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Due to high safety and cost-effective resources, sodium-ion batteries show great prospect in large-scale energy storage systems. Owing to wide structural framework and high theoretical capacity, Na-based layered oxide with prismatic P2-type structure is an ideal cathode candidate for sodium-ion batteries. However, Na-based layered compounds always suffer from detrimental multiple phase transitions during cycle period, resulting in structure deterioration and rapid capacity decay. Herein, stable P2-Na0.67Ni0.33Mn0.67-xTixO2cathodes are constructed through substituting Mn with Ti. In-situXRD demonstrates that Ti substitution effectively successfully suppresses the undesired phase transition of P2-O2, and in turn appears a stable Z-symbiotic intermediate phase with minimal volume change. Hence, designed P2-Na0.67Ni0.33Mn0.37Ti0.3O2cathode shows better structural stability and long-term cycling reliability, yielding a high reversible capacity of 123 mAh g−1and a high capacity retention of 89 % after 100 cycles at 0.2C. More importantly, a high working voltage of 3.5 V and a boosted energy density of 400 Wh kg−1is achieved in the NNMTO-3 || HC full cell, which further strengthens the competence of Ti-substituted Na-based layered oxide cathodes. This work provides a feasible design insight for long-life, high-voltage sodium-ion batteries, which can be applied to construct more stable layered compounds and further improves their electrochemical properties.

The commercialization of sodium-ion batteries (SIBs) is tremendously limited as performance decline in extreme environments (such as below 0 ℃). This work proposes remarkable low-temperature (low-T) SIBs based on 0.5 M NaPF6diglyme electrolyte. By a series of in-depth analysis, including XPS, temperature-dependent EIS, in- and ex-situ XRD, etc., finding that the weakly solvated structure of Na+-diglyme is beneficial to accelerating diffusivity of Na+and reducing charge transfer energy (from 461.9 to 158.6 meV), which is effectively to ameliorate the low-T sodium storage mechanism. Moreover, The NaF-rich cathode electrolyte interphase (CEI) formed in diglyme is conducive to stabilize the interfacial structure and accelerate transport of Na+. All these merits collectively lead to high adaptability of low-concentration diglyme-based electrolyte to low-T. It has been verified that this system delivers 95.6% of room-temperature (RT) capacity, and full-cell delivers huge superiority in lifespan (working stably for 10,000 cycles) at − 20 ℃. Even at − 60 ℃, the battery still attains high capacity density (79.2 mAh g−1). These results provide new research ideas for improving the low-T performance of energy storage equipment.

Spinel LiNi0.5Mn1.5O4(LNMO) is widely utilized because of its high-energy-density and high-voltage. Unfortunately, there is still much research to be done for LNMO due to its poor structural stability. Here, a strategy is confirmed to stabilize LNMO via modulating interstitial sites. The interstitial 16c sites of the octahedron are partially occupied by Ni2+to suppress the migration and dissolution of manganese ions upon electrochemical cycling and stabilize lithium-ion vacancies in the state of charge. Unexpectedly, this protocol not only suppresses the phase separation restraining the phase boundary dislocations and stress but also decreases the magnitude of cell volume change during cycling, which originates from the change in Ni redox couple energy states. This two-pronged modification strategy endows the cathode material with a lower charge transfer barrier and faster Li+transfer kinetics, revealing superior electrochemical performance. The regulated cathode material remains robust after 900 cycles at 1C and its capacity retention rate is 29% higher than that of the original sample. Our research is useful for providing a concrete example of how the electrochemical performance of spinel LNMO and other high voltage cathode materials can be enhanced.

Na3V2(PO4)2F3is a promising cathode candidate for sodium ion batteries due to its high working voltage. However, an “unknown” low-voltage plateau around ~3.3 V lowers its operating voltage and puzzles researchers. Herein, the reasons for its appearance are explored. It is found that the formation of the low-voltage plateau originates from the extra-generated [VO6] induced by the fluorine loss during the synthesis. To enable better performance, a feasible strategy of stabilizing fluorine is proposed to eliminate the low-voltage plateau. It is demonstrated that this strategy can effectively guarantee the existence of fluorine and modulate the local electronic structure of V to maintain [VO4F2] in Na3V2(PO4)2F3phase, thus eliminating the undesirable plateau and achieving an operating-voltage increase of ~100 mV. Moreover, the enhanced structural stability and kinetics of the optimized Na3V2(PO4)2F3cathode are confirmed by in-situXRD. As expected, the Na3V2(PO4)2F3cathode delivers higher energy density (446.4 Wh kg−1), better rate capability and longer cycling performance (89.2%@30 C after 1000 cycles). Furthermore, its excellent adaptability to wide temperature range (−25 °C–55 °C) and strong competence in Na3V2(PO4)2F3||hard carbon full-cell are verified. These excellent properties further qualify Na3V2(PO4)2F3as a competitive cathode for sodium ion batteries.