Mitigating the corrosion reaction in alkaline silicon-air batteries through nonuniform doping

Abstract

The mass production of lithium-ion batteries used in, for instance, electric vehicles led to significant cost reductions. However, concerns are emerging about the availability of resources and the environmental impact of the production. Metal-air batteries use more resource-efficient materials and potentially have higher energy densities than lithium-ion batteries. A little over a decade ago, a promising new type of metal-air battery was discovered, the silicon-air (Si-air) battery.
The Si-air battery uses a c-Si anode and a porous air cathode, combined with a liquid electrolyte. Si-air batteries have a theoretical cell potential up to 2.2 V and a very high theoretical energy density of 8,470 Wh/kg. However, Si-air batteries that use alkaline electrolytes, such as potassium hydroxide (KOH), are not rechargeable and the Si anode suffers from passivation and corrosion. Currently, corrosion is considered as the dominant factor that limits the conversion efficiency of alkaline Si-air batteries. In this work, the role of the electrons participating in the corrosion reaction is further investigated. This is based on a model where electron transfer is considered as a rate-limiting step. The research objective of this work is the mitigation of the corrosion reaction in alkaline Si-air batteries through nonuniform doping of the Si anode.

The corrosion in the Si-air battery setup was first quantified for uniformly doped c-Si. These results function as reference to nonuniformly doped Si. The corrosion rates for uniformly doped n-type and p-type Si using 6.6M KOH were found to be constant at 1.25 µm/h and lower than the 2.2 µm/h measured in previous research. This lower corrosion rate increased the conversion efficiency from 2.0% to 3.4%, corresponding to a specific capacity of 130 mAh/gSi. Based on these results, corrosion in Si-air batteries is still significant.
Nonuniformly doped wafers were made by depositing thin impurity layers on c-Si through PECVD or epitaxial growth. Subsequently, drive-in of the impurity atoms into the c-Si was performed at a temperature of 1250°C for 36 hours. The shape and junction depth of the dopant profiles were verified by the four-point probe and spreading resistance profiling. These results show the presence of doping profiles with junction depths up to 36 µm.
In the samples made with PECVD, corrosion slightly increased in nonuniformly doped p-type and n-type Si with respect to uniformly doped Si. However, the corrosion is not clearly affected by the implemented doping profiles. For nonuniformly doped p-type Si samples made with epitaxial growth, corrosion clearly decreased to 0.82 µm/h compared to 1.25 µm/h with uniformly doped Si. Corrosion decreases by having a high boron concentration above 2·10^19 atoms/cm^3 in the Si and not through the implementation of a dopant profile.
It is concluded that nonuniform doping does not affect the corrosion in alkaline Si-air batteries and, therefore, does not mitigate the corrosion reaction.