Currently, different metal sulfides (NiS, Co 9 S 8 , FeS 2 , and CuS) have been extensively studied as alternative electrodes for rechargeable batteries that can satisfy the performance requirements for more powerful energy supply and storage technologies for various applications and industries. Among them, copper sulfides have gained significant attention as a promising electrode material in rechargeable metal-ion (Li, Mg, Na, and Al) batteries. A wide range of synthesis routes and methods have been implemented in order to prepare various stoichiometry Cu x S (1 ≤ x ≤ 2) micro-/nanostructured materials with excellent electrochemical properties. Since the bulk microsized electrode materials have almost reached their performance limits for energy devices, the introduction of nanoscale Cu x S composites is now in high demand. This review focuses on the influence of the material morphology and dimensions on their performance in secondary batteries. The structures of Cu x S materials from zero-dimensional (0D) to 3D and their preparation are discussed. The primary purpose of this work is to provide an overview of the unique electrochemical and physical properties of particular structure and dimensionality which can promote these materials' application in the energy storage field. Along with this, this work summarizes the information on various synthesis strategies and how they can manage the morphologies of Cu x S micro-/nanocomposites. In the current fast technologically advancing society, the development of the most economically profitable and efficient synthesis routes is especially encouraged and required, and this aspect is also commented on in this review.

Conductive and flexible CuS films with unique hierarchical nanocrystalline branches directly grown on three-dimensional (3D) porous Cu foam were fabricated using an easy and facile solution processing method without a binder and conductive agent for the first time. The synthesis procedure is quick and does not require complex routes. The structure and morphology of the as-deposited CuS/Cu films were characterized by X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and transmission electron spectroscopy, respectively. Pure crystalline hexagonal structured CuS without impurities were obtained for the most saturated S solution. Electrochemical testing of CuS/Cu foam electrodes showed a reasonable capacity of 450 mAhg−1 at 0.1 C and excellent cyclability, which might be attributed to the unique 3D structure of the current collector and hierarchical nanocrystalline branches that provide fast diffusion and a large surface area.

Copper sulfides (Cu x S) with different stoichiometry are considered as prospective cathode materials for lithium batteries owing to their large energy storage capability. In this work, three-dimensional Cu x S cathodes were synthesized via introducing commercially available copper foam into the solution of dimethyl sulfoxide (DMSO) and sulfur powder. The synthesis procedures were straightforward and ultrafast and did not require additional reagents, high temperature, or long processing time and can be considered as a facile one-step method. Copper sulfide materials with different stoichiometry (x = 1.8, 1.96) were obtained by changing the temperature and the residence time of the copper foam in the DMSO solution. The effects of the temperature and time on phase and morphology of Cu x S were characterized by X-ray diffraction and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. Electrochemical tests resulted in a stable cyclability of Cu 1. 8 S cathode with 100% Coulombic efficiency and capacity of approximately 250 mAh g −1 .

Currently, Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is considered a promising composite in energy conversion and storage devices due to its high electrical conductivity, chemical stability, outstanding flexibility and great electrochemical properties. Owing to its favorable mechanical and electrical properties PEDOT:PSS has wide applications in transparent electrodes, photovoltaics, fuel cells, supercapacitors and lithium-ion batteries [1-2]. Flexible free-standing sulfur @ MWCNT @ PEDOT:PSS composite films might be prepared using various techniques such as vacuum infiltration, facile solution mixing method, rod-coating technique, hydrothermal method, and doctor-blade technique [3]. In our review, we synthesized PEDOT:PSS @ sulfur@ MWCNT composite film by a doctor-blade technique onto the polypropylene (PP) substrate. First, the cathode composite was prepared by mixing multi-walled carbon nanotubes (MWCNT) composite with a sulfur content 1:1, PEDOT:PSS water-based dispersion and DMSO as a solvent to form a homogeneous slurry. The slurry was coated onto PP substrate by doctor-blade method and subsequently dried in a vacuum oven. Furthermore, sulfur @ MWCNT @ PEDOT:PSS film was structurally characterized by scanning electron microscopy (SEM) analysis. SEM images show the flexible film, which exhibits a network structure formed by interpenetration of MWCNTs and PEDOT:PSS. Sulfur nanoparticle clusters are surrounded by CNT networks, forming intimate interfaces with each other. According to EDS results, uniform distribution of each element can be observed. Acknowledgement This work was supported by the research projects АР08052143 “Development of Wearable Self-Charging Power Unit” from the Ministry of Education and Science of the Republic of Kazakhstan. References [1] Sun, K., Zhang, S., Li, P., Xia, Y., Zhang, X., Du, D., ... Ouyang, J. (2015). Review on application of PEDOTs and PEDOT:PSS in energy conversion and storage devices. Journal of Materials Science: Materials in Electronics, 26(7), 4438–4462.doi:10.1007/s10854-015-2895-5[2] Fan, X., Nie, W., Tsai, H., Wang, N., Huang, H., Cheng, Y., ... Xia, Y. (2019). PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications. Advanced Science, 1900813.doi:10.1002/advs.201900813[3] Wang, Y., Zhu, C., Pfattner, R., Yan, H., Jin, L., Chen, S., ... Bao, Z. (2017). A highly stretchable, transparent, and conductive polymer. Science Advances, 3(3), e1602076.doi:10.1126/sciadv.1602076 Figure 1