Transition Metal Dissolution Mechanisms and Impacts on Electronic Conductivity in Composite LiNi_0.5Mn_1.5O₄ Cathode Films

Abstract

The high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) spinel cathode material offers high energy density storage capabilities without the use of costly Co that is prevalent in other Li-ion battery chemistries (e.g., LiNi x Mn y Co z O 2 (NMC)). Unfortunately, LNMO-containing batteries suffer from poor cycling performance because of the intrinsically coupled processes of electrolyte oxidation and transition metal dissolution that occurs at high voltage. In this work, we use operando electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopies to demonstrate that transition metal dissolution in LNMO is tightly coupled to HF formation (and thus, electrolyte oxidation reactions as detected with operando and in situ solution NMR), indicative of an acid-driven disproportionation reaction that occurs during delithiation (i.e., battery charging). Leveraging the temporal resolution (s-min) of magnetic resonance, we find that the LNMO particles accelerate the rate of LiPF 6 decomposition and subsequent Mn 2+ dissolution, possibly due to the acidic nature of terminal Mn-OH groups. X-ray photoemission electron microscopy (XPEEM) provides surfacesensitive and localized X-ray absorption spectroscopy (XAS) measurements, in addition to X-ray photoelectron spectroscopy (XPS), that indicate disproportionation is enabled by surface reconstruction upon charging, which leads to surface Mn 3+ sites on the LNMO particle surface that can disproportionate into Mn 2+ (dissolved) and Mn 4+ (s) . During discharge of the battery, we observe high quantities of metal fluorides (in particular, MnF 2 ) in the cathode electrolyte interphase (CEI) on LNMO as well as the conductive carbon additives in the composite. Electronic conductivity measurements indicate that the MnF 2 decreases film conductivity by threefold compared to LiF, suggesting that this CEI component may impede both the ionic and electronic properties of the cathode. Ultimately, to prevent transition metal dissolution and the associated side reactions in spinel-type cathodes (particularly those that operate at high voltages like LNMO), the use of electrolytes that offer improved anodic stability and prevent acid byproducts will likely be necessary.

First-Principles Examination of Multiple Criteria of Organic Solvent Oxidative Stability in Batteries

“…60 The commonly used counterion PF 6 − has been predicted to react indirectly with the oxide via solvent decomposition, 61 H 2 O-mediated reactions, 62 and even more readily with CEI components 63 and electrode surfaces. 64 Prior to those work, it has been predicted that high Li content, e.g., Li 2 PF 6 + complexes, makes LiF formation and P−F bond breaking favorable on Li x NiO 2 surfaces 56 and that FSI also decomposes on highly lithiated Li x NiO 2 cathode surfaces via breaking of a S−F bond. 55 These work suggest that high Li content should also be considered for anion bond-breaking tendencies.…”

Full-Dimensional Analysis of Electrolyte Decomposition on the Cathode–Electrolyte Interface: Deciphering Electrolyte Degradation Mechanisms on the High-Ni LiNixMnyCo1–x–yO2 Cathode–Electrolyte Interface during the Extreme Fast Charging Process

“…A larger degree of salt decomposition was observed to be associated with increasing EC dehydrogenation on charged NMC with increasing Ni. This is due to the higher reactivity of oxygen in oxides, resulting in the dehydrogenation of more solvent molecules. , The dehydrogenated carbonate intermediates can give rise to protic species, which have the potential to react with LiPF 6 , forming oxidized products, such as PF 3 O and Li x PO y F z . ,, There are studies indicating that LNMO particles can also accelerate the rate of LiPF 6 decomposition . Therefore, further research is needed to investigate whether NMC materials themselves have an impact on salt decomposition in the absence of protic species.…”

“…The dehydrogenation of solvents could be catalyzed by Ni to form more micro-molecule organics dissolving in the electrolyte; meanwhile, LiPF 6 is attacked by protic species to generate metal fluorides and resistive LiF deposited on the cathode surface. The accelerated capacity loss of cathode with higher Ni content is probably because of much more electrolyte decomposition and severe transition-metal dissolution during the charging/discharging process. , …”

“…9,11,23 There are studies indicating that LNMO particles can also accelerate the rate of LiPF 6 decomposition. 29 Therefore, further research is needed to investigate whether NMC materials themselves have an impact on salt decomposition in the absence of protic species.…”

Probing the Formation of Cathode-Electrolyte Interface on Lithium Iron Phosphate Cathodes via In Operando Mechanical Measurements

“…14,15 CEI layer should also mitigate undesired dissolution of transition metals from the cathode electrodes. 16,17 Furthermore, mechanical properties of SEI/CEI layers should be elastic enough to accommodate large volumetric changes in the electrode particles upon Li insertion and removal. 18,19 It is crucial to understand the formation mechanism of SEI / CEI layers and their impact on the performance of Li-ion batteries.…”