Effects of Cs2CO3 Additive in KOH Electrolyte Used in Ni/MH Batteries

The effects of Cs2CO3 addition in a KOH-based electrolyte were investigated for applications in nickel/metal hydride batteries. Both MgNi-based and Laves phase-related body-centered cubic solid solution metal hydride alloys were tested as the anode active materials, and sintered β-Ni(OH)2 was used as the cathode active material. Certain amounts of Cs2CO3 additive in the KOH-based electrolyte improved the electrochemical performances compared with a conventional pure KOH electrolyte. For example, with Laves phase-related body-centered cubic alloys, the addition of Cs2CO3 to the electrolyte improved cycle stability (for all three alloys) and discharge capacity (for the Al-containing alloys); moreover, in the 0.33 M Cs2CO3 + 6.44 M KOH electrolyte, the discharge capacity of Mg52Ni39Co3Mn6 increased to 132%, degradation decreased to 87%, and high-rate dischargeability stayed the same compared with the conventional 6.77 M KOH electrolyte. The effects of Cs2CO3 on the physical and chemical properties of Mg52Ni39Co3Mn6 were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, inductively coupled plasma, and electrochemical impedance spectroscopy. The results from these analyses concluded that Cs2CO3 addition changed both the alloy surface and bulk composition. A fluffy layer containing carbon was found covering the metal particle surface after cycling in the Cs2CO3-containing electrolyte, and was considered to be the main cause of the reduction in capacity degradation during cycling. Also, the Cs2CO3 additive promoted the formations of the C–O and C=O bonds on the alloy surface. The C–O and C=O bonds were believed to be active sites for proton transfer during the electrochemical process, with the C–O bond being the more effective of the two. Both bonds contributed to a higher surface catalytic ability. The addition of 0.33 M Cs2CO3 was deemed optimal in this study.