Analysis of SEI Formation and Fast Charging on Lithium Loss in NMC/Graphite Cells

Abstract

Fast charging of a Li-ion battery is a key requirement for the acceptance of electric vehicles. However, charging at high currents is known to accelerate battery degradation when not conducted in an appropriate operational window. One way to influence the viability of fast charging is to optimize the cell formation procedure. In this study, we investigated the effect of an accelerated formation protocol which was previously shown to benefit rate capability vs. a reference protocol [1]. We analyzed the effects of formation methods and charge rates on electrode surfaces with methods including Li nuclear reaction analysis (Li-NRA) [2,3,4] and secondary ion mass spectroscopy (SIMS) as well as electrochemical experiments using three-electrode cells.The two SEI formation methods were applied and then the NMC/graphite cells were cycled at charge rates of 1C, 3C, and 5C for 100 cycles. Normalized capacity retention and post-mortem electrode analysis revealed that performance degradation is not merely attributable to lithium aggregation at the graphite anode surface (as has been suggested previously), but also to accelerated cathode degradation. The correlations of formation methods and charge rates on capacity fading, resistance, and Li concentration depth profiles on the anode surfaces by Li-NRA and SIMS analyses point to a small, but nonetheless significant role of cell formation on long term performance.Two NRA methods, proton induced gamma ray emission (PIGE) and deuteron induced particle emission (DIPE), and SIMS were employed to quantify the prevalence of lithium at the anode interface. Each showed a dependence of lithium amount on the charge rate, and to a lesser extent the formation procedure, even if the capacity retention for the two fastest rates was similar after 100 cycles. Such physical results illustrate that though the aggregation of lithium at the anode surface is important, degradation of the cathode may play a greater role in sustained electrochemical function.Using a three-electrode cell, we also deconvoluted the resistance at each electrode and found that the DC resistance of the cathode was more correlated with an observed increase in full cell resistance than that of the anode. We observed that aggregation of lithium at the anode under accelerated charging conditions alters the amount of reversible lithium inventory and thus the operating potential range of the cathode, yielding greater degradation.[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.[3] A. Schulz, D. DeRosa, S. Higashiya, M. Rane-Fondacaro, H. Bakhru, P. Haldar, J. Power Sources 360 (2017), 129-135.[4] M. Chebuske, S. Higashiya, S. Flottman, H. Bakhru, B. Antonopoulos, O. Paschos, F. S. Gittleson and H. Efstathiadis, Chem. Commun. 56 (2020), 14665–14668.