Na₃V(PO₄)₂: A New Layered-Type Cathode Material with High Water Stability and Power Capability for Na-Ion Batteries

We introduce Na₃V­(PO₄)₂ as a new cathode material for Na-ion batteries for the first time. The structure of Na₃V­(PO₄)₂ was determined using X-ray diffraction and Rietveld refinement, and its high water stability was clearly demonstrated. The redox potential of Na₃V­(PO₄)₂ (∼3.5 V vs Na/Na⁺) was shown to be sufficiently high to prevent the side reaction with water (Na extraction and water insertion), ensuring its water stability in ambient air. Na₃V­(PO₄)₂ also exhibited outstanding power capability, with ∼79% of the theoretical capacity being delivered at 15C. First-principles calculation combined with electrochemical experiments linked this high power capability to the low activation barrier (∼433 meV) for the well-interconnected two-dimensional Na diffusion pathway. Moreover, outstanding cyclability of Na₃V­(PO₄)₂ (∼70% retention of the initial capacity after 200 cycles) was achieved at a reasonably fast current rate of 1C

Theoretical Evidence for Low Charging Overpotentials of Superoxide Discharge Products in Metal–Oxygen Batteries

Li–oxygen and Na–oxygen batteries are some of the most promising next-generation battery systems because of their high energy densities. Despite the chemical similarity of Li and Na, the two systems exhibit distinct characteristics, especially the typically higher charging overpotential observed in Li–oxygen batteries. In previous theoretical and experimental studies, this higher charging overpotential was attributed to factors such as the sluggish oxygen evolution or poor transport property of the discharge product of the Li–oxygen cell; however, a general understanding of the interplay between the discharge products and overpotential remains elusive. Here, we investigated the charging mechanisms with respect to the oxygen evolution reaction (OER) kinetics, charge-carrier conductivity, and dissolution property of various discharge products reported in Li–oxygen and Na–oxygen cells. The OER kinetics were generally faster for superoxides (i.e., LiO₂ and NaO₂) than for peroxides (i.e., Li₂O₂ and Na₂O₂). The electronic and ionic conductivities were also predicted to be significantly higher in superoxide phases than in peroxide phases. Moreover, systematic calculations of the dissolution energy of the discharge products in the electrolyte, which mediate a solution-based OER reaction, revealed that the superoxide phases, particularly NaO₂, exhibited markedly low dissolution energy compared with the peroxide phases. These results imply that the formation of superoxides instead of peroxides during discharge may be the key to improving the energy efficiency of metal–oxygen batteries in general

Simple and Effective Gas-Phase Doping for Lithium Metal Protection in Lithium Metal Batteries

Increasing demands for advanced lithium batteries with higher energy density have resurrected the use of lithium metal as an anode, whose practical implementation has still been restricted, because of its intrinsic problems originating from the high reactivity of elemental lithium metal. Herein, we explore a facile strategy of doping gas phase into electrolyte to stabilize lithium metal and suppress the selective lithium growth through the formation of stable and homogeneous solid electrolyte interphase (SEI) layer. We find that the sulfur dioxide gas additive doped in electrolyte significantly improves both chemical and electrochemical stability of lithium metal electrodes. It is demonstrated that the cycle stability of the lithium cells can be remarkably prolonged, because of the compact and homogeneous SEI layers consisting of Li–S–O reduction products formed on the lithium metal surface. Simulations on the lithium metal growth process suggested the homogeneity of the protective layer induced by the gas-phase doping is attributable for the effective prevention of the selective growth of lithium metal. This study introduces a new simple approach to stabilize the lithium metal electrode with gas-phase doping, where the SEI layer can be rationally tunable by the composition of gas phase

High-Power Hybrid Solid-State Lithium–Metal Batteries Enabled by Preferred Directional Lithium Growth Mechanism

Solid electrolytes are revolutionizing the field of lithium–metal batteries; however, their practical implementation has been impeded by the interfacial instability between lithium metal electrodes and solid electrolytes. While various interlayers have been suggested to address this issue in recent years, long-term stability with repeated lithium deposition/stripping has been challenging to attain. Herein, we successfully operate a high-power lithium–metal battery by inducing the preferred directional lithium growth with a rationally designed interlayer, which employs (i) crystalline-direction-controlled carbon material providing isotropic lithium transports, with (ii) prelithium deposits that guide the lithium nucleation direction toward the current collector. This combination ensures that the morphology of the interlayer is mechanically robust while regulating the preferred lithium growth underneath the interlayer without disrupting the initial interlayer/electrolyte interface, enhancing the durability of the interface. We illustrate how these material/geometric optimizations are conducted from the thermodynamic considerations, and its applicability is demonstrated for the garnet-type Li7–xLa3–aZr2–bO12 (LLZO) solid electrolytes paired with the capacity cathode. It is shown that a lithium–metal cell with the optimized amorphous carbon interlayer with prelithium deposits exhibits outstanding room-temperature cycling performance (99. 6% capacity retention after 250 cycles), delivering 4.0 mAh cm–2 at 2.5 mA cm–2 without significant degradation of the capacity. The successful long-term operation of the hybrid solid-state cell at room temperature (approximately a cumulative deliverable capacity of over 1000 mAh cm–2) is unprecedented and records the highest performance reported for lithium–metal batteries with LLZO electrolytes until date

Native Defects in Li₁₀GeP₂S₁₂ and Their Effect on Lithium Diffusion

Defects in crystals alter the intrinsic nature of pristine materials including their electronic/crystalline structure and charge-transport characteristics. The ionic transport properties of solid-state ionic conductors, in particular, are profoundly affected by their defect structure. Nevertheless, a fundamental understanding of the defect structure of one of the most extensively studied lithium superionic conductors, Li₁₀GeP₂S₁₂, remains elusive because of the complexity of the structure; the effects of defects on lithium diffusion and the potential to control defects by varying synthetic conditions also remain unknown. Herein, we report, for the first time, a comprehensive first-principles study on native defects in Li₁₀GeP₂S₁₂ and their effect on lithium diffusion. We provide the complete defect profile of Li₁₀GeP₂S₁₂ and identify major defects that are easily formed regardless of the chemical environment while the presence of path-blocking defects is sensitively dependent on the synthetic conditions. Moreover, using <i>ab initio</i> molecular dynamics simulation, it is demonstrated that the major defects in Li₁₀GeP₂S₁₂ significantly alter the diffusion process. The defects generally facilitate lithium diffusion in Li₁₀GeP₂S₁₂ by enhancing the charge carrier concentration and flattening the site energy landscape. This work delivers a comprehensive picture of the defect chemistry and structural insights for fast lithium diffusion of Li₁₀GeP₂S₁₂-type conductors

Highly Stable Iron- and Manganese-Based Cathodes for Long-Lasting Sodium Rechargeable Batteries

The development of long-lasting and low-cost rechargeable batteries lies at the heart of the success of large-scale energy storage systems for various applications. Here, we introduce Fe- and Mn-based Na rechargeable battery cathodes that can stably cycle more than 3000 times. The new cathode is based on the solid-solution phases of Na₄­Mn_<i>x</i>­Fe_3–<i>x</i>­(PO₄)₂­(P₂O₇) (<i>x</i> = 1 or 2) that we successfully synthesized for the first time. Electrochemical analysis and <i>ex situ</i> structural investigation reveal that the electrodes operate via a one-phase reaction upon charging and discharging with a remarkably low volume change of 2.1% for Na₄MnFe₂(PO₄)­(P₂O₇), which is one of the lowest values among Na battery cathodes reported thus far. With merits including an open framework structure and a small volume change, a stable cycle performance up to 3000 cycles can be achieved at 1C and room temperature, and almost 70% of the capacity at C/20 can be obtained at 20C. We believe that these materials are strong competitors for large-scale Na-ion battery cathodes based on their low costs, long-term cycle stability, and high energy density

<i>In Situ</i> Tracking Kinetic Pathways of Li⁺/Na⁺ Substitution during Ion-Exchange Synthesis of Li_<i>x</i>Na_{1.5–<i>x</i>}VOPO₄F_0.5

Ion exchange is a ubiquitous phenomenon central to wide industrial applications, ranging from traditional (bio)­chemical separation to the emerging <i>chimie douce</i> synthesis of materials with metastable structure for batteries and other energy applications. The exchange process is complex, involving substitution and transport of different ions under <i>non-equilibrium</i> conditions, and thus difficult to probe, leaving a gap in mechanistic understanding of kinetic exchange pathways toward final products. Herein, we report <i>in situ</i> tracking kinetic pathways of Li⁺/Na⁺ substitution during solvothermal ion-exchange synthesis of Li_<i>x</i>Na_{1.5–<i>x</i>}VOPO₄F_0.5 (0 ≤ <i>x</i> ≤ 1.5), a promising multi-Li polyanionic cathode for batteries. The <i>real-time</i> observation, corroborated by <i>first-principles</i> calculations, reveals a selective replacement of Na⁺ by Li⁺, leading to peculiar Na⁺/Li⁺/vacancy orderings in the intermediates. Contradicting the traditional belief of facile topotactic substitution via solid solution reaction, an abrupt two-phase transformation occurs and predominantly governs the kinetics of ion exchange and transport in the 1D polyanionic framework, consequently leading to significant difference of Li stoichiometry and electrochemical properties in the exchanged products. The findings may help to pave the way for rational design of ion exchange synthesis for making new materials

Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst

A highly active and stable non-Pt electrocatalyst for hydrogen production has been pursued for a long time as an inexpensive alternative to Pt-based catalysts. Herein, we report a simple and effective approach to prepare high-performance iron phosphide (FeP) nanoparticle electrocatalysts using iron oxide nanoparticles as a precursor. A single-step heating procedure of polydopamine-coated iron oxide nanoparticles leads to both carbonization of polydopamine coating to the carbon shell and phosphidation of iron oxide to FeP, simultaneously. Carbon-shell-coated FeP nanoparticles show a low overpotential of 71 mV at 10 mA cm^–2, which is comparable to that of a commercial Pt catalyst, and remarkable long-term durability under acidic conditions for up to 10 000 cycles with negligible activity loss. The effect of carbon shell protection was investigated both theoretically and experimentally. A density functional theory reveals that deterioration of catalytic activity of FeP is caused by surface oxidation. Extended X-ray absorption fine structure analysis combined with electrochemical test shows that carbon shell coating prevents FeP nanoparticles from oxidation, making them highly stable under hydrogen evolution reaction operation conditions. Furthermore, we demonstrate that our synthetic method is suitable for mass production, which is highly desirable for large-scale hydrogen production

Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction

Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a “dual purpose” N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is <i>in situ</i> formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From <i>in situ</i> XANES, EDS, and first-principles calculations, we confirmed that an ordered fct-PtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells