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.     

Novel inorganic ionic liquids possessing low melting temperatures and wide electrochemical windows: Binary mixtures of alkali bis(fluorosulfonyl)amides

Thermal properties of alkali bis(fluorosulfonyl)amides, MFSI (M = Li, Na, K, Rb, Cs), have been investigated. Binary phase diagrams of LiFSI–KFSI and NaFSI–KFSI systems have been constructed. Eutectic point for LiFSI–KFSI is 338 K at (x_Li, x_K) = (0.45, 0.55) and, that for NaFSI–KFSI is 330 K at (x_Na, x_K) = (0.45, 0.55). The electrochemical window of the eutectic LiFSI–KFSI is as wide as 6.0 V at 348 K with the cathode limit being lithium metal deposition. The electrochemical window of the eutectic NaFSI–KFSI is 5.0 V at 340 K with sodium metal deposition at the cathode limit. These new inorganic ionic liquids are highly promising for various electrochemical applications.     

Intermediate phases during alkali metal intercalation in HfNCl

The mechanism of the chemical and electrochemical alkali metal intercalation reactions in β-HfNCl has been investigated through electrochemical potential spectroscopy (EPS), in-situ powder X-ray diffraction during electrochemical intercalation and room temperature chemical intercalation experiments. EPS experiments in lithium cells reveal the presence of a plateau, at 1.8 V vs. Li⁺/Li⁰ accounting for ca. 0.14 mol Li, that indicates the formation of a new intermediate phase, and then a gradual decrease of potential with composition that extends up to very high lithium contents (ca. 1.1 per formula), consistent with the formation of a solid solution. Sodium electrochemical intercalation experiments showed a relatively similar behaviour with a plateau at 1.4 V vs. Na⁺/Na⁰, corresponding to ca. 1.7 V vs. Li⁺/Li⁰. In-situ monitored powder X-ray diffraction electrochemical intercalation experiments showed that the electrolyte solvent (ethylene carbonate/dimethyl carbonate, EC/DMC or propylene carbonate, PC) co-intercalated with the alkaline atom. This leads to a large expansion of the interlayer spacing that reaches a value of 21.06 Å in the lithium co-intercalated phase with EC/DMC, Li_x(EC/DMC)_yHfNCl, and 22.01 Å in the sodium co-intercalated phase with PC, Na_x(PC)_yHfNCl. Chemical intercalation using naphthyl-sodium solutions in tetrahydrofuran (THF) leads to solvent-free, multiple-phase samples showing in different proportions the pristine and the superconducting stage 2 and stage 1 phases. The composition of the intercalated samples depends on the pristine sample, the concentration of the naphthyl-sodium solution, the ratio Na:HfNCl and the reaction time. Pristine samples exhibiting low lithium intercalation degree upon electrochemical reduction gave the second stage as the major phase when treated with short reaction times or using low Na:HfNCl ratios, coexisting either with the host or with the first stage phase, whereas stage 1 is obtained as the major phase from pristine samples showing high electrochemical capacities. The staging behaviour and the multiphase nature of these samples account for the wide superconducting transitions and the different critical temperatures observed in these superconductors.     

Raman spectra of carriers in ionic-liquid-gated transistors fabricated with poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)

We observed the Raman spectra of carriers, positive polarons and bipolarons, generated in a poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14) film by FeCl₃ vapor doping. Electrical conductivity and Raman measurements indicate that the dominant carriers in the conducting state were bipolarons. We identified positive polarons and bipolarons generated in an ionic-liquid-gated transistor (ILGT) fabricated with PBTTT-C14 as an active semiconductor and an ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [BMIM][TFSI] as a gate dielectric using Raman spectroscopy. The relationship between the source−drain current (I_D) at a constant source−drain voltage (V_D) and the gate voltage (V_G) was measured. I_D increased above −V_G = 1.1 V and showed a maximum at −V_G = 2.0 V. Positive polarons were formed at the initial stage of electrochemical doping (−V_G = 0.8 V). As I_D increased, positive bipolarons were formed. Above V_G = −2.0 V, bipolarons were dominant. The charge density (n), the doping level (x), and the mobility of the bipolarons were calculated from the electrochemical measurements. The highest mobility (μ) of bipolarons was 0.72 cm² V⁻¹ s⁻¹ at x = 110 mol%/repeating unit (−V_G = 2.0 V), whereas the highest μ of polarons was 4.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ at x = 10 mol%.     

Electrochemical characterization on cobalt sulfide for electrochemical supercapacitors

High capacitance at a high charge–discharge current density of 50 mA/cm² for a new type of electrochemical supercapacitor cobalt sulfide (CoS_x) have been studied for the first time. The CoS_x was prepared by a very simply chemical precipitation method. The electrochemical capacitance performance of this compound was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests with a three-electrode system. The results show that CoS_x has excellent electrochemical capacitive characteristic with potential range −0.3 ∼ 0.35 V (versus SCE) in 6 M KOH solution. Charge–discharge behaviors have been observed with the highest specific capacitance values of 475 F/g at the current density of 5 mA/cm², even at the high current density of 50 mA/cm², CoS_x also shows the high specific capacitance values of 369 F/g.     

Comparison of the electrochemical properties of metallic layered nitrides containing cobalt,nickel and copper in the 1 V–0.02 V potential range

We report the first example of an intercalation compound based on the nitrogen framework in which lithium can be intercalated and deintercalated. A comparison of the structural and electrochemical properties of the ternary lithium cobalt, nickel and copper nitrides is performed. Vacancy layered structures of ternary lithium nitridocobaltates Li_3−2xCo_xN and nitridonickelates Li_3−2xNi_xN with 0.10 ⩽ x ⩽ 0.44 and 0.20 ⩽ x ⩽ 0.60, respectively, are proved to reversibly intercalate Li ions in the 1 V–0.02 V potential range. These host lattices can accommodate up to 0.35 Li ion par mole of nitride. Results herein obtained support Li insertion in vacancies located in Li₂N⁻ layers while interlayer divalent cobalt and nickel cations are reduced to monovalent species. No structural strain is induced by the insertion–extraction electrochemical reaction which explains the high stability of the capacity in both cases. For the Li_1.86Ni_0.57N compound, a stable faradaic yield of 0.30 F/mol, i.e. 130 mAh/g, is maintained at least for 100 cycles. Conversely, the ternary copper nitrides corresponding to the chemical composition Li_3−xCu_xN with 0.10 ⩽ x ⩽ 0.40 do not allow the insertion reaction to take place due to the presence of monovalent copper combined with the lack of vacancies to accommodate Li ions. In the latter case, the discharge of the lithium copper nitrides is not reversible.     

Sodium vanadium nitridophosphate Na3V(PO3)3N as a high-voltage positive electrode material for Na-ion and Li-ion batteries

We report the preparation and electrochemical properties of Na₃V(PO₃)₃N made by ammonolysis. Na₃V(PO₃)₃N is reversibly oxidized to Na₂V(PO₃)₃N at high voltage (4.0 V vs. Na⁺/Na⁰ and 4.1 V vs. Li⁺/Li⁰) with an unusually small difference in the insertion/extraction voltage between both alkali metal reference electrodes. In both cases, the voltage hysteresis is extremely small (~ 0.035 V vs. sodium and ~ 0.065 V vs. lithium), which suggests facile migration of alkali cations within the structure. Further oxidation to NaV(PO₃)₃N is predicted to occur beyond the voltage stability window of the electrolytes.     

Investigation of the LiCo1−xMgxPO4 (0 ≤ x ≤ 0.1)–graphitic carbon foam composites

LiCo_1−xMg_xPO₄–graphitic carbon foam (LCMP–GCF with 0 ≤ x ≤ 0.1) composites are prepared by Pechini-assisted sol-gel method and annealed with the 2-steps annealing process (T = 300 °C for 5 min in flowing air, then at T = 730 °C for t = 12 h in flowing nitrogen). The XRD analysis, performed on powders reveals LiCoPO₄ as major crystalline phase, Co₂P and Co₂P₂O₇ as secondary phases. The morphological investigation revealed the formation and growth of microcrystalline “islands” which consist of acicular crystallites with different dimensions (typically 5–50 μm). By addition of Mg-ions, CV-curves of LCMP–GCF composites show a decrease of the surface between anodic and cathodic sweeps by cycling and a stark contribution of faradaic processes due to the graphitic structured foam. The electrochemical measurements, at a discharge rate of C/10 at room temperature, show the decrease of the discharge specific capacity from 100 mAh g⁻¹ for x = 0.0 to ∼35 mAh g⁻¹ for 0.025 ≤ x ≤ 0.05, then an increase to 69 mAh g⁻¹ for x = 0.1. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity at high Mg-ions content into the LiCoPO₄ phase (x ≥ 0.025).     

Microstructures and electrochemical hydrogen storage properties of novel Mg–Al–Ni amorphous composites

The amorphous Mg–Al–Ni composites were prepared by mechanical ball-milling of Mg₁₇Al₁₂ with x wt.% Ni (x = 0, 50, 100, 150, 200). The effects of Ni addition and ball-milling parameters on the electrochemical hydrogen storage properties and microstructures of the prepared composites have been investigated systematically. For the Mg₁₇Al₁₂ ball-milled without Ni powder, its particle size decreases but the crystal structure does not change even the ball-milling time extending to 120 h, and its discharge capacity is less than 15 mAh g^?1. The Ni addition is advantageous for the formation of Mg–Al–Ni amorphous structure and for the improvement of the electrochemical characteristics of the composites. With the Ni content x increasing, the composites exhibit higher degree of amorphorization. Moreover, the discharge capacity of the composite increases from 41.3 mAh g^?1 (x = 50) to 658.2 mAh g^?1 (x = 200) gradually, and the exchange current density I₀ increases from 67.1 mA g^?1 (x = 50) to 263.8 mA g^?1 (x = 200), which is consistent with the variation of high-rate dischargeability (HRD). The ball-milled Mg₁₇Al₁₂ + 200 wt.% Ni composite has the highest cycling discharge capacity in the first 50 cycles.     

Study on the effect of Li doping in spinel Li4+xTi5−xO12 (0 ⩽ x ⩽ 0.2) materials for lithium-ion batteries

The effect of Li doping in spinel Li_4+xTi_5−xO₁₂ (0 ⩽ x ⩽ 0.2) materials on the structural and electrochemical properties were investigated. The ratio of the capacity in the voltage plateau (1.5 V) to the overall discharge capacity for Li_4.1Ti_4.9O₁₂ (x = 0.1) and Li_4.2Ti_4.8O₁₂ (x = 0.2) were higher than that of Li₄Ti₅O₁₂ especially at high current rates due to their enhanced lithium-ion and electronic conductivity by the substitution of Ti atoms by Li atoms. With the increasing of Li doping amount, lithium-ion and electronic conductivity of Li_4+xTi_5−xO₁₂ increased, however its cycling stability was depressed when the Li doping was of x = 0.2. The Li doping of x = 0.1, the appropriate Li doping amount, showed improved rate capability and better high rate performance comparing to undoped Li_4+xTi_5−xO₁₂ (x = 0).     

Plastic–polymer composite electrolytes: Novel soft matter electrolytes for rechargeable lithium batteries

We present here a soft matter solid composite electrolyte obtained by inclusion of a polymer in a semi-solid organic plastic lithium salt electrolyte. Compared to lithium bis-trifluoromethanesulfonimide-succinonitrile (LiTFSI-SN), the (100 − x)%-[LiTFSI-SN]: x%-P (P: polyacrylonitrile (PAN), polyethylene oxide (PEO), polyethylene glycol dimethyl ether (PEG)) composites possess higher ambient temperature ionic conductivity, higher mechanical strength and wider electrochemical window. At 25 °C, ionic conductivity of 95%-[0.4 M LiTFSI-SN]: 5%-PAN was 1.3 × 10⁻³ Ω⁻¹ cm⁻¹ which was twice that of LiTFSI-SN. The Young’s modulus (Y) increased from Y → 0 for LiTFSI-SN to a maximum ∼1.0 MPa for (100 − x)%-[0.4 M LiTFSI-SN]: x%-PAN samples. The electrochemical voltage window for composites was approximately 5 V (Li/Li⁺). Excellent galvanostatic charge/discharge cycling performance was obtained with composite electrolytes in Li|LiFePO₄ cells without any separator.     

Crystal structures,site occupations and phase equilibria in the system V–Zr–Ge

The V–Zr–Ge system was studied for two isothermal sections at 900 and 1200 °C. Three ternary compounds VZrGe (tI12, I4/mmm, CeScSi-type), V_xZr_5?xGe₄ (oP36, Pnma, Sm₅Ge₄-type) and V_4+xZr_2?xGe₅ (oI44, Ibam, Si₅V₆-type) were structurally characterized. Optical microscopy and powder X-ray diffraction (XRD) were used for initial sample characterization and electron probe microanalysis (EPMA) of the annealed samples was used to determine the exact phase compositions. The variation of the cell parameters of the various ternary solid solutions with the composition was determined. The three ternary phases were structurally characterized by means of single crystal and powder XRD. While VZrGe is almost a line compound, V_xZr_5?xGe₄ (0.2 ≤ x ≤ 3.0) and V_4+xZr_2?xGe₅ (0.06 ≤ x ≤ 1.2) are forming extended solid solution ranges stabilized by differential fractional site occupancy of V and Zr on the metal sites.     

Determination of the deuterium/hydrogen ratio in gas reaction products by laser-induced breakdown spectroscopy

This paper reports the first application of laser-induced breakdown spectroscopy technique (LIBS) to the determination of deuterium/hydrogen numeric ratio (β) in the headspace gases, essentially HD + H₂, that are generated by the hydrolysis of NaBD₄–NaBH₄ mixtures (molar fraction of NaBD₄, x = 50–100%) in acidic H₂O media (0 < pH < 1). The LIBS measurement of β can be easily achieved with a coefficient of variation better than 5% (over four replicates). The value of β allowed the calculation of the molar fraction of NaBD₄, x_LIBS, with a coefficient of variation better than 2.5%. The comparison of x vs. x_LIBS gives results that are in good agreement, within an average deviation of 3%, for x in the range of 50–100%. The best performances are obtained for β close to unit, which makes LIBS perfectly suited for the detection of H–D exchange taking place during aqueous hydrolysis of NaBD₄ or NaBH₄.     

Li(4−x)/3Ti(5−2x)/3CrxO4 (0 ≤ x ≤ 0.9) spinels: New negatives for lithium batteries

Members of the spinel solid solution between Li_4/3Ti_5/3O₄ and LiCrTiO₄, i.e., Li_(4−x)/3Ti_(5−2x)/3Cr_xO₄ (0 ≤ x ≤ 0.9), have been investigated as possible negative electrodes for future lithium-ion batteries. Electrochemical behaviour have been studied over the potential range 1–3.5 V vs Li⁺/Li. Results are promising with anodic capacities between 129 and 163 mA h/g with a flat operating voltage at about 1.5 V, which is attributed to the pair Ti⁴⁺/Ti³⁺. The inclusion of Cr³⁺ in the spinel structure enhances the specific capacity. In-situ X-ray diffraction experiments confirm that the reaction proceeds in a topotactic manner.     

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.     

Controllable thermal expansion and phase transition in Yb2−xCrxMo3O12

The thermal expansion and phase transition of solid solutions Yb_2?xCr_xMo₃O₁₂ have been investigated by X-ray powder diffraction and differential thermal analysis. The XRD patterns and the results of Rietveld refinement of Yb_2?xCr_xMo₃O₁₂ indicate that the solid solution limit was in the composition range of 0.0 ≤ x ≤ 0.4 and 1.7 ≤ x ≤ 2.0. Yb_2?xCr_xMo₃O₁₂ (0.0 ≤ x ≤ 0.4) has an orthorhombic structure and exhibits negative thermal expansion between 200 °C and 800 °C. Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) crystallizes in monoclinic below the phase transition and above, transforms to orthorhombic. Both monoclinic and orthorhombic compounds Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) present positive thermal expansion. Orthorhombic Yb_2?xCr_xMo₃O₁₂ exhibit anisotropic thermal expansion with the contraction of a and c axes, and the linear thermal expansion coefficients range from negative to positive with increasing chromium content. Partial substitution of Yb³⁺ for Cr³⁺ exhibits depressed monoclinic to orthorhombic phase transition.     

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.     

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.     

XPS/EXAFS study of cycleability improved LiMn2O4 thin film cathodes prepared by solution deposition

The influence of Sn substitution in LiMn₂O₄ thin films as a cathode has been studied via solution deposition to improve the electrochemical performance of thin film lithium batteries. LiSn_0.025Mn_1.95O₄ thin films showed the most promising performance, i.e. a high capacity retention of 77% at 10 C after the 500th cycle, due to the increased average Mn valence state. The thin films of LiSn_x/2Mn_2?xO₄ (x ? 0.10) showed significant precipitation of SnO₂ and SnO after the cycling evaluation.     

Achieving high-rate capacitance of multi-layer titanium carbide (MXene) by liquid-phase exfoliation through Li-intercalation

The stacks of multi-layer Ti₃C₂T_x and other types of MXene materials limit their electrochemical performance. Herein, we report a facile exfoliation technique to improve the exfoliation efficiency through Li-intercalation into Ti₃C₂T_x interlayers in isopropyl alcohol (IPA) with LiOH as intercalant. This de-intercalation method presented here not only effectively delaminates the stacked Ti₃C₂T_x multi-layers into separate few-layer MXene sheets, but also achieves high-rate supercapacitive performance of Ti₃C₂T_x electrode. The as-produced delaminated Ti₃C₂T_x shows highly improved electrochemical capacitive properties from 47 to 115 F g^− 1 at 200 mV s^− 1. Even at extremely high scan rate of 1000 mV s^− 1, a specific capacitance of 82 F g^− 1 is still obtained. The high-rate capability can be attributed to improved ions accessibility into the few-layer structures. This study offers a new and simple exfoliation pathway for MXenes materials to exploit their full potential in energy storage applications.