A view of the operando cell in the sample chamber during the measurements at the SISSY Endstation. Credit: E Kataev/ HZB. CC BY-SA.

Research at Germany’s BESSY II synchrotron radiation source shows how intrinsic oxygen shortens the lifespan of solid-state batteries.

Scientists have analysed a TiS₂|Li₃YCl₆ half-cell in operando at BESSY II using a special sample environment, with the results expected to help improve the design and handling of solid-state batteries (SSBs).

Although SSBs demonstrate high performance and are intrinsically safe, their capacity currently declines rapidly.

A team from the TU Wien, Humboldt-University Berlin and HZB has now analysed a TiS₂|Li₃YCl₆ solid-state half-cell in operando at BESSY II using a special sample environment that allows for non-destructive investigation under real operating conditions.

Data obtained by a combination of soft and hard X-ray photoelectron spectroscopy (XPS and HAXPES) revealed a new degradation mechanism that had not previously been identified in solid-state batteries.

They have gained some surprising insights, particularly regarding the harmful role played by intrinsic oxygen.

This study provides valuable information for improving the design and handling of such batteries.

Advantages

SSBs offer several advantages over conventional batteries, including higher energy and power densities, as well as greater safety, as they do not contain flammable liquid electrolytes.

However, since lithium ions migrate between the working electrode and the counter-electrode during operation, the solid material can suffer by volume changes, which can lead to cracks.

In order to maintain contact between electrodes and electrolyte, SSBs must be operated under high pressure.

Volume changes, as well as degradation processes at the interfaces, often limit the lifespan of these batteries.

Until now, it has been virtually impossible to observe these processes experimentally, particularly due to the high stacking pressure required during operation.

However, Dr Elmar Kataev, a scientist at HZB, has now developed a sample environment that enables operando analysis of SSBs under high pressure using two-colour – soft and hard – X-ray photoelectron spectroscopy (XPS and HAXPES) at the SISSY endstation at BESSY II.

These conditions of combining two different energies of X-rays (hard for bulk sensitivity and soft – for surface) hitting the same spot is exclusively available at EMIL beamline.

Collaboration

In collaboration with Dr Katherine Mazzio of the TU Wien, the team has now, for the first time, been able to distinguish between reactions at the surface and at buried interfaces, and analyse the degradation mechanisms of TiS₂|Li₃YCl₆ half-cells in greater detail.

Mazzio said: “We have gained some surprising insights, particularly regarding the harmful role played by intrinsic oxygen.

“We observed that during cycling oxygen-containing species migrate towards the cathode current collector, where they react with the active electrode material near the current collector interface.

“This results in the formation of an amorphous layer rich in titanium oxides. This is a major cause of the rapid loss of capacity.”

These findings are extremely important for the further development of SSBs.

This is because the ingress of oxygen into the battery cells should be reduced or even prevented during production of the constituent materials, prioritising its manufacturing in an inert gas atmosphere.

The research is published in ACS Energy Letters.