High nickel content layered cathodes, represented by NCM (LiNi x Co y Mn z O 2 ,x + y + z = 1), are now widely employed in the market of electric vehicles, owing to their high energy density. With the gradual increase of nickel content and capacity, the issues on cycling life and safety become more serious. In this review, various strategies for improving the performance of high nickel NCM are summarized on the aspects of surface coating, ionic doping, and singlecrystal NCM. The coating strategy was separately described according to the physical property of coating species, including inert material coating, Li +conductor coating, electronic conductor coating, and mixed conductor coating. These coating species help to suppress the interfacial oxidation of electrolytes by NCM, improving the cycling life and safety. The elemental doping in the crystal lattice of NCM is then presented in the aspects of cation, anion, and mixed-ion doping, which are beneficial to stabilize the layered structure during charge-discharge and so promote the electrochemical performance. In quite recent years, the strategy of single-crystal NCM was demonstrated to be a promising pathway, owing to the dramatically reduced surface area and grain boundary. Finally, the remaining unsolved challenges and future strategies for further development of NCM cathode materials are outlined.

is still the most popular cathode material in the LIBs for 3C products, mainly owing to its extraordinarily high tap density, [2] since there is a stricter requirement on the volumetric energy density. To reduce the amount of high-cost Co, part of Co was substituted by Ni and Mn with the generation of Li(Ni x Co y Mn z )O 2 (NCM, x + y + z = 1), which not only improves the structural and thermal stability but also allows more reversible extraction-intercalation of Li + and so endows more capacity, making it currently the dominant cathode materials in the market of power batteries of EVs. [3] Currently, compared with internal combustion engine vehicles, the main concerns for the popularization of EVs are the higher price, lower continue voyage course, and heavier safety risk, mainly resulting from the LIBs.For NCM cathodes, the capacities increase gradually with the increment of Ni contents; for instance, from Li(Ni 0.5 Co 0.2 Mn 0.3 )O 2 (NCM523) to Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 (NCM811), the capacity rises from 165 to 200 mAh g −1 , around 20% promotion. [4] However, with the increment of capacity, more Li + will reversibly intercalate into and extract from the NCM layered structure, resulting in larger volumetric change, more intergranular microcracks, and heavier interfacial reactions. [5] In addition, with the increase of Ni content, more superficial Ni 4+ ions lead to heavier catalytic oxidation of electrolytes, causing more serious capacity fading and safety risks. [6] To reduce the intergranular microcracks

Na3V2(PO4)3 (NVP) is one of the most widely studied structures as a cathode of sodium-ion batteries, although the relatively high cost of vanadium and low voltage need to be further improved compared with the counterpart of cathode materials of lithium-ion batteries. Herein, vanadium (V) of NVP was partially substituted by Al to exploit the sodium storage capability using the valence change of V4+/V5+ in higher voltage, without loss in the capacity. Since Al is lighter and less expensive, the theoretical capacity will be slightly increased and the energy density will be enhanced with the increased voltage. A family of carbon-coated Na3Al x V2–x (PO4)3 (NAVP@C, x = 0, 1/3, 1/2, 2/3, 3/4, 1) composites were prepared to comprehensively investigate the effect of Al content on extraction/intercalation of Na+ in NAVP@C. In every NAVP that contains Al, a charge/discharge voltage plateau at 3.9–4.1 V along with the common one at 3.4 V was observed, indicating that the redox of a higher valence than V4+ was made use of. However, the NAVP could not fully extract/intercalate two Na+ ions with the introduction of Al, owing to the shrinking crystal size with the Al substitution. Among all these members, Na3Al x V2–x (PO4)3 (x = 1/3) exhibited the largest reversible charge/discharge capacity around 113.82 mAh g–1, corresponding to an energy density of 405.99 Wh kg–1, calculated only from the cathode. All family members of NAVP exhibit noteworthy cycling stability and C-rate performance.

No abstract