Observation of dynamic interfacial layers in Li-ion and Li-O2 batteries by scanning electrochemical microscopy
The requirements of high energy density in modern batteries dictate the use of very high (oxidizing) or very low (reducing) potential for negative and positive electrode materials. These extreme potentials can cause molecular compounds to undergo electron transfer reactions at the interfaces. This is well documented for lithium-ion batteries, where a solid electrolyte interphase (SEI) between the lithiated graphite electrode and the electrolyte is formed by the decomposition of electrolyte components mainly during the first charging process. Characterization of the SEI is a challenge because of the variety of chemically similar components and enclosed electrolyte species. Furthermore, ex situ analysis of the SEI requires separation and isolation of the SEI, which may change the content and the structure of the SEI. Scanning electrochemical microscopy (SECM) provides in situ analysis of passivating layers formed at battery electrodes. Such approaches must deal with continuous changes of the studied interfaces. This is illustrated for the in situ investigation of the electron transport at SEI-covered lithiated graphite using 2,5-di-tert-butyl-1,4-dimethoxy benzene as SECM mediator in an inert atmosphere. With this setup, the influence of rinsing protocols on the passivating properties of the SEI was studied. An extensive rinsing compared to our previous studies [DOI 10.1002/anie.201403935] leads to much higher local variation of the SEI passivation properties which continue over the entire observation time of 54 h. The second example uses a SECM generation-collection experiment to detect gas permeation through a gas-diffusion electrode (GDE) of a Li-O-2 cell into a Li+-containing organic electrolyte. The passivation of the microelectrode was counteracted by pulse amperometric detection allowing to distinguish pore blocking vs. electrode coating by lithium oxides during the discharge. The GDE was positioned between an Ar-O-2 and an Ar atmosphere. Imaging of GDE indicated a reduced but not a complete suppression of O-2 permeation through the GDE.