Dynamic Locking of Interfacial Side Reaction Sites Promotes Aluminum‐Air Batteries Close to Theoretical Capacity

Abstract

Aluminum metal has been regarded as a promising anode material for aqueous metal‐air batteries. However, the stable cycling of Al anodes is challenging due to the severe parasitic corrosion of Al metal in alkaline electrolytes. Here, a novel additive, n‐octylphosphonic acid (OPA), is introduced into the typical NaOH electrolyte system to improve the interfacial stability of Al anodes and thus promote high‐performance Al‐air batteries (AABs). Combining several experimental characterizations and theoretical calculation, it is proved that OPA molecules in an NaOH aqueous environment can modify the Al anode/electrolyte interface and alter the stacking of the discharge product. The electrolyte engineering is capable of anchoring dynamically to restrain side reactions through hydrogen bonds (H···O), homogenizing the dissolution of Al metal and avoiding precipitation agglomeration. As a proof of concept, AABs full cells with an electrolyte containing OPA achieve higher potential plateau and discharge capacity than those with a pure NaOH electrolyte. It paves a way to develop highly‐efficient and eco‐friendly electrolyte additive strategies for high‐performance AABs devices and advance the current understanding of organic additive mechanisms in AABs.

Electrolyte Solvation Structure Regulation Promotes Aluminum–Air Batteries to Approach Theoretical Discharge Capacity

“…Previous studies have shown that negatively charged O atoms may form chemical bonds with the surface of Al . Herein, 5 ps AIMD simulations were performed to investigate the interaction between HCOO – and Al(111), Al(220) as well as Al(200), because the exposure of these three Al facets was observed in our experiments.…”

“…As shown in Figure 2c, the Al foil after soaking in 4 M NaOH exhibits two peaks at 531.2 and 531.7 eV, corresponding to Al−O (Al 2 O 3 ) and O−H (Al(OH) 3 ), respectively. 25,50 By contrast, the Al foil after soaking in 4 M HCOONa−4 M NaOH presents two other peaks at 535.8 and 530.0 eV, which can be assigned to HCOO − and Al−O−C, respectively (Figure 2d). 48,51 This provides the direct evidence for the existence of HCOO − selfassembled film on the Al surface.…”

“…Previous studies have shown that negatively charged O atoms may form chemical bonds with the surface of Al. 25 Herein, 5 ps AIMD simulations were performed to investigate the interaction between HCOO − and Al(111), Al(220) as well as Al(200), because the exposure of these three Al facets was observed in our experiments. Figure 3d Hence, HCOONa not only forms a self-assembled passivation layer on the Al surface but also remolds the solvated structure of the NaOH electrolyte.…”

“…31 Inorganic inhibitors (Na 2 SnO 3 , In(OH) 3 , 21 ZnO, 22 etc. ), organic inhibitors containing N, O, P, and S groups (cationic surfactant, 23 polypyrrole, thiourea, 24 n-octyl phosphonic acid, 25 etc.) and hybrid inhibitors 26 can form a protective metal layer or adsorbed molecular layer on the Al metal surface, thus greatly increasing the hydrogen evolution potential and boosting the discharge capacity.…”

“…Given the above, the competition between electrochemical reaction and side reaction determines the overall electrochemical performance of AABs. , Hence in the past decades, numerous efforts, including the development of Al alloy electrodes (e.g., Sn, Mg, In, Zn), − electrolyte inhibitors, − nonaqueous electrolytes, hydrogel electrolyte, and oil displacement of electrolyte after discharging, have been made to increase the hydrogen evolution overpotential of Al, reduce the amount of free water, and suppress the direct contact between H 2 O and Al anodes.…”