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.

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Anisotropically grown (b-axis short) single-nano TiO2 (B), uniformly hyper-dispersed on the surface of multiwalled carbon nanotubes (MWCNT), was successfully synthesized via an in situ ultracentrifugation (UC) process coupled with a follow-up hydrothermal treatment. The uc-TiO2 (B)/MWCNT composite materials enable ultrafast Li(+) intercalation especially along the b-axis, resulting in a capacity of 235 mA h g(-1) per TiO2 (B) even at 300C (1C = 335 mA g(-1) ).

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1. Introduction The next step in the high energy density Nanohybrid™ Capacitor (nanocrystalline Li4Ti5O12/activated carbon hybrid supercapacitors)1 is to substitute the negative electrode with other potential candidates with higher voltage (lower reaction potential) and high capacity. In the presentation, we focus on bronze-type TiO2 (TiO2(B)) which has a much higher electric conductivity (~10-2 S cm-1) compared to other TiO2 polymorphs such as anatase and rutile (10-14 ~10-13 S cm-1).2 TiO2(B) shows a theoretical capacity of 335 mAh g-1 during Li+ ion intercalation, where Li+ diffusion proceeds along the b-axis tunnel giving poor Li+ diffusion coefficient of 10-14 ~ 10-16 cm2 s-1.3 Using our original ultracentrifugation (UC) treatment, we have successfully synthesized the anisotropic-grown (b-axis short), single-nano TiO2(B), which is uniformly and highly dispersed within carbon matrix. Electrochemical performances of the hybrid supercapacitor composed of uc-TiO2(B) negative and activated carbon positive electrodes were tested and compared with those for the conventional EDLC and the uc-Li4Ti5O12-based hybrid supercapacitors. 2. Experimental In order to optimize the uc-TiO2(B) hybrid supercapacitor system, we prepared three uc-TiO2(B) negative electrodes under different conditions; i) untreated uc-TiO2(B) (case 1)), ii) pre-cycled uc-TiO2(B) to cancel its irreversible capacity (case 2)), and iii) uc-TiO2(B) with Li pre-doping (case 3)). Then, the laminate-type test cells (3 cm * 4 cm) were assembled using the uc-TiO2(B) negative electrodes combined with activated carbon (AC) positive electrodes, and the additional Ag reference electrode was used when necessary. The electrolyte was 1M LiPF4 in propylene carbonate. Test cells of EDLC and uc-Li4Ti5O12-based hybrid supercapacitors were assembled using the same AC positive electrode and electrolyte as uc-TiO2(B) system. Electrochemical characterizations (charge-discharge curves, rate capability, and cycleability etc…) were performed on the assembled test cells. 3. Results and Discussion The hybrid supercapacitor case 1) did not reach the targeted cell voltage (3.0V) appropriately and its capacity degraded within few cycles. Charge discharge profiles of positive and negative electrodes for the case 1) showed that the AC positive electrode was overcharged due to the characteristic behavior of the untreated uc-TiO2(B) negative electrode, its irreversible capacity during initial cycling and large voltage hysteresis especially in the high depth of discharge (DOD). Better cell performance was obtained for the case 2), thanks to canceling the irreversible capacity of uc-TiO2(B). The case 3) showed the best cell performance among three cases, when the uc-TiO2(B) was pre-lithiated and its operation potential was confined within the range between 1.0 V and 1.5 V vs. Li/Li+. For the optimized uc-TiO2(B) hybrid supercapacitor (case 3), the estimated volume energy density was tripled from that of EDLC and higher compared to the uc-Li4Ti5O12 hybrid supercapacitor. References 1) K. Naoi, S. Ishimoto, J. Miyamoto and W. Naoi., Energy & Environ. Sci., 5, (2012) 9363. 2) N. Taniguchi, M. Kato and K. Hirota, J. Jpn. Soc., 2012, 59, 326. 3) C. W. Mason, I. Yeo, K. Saravanan and P. Balaya, RSC Adv., 2013, 3, 2935.