Lithium ion (Li-ion) batteries with high energy density have been the center of paramount attention and concerted research efforts because of the high demand for powering portable electronic devices, electric tools and electric vehicles. However, the low energy density of Li-ion batteries compromises their merits, and the exploration of new electrochemistry that could meet the high capacity of energy, safety and environmental consideration for electric vehicles is imperative. Lithium sulfur (Li-S) batteries have been regarded as a promising candidate to replace the current lithium ion batteries. They are featured by the high theoretical energy density (2567 Wh kg^-1), environmental benignity, low cost (~$150 per ton) and abundance of sulfur on earth. However, they are still chall...[ Read more ]

Lithium ion (Li-ion) batteries with high energy density have been the center of paramount attention and concerted research efforts because of the high demand for powering portable electronic devices, electric tools and electric vehicles. However, the low energy density of Li-ion batteries compromises their merits, and the exploration of new electrochemistry that could meet the high capacity of energy, safety and environmental consideration for electric vehicles is imperative. Lithium sulfur (Li-S) batteries have been regarded as a promising candidate to replace the current lithium ion batteries. They are featured by the high theoretical energy density (2567 Wh kg^-1), environmental benignity, low cost (~$150 per ton) and abundance of sulfur on earth. However, they are still challenged by various drawbacks found in the current system, which include the low ionic and electronic conductivity of sulfur and Li₂S, the large volume variation of sulfur and the dissolution of polysulfides formed during charge/discharge, etc. Such problems lead to the shuttle effect, loss and a low utilization rate of the active material, electrode failure, poor cycling and fast decay of the cells. Besides the design of electrodes in Li-S batteries, the separator modification is also confirmed effective in containing polysulfides and elevating cycling stability of Li-S cells. In this thesis, several carbon-based interlayers are proposed and tested. The integrated Li-S cells delivered decent improvements in localizing polysulfides and reactivating the trapped active materials, leading to significantly elevated cycling performance.

The first work employed ultrathin Tortech paper with a thickness of around 5 μm as an interlayer inserted between the sulfur cathode and the commercial separator. This Tortech paper consists of bundled carbon nanotubes (diameter: 1-2 nm) that are interlinked together and form a dense and tight-knit interlayer. The modified cell achieved improved discharge capacity and reduced internal resistance, resulted from the blocking function and activation effects of the tight-knit interlayer. In addition, the Li anode was also indirectly stabilized by the suppression of diffusing polysulfides.

To fully exploit the potential of carbon materials with high conductivity and S-immobilizing capability, a novel carbon nanofiber (CNF) based composite with interspersed TiO₂ nanoparticles within the nitrogen-doped shell of carbonized polydopamine (C-PDA) was fabricated as a flexible interlayer (denoted as CTC) in Li-S cells. With the protection of this unique interlayer, the lithium sulfur cells achieved a long cycling stability with a decay rate of 0.06% per cycle (500 cycles at 2C). The stabilized electrodes and improved electrochemical performance of the CTC-modified cells corroborate the effectiveness of CTC integration in elevating the performance of the cell by physically restraining the shuttle of polysulfides and chemically localizing polysulfides via TiO₂ and heteroatoms in C-PDA. In addition, the use of C-PDA shell can also help to maintain the intact structure of CTC by preventing it from getting detached from the CNF skeleton.

CNFs were further employed to form a carbon-based framework which was interposed by CoS sheets and embedded by Ketjen black (KB) nanoparticles. CoS, although with a low conductivity (around 25 S m^-1), was examined by the visualized adsorption of Li₂S₄ and XPS inspection, which, for the first time, unraveled the active interaction between lithium polysulfides and CoS. The obtained CNF/CoS/KB interlayer has a high conductivity granted by the carbon matrix, and effective polysulfide-immobilizing ability endowed by CoS sheets. As a result, a much reduced internal resistance (one fifth of the unprotected cell) of the cell with CNF/KB/CoS modification were observed. Moreover, significantly enhanced cycling performance of the cell cycled at 1C was obtained with a capacity decay rate of 0.076% per cycle for 760 cycles, suggesting the superiority of this integrated CNF/KB/CoS coating compared with the cells protected by CNF, CNF/KB, CNF/CoS interlayers.

Following this strategy, a net-structured filter composed of Co(OH)₂-anchored CNFs and KB nanoparticles (denoted as Co(OH)₂/CNF/KB) was prepared as an effective inhibitor of polysulfide intermediate diffusion with physical flexibility and robustness on the commercial separator. This unique integration could successfully block soluble polysulfides to the cathode-side separator via the filtering function of the framework and the interaction between polysulfides and the copious polar/hydrophilic groups in Co(OH)₂. Employing this functionalized separator, the cell delivered a capacity of 1394 mA h g^-1 at 0.1C and obtained a capacity decay of 0.10% per cycle at 2C for 450 cycles, contributed by the polysulfide-filtering capability of the unique dense reticular structure, S-immobilizing effect of hydroxides, and S-reactivating ability of conductive CNF and KB.

The Tortech interlayer employed its dense and tight-knit structure to intercept S-related species, which is only physical filtering. To take the advantage of S-immobilizing materials, TiO₂ interspersed in the heteroatom-doped carbonized polydopamine was integrated on the carbon nanofibers in the second work to enhance its ability by both physically and chemically localizing sulfur species. In the third and fourth work, a better integrated work of carbon matrix, porous Ketjen black and sulfur immobilizers were adopted to achieve the intensified and structurally integrated interlayer. These rational designs based on carbon and S-immobilizing materials offer new opportunities for practical application and commercialization of Li-S batteries.