No abstract

A combination of neutron diffraction (ND), 6Li magic-angle spinning NMR, electrochemistry, and first principles calculations have been used to determine and rationalize the structural changes that occur during cycling of the layered material Li x (Ni0.5Mn0.5)O2 (x = 1), synthesized via the hydroxide route. ND and 6Li NMR experiments confirm that Li is lost from the transition metal (TM) layers, very early on in the charge process. On charging to higher voltages (above 4.5 V), the Li is lost from the tetrahedral and residual Li octahedral sites in the Li layers. This process is accompanied by a migration of more than 75% of the Ni ions originally present in the Li layers into the TM layers, to occupy the sites vacated by Li. Calculations suggest that (i) these Ni migrations occur via the tetrahedral sites, (ii) activation energies for migration depend strongly on the original position of the Ni ions in the Li layers though the driving force for migration is large (>1 eV), and (iii) because neither Ni3+ nor Ni4+ is stable in the tetrahedral site, migration will not occur once the Ni ions in the Li layers are oxidized to Ni3+ or Ni4+. Electrochemical measurements (open circuit voltage, OCV, and galvanostatic mode) are consistent with a high voltage process (approximately 4.6 V) associated with a large activation energy. The new Ni sites in the TM layers are not necessarily stable, and on discharge, 60% of the ions return to the Li layers. In particular, Ni ions surrounded by six Mn4+ ions are found (in the calculations) to be the least stable. Because the Li ions originally in the TM layers in the as-synthesized sample are predominantly in this environment, this is consistent with the Ni migration observed experimentally. Materials charged to 5.3 V can be cycled reversibly with stable capacities of over 180 mAh g-1.

Selected area electron diffraction patterns were collected from pristine LiNi 0.5 Mn 0.5 O 2 and cycled Li x -Ni 0.5 Mn 0.5 O 2 samples (to either 4.5 V or 5.3 V) in the charged and discharged states. Superlattice reflections characteristic of the 3a Hex. × 3a Hex. × c Hex. supercell, which are associated with ordering of Li-rich and Li-deficient sites in the transition metal layer of the pristine sample, were weakened considerably or disappeared completely in the charged samples, indicating a reduction of this long-range ordering. Detailed analysis revealed not only a considerable amount of Ni migration from the Li layer to the transition metal layer upon charging to 4.5 V but also that a complete removal of Ni from the Li layer might be possible upon charging to 5.3 V as evidenced by the detection of the O1 phase with a hexagonal-closepacked oxygen array. The Ni migration was in part reversible upon discharge as the fractions of crystals exhibiting the 3a Hex. × 3a Hex. × c Hex. superlattice reflections were considerably higher in the discharged samples than in the charged samples. Additional superlattice reflections that could not be indexed to the 3a Hex. × 3a Hex. × c Hex. supercell were observed in some crystallites of the cycled samples, the extent of ordering varying from crystal to crystal. This new long-range ordering was attributed to a nonrandom distribution of Li, Ni, and vacancies in the tetrahedral and/or octahedral sites of the Li layer. Although the nature of this long-range ordering is not fully understood, it is proposed that the Li, Ni, and vacancies order on the tetrahedral sites of the Li layer resulting in a 2a Hex. × 2a Hex. × c Hex. supercell with space group R3 hm, in the charged samples, while they order on the tetrahedral and octahedral sites of the Li layer in an a Mon. × a Mon. × c Mon. cell having space group P2/m, in the discharged samples. The appearance of long-range ordering in the Li layer of the cycled samples is likely due to electrostatic repulsion of cations, which might play an important role in the stability of the O3 layered structure and lithium diffusion in the layered structure.