230
J. Zhu et al.
promote the generation of Li 2 CO 3 . However, they discovered unexpectedly in the
research process that high dielectric constant electrolyte such as dimethyl sulfoxide
(DMSO) can make Li 2 CO 3 reversibly react. This important discovery means that
this kind of electrolyte can stabilize the cycle performance of the battery and reduce
the influence of CO 2 in the air on the battery, because the formation of Li 2 CO 3 in
the lithium-air battery is inevitable.
(2) Electrolyte for lithium-air battery.
When discharging the positive electrode of the water-soluble electrolyte type lithiumair battery, O 2 is reduced at the three-phase interface of the air electrode, and the
reduced product forms LiOH with Li
+ , which can prevent the air electrode from
blocking so as to reduce the overpotential of O 2 . However, it is difficult to completely
decompose and precipitate LiOH during charging, resulting in low energy density
and poor cycle performance of the battery. In addition, N 2 , CO 2 , H 2 O, etc. contained
in the air will produce a series of side reactions on the electrodes, affecting the cycle
performance of the battery. The metal lithium cathode of water-based lithium-air
battery needs to be covered with a protective layer [44] with Li
+ conductivity to
prevent self-discharge caused by reaction of metal lithium with H 2 O. However, the
limited service life of the protective layer is difficult to prevent the formation of
lithium dendrites during long-term charging and discharging of the battery.
In the nonaqueous solvent electrolyte, O 2 is reduced by dissolving in the electrolyte and then diffusing to the air electrode during discharge. Secondary lithiumair batteries mostly use nonaqueous solvent electrolyte. Unlike nonaqueous solvent
electrolyte for lithium ion batteries, nonaqueous solvent electrolyte for secondary
lithium-air batteries must be resistant to oxidation of active intermediate ions such
as O
2− , O 2
2− , etc. generated by reduction of air electrode O 2 . In addition, the ionic
conductivity and viscosity of electrolyte, the degree of dissolution of O 2 , the compatibility of electrolyte solvent polarity with porous carbon carrier materials, and the
partial solubility of discharge products Li 2 O 3 and Li 2 O have significant effects on
the performance of lithium-air batteries [45–48]. The ideal electrolyte for secondary
lithium-air batteries, first, should remain relatively stable in the process of battery
charging and discharging, and the active intermediate O
2− or O 2
2− which is not easy
to be reduced by oxygen is oxidized to form Li 2 CO 3 on the air electrode. Second,
the electrolyte should have large ionic conductivity and low viscosity to reduce the
internal resistance of the battery. Third, the solubility of the electrolyte to O 2 is larger,
and the diffusion rate of O 2 in the electrolyte is faster, thus reducing the concentration
polarization of the electrode. Fourthly, the electrolyte solvent has good wettability to
porous carbon materials and can reduce the mass transfer resistance in the O 2 reduction process. Fifth, the electrolyte has certain solubility to the discharge products
Li 2 O and Li 2 O can reduce the blockage of air electrode channels, thus improving
the air diffusivity and further prolonging the single discharge quantity. In addition,
the volatility and water absorption of electrolyte are also problems that cannot be
ignored in electrolyte selection. Low volatility and nonwater absorption electrolyte
J. Zhu et al.
promote the generation of Li 2 CO 3 . However, they discovered unexpectedly in the
research process that high dielectric constant electrolyte such as dimethyl sulfoxide
(DMSO) can make Li 2 CO 3 reversibly react. This important discovery means that
this kind of electrolyte can stabilize the cycle performance of the battery and reduce
the influence of CO 2 in the air on the battery, because the formation of Li 2 CO 3 in
the lithium-air battery is inevitable.
(2) Electrolyte for lithium-air battery.
When discharging the positive electrode of the water-soluble electrolyte type lithiumair battery, O 2 is reduced at the three-phase interface of the air electrode, and the
reduced product forms LiOH with Li
+ , which can prevent the air electrode from
blocking so as to reduce the overpotential of O 2 . However, it is difficult to completely
decompose and precipitate LiOH during charging, resulting in low energy density
and poor cycle performance of the battery. In addition, N 2 , CO 2 , H 2 O, etc. contained
in the air will produce a series of side reactions on the electrodes, affecting the cycle
performance of the battery. The metal lithium cathode of water-based lithium-air
battery needs to be covered with a protective layer [44] with Li
+ conductivity to
prevent self-discharge caused by reaction of metal lithium with H 2 O. However, the
limited service life of the protective layer is difficult to prevent the formation of
lithium dendrites during long-term charging and discharging of the battery.
In the nonaqueous solvent electrolyte, O 2 is reduced by dissolving in the electrolyte and then diffusing to the air electrode during discharge. Secondary lithiumair batteries mostly use nonaqueous solvent electrolyte. Unlike nonaqueous solvent
electrolyte for lithium ion batteries, nonaqueous solvent electrolyte for secondary
lithium-air batteries must be resistant to oxidation of active intermediate ions such
as O
2− , O 2
2− , etc. generated by reduction of air electrode O 2 . In addition, the ionic
conductivity and viscosity of electrolyte, the degree of dissolution of O 2 , the compatibility of electrolyte solvent polarity with porous carbon carrier materials, and the
partial solubility of discharge products Li 2 O 3 and Li 2 O have significant effects on
the performance of lithium-air batteries [45–48]. The ideal electrolyte for secondary
lithium-air batteries, first, should remain relatively stable in the process of battery
charging and discharging, and the active intermediate O
2− or O 2
2− which is not easy
to be reduced by oxygen is oxidized to form Li 2 CO 3 on the air electrode. Second,
the electrolyte should have large ionic conductivity and low viscosity to reduce the
internal resistance of the battery. Third, the solubility of the electrolyte to O 2 is larger,
and the diffusion rate of O 2 in the electrolyte is faster, thus reducing the concentration
polarization of the electrode. Fourthly, the electrolyte solvent has good wettability to
porous carbon materials and can reduce the mass transfer resistance in the O 2 reduction process. Fifth, the electrolyte has certain solubility to the discharge products
Li 2 O and Li 2 O can reduce the blockage of air electrode channels, thus improving
the air diffusivity and further prolonging the single discharge quantity. In addition,
the volatility and water absorption of electrolyte are also problems that cannot be
ignored in electrolyte selection. Low volatility and nonwater absorption electrolyte