01Research background As an innovative technology, rechargeable batteries have greatly changed the development process of modern society. High-efficiency energy storage technology at high temperatures requires advanced battery systems to meet growing application scenarios such as

01 research background

Rechargeable battery as an innovative technology has greatly changed the development process of modern society. High-efficiency energy storage technology at high temperatures requires advanced battery systems to meet growing application scenarios such as mining/drilling, industrial manufacturing and aerospace, etc., and to ensure high safety under these harsh working conditions. The zinc-air battery has the advantages of high theoretical energy density, environmental friendliness and low cost, showing great potential as the next generation battery energy storage system. And zinc-air batteries use non-flammable aqueous electrolytes and are inherently safe at high temperatures. Based on these considerations, zinc-air batteries are expected to achieve efficient and safe energy storage performance at high temperatures. However, there are still few studies on high-temperature zinc-air batteries, and the feasibility, high-temperature performance and technical bottlenecks of zinc-air batteries working at high temperatures need to be studied.

02 Results Introduction

Recently, Zhang Qiang of Tsinghua University and Beijing Institute of Technology Li Boquan (co-corresponding author ) published a paper entitled Working Zinc–Air Batteries at 80℃ on Angew. Chem. Int. Ed. thesis. This work comprehensively studied the feasibility of zinc-air batteries working at high temperatures, and summarized the technical bottlenecks that limit their high-temperature performance, pointing out the direction for the future development of zinc-air batteries.

03 Research Highlights

1) At high temperatures, the electrolyte of zinc-air batteries (6.0 M KOH + 0.20 M Zn(Ac)2) is conducive to efficient ion diffusion and has high ion conductivity ;

2) positive electrode electrochemistry The kinetics of reaction ORR/OER are improved at high temperatures, and the zinc anode has anti-passivation properties at high temperatures ;

3) The competitive reaction between zinc deposition and HER results in a reduction in the faradaic efficiency (FE) of the anode during charging, which It is the main bottleneck that limits the performance improvement of zinc-air batteries.

04 Graphic introduction

Figure 1 Properties of electrolytes: (a) vapor pressure (p(H2O)); (b) viscosity (μ); (c) conductivity (κ) of electrolytes at different temperatures (T); (d) ) MD simulation of electrolyte at 80°C; (e) K+ diffusion coefficient (D) based on MSD analysis; (f) Comparison of D values ​​of K+ and OH- at different temperatures.

First, the authors studied the properties of ZAB electrolyte (6.0 M KOH+0.20 M Zn(Ac)2) at high temperatures. Due to the colligative property of the solution, the boiling point of the electrolyte is higher than that of pure water, which is 115°C (vapor pressure of the electrolyte: Figure 1a), which means that ZAB can work in high temperature environments above 100°C. At the same time, the viscosity (μ) of the electrolyte dropped from 3.01 mPa s at 20°C to 2.08 mPa s at 80°C ( Figure 1b), which indicates that high temperature is beneficial to ion diffusion in the electrolyte and achieves higher ionic conductivity. As shown in Figure 1c, at 80°C, the electrolyte reaches an ultra-high conductivity of 997 mS cm-1, which is almost twice that at 20°C.

The authors further used molecular dynamics (MD) simulation to deeply study the ion diffusion mechanism at different temperatures ( Figure 1d). At high temperatures, both K+ and OH- exhibit higher mean square displacements (〈|r(t)−r(0)|2〉, MSDs, Figure 1e), corresponding to Einstein’s diffusion law (lim( t→∞)〈|r(t)−r(0)|2〉=6Dt) high ion diffusion coefficient (d) ( Figure 1f), and based on the Einstein-Smolokovsky equation High ionic conductivity (D=RTu/zF, u represents ion mobility). In summary, ZAB electrolyte can withstand temperatures exceeding 100°C, and its ionic conductivity increases at high temperatures, providing minimal internal resistance and good battery performance.

Figure 2 Positive electrode ORR/OER performance: (a) LSV curves of Pt/C+Ir/C electrocatalyst tested in 0.10 M KOH at 20℃ and 80℃; (b) φ⊖(O2 at different temperatures /H2O), E1/2 and E10; (c) Arrhenius curves of ORR and OER; (d) ΔE recorded in different electrolytes. (e) LSV curves of air cathode based on Pt/C+Ir/C electrocatalyst tested in 6.0 M KOH+0.20 M Zn(Ac)2 at 20°C and 80°C.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the discharge reaction and charging reaction of the zinc-air battery cathode respectively. Due to its slow kinetics, Pt/C+Ir/C electrocatalysts are widely used to accelerate the reaction kinetics of the cathode. First, the authors used a standard system for testing. The system uses a rotating ring-disk electrode (RRDE) loaded with Pt/C+Ir/C electrocatalyst and 0.10 M KOH electrolyte. The linear scan voltammetry (LSV) curve is shown in Figure 2a. In order to determine the ORR and OER performance at different temperatures, the traditional potential indicators (E1/2: the potential reaching half the ORR limiting current density; E10: the potential reaching 10mAcm-2 OER current density) were calibrated to the overpotential ( Figure 2b, Figure 2a inset ).

According to the Arrhenius curve ( Figure 2c), the author further calculated that the apparent activation energies (Ea) of ORR and OER are 27.7 and 30.2 kJ·mol-1 respectively. The ΔE index, defined as the difference between E10 and E1/2, can quantitatively reflect the performance of ORR and OER simultaneously. The ΔE index dropped from 0.804 V at 20°C to 0.673 V at 80°C, further proving that high temperature is beneficial to cathode kinetics ( Figure 2d). In addition, in order to reveal the cathode performance more truly, the LSV curve of the air cathode in 6.0 M KOH+0.20 M Zn(Ac)2 electrolyte was also recorded ( Figure 2e). Similar to the above results, ORR and OER kinetics accelerated at high temperatures, quantitatively reflected as a decrease in the voltage difference between the onset potentials of ORR and OER ( Figure 2d). In short, the rate of cathode reaction and gas diffusion process is accelerated at high temperature, which alleviates the kinetic problems of ZAB cathode to a certain extent.

Figure 3 Negative electrode discharge performance: (a) LSV curve of zinc electrode at 20℃ and 80℃ and (b) corresponding Tafel curve; (c) Arrhenius diagram of zinc dissolution used for Ea calculation; (d) jmax at different temperatures; (e) Use chronopotentiometry to record the passivation time (τ); (f) τ at different temperatures.

Author studied the high-temperature electrochemical properties of zinc anodes. Considering that the main challenges faced by the negative electrode during the charge and discharge process are different, the author studied the charge and discharge behavior of the negative electrode, and based on the zinc dissolution LSV curve in Figure 3a, the following three conclusions were drawn:

1) At different temperatures, zinc The equilibrium potential of the electrode is very stable;

2) The higher current response and j0 indicate that high temperature is conducive to accelerating zinc dissolution kinetics ( Figure 3b), and the Ea of zinc dissolution is 14.5 kJ·mol-1 ( Figure 3c;

3) The insulating zinc oxide on the surface of the zinc electrode can cause passivation of the zinc anode, which is suppressed at high temperatures.

Research found that the maximum current density (jmax) and temperature (T) are positively correlated ( Figure 3d). Under galvanostatic conditions with a current density of 175 mA cm-2, the authors further evaluated the passivation behavior of the zinc anode using chronopotentiometry. Under this condition, electronically insulating zinc oxide is continuously precipitated on the zinc electrode, eventually leading to a sharp increase in overpotential ( Figure 3e), corresponding to the passivation time (τ). τ increases significantly with the increase of temperature (at 20°C, τ is 208 s; at 80°C, τ is 509 s). This indicates that zinc anode passivation at high temperatures is inhibited. The anti-passivation properties of zinc anode at high temperatures enable ZABs to have excellent rate performance and high areal capacity.

Figure 4 Negative electrode charging performance: (a) schematic diagram and (b) LSV curve of zinc deposition on zinc electrode and HER on zinc and platinum electrodes; (c) zinc deposition and E10 of HER at different temperatures; (d) using Ah Lennius curve calculation of Ea of HER on zinc electrode; (e) FE of negative electrode charging at different temperatures.

During the charging process of the zinc anode, the hydrogen evolution reaction (HER) and zinc deposition are competing reactions. Zinc deposition on is thermodynamically unfavorable but kinetically favorable compared to HER. Figure 4a shows three LSV curves, namely HER (low overpotential, purple line) on the kinetically favorable platinum electrode, and HER (large overpotential, green line) on the kinetically unfavorable zinc electrode. ), as well as zinc deposition on the zinc electrode and the side reaction HER (red line).

In order to decouple zinc deposition and HER, the authors further evaluated the above LSV curves at different temperatures ( Figure 4b) and came to the following conclusions: 1) As can be seen from the slightly enhanced E10 ( Figure 4c), zinc deposition The kinetics of HER are slightly enhanced at high temperatures; 2) It can be seen from the increased HER current on the zinc electrode and the corresponding enhanced E10 ( Figure 4c) that the HER kinetics on metallic zinc are significantly promoted at high temperatures. The Ea of HER on metallic zinc is 31.2 kJ mol-1 ( Figure 4d); 3) Although the kinetics of the above two reactions are enhanced at high temperatures, the promotion of HER kinetics is more obvious than that of zinc deposition ( Figure 4c). The competitive reaction between zinc deposition and HER results in a decrease in the faradaic efficiency (FE) of the anode during charging ( Figure 4e). The author tested at a current density of 25mA cm-2 and found that FE was only 56.7% at 80℃, which cannot be compared with 88.2% at 20℃. In summary, the Faradaic efficiency of the anode is severely degraded at high temperatures due to accelerated HER kinetics, which is not conducive to the improvement of cycle performance and durability of zinc-air batteries.

Figure 5 High temperature performance of ZABs: (a) Effect of high temperature on different components of ZABs; (b) LSV curves and (c) rate performance of ZAB at 20℃ and 80℃; (c) ZABs at 25mA at 80℃ Galvanostatic cycle curve at cm2.

According to the above research, high temperature is conducive to improving the electrolyte conductivity, positive and negative electrode kinetics, and positive electrode anti-passivation performance of ZABs, but is not conducive to improving the Faradaic efficiency of the negative electrode. This is due to the occurrence of the side reaction HER ( Figure 5a). In order to fully prove the feasibility of ZABs working at high temperatures, the authors evaluated the electrochemical performance of ZABs at 80°C. According to the LSV curve and rate performance ( Figures 5b and 5c), it can be seen that the degree of polarization of the charge and discharge curves decreases at high temperatures. This is due to the accelerated anode and cathode dynamics that reduce the electrochemical polarization and faster ion diffusion. The rate reduces concentration polarization and the enhanced electrolyte conductivity provides lower ohmic polarization. At a high current density of 25 mA cm-2, galvanostatic cycling tests demonstrated that ZABs have good cycling performance at high temperatures ( Figure 5d). The working performance of ZABs at high temperatures shows that the adverse effects of negative electrode Faradaic efficiency and electrolyte volatilization on battery performance are serious, but not fatal. This shows that ZABs can work at high temperatures.

05 Summary and Outlook

This paper systematically studies the working performance of ZABs under high temperature conditions. High temperature is beneficial to improving the electrolyte conductivity, positive and negative electrode kinetics, and anti-passivation performance of the negative electrode of ZABs. However, high temperature also accelerates HER side reactions, causing the faradaic efficiency of the negative electrode to decrease, which is the main bottleneck affecting the development of high-temperature ZABs. This points out future research directions for improving the high-temperature performance of ZABs. The authors demonstrated the performance of ZABs at 80°C, fully demonstrating the feasibility of ZABs as a safe high-temperature energy storage system. This work expands the application scenarios of ZABs and provides an example for battery research under complex operating conditions.

06 Literature link

Working Zinc–Air Batteries at 80℃. (Angew. Chem. Int. Ed. 2022, DOI: 10.1002/anie.202208042.)

original Article link:

https://doi.org/10.1002/anie.202208042