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German scientists, for the first time, have peered inside a working lithium-ion battery without dismantling it, observing its degradation over thousands of charge cycles. This "operando" experiment, conducted by a German consortium led by the Helmholtz-Zentrum Berlin (HZB) and the Physikalisch-Technische Bundesanstalt (PTB), has unveiled a crucial insight into why batteries lose their capacity over time. They have identified manganese loss as the initial and most damaging step in this process.

The research team employed a sophisticated confocal micro-X-ray fluorescence (碌XRF) spectrometer at the BLiX lab of TU Berlin. This instrument enabled them to scan the intact battery every few hours during normal operation, generating a detailed 3-D chemical map with an impressive 10-micrometer depth resolution. This level of detail allowed them to clearly distinguish the different layers within the battery, including the nickel-manganese-cobalt oxide (NMC) cathode, the graphite anode, and the separator in between.

The continuous monitoring revealed a two-stage aging mechanism. In the initial phase, spanning roughly the first 200 charge-discharge cycles (about three weeks in their test), manganese atoms leached from the NMC cathode's structure and migrated through the separator, eventually depositing on the graphite anode. Notably, only minimal amounts of nickel and cobalt moved during this early stage.

The researchers discovered that once a significant amount of manganese had migrated, the dissolution process within the cathode slowed down. However, this initial manganese loss triggered further electrochemical reactions in the separator and current-collector layers, leading to a more widespread breakdown of the battery's internal chemistry and a decline in its overall performance.

For years, understanding the intricate processes that lead to battery failure has been a challenge. Researchers were limited to analyzing battery components only after the cell had been disassembled, providing static snapshots rather than a dynamic view of the aging process. This new technique overcomes this limitation by allowing scientists to monitor the chemical changes occurring within a commercial coin cell in real time as it charges and discharges.

"Everything happened non-destructively," explained Dr. Ioanna Mantouvalou of HZB, the first author of the study published in the journal Small. "We could quantify where each element migrated while the cell was still working."

This finding carries significant implications for the battery industry, particularly because manganese is an increasingly vital component in high-energy cathodes due to its lower cost compared to other metals like cobalt and nickel. "Knowing that Mn is the earliest mover lets manufacturers target coatings or electrolyte additives to pin it down," Dr. Mantouvalou pointed out. By focusing on preventing this initial manganese migration, battery manufacturers can develop strategies to significantly extend the lifespan of future high-energy batteries.