Triple-Function Electrolyte Regulation toward Advanced Aqueous Zn-Ion Batteries

enhance ZIB performance because of the aggravated competing H 2 evolution. To address Zn issues, a one-off electrolyte strategy is reported by introducing the triple-function C 3 H 7 Na 2 O 6 P, which can take effects during the shelf time of battery. It regulates H + concentration and reduces free-water activity, inhibiting H 2 evolution. A self-healing solid/electrolyte interphase (SEI) can be triggered before battery operation, which suppresses O 2 adsorption corrosion and dendritic deposition. Consequently, a high Zn reversibility of 99.6% is achieved under a high discharge depth of 85%. The pouch full-cell with a lean electrolyte displays a record lifespan with capacity retention of 95.5% after 500 cycles. This study not only looks deeply into Zn behavior in aqueous media but also underscores rules for the design of active metal anodes, including Zn and Li metals, during shelf time toward real applications. adsorption corrosion beside HER and dendrite growth. Differing from traditional aqueous Li/Na batteries, however, removing O 2 cannot enhance the battery performance. The functional additive of SG was introduced in the electrolyte to solve Zn problems. It regulated the H + concentration and diminished free-water activity, significantly inhibiting the H 2 evolution. The hydrolysis of C 3 H 7 O 6 P 2 − distributed in the inner Helmholtz layer of Zn electrode also triggers the formation of self-repairing SEI with a thickness of ≈ 217 nm before battery operation, which inhibits O 2 corrosion and dendrite growth. Thus, the high Zn reversibility was achieved even under a high DOD of 85%, which also guarantees the excellent performance of ZIBs in both coin cells and pouch cells. Under the lean electrolyte, the PANI/Zn batteries displayed excellent cycling performance for 1400 and 500 cycles in coin cell and pouch cell, respectively. This work reveals Zn behavior in water-based electrolytes and guides future research to achieve the highly efficient utilization of active metal anodes for future commercialization.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202206963.

DOI: 10.1002/adma.202206963
devices benefiting from their advantages in low manufacturing costs and safety performance. [1] Nevertheless, aqueous batteries still face huge challenges because of their limited energy density (<40 Wh kg −1 ) as a result of the narrow working window of 1.23 V. [2] Thanks to its high overpotential to the water-induced hydrogen (H 2 ) evolution, [3] metallic Zn with a high theoretical capacity of 820 mA h g −1 can be directly employed as an anode in mild aqueous media. [4] This can not only theoretically broaden the voltage window of Zn-ion batteries (ZIBs) to 1.79 V, but also significantly simplify the battery manufacturing technology, making ZIBs a promising candidate among aqueous batteries. [5] Although the utilization of Zn brings a breakthrough for aqueous batteries, Zn anodes are not perfectly satisfactory even in mild electrolytes due to the widely-reported dendrite growth and H 2 evolution reaction (HER). [6] These issues significantly decrease the Coulombic efficiency (CE) of Zn electrodes and shorten the battery lifespan. [7] To address HER and dendrite growth, the considerable efforts have been made. [8] From the perspective of Zn electrodes, various solid/electrolyte interphase (SEI) layers have been ex situ/in situ built on Zn surface. [6,9] These layers suppress the water-induced H 2 evolution by blocking the electrolyte from Zn electrode surface, and facilitate the dendrite-free deposition via providing channels for Zn 2+ transportation. [10] Unfortunately, the Zn metal experiences significant volume change upon cycling, which easily causes cracking and shedding of the artificial interphase. [11] These artificial SEI layers without self-healing ability would gradually lose their functions after long-term cycling. [12] From the perspective of electrolyte, the first-priority strategy is the use of highly-concentrated electrolyte to reduce the water activity and to regulate the Zn deposition. [5,13] Nevertheless, highlyconcentrated electrolytes sharply increase costs, which discourages their widespread application. [14] Alternatively, a series of organic electrolyte additives have been proposed. [15] Despite some improvement in Zn reversibility, these additives likely increases the flammable risk of electrolyte, which compromises the safety advantages of aqueous ZIBs. [16] Thus, designing low-cost and safe strategy to solve Zn problems is highly desirable for the development of the next-generation ZIBs.
It is worth noting that most of Zn anode studies mainly paid attention to the macroscopic improvements in the reversibility The poor Zn reversibility has been criticized for limiting applications of aqueous Zn-ion batteries (ZIBs); however, its behavior in aqueous media is not fully uncovered yet. Here, this knowledge gap is addressed, indicating that Zn electrodes face a O 2 -involving corrosion, besides H 2 evolution and dendrite growth. Differing from aqueous Li/Na batteries, removing O 2 cannot enhance ZIB performance because of the aggravated competing H 2 evolution. To address Zn issues, a one-off electrolyte strategy is reported by introducing the triple-function C 3 H 7 Na 2 O 6 P, which can take effects during the shelf time of battery. It regulates H + concentration and reduces free-water activity, inhibiting H 2 evolution. A self-healing solid/electrolyte interphase (SEI) can be triggered before battery operation, which suppresses O 2 adsorption corrosion and dendritic deposition. Consequently, a high Zn reversibility of 99.6% is achieved under a high discharge depth of 85%. The pouch full-cell with a lean electrolyte displays a record lifespan with capacity retention of 95.5% after 500 cycles. This study not only looks deeply into Zn behavior in aqueous media but also underscores rules for the design of active metal anodes, including Zn and Li metals, during shelf time toward real applications.

Introduction
As alternatives to Li-ion batteries, aqueous batteries have been regarded as the next-generation large-scale energy storage and lifespan of Zn electrode; however, a little attention was paid to revealing the fundamental issues faced by Zn metal. [7b] To date, challenges still exist in achieving a complete understanding of Zn side reactions in aqueous media. For example, does the metallic Zn electrode experience the oxygen (O 2 ) adsorption corrosion to consume active Zn? Is it necessary to remove O 2 in the electrolyte before assembling ZIBs? These fundamental issues need to be figured out prior designing high-performance Zn anodes. In addition, another challenge is the spontaneity of the side reactions, leading to the active Zn consumption and by-product accumulation during the shelf time or transportation of battery. Unfortunately, these side reactions would not stop until the active Zn is completely consumed. Thus, inhibiting side reactions of Zn anode during battery rest is of great importance, which has not attracted attention in previous studies.
Here, we studied Zn behavior using a moisture-tolerable glovebox, revealing that the metallic Zn constantly faces O 2 adsorption corrosion, leading to the sustained consumption of Zn electrodes. Unlike traditional aqueous Li/Na batteries, removing O 2 cannot improve the battery performance, since the competitive reaction of HER will be significantly aggravated. To effectively address these issues, a triple-benefit electrolyte additive of sodium glycerophosphate (C 3 H 7 Na 2 O 6 P, SG) was proposed. Differing from traditional strategies, this additive can take effect before battery operation by regulating pH value and diminishing free-water activity, which helps to suppress HER and the parasitic by-product accumulation. Meanwhile, metallic Zn could trigger the decomposition of C 3 H 7 O 6 P 2− in the inner Helmholtz plane to in situ form a SEI layer on its surface. When the SEI layer fully covered Zn surface, the decomposition of C 3 H 7 O 6 P 2− would be stopped until the fresh Zn was exposed, which makes the SEI self-repairable. Accordingly, a side-reaction-free and dendrite-free Zn anode was achieved, resulting in a highly reversible Zn anode with a CE of ≈99.6% under a high depth of discharge (DOD) of 85%. Even with a lean electrolyte, both polyaniline (PANI)/Zn coin and pouch cells feature excellent cycling life (1400 and 500 cycles, respectively).
The pure water and ZnSO 4 solution were selected as typical aqueous media to reveal the common issues faced by all aqueous Zn-based systems. Accordingly, four media were prepared: pure water, pure water without O 2 , 2 m ZnSO 4 solution, and 2 m ZnSO 4 without O 2 using a moisture-tolerable and O 2free glovebox. Subsequently, Zn foils with uniform size were soaked in four electrolytes for one week, which were marked as S-1, S-2, S-3, and S-4, respectively (Figure 1a). As presented in Figure S1, Supporting Information, both S-1 and S-2 electrodes in pure water generate the white by-products, which were collected to perform Raman measurements (Figure 1b). Results show two strong bands at 367 and 381 cm −1 attributed to the symmetric ZnO stretching of the ZnO 4 tetrahedron, [17] and one weak band at 479 cm −1 assigned to the translational mode of OH vibration, manifesting the by-product of Zn(OH) 2 . [18] It is worth noting that gas bubbles were generated on Zn electrodes during the shelf time no matter whether in pure water or ZnSO 4 electrolyte ( Figure S2, Supporting Information), indicating the occurrence of HER evidenced by gas chromatography (GC) analysis ( Figure 1c). Interestingly, the H 2 amount was significantly higher in both water and ZnSO 4 systems after removing O 2 (from 15 299 to 58 421 ppm in water, and from 63 606 to 177 513 ppm in ZnSO 4 , Figure 1d). These results show that O 2 in aqueous electrolytes affects the Zn behavior and HER. The side reactions of Zn electrodes in aqueous media with/without O 2 can be expressed as follows, The O 2 -involving side reaction can be defined as O 2 adsorption corrosion of Zn electrodes, which consumes the active Zn. When the electrolyte contains O 2 , Zn electrodes will undergo O 2 adsorption corrosion (Equation (1)). Once the O 2 in electrolyte is consumed, the Zn electrode will directly react with water to generate H 2 (Equation (2)). Thus, the O 2 adsorption corrosion of Zn electrode can also be regarded as a competitive reaction of HER.
To identify the by-product species generated in the ZnSO 4 electrolyte, Raman spectra were collected (see Figure 1e). Differing from Zn(OH) 2 by-product in water, another by-product of Zn 4 SO 4 (OH) 6 ·xH 2 O was formed in ZnSO 4 electrolyte, [19] as confirmed by Raman and XRD results (Figures 1e,f). This also confirms that Zn(OH) 2 is unstable in the ZnSO 4 electrolyte, and transform into Zn 4 SO 4 (OH) 6 The by-product accumulation of Zn(OH) 2 in pure water and Zn 4 SO 4 (OH) 6 ·xH 2 O in the ZnSO 4 electrolyte are further evidenced by Fourier transform infrared (FTIR) measurements, revealing the thermodynamic instability of Zn electrodes in all water-based electrolytes ( Figure S3, Supporting Information). As summarized in Figure 1g, O 2 adsorption corrosion, HER, and their parasitic by-product accumulation significantly affect the Zn stability in aqueous electrolytes during battery rest.
The Zn surface evolution caused by O 2 effects were studied by laser confocal scanning microscopy and scanning electron microscope (SEM). In pure water with O 2 , the 3D confocal image of the S-1 electrode shows abundant needlelike by-products (Figure 1h). After the removal of O 2 , less visible by-products are generated on the surface of S-2 ( Figure 1i), indicating that the existence of O 2 in water boosts the accumulations of Zn(OH) 2 . Interestingly, the tendency is different in ZnSO 4 electrolyte ( Figure 1j,k), which is confirmed by SEM images (Figure 1l-o). In this case, the accumulation of Zn(OH) 2 fine particles on Zn electrodes is loose. They are easily peeled off from Zn substrates due to the poor adhesion, as evidenced by optical image after two-month soaking ( Figure S4, Supporting Information). Thus, Zn(OH) 2 by-products cannot stop the corrosion reactions of Zn electrodes in pure water, which is similar to loose Zn 4 SO 4 (OH) 6 ·xH 2 O by-product sheets ( Figure S5, Supporting Information). Thus, without protection, the O 2 adsorption corrosion and HER continuously consume Zn until it is fully reacted.
In addition to the state of battery rest, O 2 effects on Zn behavior were also studied during battery operation because the necessity of eliminating O 2 in aqueous ZIBs is always controversial. [8a] Figure 2a shows Zn stripping/plating curves in 2 m ZnSO 4 with O 2 under 2.5 mA cm −2 and according in situ GC curves. Results reveal a stable polarization value of Zn plating/stripping with the increased H 2 evolution upon cycling (see Figure 2a). The intensity of H 2 evolution increases from 18.6 to 40.1 ppm after 3.5 cycles ( Figure S6, Supporting Information). After removing O 2 in ZnSO 4 electrolyte ( Figure S7, Supporting Information), the cell shows a fluctuated voltage polarization of Zn plating/stripping with the aggravated H 2 evolution from 22.5 ppm at the beginning to 50.2 ppm after 3.5 cycles (Figure 2b). Hence, removing O 2 exacerbates HER in ZIBs and further affects the performance of aqueous ZIB system. This finding reveals that O 2 effects on aqueous ZIBs are different from the traditional aqueous Li/Na batteries. [1a] Figure 2c shows the rotating disk electrode (RDE) measurements for Zn electrodes in ZnSO 4 electrolyte with/without O 2 under a rotation rate of 1600 rpm. No current polarization can be recorded for the Zn electrode in ZnSO 4 without O 2 in the whole potential window from 0.2 to −0.6 V versus saturated calomel electrode (SCE), however, there is an obvious current polarization at −0.2 V versus SCE, which corresponds to the O 2 reduction. This elucidates that O 2 involves side reactions of Zn electrodes during battery operation.
Currently, aqueous ZIBs are generally assembled in air, which helps to reduce the manufacturing costs. To truthfully reflect overall effects of O 2 adsorption, HER, and dendrite growth on Zn electrodes, the Cu/Zn cells assembled in air were tested under different DODs (with a commonly preferred 100 µm). With a super low DOD of 0.85% (1 mA cm −2 and 0.5 mA h cm −2 ), the cell shows a conspicuous CE fluctuation after ≈170 cycles at 30 °C, indicating a battery failure (Figure 2d). [21] Moreover, the Zn plating/stripping shows a low initial CE of 67.4% and average CE of 96.9%, indicating its low reversibility induced by the O 2 corrosion, HER, and dendrite formation. After removing the O 2 , however, the CE and cycling life of Zn electrodes in 2 m ZnSO 4 electrolyte were reduced to 94.6% and 110 cycles, respectively ( Figure S8  spectrum of the by-product generated in pure water. c) GC curves to test the H 2 evolution in different aqueous media after one week soaking. d) The specific H 2 amount generated in four media. e) Raman spectra of Zn foils after soaking in different media. f) XRD patterns of Zn foils soaked in various media. g) Schematic diagram illustrating issues faced by the Zn electrode to affect its stability during battery rest. h-k) 3D confocal images for Zn electrodes after one-week soaking in pure water (h), pure water without O 2 (i), 2 m ZnSO 4 (j), and 2 m ZnSO 4 without O 2 (k). l-o) SEM images for Zn electrodes after one-week soaking in pure water (l), pure water without O 2 (m), 2 m ZnSO 4 (n), and 2 m ZnSO 4 without O 2 (o).
Information). Such limited lifespan and reversibility of Zn electrodes with low Zn utilizations are far from satisfactory to meet the demands of aqueous ZIBs for practical applications.
For an ideal situation, side reactions of Zn anodes should be prevented before battery operation. Otherwise, they will unremittingly consume the active Zn anode and damage the cell during battery rest or transportation. Unfortunately, previous electrolyte strategies have not paid much attention on addressing Zn issues before battery operation. [8b,12b] Here, we propose a triple-function electrolyte additive of SG to solve Zn battery issues during the shelf time. To understand how the SG additive works on Zn electrodes during battery rest, the pH values of ZnSO 4 electrolyte, pure SG solution, and hybrids were measured (Figure 3a). These measurements show that 2 m ZnSO 4 electrolyte is slightly acidic with a pH value of 4.72. For the pure 0.5 m SG solution, a higher pH value of 8.30 is achieved, therefore SG can function as a pH regulator. For the ZnSO 4 electrolyte with 0.01 m SG, the pH value increases to 5.13, indicating the H + concentration is reduced. The pH value of the electrolyte solution raises to 5.18, 5.20, and 5.23 along with increasing SG concentration to 0.05, 0.1, and 0.5 m, respectively, which dynamically benefits the suppression of H 2 evolution corrosion. Moreover, SG features abundant oxygen-containing functional groups based on its unique structure ( Figure S9, Supporting Information), which can form the H-bond with water molecules in the ZnSO 4 electrolyte to reduce the water activity and further suppress HER.
A series of Zn electrodes were soaked in 2 mL of 2 m ZnSO 4 with/without 0.05 m SG to compare the Zn surface evolution along with soaking time from 2 days to 20 days ( Figure S10, Supporting Information). In the pure ZnSO 4 electrolyte, the Zn surface gradually changes from the bright metallic color into dark gray, revealing the progressively aggravated corrosion reactions (Figure 3b upper). However, Zn metal surface maintains a light gray color with 0.05 m additive. To determine the species generated on Zn electrodes, the ex situ XRD patterns were conducted. As can be seen from Figure 3c, the pattern of Zn electrode soaked in pure ZnSO 4 electrolyte for 2 days shows the main Zn peaks at 36.3°, 39.0°, 43.2°, and 54.3°. After soaking for 6 days, the peak of Zn 4 SO 4 (OH) 6 ·xH 2 O by-product at 8.9° is remarkable ( Figure S11a, Supporting Information), and its intensity increases sharply along with prolonged soaking time. [22] After 20 days, the (002) peak intensity of Zn 4 SO 4 (OH) 6 ·xH 2 O is much higher than those of Zn metal, indicating that side reactions ruin the Zn electrode. In comparison, patterns of Zn soaked in the SG-containing electrolyte mainly show Zn peaks without obvious by-product formation (Figure 3d). The zoom-in pattern in Figure S11b, Supporting Information shows a peak at 6.5° with a low intensity after 10 days soaking, corresponding to the (002) face of Zn 3 (PO 4 ) 2based product. This indicates the spontaneous formation of SEI on the Zn surface during the shelf time, which was also confirmed by FTIR spectra (Figure 3e). [23] Importantly, after prolonging the soaking time to 20 days, there is no visible intensity changes in the SEI peak on ex situ XRD patterns ( Figure S11b, Supporting Information), indicating its stable thickness.
The thickness is one of vital parameters for SEI layers; however, challenges still exist in building a thin SEI layer allowing fast Zn 2+ transfer. [8a] To assess the thickness of our SEI layer, the focused ion beam (FIB) was conducted on the soaked Zn electrode (20 days) to obtain the smooth cross-section (Figure 3f). A Pt layer was introduced to protect the artificial  layer since the high-energy beamline could possibly damage it. [24] Energy-dispersive spectroscopy (EDS) results in Figure S12, Supporting Information show that the SEI layer contains elements of Zn, O, and P. The thickness of SEI is ≈217 nm, much thinner than the reported SEI layers on Zn electrodes. [8d,25] 2D confocal images of Zn electrodes after soaking for 20 days in ZnSO 4 electrolyte with/without 0.05 m SG were compared. In pure ZnSO 4 electrolyte, massive by-product plates are generated on the Zn surface due to serious side reactions (Figure 3g). However, the Zn surface is clean and homogenous in the SG-containing electrolyte, indicating that side reactions are effectively inhibited (Figure 3h). Moreover, Zn/Zn pouch cells were compared to assess the effectiveness of SG additive during the shelf time ( Figure S13, Supporting Information). For the pure ZnSO 4 electrolyte, the pouch cell shows an obvious swelling after resting for two months. In contrast, the  Zn/Zn pouch cell with the SG additive-containing electrolyte does not show obvious change, indicating that side reactions can be effectively suppressed under the state of battery rest, which paves the way toward practical applications of ZIBs.
To understand the mechanism of SEI formation on Zn electrodes in the electrolyte of ZnSO 4 with SG, the density functional theory (DFT) calculations were conducted to reveal the ion distribution in the inner Helmholtz plane of Zn electrodes. The components of the inner Helmholtz plane are strongly related to the SEI layer formation on the electrode surface. [26] Figure S14 (1.99 Å), indicating that C 3 H 7 O 6 P 2− ions mainly dominate in the inner Helmholtz plane of the Zn electrode (Figure 3l right). Notably, the charge density of C 3 H 7 O 6 P 2− anion significantly shifts after immobilization on the Zn slab, as evidenced by images of differential charge density ( Figure S15a,b, Supporting Information). The re-distribution of charge density triggers the hydrolysis of the adsorbed C 3 H 7 O 6 P 2− into PO 4 2− and glycerin (C 3 H 6 O 3 ) ( Figure S15c, Supporting Information). C 3 H 6 O 3 can reduce the water activity in the Helmholtz plane to inhibit the H 2 evolution through the formation of H-bond. [27] Simultaneously, PO 4 2− -based SEI can be formed on the Zn surface, which blocks OH − and water from the Zn surface to stop the O 2 corrosion and HER. Once the Zn surface is fully covered by the SEI layer, the hydrolysis of C 3 H 7 O 6 P 2− would be terminated until the fresh Zn is exposed. As a result, this SEI is heal-repairable with the preserved thickness, as evidenced by previous ex situ XRD results. Once this solid SEI layer cracks or damages, the electrolyte would contact the fresh Zn to repair the SEI layer, which enables its self-healing capability.
The reversibility of Zn stripping/plating in ZnSO 4 electrolytes with different SG concentrations is compared in Cu/Zn cells ( Figure S16, Supporting Information and Figure 4a). Under a low DOD of 0.85% (1 mA cm −2 and 0.5 mA h cm −2 ), the Cu/Zn cell with 0.01 m SG delivers a high initial CE of 86.4%, however, it experiences dramatic fluctuations after 490 cycles caused by the short circuit. [28] When the concentration of additive increases to 0.05 m, the Cu/Zn cell displays a super long cycling life of 1000 cycles and a high average CE of 99.1%. This performance is overwhelming compared to that of cell with pure ZnSO 4 electrolyte (170 cycles with CE of 96.9%). When the additive concentration increases to 0.1, 0.5, and 1.0 m, however, initial CE values and cycling life for Zn plating/stripping are gradually reduced with higher polarization of Zn plating/stripping ( Figure S17, Supporting Information). The optimal concentration also prolongs the lifespan of Zn/Zn cell from ≈200 h to over 1000 h under 1 mA cm −2 and 1 mA h cm −2 ( Figure S18, Supporting Information). To prove the feasibility of functional electrolyte under strictly practical conditions, a super high DOD of 85% was applied by using a thin Zn electrode (10 µm) (Figure 4b). Under this high DOD, the Cu/Zn cell shows the CE fluctuations after only 30 cycles in 2 m ZnSO 4 , which demonstrates the limited Zn lifespan due to the rapid Zn consumption. During 30 cycles, the average CE is only 98.3%. In comparison, the Cu/Zn cell with 10 µm Zn foil shows a prolonged cycling life for over 100 cycles with a high CE of 99.6% in the SG-containing electrolyte, which demonstrates the effectiveness of SG under a strict working standard.
To verify the practicability of SG additive under battery operation, multiple measurements were conducted. Figure S19, Supporting Information shows RDE measurements of Zn electrode in ZnSO 4 electrolyte with/without SG additive. As mentioned previously, the Zn electrode experiences an obvious O 2 adsorption reaction in pure ZnSO 4 electrolyte. Whereas the current polarization at −0.2 V versus SCE is significantly reduced in 2 m ZnSO 4 + 0.05 m SG, indicating that O 2 adsorption reaction is effectively suppressed. [29] Figure 4c presents in situ GC results in Zn/Zn cell with the SG-containing electrolyte upon Zn plating/stripping. There is no obvious H 2 signal during the whole 3.5 cycles, which suggests that HER is suppressed during battery operation as well ( Figure S20, Supporting Information). The suppressed HER is mainly caused by decreasing H + concentration, the water-activity reduction, and self-healing SEI layer formation induced by SG additive. Figure 4d,e shows in situ pH measurements of the electrolyte during battery operation. In the pure ZnSO 4 , the pH value near the counter electrode sharply increases once the Zn plating occurs, indicating the synchronization of HER. The pH value of electrolyte is immediately reduced as the stripping of Zn proceeds, then it gradually increases during the striping process, which manifests the fluctuation of OH − caused by HER during the whole plating/stripping. [30] When the electrolyte contains SG additive, the pH value is almost the same due to the nonfluctuation of H + concentration, which further evidences the inhibition of HER.
The morphology of Zn electrodes after 50 cycles in Cu/Zn cells is compared in Figure 4f,g. In the pure ZnSO 4 electrolyte, the excessive by-product sheets are generated on the Zn surface, which not only consumes the active Zn but also shortens the battery lifespan ( Figure S21, Supporting Information). In the SG-containing electrolyte, however, side reactions are effectively suppressed. Moreover, the suppressed accumulation of Zn 4 SO 4 (OH) 6 ·xH 2 O by-product on the cycled Zn electrode is further confirmed by the 3D confocal images ( Figure S22, Supporting Information) and XRD patterns ( Figure S23, Supporting Information). To identify the chemical environment on the surface of cycled Zn in different electrolytes, X-ray photo electron spectroscopy (XPS) analysis with depth profiling was performed. No obvious peak shift occurs after depth profiling in Zn 2p curves of Zn electrodes cycled in ZnSO 4 electrolyte ( Figure S24, Supporting Information). Whereas the Zn LMM spectra change obviously with new peaks appearance at ≈992.6 and ≈996 eV along with etching, which are indexed to the metallic Zn 0 . However, the Zn 2+ (≈986.8 eV) peak ascribed to Zn 4 SO 4 (OH) 6 ·xH 2 O remains during the whole etching process, inducing the massive by-product accumulation (Figure 4h). [31] The S 2p scan is presented in Figure 4i, showing that the strong SO 4 2− peak (≈169.3 eV) is gradually reduced with increasing etching depth. Nevertheless, the peak of by-product is still visible even after 250 s etching. In contrast, the P 2p (≈134.4 eV) instead of S 2p signal is detected on the surface of Zn cycled in the electrolyte with SG additive (Figure 4j), indicating Zn 4 SO 4 (OH) 6 ·xH 2 O by-product is effectively suppressed. The P 2p signal gradually decreases along with etching, which matches well with the evolution of the Zn LMM (Figure 4k), indicating the formation of thin SEI layer.
Specially designed Cu/Zn cells were tested to in situ monitor the morphology of deposited Zn in ZnSO 4 electrolytes with/ without SG additive ( Figure S25, Supporting Information). In pure ZnSO 4 electrolyte, uneven Zn dendrites along with the serious H 2 evolution appear upon Zn plating, as presented in Video S1, Supporting Information. These dendrites and bubbles remain in the following stripping process, leading to the low Zn reversibility. Typical snapshots taken during the plating/ stripping are shown in Figure 4l. The heterogeneous Zn deposition is visible with a bubble after 200 s plating, indicating the dendrite formation and HER. Some protrusions gradually turn into Zn dendrites with further plating (600 s). Upon stripping, the deposited Zn is gradually removed from the Cu electrode, however, obvious HER and residual Zn remain on the Cu electrode even after 600 s stripping. In comparison, the Zn plating on the Cu electrode is smooth and homogeneous in the SG-based ZnSO 4 electrolyte, as illustrated in Video S2, Supporting Information. No obvious Zn dendrites and H 2 evolution are generated after 600 s plating (Figure 4m). After 600 s stripping, the deposited Zn is completely removed from the Cu electrode, which affirms the high reversibility of side-reactionfree and dendrite-free Zn plating/stripping. PANI/Zn batteries were assembled to assess the function of SG additive on the full-cell performance, where the PANI cathode was in situ grown on the carbon cloth. After the polymerization reaction, these carbon fibers are fully covered by abundant arrays, indicating the formation of PANI ( Figure S26, Supporting Information). The galvanostatic discharge-charge curves of the PANI/Zn coin cell were performed under 1 A g −1 , as presented in Figure 5a. In the pure ZnSO 4 electrolyte, the cell displays two discharging plateaux, which indicates the H + /Zn 2+ co-working mechanism of PANI cathode. In contrast, the second plateau is distinct when the electrolyte contains 0.05 m SG additive, demonstrating the storage of a large amount of Zn 2+ . Thus, the discharge capacity is slightly enhanced from ≈188.2 to ≈192.3 mA h g −1 . Then the cycling performance in coin cells was tested by controlling the electrolyte addition and compared under a high current density of 5 A g −1 , in which 30 µL electrolyte was added to just infiltrate separator (Figure 5b). In the pure ZnSO 4 electrolyte, the PANI/ Zn coin cell shows a rapid capacity fading after 300 cycles with Adv. Mater. 2022, 34, 2206963 Figure 5. Full-cell characterization. a) Charge-discharge curves for PANI/Zn coin-cells with 2 m ZnSO 4 and 2 m ZnSO 4 + 0.05 SG under a low current density of 1 A g −1 . b) Cycling stability of PANI/Zn coin cells at a high current density of 5 A g −1 with different electrolytes, the electrolyte addition is 30 µL. c) Charge-discharge curves for PANI/Zn pouch-cells with a lean electrolyte addition of 2 µL mg −1 under a current of 50 mA. d) Cycling stability of PANI/ Zn pouch cells at a high current of 80 mA with a lean electrolyte addition of 2 µL mg −1 . e-g) The practicality evaluation of the PANI/Zn pouch cells: e) by illuminating nine blue LEDs, f) by illuminating nine blue LEDs after cutting one cell, indicating the safety performance, and g) by powering an iPhone. the capacity retention of 62.5%. However, the good cycling stability in the electrolyte containing 0.05 m SG is pronounced. After 1400 cycles, a high capacity of 99.5 mA h g −1 remains, corresponding to the retention of ≈81.7%.
It has been widely accepted that the performance evolution under the lean electrolyte is one of the most important steps to fulfil the goal of battery commercialization. [32] Instead of coin cells, the PANI/Zn pouch cells with 0.05 m SG additive were assembled under the strict environment of the lean electrolyte addition (2 µL mg −1 ) ( Figure S27, Supporting Information). Under the current of 50 mA, the pouch cell with 0.05 m SG additive displays similar charge-discharge curves to these in coin cell, indicating the uniformity of electrochemical performance when the battery was scaled up (Figure 5c). The cycling stability of the pouch cell measured under a high current density of 80 mA is shown in Figure 5d. It can be found that this pouch cell shows a high reversibility of ≈100% CE. After 500 cycles, the capacity remains 95.5%, which is also superior to that reported for PANI/Zn pouch cells. [33] The efficacy of the pouch cell is demonstrated by using it to power different devices. Three small pouch cells in series can illuminate nine blue light-emitting diodes (LEDs, 3.0 V), as shown in Figure 5e. Importantly, these cells can still power the LEDs after cutting one corner of the cell (Figure 5f), indicating its advantages of high safety performance. Pouch cells in series can also power other devices, such as a rotating fan ( Figure S28, Supporting Information) and charging an iPhone (Figure 5g), which demonstrates the high potential of ZIBs with SG additive in practical applications.
In summary, Zn behavior was thoroughly studied in aqueous media, indicating that Zn anodes also face Zn-consuming O 2 adsorption corrosion beside HER and dendrite growth. Differing from traditional aqueous Li/Na batteries, however, removing O 2 cannot enhance the battery performance. The functional additive of SG was introduced in the electrolyte to solve Zn problems. It regulated the H + concentration and diminished free-water activity, significantly inhibiting the H 2 evolution. The hydrolysis of C 3 H 7 O 6 P 2− distributed in the inner Helmholtz layer of Zn electrode also triggers the formation of self-repairing SEI with a thickness of ≈217 nm before battery operation, which inhibits O 2 corrosion and dendrite growth. Thus, the high Zn reversibility was achieved even under a high DOD of 85%, which also guarantees the excellent performance of ZIBs in both coin cells and pouch cells. Under the lean electrolyte, the PANI/Zn batteries displayed excellent cycling performance for 1400 and 500 cycles in coin cell and pouch cell, respectively. This work reveals Zn behavior in water-based electrolytes and guides future research to achieve the highly efficient utilization of active metal anodes for future commercialization.

Experimental Section
Experimental details can be found in the Supporting Information.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.