Development of a safe and eco-friendly bipolar battery based on NiZn for supplying a robot – ROBOBAT

Techniques and materials necessary to manufacture nickel and zinc electrodes are well known thanks to the development works made previously by SCPS. Current collectors are made of metallic foam, nickel for the positive, and copper for the negative.
- Binding cell walls/electrodes: metallic walls electrically connected to electrodes but non porous to avoid electrolyte linkage are preferably metallic foils, nickel and copper. Resistance welding is the preferred option to bind electrodes and foils, but brazing can be an option.
- Connection between cells: electrodes with opposite polarity and welded to foils are connected by welding or gluing with conductive glue.
- Sealing of cells: electrolyte filling the cell must be kept inside. Plastic frames with o-rings gaskets will be used in a first step with tie bars to compress the stack (filter-press design). Lack of linkage is essential to prevent shunts and shorts and avoid loss of electrolyte which is added in small quantity.
- Pressure vent: connected to each cells, it will avoid overpressure and help gas recombination.
- Amount of electrolyte: it has to be large enough to minimize the ohmic drop, but small enough to facilitate the oxygen access to zinc recombination sites. Gel electrolytes are an option.
- Depending on the results obtained, over-molding of cells is possible to reduce the mass and simplified the battery manufacture.
- All the solutions experimented will take into account the constraint of a mass production of batteries.

At the end of the first year, several problems have been identified and partially solved:
- Sealing: cells linkage was solved by using to o-rings located on each frame.
- Gas blocked between zinc electrode and separator during battery formation: solved by making a small opening on the edge of separator.
- Electrodes/foils assembling: compression of nickel electrodes already welded to foil is too critical. It is preferred to build nickel electrode first and to create welding zones to assemble electrode and foil. Linkages appeared on the welded spots of bronze foil because of its low thickness (80 µm), leading to a rapid decrease of capacity of a two cells stack. Use of 125 µm foil solved the problem and capacity is stable over 100 cycles, even if linkages also appeared on nickel foils as well. Sealing at the level of o-rings is no more an issue.
- Actions undertaken: thicker nickel foil, other welding techniques than resistance welding such as laser, ultra-sounds are under progress, electrochemical welding of zinc electrode onto bronze foil. Results obtained so far with this technique are very positive: its avoid electrical welding and occurred during the electrochemical formation of the battery.
- Tests are also in progress with thick electrodes to increase surfacic capacity and consequently energy at a given voltage. Power available meets specifications.
- A larger cell is under construction to be compliant with the new specifications.

Results obtained at T0+15 makes possible much rapid progress in the coming months.
In case of success, bipolar technology would be a significant advantage at a time where large industrial groups such as Enersys or NGK planned to commercialize nickel-zinc batteries in the coming 2 or 3 years.

There is no scientific production or patent so far since some of the techniques put in place recently need to be confirmed.

The main objective of the project is to develop a bipolar battery by using safe materials in compliance with environmental regulations. Because of the performances achieved so far, the high cyclability the Ni-Zn battery technology developed by SCPS was chosen.
The bipolar Ni-Zn battery can meet the required specifications to power a robot, while offering enough design flexibility to meet other constraints.