Optimized Solvent-in-Salt Electrolytes for High-Energy Rechargeable Metallic Lithium Batteries

Lithium-based batteries as a highly efficient energy storage device has long been considered as promising power supply in various electric vehicles (EVs). However, present available lithium ion technology could not satisfy the increasing demand on energy density. Metallic lithium batteries still exhibit the highest energy density among the secondary batteries. For instance, Li-S and Li-O₂ batteries have high theoretical specific energy density of 2654 Wh/kg and 3500 Wh/kg, respectively. However, metallic lithium batteries inevitably suffer from the safety concern due to the formation of lithium dendrite during cycling. Generally, the employed electrolyte plays an important role in influencing the interfacial properties and morphology of the metallic lithium anode. Thus, in order to make the metallic lithium anode stable in long cycling process, here, we show a new class of “Solvent-in-Salt” (SIS) electrolyte with ultrahigh lithium salt concentration, in which salt holds a dominant position in whole lithium ion transport system (1,2). This new electrolyte remarkably enhances cyclic and safety performance of rechargeable metallic lithium batteries via effectively suppressing the formation of lithium dendrites and greatly reducing the degree of damage of metallic lithium anode.

  In this presentation we will take lithium-sulfur battery as an example to show the power of SIS. Compared with traditional non-aqueous electrolyte (usually 1-2 mol/L), the SIS electrolyte (5-7 mol/1 L) exhibits the unique physicochemical properties: high lithium transference number (0.73), high viscosity (72 cP) and acceptable ionic conductivity (0.814 mS/cm @ room-temperature) as well as low glass transition temperature of -77.3oC. When used in Li-S batteries, the Li storage performance is significantly improved. It shows an initial specific discharge capacity of 1041 mAh/g at a rate of 0.2C and maintains a reversible capacity of  770 mAh/g with capacity retention of 74% after 100 cycles. More importantly, the coulombic efficiency reaches nearly 100% after the first cycle (the first one is 93.7%). The improved performance can be ascribed to 1) the dissolution of polysulfides in electrolyte is fully inhibited and 2) the corrosion of lithium anode is greatly reduced and the formation of lithium dendrites is effectively suppressed in this ultrahigh salt concentration electrolyte. Furthermore, we optimized the composition of the SIS electrolyte and carbon supports and further improved the performance (3). We also applied this new SIS electrolyte into the soft package of Li-S battery whose performance will be shown and discussed.

References

[1] A Chinese patent (application number: 201210017535.0) was filed on Jan. 19, 2012.

[2] L. M. Suo, Y.-S. Hu, H. Li, M. Armand, L. Q. Chen, Nature Communications, 2013, 4, 1481.

[3] Z. Fang, L. M. Suo, Y.-S. Hu, H. Li, L. Q. Chen, 2013, in preparation.

Figure 1 A soft package of Li-S battery using our Solvent-in-Salt electrolyte