Lithium-ion batteries are the most utilized batteries today for portable electronic devices due to its high energy density, long lifecycle and light weight. However, they still have limitations on applicability for electric vehicles and high capacity storage systems. In order to satisfy the fast-growing demands in the modern society for high-capacity storage and fast charge-discharge processes, substantial investigations focused on the development of new cathode materials such as metal oxides, sulfides, fluorides and others to overcome the limits of conventional lithium-metal-oxide cathode materials (LiCoO2, LiFePO4, and LiMn2O4) [1, 2].
Copper sulfides are particularly gaining attention because of their perspective for exhibiting high theoretical capacity (560 mAh g-1 for CuS and 337 mAh g-1 for Cu2S) with flat charge/discharge plateaus and good electronic conductivities. However, they are suffering from poor capacity retention and therefore have a short lifetime [3]. The exact determination of the reaction mechanism and the reasons for poor reversibility have known to be difficult due to the complicated phase stoichiometry of copper sulfides during the charge-discharge processes [4, 5].
In this work, we prepared stoichiometric Cu2S by spray pyrolysis (SP) followed by heat treatment and investigated its physical and electrochemical properties.
Experimental
Copper (I) sulfide powder was prepared from the precursor solution by dissolving correct amount of copper (II) nitrate trihydrate (Cu (NO3)2·3H2O) and thiourea (CS (NH2)2) where the Cu: S molar ratio varied from 2 to 0.5 with a fixed cupper concentration of 0.05 mol L-1. All chemicals were purchased from Wako Pure Chemical Industries Ltd. The sprayed droplets were carried to a laminar flow aerosol reactor by N2+3%H2 gas at a flow rate of 2 L min-1. The reactor temperature was varied from 400 to 700 ºC. As-prepared samples were annealed in the temperature range from 400 to 600 ºC in a N2+3%H2 gas atmosphere for 2 h.
The positive electrode was prepared by mixing 80 wt.% Cu2S as-prepared powder, 10 wt.% polyvinylidene fluoride (PVdF) as a binder and 10 wt.% acetylene black (AB) as a conducting agent in 1-methyl-2-pyrrolidinone (NMP) and then coating them on a copper foil current collector. The coin-type cells (CR2032) were assembled with 1 M LiTFSI in 1:1 w/w in DOL/DME electrolyte and lithium metal foil as a reference electrode in a high purity Ar gas atmosphere (99.9995% purity). The cell performance test was performed for charge/discharge cyclability in a voltage range of 1.0-3.0 V. The current density and specific capacity were calculated based on the weight of Cu2S in the electrode and 1C used as 337 mA g-1.
Results and discussion
The XRD peaks of the as-prepared Cu2S powder after SP at 400 ºC with Cu:S=1 molar ratio and its further annealing in the temperature range of 400-600 ºC are demonstrated in Fig. 1a. The diffraction peaks of all samples can be indexed to the monoclinic Cu2S structure with a space group P21/c. The relationship between observed Cu/S elemental ratio in the samples and the annealing temperature is shown in Fig. 1b. As seen from the figure, the desired Cu2S with Cu/S≈2 elemental ratio can be synthesized after annealing at 460 ºC for 2 h of as-prepared SP sample at 400 ºC in N2+3%H2 gas atmosphere. Thus, the electrochemical investigations were carried out for the Cu2S with Cu/S≈2 elemental ratio.
In Fig. 2a cycling performance of Cu2S is demonstrated and the cell exhibits a 1st discharge capacity of 320 mAh g-1 (95% of its theoretical capacity) at 1C rate. The discharge capacity maintains at approximately 250 mAh g-1 up to 100 cycles with about 100% Coulombic efficiency. The rate capability testing of the Cu2S in the range of current rates from 0.1 to 30 C is illustrated in Fig. 2b. The cell releases stable capacity from 0.5C to 30C at 230 mAh g-1 with an excellent reversibility indicated by Coloumbic efficiencies.
References
[1] M.M. Thackeray, C. Wolverton, E.D. Isaacs, Energy Environ. Sci., 5, 7854(2015).
[2] F. Han, W.-C. Li, D. Li, A.-H. Lu, ChemElectroChem, 1, 733(2014).
[3] B. Jache, B. Mogwitz, F. Klein, P. Aldelhelm, J. Power Sources, 247, 703(2014).
[4] X. Meng, S.-C. Riha, J.-A. Libera, Q. Wu, H.-H. Wang, A.-B.F. Martinson, J.-W. Elam, J. Power Sources, 280, 621(2015).
[5] Y. Fu, A. Manthiram, Electrochim. Acta, 109, 716(2013).
