An increasing penetration of renewable energy sources in the global energy market necessitates an increasing amount of energy storage. Batteries are an excellent source of short term storage, used in vehicles, home storage and mobile applications. However modern technologies such as Li-ion batteries face resource scarcity and hence introduce geopolitical dependence. To this end Si-air batteries were recently, in 2009, explored in an attempt to devise a battery with high energy density and abundant materials.
The Si-air battery makes use of a silicon anode and an porous carbon cathode, to allow the circulation of air into the battery. Two electrolytes have been looked at in the past, KOH and the Room Temperature Ionic Liquid (RTIL) EMIm(HF)2.3F. The Si-air battery has excellent energy density in theory, with volumetric density being theoretically as high as 1 · 104 Wh/L. However, this theoretical energy density is as of yet far from reached, current research uses electrolytes which have a parasitic corrosion reaction with the silicon anode. This work aims to explore the usage of a new RTIL, BMPyr[NTf2], as the electrolyte in this battery. This RTIL has seen usage in Zn-air batteries, however it is yet untested for Si-air. The objective is to determine the discharge characteristics for a Si-air battery using this RTIL through comparison with KOH, with focus on the conductivity.
To do so first the relationship between conductivity and discharge potential for KOH was determined experimentally. The result found is that the discharge potential using KOH decreases with decreasing conductivity, however, this decrease is much larger than what can be solely attributed to the conductivity. Next the chosen RTIL BMPyr[NTf2], having a conductivity of 2.2 mS/cm at room temperature in a pristine state, was used in the battery discharges, where it was established to have an OCP of 0.7-0.8 V. From measurements no detectable consumption of Si was found after 1 hour of OCP. Further during the discharges it was found that the potential drops rapidly when discharge current is equal or greater to 2.5 μA. In an attempt to decrease the resistance in the battery cell a new design was created where the distance between the two electrodes is
decreased from 2 cm to 0.8 cm, using this second design a maximum OCP of 1.1 V was measured.
Finally by mixing in 1wt% and 3wt% water into the RTIL two mixtures are obtained with conductivity of 2.6 mS/Cm and 3.0 mS/cm at room temperature. Discharging these two mixtures at 20 nA, 100 nA, 500 nA and 2.5 μA it was found that for the 1 wt% the highest potential was found for 20 nA at 0.8 V. Meanwhile for the 3 wt% mixture the 20 nA discharge exhibited significantly lower potential at 0.4 V. For the 100 nA, 500 nA and 2.5 μA increasing conductivity led to increased potential, however similar to the KOH experiment, this difference in potential is larger than what is to be expected from purely conductivity changes. Finally, reproducibility of these experiments is low as a series of discharges with the same materials and current showed different potentials. These results combined lead to the conclusion of this work, thatthe relationship between conductivity and potential for the RTIL BMPyr[NTf2] is inconclusive, there are unknown factors influencing the discharge potential.