One-dimensional Tunnels Research Articles

Effect of pre-intercalation on Li-ion diffusion mapped by topochemical single-crystal transformation and operando investigation.

Limitations in electrochemical performance as well as supply chain challenges have rendered positive electrode materials a critical bottleneck for Li-ion batteries. State-of-the-art Li-ion batteries fall short of accessing theoretical capacities. As such, there is intense interest in the design of strategies that enable the more effective utilization of active intercalation materials. Pre-intercalation with alkali-metal ions has attracted interest as a means of accessing higher reversible capacity and improved rate performance. However, the structural basis for improvements in electrochemical performance remains mostly unexplored. Here we use topochemical single-crystal-to-single-crystal transformations in a tunnel-structured ζ-V2O5 positive electrode to illustrate the effect of pre-intercalation in modifying the host lattice and altering diffusion pathways. Furthermore, operando synchrotron X-ray diffraction is used to map Li-ion site preferences and occupancies as a function of the depth of discharge in pre-intercalated materials. Na- and K-ion intercalation 'props open' the one-dimensional tunnel, reduces electrostatic repulsions between inserted Li ions and entirely modifies diffusion pathways, enabling orders of magnitude higher Li-ion diffusivities and accessing higher capacities. Deciphering the atomistic origins of improved performance in pre-intercalated materials on the basis of single-crystal-to-single-crystal topochemical transformation and operando diffraction studies paves the way to site-selective modification approaches for positive electrode design.

One-dimensional tunnel VO2(B) cathode materials for high-performance aqueous zinc ion batteries: a mini review of recent advances and future perspectives

Aqueous zinc ion batteries (ZIBs) are gaining attention for safety, affordability, and eco-friendliness, emerging as a promising energy storage system. VO2(B), with its unique crystal structure, is a research hotspot for AZIB cathode material.

Tuning the Composition, Crystal Structure and Morphology of Manganese(III/IV) Oxide for High-Power Storage Applications

If inherently intermittent renewable energy technologies are to offer a practical alternative to conventional heat engines, it is critically important that efficient, cost-effective and sustainable energy storage solutions are developed to decouple supply and demand profiles. A promising strategy is to pair the competitive energy densities of batteries with the superior cycling lifetimes and power densities of supercapacitors,[1] employing an appropriate maximum power point tracking (MPPT) system to optimise energy exchange between these complementary devices.[2] Mixed-valence manganese(III/IV) oxides (MnOz, where z ranges between 1.5 and 2.0) have emerged as foremost candidates for the electroactive material in supercapacitor cathodes due to their low toxicity, minimal cost,[3] ready availability (manganese appears at an average concentration of ca. 1,000 ppm in the Earth’s crust[4]), wide electrochemical stability window (ca. 1.0 V in aqueous electrolytes[5]), and high maximum theoretical specific capacitance (ca. 1,370 F g-1, based on the reduction of Mn4+ ions by a single electron[6]). Charge storage in MnOz is predominantly mediated by the pseudocapacitive intercalation of cations from the surrounding electrolyte in each discharge phase, followed by the release of these guest ions during charging.Of the various crystallographic phases of MnOz, cryptomelane (α-MnOz) and birnessite (δ-MnOz) polymorphs are deemed two of the most promising candidates for storage applications because they possess characteristically wide channels for intercalation/de-intercalation:[8,9] α-MnOz consists of ca. 4.6 Å diameter one-dimensional tunnels enclosed between corner-sharing MnO6 octahedra, whereas δ-MnOz comprises two-dimensional layers of MnO6 octahedra separated by ca. 7.0 Å. Herein, a scalable, low-cost synthesis procedure is discussed that enables the production of highly pseudocapacitive birnessite and cryptomelane electrodes with tuneable material characteristics, including particle morphology, crystallographic structure, average Mn oxidation state, and the level of pre-intercalation by Na+ and K+ ions.[10] In addition to comparing the electrochemical properties of these NaxKyMnOz products in aqueous conditions, their application within an asymmetric supercapacitor is showcased: by optimising the relative masses of a NaxKyMnOz cathode and activated carbon anode in a 100 cm2 aqueous full-cell configuration, discharge capacitances in the range 10-30 F g-1 can be achieved over a voltage span of 1.8 V at current densities as high as 10 A g-1, with Coulombic efficiency exceeding 80% up to ca. 20 A g-1. With these competitive performance characteristics, the developed NaxKyMnOz materials are a promising option for high-power supercapacitor cathodes, helping to pave the way towards more efficient and sustainable energy storage.

Rationally designed tungsten trioxide nanosheets for high-efficiency aqueous battery application

Aqueous battery (AB) with non-metallic charging carriers is a viable candidate for grid energy storage devices owing to its comparatively low cost and high safety. However, developing practical electrode materials with remarkable electrochemical performance remains a great challenge. In this study, tungsten trioxide nanosheets (WONSs) with one-dimensional tunnels were facilely synthesized through a hydrothermal reaction. When used as the cathode material in aqueous batteries, the as-prepared WONS exhibits excellent long-term cycling stability and impressively high rate capability due to the nearly zero-strain structural characteristics of super-fast proton storage. Notably, a stable cycling performance with 89% capacity retention after 10000 cycles was achieved with an increased mass loading of WONS at around 12 mg cm−2. Our work points to a new direction of promoting high-efficiency AB applications with protons as charge carriers in a mild electrolyte.

Enabling the Electrochemical Performance of Maricite-NaMnPO4 and Maricite-NaFePO4 Cathode Materials in Sodium-Ion Batteries

NaMnPO4 and NaFePO4, polyanion cathode materials, exist in two different phases maricite/natrophilite and maricite/olivine, respectively. Both natrophilite NaMnPO4 and olivine NaFePO4 are electrochemically active and possess a one-dimensional tunnel for sodium-ion migration; however, these two phases are thermodynamically unstable. Therefore, they can be synthesized through an electrochemical route. On the contrary, maricite (m)-NaMnPO4 and maricite (m)-NaFePO4 are thermodynamically stable forms but have a huge activation energy of their diffusion pathways for sodium extraction and insertion in the crystal structure, which hinders electrochemical reactions. Therefore, the electrochemical behaviour of commercial m-NaMnPO4 and m-NaFePO4 has been studied to find a way for enabling them electrochemically. Ball milling and thermal/mechanical carbon coating are employed to reduce the particle size to enhance the electrochemical performance and shorten the diffusion pathway. Moreover, ball milling leads to defects and partial phase transformation. The electrochemical performance of milled-coated NaMnPO4 and NaFePO4 has been thoroughly investigated and compared. The phase transition of NaFePO4 is revealed by a differential scanning calorimeter. As a result, the achievable capacities of both cathode materials are significantly enhanced up to ∼50 mAh.g−1 via the particle size reduction as well as by carbon coating. However, the side reactions and agglomeration problems in such materials need to be minimized and must be considered to enable them for applications.

Tunnel-Structured ζ-V2O5 as a Redox-Active Insertion Host for Hybrid Capacitive Deionization

Much of the earth's water has a salt content that is too high for human consumption or agricultural use. Enhanced oil recovery operations generate massive volumes of produced water waste with a high mineral content that can substantially exacerbate water distress. Current deionization techniques such as reverse osmosis function by removing the water (majority phase) from the salt (minority phase) and are thus exceedingly energy-intensive. Furthermore, these methods are limited in their ability to selectively extract high-value ions from produced water waste and brine streams. Hybrid capacitive deionization holds promise for enabling both desalination and resource recovery. In this work, we demonstrate the construction of a hybrid capacitive deionization cell that makes use of tunnel-structured ζ-V2O5 as a redox-active positive electrode material. By augmenting surface adsorption with Faradaic insertion processes, a 50% improvement in the ion removal capacity for K and Li ions is obtained as compared to a capacitive high-surface-area carbon electrode. The extracted ions are accommodated in surface sites and interstitial sites within the one-dimensional tunnel framework of ζ-V2O5. The kinetics of ion removal depend on the free energy of hydration, which governs the ease of desolvation at the electrode/electrolyte interface. The overall ion removal capacity additionally depends on the solid-state diffusion coefficient. ζ-V2O5 positive electrodes show substantial selectivity for Li+ removal from mixed flow streams and enrichment of the Li-ion concentration from produced water waste derived from the Permian Basin.

Arsenotungstate-Nanostructure-Based Derivatives with One-Dimensional Tunnels for Electrochemical Capacitors and Electrocatalytic Hydrogen Evolution

Arsenotungstate-Nanostructure-Based Derivatives with One-Dimensional Tunnels for Electrochemical Capacitors and Electrocatalytic Hydrogen Evolution

Vat Orange 7 as an organic electrode with ultrafast hydronium-ion storage and super-long life for rechargeable aqueous zinc batteries

Organic electrode materials (OEMs) with various advantages are attracting much attention. However, the serious dissolution problem of OEMs has puzzled people for many years without being fundamentally addressed. Here, an aqueous Zn-ion battery using organic bisbenzimidazo[2,1-b:2′,1′-i]benzo[lmn][3,8]phenanthroline-8,17-dione (BBPD, also known as Vat Orange 7) as cathode material is developed. Due to the special H3O+ charge carrier insertion mechanism, both the BBPD and its discharged product H2BBPD as covalent compounds are not soluble in aqueous electrolytes, which make active materials integrity during long term cycles. Moreover, the special one-dimensional (1-D) square ion tunnels structure of the BBPD enables fast ion and electron transfer abilities in three dimensional directions, leading to an outstanding rate capability and power density. The assembled aqueous Zn-BBPD battery displays high rate performance (76.4 % capacity retention from 0.78 C to 153.85 C), excellent power density (13.4 kW kg−1) and capacity retention of 90.7 % after 10,000 cycles at 153.85 C. The kinetic and thermodynamic investigations on the exact dissolution mechanism of OEMs are first carried out in this work.

Tunneling maps of interacting electrons in real time: Anomalous tunneling in quantum point-contacts beyond the steady state regime

Strongly correlated transport of interacting electrons through the one-dimensional tunnel contact is considered within the Luttinger liquid model of one-dimensional electrodes on arbitrary time scales t>ħ/Λg. Using previous results, namely, the Self-equilibration theorem proven in Ref. [20] the exact distribution functions P(N,t) for the probabilities of N electrons to tunnel through the contact by the time t are derived and plotted as 3D intensity graphs called real-time tunneling maps in a whole available region of Luttinger liquid correlation parameter values, bias voltages, and temperatures. The tunneling maps being obtained reveal tunneling anomalies - unusually high tunneling probabilities for the small numbers of electrons on the short-time scales beyond the steady state regime of electron tunnel transport. Especially, this concerns the case of strong short-range electron-electron interaction in the leads at g≲1/2. The effect being found can be treated as the direct consequence of the self-equilibration phenomenon. It is shown that features of such an anomalous electron tunneling beyond the steady state regime can be used for precise experimental measurements of small differences between Luttinger liquid correlation parameter values in different one-dimensional quantum-point contacts.

Synthesis, characterization, and electronic structure of SrBi2S4

Black-colored crystals and a pure polycrystalline phase of SrBi2S4 were synthesized using the high-temperature sealed tube method. A single-crystal and powder X-ray diffraction studies established the crystal structure and phase purity of SrBi2S4, respectively. It crystallizes in the hexagonal C6h2−P63/m (176) space group with lattice parameters of a ​= ​b ​= ​24.9335(6) Å and c ​= ​4.0946(1) Å. The structure is made up of three-dimensional 3∞ [Bi2S4]4− anionic framework with one-dimensional tunnels which are occupied by Sr atoms. The optical bandgap and temperature-dependent resistivity studies indicated semiconducting nature for polycrystalline SrBi2S4 in agreement with the theoretical DFT study. The thermal conductivity (κ) of a cold-pressed sintered disk of SrBi2S4 decreases on heating the sample. The κ values vary from ∼0.65 ​W/mK at 300 ​K to ∼0.55 ​W/mK at 773 ​K. The COHP analysis is performed to estimate the relative strength of chemical bonding between Bi–S and Sr–S pairs.

Tungsten Bronzes ΑxWO3 (A=Li, Na or K) As Electrodes for Fast Aqueous Batteries

Aqueous batteries, asymmetric supercapacitors and hybrid devices using oxides as electrodes have been envisioned for stationary applications due to their low environmental impact and the easiness to recycle oxides when operated in a water based electrolyte [1]. This is especially true when neutral pH electrolytes are used. Quite a lot of positive electrode oxides have been proposed up to now but it seems more difficult to combine high capacity and low potential electrochemical window for negative electrodes [2]. In the present study we propose tungsten oxide WO3 as a potential candidate for negative electrode of mild aqueous electrolyte energy storage device. More specifically the metastable phase of hexagonal bronze (h-WO3) was targeted. This phase cannot be obtained by purely structural transformation upon temperature [3]. This structure possesses a rigid tungsten-oxygen framework built up of layers containing corner-shared WO6 octahedra that are arranged in six-membered rings. The layers are stacked along the [00l] axis, giving rise to one-dimensional tunnels being able to host different alkali cations such as Li+, Na+, and K+ [4]. This arrangement can result in interesting electrochemical properties when used as electrode materials for energy storage devices [5].Hexagonal tungsten oxides with the general formula AxWO3 (A= Li, Na, or K and 0 < x < 0.33) were successfully synthesized by hydrothermal synthesis. The materials were characterized by X-ray Diffraction, Scanning and Transmission Electron Microscopy, Energy dispersive X-ray spectroscopy, and BET specific surface area measurements. Their electrochemical behaviors in different aqueous electrolytes such as LiNO3 (5M and 1M), Li2SO4, Na2SO4, and K2SO4. (0.5 M) were investigated and specific capacities up to ~80 C g-1 were measured. In this communication we will detail the electrochemical response of the different tungsten bronzes and we will give some more fundamental insights about the charge storage mechanisms using advanced characterization techniques.

A robust numerical method for modeling ventilation through long tunnels in high temperature regions based on 1D pipe model

A robust numerical model, which can simulate the coupled air flow and heat transfer processes in long tunnels, is essentially required to design and operate the mechanical ventilation system in long tunnels surrounded by high temperature rocks. Due to the extreme scale disparity between tunnel diameter and length (~m vs. ~km), very fine mesh is required to accurately represent the highly-dynamic surrounding rocks near the tunnel, and thus the conventional numerical method is usually time consuming. To improve the computational efficiency without losing accuracy, a simplified one-dimensional tunnel model considering heat convection and conduction along tunnel axis and heat transfer between air in the tunnel and the surrounding rocks in the radial direction is proposed. A laboratory test of a high temperature tunnel with longitudinal ventilation is used to examine the reasonability and efficiency of the proposed modeling method. The good agreement between numerical and experimental results demonstrates that the developed ventilation modeling method is a robust simulation tool for design and management of mechanical ventilation system in a long tunnel. The effects of mechanical ventilation parameters and insulation layer thickness on the temperatures of air in the tunnel and surrounding rocks around the tunnel are investigated based on a case study of the planned Zheduo mountain tunnel, China.

Wells-Dawson Arsenotungstate Porous Derivatives for Electrochemical Supercapacitor Electrodes and Electrocatalytically Active Materials.

Two Wells-Dawson arsenotungstate coordination polymers, [{CuII(bim)2}3(As2W18O62)] (1) and [(CuI10pz10Cl4)(As2W18O62)] (bim = 2,2'-biimidazole; pz = pyrazine), have been assembled via a hydrothermal method and fully characterized. Compound 1 exhibits a 2,6-connected two-dimensional hybrid layer based on asymmetrically modified {As2W18} anions and {Cu(bim)2} linkers, which is extended to a three-dimensional network with a special interlayer structure and a one-dimensional tunnel. Compound 2 is a host-guest framework that consists of a Cu-pz-Cl network with 20-member square rings, 16-member irregular rings, and embedded eight-node {As2W18} guest molecules. Compounds 1 and 2 show uncommon specific capacitance (834.8 and 960.1 F g-1, respectively, at a current density of 2.4 A g-1), enduring cycling stability (capacitance retention rates of 89.3% and 91.9%, respectively, after 5000 cycles), and good electrical conductivity, which are superior to those of the unmodified zero-dimensional Dawson arsenotungstate compound and most reported electrode materials in terms of their stable structure, special layer spacing, and orderly channels. Moreover, the title compounds exhibit excellent electrocatalytic activity for oxidizing ascorbic acid and reducing nitrite.

Lanthanide-Aromatic Iminodiacetate Frameworks with Helical Tubes: Structure, Properties, and Low-Temperature Heat Capacity.

A series of lanthanide coordination polymers [LnL(H2O)2]n [Ln = Pr (1), Nd (2), Sm (3), Eu (4), and Gd (5), H3L = N-(4-carboxy-benzyl)iminodiacetic acid] was hydrothermally prepared and structurally characterized. All the five compounds have been confirmed as 3D Ln-CPs with one-dimensional helical tunnels composed of four helical chains, although there are different coordination geometries around Ln3+. Enantiomeric helixes in 1–3, and absolute left-handed and right-handed helical chains in 4 and 5, respectively, lead to different tunnel spaces. Their conformations can also be featured by different space groups and unit cell dimensions. Photoluminescence measurement on 3 and 4 show characteristic emission peaks of Sm3+ and Eu3+ ions, respectively. The low-temperature heat capacity of 1–4 has been investigated in the temperature range of 1.9–300 K. Their heat capacity values are nearly equal below 10 K and display a crossover with the value order Cp,m(2) > Cp,m(1) ≈ Cp,m(4) > Cp,m(3) above 10 K. The measured heat capacities have been fitted, and the corresponding thermodynamic functions were consequently calculated based on the fitting parameters. The standard molar entropies at 298.15 K have been determined to be (415.71 ± 4.16), (451.32 ± 4.51), (308.53 ± 3.09), and (407.62 ± 4.08) J·mol–1·K–1 for 1, 2, 3, and 4, respectively.

Syntheses, crystal structures, optical, and theoretical study of two ternary chalcogenides CsSc5Te8 and Cs0.6(1)Ti6Se8 with tunnel structures

Single crystals of two new chalcogenides CsSc5Te8 and Cs0.6(1)Ti6Se8 were synthesized by the high-temperature solid-state sealed tube method using a reactive flux (CsCl) at 1223 K. The crystal structures of CsSc5Te8 and Cs0.6(1)Ti6Se8 were determined by single-crystal X-ray diffraction method. The compound CsSc5Te8 crystallizes in the monoclinic space group C2h3−C2/m with the lattice parameters a = 21.3376(15) Å, b = 4.1434(3) Å, c = 10.2853(7) Å, and β = 103.925(2)° having the two formula units (Z = 2). The asymmetric unit of CsSc5Te8 contains eight crystallographically independent atomic sites: Cs1 (site symmetry: 2/m), Sc1 (m), Sc2 (m), Sc3 (2/m), Te1 (m), Te2 (m), Te3 (m), and Te4 (m). The structure of CsSc5Te8 is built up of the three-dimensional anionic framework of 3∞[Sc5Te8]1− where the Sc atoms are octahedrally coordinated with six Te atoms forming the one-dimensional tunnels approximately along the b-axis where the Cs+ cations are present. The compound Cs0.6(1)Ti6Se8 crystallizes in the Nb3Te4 structure type with Z = 1, in the hexagonal space group C6h2−P63/m having cell dimensions of a = b = 9.9520(1) Å and c = 3.5710(1) Å. The asymmetric unit of Cs0.6(1)Ti6Se8 structure is composed of four crystallographically independent sites with atoms Cs1 (site symmetry: 6‾..), Ti1 (m..), Se1 (m..), and Se2 (6‾..). The Ti atoms are making distorted octahedral units by coordinating with six Se atoms. These TiSe6 distorted octahedra share edges and corners with the adjacent TiSe6 units to form three-dimensional anionic networks that generate one-dimensional tunnels approximately along the c-direction. The optical absorption measurements show that CsSc5Te8 is a semiconductor having a direct bandgap of 1.2(1) eV at room temperature consistent with the DFT studies.

Polymorphic Consideration in Conversion Reaction-Based MnO2 Anode Materials for Next-Generation Li Rechargeable Batteries

Beginning with the commercialization by Sony in the 1990s, Li-ion batteries (LIBs) have been used in many electronic devices. In commercialized LIBs, insertion reaction-based electrode materials, such as LiCoO2, LiFePO4, LiMn2O4 for the cathode and graphite, Li4Ti5O12, TiO2 for the anode, have been dominantly used as active materials. In these materials, Li-ions can reversibly be stored and extracted at the specific interstitial site in the crystal without severely collapsing the initial crystal structure. As so, the crystallographic phase significantly affects the electrochemical performances because the local environments around the interstitial site are all different among these phases. For example, LiFePO4 has stable FePO4 framework even after most Li-ions have been extracted, although some Li-ions confined in the tunnel block the migration paths due to the one-dimensional tunnel inside the crystal. In contrast, Li-ions can move easier in LiCoO2 when comparing with LiFePO4 even if the Li-ions are trapped in the Li slab, but over lithiation results in the collapse of the layered structure leading to capacity fading. Even in the same composition, the electrochemical Li storage behavior varies much by the polymorphs. For instance, the layered LiCoO2 exhibit larger reversible capacity and higher redox potential compared to spinel structured Li2Co2O4 [1], and in case of TiO2, despite the similar amount of Li-ion storage, in detail, the voltage profiles are different in anatase, rutile and brookite TiO2 [2].Since the demand for high capacity energy storage devices is continuously growing, there have been extensive researches on high energy density electrode materials based on alloying and conversion reaction due to the limited capacity of insertion-based electrode materials. Manganese dioxide (MnO2) has been considered as a potential candidate due to its low cost, environmentally benignity, and especially large theoretical capacity of ~1233 mAh g-1. Accordingly, various types of polymorphs in MnO2 (α, β, γ, δ, ε, λ, etc.) have been adopted as the anode materials in LIBs. However, unlike the insertion-based electrode, the effect of crystal structure in electrochemical conversion reaction was not clearly identified because it is hard to identify the detailed phase transition process as the initial crystalline structures transformed into the amorphous state after lithiation. Recently, some researchers compared the Li storage performance of different polymorphs of MnO2 [3,4], but the morphologies of the materials are quite different or the reversible capacities are far below the theoretical value based on the conversion reaction, which could not sufficiently explain the polymorphic effects in conversion reaction. In this work, we have prepared 4 different polymorphs of MnO2 (α, β, γ, and λ) in similar hollow urchin-like morphology and compared the Li storage behaviors to discover the influence of polymorphs in conversion-based electrode materials. Among those 4 polymorphs, λ-MnO2 exhibits the highest reversible capacity of ~1200 mAh g-1 and the fastest reaction kinetics. To demonstrate the structural advantages of λ-MnO2, we observed structural changes during electrochemical reactions using synchrotron-based X-ray absorption spectroscopy and X-ray crystallography. As a result, we discovered that the overall Li storage behaviors in the 4 different polymorphs are nearly the same. However, the structural similarity between the initial polymorphs and the final polymorph after reversible conversion reaction affects the electrochemical performance. A more detailed discussion will be presented at the time of the meeting.

The Effects of Vanadium Substitution on One-dimensional Tunnel Structures of Cryptomelane: Combined TEM and DFT Study

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Polymorphic Consideration in Conversion Reaction-Based Anode Materials for Next-Generation Li Rechargeable Batteries

Beginning with the commercialization by Sony in the 1990s, Li-ion batteries (LIBs) have been used in many electronic devices. In commercialized LIBs, insertion reaction-based electrode materials, such as LiCoO2, LiFePO4, LiMn2O4 for the cathode and graphite, Li4Ti5O12, TiO2 for the anode, have been dominantly used as active materials. In these materials, Li-ions can reversibly be stored and extracted at the specific interstitial site in the crystal without severely collapsing the initial crystal structure. As so, the crystallographic phase significantly affects the electrochemical performances because the local environments around the interstitial site are all different among these phases. For example, LiFePO4 has stable FePO4 framework even after most Li-ions have been extracted, although some Li-ions confined in the tunnel block the migration paths due to the one-dimensional tunnel inside the crystal. In contrast, Li-ions can move easier in LiCoO2 when comparing with LiFePO4 even if the Li-ions are trapped in the Li slab, but over lithiation results in the collapse of the layered structure leading to capacity fading. Even in the same composition, the electrochemical Li storage behavior varies much by the polymorphs. For instance, the layered LiCoO2 exhibit larger reversible capacity and higher redox potential compared to spinel structured Li2Co2O4 [1], and in case of TiO2, despite the similar amount of Li-ion storage, in detail, the voltage profiles are different in anatase, rutile and brookite TiO2 [2].Since the demand for high capacity energy storage devices is continuously growing, there have been extensive researches on high energy density electrode materials based on alloying and conversion reaction due to the limited capacity of insertion-based electrode materials. Manganese dioxide (MnO2) has been considered as a potential candidate due to its low cost, environmentally benignity and especially large theoretical capacity of ~1233 mAh g-1. Accordingly, various types of polymorphs in MnO2 (α, β, γ, δ, ε, λ, etc.) have been adopted as the anode materials in LIBs. However, unlike the insertion-based electrode, the effect of crystal structure in electrochemical conversion reaction was note clearly identified because it is hard to identify the detailed phase transition process as the initial crystalline structures transformed to the amorphous state after lithiation. Recently, some researchers compared the Li storage performance of different polymorphs of MnO2 [3,4], but the morphology of the materials are quite different or the reversible capacities are far below the theoretical value based on the conversion reaction, which could not sufficiently explain the polymorphic effects in conversion reaction. In this work, we have prepared 4 different polymorphs of MnO2 (α, β, Ramsdellite, and λ) in similar hollow urchin-like morphology and compared the Li storage behaviors to discover the influence of polymorphs in conversion-based electrode materials. Among those 4 polymorphs, λ-MnO2 exhibits the highest reversible capacity of ~1200 mAh g-1 and the fastest reaction kinetics. To demonstrate the structural advantages of λ-MnO2, we observed structural changes during electrochemical reactions using synchrotron-based X-ray absorption spectroscopy and X-ray crystallography. As a result, we discovered that the overall Li storage behaviors in the 4 different polymorphs are nearly the same, however, the structural similarity between the initial polymorphs and the final polymorph after reversible conversion reaction affects the electrochemical performance. More detailed discussion will be presented at the time of meeting.

Hollandites’ crystal chemistry, properties, and processing: a review

ABSTRACT This review provides a broad overview of the structural characteristics, compositional flexibility, and structure–property relationships of hollandite materials. Hollandites have a general formula AxB 8O16, x ≤ 2, with ‘A’ cations located in one-dimensional tunnels formed by a framework of ‘B’–O octahedra. With numerous possibilities for chemical and structural modifications, hollandite family provides many opportunities to manipulate its properties for specific applications. First, we review the chemistry, structure–property relationship, and processing techniques for various applications. The primary focus is on the cumulative effects of A- and B-cation interaction, and the resultant parameters including unit cell symmetry, cation order–disorder, electronic and/or magnetic coupling, that dictate the material's applicability. Then, selected applications, such as crystalline hosts for radioactive caesium disposal, electrode material for Li-ion batteries, and ferromagnetic materials, are outlined from a structure–property relationship perspective. Finally, processing strategies in correlation with structural evolution and applications are briefly addressed.

The effects of vanadium substitution on one-dimensional tunnel structures of cryptomelane: Combined TEM and DFT study

Rational design of energy storage materials requires mechanistic understanding which relies on atomic scale characterization of structure and defects. Here we utilize vanadium (V)-substituted one-dimensional cryptomelane as a model material to study structural features critical to electrochemical reversibility of the cathode in lithium-ion batteries. We found the substitution of manganese (Mn) atoms in the tunnel wall with V enhances structural uniformity in the parent KMn8O16 material by inhibiting formation of irregular (2 × 3 and 2 × 1) manganese oxide tunnel structures. Upon lithiation, both transmission electron microscopy (TEM) and density functional theory (DFT) reveal significant structural evolution, eventually resulting in phase separation and formation of highly reduced manganese oxide. During this process, Mn4+ accepts electrons donated by intercalated Li, while the V4+ substituent is not electrochemically reduced but acts to stabilize the tunnel framework structure. As a result, the V substituted material shows higher operating voltage and improved electrochemical reversibility. Our results highlight the benefits of V-substitution in cryptomelane, and the significance of integrating characterization with simulation for understanding and developing design principles for lithium ion battery materials.