Calcium metaborate as a cathode additive to improve the high-temperature properties of nickel hydroxide electrodes for nickel–metal hydride batteries

• Ca(BO2)2 is proposed as a cathode additive to improve the Ni–MH cell performance.
• Ca(BO2)2 facilitates a more uniform distribution of Ca than other calcium additives.
• The cells with Ca(BO2)2 show excellent electrochemical performance at 25 and 70 °C.

In this paper, a novel additive, calcium metaborate (CMB), is proposed to improve the high-temperature characteristics of the nickel electrodes for nickel–metal hydride batteries. As a soluble calcium salt, CMB can easily and uniformly be dispersed in the nickel electrodes. The effects of CMB on the nickel electrode are investigated via a combination of cyclability, capacity retention, electrochemical impedance spectroscopy, scanning electron microscope and X-ray diffraction. Compared with conventional nickel electrodes, the electrode containing 0.5 wt.% CMB exhibits superior electrode properties including enhanced discharge capacity, improved high-rate discharge ability and excellent cycle stability at an elevated temperature (70 °C). The improved cell performance of the nickel electrode containing CMB additives can be attributable to the increased oxygen evolution overvoltage and slower oxygen evolution rate. Compared with insoluble calcium salts, such as Ca(OH)2, CaCO3, and CaF2, CMB is more effective as a cathode additive to improve the high-temperature performance of Ni–MH batteries.