PREPARATION AND ELECTROCHEMICAL BEHAVIORS OF Co-B ALLOY POWDERS

Abstract

Alkaline rechargeable batteries,such as Ni/Cd batteries and Ni/MH batteries have been widely used as power sources,however,their further applications are limited due to contamination of Cd in Ni/Cd batteries and lower discharge capacities of Ni/MH batteries.Some metal borides, such as Co-B,Ni-B,Fe-B,V-B and Ti-B,have been known to own very high discharge capacity in alkaline aqueous solution,in which Co-B alloy exhibits the highest reversible discharge capacity and the best cyclic stability.However,the reversible capacity of Co-B alloy prepared by arc melting process is usually less than 250 mA·h/g,which is only one fourth of the theoretical capacity of the alloy electrode(908 mA·h/g).In the present study,chemical reduction method was used to prepare Co-B alloy to further enhance the electrochemical capacity of the alloy.A series of ultrafine powders of amorphous Co-B alloys,Co_(0.68)B_(0.32),Co_(0.55)B_(0.45) and Co_(0.50)B_(0.50),were prepared by reducing CoSO_4 with solution of NaBH_4.Electrochemical measurements indicate that the prepared alloys exhibit excellent electrochemical properties.At a high current density of 300 mA/g,the initial discharge capacities of these alloys are 510.6,666.4 and 667.2 mA·h/g,respectively,their discharge capacities still keep 331.6,379.5 and 390.5 mA·h/g after 60 cyc,respectively.Even at a discharge current density as high as 1200 mA/g,the three alloys still deliver reversible capacities of 336.2,373.4 and 390.1 mA·h/g,respectively.In the Co-B alloy electrodes,the boron atoms have two functions.First, boron can be oxidized to BO_3~(3-),thereby partly contributes to the discharge capacity.Second,most importantly,the gradual dissolution of boron into the electrolyte(in the form of BO_3~(3-)) during the charging/discharging creates a new surface in the electrodes,which can effectively increase the surface area of the active material in contact with the electrolyte.The alloy with higher boron content thereby can produce a larger reaction surface area by boron dissolution than the alloy prepared with lower B content.So the higher B content in the Co-B alloys can be helpful for improving their electrochemical properties.