Tracking lithium loss in silicon-containing batteries

Together with VARTA AG and BASF, international researchers including the Heinz Maier-Leibnitz Zentrum have tracked what happens to lithium inside large, industry-relevant battery cells as they age. Their findings help to explain why silicon-containing batteries gradually lose capacity. By combining electrochemical testing with advanced neutron-based techniques the researchers show that repeated charging and discharging causes a steady loss of cyclable lithium in these battery cells, leading to reduced battery capacity over time.

Dr. Thien An Pham in the Heinz Maier-Leibnitz Zentrum’s physics lab. © Bernhard Ludewig, FRM II / TUM

Dr. Thien An Pham in the Heinz Maier-Leibnitz Zentrum’s physics lab. © Bernhard Ludewig, FRM II / TUM

Why does silicon promise more energy but age faster?

Silicon-containing anodes are attractive because silicon can store far more lithium than the graphite used in today’s batteries, opening the door to higher energy density for electric vehicles and stationary energy storage. However, this advantage comes with a drawback. As lithium enters and leaves silicon during battery operation, the material repeatedly expands and contracts. This triggers unwanted side reactions and mechanical damage, which consume lithium and gradually disconnect parts of the anode from the electrical network.

Looking inside real cells with neutrons

To ensure their conclusions apply to real-world batteries, the team around Dr. Thien An Pham with Dr. habil Ralph Gilles from the Advanced Materials Group at the research neutron source Heinz Maier-Leibnitz (FRM II), focused on full-size 21700-type cylindrical cells, an industry-relevant format widely used in applications such as electric vehicles, power tools, and e-bikes. The cells were made by VARTA AG in Ellwangen and used a nickel-rich cathode from BASF plus a graphite anode mixed with a bit of silicon oxide.

The examined cells were of the type 21700, which are widely used. © Andreas Heddergott, TUM

The examined cells were of the type 21700, which are widely used. © Andreas Heddergott, TUM

The non-destructive in operando neutron diffraction technique, carried out at the Swiss Spallation Neutron Source SINQ (Paul Scherrer Institute, PSI), allowed the researchers to follow subtle structural changes in the cathode and anode during charging and discharging. Over many cycles, these changes became noticeably smaller at the cathode, indicating it was no longer taking up and releasing lithium over the same range as before.

To locate that “missing” lithium, the researchers used neutron depth profiling at the CANAM facility in Řež, Czech Republic, to map lithium concentration through electrode layers. These profiles showed clear cathode lithium depletion and increased lithium accumulation near the anode surface, consistent with lithium being consumed into the solid-electrolyte interphase (SEI) or becoming trapped in inactive lithium-silicon phases.

Dr. Thien An Pham (left) with Dr. habil. Ralph Gilles at the MLZ User Meeting in front of his poster that details his research on batteries with VARTA AG and BASF. © Laura Richter, FRM II / TUM

Dr. Thien An Pham (left) with Dr. habil. Ralph Gilles at the MLZ User Meeting in front of his poster that details his research on batteries with VARTA AG and BASF. © Laura Richter, FRM II / TUM

Less lithium available

The result is less lithium available for the cathode during discharge and a reduced usable capacity of the battery. This loss of cyclable lithium accounts for most of the capacity fade observed in the cells. The team also points to transition-metal migration as a known degradation mechanism in similar battery systems, which can further accelerate unwanted reactions. As Thien An Pham summarizes: “It seems that macroscopic changes in the battery cell are not the crucial factor leading to degradation; rather microscopic processes are.”

Goal: Extending lifetime of batteries

By showing how these degradation mechanisms can be identified in large-format cells using non-destructive techniques, this study gives engineers practical guidance for designing longer-lasting batteries without sacrificing energy density.
The partners involved in this work, together with additional new industry partners, will continue their collaboration within a new research project called NERO.