Integrating photoelectrochemical charging into lithium batteries for use as a single multifunctional device

Abstract

Solar energy is a promising renewable energy resource. The energy from the sun is typically converted by solar cells and collected into a battery in the form of chemicals. It requires at least two devices for energy conversion and storage. Rechargeable lithium-ion batteries are widely used nowadays because of their high energy density. Lithium manganese oxide, particularly LiMn₂O₄, is commonly used as positive electrodes for lithium batteries. Due to its semiconducting property, it is intriguing to integrate photoelectrochemical charging into lithium-ion batteries for use as a single multifunctional device. Lithium manganospinel was synthesised using sol- gel methods. The effects of calcination temperatures and the molar ratio of Li/Mn defined the lithium manganospinels in the binary LiMn₂O₄–Li₂Mn₄O₉ and the ternary LiMn₂O₄–Li₂Mn₄O₉–Li₄Mn₅O₁₂ systems. To allow the light transmission to interact with LiMn₂O₄ at the electrode/electrolyte interface, LiMn₂O₄ was prepared by spin-coating on a transparent conducting oxide-coated glass substrate. The sol-gel method using methanol as a solvent provided a homogeneous and good-quality film. The film of pure defect spinel Li₀.₉₅₅Mn₁.₉₁₀O₄ calcined at 400 ̊C was mainly studied in this work. The 200 nm thick Li₀.₉₅₅Mn₁.₉₁₀O₄ film coated on F-doped SnO₂ glass substrate possessed a band gap energy of 2.7 eV. Photoelectrochemical cells were built to test the films of Li₀.₉₅₅Mn₁.₉₁₀O₄ in LiNO₃ solution as an aqueous electrolyte and 1 M LiPF₆ in EC/DEC (1:1 w/w) as a non-aqueous electrolyte. The cells were designed to allow UV light to pass through the cells for photoelectrochemical measurement. Cyclic voltammetry, chronopotentiometry, chronoamperometry, and open-circuit potential measurements were carried out to study and understand the photoelectrochemical properties of the films. The results reveal that light irradiation on the cell facilitated the generation of electrons and holes. The findings showed an increase in current and a decrease in potential with light irradiation. Photoelectrochemical responses of the Li₀.₉₅₅Mn₁.₉₁₀O₄ film at a potential of 2.6 V and 3.7 V (vs Li/Li+) corresponded to lithium insertion/extraction at octahedral and tetrahedral sites, respectively. The mechanisms in the photoelectrochemical process were proposed, which agree with the electrochemical results.