Insights into the Nature and Evolution upon Electrochemical Cycling of Planar Defects in the β‑NaMnO₂ Na-Ion Battery Cathode: An NMR and First-Principles Density Functional Theory Approach

β-NaMnO₂ is a high-capacity Na-ion battery cathode, delivering ca. 190 mAh/g of reversible capacity when cycled at a rate of C/20. Yet, only 70% of the initial reversible capacity is retained after 100 cycles. We carry out a combined solid-state ²³Na NMR and first-principles DFT study of the evolution of the structure of β-NaMnO₂ upon electrochemical cycling. The as-synthesized structure contains planar defects identified as twin planes between nanodomains of the α and β forms of NaMnO₂. GGA+U calculations reveal that the formation energies of the two polymorphs are within 5 meV per formula unit, and a phase mixture is likely in any NaMnO₂ sample at room temperature. ²³Na NMR indicates that 65.5% of Na is in β-NaMnO₂ domains, 2.5% is in α-NaMnO₂ domains, and 32% is close to a twin boundary in the as-synthesized material. A two-phase reaction at the beginning of charge and at the end of discharge is observed by NMR, consistent with the constant voltage plateau at 2.6–2.7 V in the electrochemical profile. GGA+U computations of Na deintercalation potentials reveal that Na extraction occurs first in α-like domains, then in β-like domains, and finally close to twin boundaries. ²³Na NMR indicates that the proportion of Na in α-NaMnO₂-type sites increases to 11% after five cycles, suggesting that structural rearrangements occur, leading to twin boundaries separating larger α-NaMnO₂ domains from the major β-NaMnO₂ phase

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, limiting 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 behavior. Here, we use ex situ 7Li NMR measurements in combination with density functional theory (DFT) calculations to examine Jahn–Teller distortions and antisite 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 antisite defects, and in combination with DFT calculations of different configurations, we show that the 7Li resonance at approximately −87 ppm is characteristic of a subset of Li–Ni antisite defects, and more specifically, a Li+ ion in the Ni layer that does not have an associated Ni ion in the Li layer in its 2nd cation coordination shell. Via ex situ 7Li MAS NMR, X-ray diffraction, and electrochemical experiments, we identify the 7Li 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

Unraveling the Complex Delithiation and Lithiation Mechanisms of the High Capacity Cathode Material V₆O₁₃

V₆O₁₃ is a promising Li-ion battery cathode material for use in the high temperature oil field environment. The material exhibits a high capacity, and the voltage profile contains several plateaus associated with a series of complex structural transformations, which are not fully understood. The underlying mechanisms are central to understanding and improving the performance of V₆O₁₃-based rechargeable batteries. In this study, we present <i>in situ</i> X-ray diffraction data that highlight an asymmetric six-step discharge and five-step charge process, due to a phase that is only formed on discharge. The Li_<i>x</i>V₆O₁₃ unit cell expands sequentially in <i>c</i>, <i>b</i>, and <i>a</i> directions during discharge and reversibly contracts back during charge. The process is associated with change of Li ion positions as well as charge ordering in Li_<i>x</i>V₆O₁₃. Density functional theory calculations give further insight into the electronic structures and preferred Li positions in the different structures formed upon cycling, particularly at high lithium contents, where no prior structural data are available. The results shed light into the high specific capacity of V₆O₁₃ and are likely to aid in the development of this material for use as a cathode for secondary lithium batteries

Electrochemical Performance of Nanosized Disordered LiVOPO₄

ε-LiVOPO₄ is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO₄, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by ⁷Li and ³¹P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO₄ at C/50 (1 C = 153 mA h g^–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO₄/Li₃VO₄/acetylene black composite yields the high cycling capacity of 250 mA h g^–1 at C/5 for over 70 cycles

Identifying the Distribution of Al³⁺ in Li­Ni_0.8­Co_0.15­Al_0.05­O₂

The doping of Al into layered Li transition metal (TM) oxide cathode materials, LiTMO₂, is known to improve the structural and thermal stability, although the origin of the enhanced properties is not well understood. The effect of aluminum doping on layer stabilization has been investigated using a combination of techniques to measure the aluminum distribution in layered Li­Ni_0.8­Co_0.15­Al_0.05­O₂ (NCA) over multiple length scales with ²⁷Al and ⁷Li MAS NMR, local electrode atom probe (APT) tomography, X-ray and neutron diffraction, DFT, and SQUID magnetic susceptibility measurements. APT ion maps show a homogeneous distribution of Ni, Co, Al, and O₂ throughout the structure at the single particle level in agreement with the high-temperature phase diagram. ⁷Li and ²⁷Al NMR indicates that the Ni³⁺ ions undergo a dynamic Jahn–Teller (JT) distortion. ²⁷Al NMR spectra indicate that the Al reduces the strain associated with the JT distortion, by preferential electronic ordering of the JT lengthened bonds directed toward the Al³⁺ ion. The ability to understand the complex atomic and orbital ordering around Al³⁺ demonstrated in the current method will be useful for studying the local environment of Al³⁺ in a range of transition metal oxide battery materials