(336f) Enhanced Lithium-Sulfur Battery By Amine-Functionalized Carbon Nanotube Cathode

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 (Li₂S_x,
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.

 (a)
(b)

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.