Titanium substitution induces a drastic decrease in high potential electrochemical capacity, whereas the capacity is maintained and the kinetics are even improved in the presence of ruth
Trang 1Contents lists available atScienceDirect Electrochimica Acta
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / e l e c t a c t a
Influence of the tetravalent cation on the high-voltage electrochemical activity of
My-Loan-Phung Lea,b,c, Pierre Strobela,∗, Fannie Alloinb, Thierry Pagnierb
a Institut Néel, CNRS/Université Joseph Fourier, BP 166, 38042 Grenoble Cedex 9, France
b Laboratoire d’Electrochimie et de Physicochimie des Matériaux et Interfaces (LEPMI), Grenoble Institute of Technology, BP 75, 38402 Saint-Martin-d’Hères, France
c VietNam National University, VNU-HCM, Ho Chi Minh City, VietNam
a r t i c l e i n f o
Article history:
Received 28 May 2010
Received in revised form 31 August 2010
Accepted 2 September 2010
Available online 15 September 2010
Keywords:
Lithium batteries
High-voltage cathodes
Spinel
Lithium–manganese–nickel oxide
a b s t r a c t
The electrochemical properties of substituted LiNi0.5Mn1.5 −xMxO4spinels at high potential (>4 V vs Li+/Li) have been investigated for M = Ti and Ru, in order to determine the role of the tetravalent cation in such systems where nickel is a priori the only electroactive species These systems are found to form extended solid solutions (up to x = 1.3 and x = 1.0 for Ti and Ru, respectively) that were characterized by X-ray diffraction and Raman spectroscopy Titanium substitution induces a drastic decrease in high potential electrochemical capacity, whereas the capacity is maintained and the kinetics are even improved in the presence of ruthenium These results are completed by new results on the Li4 −2xNi3xTi5 −xO12spinel system, which shows not any high potential activity in spite of the presence of up to 0.5 Ni2+per spinel formula unit on the octahedral site Taking into account previous data on LiNi0.5Ge1.5O4, we clearly show that even if the tetravalent cation does not participate in the overall redox reaction, electrochemical activity is only possible when nickel is surrounded by tetravalent cations able to accept a local variation
of valence (Mn, Ru), whereas full-shell cations such as Ti4+and Ge4+block the necessary electron transfer pathways in the spinel oxide electrode
© 2010 Elsevier Ltd All rights reserved
1 Introduction
The ability of manganese-based based oxides to insert reversibly
lithium at high voltage (>4.7 V) was discovered in the late 1990s
[1–3] This property opened the way for new perspectives for
lithium batteries, such as an increase in power density with respect
to LiCoO2 and other “4V” materials, and the possibility to use
negative electrodes working significantly above 0 V while
pre-serving a high power density The high-voltage plateau is mainly
present in spinels where the B-site (often conventionally indicated
by square brackets) contains a combination of Mn and another
transition metal M, as in formula Li[MxMn2−x]O4(M = Cr, Fe, Co,
Ni, Cu), where x is close to the maximum value x = 0.5 [4,5]
The high-voltage capacity depends on the nature and
concentra-tion of M; numerous studies showed that best performances are
achieved for M = Ni in compositions equal or close to LiNi0.5Mn1.5O4
[3–5] This compound contains Ni2+ and Mn4+ions exclusively
The superior performances of LiNi0.5Mn1.5O4 compared to other
compositions was analyzed considering energy diagrams, showing
that the Ni2+/3+ and Ni3+/4+ 3d bands have the least overlap with
the O2 −2p band[6] As a result, the high-potential redox reaction
∗ Corresponding author Tel.: +33 476 887 940; fax: +33 476 881 038.
E-mail address: Pierre.strobel@grenoble.cnrs.fr (P Strobel).
in LiNi0.5Mn1.5O4does not involve oxygen loss and allows stable cycling, unlike most other compositions, including those involving the hypothetical Mn4+/5+redox couple[7,8]
There is indeed strong experimental evidence (mostly from X-ray absorption near edge spectroscopy studies) that the high-potential redox mechanism involves only nickel via the Ni2+/3+/4+
redox states[9–11] The generally accepted reaction mechanism is:
LiMn1 ,54+Ni0 ,52+O4⇔ Li + Mn1 ,54+Ni0 ,54+O4 (I)
A confirmation is provided by the existence of a double step at
ca 4.7 V vs Li/Li+corresponding to successive steps involving the
Ni2+/3+and Ni3+/4+redox couples Within this reaction scheme, the
Mn4+cation does not participate in the reaction; in fact, manganese manifests itself mainly in an additional 4 V plateau appearing when the material contains a fraction of Mn3+due to non-stoichiometry [2,3,12] According to some authors, the electrochemical inactiv-ity of manganese could be a factor of stabilinactiv-ity of the structure on cycling[12]
Assuming reaction I to be correct, similar properties should be expected if Mn4+is replaced by another tetravalent cation in the LiNi2+
0.5M4+
1.5O4 formula Kawai et al.[2] reported the absence
of electrochemical activity in LiNi0.5Ge1.5O4 and LiNi0.5Ti1.5O4, although the latter showed a complex XRD pattern that could not
be indexed as a single-phase spinel Two recent studies on the 0013-4686/$ – see front matter © 2010 Elsevier Ltd All rights reserved.
Trang 2LiNi0.5Mn1.5−xTixO4 system (0 < x < 1.5) showed a severe drop in
reversible capacity for large titanium contents[12,13] Another
peculiarity of LiNi0.5Mn1.5O4 is the occurrence of Ni:Mn cation
ordering on the spinel B-site, that results in a lowering of
sym-metry from face-centered cubic (space group Fd-3m) to primitive
cubic (space group P4332) [14] This effect is present in some
LiM0.5Mn1.5O4 phases (M = Cu, Ni) and not in others (M = Co),
depending on the ionic radius difference between M2+and Mn4+
[15] It seems not to be a major factor in electrochemical properties
[3,16]
In this paper, we re-examine the role of the tetravalent cation on
the high-voltage electrochemical behaviour in LiNi0.5Mn1.5 −xMxO4
and LiNi0.5M1.5O4 (M /= Mn) compounds This study
encom-passes the LiNi0.5Mn1.5−xTixO4 and LiNi0.5Mn1.5−xRuxO4 spinel
solid solutions In view of the non-existence of spinel-type
“LiNi0.5Ti1.5O4, a third system has been studied in an attempt
to reach this stoichiometry by introducing nickel in Li4Ti5O12,
i.e making the solid solution Li4−2xNi3xTi5 −xO12, which yields
“Li3Ni1.5Ti4.5O12” = “LiNi0.5Ti1.5O4” for x = 0.5 The Li–Ni–Ti–O
spinel solid solution has not been studied previously to our
knowl-edge Although ruthenium compounds are not realistic choices for
battery materials because of the cost of ruthenium, Ru4+ is one
of few substitution candidates as a tetravalent cation in spinels
We are aware of only one recent study of the Li–Ni–Mn–Ru–O
spinel system, where substitution by ruthenium was attempted
not on tetravalent manganese, but on nickel, assuming
question-able cation-deficient formulas “LiNiyMn1.5Ru0.05O4” with y = 0.35
and 0.4[17] No higher doping level was attempted The
avail-ability of electrochemical data on all these systems will allow
drawing systematic conclusions about the role of the “inactive”
tetravalent cation on the high-voltage electrochemical reaction in
LiNi0.5M1.5O4spinel oxides
2 Experimental
2.1 Synthesis procedure
All compositions were prepared by solid state reaction Starting
reagents were Li2CO3 or LiCH3COO (Merck), Ni(CH3COO)2.4H2O,
TiO2, RuO2 and MnCO3 (Aldrich) Samples with appropriate
sto-ichiometry ratios were mixed with mortar and pestle and fired
initially in air at 600–700◦C for 20 h to decompose all of the
car-bonate, nitrate and acetates The materials were then re-grounded,
pressed into pellets and reacted repeatedly 24 h at 900◦C, with
furnace cooling (ca 200◦C/h) For titanium-rich and
ruthenium-containing compositions, higher temperatures were necessary:
1000◦C (Ti) or 1200◦C (Ru) for the final annealing
2.2 Structural characterization
Samples were analyzed by powder X-ray diffraction (XRD) using
a Siemens D-5000 diffractometer with Co K␣ radiation, 0.02◦step
and 20 s/step counting time to minimize noise Lattice parameters
were determined by a least squares method (CELREF software) The
morphology and the distribution of grain size were determined
using a LEO S440 scanning electron microscopy (SEM) instrument
equipped with EDX spectroscopy analysis
Raman spectroscopy (RS) measurements were carried out with
a Renishaw’s InVia Raman Spectrometer Spectra were obtained
with the red line of a laser (785 nm) in micro-Raman
configura-tion (objective x 50) In the Li–Ni–Ti–O case, the structure stability
and cation arrangement were also probed at low temperature and
low frequency The spectra were measured on a Jobin-Yvon T64000
Raman spectrometer equipped with the green line of an Ar-ion
laser (514.5 nm) as excitation source, a vacuum bench apparatus
Fig 1 XRD patterns in the LiNi0.5Mn1.5−xTi x O 4 series as a function of titanium con-tent x (values of x are indicated on left side) Inset: variation of the spinel cell parameter with x.
and temperature regulation The powder was placed on the sub-strate of a laboratory-made cell under vacuum and cooled by liquid nitrogen[18]
2.3 Electrochemical measurements Electrochemical test was carried out in liquid electrolyte at room temperature using Swagelok-type batteries Cathodic pastes were prepared by intimately mixing the oxide powder with carbon black and PTFE emulsion in weight ratio 70:20:10 This paste was rolled down to 0.1 mm thickness, cut into pellets with diameter 10 mm and dried at 130◦C under vacuum Typical active material masses used were 15–20 mg/cm2 The electrolyte was a 1 M solution of LiPF6 in EC-DMC 1:2 (Merck Co.) Negative electrodes were
200-m thick lithium foil (Metall Ges., Germany) Cells were assembled
in a glove box under argon with ≤2 ppm H2O Electrochemical studies were carried out using MacPile Controller (Bio-Logic, Claix, France) in the potential window 3.50–4.85 V vs Li/Li+, in either galvanostatic mode at C/20–C/25 regime or by step-potential elec-trochemical spectroscopy (SPES)[19], using typically 10–20 mV/1 h steps
3 Results and discussion
3.1 Structural characterization 3.1.1 LiNi0.5Mn1.5 −xTixO4solid solution Ten different titanium contents covering the whole range of compositions (0≤ x ≤ 1.5) were prepared by solid-state reactions
at 900–1000◦C Spinel phases are found to form up to x = 1.3 (see Fig 1) The terminal “LiNi0.5Ti1.5O4” compositions, however, does not yield a spinel phase, but is multiphase with a new hexagonal compound Li4Ni3Ti8O21as major component; the crystal structure
of this new phase has been solved recently using electron diffrac-tion[20] In the spinel solid solution, the cubic lattice parameter a increases linearly as a function of titanium content x, in agreement with previous results[12,21](seeFig 1, inset) This is consistent with the increase in octahedral ionic radius between Mn4+and Ti4+
(0.53 and 0.61 ˚A, respectively[22]) A detailed structural analysis
is given elsewhere, showing more complex features such as the disappearance of Ni:Mn cation ordering with titanium substitu-tion, partial cation inversion and the presence of a second, minor
Trang 3phase identified as Ni1−xLixO[23] The latter seems to be a rather
systematic property of LiNi0.5Mn1.5O4, and was reported for
var-ious syntheses routes[1,2,16,24–28] The expulsion of a fraction
of nickel from the spinel phase induces a nickel deficiency in the
spinel phase, as in formula LiNi0.5−ı(Mn,Ti)1.5+ıO4 A significant
consequence is the presence of a fraction of Mn3+: charge
compen-sation yields n(Mn3+) = 2ı per spinel formula Values of ı have been
estimated as 0.06± 0.02 from structural analysis, corresponding to
0.12± 0.04 Mn3+per spinel formula unit[23]
The morphology of LiNi0.5Mn1.5−xTixO4samples was studied by
scanning electron microscopy (SEM) Unsubstituted LiNi0.5Mn1.5O4
prepared by solid-state reaction exhibits a wide distribution of
par-ticle size (Fig 2) The presence of titanium seems to promote grain
growth, as shown by the fairly large, well-faceted octahedral
parti-cles for x = 0.3–0.45 (seeFig 2b and c) A similar effect has already
been reported in titanium-substituted LiMn2O4[13,29]
3.1.2 LiNi0.5Mn1.5−xRuxO4solid solution
As shown inFig 3, single-phase spinels were obtained in this
system up to x = 1 At higher contents, unreacted RuO2 remains
present, even after repeated firings at 1200–1300◦C Samples were
furnace-cooled (ca 100◦C/h) to ensure a stoichiometric oxygen
con-tent Electrochemical measurements (see Section3.3) will indeed
show that the Mn3+content in ruthenium-substituted samples, that
is directly correlated to the 4 V capacity, is similar in Ti- and in
Ru-containing spinels
Attempts to prepare the terminal phase “LiNi0.5Ru1.5O4failed,
yielding mixtures of the rocksalt-type phase Li(Mn,Ru)O2and of
RuO2 The structural features are very similar to those of the
tita-nium case: disappearance of B-site cation ordering, presence of the
minor rocksalt-type phase for all compositions and lattice
parame-ter increase with ruthenium content The increase in a, however, is
much smaller than in the titanium case (compare y-scales in insets
ofFigs 1 and 3), in spite of the neighbouring ionic radii of Ti4+
(0.61 ˚A) and Ru4+(0.62 ˚A)[22] This could be due to differences in
cation distribution, especially partial inversion detected in the
tita-nium case We also note a broadening of X-ray reflections that may
reflect a lower cristallinity and/or a higher cationic disorder in the
ruthenium case
3.1.3 Li4 −2xNi3xTi5 −xO12solid solution
The syntheses in this system started from the Li[Li1/3Ti5/3]O4
spinel formula and aimed at introducing nickel on the octahedral
sites Charge balance imposes a double substitution of 3Ni2+for
2Li++ 1 Ti4+ The extent of this solid solution is theoretically limited
by the amount of octahedral lithium (1/3 per spinel formula or 1 per
4-5-12 formula unit), yielding a maximum x value of 0.5, where all
octahedral lithium atoms are substituted XRD diagrams and the
evolution of cell parameter (Fig 4) show that the solid solution
range is limited to x≈ 0.3 Up to this value, the lattice parameter a
increases as expected For x > 0.3, a decreases and additional
diffrac-tion peaks appear The terminal composidiffrac-tion (x = 0.5) is confirms
the non-existence of the “LiNi0.5Ti1.5O4” spinel phase
3.2 Raman spectroscopy
Raman spectroscopy is a local probe that is very sensitive to
local symmetry changes It is used here to detect possible
octahe-dral cation ordering that is known to occur in LiNi0.5Mn1.5O4[14]
In this stoichiometric compound, the 1:3 ratio of Ni2+:Mn4+cations
with different ionic radii induces an ordered cation distribution of
Ni2+and Mn4+in two different crystallographic sites As a result,
the symmetry is lowered from space group (SG) Fd-3m to P4332
Such ordering is difficult to detect by X-ray diffraction because of
the weak chemical contrast between Mn and Ni We showed
pre-viously[14,15]that this symmetry change can be easily detected
Fig 2 SEM micrographs of the different spinels (initial magnification 4000× (5000× for x = 0); x = 0 (a), x = 0.3 (b), 0.45 (c).
by neutron diffraction and by vibration spectroscopy Group the-ory predicts five Raman active modes for a normal AB2O4spinel with SG Fd-3m (Oh7), while the lower-symmetry P4332 structure has 42 Raman active modes[26,30] As showed previously[15,31], cation ordering shows up in Raman spectra by a narrowing of Raman bands and important changes such as the appearance of additional bands at 166 and 411 cm−1and a doublet near 600 cm−1 These features are illustrated inFig 5, where the two upper spec-tra correspond to cation-ordered LiNi0.5Mn1.5O4 and disordered LiNi0.4Mn1.6O4 In addition, we checked that low-temperature Raman spectra (acquired at 128 K) show no narrowing of the
Trang 4cation-Fig 3 XRD patterns in the LiNi0.5Mn1.5−xRu x O 4 series as a function of nominal
ruthe-nium content x (from bottom to top: x = 0.25, 0.50, 0.75, 1.0) Inset: variation of the
spinel cell parameter with x.
disordered Raman bands This clearly indicates that disorder is
static, probably due to local variations of the cation–cation
inter-actions.Fig 5shows that substitution by titanium, and to a lesser
extent by ruthenium, induces a considerable broadening of Raman
lines and suppresses the 600 cm−1doublet, i.e present clear
evi-dence of cation disorder This has been confirmed by neutron
diffraction on Ti-substituted samples[23]
Regarding the Li4 −2xNi3xTi5 −xO12system, Raman spectra for
dif-ferent compositions (0≤ x ≤ 0.5) are shown inFig 6 For x≤ 0.25,
a single phase is observed and the Raman spectrum is that of a
cation disordered spinel[32] For x = 0.375, however, Raman
spec-tra recorded at several sample positions are different, showing the
presence of two phases in this sample These spectra can be
con-sidered as combinations of the x = 0.25 and x = 0.50 spectra The
solid solution limit lies therefore between x = 0.25 and x = 0.375,
in agreement with the crystallographic data For x = 0.50, a single
Raman spectrum is obtained whatever the point illuminated This
strongly suggests that the second phase, observed in X-ray
diffrac-Fig 4 XRD patterns in the Li4−2xNi 3x Ti 5−x O 12 series as a function of titanium content
Fig 5 Raman spectra of different Li–Ni–Mn–O, Li–Ni–Mn–Ti–O and Li–Ni–Mn–Ru–O spinels (formulas indicated).
tion experiments, is minor and/or that the Raman spectrum of the second phase is weak or absent, as expected for a nearly cubic per-ovskite We checked that the Raman spectrum of “LiNi0.5Ti1.5O4
obtained in this series for x = 0.5 is identical to that of the sample prepared by Gemmi et al.[20] We can thus attribute this Raman spectrum to the new phase Li4Ti8Ni3O21
Fig 6 Raman spectra of different Li4−2xNi 3x Ti 5−x O 12 compositions (x values
Trang 5indi-Fig 7 Slow-scanning voltammetry (step-potential electrochemical spectroscopy)
of LiNi 0.5 Mn 1.5 O 4 and LiNi 0.5 Mn 1.35 Ti 0.15 O 4 (3rd cycle, sweeping rate 10 mV/h; data
normalized to 0.1 mmol active material).
3.3 Electrochemical behaviour
Fig 7 shows SPES voltammetry scans of unsubstituted
LiNi0.5Mn1.5O4 (a), and of samples substituted by 0.5 Ru (b) and
0.15 Ti (c) vs lithium metal, all measured at same potential
sweep-ing rate All show a main, reversible, high-potential reaction around
4.75 V vs Li/Li+and a slight capacity around 4 V that is ascribed to the
Mn3+fraction resulting from non-stoichiometry in the spinel phase
Table 1
Main oxidation and reduction potentials (maximum current peak values vs Li/Li + ) measured by slow scanning voltammetry (10 mV/h).
Formula E red /V (±0.01) E ox /V (±0.01) LiNi 0.5 Mn 1.5 O 4 4.70 4.77 LiNi 0.5 MnRu 0.5 O 4 4.72 4.75 LiNi 0.5 Mn 1.35 Ti 0.15 O 4 4.76 4.79 LiNi 0.5 Mn 1.2 Ti 0.3 O 4 4.77 4.79 LiNi 0.5 Mn 1.05 Ti 0.45 O 4 4.76 4.78 LiNi 0.5 Mn 0.9 Ti 0.6 O 4 4.78 4.80
The LiNi0.5Mn1.5O4main redox peak is split into two components attributable to the Ni2+/3+and Ni3+/4+redox couples[33] The Ru-substituted peak has a very similar shape, with sharper peaks giving
a better resolution of the two components, and a lower potential difference between anodic and cathodic current peak maxima On the contrary, in the Ti-substituted case, the main current peak is considerably broader In addition, the reduction peak maximum
is shifted to higher potential: from 4.69 to 4.76 V vs Li+/Li This value (4.76± 0.01 V) is found to be constant in the whole range
of titanium compositions (0.15≤ x ≤1.30) Peak maxima data are summarized inTable 1, showing that the tetravalent cation is not as
“inert” as expected in the electrochemical reaction The fact that the
Ti or Ru substitution modifies the current peak width, the reaction potential and the reduction–oxidation peak difference indicates that the nature of the tetravalent cation influences both reaction kinetics and octahedral cation–anion bond strength The remark-able influence of ruthenium on kinetics confirms the recent report
by Wang et al.[17] The effect of substitution on electrochemical capacity can be seen inFig 8 The effect of titanium substitution is drastic and unex-pected in view of reaction I cited in the introduction: the capacity drops steeply with titanium substitution beyond ca 0.3 Ti per spinel formula, in spite of the fact that the nickel content is unchanged The oscillations measured at the end of discharge for x(Ti)≥ 0.6 indi-cate a severe deterioration of intercalation reaction kinetics in these materials In addition, a detailed analysis of the charge–discharge curves shows that the capacity decrease is entirely due to the high-voltage reaction: the 4 V plateau capacity remains constant (see Fig 9), meaning that the Mn3+/4+redox process is not influenced
by substitution, whereas the Ni2+/3+/4+is The capacities are stabi-lized after a few cycles and the effect of titanium is constant on extended cyclings (Fig 9b) These results confirm previous reports
Fig 8 3rd cycle galvanostatic charge-discharge curves of LiNi0.5Mn 1.5−x Ti x O 4
Trang 6Fig 9 (a) 4 V and 5 V capacity of LiNi0.5Mn 1.5−x Ti x O 4 spinels as a function of x (3rd
cycle data, galvanostatic cycling at C/20 between 3.75 and 4.85 V (b) Evolution of
the capacity with cycle number for different titanium contents.
about the detrimental effect of titanium substitution[12,13]and
will be discussed in more detail in Section4
As shown inFig 10, the capacity drop is much smaller in the
Li–Ni–Mn–Ru–O system, in spite of an increase in molar mass that
decreases the specific capacity at constant lithium intercalation
level in the ruthenium case On the contrary, it could be expected
that the lower atomic mass of titanium with respect to manganese
would induce a gain in specific capacity, but the opposite trend is
observed The inset inFig 10also shows the high stability on cycling
of the ruthenium-substituted spinel
Turning now to the Li4−2xNi3xTi5−xO12 system, we showed
in Section 3.1.3 that single-phase spinels were obtained
up to x = 0.25, corresponding to a spinel composition
Li3.5Ni0.75Ti4.75O12= Li[Li0.167Ni0.25Ti1.583]O4 A priori, the reaction
mechanism I should apply, at least partially (since the spinel
formula contains only 0.25 Ni2+), as follows:
Li[Li0.167+Ni0.252+Ti1.5834+]O4⇔ 0.5Li
+ Li0.5[Li0.167+Ni0.254+Ti1.5834+]O
Fig 10 3rd cycle charge–discharge curves of LiNi0.5Mn 1.5−x M x O 4 spinels for similar contents of Ti and Ru Inset: Evolution of the capacity of LiNi 0.5 MnRu 0.5 O 4 with cycle number (conditions as in Fig 9 ).
with a theoretical capacity of 0.5Li per spinel formula Yet exper-imental results show the absence of any significant high potential capacity in this material (seeFig 11) This negative result is not due
to experimental artefacts, since this material behaves quite nor-mally in the potential range 1–2 V, where the Ti4+/3+redox couple
is electrochemically active and gives a clean, reversible plateau at 1.5 V vs Li/Li+(seeFig 11, inset)
3.4 Discussion: high-potential reaction mechanism XANES studies of LiNi0.5Mn1.5O4showed that nickel is the only electro-active species in the high-potential (“5 V”) reaction Assum-ing this model, where Mn4+ is inactive, it should be possible to modify the tetravalent cation composition without interfering with the nickel redox behaviour Titanium substitution presents the additional interest to slightly lower the spinel molar mass, result-ing in a higher theoretical specific capacity Our study showed that, contrary to expectations, substitutions on “inert” Mn4+do affect the electrochemical performances, in a positive and negative way
Fig 11 Charge–discharge cycles on Li3.75Ni 0.375 Ti 4.875 O 12 in the potential window 3.75–4.85 V (C/22 regime) Inset: charge–discharge behaviour in the 1–2 V range.
Trang 7Fig 12 Environment of a Ni cation in the LiNi0.5Mn 1.5 O 4 structure.
Table 2
Electrochemical activity and electronic properties of tetravalent cations found in
Li–Ni spinel oxides.
M 4+ High-potential activity Electron
configuration
Mixed-valence possibility (3–5 V)
(Sn) (Spinel not obtained)
with ruthenium and titanium, respectively The detrimental effect
of titanium is confirmed by studies in the Li–Ni–Ti–O spinel
sys-tem, which does not give any electrochemical activity in the high
potential range (Fig 12)
If we add to the set of available data in this study the
LiNi0.5Ge1.5O4case, that was found to be electrochemically inactive
in the range 3.5–5 V[3], the electrochemical properties of
vari-ous LiNi0.5M1.5O4spinels are summarized inTable 2 For the sake
of completeness, we may add that we attempted to synthesize a
Sn4+-containing spinel with hypothetical formula “LiNi0.5Sn1.5O4
All syntheses of such a compound, including attempts under high
pressure (6 GPa), failed, and produced mixtures of LiNiO2, NiSnO3
and SnO2
Table 2shows an obvious correlation between the electronic
configuration of the tetravalent cation and the high-potential
elec-trochemical activity: the elecelec-trochemical reaction seems to require
the possibility of mixed valence on the tetravalent cation
intercalation–deintercalation mechanism of lithium in the spinel
host structure This requires three main steps: (1) migration of
lithium in or out of the host lattice, (2) electron exchange (redox
reaction) on nickel, (3) transfer of these electrons from/to the
surface of the spinel electrode In predominantly ionic compounds
such as transition metal oxides, this last step is expected to take
place via an electron hopping mechanism
Considering now the environment of a nickel atom in the spinel
structure (seeTable 3), structural data show that, in the
cation-ordered structure of LiNi0.5Mn1.5O4, the first coordination shell of
Table 3
Interatomic distances (in nm) in ordered and disordered LiNi 0.5 Mn 1.5 O 4 spinels.
Coordination shell Ordered spinel
(P4 3 32)
Disordered spinel (Fd-3m) 1st neighbours 6 O 0.205 6 O
2nd neighbours 6 Mn 0.292 6 M
3rd neighbours 6 Li 0.334 6 Li 0.334
Next Ni neighbour >0.5
nickel is oxygen, and the second one consists of tetravalent cations only at a distance a/√8; nickel–nickel distances are exceedingly high (>0.5 nm) In the absence of cation ordering (as in Ti- and Ru-substituted spinels), this scheme is not strictly applicable, but electrostatic and steric considerations still favour a B-site cation distribution with Ni2+locally surrounded by M4+cations The major hopping conduction pathways will then be Ni–O–M–O– ., and such a transfer is only possible if M cations can accept a local varia-tion of valence, whereas full-shell cavaria-tions such as Ti4+or Ge4+will act as blocking elements for the electron transfer This scheme is fully confirmed by experimental results: the best electron transfer takes place in the presence of Ru, a 2nd-transition row element with larger electron delocalization than 1st transition row elements; ruthenium is indeed notorious for forming numerous conducting (and even superconducting) oxides[34,35] In the Li–Ni–Mn–Ti–O system, which is the most studied to date, an abrupt capacity decrease with increasing Ti content at high potential was already noted, in spite of chemical diffusion determinations (from GITT measurements) giving an increase in the lithium chemical diffu-sion coefficient with Ti substitution[12] This confirms that the blocking effect of the d0Ti4+cations is the major factor limiting the redox intercalation–deintercalation mechanism
4 Conclusions
In this study, we investigate the influence of different tetrava-lent cations M4+on the high-potential redox reaction involving nickel in oxide spinels Most possible M elements have been con-sidered: titanium manganese, germanium, ruthenium and tin Ti and Ru give rise to extended solid solutions in the nickel-containing LiNi0.5(Mn1.5 −xMx)O4systems, although this solid solution does not extend to the terminal phases ‘LiNi0.5Ti1.5O4’ and ‘LiNi0.5Ru1.5O4’ Although the tetravalent cation is a priori “inert” in the redox reaction, a clear correlation is found between the feasibility of reversible lithium electrochemical intercalation involving Ni2+/4+
oxydo-reduction and the ability of the neighbouring tetravalent cations in the structure to allow the necessary electron trans-fer to and from nickel atoms This parameter proves to be a key requirement for the use of nickel as an active redox species in lithium-containing spinels Best results are obtained with mixed-valence neighbouring elements such as Mn and Ru, with the latter improving electrochemical kinetics probably due to its larger elec-tron delocalization
Acknowledgment
The authors thank région Rhône-Alpes for financial support of
My Loan Phung Le
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