Deciphering the structural and kinetic factors in lithium titanate for enhanced performance in Li+/Na+ dual-cation electrolyte

The widespread application of Li4Ti5O12 (LTO) anode in lithium-ion batteries has been hindered by its relatively low energy density. In this study, we investigated the capacity enhancement mechanism of LTO anode through the incorporation of Na+ cations in an Li+-based electrolyte (dual-cation electrolyte). LTO thin film electrodes were prepared as conductive additive-free and binder-free model electrodes. Electrochemical performance assessments revealed that the dual-cation electrolyte boosts the reversible capacity of the LTO thin film electrode, attributable to the additional pseudocapacitance and intercalation of Na+ into the LTO lattice. Operando Raman spectroscopy validated the insertion of Li+/Na+ cations into the LTO thin film electrode, and the cation migration kinetics were confirmed by ab initio molecular dynamic (AIMD) simulation and electrochemical impedance spectroscopy, which revealed that the incorporation of Na+ reduces the activation energy of cation diffusion within the LTO lattice and improves the rate performance of LTO thin film electrodes in the dual-cation electrolyte. Furthermore, the interfacial charge transfer resistance in the dual-cation electrolyte, associated with ion de-solvation processes and traversal of the cations in the solid-electrolyte interphase (SEI) layer, are evaluated using the distribution of relaxation time, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Our approach of performance enhancement using dual-cation electrolytes can be extrapolated to other battery electrodes with sodium/lithium storage capabilities, presenting a novel avenue for the performance enhancement of lithium/sodium-ion batteries.

Modeling High Concentration Bisalt-in-Sulfolane Electrolytes and the Observation of Ligand-Bridged Cation-Pair Complexes

Despite the abundance of sodium over lithium in Earth's crust and the copious amounts of expensive lithium salt required to make Li-ion high-concentration electrolytes (HCEs), studies of HCEs made from sodium salts remain sparse. A comparative molecular-level study of Li- and Na-ion HCEs and mixed cation or bisalt HCEs in an organic solvent is missing. To fill this gap, we studied model HCEs of pure and mixed Li and Na salts of bis(fluorosulfonyl)amide (FSI) in sulfolane using a confluence of classical molecular dynamics (MD), ab initio MD (AIMD) simulations, and quantum chemical cluster calculations. While Li-ion HCEs display transport properties superior to those of Na-ion HCEs, the latter's performance can be considerably improved by replacing even 25% of Na-ions with Li-ions. While the effects of doping are largely systemic, a larger sensitivity of the identity of solvation shells of Li-ions to the Li-content of the HCE is observed; in contrast, those of Na-ions are more oblivious to it. Fascinating ligand-bridged, short-distance cation pairs observed in the classical MD simulations are confirmed using density functional theory-based AIMD simulations. Quantum chemical calculations in the gas phase reveal the thermodynamic stability of such cation pairs complexed with multiple anions and solvent molecules.

Dual-Ion Intercalation Chemistry Enabling Hybrid Metal-Ion Batteries

To outline the role of dual-ion intercalation chemistry to reach sustainable energy storage, the present Review aimed to compare two types of batteries: widely accepted dual-ion batteries based on cationic and anionic co-intercalation versus newly emerged hybrid metal-ion batteries using the co-intercalation of cations only. Among different charge carrier cations, the focus was on the materials able to co-intercalate monovalent ions (such Li⁺ and Na⁺ , Li⁺ and K⁺ , Na⁺ and K⁺ , etc.) or couples of mono- and multivalent ions (Li⁺ and Mg²⁺ , Na⁺ and Mg²⁺ , Na⁺ and Zn²⁺ , H⁺ and Zn²⁺ , etc.). Furthermore, the Review was directed on co-intercalation materials composed of environmentally benign and low-cost transition metals (e. g., Mn, Fe, etc.). The effect of the electrolyte on the co-intercalation properties was also discussed. The summarized knowledge on dual-ion energy storage could stimulate further research so that the hybrid metal-ion batteries become feasible in near future.

Understanding the Aging Mechanism of Na-Based Layered Oxide Cathodes with Different Stacking Structures

Manganese-based layered oxides are one of the most promising cathodes for Na-ion batteries, but the prospect of their practical application is challenged by high sensitivity to ambient air. The stacking structure of materials is critical to the aging mechanism between layered oxides and air, but there remains a lack of systematic study. Herein, comprehensive research on model materials P-type Na0.50MnO2 and O-type Na0.85MnO2 reveals that the O-phase displays a much higher dynamic affinity toward moisture air compared to P-type compounds. For air-exposed O-type material, Na+ ions are extracted from the crystal lattice to form alkaline species at the surface in contact with air, accompanying by the increase of the valence state of transition metals. The series of undesired reactions result in an increase of interfacial resistance and huge capacity loss. Comparatively, the insertion of H2O into the Na layer is the main reaction during air-exposure of P-type material, and the inserted H2O can be extracted by high-temperature treatment. The H2O de/insertion process not only causes no performance degradation but also can enlarge the interlayer distance. With these understandings, we further propose a washing-resintering strategy to recover the performance of aged O-type materials and an aging strategy to build high-performance P-type materials.

Enabling high-performance all-solid-state hybrid-ion batteries with a PEO-based electrolyte

All-solid-state hybrid-ion batteries exhibiting a synergistic Na⁺/Li⁺ de/intercalation mechanism were designed and assembled, by using modified PEO-based solid polymer electrolyte, Na₂V₂(PO₄)₂O₂F cathode, and Li metal anode. The batteries exhibited a high average working voltage of 3.88 V, and an energy density of 432.37 W h kg^-1, providing a new avenue for the development of high-safety and low-cost secondary batteries.

Na3V2(PO4)2F3@bagasse carbon as cathode material for lithium/sodium hybrid ion battery

The nano-scale spherical Na₃V₂(PO₄)₂F₃ with a NASICON structure phase was prepared with a spray drying technique, and the bagasse in Guangxi, China was selected as the carbon source to prepare Na₃V₂(PO₄)₂F₃/C. The optimal preparation conditions of the composite determined using thermogravimetry, X-ray diffraction, scanning electron microscopy and electrochemical testing were: a calcination temperature of 650 °C and a 20% carbon source. The Na₃V₂(PO₄)₂F₃/C has obvious redox peaks, determined by cyclic voltammetry (CV), at 3.90 V and 3.75 V, 4.32 V and 4.15 V. These two pairs of redox peaks correspond to the escape/intercalation of the two pairs of Li⁺/Na⁺. Notably, compared with pure Na₃V₂(PO₄)₂F₃, the specific discharge capacity of Na₃V₂(PO₄)₂F₃/C-20%, which were used as a cathode material for lithium-sodium hybrid ion batteries, increased from 55 mA h g^-1 to 125 mA h g^-1, which was an improvement of twofold.

Rivalry at the Interface: Ion Desolvation and Electrolyte Degradation in Model Ethylene Carbonate Complexes of Li+, Na+, and Mg2+ with PF6 - on the Li4Ti5O12 (111) Surface

Spinel lithium titanate, Li₄Ti₅O₁₂ (LTO), emerges as a "universal" electrode material for Li-ion batteries and hybrid Li/Na-, Li/Mg-, and Na/Mg-ion batteries functioning on the basis of intercalation. Given that LTO operates in a variety of electrolyte solutions, the main challenge is to understand the reactivity of the LTO surface toward single- and dual-cation electrolytes at the molecular level. This study first reports results on ion desolvation and electrolyte solvent/salt degradation on an LTO surface by means of periodic DFT calculations. The desolvation stages are modeled by the adsorption of mono- and binuclear complexes of Li⁺, Na⁺, and Mg²⁺ with a limited number of ethylene carbonate (EC) solvent molecules on the oxygen-terminated LTO (111) surface, taking into account the presence of a PF₆ ^- counterion. Alongside cation adsorption, several degradation reactions are discussed: surface-catalyzed dehydrogenation of EC molecules, simultaneous dehydrogenation and fluorination of EC, and Mg²⁺-induced decay of PF₆ ^- to PF₅ and F^-. Data analysis allows the rationalization of existing experimentally established phenomena such as gassing and fluoride deposition. Among the three investigated cations, Mg²⁺ is adsorbed most tightly and is predicted to form a thicker fluoride-containing film on the LTO surface. Gassing, characteristic for carbonate-based electrolytes with LTO electrodes, is foreseen to be suppressed in dual-cation batteries. The latter bears promise to outperform the single-ion ones in terms of durability and safety.

Dual-Metal Electrolytes for Hybrid-Ion Batteries: Synergism or Antagonism?

The construction of hybrid metal-ion batteries faces a plethora of challenges. A critical one is to unveil the solvation/desolvation processes at the molecular level in electrolytes that ensure efficient transfer of several types of charge carriers. This study reports first results on simulations of mixed-ion electrolytes. All combinations of homo- and hetero-binuclear complexes of Li⁺ , Na⁺ and Mg²⁺ , solvated with varying number of ethylene carbonate (EC) molecules are modeled in non-polar and polar environment by means of first principles calculations and compared to the mononuclear analogues in terms of stability, spatial organization, charge distribution and solvation/desolvation behavior. The used PF₆ ^- counterion is shown to have minor impact on the geometry of the complexes. The desolvation energy penalty of binuclear complexes can be lowered by the fluoride ions, emerging upon the PF₆ ^- decay. These model investigations could be extended to rationalize the solvation structure and ionic mobility in dual-ion electrolytes.

Operation Mechanism in Hybrid Mg-Li Batteries with TiNb2O7 Allowing Stable High-Rate Cycling

We studied the structural evolution and cycling behavior of TiNb₂O₇ (TNO) as a cathode in a nonaqueous hybrid dual-salt Mg-Li battery. A very high fraction of pseudocapacitive contribution to the overall specific capacity makes the material suitable for ultrafast operation in a hybrid battery, composed of a Mg-metal anode, and a dual-salt APC-LiCl electrolyte with Li and Mg cations. Theoretical calculations show that Li intercalation is predominant over Mg intercalation into the TNO in a dual-salt electrolyte with Mg²⁺ and Li⁺, while experimentally up to 20% Mg cointercalation was observed after battery discharge. In hybrid Mg-Li batteries, TNO shows capacities which are about 40 mA h g^-1 lower than in single-ion Li batteries at current densities of up to 1.2 A g^-1. This is likely due to a partial Mg cointercalation or/and location of Li cations on alternative crystallographic sites in the TNO structure in comparison to the Li-intercalation process in Li batteries. Generally, hybrid Mg-Li cells show a markedly superior applicability for a very prolonged operation (above 1000 cycles) with 100% Coulombic efficiency and a capacity retention higher than 95% in comparison to conventional Li batteries with TNO after being cycled either under a low (7.75 mA g^-1) or high (1.55 A g^-1) current density. The better long-term behavior of the hybrid Mg-Li batteries with TNO is especially pronounced at 60 °C. The reasons for this are an appropriate cathode electrolyte interface containing MgCl₂ species and a superior performance of the Mg anode in APC-LiCl electrolytes with a dendrite-free, fast Mg deposition/stripping. This stable interface stands in contrast to the anode electrolyte interface in Li batteries with a Li anode in conventional carbonate-containing electrolytes, which is prone to dendrite formation, thus leading to a battery shortcut.

Vanadium oxide bronzes as cathode active materials for non-lithium-based batteries

Lithium as critical resource prompted interest for non-lithium-based batteries. This highlight review discusses vanadium oxide bronzes as one of the material families being considered as cathode for non-lithium-based batteries.

Influence of Crystal Disorder in MoS2 Cathodes for Secondary Hybrid Mg-Li Batteries

The full extent to which the electrochemical properties of MoS2 electrodes are influenced by their morphological characteristics, such as crystalline disorder, remains unclear. Here, we report that disorder introduced by ball-milling decreases the Faradaic component of cell capacity and leads to increasingly pseudo-capacitive behaviour. After high temperature annealing, a more battery-like character of the cell is restored, consistent with a decrease in disorder. These findings aid the optimisation of MoS2 electrodes, which show promise in several battery technologies.

Hybrid Li/Na Ion Batteries: Temperature-Induced Reactivity of Three-Layered Oxide (P3-Na2/3Ni1/3Mg1/6Mn1/2O2) Toward Lithium Ionic Liquid Electrolytes

Hybrid metal ion batteries are perceived as competitive alternatives to lithium ion batteries because they provide better balance between energy/power density, battery cost, and environmental requirements. However, their cycling stability and high-temperature storage performance are still far from the desired. Herein, we first examine the temperature-induced reactivity of three-layered oxide, P3-Na_2/3Ni_1/3Mg_1/6Mn_1/2O₂, toward lithium ionic liquid electrolyte upon cycling in hybrid Li/Na ion cells. Through ex situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses, the structural and surface changes in P3-Na_2/3Ni_1/3Mg_1/6Mn_1/2O₂ are monitored and discussed. Understanding the relevant changes occurring during dual Li⁺ and Na⁺ intercalation into P3-Na_2/3Ni_1/3Mg_1/6Mn_1/2O₂ is of crucial importance to enhance the overall performance of hybrid Li/Na ion batteries at elevated temperatures.

Structural evolution from layered Na2Ti3O7 to Na2Ti6O13 nanowires enabling a highly reversible anode for Mg-ion batteries

The development of suitable host materials for the reversible storage of divalent ions such as Mg²⁺ is still a big challenge and its progress to date has been slow compared to that of monovalent Li⁺ or Na⁺. Herein, we present the study of layered sodium trititanate (Na₂Ti₃O₇) and sodium hexatitanate (Na₂Ti₆O₁₃) nanowires as anode materials for rechargeable Mg-ion batteries. It is found for the first time that the structural evolution from layered Na₂Ti₃O₇ to Na₂Ti₆O₁₃ with a more condensate three-dimensional microporous structure enables remarkably enhanced Mg-ion storage performance. The Na₂Ti₆O₁₃ electrode can achieve a large initial discharge and charge capacity of 165.8 and 147.7 mA h g^-1 at 10 mA g^-1 with a record high initial coulombic efficiency up to 89.1%. Ex situ XRD, Raman measurements and EDX mapping were used to investigate the electrochemical reaction mechanism. It is suggested that the irreversible structure change and the formation of insoluble NaCl with high yield and large particles when Na⁺ is replaced by inserted Mg²⁺ for the Na₂Ti₃O₇ electrode could be ascribed to the rapid decline in capacity. By contrast, the Na₂Ti₆O₁₃ electrode exhibits good structure stability during the Mg-ion insertion/extraction process, leading to good rate performance and cycling stability.

Electrochemical study of TiO2 in aqueous AlCl3 electrolyte via vacuum impregnation for superior high-rate electrode performance

This communication elucidates the charge storage mechanism of a TiO2 electrode in 1 mol dm− 3 AlCl3 for use in aqueous-ion batteries. Cyclic voltammetry studies suggest a surface contribution to charge storage and that cycle life can be improved by limiting the potential ≥ − 1.0 V vs SCE. In order to enhance this surface contribution, a simple vacuum impregnation technique was employed to improve electrode-electrolyte contact. This resulted in a significant improvement in the high rate performance of TiO2, where a capacity of 15 mA h g− 1 was maintained at the very high specific current of 40 A g− 1, a decrease of only 25% from when the electrode was cycled at 1 A g− 1. The vacuum impregnation process was also applied to copper-hexacyanoferrate, envisaged as a possible positive electrode, again resulting in significant improvements to high-rate performance. This demonstrates the potential for using this simple technique for improving electrode performance in other aqueous electrolyte battery systems.

Focus on Spinel Li4 Ti5 O12 as Insertion Type Anode for High-Performance Na-Ion Batteries

Sodium-ion batteries (SIBs) toward large-scale energy storage applications has fascinated researchers in recent years owing to the low cost, environmental friendliness, and inestimable abundance. The similar chemical and electrochemical properties of sodium and lithium make sodium an easy substitute for lithium in lithium-ion batteries. However, the main issues of limited cycle life, low energy density, and poor power density hamper the commercialization process. In the last few years, the development of electrode materials for SIBs has been dedicated to improving sodium storage capacities, high energy density, and long cycle life. The insertion type spinel Li₄ Ti₅ O₁₂ (LTO) possesses "zero-strain" behavior that offers the best cycle life performance among all reported oxide-based anodes, displaying a capacity of 155 mAh g^-1 via a three-phase separation mechanism, and competing for future topmost high energy anode for SIBs. Recent reports offer improvement of overall electrode performance through carbon coating, doping, composites with metal oxides, and surface modification techniques, etc. Further, LTO anode with its structure and properties for SIBs is described and effective methods to improve the LTO performance are discussed in both half-cell and practical configuration, i.e., full-cell, along with future perspectives and solutions to promote its use.

Applications of MxSey (M = Fe, Co, Ni) and Their Composites in Electrochemical Energy Storage and Conversion

Transition-metal selenides (M_xSe_y, M = Fe, Co, Ni) and their composites exhibit good storage capacities for sodium and lithium ions and occupy a unique position in research on sodium-ion and lithium-ion batteries. M_xSe_y and their composites are used as active materials to improve catalytic activity. However, low electrical conductivity, poor cycle stability, and low rate performance severely limit their applications. This review provides a comprehensive introduction to and understanding of the current research progress of M_xSe_y and their composites. Moreover, this review proposes a broader research platform for these materials, including various bioelectrocatalytic performance tests, lithium-sulfur batteries, and fuel cells. The synthesis method and related mechanisms of M_xSe_y and their composites are reviewed, and the effects of material morphologies on their electrochemical performance are discussed. The advantages and disadvantages of M_xSe_y and their composites as well as possible strategies for improving the storage and conversion of electrochemical energy are also summarized.

Effect of Mixed Li+/Na+-ion Electrolyte on Electrochemical Performance of Na4Fe3(PO4)2P2O7 in Hybrid Batteries

The mixed sodium-iron ortho-pyrophosphate Na4Fe3(PO4)2P2O7 (NFPP) is a promising Na-containing cathode material with the highest operating voltage among sodium framework structured materials. It operates both in Na and Li electrochemical cells. When cycled in a hybrid Li/Na cell, a competitive co-intercalation of the Li+ and Na+ ions occurs at the cathode side. The present study shows that this process can be tuned by changing the concentration of the Na+ ions in the mixed Li+/Na+-ion electrolyte and current density. It is shown that if the Na concentration in the electrolyte increases, the specific capacity of NFPP also increases and its high-rate capability is significantly improved.

Novel MnO-Graphite Dual-Ion Battery and New Insights into Its Reaction Mechanism during Initial Cycle by Operando Techniques

Dual-ion battery complements lithium-ion batteries in terms of the use of inexpensive materials and ease to construct cells. To improve the safety and energy density of dual-ion battery, in this paper, a novel MnO-graphite dual-ion battery is reported for the first time. Microporous MnO materials are used as anode, which exhibits a low conversion potential and a low voltage hysteresis. The MnO-graphite dual-ion battery can deliver a capacity of 104 mAh g^-1 at 0.5C and exhibits good rate performances and cycling stability (capacity retention >93% after 300 cycles). A mechanism is proposed to explain the irreversibility in capacity during the initial cycle by using operando X-ray diffraction in combination with online electrochemical mass spectrometry and electrochemical impedance spectroscopy.

α-VPO4: A Novel Many Monovalent Ion Intercalation Anode Material for Metal-Ion Batteries

In this paper, we report on a novel α-VPO₄ phosphate adopting the α-CrPO₄ type structure as a promising anode material for rechargeable metal-ion batteries. Obtained by heat treatment of a structurally related hydrothermally prepared KTiOPO₄-type NH₄VOPO₄ precursor under reducing conditions, the α-VPO₄ material appears stable in a wide temperature range and possesses an interesting "sponged" needle-like particle morphology. The electrochemical performance of α-VPO₄ as the anode material was examined in Li-, Na-, and K-based cells. The carbon-coated α-VPO₄/C composite exhibits 185, 110, and 37 mA h/g specific capacities respectively at the first discharge and around 120, 80, and 30 mA h/g at consecutive cycles at a C/10 rate. The considerable capacity drop after the first cycle in Li and Na cells is presumably due to irreversible alkali ion consumption taking place upon alkali-ion de/insertion. The EDX analysis of the recovered electrodes revealed an uptake of ∼23% of Na after the first discharge with significant cell parameter alteration validated by operando XRD measurements. In contrast to the known β-VPO₄ anode materials, both Li and Na de/insertion into the new α-VPO₄ proceed via an intercalation mechanism with the parent structural framework preserved but not via a conversion mechanism. The dimensionality of alkali-ion migration pathways and diffusion energy barriers was analyzed by the BVEL approach. Na-ion diffusion coefficients measured by the potentiostatic intermittent titration technique are in the range of (0.3-1.0)·10^-10 cm²/s, anticipating α-VPO₄ as a prospective high-power anode material for Na-ion batteries.

Magnesium-Sodium Hybrid Battery With High Voltage, Capacity and Cyclability

Rechargeable magnesium battery has been widely considered as a potential alternative to current Li-ion technology. However, the lack of appropriate cathode with high-energy density and good sustainability hinders the realization of competitive magnesium cells. Recently, a new concept of hybrid battery coupling metal magnesium anode with a cathode undergoing the electrochemical cycling of a secondary ion has received increased attention. Mg-Na hybrid battery, for example, utilizes the dendritic-free deposition of magnesium at the anode and fast Na⁺-intercalation at the cathode to reversibly store and harvest energy. In the current work, the principles that take the full advantage of metal Mg anode and Na-battery cathode to construct high-performance Mg-Na hybrid battery are described. By rationally applying such design principle, we constructed a Mg-NaCrO₂ hybrid battery using metal Mg anode, NaCrO₂ cathode and a mixture of all-phenyl complex (PhMgCl-AlCl₃, Mg-APC) and sodium carba-closo-dodecaborate (NaCB₁₁H₁₂) as dual-salt electrolyte. The Mg-NaCrO₂ cell delivered an energy density of 183 Wh kg⁻¹ at the voltage of 2.3 V averaged in 50 cycles. We found that the amount of electrolyte can be reduced by using solid MgCl₂ as additional magnesium reservoir while maintaining comparable electrochemical performance. A hypothetical MgCl₂-NaCrO₂ hybrid battery is therefore proposed with energy density estimated to be 215 Wh kg⁻¹ and the output voltage over 2 V.

A Multi-Ion Strategy towards Rechargeable Sodium-Ion Full Batteries with High Working Voltage and Rate Capability

Sodium-ion batteries (SIBs) are a promising alternative for the large-scale energy storage owing to the natural abundance of sodium. However, the practical application of SIBs is still hindered by the low working voltage, poor rate performance, and insufficient cycling stability. A sodium-ion based full battery using a multi-ion design is now presented. The optimized full batteries delivered a high working voltage of about 4.0 V, which is the best result of reported sodium-ion full batteries. Moreover, this multi-ion battery exhibited good rate performance up to 30 C and a high capacity retention of 95 % over 500 cycles at 5 C. Although the electrochemical performance of this multi-ion battery may be further enhanced via optimizing electrolyte and electrode materials for example, the results presented clearly indicate the feasibility of this multi-ion strategy to improve the electrochemical performance of SIBs for possible energy storage applications.

Electrochemically Treated TiO₂ for Enhanced Performance in Aqueous Al-Ion Batteries

The potential for low cost, environmentally friendly and high rate energy storage has led to the study of anatase-TiO₂ as an electrode material in aqueous Al3+ electrolytes. This paper describes the improved performance from an electrochemically treated composite TiO₂ electrode for use in aqueous Al-ion batteries. After application of the cathodic electrochemical treatment in 1 mol/dm³ KOH, Mott⁻Schottky analysis showed the treated electrode as having an increased electron density and an altered open circuit potential, which remained stable throughout cycling. The cathodic treatment also resulted in a change in colour of TiO₂. Treated-TiO₂ demonstrated improved capacity, coulombic efficiency and stability when galvanostatically cycled in 1 mol·dm-3AlCl₃/1 mol·dm-3 KCl. A treated-TiO₂ electrode produced a capacity of 15.3 mA·h·g-1 with 99.95% coulombic efficiency at the high specific current of 10 A/g. Additionally, X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy were employed to elucidate the origin of this improved performance.

Multifunctional Additives Improve the Electrolyte Properties of Magnesium Borohydride Toward Magnesium-Sulfur Batteries

Highly reductive magnesium borohydride [Mg(BH₄)₂] is compatible with metallic Mg, making it a promising Mg-ion electrolyte for rechargeable Mg batteries. However, pure Mg(BH₄)₂ in ether-based solutions displays very limited solubility (0.01 M), low oxidative stability (<1.8 V vs Mg), and nucleophilic characteristic, all of which preclude its practical utilization for any battery applications. Herein, we present a multifunctional additive of tris(2 H-hexafluoroisopropyl)borate (THFPB) for preparing Mg(BH₄)₂-based electrolytes. By virtue of the strong electron-acceptor ability of the THFPB molecule, a transparent and high-concentration Mg(BH₄)₂/THFPB-diglyme (DGM) electrolyte (0.5 M, almost 50 times higher than that of the pristine Mg(BH₄)₂-DGM electrolyte) is first obtained, which shows dramatic performance improvements, including high ionic conductivity (3.72 mS cm^-1 at 25 °C) and high Mg plating/stripping Coulombic efficiency (>99%). The newly-generated active cation and anion species revealed by Raman, NMR and MS spectra, increase the electrochemical potential window from 1.8 V to 2.8 V vs Mg on stainless steel electrode, rendering electrolytes the ability to examine high voltage cathodes. More importantly, on account of the non-nucleophilicity of active electrolyte species, we present the first example of magnesium-sulfur (Mg-S) batteries using Mg(BH₄)₂-based electrolytes, which exhibit a high discharge capacity of 955.9 and 526.5 mA h g^-1 at the initial and 30th charge/discharge cycles, respectively. These achievements not only provide an efficient and specific strategy to eliminate the major roadblocks facing Mg(BH₄)₂-based electrolytes but also highlight the profound effect of functional additives on the electrochemical performances of unsatisfied Mg-ion electrolytes.

Magnesium/Lithium-Ion Hybrid Battery with High Reversibility by Employing NaV3O8·1.69H2O Nanobelts as a Positive Electrode

Recently, magnesium-ion batteries (MIBs) have been under remarkable research focus owing to their appealingly high energy density and natural abundance of magnesium. Nevertheless, MIBs exhibit a very limited performance because of sluggish solid-state Mg²⁺ ion diffusion and high polarizability, which hinder their progress toward commercialization. Herein, we report a Mg²⁺/Li⁺ hybrid-ion battery (MLIB) with NaV₃O₈·1.69H₂O (NVO) nanobelts synthesized at room temperature working as the positive electrode. In the hybrid-ion system, Li⁺ intercalates/deintercalates along with a small amount of Mg²⁺ adsorption at the NVO cathode, whereas the anode side of the cell is dominated by Mg²⁺ deposition/dissolution. As a result, the MLIB exhibits a much higher rate capability (i.e., 446 mA h g^-1 at 20 mA g^-1) than the previously reported MLIBs. MLIB maintains a high specific capacity of 200 mA h g^-1 at 80 mA g^-1 for 150 cycles, showing excellent stability. Moreover, the effect of different Li-ion concentrations (i.e., 0.5-2.0 M) in the electrolyte and cutoff voltage (ranging from 2 to 2.6 V) on the specific capacities are investigated. The current study highlights a strategy to exploit the Mg²⁺/Li⁺ hybrid electrolyte system with various electrode materials for high-performance MIBs.

Selective sodium intercalation into sodium nickel–manganese sulfate for dual Na–Li-ion batteries

Through electrochemical, diffraction and spectroscopic examination, we demonstrate that Na₂Ni_1/2Mn_1/2(SO₄)₂ is able to intercalate Na⁺ ions in a selective way.

FePO4 as an anode material to obtain high-performance sodium-based dual-ion batteries

Herein, FePO₄ was for the first time proposed to serve as an anode material (the sodium intercalation host), to obtain novel sodium-based dual-ion batteries.

High Specific Power Dual-Metal-Ion Rechargeable Microbatteries Based on LiMn2O4 and Zinc for Miniaturized Applications

Miniaturized rechargeable batteries with high specific power are required for substitution of the large sized primary batteries currently prevalent in integrated systems since important implications in dimensions and power are expected in future miniaturized applications. Commercially available secondary microbatteries are based on lithium metal which suffers from several well-known safety and manufacturing issues and low specific power when compared to (super) capacitors. A high specific power and novel dual-metal-ion microbattery based on LiMn₂O₄, zinc, and an aqueous electrolyte is presented in this work. Specific power densities similar to the ones exhibited by typical electrochemical supercapacitors (3400 W kg^-1) while maintaining specific energies in the range of typical Li-ion batteries are measured (∼100 Wh kg^-1). Excellent stability with very limited degradation (99.94% capacity retention per cycle) after 300 cycles is also presented. All of these features, together with the intrinsically safe nature of the technology, allow anticipation of this alternative micro power source to have high impact, particularly in the high demand field of newly miniaturized applications.

VO2 Nanoflakes as the Cathode Material of Hybrid Magnesium-Lithium-Ion Batteries with High Energy Density

The hybrid magnesium-lithium-ion batteries (MLIBs) combining the dendrite-free deposition of the Mg anode and the fast Li intercalation cathode are better alternatives to Li-ion batteries (LIBs) in large-scale power storage systems. In this article, we reported hybrid MLIBs assembled with the VO₂ cathode, dendrite-free Mg anode, and the Mg-Li dual-salt electrolyte. Satisfactorily, the VO₂ cathode delivered a stable plateau at about 1.75 V, and a high specific discharge capacity of 244.4 mA h g^-1. To the best of our knowledge, the VO₂ cathode displays the highest energy density of 427 Wh kg^-1 among reported MLIBs in coin-type batteries. In addition, an excellent rate performance and a wide operating temperature window from 0 to 55 °C have been obtained. The combination of VO₂ cathode, dual-salt electrolyte, and Mg anode would pave the way for the development of high energy density, safe, and low-cost batteries.

Combined use of EPR and 23Na MAS NMR spectroscopy for assessing the properties of the mixed cobalt–nickel–manganese layers of P3-NayCo1−2xNixMnxO2

EPR and ²³Na MAS NMR are used to gain insights into the structural peculiarities of the mixed cobalt–nickel–manganese layers of P3-Na_yCo_1−2xNi_xMn_xO₂.

Layered P3-NaxCo1/3Ni1/3Mn1/3O2 versus Spinel Li4Ti5O12 as a Positive and a Negative Electrode in a Full Sodium-Lithium Cell

The development of lithium and sodium ion batteries without using lithium and sodium metal as anodes gives the impetus for elaboration of low-cost and environmentally friendly energy storage devices. In this contribution we demonstrate the design and construction of a new type of hybrid sodium-lithium ion cell by using unique electrode combination (Li4Ti5O12 spinel as a negative electrode and layered Na3/4Co1/3Ni1/3Mn1/3O2 as a positive electrode) and conventional lithium electrolyte (LiPF6 salt dissolved in EC/DMC). The cell operates at an average potential of 2.35 V by delivering a reversible capacity of about 100 mAh/g. The mechanism of the electrochemical reaction in the full sodium-lithium ion cell is studied by means of postmortem analysis, as well as ex situ X-ray diffraction analysis, HR-TEM, and electron paramagnetic resonance spectroscopy (EPR). The changes in the surface composition of electrodes are examined by ex situ X-ray photoelectron spectroscopy (XPS).