Special report: Imide-Orthoborate double-salt electrolyte system inhibits lithium dendrite and enhances the efficiency of lithium metal coulombic

2021-05-02

1 Introduction

With the rapid development of advanced portable electronic products, electric vehicles, etc., higher requirements are imposed on the energy density of batteries. Metal lithium has a high theoretical specific capacity (3860 mAh/g) and an ultra-negative electrode potential of -3.04 V (relative to standard hydrogen electrodes), making it an ideal anode material for high specific energy secondary batteries. Therefore, the development of high-specific energy secondary batteries based on metal lithium anodes, such as lithium metal batteries, lithium air batteries and lithium-sulfur batteries, has attracted much attention and has become a research hotspot in the field of chemical power sources in recent years. However, when metal lithium is used as a negative electrode, problems such as chalking and dendrite growth are likely to occur during repeated charge and discharge, resulting in extremely poor cycle performance, rapid capacity decay, low coulombic efficiency, and severe polarization of the corresponding secondary battery; To the extent that lithium dendrite growth also pierces the membrane, it can cause a short circuit in the battery and can cause serious safety problems.

The research status at home and abroad shows that the improvement methods of lithium metal anode performance mainly include: lithium metal alloying; solid electrolyte; lithium metal surface structure design; organic electrolyte promotes lithium metal/electrolyte interface SEI film stability. Among them, the optimization of organic electrolyte composition and modified additives promotes the stability of lithium metal/electrolyte interface SEI membrane, which is considered to be one of the simplest and most effective ways to inhibit lithium dendrite growth and improve coulombic efficiency.

2. Results introduction

Recently, Dr. Li Xing of Southwest Petroleum University and Dr. Xu Wu, Dr. Zhang Jiguang and Dr. Zheng Jianming from the Northwest Pacific National Laboratory in the United States have used the Imide-Orthoborate double salt electrolyte system to inhibit the growth and promotion of lithium dendrites. More systematic research work has been carried out on the efficiency of lithium metal coulombic.

(1) Firstly, the combination of first-principles calculation (DFT) and experiment was used to compare lithium bis(trifluoromethanesulfonimide)-lithium dioxalate borate (LiTFSI-LiBOB) and bistrifluoromethanesulfonate. Lithium-difluorooxalate borate (LiTFSI-LiDFOB), lithium bisfluorosulfonimide-Li-disodium oxalate (LiFSI-LiBOB), lithium bisfluorosulfonimide-Lithium difluorooxalate borate (LiFSI-LiDFOB) The effect of the imide-borate double salt electrolyte system on inhibiting lithium dendrite growth and improving lithium metal coulombic efficiency. The results show that the LiTFSI-LiBOB double salt electrolyte system can achieve the best results. The research results were published in ACS Appl. under the title "Effects of Imide-Orthoborate Dual-Salt Mixtures in Organic Carbo nate Electrolytes on the Stability of Lithium metal Batteries". Mater. Inter. 2018, 10, 2469-2479 (Xing Li, Jianming Zheng (Joint), Mark H. Engelhard, Do nghai Mei, Qiuyan Li, Shuhong Jiao, Ning Liu, Wengao Zhao, Ji-Guang Zhang (Corresponding author), Wu Xu (Corresponding Author)). In addition, in order to more accurately determine the coulombic efficiency of the lithium metal negative electrode, the influence of the separator was systematically studied. The results show that the polyethylene (PE) film is the relatively stable diaphragm system. The research results are published in J. under the title "Stability of Polymeric Separators in Lithium me tal Battery under Low Voltage Environment". Mater. Chem. A 2018, DOI: 10.1039/c7ta11259a (Xing Li, Jinhui Tao, Dehong Hu, Mark H. Engelhard, Wengao Zhao, Ji-Guang Zhang (corresponding author), Wu Xu (corresponding author)).

(2) Based on the above research basis, the research work of LiPFSI-LiBOB double salt electrolyte modified by LiPF6 additive was carried out. Studies have shown that an appropriate amount of LiPF6 additive can induce the ring opening and polymerization of the EC solvent, so that the surface of the formed SEI film is rich in poly(CO3), and the surface of the SEI film becomes dense and smooth, which can effectively inhibit the growth of lithium dendrites. The research results are published in Nat. under the title "Electrolyte additive enabled fast charging and stable cycling lithium me tal batteries". Energy 2017, 2, 17012 (Jianming Zheng, Mark H. Engelhard, Do nghai Mei, Shuhong Jiao, Bryant J. Polzin, Ji-Guang Zhang (corresponding author) Wu Xu (corresponding author)). However, the coulombic efficiency of the lithium metal anode corresponding to the LiPF6 modified Imide-Orthoborate double salt electrolyte system is still not high, only about 90.6%.

(3) In order to further improve the coulombic efficiency of the corresponding lithium metal, the solvent ratio in the LiTFSI-LiBOB double salt electrolyte system was optimized, and the combined additive (LiPF6 + VC + FEC) was used, and the coulombic efficiency of the corresponding lithium metal anode was found to be improved to 98.1%. The research results were published in Adv. under the title "Dendrite-Free and Performance-Enhanced Lithium me tal Batteries through Optimizing Solvent Compositions and Adding Combinatio nal Additives". Energy Mater. 2018, 1703022 (Xing Li, Jianming Zheng (co-production), Xiaodi Ren, Mark H. Engelhard, Wengao Zhao, Qiuyan Li, Ji-Guang Zhang (corresponding author), Wu Xu (corresponding author)).

3, graphic introduction

Table 1 First-principles calculations show chemical and electrochemical stability: LiTFSI + LiBOB > Li TFSI + LiDFOB > LiFSI + LiDFOB > LiFSI + LiBOB (ACS Appl. Mater. Inter. 2018, 10, 2469-2479).

Figure 1 Corresponds to lithium metal secondary battery (NMC||Li) cycle stability: LiTFSI + LiBOB > Li TFSI + LiDFOB > LiFSI + LiDFOB > LiFSI + LiBOB (ACS Appl. Mater. Inter. 2018, 10, 2469-2479).

Figure 2. Coulomb efficiency of lithium metal anodes measured in two typical electrolytes for different types of separators. It can be seen that PE membranes exhibit relatively good stability (J. Mater. Chem. A 2018, DOI: 10.1039 /c7ta11259a).

Figure 3 is a SEM image of a lithium metal negative electrode cross section (a-c) and a surface (d-f) corresponding to different electrolytes.

a, d (LiPF6/EC-DEC), b, e (LiTFSI-LiBOB/EC-DEC), c, f (LiTFSI-LiBOB+LiPF6/EC-DEC). It can be observed from the figure that LiTFSI-LiBOB double salt electrolyte modified with LiPF6 additive can promote more stable SEI growth (Nat. Energy 2017, 2, 17012).

Figure 4 compares the coulombic efficiency of a lithium metal negative with a single additive versus a combined additive.

It can be observed from the figure that the coulombic efficiency of the lithium metal negative electrode using the combination additive (LiPF6+VC+FEC) is as high as 98.1% (Adv. Energy Mater. 2018, 1703022).

Figure 5 Surface SEM of a lithium metal anode corresponding to different additives after 100 cycles.

a. Use LiTFSI-LiBOB+LiPF6/EC-EMC electrolyte; b. Use LiTFSI-LiBOB+LiPF6+VC+FEC/EC-EMC(4:6) electrolyte; c. A LiTFSI-LiBOB+LiPF6+VC+FEC/EC-EMC (7:3) electrolyte was used.

It can be observed from the figure that lithium dendrites are hardly produced on the surface of lithium metal using the combination additive (LiPF6+VC+FEC), and the internal resistance of the lithium metal battery corresponding to the combination additive is also significantly smaller than that of using a single additive (Adv. Energy Mater. 2018, 1703022).

4, summary

The above results indicate that LiTFSI-LiBOB is a chemically and electrochemically stable double salt electrolyte system in the Imide-Orthoborate double salt electrolyte system, and can form a lithium-free dendritic, dense and stable SEI film on the surface of lithium metal; By using LiPF6 as an additive to modify the LiTFSI-LiBOB double salt system, the resulting SEI film can exhibit thinner, denser, more stable characteristics, and LiTFSI-LiBOB double salt modified with LiPF6 + VC + FEC combination additive. The system can also increase the coulombic efficiency of the corresponding lithium metal anode to about 98.1%.