Spinel LiMn2O4 active material with high capacity retention

Heating the mixture of LiMn₂O₄ and NiO at 650 °C was employed to enhance the cyclability of spinel LiMn₂O₄. The results of scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy analyses implied that a LiNi_xMn_2−xO₄ solid solution was formed on the surface of LiMn₂O₄ particles. And charge-discharge tests showed that the enhancement of the capacity retention of modified LiMn₂O₄ is significant, maintained 97.2% of the maximum capacity after 100 cycles at charge and discharge rate of C/2, while the pure one only 75.2%. The modified LiMn₂O₄ also results in a distinct improvement in rate capability, even at the rate of 12C. The improvement of electrochemical cycling stability is greatly attributed to the suppression of Jahn-Teller distortion at the surface of spinel LiMn₂O₄ particles.     

Advanced electrochemical performance of LiMn1.4Cr0.2Ni0.4O4 as 5 V cathode material by citric-acid-assisted method

Spinel LiMn₂O₄ and LiMn_1.4Cr_0.2Ni_0.4O₄ cathode materials were successfully synthesized by the citric-acid-assisted sol-gel method with ultrasonic irradiation stirring. The structure and electrochemical performance of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometer, cyclic voltamogram (CV) and the galvanostatic charge-discharge test in detail. XRD shows that all the samples have high phase purity, and the powders are well crystallized. SEM exhibits that LiMn_1.4Cr_0.2Ni_0.4O₄ has more uniform cubic-structure morphology than that of LiMn₂O₄. EDX reveals that a small amount of Mn³⁺ still exists in LiMn_1.4Cr_0.2Ni_0.4O₄. The galvanostatic charge-discharge test indicates that the initial discharge capacities for the LiMn_1.4Cr_0.2Ni_0.4O₄ and LiMn₂O₄ at 0.15 C discharge rates are 130.8 and 130.2 mAh g⁻¹, respectively. After 50 cycles, their capacity are 94.1% and 85.1%, respectively. The CV curve implies that Ni and Cr dual substitutions are beneficial to the reversible intercalation and deintercalation of Li⁺, and suppress Mn³⁺ generation at high temperatures and provide improved structural stability.     

Surface modification of LiMn2O4 thin films at elevated temperature

In order to improve the cycle stability of spinel LiMn₂O₄ electrode at elevated temperature, the LiCoO₂-coated and Co-doped LiMn₂O₄ film were prepared by an electrostatic spray deposition (ESD) technique. LiCoO₂-coated LiMn₂O₄ film shows excellent cycling stability at 55 °C compared to pristine and Co-doped LiMn₂O₄ films. The samples were studied by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The excellent performance of LiCoO₂-coated LiMn₂O₄ film can be explained by suppression of Mn dissolution. On the other hand, the LiCoO₂-layer on the LiMn₂O₄ surface allows a homogenous Li⁺ insertion/extraction during electrochemical cycles and improves its structure stability.     

Al掺杂的尖晶石型LiMn2O4的结构和电子性质

采用基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)和GGA+U方法下对尖晶石型LiMn₂O₄及其Al掺杂 的尖晶石型LiAl_0.125Mn_1.875O₄晶体的结构和电子性质进行了计算. 结果表明: 采用GGA方法得到尖晶石型LiMn₂O₄是立方晶系结构, 其中的Mn离子为+3.5价, 无法解释它的Jahn-Teller 畸变. 给出的LiMn₂O₄能带结构特征也与实验结果不符. 而采用GGA+U方法得到在低温下的LiMn₂O₄和其掺杂 体系LiAl_0.125Mn_1.875O₄的晶体都是正交结构, 与实验一致. 也能明确地确定Mn的两种价态Mn³⁺/Mn⁴⁺的分布并且能够说明Mn³⁺O₆的z方向有明显的Jahn-Teller 畸变, 而Mn⁴⁺O₆则没有畸变. LiMn₂O₄的能带结构与实验比较也能够符合. 采用GGA+U方法对Al掺杂体系的LiAl_0.125Mn_1.875O₄的研究表明, 用Al替换一个Mn不会明显地改变晶体的电子性质, 但可以有效地消除Al³⁺O₆ 八面体的Jahn-Teller畸变, 从而改善正极材料LiMn₂O₄的性能, 这与电化学实验的观察结果相一致.     

Suppression of Jahn-Teller distortion by chromium and magnesium doping in spinel LiMn2O4: A first-principles study using GGA and GGA+U

The effect of doping spinel LiMn₂O₄ with chromium and magnesium has been studied using the first-principles spin density functional theory (DFT) within generalized gradient approximation (GGA ) and GGA+U. We find that GGA and GGA+U give different ground states for pristine LiMn₂O₄ and same ground state for doped systems. For LiMn₂O₄, the body-centered tetragonal phase was found to be the ground-state structure using GGA and face-centered orthorhombic using GGA+U, while for LiM_0.5Mn_1.5O₄ (MCr or Mg) it was base-centered monoclinic and for LiMMnO₄ (MCr or Mg) it was body-centered orthorhombic in both GGA and GGA+U. We find that GGA predicts the pristine LiMn₂O₄ to be metallic while GGA+U predicts it to be insulating, which is in accordance with the experimental observations. For doped spinels, GGA predicts the ground state to be half metallic while GGA+U predicts it to be insulating or metallic depending on the doping concentration. GGA+U predicts insulator-metal-insulator transition as a function of doping in case of Cr and in case of Mg the ground state is found to go from insulating to a half metallic state as a function of doping. Analysis of the charge density and the density of states (DOS) suggest a charge transfer from the dopants to the neighboring oxygen atoms and manganese atoms. We have calculated the Jahn-Teller active mode displacement Q₃ for doped compounds using GGA and GGA+U. The bond lengths calculated from GGA+U are found to be in better agreement with experimental bond lengths. Based on the bond lengths of metal and oxygen, we have also estimated the average oxidation states of the dopants.     

Physical properties of the cubic spinel LiMn2O4

We have performed an ab initio study of structural, electronic, magnetic, vibrational and thermal properties of the cubic spinel LiMn₂O₄ by employing the density functional theory, the linear-response formalism, and the plane-wave pseudopotential method. An analysis of the electronic structure with the help of electronic density of states shows that the density of states at the Fermi level (N (E_F)) is found to be governed by the Mn 3d electrons with some contributions from the 2p states of O atoms. It is important to note that the contribution of Mn 3d states to N(_EF)$N\left(E_F\right)$ is as much as 85%. From our phonon calculations, we have obtained that the main contribution to phonon density of states (below 250 cm⁻¹) comes from the coupled motion of Mn and O atoms while phonon modes between 250 cm⁻¹ and 375 cm⁻¹ are characterized by the vibrations of all the three types of atoms. The contribution from Li increases rapidly at higher frequency (above 375 cm⁻¹) due to the light mass of this atom. Finally, the specific heat and the Debye temperature at 300 K are calculated to be 249.29 J/mol K and 820.80 K respectively.     

Novel approach to preparation of LiMn2O4 core/LiNixMn2−xO4 shell composite

Combining two methods, coating and doping, to modify spinel LiMn₂O₄, is a novel approach we used to synthesize active material. First we coated the LiMn₂O₄ particles with the nickel oxide particles by means of homogenous precipitation, and then the nickel oxide-coated LiMn₂O₄ was calcined at 750 °C to form a LiNi_xMn_2−xO₄ shell on the surface of spinel LiMn₂O₄ particles. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cyclic voltammetry (CV) and charge-discharge test were performed to characterize the spinel LiMn₂O₄ before and after modification. The experimental results indicated that a spinel LiMn₂O₄ core is surrounded by a LiNi_xMn_2−xO₄ shell. The resulting composite showed excellent electrochemical cycling performance with an average fading rate of 0.014% per cycle. This improved cycle stability is greatly attributed to the suppression of Jahn-Teller distortion on the surface of spinel LiMn₂O₄ particles during cycling.     

The effect of cation doping on spinel LiMn2O4: a first-principles investigation

The effect of the cation doping on the electronic structure of spinel LiM_yMn_2−yO₄ (M=Cr, Mn, Fe, Co and Ni) has been calculated by first-principles. Our calculation shows that new M-3d bands emerge in the density of states compared with that in LiMn₂O₄. Simultaneously, the new O-2p bands appear accordingly in almost the same energy range around the Fermi energy owing to the M-3d/O-2p interaction. It is found that the appearance of new O-2p bands in the lower energy position results in a higher intercalation voltage. Consequently, the origin of higher intercalation voltage in LiM_yMn_2−yO₄ can be ascribed to the lower O-2p level introduced by the doping cation M.     

On the observation of a huge lattice contraction and crystal habit modifications in LiMn2O4 prepared by a fuel assisted solution combustion

Two batches of poly-crystalline lithium manganate were prepared by a fuel assisted solution combustion method. LiMn₂O₄(S) was prepared using starch as the fuel and LiMn₂O₄(P) was prepared using poly vinyl alcohol (PVA) as the fuel. XRD studies indicated a significant and consistent shift in the 2θ values of all the hkl peaks to higher values in LiMn₂O₄(P) compared to LiMn₂O₄(S) indicating a lattice contraction in the former. TG/DTA studies indicated a higher formation temperature (∼25 °C higher) for LiMn₂O₄(P). The higher formation temperature most likely promotes the oxidation of some Mn³⁺ to Mn⁴⁺ with a lower ionic radius causing a lattice contraction. This hypothesis is confirmed through XPS studies which indicated the presence of a higher fraction of Mn⁴⁺ in LiMn₂O₄(P) than that present in LiMn₂O₄(S). A crystal shape algorithm was used to generate the crystal habits of lithium manganate from their XRD data leading to an understanding on the exposed hkl planes in these materials. From the atomic arrangement on the exposed hkl planes it is predicted that LiMn₂O₄(P) would be less prone to manganese dissolution and hence would possess a higher cycle life when compared to LiMn₂O₄(S).     

Facile synthesis and electrochemical properties of hierarchical MnO2 submicrospheres and LiMn2O4 microspheres

Hierarchical MnO₂ submicrospheres have been successfully synthesized by a wet chemical method. The as-prepared products were characterized by means of XRD, SEM, FTIR, TG, and TEM. With the as-prepared MnO₂ submicrospheres as precursors, LiMn₂O₄ microspheres were conveniently prepared by a simple solid-state reaction between MnO₂ and LiOH at a temperature as low as 600 °C. Electrochemical properties of the as-prepared MnO₂ submicrospheres and LiMn₂O₄ microspheres as cathode materials in lithium ion cells were investigated by galvanostatic charge/discharge tests.     

Improvement of electrochemical performance of all-solid-state lithium secondary batteries by surface modification of LiMn2O4 positive electrode

To improve the electrochemical performance of an all-solid-state In/80Li₂S⋅20P₂S₅ (electrolyte)/LiMn₂O₄ cell, a lithium-titanate thin film was used to coat LiMn₂O₄. The interfacial resistance between LiMn₂O₄ and the electrolyte (measured after initial charging) decreased when the LiMn₂O₄ particles were coated with lithium-titanate. A cell with lithium-titanate-coated LiMn₂O₄ had a higher capacity than a cell with noncoated LiMn₂O₄ for current densities in the range 0.064 to 2.6 mA cm^− 2. Additionally, a cell with coated LiMn₂O₄ retained 96% of the 10th-cycle reversible capacity at a current density of 0.064 mA cm^− 2 after 50 cycles.     

Columnar-grown porous films of lithium manganese oxide spinel (LiMn2O4) prepared by ultrasonic spray deposition

Using LiNO₃ and Mn(Ac)₂ as raw materials, ultrasonic spray deposition (USD) technique was used to fabricate LiMn₂O₄ films on platinum substrate at different substrate temperatures from 310 to 390 °C. The prepared thick films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical performance of the USD-derived films was also evaluated with LiMn₂O₄/Li cells. It is found that all of the LiMn₂O₄ films are porous and composed of orderly oriented columnar particles. The substrate temperature affects the fine microstructure of the columnar particles. The film prepared at 360 °C substrate temperature give rise to best electrochemical behavior.     

The effect of doping LiMn2O4 spinel on its use as a cathode in Li-ion batteries: neutron diffraction and electrochemical studies

A series of compounds Li_1+yMn_2−xM′_xO₄ (x≤0.1;y≤0.02), have been synthesised by doping the parent LiMn₂O₄ spinel with various metal ions of variable oxidation state. Powder neutron diffraction data has been collected on these samples alongside a series of electrochemical experiments in order to elucidate the relationship between structure on the performance of these systems as Li batteries. Doping the LiMn₂O₄ spinel with a small amount of metal ions has a remarkable effect on the electrochemical properties. Whereas the capacity of the spinels doped with trivalent ions is much greater, the cycling fading properties are much enhanced with using divalent ions as dopants. The underlying reasons for this are discussed, and it is suggested that the occupancy of the tetrahedral site with divalent ions to form a more compact structure offers an improved structural stability to support greater Li insertion/extraction, but which ultimately prevents the free movement of Li also sited on the tetrahedral site of the lattice.     

Effect of ultrasonic irradiation on structure and electrochemical properties of LiMn2O4 thin films cathode material for Li-ion micro-batteries

Two kinds of spinel LiMn₂O₄ thin film for lithium ion micro-batteries were successfully prepared on polycrystal Pt substrates by spin coating methods, which were carried out under ultrasonic irradiation (USG) and magnetic stirring (MSG), respectively. The microstructures and electrochemical performance of LiMn₂O₄ thin films were characterized by thermogravimetry analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge-discharge measurements. It was found that the crystalline structure of USG samples grew better than that of the MSG samples. At the same time, higher discharge capacity and better cycle stability were obtained for the LiMn₂O₄ thin films of USG at the current density of 50 μAh/cm² between 3.0 and 4.3 V. The 1st discharge capacity was 57.8 μAh/cm²-μm for USG thin films and 51.7 μAh/cm²-μm for MSG thin films. After 50 cycles, 91.4% and 69% of discharge capacity could be retained respectively, indicating that ultrasonic irradiation condition during spin coating was more suitable for preparing spinel LiMn₂O₄ thin films with better electrode performance for lithium ion micro-batteries.     

Atomic force microscopy study of surface morphology change in spinel LiMn2O4: Possibility of direct observation of Jahn-Teller instability

Surface morphology in 3.5 × 3.5 μm² area of spinel LiMn₂O₄, which is a typical cathode material for Li ion secondary batteries, is studied using an atomic force microscopy (AFM) with a conductive probe. Negative bias voltage is applied to the probe to attract Li⁺ ions toward LiMn₂O₄ surface during the AFM observation. Before applying the voltage (0 V), the whole LiMn₂O₄ surface is covered with scale-shaped grains. Under the negative voltage of 5.5 V, electric current abruptly increases, indicating Li⁺ ionic conduction. Simultaneously, part of the scale-shaped grains expand and flatten. Jahn-Teller phase transition, which is induced by the repulsive interaction between the Mn-e_g and O-2p electrons in Li accumulated layer, is proposed as a possible origin of these results.     

Jahn-Teller transition in Al doped LiMn2O4 spinel

Al-doped lithium manganese spinels, with starting composition Li_1.02Al_xMn_1.98−xO₄ (0.00<x≤0.06), are investigated to determine the influence of the Al³⁺ doping on the Jahn-Teller (J-T) cooperative transition temperature T_J-T. X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al³⁺ ions in the spinel phase. It is observed that Al³⁺ may distribute between the two cationic sublattices. The J-T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x>0.04 T_J-T it increases with increasing x, as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior.     

Ground-state magnetism of chromium-substituted LiMn2O4 spinel

Comparative crystal structure and magnetic properties studies have been conducted on quaternary powder spinel samples LiMn_1.82Cr_0.18O₄ obtained by two different synthesis methods, glycine-nitrate (GN) and ultrasonic spray-pyrolysis (SP). Although both samples possess the same spinel structure of the cubic space group Fd3¯m, their low-temperature magnetic properties display significant differences. While the SP sample undergoes only spin-glass transition at the freezing temperature T_f=20 K, the GN sample possesses more complicated low-temperature magnetic behavior of the reentrant spin-glass type with the Néel temperature T_N=42 K and freezing temperature T_f=22 K. High-temperature magnetic susceptibility of both samples is of the Curie–Weiss type with the effective magnetic moments in agreement with the nominal compositions. This fact together with the results of the chemical analysis discards the existence of the diversity in chemical compositions as a possible cause for the observed differences in the low-temperature magnetism. On the other hand, the crystal structure analysis done by the Rietveld refinement of the X-ray powder diffraction data points to the strong influence of the cation distribution on the ground-state magnetism of these systems. An explanation of this influence is proposed within the framework of a collective Jahn–Teller effect.     

First principles study of the electron structures of CaCu3Mn4O12 and CaCu3Ti4O12

The electronic structures of CaCu₃Mn₄O₁₂ and CaCu₃Ti₄O₁₂ are investigated from HF SCF LCAO calculation. In CaCu₃Mn₄O_12, the band and the density of states show a spin asymmetric ferrimagnetic character with a small energy gap. The Mn spin is anti-aligned with the Cu spin, and the total spin moment is 9 μ_B. Our calculation correctly reproduces the observed antiferromagnetic insulating character of CaCu₃Ti₄O₁₂. The gap in the band structure, which is 2.15 eV, reasonably agrees with the experimental value 1.5 eV. The electron density populations at different planes show clearly that the electron density has symmetric character. A tilted Mn(Ti) orbital implies a typical tilted three-dimensional network of MnO₆ (TiO₆) octahedra due to doping of the Jahn–Teller ion Cu. There is no covalency between Ca, Cu and Mn(Ti) atoms. In contrast, there are stronger bonds and somewhat likely covalency between Cu and O atoms, and also between Mn(Ti) and O atoms.     

Kinetic aspects of Li intercalation in mechano-chemically processed cathode materials for lithium ion batteries: Electrochemical characterization of ball-milled LiMn2O4

Micrometric LiMn₂O₄ particles are mechano-chemically modified by ball-milling to obtain a mixture of nano- and micro-scale particles. This mixture is tested as a potential active cathode material for rapid-charge Li ion batteries, and also as a model system for studying the detailed kinetics of Li intercalation/de-intercalation in such electrodes. Ragone plots recorded using galvanostatic measurements indicate enhanced power delivery characteristics of the ball-milled LiMn₂O₄ compared to its unprocessed counterpart. The processed material also exhibits improved resistance against electrolyte reactions and surface film formation. Due to these advantageous electrochemical attributes, the ball-milled LiMn₂O₄ serves as an adequately suited system for exploring certain fundamental aspects of Li intercalation in this material. Scan rate dependent slow scan cyclic voltammetry helps to identify the kinetic and diffusion controlled features of Li transport in mechano-chemically processed LiMn₂O₄. Electrochemical impedance spectroscopy substantiates these findings further and provides detailed kinetic parameters, including voltage dependent charge transfer resistance and diffusion coefficient of Li transport.     

Electrochemical behaviour of nano-sized spinel LiMn2O4 and LiAlxMn2−xO4 (x=Al: 0.00-0.40) synthesized via fumaric acid-assisted sol-gel synthesis for use in lithium rechargeable batteries

Pristine spinel LiMn₂O₄ and LiAl_xMn_2−xO₄ (x=Al: 0.00-0.40) with sub-micron sized particles have been synthesized using fumaric acid as chelating agent by sol-gel method. The synthesized samples were subjected to thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cyclic voltammetry (CV) and galvanostatic cycling studies. The TGA curve of the gel shows several weight-loss regions stepwise amounting to 55% till 800 °C attributed to the decomposition of the precursors. Calcination to higher temperatures (800 °C) yields pure-phase spinel (LiAl_xMn_2−xO₄), as it is evident from the high-intensity XRD reflections matching to the standard pattern. SEM and TEM studies confirm that the synthesized grains are of uniform regular surface morphology. FT-IR studies show stretching and bending vibration bands of Li-O, Li-Al-Mn-O. LiAl_0.1Mn_1.90O₄ spinel was found to deliver discharge capacity of 139 mA h/g during the first cycle with columbic efficiency of 97%. LiAl_0.1Mn_1.90O₄ spinel exhibits the high cathodic peak current indicating better electrochemical performance. Low doping (x=0.1) of Al is found to be beneficial in stabilizing the spinel structure.