Toward Low-Cost, High-Energy Density, and High-Power Density Lithium-Ion Batteries
Toward Low-Cost, High-Energy Density, and High-Power Density Lithium-Ion Batteries
0 1.-Energy and Transportation Science Division, Oak Ridge National Laboratory , One Bethel Valley Road, P.O. Box 2008, Oak Ridge, TN 37831 , USA. 2.-Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee , 418 Greve Hall, 821 Volunteer Blvd., Knoxville, TN 37996 , USA. 3.-Computer Science and Mathematics Division, Oak Ridge National Laboratory , One Bethel Valley Road, P.O. Box 2008, Oak Ridge, TN 37831, USA. 4.-
Reducing cost and increasing energy density are two barriers for widespread application of lithium-ion batteries in electric vehicles. Although the cost of electric vehicle batteries has been reduced by 70% from 2008 to 2015, the current battery pack cost ($268/kWh in 2015) is still >2 times what the USABC targets ($125/kWh). Even though many advancements in cell chemistry have been realized since the lithium-ion battery was first commercialized in 1991, few major breakthroughs have occurred in the past decade. Therefore, future cost reduction will rely on cell manufacturing and broader market acceptance. This article discusses three major aspects for cost reduction: (1) quality control to minimize scrap rate in cell manufacturing; (2) novel electrode processing and engineering to reduce processing cost and increase energy density and throughputs; and (3) material development and optimization for lithium-ion batteries with high-energy density. Insights on increasing energy and power densities of lithium-ion batteries are also addressed.
JIANLIN LI ,1,2,4 ZHIJIA DU,1 ROSE E. RUTHER,1 SEONG JIN AN,1,2
LAMUEL ABRAHAM DAVID,1 KEVIN HAYS,1 MARISSA WOOD,1
NATHAN D. PHILLIP,1,2 YANGPING SHENG,1 CHENGYU MAO,1
SERGIY KALNAUS,3 CLAUS DANIEL,1,2 and DAVID L. WOOD III1,2
Transportation currently consumes two-thirds
of total U.S. petroleum and accounts for one-third
of U.S. carbon pollution. In addition, on-road
vehicles account for 85% of transportation.1 One
potential strategy to decarbonize transport is use
of electric vehicles (EVs) thanks to the
breakthrough in battery technology.2 From 2008 to
2015, the battery pack cost has been reduced
from $1000/kWh to $268/kWh with a
simultaneous increase in battery pack energy density from
55 Wh/L to 295 Wh/L.3 Nevertheless, battery
cost needs to be further reduced to at least $150/
kWh to enable battery EVs to become
cost-competitive with internal combustion vehicles.4 The
U.S. Advanced Battery Consortium’s (USABC)
targets for battery packs for electric vehicles are
$125/kWh, 235 Wh/kg and 500 Wh/L, by 2020.5
Most of the battery cost comes from materials and
manufacturing. Based on the BatPac model from
the Argonne National Laboratory, batteries with
common chemistries such as graphite and LiNixMny
Co1 x yO2 (NMC) would not meet the USABC’s
target even in the best-case scenario (all chemistry
problems solved, unlimited performance, favorable
systems engineering, and high-volume
manufacturing).6 A moderate risk pathway to meet the $125/
kWh target is coupling silicon anode with a
highcapacity cathode. Using lithium metal as the anode
could further reduce the battery cost to<$100/kWh,
albeit at a higher risk.6 Many daunting challenges
need to be addressed for silicon and lithium anodes.
These include the dramatic volume changes7,8 and
unstable solid electrolyte interphase (SEI)9 for Si
anodes, as well as the catastrophic failures
associated with lithium dendrites forming on lithium
anodes.10 Therefore, cost reduction in the near term
could come from improvements in cell
manufacturing, learning rates for pack integration, and
capturing increasing economics of scales.11 Given the
mass production of lithium-ion batteries (LIBs),
battery recycling and minimizing environmental
impact would also contribute to cost reduction.
Improving the energy density of LIBs is another
challenge to increasing the limited range of EVs.
For vehicles with comparable cost, the driving
ranges of EVs are generally only 22% compared
with vehicles with internal combustion engines.12
This challenge will remain a problem for EVs until
fast recharging is realized. Electrode Engineering
can reduce battery cost and improve energy density
simultaneously by reducing the relative weight of
inactive components such as conductive additive,
binder, separator, and current collector. Another
approach is using active materials with higher
In this article, cost reduction from materials
processing and cell manufacturing is discussed,
specifically covering quality control during electrode
manufacturing to reduce scrap rate, novel
technologies for low-cost electrode manufacturing, electrode
engineering for high-energy and power electrodes,
and modified protocols to shorten the formation
period. The relationship among microstructure,
properties, and performance is also discussed. In
addition, recent progress in materials for
highenergy density LIBs is summarized for both anodes
and cathodes. Insights on achieving low-cost and
high-energy density LIBs are shared.
One strategy to reduce battery cost through
manufacturing is improving quality control (QC) to
reduce the scrap rate. The overall scrap rate in the
state-of-the-art LIB production is reported to be
2%.13 This does not include flawed or substandard
electrodes in cell assembly that results in a much
higher scrap rate and battery cost because of extra
costs from the subsequent value-added steps such as
the cell assembly and formation cycling. Current
technologies used in industry to detect coating
defects such as pinholes, agglomerates, nonuniform
coatings, etc. are beta transmission and
chargecoupled device (CCD) camera inspection.14 Other
techniques like Raman spectroscopy have also been
used to identify variations in coating composition
during off-line material testing.15,16 These
techniques are sensitive enough for certain defects like
nonuniform coating and pinholes but may not have
enough resolution to detect flaws like agglomerates
and metal contamination in the electrode
manufacturing process that lead to faulty cells being built,
thereby increasing production cost.
As an alternative to the beta transmission
technique, which is expensive and poses safety concerns
as a result of radiation, laser calipers have been
used to measure the wet thickness of the coating
along a line scan.17 The in-line measurement also
provides a feedback loop to adjust the coating
protocol for timely error correction. This is achieved
by using two sensors that emit laser light at an
angle and by capturing the laser profile using a
camera. The sensor calculates a corresponding
voltage output that has a linear relationship with
thickness. Figure 1a shows the laser caliper system
mounted on the coater orthogonally to the substrate
to maintain alignment when electrode thickness
changes. Precision of <±2% for in-line laser
measurement was achieved.
To detect defects effectively in-line on a
highspeed electrode coating system, infrared (IR)
thermography was employed.18 IR cameras detect the
IR energy emitted from an object as a function of
temperature. This system, as shown in Fig. 1b,
consists of an IR camera pointed at the electrode
coming out of the hot drying oven using a series of
mirrors. The thermal radiation off the coating is
captured by the camera and is analyzed. As
electrode coatings are solid objects, with good thermal
conductivity, the heat in the coating must be
transferred to the surface by internal conduction
and convection through pore. Coating defects tend
to change the thermal response. Hence, any flaws in
the coating will appear as a temperature variation
at that point. In the case of pinholes (insert in
Fig. 2) and nonuniform coating, there is less
radiation from the coating, and in the case of
agglomerates, there is more radiation.
The reduction in scrap rate and thus reduction in
manufacturing cost can further be addressed by
investigating the degree of correlation between
different types of defects and long-term
electrochemical performance of cells. If such correlation
appears to be insignificant for some types of
defects, the electrodes containing these defects
can still be used in batteries. For that purpose, a
comprehensive study has been conducted involving
intentionally introduced electrode defects of
different type and controlled size.18 Four types of defects
were considered, chosen based on likelihood of
encountering them in battery manufacturing:
pinholes, agglomerates of electrode material, and
areas uncoated with slurry. The latter was studied
in two configurations: one uncoated stripe of 3 mm
width and 3 uncoated stripes each 1 mm wide. The
influence of these defects on electrochemical
performance was evaluated in coin cells. Figure 2
summarizes the results of these tests along with
thermal or optical images of the four types of
defects. The extent of capacity fading during
longterm cycling at 2C rate worsened in the sequence
of: baseline <1 time large nonuniform
coating < pinholes <3 times nonuniform coating
< agglomerates. Microstructural analysis of
agglomerates revealed carbon-rich regions that
reduced the electrochemical performance during
Observations made in this study provide a
detailed in-line system for thickness measurement
and coating defect identification that can be
incorporated with a feedback loop to the slot-die coating
system. Electrochemical analysis of these defects
also provides insight into the role of these defects on
A new technique, active thermal scanning, has
been developed recently for in-line porosity and
areal loading characterization of battery
electrodes.19 Compared with other techniques, e.g.,
porosimetry and x-ray tomography, the thermal
scanning is nondestructive and compatible with
high-speed, roll-to-roll manufacturing of LIB
electrodes. This technique can be used for electrode
coating and calendaring processes. Together, these
different techniques for improved quality control
will reduce the scrap rate of electrodes and will
minimize the cell rejection rate after fabrication and
Although aqueous processing has been adopted in
manufacturing of graphite anodes, most composite
cathodes are manufactured through an organic
solvent process by using polyvinylidene fluoride
(PVDF) and N-methyl-2-pyrrolidone (NMP) as the
binder and the solvent, respectively.20,21 There has
been growing interest in switching cathode
manufacturing from NMP-based processing to aqueous
processing because of the multiple benefits: (
lower cost in raw materials [$1.5/L to $3.0/L NMP
versus $0.015/L water, $5.5/lb PVDF versus $1.1/lb
carboxymethyl cellulose (CMC)]22,23 and capital;24
) elimination of solvent recovery—up to 2 times
reduction in electrode processing cost including
drying and solvent recovery;23 (
environmental effect;25 and (
) easier siting and permitting of
battery manufacturing plant. Nevertheless,
replacing NMP with water also creates several processing
) agglomeration of electrode
components in aqueous suspensions as a result of the
strong interaction between colloidal particles;26 (
poor wetting of aqueous suspension on Al foils
ascribed to high surface tension of water;27 (
leaching in water suspension; (
) electrode cracking
attributed to high residual stress induced by surface
tension of water during electrode drying;28 and (
residual moisture removal.29
Fortunately, most of these problems are solvable
and great progress has been achieved. For example,
the agglomerates can be controlled by adding
dispersants20,26,30,31 or by optimizing mixing
techniques, sequences, and periods.32–34 Poor slurry
wetting on Al foils can be alleviated via increasing
the surface energy of the current collectors such as
treating the Al foils with corona plasma27 or coating
with a carbon layer. Metal leaching may or may not
be a problem. Figure 3 shows the Li+ concentration
in saturated LiFePO4 and LiNi0.5Mn0.3Co0.2O2
(NMC532) suspension at room temperature.
Although the Li+ concentration increased
dramatically from one day to one week, it has been proven
that the effect of lithium ion leaching on cyclability
of LiFePO4 is negligible because the leached lithium
ion can reintercalate into LiFePO435 and
commercialization of LiFePO4 cathodes via aqueous
processing is ongoing.36 Lithium ion leaching in
NMC532 is slightly higher than in LiFePO4 and
continues to increase with higher Ni content.
Nevertheless, comparable performance has been
observed from NMC11137 and NMC53238 via
aqueous and NMP-based processing. Cracking is induced
by capillary stresses generated during drying,39
which is more likely for thick electrodes. This
problem can be circumvented by reducing the
surface tension of the solvent; for example water
can be mixed with another solvent with lower
surface tension.28 As for residual moisture removal,
it is common that electrodes produced via aqueous
processing have higher moisture uptake as a result
of the hydrophilic nature of the water-soluble
binders. The efficiency in removing the residual
moisture depends on the chemistries and binders.
Yet, it has been reported that the residual moisture
of aqueous processed NMC532 cathodes can be
easily reduced to a similar level as NMP-based
processing through a routine secondary drying
protocol.29 The effect of residual moisture on
electrochemical performance also depends on the
chemistries, and it may not show up without longer
term cycling. For example, there is no significant
capacity fade in the LiFePO4 cathode with 500-ppm
moisture for 1000 cycles.41 In addition, a radiant
drying process has been demonstrated to remove
residual moisture effectively from LiFePO4 cathodes
in 2 min compared with the conventional 18-h to
22h drying at 80 C inside a vacuum oven, resulting in
a 500-fold decrease in the secondary drying process
and in a 68% reduction in energy consumption.41
The radiant drying could be an effective technique
for secondary drying of aqueous-processed
Although there is no concern of volatile organic
compounds (VOCs) in aqueous processing, adequate
amounts of water are needed to dissolve water
soluble binders and to keep the slurry at desirable
viscosity for electrode coating, which could be
energy intensive during electrode drying. Energy
curing provides an option toward solventless
coating where low-molecular-weight (MW) polymers/
oligomers or monomers are cured into cross-linked,
high-molecular polymers instantly under electron
beam (EB) or ultraviolet light (UV).42 Compared
with thermal oven drying, energy curing provides
three benefits: (
) less space needed by the UV/EB
unit enabling a reduced footprint, (
) lower energy
consumption for UV/EB unit, and (
throughputs ascribed to the fast curing speed from UV/EB
technology. Compared with UV curing, EB curing
does not need photoinitiators and has greater
penetration depth, which is adjustable from the
accelerating voltage of the electron beam.43 The
basic concept has recently been proven that low MW
oligomers are used in the slurry preparation and
high MW crosslinked polymers are obtained from
EB curing at high line speeds.40
It has been demonstrated that the NMC532
cathode was successfully fabricated using an
EBcured acrylated polyurethane (PU) binder.40 The
electrochemical performance was comparable with
that in conventional NMP-based processing. The
first charge capacities of electrodes using PVDF and
EB-cured acrylated PU as binders are 179 mAh/g
and 178 mAh/g with Coulombic efficiencies of 84%
and 85%, respectively. The similar overlapping of
the differential capacity curves indicates that the
EB-cured polymer functions identically to the PVDF
binder, which does not affect the electrochemical
reactions of Li+ intercalation/de-intercalation into
the NMC particles. After 100 cycles, the NMC532
cathodes via NMP-based processing and EB curing
delivered 157 mAh/g and 153 mAh/g in discharge
capacity, respectively, where the variation was
within experimental error. This demonstrates that
similar cyclability can be achieved by using the EB
curing process compared with the conventional
NMP-based processing.40 This novel processing
approach presents a promising new avenue for
mass manufacturing of high-performance, low-cost
Li-ion batteries. Further R&D activity on
highspeed scaling up is currently continuing at Oak
Ridge National Laboratory.
Shortening Formation Period
Electrolyte wetting and solid electrolyte
interphase (SEI) formation are the slowest processing
steps in cell assembly and take between 1 and
3 weeks depending on the cell chemistry and
manufacturer. This is a result of slow electrolyte wetting
and low C-rates (e.g., three to five charge–discharge
cycles with typically C/20 rates at research
laboratories and slightly higher at industry sites). The
processes cost is about $23/kWh, which is the second
most expensive after the electrode processing cost
($36/kWh).22 Large equipment space and high
energy are also necessary to maintain production
rates because all cells have to be placed in
environmental chambers (typically between 30 C and 60 C)
and connected to cyclers. Hence, it is beneficial to
reduce formation time and steps without
compromising the cell performance to realize cost reduction.
The SEI forms on the anode when the electrolyte
is reduced at low potentials, typically below 0.9 V
versus Li/Li+ for ethylene carbonate (EC) and
0.75 V versus Li/Li+ for propylene carbonate
(PC).44,45 Some additives in the electrolyte can
decompose and precipitate on the anode at a higher
voltage. For example, vinylene carbonate (VC) has
lower activation energy for reduction (13 kcal/mole)
compared with EC (25 kcal/mole) or PC (26
kcal/mole). VC decomposes and precipitates (forms SEI)
around 1.4 V versus Li/Li+.46 Hence, during a
charge cycle, VC precipitates first on the anode
and becomes a part of the SEI structure followed by
EC (or PC). The reduction potentials increase at an
elevated temperature and vary depending on anode
(graphite) surface chemistry (e.g., amount and kind
of electron donor groups) and structure (e.g., basal
plane surface or edge surface).
Having stable SEI layers on an electrode is
necessary because most common commercial,
carbonate-based electrolytes are not stable and
decompose irreversibly at a high state of charge (e.g.,
reduction reactions below 0.9 V versus Li/Li+ at
anode and oxidation reactions above 4.2 V versus
Li/Li+ at cathode). An SEI layer prevents an
electrolyte from irreversible reduction reactions
(decompositions) by hindering (ideally blocking)
electron transfers between an electrode and the
electrolyte while allowing lithium ion diffusion
through the SEI layer. Because a pristine electrode
does not have SEI layers that prevent the electron
transfers, most SEI forms during the first charge–
discharge cycle. Irreversible capacity loss resulted
from SEI formation dramatically decreases after the
first cycles because of the preformed SEI layers that
passivate electrode surfaces. Once SEI forms
properly, it insulates the electrode electronically and
prevents electrolyte consumption and loss of lithium
inventory. The proper SEI should have negligible
electrical conduction, high lithium ion selectivity
and permeability, stability in electrolyte, stability/
flexibility in volume changes of active materials
(graphite, Si, Sn, etc.), and thermal stability. To this
end, it is typically a slow process (e.g., multiple
cycles at C/20 or C/10) to form a proper SEI on
electrodes, which causes slow production speed and
consequently adds extra cost to LIBs.
It is challenging to reduce the SEI formation time
without sacrificing cell performance. A few studies
have been attempted to shorten the SEI formation
time. Increasing the C-rate is a simple method for
that purpose, but it can cause nonuniform and
discontinuous SEI.47,48 An alternative is reducing
the upper cutoff voltages during formation cycles at
the expense of high-capacity fade.49 Meanwhile, a
new protocol has been developed recently where
shallow charging–discharging at high
state-ofcharge was attempted and superior capacity
retention was maintained while reducing the formation
time by 6 times and impedance of the cells as
shown in Fig. 4.50 The shallow
charging–discharging method involved repeated cycling a few times
between 3.9 V and 4.2 V (cell voltage) after the first
charge. Then, a full discharge took place at the final
cycle. Further time reduction seems to be feasible
when the method is optimized.
SEI formation is not only affected by the C-rate
but also by the surface properties of active
materials. To have a uniform SEI layer on an electrode,
electrolyte has to be distributed uniformly before
the first charge. Complete electrolyte wetting of
small pores is slow especially in a large cell. One
common effective method to provide accelerated and
uniform wetting is to apply a vacuum during
cell sealing. Electrode surface properties
determine interactions between the electrodes and
electrolytes, which impacts electrolyte wetting
and reduction potentials and, eventually, SEI
There have been several studies on modifying
graphite surfaces such as heat and acid treatment
to control surface chemistry (e.g., oxygen and
nitrogen).54–58 Nitrogen or oxygen on graphite surfaces
can interact more with Li+ in electrolyte because
their electron density is high. Heat treatment in an
inert environment can reduce oxygen from the
graphite surface. The graphite with low oxygen
contents experienced exfoliations resulting from poor
SEI formation.59,60 Chemical treatment such as
HNO3 and (NH2)2S2O8 increases oxygen levels on
graphite surfaces, which resulted in better reversible
capacity.61 Ultraviolet light treatment also has been
used to reduce the oxygen level on graphite anodes
and has resulted in higher capacity retentions and
lower impedances as shown in Fig. 5.62 The results of
these studies seem to suggest that there are optimum
levels or types of oxygen on electrodes although their
exact functions in the wetting and SEI formation are
not well understood yet.
Optimizing cell engineering is a straightforward
approach to increase the cell energy density by
increasing the volume ratio of active materials in a
battery pack. Numerical modeling has been applied
to understand the electrochemical system for cell
engineering. The operation of a LIB follows porous
electrode theory and electrochemical reaction
thermodynamics. Newman et al. developed the
governing equations.63,64 LiFePO4 and NMC thick
electrodes have been optimized by a combined
experimental and simulation approach.65,66 Efforts
have been put forth in a variable porosity electrode,
but this has only led to marginal improvements in
energy density compared with well-designed
constant-porosity electrodes, suggesting it is more
important to decrease the tortuosity.67
Numerical simulation has also been applied to
investigate the limiting factors of the energy–power
density relationship in LiNi0.8Co0.15Al0.05O2 (NCA)/
graphite cells with thick electrodes.68 Lithium ion
depletion in the electrolyte and lithium diffusion
gradient in active material particles were found to
attenuate the advantage of thick electrodes. The
limiting factors were more profound with increasing
C-rates. Several different porosity gradients were
also modeled to determine the improvement of
energy density. Figure 6a shows the gradients with
four linear (i–iv) and six second-polynomial (v–x)
variations. Figure 6b shows the energy density of
the cell stack with a 150-lm-thick cathode under a
1.5C discharge rate. Although four gradients have
higher energy density than the baseline, the
improvement is not significant. Nevertheless, poor
porosity gradient design can decrease the energy
density, as shown by porosity gradient (v) because
the electrolyte depletion gets worse.68 It is noted
that the lithium ion diffusion length is estimated
with the Bruggeman relation, which is based on the
assumption of a low-volume fraction of insulating
phase represented by random, isotropic spheres.
The actual electrode structure is more complex
where the Bruggeman relation may be invalid,69
and the actual cell performance could be different
from the simulation results.
Li-ion diffusion in the active materials is usually
multiple orders of magnitude slower than in
common liquid electrolytes and, thus, becomes the
ratelimiting step in power performance for thin
electrodes. Nevertheless, the Li-ion transport
limitations in liquid electrolytes become
increasingly important as the electrode thickness
increases67,68,70,71 because the diffusion time in the
liquid phase is no longer negligible as a result of the
significant increase in diffusion length in the thick
porous electrodes. Consequently, it is necessary to
design thick electrode architectures to take
advantage of the increased energy density provided by the
higher active material volume ratio, while
minimizing the tortuosity and transport limitations
that can be detrimental to power performance.
Many different designs have been suggested,
including graded-porosity and hierarchical
architectures,67,72–76 and several simulations have shown
that smaller active material particles can help
decrease the polarization and capacity loss observed
at high discharge rates by shortening the Li-ion
diffusion path in the particles.68,71,77 Yet, particle
size variations also affect the packing structure of
the electrode and can result in different pore sizes
and distributions, as well as in differences in the
contact resistance between the electrode and the
current collector. These variations could be
leveraged to improve transport properties and battery
performance. Multilayer architectures have been
proposed to balance the potential advantages of
different particle sizes. Tuning the electrode
architecture using parameters such as particle size could
enable flexible cell optimization to meet the
requirements of different applications. The multilayer
architectures can be created by separate coatings
or simultaneous coatings that allow various
electrode components or formulations in each layer for a
specific purpose.78 For example, the bottom layer
could have higher binder content to account for
binder migration during electrode drying and
maintain good adhesion between the electrode and the
Tortuosity can also be minimized by using
spherical particles and obtaining a homogenous electrode
structure,79 which could be realized with
monodisperse particles. Nonspherical particles, such as
graphite, result in heterogeneous structures where
tortuosity is anisotropic.69 Aligning the direction
with the lowest tortuosity perpendicular to current
collector will benefit Li-ion diffusion from bottom to
top layers. Another method to reduce tortuosity is
through electrode processing. Introducing a pore
former that can be well controlled and eliminated
during electrode drying or sintering step is one
effective method to create straight channels (where
tortuosity is 1) in electrodes72,80 The straight
channels can also be created by laser structuring,81
coextruded electrodes with a low-density area besides
a higher density area,82 or 3D printing
interdigitated electrodes.83 Nevertheless, better
understanding on current distribution and its effect on lithium
plating is needed for these special structures. They
could also increase manufacturing cost.
PROGRESS IN HIGH-ENERGY MATERIALS
High-voltage Cathode Materials
Besides electrode engineering, another effective
way to increase the energy density of LIBs is to use
electrode materials with high-energy density.
Graphite anodes used in commercial LIBs can achieve a
reversible capacity >360 mAh/g, close to the
theoretical value of 372 mAh/g for LiC6.84–86 By
contrast, even the best commercial cathodes are limited
to gravimetric capacities around 200 mAh/g.85,87
The cell voltage is also limited by the choice of
cathode. Thus, there has been intense interest in
developing high-capacity, high-voltage cathodes to
increase the specific energy of LIBs.88,89
Additionally, cathode materials must meet several other
design criteria including electrochemical and
thermal stability, abuse tolerance, reasonable cost, good
electronic conductivity, and high-rate capability.
The first commercial LIB introduced by Sony in
1991 used LiCoO2 for the cathode active material.
Prepared at a high temperature, LiCoO2 adopts a
layered structure with the R 3m space group
(Fig. 7).90,91 LiCoO2 is limited to a reversible capacity
of 140 mAh/g,92 but layered oxides with significantly
higher capacity are achieved through partial
substitution of Co with other metal ions (Fig. 7).93–95 In
particular, Ni-rich NMC cathodes (LiNixMny
Co1 x yO2 where x > 0.6) with reversible capacities
>200 mAh/g show great promise for increasing the
energy density in Li-ion cells and are likely to replace
lower capacity chemistries for the next generation of
electric vehicles.96,97 Nonetheless, several
outstanding challenges with Ni-rich NMCs must be addressed.
High-voltage cycling (>4.4 V) will be required to
achieve a cell-level energy density>250 Wh/kg, even
if the NMC cathode is paired with an advanced anode
such as silicon-graphite composites.98 High-voltage
cycling results in impedance rise and capacity loss.99
The impedance rise is partly attributed to a rock-salt
surface reconstruction layer resulting from oxygen
loss and reduction of the transition metal oxidation
states.100 Also, high-voltage cycling results in
decomposition of current generation electrolytes and in the
buildup of a surface reaction layer.99,100 Ni4+, in
particular, is not stable in contact with
electrolyte.100,101 This complex interplay among
electrolyte degradation, phase changes, and transition
metal dissolution from the cathode surface have
spurred investigations of the cathode electrolyte
interface (CEI) in recent years.102 Electrolyte
decomposition products deposit on the cathode surface in the
form of Li2CO3, LiOH, LiF, LixPOFy, polycarbonates,
and species specific to electrolyte and cathode
compositions.103,104 Strategies to improve the surface
properties include doping,105 surface coatings,106,107 and
electrolyte additives.108 These approaches typically
result in trade-offs of maximum specific capacity and
first cycle coulombic efficiency109 for decreased
impedance rise, thermal stability, and reduced CEI buildup
to increase cell-cycle life.110
One especially promising approach to reduce
surface reactivity and prolong the cycle life of
Nirich NMCs is the development of compositionally
graded cathodes with less Ni at the
surface.96,106,107,112,113 These particles typically have a
Mn-rich surface and a Ni-rich core. Mn4+ is
electrochemically inactive. Therefore, the Mn-rich surface
stabilizes the electrode/electrolyte interface,
whereas the Ni-rich core enables a high capacity
and energy density. Cathodes with a Mn-rich
surface also have greater thermal stability compared
with their homogenous counterparts.114 Ultimately,
a combination of approaches will likely be needed to
produce Ni-rich NMCs with optimum performance.
For example, Al-doping115,116 and surface
coating106,117 are two strategies that have been used to
boost capacity retention and improve rate
performance in NMCs with concentration gradients.
Li-rich NMCs (xLi2MnO3Æ(1 x)LiMO2 where
M = Mn, Ni, Co) have also attracted significant
attention because they offer a very high capacity
(>250 mAh/g) at high voltage (2.5–4.7 V).111,118
Lirich NMCs are considered either a solid solution or a
composite of Li2MnO3 (monoclinic structure, C2/m
space group) and LiMO2 (trigonal structure, R 3m
space group).119–121 Despite their promise, these
cathodes suffer from first cycle irreversible capacity
loss, impedance rise during high-voltage cycling,
and most importantly, a significant drop in the
voltage profile (voltage fade) with cycling.122 The
voltage fade has been attributed to chemical and
structural changes including oxygen evolution, Ni
and Co migration from the surface into the bulk,
reduction of Mn4+ to Mn3+, and formation of a
spinel-like phase.123–129 The spinel-like phase is
formed by the migration of lithium ion from
octahedral lithium site to tetrahedral lithium site and
migration of TM ions from octahedral M site to
octahedral lithium site via oxygen vacancies.
Therefore, strategies to mitigate phase change are to
restrict the cation migration. The phase change also
results in a somewhat more complex CEI formation
mechanism than that of Ni-rich NMC.130,131
Strategies to stabilize the surface of Li-rich NMCs include
coatings,132–135 surface treatments,126 cycling
protocols,136 synthesis routes,137,138 and electrolyte
additives.135 Most of these efforts have failed to
stop the underlying mechanisms responsible for
voltage fade,135 although compositions that
incorporate small amounts of spinel domains into the
layered structure show some promise.139 Even if the
structure cannot be stabilized, Li-rich NMCs with
10–30% Li2MnO3 may still find use in applications
where some voltage fade is acceptable or as part of
Moving beyond layered transition metal oxides,
cathode materials capable of multielectron redox
reactions are another approach to increase
capacity.140 Candidates include sulfur cathodes,141
materials that undergo conversion reactions such as
FeF3,142 organic electrodes,143 disordered
materials,144,145 and several polyanionic chemistries.146
Although these emerging cathodes hold promise,
they face many obstacles to commercial success. No
clear winner has replace layered transition metal
oxides (such as NMCs) for the future generation of
Li-ion batteries. Regardless of the cathode
chemistry, new electrolytes or more effective additives
will be required to push cell voltages beyond
High-Energy and High-Power Density Anodes
High-energy density cathodes must be matched
with equally high-energy anodes. Although
traditional graphite-based anodes already offer a specific
capacity nearly twice that of the best cathode
materials on the market, replacing graphite with
higher capacity materials would still allow for
thinner and lighter anodes, thus, resulting in
higher energy density batteries. Graphite
electrochemically reacts with Li through intercalation to
form LiC6.147 Alternatively, elements such as Si and
Sn can electrochemically alloy with Li to form
Li15Si4148 and Li17Sn5,149 with much greater specific
capacities of 3579 mAh/g and 960 mAh/g,
respectively. At first glance, the capacities of these
alloying materials are very attractive, but they generate
a unique set of challenges deterring manufacturers
from large-scale implementation.
As opposed to graphite, which only expands 10%
because of Li intercalation,150 Li alloys such as Si
can balloon 300% in size to accommodate the
additional Li.151 Large volume changes between a
charged and a discharged state quickly results in
mechanical degradation, which leads to a loss in the
electronic and ionic pathways to the active material
of the anode.152 Thus, nano Si is usually used to
alleviate particle pulverization. Moreover, confining
the Si particle size to the nano domain helps
alleviate stress on the electrode by distributing
expansion of the particles throughout the entire
electrode framework.153 Using the alloying
nanoparticles alone is not sufficient as they have
the tendency to agglomerate and ripen during
cycling.154 Rather, composites of Si and C are used,
preventing agglomeration of the nanoparticles
while subsequently increasing the electronic
conductivity. The most simplistic approach to this is
achieved by mixing small quantities of Si
nanoparticles with graphite electrodes. A 30-wt.%
Si/graphite composite results in a theoretical specific
capacity of 1330 mAh/g. When the capacity is
restricted to 500 mAh/g, these composites operate
reliably up to 90 cycles.155,156 More complex nano Si
architectures using carbon nanotubes,157
graphene,158,159 and other carbon or oxide
supports7,160,161 have also been developed and shown
excellent capacity retention, but these have been
synthesized only on the laboratory scale and further
scale-up could be costly. In addition, the low areal
loading along with the nano Si architectures results
in low-volumetric energy density. Further
extending the cycle life of Si anodes also relies on forming a
stable SEI that must be sustained throughout
cycling. Fluorinated ethylene carbonate (FEC) has
been employed as an additive to form a more robust
SEI,162 but the jury is still out as it is likely
consumed during cycling, resulting in long-term
Despite significant progress, many challenges
remain unsolved such as cycle life. Most results in
literature are from half cells where there is
abundant lithium. Sufficient cycle life in full cells with
high Si content anodes has not been demonstrated.
Overall, the application of Si in lithium-ion
batteries is still limited to low Si content and electronic
devices that do not require a long cycle life. A major
breakthrough in passivating Si surface is needed to
meet the 1000 deep charge/discharge cycles. Besides
the aforementioned material problems, other factors
could also affect the application of Si in EVs,
including the high cost of nano Si, the complexity
in electrode manufacturing with Si addition,
challenges in cell and pack design to accommodate the
significant volume expansion, and modification to
the battery management system to accounts for the
different voltage of Si from graphite.
Other alloys such as Ge164 and Sn165 have also
been investigated for high-energy and high-power
density anode materials. Nevertheless, similar
challenges to Si remain including significant volume
expansion and capacity fade.
CONCLUSION AND OUTLOOK
After the 70% cost reduction in lithium-ion
batteries that began in 2008, further breakthroughs
in material development and cell manufacturing
have been required to meet the ultimate USABC
targets of $100/kWh to $125/kWh for advanced
electric vehicle batteries. The interrelationship
between new materials integration and new cell
and electrode processing methods is critical to
meeting the cost and performance targets. Cell
gravimetric and volumetric energy densities must
also be increased by a factor of 2.0–2.5 from 180 Wh/
kg to 220 Wh/kg and 300–400 Wh/L, respectively, to
achieve electric vehicle driving ranges that are
acceptable to the public. This combination
translates into a 5- to 6-time cost-energy factor that
industry must still realize. Next-generation
materials such as Si-based composite anodes and Ni-rich,
high-voltage composite cathodes must be combined
with low-cost, environmentally responsible
production methods such as aqueous electrode processing
or electron beam curing. Surface coatings may also
play a pivotal role in both performance and
processing of anode and cathode active materials. For
example, o enable aqueous processing of Ni-rich
NMC cathodes, a surface coating may be needed to
prevent Li and Ni leaching that also benefits a
longterm capacity fade.
Sophisticated electrode architecture designs, that
have been successfully used in other
electrochemical applications, such as polymer electrolyte fuel
cells and redox flow batteries, must be implemented
to realize the 15–20% energy density improvements
possible with high-loading electrodes (5–6 mAh/cm2
and higher). Thick electrodes with graded
architectures have the added advantage of being able to be
combined with any electrochemical couple, as well
as with high-speed coating methods such as UV and
electron-beam curing. Nondestructive evaluation
that goes beyond the current optical camera and
beta gauge techniques will also need to be combined
with next-generation production lines and
electrochemical couples that raise electrode production
yield to 99%. The combined cost reduction,
highenergy density, and high-power density will likely
come from parallel advances in the following specific
1. Solving the challenges of implementation of
high-energy anode and cathode active materials
such as high-capacity fade and surface and
2. Development of high-voltage electrolytes
3. Periodic revival/maintenance of batteries
allowing smaller batteries to meet the driving range
and cycle life requirement
4. Improved quality control and nondestructive
evaluation methods to reduce the scrape rate
5. Porous current collectors or eliminating current
collectors to increase the energy density of
6. Novel electrode manufacturing techniques such
as high-speed curing methods with low solvent
content or dry/solventless coating
7. Improved Li-ion transport for simultaneous
high-energy and high-power batteries. High
Liion transport will be achieved by highly
conductive active material with small particle size,
large solid electrolyte interfacial area, and low
tortuosity in electrodes and separator.
This research at Oak Ridge National Laboratory,
managed by UT Battelle, LLC, for the U.S.
Department of Energy (DOE) under Contract
DEAC05-00OR22725, was sponsored by the Office of
Energy Efficiency and Renewable Energy (EERE)
Vehicle Technologies Office (VTO) (Deputy Director:
David Howell) Applied Battery Research
subprogram (Program Manager: Peter Faguy). The U.S.
government retains and the publisher, by accepting
the article for publication, acknowledges that the
U.S. government retains a nonexclusive, paid-up,
irrevocable, worldwide license to publish or
reproduce the published form of this article, or allow
others to do so, for U.S. government purposes. The
Department of Energy will provide public access to
these results of federally sponsored research in
accordance with the DOE Public Access Plan (http://
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