Introduction

Miniaturized energy storage devices, such as micro-batteries (MBs) and micro-supercapacitors (MSCs), have garnered growing research interest owing to their potential applications in intelligent microelectronics, including micro-robotics, smart medical implants, intelligent electronic skins, deep-brain analysis, human activity monitoring, and wireless sensors1,2,3,4,5. Due to their high power densities, excellent charge/discharge rates, and long cycle lifetimes, MSCs are expected to play a pivotal role in these advanced fields. However, their areal energy density typically remains below 100 μWh cm−2 6,7,8, with some devices even falling below 10 μWh cm−2 9,10, significantly limiting their applications. To address these limitations, researchers are focused on developing MBs that offer higher energy density and more stable voltage output, thereby providing a reliable long-term energy supply for microelectronic devices.

Subsequently, various advanced fabrication techniques such as screen printing, imprint lithography, and holographic lithography with conventional photolithography, have been employed to construct a series of organic lithium ion MBs composed of lithium titanate||lithium iron phosphate, lithium||V2O5 and nickel-tin||lithium manganate11,12,13,14. However, the areal capacity and energy density of these organic lithium ion MBs remain below 200 μAh cm−2 and 360 μWh cm−2, respectively11,12,13,14. In addition, organic dual-ion MBs, using graphite as both the positive and negative electrode with lithium hexafluorophosphate electrolytes, achieve a high energy density of 873.3 μWh cm−2 due to their high working voltage (~5 V)15. Moreover, organic sodium ion MBs have also been assembled using mask-assisted filtration and 3D printing techniques16,17. Notably, 3D printing enables thick microelectrodes, allowing the sodium ion MBs to deliver high areal capacity of 4500 μAh cm−2 and high energy density 7330 μWh cm−2 16,17. Despite these advancements in electrochemical performance, the organic electrolytes used in these MBs present significant challenges due to their toxicity, volatility, and flammability, raising serious safety concerns and restricting their broader application.

Given the advantages of low cost, high theoretical capacity, and ease of operation in ambient conditions, researchers have increasingly turned to developing more environmentally-friendly and safer aqueous zinc-based MBs3,18,19. To enhance the energy density of these MBs, a series of advanced positive electrode materials, including MnO23,20,21,22,23,24, vanadium-based materials24,25,26, polyaniline24, NH4CuHCF27, LiMn2O428, LiFePO428, Ni/Co materials29,30 have been explored. Unfortunately, due to the lack of a reasonable design, such as inadequate load improvement, suboptimal fabrication processes, insufficient exploration of new battery reactions, and inefficient utilization of the limited microelectrode footprint, the energy density achieved by these MBs remain below 1100 μWh cm−2 3,20,21,22,23,24,25,26,27,28,29,30. In terms of areal capacity, Wang et al. optimized ink formulations and printing parameters to effectively increase the loading of polyvinylpyrrolidone-induced ammonium vanadate nanobelt positive electrodes for Zn ions MBs using 3D printing technology, achieving ahigh areal capacity of 4020 μAh cm−2 31. Furthermore, in situ electrochemical deposition and dual-plating approaches have been proposed for the direct fabrication of Zn||I2 and Zn||Br2 MBs4,32, attaining high areal capacity of 2220 μAh cm−2 and energy density of 3654 μWh cm−2. While these advances have significantly contributed to the progress of zinc-based MBs, a substantial performance gap persists when compared to the superior organic sodium-ion MBs. Moreover, for both the reported aqueous zinc-based MBs and organic lithium/sodium-ion MBs, the maximum reported areal capacity and energy density are still below 5000 μAh cm−2 and 7500 μWh cm−2, respectively (Fig. 1a)3,4,5,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32. Therefore, it remains highly challenging to develop a simple yet effective strategy to achieve high areal capacity above 15,000 μAh cm−2 and energy density of 15,000 μWh cm−2 for aqueous zinc-based MBs (Fig. 1a).

Fig. 1: Current situation of MBs and fabrication of microelectrodes.
Fig. 1: Current situation of MBs and fabrication of microelectrodes.
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a Schematic illustration of the design of aqueous zinc-based micro-batteries based on the in situ conductivity enhancement-assisted double-cell reaction strategy. b The fabrication process of Zn||Bi2O3@Ag2O-based microelectrodes. This innovative approach challenges the conventional micro-battery configuration by sequentially coupling two battery reactions within a single microdevice, thereby optimizing the use of the previously inactive reaction chamber for enhanced areal capacity/energy density. More importantly, the Ag2O conversion reaction in the first step significantly improves the discharge capacity of the subsequent conversion reaction, achieving nearly an order-of-magnitude increase compared to single Zn||Bi2O3 MBs. As a result, the total capacity of the dual-reaction microdevice exceeds 2.1 times the combined discharge capacity of two individual MBs.

In most aqueous zinc-based MBs and organic lithium/sodium-ion MBs, a single charge-discharge cycle typically corresponds to the reversible redox reaction of one battery, limiting the potential for significant breakthroughs in capacity and energy output per unit area (Fig. 1a). To meet higher energy demands, these MBs must be connected in series or parallel, which increases the overall footprint of microdevices, reduces space efficiency, and leads to excessive material waste. The strategy of integrating double-cell electrochemical reactions within a single battery offers a promising solution to this challenge. However, the successful implementation of this approach requires the fulfillment of several critical conditions. First, the electrolyte: the two electrochemical reactions must either share a common electrolyte or use two separate electrolytes that are compatible and can be well-mixed without interfering with each other. Second, the electrodes: both electrodes must be stable in air, compatible with each other, and non-reactive under operational conditions, allowing for the construction of microelectrodes using various fabrication techniques. Third, the electrochemical window: both reactions must be able to operate stably within the same electrochemical window without mutual interference. Ideally, the redox reaction sites should be distinct, with sequential reactions being the most favorable. This allows for the straightforward verification of the successful integration of the double-reaction system through direct analysis of cyclic voltammetry or charge/discharge profiles. Fourth, the reaction environment: the electrochemical conditions for both reactions, including solution concentration, pH, and reaction temperature, should be either identical or closely aligned. Fifth, the reaction products: the products generated by the first reaction must not interfere with or degrade the second reaction, and vice versa. Ideally, the products of the first reaction should enhance the conductivity of the second reaction system, thereby improving its overall capacity. Achieving such a design will be optimal, but it also poses significant challenges in terms of materials compatibility and reaction dynamics.

Bismuth oxide (Bi2O3), due to its low cost and non-toxicity, has been widely studied in photocatalysis33. More recently, it has been explored in electrochemical systems: as an additive to Zn negative electrodes, Bi2O3 improves cycling stability by enhancing conductivity and suppressing hydrogen evolution34,35,36, while Bi and Bi2O3 have also been employed as negative electrodes in various supercapacitors and batteries37,38,39,40,41,42,43. Notably, Bi2O3 has been utilized as a positive electrode material in aqueous Zn-based batteries43,44,45,46, where it delivers promising electrochemical performance. When coupled with Ni foam, Bi2O3 exhibits excellent mass and areal capacities43, underscoring its potential as a high-performance electrode material. The reversible conversion between Bi2O3 and Bi involves the transfer of up to six electrons, enabling high theoretical capacity and rendering it attractive for next-generation batteries. Unlike conventional batteries, which can employ porous metallic current collectors to maintain conductivity at high active material loadings, MBs must simultaneously address microfabrication constraints, limited material loading at the microscale, and the integration of two distinct microelectrodes on a single substrate4,32. Another key limitation lies in the intrinsically poor conductivity of Bi2O3, which makes it difficult to achieve high areal capacity and energy density in Zn||Bi2O3 MBs simply by increasing the material loading. In contrast, while pure silver oxide (Ag2O) electrodes also exhibit limited conductivity, the conversion product, silver, serves as an excellent conductive agent, offering a potential solution to overcome the electrochemical performance limitations associated with Bi2O3.

Herein, based on the principles for constructing MBs with dual-cell reactions discussed above, we propose an in situ conductivity enhancement-assisted double-cell reaction strategy to construct high-performance Zn||Bi2O3@Ag2O MBs that integrate the two conversion reactions of Bi2O3 and Ag2O within a single microdevice. This strategy significantly reduces the reliance on inactive battery materials such as substrates and packaging materials, substantially enhancing the space efficiency of the micro-battery. Notably, in the first conversion reaction, Ag2O with relatively low conductivity is converted into highly conductive metallic silver. Simultaneously, the Bi2O3@Ag2O composite is transformed into Bi2O3@Ag, with metallic silver tightly surrounding Bi2O3, leading to a substantial increase in the conductivity of the composite. Unlike simply combining Zn||Ag2O and Zn||Bi2O3 MBs, during the second conversion reaction in the Zn||Bi2O3@Ag2O MBs, the in situ conductivity enhancement induced by the Ag2O conversion reaction achieves a nearly order-of-magnitude increase in the capacity contribution of Bi2O3, compared to pure Zn||Bi2O3 MBs at the same current density. Moreover, the total capacity of Zn||Bi2O3@Ag2O-based MBs exceeds above 2.1 times of the combined discharge capacities of the individual Zn||Bi2O3 and Zn||Ag2O MBs. Consequently, the constructed microdevice delivers a high areal capacity of 16,561.5 μAh cm−2 at a high current density of 12 mA cm−2. Even at the current density of 30 mA cm−2, the microdevice retains the areal capacity of 3473.8 μAh cm−2. This robust rate capability enables the Zn||Bi2O3@Ag2O MBs to deliver a high areal energy density of ~19,000 μWh cm−2, even at a high-power density of ~13,550 μW cm−2. When the energy density decreases to 3410.4 μWh cm−2, the microdevice achieves a high power density of 23,231.8 μW cm−2, rivaling the performance of state-of-the-art MSCs. Additionally, a single Zn||Bi2O3@Ag2O microdevice can power a timer for over 3780 min (more than 2.5 days) without any loss in brightness. Moreover, just two MBs connected in series are sufficient to power 200 light-emitting diodes in red, yellow, green, blue, or white. Additionally, we demonstrate the intelligent integrated application of the Zn||Bi2O3@Ag2O MBs by powering a commercial wireless smart sensor with direct mobile phone connectivity, enabling the monitoring of the motion states of toy cars and human bodies.

Results

Preparation process and structural characterization of microelectrodes

The fabrication process of the highly conductive Zn||Bi2O3@Ag2O microelectrodes is schematically illustrated in Fig. 1b. The high-crystalline Bi2O3 electrode materials are synthesized using a straightforward precipitation method at room temperature (Fig. S1). Flexible graphite paper, known for its excellent conductivity, is precisely patterned into interdigital microelectrodes within seconds using laser etching technology based on a pre-designed pattern (Figs. 1b and S2). Next, the interdigital negative electrode and positive electrode are prepared by applying inks containing zinc powder, Bi2O3, Ag2O, polyvinylidene fluoride, super P, and N-methyl-pyrrolidone onto the graphite-based microelectrodes. The Bi2O3@Ag2O positive electrode and Zn negative electrode on graphite paper are carefully aligned on a customized stainless-steel template attached to polyethylene terephthalate (PET) tape (Fig. 1b). After the template is removed, the PET-supported Zn||Bi2O3@Ag2O microelectrodes are obtained (Fig. 1b). Notably, the zinc powder selected for the negative electrode features a spherical micro-morphology (Fig. S3).

As shown in Fig. S4, a single interdigital microelectrode is remarkably small. By taking advantage of the fabrication strategy combining high-resolution laser direct writing and template method, six microelectrodes can be easily integrated onto a flexible PET tape, which can be wrapped around the wrist and bent into various shapes (Fig. S4). Scanning electron microscopy (SEM) and elemental mapping images show that the Bi2O3@Ag2O positive electrode and Zn negative electrode are evenly and alternately integrated (Fig. S5). Notably, as PET contains abundant oxygen, elemental mapping of O across the entire microelectrode cannot accurately delineate the electrode outline. To more clearly define the electrode geometry, elemental maps of Zn, Bi, and Ag, present in the negative electrode and positive electrode but absent from the substrate, are presented in Fig. S5. In Fig. S6, silver oxide presents a morphology of interconnected nanospheres with smooth surfaces. Notably, the synthesized Bi2O3 powder appears yellow (Fig. S1), and its X-ray diffraction (XRD) pattern matches well with α-Bi2O3 (PDF# 71-0465, monoclinic) (Fig. S7). The Bi2O3 forms microrods resembling wheat-like structures (Fig. S6). Transmission electron microscopy (TEM) further confirms the microrods structure, showing a lattice spacing of approximately 0.322 nm (Fig. S8). Notably, the synthesized Bi2O3 exhibits enhanced conductivity and reduced size compared to the commercial counterpart (Figs. S6 and S9). The TEM image of Ag2O similarly displays interconnected nanospheres, with a lattice spacing of approximately 0.270 nm (Fig. S10). Considering the well-established use of commercial Ag2O in high-performance Zn||Ag2O batteries47,48, we employed commercial Ag2O in this study. Moreover, XRD analysis of the Bi2O3@Ag2O positive electrode, in comparison with the PDF standard card, verifies the successful incorporation of both Ag2O and Bi2O3 (Fig. S11).

Capacity enhancement mechanism of the in-situ conductivity enhancement-assisted double-cell reaction strategy

Then, a polyvinyl alcohol/KOH/zinc acetate gel electrolyte with high ionic conductivity is synthesized to enable efficient ion transport (Fig. S12). Thanks to its strong adhesive properties (Fig. S13), this electrolyte integrates seamlessly with various microelectrodes, facilitating the construction of Zn||Bi2O3, Zn||Ag2O, and Zn||Bi2O3@Ag2O-based MBs. As illustrated in Fig. 2a–c, the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) profiles of Zn||Bi2O3 MBs reveal a pair of redox peaks corresponding to the conversion between Bi2O3 and Bi. Although this reaction involves a six-electron transfer, the discharge capacity only reaches 614.4 μAh cm−2 at 18 mA cm−2 (Fig. 2c), suggesting that the full potential of the six-electron reaction is not fully realized, likely due to the limited electronic conductivity of Bi2O3 (Fig. 2d). In parallel, Zn||Ag2O MBs are assembled and their electrochemical performance is evaluated in Fig. S12. While this microdevice can achieve a high capacity nearing 4671 μAh cm−2 at 18 mA cm−2 (Fig. S14), significant performance improvements remain challenging due to limitations inherent in the single-cell reaction. To overcome these limitations, Zn||Bi2O3@Ag2O-based MBs featuring a double-cell reaction, are constructed (Fig. 2e). The CV curves for these microdevices exhibit two distinct pairs of redox peaks (Fig. 2f), corresponding to the Bi/Bi2O3 and Ag/Ag2O transformations. Notably, a pronounced Ag2O oxidation peak appears at 1.8 V, with no detectable oxygen evolution signal even beyond 2 V, confirming the absence of water decomposition within this voltage window. This configuration delivers two flat discharge plateaus and achieves a remarkable areal capacity of 11393.8 μAh cm−2 at 18 mA cm−2 (Fig. 2j), comparable to the capacities of Zn||Bi2O3 and Zn||Ag2O MBs (Fig. S15). Importantly, the electrochemical process involves only the reversible conversion reactions of Ag2O/Ag and Bi2O3/Bi, with no side reactions between Ag2O and Bi2O3 or their discharge products (Figs. S11 and S16). The capacity contribution of Ag2O in Zn||Bi2O3@Ag2O MBs closely aligns with that of pure Zn||Ag2O MBs. Notably, the Bi2O3 contribution in Zn||Bi2O3@Ag2O MBs reaches 6597.1 μAh cm−2, more than 10.7 times that of Zn||Bi2O3 MBs alone (Figs. 2d and  S17). Furthermore, the total areal capacity and energy density of Zn||Bi2O3@Ag2O MBs exceeds 2.1-fold of the combined value of the two individual MBs (Fig. S18). For the Zn||Bi2O3@Ag2O MBs, the Bi2O3 mass loading is approximately 3.6 mg, yielding a theoretical discharge capacity of 1.24 mAh, corresponding to a theoretical specific capacity of 345 mAh g−1 (Fig. S19). Benefiting from the in situ conductivity enhancement provided by Ag generated from Ag2O during the discharging process, the Bi2O3 component in Zn||Bi2O3@Ag2O MBs delivers 1.18 mAh (328 mAh g−1) even at ahigh areal loading of 40 mg cm−2 (Fig. S19a), achieving ~95% of the theoretical specific capacity. In contrast, without Ag2O, Bi2O3 exhibits a measured capacity of only 0.128 mAh (35.5 mAh g−1) under the same loading (Fig. S19b), corresponding to merely 10.3% of the theoretical value. These results demonstrate that the in situ conductivity enhancement-assisted double-cell reaction strategy enabled by Ag2O incorporation is crucial for boosting the capacity of Bi2O3 by an order of magnitude even under high loading, thereby enabling the Zn||Bi2O3@Ag2O MBs to achieve high areal/mass capacity and energy density. Notably, as the Bi2O3 mass loading increases, conventional conductive agents alone cannot effectively improve internal electron transport within the electrode (Fig. S20a), explaining the low discharge capacity observed in pure Zn||Bi2O3 MBs containing Super P. To explore the underlying enhancement, we examine the reaction mechanism in the Zn||Bi2O3@Ag2O-based MBs (Fig. 2d). By incorporating the Ag2O electrode into the Bi2O3 system, the Ag produced during the first discharge reaction effectively surrounds the Bi2O3 (Figs. 4 and 5 for further details). This not only mitigates the inherent conductivity limitations of Bi2O3 but also facilitates the utilization of the six-electron transfer reaction, thereby enhancing the overall reaction kinetics (Figs. 2d and S20b). To verify this mechanism, we prepared Bi2O3, Bi2O3@Ag2O, and Bi2O3@Ag microelectrodes with the same electrode loading as used in MB measurements (Fig. S21), rather than directly coating Bi2O3, Bi2O3@Ag2O, and Bi2O3@Ag onto graphite paper without matching the loading to that of the microelectrodes. The resistance measurements of the microelectrodes show that incorporating Ag into Bi2O3 significantly reduces electrode resistance compared with both pure Bi2O3 and Bi2O3@Ag2O (Fig. S21), further confirming that Ag enhances conductivity and then electrochemical performance. Additionally, the electrical resistance, resistivity, and conductivity of Bi2O3, Bi2O3@Ag2O, and Bi2O3@Ag composites are further measured by four-probe instrument (Fig. S22), revealing that the Bi2O3@Ag composite exhibits the highest conductivity, along with the lowest resistance and resistivity.

Fig. 2: The capacity enhancement mechanism of the in situ conductivity enhancement-assisted double-cell reaction strategy.
Fig. 2: The capacity enhancement mechanism of the in situ conductivity enhancement-assisted double-cell reaction strategy.
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a The schematic diagram, b CV curves at 0.6 mV s−1 and c) GCD profiles at 18 mA cm−2 of Zn||Bi2O3 MBs. d Schematic diagram of the mechanism of conductivity enhancement of Bi2O3@Ag2O composite. e The schematic diagram, f CV curves at 0.6 mV s−1 and g) GCD profiles at 18 mA cm−2 of Zn||Bi2O3@Ag2O MBs. h The ex-situ EIS curves under different charging and discharging conditions of Zn||Bi2O3@Ag2O batteries. i GITT curves of Zn||Bi2O3 MBs. The GITT curve and the calculated diffusion coefficients during j charging and k discharging processes of Zn||Bi2O3@Ag2O MBs.

Further validation is obtained through ex situ electrochemical impedance spectroscopy (EIS) on Zn||Bi2O3@Ag2O MBs (Figs. 2h and S23). When the discharge voltage reaches ~1.2 V (as Bi2O3@Ag2O transitions to Bi2O3@Ag), the charge-transfer resistance (Rct) of Zn||Bi2O3@Ag2O MBs declines from 16 to 9.8 Ω. With further discharge to 0.2 V (Bi2O3@Ag converts to Bi@Ag), Rct decreases further to ~6 Ω (Fig. S23). Upon charging from 0.2 to 1.5 V (Bi@Ag reverts to Bi2O3@Ag), resistance increases modestly but remains lower than when charged to 2.1 V (Bi2O3@Ag returns to Bi2O3@Ag2O). These reversible EIS changes support the conclusion that the high conductivity of Ag generated in the initial step is crucial for maximizing capacity in the second step (Figs. 2h and S23). Additionally, we conduct galvanostatic intermittent titration technique (GITT) measurements to analyze the reaction kinetics in Zn||Bi2O3 and Zn||Bi2O3@Ag2O MBs (Figs. 2i–k and  S24). In Fig. 2i, the GITT curves of pure Zn||Bi2O3 MBs reveal only three relaxation cycles during both charge and discharge, with a discharge duration of ~0.5 h. This microdevice also exhibits pronounced polarization and a slow relaxation recovery rate, primarily due to the high mass loading that impedes electron transport within the Bi2O3 (Fig. S25). In contrast, Zn||Bi2O3@Ag2O MBs display well-defined GITT curves with a discharge duration of ~4.5 h, of which ~2 h originates from the Bi2O3 (Fig. 2j, k), which is substantially longer than that in pure Zn||Bi2O3 MBs. Similarly, compared to pure Zn||Bi2O3 MBs, Zn||Bi2O3@Ag2O MBs show a lower polarization and better relaxation recovery (Figs. 2i–k and S18), which can be attributed that the in situ formation of Ag during the initial cycles creates percolating conductive networks that enhance electron transport within the thick Bi2O3 electrode (Fig. S25), thereby improving kinetic performance. These findings provide further evidence for the enhanced capacity of Bi2O3 in the Zn||Bi2O3@Ag2O MBs, validating our proposed mechanism.

The electrochemical performance of Zn||Bi2O3@Ag2O MBs

Figure 3a illustrates the reaction mechanism during the charge/discharge process of Zn||Bi2O3@Ag2O MBs. The first and second steps represent the conversion reactions of Ag2O to Ag and Bi2O3 to Bi, respectively. Various gel electrolytes with different concentrations are used to optimize the microdevice (Fig. S26), with the PVA/3 m KOH/zinc acetate gel electrolyte emerging as the optimal candidate due to its higher ionic conductivity (Fig. S27). Notably, the optimum mass ratio of Ag2O to Bi2O3 in Zn||Bi2O3@Ag2O MBs is 2:1 (Fig. S28). At a current density of 12 mA cm−2, the fabricated Zn||Bi2O3@Ag2O MBs deliver a maximum areal capacity of 16,561.5 μAh cm−2 (Fig. 3b, c). Even at a high current density of 20 mA cm−2 and 30 mA cm−2, these microdevices achieve a high areal capacity of 9602.02 μAh cm-2 and 4406.12 μAh cm-2, respectively, demonstrating high-rate performance. Due to the in situ conductivity enhancement-assisted double-cell reaction mechanism, these capacities significantly exceed those of Zn||Bi2O3 MBs (Fig. S29a, b). Furthermore, the Bi2O3 capacity contribution in Zn||Bi2O3@Ag2O MBs ranges from 7131 μAh cm−2 at 12 mA cm−2 to 3473.8 μAh cm−2 at 30 mA cm−2, representing nearly an order of magnitude increase than that of pure Zn||Bi2O3 MBs at the same current density (Fig. S29c). This enhanced performance arises from the high conductivity of Ag generated during the first-step discharge in the Zn||Ag2O battery, which markedly improves the conductivity of the Zn||Bi2O3 system in the second step, thereby boosting capacity. After 160 cycles, the Zn||Bi2O3@Ag2O MBs can deliver a high areal capacity of 3741.6 μAh cm-2, corresponding to 70.1% of the initial value (Fig. 3d). The inflection points of GCD curves are different for the three cycle points in Fig. 3d, which can be attributed to the dynamic evolution of the composite electrode materials during cycling. Initially, the mass loading of Ag2O is approximately twice that of Bi2O3, and complete penetration and interaction with the electrolyte occur gradually, resulting in a progressive activation process. In the second stage, Ag2O reaches peak electrochemical activity, undergoing nearly complete transformation; however, this extensive reaction limits electrolyte access to portions of Bi2O3, slightly reducing its capacity contribution. In the third stage, excessive Ag2O consumption in the alkaline electrolyte leads to a decline in its capacity contribution (Fig. S30), while previously isolated Bi2O3 regions gain electrolyte access and become electrochemically active, causing a moderate capacity increase relative to the second stage. Beyond 160 cycles, up to 255 cycles, the Bi2O3 contribution remains stable (Fig. S30), in agreement with SEM, XRD, and TEM results showing minimal structural change (Fig. S30). In contrast, Ag2O exhibits pronounced morphological and structural depletion after 160 cycles (Fig. S30), resulting in a markedly reduced capacity contribution compared with the initial stage. Notably, similar capacity attenuation is common in previously reported Zn||Ag2O batteries, whose cycle life is typically limited to ~120 cycles47,49,50,51. Furthermore, the corrosion of the zinc negative electrode in alkaline electrolytes is an inherent phenomenon in alkaline zinc-based batteries and represents a key factor on capacity decay during prolonged cycling52. Based on the cycling curves of Zn||Bi2O3@Ag2O MBs, the average Ag/Ag2O conversion rate is estimated at ~94.1% (Fig. S31), underscoring the good reversibility of the electrochemical reactions during cycling. Due to the reasonable structural designs, ahigh areal capacity of 16,561.5 μAh cm-2 of the Zn||Bi2O3@Ag2O MBs is obtained at 12 mA cm−2, competitive with all reported aqueous Zn-based MBs, including Zn-manganese based MBs3,20,21,22,23,24,53,54,55,56, Zn-vanadium oxides based MBs24,25,26,57, alkaline zinc-based MBs29,30,58, Zn-halogen based MBs4,32 and other Zn-ion MBs24,27,31,59 as well as state-of-the-art aqueous K/Na ion MBs60,61, and organic MBs including Li ion MBs12,62, Na ion MBs16,17 and dual-ions MBs15 (Fig. 3e and Table S1). Unlike other aqueous zinc-based MBs, which typically reach maximum areal capacity below 2 mA cm−2, only a few exceed 2 mA cm−2 (Fig. S32). Operating at such high current densities also avoids the water-related side reactions under high voltage. Owing to its optimized design, the Zn||Bi2O3@Ag2O MBs reach maximum areal power density, significantly higher than all reported Zn-based micro-batteries3,4,20,21,22,23,24,25,26,27,29,30,31,32,53,54,55,56,57,58,59 (Fig. S33). This ability to withstand high current density enables the microdevices to deliver a high areal energy density of ~19,000 μWh cm−2 even at a high-power density of ~13,550 μW cm−2, comparable to the best available aqueous Zn-based MBs (Fig. 3f and Table S1)3,4,20,21,22,23,24,25,26,27,29,30,31,32,53,54,55,56,57,58,59. Even when compared with the reported top-performing organic MBs, the areal energy density of our microdevices is at least 2.5 times higher (Fig. 3f and Table S1)12,15,16,17,62. Notably, even when the inactive gap area is included in the calculations, the maximum areal capacity and energy density of our Zn||Bi2O3@Ag2O MBs still surpass those of all previously reported high-performance MBs (Fig. S34). When the energy density decreases to 3410.43 μWh cm−2, a high power density of 23,231.8 μW cm−2 is achieved, comparable to MSCs63,64,65,66,67,68,69,70 and state-of-the-art Zn or Li ion MSCs including Zn||activated carbon MSCs (≤3900 μW cm−2)71,72, Zn||CNT MSCs (8000 μW cm−2)73, Zn ion micro-capacitors with activated carbon negative electrode and VO2 (B) positive electrode (753.12 μW cm−2)74, Zn||MXene/silver-nanowires MSCs (≤4500 μW cm−2)75,76, Li ion MSCs (<2990 μW cm−2)77, and Na ion MSCs (900 μW cm−2)78 (Fig. S35 and Table S2). Volumetrically, when accounting for the combined thickness of the positive electrode, negative electrode, and graphite paper current collector (Fig. S36), the microdevice achieves a maximum volumetric energy density of 393.8 mWh cm−3 at a power density of 285.4 mW cm−3. This performance surpasses that of most of the reported high-performance Zn-based MBs53,54,55,56,57,58, organic Li/Na/double ions MBs14,15,16 and other aqueous MBs61, offering more than 50- and 500-fold improvements in energy density over Li thin-film batteries (4 V/500 μAh)79 and commercial supercapacitors (5.5 V/100 mF and 2.75 V/44 mF)80, respectively (Fig. S36). Moreover, compared with most fiber-shaped MBs, Zn||Bi2O3@Ag2O MBs exhibit both higher volumetric capacity and superior energy density (Fig. S37). Even after 12 h of rest in a fully charged state, the fabricated MBs can still deliver a high areal capacity at a high current density of 18 mA cm−2, competitive with that of other reported high-performance aqueous Zn-based MBs without resting (Fig. S38). Notably, the electrochemical performance of our MBs, specifically areal capacity and areal energy, is compared with the reported MBs under conditions where the total microelectrode area, including both the positive and negative electrode is comparable (Fig. S39). As a summary, the high electrochemical performance of the Zn||Bi2O3@Ag2O MBs can be attributed to several critical factors. First, the PVA/KOH/zinc acetate gel electrolytes with high ion conductivity provide sufficient ions for continuous redox reactions within the Zn||Bi2O3@Ag2O MBs. Second, combining two battery reactions within a single microdevice maximizes space utilization without increasing inactive areas, significantly enhancing discharge capacity and energy density. Third, incorporating Ag2O markedly improves the reaction kinetics and discharge capacity of Bi2O3, delivering nearly a tenfold increase over Zn||Bi2O3 MBs. This improvement is due to the conductive Ag generated during the initial conversion step, which establishes an effective electron-conducting pathway between Bi2O3 and the graphite paper current collector, thereby facilitating the six-electron Bi2O3 conversion reaction. Furthermore, the correlation between the peak current density (ip) of both the anodic and cathodic reactions versus the square roots of can rates (v1/2) is obtained in Fig. S39, derived from CV curves of each electrochemically active species under different scan rates. The result shows that the plots of ip vs. v1/2 is a linear relationship, confirming that redox reaction in our fabricated Zn||Bi2O3@Ag2O MBs is mainly diffusion-controlled (Fig. S40)81,82. Based on the established method for determining b values from CV curves83,84, both reduction peaks exhibit b values close to 0.5 (Fig. S40), further verifying that the charge-storage process of our MBs are almost entirely governed by ion diffusion-controlled battery behavior. Lastly, the well-designed structure supports the reversible and stable electrochemical reactions of zinc with both Ag2O and Bi2O3, ensuring the sustained high performance of these MBs.

Fig. 3: Electrochemical performance of Zn||Bi2O3@Ag2O MBs.
Fig. 3: Electrochemical performance of Zn||Bi2O3@Ag2O MBs.
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a The Schematic diagram of structure reaction mechanism, b GCD curves c Areal capacity under the different current density, d Cycle performance of Zn||Bi2O3@Ag2O MBs. e The comparison of the areal capacity of the constructed Zn||Bi2O3@Ag2O MBs with that of all reported aqueous Zn-based MBs, including Zn-manganese based MBs3,20,21,22,23,24,53,54,55,56, Zn-vanadium oxides based MBs24,25,26,57, alkaline zinc-based MBs29,30,58, Zn-halogen based MBs4,32 and other Zn ion MBs24,27,31,59 as well as state-of-the-art aqueous K/Na ion MBs60,61, and organic MBs including Li ion MBs12,62, Na ion MBs16,17 and dual-ions MBs15. f The comparison of maximum areal energy density and its corresponding power density of the constructed microdevice with that of all reported aqueous Zn-based MBs3,4,20,21,22,23,24,25,26,27,29,30,31,32,53,54,55,56,57,58,59, state-of-the-art aqueous K/Na ion MBs60,61, and organic MBs12,15,16,17,62.

The reaction mechanism of batteries

The working mechanism of Zn||Bi2O3@Ag2O MBs is investigated in Fig. 4a–e. As the discharge voltage decreases, the large, smooth Ag2O spheres in the Bi2O3@Ag2O composite first convert into small, rough Ag particles, while the morphology of Bi2O3 remains essentially unchanged (Fig. 4a, b). Notably, during this process, the smooth Bi2O3 microrods become encased in the highly conductive Ag particles formed from Ag2O, which significantly enhances the conductivity of the entire composite. This enhancement explains the substantially higher capacity contribution of Bi2O3 in the Zn||Bi2O3@Ag2O system compared to that of pure Zn||Bi2O3 MBs. As the discharge voltage further drops to 0.2 V, the smooth Bi2O3 microrods transform into rough, hollow Bi, while the rough Ag particles remain unchanged (Fig. 4c). Upon recharging to a fully charged state, the Bi@Ag composite first reverts to Bi2O3@Ag (Fig. 4d) and then to Bi2O3@Ag2O (Fig. 4e). Notably, upon completion of battery discharge, the microrod structure of Bi2O3 was almost entirely lost in Fig. 4c. As charging proceeded, this micro-rod morphology gradually re-emerged (Fig. 4d, e). Compared with the vanished structure observed in Fig. 4c, the pronounced microrod features in Fig. 4d, e clearly demonstrate the regeneration of Bi2O3, demonstrating the reversibility of the entire charge–discharge process. Furthermore, TEM images of the fully discharged state reveal that the produced Bi and Ag exhibit distinctly different morphologies from the smooth Bi2O3 microrods and interconnected smooth Ag2O nanospheres (Figs. 4f, g, S8 and S10). Ex situ X-ray photoelectron spectroscopy (XPS) analysis further investigates the changes in Bi2O3 and Ag2O across various charge and discharge states (Fig. 4h, i). From the initial state (I) to stage II of discharge (Fig. 4h), the Ag 3d peaks shift upward, while the Bi 4f peaks remain stable, indicating the transformation of Bi2O3@Ag2O to Bi2O3@Ag. With further discharge to stage III, the Bi2O3 peak shifts downward, and a new peak appears (Fig. 4j), closely aligning with the XPS peak of commercial pure Bi powder (Fig. S41), indicating the complete conversion of Bi2O3 to Bi. Throughout this stage, the Ag 3d peak remains essentially unchanged, functioning primarily as a conductivity enhancer. Thus, between stages II and III, Bi2O3@Ag2O is transformed into Bi@Ag. As charging proceeds from the fully discharged state (III) to stages IV and V (fully charged state), the Ag 3d and Bi 4f peaks gradually revert to their original positions and shapes (Fig. 4h–j), corresponding to the transformation from Bi@Ag back to Bi2O3@Ag and ultimately to Bi2O3@Ag2O, respectively.

Fig. 4: Ex situ testing and analysis.
Fig. 4: Ex situ testing and analysis.
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a–e The ex situ SEM images of the positive electrodes of Zn||Bi2O3@Ag2O MBs under the different discharging and charging stages at 0.5 A g−1. The TEM images and corresponding element mapping of f Bi and g Ag under the fully discharged state at 0.5 A g−1. h The discharging and charging curves and corresponding ex situ XPS curves of i Ag 3d and j Bi 4f of Zn||Bi2O3@Ag2O MBs at 0.5 A g−1.

Moreover, the ex-situ fourier transform infrared spectroscopy (FT-IR) was employed to track peak variations of the Bi2O3@Ag2O composite under different charge and discharge states (Fig. S42). At the beginning of discharge, the Bi-O-Bi stretching vibrations between 800 and 900 cm−1 and the Bi-O stretching band at 1390 cm−1 remain largely unchanged, as the initial reaction is dominated by the reduction of Ag2O to Ag (Fig. S42). In addition, because the characteristic peaks of Ag2O and Bi2O3 in the 400–700 cm−1 region strongly overlap with the hydroxyl band near 1650 cm−1 (arising from surface-adsorbed water of Ag2O/Bi2O3), spectral variations in these regions are also minimal (Fig. S42). As discharge progresses, the Bi-O-Bi stretching vibrations between 800 and 900 cm−1 disappear, and the intensity of the Bi-O stretching band at 1390 cm−1 decreases significantly (Fig. S42), indicating the reduction of Bi2O3 to Bi. By the end of discharge, the peaks corresponding to Ag-O bonds and [BiO6] units (400–700 cm-1), together with the hydroxyl band near 1650 cm−1, also vanish, confirming that both Ag2O and Bi2O3 are reduced to Ag and Bi (Fig. S42). Upon charging, the characteristic peaks of Ag2O and Bi2O3 re-emerge, highlighting the high reversibility of the overall redox process. It is worth noting that although the strong overlap of Bi2O3 and Ag2O peaks obscures direct observation of the Ag2O/Ag conversion in ex situ FT-IR, this transformation is unambiguously confirmed by ex situ SEM, ex situ XPS analyses and in situ XRD (See Fig. 5). Based on ex situ SEM, TEM, XPS, and FT-IR results, a schematic of the reversible transformation mechanism in Zn||Bi2O3@Ag2O MBs is depicted in Fig. 5a. Meanwhile, the energy-level evolutions during the conversion processes of Ag2O/Ag and Bi2O3/Bi are also studied by density functional theory (DFT) calculations (Fig. S43 and Supplementary Data 1). To gain deeper insights into this reversible reaction mechanism, in situ X-ray diffraction (XRD) analysis was performed to monitor the positive electrode transformations during charge and discharge process (Fig. 5b–i and S44). As seen in Fig. 5b–g, during discharge, specific Bi2O3 crystal planes, including (−121), (−202), (221), and (−322), engage in the reaction. This is confirmed by in situ XRD patterns showing the gradual disappearance of the characteristic peaks of Bi2O3, alongside the appearance of new peaks at 27.26° (012), 39.6° (101), 45.9° (006), 48.69° (202), 56° (−242), and 62.2° (116) for Bi (a region where the color turns red). Upon charging, these Bi peaks fade, and new peaks at 27.38°, 33.2°, 46.34°, and 52.37° signify the reformation of Bi2O3. Simultaneously, the transformation of Ag2O was also tracked in situ. At the start of discharge, the Ag2O characteristic peak at 54.8° (220) gradually diminishes, giving rise to prominent Ag peaks at 38.07° (111) and 64.5° (220). During charging, the Ag peaks recede, and Ag2O is regenerated. This comprehensive analysis demonstrates that the conversion reactions between Bi2O3@Ag2O and Bi@Ag are highly reversible, with the electrode reactions described as follows:

$${{{\rm{Positive}}}}\; {{{\rm{electrode}}}}:\,{{{\rm{A}}}}{{{{\rm{g}}}}}_{2}{{{\rm{O}}}}+{{{{\rm{H}}}}}_{2}{{{\rm{O}}}}+2{{{{\rm{e}}}}}^{-}\leftrightharpoons 2{{{\rm{Ag}}}}+{2{{{\rm{OH}}}}}^{-}$$
(1)
$${{{{\rm{Bi}}}}}_{2}{{{{\rm{O}}}}}_{3}+{3{{{\rm{H}}}}}_{2}{{{\rm{O}}}}+{6{{{\rm{e}}}}}^{-}\leftrightharpoons 2{{{\rm{Bi}}}}+{6{{{\rm{OH}}}}}^{-};$$
(2)
$${{{\rm{Negative\; electrode}}}}:{{{\rm{Zn}}}}+{4{{{\rm{OH}}}}}^{-}\leftrightharpoons {{{\rm{Zn}}}}{({{{\rm{OH}}}})}_{{4}^{2-}}+{2{{{\rm{e}}}}}^{-}$$
(3)
Fig. 5: Mechanism analysis.
Fig. 5: Mechanism analysis.
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a Schematic diagram of reaction process of Zn||Bi2O3@Ag2O MBs. b–i in situ XRD during charge/discharge of Zn||Bi2O3@Ag2O batteries at 0.2 A g−1.

Potential application and wirelessly intelligent integration of the constructed MBs

To visually highlight the high performance of the Zn||Bi2O3@Ag2O MBs, a single microdevice is used to continuously power a timer. Remarkably, a single Zn/ Bi2O3@Ag2O MB maintains uninterrupted power to the timer for 3780 minutes (over 2.5 days) without any loss in brightness (Fig. 6a and Supplementary Movie 1). Notably, the mass of the constructed microdevice is 0.44% and 2.4% that of the timer and a commercial battery, respectively (Fig. S45). To meet the increasing demands of integrated circuits, higher voltages and capacities are readily achievable by connecting multiple Zn||Bi2O3@Ag2O MBs in series or parallel. As expected, two Zn||Bi2O3@Ag2O MBs connected in series provide a high platform voltage of ~3 V while retaining a similar areal discharge capacity to a single device (Fig. 6b). Connecting two MBs in parallel doubles the discharge capacity at the same current density (Fig. 6c). Two series-connected Zn||Bi2O3@Ag2O MBs can power 200 red, yellow, green, blue and white light-emitting diodes (LEDs) in parallel (Fig. 6d, e and Supplementary Movie 2). This LED count is higher than those powered by previously reported micro-battery system, including high-voltage and high-performance organic NiSn/LiMnO2 Li ion MBs (1 MB, 1 red LED)14, organic dual-ion MBs (1 MB, 10 red or blue or yellow or white LEDs)15, organic Na ion MBs (3 MBs, 42 yellow LEDs)16,17, aqueous K ion MBs (2 MBs, 1 red LED)60, aqueous Na ion MBs (2 MBs, 1 red LED)61, lithium titanate||lithium iron phosphate Li ion MBs (3 MBs, 42 yellow LEDs)85, organic Li||LiFePO4 MBs (1 MB, 1 white LED)86, and the reported advanced aqueous Zn-based MBs24,25,26,30,31,53,54,55,56,59,87,88 (Fig. 6f). Moreover, two series-connected microdevices can power white LEDs continuously for approximately 40 minutes (Supplementary Movie 2). To our knowledge, this achievement marks a significant milestone, underscoring the tremendous potential application value of Zn||Bi2O3@Ag2O MBs. When integrated multiple Zn||Bi2O3@Ag2O microelectrodes into an array on a flexible PET substrate, the device demonstrates remarkable flexibility and can be bent to a large degree (Fig. S46).The fabricated Zn||Bi2O3@Ag2O MBs array can stably power a 4.5 V LED under normal conditions, and the LED remains fully operational even when the array is bent into various shapes (Fig. S46 and Supplementary Movie 3). These results highlight the strong potential of our MBs design for developing flexible arrays capable of delivering stable energy output under a wide range of bending conditions.

Fig. 6: Series/parallel testing and the applicability in high-energy-demand scenarios of Zn||Bi2O3@Ag2O MBs.
Fig. 6: Series/parallel testing and the applicability in high-energy-demand scenarios of Zn||Bi2O3@Ag2O MBs.
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a Photograph showing that a single microdevice can uninterrupted power for the timer for over 3780 minutes (over 2.5 days) without any loss in brightness. b GCD curves of a single and two Zn||Bi2O3@Ag2O MBs (b) in series and (c) in parallel. d, e Photograph showing that two series-connected Zn||Bi2O3@Ag2O MBs can easily power 200 red, yellow, green, blue and white light-emitting diodes (LEDs) in parallel. f Comparison of the number of LEDs powered by two series-connected Zn||Bi2O3@Ag2O MBs with those powered by other high-performance double ion, Li/Na/K ion MBs14,15,16,17,60,61,85,86 as well as aqueous Zn-based MBs24,25,26,30,31,53,54,55,56,59,87,88.

At present, intelligent wireless sensors are becoming increasingly prevalent in everyday life and work environments. However, these sensors are typically powered by traditional button lithium batteries, which present notable limitations: (1) they require heavy stainless-steel casings, elastic sheets, and gaskets for secure sealing, adding significant weight and volume to the device; (2) they contain flammable organic electrolytes, which pose potential safety hazards. This limitation arises primarily from the high energy demands of Bluetooth transmitters, which are essential for wireless communication with mobile phones, as well as the power required for data processing functions of the sensor. Consequently, developing high-performance aqueous Zn-based MBs capable of powering such wireless sensors remains a challenge. Here, for the first time, we demonstrate the use of aqueous Zn||Bi2O3@Ag2O MBs to power a commercial wireless smart sensor with direct mobile connectivity for monitoring the motion states of toy cars and human bodies (Fig. 7a). As shown in Fig. 7b, c and Supplementary Movie 4, when the Zn||Bi2O3@Ag2O MBs-powered sensor rotates in any of the X, Y, or Z directions, its deflection direction and state can be observed in real time on a mobile phone receiving the wireless signal of sensors. The sensor is then integrated into a toy car, accurately and stably tracking its acceleration and angular velocity during motion and rotation (Fig. 7d, e, Fig. S47 and Supplementary Movie 5). When the car travels over different terrains, such as a smooth plastic track versus uneven grass, the sensor captures distinct acceleration profiles, with greater fluctuation observed on the grass (Fig. 7f and Supplementary Movie 5). Additionally, the sensor can be worn on a person’s arm, wirelessly transmitting data to a mobile phone to monitor different motion states in real time, such as walking, standing still, and running (Fig. 7g and Supplementary Movie 6). Collectively, these results demonstrate that our fabricated Zn||Bi2O3@Ag2O MBs with high energy density can be effectively integrated into smart wearable devices, highlighting their promising application potential.

Fig. 7: Wireless intelligent integration of Zn||Bi2O3@Ag2O MBs for sensor applications.
Fig. 7: Wireless intelligent integration of Zn||Bi2O3@Ag2O MBs for sensor applications.
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a Schematic of the integration of Zn||Bi2O3@Ag2O MBs with sensors, enabling wireless data transmission to the mobile phone. b Optical photograph of the Zn||Bi2O3@Ag2O MBs connected to the sensors. c Mobile phone wirelessly receiving signals from sensors powered by Zn||Bi2O3@Ag2O MBs. The sensor monitors the motion states of toy cars in various conditions: d regular movement, e rotational motion, and f across different surfaces. g Real-time monitoring of human motion states, such as walking, standing still, and running, using a sensor powered by Zn||Bi2O3@Ag2O MBs integrated into wearable devices.

Discussion

We propose a double-cell reaction strategy, assisted by an in situ conductivity enhancement mechanism, to fabricate high-performance Zn||Bi2O3@Ag2O MBs by integrating two conversion reactions within a single microdevice. This approach eliminates the need for additional inactive connecting materials, thereby optimizing the space utilization of MBs. In the first conversion reaction, the Bi2O3@Ag2O composite is transformed into Bi2O3@Ag, with metallic silver tightly surrounding the Bi2O3, resulting in a substantial increase in the conductivity of the composite. Unlike a simple combination of Zn||Ag2O and Zn||Bi2O3 MBs, the in situ conductivity enhancement induced by the Ag2O conversion reaction achieve a nearly order-of-magnitude increase in the capacity contribution of Bi2O3 in the Zn||Bi2O3@Ag2O MBs compared to pure Zn||Bi2O3 MBs, resulting in a high total capacity that is above 2.1 times higher than the combined discharge capacities of two individual MBs. As a result, the Zn||Bi2O3@Ag2O MBs deliver a high areal capacity of 16,561.5 μAh cm−2, competitive with all reported aqueous Zn-based MBs, as well as high-performance dual-ion and Li/K/Na-ion MBs. More importantly, even at a power density of ~13,550 μW cm−2, they provide a high areal energy density of ~19,000 μWh cm−2, which can represent up to one order of magnitude higher than some recent aqueous MBs reported. Furthermore, the constructed microdevice can maintain a MSC-level or even higher power density (up to 23,231.8 μW cm−2. This work offers a facile and effective strategy for the development of miniaturized batteries with unprecedented areal capacity and energy density, paving the way for next-generation intelligent integrated electronic devices.

Methods

Synthesis of Bi2O3

Typically, 2.91 g of Bi(NO3)3·5H2O (99.9%, 3AChem) and 1.28 g of Na2SO4 (99%, 3AChem) were dissolved in 120 mL of deionized water and stirred at room temperature for 1 h. Next, 120 mL of 0.45 M NaOH (98.0%, Aladdin) solution was added dropwise to the mixture. The resulting white dispersion was then heated in an oil bath at 60 °C for 2 h, during which the dispersion transitioned from white to light yellow. The mixture was left to stand for a brief period, allowing the formation of a yellow precipitate. The precipitate was washed with deionized water until the pH reached 7, and then dried in an oven at 80 °C overnight to obtain Bi2O3 powder.

The preparation of PVA-KOH gel electrolyte

Electrolytes were prepared by first formulating KOH/zinc acetate (Zn(Ac)2) solutions at concentrations of 2 m, 6 m, and 10 m (m: mol kgwater−1) with a fixed KOH:Zn(Ac)2 molar ratio of 20:1. Polyvinyl alcohol (PVA, 0.44 g) was dissolved in 2 mL of deionized water at 80 °C under continuous stirring until a clear, homogeneous solution was obtained. Subsequently, 2 mL of the KOH/Zn(Ac)2 solutions (2 m, 6 m, and 10 m) were slowly added to the PVA solution under stirring until homogeneous. The mixtures were then left to stand at room temperature for 0.5–1 h, yielding PVA/KOH/Zn(Ac)2 hydrogel electrolytes with final concentrations of approximately 1 m, 3 m, and 5 m, respectively.

The fabrication of Zn||Bi2O3@Ag2O microelectrodes and micro-batteries

First, conductive graphite paper (thickness: 0.05 mm) was precisely patterned into interdigital microelectrodes using laser direct-writing technology, based on a pre-designed pattern. The dimensions of the microelectrodes were shown in Fig. S48 (width: 0.7 mm; gap: 2.5 mm). Next, commercial Ag2O (99.99%, Aladdin) was mixed with Bi2O3 in a mass ratio of 2:1 (The loading mass of Ag2O and Bi2O3 is 80 mg cm−2 and 40 mg cm−2, respectively). The resulting composite was then blended with Super P (TIMCAL) and PVDF (Arkema Kynar) in a mass ratio of 8:1:1, using N-methyl-2-pyrrolidone (NMP, 99.0%, Aladdin) as the solvent. This mixture was coated onto the interdigital graphite paper microelectrodes to form the positive electrodes, with a total loading of approximately 120 mg cm−2. Zinc powder (99.99%, Aladdin) was then mixed with Super P and PVDF in the same mass ratio (8:1:1), using NMP as the solvent, and coated onto the interdigital microelectrodes as the negative electrodes (The loading mass of Zn negative electrode: 50 mg cm−2). The positive and negative electrodes were carefully positioned onto a custom stainless-steel template attached to polyimide tape, resulting in the fabrication of the Zn||Bi2O3@Ag2O microelectrodes. Finally, flexible micro-batteries were assembled by directly applying the PVA/KOH/Zn(Ac)2 hydrogel electrolyte onto the interdigital microelectrodes.

The fabrication of Zn||Ag2O microelectrodes/micro-batteries

First, Ag2O was blended with Super P and PVDF in a mass ratio of 8:1:1, using NMP as the solvent. This mixture was coated onto the interdigital graphite paper microelectrodes to form the positive electrodes, with a total loading of approximately 80 mg cm−2. The positive electrode and zinc power negative electrode were positioned onto a custom stainless-steel template attached to polyimide tape, resulting in the fabrication of the Zn||Ag2O microelectrode. Finally, Zn||Ag2O micro-batteries were assembled by directly applying the PVA/3 m KOH/Zn(Ac)2 gel electrolyte onto the interdigital microelectrodes.

The fabrication of Zn||Bi2O3 microelectrodes/micro-batteries

Zn||Bi2O3 microelectrode was similarly fabricated by mixing Bi2O3, Super P and PVDF in the mass ratio of 8:1:1, with NMP as the solvent. The, the mixture was coated onto the interdigital graphite paper microelectrodes to form the positive electrodes, with a total loading of approximately 40 mg cm−2. The positive electrode and zinc power negative electrode were positioned onto a custom stainless-steel template attached to polyimide tape, resulting in the fabrication of the Zn||Bi2O3 microelectrode. Finally, Zn||Bi2O3 micro-batteries were assembled by directly applying the PVA/3 m KOH/Zn(Ac)2 gel electrolyte onto the interdigital microelectrodes.

Characterizations

The morphologies of the samples were characterized using a scanning electron microscope (SEM, JEOL JSM-7900F; Voltage: 5.0 kV; Probe current: 76.2 μA) and a transmission electron microscope (TEM, Tecnai G20). X-ray diffraction (XRD) tests of the active materials were conducted on a UItimal IV system with Cu Kα radiation. The test range was 20–80° and the scan rate was 8°/min. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher EscaLab 250Xi system (Beam energy: 1486.8 eV; Spot area: 30–500 μm). For the convenience of XRD and XPS characterization, CR2032 coin cells were assembled for battery testing. Graphite paper (thickness: 0.05 mm; diameter: 10 mm) was used as the cathode current collector, glass fiber (diameter: 16 mm, GF/D, Whatman) served as the separator, zinc foil (thickness: 0.02 mm; diameter: 10 mm, 99.99%, Canrd) was employed as the anode, and the electrolyte volume was fixed at 200 μL. Fourier-transform infrared (FT-IR) spectra were recorded using a BRUKER INVENIO spectrometer, and the wavelength range tested was between 400 and 2000 cm−1 with a resolution of 4 cm−1, scan rate of 60 cm−1 s−1. The electrical conductivity of the active material powders was measured using a TH2515 DC resistance tester. In situ XRD tests were performed using a powder X-ray diffractometer (Bruker D8 Advance, Germany) with Cu Kα radiation. The diffraction angle range was between 20 and 80°, and the cells were assembled with a beryllium window serving as the current collector.

Electrochemical measurements

All electrochemical measurement were performed using MBs. The MBs were connected to copper foil (thickness: 0.01 mm) using conductive silver glue. All tests were carried out in an open-air environment at average temperature of 28 ± 2 °C. The PVA/KOH gel electrolyte amount used in each MB was 400 μL, unless otherwise specified.

The electrochemical performance of the fabricated batteries was evaluated through galvanostatic cycling at various current densities and galvanostatic intermittent titration technique (GITT) using a LANHE CT3002A cycler. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using a Solartron Energylab (Ametek) system.

For Zn||Bi2O3@Ag2O MBs, CV was tested in the voltage range of 0.2–2.1 V at different scan rates. GCD profile was tested in the voltage range of 0.2–2.1 V at different current densities. EIS spectra were recorded with a frequency ranging from 0.1 Hz to 1000 kHz (10 data points per decade of frequency). GITT curves were recorded with taking ten data points per second. For Zn||Bi2O3 MBs, CV was tested in the voltage range of 0.2–1.5 V at 0.6 V s−1. GCD profile was tested in the voltage range of 0.2–1.5 V at different current densities. Zn||Ag2O MBs, CV was tested in the voltage range of 1.2–2.1 V at 0.6 V s−1. GCD profile was tested in the voltage range of 1.2–2.1 V at current density of 18 mA cm−2. Gravimetric specific capacity and energy density were calculated based on the mass of the active materials, Bi2O3 and Ag2O. For the Zn||Bi2O3@Ag2O micro-batteries, the total area (0.18 cm2) of both the positive and negative electrode was considered in the calculations (Note: this area corresponds to the contact area between the electrolyte and electrodes, as shown in Fig. S49). Volumetrically, the microdevice has a volume of approximately 0.00855 cm3, which accounts for the combined thickness of the positive electrode, negative electrode, and the graphite paper current collector. The ionic conductivity of the gel electrolyte was determined as follows. The gel electrolyte was sandwiched between two titanium foil sheets (thickness: 0.06 mm), which served as electrodes. Titanium foil was chosen for its chemical stability in alkaline environments and good electrical conductivity. EIS was performed using a two-electrode configuration, with the titanium foils connected to the impedance analyzer. The real contact area (A) between the gel electrolyte and the titanium foils, as well as the electrode spacing (corresponding to the gel thickness L), were measured with calipers to minimize error. The ionic cownductivity (\(\sigma\)) of the gel electrolyte was calculated according to the equation4,32:

$$\sigma=\frac{L}{{RA}}$$
(4)

where \(L\) is the gel thickness, \(R\) is the ohmic resistance obtained from the x-axis intercept of the Nyquist plot in EIS measurements, and \(A\) is the real contact area between the gel electrolyte and the titanium foils. The authors affirm that human research participants provided inform consent for publication of the images in Fig. 7g.