In the past decades, research efforts on polyanion-type cathode materials by the scientific community intensified significantly. This paper reviews the latest advances in the exploration and development of polyanion-type compounds as high performance cathode materials for Li-ion batteries. It focuses on the synthesis, structure and physicochemical (especially electrochemical) properties of several classes of polyanion compounds. The relationship between composition–structure–performance of the novel electrode materials is also summarized and analyzed. The main approaches, achievements and challenges in this field are briefly commented and discussed.

Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable batteries. The promising P2-layered sodium transition metal oxides (P2-Na_xTmO₂) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-Na_xTmO₂ have not yet been clearly elucidated. Herein, taking P2-Na_0.67MnO₂ and P2-Na_0.67Ni_0.33Mn_0.67O₂ as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-Na_xTmO₂ in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-Na_xTmO₂ is determined by the amount of extracted Na⁺, which is mainly compensated by Na⁺/H⁺ exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-Na_xTmO₂ is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable Na_xTmO₂ cathodes.

High nickel content in LiNi_xCo_yMn_zO₂ (NCM, x ≥ 0.8, x + y + z = 1) layered cathode material allows high specific energy density in lithium-ion batteries (LIBs). However, Ni-rich NCM cathodes suffer from performance degradation, mechanical and structural instability upon prolonged cell cycling. Although the use of single-crystal Ni-rich NCM can mitigate these drawbacks, the ion-diffusion in large single-crystal particles hamper its rate capability. Herein, we report a strategy to construct an in situ Li_1.4Y_0.4Ti_1.6(PO₄)₃ (LYTP) ion/electron conductive network which interconnects single-crystal LiNi_0.88Co_0.09Mn_0.03O₂ (SC-NCM88) particles. The LYTP network facilitates the lithium-ion transport between SC-NCM88 particles, mitigates mechanical instability and prevents detrimental crystalline phase transformation. When used in combination with a Li metal anode, the LYTP-containing SC-NCM88-based cathode enables a coin cell capacity of 130 mAh g⁻¹ after 500 cycles at 5 C rate in the 2.75-4.4 V range at 25 °C. Tests in Li-ion pouch cell configuration (i.e., graphite used as negative electrode active material) demonstrate capacity retention of 85% after 1000 cycles at 0.5 C in the 2.75-4.4 V range at 25 °C for the LYTP-containing SC-NCM88-based positive electrode.

ConspectusLithium-ion batteries (LIBs) are ubiquitous in all modern portable electronic devices such as mobile phones and laptops as well as for powering hybrid electric vehicles and other large-scale devices. Sodium-ion batteries (NIBs), which possess a similar cell configuration and working mechanism, have already been proven as ideal alternatives for large-scale energy storage systems. The advantages of NIBs are as follows. First, sodium resources are abundantly distributed in the earth's crust. Second, high-performance NIB cathode materials can be fabricated by using solely inexpensive and noncritical transition metals such as manganese and iron, which further reduces the cost of the required raw materials. Recently, the unprecedented demand for lithium and other critical minerals has driven the cost of these primary raw materials (which are utilized in LIBs) to a historic high and thus triggered the commercialization of NIBs.Sodium layered transition metal oxides (Na_xTMO₂, TM = transition metal/s), such as Mn-based sodium layered oxides, represent an important family of cathode materials with the potential to reduce costs, increase energy density and cycling stability, and improve the safety of NIBs for large-scale energy storage. However, these layered oxides face several key challenges, including irreversible phase transformations during cycling, poor air stability, complex charge-compensation mechanisms, and relatively high cost of the full cell compared to LiFePO₄-based LIBs. Our work has focused on the techno-economic analysis, the degradation mechanism of Na_xTMO₂ upon cycling and air exposure, and the development of effective strategies to improve their electrochemical performances and air stability. Correlating structure-performance relationships and establishing general design strategies of Na_xTMO₂ must be considered for the commercialization of NIBs.In this Account, we discuss the recent progress in the development of air-stable, electrochemically stable, and cost-effective Na_xTMO₂. The favorable redox-active cations for Na_xTMO₂ are emphasized in terms of abundance, cost, supply, and energy density. Different working mechanisms related to Na_xTMO₂ are summarized, including the electrochemical reversibility, the main structural transformations during the charge and discharge processes, and the charge-compensation mechanisms that accompany the (de)intercalation of Na⁺ ions, followed by discussions to improve the stability toward ambient air and upon cycling. Then the techno-economics are presented, with an emphasis on cathodes with different chemical compositions, cost breakdown of battery packs, and Na deficiency, factors that are critical to the large-scale implementation. Finally, this Account concludes with an overview of the remaining challenges and new opportunities concerning the practical applications of Na_xTMO₂, with an emphasis on the cost, large-scale fabrication capability, and electrochemical performance.