Anomalous capacity and cycling stability of xLi2MnO3 · (1 − x)LiMO2 electrodes (M = Mn,Ni, Co) in lithium batteries at 50 °C

Transition metal oxides with composite xLi₂MnO₃ ·  (1 − x)LiMO₂ rocksalt structures (M = Mn, Ni, Co) are of interest as a new generation of cathode materials for high energy density lithium-ion batteries. After electrochemical activation to 4.6 or 4.8 V (vs. Li⁰) at 50 °C, xLi₂MnO₃ · (1 − x)LiMn_0.33Ni_0.33Co_0.33O₂ (x = 0.5, 0.7) electrodes deliver initial discharge capacities (>300 mAh/g) at a low current rate (0.05 mA/cm²) that exceed the theoretical values for lithiation back to the rocksalt stoichiometry (240–260 mAh/g), at least during the early charge/discharge cycles of the cells. Attention is drawn to previous reports of similar, but unaccounted and unexplained anomalous behavior of these types of electrode materials. Possible reasons for this anomalous capacity are suggested. Indications are that electrodes in which M = Mn, Ni and Co do not cycle with the same stability at 50 °C as those without cobalt.     

Electrochemical properties of Li(Li(1−x)/3CoxMn(2−2x)/3)O2 (0 ⩽ x ⩽ 1) solid solutions prepared by poly-vinyl alcohol (PVA) method

The whole range of solid solutions Li(Li_(1−x)/3Co_xMn_(2−2x)/3)O₂ (0 ⩽ x ⩽ 1) was firstly synthesized by an aqueous solution method using poly-vinyl alcohol as a synthetic agent to investigate their structure and electrochemical properties. X-ray diffraction results indicated that the synthesized solid solutions showed a single phase without any detectable impurity phase and have a hexagonal structure with some additional peaks caused by monoclinic distortion, especially in the solid solutions with a low Co amount. In the electrochemical examination, the solid solutions in the range between 0.2 ⩽ x ⩽ 0.9 showed higher discharge capacity and better cyclability than LiCoO₂ (x = 1) on cycling between 2.0 and 4.6 V with 100 mA g⁻¹ at 25 °C. For example, Li(Li_0.2Co_0.4Mn_0.4)O₂ (x = 0.4) exhibited a high discharge capacity of 180 mA h g⁻¹ at the 50th cycle. By synthesizing the solid solution between Li₂MnO₃ and LiCoO₂, the electrochemical properties of the end members were improved.     

ZrO2- and Li2ZrO3-stabilized spinel and layered electrodes for lithium batteries

Strategies for countering the solubility of LiMn₂O₄ (spinel) electrodes at 50 °C and for suppressing the reactivity of layered LiMO₂ (M=Co, Ni, Mn, Li) electrodes at high potentials are discussed. Surface treatment of LiMn₂O₄ with colloidal zirconia (ZrO₂) dramatically improves the cycling stability of the spinel electrode at 50 °C in Li/LiMn₂O₄ cells. ZrO₂-coated LiMn_0.5Ni_0.5O₂ electrodes provide a superior capacity and cycling stability to uncoated electrodes when charged to a high potential (4.6 V vs Li⁰). The use of Li₂ZrO₃, which is structurally more compatible with spinel and layered electrodes than ZrO₂ and which can act as a Li⁺-ion conductor, has been evaluated in composite 0.03Li₂ZrO₃ · 0.97LiMn_0.5Ni_0.5O₂ electrodes; glassy Li_xZrO_2 + x/2 (0<x⩽2) products can be produced from colloidal ZrO₂ for surface coatings.     

Lithium–manganese–nickel-oxide electrodes with integrated layered–spinel structures for lithium batteries

A series of lithium–manganese–nickel-oxide compositions that can be represented in three-component notation, xLi[Mn_1.5Ni_0.5]O₄ · (1 − x){Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂}, in which a spinel component, Li[Mn_1.5Ni_0.5]O₄, and two layered components, Li₂MnO₃ and Li(Mn_0.5Ni_0.5)O₂, are structurally integrated in a highly complex manner, have been evaluated as electrodes in lithium cells for x = 1, 0.75, 0.50, 0.25 and 0. In this series of compounds, which is defined by the Li[Mn_1.5Ni_0.5]O₄–{Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂} tie-line in the Li[Mn_1.5Ni_0.5]O₄–Li₂MnO₃–Li(Mn_0.5Ni_0.5)O₂ phase diagram, the Mn:Ni ratio in the spinel and the combined layered Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂ components is always 3:1. Powder X-ray diffraction patterns of the end members and the electrochemical profiles of cells with these electrodes are consistent with those expected for the spinel Li[Mn_1.5Ni_0.5]O₄ (x = 1) and for ‘composite’ Li₂MnO₃ · Li(Mn_0.5Ni_0.5)O₂ layered electrode structures (x = 0). Electrodes with intermediate values of x exhibit both spinel and layered character and yield extremely high capacities, reaching more than 250 mA h/g with good cycling stability between 2.0 V and 4.95 V vs. Li° at a current rate of 0.1 mA/cm².     

Structural and magnetic properties of orthorhombic LixMnO2

Rietveld refinement of the crystal and magnetic structures of Li_xMnO₂ (x = 0.98, 1.00, 1.02) are performed using neutron and X-ray measurements. A significant structural disorder due to the presence of manganese ions in lithium positions (Mn_Li) and lithium ions in manganese ones (Li_Mn) is found to be a common feature of Li_0.98MnO₂, Li_1.00MnO₂, and Li_1.02MnO₂.An essential anisotropy of the thermal-expansion coefficients of the lithium manganese oxides is observed in the temperature range of 1.5–300 K. Furthermore, the distortion of the oxygen octahedral environment around the manganese ions decreases when the temperature lowers. This is attributed to the strong exchange interactions between parallel exchange-coupled Mn chains. First-principles calculations of the effective exchange-interaction parameters in Li₁₆Mn₁₆O₃₂ confirm the essential antiferromagnetic interactions between the chains. In addition, a hypothetical (Li₁₅Mn)Mn₁₆O₃₂ structure where a lithium atom located between the Mn double layers is replaced by a manganese atom is considered. The calculations reveal that the presence of such defects results in appearance of a ferromagnetic component that agrees with the magnetic measurements.     

Enhancing the rate capability of high capacity xLi2MnO3 · (1 − x)LiMO2 (M = Mn,Ni, Co) electrodes by Li–Ni–PO4 treatment

The rate capability of high capacity xLi₂MnO₃ · (1 ? x)LiMO₂ (M = Mn, Ni, Co) electrodes for lithium-ion batteries has been significantly enhanced by stabilizing the electrode surface by reaction with a Li–Ni–PO₄ solution, followed by a heat-treatment step. Reversible capacities of 250 mAh/g at a C/11 rate, 225 mAh/g at C/2 and 200 mAh/g at C/1 have been obtained from 0.5Li₂MnO₃ · 0.5LiNi_0.44Co_0.25Mn_0.31O₂ electrodes between 4.6 and 2.0 V. The data bode well for their implementation in batteries that meet the 40-mile range requirement for plug-in hybrid vehicles.     

LiMn2O4 hollow nanosphere electrode material with excellent cycling reversibility and rate capability

Lithium-rich Li_1.05Mn₂O₄ hollow nanospheres have been successfully prepared by air-calcining lithiated MnO₂ precursor at a low temperature of 550 °C, which was synthesized by chemical lithiation of hollow MnO₂ nanospheres with LiI at 70 °C for 12 h. The lithium-rich Li_1.05Mn₂O₄ hollow nanospheres exhibit an excellent cycling stability and rate capability as a cathode material for rechargeable lithium batteries: it maintains 90% of its initial capacity after 500 cycles, and keeps 70% of the reversible capacity at 0.1 C rat, even at 15 C rate.     

Structures,thermal expansion properties and phase transitions of ErxFe2−x(MoO4)3 (0.0 ≤ x ≤ 2.0)

Structures, thermal expansion properties and phase transitions of Er_xFe_2−x(MoO₄)₃ (0.0 ≤ x ≤ 2.0) have been investigated by X-ray diffraction and differential thermal analysis. The partial substitution of Er³⁺ for Fe³⁺ induces pronounced decreases in the phase transition temperature from monoclinic to orthorhombic structure. Rietveld analysis of the XRD data shows that both the monoclinic and orthorhombic Fe₂(MoO₄)₃, as well as the orthorhombic Er_xFe_2−x(MoO₄)₃ (x ≤ 0.8) have positive thermal expansion coefficients. However, the linear thermal expansion coefficients of Er_xFe_2−x(MoO₄)₃ (x = 0.6–2.0) decrease with increasing content of Er³⁺ and for x ≥ 1.0, compounds Er_xFe_2−x(MoO₄)₃ show negative thermal expansion properties. Attempts for making zero thermal expansion coefficient materials result in that very low negative thermal expansion coefficient of −0.60 × 10⁻⁶/°C in Er_1.0Fe_1.0(MoO₄)₃ is observed in the temperature range of 180–400 °C, and zero thermal expansion is observed in Er_0.8Fe_1.2(MoO₄)₃ in the temperature range of 350–450 °C. In addition, anisotropic thermal expansions are found for all the orthorhombic Er_xFe_2−x(MoO₄)₃ compounds, with negative thermal expansion coefficients along the a axes.     

XAFS characterization of La1−xSrxMnO3±δ catalysts prepared by Pechini’s method

XAFS experiments at the Mn and Sr K-edges were carried out in order to investigate the short-range arrangement of Mn and Sr sites on La_1−xSr_xMnO_3±δ highly doped perovskites (x = 0, 0.2, 0.4 and 0.6). The Mn K-edge EXAFS spectra show a static Jahn–Teller distortion of the MnO₆ for x = 0 and 0.2, which is drastically reduced as x increases. The distortion of perovskite, characterized by the Mn–O–Mn tilt angle, progressively decreases with increasing Sr contents. Sr K-edge results indicated a decrease on the Sr–Mn coordination number upon Sr doping. Based on this and TPD results, a charge compensating mechanism is proposed suggesting a partial Mn oxidation and formation of Mn defect vacancies due to the introduction of Sr.     

Comments on stabilizing layered manganese oxide electrodes for Li batteries

An electrochemical study of structurally-integrated xLi₂MnO₃•(1 −x)LiMn_0.5Ni_0.5O₂ ‘composite’ materials has been undertaken to investigate the stability of electrochemically-activated electrodes at the Li₂MnO₃-rich end of the Li₂MnO₃–LiMn_0.5Ni_0.5O₂ tie-line, i.e., for 0.7 ≤ x ≤ 0.95. Excellent performance was observed for x = 0.7 in lithium half-cells; comparable to activated electrodes that have significantly lower values of x and are traditionally the preferred materials of choice. Electrodes with higher manganese content (x ≥ 0.8) showed significantly reduced performance. Implications for stabilizing low-cost, manganese-rich, layered lithium-metal-oxide electrode materials are discussed.     

Li0.93[Li0.21Co0.28Mn0.51]O2 nanoparticles for lithium battery cathode material made by cationic exchange from K-birnessite

Li_0.93[Li_0.21Co_0.28Mn _0.51]O₂ nanoparticles with an R-3m space group is hydrothermally prepared from Co_0.35Mn_0.65O₂ obtained from an ion-exchange reaction with K-birnessite K_0.32MnO₂ at 200 °C. Even at a hydrothermal reaction temperature of 150 °C, the spinel (Fd3m) phase is dominant, and a layered phase became dominant by combining an increase in the temperature to 200 °C with an increase in lithium concentration. The as-prepared cathode particle has plate-like hexagonal morphology with a size of 100 nm and thickness of 20 nm. The first discharge capacity of the cathode is 258 mAh/g with an irreversible capacity ratio of 22%, and the capacity retention after 30 cycles is 95% without developing a plateau at ∼3 V. Capacity retention of the cathode discharge is 84% at 4C rate (=1000 mA/g) and shows full capacity recovery when decreasing the C rate to 0.1 C.     

Enhancement of lithium ion conduction in the cubic rare earth oxide

A new type of lithium ion conducting solid electrolyte based on a cubic rare earth oxide was developed by co-doping LiNO₃ and KNO₃ into a (Gd_1−xNd_x)₂O₃ solid, which possesses large interstitial open spaces within the structure. Among the samples prepared, 0.6(Gd_0.4Nd_0.6)₂O₃–0.16LiNO₃–0.24KNO₃ exhibits the highest lithium ion conductivity of 8.05 × 10⁻² and 1.35 × 10⁻³ S cm⁻¹ at 400 and 100 °C, respectively, which is comparable to that of the LISICON materials. Pure Li⁺ ion conduction was successfully demonstrated by the dc electrolysis method.     

Li2CuTi3O8–Li4Ti5O12 double spinel anode material with improved rate performance for Li-ion batteries

Ti-based anode materials with the nominal compositions Li₄Ti₅Cu_xO_12 + x (x = 0, 0.075, 0.15, 0.3, 0.6, 1.20 and 1.67) were synthesized at 800 °C by a solid-state reaction process. X-ray diffraction analysis indicated that the sintered samples were composed of intergrown spinel-type Li₄Ti₅O₁₂ and Li₂CuTi₃O₈, and a small amount of Li₂O. Scanning electron microscopy, electrical resistance measurement and galvanostatic cell cycling were also employed to characterize the structure and properties of the double spinel samples. It is proposed that the first lithiation of the component Li₂CuTi₃O₈ leads to the in situ production of Cu that can significantly improve the rate performance of Li₄Ti₅Cu_xO_12 + x. The optimal nominal composition is Li₄Ti₅Cu_0.15O_12.15.     

A new carbon fuel cell with high power output by integrating with in situ catalytic reverse Boudouard reaction

Solid carbon was investigated as the fuel for an intermediate-temperature solid oxide fuel cell (IT-SOFC). An innovative, indirect operating method involving internal catalytic gasification of carbon to gaseous carbon monoxide via the reverse Boudouard reaction (C(s) + CO₂(g) → 2CO(g)) was proposed. The carbon gasification reaction rate was greatly enhanced by adopting Fe_mO_n–M_xO (M = Li, K, Ca) as a catalyst. A peak power density of ～297 mW cm^?2 was achieved at 850 °C for an anode-supported SOFC with scandium-stabilized zirconia electrolyte and a La_0.8Sr_0.2MnO₃ cathode by applying a catalyst-loaded, activated carbon as fuel. This peak power density was only modestly lower than that obtained using gaseous hydrogen as the fuel.     

Effects of Fe doping on the electrochemical performance of LiCoPO4/C composites for high power-density cathode materials

LiCo_1?xFe_xPO₄/C composites with various amounts of Fe (x = 0, 0.05 and 0.1) were synthesized by vibrant type ball-milling coupled with microwave heating to investigate the role of doped Fe²⁺ in LiCo_1?xFe_xPO₄/C composites. The initial charge–discharge curves and cyclic voltammetry profiles of LiCo_1?xFe_xPO₄/C composites apparently featured an improved kinetic property compared to LiCoPO₄. It was observed that the initial discharge capacity (120 mA hg^?1) of LiCo_0.95Fe_0.05PO₄ is higher than that (108 mA hg^?1) of LiCoPO₄ and the difference between the oxidation–reduction peaks is getting smaller with the increase of Fe doping. The electrochemical improvement in LiCo_1?xFe_xPO₄/C composites could be attributed to the enhanced Li⁺ diffusivity induced by the enlargement of 1D channel in polyanion structure of LiCoPO₄.     

Doping effects of Yb3+ on the crystal structures,nanoparticle properties and electrical behaviors of ZrO2 derived from a facile urea-based hydrothermal route

Weakly-agglomerated nanocrystalline (ZrO₂)_1−x(Yb₂O₃)_x (x=0.02–0.2) powders with high surface area (109–151 m² g⁻¹) were synthesized by a two-step hydrothermal process in the presence of urea: a stock solution of metal nitrates and urea was heated at 80 °C for 24 h and then at 180 °C for 48 h. For x=0.04–0.2, the as-derived powders were an assembly of uniform nanoparticles with well-defined edges in the size between 6.1–8.4 nm. Before and after calcination at 800 °C, the lattice parameters, microstrain and surface area of the (ZrO₂)_1−x(Yb₂O₃)_x samples tended to increase with Yb³⁺ concentration; while, the average crystallite size decreased correspondingly. In the Arrhenius plots over the measurement temperature range of 400–800 °C, the bulk ionic conductivity of the compacts sintered at 1400 °C for 24 h showed a maximum value at the composition of x=0.08 in cubic structure, with an activation energy of 0.89 eV. At 800 °C, σ_b=0.049 S cm⁻¹ for x=0.08.     

Synthesis of nano-crystalline LiFeO2 material with advanced battery performance

LiFeO₂ has been synthesized at low temperature (150 °C) using the solid-state method. It was composed of orthorhombic LiFeO₂ and small amount of spinel LiFe₅O₈ phases. A Li/LiFeO₂ cell showed not only a fairly high initial discharge capacity of over 150 mAh/g, but also a good cycle retention rate at room temperature. During the cycling test, the Li/LiFeO₂ cell exhibited a unique abrupt capacity drop near the 13th cycle and continuously showed an excellent cycling performance of over 99% for 25 cycles. We found that the orthorhombic LiFeO₂ underwent a structural change to the spinel phase during the charge/discharge process which resulted in the capacity decline during the long-term cycling.     

Structural change and capacity loss mechanism in orthorhombic Li/LiFeO2 system during cycling

Orthorhombic LiFeO₂ was synthesized at low temperature (150 °C) using a solid-state method. The Li/LiFeO₂ cell presented not only a high initial capacity of over 150 mAh/g, but also fairly good cycle retention of 73% after 50 cycles within a voltage range between 1.5 and 4.5 V. It was found that the orthorhombic phase of the LiFeO₂ material underwent a structural change to the spinel phase during cycling. Especially it showed severe structural changes during the first charge/discharge process, which might be the main reason to induce the capacity loss of the Li/LiFeO₂ system. We reported a new observation about the structural change mechanism of the orthorhombic Li/LiFeO₂ cell during cycling using in situ XRD and TEM analyses.     

LixV2O5 nanobelts for high capacity lithium-ion battery cathodes

5–10 μm long, typically 200–300 nm wide, and several nanometers thick Li_xV₂O₅ (× ～ 0.8) nanobelts with the δ-type crystal structure were synthesized by a hydrothermal treatment of Li⁺-exchanged V₂O₅ gel. When dried at 200 °C under vacuum prior to electrochemical testing, the as-prepared nanobelts underwent the well-known δ → ε → γ-phase transition giving a mixture of ε and γ phases as a nanocomposite electrode material. Such a simple preparation procedure guarantees a yield of material with drastically enhanced initial discharge specific capacity of 490 mAh/g and great cyclability. The enhanced electrochemical performance is attributed to the complex of experimental procedures including post-synthesis treatment of the single-crystalline Li_xV₂O₅ nanobelts.     

High-energy and high-power Li-rich nickel manganese oxide electrode materials

A lithium-rich nickel-manganese oxide compound Li_x(Ni_0.25Mn_0.75)O_y (x > 1) was synthesized from layered Na_0.9Li_0.3Ni_0.25Mn_0.75O_δ precursor using a lithium ion-exchange reaction. The electrochemical behavior of the material as a cathode for lithium batteries, and a preliminary discussion of its structure are reported. The product Li_1.32Na_0.02Ni_0.25Mn_0.75O_y (IE-LNMO) shows broad X-ray diffraction peaks, but possesses a high intensity sharp (003) layering peak and multiple peaks with intensity in the 20–23° 2θ region which suggest Ni–Mn ordering in the transition metal layer (TM). Li/IE-LNMO cells demonstrate very stable reversible capacities of 220 mAh/g @ 15 mA/g and possess extremely high power of 150 mAh/g @ 1500 mA/g (15C). The Li/IE-LNMO cell dQ/dV plot exhibits three reversible electrochemical processes due to Ni/Mn redox behavior in a layered component, and Mn redox exchange in a spinel component. No alteration in the dQ/dV curves and no detectable change in the voltage profiles over 40 cycles were observed, thus indicating a stable structure for lithium insertion/extraction. This new material is attractive for demanding Li-ion battery applications.