Yun-Sung Lee

Flexible energy storage devices based on graphene paper

Recently, great interest has been aroused in flexible/bendable electronic equipment such as rollup displays and wearable devices. As flexible energy conversion and energy storage units with high energy and power density represent indispensable components of flexible electronics, they should be carefully considered. However, it is a great challenge to fabricate flexible/bendable power sources. This is mainly due to the lack of reliable materials that combine both electronically superior conductivity and mechanical flexibility, which also possess high stability in electrochemical environments. In this work, we report a new approach to flexible energy devices. We suggest the use of a flexible electrode based on free-standing graphene paper, to be applied in lithium rechargeable batteries. This is the first report in which graphene paper is adopted as a key element applied in a flexible lithium rechargeable battery. Moreover graphene paper is a functional material, which does not only act as a conducting agent, but also as a current collector. The unique combination of its outstanding properties such as high mechanical strength, large surface area, and superior electrical conductivity make graphene paper, a promising base material for flexible energy storage devices. In essence, we discover that the graphene based flexible electrode exhibits significantly improved performances in electrochemical properties, such as in energy density and power density. Moreover graphene paper has better life cycle compared to non-flexible conventional electrode architecture. Accordingly, we believe that our findings will contribute to the full realization of flexible lithium rechargeable batteries used in bendable electronic equipments.

LiMnPO4 – A next generation cathode material for lithium-ion batteries

Development of an eco-friendly, low cost and high energy density (∼700 W h kg−1) LiMnPO4 cathode material became attractive due to its high operating voltage ∼4.1 V vs. Li falling within the electrochemical stability window of conventional electrolyte solutions and offers more safety features due to the presence of a strong P–O covalent bond. The vacancy formation energy for LiMnPO4 was 0.19 eV higher than that for LiFePO4, resulting in a 10−3 times-diluted complex concentration, which represents the main difference between the kinetics in the initial stage of charging of two olivine materials. This review highlights the overview of current research activities on LiMnPO4 cathodes in both native and substituted forms along with carbon coating synthesized by various synthetic techniques. Further, carbon coated LiMnPO4 was also prepared by a solid-state approach and the obtained results are compared with previous literature values. The challenges and the need for further research to realize the full performance of LiMnPO4 cathodes are described in detail.

Electric double layer capacitor and its improved specific capacitance using redox additive electrolyte

Halogen (iodide, I−) added aqueous electrolyte facilitates the capacitive behaviour of biomass derived activated carbon based electric double layer capacitors. To produce economically viable electrodes in large scale for supercapacitors (SCs), the activated carbons (ACs) prepared from Eichhornia crassipes (common water hyacinth) by ZnCl2 activation. The prepared ACs were characterized by XRD, Raman, FT-IR and surface area, pore size and pore volume analysis. The electrochemical properties of the SCs were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycling stability. The 3I−/I3−, 2I−/ I2, 2I3−/3I2 and I2/IO3− pairs produce redox peaks in CV and a large Faradaic plateau in charge–discharge curves. Similarly, I− ions improves the good ionic conductivity (lower charge transfer resistance) at the electrode/electrolyte interface which was identified through EIS studies. The calculated specific capacitance and energy density was 472 F g−1 and 9.5 W h kg−1 in aqueous solution of 1 M H2SO4. Interestingly, nearly two-fold improved specific capacitance and energy density of 912 F g−1 and 19.04 W h kg−1 were achieved when 0.08 M KI was added in 1 M H2SO4 electrolyte with excellent cycle stability over 4000 cycles. Subsequently, this improved specific capacitance and energy density was compared with 0.08 M KBr added to 1 M H2SO4 (572 F g−1, 11.6 W h kg−1) and 0.08 M KI added to 1 M Na2SO4 (604 F g−1, 12.3 W h kg−1) as electrolytes.

Preparation and characterization of nano-crystalline LiNi0.5Mn1.5O4 for 5 V cathode material by composite carbonate process

Abstract LiNi 0.5 Mn 1.5 O 4 has been synthesized using two different synthetic methods; a sol–gel method and a composite carbonate process. LiNi 0.5 Mn 1.5 O 4 obtained by the sol–gel method showed a nickel oxide impurity in the XRD diagram and two voltage plateaus at 4.1 and 4.7 V upon cycling. However, the LiNi 0.5 Mn 1.5 O 4 compound obtained by the composite carbonate process exhibited a pure cubic spinel structure ( Fd 3 m ) without any impurities and only one voltage plateau at 4.7 V in the charge/discharge curves. Furthermore, it showed an excellent cycling retention rate of over 96% in the high temperature test. The well-developed LiNi 0.5 Mn 1.5 O 4 obtained by the composite carbonate process contained many spherical particles of about 3–4 μm, made up of small nano-sized particles (50–100 nm). It was a unique powder characterization and these nano-sized particles improved the cycling performance of the LiNi 0.5 Mn 1.5 O 4 obtained by composite carbonate process.

Synthesis and characterization of lithium aluminum-doped spinel (LiAlxMn2−xO4) for lithium secondary battery

Abstract LiAl x Mn 2− x O 4 has been synthesized using various aluminum starting materials, such as Al(NO 3 ) 3 , Al(OH) 3 , AlF 3 and Al 2 O 3 at 600–800°C for 20xa0h in air or oxygen atmosphere. A melt-impregnation method was used to synthesize Al-doped spinel with good battery performance in this research. The Al-doped content and the intensity ratio of (3xa01xa01)/(4xa00xa00) peaks can be important parameters in synthesizing Al-doped spinel which satisfies the requirements of high discharge capacity and good cycleability at the same time. The decrease in Mn 3+ ion by Al substitution induces a high average oxidation state of Mn ion in the LiAl x Mn 2− x O 4 material. The electrochemical behavior of all samples was studied in Li/LiPF 6 -EC/DMC (1:2 by volume)/LiAl x Mn 2− x O 4 cells. Especially, the initial and last discharge capacity of LiAl 0.09 Mn 1.97 O 4 using LiOH, Mn 3 O 4 and Al(OH) 3 complex were 128.7 and 115.5xa0mAh/g after 100 cycles. The Al substitution in LiMn 2 O 4 was an excellent method of enhancing the cycleability of stoichiometric spinel during electrochemical cycling.

Graphene–Nanotube–Iron Hierarchical Nanostructure as Lithium Ion Battery Anode

In this study, we report a novel route via microwave irradiation to synthesize a bio-inspired hierarchical graphene--nanotube--iron three-dimensional nanostructure as an anode material in lithium-ion batteries. The nanostructure comprises vertically aligned carbon nanotubes grown directly on graphene sheets along with shorter branches of carbon nanotubes stemming out from both the graphene sheets and the vertically aligned carbon nanotubes. This bio-inspired hierarchical structure provides a three-dimensional conductive network for efficient charge-transfer and prevents the agglomeration and restacking of the graphene sheets enabling Li-ions to have greater access to the electrode material. In addition, functional iron-oxide nanoparticles decorated within the three-dimensional hierarchical structure provides outstanding lithium storage characteristics, resulting in very high specific capacities. The anode material delivers a reversible capacity of ~1024 mA · h · g(-1) even after prolonged cycling along with a Coulombic efficiency in excess of 99%, which reflects the ability of the hierarchical network to prevent agglomeration of the iron-oxide nanoparticles.

Microwave synthesis of graphene/magnetite composite electrode material for symmetric supercapacitor with superior rate performance

Pristine Fe3O4 and Fe3O4–graphene composites were synthesized by using a green and low cost urea-assisted microwave irradiation method and were utilized as electrode materials for symmetric supercapacitor applications. The Fe3O4–graphene symmetric cell exhibited a better electrochemical performance than that of the Fe3O4 cell with enhanced rate performances. The Fe3O4–graphene symmetric cell delivered a stable discharge capacitance, energy and power densities of about 72 F g−1, 9 Wh kg−1 and 3000 W kg−1, respectively at 3.75 A g−1 current density over 100u2006000 cycles between 0–1 V. The impedance studies also suggested that the Fe3O4–graphene symmetric cell showed lower resistance and high conductivity due to the small particle size, large surface area and good interaction between Fe3O4 particles and graphene layers.

Synthesis and improved electrochemical performances of nano β-NiMoO4–CoMoO4·xH2O composites for asymmetric supercapacitors

Nano-sized β-NiMoO4–CoMoO4·xH2O composites were synthesized by a solution combustion synthesis (SCS) technique. The effect of weight ratio of transition metal on the electrochemical capacitive performance of the nanocomposites was investigated by cyclic voltammetry and galvanostatic charge–discharge methods. The NiMoO4–CoMoO4·xH2O nanocomposite with weight ratio of 3:1 (Ni:Co) exhibits enhanced capacitive behaviour relative to other composites and delivered a maximum specific capacitance of 1472 Fg−1 at a current density of 5 mAcm−2. The enhancement in specific capacitance is due to the small particle size, uniform size distribution, high surface area and high weight fraction of Ni. The synergistic effect of nickel and cobalt improves the electrochemical behaviour relative to pure nickel and cobalt molybdates. A full cell was fabricated using the β-NiMoO4–CoMoO4·xH2O nanocomposite (3:1) and activated carbon (AC) as a positive and negative electrode, respectively. The cell delivered high capacitance (80 Fg−1) and energy density (28 Wh kg−1) and good cycling stability up to 1000 cycles.

Structural and electrochemical characterization of lithium excess and Al-doped nickel oxides synthesized by the sol–gel method

Abstract The effects of excess lithium and aluminum doping in nickel oxide were investigated in an attempt to improve electrochemical properties of the layered LiNiO 2 . Li 1+ x NiO 2 ( x =0–0.02) and LiAl y Ni 1− y O 2 ( y =0–0.3) powders were synthesized by a sol–gel method using adipic acid as a chelating agent. The electrochemical properties of the synthesized materials were explored at room and high temperatures. Gas analysis during decomposition of gel precursors revealed that oxygen might play an important role in the synthesis of highly crystallized LiNiO 2 . Although the electrochemical test of the Al-doped samples showed a low initial discharge capacity of about 140 mAh g −1 , the capacity loss with repeated cycling was very small at room temperature. Furthermore the fade in capacity of this cell at high temperature (50°C) was almost negligible. The Al-doping of the LiNiO 2 cathode material was very effective in improving cycle performance at high temperature due to the enhanced stability of LiNiO 2 structure.

Cycle characterizations of LiMxMn2-xO4 (M = Co, Ni) materials for lithium secondary battery at wide voltage region

LiMxMn2−xO4 (M=Co, Ni) materials have been synthesized by a melt-impregnation method using γ-MnOOH as the manganese source. Highly crystallized LiMxMn2−xO4 compounds were synthesized at a calcination temperature of 800°C for 24 h in air. All compounds show a single phase except for LiNi0.5Mn1.5O4 based on the X-ray diffraction (XRD) diagram. With the increase of the doping content from 0.1 to 0.5, the capacity of doping materials decreases mainly in the 4 V region. n nAlthough LiM0.5Mn1.5O4 (M=Co, Ni) compound shows a small capacity in the (3+4) V region compared with parent LiMn2O4, it is a very effective material in reducing capacity loss in the 3 V region that is caused by the Jahn–Teller distortion. The doping of Co and Ni ions in the LiMn2O4 cathode material promotes the stability of this structure and provides an excellent cyclability.