Zinc particles coated with bismuth oxide based glasses as anode material for zinc air batteries with improved electrical rechargeability

Highlights

• Functionalized zinc particles for rechargeable zinc air batteries.

• Coating of zinc particles with bismuth oxide based glasses.

• Zinc degradation was reduced by the coating forming an interpenetrating network.

• Cumulated zinc utilization could be increased up to 465% (uncoated zinc: 85%).

Abstract

Zinc particles are commonly used as anode material in primary zinc air batteries. One of the reasons for irreversibility of this battery system is degradation of zinc metal, such as zinc passivation by formation of ZnO layers in alkaline solution. A promising approach for reducing zinc degradation is modification of zinc with protective coatings. But up to now, no electrical rechargeability of coated zinc particles could be enabled after complete discharge. In this work, bismuth oxide based coatings are introduced which should improve cyclic stability. Bi₂O₃-ZnO-CaO and Bi₂O₃-ZnO-SiO₂ glasses were prepared and their chemical stability as well as swellability in 6 M KOH were discussed. The coating of zinc particles was realized by mechanical method. The electrochemical performance of coated zinc particles was investigated by applying complete discharge and charge steps, and compared with the behavior of uncoated zinc particles. The results pointed out, that Bi₂O₃-ZnO-CaO-coated zinc particles enabled improved rechargeability. A cyclic stability of 20 full cycles was realized in excess electrolyte, whereas uncoated zinc achieved just one complete discharge. The cumulated zinc utilization of coated zinc particles could be improved to 465%, as compared to uncoated zinc showing just 85% in test cells with electrolyte limitation.

Introduction

Zinc metal is widely known as anode material for primary zinc air batteries. This battery system provides high theoretical energy density, high safety and low cost, making it an interesting candidate for mobile applications. Nevertheless, restrictions in terms of anode material degradation in alkaline electrolyte as well as poor bifunctional catalysts for the oxygen evolution on the cathode, limit its application to a primary battery up to now [1], [2], [3], [4]. The degradation of zinc metal is mainly attributed to zinc passivation. During discharge in aqueous KOH electrolyte, zinc metal is oxidized to zincate ions, which get dissolved into the electrolyte. The reaction during discharge with standard potential $E_a^0$ is shown in Eq. (1) and discussed in detail in Ref. [5]:

$$\mathrm{Zn+4OH}\to {\left[\mathrm{Zn}{\left(\mathrm{OH}\right)}_4\right]}^{\mathrm{2}-}+2{\mathrm{e}}^{-}\left(E_a^0=-1.199\mathrm{V}\mathrm{vs}\mathrm{.}\mathrm{H/}{\mathrm{H}}^{\mathrm{+}},\mathrm{pH=14}\right)$$

Precipitation of supersaturated zincate ions results in formation of ZnO, as seen in Eq. (2) [5]:

[Zn(OH)₄]²⁻ → ZnO + H₂O + 2OH⁻

ZnO passivation layers on top of the zinc surface lead to a decrease in electrical conductivity [6], [7]. In order to achieve electrochemical accessibility and rechargeability, the zincate ions should be captured inside the anode without supersaturation to passive layers. Furthermore, morphology changes of the anode reduce life time: shape change of the zinc anode decreases the electrochemically active surface area during cycling and formation of zinc dendrites can result in short cut of the battery [1], [2], [3], [4].

Zinc air primary batteries are commonly applied as coin cells in hearing aids. State of the art of zinc anode materials are zinc powders with average diameter of 100–400 μm. The zinc particles contain alloying elements, mainly Bi or In in ppm range, which reduce zinc corrosion [3], [8]. In order to improve rechargeability, different electrode concepts have been discussed, apart from commercially available zinc powders used in primary zinc air cells. This includes 3D structuring of the anode (zinc foam) with addition of electrolyte and electrode additives showing improved rechargeability if cycled to 40% depth of discharge (DOD) [9], [10]. Moreover, enhanced rechargeability could be obtained by electrodes composed of ZnO composites [11] or ZnO nanopowders together with electrically conductive additives [12], [13], [14].

In this work, battery grade zinc particles were used as substrate material, since the processing of coin cells with particulate zinc as anode is already an established industrial process. Further advantage by using particulate zinc is their higher theoretical volumetric energy density, as compared to zinc foams: The bulk density of a conventional zinc powder bed (3–3.5 g cm⁻³ [3]) is almost 3 times higher, compared to the 3D anode reported in Ref. [10] (1.29 g cm⁻³). Approaches to reduce the degradation of conventional zinc powder were described by additives into the electrode or electrolyte [15], [16], [17], [18], [19], [20], [21] as well as by coatings on zinc particles [22], [23], [24], [25].

However, there have been no reports up to now, which show rechargeability and a certain cyclic stability of a zinc powder bed as anode material by applying full discharge and charge steps by coating zinc with one single material without additional additives. The aim of this work is to introduce a possibility to improve cyclic stability by coating zinc particles with bismuth oxide based glass powder. Glasses were chosen due to homogenous distribution of the different cations inside the amorphous oxide network. Typically, amorphous materials are swellable in aqueous KOH by chemical binding between cations of the glass and OH⁻ ions [26]. The OH⁻ ions should be able to penetrate through the amorphous network, because this is necessary to enable electrochemical reactions. As a consequence a gel is formed. Bi₂O₃ was chosen as network former of the glass system. In order to create an amorphous network, ZnO as intermediate oxide was used. Firstly, the increase in active material and secondly, the formation of Zn²⁺ pathways were reasons for using ZnO as intermediate oxide. A comparison between the glass systems Bi₂O₃-ZnO-CaO [27] and Bi₂O₃-ZnO-SiO₂ is shown, both with the composition 40-25-35 mol %. CaO was used as network modifier, SiO₂ as network former.

Improvements in electrochemical performance were discussed by comparing the results of coated zinc particles with non-modified particulate zinc, which were cycled under same conditions. Complete discharge and charge steps were applied in order to investigate zinc degradation under maximum utilization and to point out the influence of the coating materials.