Design and fabrication of N-doped graphene decorated LiFePO4@C composite as a potential cathode for electrochemical energy storage

Olivine-structured LiFePO₄@C nanoparticles grown on N-doped graphene (NG) sheets have been fabricated by using a simple sol-gel method with post calcinations. During the synthesis process, both the carbon film and NG sheets can inhibit the growth of LiFePO₄ particles. Meanwhile, the constructed conductive network between the NG and carbon film can greatly enhance the electronic conductivity of LiFePO₄ material. These unique properties lead to markedly improved lithium storage performance. The NG-decorated LiFePO₄@C (NG-LiFePO₄@C) composite presents high specific capacity (163.1 mAh g^?1, 0.1 C) and excellent rate capability (118.6 mAh g^?1, 10 C). Therefore, this NG-LiFePO₄@C composite can be regarded as a potential electrode for electrochemical energy storage.     

One-step hydrothermal synthesis of 3D porous microspherical LiFePO4/graphene aerogel composite for lithium-ion batteries

Three-dimensional (3D) porous LiFePO₄/graphene aerogel (LFP/GA) composite was successfully prepared by an in-situ hydrothermal process. In this composite, the LiFePO₄ microspheres assembled by nanoparticles were embedded in a three-dimensional framework intertwined with the graphene sheets, which acts as a bridge for transfer of electron and diffusion of lithium ion. The large specific surface of the composite structure enables the increased infiltration area and utilization of the active material. The content of the graphene sheet is analyzed and is found important for the Li-storage characteristics of LiFePO₄. An aerogel composite with 10% of graphene displays the best electrochemical performance, with the specific discharge capacities of 168 mAh g⁻¹ and 155 mAh g⁻¹ at respectively 0.1C and 1C, and the capacity retains 96.3% for up to 800 cycles. This novel 3D porous aerogel composite is identified as a promising cathode material for the rechargeable Li battery, and the simple strategy may be applied to construct other high performing composite structure and materials.     

LiFePO4 batteries with enhanced lithium-ion-diffusion ability due to graphene addition

In this study, graphene was added to LiFePO₄ via a hydrothermal method to improve the lithium-ion-diffusion ability of LiFePO₄. The influence of graphene addition on LiFePO₄ was studied by X-ray diffraction (XRD), field emission scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, cycling test, and AC impedance analysis. The addition of graphene to LiFePO₄ resulted in the formation of a LiFePO₄–graphene composite; XRD observations revealed the composite to have a single phase with an olivine-type structure. Furthermore, LiFePO₄ particles in the composite were stacked on the graphene sheet surface, thereby enabling the composite to form an effective conducting network and facilitate the penetration of the surface of active materials by an electrolyte. The lithium-ion-diffusion ability of the LiFePO₄–graphene composite was greater than that of pure LiFePO₄. Of a number of materials studied [namely, pure LiFePO₄, LiFePO₄–graphene (1 %), LiFePO₄–graphene (5 %), and LiFePO₄–graphene (8 %)], LiFePO₄–graphene (5 %) delivered the best electrochemical performance with a lithium-ion-diffusion coefficient of 8.18 × 10^?12 cm² s^?1 and the highest specific discharge capacity of 149 mAh g^?1 at 0.17 C; in contrast, the corresponding values for pure LiFePO₄ were 3.01 × 10^?12 cm² s^?1 and 109 mAh g^?1, respectively. Further, LiFePO₄–graphene (5 %) showed a very high specific discharge capacity of 170 mAh g^?1 at 0.1 C, which is equal to the theoretical capacity of LiFePO₄.     

(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.     

Porous spherical LiFePO4·LiMnPO4·Li3V2(PO4)3@C@rGO composites as a high-rate and long-cycle cathode for lithium ion batteries

The porous spherical LiFePO₄·LiMnPO₄·Li₃V₂(PO₄)₃@C@rGO (Sample-G) composites are prepared via a spray drying process. The results show that the composites consist of orthorhombic olivine-type LiFe_0.5Mn_0.5PO₄ and monoclinic Li₃V₂(PO₄)₃, which are evenly distributed. In particular, nanoparticles are embedded in graphene nanosheets, which are interconnected and stacked to form a porous sphere structure with an interior three-dimensional conductive network, resulting in the huge improvement on electrochemical performance and structural stability. Due to the increased Li⁺ diffusion coefficient, the composite possesses 98.6 and 82.9 mAh g⁻¹ with capacities retention of 81.6% and 71.8% at 10 and 20C after 1000 cycles, respectively. The mutual cross-doping effect between LFP·LMP·LVP and a porous sphere structure with a 3D conductive network inside provides a practical method for improving the cycling and rate performance.     

S-doped crosslinked porous Si/SiO2 anode materials with excellent lithium storage performance synthesized via disproportionation

The volume expansion during cycling and low electrical conductivity of a Si anode limit its commercial development. Nanostructure can effectively alleviate the volume expansion and doping can increase the electrical conductivity of silicon. Hence, in this paper, uniformly S-doped crosslinked porous Si/SiO₂ (S-doped pSi/SiO₂) were prepared by the disproportionation reaction of SiO at a high temperature. As a bifunctional additive, sulphur can be used to prepare crosslinked porous silicon by a silicon-sulphur reaction. Furthermore, sulphur can improve the conductive properties of the bulk Si via doping. At the same time, residual SiO₂ can also be used as a buffer material. This strategy not only provides space for the volume expansion of silicon, but also enhances its electrical conductivity and improves charge transfer. Consequently, the S-doped pSi/SiO₂ anode exhibits superior cycling capacity and rate performance (1035 mAh·g^?1 at 1 A g^?1 after 300 cycles and an exceptional rate performance of 1233 mAh·g^?1 at 2 A g^?1). Moreover, the electrochemical performance of the S-doped pSi/SiO₂//LiFePO₄ full cell was also evaluated, which exhibits favourable lithium storage performance.     

A novel synthesis of Fe2P–LiFePO4 composites for Li-ion batteries

Fe₂P–LiFePO₄ composites were synthesized by a novel method consisting of co-precipitation modified with in situ polyacrylamide (PAM) formation. Simultaneous thermogravimetric-differential scanning calorimetric analysis indicated that the in situ PAM precursor exhibited a moderate continuous weight loss rather than a sharp mass loss process. The best Fe₂P–LiFePO₄ composite was obtained from 14 wt% in situ PAM precursor under a sintering temperature of 750 °C for 20 h, which delivered a discharge capacity of 131 mAh g^?1 at a rate of C/5 and 110 mAh g^?1 at 1 C and sustained 30 cycles with almost no capacity fading. The relatively good electrochemical performance originates mainly from the well-mixed gelation precursor and conductive Fe₂P phase with better electronic conductivity. This novel method verified that the electrochemical performance was improved compared to the conventional LiFePO₄ without in situ PAM. It can be anticipated that the same process should be readily extendable to other olivines, such as LiMnPO₄ and LiCoPO₄, and also to other phosphates.     

Synthesis and characterization of Pt-doped LiFePO4/C composites using the sol–gel method as the cathode material in lithium-ion batteries

LiFePO₄/C and LiFe_0.96Pt_0.04PO₄/C nanocomposite cathode materials were synthesized using the sol–gel method in a nitrogen atmosphere. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Their electrochemical properties were investigated using galvanostatic charge/discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The XRD results indicate that substituting iron with platinum does not destroy the structure of LiFePO₄, but expands the lattice parameters and enlarges the cell volume. The electrochemical results show that platinum doping improves the electrochemical performance of LiFePO₄/C particles owing to the expansion of the lattice structure, which provides more space for Li ion diffusion. The, larger lattice structure parameters of the LiFe_0.96Pt_0.04PO₄/C material result in a high discharge capacity of 166, 156, 142 and 140 mAh g^?1 at rates of 0.2, 1, 5, and 10 C, respectively, as compared to 164, 150, 120, and 105 mAh g^?1 for undoped LiFePO₄/C.     

Synthesis of spindle-shaped nanoporous C–LiFePO4 composite and its application as a cathodes material in lithium ion batteries

In this work, LiFePO₄/C composites were prepared in hydrothermal system by using iron gluconate as iron source, and two feeding sequences during the preparation were comparatively studied. The morphology, crystal structure and charge–discharge performance of the prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and galvanostatic charge–discharge testing. The results showed that the feeding sequences and iron gluconate seriously affected the microstructures and electrochemical properties of the resulting LiFePO₄ cathodes in lithium ion batteries. The spindle-shaped LiFePO₄ with hierarchical microporous structure self-assembled by nanoparticles has been successfully synthesized by synthesis route B. In addition, the cell performance of the synthesized LiFePO₄ by synthesis route B was better than that of LiFePO₄ by synthesis route A. Specially at high rates, the superior rate performance of the spindle-shaped LiFePO₄/C microstructure (LFP/C-B) was revealed. And special reversible capacities of ∼118 and ∼95 mAh g⁻¹ were obtained at rates of 2 C and 5 C, comparing to ∼96 and ∼68 mAh g⁻¹ for LFP/C-A.     

Synthesis of CuGeO3/reduced graphene oxide nanocomposite by hydrothermal reduction for high performance Li-ion battery anodes

Nowadays, Lithium-ion batteries (LIBs) are prevalently applied in numerous areas, leading to increasing demand of innovative electrodes with high specific capacities. An advanced CuGeO₃/reduced graphene oxide (rGO) structure is designed and fabricated as the anode material taking the advantage of considerable capacity offered by CuGeO₃ and stable framework constructed by rGO. The as-prepared CuGeO₃ with 30 wt% GO addition exhibits the best electrochemical performance. Specifically, a reversible charge capacity of 909 mAh·g⁻¹ with high coulombic efficiency of 91.49% at the current density of 100 mA g⁻¹ after 200 cycles is demonstrated, and the rate capacity retains 747.6 mAh·g⁻¹ with 91.59% capacity retention. These results indicate that the CuGeO₃/rGO composite holds great potential in next-generation LIBs.     

Mesoporous silicon nanocubes coated by nitrogen-doped carbon shell and wrapped by graphene for high performance lithium-ion battery anodes

Silicon materials have received widespread attention due to their inherent high theoretical specific capacity. However, large volumetric expansion and poor electrical conductivity hinder the large-scale application of silicon materials. To address these issues, we synthesize mesoporous silicon nanocubes coated by nitrogen-doped carbon shell (MSC@C) and wrapped by graphene (MSC@rGO) respectively. The ordered mesoporous silica nanocubes are obtained via a hydrolysis reaction of Tetraethyl Orthosilicate (TEOS) and further reduced by a magnesiothermic reduction to prepare mesoporous silicon nanocubes (MSC). The porous structure of MSC not only speeds up the transfer of ions and electrons, but also buffers the internal stress triggered by the volume expansion of the electrode material. Moreover, in addition to providing additional lithium storage sites and high conductivity, the graphene or nitrogen-doped carbon shell also effectively prevents aggregation and cracking of the mesoporous silicon, greatly promoting the stability of the entire electrode structure. Therefore, the electrochemical properties of composite materials are significantly enhanced by the combination of the mesoporous structure and the nitrogen-doped carbon shell or graphene. MSC@C can deliver the initial discharge specific capacity of 2852.7 mAh·g^?1 and the initial Coulombic efficiency (CE) of 83.74%. After 100 cycles, the MSC@C and MSC@rGO composite materials exhibit reversible specific capacities of 1070.5 mAh·g^?1 and 738.2 mAh·g^?1 at 0.1 A g^?1, respectively.     

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 LiFePO4–carbon nanofiber–carbon nanotube composites prepared by electrospinning and thermal treatment as a cathode material for lithium‐ion batteries

Binder‐free LiFePO₄–carbon nanofiber (CNF)–multiwalled carbon nanotube (MWCNT) composites were prepared by electrospinning and thermal treatment to form a freestanding conductive web that could be used directly as a battery cathode without addition of a conductive material and polymer binder. The thermal decomposition behavior of the electrospun LiFePO₄ precursor–polyacrylonitrile (PAN) and LiFePO₄ precursor–PAN–MWCNT composites before and after stabilization were studied with thermogravimetric analysis (TGA)/differential scanning calorimetry and TGA/differential thermal analysis, respectively. The structure, morphology, and carbon content of the LiFePO₄–CNF and LiFePO₄–CNF–MWCNT composites were determined by X‐ray diffraction, high‐resolution transmission electron microscopy, Raman spectroscopy, scanning electron microscopy, and elemental analysis. The electrochemical properties of the LiFePO₄–CNF and LiFePO₄–CNF–MWCNT composite cathodes were measured by charge–discharge tests and electrochemical impedance spectroscopy. The synthesized composites with MWCNTs exhibited better rate performances and more stable cycle performances than the LiFePO₄–CNF composites; this was due to the increase in electron transfer and lithium‐ion diffusion within the composites loaded with MWCNTs. The composites containing 0.15 wt % MWCNTs delivered a proper initial discharge capacity of 156.7 mA h g^?1 at 0.5 C rate and a stable cycle ability on the basis of the weight of the active material, LiFePO₄. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43001.     

Understanding the influence of conductive carbon additives surface area on the rate performance of LiFePO4 cathodes for lithium ion batteries

Conductive carbon additives with different surface area and particle size, alone or in different combinations, were tested as conductive additives for LiFePO₄ cathode materials in lithium ion batteries. Their influence on the conductivity, rate capability as well as the structure of the resulting electrodes was investigated. Mercury porosimetry was carried out to define the porosity and pore size distribution of electrodes, and scanning electron microscopy was used to image their morphology. By comparing the discharge capacity, especially at higher rates, it can be concluded that the electrochemical performance of LiFePO₄ cathode material is significantly affected by the surface area, particle size and morphology of the used carbon additives. The best rate performance is achieved with the electrode containing a carbon additive with a specific surface area of 180 m² g⁻¹. This work reveals that the choice of conductive additive influences discharge capacity of LiFePO₄ Li-ion battery cells by as much as 20–30%. This is due to conductive additive’s influence on both electronic conductivity and porosity (which determines ionic conductivity) of LiFePO₄ electrodes. A system approach to lithium ion battery material research should always consider inactive materials, such as conductive additives and binders, in addition to active materials.     

Si/CNTs@melamine-formaldehyde resin-based carbon composites and its improved energy storage performances

In this paper, Si/carbon nanotubes@melamine-formaldehyde resin (MFR)-based carbon (Si/CNTs@C) composites have been fabricated by surface modification, electrostatic self-assembly, cross-linking of MFR under hydrothermal treatment and further carbonization. The microstructure of the Si/CNTs@C composites was characterized, and the effects of CNTs content in Si/CNTs@C composites on their electrochemical performances were also investigated in detail. The results indicate Si/CNTs@C composites as anode materials of Li-ion batteries exhibit better high-rate and cycling performances compared to Si and Si@MFR-based carbon composites. Notably, Si/CNTs@C composites with 10.4 wt% CNTs show specific capacities of 1900, 1879, 1,688, 1,394, 1,189 mAh·g⁻¹ at 0.2, 0.5, 1, 2, and 3 A·g⁻¹, respectively. Even at 4 and 5 A·g⁻¹, their capacities still reach 970 and 752 mAh·g⁻¹, respectively. Moreover, they deliver a reversible capacity of 1,184 mAh·g⁻¹ at 0.5 A·g⁻¹ after 100 cycles. Therefore, the reasonable structure is of great significance for enhancing the electrochemical performances of Si-based composites.     

Enhanced electrochemical performance of carbon nanospheres-LiFePO4 composite by PEG based sol-gel synthesis

The carbon nanospheres-LiFePO₄ (CNSs-LiFePO₄) composite has been synthesized by PEG (polyethylene glycol, mean molecular weight of 30,000) based sol-gel route. Highly conductive CNSs (30-40 nm) were adopted to improve the electronic conductivity of LiFePO₄. PEG was used to promote the dispersion of CNSs with the surface functionalization of CNSs, which could facilitate the coating of CNSs on the surface of the LiFePO₄ particles. The sample was characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Raman scattering. Electrochemical performance of the CNSs-LiFePO₄ composite was characterized by the charge-discharge test and electrochemical impedance spectra measurement. The results indicated that LiFePO₄ particles were well coated with the conductive CNSs to overcome the intrinsic low electronic conductivity problem of LiFePO₄. The CNSs-LiFePO₄ composite delivered an enhanced rate capability (146, 128 and 113 mAh g⁻¹ at 0.1 C, 1 C and 5 C rate). The PEG based sol-gel route enables LiFePO₄ networked with CNSs, which offered a higher electrochemical performance.     

基于Li_3PO_4水热法制备LiFePO_4及其改性

以自制Li3PO4为前驱体,在水热条件下与FeSO4.7H2O反应制备得到纯相LiFePO4,并通过碳包覆和Cu2+掺杂对其进行了有效改性,获得了适合高电流密度放电的LiFePO4正极材料。采用X射线衍射(XRD)、透射电子显微镜(TEM)和扫描电子显微镜(SEM)对产物进行了物相和形貌表征。实验研究了水热反应温度对产物的形貌及其电化学性能的影响,同时探讨了掺杂Cu2+对材料常温和低温电化学性能的影响。结果表明:在200℃、24h水热条件下制得的产物,经碳包覆后的复合材料LiFePO4/C(LFP200/C),以1C(150mA.g-1)电流放电,放电比容量达140.7mAh.g-1;对材料进行Cu2+掺杂得到的Cu-LFP200/C材料的放电比容量及倍率性能得到进一步提高,常温下1C倍率的放电比容量为150.3mAh.g-1,10C倍率的放电比容量为108.7mAh.g-1,在-30℃条件下的放电比容量依然达到97mAh.g-1。     

Aqueous slurry of S-doped carbon nanotubes as conductive additive for lithium ion batteries

S-doped carbon nanotubes (SCNTs) obtained by a post treatment approach are used as conductive additive for LiFePO₄ (LFP) cathodes in Lithium ion batteries (LIBs). The SCNTs exhibit higher specific surface area, higher conductivity and better hydrophily as compared to the pristine CNTs because of S doping. Thus the SCNTs can be stably dispersed in water, forming an aqueous conductive slurry. The LFP cathode using the aqueous SCNTs slurry as conductive additive exhibits excellent electrochemical performances in terms of capacity (143 mA h g⁻¹ at 2 C), rate capability and cycling stability (99.6% of initial capacity after 200 cycles) due to the uniform dispersibility of SCNTs in the bulk of electrodes forming a continuous conductive network. The full cell configuration with graphite as anode, affords a high reversible capability (150 mA h g⁻¹ at 0.2 C), good cycling stability (capacity retention of 87.6% at 2 C), ultrahigh energy density of 163.7 W h kg⁻¹ and power density of 296.8 W kg⁻¹. Our results provide an easy approach to prepare high performance LIB cathodes using water as solvent, thus leading to lower cost and more secure for the electrode production.     

Densely-stacked N-doped mesoporous TiO2/carbon microsphere derived from outdated milk as high-performance electrode material for energy storages

Spherical N-doped mesoporous TiO₂/C (MTC) composite micro-particles are produced by spray drying (SD) and carbonization process. The particle size of MTC microsphere is between 2 and 3.4?µm, and the N-doped amorphous C around TiO₂ could provide a conductive matrix, and buffer the volume change. When evaluated as electrodes for Li-ion batteries (LIBs) and Li-S batteries (LSBs), the MTC microsphere exhibits relatively high discharge-voltage plateau, excellent capacity retention and rate capability. As anode for LIBs, after 200 cycles, a reversible capacity more than 230?mA?h?g^?1 can achieved at 1?C. And for LSBs, a specific capacity of 1317.7?mA?h?g^?1 at 1?C and the capacity retention of 73.8% after 500 cycles. The superior electrochemical performance is ascribed to robust scaffolding architecture and conductive N-doped carbon matrix. The excellent electrochemical performance and process ability of the MTC microspheres make them very attractive as electrode materials for use in high rate battery application.     

Embedded Si/Graphene Composite Fabricated by Magnesium-Thermal Reduction as Anode Material for Lithium-Ion Batteries

Embedded Si/graphene composite was fabricated by a novel method, which was in situ generated SiO₂ particles on graphene sheets followed by magnesium-thermal reduction. The tetraethyl orthosilicate (TEOS) and flake graphite was used as original materials. On the one hand, the unique structure of as-obtained composite accommodated the large volume change to some extent. Simultaneously, it enhanced electronic conductivity during Li-ion insertion/extraction. The MR-Si/G composite is used as the anode material for lithium ion batteries, which shows high reversible capacity and ascendant cycling stability reach to 950 mAh·g^?1 at a current density of 50 mA·g^?1 after 60 cycles. These may be conducive to the further advancement of Si-based composite anode design.