Perspective on gallium-based room temperature liquid metal batteries

Abstract

Recent years have witnessed a rapid development of deformable devices and epidermal electronics that are in urgent request for flexible batteries. The intrinsically soft and ductile conductive electrode materials can offer pivotal hints in extending the lifespan of devices under frequent deformation. Featuring inherent liquidity, metallicity, and biocompatibility, Ga-based room-temperature liquid metals (GBRTLMs) are potential candidates to fulfill the requirement of soft batteries. Herein, to illustrate the glamour of liquid components, high-temperature liquid metal batteries (HTLMBs) are briefly summarized from the aspects of principle, application, advantages, and drawbacks. Then, Ga-based liquid metals as main working electrodes in primary and secondary batteries are reviewed in terms of battery configurations, working mechanisms, and functions. Next, Ga-based liquid metals as auxiliary working electrodes in lithium and nonlithium batteries are also discussed, which work as functional self-healing additives to alleviate the degradation and enhance the durability and capacity of the battery system. After that, Ga-based liquid metals as interconnecting electrodes in multi-scenarios including photovoltaics solar cells, generators, and supercapacitors (SCs) are interpreted, respectively. The summary and perspective of Ga-based liquid metals as diverse battery materials are also focused on. Finally, it was suggested that tremendous endeavors are yet to be made in exploring the innovative battery chemistry, inherent reaction mechanism, and multifunctional integration of Ga-based liquid metal battery systems in the coming future.

Graphical abstract

Keywords

liquid metals / soft electrodes / flexible batteries / deformable energy supply devices / epidermal electronics

1 Introduction

The current advancements in deformable devices such as wearable epidermal electronics, sensors, and soft robotic systems have given impetus to the exploration for flexible energy supply devices [1]. A pretty rapidly growing sector of energy economy is that of electrical energy generation [2–4], which is currently the most versatile and convenient form due to its high conversion efficiency and absence of gaseous exhausts [5]. Batteries, referring to the device that provides electric energy through electrochemical reaction in the traditional sense, are one of the most common and familiar electrical energy supply devices. Nevertheless, conventional rigid electrical batteries have emerged as a critical bottleneck for the development of deformable electronics [6]. Over the last decades, a series of emerging technologies such as multi-form nanogenerators [7,8], supercapacitors (SCs) [9,10], and photovoltaic solar cells [11–13] have also been exhaustingly studied for powering, which should not be overlooked. Herein, the concept of batteries is extended from electrochemical cells to electric supply devices to cater to technological developments.

To meet the requirements of deformable devices, meticulously designed structures and elaborately operated processes are explored for batteries. Generally, coating a thin layer of hard material onto an elastomeric substrate, or connecting the hard material with a specially designed structure on a soft substrate are two universal strategies for flexible batteries. Structures like origami [14], kirigami [15], spring [16], serpentine [17], and island-bridge [18] have appeared in batteries. Complex fabrication processes like electron beam evaporation [17] and photolithography [14] usually set a high demand of time and investment. However, despite the delicateness of the structures and processes, the originally rigid materials cannot endure frequent deformation and movement. Only materials that are inherently flexible are perfectly suited for deformable devices.

Most metals and their alloys are solid at room temperature. Alkali metals and their alloys, mercury (Hg), and gallium (Ga) and their alloys, are in liquid state around ambient temperature (melting point<60°C). Alkali metals and their alloys are so active in air that they are not suitable for powering skin electronics. Hg is restricted because of the high toxicity and surface energy, while Ga and Ga-based room-temperature liquid metals (GBRTLMs), enumerated in Table 1, have shown sufficient stability in a natural environment and have been proven to have biocompatibility with human beings. GBRTLMs, with dual properties of metal and liquid, are emerging intriguing functional materials. They have recently been widely investigated in thermal management [23–28], flexible and reconfigurable electronics [29–32], microfluidic technology [33–36], soft robots [37–39], and biomedical engineering [40–42]. Moreover, GBRTLMs have been used in a variety of energy applications, including energy production [43–46], conversion [6,47,48], and storage [49–52]. They are competitive candidates for the realization of deformable batteries. Guo et al. [53] analyzed the design principles and relevant designs of easily fused metal- (<183°C) based batteries. Li et al. [54] reviewed recent research progresses of Hg, Ga, and In metals in Li, Na, K, and multi-valent metal batteries. Zhang et al. [55] summarized the remaining challenges and current progress of room-temperature liquid metal batteries from the aspects of wettability behavior, volume changes, and interface formation.

Nevertheless, GBRTLMs already have some applications as soft and self-healing electrodes in batteries which have not been systematically discussed. In this paper, the development process and characteristics of high-temperature liquid metal batteries (HTLMBs) are briefly reviewed to illustrate the attraction of liquid components and the limitations of HTLMBs. Subsequently, properties of GBRTLMs which enable them to be proper electrodes are summarized. Then the applications of GBRTLMs as electrodes are introduced in detail, classified as main reacting electrodes, auxiliary working electrodes, and interconnecting electrodes (Fig. 1). Additionally, the perspective of GBRTLMs is also outlined.

2 Properties of liquid metal batteries

2.1 Development and properties of HTLMBs

The history of HTLMBs can be traced back to nearly a century ago when ultrahigh-purity Al was produced. Later, liquid metal batteries have experienced a long period of alternating development and halt until the widespread use of intermittent renewable energy [56,57] which has dramatically increased the demand for low-cost [58], long-lifespan, and grid-scale energy storage devices. Hence, liquid metal batteries once again received attention.

The HTLMBs consist of two liquid metal electrodes, the middle of which is separated by molten salt electrolyte. Due to different densities and immiscibility, the three layers are spontaneously self-segregated into the upper (negative electrode), middle, and bottom (positive electrode) layers. The strong interaction between two electrodes metals provides a thermodynamic driving force for liquid metal batteries. During discharging process, the negative metal is oxidized, and the layer thickness decreases. The oxidized metal ions pass through the molten salt electrolyte to form an alloy with the positive metal. At the same time, the thickness of the positive metal layer increases. The charging process is the opposite of discharging process (Fig. 2(a)). The candidates of the two electrodes are highlighted in Periodic Table [59] (Fig. 2(b)).

Since 2006, Massachusetts Institute of Technology (MIT) has restarted developing HTLMBs. A series of significant works have been reported [59–61]. Sodium-bismuth (Na||Bi) liquid metal battery were first tested with NaF-NaCl-NaI eutectic salt electrolytes at 560°C [59]. Limited by the comparatively higher self-discharge current and higher price of Bi, the researchers shifted their focus on magnesium-antimony (Mg||Sb) liquid metal batteries incorporating a molten salt NaCl-KCl-MgCl₂ electrolyte at 700°C (Fig. 2(c)) [60]. Subsequently, a lithium-antimony-lead (Li||Sb-Pb) liquid metal battery using molten lithium halide electrolyte LiF-LiCl-LiI with an operating temperature of about 450°C were studied [61]. Additionally, it was found that self-healing Li||Bi HTLMBs can be discharged deeper with a reversible solid intermetallic compound formed in the positive electrode. Three orders of magnitude prototype cells were assembled to demonstrate the novel self-healing concept as illustrated in Fig. 2(d) [62].

Further research into liquid metal batteries has focused on exploration of new battery chemistries [62–65], reasonable control of interfaces [66,67], efficient management of battery [68,69], and reduction in the operating temperature [70–72]. Li et al. [65] reported a battery employing alloyed tellurium-tin (Te-Sn) as positive electrode. The introduction of Sn not only enhanced the electronic conductivity of Te electrodes, but also suppressed the solubility of Te in molten salt electrolyte. The Li||Te-Sn cell presented a high discharge voltage and energy density (Fig. 2(e)).

Compared with presently available batteries, the specialties of HTLMBs can be attributed to performance, lifespan, cost, and application fields. In terms of performance, the liquidity of both electrodes and electrolytes endows an excellent kinetic. Regarding lifespan, all liquid active materials are unsusceptible to mechanical failures so that some degradation mechanisms such as volume expansion and dendrite growth can be avoided. In respect of cost, low-budget can be achieved through proper selection of materials which are earth-abundant and inexpensive. So far as application is concerned, liquid metals are sensitive to motion and have magneto hydrodynamic instabilities [73,74], which may cause short circuit. In other words, unlike other batteries which can be used in portable devices and flexible electronics, HTLMBs are only suitable for static energy storage. In addition, other disadvantages of HTLMBs such as the need for high temperature to ensure the molten state of the materials, and the resulting requirements for rigorous thermal management, hermetic seal, and stringent corrosion protection, are summarized in Table 2.

Grid-level storage devices are not suitable for wearable applications. The low-cost and scalability are important for the former while the miniaturization and adaptability are more critical in the latter. GBRTLMs are potential cathode material candidates in principle. Nevertheless, their relatively high market prices hinder their use in large-scale energy storage [20,59]. The utilization of GBRTLMs can reduce the investment of thermal management, sealing, and corrosion protection. In this regard, they can be attractive candidates for smaller device designs where thermal management is critical [75]. On the other hand, the flexibility and stretchability make them suitable as energy supply devices for bendable wearable devices [76]. The main source of Ga is the by-product of the extraction of other metals. Therefore, the price of Ga is volatile, depending on the market of the main products. The preponderance of the relatively high crustal abundance, which is smaller than that of Li, comparable with copper (Cu) and nickel (Ni), and larger than cobalt (Co) [77], indicates the potential great supply and low cost in the future [76]. Additionally, the cost can be further reduced by alloying Ga with cheaper metals such as Sn. Besides, Ga-Sn alloy-based electrode is demonstrated to have comparable electrochemical performance [76].

2.2 Properties of gallium-based liquid metals

2.2.1 Liquidity

Post transition metals with low melting and high boiling points are at a liquid state over a wide temperature range, among which gallium has a melting point of 29.8°C and a high boiling point of 2403°C [20]. The low melting point of gallium attributes to the weak binding of atoms, which is induced by the relatively large interatomic distance. This facilitates the breakup of crystal structure at a low temperature compared to most metals [20]. The melting point can be tuned by introducing other metallic elements like In, Sn, and so on, which causes disordered structure and broadens the atomic distance between metallic atoms [78]. Through adjusting the composition according to the temperature demand, the alloy method makes the extensive application scenarios in a wider range of temperature possible [79]. Ga_75.5In_24.5 (EGaIn) and Ga_68.5In_21.5Sn₁₀ (Galinstan), having a melting point of 15.4°C [80] and 13.2°C, respectively [20], are two typical alloys that are most studied. Besides, GBRTLMs exhibits a strong tendency to be subcooled below the melting point [48,81]. It is worth noting that Galinstan will not freeze until −19°C due to supercooling which is different from melting [20].

Liquidity has a series of advantages such as storage convenience [50], unprecedented lifespan, and superior kinetics transport [59]. However, GBRTLMs possess extraordinary surface tension which is approximately 7–10 times larger than that of water [48]. Because of this, they tend to agglomerate into a sphere, greatly reducing the effective surface area of the electrode. Nevertheless, just because of the large surface tension, GBRTLMs are one of the best candidates as functional battery materials to realize self-healing [5] and the ability to repair damage [82]. Liquid metal electrodes can return to liquid state even after experiencing a solid mesophase during the reaction. As soon as the liquid phase forms, the liquid metal droplets will merge. This feature is enamored by batteries because the rapid development of deformable devices has put forward unprecedented mechanical stretchability requirements. Self-healing would significantly avoid the battery failure induced by large volume change such as fracture and crack formation, thus improving the lifespan, durability, and safety of devices [83].

Liquid gallium can also behave in a completely different solid-like way [84]. As soon as gallium contacts with oxygen, a concentration even as low as a few ppm is sufficient [20], and a passive layer composed of gallium oxides with a thickness of 0.5–3 nm [84,85] forms on the surface. The oxide layer skin will be easily dissolved by acidic or alkaline solutions. It was established that about 0.2 V of the total polarization could be attributed to the drop in voltage across a suddenly formed passive layer on the surface of the gallium [50]. The presence of this skin totally changes the viscosity, surface tension, contact angle, and wetting behavior [86] of GBRTLMs, among which wetting behavior is also an important issue in the batteries. The rapid electrochemical reaction of the batteries requires a good contact between the collector, reactive materials, and electrolyte. The skin facilitates the adherence of GBRTLMs with surrounding environment and enables liquid metals to be patterned in arbitrary shapes [87], making it possible for GBRTLMs to be freestanding electrodes in all sorts of substrates [88].

2.2.2 Metallicity

Metals can be in both solid and liquid states because metallic bonds exhibit a low directionality. Metallic atoms are less electronegative and tend to lose electrons. This means that the outermost electrons are easily separated from the atom, and the electron-deprived atom is immersed in a “cloud” or “sea” of electrons shared by each atom. The liquid metal and the solid metal are different in that the atoms of the liquid metal can also move [20]. Theoretical research on the eutectic alloy Galinstan have been conducted. The results showed that most of the density of free electrons exists around Ga atoms, thus the conclusion could be drawn that the conduction is mainly centered on Ga atoms [78]. The electrical conductivity of GBRTLMs is approximately 3.4 × 10⁶ S/m at 295 K, an order of magnitude lower than Cu and orders of magnitude larger than most liquids [89,90]. In epidermal electronics, the thickness of circuit should be extremely thin (<10 µm) to make conformability possible and reduce the strain applied to the devices or modulus [91]. A series of methods have been developed to patterned GBRTLMs at a high-resolution, such as spraying [92,93], screen-printing [94], transfer printing [95], and GBRTLMs-polymer ink preparing [96,97]. The resistivity of patterned GBRTLMs with different thicknesses can be calculated according to the resistance calculation formula [89]

(1)R\mathrm{=}{\displaystyle \frac{\rho l}{A}},

where R is the resistance, l is the length of the patterned GBRTLMs, and A is the cross-sectional area. For GBRTLMs patterned with different thickness, the variation in resistance is mainly due to the influence of cross-sectional areas. When the width and the length are constant, the resistance is proportional to the reciprocal of the thickness. The experimental results indicated that the resistance of a 100 µm wide and 50 µm thick liquid metal line had a resistance of about 1 Ω [93]. It can be roughly estimated that the resistance decreases to about 0.5 Ω as the thickness increases to 100 µm and to 5 Ω as the thickness decreases to 10 µm. GBRTLMs can maintain a superb metallic conductivity while meeting the thickness requirements of flexible electronics. Hence, when flexibility and adaptability is of utmost importance such as epidermal electronics [98], it is time for GBRTLMs to appear on the stage. Like ordinary metals, GBRTLMs also exhibit an excellent thermal conductivity. For example, Ga has a thermal conductivity of 29.8 W/(m·K) of over 40 times higher than that of water (0.599 W/(m·K)) at 20°C [99].

The metallicity of liquid metals allows them to dissolve most metal elements at relatively high concentrations [20]. As a result, they can be applied as reaction media for metal batteries [75,100]. Taking Li as an example, when Ga is fully embedded with Li, per Ga atom hosting 2 Li atoms, Li₂Ga alloy will be formed with a theoretical capacity of 769 mA·h/g [101], showing a discharge potential close to the Li/Li⁺ reaction [75]. From electrochemical aspect, Ga has a relatively negative standard reference voltage of about −1.22 V versus standard hydrogen electrode (SHE) for the equilibrium [50]

(2){\mathrm{Ga\; +\; 4OH}}^{-}\leftrightarrow {\mathrm{H}}_{\mathrm{2}}{\mathrm{GaO}}_{\mathrm{3}}{}^{-}{\mathrm{+\; H}}_{\mathrm{2}}{\mathrm{O\; +\; 3e}}^{-}.

Although Ga has a low equivalent weight of 23 g/F, another property that is noticeable is the high polarization of Ga relative to hydrogen [102], leading to its high corrosion resistance. Pure Ga does not corrode easily, and the corrosion rate is low even at high temperatures. It is measured that the amount of Ga that dissolves corresponding to a corrosion current is about 8 mA/cm² in 6 mol/L of KOH solution without flow of current [50].

2.2.3 Safety

Safety is always of the utmost importance when powering flexible epidermal electronics. Alkali metals have been extensively studied in batteries. However, there are inevitably a series of safety problems, such as thermal runaway, dendrite formation, and inflation, which may occur instantaneously and cause catastrophic safety accidents [103]. For example, Li metal is an attractive anode, but its practical application has been prevented by the uncontrollable dendrite formation over 40 years [104]. Unlike alkali metals, GBRTLMs generally are biologically safe [105] and own a very small vapor pressure (<10⁻⁶ Pa at 500°C [106]). It was experimentally verified that GBRTLMs did not have a general cytotoxicity [107,108] and biotoxicity [106,109]. In addition, a growing number of experiments have been conducted in vivo and in vitro to show the good biocompatibility of GBRTLMs [41,42,105,110–112]. Moreover, GBRTLMs are stable and nonflammable in air. These features make GBRTLMs very convenient to be handled.

It is worth noting that the contact of GBRTLMs can cause severe embrittlement in normally ductile solid metals, such as Al and Cu [113]. The chemical reaction between the Ga-In eutectic alloy with Al and the Al-base alloy have been studied [114]. The experimental results show that liquid metal diffuses on the surface, grain-boundary, and volume of Al. The surface diffusion destroys the oxide film and reduces the surface energy of Al. The grain-boundary diffusion and volume diffusion cause the change of composition in Al and lead to embrittlement [114,115], which has been exploited to achieve instant hydrogen production [116,117] for hydrogen fuel cells. Nevertheless, when GBRTLMs are designed as the electrode, the structural material of the battery should be properly screened to avoid corrosion.

With an intrinsically excellent liquidity and metallicity, adequate safety, relatively negative standard reference voltage, self-healing capability, and ability to be patterned in arbitrary shape, GBRTLMs have the potential to be flexible electrodes in batteries.

3 Gallium-based liquid metals in batteries

3.1 Gallium-based liquid metals as main reacting electrodes in batteries

At this stage, GBRTLMs as main reacting components in batteries have not been widely explored. In this part, several existing studies, listed in Table 3, are summarized and discussed in terms of battery configurations, working mechanisms, and battery functions. Further, there are also proposed candidates in patents to realize various prototyped room temperature liquid metal batteries for industrial trials [118,119].

3.1.1 Battery configurations

Designing of electrodes, electrolyte, and cell structure are three key elements of battery configuration. When GBRTLMs are designed as the main reacting electrodes, several issues need to be carefully addressed, including melting point, surface tension, density, and reacting rate. The melting point of pure Ga is slightly higher than room temperature which is not convenient for experiment and application. Thus, Ga-based alloys with lower melting points such as GaIn₁₀ [120], Ga_75.5In_24.5 [121], and Ga_68.5In_21.5Sn₁₀ [51] are usually used as the main reacting liquid electrodes. There are several advantages when other metallic elements are introduced. It maintains the electrodes in the liquid state, enhances the capacity [122], and inhibits the corrosion of Ga in electrolyte due to the high hydrogen evolution over potential [120]. Moreover, the cost of electrodes can be reduced by introducing cheaper elements [76]. The high surface tension and density of GBRTLMs are another two key impediments. To decrease the surface tension, functional additives [123] and special structural designs [51,120,121] have been explored. It is proved that calcium chloride (CaCl₂) additives (Fig. 3(a)) can improve the interface reaction activity, thus enhancing battery performance [123]. Unique structure such as Cu-coated carbon fibers is prepared to avoid the influence of surface tension as depicted in Fig. 3(b) [51]. This structure combines the intermetallic alloying method to realize a better physical wetting of GBRTLMs and carbon fibers. The GBRTLMs are finally successfully dispersed on the supported substrate with a large superficial area to increase the active electrode area. To overcome the limitation caused by the high density and macroscopic interface of GBRTLMs, Gao et al. [121] introduced decomposition agent alkali bicarbonate to foam them. The preparation process of liquid metal foam (LMF) anode is illustrated in Fig. 3(c). The Ga is kept in a semisolid state by placing it in a thermostat water bath at a temperature slightly above the melting point of Ga. Then, NaHCO₃ particles are stirred into the beaker. Ga and NaHCO₃ composites (LMC) are placed on the heating platform, and the foaming process of LMF is based on the thermal decomposition reaction of NaHCO₃ particles. In GBRTLMs-air cells, the reaction rate of the cathode is usually limited. To improve the oxygen reduction reaction rate of the cathode, classical catalysts, such as Pt [120] and MnO₂ [124], are usually incorporated. Liu et al. [120] coated a thin layer of Pt nanoflowers array on the surface of the carbon fiber yarn by using the electrodeposition method to fabricate cathode electrode. This electrode enhanced the discharge performance of the battery by about 1.5 times, with a power density of 0.383 mW/cm² at 1.5 V.

The properties of GBRTLMs make the requirements of electrolyte both loose and rigorous. The loose property can be attributed to the liquid state which allows it to be molded into any shape, so the electrolyte can be filter paper [51], hydrogel [120,123] or any other forms as needed. It is rigorous because the easily formed passive skin on the surface of the GBRTLMs need to be removed in the electrolyte otherwise it will hinder the reaction. Therefore, most of the studies conducted have used the alkali solution as electrolyte [51,120,121]. Moreover, with a lower donor strength than water, the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]) has also been explored to shift the redox potential of Ga(III)-Ga toward a more stable electrochemical potential window [52]. The ionic liquid can stabilize charged species by effectively shielding charges [125].

The utilization of GBRTLMs as electrodes do not have much restriction on the structure of batteries. Both sandwich-shaped (Figs. 4(a), 4(b), 4(d)) [51,121,123], 3D-printed (Fig. 4(c)) [126], and the cable-shaped (Fig. 4(e)) [120] batteries were constructed and demonstrated to be feasible. Poly(dimethylsiloxane) (PDMS), polyvinyl Chloride (PVC) tape, and thermoplastic polyurethane (TPU) were used as packaging materials in these studies. Using a Na-K alloy anode and a Ga-based alloy cathode as shown in Fig. 4(d), working in a similar way to HTLMBs, the all-liquid metal battery can be operated at room temperature. The temperature can even drop to −13°C when using Galinstan as cathode [76]. Recently, a shape-variable secondary liquid metal battery is developed using Ga₆₈In₂₂Sn₁₀ as liquid anode and a conductive polymer polyaniline (PANI) as cathode [127]. The battery can be deformed with a several millinewtons force without any capacity loss and realize a shape-adjustable battery construction among 1-D fiber, 2-D sheet, and 3-D spherical.

3.1.2 Working mechanisms

GBRTLMs as main reaction electrodes have been researched in both primary and secondary batteries. In the GBRTLMs-air galvanic cell, the GBRTLMs act as the negative electrode and lose electrons to be oxidized, while the oxygen in the air gains electrons to be reduced. The existing several works basically adopt such a principle [50,51,120,121]. Liu et al. [120] clearly elucidated this process using experiments, energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) (Fig. 5(a)). They assembled an open-ended battery with GaIn₁₀ as the anode and Pt coated carbon fiber yarn as the cathode in a glass beaker with 37.5% KOH solution as electrolyte. Three products, indium, In(OH)₃, and GaOOH were found at the bottom of the beaker after one week of short-circuit current discharge. This meant all the Ga and a portion of the In participated in the discharge reaction and the remainder of the In was solidified into an In metal block. Hence, the working mechanism included five reactions, elaborately illustrated in Figs. 5(b)–5(c) [120]. The effective reactions in the anode and cathode are,

Anode:

(3)\mathrm{G}\mathrm{a}-\mathrm{3}{\mathrm{e}}^{-}\to \mathrm{G}{\mathrm{a}}^{\mathrm{3}+},

(4)\mathrm{I}\mathrm{n}-\mathrm{3}{\mathrm{e}}^{-}+\mathrm{3}\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{I}\mathrm{n}{(\mathrm{O}\mathrm{H})}_{\mathrm{3}}.

Cathode:

(5){\mathrm{O}}_{\mathrm{2}}+\mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+\mathrm{4}{\mathrm{e}}^{-}\to \mathrm{4}\mathrm{O}{\mathrm{H}}^{-}.

GBRTLMs can be used as either anodes or cathodes in secondary batteries. When used as positive electrodes, GBRTLMs mainly matched with the metal with higher activity, such as Li [72,75,128] and Na-K [76] alloy, based on the principle of HTLMBs. When used as negative electrodes, the batteries operate via a hybrid mechanism of stripping and plating of Ga³⁺ and transformation reaction of paired electrodes such as Cl⁻ [127] and MnO₂ [123]. Liu and his coworkers [123] constructed a rechargeable battery system using Ga₇₅In₂₅ anode and MnO₂ cathode and analyzed the working mechanism (Figs. 6(a), 6(b)) of the rechargeable battery system through calculation and testing. Specifically, EDS mapping for the anode reactants in four states were taken respectively, including the pristine state, the state after the first discharging process, the state after first charging process, and the state after 100 discharge-charge cycles (Fig. 6(c)). The rechargeability of battery was accomplished through a mechanism that involved reversible stripping and plating of gallium along with MnO₂ chemical conversion.

Anode:

(6)\mathrm{G}\mathrm{a}-\mathrm{3}{\mathrm{e}}^{-}+\mathrm{6}\mathrm{O}{\mathrm{H}}^{-}\leftrightarrow \mathrm{G}\mathrm{a}{\mathrm{O}}_{\mathrm{3}}^{\mathrm{3}-}+\mathrm{3}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}.

Cathode:

(7)\mathrm{3}\mathrm{\beta }\mathrm{-}\mathrm{M}\mathrm{n}{\mathrm{O}}_{\mathrm{2}}+\mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+\mathrm{4}{\mathrm{e}}^{-}\leftrightarrow \mathrm{M}{\mathrm{n}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{4}}+\mathrm{4}\mathrm{O}{\mathrm{H}}^{-}.

GBRTLMs as the main reaction anode electrodes material in rechargeable batteries have not been exhaustingly explored, such as the selection of matching electrodes, suitable electrolyte, and rechargeable chemistries.

3.1.3 Battery functions

GBRTLMs based batteries are supplement to the vacancy of HTLMBs in the field of portable devices and epidermal electronics. Wearable devices are required to deform smoothly with the body. Utilizing Ga₇₅In₂₅ and MnO₂ as anode and cathode, alkaline hydrogel as electrolyte, and soft elastomer as package, an all-soft stretchable battery was constructed [123]. The battery is highly flexible with a stretchability of up to 100% tensile strain and a bending radius of less than 2 mm without obvious structural damage. In practical application, the battery can continuously power an light emitting diode (LED) under a dynamic loading and a strain gauge while being attached and stretched on skin (Fig. 7(a)). The liquidity of GBRTLMs electrode endows the battery more unique properties. It can be a renewal anode simply with a syringe injecting and extracting. The discharge current can be easily controlled by adjusting the amount of GBRTLMs or the applied pressure of finger (Fig. 7(b)) [120]. In addition, the GBRTLMs-based batteries have the potential to be deformable energy devices at low temperatures and demanding environments [76,127]. Fu et al. [127] reported that a GBRTLMs-based battery delivered a capacity of 67.8 mA·h/g at 0.2 A/g with 100% of elasticity at approximately −5°C.

Gallium-based liquid metals have also been extensively studied in printed flexible circuits such as the DIY “Christmas tree” in Fig. 8(a). This feature can also be combined with flexible batteries to achieve fully flexible integrated systems. Additionally, with the further efforts of scholars, multiple functions like signal transducing and sensing have also been further integrated into GBRTLM battery as illustrated in Fig. 8(b) [51]. The humidity is converted into low and high-potential signals. Only when the humidity goes beyond the set threshold, will the digital switch turn the LED light on. The sensing function is achieved by the different responses of batteries to ethyl alcohol, methyl alcohol, and deionized water (DI) water, which is enabled by the sensitivity of Ga oxide layer. Devices integrated with energy storage, signal conversion, and sensing may have great potentials in the field of flexible electronics to realize all-in-one design [91].

3.2 Gallium-based liquid metals as auxiliary working electrodes in batteries

Having superior theoretical energy densities, electrochemically active metal anodes such as Li and Na have aroused much interest in development of advanced rechargeable batteries [129]. However, the failure mechanisms (Fig. 9(a)), including the growth and decomposition of solid electrolyte interface (SEI), separation of active material and collector, particle fracture and isolation, and random growth of dendrites need to be analyzed and solved [129,130]. A great deal of effort has been made to increase the safety of batteries. Just like the autonomous ability of positive feedback regulation biological system, researchers have dedicated for the autonomous materials for battery systems. Controllable release of microencapsulated functional additives and integration of self-healing interlayers are two effective methods [130]. Just as listed in Table 4, GBRTLMs have been widely studied and utilized in lithium and non-lithium batteries as self-healing auxiliary electrodes.

3.2.1 Lithium batteries

Lithium metal is a prospective anode owing to its low density (0.59 g/cm³), high theoretical specific capacity (3860 mA·h/g) and lowest negative electrochemical potential ( − 3.04 V versus SHE) [131,132]. However, Li-metal has such a high chemical reactivity that a series of problems have yet to be solved such as uncontrollable growth of Li dendrites and huge volume expansion. Metallic substrate can regulate Li deposition behaviors to inhibit the dendrite growth [133]. GBRTLMs such as Ga [131], GaSn [134], and GaInSnZn [135–137] have been used in lithium metal batteries (LMBs) and lithium-ion batteries (LIBs) to realize multifunctional regulation owing to their self-healing abilities [100,138]. As can be seen from the phase diagram [139] in Fig. 9(b)-i, the process of lithiation will lead to a rapid solidification of Ga. The phase diagram illustrates the preferred phase of Li-Ga alloy at different compositions at room temperature [53]. In the charging and discharging cycle at 40°C, as shown in Fig. 9(b)-ii, multiple voltage plateaus corresponding to the phase diagram of the intermetallic Li₂Ga₇, LiGa, and Li₂Ga are formed. Deshpande et al. observed that the cracks of the metal could be repaired after complete delithiation [75], thus achieving self-healing. By means of being coated onto current collector, Li metal surface, and Mxene framework, GBRTLMs are demonstrated to regulate the nucleation barrier and thus realize isotropic Li deposition, stable C layer formation, and higher Li⁺ diffusion coefficient (Fig. 9(c)) [131,134–137].

Si [140], Ge [141], and Sn [142] are promising anode materials for LIBs whose theoretical capacities are several times larger than those of the commercially used graphite (372 mA·h/g) negative electrode. However, they suffer from the short cycle life induced by volume expansion and contraction during the charging and discharging process [75]. GaInSn liquid alloy was utilized to spontaneously repair Si anode for Li-ion battery [143]. It was found that the mechanical stress induced by volume change could be absorbed by GBRTLMs. In addition, the fluidity of liquid metal ensured the eternal contact between Si and conducting network. Luo et al. [101] have studied the self-healing behavior of Ga thin film as an anode material for LIB in liquid electrolyte. The experimental results demonstrated that the self-healing ability of Ga films was limited. As the cycle went on, the effective self-healing areas decreased gradually, and the reversible capacity of the battery reduced seriously. After 25 cycles, large-size cracks of 10 µm appeared on the film electrode, and the reversible capacity declined to 245.7 mA·h/g, with a capacity retention rate of only 44.2%. The decreased self-healing ability of Ga may be related to the formation of the SEI film, which may attach to the crack surface and isolate Ga in different areas. To achieve a better self-healing ability, Ga-Sn liquid alloy was stabilized in a reduced graphene oxide (RGO)/ carbon nanotube (CNT) skeleton to be the anode for LIBs [5]. The RGO/CNT skeleton improved the electrical conductivity and prevented the GBRTLMs detaching from the current collector. The new anode ended up exhibiting an excellent cycle performance, retaining nearly a capacity of 100% within 4000 cycles. Moreover, researchers have confined GBRTLMs on Mxene paper carrier [144], interwoven carbon nanofiber [145], and porous carbon matrix [100] to avoid the influence of surface tension and fabricate freestanding electrodes.

3.2.2 Non-lithium batteries

GBRTLMs also play important roles in ameliorating the interfacial issues in non-lithium batteries, such as zinc-ion batteries (ZIBs) and aluminum-ion batteries (AlIBs). Attributed to their high theoretical specific capacity (820 mA·h/g) and non-flammability in aqueous electrolytes, ZIBs are one of the promising alternatives for meeting the requirements of high energy density and safety [144–146]. Similar to lithium batteries, ZIBs are also faced with serious dendrite problems, accompanied with inevitable corrosion of zinc anode, which greatly affects the lifespan of ZIBs [147]. Through Ga-In-Zn phase diagrams, Liu et al. [146] designed an alloying liquid interlayer by coating liquid Ga-In alloy on Zn (GaIn@Zn) anode. Through this process, a liquid-liquid interface (about 30 µm thick) was established to regulate the deposition behaviors of Zn anode. It was experimented that the charge capacity of GaIn@Zn anode can reach more than 12 mA·h/cm² at 1 Ma/cm², while that of bare Zn was less than 6 mA·h/cm². Moreover, the Tafel curve suggested that the overpotential of the modified electrode was improved effectively and thus helped resist the corrosion in electrolyte. These results illustrated that the introduction of GBRTLMs not only accelerated the mass transport but also enhanced the long-term cycling stability of electrodes (Fig. 10(a)). The advantages of AlIBs mainly lie in their low-cost, high-energy density, and safety [148]. Meanwhile, Al anodes are faced with critical problems, including dendrite, corrosion, and pulverization. Jiao et al. [149] introduced liquid Ga as the negative electrodes of Al batteries. When Al foil was used as the negative electrode of the battery, it corroded and pulverized after 28 cycles, while Ga maintained in liquid state after 100 cycles as illustrated in Fig. 10(b). But the unstable liquid-liquid interface between Ga and electrolyte determined that it was only suitable for stationary energy storage.

3.3 Gallium-based liquid metals as interconnecting electrodes in multi-scenarios

In addition to electrochemical batteries, devices such as photovoltaics solar cells, generators, and SCs have also been widely exploited. Similarly, rigid and fragile materials have been phased out in order to meet the needs of deformable devices. Among them, connecting electrodes is a prerequisite for ensuring normal work under deformation. In virtue of high stretchability and conductivity, GBRTLMs are born with the ability to be implemented as flexible and self-healable interconnecting electrodes.

3.3.1 Photovoltaics solar cells

Eutectic Ga-In liquid metal has been used in organic [12], organic-inorganic [159], perovskite [13] photovoltaics solar cells, which allows high mechanical stability of the device without any degradation of performance over repeated bending. In organic solar cells, organic semi-conductors have a relatively low charge mobility, thus an electrical contact to the cell must cover the active surface without shorting the device and provide uniform contact to the organic film [160]. Aluminum, having a work function of 4.3 eV, is most widely used in organic cells as a top metal contact through the method of costly, time-consuming vacuum thermal metal evaporation. The work function of GaIn₂₅ is very close to Al. Besides, it is non-toxic and in liquid state at room temperature, which greatly facilitates the process of depositing under an environmental condition [161]. This offers a simple vacuum-free and inexpensive method compared with the Al electrodes, making GaIn₂₅ a good candidate for replacing the Al as illustrated in Fig. 11(a). GaIn₂₅ has gradually been used as top contact in the field of testing the photovoltaic characteristics of organic solar cells [11,12,160–163]. Core-shell-structured Ga-In-Sn-Zn eutectic alloy microcapsules were used as self-healing conductors for sustainable and flexible perovskite solar cells [13]. Pressing or cutting causes the capsule to break, and liquid metal inside can flow out to repair the damaged parts of the wire (Fig. 11(b)). The results exhibited a power conversion efficiency retention (PCE) of 99% relative to the initial value. An intrinsic mechanically recoverable organic-inorganic perovskite solar cell utilizing Ga-In alloy as stretchable electrodes was presented in Fig. 11(c). It successfully prevented mechanical damage of the perovskite layer during bending and crumpling. After recovery from crumpling, the PCE of the device dropped to 6.1% from the initial value of 10.2% [159]. The development of self-healing and shape-recoverable solar cells is expected to provide a wearable energy source for practical applications.

3.3.2 Generators

GBRTLMs have been exploited in a series of electrical generators. The present laboratory had ever [43,164] proposed different liquid metal based magnetohydrodynamics generators to harvest human energy and power for wearable micro/nano devices. Such strategy also works for harvesting waste heat to power thermoelectric generator [165]. Recently, all kinds of flexible nanogenerators enabled by soft GBRTLMs electrodes have been further studied, including triboelectric nanogenerators (TENGs) [8,166,167], thermoelectric nanogenerators (TEGs) [7,168,169], and piezoelectric nanogenerators (PENGs) [170,171].

TENGs, based on the coupling of the triboelectric effect and the electrostatic induction phenomenon, have four basic operating modes, including the vertical contact-separation mode, the lateral sliding mode, the single-electrode mode, and the freestanding triboelectric-layer mode [172]. Conventional TENGs mainly uses solid electrode materials such as Al and Cu. Therefore, the contact efficiency is greatly affected by the roughness between the two layers [173]. Tang et al. [167] developed a GBRTLMs-based TENG as shown in Fig. 12(a). It had a contact area of 15 cm² and could generate a voltage of 679 V and a current of 9 µA. Moreover, compared with solid-solid contact TENGs, it can achieve over four times more output charge density (430 °C/m²) and higher energy conversion efficiency (70.6%). Intrinsically stretchable and self-healable GBRTLMs not only guarantee a total electrical contact, but also maintain a low resistance even under large deformation and multiple freedom degrees. Using the silicone rubber layer as a triboelectric and encapsulation material, a GBRTLM-electrode-based stretchable TENG was designed to harvest energy from irregular and low-frequency human motions through a patch or integrated into clothing to drive wearable electronic devices [174]. GBRTLMs can also give TENGs more features. They can act as phase change materials with melting points in the comfortable temperature range of human skin, realizating a self-powered thermoregulating electronic skin (TE-skin) (Fig. 12(b)) [166].

Just like TENGs, GBRTLMs have been explored to achieve a complete contact in soft PENGs [171] and TEGs [7,44,169]. It is worth mentioning that in TEGs, GBRTLMs is generally composited with elastomers. GBRTLMs can be effective additives to transform elastomers into multifunctional composites [19,170]. The most common method is to enclose GBRTLMs in an elastomer, or dope with other functional materials like graphene nanoplatelets at the same time. Zadan et al. [169] proposed a soft and stretchable TEGs utilizing GBRTLMs embedded elastomer as material interface. The elastomer contained a mechanically sintered pattern to provide electrical connections between p-type and n-type Bi₂Te₃ semiconductors. Owing to the ductility of liquid metal and elastomer, they did not electrically or mechanically fail when stretched to strain above 50% (Fig. 12(c)). Moreover, flexible connection provides a pathway for the realization of seamless integration. PENG and TENG were reasonably integrated by Yang et al. [8]. A stretchable piezoelectric-enhanced TENG was fabricated to achieve a higher output performance as illustrated in Fig. 12(d).

3.3.3 SCs

SCs, known as electrochemical capacitors, are different from typical batteries in materials, structures, and charge storage mechanisms. Typical batteries storage charge in bulk electrodes while SCs via surface. The different mechanisms determine their different characteristics [175]. SCs have noteworthy priorities such as long cycle life, simple structure, and fast rates of charge and discharge [9]. Traditional SCs are made of hard and brittle materials, which are ubiquitous for electrodes and connectors. The development of flexible integrated device microsystem poses new challenges to SCs. They should not lose performance or even fail in multiple mechanical deformations [10]. GBRTLMs could be potential candidates as soft electrode materials for flexible SCs because of their insulating surface oxide layer for electric double layer formation [176]. Kim et al. [10] used Ga-In alloy integrated with oxygen functionalized CNTs as electrodes to construct all-soft SCs for soft microsystems (Fig. 13(a)). The constructed SCs exhibited an area capacitance of 12.4 mF/cm² which remained nearly unchanged under an applied strain of 30%. With the increased investigation into the integrated flexible and foldable SCs [177–179], the mechanically stable interconnections between the devices have also become more intractable. Special structures like serpentine are designed to solve the connection problem, which often require complicated processing [180]. Maintaining a superior conductivity and conformability even under high deformability, GBRTLMs are proper alternatives for flexible electrical connections which can achieve desired functions through simple fabricate processes [32]. With GBRTLMs patterned as interconnections on paper as substrate, a foldable and deformable sensor system was constructed driven by integrated micro-SCs (Fig. 13(b)) [9]. The system showed a mechanically stable ultraviolet (UV) light sensing even under repetitive folding cycles.

4 Summary and perspective

In summary, GBRTLMs are intrinsically excellent combinations of liquidity, metallicity, and biocompatibility. The metallicity is the premise for their applications as electrodes. The liquidity is the guarantee of the fast kinetics and the self-healing characteristic. The biocompatibility ensures their extensive use in daily life. In these regards, GBRTLMs demonstrate potentials to be battery materials in the field of portable devices and epidermal electronics. Nevertheless, it is worth noting that some issues must be taken into consideration. First, their prices are not competitive, which determine that they are not suitable for large-scale energy storage. Then, corrosion may occur when GBRTLMs are in contact with Al and Cu. Thus, the construction materials of the batteries need to be screened. Besides, the high surface tension of GBRTLMs may cause poor contact among current collector, electrodes, and electrolyte. Therefore, extra special treatment is required for GBRTLMs to realize a better wetting.

As the main reacting electrodes, GBRTLMs have given the battery unique functions such as flexibility, wearability, and printability. Moreover, the batteries are promising as deformable energy storage devices in harsh environments at low temperatures. However, most of the flexible GBRTLMs-based batteries reported now are primary batteries. Novel electrode materials which may have a higher theoretical capacity and energy density still need to be identified.

In terms of the auxiliary functional electrodes, the self-healing property of GBRTLMs have significantly extended the performance and lifespan of batteries. Nevertheless, the performance of GBRTLMs self-healing electrodes is still not completely satisfactory, especially in the aspect of capacity. In addition, the self-healing mechanism of liquid Ga has not been clearly studied.

For interconnecting electrodes, GBRTLMs have been applied in multi-scenarios, making flexible integrated systems possible. Soft batteries can be flawlessly incorporated into sensing systems without cumbersome external battery power, thus meeting the micro and light needs of wearable devices.

At present, most of the reported GBRTLMs based batteries are still in the prototype stage. There is still a long way to go before these batteries can be commercialized on epidermal electronic occasions. This depends on the cross-application and integration of interdisciplinary knowledge, including electrochemistry, flexible electronics, sensing, materials science, and so on.

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