Ni-MH batteries

What types of Ni-MH batteries electrolytes are there?

A. Liquid electrolyte

The selection of solvents is mainly based on three aspects of property requirements, namely dielectric constant, viscosity and the electron donor properties of the solvent. Generally speaking, a high dielectric constant is conducive to the dissociation of lithium salts, and a strong electron donor ability will be conducive to the dissolution of electrolyte salts. The so-called electron donor property of the solvent is the inherent electron loss ability of the solvent molecule, and its ability determines the solvation ability of the electrolyte cation. Low viscosity can increase the mobility of ions and help improve conductivity.

At present, binary and multi-component mixed solvents composed of two or more solvents are usually used. Common organic solvents include ether, alkyl carbonate, lactone, ketal, etc.

Lithium salts are mainly used to supply effective carriers. The following principles are generally followed when selecting lithium salts:

Good stability (compatibility) with positive and negative electrode materials, that is, during storage, the electrochemical reaction rate of the electrolyte and the active material interface is low, so that the self-discharge capacity loss of the battery is minimized; high specific conductivity, small ohmic voltage drop of the solution; high safety performance, non-toxic and pollution-free.

Commonly used lithium salts are as follows: lithium hexafluoroarsenate (LiPF6), LAsF6 releases toxic arsenide during charging and discharging, and is relatively expensive. Lithium hexafluorophosphate (LiPF6), which has been widely used in commercial batteries, has high conductivity and good compatibility with carbon materials. The disadvantage is that it is relatively expensive, has poor stability in solid state, and is very sensitive to water. Lithium trifluoromethanesulfonate LiCF3SO2 has good stability, but its conductivity is only half of that of liquid electrolyte based on LiPF6. Lithium tetrafluoroborate (LibF4) and lithium perchlorate (LiCl04) are widely used salts. However, lithium perchlorate-containing imide lithium salts, typically lithium bis(fluorosulfonyl)imide (LiN(CF3SO2)2, have conductivity comparable to that of very dry LiPF6 electrolytes and are more stable than FLiCF3SO2.

b. Solid electrolytes

Solid electrolytes, also known as superionic conductors or fast ionic conductors, refer to a class of solid ion conductive materials whose ion conductivity is close to (or in some cases exceeds) that of molten and electrolyte solutions. It is a unique solid material between solid and liquid, an abnormal state of matter, in which some atoms (ions) have a mobility close to that of liquids, while other atoms maintain their spatial structure (arrangement). This liquid-solid dual phase, as well as its broad application prospects in various fields such as energy (including generation, storage and energy saving), metallurgy, environmental protection, and electrochemical devices, have attracted widespread attention from physicists, chemists, and materials scientists.

Polymer solid electrolytes are solid electrolyte materials formed by the complexation of polymers containing solvatable polar groups with salts. In addition to showing the properties of common conductive systems such as semiconductors and ionic solutions, it also has plasticity that inorganic solid electrolytes cannot match. This feature makes polymer solid electrolytes show three advantages in use:

Thin films of any shape and thickness. Therefore, although the room temperature conductivity of polymer electrolytes is not high, 2 to 3 orders of magnitude lower than that of inorganic ones, the internal resistance of the battery is greatly reduced due to the production of very thin films, so that the low conductivity can be compensated by increasing the area/thickness ratio; tightness-complete contact with the electrode to increase the charge and discharge current; adaptability-can withstand pressure changes well during the charge and discharge process and adapt to changes in electrode volume. The light weight, pressure resistance, shock resistance, fatigue resistance, non-toxicity and non-corrosion of polymer solid electrolytes, as well as the electrochemical stability shown when forming batteries with electrodes, have opened up a wider prospect for their use. At present, scientists at home and abroad are committed to making it usable in energy storage, electrochemical components, sensors and other aspects of research, and have become the strongest competitor in the development of high-energy lithium-ion batteries.

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