Electrolytes: In a battery, electrolyte serves as a carrier of ions enabling ion transfer from one side to another. The important electrolyte properties to consider:
Solubility: In the aprotic (‘‘non-aqueous’’) solvents, the absence of acidic protons implies that the dissolution of a Li-salt happens as: (i.) the dissociation of Li+ from the anion, overcoming the lattice energy of the salts, and (ii.) the consequential formation of coordination bonds between Li+ and electron lone-pairs of the solvent molecules. Due to this, many simple Li-salts such as LiCl, LiF, Li2O, etc., are excluded from electrolyte usage since their strong cation–anion interactions result in high lattice energies & poor solubility.
Ionic conductivity: The Li+ cation conductivity (σLi+) originate from both the total ionic conductivity and the cation transference number (tLi+). Given that the tLi+ in non-aqueous solvents is usually smaller than 0.5, the ionic conductivity plays a critical role in the performance. The conductivity of any aprotic Li-salt based electrolyte depends on salt concentration, anion, the solvent composition, and the temperature.
Donor number: The Gutmann donor number (DN) of solvents depicts the electron donating properties and hence the ability to interact with acceptors such as protons, Li+, and other cations. The higher DN, the higher basicity of the solvent, resulting in strong interaction with hard Lewis acids such as Li+. Most anions have similar and relatively small DN except Tf.
Electrochemical stability window (ESW): The electrolyte ESW should be located beyond the reduction and oxidation potential window of the anode and the cathode, respectively. The HOMO-LUMO difference defines the theoretical ESW.. Usually. the thermodynamic potentials of cell reactions are below 3 V vs. Li+ /Li0, which is within the ESW of most aprotic electrolytes but overpotential due to poor conductivity can increase the side reactions.
Thermal stability: LiPF6 has poor thermal stability due to the weak P–F bond and the auto-decomposition reaction of PF6¯ = >PF5 + F¯, which results in formation of toxic gases, HF and several other decomposition products. This can be resolved by: (i) replacing LiPF6 by Li-salts with higher thermal stabilities, (ii) using a co-salt to improve the thermal stability of LiPF6-based cells, (iii) adding additives to increase the thermal safety of LiPF6-based cells.
Chemical stability in contact with reaction products: Electrolytes should be either (i) thermodynamically stable or (ii) kinetically stabilized in contact with battery components including electrodes, as discussed above. Likewise, electrolytes should be stable in contact with reaction products formed during cycling.
doi:10.1039/C5EE01215E
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