The safety problems of lithium ion
batteries (LIBs) have been the
main obstacles that hinder their broad applications in portable electronic
devices, electric vehicles, and energy storage. Such problems originate
from flammable solvent-containing liquid electrolytes that could be
easily oxidized upon excessive heat, leading to further heat accumulation
and, subsequently, thermal runaway. The design strategies of a safe
electrolyte could control the flammability and volatility of the liquid
electrolyte, might prevent the thermal runaway, and ultimately ensure
the risk-free and fire-free operation of LIBs. This work is to explore
the mechanism of thermal runaway and review the state-of-the-art of
the designs of a safe electrolyte for LIBs, including the additions
of flame retardant additives, overcharge additives, and stable lithium
salts and the adoption of solid-state electrolytes, ionic liquid electrolytes,
and thermosensitive electrolytes. The features, advantages, and drawbacks
of these strategies are systematically summarized, compared, and discussed,
while the development direction of a safer electrolyte for future
LIBs is proposed in the end.
Developing
safe and high-energy-density lithium metal batteries
(LMBs) is considered to be the focus of next-generation rechargeable
batteries. However, the inevitable lithium reaction with the liquid
electrolyte and the subsequent formation of Li dendrites must be overcome,
and upgrading traditional liquid electrolytes is a key strategy for
achieving this goal. Here, we report a nano-SiO2-supported
gel polymer electrolyte (SiO2-GPE) with a hierarchical
structure fabricated via in situ gelation of a traditional organic
liquid electrolyte supported on a functionally modified SiO2 layer, which displayed high ionic conductivity (1.98 × 10–3 S cm–1 at 25 °C) and wide
electrochemical window (>4.9 V vs Li/Li+). The LiFePO4/SiO2-GPE/Li cells exhibited a high capacity of
125.5 mAh g–1 at 1 C with capacity retention of
88.42% after 700 cycles. The superior electrochemical performance
is mainly due to the highly compatible electrode/electrolyte interface
and the effective inhibition of Li dendrite growth provided by the
synergistic effects of this SiO2-GPE membrane.
Decay in electrochemical performance resulting from the “shuttle effect” of dissolved lithium polysulfides is one of the biggest obstacles for the realization of practical applications of lithium–sulfur (Li–S) batteries. To meet this challenge, a 2D g‐C3N4/graphene sheet composite (g‐C3N4/GS) was fabricated as an interlayer for a sulfur/carbon (S/KB) cathode. It forms a laminated structure of channels to trap polysulfides by physical and chemical interactions. The thin g‐C3N4/GS interlayer significantly suppresses diffusion of the dissolved polysulfide species (Li2Sx; 2
A novel strategy has been proposed
to produce in situ Li2S at the interfacial layer between
lithium anode and the solid electrolyte,
by using an amorphous-sulfide–LiTFSI–poly(vinylidene
difluoride) (PVDF) composite solid electrolyte (SLCSE). Besides retarding
the decomposition of PVDF in CSE, the Li2S-modified interfacial
layer (SMIL) also improves the wettability between lithium metal and
SLCSE which in turn optimizes the lithium deposition process. Our
density functional theory calculation results reveal that the migration
energy barrier of Li passing through SMIL is much lower than that
of Li passing through LiF-modified interfacial layer (FMIL) formed
from the decomposition of PVDF. The as-prepared SLCSE shows a Li ionic
transference number of 0.44 and Li ion conductivity of 3.42 ×
10–4 S/cm at room temperature, and the Li||SLCSE||LiFePO4 cell exhibits an outstanding rate performance with a capacity
of 153, 144, 131, and 101 mAh/g at a current density of 0.05, 0.10,
0.25, and 0.50 mA/cm2, respectively.
Lithium−sulfur batteries are considered as the most promising candidate for nextgeneration energy storage devices. However, they are subjected to the "shuttle effect" of soluble lithium polysulfides (LiPSs). Herein, a free-standing membrane composed of two-dimensional MXene material (Ti 3 C 2 T x ) and graphene oxide (GO) is synthesized by a simple vacuum-filtration method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy are carried out to determine structure, morphology, and composition of the Ti 3 C 2 T x /GO composite membrane, respectively. As a functional layer of trapping LiPS species, the Ti 3 C 2 T x /GO composite membrane and commercial polypropylene (PP) are successfully assembled to be a hybrid separator, Ti 3 C 2 T x /GO@ PP, to suppress the shuttle effect of LiPSs. The porous and rough surface of the Ti 3 C 2 T x /GO composite membrane is beneficial to improve the wettability of the commercial separator in an etherbased electrolyte. The cells with the Ti 3 C 2 T x /GO@PP hybrid separator exhibit a low polarization potential of 0.26 V in the conversion from Li 2 S 4 to Li 2 S 2 /Li 2 S and deliver a discharge capacity of 640.0 mA h g −1 for 5 C rate, indicating that the hybrid separator benefits the rate performance. According to the results of electrochemical impedance spectroscopy, increased discharge capacity is attributed to the reduced internal resistance and intensified Li + diffusion. The results of X-ray photoelectron spectroscopy focusing on the surfaces of both sides of the hybrid separator indicate that the shuttle effect of LiPSs is suppressed through a coefficient of the terminated groups' catalytic conversion on long-chain LiPSs and the titanium-reactive centers' Lewis acid−base pairs on short-chain LiPSs. Combining with digital photographs of the H-type electrolytic cell, the results of UV−visible absorption spectroscopy suggest that the concentration of long-chain polysulfides declines instantly under the redox effect of the terminated groups on Ti 3 C 2 T x surfaces and then infiltrate through the hybrid separator by virtue of concentration difference impetus. Generally, a Ti 3 C 2 T x /GO@PP hybrid separator restrains LiPS diffusion and improves the rate performance of Li−S batteries.
Despite a high energy density and specific capacity, the commercial implementation of lithium‐sulfur batteries still suffers from a severe polysulfide shuttle. Numerous efforts have been made to confine polysulfide species through physical adsorption and chemical bonding. Nevertheless, polysulfide accumulation is also ascribed to the slow redox kinetics. Herein, we design a kind of MoO3‐x nanobelt with abundant engineered oxygen defects (ODs) on the surface to promote the redox kinetics as a cathode matrix for Li−S batteries. On one hand, engineered ODs exhibit considerable electrocatalytic activity for the conversion of polysulfide in kinetic processes, achieving distinctly improved capacities at large current densities. On the other hand, they enhance the interaction between MoO3 nanobelts and polysulfide molecules from a thermodynamic perspective, leading to an ameliorative cycling stability. This implementation of ODs in Li−S batteries substantially improves electrochemical performances and provides a novel method to introduce engineered defects into matrices for Li−S batteries.
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