Researchers demonstrate alternate approaches to building Li-S batteries for improved capacity retention
11 February 2013
|Cycling performance of the different samples (the specific capacity was calculated by using the active material mass (sulfur) of the composites, given in mA h g-1). Demir-Cakan et al. Click to enlarge.|
A team from the University of Picardie Jules Verne (France) and Alistore ERI (European Research Institute) has demonstrated new approaches to lithium-sulfur (Li-S) rechargeable batteries (earlier post) with improved capacity retention. An open access paper on their work was recently published in the RSC journal Energy and Environmental Science.
Li-S batteries face a number of commercialization challenges, including electrolyte-soluble polysulfides. To counter the solubility of polysulfides, other teams have pursued confinement approaches aiming to trap sulfur within the cathode side; however, success has been limited, the French team noted. Instead, they “drastically deviate[d] from this approach” and used a liquid cathode obtained by dissolving polysulfides within the electrolyte (catholytes) and also placed sulfur powders in contact with the Li negative electrode (“SLi”).
Such approaches resulted in greater performance than confinement approaches, they found. The strategy eliminates the detrimental Li2S formation inside a porous carbon matrix and moreover leads to the formation of a protective SEI layer at the Li electrode, which seems beneficial to the cell cycling performance.
The lithium–sulphur (Li–S) battery system is one of the viable options for electric vehicles to achieve a long driving range (i.e. >300 km) since the present prototypes effectively offer higher specific energy density than conventional lithium ion batteries. Despite such an appeal, Li–S batteries are not yet commercialized, the reason being that they suffer from rapid capacity fading. This poor performance comes from each cell compartment. Among them are: (i) the use of a Li metal anode which essentially brings safety problems with liquid electrolytes, (ii) the low active material utilization due to the insulating nature of sulphur, (iii) the soluble polysulphide species generated during the battery operation, which diffuse throughout the separator and deposit on the Li electrode resulting in a loss of active material, and (iv) the irreversible deposition of non-soluble lithium sulphide (Li2S) both at the cathode and Li anode.
These issues are not new as this battery technology has been extensively studied over the past five decades, nevertheless they partially remain despite intense research efforts. Recently, most of the studies have been aiming at finding the best cathode configuration via a precise control of its porosity so as to retain part of the electrochemically generated polysulphide species generated at the cathode side.
In this paper we deviate from these approaches and explore the use of chemically synthesized polysulphide species as an active material rather than sulphur impregnated composites in order to eliminate the formation of Li2S within a porous structure. Cells that have polysulphides acting as active materials are shown to present superior performance to the conventional Li–S cell configuration. Moreover, we found that these performances can even be further enhanced when sulphur is directly deposited on the Li negative electrode.—Demir-Cakan et al.
For the study, the team used 1 M lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) containing tetra-methylene sulfone (TMS) as the electrolyte. Polysulphides with a nominal formula of Li2Sx (x>2) were chemically synthesized and dissolved in the electrolyte (catholyte) and used directly as an active material. Sulfur loading in the polysulfide was carefully tuned.
In the study, the researchers compared the performance of two different catholytes made by dissolving polysulfides of different lengths (Li2S2 and Li2S8); the SLi approach; and a mesoporous C–S composite.
Targeting the origin of the rapid capacity decay in Li–S batteries is a must if we ever want this system to become a reality for load leveling and transport. To address this issue we report on two different approaches enlisting the use of either polysulphides as active materials or S deposited on Li, both aiming to eliminate the detrimental formation of Li2S at the porous carbon matrix. Besides leading to performance improvements in terms of capacity retention, these approaches have also led to better insights regarding the impact of sulfur deposited on the Li surface. Via the use of EIS spectroscopy, we have shown evidence for the growth of a specific SEI which can act as a self-limiting barrier for chemical reactions while enabling to carry Li-ions upon application of an electrical field.
To conclude, this SEI layer seems to combine attractive features, hence the crucial importance to understand both its nature and composition and to pursue more intensive chemical/physical analysis enlisting combined XPS-NMR surface analytical techniques. Although not fully understood, such a finding, which somewhat mimics what happens in Li–thionylchloride primary cells, holds some promises regarding the feasibility to build Li–S cells differently for sustainable performance.—Demir-Cakan et al.
Rezan Demir-Cakan, Mathieu Morcrette, Gangulibabu, Aurélie Guéguen, Rémi Dedryvère and Jean-Marie Tarascon (2012) Li–S batteries: simple approaches for superior performance. Energy Environ. Sci., 6, 176-182 doi: 10.1039/C2EE23411D
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