Thermoelectric materials are one kind of functional materials which can directly realize the interconversion of thermal energy and electrical energy and have a promising application prospect in the fields of thermoelectric refrigeration and power generation. Currently, the low conversion efficiency limits the applications of thermoelectric materials. How to improve and optimize the thermoelectric figure of merit becomes very important in the current research. Cu₂Se-based phonon liquid thermoelectric materials are one new type of high performance thermoelectric materials with extremely low thermal conductivity by their special crystal structure, the researches of which have greatly promoted the development of thermoelectric materials. Cu₂Se undergoes a structural phase transition at high temperature, where the Cu atoms become the freely migrating liquid-like Cu ions. This special liquid-like behavior of Cu ions leads to the lower lattice thermal conductivity of materials by strong phonon scattering. In this paper, the Cu₂Se-based phonon liquid thermoelectric materials are focused on. The basic properties, special crystal structure and applications of Cu₂Se are summarized. The research achievements, preparation methods as well as the performance optimization means of Cu₂Se-based thermoelectric materials are introduced elaborately. Finally, the future research aspects and new ideas for improving the performance of phonon liquid thermoelectric materials are also analyzed and prospected.
Contents
1 Introduction
2 Properties of Cu₂Se and Cu₂Se-based phonon liquid thermoelectric materials
2.1 Properties of Cu₂Se
2.2 The concept of Cu₂Se-based phonon liquid thermoelectric materials
3 Development of Cu₂Se-based phonon liquid thermoelectric materials
3.1 Preparation methods of Cu₂Se-based phonon liquid thermoelectric materials
3.2 Performance optimization means of Cu₂Se-based phonon liquid thermoelectric materials
4 Conclusion

Nickel-cobalt bimetallic sulfide (NiCo₂S₄) has a typical AB₂O₄ spinel structure. The conductivity of NiCo₂S₄ is two orders of magnitude higher than that of NiCo₂O₄, and its conductivity is 1.25×10⁶ S·m^-1at room temperature. In addition, NiCo₂S₄ provides a more efficient redox reaction than the corresponding one-component sulfide, and has great potential for its unique nanostructures and electrochemical properties. In this paper, the preparation of NiCo₂S₄ nanostructures and its application in electrochemical energy conversion and storage are reviewed. The morphology, physicochemical properties and synthesis methods of NiCo₂S₄ nano materials are introduced. Pretreatment conditions, preparation methods and growth matrix will have an effect on the morphology and properties of NiCo₂S₄ nanostructures. NiCo₂S₄ with different nanostructures (such as nanoneedles, nanowires, nanorods, nanotubes, nanocapses, nanosheets, nanostructures and hierarchical structures) can be prepared by a variety of methods (such as hydrothermal method and solvent heat method, low temperature synthesis method, anion exchange method, steam conversion method, electrodeposition method, coprecipitation method and self-assembly, etc.). Among them, hydrothermal and solvothermal are the most commonly used methods because they have the characteristics of low cost, easy handling and suitable for large scale manufacturing. The application status of NiCo₂S₄ nano materials in electrocatalysis, supercapacitors and lithium ion batteries is summarized. This paper analyzes and compares the preparation process, method and application of different nanostructures, hoping to promote the development of NiCo₂S₄ nano materials in the field of electrochemical energy conversion and storage. The development and application direction of NiCo₂S₄ nano materials are proposed.
Contents
1 Introduction
2 Preparation of NiCo₂S₄ nanostructures
3 Electrochemical application of NiCo₂S₄ nanostructures
3.1 Electrochemical catalysis:bifunctional electrocatalysts
3.2 Pseudocapacitive properties:supercapacitors
3.3 New electrode materials:anode material for Li-ion batteries
4 Conclusion