In-situ Electrochemical Polymerization to Enhance the Electrochemical Performances of Selenium-carbon Composite Cathode

Abstract

The recent interest in higher gravimetric/volumetric capacity of lithium ion batteries (LiBs) has led to the investigation of chalcogen-based cathodes since chalcogens including include sulfur (S), selenium (Se), and tellurium (Te) exhibit higher gravimetric/volumetric capacities (1672, 672, and 420 mAh g-1 and 3467, 3253, and 2621 mAh cm-3, respectively) than the metal oxide-based cathodes (e.g., 148 mAh g-1 and 1363 mAh cm-3 for LiCoO2). [1] However, the shuttle effect, which induces dissolution of active materials into an electrolyte, has been the main barrier to use chalcogen-based cathodes practically. [1] The surface modification of chalcogen-based cathode has been a successful strategy to mitigate the shuttle effect. [2-3] For example, in our previous study, we synthesized various heteroatom-doped carbon frames and used these as host material to trap Se and selenides. [3] In particular, the Se/nitrogen-doped ordered mesoporous carbon (Se/NOMC) exhibited the highest capacity retention of 99.99% during 500 cycles. However, to synthesize the heteroatom-doped carbons is complicated and thus expensive. From the point of view, a facile and cost-effective method to alleviate the shuttle effect and hence enhance the electrochemical performances of chalcogen-based cathodes is necessary.

In this work, we introduced a facile and cost-effective coating method, in-situ electrochemical polymerization (EP), to enhance the electrochemical performance of chalcogen-based cathodes. As a coating layer, conducting polymer (i) decreases the contact between electrode and electrolyte thus suppress the dissolution of chalcogens, (ii) facilitates the contact between particles increasing the electrical conductivity of chalcogens (iii), and traps the chalcogenides due to its porous structure or functional group. [4] From the points of view, coating a conducting polymers on cathode by the in-situ EP can be an easy and cost-effective strategy to ameliorate the electrochemical performance of cathodes. The in-situ EP is followed by the operation of cell employing aniline monomers as an electrolyte additive. We intended that the aniline monomers are electrochemically polymerized into polyanilines (PANis) on the surface of Se/C cathode during initial formation cycle (Fig. 1a). In case of the cell employing aniline in the electrolyte, the oxidative current curve was presented in galvanostatic test. It indicates that the oxidation of the anilines occurs on the cathode side (Fig. 1a). After the electrochemical treatment, the electrochemical performances were investigated by galvanostatic charge-discharge test (Fig. 1b-c). In the rate-capability tests, Se/C-EP retains 53% capacity of that at C/10 (one without EP is only 0.06 %) and shows a 10% higher capacity retention during 50 cycles than the untreated one. To elucidate the enhanced electrochemical performance of Se/C-EP, the material characterization was conducted using FT-IR (Fig. 2). In the Se/C-EP, N-B-N (B: benzenoid) and N=Q=N (Q: quinoid) units were observed, but not in one without the aniline. These units constitute emeraldine PANi, which is the only conducting state among other states of PANis (electrical conductivities of leucomeraldine and pernigraniline are 10 orders of magnitude smaller than emeraldine PANi). [5] In particular, it is supposed that the emeraldine PANi coated 3D network layer formed between Se/C active materials can enhance both electrical and ionic conductivities of Se/C cathode, leading to the high rate-capability. In addition, it is inferred that the enhanced cyclic stability of Se/C-EP is due to the heteroatomic C–N and C=N bonds of PANi, which have strong affinity for the soluble intermediates.