The science behind the operation of a lithium-ion battery is extraordinarily complex. In batteries, electric charge is carried by positive lithium ions. During charging or discharging of the battery, the lithium ions are shuttled between two electrodes. To understand this process, researchers examined lithium in a single crystal of olivine lithium iron phosphate, an important material in electrodes. The results showed the critical role that crystal defects play in the operation of the battery. The defects were iron atoms occupying lithium sites. The scientists suggested that a few defects in the electrode material may actually improve the battery performance by enhancing the overall speed at which lithium travels.
The study offers insights into how tiny structural changes evolve in common lithium-ion battery electrodes. This new knowledge provides a basis to update design rules for lithium-ion batteries. Specifically, design rules should include the particle shape, defects, and surface reactions. The result will be designed cathode materials and better batteries.
Olivine lithium iron phosphate (LiFePO4) is an important electrode material used for lithium-ion batteries. Single crystalline micro-particles of the electrode material have been used as a model system for studying lithium intercalation (migration, uptake, and release of lithium during battery charging and use) and phase transformation mechanisms for commercial battery electrodes. Using operando transmission x-ray microscopy, researchers came to a surprising conclusion about lithium transport and phase boundary migration. The researchers confirmed that the presence of more than 3 percent anti-site defects in the electrode is sufficient to change lithium transport from the 1-D diffusivity predicted for perfect LiFePO4 crystals, to 2-D behavior. In this material, an anti-site defect occurs when an iron atom occupies a lithium site. Because the electrode material commonly used in commercial batteries has many defects, the conclusion from this study suggests that practical, defect-containing LiFePO4 has a much larger active surface area. This will lead to more intercalation of lithium during charge and discharge than previously thought. Corroborated by phase-field simulations, the operando observations reveal a new hybrid-mode phase transformation mechanism. In this mechanism, the phase boundary exhibits a fast surface-reaction-limited propagation along particle surfaces and a slow diffusion-limited propagation perpendicular to the surface. This study offers new insights on how defects and particle morphology should be tailored to improve battery electrode performance. The results will have broad implications for the use of other lithium intercalation materials as electrodes for batteries.