Electrolyte capable of realizing lithium intercalation in graphite negative electrode and lithium plating on surface of graphite negative electrode

Abstract

The invention discloses an electrolyte capable of realizing lithium intercalation inside a graphite negative electrode and lithium plating on the surface of the graphite negative electrode, and relates to a battery electrolyte. The invention aims to solve the technical problems of lithium separation caused by rapid charging or long-term circulation and sudden capacity reduction during rapid charging of a graphite negative electrode due to slow ion transmission in a solid-liquid interface and an electrolyte phase in the existing electrolyte. The electrolyte is obtained by dissolving a lithium salt into a solvent according to the concentration of the lithium salt being 0.6-1.0 mol / L; wherein the solvent is formed by mixing a weak solvation cosolvent, ethylene carbonate and diethyl carbonate; the weak solvation cosolvent is formed by mixing methyl propionate, ethyl propionate and ethyl acetate. The lithium ion transference number of the electrolyte is 0.28 and is 42.8% higher than that of a commercial electrolyte. And the ionic conductivity at 25 DEG C is 17.74 mS cm <-1 >, which is 18% higher than that of a commercial electrolyte. The invention can be used in battery field.

Description

Technical Field

[0001] The invention relates to a method for preparing an electrolyte. Background Art

[0002] Due to its high performance and low cost, graphite has become a universal negative electrode material for lithium-ion batteries. The excellent stability and long cycle life of graphite negative electrodes have also made it the mainstream of lithium-ion energy storage. However, the theoretical specific capacity of graphite negative electrodes is low (372mAh / g), which can no longer meet the urgent needs of various emerging fields for higher energy density. On the contrary, lithium metal negative electrodes have extremely high theoretical specific capacity (3860mAh / g) and are the first choice for realizing 500Wh / kg high-energy batteries. The composite lithium negative electrode prepared from graphite and lithium can promote a uniform electrochemical reaction of lithium on the surface of the negative electrode, inhibiting the formation and growth of lithium dendrites. However, in the existing electrolyte composed of ester solvents and lithium salts dissolved therein, dead lithium and solid-liquid interface degradation exist. Due to the difficulty in ion transmission at the solid-liquid interface and in the electrolyte liquid phase, lithium precipitation occurs during rapid charging or long-term cycling, shortening the cycle life. At the same time, the capacity of the graphite negative electrode drops sharply during rapid charging, limiting the promotion and application of lithium metal negative electrodes. Therefore, it is urgent to optimize the negative electrode energy storage configuration mode and adjust the electrolyte system. Summary of the invention

[0003] The present invention aims to solve the technical problems of slow ion transmission in the solid-liquid interface and the electrolyte liquid phase in the existing electrolyte, which leads to lithium deposition during rapid charging or long-term circulation and a sudden decrease in the capacity of the graphite negative electrode during rapid charging, and to provide an electrolyte that can realize lithium embedding inside the graphite negative electrode and lithium plating on the surface.

[0004] The electrolyte capable of realizing internal lithium insertion and surface lithium plating of a graphite negative electrode is obtained by dissolving a lithium salt in a solvent at a lithium salt concentration of 0.6-1.0 mol / L; wherein the solvent is a mixture of a weak solvating co-solvent, ethylene carbonate and diethyl carbonate at a volume ratio of 1:1:(0.1-0.3); and the weak solvating co-solvent is a mixture of methyl propionate, ethyl propionate and ethyl acetate at a volume ratio of (0.5-1):(0.3-0.5):1.

[0005] Furthermore, the lithium salt is lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium bis(fluorosulfonyl)imide (LiFSI).

[0006] Due to the problems of solvent co-embedding and interfacial chemistry failure in graphite negative electrodes and lithium metal negative electrodes, the use of a mixed negative electrode of graphite / lithium metal has great potential in terms of cycle stability and energy density. Conventional weakly solvated electrolytes exhibit weak ion-dipole interactions, which promote rapid desolvation of ions at the solid-liquid interface, but the bulk ion transport kinetics are slow, thereby inducing high overpotentials and lithium dendrite growth. The present invention uses methyl propionate, ethyl propionate, and ethyl acetate to prepare a weakly solvated co-solvent, and adjusts the solvation shell with loose coordination characteristics through anion-solvent interactions. It has the characteristics of weak lithium ion-anion interaction and strong lithium ion-solvent interaction, while allowing a large number of anions to participate in coordination, and promoting electrolyte liquid phase ion transport through anion dissociation. These characteristics allow lithium metal to be reversibly deposited on the surface of lithiated graphite, thereby achieving capacity compensation and increasing the effective capacity of the negative electrode. The electrolyte described in the present invention can simultaneously achieve internal lithium insertion and surface lithium plating of the graphite negative electrode, which is helpful for constructing a high energy density battery system.

[0007] The lithium ion migration number of the electrolyte of the present invention is 0.28, which is 42.8% higher than that of the lithium ion migration number in the commercial electrolyte. The ionic conductivity at 25°C is 17.74 mS cm -1 , the ion conductivity is 18% higher than that of commercial electrolytes. It can be used in the battery field. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a graph of the ion migration number of the electrolyte prepared in Example 1;

[0009] Figure 2 It is a graph of ion mobility in commercial electrolytes;

[0010] Figure 3 is a graph showing the relationship between ionic conductivity and temperature in the electrolyte prepared in Example 1 and a commercial electrolyte;

[0011] Figure 4 is a graph of ion migration activation energy of the electrolyte prepared in Example 1 and a commercial electrolyte;

[0012] Figure 5 It is the electrode cycle performance diagram in the electrolyte prepared in Example 1 and the commercial electrolyte;

[0013] Figure 6 The graphs are the cycle performance of the electrodes using the electrolyte of Example 1 and the commercial electrolyte;

[0014] Figure 7 This is the full battery cycle performance curve using the electrolyte of Example 1. DETAILED DESCRIPTION

[0015] The beneficial effects of the present invention are verified by the following examples.

[0016] Embodiment 1: The electrolyte that can form the graphite / lithium metal mixed negative electrode of this embodiment is obtained by dissolving lithium hexafluorophosphate in a solvent at a concentration of 1.0 mol / L; wherein the solvent is a mixture of a weak solvating co-solvent, ethylene carbonate and diethyl carbonate in a volume ratio of 1:1:0.3; and the weak solvating co-solvent is a mixture of methyl propionate, ethyl propionate and ethyl acetate in a volume ratio of 0.6:0.3:1.

[0017] A commercial electrolyte was used as a comparison, the composition of which was: 1M LiPF6 dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1.

[0018] The ion migration number of the electrolyte prepared in Example 1 and the commercial electrolyte was tested, and the lithium ion migration number was measured by combining AC impedance and DC polarization in the assembled Li symmetric battery. The impedance curve before and after polarization and the current-time curve of the electrolyte prepared in Example 1 are shown in FIG. Figure 1 The impedance curves and current-time curves before and after polarization in commercial electrolyte are shown in Figure 2 shown; from Figure 1 It can be seen that the lithium ion migration number in the electrolyte prepared in Example 1 is 0.40. Figure 2 It can be seen that the lithium ion migration number in the commercial electrolyte is 0.28. The lithium ion migration number in the electrolyte prepared in Example 1 is 42.8% higher than that in the commercial electrolyte.

[0019] The ionic conductivity of the electrolyte prepared in Example 1 and the commercial electrolyte were tested, and the relationship between the ionic conductivity and temperature of the assembled stainless steel symmetrical battery test was shown in the following figure: Figure 3 As shown, from Figure 3 It can be seen that the ionic conductivity of the electrolyte prepared in Example 1 at 25°C is 17.74 mS cm -1 , while the ionic conductivity of commercial electrolyte at this temperature is 14.99 mS cm -1 The ion conductivity of the electrolyte prepared in Example 1 is 18% higher than that of the commercial electrolyte.

[0020] Figure 4 The ion migration activation energy curves of the electrolyte prepared in Example 1 and the commercial electrolyte are shown in FIG. Figure 4 It can be seen that the ion migration activation energy of the electrolyte prepared in Example 1 is 15.05 kJ mol -1 The activation energy of ion migration in commercial electrolyte is 18.14 kJ mol -1The ion migration activation energy of the electrolyte prepared in Example 1 is 17% lower than that of the commercial electrolyte.

[0021] Control the surface loading of graphite anode to 1 mAh cm -2 When the electrolyte of Example 1 and the commercial electrolyte were used respectively, the electrode cycle performance was tested at 4C current; the constant current discharge-constant capacity cut-off-constant current charge-constant voltage cut-off test procedure was used, and the obtained electrode cycle performance diagram was as shown in FIG. Figure 5 As shown, from Figure 5 It can be seen that no matter the electrolyte of Example 1 or the commercial electrolyte, there is an obvious voltage platform (>0V vs.Li + / Li), indicating that lithium ions are embedded in the graphite phase, ② the voltage is less than 0V (vs.Li + / Li), indicating that lithium ions exist on the graphite surface in the form of lithium metal; the electrolyte of Example 1 can achieve 1500 cycles of graphite / lithium metal negative electrode with an average coulombic efficiency of 99.8%, while under the same conditions, the commercial electrolyte fails quickly.

[0022] Control the surface loading of graphite anode to 3 mAh cm -2 When the electrolyte of Example 1 and the commercial electrolyte were used respectively, the electrode cycle performance was tested at 1C current, and the obtained electrode cycle performance diagram is as follows Figure 6 As shown, from Figure 6 It can be seen that the electrolyte of Example 1 can achieve 120 cycles of graphite / lithium metal negative electrode with an average coulombic efficiency of 98.6%, while the commercial electrolyte fails quickly.

[0023] The electrolyte of Example 1 was used, graphite was used as the negative electrode, NCM811 was used as the positive electrode, and a full battery was assembled. The N / P ratio was controlled to be 0.7. The full battery cycle performance when the N / P ratio was 0.7 was tested. The full battery cycle performance curve obtained is as follows: Figure 7 As shown, from Figure 7 It can be seen that the electrolyte of Example 1 can achieve 100 cycles with a capacity retention rate of 85%.

[0024] In the electrolyte of the present invention, the weak solvating co-solvent can reduce the coordination number of ethylene carbonate and diethyl carbonate, induce the sodium salt anion to participate in the coordination, and the strong solvent-lithium ion interaction coupled with the weak anion-lithium ion interaction allows the lithium ions to achieve rapid bulk phase migration, has high ionic conductivity, and can simultaneously achieve rapid migration of lithium ions in the electrolyte liquid phase and at the solid-liquid interface. The electrolyte can simultaneously achieve lithium ion embedding between graphite layers and deposition on the graphite surface, establish a graphite / lithium metal mixed negative electrode, and thus improve the energy density of the battery.

Claims

1. An electrolyte capable of realizing lithium insertion inside a graphite negative electrode and lithium plating on the surface, characterized in that: The electrolyte is obtained by dissolving lithium salt in a solvent at a lithium salt concentration of 0.6-1.0 mol / L; the solvent is a mixture of a weak solvating cosolvent, ethylene carbonate and diethyl carbonate in a volume ratio of 1:1:(0.1-0.3); the weak solvating cosolvent is a mixture of methyl propionate, ethyl propionate and ethyl acetate in a volume ratio of (0.5-1):(0.3-0.5):

1.

2. The electrolyte capable of realizing internal lithium insertion and surface lithium plating of a graphite negative electrode according to claim 1, characterized in that: The lithium salt is lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide or lithium bis(fluorosulfonyl)imide.

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