Hybrid metal-ion capacitors (MICs) (M stands for Li or Na) are designed to deliver high energy density, rapid energy delivery, and long lifespan. The devices are composed of a battery anode and a supercapacitor cathode, and thus become a tradeoff between batteries and supercapacitors. In the past two decades, tremendous efforts have been put into the search for suitable electrode materials to overcome the kinetic imbalance between the battery-type anode and the capacitor-type cathode. Recently, some transition-metal compounds have been found to show pseudocapacitive characteristics in a nonaqueous electrolyte, which makes them interesting high-rate candidates for hybrid MIC anodes. Here, the material design strategies in Li-ion and Na-ion capacitors are summarized, with a focus on pseudocapacitive oxide anodes (Nb2 O5 , MoO3 , etc.), which provide a new opportunity to obtain a higher power density of the hybrid devices. The application of Mxene as an anode material of MICs is also discussed. A perspective to the future research of MICs toward practical applications is proposed to close.

Nonaqueous Hybrid Lithium-Ion and Sodium-Ion Capacitors

In this critical Review we focus on the evolution of the hybrid ion capacitor (HIC) from its early embodiments to its modern form, focusing on the key outstanding scientific and technological questions that necessitate further in-depth study. It may be argued that HICs began as aqueous systems, based on a Faradaic oxide positive electrode (e.g., Co3O4, RuOx) and an activated carbon ion-adsorption negative electrode. In these early embodiments HICs were meant to compete directly with electrical double layer capacitors (EDLCs), rather than with the much higher energy secondary batteries. The HIC design then evolved to be based on a wide voltage (∼4.2 V) carbonate-based battery electrolyte, using an insertion titanium oxide compound anode (Li4Ti5O12, LixTi5O12) versus a Li ion adsorption porous carbon cathode. The modern Na and Li architectures contain a diverse range of nanostructured materials in both electrodes, including TiO2, Li7Ti5O12, Li4Ti5O12, Na6LiTi5O12, Na2Ti3O7, graphene, hard carbon, soft carbo...

Review of Hybrid Ion Capacitors: From Aqueous to Lithium to Sodium.

Transition Metal Sulfides Based on Graphene for Electrochemical Energy Storage

In this paper, we wish to present an overview of the research carried out in our laboratories with low-cost transition metal oxides (manganese dioxide, iron oxide and vanadium oxide) as active electrode materials for aqueous electrochemical supercapacitors. More specifically, the paper focuses on the approaches that have been used to increase the capacitance of the metal oxides and the cell voltage of the supercapacitor. It is shown that the cell voltage of an electrochemical supercapacitor can be increased significantly with the use of hybrid systems. The most relevant associations are Fe3O4 or activated carbon as the negative electrode and MnO2 as the positive. The cell voltage of the Fe3O4/MnO2 device is 1.8 V and this value was increased to 2.2 V by using activated carbon instead of Fe3O4. These two systems have shown superior behavior compared to a symmetric MnO2/MnO2 device which only works within a 1 V potential window in aqueous K2SO4. Furthermore, the activated carbon/MnO2 hybrid device exhibits a real power density of 605 W/kg (maximum power density =19.0 kW/kg) with an energy density of 17.3 Wh/kg. These values compete well with those of standard electrochemical double layer capacitors working in organic electrolytes.

http://ma.ecsdl.org/content/MA2005-01/35/1348.full.pdf

Nanostructured Transition Metal Oxides for Aqueous Hybrid Electrochemical Supercapacitors

Commercial lithium-ion (Li-ion) batteries built with Ni- and Co-based intercalation-type cathodes suffer from low specific energy, high toxicity and high cost. A further increase in the energy storage characteristics of such cells is challenging because capacities of such intercalation compounds approach their theoretical values and a further increase in their maximum voltage induces serious safety concerns. The growing market for portable energy storage is undergoing a rapid expansion as new applications demand lighter, smaller, safer and lower cost batteries to enable broader use of plug-in hybrid and pure-electric vehicles (PHEVs and EVs), drones and renewable energy sources, such as solar and wind. Conversion-type cathode materials are some of the key candidates for the next-generation of rechargeable Li and Li-ion batteries. Continuous rapid progress in performance improvements of such cathodes is essential to utilize them in future applications. In this review we consider price, abundance and safety of the elements in the periodic table for their use in conversion cathodes. We further compare specific and volumetric capacities of a broad range of conversion materials. By offering a model for practically achievable volumetric energy density and specific energy of Li cells with graphite, silicon (Si) and lithium (Li) anodes, we observe the impact of cathode chemistry directly. This allows us to estimate potentials of different conversion cathodes for exceeding the energy characteristics of cells built with state of the art intercalation compounds. We additionally review the key challenges faced when using conversion-type active materials in cells and general strategies to overcome them. Finally, we discuss future trends and perspectives for cost reduction and performance enhancement.

Conversion cathodes for rechargeable lithium and lithium-ion batteries

Developing high-active and low-cost bifunctional materials for catalyzing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) holds a pivotal role in water splitting. Therefore, we present a new strategy to form NiS/Ni2P heterostructures. The as-obtained NiS/Ni2P/carbon cloth (CC) requires overpotentials of 111 mV for the HER and 265 mV for the OER to reach a current density of 20 mA cm-2, outperforming their counterparts such as NiS and Ni2P under the same conditions. Additionally, the NiS/Ni2P/CC electrode requires a 1.67 V cell voltage to deliver 10 mA cm-2 in a two-electrode electrolysis system, which is comparable to the cell using the benchmark Pt/C||RuO2 electrode. Detailed characterizations reveal that strong electronic interactions between NiS and Ni2P, abundant active sites, and smaller charge-transfer resistance contribute to the improved HER and OER activity.

/pdf/engineering-nis-ni2p-heterostructures-for-efficient-k0yqb912vt.pdf

Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting

This work reports on porous Li4Ti5O12 (LTO)–TiO2 nanosheet arrays prepared via a versatile hydrothermal method for lithium ion batteries, exhibiting a high initial discharge capacity of 184.6 mA h g−1 at 200 mA g−1 and possessing excellent electrochemical stability with only 8.3% loss of specific capacity at 1 A g−1 in a prolonged charge–discharge process (1000 cycles). The excellent electrochemical performance can be attributed to the interconnected mesoporous/macroporous structures and the abundant grain boundaries generated by the existing multiphase materials, as well as the direct connection between the nanoarrays and conductive Ti substrates which facilitates the lithium ion and electron transportation. A flexible lithium ion battery has been further designed by using the nanosheet LTO–TiO2 arrays as the anode and LiCoO2 as the cathode, enabling it to reliably power an LED light under severe mechanical bending. This is very promising for future potential application in high performance flexible energy storage devices.

Porous Li4Ti5O12–TiO2 nanosheet arrays for high-performance lithium-ion batteries

Li4Ti5O12-TiO2 nanowire arrays constructed with stacked nanocrystals for high-rate lithium and sodium ion batteries

Herein, we report on rutile TiO2 decorated hierarchical Li4Ti5O12 nanosheet arrays for lithium ion hybrid supercapacitor application for the first time. It is noted that the self-supported arrays manifest impressive rate capability and cycling stability, showing a reversible specific capacity of 142.9 mA h g−1, and retaining 92.3% of their initial capacity over 3000 cycles at a rate of 30C. The lithium ion hybrid supercapacitor, constructed with Li4Ti5O12 nanosheet arrays and nitrogen doped carbon nanotubes (N-CNTs), exhibits an ultrahigh energy density of 74.85 W h kg−1 at a power density of 300 W kg−1. The hierarchical 3D interconnected nanostructure of the self-supported Li4Ti5O12–rutile TiO2 nanosheet arrays, as well as the efficient lithium diffusion along the [011] direction for Li4Ti5O12 and [001] for rutile TiO2, play an important role in the outstanding energy storage performance.

Rutile-TiO2 decorated Li4Ti5O12 nanosheet arrays with 3D interconnected architecture as anodes for high performance hybrid supercapacitors

In this study, we report hierarchical TiO2 sphere–sulfur frameworks assisted with graphene as a cathode material for high performance lithium–sulfur batteries. With this strategy, the volume expansion and aggregation of sulfur nanoparticles can be effectively mitigated, thus enabling high sulfur utilization and improving the specific capacity and cycling stability of the electrode. Modification of the TiO2–S nanocomposites with graphene can trap the polysulfides via chemisorption and increase the electronic connection among various components. The graphene-assisted TiO2–S composite electrodes exhibit high specific capacity of 660 mA h g−1 at 5C with a capacity loss of only 0.04% per cycle in the prolonged charge–discharge processes at 1C.

/pdf/hierarchical-tio2-spheres-assisted-with-graphene-for-a-high-50d88g2d65.pdf

Lin Gao

Papers

Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting

Porous Li4Ti5O12–TiO2 nanosheet arrays for high-performance lithium-ion batteries

Li4Ti5O12-TiO2 nanowire arrays constructed with stacked nanocrystals for high-rate lithium and sodium ion batteries

Rutile-TiO2 decorated Li4Ti5O12 nanosheet arrays with 3D interconnected architecture as anodes for high performance hybrid supercapacitors

Hierarchical TiO2 spheres assisted with graphene for a high performance lithium–sulfur battery