Improving Cycle Life of Zinc–Air Batteries with Calcium Ion Additive in Electrolyte or Separator

The electrolyte carbonation and the resulting air electrode plugging are the primary factors limiting the cycle life of aqueous alkaline zinc–air batteries (ZABs). In this work, calcium ion (Ca2+) additives were introduced into the electrolyte and the separator to resolve the above issues. Galvanostatic charge–discharge cycle tests were carried out to verify the effect of Ca2+ on electrolyte carbonation. With the modified electrolyte and separator, the cycle life of ZABs was improved by 22.2% and 24.7%, respectively. Ca2+ was introduced into the ZAB system to preferentially react with CO32− rather than K+ and then precipitated granular CaCO3 prior to K2CO3, which was deposited on the surface of the Zn anode and air cathode to form a flower-like CaCO3 layer, thereby prolonging its cycle life.


Introduction
Zinc-air batteries (ZABs) have been considered for a stationary energy storage application due to their high theoretical energy densities, abundant resources, low cost, and the environmental compatibility of Zn [1][2][3][4][5][6]. Its semi-open structure, however, inevitably led to the carbonation of electrolyte, and carbon dioxide came from the air as well as the oxidization of C element in air cathode at a high discharge voltage according to Equations (1) and (2) [7][8][9]. It would significantly reduce the cycle life of ZABs and restrict further commercialization [7,8,10,11].
2KOH (aq) + CO 2 (g) = K 2 CO 3 (s) (2) Therefore, mitigating the carbonation of alkaline electrolytes is crucial for the application of alkaline ZABs, and numerous attempts have been carried out to resolve the above issues. An aqueous chloride-based electrolyte has been provided to obtain a long cycle life of over 400 h but with a relatively low overpotential of less than 1.1 V [12]. Various near-neutral electrolytes have also been tested to exhibit better recyclability but suffer from a lower current density and battery capacity due to the inhibition of oxygen evolution reactions (OERs) and the hydrogen evolution reaction (HER) [13][14][15][16][17][18][19][20]. A "water-in-salt" (WIS) gel polymer electrolyte was proposed to achieve a long cycle life, but only with lower operating power [21]. Additionally, both H 2 SO 4 and ZnSO 4 were believed to induce the abrupt loss of battery capacity [22][23][24].  Figure 1 shows the schematic of the separator preparation mold, which consists of two parts, as shown in Figure 1a, and Figure 1b exhibits the schematic after assembly. The slurry was dropped onto polypropylene (pp) film on the mold surface and then dried and turned over to repeat the above process until a satisfactory separator was obtained. croscopy (SEM). Electrochemical measurements were also cond performance evolution of the air cathode after cycling. In additio electrolyte and the concentration of CO3 2− were also measured to the mechanism of additives.

Materials
The Zn anode was an 80 × 40 × 1 mm zinc sheet, ground sandpapers in sequence, cleaned and dried with deionized wate air cathode with a 4.5 mg cm −2 mass-loading of submicron-scaled was selected, where carbon black (CB, XC-72R) powders and (PTFE) were used as the conductive additive and the binders, r electrolyte consisting of 6 M KOH and 0.2 M ZnO with 0.2 M C additive. In addition, Ca(OH)2 was added to the α-Al2O3 ceram modified separator, and the detailed composition is listed in Tab Figure 1 shows the schematic of the separator preparation two parts, as shown in Figure 1a, and Figure 1b exhibits the schem slurry was dropped onto polypropylene (pp) film on the mold su turned over to repeat the above process until a satisfactory separ

Design and Assembly of ZAB
A typical ZAB mold was applied in this work with the distance between the air cathode and the Zn anode of 1 cm and the active area of the electrode of 4.5 cm 2 [38][39][40][41][42][43]. In addition, another set of electrolytic cells was installed to explore the effect of the separator on the cycle life of ZABs, as shown in Figure 2, which consisted of an electrolytic cell and a separator fixing plate (Figure 2a). When installing (Figure 2b), the separator was placed into the electrolytic cell and then pressed tightly with the separator fixing plate. When assembling the battery, both sides of the electrolytic cell were fastened to ensure it was completely sealed without liquid leakage during the test.

Design and Assembly of ZAB
A typical ZAB mold was applied in this work with the dista ode and the Zn anode of 1 cm and the active area of the electro addition, another set of electrolytic cells was installed to explore t on the cycle life of ZABs, as shown in Figure 2, which consisted a separator fixing plate (Figure 2a). When installing (Figure 2b), into the electrolytic cell and then pressed tightly with the sepa assembling the battery, both sides of the electrolytic cell were f completely sealed without liquid leakage during the test.

Methods
Galvanostatic charge-discharge cycle tests were measured mA cm −2 with a cut-off voltage higher than 5 V or lower than 0 V system, CT2001A (LanHe Instrument Technology Co., Ltd., W voltammetry (CV), electrochemical impedance spectroscopy (EIS ammetry (LSV) were performed at a scan rate of 2 mV s −1 with a V vs. the saturated calomel electrode (SCE) in the solution of 1 M which were carried out by electrochemical workstation (PARSTA plied Research, Oak Ridge, TN, USA). All the tests were carried (25 °C) in the atmosphere environment.
X-ray diffraction (XRD, Bruker D8 advanced, Bremen, Germ the phase composition of the electrodes after cycle testing with a to 90°. Surface morphologies of the electrodes were characterize ning electron microscopy (SEM, JSM-7800F, with EDS, JEOL , Tok the composition was analyzed using a Fourier-transform infrared jin Guangdong Co., Ltd., Tianjin, China) with a wavelength ran and a resolution of 4 cm −1 .
The concentration of carbonate ion (CO3 2− ) in the electrolyte ical titration. The conductivity of the electrolyte was tested by a tivity meter with a platinum electrode of 5 × 5 mm at 28.6 °C (ob tivity meter) in the atmospheric environment of 101.1 kPa.

Methods
Galvanostatic charge-discharge cycle tests were measured at a current density of 10 mA cm −2 with a cut-off voltage higher than 5 V or lower than 0 V, using a battery testing system, CT2001A (LanHe Instrument Technology Co., Ltd., Wuhan, China). The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV) were performed at a scan rate of 2 mV s −1 with a voltage range of 0 to ±1 V vs. the saturated calomel electrode (SCE) in the solution of 1 M KOH with O 2 saturated, which were carried out by electrochemical workstation (PARSTAT 4000A, Princeton Applied Research, Oak Ridge, TN, USA). All the tests were carried out at room temperature (25 • C) in the atmosphere environment.
X-ray diffraction (XRD, Bruker D8 advanced, Bremen, Germany) was used to analyze the phase composition of the electrodes after cycle testing with a scanning angle from 10 • to 90 • . Surface morphologies of the electrodes were characterized by field-emission scanning electron microscopy (SEM, JSM-7800F, with EDS, JEOL, Tokyo, Japan). Additionally, the composition was analyzed using a Fourier-transform infrared (FTIR) (FTIR-650, Tianjin Guangdong Co., Ltd., Tianjin, China) with a wavelength range from 4000 to 400 cm −1 and a resolution of 4 cm −1 .
The concentration of carbonate ion (CO 3 2− ) in the electrolyte was measured by chemical titration. The conductivity of the electrolyte was tested by a REX DDSJ-308F conductivity meter with a platinum electrode of 5 × 5 mm at 28.6 • C (obtained from the conductivity meter) in the atmospheric environment of 101.1 kPa.

Charge-Discharge Cycle Performance
To investigate the effect of Ca 2+ on the cycle life, ZABs with a modified electrolyte and separator were evaluated with galvanostatic charge-discharge cycle tests; the corresponding results are shown in Figure 3. In addition, a reference ZAB without additives was subjected to the same test as the experimental group. ZABs with a modified electrolyte and separator cycled for 99 h and 101 h, respectively, while the reference one only ran for 81 h, indicating that the addition of the Ca(OH) 2 additive into the electrolyte and separator improved the cycle life by 22.2% and 24.7%, respectively [11,37,44,45]. It should be noted that the improvement effect of the modified electrolyte is similar to that of the modified separator, owing to the solubility limitation of Ca(OH) 2 in the aqueous electrolyte.

Morphology and Structure of ZAB Components
To better understand the improvement mechanism of the Ca life of ZABs, the batteries were disassembled to characterize their the Zn anode, air cathodes, electrolyte, and separator.
Surface morphologies of cycled Zn anodes with a modified e are shown in Figures 4 and 5, respectively, with the correspond surface of the Zn anodes after cycling was relatively flat with a un vex structure (Figures 4a and 5a). After partial amplification, it c that the convex structure was a flower-like structure, while the 3D was formed by alternating stacking of nanoparticle and lamellar Figure 5b. The corresponding element distributions of cycled Zn a ures S1 and S2, where the element of Ca was uniformly distribute and K elements were locally enriched at the surface bumps. The d results are listed in Tables S1 and S2, and it could be found that element on the surface of the Zn anode was relatively low, consiste of Ca 2+ additive in the modified electrolyte and separator. In

Morphology and Structure of ZAB Components
To better understand the improvement mechanism of the Ca 2+ additive on the cycle life of ZABs, the batteries were disassembled to characterize their components, including the Zn anode, air cathodes, electrolyte, and separator.
Surface morphologies of cycled Zn anodes with a modified electrolyte and separator are shown in Figures 4 and 5, respectively, with the corresponding XRD patterns. The surface of the Zn anodes after cycling was relatively flat with a uniformly distributed convex structure (Figures 4a and 5a). After partial amplification, it can be seen in Figure 4b that the convex structure was a flower-like structure, while the 3D microporous structure was formed by alternating stacking of nanoparticle and lamellar structures, as shown in Figure 5b. The corresponding element distributions of cycled Zn anodes are shown in Figures S1 and S2, where the element of Ca was uniformly distributed while elements of Zn and K elements were locally enriched at the surface bumps. The detailed element content results are listed in Tables S1 and S2, and it could be found that the concentration of Ca element on the surface of the Zn anode was relatively low, consistent with the low content of Ca 2+ additive in the modified electrolyte and separator. In Figures 4c and 5c, the peaks are found to correspond quite well with Zn at 36.  [46], confirming the corrosion and passivation of Zn anode during galvanostatic charge-discharge process [47]. Important to note that characteristic peaks of CaCO 3 [26][27][28]. It could be concluded that CO 3 2was adsorbed and combined with Ca 2+ to form CaCO 3 during cycling when adding Ca(OH) 2 into the electrolyte and separator [26][27][28][29][30], which generated flower-like CaCO 3 layer to provide the transport channel of OH − , thereby improving the cycle life of ZABs [48].
Surface morphologies of the air cathode after cycling with a modified electrolyte and separator are shown in Figures 6 and 7, respectively, with the corresponding XRD patterns. The lamellar convex structure was evenly distributed on the surface of the air cathode, as shown in Figures 6a and 7a, and the lamellar convex structure and fine particles were alternately distributed, as shown in Figures 6b and 7b. According to the element distribution characterization (Figures S3 and S4) and the detailed element content (Tables S3 and S4), the element of K was densely distributed on the surface of the air cathode with relatively higher coverage, while the element of Ca was enriched around spherical particles in a small amount. In addition, elements of C and O were accumulated at the enrichment sites of K and Ca, and the element of Co was only observed in local areas. In Figures 6c and 7c . 76-0606). It could be demonstrated that K 2 CO 3 and CaCO 3 were generated during charge-discharge cycling [7,8,11,[26][27][28]37]. Considering the voltage loss (i.e., iR drop) caused by the electrolyte solution between the working electrode and the reference electrode, corrected ORR and OER performance of the cycled air cathode by iR compensation was investigated in Figure 8, and both of them were significantly decreased after cycling, and its performance with a modified electrolyte deteriorated more than the one with a modified separator [8,11,[26][27][28][29][30]49,50]. After cycling, the micropores of the air cathode could easily be blocked by the generated lamellar K 2 CO 3 , hindering the diffusion of O 2 through the air cathode to participate in ORR and OER reaction, resulting in the remarkable decrease in ORR and OER performance of air cathode. In comparison, the flower-like porous structure formed by granular CaCO 3 on the surface of the air cathode could provide gas diffusion channels, and thus both ORR and OER activity of the air cathode could be improved to a certain extent, exhibiting a relatively low decrease in the performance.     Surface morphologies of the air cathode after cycling with a modified separator are shown in Figures 6 and 7, respectively, with the correspond The conductivity and concentration of CO 3 2− in the electrolyte were measured to clarify the effect of Ca 2+ , and the detailed values are summarized in Tables 2 and 3. Prior cycling, the conductivity increased by 2.5% from 63.26 × 10 −2 S cm −1 to 64.85 × 10 −2 S cm −1 when adding 0.2 mol L −1 Ca(OH) 2 . After cycling, the conductivity of the modified electrolyte decreased by 36.1% to 41.46 × 10 −2 S cm −1 , while the responding value with the modified separator decreased by 39.3% to 38.43 × 10 −2 S cm −1 , which was attributed to the increased resistance caused by the consumption of conductive ions as well as the production of CaCO 3 [26][27][28]. Remarkable changes were detected in the concentration of CO 3 2− when adding Ca(OH) 2 , which significantly decreased from 6.182 mol L −1 to 0.800 mol L −1 or 0.547 mol L −1 after cycling with the modified electrolyte or separator, respectively, as shown in Table 3 [8,11,44]. Ca 2+ was introduced into the ZAB system to preferentially react with CO 3 2− rather than K + , resulting in the precipitation of granular CaCO 3 prior to K 2 CO 3 [51], alleviating the blocking effect of micropores on the air electrode surface induced by the carbonation of alkaline electrolyte. Moreover, the modified separator would continuously release Ca 2+ to the electrolyte to consume CO 3 2− during the charge-discharge process, maintaining a relatively low concentration of CO 3 2− in the electrolyte to achieve sustained long-term improvement [48].
Surface morphologies of the modified separator on both sides were investigated before and after cycling, as shown in Figure 9, and the corresponding XRD patterns are exhibited in Figure 10. Prior to cycling, the fresh separator (Figure 9a) is relatively flat, with micropores, whiskers, and nano-spherical structures scattered and uniformly distributed on its surface. After cycling, small spherical structures were observed on the separator when contacting the Zn anode (Figure 9b), forming a loose 3D structure with micropores and whiskers alternately appearing and uniformly distributed on the surface, while at the side contacting the air cathode (Figure 9c), a relatively smoother surface could be found with layered and nano-spherical structures. Combining with the element distribution characterization (Figures S5-S7) and the detailed element content (Tables S5-S7), elements of C, O, Al, K, Ca, and Zn were uniformly distributed on both sides of the separator. Additionally, elements of Ti and Zr were enriched in the surface bumps, and the distribution of Ca, C, and O elements overlapped, indicating the formation of CaCO 3 . According to XRD results in Figure 10, the peaks detected at 31.  [26][27][28]. By comparing the infrared spectrum of Ca(OH) 2 and CaCO 3 in Figure S8, it could be found that the peak of OH − at 3642 cm −1 became weaker with enhanced peaks of C-O at 1432 cm −1 and 876 cm −1 after cycling, implying the formation of CaCO 3 to consume CO 2 by the absorption of Ca 2+ [29,31,33,34,36,48].  (1 2 4), and No. 76-0606). It could be demonstrated that K2CO3 and CaCO3 wer charge-discharge cycling [7,8,11,[26][27][28]37]. Considering the voltage caused by the electrolyte solution between the working electrode an trode, corrected ORR and OER performance of the cycled air cathode was investigated in Figure 8, and both of them were significantly dec and its performance with a modified electrolyte deteriorated more modified separator [8,11,[26][27][28][29][30]49,50]. After cycling, the micropores could easily be blocked by the generated lamellar K2CO3, hindering through the air cathode to participate in ORR and OER reaction, resu able decrease in ORR and OER performance of air cathode. In compar porous structure formed by granular CaCO3 on the surface of the air vide gas diffusion channels, and thus both ORR and OER activity of t be improved to a certain extent, exhibiting a relatively low decrease i     The conductivity and concentration of CO3 2− in the electrolyte were measured to ify the effect of Ca 2+ , and the detailed values are summarized in Tables 2 and 3. Pri cling, the conductivity increased by 2.5% from 63.26 × 10 −2 S cm −1 to 64.85 × 10 −2 S when adding 0.2 mol L −1 Ca(OH)2. After cycling, the conductivity of the modified el lyte decreased by 36.1% to 41.46 × 10 −2 S cm −1 , while the responding value with the m fied separator decreased by 39.3% to 38.43 × 10 −2 S cm −1 , which was attributed to t creased resistance caused by the consumption of conductive ions as well as the produ of CaCO3 [26][27][28]. Remarkable changes were detected in the concentration of CO3 2−   Table 3. Concentration of CO 3 2− in ZAB electrolyte after cycling.

Without Ca 2+ Additive Modified Electrolyte Modified Separator
Concentration of CO 3 2− /mol L −1 6.182 0.800 0.547 on its surface. After cycling, small spherical structures were observed on the separator when contacting the Zn anode (Figure 9b), forming a loose 3D structure with micropores and whiskers alternately appearing and uniformly distributed on the surface, while at the side contacting the air cathode (Figure 9c), a relatively smoother surface could be found with layered and nano-spherical structures. Combining with the element distribution characterization (Figures S5-S7) and the detailed element content (Tables S5-S7), elements of C, O, Al, K, Ca, and Zn were uniformly distributed on both sides of the separator. Additionally, elements of Ti and Zr were enriched in the surface bumps, and the distribution of Ca, C, and O elements overlapped, indicating the formation of CaCO3. According to XRD results in Figure 10,  [26][27][28]. By comparing the infrared spectrum of Ca(OH)2 and CaCO3 in Figure S8, it could be found that the peak of OH − at 3642 cm −1 became weaker with enhanced peaks of C-O at 1432 cm −1 and 876 cm −1 after cycling, implying the formation of CaCO3 to consume CO2 by the absorption of Ca 2+ [29,31,33,34,36,48].

Mechanism of Ca 2+ Additive to Improve the Cycle Life of ZABs
To clarify the mechanism of the Ca 2+ additive in improving the cycle life of ZABs, the morphologies of the CaCO 3 crystal on the surface of the Zn anode and the air cathode were further characterized by the flower-like structures [48] shown in Figure 11b,c, respectively, exhibiting significant differences from that of the CaCO 3 calcite particles (Figure 11a). It could be concluded that such flower-like-shaped CaCO 3 films with large gaps between the crystals provided ion transport channels for electrochemical reactions during the charge and discharge processes, which prevented the blockage of the air cathode by lamellar K 2 CO 3 crystals [26][27][28][29][30]. This phenomenon occurred mainly due to the strong electrostatic attraction coordinating unsaturated Ca 2+ in the surface of CaCO 3 to CO 3 2− [25]. Besides that, CaCO 3 exhibited relatively lower solubility compared with that of K 2 CO 3 , resulting in the preferential combination of Ca 2+ with CO 3 2− in the electrolyte to produce CaCO 3 prior to K 2 CO 3 [52]. It could alleviate the carbonation of the electrolyte as well as the blockage of the air cathode [26][27][28][29][30], effectively improving the cycle life of ZABs [48]. nomaterials 2023, 13, x FOR PEER REVIEW Figure 10. XRD patterns of the fresh separator (black line) before galvanostatic cha cycles of ZAB, and side to Zn anode (red line) and side to air cathode (blue line) after

Mechanism of Ca 2+ Additive to Improve the Cycle Life of ZABs
To clarify the mechanism of the Ca 2+ additive in improving the cycle life morphologies of the CaCO3 crystal on the surface of the Zn anode and the were further characterized by the flower-like structures [48] shown in Figu spectively, exhibiting significant differences from that of the CaCO3 calcite p ure 11a). It could be concluded that such flower-like-shaped CaCO3 films wi between the crystals provided ion transport channels for electrochemical reac the charge and discharge processes, which prevented the blockage of the ai lamellar K2CO3 crystals [26][27][28][29][30]. This phenomenon occurred mainly due to the trostatic attraction coordinating unsaturated Ca 2+ in the surface of CaCO3 t Besides that, CaCO3 exhibited relatively lower solubility compared with th resulting in the preferential combination of Ca 2+ with CO3 2− in the electrolyt CaCO3 prior to K2CO3 [52]. It could alleviate the carbonation of the electroly the blockage of the air cathode [26][27][28][29][30], effectively improving the cycle life of

Mechanism of Ca 2+ Additive to Improve the Cycle Life of ZABs
To clarify the mechanism of the Ca 2+ additive in improving the cycle life of ZABs, the morphologies of the CaCO3 crystal on the surface of the Zn anode and the air cathode were further characterized by the flower-like structures [48] shown in Figure 11b,c, respectively, exhibiting significant differences from that of the CaCO3 calcite particles (Figure 11a). It could be concluded that such flower-like-shaped CaCO3 films with large gaps between the crystals provided ion transport channels for electrochemical reactions during the charge and discharge processes, which prevented the blockage of the air cathode by lamellar K2CO3 crystals [26][27][28][29][30]. This phenomenon occurred mainly due to the strong electrostatic attraction coordinating unsaturated Ca 2+ in the surface of CaCO3 to CO3 2− [25] Besides that, CaCO3 exhibited relatively lower solubility compared with that of K2CO3 resulting in the preferential combination of Ca 2+ with CO3 2− in the electrolyte to produce CaCO3 prior to K2CO3 [52]. It could alleviate the carbonation of the electrolyte as well as the blockage of the air cathode [26][27][28][29][30], effectively improving the cycle life of ZABs [48]. Figure 11. Morphologies of (a) CaCO3 calcspar and the CaCO3 on the surface of (b) Zn anode and (c air cathode after galvanostatic charge-discharge cycles of ZAB with modified electrolyte. Figure 11. Morphologies of (a) CaCO 3 calcspar and the CaCO 3 on the surface of (b) Zn anode and (c) air cathode after galvanostatic charge-discharge cycles of ZAB with modified electrolyte.

Conclusions
In order to alleviate the carbonation of electrolytes to improve the cycle life of ZABs, Ca 2+ was introduced to modify the electrolyte and the separator. Galvanostatic chargedischarge cycling tests were conducted with morphology and structure analysis of various battery components; subsequently, it could be found that the cycle life of ZABs was improved obviously by 22.2% and 24.7% with the Ca(OH) 2 -modified electrolyte and separator, respectively. When adding Ca(OH) 2 into the electrolyte and the separator, CO 3 2− was adsorbed to combine with Ca 2+ to form CaCO 3 during cycling. The generated CaCO 3 deposited on the Zn anode and air cathode to produce a flower-like CaCO 3 layer, which provided ion transport channels for electrochemical reactions during charge-discharge cycles and alleviated the blockage of the air cathode by lamellar K 2 CO 3 crystals, thereby improving the cycle life of ZABs.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13121864/s1. Figure Figure S8: FT-IR patterns of the modified separator (a) before and (b) after cycle testing of ZAB. Tabel S1: Element content of Zn anode after cycle testing of ZAB with modified electrolyte. Tabel S2: Element content of Zn anode after cycle testing of ZAB with modified separator. Tabel S3: Element content of air cathode after cycle testing of ZAB with modified electrolyte. Tabel S4: Element content of air cathode after cycle testing of ZAB with modified separator. Tabel S5: Element content of the modified separator before cycle testing of ZAB. Tabel S6: Element content of the modified separator at the side contacting with Zn anode after cycle testing of ZAB. Tabel S7: Element content of the modified separator at the side contacting with air cathode after cycle testing of ZAB.