Characterization of structure and electrochemical properties of lithium manganese oxides for lithium secondary batteries hydrothermally synthesized from δ-KxMnO2

Lithium manganese oxides have attracted much attention as cathode materials for lithium secondary batteries in view of their high capacity and low toxicity. In this study, layered manganese oxide (δ-K_xMnO₂) has been synthesized by thermal decomposition of KMnO₄, and four lithium manganese oxide phases have been synthesized for the first time by mild hydrothermal reactions of this material with different lithium compounds. The lithium manganese oxides were characterized by powder X-ray diffraction (XRD), inductively coupled plasma emission (ICPE) spectroscopy, and chemical redox titration. The four materials obtained are rock salt structure Li₂MnO₃, hollandite (BaMn₈O₁₆) structure α-MnO₂, spinel structure LiMn₂O₄, and birnessite structure Li_xMnO₂. Their electrochemical properties used as cathode material for secondary lithium batteries have been investigated. Of the four lithium manganese oxides, birnessite structure Li_xMnO₂ demonstrated the most stable cycling behavior with high Coulombic efficiency. Its reversible capacity reaches 155 mAh g⁻¹, indicating that it is a viable cathode material for lithium secondary batteries.     

Lithium fuel cells:: I. Lithium/poly(organophosphazene) membrane anodes in KOH and seawater

The main goal of our research project is to design safe, high energy and power density lithium/water systems. We explored the feasibility of substituting the natural bilayer (formed on the lithium surface when lithium is in contact with water), for a thin polymeric film. By substituting the natural bilayer film we hope to reduce the parasitic reactions occurring at the lithium/water interface, thus yielding an increase in the anodic efficiency. We investigated the effect of placing or casting a thin, (lithium/ion-conducting) polymer layer on the lithium metal surface. This paper is part one in a series of two papers. Paper I presents the results obtained with a lithium/polymer system, where the polymer was a monolayer of a polyphosphazene with 90% trifluoromethylphenoxy and 10% lithium carboxyphenoxy side groups (Polymer 4), or a multilayer film formed of one layer of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) and one to three layers of Polymer 4 containing from 0 to 75 wt.% of lithium triflate salts. Paper II presents results obtained when the polymer layers were prepared using a polymer with equal amounts of methoxyethoxyethoxy and phenoxy side groups containing from 0 to 75 wt.% of lithium triflate salts. Phosphazene membranes have been designed and tailored to allow lithium ion conduction and prevent water migration to the surface of lithium metal. The phosphazene membranes enhance the safety of an aqueous lithium cell by inhibiting (or reducing) the reaction of lithium with water that evolves hydrogen at the anode. Original tests of lithium/phosphazene systems led to unpredictable open circuit voltages (OCVs). When the adhesion of the membrane to the lithium metal was improved, the OCV stabilized. The OCVs for the half-cell of lithium polymer aqueous electrolytes varies between −3.1 and −2.8 V_SCE, depending on the membrane. The current densities for this polymer system are in the range of 10⁻⁶-10⁻³ A/cm². The Columbic anodic efficiency is assumed to be near 100%—as hydrogen evolution is not measurable. Some of the polymeric membranes developed pinholes with use. Layered systems have also been designed to avoid the development of pinholes over time. In this paper, we present the results obtained by using polyphosphazenes with a 9:1 ratio of trifluoromethylphenoxy and p-carboxyphenoxy side groups and the lithium salt of the carboxylate function. Poly(organophosphazene) membranes with a single layer and a multilayer structure were tested in 8 M KOH or synthetic seawater for up to 5 days.     

A study of lithium insertion into MnO2 containing TiS2 additive a battery material in aqueous LiOH solution

The electrochemical behavior and surface characterization of manganese dioxide (MnO₂) containing titanium disulphide (TiS₂) as a cathode in aqueous lithium hydroxide (LiOH) electrolyte battery have been investigated. The electrode reaction of MnO₂ in this electrolyte is shown to be lithium insertion rather than the usual protonation. MnO₂ shows acceptable rechargeability as the battery cathode. The influence of TiS₂ (1, 3 and 5 wt%) additive on the performance of MnO₂ as a cathode has been determined. The products formed on reduction of the cathode material have been characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), fourier transform infrared spectroscopy (IR) and transmission electron microscopy (TEM). It is found that the presence of TiS₂ to ≤3 wt% improves the discharge capacity of MnO₂. However, increasing the additive content above this amount causes a decrease in its discharge capacity.     

Surface structure effects in nanocrystal MnO2 and Ag/MnO2 catalytic oxidation of CO

Pure-phase α-MnO₂ and β-MnO₂ nanowires/nanorods were synthesized through an easy solution-based hydrothermal method, and the effect of the manganese dioxide phase on the activity of MnO₂ and Ag/MnO₂ for the oxidation of CO was investigated. MnO₂ is an effective catalyst in CO oxidation, and its activity depends on the crystal phase of MnO₂. α-MnO₂ exhibits a higher activity than β-MnO₂, because the α-MnO₂ nanowires can be reduced more easily than the β-MnO₂ nanorods. Moreover, when Ag was introduced to MnO₂, a strong interaction occurred between Ag and MnO₂. The catalytic activity clearly correlates with this interaction, which is determined by crystal phase and surface structure.     

Cycling of lithium/metal oxide cells using composite electrolytes containing fumed silicas

The effect on cycle capacity is reported of cathode material (metal oxide, carbon, and current collector) in lithium/metal oxide cells cycled with fumed silica-based composite electrolytes. Three types of electrolytes are compared: filler-free electrolyte consisting of methyl-terminated poly(ethylene glycol) oligomer (PEGdm, M_w=250)+lithium bis(trifluromethylsufonyl)imide (LiTFSI) (Li:O=1:20), and two composite systems of the above baseline liquid electrolyte containing 10-wt% A200 (hydrophilic fumed silica) or R805 (hydrophobic fumed silica with octyl surface group). The composite electrolytes are solid-like gels. Three cathode active materials (LiCoO₂, V₆O₁₃, and Li_xMnO₂), four conducting carbons (graphite Timrex® SFG 15, SFG 44, carbon black Vulcan XC72R, and Ketjenblack EC-600JD), and three current collector materials (Al, Ni, and carbon fiber) were studied. Cells with composite electrolytes show higher capacity, reduced capacity fade, and less cell polarization than those with filler-free electrolyte. Among the three active materials studied, V₆O₁₃ cathodes deliver the highest capacity and Li_xMnO₂ cathodes render the best capacity retention. Discharge capacity of Li/LiCoO₂ cells is affected greatly by cathode carbon type, and the capacity decreases in the order of Ketjenblack>SFG 15>SFG 44>Vulcan. Current collector material also plays a significant role in cell cycling performance. Lithium/vanadium oxide (V₆O₁₃) cells deliver increased capacity using Ni foil and carbon fiber current collectors in comparison to an Al foil current collector.     

In-situ exsolution of PrO2−x nanoparticles boost the performance of traditional Pr0.5Sr0.5MnO3-δ cathode for proton-conducting solid oxide fuel cells

By synthesizing the nominal Pr_xSr_0.5MnO_3-δ materials (x = 0.5, 0.6, 0.7, 0.8), new Pr_0.5Sr_0.5MnO_3-δ (PSM50)+PrO_2−x composite cathodes for proton-conducting solid oxide fuel cells (SOFCs) were developed. The structure analysis and morphology observations verified the exsolution of PrO_2−x particles, and the amount of exsolved PrO_2−x increased with the amount of Pr in Pr_xSr_0.5MnO_3-δ. An H-SOFC with a Pr_0.7Sr_0.5MnO_3-δ (PSM70) cathode enabled the highest reported fuel cell output for H-SOFCs with manganate cathodes. The construction of a PSM50/PrO₂ heterostructure interface can reduce the formation energy of oxygen vacancies, hence accelerating the cathode oxygen reduction reaction (ORR) kinetics, as confirmed by oxygen diffusion and surface exchange experiments. The excellent electrochemical performance was combined with its good chemical stability against CO₂ and H₂O, allowing a stable operation of the cell for over 100 h, indicating that PSM70, which was in fact PSM50 +PrO_2−x, was a highly efficient and durable cathode material for H-SOFCs.     

Electrolytes for hybrid carbon-MnO2 electrochemical capacitors

Different aqueous-based electrolytes have been tested in order to improve the electrochemical performance of hybrid (asymmetric) carbon/MnO₂ electrochemical capacitor (EC). Chloride and bromide aqueous solutions lead to the formation of Cl₂ and Br₂ respectively upon oxidation of the corresponding salt, thus limiting the useful electrochemical window of the MnO₂ electrode and producing gas evolution (in the case of chloride salts) detrimental to the cycling ability of an hybrid device. For sulfate and nitrate salts, MnO₂ electrode exhibits a 20% increase in capacitance when lithium is used as the cation compared to sodium or potassium salts, probably due to partial lithium intercalation in the tunnels of α-MnO₂ structure. The higher ionic conductivity and solubility of LiNO₃ has led to the investigation of this electrolyte in carbon/MnO₂ supercapacitor compared to standard hybrid cell using K₂SO₄. A lower resistance increase was evidenced when the temperature was decreased down to −10 °C. Long term cycling ability of carbon/MnO₂ supercapacitor was also evidenced with 5 M LiNO₃ electrolyte.     

Synthesis of hollandite-type LixMnO2 by Li ion-exchange in molten salt and lithium insertion characteristics

The Li⁺ ion-exchange reaction of K⁺-type α-K_0.14MnO_1.93·nH₂O containing different amounts of water molecules (n = 0-0.15) with a large (2 × 2) tunnel structure has been investigated in a LiNO₃-LiCl molten salt at 300 °C. The Li⁺ ion-exchanged products were examined by chemical analysis, X-ray diffraction, and transmission electron microscopy measurements. The K⁺ ions and the hydrogens of the water molecules in the (2 × 2) tunnels of α-MnO₂ were exchanged by Li⁺ ions in the molten salt, resulting in the Li⁺-type α-MnO₂ containing different amounts of Li⁺ ions and lithium oxide (Li₂O) in the (2 × 2) tunnels with maintaining the original hollandite structure.The electrochemical properties and structural variation with initial discharge and charge-discharge cycling of the Li⁺ ion-exchanged α-MnO₂ samples have been investigated as insertion compounds in the search for new cathode materials for rechargeable lithium batteries. The Li⁺ ion-exchanged α-MnO₂ samples provided higher capacities and higher Li⁺ ion diffusivity than the parent K⁺-type materials on initial discharge and charge-discharge cyclings, probably due to the structural stabilization with the existence of Li₂O in the (2 × 2) tunnels.     

The influences of sodium sources on the structure evolution and electrochemical performances of layered-tunnel hybrid Na0.6MnO2 cathode

Manganese (Mn) based oxide materials are regarded as promising cathodes for sodium ion batteries (SIBs) due to their high energy density, low-cost and environmental benignity. Here, we focus on the influences of various sodium sources on the structure diversity and electrochemical performances changes of layered-tunnel hybrid Na_0.6MnO₂ cathode. The Na_0.6MnO₂ cathodes were prepared by precipitation method followed by grinding with different sodium sources and annealing in air. The XRD results evidenced that the mass ratio of layered and tunnel components would be markedly influenced by sodium source. Electrochemical test results also demonstrate distinctive performances of Na_0.6MnO₂ cathodes with various sodium sources. Na_0.6MnO₂ cathode with Na₂C₂O₄ exhibited the best performances with 90 mAh g⁻¹ retained after 100 cycles at 1.0C. Superior rate performance with average discharge capacities of 180, 159, 143, 126, 112 and 93 mAh g⁻¹ at 0.1, 0.5, 1.0, 2.0, 4.0 and 8.0C was also observed. Furthermore, the EIS demonstrate that Na_0.6MnO₂ cathode with Na₂C₂O₄ displayed smaller charge transfer and fast Na⁺ diffusion rate, which indicated enhanced electrochemical reaction kinetics. The excellent electrochemical performance of Na_0.6MnO₂ with Na₂C₂O₄ is mainly due to the appropriate proportion of layered-tunnel component and their synergistic effects, which are influenced by sodium sources.     

The effect of B4C addition to MnO2 in a cathode material for battery applications

Boron carbide (B₄C) added manganese dioxide (MnO₂) used as a cathode material for a Zn-MnO₂ battery using aqueous lithium hydroxide (LiOH) as the electrolyte is known to have higher discharge capacity but with a lower average discharge voltage than pure MnO₂ (additive free). The performance is reversed when using potassium hydroxide (KOH) as the electrolyte. Herein, the MnO₂ was mixed with 0, 5, 7 and 10 wt.% of boron carbide during the electrode preparation. The discharge performance of the Zn|LiOH|MnO₂ battery was improved by the addition of 5-7 wt.% boron carbide in MnO₂ cathode as compared with the pure MnO₂. However, increasing the additive to 10 wt.% causes a decrease in the discharge capacity. The performance of the Zn|KOH|MnO₂ battery was retarded by the boron carbide additive. Transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy analysis (EDS) results show evidence of crystalline MnO₂ particles during discharging in LiOH electrolyte, whereas, manganese oxide particles with different oxygen and manganese counts leading to mixture of phases is observed for KOH electrolyte which is in agreement with X-ray diffraction (XRD) data. The enhanced discharge capacity indicates that boron atoms promote lithium intercalation during the electrochemical process and improved the performance of the Zn|LiOH|MnO₂ battery. This observed improvement may be a consequence of B₄C suppressing the formation of undesirable Mn(III) phases, which in turn leads to enhanced lithium intercalation. Too much boron carbide hinders the charge carrier which inhibits the discharge capacity.