RESUMO
Given its exceptional theoretical energy density (over 2000 Wh kg-1), lithium||carbon fluoride (Li||CFx) battery has garnered global attention. N-methylpyrrolidone (NMP)-based electrolyte is regarded as one promising candidate for tremendously enhancing the energy density of Li||CFx battery, provided self-discharge challenges can be resolved. This study successfully achieves a low self-discharge (LSD) and desirable electrochemical performance in Li||CFx batteries at high temperatures by utilizing NMP as the solvent and incorporating additional ingredients, including vinylene carbonate additive, as well as the dual-salt systems formed by LiBF4 with three different Li salts, namely lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, and LiNO3. The experimental results unfold that the proposed methods not only minimize aluminum current collector corrosion, but also effectively passivate the Li metal anode. Among them, LiNO3 exhibits the most pronounced effect that achieves an energy density of ≈2400 Wh kg-1 at a current density of 10 mA g-1 at 30 °C, nearly 0% capacity-fade rate after 300 h of storage at 60 °C, and the capability to maintain a stable open-circuit voltage over 4000 h. This work provides a distinctive perspective on how to realize both high energy density and LSD rates at high temperature of Li||CFx battery.
RESUMO
In this work, Li-rich Li1.2Mn0.43+Mnx4+Ti0.4-xO2 (LMMxTO, 0 ≤ x ≤ 0.4) oxides have been studied for the first time. X-ray diffraction (XRD) patterns show a cation-disordered rocksalt structure when x ranges from 0 to 0.2. After Mn4+ substitution, LMM0.2TO delivers a high specific capacity of 322 mAh g-1 at room temperature (30 °C, 30 mA g-1) and even 352 mAh g-1 (45 °C, 30 mA g-1) with an energy density of 1041 Wh kg-1. The reason for such a high capacity of LMM0.2TO is ascribed to the increase of both cationic (Mn) and anionic (O) redox after Mn4+ substitution, which is proved by dQ/dV curves, X-ray absorption near edge structure, DFT calculations, and in situ XRD results. In addition, the roles of Mn3+ and Ti4+ in LMM0.2TO are also discussed in detail. A ternary phase diagram is established to comprehend and further optimize the earth-abundant Mn3+-Mn4+-Ti4+ system. This work gives an innovative strategy to improve the energy density, broadening the ideas of designing Li-rich materials with better performance.
RESUMO
In this work, we revisited the stability of the Cr4+/Cr3+ redox couple in a sodium superionic conductor (NASICON)-type compound, Na2TiCr(PO4)3. Experimental results showed that the Na2TiCr(PO4)3 compound exhibited a specific capacity of 49.9 mA h g-1 at 20 mA g-1, about 80% of its theoretical capacity of 62.2 mA h g-1 with one Na+ insertion/deinsertion per formula Na2TiCr(PO4)3. The redox couple was found to be stable against cycling with some 90.3% capacity retention after 300 cycles within the voltage range between 2.5 and 4.7 V. With a wider voltage range between 2.5 and 5.0 V, the capacity retention was about 76.6% after 1000 cycles, indicating the redox couple is stable against overvoltage. In addition, the effect of Ti/Cr ratio on the reversibility of the redox couple was studied by varying x in Na1+xTi2-xCrx(PO4)3 (where x = 0.6, 0.8, 1.0, 1.2, 1.4, 2.0). It was confirmed that x = 1 is optimal for balancing the electrode stability and the capacity. The obtained optimal content of Cr in the compound provides useful guidance for designing new Cr-based NASICON-type cathode materials. Furthermore, in situ X-ray diffraction (XRD) analysis of compound Na2TiCr(PO4)3 indicated a two-phase sodium-ion storage mechanism, which is different from the previously reported one-phase mechanism. Rietveld refinement XRD analysis showed a small volume change of the compound during cycling (about 2.6%), indicating good structural stability.
