# Improving the Performance of Lithium-Ion Batteries by Using Spinel Nanoparticles

Authors: J. C. Arrebola; A. Caballero; L. Hernán; J. Morales
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

In this work, we examined the use of nanospinels to construct batttery electrodes. We chose two spinels suitable as cathode materials (LiMn2O4 and LiNi0.5Mn1.5O4, which are representative of 4 and 5 V versus Li metal, resp.) and one providing good results as anode (Li4Ti5O12). In order to ensure good cell performance, nanometric particles must meet another requirement; thus they should contain few surface or bulk defects (i.e., they should be highly crystalline). Because the synthesis of such spinels usually requires a thermal treatment, ensuring that they will meet both requirements entails accuratly controlling in the synthesis conditions. Thermal decomposition of nanooxalate in the spinel-conaining elements obtained by mechanochenical activation in the presence of polymers provides a simple, effective route for this purpose. We prepared two types of hybrid lithium-ion batteries using LiMn2O4 and LiNi0.5Mn1.5O4 as cathode materials, and Li4Ti5O12 as anode material. The electrochemical properties of these cells were compared with those of a similar configuration made from micrometric particles. The nano-nano configuration exhibited higher reversibility and better performance than the micro-micro configuartion in both types of cells, possibly as a result of lithium ions in the former being able to migrate more easily into the electrode material.


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## Body

## 1. Introduction

Some properties of nanometric materials
differ markedly from those  of micrometric materials of identical composition by
effect of mere simple size effects or differences in electronic structure. This
has aroused great expectations in many fields including the production of
electrochemical energy storage systems. Thus, the use of nanometric materials to
prepare electrodes for lithium-ions batteries has gained substantial interest
in the last few years [1–6]. Trivial size effects (e.g., an increased
surface-to-volume ratio and shorter length displacements for lithium ions) can
lead to an improved rate capability and cycle life of cells.On the other hand, the increased surface area
of the electrode and also the increased area of the electrode/electrolyte
interface provide additional advantages for nanometric particles over the bulk
material [7] such as (i) an improved reaction kinetics that increases the
extent of development and reversibility of the lithium insertion/extraction
process and results in increased cell capacity and better capacity retention, and
(ii) the ability to use high charge/discharge rates. However, the increased
reactivity of nanometric materials can raise some problems derived from a
greater vulnerability to attack by the electrolyte (particularly at low
charge/discharge rates). Under these conditions, the lithium
insertion/extraction process takes a long time and contact between the active
material and electrolyte prolonged. This situation can be detrimental when the
electrolyte is capable of decomposing the electrode. Such is the case with the
release of HF from LiPF6 promoted by the presence of water traces. Coating
particle surfaces with a protective agent can help overcome this drawback [8]. However,
the main disadvantage of nanometric materials is probably the typical labour
intensity and high cost of their synthetic procedures. Developing simple affordable
synthetic methods will be one of the keys for nanometric materials to prevail in
the design of electrochemical devices for energy storage and conversion.Another valuable property of lithium-ion batteries is their ability to operate
at the highest available voltages. Provided cells retain their capacity, this
can be used to increase their specific energy. Cell voltage is determined by
the difference in Li chemical potential between the two insertion electrodes.
The greater the difference is, the higher will be the cell voltage. This can be
accomplished by using anodic and cathodic materials with the lowest and highest
potentials versus the lithium reference electrode, respectively. The presence
of Li ions that can be extracted during charging and inserted on discharging
the cell is an indispensable condition for cathodic materials to be active in
this respect. The most attractive and widely studied compounds in this context are
the layered oxideLiCoO2 [9] (commercial lithium-ion batteries are
based on it, but its high cost and toxicity can compromise its leading position),
and the spinel LiMn2O4, which is more inexpensive and environmentally
benign than LiCoO2 but has poorer cycling properties [10]. The spinel
gains stability when Mn is partly replaced with other elements such as V [11],
Cr [12], Fe [13], Co [14], Cu [15], or Ni [16]. A spinel of composition LiNi0.5Mn1.5O4 has been prepared which affords complete reversible extraction of Li+ at ca. 4.7 V (i.e., more than 0.5 V above the potential for the unsubsituted
spinel). This compound has started a new generation of lithium-ion batteries capable
of operating at higher voltages.All these compounds share
a common feature: introductingLi+ in their structures usually
requires a thermal treatment. Hence, the most common procedure used for their
synthesis in nanometric size is based on combustion [17]. There is, however, an
alternative procedure alike to a ceramic method which starts from nanometric
precursors and involves a rapid heating. The preparation of nanometric
precursors, which can be obtained by mechanochemical activation at room
temperature [18], is a key factor. Activation in the presence of a polymer
provides highly crystalline nanoparticles [19], otherwise it is difficult to obtain
with other methods that require prolonged heating and/or higher heating
temperatures.Polymer-assisted
nanoparticles possess significant advantages, namely, (i) they are more
resistant to electrolyte attack than are those obtained in the absence of
polymer, probably because of lower crystallinity, and (ii) they exhibit a
better electrochemical response when operated in the voltage window close to 5
V. All these factors combine to give nanocompounds which can be cycled at high
currents [20]. This was comfirmed here by highly crystalline Li-containing
nanospinels (viz. LiMn2O4 and LiNi0.5Mn1.5O4,
which were used as cathode materials and Li4Ti5O12 as anode material). The hybrid lithium batteries made from these compounds were
found to exhibit good performance under a wide range of charge/discharge rates.


## 2. Experimental

The above-describe synthetic method is effective for any Li-containing
oxide. We prepared the following spinels in this work:LiMn2O4 (LMO), Li4Ti5O12 (LTO), and LiNi0.5Mn1.5O4 (LNMO). In previous work [21], the method was used to prepare of LiCoO2 and LiNi0.5Mn0.5O2 layered oxides. The method is based on the synthesis of nanometric oxalates
followed by rapid calcination up to 800°C at a high heating rate (ca. 20°C/min). Then, the furnace is
shutoff, and the material is allowed to cool to room temperature inside. The
formation of nanometric oxalate is boosted by the presence of highly hydrated
precursors in excess oxalic acid dihydrate. Mechanical activation was done in a
planetary high-energy ball mill in the air at room temperature in two steps,
first under dryness for 15 minutes and then in ethanol for another 15 minutes. This
sufficed to obtain oxalate particles less than 10 nm in size.Mechanical
activation can be done in the presence of a polymer in order to tailor particle
size and shape. Two polymers were tested: polyethyleneglycol (PEG) in molecular
weight 400 and polymethylmethacrylate (PMMA). Mechanical activation for 1 hour sufficed
to obtain a homogeneous mixture of polymer and precursors. Further heating at 800°C yielded highly
crystalline nanoparticles.X-ray diffraction (XRD) patterns were recorded on a Siemens D5000 X-ray
diffractometer using nonmonochromated Cu Kα radiation and a graphite monochromator for the
diffracted beam. Thermogravimetric measurements were made under ambient
conditions, using a
Cahn 2000 thermobalance at a heating rate of 10°C/min. Transmission electron microscopy (TEM) images were obtained with a
Phillips TEM operating at 100 keV and SEM images with a Jeol 6400 scanning
electron microscope.Electrochemical measurements were
carried out with CR2032 two-electrodecoin cells supplied
by Hohsen Corporation. Powdered pellets 13 mm in diameter were prepared by pressing, in a stainless steel grid, ca. 10 mg of active material with acetylene black (15 wt%).
The electrolyte, supplied by Merck, was 1 M anhydrousLiPF6 in a 1 : 1 mixture
of ethylene carbonate and dimethyl carbonate. Cells were assembled in an
M-Braun glovebox. Cycling tests were performed on a McPile II (biologic)
potentiostat-galvanostat system under a galvanostatic regime. The
Li-ion batteries were anode limited [22], and the specific capacity and rate of
the batteries referred to the mass of the negative electrode.


## 3. Results and Discussion

### 3.1. Morphological
Properties

As
stated above, thermal treatment of an appropriate precursor for a short time provides
a simple route for obtaining an Li-containing oxide of nanometric size. The suitability of the precursor is a
key factor here. We started from hydrated acetates (except for the synthesis ofLi4Ti5O12, where the Ti source was
acetylacetonate). On contact with oxalic acid dihydrate and mechanochemical
activation, the acetates become nanometric oxalates (see Figure 1). The
increased reactivity of these oxalates compared with micrometric particles
affords the use of milder thermal conditions to obtain the spinel framework.
The combination of TG curves and XRD patterns revealed that the spinel is
formed at 400°C, albeit with low crystallinity. Heating at 800°C for a short time
results in improved particle crystallinity as revealed the reduction in the
broadening of the X-ray diffraction lines. Figure 2 shows some XRD patterns
representative of the different conditions used in the spinel synthesis. At
this temperature, the spinel peaks are quite well resolved, and peaks assigned
to transition metal oxide-based impurities are hardly perceptible. The role
played by oxalic acid is shown in Figures 1(c) and 1(d). The presence of oxalic
acid dihydrate in the medium plays a key role in the final particle size of the
mixed oxide by means of the formation of oxalate nanocrystals on grinding (see Figure 1(b)). Nucleation and growth of these reaction products are facilitated by the
presence of hydration water. Thus, this precursor-like route enhances the
uniform distribution of the elements resulting in a well-defined morphology and
quite homogeneous particle size. By contrast, direct synthesis from the
acetates in the absence of oxalic acid resulted in wider particle size
distribution and nanoparticles coexisted with microparticles (see Figure 1(c)).Figure 1
Steps of the method used to synthesize the nanospinels.Figure 2
XRD patterns for the
spinels. Samples: (a) and (b)LiMn2O4 in the absence and
presence of oxalic acid, (c) LiNi0.5Mn1.5O4 in the presence of oxalic acid and PEG400.The use of polymers in the synthetic procedure has two main effects,
namely, (i) particle size is reduced (compare Figures1(d), 1(e), and 1(f)) and (ii) an improvement in spinel crystallinity, as shown below. The polymer helps to
adopt a more homogeneous distribution of metal ions on an atomic scale as a
result of its ability to coordinate to the metal ions forming the spinel. The
functional groups of the polymer capable of acting as Lewis basic sites—atoms
with nonbonding electron pairs—may bind to the ions, thereby bringing them
closer and shortening diffusion paths to adopt the spinel framework. This model
is consistent with the results obtained by using a polymer such as poly(divydilene fluoride) (PVDF), where no functional oxygen atoms are present, with which the
spinel synthesis failed. Thus, the functional groups of the polymer are key
factors for the spinel synthesis and should be able to approach and stabilize
the metal ions via their chelating properties. Thus, the presence of the
polymer, in addition to tailoring the particle shape, helps maintain
connectivity between nanocrystals, thereby facilitating the release of strains
as the temperature is raised. Nanoparticles adopt a well-defined octahedral
morphology typical of the spinel structure (see Figure 3(a)). Also, lattice defects
are insignificant as revealed by HREM images (see Figure 3(b)). The interplanar spacing was calculated to be ca. 0.46,
which corresponds to the orientation of (111) atomic planes. The high
crystallinity observed by TEM, a highly localized technique, was confirmed from
X-ray broadening analysis, more suitable to examine bulk properties of a
material. The lattice distortion was estimated from the full width at half
maximum (FWHM), using the Williamson and Hall equation [23]:(1)βcosθ=2〈e〉senθ+0.9λD,where β is
the integral breadth after correction for instrumental broadening from highly
crystalline quartz and kα2 elimination using the Rachinger
method [24], 〈e〉 denotes local strains (defined as Δd/d, where d is the interplanar
space), and D is crystallite size. Equation (1) was applied to the
strongest reflections that do not overlap with other reflections. Figure 4
shows the plots of the above equation for different spinels, and Table 1 lists
the crystallite sizes and strains calculated from the intercepts and slopes,
respectively. Both the presence of oxalic acid and especially polymer in the
synthesis improved particle crystallinity as microstrain content decreased
under these conditions. The lower crystallite size for the LiMn2O4 spinel prepared in the absence of oxalic acid compared with that of the
same spinel obtained in the presence of oxalic acid could be due to the wider
particle size distribution of the former sample. Figure 1(c) shows the presence
of ill-defined particles of few tens nanometer with others exceeding 200 nm. In
any case, highly crystalline particles of small size are quite appropriate to
cycle at high charge/discharge rates as Li-ion displacement is enhanced by
effect of the reduced probability to find defects potentially truncating their
movement.Table 1
Crystallite size and
microstrain content of Li-Mn spinels.
SampleD (nm)Microstrain(×10−4)LiMn2O4(a)9219.3LiMn2O4(b)11610.7LiNi0.5Mn1.5O4(c)792.1Synthesis conditions:(a)absence of oxalic acid, (b,c)with oxalic acid, (c)with PEG400.(a) TEM and (b) HREM images ofLiNi0.5Mn1.5O4 nanocrystals synthesized in the presence of PEG400.
(a)(b)Figure 4
Plots of the Williamson
and Hall equation for various (hkl) reflections. Samples calcined at 800°C:
LiMn2O4 in the absence (a,○) and presence (b,•) of oxalic acid, LiNi0.5Mn1.5O4 in the presence (c, ■) of oxalic
acid and PEG400.


### 3.2. Electrochemical
Properties

#### 3.2.1. NanometricLiMn2O4 versus Li

The essential condition for a
material to be useful in lithium-ion battery cathodes is that it should be able
to react in a reversible manner with lithium at high potentials. This requirement
is met by the spinelLiMn2O4, for which a vast amount of
literature is available [25]. This spinel can extract and insert lithium above
4 V versus Li. Figure 5 shows
the variation of the specific capacity delivered by half-cells made from
nanometric particles as a function of the number of cycles. Galvanostatic data
were recorded at different charge/discharge rates from C/4 to 4C (C representing 1 Li+ ion exchanged in 1 hour, equivalent
to 148 mA/g). The initial capacities obtained ranged from 108 at C/4 to 100 mAh/g at 4C.
The difference in capacity was in fact rather small (8%) in spite of the
significant difference in rate (16 times). On further cycling, the half cells exhibited good
capacity retention, irrespective of the rate capability used. With the
electrode made from micrometric particles, the initial capacity delivered by
the half-cell was 107 mAh/g at C/4 and 95 mAh/g at 1C, which is close to the value
for the electrode made from nanometric particles. However, capacity fading with
cycling was more pronounced than in the half cell made from nanometric particles.
The capacity loss was particularly significant at 1C. Under these cycling
conditions, the capacity fell from 95 to 40 mAh/g over the first twenty cycles;
in the half-cell made from nanometric particles, however, the capacity only
decreased from 105 to 100 mAh/g. The origin of this striking behavior may be associated to the way lithium diffusion is affected by particle size. The time τ taken to
travel a distance r can be calculated
from the  equation:(2)τ=r2πD,
where D is the diffusion coefficient
of Li+. Thus, for nanoscale materials (viz. those having a size of
1−100 nm in at least one direction), Li ions take few minutes to cross
nanoparticles (the average value of D for Li+ is ca. 10−4cm2/s). Thus, reducing the average diffusion distances delays
concentration polarization and allows cells to operate at high currents and
hence electrode rate capabilities to be improved [7]. Obviously, reducing the
distance for Li+ ions travel must be also beneficial when the cell
anode is made from an insertion electrode, as shown below.Figure 5
Variation of cell
capacity inLiMn2O4/Li half-cells as a
function of the number of cycles. Rate: (Δ) C4,
(○) 1C, (□) 4C, and m‐LiMn2O4/Li
half-cell at (•) 1C.


### 3.2.2.  NanometricLiMn2O4 versus Li4Ti5O12
(Hybrid Batteries)

In most commercial batteries the
positive and negative electrodes are made fromLiCoO2 and graphite,
respectively. These components have two major shortcomings. Graphite operates
at a very low voltage (ca. 50 mV versus Li). Under these conditions, the
electrolyte is thermodynamically unstable and can decompose releasing gases
which detract from battery safety. On the other hand, cobalt is a toxic and
expensive element. In this context, Li-based spinels provide interesting
prospects for circumventing these shortcomings. The spinels Li4Ti5O12 and LiMn2O4 are good choices for this purpose as they can
act as anode and cathode materials, respectively, for batteries. LTO is one of
the few lithium intercalation compounds known that undergoes little expansion
or contraction during the lithium insertion/reinsertion process [26]. We have found
battery performance to be significantly improved by using both compounds as
nanoparticles [27]. Hybrid batteries made from such particles exhibit enhanced
rate capabilities and surpass batteries made from larger particles
(microparticles) in performance.Figure6 shows
the charge/discharge curves for two batteries made from the spinels obtained as
nano(n)- and micropaticles (m). The curve shapes are typical for LiMn2O4, with two
pseudoplateaux consistent with the two-step mechanism for lithium extraction
and insertion [28]. The capacity values delivered by the cells in the 1st and
10th cycle as measured at C/4 and 1C are shown in Table 2.Table 2
Capacity values delivered by the hybrid cells (mAh/g).
Battery1st discharge10th
dischargeC/41CC/41Cn-LMO–n-LTO134120133108m-LMO–m-LTO107769666Galvanostatic profiles for
the first and tenth charge/discharge of (a)m‐LiMn2O4/m‐Li4Ti5O12 and (b) n‐LiMn2O4/n‐Li4Ti5O12 hybrid batteries. Rates: C4 (solid line) and 1C (dash line).
(a)(b)The initial capacity delivered by the cell was
higher when the electrodes were made from nanoparticles, irrespective of the discharge
rate used. However, the greater differences were observed at 1C, consistent with the better
electrochemical response of nanometric particles cycled at high rates. On
cycling at a low rate (C/4), capacity retention of this cell was quite good. By
contrast, the capacity of the cell made from microparticles decreased by about
a 10%. At higher rates (1C),
the capacity of the two batteries faded with cycling, but capacity retention in
the battery made from nanoparticles was better (its capacity loss was 10% versus 26%). Clearly, as previously
found for the half cell, a nanometric size helps in improving  battery
performance.


### 3.2.3. NanometricLiNi0.5Mn1.5O4 versus Li

The highest cathode potentials achieved
to date are in the region of 5 V versus Li, value which is on the verge of the potential
window for liquid-based electrolytes. Some Li-based spinels (e.g.,LiNi0.5Mn1.5O4)
possess this property and are the best candidates for future applications [22].
Li is removed from this compound between 4.5 and 4.8 V, and charge neutrality is
maintained by oxidizing Ni2+ to Ni4+ [29]. In order to ensure good reversibility in the
electrochemical reaction at high voltages, the electrode must be made from a
highly crystalline spinel. The conditions used to obtain nanometric particles usually
provide solids with a high defect content. In other words, particles are poorly
crystalline as revealed by X-ray line broadening analysis [19]. This is so because
the heating temperature cannot be too high and the heating time should be as
short as possible in order to avoid sintering. Spinels prepared as
microparticles are not subject to this drawback and possess a high
crystallinity. As stated above, however, microparticles exhibit a limited
electrochemical response in lithium cells, particularly at high rates. One way
of overcoming this restriction is by using composite electrodes made from a
mixture of nano- and micrometric particles. In this way, cells can exploit the
advantages of nanoparticles (high rate capabilities) and microparticles (good
reversibility in the electrochemical reaction).Figure7 shows the capacity
delivered by various half cells as a function of the number of cycles. Two
types of electrodes were studied that were prepared from (i) microparticles and
(ii) a 50 : 50 w/w mixture of micro- and nanoparticles (composite electrode)
obtained by mechanochemical activation for 15 minutes [30]. Tests were conducted
at two different rates (C/4 and 2C).
As can be seen, the best performance was obtained for the composite electrode, irrespective
of the charge/discharge rate used. This electrode delivered a higher capacity over
the one hundred cycles measured. Differences in textural properties of the
electrodes rather than in the structural properties of the spinels seem to be
the origin of the enhanced performance of the composite electrode. As can be seen in the insets of Figure 8, the
particles maintained the spinel structure with similar unit cell dimensions to
those of the pristine compounds on cycling. However, the SEM images (see Figure 8) revealed significant differences between the two electrodes. Thus, the composite electrode was
the better at retaining its integrity. Also, it was the electrode exhibiting
the best electrochemical performance (see Figure 7(a)). The appearance of the electrode made
from the m-spinel (see Figure 7(b)) was quite
different. As expected, particles were clearly larger and not so closely bound
as in the composite electrode. The nanoparticles probably occupy the
cavities formed upon compaction of the microparticles. Therefore,
the nanoparticles seem to act as binders between the microparticles, thereby
improving connectivity between particles and facilitating transport of
charge carriers. This must decrease cell polarization
and increase the reversibility of the reaction with lithium, thereby improving
cell performance.Variation of cell
capacity as a function of the number of cycles in half-cells made fromm‐LiNi0.5Mn1.5O4 spinel (■) and a mixture of
50/50 wt% n‐LiNi0.5Mn1.5O4/m‐LiNi0.5Mn1.5O4 (○). Charge/discharge rate: (a)
C/4, (b) 2C.
(a)(b)SEM images of electrodes tested at C/4 after the hundredth discharge. (a)
Micrometric spinel and (b) 50/50 composite electrode.
(a)(b)As stated above, one alternative
route for improving cell performance involves preparing electrodes from highly
crystalline nanoparticles. The use of polymers in the synthetic procedure has proved
an excellent method for obtaining highly crystalline nanosized spinels. The
beneficial effect on their electrochemical properties is apparent from Figures9(a) and 9(b), which compare the performance of half cells made from nanoparticles of LiNi0.5Mn1.5O4 obtained in the presence and absence of polymer (PEG800). The influence of the charge/discharge
rate on the capacity delivered by the half-cells is weaker when the electrode
is prepared from nanoparticles obtained in the presence of polymer. The initial
capacity values range from 120 at C/4 to 100 mAh/g at 15C. In the electrode made from
the nanospinel obtained in the absence of polymer, the delivered capacity
shifts from 120 to 60 mAh/g. Although particle size was similar for the two
spinels, the use of a polymer facilitated the release of defects and internal strains,
thereby incrasing the crystallinity of the nanoparticles [31].Variation of cell capacity as a
function of the number of cycles at a variable charge/discharge rate. Half-cells
made from then‐LiNi0.5Mn1.5O4 spinel prepared in (a) the presence and (b) the absence of PEG-800.
Charge/discharge rate: C/4 (■), 2C (○), 8C (▲), 15C (⊳).
(a)(b)Another interesting property of the cells made from nanometricLiNi0.5Mn1.5O4 spinel is their ability to be charged at high current densities and discharged
slowly. This is interesting as they approach the requirements of commercial
batteries. This
property was tested in a spinel prepared in the presence of PEG and calcined at
800°C;
cells were charged under two fast regimes (2C and 4C) and further discharged under one slow regime
(C/4). Figure 10(a) shows the variation of the capacity as a function of the
number of cycles. As can be seen, capacity retention by the cells was quite
good (the capacity faded less than 5% after fifty cycles). Moreover, the
coulombic efficiency approached 100% upon cycling (see Figure 10(b)). Therefore,
the spinel exhibited an excellent electrochemical response under such drastic
operating conditions.(a) Variation of the specific capacity delivered by the half-cell made fromn‐LiNi0.5Mn1.5O4 spinel prepared in the
presence of PEG-800 under different charge/discharge regimes. (○) Charge 2C, discharge C/4; (■) charge 4C, discharge C/4. (b) Coulombic
efficiency.
(a)(b)


### 3.2.4. NanometricLiNi0.5Mn1.5O4 versus Li4Ti5O12
(Hybrid Batteries)

The electrochemical properties ofLiNi0.5Mn1.5O4 spinel versus Li4Ti5O12 were also studied. Figure 11 shows the charge/discharge curves obtained
in the 1st, 5th, and 10th cycles for the two types of hybrid batteries
examined, with electrodes made from nano (n)- and microparticles (m). All cells
were cycled at C/4. The shape of the curves of the cells made from nanometric
particles was similar to that observed for the LiNi0.5Mn1.5O4/Li half cell (viz. two pseudoplateaux between 3.0 and 3.3 V associated to the Ni2+→Ni4+ process, the mechanism of
which involves two cubic/cubic two-phase reactions [32]). By contrast, the first
charge and discharge curve for the micrometric sample exhibited a third plateau
at ca. 2.5 V. This signal is
comparable to that for the hybrid cell made from LiMn2O4 (see Figure 6). For this reason, it can be assigned to the oxidation of Mn3+ to Mn4+. The XRD pattern (not shown here) revealed the presence of NiO
related impurities; this means that not all Ni substitutes Mn in the spinel
framework, hence the presence of some Mn3+ in the spinel framework.
The presence of impurities in this spinel is a common finding when the solid is
obtained using a conventional ceramic method [33]. The low-voltage plateau
disappeared after the second cycle because of the excess of cathode material, and
the curve shape was comparable with that for the cells made from nanometric
particles except that the two pseudoplateaux were more ill-defined.Galvanostatic profiles
for the first (solid line), fifth (dash line), and tenth (dot line) charge/discharge
of (a)m‐LiNi0.5Mn1.5O4/m‐Li4Ti5O12 and (b) n‐LiNi0.5Mn1.5O4/n‐Li4Ti5O12 hybrid batteries.
(a)(b)The discharge capacities delivered by
the cells are shown in Table3. Except for the capacity value obtained in the
first discharge for the cell made from nanometric materials (110 mAh/g which is
somewhat lower than that calculated from the spinel stoichiometry) the
remaining data exhibited a well-defined trend. Thus, the capacity of the n-LNMO−n-LTO hybrid battery was retained on cycling (at
least over the ten cycles measured). This behaviour is similar to that of the
n-LMO−n-LTO hybrid battery at the same rate (C/4)
(see Table 2). By contrast, the trend exhibited by the capacity of the cell
made from micrometric particles is to significantly fade on cycling, consistent
with the results for the m-LMO−m-LTO hybrid battery (see Table 2). Again, the
performance of the cell made from nanometric particles was better than that of
the cell obtained from micrometric particles.Table 3
Capacity values delivered by the hybrid batteries (mAh/g).
Battery1st discharge5th
discharge10th
dischargen-LNMO−n-LTO110116111m-LNMO−m-LTO12812095


## 3.1. Morphological
Properties

As
stated above, thermal treatment of an appropriate precursor for a short time provides
a simple route for obtaining an Li-containing oxide of nanometric size. The suitability of the precursor is a
key factor here. We started from hydrated acetates (except for the synthesis ofLi4Ti5O12, where the Ti source was
acetylacetonate). On contact with oxalic acid dihydrate and mechanochemical
activation, the acetates become nanometric oxalates (see Figure 1). The
increased reactivity of these oxalates compared with micrometric particles
affords the use of milder thermal conditions to obtain the spinel framework.
The combination of TG curves and XRD patterns revealed that the spinel is
formed at 400°C, albeit with low crystallinity. Heating at 800°C for a short time
results in improved particle crystallinity as revealed the reduction in the
broadening of the X-ray diffraction lines. Figure 2 shows some XRD patterns
representative of the different conditions used in the spinel synthesis. At
this temperature, the spinel peaks are quite well resolved, and peaks assigned
to transition metal oxide-based impurities are hardly perceptible. The role
played by oxalic acid is shown in Figures 1(c) and 1(d). The presence of oxalic
acid dihydrate in the medium plays a key role in the final particle size of the
mixed oxide by means of the formation of oxalate nanocrystals on grinding (see Figure 1(b)). Nucleation and growth of these reaction products are facilitated by the
presence of hydration water. Thus, this precursor-like route enhances the
uniform distribution of the elements resulting in a well-defined morphology and
quite homogeneous particle size. By contrast, direct synthesis from the
acetates in the absence of oxalic acid resulted in wider particle size
distribution and nanoparticles coexisted with microparticles (see Figure 1(c)).Figure 1
Steps of the method used to synthesize the nanospinels.Figure 2
XRD patterns for the
spinels. Samples: (a) and (b)LiMn2O4 in the absence and
presence of oxalic acid, (c) LiNi0.5Mn1.5O4 in the presence of oxalic acid and PEG400.The use of polymers in the synthetic procedure has two main effects,
namely, (i) particle size is reduced (compare Figures1(d), 1(e), and 1(f)) and (ii) an improvement in spinel crystallinity, as shown below. The polymer helps to
adopt a more homogeneous distribution of metal ions on an atomic scale as a
result of its ability to coordinate to the metal ions forming the spinel. The
functional groups of the polymer capable of acting as Lewis basic sites—atoms
with nonbonding electron pairs—may bind to the ions, thereby bringing them
closer and shortening diffusion paths to adopt the spinel framework. This model
is consistent with the results obtained by using a polymer such as poly(divydilene fluoride) (PVDF), where no functional oxygen atoms are present, with which the
spinel synthesis failed. Thus, the functional groups of the polymer are key
factors for the spinel synthesis and should be able to approach and stabilize
the metal ions via their chelating properties. Thus, the presence of the
polymer, in addition to tailoring the particle shape, helps maintain
connectivity between nanocrystals, thereby facilitating the release of strains
as the temperature is raised. Nanoparticles adopt a well-defined octahedral
morphology typical of the spinel structure (see Figure 3(a)). Also, lattice defects
are insignificant as revealed by HREM images (see Figure 3(b)). The interplanar spacing was calculated to be ca. 0.46,
which corresponds to the orientation of (111) atomic planes. The high
crystallinity observed by TEM, a highly localized technique, was confirmed from
X-ray broadening analysis, more suitable to examine bulk properties of a
material. The lattice distortion was estimated from the full width at half
maximum (FWHM), using the Williamson and Hall equation [23]:(1)βcosθ=2〈e〉senθ+0.9λD,where β is
the integral breadth after correction for instrumental broadening from highly
crystalline quartz and kα2 elimination using the Rachinger
method [24], 〈e〉 denotes local strains (defined as Δd/d, where d is the interplanar
space), and D is crystallite size. Equation (1) was applied to the
strongest reflections that do not overlap with other reflections. Figure 4
shows the plots of the above equation for different spinels, and Table 1 lists
the crystallite sizes and strains calculated from the intercepts and slopes,
respectively. Both the presence of oxalic acid and especially polymer in the
synthesis improved particle crystallinity as microstrain content decreased
under these conditions. The lower crystallite size for the LiMn2O4 spinel prepared in the absence of oxalic acid compared with that of the
same spinel obtained in the presence of oxalic acid could be due to the wider
particle size distribution of the former sample. Figure 1(c) shows the presence
of ill-defined particles of few tens nanometer with others exceeding 200 nm. In
any case, highly crystalline particles of small size are quite appropriate to
cycle at high charge/discharge rates as Li-ion displacement is enhanced by
effect of the reduced probability to find defects potentially truncating their
movement.Table 1
Crystallite size and
microstrain content of Li-Mn spinels.
SampleD (nm)Microstrain(×10−4)LiMn2O4(a)9219.3LiMn2O4(b)11610.7LiNi0.5Mn1.5O4(c)792.1Synthesis conditions:(a)absence of oxalic acid, (b,c)with oxalic acid, (c)with PEG400.(a) TEM and (b) HREM images ofLiNi0.5Mn1.5O4 nanocrystals synthesized in the presence of PEG400.
(a)(b)Figure 4
Plots of the Williamson
and Hall equation for various (hkl) reflections. Samples calcined at 800°C:
LiMn2O4 in the absence (a,○) and presence (b,•) of oxalic acid, LiNi0.5Mn1.5O4 in the presence (c, ■) of oxalic
acid and PEG400.


## 3.2. Electrochemical
Properties

### 3.2.1. NanometricLiMn2O4 versus Li

The essential condition for a
material to be useful in lithium-ion battery cathodes is that it should be able
to react in a reversible manner with lithium at high potentials. This requirement
is met by the spinelLiMn2O4, for which a vast amount of
literature is available [25]. This spinel can extract and insert lithium above
4 V versus Li. Figure 5 shows
the variation of the specific capacity delivered by half-cells made from
nanometric particles as a function of the number of cycles. Galvanostatic data
were recorded at different charge/discharge rates from C/4 to 4C (C representing 1 Li+ ion exchanged in 1 hour, equivalent
to 148 mA/g). The initial capacities obtained ranged from 108 at C/4 to 100 mAh/g at 4C.
The difference in capacity was in fact rather small (8%) in spite of the
significant difference in rate (16 times). On further cycling, the half cells exhibited good
capacity retention, irrespective of the rate capability used. With the
electrode made from micrometric particles, the initial capacity delivered by
the half-cell was 107 mAh/g at C/4 and 95 mAh/g at 1C, which is close to the value
for the electrode made from nanometric particles. However, capacity fading with
cycling was more pronounced than in the half cell made from nanometric particles.
The capacity loss was particularly significant at 1C. Under these cycling
conditions, the capacity fell from 95 to 40 mAh/g over the first twenty cycles;
in the half-cell made from nanometric particles, however, the capacity only
decreased from 105 to 100 mAh/g. The origin of this striking behavior may be associated to the way lithium diffusion is affected by particle size. The time τ taken to
travel a distance r can be calculated
from the  equation:(2)τ=r2πD,
where D is the diffusion coefficient
of Li+. Thus, for nanoscale materials (viz. those having a size of
1−100 nm in at least one direction), Li ions take few minutes to cross
nanoparticles (the average value of D for Li+ is ca. 10−4cm2/s). Thus, reducing the average diffusion distances delays
concentration polarization and allows cells to operate at high currents and
hence electrode rate capabilities to be improved [7]. Obviously, reducing the
distance for Li+ ions travel must be also beneficial when the cell
anode is made from an insertion electrode, as shown below.Figure 5
Variation of cell
capacity inLiMn2O4/Li half-cells as a
function of the number of cycles. Rate: (Δ) C4,
(○) 1C, (□) 4C, and m‐LiMn2O4/Li
half-cell at (•) 1C.


## 3.2.1. NanometricLiMn2O4 versus Li

The essential condition for a
material to be useful in lithium-ion battery cathodes is that it should be able
to react in a reversible manner with lithium at high potentials. This requirement
is met by the spinelLiMn2O4, for which a vast amount of
literature is available [25]. This spinel can extract and insert lithium above
4 V versus Li. Figure 5 shows
the variation of the specific capacity delivered by half-cells made from
nanometric particles as a function of the number of cycles. Galvanostatic data
were recorded at different charge/discharge rates from C/4 to 4C (C representing 1 Li+ ion exchanged in 1 hour, equivalent
to 148 mA/g). The initial capacities obtained ranged from 108 at C/4 to 100 mAh/g at 4C.
The difference in capacity was in fact rather small (8%) in spite of the
significant difference in rate (16 times). On further cycling, the half cells exhibited good
capacity retention, irrespective of the rate capability used. With the
electrode made from micrometric particles, the initial capacity delivered by
the half-cell was 107 mAh/g at C/4 and 95 mAh/g at 1C, which is close to the value
for the electrode made from nanometric particles. However, capacity fading with
cycling was more pronounced than in the half cell made from nanometric particles.
The capacity loss was particularly significant at 1C. Under these cycling
conditions, the capacity fell from 95 to 40 mAh/g over the first twenty cycles;
in the half-cell made from nanometric particles, however, the capacity only
decreased from 105 to 100 mAh/g. The origin of this striking behavior may be associated to the way lithium diffusion is affected by particle size. The time τ taken to
travel a distance r can be calculated
from the  equation:(2)τ=r2πD,
where D is the diffusion coefficient
of Li+. Thus, for nanoscale materials (viz. those having a size of
1−100 nm in at least one direction), Li ions take few minutes to cross
nanoparticles (the average value of D for Li+ is ca. 10−4cm2/s). Thus, reducing the average diffusion distances delays
concentration polarization and allows cells to operate at high currents and
hence electrode rate capabilities to be improved [7]. Obviously, reducing the
distance for Li+ ions travel must be also beneficial when the cell
anode is made from an insertion electrode, as shown below.Figure 5
Variation of cell
capacity inLiMn2O4/Li half-cells as a
function of the number of cycles. Rate: (Δ) C4,
(○) 1C, (□) 4C, and m‐LiMn2O4/Li
half-cell at (•) 1C.


## 3.2.2.  NanometricLiMn2O4 versus Li4Ti5O12
(Hybrid Batteries)

In most commercial batteries the
positive and negative electrodes are made fromLiCoO2 and graphite,
respectively. These components have two major shortcomings. Graphite operates
at a very low voltage (ca. 50 mV versus Li). Under these conditions, the
electrolyte is thermodynamically unstable and can decompose releasing gases
which detract from battery safety. On the other hand, cobalt is a toxic and
expensive element. In this context, Li-based spinels provide interesting
prospects for circumventing these shortcomings. The spinels Li4Ti5O12 and LiMn2O4 are good choices for this purpose as they can
act as anode and cathode materials, respectively, for batteries. LTO is one of
the few lithium intercalation compounds known that undergoes little expansion
or contraction during the lithium insertion/reinsertion process [26]. We have found
battery performance to be significantly improved by using both compounds as
nanoparticles [27]. Hybrid batteries made from such particles exhibit enhanced
rate capabilities and surpass batteries made from larger particles
(microparticles) in performance.Figure6 shows
the charge/discharge curves for two batteries made from the spinels obtained as
nano(n)- and micropaticles (m). The curve shapes are typical for LiMn2O4, with two
pseudoplateaux consistent with the two-step mechanism for lithium extraction
and insertion [28]. The capacity values delivered by the cells in the 1st and
10th cycle as measured at C/4 and 1C are shown in Table 2.Table 2
Capacity values delivered by the hybrid cells (mAh/g).
Battery1st discharge10th
dischargeC/41CC/41Cn-LMO–n-LTO134120133108m-LMO–m-LTO107769666Galvanostatic profiles for
the first and tenth charge/discharge of (a)m‐LiMn2O4/m‐Li4Ti5O12 and (b) n‐LiMn2O4/n‐Li4Ti5O12 hybrid batteries. Rates: C4 (solid line) and 1C (dash line).
(a)(b)The initial capacity delivered by the cell was
higher when the electrodes were made from nanoparticles, irrespective of the discharge
rate used. However, the greater differences were observed at 1C, consistent with the better
electrochemical response of nanometric particles cycled at high rates. On
cycling at a low rate (C/4), capacity retention of this cell was quite good. By
contrast, the capacity of the cell made from microparticles decreased by about
a 10%. At higher rates (1C),
the capacity of the two batteries faded with cycling, but capacity retention in
the battery made from nanoparticles was better (its capacity loss was 10% versus 26%). Clearly, as previously
found for the half cell, a nanometric size helps in improving  battery
performance.


## 3.2.3. NanometricLiNi0.5Mn1.5O4 versus Li

The highest cathode potentials achieved
to date are in the region of 5 V versus Li, value which is on the verge of the potential
window for liquid-based electrolytes. Some Li-based spinels (e.g.,LiNi0.5Mn1.5O4)
possess this property and are the best candidates for future applications [22].
Li is removed from this compound between 4.5 and 4.8 V, and charge neutrality is
maintained by oxidizing Ni2+ to Ni4+ [29]. In order to ensure good reversibility in the
electrochemical reaction at high voltages, the electrode must be made from a
highly crystalline spinel. The conditions used to obtain nanometric particles usually
provide solids with a high defect content. In other words, particles are poorly
crystalline as revealed by X-ray line broadening analysis [19]. This is so because
the heating temperature cannot be too high and the heating time should be as
short as possible in order to avoid sintering. Spinels prepared as
microparticles are not subject to this drawback and possess a high
crystallinity. As stated above, however, microparticles exhibit a limited
electrochemical response in lithium cells, particularly at high rates. One way
of overcoming this restriction is by using composite electrodes made from a
mixture of nano- and micrometric particles. In this way, cells can exploit the
advantages of nanoparticles (high rate capabilities) and microparticles (good
reversibility in the electrochemical reaction).Figure7 shows the capacity
delivered by various half cells as a function of the number of cycles. Two
types of electrodes were studied that were prepared from (i) microparticles and
(ii) a 50 : 50 w/w mixture of micro- and nanoparticles (composite electrode)
obtained by mechanochemical activation for 15 minutes [30]. Tests were conducted
at two different rates (C/4 and 2C).
As can be seen, the best performance was obtained for the composite electrode, irrespective
of the charge/discharge rate used. This electrode delivered a higher capacity over
the one hundred cycles measured. Differences in textural properties of the
electrodes rather than in the structural properties of the spinels seem to be
the origin of the enhanced performance of the composite electrode. As can be seen in the insets of Figure 8, the
particles maintained the spinel structure with similar unit cell dimensions to
those of the pristine compounds on cycling. However, the SEM images (see Figure 8) revealed significant differences between the two electrodes. Thus, the composite electrode was
the better at retaining its integrity. Also, it was the electrode exhibiting
the best electrochemical performance (see Figure 7(a)). The appearance of the electrode made
from the m-spinel (see Figure 7(b)) was quite
different. As expected, particles were clearly larger and not so closely bound
as in the composite electrode. The nanoparticles probably occupy the
cavities formed upon compaction of the microparticles. Therefore,
the nanoparticles seem to act as binders between the microparticles, thereby
improving connectivity between particles and facilitating transport of
charge carriers. This must decrease cell polarization
and increase the reversibility of the reaction with lithium, thereby improving
cell performance.Variation of cell
capacity as a function of the number of cycles in half-cells made fromm‐LiNi0.5Mn1.5O4 spinel (■) and a mixture of
50/50 wt% n‐LiNi0.5Mn1.5O4/m‐LiNi0.5Mn1.5O4 (○). Charge/discharge rate: (a)
C/4, (b) 2C.
(a)(b)SEM images of electrodes tested at C/4 after the hundredth discharge. (a)
Micrometric spinel and (b) 50/50 composite electrode.
(a)(b)As stated above, one alternative
route for improving cell performance involves preparing electrodes from highly
crystalline nanoparticles. The use of polymers in the synthetic procedure has proved
an excellent method for obtaining highly crystalline nanosized spinels. The
beneficial effect on their electrochemical properties is apparent from Figures9(a) and 9(b), which compare the performance of half cells made from nanoparticles of LiNi0.5Mn1.5O4 obtained in the presence and absence of polymer (PEG800). The influence of the charge/discharge
rate on the capacity delivered by the half-cells is weaker when the electrode
is prepared from nanoparticles obtained in the presence of polymer. The initial
capacity values range from 120 at C/4 to 100 mAh/g at 15C. In the electrode made from
the nanospinel obtained in the absence of polymer, the delivered capacity
shifts from 120 to 60 mAh/g. Although particle size was similar for the two
spinels, the use of a polymer facilitated the release of defects and internal strains,
thereby incrasing the crystallinity of the nanoparticles [31].Variation of cell capacity as a
function of the number of cycles at a variable charge/discharge rate. Half-cells
made from then‐LiNi0.5Mn1.5O4 spinel prepared in (a) the presence and (b) the absence of PEG-800.
Charge/discharge rate: C/4 (■), 2C (○), 8C (▲), 15C (⊳).
(a)(b)Another interesting property of the cells made from nanometricLiNi0.5Mn1.5O4 spinel is their ability to be charged at high current densities and discharged
slowly. This is interesting as they approach the requirements of commercial
batteries. This
property was tested in a spinel prepared in the presence of PEG and calcined at
800°C;
cells were charged under two fast regimes (2C and 4C) and further discharged under one slow regime
(C/4). Figure 10(a) shows the variation of the capacity as a function of the
number of cycles. As can be seen, capacity retention by the cells was quite
good (the capacity faded less than 5% after fifty cycles). Moreover, the
coulombic efficiency approached 100% upon cycling (see Figure 10(b)). Therefore,
the spinel exhibited an excellent electrochemical response under such drastic
operating conditions.(a) Variation of the specific capacity delivered by the half-cell made fromn‐LiNi0.5Mn1.5O4 spinel prepared in the
presence of PEG-800 under different charge/discharge regimes. (○) Charge 2C, discharge C/4; (■) charge 4C, discharge C/4. (b) Coulombic
efficiency.
(a)(b)


## 3.2.4. NanometricLiNi0.5Mn1.5O4 versus Li4Ti5O12
(Hybrid Batteries)

The electrochemical properties ofLiNi0.5Mn1.5O4 spinel versus Li4Ti5O12 were also studied. Figure 11 shows the charge/discharge curves obtained
in the 1st, 5th, and 10th cycles for the two types of hybrid batteries
examined, with electrodes made from nano (n)- and microparticles (m). All cells
were cycled at C/4. The shape of the curves of the cells made from nanometric
particles was similar to that observed for the LiNi0.5Mn1.5O4/Li half cell (viz. two pseudoplateaux between 3.0 and 3.3 V associated to the Ni2+→Ni4+ process, the mechanism of
which involves two cubic/cubic two-phase reactions [32]). By contrast, the first
charge and discharge curve for the micrometric sample exhibited a third plateau
at ca. 2.5 V. This signal is
comparable to that for the hybrid cell made from LiMn2O4 (see Figure 6). For this reason, it can be assigned to the oxidation of Mn3+ to Mn4+. The XRD pattern (not shown here) revealed the presence of NiO
related impurities; this means that not all Ni substitutes Mn in the spinel
framework, hence the presence of some Mn3+ in the spinel framework.
The presence of impurities in this spinel is a common finding when the solid is
obtained using a conventional ceramic method [33]. The low-voltage plateau
disappeared after the second cycle because of the excess of cathode material, and
the curve shape was comparable with that for the cells made from nanometric
particles except that the two pseudoplateaux were more ill-defined.Galvanostatic profiles
for the first (solid line), fifth (dash line), and tenth (dot line) charge/discharge
of (a)m‐LiNi0.5Mn1.5O4/m‐Li4Ti5O12 and (b) n‐LiNi0.5Mn1.5O4/n‐Li4Ti5O12 hybrid batteries.
(a)(b)The discharge capacities delivered by
the cells are shown in Table3. Except for the capacity value obtained in the
first discharge for the cell made from nanometric materials (110 mAh/g which is
somewhat lower than that calculated from the spinel stoichiometry) the
remaining data exhibited a well-defined trend. Thus, the capacity of the n-LNMO−n-LTO hybrid battery was retained on cycling (at
least over the ten cycles measured). This behaviour is similar to that of the
n-LMO−n-LTO hybrid battery at the same rate (C/4)
(see Table 2). By contrast, the trend exhibited by the capacity of the cell
made from micrometric particles is to significantly fade on cycling, consistent
with the results for the m-LMO−m-LTO hybrid battery (see Table 2). Again, the
performance of the cell made from nanometric particles was better than that of
the cell obtained from micrometric particles.Table 3
Capacity values delivered by the hybrid batteries (mAh/g).
Battery1st discharge5th
discharge10th
dischargen-LNMO−n-LTO110116111m-LNMO−m-LTO12812095



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*Source: 659397-2008-05-22.xml*