原子尺度锂离子电池电极材料的近平衡结构

锂离子电池充放电过程中电极材料的结构变化与材料的电化学反应机理和性能密切相关.通过在原子尺度上直接观察脱/嵌锂前后电极材料的近平衡微观结构,有助于从更深层次认识电极反应机理和性能演化规律,对于全面理解材料的电化学行为以及改善锂离子电池性能具有重要的指导意义.本文详述了球差校正扫描透射成像技术在研究电极材料表界面结构及反应机理方面的进展,探讨了未来建立电极材料原子尺度结构与性能相关联可能的研究方向.     

碳气凝胶活化对于电极嵌锂性能的影响

碳气凝胶由于其对于可充电锂离子电池的高能嵌锂特性, 近年来受关注程度逐渐增加. 碳气凝胶以间苯二酚-甲醛在碳酸钠催化下, 通过溶胶-凝胶工艺、常压干燥技术、碳化、活化后制得. 经CO₂气体活化后的碳气凝胶结合了无定型和纳米多孔结构的优点, 在材料原有基础上丰富了多孔结构, 增加了嵌锂点位. 其中, 微孔提供了高比表面积和孔体积以容纳锂及其化合物; 介孔则提供了锂离子大量传输的通道, 从而使得电极具有更高的离子电导率. 这些微结构的优化使材料获得了更高的嵌锂比容量. 此外, 活化碳气凝胶显示了2032 m²·g^-1的比表面积. X射线衍射(XRD)和扫描电子显微镜(SEM)的测试结果分别表明了其无定型特质以及纳米颗粒的网络状骨架. 该材料在首次和第50次恒流充放电(50 mA·g^-1)循环的嵌锂容量分别为3870和352 mAh·g^-1, 对应的可逆容量分别为658 和333 mAh·g^-1. 表明了CO₂活化对于改善碳气凝胶嵌锂性能的可行性, 且对于其它多孔电极材料的制备及特性优化提供了一种途径.     

层状钛硅酸盐化合物作为锂离子电池负极储能材料

锂离子二次电池是手提设备的重要电力来源。近年来,人们为了寻找更新颖更好的锂离子电极材料,开始研究晶形离子交换材料,这种材料具有开放孔道,能够让离子在多孔框架里自由的进出。一种具有层状结构的钛硅酸盐Na-JDF-L1(Na₄Ti₂Si₈O₂₂·4H₂O)经过离子交换后被用作锂离子负极材料。它在循环200次后放电容量保持在364 mAh·g^-1,并且库伦效率约为100%。通过将TiO₂引入Li(Na)-JDF-L1中,有效的提高了材料的首次库伦效率和倍率放电性能。     

层状钛硅酸盐化合物作为锂离子电池负极储能材料

锂离子二次电池是手提设备的重要电力来源。近年来, 人们为了寻找更新颖更好的锂离子电极材料, 开始研究晶形离子交换材料, 这种材料具有开放孔道, 能够让离子在多孔框架里自由的进出。一种具有层状结构的钛硅酸盐Na-JDF-L1(Na₄Ti₂Si₈O₂₂·4H₂O)经过离子交换后被用作锂离子负极材料。它在循环200次后放电容量保持在364 mAh·g^-1, 并且库伦效率约为100%。通过将TiO₂引入Li(Na)-JDF-L1中, 有效的提高了材料的首次库伦效率和倍率放电性能。     

LiCoO2电极/电解液界面特性的电化学阻抗谱研究

运用电化学阻抗谱研究了LiCoO₂电极在电解液中的贮存和首次脱锂过程. 发现LiCoO2电极在电解液中, 随浸泡时间延长其表面SEI膜不断增厚, 归结为LiCoO₂电极与电解液之间的自发反应导致生成一些高介电常数的有机碳酸锂化合物. 研究结果指出LiCoO₂电极首次脱锂过程中, SEI膜在3.8 ~ 3.95 V电位区间发生可逆坍塌, 对应其可逆溶解; 由于过充反应, 当电位大于4.2 V SEI膜迅速增厚. 研究结果同时表明, Li/LiCoO₂电池体系的感抗来源于充放电过程中LiCoO₂电极中存在LiCoO₂/Li_1-xCoO₂局域浓差电池. 发现锂离子在LiCoO₂电极中的嵌脱过程可较好地用Langmuir嵌入等温式和Frumkin嵌入等温式描述, 测得LiCoO₂电极中锂离子嵌脱过程中电荷传递反应的对称因子α = 0.5.     

脉冲激光沉积NiCo2S4薄膜及其电化学特征

采用脉冲激光沉积法制备了NiCo₂S₄薄膜,利用恒流充放电和循环伏安测试研究了NiCo₂S₄薄膜作为锂离子电池负极材料的电化学性能和充放电机理。采用高分辨电子显微镜和选区电子衍射(TEM&SAED)表征了NiCo₂S₄薄膜首次循环过程中的组成与结构变化。恒流充放电测试结果显示NiCo₂S₄薄膜在3 μA·cm^-2的放电电流下,0~3 V(vs Li⁺/Li)范围内,薄膜的首次放电容量为698 mAh·g^-1,经过200次循环之后的放电容量为365 mAh·g^-1;在循环伏安测试中得到了分步反应的可逆氧化还原峰。TEM和SAED分析结果揭示了NiCo₂S₄薄膜与Li的电化学反应机理：首次放电过程中NiCo₂S₄与Li发生转化反应生成了Li₂S、Ni和Co,充电后生成了CoS和NiS复合薄膜。后续循环为CoS和NiS复合薄膜的可逆分解与形成。研究表明NiCo₂S₄是一种有潜在应用价值的锂离子电池负极材料。     

脉冲激光沉积NiCo2S4薄膜及其电化学特征研究

采用脉冲激光沉积法制备了NiCo₂S₄薄膜,利用恒流充放电和循环伏安测试研究了NiCo₂S₄薄膜作为锂离子电池负极材料的电化学性能和充放电机理。采用高分辨电子显微镜和选区电子衍射(TEM&SAED)表征了NiCo₂S₄薄膜首次循环过程中的组成与结构变化。恒流充放电测试结果显示NiCo₂S₄薄膜在3 μA·cm^-2的放电电流下,0~3 V(vs Li⁺/Li)范围内,薄膜的首次放电容量为698 mAh·g^-1,经过200次循环之后的放电容量为365 mAh·g^-1;在循环伏安测试中得到了分步反应的可逆氧化还原峰。TEM和SAED分析结果揭示了NiCo₂S₄薄膜与Li的电化学反应机理：首次放电过程中NiCo₂S₄与Li发生转化反应生成了Li₂S、Ni和Co,充电后生成了CoS和NiS复合薄膜。后续循环为CoS和NiS复合薄膜的可逆分解与形成。研究表明NiCo₂S₄是一种有潜在应用价值的锂离子电池负极材料。     

锂离子电池高比容量FeSn2-C复合负极材料的合成与性能

Sn基合金负极材料具有高达990 mAh·g^-1的理论比容量,但其也存在因脱嵌锂过程发生巨大的体积变化而导致循环性能较差的问题. 本文以Sn、Fe、石墨为原料利用简易的高能球磨法成功制备了具有核壳结构的FeSn₂-C复合物,系统研究了球磨时间、FeSn₂相含量对材料物相结构及电化学性能的影响,并分析了电极的失效机理. 研究表明,球磨时间的增加有利于FeSn₂金属间化合物相的形成及材料颗粒的细化,进而有利于材料比容量的增加及循环性能的提升;FeSn₂相含量的增加能够提高FeSn₂-C材料的比容量,但会降低FeSn₂-C电极的循环稳定性. 经工艺优化及组分调节,球磨24 h合成的Sn₂₀Fe₁₀C₇₀材料具有最优的电化学性能,材料的比容量在540 mAh·g^-1左右,并能稳定循环100次,是一种非常有发展前途的锂离子电池高比容量负极材料.     

可充电池的磁共振研究

快速增长的对安全能源的需求,促使科研工作者不断探索高能量密度的可充锂离子电池(LIBs)。发展原位表征技术能更好地研究电池工作中的锂离子镶嵌机制和电池失效因素。固体核磁共振(NMR)能有效的测试短程化学环境:通过对~1H、~(6,7)Li、~(11)B、~(13)C、~(17)O、~(19)F、~(23)Na和~(31)P等同位素来探测电池材料的微观结构。除了魔角旋转(MAS)高分辨NMR谱图研究电池材料的精细结构之外,核磁共振还能无损地捕获、研究电池材料在充放电循环中的演化。因此,原位核磁共振NMR及成像(MRI)可拓展到电池充放电循环中的锂离子的动态演化以及锂离子浓度的时空分布信息。互为补充地,电子顺磁共振(EPR)及成像(EPRI)能有效地跟踪和捕获电极过渡金属、阴氧离子(O_2~(n-))的氧化还原过程。这些实时捕获的动态信息能更好地指导电极材料的构效、微观设计和电池组装的改进,最终获得优异的电化学性能。     

多孔锂-硅薄膜锂离子电池负极材料的电沉积制备及其电化学性能

采用多步恒电流沉积技术, 在铜箔上电沉积制备了多孔锂-硅薄膜电极(LSF). 用X射线衍射(XRD)和扫描电镜(SEM)测试手段研究了该电极的结构和表面形貌. 作为锂离子电池负极材料, 电化学测试结果表明锂-硅薄膜电极具有较好的循环稳定性, 通过改变电沉积条件, 可有效调控该电极的嵌脱锂容量及首次循环效率. 譬如, 在0.5 mol·L^-1四氯化硅+0.7 mo·L^-1高氯酸锂的碳酸丙烯酯电解液中, 首先以-3.82 mA·cm^-2的恒定电流密度沉积600 s, 再将电流密度恒定为-1.27 mA·cm^-2, 继续电沉积7200 s, 制得锂-硅薄膜电极(LSF^-3), 该电极以12.7 μA·cm^-2的电流密度预循环2次, 其首次循环库仑效率高达97.1%. 预循环2次后, 电流密度增加到25.5 μA·cm^-2, 此时,锂-硅薄膜电极充电质量比容量和面积比容量分别为1410 mAh·g^-1及240.6 μAh·cm^-2; 50次循环后充电比容量为179 μAh·cm^-2 (1049 mAh·g^-1), 容量保持率为74.4%. 锂-硅薄膜电极中的活性锂组分可补偿首次循环时不可逆容量损失, 同时薄膜电极中的多孔结构可缓解电极材料的体积效应并改善其循环性能.     

Structure, stoichiometry and transport properties of lithium copper nitride battery materials: combined NMR and powder neutron diffraction studies

A combined NMR and neutron diffraction study has been carried out on three Li(3-x-y)Cu(x)N materials with x=0.17, x=0.29 and x=0.36. Neutron diffraction indicates that the samples retain the P6/mmm space group of the parent Li(3)N with Cu located only on Li(1) sites. The lattice parameters vary smoothly with x in a similar fashion to Li(3-x-y)Ni(x)N, but the Li(2) vacancy concentration for the Cu-substituted materials is negligible. This structural model is confirmed by wideline (7)Li NMR spectra at 193 K which show three different local environments for the Li(1) site, resulting from the substitution of neighbouring Li atoms in the Li(1) layer by Cu. Since the Cu-substituted materials are only very weakly paramagnetic, variable temperature (7)Li wideline NMR spectra can be used to measure diffusion coefficients and activation energies. These indicate anisotropic Li(+) diffusion similar to the parent Li(3)N with transport confined to the [Li(2)N] plane at low temperature and exchange between Li(1) and Li(2) sites dominant at high temperature. For the intra-layer process the diffusion coefficients at room temperature are comparable to Li(3)N and Li(3-x-y) Ni(x)N, while E(a) decreases as x increases in contrast to the opposite trend in Ni-substituted materials. For the inter-layer process E(a) decreases only slightly as x increases, but the diffusion coefficients at room temperature increase rapidly with x.     

Short- and long-range order in the positive electrode material, Li(NiMn)0.5O2: a joint X-ray and neutron diffraction, pair distribution function analysis and NMR study

The local environments and short-range ordering of LiNi(0.5)Mn(0.5)O(2), a potential Li-ion battery positive electrode material, were investigated by using a combination of X-ray and neutron diffraction and isotopic substitution (NDIS) techniques, (6)Li Magic Angle Spinning (MAS) NMR spectroscopy, and for the first time, X-ray and neutron Pair Distribution Function (PDF) analysis, associated with Reverse Monte Carlo (RMC) calculations. Three samples were studied: (6)Li(NiMn)(0.5)O(2), (7)Li(NiMn)(0.5)O(2), and (7)Li(NiMn)(0.5)O(2) enriched with (62)Ni (denoted as (7)Li(ZERO)Ni(0.5)Mn(0.5)O(2)), so that the resulting scattering length of Ni atoms is null. LiNi(0.5)Mn(0.5)O(2) adopts the LiCoO(2) structure (space group Rm) and comprises separate lithium layers, transition metal layers (Ni, Mn), and oxygen layers. NMR experiments and Rietveld refinements show that there is approximately 10% of Ni/Li site exchange between the Li and transition metal layers. PDF analysis of the neutron data revealed considerable local distortions in the layers that were not captured in the Rietveld refinements performed using the Bragg diffraction data and the LiCoO(2) structure, resulting in different M-O bond lengths of 1.93 and 2.07 Angstroms for Mn-O and Ni/Li-O, respectively. Large clusters of 2400-3456 atoms were built to investigate cation ordering. The RMC method was then used to improve the fit between the calculated model and experimental PDF data. Both NMR and RMC results were consistent with a nonrandom distribution of Ni, Mn, and Li cations in the transition metal layers; both the Ni and Li atoms are, on average, close to more Mn ions than predicted based on a random distribution of these ions in the transition metal layers. Constraints from both experimental methods showed the presence of short-range order in the transition metal layers comprising LiMn(6) and LiMn(5)Ni clusters combined with Ni and Mn contacts resembling those found in the so-called "flower structure" or structures derived from ordered honeycomb arrays.     

LiCuS,an intermediate phase in the electrochemical conversion reaction of CuS with Li: A potential environment-friendly battery and solar cell material

The crystal structure of a ternary sulfide with the approximate composition LiCuS, which is a promising candidate for environment-friendly battery and solar cell materials is reported. The crystal structure was solved by a combination of neutron and X-ray powder diffraction data, and ⁷Li solid-state NMR analysis. A yellow powder, Li_1.1Cu_0.9S, was obtained by the reaction of CuS with a slight excess of Li metal. The compound crystallizes in the Na₃AgO₂ structure type in the space group Ibam. An idealized crystal structure of Li_1.1Cu_0.9S can be derived from the cubic Li₂S structure by moving a part of the Li along the c axis so that these Li atoms become linearly coordinated by S. All the metal sites are occupied by randomly mixed Li and Cu atoms; however, there is a strong preference for linear coordination by Cu. The density functional theory calculations show that Li_1.1Cu_0.9S is a direct band-gap semiconductor with an energy gap of 1.95 eV in agreement with experimental data.     

Electronic structure and properties of isoreticular metal-organic frameworks: the case of M-IRMOF1 (M = Zn, Cd, Be, Mg, and Ca)

We investigate the possibility of tailoring the electronic properties of isoreticular metal-organic materials by replacing the metal atom in the metal-organic cluster and by doping. The electronic structure of M-IRMOF1, where IRMOF1 stands for isoreticular metal-organic framework 1 and M = Be, Mg, Ca, Zn, and Cd, was examined using density-functional theory. The results show that these materials have similar band gaps (ca. 3.5 eV) and a conduction band that is split into two bands, the lower of which has a width that varies with metal substitution. This variation prompted us to investigate whether doping with Al or Li could be used to tailor the electronic properties of the Zn-IRMOF1 and Be-IRMOF1 materials. It is shown that replacing one metal atom with Al can effectively be used to create IRMOFs with different metallic properties. On the other hand, adding Li produces structural changes that render this approach less suitable.     

ANiN(A=Li,Na,Mg,Ca)的结构、热力学、弹性和电子性质的第一性原理研究

三元过渡金属氮化物ANiN(A=Li,Na,Mg,Ca)是潜在的可充放电池的电极材料.物理性质,比如热稳定性、电子能隙以及弹性稳定性等,对于这些材料的电池应用都是非常重要的.本文使用第一原理方法,对比研究了 ANiN这些材料的结构、动力学、弹性和电子结构性质.对状态方程和声子谱的计算被用来确定体系的稳定结构.对最稳定结...     

6/7Li NMR study of the Li1-zNi1+zO2 phases

A series of Li1-zNi1+zO2 materials have been synthesised by the coprecipitation route. An X-ray diffraction study was carried out on these materials using the Rietveld method to determine the departure from the ideal stoichiometry z, which ranges from 0 to 0.138. The actual Li/Ni ratio was also checked by chemical analyses using inductively coupled plasma (ICP) for each sample. The stoichiometric sample (z approximately 0) was obtained using a 15% Li excess. (6/7)Li NMR results from LiNiO2 (z approximately 0) show that the asymmetric shape of the NMR signal is due to anisotropy. Calculations give evidence that the paramagnetic dipolar interaction from the electron spins carried by Ni is anisotropic but does not completely explain the experimental anisotropy. (6)Li MAS NMR (magic angle spinning NMR) experiments and temperature standardisation NMR measurements unambiguously assign the isotropic position at +726 ppm. The static-echo NMR spectra of the non-stoichiometric Li1-zNi1+zO2 phases also exhibit an asymmetric shape whose width increases with the departure from the ideal stoichiometry z. (6/7)Li static and MAS NMR show that the 2zNi(2+) ions thus formed modify the dipolar interaction within the materials and also affect the Fermi contact interaction, since a distribution of Li environments is observed using (6)Li NMR for non-stoichiometric samples.     

Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

Amides and imides of alkali metals are a very promising class of materials for use as a hydrogen‐storage system, as they are able to store and release hydrogen via a chemical route at controllable temperatures and pressures. We critically revise the present picture of the atomic structure of the lightest member (LiNH₂/Li₂NH) by using a combined computational and experimental approach. Specifically, ab initio path integral molecular dynamics simulations and solid‐state ¹H NMR techniques are combined. The results show that the presently assumed local structure might be inconsistent or at least incomplete and needs considerable revision. In particular, the Li atoms turn out to be more mobile and more disordered than suggested by structural data obtained from X‐ray scattering. Also, the configuration of the hydrogen atoms, which is accessible via the NMR experiment and the corresponding first‐principles calculations, is different from the previously assumed data. The computed and experimentally observed ¹H NMR parameters are in very good mutual agreement and illustrate the unusual chemical environment of the hydrogen atoms in this system. Incorporating our results on the new lithium data, we show that the effect of nuclear quantum delocalization for the hydrogen atoms is considerably reduced compared to the perfect crystal structure.     

Ab initio investigation on a new class of binuclear superalkali cations M2Li(2k+1)+ (F2Li3+, O2Li5+, N2Li7+, and C2Li9+)

Superalkalies with low ionization potentials (IPs) can exhibit behaviors reminiscent of alkali atoms and hence be considered as potential building blocks for the assembly of novel nanostructured materials. A new series of binuclear superalkali cations M(2)Li(2k+1)(+) (M = F, O, N, C) has been studied using ab initio methods. The structural features of such cations are found to be related to the central atoms. In the preferred structures of F(2)Li(3)(+), O(2)Li(5)(+), and N(2)Li(7)(+), two central atoms are bridged by lithium atoms. While in the global minima of C(2)Li(9)(+), two central carbon atoms directly link each other and the C-C unit extends to the surface of the whole system. These M(2)Li(2k+1)(+) species exhibit very low vertical electron affinities of 2.74-4.61 eV at the OVGF/6-311+G(3df) level and hence should be classified as superalkali cations.     

Solid-state 23Na and 7Li NMR investigations of sodium- and lithium-reduced mesoporous titanium oxides

Mesoporous titanium oxide synthesized using a dodecylamine template was treated with 0.2, 0.6, and 1.0 equiv of Li- or Na-naphthalene. The composite materials were characterized by nitrogen adsorption, powder X-ray diffraction, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis, and solid-state 23Na and 7Li NMR spectroscopy. In all cases the wormhole mesoporosity was retained as evidenced by BET surface areas from 400 to 700 m(2)/g, Horvath-Kawazoe pore sizes in the 20 Angstroms range, and a lack of hysteresis in the nitrogen adsorption isotherms. Variable-temperature conductivity studies show that the Li-reduced materials are semiconductors, with conductivity values 3 orders of magnitude higher than those of the Na-reduced materials. Electrochemical measurements demonstrate reversible intercalation/deintercalation of Li+ ions into pristine mesoporous Ti oxides with good cycling capacity. Solid-state 23Na NMR reveals two distinct Na environments: one corresponding to sodium ions in the mesoporous channels and the other corresponding to sodium ions intercalated into the metal framework. 23Na NMR spectra also indicate that the relative population of the framework site increases with increased reduction levels. Solid-state 7Li NMR spectra display a single broad resonance, which increases in breadth with increased reduction levels, though individual resonances inferring the presence of channel and framework Li species are not resolved. Comparisons of the lithium chemical shifts with published values suggests an "anatase-like structure" with no long-range order in the least-reduced samples but a "lithium titanate-like structure" with no long-range order in the higher reduced materials.     

Nuclear magnetic resonance for interfaces in rechargeable batteries

Nuclear Magnetic Resonance (NMR) is a powerful technique to probe the local environment of atoms bearing a nuclear spin. Interfaces in a rechargeable battery, within multi-component electrode or electrolytes or between the electrodes and the electrolyte, are key to its function and lifetime. NMR spectroscopy of the solid phases in the battery participate in the understanding of the processes at these interfaces. The solid-state NMR community is still highly active for ex situ measurements. Dynamic Nuclear Polarization attracted interest thanks to its enhanced sensitivity. In situ spectroscopy and imaging prospered in the context of metallic Li or Na deposition, either as an ageing process in conventional Li or Na batteries, or as the primary process in a metal battery.