Lithium iron phosphate/carbon nanocomposite film cathodes for high energy lithium ion batteries

This paper reports sol–gel derived nanostructured LiFePO₄/carbon nanocomposite film cathodes exhibiting enhanced electrochemical properties and cyclic stabilities. LiFePO₄/carbon films were obtained by spreading sol on Pt coated Si wafer followed by ambient drying overnight and annealing/pyrolysis at elevated temperature in nitrogen. Uniform and crack-free LiFePO₄/carbon nanocomposite films were readily obtained and showed olivine phase as determined by means of X-Ray Diffractometry. The electrochemical characterization revealed that, at a current density of 200 mA/g (1.2 C), the nanocomposite film cathodes demonstrated an initial lithium-ion intercalation capacity of 312 mAh/g, and 218 mAh/g after 20 cycles, exceeding the theoretical storage capacity of conventional LiFePO₄ electrode. Such enhanced Li-ion intercalation performance could be attributed to the nanocomposite structure with fine crystallite size below 20 nm as well as the poor crystallinity which provides a partially open structure allowing easy mass transport and volume change associated with Li-ion intercalation. Moreover the surface defect introduced by carbon nanocoating could also effectively facilitate the charge transfer and phase transitions.     

Facile synthesis and electrochemical properties of carbon-coated ZnO nanotubes for high-rate lithium storage

ZnO is an important functional material, and a nanotube structure is beneficial for various applications. Here, we report the facile synthesis and electrochemical properties of carbon-coated ZnO nanotube materials as Li rechargeable battery anodes. ZnO nanorod was first synthesized via a simple hydrothermal method. Subsequently, the material was annealed with a carbon precursor, forming free-standing, carbon-coated ZnO nanotubes. The carbon-coated nanotube structure is beneficial to alleviate volume changes of the ZnO active material during Li insertion and extraction processes as well as to improve the electrochemical reaction kinetics. Electrochemical test results demonstrate that the carbon-coated ZnO nanotube electrodes deliver improved the cycling performance compared with ZnO nanorod electrodes. Better rate performance than carbon-coated ZnO nanoparticle electrodes was also achieved.     

Ultrathin carbon layer enhanced Cu2Nb34O87 nanowires as high-performance lithium host

Cu₂Nb₃₇O₈₇ exhibits excellent electrochemical properties, high theoretical capacity (401 mAh g^?1), safe working voltage (~1.7 V) and outstanding rate performance for lithium-ion batteries. However, poor electrical conductivity inhibits its further development. In this study, Cu₂Nb₃₇O₈₇@C nano-wires are prepared by combining electrospinning and carbon-coating techniques to mitigate this issue, which breaks through the barrier of low conductivity, improves the ion diffusion rate and relieves the change of crystal volume. Besides, the sample is tested by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy and most importantly, the mechanism of lithium-ion storage is explored by an in-situ X-ray diffraction analysis. Moreover, according to a sequence of electrochemical tests, it is clarified that the electronic conductivity and electrochemical activity of Cu₂Nb₃₇O₈₇ are enhanced significantly. All these are inseparable from the synergistic effect of the nano-crystallisation and carbon coating. Therefore, nano-structures and surface cladding provide effective tactics to construct effective ion-migration interfaces and enhance conductivity for the further study of Cu₂Nb₃₇O₈₇ and other electrode materials.     

Enhancement of electrochemical ion/electron-transfer reaction at solid|liquid interface by polymer coating on solid surface

Controlling the ion/electron-transfer reaction at the electrode|electrolyte (solid/liquid) interface is a necessary and vitally important point for improving the operation of various electrochemical devices. In this paper, we propose a polymer coating technique which enhances the Li⁺ ion-transfer reaction at the LiCoO₂|electrolyte interface, and confirm the effects of the polymer coating by cyclic voltammetry (CV) and AC impedance techniques. The results from the experiments indicated that the application of an F-introduced polymer on the LiCoO₂ (LCO) surface decreased the activation barrier for Li⁺ ion transfer. Besides the electrochemical studies, the present computational results indicated that the Li⁺ exchange process between two states, which are both solved with ethylene carbonate (EC) and coordinated with an F-introduced polymer, might occur due to the close energy levels of Li⁺ stability. Accordingly, we inferred that the transition state of this exchange process promotes the observed decrease in activation energy for the interfacial Li⁺-transfer reaction.     

Li0.95Na0.05MnPO4/C nanoparticles compounded with reduced graphene oxide sheets for superior lithium ion battery cathode performance

Sodium-substituted LiMnPO₄/C/reduced graphene oxide (LNMP@rGO) was synthesized in this study via freeze drying and carbon thermal reduction method with graphene oxide as carbon source. Sodium ion doping is optimized and rGO effects are evaluated by XRD, SEM, TEM, BET, Raman, and electrochemical performance measurements. Well-distributed nanoparticles with average size of ~50?nm are evenly distributed on the surface or intercalation between rGO layers, resulting in a porous ion/electronic conductive network. Compared to 122.3?mA?h?g^?1 in unmodified LNMP, the best LNMP@rGO (20?mg rGO) exhibits an excellent initial discharge capacity of 150.4?mA?h?g^?1 at 0.05?C at 122.9% of the initial capacity. The capacity retention rate is 95.8% of the initial capacity after 100 cycles at 1?C. Capacity of 101.2?mA?h?g^?1 is preserved even at rates as high as 10?C.     

Facile synthesis of carbon-coated LiVO3 with enhanced electrochemical performances as cathode materials for lithium-ion batteries

LiVO₃ has been considered as a promising cathode material owing to the high specific capacity. But it suffers from the poor rate capability and cyclability. Carbon coating is an effective approach to improve the electrochemical performance, but the synthesis of carbon-coated LiVO₃ has not been reported. Herein, we propose a novel method to synthesize carbon-coated LiVO₃ (C@LVO) using a simple solution evaporation of LiNO₃, VOC₂O₄ and resol precursors followed by a sync-carbonization strategy. In this approach, VOC₂O₄ is utilized as the precursor for the first time. Carbon layers and encapsulated LVO are simultaneously generated. An amorphous carbon layer with thickness around 10 nm is observed on the surface of LVO particles using TEM. Compared to bare LVO, C@LVO shows a higher rate capability and more stable cyclability. C@LVO exhibits initial charge and discharge capacities of 281.3 and 339.5 mA h g⁻¹ and features long-term cyclability (125.2 and 125.4 mA h g⁻¹ at 200 mA g⁻¹ after 120 cycles). They possess lower charge-transfer resistance in comparison with bare LVO due to enhanced conductivity of the carbon layer. The higher specific capacity, improved cyclability and rate capability can be greatly attributed to the coated carbon layer, which resists the aggregation of LVO particles, and prevents the side reaction with electrolyte.     

Ultrathin CeO2 coating for improved cycling and rate performance of Ni-rich layered LiNi0.7Co0.2Mn0.1O2 cathode materials

In this study, we have successfully coated the CeO₂ nanoparticles (CeONPs) layer onto the surface of the Ni-rich layered LiNi_0.7Co_0.2Mn_0.1O₂ cathode materials by a wet chemical method, which can effectively improve the structural stability of electrode. The X-ray powder diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) are used to determine the structure, morphology, elemental composition and electronic state of pristine and surface modified LiNi_0.7Co_0.2Mn_0.1O₂. The electrochemical testing indicates that the 0.3?mol% CeO₂-coated LiNi_0.7Co_0.2Mn_0.1O₂ demonstrates excellent cycling capability and rate performance, the discharge specific capacity is 161.7?mA?h?g^?1 with the capacity retention of 86.42% after 100 cycles at a current rate of 0.5?C, compared to 135.7?mA?h?g^?1 and 70.64% for bare LiNi_0.7Co_0.2Mn_0.1O₂, respectively. Even at 5?C, the discharge specific capacity is still up to 137.1?mA?h?g^?1 with the capacity retention of 69.0%, while the NCM only delivers 95.5?mA?h?g^?1 with the capacity retention of 46.6%. The outstanding electrochemical performance is assigned to the excellent oxidation capacity of CeO₂ which can oxidize Ni²⁺ to Ni³⁺ and Mn³⁺ to Mn⁴⁺ with the result that suppress the occurrence of Li⁺/Ni²⁺ mixing and phase transmission. Furthermore, CeO₂ coating layer can protect the structure to avoid the occurrence of side reaction. The CeO₂-coated composite with enhanced structural stability, cycling capability and rate performance is a promising cathode material candidate for lithium-ion battery.     

Electrochemical performance of Li[Co0.1Ni0.15Li0.2Mn0.55]O2 modified by carbons as cathode materials

To enhance specific capacity, cycle performance and rate-capability of lithium-ion battery cathode materials, the Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂ (LCMNO) is modified by coating them with amorphous carbons and by preparing nanocomposites with nanostructured carbons (carbon nanotube and graphene). The carbon-treated LCMNO powders and their cathodes are characterized by morphological observation, crystalline property analysis, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The LCMNO nanocomposite shows a superior discharge capacity of ca. 290 mAh g⁻¹ at low C-rates, due to a greater number of active sites embedded by nanostructured carbon species. In contrast, the carbon-coated LCMNO shows higher discharge capacity in high rate regions due to the carbon-coated layer in the carbon-coated LCMNO, suppressing the side reactions and enhancing the electrical conductivity.     

Effects of carbon surface area on performance of lithium sulfur battery cathodes

The effect of carbon surface area on capacity is investigated in cathodes for lithium sulfur batteries. Carbon additives help overcome the low electrical conductivity of sulfur. Cathodes were prepared at 30 wt% sulfur on different activated carbons having unloaded BET surface areas of 1200–3200 m²/g. Sulfur utilization ranged from 33% to 83% of the theoretical capacity (1672 mAh/g) with a strong correlation to the accessible pore volumes having pore widths between 1 and 5 nm. Additionally, cathodes prepared at 12.5–68 wt% on an activated carbon having unloaded BET surface area of 3200 m²/g showed excessive sulfur loading provided little additional capacity.     

Electrochemical behavior of carbon-coated SnS2 for use as the anode in lithium-ion batteries

Carbon-coated SnS₂ nanoparticles were prepared by a simple solvothermal route at low temperature. A carbon coating with a thickness of about 5 nm was deposited on nano-sized SnS₂ particles to serve as the anode in lithium-ion batteries. Both the nanostructure and the morphology of the SnS₂ powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The coated samples were used as active anode materials for lithium-ion batteries, and their electrochemical properties were examined by constant current charge-discharge cycling, cyclic voltammetry and electrochemical impedance spectroscopy. The reversible capacity of the carbon-coated SnS₂ after 50 cycles was 668 mAh/g, which was much higher than that of the uncoated SnS₂ (293 mAh/g). The carbon-coated SnS₂ also had a better rate capability than the uncoated SnS₂ in the range of 0.008-1 C. The capacity retention of the carbon-coated SnS₂ was improved due to its good conductivity and the effective buffer matrix that alleviated volume expansion during the charge-discharge process.     

Sc3+-doping effects on porous Li3V2(PO4)3/C cathode with superior rate performance and cyclic stability

An enhanced sol-gel combustion method was used to synthesize different porous Sc³⁺-doped Li₃V_2-xSc_x(PO₄)₃/C (x = 0.00, 0.05, 0.10 and 0.15) compounds. The substitution of Sc³⁺ into the V³⁺ sites of Li₃V_2-xSc_x(PO₄)₃/C expands the lattice volume along with the enlargement of Li⁺ diffusion channel, which is beneficial for Li⁺ transportation and ionic conductivity improvement. Besides, the Sc³⁺ doping content exhibits a great impact on the morphology of Li₃V_2-xSc_x(PO₄)₃/C composite. The pristine Li₃V₂(PO₄)₃/C are constituted of porous particles and nanorods, and the ratio of nanorods to particles can be controlled by adjusting the amount of Sc³⁺ doping since the ratio of nanorods to particles decreases with increasing Sc³⁺ doping content. When Sc³⁺ doping content increases to a certain level (x = 0.15, Li₃V_1.85Sc_0.15(PO₄)₃/C), the nanorods are hardly seen. Li₃V_1.90Sc_0.10(PO₄)₃/C with higher tapped density, better reversibility, smaller resistance and larger Li⁺ diffusion coefficient demonstrates outstanding rate performance and cyclic stability, together with high specific discharge capacities of 130.2 and 92.9 mAh g⁻¹ at 0.5 and 20 C, respectively. Furthermore, a superior specific discharge capacity of 85.8 mAh g⁻¹ was retained at 20 C following 1000 cycles. Overall, a novel approach for the preparation of high-performance Li₃V_2-xSc_x(PO₄)₃/C cathodes with different morphologies for lithium-ion batteries is provided.     

Electrochemical impedance study of polyaniline electrocoated porous carbon foam

Electropolymerization of aniline on mesophase pitch based carbon foam has been studied in order to evaluate the influence of conductive polymer coating on the properties of carbon foam. The surface morphology of the coating was determined by scanning electron microscopy (SEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical properties of resulting modified carbon foam samples. Polyaniline (PANI) electrocoated-mesophase pitch based carbon foam showed good capacitor behavior in 0.5 M H₂SO₄. Better capacitive behavior is obtained for 100 and 150 mV/s compared to other scan rates, under these faster scan rates thinner films of PANI coatings were combined with more porous structure of carbon foam. Conductivity of the carbon foam was increased from 9.23 to 13.73 S/cm by electrocoating of PANI.     

An optimized strategy toward multilayer ablation coating for SiC-coated carbon/carbon composites based on experiment and simulation

Carbon/carbon (C/C) composites are widely used as thermal protection systems for atmospheric re-entry, where they are subjected to strong oxidation and mechanical denudation. Sublayer thickness of multilayer coating has considerable influence on its stress, which further governs their service life in critical environments. In this study, a multilayer coating with different sublayer thicknesses was fabricated on SiC-coated C/C composites using plasma spraying. Prior to the fabrication, finite element analysis (FEA) was firstly established to investigate the relationship between sublayer thickness and thermal stress. Thereafter, the coatings with typical sublayer thickness were verified through practical experiments. Raman spectra and ablated appearances showed well coincidence with the FEA results, pointing out close relationship among sublayer thickness, residual stress and ablation behavior. After testing for 90 s, the sample with optimized thickness owned the least stress (294 MPa) and lowest ablation rates (?0.467 µm/s and ?0.343 mg/s) as compared to other coated ones.     

Electrochemical performance of Li3V2(PO4)3/C cathode materials using stearic acid as a carbon source

The Li₃V₂(PO₄)₃/C cathode materials are synthesized by a simple solid-state reaction process using stearic acid as both reduction agent and carbon source. Scanning electron microscopy and transmission electron microscopy observations show that the Li₃V₂(PO₄)₃/C composite synthesized at 700 °C has uniform particle size distribution and fine carbon coating. The Li₃V₂(PO₄)₃/C shows a high initial discharge capacity of 130.6 and 124.4 mAh g⁻¹ between 3.0 and 4.3 V, and 185.9 and 140.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.1 and 5 C, respectively. Even at a charge–discharge rate of 15 C, the Li₃V₂(PO₄)₃/C still can deliver a discharge capacity of 103.3 and 112.1 mAh g⁻¹ in the potential region of 3.0–4.3 V and 3.0–4.8 V, respectively. Based on the analysis of cyclic voltammograms and electrochemical impedance spectra, the apparent diffusion coefficients of Li ions in the composites are in the region of 1.09 × 10⁻⁹ and 4.95 × 10⁻⁸ cm² s⁻¹.     

Elevated Li+ diffusivity in Ni-rich layered oxide by precursor pre-oxidation

Ni-rich layered oxide possesses a high theoretical capacity, high working voltage, and low cost; hence, categorized as a potential cathode material for high energy-density Li-ion batteries (LIBs). However, poor cycling performance, voltage fading, and thermal instability are some major issues that need to be addressed. Herein, a successful synthesis strategy is utilized to form a ternary oxide by sintering the hydroxide precursor at 500 °C for 6 h, followed by the lithiation of oxide under different sintering temperatures. The precursor pre-oxidation mitigates the cation mixing, removes residual lithium compounds, and enhances Li⁺ transportation. Moreover, the influence of sintering temperature on the electrochemical performance reveals that 750 °C for 15 h (NCAO-2) is the optimum temperature for oxide precursor. At this temperature, NCAO-2 holds enhanced cycling stability with retention of 89.8% after 100 cycles and 71% after 200 cycles. In contrast, the cathode synthesized with hydroxide precursor maintains 66.6% and 55.4% after 100th and 200th cycles at 1 C under the voltage range of 2.7–4.3 V. Furthermore, the rate capability, lithium diffusivity, and the thermal stability increase for oxide precursor NCAO-2. Consequently, the sintering of hydroxide precursors protects the particle from the inner to the outer surface. Hence, it is helpful to use oxide precursors to modify Ni-rich layered oxides further to attain high discharge capacity and enhanced cycling performance for their usage in high energy-density LIBs.     

Dissolution-regrowth synthesis of SiO2 nanoplates and embedment into two carbon shells for enhanced lithium-ion storage

In this work, SiO_2 nanoplates with opened macroporous structure on carbon layer(C-mSiO_2) have been obtained by dissolving and subsequent regrowing the outer solid SiO_2 layer of the aerosol-based C-SiO_2 double-shell hollow spheres. Subsequently, triple-shell C-mSiO_2-C hollow spheres were successfully prepared after coating the Cm SiO_2 templates by the carbon layer from the carbonization of sucrose. When being applied as the anode material for lithium-ion batteries, the C-mSiO_2-C triple-shell hollow spheres deliver a high capacity of 501 mA ·h·g~(-1) after100 cycles at 500 m A·g~(-1)(based on the total mass of silica and the two carbon shells), which is higher than those of C-mSiO-12(391 m A·h·g~(-1)) spheres with an outer porous SiO_2 layer, C-SiO_2-C(370 m A·h·g) hollow spheres with a middle solid SiO_2 layer, and C-SiO_2(319.8 m A·h·g~(-1)) spheres with an outer solid SiO_2 layer. In addition,the battery still delivers a high capacity of 403 m A·h·g~(-1) at a current density of 1000 m A·g~(-1) after 400 cycles.The good electrochemical performance can be attributed to the high surface area(246.7 m~2·g~(-1)) and pore volume(0.441 cm~3·g~(-1)) of the anode materials, as well as the unique structure of the outer and inner carbon layer which not only enhances electrical conductivity, structural stability, but buffers volume change of the intermediate SiO_2 layer during repeated charge–discharge processes. Furthermore, the SiO_2 nanoplates with opened macroporous structure facilitate the electrolyte transport and electrochemical reaction.     

Ti2Nb10O29@C hollow submicron ribbons for superior lithium storage

Titanium niobate prepared by traditional techniques has the shortcomings of low ion diffusion coefficient as well as poor electrical conductivity, which drastically reduce its applicability. In this work, we prepare carbon coated Ti₂Nb₁₀O₂₉ hollow submicron ribbons (Ti₂Nb₁₀O₂₉@C HSR) using a simple electrospinning procedure. As anode material for lithium-ion batteries (LIBs), it delivers a high charge capacity of 259.7 mAh g^?1 at 1 C with low capacity loss of 0.013% in long-term cycles. Increased the current density to 5 C, Ti₂Nb₁₀O₂₉@C HSR can maintain a reversible capacity of 189.9 mAh g^?1, indicating its good rate performance. Additionally, this work uses in-situ X-ray diffraction (XRD) to provide an explanation for the lithium storage process in Ti₂Nb₁₀O₂₉@C HSR, demonstrating the high reversibility during charge/discharge cycles. Therefore, Ti₂Nb₁₀O₂₉@C HSR has outstanding cycle adaptability and structural reversibility to be a promising anode for LIBs.     

Surfactant carbonization to synthesize pseudocubic α-Fe2O3/C nanocomposite and its electrochemical performance in lithium-ion batteries

Pseudocubic α-Fe₂O₃/C nanocomposites with sizes of 30–35 nm were synthesized via in situ carbonization of surfactants under Ar atmosphere using oleic acid-capped α-Fe₂O₃ nanoparticles as precursors. The transformation from oleic groups to carbon was evidenced in detail by Fourier transform infrared spectroscopy, elemental analysis and thermogravimetric analysis. The high resolution transmission electron microscope further confirmed that a thin carbon layer of 1–2 nm in thickness tightly coated α-Fe₂O₃ nanocrystals. Compared with bare α-Fe₂O₃ nanoparticles, α-Fe₂O₃/C composites exhibited excellent cycling performance (a reversible capacity of 688 mAh/g after 50 cycles at 0.2 C rate) and rate capability (370 mAh/g after 20 cycles at 2 C) as anode materials for lithium-ion batteries. The remarkably improved electrochemical performance of α-Fe₂O₃/C nanocomposites was attributed to in situ carbon coating, which prevented nanoparticle aggregation, increased electronic conductivity and stabilized solid electrolyte interface (SEI) films.     

隔膜中残油量对锂电池性能的影响

锂离子电池隔膜在锂离子电池中主要起隔绝正负极防止短路和允许锂离子导通的作用[1],因此,锂离子电池隔膜品质的优劣直接影响到锂离子电池的应用性能。在锂电池隔膜制备的过程中,以石蜡油作为造孔剂,用热致相分离法制备聚乙烯微孔膜,石蜡油的残留不可避免。本文研究了隔膜中石蜡油的残留量对电池自放电、倍率放电、高温存储、循环性能等应用性能的影响。研究结果表明,残油量越低锂电池的应用性能越优。     

LiFePO4/(C+Cu) composite with excellent cycling stability as lithium ion battery cathodes synthesized via a modified carbothermal reduction method

In order to improve the electronic conductivity of lithium iron phosphate (LiFePO₄), copper was added to modify LiFePO₄/C composite as cathode material for lithium-ion battery, which was successfully synthesized via a modified carbothermal reduction method using a low cost Fe³⁺ salt (Fe (NO₃)₃) as iron source. The morphology, particle size and electrochemical performances of olivine LiFePO₄ modified with carbon and copper were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and galvanostatics tests. The results show that the prepared composites were irregularity sphere with size of 100–200?nm, which were relatively even-distributed copper particles coated with a 3?nm carbon layer. The as-prepared composites exhibit an initial discharge capacity of 160.7?mA?h?g^-1 at current density of 0.1?C-rate, the capacity retention ratio is 98.6% at 0.5?C-rate after 200 cycles, and 96% at 5?C-rate after 500 cycles, respectively. The impedance tests reveal that the addition of copper further reduces the electric resistance of LiFePO₄/C. The well-designed synthesis method is hopeful to be applied to prepare other electrode materials of LIBs.