"Win-Win" Modification of LiCoO2 Enables Stable and Long-Life Cycling of Sulfide-Based All Solid-State Batteries

Interfacial side reactions and space charge layers between the oxide cathode material and the sulfide solid-state electrolytes (SSEs), along with the structural degradation of the active material, significantly compromise the electrochemical performance of all-solid-state batteries (ASSLBs). Surface coating and bulk doping of the cathodes are considered the most effective approaches to mitigate the interface issues between the cathode and SSEs and enhance the structural integrity of composite cathodes. Here, a one-step low-cost means is ingeniously designed to modify LiCoO2 (LCO) with heterogeneous Li2 TiO3 /Li(TiMg)1/2 O2 surface coating and bulk gradient Mg doping. When applied in Li10 GeP2 S12 -based ASSLBs, the Li2 TiO3 and Li(TiMg)1/2 O2 coating layers effectively suppress interfacial side reactions and weaken space charge layer effect. Furthermore, gradient Mg doping stabilizes the bulk structure to mitigate the formation of spinel-like phases during local overcharging caused by solid-solid contact. The modified LCO cathodes exhibit excellent cycle performance with a capacity retention of 80 % after 870 cycles. This dual-functional strategy provides the possibility for large-scale commercial implementation of cathodes modification in sulfide based ASSLBs in the future.

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.

Distinguishing the Effects of the Space-Charge Layer and Interfacial Side Reactions on Li10GeP2S12-Based All-Solid-State Batteries with Stoichiometric-Controlled LiCoO2

All-solid-state lithium batteries (ASSLBs) with high volumetric energy density and enhanced safety are considered one of the most promising next-generation batteries. Elucidating the capacity-fading mechanism caused by the space-charge layer (SCL) and the interfacial side reaction (ISR) is crucial for the future development of high-energy-density ASSLBs with a longer cycle life. Here, a systematic study to probe the electrochemical performance of Li₁₀GeP₂S₁₂-based ASSLBs with stoichiometric-controlled Li_xCoO₂ was performed with the aid of density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS), focused ion beam-field emission scanning electron microscopy (FIB-SEM), and solid-state nuclear magnetic resonance (NMR) spectroscopy. We discovered that the overstoichiometric Li_1.042CoO₂ shows a high capacity at first cycle with the smallest overpotential, but the capacity gradually decreases, which is ascribed to the weak SCL effect and strong interfacial side reactions. On the contrary, the lithium-deficient Li_0.945CoO₂ achieves the best cycling stability with a very low capacity associated with the strongest SCL effect and weak interfacial side reactions. The SCL effect is indeed coupled with ISR, which eventually leads to capacity fading in long-term operation. We believe that the new insights gained from this work will accelerate the future development of LiCoO₂/LGPS-based ASSLBs with both a mitigated SCL effect and a longer cycle life.

Solid-State NMR and MRI Spectroscopy for Li/Na Batteries: Materials, Interface, and In Situ Characterization

Enhancing the electrochemical performance of batteries, including the lifespan, energy, and power densities, is an everlasting quest for the rechargeable battery community. However, the dynamic and coupled (electro)chemical processes that occur in the electrode materials as well as at the electrode/electrolyte interfaces complicate the investigation of their working and decay mechanisms. Herein, the recent developments and applications of solid-state nuclear magnetic resonance (ssNMR) and magnetic resonance imaging (MRI) techniques in Li/Na batteries are reviewed. Several typical cases including the applications of NMR spectroscopy for the investigation of the pristine structure and the dynamic structural evolution of materials are first emphasized. The NMR applications in analyzing the solid electrolyte interfaces (SEI) on the electrode are further concluded, involving the identification of SEI components and investigation of ionic motion through the interfaces. Beyond, the new development of in situ NMR and MRI techniques are highlighted, including their advantages, challenges, applications and the design principle of in situ cell. In the end, a prospect about how to use ssNMR in battery research from the perspectives of materials, interface, and in situ NMR, aiming at obtaining deeper insight of batteries with the assistance of ssNMR is represented.

Direct 17O Isotopic Labeling of Oxides Using Mechanochemistry

While ¹⁷O NMR is increasingly being used for elucidating the structure and reactivity of complex molecular and materials systems, much effort is still required for it to become a routine analytical technique. One of the main difficulties for its development comes from the very low natural abundance of ¹⁷O (0.04%), which implies that isotopic labeling is generally needed prior to NMR analyses. However, ¹⁷O-enrichment protocols are often unattractive in terms of cost, safety, and/or practicality, even for compounds as simple as metal oxides. Here, we demonstrate how mechanochemistry can be used in a highly efficient way for the direct ¹⁷O isotopic labeling of a variety of s-, p-, and d-block oxides, which are of major interest for the preparation of functional ceramics and glasses: Li₂O, CaO, Al₂O₃, SiO₂, TiO₂, and ZrO₂. For each oxide, the enrichment step was performed under ambient conditions in less than 1 h and at low cost, which makes these synthetic approaches highly appealing in comparison to the existing literature. Using high-resolution solid-state ¹⁷O NMR and dynamic nuclear polarization, atomic-level insight into the enrichment process is achieved, especially for titania and alumina. Indeed, it was possible to demonstrate that enriched oxygen sites are present not only at the surface but also within the oxide particles. Moreover, information on the actual reactions occurring during the milling step could be obtained by ¹⁷O NMR, in terms of both their kinetics and the nature of the reactive species. Finally, it was demonstrated how high-resolution ¹⁷O NMR can be used for studying the reactivity at the interfaces between different oxide particles during ball-milling, especially in cases when X-ray diffraction techniques are uninformative. More generally, such investigations will be useful not only for producing ¹⁷O-enriched precursors efficiently but also for understanding better mechanisms of mechanochemical processes themselves.

The direct ¹⁷O enrichment of s-, p-, and d-block metal oxides is achieved with high efficiency using mechanochemistry. Atomic-level insight into the enrichment process is obtained using high-resolution solid-state ¹⁷O NMR and dynamic nuclear polarization analyses, which demonstrate that enriched oxygen sites are present both at the surface and within the oxide particles. Moreover, it is demonstrated how these labeling schemes allow the study of unique aspects of mechanochemical reactions between oxides by ¹⁷O NMR.