Enhanced Electrochemical Performance of Ultracentrifugation-Derived nc-Li3VO4/MWCNT Composites for 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

Cation-Disordered Li3VO4: Reversible Li Insertion/Deinsertion Mechanism for Quasi Li-Rich Layered Li1+x[V1/2Li1/2]O2 (x = 0–1)

The reversible lithiation/delithiation mechanism of the cation-disordered Li3VO4 material was elucidated, including the understanding of structural and electrochemical signature changes during cycling. The initial exchange of two Li induces a progressive and irreversible migration of Li and V ions from tetrahedral to octahedral sites, confirmed by the combination of in situ/operando X-ray diffraction and X-ray absorption fine structure analyses. The resulting cation-disordered Li3VO4 can smoothly and reversibly accommodate two Li and shows a Li+ diffusion coefficient larger by 2 orders of magnitude than the one of pristine Li3VO4, leading to improved electrochemical performance. This cation-disordered Li3VO4 negative electrode offers new opportunities for designing high-energy and high-power supercapacitors. Furthermore, it opens new paths for preparing disordered compounds with the general hexagonal close-packing structure, including most polyanionic compounds, whose electrochemical performance can be easily improved by simple cation mixing

Dual-Cation Electrolytes for High-Power and High-Energy LTO//AC Hybrid Capacitors

Dual-cation electrolyte systems, which contain two cations [Li+ and spiro-1,1′-bipyrrolidinium (SBP+), are proposed to enhance the power capability of hybrid capacitors composed of thick Li4Ti5O12 (LTO) negative (200 μm) and activated carbon (AC) positive electrodes (400 μm), which thus reduces the resistive overvoltage in the system. Detailed studies of the mass transport properties based on the combination of spectroscopy and electrochemical analysis have shown that the presence of SBP+, despite slower Li+ transport in the electrolyte bulk, further reduces overvoltage associated with migration limitation in the thick LTO electrode macropores. This study on the dual-cation electrolyte quantifies the influence of the addition of a supporting electrolyte and shows interest in SBPBF4 addition for increasing the output power density of hybrid capacitors with a thick electrode configuration

Ultrafast charge–discharge characteristics of a nanosized core–shell structured LiFePO4 material for hybrid supercapacitor applications

Highly 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

Stabilizing the Structure of LiCoPO4 Nanocrystals via Addition of Fe3+: Formation of Fe3+ Surface Layer, Creation of Diffusion-Enhancing Vacancies, and Enabling High-Voltage Battery Operation

Factors affecting the cyclability of the Fe-substituted LiCoPO4 (LiCo0.8Fe0.2PO4, LCFP) material were elucidated, including both the structural and electrode/electrolyte stability. Electrochemical characterization of the synthesized LCFP nanoparticles lends clear evidence for improved electrochemical stability of LCP, as well as enhanced rate capability, with Fe3+ substitution. Surface analysis using X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) suggest that Fe enrichment on the surface of LCFP occurs through the oxidation of Fe2+ into Fe3+ in the synthesis process. The Fe3+-rich phase on the LCP surface enhances the stability of the delithiated phase, preventing oxidative reactions with electrolytes during high-voltage operation. This surface protection persists as long as the electrochemical reduction of Fe3+ is avoided by ensuring that the full range of operating voltages lie above the Fe3+/Fe2+ redox potential. Our findings may offer new approaches to stabilize the structure of LCP and other high-voltage positive electrodes for use in 5 V-class Li-ion batteries

Noncrystalline Nanocomposites as a Remedy for the Low Diffusivity of Multivalent Ions in Battery Cathodes

Rechargeable batteries using multivalent metals are among the most promising next-generation battery systems due to their high capacity, high safety, and low cost compared with lithium-ion batteries. However, strong cation−anion interaction degrades diffusion in solid cathodes, an effect that must be mitigated to yield practical multivalent metal batteries. We show that a highly defective iron phosphate−carbon composite prepared by ultracentrifugation serves as a reversible insertion/deinsertion for magnesium ions with, and operates beyond, a 2-V cell voltage at room temperature. A composite of noncrystalline particles that embeds the surrounding carbon structure enhances the magnesium-ion diffusion in the solid phase with stability for cycle life. X-ray absorption spectroscopy, transmission electron microscopy with energy-dispersive X-ray spectroscopy, and high-energy X-ray scattering measurements demonstrate magnesium-ion insertion and extraction in the defective iron phosphate without conversion reactions. This work suggests promising applications for highly defective structures as intercalation hosts for multivalent ions

New Supercapacitors of Hybrid Configurations

In recent years, the improvement of the energy density of nano-composite battery materials has been object of great study. Hybridizing battery and capacitor materials overcome the energy density limitation of existing generation-I capacitors without much sacrificing the cycling performances. Normal battery-capacitor hybrids employ high-energy & sluggish redox electrode and low-energy & fast double-layer electrodes, possibly producing a larger working voltage and higher over-all capacitance. In order to smoothly operate such asymmetric systems, however, the rates of the two different electrodes must be highly balanced. Especially, the redox rates of the battery electrodes must be substantially increased to the levels of double-layer process. In this report, we attempt to identify the essential issues for the realizable hybrids and suggest ways to overcome the rate enhancement by exemplifying ultrafast performance of the Li4Ti5O12 nanocrystal prepared via a unique in-situ material processing technology under ultra-centrifuging.En los últimos años, la mejora de la densidad energética de los nanocompositos empleados en baterías ha sido objeto de un intenso estudio. La unión de un material de baterías y uno de condensadores resuelve las limitaciones de los condensadores de primera generación desde el punto de vista de la densidad energética, sin sacrificar demasiado su comportamiento frente al ciclado. Los híbridos batería-condensador usuales emplean un electrodo redox de alta energía pero lento y otro de doble capa con baja energía y rápido, que posiblemente permiten un mayor voltaje de trabajo y poseen una capacidad global más elevada. Sin embargo, para operar de forma adecuada estos sistemas asimétricos, las velocidades de estos dos electrodos deben ser compensadas. Especialmente, se deben aumentar sustancialmente las velocidades de los procesos redox del electrodo de batería hasta alcanzar las velocidades del proceso de doble capa. En el presente estudio se intentan identificar los aspectos clave para conseguir sistemas híbridos y se sugieren formas de aumentar la velocidad, mostrando como ejemplo el funcionamiento ultrarrápido de nanocristales de Li4Ti5O12 preparados mediante una tecnología única in-situ aplicando ultracentrifugación

New Supercapacitors of Hybrid Configurations

[EN] In recent years, the improvement of the energy density of nano-composite battery materials has been object of great study. Hybridizing battery and capacitor materials overcome the energy density limitation of existing generation-I capacitors without much sacrificing the cycling performances. Normal battery-capacitor hybrids employ high-energy & sluggish redox electrode and low-energy & fast double-layer electrodes, possibly producing a larger working voltage and higher over-all capacitance. In order to smoothly operate such asymmetric systems, however, the rates of the two different electrodes must be highly balanced. Especially, the redox rates of the battery electrodes must be substantially increased to the levels of double-layer process. In this report, we attempt to identify the essential issues for the realizable hybrids and suggest ways to overcome the rate enhancement by exemplifying ultrafast performance of the Li4Ti5O12 nanocrystal prepared via a unique in-situ material processing technology under ultra-centrifuging.[ES] En los últimos años, la mejora de la densidad energética de los nanocompositos empleados en baterías ha sido objeto de un intenso estudio. La unión de un material de baterías y uno de condensadores resuelve las limitaciones de los condensadores de primera generación desde el punto de vista de la densidad energética, sin sacrificar demasiado su comportamiento frente al ciclado. Los híbridos batería-condensador usuales emplean un electrodo redox de alta energía pero lento y otro de doble capa con baja energía y rápido, que posiblemente permiten un mayor voltaje de trabajo y poseen una capacidad global más elevada. Sin embargo, para operar de forma adecuada estos sistemas asimétricos, las velocidades de estos dos electrodos deben ser compensadas. Especialmente, se deben aumentar sustancialmente las velocidades de los procesos redox del electrodo de batería hasta alcanzar las velocidades del proceso de doble capa. En el presente estudio se intentan identificar los aspectos clave para conseguir sistemas híbridos y se sugieren formas de aumentar la velocidad, mostrando como ejemplo el funcionamiento ultrarrápido de nanocristales de Li4Ti5O12 preparados mediante una tecnología única in-situ aplicando ultracentrifugación.The authors are indebted to K. Hata, AIST Japan and S. Iijima (Meijo University, Japan) for supplying SGCNT through NEDO project, Carbon Nanotube Capacitor Project (2006-2011). K.N. is grateful to Y. Gogotsi and B. Dyatkin (Drexel University, U.S.A.) and P. Simon (Paul Sabatier University, France) for valuable discussions. Special thanks to S. Ishimoto (Nippon Chemi-con Corp.) for fabricating test capacitor cells for obtaining supercapacitor performances (Ragone plots).Peer reviewe

Electrochemical kinetics of nanostructure LiFePO4/graphitic carbon electrodes

Lithium cation insertion/deinsertion reaction kinetics in a LiFePO4 (LFP)/graphitic carbon composite material were electrochemically studied with a cavity microelectrode (CME). The LFP/graphitic carbon composite has a core LFP (crystalline/amorphous)/graphitic carbon shell structure. In the crystalline and amorphous LFP phase, different reaction mechanisms were observed and characterized. While the reaction mechanism in the crystalline LFP phase is controlled by Li+ diffusion, the amorphous LFP phase shows a fast, surface-controlled, pseudocapacitive charge-storage mechanism. This pseudocapacitive behavior is extrinsic in origin since it comes from the presence of Fe3+ defects in the structure. These features explain the ultrafast performance of the material which offers interesting opportunities as a positive electrode for assembling high power and high energy hybrid supercapacitors. Keywords: Defective lithium iron phosphate, Core/shell structure, Pseudocapacitive behavior, Ultrafast performance, Hybrid supercapacitor, Cavity microelectrod