Mixed Polyanionic Compounds as Positive Electrodes for Low‐Cost Electrochemical Energy Storage

Rational Design of High-Loading Electrodes with Superior Performances Toward Practical Application for Energy Storage Devices

High-loading electrodes play a crucial role in designing practical high-energy batteries as they reduce the proportion of non-active materials, such as current separators, collectors, and battery packaging components. This design approach not only enhances battery performance but also facilitates faster processing and assembly, ultimately leading to reduced production costs. Despite the existing strategies to improve rechargeable battery performance, which mainly focus on novel electrode materials and high-performance electrolyte, most reported high electrochemical performances are achieved with low loading of active materials (<2 mg cm-2). Such low loading, however, fails to meet application requirements. Moreover, when attempting to scale up the loading of active materials, significant challenges are identified, including sluggish ion diffusion and electron conduction kinetics, volume expansion, high reaction barriers, and limitations associated with conventional electrode preparation processes. Unfortunately, these issues are often overlooked. In this review, the mechanisms responsible for the decay in the electrochemical performance of high-loading electrodes are thoroughly discussed. Additionally, efficient solutions, such as doping and structural design, are summarized to address these challenges. Drawing from the current achievements, this review proposes future directions for development and identifies technological challenges that must be tackled to facilitate the commercialization of high-energy-density rechargeable batteries.

Recent Advances and Perspectives on the Promising High-Voltage Cathode Material of Na3 (VO)2 (PO4 )2 F

Na3 (VO)2 (PO4 )2 F (NVOPF) has emerged as one of the most promising cathode materials for sodium-ion batteries (SIBs) attributed to its high specific capacity (130 mAh g-1 ), high operation voltage (>3.9 V vs Na+ /Na), and excellent structural stability (<2% volume change). However, the comparatively low intrinsic electronic conductivity (≈10-7 S cm-1 ) of NVOPF leads to unsatisfactory electrochemical performance, especially at high rates, limiting its practical applications. To improve the conductivity and enhance Na storage performance, many efforts have been devoted to designing NVOPF, including morphology optimization, hybridization with conductive materials, metal-ion doping, Na-site regulation, and F/O ratio adjustment. These attempts have shown some encouraging achievements and shed light on the practical application of NVOPF cathodes. This work aims to provide a general introduction, synthetic methods, and rational design of NVOPF to give a deeper understanding of the recent progress. Additionally, the unique microstructure of NVOPF and its relationship with Na storage properties are also described in detail. The current status, as well as the advances and limitations of such SIB cathode material, are reported. Finally, future perspectives and guidance for advancing high-performance NVOPF cathodes toward practical applications are presented.

Pearl-Structure-Enhanced NASICON Cathode toward Ultrastable Sodium-Ion Batteries

Based on the favorable ionic conductivity and structural stability, sodium superionic conductor (NASICON) materials especially utilizing multivalent redox reaction of vanadium are one of the most promising cathodes in sodium-ion batteries (SIBs). To further boost their application in large-scale energy storage production, a rational strategy is to tailor vanadium with earth-abundant and cheap elements (such as Fe, Mn), reducing the cost and toxicity of vanadium-based NASICON materials. Here, the Na_3.05 V_1.03 Fe_0.97 (PO₄ )₃ (NVFP) is synthesized with highly conductive Ketjen Black (KB) by ball-milling assisted sol-gel method. The pearl-like KB branch chains encircle the NVFP (p-NVFP), the segregated particles possess promoted overall conductivity, balanced charge, and modulated crystal structure during electrochemical progress. The p-NVFP obtains significantly enhanced ion diffusion ability and low volume change (2.99%). Meanwhile, it delivers a durable cycling performance (87.7% capacity retention over 5000 cycles at 5 C) in half cells. Surprisingly, the full cells of p-NVFP reveal a remarkable capability of 84.9 mAh g^-1 at 20 C with good cycling performance (capacity decay rate is 0.016% per cycle at 2 C). The structure modulation of the p-NVFP provides a rational design on the superiority of others to be put into practice.

N-Doped Carbon Fibers Derived from Porous Wood Fibers Encapsulated in a Zeolitic Imidazolate Framework as an Electrode Material for Supercapacitors

Developing highly porous and conductive carbon electrodes is crucial for high-performance electrochemical double-layer capacitors. We provide a method for preparing supercapacitor electrode materials using zeolitic imidazolate framework-8 (ZIF-8)-coated wood fibers. The material has high nitrogen (N)-doping content and a specific surface area of 593.52 m² g^-1. When used as a supercapacitor electrode, the composite exhibits a high specific capacitance of 270.74 F g^-1, with an excellent capacitance retention rate of 98.4% after 10,000 cycles. The symmetrical supercapacitors (SSCs) with two carbon fiber electrodes (CWFZ2) showed a high power density of 2272.73 W kg^-1 (at an energy density of 2.46 W h kg^-1) and an energy density of 4.15 Wh kg^-1 (at a power density of 113.64 W kg^-1). Moreover, the SSCs maintained 81.21% of the initial capacitance after 10,000 cycles at a current density of 10 A g^-1, which proves that the SSCs have good cycle stability. The excellent capacitance performance is primarily attributed to the high conductivity and N source provided by the zeolite imidazole framework. Because of this carbon material's unique structural features and N-doping, our obtained CWFZ2 electrode material could be a candidate for high-performance supercapacitor electrode materials.

Borate-Based Compounds as Mixed Polyanion Cathode Materials for Advanced Batteries

Rational design of new and cost-effective advanced batteries for the intended scale of application is concurrent with cathode materials development. Foundational knowledge of cathode materials’ processing−structure−properties−performance relationship is integral. In this review, we provide an overview of borate-based compounds as possible mixed polyanion cathode materials in organic electrolyte metal-ion batteries. A recapitulation of lithium-ion battery (LIB) cathode materials development provides that rationale. The combined method of data mining and high-throughput ab initio computing was briefly discussed to derive how carbonate-based compounds in sidorenkite structure were suggested. Borate-based compounds, albeit just close to stability (viz., <30 meV at−1), offer tunability and versatility and hence, potential effectivity as polyanion cathodes due to (1) diverse structures which can host alkali metal intercalation; (2) the low weight of borate relative to mature polyanion families which can translate to higher theoretical capacity; and a (3) rich chemistry which can alter the inductive effect on earth-abundant transition metals (e.g., Ni and Fe), potentially improving the open-circuit voltage (OCV) of the cell. This review paper provides a reference on the structures, properties, and synthesis routes of known borate-based compounds [viz., borophosphate (BPO), borosilicate (BSiO), and borosulfate (BSO)], as these borate-based compounds are untapped despite their potential for mixed polyanion cathode materials for advanced batteries.

Elastic Buffering Layer on CuS Enabling High-Rate and Long-Life Sodium-Ion Storage

The latest view suggests the inactive core, surface pulverization, and polysulfide shuttling effect of metal sulfides are responsible for their low capacity and poor cycling performance in sodium-ion batteries (SIBs). Whereas overcoming the above problems based on conventional nanoengineering is not efficient enough. In this work, erythrocyte-like CuS microspheres with an elastic buffering layer of ultrathin polyaniline (PANI) were synthesized through one-step self-assembly growth, followed by in situ polymerization of aniline. When CuS@PANI is used as anode electrode in SIBs, it delivers high capacity, ultrahigh rate capability (500 mAh g^-1 at 0.1 A g^-1, and 214.5 mAh g^-1 at 40 A g^-1), and superior cycling life of over 7500 cycles at 20 A g^-1. A series of in/ex situ characterization techniques were applied to investigate the structural evolution and sodium-ion storage mechanism. The PANI swollen with electrolyte can stabilize solid electrolyte interface layer, benefit the ion transport/charge transfer at the PANI/electrolyte interface, and restrain the size growth of Cu particles in confined space. Moreover, finite element analyses and density functional simulations confirm that the PANI film effectively buffers the volume expansion, suppresses the surface pulverization, and traps the polysulfide.

Unusual Size Effect in Ion and Charge Transport in Micron-Sized Particulate Aluminum Anodes of Lithium-Ion Batteries

Aluminum is a promising anode material for lithium-ion batteries owing to its high theoretical capacity, excellent conductivity, and natural abundance. An anomalous size effect was observed for micron-sized aluminum powder electrodes in this work. Experimental and theoretical investigations reveal that the insulating oxide surface layer is the underlying cause, which leads to poor electrical conductivity and limited capacity utilization when the particle is too small. Additionally, poor electrolyte wettability also accounts for the hindered reaction kinetics due to the weak polarity feature of the oxide layer. Surface grafting of polar amino groups was demonstrated to be an effective strategy to improve electrolyte wettability. The present work revealed the critical limitations and underlying mechanisms for the aluminum anode, which is crucial for its practical application. Our results are also valuable for other metallic anodes with similar issues.

Perovskite-derived structure modulation in the iron sulfate family

We report the first example of a perovskite sulfate [Na₃(H₂O)]Fe(SO₄)₃. Further structure modulation, by dimensional reduction or ligand extension, has resulted in two related layered perovskite-like compounds Na₆Fe(SO₄)₄ and Na₁₂Fe₃(SO₄)₆F₈. Taken together, these results open up a more general strategy for the future design of more complex perovskite-related materials.

A Chlorine-Based Redox Electrochemical Capacitor

Electrochemical capacitors are under the spotlight due to their high power density, but they have a low energy density. Redox electrolytes have emerged as a promising approach to design high-energy electrochemical energy storage devices. Herein, a chlorine-based redox electrochemical capacitor is reported in an ionic liquid electrolyte. The commercial activated carbon is employed as the working electrode to render the reversible redox of chloride ions in an ionic liquid, by the restriction of micropores on neutral chlorine. The carbon material can simultaneously provide electrical double-layer capacitance. The effective integration of a chlorine redox reaction and electrical double layer allows for high-energy electrochemical capacitors. By this means, a rechargeable chlorine-based redox electrochemical capacitor with reversible capacity and good rate capability and cycling stability is obtained. This work offers a solution for a new type of high-energy electrochemical capacitors.

Synergistic Effect, Structural and Morphology Evolution, and Doping Mechanism of Spherical Br-Doped Na3 V2 (PO4 )2 F3 /C toward Enhanced Sodium Storage

Na₃ V₂ (PO₄ )₂ F₃ has attracted wide attention due to its high voltage platform, and stable crystal structure. However, its application is limited by the low electronic conductivity and the ease formation of impurity. In this paper, the spherical Br-doped Na₃ V₂ (PO₄ )₂ F₃ /C is successfully obtained by a one-step spray drying technology. The hard template polytetrafluoroethylene (PTFE) supplements the loss of fluorine, forming porous structure that accelerates the infiltration of electrolyte. The soft template cetyltrimethylammonium bromide (CTAB) enables doping of bromine and can also control the fluorine content, meanwhile, the self-assembly effect strengthens the structure and refines the size of spherical particles. The loss, compensation, and regulation mechanism of fluorine are investigated. The Br-doped Na₃ V₂ (PO₄ )₂ F₃ /C sphere exhibits superior rate capability with the capacities of 116.1, 105.1, and 95.2 mAh g^-1 at 1, 10, and 30 C, and excellent cyclic performance with 98.3% capacity retention after 1000 cycles at 10 C. The density functional theory (DFT) calculation shows weakened charge localization and enhanced conductivity, meanwhile the diffusion energy barrier of sodium ions is reduced with Br doping. This paper proposes a strategy to construct fluorine-containing polyanions cathode, which enables the precise regulation of structure and morphology, thus leading to superior electrochemical performance.

A Stabilized Polyacrylonitrile-Encapsulated Matrix on a Nanolayered Vanadium-Based Cathode Material Facilitating the K-Storage Performance

Layered vanadium-based metal oxides were regarded as promising cathode materials accounting for suitable K⁺ transport channels as well as high work potential in K-ion batteries. Nevertheless, because of the large radius of K⁺ and the rigid structure of inorganic materials, the typical K_0.486V₂O₅ suffers from volume expansion seriously in the repeated charging and discharging processes along with poor ionic and electronic conductivity, consequently determining inevitably poor electrochemical properties. Herein, we proposed a stabilized polymer (PAN) matrix on K_0.486V₂O₅ nanobelts by a liquid-assisted methodology and further electrospinning technology. As a result, a nanocomposite containing a 3D conductive and interconnected mesh structure was thus constructed. By avoiding the full carbonization of polyacrylonitrile (PAN) with appropriate thermal treatment, the elastic properties of the PAN precursor can be retained, effectively inhibiting the volume effect, and the stabilized PAN-encapsulated matrix can also greatly accelerate transport rates of K⁺ and electrons at a high rate as well as restrict the decomposition of organic electrolytes and side reactions. This work can supply significant basic scientific value of the polymer surface coating methodology for the far-reaching development of inorganic cathode materials in K-ion batteries.

Regulated Synthesis of α-NaVOPO4 with an Enhanced Conductive Network as a High-Performance Cathode for Aqueous Na-Ion Batteries

The low-cost and profusion of sodium reserves make Na-ion batteries (NIBs) a potential candidate to lithium-ion batteries for grid-scale energy storage applications. NaVOPO₄ has been recognized as one of the most promising cathodes for high-energy NIBs, owing to their high theoretical capacity and energy density. However, their further application is hindered by the multiphase transition and conductivity confinement. Herein, we proposed a feasible, one-step hydrothermal synthesis to regulate the synthesis of α-NaVOPO₄ with controlled morphologies. The electrochemical properties of the NaVOPO₄ electrode can be significantly enhanced taking Ketjen black (KB) as the optimized conductive carbon. Besides, combining with the nanocrystallization and construction of the conductive framework via high-energy ball milling, taking KB as the conductive carbon, the as-prepared NaVOPO₄/5%KB exhibits superior Na-storage performance (140.2 mA h g^-1 at 0.1 C and a capacity retention of 84.8% over 1000 cycles at 10 C) to the original NaVOPO₄ (128.5 mA h g^-1 at 0.1 C and a capacity retention of 83.1% over 1000 cycles at 10 C). Moreover, the aqueous full cell with NaTi₂(PO₄)₃ as the anode delivers a capacity of 114.7 mA h g^-1 at 0.2 C (141 W h kg^-1 energy density) and 80.6% capacity retention over 300 cycles at 5 C. The excellent electrochemical performance can be attributed to the nanosized structural and enhanced interfacial effect, which could be rewarding to construct electron transportation tunnels, thus speeding up the Na⁺-diffusion kinetics. The modified strategy provides an efficient approach to intensify the electrochemical performance, which exhibits potential application of the NaVOPO₄ cathode for NIBs.

MoO42--mediated engineering of Na3V2(PO4)3 as advanced cathode materials for sodium-ion batteries

Sodium vanadium phosphate [Na₃V₂(PO₄)₃] with high voltage platform, low cost and environment friendliness has been considered as one of the most promising candidates as cathodes for high-performance sodium-ion batteries. However, the sodium storage property of Na₃V₂(PO₄)₃ is limited because of its low electronic conductivity and poor kinetic performance. Herein, MoO₄^2--doped Na_(3+2x)V₂(PO₄)_(3-x)MoO_4(x) [NVP-MoO₄ (x), x = 0, 0.05, 0.10, 0.15] have been developed and prepared by a feasible solid-state reaction. The optimal NVP-MoO₄ (0.10) delivers a high initial capacity of 108.9 mA h g^-1 and presents an excellent capacity retention of 91.5% at 1 C after 150 cycles. In addition, the NVP-MoO₄ (0.10) shows a good rate capability, delivering a relatively high capacity of 84.2 mA h g^-1 at 50 C. The results of sodium storage measurement and density of states calculation indicate that MoO₄^2- doping can significantly enhance the structural stability, promote the kinetics behavior and boost the electronic conductivity of the materials. In-situ XRD test reveals that the electrochemical reaction of the NVP-MoO₄ (0.10) exhibits a highly reversible phase transition process. This work provides a new insight for the design of advanced cathodes for high-performance sodium-ion batteries by the strategy of unique anion doping.

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.

Three-Dimensional Self-Supporting Ti3C2 with MoS2 and Cu2O Nanocrystals for High-Performance Flexible Supercapacitors

The three-dimensional (3D) architecture of electrode materials with excellent stability and electrochemical activity is extremely desirable for high-performance supercapacitors. In this study, we develop a facile method for fabricating 3D self-supporting Ti₃C₂ with MoS₂ and Cu₂O nanocrystal composites for supercapacitor applications. MoS₂ was incorporated in Ti₃C₂ using a hydrothermal method, and Cu₂O was embedded in two-dimensional nanosheets by in situ chemical reduction. The resulting composite electrode showed a synergistic effect between the components. Ti₃C₂ served as a conductive additive to connect MoS₂ nanosheets and facilitate charge transfer. MoS₂ acted as an active spacer to increase the interlayer space of Ti₃C₂ and protect Ti₃C₂ from oxidation. Cu₂O effectively prevented the collapse of the lamellar structure of Ti₃C₂-MoS₂. Consequently, the optimized composite exhibited an excellent specific capacitance of 1459 F g^-1 at a current density of 1 A g^-1. Further, by assembling an all-solid-state flexible supercapacitor with activated carbon, a high energy density of 60.5 W h kg^-1 was achieved at a power density of 10³ W kg^-1. Additionally, the supercapacitor exhibited a capacitance retention of 90% during 3000 charging-discharging cycles. Moreover, high mechanical robustness was retained after bending at different angles, thereby suggesting significant potential applications for future flexible and wearable devices.

Novel Lamellar Tetrapotassium Pyromellitic Organic for Robust High-Capacity Potassium Storage

Redox-active organics are investigation hotspots for metal ion storage due to their structural diversity and redox reversibility. However, they are plagued by limited storage capacity, sluggish ion diffusion kinetics, and weak structural stability, especially for K⁺ ion storage. Herein, we firstly reported the lamellar tetrapotassium pyromellitic (K₄ PM) with four active sites and large interlayer distance for K⁺ ion storage based on a design strategy, where organics are constructed with the small molecular mass, multiple active sites, fast ion diffusion channels, and rigid conjugated π bonds. The K₄ PM electrode delivers a high capacity up to 292 mAh g^-1 at 50 mA g^-1 , among the best reported organics for K⁺ ion storage. Especially, it achieves an excellent rate capacity and long-term cycling stability with a capacity retention of ≈83 % after 1000 cycles. Incorporating in situ and ex-situ techniques, the K⁺ ion storage mechanism is revealed, where conjugated carboxyls are reversibly rearranged into enolates to stably store K⁺ ions. This work sheds light on the rational design and optimization of organic electrodes for efficient metal ion storage.

Locally Ordered Graphitized Carbon Cathodes for High-Capacity Dual-Ion Batteries

Dual-ion batteries (DIBs) inherently suffer from limited energy density. Proposed here is a strategy to effectively tackle this issue by employing locally ordered graphitized carbon (LOGC) cathodes. Quantum mechanical modeling suggests that strong anion-anion repulsions and severe expansion at the deep-charging stage raise the anion intercalation voltage, therefore only part of the theoretical anion storage sites in graphite is accessible. The LOGC interconnected with disordered carbon is predicted to weaken the interlaminar van der Waals interactions, while disordered carbons not only interconnect the dispersed nanographite but also partially buffer severe anion-anion repulsion and offer extra capacitive anion storage sites. As a proof-of-concept, ketjen black (KB) with LOGC was used as a model cathode for a potassium-based DIB (KDIB). The KDIB delivers an unprecedentedly high specific capacity of 232 mAh g^-1 at 50 mA g^-1 , a good rate capability of 110 mAh g^-1 at 2000 mA g^-1 , and excellent cycling stability of 1000 cycles without obvious capacity fading.

Pseudocapacitance multiporous vanadyl phosphate/graphene thin film electrode for high performance electrochemical capacitors

Supercapacitors are being considered as promising electricity storage devices with green sustainable energy conversion. To efficiently develop and optimize pseudocapacitive material of vanadyl phosphate, herein, multiporous vanadyl phosphate/graphene (denoted as MP-VOPO₄@rGO) is fabricated for the first time with phytic acid as a phosphorus source by extremely simple sol-gel and drop coating methods, and used as the free binder thin film electrode of supercapacitors. The smart combination of honeycomb-like architecture and graphene incorporation results in more active sites and low internal resistance, significantly improving energy storage performance. The effect of introducting polystyrene (denoted as PS) template and rGO on the performance of the nanocomposite is systematically analyzed by comparing the performance of the corresponding thin film electrodes. The MP-VOPO₄@rGO thin film electrode delivers superior pseudocapacitive performance of 672 F g^-1 at 1 A g^-1 as well as a remarkable rate capability of 552 F g^-1 at 5 A g^-1, and it presents a remarkable longterm cycling stability, with a capacitance retention of 83.5% after 5000 cycles. Very interestingly, the results of surface capacitance contribution dominance clearly demonstrates its rapid capacitive response. In addition, based on MP-VOPO₄@rGO thin film as positive and negative electrodes, the corresponding assembled symmetric supercapacitors exihibits outstanding energy density of 26.3 Wh kg^-1 at power density of 249.9 W kg^-1. This investigation can not only provide a versatile strategy to design other thin film electrode materials but also open up a new insight into the development of polyanion phosphate composites for next-generation high performance energy storage systems.

Tilting and twisting in a novel perovzalate, K3NaMn(C2O4)3

A unique variant on the perovskite structure, K₃NaMn(C₂O₄)₃, has been identified with unconventional octahedral tilting, interpenetration of two topologically identical perovskite-like frameworks and an unusual, twisted oxalate ligand.

Recent Advances and Perspectives on Calcium-Ion Storage: Key Materials and Devices

The urgent demand for cost-effective energy storage devices for large-scale applications has led to the development of several beyond-lithium energy storage systems (EESs). Among them, calcium-ion batteries (CIBs) are attractive due to abundant calcium resources, excellent volumetric and gravimetric capacities of Ca metal anode, and potential high energy density coming from the multivalent feature of Ca-ion. Therefore, the exploration of CIBs electrode materials and the construction of CIBs devices are gaining increasing research interest. Relevant publications cover a wide range of materials by both theoretical and experimental investigations, whereas the performances of rocking-chair CIBs have been unsatisfactory. Meanwhile, multi-ion strategies using more than one ion as the charge carrier have been demonstrated to be feasible and promising options in realizing room temperature CIBs. The summary and reflection of previous studies would provide useful information for future exploration and optimization. In this circumstance, this paper overviews the reported CIBs electrode materials, including both anode and cathode, and presents the latest progress of multi-ion strategies in CIBs. Fundamental challenges, potential solutions, and opportunities are accordingly proposed, mimicking other more mature EESs. This review may promote the development of electrode materials and accelerate the construction of low-cost and high-performance CIBs.

Synthesis, Structure, and Electrochemical Properties of Some Cobalt Oxalates

Transition-metal oxalates have wide applications in magnetics, photoemission, electrochemistry, etc. Herein, using hydrothermal reactions, five cobalt(II) oxalates, Na₂Co₂(C₂O₄)₃·2H₂O (I), Na₂Co(C₂O₄)₂·8H₂O (II), KLi₃Co(C₂O₄)₃ (III), Li₄Co(C₂O₄)₃ (IV), and (NH₄)₂Co₂(C₂O₄)F₄ (V) have been synthesized, and their structures are determined from single-crystal X-ray diffraction or Rietveld refinement of powder X-ray diffraction data. Notably, IV and V are identified for the first time. The structures of these cobalt oxalates are versatile, covering 0D, 1D, 2D, and 3D frameworks, while the coordination environments of Co²⁺ centers are uniquely distorted octahedra. As representative examples, I and III are investigated as cathode materials for secondary batteries. Both exhibited electrochemical activity despite large cell polarization. The present study enriches the transition-metal oxalate family and provides new options for energy storage materials.

In Situ Two-Step Activation Strategy Boosting Hierarchical Porous Carbon Cathode for an Aqueous Zn-Based Hybrid Energy Storage Device with High Capacity and Ultra-Long Cycling Life

Aqueous Zn-based hybrid energy storage devices (HESDs) exhibit great potential for large-scale energy storage applications for the merits of environmental friendliness, low redox potential, and high theoretical capacity of Zn anode. However, they are still subjected to low specific capacities since adsorption-type cathodes (i.e., activated carbon, hard carbon) have limited capability to accommodate active ions. Herein, a hierarchical porous activated carbon cathode (HPAC) is prepared via an in situ two-step activation strategy, different from the typical one-step/postmortem activation of fully carbonized precursors. The strategy endows the HPAC with a high specific surface area and a large mesoporous volume, and thus provides abundant active sites and fast kinetics for accommodating active ions. Consequently, pairing the HPAC with Zn anode yields an aqueous Zn-based HESD, which delivers a high specific capacity of 231 mAh g^-1 at 0.5 A g^-1 and excellent rate performance with a retained capacity of 119 mAh g^-1 at 20 A g^-1 , the best result among previously reported lithium-free HESDs based on carbon cathodes. Further, the aqueous Zn-based HESD shows ultra-long cycling stability with a capacity retention of ≈70% after 18 000 cycles at 10 A g^-1 , indicating great potential for environmentally friendly, low-cost, and high-safety energy storage applications.