Super-hierarchical Nickel-Vanadia Nanocomposites for Lithium Storage

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Super-hierarchical Nickel-Vanadia Nanocomposites for Lithium Storage Yuan Yue, Daniel Juarez-Robles, Partha Mukherjee, and Hong Liang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00163 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Applied Energy Materials

Super-hierarchical Nickel-Vanadia Nanocomposites for Lithium Storage Yuan Yue 1, Daniel Juarez-Robles 2, 3, Partha Mukherjee 2, 3, and Hong Liang 1, 2, * 1

Department of Materials Science and Engineering, Texas A&M University, 3003 TAMU, College Station, Texas 77843-3003, USA 2

Department of Mechanical and Engineering, Texas A&M University, 3123 TAMU, College Station, Texas 77843-3123, USA 3

School of Mechanical Engineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47907, USA

* Email: [email protected]

Abstract New materials are critically needed for advanced energy storage devices due to the limited performance of currently-used electrode materials. We report an alternative approach to fabricate a novel type of nanostructured cathodes with a three-dimensional configuration that shows superior performance. A super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite is designed and synthesized using a simple electrodeposition process followed by a hydrothermal treatment. Hierarchical V2O5 nanostructures are deposited directly on a Ni micro-channeled current collector. Morphological characterization shows that two-dimensional V2O5 nanosheets are uniformly distributed on the porous Ni substrate. A peony-like V2O5 microstructure arises having a diameter of ~4 µm. The super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite exhibits superior electrochemical performance as a binder-free cathode. Its maximum reversible discharge capacity reaches 165.6 mAh g−1 at 0.2 C, which is higher than the theoretical capacity of bulk V2O5 cathodes. The capacity retains 90.9% and 72.4% after 100 cycles at 0.2 C and 500 cycles at 3.0 C, respectively. The stable rate capability is also confirmed. Our analysis indicates that such high-performance is attributed to the synergistic effects of: the hierarchical structure, micro-channeled Ni current collectors, two-dimensional V2O5 nanostructured active materials, and the binder-free processing. This research shows significant promise for use of super-hierarchical structures in future of rechargeable batteries. Keywords Hierarchical structures, binder-free electrodes, vanadium pentoxide, lithium-ion batteries, electrochemical energy storage

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1. Introduction Energy consumption has become a critical issue with the increase in world population and industrialization. Reliable, effective, portable, and durable means to store energy is a significant part of solutions to energy needs.1-3 Rechargeable electrochemical energy storage devices present conventional approaches to the effective storage of energy. Among these devices, lithium-ion batteries (LIBs) have attracted great attention. LIBs have occupied most of the commercial market of portable energy storage devices since their introduction over 25 years ago.4 Conventional lithium transition metallic oxide electrodes for LIBs include lithium cobalt oxide (LiCoO2),5 lithium iron phosphate (LiFePO4),6 lithium manganese oxide (LiMnO2),7 and lithium nickel cobalt manganese oxide (LiNi0.33Co0.33Mn0.33O2).8 Nevertheless, most of these cathode materials suffer from low specific capacity and a relatively short lifespan.9 Recently, transition metallic oxides have started attracting attention.10 Materials such as CuO,11 NiO,12 ZnO,13 Fe2O3,14 Co3O4,15 MnO2,16 TiO2,17-19 SnO2,20 and V2O5 21-23 with micro- or nano-structures are being explored as possible electrodes for LIBs. Research on V2O5 as LIB cathodes started nearly three decades ago.24-32 The advantages of V2O5 cathodes have four aspects Firstly, V2O5 has a stable two-dimensional lamellar crystalline structure with interlayered spaces to store lithium ions.25 Secondly, V2O5 is low-cost with abundant sources than lithium metal oxide or phosphate in comparison with commercial LiCoO2 that contains expensive and scarce Co.28 Thirdly, V2O5 is easy to fabricate with various morphologies.28 Fourthly, the multiple oxidation states of vanadium (2+, 3+, 4+, and 5+) result in tunable insertion modes of lithium ions.23, 26 The multiple lithium-ion-insertion modes of V2O5 cathodes are justified by different ranges of potential windows. 33-35 In ideal cases, three, two, one lithium ion(s) participate the electrochemical reaction successively to form the phases of ω-, γ-, and δ-LixV2O5.23 This has been discussed in our recent report about V2O5 electrodes.23 In one of our earlier works,23 we reported that the novel nanostructure of V2O5 cathodes offers an interesting architecture for enhanced electrochemical performance. To date, two-dimensional (2D) nanostructures have attracted increasing research interest due to their large surface area, novel electronic structure, and high area-to-volume ratio.36-40 Those features are beneficial for the effective electrochemical reactions in a battery cell. The facile fabrication of 2D V2O5 nanosheets or nanoplatelets provides a feasible approach owing to the layered crystalline feature of V2O5 unit cells.41 Besides the design of active materials with 2D nanostructures,39 another strategy for the design of advanced battery electrodes is a 3D hierarchical architecture for the whole electrode. Despite the large number of reports, most fabricated 3D nanoarchitectures are for active materials.42-44 As-fabricated 3D active nanomaterials need to be mixed with a polymeric binder and conductive additives, and then cast on a flat current collector. Such a conventional slurry-casting-cell-assembly approach results in the severe agglomeration of active nanomaterials interrupting ion transport pathways. This results in an impediment to fully exploit advantages of active materials from their 3D nanostructures.45 Moreover, the addition of polymeric binders can also reduce the specific capacity and increase the risk of damage to electrodes at elevated temperature.46


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Potential solutions to this problem include binder-free cell assemblies and/or 3D hierarchical current collectors, as suggested in our recent studies.4, 47 In the current research, we designed and synthesized novel 3D super-hierarchical cathodes through a binder-free process for LIBs. The binder-free process is fast and costeffective. Micro-electrodes that we have developed include two parts.48, 49 Firstly, a porous Ni current collector with vertically-aligned micro-channels was grown through electrodeposition. Then, the active material, ultrathin V2O5 nanoparticles, was directly deposited onto the micro-channeled Ni current collector using a hydrothermal method. The morphology of the V2O5 nanostructures includes 2D ultrathin nanosheets and 3D multilayered micro-peonies. This is the result of 2D crystal growth under supercritical hydrothermal conditions. For battery tests, the super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite was directly encapsulated into a lithium metal half-cell. Neither binders nor additives were used. The electrochemical performance of the super-hierarchical Ni/Porous-Ni/V2O5 cathode was found to be promising. The maximum reversible discharge capacity was 165.6 mAh g−1 and the capacity retention was 90.9% after 100 cycles at 0.2 C. After testing at 3.0 C for 500 cycles, the retained capacity was 96.5 mAh g−1. This was 72.4% of the maximum reversible value of 133.2 mAh g−1. Furthermore, the rate capability of the super-hierarchical Ni/Porous-Ni/V2O5 cathode was outstanding. The reason was because of the small value of charge transfer resistance after cycling. This was due to the formation of thinner and more uniform solid electrolyte interphase (SEI) layers. The favorable electrochemical performance could be ascribed to a triple synergistic effect among porous micro-channeled Ni current collectors, ultrathin V2O5 nano-active-material, and binder-free fabrication. This synergetic effect promotes several advantages over reported LIBs, including high specific surface area, increased deposition of active material, shorter ion diffusion path, reliable accommodation of the volume change during cycling, and well-organized electron transportation. All of these features facilitate the excellent and long-lasting performance of super-hierarchical Ni/Porous-Ni/V2O5 cathodes. 2. Experimental Methods 2.1.Fabrication of Ni/Porous-Ni/V2O5 nanocomposites For the fabrication of Ni/Porous-Ni/V2O5 nanocomposites, the general route is similar to our previous work.49 The substrate of Ni/Porous-Ni was prepared first. An electrodeposition process was used to grow a micro-channeled porous layer of Ni on Ni sheet (commercial 200 Ni sheet, 99.0% purity, McMaster-Carr). A uniformly-stirred aqueous mixture of 0.2 M NiCl2 (powder, 98%, Sigma-Aldrich) and 4.5 M NH4Cl (crystallized powder, ≥99.5%, Sigma-Aldrich) was the electrolyte. The as-cleaned Ni sheet and graphite rod (99.995% carbon, 6 mm in diameter, Sigma-Aldrich) were immersed into the electrolyte as working electrode and counter electrode, respectively. The electrodeposition was galvanostatic at 0.4 A cm−2. The duration of electrodeposition was 10 minutes. After electrodeposition, the Ni/Porous-Ni sample was rinsed in ethanol, cut into square pieces of 1 × 1 cm2, and vacuum dried overnight at 60 °C.



The V2O5 nanostructure was directly synthesized and deposited on the surface of Ni/Porous-Ni substrate using hydrothermal treatment. The hydrothermal precursor solution contained 0.33 mmol (60.6 mg) V2O5 (powder, 99.99%, trace metals basis, Sigma-Aldrich), 22.75 mL DI water (Millipore), and 2.25 mL H2O2 solution (30 % H2O2 w/w in H2O, contains stabilizer, Sigma-Aldrich), and was mixed uniformly through magnetic stirring for 1 hour at room temperature. A clear orange-color precursor solution was obtained with some suspended bubbles. Next, the precursor solution and the as-cut Ni/Porous-Ni substrate were placed into a Teflon hydrothermal autoclave with firm sealing operating at 180 °C for 7 hours. After the hydrothermal process, the dark-green square samples were rinsed in DI water, and dried at 70 °C overnight. Finally, annealing in Ar was accomplished at 350 °C for 30 minutes to increase the crystallinity of the V2O5 nanostructure. The as-fabricated Ni/Porous-Ni/V2O5 nanocomposites appeared darkorange or black. For the comparison of the electrochemical performance, a reference set of samples of Ni/V2O5 nanocomposite were also fabricated. These samples used the ascleaned Ni sheet as the substrate to hydrothermally deposit V2O5 nanostructures. All other experimental parameters were the identical to those of the Ni/Porous-Ni/V2O5 nanocomposite. The deposited mass of the as-annealed V2O5 nanostructure was measured to be 1.8-2.1 mg cm−2. The mass density of the Ni/Porous-Ni/V2O5 nanocomposite was measured as 5.33 ± 0.04 g cm−3. 2.2. Characterizations X-ray diffraction (XRD) measurements were taken with a Bruker D8 Focus BraggBrentano short-arm powder diffractometer with a Cu Kα (λ = 1.540598 Å) X-ray source. A Schottky field-emission scanning electron microscope (SEM) (FERA-3 Focused Ion Beam Microscope, Tescan, Inc.) was used for microstructure imaging. The acceleration voltage was fixed at 10 kV. The porous volume and Brunauer–Emmet–Teller (BET) surface area of samples were measured through the nitrogen (N2) adsorption–desorption process using ASAP 2010 (Micromeritics Instrument Corporation) at 77 K. All samples were degassed in a high vacuum at 100 °C overnight before adsorption to remove adsorbed gases and other impurities. 2.3. Electrochemical measurements All electrochemical measurements in this research were performed using CR-2032 coin-type half-cells. The Ni/Porous-Ni/V2O5 nanocomposite and Ni/V2O5 nanocomposite were named as set 1 and 2, respectively. Both sets of samples were directly used as the working electrode, i.e. cathode, of the half-cell without binders and conductive additives. The counter electrode (anode) was lithium metal (99.9% Li, trace metals basis, ribbon, Sigma-Aldrich). It was worth to mention that the usage of a half-cell with lithium metal anode was to serve the purpose of characterization. Electrochemical performance of the Ni/Porous-Ni/V2O5 cathode measured by half-cells was meant to be used as cathodes in LIBs. In practical application of LIBs, the currently used lithium metal anode was replaced by non-lithium-metal anodes. The coin-cell assembly experiments were operated in an Ar-filled glove box with moisture and oxygen concentrations below 0.4 ppm. The


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solvent was 1 M LiPF6 in a 1:1 (v/v) mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (BASF SE) as the battery electrolyte. The as-assembled V2O5/Li half-cell had an open-circuit voltage (OCV) at ~3.05 V. Tests of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) had a scan rate of 0.1 mV/s over the range of 2.0–3.2 V (vs. Li+/Li). This was to ensure the insertion of one lithium ion without possible structural damage of V2O5 ultrathin nanosheets under high potential. The alternating current (AC) perturbation signal for EIS testing was ± 10 mV over the frequency range of 10 mHz – 1 MHz. Charge-discharge cyclic tests were accomplished at the same potential window at variable C-rates (1 C = 147 mA g−1). Each chargedischarge test was repeated for three times. To ensure the initial lithiation of V2O5 active materials, the half-cell was firstly discharged from the OCV to 2.0 V before the beginning of the cycling test.

3. Results and Discussion 3.1. Physical characterizations The as-fabricated Ni/Porous-Ni/V2O5 nanocomposite possesses well-defined crystallographic structures. Figure 1 shows the corresponding XRD pattern after annealing. Through the comparison of high peaks between the measured pattern and the standard peaks in Figure 1, the crystallographic character of the Ni/Porous-Ni/V2O5 nanocomposites is determined. There are three sharp peaks at 44.5°, 51.7°, and 76.5° in the measured pattern. The locations of these peaks correspond to the three highest peaks of FCC Ni (a = 3.5238 Å, JCPDS No. 87-0712) from the (111), (200), and (220) planes. This suggests that the electrodeposited porous Ni substrate has an FCC crystal structure. Secondly, for the hydrothermally deposited V2O5 nanostructures, five apparent peaks at 20.4°, 21.7°, 26.2°, 31.0°, and 41.3° match peaks of the (001), (101), (110), (400), and (002) planes of the orthorhombic V2O5 crystal (a = 11.510 Å, b = 3.563 Å, c = 4.369 Å, JCPDS No. 41-1426). This suggests that the V2O5 nanostructure of these materials contain a layered orthorhombic phase with distorted [VO5] pyramids.23 The morphology of as-fabricated Ni/Porous-Ni/V2O5 nanocomposites is superhierarchical at the micro- and nano-scale. Figure 2 shows SEM images of Ni/PorousNi/V2O5 and Ni/V2O5 samples. From Figure 2a, a compact, uniform, and multilayered distribution of nanosized 2D nanosheets of V2O5 deposit is observed on Ni/Porous-Ni substrate. After comparing Figure 2a with the SEM images of the bare Ni/Porous-Ni substrate (see Figure S1, Supporting Information), it is clear that ultrathin V2O5 nanosheets completely cover both outer surfaces and inner walls of the micro-channels of the Ni/Porous-Ni substrate. Meanwhile, some 2D V2O5 nanosheets are assembled into 3D multilayered structures that resemble peony blossoms. The high-magnification SEM image of Figure 2c highlights the detailed shape of one “micro-peony”. Several red dashed circles in Figure 2a mark this type of “micro-peony”. In this paper, the term “super-hierarchical” is used to describe the morphology of multilayered hierarchical nanostructures grown on porous substrates. Similar super-hierarchical nanoarchitectures



can be found in as-hydrothermal Ni/Porous-Ni/V2O5 nanocomposites before annealing (Figure 2b). Furthermore, the 2D V2O5 nanosheets and 3D micro-peonies also form if the substrate is a plain Ni sheet. Figure 2d is an SEM image of a Ni/V2O5 hierarchical nanocomposite. Some 3D micro-peonies are marked by orange dashed circles in Figure 2b and d. The ultrahigh surface area and porosity of super-hierarchical Ni/PorousNi/V2O5 nanocomposites can be observed in Figure 2a-c. Furthermore, the morphology of Ni/Porous-Ni/V2O5 nanocomposites is the result of optimization of the mass loading of V2O5 deposited on Ni porous substrate. The loading mass of 1.8-2.1 mg cm−2 of V2O5 deposit is a balance of deposition of V2O5, elimination of exaggerated agglomeration of V2O5, and the prevention of blocking in porous Ni channels. The BET surface areas of Ni/Porous-Ni/V2O5 and Ni/V2O5 samples are determined to be 15.4 m2 g−1 and 10.1 m2 g−1, respectively. The large volumetric porosity of these are also determined to be 55.3% and 54.5%, respectively. These results suggest that the super-hierarchical Ni/Porous-Ni/V2O5 nanocomposites have favorable specific surface area and porosity. The morphological change introduced by annealing is mainly shrinkage in the 2D dimension of the V2O5 nanosheets and micro-peonies. From statistical results of size measurements (detailed histogram plots are shown in Figure S2, Supporting Information), the lateral sizes of the 2D V2O5 nanosheets and diameters of the 3D micro-peonies decrease significantly during annealing. The mean value of lateral size and diameter reduce from ~6.0 µm to ~2.5 µm and from ~12.0 µm to ~5.0 µm, respectively, whereas, the mean value of thickness of the 2D V2O5 nanosheets remains almost constant during annealing (from 52.8 nm to 48.6 nm). This suggests that annealing results in shortening the 2D structure of the as-fabricated V2O5 nanosheets. This eventually leads to the crystallization of the poorly-crystalline structure of unannealed V2O5 (XRD pattern is shown in Figure S3, Supporting Information). 3.2. Electrochemical performance The hydrothermally-deposited 2D V2O5 nanosheets and 3D micro-peonies have stable electrochemical reaction kinetics with lithium ions. Figure 3a and b demonstrate the cyclic voltammetry curves of the first three cycles for set 1 (super-hierarchical Ni/Porous-Ni/V2O5) and set 2 (hierarchical Ni/V2O5) as cathodes for lithium-ion batteries, respectively. Both sets of curves have one pair of peaks. According to the mechanism of the intercalation reaction of lithium ions into pristine α-V2O5 expressed as Equation (1),23, 31 one lithium ion intercalated into one α-V2O5 structure during discharging process and δ-LiV2O5 (x=1) formed subsequently. Meanwhile, the CV curves in both panels of Figure 3 have highly-overlapped positions of reduction (discharging) and oxidation (charging) peaks without severe shifts. This suggests that the formation of an SEI film during the first cycle is thin and stable and does not significantly influence the intercalation reaction. For set 1 and 2, the reduction/oxidation peaks are located at ~2.59/~2.71 V and ~2.60/~2.73 V vs. Li/Li+, respectively. This result is consistent with several published results on V2O5 electrodes.50-54 The similar peak locations between set 1 and 2 reveal


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constant intercalation kinetics of V2O5 nanostructures regardless of the type of current collector.

α - V2O5 + xLi+ + xe− ↔ Lix V2O5

(1)

The super-hierarchical Ni/Porous-Ni/V2O5 cathode reveals excellent capacity performance and stable rate capability. The results of capacity performance of set 1 and 2 are placed in four panels in Figure 4. The charge-discharge cycling profile curves of set 1 and 2 at low C-rate of 0.2 C (29.4 mA g−1) are shown in Figure 4a and b. A series of constant potential plateaus are located at ~2.6 V and ~2.7 V for the discharging and charging processes, respectively. Those plateaus are consistent with the CV peaks in Figure 3a and b, which imply stable intercalation reactions extended to 100 cycles for both sets of cathodes. The comparison of discharge specific capacities and Coulombic efficiencies between set 1 and 2 during 100 cycles at 0.2 C is depicted as Figure 4c. For the discharge specific capacity, set 1 has an initial value of 169.2 mAh g−1. After 100 cycles, the discharge specific capacity still has a retention of 150.6 mAh g−1. This is 90.9% of the maximum reversible capacity (165.6 mAh g−1 at the second cycle), which is equivalent to only 0.091% capacity fading per cycle. Differently, the initial discharge specific capacity, eventual discharge specific capacity, capacity retention, and capacity fading per cycle of set 2 are 124.5, 102.9, 85.6%, and 0.145%, respectively. Therefore, for the case of low C-rate, set 1 possesses the larger capacity, higher retention, and weaker capacity fading. Moreover, it is worth noticing that the eventual capacity of set 1 as 150.6 mAh g−1 is even higher than the theoretical specific capacity of bulk δ-LiV2O5 powdered electrodes (147.0 mAh g−1).28 This highlights the promising electrochemical performance of super-hierarchical Ni/Porous-Ni/V2O5 cathodes. For the performance of the Coulombic efficiency, two trends in Figure 4c illustrate that both set 1 and 2 have a stable reversibility of capacity of nearly 100% at low C-rate. The set 1 sample also has a more favorable capacity performance than set 2 during high-speed cycling. In this research, 500 cycles at 3.0 C (441 mA g−1) was set as the high-speed test protocol. Figure 4d exhibits the performance of capacities and Coulombic efficiencies of sets 1 and 2. For this protocol, the initial discharge specific capacity of set 1 is 137.2 mAh g−1, while the value of set 2 is 92.7 mAh g−1. The 500th discharge capacities of sets 1 and 2 are 96.5 and 52.0 mAh g−1, respectively. These values are 72.4% and 58.4% of the corresponding maximum reversible capacity at the second cycle. Hence, the capacity fading per cycle of sets 1 and 2 are 0.055% and 0.084%, respectively. Moreover, the Coulombic efficiencies of both set 1 and set 2 fluctuate near to the value of 100% during the full 500 cycles. Set 1 shows virtually similar Coulombic efficiencies with set 2. The rate capability is another significant part of the electrochemical characterization for electrodes. The rate capability of set 1 was tested due to it having better capacity performance than set 2. The protocol was set to have the initial 3 cycles at 0.1 C and then 10 cycles at 0.2 C, 0.5 C, 1.0 C, 2.0 C, 3.0 C, and 0.1 C, respectively. Figure 5a illustrates the charge-discharge profile curves at each C-rate. The constant levels of



charge/discharge plateaus at ~2.6 V and ~2.7 V imply stable intercalation kinetics of V2O5 nanostructures at changing C-rates. The discharge specific capacities and corresponding Coulombic efficiencies are plotted in Figure 5b. The discharge specific capacities at the 3rd (at 0.1 C), 13th (at 0.2 C), 23rd (at 0.5 C), 33rd (at 1.0 C), 43rd (at 2.0 C), and 53rd (at 3.0 C) cycles are 172.2, 165.9, 145.4, 123.7, 101.5, and 88.8 mAh g−1. The eventual value of 173.5 mAh g−1 is still 95.3% of the maximum reversible capacity at the second cycle. Simultaneously, the Coulombic efficiencies maintained at approximately 100% throughout the test. These results convincingly reveal the outstanding tolerance to the large change of C-rates exhibited by the super-hierarchical Ni/Porous-Ni/V2O5 cathode. In brief, the super-hierarchical Ni/Porous-Ni/V2O5 cathode has greater capacity and retention than the hierarchical Ni/V2O5 cathode at both low and high C-rates. Moreover, the rate capability of super-hierarchical Ni/Porous-Ni/V2O5 cathode is very reliable. The microscopic morphologies of sets 1 and 2 have a stable maintenance after longterm charge-discharge cycling. Figure 6 shows the comparison of the SEM images of set 1 (super-hierarchical Ni/Porous-Ni/V2O5 cathode) before and after 100 electrochemical cycles at 0.2 C. Obviously, the overall structure of the Ni micro-channels as well as the firm deposition of V2O5 nanosheets are stably maintained. Meanwhile, the lateral size of V2O5 nanosheets remains constant at 2.5-3.0 µm throughout the cycling. The thickness of V2O5 nanosheets expands 2-3 times after long-term cycles. For set 2 (hierarchical Ni/V2O5 cathode), similar morphological features can also be found in Figure S4 (Supporting Information). In brief, the structural stability of both sets of cathodes is maintained during repetitive electrochemical reactions.

3.3. Analysis of the electrochemical performance The favorable electrochemical performance of the super-hierarchical Ni/PorousNi/V2O5 cathode is validated through the EIS test results. The Nyquist plot is an effective tool to characterize the ion transport/diffusion during lithium ion intercalation.55 A typical Nyquist plot depicts the real part versus imaginative part of the electrochemical impedance for different frequencies. Figure 5c and d show comparisons of Nyquist plots between sets 1 and 2 in two charge-discharge modes. All Nyquist curves in Figure 5 include positive gradient straight lines at low frequencies and semicircular arcs at high frequencies. These are characteristics of lithium ion diffusion passing through the surface of the electrode and the resistance introduced by charge transfer.56, 57 According to these characteristics, an equivalent electrochemical circuit shown in the inserted panel of Figure 5c is constructed to interpret the Nyquist curves.58 The fitted values of resistances of these four EIS tests are listed in Table 1. Through the comparisons, set 1 has a much smaller value of equivalent charge transfer resistance (Rct) than set 2 after both low and fast C-rate cycling. The minimum value of Rct of set 1 is 6.38 Ω after 100 cycles at 0.2 C. This value is only 71.8% of set 2 after the same cycling process and 32.7% of set 1 after 500 cycles at 3.0 C. The promising small values of Rct for both sets of samples


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(particularly set 1) indicate the effective transport of electrons through the interface between the surface of V2O5 nanosheets and liquid electrolyte. Because the characteristics of thick SEI layers are high lithium ions conductivity and low electric conductivity,59, 60 the small values of Rct suggests that the super-hierarchical nanoarchitecture owned by set 1 can effectively suppress the formation of thick SEI layers and pulverizations of active material particles. In addition, the stability of 2D layout, surface morphology, and structural integrity observed from the SEM images after 500 cycles for both samples (Figures 6 and S4) are the evidences of the suppression of thick growth of SEI layers. This is a critical advantage of the super-hierarchical Ni/Porous-Ni/V2O5 cathode. Furthermore, a comprehensive comparison of the electrochemical performance among recent results on V2O5 nano-cathodes with one lithium ion insertion, is tabulated in Table 2. The super-hierarchical Ni/Porous-Ni/V2O5 cathode in this research obviously has competitive electrochemical performance. The high maximum capacity (exceeding the theoretical capacity), tiny capacity fading, and small charge transfer resistance are the main advantages. There is a triple synergistic effect for porous micro-channeled Ni current collectors, ultrathin V2O5 nano-active-materials, and binder-free fabrication. This effect is the origin of excellent electrochemical performance of the super-hierarchical Ni/Porous-Ni/V2O5 cathode during cycling. A set of schematic sketches in Figure 7 is used to visualize the synergistic effect. From Figure 7a and b, the contributions made by the speciallydesigned current collector of porous and micro-channeled Ni are described. In previous section, it was clear that both sets of samples shared the identical morphology of active V2O5 and charge-discharge potential window. The difference between both samples was the participation of porous Ni micro-channeled current collectors in set 1. Therefore, the striking improvement in the capacity performance from set 2 to set 1, including the capacity of set 1 exceeding the theoretical values of bulk V2O5 cathodes, is attributed to the positive impact by Ni micro-channeled structure. The enhancement from the microstructure of porous Ni micro-channels has four parts: 1) The improved specific surface area of the highly porous morphology of Ni deposits accommodates more deposition of the active nano-material. This increases the specific capacity in the current collector per unit volume. 2) The vertically-aligned micro-channels benefit full wetting surface by the liquid electrolyte.61 This ensures that all of the deposited active material participates in electrochemical reactions. 3) Numerous micro-channels supply abundant space for active material. This eliminates the negative effect of the volume expansion/extraction of active material during cycling. 4) The vertically-aligned distribution of micro-channels promotes electron transport within the current collector. This will increase the transport efficiency during cycling. Purple dashed arrows in Figure 7a mark the transport pathways. These four advantages lead to enhancement of electrochemical performance. In this research, the maximum reversible capacity improves 38% when using porous micro-channeled Ni current collectors. Similarly, a number of published results62-67 have demonstrated that experimental capacities exceeding the theoretical values of corresponding bulk active materials.



To better explain the contribution from V2O5 2D nanosheets and assembled 3D micro-peonies, two sketches are presented in Figure 7c and d. The major morphological features of 2D nanosheets and 3D micro-peonies are ultrathin (~50 nm) layers, high crystallinity, large (micro-sized) 2D facets, optimal density, and multilayered stacking. These favorable features result in four aspects of enhancement: 1) The ultrahigh specific surface area and area-to-volume ratio increase the number of available sites for electrochemical reaction. This directly increases the capacity of active material. 2) The ultrathin morphology shortens the lithium ion diffusion pathway. This improves the reaction kinetics by significantly increasing the diffused volume ratio of V2O5 nanosheets. 3) The hydrothermal 2D growth promotes the formation of excellent crystallinity.68, 69 This is critical for the reversible phase transformation before and after lithium ion intercalation. The SEM results in Figures 6 and S4 confirm the structural integrity of the V2O5 nanosheets during cycles. 4) The number density of deposited 2D nanosheets is adjusted by the hydrothermal conditions and chemical concentrations. This manipulates the space between adjacent nanosheets to the appropriated, which prevents the buffering space for the cyclic volume changes of nanosheets. In summary, the super-hierarchical structure made of V2O5 2D nanosheets and 3D micro-peonies play a significant role to improve their capacities. Their ultrahigh specific surface area and the ultrathin 2D morphology promote electrochemical reaction sites of V2O5 and lithium ions. As a result, the values of discharge capacities being greater than the theoretical one is clearly observed. There are several published results of V2O5 and other nanomaterials as LIB electrodes exhibiting superior capacity performance.70-72 Our results are consistent to published data. The binder-free process was carried out in order to join porous Ni current collectors and multilayered V2O5 active materials. This approach is significant for the high performance of super-hierarchical Ni/Porous-Ni/V2O5 cathodes: 1) The binder-free design distributes the V2O5 nanosheets more uniformly compared to conventional binder approaches. This avoids the severe aggregation of dried nanoparticles during the process of slurry-casting. 2) The direct deposition of active material facilitates fast and continuous ion transport from the active material to the current collector without the assistance of carbon additives. 3) The firm contact at the Ni–V2O5 interface provided by the binder-free deposition avoids the detachment of the V2O5 active material after cycling. Figures 6 and S4 reveal this firm contact between the Ni current collector and V2O5 active material. Ultimately, the outstanding electrochemical performance of superhierarchical Ni/Porous-Ni/V2O5 cathodes is the result of the manifestation of eleven advantages. These features result in super performance and stable capacity output of super-hierarchical Ni/Porous-Ni/V2O5, including the capacity exceeding the theoretical value at low C-rate. Overall, a new aspect of this research is to introduce superhierarchical morphological characteristics to electrodes resulting significantly improved capacity. Finally, it is worth mentioning that the super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite possess a maximum specific energy as ~530 mWh g−1. When the active


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mass of V2O5 is exclusively considered, the volumetric energy density is ~1780 Wh L−1. As a result the volumetric energy density will be ~24.7 Wh L−1 including materials involved, Ni/Porous-Ni/V2O5 nanocomposite. This is due to the high volumetric occupancy of the Ni porous current collector. This indicates that the 3D hierarchical current collector to deposit V2O5 active materials need to be optimized that need to be done in future. The super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite in this study was focused on one-lithium-ion-insertion mode. Due to the multiple insertion modes of V2O5 cathodes, the adjustment of cut-off potentials (both upper and lower ones) of the charge-discharge window will have impact on the electrochemical activity of V2O5 cathodes. It has been reported that the negative influence on capacity retention is caused by broadening the potential window with smaller lower cut-off (i.e. 1.5 V).33, 73-75 The irreversible formation of the stable lithium-inserted ω-LixV2O5 phase to restrict the repeated transport of lithium ions is responsible for this behavior. In the current study of super-hierarchical Ni/Porous-Ni/V2O5 nanocomposites, the expansion of the potential window is expected to affect the insertion mode and capacity performance. It is anticipated that the capacity retention will be lower if the cut-off potential is extended to 1.5 V because of the formation of ω-LixV2O5 phase. Alternatively, the improvement of the capacity performance because of hierarchical structure has been confirmed in this research. Herewith we report the new findings and focus on the hierarchical structural design rather than the effect of various potential windows in this research. The changing windows in cut-off potentials will be reported in future.

4. Summary In this research, three-dimensional cathodes were successfully designed and fabricated using cost-effective binder-free processing steps. A novel super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite was developed as a cathode for lithium-ion batteries. This nanocomposite consists of V2O5 2D nanosheets and 3D micro-peonies on a porous Ni substrate. The mean value of the thickness of the V2O5 2D nanosheets is approximately 50 nm while their lateral sizes are ~1.5 µm. The annealed V2O5 nanosheets and micro-peonies have well-defined orthorhombic crystalline structure. The super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite possesses excellent electrochemical performance as a binder-free cathode for lithium-ion batteries. Owing to the stable single pair of reduction/oxidation peaks at ~2.6 V/~2.7 V, the superhierarchical Ni/Porous-Ni/V2O5 nanocomposite has a large capacity density, reliable retention, long lifespan, and excellent tolerance to multiple C-rates. Its maximum reversible discharge capacity is 165.6 mAh g−1 at the low C-rate of 0.2 C. This value exceeds the theoretical value of V2O5 with one lithium ion insertion (147.0 mAh g−1) for 12.7%. The capacity retention is as high as 90.9% after 100 cycles. For a high C-rate of 3.0 C, the super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite exhibits the high discharge capacity at 137.2 mAh g−1 initially, which is 93.3% of the theoretical value. There remains 72.4% of capacity after 500 cycles. Meanwhile, the structural integrity of



super-hierarchical Ni/Porous-Ni/V2O5 cathode is maintained after long-term cycles. These advantages are due to the synergistic effects of: hierarchical structure, active V2O5 nanosheets, and binder-free cell-assembly. Excellent electrochemical activity results from a high specific surface area, increased deposition of active material, reduced ion diffusion pathway, efficient buffering of the volume change during cycling, and well-organized electron transport. The integration of these features enables the super-hierarchical Ni/Porous-Ni/V2O5 nanocomposite to be a competitive electrode for next-generation rechargeable electrochemical energy storage devices.

Acknowledgement The authors thank Dr. Winson C.H. Kuo of the Materials Characterization Facility of Texas A&M University for his assistance in SEM analysis. The efforts by J. Sun and Dr. H. Jeong for their assistance with surface area measurement is much appreciated. The beneficial input to manuscript editing by Dr. K.T. Hartwig is gratefully acknowledged. This research was partially sponsored by the Turbomachinery Laboratory at the Texas A&M University and the Texas A&M University’s Strategic Initiative seed grant program.

Associated Content Supporting information available: 1) The SEM images of the Ni/Porous-Ni substrate. 2) The statistical histograms of the dimensions of Ni/Porous-Ni/V2O5 nanocomposites before and after annealing. 3) The XRD pattern of as-fabricated V2O5 nanosheets without annealing. 4) The SEM images of hierarchical Ni/V2O5 cathodes before and after cycling.


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Figure 1: XRD patterns of Ni/Porous-Ni/V2O5 nanocomposites after annealing. For reference, the standard patterns of V2O5 (PDF #41-1426) and Ni (PDF #87-0712) are marked.



Figure 2: SEM images of samples of super-hierarchical Ni/Porous-Ni/V2O5 and hierarchical Ni/V2O5 nanocomposites. (a) The low-magnified image of an annealed Ni/Porous-Ni/V2O5 sample. (b) The low-magnified image of an unannealed Ni/Porous-Ni/V2O5 sample. (c) The local high-magnified image of the annealed Ni/Porous-Ni/V2O5 sample shown in (a). (d) The lowmagnified image of an unannealed Ni/V2O5 sample.


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Figure 3: The cyclic voltammetry plots of set 1 and set 2 samples for the first three cycles. (a) Reduction and oxidation peaks at ~2.59 V and ~2.71 V for set 1 (super-hierarchical Ni/PorousNi/V2O5). (b) Reduction and oxidation peaks at ~2.60 V and ~2.73 V for set 2 (hierarchical Ni/V2O5).

Figure 4: Results of charge-discharge cycle tests of set 1 and set 2 samples. The charge-discharge profile plots of 1st, 2nd, 5th, 10th, 25th, 50th, and 100th cycles at 0.2 C for (a) set 1 and (b) set 2. (c) Comparison of the discharge specific capacity and Coulombic efficiency between set 1 and 2 for the 100 cycle case at 0.2 C. (d) Comparison of the discharge specific capacity and Coulombic efficiency between set 1 and 2 for 500 cycles at 3.0 C.



Figure 5: The rate performance test and EIS characterization results of set 1 and set 2 samples. (a) Charge-discharge profile plots of 2nd (at 0.1 C), 12th (at 0.2 C), 22nd (at 0.5 C), 32nd (at 1.0 C), 42nd (at 2.0 C), and 52nd (at 3.0 C) cycles for set 1. (b) Plot of the discharge specific capacity and Coulombic efficiency of set 1 for 63 cycles of rate testing. (c) Comparison of the Nyquist plot between set 1 and 2 after cycling 100 times at 0.2 C. (d) Comparison of the Nyquist plot between set 1 and 2 after cycling 500 times at 3.0 C.


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Figure 6: Comparison of SEM images of the super-hierarchical Ni/Porous-Ni/V2O5 cathode (set 1). (a) Before cycling. (b) After 100 cycles at 0.2 C.



Figure 7: Illustration of the process of lithium ion intercalation and deintercalation of superhierarchical Ni/Porous-Ni/V2O5 cathodes. (a) Microscopic structure of the cathode. (b) Localized nanostructure of the cathode. (c) The process of lithium ion intercalation during discharging. (d) The process of lithium ion deintercalation during charging.


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Table 1: Comparison of the electrolyte resistance (Rs) and charge transfer resistance (Rct) of set 1 and 2 according to EIS tested results. Sample Name

Electrolyte resistance (Rs, Ω)

Set 1 after 100 cycles at 0.2 C Set 2 after 100 cycles at 0.2 C Set 1 after 500 cycles at 3.0 C Set 2 after 500 cycles at 3.0 C

3.19 4.71 4.62 7.16

Charge transfer resistance (Rct, Ω) 6.38 8.89 19.52 36.75

Table 2: Comparison of the electrochemical performance among recently-published V2O5 nanocathode results with one lithium ion insertion.

Morphology

3D nanosheet-assembled hollow microspheres 3D porous hierarchical microspheres 2D ultrathin nanosheets 3D hierarchical and porous microspheres 3D hierarchical microplates 3D porous octahedrons 3D interconnected nanonetwork Super-hierarchical Ni/Porous-Ni/V2O5 cathode (binder-free) Super-hierarchical Ni/Porous-Ni/V2O5 cathode (binder-free)

Max. Specific Capacity (mAh g−1)

Cycle Number

Capacity Fading Per Cycle (%)

Measured Current Density (mA g−1)

Minimum measured value of Rct (Ω)

Reference

137

50

0.12

300

N/A

76

146

100

0.11

75

46

77

146

50

0.08

147

N/A

78

142

100

0.28

75

N/A

79

146

50

0.12

100

N/A

80

135

500

-0.008

100

202

81

149

100

0.03

100

155

71

166

100

0.091

29.4

6.38

This work

133

500

0.055

441

19.52

This work



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TOC Graphic



Figure 1 493x377mm (72 x 72 DPI)


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Figure 2 361x293mm (109 x 109 DPI)



Figure 3 1005x370mm (72 x 72 DPI)


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Figure 4 revised 2nd 1079x687mm (72 x 72 DPI)



Figure 5 revised 2nd 1072x723mm (72 x 72 DPI)


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Figure 6 88x145mm (109 x 109 DPI)



Figure 7 523x396mm (96 x 96 DPI)


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Graphical Abstract revised 564x396mm (72 x 72 DPI)


Super-hierarchical Nickel-Vanadia Nanocomposites for Lithium Storage

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