NMR studies of cathode materials for lithium-ion rechargeable batteries

Ion Dynamics at the Intermediate Charging State of the Sodium Vanadium Fluorophosphate Cathode

Na super ionic conductor (NASICON)-type polyanionic vanadium fluorophosphate Na3V2O2(PO4)2F (NVOPF) is a promising cathode material for high-energy sodium-ion batteries. The dynamic diffusion and exchange of sodium ions in the lattice of NVOPF are crucial for its electrochemical performance. However, standard characterizations are mostly focused on the as-synthesized material without cycling, which is different from the actual battery operation conditions. In this work, we investigated the hopping processes of sodium in NVOPF at the intermediate charging state with 23Na solid-state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) calculations. Our experimental characterizations revealed six distinct sodium coordination sites in the intermediate structure and determined the exchange rates among these sites at variable temperatures. The theoretical calculations showed that these dynamic processes correspond to different ion transport pathways in the crystalline lattice. Our combined experimental and theoretical study uncovered the underlying mechanisms of the ion transport in cycled NVOPF and these understandings may help the optimization of cathode materials for sodium-ion batteries.

Li3V2(PO4)3 Cathode Material: Synthesis Method, High Lithium Diffusion Coefficient and Magnetic Inhomogeneity

Li3V2(PO4)3 cathodes for Li-ion batteries (LIBs) were synthesized using a hydrothermal method with the subsequent annealing in an argon atmosphere to achieve optimal properties. The X-ray diffraction analysis confirmed the material's single-phase nature, while the scanning electron microscopy revealed a granular structure, indicating a uniform particle size distribution, beneficial for electrochemical performance. Magnetometry and electron spin resonance studies were conducted to investigate the magnetic properties, confirming the presence of the relatively low concentration and highly uniform distribution of tetravalent vanadium ions (V4+), which indicated low lithium deficiency values in the original structure and a high degree of magnetic homogeneity in the sample, an essential factor for consistent electrochemical behavior. For this pure phase Li3V2(PO4)3 sample, devoid of any impurities such as carbon or salts, extensive electrochemical property testing was performed. These tests resulted in the experimental discovery of a remarkably high lithium diffusion coefficient D = 1.07 × 10-10 cm2/s, indicating excellent ionic conductivity, and demonstrated impressive stability of the material with sustained performance over 1000 charge-discharge cycles. Additionally, relithiated Li3V2(PO4)3 (after multiple electrochemical cycling) samples were investigated using scanning electron microscopy, magnetometry and electron spin resonance methods to determine the extent of degradation. The combination of high lithium diffusion coefficients, a low degradation rate and remarkable cycling stability positions this Li3V2(PO4)3 material as a promising candidate for advanced energy storage applications.

Insights into the Phase Purity and Storage Mechanism of Nonstoichiometric Na3.4 Fe2.4 (PO4 )1.4 P2 O7 Cathode for High-Mass-Loading and High-Power-Density Sodium-Ion Batteries

Mixed-anion-group Fe-based phosphate materials, such as Na4 Fe3 (PO4 )2 P2 O7 , have emerged as promising cathode materials for sodium-ion batteries (SIBs). However, the synthesis of pure-phase material has remained a challenge, and the phase evolution during sodium (de)intercalation is debating as well. Herein, a solid-solution strategy is proposed to partition Na4 Fe3 (PO4 )2 P2 O7 into 2NaFePO4  ⋅ Na2 FeP2 O7 from the angle of molecular composition. Via regulating the starting ratio of NaFePO4 and Na2 FeP2 O7 during the synthesis process, the nonstoichiometric pure-phase material could be successfully synthesized within a narrow NaFePO4 content between 1.6 and 1.2. Furthermore, the proposed synthesis strategy demonstrates strong applicability that helps to address the impurity issue of Na4 Co3 (PO4 )2 P2 O7 and nonstoichiometric Na3.4 Co2.4 (PO4 )1.4 P2 O7 are evidenced to be the pure phase. The model Na3.4 Fe2.4 (PO4 )1.4 P2 O7 cathode (the content of NaFePO4 equals 1.4) demonstrates exceptional sodium storage performances, including ultrahigh rate capability under 100 C and ultralong cycle life over 14000 cycles. Furthermore, combined measurements of ex situ nuclear magnetic resonance, in situ synchrotron radiation diffraction and X-ray absorption spectroscopy clearly reveal a two-phase transition during Na+ extraction/insertion, which provides a new insight into the ionic storage process for such kind of mixed-anion-group Fe-based phosphate materials and pave the way for the development of high-power sodium-ion batteries.

Development of in situ high resolution NMR: Proof-of-principle for a new (spinning) cylindrical mini-pellet approach applied to a Lithium ion battery

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a powerful technique for characterizing the local structure and dynamics of battery and other materials. It has been widely used to investigate bulk electrode compounds, electrolytes, and interfaces. Beside common ex situ investigations, in situ and operando techniques have gained considerable importance for understanding the reaction mechanisms and cell degradation of electrochemical cells. Herein, we present the recent development of in situ magic angle spinning (MAS) NMR methodologies to study batteries with high spectral resolution, setting into context possible advances on this topic. A mini cylindrical cell type insert for 4 mm MAS rotors is introduced here, being demonstrated on a Li/VO2F electrochemical system, allowing the acquisition of high-resolution 7Li MAS NMR spectra, spinning the electrochemical cell up to 15 kHz.

Structural Evolution of Lithium-Exchanged Na3(VO)2(PO4)2F Cathode under Operation in Sodium Ion Batteries

Na superionic conductor (NASICON)-type Na3(VO)2(PO4)2F (NVOPF) exhibits excellent cycling stability for high-voltage sodium ion batteries. Various strategies have been developed to form ion-exchanged NVOPF which can enhance the ionic and electronic conductivity. However, the underlying ion transport mechanism and complex structural transitions during battery operation remained uninvestigated. In this work, we prepared lithium-exchanged NVOPF (namely NLVOPF) which shows improved ionic conductivity and increased capacity at high discharging rates. Solid-state nuclear magnetic resonance (SSNMR) revealed the distinctive presence of two kinds of Li-exchanged sites in the NLVOPF, which are attributed to the occupied lithium ions at the Na1 and Na2 sites (namely Li1 and Li2, respectively). The Li1 site was metastably replaced in the first cycle, yet the Li2 site participated in ion insertion/extraction in the subsequent cycles. Our characterizations show that the dynamic doping of lithium in NLVOPF could contribute to the improved cycling stability and capacity retention.

Paramagnetic Nuclear Magnetic Resonance Shifts for Triplet Systems and Beyond with Modern Relativistic Density Functional Methods

An efficient framework for the calculation of paramagnetic NMR (pNMR) shifts within exact two-component (X2C) theory and (current-dependent) density functional theory (DFT) up to the class of local hybrid functionals (LHFs) is presented. Generally, pNMR shifts for systems with more than one unpaired electron depend on the orbital shielding contribution and a temperature-dependent term. The latter includes zero-field splitting (ZFS), hyperfine coupling (HFC), and the g-tensor. For consistency, we calculate these three tensors at the same level of theory, i.e., using scalar-relativistic X2C augmented with spin-orbit perturbation theory. Results for pNMR chemical shifts of transition-metal complexes reveal that this X2C-DFT framework can yield good results for both the shifts and the individual tensor contributions of metallocenes and related systems, especially if the HFC constant is large. For small HFC constants, the relative error is often large, and sometimes the sign may be off. 4d and 5d complexes with more complicated structures demonstrate the limitations of a fully DFT-based approach. Additionally, a Co-based complex with a very large ZFS and pronounced multireference character is not well described. Here, a hybrid DFT-multireference framework is necessary for accurate results. Our results show that X2C is sufficient to describe relativistic effects and computationally cheaper than a fully relativistic approach. Thus, it allows use of large basis sets for converged HFCs. Overall, current-dependent meta-generalized gradient approximations and LHFs show some potential; however, the currently available functionals leave a lot to be desired, and the predictive power is limited.

Insights into Reversible Sodium Intercalation in a Novel Sodium-Deficient NASICON-Type Structure:Na3.40 □0.60 Co0.5 Fe0.5 V(PO4 )3

The rational design of novel high-performance cathode materials for sodium-ion batteries is a challenge for the development of the renewable energy sector. Here, a new sodium-deficient NASICON phosphate, namely Na3.40 □0.60 Co0.5 Fe0.5 V(PO4 )3 , demonstrating the excellent electrochemical performance is reported. The presence of Co allows a third Na+ to participate in the reaction thus exhibiting a high reversible capacity of ≈155 mAh g-1 in the voltage range of 2.0-4.0 V versus Na+ /Na with a reversible single-phase mechanism and a small volume shrinkage of ≈5.97% at 4.0 V. 23 Na solid-state nuclear magnetic resonance (NMR) combined with ex situ X-ray diffraction (XRD) refinements provide evidence for a preferential Na+ insertion within the Na2 site. Furthermore, the enhanced sodium kinetics ascribed to Co-substitution is also confirmed in combination with electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT), and theoretical calculation.

Origin and characterization of the oxygen loss phenomenon in the layered oxide cathodes of Li-ion batteries

Li-ion batteries have been widely applied in the field of energy storage due to their high energy density and environment friendliness. Owing to their high capacity of ∼200 mA h g-1 and high cutoff voltage of ∼4.6 V vs. Li+/Li, layered lithium transition metal oxides (LLMOs) stand out among the numerous cathode materials. However, the oxygen loss of LLMO cathodes during cycling hampers the further development LLMO cathode-based Li-ion batteries by inducing a dramatic decay of electrochemical performance and safety issues. In this regard, the oxygen loss phenomenon of LLMO cathodes has attracted attention, and extensive efforts have been devoted to investigating the origins of oxygen loss in LLMO cathodes by various characterization methods. In this review, a comprehensive overview of the main causes of oxygen loss is presented, including the state of charge, side reactions with electrolytes, and the thermal instability of LLMO cathodes. The characterization methods used in the scope are introduced and summarized based on their functional principles. It is hoped that the review can inspire a deeper consideration of the utilization of characterization techniques in detecting the oxygen loss of LLMO cathodes, paving a new pathway for developing advanced LLMO cathodes with better cycling stability and practical capabilities.

Structure Determination and Refinement of Paramagnetic Materials by Solid-State NMR

Paramagnetism in solid-state materials has long been considered an additional challenge for structural investigations by using solid-state nuclear magnetic resonance spectroscopy (ssNMR). The strong interactions between unpaired electrons and the surrounding atomic nuclei, on the one hand, are complex to describe, and on the other hand can cause fast decaying signals and extremely broad resonances. However, significant progress has been made over the recent years in developing both theoretical models to understand and calculate the frequency shifts due to paramagnetism and also more sophisticated experimental protocols for obtaining high-resolution ssNMR spectra. While the field is continuously moving forward, to date, the combination of state-of-the-art numerical and experimental techniques enables us to obtain high-quality data for a variety of systems. This involves the determination of several ssNMR parameters that represent different contributions to the frequency shift in paramagnetic solids. These contributions encode structural information on the studied material on various length scales, ranging from crystal morphologies, to the mid- and long-range order, down to the local atomic bonding environment. In this perspective, the different ssNMR parameters characteristic for paramagnetic materials are discussed with a focus on their interpretation in terms of structure. This includes a summary of studies that have explored the information content of these ssNMR parameters, mostly to complement experimental data from other methods, e.g., X-ray diffraction. The presented overview aims to demonstrate how far ssNMR has hitherto been able to determine and refine the structures of materials and to discuss where it currently falls short of its full potential. We attempt to highlight how much further ssNMR can be pushed to determine and refine structure to deliver a comprehensive structural characterization of paramagnetic materials comparable to what is to date achieved by the combined effort of electron microscopy, diffraction, and spectroscopy.

The applications of solid-state NMR and MRI techniques in the study of rechargeable sodium-ion batteries

In order to develop new electrode and electrolyte materials for advanced sodium-ion batteries (SIBs), it is crucial to understand a number of fundamental issues. These include the compositions of the bulk and interface, the structures of the materials used, and the electrochemical reactions in the batteries. Solid-state NMR (SS-NMR) has unique advantages in characterizing the local or microstructure of solid electrode/electrolyte materials and their interfaces-one such advantage is that these are determined in a noninvasive and nondestructive manner at the atomic level. In this review, we provide a survey of the recent advances in the understanding of the fundamental issues of SIBs using advanced NMR techniques. First, we summarize the applications of SS-NMR in characterizing electrode material structures and solid electrolyte interfaces (SEI). In particular, we elucidate the key role of in-situ NMR/MRI in revealing the complex reactions and degradation mechanisms of SIBs. Next, the characteristics and shortcomings of SS-NMR and MRI techniques in SIBs are also discussed in comparison to similar Li-ion batteries. Finally, an overview of SS-NMR and MRI techniques for sodium batteries are briefly discussed and presented.

O3 to O1 Phase Transitions in Highly Delithiated NMC811 at Elevated Temperatures

Nickel-rich layered oxide cathodes such as NMC811 (Li_xNi_0.8Mn_0.1Co_0.1O₂) currently have the highest practical capacities of cathodes used commercially, approaching 200 mAh/g. Lithium is removed from NMC811 via a solid-solution behavior when delithiated to x_Li > 0.10, maintaining the same layered (O3 structure) throughout as observed via operando diffraction measurements. Although it is possible to further delithiate NMC811, it is kinetically challenging, and there are significant side reactions between the electrolyte and cathode surface. Here, small format, NMC811-graphite pouch cells were charged to high voltages at elevated temperatures and held for days to access high states of delithiation. Rietveld refinements on high-resolution diffraction data and indexing of selected area electron diffraction patterns, both acquired ex situ, show that NMC811 undergoes a partial and reversible transition from the O3 to the O1 phase under these conditions. The O1 phase fraction depends not only on the concentration of intercalated lithium but also on the hold temperature and hold time, indicating that the phase transition is kinetically controlled. ¹H NMR spectroscopy shows that the proton concentration decreases with O1 phase fraction and is not, therefore, likely to be driving the O3-O1 phase transition.

Quantifying Dissolved Transition Metals in Battery Electrolyte Solutions with NMR Paramagnetic Relaxation Enhancement

Transition metal dissolution is an important contributor to capacity fade in lithium-ion cells. NMR relaxation rates are proportional to the concentration of paramagnetic species, making them suitable to quantify dissolved transition metals in battery electrolytes. In this work, ⁷Li, ³¹P, ¹⁹F, and ¹H longitudinal and transverse relaxation rates were measured to study LiPF₆ electrolyte solutions containing Ni²⁺, Mn²⁺, Co²⁺, or Cu²⁺ salts and Mn dissolved from LiMn₂O₄. Sensitivities were found to vary by nuclide and by transition metal. ¹⁹F (PF₆^-) and ¹H (solvent) measurements were more sensitive than ⁷Li and ³¹P measurements due to the higher likelihood that the observed species are in closer proximity to the metal center. Mn²⁺ induced the greatest relaxation enhancement, yielding a limit of detection of ∼0.005 mM for ¹⁹F and ¹H measurements. Relaxometric analysis of a sample containing Mn dissolved from LiMn₂O₄ at ∼20 °C showed good sensitivity and accuracy (suggesting dissolution of Mn²⁺), but analysis of a sample stored at 60 °C showed that the relaxometric quantification is less accurate for heat-degraded LiPF₆ electrolytes. This is attributed to degradation processes causing changes to the metal solvation shell (changing the fractions of PF₆^-, EC, and EMC coordinated to Mn²⁺), such that calibration measurements performed with pristine electrolyte solutions are not applicable to degraded solutions-a potential complication for efforts to quantify metal dissolution during operando NMR studies of batteries employing widely-used LiPF₆ electrolytes. Ex situ nondestructive quantification of transition metals in lithium-ion battery electrolytes is shown to be possible by NMR relaxometry; further, the method's sensitivity to the metal solvation shell also suggests potential use in assessing the coordination spheres of dissolved transition metals.

Anion redox as a means to derive layered manganese oxychalcogenides with exotic intergrowth structures

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr₂MnO₂Cu_1.5Ch₂ (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu_1.5Ch₂]^2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr₂MnO₂Ch₂ slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

Computational NMR of the iron pyrazolylborate complexes [Tp2Fe]+ and Tp2Fe including solvation and spin-crossover effects

Transition metal complexes have important roles in many biological processes as well as applications in fields such as pharmacy, chemistry and materials science. Paramagnetic nuclear magnetic resonance (pNMR) is a valuable tool in understanding such molecules, and theoretical computations are often advantageous or even necessary in the assignment of experimental pNMR signals. We have employed density functional theory (DFT) and the domain-based local pair natural orbital coupled-cluster method with single and double excitations (DLPNO-CCSD), as well as a number of model improvements, to determine the critical hyperfine part of the chemical shifts of the iron pyrazolylborate complexes [Tp₂Fe]⁺ and Tp₂Fe using a modern version of the Kurland-McGarvey theory, which is based on parameterising the hyperfine, electronic Zeeman and zero-field splitting interactions via the parameters of the electron paramagnetic resonance Hamiltonian. In the doublet [Tp₂Fe]⁺ system, the calculations suggest a re-assignment of the ¹³C signal shifts. Consideration of solvent via the conductor-like polarisable continuum model (C-PCM) versus explicit solvent molecules reveals C-PCM alone to be insufficient in capturing the most important solvation effects. Tp₂Fe exhibits a spin-crossover effect between a high-spin quintet (S = 2) and a low-spin singlet (S = 0) state, and its recorded temperature dependence can only be reproduced theoretically by accounting for the thermal Boltzmann distribution of the open-shell excited state and the closed-shell ground-state occupations. In these two cases, DLPNO-CCSD is found, in calculating the hyperfine couplings, to be a viable alternative to DFT, the demonstrated shortcomings of which have been a significant issue in the development of computational pNMR.

Solid-State NMR Spectra of Protons and Quadrupolar Nuclei at 28.2 T: Resolving Signatures of Surface Sites with Fast Magic Angle Spinning

Advances in solid-state nuclear magnetic resonance (NMR) methods and hardware offer expanding opportunities for analysis of materials, interfaces, and surfaces. Here, we demonstrate the application of a very high magnetic field strength of 28.2 T and fast magic-angle-spinning rates (MAS, >40 kHz) to surface species relevant to catalysis. Specifically, we present as case studies the 1D and 2D solid-state NMR spectra of important catalyst and support materials, ranging from a well-defined silica-supported organometallic catalyst to dehydroxylated γ-alumina and zeolite solid acids. The high field and fast-MAS measurement conditions substantially improve spectral resolution and narrow NMR signals, which is particularly beneficial for solid-state 1D and 2D NMR analysis of ¹H and quadrupolar nuclei such as ²⁷Al at surfaces.

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.

Influence of Transition-Metal Order on the Reaction Mechanism of LNMO Cathode Spinel: An Operando X-ray Absorption Spectroscopy Study

An operando dual-edge X-ray absorption spectroscopy on both transition-metal ordered and disordered LiNi_0.5Mn_1.5O₄ during electrochemical delithiation and lithiation was carried out. The large data set was analyzed via a chemometric approach to gain reliable insights into the redox activity and the local structural changes of Ni and Mn throughout the electrochemical charge and discharge reaction. Our findings confirm that redox activity relies predominantly on the Ni^2+/4+ redox couple involving a transient Ni³⁺ phase. Interestingly, a reversible minority contribution of Mn^3+/4+ is also evinced in both LNMO materials. While the reaction steps and involved reactants of both ordered and disordered LNMO materials generally coincide, we highlight differences in terms of reaction dynamics as well as in local structural evolution induced by the TM ordering.

Elastic Lattice Enabling Reversible Tetrahedral Li Storage Sites in a High-Capacity Manganese Oxide Cathode

The key to breaking through the capacity limitation imposed by intercalation chemistry lies in the ability to harness more active sites that can reversibly accommodate more ions (e.g., Li⁺ ) and electrons within a finite space. However, excessive Li-ion insertion into the Li layer of layered cathodes results in fast performance decay due to the huge lattice change and irreversible phase transformation. In this study, an ultrahigh reversible capacity is demonstrated by a layered oxide cathode purely based on manganese. Through a wealth of characterizations, it is clarified that the presence of low-content Li₂ MnO₃ domains not only reduces the amount of irreversible O loss; but also regulates Mn migration in LiMnO₂ domains, enabling elastic lattice with high reversibility for tetrahedral sites Li-ion storage in Li layers. This work utilizes bulk cation disorder to create stable Li-ion-storage tetrahedral sites and an elastic lattice for layered materials, with a reversible capacity of 600 mA h g^-1 , demonstrated in th range 0.6-4.9 V versus Li/Li⁺ at 10 mA g^-1 . Admittedly, discharging to 0.6 V might be too low for practical use, but this exploration is still of great importance as it conceptually demonstrates the limit of Li-ions insertion into layered oxide materials.

Generalized synthesis of NaCrO2 particles for high-rate sodium ion batteries prepared by microfluidic synthesis in segmented flow

NaCrO₂ particles for high-rate sodium ion batteries were prepared on a multigram scale in segmented flow from chromium nitrate and sodium nitrate using a segregated flow water-in-oil emulsion drying process. Microfluidic processing is an environmentally friendly and rapid synthetic method, which can produce large-scale industrial implementation for the production of materials with superior properties. The reaction time for NaCrO₂ particles was reduced by almost one order of magnitude compared to a normal flask synthesis and by several orders of magntitude compared to a conventional solid-state reaction. In addition, it allows for an easy upscaling and was generalized for the synthesis of other layered oxides NaMO₂ (M = Cr, Fe, Co, Al). The automated water-in-oil emulsion approach circumvents the diffusion limits of solid-state reactions by allowing a rapid intermixing of the components at a molecular level in submicrometer-sized micelles. A combination of Raman and nuclear magnetic resonance spectroscopy (¹H, ²³Na), thermal analysis, X-ray diffraction and high resolution transmission electron microscopy provided insight into the formation mechanism of NaCrO₂ particles. The new synthesis method allows cathode materials of different types to be produced in a large scale, constant quality and in short reaction times in an automated manner.

Monitoring electron spin fluctuations with paramagnetic relaxation enhancement

The magnetic interactions between the spin of an unpaired electron and the surrounding nuclear spins can be exploited to gain structural information, to reduce nuclear relaxation times as well as to create nuclear hyperpolarization via dynamic nuclear polarization (DNP). A central aspect that determines how these interactions manifest from the point of view of NMR is the timescale of the fluctuations of the magnetic moment of the electron spins. These fluctuations, however, are elusive, particularly when electron relaxation times are short or interactions among electronic spins are strong. Here we map the fluctuations by analyzing the ratio between longitudinal and transverse nuclear relaxation times T₁/T₂, a quantity which depends uniquely on the rate of the electron fluctuations and the Larmor frequency of the involved nuclei. This analysis enables rationalizing the evolution of NMR lineshapes, signal quenching as well as DNP enhancements as a function of the concentration of the paramagnetic species and the temperature, demonstrated here for LiMg_1-xMn_xPO₄ and Fe(III) doped Li₄Ti₅O₁₂, respectively. For the latter, we observe a linear dependence of the DNP enhancement and the electron relaxation time within a temperature range between 100 and 300 K.

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co)

⁷Li NMR shifts and magnetic properties have been determined for three so-called ate complexes [LiM{N(SiMe₃)₂}₃] (M²⁺ = Mn, Fe, Co; e.g., named lithium-tris(bis(trimethylsilylamide))-manganate(II) in accordance with a formally negative charge assigned to the complex fragment [M{N(SiMe₃)₂}₃]^-, which comprises the transition metal). They are formed by addition reactions of LiN(SiMe₃)₂ and [M{N(SiMe₃)₂}₂] and stabilized by Lewis base/Lewis acid interactions. The results are compared to those of the related "ion-separated" complexes [Li(15-crown-5)][M{N(SiMe₃)₂}₃]. The ate complexes with the lithium atoms connected to the 3d metal atoms manganese, iron, or cobalt via μ₂ nitrogen bridges reveal strong ⁷Li NMR paramagnetic shifts of about -75, 125, and 171 ppm, respectively, whereas the shifts for the lithium ions coordinated by the 15-crown-5 ether are close to zero. The observed trends of the ⁷Li NMR shifts are confirmed by density-functional theory calculations. The magnetic dc and ac properties display distinct differences for the six compounds under investigation. Both manganese compounds, [LiMn{N(SiMe₃)₂}₃] and [Li(15-crown-5)][Mn{N(SiMe₃)₂}₃], display almost pure and ideal spin-only paramagnetic behavior of a 3d⁵ high-spin complex. In this respect slightly unexpected, both complexes show slow relaxation behavior at low temperatures under applied dc fields, which is especially pronounced for the ate complex [LiMn{N(SiMe₃)₂}₃]. Dc magnetic properties of the iron complexes reveal moderate g-factor anisotropies with small values of the axial magnetic anisotropy parameter D and a larger E (transversal anisotropy). Both complexes display at low temperatures and, under external dc fields of up to 5000 Oe, only weak ac signals with no maxima in the frequency range from 1 to 1500 s^-1. In contrast, the two cobalt complexes display strong g-factor anisotropies with large values of D and E. In addition, in both cases, the ac measurements at low temperatures and applied dc fields reveal two, in terms of their frequency range, well separated relaxation processes with maxima lying for the most part outside of the measurement range between 1 and 1500 s^-1.

Challenges and Recent Advances in High Capacity Li-Rich Cathode Materials for High Energy Density Lithium-Ion Batteries

Li-rich cathode materials have attracted increasing attention because of their high reversible discharge capacity (>250 mA h g^-1 ), which originates from transition metal (TM) ion redox reactions and unconventional oxygen anion redox reactions. However, many issues need to be addressed before their practical applications, such as their low kinetic properties and inefficient voltage fading. The development of cutting-edge technologies has led to cognitive advances in theory and offer potential solutions to these problems. Herein, a recent in-depth understanding of the mechanisms and the frontier electrochemical research progress of Li-rich cathodes are reviewed. In addition, recent advances associated with various strategies to promote the performance and the development of modification methods are discussed. In particular, excluding Li-rich Mn-based (LRM) cathodes, other branches of the Li-rich cathode materials are also summarized. The consistent pursuit is to obtain energy storage devices with high capacity, reliable practicability, and absolute safety. The recent literature and ongoing efforts in this area are also described, which will create more opportunities and new ideas for the future development of Li-rich cathode materials.

SiOx Anode: From Fundamental Mechanism toward Industrial Application

Silicon monoxide (SiO) has been explored and confirmed as a promising anode material of lithium-ion batteries. Compared with pure silicon, SiO possesses a more stable microstructure which makes better comprehensive electrochemical properties. However, the lithiation mechanism remains in dispute, and problems such as poor cyclability, unsatisfactory electrical conductivity, and low initial Coulombic efficiency (ICE) need to be addressed. Additionally, more attention needs to be paid on the internal relationship between electrochemical performances and structures. In this review, the different preparation processes, the derived microstructure of the SiO_x , the corresponding lithiation mechanism, and electrochemical properties are summarized. Researches about disproportionation reaction which is regarded as a key point and other modifications are systematically introduced. Closely linked with structure, the advantages and disadvantages of various SiO_x anode materials are summarized and analyzed, and the possible directions toward the practical applications of SiO_x anode material are presented. In a word, from the preparation and reaction mechanism of the material to the modifications and future development, a complete and systematical review on SiO_x anode is presented.

Quantification of the Li-ion diffusion over an interface coating in all-solid-state batteries via NMR measurements

A key challenge for solid-state-batteries development is to design electrode-electrolyte interfaces that combine (electro)chemical and mechanical stability with facile Li-ion transport. However, while the solid-electrolyte/electrode interfacial area should be maximized to facilitate the transport of high electrical currents on the one hand, on the other hand, this area should be minimized to reduce the parasitic interfacial reactions and promote the overall cell stability. To improve these aspects simultaneously, we report the use of an interfacial inorganic coating and the study of its impact on the local Li-ion transport over the grain boundaries. Via exchange-NMR measurements, we quantify the equilibrium between the various phases present at the interface between an S-based positive electrode and an inorganic solid-electrolyte. We also demonstrate the beneficial effect of the LiI coating on the all-solid-state cell performances, which leads to efficient sulfur activation and prevention of solid-electrolyte decomposition. Finally, we report 200 cycles with a stable capacity of around 600 mAh g^-1 at 0.264 mA cm^-2 for a full lab-scale cell comprising of LiI-coated Li₂S-based cathode, Li-In alloy anode and Li₆PS₅Cl solid electrolyte.

High-Temperature Pulsed-Field-Gradient 7Li-NMR Measurements of Li2CO3 over 700 K

Understanding the transport property of Li ions is crucial for improving the performance of Li-ion batteries. To investigate the ion diffusion at high temperatures, we constructed a high-temperature pulsed-field-gradient (PFG) nuclear magnetic resonance (NMR) probe capable of measurements at temperatures >700 K. The accuracy of the sample temperature was confirmed via ⁷⁹Br NMR measurements of KBr. ⁷Li PFG-NMR measurements of Li₂CO₃ were performed at temperatures up to 724 K using the probe. The apparent diffusion coefficient of Li ions in Li₂CO₃ was obtained experimentally.

Separation of quadrupolar and paramagnetic shift interactions in high-resolution nuclear magnetic resonance of spinning powders

Separation and correlation of the shift anisotropy and the first-order quadrupolar interaction of spin I = 1 nuclei under magic-angle spinning (MAS) are achieved by the phase-adjusted spinning sideband (PASS) nuclear magnetic resonance (NMR) experiment. Compared to methods for static samples, this approach has the benefit of higher sensitivity and resolution. Moreover, the PASS experiment has the advantage over previous MAS sequences in the ability to completely separate the shift anisotropy and first-order quadrupolar interactions. However, the main drawback of the pulse sequence is the lower excitation bandwidth. The sequence is comprehensively evaluated using theoretical calculations and numerical simulations and applied experimentally to the ²H NMR of a range of paramagnetic systems: deuterated nickel(II) acetate tetrahydrate, deuterated copper(II) chloride dihydrate, and two forms of deuterated oxyhydride ion conductor BaTiO_3-xH_y. Our results show that despite the issue with broadband excitation, the extracted shift and quadrupolar interaction tensors and the Euler angles relating the two tensors match well with the NMR parameters obtained with static NMR methods. Therefore, the new application of the PASS experiment is an excellent addition to the arsenal of NMR experiments for ²H and potentially ¹⁴N in paramagnetic solids.

How Li diffusion in spinel Li[Ni1/2Mn3/2]O4 is seen with μ ±SR

The diffusive behavior in a spinel-type Li+ ion battery material, Li[Ni1/2Mn3/2]O4, has been studied with positive and negative muon spin rotation and relaxation (μ ±SR) measurements in the temperature range between 200 and 400 K using a powder sample. The implanted μ + locates at an interstitial site near O2− ion so as to form a O–H like bond, while the implanted μ − is mainly captured by an oxygen nucleus, resulting in the formation of muonic oxygen. This means that local magnetic environments in Li[Ni1/2Mn3/2]O4 were investigated from the two different sites in the lattice, i.e., one is an interstitial site for μ +SR and the other is an oxygen site for μ −SR. Since both μ +SR and μ −SR detected an increase in the fluctuation rate of a nuclear magnetic field for temperatures above 200 K, the origin of this increase is clearly confirmed as Li diffusion. Assuming a random walk process with the hopping of thermally activated Li+ between a regular Li site and the nearest neighboring vacant octahedral sites, a self-diffusion coefficient of Li+ was found to range above 10−11 cm2/s at temperatures above 250 K with an activation energy of about 0.06 eV.

Surface acidity of tin dioxide nanomaterials revealed with 31P solid-state NMR spectroscopy and DFT calculations

Tin dioxide (SnO₂) nanomaterials are important acid catalysts. It is therefore crucial to obtain details about the surface acidic properties in order to develop structure–property relationships. Herein, we apply ³¹P solid-state NMR spectroscopy combined with a trimethylphosphine (TMP) probe molecule, to study the facet-dependent acidity of SnO₂ nanosheets and nanoshuttles. With the help of density functional theory calculations, we show that the tin cations exposed on the surfaces are Lewis acid sites and their acid strengths rely on surface geometries. As a result, the (001), (101), (110), and (100) facets can be differentiated by the ³¹P NMR shifts of adsorbed TMP molecules, and their fractions in different nanomaterials can be extracted according to deconvoluted ³¹P NMR resonances. The results provide new insights on nanosized oxide acid catalysts.

Facet-dependent acidity of SnO₂ nanosheets and nanoshuttles is revealed with TMP-assisted ³¹P solid-state NMR spectroscopy and DFT calculations.

Role of Lithium Doping in P2-Na0.67Ni0.33Mn0.67O2 for Sodium-Ion Batteries

P2-structured Na_0.67Ni_0.33Mn_0.67O₂ (PNNMO) is a promising Na-ion battery cathode material, but its rapid capacity decay during cycling remains a hurdle. Li doping in layered transition-metal oxide (TMO) cathode materials is known to enhance their electrochemical properties. Nevertheless, the influence of Li at different locations in the structure has not been investigated. Here, the crystallographic role and electrochemical impact of lithium on different sites in PNNMO is investigated in Li _x Na_0.67-y Ni_0.33Mn_0.67O_2+δ (0.00 ≤ x ≤ 0.2, y = 0, 0.1). Lithium occupancy on prismatic Na sites is promoted in Na-deficient (Na < 0.67) PNNMO, evidenced by ex situ and operando synchrotron X-ray diffraction, X-ray absorption spectroscopy, and ⁷Li solid-state nuclear magnetic resonance. Partial substitution of Na with Li leads to enhanced stability and slightly increased specific capacity compared to PNNMO. In contrast, when lithium is located primarily on octahedral TM sites, capacity is increased but at the cost of stability.

Structural origin of the high-voltage instability of lithium cobalt oxide

Layered lithium cobalt oxide (LiCoO₂, LCO) is the most successful commercial cathode material in lithium-ion batteries. However, its notable structural instability at potentials higher than 4.35 V (versus Li/Li⁺) constitutes the major barrier to accessing its theoretical capacity of 274 mAh g^-1. Although a few high-voltage LCO (H-LCO) materials have been discovered and commercialized, the structural origin of their stability has remained difficult to identify. Here, using a three-dimensional continuous rotation electron diffraction method assisted by auxiliary high-resolution transmission electron microscopy, we investigate the structural differences at the atomistic level between two commercial LCO materials: a normal LCO (N-LCO) and a H-LCO. These powerful tools reveal that the curvature of the cobalt oxide layers occurring near the surface dictates the structural stability of the material at high potentials and, in turn, the electrochemical performances. Backed up by theoretical calculations, this atomistic understanding of the structure-performance relationship for layered LCO materials provides useful guidelines for future design of new cathode materials with superior structural stability at high voltages.

Unravelling the Fast Alkali-Ion Dynamics in Paramagnetic Battery Materials Combined with NMR and Deep-Potential Molecular Dynamics Simulation

Solid-state nuclear magnetic resonance (ssNMR) has received extensive attention in characterizing alkali-ion battery materials because it is highly sensitive for probing the local environment and dynamic information of atoms/ions. However, precise spectral assignment cannot be carried out by conventional DFT for high-rate battery materials at room temperature. Herein, combining DFT calculation of paramagnetic shift and deep potential molecular dynamics (DPMD) simulation to achieve the converged Na⁺ distribution at hundreds of nanoseconds, we obtain the statistically averaged paramagnetic shift, which is in excellent agreement with ssNMR measurements. Two ²³ Na shifts induced by different stacking sequences of transition metal layers are revealed in the fast chemically exchanged NMR spectra of P2-type Na_2/3 (Mg_1/3 Mn_2/3 )O₂ for the first time. This DPMD simulation auxiliary protocol can be beneficial to a wide range of ssNMR analysis in fast chemically exchanged material systems.

Bridging Structural Inhomogeneity to Functionality: Pair Distribution Function Methods for Functional Materials Development

The correlation between structure and function lies at the heart of materials science and engineering. Especially, modern functional materials usually contain inhomogeneities at an atomic level, endowing them with interesting properties regarding electrons, phonons, and magnetic moments. Over the past few decades, many of the key developments in functional materials have been driven by the rapid advances in short-range crystallographic techniques. Among them, pair distribution function (PDF) technique, capable of utilizing the entire Bragg and diffuse scattering signals, stands out as a powerful tool for detecting local structure away from average. With the advent of synchrotron X-rays, spallation neutrons, and advanced computing power, the PDF can quantitatively encode a local structure and in turn guide atomic-scale engineering in the functional materials. Here, the PDF investigations in a range of functional materials are reviewed, including ferroelectrics/thermoelectrics, colossal magnetoresistance (CMR) magnets, high-temperature superconductors (HTSC), quantum dots (QDs), nano-catalysts, and energy storage materials, where the links between functions and structural inhomogeneities are prominent. For each application, a brief description of the structure-function coupling will be given, followed by selected cases of PDF investigations. Before that, an overview of the theory, methodology, and unique power of the PDF method will be also presented.

In-Plane Cation Ordering and Sodium Displacements in Layered Honeycomb Oxides with Tetravalent Lanthanides: Na2LnO3 (Ln = Ce, Pr, and Tb)

The detailed structural characterization of "213" honeycomb systems is a key concern in a wide range of fundamental areas, such as frustrated magnetism, and technical applications, such as cathode materials, catalysts, and thermoelectric materials. Na₂LnO₃ (Ln = Ce, Pr, and Tb) are an intriguing series of "213" honeycomb systems because they host tetravalent lanthanides. "213" honeycomb materials have been reported to adopt either a cation-disordered R3̅m subcell, a cation-ordered trigonal (P3₁12), or monoclinic (C2/c or C2/m) supercell. On the basis of analysis of the average (synchrotron diffraction) and local [pair distribution function (PDF) and solid-state NMR] structure probes, cation ordering in the honeycomb layer of Na₂LnO₃ materials has been confirmed. Through rationalization of the ²³Na chemical shifts and quadrupolar coupling constants, the local environment of Na atoms was probed with no observed evidence of cation disorder. Through these studies, it is shown that the Na₂LnO₃ materials adopt a C2/c supercell derived from symmetry-breaking displacements of intralayered Na atoms from the ideal crystallographic position (in C2/m). The Na displacement is validated using distortion index parameters from diffraction data and atomic displacement parameters from PDF data. The C2/c supercell is faulted, as evidenced by the increased breadth of the superstructure diffraction peaks. DIFFaX simulations and structural considerations with a two-phase approach were employed to derive a suitable faulting model.

Phase-sensitive γ-encoded recoupling of heteronuclear dipolar interactions and 1H chemical shift anisotropy

γ-encoded recoupling sequences are known to produce strong amplitude modulations that lead to sharp doublets when Fourier transformed. These doublets depend very little on the recoupled tensor asymmetry and thus enable for the straightforward determination of dynamic order parameters. It can, however, be difficult to measure small anisotropies, or small order parameters, using such sequences; the resonances from the doublet may overlap with each other, or with the zero-frequency glitch. This limitation has prevented the widespread use of ¹H chemical shift anisotropy (CSA) for the measurement of dynamics, particularly for CH protons which typically have CSAs of only a few ppm when immobile. Here, we introduce a simple modification to the traditional ¹H CSA and proton-detected local field pulse sequences that enables the acquisition of a hypercomplex dataset and the removal of the uncorrelated magnetization that results in the zero-frequency glitch. These new sequences then yield a frequency shift in the indirect dimension, rather than a splitting, which is easily identifiable even in cases of weak interactions.

Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries

Ni-rich layered cathode materials are among the most promising candidates for high-energy-density Li-ion batteries, yet their degradation mechanisms are still poorly understood. We report a structure-driven degradation mechanism for NMC811 (LiNi_0.8Mn_0.1Co_0.1O₂), in which a proportion of the material exhibits a lowered accessible state of charge at the end of charging after repetitive cycling and becomes fatigued. Operando synchrotron long-duration X-ray diffraction enabled by a laser-thinned coin cell shows the emergence and growth in the concentration of this fatigued phase with cycle number. This degradation is structure driven and is not solely due to kinetic limitations or intergranular cracking: no bulk phase transformations, no increase in Li/Ni antisite mixing and no notable changes in the local structure or Li-ion mobility of the bulk are seen in aged NMCs. Instead, we propose that this degradation stems from the high interfacial lattice strain between the reconstructed surface and the bulk layered structure that develops when the latter is at states of charge above a distinct threshold of approximately 75%. This mechanism is expected to be universal in Ni-rich layered cathodes. Our findings provide fundamental insights into strategies to help mitigate this degradation process.

Mitigation of Jahn-Teller distortion and Na+/vacancy ordering in a distorted manganese oxide cathode material by Li substitution

Layered manganese-based oxides are promising candidates as cathode materials for sodium-ion batteries (SIBs) due to their low cost and high specific capacity. However, the Jahn–Teller distortion from high-spin Mn³⁺ induces detrimental lattice strain and severe structural degradation during sodiation and desodiation. Herein, lithium is introduced to partially substitute manganese ions to form distorted P′2-Na_0.67Li_0.05Mn_0.95O₂, which leads to restrained anisotropic change of Mn–O bond lengths and reinforced bond strength in the [MnO₆] octahedra by mitigation of Jahn–Teller distortion and contraction of MnO₂ layers. This ensures the structural stability during charge and discharge of P′2-Na_0.67Li_0.05Mn_0.95O₂ and Na⁺/vacancy disordering for facile Na⁺ diffusion in the Na layers with a low activation energy barrier of ∼0.53 eV. It exhibits a high specific capacity of 192.2 mA h g⁻¹, good cycling stability (90.3% capacity retention after 100 cycles) and superior rate capability (118.5 mA h g⁻¹ at 1.0 A g⁻¹), as well as smooth charge/discharge profiles. This strategy is effective to tune the crystal structure of layered oxide cathodes for SIBs with high performance.

Li-Substitution in P′2-Na_0.67MnO₂ mitigates the anisotropic change of Mn–O bonds and Na/vacancy ordering, and hence significantly promotes its cycling stability and rate capability as a cathode material for sodium-ion batteries.

Separation of quadrupolar and paramagnetic shift interactions with TOP-STMAS/MQMAS in solid-state lighting phosphors

A new approach for processing satellite-transition magic-angle spinning (STMAS) and multiple-quantum magic-angle spinning (MQMAS) data, based on the two-dimensional one-pulse (TOP) method, which separates the second-rank quadrupolar anisotropy and paramagnetic shift interactions via a double shearing transformation, is described. This method is particularly relevant in paramagnetic systems, where substantial inhomogeneous broadening may broaden the lineshapes. Furthermore, it possesses an advantage over the conventional processing of MQMAS and STMAS spectra because it overcomes the limitation on the spectral width in the indirect dimension imposed by rotor synchronization of the sampling interval. This method was applied experimentally to the 27 Al solid-state nuclear magnetic resonance of a series of yttrium aluminum garnets (YAGs) doped with different lanthanide ions, from which the quadrupolar parameters of paramagnetically shifted and bulk unshifted sites were extracted. These parameters were then compared with density functional theory calculations, which permitted a better understanding of the local structure of Ln substituent ions in the YAG lattice.

Operando NMR of NMC811/Graphite Lithium-Ion Batteries: Structure, Dynamics, and Lithium Metal Deposition

Lithium-ion batteries (LIBs) are of tremendous importance for our society, but their limited lifetime still poses a great challenge. For a better understanding of battery cycling and degradation, operando analytical measurements are invaluable. In this work, we demonstrate that operando ⁷Li nuclear magnetic resonance (NMR) spectroscopy can be applied to full LIBs. We exemplify this on LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811)/graphite cells, which are typical high-energy LIBs. Employing industry-standard electrodes, our operando cells show realistic cycling performance at practical rates, which allows us to conduct experiments at different rates and temperatures and to draw conclusions on the performance of LIBs. The NMR experiments monitor processes in both electrodes individually, including Li-ion mobility and its changes with temperature. Moreover, Li metal deposition on graphite is observed at low temperature, which is an important degradation mechanism in LIBs and a severe safety hazard. Our experiments offer unique insights into this Li metal deposition process under different charging conditions.

Fully Exploited Oxygen Redox Reaction by the Inter-Diffused Cations in Co-Free Li-Rich Materials for High Performance Li-Ion Batteries

To meet the growing demand for global electrical energy storage, high-energy-density electrode materials are required for Li-ion batteries. To overcome the limit of the theoretical energy density in conventional electrode materials based solely on the transition metal redox reaction, the oxygen redox reaction in electrode materials has become an essential component because it can further increase the energy density by providing additional available electrons. However, the increase in the contribution of the oxygen redox reaction in a material is still limited due to the lack of understanding its controlled parameters. Here, it is first proposed that Li-transition metals (TMs) inter-diffusion between the phases in Li-rich materials can be a key parameter for controlling the oxygen redox reaction in Li-rich materials. The resulting Li-rich materials can achieve fully exploited oxygen redox reaction and thereby can deliver the highest reversible capacity leading to the highest energy density, ≈1100 Wh kg-1 among Co-free Li-rich materials. The strategy of controlling Li/transition metals (TMs) inter-diffusion between the phases in Li-rich materials will provide feasible way for further achieving high-energy-density electrode materials via enhancing the oxygen redox reaction for high-performance Li-ion batteries.