Overlithiation/delithiation Reaction of Lithium Manganese Oxide Spinel for Energy Storage of Dye-sensitized Photo-rechargeable Batteries

Abstract

Photo-rechargeable batteries (PRBs) have been developed as all-in-one energy devices having a merit that both energy harvesting and storage are realized in a single device. A major portion of the PRBs are based on non-faradaic processes (capacitive PRBs) for their energy storage. On the other hand, several works have focused on using faradaic processes (faradaic PRBs) because the faradaic PRBs are much superior in terms of energy densities. However, for the spontaneous electron transfer between the photo-electrode and the storage-electrode, the energy levels of the two active materials should be matched well, so that there are not so many storage materials having faradaic reaction. In addition, dim-light performances of the PRBs for indoor applications have not been illuminated. In this research, an outstanding light-to-charge energy efficiency (ηoverall) of 13.2% at 500 lux irradiations was achieved by using an external-power free single-structured PRB named dye-sensitized photo-rechargeable battery (DSPB). The DSPB was designed to be photo-charged by the photo-anodic process of dye-sensitized solar cell (DSSC) and the cathodic process of lithium manganese oxide (LiMn2O4 or shortly LMO). Overlithiation/delithiation reaction of lithium manganese oxide spinel was adopted to accept electrons from the photo-electrode, and their stable reversibility is one of the most important factors to construct the DSPB. The performances of DSPBs were strongly dependent on the thermodynamic and kinetic parameters of redox mediators. At one sun condition, the kinetics of mediator determined the light-to-charge energy efficiency (ηoverall). A kinetically-fast but thermodynamically-unfavorable (can make smaller cell voltage) mediator (I-/I3-) showed the best results in terms of photo-charging current (JCh) and discharge capacity (QdCh). However, in dim-light condition (200 ~ 2000 lux), a thermodynamically-favorable (can make larger cell voltage) mediator (Cu+/2+(dmp)2, dmp = 2,9-dimethyl-1,10-phenanthroline) delivered the highest photo-charging energy density (EDCh) corresponding to ηoverall of 11.5 % because kinetic limitation becomes negligible. Furthermore, the crystallite structure of the storage material, lithium manganese oxide (LMO) spinel, also affected to the ηoverall of DSPBs in dim-light condition. To obtain the stable reversibility of the overlithiation/delithiation reaction, LMO was treated with the graphite for making LMO@Gn with a few layers of graphene on the surface. As a result, the desired reversibility was secured, while the crystallite was polycrystallized to create many grain boundaries which could degrade the rate capability due to slow lithium ion transport between different-orientation domains of small crystallites. The crystallite of LMO ordered by electrochemical stimuli, resulting in improved kinetics for lithium insertion into the LMO particle, which leads an increase in the charging capacity for a limited charging time. The structure ordering seems to be occurred by the phase transition between cubic and tetragonal during the overlithiation/delithiation reaction. Pristine LMO became Li2Mn2O4 (L2MO) in which all Mn ions had trivalent theoretically. In an environment with more trivalent Mn ions in the L2MO, Mn would have been easier to move from Mn-O bond, helping to reconstruct the crystal structure. Through the LMO’s crystallite ordering, the performance of DSPB at 500 lux can be enhanced from 11.5% to 13.2% of light-to-charge energy efficiency (ηoverall). It should be emphasized that the ηoverall of 13.2% obtained at 500 lux is the highest overall efficiency of PRBs ever reported at indoor lights. Finally, a temperature sensing IoT device was successfully demonstrated by using DSPBs charged with 500 lux photo-charging for 10 min. The actual operation of IoT sensor by DSPBs opens the possibility of realizing indoor-light-harvesting PRBs. Considering that buildings’ energy consumption accounts for 40% of the total, and 10% of buildings are consumed by lighting, DSPBs will be a big step toward reusing the energy wasted after lighting.