A review of recent developments in the surface modification of LiMn2O4 as cathode material of power lithium-ion battery

LiMn₂O₄ (LMO) is a very attractive choice as cathode material for power lithium-ion batteries due to its economical and environmental advantages. However, LiMn₂O₄ in the 4-V region suffers from a poor cycling behavior. Recent research results confirm that modification by coating is an important method to achieve improved electrochemical performance of LMO, and the latest progress was reviewed in the paper. The surface treatment of LMO by coating oxides and nonoxide systems could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling test. At present, LiMn₂O₄ is the mainstreaming cathode material of power lithium-ion battery, and, especially the modified LMO, is the trend of development of power lithium-ion battery cathode material in the long term.     

Synthesis of Co-coated lithium manganese oxide and its characterization as cathode for lithium ion battery

Co-coated LiMn₂O₄ was synthesized by electroless plating. The phase identification, surface morphology, and electrochemical properties of the synthesized powders were studied by X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy, and galvanostatic charge–discharge experiments, respectively. The result shows that Co-coated LiMn₂O₄ particle has a coarse surface with a lot of holes. The specific capacity of Co-coated LiMn₂O₄ is 118 mAh g⁻¹, which is a bit less than 123 mAh g⁻¹ for the uncoated LiMn₂O₄. The capacity retention of Co-coated LiMn₂O₄ is 11% higher than the uncoated LiMn₂O₄ when the electrode is cycled at room temperature for 20 times. When cycled at the temperature of 55 °C, the capacity retention of Co-coated LiMn₂O₄ becomes 15% higher than the uncoated one.     

A study on the blending effect of polyvinyledene fluoride in the ionic transport mechanism of plasticized polyvinyl chloride + lithium perchlorate gel polymer electrolytes

The gel polymer electrolytes composed of the blend of polyvinylchloride (PVC) and polyvinylidene fluoride (PVdF) as host polymers, the mixture of ethylene carbonate (EC) and propylene carbonate (PC) as a plasticizer, and LiClO₄ as a salt was studied. An attempt was made to investigate the effect of PVdF in the plasticized PVC + LiClO₄ system in three blend ratios. The differential scanning calorimetry study confirms the formation of polymer–salt complex and miscibility of the PVC and PVdF. The X-ray diffraction results of plasticized PVC (S1, S2, S3) and PVdF-blended films (S4, S5, S6) were compared, in that an increase in PVC concentration decreases the degree of crystallinity for S1 and S3, respectively, but drastically increases for PVC (S2). The increase in PVC content has not accounted in the conductivity studies also noted. However, the blending effect of PVdF showed decreases in crystallinity homogeneously for (S6 > S5 > S4), which were reflected in ionic conductivity measurements. The surface morphology of the films were also studied by scanning electron microscope, and it corroborates the same. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Improvement of lithium battery performance through composite electrode microstructure optimization

The lithium trivanadate Li_1.2V₃O₈ has been investigated during the past decade as a very promising positive electrode material for lithium batteries due to its high theoretical capacity of 360 mAh/g. However, the experimental capacity remains generally much lower than (about half) the theoretical value. To increase electrode cycling performance in batteries, most researchers generally focus their work on the active material optimisation. Here we show that the polymeric binder of the composite electrode may have an important role on the electrode performance. We describe a new tailored polymeric binder combination with controlled polymer-filler (carbon black) interactions that allows the preparation of new and more efficient electrode architecture. Using this polymeric binder, composite electrodes based on Li_1.2V₃O₈ display a room-temperature cycling capacity of 280 mAh/g (C/5 rate, 3.3-2V) instead of 180 mAh/g using a Bellcore-type composite electrode (PLIon^TM technology). We have coupled SEM observations, galvanostatic cycling and electrochemical impedance spectroscopy in order to define and understand the impact of the microstructure of the composite electrode on its electrochemical performance. Derived from these studies, the main key factors that provide efficient charge carrier collection within the composite electrode complex medium will be discussed. Present findings open up new and attractive prospects for electrode performance optimisation. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14–18, 2004.     

An investigation of PVdF/PVC-based blend electrolytes with EC/PC as plasticizers in lithium battery applications

Solid polymer electrolytes (SPEs) composed of poly(vinylidene fluoride) (PVdF)-poly(vinyl chloride) (PVC) complexed with lithium perchlorate (LiClO₄) as salt and ethylene carbonate (EC)/propylene carbonate (PC) as plasticizers were prepared using solvent-casting technique, with different weight ratios of EC and PC. The amorphicity and complexation behavior of the polymer electrolytes were confirmed using X-ray diffraction (XRD) and FTIR studies. TG/DTA and scanning electron microscope (SEM) studies explained the thermal stability and surface morphology of electrolytes, respectively. The prepared thin films were subjected to AC impedance measurements as a function of temperature ranging from 302 to 373 K. The temperature-dependence conductivity of polymer films seems to obey VTF relation.     

Origin of self-discharge mechanism in LiMn2O4-based Li-ion cells: A chemical and electrochemical approach

LiMn₂O₄-based Li-ion cells suffer from a limited cycle-life and a poor storage performance at 55 °C, both in their charged and discharged states. To get some insight on the origin of the poor 55 °C storage performance, the voltage distribution through plastic Li-ion cells during electrochemical testing was monitored by means of 3-electrode type measurements. From these measurements, coupled with chemical analysis, X-ray diffraction and microscopy studies, one unambiguously concludes that the poor performance of LiMn₂O₄/C-cells at 55 °C in their discharged state is due to enhanced Mn dissolution that increases with increasing both the temperature and the electrolyte HF content. These results were confirmed by a chemical approach which consists in placing a fresh LiMn₂O₄ electrode into a 55 °C electrolyte solution. A mechanism, based on an ion-exchange reaction leading to the Mn dissolution is proposed to account for the poor storage performance of LiMn₂O₄/C Li-ion cells in their discharged state. In order to minimize the Mn dissolution, two surface treatments were performed. The first one consists in applying an inorganic borate glass composition to the LiMn₂O₄ surface, the second one in using an acetylacetone complexing agent. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Electrochemical preparation of nanosized manganese dioxides from manganese chloride solutions

Manganese dioxides were fabricated by electrodeposition from MnCl₂, MgCl₂/MnCl₂, and HCl/MnCl₂ aqueous solutions at 100 °C, respectively. Oxidation behaviors of Mn(II) on titanium plate were studied by cyclic voltammetry. X-ray diffractometer, scanning electron microscopy, and BET measurements were used to characterize manganese dioxide crystal structures, micromorphologies, and specific surface area. The effects of electrolyte composition and potential on manganese dioxide crystal structure and micromorphology were investigated. Manganese dioxide structure type and micromorphology were controlled by adjusting electrolyte composition. γ-MnO₂ aggregates consisting of nanosheets were electrodeposited from 0.05 mol L⁻¹ MnCl₂ aqueous solution. Crystallinity and the size of γ-MnO₂ nanosheet were increased by adding Mg(II) into electrolyte. Nanosized rod-like α-MnO₂ with higher specific surface area was prepared by adding 2.0 mol L⁻¹ hydrochloric acid into manganese chloride solution.     

Preparation and characterization of Li2CoMn3O8 thin film cathodes for high energy lithium batteries

An ion layer gas reaction (ILGAR) dip-coating process for the deposition of homogeneous spinel structured Li₂CoMn₃O₈ thin layers has been developed. Thin film cathodes for use in high-energy density lithium batteries with thicknesses of about 200 nm have been prepared. The films were found to be X-ray amorphous after preparation. After annealing at 700°C in air for 2 h, the spinel structure of Li₂CoMn₃O₈ was observed by X-ray diffraction analysis. The composition of the surface was studied by XPS, which indicated enhanced Li and Mn concentrations as a result of the rinsing process and different solubilities of the precursor salts. The electrochemical behavior was investigated by separating the annealed electrode sample from a conventional organic lithium ion-conducting electrolyte by a layer of LiPON solid electrolyte and using elemental lithium as counter electrode. A capacity of 110.8 mAh/g was observed which is related to the valence changes of Mn and Co in the spinel structure.     

Porous carbon-coated graphite electrodes for energy production from salinity gradient using reverse electrodialysis

Performance of graphite foil electrodes coated by porous carbon black (i.e., Vulcan) was investigated in comparison with metal electrodes for reverse electrodialysis (RED) application. The electrode slurry that was used for fabrication of the porous carbon-coated graphite foil is composed of 7.2 wt% of carbon black (Vulcan X-72), 0.8 wt% of a polymer binder (polyvinylidene fluoride, PVdF), and 92.0 wt% of a mixing solvent (dimethylacetamide, DMAc). Cyclic voltammograms of both the porous carbon (i.e., Vulcan)-coated graphite foil electrode and the graphite foil electrode without Vulcan showed good reversibility in the hexacyanoferrate(III) (i.e., Fe(CN)₆³⁻) and hexacyanoferrate(II) (i.e., Fe(CN)₆⁴⁻) redox couple and 1 M Na₂SO₄ at room temperature. However, anodic and cathodic current of the Vulcan-coated graphite foil electrode was much higher than those of the graphite foil electrode. Using a bench-scale RED stack, the current–voltage polarization curve of the Vulcan-coated graphite electrode was compared to that of metal electrodes such as iridium (Ir) and platinum (Pt). From the results, it was confirmed that resistance of four different electrodes increased with the following order: the Vulcan-coated graphite foil<the Ir-coated titanium (Ti) mesh<the Pt-coated Ti plate<the graphite foil. Moreover, the Vulcan-coated graphite foil showed 5–10% higher power density than the metal mesh electrodes. From the polarization curve of the Vulcan-coated graphite foil electrode, it was found that total resistance decreased as thickness and geometric surface area of the electrode increased.     

表面效应对锂离子电池正极材料LiMn2O4性能的影响

应用基于自旋极化和广义梯度近似(generalized gradient approximation,GGA)的密度泛函理论计算,研究了锂离子电池正极材料LiMn₂O₄ (001)表面原子和电子结构.发现表面和亚表面附近的原子在垂直于(001)面的方向上具有非常大的弛豫,这对LiMn₂O₄材料在锂离子电池中应用时发现的表面Mn的溶解现象有很大关联.由于表面效应,在LiMn₂O₄ (001) 表面只有三价Mn³⁺离子存在,而这些三价锰离子非常活跃,在该材料电极/电解液界面很容易发生歧化反应,从而加速了Mn的溶解.其他计算结果也和实验观察相符合. 关键词： 锂二次电池 表面弛豫 从头算     

Operation of PEFC using composite sheets composed of phosphosilicate gels and thermally stable organic polymers

Thermally stable, flexible, and proton conductive composite sheets have been prepared from phosphosilicate (P₂O₅–SiO₂, P/Si = 1 in mole ratio) gel powder and several kinds of organic polymers. The organic polymers examined were polyimide (PI), polyamideimide (PAI), polyvinylidene fluoride (PVdF) and styrene–ethylene–butylenes–styrene elastomer (SEBS). PI and PAI effectively improved the chemical durability of the composite sheets by depressing the leaching of phosphoric acid from the gel. PVdF and SEBS provided the composite sheets with an excellent flexibility. Fuel cells using the membrane electrode assembly (MEA) composed of the composite sheet as an electrolyte and Pt-loaded carbon paper sheets as electrodes operated at temperatures in the range from room temperature to medium temperatures (∼ 150 °C).     

Cathodic characteristics of amorphous manganese oxides as lithium secondary battery

The amorphous manganese dioxides were prepared by reduction method using various reagents. The particle size and morphology depended on reducing agents, some of which were aggregated round particles of sub micron order, and the others were small fine fibers like of a few nm in width and several tens nm in length. The former showed low Li-insertion capacity of around 200 mAhg⁻¹, while the latter material showed high capacity of 500 mAhg⁻¹ in the OCV measurements until 1 V vs. Li. The as-prepared MnO₂ contains some amount of water in the structure. The H₂O-free MnO₂ obtained by heat-treatment under high oxygen pressure was inferior in electrode property to MnO₂ containing water in the structure. The H₂O content related to the electrode performance. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Effect of sulfolane and lithium bis(oxalato)borate-based electrolytes on the performance of spinel LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathodes at 55 °C

To seek a promising candidate electrolyte at elevated temperature for lithium manganese oxide (LiMn₂O₄)/Li cells, the electrochemical performance of 0.7 mol L^?1 LiBOB (lithium bis(oxalate)borate)-SL (sulfolane)/DEC (diethyl carbonate) (1:1, in volume) electrolyte was studied at 55 °C. The Mn dissolution in electrolyte was analyzed by inductively coupled plasma (ICP) analysis. AC impedance measurement and scanning electron microscopy (SEM) analysis were used to analyze the formation of the surface film on the LiMn₂O₄ electrode. The results demonstrate that the LiBOB-SL/DEC electrolyte can slow down the dissolution and erosion of Mn ions, and decrease the interface impedance. Moreover, the LiBOB-SL/DEC electrolyte could obviously improve the capacity retention, the operating voltage (4.05 V), and the rate performance of LiMn₂O₄/Li cells.     

Optical investigation of LaMnO<Subscript>3</Subscript> thin films: a study of the 2-eV band

We measure the transmittance of LaMnO₃ (LMO) thin films in the spectral range from 10 000 to 38 000 cm^-1 as a function of temperature. The optical conductivity obtained from the transmittance data is decomposed into five Drude-Lorentz contributions: three of them show evident changes below the Néel temperature T _N @\simeq 140 K. We discuss the band assignment in the framework of the tight-binding and of the orbitally degenerate Hubbard models. Our results ascribe the overall low-frequency spectrum of LMO to charge-transfer mechanisms involving the t _2g and e _g electrons of the Mn ions, without invoking the presence of excitonic contributions. This finding definitely promotes a Mott-Hubbard scenario for LMO and provides an estimate of the Hamiltonian parameters.     

Magnetic properties of high manganese aluminium austenitic steels

Mössbauer spectroscopy and magnetic susceptibilty measurements show that the Fe-(29–40)Mn-5Al austenitic alloys undergo an antiferromagnetic transformation similar to that observed in the Fe-(30-44)Mn austenitic steels. The Néel temperature,T_N,increases as a function of the Mn content (from 331K to 376K) while aluminium additions(5%wt.) induce a sharp decrease of T_N(–100K) but a small increase of the hyperfine field at OK (from 40 to 47 kOe for 34Mn alloys).     

Enhancement of the electrochemical performance of silicon/carbon composite material for lithium ion batteries

The silicon/carbon (Si/C) composite material was prepared, and the electrochemical performance was investigated as a promising anode material for lithium ion batteries. The results show that the binder in the electrode acts as an important role for improving the reversible capacity of the Si/C materials during cycling. The Si/C electrode with CMC/SBR binder possesses a better cycle performance than that with PVDF binder. The Si/C composite material shows an initial reversible capacity of more than 700 mAh∙g⁻¹ and remains a reversible capacity of 597 mAh g⁻¹ after 40 cycles.     

Correlation between Li+ adsorption capacity and the preparation conditions of spinel lithium manganese precursor

Spinel lithium manganese oxides can be used as Li⁺ adsorbent with topotactical extraction of lithium. In this paper, the solid state methods were introduced to prepare spinel lithium manganese precursors with Li₂CO₃ and LiOH·H₂O as different Li sources. The Li⁺ uptake was studied to clarify the correction between Li⁺ adsorption capacity and the preparation conditions of precursors, including different Li sources, Li/Mn mole ratios and heating time. The results indicated that the Li⁺-extracted materials prepared with LiOH·H₂O and MnCO₃ usually have higher Li⁺ adsorption capacity than Li₂CO₃ and MnCO₃, and an ascending trend was found in Li⁺ uptake with increasing Li/Mn mole ratio in the preparation of the precursor, but it is not proportional. The Mn₂O₃ impurities could be the primary reason for decreasing Li⁺ adsorption capacity. Furthermore, it is concluded that the Li⁺-extracted materials obtained from spinel manganese oxides synthesized with Li/Mn = 1.0 can serve as selective Li⁺ absorbents due to its high selectivity and large adsorption capacity.     

Room-temperature ferromagnetism in pure and Mn doped SnO2 powders

We report here observation of ferromagnetism in pure and Mn doped SnO₂ powder with different Mn contents. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism (RTFM), which is identified as an intrinsic characteristic. The RTFM has been observed in the pure SnO₂ powder, which is believed to be defect induced, with a saturation magnetization of ～0.017 emu/g. The RTFM was enhanced considerably in the Mn doped samples and the magnetic properties strongly depend on doping content. A sample with 1% of Mn is ferromagnetic at room temperature with a saturation magnetization of ～0.98 emu/g, a remanent magnetization of ～27%, and a coercivity of ～270 Oe. The average magnetic moment per Mn atom decreases with increasing Mn content. Our results reveal that the large RTFM observed in Mn doped SnO₂ powder originates from a combination effect of oxygen vacancies and transition metal doping.     

Electrode reaction of Sr1−xLaxCo0.8Fe0.2O3−δ with x = 0.1 and 0.6 on Ce0.9Gd0.1O1.95 at 600 ≤ T ≤ 800 °C

The electrode reaction of the perovskite phases Sr_1−xLa_xCo_0.8Fe_0.2O_3−δ (x = 0.1 and 0.6) on Ce_0.9Gd_0.1O_1.95 has been investigated by impedance spectroscopy in the temperature range 600 ≤ T ≤ 800 °C. Thick porous electrodes (t ∼ 20 μm) were sprayed on Ce_0.9Gd_0.1O_1.95 and ac impedance spectra were recorded on symmetrical cells at the equilibrium. The analysis of the complex impedance diagrams clearly indicates the presence of two contributions. The low frequency one was assigned to the gas phase oxygen diffusion through the porous electrode and a finite length diffusion (Warburg) impedance was used to describe the high frequency (HF) data. The polarization resistance of the HF impedance contribution (R_w) is higher for x = 0.1 while the activation energy of R_w is higher for x = 0.6. The variations of R_w versus the La content, temperature and thickness indicate that the Warburg-type impedance contains information of both bulk oxygen diffusion and surface processes.