7.4 Electrochemistry of the R, M and N Phases
103
such that the manganese could then be reduced from its initial 4 + state during the
next discharge, thereby activating it. More recently, Koga et al. [15] and Sathiya
et al. [16] have demonstrated oxygen participation in the redox process in some
lithium-rich oxides—a process that is accompanied with a phase transformation.
Koga demonstrated that Li–Co–Mn–Ni–O layered materials with excess lithium
convert to a two-phase material after the high voltage plateau, and this was attributed
to the shell of particles losing oxygen while the core did not, such that much of the
high voltage plateau can be attributed to oxygen redox in the core of the particles.
Fell et al. [87] recently found this two-phase behavior in Li 1.2 Ni 0.2 Mn 0.6 O 2 and used
Rietveld refinement to determine that one of the phases was oxygen deficient while
the other was not. At this moment, it is therefore unclear how much oxygen is lost
and how much oxygen redox takes place in the M material discussed here during the
high voltage plateau and remains an important question to answer. Regardless of the
ongoing debate about the nature of the high-voltage plateau, both the nickel redox
and the high voltage plateau can be seen as peaks in the dQ/dV curve during the first
charge of the M material in Fig. 7.6 The new peak appearing at 3.1 V in the second
cycle was consistent with manganese redox. After the cycles shown in Fig. 7.6, the
M phase was tested between 2–4.8 V to test its long-term cycling performance. The
long-term cycling capacities were 245.0 mAh/g during charge and 236.5 mAh/g
during discharge initially, and 244.2 mAh/g and 237.4 mAh/g respectively 40 cycles
later, demonstrating stable cycling.
By contrast to the highly ordered M-layered phase, the N-layered structure has a
disordered lithium layer with 30 % nickel occupation that apparently prevents some
lithium from diffusing out of the material, since only 50 % of the 201 mAh/g total
theoretical capacity was obtained during the first charge shown in Fig. 7.6. The capacity also faded rapidly such that this material is not attractive for Li-ion batteries.
The cubic rocksalt phase, R, did have some ordering of lithium, manganese and
metal site vacancies on the cubic lattice as discussed in Chaps. 5 and 6. Nonetheless,
a capacity of only 20 mAh/g was achieved on first charge even though the theoretical capacity was 138 mAh/g, demonstrating that most lithium diffusion paths were
blocked or severely hindered.
A consequence of the poor performance of the N and R phases is that synthesis
conditions and compositions that give rise to these phases must be avoided. This
limits the region of interest for layered–spinel composite electrodes to the area to the
right of the M–S line in Fig. 7.1 (b), with the M–S line being of particular interest
since both the M and S phases are decent electrode materials. Furthermore, since
the R phase appears in both three-phase regions during slow cooling, the effect of
quenching on composite electrodes must be studied. The knowledge gained about
the phase diagram also affects core-shell materials [88], where understanding the
equilibrium phases is necessary to determine whether or not ion mobility at a given
temperature is sufficient to destroy the desired core-shell structure.
The poor performance of the N-phase also impacts the so-called layered–layered
nano-composites that have long been searched for along the Li-rich layered line with
inconclusive results [44, 46]. The phase diagram suggests that samples showing
this short-range phase separation should appear elsewhere in the triangle, namely
103
such that the manganese could then be reduced from its initial 4 + state during the
next discharge, thereby activating it. More recently, Koga et al. [15] and Sathiya
et al. [16] have demonstrated oxygen participation in the redox process in some
lithium-rich oxides—a process that is accompanied with a phase transformation.
Koga demonstrated that Li–Co–Mn–Ni–O layered materials with excess lithium
convert to a two-phase material after the high voltage plateau, and this was attributed
to the shell of particles losing oxygen while the core did not, such that much of the
high voltage plateau can be attributed to oxygen redox in the core of the particles.
Fell et al. [87] recently found this two-phase behavior in Li 1.2 Ni 0.2 Mn 0.6 O 2 and used
Rietveld refinement to determine that one of the phases was oxygen deficient while
the other was not. At this moment, it is therefore unclear how much oxygen is lost
and how much oxygen redox takes place in the M material discussed here during the
high voltage plateau and remains an important question to answer. Regardless of the
ongoing debate about the nature of the high-voltage plateau, both the nickel redox
and the high voltage plateau can be seen as peaks in the dQ/dV curve during the first
charge of the M material in Fig. 7.6 The new peak appearing at 3.1 V in the second
cycle was consistent with manganese redox. After the cycles shown in Fig. 7.6, the
M phase was tested between 2–4.8 V to test its long-term cycling performance. The
long-term cycling capacities were 245.0 mAh/g during charge and 236.5 mAh/g
during discharge initially, and 244.2 mAh/g and 237.4 mAh/g respectively 40 cycles
later, demonstrating stable cycling.
By contrast to the highly ordered M-layered phase, the N-layered structure has a
disordered lithium layer with 30 % nickel occupation that apparently prevents some
lithium from diffusing out of the material, since only 50 % of the 201 mAh/g total
theoretical capacity was obtained during the first charge shown in Fig. 7.6. The capacity also faded rapidly such that this material is not attractive for Li-ion batteries.
The cubic rocksalt phase, R, did have some ordering of lithium, manganese and
metal site vacancies on the cubic lattice as discussed in Chaps. 5 and 6. Nonetheless,
a capacity of only 20 mAh/g was achieved on first charge even though the theoretical capacity was 138 mAh/g, demonstrating that most lithium diffusion paths were
blocked or severely hindered.
A consequence of the poor performance of the N and R phases is that synthesis
conditions and compositions that give rise to these phases must be avoided. This
limits the region of interest for layered–spinel composite electrodes to the area to the
right of the M–S line in Fig. 7.1 (b), with the M–S line being of particular interest
since both the M and S phases are decent electrode materials. Furthermore, since
the R phase appears in both three-phase regions during slow cooling, the effect of
quenching on composite electrodes must be studied. The knowledge gained about
the phase diagram also affects core-shell materials [88], where understanding the
equilibrium phases is necessary to determine whether or not ion mobility at a given
temperature is sufficient to destroy the desired core-shell structure.
The poor performance of the N-phase also impacts the so-called layered–layered
nano-composites that have long been searched for along the Li-rich layered line with
inconclusive results [44, 46]. The phase diagram suggests that samples showing
this short-range phase separation should appear elsewhere in the triangle, namely