6.5 Lithium-Sulfur Battery
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6.5.5.2 Lithium Metal Anode Modification
Researchers have developed different methods to inhibit the growth of lithium
dendrites. In addition to developing different electrolyte solvents, salts, and additives to form a stable SEI with lithium metal, many novel methods have recently
been developed.
Manual SEI
SEI is generated by the reaction of lithium metal and electrolyte (including solvents,
salts, additives, etc.), and its chemical composition is complex and unstable. Active
material and the electrolyte are continuously consumed during cycling because of the
SEI damage which caused low Coulombic efficiency and cycle life. Using in situ or
ex situ methods can generate artificial SEI as a protective layer, thereby protecting the
lithium metal anode and inhibiting the growth of lithium dendritic crystals. The ideal
artificial SEI should have the following properties: (1) Chemically stable during the
cycle; (2) It has excellent ion conductivity, which provides a channel for lithium-ion
diffusion; (3) Electronic insulation, allows lithium ions to diffuse through the SEI
and deposit on the lithium metal surface rather than the SEI surface; (4) Sufficient
mechanical strength to inhibit lithium dendrite piercing (Fan et al. 2017).
Nano-Passivation Protection Layer
The nanometer-scale passivation layer can also effectively inhibit the growth of
lithium dendrites other than artificial SEI, including hollow nanospheres, twodimensional boron nitride, and graphene materials. These materials can facilitate the
formation of stable SEI and the deposition of metallic lithium, thereby improving the
Coulombic efficiency and cycling stability of the battery. The protection of lithium
metal anodes is particularly important in Lithium-sulfur batteries. Due to the shuttling effect of lithium polysulfide, the sulfur that diffuses to the lithium metal anode
reacts with lithium metal to produce solid lithium-sulfur compounds, which results
in a decrease in battery capacity. However, using hexamethylditin and trimethylsilyl
chloride as passivation protective films for the metal lithium negative electrode can
effectively prevent polysulfide from reacting with the lithium metal and increase the
cycling performance of lithium-sulfur batteries (Wu et al. 2016a,b).
Homogenization of Lithium-Ion Current
The uneven lithium-ion spatial distribution on the surface of the electrode also directly
leads to the production of lithium dendrites, so homogenizing the lithium-ion flow
has important significance for inhibiting the growth of lithium dendrites. Dispersing
the current density by increasing the surface area of electrode is a valid method of
homogenizing the flow of lithium ions (Lin et al. 2017). For example, a 3D porous
copper mesh current collector/metal lithium composite negative electrode prepared
by a mechanical pressing method is used. The 3D porous copper current collector
has a large surface area that can homogenize lithium-ion flow, reduce local current
density, and increase electrochemical reaction activity. In addition, 3D porous copper
mesh can also meet the volumetric expansion of metallic lithium (Li et al. 2017). The
183
6.5.5.2 Lithium Metal Anode Modification
Researchers have developed different methods to inhibit the growth of lithium
dendrites. In addition to developing different electrolyte solvents, salts, and additives to form a stable SEI with lithium metal, many novel methods have recently
been developed.
Manual SEI
SEI is generated by the reaction of lithium metal and electrolyte (including solvents,
salts, additives, etc.), and its chemical composition is complex and unstable. Active
material and the electrolyte are continuously consumed during cycling because of the
SEI damage which caused low Coulombic efficiency and cycle life. Using in situ or
ex situ methods can generate artificial SEI as a protective layer, thereby protecting the
lithium metal anode and inhibiting the growth of lithium dendritic crystals. The ideal
artificial SEI should have the following properties: (1) Chemically stable during the
cycle; (2) It has excellent ion conductivity, which provides a channel for lithium-ion
diffusion; (3) Electronic insulation, allows lithium ions to diffuse through the SEI
and deposit on the lithium metal surface rather than the SEI surface; (4) Sufficient
mechanical strength to inhibit lithium dendrite piercing (Fan et al. 2017).
Nano-Passivation Protection Layer
The nanometer-scale passivation layer can also effectively inhibit the growth of
lithium dendrites other than artificial SEI, including hollow nanospheres, twodimensional boron nitride, and graphene materials. These materials can facilitate the
formation of stable SEI and the deposition of metallic lithium, thereby improving the
Coulombic efficiency and cycling stability of the battery. The protection of lithium
metal anodes is particularly important in Lithium-sulfur batteries. Due to the shuttling effect of lithium polysulfide, the sulfur that diffuses to the lithium metal anode
reacts with lithium metal to produce solid lithium-sulfur compounds, which results
in a decrease in battery capacity. However, using hexamethylditin and trimethylsilyl
chloride as passivation protective films for the metal lithium negative electrode can
effectively prevent polysulfide from reacting with the lithium metal and increase the
cycling performance of lithium-sulfur batteries (Wu et al. 2016a,b).
Homogenization of Lithium-Ion Current
The uneven lithium-ion spatial distribution on the surface of the electrode also directly
leads to the production of lithium dendrites, so homogenizing the lithium-ion flow
has important significance for inhibiting the growth of lithium dendrites. Dispersing
the current density by increasing the surface area of electrode is a valid method of
homogenizing the flow of lithium ions (Lin et al. 2017). For example, a 3D porous
copper mesh current collector/metal lithium composite negative electrode prepared
by a mechanical pressing method is used. The 3D porous copper current collector
has a large surface area that can homogenize lithium-ion flow, reduce local current
density, and increase electrochemical reaction activity. In addition, 3D porous copper
mesh can also meet the volumetric expansion of metallic lithium (Li et al. 2017). The