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A novel P2/O3-Na0.7Li0.1Ni0.1Fe0.2Mn0.6O2 cathode is prepared via Li/Ni co-doping, and delivers attractive cycling and rate performances due to the high Na+ diffusion coefficient and the complete suppression of the high-voltage P2–Z phase transition.

The interfacial stability is highly responsible for the longevity and safety of sodium ion batteries (SIBs). However, the continuous solid-electrolyte interphase(SEI) growth would deteriorate its stability. Essentially, the SEI growth is associated with the electron leakage behavior, yet few efforts have tried to suppress the SEI growth, from the perspective of mitigating electron leakage. Herein, we built two kinds of SEI layers with distinct growth behaviors, via the additive strategy. The SEI physicochemical features (morphology and componential information) and SEI electronic properties (LUMO level, band gap, electron work function) were investigated elaborately. Experimental and calculational analyses showed that, the SEI layer with suppressed growth delivers both the low electron driving force and the high electron insulation ability. Thus, the electron leakage is mitigated, which restrains the continuous SEI growth, and favors the interface stability with enhanced electrochemical performance.

In Li-ion batteries, functional cosolvents could significantly improve the specific performance of the electrolyte, for example, the flame retardancy. In case the cosolvent shows strong Li

Solid-liquid hybrid electrolytes (SLHEs) are promising electrolyte candidates for Li-metal batteries. However, most of the components of SLHE are flammable, posing safety risks. Here, a non-flammable SLHE was proposed by in-situ encapsulating a flame-retardant liquid phosphate into a robust solid polycarbonate matrix. The in-situ solidified SLHE simultaneously features high Li+ conductivity (4.4 mS cm−1), Young's modulus (12.4 GPa), Li+ transference number (0.76) and a wide electrochemical window (0-4.9 V vs. Li+/Li), which help to effectively suppress dendrites and unfavorable side reactions at the anode and provide compatibility with the high-voltage cathode. By employing non-flammable SLHE, a prototype Li||LiNi0.8Co0.1Mn0.1O2 cell retains 87.7% of the initial capacity after 200 cycles, and the Ah level Li||LiNi0.8Co0.1Mn0.1O2 pouch cells showed enhanced safety by passing the nail test by authorized third parties. This study inspires the optimal design of SLHEs towards practical realization of safe and stable Li-metal batteries.

Rechargeable lithium-metal batteries with a cell-level specific energy of >400 Wh kg-1 are highly desired for next-generation storage applications, yet the research has been retarded by poor electrolyte-electrode compatibility and rigorous safety concerns. We demonstrate that by simply formulating the composition of conventional electrolytes, a hybrid electrolyte was constructed to ensure high (electro)chemical and thermal stability with both the Li-metal anode and the nickel-rich layered oxide cathodes. By employing the new electrolyte, Li∥LiNi0.6 Co0.2 Mn0.2 O2 cells show favorable cycling and rate performance, and a 10 Ah Li∥LiNi0.8 Co0.1 Mn0.1 O2 pouch cell demonstrates a practical specific energy of >450 Wh kg-1 . Our findings shed light on reasonable design principles for electrolyte and electrode/electrolyte interfaces toward practical realization of high-energy rechargeable batteries.

Li+-conductive ceramic oxide electrolytes, such as garnet-structured Li7La3Zr2O12, have been considered as promising candidates for realizing the next-generation solid-state Li-metal batteries with high energy density. Practically, the ceramic pellets sintered at elevated temperatures are often provided with high stiffness yet low fracture toughness, making them too brittle for the manufacture of thin-film electrolytes and strain-involved operation of solid-state batteries. The ceramic powder, though provided with ductility, does not yield satisfactorily high Li+ conductivity due to poor ion conduction at the boundaries of ceramic particles. Here we show, with solid-state nuclear magnetic resonance, that a uniform conjugated polymer nanocoating formed on the surface of ceramic oxide particles builds pathways for Li+ conduction between adjacent particles in the unsintered ceramics. A tape-casted thin-film electrolyte (thickness

Li metal possesses a high theoretical specific capacity, high electronic conductivity, and a low electrochemical potential, making it a promising anode material for building next-generation rechargeable metal batteries. In case conventional liquid electrolytes were used, and the anode using Li metal has been hindered by unstable(electro)chemistry at Li/electrolyte interface and the accompanied dendrite issue. Specifically, for the Li-Se batteries, the dissolution and shuttle of polyselenide intermediates lead to the deposition of poorly-conductive species on the anode, which further aggravates the chemical environment at the anode. In this work, we proposed to stabilize the Li-Se electrochemistry by constructing a gel polymer electrolyte via in situ gelations of conventional ether-based electrolytes at room temperature. The results demonstrate that the in situ gelated electrolyte helps to build electrochemically stable electrode/electrolyte interfaces and promote the efficient transfer of charge carriers across the interface. Compared with the liquid electrolytes, the gelated electrolyte shows improved chemical compatibility with the Li metal anode, which effectively alleviates the unfavorable side reactions and dendrite formation at the anode/electrolyte interface, and the polyselenide shuttle from the cathode to the anode. As a result, the Li-Se battery shows a higher Coulombic efficiency and improved cycling performance.

The development of Na-ion full cells (NIFCs) suffers from the issue that the solid electrolyte interphase formation on the carbon anode consumes the limited sodium from cathode and thus incurs the decreased energy density and poor cyclic stability. To address these issues, we herein report that Na2O2 could be used as a sacrificial Na source through spraying its slurry on the surface of cathode, and investigate its stability as well as electrochemical behavior toward NIFCs. The results show that Na2O2 has good chemical and storage stability under a dry atmosphere and has no negative effect on the electrochemical performance of the cathode. Compared with the pristine cathode, the Na2O2-decorated cathode exhibits higher discharge capacity, superior capacity retention, and rate capability in a full cell with a carbon anode. Our cathode Na compensation strategy provides an effective avenue to make up for the irreversible Na+ loss cause by the formation of solid electrolyte interphase on the anode, thereby promoting the electrochemical performance and energy density of NIFCs toward the large-scale application.

Phenolic resin as a typical hard carbon precursor has attracted much attention towards sodium-ion batteries (SIBs) due to its high carbon yield and large reversible capacity. However, phenolic resin-based hard carbon commonly suffers from limited cycling performance and low initial Coulombic efficiency. Herein, we synthesized a hard carbon material from phenolic resin and manipulated its microstructure through tuning the pyrolysis temperature and pre-oxidation procedure. In particular, hard carbon materials with pre-oxidation deliver much improved capacity retention in contrast with those without pre-oxidation. Further analysis shows that with the pre-oxidation procedure, the enlarged d002 spacing, the much disordered microstructure, and the increased content of oxygen-containing functional groups are responsible for the performance enhancement of hard carbon. This study provides an in-depth understanding of the microstructure feature of phenolic resin-based hard carbon and opens up a feasible avenue for the design of high-performance carbon anode materials for their application in SIBs.

Recycling of spent lithium-ion batteries has recently become a critical issue based on environmental concerns and a desire to reutilize resources. Among the existing recycling strategies, direct regeneration is largely encouraged from an economic and environmental perspective. However, current procedures used to separate the active cathode materials from the aluminum foil have some limitations for direct regeneration because they either destroy the structure of the cathode or use too many toxic and expensive reagents. Hence, we conducted comprehensive research on the microstructural evolution of the LiNi1−x−yCoxMnyO2 degraded electrode and then proposed a targeted method to recycle the spent cathode materials based on the increased residual lithium compounds. This separation process involves no other reagents but water, and toxic organic solvents, complicated processes, and waste treatments are unnecessary compared with the existing pretreatment strategies. Moreover, the separated cathodes are suitable for direct regeneration. Satisfactory capacity recovery of the cathode was achieved via simple sintering. Such a recycling process enables a sustainable closed-loop for the spent cathodes and provides new inspiration for the design of LIB recycling.

Due to high ionic conductivity, favorable mechanical plasticity, and non-flammable properties, inorganic sulfide solid electrolytes bring opportunities to the practical realization of rechargeable lithium–metal batteries with high energy, yet their use was impeded by an electrochemically unstable Li–electrolyte interface. Herein, we propose to address the issue via a Li+-conductive gel polymer interlayer, which is derived in situ from a conventional liquid ether electrolyte during the cell fabrication process. The gel polymer interlayer not only enables intimate solid–solid contact and uniform Li-ion flux at the heterointerface but also effectively inhibits interfacial reactions and Li dendrite growth. With improved interfacial stability, a Li–Li symmetric cell with the gel polymer interlayer demonstrates an ultra-stable Li plating/stripping performance of over 1300 hours at 0.1 mA cm−2 and 350 hours at 0.5 mA cm−2 at room temperature, and a high critical current density of >5 mA cm−2. This work offers general insights into a reasonable design of an anode/electrolyte interface for high-energy rechargeable Li–metal batteries.

As two stable hydrogen isotopes, protium and deuterium show magnified isotope effects in physicochemical properties due to the significantly varied atomic masses. In this work, aqueous electrolytes based on heavy water (D

With the ever increasing demand for low-cost and economic sustainable energy storage, Na-ion batteries have received much attention for the application on large-scale energy storage for electric grids because of the worldwide distribution and natural abundance of sodium element, low solvation energy of Na

Layered oxide cathodes usually exhibit high compositional diversity, thus providing controllable electrochemical performance for Na-ion batteries. These abundant components lead to complicated structural chemistry, closely affecting the stacking preference, phase transition and Na

After the past decade’s rapid development, the commercial demands for sodium ion batteries (SIBs) have been put on the schedule for large-scale energy storage. Even though the electrode-electrolyte...

Carbon-coated SiOx microparticles (SiOx/C) demonstrate attractive potential for anode use in high-energy-density lithium-ion batteries due to high capacity and proper cycling stability. However, the excessive irreversible consumption of Li ions during the initial cycling remains a serious challenge arising from the limited lithium in full cells. Here, we endow SiOx/C anode with high initial Coulombic efficiency using the chemical pre-lithiation strategy. The lithium silicate is uniformly pregenerated in SiOx/C microparticles, which could effectively counteract the irreversible consumption of Li ions and avoid the complicated pre-lithiation process. Moreover, this strategy guarantees the structural integrity and processability of anode materials because of the homogeneous Li-organic complex solution pre-lithiation and high-temperature calcination process. The obtained SiOx/C microparticles can be applied as anode materials by directly mixing with commercial graphite, which demonstrates proper specific capacity, high initial Coulombic efficiency, and excellent cycling performance. Furthermore, the pouch cells using LiNi0.8Co0.1Mn0.1O2 cathodes and the as-prepared anodes exhibit high energy density (301 Wh kg-1) and satisfactory cycling stability (93.3% capacity retention after 100 cycles).

Rechargeable Li metal batteries are one of the most attractive energy storage systems due to their high energy density. However, the hostless nature of Li, the excessive dendritic growth, and the accumulation of nonactive Li induce severe volume variation of Li anodes. The volume variation can give rise to a fracture of solid electrolyte interphase, continuous consumption of Li and electrolytes, low Coulombic efficiency, fast performance degradation, and finally short cycle life. This Outlook provides a comprehensive understanding of the origin and consequences of Li volume variation. Recent strategies to address this challenge are reviewed from liquid to gel to solid-state electrolyte systems. In the end, guidelines for structural design and fabrication suggestions for future long-life Li composite anodes are presented., This Outlook focuses on the recent progress in stabilizing the electrode volume of the Li anode from liquid to gel to solid-state electrolyte systems.

Anode materials are key components of batteries that significantly impact their specific energy and power. Li metal is considered as the ultimate anode due to its high theoretical capacity (∼3860 mA h/g) and low redox potential (−3.04 V vs. standard hydrogen electrode). Specifically, rechargeable Li metal batteries (LMBs) with enabled safety promise to surpass the energy density of current Li-ion batteries. Unfortunately, the apparently inevitable growth of dendritic Li, electrolyte consumption, the severe volume changes and the connected potential safety risks of LMBs limit their practical application. Recent strategies based on manipulation of electrolyte chemistry, interface engineering, and structure modification of Li host have reportedly achieved improvement. At the moment, the trend is to move towards all-solid-state LMBs. However, there are serious challenges in terms of low ionic conductivity, poor interfacial contact, and sluggish kinetics. While there are excellent reviews available, this review emphasizes problems and provides additional insight in advanced strategies for stabilizing Li metal anodes in liquid, polymer, ceramic and composite electrolytes. New approaches and novel materials to overcome the above challenges are referred. This review aims at raising relevant questions and outlining future strategies for next-generation high-energy storage systems.

A hybrid solid/liquid electrolyte with superior security facilitates the implementation of high-energy-density storage devices, but it suffers from inferior chemical compatibility with cathodes. Herein, an optimal lithium difluoro(oxalato)borate salt was introduced to build in situ an amorphous cathode electrolyte interphase (CEI) between Ni-rich cathodes and hybrid electrolyte. The CEI preserves the surface structure with high compatibility, leading to enhanced interfacial stability. Meanwhile, the space-charge layer can be prominently mitigated at the solid/solid interface via harmonized chemical potentials, acquiring promoted interfacial dynamics as revealed by COMSOL simulation. Consequently, the amorphous CEI integrates the bifunctionality to provide an excellent cycling stability, high Coulombic efficiency, and favorable rate capability in high-voltage Li-metal batteries, innovating the design philosophy of functional CEI strategy for future high-energy-density batteries.

The pursuit for batteries with high specific energy provokes the research of high-voltage/capacity cathode materials with superior stability and safety as the alternative for lithium iron phosphate. Herein, using the sol-gel method, a lithium vanadium phosphate with higher average discharge voltage (3.8 V, vs. Li+/Li) was obtained from a single source for Mg2+ and Cl− co-substitution and uniform carbon coating, and a nearly theoretical capacity (130.1 mA h g−1) and outstanding rate performance (25 C) are acquired together with splendid capacity retention (80%) after 650 cycles. This work reveals that the well-sized anion and cation substitution and uniform carbon coating are of both importance to accelerate kinetic performance in the context of nearly un-disturbed crystal structure for other analogue materials. It is anticipated that the electrochemistry comprehension will shed light on preparing cathode materials with high energy density in the future.

The capacity degradation mechanism of a spent LiFePO4 (LFP) cathode was investigated systematically and the occurrence of FePO4 in the cathode due to loss of Li+ was found to be mainly responsible for the short cycle life of LFP batteries. Given this, an easy, low-cost, and green cathode regeneration method was proposed. This work provides a new perspective for the recycling of spent batteries.

Rational design of high-capacity nanosized composites as anode nanomaterials is crucial to boosting electrochemical performances towards large-scale application for lithium- and sodium-ion batteries (LIBs and SIBs). The small sizes and homogeneous dimensional size distributions are achieved typically by either the surface confinement on the underlying supports, or the encapsulation confinement within the precursors (such as metal-organic frameworks). Herein, we report the ultrasmall NiS

The key issue holding back the application of solid polymeric electrolytes in high-energy density lithium metal batteries is the contradictory requirements of high ion conductivity and mechanical stability. In this work, self-healable solid polymeric electrolytes (SHSPEs) with rigid-flexible backbones and high ion conductivity are synthesized by a facile condensation polymerization approach. The all-solid Li metal full batteries based on the SHSPEs possess freely bending flexibility and stable cycling performance as a result of the more disciplined metal Li plating/stripping, which have great implications as long-lifespan energy sources compatible with other wearable devices.

Sodium-ion batteries (SIBs) are currently the most promising candidates for large-scale energy storage devices owing to their low cost and abundant resources. Titanium-based layered oxides have attracted widespread attention as promising anode materials due to delivering a safe potential of about 0.7 V (vs Na

Chemical modification of electrode materials by heteroatom dopants is crucial for improving storage performance in rechargeable batteries. Electron configurations of different dopants significantly influence the chemical interactions inbetween and the chemical bonding with the host material, yet the underlying mechanism remains unclear. We revealed competitive doping chemistry of Group IIIA elements (boron and aluminum) taking nickel-rich cathode materials as a model. A notable difference between the atomic radii of B and Al accounts for different spatial configurations of the hybridized orbital in bonding with lattice oxygen. Density functional theory calculations reveal, Al is preferentially bonded to oxygen and vice versa, and shows a much lower diffusion barrier than B

Na-ion cathode materials operating at high voltage with a stable cycling behavior are needed to develop future high-energy Na-ion cells. However, the irreversible oxygen redox reaction at the high-voltage region in sodium layered cathode materials generates structural instability and poor capacity retention upon cycling. Here, we report a doping strategy by incorporating light-weight boron into the cathode active material lattice to decrease the irreversible oxygen oxidation at high voltages (i.e., >4.0 V vs. Na+/Na). The presence of covalent B–O bonds and the negative charges of the oxygen atoms ensures a robust ligand framework for the NaLi1/9Ni2/9Fe2/9Mn4/9O2 cathode material while mitigating the excessive oxidation of oxygen for charge compensation and avoiding irreversible structural changes during cell operation. The B-doped cathode material promotes reversible transition metal redox reaction enabling a room-temperature capacity of 160.5 mAh g−1 at 25 mA g−1 and capacity retention of 82.8% after 200 cycles at 250 mA g−1. A 71.28 mAh single-coated lab-scale Na-ion pouch cell comprising a pre-sodiated hard carbon-based anode and B-doped cathode material is also reported as proof of concept., The irreversible oxygen redox reaction during charging to the high-voltage region causes cathode structural degradation and Na-ion cell capacity fading. Here, the authors report a B-doped cathode active material to mitigate the irreversible oxygen oxidation and increase the cell capacity.

Transition-metal layered oxides (such as P2-Na2/3Ni1/3Mn2/3O2) are suggested as one type of the most potential cathode candidates for sodium ion batteries (SIBs) owing to their high capacity and low cost; however, they suffer from the structural damage and sluggish Na+ kinetics resulting from the undesirable phase transformation of P2−O2 and the Na+/vacancy ordering, respectively. Herein, a Mg/Ti co-doped P2-Na0.67Ni0.28Mg0.05Mn0.62Ti0.05O2 layered oxide is demonstrated as a high-efficiency cathode material for SIBs. The cathode delivers a high reversible capacity of 135.5 mAh g−1, good cycling stability (82.7 mAh g−1 upon 100 cycles at 0.1C), and an attractive energy density of 479.4 Wh Kg−1. Furthermore, the phase transition from the undesirable P2−O2 to the reversible P2−OP4 demonstrated by in situ XRD and the partially suppressed Na+/vacancy ordering as well as the improved electronic and ionic conductivities all give rise to the enhancement. These results show the important role of cationic co-doping in designing and preparing high-efficiency layered oxide cathode materials for SIBs.

Lithium (Li) metal is one of the most promising anodes for the high-energy density lithium batteries. Nevertheless, it is still a great challenge to construct a dendrite-free Li anode with stable solid electrolyte interphase (SEI) by adopting environmentally friendly approaches. Herein, a green artificial Li polylactic acid (LiPLA) SEI layer with biodegradability and highly rebound resilience is fabricated via an in situ reaction to regulate Li metal plating/stripping behavior. Guided by this stable environmentally friendly LiPLA SEI, the Li anode shows excellent stability with suppressive dendrites as demonstrated by a stable cycling of 850 h in LiPLA-Li/LiPLA-Li symmetric batteries and a significant capacity retention rate enhancement of 18% in LiPLA-Li/LiFePO4 full batteries and 25% in LiPLA-Li/LiNi3/5Co1/5-Mn1/5O2 full batteries. This proposed strategy provides a green and facile way to ameliorate the stability of the Li anode for safe and long-life lithium metal batteries.

Metallic lithium affords the highest theoretical capacity and lowest electrochemical potential and is viewed as a leading contender as an anode for high-energy-density rechargeable batteries. However, the poor wettability of molten lithium does not allow it to spread across the surface of lithiophobic substrates, hindering the production and application of this anode. Here we report a general chemical strategy to overcome this dilemma by reacting molten lithium with functional organic coatings or elemental additives. The Gibbs formation energy and newly formed chemical bonds are found to be the governing factor for the wetting behavior. As a result of the improved wettability, a series of ultrathin lithium of 10–20 μm thick is obtained together with impressive electrochemical performance in lithium metal batteries. These findings provide an overall guide for tuning the wettability of molten lithium and offer an affordable strategy for the large-scale production of ultrathin lithium, and could be further extended to other alkali metals, such as sodium and potassium., Molten lithium cannot spread onto the lithiophobic substrate, hindering the production and application of lithium metal anodes. Here, the authors show a general chemical strategy to tune the wettability by forming new chemical bonds through the reactions with various organic coatings or elemental additives.

Silicon-based electrodes are promising and appealing for futuristic Li-ion batteries because of their high theoretical specific capacity. However, massive volume change of silicon upon lithiation and delithiation, accompanied by continual formation and destruction of the solid-electrolyte interface (SEI), leads to low Coulombic efficiency. Prelithiation of Si-based anode is regarded as an effective way for compensating for the loss of Li+ in the first discharging process. Here, a high-performance lithiated SiOx anode was prepared by using a controllable, efficient, and novel prelithiation strategy. The lithiation of SiOx is homogeneous and efficient in bulk due to well-improved Li+ diffusion in SiOx. Moreover, the in situ formed SEI during the process of prelithiation reduces the irreversible capacity loss in the first cycle and thus improves the initial Coulombic efficiency (ICE). Half-cells and full cells based on the as-prepared lithiated SiOx anode prominently increase the ICE from 79 to 89% and 68 to 87%, respectively. It is worth mentioning that the homogeneously lithiated SiOx anode achieves stable 200 cycles in NCM622//SiOx coin full cells. These exciting results provide applicable prospects of lithiated SiOx anode in the next-generation high-energy-density Li-ion batteries.

SiOx-based anodes have received intensive attention with the effort on increasing the energy density of lithium-ion batteries (LIBs) for applications such as electric and hybrid electric vehicles. To ensure the security and energy density of LIBs, it is prerequisite to address the cell deformation arising from the huge volume variation of SiOx-based anodes during battery operation. We herein reported the compacted SiOx/G/C granules constructed by the interfacial adhesion between SiOx nanoparticles and thin-layer graphite under the collaborative auxiliary of binder pitch, which contributes to preserving intact conduction pathway and structure integrity of anodes during the repeated lithiation/delithiation process. The as-prepared SiOx/G/C granules deliver superior cycling stability, high initial Coulombic efficiency and good rate capability. In particular, low deformation (13.7% thickness expansion), which is comparable with that of graphite anodes, could be attained in high capacity SiOx/G/C anode (653 mA h g−1). The excellent properties of SiOx/G/C anodes, especially in maintaining structure integrity and lowering deformation, provide insights into the rational design of high capacity electrode materials with huge volume variation.

Solid-state batteries provide a safe and high energy density solution to next-generation energy storage devices. However, unsatisfactory power capability and cycling stability raised from lithium diffusion resistance and catalytic reaction at the electrode/electrolyte interface critically restrict their development. In this study, these interfacial issues are ameliorated by LiBO2 coating to achieve the successful implementation of nickel-rich LiNi0.6Co0.2Mn0.2O2 in poly(ether-acrylate) based solid-state batteries. The continuous coating acts not only as a physical barrier hindering the decomposition of polymer electrolyte but also as a buffer layer promoting lithium diffusion across the space charge layer owing to its electronic insulativity and ionic conductivity. In consequence, the LiBO2 coated nickel-rich cathode delivers superior cycling stability with a capacity retention of 84.3% during 150 cycles at 0.5C, remarkably improved lithium diffusion kinetics with an order of magnitude increase in apparent diffusion coefficient and prominent power performance with 97 mA h g−1 at 2C. The effectiveness of inserting such functional interlayer emphasizes the essentiality of interfacial chemistry in building high-performance solid-state Li metal batteries.

Safety concerns are impeding the applications of lithium metal batteries. Flame-retardant electrolytes, such as organic phosphates electrolytes (OPEs), could intrinsically eliminate fire hazards and improve battery safety. However, OPEs show poor compatibility with Li metal though the exact reason has yet to be identified. Here, the lithium plating process in OPEs and Li/OPEs interface chemistry were investigated through ex situ and in situ techniques, and the cause for this incompatibility was revealed to be the highly resistive and inhomogeneous interfaces. Further, a nitriding interface strategy was proposed to ameliorate this issue and a Li metal anode with an improved Li cycling stability (300 h) and dendrite-free morphology is achieved. Meanwhile, the full batteries coupled with nickel-rich cathodes, such as LiNi0.8 Co0.1 Mn0.1 O2 , show excellent cycling stability and outstanding safety (passed the nail penetration test). This successful nitriding-interface strategy paves a new way to handle the incompatibility between electrode and electrolyte.

As high capacity cathodes, O3-type Na-based oxides always suffer from a series of monoclinic transitions upon sodiation/desodiation, mainly caused by Na+/vacancy ordering and Jahn–Teller (J−T) dist...

The fundamental understanding of electrocatalytic reaction process is anticipated to guide electrode upgradation and acquirement of high-performance vanadium redox flow batteries (VRFBs). Herein, a carbon fiber prototype system with a heteroatom gradient distribution has been developed with enlarged interlayer spacing and a high graphitization that improve the electronic conductivity and accelerate the electrocatalytic reaction, and the mechanism by which gradient-distributed heteroatoms enhance vanadium redox reactions was elucidated with the assistance of density functional theory calculations. All these contributions endow the obtained electrode prominent redox reversibility and durability with only 1.7% decay in energy efficiency over 1000 cycles at 150 mA cm

Na-ion batteries (NIBs) have been suggested as one of the most promising technologies for low-cost rechargeable batteries due to the earth-abundance resource of sodium. However, the delivery of high-performance cathode is still facing a great challenge especially for the development of Na-ion full batteries towards practical application. Herein, we present a layered biphasic cathode with P2 and P3 integrated structures, which is clearly elucidated by X-ray diffraction refinement, high resolution transmission electron microscopy and selected area electron diffraction. Combining the respective characteristics of P2 and P3 phases and highly reversible P2/P3–P2/OP4 structural evolution, the layered composite delivers a high reversible capacity of 119 mA h g−1 with a superior initial Coulombic efficiency of 94.8%, a high operating voltage of 3.53 V vs Na+/Na based on Ni2+/Ni4+ redox couple, and an outstanding rate performance (capacity retention of 85.5% at 5C compared with 0.2C). Furthermore, the assembled Na-ion full battery of P2/P3-Na0.7Li0.06Mg0.06Ni0.22Mn0.67O2//hard carbon exhibits a high average voltage of 3.36 V with a calculated energy density up to 218 W h kg−1. Our contribution paves a smart way for designing sodium-ion full batteries with both high energy density and good cycling stability toward practical application.

Li