Preparation of Li2S-GeSe2-P2S5 electrolytes by a single step ball milling for all-solid-state lithium secondary batteries

Glass-ceramic and glass Li₂S-GeSe₂-P₂S₅ electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) with/without heat treatment (HT) from x = 0.55 to x = 1.00 were systematically investigated. Structural analysis by X-ray diffraction (XRD) showed gradual increase of the lattice constant followed by significant phase change with increasing GeSe₂. HT also affected the crystallinity. Incorporation of GeSe₂ in Li₂S-P₂S₅ kept high conductivity with a maximum value of 1.4 × 10⁻³ S cm⁻¹ at room temperature for x = 0.95 in Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) without HT. All-solid-state LiCoO₂/Li cells using Li₂S-GeSe₂-P₂S₅ as solid-state electrolytes (SE) were tested by constant-current constant-voltage (CCCV) charge-discharge cycling at a current density of 50 μA cm⁻² between 2.5 and 4.3 V (vs. Li/Li⁺). In spite of the extremely high conductivity of the SE, LiCoO₂/Li cells showed a large irreversible reaction especially during the first charging cycle. LiCoO₂ with SEs heat-treated at elevated temperature exhibited a capacity over 100 mAh g⁻¹ at the second cycle and consistently improved cycle retention, which is believed to be due to the better interfacial stability.     

All-solid-state lithium secondary batteries using LiCoO2 particles with pulsed laser deposition coatings of Li2S-P2S5 solid electrolytes

Electrode-electrolyte composite materials were prepared by coating a highly conductive Li₂S-P₂S₅ solid electrolyte onto LiCoO₂ electrode particles using pulsed laser deposition (PLD). Cross-sections of the composite electrode layers of the all-solid-state cells were observed using a transmission electron microscope to investigate the packing morphology of the LiCoO₂ particles and the distribution of solid electrolyte in the composite electrode. All-solid-state cells based on a composite electrode composed entirely of solid-electrolyte-coated LiCoO₂ were fabricated, and their performance was investigated. The coating amounts of Li₂S-P₂S₅ solid electrolytes on LiCoO₂ particles and the conductivity of the coating material were controlled to increase the capacity of the resulting all-solid-state cells. All-solid-state cells using LiCoO₂ with thick solid electrolyte coatings, grown over 120 min, had a capacity of 65 mAh g⁻¹, without any addition of Li₂S-P₂S₅ solid electrolyte particles to the composite electrode. The capacity of the all-solid-state cell increased further after increasing the conductivity of the Li₂S-P₂S₅ solid electrolyte coating by heat treatment at 200 °C. Furthermore, an all-solid-state cell based on a composite electrode using both a solid electrolyte coating and added solid electrolyte particles was fabricated, and the capacity of the resulting all-solid-state cell increased to 95 mAh g⁻¹.     

All-solid-state rechargeable lithium batteries with Li2S as a positive electrode material

The Li₂S–Cu composite electrode materials were prepared by mechanical milling and applied to all-solid-state lithium cells using the Li₂S–P₂S₅ glass–ceramic electrolyte. The addition of Cu and the mechanical activation improved the electrochemical performance of Li₂S in all-solid-state cells. The In/Li₂S–Cu cells were charged and then discharged at room temperature, suggesting that Li₂S was utilized as a lithium source. The cells exhibited high discharge capacity of about 490 mAh g⁻¹ at the 1st cycle. The SEM and EDX analyses suggested that the amorphous Li_xCuS domain was partially formed by milling, and the domain played an important role in the improvement of capacity. The electrochemical reaction mechanism of the Li₂S–Cu composites was discussed on the basis of the mechanism of the S–Cu composite electrode.     

Fabrication of all-solid-state lithium battery with lithium metal anode using Al2O3-added Li7La3Zr2O12 solid electrolyte

Li₇La₃Zr₂O₁₂ (LLZ) solid electrolyte is one of the promising electrolytes for all-solid-state battery due to its high Li ion conductivity and stability against Li metal anode. However, high calcination temperature for LLZ preparation promotes formation of La₂Zr₂O₇ impurity phase. In this paper, an effect of Al₂O₃ addition as sintering additive on LLZ solid electrolyte preparation and electrochemical properties of Al₂O₃-added LLZ were examined. By the Al₂O₃ addition, sintered LLZ pellet could be obtained after 1000 °C calcination, which is 230 °C lower than that without Al₂O₃ addition. Chemical and electrochemical properties of the Al₂O₃-added LLZ, such as stability against Li metal and ion conductivity, were comparable with the LLZ without Al₂O₃ addition, i.e. σ_bulk and σ_total were 2.4 × 10⁻⁴ and 1.4 × 10⁻⁴ S cm⁻¹ at 30 °C, respectively. All-solid-state battery with Li/Al₂O₃-added LLZ/LiCoO₂ configuration was fabricated and its electrochemical properties were tested. In cyclic voltammogram, clear redox peaks were observed, indicating that the all-solid-state battery with Li metal anode was successfully operated. The redox peaks were still observed even after one year storage of the all-solid-state battery in the Ar-filled globe-box. It can be inferred that the Al₂O₃-added LLZ electrolyte would be a promising candidate for all-solid-state battery because of facile preparation by the Al₂O₃ addition, relatively high Li ion conductivity, and good stability against Li metal and LiCoO₂ cathode.     

Electrochemical performance of all-solid-state lithium secondary batteries improved by the coating of Li2O-TiO2 films on LiCoO2 electrode

All-solid-state lithium secondary batteries using LiCoO₂ particles coated with amorphous Li₂O-TiO₂ films as an active material and Li₂S-P₂S₅ glass-ceramics as a solid electrolyte were fabricated; the electrochemical performance of the batteries was investigated. The interfacial resistance between LiCoO₂ and solid electrolyte was decreased by the coating of Li₂O-TiO₂ films on LiCoO₂ particles. The rate capability of the batteries using the LiCoO₂ coated with Li₂Ti₂O₅ (Li₂O·2TiO₂) film was improved because of the decrease of the interfacial resistance of the batteries. The cycle performance of the all-solid-state batteries under a high cutoff voltage of 4.6 V vs. Li was highly improved by using LiCoO₂ coated with Li₂Ti₂O₅ film. The oxide coatings are effective in suppressing the resistance increase between LiCoO₂ and the solid electrolyte during cycling. The battery with the LiCoO₂ coated with Li₂Ti₂O₅ film showed a large initial discharge capacity of 130 mAh/g and good capacity retention without resistance increase after 50 cycles at the current density of 0.13 mA/cm².     

High rate capabilities of all-solid-state lithium secondary batteries using Li4Ti5O12-coated LiNi0.8Co0.15Al0.05O2 and a sulfide-based solid electrolyte

Rate capability of LiNi_0.8Co_0.15Al_0.05O₂ in solid-state cells was investigated with 70Li₂S-30P₂S₅ glass-ceramics as a sulfide solid electrolyte. It showed higher rate capability than LiCoO₂; discharge capacity observed at a current density of 10 mA cm⁻² was ca. 70 mAh g⁻¹. Surface coating with Li₄Ti₅O₁₂ onto the LiNi_0.8Co_0.15Al_0.05O₂ particles further improved the high-rate performance to give ca. 110 mAh g⁻¹. The rate capability promises to realize all-solid-state lithium secondary batteries with very high performance.     

Synthesis and electrochemical performance of Li2CoSiO4 as cathode material for lithium ion batteries

Li₂CoSiO₄ has been prepared successfully by a solution route or hydrothermal reaction for the first time, and its electrochemical performance has been investigated primarily. Reversible extraction and insertion of lithium from and into Li₂CoSiO₄ at 4.1 V versus lithium have shown that this material is a potential candidate for the cathode in lithium ion batteries. At this stage reversible electrochemical extraction was limited to 0.46 lithium per formula unit for the Li₂CoSiO₄/C composite materials, with a charge capacity of 234 mAh g⁻¹ and a discharge capacity of 75 mAh g⁻¹.     

A novel high temperature stable lithium salt (Li2B12F12) for lithium ion batteries

Basic properties and battery performances of the novel high temperature stable lithium salt (Li₂B₁₂F₁₂, Dilithium Dodecafluorododecaborate; Li₂DFB) were studied using a Mn-based cathode and anode composed of a hard carbon and graphite mixture. The effect of co-solvents (mainly linear carbonate in electrolyte formulation of PC/EC/co-solvent (5/30/65 vol% mixture)) on conductivity, viscosity, charge-discharge capacities, rate performance, temperature performance, cycle life and storage life at 60 °C was investigated. Conductivity of Li₂DFB electrolyte increased with reducing its viscosity by changing co-solvent and increasing the volume of the higher dielectric solvent. Li₂DFB electrolytes showed comparable discharge capacity and columbic efficiency against LiPF₆ electrolyte. Li₂DFB electrolytes improved the storage life and cycle life of a Mn-based cell at 60 °C.     

Synthesis and characterization of Li2MnSiO4/C nanocomposite cathode material for lithium ion batteries

A high capacity Li₂MnSiO₄/C nanocomposite cathode material with good rate performance for lithium ion batteries through a solution route has been successfully prepared. The material is able to deliver a reversible capacity of 209 mAh g⁻¹ in the first cycle, i.e. more than one electron exchange can be reversible cycled in the materials. The highly dispersion of nanocrystalline Li₂MnSiO₄ which was surround by a thin film of carbon was attributed to the cause of excellent performance of the materials. Ex situ XRD and IR results show that poor cycling behavior of Li₂MnSiO₄ might be due to an amorphization process of the materials.     

Electrochemical properties of Li1.4Al0.4Ti1.6(PO4)3 synthesized by a co-precipitation method

Sub-micron Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) ceramic powder is synthesized by a co-precipitation method which can be applied for mass production. A pure Nasicon phase is confirmed by X-ray diffraction analysis and the primary particle size of the product is 200-500 nm. The sinterability of LATP is investigated and the relative density of 97% reached at a sintering temperature as low as 900 °C for 6 h. The bulk lithium ionic conductivity of the sintered pellet is 2.19 × 10⁻³ S cm⁻¹, and a total conductivity of 1.83 × 10⁻⁴ S cm⁻¹ is obtained.     

A study on lithium/air secondary batteries—Stability of the NASICON-type lithium ion conducting solid electrolyte in alkaline aqueous solutions

The stability of the high lithium ion conducting glass ceramics, Li_1+x+yTi_2−xAl_xSi_yP_3−yO₁₂ (LTAP) in alkaline aqueous solutions with and without LiCl has been examined. A significant conductivity decrease of the LTAP plate immersed in 0.057 M LiOH aqueous solution at 50 °C for 3 weeks was observed. However, no conductivity change of the LTAP plate immersed in LiCl saturated LiOH aqueous solutions at 50 °C for 3 weeks was observed. The pH value of the LiCl-LiOH-H₂O solution with saturated LiCl was in a range of 7-9. The molarity of LiOH and LiCl in the LiOH and LiCl saturated aqueous solution were estimated to be 5.12 and 11.57 M, respectively, by analysis of Li⁺ and OH⁻. The high concentration of LiOH and the low pH value of 8.14 in this solution suggested that the dissociation of LiOH into Li⁺ and OH⁻ is too low in the solution with a high concentration of Li⁺. These results suggest that the water stable LTAP could be used as a protect layer of the lithium metal anode in the lithium/air cell with LiCl saturated aqueous solution as the electrolyte, because the content of OH⁻ ions in the LiCl saturated aqueous solution does not increase via the cell reaction of Li + 1/2O₂ + H₂O → 2LiOH, and LTAP is stable under a deep discharge state.     

Surface modification of Li[Ni0.3Co0.4Mn0.3]O2 cathode by Li-La-Ti-O coating

A Li[Ni_0.3Co_0.4Mn_0.3]O₂ cathode is modified by applying a Li-La-Ti-O coating using the hydrothermal method. The coated Li[Ni_0.3Co_0.4Mn_0.3]O₂ is characterized by X-ray diffraction analysis, scanning electron microscopy, energy-dispersive spectrometry, transmission electron microscopy, and differential scanning calorimetry. The Li-La-Ti-O coating layer is formed as crystalline (perovskite structure) or amorphous phase depending on the heating temperature. The Li-La-Ti-O coated Li[Ni_0.3Co_0.4Mn_0.3]O₂ electrode has better rate capability than the pristine electrode. In particular, the rate capability is significantly associated with heating temperature; this is probably due to the phase of the coating layer. It appears that the Li-La-Ti-O coating of amorphous phase is superior to that of crystalline phase for obtaining enhanced rate capability of the coated samples. The thermal stability and cyclic performance of the Li[Ni_0.3Co_0.4Mn_0.3]O₂ electrode are also improved by Li-La-Ti-O coating. These improvements indicate that the Li-La-Ti-O coating is effective in suppressing the chemical and structural instabilities of Li[Ni_0.3Co_0.4Mn_0.3]O₂.     

Influence of conductive additives and surface fluorination on the charge/discharge behavior of lithium titanate (Li4/3Ti5/3O4)

Effect of conductive additives and surface modification with NF₃ and ClF₃ on the charge/discharge behavior of Li_4/3Ti_5/3O₄ (≈4.6 μm) was investigated using vapor grown carbon fiber (VGCF) and acetylene black (AB). VGCF and mixtures of VGCF and AB increased charge capacities of original Li_4/3Ti_5/3O₄ and those fluorinated with NF₃ by improving the electric contact between Li_4/3Ti_5/3O₄ particles and nickel current collector. Surface fluorination increased meso-pore with diameter of 2 nm and surface area of Li_4/3Ti_5/3O₄, which led to the increase in first charge capacities of Li_4/3Ti_5/3O₄ samples fluorinated by NF₃ at high current densities of 300 and 600 mA g⁻¹. The result shows that NF₃ is the better fluorinating agent for Li_4/3Ti_5/3O₄ than ClF₃.     

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.     

High-strength all-solid lithium ion electrodes based on Li4Ti5O12

A Li₄Ti₅O₁₂-Li_0.29La_0.57TiO₃-Ag electrode composite was fabricated via sintering the corresponding powder mixture. The process achieved a final relative density of 97% the theoretical. Relatively thick, ∼100 μm, electrodes were fabricated to enhance the energy density relatively to the traditional solid-state thin film battery electrodes. The sintered electrode composite delivered full capacity in the first discharge at C/40 discharge rate. Full capacity utilization resulted from the 3D percolated network of both solid electrolyte and metal, which provide paths for ionic and electronic transport, respectively. The electrodes retained 85% of the theoretical capacity after 10 cycles at C/40 discharge rate. The tensile strength and the Young's modulus of the sintered electrode composite are the highest reported values to date, and are at least an order of magnitude higher than the corresponding value of traditional tapecast “composite electrodes”. The results demonstrate the concept of utilizing thick all-solid electrodes for high-strength batteries, which might be used as multifunctional structural and energy storage materials.     

Effects of Fe2P and Li3PO4 additives on the cycling performance of LiFePO4/C composite cathode materials

In this study, a solution method was employed to synthesize LiFePO₄-based powders with Li₃PO₄ and Fe₂P additives. The composition, crystalline structure, and morphology of the synthesized powders were investigated by using ICP-OES, XRD, TEM, and SEM, respectively. The electrochemical properties of the powders were investigated with cyclic voltammetric and capacity retention studies. The capacity retention studies were carried out with LiFePO₄/Li cells and LiFePO₄/MCMB cells comprised LiFePO₄-based materials prepared at various temperatures from a stoichiometric precursor. Among all of the synthesized powders, the samples synthesized at 750 and 775 °C demonstrate the most promising cycling performance with C/10, C/5, C/2, and 1C rates. The sample synthesized at 775 °C shows initial discharge capacity of 155 mAh g⁻¹ at 30 °C with C/10 rate. From the results of the cycling performance of LiFePO₄/MCMB cells, it is found that 800 °C sample exhibited higher polarization growth rate than 700 °C sample, though it shows lower capacity fading rate than 700 °C sample. For Fe₂P containing samples, the diffusion coefficient of Li⁺ ion increases with increasing amount of Fe₂P, however, the sample synthesized at 900 °C shows much lower Li⁺ ion diffusion coefficient due to the hindrance of Fe₂P layer on the surface of LiFePO₄ particles.     

Fabrication of all-solid-state rechargeable lithium-ion battery using mille-feuille structure of Li0.35La0.55TiO3

A mille-feuille structure, which comprises both sides of dense layer are sandwiched by porous layers, is one of the promising structures for 3-dimensional (3D) all-solid-state battery. The porous layers should have 3-dimensionally ordered macroporous structure to obtain large contact area between electrolyte and electrode. Li_0.35La_0.55TiO₃ (LLT) solid electrolyte with the mille-feuille structure was fabricated by the suspension filtration method. The dense layer was sintered well, no grain boundary was observed. The porous layers contacted well with dense layer. Thicknesses of dense and porous layers were 30 and 26 μm, respectively. To check compatibility of the mille-feuille LLT with all-solid-state Li ion battery, chronopotentiometry of symmetric cell with LiMn₂O₄/mille-feuille LLT/LiMn₂O₄ configuration was measured. Charge and discharge currents were clearly observed, indicating that the cell was successfully operated.     

Synthesis and characterization of Li2Fe0.97M0.03SiO4 (M = Zn, Cu, Ni) cathode materials for lithium ion batteries

Attempts to dope Zn²⁺, Cu²⁺ or Ni²⁺ are made for Li₂FeSiO₄. The effects of dopant on the physical and electrochemical characteristics of Li₂FeSiO₄ were investigated. Zn²⁺ successfully entered into the lattice of Li₂FeSiO₄ and induced the change of lattice parameters. Compared with the undoped Li₂FeSiO₄, Li₂Fe_0.97Zn_0.03SiO₄ has higher discharge capacity, better electrochemical reversibility and lower electrode polarization. The improved electrochemical performance of Li₂Fe_0.97Zn_0.03SiO₄ can be attributed to the improved structural stability and the enhanced lithium ion diffusivity brought about by Zn²⁺ doping. However, Ni²⁺ and Cu²⁺ cannot be doped into the lattice of Li₂FeSiO₄. Cu and NiO are formed as impurities in the Cu- and Ni-containing samples, respectively. Compared with the undoped Li₂FeSiO₄, the Cu- and Ni-containing samples have lower capacities and higher electrochemical polarization.     

A fast synthesis of Li3V2(PO4)3 crystals via glass-ceramic processing and their battery performance

A synthesis of Li₃V₂(PO₄)₃ being a potential cathode material for lithium ion batteries was attempted via a glass-ceramic processing. A glass with the composition of 37.5Li₂O-25V₂O₅-37.5P₂O₅ (mol%) was prepared by a melt-quenching method and precursor glass powders were crystallized with/without 10 wt% glucose in N₂ or 7%H₂/Ar atmosphere. It was found that heat treatments with glucose at 700 °C in 7%H₂/Ar can produce well-crystallized Li₃V₂(PO₄)₃ in the short time of 30 min. The battery performance measurements revealed that the precursor glass shows the discharge capacity of 14 mAh g⁻¹ at the rate of 1 μA cm⁻² and the glass-ceramics with Li₃V₂(PO₄)₃ prepared with glucose at 700 °C in 7%H₂/Ar show the capacities of 117-126 mAh g⁻¹ (∼96% of the theoretical capacity) which are independent of heat treatment time. The present study proposes that the glass-ceramic processing is a fast synthesizing route for Li₃V₂(PO₄)₃ crystals.     

Li3V2(PO4)3/C composite as an intercalation-type anode material for lithium-ion batteries

The carbon coated monoclinic Li₃V₂(PO₄)₃ (LVP/C) powder is successfully synthesized by a carbothermal reduction method using crystal sugar as the carbon source. Its structure and physicochemical properties are investigated using X-ray diffraction (XRD), scanning electron microscopy, high-resolution transmission electron microscopy and electrochemical methods. The LVP/C electrode exhibits stable reversible capacities of 203 and 102 mAh g⁻¹ in the potential ranges of 3.0-0.0 V and 3.0-1.0 V versus Li⁺/Li, respectively. It is identified that the insertion/extraction of Li⁺ undergoes a series of two-phase transition processes between 3.0 and 1.6 V and a single phase process between 1.6 and 0.0 V. The ex situ XRD patterns of the electrodes at various lithiated states indicate that the monoclinic structure can still be retained during charge-discharge process and the insertion/deinsertion of lithium ions occur reversibly, which provides an excellent cycling stability with high energy efficiency.