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.     

Properties of Li-graphite and LiFePO4 electrodes in LiPF6–sulfolane electrolyte

Sulfolane (also referred to as tetramethylene sulfone, TMS) containing LiPF₆ and vinylene carbonate (VC) was tested as a non-flammable electrolyte for a graphite |LiFePO₄ lithium-ion battery. Charging/discharging capacity of the LiFePO₄ electrode was ca. 150 mAh g⁻¹ (VC content 5 wt%). The capacity of the graphite electrode after 10 cycles establishes at the level of ca. 350 mAh g⁻¹ (C/10 rate). In the case of the full graphite |1 M LiPF₆ + TMS + VC 10 wt% |LiFePO₄ cell, both charging and discharging capacity (referred to cathode mass) stabilized at a value of ca. 120 mAh g⁻¹. Exchange current density for Li⁺ reduction on metallic lithium, estimated from electrochemical impedance spectroscopy (EIS) experiments, was j_o(Li/Li⁺) = 8.15 × 10⁻⁴ A cm⁻². Moreover, EIS suggests formation of the solid electrolyte interface (SEI) on lithium, lithiated graphite and LiFePO₄ electrodes, protecting them from further corrosion in contact with the liquid electrolyte. Scanning electron microscopy (SEM) images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. No graphite exfoliation was observed. The main decomposition peak of the LiPF₆ + TMS + VC electrolyte (TG/DTA experiment) was present at ca. 275 °C. The LiFePO₄(solid) + 1 M LiPF₆ + TMS + 10 wt% VC system shows a flash point of ca. 150 °C. This was much higher in comparison to that characteristic of a classical LiFePO₄ (solid) + 1 M LiPF₆ + 50 wt% EC + 50 wt% DMC system (T_f ≈ 37 °C).     

Effect of fluoroethylene carbonate on high temperature capacity retention of LiMn2O4/graphite Li-ion cells

In order to overcome severe capacity fading of LiMn₂O₄/graphite Li-ion cells at high temperature at 60 °C, fluoroethylene carbonate (FEC) was newly evaluated as an electrolyte additive. With 2 wt.% FEC addition into the electrolyte (EC/DEC/PC with 1 M LiPF₆), the capacity retention at 60 °C after 130 cycles was significantly improved by about 20%. To understand the underlying principle on the capacity retention enhancement, the electrochemical properties of the cells including cell performance, impedance behavior as well as the characteristics of the interfacial properties were examined. Based on these results, it is suggested that the improved capacity retention of LiMn₂O₄/graphite Li-ion cells with addition of FEC especially at high temperature is mainly originated from the thin and stable SEI layer formed on the graphite anode surface.     

Electrochemical behavior of LiCoO2 in a saturated aqueous Li2SO4 solution

The electrochemical behavior of a commercial LiCoO₂ with spherical shape in a saturated Li₂SO₄ aqueous solution was investigated with cyclic voltammetry and electrochemical impedance spectroscopy. Three redox couples at E_SCE = 0.87/0.71, 0.95/0.90 and 1.06/1.01 V corresponding to those found at ELi/Li⁺=4.08/3.83, 4.13/4.03 and 4.21/4.14 V in organic electrolyte solutions were observed. The diffusion coefficient of lithium ions is 1.649 × 10⁻¹⁰ cm² s⁻¹, close to the value in organic electrolyte solutions. The results indicate that the intercalation and deintercalation behavior of lithium ions in the Li₂SO₄ solution is similar to that in the organic electrolyte solutions. However, due to the higher ionic conductivity of the aqueous solution, current response and reversibility of redox behavior in the aqueous solution are better than in the organic electrolyte solutions, suggesting that the aqueous solution is favorable for high rate capability. The charge transfer resistance, the exchange current and the capacitance of the double layer vary with the charge voltage during the deintercalation process. At the peak of the oxidation (0.87 V), the charge transfer resistance is the lowest. These fundamental results provide a good base for exploring new safe power sources for large scale energy storage.     

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.     

Ionic liquid-based gel polymer electrolyte for LiMn0.4Fe0.6PO4 cathode prepared by electrospinning technique

Two polymer electrolytes (PEs), one consisting of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (PE_EC/DMC) and the other consisting of LiTFSI in room temperature ionic liquid (RTIL), 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI) (PE_IL), were prepared by using electrospun P(VdF-HFP) membranes. The PEs showed typical impedance spectroscopic responses with high conductivity and good anodic stability. The PEs were applied with carbon coated LiMn_0.4Fe_0.6PO₄ cathode material prepared by sol-gel method. The charge-discharge kinetics of LiMn_0.4Fe_0.6PO₄ cathode cells were studied by electrochemical impedance spectroscopy. The excellent performance with high capacity and good cycle stability was observed for both the cells. The cell comprising of PE_IL showed a better performance than the other cell. The cells having PE_EC/DMC and PE_IL delivered discharge capacities of 150 and 141 mAh g⁻¹, and 168 and 162 mAh g⁻¹, respectively, after cycle 1 and 50. The differences in the performance of the PEs originate from the differences in viscosity, ionic conductivity and also from the different levels of interactions of a RTIL and EC/DMC with the polymer. The evaluation of lithium ion diffusion coefficients shows its fast diffusion in both the cases, the trend of which changed with the increase in the number of cycles.     

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.     

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.     

Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties

The lithium deposition-dissolution process in solid polymer electrolytes containing Al₂O₃ filler treated under different conditions has been investigated comparing with the ionic conduction behavior of the electrolyte. The composite electrolytes were prepared from poly(ethylene oxide) (PEO), LiBF₄ and α-Al₂O₃ filler by using a dry process, where the surface of α-Al₂O₃ was beforehand modified by a wet process. The exchange current densities, i₀, of the lithium electrode process in P(EO)₂₀LiBF₄ with and without Al₂O₃ filler were determined by a micro-polarization method. The temperature dependence of i₀ provided similar values for activation energy, ca. 25 and 70 kJ mol⁻¹ in both temperature regions above and below 60 °C, respectively. The effect of the surface treatment of the filler on the lithium electrode process gave a different tendency from that on the ionic conductivity. The Al₂O₃ surface treated by alkali solution enhanced the electrode process to the largest extent among the fillers used here, while it led to rather poor cycling stability in voltammetry. The enhanced reaction rate at the lithium electrode/solid polymer electrolyte interface has probably resulted in the improved ion dissociation by the surface groups of the Al₂O₃ filler.     

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.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Electrolytes for hybrid carbon-MnO2 electrochemical capacitors

Different aqueous-based electrolytes have been tested in order to improve the electrochemical performance of hybrid (asymmetric) carbon/MnO₂ electrochemical capacitor (EC). Chloride and bromide aqueous solutions lead to the formation of Cl₂ and Br₂ respectively upon oxidation of the corresponding salt, thus limiting the useful electrochemical window of the MnO₂ electrode and producing gas evolution (in the case of chloride salts) detrimental to the cycling ability of an hybrid device. For sulfate and nitrate salts, MnO₂ electrode exhibits a 20% increase in capacitance when lithium is used as the cation compared to sodium or potassium salts, probably due to partial lithium intercalation in the tunnels of α-MnO₂ structure. The higher ionic conductivity and solubility of LiNO₃ has led to the investigation of this electrolyte in carbon/MnO₂ supercapacitor compared to standard hybrid cell using K₂SO₄. A lower resistance increase was evidenced when the temperature was decreased down to −10 °C. Long term cycling ability of carbon/MnO₂ supercapacitor was also evidenced with 5 M LiNO₃ electrolyte.     

Synthesis of spinel LiMn2O4 with manganese carbonate prepared by micro-emulsion method

Micro-spherical particle of MnCO₃ has been successfully synthesized in CTAB-C₈H₁₈-C₄H₉OH-H₂O micro-emulsion system. Mn₂O₃ decomposed from the MnCO₃ is mixed with Li₂CO₃ and sintered at 800 °C for 12 h, and the pure spinel LiMn₂O₄ in sub-micrometer size is obtained. The LiMn₂O₄ has initial discharge specific capacity of 124 mAh g⁻¹ at discharge current of 120 mA g⁻¹ between 3 and 4.2 V, and retains 118 mAh g⁻¹ after 110 cycles. High-rate capability test shows that even at a current density of 16 C, capacity about 103 mAh g⁻¹ is delivered, whose power is 57 times of that at 0.2 C. The capacity loss rate at 55 °C is 0.27% per cycle.     

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.     

Impact of “Super Acid” like filler on the properties of a PEGDME/LiClO4 system

Composite polymer electrolyte based on a new class of filler added to a PEGDME/LiClO₄ model system has been investigated. “Ceramic super acids” used consist of grafted SO₄²⁻ groups on Al₂O₃ particles surface obtained by calcinations route. Conductivity, DSC and FT-IR measurements performed on such composite electrolytes, when compared to the model PEGDME/LiClO₄ electrolyte, showed only slight improvement of their inner characteristics. In contrary, Li/Li symmetric cells study, by means of impedance spectroscopy, has presented a spectacular decrease of the interfacial resistance compare to the model electrolyte. This result opens a new pathway of investigation to master the lithium metal/polymer electrolyte interface.     

Cathodic performance of LiMn1−xMxPO4 (M = Ti, Mg and Zr) annealed in an inert atmosphere

To improve the cathodic performance of olivine-type LiMnPO₄, we investigated the optimal annealing conditions for a composite of carbon with cation doping. Nanocrystalline and the cation-doped LiMn_1−xM_xPO₄ (M = Ti, Mg, Zr and x = 0, 0.01, 0.05 and 0.10) was synthesized in aqueous solution using a planetary ball mill. The synthesis was performed at the fairly low temperature of 350 °C to limit particle size. The obtained samples except for the Zr doped one consisted of uniform and nano-sized particles. The performance of LiMnPO₄ was much improved by an annealing treatment between 500 and 550 °C with carbon in an inert atmosphere. A small amount of metal-rich phosphide (Mn₂P) was detected in the sample annealed at 900 °C. In addition, 1 at.% Mg doping for Fe enhanced the rate capability in our doped samples. The discharge capacity of LiMn_0.99Mg_0.01PO₄/C was 146 mAh/g at 0.1 mA/cm² and 125 mAh/g even at 2.0 mA/cm².     

Electrochemical properties of TiP2O7 and LiTi2(PO4)3 as anode material for lithium ion battery with aqueous solution electrolyte

Some polyanionic compounds, e.g. TiP₂O₇ and LiTi₂(PO₄)₃ with 3D framework structure were proposed to be used as anodes of lithium ion battery with aqueous electrolyte. The cyclic voltammetry properties TiP₂O₇ and LiTi₂(PO₄)₃ suggested that Li-ion de/intercalation reaction can occur without serious hydrogen evolution in 5 M LiNO₃ aqueous solution. The TiP₂O₇ and LiTi₂(PO₄)₃ give capacities of about 80 mAh/g between potentials of −0.50 V and 0 V (versus SHE) and 90 mAh/g between −0.65 V and −0.10 V (versus SHE), respectively. A test cell consisting of TiP₂O₇/5 M LiNO₃/LiMn₂O₄ delivers approximately 42 mAh/g (weight of cathode and anode) at average voltage of 1.40 V, and LiTi₂(PO₄)₃/5 M LiNO₃/LiMn₂O₄ delivers approximately 45 mAh/g at average voltage of 1.50 V. Both as-assembled cells suffered from short cycle life. The capacity fading may be related to deterioration of anode material.     

Investigation of lithium transport through LiMn2O4 film electrode in aqueous LiNO3 solution

Lithium transport through LiMn₂O₄ film electrode was investigated in aqueous saturated LiNO₃ solution by analyses of the potentiostatic current transient and ac-impedance spectra. It was found that the current transient hardly shows the Cottrell behaviour, and the initial current is linearly proportional to the potential step. This strongly suggests that lithium transport through the film electrode proceeds in aqueous LiNO₃ solution by the same mechanism involving the cell-impedance-controlled constraint, as does lithium transport in non-aqueous organic solution such as LiClO₄ in propylene carbonate (PC). However, the cell-impedance in aqueous LiNO₃ solution was determined to be much smaller by more than one order in value than that cell-impedance in non-aqueous LiClO₄-PC solution over the whole potential range, indicating lithium transport is markedly enhanced in aqueous electrolyte. From the comparison between the ac-impedance spectra obtained in aqueous and non-aqueous electrolytes, the reduced cell-impedance in aqueous electrolyte can be accounted for by the kinetic facility for the interfacial charge-transfer reaction in the absence of the resistive surface film as well as by the high conductivity of the electrolyte itself.     

Electrochemical performance of single crystalline spinel LiMn2O4 nanowires in an aqueous LiNO3 solution

Single crystalline cubic spinel LiMn₂O₄ nanowires were synthesized by hydrothermal method and the precursor calcinations. The phase structures and morphologies were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). Galvanostatic charging/discharging cycles of as-prepared LiMn₂O₄ nanowires were performed in an aqueous LiNO₃ solution. The initial discharge capacity of LiMn₂O₄ nanowires was 110 mAh g⁻¹, and the discharge capacity was still above 100 mAh g⁻¹ after 56 cycles at 10C-rate, and then 72 mAh g⁻¹ was registered after 130 cycles. This is the first report of a successful use of single crystalline spinel LiMn₂O₄ nanowire as cathode material for the aqueous rechargeable lithium battery (ARLB).     

An investigation of cell-impedance-controlled lithium transport through LiCoO2/Li1−δMn2O4 bilayer film electrode prepared by rf magnetron sputtering

Lithium transport through LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode prepared by radio-frequency (rf) magnetron sputtering was investigated in a 1 M solution of LiClO₄ in propylene carbonate. From the analyses of the AC-impedance spectra experimentally measured from the Li_1−δMn₂O₄ single-layer and LiCoO₂/Li_1−δMn₂O₄ bilayer film specimens, the internal cell resistance of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode was determined to be smaller in value than that of the Li_1−δMn₂O₄ single-layer film electrode over the whole potential range, which can be accounted for by the kinetic facility for the interfacial charge-transfer reaction in the presence of the more conductive LiCoO₂ surface film. Moreover, from the analyses of the anodic current transients measured from both the film specimens, it was suggested that the cell-impedance-controlled constraint at the electrode surface is changed to the diffusion-controlled constraint simultaneously characterised by the large potential step and the small amount of lithium transferred during lithium transport. In addition, in the case of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode, it was found that the critical value of the applied potential step needed for the mechanism transition is reduced, which strongly indicates that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport. Furthermore, from the comparison of the cathodic current transients measured on the Li_1−δMn₂O₄ single-layer film specimens with various thicknesses, it was experimentally verified that the diffusion resistance is explicitly distinguished from the internal cell resistance.