Numerous benefits of porous electrode materials for lithium ion batteries (LIBs) have been demonstrated, including examples of higher rate capabilities, better cycle lives, and sometimes greater gravimetric capacities at a given rate compared to nonporous bulk materials. These properties promise advantages of porous electrode materials for LIBs in electric and hybrid electric vehicles, portable electronic devices, and stationary electrical energy storage. This review highlights methods of synthesizing porous electrode materials by templating and template‐free methods and discusses how the structural features of porous electrodes influence their electrochemical properties. A section on electrochemical properties of porous electrodes provides examples that illustrate the influence of pore and wall architecture and interconnectivity, surface area, particle morphology, and nanocomposite formation on the utilization of the electrode materials, specific capacities, rate capabilities, and structural stability during lithiation and delithiation processes. Recent applications of porous solids as components for three‐dimensionally interpenetrating battery architectures are also described.

Compared to rechargeable batteries, electrochemical double-layer capacitors (EDLCs) are normally considered to be higher power but lower electrical energy density charge storage devices. To increase the energy density, one can enlarge the interfacial area between electrodes and electrolyte through the introduction of nanopores and employ electrolytes that are stable over wider voltage ranges, such as ionic liquids. However, due to the relatively high viscosity of ionic liquids and large ion sizes, these measures can result in diminished power performance. Here, we describe the synthesis of carbon electrodes that overcome these limitations and simultaneously provide high specific energies and high specific powers in EDLCs using the ionic liquid EMI-TFSI as an electrolyte. A colloidal crystal templating method was optimized to synthesize three-dimensionally ordered mesoporous (3DOm) carbons with well-defined geometry, three-dimensionally interconnected pore structure and tunable pore size in the range from 8 to 40 nm. To achieve precise control over the pore sizes in the carbon products, parameters were established for direct syntheses or seed growth of monodisperse silica nanospheres with specific sizes, using L-lysine-assisted hydrolysis of silicon alkoxide precursors. Porous carbons were then templated from these materials using phenol−formaldehyde (PF) or resorcinol− formaldehyde (RF) precursors. The pore structures of the nanoporous carbon products were characterized in detail, and the materials were tested as electrodes for EDLCs. Optimal pore sizes were identified that provided a large interface between the electrode and the electrolyte while maintaining good ion transport through the relatively viscous electrolyte. 3DOm PF-carbons with pore diameters in the 21−29 nm range exhibited similar high specific capacitance values (146−178 F g −1 at 0.5 A g −1 , with respect to the mass of carbon in a single electrode) as typical large-scale activated-carbon-based EDLCs but showed significantly better high-rate performance (80−123 F g −1 at 25 A g −1 ), a result of the more accessible pore space in which ion diffusion was less restricted.

Monolithic, three-dimensionally ordered macroporous and meso-/microporous (3DOM/m) LiFePO 4 /C composite cathodes for lithium ion batteries were synthesized by a multiconstituent, dual templating method. Precursors containing sources for lithium, iron, and phosphate, as well as a phenol-formaldehyde sol and a nonionic surfactant were infiltrated into a colloidal crystal template. Millimeter-sized monolithic composite pieces were obtained, in which LiFePO 4 was dispersed in a carbon phase around an interconnected network of ordered macropores. The composite walls themselves contained micropores or small mesopores. The carbon phase enhanced the electrical conductivity of the cathode and maintained LiFePO 4 as a highly dispersed phase during the synthesis and during electrochemical cycling. Monoliths containing 30 wt % C were electrochemically cycled in a 3-electrode cell with lithium foil as counter and reference electrodes. No additional binder or conductive agent was used. The capacity was as high as 150 mA h g À1 at a rate of C/5, 123 mA h g À1 at C, 78 mA h g À1 at 8C, and 64 mA h g À1 at 16C, showing no capacity fading over 100 cycles. In spite of the low electronic conductivity of bulk LiFePO 4 (10 À9 À10 À10 S cm À1 ), the monolithic LiFePO 4 /C composite was able to support current densities as high as 2720 mA g À1 .

Free-standing, flexible V 2 O 5 -graphene composite films were prepared by simple filtration of aqueous dispersions of V 2 O 5 nanowires and graphene sheets. V 2 O 5 nanowires were preferentially oriented along the plane of the film as they were sandwiched between stacked graphene sheets in the composite film. The V 2 O 5 content in the composites was adjusted simply by varying the relative amount of the dispersions. Thermal annealing at 300 • C increased the conductivity of the composite films. Due to their integrated structure and high flexibility, the composite films were directly usable as flexible electrodes. In binder-free, sandwich-type lithium battery cells, a composite film containing 75.8 wt% V 2 O 5 delivered discharge capacities of 283 mAh g −1 and 252 mAh g −1 at a current density of 50 mA g −1 during the first and 50 th cycles, respectively. Owing to its higher conductivity, a composite film containing 42.8 wt% V 2 O 5 delivered a discharge capacity of 189 mAh g −1 at 750 mA g −1 . The good performance resulted from the integrated structure of the V 2 O 5 network embedded in stratified graphene nanosheets, with V 2 O 5 as the Li + host and graphene nanosheets providing the electron pathway. These composite films are promising candidates for electrical energy storage applications that require flexible electrodes.

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