Development of high-energy density lithium metal batteries by constructing a robust SEI layer with Lithium hexafluorophosphate and lithium nitrate

To solve these challenges of high energy density battery development, lithium hexafluorophosphate (LiPF6) and lithium nitrate (LiNO3) are used as F-donating additives and N-donating additives, respectively, to stabilize the interfacial reaction of metal-rich electrodes Li and Ni. It proved a potential of Li metal anode for practical use and potential for developing advanced batteries with higher energy density. Theoretical capacities of various candidate materials for cathode and anode [1]. a) schematic of the formation of the unwanted interphase of Li metal caused by the dendritic growth of Li metal; (b) undesirable interfacial reaction between trace gas and Li metal leading to battery safety problems [8];.

Cycle efficiency of Li metal in dilute electrolyte, highly concentrated electrolytes (HCE) and localized highly concentrated electrolytes (LHCE) [21]. a) Li migration energy and (b) electronic band gap for SEI components. The cell is pre-cycled with 2M LiFSI DME (b) Mechanism of how oxidation degradation of DME corrodes Li metal. The cells are pre-cycled at 0.1 C rate and 0.5 C rate 3 times, then cycle at 0.9 C rate. a) Voltage profile of precycles, (b) Cycle performance of electrolytes in NCM|Li full cell and (c) CEs of electrolytes. a) The voltage profile of NCM|Cu precycled at 0.1C rate with the cathodes retrieved after being precycled in NCM|Li with each electrolyte.

After pre-cycle, the cell is charged to 4.2V at 0.1C rate and maintains constant voltage for 10 hours. SEM image of cathodes cycled 40 times with each electrolyte at 0.9C rate in NCM|Li full cell. Li metals are pre-cycled at 0.1C rate and cycle 3 times at 0.5C rate in NCM|Li cells at 25oC.

SEM images of Li metal anode taken after pre-cycling with each electrolyte in NCM811|Li.

Introduction of Li metal anodes and Ni-rich cathodes

Challenges of Li metal anodes

Since SEI cracking causes additional electrolyte decomposition and more uneven SEI layer, additional dendritic growth of Li, electrolyte depletion, formation of resistive SEI, and ultimately reduced cycle performance may occur. Due to the chemical instability of Li metal, traces of water or oxygen trapped in the battery or formed by electrolyte decomposition side reactions can react with Li metal to form hazardous gas such as H2. Although some of the product gases formed by the reaction of traces of air or organic electrolyte are not flammable, they can increase the internal pressure and deform the batteries (Figure 3c) leading to battery explosions.

To use the strength of Li metal and develop LMB with high energy density, the amount of excess Li should be reduced to the minimum amount because it occupies significant volume, increases the weight and cost of batteries (Figure 4). However, as the Li excess decreases, the volume expansion of the Li anode during cycling increases and the remaining active Li, which can compensate for the irreversible capacity of the Li metal anode, decreases. As a result, the cycle life of high energy density LMBs with less excess Li may be worse than ordinary LiBs, so it is necessary to increase the reversibility of Li metal in order to can be used with less excess Li.

Ultimately, it should be able to be applied without redundant Li anode, overcoming all the problems of Li metal described above. a) the schematic of the formation of unwanted interphase of Li metal caused by dendritic growth of Li metal; (b) unwanted interfacial reaction between tracer gas and Li metal leading to safety problem of the batteries [8];.

Challenges of Ni-rich cathodes

Strategies for electrolytes of LMBs

It is obvious that high-concentration electrolytes can improve the potential of LMBs than conventional electrolytes, but the performance of batteries needs to be improved more because there is limitation of the salt concentration in the electrolyte, and high concentration causes high viscosity and high cost. Not only suppressing the oxidative/reductive decomposition of the organic solvent in the electrolytes, there are also other strategies using fluorinated solvents, which can form LiF-rich inorganic SEI layer as a product of their reductive decomposition. Fluorinated solvents such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrapropyl ether (TTE) [18] or fluorinated carbonate (FEC) [19] and bis(2,2,2-trifluoroethyl) ether ( BTFE) [20] is used as a co-solvent and shows meaningful stabilization effects of the co-solvents on the Li-metal interface, improving cycle performance and additional physical properties such as volatility and non-flammability.

In addition, highly fluorinated solvents are reported to have a dilution effect while maintaining a highly concentrated solvation structure composed of Li ions, major solvent molecules, and salt anions, as highly fluorinated solvents do not participate in solvation [20][21]. This means that if we add a lot of highly fluorinated solvents, we can take advantage of the electrolyte's highly concentrated solvation structure and low viscosity. Cycling performance of Li metal in dilute electrolyte, highly concentrated electrolytes (HCE) and localized highly concentrated electrolytes (LHCE) [21].

Experimental

Electrochemical measurements

2032 button cells with 40 μm PE separator and NCM811 cathodes were assembled in an Ar-filled glove box. After pre-cycling, NCM811|Li full cells were cycled 3 times at a rate of 0.5 C and then at a rate of 0.9 C. For the electrochemical drift test, the cells were fully charged after the pre-cycling steps and kept in constant voltage mode for 10 hours ( CV).

2016 coin cells with 0.5T spacers or 2032 coin cells with 1T spacers mounted with 40 µm used metal anode, PE spacer and Cu substrate in an Gold filled glove box. The Type 2016 coin with 40 μm Li metal anodes with 1T spacers, PE spacers and 40 μm Li metal anodes were mounted in a Gold-filled glove box. After pre-cycling, cells were cycled at a rate of 0.5C 3 times, then cycled at a rate of 0.9C with the battery measurement system (WonATech WBCS 3000).

Characterization

Results and Discussion

• Electrochemical performance of the electrolytes in Li|Cu and Li|Li cells
• Electrochemical performance of LiPF 6 and LiNO 3 additives in NCM|Cu and
• Synergetic effect of LiPF 6 and LiNO 3 on electrochemical performance of
• Effect of LiPF 6 and LiNO 3 on NCM811 cathodes
• Effect of LiPF 6 and LiNO 3 on Li metal

In Figure 14a, it is shown that the Li reversibility and Coulombic efficiencies (CEs) of the cells increased when LiPF6 and LiNO3 additives were added, respectively. This means that it is important to control the amount of LiF-forming additive to maximize the reversibility of Li. In Figure 17a-c, it is shown that without excess Li, the ICE of the electrolyte decreased about 23%.

However, at an operating voltage of 1V-0V, the irreversible capacity of the electrolyte without additives was only 5%. Figure 18c,d shows that none of the additive electrolytes had poor CEs in NCM|Cu compared to NCM|Li, which means that the CEI of the cathode could not suppress the continuous oxidative decomposition of DME in the cycles, so the oxidation byproducts of the reaction is continuously caused irreversible reactions at the Li metal anode. However, it is suggested in Figure 15 that the additions of LiPF6 and LiNO3 have synergistic effects on the lithium anode and cathodes, so that they can improve the performance of Li-metal batteries.

Figure 21 shows that the addition of LiPF6 greatly improved the cycle performance of the cells, reaching 397 cycles for 80% retention in 3% LiNO3 + 1.5% LiPF6. Since the CE of Li-metal batteries means the CE of the cathode [28], it can be expected that the amount of LiPF6. To reconfirm that the CEI of the cathode improves as the amount of LiPF6 increases, cathodes pretreated in NCM|Li with each electrolyte were recovered and repretreated in NCM|Cu with 2M LiFSI DME electrolytes to see the ICE of each cathodes (Figure 22). ).

It is shown that the precycled cathode with two electrolytes LiNO3 + LiPF6 had the highest ice among the electrolytes, and the cathode of 3% LiNO3 + 3% LiPF6 electrolyte had the highest discharge capacity, which means that one of the significant roles of LiPF6 in LiNO3 . + LiPF6 is to improve the interphase stability of the cathode. Electrolyte cycling performance of NCM811|Li full cells with 2032 type coin cell. a) Precycle voltage profile, (b) Electrolyte cycling performance of NCM|Li full cell and (c) CE of electrolytes. a) Voltage profile of NCM|Cu precycled at 0.1C rate with cathodes taken after they were precycled to NCM|Li with each electrolyte. To see the correct electrolyte cycling performance without the effect of excess Li, NCM\Cu cells were cycled with each electrolyte (Figure 23).

It means that the decrease of CEs indicates the consumption of this Li metal from the reservoir in the cells, and the reason for higher CEs of 3% LiNO3 + 1.5% LiPF6 in the decreasing range of CEs (10th cycle ~ 25th cycle) is that it consumed the Li reservoir. slower due to the higher Li reversibility than that of 3% LiNO3 + 3% LiPF6 (Figure 14a, 15). a) Stress profile of NCM|Cu, pre-cycled at a rate of 0.1 C, (b) cycle performance of NCM|Cu, cycled three times at a rate of 0.5 C, then cycled at a rate of 0.9 C, (c) CEs of the NCM|Cu cells. Oxidative stability of the electrolytes in (a) SUS|Li coin cells and (b) Al|Li coin cells. The SEM images of the cathodes retrieved after a cycle with each electrolyte are shown in Figure 27 and Figure 28.

To analyze the chemical composition of the SEI layer, XPS analysis was performed on the Li metal of NCM|Li full cells after pre-cycle and 3 cycles at a rate of 0.5 C (Figure 29). The structure of the SEI electrolyte LiNO3 + LiPF6 was analyzed in more detail by TOF-SIMS analysis.

Conclusion

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