The proposal won President Undergraduate Research Awards of Spring 2020

1. Aluminum Oxide Composite and Carbon Nanotube-Based Bifunctional Separators for
High-Performance Lithium Sulfur Batteries
Applicant: Jiankun Pu
Co-mentor: Kyungbin Lee
Advisor: Dr. Seung Woo Lee
Overview
Despite the high theoretical specific capacity (1675 mAhg-1
) and low redox
potential, the utilization of Li metal batteries (LMBs), as shown in Figure
1, still faces tremendous challenges. The practical application of LMBs,
especially lithium sulfur batteries (LSBs), is hindered by severe capacity
fading, low Coulombic efficiency and safety issues due to polysulfides
shuttling effect and Li dendrite formation [1]
. In this study, we will develop
a bifunctional separator using glass fiber (GF) coated with aluminum
oxides and carbon nanotubes (CNTs) to improve the cycling stability and
the safety of LSBs.
Objectives
Among various methods to suppress Li dendrite growth, atomic layer deposition (ALD) of Al2O3 onto Li metal
anode surface has recently been demonstrated to be a promising solution, showing 2 times longer cycling life
than pristine Li-Cu coin cell, and a Coulombic efficiency of ~98% at a 1 mAcm-2
current density [2]
. To suppress
the migration of polysulfides in the electrolyte, Graphene/Carbon Nanotubes (G/CNTs) separator has been proved
to retain the Coulombic efficiency of 99% and a reversible capacity of 815 mAhg-1
at a current density of 0.2 C
after 100 cycles [3]
. Moreover, glass fiber (GF) has been considered as a preferred separator for LSBs, and the
cells using glass fiber can retain a capacity of 617 mAhg-1
after 100 cycles at a current density of 0.2 C, which is
42% higher than that of the cells using microporous polypropylene (PP) separator [4]
.
To combine all the advantages of Al2O3, CNTs and GF, a bifunctional separator using GF coated with Al2O3 on
one side and CNTs on the other side will be fabricated to improve the electrochemical energy storage
performance of LSBs. Al2O3 coating will be faced towards the Li metal anode to suppress the dendrite growth,
while CNTs coating will be faced towards the sulfur cathode to capture and block the polysulfides in the
electrolyte. This study can provide insights on designing high performance and stable LSBs as the next-
generation secondary batteries that can be applied to various energy demanding products, such as electrical
vehicles and energy storage system for solar and wind power plant.
Background Information
LMBs have been studied in various research groups to alternate the Lithium-ion batteries (LIBs) in the future.
However, the poor cycle life and the severe capacity fading due to Li dendrite formation and shuttling effect of
polysulfides slow down the adoption of LSBs. Currently, solid polymer electrolytes (SPEs) and ceramic layers
are two major solutions to suppress Li dendrite growth (Figure 2a) [5,6,7]
. Both solutions show the ability to inhibit
Li dendrite growth. However, solid polymer electrolytes usually have low Li-ion conductivity at room
temperature, while a ceramic protection layer shows poor mechanical strength at high temperature [8]
. To address
this issue, some researchers recently have tried to develop a protection layer with better mechanical properties.
Al2O3 coating can provide sufficient stiffness to the protection layer and the high ionic conductivity property can
also be beneficial for the fast ion diffusion [2,8]
.
Figure 1. Schematic of a LMB battery

2. On the other hand, CNTs have been long studied and recognized as a
solution for the shuttle effect of LSBs. The shuttle effect is a result of
the dissolution and migration of polysulfides (Figure 2b). During the
discharge process, the solid sulfur is reduced and forms high order
polysulfides (Li2Sx, 4 < x < 8) that is dissolvable in the electrolyte.
Driven by the concentration gradient, Li2S8 will diffuse toward the Li
electrode. The polysulfides will react with Li and form low order
polysulfides (Li2Sx, x < 4), which causes self-discharge. During this
process, the utilization and capacity of the active material are
reduced, thereby reducing the Coulombic efficiency of the battery.
However, applying CNTs on separators can effectively capture or
block the high order polysulfides, which reduces the amount of the
high order polysulfides from diffusing to Li anode. As a result, it
promotes the Coulombic efficiency and retains the capacity of LSBs
[9]
.
Lastly, the glass fiber is considered as an excellent separator for LSBs
because of its superior thermal stability, higher ionic conductivity,
and better wettability compared to the PP separator. The higher
porous density provided bare GF membrane with enhanced
electrochemical kinetics, which results in more stable capacity and
better rate capability [4]
.
Figure 3 shows a schematic of the proposed bifunctional separator. In
this experiment, CNTs-based slurry will be pasted on the GF surface
using Doctor Blade method. After then, Al2O3 powders will be coated
on the opposite side of the GF separator with using vacuum filtration
process.
Method
Carbon nanotubes (Sigma-Aldrich) and the Polyvinylidene fluoride (PVDF) binder (weight ratio of 90:10) will
be mixed in N-Methyl-2-Pyrrolidone (NMP, Sigma-Aldrich). As-prepared slurry form will be pasted on the GF
separator. After drying in the vacuum oven at 65 ℃ for overnight to remove the solvent, Al2O3 powders (Sigma-
Aldrich) dispersed in NMP will be filtered on the opposite side of the GF separator. The modified bifunctional
separator will be dried in the vacuum oven at 65℃ for 24 h and then punched into ø15 mm for the coin cell
assembly. The sulfur electrode will be prepared by pasting the slurry of mixed sulfur powder (65 wt%), carbon
black (30 wt%), and PVDF (5 wt%) in NMP on the Al foils. The sulfur electrode will be dried in the vacuum
oven at 65℃ for 24 h and punched into ø10 mm.
The CR2032 coin-type cells, consisting of the Li metal anode, Al2O3|GF|CNTs separator, and sulfur cathode, will
be assembled in Ar-filled glove box (<0.1 ppm of H2O and <0.1 ppm of O2). The electrolyte is a 1M LiTFSI in
DOL/DME (1:1 volume ratio) solution with 5 wt% of LiNO3. The galvanostatic charge/discharge test will be
conducted in the voltage range of 1.5V and 3.0 V at different C-rates from 0.05 C to 10 C and will be returned to
0.1 C in the last cycle (1C= 1675 mAhg-1
).
Figure 2 Two major challenges facing
LSBs. (a) Li dendrite formation, (b)
shuttling effect of polysulfide [9]
Figure 3. Schematic illustration showing
preparation of the Al2O3|GF|CNT separator

3. Reference
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Shahbazian-Yassar, R., Libera, J., Mane, A. and Elam, J. (2017). Lithium metal protected by atomic
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