64 65 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Trapped oxygenmolecules provide an energy storagemechanism in lithium-ion battery cathodes Related publication: House R. A., Rees G. J., Pérez-OsorioM. A., Marie J. J., Boivin E., Robertson A.W., Nag A., Garcia-Fernandez M., Zhou K. J. & Bruce P. G. First-cycle voltage hysteresis in Li-rich 3d cathodes associatedwithmolecular O2 trapped in the bulk. Nat. Energy 5 , 777–785 (2020). DOI: 10.1038/s41560-020-00697-2 Publication keywords: Li-ion Batteries; CathodeMaterials; Resonant Inelastic X-ray Scattering L ithium-rich cathode materials exhibit a pronounced drop in voltage between the charging and discharging processes (known as ‘voltage hysteresis’) that limits the available energy density. This needs to be overcome for themto be used innext-generation battery applications. In lithium-richmaterials, the charging and discharging processes are accompanied by chemical changes to the oxide ions that form part of the cathode’s crystal structure. The reversibility of these chemical processes is critical in determining how well a cathode material functions. The voltage hysteresis seen in the lithium-rich materials implies the oxide ions undergo chemical changes that are not fully reversible, but the mechanismbehind this was not well understood. Researchers used the Resonant Inelastic X-ray Scattering (RIXS) beamline (I21) at Diamond Light Source to study the chemical changes to the oxide ions at different stages of the battery charge-discharge cycle using RIXS. The measurements acquired from this technique offer a direct probe of the electronic structure on oxygen. Using high-resolution RIXS, they detected underlying vibrational fine structure that revealed the identity of the oxidised oxide ions as molecular oxygen. These oxygen molecules are trapped within the charged cathode material, but they can be reduced back to oxide ions on discharge. However, this process takes place at a lower voltage, giving rise to the voltage hysteresis. Researchers can now devise strategies to suppress the release of oxygen, or prevent its formation, in next-generation lithium-ion (Li-ion) batteries with higher energy density. Higher energy density batteries will extend the driving range of electric vehicles and increase the battery life of portable electronics between charges. Lithium-rich layered transition metal oxide materials, such as Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 (Fig. 1a) are closely being considered as high energy density cathodes for next generation Li-ion batteries; they can store and release more charge than conventional cathodes by utilising redox chemistry of the oxide anions in addition to that of the transition metal cations 1 . This so- called oxygen redox (or O-redox ) process is, however, accompanied by complex changes to the cathode crystal structure, gas evolution from the surface and a severe loss of voltage between the first charge and discharge (known as voltage hysteresis ), (Fig. 1b) 2 .This reduces the practical energy density that can be achieved and represents a key challenge preventing their full exploitation. Understanding how the O-redox process works is crucial to unlocking the potential of these materials but, uncovering the mechanism has proved difficult. Since the discovery of lithium-rich materials in the early 2000s, it has been established that dimerisation of oxidised oxide ions in the lattice to form O-O species takes place along with the migration of transition metal ions within the structure 3,4 , but there is still debate on the nature of the O-O dimer and how these processes translate into voltage hysteresis. In the study of O-redox chemistry, Resonant Inelastic X-ray Scattering (RIXS) has proven to be an insightful tool 5 . To examine the O-redox process in depth in Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , RIXS measurements were performed at the I21 beamline at Diamond on cathode samples extracted frombatteries at different stages of the charge-discharge cycle. RIXS emission spectra were collected in increments of excitation energy across the oxygen K-edge, generating a RIXS map that revealed the valence states on oxygen, (Fig. 2a). The state-of-the- art capabilities at beamline I21 enabled this RIXS data following the O-redox process to be obtained with much higher resolution than previously possible 5 . The results showed the appearance of a series of sharp, intense peaks in emission energy in the charged cathodes at an excitation energy of 531 eV. Closer analysis of this peak progression revealed a regularly decreasing spacing between the peaks consistent with the spacing between vibrational energy levels of a diatomic O 2 molecule, (Fig. 2b).Tracking the evolution of this feature over the charge-discharge cycle confirmed the molecular O 2 was formed as a direct result of the oxidation of lattice oxide ions and that, once formed, it could be reduced back into oxide during discharge. This demonstrates that molecular O 2 partakes reversibly in the energy storage mechanism of these MagneticMaterials Group Beamline I21 high energy density O-redox cathodes, albeit accompanied by voltage hysteresis. Since the cathodes were measured under ultra-high vacuum conditions within the RIXS spectrometer, the O 2 molecules that were detected must have been trapped within the crystal structure of the material, (Fig. 2c). The close match of the peak spacing to that of free, gaseous O 2 , suggests there is minimal bonding interaction between the O 2 molecules and their surrounding crystal lattice. Furthermore, careful beam exposure studies were undertaken to critically examine the influence of the measurement conditions upon the spectroscopic features. Even under reduced temperature, lower photon flux and shorter exposure times, very little change was observed in the signal, indicating the O 2 was intrinsic to the cathode and not radiation-induced. To independently verify the formation and reduction of trapped O 2 and investigate its local chemical environment, a 17 O Nuclear Magnetic Resonance (NMR) study was performed on 17 O-labelled material. The NMR experiments detected a new, strongly paramagnetic O environment in the charged cathode with a shift and sideband manifold width consistent with previous measurements on O 2 in the condensed state. The sharp sideband peaks indicated that the molecular O 2 was rigidly trapped within the lattice, which, together with the vibrational RIXS data, suggests physical confinement rather than chemical bonding within sub-nano-sized void defects. In line with the RIXS results, this new O environment corresponding to O 2 disappeared during subsequent discharge demonstrating the reversible nature of the O 2- /O 2 charge storage mechanism. Despite its Coulombic reversibility, the formation and reduction of molecular O 2 is accompanied by pronounced voltage hysteresis, resulting in a loss of energy density and poor efficiency. To uncover the origin of this hysteresis, density functional theory calculations were employed using structural models for Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 at different states of charge. In some of the fully charged models investigated, where the transition metal ions had been permitted to migrate, O 2 molecules were found to form spontaneously during structural relaxation leading to substantially more stable, lower energy structures. Once these structures were re-populated with lithium ions, the O 2 molecules were reformed into O 2- ions, however the new, disordered arrangement of transition metal ions resulted in a higher energy structure than the original, pristine ordered one. Thus, it was determined that the irreversible structural changes triggered by the formation of molecular O 2 were to blame for the electrochemical voltage hysteresis observed. Thisnewunderstanding,inwhichtransitionmetalmigrationandmolecular O 2 formation are both implicated in the voltage hysteresis mechanism of lithium-rich cathodes, lays a foundation for researchers to devise strategies to improve these materials. This study has also demonstrated the power of high resolution RIXS as a characterisation tool for tracking redox processes in energy storage materials. The capability of RIXS to detect homonuclear diatomic molecules trapped within the bulk of an inorganic material, is a unique strength and one which may find useful application beyond the battery field. References: 1. Koga H. et al. Reversible Oxygen Participation to the Redox Processes Revealed for Li 1.20 Mn 0.54 Co 0.13 Ni 0.13 O 2. J. Electrochem. Soc. 160 (2013). DOI: 10.1149/2.038306jes 2. Lu Z. et al. Understanding the Anomalous Capacity of Li/Li[Ni x Li (1/3−2x/3) Mn (2/3−x/3) ]O 2 Cells Using In Situ X-Ray Diffraction and Electrochemical Studies. J. Electrochem. Soc. 149 (2002). DOI: 10.1149/1.1480014 3. Sathiya M. et al. Reversible anionic redox chemistry in high-capacity layered-oxide electrodes. Nat. Mater. 12 , 827–835 (2013). DOI: 10.1038/ nmat3699 4. Saubanère M. et al. The intriguing question of anionic redox in high- energy density cathodes for Li-ion batteries. Energy Environ. Sci. 9 , 984–991 (2016). DOI: 10.1039/c5ee03048j 5. Gent W. E. et al. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides. Nat. Commun. 8 , 2091 (2017). DOI: 10.1038/s41467-017-02041-x Funding acknowledgement: P.G.B. is indebted to the EPSRC, including the SUPERGEN programme (EP/L019469/1), the Henry Royce Institute for Advanced Materials (EP/ R00661X/1, EP/S019367/1 and EP/R010145/1) and the Faraday Institution (FIRG007 and FIRG008) for financial support. We acknowledge Diamond Light Source for time on I21 under proposal MM23889-1. Support from the EPSRC (EP/K040375/1 ‘South of England Analytical Electron Microscope’) is also acknowledged. We acknowledge the resources provided by the Cambridge Tier-2 system operated by the University of Cambridge Research Computing Service (http://www.hpc.cam.ac.uk ) funded by EPSRC Tier-2 capital grant EP/P020259/1, via the BATTDesign and AMAiB projects. The UK 850 MHz solid-state NMR Facility used in this research was funded by EPSRC and BBSRC (contract reference PR140003), as well as the University of Warwick, which included via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by AdvantageWest Midlands (AWM) and the European Regional Development Fund (ERDF). Corresponding author: Dr Robert A. House, Department of Materials, University of Oxford, email@example.com Figure 1: (a) Crystal structure for lithium-rich cathode Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , pale blue, Li; purple, Mn; grey, Ni; dark blue, Co. (b) First-cycle charge-discharge voltage profile for Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 in a Li-ion battery. Figure 2: (a) RIXS map and XAS spectrum for Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 in the charged state. (b) RIXS emission spectrum recorded at 531 eV in the pristine, charged and discharged states. Inset shows close-up of peak progression detected in the charged cathode with peak spacing closely corresponding to the spacing between vibrational energy levels of molecular O 2 . (c) Structural schematic for the charged cathode with trapped O 2 molecules.