Si modified by Zn and Fe as anodes in Si-air batteries with ameliorative properties

Highlights

• The adsorption energies of the oxide layer on the Zn and Fe modified Si electrode are larger.

• The band gaps of the Si/SiO₂ interface can be reduced by the Zn and Fe dopants.

• Currents across the interface under voltage bias can be enlarged significantly by the Zn and Fe dopants.

Abstract

Si-air batteries have been gradually investigated in recent ten years due to their high theoretical capacity and ideal safety. In this work, to ameliorate the properties of Si-air batteries, models of Si electrodes were doped by Zn and Fe atoms respectively and density functional theory (DFT) calculations were performed. By constructing various models of Si-Zn/SiO₂ and Si-Fe/SiO₂ interfaces, the adsorption energies of SiO₂ units on Zn or Fe-doped Si electrodes were first calculated. Meanwhile, the electrostatic difference potentials (EDP) and electron densities vary apparently across the interfaces. Additionally, the local device density of states (LDDOS) exhibit high intensities in the region of interfaces. Moreover, Zn and Fe dopants would introduce energy levels in the band gap and reduce the band gap of the interfaces according to the device density of state (DDOS) and projected local density of states (PLDOS). Finally, the I-V curves show that the current of the Si/SiO₂/Si device can be enhanced by the introduction of Zn and Fe in the Si electrode. This work provides a method of surface modification to ameliorate the properties of Si-air batteries and assists to construct the Si-Zn and Si-Fe composite anodes in air batteries.

Introduction

Commercial lithium ion batteries (LIBs) have been extensively and indispensably applied in various electronic devices and electric vehicles in recent 30 years [1], [2], [3]. With the booming requirements for large-scale grid energy storage systems with high energy density, the applications of LIBs have encountered a bottleneck due to their limited theoretical energy densities (250–300 Wh/kg) and high costs [4], [5]. Compared with the LIBs as closed energy systems, metal-air batteries (MABs) can breathe oxygen from the atmosphere and do not have to store cathode reactants, thus owning notably high energy densities [6], [7]. Metals that are suitable to serve as anodes in MABs can be divided into monovalent (e.g., Li and Na) and multivalent (e.g., Zn, Fe, Al and Mg) [8], [9], [10], [11]. Among them, Li is one of the most attractive metal electrodes because of its high theoretical specific energy (~11,430 Wh/kg). However, great efforts should be made to address the security risks to realize its commercialization [12], [13], [14]. Multivalent metals such as Zn and Fe are more abundant on the earth and less reactive compared with Li. Zn-air batteries owning lower costs and ideal safety with a high specific energy of ~1352 Wh/kg have already been maturely applied in hearing aid devices [15], [16], [17]. Fe-air batteries with a specific energy of ~1229 Wh/kg are more resistant to corrosion with a low self-discharge rate and easier to recharge with no dendrite formation that suits the electric vehicle environment [18], [19].

Besides various metal anodes, Si as the second most abundant element on the earth also exhibits the application potentiality in air batteries [18]. Since 2009 when first Si-air battery was reported by Ein-Eli’s group, a few attempts have been made to investigate the discharge mechanisms of Si-air batteries and improve their electrochemical properties, yielding an average working potential of ~1.1 V and a discharge time of hundreds of hours [20], [21], [22], [23]. The theoretical specific energy of the Si-air battery is 8461 Wh/kg, which is compared to that of the Li-air battery. Moreover, Si-air battery possesses an extremely high energy density of 19748 Wh/L. For comparison, Li, Zn, Fe exhibit energy densities in the range from 6104 to 9677 Wh/L [18]. In addition, as a kind of semiconductor, Si is an indispensable candidate for microelectronic industry. In practice, Si wafers as anodes are usually doped by different dopants (e.g., As, Sb and B) to enhance the performances of Si-air batteries [23], [24]. Actually, Si-air, Zn-air and Fe-air batteries can all be operated with the alkaline electrolyte and the corresponding cell reactions are summarized in Table 1. Consequently, it is reasonable and feasible to construct composite Si-Zn and Si-Fe anodes to ameliorate properties of Si-air batteries.

During discharging, the Si anode surface would be passivated by the insulating SiO₂ layer which can retard the electron-transfer dynamics and restrain the power density of the cell. In this work, the surface of Si model was modified by Zn and Fe atoms respectively and various models of Si-Zn/SiO₂ and Si-Fe/SiO₂ interfaces were constructed and analyzed by DFT. According to the calculated adsorption energies of SiO₂ units on Zn or Fe-modified Si electrodes, it can be concluded that Zn and Fe dopants can effectively restrain the surface passivation. Meanwhile, the EDP and electron densities vary apparently across the interfaces and the LDDOS exhibit high intensities in the region of interfaces. Moreover, Zn and Fe dopants would introduce energy levels near Fermi level and reduce the band gap of the SiO₂ layer according to the DDOS and PLDOS. Finally, the I-V curves show that the introduction of Zn and Fe in the Si electrode can enhance the current of the Si/SiO₂/Si device. This work provides a method of surface modification to ameliorate the properties of Si-air batteries and assists to design the Si-Zn and Si-Fe composite anodes in air batteries.