The Catalyst Review September 2019 - 14

EXPERIMENTAL
Enhanced Lithium-Ion Conductivity of Polymer Electrolytes by Selective Introduction of Hydrogen
into the Anion...
Conventional lithium-ion battery technology has reached the threshold of its theoretical energy density (ca. 300 Whkg-1), a value
insufficient to meet the requirements of next-generation batteries (>400 Whkg-1). However, rechargeable Li metal (Li0) batteries
(LMB) are emerging as promising alternatives; although, the formation of dendritic lithium and the low Coulombic efficiency during
repeated plating/stripping cycles is detrimental to the safety and energy density aspects of these materials. Current interest centers
on the use of solid polymer electrolytes (SPEs) in an all-solid-state assembly of lithium metal batteries (ASSLMB) due to ease of
processing, cost-effectiveness, excellent flexibility, lightweight, and the absence of highly flammable carbonate solvents. Herein, the
authors describe a new strategy for boosting the Li-ion conductivity of SPEs via the introduction of protons able to form hydrogen
bonds into the anionic structure of the lithium salt (Figure 1a). Figure 1b shows that the dissociation of Li+ cations from sulfonimide
anions becomes more difficult with the increasing number of H atoms in the anion. Meanwhile, the adverse effect that the
introduction of H atoms has on the anodic stability of the sulfonimide anions is clearly evident in Figure 1c.
The authors went on to find that
ionic conductivity decreases with
increasing number of H atoms in
the anions: LiTFSI/PEO > LiDFTFSI/
PEO > LiMTFSI/PEO LiMSI/ PEO
(PEO= polyethylene oxide (Figure
2) - ascribed to the increase of
dissociation energy as pointed
out by the DFT calculations and
the lowered ion mobility, which
is a result of the Lewis acid-base
interactions between the anion and
the PEO.

Figure 1. a) Chemical structure of the studied lithium
sulfonimide salts. b) Calculated dissociation energy
(∆Ed) and c) oxidation potential (Eox) vs. the anionic
structure of the lithium salts.

Figure 2. a) Arrhenius plots of the ionic conductivity for
the LiX/PEO electrolytes (X=TFSI, DFTFSI, MTFSI, and
MSI). Note that the ionic conductivities of LiMSI/PEO are
raised by a factor of 100. b) Total and Li-ion conductivity
(left y axis) and calculated dissociation energy (right y
axis) vs. type of lithium salt for the LiX/PEO electrolytes
(X=TFSI, DFTFSI, MTFSI and MSI). TLi+ of the LiMSI/PEO
electrolyte is assumed as 1.0.

Partial substitution of F atoms in
TFSI-with H atoms can effectively
enhance the Li-ion conductivity
at a low expense of the total ionic
conductivity. DFT calculations
combined with experimental
results suggest that the anodic
stability of such H-containing
anions is lower than that of TFSI-, but they are still electrochemically stable enough,
considering the different voltage window of each kind of ASSLMB. The results indicate
that a tailored anionic structure with consideration of the nature of the polymer
matrices and electrodes should certainly boost the properties of SPEs and improve
the electrochemical performance of ASSLMBs in the future. Source: Zhang H, Oteo U,
Zhu H, et al. (2019). Angew. Chem. Int. Ed., 58: 7829-7834.
Dynamic Frustrated Lewis Pairs on Ceria for Direct Nonoxidative Coupling of Methane...
Frustrated Lewis pairs (FLPs), sterically encumbered Lewis acid and Lewis base combinations, have found increasing applications
in the field of homogeneous catalysis. However, the problematic and costly recycling of these materials hinders the large-scale
application of soluble FLPs. For this reason, considerable attention has been directed at developing heterogeneous FLP catalysts,
which can be classified into two categories: semisolid FLPs and all-solid FLPs. Herein, the authors describe the results of their
investigation of the formation rules and dynamic behaviors of solid FLPs on CeO2(110) surfaces using combined static DFT and
AIMD calculations (Figure 1). Building upon this base, they went on to study their stability and dynamic behaviors under reaction
conditions, and then explored the methane activation and the C−C coupling behaviors of solid FLPs under nonoxidative conditions
(Figure 2).
The main conclusions arising from this work are: (1) The formation of stable FLPs on CeO2(110) is dependent on the number of
surface oxygen vacancies (Vos). The FLPs constructed by three or more oxygen vacancies are naturally stable in thermodynamics.
14

The Catalyst Review

September 2019

The Catalyst Review September 2019

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