Internal Morphological Evolution of Negative Electrodes Materials in Secondary Alkaline Zn/Ni Batteries
Authors: Caldeira Vincent , Thiel Julien , Lacoste François , Chatenet Marian , Dubau Laetitia 
Published: February 2020

Since the discovery of zinc-based alkaline secondary batteries [1] many investigations were performed on this systems to provide a reliable energy storage device. For example, the Zn/Ni or Zn/Air technologies are excellent candidates in terms of energy-density, power-density and environmental friendliness (they are essentially based on abundant elements, non-toxic materials and processes). Despite their many advantages, the development of alkaline storage technologies is however slowed down by some drawbacks of their negative zinc electrode: dendrite growth, shape change or solubility of zinc discharged products in high concentrated KOH electrolyte, are all responsible for a severe and gradual capacity loss [2]. Intensive research has been conducted concerning anode additives [3–7], to improve performances by reducing those detrimental and consequently non-desirable phenomena.
On the one hand, one of the most common used additives is calcium hydroxide, which reacts with zinc oxide during discharge [8,9], and leads to the in situ formation of calcium zincate crystals, following equation (1):
2 ZnO + Ca(OH)2 + 4 H2O ↔ Ca(OH)2,2 Zn(OH)2,2 H2O (1)
During this reaction, 4 water molecules are consumed, which can lead to a variation of KOH concentration (and therefore of local pH), especially in starved electrolyte systems, and reduce the battery charge/discharge cycle life.
On the other hand, the strategy consisting of directly using calcium zincate as active material instead of adding calcium hydroxide to the zinc oxide electrode has also been developed and investigated. One of the aims was to overcome the problem of non-uniform conversion of calcium hydroxide and zinc oxide into calcium zincate, that occurs during the first few cycles of in situ formation, by improving the distribution of zinc and calcium among the electrode [10]. Another advantage of this strategy is to avoid the electrochemical activation needed to form calcium zincate and the subsequent consumption of electrolyte. It was found [11] that calcium zincate exhibits better electrochemical performance in term of cycle life in comparison with zinc oxide or a mixture of zinc oxide and calcium hydroxide. But the effect is unclear, to date. Indeed, lower solubility of calcium zincate compared to zinc oxides in strong KOH electrolyte shall clearly improve the cycle life of zinc negative electrodes, by drastically reducing the zinc redistribution [8].
In the present study, investigations were undertaken on smalls Zn/Ni homemade Swagelok systems (10 mAh) using negative electrode made using calcium zincate synthesized through hydro-micromechanical synthesis method. Based on a complete set of experimental tools (X-ray radiography, cross section photography, and Scanning Electron Microscopy), the distribution of zinc, calcium and oxygen elements within the electrode was characterized during a prolonged charge/discharge sequence. Relevant electrode degradation mechanism was proposed, a first step towards mitigation strategies of the shape change of the zinc electrode.
Fig. Highlighting of zinc segregation inside the thickness of the negative electrode in function of state of charge in Zn/Ni battery
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