Rechargeable Aqueous Zinc-ion Batteries Research Articles

Releasing plating-induced stress for highly reversible aqueous Zn metal anodes

Rechargeable aqueous zinc-ion batteries (ZIBs) have become one of the most potential technologies for grid-scale energy storage systems. The practical application, however, has been severely plagued by the uncontrollable dendrite growth and side reactions on ZIB anodes with planar structure, making it urgent to develop efficient strategies to stabilize Zn anodes. Herein, it is revealed that the Zn dendrite growth is strongly associated with the internal stress accumulation during Zn plating. Moreover, a Zn metal anode with patterned micro-grooves structure that can effectively release the plating-induced stress and inhibit the dendrite growth. Notably, the groove-patterned Zn anodes coupling with Nafion film coating could effectively depress the side reactions and present ultra-stable cycling over 1200 h at a high current density of 10 mA cm−2, a low electrochemical polarization for the rate performance range from 1 to 20 mA cm−2 as well as a stable voltage hysteresis in deep discharge/charge performance at 10 mA cm−2 and 10 mAh cm−2. The feasibility of the proposed strategy is further confirmed and implemented in the Zn|MnO2 flexible pouch cell which delivers an impressive capacity of 186 mAh g−1, high electrochemical stability, and outstanding mechanical flexibility. The finding of this study is expected to provide a deep insight into the design of highly reversible aqueous Zn-ion batteries.

Structural engineering of α-MnO2 cathode by Ag+ incorporation for high capacity aqueous zinc-ion batteries

Rechargeable aqueous zinc-ion batteries (ZIBs) are considered as renewable alternatives to lithium-ion batteries (LIBs) for large-scale energy storage applications due to their intrinsic safety, cost-effectiveness, and environmental friendliness. The design and development of cathode materials are the main challenges to realizing the full potential of aqueous ZIBs. Here, we design Ag0.4Mn8O16 nanorods by structural engineering of α-MnO2 via Ag + incorporation for ZIBs cathode material. The sliver species are well dispersed in the tunnels of α-MnO2, which results in higher surface area, smaller nanorods, and oxygen vacancies in the Ag0.4Mn8O16 structure. Along with the redox activity of Ag+, reversible co-insertion/extraction of H+/Zn2+ ions in Ag0.4Mn8O16 followed by the formation of ZnxAg0.4-xMn8O16/MnOOH are observed. The generation of in-situ silver matrix greatly enhances the electronic conductivity, which, together with the reversible Ag0.4Mn8O16 structure, reveals an excellent electrochemical performance such as excellent rate capability, the high specific capacity of 306 mAh g−1 at 0.1 A g−1, and long-term cycling life over 800 cycles at 1 A g−1 compared to α-MnO2 (227 mAh g−1 at 0.1 A g−1). This work provides the design of cathode materials and the corresponding energy storage mechanism options for rechargeable aqueous ZIBs.

Effect of heat treatment on the electrochemical performance of V2O5·nH2O as a cathode material for aqueous rechargeable zinc ion batteries

The influence of heat treatment (100 °C, 150 °C, 200 °C, 300 °C, and 400 °C) on the structure and the zinc ion storage performance of V2O5·nH2O is explored. The results show that the interlayer water content, interlayer distance, and V4+ content in V2O5·nH2O can be regulated by heat treatment. Electrochemical characterization demonstrates that the as-prepared vanadium pentoxide without heat treatment shows rather poor cycling stability (only 56 m Ah g−1 after 600 cycles at 0.5 A/g). After being heat treated in the temperature range of 100 to 200 °C, the zinc ion storage performance of vanadium pentoxide can be enhanced dramatically. Particularly, the vanadium pentoxide after being heat-treated at 150 °C exhibits the best cycling stability (179 mA h g−1 after 600 cycles at 0.5 A/g), superior high-rate capability (117 mA h g−1 at 3.0 A/g), and faster electrochemical reaction kinetics. The results reported in this work could provide clues for regulating and optimizing the electrochemical performance of vanadium pentoxide as a cathode material for ZIBs.

In-situ oriented oxygen-defect-rich Mn[sbnd]N[sbnd]O via nitridation and electrochemical oxidation based on industrial-scale Mn2O3 to achieve high-performance aqueous zinc ion battery

As a general problem in the field of batteries, materials produced on a large industrial scale usually possess unsatisfactory electrochemical performances. Among them, manganese-based aqueous rechargeable zinc-ion batteries (ARZBs) have been emerging as promising large-scale energy storage systems owing to their high energy densities, low manufacturing cost and intrinsic high safety. However, the direct application of industrial-scale Mn2O3 (MO) cathode exhibits poor electrochemical performance especially at high current rates. Herein, a highly reversible Mn-based cathode is developed from the industrial-scale MO by nitridation and following electrochemical oxidation, which triples the ion diffusion rate and greatly promotes the charge transfer. Notably, the cathode delivers a capacity of 161 mAh g−1 at a high current density of 10 A g−1, nearly-three times the capacity of pristine MO (60 mAh g−1). Impressive specific capacity (243.4 mAh g−1) is obtained without Mn2+ additive added in the electrolyte, much superior to the pristine MO (124.5 mAh g−1), suggesting its enhanced reaction kinetics and structural stability. In addition, it possesses an outstanding energy output of 368.4 Wh kg−1 at 387.8 W kg−1, which exceeds many of reported cathodes in ARZBs, providing new opportunities for the large-scale application of high-performance and low-cost ARZBs.

Pulsed electro-synthesized tunable crystallite sizes ZnMn2O4/Mn2O3 nanocomposite as high-performance cathode material for aqueous zinc-ion batteries

Rechargeable aqueous zinc ion batteries (ZIBs) are one of the most promising candidates for large-scale energy storage, due to their high capacity, high hydrogen evolution potential, safe and cost-effective components while benefiting from the abundance of active materials. However, ZIBs still suffer from suitable cathode materials with high capacity and superior cycling stability. Herein, we study a new strategy for the synthesis of tunable crystallite sizes ZnMn2O4/Mn2O3 nanocomposite using the pulsed potential method. The effect of synthesis potential on structural, morphological, and electrochemical properties of nanocomposite was investigated via different characterization techniques. According to our results, the functional and electrochemical properties of ZnMn2O4/Mn2O3 nanocomposite are profoundly affected by the synthesis potential. The crystallite sizes of synthesized nanocomposites can be tuned from 20.2 nm to 29.3 nm, depending on the applied potential. However, the exposed surface area of nanomaterials is ranged between 50.3 m2 g−1 and 132.0 m2 g−1. The developed nanocomposite was investigated as the cathode material for aqueous zinc ion batteries. The ZnMn2O4/Mn2O3 nanocomposite exhibited an excellent discharge capacity of 216.8 mAh g−1 at 0.2 A g−1 after 200 cycles. In addition, the developed cathode showed to preserve 97.8% of its initial capacity for 2000 cycles at 2 A g−1. Apart from the high specific capacity and long cycle stability, the facile synthesis method can offer the ZnMn2O4/Mn2O3 nanocomposite as a high-performance cathode material for durable zinc ion batteries.

Dendrite suppression with zirconium (IV) based metal–organic frameworks modified glass microfiber separator for ultralong-life rechargeable zinc-ion batteries

Metal–organic frameworks (MOFs) are increasingly employed in designing an active material for various battery systems due to the variety of structures, morphology, and controllable composition at a molecular level. Moreover, their high porosity enables a facile electrolyte penetration and ion transportation. In this work, we focus on the performance and life cycle of aqueous rechargeable zinc-ion batteries (ZIBs) using zirconium (IV) based MOFs, including UiO-66 and MOF-808 modified glass microfiber separators. The results demonstrated that the symmetrical cell using MOF-808 modified separator extended a relatively stable voltage plateau of symmetric Zn battery over 350 h owing to improved uniformity of Zn ion flux during striping/plating processes. For the capacity retention at the current density of 1.0 A g −1 , the ZIBs using UiO-66@GFC and MOF-808@GFC exhibited higher specific capacities than that using the pristine separator. In addition, the ZIBs with MOF-808 modified separators could prolong the cycle life to as high as 8000 cycles because the modified separators could inhibit non-uniform dendritic zinc growth on a zinc anode surface according to scanning electron micrographs. Hence, the UiO-66 and MOF-808 modified glass microfibers as separators are promising materials for high-performance aqueous zinc-ion batteries with long-life cycles. • No short-circuit of Zn|MnO 2 is achieved for over 8000 cycles at 1 A g −1 . • MOFs modified separators exhibit uniform Zn flux on striping/plating processes after cycling for 1000 cycles. • The percentage of capacitive retention of MOFs modified separators retain 50% after 3000 cycles. • MOFs modified separators were successfully by the one-step dip-coating process.

Inorganic manganese oxide/quinone coupling for high-capacity aqueous Zn-ion battery

Rechargeable aqueous Zinc-ion batteries belong to a hot research topic on energy storage due to its high safety and low cost. However, its practical application is still remarkably limited by the fact that cathode capacity is much lower than that of zinc anode, leading to unsatisfactory energy density far away from the commercial standard. In this context, we have proposed an inorganic oxide/organic quinone coupling strategy to prepare MnO2@PDAAQ composite cathode. Significantly enhanced performance superior to most of conventional cathode materials was identified in this MnO2@PDAAQ with an impressive high capacity of 414 mAh g−1 and energy density of 537.9 Wh kg−1. We clarified such a coupling from MnO2 and quinone results in multiple energy storage mechanisms including Zn2+/H+ co-insertion in MnO2 and two-electron dissolution/deposition mechanism of Mn4+/Mn2+ redox reaction in this composite. The proposed inorganic oxide/organic quinone coupling strategy opens up a new route for the electrode design of high-performance aqueous batteries.

Rechargeable aqueous zinc ion battery based on NaV6O15 nanorods by surfactant-assisted method

Vanadium oxides have a wide range of applications in aqueous zinc ion batteries due to their abundant resources and multiple valence states. However, the inherent low conductivity and structural instability of Vanadium-based materials often lead to the rapid degradation of their capacity. Consequently, stable microstructure and coordinated ion transport channels are essential to improve electrochemical performance. In this work, we report a surfactant-assisted hydrothermal method to obtain NaV6O15 with rod-like morphology and good dispersion. The surfactant-assisted NaV6O15 (NVO-I) electrode exhibited better cycle stability than the electrode without surfactant (NVO-II) during long cycles. The ordered morphology facilitated contact between the electrode and electrolyte, so the NVO-I electrode showed high electrochemical performance. The reversible capacity was 290.2 mAh g−1 at 0.1 A g−1, and the capacity retention was 95 % after 900 cycles at 1.0 A g−1. The surfactant-assisted hydrothermal strategy provides more options for developing stable and efficient AZIBs than other techniques.

Enhanced charge transfer and reaction kinetics of vanadium pentoxide for zinc storage via nitrogen interstitial doping

Rechargeable aqueous zinc-ion batteries (ZIBs) are the prospective substitution for lithium-ion batteries applied in large scale energy storage system due to their low-cost, environmentally friendliness, and high safety. However, the development of cathodes in aqueous ZIBs suffers from sluggish Zn2+ migration. Herein, nitrogen doped V2O5 is introduced to resolve the above problem. N-doping lowers the bandgap energy of V2O5 to improve its electronic conductivity, and weakens the forces between Zn2+ and V2O5 to fasten Zn2+ diffusion. Further density functional theory (DFT) calculation testifies that N-doping reduces diffusion energy barrier and changes Zn2+ diffusion pathway from the vertical interlayer diffusion to planer intralayer diffusion. Meanwhile, the structural stability of electrode material also benefits from the N-doping, which can prevent the interlayer V2O5 from gliding or exfoliation during cycling. Profiting from these merits, N-doping V2O5 exhibits the outstanding electrochemical properties, such as high rate capability (116.8 mAh/g at 6 A/g) and long cycling performance (3000 cycles at 10 A/g). Dynamics and post-cycling analyses reveal the high capacitive ratio and the stable N distribution in N-doped V2O5 during charging/discharging.

Rose-like VS2 Nanosheets Chemically Anchored on Carbon Nanotubes for Flexible Zinc-Ion Batteries with Enhanced Properties.

Rechargeable aqueous zinc-ion batteries (ZIBs) are an attractive alternative for flexible energy storage devices due to their excellent safety and low cost. One of the main challenges that plagues their practical applications is the restricted variety of cathode materials with fast reaction kinetics and good mechanical properties. Herein, we prepared rose-like VS2 nanosheets which have decent specific capacities, metallic conductivity, and open-enough channels and further incorporated them into a single-walled carbon nanotube (SWCNT) network, achieving a C-V chemical-bonded freestanding VS2@SWCNT (C-VS2) composite. Such chemical bonding in the composites builds a bridge for rapid electron transfer and ion diffusion in the longitudinal direction from one layer to another layer. As a result, the reversible Zn/C-VS2 system in core cells exhibits a high specific capacity (205.3 mA h g-1 at 0.1 A g-1), an excellent cyclic stability (115.4 mA h g-1 was obtained after 1500 cycles at 5 A g-1), and a remarkable rate capability (135.4 mA h g-1 at 10 A g-1). Furthermore, the freestanding C-VS2 films with good flexibility and conductivity can serve as a flexible cathode to assemble soft-packaged ZIBs. Meanwhile, the soft-packaged ZIB has good electrochemical stability even under different bending conditions (the discharge capacity dropped by only 2.1 mA h g-1 after bending). This work offers insights into the rational design of zinc-ion hosts throughout chemical bond engineering.

Calcium-organic frameworks cathode for high-stable aqueous Zn/organic batteries

Rechargeable aqueous zinc-ion batteries (AZIBs) are attracting tremendous attention because of their intrinsic merits such as high safety and low cost. Cathode plays a critical role in enhancing the electrochemical performance of AZIBs. However, it is difficult to design a robust and high-efficiency cathode material and further implement the commercialization of AZIBs. Metal-organic frameworks (MOFs) electroactive compounds are attractive to serve as the cathode of AZIBs due to their unique porosity and crystal structures, resource renewability and structural diversity. In this work, a calcium-pure terephthalates acid framework (Ca-PTA·3H2O) was synthesized by facile hydrolysis and cationic exchange method, then explored as a novel cathode for AZIBs. The results highlight a high specific capacity of 431 mAh/g (0.51 mAh/cm2) at a current density of 50 mA/g, and excellent cycle performance with capacity retention of ∼90% after 2700 cycles at 500 mA/g. The following up characterizations investigate the reversible zinc storage mechanism in detail. This experiment made a specific contribution to the exploration of the new MOF as a competitive cathode for AZIBs.

An artificial zinc phosphide interface toward stable zinc anodes

Rechargeable aqueous zinc-ion batteries with the characteristics of low cost and high capacity represent a promising device for the grid-scale energy storage system. However, the deleterious dendrite growth and side reactions of the Zn metal anode are hindering its development. Herein, artificial Zn3P2 interphase is constructed on Zn metal to guide rapid and homogeneous Zn deposition via the electrochemical anodic oxidation combined with the phosphating strategy for highly reversible Zn metal batteries. The in-situ construction of artificial interphase could kinetically homogenize Zn nucleation and protect the zinc metal from corrosion and side reactions. As expected, a symmetrical cell with the Zn3P2-armed Zn anode delivers highly reversible Zn plating/stripping with a lifespan of over 1000 h. Moreover, the protected Zn||MnO2 full cells demonstrate remarkable durability with 91.8% capacity retention after 200 cycles. This work paves the way to rationally design artificial interphases for highly reversible aqueous Zn metal batteries.

Proton-assisted mixed-valence vanadium oxides cathode with long-term stability for rechargeable aqueous zinc ion batteries

In this work, we developed a rGO assisted vanadium oxide xerogel (abbreviate as VOHG) as cathode material for rechargeable aqueous zinc ion batteries (RA-ZIBs). The VOHG cathode exhibits an ultra-high specific capacity of 625 mA h g−1 at current density of 0.1 A g−1, superior reversibility and a long cycle life with a coulombic efficiency of 100.0% after 10,000 cycles. The VOHG material has mixed valent vanadium ions V4+/V5+and a layer structure with interlayer water. The interlayer water acts as soft support for the VO6 octahedron, forming a large space in the layers for H+/Zn2+ intercalation, which improves the migration rate of Zn2+ due to its charge shielding effect. Protons can move quickly through the interlayer water network by Grotthuss proton transport mechanism. The application of the unique structure of VOHG makes zinc ions battery will be a promising candidate of energy storage device with high-capacity and long cycle stability.

Tannic acid assisted metal-chelate interphase toward highly stable Zn metal anodes in rechargeable aqueous zinc-ion batteries.

Aqueous zinc-ion batteries (AZIBs) have attracted extensive attention because of their eco-friendliness, intrinsic safety, and high theoretical capacity. Nevertheless, the long-standing Zn anode issues such as dendrite growth, hydrogen evolution, and passivation greatly restrict the further development of AZIBs. Herein, a metal–chelate interphase with high Zn affinity is constructed on the Zn metal surface (TA@Zn) via dipping metallic Zn into a tannic acid (TA) solution to address the aforementioned problems. Benefiting from the abundant hydrophilic and zincophilic phenolic hydroxyl groups of TA molecules, the metal–chelate interphase shows strong attraction for Zn2+ ions, guiding uniform zinc deposition as well as decreasing Zn2+ migration barrier. Therefore, the TA@Zn anode displays an extended lifespan of 850 h at 1 mA cm−2, 1 mAh cm−2 in the Zn|Zn symmetrical cell, and a high Coulombic efficiency of 96.8% in the Zn|Ti asymmetric cell. Furthermore, the Zn|V2O5 full cell using TA@Zn anode delivers an extremely high capacity retention of 95.9% after 750 cycles at 2 A g−1. This simple and effective strategy broadens the interfacial modification scope on Zn metal anodes for advanced rechargeable Zn metal batteries.

Flexible hydrogel compound of V2O5/GO/PVA for enhancing mechanical and zinc storage performances

A highly flexible hydrogel cathode applied in rechargeable aqueous zinc-ion batteries (ZIBs) fabricated by polyvinyl alcohol (PVA), vanadium pentoxide (V2O5) and graphene oxide (GO) has been successfully synthesized by repeated freeze-thaw method. The V2O5/GO hydrogel (VOGH) presented complex 3D cross-linking system and excellent flexibility through hydrogen bonding and inorganic nanosheets reinforcement. The mechanical and electrochemical properties of VOGH are balanced and achieved by adjusting the content of GO (5%), V2O5 (30%), PVA (25%) and water (40%) of the total mass, respectively. From the electrochemical result, the flexible hydrogel cathode displayed a high capacity (240.5 mA h g–1) and a good cyclic stability after 200 cycles at 1 A g–1. Moreover, the electrochemical kinetics exhibite the capacitive control process, which confirm the excellent ion transport to enhance storage performance of ZIBs. The soft package ZIBs using VOGH as cathode and Zn foil as anode display good cycle performance (191.7 mA h g–1 at 1 A g–1 after 1000 cycles) and provide stable power supply at different bending angles.

Defect Engineered Ternary Spinel: An Efficient Cathode for an Aqueous Rechargeable Zinc-Ion Battery of Long-Term Cyclability.

The rational defect engineering of Mn-based spinel cathode materials by metal-ion doping and vacancy creation fosters reversible intercalation/deintercalation of charge carriers and boosts the charge storage performance of an aqueous rechargeable zinc-ion battery (ZIB). Herein, we demonstrate the Zn2+ ion storage performance of a defect-engineered ternary spinel cathode based on Zn, Ni, and Mn. The defect engineering of ZnMn2O4 is achieved by Ni2+ doping and creating a cation (Mn3+ and Zn2+) deficiency. The engineered cathode material has cubic spinel structure in contrast to the defect-free ZnMn2O4. The DFT studies show that the defect engineering modifies the electronic structure and improves the electronic conductivity. An aqueous rechargeable ZIB is fabricated by using the spinel cathode, and its performance is evaluated in terms of charge-discharge cycling stability, specific capacity, and so on. The ternary spinel-based ZIB has a very long charge-discharge cycling stability with a specific capacity as high as 265 mAh g-1 (at 100 mA g-1). A 2-fold enhancement in the specific capacity is observed after 5000 cycles. Ni doping affords ultralong cycling stability. The self-discharge studies for a year show that the device retains 63% of the initial performance.

Phase transformation mechanism of MnCO3 as cathode materials for aqueous zinc-ion batteries.

Aqueous rechargeable zinc-ion batteries (ZIBs) have been given more and more attention because of their high specific capacity, high safety, and low cost. The reasonable design of Mn-based cathode materials is an effective way to improve the performance of ZIBs. Herein, a square block MnCO3 electrode material is synthesized on the surface of carbon cloth by a one-step hydrothermal method. The phase transition of MnCO3 was accompanied by the continuous increase of specific capacity, and finally maintained good cycle stability in the charge–discharge process. The maximum specific capacity of MnCO3 electrode material can reach 83.62 mAh g−1 at 1 A g−1. The retention rate of the capacity can reach 85.24% after 1,500 cycles compared with the stable capacity (the capacity is 61.44 mAh g−1 under the 270th cycle). Ex situ characterization indicates that the initial MnCO3 gradually transformed into MnO2 accompanied by the embedding and stripping of H+ and Zn2+ in charge and discharge. When MnCO3 is no longer transformed into MnO2, the cycle tends to be stable. The phase transformation of MnCO3 could provide a new research idea for improving the performance of electrode materials for energy devices.

A high-performance silver-doped manganese oxide nanowire cathode based on combination displacement/intercalation reaction and deposition-dissolution mechanism

Rechargeable aqueous zinc ion batteries (ZIBs) have attracted increasing interest in large-scale energy storage systems due to their low cost, high safety, and high volumetric energy density. Manganese-based materials are one of most promising aqueous zinc ion battery cathode materials while their practical applications in ZIBs are limited by their low electrical conductivity, poor cycling stability and controversial energy storage mechanism. In this work, Ag0.8Mn8O16, a kind of manganese-based cathode material with combination displacement/intercalation reaction was prepared by using a hydrothermal method. Due to combination displacement/intercalation reaction mechanism, the generation of metallic silver during the discharge process greatly enhances the electronic conductivity of the material and thus the Ag0.8Mn8O16 exhibits excellent rate performance and a reversible capacity of 198 mA h g−1 at 2 A g−1 after 1000 cycles. In addition, Ag0.8Mn8O16 shows an interesting capacity rise at low current density and through a series of comparative experiments, the existence of deposition dissolution mechanism in Zn/Ag0.8Mn8O16 battery was confirmed to be a critical factor of capacity increase.

In situ constructing amorphous V2O5@Ti3C2Tx heterostructure for high-performance aqueous zinc-ion batteries

Rechargeable aqueous zinc-ion batteries are emerged as fascinating “beyond Li-ion” battery technologies for large-scale energy storage due to low cost, high theoretical capacity, high safety and environmental friendliness. However, developing cathode materials with outstanding electrochemical performance are prerequisites. Herein, the a-V2O5@Ti3C2Tx heterostructure is prepared by in situ incorporating amorphous V2O5 with Ti3C2Tx MXene to enhance the rechargeable aqueous Zn-ion batteries performance. Benefitting from the distinctive amorphous structure of a-V2O5, outstanding conductivity of Ti3C2Tx MXene and the strong synergistic effect between two components, the a-V2O5@Ti3C2Tx heterostructure presents isotropic ion diffusion paths, plentiful ion storage sites and splendid conductivity. As a consequence, the a-V2O5@Ti3C2Tx heterostructure displays excellent specific capacity of 544 mAh g−1 at 0.5 A g−1 and outstanding rate performance. Particularly, even at a fairly high current density of 30 A g−1, the a-V2O5@Ti3C2Tx heterostructure shows an impressive specific capacity of 135 mAh g−1 along with about 100% coulombic efficiency after 1000 cycles. Furthermore, the mechanism related is expounded via comprehensive characterizations. Therefore, this work offers a way for developing high-performance vanadium-based cathode materials for aqueous zinc-ion batteries.

A strategy to control crystal water content in hydrated vanadium oxide cathode for promoting aqueous rechargeable zinc-ion batteries

A simple hydrothermal route is used to prepare 3D porous hydrated vanadium oxide porous microspheres with different crystal water content. It is the first report that too much water molecule can lead to poor Zn storage performance and optimal structural water content is needed to improve electrochemical performance. By tuning the ratio of Zn to V during the synthesized process, Zn0.146V2O5∙0.579 H2O porous microspheres with optimal structural water content exhibit superior electrochemical performance for a promising aqueous rechargeable Zn-ion batteries. At the current density of 0.1 A g−1, it can register a high reversible discharge capacity of 416 mAh g−1. An initial discharge capacity can reach 192 mAh g−1 at the current density of 10.0 A g−1, the specific capacity remains at 202 mAh g−1 and the capacity retention is 105% after 10,000 cycles. Optimal structural water content as well as synergistic effect of pre-intercalated Zn2+ ions should be responsible for excellent electrochemical performance of cathode materials.