Enhanced charging capability of lithium metal batteries based on lithium bis(trifluoromethanesulfonyl)imide-lithium bis(oxalato)borate dual-salt electrolytes
Graphical abstract
Introduction
Continuous development of portable electronics, electric vehicles, and the smart grid requires energy storage systems that have an energy density that is higher than the state-of-the-art lithium (Li)-ion batteries (LIBs). To this end, Li metal batteries (LMBs) are considered the “holy grail” of energy storage systems because of Li metal's extremely high theoretical specific capacity (3860 mAh g−1) and the lowest redox potential (−3.040 V vs. standard hydrogen electrode) [1]. The Li metal anode is superior in specific energy to the conventional graphite anode (whose theoretical specific capacity is only 372 mAh g−1) used in LIBs [2]. Therefore, Li metal has been widely used for the anodes in Li-sulfur batteries [3], [4] and Li-air batteries [5], [6]. Compared to the problematical sulfur or air cathodes, LMBs using intercalation compounds widely used in LIBs or high-voltage conversion compounds as cathode materials could be more promising in meeting the increasing need for electrochemical energy storage systems that have high energy densities [7]. As reported by Gallagher et al., recently, the LMBs with Li metal anodes and Li-rich layered oxide cathodes have a theoretical energy density of ca. 900 Wh Kg−1, which is twice that of the graphite anode and LiNi1/3Co1/3Mn1/3O2 cathode (ca. 400 Wh Kg−1) [8].
Efforts to develop LMBs before the 1980s proved fruitless mainly because of the safety issues induced by the growth of Li dendrites during repeated charge/discharge cycles [8], [9], [10], [11]. Recently, technologies that suppress the growth of Li dendrites have been widely investigated and developed; e.g., use of polymer or solid-state electrolytes [12], [13], highly concentrated electrolytes [14], [15], [16], self-healing electrostatic shield electrolyte additives [17], [18], and protective layers coated on Li metal anodes [19], [20]. It should be noted that the cycling stability of Li metal anodes with high Coulombic efficiency and capacity retention, especially at relatively high charge/discharge current densities, needs to be guaranteed in advance to ensure successful commercialization of LMBs. Lv et al. reported that in Li||LiNi0.8Co0.15Al0.05O2 (NCA) cells with the conventional LiPF6-carbonate-based electrolyte, fast capacity fading was observed during charging at high current densities due to the quick formation of a highly resistive solid electrolyte interphase (SEI) entangled with “dead” Li metal particles [21]. In Li||LiCoO2 cells, the ionic liquid (IL)-based electrolyte containing a high concentration of 3.2 mol kg−1 lithium bis(fluorosulfonyl)imide [LiFSI, LiN(SO2F)2] in N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide exhibited excellent rate capability, in spite of its significantly higher viscosity and lower conductivity [22].
Compared to the use of high-cost ILs, the reformulation of electrolytes with different Li salts is an easy and cost-effective approach. Recently, Miao et al. explored a dual-salt electrolyte composed of LiFSI and lithium bis(trifuoromethanesulfonyl)imide [LiTFSI, LiN(SO2CF3)2] in mixed ether solvents in order to simultaneously cope with the low cycle efficiency and Li dendrite formation on the Li metal anode during charge/discharge processes [23]. Our team also investigated the effects of the highly concentrated electrolytes of LiFSI or LiTFSI in ethers on improving Li Coulombic efficiency (CE) and suppressing the growth of Li dendrites [14]. With the unique protection of SEI films and the improved Li growth pattern, a high CE of ca. 99% and dendrite-free Li deposition have been achieved. Moreover, the excellent cycling performance and favorable Li morphology can be retained even at a high current density of 10 mA cm−2. However, the ether-based electrolytes normally are not electrochemically stable at voltages around 4 V, so they cannot be used in batteries that need to be charged to 4 V and above.
Basically, it is more practical to reformulate electrolytes based on the state-of-the-art LiPF6-carbonate electrolytes to meet the requirements for LMBs with high-voltage intercalation or conversion cathode materials and at high current densities during cycling. Compared to LiPF6, which is sensitive to moisture and heat, LiTFSI is thermally stable and insensitive to moisture [24], [25]. Xu stated an empirical rule concerning the resistance of the SEI on the Li metal anode: an electrolyte with higher bulk ion conductivity usually results in an SEI of lower impedance [24]. Fast charging of LMBs using the LiTFSI-based electrolytes may be enabled by the highly conductive SEI formed on the Li electrode. The main limitation of LiTFSI is its corrosion of the aluminum (Al) current collector at voltages above 3.7 V [26], [27], [28]. Recently, we reported the effects of the dual salts of LiTFSI and lithium bis(oxalato)borate [LiBOB, LiB(C2O4)2] on the suppression of Al corrosion and the improvement in cell performance of a Li||LiFePO4 (LFP) battery system with a low LFP loading [29]. In this work, the effects of LiTFSI-LiBOB dual-salt electrolytes on the charge rate of LMBs with a relatively high loading NCA electrode are reported.
Section snippets
Experimental
LiTFSI, LiPF6, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) of battery grade were ordered from BASF Battery Materials. LiBOB of battery grade was obtained from Chemetall with no charge. All of the chemicals were stored in an MBraun glove box filled with purified argon for the preparation of electrolytes. The dual-salt electrolyte was composed of 0.6 M LiTFSI and 0.4 M LiBOB (or LiTFSI0.6-LiBOB0.4) in EC-EMC (4:6 by wt.). For comparison, the control
Results and discussion
Firstly, in order to evaluate the fast-charging properties of the NCA electrode, the cycling performances of the graphite||NCA cells and the Li||NCA cells with the conventional LiPF6/EC-EMC electrolyte at 1.50 mA cm−2 (equal to 1C charge rate) were tested and compared. Prior to the 1C cycling, all cells were conditioned through two formation cycles to allow the SEI layers to be generated on both the anodes and cathodes: the Li||NCA cells were charged and discharged at 0.15 mA cm−2 (i.e., 0.1C
Conclusions
The fast chargeability of the Li||NCA cells with two carbonate-based electrolytes was systematically investigated. The fast capacity fading in the conventional LiPF6 electrolyte is related to the thick interphase layer with high impedance covered on Li metal during fast charging. However, the LiTFSI-LiBOB dual-salt electrolyte exhibited good cycling stability during fast charging, because of the ability of this electrolyte to form a good film on Li metal and the highly conductive nature of the
Acknowledgements
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research (BMR) programs of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231, Subcontract No. 18769. The microscopic images and spectroscopic measurements were conducted at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL)—a national scientific user facility located at PNNL, which is sponsored
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