Fu Junjie, Wang Kuan, Liu Danmin, Zhang Zhenlu, Sui Manling, Yan Pengfei
Beijing Key Laboratory of Microstructure and Properties of Solids, Faculty of Materials and Manufacturing, Beijing University of Technology, Beijing 100124, China.
School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China.
ACS Appl Mater Interfaces. 2020 Sep 2;12(35):39245-39251. doi: 10.1021/acsami.0c10838. Epub 2020 Aug 21.
Phase boundary movement accomplishing reversible LiFePO/FePO biphasic transition is a fundamental Li-ion intercalation/deintercalation mechanism for LiFePO cathode. Phase boundary energetically favors crack nucleation and propagation; thus, postmortem observation on cracks becomes a feasible approach to investigate the phase-transition behavior and the Li-ion diffusion mechanism. The previously observed (200) plane cracks facilitate the "domino" diffusion model. Herein, our microscopic observations reveal another type of cracks along the (020) planes in a commercial LiFePO cathode cycled at moderate rates (0.1C, 0.33C, and 1C). Such (020) plane cracks are more detrimental to electrochemical performance because they can cut off the Li-ion diffusion pathway, causing inactive segments of LiFePO. The (020) plane cracks indicate the LiFePO/FePO phase boundary is along the (020) plane and moving along the -axis during battery operation, which is a typical bulk diffusion-limited Li-ion diffusion behavior. Our observations stress that large LiFePO primary particle (>200 nm) not only aggravates cracking degradation but also switches the Li-ion diffusion mode to a slow bulk diffusion mechanism, plunging the overall battery performance.
实现可逆的LiFePO₄/FePO₄双相转变的相界移动是LiFePO₄正极基本的锂离子嵌入/脱嵌机制。相界在能量上有利于裂纹的形核和扩展；因此，对裂纹的死后观察成为研究相转变行为和锂离子扩散机制的一种可行方法。先前观察到的（200）面裂纹促进了“多米诺”扩散模型。在此，我们的微观观察揭示了在以中等速率（0.1C、0.33C和1C）循环的商用LiFePO₄正极中，沿着（020）面存在另一种类型的裂纹。这种（020）面裂纹对电化学性能更有害，因为它们会切断锂离子扩散路径，导致LiFePO₄出现非活性部分。（020）面裂纹表明LiFePO₄/FePO₄相界沿着（020）面，并且在电池运行期间沿着z轴移动，这是一种典型的体扩散限制的锂离子扩散行为。我们的观察强调，大尺寸的LiFePO₄初级颗粒（>200 nm）不仅会加剧裂纹退化，还会将锂离子扩散模式转变为缓慢的体扩散机制，从而降低整体电池性能。
