Novel Method for Monitoring the Electrochemical Capacitance by In Situ Impedance Spectroscopy as Indicator for Particle Cracking of Nickel-Rich NCMs: Part I. Theory and Validation

Abstract

Nickel-rich NCM (LiMO2, with M = Ni, Co, and Mn) cathode active materials for lithium-ion batteries are being increasingly commercialized due to their high specific capacity. However, their capacity retention upon cycling is impaired by crack formation of NCM secondary agglomerates induced by the volume change upon repeated (de)lithiation that depends on the nickel content and the cutoff potential. Particle cracking leads to loss of electrical contact and enhanced side reactions caused by an increased surface area. Here, we introduce a novel method based on electrochemical impedance spectroscopy (EIS) in blocking conditions to quantify the increase in the active material’s surface area upon cycling, utilizing the correlation between the surface area of the electrode and the electrochemical double-layer capacitance that is validated experimentally by comparing the capacitance and BET surface area increase of NCM electrodes upon mechanical compression. To quantify the cracking of the particles upon 200 charge/discharge cycles, we perform in situ EIS measurements utilizing a micro-reference electrode and monitor the cathode’s impedance response. In addition, the crack formation of cycled NCM particles is validated visually by post mortem FIB-SEM. The effect of volume change on cracking is illuminated through the analysis of LFP and LTO as model materials.

Increased Moisture Uptake of NCM622 Cathodes after Calendering due to Particle Breakage

“…The better performance of the higher density cathodes at low C-rates is in accordance with Oswald et al 22 , who detected an increasing electrode capacity with rising compression and explained this by the increased electrochemically active surface area. Other researchers investigated crack formation in secondary NCM particles during the first charge and also detected an improved electrochemical performance due to increased electrochemically active surface areas and the associated reduced effective particle size.…”

“…This type of particle breakage due to compression has already been described in literature. 15,22,25,26,42 Furthermore, Oswald et al 22 observed that the particle breakage in NCM622 cathodes caused by compression led to a higher specific surface area, whereby the area increased with increasing compression. Thus, it can be assumed that the increasing moisture uptake with increasing density is mainly caused by the presence of more particle cracks and breakages.…”

“…It was observed that the moisture content of the cathodes increased with rising density after calendering. SEM images showed the well-known secondary NCM622 particle breakage after calendering, 15,22,25,26 which became even more pronounced with increasing density. Sorption measurements for the non-calendered (2.6 g cm −3 ) and the high density (3.2 g cm −3 ) cathode proved an increased moisture uptake of the high density cathode.…”

The influence of electrochemical cycling protocols on capacity loss in nickel-rich lithium-ion batteries

“…To rationalize this link, a number of interconnected processes must be considered. First, the structural changes induced by cycling have been demonstrated to promote inter-particle cracking 4,6,7,43–45 (evident in the charged state even in the first cycle), 46–48 which leads to increased electrolyte accessibility within the secondary particle and enabling the formation of an O-depleted resistive surface layer (and reduced surface layer) on the primary particle surface, as proposed recently by Friedrich et al 54 and Zou et al 15 This layer hinders the transport of lithium ions and/or electrons into the particle resulting in impedance rise. 12,49 Our analysis of the particle cracking and the reduced surface layer is discussed below.…”

“…Recently, several groups have shown evidence of microscale cracking of NMC particles imaged in the charged state in the first cycle. 46–48 After the first discharge, however, the cracks are no longer observed 46 suggesting a reversible (at least initially) “breathing” of the NMC particles. In this work, electrochemically induced cracks are not visible in NMC particles imaged in the discharged state ( Fig.…”