KOBIBATT: Innovative fully welded bipolar battery

The main objective of the “KOBIBATT“ project is to develop a battery system with higher energy density and greater safety at lower costs. In battery research, these goals have so far been seen as contradictory and incompatible. For the first time, “KOBIBATT“ addresses all three goals simultaneously. The central challenges are as follows: (1) the development of an innovative and fully welded polymer-based bipolar battery, (2) research into a constructive design, (3) the development of cost- and resource-efficient roll-to-roll manufacturing and joining processes. The design for the bipolar battery will be developed and demonstrated using a high-energy-density lithium ion bipolar battery for mobile applications and a long-life nickel-iron bipolar battery with cycle stability for stationary applications.

Challenges

The technical challenges of the project are the development of electrochemically stable, resource-saving and cost-effective bipolar electrodes and their assembly into perfectly sealed battery stacks. The innovative project approach pursued by the participating researchers aims to develop weldable, highly conductive polymer-based bipolar films that are produced in one process with polymer solid-state electrolytes and composite electrodes and welded together to form a complete battery.

The structure and manufacturing process of current battery systems are highly complex. In the past, a battery system required the interconnection of many individual cells with low power, which resulted in low energy density due to the large number of inactive envelope components. High material consumption and difficult-to-automate single-cell assembling (assembly of the different components) are further disadvantages of current battery systems. In order to further increase the energy density, considerably larger individual cells would have to be used, which, however, increase the risk of hot spot formation at connectors and conductors and thus the risk of fire due to the high current load.

The bipolar battery design completely eliminates these problems: The battery system to be developed in the project consists of only one battery stack, which can be completely automated. At the same time, the area-related current load is much lower and more uniform, which enables longer service lives.

Space-saving, scalable and seal-free

In the bipolar battery structure to be developed, follower cells are attached to both sides of the bipolar foil, anode, and cathode, so that the foil simultaneously forms the mechanical separation and electrical connection for the cells in the stack. The structure dispenses with separately enclosed individual cells and requires neither individual current conductors nor electrical cell connections. Due to the bipolar structure, the cell size can be scaled at will. This new approach significantly increases the energy density in the battery system and, by reducing the internal cell resistance, avoids the formation of hot spots and thus the risk of fire.

Since production in cost-effective, fully automated roll-to-roll processes is possible with less material input, costs and resources can be saved during production. The bipolar battery is fully welded so that seals can be dispensed with completely. The base material is a thermoplastic system which allows for a polymer-based battery construction or a fully plastic battery.

Mobile and stationary applications

With the project for the fully welded bipolar battery, the Fraunhofer consortium is developing a new type of battery structure in the “KOBIBATT“ project that can be applied to practically all battery chemicals. The technological leap in quality is aimed at the market needs for higher energy density with greater safety and, at the same time, lower costs.

The Fraunhofer scientists develop and demonstrate lithium-ion batteries for mobile applications and nickel-iron batteries for stationary applications on the basis of the bipolar battery concept. Mobility batteries are used in vehicle and aircraft construction, and shipbuilding.

There is great demand for stationary bipolar batteries, especially from energy suppliers, who are increasingly using mass storage due to fluctuating feeds from renewable energy sources in order to guarantee grid stability.

• Institute for Ceramic Technologies and Systems IKTS
• Fraunhofer Institute for Process Engineering and Packaging IVV
• Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM
• Fraunhofer Institute for Applied Polymer Research IAP