Serena Ann Corr

Describing lithium-based battery positive electrodes based on different transition metal or oxygen-redox regimes can cause confusion in understanding metal–ligand hybridization, oxygen dimerization and degradation processes. Therefore, it is urgent to investigate the electronic structure of these materials and identify the role each cation and anion has in charge compensation at the subnanoscale. Here, using X-ray resonance photoemission spectroscopy, single-impurity Anderson models, spectral simulations and theoretical calculations, we examine redox mechanisms in positive electrodes during lithium-based battery operation. This approach reconciles the redox description of two positive electrode active materials—LiMn _0.6 Fe _0.4 PO ₄ and LiNiO ₂ —in terms of varying degrees of charge transfer using the Zaanen–Sawatzky–Allen framework. In LiMn _0.6 Fe _0.4 PO ₄ , the lack of strong hybridization indicates that the capacity results from the depopulation of metal 3 d states, that is, conventional metal redox. However, in cells with LiNiO ₂ -based positive electrodes, negative charge transfer dominates, and redox occurs through the formation and elimination of ligand-hole states. These results clarify the role of oxygen in Ni-rich systems and provide a framework to explain how the charge/discharge capacities are linked to oxygen-dominated states in highly covalent systems, without the need to consider oxygen dimerization.

Reliably benchmarking the performance of new cathode materials is an ongoing challenge in the development of high-capacity lithium-ion batteries. {Small variations in the formulation or processing of a cathode can severely affect the performance of the resulting battery cell.} Understanding and controlling interactions between the binder and other slurry components will enable faster, more rationalised battery cell optimisation. For the first time, this work systematically varies the viscosity of polyvinylidene fluoride (PVDF) binder solutions across a decade (15 to 320 mPa·s) whilst holding constant the solid loading of carbon black (CB) and cathode active material LiNi_0.8Mn_0.1Co_0.1O₂; NMC811). Steric stabilisation, rather than depletion or flocculation, is shown to be the mechanism by which PVDF modulates the structure of CB colloidal particles in a slurry. Gelled CB structures, evident in shear-thinning behaviour, are observed for all slurries, but two distinct CB gel regimes are evident: percolated and dispersed. The percolated regime occurs when the ratio of dissolved PVDF to free CB is reduced, e.g. by the incomplete dissolution of the PVDF. These samples exhibit thixotropic flow behaviour and lead to more porous cathodes. Well-dispersed CB gels exhibit reversible flow behaviour and form denser cathodes. The mass loading of cathodes increases with solution viscosity, and above a viscosity of 128 ± 46 mPa·s, the mass loading across the cathode tape becomes uniform. The first discharge capacity of battery cells increases monotonically with the viscosity of the PVDF solution, but no correlation is evident after 100 cycles. Process models that incorporate these relationships will improve the reproducibility of cathode coatings, enable higher coating weights and reduce materials losses leading to lower manufacturing costs and higher capacity batteries.

The effect of dual cation magnesium/tungsten substitution in ultra-Ni-rich LiNiO ₂ cathodes was investigated. Combined bulk and surface stabilisation afforded through this coat-doping method improved the cyclability of these cathode materials.

The electrochemical degradation of Li-ion batteries occurs over different spatial and temporal scales. Operando XRD mapping is able to follow this process inside full cells.

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.

The electrochemical degradation of Li-ion batteries occurs at different spatial and temporal scales. This work demonstrates how operando synchrotron X-ray diffraction can be used to study degradation in Li-ion battery materials by mapping component-specific crystallographic structure evolution across a single-layer pouch full cell in real-time, with 100 micrometre spatial resolution and 10 millisecond data collection time per pixel. It enables the quantification of structural degradation processes in Li-ion battery cells from the atomic to macroscopic scale.

Professor Serena A. Cussen introduces Journal of Materials Chemistry A Editor's choice collection on Li-metal batteries.

Energy storage technologies such as lithium‐ion batteries (LIBs) incorporate carbon components key to their function. Graphite and carbon binder components in LIB electrodes are engineered to deliver critical electrical and mechanical properties, as are the surface chemistry and morphology of carbon blacks (CBs) in LIBs and catalysts. The challenge of relating surface chemistry to morphology is complicated by the numerous forms of carbon bonding and potential for surface functional groups. Furthermore, materials processing can influence bonding and structure of carbon at multiple length scales, as seen in mechanochemical functionalization of CBs. To understand the nature of carbon surfaces, secondary electron hyperspectral imaging (SEHI) is introduced as a spatially resolved analysis bridging the nano to microscale. The ability to provide novel insights is demonstrated three example applications: observation of nanoscale “satellite” particles of amorphous hydrogenated carbon on graphitic CB particles, differentiation between graphitic and amorphous hydrogenated nano‐thickness carbon coatings on particles of lithium iron phosphate, and differentiation between graphitic carbon active material and carbon binder domain in a LIB anode material. SEHI analysis using peak fitting models for graphitic and disordered carbons is developed based on reference materials and standard spectroscopic methods: Raman spectroscopy and X‐ray photoelectron spectroscopy.