Insight into the Electrode Mechanism in Li-S Batteries with Microporous Carbon Confined Sulfur As Cathode

Introduction

In lithium-sulfur batteries, small S_2-4 molecules show very different electrochemical responses from the traditional S₈ material. Hitherto, their exact lithiation/delitiation mechanism is not clear, and how to select proper electrolytes for the S_2-4 cathodes is also ambiguous. In this work, S_2-4 and S₈/S_2-4 composites with highly ordered microporous carbon (MicroC) as confining matrix are fabricated, and the electrode mechanism of the S_2-4 cathode is investigated by comparing the electrochemical performances of the S_2-4 and S_2-4/S₈ electrodes in various electrolytes combined with theoretical calculation. Experimental results show that the electrolyte and microstructure of carbon matrix play important roles in the electrochemical performance. If the micropores of carbon are small enough to prevent the penetration of the solvent molecules, the lithiation/delithiation for S_2-4 will occur as a solid-solid process. Both the irreversible chemically reactions between the polysulfudes and carbonates, and the dissolution of the polysulfides into the ethers can be effectively avoided due to the steric hindrance. Under this condition, the confined S_2-4show high adaptability to the electrolytes. The sulfur cathode based on this strategy exhibits excellent rate capability and cycling stability.

 

Results and Discussion

Fig. 1 shows the SEM images of MicroC/S-60 and MicroC/S-40. It can be seen that a certain amount of element sulfur adheres on the surface of the carbon particles in the MicroC/S-60 sample (Fig. 1a), while for MicroC/S-40, the composite particles almost have the same shape with the pure MicroC particles, and no extra sulfur is observed (Fig. 1b). In Fig. 1c and d, TEM images of MicroC/S-40 show that, when composed with sulfur, the carbon matrix still maintains the original porous structure.

Fig. 2 shows the electrochemical behaviors of MicroC/S-60 and MicroC/S-40 in two typical electrolytes, respectively. For the ether-based electrolyte (fig 2a, b), the discharge capacities of MicroC/S-60 and MicroC/S-40 are both higher than 900 mAh/g. The charge-discharge plateaus at different voltages correspond to different electrochemical reactions. For the carbonate-based electrolyte (fig 2c), the MicroC/S-40 cathode shows almost the same discharge-charge capacity and plateaus as in the ether-based electrolyte, whereas the situation of MicroC/S-60 is totally different. For MicroC/S-60, the discharge capacity in the 2nd cycle decreases from 900 mAh/g in the ether-based electrolyte to less than 600 mAh/g in the carbonate-based electrolyte, and the plateaus at about 2.3 V and 2.0 V disappear. From the cycle performance comparison (fig. 2d), it is found that the combination of the small S_2-4molecules (MicroC/S-40) and the carbonate-based electrolyte delivers better cyclability.

Theoretical calculation brings some new insights into the sulfur electrode process in a nanometer scale. Fig. 3 displays the schematic of the lithiation process in MicroC/S cathode. The diameters of the solvent molecules are calculated larger than the size of the micropores of the MicroC (~ 0.46 nm). Thus, the micropores of MicroC, which confines the small S_2-4 molecules and the polysulfides formed during the charge-discharge process, prevents the penetration of carbonate molecules. The irreversible reactions between the carbonates and the confined polysulfides can be avoided due to the physical barrier (Fig. 3). Accordingly, the lithiation/delithiation for S_2-4occurs as a solid-solid process whether in the ether-based electrolyte or the carbonate-based electrolyte.

In addition, our work proposes a simple strategy of electrolyte selection for sulfur cathode. The ether-based electrolyte is proper for open-type composite cathodes, and the carbonate-based electrolyte is only suitable for microporous structured sulfur composite cathodes.