Lithium-ion Battery Electrode Nano-materials Research

The lithium ion battery is now prevailing in many portable electronic devices such as cell phones, digital cameras/videos and laptops due to its longer cycling life and higher energy density than other rechargeable batteries. However, its relatively low charge/discharge rate and safety concerns have limited its use in new applications such as regenerative braking in hybrid electric vehicles (HEVs), power backups, power sources for EVs, and portable power tools, which require both high energy and high power densities. The rate limitations result from a number of factors including low ionic (Li+) and electronic conductivities of electrode materials, and slow insertion/extraction rate of Li+ into the active materials.Recent studies show that decreasing the particle size and enlarging the surface area of electrode materials are effective ways to improve the electrochemical performance, especially the rate capability of electrode materials. So, in my dissertation, the work is focused on the nanostructured materials:the preparation, characterization and electrochemical performace of varies nanostructured electrode materials are discussed.1.Single crystallineα-MoO3 nanobelts are synthesized with a facile hydrothermal method.Theα-MoO3 nanobelts grow along the direction of [001],with the length of 5-10μm,width of 200～500 nm and thickness of about 50 nm.When used as cathode materials for rechargeable lithium battery,they exhibit good rate performance.At a low current density,30 mA/g, they deliver a discharge capacity of 264 mAh/g in the voltage of 1.5-3.5 V vs. Li+/Li.At a current density of 5000 mA/g (9000 W/kg), they deliver a capacity of 176 mAh/g, and preserve 114 mAh/g after 50 cycles.2.A series of MoO2 nanomaterials have been synthesized.Firstly, monoclinic MoO2 nanoparticles are synthesized through reduction of MoO3 with ethanol vapor at 400℃During the reduction process, the starting material (MoO3) collapsed into nanoparticles(～100 nm), and on the nanoparticles remains a semi-graphite carbon layer from ethanol decomposition.As anode materials for lithiumn ion battery, the carbon coated MoO2 nanoparticles displays a reversible capacity of about 318 mAh/g in the initial cycle, with capacity retention of 100% after 20 cycles in the range of 0.01～3.00 V vs. lithium metal at a current density of around 500 mA/g. However, at the same discharge/charge condition, the MoO2 microparticles obtained from reduction of MoO3 with H2 only deliver a reversible capacity less than 175 mAh/g. Secondly, carbon coated monoclinic MoO2 nanobelts (MoO2@C nanobelts) are synthesized withα-MoO3 nanobelts as precursor, ethanol as reducer and glucose as protector. In the reduction procedure, glucose decomposes on the surface ofα-MoO3 nanobelts, forming an caramel layer which protects the nanobelts from collapse. The MoO2@C nanobelts delivers a reversible capacity of around 600 mAh/g in the initial cycle, and preserve 550 mAh/g after 30 cylces, at a current density of 500 mAh/g in the range of 0.01-3.00 V vs. lithium metal.When the current density increases to 1000 mA/g, the reversible capacity is close to 550 mAh/g, and retains around 300 mAh/g after 30 cycles.Moreover, tremella-like hexagonal MoO2 was obtained via a Fe2O3-assisted hydrothermal reduction of MoO3 in ethylenediamine aqueous solution.The "tremella" consists of ultrathin MoO2 nanosheets (about 10 nm in thickness),with residual Fe2O3 resting in its body. This structured MoO2 shows a reversible capacity up to 650 mAh/g at a current density of 70 mAh/g in the range of 0.01-3.00 V, and preseves around 300 mAh/g when the current density increase to 70 mAh/g.The above synthesized nanostructured MoO2 show much better rate capability compared with their micro-sized counterpart, which is highly related with their decreased size, and increased surface areas. And among them, MoO2@C nanobelts present the best electrochemical performance, which may because they have optimum size, one-dimensional morphology and perfect carbon coating.3.Disordered mesoporous Ge was prepared by mechanochemical reaction of GeO2 and Mg powders followed by an etching process with HC1 solution.With a pore-distribution concentrated around 10 nm, the product presents a BET surface area of 49.98 m2/g. When using as an anode material for lithium ion battery, the mesoporous Ge exhibits a reversible capacity of 950 mAh/g and retains a capacity of 789 mAh/g after 20 cycles at a current density of 150 mA/g. The cycleability is significantly improved compared with nonporous Ge.4.A low-cost and practical route to prepare carbon nanospheres (CNSs) from pyrolysis of polyacrylonitrile (PAN) nanospheres is introduced.A layer of titanium phosphate is coated on the surface of PAN nanospheres before the carbonization process, which effectively prevents the crosslinking and aggregation of PAN nanoparticles under high temperature. After removal of the coating following the carbonization, CNSs with average size of 50 nm are obtained.The CNSs, with the Li+ diffusion coefficient of 1.59×10-9 cm2/s, present better rate capability compared with carbonaceous mesophase spheres (CMS) as anode material for lithium ion battery.5.Besides the synthesis of nanostructured electrode materials, CMS@MoO3 composite was prepared by coating MoO3 on CMS in order to improve the performance of graphite as an anode for lithium ion battery in propylene carbonate (PC)-based electrolyte. The coated MoO3 layer acts as a protective layer which separates graphite from PC-based electrolyte solution.Cyclic voltammograms and discharge/charge measurement suggest that the cointercalation of PC is suppressed and lithium ions can reversibly intercalate into and deintercalate from the CMS@MoO3.CMS with the optimum amount of MoO3(14.4 wt.% of the composite) presents the best electrochemical performance:it delivers a reversible capacity of 380 mAh/g in 1 M LiClO4 solution of PC/DMC (1:1,v/v), and preserves above 300 mAh/g after 18 cycles.