Transient Rechargeable Batteries Triggered by Cascade Reactions

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Transient Rechargeable Batteries Triggered by Cascade Reactions Kun Fu, Zhen Liu, Yonggang Yao, Zhengyang Wang, Bin Zhao, Wei Luo, Jiaqi Dai, Steven David Lacey, Lihui Zhou, Fei Shen, Myeongseob Kim, Laura Swafford, Louise Sengupta, and Liangbing Hu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b01451 • Publication Date (Web): 17 Jun 2015 Downloaded from http://pubs.acs.org on June 22, 2015

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Transient Rechargeable Batteries Triggered by Cascade Reactions Kun Fua, ‡, Zhen Liua, ‡, Yonggang Yaoa, ‡, Zhengyang Wanga, Bin Zhaoa, Wei Luoa, Jiaqi Daia, Steven D. Laceya, Lihui Zhoua, Fei Shena, Myeongseob Kimb, Laura Swaffordb, Louise Senguptab, Liangbing Hua,* a

Department of Materials Science and Engineering, University of Maryland, College Park, MD

20742, USA b

Advanced Sensors, SPX-Technology Solutions, BAE Systems Inc., Columbia, MD, 21046,

USA ‡

These authors contributed equally to this work.

*

Corresponding author.

ACS Paragon Plus Environment

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ABSTRACT

Transient battery is a new type of technology that allows the battery to disappear by an external trigger at any time. In this work, we successfully demonstrated the first transient rechargeable batteries based on dissoluble electrodes including V2O5 as the cathode and lithium metal as the anode as well as a biodegradable separator and battery encasement (PVP and sodium alginate, respectively). All the components are robust in a traditional Lithium-ion Battery (LIB) organic electrolyte and disappear in water completely within minutes due to triggered cascade reactions. With a simple cut-and-stack method, we designed a fully transient device with an area of 0.5 cm by 1 cm and total energy of 0.1 joules. A shadow-mask technique was used to demonstrate the miniature device, which is compatible with transient electronics manufacturing. The materials, fabrication methods, and integration strategy discussed will be of interest for future developments in transient, self-powered electronics. The demonstration of a miniature Li battery shows the feasibility towards system integration for all transient electronics.

KEYWORDS: Transient electronics, Cascade reactions, Rechargeable batteries, vanadium oxide (V2O5)


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A transient device is one which can chemically or physically disappear by means of an external trigger after a specific amount of time.1–6 Like traditional devices, transient devices should have a solid structure and maintain reliable high-performance operation over a long lifespan. However, the advantage over traditional devices is the transience that enables the device to degrade in a controlled manner once an external trigger commences. The trigger can be any kind of external stimuli, including pH, light, temperature, mechanical forces, as well as the addition of gases or liquids, that will physically and/or chemically deactivate electronics.1,6 Once triggered, the transient device is able to degrade partially or completely and permanently become inoperative with time scales for transience ranging from minutes to days. Potential applications that could exploit this transience concept and strategy include information-sensitive applications. For example, transience technology can enable small electronic devices like remote sensors, which are typically impossible to track and recover, to physically disappear when no longer needed. In addition, selecting a simple trigger stimulus is also important to cause the device to disappear. Therefore, a fully transient device with a simple trigger stimulus invokes the basic requirement for the development and implementation of transience technology. Recently, nearly all transience technology efforts have focused on the design of transient electronics, including metal-oxide-semiconductor inverters, sensors, silicon solar cells, mechanical energy harvesters and actuators, digital-imaging devices, and wireless powerscavenging systems.1–4,7–9 Water is the typical trigger for the dissolution of the constituent materials that make up these electronic devices. In recent years, transience technology has


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become more prominent in small electronic devices, which motivates us to adopt this transience concept and extend it to energy storage devices. To design a transient battery, the proper galvanic materials need to be selected with both high potential and capacity for the battery, and the selected electrodes and packaging materials should be able to dissolve rapidly in a particular solution for the transience. Based on our understanding, a good transient battery should demonstrate the following characteristics: 1) all the constituent materials must physically disappear, 2) fast transience time, 3) high voltage and capacity, 4) proper battery size and mass and 5) flexible design of battery to meet various voltage and capacity levels. The previous work from Rogers’ group has touched on only a few of these important characteristics.10 The transient lithium-ion battery (LIB) developed in this work addresses all of these important characteristics and employs a cascade reaction to fully and rapidly dissolve the battery. LIB is a mature technology for energy storage applications and is a promising strategy to integrate with transient electronics due to its implementation in areas that require small size but high energy.11–13 Inspired by the recent development of LIBs and transience technology, we present a rechargeable transient battery that can deliver a stable battery performance with both high voltage and capacity for repeatable uses, and once triggered, the battery can be fully dissolved in water with a transience time within minutes. Vanadium oxide (V2O5) is selected as the cathode due to its comparatively high theoretical capacity of 294 mAh/g when voltage discharges at 2 V vs. Li/Li+.14–19 In addition to the specific capacity, V2O5 is also an electrically conductive material, which is necessary for a cathode.20 More importantly, V2O5 can be dissolved in the alkali solution that forms when the lithium metal anode reacts with water to form LiOH.21 In addition to the transient electrodes, other battery constituent materials including


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separators and battery packages need to be fast transient as well. Two kinds of polymers, polyvinylpyrrolidone (PVP) and sodium alginate (Na-AG), are selected to fabricate separators and battery encasements. The separator is prepared by electrospinning nanofibers into a nonwoven membrane.22,23 The battery encasement is a Na-AG film deposited with a prepatterned metal serving as current collector. This bio-derived polymer has good mechanical and chemical properties, making it a superior battery packaging material.24 Since PVP and Na-AG are water-soluble polymers, the separator and encasement made out of them are transient in water once exposed by the trigger. A combination of cut-and-stack and shadow mask metal depositions are used to fabricate miniature devices where one single transient microbattery (0.5 cm by 1.0 cm; total mass of 0.1 g) can deliver an energy of 0.1 joule. The transient battery demonstrates excellent performance while remaining as stable as non-transient batteries. When triggered by water, a series of cascade reactions occur causing all the components including the anode, cathode, separator, metal contacts, substrate, and encapsulate to vanish completely to the naked eye. Figure 1 shows the transience mechanism of a typical transient battery fabricated in this study. The transient battery consists of Li metal as anode, V2O5 as cathode, and PVP nanofiber membrane as separator, shown in Figure 1a. The current collectors, aluminum (Al) and copper (Cu), were evaporated via shadow mask metal deposition technique. Sodium alginate is used for the battery encasement. These materials were chosen based on its ability to dissolve in water and its stability in conventional organic electrolytes.24–26 Dense Na-AG film was used as the substrate to make current collector and thin Na-AG film was used as top cover to confine the electrolyte. All the components are compatible with the conventional LiPF6 organic electrolyte used in traditional LIBs. The schematic of the process of triggered dissolution is shown in Figure 1b, the


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LIB fully disintegrates and vanishes by the addition of water through cascade reactions. First, the water droplets dissolve the alginate substrate. The water then reaches and reacts with the Li metal anode generates hydrogen gas to facilitate the diffusion of water into the other battery layers. LiOH forms in water and rapidly reacts with the V2O5 cathode due to the one-dimensional nanostructure. After a few minutes, all the components disappear and become invisible to the naked eye. The chemical reactions responsible for the dissolution of each component are shown in Figure 1c. There are several advantages of our transient energy storage device: (1) the battery has a high energy density since it is composed of both a high capacity anode and cathode as well as a traditional LIB organic electrolyte; (2) all the components are transient; (3) the transient behavior can be programmable by controlling the thickness of the encapsulating films; (4) it is easy to manufacture due to the combination of a shadow mask technique and a simple cut-and-stack strategy. Figure 2 shows scanning electron microscopy (SEM) images of the transient battery materials, including the V2O5 cathode, the PVP membrane separator, and the alginate substrate. V2O5 was synthesized by a simple hydrothermal treatment of commercial granular V2O5 powders and H2O2 while the freestanding V2O5 electrode was prepared by vacuum filtration method. The details of the synthesis process are given in the Experimental section. In Figure 2a, the electrode has a wrinkled surface, which is a result of shrinking of the V2O5 nanofiber network during electrode preparation. Figure 2b shows a magnified view of the V2O5 electrode surface, revealing a highly porous structure consisting of randomly self-entangled nanofibers. Each nanofiber has a length over 10 μm long with an average diameter around 100 nm. The pores and nanosized V2O5 structure are two important characteristics for transient batteries, because they facilitate fast


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V2O5 dissolution in the alkaline solution. Additionally, the porous structure and nanosized V2O5 can help improve the electrochemical performance in LIBs, since the electrode itself has a high electrolyte uptake and a short lithium ion transfer distance. The PVP membrane separator shown in Figure 2c was prepared by an electrospinning method. The nanofiber has an average diameter of 600 nm and forms a porous nonwoven structure. PVP is a good transient material since it is a water-based polymer and can be dissolved in water with ease. Figure 2d depicts the film surface of the sodium alginate substrate. Na-AG is a good material for employing in lithium-ion batteries since it has been proven to be polymer-electrolyte stable in an organic electrolyte.24 The inset is a portion of the as-prepared alginate film which exhibits the film’s transparency and flexibility. The alginate film was prepared by drying water off from an alginate/water solution in a plastic petri dish. After drying, the Na-AG film can be easily peeled off from the plastic petri dish due to the hydrophobicity of the surfaces. Preparation of the transient current collector is shown in Figure 3. The as-prepared sodium alginate film is used as the substrate to deposit the copper and aluminum current collectors onto via a metal sputtering technique (Figure 3a). First, a piece of a hollowed out polyethylene terephthalate (PET) film is prepared. The hollowed PET mask is used to cover the objective substrate (Na-AG) before using the “shadow mask metal sputtering” method. Through shadowing the undesired area, copper and aluminum can be selectively deposited onto the sodium alginate film and form the desired current collector pattern. Figure 3b shows the PET film with the shadow-mask pattern. The pattern, which copies the size of the objective substrate, is directly printed onto the PET film by using a conventional office laser printer. After removing the undesired portion of the pattern, the hollowed mask is prepared. Figure 3c depicts the hollowed mask after metal sputtering. The brown portion is the copper coating for the lithium


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metal anode, and the white portion is the aluminum coating for the V2O5 cathode. Copper was deposited at a rate of 0.2 nm/s by thermal evaporation while aluminum was deposited at 0.4 nm/s using electron beam evaporation. The thickness of both metal coatings was 100 nm. When one metal is depositing, the other portion of the substrate needs to be protected by an additional film to prevent contamination. After metal deposition, the shadowing region remains clean while the area exposed to the metal sputtering has well-patterned copper and aluminum coatings on the surface. Figure 3d shows the sodium alginate-based current collector in a parallel configuration. The as-prepared current collector can be used directly for multi-cells (i.e. several single cells arranged in a parallel configuration) or be cut off to obtain one single cell for individual demonstrations. A demonstration of the as-prepared current collector in a serial configuration is shown in the inset (Figure 3d). The multi-cell configuration can be freely printed according to desired battery power and energy requirements. Since the sodium alginate film has good flexibility, its current collector also exhibits good bending resilience as shown in Figure 3e. To examine the stability of the metal coating after several bends, SEM was used to inspect the bent areas. Figure 3f demonstrates that no surface cracks are present and the copper coating remains smooth. Similar behavior was evident for the aluminum coating after several bends as well. To check the current collector’s stability in electrolyte, the current collectors were soaked in a traditional LiPF6 electrolyte. After 24 hours, both coatings and the sodium alginate substrate remained visibly intact, which demonstrates the long lifetimes of the current collectors and the packaging. The transience of battery components, including the sodium alginate film, PVP membrane separator, current collector, and V2O5 electrode, are demonstrated by evaluating their dissolution capability in water or a base solution (Figure 4). As the top battery covering, the sodium alginate


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film needs to be thin enough to get dissolved in water and trigger the cascade chemical dissolutions of the remaining battery materials. The sodium alginate film shown in Figure 4a has a thickness of ~0.04 cm, which enables a very short dissolution time (i.e. less than 5 min. in water). While this thickness is sufficient for the top cover, the bottom alginate support layer needs to be thicker to support all the battery components Since sodium alginate has a low solubility, which is normally no more than 2 wt.% in water, its high viscosity makes thick sodium alginate film difficult to get fully dissolved, which needs longer time to be dissolved. Figure 4b shows the dissolution of the PVP membrane separator. Since PVP is a water-soluble polymer, its porous nonwoven structure enables it to dissolve quickly in the presence of water. It can dissolve completely in water in 10 min. In Figure 4c, the dissolution of the sodium alginatebased current collector is demonstrated by using a LiOH base solution. The LiOH base solution (concentration of ~4 M) was prepared by direct chemical reaction between lithium and water. A small piece of lithium foil with a mass of ~0.5 mg was put into 5 mL water to simulate the alkaline environment in the battery dissolution process. Note that this amount of lithium foil is sufficient to provide LiOH environment to dissolve V2O5. In the dissolution process, the thick sodium alginate film (0.1 mm) requires a longer time (~10 min.) to fully dissolve in water. The aluminum coating immediately shrinks when the LiOH base solution is added. After 10 min., most of the aluminum current collector is dissolved, which successfully demonstrates the dissolution of aluminum via an external trigger. When the sodium alginate support film swells through the addition of water then dissolves completely, the copper coating has no substrate to support itself and naturally breaks apart. This state can be considered to be “dissolved” or “transient” even though it is still visible because it is no longer in a usable format and it can no longer be related to the initial structure. The V2O5 dissolution in LiOH solution is shown in


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Figure 4d. The color of the V2O5 changes from yellow to a more pale yellow before it completely dissolves in LiOH to produce a colorless salt, Li3VO4.21 Even though V2O5 dissolves slightly in water, which is unlike most metal oxides, its dissolution rate is not sufficient enough to meet the “transient” concept of batteries. Therefore, the LiOH base solution provided by the reaction of lithium and water is essential to accelerate the V2O5 electrode dissolution, as described by the above-mentioned chemical reactions. In short, all the materials used to fabricate the transient battery meet the important transient requirement described previously. Figure 5 shows the performance of the V2O5 transient battery. In Figure 5a-b, the electrical resistance of the aluminum and copper coatings were measured using a multimeter at a length of 1 cm: 3.8 Ω and 2.0 Ω respectively. These measurements demonstrate the metal sputtering technique can deposit highly conductive coatings on the sodium alginate film and be used as transient current collectors. The electrical resistance of the alginate-based current collector was measured after soaking the current collector in electrolyte for 24 hours and showed a very slight increase to 4.1 Ω and 2.2 Ω for the aluminum and copper coatings respectively. A prototype of the transient battery is shown in Figure 5c. The battery was assembled by stacking each component and sealing the device with epoxy glue. Note that extra electrolyte is required to fully soak the PVP membrane separator and V2O5 electrode before being sealed. In the battery, the V2O5 electrode is 4 mm by 5 mm and the lithium foil is 4 mm by 7 mm to maximize the overlap area on both sides of the PVP separator. The PVP separator is 6 mm by 6 mm to prevent short circuiting and to accommodate the electrolyte. The total weight of the stacked V2O5 battery is about 0.1 g. Figure 5d demonstrates that a conventional LED (threshold voltage: 1.6 V) can be lit up using the transient battery. The electrochemical performance of the transient battery is shown


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in Figure 5e-f. The transient battery was tested in ambient environment with a measured humidity of 60%. The battery was tested in the voltage window of 2.0-4.0 V at a current density of 100 mA/g. The battery has a working voltage of ~2.8 V, and can deliver an energy of ~0.29 mWh. The energy density of this transient cell is ~480 Wh/kg based on the components of the electrodes compared to the theoretical energy density of 760 Wh/kg of Li-V2O5 batteries. In the first cycle, the battery delivered a discharge capacity of 131.3 mAh/g with a high coulombic efficiency of 99%. The performance of 131.3 mAh/g was referring to the specific capacity of V2O5 electrode. In the following several cycles, discharge capacities increased to 136.5 mAh/g with nearly 100% coulombic efficiency. The battery’s discharge-charge curves are stable. The free-standing V2O5 membrane without any conductive agents already delivered a specific capacity of 131 mAh/g which is half of the theoretical value. This is likely due to the fact that V2O5 is a semiconductor with a reasonable electrical conductivity, indicated by its yellowish color. Figure 5g demonstrates the transience cascade of the entire V2O5 battery in water at room temperature. When a small amount of water is added to the battery, the top cover, composed of a thin Na-AG film, rapidly dissolves due to the triggered chemical reactions. Meanwhile, the PVP membrane separator dissolves, and the reaction between lithium and water forms H2 gas and LiOH. Due to the rapid dissolution of the top of the battery casing, the open space allows the H2 gas and the generated heat to be released. The LiOH product creates an alkaline solution that dissolves the V2O5 electrode as well as the aluminum current collector according to the abovementioned reactions. When the thick sodium alginate substrate swells and gradually dissolves in the solution, the copper will disintegrate into small pieces. After 10 sec, the top case dissolves and creates an open hole in the battery, allowing water to reach the separator for base generation


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while simultaneously allowing the generated H2 to escape. The base generation then triggers dissolution of the aluminum coating. At 1 min., the aluminum almost dissolves completely and the structure of the entire battery is completely dissolved, which demonstrates the success of the transient battery as a whole. Note that even though our current transient battery design can only be deployed in dry environment, in the future study will be focused on the battery packaging design that can tolerate moderate level of humidity to allow ambient (humidity) operation of transient batteries.

Conclusions In summary, we successfully demonstrated the first transient rechargeable batteries inspired by the Li-ion battery technology and transience technology that can deliver a stable battery performance with both high voltage and capacity for repeatable uses, and once triggered, the battery can be fully dissolved in water with a transience time within minutes. The transient battery is made up of dissoluble electrodes (V2O5 cathode and lithium metal anode), and dissoluble polymer-based separator and cell enclosure. All the components are robust in a traditional LIB organic electrolyte and disappear in water completely within minutes due to triggered cascade reactions. This transient battery meets the following characteristics that 1) all the constituent materials must physically disappear, 2) show fast transience time, 3) exhibit high voltage and capacity, 4) have proper battery size and mass and 5) flexible design of battery to meet various voltage and capacity levels. The materials, fabrication methods, and integration strategy presented in this work are of interest for further developments in transient energy storage and other transience technology.


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Experimental Methods V2O5 nanofiber synthesis. Vanadium oxide (V2O5) powder (≥99.6%, Sigma-Aldrich) was used directly without further purification. First, 0.8 g V2O5 was dispersed into 60 mL DI water with magnetic stirring for 1 hour to form a yellow solution. Then 10 mL hydrogen peroxide (H2O2) (30%) was added to the V2O5 solution and stirred for 2 hours to obtain a transparent orange solution. The solution was then heated in an autoclave oven at 210 oC for 100 hours to induce a hydrothermal reaction. After the treatment, the V2O5 was washed several times with DI water and vacuum filtrated into a film. After drying at 80 oC in a vacuum oven overnight, a freestanding V2O5 film was obtained. PVP membrane separator preparation. Polyvinylpyrrolidone (PVP) powder (average Mw=1,3000,000, Alfa Aesar) was used without further purification. 30 mg PVP powder was dissolved in 15 mL ethanol and magnetically stirred overnight to form a clear solution for electrospinning. A high voltage power supply was used to provide a voltage of 10 kV. The feeding rate was 10 µL/min., and the tip-to-collector distance was 15 cm. After 2 hours of electrospinning, the PVP membrane was dried at 60oC overnight to remove the residual solvent. Sodium alginate-based current collector preparation. Sodium alginate (Na-AG) powder (MP Biomedical LLC) was used without further purification for the preparation of the Na-AG film. 2 mg Na-AG was dissolved in 100 mL DI water and magnetically stirred at 70 oC overnight. The as-prepared Na-AG solution was then transferred to a plastic petri dish and dried in an air-flow environment until a Na-AG film was formed. The thickness of the Na-AG film was controlled by varying the amount of Na-AG solution placed in the petri dish. After drying at 60oC, the Na-AG


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film can be easily peeled off of the plastic petri dish due to the difference in surface hydrophobicity. Copper and aluminum were deposited by a shadow mask metal sputtering method on the as-prepared Na-AG film to create the current collector. A mask was prepared by printing the desired pattern on a polyethylene terephthalate (PET) paper via an office laser printer and then cutting it into a hollowed mask. After covering the top surface of the Na-AG film with the mask, 100 nm of Cu and Al were deposited using a Metra Thermal Evaporator at a rate of 0.2 nm/s and a Denton E-bean Evaporator at a rate of 0.4 nm/s respectively. When one metal was being deposited, the other side of the film was covered for protection. Characterization methods. The morphology of battery materials, including electrodes, separator, and current collector was examined using scanning electron microscopy (SEM, JEOL 2100F). The transience of each battery component was evaluated by recording dissolution time in both water and 4M LiOH solution. Battery performance evaluation. The transient battery was composed of a V2O5 cathode, a PVP membrane separator, lithium foil anode, and 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 vol%) as the electrolyte. Thin lithium film was prepared by pressing a lithium granule in a hydraulic press. The thin lithium film has a thickness of 0.25 mm. The cathode and anode dimensions were 4 mm × 5 mm and 4 mm × 7 mm, respectively. The PVP separator was 6 mm × 6 mm. The battery was assembled by stacking electrodes and the separator on the as-prepared current collector in an argon-filled glove-box. Epoxy adhesive was used to seal the electrodes and separators in between the alginate films. The galvanostatic charge-discharge tests were conducted in a voltage range of 4.0 to 2.0 V versus Li/Li+ with current density of 100 mA/g using a LAND-CT 2001A battery test system. The actual material loading is 0.7 mg.


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FIGURES Figure 1. Transient batteries triggered by cascade reactions. (a) Fabrication of transient batteries. (b) Schematic process of triggered battery dissolution. (c) The chemical reactions responsible for the dissolution of each battery component. Figure 2. SEM images of the transient battery components. (a-b) The V2O5 electrode is a nanofiber network with a wrinkled surface morphology. (c) Electrospun PVP membrane separator. (d) Alginate substrate with a dense and uniform surface. The inset image shows a transparent alginate substrate with good flexibility. Figure 3. Preparation of the transient current collector. (a) Schematic showing the metal sputtering process on the sodium alginate substrate. (b) The printed pattern of the current collector mask on the PET film. (c) The hollowed current collector mask after metal sputtering. The brown and white portions are copper and aluminum, respectively. (d) The sodium alginate substrate with well-patterned copper and aluminum coatings on the surface. (e) The flexibility of the sodium alginate-based current collector. (f) SEM image of the metal coating; no cracking was observed on the bent areas of the current collector. Figure 4. Transience of battery components. (a) Dissolution of the sodium alginate film in water. (b) Dissolution of the PVP membrane separator in water. (c) Dissolution of the sodium alginate-based current collector in the LiOH solution. (d) Dissolution of V2O5 in the LiOH solution. Figure 5. V2O5 transient battery performance. (a-b) resistance measurements of Al and Cu current collector, respectively. (c) Digital image of a single transient battery. (d) An LED powered by a V2O5 transient battery. (e) Charge-discharge curves and (f) cycling performance


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of the V2O5 transient battery. (g) Dissolution of the transient battery in water at room temperature.

AUTHOR INFORMATION Corresponding Author Liangbing Hu Assistant Professor Department of Materials Science and Engineering University of Maryland College Park, MD 20742 Email: [email protected] Phone: +1-301-405-9303 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT We acknowledge the support of the Defense Advanced Research Projects Agency (DARPA) and BAE Systems.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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Transient Rechargeable Batteries Triggered by Cascade Reactions

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