New method of synthesizing sulfur-impregnated carbon nanotube cathodes for Li-S batteries results in superior cyclability and Coulombic efficiency; benefits of high-temperature heat-treatment
|Cycling stability (left) and Coulombic efficiencies (right) of SDCNT cathodes. Credit: ACS, Guo et al. Click to enlarge.|
Researchers at the University of Maryland have devised a new method of impregnating sulfur into disordered carbon nanotubes (DCNTs) as cathode material for Li-S batteries. In a paper in the ACS journal Nano Letters, they report that the obtained sulfur–carbon tube cathodes demonstrate superior cyclability and Coulombic efficiency.
The team suggests that the improvement was induced by their high-temperature heat treatment of SDCNTs (sulfur disordered carbon nanotubes) in a vacuum environment, and further suggests that the resulting electrochemical characterization indicates a new stabilization mechanism of sulfur in carbon.
Lithium-sulfur is an attractive battery technology, offering a very high theoretical specific energy density (2,500 Wh kg-1), attributed to the 1,675 mAh g-1 theoretical capacity of the sulfure cathode. (Earlier post.) However, a number of complex barriers remain to commercialization.
Li-S batteries are based on the reversible redox reaction between lithium and sulfur with lithium sulﬁde (Li2S) as the final product of sulfur reduction (discharge). During the redox reaction between lithium and sulfur, there are several intermediate reactions with lithium polysulfide intermediate products (Li2Sn).
Lithium polysulﬁdes are electrical insulators so that in-depth discharge of sulfur could be diﬃcult, leading to low utilization of sulfur and low rate capacity.
In their paper, Guo et al. note that this problem could be alleviated by using electrolyte with polar organic solvents that could dissolve lithium polysulﬁdes to some extent. However, the dissolved lithium polysulﬁdes can diﬀuse to the anode to directly react to the Li metal, forming lower order polysulﬁdes including insoluble Li2S2 and Li2S, which will deposit on the Li anode, leading to capacity fading. The lower order polysulfides can be reoxidized to higher order forms, creating an internal “shuttle mechanism” resulting in severe low Coulombic efficiency— i.e., charge capacity higher than the corresponding discharge capacity, especially at low charge/discharge rate.
It is well recognized nowadays that the problem of polysulfide dissolution has to be solved to realize LiS technology. Current methodologies to alleviate this problem could be sorted into two categories: The first one was to physically restrain polysulfide dissolution using barrier materials. The physical restraint methods included carbon coating on sulfur, high surface-area carbon additives, and polymeric electrolytes.
...The second methodology was to use mesoporous silica as intermediate polysulﬁdes absorber through weak bonding, or using metal (such as copper and nickel) as sulfur absorber by forming metalS alloys.
...Demonstrated by previous studies, it is clear that both methodologies have limitations. The ideal scenario is that the sulfur should be locked and bound in certain carbon structures and not be in direct contact with the electrolyte. Therefore, the redox reaction between sulfur and Li mostly takes place through a carbon barrier that is both electron and Li ion conductive, without direct contact with electrolyte. Also, the cyclic octatomic S8 molecule should be broken into smaller structures (such as S6 or S2) so that soluble high-order polysulﬁdes could be eliminated even if the liquid electrolyte penetrates into carbon and reacts with sulfur. Unfortunately, these perspectives have not been characterized to date.
In this study, we validated the above hypothesis by carrying out a new method of impregnating sulfur into disordered carbon nanotubes (DCNTs) as cathode material for LiS batteries.—Guo et al.
After initial sulfur impregnation, the materials were further processed by heating the SDCNTs in a vacuum-sealed quartz tube under three different temperatures: SDCNT-160 was obtained by being heated at 160 °C for 10 h. SDCNT-300 and SDCNT-500 were obtained by being further heated at 300 and 500 °C for 3 h more, respectively.
The disordered DCNTs possess graphitic clusters and amorphous carbon structures that were accessible by sulfur vapor, thus serving as the sulfur host and preventing liquid electrolyte penetration. Most of the previous studies incorporated sulfur into porous carbon by heating the sulfurcarbon mixture at around 155 °C under the protection of inert gas. The reason was that elemental sulfur (S8) became liquid and had the lowest viscosity at 155 °C so that the liquid sulfur could infuse into the host structure.
A potential problem of this method was that liquid electrolyte could still reach into the sites where liquid sulfur could diffuse into. On the contrary, sulfur can be vaporized at elevated temperature in vacuum so that sulfur vapor could intercalate into carbon voids and even into graphite layers of graphitic clusters depending on temperature. Therefore, we heated the sulfur-impregnated DCNTs at 160, 300, and 500 °C in vacuum-sealed quartz tubes (denoted as SDCNT-160, SDCNT-300, and SDCNT-500, respectively).—Guo et al.
The researchers assembled two-electrode coin cells with lithium foil as the counter electrode in an argon-filled glovebox for the electrochemistry analysis.
Among their findings were that the SDCNT-160 had the fastest capacity fading of 34.8% retention after 100 cycles. The SDCNT- 300 cathode showed better stability of 53.3% retention after 100 cycles except the first one, and the SDCNT-500 cathode had the best performance among all three with 72.9% retention after 100 cycles except the first one: after 30 cycles, the capacity of SDCNT-500 stopped decreasing completely.
The large capacity drop between the first and second cycle for SDCNT-300 and SDCNT-500 could be attributed to the superficial sulfur deposited on the surface of disordered carbon nanotubes during the cooling process after the heating. These observations indicated that the heat treatment had a profound eﬀect on the performance of SDCNTs cathodes.—Guo et al.
They also found that the Coulombic efficiency was improved with increased heating temperature. The Coulombic efficiencies for SDCNT-300 and SDCNT-500 at 0.25C rate were pm average at 89% and 96% during 100 cycles, respectively. The 96% Coulombic efficiency of SDCNT-500 at 0.25C is the highest value in open literature for a sulfur cathode, they said.
The elimination of the polysulfide shuttle mechanism in SDCNT-300 and SDCNT-500 cathodes indicated the possibility that the sulfur was incorporated in the voids/graphite layers in the partially graphitized DCNTs carbon structure, which could not be directly contacted by electrolyte.
...We believe such improvement was induced by the high temperature heat treatment of SDCNTs in a vacuum environment. The hypothesis is that the vaporized sulfur can be incorporated into graphitized carbon layers and smaller voids/defects in amorphous carbon that liquid electrolyte cannot directly reach. Moreover, the heat treatment could break down the S8 molecule to S6 or S2 and enable sulfurcarbon bonding so that the conventional LiS8 reaction with dissolvable polysulfide intermediate products might be altered.—Guo et al.
Juchen Guo, Yunhua Xu and Chunsheng Wang (2011) Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium–Sulfur Batteries. Nano LettersArticle ASAP DOI: 10.1021/nl202297p