Electrolyte for a lithium battery and lithium battery comprising the same

Abstract

The present invention relates to an electrolyte that has been developed specifically for lithium batteries. The electrolyte includes a solvent system comprising an alkyl phosphate ester solvent in an amount of 65-75% by volume based on a total volume of the solvent system and a fluorinated carbonate solvent in an amount of 25-35% by volume based on the total volume of the solvent system, and a lithium salt. The invention also relates to a lithium battery comprising the said electrolyte. The lithium battery, equipped with the electrolyte described in the present invention, achieves complete non-flammability without compromising the battery's optimal performance.

Description

[0001]ELECTROLYTE FOR A LITHIUM BATTERY AND LITHIUM BATTERY COMPRISING THE SAME TECHNICAL FIELD The present invention generally relates to a liquid electrolyte that has been developed to enhance thermal stability and safety of lithium batteries. More specifically, it focuses on an electrolyte that incorporates a newly developed solvent system, which is capable of producing a completely non-flammable liquid electrolyte solution while simultaneously maintaining optimal performance of lithium batteries. Furthermore, the invention relates to a lithium battery that includes the aforementioned electrolyte. BACKGROUND OF THE INVENTION Lithium batteries play a crucial role in powering a wide range of electronic devices in today's world, including handheld devices like power banks, mobile phones, and notebook computers, as well as electric vehicles. However, the use of these batteries poses a significant safety risk due to the highly flammable nature of the liquid electrolytes they contain. Traditional electrolytes, typically composed of carbonate compounds, exhibit high flammability and low flash points, presenting a substantial risk of hazardous incidents. The combustion of such electrolytes is triggered by exothermic reactions that occur when exposed to high temperatures. Additionally, electrical and mechanical abuses can cause internal short circuits, generating heat that can ignite electrolytes. This can lead to thermal runaway, resulting in battery fires and explosions. Thus, ensuring the safety of battery usage, particularly in electronic devices relying on high-capacity lithium batteries, is of utmost importance. In particular, batteries that utilize cathodes comprising Ni-rich materials, such as NCA and NMC types, with a Ni ratio of ≥ 80%, or anodes comprising Si-graphite and lithium metals, are known to compromise the thermal stability of the battery. As a result, researchers are currently placing greater emphasis on the exploration of alternatives to enhance the safety and operational efficiency of lithium batteries within this specific context. The development of a novel electrolyte with reduced flammability is considered a viable solution to tackle the aforementioned issues. The pursuit of non-flammable electrolytes has become crucial in mitigating safety hazards associated with batteries, particularly through the exploration of solid-state electrolytes. However, these advancements still encounter challenges, such as lower ionic conductivity compared to liquid electrolytes and the instability of the solid electrolyte interface (SEI) layers. Thus, there exists a continued need to develop innovative liquid electrolytes capable of significantly reducing the risk of lithium battery ignition or even achieving complete non-flammability. Several patent documents have addressed the development of liquid electrolytes. For instance: US 8,372,549 B2 discloses an electrolyte specifically designed to enhance the safety of lithium batteries, as well as a lithium secondary battery incorporating the same. The electrolyte comprises a solvent system that includes a fluoroethylene carbonate solvent and a linear ester solvent chosen from the group consisting of ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl acetate, methyl propionate, and a combination of two or more of these solvents. US 6,210,835 B1 discloses a method to reduce lithium battery electrolyte flammability by using fluorinated ether with a carbonate solvent. The fluorinated ether options include semifluoroalkane, perfluoroalkane, perfluorobutyl methyl ether, perfluorobutyl ethyl ether, and mixtures thereof. The carbonate solvent options comprise dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (MTFEC), dimethoxyethane (DME), diglyme (DGM), triglyme (TGM), ethylene carbonate (EC), propylene carbonate (PC), trifluoropropylene carbonate (TFPC), chloroethylene carbonate (CIEC), vinylene carbonate (VC), butylene carbonate (BC), dimethylvinylene carbonate (DMVC), and mixtures thereof. This patent offers a solution to enhance lithium battery electrolyte safety using specific combinations of fluorinated ether and carbonate solvents. Despite numerous efforts to develop liquid electrolytes with improved flammability, as evidenced by the aforementioned patent documents, there is still a need to advance the field by developing an electrolyte that goes beyond the current state of the art. Specifically, there is a demand for an electrolyte that achieves the significant milestone of complete non-flammability. SUMMARY OF THE INVENTION The present invention has been conceived with the aforementioned issues in mind, with the aim of resolving the problems encountered in prior arts. An objective of the present invention is to provide a solvent system specifically tailored for use in electrolytes for lithium batteries, particularly lithium-ion and lithium metal batteries. The developed electrolyte, as disclosed herein, incorporates the solvent system comprising an alkyl phosphate ester solvent and a fluorinated carbonate solvent in a specific ratio. Additionally, the electrolyte includes a lithium salt. This distinctive combination effectively enhances the thermal stability of the electrolyte, addressing the objectives of the present invention. Another objective of the present invention is to provide an alternative liquid electrolyte that improves the safety characteristics of lithium batteries, as well as a lithium battery incorporating the same. In the first aspect, the present invention discloses an electrolyte for a lithium battery, which comprises a solvent system and a lithium salt. The solvent system comprises an alkyl phosphate ester solvent in an amount of 65-75% by volume based on a total volume of the solvent system and a fluorinated carbonate solvent in an amount of 25-35% by volume based on the total volume of the solvent system. In the second aspect, the present invention discloses a lithium battery that comprises an anode, a cathode, and an electrolyte. The electrolyte embodies the characteristics described in the first aspect, including the specific solvent system composition. Thus, the present invention provides an electrolyte formulation as described in the first aspect, which is applicable for use in a lithium battery as described in the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) illustrates the combustion mechanism of an electrolyte that employs a carbonate compound as a solvent without the addition of a flame retardant, and Fig. 1(b) illustrates the combustion mechanism of a carbonate electrolyte that incorporates sufficient triethyl phosphate (TEP) as a flame retardant. Fig. 2(a) illustrates the flammability test of an electrolyte (1.0 M LiPF6 in EC / DEC / EMC solvent (1:1:1 v / v) without any alkyl phosphate esters, Fig. 2(b) illustrates the flammability test of an electrolyte containing 1.0 M LiPF6in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of TEP, Fig. 2(c) illustrates the flammability test of an electrolyte containing 1.0 M LiPF6 in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of TBP, and Fig. 2(d) illustrates the temperature evolution after impact of the graphite / NCA90 cells with electrolytes containing 0% v / v, 30% v / v of TEP, and 30% v / v of TBP, respectively. Fig. 3(a) illustrate the initial charge-discharge curve at 0.1 C of the graphite / NCA90 cells, Fig. 3(b) illustrates the different capacity as a function of cell voltage of the graphite / NCA90 cells, Fig.3(c) illustrates the cyclic performance of the graphite / NCA90 cells at 0.5C charge, 1.0C discharge between 3.0-4.2 V in term of the capacity retention, and Fig. 3(d) illustrates the cyclic performance of the graphite / NCA90 cells at 0.5C charge, 1.0C discharge between 3.0-4.2 V in term of the differential voltage 1 C = 2,200 mA. Fig.4 illustrates the DFT calculation results of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energies of each solvent including ethylene carbonate (EC), diethylene carbonate (DEC), triethyl phosphate (TEP), and fluoroethylene carbonate (FEC). Fig. 5(a) illustrates the depth profiles of graphite anode disassembled from the graphite / NCA90 cells with an electrolyte containing 1.0 M LiPF6in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of TEP, showing C 1s, F 1s, and P 2s XPS results of graphite electrodes before 300 cycles, and Fig. 5(b) illustrates the depth profiles of graphite anode disassembled from the graphite / NCA90 cells with an electrolyte containing 1.0 M LiPF6 in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of TEP, showing C 1s, F 1s, and P 2s XPS results of graphite electrodes after 300 cycles. Fig. 6 illustrates XPS element analysis results of accumulated elements on the graphite electrode surface before and after 300 cycles in the electrolyte containing 1.0 M LiPF6in 70% v / v of EC / DEC / EMC (1:1:1 v / v) solvent and 30% v / v of TEP. Fig. 7(a) illustrates the results obtained from differential electrochemical mass spectroscopy (DEMS) or online gas detection of graphite / NCA90 18650 cells with the electrolyte containing 1.0 M LiPF6in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of TEP, and Fig.7(b) illustrates the voltage profile of graphite / NCA9018650 cells with the electrolyte containing 1.0 M LiPF6 in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of TEP. Fig. 8(a) illustrates the initial voltage profile of graphite / NCA90 jelly roll with 1.0 M LiPF6 in 20% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 80% v / v of TEP, Fig. 8(b) illustrates the initial voltage profile of graphite / NCA9018650 full cells with 1.0 M LiPF6in 20% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 80% v / v of TEP, and Fig.8(c) illustrates the results obtained from differential electrochemical mass spectroscopy (DEMS) or online gas detection of graphite / NCA9018650 cells with the electrolyte containing 1.0 M LiPF6 in 20% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 80% v / v of TEP. Fig. 9 illustrates the viscosity versus shear rate of the electrolytes with different ratios of TEP to FEC at 25ºC via Rheometer. Fig.10(a) illustrates the formation cycle at 0.025C of the graphite / NCA9018650 cells with the electrolyte containing different ratios of TEP to FEC, Fig. 10(b) illustrates the cell capacity at 0.025C of the graphite / NCA9018650 cells with the electrolyte containing different ratios of TEP to FEC, Fig. 10(c) illustrates the initial charge-discharge cycle at 0.1C of the graphite / NCA90 18650 cells with the electrolyte containing different ratios of TEP to FEC, and Fig. 10(d) illustrates the different capacity as a function of cell voltage of the graphite / NCA9018650 cells with the electrolyte containing different ratios of TEP to FEC. Fig. 11(a) illustrates the formation cycle at 0.025C of the graphite / NCA9018650 cells with the electrolyte containing different concentrations of LiPF6, Fig. 11(b) illustrates the cell capacity at 0.025C of the graphite / NCA9018650 cells with the electrolyte containing different concentrations of LiPF6, Fig. 11(c) illustrates the initial charge-discharge cycle at 0.1C of the graphite / NCA9018650 cells with the electrolyte containing different concentrations of LiPF6, and Fig. 11(d) illustrates the different capacity as a function of cell voltage of the graphite / NCA9018650 cells with the electrolyte containing different concentrations of LiPF6. Fig. 12(a) illustrates the ignition period at the beginning of the flammability test, and Fig.12(b) illustrates the immediate disappearance of the flame as the flammability test for the electrolyte (1.2 M LiPF6 in 70% v / v of TEP and 30% v / v of FEC) progresses. Fig. 13 illustrates the over-temperature test of the graphite / NCA90 18650 cells with 1.2 M LiPF6 in 70% v / v of TEP and 30% v / v of FEC, and the commercial cell (NCR18650B). Furthermore, small upper and lower pictures illustrating the tested cells in between (a) the commercial cell (NCR18650B) and (b) graphite / NCA90 18650 cells with 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC, respectively. Fig. 14(a) illustrates the depth profiles of graphite anode disassembled from the graphite / NCA90 cells with an electrolyte containing 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC, showing C 1s, F 1s, and P 2s XPS results of graphite electrodes before 500 cycles, and Fig.5(b) illustrates the depth profiles of graphite anode disassembled from the graphite / NCA90 cells with an electrolyte containing 1.2 M LiPF6 in 70% v / v of TEP and 30% v / v of FEC, showing C 1s, F 1s, and P 2s XPS results of graphite electrodes after 500 cycles. Fig. 15 illustrates XPS element analysis results of accumulated elements on the graphite electrode surface before and after 500 cycles in the electrolyte containing 1.2 M LiPF6 in 70% v / v of TEP and 30% v / v of FEC. Fig. 16(a) illustrates the step formation cycle of the graphite / NCA9018650 cells with the electrolyte containing 1.2 M LiPF6 in 70% v / v of TEP and 30% v / v of FEC from 0.025C-0.08C, Fig. 16(b) illustrates the initial charge-discharge cycle at 0.1C of the graphite / NCA9018650 cells with the electrolyte containing 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC, Fig.16(c) illustrates the different capacity as a function of cell voltage of the graphite / NCA90 18650 cells with the electrolyte containing 1.2 M LiPF6 in 70% v / v of TEP and 30% v / v of FEC, Figs. 16(d)-(f) illustrate the cycling performance of the graphite / NCA9018650 cells with the electrolyte containing 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC at 0.5C charge, 1C discharge between 3.0-4.2 V specific capacity, and Figs. 16(g)-(i) illustrate the rate performance of the graphite / NCA90 18650 cells with the electrolyte containing 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC at different discharge C-rates, 1C = 1,800 mA. Figs. 17(a) and 17(b) illustrate the cycling performance of the graphite / NCA9018650 cells with 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC compared to that of commercial 18650 cells from representative manufacturers. at 0.5C charge, 1C discharge between 3.0-4.2 V specific capacity. DETAILED DESCRIPTION The features of the present invention will now be described in more detail through an illustrative embodiment. It should be noted that this example is provided for illustrative purposes and should not be interpreted as limiting the scope of the invention. Unless otherwise specified, the aspects depicted herein shall also encompass their application to other aspects of the present invention. Unless otherwise indicated, technical and scientific terms used herein have meanings that are commonly understood by a person skilled in the relevant field. The term "about" or “approximate” used throughout the present invention indicates that the values presented or disclosed herein may exhibit slight variations or deviations. These variations or deviations may arise from errors in the equipment or methods employed to determine such values. The terms "consist(s) of," "consisting of," "consisted of," "comprise(s)," "comprising," "comprised," "has / have / having," "include(s)," "including," and "included" used herein are open-ended verbs. For instance, any methods that "consist of," "comprise," "have," or "include" one or more components or steps are not limited solely to those mentioned components or steps but also encompass additional components or steps not explicitly stated. The terms "a," "an," and "the," when used to refer to a singular form, are intended to encompass the plural form of the noun, unless otherwise specified. Any tools, equipment, methods, materials, or chemicals mentioned herein, unless otherwise indicated, refer to the tools, equipment, methods, materials, or chemicals generally utilized or practiced by a person skilled in the relevant field. Fig. 1(a) illustrates the combustion mechanism of an electrolyte that employs a carbonate compound as a solvent without the addition of a flame retardant. In the carbonate electrolyte system without an addition of the flame retardant compound, at high temperatures, carbonate molecules generate hydrogen radicals (H•). These radicals then react with released oxygen gas, resulting from both lattice oxygen and the thermal decomposition of the electrolyte. This reaction leads to the formation of hydroxide radicals (OH•) and oxygen radicals (O2•). These reactions thus far are endothermic. However, when these radicals combine to form water molecules, an exothermic reaction occurs, releasing heat. This heat can further excite other hydrogen atoms, resulting in the generation of additional hydrogen radicals. This chain reaction continues until all the fuel within the battery, particularly in the electrolyte, is completely consumed. Fig. 1(b) illustrates the combustion mechanism of a carbonate electrolyte that incorporates an alkyl phosphate ester, i.e., triethyl phosphate (TEP) in this case, as a flame retardant. When exposed to high temperatures, the alkyl phosphate ester molecules undergo excitation, leading to the formation of phosphorus radicals (P•) and phosphorus oxide radicals (PO•) due to the presence of the phosphate functional group. These radicals (P• and PO•) have the ability to react with hydrogen radicals (H•) generated from carbonate molecules, effectively hindering the formation of water molecules that could trigger an exothermic reaction. By utilizing sufficient amounts of alkyl phosphate esters, the combustion reaction of the electrolyte can be effectively suppressed. Thus, the novel alternative liquid electrolyte of the present invention is designed with the incorporation of an alkyl phosphate ester to capture the generated hydrogen radicals (H•), thereby inhibiting the production of hydroxide radicals (OH•) and oxygen radicals (O2•) while preventing the formation of water molecules within the electrolyte system. The present invention introduces a liquid electrolyte that has been developed by combining alkyl phosphate esters with fluorinated carbonates as co-solvents. This unique combination enhances the thermal stability of the electrolyte while simultaneously preserving optimal battery performance. In a preferred embodiment, the electrolyte of the present invention comprises: - a solvent system comprising an alkyl phosphate ester solvent in an amount of 65-75% by volume based on a total volume of the solvent system, and a fluorinated carbonate solvent in an amount of 25-35% by volume based on the total volume of the solvent system; and - a lithium salt. Preferably, the alkyl phosphate ester employed in this invention has carbon atom in a range of 3-12. For instance, triethyl phosphate, tributyl phosphate, or a mixture thereof may be used as examples of suitable alkyl phosphate esters. Examples of the fluorinated carbonate that can be used in this invention may be selected from the group consisting of monofluoroethylene carbonate, difluoro ethylene carbonate, and a mixture thereof. The lithium salt may be selected from the group consisting of lithium hexafluoro phosphate (LiPF6), lithium oxalyldifluoroborate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), and a mixture thereof. Preferably, the lithium salt is comprised at a concentration of 1.0-1.6 mol / L. The electrolyte of the present invention is suitable for use in a wide range of lithium batteries, including but not limited to lithium-ion and lithium metal batteries. In the second aspect of the invention, a lithium battery is disclosed. According to the present invention, the lithium battery comprises an anode, a cathode, and an electrolyte. The electrolyte comprises: - a solvent system comprising an alkyl phosphate ester solvent in an amount of 65-75% by volume based on a total volume of the solvent system, and a fluorinated carbonate solvent in an amount of 25-35% by volume based on the total volume of the solvent system; and - a lithium salt. The alkyl phosphate ester, fluorinated carbonate, and lithium salt encompass the specific properties and preferred selections as previously described in the first aspect regarding the electrolyte. In the cathode, as per a preferred embodiment, Ni-rich materials with a Ni ratio of ≥ 80% are utilized. In a specific embodiment, the cathode comprises the active material represented by Formula 1 or Formula 2: Li(NiaCobAlc)O2 Formula 1, Li(NiaMnbCoc)O2Formula 2 wherein a ≥ 0.8, 0 < b < 1, 0 < c < 1, and the sum of a, b, and c is 1. Examples of the Ni-rich materials, that can be used in this invention, include but not limited to the lithium nickel cobalt aluminium oxide (NCA) type materials and the lithium nickel manganese cobalt oxide (NMC) type materials. For example, LiNi0.88Co0.09Al0.03O2(NCA88), LiNi0.90Co0.09Al0.01O2 (NCA90), LiNi0.80Mn0.10Co0.10O2 (NMC80), LiNi0.90Mn0.09Co0.01O2 (NMC90), and the like. Further, the anode comprises an active material comprising at least one of selected from graphite, Si-graphite composite, and a mixture thereof, or anode free cell without any anode active material. Examples Hereinafter, the present invention will be further described through the following examples. However, it should be noted that the present invention is not restricted to or by the examples provided. 1. Preparation and characterization 1.1) Preparation of electrolytes Samples of electrolytes containing different amounts of alkyl phosphate esters and carbonates were prepared by using components as shown in Table 1. All electrolyte samples were prepared within an MBRAUN (Germany) glove box under high-purity Ar (Argon) gas. Table 1 Electrolyte solvent composition Concentration Sample (% by volume based on the total volume of the solvent system) of LiPF6No. EC / DEC / EMC (M) FEC TEP TBP (1:1:1 v / v) 1 1.0 100 - - - 2 1.0 70 - 30 - 3 1.0 70 - - 30 4 1.0 30 - 70 - 5 1.0 20 - 80 - 6 1.2 - 100 - - 7 1.2 - 40 60 - 8 1.2 - 30 70 - 9 1.2 - 20 80 - 10 1.2 - 10 90 - 11 1.2 - - 100 - 12 1.0 - 30 70 - 13 1.4 - 30 70 - 14 1.6 - 30 70 - Remark: EC is ethylene carbonate, DEC is diethyl carbonate, EMC is ethyl methyl carbonate, FEC is fluoroethylene carbonate, TEP is triethyl phosphate, and TBP is tributyl phosphate. 1.2) Preparation of 18650 cells Exemplary 18650 cells were fabricated using a commercially available natural graphite anode and an NCA cathode, specifically NCA90 (LiNi0.90Co0.09Al0.01O2), in addition to a binder and conductive carbon. The anode composition consisted of 95.4 wt.% graphite, 1.0 wt.% carbon black (Super P), 1.2 wt.% sodium carboxymethyl cellulose (CMC), and 2.4 wt.% styrene-butadiene rubber (SBR). The cathode comprised 95.2 wt.% NCA material, 2.4 wt.% polyvinylidene fluoride, and 2.4 wt.% carbon black (Super P). The assembly of the 18650 cells was carried out using an in-house process supervised by MTI (USA) in a pilot plant operating under dry-room conditions (with a dew point below -25°C). The mass loadings of the graphite anode and NCA cathodes were 9.8 g and 12.4 g, respectively. The N to P ratio was set at 1.24, calculated based on the manufacturing specifications, which indicated 360 mAh / g for graphite and 203.7 mAh / g for NCA material. Each cell contained approximately 5 g of electrolyte under all conditions. 1.3) Characterization of powdery materials and fabricated electrodes The NCA powder (i.e., NCA90) and electrodes were examined using a field emission scanning electron microscope (FE-SEM, JEOL-JSM-7610F, JOEL Ltd.) equipped with a backscattered electron detector at an accelerating voltage of 15kV. Prior to analysis, the fabricated electrodes were cut into small pieces, while the graphite and NCA powder were placed on carbon tape for direct characterization. Cycled electrodes were obtained by disassembling the exemplary 18650 cells inside an argon-filled glove box, followed by cleaning with battery-grade dimethyl carbonate to remove any residual Li-salt or solvent. Before FE-SEM characterization, the samples were dried at 25°C in a glove box for 12 hours. X-ray photoelectron spectroscopy (XPS, JPS-9010MC, JEOL) was performed on graphite electrodes using an Mg-Kα radiation source (hʋ = 1253.6 eV) at 12 kV and 25 mA under a high vacuum pressure of 10–7Pa at room temperature. The objective was to compare the surface chemistry of graphite before and after testing or use. For the XPS depth profile, Ar+ etching was performed for 20, 40, and 60 seconds. The binding energy was calibrated using the C1s peak (284.0 eV) of C-C in graphite. To avoid contamination or side reactions with atmospheric moisture and oxygen, the dismantled samples were carefully transferred from the glove box to the XPS chamber using an air-sensitive holder. Additionally, the gas evolution of sealed 18650 NCA88 (or NCA90) / / graphite jelly rolls with different electrolytes was investigated in an Ar-filled glove box (MBRAUN, Germany) using HPR-40 DEMS (Hidden Analytical, UK) with Hiden's professional software. 1.4) Electrochemical measurements The cycling and rate performance of 18650 full cells were evaluated at room temperature using a battery tester from GELON (China). Before testing, the cells underwent a five-step formation process involving cycles ranging from C / 40 to C / 12.5 between 3.0 - 4.3 V to establish a stable solid electrolyte interface (SEI). To ensure consistency and prevent biased selection, three samples from each experiment were individually tested for cycling and rate performance. For cyclic performance evaluation, the cells were subjected to a 0.5C charge and 1.0C discharge within the voltage range of 3.0 - 4.2 V. Regarding rate performance assessment, the cells were charged at a rate of 0.5C and discharged at various C rates, ranging from 0.25C to 3C. Safety analysis Flammability tests of electrolyte samples were conducted on CR3032 coin cell caps using a gas igniter with mixed Li-salts and solvents in their as-used form. Safety tests for the 18650 cells were carried out using Battery Testing Equipment from DGBELL (China) in accordance with UN 38.3 requirements. An impact test was performed by placing a steel column with a diameter of 15.8 mm at the center of the battery and dropping a 9.1 kg weight from a height of 610 mm onto the column, simulating an impact situation and mechanical abuse. Furthermore, an over-temperature test was conducted using an in-house method. This involved using a temperature-controlled hot plant equipped with a wireless 915i temperature sensor from Testo (Germany) to measure the onset and peak temperature. The method employed is a modified version of Accelerating Rate Calorimetry (ARC), where the battery's temperature is rapidly increased in a controlled manner, and the heat generated by the battery is recorded as a function of time. 1.5) Computational method All calculations were performed using the Gaussian 09 package. The equilibrium structures were optimized using the B3LYP / 6-31 G(d) level. Additionally, frequency calculations were performed at the same level to confirm that the optimized structure corresponded to a stationary state. 2. Experimental results 2.1) Combustion and flame retardant mechanism To confirm the anti-combustion mechanism of alkyl phosphate esters as proposed in Fig. 1(b), the combustion reaction of the electrolyte was investigated by using the computational method, i.e., density functional theory (DFT) calculation. Based on the DFT calculation, it was found that the proposed form of O• does not meet the criteria for an oxygen radical, where the spin multiplicity should be at least two. Consequently, the proposed form of an oxygen radical was changed to O2•, which met the criteria, as shown in Table 2. Table 2 shows the calculation of the binding energy for the combustion mechanism of conventional carbonate-based electrolyte and the calculation parameter to confirm radical atom in the reaction. Table 2 Electronic energy PO+H• P+H• O2+H• OH+H2 O+H2 1 -416.53 -341.194 -150.257 -75.723 -74.9574 627.51 Kcal / mol / Hartree 2 -0.50027 -0.50027 -0.50027 -1.17548 -1.17548 3 (Combined atom) -417.138 -341.824 -150.825 -76.8998 -76.409 Binding energy -0.107919 -0.129430 -0.067540 -0.000894 -0.276073 3-(1+2) (Hartree) Binding energy -67.7203 -81.2186 -42.382 -0.56099 -173.239 (Kcal / mol) Radical Charge / Spin multiplicity Frequencies S2 H• 0 / 2 - 0.75000 OH• 0 / 2 3644.98 0.75197 P• 0 / 2 - 0.75193 PO• 0 / 2 1315.10 0.75254 O• 0 / 1 - 0 O2• -1 / 2 1212.51 0.75283 In addition, the binding energy of the flame retardant reaction was calculated (-81.22 eV and -67.72 eV), which was significantly more negative than the reaction between hydrogen radical (H•) and oxygen gas (-42.38 eV), as summarized in Table 3. These results indicate that the flame retardant mechanism is more likely to occur, which confirms the proposed anti-combustion mechanism (the mechanism according to Fig.1(b)). Table 3 Condition Without or insufficient Sufficient TEP TEP Solvent evaporation RHliquid → RHgas RHliquid → RHgas (Endothermic) TEPliquid→ TEPgasRadical excitation TEPgas→ P• TEPgas→ PO• Competitive reaction H• + O2 (- 42.38 eV) (- 81.22 eV) [PO]• + H• → [PO] H (- 67.72 eV) Water formation (- 0.56 eV) ½ O2• + H2 → HO• + H• → H2O (-173.24 eV) Consequent Fire and explosion Safety (Without ΔH°H2O= -285.83 kJ / mol exothermic reaction) (Exothermic) 2.2) Effect of alkyl phosphate esters on flammability of conventional carbonate- based electrolytes (1) Use of alkyl phosphate esters as additives or co-solvents a) Safety analysis Considering a commercially conventional carbonate-based electrolyte, Sample 1 in Table 1, which is the solution of 1.0 M LiPF6 in EC / DEC / EMC solvent (1:1:1 v / v), was subjected to an in-house flame test. From the test results, it was observed that the commercial carbonate-based electrolyte without alkyl phosphate esters exhibited flammability characteristics, as shown in Fig. 2(a). Further, the graphite / NCA90 cells with this electrolyte also exhibited fire and explosion in the impact test following the UN38.3 standard. Similarly, Samples 2 and 3 in Table 1, the electrolyte which is the solution of 1.0 M LiPF6 in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of alkyl phosphate solvents (triethyl phosphate (TEP) or tributyl phosphate (TBP)), could still ignite and burn as shown in Fig. 2(b) and 2(c). Moreover, the graphite / NCA90 cells with these electrolytes exhibited fire and explosion in the impact test following the UN38.3 standard. From above, it is evident that alkyl phosphate esters present in the solvent system, at a concentration of ≤ 30% v / v, are unable to inhibit the combustion reaction of the electrolyte. However, the temperature evolution after impact, as shown in Fig. 2(d), demonstrates that graphite / NCA90 cells with 30% v / v of alkyl phosphate esters (TEP or TBP) can reduce the severity of fire and explosion by lowering the peak temperature after impact. b) Electrochemical performance The electrochemical behavior of electrolytes (Samples 1-3 in Table 1), and the graphite / NCA90 cells containing the same were tested. Figs. 3(a)-(d) illustrate the electrochemical behavior of the graphite / NCA90 cells with 30% v / v of alkyl phosphate esters (TEP or TBP) as co-solvents, and without alkyl phosphate esters. The experimental results show that 30% v / v of alkyl phosphate esters would negatively impact the cell's capacity and cycling performance. That is, the graphite / NCA90 cells containing 30% v / v of TEP or TBP (Samples 2 and 3) show only 60% retention after approximately 300 cycles, whereas the same configuration cell without alkyl phosphate esters reached 60% retention around 600 cycles. Moreover, dV, which represents the cell's Ohmic drop, was twice as high in the graphite / NCA90 cells with 30% v / v of TEP or TBP and tended to increase more rapidly than that of the graphite / NCA90 cells without alkyl phosphate esters. Further, DFT calculations were performed to calculate the HOMO and LUMO of each solvent (Fig. 4). From DFT calculations, TEP exhibits a relatively lower low-lying LUMO, indicating its inclination to accept electrons and cause reductive decomposition on the surface of graphite. To confirm this, XPS analysis and element analysis were performed to determine the surface chemistry of the graphite electrode before and after 300 cycles. The results from XPS analysis, Figs. 5(a) and 5(b), show that the depth profiles of the phosphorus species of P 2p were thicker than 60 nm until after the formation cycle. Further, from the element analysis in Fig.6, P 2p at the outer surface significantly increased after 300 cycles. These results indicate that a significant reductive decomposition of TEP occurred during the formation and continued after extended cycling. Therefore, no stable SEI layer is formed on the surface of graphite electrode. In addition, differential electrochemical mass spectroscopy (DEMS) or online gas detection was performed on graphite / NCA90 jelly roll using in-house mimic 18650 glass cells. The gas product of TEP decomposition had already been studied via experiments and Computational simulations, and the results pointed out ethylene (C2H2) for both methods. Figs.7(a) and (b) show the gas evolution of graphite / NCA9018650 cells with 1.0 M LiPF6in 70% v / v of EC / DEC / EMC solvent (1:1:1 v / v) and 30% v / v of TEP. From the experimental results, ethylene (C2H2) is the dominant gas that could be detected during the formation cycle and abuse cycles to 4.5 V at C / 10. This indicates the decomposition of TEP, aligning with the XPS results. As mentioned above, the utilization of alkyl phosphate esters as additives or co-solvents with conventional carbonate-based solvents compromises lithium battery performance while still being flammable and retaining the risk of fire and explosion. (2) Use of alkyl phosphate esters as major solvents a) Safety analysis The samples of electrolyte using alkyl phosphate esters as a major solvent with conventional carbonate-based electrolyte solvents were prepared and subjected to the flammability test. Based on the experimental results, Sample 4, which is the solution of 1.0 M LiPF6in 70% v / v of TEP and 30% v / v of EC / DEC / EMC solvent (1:1:1 v / v), exhibited favorable ignition and combustion characteristics. Whereas Sample 5, which is the solution of 1.0 M LiPF6 in 80% v / v of TEP and 20% v / v of EC / DEC / EMC solvent (1:1:1 v / v) was found to be non-flammable. b) Electrochemical performance The electrochemical behavior of graphite / NCA90 full cells and jelly roll with the solution of 1.0 M LiPF6 in 80% v / v of TEP and 20% v / v of EC / DEC / EMC solvent (1:1:1 v / v) were further investigated. For graphite / NCA90 jelly roll, Fig.8(a) shows that a significant number of gas bubbles were observed on the surface of the electrolyte at the initial stage of the formation cycle (3.3 V). Consequently, when the voltage reached 3.6 V, the mimic glass case ruptured under the accumulated pressure within the tube. Similarly, Fig. 8(b) also shows that samples of graphiteNCA90 18650 full cells failed during the first formation cycle. These results are consistent with the analysis results obtained from differential electrochemical mass spectroscopy (DEMS) as shown in Fig.8(c). However, with the rupture of the glass mimic cell, the pressure was released, rendering the measurement unable to proceed. This indicates a substantial electrolyte decomposition occurring during the formation cycle. Therefore, the utilization of alkyl phosphate esters as major solvents with conventional carbonate-based solvents for achieving non-flammable electrolytes is not possible because it would heavily decompose until the first formation cycle. 2.3) The proposed non-flammable electrolyte formulation of lithium salts in the alkyl phosphate esters and fluorinated carbonates solvents a) Optimization of solvent ratios To overcome the challenges encountered with the previous electrolyte formulation, the non-flammable electrolyte of this invention was formulated by introducing fluorinated carbonate solvents into the flame retardant, particularly alkyl phosphate esters. Samples of the electrolyte formulations of 1.2 M LiPF6in the solvent system with various ratios of triethyl phosphate (TEP) to fluoroethylene carbonate (FEC) were prepared (Samples 6-11 in Table 1). Then, the viscosity of each sample was measured at various shear rates at 25ºC using a Rheometer, and it found that the electrolyte solution of 1.2 M LiPF6in 100% FEC has the highest viscosity while the electrolyte solution of 1.2 M LiPF6in 100% TEP has the least viscosity. Further, the viscosity of electrolytes increases with the increased amount of FEC in the mixed solvent system (TEP / FEC solvent), as shown in Fig.9. Furthermore, the electrolyte samples were subjected to additional electrochemical performance tests, as shown in Fig.10(a)-(d). From the test results, at 10% v / v and 20% v / v of FEC, the electrolytes exhibited a notably decrease in capacity. This indicates that the FEC content comprised in the electrolyte is insufficient to prevent the reductive decomposition of TEP. Further, upon increasing the FEC content to 30% v / v, the capacity of the electrolyte demonstrated an increase. However, with a further increase in the FEC content to 40% v / v, the electrochemical performance of the electrolyte remained stable and comparable to that of the electrolyte containing 30% v / v of FEC. This demonstrates that higher amounts of FEC (> 30% v / v) did not have effect on electrochemical performance of the battery. Adversely, it did lead to an increase in the viscosity of the electrolyte, consequently resulting in reduced lithium conductivity. b) Optimization of lithium salt concentrations Samples of the electrolyte formulations with different concentrations of LiPF6 in 70% v / v of TEP and 30% v / v of FEC were prepared (Samples 8, 12-14 in Table 1). The obtained electrolytes were subjected to additional viscosity measurement and electrochemical performance tests, as shown in Fig. 11(a)-(d). From the experimental results, it was found that an increased concentration of lithium salts (LiPF6) resulted in an exponential rise in the viscosity, leading to a deterioration in the lithium conductivity of the electrolyte. Through this optimization process, the optimized concentration of LiPF6for a favorable non-flammable electrolyte formulation in the mixture of 70% v / v of TEP and 30% of FEC solvents falls within the range of 1.0-1.6 M, aiming to strike a balance between enhanced safety and adequate electrochemical performance. Specifically, the preferred concentration of LiPF6is about 1.2 M. c) Safety analysis of the proposed non-flammable electrolyte formulation The flammability test of the proposed electrolyte formulation, which is the solution of 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC (Sample 8 in Table 1), exhibited completely non-flammable characteristics as shown in Figs.12(a) and 12(b). Further, the fabricated graphite / NCA9018650 cells using this electrolyte sample was prepared and subjected to UN38.3 impact test by using 9.1 kg weight fell from 610 mm height on the cell. The test results show that there were no signs of fire and explosion resulting from the impact test. Additionally, an overtemperature test of the fabricated graphite / NCA90 18650 cells was performed to measure the onset and peak temperatures of the cell during thermal abuse. A comparison was made between the commercially available cells and the fabricated graphite / NCA90 18650 cells. In Fig. 13, as the temperature gradually increased, the surface temperature of both samples increased. The commercial cells abruptly exploded when the surface temperature reached around 180 °C, as shown in the small upper picture (a) in Fig.13. Then, the surface temperature of the commercial cells reached 450 °C, the maximum detectable value of the thermal sensor. In contrast, the fabricated graphite / NCA9018650 cells with the solution of 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC as the electrolyte survived thermal abuse until the hot plate reached its maximum temperature, as shown in the small lower picture (b) in Fig.13. To elucidate the superior stability of the electrolyte (Sample 8 in Table 1), XPS was conducted on graphite electrodes to investigate the reductive decomposition of alkyl phosphate esters (TEP). After the formation cycle, the depth profiles in Fig. 14(a) and (b) show that the thickness of P 2s was less than 20 nm, and the dominant species was F-P-O, which likely originated from the decomposition of LiPF6. Moreover, after 500 cycles, the thickness of P 2s increased only to 40 nm, and F-P-O remained the dominant species. Additionally, from the elemental quantitative analysis in Fig. 15, the ratio of P 2s species decreased after 500 cycles. As shown in Fig.4, FEC has a relatively lower low-lying LUMO than TEP. Consequently, this could result from a stable layer of LiF detected by XPS, produced by FEC. Furthermore, differential electrochemical mass spectroscopy (DEMS) or online gas detection was performed on graphite / NCA9018650 jelly roll with the solution of 1.2 M LiPF6 in 70% v / v of TEP and 30% v / v of FEC to confirm XPS results. From the DEMS results shown in Fig.11, there was no dominant signal of C2H2(labeled in red), during the formation cycle or even during abuse cycles to 4.5 V at C / 10, indicating that 30% FEC could effectively inhibit and significantly reduce TEP decomposition. d) Electrochemical performance of the proposed non-flammable electrolyte formulation The electrochemical behavior of the fabricated graphite / NCA9018650 cells using the non-flammable electrolyte according to the present invention and the comparative commercial cells (i.e., commercial A and commercial B samples) were further investigated and the results are shown in Figs.16(a)-(i) and Figs 17(a) and 17(b). From the experimental results (Fig. 16), the graphite / NCA90 cell using the solution of 1.2 M LiPF6in 70% v / v of TEP and 30% v / v of FEC as electrolyte exhibited a satisfactory initial coulombic efficiency (ICE) of 91.80%, comparable to conventional carbonate-based electrolytes. As seen in the dQ / dV plot, there were no clear and separate peaks, possibly due to the kinetic sluggishness of alkyl phosphate esters (TEP) and FEC. Nevertheless, the capacity retention of the graphite / NCA90 cell was maintained 71.63% after 500 cycles comparing to 67.70 % from commercial A and 23.20 % from commercial B at the same charge-discharge protocol (Fig.17). In addition, the experimental results showed a maximum operating C-rate of 2C from the rate performance test. These results indicate acceptable electrochemical performance that is comparable and superior to commercial cells, particularly in terms of capacity retention.

Claims

WHAT IS CLAIMED IS:

1. An electrolyte for a lithium battery, comprising: - a solvent system comprising an alkyl phosphate ester solvent in an amount of 65-75% by volume based on a total volume of the solvent system, and a fluorinated carbonate solvent in an amount of 25-35% by volume based on the total volume of the solvent system; and - a lithium salt.

2. The electrolyte of claim 1, wherein the alkyl phosphate ester has carbon atom in a range of 3-12.

3. The electrolyte of claim 1, wherein the alkyl phosphate ester is triethyl phosphate, tributyl phosphate, or a mixture thereof.

4. The electrolyte of claim 1, wherein the fluorinated carbonate is selected from the group consisting of monofluoroethylene carbonate, difluoroethylene carbonate, and a mixture thereof.

5. The electrolyte of claim 1, wherein the lithium salt is selected from the group consisting of lithium hexafluorophosphate (LiPF6), lithium oxalyldifluoroborate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoro methanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), and a mixture thereof.

6. The electrolyte of claim 1 or 5, wherein the lithium salt is comprised at a concentration of 1.0-1.6 mol / L.

7. A lithium battery comprising an anode, a cathode, and an electrolyte, wherein the electrolyte comprises: - a solvent system comprising an alkyl phosphate ester solvent in an amount of 65-75% by volume based on a total volume of the solvent system, and a fluorinated carbonate solvent in an amount of 25-35% by volume based on the total volume of the solvent system; and - a lithium salt.

8. The lithium battery of claim 7, wherein the alkyl phosphate ester has carbon atom in a range of 3-12.

9. The lithium battery of claim 7, wherein the alkyl phosphate ester is triethyl phosphate, tributyl phosphate, or a mixture thereof.

10. The lithium battery of claim 7, wherein the fluorinated carbonate is selected from the group consisting of monofluoroethylene carbonate, difluoroethylene carbonate, and a mixture thereof.

11. The lithium battery of claim 7, wherein the lithium salt is selected from the group consisting of lithium hexafluorophosphate (LiPF6), lithium oxalyldifluoroborate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoro methanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), and a mixture thereof.

12. The lithium battery of claim 7 or 11, wherein the lithium salt is comprised at a concentration of 1.0-1.6 mol / L.

13. The lithium battery of claim 7, wherein the cathode comprises an active material represented by Formula 1 or Formula 2: Li(NiaCobAlc)O2Formula 1, Li(NiaMnbCoc)O2 Formula 2 wherein a ≥ 0.8, 0 < b < 1, 0 < c < 1, and the sum of a, b, and c is 1.

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