Enhanced Lithium-Sulfur battery by amine-functionalized carbon nanotube cathode
The rechargeable Lithium-Sulfur (Li-S) battery is an attractive platform for high-energy, low-cost electrochemical energy storage due to the low cost of sulfur ($0.02/g) and the high theoretical energy density (2500 Wh/kg or 2800 Wh/L) of the sulfur cathode. Practical Li-S cells are limited by several fundamental issues, which derive from the complex solid-state and solution physical chemistry of the electrodes and electrolyte. The poor ionic and electronic conductivity of sulfur and its reduction compounds with lithium leads to poor electrode kinetics and active material utilization. Dissolution of long-chain lithium polysulfides (Li2Sx, 2 < x < 8) (LiPS) into the electrolyte and the shuttling of polysulfides between cathode and anode consume the active material in a parasitic process that ultimately ends in premature cell failure. A variety of methods have been applied to prevent the dissolution of LiPS, the most effective of which focus on synergetic benefits of nanoengineered carbons to simultaneously facilitate electron transport and to sequester soluble species in the cathode. However, the effective barrier such materials present to dissolution of LiPS are now understood to be kinetic; a soluble LiPS species trapped by the host will eventually leach into the electrolyte. It is possible to augment interactions between cathode components and LiPS by using polar additives or oxides, but additives or oxides are insulator, which will lower the conductivity of the electrode, resulting in limited utilization of the active materials.
In this talk, we report on an approach that allows high-performance sulfur-carbon cathodes to be designed and synthesized. Specifically, Polyethylenimine (PEI) polymers bearing a large amount of amine groups in every molecular unit are attached to the cathode by reaction with hydroxyl and carboxyl functionalized carbon nanotubes (Figure 1). The covalent attachment is confirmed by XPS and Raman spectroscopy. And, the strong affinity of LiPS to PEI-functionalized CNT is verified by density functional theoretical (DFT) analysis, which shows a high binding energy of 1.23 eV, which is much higher than that of 0.34eV between LiPS and graphene and 0.83eV between LiPS and PVDF. The interaction is also confirmed by FTIR, XPS, and other characterization techniques. Significantly, we also show that there is also strong bonding between elemental sulfur and the composite, which is expected to further stabilize the active materials in the Li-S cathode. With the merits of CNT such as good conductivity and robust mechanical properties, the nanocomposite of amine-functionalized CNT and sulfur exhibits excellent electrochemical properties, including stable cycling performance with high capacities at rates up to 3.35 mA/cm2 or 2C (Figure 2).
Figure 1. Schematic illustrating modification of CNT with PEI and structure of PEI deduced from DFT analysis of the binding energy between PEI and Li-S. species.
Figure 2. (a) Discharge capacity as a function of cycle number for Li-S cells cycled galvanostatically at rates from 0.84 mA/cm2 (0.5C) to 3.35 mA/cm2 (2C)(70% sulfur in the composite). The blank circles correspond to the Coulombic efficiency. (b) Voltage profile for Li-S cells at different current rates.
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