Boosting Sodium Compensation Efficiency via a CNT/MnO2 Catalyst toward High-Performance Na-Ion Batteries.

The formation of a solid electrolyte interphase on carbon anodes causes irreversible loss of Na+ ions, significantly compromising the energy density of Na-ion full cells. Sodium compensation additives can effectively address the irreversible sodium loss but suffer from high decomposition voltage induced by low electrochemical activity. Herein, we propose a universal electrocatalytic sodium compensation strategy by introducing a carbon nanotube (CNT)/MnO2 catalyst to realize full utilization of sodium compensation additives at a much-reduced decomposition voltage. The well-organized CNT/MnO2 composite with high catalytic activity, good electronic conductivity, and abundant reaction sites enables sodium compensation additives to decompose at significantly reduced voltages (from 4.40 to 3.90 V vs Na+/Na for sodium oxalate, 3.88 V for sodium carbonate, and even 3.80 V for sodium citrate). As a result, sodium oxalate as the optimal additive achieves a specific capacity of 394 mAh g-1, almost reaching its theoretical capacity in the first charge, increasing the energy density of the Na-ion full cell from 111 to 158 Wh kg-1 with improved cycle stability and rate capability. This work offers a valuable approach to enhance sodium compensation efficiency, promising high-performance energy storage devices in the future.

Bismuth-doping boosting Na+ diffusion kinetics of layered oxide cathode with radially oriented {010} active lattice facet for sodium-ion batteries.

O3-type layered oxide cathodes (NaxTMO2) for sodium-ion batteries (SIBs) have attracted significant attention as one of the most promising potential candidates for practical energy storage applications. The poor Na+ diffusion kinetics is, however, one of the major obstacles to advancing large-scale practical application. Herein, we report bismuth-doped O3-NaNi0.5Mn0.5O2 (NMB) microspheres consisting of unique primary nanoplatelets with the radially oriented {010} active lattice facets. The NMB combines the advantages of the oriented and exposed electrochemical active planes for direct paths of Na+ diffusion, and the thick primary nanoplatelets for less surface parasitic reactions with the electrolyte. Consequently, the NMB cathode exhibits a long-term stability with an excellent capacity retention of 72.5% at 1C after 300 cycles and an enhanced rate capability at a 0.1C to 10C rate (1C = 240 mA g-1). Furthermore, the enhancement is elucidated by the small volume change, thin cathode-electrolyte-interphase (CEI) layer, and rapid Na+ diffusion kinetics. In particular, the radial orientation-based Bi-doping strategy is demonstrated to be effective at boosting electrochemical performance in other layered oxides (such as Bi-doped NaNi0.45Mn0.45Ti0.1O2 and NaNi1/3Fe1/3Mn1/3O2). The results provide a promising strategy of utilizing the advantages of the oriented active facets of primary platelets and secondary particles to develop high-rate layered oxide cathodes for SIBs.

Fast Na+ Kinetics and Suppressed Voltage Hysteresis Enabled by a High-Entropy Strategy for Sodium Oxide Cathodes.

O3-type layered transition metal cathodes are promising energy storage materials due to their sufficient sodium reservoir. However, sluggish sodium ions kinetics and large voltage hysteresis, which are generally associated with Na+ diffusion properties and electrochemical phase transition reversibility, drastically minimize energy density, reduce energy efficiency, and hinder further commercialization of sodium-ion batteries (SIBs). Here, this work proposes a high-entropy tailoring strategy through manipulating the electronic local environment within transition metal slabs to circumvent these issues. Experimental analysis combined with theoretical calculations verify that high-entropy metal ion mixing contributes to the improved reversibility of redox reaction and O3-P3-O3 phase transition behaviors as well as the enhanced Na+ diffusivity. Consequently, the designed O3-Na0.9Ni0.2Fe0.2Co0.2Mn0.2Ti0.15Cu0.05O2 material with high-entropy characteristic could display a negligible voltage hysteresis (<0.09 V), impressive rate capability (98.6 mAh g-1 at 10 C) and long-term cycling stability (79.4% capacity retention over 2000 cycles at 5 C). This work provides insightful guidance in mitigating the voltage hysteresis and facilitating Na+ diffusion of layered oxide cathode materials to realize high-rate and high-energy SIBs.

Refined Electrolyte and Interfacial Chemistry toward Realization of High-Energy Anode-Free Rechargeable Sodium Batteries.

Anode-free rechargeable sodium batteries represent one of the ultimate choices for the 'beyond-lithium' electrochemical storage technology with high energy. Operated based on the sole use of active Na ions from the cathode, the anode-free battery is usually reported with quite a limited cycle life due to unstable electrolyte chemistry that hinders efficient Na plating/stripping at the anode and high-voltage operation of the layered oxide cathode. A rational design of the electrolyte toward improving its compatibility with the electrodes is key to realize the battery. Here, we show that by refining the volume ratio of two conventional linear ether solvents, a binary electrolyte forms a cation solvation structure that facilitates flat, dendrite-free, planar growth of Na metal on the anode current collector and that is adaptive to high-voltage Na (de)intercalation of P2-/O3-type layered oxide cathodes and oxidative decomposition of the Na2C2O4 supplement. Inorganic fluorides, such as NaF, show a major influence on the electroplating pattern of Na metal and effective passivation of plated metal at the anode-electrolyte interface. Anode-free batteries based on the refined electrolyte have demonstrated high coulombic efficiency, long cycle life, and the ability to claim a cell-level specific energy of >300 Wh/kg.

Mitigating Electron Leakage of Solid Electrolyte Interface for Stable Sodium-Ion Batteries.

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.

Noncoordinating Flame-Retardant Functional Electrolyte Solvents for Rechargeable Lithium-Ion Batteries.

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+-coordinating ability, it could adversely influence the electrochemical Li+-intercalation reaction of the electrode. In this work, a noncoordinating functional cosolvent was proposed to enrich the functionality of the electrolyte while avoiding interference with the Li storage process. Hexafluorocyclotriphosphazene, an efficient flame-retardant agent with proper physicochemical properties, was chosen as a cosolvent for preparing functional electrolytes. The nonpolar phosphazene molecules with low electron-donating ability do not coordinate with Li+ and thus are excluded from the primary solvation sheath. In graphite-anode-based Li-ion batteries, the phosphazene molecules do not cointercalate with Li+ into the graphite lattice during the charging process, which helps to maintain integral anode structure and interface and contributes to stable cycling. The noncoordinating cosolvent was also applied to other types of electrode materials and batteries, paving a new way for high-performance electrochemical energy storage systems with customizable functions.

Layered Oxide Cathode-Electrolyte Interface towards Na-Ion Batteries: Advances and Perspectives.

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+ ion in the electrolyte and the low cost of Al as current collectors. Starting from a brief comparison with Li-ion batteries, this review summarizes the current understanding of layered oxide cathode/electrolyte interphase in NIBs, and discusses the related degradation mechanisms, such as surface reconstruction and transition metal dissolution. Recent advances in constructing stable cathode electrolyte interface (CEI) on layered oxide cathode are systematically summarized, including surface modification of layered oxide cathode materials and formulation of electrolyte. Urgent challenges are detailed in order to provide insight into the imminent developments of NIBs.

NiS2 nanodots on N,S-doped graphene synthesized via interlayer confinement for enhanced lithium-/sodium-ion storage.

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 NiS2 nanodots on reduced graphene oxide (NiS2/N,S-rGO) synthesized via interlayer confinement as anode nanomaterials for LIBs and SIBs. The composite is synthesized by pyrolyzing a host/guest precursor of sodium dodecyl sulfate ion/[NiEDTA]2- anions co-intercalated MgAl-layered double hydroxide LDH host, without additional sulfur source. The host/guest-derived interlayer nanoconfinement enables the composite to integrate the advantageous features: low-content active NiS2 nanodots (11.0 wt%) with a mean size of 3.8 ± 0.5 nm, high-content N,S-rGO (89.0 wt%), as well as a large specific surface area and mesopore size distribution. The composite used as anode nanomaterial exhibits reversible capacities of 801.2 mAh g-1 after 100 cycles at 100 mA g-1 for LIBs, and 207.7 mAh g-1 after 200 cycles at 0.1 A g-1 for SIBs, which are greatly higher than those of the pristine N,S-rGO without NiS2 nanodots. The enhancement is experimentally supported by the low charge transfer resistance, high capacitive-controlled contribution, and good structural stability. Our guest/host-based interlayer nanoconfinement can promise an effective synthesis strategy for designing various nanosized anodes for electrochemical energy storage.

A Rational Biphasic Tailoring Strategy Enabling High-Performance Layered Cathodes for Sodium-Ion Batteries.

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+ kinetics. With this perspective, we explore the thermodynamically stable phase diagram of various P2/O3 composites based on a rational biphasic tailoring strategy. Then a specific P2/O3 composite is investigated and compared with its monophasic counterparts. A highly reversible structural evolution of P2/O3-P2/O3/P3-P2/P3-P2/Z/O3'-Z/O3' based on the Ni2+ /Ni3.5+ , Fe3+ /Fe4+ and Mn3.8+ /Mn4+ redox couples upon sequential Na extraction/insertion is revealed. The reduced structural strain at the phase boundary alleviates the phase transition and decreases the lattice mismatch during cycling, endowing the biphasic electrode a large reversible capacity of 144 mAh g-1 with the energy density approaching 514 Wh kg-1 .

Hydrogen Isotope Effects on Aqueous Electrolyte for Electrochemical Lithium-Ion Storage.

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 (D2 O) and light water (H2 O) were prepared to reveal the electrochemical isotope effects between the hydrogen isotopes. The covalent hydrogen-oxygen bond and intermolecular hydrogen bond in D2 O are much stronger than those in H2 O, making them thermodynamically more stable. Compared with the H2 O-based electrolyte, the D2 O-based electrolyte shows a broader electrochemical window, a higher percentage of coordinated water and a longer lifetime of hydrogen bond. Because of the above electrochemical isotope effects, the D2 O-based electrolyte shows high anodic stability against operation of high-voltage layered oxide cathode materials including LiCoO2 and LiNi0.8 Co0.1 Mn0.1 O2 , which enables long cycle life and favorable rate performance of aqueous Li-ion batteries.

Competitive Doping Chemistry for Nickel-Rich Layered Oxide Cathode Materials.

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 BIII . In the case of Al-preoccupation, the bulk diffusion of BIII is hindered. In this way, a B-rich surface and Al-rich bulk is formed, which helps to synergistically stabilize the structural evolution and surface chemistry of the cathode.

O3-Type Na2/3Ni1/3Ti2/3O2 Layered Oxide as a Stable and High-Rate Anode Material for Sodium Storage.

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+/Na) and a small volume contraction during cycles; P2-type Ti-based layered oxides are typically reported, due to the challenging synthesis of the O3-type counterpart resulting from the high percentage of unstable Ti3+. Herein, we report an anomalous O3-Na2/3Ni1/3Ti2/3O2 layered oxide as an ultrastable and high-rate anode material for SIBs. The anode material delivers a reversible capacity of 112 mA h g-1 after 300 cycles at 0.1 C, a good capacity retention rate of 91% after 1400 cycles at 2 C, and, in particular, a capacity of 52 mA h g-1 even at a high rate of 20 C (1780 mA g-1). Furthermore, the in situ X-ray diffraction monitoring reveals no phase transitions and almost zero strain both underlie the good long-cycle stability. The measured high apparent Na+ diffusion coefficient (2.06 × 10-10 cm2 s-1) and the low migration energy barrier (0.59 eV) from density functional theory calculations are responsible for the superior rate capability. Our results promise advanced high-performance O3-type Ti-based layered oxides as promising anode materials toward application for SIBs.

Boron-doped sodium layered oxide for reversible oxygen redox reaction in Na-ion battery cathodes.

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.

Formulating the Electrolyte Towards High-Energy and Safe Rechargeable Lithium-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.

Bridging Interparticle Li+ Conduction in a Soft Ceramic Oxide Electrolyte.

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: <10 µm), prepared from the polymer-coated ceramic particles, exhibits sufficient ionic conductivity, a high Li+ transference number, and a broad electrochemical window to enable stable cycling of symmetric Li/Li cells and all-solid-state rechargeable Li-metal cells.

P3/O3 Integrated Layered Oxide as High-Power and Long-Life Cathode toward Na-Ion Batteries.

Low-cost and stable sodium-layered oxides (such as P2- and O3-phases) are suggested as highly promising cathode materials for Na-ion batteries (NIBs). Biphasic hybridization, mainly involving P2/O3 and P2/P3 biphases, is typically used to boost their electrochemical performances. Herein, a P3/O3 intergrown layered oxide (Na2/3 Ni1/3 Mn1/3 Ti1/3 O2 ) as high-rate and long-life cathode for NIBs via tuning the amounts of Ti substitution in Na2/3 Ni1/3 Mn2/3- x Tix O2 (x = 0, 1/6, 1/3, 2/3) is demonstrated. The X-ray diffraction (XRD) Rietveld refinement and aberration-corrected scanning transmission electron microscopy show the co-existence of P3 and O3 phases, and density functional theory calculation corroborates the appearance of the anomalous O3 phase at the Ti substitution amount of 1/3. The P3/O3 biphasic cathode delivers an unexpected rate capability (≈88.7% of the initial capacity at a high rate of 5 C) and cycling stability (≈68.7% capacity retention after 2000 cycles at 1 C), superior to those of the sing phases P3-Na2/3 Ni1/3 Mn2/3 O2 , P3-Na2/3 Ni1/3 Mn1/2 Ti1/6 O2 , and O3-Na2/3 Ni1/3 Ti2/3 O2 . The highly reversible structural evolution of the P3/O3 integrated cathode observed by ex situ XRD, ex situ X-ray absorption spectra, and the rapid Na+ diffusion kinetics, underpin the enhancement. These results show the important role of P3/O3 biphasic hybridization in designing and engineering layered oxide cathodes for NIBs.

Insights on Electrochemical Behaviors of Sodium Peroxide as a Sacrificial Cathode Additive for Boosting Energy Density of Na-Ion Battery.

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.

Large-Scale Synthesis of the Stable Co-Free Layered Oxide Cathode by the Synergetic Contribution of Multielement Chemical Substitution for Practical Sodium-Ion Battery.

The O3-type layered oxide cathodes for sodium-ion batteries (SIBs) are considered as one of the most promising systems to fully meet the requirement for future practical application. However, fatal issues in several respects such as poor air stability, irreversible complex multiphase evolution, inferior cycling lifespan, and poor industrial feasibility are restricting their commercialization development. Here, a stable Co-free O3-type NaNi0.4Cu0.05Mg0.05Mn0.4Ti0.1O2 cathode material with large-scale production could solve these problems for practical SIBs. Owing to the synergetic contribution of the multielement chemical substitution strategy, this novel cathode not only shows excellent air stability and thermal stability as well as a simple phase-transition process but also delivers outstanding battery performance in half-cell and full-cell systems. Meanwhile, various advanced characterization techniques are utilized to accurately decipher the crystalline formation process, atomic arrangement, structural evolution, and inherent effect mechanisms. Surprisingly, apart from restraining the unfavorable multiphase transformation and enhancing air stability, the accurate multielement chemical substitution engineering also shows a pinning effect to alleviate the lattice strains for the high structural reversibility and enlarges the interlayer spacing reasonably to enhance Na+ diffusion, resulting in excellent comprehensive performance. Overall, this study explores the fundamental scientific understandings of multielement chemical substitution strategy and opens up a new field for increasing the practicality to commercialization.

High-Efficiency Cathode Sodium Compensation for Sodium-Ion Batteries.

Sodium-ion batteries have gained much attention for their potential application in large-scale stationary energy storage due to the low cost and abundant sodium sources in the earth. However, the electrochemical performance of sodium-ion full cells (SIFCs) suffers severely from the irreversible consumption of sodium ions of cathode during the solid electrolyte interphase (SEI) formation of hard carbon anode. Here, a high-efficiency cathode sodiation compensation reagent, sodium oxalate (Na2 C2 O4 ), which possesses both a high theoretical capacity of 400 mA h g-1 and a capacity utilization as high as 99%, is proposed. The implementation of Na2 C2 O4 as sacrificial sodium species is successfully realized by decreasing its oxidation potential from 4.41 to 3.97 V through tuning conductive additives with different physicochemical features, and the corresponding mechanism of oxidation potential manipulation is analyzed. Electrochemical results show that in the full cell based on a hard carbon anode and a P2-Na2/3 Ni1/3 Mn1/3 Ti1/3 O2 cathode with Na2 C2 O4 as a sodium reservoir to compensate for sodium loss during SEI formation, the capacity retention is increased from 63% to 85% after 200 cycles and the energy density is improved from 129.2 to 172.6 W h kg-1 . This work can provide a new avenue for accelerating the development of SIFCs.

An Outlook on Low-Volume-Change Lithium Metal Anodes for Long-Life Batteries.

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.