Electrolytic Sn/Li2O coatings for thin-film lithium ion battery anodes

Sn/Li₂O composite coatings on stainless steel substrate, as anodes of thin-film lithium battery are carried out in SnCl₂ and LiNO₃ mixed solutions by using cathodic electrochemical synthesis and subsequently annealed at 200 °C. Through cathodic polarization tests, three major regions are verified: (I) O₂ + 4H⁺ + 4e⁻ → 2H₂O (∼0.25 to −0.5 V), (II) 2H⁺ + 2e⁻ → H₂, Sn²⁺ + 2e⁻ → Sn, and NO₃⁻ + H₂O + 2e⁻ → NO₂⁻ + 2OH⁻ (−0.5 to −1.34 V), and (III) 2H₂O + 2e⁻ → H₂ + 2OH⁻ (−1.34 to −2 V vs. Ag/AgCl). The coated specimens are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and charge/discharge tests. The nano-sized Sn particles embedded in Li₂O matrix are obtained at the lower part of region II such as −1.2 V, while the micro-sized Sn with little Li₂O at the upper part, such as −0.7 V. Charge/discharge cycle tests elucidated that Sn/Li₂O composite film showed better cycle performance than Sn or SnO₂ film, due to the retarding effects of amorphous Li₂O on the further aggregation of Sn particles. On the other hand, the one tested for cut-off voltage at 0.9 V (vs. Li/Li⁺) is better than those at 1.2 and 1.5 V since the incomplete de-alloy at lower cut-off voltage may inhibit the coarsening of Sn particles, revealing capacity 587 mAh g⁻¹ after 50 cycle, and capacity retention ratio C50/C2 81.6%, higher than 63.5% and 49.1% at 1.2 and 1.5 V (vs. Li/Li⁺), respectively.     

Mixed-metal Li3N-based systems for hydrogen storage: Li3AlN2 and Li3FeN2

The hydrogen storage systems Li₃AlN₂ and Li₃FeN₂ were synthesized mechanochemically by two different routes. In each case an intermediate material formed after milling, which transformed into Li₃MN₂ (M = Al or Fe) upon annealing. The synthesis route had a measurable effect on the hydrogen storage properties of the material: Li₃AlN₂ prepared from hydrogenous starting materials (LiNH₂ and LiAlH₄) performed better than that synthesized from non-hydrogenous materials (Li₃N and AlN). For both Li₃AlN₂ materials, the hydrogen storage capacity and the absorption kinetics improved significantly upon cycling. Ti-doped Li₃AlN₂ synthesized from LiNH₂ and LiAlH₄ showed the best hydrogen storage characteristics of all, with the best kinetics for hydrogen uptake and release, and the highest hydrogen storage capacity of 3.2 wt.%, of which about half was reversible. Meanwhile, Li₃FeN₂ synthesized from Li₃N and Fe displayed similar kinetics to that synthesized from Li₃N and Fe_xN (2 ≤ x ≤ 4), but demonstrated lower gravimetric hydrogen storage capacities. Li₃FeN₂ displayed a hydrogen uptake capacity of 2.7 wt.%, of which about 1.5 wt.% was reversible. For both Li₃AlN₂ and Li₃FeN₂, doping with TiCl₃ resulted in enhancement of hydrogen absorption kinetics. This represents the first study of a ternary lithium-transition metal nitride system for hydrogen storage.     

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.     

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

Na-doped Li_3−xNa_xV₂(PO₄)₃/C (x = 0.00, 0.01, 0.03, and 0.05) compounds have been prepared by using sol-gel method. The Rietveld refinement results indicate that single-phase Li_3−xNa_xV₂(PO₄)₃/C with monoclinic structure can be obtained. Among three Na-doped samples and the undoped one, Li_2.97Na_0.03V₂(PO₄)₃/C sample has the highest electronic conductivity of 6.74 × 10⁻³ S cm⁻¹. Although the initial specific capacities for all Na-doped samples have no much enhancement at the current rate of 0.2 C, both cycle performance and rate capability have been improved. At the 2.0 C rate, Li_2.97Na_0.03V₂(PO₄)₃/C presents the highest initial capacity of 118.9 mAh g⁻¹ and 12% capacity loss after 80 cycles. The partial substitution of Li with Na (x = 0.03) is favorable for electrochemical rate and cyclic ability due to the enlargement of Li₃V₂(PO₄)₃ unit cells, optimizing the particle size and morphology, as well as resulting in a higher electronic conductivity.     

Synthesis, phase relation and electrical and electrochemical properties of ruthenium-substituted Li2MnO3 as a novel cathode material

Layered oxides, ruthenium-substituted Li₂MnO₃, were synthesized at 800 °C and 1200 °C. Their phase relation and electrical and electrochemical properties were investigated. Li₂Mn_1−xRu_xO₃ synthesized at 800 °C clearly separated into two phases, manganese-rich and ruthenium-rich phases, except for the narrow composition range of 0 ≤ x ≤ 0.05, while Li₂Mn_1−xRu_xO₃ synthesized at 1200 °C formed two solid solutions in the whole composition range across a structural transition between x = 0.6 and 0.8. The electrical resistivity of Li₂Mn_1−xRu_xO₃ decreased with increasing ruthenium content. Li₂Mn_0.2Ru_0.8O₃ (x = 0.8) synthesized at 1200 °C showed the lowest resistivity of 5.7 × 10² Ω cm at room temperature. The discharge capacity and cycling performance were improved by the ruthenium substitution. Li₂Mn_0.4Ru_0.6O₃ (x = 0.6) exhibited a discharge capacity of 192 mAh g⁻¹ in the initial cycle and 169 mAh g⁻¹ in the tenth cycle with high and almost constant charge-discharge efficiencies of 99% from the second to tenth cycle at a current rate of 1/10C. The ruthenium substitution to Li₂MnO₃ is quite effective to improve electrical conductivity and charge-discharge performance.     

Characteristics of Sn/Li2O multilayer composite anode for thin film microbattery

Sputtering growth of a Sn/Li₂O multilayer composite thin film is conducted to produce an anode thin film with less capacity fading than that of a pure SnO₂ film for a thin-film battery. The structural properties of the Sn/Li₂O multilayer are examined. In addition, the electrochemical characteristics of the Sn/Li₂O and pure SnO₂ thin films are compared. X-ray diffraction and transmission electron microscopy measurements reveal a Sn crystalline peak only and a Sn–Li₂O multilayer structure, respectively, in the Sn/Li₂O thin film. A SnO₂ thin film with a polycrystalline phase shows an irreversible side-reaction at 0.8 V versus Li/Li⁺, an initial charge retention of about 29%, and poor cycleability in the cut-off voltage range from 1.2 to 0 V versus Li/Li⁺. By contrast, no irreversible side-reaction is found in the Sn/Li₂O multilayer composite thin film while there is an initial charge retention of 49% and better cycleability (more than twice) than that of pure SnO₂ film after about 150 cycles. These results indicate that the Sn/Li₂O multilayer composite thin film can be used for tin-based, thin-film, microbatteries and provide motivation to pursue fabrication of Sn–Li₂O anode powder for bulk type batteries.     

Synthesis and electrochemical performances of Li3V2(PO4)3/(Ag + C) composite cathode

Li₃V₂(PO₄)₃, Li₃V₂(PO₄)₃/C and Li₃V₂(PO₄)₃/(Ag + C) composites as cathodes for Li ion batteries are synthesized by carbon-thermal reduction (CTR) method and chemical plating reactions. The microstructure and morphology of the compounds are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Li₃V₂(PO₄)₃/(Ag + C) particles are 0.5-1 μm in diameters. As compared to Li₃V₂(PO₄)₃, Li₃V₂(PO₄)₃/C, the Li₃V₂(PO₄)₃/(Ag + C) composite cathode exhibits high discharge capacity, good cycle performance (140.5 mAh g⁻¹ at 50th cycle at 1 C, 97.3% of initial discharge capacity) and rate behavior (120.5 mAh g⁻¹ for initial discharge at 5 C) for the fully delithiated (3.0-4.8 V) state. Electrochemical impedance spectroscopy (EIS) measurements show that the carbon and silver co-modification decreases the charge transfer resistance of Li₃V₂(PO₄)₃/(Ag + C) cathode, and improves the conductivity and boosts the electrochemical performance of the electrode.     

Fe content effects on electrochemical properties of Fe-substituted Li2MnO3 positive electrode material

Fe-substituted Li₂MnO₃ including a monoclinic layered rock-salt structure (C2/m), (Li_1+x(Fe_yMn_1−y)_1−xO₂, 0 < x < 1/3, 0.1 ≤ y ≤ 0.5) was prepared by coprecipitation-hydrothermal-calcination method. The sample was assigned as two-phase composite structure consisting of the cubic rock-salt () and monoclinic ones at high Fe content above 30% (y ≥ 0.3), while the sample was assigned as a monoclinic phase (C2/m) at low Fe content less than 20%. In the monoclinic Li₂MnO₃-type structure, the Fe ion tends to substitute a Li (2b) site, which corresponds to a center position of Mn⁴⁺ hexagonal network in Mn-Li layer. The electrochemical properties including discharge characteristics under high current density (<3600 mA g⁻¹ at 30 °C) and low temperature (<−20 °C at 40 mA g⁻¹) were severely affected by chemical composition (Fe content and Li/(Fe + Mn) ratio), crystal structure (monoclinic phase content) and powder property (specific surface area). Under the optimized Fe content (0.2 < y < 0.4), the Li/sample cells showed high initial discharge capacity (240-300 mAh g⁻¹) and energy density (700-950 mWh g⁻¹) between 1.5 and 4.8 V under moderate current density, 40 mA g⁻¹ at 30 °C. Results suggest that Fe-substituted Li₂MnO₃ would be a non-excludable 3 V positive electrode material.     

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.     

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.     

Formation of the high lithium ion conducting phase from mechanically milled amorphous Li2S-P2S5 system

The fast ionic conducting structure similar to thio-Lithium Super Ionic Conductor (LISICON) phase is synthesized in the Li₂S-P₂S₅ system. The Li₂S-P₂S₅ glass-ceramics with the composition of xLi₂S·(100−x)P₂S₅ (75 ≤ x ≤ 80) are prepared by the heat-treatment of mechanically milled amorphous sulfide powders. In the binary Li₂S-P₂S₅ system, 78.3Li₂S·21.7P₂S₅ glass ceramic prepared by mechanical milling and subsequent heat-treatment at 260 °C for 3 h shows the highest conductivity of 6.3 × 10⁻⁴ S cm⁻¹ at room temperature and the lowest activation energy for conduction of 30.5 kJ mol⁻¹. The enhancement of conductivity with increasing x up to 78.3 is probably caused by the introduction of interstitial lithium ions at the Li sites which affects the Li ion distribution. The prepared electrolyte exhibits the lithium ion transport number of almost unity and voltage stability of 5 V vs. Li at room temperature.     

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.     

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.     

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.     

Influence of surface modification of LiCoO2 by organic compounds on electrochemical and thermal properties of Li/LiCoO2 rechargeable cells

LiCoO₂ is the most famous positive electrode (cathode) for lithium ion cells. When LiCoO₂ is charged at high charge voltages far from 4.2 V, cycleability of LiCoO₂ becomes worse. Causes for this deterioration are instability of pure LiCoO₂ crystalline structure and an oxidation of electrolyte solutions LiCoO₂ at higher charge voltages. This electrolyte oxidation accompanies with the partial reduction of LiCoO₂. We think more important factor is the oxidation of electrolyte solutions. In this work, influence of 10 organic compounds on electrochemical and thermal properties of LiCoO₂ cells was examined as electrolyte additives. As a base electrolyte solution, 1 M (M: mol L⁻¹) LiPF₆-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) was used. These compounds are o-terphenyl (o-TP), Ph-X (CH₃)_n (n = 1 or 2, X = N, O or S) compounds, adamantyl toluene compounds, furans and thiophenes. These additives had the oxidation potentials (Eox) between 3.4 and 4.7 V vs. Li/Li⁺. These Eox values were lower than that (6.30 V vs. Li/Li⁺) of the base electrolyte. These additives are oxidized on LiCoO₂ during charge of the LiCoO₂ cells. Oxidation products suppress the excess oxidation of electrolyte solutions on LiCoO₂. As a typical example of these organic compounds, o-TP (Eox: 4.52 V) was used to check the fundamental properties of these organic additives. Charge-discharge cycling tests were carried out for the Li/LiCoO₂ cells with and without o-TP. Constant current charge at 4.5 V is mainly used as a charge method. Cells with 0.1 wt.% o-TP exhibited slightly better cycling performance and lower polarization than those without additives. Lower polarization arises from a decrease in a resistance of interface between electrolyte solutions and LiCoO₂ by surface film formation resulted from oxidation of o-TP. Oxidation products were found by mass spectroscopy analysis to be mixture of several polycondensation compounds made from two to four terphenly monomers. Thermal stability of LiCoO₂ with electrolyte solutions did not improve by addition of o-TP. Slightly better charge-discharge cycling properties were obtained by using organic modifiers. However, when industrial applications were considered, drastic improvements have not been obtained yet. One of reasons may be too large influence of the deterioration of stability of pure LiCoO₂ structure at high voltage charging for industrial use. We hope to realize the tremendous improvements of high energy, long cycle life and safe lithium cells by the combination of both LiCoO₂ with more stable structure such as LiCoO₂ treated with MgO and new organic additives with molecular structure more carefully designed.     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

In situ lithiation of TiS2 enabled by spontaneous decomposition of Li3N

We report on a novel method for in situ lithiation of lithium free TiS₂ using Li₃N in an all-solid-state battery configuration. This method was tested using a Li₃N-TiS₂-80Li₂S:20P₂S₅ composite positive electrode and an indium metal negative electrode. It is shown that approximately 37% of Li₃N spontaneously decomposes during composite preparation regardless of the composition. Solid-state battery cells built with a 3:1 stoichiometric ratio of Li:Ti demonstrated a high 1st cycle charge capacity of 287 mAh g⁻¹, 20% greater than the theoretical capacity of TiS₂ at 239 mAh g⁻¹. The difference provides an excess capacity in the indium metal negative electrode.     

Effects of reactant dispersion on the structure and electrochemical performance of Li1.2V3O8

The pure-phase Li_1.2V₃O₈ was synthesized by ultrasonically dispersing Li₂CO₃ and NH₄VO₃ reactants. Its structure and morphology compared with the pristine Li_1+xV₃O₈ obtained from the solid-state reaction were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the compound synthesized at 570 °C from the precursor obtained by ultrasonic treatment in anhydrous ethanol has low crystallinity and homogeneous morphology with bar-like shape. Charge–discharge cycling, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments indicate that this sample has relatively high initial discharge capacity and good cycle ability, and it is beneficial to the reversible insertion/extraction of Li⁺ ions because of the low kinetic resistance. Its discharge capacity reaches 270 mAh g⁻¹ in the 2nd cycle at 0.2 C discharge rate and still retains 210 mAh g⁻¹ in the 100th cycle in the range of 2.0–4.0 V.     

Synthesis of plate-like Li3V2(PO4)3/C as a cathode material for Li-ion batteries

Plate-like Li₃V₂(PO₄)₃/C composite is synthesized via a solution route followed by solid-state reaction. The Li₃V₂(PO₄)₃/C plates are 40-100 nm in thicknesses and 2-10 μm in lengths. TEM images show that a uniform carbon layer with a thickness of 5.3 nm presents on the surfaces of Li₃V₂(PO₄)₃ plates. The apparent Li-ion diffusion coefficient of the plate-like Li₃V₂(PO₄)₃/C is calculated to be 2.7 × 10⁻⁸ cm² s⁻¹. At a charge-discharge rate of 3 C, the plate-like Li₃V₂(PO₄)₃/C exhibits an initial discharge capacity of 125.2 and 133.1 mAh g⁻¹ in the voltage ranges of 3.0-4.3 and 3.0-4.8 V, respectively. After 500 cycles, the electrodes still can deliver a discharge capacity of 111.8 and 97.8 mAh g⁻¹ correspondingly, showing a good cycling stability.     

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.