Effect of Anions and Dipole Molecules in Electrolytes on the Electrochemical Performance of Energy Storage Devices

電解液中的陰離子及極性小分子對儲能器件電化學行為的影響

Student thesis: Doctoral Thesis

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Award date24 Aug 2021

Abstract

The key to develop high-energy energy storage devices is to improve the specific capacity and output voltage of the electrode, and to expand the electrochemical stability window of the electrolyte, so as to ensure the reversible redox reaction at high potential. This requires pay attention not only to the electrochemical behavior of cationic carriers and anode/cathode materials, but also to electrolyte regulation and anion chemistry in electrolyte. The anions in electrolytes play a nonnegligible role in various components of energy storage device, and the small dipole molecules in electrolytes have an important effect on the electrochemical stability of the electrolyte and the mass transfer kinetics of charge carriers. This thesis is devoted to the study of the effects of anions and small dipole molecules in electrolytes on various electrochemical properties of energy storage system.

At every beginning, we summarized the effects of anions in the electrolytes on the electrochemical performance of various energy storage fields, including supercapacitors, metal-ion rechargeable batteries, anion shuttle batteries, dual-ion batteries, and metal-oxygen group elements batteries. Specifically, we summarized the development trend of anion chemistry research in these fields. Then, an essential overview about the influence of anions on surface-interface chemistry and mass transfer kinetics have been presented, including the anion-originated solid-electrolyte interphase membrane, anion-induced deposition, and mass transfer of carriers. Subsequently, we reviewed the research focus on the effects of anions on the electrochemical stability of solvent molecules and the solvation sheath structure. Finally, we introduced two effective strategies to expand the electrochemical stability window of electrolyte, including preparation of highly concentrated electrolyte salts and introducing small dipole molecules.

Subsequently, we have explored the effects of anionic carriers on the capacitance and anti-self-discharge ability of zinc ion capacitor. Introducing pseudocapacitive behavior and ion hybrid capacitor technique were deemed effective to improve the energy density of supercapacitors. However, the current research about ion hybrid capacitor only considered the reaction of cations during the electrochemical process, leading to a flawed mechanism understanding. In Chapter 2, the effects of various anions carriers on the electrochemical behaviors of titanium nitride-based zinc ion capacitor (Zn-TiN capacitor) have been carefully explored. Density functional theory (DFT) calculation results revealed the stable structure of TiN-SO4 after adsorbed process, enabling SO42- participate in the electrochemical process and construct a two-step adsorption and intercalation energy storage mechanism, which greatly improved the capacitance and anti-self-discharge ability of Zn-TiN capacitor. Zn-TiN capacitor delivered an ultrahigh capacitance of 489.8 F g-1 and remained 83.92% of capacitance even after 500 h resting time. It is believed that designing an appropriate energy storage system to involve anions in the electrochemical process can significantly improve the capacitance and anti-self-discharge ability of ion hybrid capacitors.

Aqueous graphite-based dual-ion batteries own unique superiorities in stationary energy storage systems due to their non-transition metal configuration and safety properties, besides, the potential of graphite cathode can be manipulated by intercalation energy and hydration energy. However, there is an absence of thorough study about the interactions between anions and water molecules and between anions and electrode materials, which is the key to obtain a high output voltage. In Chapter 3, we revealed the four-stages intercalation process and energy conversion in graphite cathode of anions with different configurations. The difference between the intercalation energy and hydration energy of bis (trifluoromethanesulfonimide) (TFSI-) can make the best use of the electrochemical stability window of its electrolyte and delivered an outstanding intercalation potential, while BF4- and CF3SO3- (Otf-) have not exhibited a high potential since the graphite intercalation potential of BF4- is inferior and the graphite intercalation potential of Otf- exceeds the electrochemical stability window of its electrolyte. The theoretical voltage plateaus calculated through density functional theory are well matched with the experimental results, demonstrating the rationality and feasibility of the voltage manipulation design. An aqueous DIB based on the intercalation behaviors of TFSI- anions into graphite cathode exhibited an ultrahigh voltage of 2.2 V together with excellent energy density of 242.74 Wh kg-1. This work provides a clear guidance for the voltage plateau manipulation of anions intercalation into two-dimensional materials.

High-voltage aqueous rechargeable batteries are promising competitors for the next-generation energy storage system with safety and high specific energy, while limited by the absence of low-cost aqueous electrolytes with wide electrochemical stability window. The decomposition of aqueous electrolytes is mainly facilitated by the hydrogen bond network between water molecules and the water molecules in solvation sheath. In Chapter 4, we report three types of small dipole molecules (glycerol, erythritol and acrylamide) to develop aqueous electrolytes with high safety and wide electrochemical stability window (over 2.5 V) for aqueous lithium, sodium and zinc-ion batteries, respectively. The solvation sheath structures are explored by ab initio MD simulations, demonstrating that three types of dipole molecules deplete the water molecules in the solvation sheath of charge carrier and break the hydrogen bond network between water molecules, thus effectively expand the electrochemical stability window. A battery construct by lithium titanate and lithium manganate in glycerol-containing electrolyte exhibits an output voltage of 2.45 V and remains a specific capacity 119.6 mAh g-1 after 400 cycles. This work provides another strategy for exploiting low-cost high-voltage electrolytes for aqueous energy storage system.

In summary, the function and influence of anions to electrochemical performance of aqueous energy storage devices have been explored and novel high-voltage aqueous electrolytes have been exploited to obtain high-output-voltage aqueous batteries. Superior attentions have been paid to various aspects from capacitance, anti-self-discharge ability, reaction mechanisms, cell voltage, which targeted at figuring out the selection criteria and design principle of anions in electrolytes and battery materials. In addition, the effects of small molecule on the electrochemical stability, ion transport mechanism of aqueous electrolytes have been carefully explored. The existing form of small molecules in electrolyte was determined by DFT calculation, and their solvent-assisted hopping ion transport mechanism has been revealed.