Energy density is the main property of rechargeable batteries that has driven the entire technology forward in past decades. Lithium-ion batteries (LIBs) now surpass other, previously competitive battery types (for example, lead–acid and nickel metal hydride) but still require extensive further improvement to, in particular, extend the operation hours of mobile IT devices and the driving mileages of all-electric vehicles. In this Review, we present a critical overview of a wide range of post-LIB materials and systems that could have a pivotal role in meeting such demands. We divide battery systems into two categories: near-term and long-term technologies. To provide a realistic and balanced perspective, we describe the operating principles and remaining issues of each post-LIB technology, and also evaluate these materials under commercial cell configurations. Post-lithium-ion batteries are reviewed with a focus on their operating principles, advantages and the challenges that they face. The volumetric energy density of each battery is examined using a commercial pouch-cell configuration to evaluate its practical significance and identify appropriate research directions.

Promise and reality of post-lithium-ion batteries with high energy densities

Due to their high energy density and low material cost, lithium–sulfur batteries represent a promising energy storage system for a multitude of emerging applications, ranging from stationary grid storage to mobile electric vehicles. This review aims to summarize major developments in the field of lithium–sulfur batteries, starting from an overview of their electrochemistry, technical challenges and potential solutions, along with some theoretical calculation results to advance our understanding of the material interactions involved. Next, we examine the most extensively-used design strategy: encapsulation of sulfur cathodes in carbon host materials. Other emerging host materials, such as polymeric and inorganic materials, are discussed as well. This is followed by a survey of novel battery configurations, including the use of lithium sulfide cathodes and lithium polysulfide catholytes, as well as recent burgeoning efforts in the modification of separators and protection of lithium metal anodes. Finally, we conclude with an outlook section to offer some insight on the future directions and prospects of lithium–sulfur batteries.

Designing high-energy lithium–sulfur batteries

Poly(ethylene oxide) (PEO) based materials are widely considered as promising candidates of polymer hosts in solid-state electrolytes for high energy density secondary lithium batteries. They have several specific advantages such as high safety, easy fabrication, low cost, high energy density, good electrochemical stability, and excellent compatibility with lithium salts. However, the typical linear PEO does not meet the production requirement because of its insufficient ionic conductivity due to the high crystallinity of the ethylene oxide (EO) chains, which can restrain the ionic transition due to the stiff structure especially at low temperature. Scientists have explored different approaches to reduce the crystallinity and hence to improve the ionic conductivity of PEO-based electrolytes, including blending, modifying and making PEO derivatives. This review is focused on surveying the recent developments and issues concerning PEO-based electrolytes for lithium-ion batteries.

Poly(ethylene oxide)-based electrolytes for lithium-ion batteries

Owing to high specific energy, low cost, and environmental friendliness, lithium–sulfur (Li–S) batteries hold great promise to meet the increasing demand for advanced energy storage beyond portable electronics, and to mitigate environmental problems. However, the application of Li–S batteries is challenged by several obstacles, including their short life and low sulfur utilization, which become more serious when sulfur loading is increased to the practically accepted level above 3–5 mg cm−2. More and more efforts have been made recently to overcome the barriers toward commercially viable Li–S batteries with a high sulfur loading. This review highlights the recent progress in high-sulfur-loading Li–S batteries enabled by hierarchical design principles at multiscale. Particularly, basic insights into the interfacial reactions, strategies for mesoscale assembly, unique architectures, and configurational innovation in the cathode, anode, and separator are under specific concerns. Hierarchy in the multiscale design is proposed to guide the future development of high-sulfur-loading Li–S batteries.

Review on High-Loading and High-Energy Lithium–Sulfur Batteries

Ultrathin two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDs), such as MoS2, TiS2, TaS2, WS2, MoSe2, WSe2, etc., are emerging as a class of key materials in chemistry and electronics due to their intriguing chemical and electronic properties. The ability to prepare these TMD nanosheets in high yield and large scale via various methods has led to increasing studies on their hybridization with other materials to create novel functional composites, aiming to engineer their chemical, physical and electronic properties and thus achieve good performance for some specific applications. In this critical review, we will introduce the recent progress in hybrid nanoarchitectures based on 2D TMD nanosheets. Their synthetic strategies, properties and applications are systematically summarized and discussed, with emphasis on those new appealing structures, properties and functions. In addition, we will also give some perspectives on the challenges and opportunities in this promising research area.

Two-dimensional transition metal dichalcogenide nanosheet-based composites.

An excess of elemental sulfur is generated annually from hydrodesulfurization in petroleum refining processes; however, it has a limited number of uses, of which one example is the production of sulfuric acid. Despite this excess, the development of synthetic and processing methods to convert elemental sulfur into useful chemical substances has not been investigated widely. Here we report a facile method (termed ‘inverse vulcanization’) to prepare chemically stable and processable polymeric materials through the direct copolymerization of elemental sulfur with vinylic monomers. This methodology enabled the modification of sulfur into processable copolymer forms with tunable thermomechanical properties, which leads to well-defined sulfur-rich micropatterned films created by imprint lithography. We also demonstrate that these copolymers exhibit comparable electrochemical properties to elemental sulfur and could serve as the active material in Li–S batteries, exhibiting high specific capacity (823 mA h g−1 at 100 cycles) and enhanced capacity retention. A polymerization method for converting elemental sulfur into a chemically stable, processable and electrochemically active copolymer has been described. This methodology — termed inverse vulcanization — is conducted by a one-step process using liquid sulfur, as both reaction medium and reactant, and vinylic comonomers to form polymeric materials with a high content of sulfur (50–90 wt%).

The use of elemental sulfur as an alternative feedstock for polymeric materials

A shape-programmable liquid crystal elastomer comprises polymerized, nematic monomers. The monomers are organized into a plurality of voxels with each voxel having a director orientation.

http://science.sciencemag.org/content/sci/347/6225/982.full.pdf

Voxelated liquid crystal elastomers

A supramolecular polymer blend, formed via π–π interactions between a π-electron rich pyrenyl end-capped oligomer and a chain-folding oligomer containing pairs of π-electron poor naphthalene-diimide (NDI) units, has been reinforced with cellulose nanocrystals (CNCs) to afford a healable nanocomposite material. Nanocomposites with varying weight percentage of CNCs (from 1.25 to 20.0 wt %) within the healable supramolecular polymeric matrix have been prepared via solvent casting followed by compression molding, and their mechanical properties and healing behavior have been evaluated. It is found that homogeneously dispersed films can be formed with CNCs at less than 10 wt %. Above 10 wt % CNC heterogeneous nanocomposites were obtained. All the nanocomposites formed could be rehealed upon exposure to elevated temperatures although, for the homogeneous films, it was found that the healing rate was reduced with increasing CNC content. The best combination of healing efficiency and mechanical properties was obt...

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High-Strength, Healable, Supramolecular Polymer Nanocomposites

The demand for soft robots urges the development of new light-responsive materials for remotely powered actuation. Here, Wie et al. show directional motion over centimeter scales using azobenzene-functionalized liquid crystalline polymer films upon continuous radiation from ultraviolet to visible light.

Photomotility of polymers.

High porosity combined with mechanical durability in conductive materials is in high demand for special applications in energy storage under limiting conditions, and it is fundamentally important for establishing a relationship between the structure/chemistry of these materials and their properties. Herein, polymer-assisted self-assembly and cross-linking are combined for reduced graphene oxide (rGO)-based aerogels with reversible compressibility, high elasticity, and extreme durability. The strong interplay of cross-linked rGO (x-rGO) aerogels results in high porosity and low density due to the re-stacking inhibition and steric hinderance of the polymer chains, yet it makes mechanical durability and structural bicontinuity possible even under compressive strains because of the coupling of directional x-rGO networks with polymer viscoelasticity. The x-rGO aerogels retain >140% and >1400% increases in the gravimetric and volumetric capacitances, respectively, at 90% compressive strain, showing reversible change and stability of the volumetric capacitance under both static and dynamic compressions; this makes them applicable to energy storage devices whose volume and mass must be limited.

Jeong Jae Wie

Papers

The use of elemental sulfur as an alternative feedstock for polymeric materials

Voxelated liquid crystal elastomers

High-Strength, Healable, Supramolecular Polymer Nanocomposites

Photomotility of polymers.

Reversibly Compressible, Highly Elastic, and Durable Graphene Aerogels for Energy Storage Devices under Limiting Conditions