Degradation in Ni-Rich LiNi_1–x–yMn_xCo_yO₂/Graphite Batteries: Impact of Charge Voltage and Ni Content

Abstract

Ni-rich LiNi 1−x−y Mn x Co y O 2 (NMC) materials are attractive as cathodes for Li-ion batteries due to their high energy density and low Co content. However, these materials may display poor electrochemical reversibility relating to structural and interfacial instabilities. The influence of Ni content and level of delithiation during charge on degradation mechanisms and relevance to electrochemical cycling behavior are probed for LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathode materials in a full cell configuration under two upper voltage limits (4.1 and 4.3 V). The combined use of dQ/dV analysis of electrochemical voltage profiles, operando XRD, and postcycling scanning electron microscopy (SEM) measurements indicates that a major contributor to capacity fade is the large anisotropic volume change from an H2 ⇄ H3 phase transition and associated mechanical degradation particularly for NMC811. Notably, transition metal dissolution and deposition on the negative electrode are found to correlate with the structural changes occurring in the cathode under high voltage charge. X-ray photoelectron spectroscopy (XPS) analyses of the cycled cathodes reveal a more organic-rich cathode−electrolyte interphase (CEI) when cycling to 4.3 V with lower relative amounts of LiF. Surface reconstruction is not a significant factor under these cycling conditions as determined by soft X-ray absorption spectroscopy (sXAS) analysis. The results emphasize the opportunity to match the electrochemical test parameters with the specific NMC active material to mitigate degradation and extend cycle life.

Improving the Performance of LiNi0.9Co0.05Mn0.05O2 via Atomic Layer Deposition of ZnxOy Coating

“…Additionally, parasitic reactions such as electrolyte decomposition at higher de-lithiated states of NMC90 consequently leads to severe irreversible capacity loss in the case of the formation of uncontrolled CEI in the pristine full cells. [47][48][49] This phenomenon of lithium-ion depletion increases the voltage required for delithiation in the untreated NMC90, whereas in the Zn x O y @NMC90 full cells, this behavior is partially mitigated because the ACEI layer prevents electrolyte decomposition on the electrode at higher voltages, thereby improving the lithium-ion inventory over long term cycling. [50][51][52][53] Figure 4a depicts the overlayed voltage profiles of the formation cycles for pristine NMC90 and Zn x O y @NMC90 full cells.…”

Paradoxical role of structural degradation of nickel-rich layered oxides in capacity retention upon storage of lithium-ion batteries

“…Indeed, storage-induced capacity fading shows a peculiar dependence on SoCs that is incomprehensible in the conventional viewpoint of cycle aging mechanisms. For instance, while cycling up to high SoC regions generally accelerates cycle aging, 26–29 storage at high SoCs (90–100%) shows superior capacity retention to that at intermediate SoCs (60–80%). 30–34 Although LIBs in EVs often stay at high SoCs, the degradation of electrode materials during storage and its impact on battery performance has been largely overlooked because performance degradation is not apparent with well-maintained capacity after storage at high SoCs.…”

“…Moreover, NCMA8/graphite cells showed comparable or slightly better capacity retention during storage than NCM6/graphite cells, although the CAM with higher Ni composition is generally considered to be more unstable at high SoCs. 28,40 O perando X-ray diffraction (XRD) of aged full-cells combined with advanced post-mortem analyses revealed the origin of the counterintuitive capacity fading upon storage. Although conventional degradation phenomena—Li/Ni cation mixing, SRL formation, and the presence of fatigued phases—also occurred upon SoC70 storage, non-invasive operando XRD directly captured the severe electrode slippage in SoC70-stored cells, indicating that capacity fading upon storage at intermediate SoCs is predominantly due to Li inventory loss.…”