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

Diagnostic Protocols for Evaluating the Degradation Mechanisms in Gel-Polymer Lithium Batteries

Gel polymer electrolytes (GPEs) present a promising alternative to standard liquid electrolytes (LE) for Lithium-ion Batteries (LIBs) and Lithium Metal Batteries bridging the advantages of both liquid and solid polymer electrolytes. However, their cycle life still lags behind that of standard LIBs, and their degradation mechanisms remain poorly understood. A significant challenge is the need for specific diagnostic protocols to systematically study the degradation mechanisms of GPE-based cells. Challenges include the separation of cell components and effective washing, as well as the study of the solid electrolyte interfaces, all complicated by the semi-solid nature of GPEs. This paper provides a brief review of existing literature and proposes a comprehensive set of diagnostic tools for dismantling and evaluating the degradation of GPE-based LIBs. Finally, these methods and recommendations are applied to LiNi0.5Mn1.5O4 (LNMO)-graphite cells, revealing electrolyte oxidation as a major source of cell degradation.

Morphology characterization of dendrites on lithium metal electrodes by NMR spectroscopy

Despite the considerable potential offered by lithium metal's high capacity for rechargeable batteries, challenges such as dendrite formation and safety concerns persist. As strategies continue to advance in dendrite management, the demand for efficient monitoring tools becomes increasingly pronounced. In this study, we delve into the characterization of dendrites, elucidating the influence of microstructure morphology on the NMR spectrum using a combination of simulations and experiments. Systematic variations in various geometrical parameters highlight dendrite density as a pivotal distinguishing feature. Furthermore, the investigation explores the effectiveness of a pulse sequence in selectively exciting microstructures over the bulk, providing valuable insights into mitigating dendrite-related challenges in lithium-metal batteries.

SCREENES: Enhancing non-uniform sampling reconstruction for symmetrical NMR spectroscopy

BACKGROUND

Symmetrical NMR spectroscopy, such as Total Correlation Spectroscopy (TOCSY) and other homonuclear spectroscopy, displays symmetry in chemical shift but are generally not symmetrical in terms of intensity, which constitutes a pivotal branch of multidimensional NMR spectroscopy and offers a robust tool for elucidating the structures and dynamics of complex samples, particularly in the context of biological macromolecules. Non-Uniform Sampling (NUS) stands as a critical technique for accelerating multidimensional NMR experiments. However, symmetrical NMR spectroscopy inherently presents dynamic peak intensities, where cross peaks tend to be substantially weaker compared to diagonal peaks. Recovering these weaker cross peaks from NUS data poses a significant challenge, often resulting in compromised data quality.

RESULTS

We enhance the reconstruction quality of NUS symmetrical NMR spectroscopy based on the assumption that the asymmetry in intensity is mild. Regarding the sampling schedule, we employ the symmetrical sampling structure integrated with Poisson sampling schedule to enhance the efficiency of data acquisition. In term of the reconstruction algorithm, we propose the new method by incorporating hard and soft symmetrical constraints into our recently developed L1-norm-based Compressed Sensing (CS) method known as Sparse Complex-valued REconstruction Enabled by Newton method (SCREEN). Additionally, we propose a two-step reconstruction strategy that separately addresses diagonal and cross peaks. In this two-step strategy, cross peaks are effectively reconstructed by excluding the stronger diagonal peaks. Extensive experimental results validate the effectiveness of our proposed methodology.

SIGNIFICANCE

This method enhances the overall quality of the reconstructed NUS symmetrical NMR spectra, especially in terms of cross peaks, thereby enriching the interpretation of spectral information. Furthermore, it boosts the robustness towards regularization parameters, facilitating a user-friendly experience.

A New Method for In-Situ Characterization of Solid-State Batteries Based on Optical Coherence Tomography

With the in-depth study of solid-state batteries (SSBs), various in situ and ex situ characterization technologies have been widely used to study them. The performance and reliability of SSBs are limited by the formation and evolution of lithium dendrites at the interfaces between solid electrodes and solid electrolytes. We propose a new method based on optical coherence tomography (OCT) for in situ characterization of the internal state of solid-state batteries. OCT is a low-loss, high-resolution, non-invasive imaging technique that can provide real-time monitoring of cross-sectional images of internal structures of SSBs. The morphology, growth, and evolution of lithium dendrites at different stages of cycling under various conditions can be visualized and quantified by OCT. Furthermore, we validate and correlate the OCT results with scanning electron microscopy (SEM) and XPS, proving the accuracy and effectiveness of the OCT characterization method. We reveal the interfacial phenomena and challenges in SSBs and demonstrate the feasibility and advantages of OCT as a powerful tool for in situ and operando imaging of battery interfaces. This study provides new insights into the mechanisms and factors that affect SSB performance, safety, and lifetime, and suggests possible solutions for improvement and application in the field of applied energy.

Mechanisms of the Accelerated Li+ Conduction in MOF-Based Solid-State Polymer Electrolytes for All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) for lithium metal batteries have garnered considerable interests owing to their low cost, flexibility, lightweight, and favorable interfacial compatibility with battery electrodes. Their soft mechanical nature compared to solid inorganic electrolytes give them a large advantage to be used in low pressure solid-state lithium metal batteries, which can avoid the cost and weight of the pressure cages. However, the application of SPEs is hindered by their relatively low ionic conductivity. In addressing this limitation, enormous efforts are devoted to the experimental investigation and theoretical calculations/simulation of new polymer classes. Recently, metal-organic frameworks (MOFs) have been shown to be effective in enhancing ion transport in SPEs. However, the mechanisms in enhancing Li+ conductivity have rarely been systematically and comprehensively analyzed. Therefore, this review provides an in-depth summary of the mechanisms of MOF-enhanced Li+ transport in MOF-based solid polymer electrolytes (MSPEs) in terms of polymer, MOF, MOF/polymer interface, and solid electrolyte interface aspects, respectively. Moreover, the understanding of Li+ conduction mechanisms through employing advanced characterization tools, theoretical calculations, and simulations are also reviewed in this review. Finally, the main challenges in developing MSPEs are deeply analyzed and the corresponding future research directions are also proposed.

Advancements in Battery Monitoring: Harnessing Fiber Grating Sensors for Enhanced Performance and Reliability

Batteries play a crucial role as energy storage devices across various industries. However, achieving high performance often comes at the cost of safety. Continuous monitoring is essential to ensure the safety and reliability of batteries. This paper investigates the advancements in battery monitoring technology, focusing on fiber Bragg gratings (FBGs). By examining the factors contributing to battery degradation and the principles of FBGs, this study discusses key aspects of FBG sensing, including mounting locations, monitoring targets, and their correlation with optical signals. While current FBG battery sensing can achieve high measurement accuracies for temperature (0.1 °C), strain (0.1 με), pressure (0.14 bar), and refractive index (6 × 10-5 RIU), with corresponding sensitivities of 40 pm/°C, 2.2 pm/με, -0.3 pm/bar, and -18 nm/RIU, respectively, accurately assessing battery health in real time remains a challenge. Traditional methods struggle to provide real-time and precise evaluations by analyzing the microstructure of battery materials or physical phenomena during chemical reactions. Therefore, by summarizing the current state of FBG battery sensing research, it is evident that monitoring battery material properties (e.g., refractive index and gas properties) through FBGs offers a promising solution for real-time and accurate battery health assessment. This paper also delves into the obstacles of battery monitoring, such as standardizing the FBG encapsulation process, decoupling multiple parameters, and controlling costs. Ultimately, the paper highlights the potential of FBG monitoring technology in driving advancements in battery development.

From Oxygen Redox to Sulfur Redox: A Paradigm for Li-Rich Layered Cathodes

The utilization of anionic redox chemistry provides an opportunity to further improve the energy density of Li-ion batteries, particularly for Li-rich layered oxides. However, oxygen-based hosts still suffer from unfavorable structural rearrangement, including the oxygen release and transition metal (TM)-ion migration, in association with the tenuous framework rooted in the ionicity of the TM-O bonding. An intrinsic solution, by using a sulfur-based host with strong TM-S covalency, is proposed here to buffer the lattice distortion upon the highly activating sulfur redox process, and it achieves howling success in stabilizing the host frameworks. Experimental results demonstrate the prolonged preservation of the layered sulfur lattice, especially the honeycomb superlattice, during the Li+ extraction/insertion process in contrast to the large structural degeneration in Li-rich oxides. Moreover, the Li-rich sulfide cathodes exhibited a negligible overpotential of 0.08 V and a voltage drop of 0.13 mV/cycle, while maintaining a substantial reversible capacity upon cycling. These superior electrochemical performances can be unambiguously ascribed to the much shorter trajectories of sulfur in comparison to those of oxygen revealed by molecular dynamics simulations at a large scale (∼30 nm) and a long time scale (∼300 ps) via high-dimensional neural network potentials during the delithiation process. Our findings highlight the importance of stabilizing host frameworks and establish general guidance for designing Li-rich cathodes with durable anionic redox chemistry.

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.

Measuring T1 relaxation in paramagnetic solids with solid-state NMR: a case study on the milling induced phase transition in Li6CoO4

Solid-state NMR has been a vital tool for the study of structural evolution of cathodes in lithium-ion and sodium-ion batteries. However, the differentiation of relaxation parameters for certain sites is difficult owing to limited spectral resolution associated with strong anisotropic hyperfine interaction. Here we propose a novel IR-pjMATPASS method that can measure T₁ relaxation with site-specific resolution for paramagnetic solids. We apply this method to the characterization of ball-milling induced order-disorder phase transition in Li₆CoO₄ as a case study. The quasi-quantitate ⁷Li NMR enables the synthetic optimization of high energy ball-milling conditions to harvest a disordered cubic phase through site-specific ⁷Li T₁ measurements. The example study shown here provides a quantitative strategy for NMR studies of paramagnetic solids.

Understanding the failure process of sulfide-based all-solid-state lithium batteries via operando nuclear magnetic resonance spectroscopy

The performance of all-solid-state lithium metal batteries (SSLMBs) is affected by the presence of electrochemically inactive (i.e., electronically and/or ionically disconnected) lithium metal and solid electrolyte interphase (SEI), which are jointly termed inactive lithium. However, the differentiation and quantification of inactive lithium during cycling are challenging, and their lack limits the fundamental understanding of SSLMBs failure mechanisms. To shed some light on these crucial aspects, here, we propose operando nuclear magnetic resonance (NMR) spectroscopy measurements for real-time quantification and evolution-tracking of inactive lithium formed in SSLMBs. In particular, we examine four different sulfide-based solid electrolytes, namely, Li₁₀GeP₂S₁₂, Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, Li₆PS₅Cl and Li₇P₃S₁₁. We found that the chemistry of the solid electrolyte influences the activity of lithium. Furthermore, we demonstrate that electronically disconnected lithium metal is mainly found in the interior of solid electrolytes, and ionically disconnected lithium metal is found at the negative electrode surface. Moreover, by monitoring the Li NMR signal during cell calendar ageing, we prove the faster corrosion rate of mossy/dendritic lithium than flat/homogeneous lithium in SSLMBs.

The chemical evolution of solid electrolyte interface in sodium metal batteries

The solid electrolyte interface (SEI) formed on the anode is one of the key factors that determine the life span of sodium metal batteries (SMBs). However, the continuous evolution of SEI during charging/discharging processes complicates the fundamental understanding of its chemistry and structure. In this work, we studied the underlying mechanisms of the protection effect offered by the SEI derived from sodium difluoro(oxalato)borate (NaDFOB). In situ nuclear magnetic resonance (NMR) shows that the prior reduction of DFOB anion contributes to the SEI formation, and it suppresses the decomposition of carbonate solvents. Depth-profiling x-ray photoelectron spectroscopy and high-resolution solid-state NMR reveal that the DFOB anion is gradually turned into borate and fluoride-rich SEI with cycling. The protection effect of SEI reaches the optimum at 50 cycles, which triples the life span of SMB. The detailed investigations provide valuable guidelines for the SEI engineering.

Lithium Vacancy-Tuned [CuO4 ] Sites for Selective CO2 Electroreduction to C2+ Products

Electrochemical CO₂ reduction to valuable multi-carbon (C₂₊ ) products is attractive but with poor selectivity and activity due to the low-efficient CC coupling. Herein, a lithium vacancy-tuned Li₂ CuO₂ with square-planar [CuO₄ ] layers is developed via an electrochemical delithiation strategy. Density functional theory calculations reveal that the lithium vacancies (V_Li ) lead to a shorter distance between adjacent [CuO₄ ] layers and reduce the coordination number of Li⁺ around each Cu, featuring with a lower energy barrier for COCO coupling than pristine Li₂ CuO₂ without V_Li . With the V_Li percentage of ≈1.6%, the Li_{2- x} CuO₂ catalyst exhibits a high Faradaic efficiency of 90.6 ± 7.6% for C₂₊ at -0.85 V versus reversible hydrogen electrode without iR correction, and an outstanding partial current density of -706 ± 32 mA cm^-2 . This work suggests an attractive approach to create controllable alkali metal vacancy-tuned Cu catalytic sites toward C₂₊ products in electrochemical CO₂ reduction.

Dynamic Nuclear Polarization in battery materials

The increasing need for portable and large-scale energy storage systems requires development of new, long lasting and highly efficient battery systems. Solid state NMR spectroscopy has emerged as an excellent method for characterizing battery materials. Yet, it is limited when it comes to probing thin interfacial layers which play a central role in the performance and lifetime of battery cells. Here we review how Dynamic Nuclear Polarization (DNP) can lift the sensitivity limitation and enable detection of the electrode-electrolyte interface, as well as the bulk of some electrode and electrolyte systems. We describe the current challenges from the point of view of materials development; considering how the unique electronic, magnetic and chemical properties differentiate battery materials from other applications of DNP in materials science. We review the current applications of exogenous and endogenous DNP from radicals, conduction electrons and paramagnetic metal ions. Finally, we provide our perspective on the opportunities and directions where battery materials can benefit from current DNP methodologies as well as project on future developments that will enable NMR investigation of battery materials with sensitivity and selectivity under ambient conditions.