Reaching the Energy Density Limit of Layered O3‐NaNi_0.5Mn_0.5O₂ Electrodes via Dual Cu and Ti Substitution

Abstract

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.

Understanding the Capacity Fading Mechanisms of O3‐Type Na[Ni0.5Mn0.5]O2 Cathode for Sodium‐Ion Batteries

“…The poor cycling stability of Na[Ni 0.5 Mn 0.5 ]O 2 is considered to be related to the severe structural changes arising from multiple phase transitions. [ 28–30 ] In order to clearly investigate the effect of phase transitions on the cycling stability, the Na[Ni 0.5 Mn 0.5 ]O 2 cathode was additionally tested with upper cut‐off voltages limited to 3.35 and 3.58 V below the voltage where the last two phases of P3′ and O3′ were intentionally avoided. The discharge capacity decreased as the cut‐off voltage was lowered, resulting in 125.4 and 108.9 mAh g −1 for 3.58 and 3.35 V, respectively.…”

“…in the P′3 monoclinic phase between 3.23 and 3.5 V. [ 25 ] Above 3.6 V, the P3′ to O3′ phase transition occurred, which is corroborated by the slight shift of the (003) peak to a higher angle and the appearance of the (104) O3′ peak located at ≈42.5°. [ 30,32 ] Upon following discharging from 4.0 to 2.0 V, the cathode reversely underwent a series of phase transitions from the hexagonal O3′ to the initial O3 phase and all of the peaks in the O3 phase were completely recovered after full discharge to 2.0 V (see Figure S5, Supporting Information). Figure 3b shows the lattice parameters obtained from the in situ XRD data.…”

Hierarchical Engineering of Porous P2‐Na2/3Ni1/3Mn2/3O2 Nanofibers Assembled by Nanoparticles Enables Superior Sodium‐Ion Storage Cathodes

“…[1][2][3][4] However, the pursuit of applicable Na-ion storage electrode materials seems harder than the exploration of their Li-ion counterparts due to the larger size and heavier mass of Na + versus Li + . [7,8] Among the currently proposed cathode candidates, such as transition metal oxides (TMOs), [9][10][11][12][13][14][15][16][17] polyanion-type compounds, [18][19][20][21][22][23] Prussian blue analogues, [24,25] and organic salts, [26,27] layered transition metal oxides are particularly intriguing because of their 2D frameworks offering free Na-diffusion channels. [7,8] Among the currently proposed cathode candidates, such as transition metal oxides (TMOs), [9][10][11][12][13][14][15][16][17] polyanion-type compounds, [18][19][20][21][22][23] Prussian blue analogues, [24,25] and organic salts, [26,…”

Large-Scale Synthesis of the Stable Co-Free Layered Oxide Cathode by the Synergetic Contribution of Multielement Chemical Substitution for Practical Sodium-Ion Battery

“…Namely, the rationally selected multielement chemical substitution of Ti for Mn and Cu for Ni could not only increase the ionicity of the crystal lattice and the redox potential as well as restrain the unfavorable multiphase transformation but also improve the air stability because of the suppression of spontaneous Na extraction and enhanced antioxidizability. Furthermore, the inactive magnesium element also shows a pinning effect in improving the high structural reversibility and enlarges the interlayer spacing reasonably to enhance rate capability [ 55 – 57 ].…”