Relocation of Cobalt Ions in Electrochemically Delithiated LiCoPO4 Cathode Materials
Relocation of Cobalt Ions in Electrochemically Delithiated LiCoPO4 Cathode Materials
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DOI：
10.1021/cm501452p
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发表时间：
2014-05-13
影响因子：
8.6
通讯作者：
Honma, Itaru
中科院分区：
文献类型：
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作者：
Quang Duc Truong;Deyaraju, Murukanahally Kempaiah;Honma, Itaru
晶格中的点缺陷包括反位阳离子交换、掺杂剂和原子空位等，一直是固体物理和化学领域的研究热点。1−3晶格中这些缺陷的浓度和分布严重影响了材料的光学性质、电导率、离子扩散和由此产生的化学性质，以及质量和电荷输运行为，这引发了引入故意缺陷以提供器件最佳性能的大量努力。在锂嵌入化合物中，锂沿通道的扩散是高度各向异性的，这强烈依赖于晶格内阳离子的顺序。5,6阳离子交换紊乱7,8，即过渡金属占据锂离子的位置，不可避免地阻断了锂离子的扩散途径，从而直接影响锂离子电池的阴极性能。理论计算、中子衍射和先进电子显微镜的直接观察清楚地证实了锂插层过渡金属氧化物和磷酸盐中反位缺陷的存在。7−10我们现在转向研究充放电循环过程中的局部变化、过渡金属迁移、结构变化及其与电压/容量衰减的关系。11−22利用先进的电子显微镜对层状镍锰氧化物在合成或电化学循环过程中的结构变化进行了研究，发现过渡金属离子（Ni）的迁移和偏析可能引发相变并抑制电池的充放电速率。16−20然而，金属离子在有序橄榄石磷酸金属锂的插脱反应中的重新定位仍然是未知的，尽管这种迁移无疑影响了橄榄石材料的质量和电荷输运行为。本文中，我们使用像差校正扫描透射电镜观察了在电化学分解过程中，橄榄石磷酸钴锂中钴离子从M2位到空位M1位的局部变化。这一发现为进一步了解LiCoPO4正极材料的容量衰减机理提供了新的思路。
The point defects in crystal lattices including antisite cation exchange, dopants, and atomic vacancies have been the topic of extraordinary research interest in solid state physics and chemistry. 1− 3 The optical properties, electrical conductivity, ionic diffusion and resulting chemical properties, and mass and charge transport behavior of the materials are heavily affected by the concentration and distribution of these defects in the crystal lattices, which has triggered considerable efforts in introducing the intentional defects to provide optimal performance in devices. 4In lithium intercalation compounds, the lithium diffusion along the channel is highly anisotropic, which is strongly dependent on the cation ordering within the crystal lattices. 5, 6 The cation exchange disorder, 7, 8 namely, the occupation of Li sites by transition metals, inevitably blocks the lithium ion diffusion pathway and, thus, directly affects the cathode performance in lithium-ion batteries. The presence of antisite defects in lithium-intercalated transition-metal oxides and phosphates has been clearly confirmed by theoretical calculations, neutron diffraction, and direct observation by advanced electron microscopy. 7− 10 We now turn to the research on the local variations, transition metal relocation, structural change, and their correlation to the voltage/capacity fading during the charge/discharge cycling. 11− 22 The investigation by advanced electron microscopy on the structural change of the layered nickel manganese oxides during the synthesis or electrochemical cycling reveals that the migration and segregation of transition metal ions, ie, Ni, may initiate the phase transformation and inhibit the battery charge/discharge rate. 16− 20 However, the relocation of metal ions in ordered olivine lithium metal phosphates during the intercalation− deintercalation reactions remains unknown, although this migration undoubtedly influences the mass and the charge transport behavior of the olivine materials. Herein, we observed the local variation of cobalt ions from M2 sites to vacancy M1 sites in olivine lithium cobalt phosphates upon the electrochemical delithiation process using aberration-corrected scanning transmission electron microscopy. The finding provides the insight into the capacity fading mechanism of the LiCoPO4 cathode materials.