Low current is considered one of the biggest hurdles in the development of solid-state batteries; it is the reason why the batteries take a relatively long time to charge fully—some 10 to 12 hours for a solid-state battery. Now, researchers at Jülich Forschungzentrum have introduced a new concept battery that allows currents up to ten times greater during charging and discharging than previously described in the literature. Their battery takes less than an hour to recharge.

The battery, described in a paper in the journal ACS Applied Materials and Interfaces, uses a monolithic all-phosphate concept based on screen-printed thick LiTi₂(PO₄)₃ anode and Li₃V₂(PO₄)₃ cathode composite layers on a densely sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ solid electrolyte.

With the concepts described to date, only very small charge and discharge currents were possible due to problems at the internal solid-state interfaces. This is where our concept based on a favorable combination of materials comes into play, and we have already patented it.

—Dr. Hermann Tempel, group leader at the Jülich Institute for Energy and Climate Research (IEK-9)

In conventional lithium-ion batteries, a liquid electrolyte is used, which usually contacts the electrodes very well. With their textured surfaces, the electrodes soak up the liquid like a sponge, creating a large contact area. In principle, two solids cannot be joined together seamlessly. The contact resistance between the electrodes and the electrolyte is correspondingly high.

In order to allow the largest possible flow of current across the layer boundaries, we used very similar materials to produce all components. The anode, cathode, and electrolyte were all made from different phosphate compounds to enable charging rates greater than 3C (at a capacity of about 50 mAh/g). This is ten times higher than the values otherwise found in the literature.

—Hermann Tempel

The solid electrolyte serves as a stable carrier material to which phosphate electrodes are applied on both sides using the screen printing process. The materials used are reasonably priced and relatively easy to process. Unlike conventional lithium-ion batteries, the new solid-state battery is also largely free of toxic or harmful substances.

In initial tests, the new battery cell was very stable over 500 charge and discharge cycles and retained over 84 percent of its original capacity. There is still room for improvement here. Theoretically, a capacity loss of less than 1 percent should even be feasible,

—Dr. Shicheng Yu

Yu developed and tested the battery as part of a China Scholarship Council (CSC) funding programme at the Jülich Institute for Energy and Climate Research (IEK-9).

The energy density is already very high at around 120 mAh/g, even if it is still slightly below that of today’s lithium-ion batteries.

—Institute director Prof. Rüdiger-A. Eichel

In addition to the development for electromobility, Professor Eichel, the spokesman for the “battery storage” topic in the Helmholtz Association, believes solid-state batteries will also be used in other areas in future.

Solid-state batteries are currently being developed with priority as energy storage for next-generation electric vehicles. But we also believe that solid-state batteries will prevail in other fields of application that require a long service life and safe operation, such as medical technology or integrated components in the smart home area. Because they do not overheat or even explode when damaged, unlike lithium-ion batteries, they would also be a safe alternative for smartphones and other mobile computers.

—Rüdiger-A. Eichel

Resources

• Shicheng Yu, Andreas Mertens, Hermann Tempel, Roland Schierholz, Hans Kungl, and Rüdiger-A. Eichel (2018) “Monolithic All-Phosphate Solid-State Lithium-Ion Battery with Improved Interfacial Compatibility” ACS Appl. Mater. Interfaces doi: 10.1021/acsami.8b05902