What factors contribute most to solubility of ionic substances in highly polar solvents, esp. ionic liquids and DES?

I am trying to determine a method to dissolve a mineral salt with a lattice energy of ≈18000 kJ/mol. I know that lattice energy is the most significant factor affecting solubility of an ionic substance in highly polar solvents, and my general understanding is that the more polar (or locally polar), the greater the ability of the solvent to dissolve materials with higher lattice energies, by contributing more hydrogen bonding and/or more efficient charge distribution. 


Hence I have been testing solubility in an array of Deep Eutectic Solvents (DES) and doing research into solubility of this and similar compounds in other ionic liquids, and reached out to an expert in the field. 


I have found that while these solvents are typically able to dissolve compounds with lattice energies unto around 12000 kJ/mol. in great quantity, solubility quickly drops off around 15000 kJ/mol. and the professor I contacted said that their research indicated that without other agents, this material would be insoluble in the ionic liquids they worked with because of it's high lattice energy. 


**My question then is** what other factors would be most important in increasing the solubility of this compound in ionic liquids, anything from adding highly soluble cations, chelating agents, type of ionic liquid I might use, or any other other factors that play into solubility, and could be tuned to optimal conditions?


