Pristine LiMn2O4 Research Articles

Freestanding Binary Graphene/LiMn2O4 Nanorod Cathode Electrodes for High Energy Storage Applications

The transition from ‘fossil ‘economy to a greener and sustainable economy cannot be achieved without efficient energy storage systems. The recovery of energy from renewable sources such as solar or wind power has enormous potential to meet current and future energy needs and to lead to a better preservation of nature and the environment. In the United States for example, the combustion of fossil fuel results in more than 90% of the greenhouse gas emissions, which is also the main cause of global warming. This noticeable climate changes have urged major vehicles producers to develop zero-emission vehicles (electric or hybrid vehicles). To achieve the aim to develop a suitable energetic solutions for mobile and stationary applications, an efficient and low cost energy storage system is needed. The lithium-ion batteries with high energy density have long been effective solution to meet these demands. Nano structured electrode materials have shown improved electrochemical performance due to the large surface area which facilitates the electrode-electrolyte contact. Tailoring the electrode morphology in the forms of nanowires and nanorods are particularly attractive since they provide large surface area and efficient one-dimensional electron transport pathways and facile strain relaxation during battery charge and discharge. Spinel LiMn2O4 based cathode electrode materials are one of the most promising alternative cathode materials for Li-ion power batteries, which is due to the advantages such as low cost, good environmental compatibility, and good thermal stability. It has been reported that the charged LiMn2O4 material shows obviously higher thermal stability in the electrolyte at high temperature than LiCoO2 and LiNiO2. The good thermal stability of spinel LiMn2O4 is beneficial for its use in high power batteries. Although many reports revealed that the LiMn2O4 based electrodes offer a potentially attractive alternative to the presently commercialized LiCoO2, there are still some issues prohibiting LiMn2O4 from commercialization is its severe capacity and cycling performance fading during cycling. It is reported that both the particle size and its size distribution play important roles in the electrochemical performance. To overcome this problem, many researchers are focused on nanocomposite electrodes. Mats or so-called freestanding electrode that contains high concentrations of graphene nanosheets have the potential to form strong and light weight composite materials. The formation of intense surface films may increase the electrode impedance and even electrically isolate part of the active mass. In this study, LiMn2O4 nanorods were produced via cost effective and facile method by chemically converting a-MnO2 structure. a-MnO2 nanorods were produced via microwave hydrothermal synthesis method KMnO4 and manganese sulfate MnSO4.H2O as the starting precursors. LiMn2O4 nanorods were then chemically obtained by using a-MnO2 nanorods by a simple solid-state reaction. As-synthesized LiMn2O4 nanorods were then decorated between graphene nanosheets through a vacuum filtration process and freestanding cathode electrodes were obtained. Graphene/LiMn2O4 cathode electrode exhibited higher rate capability, specific capacity and cycle performance when compared with pristine LiMn2O4 electrode. This is due high surface area of LiMn2O4 nanoparticles and good electronic conductivity of graphene. The improved electrochemical performance of the graphene coated LiMn2O4 electrode was attributed to decreasing Mn dissolution into electrolyte. The improved electrochemical performance of the LiMn2O4/graphene nanocomposite makes it the promising cathode material for high-performance lithium-ion batteries. Keywords: LiM2O4 nanorods, graphene, freestanding electrodes, Li ion batteries.

The effect of LaMnO3 with high electronic conductivity on the high rate charge-discharge performance of LiMn2O4

The effect of LaMnO3 with high electronic conductivity on the fast charge-discharge rate performance of LiMn2O4 is studied. X-ray diffraction patterns confirm the existence of LaMnO3 and also indicate LaMnO3 has no influence on the crystal structure of pristine LiMn2O4. The transmission electron microscopy (TEM) images indicate that LaMnO3 coating layer, about 15nm thickness, covers the surface of LiMn2O4 well. The electrochemical performances are evaluated by galvanostatic charge/discharge tests and electrochemical impedance spectroscopy (EIS). At 0.5C/0.5C, LaMnO3 coated LiMn2O4 delivers an initial capacity of about 114mAhg−1 along with the coulombic efficiency of 95.0%, which are higher than those of uncoated LiMn2O4 (106mAhg−1 and 89.1%). Furthermore, LaMnO3 coated LiMn2O4 can exhibit higher capacities at high charge-discharge rates than uncoated LiMn2O4. It can deliver about 90.6mAhg−1 at 10C/10C and 68.0mAhg−1 at 20C/20C, but there are only 53.6mAhg−1 and 43.3mAhg−1 for bare LiMn2O4. Electrochemical impedance spectroscopy (EIS) demonstrates that LaMnO3 coating layer can effectively reduce the electrodes' resistances and improve the kinetics of electrodes. The improved high rate properties of LaMnO3 coated LiMn2O4 are ultimately ascribed to the easier phase conversion from λ-MnO2 to Li0·5Mn2O4 which is related to LaMnO3 coating layer with high electronic conductivity.

ZnO-coated LiMn2O4 cathode material for lithium-ion batteries synthesized by a combustion method

ZnO-coated LiMn2O4 cathode materials were prepared by a combustion method using glucose as fuel. The phase structures, size of particles, morphology, and electrochemical performance of pristine and ZnO-coated LiMn2O4 powders are studied in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge test, and X-ray photoelectron spectroscopy (XPS). XRD patterns indicated that surface-modified ZnO have no obvious effect on the bulk structure of the LiMn2O4. TEM and XPS proved ZnO formation on the surface of the LiMn2O4 particles. Galvanostatic charge/discharge test and rate performance showed that the ZnO coating could improve the capacity and cycling performance of LiMn2O4. The 2 wt% ZnO-coated LiMn2O4 sample exhibited an initial discharge capacity of 112.8 mAh g−1 with a capacity retention of 84.1 % after 500 cycles at 0.5 C. Besides, a good rate capability at different current densities from 0.5 to 5.0 C can be acquired. CV and EIS measurements showed that the ZnO coating effectively reduced the impacts of polarization and charge transfer resistance upon cycling.

Plasma-Assisted Sulfur Doping of LiMn2O4 for High-Performance Lithium-Ion Batteries

Though considered as one of the most promising materials for rechargeable Li-ion batteries, spinel LiMn2O4 suffers from fast capacity fading during cycling due to the structural instability, Jahn–Teller distortion, and Mn dissolution into the electrolyte. In order to improve the electrochemical performance, in this work, we, for the first time, realize the sulfur doping by the plasma-assisted method in LiMn2O4. Physical properties of the synthesized materials LiMn2O4–xSx are measured by XRD, SEM, and EDS, which confirm that S atoms have been successfully doped into the structure of LiMn2O4 (LiMn2O4–xSx) with the high crystalline and uniform morphology. Compared to the pristine LiMn2O4 prepared by the conventional method (800 °C, 8 h), the LiMn2O4–xSx prepared by the plasma-assisted method shows superior performance with higher capacity (125.3 mAh·g–1) and significantly improved cycling stability (maintaining 97.76% of its initial discharge capacity after 60 cycles). In addition, the sulfur-doped LiMn2O4 d...

Improved cycle performance of LiMn2O4 cathode material for aqueous rechargeable lithium battery by LaF3 coating

LaF3-coated LiMn2O4 composites were prepared by a simple co-precipitation method using pristine LiMn2O4, La(NO3)3·6H2O, NH4F and polyvinyl pyrrolidone (PVP) as raw materials and the physical and electrochemical properties of the composites were investigated. The results indicate that LaF3 coating is beneficial to improving the electrochemical performance of LiMn2O4 used as a cathode material for aqueous rechargeable lithium battery (ARLB) in 1 mol/L LiNO3 solution. The optimal LaF3-coated LiMn2O4 composite (L-3-LMO) containing 3 wt% LaF3 exhibits much improved electrochemical performance as compared with the pristine LiMn2O4. When galvanostatically charged and discharged at 2 C (296 mA g−1), L-3-LMO displays initial discharge capacity of 118.4 mAh g−1 and capacity retention of 99.7% after 50 cycles, while the pristine LiMn2O4 presents initial discharge capacity of 109.2 mAh g−1 and capacity retention of 89.4%. Even at high current density of 10 C, L-3-LMO delivers discharge capacity of 109.5 mAh g−1 and the capacity remains 107.9 mAh g−1 after 100 cycles, while the highest discharge capacity of pristine LiMn2O4 is only 101.6 mAh g−1 and the capacity is as low as 86.3 mAh g−1 after 100 cycles. The effects of LaF3 coating on Li+ diffusion coefficient, dissolution of manganese ion of electroactive materials are also investigated.

Improvement of the overall performances of LiMn2O4 via surface-modification by polypyrrole

Polypyrrole (PPy) is an excellent conductive polymer and the study on its utilization in the surface modification of the LiMn2O4 (LMO) is few. In this work, the structure, morphology and electrochemical performance of surface-modified LiMn2O4 composites with PPy and polyimides (PI) were discussed. The crystal structure, chemical bonds and morphology were characterized through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), respectively. Moreover, the specific power and cycling performance were tested at room and high (55°C) temperature. The PPy@LMO (surface-modified LMO composites with PPy) shows better performances than the pristine LMO. The addition of PPy not only weakens the corrosion caused by electrolyte, but also improves the discharge capacity at higher rates. The charge transfer resistance of the PPy@LMO is much lower than that of the pristine LMO after cycling.

Solution-combustion synthesized aluminium-doped spinel (LiAl x Mn2−x O4) as a high-performance lithium-ion battery cathode material

High-performing LiAl x Mn2−x O4 (x = 0, 0.125, 0.25, 0.375, and 0.5) spinel cathode materials for lithium-ion battery were developed using a solution combustion method. The as-synthesized cathode materials have spinel cubic structure of LiMn2O4 without any impurity peak and accompanied with peak shift as doping with aluminium. LiAl0.375Mn1.625O4 (first cycle capacity = 113.1 mAh g−1) retains 85 % (96.2 mAh g−1), while pristine LiMn2O4 electrode (first cycle capacity = 135.8 mAh g−1) fades quickly and retains only 54 % (73.9 mAh g−1) after 50 cycles. The electrochemical performance of all the cathode samples prepared using the SCM is comparable to those reported for Al-doped LiMn2O4 spinel cathode materials. The experimental lattice parameter of LiAl x Mn2−x O4 was validated by ab initio calculations and correlated with the first cycle capacity of materials. The variation in lattice parameter as a result of Al doping greatly enhanced the cyclability of discharge capacity of the LiMn2O4 spinel.

Synthesis of a Co–Ni doped LiMn2O4 spinel cathode material for high-power Li-ion batteries by a sol–gel mediated solid-state route

A Co–Ni doped LiMn2O4 spinel cathode material with a nominal stoichiometry of LiMn1.95Co0.025Ni0.025O4 was synthesized by a sol–gel mediated solid-state route at a low temperature of 650°C, using a highly dispersed ultra-fine Mn3O4 as the Mn source. Also, a pelletizing process was adopted to optimize its morphology. It was revealed that the Co–Ni doped LiMn2O4 obtained consisted of porous nanoparticles-constructed LiMn1.95Co0.025Ni0.025O4 granules with a size of 30–50μm, which combined the favorable characteristics of both nano-sized and bulk materials, i.e. considerably large specific surface area and high tap density. The cathode material exhibited excellent electrochemical performance. Notably, its rate capability was extraordinarily high, which was superior to that of pristine or doped LiMn2O4 materials ever reported. It showed a discharge capacity of 119mAhg−1 at a current rate of 0.2C (1C=148mAg−1), and retained a capacity of 111mAhg−1 at 10C, presenting a 93% capacity retention. After 200 cycles at 1C and 25°C, it delivered a capacity of 112mAhg−1, retaining 97% of its initial capacity. After 100 cycles at 1C and 55°C, it showed a capacity of 110mAhg−1, preserving 96% of its initial capacity. The excellent electrochemical performance together with the facile synthesis process allowed the synthesized LiMn1.95Co0.025Ni0.025O4 to be a promising cathode material for high-power Li-ion batteries.

Graphene/LiMn2O4 nanocomposites for enhanced lithium ion batteries with high rate capability

Composites of LiMn2O4 nanoparticles in a graphene matrix were prepared to compensate for the low electronic conductivity of the LiMn2O4 cathode. The LiMn2O4 nanoparticles were homogeneously dispersed and attached on the graphene surface by the hydrothermal method. The graphene/LiMn2O4 electrode showed higher discharge capacity and rate capability than a pristine LiMn2O4 electrode. This is attributed to the high surface area of LiMn2O4 nanoparticles and good electronic conductivity because of the presence of graphene. The composites of LiMn2O4 nanoparticles with graphene were also effective in stabilizing the cyclic performance of the LiMn2O4 nanoparticle cathode.

Synergetic action of doping and coating on electrochemical performance of lithium manganese spinel as an electrode material for lithium-ion batteries

Spinel LiMn2O4 and multidoped spinel LiMn1.9Co0.025Cr0.025Ni0.025Fe0.025O4 were synthesized by the glycine-nitrate method and coated with lithium borosilicate (LBS) in order to enhance the electrochemical performance at room temperature. The structure and electrochemical performance of all samples were characterized by inductively coupled plasma-mass spectrometer (ICP-MS), X-ray diffraction (XRD), differential thermal analysis/thermogravimetry (DTA/TG), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), atomic force microscopy (AFM) and galvanostatic charge–discharge measurements. The XRD analysis shows that the samples exhibit a pure spinel phase. The SEM results indicated that LBS particles had encapsulated the surface of the undoped and multidoped LiMn2O4 without causing any structural change. The charge–discharge tests showed that LiMn1.9Co0.025Cr0.025Ni0.025Fe0.025O4 displays better cycling performance than the pristine LiMn2O4 at room temperature. However, in the same conditions, LBS-coated LiMn1.9Co0.025Cr0.025Ni0.025Fe0.025O4 and LiMn 2O4 have better cycling performance than uncoated samples. The results suggest that multidoped and LBS-coated LiMn2O4 could develop into a promising cathode material for lithium ion batteries.

Enhanced cycling stability of spinel LiMn2O4 cathode by incorporating graphene sheets

LiMn2O4-graphene nanocomposites with different weight ratios of LiMn2O4/graphene were successfully prepared via a simple method by ball-milling of commercially available LiMn2O4 particles and graphene nanosheets. Experimental results revealed that the spinel LiMn2O4 particles within the as-prepared LiMn2O4-graphene nanocomposites were well distributed onto the flexible graphene sheets, and the nano-composites with a higher graphene content were favorable to form more uniform composite materials. Compared to the pristine spinel LiMn2O4 particles, the as-prepared LiMn2O4-graphene nanocomposites exhibited lower initial discharge capacities owing to the reduced amount of active materials (LiMn2O4 particles) in the nanocomposites. However, their electrochemical cycling performance was significantly enhanced, high-lighting the advantages of anchoring LiMn2O4 particles on graphene sheets. The enhanced cycling performance could be ascribed to the fact that the graphene nanosheets within the LiMn2O4-graphene nanocomposites could provide a 3D conducting scaffold, which could not only alleviate the aggregation of LiMn2O4 particles and accommodate the volume changes of LiMn2O4 particles, but also enhance the ionic conductivity and charge transfer during the lithiation/delifhiation process.

Palladium‐Gold Nanoalloy Surface Modified LiMn2O4 Cathode forEnhanced Li‐Ion Battery

Au with Pd nanoparticles were synthesized and coated onto the spinel LiMn2O4 via a coprecipitation calcination method with the objective to improve the microstructure, conductivity, and electrochemical activities of pristine LiMn2O4. The novel Li[PdAu]xMn2−xO4 composite cathode had high phase purity, well crystallized particles, and more regular morphological structures with narrow size distributions. At enlarged cycling potential ranges the Li[PdAu]xMn2−xO4 sample delivered 90 mAh g−1 discharge capacity compared to LiMn2O4 (45 mAh g−1). It was concluded that even a small amount of the Pd and Au enhanced both the lithium diffusivity and electrochemical conductivity of the host sample due to the beneficial properties of their synergy.

Enhanced cycle stability at high rate and excellent high rate capability of La0.7Sr0.3Mn0.7Co0.3O3-coated LiMn2O4

La0.7Sr0.3Mn0.7Co0.3O3-coated spinel LiMn2O4 with excellent cycle stability and high rate capability is successfully prepared by a sol–gel method. The 3 wt.% La0.7Sr0.3Mn0.7Co0.3O3-coated LiMn2O4 shows the optimum electrochemical performance. It can deliver 101 mAh g−1 at 10 C even after 100 cycles with a capacity retention of 93.5%. In contrast, the bare LiMn2O4 delivers 83.6 mAh g−1 at the same condition, only 84.5% capacity left. The rate capability of 3 wt.% La0.7Sr0.3Mn0.7Co0.3O3-coated LiMn2O4 is also obviously enhanced, especially at high rates (10 C, 20 C and 30 C). It can deliver 74.3 mAh g−1 at 30 C which is much higher than that of the bare sample (47.2 mAh g−1). The bare and coated LiMn2O4 samples are studied with various techniques. Both powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements demonstrate the existence of the La0.7Sr0.3Mn0.7Co0.3O3, and it has no influence on the crystal structure of the pristine LiMn2O4. Transmission electron microscopy (TEM) shows that La0.7Sr0.3Mn0.7Co0.3O3 coating layer with good crystallinity can cover the surface of LiMn2O4 to form a core–shell structure. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) demonstrate that the coating layer can improve the kinetics of electrodes.

Zinc and aluminium: glutamic acid assisted dual-doped LiMn2O4 spinels via sol–gel method as cathode material for use in lithium rechargeable batteries

LiMn2O4 and LiZnxAlyMn2-x-yO4 (x = 0.07–0.10; y = 0.15–0.18) powders have been synthesized by sol–gel method using glutamic acid as chelating agent. The synthesized samples have been subjected to spectral studies viz., thermo gravimetric and differential thermal analysis to ascertain the phase formation, X-ray diffraction to confirm the formation of single-phase spinel material, scanning electron microscope (SEM) and transmission electron microscope (TEM) to understand the surface morphology of the synthesized materials, energy dispersive X-ray analysis (EDAX) to corroborate the elements in the exact composition of the undoped and doped spinel and electrochemical characterization. SEM images of LiMn2O4 depict majority of spherical particles are of 0.5 μm size. TEM images of spinel LiMn2O4 and LiZnxAlyMn2-x-yO4 confirm the synthesized particles are an ice cube morphology of nano-sized (25–50 nm) with good agglomeration. EDAX peaks of Zn, Al, Mn and O have been confirmed in actual compositions of LiMn2O4 and LiZnxAlyMn2-x-yO4. Charge–discharge studies of pristine spinel LiMn2O4 delivers maximum discharge capacity of 130 mA h g−1 corresponding to columbic efficiency of 74 % during the first cycle with capacity fade of 0.19 mA h g−1 cycle−1 over the investigated ten cycles. At the end of 10th cycle, it delivers maximum discharge capacity of 105 mA h g−1 with columbic efficiency of 67 % and capacity fade of 0.19 mA h g−1 cycle−1 over the investigated ten cycles. An inter alia, all dopant substitutions, (LiZn0.10Al0.15Mn1.75O4) sample exhibits the better cycling performance with good capacity retention (93 %) during the 10th cycle discharge capacity of 112 mA h g−1 comparing to undoped and other doped spinel with capacity fade of 1 mA h g−1 cycle−1.

Electrochemical performances of surface modified CePO4-coated LiMn2O4 cathode materials for rechargeable lithium ion batteries.

Improved electrochemical performances of Spinel LiMn2O4 cathode was prepared by a sol-gel coating of CePO4 to the surface of the LiMn2O4 powder and subsequent heat treatment at 600 degrees C for 8 h in air. The surface modified LiMn2O4 cathode materials were characterized by using TG/DTA, X-ray diffraction, Scanning electron microscope, X-ray photoelectron spectroscope, cyclic voltammetry techniques and charge/discharge studies. XRD and SEM results show that no significant bulk structural differences are observed between the coated and pristine LiMn2O4. 2 wt.% CePO4 coated LiMn2O4 cathode materials was enhance the particle size compared to pristine due to small layer of CePO4 particle covered on the over the surface. XPS data illustrated that, the CePO4 was completely coated over the surface of the LiMn2O4 core materials. Galvanostatic charge/discharge studies of the uncoated and CePO4-coated LiMn2O4 cathode materials were carried out in the voltage range between 3.0 and 4.5 V at elevated temperature (50 degrees C). The composition of CePO4 coating was increased from 0.0 to 3.0 Wt.%, the initial capacity of the samples decreased slightly. But the cycling stability was increased remarkably between 3-4.5 V. 2.0 Wt.% CePO4-coated LiMn2O4 cathode has satisfied the structural stability, high reversible capacity and excellent electrochemical performances. This result indicates that, the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.

The effect of Na0.44MnO2 formation in Na+-modified spinel LiMn2O4

Na0.44MnO2 impure phase is formed during the synthetic process of Na+-modified spinel LiMn2O4 by solid state reaction which is confirmed by x-ray diffraction analysis. Scanning electron microscopy, transmission electron microscopy and galvanostatic charge-discharge measurements are carried out to investigate the effect of the formation of Na0.44MnO2 impurity on the morphology and electrochemical properties of the spinel material. The results show that the spinel material with impure phase exhibits improved cyclability compared to that of the pristine LiMn2O4. The improved electrochemical performance is mainly ascribed to the improved crystallinity of the spinel particles, enhanced stability of the spinel structure and good electronic conductivity of the composite.

Surface modification of cathodes with nanosized amorphous MnO2 coating for high-power application in lithium-ion batteries

In this paper, we discuss the surface modification of spinel LiMn2O4 cathodes by MnO2 particles via a simple precipitation method at room temperature, without any heat treatment. Nanosized MnO2 particles with an amorphous structure were formed and distributed evenly on the parent cathode, without severe agglomeration. Electrochemical measurements at ambient temperature revealed that the MnO2-coated LiMn2O4 electrode exhibited superior rate capability as compared to the pristine LiMn2O4 electrode. Based on the capacity at low currents, the surface-modified electrode delivered 62% capacity, whereas the pristine LiMn2O4 electrode maintained 8% capacity during fast discharging for 3min at 20 C-rate. Surface coating with nano-MnO2 enhanced the electrochemical properties such as storage characteristics, particularly at elevated temperatures. X-ray photoelectron spectroscopy data showed that the improved performance of the coated electrode was mainly because of the suppressed side reaction between the cathode and the electrolyte.

Structure and electrochemical performances of co-substituted LiSm x La0.2-x Mn1.80O4 cathode materials for rechargeable lithium-ion batteries

Spinel LiMn2O4 and Sm, La co-substituted LiSm x La0.2-x Mn1.80O4 (x = 0.05, 0.10 and 0.15) cathode materials were synthesized by sol–gel method using aqueous solutions of metal nitrates and tartaric acid as chelating agent at 600 °C for 10 h. The structure and electrochemical properties of the synthesized materials were characterized by using thermogravimetric/differential thermal analysis, X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, charge/discharge and electrochemical impedance spectroscopy studies. XRD analysis indicated that all the prepared samples were mainly belong to cubic crystal form with Fd3m space group. LiSm0.10La0.10Mn1.80O4 exhibits capacity retention of 90 % and 82 % after 100 cycles at room temperature (30 °C) and at elevated temperature (50 °C) at a rate of 0.5-C, respectively, much higher than those of the pristine LiMn2O4 (74 % and 60 %). Among all the compositions, LiSm0.10La0.10Mn1.80O4 cathode has improved the structural stability, high-capacity retention, better elevated temperature performance and excellent electrochemical performances of the rechargeable lithium-ion batteries.

Crystal structure and multicomponent effects in Li1+xMn2−x−yAlyO4 cathode materials for Li-ion batteries

Li, Al co-doped LiMn2O4 (Li1+xMn2−x−yAlyO4, 0 ≤ x ≤ 0.12, 0 ≤ y ≤ 0.1) cathode has been synthesized via a solid-state reaction designedly using industrial raw materials in bulk scale (>20 kg). The multicomponent substitution effects on the crystal structures are examined systematically by Rietveld refinement of X-ray diffraction, and the resultant electrochemical properties for Li-ion batteries are also evaluated by galvanostatic charge–discharge and electrochemical impedance spectroscopy measurements. As a result, Li, Al co-doping significantly changes the unit cell parameter and atomic arrangement. With the increasing of doping levels, a cell dimension contracts with concomitant changes in bond length, whereby the MO6 octahedron (M = Mn/Li/Al) shrinks to provide structural integrity and the LiO4 tetrahedron expands to facilitate a fast electrochemical process. The strong spinel-framework contributes to a better structure-stabilization, resulting in a superior capacity retention ratio of 90% after 200 cycles at 0.5 C at 55 °C for the optimized composition (Li1.06Mn1.86Al0.08O4), which possesses an initial value of 102 mAh g−1. Meanwhile, the expansion of LiO4 tetrahedron leads to better high-rate performance, bringing about a capacity of 88 mAh g−1 upon cycling at 10 C at 55 °C. Further, Li1.06Mn1.86Al0.08O4 displays lower impedance than that of the pristine LiMn2O4.

Synthesis and electrochemical performance of a LiMn1.83Co0.17O4 shell/LiMn2O4 core cathode material

A shell/core Co-incorporated LiMn2O4 cathode material was synthesized by a facile modified sol–gel process, using highly dispersed Mn3O4 nanoparticles as the Mn source. Structural characterization revealed that the shell layer, which was 5–10nm thick, and composed of spinel LiMn1.83Co0.17O4, grew homogeneously on the spinel LiMn2O4 core with a size of ~50nm. Electrochemical results showed that electrochemical performance of the shell/core cathode material compared favorably with that of the Co-doped LiMn2O4 counterparts. Also its rate capability and cycling performance were apparently superior to those of pristine LiMn2O4 synthesized under the same conditions. The shell/core cathode material exhibited a discharge capacity of 127mAhg−1 at a current rate of 0.5C (where 1C=148mAg−1), and retained a capacity of 103mAhg−1 at 10C, showing 81% capacity retention. After 200 cycles at 1C and 25°C, it delivered a capacity of 123mAhg−1, retaining 98% of its initial capacity. After 100 cycles at 1C and 55°C, it showed a capacity of 118mAhg−1, preserving 94% of its initial capacity. Its excellent electrochemical performance along with the facile synthesis process allowed the shell/core cathode material to serve as a promising cathode for high-performance Li-ion batteries.