Stanford team reports 3D electrode structure addressing major limiting characteristics of sulfur cathodes for Li-S batteries
|Rate performance of the composite cathode at different C rates ranging from 0.05C to 1C. Credit: ACS, Liang et al.Click to enlarge.|
A team at Stanford University led by Prof. Yi Cui recently reported in a paper in the journal ACS Nano the development of a three-dimensional (3D) electrode structure for Li-sulfur batteries that simultaneously achieves both sulfur physical encapsulation and polysulfides binding. The composite electrode is based on hydrogen-reduced TiO2 with an inverse opal structure that is highly conductive and robust toward electrochemical cycling.
With such a TiO2-encapsulated sulfur structure, the sulfur cathode can deliver a high specific capacity of 1100 mAh/g in the beginning, with a reversible capacity of 890 mAh/g after 200 cycles of charge/discharge at a C/5 rate. Coulombic efficiency was also maintained at around 99.5% during cycling. The researchers suggested that their results showed that the inverse opal structure of hydrogen-reduced TiO2 represents an effective strategy in improving the performance of lithium sulfur batteries.
Research into anode materials in secondary [rechargeable] LIB has seen tremendous progress with the successful demonstration of high specific capacity in silicon nanostructures. Whereas the specific capacity of silicon can reach around 3000 mAh/g (with theoretical value of 4200 mAh/g), the practical specific capacity for most commercial transition metal oxide cathodes is limited to 200 mAh/g. The mismatch in the cathode/anode pairs and the relatively slow research progress on cathodes become a big hurdle in fully exploiting the potential of lithium-ion batteries.
When pairing with Li metal anodes, sulfur system can deliver a specific capacity as high as 1675 mAh/g and an energy density of 2500 Wh/kg or 2800 Wh/L.… Despite the various advantages, sulfur cathodes suffer from several inherent challenges. Sulfur itself and the discharge product lithium sulfide are both insulating (conductivity of elemental sulfur can be as low as 5 x 10-30 S/cm at room temperature), which leads to the limited utilization of active material and low rate capability along with a poor electrochemical reversibility. Sulfur cathodes face a significant volume change (up to 80%) during discharge.
Moreover, the lithium polysulfides (Li2Sn, 4 < n < 8) formed as intermediate products during the charge/discharge process are highly soluble in the organic electrolyte. Dissolved lithium polysulfides diffuse to the lithium anode and then get reduced into lower order polysulfides. These lower order polysulfides can diffuse back to the cathode during cycling to cause the “shuttle effect”, which gives rise to the fast capacity decay, low Coulombic efficiency and limited cycle performance.—Liang et al.
To address all these issues, the Stanford team decided to combine all the relevant design criteria—e.g., sulfur immobilization, hindering of polysulfides dissolution and trapping of polysulfides, and reduced complexity—into an integrated design.
|Cross-sectional SEM image of the composite structure showing sulfur particles well encapsulated by the reduced TiO2 nanospheres. Scale bar is 2 μm. Credit: ACS, Liang et al. Click to enlarge.|
They prepared the inverse opal structure using a polystyrene opal template method. An aqueous suspension of polystyrene nanoparticles was drop-casted onto an aluminum substrate. In the drying process, the polystyrene spheres self-assemble to form a close-packed layer-by-layer network; the 3D ordered structure served as a template for TiO2 growth. Amorphous TiO2 was formed on the polystyrene spheres through a low temperature atomic layer deposition (ALD) with oxygen plasma pretreatment to ensure conformal coating. The polystyrene template was removed after annealing in hydrogen atmosphere.
The resulting 3D hierarchically porous framework consist of highly ordered nano- pores (780 nm in diameter) interconnected with each other by channels.
… the 3D interconnected network along with open channels allows facile electrolyte permeation and fast ionic transport. In addition, the complex architecture and TiO2 itself effectively retain polysulfides both by physical trapping and chemical interaction. These attributes of this structure enable high specific capacity and remarkable cycle stability in the Li-S system.—Liang et al.
Hydrogen-reduced TiO2 inverse opal has following attributes:
Hydrogen treated TiO2 gains a significant increase in conductivity making this large bandgap semiconductor a promising current collector.
Rapid electron and lithium-ion transport is facilitated by the 3D framework and thin TiO2 shell (∼25 nm).
The oxygen vacancies generated during reduction process may promote the interaction between TiO2 and sulfur (S–Ti–O), which can improve the TiO2 surface adsorption of polysulfides.
The TiO2-x/sulfur composite material was fabricated by a simple melt-diffusion process. Little sulfur was deposited on the top surface; the researchers concluded that sulfur is mainly homogeneously coated onto the inner surface instead of the exterior surface.
To test the electrochemical performance of the TiO2-x/sulfur nanocomposite, the team assembled coin cells with lithium foil as counter/reference electrode.
At a low current rate of C/20, they cells delivered high specific capacity of 1250 mAh/g without any noticeable overpotential. As the current rate increased, the specific capacity was reduced slightly to 1050 mAh/g at C/5 and 990 mAh/g at C/2.
The design successfully integrates the physical confinement and chemical adsorption of polysulfides. The relatively enclosed 3D structure provides an ideal architecture for sulfur and polysulfides confinement. The openings at the top surface allow sulfur infusion into the inverse opal structure. In addition, chemical tuning of the TiO2 composition through hydrogen reduction was shown to enhance the specific capacity and cyclability of the cathode.
The reduced TiO2 has high electrical conductivity. The small dimension of the nanopores leads to a high specific surface area and thus a high utilization of active material. The hollow structure offers enough space to accommodate the volume change and relax strain during cycling.
Therefore, hydrogen reduced TiO2 inverse opal would have the potential use as a novel electrode structure for Li-S system. And this concept opens up a new avenue for constructing sulfur cathodes with metal oxides.—Liang et al.
Zheng Liang, Guangyuan Zheng, Weiyang Li, Zhi Wei Seh, Hongbin Yao, Kai Yan, Desheng Kong, and Yi Cui (2014) “Sulfur Cathodes with Hydrogen Reduced Titanium Dioxide Inverse Opal Structure,” ACS Nano 8 (5), 5249-5256 doi: 10.1021/nn501308m