Ionic transport mechanisms in inorganic solid electrolytes: Interface, NMR and DNP studies

Advancements in electronic devices such as drones, hybrid, and electric cars have prompted demand for superior next-generation battery technologies, with research focusing on solid electrolytes (SEs). SEs are categorized into organic (polymer) and inorganic (ceramic) electrolytes, with this review focusing on inorganic solid electrolytes (ISEs). The main obstacles to high-performing ISEs are poor ionic conductivity at room temperature and high impedance at the electrode-electrolyte interface. Many strategies to improve the conductivity and interface have been attempted and are highlighted in detail in this review. This review commences by detailing the ion conduction mechanisms in ISEs, including halides, phosphates (NASICON), oxides (perovskite, antiperovskite, and garnet), and sulfides (argyrodite-type, LGPS-type, and LISICON). The review further explores the influence of defect chemistry, elemental substitution, ion migration pathways, ion doping, and phase stability on ionic mobility and interface in ISEs. Theoretical calculation and experimental characterization are discussed in parallel to give a comprehensive and deep grasp of ion movement and interfaces. Additionally, various nuclear magnetic resonance (NMR) techniques have been explored, such as NMR relaxometry to examine both slow and rapid bulk ion transport in ISEs, PFG-NMR to investigate ion self-diffusivity, and 2D NMR exchange spectroscopy (2D EXSY) to study ion exchange mechanisms. The review concludes by discussing dynamic nuclear polarization (DNP) as a hyperpolarization technique to enhance NMR sensitivity for electrode-electrolyte interfacial studies, and by proposing future research directions for ISEs.