Enhancing the Kinetics of Li-Rich Cathode Materials through the Pinning Effects of Gradient Surface Na+ Doping
Enhancing the Kinetics of Li-Rich Cathode Materials through the Pinning Effects of Gradient Surface Na+ Doping
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DOI：
10.1002/aenm.201501914
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发表时间：
2016-03-23
影响因子：
27.8
通讯作者：
Guo, Yu-Guo
中科院分区：
文献类型：
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作者：
Qing, Ren-Peng;Shi, Ji-Lei;Guo, Yu-Guo
DOI：10.1002/aenm201501914促进Li_2MnO_3的电化学活性。[22-24]在Lirich层状材料中，激活Li_2MnO_3组分的传统方法主要是通过化学腐蚀Li_2MnO_3相中的Li_2O来产生结构缺陷，如硝酸或水合肼的修饰。[6，20，25]虽然Li_2MnO_3可以被激活，这提高了初始的库仑效率，但在刻蚀过程中(图S1a，支持信息)，原始的表面形态容易被破坏，从而导致在随后的循环中循环稳定性(图S1b，支持信息)和速率性能较差。据报道，用水合肼修饰的富锂层状材料具有长期的能量保持能力。[20]然而，在放电过程中出现了低于3.0V的不利平台，这损害了能量密度，特别是对于富锰基富锂正极材料(图S2，支持信息)。Xia等人报道了通过控制富锂材料Li2MnO3相中的结构缺陷来改善其电化学性能。然而，高度的结构缺陷伴随着过渡金属层中Li+的高度无序，将恶化结构的稳定性，导致循环稳定性和倍率容量较差。Na+掺杂被认为是促进富Li层状材料Li+扩散从而提高其倍率容量的有效途径。[27-30]请注意，控制Na+的掺杂量是至关重要的。低Na+掺杂的富Li层状材料的循环稳定性较差，而高Na+掺杂的材料比容量降低，能量密度降低。因此，开发一种简单有效的方法来合成动力学性能较好的富Li层状材料仍然是一个具有挑战性的方法。在Na+浓度扩散热力学的驱动下，富锂材料在熔融的氯化钠状态下，通过焙烧过程实现了表面Na+的梯度掺杂。粉末X射线衍射分析表明，在熔融的NaC l助熔剂中，富锂材料的Li2 MnO3相中形成了高度的结构缺陷，[2 6]在大颗粒富锂层状材料表面进行梯度Na+掺杂，不仅可以实现钉扎效应，稳定具有大量结构缺陷的富锂层状结构，而且有利于Li+在层状结构中的扩散。因此，合成的大颗粒富锂层状材料表现出优异的电化学性能，特别是高比容量、出色的库仑效率和令人印象深刻的循环稳定性。结构设计示意图如图1所示。
DOI: 10.1002/aenm. 201501914 facilitate the electrochemical activity of Li 2MnO 3.[22–24] Conventional approaches in activating Li 2MnO 3 component in Lirich layered materials are mainly through chemical etching of Li 2O in Li 2MnO 3 phases to generate structural defects, such as nitric acid or hydrazine hydrate modification.[6, 20, 25] Although Li 2MnO 3 could be activated, which improved the initial Coulombic efficiency, the original surface morphology is prone to being destroyed during the etching process (Figure S1a, Supporting Information), hence leading to the poor cycling stability (Figure S1b, Supporting Information) and rate performance during subsequent cycles. Li-rich layered materials modified by hydrazine hydrate was reported to have long-term energy retention.[20] However, the unfavorable platform below 3.0 V appears in the discharge process, which compromises the energy density, especially for the Mn-rich based Li-rich cathode materials (Figure S2, Supporting Information). Xia and co-workers reported to improve their electrochemical performances through controlled structure defects in Li 2MnO 3 phase of Li-rich materials.[26] However, a high degree of structure defects, accompanied with highly disordering of Li+ in the transition metal layer, would deteriorate the structural stability, leading to the poor cycling stability and the rate capacity. Na+ doping has been reported to be an effective avenue to facilitate the Li+ diffusion of Li-rich layered materials and thus to improve their rate capacity.[27–30] Note that it is vital to control the doping amount of Na+. Li-rich layered materials doped with a low amount of Na+ would exhibit the poor cycling stability whereas the material doped with a high amount of Na+ shows decreased specific capacity and deteriorates the energy density.[28] Therefore, it is still challenging to develop one simple and effective approach to synthesize Li-rich layered material with much improved kinetics.Herein, we propose a novel method to enhance the kinetics of large particle Li-rich layered materials by gradient surface Na+ doping. Driven by Na+ concentration diffusion thermodynamically, gradient surface Na+ doping are realized through the calcination process of Li-rich materials in molten NaCl state. Powder X-ray diffraction (XRD) shows that high degree of structure defects are formed in Li 2MnO 3 phase of Li-rich material in molten NaCl flux.[26] Gradient Na+ doping on the surface of large particle Li-rich layered material could not only realize the pinning effect in stabilizing the Li-rich layered structure with large amount of structural defects but also facilitate the diffusion of Li+ in the layered structure. Accordingly, the resultant large particle Li-rich layered material represents superior electrochemical performances, particularly high specific capacity, excellent Coulombic efficiency, and impressive cycling stability. The schematic illustration of the structural design is shown in Figure 1.