Carbon Nanofibers Modified By Natural Pectic Polysaccharide As the Multifunctional Interlayer for Advanced Li-SeS2 Batteries
Authors: Zhang Yin , Wu Hao , Zhang Yun 
Published: February 2020

In order to satisfy the ever-growing demand of higher energy densities for electric vehicles and consumer electronics, high-performance rechargeable batteries have attracted remarkable attentions in the last ten years. Recently, rechargeable Li-S batteries have captured intensive attention for high theoretical energy density and cost effective materials.[1] However, there is a certain distance to achieve the practical application of Li-S batteries due to some pivotal issues that are demanded of being disposed promptly: 1) the connatural poor electroconductivity of elemental sulfur and the end discharge products (Li2S/Li2S2), 2)a noticeable volume expansion in the lithiation process, 3) the shuttle effect of polysulfides (Li2Sn, 4≤n≤8).[2,3]

Recently, researchers focus on the Se cathodes owing to the high electronic conductivity (1 X 10-
3 S m-
1) and the theoretical volumetric capacity (3253 mA h cm-
3) comparable to that of S (3467 mA h cm-
3). As the congener of S , Se has a similar (de)lithiation process to sulfur. However, the theoretical gravimetric capacity of Se (675 mA h g-
1) is much lower than that of S (1675 mA h g-
1).[4] To rationally balance the opposite but complementary features of Se and S, selenium sulfide (SeS2) has been proposed as a cathode for lithium ion batteries due to its theoretical mass capacity of 1345 mA h g-
1, which is close to that of sulfur, and its advantageous electronic conductivity as compared to sulfur. Since SeS2 has similar chemical properties to S, it is expected that the strategies used for stabilizing the S cathode might be also effective for suppressing the dissolution of lithium polysulfides/polyselenides in the SeS2 cathodes. To alleviate shuttling in high-sulfur-loading cathodes, improve sulfur utilization, and accelerate the redox reaction of polysulfides, a novel and advanced cell configuration has recently been developed by introducing a conductive interlayer between the sulfur cathode and the ordinary polymer separator. This inserted interlayer not only serves as an LPS trap, stabilizing LPSs within the cathode region, but also as a second current collector to reduce cell polarization and facilitate the redox kinetics.[5]

In this contribution, pectic polysaccharide (named NPG), a natural polymer, is introduced to conductive carbon nanofibers (named CF) to build a free-standing CF-NPG composite interlayer via a solution-coating method. The pectic polysaccharide is isolated from the seed of
Nicandra physaloides
(L.) Gaertn by water extraction. The NPG have a backbone chain composed of 1→ 4 linked galacturonic acid with heavily branched glucose, galactose, and rhamnose side chains that can afford a strong binding interaction toward lithium polysulfdes and polyselenides through its rich hydroxyl, carboxyl, and ether functional groups.[6] Thus, the as-obtained CF-NPG composite interlayer can effectively impede the shuttle effect. In addition, the conductive composite interlayer, functioned as upper current collectors, can improve the SeS2 utilization, ensuring good cycling stability and rate capability. With the CF-NPG interlayers, the SeS2 cathodes with a high loading (70 wt%) demonstrat a good cyclability with a capacity retention of nearly 80% over 500 cycles. The SeS2 cathodes show excellent rate performances with capacities of 592 mA h g-1, 440 mA h g-1, and 350 mA h g-1 at 1A g-1, 5A g-1, and 10A g-1, respectively. In conclusion, a polysulfde-immobilizing polymer interlayer was firstly used for Li-SeS2 batteries, an effcient reuse of the adsorbed sulfides and selenides is realized and thus a high utilization of SeS2, which outperform most Li-SeS2 batteries reported in the literature.

Figure 1 (a) the optical photographs of raw materials (nicandra physaloides seeds) (b) the SEM images of CF-PNG, (c) the cycle performance of SeS2 cathodes with CF-PNG interlayers.
[1] Manthiram A, Fu Y, Su Y S. Challenges and prospects of lithium–sulfur batteries. Accounts of chemical research, 2012, 46(5): 1125-1134.
[2] Liang X, Hart C, Pang Q, et al. A highly efficient polysulfide mediator for lithium–sulfur batteries. Nature communications, 2015, 6: 5682.
[3] Fang R, Zhao S, Sun Z, et al. More Reliable Lithium‐Sulfur Batteries: Status, Solutions and Prospects. Advanced Materials, 2017, 29(48): 1606823.
[4] He J, Lv W, Chen Y, et al. Direct impregnation of SeS 2 into a MOF-derived 3D nanoporous Co–N–C architecture towards superior rechargeable lithium batteries. Journal of Materials Chemistry A, 2018, 6(22): 10466-10473.
[5] Guo Y, Zhang Y, Zhang Y, et al. Interwoven V 2 O 5 nanowire/graphene nanoscroll hybrid assembled as efficient polysulfide-trapping-conversion interlayer for long-life lithium–sulfur batteries. Journal of Materials Chemistry A, 2018, 6(40): 19358-19370.
[6] Niu Q F, Wang B, Li T, et al. Structure and Gel Characterizaction of Petic Polysaccharide from
Nicandra physaloides
(L.) Gaertn Seeds. Modern Food Science and Technology, 2015, 9: 68-73.

Figure 1