Influence of Dopant Type and Orientation of Silicon Anodes on Performance, Efficiency and Corrosion of Silicon–Air Cells with EMIm(HF)_2.3F Electrolyte

Abstract

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/cm 2 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. One line of development in technologies for electrical energy storage is metal-air batteries, which provide high specific energies and -when referring to Zn, Al, Fe, or Si -are at the same time resource effective with respect to the availability and price of the anode materials. The theoretical specific energy of a Si-air cell, related to the anode mass only, is 8470 Wh/kg. Using Si material in aqueous alkaline solutions, however, results in a severe corrosion reaction which is accompanied by intense hydrogen evolution.1-3 Despite the corrosion reaction, it is still feasible to build an alkaline Si-air cell at a discharge potential around 1.1 V, however, with sacrifice of huge amount of Si anode to corrosion. [4][5][6] Therefore, new approaches to establish batteries on silicon materials have been put forward using ionic liquid electrolytes. One of the possible approaches is the usage of EMIm(HF) 2.3 F electrolyte which possesses high conductivity, low viscosity and chemical stability in air.7-10 The proof of concept, that substantial discharge was possible when using EMIm(HF) 2.3 F electrolyte, was proposed in 2010 according to the following reactions: Additionally, a screening of several anode materials -As-, Sband B-doped Si wafers -was performed, in which the cell potential at intermediate current densities as determined from potentiodynamic polarization measurements, was set as major criterion. The corrosion current densities as obtained by the Tafel fits from the polarization experiments for the different wafer types were also considered for the material selection. However, owing to the low corrosion rates, it played a minor ro...

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

“…The large difference under OCV (≈ 400 mV) was lowered to 150 mV for continuous discharge experiments. Similar results have also been reported previously by potentiodynamic polarization experiments in half-cells [13,26]. As confirmed by both, galvanostatic discharge and potentiodynamic polarization experiments, B-doped Si electrodes in EMIm(HF) 2.3 F electrolyte reveal more "passive-like" electrochemical behavior, which is reflected as lower cell voltages on the discharge profiles.…”

“…confirmed by further investigations in the intermediate term discharge behavior; however, the analysis of mass losses in these experiments revealed substantially lower corrosion rates for the B-doped Si anodes [26]. Nevertheless, the specific energies under continuous discharge were higher for the ⟨100⟩ As-doped Si anodes, because their higher cell voltages overcompensated the effects from corrosion mass losses [26].…”

“…This modification might be due to the formation of a surface layer containing Si-F-B elements as shown in Fig. 5c (2nd state) [26]. In presence of carbon, the modifications are temporarily and seem to be counteracted by carbon (3rd to 4th state).…”

“…The development of Si-air batteries with EMIm(HF) 2.3 F electrolyte started in 2009, when the proof of concept was established [23] and, subsequently, the discharge capacities up to 26.7 mAh from a cell with 0.5 cm 2 anode surface area were corroborated [13]. Since then, focus of the further research was on electrochemical mechanisms, on factors influencing the end of discharge [24,25], andwith respect to material selection-on performance investigations depending on silicon anode type [26]. In general, pore clogging by silicon oxide deposits at the cathode side [13,25] and enhanced resistivity at the anode interface [24] were identified as factors contributing to the discharge limitations.…”

“…The nature of Si as being a semiconductor makes doping an essential process to improve the electronic conductivities of Si electrodes. Investigations of the dopant types have already been provided by potentiodynamic polarization experiments [13,23,26] and galvanostatic discharge experiments [26]. According to the potentiodynamic polarization experiments, the OCP and the anodic potentials are substantially higher for As-than for B-doped Si wafer anodes.…”