Probing Sources of Capacity Fade in LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622): An Operando XRD Study of Li/NMC622 Batteries during Extended Cycling

Abstract

LiNi0.6Mn0.2Co0.2O2 (NMC622) is one of the most promising Li-ion battery cathodes as it delivers high capacity at high potentials. However, high potentials also lead to decreases in capacity retention where the disintegration of the secondary particles has been implicated as a major driving force of this capacity fade. This has been attributed to anisotropic lattice changes and increased microstrain during cycling. To probe how these factors affect capacity fade, Li/NMC622 batteries were cycled from 3 to 4.3 or 4.7 V and probed with operando X-ray diffraction (XRD) over the 1st, 2nd, and 101st cycles. Further characterization with scanning electron microscopy and inductively coupled plasma-optical emission spectroscopy was also performed. The use of operando XRD over many cycles allowed for the collection of detailed structural information in real time over a time frame in which fading can be observed. During the first two cycles, the cells charged to 4.7 V exhibit increased anisotropic lattice changes as compared to the cells charged to 4.3 V. Upon the 101st cycle, when significant fade has been observed, the cells charged to 4.3 and 4.7 V show identical lattice changes to one another, while the 4.7 V charge limit induces more microstrain. This shows that elevated microstrain at high charge limits is a major driver for particle disintegration in NMC622 cathodes. This study provides important insights into the mechanisms of particle disintegration and capacity fade in NMC/Li-ion batteries, which will enable the design of NMC electrodes that deliver both higher capacities and exhibit better capacity retention.

Experimental Considerations for Operando Metal‐Ion Battery Monitoring using X‐ray Techniques

“…Pouch cells represent a versatile option, since they allow for three‐electrode measurements and the offset of electrodes, so that only the electrode of study is being measured, and pressure is ensured by vacuum. Although their use in lab‐scale instruments has been reported, [20] they are usually dedicated to synchrotron experiments as the absorption of the pouch envelop can be an issue in the Bragg‐Brentano geometry. In addition, the offset geometry may induce over‐polarization in the electrochemical response.…”

Probing and Resolving the Heterogeneous Degradation of Nickel‐Rich Layered Oxide Cathodes across Multi‐Length Scales

“…The careful analysis of operando XRD results elucidated the development of micro‐strain in NMC622 and the nonuniform phase transitions in the degraded Li/NMC622 batteries. [ 73 ] The source of capacity fade and disintegration of NMC622 particles was attributed to the evolution of lattice parameters and micro‐strain depending on the upper cut‐off voltage.…”

Cracking predictions of lithium-ion battery electrodes by X-ray computed tomography and modelling

“…The formulations for the cathode made in this work are generic formulations that are extensively employed [50,51]. The cathode slurry consists of LiNi0.6Mn0.2Co0.2O2 (NMC622, BASF), polyvinylidene fluoride (PVDF, Solvay) and C65 (Imerys) powders, which were pre-dried at 120 °C in a vacuum oven over 12 h to remove any moisture.…”

“…Furthermore, it is generally accepted that an increased rate of charge or discharge, i.e. rate of lithiation/delithiation, reduces the lifespan of the battery [50]. Discharge or charge of cells at a high rate increases the extent of concentration gradients within the particles, resulting in higher levels of particle stresses.…”