Lu Feye-Feng, Tian Hong-Kang
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan.
Program on Smart and Sustainable Manufacturing, Academy of Innovative Semiconductor and Sustainable Manufacturing, National Cheng Kung University, Tainan 70101, Taiwan.
Phys Chem Chem Phys. 2023 Jul 19;25(28):18973-18982. doi: 10.1039/d3cp02336b.
Cubic garnet LiLaZrO (c-LLZO) is a promising solid electrolyte for all-solid-state batteries, often doped with Ga, Al, and Fe to stabilize the structure and enhance Li-ion conductivity. Despite introducing the same amount of Li vacancies, these dopants with +3 classical charge yield different Li-ion conductivities by around an order of magnitude. In this study, we used density functional theory (DFT) calculations to investigate the impact of Ga, Fe, and Al dopants on Li chemical potential and Li-ion conductivity variations. We identified the energetically favorable dopant location in c-LLZO and determined the optimal value of 7.5 eV for DFT+ calculations for dopant Fe in c-LLZO. Our calculations showed that Ga or Fe doping enhances the Li chemical potential by 0.05-0.08 eV, reducing Li-ion transfer barriers and increasing Li-ion conductivity, while Al doping lowers the Li chemical potential by 0.08 eV, reducing Li-ion conductivity. To determine the cause of Li chemical potential variations, we performed a combined analysis of the projected density of states, charge density, and Bader charge. The distinct charge distribution from dopant atoms to neighboring O atoms is critical for determining the Li-ion chemical potential. Ga and Fe dopants retain more electrons, which consequently makes the adjacent O atoms acquire a more positive charge that destabilizes Li ions by reducing the restraining force acting on them, thereby enhancing Li-ion conductivity. In contrast, Al doping transfers more electrons to neighboring O atoms, resulting in greater attraction forces to Li ions and reducing Li-ion conductivity. Additionally, Fe-doped LLZO exhibits extra states in the bandgap, potentially causing Fe reduction, as observed in experiments. Our findings provide valuable insights into the design of solid electrolytes and highlight the importance of the local charge distribution around the dopant and Li atoms in determining Li-ion conductivity. This insight could serve as a guiding principle for future materials design and optimization in solid-state electrolyte systems.
立方石榴石型LiLaZrO（c-LLZO）是一种很有前景的全固态电池固体电解质，通常掺杂Ga、Al和Fe以稳定结构并提高锂离子电导率。尽管引入了相同数量的锂空位，但这些具有+3价经典电荷的掺杂剂产生的锂离子电导率相差约一个数量级。在本研究中，我们使用密度泛函理论（DFT）计算来研究Ga、Fe和Al掺杂剂对锂化学势和锂离子电导率变化的影响。我们确定了c-LLZO中能量有利的掺杂剂位置，并确定了c-LLZO中掺杂剂Fe的DFT+计算的最佳值为7.5 eV。我们的计算表明，Ga或Fe掺杂使锂化学势提高0.05-0.08 eV，降低了锂离子转移势垒并提高了锂离子电导率，而Al掺杂使锂化学势降低0.08 eV，降低了锂离子电导率。为了确定锂化学势变化的原因，我们对态密度投影、电荷密度和巴德电荷进行了综合分析。掺杂原子到相邻O原子的独特电荷分布对于确定锂离子化学势至关重要。Ga和Fe掺杂剂保留了更多电子，这使得相邻的O原子获得更正的电荷，通过降低作用在锂离子上的约束力使锂离子不稳定，从而提高了锂离子电导率。相比之下，Al掺杂将更多电子转移到相邻的O原子上，导致对锂离子的吸引力更大，降低了锂离子电导率。此外，如实验中所观察到的，Fe掺杂的LLZO在带隙中表现出额外的态，可能导致Fe还原。我们的研究结果为固体电解质的设计提供了有价值的见解，并强调了掺杂剂和锂原子周围的局部电荷分布在确定锂离子电导率方面的重要性。这一见解可为未来固态电解质系统中的材料设计和优化提供指导原则。
