The hard carbon volumetric capacity improves by decreasing the electrode porosity after calendaring (d002 decrease). The initial CCoulombic efficiency is enhanced when the specific surface area of ultra-micropores and the active sites are diminished.

counterparts, hence offering practical interest for grid applications where capabilities for smoothening fluctuations and low cost are the overriding factors. [1][2][3][4][5][6][7] Most of the Na-ion systems demonstrated till now use hard carbon (HC) as common negative electrode and either sodium layered oxides (Na x TMO 2 , 0 < x ≤ 1, TM = transition metals) or polyanionic compounds (Na 3 V 2 (PO 4 ) 2 F 3 ) as positive electrodes. [8][9][10][11] We have recently benchmarked such Na-ion full cells having sodium layered oxide electrodes with either O3 or P2 structures and TM/TM′ of different nature against Na 3 V 2 (PO 4 ) 2 F 3 /HC cells. [12] Whatever the figures of merit we have checked, such as specific energy, cyclability as well as rate capability, the 3D Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) phase outperforms the layered phases, irrespective of its high molecular weight and modest capacity (128 mAh g −1 , equivalent to 2 Na + exchange). [13] Moreover, sodium based layered oxides reported so far, whatever O or P-type structures, suffer from low redox potential, limited reversible capacity versus carbon in Na-ion full cells (hardly 50% of their theoretical capacity), Na-driven structural instability and moisture sensitivity. [14,15] So how long will this supremacy of NVPF last?To compete with NVPF, it is important to design sodium layered oxides, which can reversibly release and uptake Na + ions Although being less competitive energy density-wise, Na-ion batteries are serious alternatives to Li-ion ones for applications where cost and sustainability dominate. O3-type sodium layered oxides could partially overcome the energy limitation, but their practical use is plagued by a reaction process that enlists numerous phase changes and volume variations while additionally being moisture sensitive. Here, it is shown that the double substitution of Ti for Mn and Cu for Ni in O3-NaNi 0.5 − y Cu y Mn 0.5 − z Ti z O 2 can alleviate most of these issues. Among this series, electrodes with specific compositions are identified that can reversibly release and uptake ≈0.9 sodium per formula unit via a smooth voltage-composition profile enlisting minor lattice volume changes upon cycling as opposed to ΔV/V≈23% in the parent NaNi 0.5 Mn 0.5 O 2 while showing a greater resistance against moisture. The positive attributes of substitution are rationalized by structure considerations supported by density functional theory (DFT) calculations. Electrodes with sustained capacities of ≈180 mAh g −1 are successfully implemented into 18 650 Na-ion cells having greater performances, energy density-wise (≈250 Wh L −1 ), than today's Na 3 V 2 (PO 4 ) 2 F 3 /HC Na-ion technology which excels in rate capabilities. These results constitute a step forward in increasing the practicality of Na-ion technology with additional opportunities for applications in which energy density prevails over rate capability.