Influence of the specific surface area of Stöber silica additives on the electrochemical properties of negative electrodes in lead-acid batteries
The surface area of carbon additives has been described as a key property for the enhancement of cycling stability and dynamic charge acceptance (DCA) of negative lead-acid electrodes. However, it is still uncertain if similar performance enhancing effects can be achieved by other inorganic additives of similar structure, or if the electrochemical activity of carbon additives is mandatory for the enhancing effects. In order to elucidate the importance of structure and electrochemical activity of high DCA additives, Stöber silica with tailorable particle size and specific external surface area were prepared and incorporated into negative lead-acid electrodes, combined with the addition of carbon black to ensure high cycling stability. The properties and electrical performance of the negative active material were investigated by electrical tests in 2 V laboratory cells and different physical characterization methods. The overall electrochemical activity in terms of hydrogen evolution and double-layer capacitance was not affected by the silica additive and therefore is also independent of the external surface area. In addition, the silica particles led to a slightly reduced, though steady, mass utilization and discharge capacity. Finally the DCA could not be improved by the Stöber silica additives, which excludes Stöber silica as high DCA additive. In addition, since the DCA performance has not been increased, despite the broad specific surface area range of the additive particles utilized, this study provides a clear indication that the performance enhancement observed with carbon additives also relies on their electrical conductivity.