices to fulfil the energy and safety requirements for future flexible applications. The bottlenecks of FSSBs are how to realize high energy density with competent ionic conductivity for room-temperature (RT) flexible applications. Here, the first fabrication of RT FSSB with high energy density is reported, which is realized by in situ integration of a 20-µm-thick hybrid polymer/ceramic/ionic liquid solid-state electrolyte (SSE) between the high energy combination of anode/cathode electrodes. The in situ electrode/electrolyte interfacial integration strategy provides an ultrathin SSE layer, ultralow resistance and superior flexibility, and the SSE guarantees both high ionic conductivity and good compatibility with high-energy cathode LiNi0.8Co0.1Mn0.1O2 (NCM811). The fabricated Li4Ti5O12/NCM811 FSSB delivers super-low resistance approaching conventional liquid cells and excellent cycling stability up to 600 cycles at RT. The extension of anode to SiOx@graphite leads to a high theoretical energy density of 489.6 Wh kg−1 at material’s level, times higher than current options. In addition, the RT FSSB shows great flexibility, indicating a high performance application in future flexible electronics.

A Stretchable and Safe Polymer Electrolyte with a Protecting-Layer Strategy for Solid-State Lithium Metal Batteries.

An elastic and safe electrolyte is demanded for flexible batteries. Herein, a stretchable solid electrolyte comprised of crosslinked elastic polymer matrix, poly(vinylidene fluoride‐hexafluoropropylene) (PVDF‐HFP), and flameproof triethyl phosphate (TEP) is fabricated, which exhibits ultrahigh elongation of 450%, nonflammability and ionic conductivity above 1 mS cm−1. In addition, in order to improve the interface compatibility between the electrolyte and Li anode and stabilize the solid‐electrolyte interphase (SEI), a protecting layer containing poly(ethylene oxide) (PEO) is designed to effectively prevent the anode from reacting with TEP and optimize the chemical composition in SEI, leading to a tougher and more stable SEI on the anode. The LiFePO4/Li cells employing this double‐layer electrolyte exhibit an 85.0% capacity retention after 300 cycles at 1 C. Moreover, a flexible battery based on this solid electrolyte is fabricated, which can work in stretched, folded, and twisted conditions. This design of a stretchable double‐layer solid electrolyte provides a new concept for safe and flexible solid‐state batteries.

Ultrathin rechargeable all-solid-state batteries based on monolayer graphene

The energy and power requirements of portable electronic devices and electric vehicles are ever increasing, driving research into novel battery structures with increased volumetric energy and power densities. Existing energy storage technologies cannot satisfy both of these requirements. There are many reports on the application of graphene in batteries and supercapacitors with enhanced power and energy densities; however, few results were shown on the performance of an electrochemical energy storage device made of monolayer graphene. The energy storage capability of monolayer graphene is investigated in this paper and it can contribute an understanding of the application of graphene materials in high energy and power density batteries. In parallel, flexible solid-state batteries will relax design constraints, giving the freedom to create new device form factors. A mechanically flexible all-solid state battery can be made of monolayer graphene grown by chemical vapour deposition (CVD) directly onto copper (Cu) foil. The total thickness of the resulting battery was ∼50 μm. Such an ultrathin battery showed the highest energy density of 10 W h L−1 and the highest power density of 300 W L−1. It also shows excellent cyclic stability and sustains a discharge current density of 100 μA cm−2 over 100 cycles, maintaining energy capacity over 0.02 mA h cm−2.