Microscopy Study of Structural Evolution in Epitaxial LiCoO2 Positive Electrode Films during Electrochemical Cycling

Degradation Mechanisms of Electrodes Promotes Direct Regeneration of Spent Li-Ion Batteries: A Review

The rapid growth of electric vehicle use is expected to cause a significant environmental problem in the next few years due to the large number of spent lithium-ion batteries (LIBs). Recycling spent LIBs will not only alleviate the environmental problems but also address the challenge of limited natural resources shortages. While several hydro- and pyrometallurgical processes are developed for recycling different components of spent batteries, direct regeneration presents clear environmental, and economic advantages. The principle of the direct regeneration approach is restoring the electrochemical performance by healing the defective structure of the spent materials. Thus, the development of direct regeneration technology largely depends on the formation mechanism of defects in spent LIBs. This review systematically details the degradation mechanisms and types of defects found in diverse cathode materials, graphite anodes, and current collectors during the battery's lifecycle. Building on this understanding, principles and methodologies for directly rejuvenating materials within spent LIBs are outlined. Also the main challenges and solutions for the large-scale direct regeneration of spent LIBs are proposed. Furthermore, this review aims to pave the way for the direct regeneration of materials in discarded lithium-ion batteries by offering a theoretical foundation and practical guidance.

Overcharge-Induced Phase Heterogeneity and Resultant Twin-Like Layer Deformation in Lithium Cobalt Oxide Cathode for Lithium-Ion Batteries

Overcharging is expected to be one of the solutions to overcome the current energy density limitation of lithium-ion battery cathodes, which will support the rapid growth of the battery market. However, high-voltage charging often poses a major safety threat including fatal incendiary incidents, limiting further application. Numerous researches are dedicated to the disadvantages of the overcharging process; nonetheless, the urgent demand for addressing failure mechanisms is still unfulfilled. Herein, it is revealed that overcharging induces phase heterogeneity into layered and cobalt oxide phases, and consequent "twin-like deformation" in lithium cobalt oxide. The interplay between the uncommon cobalt(III) oxide and the deformation is investigated by revealing the atomistic formation mechanism. Most importantly, abnormal cracking is discovered in the vicinity of the cobalt oxide where structural instability induces substantial contraction. In addition, surface degradation is widely observed in the crack boundary inside the particle. As unintentional overcharging can occur due to local imbalance in state-of-charge in severe operating conditions such as fast charging, the issues on overcharging should be emphasized to large extent and this study provides fundamental knowledge of overcharge by elucidating the crack development mechanism of layered cathodes, which is expected to broaden the horizon into high voltage operation.

Nanoscale-engineered LiCoO2as a high energy cathode for wide temperature lithium-ion battery applications-role of coating chemistry and thickness

Extending the charge cutoff voltage of LiCoO₂(LCO) beyond 4.2 V is considered as a key parameter to obtain higher energy densities. Following gaps have been identified based on a thorough literature survey especially for higher cutoff voltage of nanoscale engineered LCO cathodes, (i) different metal oxides and metal fluoride surface coatings have been mostly done independently by different groups, (ii) room temperature performance was the focus with limited investigations at high temperature, (iii) nonexistence of low temperature cycling studies and (iv) no reports on high rate capability of LCO beyond 4.5 V (especially at 4.8 V) needs to be investigated. Herein, we report the effect of nanoscale engineering of LCO along with the role of coating chemistry and thickness to study its electrochemical performance at higher voltages and at wide operating temperatures. Surface coating was implemented with different metal oxides and a metal fluoride with tunable thickness. At 4.5 V, 5 wt% Al₂O₃coated LiCoO₂(LCO@Al₂O₃-5) delivered a reversible capacity of 169 mAh g^-1at 100 mA g^-1and 151 mAh g^-1at high rate of 10 C (2 A g^-1) and 72% retention at the end of 500 cycles. At 55 °C, it exhibited better stability over 500 cycles at 5 C and even at -12.5 °C it maintained 72% of its initial capacity after 100 cycles at 200 mA g^-1. At 4.8 V cut-off, LCO@Al₂O₃-5 rendered reversible capacity of 213 mAh g^-1at 100 mA g^-1, a high value compared to literatures reported for LCO. Also noted that it delivered a capacity of 126 mAh g^-1at a current density of 1 A g^-1, whereas bare could only exhibit 66 mAh g^-1under same testing conditions. Enhanced performance of LCO@Al₂O₃-5 can be ascribed to the lower charge transfer resistance derived from the stable solid solution formation on the interface.Ex situXRD andex situRaman analysis at different stages of charge/discharge cycles correlates the enhanced performance of LCO@Al₂O₃-5 with its structural stability and minimal structural degradation.

In Situ Monitoring of Thermally Induced Effects in Nickel-Rich Layered Oxide Cathode Materials at the Atomic Level

The thermal stability of cathode active materials (CAMs) is of major importance for the safety of lithium-ion batteries (LIBs). A thorough understanding of how commercially viable layered oxide CAMs behave at the atomic length scale upon heating is indispensable for the further development of LIBs. Here, structural changes of Li(Ni_0.85Co_0.15Mn_0.05)O₂ (NCM851005) at elevated temperatures are studied by in situ aberration-corrected scanning transmission electron microscopy (AC-STEM). Heating NCM851005 inside the microscope under vacuum conditions enables us to observe phase transitions and other structural changes at high spatial resolutions. This has been primarily possible by establishing low-dose electron beam conditions in STEM. Specific focus is put on the evolution of inherent nanopore defects found in the primary grains, which are believed to play an important role in LIB degradation. The onset temperature of structural changes is found to be ∼175 °C, resulting in phase transformation from a layered to a rock-salt-like structure, especially at the internal interfaces, and increasing intragrain inhomogeneity. The reducing environment and heat application lead to the formation and subsequent densification of {003}- and {014}-type facets. In the light of these results, postsynthesis electrode drying processes applied under reducing environment and heat, for example, in the preparation of solid-state batteries, should be re-examined carefully.

Plane-Selective Coating of Li2SnO3 on Li[NixCo1-x]O2 for High Power Li ion Batteries

Interphase engineering is becoming increasingly important in improving the electrochemical performance of cathode materials for rechargeable batteries, including Li ion, Li metal, and all-solid-state batteries, because irreversible surface reactions, such as electrolyte decomposition, and transition metal dissolution, constitute one of these batteries' failure modes. In this connection, various surface-engineered cathode materials have been investigated to improve interfacial properties. No synthesis methods, however, have considered a plane-selective surface modification of cathode materials. Herein, we introduce the basal-plane-selective coating of Li₂SnO₃ on layered Li[Ni_xCo_1-x]O₂ (x = 0 and 0.5) using the concept of the thermal phase segregation of Sn-doped Li[Ni_xCo_1-x]O₂ due to the solubility variation of Sn in Li[Ni_xCo_1-x]O₂ with respect to temperature. The plane-selective surface modification enables the formation of Li₂SnO₃ nanolayers on only the Li[Ni_xCo_1-x]O₂ basal plane without hindering the charge transfer of Li⁺ ions. As a result, the vertical heterostructure of Li[Ni_xCo_1-x]O₂-Li₂SnO₃ core-shells show promising electrochemical performance.

High-Voltage Cycling Induced Thermal Vulnerability in LiCoO2 Cathode: Cation Loss and Oxygen Release Driven by Oxygen Vacancy Migration

The release of the lattice oxygen due to the thermal degradation of layered lithium transition metal oxides is one of the major safety concerns in Li-ion batteries. The oxygen release is generally attributed to the phase transitions from the layered structure to spinel and rocksalt structures that contain less lattice oxygen. Here, a different degradation pathway in LiCoO₂ is found, through oxygen vacancy facilitated cation migration and reduction. This process leaves undercoordinated oxygen that gives rise to oxygen release while the structure integrity of the defect-free region is mostly preserved. This oxygen release mechanism can be called surface degradation due to the kinetic control of the cation migration but has a slow surface to bulk propagation with continuous loss of the surface cation ions. It is also strongly correlated with the high-voltage cycling defects that end up with a significant local oxygen release at low temperatures. This work unveils the thermal vulnerability of high-voltage Li-ion batteries and the critical role of the surface fraction as a general mitigating approach.

Pulsed Laser Deposited Films for Microbatteries

This review article presents a survey of the literature on pulsed laser deposited thin film materials used in devices for energy storage and conversion, i.e., lithium microbatteries, supercapacitors, and electrochromic displays. Three classes of materials are considered: Positive electrode materials (cathodes), solid electrolytes, and negative electrode materials (anodes). The growth conditions and electrochemical properties are presented for each material and state-of-the-art of lithium microbatteries are also reported.

Electrochemical and Spectroscopic Analysis of the Ionogel-Electrode Interface

Ionogels, pseudo-solid-state electrolytes consisting of an ionic liquid electrolyte confined in a mesoporous inorganic matrix, have attracted interest recently due to their high ionic conductivity and physicochemical stability. These traits, coupled with their inherent solution processability, make them a viable solid electrolyte for solid-state battery systems. Despite the promising properties of ionogels, there have been very few investigations of the electrode-ionogel interface. In the present study, X-ray photoelectron spectroscopy, Raman spectroscopy, and electrochemical measurements were utilized to probe the surface reactions occurring at the electrode-ionogel interface for several electrode materials. Our results indicate that the sol acidity initiates breakdown of the organic constituents of the sol and reduction of the transition metals present in the electrode materials. This chemical attack forms an organic surface layer and affects the electrode composition, both of which can impede Li⁺ access. By modifying the silica sol-gel reaction via a two-step acid-base catalysis, these interfacial reactions can be avoided. Results are shown for a LiCoO₂ electrode in which a high Li-ion capacity and stable cycling were achieved.

Unveiling the Intrinsic Cycle Reversibility of a LiCoO2 Electrode at 4.8-V Cutoff Voltage through Subtractive Surface Modification for Lithium-Ion Batteries

The thermodynamic instability of the LiCoO₂ layered structure at >0.5Li extraction has been considered an obstacle for the reversible utilization of its near theoretical capacity at high cutoff voltage (>4.6 V vs Li/Li⁺) in lithium-ion batteries. Many previous studies have focused on resolving this issue by surface modification of LiCoO₂, which has proven to be effective in suppressing phase transformation. To determine the extent to which surface protection of LiCoO₂ is effective despite its thermodynamic instability and presumably incomplete reversibility involving the O1 phase, here we verify the intrinsic reversibility of bulk LiCoO₂ with extended lithium extraction by ruling out the effect of a surface. Specifically, first, we show that, contrary to conventional belief, electrochemical cycling of LiCoO₂ at a cutoff voltage of 4.8 V (vs Li/Li⁺) results in better cycle stability and lower polarizations than those at 4.6 V. We demonstrate, using an exhaustive suite of characterization tools, that the rapid cycle degradation under high-voltage cycling is mostly caused by the formation of a surface resistive layer; however, these damaged surfaces are leached out faster than they are accumulated above a certain potential, which results in superior cyclability compared with that achieved for less oxidative 4.6-V cycling. This beneficial leaching out of the resistive surface layer serves as a "subtractive" surface modification and plays a role in enhancing the cycle stability and is distinguished from conventional "additive" surface modification such as coating. This approach allows us to decouple factors of the bulk and surface degradations that contribute to the capacity fade and leads to the finding that, in the absence of a resistive surface, the capacity retention of a LiCoO₂ electrode with 4.8-V cutoff cycling can be intrinsically high, indicating that the instability of the crystalline Li _xCoO₂ ( x < 0.5) has a limited effect on the cycle stability. Our findings also explain why the strategy of coating foreign materials on the surface of LiCoO₂ can improve the high-voltage cycling to some extent despite the expected thermodynamic instability of the highly charged phase.

A deep study of the protection of Lithium Cobalt Oxide with polymer surface modification at 4.5 V high voltage

Charging the cells above a conventional voltage of 4.2 V is a promising attempt to increase the energy density of Lithium Cobalt Oxide (LCO), however, the problem of crystal instability at high voltage that leading deterioration of cycle performance needs to be urgently resolved. In this work, as an effective and easy approach to improve the cycle performance and crystal stability of LCO cycling at 4.5 V high voltage, we demonstrate direct surface modification of a LCO cathode by poly [N,N-bis(2-cryano-ethyl)-acrylamide]. The results of SEM, TEM and XRD all indicate that the crystal structure of polymer coating LCO remains unchanged after cycling at 4.5 V high voltage for 60 times. Furthermore, the XPS study of valence of cobalt on the surface of LCO demonstrates that cobaltic ion of polymer coating LCO can be reduced to cobaltous ion after charging the cell. Thus, the activity of the crystal surface can be weakened, as a result, the stability is improved, leading to the performance improvement.

Utilizing Co2+/Co3+ Redox Couple in P2-Layered Na0.66Co0.22Mn0.44Ti0.34O2 Cathode for Sodium-Ion Batteries

Developing sodium-ion batteries for large-scale energy storage applications is facing big challenges of the lack of high-performance cathode materials. Here, a series of new cathode materials Na0.66Co x Mn0.66-x Ti0.34O2 for sodium-ion batteries are designed and synthesized aiming to reduce transition metal-ion ordering, charge ordering, as well as Na+ and vacancy ordering. An interesting structure change of Na0.66Co x Mn0.66-x Ti0.34O2 from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na0.66Co0.22Mn0.44Ti0.34O2 with a P2-type layered structure delivers a reversible capacity of 120 mAh g-1 at 0.1 C. When the current density increases to 10 C, a reversible capacity of 63.2 mAh g-1 can still be obtained, indicating a promising rate capability. The low valence Co2+ substitution results in the formation of average Mn3.7+ valence state in Na0.66Co0.22Mn0.44Ti0.34O2, effectively suppressing the Mn3+-induced Jahn-Teller distortion, and in turn stabilizing the layered structure. X-ray absorption spectroscopy results suggest that the charge compensation of Na0.66Co0.22Mn0.44Ti0.34O2 during charge/discharge is contributed by Co2.2+/Co3+ and Mn3.3+/Mn4+ redox couples. This is the first time that the highly reversible Co2+/Co3+ redox couple is observed in P2-layered cathodes for sodium-ion batteries. This finding may open new approaches to design advanced intercalation-type cathode materials.

In Situ Visualized Cathode Electrolyte Interphase on LiCoO2 in High Voltage Cycling

Charging lithium ion battery cathode materials such as LiCoO₂ to a higher voltage may simultaneously enhance the specific capacity and average operating voltage and thus improve the energy density. However, battery cycle life is compromised in high voltage cycling due to lattice instability and undesired oxidation of electrolyte. Cathode solid-electrolyte interphase (SEI), or cathode-electrolyte interphase (CEI), in situ formed at the cathode-electrolyte interface under high voltage, is critically important in understanding the cathode degradation process and crucial in improving high voltage cycle stability. Here we present in situ atomic force microscopy (AFM) investigation of CEI on LiCoO₂ at high voltage. The formation of CEI is only observed at the LiCoO₂ edge plane, not at the basal plane. The thin layer of Al₂O₃ coating completely suppresses the formation of CEI at the edge planes, and is shown to significantly improve coin cell high voltage cycle stability.