Rational design of mesoporous LiFePO4@C nanofibers as cathode materials for energy storage

In this work, the mesoporous LiFePO₄@C nanofibers have been successfully fabricated through a facile electrospinning method. The structure, morphology, chemical composition and lithium storage performance have been systematically investigated. The results reveal that the LiFePO₄ grains with particle size of ～15 nm are uniformly dispersed in the mesoporous carbon nanofibers. The LiFePO₄@C electrode presents a high reversible capacity and excellent rate performance. It delivers a discharge capacity of 107 mAh g⁻¹ and retains 105 mAh g⁻¹ over 200 cycles at 10C. The excellent electrochemical performances are attributed to the novel nanostructure where LiFePO₄ nanoparticles are embedded in the carbon fibers. This designed structure can significantly enhance the conductivity of LiFePO₄@C, accelerate the diffusion of electrolyte, and thus facilitate the transport of electrons and Li-ions.     

Improvement of electrochemical and thermal stability of LiFePO4@C batteries by depositing amorphous silicon film

Amorphous silicon (α-Si) films are deposited on LiFePO₄@C electrode by using vacuum thermal evaporation deposition technique and the effect of α-Si film on electrochemical performance of LiFePO₄@C cells is investigated systematically by the charge–discharge testing, cyclic voltammograms and AC impedance spectroscopy, respectively. The results reveal that the present of α-Si film on electrode surface could remarkably improve the electrochemical performance at high charge/discharge rate, especially at elevated temperature. This enhancement may be attributed to the amelioration of the electrochemical dynamics on the electrode/electrolyte interface resulting from the beneficial effects of α-Si film, which might significantly suppress the rise of both of the surface film resistance and charge transfer resistance.     

Enhanced rate performance of nano–micro structured LiFePO4/C by improved process for high-power Li-ion batteries

A spherical carbon-coated nano–micro structured LiFePO₄ composite is synthesized for use as a cathode material in high-power lithium-ion batteries. The composites are synthesized through carbothermal reduction with two sessions of ball milling (before and after pre-sintering of precursor) followed by spray-drying with the dispersant of polyethylene glycol added. The structure, particle size, and surface morphology of the cathode active material and the properties of the coated carbon are investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Results indicate that the LiFePO₄/C composite has a spherical micro-porous morphology composed of a large number of carbon-coated nano-spheres linked together with an ordered olivine structure. The carbon on the surface of LiFePO₄ effectively reduces inter-particle agglomeration of the LiFePO₄ particles. A galvanostatic charge–discharge test indicates that the LiFePO₄/C composites exhibit initial discharge capacities of 155 mAh g⁻¹ and 88 mAh g⁻¹ at 0.2 C and 20 C rates with the end of discharge voltage of 2.5 V, respectively. This behavior is ascribed to the unique spherical structure, which shortens lithium ions diffusion length and improves the electric contact between LiFePO₄ particles.     

Effects of neodymium aliovalent substitution on the structure and electrochemical performance of LiFePO4

LiFe_1−xNd_xPO₄/C (x = 0-0.08) cathode material was synthesized using a solid-state reaction. The synthesis conditions were optimized by thermal analysis of the precursor and magnetic properties of LiFePO₄/C. The structure and electrochemical performances of the material were studied using XRD, FE-SEM, EDS, electrochemical impedance spectroscopy and galvanostatic charge-discharge. The results show that a small amount of aliovalent Nd³⁺ ion-dopant substitution on Fe²⁺ ions can effectively reduce the particle size of LiFePO₄/C. Cell parameters of LiFe_1−xNd_xPO₄ (x = 0.04-0.08) were calculated, and the results showed that LiFe_1−xNd_xPO₄/C had the same olivine structure as LiFePO₄. LiFe_0.4Nd_0.6PO₄/C delivers the discharge capacity of 165.2 mAh g⁻¹ at rate of 0.2 C and the capacity retention rate is 92.8% after 100 cycles. Charge-transfer resistance decreases with the addition of glucose and Nd³⁺ ions. Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) was synthesized and PZS nanorods were used as a carbon source to coat LiFePO₄. All of the results show that aliovalent doping substitution of Fe in LiFePO₄ is well tolerated.     

锂硫电池石墨烯/纳米硫复合正极材料的制备及电化学性能

利用热解还原将Hummers法制得的氧化石墨烯还原为石墨烯,并采用化学沉淀法将纳米硫成功负载到石墨烯片层上,获得石墨烯/纳米硫（RGO/nano-S）正极复合材料。利用FT-IR、XRD、SEM、TEM和Raman对所制备复合材料的微观结构、形貌等进行表征,采用恒流充放电、循环伏安法和交流阻抗法对复合材料的电化学性能进行研究。研究结果表明,热还原所得石墨烯褶皱的表面形成容纳硫及多硫离子的空间,有助于缓解活性物质溶解和抑制多硫离子迁移;同时,均匀分布的纳米硫能更好地与电解液接触,在石墨烯的导电网络上增大了电化学反应面积,进而改善了该材料作为锂硫电池的实际放比电容量和倍率循环性能。     

Synthesis of Mn3O4/N-doped graphene hybrid and its improved electrochemical performance for lithium-ion batteries

Mn₃O₄/N-doped graphene (Mn₃O₄/NG) hybrids were synthesized by a simple one-pot hydrothermal process. The scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray powder diffraction (XRD), Thermogravimetric analysis (TG), Raman Spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the microstructure, crystallinity and compositions. It is demonstrated that Mn₃O₄ nanoparticles are high-dispersely anchored onto the individual graphene nanosheets, and also found that, in contrast with pure Mn₃O₄ obtained without graphene added, the introduction of graphene effectively restricts the growth of Mn₃O₄ nanoparticles. Simultaneously, the anchored well-dispersed Mn₃O₄ nanoparticles also play a role as spacers in preventing the restacking of graphene sheets and producing abundant nanoscale porous channels. Hence, it is well anticipated that the accessibility and reactivity of electrolyte molecules with Mn₃O₄/NG electrode are highly improved during the electrochemical process. As the anode material for lithium ion batteries, the Mn₃O₄/NG hybrid electrode displays an outstanding reversible capacity of 1208.4 mAh g⁻¹ after 150 cycles at a current density of 88 mA g⁻¹, even still retained 284 mAh g⁻¹ at a high current density of 4400 mA g⁻¹ after 10 cycles, indicating the superior capacity retention, which is better than those of bare Mn₃O₄, and most other Mn₃O₄/C hybrids in reported literatures. Finally, the superior performance can be ascribed to the uniformly distribution of ultrafine Mn₃O₄ nanoparticles, successful nitrogen doping of graphene and favorable structures of the composites.     

Synthesis and characterization of sulfur-doped carbon decorated LiFePO4 nanocomposite as high performance cathode material for lithium-ion batteries

This is the first report where crystalline sulfur-doped carbon decorated LiFePO₄ nanocomposite are employed as cathode material for lithium-ion batteries. The electrode has been synthesized via a sol–gel route, in which benzyl disulfide and oxalic acid are used as the sulfur and carbon source, respectively. Meanwhile, the as-synthesized sample is characterized by XRD, SEM, EDS mapping, TEM, Raman spectra, XPS and electrochemical techniques. The results reveal that the sulfur-doped carbon is uniformly coated on the surface of LiFePO₄ without destroying the crystal structure of the bulk material. Moreover, both the electronic conductivity and defect level of the carbon clearly increase, as a consequence, the electron and Li-ion diffusion of the electrode is further improved. As a cathode material for lithium-ion batteries, it exhibits a more outstanding electrochemical performance, especially the rate capability and long cyclic performance. Thus, it can be drawn a conclusion that the sulfur-doped carbon coating approach is an effective method to improve the electrochemical performance of LiFePO₄ and could be extended to modify other electrode materials for lithium-ion batteries.     

(010) facets dominated LiFePO4 nano-flakes confined in 3D porous graphene network as a high-performance Li-ion battery cathode

Olivine-structured LiFePO₄ (LFP) has been widely considered as one of the most promising and safest high-power positive electrode materials for lithium-ion batteries (LIBs) as a power source in the electric transportation. However, the electrochemical behavior of LFP for lithium-storage is seriously restrained by its intrinsic feature of low electrical conductivity and poor lithium-ion diffusion ability. In this research, LFP nano-flakes with oriented (010) facets were prepared through the solvothermal method, and 3D porous composite of LFP nano-flakes confined on graphene (LFP@G) was synthesized by freeze-drying concentrated graphene-oxide-gel containing LFP nano-flakes followed by a heat-treatment process. As the cathode materials for LIBs, LFP@G composite can release a reversible specific capacity of 129 mAh g^?1 at a high current rate of 20?C. Meanwhile, a long cycling stability for LFP@G composite with a capacity of 139.8 mAh g^?1 over 600 cycles up to 10?C can be achieved. The superior electrochemical Li-storage properties of LFP@G composite can be ascribed to the fast lithium-ion transfer channels of LFP originated from the exposed (010) planes, shortened lithium-ion diffusion distance, and the excellent two-phase electric contact between LFP and graphene in the 3D porous graphene conductive network for fast electron and lithium-ion transport.     

Insight into the improvement of rate capability and cyclability in LiFePO4/polyaniline composite cathode

A simple and efficient strategy was employed to enhance high-rate performance for carbon-coated LiFePO₄ (C-LFP) by incorporating with conducting polymer polyaniline (PANI). C-LFP was synthesized via a solid state reaction whereas PANI was formed in situ by chemical oxidative polymerization of aniline with ammonium persulfate as an oxidizer to achieve the C-LFP/PANI composites. Specific capacities as high as 165 mAh g⁻¹ at 0.2 C, 133 mAh g⁻¹ at 7 C and 123 mAh g⁻¹ at 10 C were obtained in C-LFP/7 wt.% PANI composite. Moreover, the composite exhibits remarkably improved cyclability as compared with the parent C-LFP. The mechanism has been carefully investigated for the improvement in the electrochemical performance. Experimental results show that the charge transfer impedance decreases significantly and the cathode surface becomes much smooth over cycling with modification of conductive PANI. The incorporated PANI can work not only as an additional host for Li⁺-ion insertion/extraction, but also as a binder to modify the electrode surface and a container for electrolyte to penetrate into C-LFP particles.     

An investigation of polypyrrole-LiFePO4 composite cathode materials for lithium-ion batteries

A series of polypyrrole-LiFePO₄ (PPy-LiFePO₄) composites were synthesised by polymerising pyrrole monomers on the surface of LiFePO₄ particles. AC impedance measurements show that the coating of polypyrrole significantly decreases the charge-transfer resistance of LiFePO₄ electrodes. The electrochemical reactivity of polypyrrole and PPy-LiFePO₄ composites for lithium insertion and extraction was examined by charge/discharge testing. The PPy-LiFePO₄ composite electrodes demonstrated an increased reversible capacity and better cyclability, compared to the bare LiFePO₄ electrode.     

Mg2+-doped Na3V2(PO4)3/C decorated with graphene sheets: An ultrafast Na-storage cathode for advanced energy storage

NASICON-type Na₃V₂(PO₄)₃ is one of the most promising cathode materials for sodium-ion batteries, delivering about two Na⁺-ions extraction/insertion from/into the unit structure. However, the low electronic conductivity which leads to bad rate capability and poor cycle performance, limits its practical application for sodium-ion batteries. To overcome the kinetic problem, we attempt to prepare the carbon-coated Na₃V₂(PO₄)₃ nanocrystals further decorated by graphene sheets and doped with Mg²⁺ ion via the two steps of sol-gel process and solid-state treatment for the first time. Such architecture synergistically combines the advantages of two-dimensional graphene sheets and 0-dimensional Mg²⁺-doped Na₃V₂(PO₄)₃/C nanoparticles. It greatly increases the electron/Na⁺-ion transport kinetics and assures the electrode structure integrity, leading to attractive electrochemical performance. When used as sodium-ion batteries cathode, the hybrid composite delivers an initial discharge capacity of 115.2 mAh g⁻¹ at 0.2 C rate, and retains stable discharge capacities of 113.1, 109.0, 102.4, 94.0 and 85.2 mAh g⁻¹ at high current rates of 1, 2, 5, 10 and 20 C rate, respectively. Thus, this nanostructure design provides a promising pathway for developing high-performance Na₃V₂(PO₄)₃ material for sodium-ion batteries.     

Long-term cyclability of LiFePO4/carbon composite cathode material for lithium-ion battery applications

A simple high-energy ball milling combined with spray-drying method has been developed to synthesize LiFePO₄/carbon composite. This material delivers an improved tap density of 1.3 g/cm³ and a high electronic conductivity of 10⁻² to 10⁻³ S/cm. The electrochemical performance, which is especially notable for its high-rate performance, is excellent. The discharge capacities are as high as 109 mAh/g at the current density of 1100 mA/g (about 6.5C rate) and 94 mAh/g at the current density of 1900 mA/g (about 11C rate). At the high current density of 1700 mA/g (10C rate), it exhibits a long-term cyclability, retaining over 92% of its original discharge capacity beyond 2400 cycles. Therefore, the as-prepared LiFePO₄/carbon composite cathode material is capable of such large-scale applications as hybrid and plug-in hybrid electric vehicles.     

An effective and simple way to synthesize LiFePO4/C composite

The cathode material is synthesized from FeC₂O₄·2H₂O and LiH₂PO₄ by a solid-state reaction using citric acid as a carbon source. The electric conductivity of the synthesized LiFePO₄ has been raised by eight orders of magnitude from 10⁻⁹ S cm⁻¹. The LiFePO₄/C composite shows a greatly enhanced rate performance and the cyclic stability at room temperature. It delivers an initial discharge capacity of 128 mAh g⁻¹ at 4C, which is retained as high as 92% after 1000 cycles. In addition, the tested low temperature character is attractive. At −20 °C, the composite exhibits a discharge capacity of 110 mAh g⁻¹ at 0.1C. The homogenous morphology, the porous surface, the small particles inside and the conductive carbon observed contribute much to obtain the favorable electrochemical performance.     

Synthesis and electrochemical characterizations of nano-scaled Zn doped LiMn2O4 cathode materials for rechargeable lithium batteries

The LiZn_xMn_2−xO₄ (x = 0.00-0.15) cathode materials for rechargeable lithium-ion batteries were synthesized by simple sol-gel technique using aqueous solutions of metal nitrates and succinic acid as the chelating agent. The gel precursors of metal succinates were dried in vacuum oven for 10 h at 120 °C. After drying, the gel precursors were ground and heated at 900 °C. The structural characterization was carried out by X-ray powder diffraction and X-ray photoelectron spectroscopy to identify the valance state of Mn in the synthesized materials. The sample exhibited a well-defined spinel structure and the lattice parameter was linearly increased with increasing the Zn contents in LiZn_xMn_2−xO₄. Surface morphology and particle size of the synthesized materials were determined by scanning electron microscopy and transmission electron microscopy, respectively. Electrochemical properties were characterized for the assembled Li/LiZn_xMn_2−xO₄ coin type cells using galvanostatic charge/discharge studies at 0.5 C rate and cyclic voltammetry technique in the potential range between 2.75 and 4.5 V at a scan rate of 0.1 mV s⁻¹. Among them Zn doped spinel LiZn_0.10Mn_1.90O₄ has improved the structural stability, high reversible capacity and excellent electrochemical performance of rechargeable lithium batteries.     

Facile synthesis and electrochemical performances of hollow graphene spheres as anode material for lithium-ion batteries

The hollow graphene oxide spheres have been successfully fabricated from graphene oxide nanosheets utilizing a water-in-oil emulsion technique, which were prepared from natural flake graphite by oxidation and ultrasonic treatment. The hollow graphene oxide spheres were reduced to hollow graphene spheres at 500°C for 3 h under an atmosphere of Ar(95%)/H₂(5%). The first reversible specific capacity of the hollow graphene spheres was as high as 903 mAh g^-1 at a current density of 50 mAh g^-1. Even at a high current density of 500 mAh g^-1, the reversible specific capacity remained at 502 mAh g^-1. After 60 cycles, the reversible capacity was still kept at 652 mAh g^-1 at the current density of 50 mAh g^-1. These results indicate that the prepared hollow graphene spheres possess excellent electrochemical performances for lithium storage. The high rate performance of hollow graphene spheres thanks to the hollow structure, thin and porous shells consisting of graphene sheets.

PACS

81.05.ue; 61.48.Gh; 72.80.Vp     

Enhanced electrochemical performance of LiFePO4/C via Mo-doping at Fe site

A series of Mo-doped LiFe_1−3xMo_xPO₄/C (x = 0.000, 0.025, 0.050, 0.100, 0.150) cathode materials are synthesized by sol–gel method. XRD, ICP and Rietveld refinement results reveal that Mo doped in the crystal lattice and probably occupied Fe site. The structure benefits the transportation of Li⁺ and the diffusion of Li⁺ in the doped materials are enhanced remarkably than that of the undoped one, which leads to excellent electrochemical performance. The doped sample with x = 0.025 exhibits the best electrochemical performance, with the initial discharge capacity of 162.3 mAh g⁻¹ at 0.1 C rate.     

Microwave solid-state synthesis and electrochemical properties of carbon-free Li3V2(PO4)3 as cathode materials for lithium batteries

Monoclinic Li₃V₂(PO₄)₃ can be rapidly synthesized at 750 °C for 5 min (MW5m) by using microwave solid-state synthesis method. The refined cell parameters and atomic coordination of the sample MW5m show some deviations compared with those of the sample synthesized in conventional solid-state synthesis method, especially the coordinate of Li atoms. Compared with the electrochemical properties of the carbon-coating sample Li₃V₂(PO₄)₃, the carbon-free sample MW5m presents well electrochemical properties. In the cut-off voltage of 3.0-4.3 V, MW5m sample presents a specific charge capacity of 132 mAh g⁻¹, almost equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹), and specific discharge capacity of 126.4 mAh g⁻¹. In the cut-off voltage of 3.0-4.8 V, MW5m shows an initial specific discharge capacity of 183.4 mAh g⁻¹ at 0.1 C, near the theoretical discharge capacity. In the cycle performance, the capacity fade of Li₃V₂(PO₄)₃ is dependent on the cut-off voltage and the preparation method, more capacity lost at relatively higher charge/discharge voltage. The reasons for the excellent electrochemical properties of Li₃V₂(PO₄)₃ rapidly synthesized in microwave field are discussed in detail.     

Morphological solution for enhancement of electrochemical kinetic performance of LiFePO4

LiFePO₄/C nanosheet composite has been prepared via a low-temperature solvothermal reaction followed by high-temperature treatment. The as-prepared sample is characterized by XRD, FTIR, Raman, SEM, and TEM. It is confirmed that the nanosheets are composed of ca. 50 nm thickness of crystalline LiFePO₄-core coated with ca. 10 nm thickness of carbon-shell. The charge-discharge tests show that the as-fabricated LiFePO₄/C nanosheet cathode in lithium-ion cell demonstrates high reversible capacity (164 mAh g⁻¹ at 0.1 C) and good cycle stability (columbic efficiency 100% during 100 cycles). The cyclic voltammetric analysis indicates Li⁺ diffusion determines the whole electrode reaction kinetics, and the diffusion coefficient estimated by EIS is comparable to the reported data. The enhanced kinetic behavior of the as-fabricated cathode is actually originated from the nano-dimensional sheet-like morphology, which facilitates Li⁺ migration due to the shortened diffusion distance, and simultaneously increased exchangeable Li⁺ amount considering more accessible active surface. In addition, the uniformly coated thin conductive carbons contribute a lot for this enhancement because of considerably improved electronic conductivity.     

Effect of CeO2-coating on the electrochemical performances of LiFePO4/C cathode material

The effect of CeO₂ coating on LiFePO₄/C cathode material has been investigated. The crystalline structure and morphology of the synthesized powders have been characterized by XRD, SEM, TEM and their electrochemical performances both at room temperature and low temperature are evaluated by CV, EIS and galvanostatic charge/discharge tests. It is found that, nano-CeO₂ particles distribute on the surface of LiFePO₄ without destroying the crystal structure of the bulk material. The CeO₂-coated LiFePO₄/C cathode material shows improved lithium insertion/extraction capacity and electrode kinetics, especially at high rates and low temperature. At −20 °C, the CeO₂-coated material delivers discharge capacity of 99.7 mAh/g at 0.1C rate and the capacity retention of 98.6% is obtained after 30 cycles at various charge/discharge rates. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.     

Improved Li-storage performance of CNTs-decorated LiVPO4F/C cathode material for electrochemical energy storage

Lithium vanadium fluorophosphate (LiVPO₄F) has been attracted increasing attention as an advanced cathode for Li-ion batteries because of its excellent thermal stability and high operating voltage. Nevertheless, the pure LiVPO₄F possesses a low electrical conductivity which prevents its usage for practical application in energy storage. In this work, the CNTs-decorated LiVPO₄F/C (CNTs@LiVPO₄F/C) nanocomposite has been prepared via a conventional sol-gel approach. The XRD results reveal that all the diffraction peaks obtained for CNTs@LiVPO₄F/C are indexed to the triclinic structure. TEM images show that the conductive CNTs are distributed homogeneously over the LiVPO₄F/C particles. Benefiting from the enhanced conductivity, the as-prepared CNTs@LiVPO₄F/C electrode exhibits outstanding electrochemical performance with high reversible capacity of 121.1 mAh g^?1 at a high current rate of 10 C. Therefore, the novel CNTs@LiVPO₄F/C cathode material developed from this investigation with superior Li-storage performance has promising practical applications in electrochemical energy storage systems.