Superstructure and Correlated Na+ Hopping in a Layered Mg-Substituted Sodium Manganate Battery Cathode are Driven by Local Electroneutrality

Acknowledgments ARTICLE SECTIONSJump To E.N.B. acknowledges funding from the Engineering Physical Sciences Research Council (EPSRC) via the National Productivity Interest Fund (NPIF) 2018 (EP/S515334/1). J.D.B. acknowledges funding from the Faraday Institution (EP/S003053/1, FIRG016). The authors also thank the Science and Technology Facilities Council (STFC) and ISIS Neutron and Muon source for neutron data (experiment no.: RB2010350). Additional thanks are given to the staff scientists at beamline I11 of the Diamond Light Source for synchrotron data using block allocation group time under proposal CY34243. This work also utilized the ARCHER UK National Supercomputing Service via our membership in the UK’s HEC Materials Chemistry Consortium, funded by the EPSRC (EP/L000202). The research was also carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, through the U.S. Department of Energy, Office of Basic Energy Sciences, Contract DE-AC02-98CH10866. E.N.B. would also like to thank A. Van der Ven and M.A. Jones for illuminating discussions.Peer reviewedPublisher PD

17O NMR Spectroscopy in Lithium-Ion Battery Cathode Materials: Challenges and Interpretation.

Modern studies of lithium-ion battery (LIB) cathode materials employ a large range of experimental and theoretical techniques to understand the changes in bulk and local chemical and electronic structures during electrochemical cycling (charge and discharge). Despite its being rich in useful chemical information, few studies to date have used 17O NMR spectroscopy. Many LIB cathode materials contain paramagnetic ions, and their NMR spectra are dominated by hyperfine and quadrupolar interactions, giving rise to broad resonances with extensive spinning sideband manifolds. In principle, careful analysis of these spectra can reveal information about local structural distortions, magnetic exchange interactions, structural inhomogeneities (Li+ concentration gradients), and even the presence of redox-active O anions. In this Perspective, we examine the primary interactions governing 17O NMR spectroscopy of LIB cathodes and outline how 17O NMR may be used to elucidate the structure of pristine cathodes and their structural evolution on cycling, providing insight into the challenges in obtaining and interpreting the spectra. We also discuss the use of 17O NMR in the context of anionic redox and the role this technique may play in understanding the charge compensation mechanisms in high-capacity cathodes, and we provide suggestions for employing 17O NMR in future avenues of research.E.N.B. acknowledges funding from the Engineering Physical Sciences Research Council (EPSRC) via the National Productivity Interest Fund (NPIF) 2018. E.N.B. would also like to thank K.R. Bassey for assistance with figure preparation and invaluable advice and discussions

Probing Jahn-Teller distortions and antisite defects in LiNiO2 with 7Li NMR spectroscopy and Density Functional Theory

The long- and local-range structure and electronic properties of the high-voltage lithium-ion cathode material for Li-ion batteries, LiNiO2, remain widely debated, as are the degradation phenomena at high states of delithiation, which limit the more widespread use of this material. In particular, the local structural environment and the role of Jahn-Teller distortions are unclear, as are the interplay of distortions and point defects, and their influence on cycling behaviour. Here, we use ex-situ 7Li NMR measurements in combination with density functional theory (DFT) calculations to examine Jahn-Teller distortions and anti-site defects in LiNiO2. We calculate the 7Li Fermi contact shifts for the Jahn-Teller distorted and undistorted structures, the experimental 7Li room temperature spectrum being ascribed to an appropriately weighted time-average of the rapidly fluctuating structure comprising, collinear, zigzag and undistorted domains. The 7Li NMR spectra are sensitive to the nature and distribution of anti-site defects, and in combination with DFT calculations of different configurations, we show that the 7Li resonance at approximately -89 ppm is characteristic of separated Li – Ni antisite defects, these and the anti-site defect with Li in next-nearest Li-Ni neighbour configurations accounting for approximately 1/3 and 2/3, respectively of the total anti-site defect concentrations at room temperature in the pristine material. Via ex-situ 7Li MAS NMR, X-ray diffraction and electrochemical experiments, we identify the 7 Li spectral signatures of the different crystallographic phases on delithiation. The results imply fast Li-ion dynamics in the monoclinic phase and indicate that the hexagonal H3 phase near the end of charge is largely devoid of Li.This work was supported by the Faraday Institution degradation project (FIRG011, FIRG020). This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 957189 (BIGMAP). The project is part of BATTERY 2030+, the large-scale European research initiative for inventing the sustainable batteries of the future, funded by the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 957213. A.R.G.-S. gratefully acknowledges funding from the German National Academy of Sciences Leopoldina. We thank Teresa Insinna for fruitful discussions. Generous computing resources were provided by the Sulis HPC service (EP/T022108/1)

Structural Origins of Voltage Hysteresis in the Na-Ion Cathode P2-Na_0.67[Mg_0.28Mn_0.72]O₂: A Combined Spectroscopic and Density Functional Theory Study.

P2-layered sodium-ion battery (NIB) cathodes are a promising class of Na-ion electrode materials with high Na+ mobility and relatively high capacities. In this work, we report the structural changes that take place in P2-Na0.67[Mg0.28Mn0.72]O2. Using ex situ X-ray diffraction, Mn K-edge extended X-ray absorption fine structure, and 23Na NMR spectroscopy, we identify the bulk phase changes along the first electrochemical charge-discharge cycle-including the formation of a high-voltage "Z phase", an intergrowth of the OP4 and O2 phases. Our ab initio transition state searches reveal that reversible Mg2+ migration in the Z phase is both kinetically and thermodynamically favorable at high voltages. We propose that Mg2+ migration is a significant contributor to the observed voltage hysteresis in Na0.67[Mg0.28Mn0.72]O2 and identify qualitative changes in the Na+ ion mobility

Ni-O-redox, oxygen loss and singlet oxygen formation in LiNiO2_2 cathodes for Li-ion batteries

Ni-rich cathode materials such as LiNiO$_2$ achieve high voltages in Li-ion batteries but are prone to structural instabilities and oxygen loss. Mitigating this degradation requires a comprehensive understanding of the cause and mechanism of oxygen loss, also accounting for the formation of singlet oxygen. Using ab initio molecular dynamics simulations, we observe spontaneous O$_2$ loss from the (012) surface of delithiated LiNiO$_2$, singlet oxygen forming in the process. We find that the origin of the instability lies in the pronounced oxidation of O during delithiation, i.e., a central role of O in Ni-O redox in LiNiO$_2$. For LiNiO$_2$, NiO$_2$, and the prototype rock salt NiO, a range of computational tools including density-functional theory and dynamical mean-field theory calculations based on maximally localised Wannier functions yield a Ni charge state of ca. +2, with O varying between -2 (NiO), -1.5 (LiNiO$_2$) and -1 (NiO$_2$). The O$_2$ loss route observed here consists of 2 surface O$^{.-}$ radicals combining to form a peroxide ion, which is then oxidised to O$_2$. In leaving the surface, O$_2$ leaves behind 2 O vacancies and 2 electrons, reducing the 2 nearest surface O$^{.-}$ radicals to O$^{2-}$ ions: effectively 4 O$^{.-}$ radicals disproportionate to O$_2$ and 2 O$^{2-}$ ions, forming 2 O vacancies. The reaction liberates ca. 3 eV. Singlet oxygen formation is caused by the singlet ground state of the peroxide ion, with spin conservation dictating the preferential release of $^1$O$_2$. The strongly exergonic reaction easily provides the free energy required for the formation of $^1$O$_2$ in its excited state