Lithium-deficient LiYMn2O4 spinels (0.9 ≤ Y < 1): Lithium content, synthesis temperature, thermal behaviour and electrochemical properties

Lithium-deficient Li_YMn₂O₄ spinels (LD-Li_YMn₂O₄) with nominal composition (0.9 ≤ Y < 1) have been synthesized by melt impregnation from Mn₂O₃ and LiNO₃ at temperatures ranging from 700 °C to 850 °C. X-ray diffraction data show that LD-Li_YMn₂O₄ spinels are obtained as single phases in the range Y = 0.975-1 at 700 °C and 750 °C. Morphological characterization by transmission electron microscopy shows that the particle size of LD-Li_YMn₂O₄ spinels increases on decreasing the Li-content. The influence of the Li-content and the synthesis temperature on the thermal and electrochemical behaviours has been systematically studied. Thermal analysis studies indicate that the temperature of the first thermal effect in the differential thermal analysis (DTA)/thermogravimetric (TG) curves, T_C1, linearly increases on decreasing the Li-content. The electrochemical properties of LD-Li_YMn₂O₄ spinels, determined by galvanostatic cycling, notably change with the synthesis conditions. So, the first discharge capacity, Q_disch., at C rate increases on rising the Li-content and the synthesis temperature. The sample Li_0.975Mn₂O₄ synthesized at 700 °C has a Q_disch. = 123 mAh g⁻¹ and a capacity retention of 99.77% per cycle. This LD-Li_YMn₂O₄ sample had the best electrochemical characteristics of the series.     

Ultrasonic spray pyrolysis of nano crystalline spinel LiMn2O4 showing good cycling performance in the 3 V range

Spherical lithium manganese oxide spinel was synthesized by an ultrasonic spray pyrolysis method, and has been characterized using X-ray diffraction, scanning electron microscopy, transimission electron microscopy and electrochemical cycling at 3 V regions. The LiMn₂O₄ powders were composed of about 10 nm-sized primary particles. The delivered discharge capacity of the synthesized nano-material was 125 mAh g⁻¹ between 2.4 and 3.5 V and its retention was about 96% upon 50 cycling. From the high resolution transmission electron microscopic study, it was found that structural transition of the parent material did not occur even after the 50th electrochemical cycling on the 3 V region. It seems that the reversible structural change is possible for nanocrystalline LiMn₂O₄ as observed by the X-ray diffraction and transition electron microscopic observations.     

Properties of nano-crystalline LiMn2O4 thin films deposited by pulsed laser deposition

Properties of LiMn₂O₄ thin films deposited on polished stainless steel substrates at 400 °C and 200 mTorr of oxygen by pulsed laser deposition have been characterized by electrochemical measurements and physical analyses. The film was mainly composed of nano-crystals less than 100 nm. A maximum specific capacity of 141.9 mAh/g cycled between 3.0 and 4.5 V with a current density of 20 μAh/cm² has been achieved. The film exhibited an excellent cycling stability up to 500 cycles. The low charge-transfer resistance at high potentials as revealed by AC impedance resulted in high charge/discharge potential and more capacity. The effect of overdischarge was limited and Jahn-Teller effect was overcome to a significant extent in this nano-crystalline film. Ex situ XRD, Raman and XPS provided supporting evidence in the changes in structure, reactivity and cycling stability of nano-crystalline LiMn₂O₄ film cathodes under different charge/discharge states and cycling tests. SEM images also revealed the stability of the surface topography after a long-term cycling test.     

Low-temperature solution combustion synthesis of high performance LiNi0.5Mn1.5O4

High-voltage LiNi_0.5Mn_1.5O₄ spinels were synthesized by a low temperature solution combustion method at 400 °C, 600 °C and 800 °C for 3 h. The phase composition, structural disordering, micro-morphologies and electrochemical properties of the products were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and constant current charge–discharge test. XRD analysis indicated that single phase LiNi_0.5Mn_1.5O₄ powders with disordered Fd-3m structures were obtained by the method at 400 °C, 600 °C and 800 °C. The crystallinity increased with increasing preparation temperatures. XRD and FTIR data indicated that the degree of structural disordering in the product prepared at 800 °C was the largest and in the product prepared at 600 °C was the least. SEM investigation demonstrated that the particle size and the crystal perfection of the products were increased with increasing temperatures. The particles of the product prepared at 600 °C with ~200 nm in size are well developed and homogeneously distributed. Charge/discharge curves and cycling performance tests at different current density indicated that the product prepared at 600 °C had the largest specific capacity and the best cycling performance, due to its high purity, high crystallinity, small particle size as well as moderate amount of Mn³⁺ ions.     

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.     

All-solid-state lithium battery with a three-dimensionally ordered Li1.5Al0.5Ti1.5(PO4)3 electrode

To fabricate all-solid-state Li batteries using three-dimensionally ordered macroporous Li_1.5Al_0.5Ti_1.5(PO₄)₃ (3DOM LATP) electrodes, the compatibilities of two anode materials (Li₄Mn₅O₁₂ and Li₄Ti₅O₁₂) with a LATP solid electrolyte were tested. Pure Li₄Ti₅O₁₂ with high crystallinity was not obtained because of the formation of a TiO₂ impurity phase. Li₄Mn₅O₁₂ with high crystallinity was produced without an impurity phase, suggesting that Li₄Mn₅O₁₂ is a better anode material for the LATP system. A Li₄Mn₅O₁₂/3DOM LATP composite anode was fabricated by the colloidal crystal templating method and a sol-gel process. Reversible Li insertion into the fabricated Li₄Mn₅O₁₂/3DOM LATP anode was observed, and its discharge capacity was measured to be 27 mA h g⁻¹. An all-solid-state battery composed of LiMn₂O₄/3DOM LATP cathode, Li₄Mn₅O₁₂/3DOM LATP anode, and a polymer electrolyte was fabricated and shown to operate successfully. It had a potential plateau that corresponds to the potential difference expected from the intrinsic redox potentials of LiMn₂O₄ and Li₄Mn₅O₁₂. The discharge capacity of the all-solid-state battery was 480 μA h cm⁻².     

Synthesis of compounds, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12), (Co1/12Al1/12), (Cr1/12Al1/12)) by polymer precursor method and its electrochemical performance for lithium-ion batteries

The compounds, Li(MMn_11/6)O₄ (M = Mn_1/6, Co_1/6, (Co_1/12Cr_1/12), (Co_1/12Al_1/12), (Cr_1/12Al_1/12)) are synthesised by the polymer precursor method. The structure and the morphology of the compounds are studied by the Rietveld refined X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques, respectively. Density and the Brunauer, Emmet and Teller surface area (BET) of the compounds are also studied. The cobalt doped compound, Li(Co_1/6Mn_11/6)O₄ is found to be nanosized particles in the range of 60-100 nm, when compared to the other compounds in our present study. The oxidation state and the local structure of the compounds are analysed by the X-ray absorption spectroscopy (XAS) technique. Cyclic voltammetry (CV) and the galvanostatic charge-discharge cycling (30 mA g⁻¹) studies are made in the voltage range of 3.5-4.3 V at room temperature for all the compounds under study. The bare and (Co_1/6), and (Co_1/12Cr_1/12) substituted spinels are cycled at high current rates of 1, 2 and 5C (assuming 1C∼120 mA g⁻¹). Cycling results of Co-substituted spinels show better and long-term capacity retention at all the current rates. At the end of the second cycle, Li(Co_1/6Mn_11/6)O₄ compound delivers a discharge capacity value of 100 (±3) and 87 (±3) mAh g⁻¹ for the current rate of 2 and 5C, respectively. An excellent capacity retention value of 94% is observed at the end of the 1000 cycles for both 2 and 5C rates.     

Synthesis and electrochemical properties of LiAl0.05Mn1.95O4 by the ultrasonic assisted rheological phase method

Pure-phase and well-crystallized spinel LiAl_0.05Mn_1.95O₄ powders were successfully synthesized by a simple ultrasonic assisted rheological phase (UARP) method. The structure and morphology properties of this as-prepared powder compared with the pristine LiMn₂O₄ and LiAl_0.05Mn_1.95O₄ obtained from the solid-state reaction (SSR) method were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical properties focused on the LiAl_0.05Mn_1.95O₄ by this new method have also been investigated in detail. According to these tests results, it is obviously to see that the newly prepared sample delivers a relatively high initial discharge capacity of 111.6 mAh g⁻¹, presents excellent rate capability and reversibility, and shows good cycling stability with capacity retention of 90.6% after 70 cycles. Meanwhile, the electrochemical impedance spectroscopy (EIS) investigations were employed to study the electrochemical process of Li⁺ ions with the synthesized LiAl_0.05Mn_1.95O₄ electrode in detail.     

Comparison of 4 V and 3 V electrochemical properties of nanocrystalline LiMn2O4 cathode particles in lithium ion batteries prepared by ultrasonic spray pyrolysis

Nanocrystalline LiMn₂O₄ particles were prepared by an ultrasonic spray pyrolysis method using nitrate salts at 800 °C in air atmosphere. Particle properties were characterized by the X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy. In addition, cyclic voltammetry and galvanostatic tests were performed to investigate the effects of structure on electrochemical behavior of both the 4 V and 3 V potential plateaus. Particle characterization studies show that the nanocrystalline particles have spinel structure of submicron size with spherical morphology. Particles, ranging between 75 and 1250 nm, were formed by aggregation of nanoparticles. Discharge capacity of LiMn₂O₄ particles between 3.0 and 4.5 V is 70 mA h g⁻¹ and cumulative capacity between 2.2 and 4.5 V is 111 mA h g⁻¹ at 0.5 C rate. Discharge capacity at the 4 V potential region reduces to 47% of initial capacity, whereas cumulative capacity fade is 62% after 100 cycles at 0.5 C rate. Although nanocrystalline LiMn₂O₄ cathode particles exhibit good rate capability at the 4 V plateau, capacity decreased rapidly by increasing C- rates and cycling between 2.2 and 4.5 V. The loss of capacity can be attributed to phase transformation and dissolution of electrode material. Particle characterization of used cathodes showed that nanocrystalline LiMn₂O₄ electrodes partly dissolve during electrochemical cycling.     

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.     

Isothermal calorimetry investigation of Li1+xMn2−yAlzO4 spinel

The heat generation of LiMn₂O₄, Li_1.156Mn_1.844O₄, and Li_1.06Mn_1.89Al_0.05O₄ spinel cathode materials in a half-cell system was investigated by isothermal micro-calorimetry (IMC). The heat variations of the Li/LiMn₂O₄ cell during charging were attributed to the LiMn₂O₄ phase transition and order/disorder changes. This heat variation was largely suppressed when the stoichiometric spinel was doped with excess lithium or lithium and aluminum. The calculated entropy change (dE/dT) from the IMC confirmed that the order/disorder change of LiMn₂O₄, which occurs in the middle of the charge, was largely suppressed with lithium or lithium and aluminum doping. The dE/dT values obtained did not agree between the charge and the discharge at room temperature (25 °C), which was attributed to cell self-discharge. This discrepancy was not observed at low temperature (10 °C). Differential scanning calorimeter (DSC) results showed that the fully charged spinel with lithium doping has better thermal stability.     

Enhanced electrochemical stability of Al-doped LiMn2O4 synthesized by a polymer-pyrolysis method

LiAl_xMn_2−xO₄ samples (x = 0, 0.02, 0.05, 0.08) were synthesized by a polymer-pyrolysis method. The structure and morphology of the LiAl_xMn_2−xO₄ samples calcined at 800 °C for 6 h were investigated by powder X-ray diffraction and scanning electron microscopy. The results show that all samples have high crystallinity, regular octahedral morphology and uniform particle size of 100-300 nm. The electrochemical performances were tested by galvanostatic charge-discharge and cyclic voltammetry. The results demonstrate that the Al-doped LiMn₂O₄ can be very well cycled at an elevated temperature of 55 °C without severe capacity degradation. In particular, the LiAl_0.08Mn_1.92O₄ sample demonstrates excellent capacity retention of 99.3% after 50 cycles at 55 °C, confirming the greatly enhanced electrochemical stability of LiMn₂O₄ by a small quantity of Al-doping.     

High voltage nickel manganese spinel oxides for Li-ion batteries

High voltage spinel oxides with composition LiMn_2 − xM_xO₄ (M, a transition metal element) have remarkable properties such as high potential, high energy density and high rate capability. We believe that these positive electrode materials could replace the widespread commercial layered nickel cobalt oxides in some applications. The present assessment highlights electrochemical performance of optimized LiNi_0.5Mn_1.5O₄ and substituted counterparts, all having a spinel structure (cubic close-packed oxygen array) similar to the relative LiMn₂O₄. To fully emphasize the benefit from high potential spinel oxides, tests have been performed versus lithium metal, Li₄Ti₅O₁₂ and graphite, using various electrode loadings (0.3-4.5 mAh cm⁻²) and cycling rates (from C/20 to 60C rate). Steady capacity retention (130-140 mAh g⁻¹ for nearly 500 cycles) and flat voltage (4.7 V vs. Li⁺/Li) have been obtained at C/5 rate at room temperature. Effect of cycling at high temperature has been shown to be less critical than for LiMn₂O₄. High voltage spinel oxides still sustain 100 mAh g⁻¹ and over after 400 cycles at 55 °C at 1C rate. Rate capability is also excellent, with only 4% loss of capacity when comparing C/8 and 8C rates (thin electrodes).     

Structural, magnetic and electrochemical properties of LiNi0.5Mn0.5O2 as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.5O₂ was synthesized by wet-chemical method and characterized by X-ray diffraction and SQUID magnetometry. The powders adopted the α-NaFeO₂ structure. The ferromagnetism observed below Tc = 140 K is attributed to the linear Ni²⁺(3a)-O-Mn⁴⁺(3b)-O-Ni²⁺(3a) magnetic paths, from which we derive that 7% of the nickel occupies the (3a) Wyckoff position in place of Li, constituting a Ni²⁺(3a) defect. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie-Weiss law agrees well with the combination of Ni²⁺ (S = 1) and Mn⁴⁺ (S = 3/2) spin-only values. Results of structural and magnetic properties of chemically delithiated sample are consistent with the electronic state Mn⁴⁺/Ni⁴⁺ and the high-spin configuration for Ni⁴⁺ ions. The LiNi_0.5Mn_0.5O₂ electrode sintered at 900 °C delivers a capacity 166 mAh/g at 0.1C rate which is capacity retention of 95%.     

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 delithiated LiNi0.65Co0.25Mn0.10O2 electrode material: A structural, magnetic and electrochemical study

A crystalline LiNi_0.65Co_0.25Mn_0.10O₂ electrode material was synthesized by the combustion method at 900 °C for 1 h. Rietveld refinement shows less than 3% of Li/Ni disorder in the structure. Lithium extraction involves only the Ni²⁺/Ni⁴⁺ redox couple while Co³⁺ and Mn⁴⁺ remain electrochemically inactive. No structural transition was detected during cycling in the whole composition range 0 < x < 1.0. Furthermore, the hexagonal cell volume changes by only 3% when all lithium was removed indicating a good mechanical stability of the studied compound. LiNi_0.65Co_0.25Mn_0.10O₂ has a discharge capacity of 150 mAh/g in the voltage range 2.5-4.5 V, but the best electrochemical performance was obtained with an upper cut-off potential of 4.3 V. Magnetic measurements reveal competing antiferromagnetic and ferromagnetic interactions - varying in strength as a function of lithium content - yielding a low temperature magnetically frustrated state. The evolution of the magnetic properties with lithium content confirms the preferential oxidation of Ni ions compared to Co³⁺ and Mn⁴⁺ during the delithiation process.     

Synthesis of spherical spinel LiMn2O4 with commercial manganese carbonate

Spherical spinel LiMn₂O₄ particles were successfully synthesized from a mixture of manganese compounds containing commercial manganese carbonate by sintering of the spray-dried precursor. Different preparation routes were investigated to improve the tap density and to enhance the electrochemical performance of LiMn₂O₄. The structure and morphology of the LiMn₂O₄ particles were confirmed by X-ray diffraction (XRD) and scanning electron microscopy. The results showed that hollow spherical LiMn₂O₄ particles could be obtained when only commercial MnCO₃ was used as the manganese source. These particles had a low tap density (ca.0.8 g/cm³). Perfect micron-sized spherical LiMn₂O₄ particles with good electrochemical performance were obtained by spray-drying a slurry composed of MnCO₃, Mn(CH₃CHOO)₂ and LiOH, followed by a dynamic sintering process and a stationary sintering process. The as-prepared spherical LiMn₂O₄ particles comprised hundreds of nanosize crystal grains and had a high tap density(ca. 1.4 g/cm³). The galvanostatic charge-discharge measurements indicated that the spherical LiMn₂O₄ particles had an initial capacity of 121 mAh/g between 3.0 and 4.2 V at 0.2 C rate and still delivered a reversible capacity of 112 mAh/g at 2 C rate. The retention of capacity after 50 cycles was still 96% of its initial capacity at 0.2 C. All the results showed that the as-prepared spherical LiMn₂O₄ particles had an excellent electrochemical performances. The methods we used for preparing spherical LiMn₂O₄ are energy-saving and suitable for industrial application.     

Improvement of electrochemical stability of LiMn2O4 by CeO2 coating for lithium-ion batteries

CeO₂-coated LiMn₂O₄ spinel cathode was synthesized using two-step synthesis method. All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. The results of the electrochemical performances on CeO₂-coated electrode are compared to those of electrodes based on LiMn₂O₄ spinel without CeO₂ coating. CeO₂-coated LiMn₂O₄ cathode improved the cycling stability of the electrode. The capacity retention of 2 wt% CeO₂-coated LiMn₂O₄ was more than 82% after 150 cycles between 3.0 and 4.4 V at room temperature and 82% after 40 cycles at elevated temperature of 60 °C. The amounts of dissolved manganese-ions in CeO₂-coated LiMn₂O₄ significantly are smaller than pristine LiMn₂O₄ systems especially at elevated temperatures. Surface-modified LiMn₂O₄ can suppress the dissolution reaction of manganese-ions at elevated temperature and clearly improve the cyclability of the spinel LiMn₂O₄ cathode materials.     

Electrochemical insertion and extraction of lithium-ion at nano-sized LiMn2O4 particles prepared by a spray pyrolysis method

Nano-sized LiMn₂O₄ particles were prepared at 1023 K by electrospray pyrolysis in which they were directly deposited on a Pt substrate in gas phase. Cyclic voltammetry gave very sharp and symmetrical redox peaks at ca. 4.0 and 4.1 V vs. Li/Li⁺ owing to the insertion and extraction of lithium-ion at LiMn₂O₄. However, the redox peaks broadened and their peak separation in an electrode potential increased when aggregated nano-sized LiMn₂O₄ particles were used. In Nyquist plots, a semi-circle due to lithium-ion transfer resistance appeared at potentials above 3.90 V. The values of the lithium-ion transfer resistances were small for dispersed nano-sized LiMn₂O₄ particles. On the other hand, the lithium-ion transfer resistances increased and the Warburg impedance became obvious as the nano-sized LiMn₂O₄ particles aggregated. These results clearly indicate that the apparent rapid diffusion of lithium-ion can be attained using well-dispersed nano-sized particles of electroactive materials.     

Electrochemical properties of nano- and micro-sized LiNi0.5Mn1.5O4 synthesized via thermal decomposition of a ternary eutectic Li-Ni-Mn acetate

Nano- and micro-sized LiNi_0.5Mn_1.5O₄ particles are prepared via the thermal decomposition of a ternary eutectic Li-Ni-Mn acetate. Lithium acetate, nickel acetate and manganese acetate can form a ternary eutectic Li-Ni-Mn acetate below 80 °C. After further calcination, nano-sized LiNi_0.5Mn_1.5O₄ particles can be obtained at an extremely low temperature (500 °C). When the sintering temperature goes above 700 °C, the particle size increases, and at 900 °C micro-sized LiNi_0.5Mn_1.5O₄ particles (with a diameter of about 4 μm) are obtained. Electrochemical tests show that the micro-sized LiNi_0.5Mn_1.5O₄ powders (sintered at 900 °C) exhibit the best capacity retention at 25 °C, and after 100 cycles, 97% of initial discharge capacity can still be reached. Nano-sized LiNi_0.5Mn_1.5O₄ powders (sintered at 700 °C) perform the best at low temperatures; when cycled at −10 °C and charged and discharged at a rate of 1 C, nano-sized LiNi_0.5Mn_1.5O₄ powders can deliver a capacity as high as 110 mAh g⁻¹.