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Electrochemical characteristics of nanostructured NiO plates hydrothermally treated on nickel foam for Li-ion storage
Accepted Manuscript Title: Electrochemical characteristics of nanostructured NiO plates hydrothermally treated on nickel foam for Li-ion storage Author: Zhiyuan Ma Hong Zhang Yu Zhang Jia Zhang Zhicheng Li PII: DOI: Reference:
S0013-4686(15)30226-7 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.161 EA 25443
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
23-5-2015 8-7-2015 28-7-2015
Please cite this article as: Zhiyuan Ma, Hong Zhang, Yu Zhang, Jia Zhang, Zhicheng Li, Electrochemical characteristics of nanostructured NiO plates hydrothermally treated on nickel foam for Li-ion storage, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.161 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical characteristics of nanostructured NiO plates hydrothermally treated on nickel foam for Li-ion storage Zhiyuan Ma 1, Hong Zhang 1,2, Yu Zhang 1, Jia Zhang 1, Zhicheng Li 1,* 1
School of Materials Science and Engineering, Central South University, Changsha 410083, P.R. China
2
Institute for Materials Microstructure, Central South University, Changsha 410083, P.R. China
Abstract
Nanostructured NiO plates on 3-dimensional porous nickel foam were prepared by hydrothermally treating the commercial nickel foam in an alkaline (NH4)2S2O8 solution followed by annealing in the air. The microstructures and electrochemical performance as well as electrochemically induced phase evolution of the nanostructured NiO electrodes were investigated. Working with a Li metal counter electrode, the hierarchically nanostructured NiO electrode presents a high reversible specific capacity of 604 mA h g-1 at 1.25 C even after 100 cycles and excellent rate performance with a specific capacity of 320 mA h g-1 at 8 C. TEM investigations conducted at several representative charge/discharge statuses reveal that an intermediate phase of Li2NiO2 produced during the redox conversion between metallic Ni and NiO. Therefore, the reactions of NiO electrode for Li-ion storage can be described as NiO ↔ 0.5Ni + 0.5Li2NiO2 ↔ Ni + Li2O.
Keywords: Nickel foam; NiO; Electrochemical performance; Redox conversion; TEM investigation
1. Introduction Quick development of the electric and hybrid vehicles has recently met an urgent demand for the energy storage-conversion devices such as lithium-ion batteries (LIBs) with both high energy density and power density. The high-performance LIBs rely on *Corresponding
author, Tel: +86-731-8887 7740; E-mail: [email protected] （Z.C. Li）
the electrode materials with high specific capacity and excellent rate performance [1-5]. Transition metal oxides have attracted much attention for their high capacities and are considered to be the candidates as anode materials to replace the conventional commercial graphite that suffers from the low specific capacity (theoretical capacity of 372 mAh g-1 for graphite), poor Li+ intercalation kinetics and safety concern [2, 4, 6-11]. Nanostructured materials, especially nanostructured thin films, show their unique merit to acquire the favorable cycle stability and rate performance for they are useful to reduce the electronic/lithium-ionic migration pathways and to accommodate volume changes during the charge/discharge processes [12-16]. Three-dimensional (3D) NiO anode materials supported by nickel foam have recently drawn much attention, for the 3D porous structure feature provides high specific surface of active materials and large contacting area between electrolyte and active materials [17-23]. Wang et al. fabricated nickel foam-supported nano-structured NiO electrodes via a direct thermal treatment of nickel foams at 400 ºC in air [18], and the electrode delivered a capacity higher than 375 mA h g-1 at 10 C, which can be attributed to the good adhesion and electrical contact between NiO and Ni skeleton. While, the higher treatment temperature might result in coarse grains and poorer electrochemical performances, especially rate capability. The electrode obtained at 500 ºC showed only about 200 mA h g-1 at current density less than 2 C [19]. Ni et al. directly grafted NiO nanoflakes on Ni foam via a chemical liquid deposition process followed by annealing treatment [20]. The acquired electrode maintained a high specific capacity of more than 1.3 mA h cm-2 at 0.4 C after 140 electrochemical cycles and
delivered a high discharge capacity retention of 70% (comparing with that of 0.2 C) at 2.5 C. Yan et al. constructed a 3D porous nano-Ni/NiO nanoflake composite film by the combination of hydrogen-bubble template assisted electrodeposition and chemical bath deposition methods, and the electrodes presented a specific capacity of 601 mAh g-1 after 50 cycles at about 2 C [21]. Ni et al. prepared NiO nanowalls on nickel foam via a facile method of the electrochemical corrosion followed by annealing. The related electrode maintained a reversible capacity of 704 mAh g-1 after 100 cycles at 0.15 C [22]. In terms of the lithium-ion storage mechanisms, the researchers have reached the consensus that the electrochemical processes of NiO can be classified as a conversion (redox)
reaction,
which
is
different
from
those
based
on
lithium
intercalation/deintercalation reactions as in graphite and alloying/dealloying reactions as in Li-Si. Understanding the electrochemical processes are of importance for providing strategies to optimize and improve the active materials and related lithium storage performance. The NiO electrode always shows double voltage plateaus in charge process, and the dominant opinion ascribed the oxidation voltage plateau at about 1.5-1.7 V to the partial decomposition of solid electrolyte interface (SEI) [17-29], but the detailed reactions are still waiting for clarifying. In this work, nickel foam-supported nanostructured NiO flakes are fabricated by nickel foams via a facile hydrothermal method. The NiO electrodes exhibit high electrochemical performances. The investigations by transmission electron microscopy (TEM) were conducted to study the phase evolution during charge and discharge
processes and to reveal out the conversion reaction products for the NiO electrode. An intermediate phase of Li2NiO2 produced during the redox reactions was determined, and the electrochemical reactions of NiO electrodes for LIBs were proposed to be NiO + Li+ + e- ↔ 0.5Ni + 0.5Li2NiO2 and 0.5Li2NiO2 + Li+ + e- ↔ 0.5Ni + Li2O.
2. Experimental Commercial nickel foam was ultrasonically cleaned successively in diluted hydrochloric acid, deionized water and ethyl alcohol. In a typical procedure, 1.5 g KOH and 0.5 g (NH4)2S2O8 were dissolved in 30 mL deionized water in a 50 mL beaker. The nickel foam was immersed into the solution and then was transferred into a 40 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 180 ºC for 8 h in an oven. The hydrothermally treated nickel foam was cleaned and annealed at 350 ºC for 5 h in air. The masses of the nickel foam were weighed before hydrothermal treatment and after annealing. The weights of produced NiO on the nickel foam were calculated by the formula mNiO = ∆m × 74.69/16, where, ∆m is the mass difference before and after the experiment treatments. Each tested electrode has the mass of active material about 1.3-1.4 mg/cm2. For the TEM investigations, the nanostructured NiO was hydrothermally treated the nickel foil instead of nickel foam by using the same treatment processes as that on the nickel foam. Li/electrolyte/NiO half-cells (2016 coin cell) were assembled in a glove box filled with ultra-high purity argon. Polypropylene membrane (Celgard 3501) was used as a separator, lithium metal foil was used as a reference and counter electrode, and 1.0 mol
L-1 LiPF6 in mixed ethylene carbonate (EC) and diethyl carbonate (DEC) (the volume ratio of EC to DEC is 1:1) as the electrolyte. The galvanostatic charging/discharging of the NiO electrode was performed with a battery measurement system (Land CT2001A, China) at room temperature. Cyclic voltammetry (CV) and alternating current (AC) impedance tests were conducted with an electrochemical workstation (Gamry Reference600, USA). X-ray diffraction (XRD) patterns of the NiO electrodes were collected by a Rigaku D/MAX 2500 diffractometer using Cu-Kα radiation (λ = 0.154056 nm) with a step of 0.02o to analyze the phase composition. Field-emission scanning electron microscopy (FE SEM) (FEI Sirion 200) was employed to characterize the morphology of NiO on the nickel foam. The phase component and microstructure of the NiO electrodes at various voltages, at which the cells had been charged or discharged until the current is less than 10 µA, were investigated by using a TEM (FEI Tecnai G2 F20). The nickel foil-supported electrodes were rinsed with anhydrous dimethyl carbonate (DMC) to eliminate the residual Li salts after being taken out from the disassembled cell, and were prepared by a conventional ion milling way with a precision ion milling system (Gatan 691).
3. Results and discussion 3.1 Phase and microstructure of as-prepared product Fig. 1 shows the XRD patterns of as-received nickel foam and hydrothermally treated ones before and after being annealed. Three extremely strong peaks at 44.4o,
51.8o and 76.2o shared by all the three samples are in good agreement with the ones from {111}, {002} and {022} crystalline planes of metallic Ni, respectively (PDF no. 70-1849, space group of Fm 3 m, lattice parameter a = 0.3525 nm). Compared to the as-received nickel foam, the hydrothermally treated one displays several additional diffraction peaks at 19.2o, 38.5o, 58.8o and 62.6o, which can be indexed to the hexagonal β-Ni(OH)2 {001}, {101}, {110} and {111} crystalline planes, respectively (PDF no. 74-2075, space group of P 3 m1, lattice parameters a = 0.3130 nm, c = 0.4630 nm). This result is consistent with the β-Ni(OH)2 nanowalls hydrothermal synthesized in aqueous circumstance at various temperatures as reported by Ni et al. [30]. After being annealed at 350 ºC, the characteristic XRD peaks of Ni(OH)2 disappeared, and three peaks from cubic NiO (PDF no. 75-0269, space group of Fm 3 m, lattice parameter a = 0.4176 nm) can be detected at 37.1o, 43.2o and 62.7o, which match well with the diffractions from {111}, {002} and {022} planes, respectively. The related reactions during the synthesis processes can be described as Eqs. (1) and (2). S2O82- + Ni + 2OH- → 2SO42- + Ni(OH)2
(1)
Ni(OH)2 → NiO + H2O
(2)
Figs. 2a~2c show the SEM images of surface morphologies for the nickel foams before and after hydrothermal treatment and being annealed. The as-received nickel foam shows a smooth surface (see in Fig. 2a) with a three-dimensional porous skeleton (upper-right inset in Fig. 2a). After hydrothermal treatment, one can see, in Fig. 2b, that the 3D skeleton is well preserved and that plates grew on the nickel foam surface. High-magnification image (upper-right inset in Fig. 2b) shows that the morphology of
Ni(OH)2 phase possesses a 2D characteristic with the length of about 1 µm and thickness of tens of nanometers. After being annealed, as seen in Fig. 2c, the NiO plates do not show much change in morphology compared to the ones of Ni(OH)2 as shown in Fig. 2b. The right-upper inset in Fig. 2c is a cross-sectional SEM observation of nickel foam hydrothermal treated followed by annealing, demonstrating that the thickness of NiO layer on nickel foam is about 1.5 µm. Fig. 2d shows a typical TEM observation of the nickel foil after hydrothermal treatment and annealing. In the high resolution TEM (HRTEM) image, nanostructured NiO crystals with the size of several nanometers can be observed obviously. The bottom-left inset in Fig. 2d displays the bright field TEM image, showing that each particle consists of multiple nanometered crystals. The upper-right inset is a selected area electron diffraction (SAED) pattern of the treated Ni foam, which can be indexed to be the NiO {111}, {002}, {022} and {113} planes, respectively. Based on the analysis, one can get that the porous 3D skeleton of nickel foam and the 2D NiO plates consisting of lots of nanocrystals, which grew on the nickel foam surface, composing a hierarchical structure feature. This kind of structure feature can greatly enlarge the specific surface area of an electrode, enormously facilitate the charger carriers exchange between electrolyte and electrode, and thus may take full advantages of the electrochemical performance of the NiO electrode on nickel foam.
3.2 Electrochemical performance The cyclic voltammetric (CV) curves of the obtained NiO electrode tested over a voltage range from 0.01 to 3.0 V (versus Li/Li+) at a scan rate of 0.1 mV s 1 are shown −
in Fig. 3a. Only one strong peak is found located at 0.37 V in the first reduction process, corresponding to the reaction NiO + Li+ + 2e- → Ni + Li2O and the formation of the SEI film [17]. In the second CV cycle, the reduction peak shifted to a higher voltage and split into two overlapped peaks (marked by 1 and 2 in Fig. 3a) located at about 1.15 V and 0.9 V, respectively. As can be seen in the 3rd and 30th cycles, peaks 1 and 2 overlapped obviously and shifted to lower voltages with the test proceeding. On the other hand, two obvious peaks (marked by 3 and 4 in Fig. 3a) are at 1.55 V and 2.25 V, respectively, in the first oxidation process. In the subsequent cycles such as the 2nd, 3rd and 30th cycles, the peaks 3 and 4 are repeatable, implying high reversibility of the electrochemical reactions. Fig. 3b displays the charge-discharge profiles of the nickel foam-supported NiO electrode in the 1st, 2nd, 30th, 50th and 100th in the voltage range of 0.01 to 3.0 V (versus Li/Li+) at a current rate of 1.25 C (1 C = 718 mA g-1). The electrochemical processes reflected from the voltage-capacity curves are in good agreement with those in the CV curves. A long voltage plateau blow 0.5 V in the initial discharge process coincides with the one in CV. In the subsequent cycles, the discharge voltage plateaus between 0.8 V and 1.4 V show a good repeatability, corresponding to the peaks 1 and 2 in CV tests. In contrast, the charge processes exhibit two stages, below and above 2 V, which correspond to the peaks 3 and 4 in the CV curves, respectively. Additionally, the capacities attributed to the two stages are almost equal, and the amount of extracted Li+ from the electrode corresponding to per mol NiO active material is about 0.8 mol for each stage. This phenomenon can also be perceived from previous studies, which
adopted various synthesis methods and obtained various electrode microstructures with different specific surface area [17-33]. These stepwise charge and discharge processes should result from the formation of various electrochemically induced products, which will be discussed in detail in section 3.3. The cycling performance of the nickel foam-supported NiO electrode in the voltage range of 0.01 to 3.0 V (versus Li/Li+) at a current rate of 1.25 C is given in Fig. 3c. The first discharge and charge capacities are 1023 mAh g-1 and 576 mA h g-1, respectively. The initial irreversible capacity is mainly due to the SEI formation [25], leading to an initial coulombic efficiency of 56.3%. Since the 2nd cycle, the capacities increasing in the following few cycles should be attributed to the electrochemical activation of electrode [34, 35], which will be further verified by the AC impedance analysis as shown in Fig. 4. After 100 cycles, a capacity of 604 mA h g-1 is retained. The favorable specific capacity should be attributed to the high electrochemical activity of NiO resulting from the hierarchical structure feature, while the high cycling stability mainly benefits from the firm directly adhesion between nickel substrate and NiO plates. To better understand the superiority of the nickel foam-supported NiO electrodes, the test cells were cycled at various rates from 0.32 C to 8 C, as shown in Fig. 3d. The reversible specific capacities are 700, 678, 589, 448 and 320 mA h g-1 at the charge-discharge rates of 0.32 C, 0.8 C, 1.6 C, 4 C and 8 C, respectively, in the 2nd cycle. As the current density is reset to the initial value of 0.32 C, the electrode restores a specific capacity of 728 mA h g-1 after 10 cycles. These properties are comparable to
those of the recently reported works [36-38]. The excellent rate performance should be ascribed to the convenient charge migration produced by the porous 3D electrode skeleton and the hierarchical 2D structure of the NiO active material, which reduces the electrode overpotential caused by kinetic effects [18, 39]. AC impedance tests were conducted on the cells as assembled, after being electrochemically cycled at 1.25 C for 10 and 100 cycles, respectively, with a frequency range from 1 Hz to 1 MHz. The Nyquist plots are shown in Fig. 4. Each plot consists of a depressed semicircle in the high frequency region (left part) and a line tail in the low frequency region. The depressed semicircle is related to the charge transfer resistance in the electrode. The low frequency tail is associated with the diffusion effect of Li-ion on the interface. The charge transfer resistance in the NiO active material can be estimated by the diameter of the depressed semicircle on the Z' axis [27, 40]. The charge transfer resistance is 105 Ω in the fresh cell, and reduced to 80 Ω after 10 cycles, indicating the activation process took place in the electrode during the initial electrochemical cycles. The charge transfer resistance after 100 cycles is quite approximate to that after 10 cycles, suggesting a firm connection between NiO nanoflakes and Ni foam substrate.
3.3 Electrochemically induced phase evolution Strategies to improve the lithium storage performance require a better understanding of the electrochemical behavior of the NiO electrodes, so the intensive investigations on the phase evolution during electrochemical processes are interesting. TEM provides another approach to observing directly the products and intermediates
formed at various charge-discharge statuses in the cell. Meanwhile, for the advantages of high resolution, instantaneous data collection, visualized/digital morphology and microstructure, etc., TEM investigations have shown to be an effective method for characterizing the phase, microstructure and related evolution of the crystalline materials and even amorphous materials [5, 27-30, 34, 35, 41, 42]. Here, TEM investigations were performed when the electrodes were discharged to 0.01 V and charged to 1.7 V and 3.0 V in the 11th cycle, respectively. Fig. 5a is a typical HRTEM image of the NiO electrode discharged to 0.01 V in the 11th cycle. The reduction products still have a grain size of several nanometers. The lattice fringes marked with white lines have a lattice spacing of 0.20 nm and belong to the Ni {111} plane. The bright field image as given in the upper-right inset shows the morphology of the fully reduced electrode is similar to that of the as-prepared NiO electrode as shown in Fig. 2d. Fig. 5b shows the related SAED pattern. Four relatively bright diffraction rings can be assigned to Ni {111}, {002}, {022} and {113} crystal planes, while no diffraction rings or spots from NiO phase were detected. The results reveal that NiO has been fully reduced to metallic Ni when the electrode is discharged to 0.01 V. The related electrochemical reaction can be described as Eq. (3): NiO + 2Li+ + 2e- → Ni + Li2O
(3)
Figs. 5c and 5d present the typical observations of the electrode charged to 1.7 V. The bright field image (upper-right inset in Fig. 5c) shows that the morphology at this stage is also alike with that of the NiO electrode. In the HRTEM image (see in Fig. 5c), apart from the Ni {111} lattice fringes of 0.2 nm (in the right-down region), a lattice
spacing of 0.45 nm can be detected in the upper-middle part. This value is consistent with Li2NiO2 (PDF no. 73-2422, space group of Immm, lattice parameters a = 0.3743 nm, b = 0.2779 nm, c = 0.9026 nm) {002} plane spacing. By analyzing the SAED pattern shown in Fig. 5d, the diffraction rings resulted from Ni {111}, {002}, {022} and {113} planes remain the most evident, but many diffraction spots emerge. The interplanar spacings calculated from these diffraction spots are identical to {002}, {101}, {112}, {200}, {211}, {204}, {022}, {121}, {303} and {220} planes of Li2NiO2, respectively. No diffractions from NiO could be found in the SAED pattern or the HRTEM image. These results prove that an exclusive reaction exists on the electrode when it is charged from 0.01 V to 1.7 V and produces an intermediate phase Li2NiO2 as described in Eq. (4): 0.5Ni + Li2O - e- → Li+ + 0.5Li2NiO2
(4)
The phase of Li2NiO2 has been investigated as a cathode additive for overcharge protection in LIBs [43-45]. As reported by Kim et al. on the Li2NiO2 electrode cycled between 1.5 V and 4.3 V versus metal Li, Li2NiO2 exhibited a short charge plateau at the voltage range of 2-2.5 V since the second cycle, corresponding to its first delithiation step and after that Li2NiO2 would be transformed to the layered LiNiO2 [44]. However, in the case of the present NiO electrode, during the charge process, metallic Ni was partially oxidized to Li2NiO2. Since metallic Ni and LiNiO2 cannot coexist because of the comproportionation reaction Ni + Ni3+ → Ni2+. In the continuous charge process, Li2NiO2 would not be transformed to the layered LiNiO2. Instead metallic Ni will be oxidized to NiO with the delithiation of Li2NiO2 until the delithiation process
finished. Fig. 5e shows a typical HRTEM image of the electrode charged to 3.0 V. The nanoscaled grains are preserved after the 11th complete electrochemical cycle. The lattice fringes with spacing of 0.24 nm can be recognized, corresponding to NiO {111} plane. The upper-right inset gives a bright field image. In comparison with the as-prepared NiO electrode, the electrode at recharged state varied little in morphology. Fig. 5f is a SAED pattern recorded from the area as shown in Fig. 5e. The strong diffraction rings can be assigned to NiO {111}, {002}, {022}, {113} and {222} crystal planes. No diffraction ring or spot from other phases is found. This reveals that intermediate product Li2NiO2 has completely changed to NiO phase and that the residual Ni has also been oxidized to NiO phase. Hence, the reaction in the active materials electrode from 1.7 V to 3 V can be described as in Eq. (5): 0.5Ni + 0.5Li2NiO2 - e- → Li+ + NiO
(5)
The microstructures of the NiO electrode after 100 electrochemistry cycles were observed and are presented in Fig. 6. As can be seen from the SEM image in Fig. 6a, the microstructural characteristic of the nickel foam electrode with the 3D skeleton, flake-like structure and near homogenous distribution of active materials are preserved. The NiO electrode does not show obvious change even after 100 cycles at a current density of 1.25 C. Fig. 6b displays the TEM observations of the nickel foil-supported NiO electrode after 100 cycles. The bright field image in the bottom-left inset shows the particle consisting of nano-meter grains. The HRTEM image and the right-upper inset SAED pattern in Fig. 6b testify that the active material electrode is still composed of
nanocrystallized NiO after cycling. This kind of hierarchical microstructure characteristic is similar with that of the as-treated NiO as shown in Fig. 2, indicating a firm connection between NiO active materials and Ni foam substrate. According to the above discussion, it is confirmed that the lithium storage capability of the NiO electrode bases on the redox conversion between Ni metal and NiO. The completed redox reaction declares that the hierarchical nanostructured NiO electrode possesses favorable electrochemical activity and kinetics, resulting in the high specific capacity and excellent rate performance. The detection of the intermediate product Li2NiO2 well explains the stepwise charge/discharge processes. The electrochemical reactions taken place in the nanostructured NiO electrodes can be summarized as NiO + Li+ + e- ↔ 0.5Ni + 0.5Li2NiO2 and 0.5Li2NiO2 + Li+ + e- ↔ 0.5Ni + Li2O.
4. Conclusions Nanostructured NiO plates on the nickel foam have been directly fabricated by a hydrothermal synthesis and subsequent heat treatment in the air. The 3D hierarchical nanostructured NiO electrodes for Lithium-ion storage exhibit a high specific capacity, superior cycling stability and excellent rate performance. The advantageous electrochemical performance results from the 3D porous architecture, nanostructured NiO plates and firm connection between the NiO and nickel foam substrate, which provide the electrode with large specific surface area, short charge transfer pathways and a good ohmic contact. Detailed TEM investigations on the phase evolution during
the charge-discharge process demonstrate that the stepwise charge/discharge processes are due to the presence of an intermediate product of Li2NiO2 phase, and the electrochemical reactions can be summarized as NiO ↔ 0.5Ni + 0.5Li2NiO2 ↔ Ni + Li2O during the lithiation/delithiation processes.
Acknowledgment The authors acknowledge the support of the National Nature Science Foundation of China (No. 51172287).
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Figures caption
Figure 1 XRD patterns of the as-received nickel foam, before and after annealing after being hydrothermally treated, respectively. Figure 2 Microstructure investigations, (a) SEM observations of the as-received nickel foam and the upper-right inset of a low magnification showing a 3D porous architecture, (b) morphology of a as hydrothermally treated nickel foam and the upper-right inset of an enlarged image, (c) morphology of the surface of a nickel foam treated by hydrothermally treated followed by 350 ºC annealing, and the upper-right inset of a cross-sectional observation, (d) TEM investigations of a nickel foil treated by hydrothermally treated followed by 350 ºC annealing, the HRTEM image and down-left inset bright-field image showing a nanostructured microstructure, and the upper-right inset SAED revealed the phase of cubic NiO. Figure 3 Electrochemical performance of the NiO electrodes, (a) cyclic voltammetry curves in the initial three cycles and the 30th cycles induced at 0.1 mV s-1, (b) charge-discharge profiles of different cycles at 1.25 C, (c) cycling performance at 1.25 C, (d) rate performance. Figure 4 Nyquist plots of the cell assembled with 3D NiO electrodes at various statuses, the
cells as fresh, after being electrochemically cycled 10 and 100 cycles, respectively. Figure 5 TEM observations and SAED patterns of NiO electrodes discharged-charged to various stages in the 11th electrochemical cycle, (a) and (b) discharged to 0.01 V, (c) and (d) charged to 1.7 V, (e) and (f) charged to 3.0 V. Figure 6 Microstructure investigations of NiO electrodes after 100 electrochemical cycles at 1.25 C, (a) morphology of the nickel foam-supported NiO electrode and the upper-right inset of an enlarged image, (b) TEM investigations of a nickel foil-supported NiO
electrode.
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S0013-4686(15)30226-7 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.161 EA 25443
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
23-5-2015 8-7-2015 28-7-2015
Please cite this article as: Zhiyuan Ma, Hong Zhang, Yu Zhang, Jia Zhang, Zhicheng Li, Electrochemical characteristics of nanostructured NiO plates hydrothermally treated on nickel foam for Li-ion storage, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.161 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical characteristics of nanostructured NiO plates hydrothermally treated on nickel foam for Li-ion storage Zhiyuan Ma 1, Hong Zhang 1,2, Yu Zhang 1, Jia Zhang 1, Zhicheng Li 1,* 1
School of Materials Science and Engineering, Central South University, Changsha 410083, P.R. China
2
Institute for Materials Microstructure, Central South University, Changsha 410083, P.R. China
Abstract
Nanostructured NiO plates on 3-dimensional porous nickel foam were prepared by hydrothermally treating the commercial nickel foam in an alkaline (NH4)2S2O8 solution followed by annealing in the air. The microstructures and electrochemical performance as well as electrochemically induced phase evolution of the nanostructured NiO electrodes were investigated. Working with a Li metal counter electrode, the hierarchically nanostructured NiO electrode presents a high reversible specific capacity of 604 mA h g-1 at 1.25 C even after 100 cycles and excellent rate performance with a specific capacity of 320 mA h g-1 at 8 C. TEM investigations conducted at several representative charge/discharge statuses reveal that an intermediate phase of Li2NiO2 produced during the redox conversion between metallic Ni and NiO. Therefore, the reactions of NiO electrode for Li-ion storage can be described as NiO ↔ 0.5Ni + 0.5Li2NiO2 ↔ Ni + Li2O.
Keywords: Nickel foam; NiO; Electrochemical performance; Redox conversion; TEM investigation
1. Introduction Quick development of the electric and hybrid vehicles has recently met an urgent demand for the energy storage-conversion devices such as lithium-ion batteries (LIBs) with both high energy density and power density. The high-performance LIBs rely on *Corresponding
author, Tel: +86-731-8887 7740; E-mail: [email protected] （Z.C. Li）
the electrode materials with high specific capacity and excellent rate performance [1-5]. Transition metal oxides have attracted much attention for their high capacities and are considered to be the candidates as anode materials to replace the conventional commercial graphite that suffers from the low specific capacity (theoretical capacity of 372 mAh g-1 for graphite), poor Li+ intercalation kinetics and safety concern [2, 4, 6-11]. Nanostructured materials, especially nanostructured thin films, show their unique merit to acquire the favorable cycle stability and rate performance for they are useful to reduce the electronic/lithium-ionic migration pathways and to accommodate volume changes during the charge/discharge processes [12-16]. Three-dimensional (3D) NiO anode materials supported by nickel foam have recently drawn much attention, for the 3D porous structure feature provides high specific surface of active materials and large contacting area between electrolyte and active materials [17-23]. Wang et al. fabricated nickel foam-supported nano-structured NiO electrodes via a direct thermal treatment of nickel foams at 400 ºC in air [18], and the electrode delivered a capacity higher than 375 mA h g-1 at 10 C, which can be attributed to the good adhesion and electrical contact between NiO and Ni skeleton. While, the higher treatment temperature might result in coarse grains and poorer electrochemical performances, especially rate capability. The electrode obtained at 500 ºC showed only about 200 mA h g-1 at current density less than 2 C [19]. Ni et al. directly grafted NiO nanoflakes on Ni foam via a chemical liquid deposition process followed by annealing treatment [20]. The acquired electrode maintained a high specific capacity of more than 1.3 mA h cm-2 at 0.4 C after 140 electrochemical cycles and
delivered a high discharge capacity retention of 70% (comparing with that of 0.2 C) at 2.5 C. Yan et al. constructed a 3D porous nano-Ni/NiO nanoflake composite film by the combination of hydrogen-bubble template assisted electrodeposition and chemical bath deposition methods, and the electrodes presented a specific capacity of 601 mAh g-1 after 50 cycles at about 2 C [21]. Ni et al. prepared NiO nanowalls on nickel foam via a facile method of the electrochemical corrosion followed by annealing. The related electrode maintained a reversible capacity of 704 mAh g-1 after 100 cycles at 0.15 C [22]. In terms of the lithium-ion storage mechanisms, the researchers have reached the consensus that the electrochemical processes of NiO can be classified as a conversion (redox)
reaction,
which
is
different
from
those
based
on
lithium
intercalation/deintercalation reactions as in graphite and alloying/dealloying reactions as in Li-Si. Understanding the electrochemical processes are of importance for providing strategies to optimize and improve the active materials and related lithium storage performance. The NiO electrode always shows double voltage plateaus in charge process, and the dominant opinion ascribed the oxidation voltage plateau at about 1.5-1.7 V to the partial decomposition of solid electrolyte interface (SEI) [17-29], but the detailed reactions are still waiting for clarifying. In this work, nickel foam-supported nanostructured NiO flakes are fabricated by nickel foams via a facile hydrothermal method. The NiO electrodes exhibit high electrochemical performances. The investigations by transmission electron microscopy (TEM) were conducted to study the phase evolution during charge and discharge
processes and to reveal out the conversion reaction products for the NiO electrode. An intermediate phase of Li2NiO2 produced during the redox reactions was determined, and the electrochemical reactions of NiO electrodes for LIBs were proposed to be NiO + Li+ + e- ↔ 0.5Ni + 0.5Li2NiO2 and 0.5Li2NiO2 + Li+ + e- ↔ 0.5Ni + Li2O.
2. Experimental Commercial nickel foam was ultrasonically cleaned successively in diluted hydrochloric acid, deionized water and ethyl alcohol. In a typical procedure, 1.5 g KOH and 0.5 g (NH4)2S2O8 were dissolved in 30 mL deionized water in a 50 mL beaker. The nickel foam was immersed into the solution and then was transferred into a 40 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 180 ºC for 8 h in an oven. The hydrothermally treated nickel foam was cleaned and annealed at 350 ºC for 5 h in air. The masses of the nickel foam were weighed before hydrothermal treatment and after annealing. The weights of produced NiO on the nickel foam were calculated by the formula mNiO = ∆m × 74.69/16, where, ∆m is the mass difference before and after the experiment treatments. Each tested electrode has the mass of active material about 1.3-1.4 mg/cm2. For the TEM investigations, the nanostructured NiO was hydrothermally treated the nickel foil instead of nickel foam by using the same treatment processes as that on the nickel foam. Li/electrolyte/NiO half-cells (2016 coin cell) were assembled in a glove box filled with ultra-high purity argon. Polypropylene membrane (Celgard 3501) was used as a separator, lithium metal foil was used as a reference and counter electrode, and 1.0 mol
L-1 LiPF6 in mixed ethylene carbonate (EC) and diethyl carbonate (DEC) (the volume ratio of EC to DEC is 1:1) as the electrolyte. The galvanostatic charging/discharging of the NiO electrode was performed with a battery measurement system (Land CT2001A, China) at room temperature. Cyclic voltammetry (CV) and alternating current (AC) impedance tests were conducted with an electrochemical workstation (Gamry Reference600, USA). X-ray diffraction (XRD) patterns of the NiO electrodes were collected by a Rigaku D/MAX 2500 diffractometer using Cu-Kα radiation (λ = 0.154056 nm) with a step of 0.02o to analyze the phase composition. Field-emission scanning electron microscopy (FE SEM) (FEI Sirion 200) was employed to characterize the morphology of NiO on the nickel foam. The phase component and microstructure of the NiO electrodes at various voltages, at which the cells had been charged or discharged until the current is less than 10 µA, were investigated by using a TEM (FEI Tecnai G2 F20). The nickel foil-supported electrodes were rinsed with anhydrous dimethyl carbonate (DMC) to eliminate the residual Li salts after being taken out from the disassembled cell, and were prepared by a conventional ion milling way with a precision ion milling system (Gatan 691).
3. Results and discussion 3.1 Phase and microstructure of as-prepared product Fig. 1 shows the XRD patterns of as-received nickel foam and hydrothermally treated ones before and after being annealed. Three extremely strong peaks at 44.4o,
51.8o and 76.2o shared by all the three samples are in good agreement with the ones from {111}, {002} and {022} crystalline planes of metallic Ni, respectively (PDF no. 70-1849, space group of Fm 3 m, lattice parameter a = 0.3525 nm). Compared to the as-received nickel foam, the hydrothermally treated one displays several additional diffraction peaks at 19.2o, 38.5o, 58.8o and 62.6o, which can be indexed to the hexagonal β-Ni(OH)2 {001}, {101}, {110} and {111} crystalline planes, respectively (PDF no. 74-2075, space group of P 3 m1, lattice parameters a = 0.3130 nm, c = 0.4630 nm). This result is consistent with the β-Ni(OH)2 nanowalls hydrothermal synthesized in aqueous circumstance at various temperatures as reported by Ni et al. [30]. After being annealed at 350 ºC, the characteristic XRD peaks of Ni(OH)2 disappeared, and three peaks from cubic NiO (PDF no. 75-0269, space group of Fm 3 m, lattice parameter a = 0.4176 nm) can be detected at 37.1o, 43.2o and 62.7o, which match well with the diffractions from {111}, {002} and {022} planes, respectively. The related reactions during the synthesis processes can be described as Eqs. (1) and (2). S2O82- + Ni + 2OH- → 2SO42- + Ni(OH)2
(1)
Ni(OH)2 → NiO + H2O
(2)
Figs. 2a~2c show the SEM images of surface morphologies for the nickel foams before and after hydrothermal treatment and being annealed. The as-received nickel foam shows a smooth surface (see in Fig. 2a) with a three-dimensional porous skeleton (upper-right inset in Fig. 2a). After hydrothermal treatment, one can see, in Fig. 2b, that the 3D skeleton is well preserved and that plates grew on the nickel foam surface. High-magnification image (upper-right inset in Fig. 2b) shows that the morphology of
Ni(OH)2 phase possesses a 2D characteristic with the length of about 1 µm and thickness of tens of nanometers. After being annealed, as seen in Fig. 2c, the NiO plates do not show much change in morphology compared to the ones of Ni(OH)2 as shown in Fig. 2b. The right-upper inset in Fig. 2c is a cross-sectional SEM observation of nickel foam hydrothermal treated followed by annealing, demonstrating that the thickness of NiO layer on nickel foam is about 1.5 µm. Fig. 2d shows a typical TEM observation of the nickel foil after hydrothermal treatment and annealing. In the high resolution TEM (HRTEM) image, nanostructured NiO crystals with the size of several nanometers can be observed obviously. The bottom-left inset in Fig. 2d displays the bright field TEM image, showing that each particle consists of multiple nanometered crystals. The upper-right inset is a selected area electron diffraction (SAED) pattern of the treated Ni foam, which can be indexed to be the NiO {111}, {002}, {022} and {113} planes, respectively. Based on the analysis, one can get that the porous 3D skeleton of nickel foam and the 2D NiO plates consisting of lots of nanocrystals, which grew on the nickel foam surface, composing a hierarchical structure feature. This kind of structure feature can greatly enlarge the specific surface area of an electrode, enormously facilitate the charger carriers exchange between electrolyte and electrode, and thus may take full advantages of the electrochemical performance of the NiO electrode on nickel foam.
3.2 Electrochemical performance The cyclic voltammetric (CV) curves of the obtained NiO electrode tested over a voltage range from 0.01 to 3.0 V (versus Li/Li+) at a scan rate of 0.1 mV s 1 are shown −
in Fig. 3a. Only one strong peak is found located at 0.37 V in the first reduction process, corresponding to the reaction NiO + Li+ + 2e- → Ni + Li2O and the formation of the SEI film [17]. In the second CV cycle, the reduction peak shifted to a higher voltage and split into two overlapped peaks (marked by 1 and 2 in Fig. 3a) located at about 1.15 V and 0.9 V, respectively. As can be seen in the 3rd and 30th cycles, peaks 1 and 2 overlapped obviously and shifted to lower voltages with the test proceeding. On the other hand, two obvious peaks (marked by 3 and 4 in Fig. 3a) are at 1.55 V and 2.25 V, respectively, in the first oxidation process. In the subsequent cycles such as the 2nd, 3rd and 30th cycles, the peaks 3 and 4 are repeatable, implying high reversibility of the electrochemical reactions. Fig. 3b displays the charge-discharge profiles of the nickel foam-supported NiO electrode in the 1st, 2nd, 30th, 50th and 100th in the voltage range of 0.01 to 3.0 V (versus Li/Li+) at a current rate of 1.25 C (1 C = 718 mA g-1). The electrochemical processes reflected from the voltage-capacity curves are in good agreement with those in the CV curves. A long voltage plateau blow 0.5 V in the initial discharge process coincides with the one in CV. In the subsequent cycles, the discharge voltage plateaus between 0.8 V and 1.4 V show a good repeatability, corresponding to the peaks 1 and 2 in CV tests. In contrast, the charge processes exhibit two stages, below and above 2 V, which correspond to the peaks 3 and 4 in the CV curves, respectively. Additionally, the capacities attributed to the two stages are almost equal, and the amount of extracted Li+ from the electrode corresponding to per mol NiO active material is about 0.8 mol for each stage. This phenomenon can also be perceived from previous studies, which
adopted various synthesis methods and obtained various electrode microstructures with different specific surface area [17-33]. These stepwise charge and discharge processes should result from the formation of various electrochemically induced products, which will be discussed in detail in section 3.3. The cycling performance of the nickel foam-supported NiO electrode in the voltage range of 0.01 to 3.0 V (versus Li/Li+) at a current rate of 1.25 C is given in Fig. 3c. The first discharge and charge capacities are 1023 mAh g-1 and 576 mA h g-1, respectively. The initial irreversible capacity is mainly due to the SEI formation [25], leading to an initial coulombic efficiency of 56.3%. Since the 2nd cycle, the capacities increasing in the following few cycles should be attributed to the electrochemical activation of electrode [34, 35], which will be further verified by the AC impedance analysis as shown in Fig. 4. After 100 cycles, a capacity of 604 mA h g-1 is retained. The favorable specific capacity should be attributed to the high electrochemical activity of NiO resulting from the hierarchical structure feature, while the high cycling stability mainly benefits from the firm directly adhesion between nickel substrate and NiO plates. To better understand the superiority of the nickel foam-supported NiO electrodes, the test cells were cycled at various rates from 0.32 C to 8 C, as shown in Fig. 3d. The reversible specific capacities are 700, 678, 589, 448 and 320 mA h g-1 at the charge-discharge rates of 0.32 C, 0.8 C, 1.6 C, 4 C and 8 C, respectively, in the 2nd cycle. As the current density is reset to the initial value of 0.32 C, the electrode restores a specific capacity of 728 mA h g-1 after 10 cycles. These properties are comparable to
those of the recently reported works [36-38]. The excellent rate performance should be ascribed to the convenient charge migration produced by the porous 3D electrode skeleton and the hierarchical 2D structure of the NiO active material, which reduces the electrode overpotential caused by kinetic effects [18, 39]. AC impedance tests were conducted on the cells as assembled, after being electrochemically cycled at 1.25 C for 10 and 100 cycles, respectively, with a frequency range from 1 Hz to 1 MHz. The Nyquist plots are shown in Fig. 4. Each plot consists of a depressed semicircle in the high frequency region (left part) and a line tail in the low frequency region. The depressed semicircle is related to the charge transfer resistance in the electrode. The low frequency tail is associated with the diffusion effect of Li-ion on the interface. The charge transfer resistance in the NiO active material can be estimated by the diameter of the depressed semicircle on the Z' axis [27, 40]. The charge transfer resistance is 105 Ω in the fresh cell, and reduced to 80 Ω after 10 cycles, indicating the activation process took place in the electrode during the initial electrochemical cycles. The charge transfer resistance after 100 cycles is quite approximate to that after 10 cycles, suggesting a firm connection between NiO nanoflakes and Ni foam substrate.
3.3 Electrochemically induced phase evolution Strategies to improve the lithium storage performance require a better understanding of the electrochemical behavior of the NiO electrodes, so the intensive investigations on the phase evolution during electrochemical processes are interesting. TEM provides another approach to observing directly the products and intermediates
formed at various charge-discharge statuses in the cell. Meanwhile, for the advantages of high resolution, instantaneous data collection, visualized/digital morphology and microstructure, etc., TEM investigations have shown to be an effective method for characterizing the phase, microstructure and related evolution of the crystalline materials and even amorphous materials [5, 27-30, 34, 35, 41, 42]. Here, TEM investigations were performed when the electrodes were discharged to 0.01 V and charged to 1.7 V and 3.0 V in the 11th cycle, respectively. Fig. 5a is a typical HRTEM image of the NiO electrode discharged to 0.01 V in the 11th cycle. The reduction products still have a grain size of several nanometers. The lattice fringes marked with white lines have a lattice spacing of 0.20 nm and belong to the Ni {111} plane. The bright field image as given in the upper-right inset shows the morphology of the fully reduced electrode is similar to that of the as-prepared NiO electrode as shown in Fig. 2d. Fig. 5b shows the related SAED pattern. Four relatively bright diffraction rings can be assigned to Ni {111}, {002}, {022} and {113} crystal planes, while no diffraction rings or spots from NiO phase were detected. The results reveal that NiO has been fully reduced to metallic Ni when the electrode is discharged to 0.01 V. The related electrochemical reaction can be described as Eq. (3): NiO + 2Li+ + 2e- → Ni + Li2O
(3)
Figs. 5c and 5d present the typical observations of the electrode charged to 1.7 V. The bright field image (upper-right inset in Fig. 5c) shows that the morphology at this stage is also alike with that of the NiO electrode. In the HRTEM image (see in Fig. 5c), apart from the Ni {111} lattice fringes of 0.2 nm (in the right-down region), a lattice
spacing of 0.45 nm can be detected in the upper-middle part. This value is consistent with Li2NiO2 (PDF no. 73-2422, space group of Immm, lattice parameters a = 0.3743 nm, b = 0.2779 nm, c = 0.9026 nm) {002} plane spacing. By analyzing the SAED pattern shown in Fig. 5d, the diffraction rings resulted from Ni {111}, {002}, {022} and {113} planes remain the most evident, but many diffraction spots emerge. The interplanar spacings calculated from these diffraction spots are identical to {002}, {101}, {112}, {200}, {211}, {204}, {022}, {121}, {303} and {220} planes of Li2NiO2, respectively. No diffractions from NiO could be found in the SAED pattern or the HRTEM image. These results prove that an exclusive reaction exists on the electrode when it is charged from 0.01 V to 1.7 V and produces an intermediate phase Li2NiO2 as described in Eq. (4): 0.5Ni + Li2O - e- → Li+ + 0.5Li2NiO2
(4)
The phase of Li2NiO2 has been investigated as a cathode additive for overcharge protection in LIBs [43-45]. As reported by Kim et al. on the Li2NiO2 electrode cycled between 1.5 V and 4.3 V versus metal Li, Li2NiO2 exhibited a short charge plateau at the voltage range of 2-2.5 V since the second cycle, corresponding to its first delithiation step and after that Li2NiO2 would be transformed to the layered LiNiO2 [44]. However, in the case of the present NiO electrode, during the charge process, metallic Ni was partially oxidized to Li2NiO2. Since metallic Ni and LiNiO2 cannot coexist because of the comproportionation reaction Ni + Ni3+ → Ni2+. In the continuous charge process, Li2NiO2 would not be transformed to the layered LiNiO2. Instead metallic Ni will be oxidized to NiO with the delithiation of Li2NiO2 until the delithiation process
finished. Fig. 5e shows a typical HRTEM image of the electrode charged to 3.0 V. The nanoscaled grains are preserved after the 11th complete electrochemical cycle. The lattice fringes with spacing of 0.24 nm can be recognized, corresponding to NiO {111} plane. The upper-right inset gives a bright field image. In comparison with the as-prepared NiO electrode, the electrode at recharged state varied little in morphology. Fig. 5f is a SAED pattern recorded from the area as shown in Fig. 5e. The strong diffraction rings can be assigned to NiO {111}, {002}, {022}, {113} and {222} crystal planes. No diffraction ring or spot from other phases is found. This reveals that intermediate product Li2NiO2 has completely changed to NiO phase and that the residual Ni has also been oxidized to NiO phase. Hence, the reaction in the active materials electrode from 1.7 V to 3 V can be described as in Eq. (5): 0.5Ni + 0.5Li2NiO2 - e- → Li+ + NiO
(5)
The microstructures of the NiO electrode after 100 electrochemistry cycles were observed and are presented in Fig. 6. As can be seen from the SEM image in Fig. 6a, the microstructural characteristic of the nickel foam electrode with the 3D skeleton, flake-like structure and near homogenous distribution of active materials are preserved. The NiO electrode does not show obvious change even after 100 cycles at a current density of 1.25 C. Fig. 6b displays the TEM observations of the nickel foil-supported NiO electrode after 100 cycles. The bright field image in the bottom-left inset shows the particle consisting of nano-meter grains. The HRTEM image and the right-upper inset SAED pattern in Fig. 6b testify that the active material electrode is still composed of
nanocrystallized NiO after cycling. This kind of hierarchical microstructure characteristic is similar with that of the as-treated NiO as shown in Fig. 2, indicating a firm connection between NiO active materials and Ni foam substrate. According to the above discussion, it is confirmed that the lithium storage capability of the NiO electrode bases on the redox conversion between Ni metal and NiO. The completed redox reaction declares that the hierarchical nanostructured NiO electrode possesses favorable electrochemical activity and kinetics, resulting in the high specific capacity and excellent rate performance. The detection of the intermediate product Li2NiO2 well explains the stepwise charge/discharge processes. The electrochemical reactions taken place in the nanostructured NiO electrodes can be summarized as NiO + Li+ + e- ↔ 0.5Ni + 0.5Li2NiO2 and 0.5Li2NiO2 + Li+ + e- ↔ 0.5Ni + Li2O.
4. Conclusions Nanostructured NiO plates on the nickel foam have been directly fabricated by a hydrothermal synthesis and subsequent heat treatment in the air. The 3D hierarchical nanostructured NiO electrodes for Lithium-ion storage exhibit a high specific capacity, superior cycling stability and excellent rate performance. The advantageous electrochemical performance results from the 3D porous architecture, nanostructured NiO plates and firm connection between the NiO and nickel foam substrate, which provide the electrode with large specific surface area, short charge transfer pathways and a good ohmic contact. Detailed TEM investigations on the phase evolution during
the charge-discharge process demonstrate that the stepwise charge/discharge processes are due to the presence of an intermediate product of Li2NiO2 phase, and the electrochemical reactions can be summarized as NiO ↔ 0.5Ni + 0.5Li2NiO2 ↔ Ni + Li2O during the lithiation/delithiation processes.
Acknowledgment The authors acknowledge the support of the National Nature Science Foundation of China (No. 51172287).
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Figures caption
Figure 1 XRD patterns of the as-received nickel foam, before and after annealing after being hydrothermally treated, respectively. Figure 2 Microstructure investigations, (a) SEM observations of the as-received nickel foam and the upper-right inset of a low magnification showing a 3D porous architecture, (b) morphology of a as hydrothermally treated nickel foam and the upper-right inset of an enlarged image, (c) morphology of the surface of a nickel foam treated by hydrothermally treated followed by 350 ºC annealing, and the upper-right inset of a cross-sectional observation, (d) TEM investigations of a nickel foil treated by hydrothermally treated followed by 350 ºC annealing, the HRTEM image and down-left inset bright-field image showing a nanostructured microstructure, and the upper-right inset SAED revealed the phase of cubic NiO. Figure 3 Electrochemical performance of the NiO electrodes, (a) cyclic voltammetry curves in the initial three cycles and the 30th cycles induced at 0.1 mV s-1, (b) charge-discharge profiles of different cycles at 1.25 C, (c) cycling performance at 1.25 C, (d) rate performance. Figure 4 Nyquist plots of the cell assembled with 3D NiO electrodes at various statuses, the
cells as fresh, after being electrochemically cycled 10 and 100 cycles, respectively. Figure 5 TEM observations and SAED patterns of NiO electrodes discharged-charged to various stages in the 11th electrochemical cycle, (a) and (b) discharged to 0.01 V, (c) and (d) charged to 1.7 V, (e) and (f) charged to 3.0 V. Figure 6 Microstructure investigations of NiO electrodes after 100 electrochemical cycles at 1.25 C, (a) morphology of the nickel foam-supported NiO electrode and the upper-right inset of an enlarged image, (b) TEM investigations of a nickel foil-supported NiO
electrode.
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