The use of the sulfur electrode in liquid electrolyte is still limited by the formation of soluble polysulfide Li₂S_x (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. Li₂S_x) 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 Li_xSn-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 g_S⁻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 g_S⁻¹ at an average voltage of 1.5 V, hence with a theoretical energy density ranging from 2700 to 750 Wh kg_S⁻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)

Resources

• 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