Zinc particles coated with bismuth oxide based glasses as anode material for zinc air batteries with improved electrical rechargeability
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 is shown in Eq. (1) and discussed in detail in Ref. [5]:
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.
Section snippets
Preparation of Bi2O3-ZnO-CaO and Bi2O3-ZnO-SiO2 glasses
Bismuth oxide based functional glasses (Bi2O3-ZnO-CaO and Bi2O3-ZnO-SiO2) with the composition 40-25-35 mol % were prepared (82.34 wt % Bi2O3, 8.99 wt % ZnO and 8.67 wt % CaO). As starting materials Bi2O3 (Alfa Aesar, 99.975%) as network former, ZnO (Chempur, 99.9%) as intermediate oxide, and CaO (Sigma Aldrich, reagent grade) as network modifier or SiO2 (Chempur, 99.9%) as network former, respectively, were mixed homogenously in a tubular mixer (T2F Turbula, Willy A. Bachofen) for 30 min. The
Glass system Bi2O3-ZnO-CaO vs. powder mixture Bi2O3+ZnO+CaO
The chemical stability of the glass powder Bi2O3-ZnO-CaO (40-25-35 mol %), compared to the powder mixture Bi2O3+ZnO+CaO of the same composition, was analyzed by ICP-AES after different swelling times in 6 M KOH (Fig. 1). After a soaking time of 5 h, nearly no Zn remained inside the powder mixture Bi2O3+ZnO+CaO, because ZnO dissolved into 6 M KOH by formation of [Zn(OH)4]2-. In contrast, around 50% of the initial amount of Zn could still be analyzed inside the glass system after the same
Discussion
The coating of particulate zinc with Bi2O3-ZnO-CaO glasses resulted in improved rechargeability and cyclic stability as compared to uncoated zinc powder. The glass coating enabled an interpenetrating network during cycling:
- (i)
The functional oxides were able to swell in aqueous KOH. This led to formation of a gel, which immobilized the discharge products near the zinc surface.
- (ii)
Bi phases inside the coating on zinc particles were formed, improving electrical conductivity. Irreversible passivation of
Conclusion
In this work, the electrochemical behavior of battery grade zinc particles (d50 = 250 μm) coated with bismuth oxide based glasses was reported. The coating process was performed by ball milling. The glass system Bi2O3-ZnO-CaO (40-25-35 mol %) was chosen as coating material due to swellability in 6 M KOH. The electrochemical performance of zinc particles coated with 3.2 wt % glass was investigated by applying complete discharge and charge steps, in order to characterize the passivation behavior
Acknowledgement
Financial support of Bavarian Research Foundation for M. Schmid (Grant No. 1025-12) is gratefully acknowledged. The authors would like to thank all project partners at VARTA Microbattery GmbH, Fraunhofer ISC (Würzburg) and Eckart GmbH. Furthermore, the ICP-AES is a financial support of the Bavarian State Ministry of Education, Science and the Arts within the framework TechnologieAllianzOberfranken (TAO). The authors gratefully acknowledge the funding.
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The present work is dedicated to Prof. Dr. Monika Willert-Porada with deep gratitude. Prof. Willert-Porada passed away unexpectedly on December 11, 2016 at age of 61. We lost an open mind, highly interdisciplinary and innovative scientist and supervisor.