Post Mortem and Operando XPEEM: a Surface-Sensitive Tool for Studying Single Particles in Li-Ion Battery Composite Electrodes

Converting the CHF3 Greenhouse Gas into Nanometer-Thick LiF Coating for High-Voltage Cathode Li-ion Batteries Materials

Solving the surface (electro-)chemical instability and the fading behavior of high voltage cathode materials cycled above 4.3 V vs Li+/Li remains a major challenge for the next generation of high energy density Li-ion batteries. Here, we present a facile, environmentally friendly, cost effective and scalable method to address this problem by uniformly fluorinating the surface of cathode materials with a mild fluorinating agent (CHF3) using a gas flow-type reactor. CHF3, well known as a potent greenhouse gas, is successfully transformed into a stable ~2 nm LiF homogenous layer by converting the adventitious Li2CO3 layer covering the surface of the vast majority of layered-oxide cathode materials. The fluorination mechanism and the interface stability of the LiF coating layer is systematically studied on LiNi0.8Co0.15Al0.05O2 using synchrotron surface spectroscopy techniques, operando XRD and TEM. In addition, we demonstrate improved electrochemical cycling performance of the LiF coated LiNi0.8Co0.15Al0.05O2 when cycled up to 4.5 V where the impedance and overpotential decrease by 30 % and 100 mV respectively after 100 cycles, with a capacity retention better than 94 % and improved rate performance at high current density. Furthermore, the universality of the fluorination approach is validated further on Ni-rich LiNi0.85Co0.1Mn0.05O2 cathode material cycled up to 4.3 and 4.8 V vs Li+/Li.

Nanoscale Surface and Bulk Electronic Properties of Ti3C2Tx MXene Unraveled by Multimodal X-Ray Spectromicroscopy

2D layered materials, such as transition metal carbides or nitrides, known as MXenes, offer an ideal platform to investigate charge transfer processes in confined environment, relevant for energy conversion and storage applications. Their rich surface chemistry plays an essential role in the pseudocapacitive behavior of MXenes. However, the local distribution of surface functional groups over single flakes and within few- or multilayered flakes remains unclear. In this work, scanning X-ray microscopy (SXM) is introduced with simultaneous transmission and electron yield detection, enabling multimodal nanoscale chemical imaging with bulk and surface sensitivity, respectively, of individual MXene flakes. The Ti chemical bonding environment is found to significantly vary between few-layered hydrofluoric acid-etched Ti3C2Tx MXenes and multilayered molten salt (MS)-etched Ti3C2Tx MXenes. Postmortem analysis of MS-etched Ti3C2Tx electrodes cycled in a Li-ion battery further illustrates that simultaneous bulk and surface chemical imaging using SXM offers a method well adapted to the characterization of the electrode-electrolyte interactions at the nanoscale.

Transition Metal Dissolution Mechanisms and Impacts on Electronic Conductivity in Composite LiNi0.5Mn1.5O4 Cathode Films

The high-voltage LiNi0.5Mn1.5O4 (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., LiNixMnyCozO2 (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 LiPF6 decomposition and subsequent Mn2+ dissolution, possibly due to the acidic nature of terminal Mn-OH groups. X-ray photoemission electron microscopy (XPEEM) provides surface-sensitive 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 Mn3+ sites on the LNMO particle surface that can disproportionate into Mn2+(dissolved) and Mn4+(s). During discharge of the battery, we observe high quantities of metal fluorides (in particular, MnF2) in the cathode electrolyte interphase (CEI) on LNMO as well as the conductive carbon additives in the composite. Electronic conductivity measurements indicate that the MnF2 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.

Operando resonant soft X-ray emission spectroscopy of LiMn2O4 cathode using an aqueous electrolyte solution

The Mn 3d electronic-structure change of LiMn2O4 cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES). The Mn L3 RXES...

Reactivity and Potential Profile across the Electrochemical LiCoO2-Li3PS4 Interface Probed by Operando X-ray Photoelectron Spectroscopy

All-solid-state lithium batteries are a promising alternative for next-generation safe energy storage devices, provided that parasitic side reactions and the resulting hindrances in ionic transport at the electrolyte-electrode interface can be overcome. Motivated by the need for a fundamental understanding of such an interface, we present here real-time measurements of the (electro-)chemical reactivity and local surface potential at the electrified interface (Li₂S)₃-P₂S₅ (LPS) and LiCoO₂ (LCO) using operando X-ray photoelectron spectroscopy (XPS) supplemented by X-ray photoemission electron microscopy (XPEEM). We identify three main degradation mechanisms: (i) reactivity at open circuit potential leading to the formation of reduced Co in the +2 oxidation state at the LCO surface, detected in the Co L-edge, which is further increased upon cycling, (ii) onset of electrochemical oxidation of the LPS at 2.3 V vs InLi_x detected in the S 2p and P 2p core levels, and (iii) Co-ion diffusion into the LPS forming CoS_x species at 3.3 V observed in both S 2p and Co 2p core levels. Concurrently, a local surface overpotential of 0.9 V caused by a negative localized charge layer is detected at the LPS-LCO interface. Furthermore, in agreement with previous theoretical results, the presence of a sharp potential drop at the interface between active materials and solid electrolyte is demonstrated in all-solid-state batteries.

Unveiling the Complex Redox Reactions of SnO2 in Li-Ion Batteries Using Operando X-ray Photoelectron Spectroscopy and In Situ X-ray Absorption Spectroscopy

We experimentally determine the redox reactions during (de-)lithiation of the SnO₂ working electrode cycled in (Li₂S)₃-P₂S₅ solid electrolyte by combining operando X-ray photoelectron spectroscopy and in situ X-ray absorption spectroscopy. Specifically, we have accurately determined the composition changes in the SnO₂ working electrode upon cycling and identified the onset voltage formation of the various phases. Starting from the open-circuit potential, we find that, on lithiation, the Sn M-edge absorption spectra reveal unequivocally the formation of SnO_x (x ≤ 1) and Li₂SnO₃ already at a potential of 1.6 V vs Li⁺/Li, while Sn 3d/Sn 4d, O 1s, and Li 1s core-level spectra show the formation of Sn⁰ and Li₂O along the first potential plateau at 0.8 V vs Li⁺/Li and of Li₈SnO₆ at lower potentials. Below 0.6 V vs Li⁺/Li, an alloying reaction takes place until the end of the lithiation process at 0.05 V vs Li⁺/Li, as shown by the formation of Li_xSn. During delithiation, both the conversion and alloying reactions are found to be partially reversible, starting by the re-formation of Sn⁰ at 0.3 V vs Li⁺/Li and followed by the re-formation of Li₈SnO₆ and SnO_x above 0.5 V vs Li⁺/Li. The conversion and alloying reactions are found to overlap during both lithiation and delithiation. Finally, we validate the theoretical prediction for the SnO₂ conversion and alloy (de-)lithiation reactions and clarify the open questions about their reaction mechanism.

Tunable deep ultraviolet laser based near ambient pressure photoemission electron microscope for surface imaging in the millibar regime

A newly developed instrument comprising a near ambient pressure (NAP) photoemission electron microscope (PEEM) and a tunable deep ultraviolet (DUV) laser source is described. This NAP-PEEM instrument enables dynamic imaging of solid surfaces in gases at pressures up to 1 mbar. A diode laser (976 nm) can illuminate a sample from the backside for in situ heating in gases up to 1200 K in minutes. The DUV laser with a tunable wavelength between 175 nm and 210 nm is perpendicularly incident onto the sample surface for PEEM imaging of a wide spectrum of solids with different surface work functions. Using this setup, we have first demonstrated spatiotemporal oscillation patterns of CO oxidation reaction on Pt(110) from high vacuum to NAPs and gas-induced restructuring of metal nanostructures in millibar gases. The new facility promises important applications in heterogeneous catalysis, electrochemical devices, and other surface processes under nearly working conditions.

Multi-length-scale x-ray spectroscopies for determination of surface reactivity at high voltages of LiNi0.8Co0.15Al0.05O2 vs Li4Ti5O12

The surface evolution of LiNi_0.8Co_0.15Al_0.05O₂ (NCA) and Li₄Ti₅O₁₂ (LTO) electrodes cycled in a carbonate-based electrolyte was systematically investigated using the high lateral resolution and surface sensitivity of x-ray photoemission electron microscopy combined with x-ray absorption spectroscopy and x-ray photoelectron spectroscopy. On the cathode, we attest that the surface of the pristine particles is composed of adventitious Li₂CO₃ together with reduced Ni and Co in a +2 oxidation state, which is directly responsible for the overpotential observed during the first de-lithiation. This layer decomposes at 3.8 V vs Li⁺/Li, leaving behind a fresh surface with Ni and Co in a +3 oxidation state. The charge compensation upon Li⁺ extraction takes place above 4.0 V and is assigned to the oxidation of both Ni and oxygen, while Co remains in a +3 oxidation state during the whole redox process. We also identified the formation of an inactive surface layer already at 4.3 V, rich in reduced Ni and depleted in oxygen. However, at 4.9 V, NiO-like species are detected accompanied with reduced Co. Despite the highly oxidative potential, the surface of the cathode after long cycling is free of oxidized solvent byproducts but contains traces of LiPF₆ byproducts (LiF and PO_xF_y). On the LTO counter electrode, transition metals are detected only after long cycling vs NCA to 4.9 V as well as PVdF and LiPF₆ byproducts originating from the cathode. Finally, harvested cycled electrodes prove that the influence of the crosstalk on the electrochemical performance of LTO is limited.