Alkali Metal Layer Research Articles

Revealing Gliding-Induced Structural Distortion in High-Nickel Layered Oxide Cathodes for Lithium-Ion Batteries.

After charging to a high state-of-charge (SoC), layered oxide cathodes exhibit high capacities but suffer from gliding-induced structural distortions caused by deep Li depletion within alkali metal (AM) layers, especially for high-nickel candidates. In this study, we identify the essential structure of the detrimental H3 phase formed at high SoC to be an intergrowth structure characterized by random sequences of the O3 and O1 slabs, where the O3 slabs represent Li-rich layers and the O1 slabs denote Li-depleted (or empty) layers that glide from the O3 slabs. Moreover, we adopt two doping strategies targeting different doping sites to eliminate the formation of Li-vacant O1 slabs. First, we introduce direct transition metal (TM) pillars between TMO2 slabs achieved through dopants (e.g., Nb) positioned within AM layers, significantly improving the cycling stability. Second, we introduce indirect Li pillars achieved through dopants located at TM layers to adjust the Li-O bond strength. While this strategy can regulate the uniformity of Li at the slab level, it results in an uneven Li distribution at the particle scale, ultimately failing to enhance the electrochemical performance. Our established research strategy facilitates the realization of diverse pillars between TMO2 slabs through doping, thereby offering guidance for stabilizing high-capacity layered oxide cathodes at high SoC.

First-Principle Insight of Synergistic Modification Mechanism in P2-Type Layered Oxide Cathode Material with Anionic Redox Activity for Sodium-Ion Batteries

Layered oxides exhibiting anionic redox activity have gained attention as a promising material in both Na-ion batteries owing to their additional capacity provided through oxygen redox chemistry. However, unfavorable side effects such as irreversible phase transformation, hysteresis, and gas evolution, usually arise during the stabilization process of the unstable electron holes generated by the redox reaction of oxygen. To address these challenges, we co-dope Cu and Ca in the pristine Nax[Mg0.28Mn0.72]O2 (NMMO) and propose the P2-type NaxCa0.03Mg0.22Cu0.11Mn0.67O2 (NCaMCMO) Na-deficient cathode material which demonstrates high specific capacity with minimal phase change. In this work, we employed the density functional theory (DFT) calculations to investigate the underlying mechanisms of the synergistic modification strategy in both transition metal and alkali metal layers. The substitution of electrochemically active Cu induced variations in the electronic structure near the Fermi level, indicating an inherent change in redox chemistry. The computational results of the local structure provide a deeper understanding of the oxygen-stabilizing effect of Cu. Furthermore, we analyze the nature of the Ca insertion as a pillar in an alkali-metal layer, which mitigates phase transformation to the kinetically unfavorable O2 phase through phase energy comparison. The calculations manifest that Ca doping effectively extends the P2 phase into a deeper charging state. This work elucidates fundamental mechanisms for enhancing cathode materials in future battery design.

Deep insight on Li interlayer migration in O3-type NaLi1/3Mn2/3O2 as a cathode material for Na-ion batteries

Layered manganese transition metal oxides, such as NaMnO2, have attracted great interest due to the low cost and high capacity. However, complex phase transitions in NaMnO2 lead to poor cycling stability. The introduction of Li doping has been confirmed to improve the performance of NaMnO2. O3-type NaLi1/3Mn2/3O2 (NLMO), synthesized in 2021, has demonstrated excellent electrochemical performance. Notably, irreversible Li interlayer migration (Li migrates from the transition metal layer to the alkali metal layer) has been observed during cycling, which is related to the electrochemical performance. Therefore, it is crucial to understand the mechanism underlying Li interlayer migration in O3-NLMO. However, the environment of Li interlayer migration on cycling is complex and involves interlayer spacing, Na-ion concentration, the degree of O-ion oxidation, and phase transition. Here, in this work, we utilized the first-principles method to decouple the coupling factors influencing the Li interlayer migration. Through analyzing the impact of the single-factor on Li interlayer migration, we aim to identify the crucial factors affecting this process. Our results show that a decrease in Na-ion concentration and an increase in O-ion oxidation degree promote the Li interlayer migration, while the O–P phase transition suppresses the Li interlayer migration. Interlayer spacing was found to play a less influential role in Li interlayer migration. Our investigations provide effective strategies for the subsequent regulation of Li interlayer migration.

Quasi-Homoepitaxial Growth of Highly Strained Alkali-Metal Ultrathin Films on Kagome Superconductors.

Applying lattice strain to thin films, a critical factor to tailor their properties such as stabilizing a structural phase unstable at ambient pressure, generally necessitates heteroepitaxial growth to control the lattice mismatch with substrate. Therefore, while homoepitaxy, the growth of thin film on a substrate made of the same material, is a useful method to fabricate high-quality thin films, its application to studying strain-induced structural phases is limited. Contrary to this general belief, here the quasi-homoepitaxial growth of Cs and Rb thin films is reported with substantial in-plane compressive strain. This is achieved by utilizing the alkali-metal layer existing in bulk crystal of kagome metals AV3Sb5 (A=Cs and Rb) as a structural template. The angle-resolved photoemission spectroscopy measurements reveal the formation of metallic quantum well states and notable thickness-dependent quasiparticle lifetime. Comparison with density functional theory calculations suggests that the obtained thin films crystalize in the face-centered cubic structure, which is typically stable only under high pressure in bulk crystals. These findings provide a useful approach for synthesizing highly strained thin films by quasi-homoepitaxy, and pave the way for investigating many-body interactions in Fermi liquids with tunable dimensionality.

Destabilization of Oxidized Lattice Oxygen in Layered Oxide Cathode.

Integrating anion-redox capacity with orthodox cation-redox capacity is deemed as a promising solution for high-energy-density battery cathodes surmounting the present technical bottlenecks. However, the evolution of oxidized oxygen species during the electrochemical or chemical process easily jeopardizes the reversibility of oxygen redox and remains poorly understood. Herein, we showcase the gradual conversion of the π-interacting oxygen (localized hole states on O) to the σ-interacting oxygen upon resting at a high voltage for P3-type Na0.6Li0.2Mn0.8O2 with nominally stable ribbon-like superstructure, accompanied by the O-O dimerization and the local structural reorganization. We further pinpoint an abnormal Li+ migration process from the alkali-metal layer to the transition-metal layer for desodiated P3-Na0.6Li0.2Mn0.8O2, thereby leading to a partial reconstruction of the ribbon superstructure. The high-voltage plateau of oxygen-redox cathodes is concluded to be exclusively controlled by the oxygen stabilization mechanism rather than the superstructure ordering. In addition, there exists a kinetic competition between π and σ interaction during the uninterrupted electrochemical process.

Promoting Na-ion storage in P2-Na0.67Ni0.33Mn0.67O2 cathode via synergetic Ti and Zn Co-incorporation

P2-Na0.67Ni0.33Mn0.67O2 (NNMO) is regarded as a promising cathode candidate for sodium-ion batteries due to its high energy density. However, the electrochemical performance is hindered by Na+/vacancy order, irreversible P2–O2 phase transition at high voltage (>4.2 V), and harmful oxygen evolution. Herein, a synergetic Zn and Ti co-incorporation tactic is proposed for designing a Na0.67Ni0.29Zn0.04Mn0.63Ti0.04O2 (NNZMTO) cathode to overcome the above-mentioned challenges. First, the incorporated Ti heteroatom could break down Na+/vacancy order of NNMO by taking advantage of a similar ionic radius and substantially different Fermi levels with host Mn atom. Subsequently, the introduced Zn heteroatom could induce local Na–O–Zn configurations, buffer interlayer O2−–O2− electrostatic repulsion, as well as inhibit unfavorable phase transition. Moreover, the d10 band of Zn is lower than the oxygen states, and the Zn behaves like an s/p metal with oxygen, thus avoiding O2 release. Notably, in comparison with highly oxidized (Ni4+/Mn4+O6)δ− octahedron, the partial Na+ for charge neutrality in alkali metal layers could be well maintained in the as-designed (Zn2+/Ti4+O6)δ′−, which could be served as “pillars” to avoid layer gliding and structural collapse in the c-direction. As a result, an excellent electrochemical performance with high specific capacity of 90.9 mA h g−1 at 7 C could be retained for NNZMTO thanks to the synergetic effect from Ti and Zn incorporation. This study provides deep insights for designing superior layered cathode via conducting a rational cations co-incorporation strategy.

Significant improvement in cycling and rate ability originating from regulation of bonding and transport channel via Ba2+ decoration in P2 layer oxide cathode

Nowadays, commercialization of layered transition metal oxides is hindered by irreversible phase transition during charging and discharging, low-capacity retention rate at high rate and slow Na+ diffusion kinetics. In this work, Ba2+ was introduced into the alkali metal layer to improve the electrochemical performance of layered transition metal oxides. XRD refinement and first principle calculations demonstrate that the introduced Ba2+ can effectively enlarge the distance between Na layers and reduce the potential barrier of sodium ion migration, thus leading to faster sodium ion diffusion kinetics. Simultaneously, charge distribution density calculation indicates that the introduced Ba2+ enhances overlap of electron clouds between transition metal (TM) and O, which result in more obvious covalence of TM-O bond, bigger electrostatic attraction between TM and O, and stronger bond energy of TM-O. These microstructures are beneficial to maintain structural stability and suppress the formation of OP4 phase during charging, which is demonstrated by XRD during charging and discharging process. The optimized Na0.6Ba0.012Mn0.62Ni0.22Fe0.16O2 provides reversible specific capacity of 133.8mAh g−1 at 0.5C (1C = 150mA g−1) and 122.1mAh g−1 at 2C. The strategy of regulating chemical bond environment by introducing Ba2+ achieves excellent the electrochemical performances and promotes the commercial development of sodium ion batteries.

One-step electrochemical in-situ Li doping and LiF coating enable ultra-stable cathode for sodium ion batteries

Despite of the higher energy density and inexpensive characteristics, commercialization of layered oxide cathodes for sodium ion batteries (SIBs) is limited due to the lack of structural stability at the high voltage. Herein, the one-step electrochemical in-situ Li doping and LiF coating are successfully achieved to obtain an advanced Na0.79Lix[Li0.13Ni0.20Mn0.67]O2@LiF (NaLi-LNM@LiF) cathode with superlattice structure. The results demonstrate that the Li+ doped into the alkali metal layer by electrochemical cycling act as “pillars” in the form of Li-Li dimers to stabilize the layered structure. The supplementation of Li to the superlattice structure inhibits the dissolution of transition metal ions and lattice mismatch. Furthermore, the in-situ LiF coating restrains side reactions, reduces surface cracks, and greatly improves the cycling stability. The electrochemical in-situ modification strategy significantly enhances the electrochemical performance of the half-cell. The NaLi-LNM@LiF exhibits high reversible specific capacity (170.6 mA h g−1 at 0.05 C), outstanding capacity retention (92.65% after 200 cycles at 0.5 C) and excellent rate performance (80 mA h g−1 at 7 C) in a wide voltage range of 1.5–4.5 V. This novel method of in-situ modification by electrochemical process will provide a guidance for the rational design of cathode materials for SIBs.

The synergy of dis‐/ordering ensures the superior comprehensive performance of P2‐type Na‐based layered oxide cathodes

AbstractTwo kinds of crystal orderings in layered oxides typically exhibit opposite influences on performances: Na+/vacancy ordering in alkali metal layers with an unfavorable effect on electrochemical performance and the cation ordering in transition metal layers with a positive effect on air stability. However, because the two kinds of orderings are associated with each other and often occur at the same time, it is difficult to achieve an excellent comprehensive performance. Herein, we propose a strategy of introducing a new cation ordering to construct the coexistence of Na+ disordering and transition metal ordering. An absolute solid‐solution reaction mechanism is realized in the Na+ disordered system, resulting in a superior cycling stability of 90.4% retention after 150 cycles and a rate performance of 82.7 mAh g−1 capacity at 10C, much higher than the original 81.3% and 66.4 mAh g−1. Simultaneously, the cation ordering strengthens the interlayer interaction and inhibits the insertion of water molecules from the air, ensuring stable lattice stability and thermostability after air exposure. The synergy of dis‐/ordering configuration provides new insights to design high‐performance layered oxide cathode materials for secondary‐ion batteries.

Oxygen Redox Activation at Initial Cycle to Improve Cycling Stability for the Na0.83Li0.12Ni0.22Mn0.66O2 System.

Oxygen reactions are commonly used to increase the specific capacities of Na-ion batteries, especially for the NaxLiyTMO2 systems. Previous research focused on improving the stabilities of oxygen reactions to enhance cycling stability. However, the effects of oxygen reactions on the distribution of Li ions in the transition metal (TM) and alkali metal (AM) layers for the Na-ion battery are relatively unexplored and rarely employed. In this study, we employ a layered P2-Na0.83Li0.12Ni0.22Mn0.66O2 cathode to control the effects of the oxygen reactions on the distributions of Li ions in two layers. With oxygen-redox-activation-at-first-cycle (ORAFIC)-cycling, which cycled first within 2.0-4.6 V to activate oxygen redox and then cycled within 2.0-4.2 V, this cathode exhibited better cycling stability compared to low-voltage (LV)-cycling of 2.0-4.2 V and high-voltage (HV)-cycling of 2.0-4.6 V. Using nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, inductively coupled plasma experiments, and X-ray diffraction, it is confirmed that ORAFIC-cycling stabilizes the crystal structure and distributions of Li ions in the TM and AM layers and reduces Li-ion loss, thus improving the cycling stability.

Ion-Migration Mechanism: An Overall Understanding of Anionic Redox Activity in Metal Oxide Cathodes of Li/Na-Ion Batteries.

The anionic redox reaction (ARR) has attracted extensive attention due to its potential to enhance the reversible capacity of cathode materials in Li/Na-ion batteries (LIBs/SIBs). However, the understanding of its activation mechanism is still limited by the insufficient mastering of the underlying thermodynamics and kinetics. Herein, a series of Mg/Li/Zn-substituted Nax MnO2 and Lix MnO2 cathode materials are designed to investigate their ARR behaviors. It is found that the ARR can be activated in only Li-substituted Lix MnO2 and not for Mg- and Zn-substituted ones, while all Mg/Li/Zn-substituted Nax MnO2 cathode materials exhibit ARR activities. Combining theoretical calculations with experimental results, such a huge difference between Li and Na cathodes is closely related to the migration of substitution ions from the transition metal layer to the alkali metal layer in a kinetic aspect, which generates unique Li(Na)-O-□TM and/or □Li/ Na -O-□TM configurations and reducing reaction activation energy to trigger the ARR. Based on these findings, an ion-migration mechanism is proposed to explain the different ARR behaviors between the Nax MnO2 and Lix MnO2 , which can not only reveal the origin of ARR in the kinetic aspect, but also provide a new insight for the development of high-capacity metal oxide cathode materials for LIBs/SIBs.

Capturing surface interlayer cation migration in Na0.6[Li0.2Mn0.8]O2 layered cathode materials for sodium battery

Anionic redox of charge compensation is an effective strategy to release the excess capacity of the cathode material for sodium ion batteries. Nonetheless, the development of cathode materials with this electrochemical reaction mechanism faces great challenges in terms of voltage attenuation and irreversible loss of capacity. As a result of the complexity of its structure, the failure mechanism is not well understood. Here we conduct STEM coupled with EELS on a typical anionic redox material of sodium ion batteries (Na 0.6 [Li 0.2 Mn 0.8 ]O 2 ) after the electrochemical cycle. Surprisingly, it was found that the transition metal ions on the surface of the particles migrated into the alkali metal layer, forming a spinel structure, and resulting in a decrease in the valence of the transition metal on the surface. The possibility of this kind of migration is confirmed, and the most likely migration path is calculated by DFT. Combining the electrochemical curve and XPS spectroscopy, we found the evolution of the charge transfer mechanism, which gradually changes from anion to cation with the progress of the electrochemical cycle. The structural reconstruction of the particle surface and the evolution of the charge transfer mechanism lead to the voltage hysteresis and capacity loss of the cathode. The new failure mechanism provides an important basis for solving the problems of this type of material for sodium cathode.

Synergetic stability enhancement with magnesium and calcium ion substitution for Ni/Mn-based P2-type sodium-ion battery cathodes.

The conventional P2-type cathode material Na0.67Ni0.33Mn0.67O2 suffers from an irreversible P2–O2 phase transition and serious capacity fading during cycling. Here, we successfully carry out magnesium and calcium ion doping into the transition-metal layers (TM layers) and the alkali-metal layers (AM layers), respectively, of Na0.67Ni0.33Mn0.67O2. Both Mg and Ca doping can reduce O-type stacking in the high-voltage region, leading to enhanced cycling endurance, however, this is associated with a decrease in capacity. The results of density functional theory (DFT) studies reveal that the introduction of Mg2+ and Ca2+ make high-voltage reactions (oxygen redox and Ni4+/Ni3+ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg2+ and Ca2+ ions, the adverse effects on high-voltage reactions involving Ni–O bonding are limited, and the structural stability is further enhanced. The finally obtained P2-type Na0.62Ca0.025Ni0.28Mg0.05Mn0.67O2 exhibits a satisfactory initial energy density of 468.2 W h kg−1 and good capacity retention of 83% after 100 cycles at 50 mA g−1 within the voltage range of 2.2–4.35 V. This work deepens our understanding of the specific effects of Mg2+ and Ca2+ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.

Engineering Na+-layer spacings to stabilize Mn-based layered cathodes for sodium-ion batteries

Layered transition metal oxides are the most important cathode materials for Li/Na/K ion batteries. Suppressing undesirable phase transformations during charge-discharge processes is a critical and fundamental challenge towards the rational design of high-performance layered oxide cathodes. Here we report a shale-like NaxMnO2 (S-NMO) electrode that is derived from a simple but effective water-mediated strategy. This strategy expands the Na+ layer spacings of P2-type Na0.67MnO2 and transforms the particles into accordion-like morphology. Therefore, the S-NMO electrode exhibits improved Na+ mobility and near-zero-strain property during charge-discharge processes, which leads to outstanding rate capability (100 mAh g−1 at the operation time of 6 min) and cycling stability (>3000 cycles). In addition, the water-mediated strategy is feasible to other layered sodium oxides and the obtained S-NMO electrode has an excellent tolerance to humidity. This work demonstrates that engineering the spacings of alkali-metal layer is an effective strategy to stabilize the structure of layered transition metal oxides.

Bulk O2 formation and Mg displacement explain O-redox in Na0.67Mn0.72Mg0.28O2

Summary O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O2 on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na0.67[Mn0.72Mg0.28]O2, which contains Mg2+ on the TML. The molecular O2 is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O2 is trapped in voids that are formed by Mg2+ out-of-plane displacement and Mn4+ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na0.67[Mn0.72Mg0.28]O2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering.

Achieving stable anionic redox chemistry in Li-excess O2-type layered oxide cathode via chemical ion-exchange strategy

Triggering oxygen redox activity has been regarded as a promising strategy to boost the output capacity of cathode materials for Li/Na-ion batteries. However, irreversible loss of lattice oxygen aggravates a structural distortion to a spinel phase, which leads to severe voltage decay and capacity degeneration. Herein, via chemical ion exchange procedure, the sodium within the alkali metal layer of a P2-type oxide precursor has been substituted by Li while the transition metal layer can be well preserved, resulting in the formation of a Li-excess O2-type layered oxide cathode, Li0.66[Li0.12Ni0.15Mn0.73]O2. Through systematic in/ex-situ and surface/bulk characterization (hard X-ray absorption spectroscopy, operando Raman/XRD and differential electrochemical mass spectroscopy, etc.), the redox reversibility and structural stability has been comprehensively demonstrated. Moreover, being evolved from the same sodium-based precursor, a similar O2-type compound generated by electrochemical ion exchange procedure presents severely irreversible behavior on both structural and redox processes. These findings elucidated that the chemical ion exchange strategy can be regarded as an efficient way to design high-capacity cathode candidates possessing stable anionic/cationic redox activities and enhanced structural stability.

Stabilizing Anionic Redox Chemistry in a Mn-Based Layered Oxide Cathode Constructed by Li-Deficient Pristine State.

Li-rich cathode materials are of significant interest for coupling anionic redox with cationic redox chemistry to achieve high-energy-density batteries. However, lattice oxygen loss and derived structure distortion would induce serious capacity loss and voltage decay, further hindering its practical application. Herein, a novel Li-rich cathode material, O3-type Li0.6 [Li0.2 Mn0.8 ]O2 , is developed with the pristine state displaying both a Li excess in the transition metal layer and a deficiency in the alkali metal layer. Benefiting from stable structure evolution and Li migration processes, not only can high reversible capacity (≈329 mAh g-1 ) be harvested but also irreversible/reversible anionic/cationic redox reactions are comprehensively assigned via the combination of in/ex situ spectroscopies. Furthermore, irreversible lattice oxygen loss and structure distortion are effectively restrained, resulting in long-term cycle stability (capacity drop of 0.045% per cycle, 500 cycles). Altogether, tuning the Li state in the alkali metal layer presents a promising way for modification of high-capacity Li-rich cathode candidates.

Nocturnal and Seasonal Variation of Na and K Layers Simultaneously Observed in the MLT Region at 23°S

AbstractThe present work shows for the first time the study of morphology of the mesopause sodium (Na) and potassium (K) layers simultaneously observed by a dual‐beam LIDAR in the Southern Hemisphere. We analyze these two alkali metal layers from November 2016 to February 2019 measured at São José dos Campos (23.1°S, 45.9°W) and present their nocturnal and seasonal behavior. The night of 25 April 2017 was investigated as a representative of the vertical descending structure frequently observed in the metal density data at this location. We discuss here the chemical and dynamical contribution in the formation of these distinct layers, using simultaneous meteor radar wind and ionosonde data. These downward structures are seen in 66% of Na/K LIDAR data and seem to occur preferentially around April. The good agreement with diurnal tide and the presence of Es layer, suggest a combining mechanism in the formation of these structures. Moreover, semiannual variations are observed in both layers; however, they present different maxima location. The Na density presents its maxima around May and September; whereas, K density shows a strong maximum around July and a weaker one around December. Likewise, semiannual variation with maxima at the equinoxes is observed in the centroid height for both layers. However, the column abundances of these two metals show distinct seasonal variation: Annual variation peaking from one equinox to the other is observed in Na and semiannual with maxima in the solstices in K. The same behavior of centroid height for both layers indicates the same mechanism acting in the seasonal variation, which is not yet completely understood.

Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.

In conventional intercalation cathodes, alkali metal ions can move in and out of a layered material with the charge being compensated for by reversible reduction and oxidation of the transition metal ions. If the cathode material used in a lithium-ion or sodium-ion battery is alkali-rich, this can increase the battery's energy density by storing charge on the oxide and the transition metal ions, rather than on the transition metal alone1-10. There is a high voltage associated with oxidation of O2- during the first charge, but this is not recovered on discharge, resulting in reduced energy density11. Displacement of transition metal ions into the alkali metal layers has been proposed to explain the first-cycle voltage loss (hysteresis)9,12-16. By comparing two closely related intercalation cathodes, Na0.75[Li0.25Mn0.75]O2 and Na0.6[Li0.2Mn0.8]O2, here we show that the first-cycle voltage hysteresis is determined by the superstructure in the cathode, specifically the local ordering of lithium and transition metal ions in the transition metal layers. The honeycomb superstructure of Na0.75[Li0.25Mn0.75]O2, present in almost all oxygen-redox compounds, is lost on charging, driven in part by formation of molecular O2 inside the solid. The O2 molecules are cleaved on discharge, reforming O2-, but the manganese ions have migrated within the plane, changing the coordination around O2- and lowering the voltage on discharge. The ribbon superstructure in Na0.6[Li0.2Mn0.8]O2 inhibits manganese disorder and hence O2 formation, suppressing hysteresis and promoting stable electron holes on O2- that are revealed by X-ray absorption spectroscopy. The results show that voltage hysteresis can be avoided in oxygen-redox cathodes by forming materials with a ribbon superstructure in the transition metal layers that suppresses migration of the transition metal.

Proton intercalation into different CoO2 layer matrices

The hexagonal CoO2 layer is an integral part of many important functional materials such as the LixCoO2 battery electrode, the NaxCoO2 and [CoCa2O3]0.62CoO2 thermoelectrics, and the Na0.3CoO2(H2O)1.3 superconductor. However, the CoO2-layer piling is prone to incorporate not only positively-charged metal and metal oxide layers but also protons between successive CoO2 layers. Here we systematically investigate the de-intercalation of the intervening alkali metal and metal oxide layers, and the subsequent intercalation of protons for an extensive series of LiCoO2, NaxCoO2 (x = 0.6–0.9) and [CoCa2O3]0.62CoO2 samples. First, we demonstrate the partial extraction of sodium from NaxCoO2 through immersion in an aqueous solution of either n-butylamine or NH4NO3. Such a treatment results in an appreciably low Na content of x = 0.37 or 0.10, respectively, but a simultaneous incorporation of certain amounts of charge balancing protons into the crystal lattice. Then, using a newly developed hydrothermal route we demonstrate the complete de-intercalation of the alkali and alkaline earth metal species from LiCoO2, NaxCoO2 and [CoCa2O3]0.62CoO2, and the intercalation of protons between the CoO2 layers. Finally we synthesize and characterize a water-containing NaxCoO2(H2O)z sample for comparison. As the protonation is found to be a common tendency of all the studied multilayered functional materials based on the CoO2 layer we believe that our work is not only of fundamental scientific interest but also of more practical relevance.