Relative stability of normal vs. inverse spinel for 3d transition metal oxides as lithium intercalation cathodes

Abstract

Spinel oxides represent an important class of cathode materials for Li-ion batteries. Two major variants of the spinel crystal structure are normal and inverse. The relative stability of normal and inverse ordering at different stages of lithiation has important consequences in lithium diffusivity, voltage, capacity retention and battery life. In this paper, we investigate the relative structural stability of normal and inverse structures of the 3d transition metal oxide spinels with first-principles DFT calculations. We have considered ternary spinel oxides LixM2O4 with M = Ti, V, Cr, Mn, Fe, Co and Ni in both lithiated (x = 1) and delithiated (x = 0) conditions. We find that for all lithiated spinels, the normal structure is preferred regardless of the metal. We observe that the normal structure for all these oxides has a lower size mismatch between octahedral cations compared to the inverse structure. With delithiation, many of the oxides undergo a change in stability with vanadium in particular, showing a tendency to occupy tetrahedral sites. We find that in the delithiated oxide, only vanadium ions can access a +5 oxidation state which prefers tetrahedral coordination. We have also calculated the average voltage of lithiation for these spinels. The calculated voltages agree well with the previously measured and calculated values, wherever available. For the yet to be characterized spinels, our calculation provides voltage values which can motivate further experimental attention. Lastly, we observe that all the normal spinel oxides of the 3d transition metal series have a driving force for a transformation to the non-spinel structure upon delithiation.

Surface phase diagram and stability of (001) and (111)LiMn2O4spinel oxides

“…[2][3][4][11][12][13][14][15][16][17][18][19][20][21][22][23] The LMO surface structure was not investigated until very recent years using DFT, 2 and there are notable discrepancies in the reported LMO surface energies. [2][3][4] For instance, the reported energies for the same (001) Li-terminated surface vary from 0.26 to 0.96 J/m 2 .…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] However, there are several drawbacks of LMO including a severe capacity fade during cycling due to a Jahn-Teller (JT) distortion in the oxide material, as well as dissolution of Mn in the electrolyte. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Since the dissolution can initiate from the surfaces of LMO particles, a complete understanding of the surface structure and stability is the key to suppress the Mn loss and to overcome the current limitations of the spinel LMO as a Li-ion battery cathode. [2][3][4]11,12 The electrochemical performance of batteries with LMO cathodes depends on multiple factors (e.g.…”

Ab Initio Study of Sodium Insertion in the λ-Mn2O4 and Dis/Ordered λ-Mn1.5Ni0.5O4 Spinels

“…19,63 Bhattacharya and colleagues methodically studied the normal vs inverse spinel stability for Li-ion batteries of various spinel oxides at 0 K, including (de)lithiated Mn 2 O 4 . 62 With regard to the lithiated phase, the normal spinel is reported to be thermodynamically much more favorable (∼1 eV/Li 1 Mn 2 O 4 ). 62 Here, similar to its Li counterpart, the normal spinel is found to be much more stable (0.97 eV/Na 1 Mn 2 O 4 ) compared to the inverse spinel of Na 1 Mn 2 O 4 stoichiometry, as shown in Figure 4e.…”

“…Additionally, it should be noted that with regard to the delithiated Mn 2 O 4 phase, the normal spinel is reported as only 23 meV lower in energy than the inverse spinel, raising concerns on the stability of the delithiated/desodiated state. 62 …”

“…This demonstrates the superior kinetics of the normal spinels, confirming that a network of tetrahedral sites offers faster ion transport than a network of octahedral sites. 62,72 The dashed cyan circle in Figure 11b represents a tetrahedral 8a site that was partially occupied by Ni during the MD simulation, with Ni being able to hop back to its original transition metal octahedral site. It is also observed (Figure 11b), that when Na resides close to the normal/inverse boundary (dashed cyan circle) of the unit cell, it spends most of the time near a 16c octahedral environment.…”