Olivine lithium iron phosphate is a technologically important electrode material for lithium-ion batteries and a model system for studying electrochemically driven phase transformations. Despite extensive studies, many aspects of the phase transformation and lithium transport in this material are still not well understood. Here we combine operando hard X-ray spectroscopic imaging and phase-field modeling to elucidate the delithiation dynamics of single-crystal lithium iron phosphate microrods with long-axis along the [010] direction. Lithium diffusivity is found to be two-dimensional in microsized particles containing ~3% lithium-iron anti-site defects. Our study provides direct evidence for the previously predicted surface reaction-limited phase-boundary migration mechanism and the potential operation of a hybrid mode of phase growth, in which phase-boundary movement is controlled by surface reaction or lithium diffusion in different crystallographic directions. These findings uncover the rich phase-transformation behaviors in lithium iron phosphate and intercalation compounds in general and can help guide the design of better electrodes.

Single-crystalline nickel-rich cathodes are a rising candidate with great potential for high-energy lithium-ion batteries due to their superior structural and chemical robustness in comparison with polycrystalline counterparts. Within the single-crystalline cathode materials, the lattice strain and defects have significant impacts on the intercalation chemistry and, therefore, play a key role in determining the macroscopic electrochemical performance. Guided by our predictive theoretical model, we have systematically evaluated the effectiveness of regaining lost capacity by modulating the lattice deformation via an energy-efficient thermal treatment at different chemical states. We demonstrate that the lattice structure recoverability is highly dependent on both the cathode composition and the state of charge, providing clues to relieving the fatigued cathode crystal for sustainable lithium-ion batteries.

Ni-rich layered oxides (LiNi_xCo_yMn_1−x−yO₂, x > 0.8, NCM) are technologically important cathode (i.e., positive electrode) materials for next-generation high-energy batteries. However, they face challenges in cycle stability and durability due to internal strain accumulation and particle fracture as the batteries cycle. Here we report a simple molten-salt-assisted synthesis route to introduce gradiently distributed pores into the polycrystalline NCM secondary particles. The gradient porous strategy creates void spaces to buffer the anisotropic volume change of the primary particles, effectively mitigating the intergranular fracture and limiting the impedance growth. It not only increases the maximum accessible capacity of the NCM cathodes but also greatly enhances their cycle stability in practical pouch-type batteries and all-solid-state-batteries. It further enables a high nickel, low cobalt cathode (LiNi_0.96Co_0.02Mn_0.02O₂) with a combination of high specific energy (941.2 Wh kg⁻¹ based on cathode weight at 0.1 C and 25 °C, 1 C = 245 mA g⁻¹) and high stability during cycling (80.5% capacity retention after 800 cycles at 1 C relative to that of the first cycle) and high-temperature storage (reversible capacity retention >95.5% after 42-day storage at 60 °C at the fully charged state) in pouch cells.