The present invention relates to an electrolyte additive for a lithium-ion battery, an electrolyte including the same, and a lithium-ion battery including the same.
The successful commercialization and widespread application of ion batteries (LIBs) have had a profound impact on our daily lives, from portable electronics to electric vehicles to smart grids.
While the demand for high energy density LIB has increased significantly in recent years, existing LIBs based on graphite anodes (372 mAh g-1) seem to have reached their limits in terms of energy storage capacity, so they cannot keep up with this demand. In this situation, lithium metal is considered the ultimate positive electrode due to its low density (0.59 g cm-3), low electrode potential (−3.04V vs. standard hydrogen electrode), and extremely high energy density (3,860 mAh g-1).
This is because the energy density (500 Wh kg-1) can be doubled compared to the current LIB when paired with a high voltage negative electrode material such as LiNixCoyMn1−x−yO2 (Ni-rich NCM, Ni≥60%). However, the lithium metal positive electrode (LMA) particularly has thermodynamic instability with respect to carbonate, and causes problems such as lithium metal and electrolyte consumption, non-uniform and mechanically weak solid electrolyte interface (SEI) and lithium dendrite formation, and thus, low cycle life and coulombic efficiency.
The SEI layer on the surface of the lithium metal anode obtained by reduction of the solvent and the lithium salt can stabilize the lithium metal anode by minimizing contact with the electrolyte and side reactions. However, the organic rich SEI layer containing RCO2Li arising from carbonates results in a large volume change of the lithium metal anode due to its strong interaction with the lithium metal anode surface and low interfacial energy, and unavoidable cracking of the surface during cycling and eventually reducing the coulombic efficiency.
That is, the carbonate electrolyte has been successfully applied to existing LIBs due to the compatibility of the graphite anode and the organic material-rich SEI under a small volume change (˜13%) during (di) intercalation, but low CE (<90%) and lithium filament formation in the carbonate electrolyte is problematic due to the formation of a porous thick and fragile SEI layer having a low shear modulus of less than 1 GPa when paired with a lithium metal anode.
This is due to the presence of lithium alkyl carbonate (ROCO2Li) in the reduction of the solvent, and it is very important to control the thickness, composition, and characteristics of the SEI in applying the lithium metal anode to the LIB, accordingly.
As additives that are materials added to the carbonate electrolyte, ionic liquids, fluoroethylene carbonate (FEC), vinylene carbonate (VC), amide derivatives, and the like have been studied. Such additives have been studied to promote the formation of an inorganic-rich SEI layer containing LiF and Li2CO3 to improve the CE stability of carbonate electrolytes. However, organic by-products resulting from the decomposition of additives are still problematic.
Compared to carbonate based electrolytes, ether based electrolytes are more stable, suitable for Li anodes, and promote the formation of Li20, which is thicker and has less Dentrite phase.
However, the use of an ether-based electrolyte in the high voltage LMB is problematic due to low cathode stability (<4V). That is, although improved CE can be obtained, it is difficult to maintain stability for a long cycle due to the formation of organolithium alkoxy (ROLi) species. Therefore, there is a need to develop new electrolyte additives to optimize the SEI layer composition and improve CE stability.
Accordingly, an object of the present invention is to provide a new electrolyte material and composition for a Lithium Metal Anode (LMA) battery, optimizing the SEI layer and achieving CE stability.
According to an aspect of the present invention, there is provided an electrolyte additive for a lithium-ion battery, wherein the electrolyte additive has a minimum electrostatic potential (ESP) of about-151 KJ mol-1 to about-100 KJ mol-1.
In one example of the present invention, the anode of the lithium-ion battery above includes a lithium metal.
The electrolyte additive may include at least one selected from the group consisting of bis(2,2,2-trifluoroethoxy)methane (BTFM), 1,1,1-trifluoro-2-methoxyethane, bis(2,2,3,3-pentafluoropropoxy)methane, and 1,1,1,3,3,3-hexafluoro-2-((2,2,2-trifluoroethoxy)methoxy)propane.
The present invention provides an electrolyte for a lithium-ion battery including the electrolyte additive for a lithium-ion battery described above.
In an exemplary embodiment of the present invention, the electrolyte for a lithium-ion battery includes: the additive described above; an electrolyte salt; and an ether-based or carbonate-based solvent.
In one embodiment of the present invention, the additive includes at least one selected from the group consisting of bis(2,2,2-trifluoroethoxy)methane (BTFM), 1,1,1-trifluoro-2-methoxyethane, bis(2,2,3,3,-pentafluoropropoxy)methane, and 1,1,1,3,3,3-hexafluoro-2-((2,2,2-trifluoroethoxy)methoxy)propane, and the volume ratio of the additive and the solvent is 8:1 to 1:1.
The electrolyte salt may be lithium bis(fluorosulfonyl)imide (LiFSI), and the anode of the lithium-ion battery may be lithium metal.
The present invention also provides a lithium-ion battery including the electrolyte additive described above, and in an exemplary embodiment of the present invention, the additive is bis(2,2,2-trifluoroethoxy)methane (BTFM).
Li2O dominates the SEI layer of the lithium-ion battery, and an electrolyte salt of the electrolyte of the lithium-ion battery is (lithium bis(fluorosulfonyl)imide (LiFSI)).
In one embodiment of the present invention, the solvent of the electrolyte is 1,2-dimethoxyethane (1,2-dimethoxyethane, DME).
The present invention provides the electrolyte additive characterized in that the minimum electrostatic potential (ESP) of the electrolyte additive is-150-151 KJ mol-1 to −1200 KJ mol-1. That is, a component including a minimum electrostatic potential (ESP) of −151 KJ mol-1 to −100 KJ mol-1 was used as an electrolyte, thereby achieving a very high Li2O content (63%) in SEI with uniform phase distribution. In addition, high coulombic efficiency (CE) of 99.72% is possible together with mechanical stability, and excellent capacity maintenance is possible at 1C (1C=1.6 mA cm-2) in a Li|LiNi0.8Co0.1Mn0.1O2 (NCM811) complete cell including a negative/positive capacity (N/P) ratio of 2.5 and a complete cell without a Cu|NCM811 anode. Therefore, the electrolyte additive according to the present invention is expected to greatly contribute to the commercialization of conventional lithium metal anode-based batteries.
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms or words used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation.
Therefore, since the embodiments described in the present specification and the configurations shown in the drawings are only the most preferred embodiments of the present disclosure and do not represent all of the technical ideas of the present disclosure, it should be understood that there may be various equivalents and modifications that may replace them at the time of filing the present application.
Due to the fundamental limitations of the conventional graphite-based anode, lithium metal has been spotlighted as a new anode electrode material for low density, low electrode potential, and extremely high specific capacity.
However, various problems such as thermal instability of a lithium metal anode with respect to an organic solvent represented by carbonate, non-uniform and mechanically weak solid electrolyte interface (SEI) layer, and lithium dendrite formation result in a short cycle life and low Coulombic efficiency.
When a carbonate-based or ether-based electrolyte is used, the RCO2Li-containing SEI layer promotes a large volume change of a lithium anode and causes a mechanical problem. Therefore, in the case of a lithium metal anode (lithium metal anode) other than a graphite anode, it is necessary to develop an additive (co-solvent) of a new electrolyte. In the present specification, the additive is a co-solvent or diluent mixed with an ether-based or carbonate-based solvent, and is referred to as an additive for convenience, but the scope of the present invention is not limited to these terms.
To solve the above-described problem, the present invention provides bis(2,2,2-trifluoroethoxy)methane (BTFM) and bis(2,2,2,3-tetrafluoropropoxy)methane (BTFPM) as an electrolyte additive characterized in that a minimum electrostatic potential (ESP) of the electrolyte additive is-151 KJ mol-1 to −100 KJ mol-1. When these additive is used, a very high Li2O content (63%) can be achieved in the SEI together with a high uniform phase distribution, thereby solving various problems such as thermal instability of the lithium metal anode with respect to the organic solvent, non-uniform and mechanically weak solid electrolyte interface (SEI) layer, lithium dendrite formation, and the like.
The excellent properties of the electrolyte including the additive according to the present invention enable high coulombic efficiency (CE) of 99.72%, which is due to the uniformly distributed high Li2O ratio, as well.
In addition, 90% capacity retention was achieved after 200 cycles in 1C of Li|LiNi0.8Co0.1Mn0.1O2 (NCM811) full cells with a negative/positive capacity (N/P) ratio of 2.5, and 80% capacity retention was achieved after 596 cycles in 3C. It also showed 64% capacity retention after 80 cycles at 1C (1C=1.6 mA cm-2) of the complete cell without the Cu|NCM811 anode.
Hereinafter, properties of the electrolyte additive according to the present invention will be described in more detail.
To prepare an electrolyte composition for solving the above-described problems, the present invention calculates and classifies the previously reported minimum electrostatic potential (ESP) values of the solvent and ether-based diluents.
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In the present invention, an ESP range (−151 KJ mol-1 to −100 KJ mol-1) between NCD and SCS is defined as a weak-coordinated diluent (WCD), and a material including an ESP in this range is provided as an additive of an electrolyte of a lithium metal anode-based battery.
Electrolytes including such additives in the ESP range between the NCD and SCS are suitable for achieving optimal anion decomposition for high Li2O content in the SEI. As such additives, the present invention provides bis(2,2,2-trifluoroethoxy)methane (BTFM), which is a fluorinated ether. However, in addition to this, 1,1,1-trifluoro-2-methoxyethane, bis(2,2,3,3,-pentafluoropropoxy)methane, and 1,1,1,3,3,3-hexafluoro-2-((2,2,2-trifluoroethoxy)methoxy)propane are also within the scope of the present invention.
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Hereinafter, the present invention will be described by using bis(2,2,2-trifluoroethoxy)methane (BTFM) as an additive component.
In an exemplary embodiment of the present invention, 1M and 2M lithium bis(fluorosulfonyl)imide (LiFSI) were dissolved in a volume ratio of 7:1 or 3:1 to prepare two types of electrolytes (hereinafter, Example 1, Example 2). Each electrolyte all showed excellent battery characteristics and formation of an SEI layer rich in inorganic materials with a high Li2O content.
In an exemplary embodiment of the present invention, the volume ratio of the additive and the solvent may be set according to the concentration of the salt, and in the case of 2M salt, the salt is not dissolved in a region larger than 3:1 (e.g., 4:1), and in the case of 1M, the volume ratio is 8:1 to 1:1. Therefore, the volume ratio of the additive and the solvent may be freely selected at a salt concentration and a level at which the salt of a used concentration may be dissolved, and a level of 8:1 to 1:1 is preferable.
An acetal functional group O—CH2-O was introduced into bis(2,2,2-trifluoroethoxy)methane (O—CH2-CH2-O), which is an electrolyte component according to an embodiment of the present invention, because Li+ affinity is lower than that of BTFM due to a stereophonic effect. Furthermore, the introduction of electron withdrawing groups (—CF3) not only increased high voltage resistance, but also reduced solvating power to achieve WCD.
Both the two types of electrolytes of Example 1 and Example 2 according to an embodiment of the present disclosure exhibited excellent oxidation resistance up to 5.5V. In particular, the 2M LiFSI-3BTFM-1DME electrolyte (Example 2) allowed the formation of inorganic SEI with remarkably high Li2O content, obtaining the highest average CE of 99.72% among the values reported so far in the Li|Cu half-cell.
Some excess Li|NCM811 (LiNi0.8Co0.1Mn0.1O2) complete cells containing a 2M LiFSI-3BTFM-1DME electrolyte with an N/P (negative/positive capacity) ratio of 2.5 exhibited excellent electrochemical performance of 90% capacity.
The capacity retention rate was 90% after 200 cycles at 1 C, and the capacity retention rate was 80% after 596 cycles at 3 C, and the Cu|NCM811 total battery without the anode maintained a 64% retention rate after 80 cycles at 1 C (1C=1.6 mA cm-2, 1C=200 mA g-1).
Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples.
A solution of 95% concentrated sulfuric acid (90 mL) and paraformaldehyde (18 g) was cooled in an ice bath and 2,2,2-trifluoroethanol (108 g) was quickly added to this solution with stirring. The mixture was then stirred in an ice bath for 10 minutes and at room temperature for 1 hour. After completion of the reaction, the upper layer was separated and poured into ice water, and the mixture was extracted with ether (2×100 mL) and dried with anhydrous magnesium sulfate. After removal of the ether, the product was collected by vacuum distillation at 35° C. (40 Torr) to synthesize BTFM. Yield: 37%. 1H NMR (400 MHZ, CDCl3) δH=4.84 (s, 2H), 3.96 (q, 4H, J=8.56 Hz) ppm; 19F NMR (376 MHZ, CDCl3) δF=−74.57 (t, J=8.17 Hz, 6F) ppm; 13C NMR (100 MHZ, CDCl3) δC=123.62 (q, J=278.03 Hz), 95.45 (s), 64.99 (q, J=35.21 Hz) ppm.
In the present specification, a case where lithium bis(fluorosulfonyl)imide (LiFSI) of 1 M and 2 M, which are electrolyte salts, is dissolved in BTFM, which is an additive, and DME, which is a solvent, at a volume ratio of 7:1 and 3:1 is referred to as 1 M LiFSI-7BTFM-1DME (Example 1) and 2 M LiFSI-3BTFM-1DME (Example 2).
The additive, BTFM, was dried using a 4 Å molecular sieve before use. For 1M LiFSI-7BTFM-1DME electrolyte (Example 1), 150 mg of LiFSI was dissolved in 0.7 mL BTFM and 0.1 mL DME. For 2M LiFSI-3BTFM-1DME electrolyte (Example 2), 374 mg of LiFSI was dissolved in 0.75 mL BTFM and 0.25 mL DME. For 1M LiFSI-DME and 4M LIFSI-DME, 187 mg and 748 mg LiFSI were dissolved in 1 mL DME, respectively. All electrolytes were stirred at ambient temperature in a glove box (O2<0.5 ppm, H2O<0.5 ppm) for 4 hours.
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Number | Date | Country | Kind |
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10-2022-0037980 | Mar 2022 | KR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/KR2023/003882 | 3/23/2023 | WO |