Nitrogen and Oxygen Co-doped Graphitized Carbon Fibers with

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Nitrogen and Oxygen Co-Doped Graphitized Carbon Fibers with Rich-Sodiophilic Sites Guide Uniform Sodium Nucleation for Ultrahigh Capacity Sodium Metal Anodes Zi-Jian Zheng, Xian-Xiang Zeng, Huan Ye, Fei-Fei Cao, and Zhengbang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10292 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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ACS Applied Materials & Interfaces

Nitrogen and Oxygen Co-Doped Graphitized Carbon Fibers with Rich-Sodiophilic Sites Guide Uniform Sodium Nucleation for Ultrahigh Capacity Sodium Metal Anodes Zijian Zheng,a Xianxiang Zeng,c Huan Ye,*b Feifei Cao,*b and Zhengbang Wang*a a

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key

Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, P. R. China b

c

College of Science, Huazhong Agricultural University, Wuhan 430070, P. R. China

College of Science, Hunan Agricultural University, Changsha, 410128, P. R. China

KEYWORDS: Sodium metal anode, Na deposition, Sodiophilic sites, Dendrite-free, Ultrahigh capacity

ACS Paragon Plus Environment

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ABSTRACT: Sodium (Na) metal is an ideal anode for high energy Na battery due to the low cost and natural abundance of Na metal. Nevertheless, issues regarding to dendritic and mossy Na metal deposits have prevented their practical application. Herein, nitrogen and oxygen codoped graphitized carbon fibers (DGCF) have been developed as the Na plating hosts to direct Na metal homogenous nucleation and suppress the growth of Na dendrite. We show experimental results as well as first-principles calculations demonstrating that the uniformly doped nitrogen and oxygen function as sodiophilic sites that direct the sodium metal nucleation to a smooth dendrite-free anode. The resultant DGCF-Na anode can be cycled stably at 1 mA cm−2 for a high areal capacity of 12.7 mA h cm−2 with an average Coulombic efficiency of 99.8%, and a Na|Na symmetrical cell can be cycled with long-term durability for more than 1200 h at 2 mA cm−2. When coupled with P2-Na2/3Ni1/3Mn1/3Ti1/3O2 and Na3V2(PO4)3 cathodes, the DGCF-Na composite demonstrates a good feasibility in full cells.

INTRODUCTION Rechargeable room-temperature Na metal battery, as an ideal electrochemistry system, has been emerged as an ideal candidate for the existing electrochemical energy storage systems because of the low cost and abundant raw resources of Na metal.1-8 As one of the most critical ingredients of metal Na batteries, Na metal demonstrates a high theoretical capacity of 1166 mA h g−1 upon repeated plating and stripping of metallic Na.9-11 However, the critical issues referring to battery safety, inferior cycling stability,

and poor plating/stripping efficiency caused by the

inhomogeneous deposition, dendritic growth, and huge volume variation of Na largely prevent their utilization in practice.12-14 With an imperative requirement of large-scale electric-energy storage, exploiting stable and safe metallic Na anodes is a quite vital but maintains a significantly challenging topic.


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To settle the problems indwelt in Na metal anodes, various methods have been adopted, including the use of alternative electrolytes,15-21 artificial electrode/electrolyte interphase films,2225

nanostructured anodes,26-32 and solid-state electrolyte.33-34 However, there has been few

researches addressing the initial nucleation process. Actually, the initial nucleating behavior has a pivotal effect on the final metal deposition morphology and systematic investigation of Na metal nucleation is essential to help exploit enabling Na metal batteries. Experimental studies confirm that the initial nucleation of metal correlates with the current density and the type of metal plating matrix.35-36 Cui’s group reported that Li nuclei size and shape are highly related to the current density.37 Zhang’s group prepared a nitrogen-doped graphene matrix decorated with lithiophilic functional groups that realize Li metal uniform nucleation and growth.35 Recently, Wang and co-workers have conducted in situ electron microscopies to investigate Na nucleation and growth behavior on amorphous carbon nanofibers.38 The Na nucleation overpotential is heavily dependent on the surface defects of carbon substrates, confirming the possibility of realizing uniform deposition by introducing abundant sodiophilic defects on the surface of Na plating matrix. Hence, designing proper defect structure on the Na plating matrix is supposed effectively to enhance the sodiophilic sites by decorating sodiophilic functional groups on the surface of matrix, which may dramatically improve the deposition behavior and the stability of the Na metal anodes. Here, we propose a nitrogen and oxygen co-doped graphitized carbon fibers (DGCF) matrix with evenly distributed sodiophilic functional groups engineered to direct Na metal homogenous nucleation and growth, forming a dendrite-free Na metal anode. Via wet chemical oxidation process, a variety of functional groups such as nitrogen, ester group (–COOR), carbonyl (–C=O), and hydroxyl (–OH) groups with strong negativity can be introduced into the DGCF. Thanks to


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the enhanced negativity of surface charge of DGCF, the DGCF shows a strong binding to the Na+, which is beneficial to a uniform Na plating.39As expected, guided by the uniformly distributed sodiophilic functional groups, Na is preferentially nucleated and plated on the DGCF eschewing the dendritic morphology (Figure 1a). Without the modifications with sodiophilic functional groups, however, Na deposition on the GCF matrix is unregulated (Figure 1b). The relatively low nucleation overpotential of 0.02 V and excellent plating/stripping stability over 1200 h of DGCF based Na metal anode also proved the enrichment effect of sodiophilic sites. Proof-of-concept full cell measurements demonstrated that the DGCF host is suitable for Na-P2Na2/3Ni1/3Mn1/3Ti1/3O2 and Na-Na3V2(PO4)3 cells. This success of engineering defect structure to boost sodiophilic sites may direct a new avenue for exploring safe and stable Na metal anodes in the foreseeable future. Experimental Section Materials Synthesis Commercial GCF of 0.5 mm in thickness (CeTech™, Taiwan) was treated by mixed H2SO4/HNO3 (V: V=3:1) for 2 h at 120 °C, then successively washed by deionized water until the pH value reaches 7, and finally dried at 80 °C in vacuum oven overnight. The treated GCF can be used as the 3D host for metallic Na, while the commercial GCF was served as the contrast electrode. Na Deposition Performance To calculate the Na deposition/ dissolution efficiency, GCF or DGCF working electrodes pairing with metallic Na disks counter electrodes, Celgard separators, 1 M sodium hexafluophosphate


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(NaPF6) (diethylene glycol dimethyl ether) electrolyte were assembled into CR2032-type coin cells. The tests were performed at 0.5, 1, and 2 mA cm–2 at room temperature. The stripping/plating efficiency refers to the stripping capacity (that is, recharged to 1.0 V) over plating capacity. For symmetric batteries, 8 mA h cm–2 of metallic Na were pre-deposited into DGCF and GCF electrodes to form two DGCF-Na and GCF-Na electrodes, respectively, and then the DGCF-Na and GCF-Na electrodes were disassembled in a glovebox and reassembled into DGCF-Na|DGCF-Na and GCF-Na|GCF-Na, forming Na|Na symmetric cells to appraise the cyclic durability and lifespan of the Na anodes. Full cells were assembled to testify the practicality of the DGCF-Na anode. The P2Na2/3Ni1/3Mn1/3Ti1/3O2 and Na3V2(PO4)3 were synthesized as reported in the previous works.40-41 The working electrodes were prepared by mixing active materials (P2-Na2/3Ni1/3Mn1/3Ti1/3O2 and Na3V2(PO4)3), conductive carbon black, and polyvinylidene binder in the ratio of 8:1:1. The loading mass of active materials was 4–5 mg cm−2. 1 M NaPF6 in diethylene glycol dimethyl was utilized as the electrolyte. DGCF-Na pre-deposited with 6 mA h cm–2 of metallic Na was used as the counter electrode. Galvanostatic cycling measurements over a voltage range of 2.5 to 4.0 V and 2.5 to 3.8 V for P2-Na2/3Ni1/3Mn1/3Ti1/3O2 and Na3V2(PO4)3 at a rate of 0.5 C at 25 °C, respectively, were conducted on a Land battery test system. Structure Characterizations SEM (SU-8020, operating at 15 kV), and Electron probe microanalyzer (EPMA) elemental mappings were applied to investigate the microstructures of the DGCF and GCF, the deposition morphology of metallic Na on DGCF and GCF electrodes. Raman spectrum (LabRAM HR Evolution (HORIBA)) was carried out with a laser wavelength of 325 nm to characterize the defects degree of DGCF and GCF. XPS was conducted on an ESCALab220i-XL (VG Scientific)


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utilizing 300 W Al Kα radiation to analyze the surface composition on the DGCF-Na and GCFNa electrodes. XRD profiles were collected from a Philips PW3710 with filtered Cu Kα radiation (Rigaku D/max-2500, λ=1.5405 Å). Before transferring the cycled electrode to a specially designed device for ex situ SEM, EPMA and XPS testing, the electrodes were firstly disconnected in a glovebox followed by repeatedly rinsed with glyme solvent to remove residual electrolytes. Computation Details The first-principles calculations were conducted on the Dmol3 module in Materials Studio 6.1 (Accelrys, USA) applying the Perdew Burke Ernzerhof (PBE) exchange-correlation functional and general gradient approximation (GGA). A zigzag nanoribbon with a 2.0 nm vacuum layer in both the slip direction and normal direction was adopted as the basic model. During the calculation, an all-electron double numerical basis set with polarization functions (DNP basis set) was used. The converged criteria adopted in geometry optimization were listed as follows: 1.0×10–5 au for energy, 2.0×10–3 au Å–1 for maximum force and 5.0×10–3 Å for maximum displacement. A k-mesh used to sample the Brillouin zone was 8×1×1. The binding energy between GCF or DGCF and Na was calculated as follows: Eb=E(carbon-Na composite)E(carbon)- E(Na). RESULTS AND DISCUSSION Features of As-prepared DGCF Owing to their excellent hydrophilicity, good flexibility and superior conductivity, graphitized carbon fibers have been developing continuously, with a focus on supercapacitor, fuel batteries,


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Li-O2 batteries and rechargeable Li batteries.42-45 However, due to the heterogeneous surface of GCF, there are graphitic microcrystallites and protuberances randomly distributed onto GCF, further driving the accumulation of highly concentrated electron/ion on these sites and finally resulting in the uneven Na deposition. Since GCF exhibits poor sodiophilic, the nucleation overpotential for plated Na metal on the surface of GCF is relatively higher than that on a Na disk. Therefore, for GCF, Na atom is more likely to plate on existing nucleus than to generate new seeds under the high driving force of overpotential in the subsequent plating, thus leading to the so-called “tip-growth”. The resulting filamentous dendritic Na will impale the separator to reach the cathode, bringing about cell failure. Besides, the filamentous Na metal dendrite would ceaselessly react with electrolyte because of its high specific area and high chemical activity, evoking a low Coulombic efficiency. When DGCF is utilized as the current collector, the surficial sodiophilic functional groups of DGCF can adsorb a certain Na+ to offset the electrostatic interactions between Na+ and protuberances, resulting in a seemingly even distribution of electric field and a dendrite-free morphology of Na deposits. In this study, the DGCF employed here was prepared by refluxing commercial GCF with oxidizing agents of H2SO4/HNO3 for 2 h at 120 °C. As shown in Figure S1, after oxidation, the DGCF still maintains its intact fibrous structure. The two X-ray diffraction (XRD) patterns of C (002) and C (100) show no significant variation after treatment as shown in Figure S2, indicating that the oxidation treatment did not impair the graphite structure of DGCF in the bulk. The processed DGCF surface emerges a lot of functional groups as confirmed by XPS. As illustrated in Figure 2a, five peaks at binding energies of 284.4 (C–C sp2), 285 (C–C sp3), 286.1 (C–OH), 287.3 (C=O), and 288.7 eV (COOR) appear in the C1s spectrum46-47 while the O1s spectrum shows corresponding peaks at 531.3 (HO–C), 532.3 (COOR), and 533.5 eV(COOR)46 (Figure


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2b). The N 1s spectrum also can be fitted into two component peaks located at 399.9 and 401.3 eV, corresponding to pyrrolic N and quaternary N35 (Figure 2c-d). Wherein, the peaks related to C=O, COOR and pyrrolic N are remarkably strengthened in DGCF than that in pristine GCF, while those characteristics of C–C decreased, confirming the successful doping of nitrogen and oxygen. Figure 2e displays the atomic percentage of carbon, oxygen, and nitrogen in GCF and DGCF. After the oxidization treatment, O percentage increased from 2.68% up to 22.49% while N percentage increased from 0.5% up to 1.76%, indicating chemical oxidization of GCF with H2SO4/HNO3 not only increased the content of oxygen but also introduced nitrogen atoms into host GCF. The structure of GCF and DGCF was also explored by means of Raman spectra (Figure S3). For GCF, the average ID/IG ratio is 0.26, while the average ID/IG for DGCF is 0.3. The higher ID/IG value of DGCF clearly proves the lower graphitization degree of DGCF than GCF, reconfirming the successful introduction of defects functional groups. First-principles Calculations To elucidate the effect of nitrogen and oxygen co-doping on the Na plating, the interactions between GCF or DGCF substrates and Na atoms were explored by the first-principles calculations. As illustrated in Figure 2f and Figure S4, the nitrogen and oxygen co-doped substrates are well-known to possess higher binding energies (pyrrolic nitrogen of –3.23 and carbonyl oxygen of –2.82 eV) than the pristine GCF substrates (graphene of –2.61 eV), indicating that the DGCF exhibits much stronger interactions with the Na atom. Chiefly, the binding energies in nitrogen and oxygen co-doped DGCF increase compared with that in GCF, which is due to the introduction of stronger electronegative N and O which can strengthen the polarity of the GCF (Figure S5). The cross-section density difference isosurface of nitrogen or oxygen doped graphitized carbon fibers reveals that the electron density is shifted toward the


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nitrogen and oxygen atoms around the carbon atoms (Figure 2g-i). This shift can be attributed to the higher electronegativity of nitrogen and oxygen compared to carbon, a difference that causes the nitrogen and oxygen atoms to be negatively polarized, which promotes the adsorbing of the Na+ in the electrolyte and the uniform distribution of Na+ on the DGCF surface. The homogeneous distribution of Na+ is deemed to the key to enabling homogeneous Na plating. To directly display the sodiophilicity of the DGCF, surface wettability tests of liquid Na on GCF and DGCF were performed (Figure S6). The molten Na can spread out completely on the surface of DGCF while displays a large contact angle on that GCF. Electrochemical performances The pristine plating behavior of metallic Na onto various hosts at 1 mA cm−2 was compared in Figure 3a to disclose the influence of the defect functional groups on Na nucleation. A sharp voltage spike at the initial Na deposition of –0.029 V at 1 mA cm−2 is recorded onto the raw GCF electrode, referring to the nucleation process. Once the first nucleation occurs, the overpotential rises to –0.01 V, which relates to the Na nuclei growth. In contrast, for the GCF decorated by sodiophilic functional groups, Na is selectively nucleated on sodiophilic sites with a nucleation overpotential of merely 0.02 V, which is lower than that of GCF. Considering the nuclei size is inversely proportional to electrochemical overpotential,37 one can boldly deduce that the lower overpotential spike for DGCF electrode is more favorable for uniformly depositing rather than forming long dendritic. Figure 3b plots the nucleation overpotentials of Na on DGCF and GCF for the same deposition capacity under different current densities ranging from 0.5 to 2 mA cm−2. The nucleation overpotentials (ηn) on DGCF electrode are seen to increase from 0.02 to 0.035 V with increasing current densities, which is relatively smaller than that on GCF electrode from 0.02 to 0.04 V, indicating the superior regulating ability of DGCF electrode on metallic Na


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nucleation at higher current densities. Further, we explored plating/stripping testing with progressing sodium loading. One can observe that the DGCF electrodes displayed better performance with negligible variation of nucleation overpotentials and growth plateau overpotentials at 1 mA cm−2 for plating 2 mA h cm−2 up to plating 12.7 mA h cm−2 (Figure 3c), with stable average Coulombic efficiency of 99.8% (Figure 3d), which shows its great potential for exploring the electrodes with high areal capacities. Doped-induced growth of metallic Na on the evenly distributed sodiophilic functional groups could generate a sleek Na anode in the 3D DGCF host. With sodiophilic functional groups as nucleation sites, Na is preferentially nucleated on the sodiophilic sites because of the small nucleation overpotential. Foreign Na tends to plate on existing nucleus than for forming new seeds due to the high driving force for overpotential.37 Thus, at the initial stage for inserting/plating 2 mA h cm−2 of Na on DGCF electrode, metallic Na preferentially formed spherical nuclei (Figure 4a, d). During ensuing Na deposition, Na grows on the existing spherical nuclei and fuses together, leading to increased Na particles size and decreased nuclei density (Figure 4b, e). With further Na deposition, the DGCF electrodes were covered with large and densely distributed Na, forming an even Na metal anode without dendrites (Figure 4c, f). The homogeneous distribution of metallic Na on the DGCF electrode is further certified by electron probe micro-analyser (EPMA) through the well overlap between Na and C elemental mappings (Figure 4g-i). In contrast, the pristine GCF cannot emerge a smooth Na metal anode due to Na deposition is not directed onto the GCF scaffold (Figure S7). Because the raw GCF shows poor Na metal wettability, Na forms nonuniform Na nucleation deposits on GCF, and subsequently grows into uneven Na dendrites (Figure S8). The uncontrolled Na dendrites would impale the


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separator and reach the cathode, giving rise to internal short circuits, and thus deteriorating the cycling lifespan and causing safety hazards. The Na deposition behavior is directly linked to its electrochemical performance. Being plated/stripped at 1.0 mA cm−2, the pure GCF shows a initial efficiency of 91.88%, manifesting large initial irreversible capacity loss (Figure 5a). However, the capacity loss for DGCF is negligible because of a significantly improved Coulombic efficiency of approaching 100% (Figure 5b). An extremely higher interfacial stability of DGCF than that of GCF after the plating process was confirmed by stable charge/discharge voltage profiles and stable Coulombic efficiency (Figure 5c-d). The plating/stripping efficiency of DGCF at 1 mA cm−2 with a cycling capacity of 8.0 mA h cm−2 remains 99.9% for near 100 cycles, while that of non-doped GCF fluctuates ranging from 93.74 to 107.32% (Figure 5a-c). The Coulombic efficiency of the metallic Na anode on the GCF electrode is greater than 100%, possibly because some preintercalated Na+ were extracted from the graphite layers and isolated Na recovers active in cycling. During the initial Na plating process (above 0 V vs Na+/Na), Na+ were preferentially intercalated into the graphite layers with a large interlayer space and then plated on the surface of GCF. However, the inserted Na+ ions were not extracted completely from the graphite layers during the first charge process (Figure 5c, d), leading to low initial Coulombic efficiency. During the subsequent cycling, some pre-intercalated Na+ were possibly extracted from the GCF to offset the Coulombic efficiency. Second, as an analogue of metallic Li, metallic Na also exhibits high electrochemical active with many kinds of organic electrolytes and can react with electrolyte to form SEI film, leading to corrosion and isolation of Na. The isolated Na might recover active in cycling, accounting for the increased Coulombic efficiency, which has been reported in previous article.32 Increasing the current density to 2.0 mA cm−2, the Coulombic


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efficiency of DGCF electrode still maintain 99.8% for more than 120 cycles without obvious fluctuation (Figure 5d). By contrast, erratic Coulombic efficiency and voltage hysteresis of the GCF electrode is observed (Figure 5d and Figure S9), indicating the generation of fractal Na dendrites on the GCF. These results demonstrate that the irreversible capacity loss and the growth of dendrites upon cycling has been effectively suppressed and highlight the importance of introducing uniform sodiophilic sites into 3D hosts for smooth Na metal anode. Nevertheless, the cycling performance of the Na metal anode on the DGCF electrode in carbonate electrolyte should get further improved due to the parasitic reaction between active Na and carbonate electrolytes (Figure S10). The Na|Na symmetric cell tests were assembled to appraise the interfacial stability of Na metal anodes during Na deposition/ dissolution processes. The GCF-Na|GCF-Na symmetrical cell shows ever-increasing overpotential at 0.5 mA cm−2 and labile voltage behavior (Figure 6a), which could be interpreted as the consumption of electrolyte and the formation unstable SEI layer due to the repeated growth/corrosion of Na dendrites. After short cycling for merely170 h, the overpotential is increased from 20 to 40 mV. Meanwhile, the GCF-Na|GCF-Na symmetrical cell exhibits obvious overpotential tips at the start and at the end of each deposition or dissolution cycle (Figure 6b), which could be ascribed to high specific kinetic hindrance for uneven Na deposition and dissolution deriving from the dynamic evolution of dendrites and pits in cycling.48-49 In contrast, the DGCF-Na|DGCF-Na symmetrical cell exhibits relatively stable voltage profile with an ultralong lifespan of 800 h at current density of 0.5 mA cm−2 and 1262 h at current density of 1 mA cm−2 (Figure 6a-c), which is close to or better than the reported literatures.16,

22, 50

Increasing the current density to 2 mA cm−2, the DGCF-Na|DGCF-Na

symmetrical cell shows stable Na plating/stripping for more than 1238 h without significant


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increment in hysteresis (Figure 6d). These results further certified a stable SEI layer generates on the DGCF due to the favorable Na deposition behavior, enabling enhanced charge-transfer kinetics and cycling stability. Electrochemical impedance spectroscopy analysis of the cells using the DGCF electrodes was conducted to evaluate the interface stability. As shown in Figure S11, the electrolyte resistance and the interfacial resistance for the DGCF-Na electrode increase gradually with cycling, indicating stable electrolyte/electrode interface and rapid kinetics, which is consistent with low voltage hysteresis of the Na anode in the DGCF. Ex situ XPS testing was carried out to characterize the surface chemical component of GCFNa and DGCF-Na electrodes after initial plating. The C1s, O1s, F1s and Na1s XPS spectra of each sample are displayed in Figure S12. In the C 1s spectrum (Figure S12a), the peaks at 284.8, 286.6, 289.2, and 293.0 eV corresponding to the −C−C−species, −C−O− species, CO32− species, and −CF3 species occur in both samples. For the O1s spectrum, the O1s peak of 530.9, 533.3 eV in DGCF-Na electrode is the formation of Na2CO3 and C−O in the SEI layer (Figure S12b). In the F1s spectrum, the peaks of NaF, F−C, NaxPFy and NaxPOyFz could be detected at 683.8, 689, and 687.3 eV, respectively (Figure S12c). Combining these results with the Na1s spectrum, we can speculate that the Na 1s peak at 1071.4 eV composes of Na−O and Na−F (Figure S12d). Besides, the content of C, F, O and Na in GCF-Na electrode is much less than that in the DGCFNa electrode demonstrating unstable SEI layer on GCF-Na. This unstable SEI layer in the GCFNa electrode could give rise to the exposure of fresh Na to the liquid electrolyte, aggravating nasty negative reaction, consuming both active Na and electrolyte, reducing the stripping/plating Coulombic efficiency of Na, and resulting in the generation of Na dendrites and “dead Na”. In addition, the anchored sites, such as O and N on the cycled DGCF electrode show high stability by exhibiting negligible change in content, which was characterized by ex situ XPS analysis


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(Figure S13). Likewise, the SEI thickness on the cycled GCF and DGCF was characterized by ex situ TEM. From the Figure S14a, one can find that a very thick amorphous surface film of 60-80 nm coated outside the GCF after 20 plating/stripping cycles, which mainly derived from the repeated reaction between metallic Na and electrolyte. This thick SEI layer in the GCF-Na electrode could lead to the increased electrochemical impedance and generation of “dead Na”, reducing the stripping/plating Coulombic efficiency of Na and decaying the cycling performance. In contrast, a uniform and thin SEI layer of less than 25 nm was detected on the cycled DGCF (Figure S14b). The content and component of SEI layer in the DGCF-Na electrode can still remain after 20 plating/stripping cycles (Figure S15), demonstrating the stable properties of the interface, which is beneficial for the uniform Na+ flux and nondendritic growth of Na metal. As a proof-of-concept, full cells with the P2-Na2/3Ni1/3Mn1/3Ti1/3O2 cathodes were assembled using GCF and DGCF pre-deposited with 6 mA h cm−2 Na as the anodes. As displayed in Figure 7a, b, the cells with DGCF-Na anodes exhibited better capacity retention compared to the cells using GCF-Na anodes (94.2% vs 87.4%), demonstrating high Na utilization of the DGCF-Na anode. In addition, full cells with Na3V2(PO4)3 cathodes were also assembled to verify the feasibility of the DGCF-Na anode in a practical battery. The reversible capacities collected from the full cells based on the DGCF-Na and the GCF-Na anodes are 97 and 85 mA h g−1 at 0.5 C, respectively. After 80 cycles, the full cells with DGCF-Na anode can still deliver a reversible capacity of 95 mA h g−1, much higher than that full cells with GCF-Na anode, demonstrating a superior suitability of the DGCF-Na anode in full cells. CONCLUSIONS


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In summary, we have demonstrated uniformly distributed sodiophilic functional groups decorated onto GCF to direct metallic Na nucleation and suppress Na dendrite growth. The firstprinciples calculations demonstrate the doping of nitrogen and oxygen can enhance the surficial negativity of DGCF, help Na metal distributing uniformly on the GCF hosts, on which Na metal are directed to form smooth Na metal anode free from dendrite obsessions. The Na deposition/dissolution properties of the DGCF, therefore, far exceeds that of the counterpart GCF electrode. It is demonstrated that a DGCF|Na cell can be cycled stably at 1 mA cm−2 for an ultrahigh areal capacity of 12.7 mA h cm−2 with an average Coulombic efficiency of 99.8%, and a Na|Na symmetrical cell exhibits an ultralong lifespan with low voltage overpotential and superior cycling stability without short-circuiting obsession for more than 1200 h. To assemble full cells sodium batteries we showcase its practical feasibility of the engineered DGCF host. This job offers an approach for future endeavors to regulate metallic Na growth at a sprouting stage for the safe and highly efficient Na metal anode for advanced energy storage application. ASSOCIATED CONTENT Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. Electron probe microanalyzer (EPMA) elemental mappings, XPS tests, SEM images, Raman spectra, electrochemical performances and comparative results of GCF and DGCF. AUTHOR INFORMATION Corresponding Author *[email protected]


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*[email protected] *[email protected] ORCID Zijian Zheng: 0000-0001-5639-3841 Xianxiang Zeng: 0000-0001-7662-2349 Huan Ye: 0000-0002-1074-528X Feifei Cao: 0000-0002-4290-2032 Zhengbang Wang: 0000-0002-4811-8943 Notes The authors declare no competing financial interest. Acknowledgments Financial supports from the National Natural Science Foundation of China (grant nos. 51703052), the Fundamental Research Funds for the Central Universities of China (2662017QD028), and the Science and Technology Department of Hubei Province (2018FB238) are gratefully acknowledged. The National Supercomputing Center in Shenzhen for providing the computational resources and materials studio (6.1, Dmol3) is also greatly appreciated. REFERENCES


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Figure 1 Schematic illustration of the Na nucleation and plating process on (a) nitrogen and oxygen co-doped graphitized carbon fibers electrode and (b) graphitized carbon fibers electrode.


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Figure 2 The XPS results of pristine GCF and DGCF. (a) The C 1s spectra, (b) the O 1s spectra, (c) the N 1s spectra, and (d) schematic diagram of DGCF decorated with sodiophilic functional groups. (e) Atomic concentration comparison of C, O, N. (f) Binding energy of a Na atom with different functional groups of DGCF. The charge density of: (g) graphene (G), (h) pyrrolic N (prN), and (i) carbonyl (C=O) in DGCF. An increase in total electron density is shown as red, a decrease as blue.


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Figure 3 (a) The voltage–time curves during Na nucleation at 1 mA cm−2 on GCF and DGCF electrodes. (b) The Na nucleation overpotentials (ηn) on GCF and DGCF electrodes at different current densities (J). (c) Galvanostatic plating/stripping of Na on DGCF performed over a range of Na loadings at 1mA cm−2. (d) Rate capability of Na on DGCF performed over a range of Na loadings at 1 mA cm−2.


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Figure 4 SEM images of metallic Na deposits at a current density of 1 mA cm−2. DGCF electrode with the inserted/plated Na of (a) (d) 2 mA h cm−2, (b) (e) 4 mA h cm−2, (c) (f) 6 mA h cm−2. EPMA elemental mappings of (g) carbon and (h) sodium for DGCF-Na anode. (i) Energy dispersive X-ray spectroscopy (EDS) for DGCF-Na anode.


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Figure 5 Typical galvanostatic discharge–charge profiles of the GCF electrode (a) and (b) DGCF electrode at 1 mA cm−2 for 8 mA h cm−2. Comparison of Coulombic efficiency of Na plating/stripping on/from the GCF electrode and DGCF electrode under a current density of (c) 1 mA cm−2, (d) 2 mA cm−2 for the same areal capacity of 8 mA h cm−2.


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Figure 6 (a) Voltage profiles of metallic Na plating/stripping in GCF and DGCF symmetric cells at 0.5 mA cm−2 for 1 mA h cm−2. (b) The detailed voltage profiles from 0 to 50 h, 100 to 150 h, and 550 to 600 h in Figure 6a. (c) Voltage profiles of metallic Na plating/stripping in GCF and


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DGCF symmetric cells at 1 mA cm−2 for 1 mA h cm−2. (d) Voltage profiles of metallic Na plating/stripping in GCF and DGCF symmetric cells at 2 mA cm−2 for 1 mA h cm−2.


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Figure 7 (a) The initial voltage profiles and (b) cycling performance of full cells pairing with P2Na2/3Ni1/3Mn1/3Ti1/3O2 cathode and GCF-Na and DGCF-Na anodes at 0.5 C (1 C=89 mA g−1). (c) The initial voltage profiles and (d) cycling performance of full cells pairing with Na3V2(PO4)3 cathode and GCF-Na and DGCF-Na anodes at 0.5 C (1 C=118 mA g−1).


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Nitrogen and Oxygen Co-doped Graphitized Carbon Fibers with

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