Materials Research Advances towards High-Capacity Battery/Fuel Cell Devices(Invited paper)

The world has entered an era featured with fast transportations,instant communications,and prompt technological revolutions,the further advancement of which all relies fundamentally,yet,on the development of cost-effective energy resources allowing for durable and high-rate energy supply.Current battery and fuel cell systems are challenged by a few issues characterized either by insufficient energy capacity or by operation instability and,thus,are not ideal for such highly-demanded applications as electrical vehicles and portable electronic devices.In this mini-review,we present,from materials perspectives,a few selected important breakthroughs in energy resources employed in these applications.Prospectives are then given to look towards future research activities for seeking viable materials solutions for addressing the capacity,durability,and cost shortcomings associated with current battery/fuel cell devices.

Recent progress in cobalt-based compounds as high-performance anode materials for lithium ion batteries

Despite carbonaceous materials are widely employed as commercial negative electrodes for lithium ion battery, an urge requirement for new electrode materials that meet the needs of high energy density, long cycle life, low cost and safety is still underway. A number of cobalt-based compounds（Co（OH）_2, Co_3O_4, CoN, CoS,CoP, NiCo_2O_4, etc.） have been developed over the past years as promising anode materials for lithium ion batteries（LIBs） due to their high theoretical capacity, rich redox reaction and adequate cyclability. The LIBs performances of the cobalt-based compounds have been significantly improved in recent years, and it is anticipated that these materials will become a tangible reality for practical applications in the near future. However, the different types of cobalt-based compounds will result in diverse electrochemical performance. This review briefly analyzes recent progress in this field, especially highlights the synthetic approaches and the prepared nanostructures of the diverse cobalt-based compounds and their corresponding performances in LIBs, including the storage capacity, rate capability, cycling stability and so on.

The roles of graphene in advanced Li-ion hybrid supercapacitors

Lithium-ion hybrid supercapacitors （LIHSs）, also called Li-ion capacitors, are electrochemical energy stor- age devices that combining the advantages of high power density of supercapacitor and high energy density of Li-ion battery. However, high power density and long cycle life are still challenges for the cul~ rent LIHSs due to the imbalance of charge-storage capacity and electrode kinetics between capacitor-type cathode and battery-type anode. Therefore, great efforts have been made on designing novel cathode materials with high storage capacity and anode material with enhanced kinetic behavior for LIHSs. With unique two-dimensional form and numerous appealing properties, for the past several years, the rational designed graphene and its composites materials exhibit greatly improved electrochemical performance as cathode or anode for LIHSs. Here, we summarized and discussed the latest advances of the state- of-art graphene-based materials for LIHSs applications. The major roles of graphene are highlighted as （1） a superior active material, （2） ultrathin 2D flexible support to remedy the sluggish reaction of the metal compound anode, and （3） good 2D building blocks for constructing macroscopic 3D pOFOUS car- bonjgraphene hybrids. In addition, some high performance aqueous LIHSs using graphene as electrode were also summarized. Finally, the perspectives and challenges are also proposed for further develop- ment of more advanced graphene-based LIHSs.

Fabrication and application of hierarchical mesoporous MoO2/Mo2C/C microspheres

Hierarchical mesoporous MoO2/Mo2C/C microspheres,which are composed of primary nanoparticles with a size of about 30 nm,have been designed and synthesized through polymer regulation and subsequent carbonization processes.The as-synthesized microspheres were characterized by XRD,Raman,SEM,TEM,XPS measurements and so on.It was found that polyethylene glycol acted as a structure-directing agent,mild reducing agent and carbon source in the formation of these hierarchical mesoporous Mo O2/Mo2C/C microspheres.Moreover,the electrochemical property of the microspheres was also investigated in this work.Evaluated as an anode material for lithium ion batteries,the hierarchical mesoporous Mo O2/Mo2C/C electrode delivered the discharge specific capacities of 665 and 588 m Ah/g after 100 cycles at current densities of 100 and 200 m A/g,respectively.The satisfactory cycling performance and controllable process facilitate the practical applications of the hierarchical mesoporous Mo O2/Mo2C/C as a potential anode material in high-energy density lithium-ion batteries.

Influence of cerium doping on structure and electrochemical properties of LiNi0.5Mn1.5O4 cathode materials

Pristine LiNi_（0.5）Mn_（1.5）O_4 and cerium doped LiCe_xNi_（0.5–x）Mn_（1.5）O_4（x=0.005, 0.01, 0.02） cathode materials were synthesized by solid-state method. The effect of Ce doping content on structure and electrochemical properties of LiNi_（0.5）Mn_（1.5）O_4 cathode material was systematically investigated. The samples were characterized by X-ray diffraction（XRD）, Fourier transformation infrared spectrometer（FT-IR）, scanning electron microscopy（SEM）, electrochemical impedance spectroscopy（EIS）, cyclic voltammetry（CV） and constant-current charge/discharge tests. The results showed that Ce doping did not change the cubic spinel structure with Fd3m space group, but effectively restrained the formation of Li_xNi_（1–x）O impurity phase. Appropriate Ce doping（x=0.005） could decrease the extent of confusion between lithium ions and transition metal ions, increase the lattice parameter and Ni/Mn disordering degree（Mn^（3＋） content）. The synergic effects of the above factors led to the optimal electrochemical performance of LiCe_（0.005）Ni_（0.495）Mn_（1.5）O_4 sample. The discharge capacity at 10 C rate could reach 115.4 mAh/g, 94.82% of that at 0.2C rate, and the capacity retention rate after 100 cycles at 1C rate could reach 94.51%. However, heavier Ce doping had an adverse effect on the electrochemical properties, which might be due to the lower disordering degree and existence of more CeO_2 secondary phase.

Micro-structured lepidocrocite-type H_(1.07)Ti_(1.73)O_(4)as anode for lithium-ion batteries with an ultrahigh rate and long-term cycling performance

The lepidocrocite-type H_(1.07)Ti_(1.73)O_(4) microsized structures with a tap density of 0.88 g·cm^(-3) were prepared through the ion exchange method with K_(0.8)Li_(0.27)Ti_(1.73)O_(4) powder as the precursor,and they exhibited good rate performance and outstanding cycle stability as an anode material for lithium ion batteries(LIB).The ion exchange method provides favorable conditions for H_(1.07)Ti_(1.73)O_(4) as an anode electrode material for LIBs.X-ray photoelectron spectroscopy(XPS)result demonstrates the existence of defects in the nonstoichiometric H1.07Ti1.73O4,which have a beneficial effect on the LIB performance.The electrochemical performance test proves that the half-cell with microsized H_(1.07)Ti_(1.73)O_(4)as the anode electrode can maintain a specific capacity of 129.5 mAh·g^(-1) after 1100 cycles and 101 mAh·g^(-1)after 3000 long cycles at high current densities of 2.0 and 5.0 A·g^(-1),respectively.In addition,the small volume change rate of 3.6%in H_(1.07)Ti_(1.73)O_(4)during Li ion insertion was confirmed by real-time in situ transmission electron microscopy(TEM).The LiFePO_(4)||H_(1.07)Ti_(1.73)O_(4)full battery exhibits a longterm cycling stability with a specific capacity of73.8 mAh·g^(-1) at a current density of 500 mA·g^(-1) after 200 cycles.

Nanocomposite LiFePO_4·Li_3V_2(PO_4)_3/C synthesized by freeze-drying assisted sol-gel method and its magnetic and electrochemical properties

Nano-sized LiFePO_4·Li_3V_2（PO_4）_3/C was synthesized via a sol-gel route combining with freeze-drying. X-ray diffraction results show that this composite mainly consists of olivine Li Fe PO4 and monoclinic Li3 V2（PO4）3 phases with small amounts of V-doped LiFePO_4 and Fe-doped Li_3V_2（PO_4）_3. The magnetic properties of LiFePO_4·Li_3V_2（PO_4）_3/C are significantly different from LiFePO_4/C. Trace quantities of ferromagnetic impurities and Fe_2P are verified in LiFePO_4/C and LiFePO_4·Li_3V_2（PO_4）_3/C by magnetic tests, respectively. LiFePO_4·Li_3 V_2（PO_4）_3/C possesses relatively better rate capacities and cyclic stabilities, especially at high charge-discharge rates.The initial discharge capacities are 136.4 and 130.0 mA h g^（-1）,and the capacity retentions are more than 98% after 100 cycles at 2C and 5C, respectively, remarkably better than those of LiFePO_4/C. The excellent electrochemical performances are ascribed to the mutual doping of V^（3＋）and Fe^（2＋）, complementary advantages of LiFePO_4 and Li_3V_2（PO_4）_3 phases, the residual high-ordered carbon and Fe_2P with outstanding electric conductivity in the nanocomposite.