Confronting Sulfur Electrode Passivation and Li Metal Electrode Degradation in Lithium-Sulfur Batteries Using Thiocyanate Anion.

Salt anions with a high donor number (DN) enable high sulfur utilization in lithium-sulfur (Li-S) batteries by inducing three-dimensional (3D) Li2 S growth. However, their insufficient compatibility with Li metal electrodes limits their cycling stability. Herein, a new class of salt anion, thiocyanate (SCN- ), is presented, which features a Janus character of electron donor and acceptor. Due to a strong Li+ coordination by SCN- and the direct interaction of SCN- with polysulfide anions, the LiSCN electrolyte has a remarkably high lithium polysulfide solubility. This electrolyte induces 3D Li2 S formation and ameliorates cathode passivation, even more than Br- , a typical high DN anion. Moreover, SCN- forms a Li3 N-enriched stable SEI layer at the surface of the Li metal electrode, enhancing cycling stability. A Li-S battery with the LiSCN electrolyte shows high current density operation (2.54 mA cm⁻2 ) with high discharge capacity (1133 mAh g⁻1 ) and prolonged cycle life (100 cycles). This work demonstrates that the cathode and anode performance in a Li-S battery can be simply and concurrently enhanced by the single salt anion.

Tuning Two Interfaces with Fluoroethylene Carbonate Electrolytes for High-Performance Li/LCO Batteries.

Various electrolytes have been reported to enhance the reversibility of Li-metal electrodes. However, for these electrolytes, concurrent and balanced control of Li-metal and positive electrode interfaces is a critical step toward fabrication of high-performance Li-metal batteries. Here, we report the tuning of Li-metal and lithium cobalt oxide (LCO) interfaces with fluoroethylene carbonate (FEC)-containing electrolytes to achieve high cycling stability of Li/LCO batteries. Reversibility of the Li-metal electrode is considerably enhanced for electrolytes with high FEC contents, confirming the positive effect of FEC on the stabilization of the Li-metal electrode. However, for FEC contents of 50 wt % and above, the discharge capacity is significantly reduced because of the formation of a passivation layer on the LCO cathodes. Using balanced tuning of the two interfaces, stable cycling over 350 cycles at 1.5 mA cm-2 is achieved for a Li/LCO cell with the 1 M LiPF6 FEC/DEC = 30/70 electrolyte. The enhanced reversibility of the Li-metal electrode is associated with the formation of LiF and polycarbonate in the FEC-derived solid electrolyte interface (SEI) layer. In addition, electrolytes with high FEC contents lead to lateral Li deposition on the sides of Li deposits and larger dimensions of rodlike Li deposits, suggesting the elastic and ion-conductive nature of the FEC-derived SEI layer.

Rational Design of Highly Packed, Crack-Free Sulfur Electrodes by Scaffold-Supported Drying for Ultrahigh-Sulfur-Loaded Lithium-Sulfur Batteries.

Despite the notable progress in the development of rechargeable lithium-sulfur batteries over the last decade, achieving high performance with high-sulfur-loaded sulfur cathodes remains a key challenge on the path to the commercialization of practical lithium-sulfur batteries. This paper presents a novel method by which to fabricate a crack-free sulfur electrode with an ultrahigh sulfur loading (16 mg cm-2) and a high sulfur content (64%). By introducing a porous scaffold on the top of a cast of sulfur cathode slurry, the formation of cracks during the drying of the cast can be prevented due to the lower volume shrinkage of the skin. The scaffold-supported sulfur cathode delivers a notably high capacities of 10.3 mAh cm-2 and 473 mAh cm-3 after a prolonged cycle, demonstrating that the crack-free structure renders more uniform redox reactions at such high sulfur loading. The highly packed, crack-free feature of the sulfur cathode is advantageous, given that it reduces the electrolyte uptake to as low as an E/S ratio of 4 µL mg-1, which additionally contributes to the high energy density. Therefore, the scaffold-supported drying fabrication method as presented here provides an effective route by which to design practically viable, energy-dense lithium-sulfur batteries.

Achieving three-dimensional lithium sulfide growth in lithium-sulfur batteries using high-donor-number anions.

Uncontrolled growth of insulating lithium sulfide leads to passivation of sulfur cathodes, which limits high sulfur utilization in lithium-sulfur batteries. Sulfur utilization can be augmented in electrolytes based on solvents with high Gutmann Donor Number; however, violent lithium metal corrosion is a drawback. Here we report that particulate lithium sulfide growth can be achieved using a salt anion with a high donor number, such as bromide or triflate. The use of bromide leads to ~95 % sulfur utilization by suppressing electrode passivation. More importantly, the electrolytes with high-donor-number salt anions are notably compatible with lithium metal electrodes. The approach enables a high sulfur-loaded cell with areal capacity higher than 4 mA h cm-2 and high sulfur utilization ( > 90 %). This work offers a simple but practical strategy to modulate lithium sulfide growth, while conserving stability for high-performance lithium-sulfur batteries.