Beyond Composition: Surface Reactivity and Structural Arrangement of the Cathode–Electrolyte Interphase

Abstract

The role of the cathode–electrolyte interphase (CEI) on battery performance has been historically overlooked due to the anodic stability of carbonate-based electrolytes used in Li-ion batteries. Yet, over the past few decades, degradation in device lifetime has been attributed to cathode surface reactivity, ion transport at the cathode/electrolyte interface, and structural transformations that occur at the cathode surface. In this review, we highlight recent progress in analytical techniques that have facilitated these insights and elucidated not only the CEI composition but also the spatial distribution of electrolyte decomposition products in the CEI as well as cathode-driven reactions that occur during battery operation. With a deeper understanding of the CEI and the processes that lead to its formation, these advanced characterization tools can unlock routes to mitigate impedance rise, particle cracking, transition metal dissolution, and electrolyte consumption, ultimately enabling longer lasting, safer batteries.

Direct NCA Cathode Active Materials Recycling from Spent Li-Ion Batteries: Solvent-Free Recovery and Healing by Heat Treatment

“…And no significant changes were observed in the particles of the cathode active material morphology after 50 cycles, demonstrating the structural stability of the repaired R 1 -NCA-900 sample. However, a coating-like is covering all the particles of the materials indicating the presence of a stable cathode–electrolyte interface (CEI). , …”

Hybrid Density Functional Theory Comparison of Oxygen Release and Solvent Decomposition Kinetics on LixNiO2 Surfaces

“…This value is likely overestimated, because it omits diffusion limitations. Then, ΔE > 0.92 eV suggests reaction times of at least 280 s at T = 300 K. In contrast, the experimental nonreactive relaxation time of 1 O 2 to the ground state 3 O 2 has been reported to be submilliseconds in ethereal solvents. 42 Nonreactive relaxation processes in other solvents are also much faster than seconds.…”

“…High-nickel-content layered oxides − have emerged as leading candidates for cathode materials in electrical vehicles. The higher voltages associated with these materials and the exposure of Ni cations on layered oxide cathode surfaces are however known to enhance degradation reactions, including oxidation of liquid electrolytes − accelerated at high voltages; ,− oxygen release (including reactive O 2 like spin-singlet 1 O 2 , superoxide anions O 2 – , and peroxide anions O 2 2– ), − transition metal ion dissolution, surface layer phase transformation, − and particle cracking. , These mechanisms are likely interrelated; for example, surface oxygen loss mostly likely contributes to rock salt transformation. , Elucidating and mitigating these degradation mechanisms will yield improvements in battery cycle life and safety and allow cost reduction.…”

Unprotected Organic Cations─The Dilemma of Highly Li-Concentrated Ionic Liquid Electrolytes

“…Lithium-ion batteries (LIBs) have emerged as the leading technology in secondary batteries, owing to their exceptional electrochemical performance and well-established technologies. − As a crucial component of electrochemical energy storage devices, the electrolyte selected for LIBs necessitates a stable electrochemical potential windowtypically determined by its oxidation and reduction potentials, to enable high operating voltages and, consequently, high energy densities. , It is worth noting that the performance of LIBs is also closely linked to the different liquid environments established during electrolyte development due to their significant impact on the interfacial properties. − Recently, increasing the concentration of Li salts in the solvents (nonaqueous and aqueous) has been intensively pursued as a strategy for enhancing and differentiating the electrochemical properties of electrolytes. − These highly concentrated electrolytes have been found to offer electrochemical advantages by reducing the number of solvating molecules. For instance, the first solvation of the Li + ion sheath in highly concentrated electrolytes harbors more anions than their diluted counterparts, forming abundant ion pairs such as contact ion pairs and aggregates (commonly referred to as CIP and AGG, respectively) .…”