The project investigated the feasibility of replacing the conventional non‑evaporable electrolyte paste (NEP) used in VARTA Microbattery’s micro‑cells with a water‑based alkaline paste that incorporates additives to improve rheology and adhesion. In load‑testing up to 3 C, the NEP reference delivered a higher capacity than the water‑based formulation, but at higher rates beyond 3 C the water‑based paste performed better. Adding the additives did not change the capacity of the water paste, indicating that the main limitation was electrode adhesion rather than the paste composition itself. The additives, however, rendered the paste shear‑rate independent, eliminated surface pitting and void formation, and produced a uniform coating. Cycle performance of the water‑based electrodes matched that of the NEP reference, although the adhesion to the NEP system remained lower, comparable only to the anode. Consequently, the load‑test performance of the water‑based cells was slightly reduced. To compensate for the lower adhesion, the design team recommended a 15 % over‑dimensioning of the anode.

The study progressed to cell testing with Generation 1 electrodes in a CoinPower CP1254 format. Electrodes supplied by ZWS were cut to width, wound with a 16 µm polyolefin separator, and assembled in a dry room. Formation and performance tests were carried out at 0.5 C/0.2 C and 1 C/1 C charge‑discharge rates, with end‑of‑discharge voltages of 3 V and 2.7 V. The load‑test data showed that reducing the end‑of‑discharge voltage from 3 V to 2.7 V increased the specific capacity benefit, rising from 1.7 % at 0.1 C to 7.9 % at 3 C. When compared with Generation 0 pouch cells, Generation 1 exhibited comparable performance at low rates but superior capacity retention at higher rates.

Solvent‑type experiments compared water‑based and organic‑solvent electrodes for both anode and cathode. In the first set of tests, the organic‑solvent cell achieved an initial capacity of about 51 mAh versus 46 mAh for the water‑based cell, a 10 % improvement. After 500 cycles at 1 C/1 C, the organic cell retained 88.4 % of its capacity, whereas the water‑based cell retained 82.8 %. At 60 °C, the capacity advantage of the organic cell decreased to roughly 4 %. A subsequent storage test at 60 °C followed by cycling at room temperature confirmed a roughly 20 % higher initial capacity for the organic cell. These results demonstrate that the choice of solvent has a significant impact on both initial performance and long‑term cycle life.

Cycle‑life data for the Generation 1 cells showed an initial capacity of about 45 mAh regardless of the end‑of‑discharge voltage. When cycled to 3 V, the cells retained 80 % of their capacity after 1,100 cycles; when cycled to 2.7 V, 70 % remained after 1,500 cycles. Some cells exhibited premature capacity loss, prompting post‑mortem investigations to identify failure mechanisms.

The project was funded by the Oekobat‑2020 programme and executed in collaboration between VARTA Microbattery, ZWS (ZSW), and Freudenberg. The research spanned the development of the water‑based paste, electrode fabrication, cell assembly, and extensive electrochemical testing, culminating in a comprehensive assessment of the viability of water‑based electrolytes for micro‑battery applications.