The extensive utilization of fossil fuels since 2nd industry revolution bears a major responsibility for climate change. The raising awareness towards sustainable and renewable energy supply calls for game-changing research and progress in field of electrochemical energy storage, among which lithium-ion batteries (LIBs) is of particular interest. The developments of LIBs, in conjunction with the revolutions in the area of semiconductor and information technologies, have triggered the rapid growth of portable electronics and electric vehicles. Particularly, the transition of gasoline-powered automobiles to electrification ones requires better LIBs with higher energy density, faster charging rate, cheaper cost and longer-lasting lifetime.
To achieve the goals, it is essential to rethink and closely examine the fundamental electrochemistry beneath the conversion between electricity and chemical reactions. The operation of batteries relies on the separation of electrons and ions in electrodes, and their subsequent respective translocation through the electronic pathways and the electrolytes. The electronic conductivity of electrodes has been improved by rational architecture design and incorporation of conductive agents. While optimizing ionic transport is more challenging since the electrode-electrolyte interface is dynamic during cycling. Variation of electrolytes would not only impact the electrochemical reactions in electrodes, but also the ohmic and concentration polarizations throughout the devices. Therefore, advances in electrolyte are vital for driving innovations in battery technologies.
Commercial liquid electrolytes, which are based on ion diffusion in fluidic medium, have merit in ionic conductivity. However, its suitability for next-generation LIBs is under dispute. Firstly, the Li+ transference number, defined as the ratio of conductivity carried by Li+ versus by Li+ and counter anions, is typically as low as 0.3, indicating an inferior transport efficiency. Such scenario is responsible for severe polarization and deterioration of the cycling life, particularly, during fast charging/discharging process. Second, liquid electrolytes are not compatible with high energy electrodes (e.g. Li anode, high voltage cathode, etc.) viewed from the aspects of electrochemical voltage window and safety. To address these issues, solid electrolytes and polymer electrolytes have been extensively explored due to their high Li+ transference number and superior safety. Yet their implementation to commercial LIBs still encounters considerable challenges from the aspects of low ionic conductivity and manufactural difficulties.
In this dissertation, a novel class of ionic conductors with biomimetic ionic channels have been developed to overcome the aforementioned limitations in liquid electrolytes. By thermal activation, porous metal-organic frameworks (MOFs) yield unsaturated metal centers which could be complexed with liquid electrolytes. The anions in liquid electrolytes can spontaneously bind with the unsaturated metal centers, forming ionic channels mimicking those of in the biologic systems and allowing effective transport of Li+. The ionic conductors built upon MOFs outperform liquid electrolytes in terms of high ionic conductivity, high transference number, broad electrochemical window and improved safety.
The dissertation research could be outlined briefly with following two parts:
1. Development of MOFs-based electrolytes with high ionic conductivity and high Li+ transfer number. This part of work firstly demonstrated the concept of biomimetic ionic channels within MOFs. Second, optimization of MOF pore structures according to infiltrated liquid electrolyte affords the synthesis of suitable MOF-based electrolytes with high Li+ ionic conductivity and low cost.
2. Integration of MOFs-based electrolytes into batteries. Three strategies were explored in this part to integrate the MOFs-based ionic conductors as following components: 1) separator; 2) electrolyte additive; 3) electrode additive.
Overall, this dissertation research has developed a new class of fast lithium ion conductors based on MOFs and commercially available liquid electrolytes, a variety of architecture designs for incorporating these fast Li+ conductors into battery device could be implemented in a cost-effective manner. By taking advantage of unsaturated metal sites in MOFs, immobilized anions and fast Li+ mobility enable superior device performances with prolonged cycling performance, especially at fast charging rate. Based on these works, one can expect the advances in electrolytes will impact the markets of lithium rechargeable batteries in the near future.