The properties of the solid electrolyte interface (SEI) are critical for safety and aging of lithium-ion batteries. The SEI is mainly built during the first few charge/discharge cycles, the so-called cell formation. Composition and morphology of the SEI are influenced by many factors, such as electrolyte composition, electrode structure, temperature, and formation protocol [1,2]. Moreover, the impact of these factors on the SEI is difficult to evaluate during the early stages of the film formation. Therefore, currently high experimental effort is needed for systematic optimization of the SEI. Hence, novel methods that allow to quickly obtain information about the SEI and its growth during the formation are vitally important.
Here, we present an approach that uses coulometry and differential voltage analysis (DVA) to gain a in operando estimate of the SEI growth during cell formation. Using DVA, distinct points (feature) in the course of the anode potential are identified. Since these points correspond to distinct degrees of lithiation, the capacity of the SEI layer can be determined by comparing capacities during formation with a reference measurement of capacities after the SEI is established. The identification of the SEI capacity curve is computed with two different methods: a time series without restrictions and an empirical SEI growth model. The two methods are compared in terms of their advantages and disadvantages. Both approaches show that the SEI capacity rises steeply during the very first part of the first charge and then continuously slows down, which is in line with our previous simulation study . Using a SEI density and the surface area of the electrode, the final SEI thickness is estimated. Finally, the method is exemplarily applied to examine the influence of two electrolytes onto the SEI growth.
Since this method provides in operando information about the film growth rate, there are several promising future applications. As presented, it can be used to validate SEI models and to review SEI growth mechanisms as well as to investigate the impact of different electrolytes with minimal experimental effort. In principle the method is also applicable to full cell voltage. This allows its application within battery cell production lines for end of line test purposes.
 Seong J. An, Jianlin Li, Claus Daniel, Debasish Mohanty, Shrikant Nagpure, David L. Wood, The State of Understanding of the Lithium-Ion-Battery Graphite Solid Electrolyte Interphase (SEI) and its Relationship to Formation Cycling, Carbon 2016, 105, 52–76, DOI: 10.1016/j.carbon.2016.04.008.
 Kang Xu, Electrolytes and Interphases in Li-Ion Batteries and Beyond, Chemical reviews 2014, 114(23), 11503–11618, DOI: 10.1021/cr500003w.
 Fridolin Röder, Richard D. Braatz, Ulrike Krewer, Multi-Scale Simulation of Heterogeneous Surface Film Growth Mechanisms in Lithium-Ion Batteries, J. Electrochem. Soc. 2017, 164 (11), E3335-E3344, DOI: 10.1149/2.0241711jes.
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