Developments of lithium-ion batteries and challenges of LiFePO<Subscript>4</Subscript> as one promising cathode material

Developments and developing trends of lithium-ion batteries (LIBs) are summarized first: it is proposed that solid thin film microbatteries and large-scale all-solid-state rechargeable LIBs are the two main developing tendencies. Meanwhile, cost and safety issues are the primary limitations to improve advanced LIBs with excellent electrochemical performance. Next, one of the most promising cathode materials, LiFePO₄, is introduced in detail. Advantages and drawbacks of LiFePO₄ as cathode active material are analyzed, then, main approaches to circumvent its drawbacks proposed by many groups are also summarized. In addition, some mechanism investigations on this cathode material presently and challenging problems waiting for solutions before LiFePO₄ can be commercialized are also discussed in this review.     

Hydrothermal synthesis of LiFePO<Subscript>4</Subscript> as a cathode material for lithium batteries

LiFePO₄ (space group: Pnma) was prepared by hydrothermal method at 170 °C. LiFePO₄ was prepared from precursor solutions consisting of FeSO₄ · 7H₂O, (NH₄)₂HPO₄, and three kinds of Li sources. LiCl, Li(CH₃COO), and LiOH · H₂O were used as Li sources. The pH of the precursor solution varied depending on Li source. The particle size, particle shape, and crystal texture of the obtained LiFePO₄ changed depending on pH. The electrochemical properties of the prepared LiFePO₄ were characterized as a cathode material for lithium batteries in an organic electrolyte at room temperature. The LiFePO₄ particle prepared from the precursor solution with Li(CH₃COO) was flake-like crystal (particle size: 1–2 μm) and had a preferred crystal orientation with a (020) texture. This LiFePO₄ exhibited a discharge capacity of 147 mA h g⁻¹, which was 85% of the theoretical capacity 170 mA h g⁻¹.     

One-step hydrothermal synthesis of Li<Subscript>2</Subscript>FeSiO<Subscript>4</Subscript>/C composites as lithium-ion battery cathode materials

Li₂FeSiO₄/C composites were one-step synthesized under hydrothermal conditions at 200 °C for 72 h using glucose as carbon source. By adjusting the quantity of added glucose, we obtained varied Li₂FeSiO₄/C composites with different size and morphology. A series of electrochemical tests demonstrate that the Li₂FeSiO₄/C nanoparticles with diameters about 20 nm have higher discharge capacity, and slower capacity fading in comparison with Li₂FeSiO₄ and other Li₂FeSiO₄/C composites. Li₂FeSiO₄/C nanoparticles deliver a discharge capacity of 136 mAh g⁻¹ at 0.2 C, and after 100 cycles, the discharge capacity remains 96.1%. Furthermore, Li₂FeSiO₄/C nanoparticles also exhibit an excellent rate capability with a capacity of about 80 mAh g⁻¹ at 10 C.     

Synthesis and electrochemical performance of spherical FeF<Subscript>3</Subscript>/ACMB composite as cathode material for lithium-ion batteries

To improve the rate capability and cyclability of FeF₃ cathode for Li-ion batteries, FeF₃ has been modified by forming FeF₃/ activated carbon microbead (ACMB) composite. The FeF₃/ACMB composite is successfully achieved via a simple chemical route. The morphology and structural properties of the samples are investigated by X-ray diffraction and scanning electron microscopy (SEM). SEM observations demonstrate that FeF₃/ACMB composite has a distinct spherical morphology. Electrochemical tests show that the FeF₃/ACMB composite cathode has higher capacity, better cycleability, and better rate capability than pristine FeF₃. Electrochemical impedance spectra indicate that the FeF₃/ACMB composite electrode has low electrochemical resistance compared with pristine FeF₃, indicating the enhanced conductivity of the FeF₃/ACMB composite.     

Synthesis and electrochemical analyses of vapor-grown carbon fiber/pyrolytic carbon-coated LiFePO<Subscript>4</Subscript> composite

A vapor-grown carbon fiber/pyrolytic carbon-coated LiFePO₄ (VGCF/PCLFP) composite has been prepared in one step through a solid-state reaction accompanied by a gas-phase decomposition process. This method leads to the formation of a conductive network composed of pyrolytic carbon layer and in situ vapor-grown carbon fiber in the composite. The amount of carbon in the composite has been determined by a modified formula based on thermogravimetric analysis to be around 3.0 wt%. The optimized electrode of VGCF/PCLFP composite can deliver 150 mAhg⁻¹ at 0.5 C rate, 137 mAhg⁻¹ at 1.0 C rate and 132 mAhg⁻¹ at 3.0 C rate. And its discharge capacity loses only ~4% at a higher rate of 3.0 C after 100 cycles. The area-specific impedance of a cell fabricated with VGCF/PCLFP composite is lower than that made of only pyrolytic carbon-coated LiFePO₄, reported here for the purpose of comparison. In comparison to the electrode made of carbon black/LiFePO₄ composite (10 wt% carbon), the charge transfer resistance of the VGCF/PCLFP composite electrode decreases from 165 to 91 Ω. This technique presents an attractive way to produce high-performance LiFePO₄ cathode material through a low-cost high-efficiency process.     

Synthesis of Ag@LiFePO4/C composite cathode material by electrodeposition method

Ag@LiFePO4/C particles were prepared by a facile and one-step electrodeposition method. This core-shell structure cathode material has excellent electrochemical performances (both high charge/discharge rates and good cyclability), which can meet the demands of many high power applications.     

LiFePO<Subscript>4</Subscript>/TiO<Subscript>2</Subscript>/Pt composite film used as effective and robust counter electrode for dye sensitized solar cells

A series of composite films based on LiFePO₄/TiO₂/Pt were synthesized and used as counter electrodes for dye sensitized solar cells (DSSCs). The composites are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET). These analysis results demonstrate that the crystal structure of LiFePO₄ in composite is not changed, and the prepared LiFePO₄/TiO₂/Pt composite films hold a rough surface and porous structure which provide more catalytic activity sites for I₃ ^? reduction and more space for I^?/I₃ ^? diffusion. The DSSC based on LiFePO₄/TiO₂/Pt composite CEs shows a high power conversion efficiency of 6.23% at a low Pt dosage of 2%, comparable to the conventional magnetron sputtering Pt CE (6.31%). The electrochemical analysis reveals that the presented composite CEs have good electrocatalytic activity and low charge transfer resistance. Furthermore, the DSSCs based on LiFePO₄/TiO₂/Pt composite CE exhibit high stability under the continuous tests condition and electrolyte soaking. The results suggest that this LiFePO₄-based composite film could be a perspective electrode for practical application of DSSCs and it maybe provide a potential for further research about photo-charging lithium-ion batteries.     

锂离子电池正极材料LiFePO4的研究现状及展望

综述了橄榄石LiFePO4的晶体结构,重点讨论了近年来各种合成LiFePO4的制备方法以及改性研究,并对其发展方向作出了展望.     

High rate performance of LiFePO<Subscript>4</Subscript> cathode materials co-doped with C and Ti<Superscript>4+</Superscript> by microwave synthesis

Nanostructured LiFePO₄ powder with a narrow particle size (ca. 100 nm) for high rate lithium-ion battery cathode application was obtained by microwave heating and using citric acid as carbon source. The microstructures and morphologies of the synthesized materials were investigated by X-ray diffraction and scanning electron microscope while the electrochemical performances were evaluated by galvanostatic charge-discharge. The carbon coating and Ti⁴⁺ could improve the conductivity both between the LiFePO₄ particles and the intrinsic electronic conductivity. The LiFePO₄ doped with 5% C and 1% Ti⁴⁺ resulted in a specific capacity of 114·95 mAh·g⁻¹ and 102·4 mAh·g⁻¹ at discharge rates of 0·3C and 1C, respectively, and the cycle performance is very good.     

Synthesis of SnW3O9/C as novel anode material for lithium-ion battery application

Journal of Materials Science: Materials in Electronics - Both SnW3O9 and SnW3O9/C microspheres were successfully synthesized by a simple solvothermal method combined with low-temperature heat...     

Synthesis of well-defined Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods/N-doped graphene for lithium-ion batteries

The geometric size and distribution of magnetic nanoparticles are critical to the morphology of graphene (GN) nanocomposites, and thus they can affect the capacity and cycling performance when these composites are used as anode materials in lithium-ion batteries (LiBs). In this work, Fe₃O₄ nanorods were deposited onto fully extended nitrogen-doped GN sheets from a binary precursor in two steps, a hydrothermal process and an annealing process. This route effectively tuned the Fe₃O₄ nanorod size distribution and prevented their aggregation. The transformation of the binary precursor was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). XPS analysis indicated the presence of N-doped GN sheets, and that the magnetic nanocrystals were anchored and uniformly distributed on the surface of the flattened N-doped GN sheets. As a high performance anode material, the structure was beneficial for electron transport and exchange, resulting in a large reversible capacity of 929 mA·h·g^–1, high-rate capability, improved cycling stability, and higher electrical conductivity. Not only does the result provide a strategy for extending GN composites for use as LiB anode materials, but it also offers a route for the preparation of other oxide nanorods from binary precursors.

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Hierarchical porous onion-shaped LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> as ultrahigh-rate cathode material for lithium ion batteries

Spinel LiMn₂O₄ is a widely utilized cathode material for Li-ion batteries. However, its applications are limited by its poor energy density and power density. Herein, a novel hierarchical porous onion-like LiMn₂O₄(LMO) was prepared to shorten the Li⁺ diffusion pathway with the presence of uniform pores and nanosized primary particles. The growth mechanism of the porous onion-like LiMn₂O₄ was analyzed to control the morphology and the crystal structure so that it forms a polyhedral crystal structure with reduced Mn dissolution. In addition, graphene was added to the cathode (LiMn₂O₄/graphene) to enhance the electronic conductivity. The synthesized LiMn₂O₄/graphene exhibited an ultrahigh-rate performance of 110.4 mAh·g^–1 at 50 C and an outstanding energy density at a high power density, maintaining 379.4 Wh·kg^–1 at 25,293 W·kg^–1. Besides, it shows durable stability, with only 0.02% decrease in the capacity per cycle at 10 C. Furthermore, the (LiMn₂O₄/graphene)/graphite full-cell exhibited a high discharge capacity. This work provides a promising method for the preparation of outstanding, integrated cathodes for potential applications in lithium ion batteries.

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新型锂电池正极材料——蒽醌/介孔炭寄生型复合物

将9,10-蒽醌（AQ）通过升华法填充在介孔炭（MC）的孔中,制备出蒽醌/介孔炭寄生型复合物。用TG/DSC、SEM、氮吸附等方法对介孔炭及复合材料进行了表征,应用恒流充放电、循环伏安法、交流阻抗等测试手段对复合材料的电化学性能进行了测试。结果表明炭/醌复合电极的比容量和循环性能较蒽醌电极有了很大程度的提高。其中蒽醌填充量为75%的复合材料首次放电比容量达到216mAh/g,50次循环之后,比容量仍有76mAh/g,是蒽醌电极比容量的2倍。     

两步碳热还原方法制备LiFePO4/C阴极复合材料

橄榄石型的LiFePO4材料是一种具有良好发展潜力的锂离子电池阴极材料。应用一种两步烧结的碳热还原方法制备出LiFePO4阴极材料,该法缩短了高温烧结阶段的时间,从而达到抑制晶粒长大的目的,并对LiFePO4进行原位碳包覆,制得LiFePO4/C复合阴极材料。对制得的材料进行0.1C恒电流充放电测试,首次放电容量为149.4mAh/g,首次放电效率可以达到93.5%。而用作对比的一步法烧结碳热还原样品在0.1C恒流充放电试验中首次容量只有99.1mAh/g,放电效率是81.4%,并对制备反应及充放电结果的机理进行了探讨。     

Ionic liquid-derived Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/carbon nano-onions composite and its enhanced performance as anode for lithium-ion batteries

In this work, a novel composite of Co₃O₄ nanoparticle and carbon nano-onions (CNOs) is synthesized by using ionic liquid as carbon and nitrogen source through a facile carbothermic reduction followed by low-temperature oxidation method. The SEM and HRTEM images reveal that the Co₃O₄ particles are homogenously embedded in the CNOs. Due to the unique nano-structure, the electrolyte contacts well with the active materials, leading to a better transfer of lithium ions. Moreover, the unique nano-structure not only buffers the volume changes but also facilitates the shuttling of electrons during the cycling process. As a result, the electrode made up of Co₃O₄/CNOs composite delivers favorable cycling performance (676 mAh g^?1 after 200 cycles) and rate capability (557 mAh g^?1 at the current of 1 C), showing a promising prospect for lithium-ion batteries as anode materials.     

Modeling effects of particle size in LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode material on peak current level in lithium-ion batteries

Mathematical simulation of the current transfer characteristics of submicron and nanodimensional objects has been used to predict some properties of a structured electrode material in lithium-ion batteries. A diffusion model of the intercalation-deintercalation (ID) process is presented. Using this model, the current density in cathode material particles (nanocrystals) has been numerically simulated and an equation for the current density in the absence of diffusion polarization is derived. The proposed model can be used to optimize the ID process for improving the functional properties of cathode materials and increasing the working life of cathodes. Results are illustrated by examples for LiMn₂O₄ cathode material.     

Synthesis and microwave absorption properties of sandwich-type CNTs/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/RGO composite with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> as a bridge

A novel sandwich-type CNTs/Fe₃O₄/RGO composite with Fe₃O₄ as a bridge was successfully prepared through a simple solvent-thermal and ultrasonic method. The structure and morphology of the composite have been characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. This new structure can effectively prevent the agglomeration of GO and the combination of CNTs/Fe₃O₄ and RGO shows a strong reflection loss (R_L) (?50 dB) at 8.7 GHz with absorber thickness of 2.5 mm. Moreover, compared with CNTs/Fe₃O₄/GO composite, it is found that the thermal treating process is beneficial to enhance the microwave absorption properties, which may be attributed to high conductivity of RGO. On this basis, the microwave absorbing mechanism is systematically discussed. All the data show that the CNTs/Fe₃O₄/RGO composite exhibits excellent microwave absorption properties with light density and is expected to have potential applications in microwave absorption.     

Kinetics of lithium deintercalation from LiFePO<Subscript>4</Subscript>

We have studied the kinetics of lithium deintercalation from lithium iron phosphate in a cathode material for batteries. The main contribution to the resistance of the cell is made by interfaces and the resistance of LiFePO₄ grains. The FePO₄ solubility in LiFePO₄ is 4.0%. The lithium deintercalation process can be described in terms of a heterogeneous grain model and its rate is controlled by the lithium diffusion across the layer of the forming product (FePO₄).     

Nonstoichiometric LiFePO4/C nanofibers by electrospinning as cathode materials for lithium-ion battery

Nonstoichiometric lithium iron phosphate/carbon (LiFePO₄/C) composite nanofibers are prepared by electrospinning and subsequent calcination. The ratio of raw materials exerts great effects on the morphology and electrochemical performance of LiFePO₄/C nanofibers, and the sample prepared using the LiH₂PO₄/FeC₆H₅O₇ ratio of 1.3 has good fibrous morphology, porous structure and high purity, thus exhibiting high capacity and stability for lithium-ion battery.