Structural and electrochemical study of the reaction of lithium with silicon nanowires

The structural transformations of silicon nanowires when cycled against lithium were evaluated using electrochemical potential spectroscopy and galvanostatic cycling. During the charge, the nanowires alloy with lithium to form an amorphous Li_xSi compound. At potentials <50 mV, a structural transformation occurs. In studies on micron-sized particles previously reported in the literature, this transformation is a crystallization to a metastable Li₁₅Si₄ phase. X-ray diffraction measurements on the Si nanowires, however, show that they are amorphous, suggesting that a different amorphous phase (Li_ySi) is formed. Lithium is removed from this phase in the discharge to form amorphous silicon. We have found that limiting the voltage in the charge to 70 mV results in improved efficiency and cyclability compared to charging to 10 mV. This improvement is due to the suppression of the transformation at low potentials, which alloys for reversible cycling of amorphous silicon nanowires.     

Electrochemical behaviour of Heusler alloy Co2MnSi for secondary lithium batteries

Electrochemical lithiation of Co₂MnSi with a Heusler structure is investigated as a candidate negative electrode (anode) material for secondary lithium batteries. The electrode maintains a reversible discharge capacity of 112 mAh g⁻¹ for 50 cycles when cycled between 0.01 and 3 V. It is proposed that the lithiation mechanism consists of two steps. Co₂MnSi transforms to Heusler-type Li₂MnSi during the first charge cycle and subsequent charge–discharge cycles involve the formation of a solid solution in Li_xMnSi. The latter compound maintains its structural integrity throughout cycling to provide steady cycling behaviour. Magnetic measurements are also employed to substantiate further the structural changes during electrochemical cycling.     

Layered lithium cobalt nitrides: A new class of lithium intercalation compounds

Lithium cobalt nitrides Li_3−2xCo_xN (0.1 ≤ x ≤ 0.44) have been prepared and investigated as negative electrode in the 1/0.02 V potential window. The evolution of the unit cell parameters and unit cell volume with the Co content show a solid solution behaviour. Whatever the Co content, all these nitrides are electroactive with a single step around 0.6 V/0.7 V for the discharge and charge processes, respectively. The electrochemical behaviour observed is typical of a Li intercalation compound and involves the Co²⁺/Co⁺ redox couple in the interlayer plane combined with the reversible accommodation of Li⁺ ions in the cation vacancies located in Li₂N⁻ layers. XRD experiments performed after discharge, charge and cycling tests clearly indicate the hexagonal layered structure of the host lattice is maintained. This intercalation process explains the excellent capacity retention found after 50 cycles. A specific capacity of 180 mAh g⁻¹ at C/20 and 130 mAh g⁻¹ at C/5 rate (100 mA cm⁻²) is achieved for Li_2.23Co_0.39N. ac impedance measurements have allowed to characterize the kinetics of the reaction.     

Temperature effect on the graphite exfoliation in propylene carbonate based electrolytes

Graphite exfoliation at a low potential has long been an issue for lithium-ion cells using a propylene carbonate (PC) based electrolyte. Two different mechanisms have been proposed in literature to explain this structural degradation. In this study, the initial lithium intercalation temperature is found to have a great impact on the extent of the graphite exfoliation. At an elevated temperature, the exfoliation can be largely suppressed and the irreversible capacity loss is reduced substantially. After the initial cycling at 50 °C, the graphite anode can be cycled in a PC-based electrolyte at room temperature without the exfoliation problem. It is also discovered that such a graphite anode gives rise to a specific capacity of over 372 mAh g⁻¹ at 50 °C and a room temperature capacity higher than that of a graphite anode with the initial lithium intercalation at room temperature. This finding sheds a new light on the exfoliation mechanism. It may lead to a simple cycling procedure that allows us to make rechargeable lithium-ion batteries with better safety and higher capacity.     

Effect of Cr doping on the structural,electrochemical properties of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Crx]O2 (x = 0, 0.02, 0.04, 0.06, 0.08) as cathode materials for lithium secondary batteries

Prospective positive-electrode (cathode) materials for a lithium secondary battery, viz., Li[Li_0.2Ni_0.2−x/2Mn_0.6−x/2Cr_x]O₂ (x = 0, 0.02, 0.04, 0.06, 0.08), were synthesized using a solid-state pyrolysis method. The structural and electrochemical properties were examined by means of X-ray diffraction, cyclic voltammetry, SEM and charge–discharge tests. The results demonstrated that the powders maintain the α-NaFeO₂-type layered structure regardless of the chromium content in the range x ≤ 0.08. The Cr doping of x = 0.04 showed improved capacity and rate capability comparing to undoped Li[Li_0.2Ni_0.2Mn_0.6]O₂. ac impedance measurement showed that Cr-doped electrode has the lower impedance value during cycling. It is considered that the higher capacity and superior rate capability of Cr-doping samples would be ascribed to the reduced resistance of the electrode during cycling.     

A transmission electron microscopy study of crack formation and propagation in electrochemically cycled graphite electrode in lithium-ion cells

Subsurface defects and local compositional changes that occurred in graphite anodes subjected to cyclic voltammetry tests (vs. Li/Li⁺, using an electrolyte consisting of 1 M LiClO₄ in a 1:1 volumetric mixture of ethylene carbonate and 1,2-dimethoxy ethane) were investigated using high-resolution transmission electron microscopy (HR-TEM). Cross-sections of anodes prepared by focused ion beam (FIB) milling indicated that graphite layers adjacent to solid electrolyte (SEI)/graphite interface exhibited partial delamination due to the formation of interlayer cracks. The SEI layer formed on the graphite surface consisted of Li₂CO₃ that was identified by {1 1 0} and {0 0 2} crystallographic planes. Lithium compounds, LiC₆, Li₂CO₃ and Li₂O, were observed on the surfaces of separated graphite layers. Deposition of these co-intercalation compounds near the crack tip caused partial closure of propagating graphite cracks during electrochemical cycling, and possibly reduced the crack growth rate. Graphite fibres that were observed to bridge crack faces likely provided an additional mechanism for the retardation of crack propagation.     

Anode properties of titanium oxide nanotube and graphite composites for lithium-ion batteries

Titanium oxide nanotube and graphite composites are prepared by adding graphite before and after a hydrothermal reaction to enhance the cyclic performance and high-rate capability of lithium-ion batteries. The composite powders, their anode electrodes, and lithium half-cells containing the anodes are characterized by means of morphological and crystalline analysis, Raman spectroscopy, cyclic voltammetry, impedance spectroscopy, and repeated discharge-charge cycling at low and high C-rates. Notably, the composite anode (R5G5-T) that concurrently uses natural graphite and rutile particles before the hydrothermal reaction shows superior high-rate capability and achieves a discharge capacity of ca. 70 mAh g⁻¹ after 100 cycles at 50 C-rate. This may be due to the high-rate supercapacitive reactions of the TiO₂ nanotube on the graphite surface caused by a diffusion-controlled or a charge-transfer process.     

Electrochemistry and safety of Li4Ti5O12 and graphite anodes paired with LiMn2O4 for hybrid electric vehicle Li-ion battery applications

A promising anode material for hybrid electric vehicles (HEVs) is Li₄Ti₅O₁₂ (LTO). LTO intercalates lithium at a voltage of ∼1.5 V relative to lithium metal, and thus this material has a lower energy compared to a graphite anode for a given cathode material. However, LTO has promising safety and cycle life characteristics relative to graphite anodes. Herein, we describe electrochemical and safety characterizations of LTO and graphite anodes paired with LiMn₂O₄ cathodes in pouch cells. The LTO anode outperformed graphite with regards to capacity retention on extended cycling, pulsing impedance, and calendar life and was found to be more stable to thermal abuse from analysis of gases generated at elevated temperatures and calorimetric data. The safety, calendar life, and pulsing performance of LTO make it an attractive alternative to graphite for high power automotive applications, in particular when paired with LiMn₂O₄ cathode materials.     

Preparation and characterization of Ti-doped LiFePO4 cathode materials for lithium-ion batteries

Olivine structured LiFePO₄ (lithium iron phosphate) and Ti⁴⁺-doped LiFe_1−xTi_xPO₄ (0.01 ≤ x ≤ 0.09) powders were synthesized via a solution route followed by heat-treatment at 700 °C for 8 h under N₂ flowing atmosphere. The compositions, crystalline structure, morphology, carbon content, and specific surface area of the prepared powders were investigated with ICP-OES, XRD, TEM, SEM, EA, and BET. Capacity retention study was used to investigate the effects of Ti⁴⁺ partial substitution on the intercalation/de-intercalation of Li⁺ ions in the olivine structured cathode materials. Among the prepared powders, LiFe_0.97Ti_0.03PO₄ manifests the most promising cycling performance as it was cycled with C/10, C/5, C/2, 1C, 2C, and 3C rate. It showed initial discharge capacity of 135 mAh g⁻¹ at 30 °C with C/10 rate. From the results of GSAS refinement for the prepared samples, the doped-Ti⁴⁺ ions did not occupy the Fe²⁺ sites as expected. However, the occupancy of the doped Ti⁴⁺ ions are still not clear, and theoretical calculations are needed for further studies. From the variation of lattice parameters calculated by the least square method without refinement, it suggested that Ti⁴⁺-doped LiFePO₄ samples formed solid solutions at low doping levels while TiO₂ was also observed with TEM in samples prepared with doping level higher than 5 mol%.     

Carbonate-modified siloxanes as solvents of electrolyte solutions for rechargeable lithium cells

Influence of mixing carbonate-modified siloxanes into LiPF₆-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) mixed solvent electrolytes on charge-discharge cycling properties of lithium was examined. As the solute, 1 M (M: mol L⁻¹) LiPF₆ was used. As siloxanes, 4-(2-trimethylsilyloxydimethylsilylethyl)-1,3-dioxolan-2-one and 4-(2-bis(trimethylsilyloxy)methylsilylethyl)-1,3-dioxolan-2-one were investigated. These siloxanes are derivatives of butylene cyclic carbonate or vinyl ethylene carbonate. Charge-discharge cycling efficiencies of lithium metal anodes improved and an impedance of anode/electrolyte interface decreased by mixing siloxanes, compared with those in 1 M LiPF₆-EC/MEC alone. Slightly better cycling behavior of natural graphite anode was obtained by adding siloxanes. Si-C/LiCoO₂ cells exhibited better anode utilization and good cycling performance by using 1 M LiPF₆-EC/MEC + siloxane electrolytes. Thermal behavior of electrolyte solutions toward graphite-lithium anodes was evaluated with a differential scanning calorimeter. By adding siloxanes, temperature starting the large heat-output of graphite-lithium anodes with 1 M LiPF₆-EC/MEC electrolyte solutions shifted to higher temperature about 100 °C. However, amount of heat-output did not decrease by adding siloxanes.     

Interfacial reactions between graphite electrodes and propylene carbonate-based solutions: Electrolyte-concentration dependence of electrochemical lithium intercalation reaction

This study examines the electrochemical reactions occurring at graphite negative electrodes of lithium-ion batteries in a propylene carbonate (PC) electrolyte that contains different concentrations of lithium salts such as, LiClO₄, LiPF₆ or LiN(SO₂C₂F₅)₂. The electrode reactions are significantly affected by the electrolyte concentration. In concentrated solutions, lithium ions are reversibly intercalated within the graphite to form stage 1 lithium–graphite intercalation compounds (Li–GICs), regardless of the lithium salt used. On the other hand, electrolyte decomposition and exfoliation of the graphene layers occur continuously in the low-concentration range. In situ analysis with atomic force microscopy reveals that a thin film (thickness of ∼8 nm) forms on the graphite surface in a concentrated solution, e.g., 3.27 mol kg⁻¹ LiN(SO₂C₂F₅)₂/PC, after the first potential cycle between 2.9 and 0 V versus Li⁺/Li. There is no evidence of the co-intercalation of solvent molecules in the concentrated solution.     

Electrochemical properties of doped lithium titanate compounds and their performance in lithium rechargeable batteries

Several substituted titanates of formula Li_4−xMg_xTi_5−xV_xO₁₂ (0 ≤ x ≤ 1) were synthesized (and investigated) as anode materials in rechargeable lithium batteries. Five samples labeled as S1–S5 were calcined (fired) at 900 °C for 10 h in air, and slowly cooled to room temperature in a tube furnace. The structural properties of the synthesized products have been investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transmission infrared (FTIR). XRD explained that the crystal structures of all samples were monoclinic while S1 and S3 were hexagonal. The morphology of the crystal of S1 was spherical while the other samples were prismatic in shape. SEM investigations explained that S4 had larger grain size diameter of 15–16 μm in comparison with the other samples. S4 sample had the highest conductivity 2.452 × 10⁻⁴ S cm⁻¹. At a voltage plateau located at about 1.55 V (vs. Li ⁺), S4 cell exhibited an initial specific discharge capacity of 198 mAh g⁻¹. The results of cyclic voltammetry for Li_4−xMg_xTi_5−xV_xO₁₂ showed that the electrochemical reaction was based on Ti⁴⁺/Ti³⁺ redox couple at potential range from 1.5 to 1.7 V. There is a pair of reversible redox peaks corresponding to the process of Li⁺ intercalation and de-intercalation in the Li–Ti–O oxides.     

Electrochemical behaviors of a Li3N modified Li metal electrode in secondary lithium batteries

A lithium conductive Li₃N film is successfully prepared on Li metal surface by the direct reaction between Li and N₂ gas at room temperature. X-ray diffraction (XRD), Auger electron spectroscopy (AES), cyclic voltammetry (CV), scanning electron microscopy (SEM), AC impedance, cathodic polarization and galvanostatic charge/discharge cycling tests are applied to characterize the film. The experimental results show that the Li₃N protective film is tight and dense with high stability in the electrolyte. Its thickness is more than 159.4 nm and much bigger than that of a native SEI film formed on the lithium surface as received. An exchange current as low as 3.244 × 10⁻⁷ A demonstrates the formation of a complete SEI film at the electrode|electrolyte interface with Li₃N modification. The SEI film is very effective in preventing the corrosion of the Li electrode in liquid electrolyte, leading to a decreased Li|electrolyte interface resistance and an average short distance of 3.16 × 10⁻³ cm for Li ion diffusion from electrolyte to Li surface. The Li cycling efficiency depends on N₂ exposing time and is obviously enhanced by the Li₃N (1 h) modification. After cycling, a dense and homogeneous Li layer deposits on the Li₃N (1 h) modified Li surface, instead of a loose and inhomogeneous layer on the Li surface as received.     

Lithium-storage and cycleability of nano-CdSnO3 as an anode material for lithium-ion batteries

Nano-CdSnO₃ is prepared by thermal decomposition of the precursor, CdSn(OH)₆ at 600 °C for 6 h in air. The material is characterized physically by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and selected-area electron diffraction (SAED) techniques. Nano-CdSnO₃ exhibits a reversible and stable capacity of 475(±5) mAh g⁻¹ (∼5 mol of cycleable Li per mole of CdSnO₃) for at least 40 cycles between 0.005 and 1.0 V at a current rate of 0.13 C. Extensive capacity fading is found when cycling in the range 0.005-1.3 V. Cyclic voltammetry studies complement galvanostatic cycling data and reveal average discharge and charge potentials of 0.2 and 0.4 V, respectively. The proposed reaction mechanism is supported by ex situ XRD, TEM and SAED studies. The electrochemical impedance spectra taken during 1st and 10th cycle are fitted with an equivalent circuit to evaluate impedance parameters and the apparent chemical diffusion coefficient (DLi⁺) of Li. The bulk impedance, R_b, dominates at low voltages (≤0.25 V), whereas the combined surface film and charge-transfer impedance (R_(sf+ct)) and the Warburg impedance dominate at higher voltages, ≥0.25 V. The DLi⁺ is in the range of (0.5-0.9) × 10⁻¹³ cm² s⁻¹ at V = 0.5-1.0 V during the 10th cycle.     

Nano-sized SnSbCux alloy anodes prepared by co-precipitation for Li-ion batteries

Nano-sized SnSbCu_x alloy anode materials are prepared by reductive co-precipitation method combining with the aging treatment in water bath at 80 °C. The microstructure, morphology and electrochemical properties of synthesized SnSbCu_x alloy powders are evaluated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and galvanostatical cycling tests. The results indicate that the average particle size is reduced and the Cu₆Sn₅, Cu₂Sb phases appear successively along with the increase of Cu content in the SnSbCu_x alloy. The reduction of average particle size, the existence of inactive element Cu and the complex multi-step reaction mechanism in SnSbCu_x alloy anodes are propitious to improve the structure stability and thus improve the cycling performance. When cycled at a constant current density of 0.1 mA cm⁻² between 0.02 and 1.50 V, the coulomb efficiency of first cycle exceeds 74% and the reversible capacity of 20th cycle attains to 490 mAh g⁻¹ in SnSbCu_0.5 alloy anode.     

Structural evolution of ramsdellite-type LixTi2O4 upon electrochemical lithium insertion–deinsertion (0 ≤ x ≤ 2)

Structural evolution during topotactical electrochemical lithium insertion and deinsertion reactions in ramsdellite-like Li_xTi₂O₄ has been followed by means of in situ X-ray diffraction techniques. The starting Li_xTi₂O₄ (x = 1) exists as a single phase with variable composition which extends in the range 0.50 ≤ x ≤ 1.33. However, beyond the lower and upper compositional limits, two other single phases, with ramsdellite-like structure, are detected. The composition of these single phases are: TiO₂ upon lithium deinsertion and Li₂Ti₂O₄ upon lithium insertion. Both TiO₂ and Li₂Ti₂O₄ are characterized by narrow compositional ranges. The close structural relationship between pristine LiTi₂O₄ and the inserted and deinserted compounds together with the relative small volume change over the whole insertion–deinsertion range (not more than 1.1% upon reduction) is a guaranty for the high capacity retention after long cycling in lithium batteries. The small changes in cell parameters well reflect the remarkable flexibility of the ramsdellite framework against lithiation and delithiation reactions.     

Imidazolium-organic solvent mixtures as electrolytes for lithium batteries

γ-Butyrolactone (BL) has been mixed to the room temperature ionic liquid (RTIL) 1-butyl 3-methyl-imidazolium tetrafluoroborate (BMIBF₄) (ratio: 3/2, v/v) in the presence of lithium tetrafluoroborate (LiBF₄) for use as electrolyte in lithium-ion batteries. This mixture exhibits a larger thermal stability than the reference electrolyte EC/DEC/DMC (2/2/1) + LiPF₆ (1 M) and can be considered as a new RTIL as no free BL molecules are present in the liquid phase. The cycling ability of this electrolyte has been investigated at a graphite, a titanate oxide (Li₄Ti₅O₁₂) and a cobalt oxide (Li_xCoO₂) electrodes. The ionic liquid is strongly reduced at the graphite electrode near 1 V and leads to the formation of a blocking film, which prevents any further cycling. The titanate oxide electrode can be cycled with a high capacity without any significant fading. Cycling of the positive cobalt oxide electrode was unsuccessfully owing to an oxidation reaction at the electrode surface, which prevents the intercalation or de-intercalation of Li ions in and from the host material. Less reactive cathode material than cobalt oxide must be employed with this RTIL.     

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.     

Synthesis of LixMnO2 by chemical lithiation in an aqueous media

Li_xMnO₂ (x = 0.302) was synthesized by chemical lithiation, using a formaldehyde reducing agent and a LiOH lithium source in an aqueous media. The electrochemical properties and structural stability of the product were characterized by X-ray diffraction and charge-discharge measurements. The chemically lithiated Li_xMnO₂ had a first charge and discharge capacity of 86.2 and 265 mAh/g, respectively, with good cycling behavior. Based on the electrochemical results of the first charge, a two-step mechanism of Li_xMnO₂ lithiation is proposed. γ-MnO₂ is first oxidized by formaldehyde, and then Li diffuses into the γ-MnO₂ lattice.     

Fabrication and lithium storage performance of three-dimensional porous NiO as anode for lithium-ion battery

Three-dimensional porous NiO is prepared on Ni foam by a thermal treatment method at various temperatures. The morphology and structure of porous NiO are characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The electrochemical properties of three-dimensional porous NiO anode are evaluated by galvanostatic discharge-charge cycling, cyclic voltammery, and impedance spectral measurements on cells with lithium as the counter and reference electrodes. Results show that porous NiO delivers a stable capacity of 520 ± 20 mAh g⁻¹ with no noticeable capacity fading up to 30 cycles when cycled in the voltage range 0.05-3.0 V at rate of 0.2 C. The porous NiO exhibits higher reversible capacity, better cycleability, as well as higher rate capability in comparison to NiO foil. The observed cyclic voltammograms and impedance spectra are analyzed and interpret a redox reaction of NiO-Ni⁰ with formation and decomposition of Li₂O. The excellent electrochemical performance of porous NiO can be attributed to its large surface area, which lowers the current density of NiO reaction interface, and then an alternative anode is provided for lithium-ion batteries.