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Articles
https://doi.org/10.1038/s41560-019-0351-0
1Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology,
Cambridge, MA, USA. 2Beijing Advanced Innovation Center for Materials Genome Engineering, Key Laboratory for Renewable Energy, Beijing Key
Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy
of Sciences, Beijing, China. 3Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, China.
4Songshan Lake Materials Laboratory, Dongguan, Guangdong, China. 5Department of Electrical Engineering and Computer Science, Massachusetts
Institute of Technology, Cambridge, MA, USA. 6Advanced Materials Lab , Samsung Advanced Institute of Technology America, Burlington, MA, USA.
7Institute of New Energy for Vehicles, School of Materials Science and Engineering, Tongji University, Shanghai, China.
*e-mail: suoliumin@iphy.ac.cn; liju@mit.edu
Anion-redox lithium–sulfur (Li–S) is one of the most prom-
ising conversion battery chemistries with high theoreti-
cal cathode energy density of 2,600 Wh kg−1 based on the
weight of Li2S, S8 + 16e− + 16Li+ = 8Li2S, several times higher than
conventional lithium-ion battery (LIB) cathodes based on transi-
tion metal cation–redox intercalation reactions1,2. Unfortunately,
the actual full-cell energy densities are a far cry from the theoretical
values resulting from the excessive use of inactive components, such
as electrolyte and conductive carbon. The electronic insulating
nature of the S8 and Li2S phases (as compared to, say, LixCoO2, with
its high Co3+ ↔ Co4+ polaron mobility) means that for the anion-
redox reaction Sα− ↔ Sβ− + (α − β)e− to proceed (where 0≤ α,β ≤ 2
is the average sulfur valence reflecting a mix of ionic and covalent
bonding, often in the physical form of Sn2−, where α = 2/n), the S8
must physically dissolve into liquid electrolytes as Sn2− (electrolyte),
transform into Sm2− (electrolyte) and eventually redeposit some-
where else as solid phases.
This sulfur mobility is a key characteristic of many Li–S batteries,
which has profound consequences on battery processes and perfor-
mances. First, the dissolution of lithium polysulfide intermediates
(LiPS) in the liquid electrolyte does help the kinetics. Even though
LiPS is often written as Li2Sn, one needs to understand that its sol-
ubilized form is 2Li+ (electrolyte) and Sn2−(electrolyte), with indi-
vidual solvation shells. A problem brought by such sulfur mobility
is that Sn2−(electrolyte) may physically cross over the separator to
the Li metal anode in a non-blocking manner (so-called ‘shuttling’
of soluble redox mediators). This causes fast capacity fading on the
cathode side, even if we do not consider the ill effects this has on the
anode3. Second, to enhance the sluggish redox kinetics of S8 ↔ Li2S,
one needs a lot more liquid electrolyte and electrocatalytical sur-
face areas in the cathode, which can be a common carbon black
(which is neither a particularly good electrocatalyst, nor a good wet-
ting substrate), or something more tailor made. Although a high
fraction of conductive carbons (>30 wt%)4 is usually required for
sufficient sulfur use in conventional C/S8 cathodes, it is equally true
that such an excessive use (compared to LIB cathodes with 5 wt%
carbon) of high-specific-area carbons gives rise to high cathode
porosity (usually >70 vol%)5, which demands a considerable amount
of electrolyte to support a satisfactory ionic conductivity6. The theo-
retical prediction for the relationship between cathode porosity and
cathode-specific energy densities in Fig. 1 shows that the porosity
greatly affects both the cathode-specific gravimetric eg and volumet-
ric ev energy densities. In most studies, a high electrolyte to active
material ratio (E/AM ratio) >15 µl mg−1 (ref. 7)(~0.3 µl mg−1 for LIB
cathode) is employed8. In other words, the most impressive high
specific capacity based on just the S8 weight is attained with a large
excess of cathode porosity and electrolyte. In this case, the full-cell
gravimetric Eg and volumetric Ev energy densities drop to an unac-
ceptably low level. For example, with E/AM ratio >15 µl mg−1, the
full-cell Eg could not be higher than 175 Wh kg−1 even if reaching
perfect sulfur utilization of 1,675 mAh g−1 (Supplementary Fig. 1).
Another key challenge, becoming increasingly known9,10 to make
Li–S batteries less interesting for important markets, is the low Ev
that is a crucial factor for many applications. The Ev evaluated from
Intercalation-conversion hybrid cathodes enabling
Li–S full-cell architectures with jointly superior
gravimetric and volumetric energy densities
WeijiangXue 1, ZheShi1, LiuminSuo 1,2,3,4*, ChaoWang1, ZiqiangWang1, HaozheWang 5,
KangPyoSo1, AndreaMaurano 6, DaiweiYu5, YumingChen1, LongQie1,7, ZhiZhu1, GuiyinXu 1,
JingKong5 and JuLi 1*
A common practise in the research of Li–S batteries is to use high electrode porosity and excessive electrolytes to boost sulfur-
specific capacity. Here we propose a class of dense intercalation-conversion hybrid cathodes by combining intercalation-type
Mo6S8 with conversion-type sulfur to realize a Li–S full cell. The mechanically hard Mo6S8 with fast Li-ion transport ability, high
electronic conductivity, active capacity contribution and high affinity for lithium polysulfides is shown to be an ideal backbone
to immobilize the sulfur species and unlock their high gravimetric capacity. Cycling stability and rate capability are reported
under realistic conditions of low carbon content (~10 wt%), low electrolyte/active material ratio (~1.2 µl mg−1), low cathode
porosity (~55 vol%) and high mass loading (>10 mg cm−2). A pouch cell assembled based on the hybrid cathode and a 2× excess
Li metal anode is able to simultaneously deliver a gravimetric energy density of 366 Wh kg−1 and a volumetric energy density
of 581 Wh l−1.
NATURE ENERGY | VOL 4 | MAY 2019 | 374–382 | www.nature.com/natureenergy
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