A comparative electrochemical study of LiMn2O4 spinel thin-film and porous laminate

Novel Electrostatic Spray Deposition (ESD) technique was used to fabricate LiMn₂O₄ spinel thin-films. Cyclic voltammograms of both the ESD and porous laminate films show the double peaks in the 4.0 V range characteristic of the LiMn₂O₄ spinel materials. The porous laminates exhibit two semicircles in the impedance spectra while the ESD films show only one single semicircle. The diffusion time constant in the laminate films was typically one order of magnitude larger than that in the ESD thin-films. The apparent lithium-ion chemical diffusion coefficient in LiMn₂O₄ was found to be of the order of 10⁻⁹ cm²/s for both the porous laminate film and the ESD films despite the difference in the diffusion time constants.     

The cell-impedance-controlled lithium transport through LiMn2O4 film electrode with fractal surface by analyses of ac-impedance spectra, potentiostatic current transient and linear sweep voltammogram

Lithium transport through the fractal LiMn₂O₄ film electrode under the cell-impedance-controlled constraint was investigated by employing ac-impedance spectroscopy, potentiostatic current transient technique and linear sweep voltammetry. For this purpose, the flat and fractal LiMn₂O₄ film electrodes were prepared on the as-deposited Pt/polished Al₂O₃ substrate and the surface modified Pt/unpolished Al₂O₃ substrate, respectively. From the analysis of the ac-impedance spectra obtained from the flat and fractal electrodes, it is found that the apparent self-similar fractal dimension reduces the charge-transfer resistance, and the phase angle of the diffusion impedance under the semi-infinite diffusion condition positively deviates in absolute value from 45° with increasing fractal dimension. All the potentiostatic current transients experimentally measured from the flat and fractal LiMn₂O₄ electrodes showed non-generalised Cottrell behaviour which resulted from the cell-impedance-controlled constraint during lithium transport. In the case of linear sweep voltammogram theoretically calculated and experimentally measured from the flat and fractal LiMn₂O₄ electrodes, the power dependence of the peak current on the scan rate hardly exhibited the generalised Randles-Sev?ik behaviour. From the analyses of the potentiostatic current transients and the linear sweep voltammograms, furthermore, it is experimentally confirmed that the internal cell resistance mainly determining the cell-impedance-controlled lithium transport strongly depends upon the fractal dimension as well as such external parameters as the applied potential step and the amount of lithium transferred during lithium transport.     

Electrochemical investigation of LiMn2O4 cathodes in gel electrolyte at various temperatures

A composite lithium battery electrode of LiMn₂O₄ in combination with a gel electrolyte (1 M LiBF₄/24 wt% PMMA/1:1 EC:DEC) has been investigated by galvanostatic cycling experiments and electrochemical impedance spectroscopy (EIS) at various temperatures, i.e. −3<T<56 °C. For analysis of EIS data, a mathematical model taking into account local kinetics and potential distribution in the liquid phase within the porous electrode structure was used. Reasonable values of the double-layer capacitance, the exchange-current density and the solid phase diffusion were found as a function of temperature. The apparent activation energy of the charge-transfer (∼65 kJ mol⁻¹), the solid phase transfer (∼45 kJ mol⁻¹) and of the ionic bulk and effective conductance in the gel phase (∼34 kJ mol⁻¹), respectively, were also determined. The kinetic results related to ambient temperature were compared to those obtained in the corresponding liquid electrolyte. The incorporated PMMA was found to reduce the ionic conductivity of the free electrolyte, and it was concluded that the presence of 24 wt% PMMA does not have a significant influence on the kinetic properties of LiMn₂O₄.     

Modeling the impedance versus voltage characteristics of LiNi0.8Co0.15Al0.05O2

The power-delivery capability of lithium-ion cells based on LiNi_0.8Co_0.15Al_0.05O₂-based positive electrodes shows a significant dependence on the cell's state-of-charge. One reason for this behavior is the variation of the positive electrode's impedance with the oxide's lithium content. In this article, an electrochemical model based on concentrated solution porous electrode theory is used to model impedance data obtained on LiNi_0.8Co_0.15Al_0.05O₂-based positive electrodes charged to potentials ranging from 3.55 to 4.55 V versus Li. The parameters obtained from model fits include the exchange-current density and Li-ion diffusion coefficients in the oxide. The variations in these parameters with oxide potential are correlated with structural changes in the material observed during Li-ion intercalation-deintercalation reactions.     

On the fractal study of LiMn2O4 electrode surface

Fractal dimension of a LiMn₂O₄ electrode prepared by sol-gel method was determined using electrochemical techniques based on the phenomenon of “diffusion towards electrode surface”. A simple discussion was made on the methodology to understand what is really estimated as the fractal dimension. It was demonstrated that the value of fractal dimension determined based on electrochemical methods is strongly dependent on the electrochemical system situation. This is generally true for all real electrodes involving insertion/extraction processes. This comes from the fact that surface morphology of the electrode is subject of significant changes during the electrochemical experiment.     

Properties of LiMn2O4 cathode in electrolyte based on N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide

LiMn₂O₄ was examined as a cathode material for lithium-ion batteries, working together with a room temperature ionic liquid electrolyte, obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂) in liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MePrPipNTf₂), with the formation of a liquid LiNTf₂-MePrPipNTf₂ system. The Li/LiMn₂O₄ cell was tested by galvanostatic charging/discharging and by impedance spectroscopy. The LiMn₂O₄ cathode showed good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate (VC) as an additive to the ionic liquid. The flash point of the LiNTf₂-MePrPipNTf₂-VC(10%) electrolyte was estimated to be above 300 °C.     

Conventional- and microwave-hydrothermal synthesis of LiMn2O4: Effect of synthesis on electrochemical energy storage performances

The LiMn₂O₄ electrode materials were synthesized by the conventional-hydrothermal and microwave-hydrothermal methods. The electrochemical performances of LiMn₂O₄ were studied as supercapacitors in LiNO₃ electrolyte and lithium-ion battery cathodes. The microwave-hydrothermal method can synthesize LiMn₂O₄ electrode materials with reversible electrochemical reaction in a short reaction time and low reaction temperature than conventional-hydrothermal route. The capacitance of LiMn₂O₄ electrode increased with increasing crystallization time in conventional-hydrothermal route. The results showed that LiMn₂O₄ supercapacitors had similar discharge capacity and potential window (1.2 V) as that of ordinary lithium-ion battery cathodes. In LiNO₃ aqueous electrolyte, the reaction kinetics of LiMn₂O₄ supercapacitors was very fast. Even, at current densities of 1 A/g and 5 A/g, aqueous electrolyte gave good capacity compared with that in organic electrolyte at a current density of 0.05 A/g.     

Electrochemical and ex situ XRD studies of a LiMn1.5Ni0.5O4 high-voltage cathode material

Sub-micro spinel-structured LiMn_1.5Ni_0.5O₄ material was prepared by a spray-drying method. The electrochemical properties of LiMn_1.5Ni_0.5O₄ were investigated using Li ion model cells, Li/LiPF₆ (EC + DMC)/LiMn_1.5Ni_0.5O₄. It was found that the first reversible capacity was about 132 mAh g⁻¹ in the voltage range of 3.60-4.95 V. Ex situ X-ray diffraction (XRD) analysis had been used to characterize the first charge/discharge process of the LiMn_1.5Ni_0.5O₄ electrode. The result suggested that the material configuration maintained invariability. At room temperature, on cycling in high-voltage range (4.50-4.95 V) and low-voltage range (3.60-4.50 V), the discharge capacity of the material was about 100 and 25 mAh g⁻¹, respectively, and the spinel LiMn_1.5Ni_0.5O₄ exhibited good cycle ability in both voltage ranges. However, at high temperature, the material showed different electrochemical characteristics. Excellent electrochemical performance and low material cost make this spinel compound an attractive cathode for advanced lithium ion batteries.     

Aging characteristics of high-power lithium-ion cells with LiNi0.8Co0.15Al0.05O2 and Li4/3Ti5/3O4 electrodes

The impedance rise that results from the accelerated aging of high-power lithium-ion cells containing LiNi_0.8Co_0.15Al_0.05O₂-based positive and graphite-based negative electrodes is dominated by contributions from the positive electrode. Data from various diagnostic experiments have indicated that a general degradation of the ionic pathway, apparently caused by surface film formation on the oxide particles, produces the positive electrode interface rise. One mechanistic hypothesis postulates that these surface films are components of the negative electrode solid electrolyte interphase (SEI) layer that migrate through the electrolyte and separator and subsequently coat the positive electrode. This hypothesis is examined in this article by subjecting cells with LiNi_0.8Co_0.15Al_0.05O₂-based positive and Li_4/3Ti_5/3O₄-based negative electrodes to accelerated aging. The impedance rise in these cells was observed to be almost entirely from the positive electrode. Because reduction products are not expected on the 1.55 V Li_4/3Ti_5/3O₄ electrode, the positive electrode impedance cannot be attributed to the migration of SEI-type fragments from the negative electrode. It follows then that the impedance rise results from mechanisms that are “intrinsic” to the positive electrode.     

Enhanced cycling stability of LiMn2O4 cathode by amorphous FePO4 coating

The cycling performance of LiMn₂O₄ at room and elevated temperatures is improved by FePO₄ modification through chemical deposition method. The pristine and FePO₄-coated LiMn₂O₄ materials are characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Their cycling performances are thoroughly investigated and compared. The 3 wt.% FePO₄-coated LiMn₂O₄ exhibits capacity losses of only 32% and 34% at room temperature and 55 °C, respectively, after 80 cycles, much better than those of the pristine material, 55% and 72%. The cyclic voltammograms at 55 °C reveal that the improvement in the cycling performance of FePO₄-coated LiMn₂O₄ electrodes can be attributed to the stabilization of spinel structures. The separation of FePO₄ between active materials and electrolyte and its interaction with SEI (solid electrolyte interphase) film are believed to account for the improved performances.     

Mixed bi-material electrodes based on LiMn2O4 and activated carbon for hybrid electrochemical energy storage devices

The performance of mixed bi-material electrodes composed of the battery material, LiMn₂O₄, and the electrochemical capacitor material, activated carbon, for hybrid electrochemical energy storage devices is investigated by galvanostatic charge/discharge and pulsed discharge experiments. Both, a high and a low conductivity lithium-containing electrolyte are used. The specific charge of the bi-material electrode is the linear combination of the specific charges of LiMn₂O₄ and activated carbon according to the electrode composition at low discharge rates. Thus, the specific charge of the bi-material electrode falls between the specific charge of the activated carbon electrode and the LiMn₂O₄ battery electrode. The bi-material electrodes have better rate capability than the LiMn₂O₄ battery electrode. For high current pulsed applications the bi-material electrodes typically outperform both the battery and the capacitor electrode.     

Cycling-induced stress in lithium ion negative electrodes: LiAl/LiFePO4 and Li4Ti5O12/LiFePO4 cells

In this work, we examined the electrochemical behaviour of lithium ion batteries containing lithium iron phosphate as the positive electrode and systems based on Li-Al or Li-Ti-O as the negative electrode. These two systems differ in their potential versus the redox couple Li⁺/Li and in their morphological changes upon lithium insertion/deinsertion. Under relatively slow charge/discharge regimes, the lithium-aluminium alloys were found to deliver energies as high as 438 Wh kg⁻¹ but could withstand only a few cycles before crumbling, which precludes their use as negative electrodes. Negative electrodes consisting solely of aluminium performed even worse. However, an electrode made from a material with zero-strain associated to lithium introduction/removal such as a lithium titanate spinel exhibited good performance that was slightly dependent on the current rate used. The Li₄Ti₅O₁₂/LiFePO₄ cell provided capacities as high as 150 mAh g⁻¹ under C-rate in the 100th cycle.     

Preparation and electrochemical properties of LiMn2O4 by the microwave-assisted rheological phase method

Pure-phase and well-crystallized spinel LiMn₂O₄ powders as cathode materials for lithium-ion batteries were successfully synthesized by a new simple microwave-assisted rheological phase method, which was a timesaving and efficient method. The physical properties of the as-synthesized samples compared with the pristine LiMn₂O₄ obtained from the rheological phase method were investigated by thermogravimetry analysis (TGA), X-ray diffraction (XRD) and scanning electronic microscope (SEM). The as-prepared powders were used as positive materials for lithium-ion battery, whose charge/discharge properties and cycle performance were examined in detail. The powders resulting from the microwave-assisted rheological phase method were pure, spinel structure LiMn₂O₄ particles of regular shapes with distribution uniformly, and exhibited promising electrochemical properties for battery. Furthermore, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the reactions of Li-ion insertion into and extraction from LiMn₂O₄ electrode.     

The study of lithium ion de-insertion/insertion in LiMn2O4 and determination of kinetic parameters in aqueous Li2SO4 solution using electrochemical impedance spectroscopy

In this work, we report a basic study on the mechanism of lithium ion de-insertion/insertion process from/into LiMn₂O₄ cathode material in aqueous Li₂SO₄ solution using electrochemical impedance spectroscopy (EIS). An equivalent circuit distinguishing the kinetic parameters of lithium ion de-insertion/insertion is used to simulate the experimental impedance data. The fitting results are in good agreement with the experimental results and the parameters of the kinetic process of Li⁺ de-insertion and insertion in LiMn₂O₄ at different potentials during charge and discharge are obtained using the same circuit. The results indicate that the de-insertion/insertion behavior of lithium ions at LiMn₂O₄ cathode in Li₂SO₄ aqueous solution is similar to that reported in the organic electrolytes. The charge transfer resistance (R_ct), warburg resistance, double layer capacitance and chemical diffusion coefficient (DLi+) vary with potentials during de-insertion/insertion processes. R_ct is lowest at the CV peak potentials and the important kinetic parameter, DLi+ exhibits two distinct minima at potentials corresponding to CV peaks during de-insertion–insertion and it was found to be between 10⁻⁸ and 10⁻¹⁰ cm² s⁻¹during lithium de-insertion/insertion processes.     

Mechanism transition of cell-impedance-controlled lithium transport through Li1−δMn2O4 composite electrode caused by surface-modification and temperature variation

The mechanism transition of lithium transport through a Li_1−δMn₂O₄ composite electrode caused by the surface-modification and temperature variation was investigated using the galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and the potentiostatic current transient technique. From the analyses of the ac-impedance spectra, experimentally measured from unmodified Li_1−δMn₂O₄ and surface-modified Li_1−δMn₂O₄ with MgO composite electrodes, the internal cell resistance of the MgO-modified Li_1−δMn₂O₄ electrode was determined to be much smaller in value than that of the unmodified electrode over the whole potential range. Moreover, from the analysis of the anodic current transients measured on the MgO-modified Li_1−δMn₂O₄ electrode, it was found that the cell-impedance-controlled constraint at the electrode surface is changed to a diffusion-controlled constraint, which is characterised by a large potential step and simultaneously by a small amount of lithium transferred during lithium transport. This strongly suggests that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport through the MgO-modified Li_1−δMn₂O₄ electrode. Furthermore, from the temperature dependence of the internal cell resistance and diffusion resistance in the unmodified Li_1−δMn₂O₄ composite electrode measured by GITT and EIS, it was concluded that which mechanism of lithium transport will be operative strongly depends on the diffusion resistance as well as on the internal cell resistance.     

Segmented bi-material electrodes of activated carbon and LiMn2O4 for electrochemical hybrid storage devices: Effect of mass ratio and C-rate on current sharing

The performance of segmented bi-material electrodes consisting of an activated carbon and LiMn₂O₄ was investigated as a function of the relative amount of the components for full slow discharge at C/5. By means of a segmented current collector the contribution of each component to the overall current has been monitored. The potential profiles for the constant current discharges showed a monotonous transition from pure capacitive behavior of the activated carbon to battery behavior of pure LiMn₂O₄ for increasing amounts of LiMn₂O₄ in the segmented bi-material electrode. Correspondingly, the current sharing measurements have shown a decreased contribution of the activated carbon to the overall current. The effect of the C-rate up to 20C and pulse discharge at 25C was investigated for a segmented bi-material electrode with a relative amount of activated carbon of 72% by weight. The current sharing showed an increased contribution of the activated carbon to the overall current at high C-rate and during pulse discharge. Moreover it could be demonstrated that the battery material recharges the capacitive material subsequent to the discharge pulse.     

Electrochemical stability of thin film LiMn2O4 cathode in organic electrolyte solutions with different compositions at 55 °C

A survey of the electrochemical stability of electrostatic spray deposited thin film of LiMn₂O₄ was performed in LiClO₄-EC-PC, LiBF₄-EC-PC, and LiPF₆-EC-PC solutions at 55 °C. The solution resistance, the surface film resistance, and the charge-transfer resistance were all found to depend on the electrolyte composition. Among the LiX-salts studied, the lowest charge transfer-resistance, and surface layer resistance were obtained in LiBF₄-EC-PC solution. There is no major influence of the electrolyte solution compositions upon lithium ion transport in the LiMn₂O₄ bulk at 55 °C. The diffusion coefficient of lithium in the solid phase varied within 10⁻¹⁰-10⁻⁸ cm² s⁻¹ in the three solutions. In general, it seems that in LiBF₄ solutions, the surface chemistry is the most stable in the three solutions examined, and hence the electrode impedance in LiBF₄ solutions was the lowest. In LiPF₆ solutions, HF seems to play an important role, and thus, the electrode impedance is relatively high due to the precipitation of surface LiF.     

Hydrothermal synthesis of LiMn2O4/C composite as a cathode for rechargeable lithium-ion battery with excellent rate capability

A spinel LiMn₂O₄/C composite was synthesized by hydrothermally treating a precursor of manganese oxide/carbon (MO/C) composite in 0.1 M LiOH solution at 180 °C for 24 h, where the precursor was prepared by reducing potassium permanganate with acetylene black (AB). The AB in the precursor serves as the reducing agent to synthesize the LiMn₂O₄ during the hydrothermal process; the excess of AB remains in the hydrothermal product, forming the LiMn₂O₄/C composite, where the remaining AB helps to improve the electronic conductivity of the composite. The contact between LiMn₂O₄ and C in our composite is better than that in the physically mixed LiMn₂O₄/C material. The electrochemical performance of the LiMn₂O₄/C composite was investigated; the material delivered a high capacity of 83 mAh g⁻¹ and remained 92% of its initial capacity after 200 cycles at a current density of 2 A g⁻¹, indicating its excellent rate capability as well as good cyclic performance.     

A comparative study on IrO2-Ta2O5 coated titanium electrodes prepared with different methods

The electrodes of IrO₂-Ta₂O₅ coated titanium were prepared using conventionally thermal decomposition procedure and polymer sol-gel (Pechini) method, respectively. The microstructure and electrochemical properties of the electrodes were studied with scanning electron microscope (SEM), energy dispersive X-ray (EDX), atomic force microscope (AFM), potentiodynamic polarization, cyclic voltammetry, electrochemical impedance spectroscopy and accelerated life test. As compared with the electrode formed using the traditional method of thermal decomposition, the oxide electrode prepared by Pechini method presents morphology of higher nano-scale roughness and more uniform surface composition with little precipitates. It also has larger electrochemically active surface area, better electrocatalytic activity for oxygen evolution and higher stability.     

Fractal study of LiMn2O4 film electrode surface for lithium batteries application

Fractal structure of a LiMn₂O₄ film electrode has been investigated and its fractal dimensions was determined using different electrochemical techniques, viz. cyclic voltammetry and chronoamperometry. The results obtained from both these methods are in good agreement indicating the reliability of the estimated D_f. The fractal study of the LiMn₂O₄ film electrode suggests a complex surface with high fractal dimension. In addition, length scales of the electrode surface were also calculated.