LiMn2–x
Ni
x
O4 spinel phases,
with their almost flat electrochemical
curves composed of two plateaus around 4.7 V vs Li+/Li separated by a voltage difference ΔV of 20–60 mV, are good candidates for high power applications.
The Ni/Mn order is one of the key parameters in understanding the
electrochemical curve shape. In this work, the Ni/Mn order in the
nickel-rich region of the spinel LiMn2–x
Ni
x
O4 solid solution
(0.38 ≤ x ≤ 0.50) has been investigated
using time-of-flight powder neutron diffraction (TOF-PND) and density
functional theory (DFT) calculations. For LiMn2–x
Ni
x
O4 solid-solution
samples prepared between 700 and 900 °C, Ni/Mn ordering was found
to be retained to room temperature by systematic broadening of diffraction
peaks with hkl indexes of mixed even/odd parity.
This broadening is due to the increasing density of a planar defect
called antiphase domain boundaries (APBs). DFT calculations performed
on several Ni/Mn defective configurations and TOF-PND Rietveld refinement
indicate that the {100} orientation of the APB boundary is the most
probable. Hence, in the whole composition range, a unique ordered
spinel phase within the space group P4332, with a single hkl-dependent parameter to represent
the APB crossing probability, gives a measure of the Ni/Mn order coherence
length. We show that this defect density is driven by the synthesis
temperature and the nickel content of the spinel phase. A correlation
between the synthesis condition effect on the local ordering and the
voltage profile is given for two Ni/Mn initial ratios (0.4/1.6 and
0.5/1.5). The influence of the synthesis temperature on these two
compositions is drastically different: for LiMn1.6Ni0.4O4, with a similar APB domain size whatever the
temperature, only a slight variation of ΔV is
observed. Reversely, for LiMn1.5Ni0.5O4, a strong increase of the ΔV with the synthesis
temperature is evidenced, concomitant with a decrease in the APB domain
size and the Ni content.