| As the demands for high energy rechargeable batteries have remarkably increased, lithium (Li) metal as an ideal anode material of rechargeable batteries has drawn much attention in recent years. This is attributed by its advantages of highest specific capacity and lowest radox potential among all the solid state anode materials. However, dendrite formation on Li metal anode during the deposition-oxidation cycles remains the most important issue that has hindered the successful commercialization of Li metal batteries. 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, capacity fading and shortened cycle life. On the other hand, Li dendrites could penetrate the separator and lead to the internal short circuit, causing safety problems of the batteries. Thus, inhibiting the growth of Li dendrites becomes a key issue of Li metal batteries research. Aiming at enhancing the cycling capability of Li metal electrode in the commonly used organic electrolytes, in this dissertation, we have systematicly 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) A Zn layer is electrodeposited on Cu disk with ultrasonic assistance, which is named as Znuae, and it is used as the substrate for Li cycling. The mechanism that Znuae is suitable as substrate for Li cycling is analyzed by galvanostatic charge-discharge, constant current pulse polarization, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and digital microscope system in detail. The cycling performance of Li on bulk metallic platinum (Pt), bulk metallic copper (Cus), bulk metallic zinc (Zns), electrodeposited Zinc (Zne) and the Zinc electrodeposited with ultrasonic assistance (Znnae) substrates are compared by constant curent charge-discharge cycling tests. Among them the Znuse has been verified as the most suitable substrate for Li deposition-oxidation cycling. Several common organic electrolyte systems are attempted, and the results show that LiBOB/EC+DMC electrolyte is the most suitable one. The coulombic efficiency (Ec) value of Li deposition-oxidation on Znuae remains as high as97%even after900cycles at a current density of0.1mA cm-2. As the current density is increased up to0.3mA cm-2, the Ec value still maintains at90%after600cycles. This improvement can be attributed to two reasons. On the one hand, the deposited Li metal can form Li-Zn alloy with zinc substrate. This process is conducive to a uniform deposition of Li and helps to slow the growth rate of solid electrolyte interface (SEI) film formed by reaction of metal Li with electrolyte, which is beneficial to form dense and uniform SEI films. On the other hand, the surface of Znuae substrate formed with ultrasonic assistance is constructed with nano-particles and nano-holes, which increases the actual surface area and reduces the actual current density during the Li metal deposition-oxidation cycling.(2) On the basis of Znuae substrate, a novel Li-Zn alloy substrate is prepared by in-situ electrodeposition of Li on Znuae. According to the test results of constant current pulse deposition, CV, EIS and SEM, the calculated exchange current density of Li electrodepositon on Li-Zn substrate is much larger than that on Li metal and Znuae substrate, and the interface impedance of the Li-Zn alloy substrate has good stability during Li cycling. The Ec value of Li deposition-oxidation on the Li-Zn alloy substrate remains as high as97%after400cycles at a current density of0.1mA cm-2. When the current density is increased up to0.2mA cm-2, the Ec value maintains at97%after250cycles. Even more, the initial Ec value of Li metal on Li-Zn substrate is higher than95%. These results can be attributed mainly to the reason that the intrinsic feature of Li-Zn alloy which classifies it as such a substrate that Li is more easy to deposite on it; the second reason is that, as the deposition rate of Li is slow during Li-Zn alloy formation, a more stable SEI film can be formed on the substrate surface that protects Li cycling. At the same time, the electrolyte effect is weakened on this Li-Zn alloy substrate, and LiClO4/EC+DMC electrolyte is found to be more suitable for Li cycling than others.(3) On the basis of Li-Zn alloy substrate, different amounts of NaC104are added into1mol L-1LiClO4/EC+DMC electrolyte to form mixed electrolytes. The cyclic performance of Li metal electrode is investigated in the mixed electrolytes with different proportions of Li+:Na+, and lmol L-1LiClO4+0.5mol L-1NaC104/EC+DMC electrolyte is verified as the most suitable electrolyte for Li metal electrode cycling. The analysis of energy dispersive X-ray spectroscopy shows that Li-Na co-deposition occurs during Li deposition in this mixed electrolyte, and ICP analysis for the electrolyte also shows that the Na in the codeposited product can be re-oxidized. The Li deposition-oxidation performance is measured by galvanostatic charge/discharge, CV, EIS and SEM. The results show that adding Na+in the electrolyte can improve the performance of the SEI film:not only improves the stability of the SEI film, but also reduces the interface impedance of the substrate surface, making the Li-Na co-deposition and co-oxidation easier to proceed. The charge-discharge test results show that when Li-Na co-deposits and co-oxidizes at a current density of0.2mA-cm-2for60min, the oxidation potential is decreased while the deposition potential is increased, and the Ec value reaches a value higher than95%and it keeps at90%after500cycles. When the Ec value decreases to83.3%, the cycle life is more than800cycles. When the deposition time is doubled, the cycle life still reaches200cycles with the Ec value of90%at the same current density. These results manifest that utilizing Li-Na co-deposition and co-oxidation reactions to replace single Li cycling in organic electrolyte is an effective way to inhibit Li dendrite growth and improve the cycling performance of Li anode. At the same time, Li-Zn alloy substrate also has a role in promoting Li-Na co-deposition and co-oxidation reactions.(4) On the basis of Li-Zn alloy substrate, the effect of Mg2+on Li metal cycling has been preliminarily studied by adding a small amount of Mg(ClO4)2into LiClO4/EC+DMC electrolyte. The test results of CV, EIS and SEM show that, adding ppm grade of Mg2+in the Li+containing electrolyte can promote the deposition-oxidation process of Li metal. The charge and discharge test results indicates that in the lmol L"1LiClO4+400ppm NaClO4/EC+DMC electrolyte, the cycle life of Li electrode reaches a level of more than500cycles with the Ec value of 83.3%at current density of0.2mA-cm2for60min. This is a significant improvement as compared with that in the electrolyte without Mg+addition. |
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