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, high experimental effort is currently required for optimizing the SEI systematically. 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 features in noisy measurement data requires smoothing of data, which is associated with uncertainty of the obtained SEI capacities. However, the obtained course of the SEI capacity is in line with our previous simulation study [3]. The results show that the SEI capacity rises steeply during the very first part of the first charge. Afterwards, SEI growth continuously slows down and shows approximately a square root over time growth, which is frequently reported. Using the SEI density and the surface area of the electrode, the final SEI thickness is estimated. Finally, various lumped SEI growth models with different growth limiting functions have been tested using the measurement data.

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. Furthermore, the impact of different electrolytes or electrode structures can be investigated with minimal experimental effort. Lastly, the method is in principle also applicable to full cell voltage. This allows its application within battery cell production lines for end of line test purposes.

References:
[1] 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.
[2] Kang Xu, Electrolytes and Interphases in Li-Ion Batteries and Beyond, Chemical reviews 2014, 114(23), 11503–11618, DOI: 10.1021/cr500003w.
[3] 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.