Study On The Improvement Of Cyclic Performance Of Lithium Metal Electrode Based On Liquid Electrolytes

Lithium metal has long been considered as an anode for next-generation electrochemical energy storage systems because of its outstanding characteristics, such as low redox potential (-3.04 V vs. standard hydrogen electrode), low density (0.59 g cm-3) and high theoretical specific capacity (3860 mAh g-1). However, the safety and cycle performance of metallic Li rechargeable batteries still need to be improved before these devices can be commercialized. The growth of Li dendrites during deposition-stripping cycles may lead to internal short circuit and incur fire or other safety problems. On the one hand, Li dendrites are easy to leave the base due to its fracture and form "dead Li", which leads to a lowering of coulombic efficiency (Ec), capacity fading and shortened cycle life. Therefore, researchers focused on restricting Li dendrite growth. The inhomogeneous deposition of Li+on substrates is a key factor that triggers Li dendrite growth. Aiming at enhancing the cycling capability of Li metal electrode in the commonly used organic electrolytes, in this dissertation, we have systematacially studied the Li electrode characteristics by taken two strategies, which is the development of a suitable substrate and the improvement of the electrolyte. The research can be summarized into four aspects:(1) Li22Sn5 material was prepared as a novel substrate of Li+ deposition for the metallic Li anode of rechargeable Li batteries. The performance of this alloy substrate was compared with those of Li, Cu, and Sn substrates. The deposition-stripping cycling performance of Li on the substrates was studied through galvanostatic charge-discharge method, cyclic voltammetry. The morphologies of the substrates before and after Li+ deposition were investigated through scanning electron microscopy and digital video microscopy. The electrochemical kinetics of Li+ electro-deposition on the different substrates was studied through galvanostatic pulse method and linear sweep voltammetry. The solid electrolyte interface films of Li deposits on the substrates were characterized through electrochemical impedance spectroscopy. Results show that Li22Sns is an excellent substrate for metallic Li electrodes. "The competitive kinetics model" was proposed as a novel mechanistic model to explain the electrodeposition behavior of Li+ on general substrates on the basis of electrochemical kinetics principles;(2) The electrochemical properties of LiODFB, LiGO4, and LiPF6 are systematacially studied and compared to demonstrate the feasibility of LiODFB as the lithium salt for lithium metal secondary batteries. Results show that the electrolyte with LiODFB has better temperature stability among them between 60? and-20? and lower deposition overpotential. More importantly, it favors the formation of stable SEI film and lowering the cell impedance, which results in the homogeneous deposition of Li+ and considerable enhancement of cycling performance. The compatibility of mixed solvent EC+DMC (1:1 vol.%) with LiODFB is studied. The electrolyte of 1 mol L-1 LiODFB/EC+DMC (1:1 vol.%) is prepared and used in the following studies;(3) The effect of Na+ additive in 1 mol L-1 LiODFB/EC+DMC (1:1 vol.%) electrolyte on the cycle behaviors and the surface characteristics of the Li22Sns electrode were investigated. The mixed electrolytes (a LiODFB+ b NaODFB/EC +DMC) are prepared via electrolysis method. Results show that lower concentrations of Na+(less than 0.25 mol L-1) contributes to the enhancement of cycling performance, while excess concentrations of Na+(more than 0.25 mol L-1) worsens. Na co-deposites and co-strips with Li, there exists concomitant phases but no LixNay phases. When excess concentration of Na ions are added in the mixed electrolyte, the nature of the metal Na and the problems itself will be highlighted, which seriously affect the deposition of Li+ cycling performance. Meanwhile, the electrodeposition characteristics of Na metal are studied according to the research method of Li metal, in order to better understand the role of Na in the mixed electrolyte. Results show that the electrodeposition characteristics of Na metal are far worse than Li metal, including the cycling performance, the stability of SEI film. Na metal easily reacts with electrolyte and has huge irreversible capacity loss. Nevertheless, this work can provide valuable data and references for the research for sodium secondary batteries;(4) The effect of K+ additive in 1 mol L-1 LiODFB/EC+DMC (1:1 vol.%) electrolyte on the cycle behaviors and the surface characteristics of the Li-Sn electrode were investigated. The morphology of the electrode is greatly improved for Li+ deposition in the electrolyte with K+ additive, and results in the enhancement of the charge-discharge cyclability. The results of ICP-AES, EDS, element mapping and CV demonstrate that K ions are not reduced nor incorporated on the Li surface during deposition, which is the most significant difference with other additive cations as reported, such as Na+, Mg2+ and so on. This phenomenon obey the reported "SHES mechanism" similar with Cs+ and Rb+. Meanwhile, the kinetic factors affect the Li+ deposition are discussed. Since the classic SHES mechanism just focuses on the thermodynamic factors (Nerst equation), the kinetic factors are ignored. So the exchange current density and polarization characteristics for Li+deposition on substrates with different electrolytes are studied. Last but not least, an improved SHES mechanism---"SHES-Kinetic mechanism" based on the experimental results of K+ additive in electrolyte is illustrated.