Electrochemical analysis and mixed potentials theory of ionic liquid based Metal–Air batteries with Al/Si alloy anodes

Aluminium and silicon, when coupled with an air cathode in an electrochemical cell may provide theoretical specific energies of up to 8146 mWh/g and 8470 mWh/g. Proof of concept for the discharge in cells with ionic liquid EMIm(HF)2.3F electrolyte had been established in 2009 for silicon and in 2015 for aluminium. The objective of the present work is the investigation of discharge behavior and corrosion in this type of cell using binary Al/Si alloys as anodes. Al/Si alloys with nine different compositions were prepared by an arc melting process and shaped to anodes. Microstructure of the anodes in the initial state was evaluated with respect to the fractions of its constituents. Al/Si–air primary full cells were investigated with respect to voltages during OCV and discharge during intermediate term (20 h) runs under current densities of 250 μA/cm2. Voltages decrease with Si-content in the alloys following trends with quantitatively different characteristics for the hypoeutectic, intermediate hypereutectic and the alloys with high Si content. SEM analysis of surface morphology of the anodes after discharge experiments indicates that for all alloys the discharge capacity results mostly from the oxidation of the aluminium. Potentiodynamic polarization measurements were conducted in order to determine corrosion potentials for the alloys and analyzed with approaches based on mixed potential theory including galvanic coupling. The results are discussed in terms of Evans diagrams; thereby approaches based on alternative scenarios for the galvanic coupling are examined

Analysis on discharge behavior and performance of As- and B-doped silicon anodes in non-aqueous Si–air batteries under pulsed discharge operation

Very high theoretical specific energies and abundant resource availability have emerged interest in primary Si–air batteries during the last decade. When operated with highly doped Si anodes and EMIm(HF)2.3F ionic liquid electrolyte, specific energies up to 1660 Wh kgSi−1 can be realized. Owing to their high-discharge voltage, the most investigated anode materials are ⟨100⟩ oriented highly As-doped Si wafers. As there is substantial OCV corrosion for these anodes, the most favorable mode of operation is continuous discharge. The objective of the present work is, therefore, to investigate the discharge behavior of cells with ⟨100⟩ As-doped Si anodes and to compare their performance to cells with ⟨100⟩ B-doped Si anodes under pulsed discharge conditions with current densities of 0.1 and 0.3 mA cm−2. Nine cells for both anode materials were operated for 200 h each, whereby current pulse time related to total operating time ranging from zero (OCV) to one (continuous discharge), are considered. The corrosion and discharge behavior of the cells were analyzed and anode surface morphologies after discharge were characterized. The performance is evaluated in terms of specific energy, specific capacity, and anode mass conversion efficiency. While for high-current pulse time fractions, the specific energies are higher for cells with As-doped Si anodes, along with low-current pulse fractions the cells with B-doped Si anodes are more favorable. It is demonstrated, that calculations for the specific energy under pulsed discharge conditions based on only two measurements—the OCV and the continuous discharge—match very well with the experimental data

Investigation of the corrosion behavior of highly As-doped crystalline Si in alkaline Si-air batteries

High corrosion rate is one of the major obstacles that have to be overcome in order to establish practical application of primary alkaline Si–air batteries. At the current state of development the theoretical specific capacity of 3820 mAh/g is reduced to 120 mAh/g in long term operable alkaline Si–air batteries, with most of the capacity losses being due to corrosion reactions. In the present work the corrosion behavior of highly As-doped oriented silicon wafers, that have proved stable performance as anode materials is summarized for a scope of conditions that may arise in battery operation. More specific, corrosion rates are presented and discussed with respect to (i) time dependence, (ii) influence of KOH electrolyte concentration, (iii) chemical vs. electrochemical corrosion, and (iv) corrosion under anodic potentials as present during the discharge of batteries. Corrosion rates were found to exhibit stable time profiles for immersion times longer than 8 h. With respect to concentration dependence, three ranges of KOH concentrations were identified. Within each range, the corrosion behavior is governed by similar mechanisms, but different limiting factors. Potentiodynamic measurements show that large part of the corrosion is chemical in nature. Under discharge conditions corrosion increases whereby the discharge potential, corrosion rates, and mass conversion efficiencies depend on KOH concentrations and discharge current densities

Influence of Dopant Type and Orientation of Silicon Anodes on Performance, Efficiency and Corrosion of Silicon–Air Cells with EMIm(HF) 2.3 F Electrolyte

Intermediate term discharge experiments were performed for Si–air full cells using As-, Sb- and B-doped Si-wafer anodes, with 〈100〉 and 〈111〉 orientations for each type. Discharge characteristics were analyzed in the range of 0.05 to 0.5 mA/cm2 during 20 h runs, corrosion rates were determined via the mass-change method and surface morphologies after discharge were observed by laser scanning microscopy and atomic force microscopy. Corresponding to these experiments, potentiodynamic polarization curves were recorded and analyzed with respect to current-potential characteristics and corrosion rates. Both, discharge and potentiodynamic experiments, confirmed that the most pronounced influence of potentials – and thus on performance – results from the dopant type. Most important, the corrosion rates calculated from the potentiodynamic experiments severely underestimate the fraction of anode material consumed in reactions that do not contribute to the conversion of anode mass to electrical energy. With respect to materials selection, the estimates of performance from intermediate term discharge and polarization experiments lead to the same conclusions, favoring 〈100〉 and 〈111〉 As-doped Si-wafer anodes. However, the losses in the 〈111〉 As-doped Si-anodes are by 20% lower, so considering the mass conversion efficiency this type of anode is most suitable for application in non-aqueous Si–air batteries