Lithium iodide
Lithium iodide is an inorganic compound with the chemical formula LiI, composed of lithium cations (Li⁺) and iodide anions (I⁻), and it serves as a source of iodide ions in various chemical applications.[1] It appears as a white crystalline solid that is highly hygroscopic and turns yellow upon exposure to air due to partial oxidation of the iodide to iodine.[2] The compound has a molar mass of 133.85 g/mol and exhibits high solubility in water (approximately 1670 g/L at 25 °C), as well as in polar solvents like methanol and ethanol.[1][3] In terms of physical properties, lithium iodide has a density of 4.08 g/cm³, a melting point of 469 °C, and a boiling point of 1171 °C.[4] It crystallizes in the cubic rock salt (halite) structure with the space group Fm-3m (No. 225), where each lithium ion is octahedrally coordinated to six iodide ions at a bond length of 2.98 Å, forming a network of corner- and edge-sharing octahedra.[5] Lithium iodide finds prominent use as a solid-state electrolyte in high-temperature batteries and implantable devices, such as lithium-iodine batteries employed in cardiac pacemakers for their long-term reliability.[6] It also serves as an additive in lithium-sulfur batteries and dye-sensitized solar cells to enhance cycle life and performance.[2] In organic synthesis, it acts as a reagent for cleaving C-O bonds, facilitating ester cleavage, decarboxylation, epoxide opening, and C-C bond formation reactions.[7] Additionally, lithium iodide is utilized as a phosphor in neutron detection devices due to the neutron-capture properties of lithium-6.[8]Properties
Physical properties
Lithium iodide has the chemical formula LiI and a molar mass of 133.85 g/mol.[9] It appears as a white to yellow to beige powder or crystalline solid.[9] Due to its highly hygroscopic nature, it readily absorbs moisture from the air, leading to deliquescence and often forming hydrates such as the common trihydrate LiI·3H₂O.[9] In its anhydrous form, lithium iodide has a melting point of 446 °C and a boiling point of 1171 °C.[9] The density of the anhydrous solid is 3.49 g/cm³ at 25 °C.[9] The trihydrate form has a density of approximately 3.49 g/cm³.[7] Lithium iodide exhibits high solubility in polar solvents. It is highly soluble in water, with a solubility of 165 g/100 mL at 20 °C, increasing to 433 g/100 mL at 80 °C.[9] It is also soluble in ethanol and acetone (42.6 g/100 g at 18 °C), as well as in methanol (343 g/100 g at 20 °C).[9] The solid-state crystal structure of lithium iodide is of the rock salt (NaCl) type, featuring a face-centered cubic lattice with Li⁺ cations octahedrally coordinated to six I⁻ anions, and vice versa.[10]Chemical properties
Lithium iodide (LiI) is an ionic compound consisting of the lithium cation (Li⁺) and iodide anion (I⁻), characterized by strong electrostatic interactions typical of alkali metal halides.[11] In aqueous solutions, it behaves as a strong electrolyte, fully dissociating into its constituent ions to facilitate high ionic conductivity. As a neutral salt derived from a strong base and strong acid, its aqueous solutions are expected to be neutral, with observed pH values of 5.2–8.8 in 10% solutions due to impurities.[11][12] The redox behavior of lithium iodide involves the iodide anion (I⁻), which can be readily oxidized to elemental iodine (I₂) under oxidizing conditions, while the lithium cation (Li⁺) remains stable and does not participate in redox reactions within the compound.[11] In the presence of excess iodine, lithium iodide forms polyiodide complexes, such as LiI·I₂ (or LiI₃), where the iodide ions coordinate with additional I₂ molecules to create extended anionic chains.[13] Lithium iodide demonstrates thermal stability up to high temperatures, remaining intact below its boiling point, but undergoes decomposition above approximately 1171 °C.Synthesis
Laboratory preparation
Lithium iodide can be synthesized in the laboratory through simple acid-base neutralization reactions or direct elemental combination, suitable for small-scale preparation. A standard method involves reacting aqueous lithium hydroxide with hydroiodic acid to form lithium iodide solution:\ce{LiOH(aq) + HI(aq) -> LiI(aq) + H2O(l)}
This reaction occurs readily at room temperature and is often used for its simplicity and availability of reagents.[14] Another approach utilizes lithium carbonate and hydroiodic acid, producing carbon dioxide gas that facilitates the reaction:
\ce{Li2CO3(aq) + 2HI(aq) -> 2LiI(aq) + H2O(l) + CO2(g)}
High-purity lithium carbonate is typically employed, with the acid added gradually under stirring to control effervescence; the solution is then filtered to remove any excess carbonate.[15][14] For preparing anhydrous lithium iodide, lithium metal is directly combined with iodine in a vigorous, exothermic reaction:
\ce{2Li(s) + I2(s) -> 2LiI(s)}
This synthesis requires an inert atmosphere, such as argon or nitrogen, to prevent lithium's reaction with atmospheric moisture or oxygen, and is conducted in a dry glovebox or sealed apparatus.[14][9] Regardless of the method, the product is commonly purified by recrystallization from ethanol or water, leveraging lithium iodide's high solubility to separate impurities effectively; multiple recrystallizations may be needed for high purity (>99%). These procedures generally provide high yields due to the favorable thermodynamics of the reactions.[16][14]