Combination of Mixed Oxides and Sulfurized Polyacrylonitrile for Li-S cathodes with Long-Term Cyclability at High C-Rates

PÁEZ JEREZ, ANA L.; DAVIES, LILIAN E.; SHAM, EDGARDO L.; VERA, MARIA L.; ALVARO TESIO; FLEXER, VICTORIA

Mar del Plata

Congreso; 34th Topical Meeting of the International Society of Electochemistry; 2023

Intenational Society of Elecrochemistry

Sulfurized polyacrylonitrile (SPAN) cathodes are an attractive prospect for the next generation of energy storage systems. Their extraordinary performance is related to the unique chemistry and distinctive characteristics, which make them completely different from the conventional Li-S battery with S8-ring containing cathodes. One of the most notable differences is that SPAN cathodes feature a single sloped voltage plateau with a solid-solid mechanism since the sulfur is present in the form of short chains (-Sn-, n ≤ 4) which are covalently bonded to the PAN backbone. This important property eliminates the harmful shuttle effect, and this ability applies both for carbonate-based electrolytes, the typical electrolyte used in Li-ion batteries, and ether-based electrolytes with LiNO3 as an additive, which is used in conventional Li-S batteries.Firstly, a ternary compound was synthesized from commercial titanium dioxide (TiO2), elemental sulfur and polyacrylonitrile through a simple ball-milling and heating process under inert atmosphere. Secondly, a quaternary compound was prepared. In this case, TiO2 doped with Y2O3 was first prepared via sol-gel technique. The as-prepared solid nanoparticles were next mixed with elemental sulfur and polyacrylonitrile, and once again a simple ball-milling and heating process under inert atmosphere followed. Both compounds were fully characterized morphologically and structurally as belonging to the family of sulfurized polyacrylonitrile compounds (SPAN), while homogeneously incorporating either TiO2 or TiO2/Y2O3 within their structures. Both the ternary and quaternary composites were incorporated as active materials at the cathode and combined with a metallic lithium anode to form Li-S batteries.The cells fitted with the ternary composite achieved high and stable capacity values at 0.5 C reaching 1885 mAh/gS for the 10th cycle and ~1600 mAh/gS after 200 cycles (498 and 422 mAh/g composite, respectively). The high capacity values observed, higher than the theoretical capacity of elemental sulfur (1675 mAh/g), are explained by the extra capacity provided by the lithiation/delithiation process of TiO2. The metallic oxide also improves the overall kinetics of the redox processes in SPAN, which helped to achieve good cycling performance at very high charge/discharge rates. At 3.3 C, a remaining capacity of 672 mAh/gS after 1400 cycles was achieved, and even at 5 C where a remaining capacity of 660 mAh/gS after 500 cycles were recorded. The cells containing the quaternary composite as a cathode, also comprised an interlayer made of TiO2/Y2O3 between cathode and separator. Upon cycling, these cells reached a stable electrochemical performance at 3.3 C (6 A g-1) in simple carbonate-based electrolytes. The performance was even superior to that previously recorded for TiO2-SPAN cathodes. The capacity for the 10th cycle was 2250 mAh/gS with about 1400 mAh/gS remaining after 850 cycles. For these cells, a better understanding of the molecular structure can be suggested through XPS analysis of the composite and the cell at different charge and discharge states. To the best of our knowledge, we are the first ones to report the combination of SPAN, TiO2, and Y2O3 to show the synergistic behaviour of these compounds.