Defect engineering unveiled: Enhancing potassium storage in expanded graphite anode.

Expanded graphite (EG) stands out as a promising material for the negative electrode in potassium-ion batteries. However, its full potential is hindered by the limited diffusion pathway and storage sites for potassium ions, restricting the improvement of its electrochemical performance. To overcome this challenge, defect engineering emerges as a highly effective strategy to enhance the adsorption and reaction kinetics of potassium ions on electrode materials. This study delves into the specific effectiveness of defects in facilitating potassium storage, exploring the impact of defect-rich structures on dynamic processes. Employing ball milling, we introduce surface defects in EG, uncovering unique effects on its electrochemical behavior. These defects exhibit a remarkable ability to adsorb a significant quantity of potassium ions, facilitating the subsequent intercalation of potassium ions into the graphite structure. Consequently, this process leads to a higher potassium voltage. Furthermore, the generation of a diluted stage compound is more pronounced under high voltage conditions, promoting the progression of multiple stage reactions. Consequently, the EG sample post-ball milling demonstrates a notable capacity of 286.2 mAh g-1 at a current density of 25 mA g-1, showcasing an outstanding rate capability that surpasses that of pristine EG. This research not only highlights the efficacy of defect engineering in carbon materials but also provides unique insights into the specific manifestations of defects on dynamic processes, contributing to the advancement of potassium-ion battery technology.

Electrolyte Chemistry toward Ultrawide-Temperature (-25 to 75 °C) Sodium-Ion Batteries Achieved by Phosphorus/Silicon-Synergistic Interphase Manipulation.

All-weather operation is considered an ultimate pursuit of the practical development of sodium-ion batteries (SIBs), however, blocked by a lack of suitable electrolytes at present. Herein, by introducing synergistic manipulation mechanisms driven by phosphorus/silicon involvement, the compact electrode/electrolyte interphases are endowed with improved interfacial Na-ion transport kinetics and desirable structural/thermal stability. Therefore, the modified carbonate-based electrolyte successfully enables all-weather adaptability for long-term operation over a wide temperature range. As a verification, the half-cells using the designed electrolyte operate stably over a temperature range of -25 to 75 °C, accompanied by a capacity retention rate exceeding 70% even after 1700 cycles at 60 °C. More importantly, the full cells assembled with Na3V2(PO4)2O2F cathode and hard carbon anode also have excellent cycling stability, exceeding 500 and 1000 cycles at -25 to 50 °C and superb temperature adaptability during all-weather dynamic testing with continuous temperature change. In short, this work proposes an advanced interfacial regulation strategy targeted at the all-climate SIB operation, which is of good practicability and reference significance.

Hybrid Binder Chemistry with Hydrogen-Bond Helix for High-Voltage Cathode of Sodium-Ion Batteries.

Since sodium-ion batteries (SIBs) have become increasingly commercialized in recent years, Na3V2(PO4)2O2F (NVPOF) offers promising economic potential as a cathode for SIBs because of its high operating voltage and energy density. According to reports, NVPOF performs poorly in normal commercial poly(vinylidene fluoride) (PVDF) binder systems and performs best in combination with aqueous binder. Although in line with the concept of green and sustainable development for future electrode preparation, aqueous binders are challenging to achieve high active material loadings at the electrode level, and their relatively high surface tension tends to cause the active material on the electrode sheet to crack or even peel off from the collector. Herein, a cross-linkable and easily commercial hybrid binder constructed by intermolecular hydrogen bonding (named HPP) has been developed and utilized in an NVPOF system, which enables the generation of a stable cathode electrolyte interphase on the surface of active materials. According to theoretical simulations, the HPP binder enhances electronic/ionic conductivity, which greatly lowers the energy barrier for Na+ migration. Additionally, the strong hydrogen-bond interactions between the HPP binder and NVPOF effectively prevent electrolyte corrosion and transition-metal dissolution, lessen the lattice volume effect, and ensure structural stability during cycling. The HPP-based NVPOF offers considerably improved rate capability and cycling performance, benefiting from these benefits. This comprehensive binder can be extended to the development of next-generation energy storage technologies with superior performance.

Advanced flame-retardant electrolyte for highly stabilized K-ion storage in graphite anode.

Although graphite anodes operated with representative de/intercalation patterns at low potentials are considered highly desirable for K-ion batteries, the severe capacity fading caused by consecutive reduction reactions on the aggressively reactive surface is inevitable given the scarcity of effective protecting layers. Herein, by introducing a flame-retardant localized high-concentration electrolyte with retentive solvation configuration and relatively weakened anion-coordination and non-solvating fluorinated ether, the rational solid electrolyte interphase characterized by well-balanced inorganic/organic components is tailored in situ. This effectively prevented solvents from excessively decomposing and simultaneously improved the resistance against K-ion transport. Consequently, the graphite anode retained a prolonged cycling capability of up to 1400cycles (245 mA h g-1, remaining above 12mon) with an excellent capacity retention of as high as 92.4%. This is superior to those of conventional and high-concentration electrolytes. Thus, the optimized electrolyte with moderate salt concentration is perfectly compatible with graphite, providing a potential application prospect for K-storage evolution.

An Advanced High-Entropy Fluorophosphate Cathode for Sodium-Ion Batteries with Increased Working Voltage and Energy Density.

Impossible voltage plateau regulation for the cathode materials with fixed active elemental center is a pressing issue hindering the development of Na-superionic-conductor (NASICON)-type Na3 V2 (PO4 )2 F3 (NVPF) cathodes in sodium-ion batteries (SIBs). Herein, a high-entropy substitution strategy, to alter the detailed crystal structure of NVPF without changing the central active V atom, is pioneeringly utilized, achieving simultaneous electronic conductivity enhancement and diffusion barrier reduction for Na+ , according to theoretical calculations. The as-prepared carbon-free high-entropy Na3 V1.9 (Ca,Mg,Al,Cr,Mn)0.1 (PO4 )2 F3 (HE-NVPF) cathode can deliver higher mean voltage of 3.81 V and more advantageous energy density up to 445.5 Wh kg-1 , which is attributed by the diverse transition-metal elemental substitution in high-entropy crystalline. More importantly, high-entropy introduction can help realize disordered rearrangement of Na+ at Na(2) active sites, thereby to refrain from unfavorable discharging behaviors at low-voltage region, further lifting up the mean working voltage to realize a full Na-ion storage at the high voltage plateau. Coupling with a hard carbon (HC) anode, HE-NVPF//HC SIB full cells can deliver high specific energy density of 326.8 Wh kg-1 at 5 C with the power density of 2178.9 W kg-1 . This route means the unlikely potential regulation in NASICON-type crystal with unchangeable active center becomes possible, inspiring new ideas on elevating the mean working voltage for SIB cathodes.

Covalent Organic Framework with Highly Accessible Carbonyls and π-Cation Effect for Advanced Potassium-Ion Batteries.

Covalent organic frameworks (COF) possess a robust and porous crystalline structure, making them an appealing candidate for energy storage. Herein, we report an exfoliated polyimide COF composite (P-COF@SWCNT) prepared by an in situ condensation of anhydride and amine on the single-walled carbon nanotubes as advanced anode for potassium-ion batteries (PIBs). Numerous active sites exposed on the exfoliated frameworks and the various open pathways promote the highly efficient ion diffusion in the P-COF@SWCNT while preventing irreversible dissolution in the electrolyte. During the charging/discharging process, K+ is engaged in the carbonyls of imide group and naphthalene rings through the enolization and π-K+ effect, which is demonstrated by the DFT calculation and XPS, ex-situ FTIR, Raman. As a result, the prepared P-COF@SWCNT anode enables an incredibly high reversible specific capacity of 438 mA h g-1 at 0.05 A g-1 and extended stability. The structural advantage of P-COF@SWCNT enables more insights into the design and versatility of COF as an electrode.

Ether-Based Electrolyte Chemistry Towards High-Voltage and Long-Life Na-Ion Full Batteries.

Although ether-based electrolytes have been extensively applied in anode evaluation of batteries, anodic instability arising from solvent oxidability is always a tremendous obstacle to matching with high-voltage cathodes. Herein, by rational design for solvation configuration, the fully coordinated ether-based electrolyte with strong resistance against oxidation is reported, which remains anodically stable with high-voltage Na3 V2 (PO4 )2 O2 F (NVPF) cathode under 4.5 V (versus Na+ /Na) protected by an effective interphase. The assembled graphite//NVPF full cells display superior rate performance and unprecedented cycling stability. Beyond that, the constructed full cells coupling the high-voltage NVPF cathode with hard carbon anode exhibit outstanding electrochemical performances in terms of high average output voltage up to 3.72 V, long-term cycle life (such as 95 % capacity retention after 700 cycles) and high energy density (247 Wh kg-1 ). In short, the optimized ether-based electrolyte enriches systematic options, the ability to maintain oxidative stability and compatibility with various anodes, exhibiting attractive prospects for application.

Aliovalent-Ion-Induced Lattice Regulation Based on Charge Balance Theory: Advanced Fluorophosphate Cathode for Sodium-Ion Full Batteries.

There are still many problems that hinder the development of sodium-ion batteries (SIBs), including poor rate performance, short-term cycle lifespan, and inferior low-temperature property. Herein, excellent Na-storage performance in fluorophosphate (Na3 V2 (PO4 )2 F3 ) cathode is achieved by lattice regulation based on charge balance theory. Lattice regulation of aliovalent Mn2+ for V3+ increases both electronic conductivity and Na+ -migration kinetics. Because of the maintaining of electrical neutrality in the material, aliovalent Mn2+ -introduced leads to the coexistence of V3+ and V4+ from charge balance theory. It decreases the particle size and improves the structural stability, suppressing the large lattice distortion during cathode reaction processes. These multiple effects enhance the specific capacity (123.8 mAh g-1 ), outstanding high-rate (68% capacity retention at 20 C), ultralong cycle (only 0.018% capacity attenuation per cycle over 1000 cycles at 1 C) and low-temperature (96.5% capacity retention after 400 cycles at -25 °C) performances of Mn2+ -induced Na3 V1.98 Mn0.02 (PO4 )2 F3 when used as cathode in SIBs. Importantly, a feasible sodium-ion full battery is assembled, achieving outstanding rate capability and cycle stability. The strategy of aliovalent ion-induced lattice regulation constructs cathode materials with superior performances, which is available to improve other electrode materials for energy storage systems.

Carbon-coating-increased working voltage and energy density towards an advanced Na3V2(PO4)2F3@C cathode in sodium-ion batteries.

One main challenge for phosphate cathodes in sodium-ion batteries (SIBs) is to increase the working voltage and energy density to promote its practicability. Herein, an advanced Na3V2(PO4)2F3@C cathode is prepared successfully for sodium-ion full cells. It is revealed that, carbon coating can not only enhance the electronic conductivity and electrode kinetics of Na3V2(PO4)2F3@C and inhibit the growth of particles (i.e., shorten the Na+-migration path), but also unexpectedly for the first time adjust the dis-/charging plateaux at different voltage ranges to increase the mean voltage (from 3.59 to 3.71 V) and energy density (from 336.0 to 428.5 Wh kg-1) of phosphate cathode material. As a result, when used as cathode for SIBs, the prepared Na3V2(PO4)2F3@C delivers much improved electrochemical properties in terms of larger specifc capacity (115.9 vs. 93.5 mAh g-1), more outstanding high-rate capability (e.g., 87.3 vs. 60.5 mAh g-1 at 10 C), higher energy density, and better cycling performance, compared to pristine Na3V2(PO4)2F3. Reasons for the enhanced electrochemical properties include ionicity enhancement of lattice induced by carbon coating, improved electrode kinetics and electronic conductivity, and high stability of lattice, which is elucidated clearly through the contrastive characterization and electrochemical studies. Moreover, excellent energy-storage performance in sodium-ion full cells further demonstrate the extremely high possibility of Na3V2(PO4)2F3@C cathode for practical applications.

Self-Supporting, Flexible, Additive-Free, and Scalable Hard Carbon Paper Self-Interwoven by 1D Microbelts: Superb Room/Low-Temperature Sodium Storage and Working Mechanism.

Hard carbon is regarded as a promising anode material for sodium-ion batteries (SIBs). However, it usually suffers from the issues of low initial Coulombic efficiency (ICE) and poor rate performance, severely hindering its practical application. Herein, a flexible, self-supporting, and scalable hard carbon paper (HCP) derived from scalable and renewable tissue is rationally designed and prepared as practical additive-free anode for room/low-temperature SIBs with high ICE. In ether electrolyte, such HCP achieves an ICE of up to 91.2% with superior high-rate capability, ultralong cycle life (e.g., 93% capacity retention over 1000 cycles at 200 mA g-1 ) and outstanding low-temperature performance. Working mechanism analyses reveal that the plateau region is the rate-determining step for HCP with a lower electrochemical reaction kinetics, which can be significantly improved in ether electrolyte.