Solid-state Li NMR with applications to the translational dynamics in ion conductors

This review focuses on the application of nuclear magnetic resonance (NMR) spectroscopy in the study of lithium and sodium battery electrolytes. Lithium-ion batteries are widely used in electronic devices, electric vehicles, and renewable energy systems due to their high energy density, long cycle life, and low self-discharge rate. The sodium analog is still in the research phase, but has significant potential for future development. In both cases, the electrolyte plays a critical role in the performance and safety of these batteries. NMR spectroscopy provides a non-invasive and non-destructive method for investigating the structure, dynamics, and interactions of the electrolyte components, including the salts, solvents, and additives, at the molecular level. This work attempts to give a nearly comprehensive overview of the ways that NMR spectroscopy, both liquid and solid state, has been used in past and present studies of various electrolyte systems, including liquid, gel, and solid-state electrolytes, and highlights the insights gained from these studies into the fundamental mechanisms of ion transport, electrolyte stability, and electrode-electrolyte interfaces, including interphase formation and surface microstructure growth. Overviews of the NMR methods used and of the materials covered are presented in the first two chapters. The rest of the review is divided into chapters based on the types of electrolyte materials studied, and discusses representative examples of the types of insights that NMR can provide.

Sulfide solid-state electrolytes (SSEs), as the most important component of all-solid-state batteries (ASSBs), have a profound impact on their performance. Among the many sulfide SSEs, Li-argyrodite SSEs have been extensively studied and are considered to be one of the most promising solid sulfide-based Li superionic conductors nowadays. However, the SSEs still have some drawbacks to be addressed, such as limited ionic conductivity at room temperature, incompatible electrode/electrolyte interface, low operating voltage window, and poor air stability. It is found that doping strategies have a non-negligible role in solving the above problems. In this review, we first introduce the crystal structures of Li-argyrodite SSEs and provide a detailed description of Li-ion transport in Li-argyrodite SSEs, followed by a detailed description of structure of the Li-sublattice. Next, the mechanism of doping strategies to enhance the ionic conductivity of Li-argyrodite SSEs is focused. Particular emphasis is provided on the ionic dynamics in doped Li-argyrodite SSEs. In addition, the effects of doping strategies (e.g., soft acid ions and metal oxides) on improving the performance (electrode/electrolyte interface and air stability) of Li-argyrodite SSEs are comprehensively presented. Finally, attractive research directions and perspectives for doped Li-argyrodite SSEs are presented, which are of great significance in guiding the energy conversion and storage of Li-argyrodite-based ASSBs.

Organic ionic plastic crystals (OIPCs) are emerging as an important material family for solid-state electrolytes and many other applications. They have significant advantages over conventional electrolyte materials, such as high ionic conductivity, non-flammability, and plasticity. Various nuclear magnetic resonance (NMR) spectroscopy techniques including solid-state NMR, pulsed-field gradient (PFG) NMR, and magnetic resonance imaging (MRI) etc., provide us a versatile toolkit to understand the fundamental level structures, molecular dynamics, and ionic interactions in these materials. This article reviews the commonly used NMR methods including solid- and solution-state NMR, PFG-NMR, dynamic nuclear polarization (DNP) and the application of these methods in revealing the microscopic level structures and ion-transport mechanisms in OIPC materials.

⁷Li NMR spectroscopy is known to be very sensitive to translational motion in solids and therefore highly suited for investigating temperature-dependent Li⁺ dynamics. A number of different NMR methods are available for choosing the dynamical range of the observed Li⁺ jump frequencies present in inorganic solid-state electrolytes. This includes ⁷Li spin-alignment echo NMR spectroscopy, static ⁷Li lineshape analysis, and ⁷Li spin-lattice relaxometry that can be used to detect Li⁺ jumps in the Hz, kHz, and MHz range, respectively. We introduce and discuss these NMR techniques with respect to their theoretical description and practical application to investigate the Li⁺ dynamics at different time scales for the two solid-state electrolytes Li₈SiP₄ and Li₁₄SiP₆. The data evaluation for all methods is discussed in detail, focusing on the determination of Li⁺ jump frequencies and activation energies for the investigated self-diffusion processes in a given structure.

Progress in new sustainable technologies depends on the development of battery materials, specifically on safer, low-cost, and higher energy density batteries. One new type of materials are the halide solid electrolytes (HSEs), which have been shown to exhibit high ionic conductivity, deformability, and oxidative stability. Here, the synthesis of Li₃InCl₆ (LIC) HSEs by ball-milling followed by dry room annealing is investigated. Crystal structure, particle size, and ionic conductivity are analyzed using a combination of X-ray diffraction, transmission electron microscopy, and electrochemical impedance spectroscopy. Dry room annealing increases the presence of impurities in the sample but also increases the Li⁺ ionic conductivity up to 1.03 mS cm⁻¹. Additional pulsed-field gradient and relaxation time NMR measurements were performed to understand the lithium diffusion in the LIC samples. Two-dimensional diffusion – T₂ relaxation correlation and T₂ relaxation exchange measurements showed that there are multiple unique Li atomic motion sites, which are correlated to different rates of diffusive, micrometer-scale motion. This work outlines a simple solid-state synthesis approach and a novel strategy for designing advanced materials, understanding the ionic conduction, as well as the challenges in scalable wet processing of halide-based cathode sheets for solid-state battery applications.

LAGP (Li_1.5Al_0.5Ge_1.5(PO₄)₃) is one of inorganic solid electrolytes (ISEs) and is relatively stable against atmospheric CO₂ and H₂O. Five LAGP pellet samples with thicknesses ranging from 0.5 to 4 mm (10⁻³ m) were investigated by pulsed field gradient (PFG) ⁷Li NMR spectroscopy. PFG was applied in the thickness direction and the Li diffusion in the same direction was observed. The apparent diffusion coefficients (D_apparent) of Li were determined over the observation time (Δ) ranging from 5 to 100 × 10⁻³ s and the distances from 0.1 to 1 × 10⁻⁶ m. The D_apparent of LAGP samples was 10⁻¹¹ to 10⁻¹³ m²s⁻¹ at 28 °C. The results showed that the Li diffusion phenomenon was thickness dependent. When Δ was short, diffusion of Li was slower for thinner samples, suggesting that the Li pathway was made to avoid surface boundaries. The Li ions were found to diffuse through preferential and longer paths of the pellet samples. As Δ became longer, the diffusion process of Li averaged out and the diffusion coefficient converged to a constant value regardless of the thickness. The PFG-NMR observes a diffusion coefficient of ⁷Li species which are marked by the first PFG and detected by the second PFG after Δ. Impedance spectroscopy, on the other hand, measures the ionic conductivity for continuous charge transfer between electrodes under electric field, and higher ionic conductivities have been reported for thinner samples.