Mario El Kazzi

Confinement and integration of magnetic impurities in silicon

Integration of magnetic impurities into semiconductor materials is an essential ingredient for the development of spintronic devices such as dilute magnetic semiconductors. While successful growth of ferromagnetic semiconductors was reported for III-V and II-VI compounds, efforts to build devices with silicon technology were hampered by segregation and clustering of magnetic impurities such as manganese (Mn). Here, we report on a surface-based integration of Mn atoms into a silicon host. Control of Mn diffusion and low-temperature silicon epitaxy lead to confined Mn δ-layers with low interface trap densities, potentially opening the door for a new class of spintronic devices in silicon.

Surface and morphological investigation of the electrode/electrolyte properties in an all-solid-state battery using a Li2S-P2S5 solid electrolyte

All-solid-state lithium-ion batteries represent a promising battery technology thanks to the replacement of the volatile and flammable state-of-the-art liquid electrolyte by a solid electrolyte. Despite the recent progress in the synthesis of sulfide based solid electrolyte with high ionic conductivity, little is known about the interface reactivity of the solid electrolyte with electrode materials. In this study, we synthesized and characterized an amorphous solid electrolyte with the nominal composition (Li2S)3(P2S5). We assessed the feasibility of using this electrolyte at the laboratory scale, and we discuss the potential challenges that govern its electrochemical performance. Galvanostatic cycling and rate performance measurements were conducted using lithium titanium oxide (Li4Ti5O12) as the negative electrode material. The electrochemical measurements were performed using two different counter electrodes, namely Li metal and an InLix alloy. The alloy counter electrode suppressed the formation of lithium dendrites, resulting in increased cycling stability and cell safety. Post mortem X-ray photoemission spectroscopy measurements reveal the reactivity of the solid electrolyte Li3PS4 with the Li4Ti5O12, lithium metal, and InLix alloy.

Solving the puzzle of Li4Ti5O12 surface reactivity in aprotic electrolytes in Li-ion batteries by nanoscale XPEEM spectromicroscopy

The safe operation and long life-span of Li-ion batteries rely on a stable electrode–electrolyte interface. However, determining the thermodynamic stability window of such an interface is challenging due to the different (electro)chemical reactivities of the electrode components. Here we demonstrate a holistic experimental and theoretical approach to elucidate the nature and origin of the multiple reactions at such complex interfaces, which remain a major obstacle for the development of next generation Li-ion batteries. We applied X-ray photoemission electron microscopy (XPEEM) on Li4Ti5O12 electrodes to solve, with nanoscale resolution, its controversial surface reactivity in carbonate-based electrolytes. Local X-ray absorption spectroscopy (XAS) is performed upon cycling on individual carbon and Li4Ti5O12 particles, while maintaining their working environment, as in the commercial-like electrode composition. Despite the theoretical prediction of a stable electrochemical interface, we find that electrolyte reduction occurs solely on Li4Ti5O12 particles during lithiation at 1.55 V vs. Li+/Li. With the support of density functional theory (DFT) calculations, we show that this behavior is caused by the solvents adsorbed on the Li4Ti5O12 outer planes driven by the Li-ion insertion. The DFT results indicate that Li-ion insertion leads to a shift of the LUMO of the adsorbed solvents to energies below the Fermi level position of lithiated Li7Ti5O12 and thus to chemical instability. Simultaneously, at the same potential, we detect a competing reaction that leads to the partial dissolution of the electrolyte by-product layer. Such a finding has to be considered for other insertion materials and needs to be addressed in surface engineering to mitigate side reactions and design safe and long-lasting batteries.

Toward high-performance Li(NixCoyMnz)O2 cathodes: facile fabrication of an artificial polymeric interphase using functional polyacrylates

Targeting the long-term performance retention of high-energy-density batteries, a facile yet effective surface coating approach for high-voltage composite cathodes was developed. Ultra-thin polyacrylate (PAA) surface coatings with an estimated thickness of less than 5 nm were integrated onto Li-rich Li(NixCoyMnz)O2 cathodes through a solution-casting method. Obvious improvements of specific charge – around 20 mA h g−1 at the 1st cycle and up to 50 mA h g−1 at the 100th cycle – as well as enhanced capacity retention were achieved post the integration of a lithium polyacrylate (LiPAA) surface coating. A fundamental understanding of the role of PAA coatings was systematically achieved by X-ray photoelectron spectroscopy (XPS), electron microscopy (TEM, SEM/EDX), Fourier transform infrared spectroscopy (FTIR) and online electrochemical mass spectrometry (OEMS). Physically adhesive coatings on the active sites of cathodes were observed, which might contribute to impeded surface reconstruction of active materials during cycling. Furthermore, in OEMS analysis reduced CO2 evolution was detected from coated electrodes, indicating that the LiPAA coating could interrupt electrolyte decomposition reactions and enhance the cathode/electrolyte interfacial stability at high anodic potentials. This functional polymer coating acts as a stable artificial polymeric interphase to mitigate electrode and electrolyte decomposition during long-term cycling.

SnO2 Model Electrode Cycled in Li-Ion Battery Reveals the Formation of Li2SnO3 and Li8SnO6 Phases through Conversion Reactions

SnO2 is an attractive negative electrode for Li-ion battery owing to its high specific charge compared to commercial graphite. However, the various intermediate conversion and alloy reactions taking place during lithiation/delithiation, as well as the electrolyte stability, have not been fully elucidated, and many ambiguities remain. An amorphous SnO2 thin film was investigated for use as a model electrode by a combination of postmortem X-ray photoelectron spectroscopy supported by density functional theory calculations and scanning electron microscopy to shed light on these different processes. The early stages of lithiation reveal the presence of multiple overlapping reactions leading to the formation of Li2SnO3 and Sn0 phases between 2 and 0.8 V vs Li+/Li. Between 0.45 V and 5 mV vs Li+/Li Li8SnO6, Li2O and Li xSn phases are formed. Electrolyte reduction occurs simultaneously in two steps, at 1.4 and 1 V vs Li+/Li, corresponding to the decomposition of the LiPF6 salt and ethylene carbonate/dimethyl carbonate solvents, respectively. Most of the reactions during delithiation are reversible up to 1.5 V vs Li+/Li, with the reappearance of Sn0 accompanied by the decomposition of Li2O. Above 1.5 V vs Li+/Li, Sn0 is partially reoxidized to SnO x. This process tends to limit the conversion reactions in favor of the alloy reaction, as also confirmed by the long-term cycling samples.