Effect of La Addition on the Electrochemical Properties of Secondary Zinc Electrodes

Abstract

The electrochemical properties of secondary zinc electrodes after La addition by the solid-phase diffusion method have been investigated. The results showed that the La addition to the zinc electrode significantly improved the cycle life. After 80 cycles at 100 mA/g the zinc electrode with 0.5 atom % La addition showed the best cyclic behavior. Its discharge capacity was more than 520 mAh/g. The linear potentiodynamic polarization results showed that suitable La addition significantly suppressed the corro-sion behavior of secondary zinc electrodes while little effect on the anodic behavior was found. The potentiostatic polarization results at Ϫ100 mV overpotential showed that the growth of zinc dendrites was effectively inhibited by the La addition. Scanning electron microscopy images showed that La addition changed the formation of zinc deposition. X-ray photoelectron spectroscopy results revealed that lanthanum oxides ͓La 2 O 3 /La͑OH) 3 ] were formed and enriched on the surface during cycling, which pre-vented the discharge products of zinc electrodes from dissolving in the alkaline electrolyte and suppressed the dendrite growth during charge. Secondary batteries employing zinc electrodes have many advan-tages, such as high specific energy density, high specific power den-sity, low cost, and low toxicity, which urged researchers to renew the interest in such batteries. However, their widespread applications in sealed batteries was hindered by short cycle lives due to zinc den-drite formation, especially after deep discharge. The dendrite growth of zinc electrodes resulting from the high solubility of zinc dis-charge products in the alkaline electrolyte was one of the main rea-sons responsible for the short cycle life. One efficient way to solve this problem was to suppress the solubility of zinc discharge prod-ucts in the electrolyte. Many attempts have been made to reduce the solubility of zinc electrode discharge products using organic and inorganic additives in the electrodes or electrolytes. 1-5 Our recent work showed that an La addition to the zinc electrode was an effective way for improving its performance, especially the cycle life. 6 It is known that La is a very active metal towards water. La easily reduces water to produce hydrogen and forms La 2 O 3 /La͑OH) 3 on the metal surface espe-cially in an alkaline solution. In our previous work, 6 La was added to zinc with the purpose of utilizing the highly active properties with the aqueous solution. If a large amount of La were added to the zinc electrode, it would cause some damage to the anticorrosion behavior of the zinc electrode. Very small amounts of La addition could sup-press the corrosion and dendrite growth of zinc because La very easily to formed an oxide layer ͓La 2 O 3 /La͑OH) 3 ] to prevent zinc discharge products from dissolving in the alkaline solution, while during the cycling process, La oxides ͓La 2 O 3 /La͑OH) 3 ] could not be reduced. However, according to our knowledge, very few papers have reported related results. Ishikura et al. used a small amount of LaNi 5 alloy as an absorber of hydrogen in a sealed Zn/Ni battery to reduce the inner pressure. 7 However, the lower hydrogen overpoten-tial of LaNi 5 alloy caused lots of hydrogen to evolve before the zinc discharge products were reduced. Biegler et al. 8 reported that me-chanically mixing ZnO with cerium oxide improved charge/ discharge performance of the zinc electrode, but no further results were reported. Furukawa et al. 9,10 found that among some oxide additives, e.g., beryllium oxide, zirconium oxide, or barium oxide, etc. a lanthanum oxide addition to the periphery of the zinc electrode made the current density distribution of the electrode more uniform and suppressed the shape change of the zinc electrode to some extent, and then extended the cycle life. Recently, Zhu et al.