Quasi‐Solid‐State Aluminum–Air Batteries with Ultra‐high Energy Density and Uniform Aluminum Stripping Behavior

Abstract Aqueous aluminum–air batteries are attracting considerable attention with high theoretical capacity, low‐cost and high safety. However, lifespan and safety of the battery are still limited by the inevitable hydrogen evolution reaction on the metal aluminum anode and electrolyte leakage. Herein, for the first time, a clay‐based quasi‐solid‐state electrolyte is proposed to address such issues, which has excellent compatibility and a liquid‐like ionic conductivity. The clay with uniform pore channels facilitates aluminum ions uniform stripping and reduces the activity of free H2O molecules by reconstructing hydrogen bonds network, thus suppressing the self‐corrosion of aluminum anode. As a result, the fabricated aluminum–air battery achieves the highest energy density of 4.56 KWh kg−1 with liquid‐like operating voltage of 1.65 V and outstanding specific capacity of 2765 mAh g−1, superior to those reported aluminum–air batteries. The principle of constructing quasi‐solid‐state electrolyte using low‐cost clay may further promote the commercialization of aluminum–air batteries and provide a new insight into electrolyte design for aqueous energy storage system.

By mixing different mass of 4 M KOH solution with kaolin powder, 1:0.5, 1:0.7 and 1:1 electrolytes were prepared, while 4 M KOH solution was marked as Blank.

Characterization:
The morphology and composition of the aluminum surface were systematically investigated by using a scanning electron microscope (SEM, Nova Nano-SEM 230), in-situ optical microscope (Nikon, SMZ25) and transmission electron microscope (TEM, JEM-2100F).Fourier transform infrared (FT-IR) spectra were obtained using a Bruker Vertex 70 FT-IR spectrophotometer.The Brunauere-Emmette-Teller (BET) specific surface areas of the kaolin were determined by micromeritics ASAP 2460 and the pore size distributions were obtained by the Barrett-Joyner-Halenda (BJH) model.X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250 Xi X-ray photoelectron spectrometer (Thermo Fisher).X-ray diffraction spectrometer (XRD) patterns were performed on Shimadzu XRD-6000.
Electrochemical measurements: Electrochemical measurement was carried out in a conventional three-electrode system by using a CHI760 electrochemical workstation, which used an aluminum alloy (10 mm × 10 mm × 3 mm) as the working electrode (WE), a Hg/HgO electrode as the reference electrode (RE) and platinum as the counter electrode (CE).The potentiodynamic polarization curves were obtained from 0.5 V to 1.5 V vs. the open circuit potential (OCP).The electrochemical impedance spectroscopy (EIS) experiments were performed at the OCP in the frequency from 100 kHz to 0.01 Hz with 5 mV amplitude.The electrochemical windows of the different electrolytes were obtained by linear sweep voltammetry (LSV) at 1 mV s −1 in the three-electrode system.

Aluminum-air full battery tests:
The aluminum-air full batteries were composed of an aluminum plate anode, electrolyte and two cathode films with a MnxOy@Ag catalyst.
The mass-specific capacity of the full battery was calculated by dividing the mass difference of the aluminum anode before and after galvanostatic discharge.
Computational details: All calculations were performed by the Material Studio 2019 software.Adsorption energy (Ead) was performed by the CASTEP, and the exchangecorrelation energy was approximately described by the Perdew-Burke-Ernzerhof (PBE) functional based on the generalized gradient approximation (GGA).The cutoff energy of the plane wave base was set in 400 eV and the surface of kaolin (001) was optimized with a 20 Å vacuum layer.The adsorption energy between molecules A and B is described as: The adsorption energy between molecules A and B is described as: EAB is the energy of [Al(H2O)6] 3+ adsorbed on the surface of kaolin ( 001 The deep pit of aluminum surface was represented by three semicircles with a radius 60 μm while the distance between them was 155 μm for blank electrolyte, as for 1:1 electrolyte, the deep pits was represented by seven semicircles with a radius 20 μm and the distance between them was 72 μm.It should be noted that the sizes in these simulations are based on the in-situ optical microscope images.In addition, a transient simulation of the process was performed in an area filled with electrolyte. ) surface (Si-O or Al-O surface).EA is the energy of [Al(H2O)6] 3+ , EB is the energy of Si-O or Al-O surface.Electric filed simulation:In order to simulate the electric field distribution at the interface between anode and electrolyte, a Finite Element Analysis (FEA) model was performed using COMSOL Multiphysics 5.4 software with the "Tertiary Current Distribution".The size of the entire two-dimensional model was set to 500 μm×500 μm.

Fig. S11 .
Fig. S11.(a-b) SEM images and in-situ optical microscope images of Al anodes