Adsorption equilibrium polysulphides seesaw: N, O-bis atoms inhibiting effect polysulfide shuttle diaphragm

Lithium-sulfur battery using sulfur as an electrode material, compared with the conventional graphite-based lithium-ion battery, its high theoretical specific capacity and energy density have attracted much attention. During the discharge, S8 Molecular Electronics and gradually gained Li ⁺ binding, undergoes a plurality of polysulfide intermediates (Li ₂ S _x [ after 123], x = 2, 4, 6, 8), eventually converted to Li 2 _{S, to complete the discharge process.}

Figure 1 a schematic view of the conversion discharge process sulfur speciesUnfortunately, shown in Figure 1, an intermediate product of a long chain polysulphide of (Li

2 _S x , x = 4, 6, 8) which is dissolved in the electrolyte, the positive electrode material from falling down through the porous separator, a negative electrode to the battery, causing the active material the loss of the electrode structure collapses. This process is known as \”polysulfide shuttle effect\”, is a key factor limiting the development of lithium-sulfur batteries. To solve this problem, many researchers dedicated to the design of sulfur cathode, electrolyte regulation and modification of the membrane thereby inhibiting \”polysulfide shuttling effect.\” Reported in the literature but the coating / interlayer modified aperture diaphragm tends to clog the membrane, to restrict the transmission of lithium ions to some extent, increase the internal resistance of the battery. Therefore, the development of new membrane materials, without affecting the transport of lithium ions, polysulfide conversion behavior regulation, a useful modified lithium-sulfur battery separator for a Class ideas. Chongqing University Professor Wei Zidong, Associate Professor Li Cun Pu and other reported the use of hard and soft acid-base reaction reversible capture polysulfide, at the same time does not inhibit migration of lithium ions lithium-sulfur battery separator [1] ⁽ Small , 2018, 14 (52): 1804277.). Recently, the team cognitive further conversion reaction of the interface, it is reported Based morpholine N, O-bis chemisorption atoms, can be effectively suppressed lithium-sulfur battery separator \”polysulfide shuttle effect\” [2] ⁽ ACS Appl. Mater. Interfaces , 2020, 10.1021 / acsami.0c04554). By elemental sulfur analysis to multi-step reduction process can be found Li2S, various types of soluble polysulfides transformation involves the reaction of a plurality of interfaces, including solid – liquid interface reaction (S8-Li2S8), liquid – liquid interface reaction (Li2S8 – Li2S4) and liquid – solid interface reaction (Li2S4- Li2S2). With reduction of sulfur cell having constant plateau voltage because loss of various types of polysulfide intermediates of sulfur will result in a subsequent reduction process interrupted, resulting in decrease in battery capacity. Therefore, it is necessary to control various types of polysulfide equivalent to achieve steady charging and discharging of the lithium-sulfur battery. Structure by know chemical analysis, polysulfides Li 2 _S x The smaller the x, Li-S bonds closer ionic bond, i.e., lithium the harder the acid, the acid itself, change in hardness of the Li-based chemical adsorption such that a single element, it is difficult to achieve the equivalent of trapping various types of polysulfide. Thereto, as shown, trapping of morpholine as polypropylene diaphragm units introduced polysulfide shuttle 2 inhibition. Morpholine containing two heteroatoms N and O, where O atoms electronegative charge greater concentration, large electron cloud hardness; electrically negative charge dispersion less N atom, the electron cloud of small hardness. Under the control effect of the two atoms to achieve an equivalent adsorption of different hardness polysulfide.

FIG. 2 (a) chemical structure of morpholine; change process (b) during discharge of the voltage polysulfide; (c) PP separator not inhibit soluble polysulfides diffuse to the negative electrode; (DF) morpholine-modified membrane can efficiently capture soluble polysulfides, and then released on demand polysulfide in a subsequent reduction process; (G) morpholine N, O-bis heteroatoms boat conformation to achieve a polysulphide reversible adsorption equivalent thereof.As shown, the use of UV-assisted oxidation-based 3 – Polymerization Encapsulation – functionalized synthetic strategy can be provided while retaining the original porous structure of the membrane, to achieve the introduction of morpholine capture unit. Since the porous structures are successfully reserved, the transfer of lithium ions is not impeded, the performance of which is essential for battery charge and discharge rate.

FIG. 3 (a) a porous membrane of polypropylene (PP) SEM image; (b-c) SEM image morpholine-modified membrane. After the modification, the porous structure may be retained; (D) TEM image of a polypropylene porous membrane; (e-f) morpholin-TEM image of modified diaphragm, N, O heteroatoms stained with heavy metals, exhibits a darker color.The principle of calculations reveal that the diaphragm structure polysulfide equivalent uniform regulation \”shuttle effect\” using the density functional theory. Study of soluble polysulfides (Li

2 _S x , x = 4, 6, 8) with piperidine (only one N atom), tetrahydrofuran (only one O atom), the binding energy between it morpholine (N, O atom), as shown in FIG, piperidine and tetrahydrofuran are monoatomic adsorption 4, the binding energy between the multiple levels of polysulfide weak, the difference is also larger; and morpholinyl may be equivalent to three kinds of adsorption polysulfide (Eb ≈-1.5 eV). When a long chain polysulphide side through morpholine, morpholine N, O heteroatoms it to arrest the boat conformation morpholine intermediate form diatomic adsorption, to prevent the loss of polysulfide, lithium-sulfur battery in order to achieve sustained and stable charging and discharging. Binding energy between the

FIG. 4 (a) polysulfide with piperidine (only the N atom), tetrahydrofuran (only the O atom), morpholine, N-bis atomic adsorption specific, single O more atomic adsorption, adsorption of diatomic morpholine deviation reduces binding energy; (b) morpholine -Li2Sx (x = 4, 6, 8) the distance between the Li-N, and Li-O system optimization; ( CE) morpholine Li2S4 (c), Li2S6 (d), Li2S8 (e) optimizing the structure of the ship are formed configuration.Thanks to the above reasons, the use of modified diaphragm morpholine coin cell at 500 2 C cyclic ring, able to maintain the 640.2 mAh · g

-1 ^{the specific capacity, there good cycle stability and reversibility. Thanks to the protective effect of electrode structure on the rate capability of a battery which also modified diaphragm far} better than the polypropylene porous membrane.

of FIG. 5 (a) different current densities, a polypropylene porous membrane (PP) and morpholine modified diaphragm (the PP-Rate capability C-St-MP) cells; (b) modified diaphragm morpholine galvanostatic charge-discharge curve of the battery at different current densities; (c) two kinds of long cycle performance of the battery separator under 2 C; and ( d) morpholine-modified membrane cell at different current densities (0.5 C, long cycle performance at 1 C, 2 C, 3 C, 5 C).

Summary: of replacement based on knowledge of the interface conversion polysulphide process, by introducing on the diaphragm with N, O atoms of the two hybrid morpholine as the polysulfide regulatory elements to achieve the equivalent chemical adsorption of soluble polysulfides, soluble polysulfides limitation on the positive side, to ensure that the Li2S8, between the liquid and Li2S6 Li2S4 – liquid-liquid and subsequent conversion of Li2S4 solid transformation can be performed effectively, to achieve a high rate lithium-sulfur battery, charge-discharge sustainable, provides a new high performance research strategies for the development of lithium-sulfur battery.

References:

[1] Dong Q, Shen R, Li C, et al Construction of soft base tongs on separator to grasp polysulfides from shuttling in lithium-sulfur batteries [J] Small… , 2018, 14 (52): 1804277. [2] Dong Q, Wang T, Gan R, et al Balancing the Seesaw:… Investigation of a Separator to Grasp Polysulfides with diatomic chemisorption [J] ACS Applied Materials & Interfaces, 2020. DOI: 10.1021 / acsami.0c04554