Encapsulation of Fe3O4 between Copper Nanorod and Thin TiO2 Film

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Encapsulation of Fe3O4 between Copper Nanorod and Thin TiO2 Film by ALD for Lithium-Ion Capacitors Yuzhu Li, Tian Liang, Rui Wang, Beibei He, Yansheng Gong, and Huanwen Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

Encapsulation of Fe3O4 between Copper Nanorod and Thin TiO2 Film by ALD for Lithium-Ion Capacitors Yuzhu Li, Tian Liang, Rui Wang, Beibei He, Yansheng Gong, Huanwen Wang* Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Material and Chemistry, China University of Geosciences, Wuhan 430074, China. E-mail: [email protected] ABSTRACT: Lithium-ion capacitors (LICs) are considered to be promising power sources due to their combination of high-rate capacitors and high-capacity batteries. However, development of a high-performance LIC is still restricted by the sluggish intercalation reaction and unsatisfied specific capacities in battery-type bulk anodes. To overcome these issues, herein we utilize two-step atomic layer deposition (ALD) to realize uniform coating of FeOx and TiO2 on CuO nanorods, which result in the formation of ternary CuO@FeOx@TiO2 composite. After further treatment in H2/Ar atmosphere, the as-derived Fe3O4 is encapsulated between conductive Cu nanorod and hollow TiO2 shell (denoted as Cu@Fe3O4ϿTiO2). Owning to the rational design, the binder-free Cu@Fe3O4ϿTiO2 electrode exhibits an ultrahigh Li-ion storage capacity (1585 mA h g−1 at 0.2 A g−1), superior rate capability and excellent cycle performance (no decay after 1000 cycles), which could efficiently boost the energy-storage capability of LICs. By employing an anode of Cu@Fe3O4ϿTiO2 and a cathode of activated carbon (AC), the as-constructed full LIC device provides high energy//powder densities (154.8 Wh kg−1 at 200 W kg−1; 66.2 Wh kg−1 at 30 kW kg−1). These superior results demonstrate that ALD-enabled architectures hold great promise for synthesizing high-capacity anodes for LICs. KEYWORDS: Fe3O4, TiO2, Cu nanorod, lithium-ion capacitor, atomic layer deposition

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1. INTRODUCTION With the wide application of various electronic products and green vehicles, construction of energy-storage devices offering both large energy//power densities and excellent cycle ability becomes increasingly significant.1-3 Among current electrochemical energy storage systems, lithium ion battery (LIB) is recognized as successful representative and has been applied in various fields due to its energy superiority. Generally, LIBs have the prominent specific energy but suffer from low charge-discharge rate and unsatisfied cycling ability due to sluggish Li-ion diffusion within the bulk phase. In contrary, supercapacitors (SCs) show high charge-discharge rates and long cycling stability via the fast electrical double layer (EDL) mechanism on the electrode surface, but their specific energy is significantly low (usually 100 times lower than typical LIBs) due to less physically stored charges on the surface.4-6 To overcome the individual shortcomings of LIBs and SCs, a novel lithium ion capacitor (LIC) that consists of a high-energy anode and a high-power cathode in organic lithium salt electrolyte, is developed in recent years.7-8 In a typical LIC, the anode provides a high specific capacity via Li+ insertion/extraction, while the cathode offers a high rate capability through fast anion (such as PF6−) adsorption/desorption on the surface electrode.9-10 In addition, the voltage window of LICs can be efficiently enlarged to 4 V (even 4.5 V) due to the asymmetrical assembly in a hybrid device,11 which further result in high energy densities. In spite of these competitive advantage, the electrochemical kinetics of Faradaic LIB-type anodes is relatively sluggish as compared to the capacitive cathode. This imbalanced kinetics between two electrodes will limit the capacity utilization of the anode.12 Constructing suitable anode materials, to overcome the kinetic discrepancy in LICs, is the key to realizing advanced LICs. Since nanostructured LiTi5O12 is explored as a LIC anode,13 various LICs with improved energy-storage ability are reported. Most reported anode materials for LICs are mainly focused on Ti, Nb, Vbased materials (such as Li4Ti5O12,14 TiO2,15 TiC,7 H2Ti6O13,16 Nb2O5,17 TiNb2O7,18 V2O5,19 Li3VO420). However, the relatively low capacity (below 300 mA h g−1) and the high potential plateau (about 1.5 V vs. Li/Li+) of the above materials will not only result in limited energy but also greatly reduce voltage output of the LIC device. In addition, the graphite,21-23 MnO,24 FeOx,25 Si,26-27 Sn,28 VN,29 MoS230-based anodes have also been reported for LICs. Although the relatively high energy values (over 90 Wh kg–1) 2


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of reported LIC devices are achieved at small rates, the sluggish redox kinetics of these anodes still limit the power and cycling performance of LICs. Therefore, rational design of an appropriate anode with both high capacity and fast electrochemical kinetics is the key to construct an advanced LIC. To date, among various high-capacity metal oxides, ferroferric oxide (Fe3O4) is recognized as a completive anode for LICs because of the large theoretical specific capacity (over 900 mA h g−1), low potential plateau (~0.8 V), wide abundance in earth, and nontoxicity.31-32 However, the capacity of the Fe3O4 electrode exhibits rapid capacity decay owing to its structural pulverization during Li+ insertion/extraction. To save this issue, nanostructure engineering and hybridization with carbonaceous materials are the common paths. For example, Fe3O4@PCM/graphene,33 Fe3O4@C,34 Fe3O4/conductive carbon,35 Fe3O4/graphene foam

36

are reported to overcome the

volume expansion of Fe3O4 during the charge-discharge process. However, introduction of carbon in Fe3O4 not only leads to the low first Coulomb Efficiency but also decreases the volumetric performance of the LIC device.37 In addition, the reported Fe3O4-based composites are typical powders, which need additional binders (5-10 wt %) or conducting additives (10–20 wt%) to make working electrodes. These inert components will reduce the electron transport between active phases and current collector in long-term cycling tests.12 Therefore, rational engineering is still highly required for accommodating the huge volume expansion for Fe3O4. In the present work, to avoid the structural pulverization of Fe3O4 during long cycles, we directly encapsulate Fe3O4 between Cu nanorods and hollow TiO2 shell (Cu@Fe3O4ϿTiO2) via ALD and subsequent thermal-reduction. The as-obtained ‘‘Cu@Fe3O4ϿTiO2’’ array electrode has several merits: (1) The Cu nanorods aligned on copper grid act as 3D current collector, which can facilitate the fast charge transport; (2) The active Fe3O4 nanolayer is strongly connected with Cu nanorods, which will lead to a significantly increased contact area; (3) The outer hollow TiO2 shell can efficiently accommodate the volume variation of Fe3O4 and further avoid exfoliation from Cu nanorods. Therefore, the as-prepared Cu@Fe3O4ϿTiO2 electrode exhibits outstanding Li-storage performance. It delivers an ultrahigh capacity of 1585 mA h g−1 in the 400th cycle at 0.2 A g−1, superior rate/cycling performance. The as-fabricated Cu@Fe3O4ϿTiO2//AC LIC full cell achieves high energy//power values (154.8 Wh kg−1 and 30,000 W kg−1). 3



2. EXPERIMENTAL SECTION 2.1 Preparation of CuO nanorod arrays. A transparent alkaline oxidative etchant solution was prepared by dissolving 1.52 g (NH4)2S2O8 and 5.3 g NaOH in 50 mL deionized water under vigorous stirring. Then, the Cu grid was dipped into the above solution for 1 h at about 25 oC. After that, the surface of Cu grid became light blue, which was denoted as Cu@Cu(OH)2. After thermal treatment at 200 oC at 1 oC min−1 in the air atmosphere, the CuO nanorod arrays/Cu grid was obtained (Cu@CuO). 2.2 Deposition of FeOx and TiO2 on CuO nanorods by ALD. The deposition of FeOx and TiO2 via ALD was performed by using a LanNano-9100 system (Ensure NanoTech (Beijing), Inc.). The FeOx was deposited on the Cu@CuO by using ferrocene (FeCp2) and O3 as the reactants at the speed of ∼0.017 nm/cycle. The four steps are involved in one ALD Fe3O4 cycle. In step 1, the Fe(Cp)2 molecule was supplied into the ALD chamber and then adsorbed on the CuO surface to form a covalent bond with the surface molecules. In step 2, the N2 was pushed into the ALD chamber to remove reaction byproduct and the unreacted Fe(Cp)2. In step 3, the O3 flowed as the oxidation agent and reacted with Fe(Cp)2 to grow a thin film of FeOx on CuO nanorods. Finally, the chamber is again purged to completely remove excess O3 and by-products from the chamber. Subsequently, the TiO2 was deposited on the surface of Cu@CuO/FeOx by using Tetrakis (dimethylamino) titanium and H2O as the reactants at the speed of ∼0.055 nm/cycle, and the Ti source was heated at 75 °C during the deposited process. The ALD TiO2 cycle is similar to FeOx but the H2O was used as the oxidation agent. The thickness of the FeOx and TiO2 was controlled by setting different cycles. The carrying gas were nitrogen with a purity of 99.999%. The ALD system was maintained at about 18002000 Pa and the temperature for FeCp2 and Tetrakis (dimethylamino) titanium were kept at 250 and 150 °C, respectively. 2.3 Preparation of Cu@Fe3O4ϿTiO2. The above Cu@CuO/FeOx/TiO2 was thermally treated at 600 °C for 2 h in the Ar/H2(5%) to form Cu@Fe3O4ϿTiO2. The heating rate was set in 1 oC min−1. For comparison, Cu@Fe3O4 was also prepared by the same procedure with Cu@Fe3O4ϿTiO2 without ALD TiO2. 2.4 Materials characterization. The as-synthesized materials were analyzed by X-ray diffraction (XRD, Bruker AXS D8-Focus, Cu Kα radiation, λ=0.1540598), field emission scanning electron microscopy (SEM, Hitachi SU8010) and transmission 4


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electron microscopy (TEM, Philips CM12 TEM/STEM). X-ray photoelectron spectroscopy (XPS) was carried out by VG Multilab2000 and the data was calibrated through the C 1s peak at 284.8 eV. 2.5 Electrochemical characterizations. The electrochemical performances of the Cu@Fe3O4 and Cu@Fe3O4ϿTiO2 electrodes were examined through standard coin cells

(CR2032)

that

were

assembled

in

the

Ar-filled

glovebox.

The

Cu@Fe3O4ϿTiO2//AC hybrid LIC was fabricated with the Cu@Fe3O4ϿTiO2 anode and the AC cathode. The mass ratio of anode (~0.8 mg cm−2) and cathode (~4 mg cm−2) was under the optimized mass rate (Fe3O4ϿTiO2: AC was set to 1 : 5). The electrolyte was 1 mol L−1 LiPF6 dissolved in ethylene carbonate/propylene carbonate (1:1 in volume) solution and the separator was the Celgard 2400. The electrochemical tests were performed in a CT2001A cell test instrument. 3. RESULTS AND DISCUSSION

Figure 1. Schematic illustration for (a) the preparation of the Cu@Fe3O4ϿTiO2 electrode and (b) The LIC device assembled by the battery-type Cu@Fe3O4ϿTiO2 anode and the EDLC-type AC cathode.

The synthetic procedure of Cu@Fe3O4ϿTiO2 was illustrated in Figure 1a. Firstly, vertical CuO nanorod arrays on highly conductive copper grid were fabricated through a simple solution oxidation (Cu + (NH4)2S2O8 + 4NaOH→ Cu(OH)2 + 2Na2SO4 + 2NH3 + 2H2O) and subsequent thermal treatment (Cu(OH)2→CuO + H2O). Secondly, amorphous FeOx nanolayer is directly deposited on CuO nanorods through the ALD 5



technique using ferrocene and ozone as the reactants, which forms a binary CuO@FeOx core/shell nanorod architecture. Thirdly, to attain stable Li+-storage, the uniform TiO2 coating is further deposited on the surface of CuO@FeOx, which generates a ternary CuO@FeOx@TiO2 composite.38-40 To attain stable Li+-storage, the uniform TiO2 coating is further deposited on the surface of CuO@FeOx, which generates a ternary CuO@FeOx@TiO2 composite. Finally, after reduction in H2/Ar atmosphere, CuO@FeOx@TiO2 was converted to Cu@Fe3O4ϿTiO2. During the reduction process (CuO→Cu), the oxygen release will produce an interesting void between the Cu@Fe3O4 and the TiO2 coating. This as-formed void could buffer the volume expansion of Fe3O4 during long-term cycling processes. Figure 1b schematizes the LIC architecture and its charge/discharge mechanisms. When the LIC is charged, Li+ ions are intercalated into Cu@Fe3O4ϿTiO2 and PF6− ions move toward the porous AC cathode under the interaction of the electric field. In the discharge process, Li+ and PF6− ions are moving in the opposite direction.

Figure 2. SEM images of (a-c) CuO, (d-f) Cu@Fe3O4ϿTiO2 and (g-i) Cu@Fe3O4 nanorod arrays grown on copper grid.

The surface microstructure of the as-synthesized samples is investigated by SEM. The pristine copper grid is weaved along the vertical and horizontal directions by the copper fiber with the diameter of 35 μm (Figure S1, Supporting Information). After oxidation, the diameter of copper fiber is increased to around 100 μm and uniform 6


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Cu(OH)2 nanorod arrays are grown on every copper fiber (Figure S2, Supporting Information). After thermal treatment at 200 oC, the as-obtained CuO still maintain the nanorod structure with the average size of 300 nm (Figure 2a-c). The colour change from light blue to black also demonstrates the formation of CuO (inset of Figure 2c). As for Cu@Fe3O4ϿTiO2, the inner Cu@Fe3O4 particles are successfully encapsulated in hollow TiO2 shell and the typical nanorod architecture is clearly presented (Figure 2d-e). The low-magnification SEM image (Figure 2f) indicates that the Cu@Fe3O4ϿTiO2 nanorod arrays are uniformly and vertically grown on each copper fiber and no freely dispersed aggregate is observed outside the fiber. In addition, the asobtained Cu@Fe3O4ϿTiO2/copper grid can be flexible and bendable due to the tightly interconnected network (inset of Figure 2f). For comparison, we also fabricated the Cu@Fe3O4 nanorods grown on copper grid by the same method but without ALD TiO2. Figure 2g-i show SEM images of Cu@Fe3O4 nanorods, in which large Fe3O4 particles are dispersed on the surface of Cu nanorods.

Figure 3. (a-d) TEM images of Cu@Fe3O4ϿTiO2. (e) Elemental mapping images of Cu@Fe3O4/TiO2 and Ti, O, Fe and Cu components. (f) XRD patterns. XPS spectrum of (g) Fe 2p and (h) Ti 2p in Cu@Fe3O4ϿTiO2. 7



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The microstructure of Cu@Fe3O4ϿTiO2 was further characterized by TEM, XRD and XPS. The TEM images in Figure 3a-b show that the diameter of the Cu@Fe3O4/TiO2 nanorods is about 300 nm, in agreement with SEM. Furthermore, the void space can be found between the Cu@Fe3O4 and the TiO2 coating. The TiO2 shell has a thickness of about 10 nm, which can act as a protective layer to guarantee superior structure stability during repeated Li+ insertion/extraction. The typical lattice fringes in Figure 3c shows the interlayer spacing of 0.297 and 0.253 nm, which are ascribed to the (220) and (311) planes of Fe3O4, respectively (JCPDS card no. 65-3107).41 Close to Fe3O4 lattices (Figure 3c), the spacing of 0.208 nm arises from the (111) plane of metal Cu (JCPDS No. 65-9026).42 The tight connection between Fe3O4 and Cu could facilitate fast electron conduction. As presented in Figure 3d, the lattice fringe of 0.35 nm corresponds to the (101) plane of anatase TiO2 (JCPDF No. 65-5714).43 The uniform distribution of Ti, O, Fe and Cu elements in the Cu@Fe3O4ϿTiO2 nanorod is demonstrated by elemental mapping images (Figure 3e). XRD patterns of copper grid, Cu(OH)2/copper

grid,

CuO/copper

grid,

Cu@Fe3O4/copper

grid

and

Cu@Fe3O4ϿTiO2/copper grid, are shown in the Figure 3f. After treatment in H2/Ar atmosphere, the diffraction peaks of CuO disappear, indicating the successful conversion of CuO to Cu. As compared to copper substrate, the peak intensity of Fe3O4 is relatively low and only the crystal plane of (400) is observed (JCPDS No.65-3107).44 The XPS spectrum (Figure S3, Supporting Information) demonstrates the presence of Cu, Fe, Ti and O elements, which agrees well with the mapping result. The fitting peaks at 710.5 and 723.4 eV are attributed to Fe 2p3/2 and Fe 2p1/2 (Figure 3g), respectively, which belong to the characteristic peaks of Fe2+. Meanwhile, the other two peaks at 712.3 (Fe 2p3/2) and 725.4 eV (Fe 2p1/2) are ascribed to the characteristic peaks of Fe3+ 31.

This suggests that the as-obtained iron oxide is Fe3O4. The XPS curve of Ti (Figure

3h) shows the typical Ti 2p3/2 and Ti 2p1/2 at 459.4 and 465.1 eV, respectively. These analysis demonstrates the successful formation of Cu@Fe3O4ϿTiO2.

8


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Figure 4. Electrochemical performances in half-cell: (a) GCD curves of Cu@Fe3O4ϿTiO2 at 0.1 A g−1. (b) GCD curves of Cu@Fe3O4ϿTiO2. (c) Rate performance of Cu@Fe3O4 and Cu@Fe3O4ϿTiO2 electrodes. (d) Cycling stabilities of Cu@Fe3O4 and Cu@Fe3O4ϿTiO2 electrodes at 0.2 A g−1. (e) The long-term cycle life of the Cu@Fe3O4ϿTiO2 electrode at 2 A g−1. Noted that the electrode is firstly activated at 0.1 A g−1 for ten cycles before long-term cycling.

The electrochemical performances of the Cu@Fe3O4ϿTiO2 and Cu@Fe3O4 electrodes were measured in the potential range of 0.01~3.0 V (vs. Li/Li+). Figure 4a displays the initial three GCD curves of Cu@Fe3O4ϿTiO2 at 0.1 A g−1. The discharge plateau at ~0.8 V corresponds to the conversion reaction of Fe3+ or Fe2+ → Fe0 and the formation of solid electrolyte interface (SEI). The charge plateau between 1.5~2.1 V belongs to the opposite conversion of Fe0 ↔ Fe3+/Fe2+.34 In the first cycle, the discharge capacity is 1409 mA h g−1, while charge capacity is 987 mA h g−1. The initial Coulombic efficiency (CE) is as high as 70%. Moreover, the almost overlapped GCD curves at the 2nd and 3th cycle indicate the high structural stability during Li+ intercalation/extraction. The GCD curves of the Cu@Fe3O4ϿTiO2 electrode at different current densities were depicted in Figure 4b. The Cu@Fe3O4ϿTiO2 electrode delivers average reversible capacities of 908, 1061, 982, 923, 808, 713 and 598 mA h g−1 at 0.1 to 0.2, 0.5, 1, 2, 5 and 10 A g−1, respectively. Particularly, when current density is increased to 15 A g−1, the capacity is still up to 489 mA h g−1 (Figure 4c). With back 9



to 0.1 A g−1, Cu@Fe3O4ϿTiO2 still releases a high average capacity of 1003 mA h g−1, indicating the good reversibility and stability. In contrast, the Cu@Fe3O4 electrode displays relatively low specific capacities of 998 and 415 mA h g−1 at 0.1 to 5 A g−1, respectively. At a high current density of 10 A g−1, the specific capacity of Cu@Fe3O4 electrode sharply decreases to only 54 mA h g−1. For further comparison, we also test the electrochemical performance of commercial Fe3O4 powder under the same mass loading on copper substrate. From Figure S4a, Supporting Information, it could be found that fast capacity decay is observed for commercial Fe3O4 electrode from 776 mA h g−1 at 0.1 A g−1 to only 88 mA h g−1 at 5 A g−1. Based on the results, the Cu@Fe3O4ϿTiO2 gives the best rate capability because of its unique electrode configuration. In addition, to investigate the effect of the Cu substrate, we measured the Li-storage performance of copper grid/Cu nanorods at 0.14 mA cm−2. As seen in Figure S4b of Supporting Information, the copper grid/Cu electrode displays an areal capacity of 0.001 mA h cm−2, much lower than that of Cu@Fe3O4ϿTiO2 (0.74 mA h cm−2). Therefore, the Cu particle in Cu@Fe3O4ϿTiO2 is just a conductive substrate. The cycling ability is very important for conversion-type Fe3O4-based anodes. Figure 4d shows the cycling stability of the Cu@Fe3O4ϿTiO2 and Cu@Fe3O4 at 0.2 Ag−1. After the initial several activation at 0.1 A g−1, the Cu@Fe3O4ϿTiO2 electrode provides an initial specific capacity of around 1060 mAh g−1 at 0.2 A g−1. At the 400th cycle, the maximum capacity of 1580 mA h g−1 has been obtained. This maximum capacity can be stable within the subsequent 100 cycles. The increase of specific capacity at a low current density can be attributed to the pulverizing of Fe3O4 nanoparticles, which will expose more active sites for lithium storage within hollow TiO2 shell. Moreover, the Fe metallic nanoparticles generated by conversion reaction will enhance the conductivity of the Fe3O4.

45

However, completely different from

Cu@Fe3O4ϿTiO2, the Cu@Fe3O4 electrode exhibits a fast capacity decay to 280 mA h g−1 at the 150th cycle, which may be due to serious structure pulverization or exfoliation from the Cu nanorod current collector. The corresponding GCD cueves of Cu@Fe3O4ϿTiO2 and Cu@Fe3O4 at different cycle numbers further demonstrate this value (Figure S5, Supporting Information). The cycling stability of the Cu@Fe3O4ϿTiO2 electrode was further tested at 2 A g−1. It is observed that a high capacity of 831 mA h g−1 is stable with no any decay within 1000 cycles at 2 A g−1 (Figure 4e). Therefore, our designed Cu@Fe3O4ϿTiO2 electrode displays a high Li10


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storage performance, which surpasses most of the Fe3O4-base Li+-storage anodes in previous reports (see Table S1 of Supporting Information).

Figure 5. Schematic illustration the different behaviors of (a) Cu@Fe3O4 and (b) Cu@Fe3O4ϿTiO2 electrodes during the Li+ insertion/extraction process. (c-e) SEM images of Cu@Fe3O4ϿTiO2 after 500 cycles.

As illustrated in Figure 5a, b, the two electrodes undergo different variations during the repeated charge-discharge process. For the Cu@Fe3O4 electrode, the repeated volume change of Fe3O4 will lead to structure pulverization and exfoliation from the 3D current collectors (Figure 5a). For the Cu@Fe3O4ϿTiO2 electrode, the TiO2 shell protection can make the Fe3O4 durably align on the Cu nanorods (Figure 5b). In order to investigate the structure change after cycling, the Cu@Fe3O4ϿTiO2 electrode is disassembled for SEM tests after 500 cycles at 0.2 A g−1. As presented in Figure 5c-e, the microstructure of original Cu@Fe3O4ϿTiO2 nanorods can be well maintained and Fe3O4 are still encapsulated into hollow TiO2 shell after long cycling. In contrast, without TiO2 protection, Fe3O4 will form large aggregates on Cu nanorods after cycling (Figure S6, Supporting Information). This comparison suggests that our ALD-enabled Cu@Fe3O4ϿTiO2 configuration has strong tolerance of Li+-insertion/extraction during long-term discharge/charge cycles. Therefore, a remarkable cycling and rate performances are achieved for Cu@Fe3O4ϿTiO2, which enable it as a promising anode 11



of LICs.

Figure 6. The hybrid LIC based on Cu@Fe3O4ϿTiO2 and AC. (a) The potential windows of the anode and the cathode in half-cells. (b) CV curves of the LIC, Cu@Fe3O4ϿTiO2 and AC. (c) GCD profiles of the LIC. (d) The rate performances of the LIC. The cycling ability at (e) 0.2 A g−1 and (f) 2 A g−1. (g) Ragone plots of the Cu@Fe3O4ϿTiO2//AC device.

A LIC full device is constructed using the Cu@Fe3O4ϿTiO2 anode and the AC cathode. Before building the device, the Cu@Fe3O4ϿTiO2 is first prelithiated to achieve high efficiency. Coupled with the AC electrode (2−4.5 V) (the details in Figure S7 of Supporting Information), an operation window of 0−4 V for the LIC device can be achieved (Figure 6a). Cyclic voltammetry (CV) curves (Figure 6b) of Li half-cells using the Cu@Fe3O4ϿTiO2 electrode and the AC electrode were performed at 2 mV s−1. The Cu@Fe3O4ϿTiO2 exhibits obvious peaks, which are ascribed to the redox reaction of Fe3O4, while the AC electrode displays a rectangular shape. Hence, both capacitive and Li+-inserted redox reactions simultaneously happen in the hybrid LIC. The CV curve of the Cu@Fe3O4ϿTiO2//AC LIC has no apparent peak but slightly deviates from the ideal rectangular shape, which is mainly caused by the insertion-type charge-storage mechanism of Fe3O4.46 In addition, the mass ratio of Cu@Fe3O4ϿTiO2 and AC is set as 1:5 to match their individual capacities. 12


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The electrochemical performances of Cu@Fe3O4ϿTiO2//AC full cell were evaluated in the range of 0–4 V. Based on show the GCD profiles (Figure 6c), the linear charge and discharge curves are nearly symmetrical, indicating good electrochemical reversibility. The LIC device delivers the 3th capacities of 77.4 and 33.1 mA h g–1 at 0.1, and 15 A g−1, respectively (Figure 6d). With 150-times increase of current density, there is still 43% capacity retention, demonstrating excellent rate capability. Figure 6e displays the cycling performance of the Cu@Fe3O4ϿTiO2//AC LIC at 0.2 A g−1, which has 86% capacity retention for 300 cycles. We further investigated the cycle ability at a high current density of 2 A g−1, in which the retention is as high as 84% after 1000 cycles (Figure 6f), indicating an excellent cycling ability. To further evaluate the performances of the LIC, the power (P) and energy densities 𝑡

(E) of the Cu@Fe3O4ϿTiO2//AC device are obtained by the equation: E = ∫𝑡2𝐼𝑉𝑑𝑡 and 1

P = E/t, where I is the specific discharge current (A g−1), t is the discharge time (s), V (V) is the discharge voltage. The results indicate that the energy density of the Cu@Fe3O4ϿTiO2//AC device is up to 154.8 W h kg−1, corresponding to a power density of 200 W kg−1. The Cu@Fe3O4ϿTiO2//AC can still provide 66.2 W h kg−1 at a high power density of 30,000 W kg−1. The obtained energy/power values of Cu@Fe3O4ϿTiO2//AC device is better than that of previously reported lithium ion capacitors, which is shown in the Ragone plot (Figure 6g), such as PF16//FRGO (148.3 Wh kg−1 at 141 W kg−1),21 VN-RGO//APDC (162 Wh kg−1 at 200 W kg−1),

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BiVO4//PRGO (152 Wh kg−1 at 384 W kg−1),47 Fe3O4-G//AC (120 Wh kg−1 at 130 W kg−1),25 TiO2-rGO//AC (42 Wh kg−1 at 800 W kg−1),48 TiNb2O7@C//CFs (110.4 Wh kg−1 at 99.58 W kg−1).18 The comparison of our Cu@Fe3O4ϿTiO2//AC device with other LICs is exhibited in Table S2 (Supporting Information). The achieved high Li+storage performances for our Cu@Fe3O4ϿTiO2//AC LIC is reasonably attributed to the advanced structural design by encapsulating high-capacity Fe3O4 between conductive Cu nanorods and hollow TiO2 shell. This electrode configuration endows the full LIC with a significantly high energy-storage performance as well as a superior cycling ability. 4. CONCLUSION In summary, we design a unique Cu@Fe3O4ϿTiO2 array on the Cu grid surface. The ultrahigh capacity of 1585 mA h g−1 at 0.2 A g−1 has been achieved for the



Cu@Fe3O4ϿTiO2. More importantly, the Cu@Fe3O4ϿTiO2 electrode exhibits superior rate capability (489 mA h g−1 at 15 A g−1) and long cycle life (1000 cycles at 2 A g−1 with no capacity loss). The remarkable Li-storage performance of the Cu@Fe3O4ϿTiO2 electrode is reasonably attributed to the ALD-enabled structure that provides void space for accommodating volume expansion of Fe3O4 and 3D Cu current collector that enhances the conductivity of the electrode, therefore obtaining superior cycle stability and reaction kinetics. The as-fabricated Cu@Fe3O4ϿTiO2//AC LIC device exhibit a high energy-storage performance (154.8 Wh kg−1//200 W kg−1, 66.2 Wh kg−1//30,000 W kg−1), and a long cycle life with 84% capacity retention after 1000 cycles. Our work will open up an important route for the ALD-preparation of superior electrode materials for hybrid Li-ion capacitors. ASSOCIATED CONTENT Supporting Information SEM images and (d) Photographs of Cu screen; SEM images and (d) Photographs of Cu@Cu(OH)2; XPS spectrum Cu@Fe3O4ϿTiO2; Rate performances of Cu@commercial Fe3O4 powder electrodes; The corresponding GCD profiles of (a) Cu@Fe3O4ϿTiO2 and (b) Cu@Fe3O4 and at 0.2 A g-1; The electrochemical performances of commercial AC cathode within the potential widow of 2.0-4.5 V (vs. Li/Li+); table of comparison of electrochemical performance. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Notes There are no conflicts to declare. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (nos. 51702295). REFERENCES (1) Wang, H.; Zhu, C.; Chao, D.; Yan, Q.; Fan, H. J., Nonaqueous Hybrid Lithium-Ion and SodiumIon Capacitors. Adv. Mater. 2017, 29, 1702093-1702110. (2) Guan, C.; Wang, H.; Zhang, Q.; Fan, Z.; Zhang, H.; Fan, H., Highly Stable and Reversible Lithium Storage in SnO2 Nanowires Surface Coated with A Uniform Hollow Shell by Atomic Layer Deposition. Nano Lett. 2014, 14, 4852-4858. (3) Li, Y.; Wang, H.; Wang, L.; Mao, Z.; Wang, R.; He, B.; Gong, Y.; Hu, X., Mesopore-Induced 14


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TOC

Two-step atomic layer deposition (ALD) is utilized to encapsulate Fe3O4 between copper nanorod and thin TiO2 shell for high-performance lithium-ion capacitors.

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Encapsulation of Fe3O4 between Copper Nanorod and Thin TiO2 Film

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