Lithium desolvation

Numerous experimental studies [104-107] indicated that the interfacial impedance could be a limiting factor for charge transport in lithium batteries at low temperature as it exceeds the resistance of the bulk electrolyte. Two major contributions to the interfacial impedance are commonly discussed (1) the low conductivity and high activation energy of SEI and (2) the resistance associated with the lithium desolvation process as it changes phases from electrolyte to SEI, from SEI to... 

The resistance to lithium motion at the interface between the electrolyte and an electrode due to structure imposed on the electrolyte by the electrode, barriers to intercalation into the electrode, lithium desolvation energy, and/or the SEI layer. 

Fig. 7.23 A snapshot of the simulation box used for calculating the free-energy barrier of the lithium desolvation energy...

The increase in sensitivity in the alkali metal ion series from lithium to cesium corresponds to the variation of the lattice energies and desolvation energies of the alkali elements. These energies decrease from [Li] to [Cs]" thus favoring the desorption of [Cs] as compared to that of [Li]. This effect is also reflected in the observation that [Cs] desorbs at a lower emitter temperature than [Li]. ... 

Desolvated lithium ions are intercalated into the crystalline lattice under discharge of solid cathodes, including oxides and chalcogenides. The charge of lithium ions remains herewith unchanged and the atoms of the active substance metal are reduced. 

Fig. 5.9 Schematic description of a solvated lithium ion s journey from solution bulk to graphene interior, and the impedance components associated with these steps, (a) Assignment by convention where the process occurring at low AC frequencies was assigned to charge transfer after Li-ion diffusion across the interface film, (b) Rationale by Abe and Ogumi et al., where the desolvation process of Li-(solvent)j complex is identified as responsible for the above impedance components (reproduced with the permission by Electrochemical Society from [42])...

Fig. 7.24 The free-energy profile for the lithium cation desolvation from EC DMC(3 7)A iPFg at 1 M electrolyte at 298 K calculated using the Li probability profile from equilibrium MD simulations and the integration of the constrained force method. Z = 0 at the position of hydrogen atoms of graphite...

Fig. 7.25 The lithium cation desolvation free-energy profile for EC/LiPFe and EC DMC(3 7)/LiPF6 1 M electrolytes next to graphite at 298 K...

This inter-layer separation remains slight (which is an essential condition for the battery to woik) because the lithium is inserted without its solvation shell formed by the organic molecules from the electrolyte. This phenomenon of desolvation takes place when lithium ions are diffused in an interfacial passivation layer present between the electrode and the... 

Even though less numerous, Raman studies of nitrate solutions in nonaqueous solvents have covered a range of metal ions (Li, Na, Ag, and Cu " ) in both protic and aprotic solvents [260-264]. Wooldridge et al. [260] carried out a vibrational study of alkali metal (lithium and sodium) nitrates dissolved in dimethylsulfoxide (0.5-2 M) at different temperatures (298-400 K). They found spectral evidence of an increase in the ionic pairing process when the temperature increases. Interestingly, in LiNOg solutions, the contact ionic pair formation promoted by the thermal increase is accompanied by a change in the ion-pair structure from monodentate to bidentate and by the partial desolvation of the alkaline ion ... 

The first equilibrium shows an enthalpy change of —3.6 kJ/mol (1 M) and —6.3 kJ/mol (2 M), whereas for the second, the AH value is 17.1 kJ/mol. Although the overall effect of an increase in temperature would be to increase the ionic association because the second equilibrium is strongly endothermic, the exothermic nature of the first step is rather surprising because it involves the loss of two DMF molecules from the first solvation sphere of the lithium ion and the formation of only one Li —N03 bond. Furthermore, in the second step, the substitution of one molecule of DMF by one nitrate ion in the Li first solvation sphere has an enthalpy cost of 17.1 kJ/mol, which demonstrates that, as expected, the desolvation of the lithium ion is an endothermic process, which is partially compensated by the likely exothermic nature of the ion-pair formation. 

Considering the promising properties, especially their cost, hazard, and the risks of these DESs series, their introduction as safer electrolytes could represent an important challenge for the realization of environmentally friendly EDLCs operating at high temperatures. The study of the original ions adsorption mechanism of desolvated lithium ions provides an interesting opportunity for fundamental smdies even beyond the applicative interest of DESs as electrolytes for supercapacitors. 

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