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Estimated cell specific energy plot (including all components except the cell housing) as a function of the specific capacity based on S and the S content of the electrode. Data reported in prior studies are marked by blue squares for comparison purposes; the data of the Berkeley Lab work are indicated by the red star. Credit: ACS, Cai et al. Click to enlarge. |
Researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) have developed nanostructured lithium sulfide/carbon (Li2S–C) composite cathodes that show promise for use in high-energy batteries. The paper on their work, published in the ACS journal Nano Letters, follows shortly after an earlier report from a Stanford team led by Yi Cui on another approach to using lithium-sulfide materials to build rechargeable batteries with specific energies of about 4 times that of current technology and approaching those of lithium-sulfur (LiS) systems, while avoiding some of the issues with those systems.
The Berkeley Lab team reported that, with a very high specific capacity of 1144 mA·h·g–1 (98% of the theoretical value) obtained at a high Li2S content (67.5 wt %), the estimated specific energy of a cell using the nanostructured composite was 610 W·h·kg–1—the highest demonstrated so far for lithium-sulfide cells. The cells also maintained good rate capability and improved cycle life.
Newly emerging technologies such as electric vehicles (EV) and advanced portable electronics are placing a strong and urgent demand on the next generation of rechargeable batteries with high specific energy. Current lithium-ion cells with oxide-based cathodes, such as LiCoO2 and LiMn2O4, have theoretical specific energies of approximately 430−570 W·h·kg−1, but their practical (or obtainable) specific energies are only in the range of 120−200 W·h·kg−1, which is insufficient for long EV driving ranges (i.e., >300 km). In this regard, the lithium/sulfur cell is considered to be a potential candidate to replace current lithium-ion cells because its theoretical specific energy and volumetric energy density are estimated to be 2600 W·h·kg−1 and 2800 W·h·L−1, respectively, based on the electrochemical reaction 16Li + S8 = 8Li2S. Additionally, the abundant availability and low price of sulfur offer the opportunity for a significant cost reduction.
However, the insulating nature of sulfur, dissolution and shuttling of lithium polysulfides during cycling, and their high reactivity with the lithium metal anode, together with significant volume change, are currently preventing the use of this promising system in practical applications.
Recently, the lithium sulfide (Li2S) cathode, with a theoretical capacity of 1166 mA·h·g−1, has received much attention due to the potential to use non-lithium anodes; other high-capacity anode materials (e.g., silicon or tin-based compounds which can form alloys with lithium) can be used as negative electrodes with improved safety. To resolve the insulating problem of Li2S that prevents the achievement of high utilization (or high capacity), various efforts have been made to improve the contact between Li2S and electronic conductive additives such as carbon and metals.
...Despite these efforts, a relatively high capacity could only be obtained when the Li2S content is lower than 50 wt %, while higher Li2S contents often resulted in very low discharge capacity (i.e., only ∼200 mA·h·g−1 at 76.8 wt %). To meet the rigorous requirements of high specific energy for EV applications, we need to dramatically increase the loading of Li2S while maintaining good electrochemical utilization and good cycle life.
—Cai et al.
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Cycling performance of Li2S−C composite electrodes. The capacity is normalized both by the weight of Li2S and sulfur. The average loading of the electrodes is 0.794 mg·cm−2, which corresponds to 0.54 mg·cm−2 of Li2S. Credit; ACS, Cai et al. Click to enlarge. |
The Berkeley team devised a cost-effective way of preparing nanostructured Li2S-carbon composite cathodes via the high-energy dry ball milling of commercially available micrometer-sized Li2S powder together with carbon additives. This overcame the difficulties in efficiently converting lithium sulfide to sulfur due to the particle size of commercial Li2S powder (between 10 and 30 μm) and its insulating nature.
After high-energy dry ball milling for 2 h, the size of Li2S particles was reduced to about 200−500 nm with some agglomeration, while carbon black was found to be uniformly dispersed and deposited onto the surface of these smaller Li2S particles. The smaller dimensions can effectively reduce the distance that Li-ions and electrons must travel during cycling in the solid state.
They then used a simple but effective electrochemical activation processto significantly improve the utilization and reversibility of the Li2S–C electrodes, which was confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. They further improved the cycling stability of the Li2S–C electrodes by adding multi-walled carbon nanotubes (MWCNT) to the nanocomposites.
With the highest specific energy (∼610 W·h·kg−1) demonstrated in this report, and with further improvement in capacity retention, this Li2S−C nanocomposite electrode may offer a significant opportunity
