Since the report of electrochemical activity of LiFePO₄ from Goodenough's group in 1997, it has attracted considerable attention as cathode material of choice for lithium‐ion batteries. It shows excellent performance such as the high‐rate capability, long cyclability, and improved safety. Furthermore, the raw materials cost of LiFePO₄ are lower and abundant compared with conventional Li‐ion battery oxides compounds. The lithium extraction from LiFePO₄ operates as biphase mechanism accompanied by a relatively large volume change of ∼6.8%, even though, nanosized LiFePO₄ shows exceptionally high‐rate capability during cycling. In the aim to explain this remarkable feature, recent reports using cutting‐edge techniques, such as in situ high‐resolution synchrotron X‐ray diffraction, explained that the origin of the observed high‐rate performance in nanosized LiFePO₄ is the absence of phase separation during battery operation at high current densities. In this review, the importance of understanding lithium insertion mechanisms towards explaining the significantly fast‐charging performance of LiFePO₄ electrode is highlighted. In particular, phase separation mechanisms, are unclear and deserve considerable attention. Several proposed models for Li diffusion and phase separation in LiFePO₄ are summarized. In addition, the crystallographic aspects, defect structures of LiFePO₄ are also reviewed. Finally, the status of development of other olivine‐type LiMPO₄ (M = Co, Mn, and Ni) are summarized.

Cycling positive electrode materials with high voltages and long lifetimes remains a challenge. On the one hand, operating electrodes at high voltages is usually accompanied by severe structural distortions and irreversible reactions. On the other hand, the significant volume variation upon Na⁺ insertion/extraction limits the long cycling life. Here, we report a defective positive electrode material with a chemical formula of Na_3.2□_0.8Co_0.5Fe_0.5V(PO_3.9F_0.1)₃ (□ represents Na vacancy) through fluorine doping with activated reversible and fast Na⁺ intercalation/deintercalation to increase the energy and power densities. Notably, this positive electrode material achieves a high reversible specific capacity of 151 mAh g⁻¹ and prominent rate performance ranging from 1.5–4.2 V vs. Na⁺/Na, as well as a long lifespan of 6000 cycles under a high rate of 5 A g⁻¹ with a capacity retention of ~94%, due to the activated cationic defect sites which reduces the transport barrier at the Na(1) site. This approach is expected to be applied to the rational design of polyanionic materials for batteries.

The rapid diffusion of renewable energy boosts the wide deployment of large-scale energy storage system. With the low cost and high crustal abundance, sodium-ion battery (SIB) technology is expected to become a dominant technology in that area in the future. Toward the practical application, novel cathode materials are urged to develop that show high energy density without sacrificing their cost and benignity to the environment. While the years of many studies, this still remains a huge challenge to battery scientists. In this review, we discuss recent breakthroughs in SIB cathode materials with high energy density, namely fluorphosphates and fluorosulfates. The design of materials, the crystal structure, the electrochemical performance, and the underlaying intercalation mechanism are systematically reviewed. Useful strategies and research directions are also provided to advance future high-energy, low-cost, and ecofriendly cathode materials for next generation SIB.