Multilayered Electride Ca2N Electrode via ... - ACS Publications

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Multi-Layered Electride Ca2N Electrode via Compression Molding Fabrication for Sodium Ion Batteries Guanghai Chen, Ying Bai, Hui Li, Yu Li, Zhaohua Wang, Qiao Ni, Lu Liu, Feng Wu, Yugui Yao, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16186 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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ACS Applied Materials & Interfaces

Multi-Layered Electride Ca2N Electrode via Compression Molding Fabrication for Sodium Ion Batteries †

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†

†

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† † Guanghai Chen, Ying Bai,*, Hui Li, Yu Li, Zhaohua Wang, Qiao Ni, Lu Liu,

‡

§

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† Feng Wu, , Yugui Yao, and Chuan Wu*, ,

†

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science

and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China

§

School of Physics, Beijing Institute of Technology, Beijing 100081, China

ABSTRACT Pursuing for novel electrode materials is significant for the progress of sodium ion batteries (SIBs). Here, a multi-layered electride prepared by simple thermal decomposition of solid Ca3N2, namely Ca2N, is introduced as a new anode material of SIBs for the first time, and a compression molding electrode fabricated by pressing Ca2N powder into nickel foam is applied to protect Ca2N from trace moisture and oxygen. The as-prepared electrode delivers an initial discharge capacity of 1110.5 mAh g-1 and a reversible discharge capacity of ~320 mAh g-1. These results suggest that Ca2N has a great potential for sodium ion batteries. Keywords: sodium ion batteries, multi-layered, electride, Ca2N, anode

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In recent years, growing demand for large-scale energy storage technology brings fresh life and new vitality to the development of sodium ion batteries (SIBs) due to abundant and wide distributed sodium resource.1 Although the intercalation of Na+ in layered structures has been investigated in 1970s, approximately the same time period as lithium ion batteries (LIBs),2 developments of SIBs lag behind that of LIBs. Previous researches demonstrate that many sodium analogues of LIBs are not for SIBs due to the sluggish Na+ kinetics.1, 3 Therefore, exploration of novel electrode materials for SIBs is of predominant significance. As is known to all, great progresses have been made on cathode materials of SIBs since 2000. For instance, transition metal oxide,4,5 poly-anionic compounds6, 7 and Prussian blue8, 9 have been widely studied as cathode materials for SIBs. Compared with cathode, the research on anode materials is not fruitful. Present sodium ion batteries anode materials including hard carbons,10,

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metal alloys,12 and Ti-based compounds13 are revealed to have

considerable capability of Na storage. However, low potential and large polarization, large volume change and poor electronic conductivity are severe concern for their application in batteries, respectively. 13, 14 Recently, two-dimensional (2D) electride, dicalcium nitride (Ca2N) electride with a rhombohedral layered structure has garnered increasing attention.15-17 As an electride in terms of [Ca2N]+·e-, Ca2N has good electronic conductive with metallic character and large interlayer distance, making intercalation of Na+ possible. Hu et al.16 theoretically investigated the electrochemical properties of electrides M2N (M = Ca,

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Sr) for SIBs anode by the first-principle calculation, which indicated that the monolayer Ca2N sheet possesses an extremely high theoretical capacity (~1138 mAh g-1) and low migration energy barrier (~0.084 eV) for Na atom, and showed great superiority than usual 2D electrode materials. However, process of preparing 2D Ca2N by ultrasonic exfoliation is cumbersome, which impedes the practical application of Ca2N in electrode materials. And the processes of drying and transferring monolayer Ca2N nanosheets bristle with difficulties because of high sensitivity of Ca2N to moisture. Herein, inspired by 2D Ca2N, for the first time, we report multi-layered Ca2N as the Na storage material, which is prepared by facile thermal decomposing of solid-state Ca3N2, and may be more likely for practical application. To avoid the contact between Ca2N and trace moisture/oxygen, the electrode is fabricated by pressing Ca2N powder into the nickel foam (named compression molding electrode). Synthesis and electrode fabrication procedures are schematically shown in Scheme 1. Detailed description is explained in the Supporting Information. X-ray diffraction (XRD) patterns of Ca2N are depicted in Figure 1a. Almost all of the characteristic peaks are indexed to Ca2N (3 (166) space group, JCPDS no. 21-0837) except some weak impure peaks. These impure peaks can be ascribed to Ca(OH)2, as hygroscopic Ca2N can react with H2O to produce Ca(OH)2 within only a few seconds when exposed to air. It is noteworthy that Ca2N exhibits a sharp characteristic peak at 2θ = 14°, which corresponds to (003) plane. As the inset of

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Figure 1a shows, the layered structure of Ca2N consisting of the sequential stacking Ca-N-Ca (003) monolayers with an interlayer distance of 0.633 nm. This interlayer distance is larger than that of carbon (0.37 nm) which is predicted by theoretical calculation to enable Na+ insertion.18 Furthermore, XRD was carried out to check the protection effect of compression molding electrode on Ca2N. As shown in Figure 1b, characteristic peaks match well with the standard card, demonstrating that the active material in compression molding electrode is still Ca2N. However, as shown in Figure S1, Ca2N fabricated by usual slurry blade coating will easily deliquesce to form Ca(OH)2. Therefore, the technology to fabricate compression molding electrode is effective for protecting Ca2N from moisture and air The field emission scanning electron microscopy (FESEM) images of Ca2N are shown in Figure 1c, d, where Ca2N has a multi-layered morphology with a single layer is less than 200 nm. Such structure is beneficial for electrolyte infiltrating into active material and sodium ions transportation. The electrochemical performances of Ca2N are evaluated by galvanostatic discharging/charging at 50 mA g-1 and cyclic voltammetry tests (CV) at the scan rate of 0.1 mV s-1 between 0-3 V. Figure 2a exhibits the discharge-charge curves of Ca2N compression molding electrode. In the first discharge curve, two slopes, respectively, in 1.3-0.6 V and in 0.6-0 V voltage ranges are observed, well agreement with the observation in the CV plots (Figure 2b). The slope above 0.6 V reveals the irreversible capacity loss, as a result of the concomitant formation of solid electrolyte interface

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(SEI) and/or the trapping of Na+ between the interlayers and at defect sites.

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Particularly, part of Na+ is supposed to be trapped in the layers of a 2D electron gas due to the attractive force between Na+ and electron, which is also the primary factor of the poor initial Coulombic efficiency. However, the subsequent discharge curves have changed greatly. The discharge and charge slopes appear in 1.0-0 V voltage range, well corresponding to a pair of humps at 0.6 V and 0.78 V in the CV plots, respectively. The cycling stability and coulombic efficiency are displayed in Figure 2c. The Ca2N compression molding electrode delivers an initial discharge capacity of 1110.5 mAh g-1 and a reversible capacity of ~320 mAh g-1 over 30 cycles. The decay of capacity is observed and can be ascribed to the Ca2N powder exfoliation from nickel foam in long-term cycling, as shown in Figure S2a. However, if the Ca2N electrode is just fabricated by usual slurry blade coating, it exhibts almost no electrochemical activity, as shown in Figure S2b. Therefore, the effect of compression molding fabrication is very effective in this work, though the cycling performance is still not perfect at present. Compared with existing anode materials in Table S1, the practical specific capacity of Ca2N is good, especially, it delivers higher discharge capacity than other metal nitrides such as Fe2N and Mo2N. Moreover, it is notalbe that Ca2N here has not been modified in any way and its electrochemical performance may be improved greatly in the future. Supposing Ca2N deliquesces to Ca(OH)2 and even ultimately converts to CaCO3, the Ca(OH)2 and CaCO3 compression molding electrodes with the same powder

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loading as Ca2N are fabricated respectively, whose electrochemical performances are conducted in the same test conditions, as shown in Figure 2d. The reversible capacities of Ca(OH)2 and CaCO3 powder electrodes are about 160 and 150 mAh g-1, respectively, much less than that of Ca2N. These results further confirm that: (1) Ca2N is highly active for Na storage; (2) Ca2N can be effectively protected from deliquesce via compression molding electrode fabrication. In addition, the Ca2N compression molding electrode has good rate performances. As shown in Figure 3a, the reversible discharge capacities are 343, 244, 219, 168, 146 and 131 mAh g-1 at the current densities of 50, 100, 200, 500, 1000 and 2000 mA g-1, respectively. Similar to previous report, the ion diffusion coefficient here is calculated based on electrochemical impedance spectroscopy (EIS). 20 As shown in Figure 3b, the high frequency semicircle is related to the charge transfer resistance, while the low frequency sloping line to the Warburg diffusion, respectively. The relationship between the Zre and ⁄ in the low frequency region is shown in Figure 3c. The as-obtained Na+ diffusion coefficients before and after the first cycle are 2.46 × 10-15 and 3.21 × 10-16 cm2 s-1, respectivity. They are approximate to the diffusion of Na+ in hard carbon anode.11 It implies the fast kinetic behavior of Na+ in the two-dimensional structure with large interlaminar space is benificial for good rate performances. In summary, the electrochemical behaviors of Ca2N in sodium ion battery are demonstrated for the first time. Ca2N is synthesized by a facile thermal decomposition of Ca3N2. However, Ca2N is highly deliquescent, making it difficult to

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fabricate the electrode through usual slurry blade coating process. To solve this problem, compression molding electrode is fabricated by pressing Ca2N powder into nickel foam. An initial discharge capacity of 1110.5 mAh g-1 at 50 mA g-1 between 0-3V is achieved, which remains 327 mAh g-1 after 30 cycles. It is helpful for developing electrides, not only Ca2N but also other alkaline-earth metallic nitrides, as a novel family of anode materials for sodium ion batteries.

ASSOCIATED CONTENT Supporting information Synthesis of Ca2N, fabrication of powder electrode, assembly of cells, characterization techniques, an additional data are available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMANTION Corresponding Authors ∗

E-mail address: [email protected]. (Y. Bai)

∗

E-mail address: [email protected]. (C. Wu)

Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENT The present work is supported by the National Basic Research Program of China (Grant No. 2015CB251100), and the Program for New Century Excellent Talents in University (Grant No. NCET-13-0033).

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Scheme 1. The schematic diagram of fabricating Ca2N compression molding electrode.

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Figure 1. XRD patterns of (a) Ca2N (inset is the structure of Ca2N), (b) compression molding electrode; SEM images of Ca2N performed in nitrogen atmosphere (c, d).

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Figure 2. (a) Charge-discharge curves at a current density of 50 mA g-1 between 0-3V, (b) cyclic voltammograms at the scan rate of 0.1mV s-1, and (c) cycling performance of Ca2N compression molding electrode; (d) cycling performance of Ca(OH)2 and CaCO3 compression molding electrodes.

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Figure 3. (a) Rate capability of Ca2N compression molding electrode at changing current densities from 50 mA g-1 to 2000 mA g-1, (b) Nyquist plots and fitting curves using the equivalent circuit (inset) before and after the first cycle, (c) the relationship between Zre and ⁄ in the low frequencies region.

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