Evidence of Transition-Metal Accumulation on Aged Graphite Anodes by SIMS

In cells containing Li 1.05 ( Ni 1 ∕ 3 Co 1 ∕ 3 Mn 1 ∕ 3 ) 0.95 O 2 -based positive and graphite-based negative electrodes, a significant portion of cell impedance rise on aging is known to be from the negative electrode. One possible reason for this impedance rise is the dissolution of transition-metal elements from the oxide electrode that accumulate and create a high-impedance layer at the negative electrode–electrolyte interface. This article details dynamic secondary ion mass spectrometry (SIMS) measurements, which provide a relative comparison of Mn, Co, and Ni contents on fresh, formed, and aged graphite electrodes. The data clearly indicate that these transition-metal elements accumulate at the electrode surface and diffuse into the electrode during cell aging.