High Active Material Utilization Research Articles

Formation of three-dimensional hierarchical pompon-like cobalt phosphide hollow microspheres for asymmetric supercapacitor with improved energy density

A novel three-dimensional (3D) hierarchical pompon-like CoP hollow microspheres are prepared via a hydrothermal method combined with a subsequent phosphatization treatment. These hollow microspheres present a high specific surface area of roughly 24.6 m2 g−1, and a plausible mechanism for their growth is also proposed. The as-obtained materials display an enhanced specific capacitance of 449.4 F g−1 at 1 A g−1 benefited from its unique 3D hierarchical pompon-like hollow structure with a large surface area and high utilization of active materials. Moreover, an asymmetric supercapacitor employing the as-synthesized CoP as the positive electrode and actived carbon (AC) as the negative electrode were successfully assembled, which delivered a high energy density of 22.2 Wh kg−1 at power density of 374.9 W kg−1 with a good cycling performance (80.9% capacitance retention after 3000 cycles). As a practical application test, a unit of three serially-connected ASCs was charged for 15 s and supplied power to, (a) a light emitting diode (LED) and (b) a small LED display screen, each of which worked for more than 35 min and 1 min, respectively. The above results demonstrate that the 3D hierarchical pompon-like CoP hollow microspheres are a promising electrode material for supercapacitor applications.

A hierarchical NiCo2S4 honeycomb/NiCo2S4 nanosheet core-shell structure for supercapacitor applications.

Transition metal sulphides are becoming one of the promising materials for energy storage applications. Particularly, an advanced electrode material architecture, which gives favourable electronic and ionic conductivity, is highly in demand. Herein, a hierarchical NiCo2S4 honeycomb/NiCo2S4 nanosheet core–shell structure is reported for supercapacitor applications. The core–shell structure was in situ grown on a nickel foam via two consecutive hydrothermal processes, followed by an electrochemical deposition process. Moreover, we tuned the deposition cycle to get abundant active sites with gaps of suitable sizes between the walls of the honeycomb structure for efficient electrolyte diffusion routes. The 3D honeycomb core structure was used as superhighway for electron transport to the current collector, while the ultrathin shell structure offered a large surface area with short electron and ion diffusion paths, thus leading to the faster kinetics and higher utilization of active materials. Thus, using the synergistic advantages of the core material and the shell material, the as-synthesized optimized electrode material came up with an excellent specific capacitance of 17.56 F cm−2 at a current density of 5 mA cm−2 and the highest cycling stability of 88.2% after 5000 cycles of charge–discharge process. Such advanced electrode architectures are highly promising for the future electrode materials.

Nitrogen-doped single walled carbon nanohorns enabling effective utilization of Ge nanocrystals for next generation lithium ion batteries

Among various carbon materials, nitrogen doped single walled carbon nanohorns (N-SWCNHs) have a unique structure of clustered conical cages (2–5 nm in diameter and 40–50 nm in length) arranged in dahlia, bud and seed-like configurations. Each conical cage has five pentagons at their tips which act as potential reactive site with their own distinct chemistry. We exploited these reactive sites of N-SWCNHs by preferentially growing germanium nanocrystals (Ge NCs) onto their conical tips using oleylamine as a mild reducing agent. Therefore, Ge decorated N-SWCNHs (Ge@N-SWCNHs) composite was used, for the first time, as active anode material for lithium ion batteries providing high and stable capacity of 1285 mAh/g at 0.1C after 100 cycles. Our results show that preferential growth of Ge Nanocrystals (NCs) on the tips of N-SWCNHs not only allows high utilization of active material but prevents the aggregation of Ge NCs after multiple cycling. Finally, we highlight the potential role of N-SWCNHs as cheap and industrially scalable conductive host for next generation lithium ion batteries.

ZnO@MOF@PANI core-shell nanoarrays on carbon cloth for high-performance supercapacitor electrodes

Abstract Hierarchical ZnO@metal-organic framework @polyaniline (ZnO@MOF@PANI) core-shell nanorod arrays on carbon cloth has been fabricated by combining electrodeposition and hydrothermal method. Well-ordered ZnO nanorods not only act as a scaffold for growth of MOF/PANI shell but also as Zn source for the formation of MOF. The morphology of ZnO@MOF@PANI composite is greatly influenced by the number of PANI electrodeposition cycles. Their structural and electrochemical properties were characterized with different techniques. The results indicate that the ZnO@MOF@PANI with 13 CV cycles of PANI deposition demonstrates the maximum specific capacitance of 340.7 F g−1 at 1.0 A g−1, good rate capability with 84.3% capacitance retention from 1.0 to 10 A g−1 and excellent cycling life of 82.5% capacitance retention after 5000 cycles at high current density of 2.0 A g−1. This optimized core-shell nanoarchitecture endows the composite electrode with short ion diffusion pathway, rapid ion/electron transfer and high utilization of active materials, which thus result in excellent electrochemical performance of the ternary composite.

Titanium dioxide-reduced graphene oxide hybrid as negative electrode additive for high performance lead-acid batteries

Abstract Sulfation at the negative electrode is one of the major failure modes of lead-acid batteries. To overcome the issues of sulfation, we herein report solvothermal synthesis of titanium dioxide- reduced graphene oxide hybrid as negative electrode additive in lead acid cell. The presence of titanium dioxide-reduced graphene oxide hybrid additive (0.5 wt. %) provides high interfacial stability, slows down hard sulfation, high active material utilization by occupying pores on the negative plate. 0.5 wt. % of titanium dioxide-reduced graphene oxide hybrid (3:1 wt. ratio) additive to the negative active mass delivers 85% increase in capacity during the first formation cycle, and thereby increases the formation efficiency from 3 cycles to 1 cycle; >20% and ∼200% increase in capacity at low discharge rates of C/20 and high rates of discharge (2C), respectively as compare to conventional lead acid cell. Besides, 3:1 wt. ratio titanium dioxide-reduced graphene oxide hybrid additive cells show ∼200% increase in high rate partial state of charge cycle life in relation to conventional cell. Titanium dioxide-reduced graphene oxide hybrid additives to the negative active mass provides increase in conductivity, controls the crystallization of lead sulphate growth, acts as a capacitor, decreases the hydrogen evolution and hence enhances the electrochemical performance.

Towards zinc-oxygen batteries with enhanced cycling stability: The benefit of anion-exchange ionomer for zinc sponge anodes

Electrically rechargeable zinc-oxygen batteries are promising for sustainable and resource-uncritical energy storage with high energy density. Their commercialization is hindered by the limited cycling stability – especially due to the zinc anode. In this study, we present a zinc anode that stands out with two advantages: The use of an energy-saving and low temperature preparation method, and the mechanical stabilization of the pore system with a lightweight anion-exchange ionomer.Our approach increases cycling stability for both the zinc-nickel oxide hydroxide cell and the zinc-oxygen cell: Electrochemical analysis in combination with operando X-ray diffraction measurements prove that the novel zinc anode is electrically rechargeable and can be cycled up to five times more often than zinc sponge anodes without anion-exchange ionomer to a depth of discharge of 35%. Scanning electron microscopy is used to evidence that the anion-exchange ionomer stabilises the entire pore system during cycling. We furthermore analyse zinc-oxygen batteries with anion-exchange ionomer at all parts – inside the zinc sponge anode, as separator and at the cathode. These cells can be electrically recharged with unprecedented high utilization of active material, and thus highlight the benefit of introducing an ion-selective coating into alkaline batteries with zinc anode.

Effective Inhibition of the Polysulfide Shuttle Effect in Lithium–Sulfur Batteries By Rgo-PEDOT:PSS Coated Separators Via Air-Controlled Electrospray

Lithium-sulfur batteries are promising battery systems for becoming the next-generation energy storage devices. However, realizing their practical application is greatly challenged by the limitation of exhibiting poor cycling performances owing to the low electrical conductivity of sulfur active material and polysulfide shuttling effects during cycling. To solve these issues, a hybrid structure of reduced graphene oxide and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) was fabricated and coated onto the surface of the conventional separator using the air-controlled electrospraying method in this work. By applying these separators to the lithium-sulfur batteries, lower polarization ensued and a combination of chemical, physical, and electrostatic interactions mitigated the polysulfide shuttling effect. Hence, using these bi-functional coatings led to high initial capacity and improved cycling performances, resulting from to the higher utilization of active material and less loss of active material. Overall, this investigation demonstrates the potential for this coating material being adopted to improve lithium-sulfur battery technology.

Metal/nanocarbon layer current collectors enhanced energy efficiency in lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries with intrinsic merits in high theoretical energy density are the most promising candidate as the next-generation power sources. The strategy to achieve a high utilization of active materials with high energy efficiency is strongly requested for practical applications with less energy loss during repeated cycling. In this contribution, a metal/nanocarbon layer current collector is proposed to enhance the redox reactions of polysulfides in a working Li-S cell. Such a concept is demonstrated by coating graphene–carbon nanotube hybrids (GNHs) on routine aluminum (Al) foil current collectors. The interfacial conductivity and adhesion between the current collector and active material are significantly enhanced. Such novel cell configuration with metal/nanocarbon layer current collectors affords abundant Li ions for rapid redox reactions with small overpotential. Consequently, the Li-S cells with nanostructured current collectors exhibit an initial discharge capacity of 1,113mAhg−1 at 0.5C, which is ∼300mAhg−1 higher than those without a GNH coating layer. The capacity retention is 73% for cells with GNH after 300 cycles. A reduced voltage hysteresis and a high energy efficiency of ca. 90% are therefore achieved. Moreover, the Al/GNH layer current collectors are easily implanted into current cell assembly process for energy storage devices based on complex multi-electron redox reactions (e.g., Li-S batteries, Li-O2 batteries, fuel cells, and flow batteries).

Incorporation of Nickel Sulfide into Sulfur Cathode for Li–S Battery

The lithium–sulfur (Li–S) battery has been attracting tremendous interest as a promising “beyond Li-ion” device, because of its high theoretical specific capacity and energy density [1-4]. However, its widespread applications, such as electric vehicles and large-scale grid-based energy storage systems, are still plagued with several issues and challenges. For example, the intrinsic insulating nature of sulfur makes it difficult to achieve high active material utilization. Additionally, the polysulfide intermediates are highly soluble in the benchmarked ether-based electrolytes, resulting in a severe redox shuttle effect between the sulfur cathode and Li anode. This undesired shuttle phenomenon can lead to a significant loss of active material, fast capacity fading and low Coulombic efficiency. In this work, we incorporated nickel sulfide into the sulfur cathode design through a facile two-step strategy. Note that nickel sulfide here can trap the polysulfide intermediates and inhibit the shuttle effect owing to its binding interactions with polysulfide. Moreover, the low resistivity of nickel sulfide is expected to facilitate the redox kinetics of the adsorbed polysulfides. As a result, this hybrid cathode with a sulfur loading of 4.0 mg/cm2 could manifest an excellent electrochemical performance at a high current density.

Improved electrochemical properties of highly porous amorphous manganese oxide nanoparticles with crystalline edges for superior supercapacitors

An electrochemical electrode, based on dual-morphology manganese-oxide nanoparticles, is fabricated via acidic reduction of potassium permanganate, which consists of amorphous, porous surface, terminated by crystalline edges. The higher surface-to-volume ratio into the pores promotes the surface redox reaction via higher utilization of active materials, whereas the crystalline edges enhance bulk insertion/de-insertion of cations/protons to promote bulk redox reactions. This combinatorial effect increases the charge storage performance of the manganese-oxide-based supercapacitor electrode considerably, with a specific capacitance ∼1000 F g−1. Additionally, removal of K+-ions within manganese-oxide matrix, via repeated rinsing steps during synthesis, enhances the charge transport properties for improved supercapacitive activity.

High areal capacity cathode and electrolyte reservoir render practical Li-S batteries

With the considerable development in sulfur cathode, Li-S batteries have recently witnessed a significant improvement, especially in the gravimetric capacity and cycling performance. However, maintaining high energy density of Li-S batteries and their commercialization relies on the high areal loading and high utilization of active material on the electrode, which are always ignored in the most fundamental research reports. For the Li-S batteries with much higher sulfur loading, except for the well-known issues about polysulfide dissolution, some new issues such as electron and ion transport in thick cathode, depletion of electrolyte and lithium dendrite growth need to be addressed. Here, a Li-S battery with a high areal capacity is proposed by a systematic strategy incorporating two approaches as follows: 1) a hierarchically porous carbon host containing graphene (G), mesoporous carbon (MPC) and super P (SP) diminishes polysulfide migration and guarantees fast electron and ion transport in thick cathode; 2) a glass-fiber (GF) membrane severs as the electrolyte reservoir to prevent the short circuit resulted from the deficiency of liquid electrolyte. With these methods, the Li-S batteries with an ultrahigh sulfur loading of 13mgcm−2 provide a high areal capacity of 14.3mAhcm−2 (1099mAhg−1) at the first cycle and stable cycling performance with a reversible capacity of 628mAhg−1 (8.16mAhcm−2) after 75 cycles at 0.1C.

Facile preparation of TiO2/C3N4 hybrid materials with enhanced capacitive properties for high performance supercapacitors

A new tendency in supercapacitor research has focused on the construction of the inexpensive electrode materials with long cycle stability. In this study, TiO2/C3N4 hybrid materials are synthesized through an efficient, low-cost, facile hydrothermal approach followed by a heat treatment for the first time. The hybrid materials are further used as electrodes for supercapacitors. Interestingly, compared to pure TiO2 nanoparticles, TiO2/C3N4 hybrid materials exhibit excellent electrochemical performance with improved specific capacitance, great rate performance and cycling stability (capacitance retention of 100% after 1000 cycles), which can be attributed to good conductivity and low polarization resulting in a high utilization of active materials. Furthermore, it was found that the capacitive properties of TiO2/C3N4 hybrid were related to the pore sizes and electrical resistances of active materials. The electrode materials with smaller pore size and resistance showed higher capacitive properties.

A 3D conductive network with high loading Li2S@C for high performance lithium–sulfur batteries

We have prepared an integrated CF–CB–Li2S@C cathode with high loading Li2S for high areal-capacity Li–S batteries. The federated conductive network and efficient carbon coating not only provide an efficient electron transport and guarantee high active material utilization but also form a durable protective shield for suppressing polysulfide dissolution.

Compatibility of Polymer Binders with Solvate Ionic Liquid Electrolytes in Lithium-Sulfur Batteries

The molten complex of tetraglyme (G4) and Li[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide), [Li(G4)][TFSA], is a solvate ionic liquid (SIL) composed of complex [Li(G4)]+ cation and [TFSA] anion.1 We reported that stable operation of Li-S batteries can be achieved using [Li(G4)][TFSA] electrolyte.2 The solubility of lithium polysulfides (Li2S m , 2 ≤ m≤ 8), which are reaction intermediates of the S cathode, is suppressed in the SIL, and the redox shuttle effect in the Li-S cell is prohibited, leading to the high coulombic efficiency of discharge/charge and long cycle life of the cell. In this study, we focus on the effects of the compatibility of polymer binders with SIL electrolytes on the electrochemical reactions of the S cathode. The S cathode is composed of sulfur, a conductive agent, and a polymer binder, and the composite cathode has a porous structure. In the porous composite cathode, the surface of active material is partially covered with the binder. Here we report that the electrochemical reaction mechanism, discharge capacity, and rate capability of S cathode change depending on the compatibility of polymer binder with the SIL electrolyte.[Li(G4)][TFSA] electrolyte was prepared by mixing Li[TFSA] and G4 in 1:1 molar ratio in an Ar-filled glove box. The ionic conductivity of [Li(G4)][TFSA] is 1.6 mS/cm at 30 °C. The S composite cathode was composed of elemental sulfur, carbon powder (Ketjenblack, specific surface area of 1270 m2/g), and a polymer. Poly(vinyl alcohol) (PVA-x) with different degrees of saponification (x%) were used as polymer binders.The mass ratio of sulfur, carbon, and polymer in the composite cathode was 60:30:10.The electrolyte uptake of PVA-x film was estimated. PVA-100 was negligibly swollen with [Li(G4)][TFSA] electrolyte. The electrolyte uptake of PVA-x increased on decreasing the degree of saponification. The weights of PVA-80 and PVA-66 films increased by 8 and 17%, respectively, owing to electrolyte uptake. [Li(G4)][TFSA] electrolyte was incorporated into the acetate moiety of the PVA-x polymer chain, and the PVA-x film was partially swollen with the electrolyte.3 Figure 1 shows the dependence of discharge capacity of Li-S cells on current density. As decreasing the saponification degree of PVA, the discharge capacities increased. The compatibility of binder also affects the charge-discharge rate capability of Li-S cells. The partial swelling of binder in the electrolyte facilitated Li+ ion conduction in the S composite cathode. This results in the high utilization of active material of cathode and the higher rate capability of the cell. 3 Acknowledgements This study was supported in part by Specially Promoted Research for Innovative Next Generation Batteries (SPRING) of the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST), and JSPS KAKENHI (No. 15H03874 and No. 15K13815) from the Japan Society for the Promotion of Science (JSPS). References K. Ueno et al., J. Phys. Chem. B, 116, 11323 (2012).K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013).T. Nakazawa, J. Power Sources, 307, 746 (2016). Figure 1

Improved Electrode Processing and Uniformity with Discrete Carbon Nanotube Aqueous Binder Additives

The US Department of Energy has set a goal of doubling the energy density of batteries for vehicle applications by 2022, to spur innovation towards extended battery run time and electric vehicle range, and to reduce cost of cells to levels where economic factors favor mass adoption of electrified vehicles[1]. Novel high-performance active materials can be used to improve energy density, but provide a unique set of challenges, such as accommodating high volume changes associated with the insertion process or matching insertion potentials to the stability of acceptable electrolytes. One of the most straightforward ways to improve energy density is to manufacture thicker electrodes still capable of high active material utilization and robust to high-power charge and discharge. This requires careful optimization of the microstructure of the finished electrode, excellent adhesion to the current collector, and the possibility of creating defect-free structures as the electrodes are processed[2]. In order to make a reliable electrode with acceptable power and energy density, the active materials and electrolyte must be paired with an appropriate binder system. In traditional Li-ion cells, PVDF in NMP has been used in the electrode slurry-making processes. To ease processing requirements, cost and environmental issues, manufacturers are gradually moving away from PVDF and using aqueous base materials such as carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR)[3-4]. Molecular Rebar®, which are discrete carbon nanotubes made by Molecular Rebar Design, LLC, are a good candidate for additives to improve mechanical properties and transport properties in the finished electrodes. While carbon nanotubes have been used in a variety of composite mesostructured applications, the production of nanotubes at the industrial scale frequently result in bundles of tubes that behave more like weak-porous-stress concentration centers than reinforcing components in the composite. Black Diamond Structures is commercializing products based on Molecular Rebar®, a technology platform produced with industrial-scale process that disentangles and functionalizes stock carbon nanotubes and opens the ends of the tubes. The process also cleans the residual carbon and the residual catalyst contamination. These tubes that are individualized and permanently dispersed in liquid medium, can then be added to lithium electrode slurries as, for example, an aqueous dispersion appropriate for meeting requirements of existing roll-to-roll manufacturing process lines. Graphite anodes produced with addition of Molecular Rebar® to both the CMC and the SBR components of the binder mixture show enhanced mechanical strength, and much cleaner processing. We observe improved pasting, uniform drying and thixotropy which facilitates binder and conductive additive retention in the bulk of the paste thus preventing migration to the surface. In addition, we observe no edge flaking or shedding during cutting or slitting and more uniform interconnectivity between graphite particles. Side-by-side micrographs in Figure 1 show the improved edge performance and reduced shedding of slit electrodes with (left) and without (right) the nanotube additives. Figure 2 shows half-cell performance of cells made with Molecular Rebar® in the CMC and SBR components of the binder, suggesting that the nanotubes improve capacity retention and uniformity at high C-rates. Coin cell data is shown, but pouch cell performance testing is underway. Performance analysis suggests that much of the improvement is due to enhanced mass transport of lithium ions through the thickness of the anode layer, resulting in a more uniform utilization of the electrode at longer discharge times. More detailed characterization work is underway. Figure 3 shows SEM micrographs taken of the electrodes after cycling, revealing a more homogeneous and uniform SEI layer in the binders containing Molecular Rebar®, suggesting improved cycleability and more uniform access to the particles during the charge and discharge processes. ®Registered Trademark of Molecular Rebar Design, LLC [1] “EV Everywhere Grand Challenge Blueprint,” U.S. Department of Energy, December 2013, http://www.energy.gov/sites/prod/files/2014/02/f8/eveverywhere_blueprint.pdf. [2] “Thick Electrodes for High Energy Lithium Ion Batteries,” Madhav Singh, Jörn Kaiser, and Horst Hahn, Journal of the Electrochemical Society, 162 (2), A1196-A1201 (2015). [3] “Effect of Carboxymethyl Cellulose on Aqueous Processing of Natural Graphite Negative Electrodes and their Electrochemical Performance for Lithium Batteries,” Jin-Hyon Lee, Ungyu Paik,, Vincent A. Hackely, and Young-Min Choi, Journal of the Electrochemical Society, 152 (9) A1763-A1769 (2005). [4] “Optimization of ratio and amount of CMC/SBR binder for a graphite anode,” Honghe Zheng, Gao Liu, Xiangyun Song, Paul Ridgeway and Vince Battaglia, #200, 218th ECS Meeting. Figure 1

Chemical modification of pristine carbon nanotubes and their exploitation as the carbon hosts for lithium-sulfur batteries

In this work, we prepared nitrogen-doped open-ended carbon nanotubes (N-O-E-CNTs) through controllable chemical cutting followed by a facile hydrothermal process. Scanning electron microscopy, X-ray diffraction, Raman and some other techniques were used to characterize the as-prepared N-O-E-CNTs, which possessed enhanced specific surface area and pore volume, and excellent conductivity. The N-O-E-CNTs were investigated as carbon host for Li–S batteries by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance, and the electrochemical testing showed that the N-O-E-CNTs/S cathode presented much better cyclability and rate capability than the carbon nanotubes (CNTs)/S and open-ended carbon nanotubes (O-E-CNTs)/S cathodes. The main reasons for the improved performance were explained as follows. On one hand, the controllable chemical cutting for CNTs can effectively increase the specific surface area and pore volume while retaining good conductivity, and the formed open-ended structure is beneficial to trapping sulfur species in the internal nanotube cavity. On the other hand, the N doping for the N-O-E-CNTs can not only recover the conductivity, but also enable the N-O-E-CNTs to possess more superior compatibility and adsorbability for sulfur species, resulting in high utilization of active material and effective alleviation of polysulfides shuttling.

Sea urchin-like NiCoO2@C nanocomposites for Li-ion batteries and supercapacitors

The rational construction of battery electrode architecture that offers both high energy and power densities on a gravimetric and volumetric basis is a critical concern but achieving this aim is beset by many fundamental and practical challenges. Here we report a new sea urchin-like NiCoO2@C composite electrode architecture composed of NiCoO2 nanosheets grown on hollow concave carbon disks. Such a unique structural design not only preserves all the advantages of hollow structures but also increases the packing density of the active materials. NiCoO2 nanosheets grown on carbon disks promote a high utilization of active materials in redox reactions by reducing the path length for Li+ ions and for electron transfer. Meanwhile, the hollow concave carbon not only reduces the volume change, but also improves the volumetric energy density of the entire composite electrode. As a result, the nanocomposites exhibit superior electrochemical performance measured in terms of high capacity/capacitance, stable cycling performance and good rate capability in both Li-ion battery and supercapacitor applications. Such nanostructured composite electrode may also have great potential for application in other electrochemical devices.

One-pot fabrication of hierarchical Ag/MnO2 nanoflowers for electrochemical capacitor electrodes

Flower-like Ag/MnO2 nanospheres with diameter of 300nm are developed by a one-pot solution-based method. The nanoflowers are assembled by numerous ultrathin nanosheets, and Ag metal is uniformly distributed in the body of MnO2. Compared to the unmodified sample, Ag/MnO2 nanoflowers exhibit an excellent electrochemical performance with improved specific capacitance, rate capability and cycling stability, which can be attributed to good conductivity and low polarization resulting in a high utilization of active materials. This simple yet convenient synthetic route may provide a low-cost approach for the fabrication of high performance electrode materials for electrochemical capacitor application on a large scale.

MnO2-Carbon nanotube composite for high-areal-density supercapacitors with high rate performance

Practical supercapacitor devices require high areal capacitance and areal power density, and thus demand high utilization of active material and good rate performance under high areal mass loading. However, ion transport in high-mass-loading electrodes can be a challenge, which leads to deteriorate specific capacitance and rate performance. In this paper, a well-dispersed porous MnO2-carbon nanotube (CNT) composite was prepared for use as a supercapacitor electrode material. The small MnO2 nanoparticles and porous CNT network facilitated fast electron/ion transfer kinetics in the electrode. With a mass loading as high as 6.4 mg cm−2 on the electrode, the MnO2-CNT composite exhibited an excellent areal capacitance of 1.0 F cm−2 at 0.2 A g−1 (1.28 mA cm−2), with a retention of 77% even at a high current density of 20 A g−1 (128 mA cm−2). The electrode exhibited a high power density of 45.2 kW kg−1 (0.29 W cm−2) while maintaining a reasonable energy density of 16.7 Wh kg−1 (106 μWh cm−2). No apparent fading was observed even after 3000 charge/discharge cycles at 1 A g−1. This porous and evenly distributed MnO2-CNT composite has great potential for practical applications in supercapacitors.

Expandable-graphite-derived graphene for next-generation battery chemistries

Lithium–sulfur and lithium–air batteries offer theoretical energy densities an order of magnitude higher than that of current lithium-ion batteries and are considered as promising candidates as the next-generation battery chemistries. For an efficient use of these new battery chemistries, careful selection of suitable electrode materials/structures is critical. Graphene, a unique two-dimensional nanomaterial, with its superior electronic conductivity, mechanical strength, and flexibility has been successfully applied in battery studies. Graphene, even with imperfect layers, will be of great interest to battery industrial applications if the manufacturing cost is reduced. Herein, we demonstrate the application of low-cost graphene sponge/sheets derived from expandable graphite in both lithium–sulfur and hybrid lithium–air batteries, respectively, as a cathode conductive matrix to accommodate the soluble polysulfides and as a catalyst for the oxygen reduction reaction. High utilization of active materials and good cycling stability are realized in lithium–sulfur and hybrid lithium–air batteries by employing this low-cost material, demonstrating its promise for use in next-generation battery chemistries.