Ge Ji

National University of Singapore, Tumasik, Singapore

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Publications (25)

  • Source
    Ge Ji · Yong Yu · Qiaofeng Yao · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: After years of development, improvement in the electrochemical properties of MoS2 through structural modifications has reached its limit. Further improvements in the Li+ storage properties of MoS2 must be based on an understanding of the Li+ storage mechanism in MoS2. On this basis, we have developed a novel ternary composite of graphene, MoS2 nanosheets and a small amount (1 wt%) of silver nanoclusters (NCs; MoS2/G/Ag). The presence of the Ag NCs in the composite is, however, instrumental and serves several purposes: immobilization of sulfur, increased association with Li+ and increased spacing between graphene sheets. The presence of this small amount of Ag NCs was able to increase the Li+ storage capacity of MoS2 by 60% (with a discharge capacity of ~1300 mAh g−1 at 0.5 A g−1 compared with 800–850 mAh g−1 for a MoS2/graphene composite without the Ag NCs (MoS2/G)). The MoS2/G/Ag composite also exhibited a very impressive rate performance: discharge capacities of 1040 and 850 mAh g−1 at the very high current densities of 1 and 5 A g−1, respectively (the corresponding values from MoS2/G without the Ag NCs are 790 and 580 mAh g−1).
    Full-text available · Article · Mar 2016
  • [Show abstract] [Hide abstract] ABSTRACT: The increasing interest in Na-ion batteries is based on their lower projected cost relative to Li-ion batteries and hence are more economically viable for the large-scale storage of electrical energy. Similar to Li-ion batteries, the capacity of Na-ion batteries is cathode-limited. Na3V2(PO4)3 (NVP), a prevalent cathode candidate and one of the most stable Na-ion host materials, still exhibits capacity losses in prolonged cycling. We report herein a method which can improve the durability of NVP in extended use. This is done by using a carbon scaffold to constrain the movement of NVP during charge and discharge reactions. The procedure consists of the sol-gel synthesis of densely aligned dense NVP nanofibers under hydrothermal conditions, followed by sucrose infiltration into the interstices of these fibers to form an interdigitated carbon scaffold after calcination. The NVP-carbon nanocomposite fabricated as such shows ultra-stable cycling performance at very high C-rates, 99.9% capacity retention at 20C for more than 10000 cycles, thereby demonstrating the effectiveness of the materials design principles behind this modification strategy.
    Article · Jan 2016 · Journal of Materials Chemistry A
  • Baihua Qu · Ge Ji · Bo Ding · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: There has been strong recent interest in using SnS2/graphene nanocomposites for reversible Li+ storage. Although SnS2 and graphene are 2D nanomaterials, many of their composites reported to date have a random 3D structure. We present here a stacked SnS2/graphene nanocomposite that retains many of the original 2D features of the graphene nanosheets. As a result, significantly improved Li+-storage properties were shown including: 1) good cycle stability (delithiation capacity of 1063 mAh g−1 for 100 cycles at 200 mA g−1 and 918 mAh g−1 for 500 cycles at 1 A g−1); 2) high first-cycle coulombic efficiency (89.7 %); 3) high reversibility of the conversion reaction SnS2+4 Li++4 e−Sn+2 Li2S (87.5 %); and 4) high rate performance (712 mAh g−1 at 5 A g−1). Such good performance corresponds well with a 2D construction, in which Sn nanoparticles are firmly held by graphene nanosheets. The conversion reaction becomes more reversible in the presence of the Sn nanoparticles to contribute to the additional storage capacity.
    Article · May 2015
  • Bo Ding · Ge Ji · Zhou Sha · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: Cost and safety considerations have driven up the interest in LiFePO4 as a lithium-ion battery cathode material. Carbon nanopainting is currently the most common approach to increase the power density of LiFePO4, but more can be done to improve the application performance further. In this study the rate performance of LiFePO4 was increased by using a conductive dual-carbon network that can extract and conduct electrons from the Li+ storage host more effectively than common pyrolyzed carbon. The dual-carbon network consists of a connected network of graphene sheets and a nanoscale continuous coating of pyrolyzed conductive carbon on the surface of the aggregated LiFePO4 nanocrystals. Such a construction supports fast electron transport between the aggregated LiFePO4 nanocrystals as well as within them. Consequently the LiFePO4/C composite fabricated as such delivered very high rate performances even at very high discharge rates (104 mAh g−1 at 50 C where 1 C=170 mA g−1).
    Article · Dec 2014 · Energy Technology
  • Dongyun Chen · Ge Ji · Bo Ding · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: Transition-metal dichalcogenides (TMDs) are a recent addition to a growing list of anode materials for the next-generation lithium-ion battery (LIB). The actual performance of TMDs is however constrained by their limited electronic conductivity. For example, MoS2, the most studied TMD, does not have adequate rate performance even in the few-layer form or after compounding with nitrogen-doped graphene (NG). WS2, a TMD with a higher intrinsic electronic conductivity, is more suitable for high rate applications but its theoretical capacity is lower than that of MoS2. Hence, we hypothesize that a composition-optimized composite of MoS2, WS2, and NG may provide high capacity concurrently with good rate performance. This is a report on the design and preparation of double transition-metal chalcogenide (MoS2/WS2)-nitrogen doped graphene composites where the complementarity of component functions may be maximized. For example the best sample in this study could deliver a high discharge capacity of 1195 mAh·g–1 at 100 mA·g–1 concurrently with good cycle stability (average of 0.02% capacity fade per cycle for 100 cycles) and high rate performance (only 23% capacity reduction with a 50 fold increase in current density from 100 mA·g–1 to 5000 mA·g–1).
    Article · Nov 2014 · Industrial & Engineering Chemistry Research
  • Bo Ding · Wei Chin Tang · Ge Ji · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: The common strategy to address the low electronic conductivity of LiFePO4 is to downsize LiFePO4 and to coat the nanocrystal with conductive carbon film. The major issues with existing carbon coating techniques are thickness and quality control. This paper reports a facile carbon coating method which can provide ultrathin, uniform and fully encapsulating carbon coating on LiFePO4. This coating method capitalizes on the redox chemistry of surface Fe3+ on solvothermally synthesized LiFePO4 nanocrystal, to deposit uniform thin films of polydopamine films. The polymer film is easily carbonized into ultrathin carbon film. The carbon coated LiFePO4 exhibits very high rate performance (143 mAh g-1 at current density of 1700 mA g-1) with excellent capacity retention.
    Article · Nov 2014 · Journal of Power Sources
  • Source
    Baihua Qu · Chuze Ma · Ge Ji · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: A layered SnS2 -reduced graphene oxide (SnS2 -RGO) composite is prepared by a facile hydrothermal route and evaluated as an anode material for sodium-ion batteries (NIBs). The measured electrochemical properties are a high charge specific capacity (630 mAh g(-1) at 0.2 A g(-1) ) coupled to a good rate performance (544 mAh g(-1) at 2 A g(-1) ) and long cycle-life (500 mAh g(-1) at 1 A g(-1) for 400 cycles).
    Full-text available · Article · Jun 2014 · Advanced Materials
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: The unusual properties of graphene and graphene-like (GL-) layered metal dichalcogenides (LMDs, MoS2, WS2 and SnS2) have stimulated strong interest in GL-LMD/graphene composites. Heterostructures which are constructed by stacking GL-LMD and graphene together are expected to extend the usability of these 2D materials beyond graphene alone. This review will focus on recent progress in the synthesis and applications of GL-LMD/graphene composites in energy storage and conversion. The remarkable electrochemical properties of GL-LMD/graphene for reversible lithium storage are highlighted in particular. The applications of these composites in electrochemical and photochemical water splitting for hydrogen generation are also discussed.
    Full-text available · Article · May 2014 · Materials Today
  • Bo Ding · Ge Ji · Yue Ma · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: The performance of lithium manganese phosphate as a lithium-ion battery cathode material is improved by collective and cooperative strategies including Fe substitution, carbon coating, and the assembly of carbon-coated LiMn1-xFexPO4 nanocrystals into a highly dense packing of monodisperse microboxes. These strategies are implemented experimentally by a facile and scalable synthesis method. The dense packing allows the conductive carbon coating to be interconnected into a continuous three-dimensional network for electron conduction. The porosity in the packed structure forms the complementary network for Li+ transport in the electrolyte. The primary particles are nanosized and Fe-substituted to improve the effectiveness of Li+ insertion and extraction reactions in the solid phase. The reduction of transport resistance external and internal to the nanocrystals yields a Li storage host with good rate performance (116 mAh g-1 at 5 C discharge rate where C = 170 mA g-1) and cycle stability (95% retention of initial capacity in 50 cycles). Electrochemical impedance spectroscopy and morphology examination of the cycled microboxes reveal a robust packed structure with stable surfaces.
    Article · Feb 2014 · Journal of Power Sources
  • Yue Ma · Bo Ding · Ge Ji · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: TiO2 nanoparticles aggregated into a regular ball-in-ball morphology were synthesized by hydrothermal processing and converted to carbon-encapsulated F-doped Li4Ti5O12 (LTO) composites (C-FLTO) by solid state lithiation at high temperatures. Through the careful control of the amount of carbon precursor (D(+)-glucose monohydrate) used in the process, LTO encapsulated with a continuous layer of nanoscale carbon was produced. The carbon encapsulation served a dual function: preserving the ball-in-ball morphology during the transformation from TiO2 to LTO and decreasing the external electron transport barriers. The fluoride doping of LTO not only increased the electron conductivity of LTO through trivalent titanium (Ti3+) generation, but also increased the robustness of the structure to repeated lithiation and de-lithiation. The best-performing composite, C-FLTO-2, therefore delivered a very satisfying performance for a LTO anode: a high charge capacity of ~ 158 mA h g-1 at the 1C rate with negligible capacity fading for 200 cycles, and an extremely high rate performance up to 140C.
    Article · Nov 2013 · ACS Nano
  • Bo Ding · Pengfei Xiao · Ge Ji · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: LiMn1-xFexPO4/C (x=0, 0.3) with uniform carbon coating and interspersed carbon particles was prepared by a high-energy ball milling (HEBM) - assisted solid-state reaction. The as-synthesized LiMn0.7Fe0.3PO4/C delivered excellent rate performance as a LiMnPO4 class of materials. Specifically the discharge capacity was 164 mAh/g (96 % of theoretical value) at the 0.05 C rate and 107 mAh/g at the 5 C rate (1 C = 170 mAh/g). Electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) measurements indicated improvements in the transport of electrons and Li+ and the emergence of a single-phase region in lithium extraction and insertion reactions.
    Article · Nov 2013 · ACS Applied Materials & Interfaces
  • Yue Ma · Ge Ji · Bo Ding · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: A scalable method has been developed to encapsulate silicon (Si) nanocrystallites in a mesoporous N-doped carbon matrix for use as the anode in rechargeable lithium-ion batteries. The deliberate use of ionic liquid as the carbon source and pore size engineering of the SiO2 precursor in the synthesis not only produced the ultrafine Si nanocrystals and the mesoporous N-doped carbon matrix, but also their tight integration into a composite (IL–C–Si–MS) with mixed-conducting (ionic/electronic) properties and effective cushioning of the Si volume change during cycling. The good cyclability and rate performance of IL–C–Si–MS demonstrates the effectiveness of this N-doped carbon encapsulation technique for addressing the instability issues of Si-based anodes.
    Article · Oct 2013
  • Ge Ji · Bo Ding · Yue Ma · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: Increasing the power density of lithium-ion batteries at the materials level is still the preferred strategy for meeting the requirements of large-scale applications such as electric vehicles and grid-scale energy storage. This paper reports a method to produce reversible Li-ion storage hosts as nanoparticles furnished with their own current collectors. The consequent decrease in interparticle electrical resistance enabled SnO2 to be used as a lithium-ion battery anode without conductive additives, even surpassing the rate performance of SnO2 with 10 wt % (typical value) carbon black conductive additive. The “current collector” in this case was a thin, conductive LiyTi1−yO2 shell on a core aggregated from SnO2 nanoparticles. Good electrical connectivity in the electrode was maintained even with the expansion and contraction of the Li-ion storage host during discharging and charging. The composite fabricated as such delivered a charge (delithiation) capacity of 743 mAh g−1 at 0.2 A g−1 in the first cycle and 518 mAh g−1 at the end of 30 cycles without conductive additives. The charge capacity after increasing the current density by a factor of five was similar: 735 mAh g−1 at 1 A g−1 in the first cycle and 505 mAh g−1 at the end of 30 cycles. Hence the design was effective in minimizing the capacity loss in cycling at high current densities. Impendence measurements indicated that the charge transfer resistance of the composite without conductive additives was even smaller than the charge transfer resistance of pristine SnO2 mixed with carbon black. The TiO2-coating method can therefore be an effective approach for the preparation of conductive additive-free high-power-density anode materials.
    Article · Oct 2013 · Energy Technology
  • [Show abstract] [Hide abstract] ABSTRACT: Fe-doped Mnx Oy with hierarchical porosity is prepared from a nanocasting technique using amine-functionalized bromomethylated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) membranes as the sacrificial template. The synergistic coupling of a percolating macroporous network, uniformly distributed mesopores, and optimal iron doping is used to improve the electronic and ionic wirings of manganese oxides for Li(+) storage via the conversion reaction. Very impressive Li(+) storage capabilities are shown.
    Article · Sep 2013 · Advanced Materials
  • Dongyun Chen · Ge Ji · Bo Ding · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: Two-dimensional nanosheets can leverage on their open architecture to support facile insertion and removal of Li(+) as lithium-ion battery electrode materials. In this study, two two-dimensional nanosheets with complementary functions, namely nitrogen-doped graphene and few-layer WS2, were integrated via a facile surfactant-assisted synthesis under hydrothermal conditions. The layer structure and morphology of the composites were confirmed by X-ray diffraction, scanning electron microscopy and high-resolution transmission microscopy. The effects of surfactant amount on the WS2 layer number were investigated and the performance of the layered composites as high energy density lithium-ion battery anodes was evaluated. The composite formed with a surfactant : tungsten precursor ratio of 1 : 1 delivered the best cyclability (average of only 0.08% capacity fade per cycle for 100 cycles) and good rate performance (80% capacity retention with a 50-fold increase in current density from 100 mA g(-1) to 5000 mA g(-1)), and may find uses in power-oriented applications.
    Article · Jul 2013 · Nanoscale
  • Hui Liu · Feng Ye · Hongbin Cao · [...] · Jun Yang
    [Show abstract] [Hide abstract] ABSTRACT: The integration of semiconductor and noble metal nanoparticles with controlled structures into a nanosystem can effectively couple various effects specific to the different domains of the nanocomposite for greater application versatility. Herein, we demonstrate the general synthesis of nanocomposites of Ag2S and noble metal nanoparticles with a hollow or cage-bell structure. The synthesis is based on the inside-out diffusion of Ag in core-shell nanoparticles. It begins with the preparation of core-shell Ag-M or core-shell-shell MA-Ag-MB nanoparticles in an organic solvent. The Ag is then removed from the core or from the internal shell and converted into Ag2S by elemental sulfur or sodium sulfide. The Ag2S forms the semiconductor domain in the nanocomposite and shares solid-state interfaces with the resultant hollow or cage-bell structured metal nanoparticle. The structural transformation from core-shell to heterogeneous nanocomposites may provide new opportunities to design and fabricate hybrid nanostructures with interesting physicochemical properties.
    Article · Jun 2013 · Nanoscale
  • Ge Ji · Bo Ding · Zhou Sha · [...] · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: The performance of SnO2 nanoparticle (NP) aggregates for reversible storage of Li(+) was improved after conformal encapsulation of individual aggregates with graphene (i.e., encapsulation without changing the underlying morphology of SnO2 aggregates). Conformal encapsulation was carried out by modifying the surface of SnO2 NP aggregates with amine terminating groups to increase their binding affinity to graphene. The thickness of the graphene encapsulation could then be varied by the amount of graphene oxide (GO) solution used in the preparation. Electron microscopy confirmed the successful coating of graphene as a thin layer on the NP aggregate surface. This unique construction method resulted in SnO2-graphene composites with a satisfying cycling performance. In particular a composite with only 5 wt% graphene could deliver, without the use of any carbon conductive additive, a charge (Li(+) extraction) capacity of 700 mA h g(-1) at the regular current density of 0.1 A g(-1) and 423 mA h g(-1) after a tenfold increase of the current density to 1 A g(-1) in the 0.005-2 V voltage window. There was evidence to suggest that the composite performance was determined by Li(+) diffusion across the basal plane of the graphene layers.
    Article · May 2013 · Nanoscale
  • Yue Ma · Ge Ji · Bo Ding · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: TiO2 microspheres with different morphologies and microstructures were synthesized by a facile solvothermal process. One of the mesoporous microspheres among the synthesized products exhibited high reversible capacity, good rate capability and long cycle life as an anode material for lithium ion batteries. The good electrochemical performance for Li+ storage could be attributed to the synergy of coupling ultra-fine anatase nanocrystallites (6–8 nm) to a uniform distribution of mesopores (pore size of 4–8 nm), leading to concurrent improvements in charge transfer kinetics and the transport of lithium ions and electrons in the material.
    Article · Nov 2012 · Journal of Materials Chemistry
  • Ge Ji · Yue Ma · Bo Ding · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: Current methods for improving the electrochemical performance of lithium-ion battery electrode materials mostly depend on materials design and synthesis. We propose that the unique electrochemical properties of spinel lithium titanate (Li4Ti5O12, LTO) make it suitable as a protective coating to improve the performance of high capacity anode materials. In this study, tin oxide was coated with LTO to reduce the initial irreversible capacity loss because of solid electrolyte interface (SEI) formation and to improve the reversibility (capacity and rate performance) of tin oxide for Li+ storage. The LTO coating was applied to porous hollow tin oxide particles by a two-step process. Experimental measurements showed that the LTO coating shielded most of the direct contact between tin oxide and the electrolyte and hence the ICL due to SEI formation was reduced to mostly that of LTO, which is much lower than tin oxide. In addition the coated tin oxide also showed notable improvements in material cyclability and rate performance.
    Article · Aug 2012 · ChemInform
  • Yue Ma · Chao Zhang · Ge Ji · Jim Yang Lee
    [Show abstract] [Hide abstract] ABSTRACT: One great challenge in designing anode materials for lithium-ion batteries is to satisfy the concurrent requirements for good capacity retention, high rate performance and low first cycle losses. We report here the design and synthesis of a nitrogen-doped carbon encapsulated Fe3O4 composite which performed very well in all these areas. The composite with the optimized carbon content not only showed a high reversible capacity of 850 mA h g−1 for 50 cycles at 100 mA g−1, but was also able to maintain a stable cycling performance at a twenty-fold increase in current density to 2000 mA g−1. More importantly, the composite significantly lowered the irreversible capacity loss in the first cycle compared with other iron oxide anodes reported in the literature. Characterization of the electrode/electrolyte interface indicated the presence of a protective solid electrolyte interface (SEI) layer in which chemically stable LiF and FeF3 were the major constituents. Thus, it is believed that the N-doped carbon coating had effectively modified the surface chemistry at the anode/electrolyte interface to increase the columbic efficiency of cycling and to reduce the secondary reactions in the first cycle of use.
    Article · Mar 2012 · Journal of Materials Chemistry