Suppressed Electrolyte Decomposition Behavior to Improve Cycling Performance of LiCoO2 under 4.6 V through the Regulation of Interfacial Adsorption Forces.

Alleviating the decomposition of the electrolyte is of great significance to improving the cycle stability of cathodes, especially for LiCoO2 (LCO), its volumetric energy density can be effectively promoted by increasing the charge cutoff voltage to 4.6 V, thereby supporting the large-scale application of clean energy. However, the rapid decomposition of the electrolyte under 4.6 V conditions not only loses the transport carrier for lithium ion, but also produces HF and insulators that destroy the interface of LCO and increase impedance. In this work, the decomposition of electrolyte is effectively suppressed by changing the adsorption force between LCO interface and EC. Density functional theory illustrates the LCO coated with lower electronegativity elements has a weaker adsorption force with the electrolyte, the adsorption energy between LCO@Mg and EC (0.49 eV) is weaker than that of LCO@Ti (0.73 eV). Meanwhile, based on the results of time of flight secondary ion mass spectrometry, conductivity-atomic force microscopy, in situ differential electrochemical mass spectrometry, soft X-ray absorption spectroscopy, and nuclear magnetic resonance, as the adsorption force increases, the electrolyte decomposes more seriously. This work provides a new perspective on the interaction between electrolyte and the interface of cathode and further improves the understanding of electrolyte decomposition.

Na3V2(PO4)3-decorated Na3V2(PO4)2F3 as a high-rate and cycle-stable cathode material for sodium ion batteries.

Since Na3V2(PO4)3 (NVP) possesses modest volume deformation and three-dimensional ion diffusion channels, it is a potential sodium-ion battery cathode material that has been extensively researched. Nonetheless, NVP still endures the consequences of poor electronic conductivity and low voltage platforms, which need to be further improved. On this basis, a high voltage platform Na3V2(PO4)2F3 was introduced to form a composite with NVP to increase the energy density. In this study, the sol-gel technique was successfully used to synthesize a Na3V2(PO4)2.75F0.75/C (NVPF·3NVP/C) composite cathode material. The citric acid-derived carbon layer was utilized to construct three-dimensional conducting networks to effectively promote ion and electron diffusion. Furthermore, the composites' synergistic effect accelerates the quick ionic migration and improves the kinetic reaction. In particular, NVP as the dominant phase enhanced the structural stability and significantly increased the capacitive contribution. Therefore, at 0.1C, the discharge capacity of the modified NVPF·3NVP/C composite is 120.7 mA h g-1, which is greater than the theoretical discharge capacity of pure NVP (118 mA h g-1). It discharged 110.9 mA h g-1 of reversible capacity even at an elevated multiplicity of 10C, and after 200 cycles, it retained 64.1% of its capacity. Thus, the effort produced an optimized NVPF·3NVP/C composite cathode material that may be used in the sodium ion cathode.

Enhancing Sodium-Ion Transport by Hollow Nanotube Structure Design of a V5S8@C Anode for Sodium-Ion Batteries.

V5S8 has received extensive attention in the field of sodium-ion batteries (SIBs) due to its two-dimensional (2D) layered structure, and weak van der Waals forces between V-S accelerate the transport of sodium ions. However, the long-term cycling of V5S8 still suffers from volume expansion and low conductivity. Herein, a hollow nanotube V5S8@C (H-V5S8@C) with improved conductivity was synthesized by a solvothermal method to alleviate cracking caused by volume expansion. Benefiting from the large specific surface area of the hollow nanotube structure and uniform carbon coating, H-V5S8@C exhibits a more active site and enhanced conductivity. Meanwhile, the heterojunction formed by a few residual MoS2 and the outer layer of V5S8 stabilizes the structure and reduces the ion migration barrier with fast Na+ transport. Specifically, the H-V5S8@C anode provides an enhanced rate performance of 270.1 mAh g-1 at 15 A g-1 and high cycling stability of 291.7 mAh g-1 with a retention rate of 90.98% after 300 cycles at 5 A g-1. This work provides a feasible approach for the structural design of 2D layered materials, which can promote the practical application of fast-charging sodium-ion batteries.

Operando Investigation of Silicon Anodes During Electrochemical Cycling in Li-ion Batteries.

Silicon (Si) is recognized as a promising anode material for next-generation anodes due to its high capacity. However, large volume expansion and active particle pulverization during cycling rapidly deteriorate the battery performance. The relationship between Si anode particle size and particle pulverization, and the structure evolution of Si particles during cycling is not well understood. In this study, a quantitative, time-resolved "operando" small angle X-ray scattering (SAXS) investigation into the morphological change of unwrapped and reduced graphene oxide (rGO) wrapped Si nanoparticles (Si@rGO) is conducted with respect to the operating voltage. The results provide a clear picture of Si particle size change and the role of nonrigid rGO in mitigating Si volume expansion and pulverization. Further, this study demonstrates the advantage of "operando" SAXS in electrochemical environments as compared to other approaches.

In Situ-Constructed Multifunctional Interface for High-Voltage 4.6 V LiCoO2.

Due to high volumetric energy density, the major market share of cathode materials for lithium-ion batteries is still dominated by LiCoO2 (LCO) at a 3C field. However, a number of challenges will be triggered if the charge voltage is increased from 4.2/4.3 to 4.6 V to further increase energy density, such as a violent interface reaction, Co dissolution, and release of lattice oxygen. Here, LCO is coated with the fast ionic conductor Li1.8Sc0.8Ti1.2(PO4)3 (LSTP) to form LCO@LSTP, while a stable interface of LCO is in situ constructed by the decomposition of LSTP at the LSTP/LCO interface. As decomposition products of LSTP, Ti and Sc elements can be doped into LCO and thus reconstruct the interface from a layered structure to a spinel structure, which improves the stability of the interface. Moreover, Li3PO4 from the decomposition of LSTP and remaining LSTP coating as a fast ionic conductor can improve Li+ transport when compared with bare LCO, and thus boost the specific capacity to 185.3 mAh g-1 at 1C. Benefited from the stable interface and fast ion conducting coating, the LCO@LSTP (1 wt %) cathode delivers a high capacity of 202.3 mAh g-1 at the first cycle (0.5C, 3.0-4.6 V), and shows a higher capacity retention of 89.0% than LCO (50.9%) after 100 cycles. Furthermore, the change of the Fermi level obtained by using a kelvin probe force microscope (KPFM) and the oxygen band structure calculated by using density functional theory further illustrate that LSTP supports the performance of LCO. We anticipate that this study can improve the conversion efficiency of energy-storage devices.

Enhanced Li-Ion Diffusion and Cycling Stability of Ni-Free High-Entropy Spinel Oxide Anodes with High-Concentration Oxygen Vacancies.

High-entropy oxide (HEO) is an emerging type of anode material for lithium-ion batteries with excellent properties, where high-concentration oxygen vacancies can effectively enhance the diffusion coefficient of lithium ions. In this study, Ni-free spinel-type HEOs ((FeCoCrMnZn)3O4 and (FeCoCrMnMg)3O4) were prepared via ball milling, and the effects of zinc and magnesium on the concentration of oxygen vacancy (OV), lithium-ion diffusion coefficient (DLi+), and electrochemical performance of HEOs were investigated. Ab initio calculations show that the addition of zinc narrows down the band gap and thus improves the electrical conductivity. X-ray photoelectron spectroscopy (XPS) results show that (FeCoCrMnZn)3O4 (42.7%) and (FeCoCrMnMg)3O4 (42.5%) have high OV concentration. During charge/discharge, the OV concentration of (FeCoCrMnZn)3O4 is higher than that of (FeCoCrMnMg)3O4. The galvanostatic intermittent titration technique (GITT) results show that the DLi+ value of (FeCoCrMnZn)3O4 is higher than that of (FeCoCrMnMg)3O4 during charge and discharge. All of that can improve its specific discharge capacity and enhance its cycle stability. (FeCoCrMnZn)3O4 achieved a discharge capacity of 828.6 mAh g-1 at 2.0 A g-1 after 2000 cycles. This work provides a deep understanding of the structure and performance of HEO.

Adjusting Crystal Orientation to Promote Sodium-Ion Transport in V5 S8 @Graphene Anode Materials for High-Performance Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have inspired the potential for widespread use in energy storage owing to the advantages of abundant resources and low cost. Benefiting from the layered structure, 2D-layered materials enable fast interlayer transport of sodium ions and thus are considered promising candidates as anodes for SIBs. Herein, a strategy of adjusting crystal orientation is proposed via a solvothermal method to improve sodium-ion transport at the edge of the interlayers in 2D-layered materials. By introducing surfactants and templates, the 2D-layered V5 S8 nanosheets are controlled to align the interlayer diffusion channels vertically to the surface, which promotes the fast transport of Na+ at the edge of the interlayers as revealed by experimental methods and ab initio calculations. Benefiting from the aligned crystal orientation and rGO coating, the vertical-V5 S8 @rGO hybrid delivers a high initial discharge capacity of 350.6 mAh g-1 at a high current density of 15 A g-1 . This work provides a strategy for the structural design of 2D-layered anode materials by adjusting crystal orientation, which demonstrates the promise for applications in fast-charging alkaline-ion batteries.

Recent Advances on Heterojunction-Type Anode Materials for Lithium-/Sodium-Ion Batteries.

Rechargeable batteries are key in the field of electrochemical energy storage, and the development of advanced electrode materials is essential to meet the increasing demand of electrochemical energy storage devices with higher density of energy and power. Anode materials are the key components of batteries. However, the anode materials still suffer from several challenges such as low rate capability and poor cycling stability, limiting the development of high-energy and high-power batteries. In recent years, heterojunctions have received increasing attention from researchers as an emerging material, because the constructed heterostructures can significantly improve the rate capability and cycling stability of the materials. Although many research progress has been made in this field, it still lacks review articles that summarize this field in detail. Herein, this review presents the recent research progress of heterojunction-type anode materials, focusing on the application of various types of heterojunctions in lithium/sodium-ion batteries. Finally, the heterojunctions introduced in this review are summarized, and their future development is anticipated.

Self-assembled GeOX/Ti3C2TX Composites as Promising Anode Materials for Lithium Ion Batteries.

High-capacity germanium-based anode materials are alternative materials for outstanding electrochemical performance lithium-ion batteries (LIBs), but severe volume variation and pulverization problems during charging-discharging processes can seriously affect their electrochemical performance. In addressing this challenge, a simple strategy was used to prepare the self-assembled GeOX/Ti3C2TX composite in which the GeOX nanoparticles can grow directly on Ti3C2TX layers. Nanoscale GeOX uniformly renucleates on the surface and interlayers of Ti3C2TX, forming the stable multiphase structure, which guarantees its excellent electrochemical performance. Electrochemical evaluation has shown that the rate capability and reversibility of GeOX/Ti3C2TX are both greatly improved, which delivers a reversible discharge specific capacity of above 1400 mAh g-1 (at 100 mA g-1) and a reversible specific capacity of 900 mAh g-1 after 50 cycles while it still maintains a stable specific capacity of 725 mAh g-1 at 5000 mA g-1. Furthermore, the composite exhibits an exceptionally superior rate capability, making it a good electrochemical performance anode for LIBs.

In Situ-Formed Hollow Cobalt Sulfide Wrapped by Reduced Graphene Oxide as an Anode for High-Performance Lithium-Ion Batteries.

Transition-metal sulfides have been considered as promising anode materials for lithium-ion batteries (LIBs) due to their high theoretical specific capacity and superior electrochemical performance. However, the large volume change during the discharge/charge process causes structural pulverization, resulting in rapid capacity decline and the loss of active materials. Herein, we report Co1-xS hollow spheres formed by in situ growth on reduced graphene oxide layers. When evaluated as an anode material for LIBs, it delivers a specific capacity of 969.8 mAh·g-1 with a high Coulombic efficiency of 96.49% after 90 cycles. Furthermore, a high reversible capacity of 527.2 mAh·g-1 after the 107th cycle at a current density of 2.5 A g-1 is still achieved. The results illustrate that in situ growth on the graphene layers can enhance conductivity and restrain volume expansion of cobalt sulfide compared with ex situ growth.

Ultrahigh-Rate Behavior Anode Materials of MoSe2 Nanosheets Anchored on Dual-Heteroatoms Functionalized Graphene for Sodium-Ion Batteries.

MoSe2 is a prospective anode material for Na-ion batteries because of its layered structure and high theoretical capacity, while the unsatisfied electrochemical performance limits its further development. Herein, we report MoSe2 nanosheets anchored on dual-heteroatoms functionalized graphene by a solvothermal method. The heteroatoms and carbon matrix coexist in the form of graphitic-N/pyridinic-N/pyrrolic-N and P-C/P═O bonds, which result in excellent electronic conductivity of the materials and provide abundant active sites for electrochemical process. Results indicated that organic intercalation increased the layer spacing of the materials to facilitate sodium-ion diffusion, and the in situ formed carbon networks improved the conductivity among the layers of the materials and alleviated volume expansion during the continued charge and discharge process. As an anode of Na-ion batteries, the nanosheets materials exhibited ultrahigh rate performance and deliver capacities of approximately 200 mAh g-1 at the current density of 10 A g-1. The ultrahigh-rate performance can be attributed to its unique nanosheets structure, the dual-heteroatoms functionalized graphene, and the considerable pseudocapacitive quality of the material.

Formation and Effect of Residual Lithium Compounds on Li-Rich Cathode Material Li1.35[Ni0.35Mn0.65]O2.

Li-rich cathode materials are regarded as ideal cathode materials, owing to their excellent electrochemical capacity. However, residual lithium compounds, which are formed on the surface of the materials by reacting with moisture and carbon dioxide in ambient atmosphere, can impair the surface structure, injure the capacity, and impede the electrode fabrication using Li-rich materials. Exposure to air atmosphere causes the formation of residual lithium compounds; the formation of such compounds is believed to be related to humidity, temperature, and time during handling and storage. In this study, we demonstrated for the first time an artificial strategy for controlling time, temperature, and humidity to accelerate exposure. The formation and effect of residual lithium compounds on Li-rich cathode material Li1.35[Ni0.35Mn0.65]O2 were systematically investigated. The residual lithium compounds formed possessed primarily an amorphous structure and were partially coated on the surface. These compounds include LiOH, Li2O, and Li2CO3. Li2CO3 is the major component in residual lithium compounds. The presence of residual lithium compounds on the material surface led to a high discharge capacity loss and large discharge voltage fading. Understanding the formation and suppressing the effect of residual lithium compounds will help prevent their unfavorable effects and improve the electrochemical performance.

Multiple Linkage Modification of Lithium-Rich Layered Oxide Li1.2Mn0.54Ni0.13Co0.13O2 for Lithium Ion Battery.

A multiple linkage modification (MLM) method was investigated to comprehensively improve the properties of lithium-rich layered oxides. MLM Li1.2Mn0.54Ni0.13Co0.13O2 was successfully synthesized via continuous and appropriate heat treatment. The synthesized Li1.2Mn0.54Ni0.13Co0.13O2 particles were coated with a Li2ZrO3 layer and doped with Zr4+ by using a Zr compound as the MLM reagent. The Li2ZrO3 coating layer could protect materials from the corrosion of hydrogen fluoride, and the structure of the base materials was stabilized due to Zr4+ doping. In addition, the formation of Li2ZrO3 captured inactive residual lithium on the surface and absorbed lithium of host materials, thereby leading to the reduction in the Li/M ratio of materials and promoting the first-cycle Coulombic efficiency. The MLM material delivered the highest initial cycle Coulombic efficiency (∼85%), the best cycle and rate performance among bare and ZrO2-coated particles. These results indicate that MLM is an important technique for improving the performance of electrode materials.

Comparative Investigation of 0.5Li2MnO3·0.5LiNi0.5Co0.2Mn0.3O2 Cathode Materials Synthesized by Using Different Lithium Sources.

Lithium-rich manganese-based cathode materials has been attracted enormous interests as one of the most promising candidates of cathode materials for next-generation lithium ion batteries because of its high theoretic capacity and low cost. In this study, 0.5Li2MnO3·0.5LiNi0.5Co0.2Mn0.3O2 materials are synthesized through a solid-state reaction by using different lithium sources, and the synthesis process and the reaction mechanism are investigated in detail. The morphology, structure, and electrochemical performances of the material synthesized by using LiOH·H2O, Li2CO3, and CH3COOLi·2H2O have been analyzed by using Thermo gravimetric analysis (TGA), X-ray diffraction (XRD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. The 0.5Li2MnO3·0.5LiNi0.5Co0.2Mn0.3O2 material prepared by using LiOH·H2O displays uniform morphology with nano particle and stable layer structure so that it suppresses the first cycle irreversible reaction and structure transfer, and it delivers the best electrochemical performance. The results indicate that LiOH·H2O is the best choice for the synthesis of the 0.5Li2MnO3·0.5LiNi0.5Co0.2Mn0.3O2 material.

CNT-Decorated Na3V2(PO4)3 Microspheres as a High-Rate and Cycle-Stable Cathode Material for Sodium Ion Batteries.

A novel cathode material, carbon nanotube (CNT)-decorated Na3V2(PO4)3 (NVP) microspheres, was designed and synthesized via spray-drying and carbothermal reduction methods. The microspheres were covered and embedded by CNTs, the surfaces of which were also covered by amorphous carbon layers. Thus, a carbon network composed of CNTs and amorphous carbon layers formed in the materials. The polarization of a 10 wt % CNT-decorated NVP (NVP/C10) electrode was much less compared with that of the electrode with pristine NVP without CNTs. The capacity of the NVP/C10 electrode only decreased from 103.2 to 76.2 mAh g-1 when the current rates increased from 0.2 to 60 C. Even when cycled at a rate of 20 C, the initial discharge capacity of the NVP/C10 electrode was as high as 91.2 mAh g-1, and the discharge capacity was 76.9 mAh g-1 after 150 cycles. The charge-transfer resistance and ohmic resistance became smaller because of CNT decorating. Meanwhile, the addition of CNTs can tune the size of the NVP particles and increase the contact area between NVP and the electrolyte. Consequently, the resulted NVP had a larger sodium ion diffusion coefficient than that of the pristine NVP.

VOPO4 nanosheets as anode materials for lithium-ion batteries.

VOPO4 nanosheets are successfully synthesized using a hydrothermal method followed by calcination. The XRD results reveal that obtained products are crystallized in the orthorhombic VOPO4 phase. SEM and TEM images demonstrate that VOPO4 products possess unique nanosheet morphology. Electrochemical tests show that the VOPO4 nanosheets exhibit an excellent electrochemical performance as anode materials. They can deliver an initial discharge capacity of 1356 mA h g(-1) at 0.1 C, and possess a favorable capacity at rates of 0.2, 1, and 2 C. The synthesized VOPO4 can be used as a good anode material; furthermore, the nanosheet structure can effectively improve the electrochemical performance of this material.

Comparative investigation of phosphate-based composite cathode materials for lithium-ion batteries.

Li3V2(PO4)3-LiVPO4F, LiFePO4-Li3V2(PO4)3, and LiFePO4-Li3V2(PO4)3-LiVPO4F composite cathode materials are synthesized through mechanically activated chemical reduction followed by annealing. X-ray diffraction (XRD) results reveal that the obtained products are pure phase, and the molar ratio of each phase in the composites is consistent with that in raw material. Transmission electron microscopy (TEM) images show that each phase coexists in the composites. The LiFePO4-Li3V2(PO4)3-LiVPO4F composites exhibit the best electrochemical performance. These composites can deliver a capacity of 164 mAh g(-1) at 0.1 C and possess favorable capacities at rates of 0.5, 1, and 5 C. The excellent electrochemical performance is attributed to the mutual modification and the synergistic effects.

Electrochemical properties of VPO4/C nanosheets and microspheres as anode materials for lithium-ion batteries.

VPO4/C nanosheets and microspheres are successfully synthesized via a hydrothermal method followed by calcinations. The XRD results reveal that the obtained products both have an orthorhombic VPO4 phase. The SEM and TEM images demonstrate that nanosheets and spherical morphology can be obtained by controlling the synthesis conditions. The samples are both uniformly coated by amorphous carbon. The electrochemical test results show that the sample with a nanosheet structure has a better electrochemical performance than the microsphere samples. The VPO4/C nanosheets can deliver an initial discharge capacity of 788.7 mAh g(-1) at 0.05 C and possessed a favorable capacity at the rates of 1, 2, and 4 C. The nanosheet structure can effectively improve the electrochemical performances of VPO4/C anode materials.