Li-ion sulfur polymer battery shows high energy density as well as safety
07 January 2015
A team from the University of Rome Sapienza has developed a rechargeable lithium-ion polymer battery based on the combination of a high capacity sulfur-carbon cathode, nanostructured LixSn-C anode and polysulfide-added PEO-based gel membrane. The cell shows very good electrochemical performances in terms of stability and delivered capacity; this electrolyte configuration allows the achievement of a stable capacity ranging from 500 to 1500 mAh gS-1, depending on the cycling rate.
Further, the use of a polymer electrolyte and the replacement of lithium metal with a Li-Sn-C nanostructured alloy for the anode should provide high safety content, they noted in their open access paper published in Nature’s Scientific Reports.
The use of the sulfur electrode in liquid electrolyte is still limited by the formation of soluble polysulfide Li2Sx (1 ≤ x ≤ 8) at the cathode and its contemporary migration in the solution, thus leading to shuttle reaction and precipitation of Li2S2 and Li2S at the anode, with consequent loss of active material and capacity fading. These issues have been recently mitigated by moving from bulk-electrodes to sulfur-carbon composites, in which the elemental sulfur is efficiently trapped within protecting carbon matrixes of various configurations. The change of the electrolyte configuration by the addition of Li-film forming salts, e.g. LiNO3, and lithium polysulfides, have been recently revealed as efficient solutions to promote the formation of a stable SEI film layer at the lithium surface, thus reducing the polysulfide shuttle effect and the cathode dissolution. Moreover, the addition of a dissolved polysulfide (i.e. Li2Sx) to liquid electrolytes, such as DOL-DME-LiTFSI and TEGDME-LiCF3SO3, has shown the most promising results in increasing the lithium-sulfur cell stability and efficiency.
However, the use of lithium metal in liquid electrolytes may lead to safety hazard associated with possible dendrite formation, cell short-circuit, heat evolution and, in presence of flammable electrolyte, to firing. Hence, alternative, not flammable electrolytes, such as inorganic glass-type lithium conducting materials or polymer membranes characterized by wide electrochemical stability window and favorable SEI film formation, are required in order to match the safety targets in batteries using lithium-metal as high capacity anode.
Furthermore, the replacement of lithium metal with alternative, high performance anodes, such as lithium alloy materials, e.g. Li-Sn and Li-Si, is considered the most suitable solution to increase the safety content of the cell. Polymer electrolytes, such as those based on PEO, still suffer from low ionic conductivity and high interphase resistance at temperature lower than 70 °C. Recent work demonstrated that PEO-based electrolytes operating at lower temperature level may be achieved by the use of various plasticizers, such as organic carbonates or glymes. This class of “gel-type” polymer electrolytes requires, however, a proper optimization, in particular in terms of cycling stability, in order to be efficiently used in lithium sulfur cell.—Agostini & Hassoun (2015)
The researchers added a polysulfide to the polymer membranes during their synthesis with the aim to reduce the cathode dissolution during operation. To improve the room-temperature ionic conductivity of the polymer electrolyte, they plasticized the LiS8 added membrane.
The resulting LixSn-C/GPS-Li2S8/S-C lithium-ion sulfur cell is characterized by a high safety level, due to the polymer configuration and the absence of lithium metal anode, and expected low cost. At the lower C-rate, the cell may stably deliver a capacity of about 1500 mAh gS−1 at an average voltage of 1.8 V, while at the higher C-rate, the cell delivers a still relevant capacity of about 500 mAh gS−1 at an average voltage of 1.5 V, hence with a theoretical energy density ranging from 2700 to 750 Wh kgS−1, respectively.
The researchers suggested that adding the polysulfide to the polymer electrolyte allowed an enhancement of the cell stability and a reduction of the polysulfide dissolution from the cathode side.
The cell here characterized may be suggested as suitable energy storage system for application requiring high energy and safety levels, such as electric vehicles motion. However, longer cycle life and further characterizations are certainly required to match the severe targets of the lithium battery community.—Agostini & Hassoun (2015)
Marco Agostini & Jusef Hassoun (2015) “A lithium-ion sulfur battery using a polymer, polysulfide-added membrane,” Scientific Reports 5, Article number: 7591 doi: 10.1038/srep07591
Another of the 1001 ways to make higher performance, lower cost future EV batteries?
When will they be mass produced and used in extended range (500 to 700 Km) BEVs?
Posted by: HarveyD | 07 January 2015 at 07:38 AM
I'm thinking the better battery will be available when Tesla puts pressure on the industry to mass produce it. The production cell and battery companies appear to be tied up with OEM car makers in joint ventures, partnerships, contract agreements, etc. and progress is proceeding on schedules dictated by the car OEM's interests; even Tesla and Panasonic are jointed together by contracts.
An example; even though Li batteries have shown incremental advancement in density, Nissan has decided to wait five years to release new cells/batteries in their cars based on a large step in density progress because it suits their profit plans.
This is just the way the car business, any large business for that matter, works in the U.S. It's all about controlling the market; not about fair competition. And, I'm afraid it's going to stay this way as long as Big Business controls the politicians in Washington.
I hope I'm wrong and next week we see where GM releases a new BEV with a 200 mile range called the "Bolt."
Posted by: Lad | 07 January 2015 at 09:55 AM
"The use of the sulfur electrode in liquid electrolyte is still limited by the formation of soluble polysulfide Li2Sx (1 ≤ x ≤ 8) at the cathode and its contemporary migration in the solution, thus leading to shuttle reaction and precipitation of Li2S2 and Li2S at the anode, .."
plus no mention of number of cycles usually spells 'not good enough'.
Posted by: kelly | 07 January 2015 at 01:56 PM
I'm starting to think there are batteries out there which are already better/cheaper and we're getting into a situation where it's "standards" slowing down rollouts.
Someone like a Nissan or Tesla needs to retool everything and be ready to change some parts which are standard on their infrastructure for production before they roll them out.
It's like seeing a new generation of hard drives on computers. There are plenty of them ready in the wings, but it has to make financial sense before some jumps up and starts production/shipping.
Posted by: DaveD | 08 January 2015 at 08:31 AM
I am not too sure it has anything to do with marketing consideration and/or conspiracies by the car companies. If there would be a significant performance increase in battery power available, the computer companies for one would jump right in, since it would give them a competitive edge. Same logic would be valid for smartphones and other mobile technologies. The fact that it hasn't happened is probably that none of these promising battery technologies are really ready for prime time. Even now Li-on batteries, after years on the market, still give trouble, witness the recent issues with the Boeing Dreamliner.
Posted by: Joe | 08 January 2015 at 01:08 PM
We are starting to see better batteries and even fast chargers in smartphones. A lot of what they are gaining is eaten up by new features like 4K displays that would have chewed up older batteries and spit them out.
Posted by: DaveD | 08 January 2015 at 03:35 PM
Oh, and I wasn't implying conspiracy, at all.
I was talking about them not having incentive to change to newer batteries in mass production until it was financially in their favor (e.g. they had written down enough cost on the old tech to justify switching and/or they had competitive pressures from other automakers).
Posted by: DaveD | 08 January 2015 at 03:38 PM