The market demand for high energy density energy storage devices is growing as portable electronic devices, electric vehicles, smart grids, and other fields develop. With graphite as the negative electrode, lithium-ion batteries have limited energy density, especially when compared to the development goals of electric vehicle power batteries (300~500 Wh kg-1). Research and development of a new generation of batteries is therefore very urgent.
Lithium metal has a high theoretical capacity (3860 mAh g-1) and the lowest electrode potential (- 3.04V vs. Standard hydrogen electrode), these two advantages greatly increase the energy density of metal lithium battery (LMBs) with lithium as negative electrode. However, in the actual cycle, there are obvious problems in the metal lithium negative electrode: uneven lithium deposition causes dendrite growth, and then punctures the diaphragm, resulting in short circuit, resulting in battery thermal failure; the lively interface leads to a side reaction between the electrode and the electrolyte, resulting in low Coulomb efficiency, electrolyte loss and other problems. Therefore, how to restrain dendrite growth, negative electrode volume expansion, build a stable electrode interface, improve safety and other issues is an important direction of LMBs research. Most of the reported studies on electrolyte additives have only studied the mechanism of electrolyte improving LMBs performance from a single point of view, and lack of systematic understanding of the electrochemical behavior of additives. For this reason, the J.M. Ma Research Group of Hunan University put forward a "Multi-factor Design Principle of Electrolyte Additives" to systematically design and study new LMBs electrolyte additives from many angles. This multi-factor "design principle" refers to making full use of various parts of additive molecules to improve the electrochemical performance of LMBs from many factors, such as electrode surface film formation, lithium ion solvation, lithium nucleation and growth.
1. Promote the film formation of positive and negative electrodes
Potassium perfluoroalkyl sulfonate is an important type of surfactant. Its high fluorine content and sulfonic acid functional group can coordinate with lithium ions make it have the potential to be used as an additive to the electrolyte of LMBs. Taking potassium perfluorohexane sulfonate (K+PFHS) as an example, this additive can synergistically improve the electrochemical performance of LMBs with multiple factors, as shown in Figure 1. Theoretical studies have found that compared to other components of the electrolyte, the additive has a lower lowest unoccupied orbital (LUMO) and a higher highest occupied orbital (HOMO). During the electrochemical cycle, the K+PFHS additive can be reduced or oxidized before the electrolyte, forming a film on the surface of the positive and negative electrodes, inhibiting further decomposition of the electrolyte, and improving the Coulombic efficiency of the battery. On the lithium metal anode side, the decomposition of K+PFHS provides a large amount of fluorine element, which increases the LiF content in the SEI film, which is beneficial to reducing the thickness of the SEI film and increasing the lithium ion migration rate; On the side of the cathode, the cathode electrolyte interphase film (CEI film) is thinner and more uniform, which not only shortens the lithium ion diffusion distance, but also inhibits the precipitation of high valence metals in the cathode material, and stabilizes the structure of the cathode material. These changes improve the Coulomb efficiency and cycle stability of LMBs.
Fig. 1 Schematic diagram of action mechanism of potassium perfluorohexanesulfonate additive at electrolyte / positive and negative interface
Sulfonate Product List
2. Controlling solvation shell structure
In the electrolyte, lithium ions are always surrounded by solvent molecules, forming a solvation shell. However, during the electrochemical reaction process, lithium ions need to be extracted from the solvation shell before subsequent intercalation reactions or deposition reactions can occur. Adjusting the solvation structure and lowering the energy barrier for lithium ion extraction can not only speed up the electrochemical reaction, but also prevent the solvation shell from being embedded in the electrode material and destroying the structure of the electrode material. The oxygen atom of the sulfonic acid functional group in K+PFHS can coordinate with lithium ions and enter the solvation shell of lithium ions to adjust its structure. The introduction of K+PFHS repels the ethylene carbonate (EC) molecules around the lithium ions and weakens the interaction between the EC molecules and the lithium ions, which is conducive to the escape of the lithium ions from the solvation shell.
3. Cation shielding
The formation process of lithium dendrites on the surface of lithium negative electrodes can be roughly divided into two steps: first, lithium ions are reduced to form tiny lithium nuclei on the surface of lithium negative electrodes, and the charge is concentrated on these nuclei; then, more lithium ions are electrically adsorbed on the surface of these nuclei and are continuously reduced, and the nuclei grow into dendritic structures. The introduction of unreduced cations adsorbs on the surface of the crystal nucleus and repels lithium ions, thus inhibiting the growth of lithium dendrites, which is called "cation shielding". The introduction of K+PFHS can provide potassium ions for the electrolyte. Although potassium ion has higher reduction potential than lithium ion, according to Nernst equation, potassium ion can have lower reduction potential than lithium ion under the condition of low concentration of potassium ion and high concentration of lithium ion. Therefore, adjusting the concentration of K+PFHS additive can form a good cationic shielding layer on the surface of lithium metal anode and inhibit the growth of lithium dendrite. XPS results show that there is no potassium signal on the surface of lithium negative electrode, which indicates that potassium ion has not been reduced, which proves the above potassium ion shielding mechanism.
