Moritz Hirsbrunner

Zn-substituted Na_0.75Mn_0.68Ni_0.25Zn_0.07O₂ has been synthesized in P3, P2, and composite P2/P3 structures and compared with unsubstituted Na_0.7Mn_0.75Ni_0.25O₂ analogues as positive electrodes for sodium-ion batteries. The synthesis temperature was shown to provide a means of controlling the phase ratio of P2 and P3 phases. Powder diffraction measurements, high-resolution transmission electron microscopy (TEM), and selected area electron diffraction (SAED) revealed that Zn substitution enhanced the ordering of the transition metal (TM) layers. Electrochemical studies combined with XAS measurements showed that after Zn substitution, Ni activity was enhanced, while the irreversible activity of O and Mn was suppressed. Structural transformations were suppressed, and the reversibility of Zn-substituted samples on cycling was improved. Among the Zn-substituted samples, Zn–P2/P3 delivers the best electrochemical performance with an initial capacity of 121 mAh g^–1 at a rate of 25 mA g^–1 and 90% capacity retention after 100 cycles in half-cells. This work reveals the intrinsic correlation among cation doping, synthesis conditions, and crystal phase compositions but also provides a reliable strategy for designing high-stability composite layered cathode materials for sodium-ion batteries.

Oxygen redox (OR) in Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LRNMC) and Na_0.67Mg_0.28Mn_0.72O₂ (NMMO) has been associated with the formation of embedded molecular O₂ due to the appearance of their distinctive features in RIXS, while it is unclear whether OR also affects the oxygen left in the lattice. Here we use high-resolution oxygen K-edge resonant inelastic X-ray scattering (RIXS) at threshold excitation (527.5 – 529.5 eV) revealing lattice responses due to OR. We find that both cathodes show pronounced multiphonon progressions, which are either altered or activated upon charging. The first progression, with a fundamental energy loss ∼67-74 meV, matches the A_1g lattice-oxygen mode observed by Raman spectroscopy, confirming its bulk origin. The second progression that exhibits a fundamental loss of ∼98 meV appears only at the highest state of charge and is resonant with the new pre-edge states at 527.5 eV. The latter mode emerges concurrently with the characteristic trapped-O₂ RIXS signal and is strongly coupled to an occupied electronic band near the Fermi level, indicating that OR may not only contribute to the formation of molecular O₂, but also perturbs lattice oxygen states, likely via polaron-forming oxidized lattice oxygen.

Li⁺, when added in small amounts as a dopant to Na–Fe–Mn–O layered oxide cathodes, disorders the transition-metal layer and stabilizes the P2-phase. Li⁺ becomes dynamic at a high state of charge and completely suppresses the P to O phase transition.

Vibrationally-resolved resonant inelastic X-ray scattering (VR-RIXS) at the O K-edge is emerging as a powerful tool for identifying embedded molecules in lithium-ion battery cathodes. Here, we investigate two known oxygen redox-active cathode materials: the commercial Li_xNi_0.90Co_0.05Al_0.05O₂ (NCA) used in electric vehicles and the high-capacity cathode material Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LRNMC) for next-generation Li-ion batteries. We report the detection of a novel vibrational RIXS signature for Li-ion battery cathodes appearing in the O K pre-peak above 533 eV that we attribute to OH-groups. We discuss likely locations and pathways for OH-group formation and accumulation throughout the active cathode material. Initial-cycle behaviour for LRNMC shows that OH-signal strength correlates with the cathodes state of charge, though reversibility is incomplete. The OH-group RIXS signal strength in long-term cycled NCA is retained. Thus, VR-RIXS offers a path for gaining new insights to oxygen reactions in battery materials.

The influence of substitution of V with Fe or Ti in the Li-rich battery cathode material Li₂VO₂F was investigated utilizing XAS and RIXS to obtain bulk sensitive information about electronic structure changes.

The rising demand for high-performance lithium-ion batteries, pivotal to electric transportation, hinges on key materials like the Ni-rich layered oxide LiNi_xCo_yAl_zO₂ (NCA) used in cathodes. The present study investigates the redox mechanisms, with particular focus on the role of oxygen in commercial NCA electrodes, both fresh and aged under various conditions (aged cells have performed >900 cycles until a cathode capacity retention of ∼80%). Our findings reveal that oxygen participates in charge compensation during NCA delithiation, both through changes in transition metal (TM)–O bond hybridization and formation of partially reversible O₂, the latter occurs already below 3.8 V vs. Li/Li⁺. Aged NCA material undergoes more significant changes in TM–O bond hybridization when cycling above 50% SoC, while reversible O₂ formation is maintained. Nickel is found to be redox active throughout the entire delithiation and shows a more classical oxidation state change during cycling with smaller changes in the Ni–O hybridization. By contrast, Co redox activity relies on a stronger change in Co–O hybridization, with only smaller Co oxidation state changes. The Ni–O bond displays an almost twice as large change in its bond length on cycling as the Co–O bond. The Ni–O₆ octahedra are similar in size to the Co–O₆ octahedra in the delithiated state, but are larger in the lithiated state, a size difference that increases with battery ageing. These contrasting redox activities are reflected directly in structural changes. The NCA material exhibits the formation of nanopores upon ageing, and a possible connection to oxygen redox activity is discussed. The difference in interaction of Ni and Co with oxygen provides a key understanding of the mechanism and the electrochemical instability of Ni-rich layered transition metal oxide electrodes. Our research specifically highlights the significance of the role of oxygen in the electrochemical performance of electric-vehicle-grade NCA electrodes, offering important insights for the creation of next-generation long-lived lithium-ion batteries.

Sodium layered oxides which display oxygen anion redox behaviour are considered promising positive electrodes for sodium-ion batteries because they offer increased specific capacities. However, they suffer from irreversible structural changes resulting in significant capacity loss and limited oxygen redox reversibility. Here the effect of Cu substitution on the electrochemical performance of P3-type sodium manganese oxide is examined by evaluating the structural and electronic structural evolution upon cycling, supported by density functional theory (DFT) calculations. Over the voltage range 1.8–3.8 V vs. Na/Na⁺, where the redox reactions of the transition metal ions contribute entirely towards the charge compensation mechanism, stable cycling performance is maintained, showing a capacity retention of 90% of the initial discharge capacity of 166 mA h g⁻¹ after 40 cycles at 10 mA g⁻¹. Over an extended voltage range of 1.8–4.3 V vs. Na/Na⁺, oxygen anion redox is invoked, with a voltage hysteresis of 110 mV and a greater initial discharge capacity of 195 mA h g⁻¹ at 10 mA g⁻¹ is reached. Ex-situ powder x-ray diffraction patterns reveal distortion of the P3 structure to P′3 after charge to 4.3 V, and then transformation to O′3 upon discharge to 1.8 V, which contributes towards the capacity fade observed between the voltage range 1.8–4.3 V. DFT with projected density of states calculations reveal a strong covalency between the copper and oxygen atoms which facilitate both the cationic and anionic redox reactions in P3-type Na_0.67Mn_0.9Cu_0.1O₂.

In this work, structural and mechanical properties of hydrogen-charged metallic glass are studied to evaluate the effect of hydrogen on early plasticity. Hydrogen is introduced into samples of a Zr-based (Vit 105) metallic glass using electrochemical charging. Nanoindentation tests reveal a clear increase in modulus and hardness as well as in the load of the first pop-in with increasing hydrogen content. At the same time, the probability of a pop-in occurring decreases, indicating that hydrogen hinders the onset of plastic instabilities while allowing local homogeneous deformation. The hydrogen-induced stiffening and hardening is rationalized by hydrogen stabilization of shear transformation zones (STZs) in the amorphous structure, while the improved ductility is attributed to the change in the spatial correlation of the STZs.