Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Applications of 2D MXenes in energy conversion and storage systems

Enabling the reversible lithium metal electrode is essential for surpassing the energy content of today’s lithium-ion cells. Although lithium metal cells for niche applications have been developed already, efforts are underway to create rechargeable lithium metal batteries that can significantly advance vehicle electrification and grid energy storage. In this Perspective, we focus on three tasks to guide and further advance the reversible lithium metal electrode. First, we summarize the state of research and commercial efforts in terms of four key performance parameters, and identify additional performance parameters of interest. We then advocate for the use of limited lithium (≤30 μm) to ensure early identification of technical challenges associated with stable and dendrite-free cycling and a more rapid transition to commercially relevant designs. Finally, we provide a cost target and outline material costs and manufacturing methods that could allow lithium metal cells to reach 100 US$ kWh–1. Li metal batteries offer much hope for the future of high-energy storage systems. Albertus et al. survey the current status of research and commercial efforts, and discuss key metrics and measurements as well as cost issues in enabling high-performing lithium metal electrodes.

Status and challenges in enabling the lithium metal electrode for high-energy and low-cost rechargeable batteries

Review on solid electrolytes for all-solid-state lithium-ion batteries

A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

The world of two-dimensional carbides and nitrides (MXenes)

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This paper describes a sintering technique for ceramics and ceramic-based composites, using water as a transient solvent to effect densification (i.e. sintering) at temperatures between room temperature and 200 °C. To emphasize the incredible reduction in sintering temperature relative to conventional thermal sintering this new approach is named the "Cold Sintering Process" (CSP). Basically CSP uses a transient aqueous environment to effect densification by a mediated dissolution-precipitation process. CSP of NaCl, alkali molybdates and V2 O5 with small concentrations of water are described in detail, but the process is extended and demonstrated for a diverse range of chemistries (oxides, carbonates, bromides, fluorides, chlorides and phosphates), multiple crystal structures, and multimaterial applications. Furthermore, the properties of selected CSP samples are demonstrated to be essentially equivalent as samples made by conventional thermal sintering.

Cold Sintering: A Paradigm Shift for Processing and Integration of Ceramics

Research on sintering of dense ceramic materials has been very active in the past decades and still keeps gaining in popularity. Although a number of new techniques have been developed, the sintering process is still performed at high temperatures. Very recently we established a novel protocol, the “Cold Sintering Process (CSP)”, to achieve dense ceramic solids at extraordinarily low temperatures of <300°C. A wide variety of chemistries and composites were successfully densified using this technique. In this article, a comprehensive CSP tutorial will be delivered by employing three classic ferroelectric materials (KH2PO4, NaNO2, and BaTiO3) as examples. Together with detailed experimental demonstrations, fundamental mechanisms, as well as the underlying physics from a thermodynamics perspective, are collaboratively outlined. Such an impactful technique opens up a new way for cost-effective and energy-saving ceramic processing. We hope that this article will provide a promising route to guide future studies on ultralow temperature ceramic sintering or ceramic materials related integration.

Cold Sintering Process: A Novel Technique for Low‐Temperature Ceramic Processing of Ferroelectrics

A new ultra-low-temperature firing microwave dielectric ceramic, Li2WO4 with the phenacite structure, was prepared via solid-state reaction method. The Li2WO4 ceramic can be well sintered at 640°–660°C, with a microwave relative permittivity ∼5.5, a Q×f value about 62 000 GHz, and a negative temperature coefficient of −146 ppm/°C at 15.7 GHz. From an X-ray diffraction analysis, the Li2WO4 ceramic was found to be chemically compatible with both silver and aluminum powders at 640°C. All the results indicate that the Li2WO4 ceramic is a promising candidate for ultra-low temperature cofired ceramic technology, especially for dielectric substrate application.

Microwave Dielectric Properties of Li2WO4 Ceramic with Ultra-Low Sintering Temperature

Co-sintering ceramic and thermoplastic polymer composites in a single step with very high volume fractions of ceramics seems unlikely, given the vast differences in the typical sintering temperatures of ceramics versus polymers. These processing limitations are overcome with the introduction of a new sintering approach, namely “cold sintering process” (CSP). CSP utilizes a transient low temperature solvent, such as water or water with dissolved solutes in stoichiometric ratios consistent with the ceramic composition, to control the dissolution and precipitation of ceramics and effect densification between room temperature and ≈200 °C. Under these conditions, thermoplastic polymers and ceramic materials can be jointly formed into dense composites. Three diphasic composite examples are demonstrated to show the overall diversity of composite material design between organic and inorganic oxides, including the microwave dielectric Li2MoO4–(C2F4 ) n , electrolyte Li1.5Al0.5Ge1.5(PO4)3–(CH2CF2 ) x [CF2CF(CF3)] y , and semiconductor V2O5–poly(3,4-ethylenedioxythiophene) polystyrene sulfonate composites. Cold sintering is more general and shall have a major impact on the processing of composite materials for many different applications, mechanical, thermal, and electronic, to mention a few possibilities. CSP concepts open up new composite material design and device integration schemes, impacting a wide variety of applications.

Cold Sintering Process of Composites: Bridging the Processing Temperature Gap of Ceramic and Polymer Materials

With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N = 3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process

Jing Guo

Papers

Cold Sintering: A Paradigm Shift for Processing and Integration of Ceramics

Cold Sintering Process: A Novel Technique for Low‐Temperature Ceramic Processing of Ferroelectrics

Microwave Dielectric Properties of Li2WO4 Ceramic with Ultra-Low Sintering Temperature

Cold Sintering Process of Composites: Bridging the Processing Temperature Gap of Ceramic and Polymer Materials

Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics