The Li-S batteries are bedeviled by the inherently low conductivity of S and its final discharge products (Li2S2/Li2S), the large volume expansion (~80％) from the oxidation state S8 to reduction state Li2S, and the dissolution and migration of the long-chain intermediate polysulfides (Sn2 , 3≤n≤8). In this work, we first report a perovskite-type host material, La0.6Sr0.4CoO3-δ (LSC) as a new high-efficiency polysulfide adsorbent. To make the best of the polysulfide immobilization capability of LSC, a novel coaxial yolk-shell structured host with carbon nanotubes (CNTs) filled with porous LSC nanofibers was designed.
Result and Discussion
The LSC/S@C nanofibers were fabricated by a simple electrospinning route followed by high-temperature calcination, and then coated with the precursor of SiO2 and resorcinol formaldehyde (RF) resin. The porous LSC nanofiber is composed of interlinked nanoparticles and the fiber length reaches several micrometres (Fig. 1a and 1b). The high-resolution TEM (HR-TEM) image in Fig. 1c demonstrates that the LSC particle has typical polycrystalline structure, and the well-resolved lattice fringes of 0.27 and 0.38 nm correspond to the d-spacing values of (110) and (100) planes of LSC, respectively. After coating with tetraethyl orthosilicate (TEOS), the average diameter of LSC@SiO2 nanofibers increases to about 120 nm (Fig. 1d). The thick SiO2 layer will generate sufficient internal void space to accommodate S, which aids to realize high S content in the cathode. The thin RF layer grown on the LSC@SiO2 nanofibers serves as carbon precursor. After carbonization, the LSC@SiO2@C coaxial nanofibers well maintain the 1D morphology with an average diameter of ~140 nm ( Fig. 1e ). The thickness of the final carbon sheath is about 20 nm (Fig. 1e and 1f). The TEM image of the sample after S loading verifies the successful encapsulation of S (Fig. 1f).
The LSC/S@C cathode were investigated with sulfur loading from 1.4 to 5.4 mgsulfur cm-2. As shown in Fig. 2a, all the cathodes obtain reversible specific capacity about 1000 mAh g-2 at 0.2 C, showing high S utilization. Fig. 2b shows the areal discharge capacity of the LSC/S@C electrodes with various S loadings. A high areal discharge capacity of 4.0 mAh cm-2 is achieved by the electrode with 3.3 mgsulfur cm-2 loading (0.05 C), and 7.8 mAh cm2 is obtained by the cathode with 5.4 mgsulfur cm-2 loading. In Fig. 2c, the electrodes with sulfur loading from 1.4 to 3.3 mgsulfur cm-2 show excellent cycling stability. Even for the cathode with a high loading of 5.4 mgsulfur cm-2, the reversible capacity fades slightly in the first 20 cycles but stabilizes in the following cycles. Fig. 2d displays the long-term cycling performance of the electrode with 5.4 mgsulfur cm-2 loading. The initial capacity is 915 mAh g-1, and the reversible capacity stabilizes at 4 mAh cm-2 and 728 mAh g-1 after 100 cycles.
The excellent cycle stability benefits from this chemical interaction between LSC and polysulfides, and the physical entrapment of the carbon shell. The theoretical calculations further demonstrate that Sr doping result in valence variation in Co along with oxygen vacancy; The Co ions with mixed valence have strong adsorption to the polysulfide ions while the existence of oxygen vacancy enhances the binding strength between Li2S4 and LSC. Moreover, the highly conductive LSC@C host and the porous interconnected fiber web-like architecture facilitate the mass transfer during charge/discharge process synchronously. Furthermore, perovskite is a big family, which component, cation valence and oxygen vacancy can be well controlled. Therefore, we believe this work will shed some new light on the discoveries of new high-efficiency polysulfide immobilizers and hence the design of high-performance sulfur cathode.