Quantitative manganese dissolution investigation in lithium-ion batteries by means of X-ray spectrometry techniques

Abstract

Each battery suffers from degradation effects which lead to capacity fading and life cycle reduction. With electrochemical methods, the battery capacity decrease can easily be monitored, but they are inappropriate to comprehend the underlying chemical and physical properties responsible for the capacity fading. In the present paper, we demonstrate the power of X-ray spectrometry techniques to investigate the transition metal dissolution (TMD) fading process for manganese in Lithium Ion Batteries (LIBs). Regarding aging processes, quantitative analyses are important to evaluate the magnitude of a certain process for the total capacity decrease. The challenge for the investigation of aged battery materials is the lack of reference materials such that techniques based on them cannot be applied. With the use of reference sample-free quantitative X-ray fluorescence analysis (XRF), we investigated the total mass deposition of manganese deposited at the anode for an aged cell. For 50 full cycles with elevated cut-off voltage, the capacity decreased by 12.5%, while up to 1.6‰ of cathodic manganese was found deposited in the anode. Furthermore, the species of manganese deposited in the anode has been studied with near-edge X-ray absorption fine structure analysis (NEXAFS) at the manganese L-edges and K-edges. In addition to a different discrimination capability at L- and K-edges, the application of significantly different excitation energies provides sensitivity to different sample depths. We determined the manganese deposition to be in di- and tetra-valent states, whereby tetravalent manganese is solely found in the anode bulk. The combination of the different methods generates a clearer understanding of the chemical processes in the battery and can therefore help to improve the performance by exploring the absolute mass deposition and the electronic structure of elements contained in enhanced material combinations.