In the 21st Century, with the increasing severity of the energy crisis and environmental pollution caused by the combustion of fossil fuels, human society is eager to find more sustainable and renewable resources. Over the past decades, many kinds of renewable energy, such as solar, wind, tidal, and geothermal, have been extensively used to replace fossil fuels. However, these renewable energies share drawbacks of discontinuity and instability. Thus, a system that can realize energy storage and conversion is strongly required. The regenerative electric power has been regarded as the most promising solution considering the reliability of energy supply, which is being widely researched.
Many energy storage systems, like batteries and electrochemical capacitors (ECs), have been hitherto achieved. However, due to the higher requirements of future systems ranging from portable electronics to hybrid electric vehicles and large industrial equipment, much more work still needs to be done to improve the power and energy density of energy storage systems. Thus, lithium-ion batteries (LIBs), characterized by high energy density and cycle stability, become the best possibility to make a breakthrough.
For improving the performance of lithium-ion batteries, the main challenge lies in the kinetics of the transport of electrons and ions, which dominates the rate performance of the devices, especially for high-loading batteries. In terms of the electrode design, massive efforts have been made to improve the transport kinetics, such as decreasing the size of the active materials to shorten the diffusion length of lithium-ions and adding conductive agents to improve the electronic conductivity. At the same time, the transport kinetics in electrolyte also plays an important role.
In lithium-ion batteries, the anions in the electrolytes generally do not participate in the lithiation reactions but exhibit higher mobility than lithium-ions, resulting in a low Li+ transference number (tLi+) of ~ 0.3. A low tLi+ gives rise to concentration polarization, reduces energy efficiency, and causes side reactions and joule heating, which shorten the cycling life, especially under fast charging/discharging condition. Although extensive efforts have been devoted to optimizing the composition of the electrolyte, simultaneously achieving high Li+ conductivity and Li+ transference number remains challenging.
In this dissertation, we designed and synthesized metal-organic frameworks (MOFs)-based electrolytes as anion-sorbent to increase the transference number and facilitate lithium-ion transport.
To find proper MOFs, three post-synthetic strategies focusing on metal cluster, ligand and pore structure, respectively, were used to prepare five typical MOFs. By heat treatment, the open metal sites (OMSs) in MOFs could be exposed to complex with the anions in solvent-filled pore channels, liberating Li+ mobility and affording high Li+ conductivity. With increased OMSs density, the transference number will increase while the electrochemical stability will decrease. By ligand modification and host-guest encapsulation, MOFs can provide free Li+ as anions, which present an increased transference number but inferior conductivity limited by the dissociation degree of the lithium-ions. Specifically, two MOFs structure, MOF-808 (tLi+ = 0.79) and UiO-66 (tLi+ = 0.62) are selected with high surface area and abundant OMSs, which can provide fast and efficient transport of lithium-ions.
For MOF-808, characterized for high OMSs density, a novel MOFs-based single-ion conducting electrolyte (SICE), 808-LiClO4, with both high transference number (0.77) and high conductivity (0.5 mS cm-1) was developed. An in-depth understanding of the mechanism of dehydration and anion adsorption process was explored to verify the significance of OMSs. This SICE can efficiently depress the generation of dendrite. A typical LFP|Li lithium metal battery (LMB) with 88% capacity retention after 400 cycles was achieved with 808-LiClO4 as electrolyte, which provides insight into the exploration of SICE for LMBs and next-generation battery devices.
For UiO-66, characterized for extremely electrochemical stability, a novel composite separators containing UiO-66 particles and PVA were fabricated by electrospinning process, producing non-woven fibrous mats with highly tunable pore size and structure. The electrospun separators show outstanding wettability and thermostability. Meanwhile, incorporating the MOF particles alleviates the decomposition of electrolytes, enhances the electrode reaction kinetics, and reduces the interface resistance between the electrolytes and electrodes. Primarily, they are the first reported separators that can increase ionic conductivity (from 0.7 mS cm–1 to 2.9 mS cm–1) and transference number (from 0.37 to 0.59) simultaneously. With the electrospun separators, the high-loading NCM|graphite cell (loading NCM: 20mg cm-2) with 73% capacity retention after 1000 cycles at high-rate (1C) was achieved.
Implementation of such MOFs-based electrolyte leads to dramatically improved power output and extended cycling lifetime, providing a new route towards better-performance lithium-ion batteries.