density and high output power is being considered indispensable for both portable electronic devices and electric vehicles. However, limited lithium reserves and increased mining difficulty cannot meet the expanded needs for LIBs in largescale energy-storage systems (EESs). [1][2][3] In recent years, sodium-ion batteries (SIBs) emerge as a promising candidate for EESs owing to the high abundance and low cost of sodium resources. In spite of a similar intercalation chemistry to LIBs, SIBs afford a lower energy density and sluggish sodium intercalation/deintercalation kinetics which are regarded as the main obstacles to its widespread use. [4][5][6][7] Hence, considerable efforts have been made to search suitable electrode materials for SIBs.Cathode materials play a dominant role in determining the energy density, cost, and safety of SIBs, consequently become the focus of the current research. Nevertheless, only several types of materials, including transition-metal oxides, [8][9][10] Prussian blue analogues, [11,12] and polyanionic compounds, [4,13,14] show their potential application in SIBs. Compared to the other two types of cathode materials, polyanionic compounds with robust open frameworks have received significant attention due to their high structural and thermal stability, which are beneficial for a long lifespan and high safety. Generally, the exploration of available polyanionic cathodes has concentrated on vanadium-and ironbased compounds. Several NASICON-type vanadium-based materials, such as Na 3 V 2 (PO 4 ) 3 , [15,16] Na 7 V 4 (P 2 O 7 ) 4 PO 4 , [17] and Na 3 (VO) 2 (PO 4 ) 2 F, [18] exhibit high operating voltage, favorable energy density, fast sodium-ion transport and long cycle life, thus making them comparable to the cathodes of LIBs. However, the use of expensive and toxic V element remains a barrier to their practical applications. [19,20] Instead, employing earth-abundant and non-toxic iron as the redox center in the polyanionic compounds can lower the manufacturing cost and advance the environmental friendliness, which will accelerate the commercialization of SIBs.The great success of the commercial application of LiFePO 4 in LIBs encouraged researchers to seek for electrochemical active Na-Fe-PO 4 material for SIBs. [21] However, unlike its lithium analogue, NaFePO 4 does not crystallize in the olivine structure, and only some unconventional synthetic Sodium-ion battery has been considered as one of the most promising power sources for large-scale energy storage systems due to its similar electrochemistry to the lithium-ion battery and the crust abundance of Na resources. Essentially, developing low-cost electrode materials along with a facile and economical synthesis procedure is critically important to promote the commercialization of sodium-ion batteries. However, applicable cathode materials capable of being massively produced are still scarcely reported to date. Herein, a green and scalable synthesis approach is developed to obtain Na 3 Fe 2 (PO 4 )P 2 O 7 (NFPP)/rGO composite by usi...

Multivalent metal ion (Mg2+, Zn2+, Ca2+, and Al3+) batteries (MMIBs) emerged as promising technologies for large-scale energy storage systems in recent years due to the abundant metal reserves in the Earth’s crust and potentially low cost. However, the lack of high-performance electrode materials is still the main obstacle to the development of MMIBs. As a newly large family of two-dimensional transition metal carbides, nitrides, and carbonitrides, MXenes have attracted growing focus in the energy storage field because of their large specific surface area, excellent conductivity, tunable interlayer spaces, and compositional diversity. In particular, the multifunctional chemistry and superior hydrophilicity enable MXenes to serve not only as electrode materials but also as important functional components for heterojunction composite electrodes. Herein, the advances of MXene-based materials since its discovery for MMIBs are summarized, with an emphasis on the rational design and controllable synthesis of MXenes. More importantly, the fundamental understanding of the relationship between the morphology, structure, and function of MXenes is highlighted. Finally, the existing challenges and future research directions on MXene-based materials toward MMIBs application are critically discussed and prospected.