Quantifying Immobile Lithium at the Graphite Anode Surface from Formation to Fast Charging

Engineering the solid electrolyte interphase (SEI) at the surface of graphite anodes in Li-ion batteries is crucial to optimizing the rate capability, cycle life, and calendar life of commercial cells. It has been reported that fast-forming SEIs on composite graphite electrodes in a high-potential region (~0.24 V vs. Li/Li+) has a positive influence on rate capability in half cell formats [1]. Potentiostatic EIS modulation applied during formation has shown similar results to the fast formation method. In this work, we evaluated the efficacy of various SEI formation methods on the rate and cycle life of full cell Li-ion batteries, and then analyzed electrode surfaces ex situ to locate and quantify immobile lithium. Lithium nuclear reaction analysis (Li-NRA) provided an effective non-destructive method to correlate irreversible electrochemistry with physical surface phenomena [2]. It is well established that high current charging can induce Li plating on the graphite anode, leading to accelerated capacity fade. Several methods have been used to quantify this phenomena including titration and secondary ion mass spectroscopy (SIMS). Here, we employ Li-NRA and a suite of complementary techniques (i.e. XPS and SIMS) to show how the amount of immobile Li and its depth profile varies with charge rate (up to 5C) and cycle number. [1] B. K. Antonopoulos, C. Stock, F. Maglia, H. E. Hoster, Electrochim. Acta, 269 (2018), 331-339. [2] A. Schulz, D. DeRosa, S. Higashiya, M. Rane-Fondacaro, H. Bakhru, P. Haldar, J. Energy Storage 14 (2017), 106-111. A. Schulz, D. DeRosa, S. Higashiya, M. Rane-Fondacaro, H. Bakhru, P. Haldar, J. Power Sources 360 (2017), 129-135.