Nanoscience and nanotechnology can provide tremendous benefits to electrochemical energy storage devices, such as batteries and supercapacitors, by combining new nanoscale properties to realize enhanced energy and power capabilities. A number of published reports on hybrid systems are systematically reviewed in this perspective. Several potential strategies to enhance the energy density above that of generation-I electric double layer capacitors (EDLC: activated carbon/activated carbon) are discussed and some fundamental issues and future directions are identified. We suggest a new hybrid supercapacitor system that is able to meet the energy and power demands for a variety of applications, ranging from microelectronic devices to electrical vehicles, which presents itself as a breakthrough improvement. Two practical hybrid supercapacitor systems, namely, a lithium-ion capacitor (LIC: graphite/activated carbon) and a nanohybrid capacitor (NHC: (nc-Li 4 Ti 5 O 12 /CNF composite)/activated carbon), are featured and compared. The proposed NHC can pave the way toward generation-II supercapacitor systems by taking advantage of a novel, high quality, high efficiency and inexpensive nanomaterial preparation procedure. With such a breakthrough in nanofabrication-nanohybridization technology, the NHC, which utilizes an ultrafast nano-crystalline Li 4 Ti 5 O 12 , is considered to be an alternative for conventional generation-I EDLCs.

To meet growing demands for electric automotive and regenerative energy storage applications, researchers all over the world have sought to increase the energy density of electrochemical capacitors. Hybridizing battery-capacitor electrodes can overcome the energy density limitation of the conventional electrochemical capacitors because they employ both the system of a battery-like (redox) and a capacitor-like (double-layer) electrode, producing a larger working voltage and capacitance. However, to balance such asymmetric systems, the rates for the redox portion must be substantially increased to the levels of double-layer process, which presents a significant challenge. An in situ material processing technology called "ultracentrifuging (UC) treatment" has been used to prepare a novel ultrafast Li4Ti5O12 (LTO) nanocrystal electrode for capacitive energy storage. This Account describes an extremely high-performance supercapacitor that utilizes highly optimized "nano-nano-LTO/carbon composites" prepared via the UC treatment. The UC-treated LTO nanocrystals are grown as either nanosheets or nanoparticles, and both have hyperlinks to two types of nanocarbons: carbon nanofibers and supergrowth (single-walled) carbon nanotubes. The spinel structured LTO has been prepared with two types of hyperdispersed carbons. The UC treatment at 75 000G stoichiometrically accelerates the in situ sol-gel reaction (hydrolysis followed by polycondensation) and further forms, anchors, and grafts the nanoscale LTO precursors onto the carbon matrices. The mechanochemical sol-gel reaction is followed by a short heat-treatment process in vacuo. This immediate treatment with heat is very important for achieving optimal crystallization, inhibiting oxidative decomposition of carbon matrices, and suppressing agglomeration. Such nanocrystal composites can store and deliver energy at the highest rate attained to this date. The charge-discharge profiles indicate a very high sustained capacity of 80 mAh g(-1) at an extremely high rate of 1200 C. Using this ultrafast material, we assembled a hybrid device called a "nanohybrid capacitor" that consists of a Faradaic Li-intercalating LTO electrode and a non-Faradaic AC electrode employing an anion (typically BF4(-)) adsorption-desorption process. The "nanohybrid capacitor" cell has demonstrated remarkable energy, power, and cycleability performance as an electrochemical capacitor electrode. It also exhibits the same ion adsorption-desorption process rates as those of standard activated carbon electrodes in electrochemical capacitors. The new-generation "nanohybrid capacitor" technology produced more than triple the energy density of a conventional electrochemical capacitor. Moreover, the synthetic simplicity of the high-performance nanostructures makes it possible to scale them up for large-volume material production and further applications in many other electrochemical energy storage devices.

International audienceHighly dispersed crystalline/amorphous LiFePO4 (LFP) nanoparticles encapsulated within hollow-structured graphitic carbon were synthesized using an in situ ultracentrifugation process. Ultracentrifugation triggered an in situ sol–gel reaction that led to the formation of core–shell LFP simultaneously hybridized with fractured graphitic carbon. The structure has double cores that contain a crystalline LFP (core 1) covered by an amorphous LFP containing Fe3+ defects (core 2), which are encapsulated by graphitic carbon (shell). These core–shell LFP nanocomposites show improved Li+ diffusivity thanks to the presence of an amorphous LFP phase. This material enables ultrafast discharge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C) as well as ultrafast charge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C). The synthesized core–shell nanocomposites overcome the inherent one-dimensional diffusion limitation in LFP and yet deliver/store high electrochemical capacity in both ways symmetrically up to 480C. Such a high rate symmetric capacity for both charge and discharge has never been reported so far for LFP cathode materials. This offers new opportunities for designing high-energy and high-power hybrid supercapacitors

Nanocrystalline Li3VO4 dispersed within multiwalled carbon nanotubes (MWCNTs) was prepared using an ultracentrifugation (uc) process and electrochemically characterized in Li-containing electrolyte. When charged and discharged down to 0.1 V vs Li, the material reached 330 mAh g(-1) (per composite) at an average voltage of about 1.0 V vs Li, with more than 50% capacity retention at a high current density of 20 A g(-1). This current corresponds to a nearly 500C rate (7.2 s) for a porous carbon electrode normally used in electric double-layer capacitor devices (1C = 40 mA g(-1) per activated carbon). The irreversible structure transformation during the first lithiation, assimilated as an activation process, was elucidated by careful investigation of in operando X-ray diffraction and X-ray absorption fine structure measurements. The activation process switches the reaction mechanism from a slow "two-phase" to a fast "solid-solution" in a limited voltage range (2.5-0.76 V vs Li), still keeping the capacity as high as 115 mAh g(-1) (per composite). The uc-Li3VO4 composite operated in this potential range after the activation process allows fast Li(+) intercalation/deintercalation with a small voltage hysteresis, leading to higher energy efficiency. It offers a promising alternative to replace high-rate Li4Ti5O12 electrodes in hybrid supercapacitor applications.

Anisotropically grown Li 3 V 2 (PO 4 ) 3 nanocrystals, which are highly dispersed and directly impregnated on the surface of a carbon nanofiber (CNF), were successfully synthesized via a two-step synthesis process: i) precipitation of nanoplated V 2 O 3 precursors (20-200 nm); ii) transformation of the V 2 O 3 precursor into Li 3 V 2 (PO 4 ) 3 nanoplates without size change. The direct attachment of the Li 3 V 2 (PO 4 ) 3 nanocrystals to the carbon surface improves the electronic conductivity and Li + diffusivity of the entire Li 3 V 2 (PO 4 ) 3 /CNF composite, simultaneously producing a mesoporous network (pore size of approximately 10 nm) that acts as an electrolyte reservoir owing to the pillar effect of the impregnated Li 3 V 2 (PO 4 ) 3 crystals. This ideal Li 3 V 2 (PO 4 ) 3 /CNF nanostructure enabled a 480C rate (7.5 seconds) discharge with 83 mA h g −1 , and 69% of capacity retention at the slowest discharge rate (1C). Such an ultrafast charge-discharge performance opens the possibility of using Li 3 V 2 (PO 4 ) 3 as a cathode material for ultrafast lithium ion batteries with a stable cycle performance over 10,000 cycles at a 10C rate, maintaining 85% of the initial capacity. In the current society, the storage of electrical energy at high charge and discharge rate is an important technological issue as it enables hybrid and plug-in hybrid electric vehicles and provides a back-up to wind and solar energies.1,2 Rechargeable lithium-ion batteries (LIBs) are considered the most advanced energy storage systems; they possess high energy but limited power compared to high-power devices such as supercapacitors.1 To further improve the performance of the LIBs, several electrode materials have been proposed and investigated so far.2-11 Commercial cells utilize the layer-structured LiCoO 2 as the positive electrode, 3 but the high cost and toxicity of cobalt prohibit its use on a large scale. Spinel-type LiMn 2 O 4 is one of the alternative materials to cobalt for high-rate use. 4 Several reports on such materials for high-rate use have been published; however, the reported discharge performance are limited within 50-150C. Owing to their easy release of oxygen, LiCoO 2 and LiMn 2 O 4 also have safety issues at overcharged states or high temperatures.3,4 Thus, to achieve a long-term and safe use of the LIBs, cathode materials other than those including layer-structured or spinel-type materials have received significant attention. Researchers have identified polyanion-type cathode materials-such as phosphate cathode materials like LiFePO 4 (LFP) 5 and Li 3 V 2 (PO 4 ) 3 (LVP)-as attractive active materials because of their high thermal stability, high cyclability, and superior safety properties provided by the stable (PO 4 ) 3− unit. 6 The presence of a phosphate with a strong P-O covalency stabilizes the antibonding M-O (M = V or Fe) energy level through an M-O-P inductive effect and generates a conveniently high redox potential for M 3+ /M 2+ . 7 In particular, monoclinic LVP has attracted much attention because o...