Reinforcing conversion of polyselenides via a bifunctional blocking layer for efficient Li-Se batteries

Lithium-selenium (Li-Se) batteries possess high volumetric capacity and have attracted considerable attention as a high energy storage system. However, the shuttling of polyselenides seriously worsens the electrochemical performance and retards their application advancement. Herein, we engineered a bifunctional membrane consisted of polyethylenimine derived carbon quantum dots (Cdots) to efficiently restrict the shuttling of polyselenides under a high Se loading (Se≈70 wt%) and promote Li-Se conversion kinetics, which can be accounted by the greatly accelerated transportation of charge carriers and dipole–dipole interactions between polar moieties and long-chain polyselenides (Li2Se4 and Li2Se6) as corroborated by theoretical calculations. Thus, the bifunctional membrane endows Li-Se batteries with a specific capacity of 658.60 mAh g−1 at 0.1 C and coulombic efficiency of 97.8% in average, and demonstrates the effectiveness of defect-rich Cdots on suppressing polyselenides shuttling and reinforcing Li-Se conversion kinetics in augmenting the battery’s durability and efficiency. • A PEI-Cdots@Super P hybrid layer modified Celgrad membrane was prepared for Li-Se batteries. • The PEI-Cdots@Super P membrane reinforces the conversion kinetics of polyselenides. • The PEI-Cdots@Super P membrane exhibits excellent ability for restricting polyselenides.


Graphical Abstract 1 Introduction
Lithium-ion batteries (LIBs) as one of the important energy storage technologies have received great attention in the fields of mobile electronics and electronic vehicles (Chen et al. 2018;Fu and Zhong 2019;Wang et al. 2020).Nevertheless, the low energy output of LIBs is a vital barrier to the ever-increasing demand for development in the future (Luo et al. 2013;Li et al. 2019).Therefore, exploring next-generation Li-metal based energy storage systems with higher energy density is of great significance (Yang et al. 2013;Zeng et al. 2020;Chen et al. 2022).Selenium (Se), an element of the same main group as sulfur (S), has attracted extensive research interest because of its comparable electrochemical properties with S (Ding et al. 2017;Zhang et al. 2017).Moreover, the conductivity of Se is much higher than that of S, and the shuttle effect of polyselenides in carbonate-based liquid electrolytes is less serious than that of polysulfides (Liu et al. 2014;Wang et al. 2022a).Even if the higher density of Se makes the theoretical specific capacity of Li-Se batteries (675 mAh g −1 ) inferior to that of Li-S batteries (Luo et al. 2013;Huang et al. 2019).Another merit of Li-Se batteries is the high volumetric energy density (3253 mAh cm −3 ), making them suitable for large-scale grid energy storage and electric vehicles (Li et al. 2014;Zhang et al. 2017).Unfortunately, the Li-Se batteries still suffer from polyselenides dissolution and shuttling, mainly ( Se 8 → Li2Se x , 4 ≤ x ≤ 8 ), and subsequent irre- versible reactions with Li metal anode (Kim et al. 2020;Zhang et al. 2020).These problems deteriorate the cycle stability and efficiency of Li-Se batteries (Jiang et al. 2014;Yang et al. 2015).
With regard to this, numerous works focus on preparing advanced cathode materials to fundamentally eliminate polyselenide dissolution and shuttling at cathode side, including carbon host materials (Qu et al. 2015;Zeng et al. 2015) metal materials (Ma et al. 2018;Sun et al. 2021;Xia et al. 2023a) or polymer/carbon composite host as cathode materials (Kundu et al. 2013;Guo et al. 2015;Zhao et al. 2018;Zhang et al. 2019a).But many works are associated with reducing the mass loading of Se, compromising their application prospect (Zhang et al. 2020).Recently, functional membrane is another effective way to solve the shuttling problems for chalcogen-based LIBs (Ghazi et al. 2017;Wang et al. 2018;Yao et al. 2018;Xia et al. 2023b).It has been proved that the nonpolar or weakly polar materials on separator show poor inhibitory effects on polyselenide (Hou et al. 2016;Zhang et al. 2019b), which is the common choice for existing reports, but too many defects may lower the conductivity of functive on membranes (Fang et al. 2019).As a result, the ideal functive should possess adequate active sites, moderate chemical binding ability with polyselenides, and high conductivity for Li-Se conversion kinetics (Seh et al. 2013;Chung and Manthiram 2014;Zhang et al. 2020).Zero-dimensional carbon dots (Cdots) derived from oligomer or polymer molecules are easy to disperse on substrate and help to anchor poly-sulfides/selenides without hindering the transportation of charge carriers at moderately high mass loading (Hu et al. 2019;Park et al. 2021).Polyethylenimine (PEI), as one of the typical branched polymers, possesses abundant amine groups that are beneficial for inhibiting polyselenides shuttling and Li + migration, if reducing its dimension to quantum dot size (Li et al. 2017;Yu et al. 2021).Hence, there is immense potential in using the PEI as precursor to synthesize Cdots for separator functionalization, and simultaneously realize the inhibition of polyselenides shuttling and acceleration of Li-Se conversion.
In this work, the PEI Cdots were blended with Super P and coated on the Celgard membrane and dried as a bifunctional layer for Li-Se batteries.Thereinto, the PEI Cdots provide abundant polar sites to restrict the shuttling of polyselenides, while Super P acts as the supporting carbon skeleton to enhance the conductivity.The electrochemical measurements proved that the bifunctional blocking layer showed excellent capability of restricting shuttle effect and accelerating charge carrier transportation for Li-Se batteries at moderately high Se loading.Furthermore, the performance of bindings and electronics in DFT calculations certified that the amino group contained in the bifunctional layer had a proper anchoring effect on long-chain polyselenides, Li 2 Se 4 and Li 2 Se 6 , due to the distinct electronic exchange between polyselenides and the nitrogen element of PEI Cdots.

Synthesis of PEI Cdots
The specific steps refer to the previous study (Hu et al. 2019). 1 g PEI (molecular weight = 600, Aladdin) in a glass bottle was sealed with Al foil, and then carbonized under 400℃ for 50 min on a heating platform.After the glass bottle was cooled to ambient temperature, the compound was mixed with distilled water and fully dispersed with vigorous sonication for 1 h.The supernatant was acquired from the dispersion solvent after being centrifuged under 8000 r/min for 15 min, and used for dialyzing with the distilled water for 24 h.Last, the PEI Cdots were obtained after removing water by lyophilisation.

Preparation of the C/Se cathode
The preparation of C/Se composite materials followed the prior study (Wang et al. 2022b).The C/Se cathode was acquired by mixing the C/Se composite materials, Super P, SBR and CMC (weight ratio = 80:10:5:5) Then, the slurry was cast onto the surface of the Al foil current collector as the C/Se cathode materials for electrochemical tests.

Preparation of PSP membrane
The PEI Cdots, Super P and PVDF (mass ratio = 0.3:3:1) were mixed by mortar, dispersed in 10 mL 1-Methyl-2-pyrrolidinone (NMP) and magnetically stirred for 1 h.After the vacuum filtration step, the cast slurry was coated onto the commercial membrane for relative tests.

Electrochemical test
The CR2032-type Li-Se batteries were assembled using the C/Se composite materials as the cathode, lithium foil as the anode, Celgard membrane or PSP membrane as the interlayer between the cathode and anode, and 1 M LiPF 6 as electrolyte.The charge-discharge measurements of Li-Se batteries were conducted using the LAND CT2001A.The EIS tests were completed on the electrochemical workstation CHI 760d (Chenhua, Shanghai).

Materials characterizations
Scanning electron microscopy (SEM) and elemental mapping images were taken by using a JEOL JSM-6701F.The FTIR of PSP and Super P were characterized by a Bruker Alpha which ranged from 3500 ~ 1000 cm −1 .The XPS analyses were performed using the Thermo Scientific K-Alpha.The XRD measurements were carried out via the SHIMADZU with the range set to 10-80° and a Cu Kα as the radiation.The TGA analysis was carried out via the METTLER under the argon atmosphere.

Computational details
All the geometry structure relaxations for first principles calculations were carried out within the density functional theory through Vienna Ab Simulation Package (VASP) (Hohenberg and Kohn 1964;Kresse and Furthmüller 1996).The Perdew-Burke-Ernzerhof type was used to represent the exchange-correlation function.The electron-ion interactions between the constituent atoms were described through the projector augmented wave (PAW) method (Hou et al. 2016).The Van Der Waals interactions between substrate materials surface and Li 2 Se n (n = 4 and 6) were considered using the optPBE-vdW functional.The energy cut off was set to 500 eV for all the calculations in this work.The convergence criterions of energy and force calculations had been achieved with 10 -5 convergence and 0.02 eV Å −1 , respectively.The Brillouin zone sampling was set to the Gamma-centered grids with a k-point mesh 3 × 3 × 1.The density of states (DOS) was also analyzed with the k-points mesh 11 × 11 × 1 in the Gammacentered grids.The vacuum layer of 15 Å was used to avoid the interactions of adjacent configurations.

Results
The PEI Cdots were carbonized by a "one-pot" method, which is a facile method for the synthesis of Cdots.Specifically, PEI Cdots were developed by being carbonized, which served as the source of the surface functional group.After that, the PEI Cdots were blended with Super P and polyvinylidene fluoride (PVDF) to form a suspension and filtered onto the Celgard separator to obtain the target membrane (PSP) (Fig. 1).During the discharge process, the polyselenides were confined within cathode side by PSP, reducing the capacity loss caused by the shuttle effect under the high Se mass loading.For comparision, the Super P and PVDF were employed to form a suspension and filtered onto the Celgard separator to obtain the SP membrane.
To observe the morphology and structure characteristics of the SP and PSP membrane, SEM characterizations were carried out.As shown in Fig. 2a-c, the Super P, as an interconnected porous skeleton, lays the foundation for the composite, which facilitates the migration of selenium ions in it, while the PEI Cdots decorate on it after filtration, and forms a conformal porous coating on the Clegard separator.Its porosity shortens the length of ion diffusion and provides a large specific surface area to facilitate the redox reaction between Se and Li + at cathode.At the same time, the polar groups on the PEI Cdots are beneficial to restrict the shuttle effect.The EDS element mappings also confirm the uniform element distribution of PSP on the Celgard separator surface (Fig. 2d).
To explore the bonds in the composite, the structure of the as-prepared PSP membrane was further investigated by Fourier transform infrared spectroscopy (FTIR) spectra.As shown in Fig. 2e, only the C-H stretching adsorption sites were observed at 2900-2700 cm −1 and 1460 cm −1 from the FTIR spectra of both SP and PSP membranes.The characteristic spectrums of amine did not appear because the content of N element in PSP was low (Boronat et al. 2009;Wepasnick et al. 2011).But from the results from X-ray photoelectron spectroscopy (XPS) of PSP, three peaks could be observed in the unrolled integration signals of C1s, corresponding to the combination of C-C binding (284.8 eV), C-N amine bonding (286.7 eV), and π-π* (291.3 eV), respectively (Fig. 2f ) (Chen et al. 2020).Meanwhile, the signals of N 1 s can be observed in Fig. 2g, corresponding to the combination of graphitic N (400.6 eV), oxidized N (402.4eV), and quaternary N (403.7 eV), respectively (Hellgren et al. 2016;Hu et al. 2019).The above results indicated that the PEI Cdots were preserved in the PSP membrane, which acted as the functional source of nitrogen and oxygen.
The high wettability of PSP membrane to electrolyte could reflect its low impedance toward mass transportation, which also contributes to low membrane resistance for charge transfer.As shown in Fig. 3a-c, the contact angles of Celgard, SP and PSP membrane were 38.0, 14.1 and 8.1, respectively.Due to the introduction of polar groups, the PSP showed the smallest contact angle than other two separators.Meanwhile, according to the Raman spectra of SP and PSP, the peaks of the defected and disordered carbon were detected at ~ 1350 cm −1 and the peaks of the sp 2 hybrid carbon were shown at ~ 1580 cm −1 (Fig. 3d).In addition, the peak intensity of I D /I G in PSP (about 1.00) was almost the same with SP (1.01), signifying the excellent electric conductivity of PSP.
To check the efficacy of PSP membrane, a high-loading C/Se cathode was prepared.As shown in Fig. 4a, there was no distinct bulky Se existing on the surface of carbon materials, and the Se and C, O elements distribute homogeneously in the C/Se composites.The mass loading of Se in the C/Se composite was measured to be 70% using thermogravimetric analysis (TGA, Fig. 4b).As shown in X-ray diffraction (XRD) patterns, the diffraction peaks of the crystalline phase Se in C/Se disappeared (Fig. 4c), it means Se element in the carbon host is amorphous.Then, the Li-Se batteries were prepared with the C/Se composite materials cathode, the metal lithium anode, and the pure Celgard (PP) separator or PSP membrane.Comparison of interfacial charge transfer capability between various membranes was conducted via electrochemical impedance spectroscopy (EIS).As shown in Fig. 5a, there is a single semicircle in the high frequency region of two kinds of battery.According to the equivalent circuit, Ohmic resistance of the ingredient for electrolyte and battery (R e ), charge transfer resistance of the electrode reaction kinetics (R ct ), the constant phase element (CPE), and the Warburg impedance (W 0 ) are involved (Fang et al. 2015).The resistance of Li-Se batteries with PSP membrane (41.20 Ω) was lower than that using PP membrane (70.03Ω), because PSP serves as the interconnected porous skeleton and endows the Celgard separator substrate with high electronic conductivity (Su and Manthiram 2012).The rate performance from the third cycle of PP and PSP membrane for Li-Se batteries was further explored with the rate from 0.1 C to 1 C (Fig. 5b-c).Different from the rapidly decreased capacity of the PP membrane (from 485.99 mAh g −1 at 0.1 C to 250.26 mAh g −1 at 1 C), the PSP membrane exhibited a superior rate capability at 0.1 C (658.60 mAh g −1 ) and maintained approximately 300 mAh g −1 when the rate was increased to 1 C. Furthermore, the voltage polarization difference of the PSP membrane (0.20 V) was smaller than that of the PP membranes (0.24 V) (Fig. 5d).More interesting, the Li-Se batteries with the PSP membrane exhibited an apparent enhancement in specific capacity, which was resulted from further conversion of polyselenides ( Li 2 Se 2 → Li 2 Se ) at the discharge plateau of approximately 1.2 V (Lei et al. 2022).Such different charge-discharge behaviour indicates that the introduction of PEI Cdots, we think, reinforces the Li-Se conversion kinetics.The cycling performance of PP and PSP membranes at various rates are exhibited in Fig. 5e.Different from the rapidly deteriorated specific capacity of the PP-based Li-Se batteries, the PSP membrane enables Li-Se batteries to exhibit superior specific capacities of 1262.00 mAh g −1 at 0.1 C and then become at 658.60 mAh g −1 , and 341.60 mAh g −1 at 1 C, which is attributed to the enhancement of interfacial kinetics with the introduction of the PEI-Cdots and Super P. Additionally, the Li-Se batteries with PSP can still maintain a reversible capacity of 330.50 mAh g −1 after 80 cycles at 0.05 C and an average coulombic efficiency of 97.78% after rate test (Fig. 5f), which displays better electrochemical performance than that of PP.
Results mentioned above were further confirmed by the calculations from electronic performance.Different from the SP, the significant energy shifts can be observed from the partial density of states (PDOS) of PSP and Li 2 Se n (n = 4, 6), before and after the binding, as shown in Fig. 6g-j.In the PDOS plots, the energy downshift of the PSP substrate and the energy upshift of Li 2 Se n can be observed during the binding, which signifies that PSP can cause charge exchange and polar-polar interaction with Li 2 Se n .The electron charge density differences in Li 2 Se n adsorbed on PSP are illustrated in Fig. 6k-n.The yellow and red areas represent the accumulation and depletion of charges.Compared with SP, there is a larger charge accumulation area in PSP, which contributes to the effective prohibition of polyselenides shuttling.

Conclusions
In conclusion, we have proposed a bifunctional blocking layer that acts as an effective barrier for Li-Se batteries to inhibit the shuttle effect of polyselenides under a selenium loading of 70 wt%.The EIS and DFT calculations confirm that the PSP can increase the conductivity and greatly promote the migration of electrons and ions, accelerating the interfacial charge transfer and maintaining a stable releasing capacity and over 97% Coulombic efficiency at low rate, when side reactions are attenuated.Besides, it is found that the PEI Cdots provides additional active sites for promoted Li-Se conversion kinetics.This study provides a facile but efficient membrane engineering strategy for efficiently inhibiting redox species shuttling and reinforcing conversion at moderately high mass loading, promoting the advancement of energy-dense metal batteries.

Fig. 1
Fig. 1 Schematic of the PSP membrane and confine mechanism for Li 2 Se n

Fig. 3
Fig. 3 Schematic and contact angle images of (a) Celgard, (b) SP and (c) PSP membranes with electrolytes (top sets are their digital images).d Raman spectra of SP and PSP membranes

Fig. 5
Fig. 5 (a) The electrochemical impedance spectra (EIS) of the Li-Se batteries with different PP and PSP membranes before cycling (the inset is the magnified image for the Li-Se batteries with PP or PSP membrane).Rate performance with PP (b) and PSP (c) membrane for Li-Se batteries.(d) The second cycle performance comparison plots at 0.1 C with various membranes.(e) Cycling performance at different rates.(f) The long-term cycling stability performance of Li-Se batteries with PSP membrane at 0.05 C

Fig. 6
Fig. 6 Optimized molecular models of the interactions of Li 2 Se 4 and Li 2 Se 6 binding on Super P (a, b), PEI Cdots (c, d), and PSP (e, f) substrate materials along with the corresponding binding energies.Partial density of states (PDOS) of Li 2 Se 4 binding on SP (g) and PSP (i) membrane and the Li 2 Se 6 binding on SP (h) and PSP (j) membrane before and after the interaction.Electron density differences of Li 2 Se 4 binding on SP (k) and PSP (m) membrane and the Li 2 Se 6 binding on SP (l) and PSP (n) membrane