One major challenge in the field of lithium-ion batteries is to understand the degradation mechanism of high-energy lithium- and manganese-rich layered cathode materials. Although they can deliver 30 % excess capacity compared with today’s commercially- used cathodes, the so-called voltage decay has been restricting their practical application. In order to unravel the nature of this phenomenon, we have investigated systematically the structural and compositional dependence of manganese-rich lithium insertion compounds on the lithium content provided during synthesis. Structural, electronic and electrochemical characterizations of LixNi0.2Mn0.6Oy with a wide range of lithium contents (0.00 ≤ x ≤ 1.52, 1.07 ≤ y < 2.4) and an analysis of the complexity in the synthesis pathways of monoclinic-layered Li[Li0.2Ni0.2Mn0.6]O2 oxide provide insight into the underlying processes that cause voltage fading in these cathode materials, i.e. transformation of the lithium-rich layered phase to a lithium-poor spinel phase via an intermediate lithium-containing rock-salt phase with release of lithium/oxygen.

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A novel crystalline lithium superionic conductor, Li 4 PS 4 I, has been discovered utilizing a solvent-based synthesis approach. It was found that the starting material Li 3 PS 4 •DME reacts with LiI in a 1:1 ratio in DME to give a precursor that results in Li 4 PS 4 I after soft heat treatment at around 200 °C in vacuum. Its crystal structure was solved ab initio by evaluating both X-ray (Mo-Kα 1 ) and neutron (TOF, GEM, ISIS) powder diffraction data, in a combined refinement (P4/nmm, Z = 2, a = 8.48284(12) Å, c = 5.93013(11) Å, wR p = 0.02973, GoF = 1.21499). The final structure model comprises, besides Li + ions, isolated PS 4 3− tetrahedra in a layer-like arrangement perpendicular to the c-axis that are held apart by I − ions. The Li + ions are distributed over five partially occupied sites residing in 4-, 5-, and 6-fold coordination environments. A topostructural analysis of the voids and channels within the PS 4 I 4− substructure suggested a threedimensional migration pathway system for the Li + ions in Li 4 PS 4 I. The Li + ion mobility was studied by temperature-dependent impedance spectroscopy as well as 7 Li solid-state nuclear magnetic resonance (NMR) spectroscopy including the measurement of spin− lattice relaxation rates T 1 −1 . The total ionic conductivity was determined to be in the range of 6.4 × 10 −5 to 1.2 × 10 −4 S•cm −1 at room temperature with activation energies (E A ) of 0.37 to 0.43 eV. The NMR analyses revealed a hopping rate of the Li + ions of τ −1 = 5 × 10 8 s −1 corresponding to a bulk conductivity of 1.3 × 10 −3 S•cm −1 at 500 K and an activation energy E A = 0.23(1) eV.