Pardeep Kumar Thakur

A correlation between depth-resolved defect chemistry and operando electrochemical response is established for compositionally graded gadolinia-doped ceria (GDC)/yttria-stabilized zirconia (YSZ) thin film electrodes.

An investigation of the electronic structure of K ₂ SiF ₆ based on GW theory and photoelectron spectroscopy reveals that Auger–Meitner decay of K 3p holes is energetically forbidden in this material.

MAX phases are an extremely versatile family of layered compounds that usually consist of an early to-mid transition metal (M-element), a main group element (mainly groups 13-15) or late transition metal (A-element) and carbon and/or nitrogen (X-element). It is therefore not too surprising that in addition to the roughly 70 compounds with 211 stoichiometry, there exist many solid solutions with mixed elements on the M- and A-site, respectively. Much less common are solid solution phases with mixed elements on both M- and A-site simultaneously (double-site solid solutions), as well as solid solutions on the X-site (carbonitride MAX phases). Challenging these restrictions in the chemical composition space, we present here for the first time (V_0.2Cr_0.8)₂(Ga_0.5Ge_0.5)(C_0.6N_0.4) as a new carbonitride member of the MAX phase family, containing solid solutions on all three lattice sites simultaneously. This triple-site solid solution MAX phase is synthesized by high-temperature solid-state methods, and we demonstrate that it is possible to use two different nitrogen-containing precursors (VN and Cr₂N), respectively. Structure, morphology and chemical composition are characterized by X-ray powder diffraction (XRD), electron microscopy (SEM/TEM), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (HAXPES).

MAX phases are a class of compounds known for having both metallic and ceramic properties, such as good electrical conductivity, oxidation resistance, and high hardness. The bulk of the research on their properties focuses on those with titanium at the M-site and metals from groups 13 to 15, e.g., aluminum, at the A-site. Here, we expand the properties repertoire with new arsenic-containing A-site solid solutions, V₂(As_1-xP_x)C and V₂(As_1-xGe_x)C. The structure and elemental composition of the solid solutions were resolved with powder X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and hard X-ray photoelectron spectroscopy. The electrical resistivity measurements show that both full series are metallic with the parent phases being the most conductive. Thermal analyses show V₂GeC is the most oxidation resistant and V₂AsC is the least, while substitutions decrease thermal stability, as oxidation resistance of the intermediate compositions shifts toward that of V₂AsC. The V₂(As_1-xGe_x)C series shows little variation in hardness across compositions, while the incorporation of phosphorus noticeably increases hardness.

Photocatalytic water splitting offers a scalable and potentially low-cost route for the production of renewable hydrogen. Recently, a state-of-the-art system based on flux-mediated Al³⁺-doped SrTiO₃, modified with Rh-Cr-based proton reduction and CoOOH water oxidation cocatalysts, achieved apparent quantum yields for unassisted water splitting of up to 93%. Herein, we focus on the role of Al³⁺ doping and Rh-Cr-based cocatalyst deposition on the accumulation and reaction dynamics of the long-lived holes required to drive water oxidation. We employ in situ and operando photoinduced absorption spectroscopy (PIAS) under water splitting conditions complemented by X-ray photoelectron spectroscopy (XPS). XPS data indicate that Al³⁺ doping suppresses surface Ti³⁺ defect states, coinciding with a 5-fold increase in the accumulation of long-lived SrTiO₃ holes observed by PIAS. Rh-Cr-based cocatalyst addition is observed to further enhance the yield and lifetime (s-10 s time scales) of these photoaccumulated holes, assigned to the efficient electron extraction by this cocatalyst. These photoaccumulated holes exhibit fast (ca. 1 s) and slow (ca. 10 s) decay phases. While the dominant fast phase is assigned to the desired water oxidation reaction, the slow phase is assigned to deeply trapped unreactive holes; the yield of these unreactive holes is suppressed by facet-selective photodeposition of cocatalysts or preillumination. These results provide key insights into how Al:SrTiO₃ functionalized by Rh-Cr-based cocatalysts accumulates oxidizing holes with lifetimes long enough to drive the kinetically challenging water oxidation reaction, thus achieving remarkably high quantum efficiencies for overall water splitting, insights which can be applied in the design of future photocatalytic materials.

LiNiO2 (LNO) cathodes are regarded as promising materials for high-energy-density applications owing to their high practical capacity and competitive cost. However, over the course of cycling, they experience rapid capacity decay when cycled above 4.2 V (vs Li/Li+) through bulk and surface mediated instabilities. Surface engineering via ultrathin aluminium oxide (AlxOy) coatings has the potential to provide an effective approach to inhibit the surface degradation pathways in LNO, but their long-term benefits remain unclear. Further, it is not well understood whether AlxOy coatings can provide any surface-to-bulk stabilisation effects. Here, we employ an ultrathin and conformal film of AlxOy on polycrystalline LNO powder via a scalable powder atomic layer deposition (ALD) method. Electrochemical performance was evaluated in high-mass-loading pouch cells (LNO vs. graphite) in a voltage window of 3.0–4.1 V using a constant-current constant-voltage (CCCV) protocol. Cells containing the ALD-coated LNO cathodes demonstrated significantly improved capacity retention (91.2%) over 100 cycles at a C/3 rate. In-house operando X-ray diffraction data on pouch cells demonstrated that after aging, ALD-coated LNO showed less kinetically hindered delithiation as compared to the uncoated LNO, confirming a surface-to-bulk effect. Postmortem surface-sensitive spectroscopy showed both the HF scavenging role of the AlxOy coatings and suppression of dense surface reconstructions in coated LNO. These findings highlight the efficacy of our scalable surface engineering approach in mitigating interfacial degradation, reducing impedance growth and improving bulk delithiation kinetics, which enhances the long-term electrochemical performance of these ultra-Ni-rich cathode materials.

A comprehensive understanding of the solid electrolyte interphase (SEI) is crucial for ensuring long‐term battery stability. This is particularly pertinent in sodium‐ion batteries (NIBs), where the SEI remains poorly understood, and investigations are typically undertaken in half‐cell configurations with sodium metal as the counter electrode. Na metal is known to be highly reactive with common carbonate‐based electrolytes; nevertheless, its effects on SEI formation at the working electrode are largely unexplored. This work investigates the evolution of the SEI in NIBs during cycling, with an emphasis on the consequences of using a sodium metal counter electrode. Advanced analytical techniques, including hard X‐ray photoelectron spectroscopy (HAXPES) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), are used to obtain depth‐resolved insights into the chemical composition and structural changes of the SEI on hard carbon anodes during cycling. The findings demonstrate that the cell configuration has a significant impact on SEI evolution and, by extension, battery performance. These findings suggest that full‐cell studies are necessary to better simulate practical operating conditions, challenging traditional half‐cell experiments.

Single‐crystalline LiNiO ₂ (SC‐LNO), a high‐energy‐density Li‐ion cathode material, suffers from poor long‐term electrochemical performance when cycled above 4.2 V (vs Li ⁺ /Li). In this study, this degradation is evaluated using SC‐LNO–graphite pouch cells electrochemically aged within a stressful voltage window (2.5–4.4 V) using a constant‐current constant‐voltage (CC‐CV) protocol. Notable capacity fade is observed after one hundred cycles at C/3 rate, in addition to an increase in the overall electrochemical cell impedance. Operando X‐ray diffraction data reveal that, despite no significant long‐range bulk structural changes, (de‐)lithiation of the aged SC‐LNO becomes kinetically hindered after 100 cycles. Aging‐induced changes in the short‐range structure and charge compensation are evaluated through a multi‐model quantitative analysis of the operando X‐ray absorption spectroscopy data. While the electrochemical aging does not result in particle cracking, soft X‐ray absorption spectroscopy data revealed the reconstruction of the cathode surface to a dense rock salt‐like layer after long‐term cycling, which acts as a kinetic trap for Li ⁺ diffusion. Therefore, even under stressful conditions, it is the surface reconstruction that dominates the overall cathode degradation by reducing the Li ⁺ mobility and leading to the capacity fade. Cathode surface engineering will therefore be key to improving the long‐term electrochemical performance of SC‐LNO cathodes.