Solid-state batteries for electric mobility

Solid-state batteries are an important building block for the electrification of mobility: They are safer and enable higher range and shorter charging times than conventional Li-Ion batteries. New cell concepts allow even higher energy densities. Fraunhofer IFAM is developing polymer and sulfide-based solid-state batteries for various electric mobility application areas. The research is oriented towards industrial battery production and ranges from development of new materials for solid electrolytes and battery components (electrodes and separators) to manufacturing process steps and cell assembly.

Advantages of solid-state batteries

The development of solid-state batteries based on lithium ion technology promises higher energy densities of up to 500 Wh/kg (or 1000 Wh/L), as well as fast-charge capability and intrinsic fire safety. This is achieved by replacing the flammable liquids (organic electrolytes) currently used in batteries with ion-conducting solid materials. Due to their chemical resistance, solid ion conductors allow the use of high-energy materials, such as metallic lithium, and also have a longer lifetime. Another advantage of batteries with solid electrolytes is the feasibility of new space- and weight-saving cell concepts. In addition to the construction of bipolar interconnected cells as a new cell design, the shape of the battery can also be freely selected. This opens up the possibility of using batteries not only as energy supply and passive structural components in mobile systems, but also of incorporating them directly as active structural elements (structural battery)

New materials for the battery

The choice of solid materials as electrolytes in solid-state batteries is wide and can be broadly divided into three groups:

• polymers or organic electrolytes
• sulfides or thiophosphates
• oxides

Fraunhofer IFAM mainly focuses on the first two material classes, since their production and processing do not require any high-temperature steps (sintering) compared to oxide based systems. Polymeric solid-state batteries are mostly based on thermoplastic systems and thus offer the advantage adapting long-established manufacturing technologies such as extrusion, injection molding, or calendering. The somewhat lower ionic conductivity and thus lower performance pose a challenge. Solid-state batteries based on sulfide ionic conductors, on the other hand, are just as conductive as liquid electrolytes and are therefore suitable for high-power applications. They are relatively soft and thus allow fabrication using conventional solvent-based coating techniques. The disadvantage of such systems is the sensitivity of the materials to moisture.

Applications in electric mobility

The variety of possible solid-state batteries allows for tailoring the battery cell precisely to the specific application. Polymer-based solid-state batteries are already being used today in public transport (buses), where a constant stress profile can be predicted due to predictable routes and low speeds.

In the future, electric cars may also be equipped with high-performance solid-state batteries. Here, batteries with both high energy density and high power density are required due to individual driving behaviour and higher driving dynamics.

For the incipient electrification and individualization of air travel (air taxi), solid-state batteries offer a space- and weight-saving alternative with a robust design. The intrinsic thermal safety of the solid-state battery also allows a reduction in thermal management.

The long lifetime and low maintenance requirements also make the solid-state battery attractive for Autonomous Guided Vehicles in the form of robots and drones.

Alternative anode concepts for safe solid-state batteries - ALANO (BMBF)

The OEM-led joint project ALANO focuses on the research and development of the lithium metal anode. The aim is to establish the lithium metal anode in solid-state cells by evaluating innovative lithium metal-based anode concepts. Particularly, in electric mobility applications, there are challenges in terms of reactivity (safety) and performance (range and acceleration).

The tasks of Fraunhofer IFAM include the development, processing, and production of lithium metal solid-state batteries. The material composition of the components, such as separators and composite cathodes, is optimized for upscaling of the production.

Then, full cells will then be assembled in the form of pouch cells with the lithium metal anodes developed by the project partners. Here, the aim is to evaluate both possible cell designs (e.g. monopolar/bipolar) and favourable operating conditions (temperature and pressure).

Process development for series production

Since 2018, Fraunhofer IFAM has been working on processes for the series production of solid-state batteries at the Project Center for Energy Storage and Systems ZESS in Braunschweig (Lower Saxony, Germany). Here, a new research building is being built. The enclosed dry room enables the evaluation of different process technologies from material synthesis to cell assembly. The focus of current activities is on the scaled production of sulfide electrolyte materials and the processing of polymer electrolytes. Here, continuous progresses in cell development constantly push the manufacturing of battery cells.