5142 Inorganic Chemistry, Vol. 48, No. 12, 2009
Cabana et al.
system,37 yet more controversy has arisen as to whether
or negative electrode17 and electrolyte18 materials in Li-ion
batteries. Interestingly, some lithium-containing oxynitrides
such as Li16M2N8O (M = Nb, Ta),19-21 Li14Cr2N8O,22,23
Li7VN4 is a member of the Li7VN4 nLi2O solid solution
3
or not.37-40
24
Li6Ca12Re4N16O3 or Li2Sr6Cr2N8O25 were discovered be-
To our knowledge, no such systematic study has been
carried out in the Li-Mn-N-O system, where the only
phase known, Li7.9MnN3.2O1.6, was recently reported by
us.41 This compound also has an antifluorite structure and
constitutes a very good example of the effect of the anionic
ratio on the properties of oxynitrides. When compared to
Li7MnN4, higher air stability was found,17 albeit a certain
sensitivity to moisture was still observed. Nonetheless,
Li7.9MnN3.2O1.6 could be stored for several weeks without
noticeable decomposition when kept in a dry atmosphere.
Lithium transition metal nitrides were first proposed in the
1990s as alternative materials for negative electrodes in
lithium-ion batteries. Among them, Li3-xCoxN shows ex-
cellent specific capacity values,42,43 that is, the amount of
charge obtained per unit mass of material used, whereas
Li7MnN4 shows smaller capacity values but notable reten-
tion upon cycling.44-47 Li7.9MnN3.2O1.6 was the first lithium
transition metal oxynitride tested for this application. Its
initial specific capacity is comparable to that of the nitride,
but it shows improved retention on cycling.17
As a logical extension of our previous work, we present
here the results of the exploration of the Li-Mn-N-O
system by changing both the Li/Mn and the N/O ratios
simultaneously through the introduction of variable amounts
of Li2O to the reaction mixture. Our aim has been to obtain
deeper insight into the chemical and structural relationships
among the phases found in this system by using techniques
such as powder X-ray diffraction (XRD), X-ray absorption
spectroscopy (XAS), and solid state Nuclear Magnetic Re-
sonance (NMR). In addition, representative samples were
tested as electrodes in lithium batteries, and the effects of the
compositional changes on the performance of these com-
pounds are also discussed.
cause of small accidental oxygen leaks in the reaction setup
intended in principle for the preparation of nitrides. Two
main methods have been described for the synthesis of these
phases. The first one (and the most usual) consists of the
ammonolysis of oxide mixtures at high temperatures.16,26-32
The other approach was proposed by Juza and co-workers in
the 1950s, when they investigated the possibility of controlling
the amount of oxygen in lithium transition metal oxynitrides
by adding variable quantities of binary oxides to a mixture of
Li3N and a transition metal, which was subsequently heated
under a nitrogen atmosphere. They used this methodology to
prepare what they described as solid solutions of a lithium
transition metal nitride having a general formula Li2n-1MNn
(where M = transition metal) and Li2O. More specifically,
systems with M = Ti,33 V,34 and Cr35 were studied. In all
cases, the corresponding ternary nitrides adopt different
antifluorite type superstructures derived from Li/M ordering
in the tetrahedral positions of the structure, whereas the
oxynitrides show a disordered antifluorite structure, with a
Fm3m space group. Recent studies by other authors22 and
ourselves36 in the Li-Cr-N-O system point out that this
solid solution might not have the ternary nitride as end
member, as suggested by the existence of Li14Cr2N8O, a
compound with an antifluorite superstructure due to Li/Cr
and N/O ordering, and its co-existence with the fully dis-
ordered Li10CrN4O2 phase depending on the initial reagent
ratio and the synthesis conditions.36 A limited solid solution
range also appears to be the case for the Li-Ti-N-O
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Experimental Section
Samples with different Li, Mn, N, and O contents were
prepared from a powder mixture of Li3N (Aldrich), manga-
nese nitride (MnxN, commercial mixture of Mn4N and
Mn2N, with x = 3.1, according to chemical analysis, Cerac
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