Nontrivial Effects of “Trivial” Parameters on the Performance of Lithium–Sulfur Batteries

Sulfur/carbon cathode material chemistry and morphology optimisation for lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) are a promising alternative to lithium-ion batteries because sulfur is highly abundant and exhibits a high theoretical capacity (1675 mA h g-1). However, polysulfide shuttle and other challenges have made it difficult for LSBs to be commercialised. Here, a sulfur/carbon (S/C) composite was synthesised and cathodes were fabricated via scalable melt diffusion and slurry casting methods. Carbon nanoparticles (C65) were used as both sulfur host and electrical additive. Various carbon ratios between the melt-diffusion step and cathode slurry formulation step were investigated. An increased amount of C65 in melt-diffusion led to increased structural heterogeneity in the cathodes, more prominent cracks, and a lower mechanical strength. The best performance was exhibited by a cathode where 10.5 wt% C65 (TC10.5) was melt-diffused and 24.5 wt% C65 was externally added to the slurry. An initial discharge capacity of ∼1500 mA h g-1 at 0.05C and 800 mA h g-1 at 0.1C was obtained with a capacity retention of ∼50% after 100 cycles. The improved electrochemical performance is rationalised as an increased number of C-S bonds in the composite material, optimum surface area, pore size and pore volume, and more homogeneous cathode microstructure in the TC10.5 cathode.

Influence of Backbone on the Performance of Pendant Polymer Electrode Materials in Li-ion Batteries

Pendant polymers are a promising class of electrode materials due to their synthetic simplicity, derivation from sustainable feedstocks, and potentially benign synthesis. These materials consist of a redox-active pendant tethered to a polymer backbone, which mitigates dissolution during electrode cycling. To date, an extensive number of pendant groups have been studied within the context of metal-ion batteries. However, the choice of the polymer backbone and its impact on the electrode performance have been relatively understudied. In this work, we use a postpolymerization modification approach to synthesize a series of viologen-bearing redox-active pendant polymers with similar molecular weights but three distinct chemical backbones, namely, polyacrylamide, polymethacrylamide, and polystyryl. By evaluating the polymers in lithium-ion batteries, we show that the polymer backbone has a significant influence on electrode performance and behavior. Specifically, the polymethacrylamide displays slower kinetics than the other two polymers, resulting in lower capacities, particularly at high cycling rates. Furthermore, the charge storage mechanism is dependent on the nature of the backbone: the polyacrylamide shows a significant capacitive contribution to charge storage, while the polystyryl does not. The difference in performance between the polymer electrode materials is ascribed to a difference in chain mobility and packing within the electrode films. Overall, this work shows that the fundamental properties of the polymer backbone are critical to the design of high-performance polymer electrodes.

Dual additive of lithium titanate and sulfurized pyrolyzed polyacrylonitrile in sulfur cathode for high rate performance in lithium-sulfur battery

Lithium-Sulfur (Li-S) batteries have attracted much attention as next-generation batteries due to their high theoretical energy density. However, lithium polysulfide generated during the discharge loses intimate electrical contact with the carbon matrix due to its high solubility in the electrolyte, causing a high charge transfer resistance and slow redox kinetics for the discharge reactions, resulting in a low rate capability. A cathode additive having a strong chemical adsorbing site toward the polysulfide can effectively inhibit their dissolution. We now report a dual additive of lithium titanium oxide (LTO) and sulfurized polyacrylonitrile (SPAN). LTO provides a rapid charge transfer and a fast Li⁺ ion transfer in the cathode. On the other hand, SPAN helps to enhance the polysulfide adsorption capability. This dual additive system synergistically supplies the cathode with a strong polysulfide adsorption capability and fast redox kinetics. As a result, the dual additive exhibits high discharge capacities of 1430 mA h g^-1 at 0.1C and 1200 mA h g^-1 at 0.5C at the high-sulfur-loading cathode of 5.0 mg cm^-2. Our findings demonstrated the manufacturing of the cathode with a strong polysulfide adsorption capability and a fast redox reaction which could then effectively improve the rate performance of the Li-S batteries.

Cryogenic electron microscopy reveals that applied pressure promotes short circuits in Li batteries

Li metal anodes are enticing for batteries due to high theoretical charge storage capacity, but commercialization is plagued by dendritic Li growth and short circuits when cycled at high currents. Applied pressure has been suggested to improve morphology, and therefore performance. We hypothesized that increasing pressure would suppress dendritic growth at high currents. To test this hypothesis, here, we extensively use cryogenic scanning electron microscopy to show that varying the applied pressure from 0.01 to 1 MPa has little impact on Li morphology after one deposition. We show that pressure improves Li density and preserves Li inventory after 50 cycles. However, contrary to our hypothesis, pressure exacerbates dendritic growth through the separator, promoting short circuits. Therefore, we suspect Li inventory is better preserved in cells cycled at high pressure only because the shorts carry a larger portion of the current, with less being carried by electrochemical reactions that slowly consume Li inventory.

Abnormal Overcharging during Lithium-Ether Co-Intercalation in a Graphite System: Formation of Shuttling Species by the Reduction of the TFSI Anion

Materializing an ultrafast charging system is one of the crucial technologies for next-generation Li-ion batteries (LIBs). Among many studies aimed at achieving fast charging systems, Li-ether solvent cointercalation into the graphite electrodes in LIB has been identified as a novel concept for achieving high power performance because this system does not consist of the sluggish desolvation step and a resistive solid-electrolyte interface (SEI) layer. Interestingly, while studying the Li-ether solvent cointercalation into graphite electrodes, employing lithium bis-trifluoromethane sulfonimide (LiTFSI) as the Li salt, we observed an abnormal overcharging phenomenon. Here, we screened the specific conditions, under which the abnormal overcharging occurred, and revealed that this abnormal overcharging was attributable to the shuttling mechanism. The formation of shuttling species could have been derived by the reduction of TFSI^- anion. With this understanding of the underlying mechanism, we efficiently suppressed the abnormal overcharging by adding LiNO₃ to the electrolyte. The shuttling and resulting overcharging could be prevented by the synergistic contributions of LiNO₃ and S_xO_y, dissolved in the electrolyte, to the formation of a dense solid LiS_xO_y SEI layer on Li-metal. We expect that this work could be a great reference in analyzing many unsolved phenomena in systems utilizing TFSI^-.