All-Solid-State Flexible Asymmetric Supercapacitor with Good Cycling Performance and Ultra-Power Density by Electrode Materials of Core-Shell CoNiO2@NiAl-Layered Double Hydroxide and Hollow Spherical α-Fe2O3
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High electrochemical performance of asymmetric supercapacitor (ASC) depends on exquisite nanostructure design and synthesis of electrodes, including structural controllable design and synthesis of high theoretical performance materials and nanocomposite materials. Herein, a template-free hierarchical core-shell nanostructure of CoNiO2@NiAl-layered double hydroxide (NiAl-LDH) and α-Fe2O3 with a hollow spherical structure composed of nanoparticles are successfully prepared. The CoNiO2@NiAl-LDH as the cathode electrode and the hollow spherical α-Fe2O3 as the anode electrode of the ASC devices exhibit superior electrochemical performance. The gel of polyvinyl alcohol (PVA) and KOH acts as the solid electrolyte and the separator to assemble into the all-solid-state flexible ASC devices. Two of the CoNiO2@NiAl-LDH//α-Fe2O3 ASC devices in series are fabricated to meet the voltage requirement of mobile energy equipment, which exhibit a maximum energy density of 65.68 Wh kg−1 at the power density of 369.45 W kg−1. Interestingly, in addition to many advantages that the solid electrolyte in ASC devices already have, we find that it can be an alternative way of solving the problem of iron oxide cycling performance, and of course it can also be used as a reference for other materials with poor cycling performance.
KeywordsAsymmetric supercapacitors Rate capability CoNiO2 NiAl-LDH α-Fe2O3 State electrolyte
Hexadecyl trimethyl ammonium bromide
Electrical double-layer capacitors
Energy dispersive X-ray
Electrochemical impedance spectroscopy
Field emission scanning electron microscopy
High-resolution transmission electron microscopy
NiAl-layered double hydroxide
Organic light-emitting diode
Transmission electron microscopy
Two liquid ASC devices in series
Two solid ASC devices in series
With the development of electric vehicles, traditional energy storage equipment has been difficult to meet the demand of high-energy density and high-power density. Although lithium-ion battery has high energy density, its intrinsic factors limit its power density, which is hard to meet the requirement of the output of high power in practical application of electric vehicle. Supercapacitor is the energy storage device between dielectric capacitor and rechargeable battery exhibiting fast charge-discharge rate, high power density, low cost and good cycling stability, etc. [1, 2, 3, 4, 5]. Supercapacitors can fall into two diverse types by the energy storage mechanism, one is the electrical double-layer capacitors (EDLCs), dominated by the adsorption and desorption of the electrostatic charge in the interface and have high power density and stable cycling characteristics but possess limited energy density; the other is pseudocapacitors, also called Faraday capacitor which is dominated by the Faradaic redox reaction and have considerable energy density . The electrode materials of EDLCs generally consist of carbon materials, such as activated carbon , activated carbon fiber , carbon nanotubes , and graphene . While the electrode materials of the pseudocapacitors are often composed of transition metal oxides, transition metal hydroxides, or layer double hydroxides, such as RuO2 , NiO , Mn2O3 , MnO2 , Co3O4 , NiCo2O4 , CoNiO2 , Ni(OH)2 , Co(OH)2 , NiAl-layered double hydroxide (NiAl-LDH) , NiCo-LDH , CoAl-LDH , and so on. One of the big obstacles to limiting the practical use of supercapacitors is that they have a much smaller energy density than the secondary battery. Generally, in the same specific surface area, the pseudocapacitors can provide more than ten times storage capacity than the electric double-layer capacitance which have attracted great interest in increasing their energy densities of the pseudocapacitors to partially replace the current commercialized two-time battery. However, the pseudocapacitors suffer from the low conductivity of active materials with poor power characteristics and poor cyclic stability caused by volumetric expansion during the charge and discharge processes.
In recent years, the pseudocapacitors have received considerable progress, especially in their energy density [23, 24, 25, 26]. The oxidized nickel-based layered double hydroxides (LDHs), also known as anionic clays, have drawn intensive attention as pseudocapacitive cathode materials because of their high theoretical capacitance, stable cycling performance, abundance in the earth, and environmentally friendly [27, 28]. However, one of the main disadvantages of LDHs is the poor conductivity, which limits their power density and the application in high-power devices in the future. For solving this problem, there is a growing number of research on the complex heterostructure nanomaterials of LDHs such as NiAl-LDHs on Ni foam , NiAl-LDHs@carbon nanoparticles , NiMn-LDHs/carbon nanotubes , NiCo2S4 Nanotube@NiMn LDHs , NiCoAl-LDH/NiCo-Carbonate hydroxide , NiAl-LDHs/graphene , and so on. Some of the above-mentioned studies have indeed increased their conductivity than individual material, but due to the introduction of conductive medias or other active materials, the energy densities of the overall active materials have a certain effect. Therefore, to find a material which has a good conductivity and can also provide a relatively better energy density to compound with LDHs is critical for LDH materials.
Many efforts have been done to focus on the anode materials of the pseudocapacitors to achieve high capacity and good cyclic behavior, including RuO2 , FeOx [36, 37, 38], ZnO , VOx [40, 41, 42], and so on. Among them, α-Fe2O3 is a promising anode material which has attracted great deal of research interest due to the appropriate potential window for rich redox chemistry, the high theoretical capacity, the relatively stable structure, low cost, and abundance in the earth. In recent advance, the reported capacity and the cycling stability of α-Fe2O3 is far below its theoretical capacity and a long way from commercial use, respectively, which can be contributed to the poor conductivity, the reduction of the active regions, and the volume changes of the electrodes during the redox reaction. In order to address these challenges, researchers have developed different nanostructures of α-Fe2O3 for improvement of electrochemical properties, which can be categorized as nanowires, nanotubes, nanosheets, nanosphere, and nanoflowers. But for all this, the transformation of crystal structure of the α-Fe2O3 is occurred in the redox reaction, which will lead to the volume changes and the reduction of the active regions. It still restrains the utilization of α-Fe2O3 in the anode of the supercapacitors. Therefore, it is very urgent to find an effective way to develop α-Fe2O3 as the anode electrode of supercapacitors with both high specific capacity and good cycling performance.
Based on the above consideration, we report a hierarchical CoNiO2@NiAl-LDHs core-shell nanosheet as the cathode electrode and the hollow sphere of α-Fe2O3 as the anode electrode of all-solid-state supercapacitor, which PVA&KOH was introduced as the solid electrolyte and separator. CoNiO2 is a kind of nickel-cobalt material oxides, which has been confirmed that it is a material with high energy density and good conductivity in our previous research work . CoNiO2 was constructed as an inner framework for supporting the outer shell of NiAl-LDHs nanosheets. Two kinds of materials in the inside and outside are based on the nickel element as the main framework, making it easy to form a whole integrated architecture. The structure of CoNiO2@NiAl-LDHs effectively reduced the contact resistance and further increased the effective active area of NiAl-LDHs. The unique 3D hollow sphere of α-Fe2O3 is made up of nanoparticles, which enables each nanoparticle to be easily in contact with the all-solid-state electrolyte and provides sufficient active region for electrochemical reactions. The as-prepared CoNiO2@NiAl-LDHs cathode electrode exhibited a high specific capacitance of 1905 F g−1 at the current density of 1 A g−1. And even at the current density of 8 A g−1, the specific capacitance can be maintained at 1555 F g−1. Meanwhile, the hollow sphere of α-Fe2O3 anode electrode displayed a remarkable electrochemical performance, which the specific capacitance can achieve 802 F g−1 at 1 A g−1 and 70.3% of the capacity retention with the high current density of 16 A g−1. Two all-solid-state asymmetric supercapacitor (ASC) devices in series are assembled by the cathode electrode material of CoNiO2@NiAl-LDHs and the anode electrode material of α-Fe2O3 with the electrolyte of PVA&KOH, which exhibits a maximum energy density of 65.68 Wh kg−1 at the power density of 369.45 W kg−1. Moreover, the system shows a prominent cycling stability of 88.8% retained over 1000 cycles at 1 A g−1.
All the reagents were of analytical grade without any further purification. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), Iron(III) chloride hexahydrate (FeCl3·6H2O), anhydrous ethanol, and hexadecyl trimethyl ammonium bromide (CTAB) were obtained from Sinopharm Chemical Reagent Co. Sodium-p-styrenesulfonate (PSS) was gained from Aladdin Chemical Reagent Co.
Preparation of the PVA&KOH
In a typical preparation of PVA&KOH, 6 g polyvinyl alcohol (PVA) was added to the 40 ml deionized water and was heated to 95 °C for 2 h with magnetic stirring. And then 4 g KOH was dissolved in 10 ml deionized water. The obtained KOH solution was added to the PVA solution dropwise. After that, continued stirring for 30 min and cooled down to the temperature of 30 °C. Finally, the PVA&KOH was formed.
Synthesis of CoNiO2
In a typical synthesis procedure, firstly, the Ni(NO3)2·6H2O and Co(NO3)2·6H2O solution of 1 mol L−1 were prepared, respectively. And then 4 ml of Ni(NO3)2·6H2O and 4 ml Co(NO3)2·6H2O were added into 32 ml deionized water with magnetic stirring for 10 min, followed by 0.12 g PSS was dissolved in the solution with magnetic stirring for 15 min. In addition, the 36 mmol of urea was poured into the above solution with continuous magnetic stirring for 15 min. The resulting transparent solution was poured into a 50 ml Teflon kettle with stainless steel autoclave for hydrothermal reaction at 90 °C for 12 h. After the reactor was cooled down to room temperature, the products were collected and washed by centrifugation and rinsed with deionized water and ethanol for several times, respectively. The obtained products of the CoNiO2 precursors were dried at 60 °C for 12 h and calcinated at 300 °C in air atmosphere for 2 h at a heating rate of 5 °C min−1.
Synthesis of CoNiO2@NiAl-LDHs
The NiAl-LDHs were grown on the prepared CoNiO2 to form the CoNiO2@NiAl-LDHs materials also by a hydrothermal method. Then, 0.2 g as-prepared CoNiO2 and 1.2 g hexadecyl trimethyl ammonium bromide (CTAB) was dispersed in the solutions of 20 ml deionized and 20 ml ethanol with ultrasonic dissolution for 30 min and magnetic stirring for 20 min. Then, 3 mmol Ni(NO3)2·6H2O and 1 mmol Al(NO3)3·9H2O were added; the mixture kept under magnetic stirring for 10 min, followed by 16 mmol urea was added with magnetic stirring for 15 min. And then the obtained solution was transferred to a 50 ml Teflon kettle with stainless steel autoclave for hydrothermal reaction at 90 °C for 12 h. When the reactor cooled down to room temperature, the products were collected and washed by centrifugation with deionized water and ethanol for several times. And then the obtained products were dried at 60 °C for 12 h in air atmosphere.
Synthesis of α-Fe2O3
In a typical procedure, 10 mmol FeCl3·6H2O was dissolved into 40 ml of anhydrous ethanol and 40 ml deionized water with magnetic stirring for 30 min. The obtained transparent solution was transferred to a 100 ml Teflon kettle with stainless steel autoclave for hydrothermal reaction at 200 °C for 12 h. The products were collected and washed by centrifugation and rinsed with deionized water and ethanol for several times, respectively. And then the as-prepared products were also dried at 60 °C for 12 h in air atmosphere.
The as-prepared materials were characterized by X-ray diffractometer (PANalytical X’Pert Pro XRD, CuKa λ = 0.15405 nm) with a scan rate of 5° min−1. Morphology of the prepared products were examined by field emission scanning electron microscopy (FESEM, Inspect F, FEI) with accelerating voltage of 20 kV. Moreover, the transmission electron microscopy (TEM), the high-resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray (EDX) spectroscopy data were acquired using an FEI Tecnai F20 electron microscope operated at accelerating voltage of 200 kV.
The symbols of C, I, Δt, m, ΔV represent the specific capacitance (F g−1) of the active material of the electrode, discharge current (A), discharge time (s), the mass loading of the active material (g), the potential window of discharge (V), respectively. In addition, the electrochemical impedance spectroscopy (EIS) was carried out with the AC voltage of 5 mV amplitude in the frequency range of 0.01 Hz–10 kHz at the open circuit potential.
Results and Discussion
The galvanostatic charge-discharge measurements of CoNiO2@NiAl-LDH electrode is conducted at the current densities of 1 A g−1, 2 A g−1, 4 A g−1, and 8 A g−1 with a potential window between 0 and 0.55 V (vs Hg/HgO), as presented in Fig. 4b. There is a distinct platform region during the charge and discharge, which suggests the faradaic pseudocapacitive behavior of the electrodes. At the same current densities, the specific capacitances are calculated by the formula (1), as shown in the top-right side of Fig. 4b, which exhibits a high specific capacitance of 1905 F g−1 at the current density of 1 A g−1 and much larger than our former research of CoNiO2 (1462 F g−1)  and NiAl-LDH (1313 F g−1) . The specific capacitance retention is up to 81.6% at the current density of 8 A g−1, implying the electrode of CoNiO2@NiAl-LDH has an excellent rate capability and it can be attributed to the core-shell nanostructure. The outstanding electrochemical performance of CoNiO2@NiAl-LDH is better than most of previously reported for bimetallic oxides and the layer double hydroxides, especially the high discharge voltage plateau, such as NiCo2O4@MnMoO4 , NiP@CoNi-LDH , NiCo-LDH/NiCoP@NiMn-LDH , MnCo2O4@Co3O4 , and so on.
The CV curves of the as-prepared α-Fe2O3 are measured at the scan rate of 5, 10, 20, 40, and 60 mV s−1 in the potential window of − 1.2 to − 0.4 V (vs. Hg/HgO) in Fig. 4c. It can be seen that the CV curves also share a similar shape with a pair of the oxidation peak and the reduction peak, indicating the behavior of the pseudocapacitance. The oxidation peak are only just shifted by 0.1 V when the scan rate is increased from 5 to 60 mV s−1, implying a very low resistance of the as-prepared α-Fe2O3 electrode. The CV curve with a slow scan rate of 1 mV s−1 is displayed in the bottom-right side of Fig. 4c. It can be observed that the oxidation peak is − 0.626 V and the reduction peak is − 0.998 V, corresponding to the conversion between Fe3+ and Fe2+ . But at the same time, the reduction peak of − 1.125 V is also observed, which probably some of the materials involved in the conversion between Fe3+ and Fe .
The galvanostatic discharge-charge tests of α-Fe2O3 electrode is carried out at the current densities of 1 A g−1, 2 A g−1, 4 A g−1, 8 A g−1, and 16 A g−1 with a potential window between − 0.1 and − 1.2 V (vs Hg/HgO), as presented in Fig. 4d. It can be also found that a distinct platform region is occurred during the charge and discharge, indicating the faradaic pseudocapacitive behavior of the as-prepared α-Fe2O3 electrodes. The specific capacitances are calculated at the current density of 1, 2, 4, 8, and 16 A g−1, as shown in the top-right side of Fig. 4d. The specific capacitance of 1 A g−1 is 802 F g−1 and only 29.7% loss of the capacitance is acquired at the current density of 16 A g−1, where the rate capability is higher than that of Fe2O3 based on previous work [51, 52, 53, 54, 55].
The cyclic voltammograms of two solid CoNiO2@NiAl-LDH//α-Fe2O3 in series are conducted at the scan rate of 5 mV s−1, 10 mV s−1, 20 mV s−1, and 40 mV s−1 with the potential window between 0 and 3.2 V, as shown in Fig. 5b. It can be observed that the CV curves share a similar shape and the position of redox peak is nearly no shift with the scan rates increased, suggesting a good rate capability of the ASC devices. And the Ragone plots of the two solid CoNiO2@NiAl-LDH//α-Fe2O3 in series ASC devices are shown in the bottom of Fig. 4b which the power density and the energy density are calculated by the CV curves at the scan rate of 5 mV s−1, 10 mV s−1, 20 mV s−1, and 40 mV s−1. The CoNiO2@NiAl-LDH//α-Fe2O3 TSASC devices exhibit a maximum energy density of 65.7 Wh kg−1 with the power density of 369.45 W kg−1 at the CV scan rate of 5 mV s−1 and even at the scan rate of 10 mV s−1, the energy density can still be remained 21.1 Wh kg−1 with the power density of 946.35 W kg−1 based on the sum of the total mass of cathode and anode active substances, indicating a considerable rate capability in the two all-solid state in series devices.
The electrochemical stabilities of the CoNiO2@NiAl-LDH//α-Fe2O3 TSASC devices and the two liquid CoNiO2@NiAl-LDH//α-Fe2O3 in series ASC (named as TLASC) devices are evaluated by the galvanostatic charge-discharge measurements with the current density of 1 A g−1 for 1000 cycles, as shown in Fig. 5c. The CoNiO2@NiAl-LDH//α-Fe2O3 TSASC devices exhibit remarkable cycling stability with 88.8% specific capacitance retention while the CoNiO2@NiAl-LDH//α-Fe2O3 TLASC devices only achieve 68.7% after 1000 cycles. Additionally, the cycling stability of CoNiO2@NiAl-LDH for the cathode materials is presented in the embedded chart of Fig. 5c, indicating an excellent cycling stability with 96.8% retention of the specific capacitance. That means the attenuation of cyclic performance for the ACS devices are mainly caused by the instability of the anode material of α-Fe2O3 in a continuous redox reaction . Compared the cycling stability of TSASC and TLASC, it can be found that the solid electrolyte of PVA&KOH can increase the stability of the anode material of α-Fe2O3, which can be probably that the solid electrolyte of PVA&KOH limits the expansion and passivation of α-Fe2O3 during the continuous redox reaction. It is equivalent to solving the problem of iron oxide cycling performance in an alternative way, and of course it can be analogue to other materials with poor cycling performance.
The comparison of galvanostatic charge-discharge tests with the current density of 1 A g−1 between the origin cycle and 1000th cycle is presented in Fig. 4d. It can be seen that both of two galvanostatic charge-discharge curves have charging and discharging platforms, suggesting a Faradaic reaction. The discharge platform of the 1st cycle is higher than that of the 1000th cycle and the discharge time of the 1st cycle is longer than that of 1000th cycle, which indicates that the electrode materials still have a certain degree of passivation, although the PVA&KOH can prevent some passivation of the electrode materials. Meanwhile, the charge platform of the 1000th cycle is higher than that of the 1st cycle, implying the increase of the internal resistance of the ASC devices.
Further studying the electrochemical properties of the CoNiO2@NiAl-LDH//α-Fe2O3 TLASC and TSASC devices, the electrochemical impedance (EIS) is carried out at the open circuit potential. Figure 5e shows the Nyquist plots of the CoNiO2@NiAl-LDH//α-Fe2O3 TLASC and TSASC devices in the frequency range of 0.01 Hz to 100 kHz with an AC perturbation of 5 mV. The Nyquist plots can be divided into two distinct parts of a semicircle in the high-frequency region and a straight line in the low-frequency region. And the intercepts of the Nyquist plots on the real axis are about 0.67 and 0.72 Ω for the CoNiO2@NiAl-LDH//α-Fe2O3 TLASC and TSASC, respectively, indicating the extremely low intrinsic resistances. The intrinsic resistances are lower than most of previous reported Fe2O3-based ASC devices [57, 58, 59, 60]. Comparing to the Nyquist plots of CoNiO2@NiAl-LDH//α-Fe2O3 TLASC and TSASC, it can be found that the small semicircle diameter of the TSASC is larger than that of TLASC, suggesting that the solid electrolyte of PVA&KOH has a certain barrier for electron transportation. It is proved that the conductivity of one-dimensional nanostructures is better than that of bulk materials . The interlaced nanowires of CoNiO2 can act as the channel for efficient electron transportation between the NiAl-LDH and the current collectors and the hollow sphere structure of α-Fe2O3, composed of nanoparticles, also possesses good conductivity and the ability to efficient transport electron. It is the reason that the Nyquist plots shows low intrinsic resistances and good charge-transfer abilities. An organic light-emitting diode (OLED) display with drive circuit is used to simulate the application of the CoNiO2@NiAl-LDH//α-Fe2O3 TSASC on the mobile phone, which the TSASC successfully powered the OLED display and the brightness of the screen is very high as shown in Fig. 5e.
In summary, a template-free hierarchical nanostructure CoNiO2@NiAl-LDH nanocomposite and α-Fe2O3 with a hollow spherical structure composed of nanoparticles were successfully prepared. The cathode electrode of CoNiO2@NiAl-LDH exhibits outstanding electrochemical performance with a high specific capacitance of 1905 F g−1 at 1 A g−1 and also possesses 1555 F g−1 (81.6% of initial capacitance) at the high current density of 8 A g−1, implying a good rate capability, which can be ascribed to efficient electron transportation channel and the high utilization rate of the active materials. The anode electrode of hollow sphere α-Fe2O3 reveals excellent electrochemical properties with a specific capacitance of 802 F g−1 at 1 A g−1 and the capacitance retention of 70.3% at the high current density of 16 A g−1 which also shows a good rate capability due to the good conductivity and the ability of the hollow spherical structure composed of nanoparticles to efficient transport electron. The PVA&KOH was utilized as the separator and the solid electrolyte of the two all-solid-state ACS devices in series which has been constructed by the cathode electrode of CoNiO2@NiAl-LDH and the anode electrode of hollow sphere α-Fe2O3. The CoNiO2@NiAl-LDH//α-Fe2O3 TSASC exhibits a maximum energy density of 65.68 Wh kg−1 at the power density of 369.45 W kg−1. Moreover, the system shows an enhanced cycling stability of 88.8% retained over 1000 cycles at the current density of 1 A g−1 which is compared to previously reported ASC devices. Moreover, in addition to many advantages that the solid electrolyte in ASC devices already have, it can be interestingly found that it can be an alternative way of solving the problem of iron oxide cycling performance, and of course it can be analogue to other materials with poor cycling performance.
We are grateful for the financial support from the National Natural Science Foundation of China (NSFC 61071027 51702037) and the Fundamental Research Funds for the Central Universities of China (ZYGX2015KYQD014).
This work was supported by the National Natural Science Foundation of China (NSFC 61071027 51702037) and the Fundamental Research Funds for the Central Universities of China (ZYGX2015KYQD014).
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