...intercalation cathodes used in current lithium-ion batteries possess an inherent theoretical capacity limit of ~300 mAh g
-¹, which is a major factor limiting the specific energy of such batteries. These inherent theoretical constraints hinder the widespread use of lithium-ion batteries in many emerging applications such as vehicle electrification, thus impelling the pursuit of next-generation cathode materials with much higher specific capacities.

Sulfur is a promising cathode material with a high theoretical capacity of 1673 mAh g^-1...However, further progress is hindered by the need for pairing with a lithium metal anode which is prone to dendrite formation and other safety-related challenges.

Compared to sulfur, fully-lithiated Li₂S (theoretical capacity 1166 mAh g^-1) represents a more attractive cathode material because it enables pairing with high-capacity lithium metal-free anodes (such as silicon or tin), hence obviating dendrite formation and safety concerns associated with metallic lithium. Moreover, the high melting point of Li₂S (unlike that of sulfur) imparts greater ease of processing in the synthesis of carbon-based composite cathode materials. Despite the inherent promise, there have only been a handful of reports on Li₂S cathodes to date.

—Seh et al.

If paired with Si anodes with 2000 mAh/g capacity, the specific energy of a Li₂S-based lithium-ion battery could be 60% higher than the theoretical limit of metal oxide/phosphate counterparts, and three times that of the current prevailing LiCoO₂/graphite system.

Although the theoretical specific energy of the Li₂S/silicon system (1550 Wh/kg) is only about 60% of that of lithium-sulfur system, practically, significantly more lithium is required in Li/S batteries due to formation of mossy lithium and the low Coulomb efficiency of lithium, the team had noted in their earlier paper. As a result, the practical specific energy of Li₂S/ silicon (930 Wh/kg) is close to that of the practical Li/S battery (1000 Wh/kg).

Li₂S could also be paired with a lithium-free anode, preventing safety concerns and low Coulomb efficiency of lithium metal in Li/S batteries.

However, as a team from UC Berkeley noted in its recent paper on lithium-sulfide batteries, the insulating nature of sulfur; dissolution and shuttling of lithium polysulfides during cycling; and their high reactivity with the lithium metal anode, together with significant volume change, are currently preventing the use of this promising system in practical applications. (Earlier post.)

Most efforts to improve lithium-sulfide batteries have focused on the active cathode material itself to attempt to improve the overall conductivity and limit polysulfide dissolution. Very little attention has been placed on electrochemically-inactive components such as binders.

Yet recent studies have shown that the use of effective binders can have a profound effect on the structural stability, kinetics and long-term cycling performance of electrode materials including silicon and sulfur. In the case of sulfur cathodes, the effect of different binders on their electrochemical performance has been well-studied,31–33 with the most commonly-used binder being poly-(vinylidene fluoride) (PVDF).

Developing an effective binder for Li₂S cathodes as opposed to sulfur requires a paradigm shift because Li₂S is ionic and highly-polar whereas sulfur is covalent and non-polar in nature. Because of this difference in bonding and chemical nature, binder materials that are known to interact strongly with sulfur particles to act as good dispersion agents might not be effective for Li₂S and vice versa.

—Seh et al.

Professor Cui and his colleagues used ab initio simulations to elucidate the interaction between Li₂S and lithium polysulfides to guide the rational selection of PVP as a binder. The PVP allows smooth distribution of the lithium ions throughout the cathode and minimizing the loss of polysulfides.

Given the simplicity of our strategy, the appropriate choice of binder can be combined with more elaborate cathode structures such as Li₂S–carbon nanocomposites to further mitigate polysulfide dissolution and capacity decay. Insight gained from this work, particularly through ab initio simulations, can be extended to other promising high-capacity electrode materials for the future development of novel binders with precisely-tailored functionalities.

—Seh et al.

Resources

• Zhi Wei Seh, Qianfan Zhang, Weiyang Li, Guangyuan Zheng, Hongbin Yao and Yi Cui (2013) Stable cycling of lithium sulfide cathodes through strong affinity with a bifunctional binder. Chem. Sci. doi: 10.1039/C3SC51476E