Thermoelectric energy conversion can be used to capture electric power from waste heat in a variety of applications. The materials that have been shown to have the best thermoelectric properties are compounds containing elements such as tellurium and antimony. These compounds can be oxidized if exposed to the high temperature air that may be present in heat recovery applications. Oxide materials have better stability in oxidizing environments, so their use enables the fabrication of more durable devices. Thus, although the thermoelectric properties of oxides are inferior to those of the compounds mentioned above, their superior stability may expand potential the high temperature application of thermoelectric energy conversion. In this paper, the thermoelectric properties of promising oxide materials are reviewed. The different types of oxides used for thermoelectric applications are compared and approaches for improving performance through doping are discussed.

Oxide materials for high temperature thermoelectric energy conversion

Although layered lithium oxides have become the cathode of choice for state-of-the-art Li-ion batteries, substantial gaps remain between the practical and theoretical energy densities. With the aim of supporting efforts to close this gap, this work reviews the fundamental operating mechanisms and challenges of Li intercalation in layered oxides, contrasts how these challenges play out differently for different materials (with emphasis on Ni–Co–Al (NCA) and Ni–Mn–Co (NMC) alloys), and summarizes the extensive corpus of modifications and extensions to the layered lithium oxides. Particular emphasis is given to the fundamental mechanisms behind the operation and degradation of layered intercalation electrode materials as well as novel modifications and extensions, including Na-ion and cation-disordered materials.

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Narrowing the Gap between Theoretical and Practical Capacities in Li‐Ion Layered Oxide Cathode Materials

This Review flows from past attempts to develop a (rechargeable) battery technology based on Ca via crucial breakthroughs to arrive at a comprehensive discussion of the current challenges at hand. The realization of a rechargeable Ca battery technology primarily requires identification and development of suitable electrodes and electrolytes, which is why we here cover the progress starting from the fundamental electrode/electrolyte requirements, concepts, materials, and compositions employed and finally a critical analysis of the state-of-the-art, allowing us to conclude with the particular roadblocks still existing. As for crucial breakthroughs, reversible plating and stripping of calcium at the metal-anode interface was achieved only recently and for very specific electrolyte formulations. Therefore, while much of the current research aims at finding suitable cathodes to achieve proof-of-concept for a full Ca battery, the spectrum of electrolytes researched is also expanded. Compatibility of cell components is essential, and to ensure this, proper characterization is needed, which requires design of a multitude of reliable experimental setups and sometimes methodology development beyond that of other next generation battery technologies. Finally, we conclude with recommendations for future strategies to make best use of the current advances in materials science combined with computational design, electrochemistry, and battery engineering, all to propel the Ca battery technology to reality and ultimately reach its full potential for energy storage.

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Achievements, Challenges, and Prospects of Calcium Batteries

Many layered oxide and sulfide intercalation compounds used in secondary batteries undergo stacking-sequence-change phase transformations during (de)intercalation. However, the underlying reasons why different intercalants result in different stacking-sequence changes are not well understood. This work reports on high-throughput density functional theory calculations on the prototype systems AxCoO2 and AxTiS2 (where A = [Li, Na, K, Mg, and Ca]), which show that a few simple rules explain the relative stability among the O1, O3, and P3 stacking sequences. First, for large intercalants (Na, K, and Ca), P3 stacking is favorable at intermediate concentrations (x ∼ 0.5) as its intercalant site topology minimizes in-plane electrostatic repulsion. At the extreme compositions (x ∼ 0 and x ∼ 1), O1 or O3 are stable, with more ionic compounds preferring O3 and covalent ones O1. These rules explain why stacking-sequence changes are much more common in Na materials than Li ones.

Stability of Prismatic and Octahedral Coordination in Layered Oxides and Sulfides Intercalated with Alkali and Alkaline-Earth Metals

n-Type ZnO-based semiconducting materials are increasingly studied for thermoelectrics (TE) in the past due to their inexpensive and non-toxic nature coupled with their high Seebeck coefficient and stability at elevated temperatures. However, their high thermal conductivity limits their prospect for TE application. In this work, a novel, simpler and faster bottom-up approach to produce thermoelectric Al-doped ZnO ceramics from nanopowders produced by an interfacial reaction (double emulsion method) followed by consolidation by spark plasma sintering (SPS) is explored. This alternative interfacial reaction yielded porous aggregates composed of nanoparticles, which exhibited distinctly enhanced TE performance after densification by SPS at low pressure, due to the notable improvement in the power factor and pronounced suppression of the lattice thermal conductivity caused by the nano/micro-structure engineered by this bottom-up synthesis technique. This encompasses grain size reduction, inclusion of Al-rich nanoprecipitates, and nanoporosity, thus enabling scattering of phonons of different mean free paths. A maximum figure of merit ZT ∼0.13 at 750 K was obtained for Zn0.97Al0.03O ceramics, a substantial enhancement to the previously reported values for the same composition synthesized by the traditional synthesis methods.

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Improvement in the thermoelectric properties of porous networked Al-doped ZnO nanostructured materials synthesized via an alternative interfacial reaction and low-pressure SPS processing

We have synthesized a single-phase sample of the compound known as CaCo2O4 and investigated its crystal structure by means of a single crystal X-ray diffraction technique The compound crystallizes in a triclinic system (space group P1) with lattice parameters of a = 4924(2) pm, b = 5683(3) pm, c = 5685(3) pm, α = 75400(9)°, β = 89974(9)° and γ = 81261(9)° at 293(2) K The structure consists of an alternate stack of a CdI2-type CoO2 triangular sheet and a layer of Ca atoms, showing great similarity to that of β-NaxCoO2 (x∼ 067) However, the Ca atom has four crystallographic sites with different occupation probabilities, leading to the structure formula Ca187(5)Co4O8 (Ca047(1)CoO2) Due to a low bulk density as well as a small number of hole carriers, equivalent to the nominal valence state of Co ions close to 306, a polycrystalline sample of the Ca-phase exhibits much higher electrical resistivity ρ = 680 μΩ m, relative to that of the Na-phase ρ = 12 μΩ m, with a large positive Seebeck coefficient of S = 145 μV/K at 300 K

Synthesis, crystal structure and physical properties of layered cobalt oxide CaxCoO2 (x∼ 0.47)

Single-crystal x-ray diffraction and high-resolution electron microscopy studies were carried out for Co-121 ([Ca2CoO3]
 p
 CoO2) and Sr-doped Co-121 grown in a KCl flux at 810°C. Typically, the samples were 2 mm × 2 mm × 0.02 mm in size. The single-crystal diffraction intensities were measured by the use of a four-circle diffractometer. Twinned super reflections, e.g., $$ 010\bar{1}/0\bar{2}04 $$
 and $$ 020\bar{2}/0\bar{1}03 $$
 were observed in the electron diffraction patterns. These super reflections were not observed in the end-member. Diffuse scattering was observed in the same reciprocal space by the single-crystal x-ray diffraction study. A discommensurate crystal model is proposed for the Sr-doped system.

Discommensuration of Doped [Ca2CoO3]pCoO2

Compounds and subsolidus phase relations in the CaO–Co3O4–CuO system

A Bi-substituted Co121 single crystal, grown by a flux growth method, is studied by electron diffraction (ED) measurement and high-resolution electron microscopy (HREM). The atomic arrangement of the Bi-substituted crystal is characterized by extended discommensurate boundaries and 3/5-type monoclinic commensurate structural units, which consist of three rock-salt (RS)-type and five CoO 2 subunits. The 3/5-type units are similarly observed in a Sr-substituted system. In the HREM image taken with the electron beam parallel to the [0010] direction, two regions, namely, the discommensurate and commensurate regions, are connected parallel to the a – c plane.

High-Resolution Electron Microscopy Study of [(Ca,Bi)2CoO3]0.62CoO2

Oxide thermoelectrics are relatively new materials that are workable at temperatures in the range of 400K≤T≤1200K. There are several types of thermoelectric oxide, namely, cobalt oxides (p-type semi-conductors), manganese oxides (n-type) and zinc oxides (n-type semi-conductors) for high temperature energy harvesting. The Seebeck coefficient of 3d metal oxide thermoelectrics is relatively high due to either high density of states at Fermi surfaces or spin entropy flow associated with the carrier flow. The spin entropy part dominates the Seebeck coefficient of 3d-metal oxides at temperatures above 300K. Introduction of impurity particles or quantum-well structures to enhance thermionic emission and energy filtering effects for the oxide semiconductors may lead to a significant improvement of thermoelectric performance.

Xiangyang Huang

Papers

Synthesis, crystal structure and physical properties of layered cobalt oxide CaxCoO2 (x∼ 0.47)

Discommensuration of Doped [Ca2CoO3]pCoO2

Compounds and subsolidus phase relations in the CaO–Co3O4–CuO system

High-Resolution Electron Microscopy Study of [(Ca,Bi)2CoO3]0.62CoO2

Thermoelectric Energy Conversion and Ceramic Thermoelectrics