|Discharge capacity (left axis) and Coulombic efficiency (right axis) of the Li2S-C cathode as a function of cycle number. A fixed current density of 200 mA g-1 was used for these measurements. Credit: ACS, Guo et al. Click to enlarge.|
Researchers at Cornell University are proposing a new scheme for cathodes for lithium-sulfide batteries (earlier post) to prevent lithium polysulfide dissolution and shuttling during electrochemical cycling. Their approach, described in a paper published in the Journal of the American Chemical Society, creates composites based on lithium sulfide uniformly dispersed in a carbon host, which serve to sequester polysulfides.
Li-sulfur batteries—which conventionally use elemental sulfur (with conductive additives) as the cathode, an aprotic liquid electrolyte, and lithium metal as the anode—are under intensive investigation by research groups worldwide because of the promise for low-cost, high-energy storage. (Earlier post.) Lithium sulfide (Li2S) is a promising cathode material for high-energy lithium ion batteries because, unlike elemental sulfur, it obviates the need for metallic lithium anodes.
Li2S also has a theoretical capacity of 1,166 mAh g-1—nearly 1 order of magnitude higher than traditional metal oxides/phosphates cathodes. If paired with Si anodes with 2,000 mAh g-1 capacity, the specific energy of a Li2S-based lithium-ion battery could be 60% higher than the theoretical limit of metal oxide/phosphate counterparts. (Earlier post.)
As with elemental sulfur, however, a successful lithium sulfide cathode requires a mechanism for preventing lithium polysulfide dissolution and shuttling during electrochemical cycling.
Lithium sulfide (Li2S), the fully lithiated sulfur product, is already under active investigation for its promise as a cathode. Because the cathode is lithiated, it can be paired with high capacity anode materials other than metallic lithium. Also, unlike sulfur that sublimes at a modest temperature, Li2S has a high decomposition temperature above 900 °C, which improves its processing in carbon composites. The particular property of Li2S we utilize in our synthesis is the capacity of the lithium ions to strongly interact with electron-donating groups in carbon-precursor polymers such as polyacrylonitrile (PAN).
Specifically, lone pair electrons in the nitrile group of PAN are capable of interacting with lithium through a coordination bond-like interaction. Thus, when lithium sulfide is mixed with PAN in a homogeneous solution, Li2S may function as a cross-linking agent, which interconnects the PAN network via lithium sulfide net-nodes. We hypothesize that, in addition to stiffening the PAN framework, such linkages favor uniform dispersion of Li2S in the PAN matrix. We show that the resultant lithium sulfide-PAN cross-linked matrix can be carbonized at elevated temperature in an inert environment to obtain an ideal Li2S-C composite cathode material in which Li2S is uniformly and completely dispersed in carbon.—Guo et al.
The Cornell team’s synthesis methodology makes use of interactions between lithium ions in solution and nitrile groups uniformly distributed along the chain backbone of a polymer precursor (e.g., polyacrylonitrile), to control the distribution of lithium sulfide in the host material.
The method involves the co-dissolution of Li2S3 salt (easily created from Li2S) and PAN in dimethylformamide (DMF). The co-dissolution promotes uniform dispersion in a high-dielectric constant DMF medium, which favors ion pair dissociation of Li2S3 and cross-linking of the polymer.
The cross-linked polymer was then treated at 100 °C for 48 h under vacuum to remove the DMF. The resultant solid material was pulverized by mechanical ball milling to yield a fine powder, which was heated in an argon-filled furnace at 300 °C for 2 h.
They evaluated the synthesized material as cathode materials in a half-cell lithium battery. Under a charge/discharge current of 200 mA g-1, the materials showed stable reversible capacities of 500 mA g-1 and Coulombic efficiencies of nearly 100%. This indicated the effectiveness of the dispersed Li2S architecture in sequestering sulfur and inhibiting shuttling reaction, the researchers suggested.
We believe that similar approaches can be used to control the distribution of other metal salts in polymer- or carbon-based composites. Preliminary results indicate that carbon-Li2S composites created using the new approach offer superior potential, in comparison to other reported methods, as cathode materials for high-energy lithium ion batteries with great cycling stability and excellent Coulombic efficiency.—Guo et al.
Juchen Guo, Zichao Yang, Yingchao Yu, Héctor D. Abruña, and Lynden A. Archer (2012) Lithium–Sulfur Battery Cathode Enabled by Lithium–Nitrile Interaction. Journal of the American Chemical Society doi: 10.1021/ja309435f