From hydrated layered-spinel lithium manganate composite to high-performance spinel LiMn2O4: A novel synthesis tuned by the concentration of LiOH

To obtain high-performance spinel LiMn₂O₄, various types of hydrated layered-spinel lithium manganate composites have been controllably synthesized through the hydrothermal process. It is found that the composition and morphology of these intermediate products can be tuned by the concentration of LiOH: Li⁺ act as the template and OH^- provide the required alkaline environment. In particular, the nanostructure varies from nanowires to nanosheets at different levels, depending on the phase ratio of the spinel phase ranging from 0% to 100%. Phase purity and the corresponding electrochemical properties of the as-prepared LiMn₂O₄ products are further tailored through the subsequent heat treatment. With the optimized LiOH concentration of 0.08 M, the resulting LiMn₂O₄ cathode material exhibits the best electrochemical performance with the initial discharge capacity of 121.7 mA h g⁻¹ at 1 C and 117.8 mA h g⁻¹ at 30 C, while a retention over 90% can be achieved after 1500 cycles. This study will help deepen understanding of the function mechanisms and further direct the novel synthesis from hydrated layered-spinel lithium manganate composites to high-performance spinel LiMn₂O₄ cathode materials.     

LiSixMn2−xO4 (x≤0.10) cathode materials with improved electrochemical properties prepared via a simple solid-state method for high-performance lithium-ion batteries

LiSi_xMn_2−xO₄ (x≤0.10) cathode materials were prepared via a simple solid-state process with tetraethylorthosilicate (TEOS) as the silicon source. The effects of Si-doping on the structure, morphology and electrochemical performance were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge-discharge tests and electrochemical impedance spectroscopy (EIS), respectively. All the Si-doped LiMn₂O₄ samples showed the intrinsic spinel structure. With the increasing of Si-doping concentration, the crystal lattice constant of LiSi_xMn_2−xO₄ samples increased and the particle size distribution becomes more uniform to some extent. Among these samples, the optimal Si-doped LiMn₂O₄ exhibited an initial discharge capacity of 134.6 mAh g⁻¹ at 0.5 C, which was higher than that of the undoped spinel. After 100 cycles, the discharge capacity could still reach up to 114.5 mAh g⁻¹ with capacity retention of 85.1%. Especially, at the high rate of 5.0 C, a high discharge capacity of 87.5 mAh g⁻¹ was obtained while the undoped spinel only exhibited 33.7 mAh g⁻¹. Such high performance indicated that doping the manganese sites with appropriate amount of silicon ions could effectively improve the specific capacity and cycling stability.     

Effect of molybdenum substitution on electrochemical performance of Li[Li0.2Mn0.54Co0.13Ni0.13]O2 cathode material

Molybdenum doping is introduced to improve the electrochemical performance of lithium-rich manganese-based cathode material. X-ray diffraction (XRD) results illustrate that the crystallographic parameters a, c and lattice volume V become larger with the increase of Mo content. The scanning electron microscope (SEM) shows that the molybdenum substitution increases the crystallinity of the primary particles. When evaluated as cathode material, the as-prepared Li[Li_0.2Mn_0.54-x/3Ni_0.13-x/3Co_0.13-x/3Mo_x]O₂ (x = 0.007) delivers a discharge capacity of 155.5 mA h g⁻¹ at 5 C (1 C = 250 mA g⁻¹) and exhibits the capacity retention of 81.8% at 1 C after 200 cycles. The results of cyclic voltammetry (CV) and electronic impedance spectroscopy (EIS) tests reflect that the molybdenum substitution is able to significantly reduce the electrode polarization and lower the charge-transfer resistance. Within appropriate amount of Mo doping, the lithium ion diffusion coefficient of the material can reach to 8.92 × 10^–15 cm² s⁻¹, which is ~ 30 times higher than that of pristine materials (2.65 × 10^–16 cm² s⁻¹).     

The effect of AlF3 modification on the physicochemical and electrochemical properties of Li-rich layered oxide

Lithium (Li)-rich layered oxides are considered promising cathode materials for Li-ion batteries because of their favorable properties. Here, we report our recent finding in the novel oxide, aluminum fluoride (AlF₃)-modified Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ (LMNCAF), which was synthesized via a facile, cost-effective and readily scalable solid-state reaction. LMNCAF possess an F and Al co-doped core structure with a LiF nano-coating on its surface which leads to considerably enhancement in the electrochemical performance of the oxide. The initial discharge capacity (at 0.05 C) increased from 212 mA h g⁻¹ for Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ to 291 mA h g⁻¹ for LMNCAF. A much higher discharge capacity of 211 mA h g⁻¹ was obtained for LMNCAF after 99 charge/discharge cycles at 0.2 C compared with that of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ (160 mA h g⁻¹). Our preliminary results suggest that AlF₃ modification is an effective strategy to tailor the physicochemical and electrochemical properties of Li-rich layered oxides.     

Ultrafine SnO2 nanoparticles as a high performance anode material for lithium ion battery

In this paper, the ultrafine tin oxides (SnO₂) nanoparticles are fabricated by a facile microwave hydrothermal method with the mean size of only 14 nm. Phase compositions and microstructures of the as-prepared nanoparticles have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the ultrafine SnO₂ nanoparticles are obtained to be the pure rutile-structural phase with the good dispersibility. Galvanostatic cycling and cyclic voltammetry results indicate that the first discharge capacity of the ultrafine SnO₂ electrode is 1196.63  mAh g⁻¹, and the reversible capacity could retain 272.63 mAh g⁻¹ at 100 mA g⁻¹ after 50 cycles for lithium ion batteries (LIBs). The excellent electrochemical performance of the SnO₂ anode for LIBs is attributed to its ultrafine nanostructure for providing active sites during lithium insertion/extraction processes. Pulverization and agglomeration of the active materials are effectively reduced by the microwave hydrothermal method.     

Effect of processing parameters on the charge storage properties of MgCo2O4 electrodes

Three morphologies of magnesium cobaltite (MgCo₂O₄), viz. cuboidal microcrystals, nanoflowers, and nanospheres, were synthesized using hydrothermal and molten salt methods and evaluated their electrochemical energy storage properties. Among them cuboidal microcrystal and nanoflowers were obtained by a facile hydrothermal route – the former with ethylene glycol and the latter with hexadecyltrimethylammonium bromide as surfactants. The cuboidal microcrystals showed layered flake microstructure with an appreciable space between the layers (~ 100 nm), which would facilitate ion movement between the flakes. The electrochemical studies of the materials revealed the superiority of MgCo₂O₄ cuboidal microcrystals as a charge storage medium over the nanoflowers and nanospheres, the reasons for this is deeply investigated and reported herewith. The specific charge stored in the MgCo₂O₄ cuboidal microcrystal electrode was ~ 345 C g⁻¹ at a specific current of 1 A g⁻¹ which was superior to nanoflowers (~ 178 C g⁻¹) and nanospheres (~ 139 C g⁻¹) at the similar current density in 3 M LiOH electrolyte. The MgCo₂O₄ cuboidal microcrystals also demonstrated superior charge retention (~ 110%) after 3000 cycles over the other electrodes demonstrating its practical utility as a charge storage material.     

Preparation and characterization of SrLi2Ti6O14@C/Ag as lithium storage anode material for rechargeable batteries

In this work, silver and carbon co-coated SrLi₂Ti₆O₁₄ is synthesized by using a solid-state assisted solution method, with glucose as carbon source and silver nitrate as Ag source. The structural and morphological properties of as-prepared samples are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), which confirm that C/Ag composite layer is uniformly coated on the surface of SrLi₂Ti₆O₁₄. Electrochemical measurements like galvanostatic charge/discharge tests, rate performance, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis are also undertaken to evaluate and compare the lithium storage capability of SrLi₂Ti₆O₁₄ before and after coating. According to the results, SrLi₂Ti₆O₁₄@C/Ag presents enhanced electrochemical capability compared with bare material. It can be found that bare SrLi₂Ti₆O₁₄ only delivers the reversible capacity of 140.32 mA h g⁻¹ with capacity retention of 90.7% at 100 mA g⁻¹ after 200 cycles. In contrast, SrLi₂Ti₆O₁₄@C/Ag presents the reversible capacity of 151.20 mA h g⁻¹ with only 6.7% capacity loss after 200 cycles. The improvement is owing to the increase of electronic conductivity and the decrease in the redox polarization after coating. In order to further investigate the structural stability of SrLi₂Ti₆O₁₄@C/Ag, in-situ XRD was performed as well. All the results prove that the C/Ag co-coating has positive effect on the electrochemical performance of SrLi₂Ti₆O₁₄.     

Synthesis and electrochemical performance of Li4Ti5O12/C composite by a starch sol assisted method

Li₄Ti₅O₁₂/C composite anode materials were synthesized by a simple starch sol assisted method using TiO₂-anatase and Li₂CO₃ as raw materials and soluble starch as carbon source. The influences of calcination temperature and starch amounts on the microstructure and electrochemical performance were systematically investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and constant-current charge/discharge cycling tests. The results showed that the Li₄Ti₅O₁₂/C composite with 10 wt.% starch synthesized at 800 °C for 6 h had homogeneous particle size distribution with an average particle size of 200–300 nm and exhibited the optimal electrochemical performance with specific discharge capacities of 168.5, 160.8, 155.1 and 141.8 mAh g^? 1 at 0.2 °C, 1 °C, 2 °C and 5 °C rates, respectively, and satisfactory cycling stability. It could be attributed to the homogeneous ultrafine particles and in situ carbon coating, which enhanced the electronic conductivity and diffusion of lithium ions in the electrode.     

One-step synthesis of Li-doped NiO as high-performance anode material for lithium ion batteries

The poor electronic conductivity and huge volume expansion of NiO are the vital barriers when used as anode for lithium ion batteries. In order to solve above issues, Li-doped NiO are prepared by a facile one-step ultrasonic spray pyrolysis method. The effects of Li doping on the morphology, structure and chemical composition of the Li-doped NiO powders are extensively studied. When used as lithium ion batteries anode, it is demonstrated that the doping of Li has significant positive effect on improving the electrochemical performance. After 100 cycles at 400 mA g⁻¹, The Li-doped NiO samples deliver a discharge capacity of 907 mAh g⁻¹, much more than that of un-doped sample (736 mAh g⁻¹). The improved electrochemical performances can be ascribed to the improved p-type conductivity and lower impedance, which are confirmed by Rietveld refinement, X-ray photoelectron spectroscopy and electron impedance spectroscopy.     

Effects of TiO2 starting materials on the synthesis of Li2ZnTi3O8 for lithium ion battery anode

Lithium zinc titanate (Li₂ZnTi₃O₈) anode materials have been successfully synthesized using rutile-TiO₂ with different particle sizes as titanium sources via a molten-salt method. Various physical and electrochemical methods are applied to characterize the effects of TiO₂ particle sizes on the structures and physicochemical properties of the Li₂ZnTi₃O₈ materials. When the particle size of TiO₂ is too small (10 nm), it is difficult to homogeneously mix TiO₂ with the other raw materials. Thus, the final product Li₂ZnTi₃O₈ has poor crystallinity, large particle size, small specific surface area, pore volume and average pore diameter, which are disadvantageous to its electrochemical performance. Using TiO₂ with the proper particle size of 100 nm as the titanium source, the Li₂ZnTi₃O₈ (R-100-LZTO) with excellent electrochemical performance can be obtained. At 1 A g⁻¹, 175.8 and 163.6 mA h g⁻¹ are delivered at the 1st and the 200th cycles, respectively. The largest capacities of 163, 133.3 and 122.5 mA h g⁻¹ are delivered at 2.5, 5 and 6 A g⁻¹, respectively. The good high-rate performance of the R-100-LZTO originates from the good crystallinity, small particle size, large specific surface area and average pore diameter, low charge-transfer resistance and high Li⁺ diffusion coefficient.     

Hollow core–shell ZnMn2O4 microspheres as a high-performance anode material for lithium-ion batteries

The hollow core–shell ZnMn₂O₄ microspheres are successfully prepared by a solvothermal carbon templating method and then a annealing process. The crystal phase and particle morphology of resultant ZnMn₂O₄ microspheres are characterized by XRD and TEM. The electrochemical properties of the ZnMn₂O₄ microspheres as an anode material are investigated for lithium ion batteries. The results show that the ZnMn₂O₄ microspheres exhibit a reversible capacity of 855.8 mA h g⁻¹ at a current density of 200 mA g⁻¹ after 50 cycles. Even at 1000 mA g⁻¹, the reversible capacity of the ZnMn₂O₄ microspheres is still kept at 724.4 mA h g⁻¹ after 60 cycles. The enhanced electrochemical performance suggests the promising potential of the hollow core–shell ZnMn₂O₄ microspheres in lithium-ion batteries.     

Phase structure and electrochemical performance of layered-spinel integrated LiNi0.5Mn0.5O2-LiMn1.9Al0.1O4 composite cathodes for lithium ion batteries

In recent years, multi-component integrated composite cathodes for lithium ion batteries have attracted considerable attention. In this work, novel layered-spinel integrated cathode materials of (1−x)LiNi_0.5Mn_0.5O₂-xLiMn_1.9Al_0.1O₄ were synthesized by a sol-gel method, and their phase structures, morphologies and electrochemical performance were investigated. The crystal structure of the (1−x)LiNi_0.5Mn_0.5O₂-xLiMn_1.9Al_0.1O₄ is changed from layered to spinel structure with increasing x. All the samples exhibit nanoscale grains with the minimum grain size of ~130 nm when x = 0.5. The composite electrode with x = 0.5 exhibits the optimal discharge capacity, presenting a large initial discharge capacity of 236 mAh g⁻¹ at the current density of 20 mA g⁻¹. Good rate capability is also obtained at the composite electrode with x = 0.5 where the electrode displays the relatively high discharge capacity of 64.9 mAh g⁻¹ at the high rate of 5 C. The improved electrochemical performance is related to the introduction of spinel structure into layered structure and small grain size. The spinel structure can stabilize the layered structure, which leads to the improvement in the electrochemical performance of the composites; and the small grain size in the sample with x = 0.5 provides short lithium ion diffusion way and thus enhances the electrochemical performance.     

Synthesis and electrochemical properties of LiV3O8 via an improved sol–gel process

LiV₃O₈ cathode material was synthesized via a hydrothermal improved sol–gel process using LiOH, NH₄VO₃ and oxalic acid as raw materials. The thermal decomposition process of the as-prepared LiV₃O₈ precursor was investigated by thermogravimetric (TG) and differential scanning calorimetry (DSC). The structure, morphology and electrochemical performance of the as-synthesized LiV₃O₈ samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM) and the galvanostatic charge–discharge test. The effects of synthesis conditions on phases, structure and electrochemical performance of the LiV₃O₈ samples were particularly discussed. Result shows that pure LiV₃O₈ sample can be obtained at 300 °C, which is much lower than that of normal citric assisted sol–gel method. The sample synthesized at 350 °C exhibits the best electrochemical performance, which can present an initial discharge capacity of 301.1 mAh/g at a current density of 50 mA/g and maintain 271.6 mA/g (about 90.2% of its initial value) after 10 cycles.     

Simple self-assembly of SnS2 entrapped graphene aerogel and its enhanced lithium storage performance

In this work, SnS₂ nanoplates entrapped graphene aerogel has been successfully prepared by simple self-assembly of reduced graphene oxide obtained through mild chemical reduction. Structural and morphological investigations demonstrated that SnS₂ nanoplates are highly dispersed in the three dimensional (3D) porous graphene matrix. When served as anode material for lithium-ion batteries, the electrochemical properties of SnS₂/graphene aerogel (SnS₂/GA) were evaluated by galvanostatic discharge–charge tests, cyclic voltammetry and impedance spectroscopy measurement. Compared with pristine SnS₂, the SnS₂/GA nanocomposite achieved a much higher initial reversible capacity (1186 mAh g⁻¹), superior cyclic stability (1004 mAh g⁻¹ after 60 cycles, corresponding to 84.7% of the initial reversible capacity), as well as better rate capability (650 mAh g⁻¹ at a current density of 1000 mA g⁻¹). This significantly improved lithium storage performance can be attributed to the good integration of SnS₂ nanoplates with 3D porous graphene network, which can not only provide much more active sites and easy access for Li ions intercalation, but also prevent the aggregation of SnS₂ nanoplates and facilitate fast transportation of Li ions and surface electrons during the electrochemical process.     

Low-cost and high-performance electrode materials based on BiCoO3 microspheres

Ternary metal oxides have great potential for chemical storage devices because of their outstanding synergistic effects as well as rich redox reactions. However, there are limited reports of 3D structure BiCoO₃ materials and relevant electrochemical properties. Meanwhile, the study of BiCoO₃ is reasonably important for underlying metal oxides researches. In this work, we have successfully developed a 3D urchin-like BiCoO₃ material without using any template and surfactant. For the supercapacitor application, the BiCoO₃ material showed a specific capacitance of 152 F g⁻¹ at the current density of 1 A g ⁻¹, and this value exhibited a rate capability of 82.3% at a high current density of 10 A g ⁻¹. Furthermore, the sample showed the ideal cycling stability (92.7% retention after 5000 times cycles at the current density of 1 A g ⁻¹ and nearly invariable specific capacitance during different current density cycles). These results suggest that the obtained urchin-like BiCoO₃ sample has superb electrochemical performances which suggest its promising applications as renewable and clean energy storage devices electrode materials in the future.     

Synthesis of cross-linked graphene aerogel/Fe2O3 nanocomposite with enhanced supercapacitive performance

In this work, cross-linked graphene aerogel (CL-GA) and its composite with Fe₂O₃ nanoparticles (NPs) were synthesized through a one-step hydrothermal procedure by using p-phenylenediamine (PPD). Structural characterizations revealed that in the preparation of the composite PPD acts as a cross-liker and provides high surface area by decreasing restacking of graphene sheets and functions as nitrogen source simultaneously. The electrochemical characteristics of the nanocomposite were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS) and Fast Fourier transform continues cyclic voltammetry (FFTCCV). The results show that cross-linked graphene aerogel/Fe₂O₃ (CL-GA/Fe₂O₃) nanocomposite displays enhanced supercapacitive performance, where it has capacitance of 445 at 1 A g⁻¹, high energy density of 63 W h Kg⁻¹, and 89% capacitance retention after 5000 cycles in 3 M KOH. Presence of PPD considerably improved supercapacitive performance of nanocomposite as a result it could be promising material in synthesis of efficient graphene/metal oxide-based electrode material for high performance supercapacitors.     

Formic acid as additive for the preparation of high-performance FePO4 materials by spray drying method

High-performance ferric phosphate (FePO₄), with well-defined ellipsoid morphology and uniform particle size distribution, is successfully fabricated via a green spray drying method with formic acid as additive. It is found that the added formic acid plays a crucial role for the formation of the well-distributed FePO₄ particles. Benefited by the outstanding structure and properties of ferric phosphate prepared above, a high performance of lithium iron phosphate (LiFePO₄) has been prepared. It exhibits high capacity, especially at high charging/discharging rate (158.4 mAh g⁻¹ at 0.2 C and 107.3 mAh g⁻¹ at 10 C), and excellent cycling stability (without capacity fading after cycling for 200cycles at 1 C). All these impressive electrochemical performance could be ascribed to the FePO₄ precursor, and further attributed to the addition of formic acid, which may play as a template, resulting in the well-defined morphology, uniform particles size distribution, hierarchical pore structure, and high surface area of the ferric phosphate.     

Enhanced electrochemical properties of LiFePO4–silicon composites as positive electrode materials fabricated using a solid-state method

LiFePO₄–silicon composites were fabricated by using a solid-state method for applying positive electrodes in lithium ion batteries. The LiFePO₄–silicon composites were characterized with X-ray diffraction and field emission scanning electron microscopy. Their electrochemical properties were investigated with cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge tests. The added silicon not only suppressed the surface corrosion caused by the decreasing H⁺ concentration in the electrolyte, but it also acted as a barrier between the LiFePO₄ particles and LiPF₆ electrolyte, thereby preventing the dissolution of Fe²⁺ in the electrode and enhancing the electrolyte/active material interactions. This resulted in improved lithium-ion transfer kinetics and excellent positive electrode performance, especially at high current densities and different operating temperatures (0, 25, and 50 °C). At 25 °C, the LiFePO₄ composite containing 2 wt% of silicon delivered the best electrochemical performance with a lithium-ion diffusion coefficient of 1.81×10⁻⁹ cm² s⁻¹, a specific discharge capacity of 143 mA h g⁻¹ for the initial cycle, and a capacity retention of 98% after 100 cycles. In contrast, the corresponding values for the pure LiFePO₄ were 1.19×10⁻¹¹ cm² s⁻¹, 115 mA h g⁻¹, and a capacity retention of 76% after 100 cycles, respectively.     

The additive-free electrode based on the layered MnO2 nanoflowers/reduced,graphene oxide film for high performance supercapacitor

The MnO₂ nanoflowers/reduced graphene oxide composite is coated on a nickel foam substrate (denoted as MnO₂ NF/RGO @ Ni foam) via the layer by layer (LBL) self-assembly technology without any polymer additive, following the soft chemical reduction. The layered MnO₂ NF/RGO composite is uniformly anchored on the Ni foam skeleton to form the 3D porous framework, and the interlayers have access to lots of ions channels to improve the electron transfer and diffusion. This special construction of 3D porous structure is beneficial to the enhancement of electrochemical property. The specific capacitance is up to 246 F g⁻¹ under the current density of 0.5 A g⁻¹. After 1000 cycles, it can retain about 93%, exhibiting excellent cycle stability. The electrochemical impedance spectroscopy measurements confirm that MnO₂ NF/RGO @ Ni foam electrode has lower R_ESR and R_CT values when compared to MnO₂ @ Ni foam and RGO @ Ni foam. This study opens a new door to the preparation of composite electrodes for high performance supercapacitor.     

Carbon-coated SnO2/graphene nanosheets as highly reversible anode materials for lithium ion batteries

A simple approach is reported to prepare carbon-coated SnO₂ nanoparticle–graphene nanosheets (Gr–SnO₂–C) as an anode material for lithium ion batteries. The material exhibits excellent electrochemical performance with high capacity and good cycling stability (757 mA h g^?1 after 150 cycles at 200 mA g^?1). The likely contributing factors to the outstanding charge/discharge performance of Gr–SnO₂–C could be related to the synergism between the excellent conductivity and large area of graphene, the nanosized particles of SnO₂, and the effects of the coating layer of carbon, which could alleviate the effects of volume changes, keep the structure stable, and increase the conductivity. This work suggests a strategy to prepare carbon-coated graphene–metal oxide which could be used to improve the electrochemical performance of lithium ion batteries.