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REVIEW
Cite this: J. Mater. Chem. A, 2016, 4,
17251
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Challenges and prospects of the role of solid
electrolytes in the revitalization of lithium metal
batteries
Alberto Varzi,ab Rinaldo Raccichini,ab Stefano Passerini*ab and Bruno Scrosati*ac
The scientific community is continuously committed to the search for new high energy electrochemical
storage devices. In this regard, lithium metal batteries, due to their very high electrochemical energy
storage capacity, appear to be a highly appealing choice. Unfortunately, the use of lithium metal as the
anode may lead to some safety hazards due to its uneven deposition upon charging, resulting in
dendrite growth and eventual shorting of the battery. This issue may be successfully addressed by using
intrinsically safer electrolytes capable of establishing a physical barrier at the electrode interface. The
Received 27th August 2016
Accepted 3rd October 2016
most promising candidates are solid electrolytes, either polymeric or inorganic. The main purpose of this
review is to describe the present status of worldwide research on these electrolyte materials together
DOI: 10.1039/c6ta07384k
with a critical discussion of their transport properties and compatibility with metallic lithium, hoping to
www.rsc.org/MaterialsA
provide some general guidelines for the development of innovative and safe lithium metal batteries.
1. Introduction
Lithium-ion batteries (LIBs) are, nowadays, the power sources of
choice for consumer electronics with a continuously expanding
market to meet the increasing sophistication of popular
devices, such as smartphones, tablets and so on.1 However,
crucial phenomena that affect our lives, such as the severe
pollution of urban areas and the gas emissions of gasolinepowered cars, call with urgency for energy renewal, both in
terms of the sustainability of energy sources and wider road
circulation of low-emission hybrid electric vehicles and, ideally,
no-emission full-electric vehicles. The successful evolution of
these emerging markets requires the availability of a battery
technology suitable to ensure stationary energy storage and
electric engine power.2 Unfortunately, despite the continuous
improvements from its rst commercialization back in 1991,
the present LIB technology based on the “Carbon–Lithium
Metal Oxide” intercalation chemistry is still not adequate for
properly performing these duties.1,3 Accordingly, there have
been many efforts to identify and develop alternative battery
chemistries with higher energy densities and lower costs.
Different approaches are presently considered to reach the goal.
In this respect, lithium metal represents the ideal anode
material and, indeed, in the initial commercial development of
lithium batteries (in the late 80s), lithium was the anode of
a
Helmholtz Institute Ulm (HIU), Electrochemistry 1, Helmholtzstrasse 11, D-89081
Ulm, Germany. E-mail: stefano.passerini@kit.edu
b
Karlsruhe Institute of Technology (KIT), P. O. Box 3640, D-76021, Karlsruhe, Germany
c
Istituto Italiano di Tecnologia, Via Morego 30, I-16163 Genova, Italy. E-mail: bruno.
scrosati@gmail.com
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choice.4 Emerging candidates employing lithium metal are, for
example, lithium–sulfur5 and lithium–air batteries.6 However,
several unsolved issues make such systems difficult for practical
exploitation. One of these is the serious safety hazard associated
with the presence of lithium metal, which was the reason
behind the commercial withdrawal of the rst lithium batteries
and motivated the shi to the safer and more reliable LIB
technology.4,7
Effectively, the use of lithium metal in a practical battery
poses serious challenges. Indeed, it readily reacts with most
electrolytes and, in particular, with liquid organic solutions
commonly adopted in LIBs, experiencing dendritic growth upon
charging with associated serious safety concerns (Fig. 1a).
This explains why most of the battery manufacturers are still
reluctant to move away from the more comfortable LIB design.
Interestingly though, there has been lately, in the battery
community, a renewed interest in the use of lithium metal in
batteries.8 The excitement is obviously motivated by the unique
specic capacity values of lithium (i.e., 3.86 A h g1 and 2.06
A h cm3) and its low electronegative standard potential of
3.040 V, which result in a very high electrochemical energy
equivalent.9 Indeed, a battery using lithium metal would have
a considerably higher energy density (both gravimetric and
volumetric) than those employing other common anode materials (see Fig. 1b). Certainly, the use of lithium metal in practical
batteries may be acceptable only if the safety of operation is
totally ensured. This essential condition can be achieved by
means of electrolytes providing thermodynamic stability against
Li or, alternatively, able to form a stable passivation layer on
their surface,10 enabling a smooth and reversible deposition
upon cycling.
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Fig. 1 (a) Schematic representation of a lithium metal battery experiencing dendrite growth upon charging. (b) Gravimetric and volumetric
energy densities achieved using Li-metal (Gen III) or other common anodes such as graphite (Gen I) and silicon–carbon composites (Gen II). The
values are taken from ref. 8 and are presumably obtained with different cathode materials (Gen I: LiCoO2, Gen II: not specified; Gen III: not
specified). (c) A possible classification of solid electrolytes which might be employed in lithium metal batteries.
Although the growth of lithium dendrites can be partially
suppressed by introducing additives in conventional liquid
electrolytes,11,12 this cannot be considered as the ultimate
answer to the safety issues of lithium metal batteries. Organic
carbonates are, indeed, highly ammable anyway and can
easily ignite in LIBs, too. The ideal strategy would involve the
complete replacement of conventional electrolytes with safer
electrolytic media establishing a physical barrier to the
growth of dendrites. The most promising category is constituted by the so-called solid electrolytes (SEs). These are
generally dened as electronically insulating solid materials
with high mobility and selective transport of charged ionic
species within their structure.13 In general, they can be
divided into two main categories: polymeric and inorganic.
Over the years, then, different classication schemes have
been proposed, according to their chemical composition,
structure, etc. (Fig. 1c).14 In this review article, we do not aim
at a systematic description of all the typologies of SEs, which
has been already done in previously published technical
reviews.13–18 Our approach will be, instead, a more critical
discussion and evaluation of the main features of such
materials. Particular attention will be devoted to their
compatibility with the lithium metal electrode and the open
challenges.
17252 | J. Mater. Chem. A, 2016, 4, 17251–17259
2.
Polymer electrolytes
Solid polymer electrolytes (SPEs), rst proposed for lithium
metal batteries in the late seventies,16,18–20 have the advantage of
combining solid-state behaviour with the ease of processing
plastic materials. The historically most well-known SPEs are
based on poly(ethylene oxide) (PEO) and a lithium salt (LiX; X ¼
ClO4, PF6, N(SO2CF3)2).19–23 In such electrolytic systems, Li+ ions
are complexed to the ether oxygens of the PEO chains, and,
therefore, their mobility is strongly affected by the motion of the
complexing polymer segments. As schematically described in
Fig. 2a, the ionic transport is caused by the motion of the
complexation sites assisted by the segmental motion of the PEO
matrix.18 PEO-based SPEs are highly appealing because of their
compatibility with the lithium metal electrode. Recent studies
have demonstrated that, similar to conventional organic
carbonates, they are not fully thermodynamically stable with
lithium metal and a passivation layer is formed at the interface.24 Nevertheless, this allows quite reversible stripping–
plating behaviour (see Fig. 2b).22 Furthermore, their plastic
nature allows, in principle, the development of safe and exible
lithium polymer batteries (LPBs). Indeed, efficient rechargeable
LPBs have been developed starting from the early eighties25 (see
a typical conguration in Fig. 2c) and power, nowadays, the full
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Journal of Materials Chemistry A
Fig. 2 (a) Schematic drawing of Li+ transport in a PEO matrix, assisted by the segmental motion of the polymeric chains.18 (b) Cyclic voltammetry
showing the reversible Li stripping–plating achievable with a PEO-based PSE. (c) Example of a typical lithium-polymer battery (LPB) configuration. (d) Schematic representation of dendrite growth suppressed by the nanocomposite SPE.36
electric vehicles produced by Bolloré.26 The famous Bluecars are
a solid reality, which proves that LPBs are safe and reliable, as
testied by the 3000 car eet that has been driven over
10 million miles without issues. Nevertheless, the large excess
of Li metal needed to ensure such performance (three-fold
excess) denitely leaves much room for improvement in terms
of energy density.27 Nowadays, the major drawback of PEObased electrolytes is their modest ionic conductivity (i.e., 0.85). As the decrease of conductivity at
the interface is one of the triggers of dendrite formation, this
class of electrolytes would be highly promising to increase the
safety of LPBs.
Usually, a common approach to address the ionic conductivity issue relies on the addition of liquid plasticizers (leading
to the so-called “gel polymer electrolytes”, i.e. GPEs), such as
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propylene carbonate (PC) or ethylene carbonate (EC), which, by
reducing the crystalline fraction of the polymer matrix, lead to
an increase of the conductivity, however, adversely accompanied by the loss of the solid-state conguration (these are
indeed gels) and compatibility with the lithium metal electrode.14,29,30 In order to ensure both reasonably high conductivity and compatibility with the lithium electrode, one of the
most successful strategies was found to be the addition of
selected ionic liquids (ILs). The presence of two salts, one
containing the organic cations (i.e., the IL, N-propyl-N-methylpyrrolidinium bis(triuoromethanesulfonyl)imide, named
PYR13TFSI) and the other the Li+ (i.e., lithium
bis(triuoromethanesulfonyl)imide, named LiTFSI) in the PEO
matrix, enables a relatively high conductivity of 104 S cm1 at
RT, with enhanced lithium-ion transfer properties compared to
the IL-free PEO–LiTFSI electrolyte.31–34 Additionally, the IL has
the benecial effect of increasing the onset for dendrite growth.
Such dendrite suppression is fundamental for ensuring long
term stability of the lithium metal anode.
A further approach to inhibit the growth of Li dendrites is
the dispersion of solid plasticizers (namely, selected ceramic
powders with a sub-micrometric particle size) into the polymeric matrix to form “nanocomposite” (or “hybrid”) polymer
electrolytes.35 Experimental results suggest that the uniformly
dispersed ceramic ller (e.g., TiO2 or Al2O3) may provide sturdier and more tortuous interfaces hampering the penetration of
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nucleated dendrites (see Fig. 2d).36 It should be furthermore
mentioned that, besides inorganic llers, cross-linked polymeric chains can also improve the cycling stability of LPBs.37
3.
Inorganic solid electrolytes
3.1
Conductivity mechanism and classic examples
Unlike liquids, where the ions move in the medium with their
solvation shells, the conductivity in inorganic solid electrolytes
(ISEs) occurs via ionic motion across crystalline lattice sites.
Furthermore, ISEs are generally single-ion conductors with
a high transference number (approaching unity), which represents a considerable advantage over aprotic electrolytes (transference number in the range 0.2–0.5),38 to eliminate
concentration polarization at electrode interfaces. ISEs are
formed by xed ionic sub-lattice offering paths (composed of
vacant or interstitial sites) along which the mobile ions can
diffuse by hopping between adjacent sites. For most of the time,
the mobile ions are localized in given lattice sites where they
vibrate along their equilibrium positions. Occasionally, the ions
leave their stable positions to move (by thermally activated
jumps) to adjacent sites, where they reside for a certain time
before continuing their motion either to another empty site or
back to their initial one (theory of casual motion). To jump across
the crystalline sites, the ions have to overcome an activation
energy barrier. For solids with highly packed structures (e.g.,
NaCl and b-AgI), having not well dened motion channels, the
activation energy is typically of the order of 1 eV (96 kJ) or
higher and, consequently, the room temperature (RT) conductivity is modest (i.e., in the order of 106 S cm1 or less).
Differently, in solids characterized by open conduction channels, the values of activation energy are considerably lower (e.g.,
in the order of 0.03 eV) and the conductivity is several orders of
magnitude higher.
A classic example of the latter is a-AgI16 (see Fig. 3a) where
the Ag+ ions, on average two per unit cell, are statistically and
randomly dispersed over a great number of interstitial sites
having a preferential coordination 4 (where Ag+ ions reside for
most of the time) but, also, coordination 3 or 2 (where the ions
move during their migration). Unfortunately, the a-AgI phase is
stable only at temperatures higher than 147 C, and this obviously reduces its practical interest. However, by “doping” silver
iodide with large ionic radius cations or anions, it is possible to
expand the unit cell, and thus to “freeze” down the a-AgI
disordered structure to ambient temperature. Typical examples
of these stabilizing ions are Rb+, K+ and (CH3)4N+ among the
cations and P2O74 and WO42 among the anions.16 The most
known electrolyte of this family is silver rubidium iodide,
RbAg4I5,39 whose structure is schematically shown in Fig. 3b.
This was indeed proposed for battery fabrication.40 However,
although proved to have a uniquely long shelf-life,41 these
batteries, due to the high cost and low voltage issues of silver,
have been of modest practical relevance.
Lithium-ion conducting inorganic solid electrolytes (LISEs)
would encounter considerably more interest since, in principle,
they may lead to the development of safer lithium batteries. In
fact, compared to conventional liquid electrolytes, LISEs offer
17254 | J. Mater. Chem. A, 2016, 4, 17251–17259
Review
a series of key advantages, such as high reliability (no leakage)
and enhanced safety (intrinsically not ammable).12,13,42,43
Unfortunately, the effective use of LISEs is still limited mainly by
(i) their low ionic mobility, which results in conductivities
considerably lower than those of conventional organic liquids,
and (ii) reactivity at the interface with the electrodes, especially at
low potentials (i.e. vs. Li or graphite). Historically, the rst
example of a LISE of practical interest is lithium iodide, LiI.16
Although it adopts a NaCl-type compact structure (see Fig. 3c)
and possesses a very low lithium ion conductivity at RT (i.e., 107
S cm1), clever manufacturing procedures, aimed at reducing its
thickness, enabled the development of thin lm lithium metal
batteries of great relevance in the medical sector. Indeed, cardiac
pacemakers, which require isothermal operation (i.e., 37 C) at
a very low rate (i.e., 10 year rate), all use LiI-based batteries.44 It
was also shown that the bulk ionic conductivity of LiI can be
considerably enhanced by the incorporation of Al2O3 powders up
to values of the order of 105 S cm1. The increase in conductivity
was proposed to be promoted by the occurrence of preferential
pathways on the surface of the aluminium-based additive.45 In
this way, the LiI–Al2O3 conductivity resulted to be sufficient for
the development of a solid-state battery, although no substantial
market impact was achieved at the time. Another classical LISE is
H-doped (1–2%) lithium nitride (Li3N).46 It is constituted by
a structure formed by Li2N layers, which are separated by additional bridging of loosely bonded Li+ ions (see Fig. 3d), leading to
a conductivity of about 103 S cm2 (at 25 C),47 i.e., a value
comparable with that of organic liquid solutions. Unfortunately,
the electrochemical stability window (ESW) of Li3N is too narrow
for use with high voltage cathodes (ca. 0.45 V), thus, preventing
its use for the development of batteries of practical relevance.
3.2
Present developments and ongoing challenges
Due to the renewed interest in high performance solid-state
lithium metal batteries (SSLMBs), tremendous research efforts
are presently underway in academia and industry to develop
advanced solid lithium ion conductors having fast ion transport
and a wide ESW.8,48 As testied by some detailed technical
reviews recently published, the research is currently focused,
mainly, on crystalline and glassy (or glass-ceramic)
compounds.13,15,49
A promising class of crystalline lithium ion conductors is
provided by garnet-like structured compounds with the
Li5La3M2O12 general formula, where M ¼ Ta, Nb, Ba or Zr50,51
(see Fig. 4a). Although these materials offer excellent compatibility with metallic lithium and most of the common lithium
cathodes, their commonly low RT conductivity (i.e., in the order
of 105 S cm1) has so far prevented their use in practical
SSLMBs. Nevertheless, it has been recently demonstrated that
the ionic conductivity of this class of LISEs can be boosted up to
104 to 103 S cm1 at RT, by stabilizing the cubic form of the
structure with small amounts of Al.52 Recently, Kanno et al. reported a superionic conductor, i.e., Li10GeP2S12 (LGPS), a solid
having mono-dimensional conductive pathways (Fig. 4b) and
fast lithium-ion transport, which showed one of the highest
ionic conductivity. Indeed, the resulting ionic conductivity
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Fig. 3
Journal of Materials Chemistry A
Classical examples of solid ionic conductors and their structures. (a) a-AgI, (b) RbAg4I5, (c) LiI and (d) Li3N.
above 102 S cm1 at 27 C (ref. 53 and 54) is sufficient for
battery use.55 Although it was experimentally shown that LGPS
possesses a wide electrochemical stability window, with no
reactions occurring between 0 and 5 V versus Li+/Li,53,56 some
computational studies predict potential instability when in
contact with both metallic lithium and cathode materials.56,57
The discrepancy between theory and experiments might indicate the presence of large overpotentials at the interfaces. In
general, reactions between the electrode materials and the solid
electrolyte are still poorly understood, and the formation of
interfacial layers causing such overpotential cannot be
excluded.56,57 Recently, Kanno's research group58 developed
a further superionic conductor (i.e., Li9.54Si1.74P1.44S11.7Cl0.3)
which exhibits even higher conductivity up to 25 mS cm1 at RT.
Such a high value is two times higher than that of Li10GeP2S12.
The authors attributed the high ionic conduction characteristics of this novel superionic conductor to the 3D structural
pathway.
A further interesting ISE is the LISICON-type crystalline
phase consisting of a Li2O–Al2O3–SiO2–P2O5–TiO2–GeO2
composite (glass-ceramic), having a RT lithium ion conductivity
This journal is © The Royal Society of Chemistry 2016
of about 103 to 104 S cm1, which was developed and
patented by Ohara Inc. in 1997.59 Being rather stable in moist air
makes it very appealing; however, its practical application in the
battery eld is still hindered by its poor chemical stability
against metallic lithium and, in part, high costs. A successful
approach to overcome the poor compatibility of ISEs with
lithium metal is the introduction of an additional interfacial
layer of lithium phosphorous oxynitride, Li2PO2N, commonly
known by LIPON.16,60 This popular glass-ceramic ISE, one of the
most explored in the last few years,13 in fact, shows negligible
reactivity with lithium metal. Furthermore, it possesses
a considerably high shear modulus (7.3 times higher than Li),
which is a fundamental requirement for suppressing dendrite
growth.61 Unfortunately, though, it shows a very low conductivity (i.e., in the order of 106 S cm1 at room temperature). For
this reason, its application is still limited to thin-layer solidstate lithium batteries (a few microns thick).60
Among glassy electrolytes, suldes offer high lithium ion
conductivity at room temperature (103 to 105 S cm1),13,62,63
due to the large ionic radius and high polarizability of sulfur
ions compared to oxide ions. Li2S–P2S5 is, probably, the most
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Fig. 4 Framework structure of (a) a generic garnet-like lithium ISE (M ¼ Ta, Nb, Ba or Zr) and (b) the recently reported superionic conductor
Li10GeP2S12 (re-drawn and re-adapted from the original manuscript of Kamaya et al.53). (c) Schematic description of the differences between
glassy and crystalline phases in ISEs. (d) Effect of IL grafting on the ionic conduction properties of the LiZnSO4F ceramic electrolyte (re-drawn and
re-adapted from the original manuscript of Barpanda et al.64). (e) Influence of electrolytes on the establishment of the electrode/electrolyte
interfaces. Differences between liquid (left panel) and solid electrolytes (right panel) are highlighted. (f) Schematic cross-section of an all-solidstate lithium metal battery showing the importance of creating percolative pathways for ions and electrons through the whole composite
electrode depth, in order to allow full utilization of the active materials.
famous example of sulde ISEs. Li2S–P2S5 glasses with high Li+
concentration have shown encouraging conductivities of over
104 S cm1.62 This typology of suldes can be produced also as
glass-ceramic electrolytes (crystallized glasses). Mechanical
milling of Li2S–P2S5 can lead to the precipitation of superionic
crystals with a structure similar to those of thio-LISICON phases.49 It is known that crystallization usually decreases the
conductivity. Glasses generally have, indeed, higher conductivity than the corresponding crystalline phase due to isotropic
17256 | J. Mater. Chem. A, 2016, 4, 17251–17259
ionic conduction and their so-called “open structure” (Fig. 4c).
However, glass-ceramic compounds are constituted by crystalline domains surrounded by an amorphous phase, resulting in
a considerably reduced grain-boundary resistance compared to
polycrystalline systems. Despite such encouraging results, the
large resistance at grain-boundaries still represents the most
serious issue hindering the development of all-solid-state
batteries. In fact, it has been proven that there are classes of
ISEs which can provide sufficient ionic conduction (bulk
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conductivity) for practical application. However, the inuence
of grain-boundaries can decrease the total conductivity by a few
orders of magnitude.49,62 Interestingly, Tarascon's group has
recently proposed a strategy which might solve such an issue.64
This involves the graing of ILs on lithium-containing ISE
particles leading to an improvement of up to 6 orders of
magnitude in conductivity, for example, in ceramic LiZnSO4F
(see Fig. 4d). It is not yet clear whether such an improvement is
due to the reduction of grain-boundary resistance or, simply, to
the IL layer providing a better percolation path for the ions.
Nevertheless, such pioneering work is surely opening up new
opportunities for the application of ISEs in batteries.
So far, great efforts have been made to increase the
conductivity and compatibility of ISEs with lithium. The above
mentioned examples prove that materials suited for all-solidstate lithium metal batteries are indeed available, and their
performance is improving day-by-day. However, further technological advances are needed to introduce ISEs into bulk-type
batteries (which are of more practical interest than thin-lm
devices). Contrarily to conventional LIBs, where the liquid
electrolyte easily penetrates into the electrode, the solid electrolyte, having no uidity, does not form continuous but pointto-point contacts with the active material (see Fig. 4e).47 As
charge transfer only occurs at the contact points, establishing
an extended electrode–electrolyte interface is necessary to allow
full utilization of the active material. Furthermore, it would be
benecial to ensure an even current distribution in the electrode. In fact, avoiding spots with large current density is
essential to suppress the formation of Li dendrites, which could
grow in the voids and propagate along the grain boundaries of
the ISE.65
In order to increase the contact area, the solid electrolyte
needs to percolate throughout the whole electrode depth (see
Fig. 4f).
Several strategies are under development to achieve this
goal, such as preparation of composites by ball milling, surface
coating of the active material and impregnation with supercooled liquid glass.62 Of course, the strategy to apply is highly
dependent on the type of ISE used. Although these methods
work with glassy materials, they might not be suitable for the
processing of crystalline electrolytes.
4. Conclusions
Lithium metal batteries, abandoned for about 40 years because
of safety issues associated with uncontrolled and hazardous
growth of dendrites upon cycling, are recently seeing a renewed
interest.66 They may indeed offer superior electrochemical
performance with respect to conventional LIBs, when used in
important applications aimed to improve the quality of our life,
such as efficient energy use and sustainable road transportation. Undoubtedly, the development of solid-state lithium
metal batteries (SSLMBs) is the most promising way to obtain
power sources combining high energy with intrinsic safety.
Unfortunately, the appropriate way to reach this goal is still
practically prevented by a series of issues, the main being the
unavailability of a solid electrolyte capable of combining good
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Journal of Materials Chemistry A
compatibility with lithium metal as well as high ionic conductivity at medium-low temperature.
As discussed in this review paper, in this respect there are
two classes of materials under consideration, one involving
polymer-based systems and the other inorganic solid
compounds. The former is possibly more promising since it has
been shown that additives such as ionic liquids and/or ceramic
llers may lead to improvements in the transport properties.
Indeed, lithium polymer batteries are currently commercially
produced to be used as power sources for electric vehicles. An
increase in conductivity can also be obtained by adding liquid
plasticizers, but this approach is not totally acceptable since it is
accompanied by a decay of the solid-state conguration, as well
as by an increase of reactivity with the lithium metal anode.
A real breakthrough in the eld may be enabled by inorganic
solid electrolytes (ISEs). These benet from a series of relevant
properties, such as non-ammability, robustness (preventing
lithium dendrite growth) and a single-ion conduction mechanism (greatly mitigating the problem of concentration polarization under high drain), which make them the most promising
electrolyte category for the next generation of lithium metal
batteries. However, the main problems of ISEs, i.e., the low ionic
conductivity resulting in a high ohmic polarization during cell
operation, and the poor interconnection of the solid to solid
electrode–electrolyte interface, which does not guarantee the
full utilization of the electrode active materials, are not yet fully
solved, still hindering their practical use. To achieve high
performance, nano-structured electrode/electrolyte interfaces
need to be used. This would help to overcome diffusion problems and provide sufficiently good contacts to all particles.
Keeping a certain degree of exibility in the cell would also be
very appealing for applications. From this point of view, polymer-based electrolytes may be a more reasonable choice.
Overall, the interest in SSLMBs is continuously increasing
and, according to the latest research records, many industrial
and academic laboratories are devoting large efforts to address
these issues. In this regard, different strategies for reaching this
goal are employed. In the case of glassy ISEs, for example, one
valid method to increase the conductivity is represented by the
electrolyte crystallization to form glass-ceramic compounds.
Clever engineering approaches, including thin lm construction and/or design of electrode and electrolyte powder mixtures,
may help to overcome the interfacial issue. Furthermore, the
low intrinsic electronic transport can be tackled by adding
a conductive component in the electrode conguration. Indeed,
some examples of practical SSLMBs are already available.8,26 The
academic and, especially, industrial interest in developing
solid-state batteries is increasing at such a fast rate that it is
reasonable to forecast important breakthroughs in the near
future.
Acknowledgements
A. V., R. R. S. P. and B. S. acknowledge the nancial support of
Helmholtz Institute Ulm (HIU) and the Karlsruhe Institute of
Technology (KIT).
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Author's personal copy
Journal of Power Sources 195 (2010) 327–334
Contents lists available at ScienceDirect
Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
Interface-mediated electrochemical effects in lithium/polymer-ceramic cells
Jitendra Kumar a,∗ , Stanley J. Rodrigues b , Binod Kumar a
a
b
Electrochemical Power Group, Metals & Ceramics Division, University of Dayton Research Institute, Dayton, Ohio 45469-0171, USA
Air Force Research Laboratory, Propulsion Directorate, WPAFB, Ohio 45433, USA
a r t i c l e
i n f o
Article history:
Received 15 June 2009
Received in revised form 29 June 2009
Accepted 30 June 2009
Available online 8 July 2009
Keywords:
Polymer-ceramic composite
Ionic transport
Charge-transfer reaction
Solid electrolyte interface
a b s t r a c t
The paper presents and discusses a method to achieve beneficial electrochemical effects mediated by
interfaces in an ionic conducting polymer matrix. The beneficial effects include enhanced ionic transport, catalysis of anodic oxidation reaction, and stabilization of the lithium-electrolyte interface in
lithium-based electrochemical cells. Polyethylene oxide (PEO) doped with LiN(SO2 CF2 CF3 )2 (LiBETI) was
chosen as the ion conducting polymer matrix. The polymer-ceramic (PC) composite electrolytes from the
PEO:LiBETI-BN and PEO:LiBETI-Li2 O systems were optimized to achieve high conductivity, reduce chargetransfer resistance, and stabilize the solid electrolyte interface (SEI) layer at the lithium anode. Both BN and
Li2 O were effective in enhancing interface-mediated lithium ion transport. The charge-transfer resistance
was reduced by orders of magnitude and the long-term stability of the cells was improved remarkably due
to the addition of BN and Li2 O in the PEO:LiBETI polymer matrix. AC impedance spectroscopy was used
to investigate the phenomenon by measuring the time- and temperature-dependent electrical behavior
of the aforementioned materials and cells. The interface-mediated effects due to the addition of BN and
Li2 O dielectrics contributed to the improved cell properties.
Published by Elsevier B.V.
1. Introduction
The term interface-mediated refers to an accumulation of local,
uncompensated charges in bulk solids. The charges may result from
the ionization and adsorption of species at interfaces and/or on
dielectric surfaces. These charges may have a major influence on
the electrical properties of solids and the electrochemical devices
fabricated from those solids.
The first demonstration of the interface-mediated ionic transport may be traced to the work of Liang [1]. In the pioneering paper,
Liang [1] reported that lithium iodide (LiI) doped with 35–45 mol%
of aluminum oxide enhanced conductivity by almost a factor of
50 at 25 ◦ C. However, the amount of aluminum oxide determined
to be soluble in LiI was insignificant. Subsequently, a number of
investigators have reported enhanced conductivity of silver in the
AgI–Al2 O3 system [2], copper in the CuCl–Al2 O3 system [3], fluorine in the PbF2 –SiO2 and PbF2 –Al2 O3 systems [4] and lithium in
polymer-ceramic composite electrolytes [5]. Three review papers
[6–8] also document the developmental history and general properties of theses heterogeneous ionic conductors.
The oxidation of lithium at the lithium electrode and the transport of lithium ions through the electrolyte are the fundamental
electrochemical processes that take place in lithium-based electro-
∗ Corresponding author. Tel.: +1 937 229 5314; fax: +1 937 229 3433.
E-mail address: kumarjit@notes.udayton.edu (J. Kumar).
0378-7753/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.jpowsour.2009.06.098
chemical cells. The performance of these cells primarily depends
upon the kinetics of the two processes. The slowest process determines the performance of the cells. With a recent surge in interest in
lithium-based electrochemical cells, it has become imperative that
materials be developed to enhance the kinetics of both processes.
Composite membranes are heterogeneous solid ionic conductors in which a dielectric phase is dispersed by design. Prior
publications [9–11] have reported the effects of the dielectric phase
on the ionic conductivity in terms of the space charge and blocking effects in the composite membranes. The ionic conductivity of
the composite membranes is of profound interest to chemists and
engineers because of its application in commercial electrochemical
devices such as batteries, fuel cells, electrolyzers, electrosynthesizers, and sensors. The requirements of these electrochemical devices
continue to evolve, and it is believed that the next generation of
these devices will require electrolytes with a much higher conductivity in a wider temperature range.
The power sources based on lithium chemistry have been of
considerable interest due to their high energy and power densities. The lithium–oxygen/air cell is perhaps the ultimate power
source among the cells derived from lithium chemistry. However, its
development has been impeded by the lack of suitable membranes.
This paper will present and discuss some of the potential membrane materials for the lithium-composite solid electrolyte cells.
The potential of these materials lies in the high ionic conductivity
and improved stability of lithium-electrolyte interface of all solid
electrochemical cells.
Author's personal copy
328
J. Kumar et al. / Journal of Power Sources 195 (2010) 327–334
2. Experimental
2.1. Processing of polymer-ceramic membranes
As-received poly(ethylene)oxide (PEO) (M.W. 2,000,000 Union
Carbide) and LiN(SO2 CF2 CF3 )2 (3 M), hereafter called LiBETI, were
used as solvent and solute, respectively, to prepare a polymer
electrolyte. The PEO and LiBETI were dried in an oven at 50 and
60 ◦ C, respectively, for 48 h. The PEO:LiBETI (8.5:1) electrolyte was
prepared by an energy milling technique which is a solvent-free
process. The chemicals were weighed inside a glove box maintained
at
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