Deformation of Lithium-Ion Battery Anodes during SEI Formation: a Probe for the Structure of the SEI

Lithium-ion batteries are ubiquitous in small, portable electronics, where the lifetime of the battery now exceeds the expected lifetime of the device (ca. 2 years).  Capacity fade over time, however, remains a significant challenge hindering the use of lithium-ion batteries in more demanding applications with long life-time requirements, such as in electric vehicles.  One of the primary contributors to capacity fade is lithium trapping within the solid-electrolyte interphase (SEI) on the anode, making understanding the SEI critical to the design of more reliable lithium-ion batteries.  The size-scale (tens of nanometers) and reactivity of the SEI, though, make characterization of the SEI difficult. 

In this presentation, we present a new method to probe the structure of the SEI that is formed on lithium-ion battery anodes.  A custom battery cell employs an unconstrained (i.e. not on a current collector) graphite composite electrode and allows optical access to the electrode during electrochemical cycling.  Electrolyte is reduced on the electrode surface potentiostatically, and the resulting deformation of the electrode is measured using digital image correlation (DIC), an optical, full-field strain measurement technique.  The amount of deformation of the electrode induced during SEI formation is related to the structure of the SEI layer.  The strain measurements are then compared to galvanostatic cycling results, and the effects of different electrolyte salts, solvents, and additives on the SEI structure and battery capacity retention are evaluated.