Hoechst Trevira has succeeded in developing a design for a continuously processed, thermally shaped, open-mesh network structure of co-knitted multi-component polyester fibres and copper filaments, to be used as lightweight electrode grid in lead-acid batteries. The inclusion of copper filaments at the knitting step yields a good electrical conductivity across the grid, thus allowing a fast processing at the subsequently performed galvanic metal coating.
In order to metallise polymer grids by electrochemical plating, conducting starting layers have been provided by coatings of carbonaceous materials. This new process, developed by TU-Graz, is characterized by a simple dip coating of the polymer grids in a carbon paste with an additional surface treatment by a graphite finish process, and replaces conventional electroless methods for metallising plastics with advantages of simple processing and machining, possible continuous processing, short process times, reliability and the use of relatively cheap and environmentally friendly materials.
Devex has developed a continuous process has been developed for the carbon coating, surface treatment and reinforcement of the conductive starting layer by electroplating, characterised by a material selective pre-cutting of the electrodes out of the continuous PNS-band, leaving the uncut copper filaments in the continuous band. This mechanical support of the electrodes is a big improvement for transporting, contacting and mechanical stability during the whole process.
Two possible solutions for the corrosion protection of the copper layer on the positive electrode have been developed by TU Graz at the laboratory scale: A valve metal layer between the copper and lead alloy layer that shows a blocking protection under corrosive conditions in the battery electrolyte and very low corrosion rate lead alloys to be used either in combination with a valve metal interlayer or as a self-sufficient copper corrosion protection which will not hinder copper corrosion but will retard the electrolyte penetration to the copper layer and therefore result in improved cell life. However, tests to implement those methods with real size PNS electrodes have been unsuccessful up to date due to mechanical cracks in the protective layers during electrode handling.
Tudor has developed new methods to test, mechanically and electrically, PNS and conventional lead grids and electrodes, and characterised all the materials developed under the project. Results obtained have allowed an optimisation of the mesh size in the PNS grid to provide the best compromise performance/ weight/ cost.
Several battery manufacturing processes had to be adapted to the characteristics of the new grid materials: New active masses with lower density and higher penetration values have led to a higher active material efficiency, taking advantage of the three-dimensional structure of the new grid. A lug fixing process has also been developed to provide the PNS grid with a compact metallic contact for current transfer without damaging the polymeric structure and a laboratory installation for lead plating the grids as well. Finally, cast on strap welding of the plates has been adapted for the group completion.
In order to demonstrate the new battery technology, all the partners have been involved in the preparation of materials to manufacture more than 2200 electrodes. Several 2V cells and 56 12-V batteries have been assembled and tested by Renault as end user and Tudor as battery manufacturer. However, the results were disappointing, as the good initial capacity values (specific energy of 38.8 Wh/kg), decreased during battery testing, leading to a premature battery failure. The failure mode analysis has demonstrated a combined failure of positive plates and a too high water loss, due to the copper release during testing from the negative grid.
Lead-acid batteries have a theoretic specific energy of 167 Wh/kg. In practice, only 25-40 Wh/kg, depending on the respective application, can be achieved. These low values are due to the high weight of the inactive masses in particular the lead-grids, and to the low utilisation of the active masses.
Substituting the heavy lead-alloy grids by lightweight polymeric network structures a remarkable weight reduction will be attained. To use such polymeric network structures polymeric network structures in advanced lead-acid batteries, they have to be conductive, corrosion resistant, and an uncomplicated processing has to be ensured.
Additional weight reduction the range of 20% to 30% is possible. To realise this without deterioration of the electrical and mechanical properties of lead-acid batteries and with cost efficient processes, is the goal of this project.
Call for proposalData not available
Funding SchemeCSC - Cost-sharing contracts
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