In Situ AFM Study of Solid Electrolyte Interface in Lithium-Ion Batteries

Chemical and morphological structure of solid electrolyte interphase (SEI) plays a vital role in lithium-ion battery (LIB), especially for its cycleability and safety. To date, research on SEI is quite limited due to the complexity of SEI and lack of effective in situ characterization techniques. Here, we present real-time views of SEI morphological evolution using electrochemical atomic force microscopy (EC-AFM). Complemented by an ex situ XPS analysis, fundamental differences of SEI formation from ethylene carbonate (EC) and fluoroethylene carbonate (FEC)-based electrolytes during first lithiation/delithiation cycle on HOPG as well as Fe 3 O 4 electrode surfaces were revealed. Our results showed that SEI layer formed by reductive decomposition of the EC/DMC electrolyte was principally composed of alkyl carbonates ROCO 2 Li, while the SEI layer formed by FEC/DMC electrolyte was mainly composed of LiF. Only trace of Li 2 CO 3 was found in the SEI layer formed from FEC/DMC electrolyte. The dense and hard nature of SEI formed by FEC/DMC electrolyte can protect the graphite against dendrite formation and could be the reason for better cyclability of FEC/DMC electrode as compared to those of EC/DMC based electrolyte. For the Fe 3 O 4 anode, it was found that Fe 3 O 4 electrode was unable to form a stable SEI layer on the electrode surface which resulting in electrolyte decomposition. FEC-based electrolyte can help to improve the performance of Fe 3 O 4 anodes in lithium ion batteries but its protective effects are far from perfect. To accelerate the application of Fe 3 O 4 or other metal oxide anodes in lithium ion batteries, better electrolytes and sophisticated carbon coating techniques are needed warrant formation of a stable SEI layer. Conventional method to evaluate electrolytes are usually complex and tedious procedures. Thus, combination of in situ electrochemical AFM and ex situ XPS could potentially serve as a fast diagnostic tool to evaluate the properties and quality of SEI formed on different electrodes from diverse electrolytes and additives.