Berkeley Lab team demonstrates high-rate, high-energy, long-life Li/S battery in the lab; looking for industry partners
Researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory have demonstrated in the laboratory a lithium-sulfur (Li/S) battery that has more than twice the specific energy of lithium-ion batteries, and that lasts for more than 1,500 cycles of charge-discharge with minimal decay of the battery’s capacity.
In a paper in the ACS journal Nano Letters, the team reported that a Li/S cell employing a sulfur-graphene oxide (S–GO) nanocomposite cathode can be discharged at rates as high as 6C (1C = 1.675 A/g of sulfur) and charged at rates as high as 3C while still maintaining high specific capacity (800 mA·h/g of sulfur at 6C), with a long cycle life exceeding 1,500 cycles and an extremely low decay rate (0.039% per cycle)—perhaps the best performance demonstrated so far for a Li/S cell.
The initial estimated cell-level specific energy of the cell was 500 W·h/kg—much higher than that of current Li-ion cells (~200 W·h/kg). Even after 1,500 cycles, the cell exhibited a very high specific capacity (740 mA·h/g of sulfur), which corresponds to 414 mA·h/g of electrode—still higher than state-of-the-art Li-ion cells. These Li/S cells with lithium metal electrodes can be cycled with an excellent Coulombic efficiency of 96.3% after 1,500 cycles, which, the team said, was enabled by its new formulation of the ionic liquid-based electrolyte.
For electric vehicles to have a 300-mile range, the battery should provide a cell-level specific energy of 350 to 400 Watt-hours/kilogram (Wh/kg), they noted. This would require almost double the specific energy (about 200 Wh/kg) of current lithium-ion batteries. The batteries would also need to have at least 1,000, and preferably 1,500 charge-discharge cycles without showing a noticeable power or energy storage capacity loss.
Lithium-sulfur batteries are attractive for electric vehicles and advanced electronic devices due to their much higher theoretical specific energy (∼2600 W·h/kg) than that of current lithium-ion cells (∼600 W·h/kg). This is due to the very high specific capacity of sulfur (1675 mA·h/g), based on a two-electron reaction (S + 2Li+ + 2e− ↔ Li2S)—significantly larger than the specific capacities of current cathode materials (130−200 mA·h/g).
Li/S batteries would be cheaper than current Li-ion batteries, and they would be less prone to safety problems that have plagued Li-ion batteries, such as overheating and catching fire.
However, the poor cycle life and rate capability have remained a grand challenge, preventing the practical application of this attractive technology. During discharge lithium polysulfides tend to dissolve from the cathode in the electrolytes and react with the lithium anode forming a barrier layer of Li2S. This chemical degradation is one reason why the cell capacity begins to fade after just a few cycles.
Another problem with Li/S batteries is that the conversion reaction from sulfur to Li2S and back causes the volume of the sulfur electrode to swell and contract up to 76% during cell operation, which leads to mechanical degradation of the electrodes. As the sulfur electrode expands and shrinks during cycling, the sulfur particles can become electrically isolated from the current collector of the electrode.
|A schematic of a lithium-sulfur battery with SEM photo of silicon-graphene oxide material. Source: Berkeley Lab. Click to enlarge.|
The prototype cell uses several electrochemical technologies to address this array of problems. For one, the S-GO cathode can accommodate the volume change of the electrode active material as sulfur is converted to Li2S on discharge, and back to elemental sulfur on recharge.
To further reduce mechanical degradation from the volume change during operation, the team used an elastomeric binder. By combining elastomeric styrene butadiene rubber (SBR) binder with a thickening agent, the cycle life and power density of the battery cell increased substantially over batteries using conventional binders.
To address the problem of polysulfide dissolution and the chemical degradation the research team applied a coating of cetyltrimethyl ammonium bromide (CTAB) surfactant that is also used in drug delivery systems, dyes, and other chemical processes. CTAB coating on the sulfur electrode reduces the ability of the electrolyte to penetrate and dissolve the electrode material.
Furthermore, the team developed a novel ionic liquid based electrolyte. The new electrolyte inhibits polysulfides dissolution and helps the battery operate at a high rate, increasing the speed at which the battery can be charged up, and the power it can deliver during discharge. The ionic liquid-based electrolyte also significantly improves the safety of the Li/S battery, as ionic liquids are non-volatile and non-flammable.
In summary, we have developed a long-life, high-rate Li/S cell with a high specific energy through a multifaceted approach by uniquely combining CTAB-modified S−GO nanocomposite with an elastomeric SBR/CMC binder and an ionic liquid-based novel electrolyte containing LiNO3 additive.… With the estimated high specific energy, long cycle life, and excellent rate capability demonstrated in this work, the Li/S cell seems to be a promising candidate to challenge the dominant position of the current Li-ion cells.—Song et al.
The team is now seeking support for the continuing development of the Li/S cell, including higher sulfur utilization, operation under extreme conditions, and scale-up. Partnerships with industry are being sought. The next steps in the development are to further increase the cell energy density, improve cell performance under extreme conditions, and scale up to larger cells.
The paper was authored by Min-Kyu Song (Molecular Foundry, Berkeley Lab), Yuegang Zhang (Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences) and Elton Cairns (Environmental Energy Technologies Division, Berkeley Lab). The research was funded by the US Department of Energy’s Office of Science and a University of California Proof of Concept Award.
The Molecular Foundry is one of five DOE Nanoscale Science Research Centers (NSRCs), national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize, and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative.
The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos national laboratories.
Min-Kyu Song, Yuegang Zhang, and Elton J. Cairns (2013) “A Long-Life, High-Rate Lithium/Sulfur Cell: A Multifaceted Approach to Enhancing Cell Performance,” Nano Letters doi: 10.1021/nl402793z