Structural and electrochemical behaviors of metastable Li2/3[Ni1/3Mn2/3]O2 modified by metal element substitution

Layered metastable lithium manganese oxides, Li_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ precursors. The Al and Co doping produced the T^#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni_1/3−xMn_2/3−yM_x+yO₂ slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T^#2-Li_2/3[Ni_1/3Mn_2/3]O₂ were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g⁻¹ in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g⁻¹ along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T^#2 into the O2 structure was observed during the initial lithium deintercalation.     

Relationship between electrochemical behavior and Li/vacancy arrangement in ramsdellite type Li2+xTi3O7

The changes of Li⁺/vacancy arrangement in Li_2+xTi₃O₇ with a ramsdellite-type structure upon topo-electrochemical Li⁺ insertion were investigated by the entropy measurement of reaction combined with the Monte Carlo simulation. The experimental entropy measurement was conducted by potentiometric and calorimetrical methods. The obtained experimental data were in good accordance with simulated results.The results indicated that the ordered Li⁺/vacancy arrangement appeared at the compositions of x ∼ 0.45 and ∼1.20, where the observed entropy of reaction humped. The ordering of Li/vacancy were also indicated at the composition x ∼ 0.24 and 1.16 in Li_2+xTi₃O₇ by the Monte Carlo simulation which considers the most stable Li/vacancy arrangement in terms of Coulombic interaction. This substantial agreement between electrochemical behaviors and computational results confirmed that the formation of superstructure arising from Li/vacancy arrangement during the electrochemical reaction deeply related to the atomic level Coulombic interactions.     

Enhanced luminescence of novel Ca3B2O6:Dy phosphors by Li-codoping for LED applications

The Ca_3−xB₂O₆:xDy³⁺ (0.0 ≤ x ≤ 0.105) and Ca_2.95−yDy_0.05B₂O₆:yLi⁺ (0 ≤ y ≤ 0.34) phosphors were synthesized at 1100 °C in air by solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation (PLE) and photoluminescence (PL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm is due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions upon 350 nm excitation is observed at 480 nm (blue) due to the ⁴F_9/2 → ⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2 → ⁶H_13/2 transitions and a weak 660 nm (red) due to ⁴F_9/2 → ⁶H_11/2 emissions, respectively. The optimal PL intensity of the Ca_3−xB₂O₆:xDy³⁺ phosphors is found to be x = 0.05. Moreover, the PL results from Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors show that Dy³⁺ emissions can be enhanced with the increasing codopant Li⁺ content till y = 0.22. By simulation of white light, the CIE of the investigated phosphors can be tuned by varying the content of Li⁺ ions, and the optimal CIE value (0.300, 0.298) is realized when the content of Li⁺ ions is y = 0.22. All the results imply that the Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors could be potentially used as white LEDs.     

Synthesis and electrochemical properties of Co-doped Li3V2(PO4)3 cathode materials for lithium-ion batteries

Co-doped Li₃V_2−xCo_x(PO₄)₃/C (x = 0.00, 0.03, 0.05, 0.10, 0.13 or 0.15) compounds were prepared via a solid-state reaction. The Rietveld refinement results indicated that single-phase Li₃V_2−xCo_x(PO₄)₃/C (0 ≤ x ≤ 0.15) with a monoclinic structure was obtained. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the cobalt is present in the +2 oxidation state in Li₃V_2−xCo_x(PO₄)₃. XPS studies also revealed that V⁴⁺ and V³⁺ ions were present in the Co²⁺-doped system. The initial specific capacity decreased as the Co-doping content increased, increasing monotonically with Co content for x > 0.10. Differential capacity curves of Li₃V_2−xCo_x(PO₄)₃/C compounds showed that the voltage peaks associated with the extraction of three Li⁺ ions shifted to higher voltages with an increase in Co content, and when the Co²⁺-doping content reached 0.15, the peak positions returned to those of the unsubstituted Li₃V₂(PO₄)₃ phase. For the Li₃V_1.85Co_0.15(PO₄)₃/C compound, the initial capacity was 163.3 mAh/g (109.4% of the initial capacity of the undoped Li₃V₂(PO₄)₃) and 73.4% capacity retention was observed after 50 cycles at a 0.1 C charge/discharge rate. The doping of Co²⁺into V sites should be favorable for the structural stability of Li₃V_2−xCo_x(PO₄)₃/C compounds and so moderate the volume changes (expansion/contraction) seen during the reversible Li⁺ extraction/insertion, thus resulting in the improvement of cell cycling ability.     

Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li_2xVO(H_2−xPO₄)₂ (0 < x < 2), have been undertaken. H⁺/Li⁺ ionic exchange from VO(H₂PO₄)₂ to Li₂VO(HPO₄)₂ leads to grain decrepitation. Further ionic exchange toward formation of Li₄VO(PO₄)₂ lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li₄VO(PO₄)₂ shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with V^V/V^IV and V^III/V^II redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of V^III to V^IV is involved.     

Preparation and characterization of novel spinel Li4Ti5O12−xBrx anode materials

Br-doped Li₄Ti₅O₁₂ in the form of Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) compounds were successfully synthesized via solid state reaction. The structure and electrochemical properties of the spinel Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) materials were investigated. The Li₄Ti₅O_12−xBr_x (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li₄Ti₅O_12−xBr_x (x = 0.2) sample presented the excellent discharge capacity of 172 mAh g⁻¹, which was very close to its theoretical capacity (175 mAh g⁻¹), while that of the pristine Li₄Ti₅O₁₂ was 123.2 mAh g⁻¹ only.     

Study of the potentiometric response of the doped spinel Li1.05Al0.02Mn1.98O4 for the optimization of a selective lithium ion sensor

In this paper, we studied the development of a selective lithium ion sensor constituted of a carbon paste electrode modified (CPEM) with an aluminum-doped spinel-type manganese oxide (Li_1.05Al_0.02Mn_1.98O₄) for investigating the influence of a doping ion in the sensor response. Experimental parameters, such as influence of the lithium concentration in the activation of the sensor by cyclic voltammetry, pH of the carrier solution and selectivity for Li⁺ against other alkali and alkaline-earth ions were investigated. The sensor response to lithium ions was linear in the concentration range 5.62 × 10⁻⁵ to 1.62 × 10⁻³ mol L⁻¹ with a slope 100.1 mV/decade over a wide pH 10 (Tris buffer) and detection limit of 2.75 × 10⁻⁵ mol L⁻¹, without interference of other alkali and alkaline-earth metals, demonstrating that the Al³⁺ doping increases the structure stability and improves the potentiometric response and sensitivity of the sensor. The super-Nernstian response of the sensor in pH 10 can be explained by mixed potential arising from two equilibria (redox and ion-exchange) in the spinel-type manganese oxide.     

Relationship between glass network structure and conductivity of Li2O-B2O3-P2O5 solid electrolyte

Solid state glass electrolyte, xLi₂O-(1 − x)(yB₂O₃-(1 − y)P₂O₅) glasses were prepared with wide range of composition, i.e. x = 0.35 - 0.5 and y = 0.17 - 0.67. This material system is one of the parent compositions for chemically and electrochemically stable solid-state electrolyte applicable to thin film battery. Lithium ion conductivity of Li₂O-B₂O₃-P₂O₅ glasses was studied in the correlation to the structural variation of glass network by using FTIR and Raman spectroscopy. The measured ionic conductivity of the electrolyte at room temperature increased with x and y. The maximum conductivity of this glass system was 1.6 × 10⁻⁷ Ω⁻¹ cm⁻¹ for 0.45Li₂O-0.275B₂O₃-0.275P₂O₅ at room temperature. It was shown that the addition of P₂O₅ reduces the tendency of devitrification and increases the maximum amount of Li₂O added into glass former without devitrification. As Li₂O and B₂O₃ contents increased, the conductivity of glass electrolyte increased due to the increase of three-coordinated [BO₃] with a non-bridging oxygen (NBO).     

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.     

Preparation and characterization of three dimensionally ordered macroporous Li4Ti5O12 anode for lithium batteries

Three dimensionally ordered macroporous (3DOM) Li₄Ti₅O₁₂ membrane (80 μm thick) was prepared by a colloidal crystal templating process. Colloidal crystal consisting of monodisperse polystyrene particles (1 μm diameter) was used as the template for the preparation of macroporous Li₄Ti₅O₁₂. A precursor sol consisting of titanium isopropoxide and lithium acetate was impregnated into the void space of template, and it was calcined at various temperatures. A macroporous membrane of Li₄Ti₅O₁₂ with inverse-opal structure was successfully prepared at 800 °C. The interconnected pores with uniform size (0.8 μm) were clearly observed on the entire part of membrane. The electrochemical properties of the three dimensionally ordered Li₄Ti₅O₁₂ were characterized with cyclic voltammetry and galvanostatic charge and discharge in an organic electrolyte containing a lithium salt. The 3DOM Li₄Ti₅O₁₂ exhibited a discharge capacity of 160 mA h g⁻¹ at the electrode potential of 1.55 V versus Li/Li⁺ due to the solid state redox of Ti^3+/4+ accompanying with Li⁺ ion insertion and extraction. The discharge capacity was close to the theoretical capacity (167 mA h g⁻¹), which suggested that the Li⁺ ion insertion and extraction took place at the entire part of 3DOM Li₄Ti₅O₁₂ membrane. The 3DOM Li₄Ti₅O₁₂ electrode showed good cycle stability.     

Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material

The chemical diffusion coefficients of lithium ions (DLi⁺) in Li₃V₂(PO₄)₃ between 3.0 and 4.8 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The DLi⁺ values are found to be dependent on the voltage state of charge and discharge. Based on the results from all the three techniques, the true diffusion coefficients measured in single-phase region are in the range of 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹. Its apparent diffusion coefficients measured in two-phase regions by CV and GITT range from 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹ and 10⁻⁸ to 10⁻¹³ cm² s⁻¹, respectively, depending on the potentials. By the GITT, the DLi⁺ varies non-linearly in a “W” shape with the charge-discharge voltage, which is ascribed to the strong interactions of Li⁺ with surrounding ions. Finally, the chemical diffusion coefficients of lithium ions measured by CV, EIS and GITT are compared to each other.     

A novel lithium intercalation compound based on the layered structure of lithium nitridonickelates Li3−2xNixN

New lithium nickel nitrides Li_3−2xNi_xN (0.20 ≤ x ≤ 0.60) have been prepared and investigated as negative electrode in the 0.85/0.02 V potential window. These materials are prepared from a Ni/Li₃N mixture at 700 °C under a nitrogen flow. Their structural characteristics as well as their electrochemical behaviour are investigated as a function of the nickel content. For the first time are reported here the electrochemical properties of a lithium intercalation compound based on a layered nitride structure. The Li_3−2xNi_xN compounds can be reversibly reduced and oxidized around 0.5 V versus Li/Li⁺ leading to specific capacities in the range 120-160 mAh/g depending on the nickel content and the C rate. Due to a large number of lithium vacancies, the structural stability provides an excellent capacity retention of the specific capacity upon cycling.     

Influence of the tetravalent cation on the high-voltage electrochemical activity of LiNi0.5M1.5O4 spinel cathode materials

The electrochemical properties of substituted LiNi_0.5Mn_1.5−xM_xO₄ spinels at high potential (>4 V vs Li⁺/Li) have been investigated for M = Ti and Ru, in order to determine the role of the tetravalent cation in such systems where nickel is a priori the only electroactive species. These systems are found to form extended solid solutions (up to x = 1.3 and x = 1.0 for Ti and Ru, respectively) that were characterized by X-ray diffraction and Raman spectroscopy. Titanium substitution induces a drastic decrease in high potential electrochemical capacity, whereas the capacity is maintained and the kinetics are even improved in the presence of ruthenium. These results are completed by new results on the Li_4−2xNi_3xTi_5−xO₁₂ spinel system, which shows not any high potential activity in spite of the presence of up to 0.5 Ni²⁺ per spinel formula unit on the octahedral site. Taking into account previous data on LiNi_0.5Ge_1.5O₄, we clearly show that even if the tetravalent cation does not participate in the overall redox reaction, electrochemical activity is only possible when nickel is surrounded by tetravalent cations able to accept a local variation of valence (Mn, Ru), whereas full-shell cations such as Ti⁴⁺ and Ge⁴⁺ block the necessary electron transfer pathways in the spinel oxide electrode.     

Optimum synthesis of Li2Fe1−xMnxSiO4/C cathode for lithium ion batteries

Li₂Fe_1−xMn_xSi0₄/C cathode materials were synthesized by mechanical activation-solid-state reaction. The effects of Mn-doping content, roasting temperature, soaking time and Li/Si molar ratio on the physical properties and electrochemical performance of the Li₂Fe_1−xMn_xSi0₄/C composites were investigated. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), charge-discharge tests and AC impedance measurements. SEM images suggest that the morphology of the Li₂Fe_1−xMn_xSi0₄/C composite is sensitive to the reaction temperature. Samples synthesized at different temperatures have different extent of agglomeration. Being charged-discharged at C/32 between 1.5 and 4.8 V, the Li₂Fe_0.9Mn_0.1Si0₄/C synthesized at the optimum conditions shows good electrochemical performances with an initial discharge capacity of 158.1 mAh g⁻¹ and a capacity retention ratio of 94.3% after 30 cycles. AC impendence investigation shows Li₂Fe_0.9Mn_0.1SiO₄/C have much lower resistance of electrode/electrolyte interface than Li₂FeSiO₄/C.     

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.     

Effect of K2O on structure-property relationships and phase transformations in Li2O-SiO2 glasses

Glass compositions with formula (71.78 − x)SiO₂-2.63Al₂O₃-(2.63 + x)K₂O-23.7Li₂O (mol.%, x = 0-10) and SiO₂/Li₂O molar ratios far beyond that of stoichiometric lithium disilicate (Li₂Si₂O₅) were prepared by conventional melt-quenching technique to investigate the influence of K₂O content on structural transformations and devitrification behaviour of glasses in the Li₂O-SiO₂ system. The scanning electron microscopy (SEM) examination of as cast non-annealed glasses revealed the presence of nanosized droplets in glassy matrices suggesting occurrence of liquid-liquid phase separation. An overall trend towards depolymerization of the silicate glass network with increasing K₂O content was demonstrated by employing magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. The distribution of structural units in the experimental glasses was estimated using ²⁹Si MAS-NMR spectroscopy suggesting the appearance of Q², enhancement of Q³ and diminishing of Q⁴ groups with increasing K₂O contents. X-ray diffraction (XRD) and differential thermal analysis (DTA) were used to assess the influence of K₂O on devitrification process and formation of lithium disilicate (Li₂Si₂O₅) and/or lithium metasilicate (Li₂SiO₃) crystalline phases.     

Ion-pairing effects and ion-solvent-polymer interactions in LiN(CF3SO2)2-PC-PMMA electrolytes: a FTIR study

FTIR spectroscopic investigations coupled with ionic conductivity and viscosity measurements on lithium imide (LiN(CF₃SO₂)₂)-propylene carbonate (PC)-poly(methyl methacrylate) (PMMA) based liquid and gel electrolytes over a wide range of salt (0.025-3 M) and polymer (5-25 wt.%) concentration range furnish a novel insight into the ion-ion and ion-solvent-polymer interactions. Vibrational spectral data for LiN(CF₃SO₂)₂-PC electrolytes reveal that the solvation of lithium ions manifests from Li⁺OC and Li⁺O (ring oxygens) interactions as the ν_s(CO), the ring breathing and the δ(CH) modes of the pentagonal solvent ring are strongly perturbed for all salt concentrations. The split of the ν(SO₂) mode (that appears at 1355 cm⁻¹ for the “free imide ion”) into two components at 1337 and 1359 cm⁻¹ confirms the existence of contact ion-pairs possessing two different stable optimized geometries wherein the Li⁺ ion coordinates in a bidentate fashion in liquid and gel electrolytes of 3 M LiN(CF₃SO₂)₂-PC strength. Perturbations observed for the ν_a(SNS) and ν_s(SNS) modes of the imide ion and the symmetric ring deformation mode of PC confirms the presence of ion-pairs in both 2 and 3 M electrolytes. Incorporation of even upto 25 wt.% of PMMA in a solution of LiN(CF₃SO₂)₂-PC of 3 M strength results in an insignificant conductivity decline (as σ₂₅>10⁻³ S cm⁻¹) which is simultaneously accompanied by a massive increase in its macroscopic viscosity (as η₂₅>10⁸ cSt). Gels containing 25 wt.% of PMMA exhibit a complex pattern of Li⁺-PMMA interactions through the carbonyl oxygen of its ester group which is evidenced from the perturbations observed for the ν_s(CO) mode of PMMA. Ionic conductivity decline that occurs at salt concentrations ≥1.25 M LiN(CF₃SO₂)₂-PC in both liquid and gel electrolytes, is therefore attributable to (i) ion-pairing phenomenon and (ii) an enhancement in the solution viscosity due to a high salt proportion.     

Determination of lithium diffusion coefficient in LiFePO4 electrode by galvanostatic and potentiostatic intermittent titration techniques

A new model of lithium-ion transport processes in the LiFePO₄ electrode is proposed. This model takes into account the phase transition LiFePO₄ ↔ FePO₄ accompanying reversible lithium intercalation into the electrode during potential or current steps. The diffusion coefficient of Li⁺ ion and its dependence on the LiFePO₄/FePO₄ phase ratio have been determined by means of processing of experimental potential and current transients in accordance with the model's equations. The results of galvanostatic and potentiostatic intermittent titration techniques are in good agreement. The value of diffusion coefficient varies within 10⁻¹⁰-10⁻¹⁶ cm² s⁻¹ depending on the lithium content in solid solution Li_XFePO₄ and Li_1−XFePO₄ (X < 0.02) or the LiFePO₄/FePO₄ phase ratio.     

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

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

Preparation and electrochemical properties of LiCoO2-LiNi0.5Mn0.5O2-Li2MnO3 solid solutions with high Mn contents

The solid solutions LiCoO₂-LiNi_1/2Mn_1/2O₂-Li₂MnO₃ with higher Mn content have been prepared by a spray drying method between 750 and 950 °C and their electrochemical performances have also been characterized. The effects of the Li content on the structure and electrochemical performance of the samples have been studied. It was found that their lattice parameters a, c and V increase with the increase in Ni content and the decrease in Co content. The solid solutions xLiCoO₂-yLiNi_1/2Mn_1/2O₂-(1−x−y)Li₂MnO₃ with x = 0.18, 0.27 and y = 0.2 have the largest discharge capacity, which is more than 200 mAh/g in the voltages of 3.0-4.6 V. It is believed that the optimum Co content x in xLiCoO₂-yLiNi_1/2Mn_1/2O₂-(1−x−y)Li₂MnO₃ is between 0.2 and 0.3 in the charge-discharge voltage range of 3.0-4.6 V. The solid solutions xLiCoO₂-yLiNi_1/2Mn_1/2O₂-(1−x−y)Li₂MnO₃ with x = 0.18-0.36 and y = 0.2 have the excellent cycling performance and the capacity retention attains to almost 100% after 50 cycles. Moreover, it is found that the discharge capacity gradually increases with the increment of cycle number especially in the initial 10 cycles. XRD showed that the layered structure has been kept all the time in 20 cycles, which is perhaps the reason why the sample has the excellent cycling performance.