Low melting point molten salt electrolyte for lithium metal batteries

[Research Background]

Lithium metal batteries (LMBs) have a higher specific energy and are an important development direction for high specific energy batteries in the future. 

However, the small molecule electrolytes commonly used in lithium metal batteries have poor thermal stability and strong volatility, 

which increases the safety risk of LMB. Ionic liquids (ILs) have low volatility, 

but the large-sized cations in them are prone to decomposition in lithium metal systems, 

which can reduce the Coulombic efficiency and cycle life of the battery system.

[Job Introduction]

Based on this, the Chibueze V. Amanchukwu team at the University of Chicago published 

a research paper titled "Low melting alkali based molar salt electrolytes for solvent free lithium metal batteries" in Matter. 

The authors introduced a solvent-free, low melting point molten salt electrolyte for the LMB system, 

which is safer than state-of-the-art small molecule electrolytes and more stable than IL electrolytes. 

These molten salt electrolytes can achieve uniform lithium deposition, high Coulombic efficiency, and a wide electrochemical stability window. 

The cycling of batteries using high-voltage positive electrodes shows 

that the performance of molten salts is superior to that of liquid electrolytes and volatile liquid electrolytes. 

This study demonstrates the enormous potential of solvent-free molten salt electrolytes in terms of safety and high-performance LMB.

[Text and Image Introduction]

The author reported that a low eutectic melting temperature (~45 ° C) molten salt electrolyte can enable LMB to operate at moderate temperatures of 80-100 ° C. 

Binary (Li0.45K0.55FSA, 45:55, mol%) and ternary (Li0.30K0.35Cs0.35FSA, 30:35:35, mol%) mixtures were prepared using lithium, potassium, 

and cesium salts of bis (fluorosulfonyl) amide anions (LiFSA, KFSA, and CsFSA), exhibiting high ionic conductivity, high lithium migration number, 

wide electrochemical stability window, and high Coulombic efficiency.

Figure 1 (A) Comparison of traditional electrolyte, high concentration electrolyte, and solvent-free electrolyte. 

(B) Comparison of polymer electrolyte (PE), inorganic solid electrolyte (ISSE), and solvent-free molten salt electrolyte (Li0.30K0.35Cs0.35FSA). 

(C) The ternary plots of melting point, 

(D) enthalpy of melting transition, 

(E) and (F) ionic conductivity as a function of salt mole fraction at 80 ° C and 100 ° C, respectively.

Compared with typical liquid electrolytes (1m LiFSA/DME), 

they exhibit similar lithium metal overpotentials (although liquid electrolytes have lower viscosity and higher conductivity), 

which can improve cycling (Figure 1A). 

Compared with non-volatile liquid salts (1 M LiFSA/Pyr14FSA [1-butyl-1-methylpyridinebis (fluorosulfonyl) amide]), polymers, and inorganic solid electrolytes, 

these molten salts support higher current densities, higher ionic conductivity, and greatly improve cycling (Figure 1B). 

The melting transition peaks (Tms) of single salt LiFSA, KFSA, and CsFSA are 142 ℃, 101 ℃, and 104 ℃, respectively. 

Consistent with other salts, Figure 1C shows that the Tms of binary and ternary mixtures of these salts are much lower 

than those of single salts with nearly equimolar binary Li0.45K0.55FSA and Li0.5Cs0.5FSA, 

with Tms of~68 ℃ and 62 ℃, respectively. The eutectic Tm of ternary Li0.30K0.35Cs0.35FSA is even lower, at~45 ℃ (melting peak). According to reports, 

the Tm of the ternary eutectic mixture of LiNO3-KNO2-CsNO3 is around 90 ° C, which is lower than that of the binary mixture (LiNO3-KNO3, 125 ° C). 

Integrating the Tm peak area can yield Δ Hm. Figure 1D shows the average enthalpy in the range of 50-90 J g − 1, with higher amounts of LiFSA resulting in larger Δ Hm. 

Figures 1E-F show the ion conductivity of molten salt at 80 ℃ and 100 ℃, respectively. 

The author pays special attention to temperatures of 80 ° C and 100 ° C, as they allow for a temperature difference of at least~30 ° C compared to Tm. 

Generally speaking, the lower the melting point of a molten salt mixture, the higher the ionic conductivity. Above 60 ℃, the ionic conductivity is about 1 mS cm − 1.

Figure 2 (A) shows the CV curve of a Li | | SS half cell using molten salt and IL electrolyte at 80 ℃ and 1mV s-1. 

(B) ICP-MS analysis of metal layer deposited on copper electrode in Li | | Cu half cell. 

(C) DSC analysis of metal layer deposited on copper electrode in Li | | Cu half cell. 

SEM images of lithium deposited on copper electrodes using

 (D) Li0.45K0.55FSA electrolyte, 

(E) Li0.30K0.35Cs0.35FSA electrolyte, 

and (F) 1 M LiFSA/Pyr14FSA at 0.5 mA cm-2 and 1 mAh cm-2, respectively. 

XPS spectra of (G) Li 1s, (H) F 1s, and (I) N 1s on the Li surface in Li | | Cu batteries using Li0.30K0.35Cs0.35FSA molten salt electrolyte, and (J) Li 1s, 

(K) F 1s, and (L) N 1s on the Li surface in Li | | Cu batteries using 1M LiFSA/Pyr14FSA electrolyte after 10 cycles.

Figure 2A shows the cyclic voltammetry (CVs) of molten salt electrolyte in a Li | | SS (stainless steel) battery, with a scan rate of 1 mV s-1 and a temperature of 80 ° C. 

From Figure 2B, it can be seen that the sediment composition is mainly Li (close to 99%), with residual salts and impurities such as K and Cs, with relatively low content. 

In addition, the thermal map of the metal deposited on copper shows a Tm peak at 180 ° C (close to the Tm of Li metal), 

while peaks for K (63.5 ° C) and Cs (28 ° C) were not observed (Figure 2C). 

For the binary Li0.45K0.55FSA electrolyte, porous and fibrous morphology of lithium deposits can be observed 

at a current density of 0.5 mA cm-2 and a capacity of 0.5 mAh cm-2, 

while batteries with capacities of 1 (Figure 2D) and 2 mAh cm-2 exhibit dense large nodular lithium particle aggregates. 

In contrast, when the current density is 0.5 mA cm-2 and all test capacities are between 0.5 and 2 mAh cm-2, 

the dense nodular structure of Li particles deposited in the battery using a ternary electrolyte is shown in Figure 2E, 

and the effect is similar to that of high-quality liquid electrolytes such as 1m LiFSA/FDMB. 

Interestingly, the lithium deposition layer using IL electrolyte (1M LiFSA/Pyr14FSA) exhibited a rough 

and uneven surface due to the formation of whisker like lithium deposition layers (Figure 2F). 

X-ray photoelectron spectroscopy (XPS) analysis revealed the chemical composition of interface phases formed 

on lithium metal surfaces under different electrolytes (Figure 2G-2L). 

Compared with IL electrolytes, the interfacial phases of LiF (56.6 eV in Li 1s and 684 eV in F 1s spectra) 

and LiNx (395.4 eV in N1s spectra) using molten salts have higher intensities, 

while the interfacial phase of IL electrolytes using organic cations has a higher intensity of Li oxygen binding (LiOH/LiOR/Linx) at 55.8 eV.

Figure 3 shows the LSV curves of molten salt electrolyte and IL electrolyte for oxidation stability testing at 1mV s-1.

The electrochemical stability window of molten salt electrolytes was characterized using linear sweep voltammetry (LSV). 

At 80 ° C and 100 ° C, the stability of molten salt to SS (stainless steel) is as high as 5.3 V (Figure 3A), 

and the stability to aluminum is as high as 6.0 V (Figure 3B). 

In contrast, IL electrolyte appears more fragile at SS (~4 V), generates higher side reaction currents, and does not passivate aluminum. 

The constant potential maintenance measurement (Figure 3C and S6) shows that the aluminum working electrode maintains an increasing potential, 

which is confirmed by the LSV measurement results of oxidation stability Li0.30K0.35Cs0.35FSA. 

The SEM in Figure 3D shows that after being placed in a Li | | Al battery for 72 hours at 5.5 V and 80 ℃, 

the Al working electrode using the ternary Li0.30K0.35Cs0.35FSA mixture did not corrode. 

On the contrary, when using 1 M LiFSA/Pyr14FSA (Figure 3E) and 1 M LiFSA/DME (Figure 3F) electrolytes, 

pores and cracks were observed on the Al surface.

Figure 4 (A) Coulombic efficiency of lithium plating/stripping cycled at different current densities and temperatures. 

(B) Li | | Cu batteries undergo long-term cycling at 0.5 mA cm-2 to 1 mAh cm-2 at 80 ° C. 

(C) The rate performance of lithium batteries increased from 0.1 to 6 mA cm-2 at different current densities at 80 ° C and 100 ° C. 

(D) Long cycle of the battery at a current density of 1mA cm-2 to 1mAh cm-2 at 80 ° C. 

(E) Summary and comparison of overpotential of lithium batteries with different types of electrolytes at 20 ° C and high temperature under different current densities.

The author studied the Coulombic efficiency of lithium deposition/stripping on lithium metal 

anodes (LMAs) using Li0.45K0.55FSA and Li0.30K0.35Cs0.35FSA molten salt electrolytes,

 as well as 1M LiFSA/Pyr14FSA IL electrolytes, at different current densities in Li | | Cu batteries at 80 ° C and 100 ° C (Figure 4A). 

At a current density of 0.5 mA cm-2 and a surface capacity of 1 mAh cm-2, 

the authors further investigated the evolution of voltage and Coulombic efficiency during the plating/stripping process of Li | | Cu batteries (Figure 4B). 

From Figure 4C, it can be seen that as the current density increases, 

the overpotentials of both binary and ternary electrolytes show an increasing trend. 

However, due to lower viscosity and faster Li+transport, the overpotential of ternary electrolytes is always lower than that of binary electrolytes. 

At 80 ° C, the ternary electrolyte exhibits low overpotentials of 55 and 160 mV at 1 mA and 3 mA cm-2, respectively. 

At 100 ° C, the overpotential at 1 mA cm-2 is only 10 mV, and at a high current density of 5 mA cm-2, the overpotential is 60 mV, 

indicating good reaction kinetics. In order to analyze the stability and compatibility of molten salt electrolytes with metallic lithium, 

the long-term cycling stability at a current density of 1mA cm-2, a capacity of 1mAh cm-2, and a temperature of 80 ℃ is shown in Figure 4D. 

The battery using ternary electrolyte showed stable lithium cycling up to 500 cycles within 1000 hours, with an increase in overpotential from 30 to 65 mV. 

Figure 4E compares the overpotentials of molten salt electrolytes with reported small molecule liquid electrolytes, 

IL electrolytes, polymer/polymer composite solid electrolytes, 

and inorganic solid electrolytes at different current densities and temperatures. 

At high temperatures, the overpotential difference between the molten salt electrolyte (100 ° C) reported

 in this article and state-of-the-art liquid (room temperature) or IL electrolyte (60 ° C cycling) can be ignored.

At a C/3 ratio, the capacity of the ternary Li0.30K0.35Cs0.35FSA electrolyte is close to 

that of the IL electrolyte and significantly higher than that of the 1M LiFSA/DME electrolyte. 

Its performance is similar to that of the binary Li0.45K0.55FSA electrolyte. 

After three cycles of formation at C/20, the recyclability of Li | | NMC811 batteries with different electrolytes was further evaluated at C/3 rate charging (Figure 5A-B). 

In order to test the actual battery conditions, the authors assembled a battery with NMC811 positive electrode (9 mg cm-2) 

and thin lithium metal negative electrode (20 μ m) N/P ratio of 2.2 (Figure 5C). 

The charge discharge curves of the battery using molten salt as the electrolyte in several selected cycles are shown in Figure 5D. 

For the thin lithium NMC811 battery, the reduction in LMB area capacity using molten salt electrolyte will not significantly accelerate capacity decay until 100 cycles. 

At the 100th cycle, the discharge capacity of both molten salt electrolytes was~170 mAh g-1, equivalent to an 81% capacity retention rate. 

In addition, the author assembled an "non-polar" Cu | | NMC811 battery and characterized it, with its capacity and Coulombic efficiency shown in Figure 5E. 

The charge discharge curve of the selected cycle at this time is shown in Figure 5F. 

In the 100th cycle, the battery using ternary Li0.30K0.35Cs0.35FSA electrolyte showed a capacity of 75 mAh g-1, equivalent to a capacity retention rate of 40%, 

while the battery using IL electrolyte rapidly decayed to the same capacity in only 30 cycles. 

Due to the higher Coulombic efficiency of lithium deposition/stripping in the molten salt Li0.30K0.35Cs0.35FSA electrolyte compared to the IL electrolyte.

Figure 5: Capacity differential curves and H2-H3 phase transition analysis of raw materials (a, d) HMPM (b, e) and LMPM (d, f).

【 Summary 】

In this study, the author introduced solvent-free, low melting point, alkali metal molten salt electrolytes for high energy density LMB. 

Base molten salts avoid the problem of easy decomposition of electrolytes containing organic cation ions 

and reduce the safety hazards of traditional liquid electrolytes. 

In addition, compared with solid-state battery systems, the interface compatibility of molten salt electrolytes is better. 

The mixture of binary Li0.45K0.55FSA and ternary Li0.30K0.35Cs0.35FSA exhibits Tms at 68 ° C and 45 ° C, good ionic conductivity of~1 mS cm-1 at 60 ° C, 

high oxidation stability up to 6 V (relative to Li/Li+), and can achieve aluminum passivation. 

These molten salt electrolytes can support up to 99.8% lithium metal coulombic efficiency, with dense and uniform lithium deposition. 

In addition, the metal interface is enriched with SEI derived from inorganic anions. 

These molten salts can support up to 5mA cm-2, have low overpotential, 

are similar to flammable high concentration electrolytes, and are superior to solid polymers and inorganic electrolytes. 

Finally, these molten salt electrolytes exhibit excellent compatibility with high-voltage cathode materials such as NMC811 at 80 ° C, 

providing a specific capacity of 150 mAh g-1 and an average Coulombic efficiency of 99.65% in 100 cycles, 

surpassing IL electrolytes (1 M LiFSA/Pyr14FSA) and traditional liquid electrolytes (1 M LiFSA/DME). 

This new type of electrolyte system has great potential for application in LMB.