The preparation procedure of Bi₂O₃ is illustrated in the scheme 1(a). Bi₂O₃ was fabricated by annealing Bi₂O₂CO₃ at 500°C for 2 h in air atmosphere. Bi₂O₂CO₃ was synthesized based on the articles published before with some minor modifications [25,26]. The detailed preparation process of Bi₂O₂CO₃ is depicted as follows. Firstly, the reagents of Bi(NO₃)₃·5H₂O (0.97 g), Na₃C₆H₅O₇·2H₂O (2.01 g), urea (0.48 g) and polyvinylpyrrolidone (1.02 g) were dispersed in turn into the mixed solvents of H₂O (60 mL) and C₂H₆O₂ (12 mL). After stirred for 45 min, above reaction liquid was treated at 180°C for 24 h by the solvothermal method. After naturally cooling, Bi₂O₂CO₃ was obtained by rinsing with H₂O and drying above precipitates.

The synthesis process for cathode CoNi-CoNiO₂@NCLDH grown on NF is presented as follows. 1 mmol of Ni(NO₃)₂·6H₂O, 2 mmol of Co(NO₃)₂·6H₂O and 320 mg of hexamethylenetetramine were first dissolved into 70 mL of H₂O. Subsequently, the pretreated NF was submerged into the above solution followed by hydrothermal treatment at 90°C for 12 h. After naturally cooling, the NF was treated with sonication and rinsed by distilled water/ethanol. Then, that NF with product was carbonized at 400°C for 1 h in flowing N₂ stream to prepare CoNi-CoNiO₂ grown on NF, and the mass of CoNi-CoNiO₂ is about 0.85 mg/cm². The NF coated by CoNi-CoNiO₂ was immersed into the identical reaction solution as described above followed by hydrothermal treatment at 90°C for 12 h. After cooling, sonication, rinsing and drying, the resulting sample was labeled as CoNi-CoNiO₂@NCLDH with the total active materials of 1.72 mg/cm². The above preparation procedure of cathode can also be observed in the scheme 1(b).

X-ray diffraction (XRD) was tested to characterize crystal structure, and elemental composition were characterized by X-ray photoelectron spectroscopy (XPS). The micromorphology of electrode active materials was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Cyclic voltammogram (CV), galvanostatic charge/discharge (GCD), cycling durability and electrochemical impedance spectroscopy (EIS), were evaluated on the electrochemical working station (CHI 660E) with the three-electrode test system. The three-electrode system mainly consists of platinum sheet as the counter electrode, Hg/HgO electrode with double salt bridge as the reference electrode, and one working electrode, in which 6 M KOH aqueous solution is adopted as the electrolyte. Mixing Bi₂O₃, acetylene black and polytetrafluoroethylene (with a ratio of 8:1:1 by weight) homogeneously into 1-methyl-2-pyrrolidone with slurry state to fabricate working electrode. Then, the slurry was coated onto NF (1 cm × 1 cm), dried under vacuum at 80°C for 12 h and then pressed under 10 MPa pressure. The average mass loading of Bi₂O₃ in the anode is about 3.52 mg/cm².

The ARNBBs was assembled with CoNi-CoNiO₂@NCLDH grown on NF as the cathode (1 cm × 1 cm), Bi₂O₃ single electrode as the anode (1 cm × 1 cm), and 6 M KOH aqueous solution as the electrolyte. The two electrodes were pre-activated at a scan rate of 50 mV/s for 100 cycles before assembly. The total active materials mass of ARNBBs is about 3.08 mg. The assembly illustration is demonstrated in the scheme 1(c).

Several characteristic peaks in Fig. 1a correspond well to the standard pattern of Bi₂O₃ (JCPDS no. 71-2274) [27], indicating that Bi₂O₃ with high purity and crystallinity can be prepared by annealing Bi₂O₂CO₃. The survey spectrum of XPS in Fig. 1b suggests anode active materials are made up of Bi and O elements. Two peaks with an energy difference of 5.3 eV indicate Bi exists as the valence of +3[23] (Fig. 1c). The fitting peak of O 1s at 529.4, 530.8 and 532.3 eV in Figure 1d can be designated to Bi-O bond, oxygen adsorbed on Bi₂O₃, together with oxygen in H₂O, respectively [12]. The above results indicate Bi₂O₃ with high purity and crystallinity can be transformed from Bi₂O₂CO₃. Bi₂O₃ exhibits ‘cookie’-like heterogeneous morphology with locally dense regions embedded in a loosely packed porous matrix as presented in Fig. 2 (a and b), and the structure is not too dense ascribed to the pores in the skeleton and cracks from adjacent skeletons, which contributes to accelerated moving of electrolyte ions and effective utilizing of Bi₂O₃ [28,29]. The porous structure of Bi₂O₃ is also verified from TEM images (Fig. 2c). The crystal plane distance of 0.195 nm agrees well with the (041) plane of Bi₂O₃ in Fig. 2d, and bright diffraction spots further validate the relatively high crystallinity of Bi₂O₃[30,31].

The CV curves of Bi₂O₃ all exhibit obvious redox peaks at series of scan rates in Fig. 3a, indicating the Faradic redox reaction occurs [3,32]. When scan rate goes up from 1 mV/s to 10 mV/s, the peak voltage between oxidation/reduction peak increase gradually, which can be put down to the internal resistance and the polarization of Bi₂O₃ anode [31,33]. Additionally, the shape of redox peaks can be well maintained and peak currents increase gradually with elevating scan rate, manifesting excellent rate capability of Bi₂O₃ [34]. The b value for Peak1, Peak2 and Peak3 is about 0.6475, 0.5088 and 0.5043, respectively, signifying the diffusion-controlled process mainly dominates electrochemical reactions of Bi₂O₃ anodes [1,35] (Fig. 3b). The apparent voltage plateau in GCD profiles in Fig. 3c further verifies presence of Faradic redox reaction [3]. The specific capacity shows a fading tendency with elevating current density in Fig. 3d, and Bi₂O₃ anode delivers the specific capacity of 266.7, 257.6, 250.2, 243.9, 243.5, 235.3, 228.6, 214.6 and 203.3 mAh/g at 2, 3, 4, 5, 6, 8, 10, 15 and 20 A/g, respectively. The Bi₂O₃ anode exhibits a smaller semicircular diameter (Rct = 1.56 Ω) and a lower real-axis intersection (R1 = 0.76 Ω) in the high-frequency region, suggesting low internal resistance [1,31] (Fig. 3e). The Rct and R1 value are lower than that of Bi₂MoO₆ (Rct = 8.6 Ω and R1 = 0.78 Ω) [31]. Bi₂O₃ displays about 62.6% capacity retention after 1900 cycles at 15 A/g and nearly 100% coulombic efficiency (Fig. 3f). The above electrochemical results indicate Bi₂O₃ is one strong anode materials for aqueous energy storage equipment.

The crystal structure of cathode active materials was studied by XRD. Fig. 4a shows that the nickel-cobalt layered double hydroxide can be transformed into the composite of CoNi alloy and CoNiO₂ after carbonization at 400°C in nitrogen atmosphere. Fig. 4b shows that NCLDH is successfully grown on the surface of CoNi-CoNiO₂. Fig. 4c demonstrates the presence of Ni, Co and O elements on the surface. In Fig. 4d, the peaks at 775.2 eV and 796 eV go well with Co⁰ [36], the peaks at 782 eV and 798.2 eV are designated to Co²⁺ [37,38], the peaks at 780.9 eV and 797 eV are apportioned to Co³⁺ [30,39], and the two peaks at 786.4 eV and 802.8 eV belong to satellite peaks [40]. Similarly, as demonstrated in Fig. 4e, the peaks at 854.8 eV and 872.8 eV belong to Ni⁰ [41], the peaks at 855.8 eV and 873.9 eV are concerned with Ni²⁺ [38,42], the peaks at 856.8 eV and 877.1 eV are in line with Ni³⁺ [38,43], and the two peaks at 880.2 eV and 861.9 eV are satellite peaks [38]. As shown in Fig. 4f, the O 1s high-resolution spectrum can be fitted into three peaks. The peaks at 531.0 eV (O1), 531.7 eV (O2) and 532.5 eV (O3) represent metal-oxygen bonds, OH^- and multiple physically/chemically adsorbed H₂O on or near the surface of the materials, respectively [38,44]. The above test results suggest that the cathode active materials CoNi-CoNiO₂@NCLDH are successfully prepared.

As observed in Supplementary Fig. 1(a~c), the NF is completely covered by grass-like CoNi-CoNiO₂, which is assembled with crossed nanosheets. The TEM images in Supplementary Fig. 1(d~e) further confirms the lamellar structure of CoNi-CoNiO₂, and plenty of pores are distributed on the nanosheets. The HRTEM images in Supplementary Fig. 1f exhibit the interplanar spacing of 0.21, 0.24 and 0.20 nm, which can be designated to the (200) and (111) crystal plane of CoNiO₂ and (111) crystal plane of CoNi alloy, respectively [44,45]. The images in Fig. 5 present microstructure of cathode active material CoNi-CoNiO₂@NCLDH. The micromorphology of CoNi-CoNiO₂@ NCLDH is similar to that of CoNi-CoNiO₂, but the arrangement of nanosheets is more disordered than CoNi-CoNiO₂ (Fig. 5(a~d)). TEM image in Fig. 5e further verifies the lamellar structure of CoNi-CoNiO₂@NCLDH, indicating that NCLDH also presents a lamellar structure when grown on the thin CoNi-CoNiO₂ substrate. A crystal plane interval of 0.27 nm in HRTEM image in Fig. 5f is delegated to the (101) crystal plane of NCLDH.

CV curves of CoNi-CoNiO₂ are shown in Supplementary Fig. 2a. Clear and distinguishable redox peaks indicate Faradic redox reaction occurs of CoNi-CoNiO₂ [38]. Even at 100 mV/s, CV curves can maintain well the initial shapes, indicating the admirable rate capability of CoNi-CoNiO₂ [34]. Supplementary Fig. 2b shows an obvious charge/discharge voltage platform in GCD profiles of CoNi-CoNiO₂. With enlarging current density, voltage platform in discharge curves moves to the low potential direction step by step. The specific discharge capacity of CoNi-CoNiO₂ is 62.2, 60.0, 58.5, 57.3, 56.3, 55.2, 53.6, 51.9, 49.2 and 46.7 mAh/g at 1, 2, 3, 4, 5, 6, 8, 10, 15 and 20 A/g, respectively, and the rate capability is 75.0% during 1~20 A/g. The above electrochemical test results show that CoNi-CoNiO₂ can be used as one promising skeleton materials to further grow active materials NCLDH. The CV curves of CoNi-CoNiO₂@NCLDH in Fig. 6a possess obvious redox peaks, indicating Faradic redox reaction occurs to store energy [32,33]. As the scan rate goes up, the oxidation/reduction peak shift to ends of the potential window, which is caused by the internal resistance of active materials and polarization during electrochemical reactions. Moreover, with promoting scan rate, the peak current of oxidation peak together with reduction peak gradually increases, and the shape of CV curve is well kept, indicating that CoNi-CoNiO₂@NCLDH has admirable rate capability [34]. The calculated b value is b1=0.561 for anodic peak and b2=0.603 for cathodic peak, indicating that energy storage behavior of CoNi-CoNiO₂@NCLDH is both affected by the diffusion process and surface capacitive effect [32]. Fig. 6c depicts GCD curves of CoNi-CoNiO₂@NCLDH. The obvious charge/discharge voltage platforms further affirm occurrence of Faraday redox reaction. With enlarging current density, the platform in discharge profile gradually moves to lower potential. As shown in Fig. 6d, the specific discharge capacity is 249.7, 241.5, 234.9, 228.4, 220.4, 211.4, 192.0, 173.8, 136.4 and 112.0 mAh/g at 1, 2, 3, 4, 5, 6, 8, 10, 15 and 20 A/g, respectively. The rate capability is 44.9% in 1~20 A/g. The specific capacity retention of CoNi-CoNiO₂@NCLDH is about 71.8% after 2000 cycles at 20 A/g (Fig. 6e). Fig. 6f shows the EIS diagram of CoNi-CoNiO₂ and CoNi-CoNiO₂@NCLDH. The lower intersection point on the X-axis indicates that CoNi-CoNiO₂@NCLDH has better conductivity as cathode active materials [25,31].

The ARNBBs were assembled with heterostructure nanoarrays of CoNi-CoNiO₂@NCLDH grown on NF as the cathode, single Bi₂O₃ electrode as the anode together with electrolyte of 6 M KOH aqueous solution. The CV profiles of single electrodes at 5 mV/s both present obvious redox peak within their individual potential window in Fig. 7a. As demonstrated in Fig. 7b, apparent redox peaks appear in the potential window (0~1.6 V) [12]. As scan rate goes up to 20 mV/s, CV curves together with redox peaks can keep the similar shape. The GCD curves also display charge/discharge voltage plateaus in Fig. 7c and d. The specific capacity of ARNBBs decreases by degrees as current density goes up (Fig. 7e). The specific capacity is 50.4, 42.8, 38.0, 34.4, 31.5, 29.2, 25.3 and 21.9 mAh/g at 1, 2, 3, 4, 5, 6, 8 and 10 A/g, respectively, also exhibiting the decaying tendency as enlarge current density. The short quasi-semi-circular arc (R = 3.08 Ω) along with lower intersection of the semicircular arc with the real axis (R1 = 0.41 Ω) in Fig. 7f indicate the low internal resistance of ARNBBs and rapid electrolyte ion diffusion rate in ARNBBs [31]. After 1800 cycles at 2 A/g, ARNBBs deliver a capacity retention of 50.7% with 98.9% coulombic efficiency (Fig. 7g). This battery possesses a maximum energy density of 34.8 Wh/kg along with power density of 691.0 W/kg at 1 A/g. A power density of 4400.5 W/kg along with energy density of 9.7 Wh/kg can be yielded at 10 A/g (Fig. 7h). One LED bulb can be lighted by two aqueous ARNBBs connected in series, indicating promising potential application of this aqueous battery system (Fig. 7i).