4-Volt cathode materials for rechargeable lithium batteries wet-chemistry synthesis, structure and electrochemistry

Lithium transition-metal oxides used as intercalation compounds for rechargeable lithium batteries are widely studied in search of structural stability and improved electrochemical performance. Cathode materials belonging to the 4-volt class electrodes were synthesized by wet-chemistry methods, i.e., sol-gel, combustion or co-precipitation techniques. It is shown that synthesis greatly affects the electrochemistry and cycle life characteristics of the cathodes. Extensive damage including local strain variation, nanodomain formation, and changes in cation ordering, has been observed by local probes such as Raman and FTIR spectroscopy. In this work we wish to show the relationship between the local cationic environment and electrochemical characteristics of the 4-volt cathodes. Materials such as LiMn₂O₄, LiCoO₂, LiNi_1−yCo_yO₂, LiNi_1−yCo_yVO₄, and LiMoVO₆ are investigated.     

Electrochemical characterization of a lithiated mixed nickel-cobalt oxide (LiNi0.5Co0.5O2) prepared by sol-gel process

Lithiated transition metal oxides having a layered structure and general formula LiMO₂, have been extensively studied as positive electrode active materials for lithium or lithium-ion batteries. In particular, lithium nickel dioxide (LiNiO₂) and lithium cobalt dioxide (LiCoO₂) present a layered structure with high diffusion coefficients for the lithium ion. This latter property is very important in order to realize practical devices having high discharge rates. LiNiO₂, compared with LiCoO₂, has the advantage to be a cheaper material with a higher specific capacity for lithium cycling, but its stability upon cycling can be greatly influenced by the displacement of Ni ions from the Ni layers to the Li planes as the content in lithium is reduced over a certain value. Recently, solid solutions such as LiNi_xCo_1−xO₂ have been proposed to offer a compromise between stability, cost and capacity. In this work we have studied LiNi_0.5Co_0.5O₂ prepared by the Complex Sol-Gel Process (CSGP). The advantage of this procedure toward the solid-state process is the high homogeneity in composition and in particle dimension of the synthesized compounds. The samples have been characterized electrochemically using chronopotentiometric, voltammetric and impedance measurements in liquid electrolyte. The results indicates that CSGP-synthesized LiNi_0.5Co_0.5O₂ shows good cyclability (after 1000 cycles about 2/3 of the initial capacity can still be cycled) only if the anodic potential is limited to about 4.2 V. The quite low values of the specific capacity (∼70 mAh/g at C/1 charge-discharge rate) can be justified by the non-complete calcination reaction, as suggested by X-ray measurements. Kinetic properties have been evaluated by Electrochemical Impedance Spectroscopy measurements, which have shown quite high values for the lithium chemical diffusion coefficient (10⁻⁷÷10⁻⁸ cm²s⁻¹) and its unexpected decrease as deintercalation proceeds from x=0.5 in LiNi_0.5Co_0.5O₂. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Structural properties of LiNi1−yCoyO2 (0≤y≤1) synthesized by wet chemistry via malic acid-assisted technique

We have synthesized LiNi_1−yCo_yO₂ powders by a sol-gel method using malic acid as a chelating agent. The dependence of physico-chemical properties of the powders (crystallinity, lattice constants, grain size) has been investigated by changing the malic acid quantity and the calcination temperature for the different LiNi_1−yCo_yO₂ oxides in the composition range 0 ≤ y ≤1. Structural studies show that a layered single phase was obtained for the y values 0.2 ≤ y ≤ 1.0. The local cationic environment has been studied by Raman and FTIR spectroscopy. Using acid-assisted LiNi_1−yCo_yO₂ powders (with compositions y=0.8 and y=0.6) calcined at 800 °C, Li//LiNi_1−yCo_yO₂ cells were assembled and tested by galvanostatic titration. These cells had an initial capacity of 140 mAh/g in the voltage range 2.8-4.2 V and showed attractive charge-discharge profiles upon cycling. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Ital, Sept. 12–19, 1999.     

Layered LT-LiNi1−yCoyO2 (0.3≤y≤0.7) cathodes for rechargeable lithium cells synthesized by co-precipitation method

Lithium nickel-cobalt oxides were synthesized at low temperature using a precipitation method involving dissolution of metal acetates and Li(COOCH₃) in oxalic acid (dicarboxylic acid). Pyrolysis of the precipitate at 400–600 °C produced single phase LiNi_1−yCo_yO₂ (0.3≤y≤0.7) with submicron-sized particles. Oxalic acid acted as a fuel, decomposed the homogeneous precipitate of metal complexes at low temperature, and yielded the free impurity LiNi_1−yCo_yO₂ compounds. The physicochemical properties of synthesized products were investigated by structural (XRD, SEM), spectroscopic (FTIR and Raman) and thermal (DTA/TG) analyses. The electrochemical properties of the LiNi_1−yCo_yO₂ cathode materials were evaluated in rechargeable Li cells by employing a non-aqueous organic electrolyte mixture of 1M LiPF₆ in EC+DMC. The cells maintained excellent cyclability at moderate charge-discharge rates. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Study of the local order in LiNi1−yCoyO2 oxides synthesized by soft chemistry

Lattice dynamics of LiNi_1−yCo_yO₂ solid solution are investigated using Raman and FTIR measurements. Evolution of the vibrational spectra, i.e., frequency shift and band broadening of the stretching modes of either (Ni_1−yCo_yO₂)O₆ or LiO₆ octahedra, are studied as a function of the calcination temperature of the LiNi_1−yCo_yO₂ solid solution. Results show that the solid solution exhibits a one-mode behavior with an increase of the (Ni_1−yCo_yO₂)_n sheet covalency upon substitution of cobalt for nickel. The change in the covalency corresponds to an increase of the Madelung constant with decreasing the c/a value. The broadening of the vibrational bands for Ni-rich compounds is the result of the cation mixing in the crystal layers. The partially disordered cation distribution appearing in lithium nickelate materials can also explain the observed broadening of the Raman spectra. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Structural and electrochemical properties of LiCoO2 and LiAlyCo1−yO2 (y=0.1 and 0.2) oxides: A comparative study of electrodes prepared by the citrate precursor route

We present the characterization and electrode behavior of LiCoO₂ and Al-doped LiAl_yCo_1−yO₂ (y=0.1 and 0.2) oxides prepared by wet-chemical method from the citrate precursor route. We study the phase evolution as a function of the aluminum substitution and the modification on the intercalation and deintercalation of Li ions. Characterization methods include XRD, SEM, and FTIR. X-ray diffraction patterns show that samples belong to the LiCoO₂-LiAlO₂ solid solution and have the layered α-NaFeO₂ structure. FT-infrared vibrational spectroscopy indicates the slight modification in the local structure related to the short-range environment of oxygen coordination around the cations in oxide lattices. The frequencies and relative intensities of the bands are sensitive to the covalency of the (Al, Co)O₂ slabs. The overall electrochemical capacity of the LiAl_yCo_1−yO₂ oxides have been reduced due to the sp metal substitution, however, a more stable charge-discharge cycling performances have been observed when electrodes are charged to 4.3 V as compared to the performances of the native oxide. Differences and similarities between LiCoO₂ and Al-substituted oxides are discussed therefrom. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Enthalpy of formation of LiNiO2, LiCoO2 and their solid solution,LiNi1−xCoxO2

LiCoO₂, LiNiO₂ and their solid solution, LiNi_1−xCo_xO₂, are important cathode materials for lithium ion batteries. Samples in this system were synthesized by solid state reaction of Co₃O₄, NiO and Li₂CO₃ or LiOH·H₂O. Their lattice parameters were determined by Rietveld refinement. High temperature drop solution calorimetry in molten 3Na₂O·4MoO₃ and 2PbO·B₂O₃ solvents at 974 K was performed to determine the enthalpy of formation from the constituent oxides plus oxygen and the enthalpy of mixing in the solid solution series. There are approximately linear correlations between the lattice parameters, the enthalpy of formation from oxides (Li₂O, NiO and CoO) plus O₂ and the Co content in the compounds. The solid solution of LiCoO₂ and LiNiO₂ is almost ideal, showing a small positive enthalpy of mixing. The enthalpy of formation of LiCoO₂ from oxides (Li₂O, NiO and CoO) and oxygen at 298 K is −142.5±1.7 kJ/mol (from sodium molybdate calorimetry) or −140.2±2.3 kJ/mol (from lead borate calorimetry). That of LiNiO₂ is −56.2±1.5 kJ/mol (from sodium molybdate calorimetry) or −53.4±1.7 kJ/mol (from lead borate calorimetry). The cobalt compound is thus significantly more stable than its nickel analogue. The phase assemblage LiCoO₂, Li₂O and CoO is seen at a lower oxygen pressure at constant temperature than the assemblage Co₃O₄/CoO, reflecting the stabilization of Co(III) in the ternary Li–Co–O system.     

Local structure and electrochemistry of LiNi y Mn y Co1 − 2y O2 electrode materials for Li-ion batteries

A series of LiNi _x Mn _y Co _z O₂ (x = y, z = 1 − 2y) oxides have been synthesized by “chimie douce” and investigated as positive electrodes in rechargeable lithium batteries. Layered LiNi _y Mn _y Co_1 − 2y O₂ materials with high homogeneity and crystallinity were synthesized using the wet-chemical method assisted by carboxylic acid as the polymeric agent. The long range and local structural properties are investigated with experiments including X-ray diffraction, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy. The evolution of the structure is discussed as a function of the cobalt content that confers layer-like behavior on the framework. Electrochemical performance of LiNi _y Mn _y Co_1 − 2y O₂ oxides is tested in cells using nonaqueous 1 M LiPF₆ dissolved in ethylene carbonate–diethyl carbonate. Charge–discharge profiles are investigated as a function of the rate capability and the voltage window. A relation is found between the gravimetric capacity and the cation disorder of the positive electrode as indicated by structural analysis. Fast lithium extraction attributed to the larger interslab space has been observed in the cobalt-rich oxides. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007.     

Surface properties of LiCoO2, LiNiO2 and LiNi1−xCoxO2

The surface composition and chemical environment of LiCoO₂, hexagonal LiNiO_2, cubic LiNiO₂, and the mixed transition metal oxide LiNi_0.5Co_0.5O₂ have been determined by Auger electron and X-ray photoelectron spectroscopies. While the LiCoO₂ surface properties can easily be extrapolated from bulk composition, the nickel-containing materials are less straightforward. Their surface concentration tends to be depleted in lithium relative to that of the bulk and shows an atypical chemical environment for the constituent elements. The Ni 2p XPS photoemission suggests a near “ NiO-like” selvedge through the XPS binding energies and satellite structure which are essentially identical to that of NiO; the spectrum appears fairly insensitive to lithium concentration. Although there is little evidence for higher binding energy Ni³⁺ species or for an electron poor Ni^2.δ+-derived band structure in the XPS, the lattice oxygen is very electron-rich and yields among the lowest binding energies reported for a transition metal oxide. The nickel-containing lithium oxide selvedge is thus not simply “NiO” and the surface lithium cations have a measurable effect on the electronic structure even in their more highly depleted levels. This is explained in the context of the charge-transfer model of the oxide band structure.     

Electrochemical studies on mixtures of LiNi0.8Co0.17Al0.03O2 and LiCoO2 cathode materials for lithium ion batteries

Detailed electrochemical investigations have been carried out on LiNi_0.8Co_0.17Al_0.03O₂ as cathode materials for lithium ion batteries in the potential range of 2.8-4.3 V. This sample showed an initial discharge capacity of 186 mAh/g which corresponds to 67% of its theoretical capacity. The effect of addition of LiCoO₂ to LiNi_0.8Co_0.17Al_0.03O₂ in the ratio 10:90, 30:70, 50:50 has been studied. The results showed that the addition of LiCoO₂ has improved the working voltage of the cell. In addition, the percentage retention (95%) of the cell is significantly increased in the composition ratio 50:50.     

Improved electrochemical properties of Sr2+-doped LiNi0.8Co0.2O2 synthesized via a sol-gel route using tartaric acid as a chelating agent

Single-phase undoped LiNi_0.8Co_0.2O₂ and Sr²⁺-doped LiNi_0.8Co_0.2O₂ were synthesized by a low temperature tartaric acid assisted sol-gel method. Small quantities of Sr²⁺ were used as dopants in order to improve the electrochemical characteristics, especially the capacity and cycling performance of LiNi_0.8Co_0.2O₂. The electrochemical performance of the undoped material was promising with a first discharge capacity of 174 mAh/g and 165 mAh/g after 10 cycles with a 100% cycling efficiency in the tenth cycle. Addition of Sr²⁺ for Li in minimum quantities with the Sr²⁺/Li⁺ dopant mole ratio ranging from 10⁻⁴ to 10⁻⁸ resulted in improved electrochemical properties for dopant mole ratio of 10⁻⁶. The first discharge capacity was 182 mAh/g and the tenth was 174 mAh/g at the 10th discharge. The synthesis of Sr²⁺-doped LiNi_0.8Co_0.2O₂ and its improved electrochemical properties have been discussed for the first time. The improved electrochemical properties of Sr²⁺-doped LiNi_0.8Co_0.2O₂ system are explained based on defect models.     

Structural, thermal and electrochemical properties of chromium-substituted LiNi0.8Co0.2O2

LiCr_yNi_0.8−yCo_0.2O₂ compositions, where y=0.000, 0.010, 0.025, 0.040, 0.050, 0.075 and 0.100, were synthesized via a conventional ceramic route. X-ray diffraction studies indicated cation mixing for the compositions with y ≥ 0.05. Cyclic voltammetric studies revealed that the systems were reversible only when y was lower than 0.05. High levels of substitutions with Cr resulted in highly irreversible systems, either due to cation mixing or the displacement of the substituent ions to the lithium inter-slab regions, or both. The charge-discharge characteristics of LiCr_yNi_0.8−yCo_0.2O₂ were similar to those of the unsubstituted material over ten cycles. All the other substituted compositions showed much lower capacities and reduced cyclability. LiCr_0.025Ni_0.775Co_0.2O₂ gave a first-cycle capacity of 169 mAh/g in the 3.0 to 4.4 V window at a 0.1 C rate, fading to 156 mAh/g in the tenth cycle. Differential scanning calorimetric studies revealed that substituting with chromium produced no benefit to thermal stability. The structural, thermal and electrochemical properties of the pristine and Cr-substituted LiNi_0.8Co_0.2O₂ compositions are discussed.     

Studies of the local structure in LixNi0.7Co0.3O2 (0.5≤x≤1.0) cathode materials

Among the Li_xNi_1−yCo_yO₂ system, LiNi_0.7Co_0.3O₂ is being considered one of the best cathode materials due to its small volume cell expansion upon charge-discharge cycling. In order to study the modifications of structural and physical properties occurring in the cathode materials during charge, different Li_xNi_0.7Co_0.3O₂ samples (0.5 ≥ x ≥ 1.0) were prepared by electrochemical lithium deintercalation in non-aqueous cells. During the first charge of the Li//Li_xNi_0.7Co_0.3O₂ cell, the structural change in the cathode lattice was followed by both x-ray powder diffraction and FTIR spectroscopy at room temperature. A good correlation is found between XRD data and the local environment of the host lattice. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998     

Iron doped lithium cobalt oxides as lithium intercalating cathode materials

Layered transition metal oxides of the formula LiMO₂ have good lithium insertion properties for which reason LiCoO₂ and LiNiO₂ have been exploited in practical lithium rocking chair batteries. Another member of the LiMO₂ series, LiFeO₂, should be an attractive cathode material considering the cheapness and environment-friendliness of iron compounds. Its rock-salt structure, however, does not allow significant amounts of lithium to be reversibly intercalated in its structure. Synthesis of layered LiFeO₂ and study of its lithium intercalating properties have been of limited success. Therefore, an attempt has been made here to study LiCo_1−yFe_yO₂ solid solutions (0 ≤ y ≤ 0.4) as prospective cathode materials. XRD, FTIR, Atomic absorption spectroscopy, Particle size and Surface area analysis were carried out in this regard towards the physical characterization of the entire series of LiCo_1−yFe_yO₂ compounds. The electrochemical discharge capacity of these materials is explained as a function of the iron content.     

Morphology and magnetic properties of Fe x Co1−x /Co y Fe3−y O4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Recently, we have demonstrated the successful synthesis of Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of Fe _x Co_1−x /Co_yFe_3−y O₄ nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.     

Structural, transport and electrochemical properties of LiNi1 − yCoyMn0.1O2 and Al, Mg and Cu-substituted LiNi0.65Co0.25Mn0.1O2 oxides

LiNi_{1 - y − z}Co_yMn_zO₂ (y = 0.25, 0.35, 0.5, 0.6; z = 0.1, 0.2), LiNi_0.63Cu_0.02Co_0.25Mn_0.1O₂, LiNi_0.65Co_0.25Mn_0.08Al_0.02O₂, LiNi_0.65Co_0.25Mn_0.08Mg_0.02O₂ and LiNi_0.65Co_0.25Mn_0.08Al_0.01Mg_0.01O₂ cathode materials were synthesized by a soft chemistry EDTA-based method. Structural and transport properties of pristine and delithiated materials (Li_xNi_0.65Co_0.25Mn_0.1O₂, Li_xNi_0.55Co_0.35Mn_0.1O₂ and LiNi_0.63Cu_0.02Co_0.25Mn_0.1O₂ oxides) are presented. In the considered group of oxides there is no correlation between electrical conductivity and the a parameter (M-M distance in the octahedra layers). The results of electrochemical performance of cathode materials are presented. The best stability during first 10 cycles was obtained for Li/Li_xNi_0.63Cu_0.02Co_0.25Mn_0.1O₂ cell due to enhanced kinetics of intercalation process.     

Structure, morphology and electrochemistry of doped lithium cobalt oxides

The structure, morphology and electrochemical performance of LiCoO₂ powders doped with various trivalent cations such as M³⁺=Ni³⁺, Al³⁺, and B³⁺ have been investigated. X-ray diffraction patterns and FTIR data show that LiCo_1−yNi_yO₂ oxides display a solid solution in the whole range 0≤y≤1, while the solubility limit of LiCo_1−yAl_yO₂ and LiCo_1−yB_yO₂ materials is about y=0.35. Electrochemical features of LiCo_1−yNi_yO₂ cathode materials show remarkable stability in their charge-discharge profiles. Aluminum substitution also stabilizes the two-dimensional framework and induces an increasing average cell potential than for LiCoO₂. The overall capacity of the LiCo_1−yB_yO₂ oxides has been reduced due to the sp metal substitution, however, a more stable charge-discharge cycling characteristic has been observed when electrodes are charged up to 4.4 V as compared to the performances of the native oxides. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Synthesis and characterization of LiNi<Subscript>y</Subscript>Co<Subscript>1−y</Subscript>PO<Subscript>4</Subscript> (y=0–1) cathode materials for lithium secondary batteries

In this work series of LiNi_yCo_1−yPO₄ (y=0, 0.2, 0.4, 0.6, 0.8 and 1) phospho olivines were synthesized by solution co-precipitation technique and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and impedance spectroscopic analysis. The XRD patterns of LiNi_yCo_1−yPO₄ (y=0.2, 0.4, 0.6 and 0.8) revealed that they are essentially single phase and have an Olivine-type XRD patterns similar to those of their parent compounds LiCoPO₄ and LiNiPO₄. An increase in wave number for most of the dominant infrared bands in PO₄ vibrational region for the substitution of Co by Ni in LiCoPO₄ indicated the strengthening of both the P-O and Li/Ni-O bonds. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

ZrO2 coating of LiNi1/3Co1/3Mn1/3O2 cathode materials for Li-ion batteries

ZrO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ materials were prepared by hydroxide precipitation. The structure and electrochemical properties of the ZrO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ were investigated using X-ray diffraction, scanning electron microscope, and charge–discharge tests, indicating that the lattice structure of LiNi_1/3Co_1/3Mn_1/3O₂ were unchanged after the coating but the cycling stability was improved. As the coating amount increased from 0.0 to 0.5 mol.%, the initial capacity of the coated LiNi_1/3Co_1/3Mn_1/3O₂ decreased slightly; however, the cycling stability increased remarkably over the cut-off voltages of 2.5~4.3 V and the capacity retention reached 99.5% after 30 cycles at the coating amount of 0.5 mol.%. ZrO₂ coating also improved the cycling stability of LiNi_1/3Co_1/3Mn_1/3O₂ over wider cut-off voltage of 2.5~4.6 V.     

Suppression of structural degradation of LiNi0.9Co0.1O2 cathode at 90 °C by AlPO4-nanoparticle coating

This study examined the electrochemical and structural stability of ∼1.5 wt.% AlPO₄-coated LiNi_0.9Co_0.1O₂. The AlPO₄-coated LiNi_0.9Co_0.1O₂ retained ∼60% of the original capacity after 50 cycles, compared with the ∼30% capacity retention of the bare LiNi_0.9Co_0.1O₂. The discharge profiles and cyclic voltammograms from 4.5 V at 90 °C for 4 h showed enhanced structural stability. Scanning electron microscopy and X-ray diffraction revealed that the AlPO₄-coated LiNi_0.9Co_0.1O₂ had less degradation than the bare LiNi_0.9Co_0.1O₂.