Tailoring Stress-Relieved Structure for SnSe Toward High Performance Potassium Ion Batteries.

Metal chalcogenides as an ideal family of anode materials demonstrate a high theoretical specific capacity for potassium ion batteries (PIBs), but the huge volume variance and poor cyclic stability hinder their practical applications. In this study, a design of a stress self-adaptive structure with ultrafine SnSe nanoparticles embedded in carbon nanofiber (SnSe@CNF) via the electrospinning technology is presented. Such an architecture delivers a record high specific capacity (272 mAh g-1 at 50 mA g-1) and high-rate performance (125 mAh g-1 at 1 A g-1) as a PIB anode. It is decoded that the fundamental understanding for this great performance is that the ultrafine SnSe particles enhance the full utilization of the active material and achieve stress relief as the stored strain energy from cycling is insufficient to drive crack propagation and thus alleviates the intrinsic chemo-mechanical degradation of metal chalcogenides.

Zero-Strain Sodium Lanthanum Titanate Perovskite Embedded in Flexible Carbon Fibers as a Long-Span Anode for Lithium-Ion Batteries.

"High-capacity" graphite and "zero-strain" spinel Li4Ti5O12 (LTO) occupy the majority market of anode materials for Li+ storage in commercial applications. Nevertheless, their intrinsic drawbacks including the unsafe potential of graphite and unsatisfactory capacity of LTO limit the further development of lithium-ion batteries (LIBs), which is unable to satisfy the ever-increasing demands. Here, a novel Na0.35La0.55TiO3 perovskite embedded in multichannel carbon fibers (NLTO-NF) is rationally designed and synthesized through an electrospinning method. It not only has the advantages of a respectable specific capacity of 265 mAh g-1 at 0.1 A g-1 and superb rate capability, but it also possesses the zero-strain characteristic. Impressively, an ultralong cycling life with 96.3% capacity retention after 9000 cycles at 2 A g-1 is achieved in the half cell, and 90.3% of capacity retention ratio is obtained after even 2500 cycles at 1 A g-1 in the coupled LiFePO4/NLTO-NF full cell. This study introduces a new member with excellent performance to the zero-strain materials family for next-generation LIBs.

Tailoring Na+ Solvation Environment and Electrode-Electrolyte Interphases with Sn(OTf)2 Additive in Non-flammable Phosphate Electrolytes towards Safe and Efficient Na-S Batteries.

Room-temperature sodium-sulfur (RT Na-S) batteries are promising for low-cost and large-scale energy storage applications. However, these batteries are plagued by safety concerns due to the highly flammable nature of conventional electrolytes. Although non-flammable electrolytes eliminate the risk of fire, they often result in compromised battery performance due to poor compatibility with sodium metal anode and sulfur cathode. Herein, we develop an additive of tin trifluoromethanesulfonate (Sn(OTf)2 ) in non-flammable phosphate electrolytes to improve the cycling stability of RT Na-S batteries via modulating the Na+ solvation environment and interface chemistry. The additive reduces the Na+ desolvation energy and enhances the electrolyte stability. Moreover, it facilitates the construction of Na-Sn alloy-based anode solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI). These interphases help to suppress the growth of Na dendrites and the dissolution/shuttling of sodium polysulfides (NaPSs), resulting in improved reversible capacity. Specifically, the Na-S battery with the designed electrolyte boosts the capacity from 322 to 906 mAh g-1 at 0.5 A g-1 . This study provides valuable insights for the development of safe and high-performance electrolytes in RT Na-S batteries.

Fine valence regulation of hydrated vanadium oxide as a novel cathode for stable potassium-ion storage.

Layered V10O24·nH2O with a large interlayer spacing of 14 Å is hydrothermally synthesized and used as a cathode for potassium-ion batteries. It exhibits a capacity of 110 mA h g-1 with a capacity retention of 99.2% over 700 cycles. Its storage mechanism is identified as pseudo-capacitive intercalation.

Designing Solid Electrolyte Interfaces towards Homogeneous Na Deposition: Theoretical Guidelines for Electrolyte Additives and Superior High-Rate Cycling Stability.

Metallic Na is a promising metal anode for large-scale energy storage. Nevertheless, unstable solid electrolyte interphase (SEI) and uncontrollable Na dendrite growth lead to disastrous short circuit and poor cycle life. Through phase field and ab initio molecular dynamics simulation, we first predict that the sodium bromide (NaBr) with the lowest Na ion diffusion energy barrier among sodium halogen compounds (NaX, X=F, Cl, Br, I) is the ideal SEI composition to induce the spherical Na deposition for suppressing dendrite growth. Then, 1,2-dibromobenzene (1,2-DBB) additive is introduced into the common fluoroethylene carbonate-based carbonate electrolyte (the corresponding SEI has high mechanical stability) to construct a desirable NaBr-rich stable SEI layer. When the Na||Na3 V2 (PO4 )3 cell utilizes the electrolyte with 1,2-DBB additive, an extraordinary capacity retention of 94 % is achieved after 2000 cycles at a high rate of 10 C. This study provides a design philosophy for dendrite-free Na metal anode and can be expanded to other metal anodes.

Realizing High-Performance Lithium Storage by Fabricating FeTiO3 Nanoparticle-Impregnated Multichannel Carbon Nanofibers with Promoted Reaction Kinetics.

In this paper, a free-standing film of ilmenite FeTiO3 nanoparticle-impregnated porous multichannel N-doped carbon nanofibers (NF-FTO) is fabricated via electrospinning technology. The as-prepared NF-FTO film is highly flexible and can be tailored to a suitable size to assemble into lithium-ion batteries. The introduction of a conductive N-doped carbon matrix is conducive to the improvement of intrinsic electronic conductivity and the acceleration of Li+ diffusion kinetics. The construction of the porous structure and highly parallel channels facilitates the transfer of electrolyte to FTO particles through the pores and shortens the transport path of lithium ions. Thus, the self-supporting electrode yields an initial charge capacity of 718.5 mAh g-1 at 50 mA g-1, a high-rate performance of 410.4 mAh g-1 at 3 A g-1, and an outstanding cycling performance with no capacity decay after 1500 cycles at 3 A g-1. By ex situ X-ray diffraction and transmission electron microscopy analysis, the reaction mechanism of NF-FTO is determined as a reversible conversion reaction. Furthermore, the assembled LiFePO4/NF-FTO full cell delivers an initial discharge capacity of 521 mAh g-1 and superb rate performance.

Structural design enabled a hypotoxic Na3.36FeV(PO4)3 cathode with ultra-fast and ultra-long sodium storage.

Among various cathode materials for sodium-ion batteries, Na3V2(PO4)3 has attracted much attention due to its outstanding electrochemical performance. However, the toxicity and expensive price of vanadium limit its practical application. Therefore, the substitution of vanadium with nontoxic and inexpensive transition metal elements is significant. We select the earth-abundant iron element to partially replace the vanadium element, and successfully synthesize Na3.36FeV(PO4)3 with a Na superionic conductor structure. Furthermore, a Na3.36FeV(PO4)3 cathode with an optimal carbon content can deliver an initial capacity of 97.6 mA h g-1 at 0.5C with a high capacity retention of 96.4% after 200 cycles. In addition, it also delivers an initial capacity of 90.4 mA h g-1 at 10C, and a capacity retention of 80% can be obtained after 5000 cycles. We also found that the lack of sodium in the material can be compensated by an electrochemical reaction. Furthermore, the in situ X-ray diffraction analysis reveals that the sodium storage process follows a pseudo-solid-solution reaction mechanism and the volume change ratio is less than 3% during charging/discharging. In order to study the practical application capability of Na3.36FeV(PO4)3, we assemble the pre-activated cathode and a hard carbon anode into a full cell, which exhibits high initial discharge capacities of 103 and 91.3 mA h g-1 at 0.5C and 10C, respectively. This work will provide new insights into the structural engineering of low-toxicity and ultralong-life NASICON-type cathode materials for SIBs.

VN and SeS2 embedded porous carbon-nanofiber film as a free-standing electrode for improved Li-SeS2 batteries.

We design a vanadium nitride (VN) modified porous carbon nanofiber film as the host to load SeS2 as the cathode (SeS2@VN/CNFs) for improving Li storage capacity. The conductive porous carbon nanofibers can accommodate active SeS2 and release the volume change. The introduced VN nanoparticles can chemically anchor the intermediate species and improve the utilization of SeS2. As a result, the SeS2@VN/CNFs cathode displays a superior electrochemical performance including a high reversible capacity of 806 mAh g-1 at 0.2 C and good long-term cycling stability in Li-SeS2 batteries.

Introducing a Pseudocapacitive Lithium Storage Mechanism into Graphite by Defect Engineering for Fast-Charging Lithium-Ion Batteries.

The extreme fast-charging capability of lithium-ion batteries (LIBs) is very essential for electric vehicles (EVs). However, currently used graphite anode materials cannot satisfy the requirements of fast charging. Herein, we demonstrate that intrinsic lattice defect engineering based on a thermal treatment of graphite in CO2 is an effective method to improve the fast-charging capability of the graphite anode. The activated graphite (AG) exhibits a superior rate capability of 209 mAh g-1 at 10 C (in comparison to 15 mAh g-1 for the pristine graphite), which is attributed to a pseudocapacitive lithium storage behavior. Furthermore, the full cell LiFePO4||AG can achieve SOCs of 82% and 96% within 6 and 15 min, respectively. The intrinsic carbon defect introduced by the CO2 treatment succeeds in improving the kinetics of lithium ion intercalation at the rate-determining step during lithiation, which is identified by the distribution of relaxation times (DRT) and density functional theory (DFT) calculations. Therefore, this study provides a novel strategy for fast-charging LIBs. Moreover, this facile method is also suitable for activating other carbon-based materials.

Ultra-stable potassium storage and hybrid mechanism of perovskite fluoride KFeF3/rGO.

Potassium-ion batteries (PIBs) are promising for large-scale energy storage due to the abundant reserves of the element potassium yet few satisfactory cathode materials have been developed due to the limitation of the large ionic radius of the potassium ion. Cubic perovskite fluorides have three-dimensional diffusion channels and a robust structure, which are favorable for ion transfer, but their poor electronic conductivity needs to be compensated. Here, we synthesized cubic KFeF3 powder by a solvothermal procedure. After the combination with reduced graphene oxide (rGO) and carbon coating, its electronic conductivity is greatly improved. In the optimal sample KFeF3/rGO-PVA-500, KFeF3 nano-particles (smaller than 50 nm) distribute on the rGO surface evenly. Owing to the special structure, KFeF3/rGO-PVA-500 provides an excellent rate performance and cycling stability. In particular, a high capacity retention of 94% is obtained after 1000 cycles at 200 mA g-1. In addition, a hybrid reaction mechanism combining mainly solid solution and partly conversion processes is revealed by employing in situ and ex situ characterization.

Active-Site-Specific Structural Engineering Enabled Ultrahigh Rate Performance of the NaLi3Fe3(PO4)2(P2O7) Cathode for Lithium-Ion Batteries.

Iron-based mixed-polyanionic cathode Na4Fe3(PO4)2(P2O7) (NFPP) has advantages of environmental benignity, easy synthesis, high theoretical capacity, and remarkable stability. From NFPP, a novel Li-replaced material NaLi3Fe3(PO4)2(P2O7) (NLFPP) is synthesized through active Na-site structural engineering by an electrochemical ion exchange approach. The NLFPP cathode can show high reversible capacities of 103.2 and 90.3 mA h g-1 at 0.5 and 5C, respectively. It also displays an impressive discharge capacity of 81.5 mA h g-1 at an ultrahigh rate of 30C. Density functional theory (DFT) calculation demonstrates that the formation energy of NLFPP is the lowest among NLFPP, NFPP, and NaFe3(PO4)2(P2O7), indicating that NLFPP is the easiest to form and the conversion from NFPP to NLFPP is thermodynamically favorable. The Li substitution for Na in the NFPP lattice causes an increase in the unit cell parameter c and decreases in a, b, and V, which are revealed by both DFT calculations and in situ X-ray powder diffraction (XRD) analysis. With hard carbon (HC) as the anode, the NLFPP//HC full cell shows a high reversible capacity of 91.1 mA h g-1 at 2C and retains 82.4% after 200 cycles. The proposed active-site-specific structural tailoring via electrochemical ion exchange will give new insights into the design of high-performance cathodes for lithium-ion batteries.

High ICE Hard Carbon Anodes for Lithium-Ion Batteries Enabled by a High Work Function.

Hard carbons (HC) derived from biomass material are most promising anodes for lithium-ion batteries (LIBs) because of their cost effectiveness and environmental friendliness. However, the low initial Coulombic efficiency (ICE) of HC anodes reduces the energy density of full cells, which seriously impedes their practical applications. Herein, we demonstrate that the ICE of HC anodes can be significantly improved by modulating the work function of a model HC derived from cotton and deliberately treated to form C-Cl bonds on its surface. By X-ray absorption near-edge structure and density functional theory (DFT) calculation studies, it is verified that the introduction of the C-Cl bond leads to the electron transfer from C to Cl and enhances the work function of the system. In addition, this Cl-doped HC anode can inhibit the reduction of solvent molecules in the electrolyte and reduce the formation of a solid electrolyte interface (SEI) film. Consequently, the ICE is improved from 64.8 to 78.1%. This study provides an effective route to reduce the formation of the SEI film and improve the ICE of hard carbon anodes for LIBs.

Manipulating the Electronic Structure of Nickel via Alloying with Iron: Toward High-Kinetics Sulfur Cathode for Na-S Batteries.

The sluggish conversion kinetics and severe shuttle effect in room-temperature Na-S (RT Na-S) batteries cause knotty issues, such as poor rate performance, fast capacity decay as well as low Coulombic efficiency, which seriously impede their practical application. Therefore, exploiting cost-effective and efficient electrocatalysts for absorbing soluble long-chain sodium polysulfides (NaPSs) and expediting NaPSs conversion is of paramount importance. Herein, catalyst mining is first conducted by density functional theory calculations, which reveal that the alloying of Fe into Ni can tailor the electronic structure, leading to lower reaction energy barrier for polysulfide conversion. Based on this, FeNi3@hollow porous carbon spheres (FeNi3@HC) as a promising sulfur host for RT Na-S batteries are rationally designed and fabricated. As expected, the S@FeNi3@HC cathode exhibits an excellent cycling stability (591 mAh g-1 after 500 cycles at 2 A g-1) and outstanding rate performance (383 mAh g-1 at 5 A g-1). Our work demonstrates an effective strategy (i.e., alloying strategy with a rich electron state) to design superior electrocatalysts for RT Na-S batteries.

Hollow-Sphere-Structured Na4Fe3(PO4)2(P2O7)/C as a Cathode Material for Sodium-Ion Batteries.

The mixed polyanionic material Na4Fe3(PO4)2(P2O7) combines the advantages of NaFePO4 and Na2FeP2O7 in capacity, stability, and cost. Herein, we synthesized carbon-coated hollow-sphere-structured Na4Fe3(PO4)2(P2O7) powders by a scalable spray drying route. The optimal sample can deliver a high discharge capacity of 107.7 mA h g-1 at 0.2C. It also delivers a capacity of 88 mA h g-1 at 10C and a capacity of retention of 92% after 1500 cycles. Ex situ X-ray diffraction analysis indicates a slight volume change (less than 3%) in the Na4Fe3(PO4)2(P2O7) lattice cell. Therefore, such a spraying-derived carbon-coated Na4Fe3(PO4)2(P2O7) powder is a very attractive cathode electrode for sodium-ion batteries.