Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage
Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage
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DOI：
10.1002/aenm.201401814
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发表时间：
2015-05-06
影响因子：
27.8
通讯作者：
Hahn, Horst
中科院分区：
文献类型：
--
作者：
Chen, Ruiyong;Ren, Shuhua;Hahn, Horst
DOI：10.1002/aenm.具有Li+/TM均匀共享阳离子位点和紧密堆积的阴离子阵列的岩盐结构可能是有效Li+储存的合理替代方案。在Li+提取后，TM可以保留在阳离子亚晶格位置，并很好地维持无序的岩盐骨架。最近的一项基于从头计算的工作表明，Li+在无序立方岩盐氧化物（Li1.211Mo0.467Cr0.3O2）中的输运是容易的。[8]结果发现，这种框架是稳定的，最大的45%的阳离子位点空缺。在本文中，我们证明了一种新的二锂无序岩盐Li 2 VO 2F插层材料可以在2.5 V（1000 Wh kg− 1）下提供高达约1.8 Li+容量/TM（420 mAh g− 1），只有很小的晶格体积变化（103%）。这种具有混合O2 −/F−阴离子环境的材料已经通过简单的球磨方法合成。两个Li+存储与V 3+/V 5+氧化还原反应（Li 2 VO 2F参与Li++2 e −+ VO 2F）导致462 mAh g− 1的高理论容量。此外，Li 2 VO 2F在快速充放电或低温操作时显示出良好的容量保持率和轻微的极化增加。通过粉末中子衍射图（图1a）和X射线衍射图（图1 B）的Rietveld结构精修，确定了球磨后Li 2 VO 2F目标相的晶体结构。改进排除了分层岩盐结构（R-3 m空间群）（图S1，支持信息）。[11]相反，衍射图案可以在无序的立方岩盐相中很好地索引（空间群Fm-3 m，表S1，支持信息）。在细化过程中，原子位移参数被约束为对所有原子相同。如图1a的插图中所示，Li和V原子随机分布在阳离子亚晶格（4个α维科夫位点）上，具有0.64：0.36的精细占据比。由于O和F对X射线和中子的散射特性的对比度较低，因此不能从衍射数据中获得O和F在阴离子亚晶格（4个B位点）上的精确占位细化。[12]在Li 2 VO 2F中，阳离子平均与4个O原子和2个F原子配位，形成（Li/V）O 4f 2八面体单元.此外，细化产生的材料密度为4.25 g cm− 3，而LiCoO 2为5 g cm− 3，LiFePO 4为3.5 g cm− 3。[13]高分辨率透射电子显微镜（HRTEM）图像显示了合成的Li 2 VO 2F的纳米晶特征（约10 nm）（图1 c）。Li 2 VO 2F的立方结构也通过相应的快速傅里叶变换（FFT）图像（图1c中的插图）证实。一些纳米晶体（在图1c中用白色圆圈标记）显示出明显的堆垛层错，这可能是由（Li/V）O xF y八面体的局部畸变引起的。能量色散谱（EDS，图1 d）移动的和固定应用对具有上级储能能力的先进阴极材料的需求很高。Li+主体的固有结构特征对电池性能至关重要。高容量转换阴极材料在结构重构的同时往往会遇到较大的电压滞后（低能效）。[1]目前商业化的正极材料仍然以具有内在结构完整性的嵌入材料为主，用于容纳Li+。[2]然而，已知的嵌入材料具有有限的理论容量（< 300 mAh g-1）。[3]此外，结构转变/退化已经常观察到常见的嵌入主机与有序的Li+/过渡金属（TM）的晶格位置。
DOI: 10.1002/aenm. 201401814 rock-salt structures, with Li+/TM evenly sharing cation sites and close-packed anion arrays, may be a reasonable alternative for efficient Li+ storage. After Li+ extraction, the TM may remain at the cation sublattice sites and uphold well the disordered rocksalt framework. A recent work based on ab initio computations revealed that Li+ transport can be facile in a disordered cubic rock-salt oxide (Li 1.211Mo 0.467Cr 0.3O 2) with Li-excess.[8] It was found that such framework is stable with a maximum of 45% of the cation sites vacant. Herein, we demonstrate that a new dilithium disordered rock-salt Li 2VO 2F intercalation material can deliver up to about 1.8 Li+ capacity per TM (420 mAh g− 1) at≈ 2.5 V (1000 Wh kg− 1) with only minor lattice volume change (≈ 3%). Such material with mixed O 2−/F− anion environment has been synthesized by a simple ball-milling method. The two Li+ storage with V 3+/V 5+ redox reactions (Li 2VO 2F↔ 2 Li++ 2e−+ VO 2F) leads to an attractively high theoretical capacity of 462 mAh g− 1. Moreover, Li 2VO 2F shows good capacity retention and minor increase in polarization upon fast charging/discharging or upon low-temperature operation. The crystal structure of the target phase for the as-milled Li 2VO 2F is determined by Rietveld structural refinement of powder neutron (Figure 1a) and X-ray (Figure 1 b) diffraction patterns. The refinement precludes a layered rock-salt structure (R-3 m space group)(Figure S1, Supporting Information).[11] Instead, the diffraction patterns can be well indexed in a disordered cubic rock-salt phase (space group Fm-3 m, Table S1, Supporting Information). The atomic displacement parameters were constrained to the same for all atoms in the course of the refinements. As illustrated in the inset of Figure 1 a, Li and V atoms distribute randomly on the cationic sublattice (4 a Wyckoff sites) with a refined occupancy ratio of 0.64: 0.36. The accurate occupancy refinements for O and F on the anionic sublattice (4 b sites) cannot be obtained from the diffraction data because O and F have low contrast in scattering properties for X-rays and neutrons.[12] On an average, cations are coordinated with four O and two F atoms to form the (Li/V) O4f 2 octahedral units in the stoichiometric Li 2VO 2F. In addition, the refinement yields a material density of 4.25 g cm− 3 as compared with 5 g cm− 3 for LiCoO 2 and 3.5 g cm− 3 for LiFePO 4.[13] High-resolution transmission electron microscopy (HRTEM) image shows the nanocrystalline character (≈ 10 nm) of the as-synthesized Li 2VO 2F (Figure 1 c). The cubic structure of Li 2VO 2F was also confirmed by the corresponding fast Fourier transform (FFT) image (inset in Figure 1 c). Some nanocrystals (marked with white circle in Figure 1 c) show clear stacking faults, arising probably from the local distortion of (Li/V) O xF y octahedra. Energy-dispersive spectroscopy (EDS, Figure 1 d)Advanced cathode materials with superior energy storage capability are highly demanded for mobile and stationary applications. The inherent structural feature of Li+ hosts is critical for the battery performance. High-capacity conversion cathode materials often encounter large voltage hysteresis (low energy efficiency) accompanied with the structural reconstruction.[1] The current commercial cathode materials are still dominated by intercalation materials with intrinsic structural integrity for accommodating Li+.[2] However, the known intercalation materials have limited theoretical capacity (< 300 mAh g− 1).[3] In addition, structural transition/degradation have often been observed for the common intercalation hosts with ordered Li+/transition metal (TM) lattice sites …