Low Average Voltage Research Articles

AB–stacked bilayer β12–borophene as a promising anode material for alkali metal-ion batteries

As the lightest 2D material, monolayer borophene exhibits a specific charge capacity of 1860 mA h g−1 for Li-ion batteries, which is four times higher than that of graphite and is one of the highest specific charge capacities ever reported for 2D anode materials. Additionally, it showed high mechanical strength and a low diffusion barrier. However, monolayer borophene suffers from stability issues in its free-standing form, which restricts its real-life applications. Inspired by the recent experimental investigations, which proved the higher stability of bilayer borophene polymorphs (BBPs) over their monolayer counterparts, in this work, we investigated the dynamical and thermodynamical stabilities of both AA– and AB–stacked BBPs in their β12 phase using first-principles calculations. Between the two stacking patterns, we found that only the AB–stacked β12–BBP is both energetically and dynamically stable, and we further investigated its potential as a high-performance anode material for alkali metal-ion batteries. Our investigations show that AB–stacked β12–BBP exhibits good electrical conductivity before and after metal atom (Li/Na/K) adsorption onto it. Further, AB–stacked β12–BBP adsorbs the metal atoms strongly with adsorption energies ranging between −0.89 to −1.44 eV, indicating that there is a lesser possibility of forming dendrites on this anode. Similarly, it has a low diffusion energy barrier (~ 0.13–0.49 eV) for metal atoms, meeting the fast charge/discharge rate requirements. Moreover, it exhibits a reasonably low average metal-insertion voltage (0.43 to 0.65 V) and a specific charge capacity of 330–413 mA h g−1 that is comparable to graphite. All the above findings suggest that the AB–stacked bilayer β12– borophene can be a potentially favorable anode material.

Unlocking the potential of Mg-ion batteries: Cu2C MXene anode with ultrahigh storage and energy density with rapid Mg diffusion

Magnesium-ion batteries hold immense promise for the future of electrical energy storage due to the abundant availability of magnesium metal and the efficient energy storage capacity of divalent magnesium ions which has the potential to revolutionize both large-scale stationary applications and portable devices. MXenes, encompassing transition metal carbides/nitrides/carbonitrides, are still considered newcomers in the domain of 2D nanomaterials, especially regarding their utilization in energy storage applications. In this work, we employed first principles based DFT calculations to comprehensively evaluate the electrochemical behavior of 2H phase Cu-based MXene (Cu2C) monolayer as promising anode material for Mg-ion batteries. The stability of the Cu2C monolayer was assessed through calculations of negative cohesive energy, positive phonon frequencies, and AIMD simulations to confirm stability at room temperature along with superior mechanical strength. The simulation outcomes show that strong affinity for Mg ions to the Cu2C monolayer, which is attributed to metallic character, suggesting good electronic conductivity and aslo, the calculated negative adsorption energy of −1.25 eV, indicating a favorable interaction for potential battery applications. The Cu2C monolayer emerges as a compelling candidate for anode materials due to its exceptional combination of high theoretical specific capacity (1541.36 mAh/g) and a low average operating voltage (0.50 V). This translates to a remarkable energy density of 2882.34 mWh/g (referenced against the standard hydrogen electrode potential), making it a strong contender for next-generation battery technologies. Furthermore, our investigation revealed rapid diffusion barriers for Mg-ion migration with exceptional properties with robust compatibility with electrolytes, makes it anode materials for Mg-ion batteries, attracting significant interest in the near future.

α-Graphyne with ultra-low diffusion barriers as a promising sodium-ion battery anode and a computational scheme for accurate estimation of theoretical specific capacity.

Sodium-ion batteries are considered as potential alternatives to conventional lithium-ion batteries. To realize their large-scale practical applications, it is essential to identify suitable anode candidates exhibiting promising electrochemical properties such as high specific capacity, low diffusion energy barrier, and excellent cyclic stability. In this work, using density functional theory (DFT) calculations and ab initio molecular dynamics simulations, we examine α-graphyne - a carbon-based 2D material - as a potential anode candidate. Our results show that AGY exhibits an ultra-low diffusion barrier of 0.23 eV along both the horizontal and vertical directions and a low average anodic voltage of 0.36 V. Our AIMD studies at 300 K show excellent thermodynamical stability with the loading of four sodium atoms, resulting in a theoretical specific capacity (TSC) of 279 mA h g-1. Doping studies show that varying the nature of acetylenic links of AGY with electron-rich (nitrogen) or electron-deficient (boron) elements, the adsorption strength and diffusion barriers for Na atoms on AGY can be tuned. Furthermore, treating AGY as a case study, we find that conventional DFT studies are expected to overestimate the TSC by a huge margin. Specific to AGY, this overestimation can be up to ∼300%. We identify that ignoring the probable formation of temperature-driven metal clusters is the main reason behind such overestimations. Furthermore, we develop a scheme to calculate TSC with higher accuracy. The scheme, which can be easily generalized to the universal class of electrodes, is evolved by concurrently employing AIMD simulations, DFT calculations and cluster analysis.

Chiolite Na5Ti3F14: A novel sodium titanium fluoride anode for low-cost and high-performance Na-ion batteries

Fluoride-based conversion reaction electrode materials offer exceptional theoretical capacity merit for Na-ion batteries. Nevertheless, it has rarely been considered as potential anode material candidate due to (i) excessive redox potential (> 3 V) and (ii) intrinsically low reaction kinetics related to sluggish structural reorganization process. In this work, we demonstrate that chiolite Na5Ti3F14/carbon nanocomposite can deliver the outstanding electrochemical performances as the promising anode for Na-ion batteries, such as a large specific capacity of ∼425 mAh g−1 at 10 mA g−1 with a low average operating voltage, the capacity retention of ∼78 % compared to the initial capacity after 300 cycle with a high Coulombic efficiency of above 99 %, etc. We demonstrate that the chiolite Na5Ti3F14 phase can store the ∼8.33 mol Na ions through the following conversion reaction; Na5Ti3F14 + 9Na + 9e−⇔ 3Ti + 14NaF, which is clearly confirmed by various ex-situ analyses using X-ray diffraction, synchrotron-based X-ray adsorption spectroscopy, etc. We expect that this research can provide guidance toward the development of a new class of low-cost and high-performance anode materials, not only for Na-ion batteries but also for other rechargeable batteries.

Reversible Electrochemical Lithium Cycling in a Vanadium(IV)- and Niobium(V)-Based Wadsley–Roth Phase

Fast charging remains one of the greatest safety challenges in Li-ion batteries due to Li-dendrite growth occurring on graphite anodes if they are lithiated too quickly. The search for high-rate anodes has highlighted materials in the Wadsley–Roth (WR) shear phase family. The relative abundance of V compared with traditional WR compositions of Nb and W makes V-based phases attractive. However, the high voltage and poor reversibility typically associated with V redox have made V-rich WR phases less studied than Nb- and W-rich phases. Here, we show that a new V-rich Wadsley–Roth phase, V7Nb6O29, achieves excellent rate capability and 80% capacity retention after 228 cycles with a relatively low average voltage of 1.76 V vs Li/Li+ compared with other V-rich WR phases. Single-crystal X-ray diffraction reveals a P4/m space group with repeating 2 × 2 × ∞ and 3 × 3 × ∞ blocks of V4+ and Nb5+ octahedra. Combined neutron pair distribution function analysis, X-ray absorption spectroscopy, and density functional theory calculations show that V redox is the primary source of capacity and that cycling stability is provided by the stable octahedral coordination adopted by V4+ in the material.

BiOIO3@Zn3(PO4)2·4H2O Heterojunction with Fast Ionic Diffusion Kinetics for Long-Life “Rocking-Chair” Zinc Ion Batteries

Increasing insertion host materials are developed as high-performance anodes of "rocking-chair" zinc ion batteries. However, most of them show unsatisfactory rate capabilities. Herein, layered BiOIO3 is reported as an excellent insertion host and a zinc ion conductor, i.e., Zn3(PO4)2·4H2O (ZPO), is introduced to construct a BiOIO3@ZPO heterojunction with a built-in electric field (BEF). Both ZPO and a BEF obviously enhance Zn2+ transfer and storage, which is proven by theoretical calculations and experimental studies. The conversion-type mechanism of BiOIO3 is revealed through ex situ characterizations. The optimized electrode exhibits a high reversible capacity of 130 mAh g-1 at 0.1 A g-1, a low average discharge voltage of 0.58 V, an ultrahigh rate performance with 68 mAh g-1 at 5 A g-1 (52% of capacity at 0.1 A g-1), and an ultralong cyclic life of 6000 cycles at 5 A g-1. Significantly, the BiOIO3@ZPO//Mn3O4 full cell shows a good cyclic life of 67 mAh g-1 over 1000 cycles at 0.1 A g-1. This work provides a new insight into the design of anodes with excellent rate capability.

From P‐type to N‐type: Peripheral fluorination of axially substituted silicon phthalocyanines enables fine tuning of charge transport

AbstractSilicon phthalocyanines (R2‐SiPcs) are a family of promising tunable materials for organic electronic applications. We report the chemistry of the synthesis of axially substituted fluorinated SiPcs (tb‐Ph)2‐FxSiPc (where X = 0, 4, 8, or 16) and explore how the degree of fluorination effects optical and electronic properties. A new treatment with boron trichloride was included to obtain Cl2‐FXSiPcs from F2‐FXSiPcs, activating the axial position for further functionalization. We observed that as the degree of fluorination increased, so did the electron affinity of the compounds, leading to a drop in frontier orbital levels, as measured by electrochemistry and ultraviolet photoelectron spectroscopy (UPS). The deeper energy levels enabled successful (tb‐Ph)2‐F4SiPc and poly [[6,7‐difluoro[(2‐hexyldecyl)oxy]‐[5,8‐quinoxalinediyl]‐2,5‐thiophenediyl]] (PTQ10) blends for organic photovoltaics and photodetectors. All four compounds were incorporated in organic thin‐film transistors (OTFTs), where the degree of fluorination influenced device operation, changing it from p‐type conduction for (tb‐Ph)2‐F0SiPc, to ambipolar for (tb‐Ph)2‐F4SiPc, and n‐type for (tb‐Ph)2‐F8SiPc and (tb‐Ph)2‐F16SiPc. The OTFT devices made with (tb‐Ph)2‐F16SiPc achieved a low average threshold voltage of 7.0 V in N2 and retained its n‐type mobility when exposed to air.

Highly stable 3D Li metal anodes enabled by a shielding/rectifying polymer layer

The practical application of Li metal anode is hindered by the uncontrollable dendrites growth, severe side reactions and huge volume fluctuation, which would result in low Coulombic efficiency (CE) and poor cycling performance. Herein, a 3D Li host is functionalized by a low-cost and environment-friendly polyvinyl alcohol (PVA) layer via a facile solution immersion method to solve the above issues. The polymer layer can act as a stable artificial SEI with high ionic conductivity during cycling, so as to isolate the direct contact between Li metal and electrolyte and then avoid interfacial side reaction. In addition, the abundant hydroxyl (−OH) polar groups in PVA, which own a strong binding energy with Li atoms, can effectively regulate Li plating/stripping behaviors on the 3D framework. As a result, the symmetric cell using PVA-Cu mesh-Li electrode delivers a superior long-term stability over 1000 h with a low average voltage hysteresis of 17 mV at 1 mA cm−2. And the PVA-Cu mesh-Li || LiFePO4 full cell can remain a high discharge capacity of 104.4 mAh g−1 and a high CE of 99.1% after 350 cycles at 5 C.

Graphene/C2N lateral heterostructures as promising anode materials for lithium-ion batteries.

Two-dimensional (2D) heterostructures have been proposed as potential anode materials for lithium-ion batteries due to their large surface areas and excellent electronic properties. In this study, we employ first-principles calculations to investigate the structural stability, electronic properties, and ion diffusion behaviors of 2D graphene/C2N lateral heterostructures. Three species of (5, 26), (11, 26), and (17, 26) heterostructures are chosen to explore the effects of graphene components on electronic properties. The results show that graphene/C2N lateral heterostructures exhibit good dynamic stability due to a small lattice mismatch and strong chemical interaction at the heterojunction interface. By introducing zero-gap graphene, these heterostructures acquire good electronic conductivity with small direct band gaps. The component ratio of graphene can significantly tune the band gap, showing a monotonic decrease as the ratio increases. Moreover, the introduction of C2N components can greatly improve the lithium capacities of heterostructures. Small diffusion energy barriers (0.257-0.273 eV) and a low average operating voltage of 0.758 V are observed in graphene/C2N heterostructures. The effects of graphene components and valence states on Li migration are discussed. Our results demonstrate that the graphene/C2N lateral heterostructure can effectively combine the advantages of graphene and the C2N monolayer, showing great promise as an anode material for lithium-ion batteries.

Two-dimensional polymers made of carbonyl-bridged heterotriangulenes are promising anode materials for Li-ion batteries

By means of first-principles calculations, we have explored the potential of two-dimensional (2D) polymers made of carbonyl-bridged triphenylamine (CTPA) and carbonyl-bridged triphenylborane (CTPB) as electrode materials for lithium-ion batteries (LIBs). Our investigations demonstrate that the carbonyl groups of 2D CTPB and CTPA are rather active to accommodate Li. Both 2D CTPA and CTPB show the transition from semiconductor to metal after combining with Li. The migration of Li through the pore space of 2D CTPB and CTPA is facilitated with a small diffusion barrier of 0.76 and 0.79 eV, respectively. 2D CTPB exhibits a high theoretical capacity of 760.86 mAh g−1 because it can accommodate Li at both the carbonyl sites and the surface sites of the skeleton, which is ascribed to the promotion of the electron-deficient B center. As a comparison, 2D CTPA can only combine with Li at the carbonyl sites and shows a capacity of 251.09 mAh g−1. With fast Li-diffusion ability, high capacity and low average operating voltage, 2D CTPA and CTPB are predicted to be promising non-metal anode materials for LIBs.

Development of boron-doped diamond (BDD) deposited on carbon nanotubes (CNT) to form BDD/CNT structures relevant for electrochemical degradation

This work presents a new class of Boron-Doped Diamond (BDD) anode for electrochemical degradation. BDD deposition on carbon nanotubes (CNT) is the base for the electrode, because the nanodiamond seeding of oxidized CNT makes diamond nucleation and onset growth very efficient. For the tests presented in this work, the CNT was grown on a carbon fiber (CF) cloth. The BDD electrodes developed with only 10 min of diamond growth (named BDD-10) were tested for brilliant green (BG) dye degradation and showed excellent electrochemical characteristics as large surface area, large potential window, low charge transfer resistance and high degradation rates (kapp). For precise kapp determination in a setup using 360 mL of solution with 100 mg/L of the BG dye, the degradation experiments operated at low current density (5 and 10 mA/cm2). Dye degradation characterization used both UV/Vis color removal and HPLC. Few experiments at higher current densities and at a fixed time of 10 min showed an increase of color removal compatible with a linear dependence of kinetics on current. The kapp found are relatively high (≈0.033 min−1 at 10 mA/cm2), and an important result is the low value of energy consumption to decrease BG dye concentration by one order-of-magnitude (EEO < 0.4 Wh/L). The low energy consumption correlated with the low average voltage during degradation and with the electrode low charge transfer resistance. FEG-SEM and Raman spectroscopy surface characterization show that BDD-10 electrode preserves their morphological and structural characteristics after BG dye degradation, showing that the short time BDD growth is enough to preserve electrode from surface deterioration. The BDD-10 outperformed all anodes of intermediary phases of electrode production: the CF, the oxidized CNT deposited on CF with and without purification to remove surface iron particles. Lifetime tests showed the electrode fails soon, after 8–9 h under a current of 25 mA/cm2. However, the analysis and characterization of the lifetime test shows that erosion of CF/CNT interfaces caused the failure. The BDD/CNT structure remains completely preserved. These findings may suggest this as an excellent and economical method to produce BDD degradation anodes.

An extendable asymmetric boost multi‐level inverter with self‐balanced capacitors

AbstractThis paper proposes a basic asymmetric boost 35‐level inverter topology that can be extended to earn higher gains and levels. The proposed structure benefits from following advantages: extendibility, voltage boosting capability, natural voltage balancing of capacitors, low average voltage stress on semiconductors and large ratio of number of steps to components. The impulse charging current of capacitor(s) is suppressed by small inductor(s), which is bypassed by a diode during discharging mode. The low magnitude (accordingly small size, low price and low weight) and balanced output powers of VX and VY sources are other important profits of proposed topology. The input DC sources can be realized by batteries or combination of PVs and dc–dc converters. This paper also proposes an algorithm for deciding the magnitude of DC sources, which maximizes the number of steps. The gain and steps can be further increased by adding more cascaded switched‐capacitor cells (CSCCs). Due to large number of levels, the output voltage quality of proposed structure is high, which helps to remove or downsize the ac‐side filter. The laboratory‐scaled prototype of converter (feeding a R–L load) has been implemented. The comparison and experimental results approve superiority and correct performance of proposed topology.

A layered nonstoichiometric lepidocrocite-type sodium titanate anode material for sodium-ion batteries

Further performance improvements of sodium-ion batteries require better-performing electrode materials, particularly anodes. The layered lepidocrocite-type sodium titanate (Na0.74Ti1.815□0.185O4·1.27H2O), showing a high Na+ storage capacity of 229 mAh g−1 at relatively low average voltage of ca. 0.6 V vs. Na+/Na, is a promising candidate anode material. A lepidocrocite-structured sodium titanate prepared by ion-exchange of a Cs-containing precursor shows promise as an anode material for sodium ion batteries, with a discharge capacity of up to 229 mAh g−1 at an average potential of about 0.6 V vs. Na+/Na. Titanium vacancies in the metal oxide layers provide additional sites for sodium intercalation in addition to interlayer sites, which accounts for the higher capacity compared to other previously reported lepidocrocite-structured titanates. By screening a series of electrolyte formulations and binders, we were able to improve the first-cycle coulombic efficiency to 81.8% and 94.7% respectively using CMC/SBR-based and binder-free electrodes in ether electrolytes. The electrochemical consequences of short-term air-exposure on the electrodes are also discussed.

First principles predictions of Na and K storage in layered SnSe2

Recent years have witnessed a flow of research in two-dimensional (2D) materials for the development of new rechargeable metal-ion battery systems due to their large surface areas and plenty accommodating sites for the adsorption of metal ions. Inspired from the experimental synthesis of 2D SnSe2 for Na ion batteries, we speculate the adsorption and diffusion mechanism of Na and K on SnSe2 by means of computational approach. Our results disclose strong adsorption strength with Na and K loading, confirming the favorable electrochemical reactions between adsorbates (Na/K) and the host SnSe2. The electronic structure of the host material is displaying the conductive behavior with Na and K loading. Additionally, the low average voltages for Na (0.68 V) and K (0.48 V), and the high theoretical capacity (387.5 mAhg−1) captivate SnSe2 monolayer for Na- and K- ion batteries. Finally, the activation barriers and the energy pathways for Na and K are calculated. The simulated activation barriers for Na and K are as low as 0.104 eV and 0.113 eV, respectively. The current study endows that Na and K storage for SnSe2 sheet could be industrious and productive for the commercialization of Na- and K- ion batteries.

Role of Electrolyte in Stabilizing the Solid Electrolyte Interface of Hard Carbon As an Anode for Sodium-Ion Batteries

Hard carbon (HC) is an attractive anode material for sodium-ion batteries (NIBs) because it can be made from biomass or plastic, has a high capacity of ~300 mAh/g, and an extremely low average voltage near 0V. When fabricated with cathodes free of Li, Ni, and Co, NIBs using HC anodes are a promising approach for grid storage because they are energy-dense, sustainable, and have a low cost of energy. In order for these batteries to become a practical long-life solution, the issues of low first coulombic efficiency and poor rate performance of HC in conventional carbonate electrolytes need to be addressed. Two mechanisms that potentially cause these issues to occur are changes to the HC structure affecting sodium storage and the formation of the solid electrolyte interface (SEI). The proper selection of electrolyte is critical for controlling these mechanisms. To observe and characterize these mechanisms, we compared HC using a conventional carbonate electrolyte, 1M NaPF6 in propylene carbonate (PC), with a high performing glyme based electrolyte, 1M NaBF4 in tetraethylene glycol dimethyl ether (TEGDME). Compared to the poor rate capability of HC in PC electrolyte, HC in TEGDME electrolyte shows an excellent rate capability of ~265 mAh/g at C/20 and ~260 mAh/g at C/3. Titration gas chromatography and Raman spectroscopy were used to show that the HC with either electrolyte had the same sodium storage mechanism with no “stuck” sodium in the HC structure after the first cycle. These observations suggest that SEI formation is responsible for the irreversible capacity differences. The SEI formation was explored with scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). The PC electrolyte’s SEI is found to be thick, fluffy, irregular and composed of polyesters, alkyl carbonates, NaxPFyOz, and NaF. The TEGDME electrolyte’s SEI is found to be thin, conformal, uniform, and composed of polyethers, sodium alkoxides, NaxBFyOz, and NaF. Additionally, the PC electrolyte’s SEI is rate dependent with more unstable products formed at high rates whereas the TEGDME electrolyte’s SEI is not rate dependent. These findings show that HC anode’s rate capability is controlled by the quality of the SEI formed by the electrolyte rather than the HC material structure. This model of rate and electrolyte dependent SEI indicates that SEI engineering is a pathway to improve HC as an anode for long-life grid-level NIBs. Figure 1

Full Concentration Gradient-Tailored Li-Rich Layered Oxides for High-Energy Lithium-Ion Batteries.

Lithium-rich layered oxides (LLOs) are prospective cathode materials for next-generation lithium-ion batteries (LIBs), but severe voltage decay and energy attenuation with cycling still hinder their practical applications. Herein, a series of full concentration gradient-tailored agglomerated-sphere LLOs are designed with linearly decreasing Mn and linearly increasing Ni and Co from the particle center to the surface. The gradient-tailored LLOs exhibit noticeably reduced voltage decay, enhanced rate performance, improved cycle stability, and thermal stability. Without any material modifications or electrolyte optimizations, the gradient-tailored LLO with medium-slope shows the best electrochemical performance, with a very low average voltage decay of 0.8mV per cycle as well as a capacity retention of 88.4% within 200 cycles at 200mA g-1 . These excellent findings are due to spinel structure suppression, electrochemical stress optimization, and Jahn-Teller effect inhibition. Further investigation shows that the gradient-tailored LLO reduces the thermal release percentage by as much as about 41% when the battery is charged to 4.4V. This study provides an effective method to suppress the voltage decay of LLOs for further practical utilization in LIBs and also puts forward a bulk-structure design strategy to prepare better electrode materials for different rechargeable batteries.

(Invited) How Does Intercalation Ion Species Determine the Electrochemical Properties of Cathode Materials for Rechargeable Batteries?

Alkali-ion intercalation compounds are widely used as cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. For many years, researchers have developed layered oxide cathode materials for Na- and K-ion batteries given that the layered oxides show high reversible capacity and high working voltage in Li-ion technology. However, our works have demonstrated that the layered oxide compounds may not be good cathode candidates for Na and K systems. There are two main issues:[1-4] (i) Na and K transition metal oxide compounds form non-stoichiometric composition (x<1.0 in A x MO2, A= Na and K). It leads practical difficulty of realizing Na- and K-ion batteries because all the Na and K ions should come from the cathode in rocking-chair batteries. (ii) As the insertion ion size increases, the voltage curve becomes much sloped. The sloped voltage curves result in low specific capacity and average voltage. Both the problems are attributable to much stronger Na+-Na+ and K+-K+ interaction than Li+-Li+ in the layered oxide structure. In this respect, polyanion compounds that have 3 dimensional arrangements of Na and K ions, resulting in longer Na+-Na+ and K+-K+ distance, will be better candidates. We proposed KVPO4F[5] as an example of polyanion compounds and it has stoichiometric composition and provides high average voltage of ~4.3 V (vs. K/K+) with a reversible capacity of ~105 mAh/g.We further investigated the effect of intercalation ion species in K x VPO4F (x~0).[6] Our work demonstrates the voltage for Na intercalation is even higher than that for Li insertion unlike the common belief that Li insertion voltage is always higher than Na insertion. The lower Li intercalation voltage is likely attributed to unstable Li site in large cavity, making less stable discharged product upon Li insertion vs. Na insertion. In addition, we found that Li intercalation is more sluggish than Na and K intercalation in K x VPO4F (x~0). Since Li ion is too small compared to cavity in the cathode, Li ions are undercoordinated in the transition state, in which a Li ion is coordinated by two anions only. Therefore, Li ion migration barrier is much higher than Na and K migration. This finding teaches us that large cavity size (or channel size) is not always good for fast alkali ion migration and we need to finely tune the cavity size suitable for each intercalating ion species.

First-principles calculations into LiAl(NH2)4 and its derivative hydrides for potential sodium storage

In this work, we have employed the first-principles quantum physics method to investigate the light-metal based LiAl(NH2)4 and its modified compounds as conversion electrode materials for sodium-ion batteries on the basis of state-of-the-art Density Functional Theory. The pure LiAl(NH2)4 possesses an average voltage of 0.294 V (versus Na+/Na0) and a theoretical specific capacity of 1093.77 mA h g−1 for sodium storage. Among the modified materials, the Li4AlB3(N4H8)4 has the most excellent electrochemical properties with a theoretical specific capacity of 1249.57 mA h g−1 and a low average voltage of 0.087 V (versus Na+/Na0) for potential anode applications. The diffusion behavior of Na-ion is also improved in Li4AlB3(N4H8)4 whether at 300 K or at 400 K, which indicates the prospective rate capability. The diffusion coefficient of Na-ion is obviously increased to 3.667 × 10−9 m2 s−1 (in modified material) from 1.500 × 10−9 m2 s−1 (in pristine material) at 400 K. The diffusion of Na-ion is calculated to be very fast in Li4AlB3(N4H8)4 with a kinetic barrier of 0.31 eV. This work will provide impetus to the quantum design and experimental development of complex hydride materials for metal-ion battery applications.

Density Functional Theory Calculations for the Evaluation of FePS3 as a Promising Anode for Mg Ion Batteries

FePS3, a classical 2D layered material with transition metal phosphorous trichalcogenides, was investigated as an anode material for Mg ion batteries. We used density functional theory to calculate the Mg storage properties of FePS3, such as Mg adsorption energy, theoretical specific capacity, average voltage, diffusion energy barriers, volume change, and electronic conductivity. The theoretical specific capacity of the FePS3 monolayer is 585.6 mA h/g with a relatively low average voltage of 0.483 V (vs. Mg/Mg2+), which is favorable to a high energy density. The slight change in volume and good electronic conductivity of bulk FePS3 are beneficial to electrode stability during cycling.

High Energy Polyanion Cathode for K-Ion Batteries: KVPO4F

The development of high-energy cathode materials is crucial for the commercialization of emerging K-ion batteries (KIBs). While layered potassium-transition-metal-oxides (K x MO2, M=transition metals) have been investigated as potential cathodes, they have two intrinsic challenges. First, most of the K-layered oxides have K-poor composition (x<1.0 in K x MO2) and it leads practical difficulty of realizing KIBs because all the K ions should come from the cathode in a rocking-chair KIB.[1-6] Second, K-layered oxides have too high a voltage slope, resulting in low specific capacity and average voltage. Both the problems are attributable to much stronger K+-K+ interaction than Na+-Na+ and Li+-Li+ in the layered oxide structure.[2, 4] In contrast, polyanion cathodes can be better alternatives because 3-dimensional arrangement of K ions in their frameworks can significantly reduce the strength of effective interaction between K ions. As a result, polyanion cathodes can have K-rich (or K-stoichiometric) composition and high working voltage.In this work, we demonstrate that KVPO4F, a polyanion compound, is a high-energy cathode of KIBs. The KVPO4F cathode delivers a reversible capacity of ~105 mAh g−1 with an average voltage of ~4.3 V (vs. K/K+) (Figure 1), which is the highest among the K-cathodes developed up to date.[7] The specific energy (a gravimetric energy) of KVPO4F reaches ~450 Wh kg−1. Ex-situ X-ray diffraction characterization and ab-initio calculations demonstrate reversible multiple phase transitions of K x VPO4F during electrochemical cycling. K x VPO4F goes through various intermediate phases at x = 0.75, 0.625, and 0.5 upon K extraction and reinsertion. We further explain the role of oxygen substitution in KVPO4+xF1−x: the oxygenation of KVPO4F leads to an anion-disordered structure which prevents the formation of K+/vacancy orderings without electrochemical plateaus and hence to a smoother voltage profile.