Effect of High Voltage on the Structure and Electrochemistry of LiNi_0.5Mn_0.5O₂:  A Joint Experimental and Theoretical Study

A combination of neutron diffraction (ND), Li-6 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 Lix(Ni0.5Mn0.5)O-2 (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).