The Fine Line between a Two-Phase and Solid-Solution Phase Transformation and Highly Mobile Phase Interfaces in Spinel Li4+xTi5O12
The Fine Line between a Two-Phase and Solid-Solution Phase Transformation and Highly Mobile Phase Interfaces in Spinel Li4+xTi5O12
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DOI：
10.1002/aenm.201601781
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发表时间：
2017-05-10
影响因子：
27.8
通讯作者：
Wagemaker, Marnix
中科院分区：
文献类型：
--
作者：
Ganapathy, Swapna;Vasileiadis, Alexandros;Wagemaker, Marnix
相变在锂离子电池电极中起着至关重要的作用，对于功率密度和循环寿命都具有决定性作用。相变的动力学性质相对尚未被探索，并且缺陷尖晶石 Li4+xTi5O12 中相变的性质引起了争议，因为与一级相变相关的非常恒定的（放电）充电电势似乎与与固溶体反应相关的异常高的速率性能相矛盾。通过目前的密度泛函理论研究，提出了一种微观机制，为这种有趣且技术相关的材料提供了更深入的见解。在尖晶石 Li4+xTi5O12 晶格中，Ti 被 Li 局部取代，稳定了锂离子插入时引入的相界。这有利于亚纳米相在平衡状态下共存，虽然与固溶体非常相似，但应被视为真正的一级相变。由于位于界面处的锂离子具有高迁移率，预计所得界面将具有很高的迁移率。这种高度流动、几乎类似液体的亚纳米相形态能够在电池运行期间对非平衡条件做出非常快速的响应，这解释了与一级相变相结合的优异倍率性能。
Phase transitions play a crucial role in Li-ion battery electrodes being decisive for both the power density and cycle life. The kinetic properties of phase transitions are relatively unexplored and the nature of the phase transition in defective spinel Li4+xTi5O12 introduces a controversy as the very constant (dis)charge potential, associated with a first-order phase transition, appears to contradict the exceptionally high rate performance associated with a solid-solution reaction. With the present density functional theory study, a microscopic mechanism is put forward that provides deeper insight in this intriguing and technologically relevant material. The local substitution of Ti with Li in the spinel Li4+xTi5O12 lattice stabilizes the phase boundaries that are introduced upon Li-ion insertion. This facilitates a subnanometer phase coexistence in equilibrium, which although very similar to a solid solution should be considered a true first-order phase transition. The resulting interfaces are predicted to be very mobile due to the high mobility of the Li ions located at the interfaces. This highly mobile, almost liquid-like, subnanometer phase morphology is able to respond very fast to nonequilibrium conditions during battery operation, explaining the excellent rate performance in combination with a first-order phase transition.