Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries

Reassessing anionic redox in conventional layered oxide cathodes for Li-ion batteries: ionic and covalent mechanisms

Efforts to improve the specific capacity and energy density of lithium nickel-cobalt-manganese oxide (NCM) cathodes focus on operating at high voltages or increasing nickel content. However, both approaches necessitate a thorough understanding of the charge compensation mechanism. Traditional ionic-bonding models which separate transition metal (TM) and oxygen redox processes prove inadequate as anionic redox becomes significant, ignoring crucial metal-oxygen interactions. In this study, we systematically investigate the charge compensation process in low-nickel and high-nickel NCMs under high-voltage conditions. Here, the involvement of oxygen is critical in redox, as it shares electrons with TM to form a strong TM-O covalent bond. Compared to low-Ni NCMs, high-Ni NCMs exhibit an oxygen dimerization stage with trapped O2, which leads to the aggregation of vacancies in the transition metal layer, thereby accelerating structural destabilization. This variation in oxygen dimerization behavior among NCMs is closely correlated with differences in elemental composition, spin states, and stacking faults. Our findings comprehensively reveal the redox behaviors of transition metals and oxygen, particularly highlighting oxygen behavior at each delithiation state, helping to optimize the utilization of oxygen redox reactions in commercial NCM compounds for high-capacity and high-energy-density lithium-ion batteries.

Recent Advances in the Application of Magnetite (Fe3O4) in Lithium-Ion Batteries: Synthesis, Electrochemical Performance, and Characterization Techniques

With the promotion of portable energy storage devices and the popularization of electric vehicles, lithium-ion battery (LiB) technology plays a crucial role in modern energy storage systems. Over the past decade, the demands for LiBs have centered around high energy density and long cycle life. These parameters are often determined by the characteristics of the active materials in the electrodes. Given its high abundance, environmental friendliness, low cost and high capacity, magnetite (Fe3O4) emerges as a promising anode material. However, the practical application of Fe3O4 faces challenges, such as significant volume expansion during cycling. To overcome these obstacles and facilitate the commercialization of Fe3O4, a comprehensive understanding of its properties and behavior is essential. This review provides an overview of recent Fe3O4 research advances, focusing on its synthesis, factors influencing its electrochemical performance, and characterization techniques. By thoroughly understanding the characteristics of Fe3O4 in LiB applications, we can optimize its properties and enhance its performance, thereby paving the way for its widespread use in energy storage applications. Additionally, the review concludes with perspectives on promoting the commercialization of Fe3O4 in LiBs and future research directions.

Solvent-mediated oxide hydrogenation in layered cathodes

Self-discharge and chemically induced mechanical effects degrade calendar and cycle life in intercalation-based electrochromic and electrochemical energy storage devices. In rechargeable lithium-ion batteries, self-discharge in cathodes causes voltage and capacity loss over time. The prevailing self-discharge model centers on the diffusion of lithium ions from the electrolyte into the cathode. We demonstrate an alternative pathway, where hydrogenation of layered transition metal oxide cathodes induces self-discharge through hydrogen transfer from carbonate solvents to delithiated oxides. In self-discharged cathodes, we further observe opposing proton and lithium ion concentration gradients, which contribute to chemical and structural heterogeneities within delithiated cathodes, accelerating degradation. Hydrogenation occurring in delithiated cathodes may affect the chemo-mechanical coupling of layered cathodes as well as the calendar life of lithium-ion batteries.

Side Reactions/Changes in Lithium-Ion Batteries: Mechanisms and Strategies for Creating Safer and Better Batteries

Lithium-ion batteries (LIBs), in which lithium ions function as charge carriers, are considered the most competitive energy storage devices due to their high energy and power density. However, battery materials, especially with high capacity undergo side reactions and changes that result in capacity decay and safety issues. A deep understanding of the reactions that cause changes in the battery's internal components and the mechanisms of those reactions is needed to build safer and better batteries. This review focuses on the processes of battery failures, with voltage and temperature as the underlying factors. Voltage-induced failures result from anode interfacial reactions, current collector corrosion, cathode interfacial reactions, overcharge, and over-discharge, while temperature-induced failure mechanisms include SEI decomposition, separator damage, and interfacial reactions between electrodes and electrolytes. The review also presents protective strategies for controlling these reactions. As a result, the reader is offered a comprehensive overview of the safety features and failure mechanisms of various LIB components.

Improving the Cyclic Reversibility of Layered Li-Rich Cathodes by Combining Oxygen Vacancies and Surface Fluorination

Layered-type Li-rich cathode materials have attracted significant attention for next-generation Li-ion batteries, but the advantage of their high capacity is eclipsed by their poor reversibility upon cycling. Irreversible oxygen redox activity and surface degradation have been deemed as the root cause and direct cause for their poor performance, respectively. We attempted to suppress surface degradation by inserting fluoride ions up to some depth on the surface. By fluorination with NH4HF2 after introducing a significant amount of oxygen vacancies in layered Li1.2Ni0.2Co0.2Mn0.4O2 by using CaH2 as a reducing agent, the reversible capacity reached 268 mAh/g, and the capacity retention after 100 cycles was about 99%. The scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) technique revealed that, in contrast to directly fluorinated samples, our materials exhibit deeper fluorine signals besides surface signals, and hard X-ray photoelectron spectroscopy (HAXPES) patterns show ionic and covalent fluorine coordination. These results indicate that the combination of oxygen deficiency introduction and surface fluorination allows some F- ions to occupy near-surface oxygen vacancy sites rather than forming only a LiF layer on the surface, suggesting a new strategy to modify cathode materials for lithium-ion batteries.

State-of-charge of individual active material particles in lithium ion batteries: a perspective of analytical techniques and their capabilities

The state-of-charge (SOC) is an essential parameter for battery management systems to reflect and monitor the remaining capacity of individual battery cells. In addition to its application at the cell level, the SOC also plays an important role in the investigation of redox processes of cathode active materials (CAMs) in lithium ion batteries (LIBs) during electrochemical cycling. These processes can be influenced by a large variety of factors such as active material properties, inhomogeneities of the electrode, degradation phenomena and the charge/discharge protocol during cycling. Consequently, non-uniform redox reactions can occur, resulting in charge heterogeneities of the active material. This heterogeneity can translate into accelerated aging of the CAM and a reduction in reversible capacity of the battery cell, since the active material is not fully utilized. To understand and monitor the SOC heterogeneity at the mesoscale, a wide range of techniques have been implemented in the past. In this perspective an overview of current state-of-the-art techniques to evaluate charge heterogeneities of CAMs in LIBs is presented. Therefore, techniques which utilize synchrotron radiation like X-ray absorption near-edge structure (XANES) and transmission X-ray spectroscopy (TXM) are presented as well as Raman spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Next to these established techniques, classification single particle inductively coupled plasma optical emission spectroscopy (CL-SP-ICP-OES) as a new approach is also discussed in this perspective. For these techniques, the areas of application, advantages as well as drawbacks are highlighted and discussed.

Multimodal electrochemistry coupled microcalorimetric and X-ray probing of the capacity fade mechanisms of Nickel rich NMC - progress and outlook

Lithium nickel manganese cobalt oxide (NMC) is a commercially successful Li-ion battery cathode due to its high energy density; however, its delivered capacity must be intentionally limited to achieve capacity retention over extended cycling. To design next-generation NMC batteries with longer life and higher capacity the origins of high potential capacity fade must be understood. Operando hard X-ray characterization techniques are critical for this endeavor as they allow the acquisition of information about the evolution of structure, oxidation state, and coordination environment of NMC as the material (de)lithiates in a functional battery. This perspective outlines recent developments in the elucidation of capacity fade mechanisms in NMC through hard X-ray probes, surface sensitive soft X-ray characterization, and isothermal microcalorimetry. A case study on the effect of charging potential on NMC811 over extended cycling is presented to illustrate the benefits of these approaches. The results showed that charging to 4.7 V leads to higher delivered capacity, but much greater fade as compared to charging to 4.3 V. Operando XRD and SEM results indicated that particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade. Operando hard XAS revealed significant Ni and Co redox during cycling as well as a Jahn-Teller distortion at the discharged state (Ni³⁺); however, minimal differences were observed between the cells charged to 4.3 and 4.7 V. Additional XAS analyses using soft X-rays revealed significant surface reconstruction after cycling to 4.7 V, revealing another contribution to fade. Operando isothermal microcalorimetry (IMC) indicated that the high voltage charge to 4.7 V resulted in a doubling of the heat dissipation when compared to charging to 4.3 V. A lowered chemical-to-electrical energy conversion efficiency due to thermal energy waste was observed, providing a complementary characterization of electrochemical degradation. The work demonstrates the utility of multi-modal X-ray and microcalorimetric approaches to understand the causes of capacity fade in lithium-ion batteries with Ni-rich NMC.

The influence of electrochemical cycling protocols on capacity loss in nickel-rich lithium-ion batteries

The transition towards electric vehicles and more sustainable transportation is dependent on lithium-ion battery (LIB) performance. Ni-rich layered transition metal oxides, such as NMC811 (LiNi0.8Mn0.1Co0.1O2), are promising cathode candidates for LIBs due to their higher specific capacity and lower cost compared with lower Ni content materials. However, complex degradation mechanisms inhibit their use. In this work, tailored aging protocols are employed to decouple the effect of electrochemical stimuli on the degradation mechanisms in graphite/NMC811 full cells. Using these protocols, impedance measurements, and differential voltage analysis, the primary drivers for capacity fade and impedance rise are shown to be large state of charge changes combined with high upper cut-off voltage. Focused ion beam-scanning electron microscopy highlights that extensive microscale NMC particle cracking, caused by electrode manufacturing and calendering, is present prior to aging and not immediately detrimental to the gravimetric capacity and impedance. Scanning transmission electron microscopy electron energy loss spectroscopy reveals a correlation between impedance rise and the level of transition metal reduction at the surfaces of aged NMC811. The present study provides insight into the leading causes for LIB performance fading, and highlights the defining role played by the evolving properties of the cathode particle surface layer.