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.

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.

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.

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.

Enabling a Durable Electrochemical Interface via an Artificial Amorphous Cathode Electrolyte Interphase for Hybrid Solid/Liquid Lithium-Metal Batteries.

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.

Engineering Janus Interfaces of Ceramic Electrolyte via Distinct Functional Polymers for Stable High-Voltage Li-Metal Batteries.

The fast-ionic-conducting ceramic electrolyte is promising for next-generation high-energy-density Li-metal batteries, yet its application suffers from the high interfacial resistance and poor interfacial stability. In this study, the compatible solid-state electrolyte was designed by coating Li1.4Al0.4Ti1.6(PO4)3 (LATP) with polyacrylonitrile (PAN) and polyethylene oxide (PEO) oppositely to satisfy deliberately the disparate interface demands. Wherein, the upper PAN constructs soft-contact with LiNi0.6Mn0.2Co0.2O2, and the lower PEO protects LATP from being reduced, guaranteeing high-voltage tolerance and improved stability toward Li-metal anode performed in one ceramic. Moreover, the core function of LATP is amplified to guide homogeneous ions distribution and hence suppresses the formation of a space-charge layer across interfaces, uncovered by the COMSOL Multiphysics concentration field simulation. Thus, such a bifunctional modified ceramic electrolyte integrates the respective superiority to render Li-metal batteries with excellent cycling stability (89% after 120 cycles), high Coulombic efficiency (exceeding 99.5% per cycle), and a dendrite-free Li anode at 60 °C, which represents an overall design of ceramic interface engineering for future practical solid battery systems.

Nonaqueous Sodium-Ion Full Cells: Status, Strategies, and Prospects.

With ever-increasing efforts focused on basic research of sodium-ion batteries (SIBs) and growing energy demand, sodium-ion full cells (SIFCs), as unique bridging technology between sodium-ion half-cells (SIHCs) and commercial batteries, have attracted more and more interest and attention. To promote the development of SIFCs in a better way, it is essential to gain a systematic and profound insight into their key issues and research status. This Review mainly focuses on the interface issues, major challenges, and recent progresses in SIFCs based on diversified electrolytes (i.e., nonaqueous liquid electrolytes, quasi-solid-state electrolytes, and all-solid-state electrolytes) and summarizes the modification strategies to improve their electrochemical performance, including interface modification, cathode/anode matching, capacity ratio, electrolyte optimization, and sodium compensation. Outlooks and perspectives on the future research directions to build better SIFCs are also provided.

Trapping Lithium into Hollow Silica Microspheres with a Carbon Nanotube Core for Dendrite-Free Lithium Metal Anodes.

Li metal anodes, which have attracted much attention for their high specific capacity and low redox potential, face a great challenge in realizing their practical application. The fatal issue of dendrite formation gives rise to internal short circuit and safety hazards and needs to be addressed. Here we propose a rational strategy of trapping Li within microcages to confine the deposition morphology and suppress dendrite growth. Microcages with a carbon nanotube core and porous silica sheath were prepared and proved to be effective for controlling the electrodeposition behavior. In addition, the insulative coating layer prevents concentrated electron flow and decreases the possibility of "hot spots" formation. Because of the Li trapper and uniform electron distribution, the electrode with delicate structure exhibits a dendrite-free morphology after plating 2 mA h cm-2 of Li. As the dendrite growth is suppressed, the as-obtained electrode maintains a high plating/stripping efficiency of 99% over 200 cycles. This work delivers new insights into the design of rational Li metal anodes and hastens the practical application of Li metal batteries.

Self-Healable Solid Polymeric Electrolytes for Stable and Flexible Lithium Metal Batteries.

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.

Nitriding-Interface-Regulated Lithium Plating Enables Flame-Retardant Electrolytes for High-Voltage Lithium 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.

Guiding Uniform Li Plating/Stripping through Lithium-Aluminum Alloying Medium for Long-Life Li Metal Batteries.

The uncontrolled growth of Li dendrites upon cycling might result in low coulombic efficiency and severe safety hazards. Herein, a lithiophilic binary lithium-aluminum alloy layer, which was generated through an in situ electrochemical process, was utilized to guide the uniform metallic Li nucleation and growth, free from the formation of dendrites. Moreover, the formed LiAl alloy layer can function as a Li reservoir to compensate the irreversible Li loss, enabling long-term stability. The protected Li electrode shows superior cycling over 1700 h in a Li|Li symmetric cell.

Dendrite-Free Li-Metal Battery Enabled by a Thin Asymmetric Solid Electrolyte with Engineered Layers.

The key bottleneck troubling the application of solid electrolyte is the contradictory requirements from Li-metal and cathode, which need high modulus to block Li-dendrite penetration and flexibility to enable low interface resistance, respectively. This study describes a thin asymmetrical design of solid electrolyte to address these shortcomings. In this architecture, a rigid ceramic-layer modified with an ultrathin polymer is toward Li-metal to accomplish dendrite-suppression of Li-anode, and a soft polymer-layer spreads over the exterior and interior of cathode to endow connected interface simultaneously. This ingenious arrangement endows solid Li-metal batteries with extremely high Coulombic efficiency and cyclability. This work will open up one avenue for realizing safe and long-life energy storage systems.

Mitigating Interfacial Potential Drop of Cathode-Solid Electrolyte via Ionic Conductor Layer To Enhance Interface Dynamics for Solid Batteries.

The rapid capacity decay caused by the poor contact and large polarization at the interface between the cathode and solid electrolytes is still a big challenge to overcome for high-power-density solid batteries. In this study, a superior Li+ conductive transition layer Li1.4Al0.4Ti1.6(PO4)3 is introduced to coat LiNi0.6Co0.2Mn0.2O2, as a model cathode, to mitigate polarization and enhance dynamic characteristics. The critical attribute for such superior dynamics is investigated by the atomic force microscopy with boundary potential analysis, revealing that the formed interfacial transition layer provides a gradual potential slope and sustain-released polarization, and endows the battery with improved cycling stability (90% after 100 cycles) and excellent rate capability (116 mA h g-1 at 2 C) at room temperature, which enlightens the comprehension of interface engineering in the future solid batteries systems.

Uniform Nucleation of Lithium in 3D Current Collectors via Bromide Intermediates for Stable Cycling Lithium Metal Batteries.

The conductive framework is generating considerable interest for lithium metal anodes to accommodate Li+ deposition, due to its ability to reduce electrode current density by increasing the deposition area. However, in most cases, the electroactive surface area is not fully utilized for the nucleation of Li in 3D current collectors, especially under high current densities. Herein, uniform nucleation of Li in the conductive skeleton is achieved by a two-step synergetic process arising from CuBr- and Br-doped graphene-like film. The modified electrode regulates Li nucleating in uniform pancake-like seeds and growing into a granular Li metal ascribed to the excellent lithiophilicity of CuBr- and Br-doping sites and the low Li diffusion barrier on the surface of generated LiBr, as confirmed by the experimental and computational results. Therefore, the modified anode endows small nucleation overpotential, a high-reversibility Li plating/stripping process, and excellent performance in full batteries with industrially significant cathode loading. This work suggests that a two-step cooperative strategy opens a viable route to the development of a Li anode with high reversibility for stable cycling Li metal batteries.

A Flexible Solid Electrolyte Interphase Layer for Long-Life Lithium Metal Anodes.

Lithium (Li) metal is a promising anode material for high-energy density batteries. However, the unstable and static solid electrolyte interphase (SEI) can be destroyed by the dynamic Li plating/stripping behavior on the Li anode surface, leading to side reactions and Li dendrites growth. Herein, we design a smart Li polyacrylic acid (LiPAA) SEI layer high elasticity to address the dynamic Li plating/stripping processes by self-adapting interface regulation, which is demonstrated by in situ AFM. With the high binding ability and excellent stability of the LiPAA polymer, the smart SEI can significantly reduce the side reactions and improve battery safety markedly. Stable cycling of 700 h is achieved in the LiPAA-Li/LiPAA-Li symmetrical cell. The innovative strategy of self-adapting SEI design is broadly applicable, providing opportunities for use in Li metal anodes.