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

Abstract

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) 2.3 F ionic liquid electrolyte, specific energies up to 1660 Wh kg Si −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.

Rechargeable Silicon Redox Batteries

“…In lieu of recent publications, primarily on active Si anode dissolution and its implementation in primary nonrechargeable cells entailing harsh alkaline aqueous solutions or concentrated fluoride-based electrolytes, [16][17][18][19] we devised a novel procedure focused on mild surface activation aimed at mitigating parasitic corrosion, previously found to be detrimental to long term storage. [19][20][21][22][23][24][25][26] The electrolytes were comprised of varied concentrations of fluoridebased organic salts in designated aprotic solvents. Beforehand, the solvents were tested in a Karl Fischer coulometric titrator and found to contain less than 20 ppm of H 2 O. Fluoro-hydrogenate ionic liquids, used as the sole source of fluoride, were incompatible with the KF titrator, and therefore were dried in an anhydrous CaF 2 stir reactor, filtered, and tested for residual calcium contamination via ICP test (<1 ppm).…”

“…[ 16,17 ] In the past decade, several publications reported the incorporation of active Si anodes in primary air‐battery designs, with 1.2 V nominal working voltage. [ 17–26 ] The anodic electrochemical dissolution of highly doped n‐type silicon in EMI∙(HF) 2.3 F ionic liquid (initiated at a potential of ‐1.4 V vs Fc/Fc + ) was ascribed to the following suggested reaction, forming dissolvable SiF 4 gas in the ionic liquid: [ 16,26 ] $$\[ \begin{array}{*{20}{c}}{{\rm{Si}} + 12{{({\rm{HF}})}_2}{{\rm{F}}^ - } \to {\rm{Si}}{{\rm{F}}_4} + 8{{({\rm{HF}})}_3}{{\rm{F}}^ - } + 4{e^ - }\;\;{\rm{E}} = - 1.4{\rm{V}}\;{\rm{vs}}\;{\rm{Fc/F}}{{\rm{c}}^ + }}\end{array} \]$$Coulombic efficiency, lack of in‐depth understanding regarding the related cell mechanism, ambiguity on the kinetics and products formation in the Si–air cells, precluded addressing the potential rechargeability of cells based on Si as an active anode. Recently, several research groups reported successful Si electrodeposition also in ionic liquids (RTILs), [ 28–49 ] providing further insight into the Si redox mechanism and kinetics in various electrolytes.…”

“…Moreover, the counter cathodic reaction must be compatible with cell chemistry and design. The single and only cathode reported thus far was limited only to oxygen reduction via air cathodes, [16][17][18][19][20][21][22][23][24][25][26] with unsuccessful (and unreported) attempts thus far achieving any reversibility in the course of Si-air battery operation. On the other hand, halide cathodes were long utilized in research of rechargeable cell chemistries, [5,9,78] due to proven reversibility in various electrolytes and well explored redox kinetics and speciation (X 2 /X -, X 3 -/X -) [X = Cl, Br, I], [72] high solubility and stability in organic solvents.…”

“…[16,17] In the past decade, several publications reported the incorporation of active Si anodes in primary air-battery designs, with 1.2 V nominal working voltage. [17][18][19][20][21][22][23][24][25][26] The anodic electrochemical dissolution of highly doped n-type silicon in EMI•(HF) 2.3 F ionic liquid (initiated at a potential of -1.4 V vs Fc/Fc + ) was ascribed to the following suggested reaction, forming dissolvable SiF 4 gas in the ionic liquid: [16,26] Si 12(HF) F SiF 8(HF) F 4 E 1.4V vs Fc/FcCoulombic efficiency, lack of in-depth understanding regarding the related cell mechanism, ambiguity on the kinetics and products formation in the Si-air cells, precluded addressing the potential rechargeability of cells based on Si as an active anode. Recently, several research groups reported successful Si electrodeposition also in ionic liquids (RTILs), providing further insight into the Si redox mechanism and kinetics in various electrolytes.Several issues plaguing previous cell designs were left unanswered: 1) highly stable passivation of Si requiring harsh electrolytes for removal and surface activation, resulting in parasitic corrosion during long term storage; [18] 2) low current densities due to poor kinetics of multivalent redox; [19] 3) hydrogen evolution reaction (HER) in protic electrolytes due to low kinetic barrier of protons with Si species; [19,43,50] 4) high overpotential necessary for Si electrochemical reduction, limiting potential electrolytes to RTILs to mitigate electrolyte decomposition.…”

“…[16,17] In the past decade, several publications reported the incorporation of active Si anodes in primary air-battery designs, with 1.2 V nominal working voltage. [17][18][19][20][21][22][23][24][25][26] The anodic electrochemical dissolution of highly doped n-type silicon in EMI•(HF) 2.3 F ionic liquid (initiated at a potential of -1.4 V vs Fc/Fc + ) was ascribed to the following suggested reaction, forming dissolvable SiF 4 gas in the ionic liquid: [16,26] Si 12(HF) F SiF 8(HF) F 4 E 1.4V vs Fc/Fc…”