Heterogeneous Catalyst as a Functional Substrate Governing the Shape of Electrochemical Precipitates in Oxygen-Fueled Rechargeable Batteries.

Lithium-oxygen batteries have the potential to become the most eminent solution for future energy storage with their theoretical energy density exceeding all existing batteries. However, the insulating and insoluble discharge product (lithium peroxide; Li2O2) impairs practical application. Conventional catalyst designs based on the electronic structure and interfacial charge transfer descriptors have not been able to overcome these limitations due to Li2O2. Herein, we revisit the role of heterogeneous catalysts as substrates to regulate Li2O2 growth and the formation of solid/solid reaction interfaces. We demonstrate that controlled solid/solid interfacial structure design is a critical performance parameter beyond the inherent electronic structure. In particular, the Cu2O substrate in this study induces a homogeneous deposition of Pd atoms, which leads to well-controlled growth of Li2O2 resolving mass and charge transport limits (i.e., the bottleneck of oxygen reduction/evolution reactions), thus improving reversibility, capacity, and durability of the cells by dissipating electrochemical and mechanical stress. We thus verified the essential role of solid/solid interfaces to regulate the nucleation and growth process of Li2O2 in lithium-oxygen batteries.

Hysteresis Induced by Incomplete Cationic Redox in Li-Rich 3d-Transition-Metal Layered Oxides Cathodes.

Activation of oxygen redox during the first cycle has been reported as the main trigger of voltage hysteresis during further cycles in high-energy-density Li-rich 3d-transition-metal layered oxides. However, it remains unclear whether hysteresis only occurs due to oxygen redox. Here, it is identified that the voltage hysteresis can highly correlate to cationic reduction during discharge in the Li-rich layered oxide, Li1.2 Ni0.4 Mn0.4 O2 . In this material, the potential region of discharge accompanied by hysteresis is apparently separated from that of discharge unrelated to hysteresis. The quantitative analysis of soft/hard X-ray absorption spectroscopies discloses that hysteresis is associated with an incomplete cationic reduction of Ni during discharge. The galvanostatic intermittent titration technique shows that the inevitable energy consumption caused by hysteresis corresponds to an overpotential of 0.3 V. The results unveil that hysteresis can also be affected by cationic redox in Li-rich layered cathodes, implying that oxygen redox cannot be the only reason for the evolution of voltage hysteresis. Therefore, appropriate control of both cationic and anionic redox of Li-rich layered oxides will allow them to reach their maximum energy density and efficiency.

Activating a Multielectron Reaction of NASICON-Structured Cathodes toward High Energy Density for Sodium-Ion Batteries.

The increasing demand to efficiently store and utilize the electricity from renewable energy resources in a sustainable way has boosted the request for sodium-ion battery technology due to the high abundance of sodium sources worldwide. Na superionic conductor (NASICON) structured cathodes with a robust polyanionic framework have been intriguing because of their open 3D structure and superior thermal stability. The ever-increasing demand for higher energy densities with NASICON-structured cathodes motivates us to activate multielectron reactions, thus utilizing the third sodium ion toward higher voltage and larger capacity, both of which have been the bottlenecks for commercializing sodium-ion batteries. A doping strategy with Cr inspired by first-principles calculations enables the activation of multielectron redox reactions of the redox couples V2+/V3+, V3+/V4+, and V4+/V5+, resulting in remarkably improved energy density even in comparison to the layer structured oxides and Prussian blue analogues. This work also comprehensively clarifies the role of the Cr dopant during sodium storage and the valence electron transition process of both V and Cr. Our findings highlight the importance of a broadly applicable doping strategy for achieving multielectron reactions of NASICON-type cathodes with higher energy densities in sodium-ion batteries.

Multifunctionalities of Graphene for Exploiting a Facile Conversion Reaction Route of Perovskite CoSnO3 for Highly Reversible Na Ion Storage.

Transition-metal oxides are promising anode materials for sodium ion batteries (SIBs) and have attracted a great deal of attention because of their natural abundance and high theoretical capacities. However, they suffer from low conductivity and large volumetric/structural variation during sodiation/desodiation processes, leading to unsatisfactory cycling stability and poor rate capability. This study proposes a novel conversion reaction using CoSnO3 (CSO) nanocubes uniformly wrapped in graphene nanosheets, which are fabricated using a wet-chemical strategy followed by low-temperature heat treatment. This optimized composite exhibits durable cyclability and high rate capability, which can be attributed to the strong interaction between reduced graphene oxide and CSO through its surface oxygen moieties. It develops a facile conversion reaction route, thereby leading to SnO2 formation during charging. This interactive phenomenon further contributes to improving the reaction kinetics and restraining the volume expansion during cycling. This study may provide a facile approach for addressing irreversible conversion of high-capacity oxide materials toward advanced SIBs.

Interface-Controlled Rhombohedral Li3V2(PO4)3 Embedded in Carbon Nanofibers with Ultrafast Kinetics for Li-Ion Batteries.

We present a unique composite assembly of rhombohedral Li3V2(PO4)3 and carbon nanofiber, which simultaneously facilitates Li-ion transport as well as electron transfer. For the synthesis of this composite, the inorganic precursors were confined in electron-spun nanofibers, and then, through controlled annealing, Na3V2(PO4)3 particulates were grown with controllable crystallite size and partially embedded into carbon nanofibers with precisely controlled diameter. The rhombohedral Li3V2(PO4)3 could be successfully obtained by ion exchange from Na to Li in the prepared Na3V2(PO4)3. The final rhombohedral Li3V2(PO4)3 particles anchored onto the carbon nanofibers exhibited excellent electrochemical performance with fast kinetics for Li-ion batteries. Suprisingly it maintains 69 and 41 mAh/g even at 100C as cathode and anode. Several advanced characterizations revealed that its ultrafast kinetics could be attributed to synergistic effect resulting from the distinctive microstructure of the composite and the structural superiority of highly symmetric rhombohedral Li3V2(PO4)3 over its monoclinic homologue for Li-ion transport.

Highly Reversible and Rapid Sodium Storage in GeP3 with Synergistic Effect from Outside-In Optimization.

The composite GeP3/C@rGO as a sodium ion battery anode material was fabricated by introducing a carbon matrix into GeP3 through high-energy ball milling, followed by encapsulating the resultant composite with graphene via a solution-based ultrasonic method. To delineate the individual role of carbon matrix and graphene, material characterization and electrochemical analyses were performed for GeP3/C@rGO and three other samples: bare GeP3, GeP3 with graphene coating (GeP3@rGO), and GeP3 with carbon matrix (GeP3/C). GeP3/C@rGO exhibits the highest electric conductivity (5.89 × 10-1 S cm-1) and the largest surface area (167.85 m2 g-1) among the four samples. The as-prepared GeP3/C@rGO delivered a reversible high capacity of 1084 mA h g-1 at 50 mA g-1, excellent rate capacity (435.4 mA h g-1 at a high rate of 5 A g-1), and long-term cycling stability (400 cycles with a reversible capacity of 823.3 mA h g-1 at 0.2 A g-1), all of which outperform the other three samples. The kinetics investigation reveals a "pseudocapacitive behavior" in GeP3/C and GeP3/C@rGO, where solely faradic reactions took place in bare GeP3 and GeP3@rGO with a typical "battery behavior". Based on ex-situ X-ray photoelectron spectroscopy and ex-situ electrochemical impedance spectroscopy, the carbon matrix serves to activate and stabilize the interior of the composite, while the graphene protects and restrains the exterior surface. Benefiting from the synergistic combination of these two components, GeP3/C@rGO achieved extremely stable cycling stability as well as outstanding rate performance.

Synergistic Control of Structural Disorder and Surface Bonding Nature to Optimize the Functionality of Manganese Oxide as an Electrocatalyst and a Cathode for Li-O2 Batteries.

An efficient way to improve the electrocatalyst and Li-O2 battery performances of metal oxide is developed by an exquisite synergistic control over structural disorder and surface bonding nature. The effects of amorphous nature and surface chemical environment on the functionalities of metal oxide are systematically investigated with well-crystalline and amorphous MnO2 nanocrystals with/without surface anchoring of highly oxidized iodate clusters. The amorphous MnO2 nanocrystal containing anchored iodate clusters shows much better performance as an oxygen evolution electrocatalyst and cathode catalyst for Li-O2 batteries than both iodate-free amorphous and well-crystalline homologues, underscoring the remarkable advantage of simultaneous enhancement of structural disorder and surface electron density. In situ X-ray absorption spectroscopic analysis demonstrates the promoted formation of double (MnO) bond, a critical step of oxygen evolution reaction, upon amorphization caused by the poor orbital overlap inside highly disordered crystallites. The beneficial effects of iodate anchoring and amorphization on electrocatalyst functionality are attributable to the alteration of surface bonding character, stabilization of Jahn-Teller active Mn3+ species, and enhanced charge transfer of interfaces. The present study underscores that fine-tuning of structural disorder and surface bonding nature provides an effective methodology to explore efficient metal oxide-based electrocatalysts.

Manganese based layered oxides with modulated electronic and thermodynamic properties for sodium ion batteries.

Manganese based layered oxides have received increasing attention as cathode materials for sodium ion batteries due to their high theoretical capacities and good sodium ion conductivities. However, the Jahn-Teller distortion arising from the manganese (III) centers destabilizes the host structure and deteriorates the cycling life. Herein, we report that zinc-doped Na0.833[Li0.25Mn0.75]O2 can not only suppress the Jahn-Teller effect but also reduce the inherent phase separations. The reduction of manganese (III) amount in the zinc-doped sample, as predicted by first-principles calculations, has been confirmed by its high binding energies and the reduced octahedral structural variations. In the viewpoint of thermodynamics, the zinc-doped sample has lower formation energy, more stable ground states, and fewer spinodal decomposition regions than those of the undoped sample, all of which make it charge or discharge without any phase transition. Hence, the zinc-doped sample shows superior cycling performance, demonstrating that zinc doping is an effective strategy for developing high-performance layered cathode materials.

High-Energy-Density Metal-Oxygen Batteries: Lithium-Oxygen Batteries vs Sodium-Oxygen Batteries.

The development of next-generation energy-storage devices with high power, high energy density, and safety is critical for the success of large-scale energy-storage systems (ESSs), such as electric vehicles. Rechargeable sodium-oxygen (Na-O2 ) batteries offer a new and promising opportunity for low-cost, high-energy-density, and relatively efficient electrochemical systems. Although the specific energy density of the Na-O2 battery is lower than that of the lithium-oxygen (Li-O2 ) battery, the abundance and low cost of sodium resources offer major advantages for its practical application in the near future. However, little has so far been reported regarding the cell chemistry, to explain the rate-limiting parameters and the corresponding low round-trip efficiency and cycle degradation. Consequently, an elucidation of the reaction mechanism is needed for both lithium-oxygen and sodium-oxygen cells. An in-depth understanding of the differences and similarities between Li-O2 and Na-O2 battery systems, in terms of thermodynamics and a structural viewpoint, will be meaningful to promote the development of advanced metal-oxygen batteries. State-of-the-art battery design principles for high-energy-density lithium-oxygen and sodium-oxygen batteries are thus reviewed in depth here. Major drawbacks, reaction mechanisms, and recent strategies to improve performance are also summarized.

Cobalt-Doped FeS2 Nanospheres with Complete Solid Solubility as a High-Performance Anode Material for Sodium-Ion Batteries.

Considering that the high capacity, long-term cycle life, and high-rate capability of anode materials for sodium-ion batteries (SIBs) is a bottleneck currently, a series of Co-doped FeS2 solid solutions with different Co contents were prepared by a facile solvothermal method, and for the first time their Na-storage properties were investigated. The optimized Co0.5 Fe0.5 S2 (Fe0.5) has discharge capacities of 0.220 Ah g(-1) after 5000 cycles at 2 A g(-1) and 0.172 Ah g(-1) even at 20 A g(-1) with compatible ether-based electrolyte in a voltage window of 0.8-2.9 V. The Fe0.5 sample transforms to layered Nax Co0.5 Fe0.5 S2 by initial activation, and the layered structure is maintained during following cycles. The redox reactions of Nax Co0.5 Fe0.5 S2 are dominated by pseudocapacitive behavior, leading to fast Na(+) insertion/extraction and durable cycle life. A Na3 V2 (PO4 )3 /Fe0.5 full cell was assembled, delivering an initial capacity of 0.340 Ah g(-1) .

Tailored surface structure of LiFePO4/C nanofibers by phosphidation and their electrochemical superiority for lithium rechargeable batteries.

We offer a brand new strategy for enhancing Li ion transport at the surface of LiFePO4/C nanofibers through noble Li ion conducting pathways built along reduced carbon webs by phosphorus. Pristine LiFePO4/C nanofibers composed of 1-dimensional (1D) LiFePO4 nanofibers with thick carbon coating layers on the surfaces of the nanofibers were prepared by the electrospinning technique. These dense and thick carbon layers prevented not only electrolyte penetration into the inner LiFePO4 nanofibers but also facile Li ion transport at the electrode/electrolyte interface. In contrast, the existing strong interactions between the carbon and oxygen atoms on the surface of the pristine LiFePO4/C nanofibers were weakened or partly broken by the adhesion of phosphorus, thereby improving Li ion migration through the thick carbon layers on the surfaces of the LiFePO4 nanofibers. As a result, the phosphidated LiFePO4/C nanofibers have a higher initial discharge capacity and a greatly improved rate capability when compared with pristine LiFePO4/C nanofibers. Our findings of high Li ion transport induced by phosphidation can be widely applied to other carbon-coated electrode materials.