Highly Reversible Li–Se Batteries with Ultra-Lightweight N,S-Codoped Graphene Blocking Layer
KeywordsLi–Se batteries N,S-codoped Graphene Ultra-lightweight Free-standing Vacuum filtration
A free-standing, ultra-lightweight, N,S-codoped graphene membrane is assembled by a simple vacuum filtration method.
The N,S-codoped graphene membrane is first used as the blocking layer for a polyselenide catholyte.
The Li–Se batteries based on the as-prepared graphene membrane exhibites excellent cycling performance and rate capability at high selenium loading (5 mg cm−2).
Rechargeable lithium–selenium (Li–Se) batteries have recently attracted considerable attention as potential energy storage devices for portable electronics and electric vehicles because Se has a high volumetric capacity (3253 mAh cm−3), which is comparable to that of sulfur (3467 mAh cm−3) [1, 2, 3, 4, 5], and has a relatively high electronic conductivity among nonmetallic materials (1 × 10−3 S m−1) [2, 6]. Despite these advantages of Li–Se batteries, great challenges regarding the cathode impede their practical applications. Similar as the Li–S batteries, these challenges of Li–Se batteries mainly involve the dissolution of polyselenide intermediates [2, 7, 8] and electrode collapse , which result in considerable loss of the active material and rapid capacity decay [9, 10, 11, 12].
To overcome these issues, a common approach, trapping selenium/sulfur in porous carbon hosts [1, 2, 8, 12, 13, 14, 15, 16, 17, 18], has been widely used. The resulting conductive and porous structure could provide electronic conductivity while hosting selenium and its discharge products . Owing to the physical confinement of selenium in porous carbon, the lithium polyselenide shuttle phenomenon could be greatly suppressed; in addition, the cathode could retain its integrity very well, leading to improved cycling stability and Coulombic efficiency. However, owing to the nonpolar nature of carbon, the interaction between carbon and lithium polyselenides is relatively weak, causing a gradual loss of the polar polyselenides during cycling. This phenomenon is even worse for cells with high active material loading in the cathodes. Thus, a more effective approach using chemical adsorption of lithium polyselenides has been reported recently [20, 21, 22, 23, 24]. For instance, our previous work using a nitrogen-doped loofah sponge carbon interlayer in Li–Se batteries illustrated that polyselenides could be effectively retained by N atoms . Wen’s group designed a conductive heterocyclic selenized polyacrylonitrile compound by the dehydrogenation/selenation method at high temperature [23, 25]. This conductive selenized polymer material could perform stably over thousands of cycles with a higher specific capacity (> 300 mAh g−1) and exhibited a considerably better rate capability compared to conventional oxides . Zhang et al.  used first-principle calculations to evaluate the influence of heteroatom doping on the electrochemical performance and confirmed that the presence of heteroatoms in the carbon framework greatly facilitates the interaction between carbon and Li2Se.
Though significantly improved cycling stability has been obtained by using these porous (heteroatom) carbon and carbon interlayers, it should be noted that the excellent cycling performance is achieved at relatively low selenium contents (less than 60%) and low selenium areal loading (< 2 mg cm−2) in the final cathode [1, 4, 8, 13, 26, 27, 28]. There is no doubt that the overall energy density of Li–Se batteries would be seriously reduced if these cathodes with low selenium content and low selenium areal loading are used. Therefore, advanced architectures and abundant functional groups/heteroatoms are needed to ensure simultaneous realization of high selenium content and high selenium areal loading with excellent electrochemical performance.
3.1 Material Synthesis
3.1.1 Preparation of N,S-G
The N,S-G was prepared according to our previous work . The detailed synthesis processes are as follows: First, 20 mg of thiocarbohydrazide (analytical reagent, Sigma-Aldrich) were added to 5 mL of a graphene oxide (GO) suspension with a concentration of 5 mg mL−1. Subsequently, the mixture was held in an oil bath at 90 °C without stirring for 30 min after 1 min of sonication. After the solution cooled naturally to room temperature, the resulting N,S-G hydrogels with cylindrical structure were immersed in deionized water for 24 h and washed several times to remove residual impurities. Finally, the hydrogels were vacuum-dried overnight for further processing.
3.1.2 Free-Standing N,S-G Membrane Preparation
N,S-G (7.5 mg) was dispersed in 5 mL of an N-methylpyrrolidone solution under sonication for 1 h. Subsequently, the homogenous N,S-G solution was vacuum-filtrated with a Celgard 2300 separator as the filter membrane. Then, the entire filter membrane was placed in a vacuum oven and dried overnight at room temperature. The N,S-G could then be peeled from the filter. After being peeled off, the N,S-G membrane was continuously vacuum-dried for another 12 h at 60 °C. The diameter of the N,S-G membrane was approximately 3.8 cm.
3.1.3 Preparation of Li2Se8 Solution
Li2Se (0.186 g, Shanghai Longjin Metallic Material Co., Ltd.) and 1.106 g of selenium (analytical reagent, Sigma-Aldrich) were added to 10 mL of 1 M lithium bis(trifluoromethanesulfonyl)imide solution, which uses 1,3 dioxolane/1,2-dimethoxyethane (1:1 v/v) as the solvent and contains 0.1 M LiNO3. Then, the mixed solution was stirred at 80 °C for 24 h to obtain the 0.2 mol L−1 Li2Se8 electrode solution. All the operations were conducted in a glove box under Ar atmosphere.
4 Material Characterization
The samples’ structures were characterized by X-ray diffraction (XRD, Model LabX-6000, Shimadzu, Japan), X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd., Manchester, UK), and Raman scattering (Renishaw Inc., Illinois, USA). The morphologies of the obtained samples were characterized by transmission electron microscopy (TEM, Tecnai 20 FEI, USA) with an acceleration voltage of 200 kV. The membrane morphologies were investigated by scanning electron microscopy (SEM, JSM-7001F, JEOL, Japan).
5 Electrochemical Measurements
The large free-standing N,S-G membrane was cut into small wafers (1.4 cm in size and approximately 1.0 mg in weight). The small wafers were used as the current collector and blocking layer. A 0.2 M Li2Se8 solution was used directly as the catholyte. To a 2032 coin cell, 40 μL of Li2Se8 solution was added to the surface of the N,S-G wafers or directly to the surface of the Celgard 2300 separator to obtain a selenium loading of ~ 5 mg cm−2 when the weight of N,S-G membrane in the electrode is considered. The charge/discharge performance of the coin cells was tested using a LAND CT-2001A instrument (Wuhan, China), and the potential range was controlled between 1.5 and 3.0 V at room temperature. A CHI 660D electrochemical workstation (CHI Instrument, Shanghai, China) was used to perform cyclic voltammetry (CV) measurements at a scan rate of 0.1 mV s−1 and a potential of 1.5–3.0 V. Electrochemical impedance spectroscopy (EIS) was also performed using the same instrument over a frequency range of 100 kHz–10 MHz. The electrical conductivity of the N,S-G membrane was determined by a four-point probe method on a resistivity measurement system (RTS-8, China).
6 Results and Discussion
Figure 5c, d shows the galvanostatic charge/discharge voltage profile of the Li–Se batteries with and without the N,S-G interlayer at different cycles at a current density of 1 C between 1.5 and 3.0 V. There are two voltage plateaus in the discharge process for the catholyte with the N,S-G interlayer (Fig. 5c): a short plateau at 2.25 V and a long plateau at 1.97 V, which could be attributed to a series of reduction reactions from high-order polyselenides to low-chain Li2Se. In addition, a long charge plateau at approximately 2.25 V and a short one at approximately 2.4 V are observed in the charge process; these plateaus correspond to the reverse reaction from Li2Se to high-order polyselenides and even to elemental selenium. Both the charge and discharge voltage plateaus are in good agreement with the CV measurements. Further, even after 500 cycles, the two voltage plateaus are still clearly observed. However, the voltage plateaus are barely observed for the catholyte without the N,S-G interlayer, as shown in Fig. 5d. In addition, the cell with the N,S-G interlayer exhibits a low polarization of 255 mV (vs. 357 mV for the cell without the N,S-G interlayer) after 100 cycles at 1 C. Further, the catholyte without the N,S-G interlayer also reveals strong overcharge in different cycles. These distinct charge/discharge characteristics further confirm the lower polarization, higher electrochemical stability, and reversibility of the polyselenide catholyte with the N,S-G interlayer .
To further demonstrate the advantages of inserting the N,S-G interlayer, the long-term cycling performance of the cells with and without the interlayer was evaluated at a rate of 1 C for 500 cycles. As shown in Fig. 6b, the cell with N,S-G interlayer has a high initial discharge capacity of 638.5 mAh g−1 and a reversible capacity of 330.7 mAh g−1 after 500 cycles. The capacity decay is as low as 0.09% per cycle, which is half that without interlayer. Further, to the best of our knowledge, this is one of the best cycling performances among cathodes with high selenium areal loading, as shown by the comparison in Table S1. In addition, the corresponding Coulombic efficiency of the cell with the N,S-G interlayer is still as high as 99.6% after 500 cycles, whereas that of the catholyte in the cell without the N,S-G interlayer is only 90.5%, demonstrating that the active materials are well confined within the N,S-G interlayer by the effective chemisorption and physical adsorption of lithium polyselenides [19, 24, 35].
After 500 charge/discharge cycles, the calculated values of σw of Li–Se cells with and without the N,S-G interlayer are 17.7 and 55.0, respectively, and the corresponding DLi value of the Li–Se cell with the interlayer is 9.6 times higher than that of the cell without the N,S-G interlayer, indicating that the N,S-G blocking layer is favorable for the redox reaction kinetics [38, 39, 40].
Moreover, the polyselenide catholyte with the N,S-G interlayer also shows a smaller Rs value (34.23 vs. 195.70, Table S2), which indicates that the N,S-G interlayer could effectively prohibit lithium polyselenide shuttling to the anode to form a Li2Se/Li2Se2 film .
A free-standing and ultra-lightweight N,S-G membrane was successfully prepared through a facile and simple vacuum filtration method. When it was used as an interlayer for a polyselenide catholyte, the corresponding Li–Se cells exhibited a high specific capacity and excellent rate and cycling performance even at a high selenium content (79 wt%) and high selenium loading (5 mg cm−2). A reversible discharge capacity of 330.7 mAh g−1 was obtained at 1 C after 500 cycles. This superior electrochemical performance compared to the cell without the N,S-G interlayer could be attributed to good dispersion of the liquid active material in the electrode, high Li+-ion accessibility, fast electronic transport in the conductive graphene framework, and strong chemical adsorption of polyselenides. This work may provide a new route toward optimization of the carbon matrix for high-energy-density Li–Se batteries.
This work was supported by the National Natural Science Foundation of China (51125001, 51172005), the NSFC-RGC Joint Research Scheme (51361165201), and the Start-up Foundation of High-level Talents in Chongqing Technology and Business University (1856008).
- 1.T. Liu, M. Jia, Y. Zhang, J. Han, Y. Li, S. Bao, D. Liu, J. Jiang, M. Xu, Confined selenium within metal–organic frameworks derived porous carbon microcubes as cathode for rechargeable lithium–selenium batteries. J. Power Sources 341, 53–59 (2017). https://doi.org/10.1016/j.jpowsour.2016.11.099 CrossRefGoogle Scholar
- 5.X. Gu, C.-J. Tong, B. Wen, L.-M. Liu, C. Lai, S. Zhang, Ball-milling synthesis of ZnO@sulphur/carbon nanotubes and Ni(OH)2@sulphur/carbon nanotubes composites for high-performance lithium-sulphur batteries. Electrochim. Acta 196, 369–376 (2016). https://doi.org/10.1016/j.electacta.2016.03.018 CrossRefGoogle Scholar
- 12.H.J. Peng, D.-W. Wang, J.-Q. Huang, X.-B. Cheng, Z. Yuan, F. Wei, Q. Zhang, Janus separator of polypropylene-supported cellular graphene framework for sulfur cathodes with high utilization in lithium–sulfur batteries. Adv. Sci. 3, 1500268–1500278 (2016). https://doi.org/10.1002/advs.201500268 CrossRefGoogle Scholar
- 15.L.C. Zeng, W.C. Zeng, Y. Jiang, X. Wei, W.H. Li, C.L. Yang, Y.W. Zhu, Y. Yu, A flexible porous carbon nanofibers-selenium cathode with superior electrochemical performance for both Li–Se and Na–Se batteries. Adv. Energy Mater. 5(4), 1401377–1401387 (2015). https://doi.org/10.1002/Aenm.201401377 CrossRefGoogle Scholar
- 17.T.Z. Zhuang, J.Q. Huang, H.J. Peng, L.Y. He, X.B. Cheng, C.M. Chen, Q. Zhang, Rational integration of polypropylene/graphene oxide/nafion as ternary-layered separator to retard the shuttle of polysulfides for lithium–sulfur batteries. Small 12(3), 381–389 (2016). https://doi.org/10.1002/smll.201503133 CrossRefGoogle Scholar
- 18.T. Wang, K. Kretschmer, S. Choi, H. Pang, H. Xue, G. Wang, Fabrication methods of porous carbon materials and separator membranes for lithium–sulfur batteries: development and future perspectives. Small Methods 1(8), 1700089–1700107 (2017). https://doi.org/10.1002/smtd.201700089 CrossRefGoogle Scholar
- 37.Y. Wei, Y. Tao, Z. Kong, L. Liu, J. Wang, W. Qiao, L. Ling, D. Long, Unique electrochemical behavior of heterocyclic selenium–sulfur cathode materials in ether-based electrolytes for rechargeable lithium batteries. Energy Storage Mater. 5, 171–179 (2016). https://doi.org/10.1016/j.ensm.2016.07.005 CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.