Study on the Structural and Surface Changes during Lithiation/Delithiation Processes in Copper Sulfide

Abstract

Rechargeable batteries have been considered as one of efficent routes for energy conversion and storage. Li-ion batteries (LIBs) have been empolyed as major power sources for both mobile and stationary and applications. However, the energy densities of conventional, intercalation-based LIBs are insufficient to meet the demands for high-energy-densities enabling long distance driving electric vehicles and large-scale energy storage [1]. Transition metal sulfides based on conversion and/or alloying reactions have drawn considerable interest due to their abundance and high specific capacities [2]. Among them, copper sulfide (CuS) has a theoretical capacity of 560 mAh g-1 (CuS + 2Li+ + 2e‒ ↔ Li2S + Cu) and good electronic conductivity of ~10-3 S cm-1 [3,4]. Nevertheless, CuS shows poor cycling stability, owing to the dissolution of lithium polysulfides (Li2Sn, 2<n≤8) in the organic electrolytes and the insulating nature of Li2S products. To improve the irreversibility issues, it is highly required to understand the electrochemical behavior of CuS during lithiation and delithiation processes. In this work, we conducted in-situ and ex-situ analysis studies to investigate the electrochemical behavior of CuS during lithiation and delithiation in a carbonate-based electrolyte in the potential range of 0.05 – 3.0 V vs. Li+/Li. The structural and phase evolution of the CuS electrode were in-situ monitored by X-ray diffraction (XRD) and Raman spectroscopy during the lithiation and delithiation processes. In addition, the ex-situ XRD measurement was carried out to probe the phase transition of the CuS during the electrochemical processes and the result was correlated to the in-situ XRD patterns for characteristic lithiation/delithiation potentials. Furthermore, the changes in the chemical binding energies and the formation of the solid-electrolyte interphase layer on the CuS surface were analyzed by ex-situ X-ray photoelectron spectroscopy. It was found that CuS experiences a two-step lithiation reaction of insertion, followed by conversion; however, from the in-situ data, the CuS appears to convert into Li2S and Cu phases without forming distinct intermediate phases, such as Cu1.8S, Cu1.96S, and Cu2S, by lithiation process, which is contrast from the ex-situ data. Based on the above results, we showed the improved battery performance in the carbonate-based electrolyte through nano-engineering of CuS electrode, demonstrating the promise as a high-capacity LIB electrode. [1] O. Grӧger, H. A. Gasteiger, and J.-P. Suchsland, J. Electrochem. Soc., 162, A2605‒A2622 (2015). [2] X. Xu, W. Liu, Y. Kim, and J. Cho, Nano Today 9, 604‒630 (2014). [3] J.-S. Chung and H.-J. Sohn, J. Power Sources 108, 226‒231 (2002). [4] A. Débart, L. Dupont, R. Patrice, and J.-M. Tarascon, Solid State Sci. 8, 640‒651 (2006).