Supercapacitor as an energy storage device between the battery and the conventional capacitor has attracted great research interest in recent years [[1], [2], [3]]. Supercapacitors which combine the advantages of batteries and conventional capacitors can not only provide much higher energy density than conventional capacitors but also provide high power density and excellent cycle stability [[4], [5], [6]]. In order to meet the practical application, it is necessary to develop high energy density supercapacitor. Different positive and negative electrode materials are used to form asymmetric supercapacitors (ASCs) so as to improve energy density. Energy density could be increased greatly by increasing operating potential according to the equation E = 1/2CV². Although a variety of ASC systems have been studied, most studies have focused on the preparation of positive electrode materials, while negative electrode materials have been less studied [7]. Metal oxides as negative electrodes such as Molybdenum oxide [[8], [9], [10]], Iron oxide [[11], [12], [13]], and Bismuth oxide [[14], [15], [16]] have been investigated because of their much higher energy density than carbon materials. Among them, Bismuth oxide has attracted extensive attention owing to its high theoretical capacitance (1370 F g^-1), suitable potential window, low cost, non-toxic to environment, and easy preparation [14]. For example, Y. Qiu et al. reported the fabrication of Bi₂O₃ nanowires with the specific capacitance of 693 F g^-1 (1A g^-1) [15], and N. M. Shinde et al. had prepared Bi₂O₃ nanosheets with a specific capacitance of 447 F g^-1 [16]. In alkaline solution, Bi₂O₃ is charged and discharged by the redox reaction between Bi₂O₃ and Bi, and it can provide high energy density since the valence state of Bi element is converted between trivalence and zero valence, which indicates the feasibility of Bismuth metal as electrode material.

However, Bi₂O₃ similar to other metal oxides has very poor electrical conductivity, which limits its electrochemical performance severely. In order to overcome the poor conductivity of Bi₂O₃, researchers have designed and prepared composites of Bi₂O₃ and carbon materials to improve electrochemical performance [[17], [18], [19]]. Bi₂O₃ and carbon materials such as GO or CNF had been adopted to form a composite electrode so that the conductivity of the electrode can be improved greatly, and the specific surface area of the Bi₂O₃ electrode can be also increased to improve the contact area between electrode and electrolyte. For instance, H. Xu et al. had prepared Bi₂O₃ nanoflower/CNF by a solvothermal method, and the specific capacitance was 545 m F cm^-2 - 312 m F cm^-2 (3 mA cm^-2 - 15 mA cm^-2) [17]. R. Liu et al. used a solvothermal method to prepare Bi₂O₃/GO/BC and the specific capacitance was 6675 m F cm^-2 or 681 F g^-1 (1 mA cm^-2) [18]. X. Li et al. had prepared Bi₂O₃/CNF/CC by a solvothermal method and the capacity is 110 mA h g^-1 (2 A g^-1) [19].

Carbon nanotube (CNT) as a kind of quasi-one-dimensional material possesses excellent electrical conductivity, mechanical property, chemical stability, and extremely high specific surface area [20] so that it is suitable to prepare metal oxide and CNT composite electrode to improve electrochemical performance [[21], [22], [23], [24], [25], [26]]. In addition, the high reducibility of carbon nanotubes can be used to prepare metal nanoparticle [[27], [28], [29]]. W. Chen et al. prepared Fe/CNT by annealing treatment of Fe₂O₃/CNT [27,28]. J. LÜ et al. prepared Co/CNT by annealing Co₃O₄/CNT [29]. Therefore, it is an excellent method to obtain metal Bi by autoreduction of Bi₂O₃/CNT.

In this work, we have prepared Bi₂O₃/CNT composite by facile solvothermal method, and core-shell Bi-Bi₂O₃/CNT with 3-dimensional neural network structure by annealing treatment of Bi₂O₃/CNT. The 3-dimensional neural network and the core-shell structure of Bi-Bi₂O₃ nanosphere provide an efficient electronic transfer path for the electrochemical redox reaction. The Bi-Bi₂O₃/CNT electrode reveals a high specific capacitance of 850 F g^-1 (1 A g^-1). The specific capacitance of Bi-Bi₂O₃/CNT could be 714 F g^-1 when the current density is 30 A g^-1 indicating excellent rate performance. Asymmetric supercapacitor is fabricated with Bi-Bi₂O₃/CNT as negative electrode and Ni(OH)₂/CNT as positive electrode revealing a high energy density of 36.7 Wh kg^-1 and a maximum power density 8000 W kg^-1. The asymmetric supercapacitor shows acceptable cycle performance of 72.9% after 1000 cycle.

Multi-walled carbon nanotubes (CNTs) was purchased from the TIME NANO (purity >98%, length 0.5-2 μm), All chemicals were purchased from Kmart Chemical Co. Ltd and were A.R. grade without further purification. Fabrication of Bi-Bi₂O₃/CNT: 0.97 g Bi(NO₃)₃·5H₂O was dissolved in a mixed solution consisting of 26 ml ethanol and 13 ml ethylene glycol. Subsequently, 100 mg CNTs was added into the solution above with 30 min magnetic stirring and 30 min ultrasonication. And then the solution was transferred into 50 ml Teflon-lined stainless steel autoclave at 160 °C for 5 h. After cooling to room temperature, the sample was washed several times with distilled water and ethanol and dried in a vacuum oven at 80 °C for 3 h. The dried powder was put in a tube furnace by annealing treatment at 300 °C for 2 h with heating rate of 1 °C min^-1 in N₂ atmosphere. For comparison, pure Bi₂O₃ and Bi₂O₃/CNT samples were prepared. Pure Bi₂O₃ was prepared by same solvothermal method above without CNTs added. Bi₂O₃/CNT was prepared by same solvothermal method without annealing treatment. In order to explore the formation mechanism of Bi-Bi₂O₃/CNT, the sample named as Bi₂O₃/CNT200 was prepared by 200 °C annealing of Bi₂O₃/CNT. The positive electrode of Ni(OH)₂/CNT is prepared by one-step solvothermal method. Briefly, 0.29 g Ni(NO₃)₂·5H₂O, 0.6 g urea and 50 mg CNTs were added into 40 ml deionized water with 30 min magnetic stirring and 30 min ultrasonication. Then, the solution was transferred into a 50 ml Teflon-lined stainless steel autoclave at 110 °C for 3 h. The sample was washed several times with distilled water and ethanol and dried in a vacuum oven at 80 °C for 3 h after cooling to room temperature.

The phases of samples were characterized by X-ray powder diffraction (XRD, D8 advance, CuKα = 0.1542 nm, 40 kV, 40 mA). The morphology, microstructure, and elemental mapping were characterized by field emission scanning electron microscopy (SEM, Hitachi S-4800, 5 kV), transmission electron microscopy (TEM, JEM-2100 F, 200 kV) and energy dispersive spectrometer (EDS, Oxford instrument). The surface analysis was obtained by X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC ESCALAB 250Xi). The nitrogen adsorption-desorption isotherm and pore distribution isotherm were measured by using Quantachrome NOVA 2200e.

Electrode fabrication and electrochemical measurements: 20 mg sample powders, 2.5 mg acetylene black, 4.2 mg PTFE (60% aqueous solution) were added to 2 ml ethanol for 30 min ultrasonication and then were rolled into films. The films were weighed and pressed on the foamed nickel on 20 MPa for 10 s. The Bi-Bi₂O₃/CNT negative electrode and the Ni(OH)₂/CNT positive electrode were assembled to an asymmetric supercapacitor device with 6 M KOH as the electrolyte and cellulose acetate membrane as the separator. Electrochemical tests including cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed with a CHI660E electrochemical workstation. The three-electrode test was based on 2 cm × 3 cm platinum plate as a counter electrode and calomel electrode (saturated KCl) as a reference electrode in 6 M KOH.

The specific capacitance of electrodes and asymmetric supercapacitor were calculated from the GCD curves according to Eq. (1).

Where C_s is specific capacitance (F g^-1), m is the mass of the active material, Δt is the discharge time, and ΔV is the operating voltage window.

The energy density and power density of the asymmetric supercapacitors were calculated according to Eqs. (3) and (4).

Where C is the specific capacitance of the asymmetric supercapacitor devices, V is the voltage change of the discharge, and Δt is the discharge time.

Fig. 1 shows the schematic diagram of the synthesis process of Bi-Bi₂O₃/CNT. Bi₂O₃/CNT was prepared by simple solvothermal method where Bi₂O₃ nanosheets grow on the surface of CNT, and then Bi-Bi₂O₃/CNT was obtained by annealing treatment. Amorphous Bi₂O₃ nanospheres begin to precipitate on the surface of Bi₂O₃ nanosheets during heating, and adhere to the surface of CNT with Bi₂O₃ nanosheets decompose gradually. Subsequently, amorphous Bi₂O₃ nanospheres on the surface of CNT convert to core-shell Bi-Bi₂O₃ by autoreduction between amorphous Bi₂O₃ and CNT. The amorphous Bi₂O₃ nanospheres on CNT surface were partially reduced to metal Bi as core to form the core-shell structure of Bi-Bi₂O₃. The core-shell Bi-Bi₂O₃ and CNT are assembled into core-shell Bi-Bi₂O₃/CNT with 3-dimensional neural network structure.

The morphology and nanostructure of pure Bi₂O₃, Bi₂O₃/CNT and Bi-Bi₂O₃/CNT are analyzed by using SEM. The morphology of pure Bi₂O₃ prepared by using solvothermal method is upright-standing and intersecting nanosheets in Fig. 2(a). This intersecting nanosheets network can provide a high surface area, which can increase the contact area with electrolyte and facilitate the continuous electron transfer, but the conductivity is still poor due to the long electron transfer path inside nanosheets network. Fig. 2(b) is the morphology of Bi₂O₃/CNT that Bi₂O₃ nanosheets grow and wrap on the surface of CNT, and this structure can facilitate the continuous electron transfer inside nanosheets, but the cross grown Bi₂O₃ nanosheets on the surface of CNT may have no great contact with CNT, and the CNT coated by Bi₂O₃ nanosheets have no good contact with the collector. Fig. S1 shows the TEM and HRTEM images of Bi₂O₃/CNT200. It can be observed that nanospheres appear on the surface of CNT and Bi₂O₃ nanosheets in Fig. S1(a). Amorphous Bi₂O₃ nanospheres inside dotted circles precipitated from Bi₂O₃ nanosheets in Fig. S1(b). Amorphous Bi₂O₃ nanosphere adhered on the surface the CNT with the gradual decomposition of Bi₂O₃ nanosheets in Fig. S1(c). Fig. 2(c) and (d) displays the structure and morphology of Bi-Bi₂O₃/CNT, in which Bi-Bi₂O₃ nanospheres grow on the surface of carbon nanotubes. The Bi-Bi₂O₃/CNT with 3-dimensional neural network structure composed of Bi-Bi₂O₃ nanospheres and CNT network is also confirmed by TEM in Fig. 2(e). It displays the Bi-Bi₂O₃ nanospheres growing on the surface of CNT and Bi-Bi₂O₃ with core-shell structure in Fig. 2(f). Notably, there is a great contact between the Bi core, Bi₂O₃ shell and CNT for more efficient electron transfer. Core-shell Bi-Bi₂O₃ nanospheres like cell bodies of neuronal cells can store energy, and exposed CNTs like synapses are more conducive to electron transfer with collector compared with Bi₂O₃/CNT. It can be observed that the core of the nanosphere is metal Bi that the lattice distance is 0.328 nm corresponding to (012) plane of metal Bi, and the shell of nanosphere is amorphous Bi₂O₃ without lattice. It can be also confirmed that the core of nanosphere with lattice is metal Bi from SEAD image inserted in Fig. 2(f), in which three obvious diffraction rings correspond to (012), (104) and (110), respectively. The presence of Bi₂O₃ is also confirmed from a high-angle annular dark field (HAADF) image of scanning transmission electron microscopy (STEM) and X-ray element mapping images in Fig. 2(g). The distribution of Bi element is approximately the same as O element corresponding to the nanospheres in the STEM image, and the distribution of C element corresponds to the CNTs in the STEM image, so it can be speculated that the shells of the nanospheres growing on the surface of CNTs are amorphous Bi₂O₃. Moreover, Fig. S2 shows the EDS and element proportion of Bi₂O₃/CNT200 and Bi-Bi₂O₃/CNT revealing the presence of C, Bi, and O element. The ratio of Bi and O element of Bi₂O₃/CNT200 is 2: 2.92 close to 2: 3 of Bi₂O₃ which indicates Bi₂O₃ has not been reduced by CNT at 200 °C in Fig. S2(a). The ratio of Bi and O element of Bi-Bi₂O₃/CNT is 2 : 1.65 due to Bi₂O₃ reduced to metal Bi partially in Fig. S2(b).

Fig. 3(a) represents the XRD patterns of CNT, pure Bi₂O₃, Bi₂O₃/CNT and Bi-Bi₂O₃/CNT. Typical (002) plane of the CNT is shown [30]. The fraction peaks of Bi₂O₃ and Bi₂O₃/CNT in Fig. 3(a) can be indexed to the cubic phase of δ-Bi₂O₃ (PDF#02-0542) with (111), (200) and (220) diffraction plane. The diffraction peaks of Bi-Bi₂O₃/CNT can be indexed to the hexagonal phase of metal Bi (PDF#01-0688) with (012), (104) and (110) diffraction plane. It indicated that Bi crystal will appear by annealing treatment of Bi₂O₃/CNT. According to the Debye-Scherrer formula, the average grain size of Bi₂O₃, Bi₂O₃/CNT, and Bi-Bi₂O₃/CNT are calculated to be about 5 nm, 8 nm and 32 nm indicating that Bi₂O₃ is not well crystallized. Fig. S3 shows the XRD comparison of Bi₂O₃/CNT, Bi₂O₃/CNT200 and Bi-Bi₂O₃/CNT. Bi₂O₃/CNT is indexed to cubic phase of δ-Bi₂O₃ same as Bi₂O₃/CNT, but the diffraction peak intensity of Bi₂O₃/CNT200 is lower than Bi₂O₃/CNT indicating that crystallinity of Bi₂O₃ can be reduced during heating. Fig. 3(b, c) show the nitrogen adsorption-desorption isotherms and pore-size distribution plots of Bi₂O₃, Bi₂O₃/CNT, Bi-Bi₂O₃/CNT. H₃-type hysteresis in the range of 0.6-1.0 P/P₀ confirms the involvement of the mesoporous type character in pure Bi₂O₃ and Bi₂O₃/CNT [16]. The specific surface area of pure Bi₂O₃, Bi₂O₃/CNT, Bi-Bi₂O₃/CNT are calculated to be 28.64, 65.98 and 20.79 m² g^-1 according to the Brunauer-Emmett-Teller (BET) method. The average pore-size distribution maximum and average pore volumes of Bi₂O₃, Bi₂O₃/CNT, Bi-Bi₂O₃/CNT are 7.86 nm (0.21 cm³ g^-1), 9.60 nm (0.37 cm³ g^-1) and 19.88 nm (0.05 cm³ g^-1), respectively. It suggests higher specific surface area, pore size and pore volume of Bi₂O₃ by adding CNT, but the specific surface area and pore volume are reduced by annealing treatment of Bi₂O₃/CNT to form Bi-Bi₂O₃/CNT suggesting the energy of δ-Bi₂O₃ reduced by decreasing its specific surface area and pore volume due to the instability of δ-Bi₂O₃ at high temperature. The typical survey XPS spectrum in Fig. 3(d) reveals the presence of C, Bi and O elements in the Bi-Bi₂O₃/CNT. The Bi 4f spectrum of Bi-Bi₂O₃/CNT is shown in Fig. 3(e), which exhibits the binding energy at 159.5 eV and 164.8 eV corresponding to Bi 4f_7/2 and Bi 4f_5/2 suggesting the presence of Bi₂O₃ [18]. The O 1s of Bi-Bi₂O₃/CNT at 530.3 eV and 532.0 eV confirms the presence of oxygen species in Fig. 3(f). Therefore, it suggests that the surface of Bi-Bi₂O₃ nanosphere is Bi₂O₃ shell.

It is interesting and worth discussing to obtain core-shell Bi-Bi₂O₃/CNT with 3-dimensional neural network structure. The δ-Bi₂O₃ synthesized by the solvothermal method is not well crystallized because complete δ-Bi₂O₃ crystal exists only above 730 °C indicating the instability of Bi₂O₃ nanosheets [31]. The metastable state of δ-Bi₂O₃ causes the structural transformation of δ-Bi₂O₃ during heating. However, the annealing temperature is not high enough to transform δ-Bi₂O₃ into α-Bi₂O₃ but enough into amorphous nanosphere to reduce its energy at high temperature. M. J. Chen et al prepared the Bi@amorphousBi₂O₃ nanospheres by prolonging solvothermal time and using glucose as reducing agent, which indicated the possibility of preparing Bi-Bi₂O₃ with core-shell structure by reduction method and forming Bi₂O₃ amorphous shell [32]. When the electrochemical reaction occurs in Bi₂O₃ electrode, one of the steps is 2H₂O+3BiO₂^2-→2BiO₂-+4OH-+Bi⁽⁰⁾, where Bi (II) exists and disproportionates into Bi (0) and Bi (III) [33]. In fact, Bi (II) is an unstable valence state and is highly prone to disproportionation: 3Bi²⁺→2Bi³⁺+Bi⁽⁰⁾ [34]. In addition, CNT has strong reducibility to metal oxides. For instance, Fe₂O₃ growing on the surface of CNT could be reduced into Fe with CO production from 600 °C to 800 °C [27,28]. Therefore it can be inferred that Bi₂O₃/CNT reacts as Eqs. (4) and (5) at 300 °C in N₂ atmosphere.

Fig. 4(a-c) shows the CV curves of the Bi₂O₃, Bi₂O₃/CNT and Bi-Bi₂O₃/CNT electrodes from -1.2 to -0.2 V at 1-20 mV s^-1 scan rates in an aqueous 6 M KOH electrolyte solution. A couple of typical characteristic redox peaks of Bi₂O₃ is observed between -1.2 V and -0.2 V, following the Faradaic reactions below [33].

Typically, the current response increases and redox peaks shift with increasing scan rate. Compared with pure Bi₂O₃, the redox processing of Bi₂O₃/CNT and Bi-Bi₂O₃/CNT show higher specific capacitance and better reversibility because both Bi₂O₃/CNT and Bi-Bi₂O₃/CNT show higher peak current density and smaller shift of redox peaks, which can be attributed to the introduction of CNT. In addition, the oxidation peak current of Bi-Bi₂O₃/CNT increases with the increasing scan rate, while the Bi₂O₃/CNT increases slightly at high scan rate suggesting better rate performance of Bi-Bi₂O₃/CNT. Fig. 4(d-f) show GCD curves of pure Bi₂O₃, Bi₂O₃/CNT and Bi-Bi₂O₃/CNT electrodes at a various current density from 1 A g^-1-30 A g^-1. The GCD curves are asymmetrical with voltage plateaus, which is attributed to a pseudocapacitive signature of Bi₂O₃. The discharge time of Bi₂O₃/CNT, Bi-Bi₂O₃/CNT are longer than pure Bi₂O₃, which indicates a high specific capacitance due to the presence of CNT. The calculated corresponding gravimetric capacitance based on GCD as a function of the current densities according to Eq. (1) are shown in Fig. 5(a). The specific capacitance of pure Bi₂O₃, Bi₂O₃/CNT and Bi-Bi₂O₃/CNT at 1 A g^-1 are calculated to be 713 F g^-1, 900 F g^-1, and 850 F g^-1. Compared with pure Bi₂O₃ (474 F g^-1) and Bi₂O₃/CNT (609 F g^-1) at 30 A g^-1, the specific capacitance of Bi-Bi₂O₃/CNT still retains 714 F g^-1. It indicates that there is still a higher specific capacitance than the pure Bi₂O₃, even though the mass of the active material decreases with CNT added. The specific capacitance of Bi-Bi₂O₃/CNT is the highest specific capacitance value compared with Bi₂O₃ based pseudocapacitor electrodes ever reported such as Bi₂O₃ nanowire (691 F g^-1) [15], Bi₂O₃ nanosheet (447 F g^-1) [16], Bi₂O₃ nanoflower/CNF (51 F g^-1) [17], Bi₂O₃/GN/BC (681 F g^-1) [18], Bi₂O₃ film (98 F g^-1) [35], Bi₂O₃ nanobelt (250 F g^-1) [36], Bi₂O₃ flower (29 F g^-1) [37], Bi₂O₃ nanosheet/CNF/CC (396 F g^-1) [19]. In addition, it shows excellent rate performance of Bi-Bi₂O₃/CNT that the specific capacitance calculated at 30 A g^-1 is 84% of the specific capacitance calculated at 1 A g^-1, which is higher than pure Bi₂O₃ (66%) and Bi₂O₃/CNT (68%). In order to clarify the excellent electrochemical performance of Bi-Bi₂O₃/CNT, the Nyquist plots of all electrodes in the 100 kHz to 0.01 Hz frequency range are presented in Fig. 5(b). All Nyquist plots consist of a straight line in the low-frequency region and an arc in the high-frequency region. As shown in the equivalent circuit diagram in Fig. 5(b), the internal resistance of the electrode includes series resistance (R_s) and charge transfer resistance (R_ct). The R_s of electrodes, which is related to the impedance of electrolyte depends on Nyquist plots intersection with X-axis, and the R_ct of electrodes, which is related to the current exchange defined by the Butler-Volmer equation depends on the arc diameter of Nyquist plots. In addition, the straight slope in the low-frequency range corresponds to ion diffusion from the electrolyte solution to the electrode interface [38]. The fitting results of equivalent circuit diagram reveal that the charge-transfer resistance of pure Bi₂O₃, Bi₂O₃/CNT and Bi-Bi₂O₃/CNT are 9.65 Ω, 1.88 Ω and 0.98 Ω respectively, which indicated that the R_ct of Bi₂O₃ can be reduced significantly with CNT added, and the R_ct of 3-dimentinal neural network of Bi-Bi₂O₃/CNT could be lower than Bi₂O₃/CNT. Moreover, the straight slope of Bi-Bi₂O₃/CNT is also steeper than pure Bi₂O₃ and Bi₂O₃/CNT, indicating more efficient ion transport of Bi-Bi₂O₃/CNT. The above analysis shows that the specific surface area of Bi-Bi₂O₃/CNT is lower than pure Bi₂O₃ and Bi₂O₃/CNT, but it has higher specific capacitance, better rate performance. and smaller R_ct, which reveals that there is a great improvement in electrochemical performance by forming core-shell Bi-Bi₂O₃/CNT with 3-dimensional neural network structure. Fig. 5(c) shows the scheme illustrating the benefits of Bi-Bi₂O₃/CNT neural network nanostructure. The metal Bi core of Bi-Bi₂O₃/CNT is reduced by CNT, so the contact between the metal Bi core and the CNT is great. In the electrochemical reduction reaction, electrons can be transferred to the Bi₂O₃ shell through CNT and metal Bi cores where R_ct can be reduced greatly by two electron transfer paths. Similarly, electrons can be transferred from the metal Bi on the surface of nanosphere to CNT through the metal Bi nanosphere in the electrochemical oxidation reaction.

Fig. 6(a) shows the CV curves of negative electrode (Bi-Bi₂O₃/CNT) and positive electrode (Ni(OH)₂/CNT), which shows higher operating potential extended to 1.6 V due to different potential ranges of the positive and the negative electrodes. It should be noted that the area of the CV curve and current density peak of the negative electrode are bigger than the positive electrode at 20 mV s^-1 indicating the high capacitance of Bi-Bi₂O₃/CNT. The CV curves of ASC device were measured at 10 mV s^-1 - 200 mV s- shown in Fig. 6(b). Clearly, well defined redox reaction peaks are observed, suggesting the capacitance of ASC device attributed to pseudocapacitance. The galvanostatic charge/discharge (GCD) curves at 1 A g^-1 - 10 A g^-1 are shown in Fig. 6(c) showing nonlinear profiles and discharge plateau at lower current densities which confirm the contribution of metal compound agreed with CV curves. The specific capacitance of the ASC device as a function of current density is shown in Fig. 6(d), which display a high capacitance with 104 F g^-1 (1 A g^-1) and 51 F g^-1 (10 A g^-1). Meanwhile, as is shown in Fig. 6(e), the ASC device exhibits acceptable electrochemical stability that the capacitive retention is still as high as 72.9% even after 1000 cycles at 1 A g^-1. The inset image in Fig. 6(e) provides the evidence that this ASC device consisting of Bi-Bi₂O₃/CNT//Ni(OH)₂/CNT could be applied to practice, in which the green LED can light at least 10 min. The Ragone plot is shown in Fig. 6(f) which reveal the energy density and power density value of this ASC device, confirming as high as 36.7 Wh kg^-1 (800 W kg^-1) and 18 Wh kg^-1 (8000 W kg^-1). In addition, the energy density and power density value of other similar system of ASC devices reported previously are observed in Fig. 6(f) and Table S1 [[9], [10], [11], [12],17,37,39,40].

In summary, we have prepared Bi₂O₃ nanosheets growing on the surface of CNT by an simple solvothermal method and Bi-Bi₂O₃/CNT with 3-dimensional neural network structure by annealing treatment of Bi₂O₃/CNT. The Bi-Bi₂O₃/CNT shows a high specific capacitance of 850 F g^-1 (1 A g^-1) and good rate performance with the 714 F g^-1 at 30 A g^-1, which is attributed to the benefits of double contact of Bi₂O₃ shell with CNT and metal Bi core. Compared with pure Bi₂O₃, Bi₂O₃/CNT and other Bi₂O₃ electrodes ever reported, Bi-Bi₂O₃/CNT possesses the highest specific capacitance at various current density except at 1 A g^-1. The ASC device included Bi-Bi₂O₃/CNT negative electrode and Ni(OH)₂/CNT positive electrode shows a high voltage window of 1.6 V, a high energy density of 36.7 Wh kg^-1 and a maximum power density of 8000 W kg^-1. This work verifies that Bi-Bi₂O₃/CNT with 3-dimensional neural network structure serve as an ideal candidate for advanced energy storage device with high performance.

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jmst.2020.02.007.