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Keeping Up with Increasing Demands for
Electrochemical Energy Storage
Jeff Sakamoto
Michigan State University.
The interest in vehicle electrification is unprecedented. Several automotive
manufacturers are producing or planning to produce hybrid electric vehicles
(HEVs), plug-in hybrid electric vehicles (PHEVs), and fully or battery electric
vehicles (BEVs). Lithium-ion (Li-ion) battery technology is the current leading
candidate to meet the near- and medium-term needs for electric vehicles. Leverag-
ing considerable growth and development from the manufacturing of batteries for
microelectronics, Li-ion technology has advanced significantly in the last decade.
However, the leap from small-scale microelectronic batteries (tens of watt hours)
to electric vehicle battery packs (tens of kilowatt hours) is not trivial. Perfor-
mance metrics such as cost/kilowatt hour, specific energy (Wh/kg), specific power
(kW/kg), safety, and cycle life are considerably more demanding for electric
vehicles than for laptops and cell phones. Electric vehicles (EVs) show promise in
minimizing reliance on fossil fuels, but their widespread use will likely require a
revolutionary advance in energy storage technology. Research in sophisticated and
efficient power electronics, battery/cell telemetry, safety, thermal management,
and schemes to recycle/reuse EV batteries can help to establish a solid foundation
for the development and use of EVs. This article provides an overview of energy
storage technology for vehicle electrification, highlights challenges, and discusses
opportunities at the frontiers of battery research.
THE NEED FOR ADVANCED ENERGY STORAGE
In terms of sustainability, minimizing dependence on fossil fuels and reduc-
ing CO2 emissions are compelling arguments to electrify vehicles. And from a
practical perspective, EVs can take advantage of existing infrastructure for elec-
41
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42 FRONTIERS OF ENGINEERING
trical power production and transport—infrastructure that will soon be bolstered
by efforts to augment renewable energy production whose primary byproduct is
electrical power (e.g., through photovoltaic cells and wind turbines).
If electrical energy becomes the preferred form of energy, electrochemical
energy storage is a natural fit. In contrast, hydrogen fuel cell technology requires
an entirely new infrastructure to efficiently produce hydrogen and then transport,
store, and reconvert it to electrical energy.
To put into perspective the amount of energy consumed by the transportation
sector, of the total 2.85 × 1016 watt-hours (1 Quad = 2.93 × 1014 watt-hours) of
energy used by the United States in 2011 27.7% (7.91 × 1015 watt-hours) went
to transportation (Figure 1).1 However, due to the relatively low chemical-to-
mechanical energy conversion efficiency of internal combustion engine (ICE)
technology, the ratio of serviceable to rejected energy is disproportionately low
compared to other energy use sectors.
If EVs can improve energy efficiency in the short term and the technology
for non-fossil-fuel-based/renewable electrical power generation can be realized
in the long term, the benefits to our country’s current and future sustainability are
clear. Assuming the latter, the following discussions focus on electrical energy
storage, specifically batteries.
CHALLENGES FOR ELECTROCHEMICAL ENERGY
STORAGE AND USE IN EVS
Defining the ideal battery for EVs is complicated because of the numerous
powertrain configurations involved in HEVs, PHEVs, and BEVs; for example,
the capacity (kWh), power (kW), and cycle life can be considerably different
for an HEV compared to a BEV (Khaligh and Li 2010). To simplify discussion,
this article focuses on BEVs with battery characteristics that can power a four-
seat vehicle for approximately 100 miles on a single charge, criteria favorable
for widespread adoption.2 Figure 2 shows the necessary performance attributes
of an effective EV battery.
Vehicle range is determined by the amount of energy stored in the battery and
the rate at which the energy is expended to propel the vehicle. A 23 kWh battery
used to power a ~70 kW electric motor is believed to be sufficient to achieve a
range of about 160 km. The mass and volume of the battery should be minimized
to reduce the vehicle mass while maximizing vehicle cabin volume, respectively.
1 hese
T data and the accompanying figure are from the Lawrence Livermore National Labo-
ratory website, https://flowcharts.llnl.gov/content/energy/energy_archive/energy_flow_2011/
LLNLUSEnergy2011.png, accessed November 9, 2012.
2 hether this BEV performance standard is specifically required to significantly impact energy
W
consumption is not yet known, but agencies and auto companies generally agree with this definition
(Bruce et al. 2012; CCC 2012; Thackeray et al. 2012).
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FIGURE 1 Energy use in the United States in 2011.
43
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44 FRONTIERS OF ENGINEERING
FIGURE 2 Battery performance criteria to power the next generation of battery electric
vehicles.
Vehicle acceleration is determined by specific power (kW/kg) or how quickly
the stored energy can be extracted per unit mass of battery. A common metric is
in the single to multi-kW/kg range (e.g., 1–3 kW/kg).
Replacement of the ICE powertrain with an electric powertrain should not
considerably add to the vehicle cost, and the cost of the battery pack should be
less than $5,000.
Ensuring consistent, long-term vehicle range requires a charging efficiency of
99.9999% such that approximately 80% of the original battery capacity is avail-
able at the end of the vehicle’s life.
Widespread use of BEVs will entail operation in dramatically different
climates, so the battery must be capable of operating at relatively low and high
ambient temperatures.
Although it is difficult to quantify how fast is fast enough, the issue of range
anxiety may be addressed if a battery pack can be charged at a charging station
as quickly as a gasoline tank can be filled at a gas station.
Last, and perhaps most importantly, the battery technology must be safe and
reliable.
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KEEPING UP WITH DEMANDS FOR ELECTROCHEMICAL ENERGY STORAGE 45
LI-ION BATTERIES
Of the battery chemistries available today (Figure 3), Li-ion has the highest
specific energy (Tarascon and Armand 2001) and is the only technology capable
of meeting the criteria shown in Figure 2. While other energy storage technologies
such as supercapacitors, flywheels, and compressed air are in development, only
Li-ion batteries are mature enough or meet the necessary criteria or both (Dunn
et al. 2011). Li-ion batteries also have the distinct advantage of both intrinsically
high cell voltage (>3 V) and the capacity to store lithium ions in the solid state,
resulting in high specific energy and low cell volume (energy density), respectively.
In a typical Li-ion cell (Figure 4), lithium ions are shuttled, with relatively
high efficiency, between the anode and cathode via a liquid Li-ion electrolyte
(EVSAE 2012). Graphite (in powder form) is by far the most common anode
that reversibly uptakes and releases lithium ions between graphene sheets. The
cathode consists of a ceramic of nominal formula LiMO2 (in powder form), where
M stands for a transition metal such as cobalt (Co), manganese (Mn), or nickel
(Ni) that can change valence states upon insertion/extraction of Li-ions. During
FIGURE 3 Comparison of battery technologies currently available and under develop-
ment (Bruce et al. 2012). Darker shading in the bars indicates the specific energy values
in laboratory-scale prototypes; lighter shading indicates the range of anticipated specific
energy values for Li-S and Li-air technologies, respectively. Cd, cadmium; Li, lithium;
MH, metal hydride; Ni, nickel; Pb, lead; Zn, zinc.
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46
FIGURE 4 Schematic of a lithium (Li)-ion cell: (A) at the atomic scale (A, amperes; reprinted with permission from Tarascon and Armand
2001) and (B) at the microscopic scale (adapted from EVSAE 2012).
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KEEPING UP WITH DEMANDS FOR ELECTROCHEMICAL ENERGY STORAGE 47
discharge, it is more energetically favorable for the graphite anode to release its
Li-ions and for the cathode uptake Li-ions to reduce the M valence charge (e.g.,
M4+ to M3+). This shuttling of lithium ions from the anode to the cathode is
accompanied by the simultaneous passing of electrons through an external circuit
to power the electric vehicle.
Since their invention in 1991 by Sony and professor John B. Goodenough,
Li-ion batteries have been integrated into cell phones, laptop computers, and
other microelectronics (Figure 5). And some of the first Li-ion-powered EVs
were not terrestrial but instead were vehicles sent to survey the surface of Mars in
2003 through NASA’s Mars Exploration Program (Huang et al. 2000). The Mars
Exploration Rover Li-ion batteries started development in 1996 and were flight
qualified and implemented in 2003, a testament to how quickly Li-ion battery
technology can progress.
In 2008, a combination of factors led to a significant push to improve vehicle
fuel efficiency, resulting in a rapid transformation of the auto industry with an
emphasis on vehicle electrification. In 2011 GM rolled out the PHEV Volt and
Nissan the BEV Leaf, and in 2012 Ford started selling the BEV Ford Focus.
These past and recent successes are impressive, but Li-ion battery packs still
require considerable reductions in cost as well as increases in specific energy to
extend vehicle range. The following section presents a materials perspective on
opportunities in electrochemical energy storage and milestones whose achieve-
ment will address these issues.
OPPORTUNITIES IN ELECTROCHEMICAL ENERGY STORAGE
Unlike lead (Pb)-acid, nickel-cadmium (Ni-Cd), and nickel–metal hydride
(Ni-MH) battery technologies, Li-ion technology performance has room for
improvement, as shown in Figure 3. Advanced electrode and cell designs and
electrode material breakthroughs (Thackeray et al. 2012) may enable a doubling in
energy density and a fourfold reduction in cost compared to available Li-ion tech-
nology. Eventually, however, Li-ion technology improvements will crest, requiring
a breakthrough in battery technology to approach the cost target (~$150/kWh) and
the range of an ICE powertrain vehicle (>400 km).
Several research and government agency reports (e.g., Bruce et al. 2012; CCC
2012) present complementary near-term roadmaps to guide battery research and
development over the next two decades. Three milestones extrapolated from these
roadmaps illustrate the frontiers of battery development, with substantial steps in
2015 and 2020 followed by a revolutionary leap in 2030.
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48
FIGURE 5 Li-ion batteries come in a variety of designs: (a) spiral wound cell, (b) button or watch cell, (c) prismatic cell, and (d) solid polymer
(electrolyte) battery (Tarascon and Armand 2001). Ah, amp-hour; Al, aluminum; Cu, copper; V, volt.
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KEEPING UP WITH DEMANDS FOR ELECTROCHEMICAL ENERGY STORAGE 49
2015 MILESTONE: OPTIMIZE CURRENT MATERIALS
AND CELL COMPONENT DESIGN
In the short term the focus is on optimization of materials and conventional
liquid electrolytes. At present, approximately 50% of a battery pack mass is dead
weight (Johnson and White 1998). For example, in the cell cross-section shown
in Figure 4b, only the graphite anode and LiMO2 particles store lithium and
therefore energy; the electrical current–collecting foils, electrolyte, separator,
and hermetic container do not store energy.
Increasing the mass/volume fraction of active material is one strategy to
improve specific energy. Making thicker, less porous electrodes is a popular
approach to achieve this, but thicker and less porous active electrode layers impede
the transport of ions in the electrolyte and thus reduce power (Buqa et al. 2005).
Furthermore, the nonuniform current in thicker electrodes can cause metallic
lithium to deposit on the anode and oxygen gas to be released from certain LiMO 2
cathodes, which can be a safety hazard in the presence of heat and flammable
electroyte solvents. These challenges can be addressed through research on
l
advanced electrode designs, powder processing, and coating technologies (DOE
2010).
Cycle life is another concern that requires attention. A passivation layer
forms on the surface of a graphitic anode particle during the solid electrolyte
interphase (SEI). As lithium intercalates and deintercalates from graphite par-
ticles, the corresponding swelling and contraction create fissures in the SEI,
resulting in the continuous and irreversible consumption of lithium and dimin-
ishing capacity retention. Again, improved electrode designs to homogenize
charge flow could address this concern, as could the development of new elec-
trolytes and/or electrolyte additives to make the SEI more robust.
Economies of scale will probably not play a significant role in minimizing
cost per kilowatt hour ($/kWh) (Bruce et al. 2012; CCC 2012) by 2015. Rather,
new materials with appreciably better performance and lower cost are needed to
bring costs down to the target of approximately $150/kWh.
2020 MILESTONE: ELECTRODE AND ELECTROLYTE
MATERIALS BREAKTHROUGHS
The 2020 milestone focuses on reducing cost rather than increasing specific
energy, although it is hoped that the latter will increase by more than a factor of
two. Once the electrode and cell design have been optimized, increases in the
specific energy will require new electrode and complementary electrolyte mate-
rials that can store more lithium or charge-per-unit mass/volume and that have
higher voltage (energy = amps × volts × time). If the new materials can be made
at comparable or lower cost, a byproduct of increased specific energy will be a
commensurate decrease in cost/kWh (Figure 3).
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50 FRONTIERS OF ENGINEERING
Alloying anodes such as silicon (Si)- or tin (Sn)-based electrodes will likely
constitute the next generation of Li-ion battery anodes (Thackeray et al. 2012).
The term “alloying” is used to describe the reversible, electrochemical reaction
between lithium and a pure element such as silicon or tin.
Specific capacity (milliamp hours per gram, or mAh/g), which refers to the
amount of lithium that an electrode can uptake and release, is commonly used
where one mole (6.94 grams) of lithium can provide 26.8 Ah of electrical charge.
Graphitic anodes have a theoretical specific capacity of 372 mAh/g, and silicon
and tin have specific capacities of 4,009 and 960 mAh/g, respectively, making the
interest in these anodes apparent.
However, a >300% change in volume accompanies the uptake and release of
lithium from silicon and tin, creating significant mechanical stresses that cause
decrepitation and poor cycle life (Deshpande et al. 2010). One solution is to reduce
the powder particle size from the typical micron scale to the nano scale and thus
decrease the magnitude of strain. Creating nano Si wires with <100 nm dimen-
sions, originally demonstrated by the Cui group (Wu et al. 2012), reduces the
overall strain to minimize decrepitation and improve cycle life. Another approach
is to increase cycle life by embedding Si or Sn particles in an elastic or compliant
carbon matrix to create an encapsulation effect (Zhao et al. 2011). Envia Systems
recently announced a 400 Wh/kg Li-ion cell pack using Si-based anodes, but it has
yet to be commercialized (Thackeray et al. 2012). Advanced materials processing
and materials engineering could play a major role in optimizing alloying electrode
performance and reducing cost.
On the cathode side, there are two promising approaches. First, the cathode sys-
tem, a composite layered structure, enables the full extraction of one molecular unit
of lithium, or x = 1 per formula unit of xLi2MnO3(1 − x)LiMO2 (M = Mn, Ni, Co)
(Thackeray et al. 2012). This type of material, developed by Thackeray and col-
leagues at Argonne National Laboratory, can deliver nearly twice the specific capac-
ity compared to conventional LiMO2 cathodes.
There are a few practical concerns associated with this material strategy, how-
ever. For example, the lithium must come from the anode (which is not the case
with conventional LiMO2 cathodes) and the charging voltage (4.6 V) is outside
the stability window of most conventional electrolytes, resulting in diminished
cycle life.
The second approach involves increasing the cathode reaction voltage from
about 4.0 V to approximately 5.0 V to result in a 20% increase in specific energy,
provided the specific capacity is comparable to that of conventional cathodes. Exam-
ples of high-voltage cathodes include LiMn1.5Ni0.5O2 and LiMPO4 (M = Co, Ni)
(Allen et al. 2011), both of which are relatively mature compared to the composite
layered cathodes described above, but the lack of stable electrolytes limits their
widespread implementation.
Higher cell voltage (cathode side) and a stable SEI (anode side) with advanced
anodes both require significant improvements in electrolytes. One approach is
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KEEPING UP WITH DEMANDS FOR ELECTROCHEMICAL ENERGY STORAGE 51
to integrate additives to conventional electrolytes to improve the high-voltage
( athode) stability. The Kang group achieved this by increasing the electrolyte
c
s
tability to enable the use of LiCoPO4 (4.8 V) cathodes (von Cresce and Kang
2011). A completely different approach involves a solid electrolyte material
breakthrough using a ceramic rather than liquid electrolyte. The advantages could
include higher stability (0 to >6 V) and perhaps safety as a flammable liquid elec-
trolyte is replaced by a highly thermal and chemically stable ceramic.
A class of ceramics referred to as “fast-ion conductors” conducts lithium ions
about as fast as a conventional liquid electrolyte. Additionally, these ceramics
have negligible electronic conductivity and the Li-ion conductivity improves with
increasing temperature. Recent examples of promising solid electrolytes include
sulfur (S)-based (Kamaya et al. 2011) and oxide-based electrolytes (Murugan et
al. 2007; Rangasamy et al. 2011) that exhibit Li-ion conductivities comparable to
conventional liquid electrolytes.
Next-generation Li-ion batteries will require new materials for anodes, cath-
odes, and electrolytes. Advanced materials and ceramic processing technology
based on lessons learned from the 2015 milestone will play a key role in achieving
the 2020 milestone. The development of new electrolyte materials, in particular,
will advance progress toward the 2030 milestone of enabling new battery chemistry
beyond Li-ion technology.
2030 MILESTONE: BEYOND LI-ION BATTERIES
If electric powertrains are to replace ICE technology, without raising con-
cerns about cost or range, a new battery technology is required (Bruce et al.
2012). Three of the most popular battery chemistries that represent the frontier
of energy storage are Li-S, Zn-air, and Li-air (the metal air batteries are actually
semifuel cells, but for brevity and consistency they are referred to as batteries).
Because the challenges related to Zn-air technology are relatively well known
(Lee et al. 2011), the focus here is on Li-S and Li-air batteries, which are not as
well understood.
Li-S is attractive because of its high theoretical energy density (2,199 Wh/l),
high theoretical specific energy (2,567 Wh/kg), and the low cost and abundance
of sulfur (Bruce et al. 2012). Factoring in the mass of the electrolyte, electrical
c
urrent–collecting foils, packaging, and other features, the practical specific
energy is reduced to approximately 600 Wh/kg, which is still considerably higher
than that of advanced Li-ion batteries. In a Li-S cell, elemental lithium and
sulfur are the reactants, a nonaqueous electrolyte shuttles lithium ions between
electrodes, and, because sulfur does not have sufficient electrical conductivity, a
specific porous carbon (Ji et al. 2009) is added to increase the effective electrical
conductivity of the S-cathode.
Two challenges remain: (1) prevention of deleterious mechanisms that result
from the formation of soluble Li-S compounds during cycling and (2) achievement
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52 FRONTIERS OF ENGINEERING
of a stable/cyclable Li-electrolyte interface, a challenge since the 1980s when it
led to the demise of rechargeable lithium metal anode batteries.
Li-air batteries are of two types: nonaqueous and aqueous (Bruce et al.
2012); the “air” in question is the source of oxygen and, for the aqueous bat-
teries, water vapor. Nonaqueous batteries involve the reaction of lithium with
oxygen gas (O2) to form Li2O2. (The reference to “air” may be a bit misleading
since both water vapor and carbon dioxide must be excluded from the reaction/
cell in the non queous configuration.) During discharge, lithium is transported
a
through a nonaqueous electrolyte and reacts with O2 in the presence of a porous
carbon network and a catalyst to form solid precipitates of Li2O2. The theoretical
energy density of this system is (3,436 Wh/l) and the theoretical specific energy
is (3,505 Wh/kg). Some of the key challenges for nonaqueous Li-air batteries are
(1) development of an oxygen-permeable membrane that excludes carbon dioxide
and water vapor, (2) development of effective cathode electrodes that prevent pore
occlusion resulting from the formation of solid byproducts during discharge, and
(3) effective integration of catalysts to improve reaction kinetics.
In the second type of Li-air battery an aqueous electrolyte is used to transport
lithium ions into a carbon cathode electrode to form lithium hydroxide (LiOH)
during discharge. At lower concentrations LiOH is soluble in the electrolyte,
whereas at higher concentrations (i.e., greater degrees of discharge) it precipitates
out as a solid. The theoretical energy density of the aqueous variant is (2,234 Wh/l)
and the theoretical specific energy is (3,582 Wh/kg). Some of the challenges that
remain for aqueous Li-air technology are technologies to (1) protect the lithium
metal anode from the aqueous electrolyte using a ceramic electrolyte membrane,
(2) prevent reactions with carbon dioxide from ambient air, and (3) prevent pore
and electrolyte interface occlusion when/if LiOH precipitates at higher depths of
discharge. Although there are few examples of advanced prototypes, the projected
specific energy for both Li-air variants is expected to be about 1,000 Wh/kg.
The majority of the challenges involve the discovery of new materials and
development of an electrolyte to enable the use of metallic lithium anodes. The
need for ceramic electrolytes that protect the lithium metal anode is one aspect
common to Li-air and Li-S technology. In addition to poor cycle stability, excess
lithium is required to counter the effects of poor cycling efficiency. For example,
two- to fourfold excess lithium may be necessary, thus reducing the energy den-
sity. One recent material breakthrough (Murugan et al. 2007) identified a new
class of ceramic oxide electrolyte that is believed to exhibit the unprecedented
combination of stability against lithium with high, room-temperature ionic con-
ductivity (Figure 6).
In addition to new electrolytes, advanced catalyst and catalyst support elec-
trodes, similar to those found in fuel cells, are required to improve rechargeability
and power.
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KEEPING UP WITH DEMANDS FOR ELECTROCHEMICAL ENERGY STORAGE 53
FIGURE 6 Ceramic electrolytes may enable the use of metallic lithium (Li) anodes. A
new ceramic electrolyte referred to as LLZO (Li7La3Zr2O12) exhibits the unprecedented
combination of high ionic conductivity and stability against metallic lithium and air. Shown
above, a prototypical LLZO membrane fabricated in the Sakamoto lab using unique powder
synthesis and sintering technology.
CONCLUSIONS
There is a compelling need for advanced electrochemical energy storage
to power the next generation of electric vehicles. Furthermore, interest in Li-
ion technology is on the rise, if growing attendance at the five-year-old annual
symposium “Beyond Li-ion” is any evidence. But although Li-ion batteries offer
substantial performance advantages over previous battery technologies, range
capacity and cost are major challenges to overcome by 2015. Better electrode,
cell, and pack design, together with advanced manufacturing and power electron-
ics, will establish a solid foundation for future EV technology.
By 2020, material and electrolyte breakthroughs are expected to provide
moderate improvements in BEV range—and dramatic reductions in cost. Anodes
that are cheap (based on Si or Sn) are expected to uptake and release more lithium
per unit mass. On the cathode side, the focus will be on increasing the voltage
and lithium uptake and release per unit mass. Developing higher-stability liquid
and solid electrolytes will complement higher-voltage cathodes and efforts to
revolutionize energy storage in the long term (2030).
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54 FRONTIERS OF ENGINEERING
Provided the necessary electrical infrastructure is in place by 2030, a break-
through in electrochemical energy storage is required if ICE technology is to be
replaced by BEVs. Li-air or Li-S batteries may be the high specific energy, low-
cost technology of the future, but significant materials and engineering challenges
must first be overcome. Solving the lithium metal anode–electrolyte interface
stability issue; developing novel catalyst/catalyst support cathodes; and creating
stable, semipermeable solid electrolytes require further research and development
if Li-air and Li-S technologies are to mature.
The frontiers of electrochemical energy storage are exciting from multiple
perspectives, and are likely to generate significant engineering research and devel-
opment opportunities in the coming decades.
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