A facile synthesis of copper sulfides composite with lithium-storage properties
Introduction
The last two decades have witnessed the rapid growth of lithium ion battery technology and its market in energy storage and electron devices application [1], [2], [3], [4], [5]. The advantages of these lithium ion batteries, compared with other battery devices, are of high energy density, long cycling life, long shelf life, etc. Today it is generally realized and accepted that each battery type has its pros and cons, lithium ion batteries are not in the stage yet to replace all others, even they have the great potentials. To further expand their market potentials, there has been strong incentive to develop low cost lithium-ion batteries with high-energy, high-power density and high-safety.
Amongst many compounds which have been studied as alternatives [6], [7], [8], [9], [10] to transition metal oxides that are commonly used in current lithium ion battery, copper sulfides of the molecular formula of CuxS stand out because of their good electrical conductivities, high theoretical capacity and flat discharge curves, for example, covellite CuS is a good conductor (10−3 S cm−1) and possesses a high capacity (560 mA⋅h⋅g−1) which is almost four times of that of commercial Li4Ti5O12 [11], [12], [13]. Copper sulfide is a common nomenclature of binary compounds composed of copper and sulfur. In fact, these two elements form a myriad of binary compounds with different stoichiometries, such as Cu2S, Cu7S4, Cu1.85S, Cu1.96S and Cu2S [14], [15]. The mechanism of copper sulfides reacting with lithium ions and electrons is different from that of transition metal oxides, rather than an insertion reaction, it is a replacement reaction in which the final product is Cu and Li2S [9], [13], [16], [17]. The formation of lithium polysulfides causes severe capacity decay during charge/discharge cycles, because lithium polysulfides can dissolve into electrolytes and drift away from the electrode [13]. Many attempts have been made to improve its capacity retention [10], [18], [19], [20]. Cu2S film obtained from direct hydrothermal growth on Cu foil reported by Ni et al. and highly ordered large-scale Cu2S nanowire arrays grown onto the copper current collector reported by Lai et al. can lead to excellent cycling performance [21], [22]. The intrigue electrochemical behavior of Cu2S materials on Cu foil inspired us to investigate the relation between the material chemistry and its electrochemical properties, and to further improve their electrochemical performance as anode for lithium ion batteries.
It is well known that electrochemically the ‘copper-excess’ copper sulfides electrodes [10], [18], [19], [20], [21], [22], [23], [24], [25], [26] exhibit significant differences to the ‘copper-deficient’ copper sulfides electrodes [9], [13]. If the molar ratio of copper to sulfur in electrodes is greater than 2:1, they are designated as ‘copper-excess’ copper sulfides electrodes, otherwise, they are ‘copper deficient’ copper sulfides electrodes. We carried out systematic work in attempt to uncover the origin of these pronounced differences. We first synthesized copper sulfides compounds with different Cu/S stoichiometries. And the influence of current collector on copper sulfides electrochemical performance is also studied in this work. In general, these ‘copper excess’ electrodes showed excellent performance compared with ‘copper deficient’ electrodes. This may be related to the excess copper can immobilize the active sulfur element to some extent, and the related mechanism is revealed in further discussion.
Section snippets
Preparation of electrodes
Copper sulfides (CuxS) were synthesized from copper powder (20–30 nm) (Aladdin, US) and sulfur powder (China National Medicines Corporation Ltd.). Different molar ratios of copper and sulfur were mixed and dispersed in N-methyl-2-pyrrolidone (NMP), and heated at 100 °C for 0.5 h before solvent evaporation. The synthesized CuxS are designated as CuxS I (x = 1) and CuxS II (x = 2), respectively.
The preparation of copper sulfides electrodes were as follows: copper sulfides powder, acetylene black,
Characterization of copper sulfides materials
The morphologies and crystal phases of the synthesized copper sulfides powders are characterized by SEM and TEM, as shown in Fig. 1. SEM images (Fig. 1a and b) indicate that the particles of these two samples are both coral-like and made of needle-like nanoparticles and all these nanoparticles are about one micrometer long and several hundred nanometers wide, even though the particle sizes of each sample are different. The needles in sample CuxS I are little thinner than that in sample CuxS II.
Conclusions
In summary, we have demonstrated that the ‘copper-excess’ copper sulfides electrodes showed excellent cycling performance and rate capability. The excellent cycling and rate performance could be contributed to the high conductivity of copper sulfides and the unique displacement reaction between Cu2S and Li2S, both of them possess similar crystal structures. We believe and demonstrate that the most critical factor for achieving such good performances in Li-CuxS batteries is that the amount of
Acknowledgment
This work was supported by grants from National Natural Science Foundation of China (21273185 and 21321062) and NFFTBS (J1310024). We thank Dr. Binbin Xu in Xiamen University for TEM test.
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X. X. Wang, Y. H. Wang and X. Li contributed equally to this work and share first authorship.