Comparison of different soft chemical routes synthesis of submicro-LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> and their influence on its electrochemical properties

A comparative study of submicro-crystalline spinel LiMn₂O₄ powders prepared by two different soft chemical routes such as hydrothermal and sol–gel methods is made. The dependence of the physicochemical properties of the spinel LiMn₂O₄ powder has been extensively investigated by using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscope, cyclic voltammogram, charge–discharge test, and electrochemical impedance spectroscopy (EIS). The results show that the electrochemical performances of spinel LiMn₂O₄ depend strongly upon the synthesis method. The LiMn₂O₄ powder prepared by hydrothermal route has higher specific capacity and better cycling performance than the one synthesized from sol–gel method. The former has the max discharge capacity of 114.36 and 99.78 mAh g⁻¹ at the 100th cycle, while the latter has the max discharge capacity of 98.67 and 60.25 mAh g⁻¹ at the 100th cycle. The selected equivalent circuit can fit well the EIS results of synthesized LiMn₂O₄. For spinel LiMn₂O₄ from sol–gel method and hydrothermal route in the first charge process R _SEI remain almost invariable, R _e and R _ct first decreasing and then increasing with the increase of polarization potential.     

Synthesis and electrochemical performances of Li4Ti4.95Zr0.05O12/C as anode material for lithium-ion batteries

Spinel Li₄Ti_5 − x Zr _x O₁₂/C (x = 0, 0.05) were prepared by a solution method. The structure and morphology of the as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The electrochemical performances including charge–discharge (0–2.5 V and 1–2.5 V), cyclic voltammetry, and ac impedance were also investigated. The results revealed that the Li₄Ti_4.95Zr_0.05O₁₂/C had a relatively smaller particle size and more regular morphology than that of Li₄Ti₅O₁₂/C. Zr⁴⁺ doping enhanced the ability of lithium-ion diffusion in the electrode. It delivered a discharge capacity 289.03 mAh g⁻¹ after 50 cycles for the Zr⁴⁺-doped Li₄Ti₅O₁₂/C while it decreased to 264.03 mAh g⁻¹ for the Li₄Ti₅O₁₂/C at the 0.2C discharge to 0 V. Zr⁴⁺ doping did not change the electrochemical process, instead enhanced the electronic conductivity and ionic conductivity. The reversible capacity and cycling performance were effectively improved especially when it was discharged to 0 V.     

Synthesis and characterization of submicron size particles of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> by microemulsion route

Among the various positive electrode materials investigated for Li-ion batteries, spinel LiMn₂O₄ is one of the most important materials. Small particles of the active materials facilitate high-rate capability due to large surface to mass ratio and small diffusion path length. The present work involves the synthesis of submicron size particles of LiMn₂O₄ in a quaternary microemulsion medium. The precursor obtained from the reaction is heated at different temperatures in the range from 400 to 900 °C. The samples heated at 800 and 900 °C are found to possess pure spinel phase with particle size <200 nm, as evidenced from XRD, SEM, and TEM studies. The electrochemical characterization studies provide discharge capacity values of about 100 mAh g⁻¹ at C/5 rate, and there is a moderate decrease in capacity by increasing the rate of charge–discharge cycling. Studies also include charge–discharge cycling and ac impedance studies in temperature range from −10 to 40 °C. Impedance data are analyzed with the help of an equivalent circuit and a nonlinear least squares fitting program. From temperature dependence of charge-transfer resistance, a value of 0.62 eV is obtained for the activation energy of Mn³⁺/Mn⁴⁺ redox process, which accompanies the intercalation/deintercalation of the Li⁺ ion in LiMn₂O₄.     

LiNi<Subscript>0.8</Subscript>Co<Subscript>0.2−x</Subscript>Ti<Subscript>x</Subscript>O<Subscript>2</Subscript> nanoparticles: synthesis,structure, and evaluation of electrochemical properties for lithium ion cell application

Layered Ti-doped lithiated nickel cobaltate, LiNi_0.8Co_{0.2 − x}Ti_xO₂ (where x = 0.01, 0.03, and 0.05) nanopowders were prepared by wet-chemistry technique. The structural properties of synthesized materials were characterized by X-ray diffraction (XRD) and thermo-gravimetric/differential thermal analysis (TG/DTA). The morphological changes brought about by the changes in composition of LiNi_0.8Co_{0.2 − x}Ti_xO₂ particles were examined through surface examination techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. Electrochemical studies were carried out using 2016-type coin cell in the voltage range of 3.0–4.5 V (vs carbon) using 1 M LiClO₄ in ethylene carbonate and diethyl carbonate as the electrolyte. Among the various concentrations of Ti-doped lithiated nickel cobaltate materials, C/LiNi_0.8Co_0.17Ti_0.03O₂ cell gives stable charge–discharge features.     

富锂层状正极材料Li_2Mn_(1-x)Ti_xO_3的合成及电化学性能

应用简单的高温固相烧结法合成了Ti掺杂改性的Li_2MnO_3材料。电子扫描显微镜、X射线衍射以及X射线光电子能谱分析表明Ti元素取代Mn离子掺入到Li_2MnO_3晶格中,且掺杂能有效地抑制一次颗粒的团聚。电化学阻抗和恒流充放电测试结果表明,在2.0~4.6 V的电压窗口下,掺杂改性的样品Li_2Mn_(0.97)Ti_(0.03)O_3的首圈放电比容量达到209 m Ah·g~(-1),库仑效率为99.5%,循环40圈后容量保持率为94%;当电流密度增大到400 m A·g~(-1)时,掺杂改性的样品仍然可以放出120 m Ah·g~(-1)比容量,远高于同等电流密度下未掺杂的Li_2MnO_3原粉的比容量(52 m Ah·g~(-1))。Ti掺杂可有效地改善Li_2MnO_3的循环稳定性和倍率性能,有利于促进该材料的商业化应用。     

富锂层状正极材料Li2Mn1-xTixO3的合成及电化学性能

应用简单的高温固相烧结法合成了Ti掺杂改性的Li₂MnO₃材料。电子扫描显微镜、X射线衍射以及X射线光电子能谱分析表明Ti元素取代Mn离子掺入到Li₂MnO₃晶格中,且掺杂能有效地抑制一次颗粒的团聚。电化学阻抗和恒流充放电测试结果表明,在2.0~4.6 V的电压窗口下,掺杂改性的样品Li₂Mn_0.9Ti_0.03O₃的首圈放电比容量达到209 mAh·g^-1,库仑效率为99.5%,循环40圈后容量保持率为94%;当电流密度增大到400 mA·g^-1时,掺杂改性的样品仍然可以放出120 mAh·g^-1比容量,远高于同等电流密度下未掺杂的Li₂MnO₃原粉的比容量（52 mAh·g^-1）。Ti掺杂可有效地改善Li₂MnO₃的循环稳定性和倍率性能,有利于促进该材料的商业化应用。     

Mechanisms for the formation of layered A<Subscript>4</Subscript>B<Subscript>3</Subscript>O<Subscript>12</Subscript> compounds from coprecipitated hydroxocarbonate and hydroxide systems

We have established and analyzed the sequences of phase transitions in synthesis of layered compounds in the A_nB_n–1O_3n family ( \textA₃^\textII\textLnB₃^\textV\textO₁₂ {\text{A}}_3^{\text{II}}{\text{LnB}}_3^{\text{V}}{{\text{O}}_{{12}}} (A^II = Ba, Sr, Ln = La, Nd, B^V = Nb, Ta) and La₄Ti₃O₁₂ with n = 4) from coprecipitated hydroxocarbonate and hydroxide systems, including steps involving the formation, solid-phase reaction, or structural rearrangement of intermediates.     

二价与四价金属离子等摩尔共掺杂的锂离子电池正极材料LiMn1.9Mg0.05Ti0.05O4的制备与表征

以氢氧化锂、乙酸锰、硝酸镁和钛酸丁酯为原料, 以柠檬酸为螯合剂, 采用溶胶-凝胶法制备了二价镁离子与四价钛离子等摩尔共掺杂的尖晶石型锂离子电池正极材料LiMn_1.9Mg_0.05Ti_0.05O₄. 采用热重分析(TGA), X射线衍射(XRD), 扫描电子显微镜(SEM), 透射电子显微镜(TEM)和电化学性能测试(包括循环伏安(CV)和电化学交流阻抗谱(EIS)测试)对所得样品的结构、形貌及电化学性能进行了表征. 结果表明: 780℃下煅烧12 h 得到了颗粒均匀细小的尖晶石型结构的LiMn_1.9Mg_0.05Ti_0.05O₄材料, 该材料具有良好的电化学性能, 在室温下以0.5C倍率充放电, 在4.35-3.30 V电位范围内放电比容量达到126.8 mAh·g^-1, 循环50 次后放电比容量仍为118.5mAh·g^-1, 容量保持率为93.5%. 在55℃高温下循环30次后的放电比容量为111.9 mAh·g^-1, 容量保持率达到91.9%, 远远高于未掺杂的LiMn₂O₄的容量保存率. 二价镁离子与四价钛离子等摩尔共掺杂LiMn₂O₄, 改善了尖晶石锰酸锂的电子导电和离子导电性能, 使其倍率性能和高温性能都得到了明显的提高.     

Oxygen non-stoichiometry,high-temperature properties,and phase diagram of CaMnO<Subscript>3–<Emphasis Type="Italic">δ</Emphasis></Subscript>

The oxygen content in CaMnO_3–δ is studied by a coulometric titration technique depending on temperature and oxygen partial pressure variations in the ambient atmosphere. The δ–T phase diagram is derived from the obtained data where single-phase fields are outlined for orthorhombic, tetragonal, and cubic structures. The thermal expansion coefficient considerably larger in the cubic phase than in the tetragonal one is related with formation of large Mn³⁺ cations at depletion of oxygen from the crystalline lattice. Negative thermopower is explained by concomitant reactions of oxygen loss and charge disproportionation, 2Mn^{4 +} = Mn^{3 +} + Mn^{5 +} {\hbox{2M}}{{\hbox{n}}^{{{4 + }}}}{\hbox{ = M}}{{\hbox{n}}^{{{3 + }}}}{\hbox{ + M}}{{\hbox{n}}^{{{5 + }}}}{ } . The forbidden energy gap in CaMnO_3–δ is evaluated to be about 0.5 eV.     

Synthesis and electrochemical properties of spinel Li4Ti5O12−x Cl x anode materials for lithium-ion batteries

Li₄Ti₅O_12−x Cl _x (0 ≤ x ≤ 0.3) compounds were synthesized successfully via high temperature solid-state reaction. X-ray diffraction and scanning electron microscopy were used to characterize their structure and morphology. Cyclic voltammetry, electrochemical impedance spectroscopy, and charge/discharge cycling performance tests were used to characterize their electrochemical properties. The results showed that the Li₄Ti₅O_12−x Cl _x (0 ≤ x ≤ 0.3) compounds were well-crystallized pure spinel phase and that the grain sizes of the samples were about 3–8 μm. The Li₄Ti₅O_11.8Cl_0.2 sample presented the best discharge capacity among all the samples and showed better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂. When the discharge rate was 0.5 C, the Li₄Ti₅O_11.8Cl_0.2 sample presented the superior discharge capacity of 148.7 mAh g⁻¹, while that of the pristine Li₄Ti₅O₁₂ was 129.8 mAh g⁻¹; when the discharge rate was 2 C, the Li₄Ti₅O_11.8Cl_0.2 sample presented the discharge capacity of 120.7 mAh g⁻¹, while that of the pristine Li₄Ti₅O₁₂ was only 89.8 mAh g⁻¹.     

Synergistic extraction of some univalent cations into nitrobenzene by using sodium dicarbollylcobaltate and hexaethyl <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">tert</Emphasis>-butylcalix[6]arene hexaacetate

From extraction experiments and γ-activity measurements, the exchange extraction constants corresponding to the general equilibrium M⁺ (aq) + NaL⁺ (nb) ⇔ ML⁺ (nb) + Na⁺ (aq) taking place in the two-phase water–nitrobenzene system (M⁺ = H₃O⁺, \textNH₄⁺ {\text{NH}}_{4}{}^{+} , Ag⁺, Tl⁺; L = hexaethyl p-tert-butylcalix[6]arene hexaacetate; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Furthermore, the stability constants of the ML⁺ complexes in nitrobenzene saturated with water were calculated; they were found to increase in the following order: \textAg ⁺   <  NH₄ ⁺   <  \textH ₃ \textO ⁺   <  \textNa ⁺   <  \textTl ⁺ . {\text{Ag}}^{ + } \, < \,\hbox{NH}_{4}{}^{ + } \, < \,{\text{H}}_{ 3} {\text{O}}^{ + } \, < \,{\text{Na}}^{ + } \, < \,{\text{Tl}}^{ + }.     

Preparation and performance comparison of LiMn2O3.95Br0.05 and LiMn2O3.95Br0.05/SiO2 cathode materials for lithium-ion battery

LiMn₂O_3.95Br_0.05 and LiMn₂O_3.95Br_0.05/SiO₂ cathode composites for lithium-ion battery are prepared by solid-state reaction methods. The crystalline structures of the as-synthesized samples are investigated by X-ray diffraction and transmission electron microscope; at the same time, the electrochemical performances are tested by cyclic voltammetry and galvanostatic cycling. The results reveal that the sample of LiMn₂O_3.95Br_0.05/SiO₂ has more excellent electrochemical performance than the sample of LiMn₂O_3.95Br_0.05. It delivers an initial discharge capacity of 145.3 mA h g⁻¹ at ambient temperature, and 138.9 mA h g⁻¹ at the higher temperature of 55 °C with good capacity retention with the voltage range of 3.0–4.35 V (vs. Li) at a current density of 0.5 C; while the sample of LiMn₂O_3.95Br_0.05 only deliver initial discharge capacity 136.5 mA h g⁻¹ at ambient temperature, and 119.2 mA h g⁻¹ at 55 °C in the same conditions; in addition, the rate performance of LiMn₂O_3.95Br_0.05/SiO₂ is excellent too, so the SiO₂ layer has improved the electrochemical behaviors of LiMn₂O_3.95Br_0.05 availably.     

Electrochemical performance of rare-earth doped LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel cathode materials for Li-ion rechargeable battery

Spinel powders of LiMn_2−x RE _x O₄ (RE = La, Ce, Nd, Sm; 0 ≤ x ≤ 0.1) have been synthesized by solid-phase reaction. The structure and electrochemical properties of these electrode materials were characterized by X-ray diffraction (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and charge–discharge experiment. The part substitution of rare-earth element RE for Mn in LiMn₂O₄ decreases the lattice parameter, resulting in the improvement of structural stability, and decreases the charge transfer resistance during the electrochemical process of LiMn₂O₄. As a result, the cycle ability, 55 °C high-temperature and high-rate performances of LiMn_2−x RE _x O₄ electrode materials are significantly improved with increasing RE addition, compared to the pristine LiMn₂O₄.     

Characteristics of methyl cellulose-NH4NO3-PEG electrolyte and application in fuel cells

We report the viability of methyl cellulose (MC) as a membrane in a polymer electrolyte membrane fuel cell (PEMFC). Methyl cellulose serves as the polymer host, ammonium nitrate (NH₄NO₃) as the doping salt and poly(ethylene glycol) (PEG) as plasticizer. Conductivity measurement was carried out using electrochemical impedance spectroscopy. The room temperature conductivity of pure MC film is ( 3.08±0.63 ) ×10 ^{- 11}S cm ^{- 1} \left( {{3}.0{8}\pm 0.{63}} \right) \times {1}{0^{ - {11}}}{\hbox{S}}\,{\hbox{c}}{{\hbox{m}}^{ - {1}}} . The conductivity increased to ( 2.10±0.37 ) ×10 ^{- 6}S cm ^{- 1} \left( {{2}.{1}0\pm 0.{37}} \right) \times {1}{0^{ - {6}}}{\hbox{S}}\,{\hbox{c}}{{\hbox{m}}^{ - {1}}} on addition of 25 wt.% NH₄NO₃. By adding 15 wt.% of PEG 200 to the highest conducting sample in the MC-NH₄NO₃ system, the conductivity was further raised by two orders of magnitude to ( 1.14±0.37 ) ×10 ^{- 4}S cm ^{- 1} \left( {{1}.{14}\pm 0.{37}} \right) \times {1}{0^{ - {4}}}{\hbox{S}}\,{\hbox{c}}{{\hbox{m}}^{ - {1}}} . The highest conducting sample containing 15 wt.% PEG was used as membrane in PEMFC and was operated at room and elevated temperatures. From voltage-current density characteristics, the short circuit current density was 31.52 mA cm⁻² at room temperature (25 °C).     

Ge4+、Sn4+掺杂尖晶石LiMn2O4的合成与电化学表征

使用Ge⁴⁺、Sn⁴⁺作为掺杂离子, 通过高温固相法制备四价阳离子掺杂改性的尖晶石LiMn₂O₄材料. X射线衍射(XRD)和扫描电子显微镜(SEM)分析表明, Ge⁴⁺离子取代尖晶石中Mn⁴⁺离子形成了LiMn_2-xGe_xO₄ (x=0.02,0.04, 0.06)固溶体; 而Sn⁴⁺离子则以SnO₂的形式存在于尖晶石LiMn₂O₄的颗粒表面. Ge⁴⁺离子掺入到尖晶石LiMn₂O₄材料中, 抑制了锂离子在尖晶石中的有序化排列, 提高了尖晶石LiMn₂O₄的结构稳定性; 而在尖晶石颗粒表面的SnO₂可以减少电解液中酸的含量, 抑制酸对LiMn₂O₄活性材料的侵蚀. 恒电流充放电测试表明, 两种离子改性后材料的容量保持率均有较大幅度的提升, 有利于促进尖晶石型LiMn₂O₄锂离子电池正极材料的商业化生产.     

Transport, thermomechanical, and electrode properties of perovskite-type {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right)

Increasing Sr²⁺ and Ti⁴⁺ concentrations in perovskite-type $ {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right) $ results in slightly higher thermal and chemical expansion, whereas the total conductivity activation energy tends to decrease. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range (10.8?C14.5)?×?10^?6?K^?1 at 373?C1,373?K, being almost independent of the oxygen partial pressure. Variations of the conductivity and Seebeck coefficient, studied in the oxygen pressure range 10^?18?C0.5?atm, suggest that the electronic transport under oxidizing and moderately reducing conditions is dominated by p-type charge carriers and occurs via a small-polaron mechanism. Contrary to the hole concentration changes, the hole mobility decreases with increasing x. The oxygen permeation fluxes through dense ceramic membranes are quite similar for all compositions due to very low level of oxygen nonstoichiometry and are strongly affected by the grain-boundary diffusion and surface exchange kinetics. The porous electrodes applied onto lanthanum gallate-based solid electrolyte exhibit a considerably better electrochemical performance compared to the apatite-type La₁₀Si₅AlO_26.5 electrolyte at atmospheric oxygen pressure, while Sr²⁺ and Ti⁴⁺ additions have no essential influence on the polarization resistance. In H₂-containing gases where the electronic transport in $ {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta $ perovskites becomes low, co-doping deteriorates the anode performance, which can be however improved by infiltrating Ni and $ {\hbox{Ce}}{{\hbox{O}}_{{\rm{2}} - }}_\delta $ v into the porous oxide electrode matrix.     

溶胶-凝胶法合成Li1-xNaxMn2O4及其作为水系锂离子电池正极材料的电化学性能

用溶胶凝胶法合成了Na⁺离子掺杂的Li_1-xNa_xMn₂O₄(x=0,0.01,0.03,0.05)。X射线衍射图表明Na⁺取代Li⁺进入Li_1-xNa_x Mn₂O₄晶格中,扫描电镜图看出产物是粒径为100~300 nm的颗粒。恒流充放电测试结果表明,Li_0.97Na_0.03Mn₂O₄在2C倍率下循环100圈后放电容量保持率比未掺杂的LiMn₂O₄从51.2%提升到84.1%。循环伏安测试表明Na⁺离子掺杂降低了材料极化且增大了锂离子扩散系数。10C倍率下Li_0.97Na_0.03Mn₂O₄仍有79.0 mAh·g^-1的放电容量,高于未掺杂样品的52.1 mAh·g^-1。Na⁺离子掺杂可以稳定材料结构并提高锂离子扩散系数,从而提高LiMn₂O₄的电化学性能,是一种可行的改性方法。     

溶胶-凝胶法合成Li_(1-x)Na_xMn_2O_4及其作为水系锂离子电池正极材料的电化学性能

用溶胶凝胶法合成了Na+离子掺杂的Li_(1-x)Na_xMn_2O_4(x=0,0.01,0.03,0.05)。X射线衍射图表明Na+取代Li+进入Li_(1-x)Na_xMn_2O_4晶格中,扫描电镜图看出产物是粒径为100~300 nm的颗粒。恒流充放电测试结果表明,Li_(0.97)Na_(0.03)Mn_2O_4在2C倍率下循环100圈后放电容量保持率比未掺杂的LiMn_2O_4从51.2%提升到84.1%。循环伏安测试表明Na+离子掺杂降低了材料极化且增大了锂离子扩散系数。10C倍率下Li0.97Na0.03Mn2O4仍有79.0 m Ah·g-1的放电容量,高于未掺杂样品的52.1 m Ah·g~(-1)。Na+离子掺杂可以稳定材料结构并提高锂离子扩散系数,从而提高LiMn_2O_4的电化学性能,是一种可行的改性方法。     

Do the O<Subscript>2</Subscript> Schumann–Runge Bands Participate in keV Collision-Induced Dissociation Experiments?

In high-energy (keV) CID experiments, oxygen has the unique ability to enhance specific ion fragmentation pathways that lie within a relatively narrow band of activation energy. It has been previously proposed that this oxygen-enhanced dissociation phenomenon is due to the participation of the O₂  B ³S_u ⁺ - X ³S_g ^- {{\hbox{O}}_{{2}}}\;{\hbox{B}}{\,^{{3}}}{\Sigma_{\rm{u}}}^{ + } - {\hbox{X}}{\,^{{3}}}{\Sigma_{\rm{g}}}^{ - } (Schumann–Runge) system in the collision complex. During the collision, oxygen is first excited to its B ³S_u ⁺ {\hbox{B}}{\,^{{3}}}{\Sigma_{\rm{u}}}^{ + } state before it returns this energy to the projectile ion. This energy drives the nonstatistical dissociation of the projectile provided there is an energetically accessible pathway in resonance with the absorbed radiation. To probe the validity of this hypothesis, a modified VG-ZAB mass spectrometer was used to observe the photon emissions from keV collisions of a selection of projectile ions with O₂ target gas. By studying the resulting collision-induced emission (CIE) spectra, a second potential mechanism came to light, one that involves the near-isoenergetic O₂ ^+. A ²Π_u→X ² Π_g state transition.     

Oxalic acid-assisted preparation of LiFePO4/C cathode material for lithium-ion batteries

LiFePO₄/C composite is synthesized by oxalic acid-assisted rheological phase method. Fe₂O₃ and LiH₂PO₄ are chosen as the starting materials, sucrose as carbon sources, and oxalic acid as the additive. The crystalline structure and morphology of the products are characterized by X-ray diffraction and field emission scanning electron microscopy. The charge–discharge kinetics of LiFePO₄ electrode is investigated using cyclic voltammetry and electrochemical impedance spectroscopy. It is found that the introduction of appropriate amount of oxalic acid leads to smaller particle sizes, more homogeneous size distribution, and some Fe₂P produced in the final products, resulting in reduced polarization, impedance, and improved Li⁺ ion diffusion coefficient. The best cell performance is delivered by the sample with R = 1.5 (R of the molar ratio of oxalic acid to LiH₂PO₄). Its discharge capacity is 154 mAh g⁻¹ at 0.2 C rate and 120 mAh g⁻¹ at 5.0 C rate. At the same time, it exhibits an excellent cycling stability; no obvious decrease even after 1,000 cycles at 1.0 C rate.