Changes in electronic structure of Li2−xCuO2

A single phase of Li₂CuO₂ was successfully prepared by solid state reaction. The crystal structure was refined by Rietveld method using a synchrotron radiation source. Electronic structures of x = 0 and 1 in Li_2−xCuO₂ were determined using the first principle calculation. We will discuss electrochemical properties of Li₂CuO₂ and changes in electronic structure of Li_2−xCuO₂, using both the information obtained from the calculation and that obtained experimentally. These results confirmed that Li-extraction of Li₂CuO₂ resulted in the oxidation of Cu²⁺ or O²⁻. The cuprate-based cathode materials have the potential showing high energy density. The calculation of electronic state will play a major role in designing the materials for Li-batteries.     

Thermal stability of Li1−yNixMn(1−x)/2Co(1−x)/2O2 layer-structured cathode materials used in Li-Ion batteries

In order to develop safe lithium-ion batteries using Ni-based cathode active materials, such as LiNi_xMn_(1−x)/2Co_(1−x)/2O₂, thermal stability is one of the most important requirements. We used XRD and TDS-MS in the first step of our study to elucidate the thermal stability and to improve it under anomalous high temperature conditions. We investigated the relationship between the thermal stability and cathode composition, especially for that of the nickel and lithium content. The XRD indicated that the crystal structure of electrochemically delithiated materials changed from a layered into a spinel structure followed by a rock-salt structure as the temperature rose. The TDS-MS indicated that these changes coincided with the release of oxygen from the cathode materials. We found that decreasing the lithium content and increasing the nickel content made the temperature of the crystal structure change and oxygen release lower, and thus, influenced the cathode composition.     

Electrochemical performances of LiFe1−xMnxPO4 with high Mn content

A series of LiFe_1−xMn_xPO₄/C materials with high Mn content (0.7 ≤ x ≤ 0.9) are synthesized by solid state reaction. The samples have mesoporous structure with an average pore size of 25 nm, particle size around 200-300 nm, crystalline size around 30 nm and specific areas around 50 m² g⁻¹. Their electrochemical performances are studied and the reversible capacity and rate performance decrease with the increase of Mn content. The redox potential of the Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox couple also shift accordingly. The overpotential value of the Mn²⁺/Mn³⁺ redox couple (80 mV) is close to that of the Fe²⁺/Fe³⁺ couple (60 mV) in all three compositions and shows a maximum (∼300 mV) in the regions of voltage transition.     

Synthesis and performance of Li3(V1−xMgx)2(PO4)3 cathode materials

In order to search for cathode materials with better performance, Li₃(V_1−xMg_x)₂(PO₄)₃ (0, 0.04, 0.07, 0.10 and 0.13) is prepared via a carbothermal reduction (CTR) process with LiOH·H₂O, V₂O₅, Mg(CH₃COO)₂·4H₂O, NH₄H₂PO₄, and sucrose as raw materials and investigated by X-ray diffraction (XRD), scanning electron microscopic (SEM) and electrochemical impedance spectrum (EIS). XRD shows that Li₃(V_1−xMg_x)₂(PO₄)₃ (x = 0.04, 0.07, 0.10 and 0.13) has the same monoclinic structure as undoped Li₃V₂(PO₄)₃ while the particle size of Li₃(V_1−xMg_x)₂(PO₄)₃ is smaller than that of Li₃V₂(PO₄)₃ according to SEM images. EIS reveals that the charge transfer resistance of as-prepared materials is reduced and its reversibility is enhanced proved by the cyclic votammograms. The Mg²⁺-doped Li₃V₂(PO₄)₃ has a better high rate discharge performance. At a discharge rate of 20 C, the discharge capacity of Li₃(V_0.9Mg_0.1)₂(PO₄)₃ is 107 mAh g⁻¹ and the capacity retention is 98% after 80 cycles. Li₃(V_0.9Mg_0.1)₂(PO₄)₃//graphite full cells (085580-type) have good discharge performance and the modified cathode material has very good compatibility with graphite.     

Pillared layered Li1−2xCaxCoO2 cathode materials obtained by cationic exchange under hydrothermal conditions

A simple method has been employed to prepare pillared layered Li_1−2xCa_xCoO₂ cathode materials by cationic exchange under hydrothermal conditions. The synthesized materials were characterized by means of X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), field emission scanning electron microscope (FE-SEM) and galvanostatic charge–discharge cycling. The XRD data of the products show that they are single phases and retain the layered α-NaFeO₂ type structure. The FE-SEM images of the materials prepared by hydrothermal method show uniform small particles, and the particle size of the materials is about 200 nm. The initial discharge specific capacities of layered LiCoO₂ and pillared layered Li_0.946Ca_0.027CoO₂ cathode materials calcined at 800 °C for 5 h within the potential range of 3.0–4.3 V (vs. Li⁺/Li) are 144.6 and 142.3 mAh g⁻¹, respectively, and both materials retain good charge–discharge cycling performance. However, with increasing upper cutoff voltage, the pillar effect of Ca²⁺ in Li_1−2xCa_xCoO₂ becomes more significant. The pillared layered Li_0.946Ca_0.027CoO₂ has a higher capacity with an initial discharge specific capacity of 177.9 and 215.8 mAh g⁻¹ within the potential range of 3.0–4.5 and 4.7 V (vs. Li⁺/Li), respectively, and retains good charge–discharge cycling performance.     

High-throughput studies of Li1−xMgx/2FePO4 and LiFe1−yMgyPO4 and the effect of carbon coating

A two-dimensional sample array synthesis has been used to screen carbon-coated Li_(1−x)Mg_x/2FePO₄ and LiFe_(1−y)Mg_yPO₄ powders as potential positive electrode materials in lithium ion batteries with respect to x, y and carbon content. The synthesis route, using sucrose as a carbon source as well as a viscosity-enhancing additive, allowed introduction of the Mg dopant from solution into the sol–gel pyrolysis precursor. High-throughput XRD and cyclic voltammetry confirmed the formation of the olivine phase and percolation of the electronic conduction path at sucrose to phosphate ratios between 0.15 and 0.20. Measurements of the charge passed per discharge cycle showed that the capacity deteriorated on increasing magnesium in Li_(1−x)Mg_x/2FePO₄, but improved with increasing magnesium in LiFe_(1−y) Mg_yPO₄, especially at high scan rates. Rietveld-refined XRD results on samples of LiFe_(1−y)Mg_yPO₄ prepared by a solid-state route showed a single phase up to y = 0.1 according to progressive increases in unit cell volume with increases in y. Carbon-free samples of the same materials showed conductivity increases from 10⁻¹⁰ to 10⁻⁸ S cm⁻¹ and a decrease of activation energy from 0.62 to 0.51 eV. Galvanostatic cycling showed near theoretical capacity for y = 0.1 compared with only 80% capacity for undoped material under the same conditions.     

Physical and electrochemical properties of spherical Li1+x(Ni1/3Co1/3Mn1/3)1−xO2 cathode materials

A (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor with an uniform, spherical morphology was prepared by coprecipitation using a continuously stirred tank reactor method. The as-prepared spherical (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor served to produce dense, spherical Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ (0 ≤ x ≤ 0.15) cathode materials. These Li-rich cathodes were also prepared by a second synthesis route that involved the use of an M₃O₄ (M = Ni_1/3Co_1/3Mn_1/3) spinel compound, itself obtained from the carbonate (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor. In both cases, the final Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ products were highly uniform, having a narrow particle size distribution (10-μm average particle size) as a result of the homogeneity and spherical morphology of the starting mixed-metal carbonate precursor. The rate capability of the Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ electrode materials, which was significantly improved with increased lithium content, was found to be better in the case of the denser materials made from the spinel precursor compound. This result suggests that spherical morphology, high density, and increased lithium content were key factors in enabling the high rate capabilities, and hence the power performances, of the Li-rich Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ cathodes.     

Minimization of the cation mixing in Li1+x(NMC)1−xO2 as cathode material

Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ layered materials were synthesized by the co-precipitation method with different Li/M molar ratios (M = Ni + Mn + Co). Elemental titration evaluated by inductively coupled plasma spectrometry (ICP), structural properties studied by X-ray diffraction (XRD), Rietveld analysis of XRD data, scanning electron microscopy (SEM) and magnetic measurements carried out by superconducting quantum interference devices (SQUID) showed the well-defined α-NaFeO₂ structure with cationic distribution close to the nominal formula. The Li/Ni cation mixing on the 3b Wyckoff site of the interlayer space was consistent with the structural model [Li_1−yNi_y]_3b[Li_x+yNi_{(1−x)/3−y}Mn_(1−x)/3Co_(1−x)/3]_3aO₂ (x = 0.02, 0.04) and was very small. Both Rietveld refinements and magnetic measurements revealed a concentration of Ni²⁺-3b ions lower than 2%; moreover, for the optimized sample synthesized at Li/M = 1.10, only 1.43% of nickel ions were located into the Li sublattice. Electrochemical properties were investigated by galvanostatic charge-discharge cycling. Data obtained with Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ reflected the high degree of sample optimization. An initial discharge capacity of 150 mAh g⁻¹ was delivered at 1 C-rate in the cut-off voltage of 3.0-4.3 V. More than 95% of its initial capacity was retained after 30 cycles at 1 C-rate. Finally, it is demonstrated that a cation mixing below 2% is considered as the threshold for which the electrochemical performance does not change for Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂.     

High lithium ionic conductivity in the garnet-type oxide Li7−X La3(Zr2−X, NbX)O12 (X = 0-2)

Lithium garnet-type oxides Li_7−XLa₃(Zr_2−X, Nb_X)O₁₂ (X = 0-2) were synthesized by a solid-state reaction, and their lithium ion conductivity was measured using an AC impedance method at temperatures ranging from 25 to 150 °C in air. The lithium ion conductivity increased with increasing Nb content, and reached a maximum of ∼0.8 mS cm⁻¹ at 25 °C. By contrast, the activation energy reached a minimum of ∼30 kJ mol⁻¹ at the same point with X = 0.25. The potential window was examined by cyclic voltammetry (CV), which showed lithium deposition and dissolution peaks around 0 V vs. Li⁺/Li, but showed no evidence of other reactions up to 9 V vs. Li⁺/Li.     

X-ray diffraction study on LixCoO2 below ambient temperature

In order to elucidate the structural change of Li_xCoO₂ with temperature (T), powder X-ray diffraction measurements have been carried out using a synchrotron radiation source in the T range between 300 and 90 K for the samples with x=1.02, 0.60, 0.56, and 0.53. The samples with x<1.02 were prepared by an electrochemical reaction in a non-aqueous lithium cell. The x=1.02 and 0.60 samples are in a rhombohedral phase () in the whole T range measured. On the other hand, the x=0.56 and 0.53 samples exhibit a structural transition around 140 K, although the both samples are in a monoclinic phase (C2/m) down to 90 K. That is, the angle between a_M- and c_M-axis (β_M) increases monotonically down to 150 K, then increases more rapidly with further lowering T. The values of and a_M/b_M, which are parameters to characterize a monoclinic distortion from the hexagonal symmetry, are and a_M/b_M<1.732 above 140 K, while and a_M/b_M≈1.732 below 140 K. This suggests that the monoclinic distortion below 140 K is mainly caused by a gliding along the basal plane.     

Lithium ion transport pathways in xLiCl−(1 − x)(0.6Li2O–0.4P2O5) glasses

xLiCl–(1 − x)(0.6Li₂O–0.4P₂O₅) systems with x = 0.1, 0.15, 0.2, 0.25, have been prepared using melt quenching method and their ionic conductivity was characterized by impedance spectroscopy. Molecular dynamics (MD) simulations for the same systems have been performed with an optimized potential, fitted to match bond lengths, coordination numbers and ionic conductivity. Based on the equilibrated configurations of these MD simulations, ion transport pathways are modelled in detail by the bond valence approach to clarify the influence of the halide dopant concentration on the glass structure and its consequence for Li ion mobility. Features of the consequential ion transport pathway models (such as volume fraction and local dimensionality of the percolating pathway) are compared to pathway models for related glassy solid electrolytes based on reverse Monte Carlo modelling of diffraction data.     

Synthesis and electrochemical properties of Li[Ni0.45Co0.1Mn0.45−xZrx]O2 (x = 0, 0.02) via co-precipitation method

Li[Ni_0.45Co_0.1Mn_0.45−xZr_x]O₂ (x = 0, 0.02) was synthesized via co-precipitation method. Partial Zr doping on the host structure of Li[Ni_0.45Co_0.1Mn_0.45]O₂ was carried out to improve the electrochemical properties. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.43Zr_0.02]O₂ was evaluated in terms of specific discharge capacity, cycling performance and thermal stability. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.45−xZr_0.02]O₂ shows the improved cycling performance and stable thermal stability. The major exothermic reaction was delayed from 252.1 °C to 289.4 °C.     

Electrochemical characteristics of Li2−xVTiO4 rock salt phase in Li-ion batteries

Li_2−xVTiO₄/C sample with a disordered rock salt structure was successfully prepared by annealing at a temperature of 850 °C. The electrochemical oxidation in the first cycle occurs at voltages above 4 V vs. metallic lithium, while the shapes of the electrochemical curves in consequent reduction-oxidation processes show a monotonous change of the potential between the selected cut-off voltages. A linear combination fit of individual XANES spectra was used for the determination of the oxidation states of as prepared sample and intermediate states during oxidation and reduction. In the as-prepared sample, vanadium was found to be in the average oxidation state of V^3.5+ and was additionally oxidized to V^3.8+ by the electrochemical charging. During the discharge process, the vanadium oxidation state was reduced to V^3.0+. In situ X-ray diffraction patterns and EXAFS analysis suggest good structural stability during oxidation and reduction, which is also reflected in the cycling stability if batteries were cycled in the voltage window between 2.0 V and 4.4 V. Extension of the lower cut-off voltage to 1.0 V doubles the capacity retention with the improved capacity stability if compared with several high capacity vanadium based materials.     

Structural and transport properties of layered Li1+x(Mn1/3Co1/3Ni1/3)1−xO2 oxides prepared by a soft chemistry method

In this work structural and transport properties of layered Li_1+x(Mn_1/3Co_1/3Ni_1/3)_1−xO₂ oxides (x = 0; 0.03; 0.06) prepared by a “soft chemistry” method are presented. The excessive lithium was found to significantly improve transport properties of the materials, a corresponding linear decrease of the unit cell parameters was observed. The electrical conductivity of Li_1.03(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ composition was high enough to use this material in a form of a pellet, without any additives, in lithium batteries and characterize structural and transport properties of deintercalated Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ compounds. For deintercalated samples a linear increase of the lattice parameter c together with a linear decrease of the parameter a with the increasing deintercalation degree occurred, but only up to 0.4-0.5 mol of extracted lithium. Further deintercalation showed a reversal of the trend. Electrical conductivity measurements performed of Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ samples (y = 0.1; 0.3; 0.5; 0.6) showed an ongoing improvement, almost two orders of magnitude, in relation to the starting composition. Additionally, OCV measurements, discharge characteristics and lithium diffusion coefficient measurements were performed for Li/Li⁺/Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ cells.     

LiFexMn1−xPO4: A cathode for lithium-ion batteries

The high redox potential of LiMnPO₄, ∼4.0 vs. (Li⁺/Li), and its high theoretical capacity of 170 mAh g⁻¹ makes it a promising candidate to replace LiCoO₂ as the cathode in Li-ion batteries. However, it has attracted little attention because of its severe kinetic problems during cycling. Introducing iron into crystalline LiMnPO₄ generates a solid solution of LiFe_xMn_1−xPO₄ and increases kinetics; hence, there is much interest in determining the Fe-to-Mn ratio that will optimize electrochemical performance. To this end, we synthesized a series of nanoporous LiFe_xMn_1−xPO₄ compounds (with x = 0, 0.05, 0.1, 0.15, and 0.2), using an inexpensive solid-state reaction. The electrodes were characterized using X-ray diffraction and energy-dispersive spectroscopy to examine their crystal structure and elemental distribution. Scanning-, tunneling-, and transmission-electron microscopy (viz., SEM, STEM, and TEM) were employed to characterize the micromorphology of these materials; the carbon content was analyzed by thermogravimetric analyses (TGAs). We demonstrate that the electrochemical performance of LiFe_xMn_1−xPO₄ rises continuously with increasing iron content. In situ synchrotron studies during cycling revealed a reversible structural change when lithium is inserted and extracted from the crystal structure. Further, introducing 20% iron (e.g., LiFe_0.2Mn_0.8PO₄) resulted in a promising capacity (138 mAh g⁻¹ at C/10), comparable to that previously reported for nano-LiMnPO₄.     

Influence of lithium content on high rate cycleability of layered Li1+xNi0.30Co0.30Mn0.40O2 cathodes for high power lithium-ion batteries

Layered Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) materials have been synthesized using citric acid assisted sol-gel method. The materials with excess lithium showed distinct differences in the structure and the charge and discharge characteristics. The rate capability tests were performed and compared on Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) cathode materials. Among these materials, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode demonstrated higher discharge capacity than that of the other cathodes. Upon extended cycling at 1C and 8C, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ showed better capacity retention when compared to other materials with different lithium content. Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ exhibited 93 and 90% capacity retention where as Li_1.05Ni_0.30Co_0.30Mn_0.40O₂, Li_1.15Ni_0.30Co_0.30Mn_0.40O₂, and Li_1.00Ni_0.30Co_0.30Mn_0.40O₂ exhibited only 84, 71, and 63% (at 1C), and 79, 66 and 40% (at 10C) capacity retention, respectively, after 40 cycles. The enhanced high rate cycleability of Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode is attributed to the improved structural stability due to the formation of appropriate amount of Li₂MnO₃-like domains in the transition metal layer and decreased Li/Ni disorder (i.e., Ni content in the Li layer).     

Synthesis and performance of carbon-coated Li3V2(PO4)3 cathode materials by a low temperature solid-state reaction

The carbon-coated monoclinic Li₃V₂(PO₄)₃ (LVP) cathode materials can be synthesized by a low temperature solid-state reaction route. The influences of different heat treatments on the LVP have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods. In the range of 3.0-4.3 V, both LVP/C electrodes present good rate capability and excellent cyclic performance. It is found that the sample (LVP1/C) prepared by the two-step heat treatment with pre-sintering at 350 °C delivers the initial discharge capacity of 99.8 mAh g⁻¹ at 10 C charge-discharge rate and still retains 95.8 mAh g⁻¹ after 300 cycles. For the sample (LVP2/C) synthesized by the one-step heat treatment, 95.9 and 90.0 mAh g⁻¹ are obtained in the 1st and 300th cycles at 10 C rate, respectively. Our results based on the XRD patterns and the SEM images suggest that the good rate capability and cyclic performance may be owing to the pure phases, small particles, large specific surface areas and residual carbon. In the range of 3.0-4.8 V, compared with the LVP2/C, the LVP1/C also exhibits better performance.     

Synthesis and electrochemical performances of core-shell structured Li[(Ni1/3Co1/3Mn1/3)0.8(Ni1/2Mn1/2)0.2]O2 cathode material for lithium ion batteries

Micro-scale core-shell structured Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ powders for use as cathode material are synthesized by a co-precipitation method. To protect the core material Li[Ni_1/3Co_1/3Mn_1/3]O₂ from structural instability at high voltage, a Li[Ni_1/2Mn_1/2]O₂ shell, which provides structural and thermal stability, is used to encapsulate the core. A mixture of the prepared core-shell precursor and lithium hydroxide is calcined at 770 °C for 12 h in air. X-ray diffraction studies reveal that the prepared material has a typical layered structure with an space group. Spherical morphologies with mono-dispersed powders are observed in the cross-sectional images obtained by scanning electron microscopy. The core-shell Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ electrode has an excellent capacity retention at 30 °C, maintaining 99% of its initial discharge capacity after 100 cycles in the voltage range of 3-4.5 V. Furthermore, the thermal stability of the core-shell material in the highly delithiated state is improved compared to that of the core material. The resulting exothermic onset temperature appear at approximately 272  °C, which is higher than that of the highly delithiated Li[Ni_1/3Co_1/3Mn_1/3]O₂ (261 °C).     

Synthesis, structure and electrochemical Li insertion behaviour of Li2Ti6O13 with the Na2Ti6O13 tunnel-structure

Li₂Ti₆O₁₃ has been prepared from Na₂Ti₆O₁₃ by Li ion exchange in molten LiNO₃ at 325 °C. Chemical analysis and powder X-ray diffraction study of the reaction product respectively indicate that total Na/Li exchange takes place and the Ti-O framework of the Na₂Ti₆O₁₃ parent structure is kept under those experimental conditions. Therefore, Li₂Ti₆O₁₃ has been obtained with the mentioned parent structure. An important change is that particle size is decreased significantly which is favoring lithium insertion. Electrochemical study shows that Li₂Ti₆O₁₃ inserts ca. 5 Li per formula unit in the voltage range 1.5-1.0 V vs. Li⁺/Li, yielding a specific discharge capacity of 250 mAh g⁻¹ under equilibrium conditions. Insertion occurs at an average equilibrium voltage of 1.5 V which is observed for oxides and titanates where Ti(IV)/Ti(III) is the active redox couple. However, a capacity loss of ca. 30% is observed due to a phase transformation occurring during the first discharge. After the first redox cycle a high reversible capacity is obtained (ca. 160 mAh g⁻¹ at C/12) and retained upon cycling. Taking into consideration these results, we propose Li₂Ti₆O₁₃ as an interesting material to be further investigated as the anode of lithium ion batteries.     

A new ternary Li4FeSb2 structure formed upon discharge of the FeSb2/Li cell

We report here the performance of FeSb₂ used as negative electrode for Li-ions batteries. A capacity of 400 mAh g⁻¹ and 3500 mAh cm⁻³ can be retained after 10 cycles at a C/10 cycling rate. We studied in particular the electrochemical mechanism during the first discharge of the FeSb₂/Li battery. Both Mössbauer spectroscopy (⁵⁷Fe and ¹²¹Sb) and in situ XRD studies show the formation of an unknown fcc-phase during the first step. A second step shows the conversion process leading to Li₃Sb at the end of the discharge. The structure of the intermediate new phase Li₄FeSb₂ is isotype to the face-centered-cubic Li₃Sb phase in which one Fe atom is substituted by two Li atoms.