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6 Nanomaterials for Batteries
which can increase the capacity and specific capacity of the battery (due to the
increased interface area), and the thin diaphragm also has lower impedance.
2. Permeability
The separator has almost no effect on electrochemical performance of the
batteries. For example, the presence of the separator can increase the resistance
of the electrolyte by 6–7 orders of magnitude but has almost no effect on performance of the batteries. The impedance coefficient generated by the electrolyte
flowing through the effective pores of the membrane is usually distinguished
from the impedance coefficient of the electrolyte, the former being known as
the MacMullin coefficient. In commercial batteries, the MacMullin coefficient
is generally 10–12.
3. Air permeability
For a given form of membrane material, its air permeability and resistance
are proportional. Lithium-ion battery separators should have good electrical
properties and low air permeability.
4. Pore volume rates
The porosity product rate and permeability are closely related, and the porosity
of lithium-ion battery separator is about 40%. For the lithium-ion battery,
controlling the porosity of separators is very important. The standard porosity
rate is an integral part of the diaphragm standard. High porosity and uniform
pore size distribution will not hinder the flow of ions. Inhomogeneous pore size
distribution will cause uneven current density and affects the activity of the
working electrode. Due to the inconsistent working load of some parts of the
electrode and other parts, eventually the damage of the battery core is faster.
5. Wettability
The separator should have fast, complete wetting characteristics in the battery
electrolyte.
6. Absorb and retain electrolyte
In lithium-ion batteries, the separator mechanically absorbs and retains the
electrolyte in the battery without causing swelling, because the absorption of
the electrolyte is the need for ion transport.
7. Chemical stability
The separators can exist stably in the battery, are chemically inert to strong
oxidation and strong reduction environments, do not degrade under the above
conditions, and do not lose mechanical strength. At temperatures up to 75 °C,
the separator should be able to withstand the oxidation of highly oxidizing
positive electrodes and corrosion of highly corrosive electrolytes. The stronger
the antioxidant capacity, the longer the life of the diaphragm in the battery.
Polyolefin-based separators (e.g., polypropylene, polyethylene, etc.) are resistant to most chemicals, have good mechanical properties, and can be used in the
medium temperature range. Polyolefin-based separators are the ideal choice for
commercial lithium-ion battery separators. In contrast, the polypropylene film
has better oxidation resistance when it is in contact with the cathode materials
of the lithium-ion batteries. Therefore, in the three-layer separator (PP/PE/PP),
6 Nanomaterials for Batteries
which can increase the capacity and specific capacity of the battery (due to the
increased interface area), and the thin diaphragm also has lower impedance.
2. Permeability
The separator has almost no effect on electrochemical performance of the
batteries. For example, the presence of the separator can increase the resistance
of the electrolyte by 6–7 orders of magnitude but has almost no effect on performance of the batteries. The impedance coefficient generated by the electrolyte
flowing through the effective pores of the membrane is usually distinguished
from the impedance coefficient of the electrolyte, the former being known as
the MacMullin coefficient. In commercial batteries, the MacMullin coefficient
is generally 10–12.
3. Air permeability
For a given form of membrane material, its air permeability and resistance
are proportional. Lithium-ion battery separators should have good electrical
properties and low air permeability.
4. Pore volume rates
The porosity product rate and permeability are closely related, and the porosity
of lithium-ion battery separator is about 40%. For the lithium-ion battery,
controlling the porosity of separators is very important. The standard porosity
rate is an integral part of the diaphragm standard. High porosity and uniform
pore size distribution will not hinder the flow of ions. Inhomogeneous pore size
distribution will cause uneven current density and affects the activity of the
working electrode. Due to the inconsistent working load of some parts of the
electrode and other parts, eventually the damage of the battery core is faster.
5. Wettability
The separator should have fast, complete wetting characteristics in the battery
electrolyte.
6. Absorb and retain electrolyte
In lithium-ion batteries, the separator mechanically absorbs and retains the
electrolyte in the battery without causing swelling, because the absorption of
the electrolyte is the need for ion transport.
7. Chemical stability
The separators can exist stably in the battery, are chemically inert to strong
oxidation and strong reduction environments, do not degrade under the above
conditions, and do not lose mechanical strength. At temperatures up to 75 °C,
the separator should be able to withstand the oxidation of highly oxidizing
positive electrodes and corrosion of highly corrosive electrolytes. The stronger
the antioxidant capacity, the longer the life of the diaphragm in the battery.
Polyolefin-based separators (e.g., polypropylene, polyethylene, etc.) are resistant to most chemicals, have good mechanical properties, and can be used in the
medium temperature range. Polyolefin-based separators are the ideal choice for
commercial lithium-ion battery separators. In contrast, the polypropylene film
has better oxidation resistance when it is in contact with the cathode materials
of the lithium-ion batteries. Therefore, in the three-layer separator (PP/PE/PP),