Unraveling the Critical Role of Ti Substitution in P₂-Na_xLi_yMn_1–yO₂ Cathodes for Highly Reversible Oxygen Redox Chemistry

Abstract

Monovalent Li-substitution has been proven to be an effective strategy to resolve the pivotal problems confronted with P2-type layered Mn oxides, such as cooperative Jahn–Teller distortions of Mn3+ ions and drastic P2-(OP4)-O2 phase transformations occurring during desodiation. However, the cycling stability of most Li+-substituted P2-Na x Li y Mn1–y O2 remains far from satisfactory. We herein develop a facile Ti-substitution method to improve the cyclability by taking Na0.72Li0.24Mn0.76O2 (NLMO) as an example. As expected, the novel layered oxide cathode Na0.72Li0.24Ti0.10Mn0.66O2 (NLMTO-0.1) is able to deliver a very high reversible capacity of 165 mA h g–1 for over 80 cycles within the voltage range of 1.5–4.5 V (vs Na metal), which is among the best for the reported Na-storage cathode materials. Moreover, the structure–property relationship of Ti4+ substitution is scrutinized by an arsenal of 23Na/7Li solid-state nuclear magnetic resonance, dual-mode electron paramagnetic resonance, and synchrotron X-ray diffraction techniques. The results unequivocally substantiate that Ti substitution can effectively reduce the Li+/Mn4+ ordering in TMO2 slabs, assist the reversible migration of Li+ during Na+ extraction/intercalation, and ultimately enhance the reversibility of the oxygen redox process. This work provides a comprehensive insight into the structure chemistry in developing high-capacity and high-stability layered oxide cathodes.

Anionic Redox Reactions in Cathodes for Sodium‐Ion Batteries

“…According to previous studies, the disorder in TM layers induced by low crystallization causes severe voltage decay and capacity reduction [14,46] because TM migration and irreversible slab gliding occur [46] . However, TM disordering caused by the substitution of d 0 (or d 10 ) ions not only eliminates the localized energy difference in Na sites but also increases reversibility [47–50] . Because d 0 and d 10 ions such as Ti 3+/4+ , Zn 2+ , Mg 2+ , and Li + are not electrochemically active in Na intercalation compounds, the structural variability that occurs during the redox of the TM−O orbital is reduced.…”

“…Because d 0 and d 10 ions such as Ti 3+/4+ , Zn 2+ , Mg 2+ , and Li + are not electrochemically active in Na intercalation compounds, the structural variability that occurs during the redox of the TM−O orbital is reduced. In addition, the larger ionic radii of these species result in more effective diffusion paths for alkali metal ions, thereby increasing the ionic conductivity of the layered structure [50] . The substitution of TMs, which can increase covalency by donating electrons to TM−O bonds, is, thus, a good strategy to regulate the anionic redox activity and provide additional cation‐redox couples [49,51–53] .…”

Recent Advances in Electrode Materials with Anion Redox Chemistry for Sodium-Ion Batteries

“…The continuous reduction of the a-axis parameter was also indicative of the gradual oxidation from O 2− to O n− (n < 2) during charge, compensating for the charge imbalance occurring in Mn 4+ . Li et al [24] observed the migration of lithium using NMR in Na 0.72 [ [20]. However, the aforementioned P2 Na x [Li y Mn 1-y ]O 2 compounds exhibited hysteresis between charge and discharge, as evident in Figure 2(a).…”

“…for which a third of the TM layers can be filled by mono-or divalent elements such as Li [17][18][19][20][21][22][23][24][25][26][27][28][29], Mg [46][47][48][49][50][51][52][53][54][55][56], and Zn [57][58][59] to induce an average oxidation state of Mn of 4+. Hence, the extraction of sodium ions in the structure is not theoretically possible because of the difficulty of the oxidation of Mn 4+ to higher valence states in an octahedral environment, in which the electrolyte does not decompose.…”

“…The selection of elements in transition metal layers is of great importance in improving the reversibility and controlling the operation voltage of the anionic redox reaction, namely, Na x [A y TM 1-y ]O 2 (A: Li [17][18][19][20][21][22][23][24][25][26][27][28][29], Na [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45], Mg [46][47][48][49][50][51][52][53][54][55][56], Zn [57][58][59][60], Ni [61][62][63][64][65][66]…”