A Shape-Variable, Aqueous, Low-Temperature Liquid Metal–Conductive Polymer Secondary Battery

A shape-variable aqueous secondary battery operating at low temperature is developed using Ga 68 In 22 Sn 10 in wt% liquid metal anode and conductive polymer (polyaniline (PANI)) cathode. While in the GaInSn alloy anode, Ga is the active constituent; Sn and In increase the acid resistance and decrease the eutectic point to -19°C. This enables the use of strongly acidic aqueous electrolytes (here, pH 0.9), thereby improving the activity and stability of the PANI cathode. Consequently, the battery exhibits remarkable excellent electrochemical performance and mechanical stability. The GaInSn–PANI battery operates via a hybrid mechanism of Ga 3+ stripping/plating and Cl − insertion/extraction and delivers a high initial capacity of over 324.6 mAh g − 1 and a 52.4% retention rate at 0.2 A g − 1 after 500 cycles, and outstanding power and energy densities of 4300 mW g − 1 and 98.7 mWh g − 1 , respectively. Because of the liquid anode, the battery without packaging can be deformed with a small force of several millinewtons without any capacity loss. Moreover, at approximately − 5°C, the battery delivers a capacity of 67.8 mAh g − 1 at 0.2 A g − 1 with 100% elasticity. Thus, the battery is promising as a deformable energy device at low temperatures and in demanding environments.


Introduction
The rapid growth of smart wearable electronics such as soft robotics 1, 2 and wearable sensors [3][4][5] has raised the bar on requirements such as elasticity, deformability, low operation temperature, and excellent safety, in addition to exibility for advanced energy storage devices. 6-15 To achieve exibility and stretchability with various types of batteries such as Li, Na, Zn, and Al-based batteries, signi cant efforts have been devoted to replacing the rigid metal-based anodes 16 with exible electrode materials such as carbon-or metal-based exible yarn/sheet electrodes, [17][18][19][20][21][22][23][24][25] spring-structured metal-ber electrodes with stretchable substrates, 19,20,23,[26][27][28] and liquid metal conductive adhesive-based wearable electrodes. 9,[29][30][31][32] Moreover, aqueous batteries are considered promising in terms of safety for wearable devices. 33 Although these studies have achieved remarkable results, solid electrodes have limited stretchability and bendability. 7 In addition, the dendrite growth on solid metal electrode surfaces poses serious safety hazards such as re and explosion, and is the main bottleneck in battery research. 34,35 Besides, at subzero temperatures, solid electrodes may detach from quasi-solid/solid electrolytes because of the difference in their coe cients of thermal expansion, resulting in the breakdown of the battery circuit.
In contrast to solid electrodes, liquid electrodes offer excellent liquidity and non-dendrite growth. 35,36 Gallium (Ga)-based liquid metals are considered ideal deformable anodes for wearable aqueous batteries owing to their metallic conductivity, high theoretical capacity (1153.4 mAh g − 1 for Ga, shown in the Supporting Information), and suitable standard electrode potential (-0.53 V vs E ϴ ). 6, 30, 37-39 Unlike largescale stationary liquid metal batteries such as Mg-Sb, 40 Mg-Bi-Sn, 41 and Sb-Pb systems, 42 Ga-based liquid metal batteries with a low eutectic point (< 30°C) obviate the necessity of high working temperatures (> 200°C) and rigorous encapsulation. Although both alkali metal alloys with organic electrolytes and toxic Hg can be employed as liquid metal electrodes at room temperature, 43 Ga-based alloys which have already been used in wearable electronics, [44][45][46][47] are more suitable liquid metal electrodes for the development of wearable aqueous batteries, due to their nontoxicity and high chemical stability in air and aqueous environments. 6 However, a possible drawback with using liquid metals is that a low operating temperature and the stripping/plating of Ga 3+ may lead to the solidi cation of a part of the liquid electrode, which leads to poor exibility and dendritic growth. This can be resolved by employing liquid metals with a wide liquid range at room temperature or a low melting point.
Further, polyaniline (PANI), a classic conductive polymer, has been widely used as a cathode in exible aqueous batteries, such as exible Zn-PANI batteries, owing to its high conductivity, stability, and reversible redox reaction in acidic electrolytes. 48, 49 The performance of conductive polymers signi cantly depends on the acidity of the aqueous electrolyte; that is, the lower the pH, the better are the electrochemical activity and structural stability of the PANI cathode. 50,51 However, a strong acid may severely corrode the metal anode. Hence, preventing the hydrogen evolution reaction (HER) of metal anodes in strongly acidic electrolytes is a major challenge in aqueous metal-PANI batteries.
In this study, Ga 68 In 22 Sn 10 is used as a liquid metal anode in a shape-variable aqueous liquid metal-PANI secondary battery owing to its high hydrogen evolution potential and low eutectic point.
Theoretically, the inert Sn and In can improve the acid resistance of the GaInSn alloy, enabling the use of a highly acidic electrolyte, which in turn improves the activity and stability of the conductive polymer cathode. Moreover, because of a low eutectic temperature (-19°C) 51 and a wide liquid window at room temperature, the GaInSn alloy maintains its liquid phase during the stripping/plating of Ga 3+ , thereby preventing the detachment of the electrode from the solid electrolyte and imparting exibility, elasticity, and shape-variability to the liquid metal battery along with a wide operating temperature range. Therefore, the GaInSn-PANI battery with various shapes (one-dimensional (1-D), two-dimensional (2-D), and threedimensional (3-D) structures) exhibits excellent electrochemical performance with remarkable wearability and deformability even at a sub-zero temperature.

Results And Discussion
Flexible liquid metal-polymer batteries assembled with a uid GaInSn alloy anode, a exible PANI cathode, and a GaCl 3 /NH 4 Cl/PVA hydrogel electrolyte and encapsulated in an elastic silicone elastomer shell were fabricated as 1-D ( ber), 2-D (sheet) and 3-D (sphere) structures, as shown in Fig. 1a and Supplementary Fig. 1. As illustrated in Fig. 1b

Effect of Electrolyte Acidity on the Properties of Ga-Based Electrodes
Metal anodes commonly self-corrode due to the HER and acid attack in aqueous electrolytes, especially, with strong acidities. In the case of the liquid metal-PANI battery, although the use of a strongly acidic electrolyte can improve the activity and stability of the PANI cathode, it will aggravate the self-corrosion of the liquid metal anode, resulting in a low open circuit voltage and severe self-discharge. 7, 52 Therefore, the degree of self-corrosion of Ga-based metal anodes in electrolytes with different pH levels was determined by Tafel extrapolation and linear scan voltammetry (LSV), and from the potential distribution during charging and discharging.
The corrosion potentials/rates of Ga, GaIn, and GaInSn as determined from the Tafel curves (Fig. 2a were 16.6% and 79.9% in the pH 0.9 electrolyte, and 31.6% and 94.9% in the pH 1.6 electrolyte, respectively. The result indicates that although the use of the pH 0.9 electrolyte led to more serious metal corrosion than the use of the pH 1.6 electrolyte, the presence of In and Sn in the GaInSn alloy signi cantly decreased the corrosion rate in each stage in both the electrolytes. The HER of Ga-based metals was measured to determine the nature of metal corrosion. As shown in Fig. 2b, the HER onset potentials of Ga, GaIn, and GaInSn are − 0.71, -0.93, and − 1.09 V vs SCE in the pH 0.9 electrolyte, and − 0.93, -1.05, and − 1.15 V vs SCE in the pH 1.6 electrolyte, respectively. The result shows that in both electrolytes (pH 0.9 and 1.6), the resistance to the HER increased in the order of Ga < GaIn < GaInSn, since the addition of Sn further increased the resistance of the alloy to the HER because of the high HER overpotentials of In and Sn. To evaluate the effect of electrolyte acidity on the chemical stability of Ga-based metal anodes in the batteries, symmetric cells employing Ga, GaIn, or GaInSn in pH 0.9 and 1.6 electrolytes were tested at a current density of 1.0 mA cm − 2 for 30 min. As shown in Fig. 2c, the overpotentials of the Ga-based electrodes were higher in the pH 0.9 electrolyte than in the pH 1.6 electrolyte because of the stronger acidity of the former; unlike the GaIn-and Ga-based cells that exhibited unstable charge-discharge voltage curves with large polarizations, the GaInSn-based cell exhibited a stable charge-discharge voltage curve over 150 h with a smaller voltage polarization in both the electrolytes. Therefore, among the Ga-based alloys examined in this study, GaInSn is the most suitable anode candidate for aqueous liquid metal batteries that use a strongly acidic electrolyte.
The cycling performance of the Ga-PANI batteries (Fig. 2d) revealed that the overall performance of the GaInSn-based battery was better in the pH 0.9 electrolyte than in the pH 1.6 electrolyte; the GaInSn-based battery with the pH 0.9 electrolyte exhibited a higher capacity of 252.6 mAh g − 1 after 500 cycles with a higher average Coulombic e ciency (CE) of 98.6% due to the higher ionic conductivity of the pH 0.9 electrolyte ( Supplementary Fig. 3) and high activity of PANI in the strongly acidic electrolyte. 53 In contrast, although the Ga-and GaIn-based batteries exhibited excellent capacities of 496.2 mAh g − 1 and 473.7 mAh g − 1 after the rst few cycles in the pH 0.9 and pH 1.6 electrolytes, respectively, their capacities signi cantly decreased to below 50 mAh g − 1 after 150 cycles because of the low corrosion resistances of Ga and GaIn in the strongly acidic electrolytes, resulting in relatively low CEs (< 89%). Therefore, the GaInSn-PANI battery exhibited the best performance when GaInSn, the most stable alloy, was used as the anode. A comparison of the cycling performances of Ga-PANI, In-PANI, and Sn-PANI batteries (Supplementary Fig. 4) revealed that due to the chemical inertness of In and Sn, the In-and Sn-based batteries delivered considerably low capacities (< 15 mAh g − 1 ) during the rst cycle, which then rapidly decreased to 5 mAh g − 1 . This indicates that Sn and In are inactive for the battery reactions upon alloying with Ga. Thus, Ga is the only active constituent of the anode in the GaIn-PANI and GaInSn-PANI batteries.
To determine whether the liquid metal anode can stably operate in the acidic electrolytes, anodic and cathodic polarization potential curves were measured from the charge-discharge cycling test. As shown in Fig. 2e and Fig. 2f, after charging, the potentials of the GaIn anode were − 1.26 and − 0.98 V vs SCE, whereas those of the GaInSn anode were − 1.08 and − 1.01 V vs SCE in the pH 0.9 and 1.6 electrolytes, respectively. This, together with the LSV result, (Fig. 2b) indicates that the GaIn anode operates only in the pH 1.6 electrolyte. This is because in the pH 1.6 electrolyte, the anodic potential of GaIn (-0.98 V) is higher than the HER onset potential (-1.05 V), whereas in the pH 0.9 electrolyte, the anodic potential of GaIn (-1.26 V) is much lower than the HER onset potential (-0.93 V); this implies that the HER occurs in the pH 0.9 electrolyte, not in the pH 1.6 electrolyte. In contrast, GaInSn stably operated in both electrolytes as its anodic potential (-1.08 and − 1.01 V, respectively) is higher than the HER onset potentials in both. In the case of the pure Ga anode, a signi cant portion of the charging curve is below the HER onset potential for both electrolytes ( Supplementary Fig. 5), which indicates that Ga is unsuitable as an anode in both electrolytes because of severe corrosion. Moreover, the PANI cathode exhibited a regularly uctuating cathodic potential in the range of -0.2 to 0.6 V vs SCE during cycling, which avoids structural or morphological damage due to the over-oxidation and over-reduction of PANI. 54,55 This is essential for the stable performance of the GaInSn-PANI battery and is further con rmed by SEM analysis (Supplementary Fig. 6).

Working Mechanism of the GaInSn-PANI Battery
The working mechanism of the rechargeable GaInSn-PANI batteries was systematically investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3a (Supplementary Fig. 7 and Supplementary Fig. 8), the Ga content of the GaInSn anode decreased from 67.68 wt% to 63.89 wt% after the 1st discharge, increased to 70.96 wt% after the 1st charge, and remained almost unchanged (70.48 wt%) after 100 charging cycles. To accurately evaluate the compositional changes in the anode, the mass fraction, i.e., Ga/Sn and In/Sn ratios (Sn being the most inert metal in the alloy) were determined; the results are shown in Fig. 3b. Because the activities of In and Sn are lower than that of Ga, In/Sn stabilized at approximately 2.07 over 100 cycles, which indicates that Sn and In are the inactive components of the liquid metal anode. In contrast, during charging-discharging, the Ga/Sn ratio (6.427) rst decreased to 5.433 and then increased to 7.501, which indicates that Ga is the only active material in the GaInSn anode with the reaction Ga − 3e − Ga 3+ . 7,8 Moreover, after 100 charging cycles, the Ga/Sn ratio remained almost constant (with a decrease of 0.52 wt% compared with that after the rst charging cycle), which indicates the su cient reversibility of the proposed anode even after 100 cycles for liquid metal secondary batteries. It is worth noting that the GaIn anode exhibits a similar working mechanism; ⇌ however, some small solid In particles appeared on surface of the GaIn anode after discharging (Supplementary Fig. 9b and Supplementary Fig. 10) because the ratio of Ga and In in the GaIn anode exceed the liquid window during stripping ( Supplementary Fig. 11). In contrast, even after long-term cycling test, the GaInSn anode still maintained the liquidity due to its wide liquid window (> 55wt% Ga maintain liquidity, shown in phase diagram in Supplementary Fig. 11).
Further, the Raman spectra ( Supplementary Fig. 12) exhibited characteristic peaks at 1166.50 and 1480.17 cm − 1 , which indicate the growth of PANI on the carbon substrate. 56 In addition, the FTIR spectrum of the initial PANI cathode (Fig. 3c)  The cation-anion hybrid mechanism of PANI cathodes has been proposed in previously reported PANI-based batteries with promising performances. 56 In addition, the PANI cathode in the GaIn-PANI battery exhibited the same working mechanism (see the XPS pro les in Supplementary Fig. 13). Therefore, as shown in Fig. 3e, the reaction mechanism of the PANI cathode can be concluded as follows: PANI oxd -(Cl) n + (n + 3m)e − +mGa 3+ ⇌ (PANI) red -Ga m + nCl − (1) Furthermore, even after 100 cycles, the FTIR spectra remained unchanged and distinct Cl and Ga XPS signals appeared; this con rms the high stability of the PANI cathode in the GaInSn-PANI batteries. In general, during discharging, the Ga atoms that stripped from the GaInSn anode oxidize to form Ga 3+ in the electrolyte, while the Cl in the PANI cathode reduces to form Cl − . In contrast, during charging, Cl − and Ga 3+ return to PANI and GaInSn, respectively (Fig. 3f). In the charge-discharge process, the Ga from the GaInSn anode and the Cl associated with the PANI cathode act as reversible ions and participate in the redox reaction of the GaInSn-PANI battery. Thus, the overall reaction is as follows: PANI oxd -(Cl) n + (1/3n + m) Ga ⇌3(PANI) red -Ga m + 1/3n Ga 3+ + nCl − (2)

Electrochemical Performance of the GaInSn-PANI Battery Under Various Temperatures and Deformability Degrees
The electrochemical performance of the GaInSn-PANI battery in the form of a 2-D sheet was evaluated.
As can be seen from the long-term cycling performance (Fig. 4a) and galvanostatic charge/discharge pro les ( Supplementary Fig. 14), the GaInSn-PANI battery delivered an initial discharge capacity of approximately 238.2 mAh g − 1 at 0.2 A g − 1 with a CE of 58.9%. The capacity increased signi cantly at the 2nd cycle (343.7 mAh g − 1 ), which is attributed to the activation of the partially oxidized PANI in the initial cycle. After 50 cycles, the capacity signi cantly decreased to approximately 223.9 mAh g − 1 , because of the morphological and structural changes in the PANI cathode, and the irreversibly Ga 3+ trapped in the CF@PANI electrode. 60-62 After 500 cycles at 0.2 A g − 1 , the GaInSn-PANI battery maintained a reversible capacity of approximately 179.8 mAh g − 1 with a capacity retention of 80.3% and an average capacity loss of 0.098 mAh g − 1 cycle − 1 (Fig. 4a, and Supplementary Table 2). Even at a high current density of 5.0 A g − 1 , the GaInSn-PANI battery maintained a capacity of 92.8 mAh g − 1 with a high CE of over 98% and an average capacity loss of 0.013 mAh g − 1 cycle − 1 after 500 cycles. This indicates the promising electrochemical performance of the GaInSn-PANI battery at a high current density.
In addition to the outstanding cycling performance, the GaInSn-PANI battery exhibited an excellent rate performance. As shown in Fig. 4b, at low current densities of 0.  Fig. 15). As can be seen from the Ragone plot (Fig. 4c), the GaInSn-PANI battery exhibits a signi cantly high energy density of 266.0 mWh g − 1 with a power density of 217.8 mW g − 1 . At a high power density of 4300 mW g − 1 , an energy density of 98.7 mWh g − 1 was achieved, which are higher than those of existing exible metal-PANI batteries. 50 contribution is the major contribution to total Ga 3+ storage. The capacitive contribution is further con rmed by the equation i = k 1 v + k 2 v 1/2 , where k 1 v and k 2 v 1/2 are the current contributions of the capacitance-controlled process and diffusion-controlled process, respectively. By quanti cation at every voltage and a certain scan rate, a simulated closed curve was obtained after integration (Fig. 4f). As can be seen, the capacitive process contributes to nearly 68.7% of the total Ga 3+ storage at a scan rate of 0.6 mV s − 1 ; this suggests that the capacitive process dominates the entire reaction. This is the reason for the high-rate performance and excellent power density of the GaInSn-PANI battery.
Because of the liquid anode, the GaInSn-PANI batteries exhibited excellent cycling performance at low temperatures. As shown in Fig. 4g and Supplementary Fig. 16 and Supplementary Video 1), the battery delivered a capacity (140.9 mAh g − 1 ) comparable to that in the pristine state (142.4 mAh g − 1 ). In addition, the battery exhibited a stable capacity during the bending (bending radius of 5 mm) and recovery processes. The Nyquist plots of the battery in various states are shown in Fig. 5b, and the simulated parameters obtained from the equivalent circuit of R e (R in Q 1 )(R ct Q 2 )W ( Supplementary Fig. 18) are summarized in Supplementary Table 3 and Supplementary Table 4. The starting point (R e ) re ects the total contact resistance of the batteries, and the rst and second arcs correspond to the interfacial resistance (R in ) and charge transfer resistance (R ct ), respectively. Under the stretched and bent states, the R in of the battery increased by 2-3.5 times and the R ct decreased by 33.7-39.2%, respectively, compared with those of the battery in the pristine state because of the increases in the interface contact area and charge transfer channels between the electrode and electrolyte. Under the combined effect of R ct and R in , the battery exhibited a stable electrochemical performance during bending and stretching.
In addition to the battery-deformation degree mentioned above, the degree of battery-deformation di culty will also be a necessary factor to be examined for wearable/deformable devices. To quantitatively exhibit the battery-deformation di culty, the force information was recorded during stretching process. As shown in Fig. 5c, an average force of 0.850 N was required to stretch the battery encapsulated in a rubber shell to 200% of its original length, while almost the same force (0.842 N) was required to stretch the packing shell without the battery to 200% of its original length ( Supplementary  Fig. 19). This suggests that the battery body without packing requires a force of only several millinewtons (8 and 11 mN for 2-D and 1-D batteries, respectively, as shown in Supplementary Fig. 19 and Supplementary Fig. 20 and Supplementary Video 2) to stretch it to 200% of its original length. This is attributed to the outstanding uidity of the GaInSn alloy at room temperature, and the special designed electrolyte and PANI cathode. Furthermore, the liquid metal battery demonstrated unique low-temperature exibility: at -4.6°C, the battery could be easily stretched to 200% of its original length and recovered its original state upon release. The results indicate the excellent deformability of the GaInSn-PANI battery at a low temperature, and thus, its promising applicability to exible devices.

Conclusion
A shape-variable and rechargeable aqueous liquid metal-conductive polymer battery operating at low temperature was fabricated using a GaInSn alloy (68 wt% Ga, 22 wt% In, and 10 wt% Sn) anode, PANI cathode, and GaCl 3 -based hydrogel electrolyte. The anode reaction with Ga as the only active element was (Ga − 3e − Ga 3+ ). And the components of Sn and In enhanced the alloy's acid resistance and decreased the eutectic point to − 19 o C, which helped not only to increase the acidity of the aqueous electrolyte to pH = 0.9 to improve activity and stability of PANI cathode, but also to maintain a stable liquid anode at − 5 o C. Relatively, due to the hybrid mechanism (the Ga 3+ stripping/plating and Cl − insertion/extraction) of PANI, the cathode reaction was PANI oxd -(Cl) n + (n + 3m)e − +mGa 3+ ⇌ (PANI) red -Ga m + nCl − . Therefore, the GaInSn-PANI battery exhibited an excellent performance, with a capacity of 179.8 mAh g − 1 and a low-capacity decay of 0.098 mAh g − 1 cycle − 1 at 0.2 A g − 1 after 500 cycles. Even at a high current density of 5.0 A g − 1 , the battery delivered a capacity of 92.8 mAh g − 1 after 500 cycles. In short, the GaInSn-PANI battery showed more outstanding power and energy densities (98.7 mWh g − 1 at 4300 mW g − 1 ) compared with other existing stretchable metal-PANI batteries. In addition the satisfying electrochemical performance, because of the liquidity of the liquid metal anode, a force of only several millinewtons was required to deform the 1-D, 2-D, and 3-D batteries without packing, and the batteries could be easily stretched to 200% of their original length, bent to 0.5 cm curvature radius, deformed into various shapes, and recovered their initial state almost without any capacity loss (142.4 mAh g − 1 ). It's worth noting that, the low eutectic temperature leads the battery to an excellent deformation even at -4.6 o C with a promising capacity of 64.0 mAh g − 1 . Although the high molecular weight of liquid metals limits the energy density in weight, the battery still exhibits wide application prospects as a shape-variable soft power source for deep sea mollusk bionic robots as well as in civilian and military elds.
Declarations guidance throughout the study and facilitated funding-nancial support.
Competing Interest     Supplementary Files