Method for guiding lithium battery electrolyte optimization based on volatile fragments

Abstract

The invention discloses a lithium battery electrolyte optimization method based on volatile fragment analysis. According to the method, in order to solve the key problem of performance degradation caused by electrolyte side reaction in circulation of the lithium battery, a differential electrochemical mass spectrometer is adopted to carry out in-situ and real-time monitoring on volatile intermediate products generated in the operation process of the battery. Key harmful steps causing performance degradation are identified by analyzing the type, time sequence and intensity change of the feature fragments and dynamically reconstructing the decomposition path and competitive reaction of the electrolyte. On the basis, side reaction product evolution maps of different electrolyte formulas (solvents, lithium salts and additives) are systematically compared, and a structure-function relationship between a side reaction path and electrolyte components is established. The method is applied to research of various complex systems, the cooperation of a raw material formula, a battery process and online diagnosis equipment is realized, traditional experience trial and error are converted into mechanism-driven accurate design, and the cycling stability and safety of the lithium battery are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery electrolyte design and interface chemistry regulation, specifically involving a lithium battery electrolyte optimization method based on volatile fragments. Background Technology

[0002] With the development of high-energy-density batteries, lithium batteries have attracted much attention due to their high theoretical specific capacity and energy density. However, their practical applications are limited by problems such as severe capacity decay and short cycle life. The core bottleneck lies in the complex side reactions at the electrode / electrolyte interface. During cycling, the electrolyte undergoes continuous reduction, oxidation, and chemical decomposition, producing a series of gaseous or volatile intermediate products. These products directly reflect the pathways and kinetics of the side reactions and are key factors leading to the loss of active materials, increased interfacial impedance, and battery failure.

[0003] Current research on electrolytes largely focuses on in-situ analysis of final products (such as XPS and SEM of electrodes after cycling) or changes in the bulk electrolyte composition, making it difficult to capture information on transient and unstable intermediate products during the reaction process. While first-principles calculations can predict reaction trends, they lack experimental verification and struggle to accurately describe the complex interfacial environments under actual operating conditions. Differential electrochemical mass spectrometry, often used for permanent gas detection, contains important reaction pathway information in the various long-chain volatile fragments generated by interfacial reactions, yet this information is often overlooked. Therefore, the lack of understanding of reaction mechanisms under actual operating conditions leads the field to rely on traditional trial-and-error methods for electrolyte selection and electrode / electrolyte interface matching, lacking efficient and precise guidance based on reaction mechanisms.

[0004] To address the aforementioned issues, there is an urgent need to develop in-situ / operating-condition characterization-assisted research on battery interface reaction mechanisms. This research aims to capture rapidly converting intermediate products in real time during actual battery cycling, analyze their reaction sequences and pathways, and provide strategic methods to directly guide electrolyte formulation design and electrode matching. Summary of the Invention

[0005] To address the current situation of poor interface stability, insufficient understanding of reaction mechanisms, and lack of mechanistic guidance for interface optimization in existing battery technologies, this invention provides a lithium battery electrolyte optimization strategy based on the analysis of volatile intermediate products. The core of this strategy lies in: dynamically reconstructing the side reaction pathways and sequences by real-time monitoring of characteristic volatile intermediate products generated by electrolyte decomposition under actual battery cycling conditions; and identifying key harmful reaction steps and beneficial interface formation processes based on the differences in the "side reaction product evolution maps" corresponding to different electrolyte formulations. This establishes a rational electrolyte selection and design principle guided by inhibiting harmful decomposition and promoting stable interfaces, thereby achieving a significant improvement in battery performance.

[0006] The basic principle of this invention is as follows: During the charging and discharging process of a lithium battery, the electrolyte (including solvent, lithium salt, and additives) undergoes complex electrochemical and chemical reactions at the interface between the positive and negative electrodes, generating a series of unstable, difficult-to-detect, and rapidly convertible volatile intermediate and final products (such as hydrocarbons like CH4, CS2, and C2H4, and long-chain decomposition fragments like HOCH2CH2OH and CH2CH2OCH2). These volatile fragments are continuously detected in real time by a differential electrochemical mass spectrometer via a microflow interface. By synchronously recording the temporal changes of electrochemical signals (voltage, current, and capacity) and characteristic fragment signals, decomposition products appearing within specific potential or capacity ranges can be identified with high sensitivity, and their generation rate and evolution patterns can be quantitatively analyzed. Through in-depth analysis of the types, order of appearance, relative intensity, and correlations of these volatile intermediate products, the specific reaction pathways, competing reactions, and key rate-determining steps of electrolyte decomposition can be deduced.

[0007] This invention provides a lithium battery electrolyte optimization method based on the analysis of volatile intermediate products. The core of this method lies in establishing a standardized testing and data analysis process to transform characteristic volatile fragment signals monitored online by differential electrochemical mass spectrometry into a quantifiable "side reaction suppression index," which is then used as a direct criterion for electrolyte formulation screening and optimization. The strategy established in this invention involves real-time acquisition and detection of volatile fragments generated by interfacial reactions during battery operation; obtaining volatile fragment signals through mass spectrometry and real-time correlation with electrochemical signals to achieve in-situ, dynamic identification and analysis of decomposition reaction pathways, intermediates, and final products. Differences in side reaction pathways and product characteristic spectra obtained from the analysis of different electrolyte formulations under the same testing conditions are compared; based on these differences, an electrolyte formulation selection and optimization strategy guided by suppressing characteristic harmful side reactions and promoting stable interface formation is formulated to effectively improve battery capacity, cycle life, and safety.

[0008] Specifically, the present invention provides the following technical solution to solve the above problems: a lithium battery electrolyte optimization method based on volatile debris analysis, comprising the following steps:

[0009] (S1) Baseline Signal Acquisition and Critical Fragment Identification: Under preset standard battery operating conditions, the baseline electrolyte formulation is tested. Volatile fragments generated are continuously monitored using differential electrochemical mass spectrometry. By analyzing the fragment occurrence potential, timing, and intensity, "critical hazardous fragments" strongly correlated with battery capacity decay are identified. For example, for systems where the solvent is carbonate, CO2 (m / z=44) and CH4 (m / z=16) are identified as critical hazardous fragments. The integral intensity of the critical hazardous fragments throughout the entire cycle is recorded as the baseline value S. 基准For example, in ether solvent systems, CH3OCH2CH2 (m / z=59) and CH3CH2O (m / z=45) can be considered key hazardous fragments. For sulfone solvents, sulfur-containing fragments such as SO2 (m / z=48, 64) and H2S (m / z=34) can be characteristic. For sulfur-containing imide salts, SO2 (m / z=48, 64) and SOF2 (m / z=67, 86), which generate sulfur-containing components through the cleavage of SF or S=O bonds, can be considered key hazardous fragments. For phosphorus-containing salts such as LiPF6, POF3 (m / z=85, 104) and HF (m / z=20) can be considered characteristic fragments. Novel borates can use boron fluoride BF3 (m / z=49) as a characteristic fragment. For more novel electrolytes and salt molecules, full-sequence m / z spectra can be acquired first, and the specific m / z signals detected can be labeled as key hazardous fragments.

[0010] (S2) Test Formulation Testing and Signal Acquisition: Under the same test conditions (including battery model, electrode material, test temperature, charge / discharge protocol, mass spectrometer parameters, etc.), the electrolyte formulation to be tested is tested, and the integrated intensity S of the key hazardous fragments is recorded. 待测 .

[0011] (S3) Define the side reaction inhibition index I 抑制 Its calculation formula is: I 抑制 =(1-S 待测 / S 基准 ) × 100%

[0012] Among them, I 抑制 The higher the value, the stronger the inhibitory ability of the tested formulation to key harmful side reactions. Through the above steps, this invention transforms the traditional trial-and-error method into a quantifiable electrolyte rational design method driven by online diagnostic signals and with a clearly defined operating procedure.

[0013] Furthermore, the lithium battery is any one of lithium-ion batteries, lithium metal batteries, and lithium-sulfur batteries.

[0014] Furthermore, by systematically comparing the "byproduct evolution maps" obtained under the same test conditions for different electrolyte formulations (such as different solvent compositions, lithium salt types, and functional additives), the ability of each formulation to suppress harmful side reactions (such as excessive dissolution of polysulfides, violent solvent decomposition, and gas generation) and its contribution to the formation of a stable solid electrolyte interface (SEI) or cathode electrolyte interface (CEI) can be intuitively evaluated. In addition, the correlation between the "suppression index" and battery performance is established. Long-cycle performance tests are conducted on multiple groups of test formulations to obtain their capacity retention rate C. 待测 Through data analysis, I can be fitted. 抑制 With C待测 By using empirical formulas or determining a screening threshold, one can determine whether a formulation possesses excellent cycling stability. Formulation optimization and selection based on quantitative criteria can rapidly screen and optimize electrolyte formulations, and based on these analytical results, a clear electrolyte optimization strategy can be formulated.

[0015] The electrolyte formulation includes a solvent, a lithium salt, and optionally, functional additives. Further, the solvent system includes ethers (such as DOL, DME), sulfones (such as TMS), esters (such as DEC, FEC, VC), and mixtures thereof; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium hexafluorophosphate (LiPF6); the functional additives include lithium nitrate (LiNO3) and lithium difluorophosphate (LiDFP).

[0016] The differential electrochemical mass spectrometer is a full m / z sequence acquisition differential electrochemical mass spectrometer, whose high chemical sensitivity and time resolution are particularly suitable for capturing transient, low-concentration and rapidly transforming volatile intermediate product signals during the reaction process.

[0017] The synergistic improvement of this invention is reflected in the close integration of three aspects: raw materials (optimized electrolyte formulation), system (high-energy-density, high-safety lithium battery electrode / electrolyte system), and research scheme (online acquisition of the full m / z sequence by differential electrochemical mass spectrometry combined with intelligent analysis by machine learning). By acquiring dynamic reaction information, and targeting the failure mechanism under specific operating conditions, it directly guides the rational design and screening of electrolytes, forming a closed-loop R&D strategy of "monitoring-analysis-optimization," thereby significantly accelerating the development process of high-performance battery systems and greatly improving battery cycle stability and safety. Compared with existing technologies, the significant advantages of this invention lie in its ability to guide electrolyte design directly from molecular-level information on side reaction pathways, transcending the traditional trial-and-error approach; it captures unstable intermediate products, providing complete dynamic evidence of the side reaction chain, and can identify competing reactions and key steps; by comparing the "side reaction product profiles" of different formulations, it introduces quantitative concepts such as "key feature fragments" and "side reaction inhibition index," enabling rapid evaluation and screening of electrolyte systems that can effectively inhibit harmful processes and promote the formation of stable interfaces from a "qualitative" to a "quantitative" perspective; and it combines advanced online diagnostic technology, failure analysis of specific battery processes, and electrolyte materials science to generate synergistic innovation and achieve a significant improvement in battery performance.

[0018] In summary, the method disclosed in this invention achieves dynamic diagnosis and mechanistic elucidation of side reaction pathways, electrolyte optimization, and performance improvement by monitoring and analyzing volatile intermediate products generated by the electrolyte during battery operation in situ. This method extends experimental observation from macroscopic performance and end products to intermediate reaction steps, providing an innovative research tool and solid scientific foundation for understanding electrolyte failure mechanisms at their source and for the rational design and screening of highly stable electrolyte systems. It has significant theoretical value and engineering guidance significance. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the electrolyte optimization strategy of the present invention.

[0020] Figure 2 This is a constant current charge-discharge curve of the battery (Example 1) and a conventional coin cell battery (Comparative Example 1) tested by mass spectrometry in this invention, at a rate of 0.05C and an operating voltage range of 1.0 V-3.0 V. The difference is that argon gas is introduced during the mass spectrometry test.

[0021] Figure 3 Examples 1-1 and their interfacial volatile fragment evolution are described.

[0022] Figure 4 The corresponding gas production data are obtained by integrating the signals of CH4 (m / z=16) and CO2 (m / z=44) over time during the electrochemical reaction process intervals of Examples 1-1 and 1-2. Detailed Implementation

[0023] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments. Unless otherwise specified, the experimental methods described in the following embodiments are conventional methods; the reagents and materials described are commercially available unless otherwise specified. A specific embodiment is given below to help understand the present invention's strategy for optimizing lithium battery electrolytes based on volatile products.

[0024] Electrode preparation method: In an argon-protected glove box (water content ≤0.1 ppm, oxygen content ≤0.1 ppm), weigh 200 mg of active material, conductive carbon black and polyvinylidene fluoride (PVDF) in a ratio of 8:1:1 using an analytical balance and place them in a sample box. Add 1-methyl-2-pyrrolidone (NMP) to the sample box to make the sample flowable. Remove the sample, slurry it to obtain a uniform slurry, coat it on the current collector coated carbon aluminum foil, and then dry it in an oven at 60 ℃ for more than 3 hours to obtain the positive electrode sheet.

[0025] Mass spectrometry battery assembly method: In an argon-protected glove box (water content ≤ 0.1 ppm, oxygen content ≤ 0.1 ppm), first place the negative electrode in the negative electrode shell, and cover the negative electrode surface with a separator, typically 20 mm in size. Add electrolyte to the separator, usually 200 μL. Make a few small holes in the positive electrode to facilitate gas diffusion, then assemble the positive electrode, close the positive electrode shell, and tighten it. The battery assembly is complete.

[0026] Test setup connection method: Use a four-way valve to connect the battery's inlet and outlet, as well as the mass spectrometer's inlet and outlet. Perform electrochemical testing on the battery while simultaneously conducting mass spectrometry measurements.

[0027] Example 1-1

[0028] The obtained sulfur-carbon positive electrode sheets were uniformly cut into circular electrode sheets with a diameter of 10 mm using a coin cell slicing machine. The electrode sheets were evenly perforated to ensure gas passage. For mass spectrometry testing, coin-type lithium-sulfur half-cells were assembled in an argon-protected glove box (water content ≤0.1 ppm, oxygen content ≤0.1 ppm) in the following order: negative electrode base - active material - glass fiber separator - electrolyte - lithium sheet - gasket - spring sheet - positive electrode shell. Constant current charge-discharge tests were conducted at a rate of 0.05C and a voltage range of 1.0 V-3.0 V using wires connected to an electrochemical workstation. A four-way valve was used to connect the mass spectrometer and the electrochemical apparatus. The gas path was connected in the following order: mass spectrometer - four-way valve - inlet to battery - outlet to battery - inlet to four-way valve - mass spectrometer. The electrolyte was (2 / 3 M LiFSI + 1 / 3 M LiTFSI dissolved in FEC (fluoroethylene carbonate)). Electrolyte decomposition fragment signals were detected after stable battery cycling, and the signals were integrated over time to obtain the corresponding gas production data. For long-cycle testing, in an argon-protected glove box (water content ≤ 0.1 ppm, oxygen content ≤ 0.1 ppm), a coin-type lithium-sulfur half-cell was assembled in the following order: positive electrode shell - active material - glass fiber separator - electrolyte - lithium sheet - gasket - spring contact - negative electrode shell. The electrochemical workstation was connected using wires at a rate of 0.05C, 1.0 V - 3.0 V. Constant current charge-discharge tests were performed within the voltage range of V. The electrolyte decomposition fragment signals were detected after the battery was stably cycled, and the CH4 (m / z=16) and CO2 (m / z=44) signals were integrated over time to obtain the corresponding gas production data. Figure 4 The electrochemical reaction process intervals of Examples 1-1 and 1-2 were obtained by integrating the signals of CH4 (m / z=16) and CO2 (m / z=44) over time to obtain the corresponding gas production data, such as... Figure 4 As shown, the dashed line represents the electrochemical reaction process interval as the integration interval, and the area of ​​the region represents the gas production.

[0029] Examples 1-2

[0030] Same as Example 1-1, except that the electrolyte is (2 / 3 M LiFSI + 1 / 3 M LiTFSI dissolved in DEC (diethyl carbonate) / FEC = 1 / 1, volume ratio).

[0031] Examples 1-3

[0032] Same as Example 1-1, except that the electrolyte is (2 / 3 M LiFSI + 1 / 3 M LiTFSI dissolved in DEC / FEC / VC (ethylene carbonate) = 5 / 4 / 1, volume ratio).

[0033] Examples 1-4

[0034] Same as Example 1-1, except that the electrolyte is (2 / 3 M LiFSI + 1 / 3 M LiTFSI dissolved in DEC / FEC / VC = 2 / 1 / 1, volume ratio).

[0035] Comparative Example 1

[0036] The obtained sulfur-carbon positive electrode sheets were uniformly cut into circular electrode sheets with a diameter of 10 mm using a coin cell slicing machine. In an argon-protected glove box (water content ≤0.1 ppm, oxygen content ≤0.1 ppm), coin-type lithium-sulfur half-cells were assembled directly in the following order: positive electrode shell - active material - glass fiber separator - electrolyte (2 / 3 M LiFSI + 1 / 3 M LiTFSI dissolved in DEC:FEC = 1:1, volume ratio) - lithium sheet - gasket - spring sheet - negative electrode shell. Argon gas was not applied, and the cell was not connected to a mass spectrometer. Constant current charge-discharge tests were conducted using an electrochemical workstation connected via wires at a rate of 0.05C within a voltage range of 1.0 V–3.0 V.

[0037] Examples 1-5

[0038] Same as Example 1-1, except that the electrolyte is (2 / 3 M LiFSI + 1 / 3 M LiTFSI dissolved in DEC / VC = 1 / 1, volume ratio).

[0039] The batteries of the above embodiments and comparative examples were tested using the same method, and the results are shown in Table 1 below.

[0040] Table 1 Relationship between battery performance and side reaction suppression index

[0041]

[0042] Example 2-1

[0043] Same as Example 1, except that the electrolyte is (1M LiTFSI dissolved in DOL / DME=1 / 1, volume ratio). The charge / discharge range is changed from 1.0 V-3.0 V to 1.8 V-3.0 V. The signals of CH3OCH2CH2 (m / z=59) and CH3CH2O (m / z=45) are integrated over time to obtain the corresponding gas production data.

[0044] Good Example 2-2

[0045] Same as Example 2-1, except that the electrolytes are (1M LiTFSI (+5wt% LiNO3) dissolved in DOL / DME=1 / 1, volume ratio).

[0046] Comparative Examples 2-3

[0047] The difference from Example 2-1 is that the electrolytes are (1M LiTFSI (+10wt% LiNO3) dissolved in DOL / DME=1 / 1, volume ratio).

[0048] The batteries of Examples 2-1 to 2-2 above were tested using the same method, and the results are shown in Table 2 below.

[0049] Table 2 Relationship between battery performance and side reaction suppression index

[0050]

[0051] Testing and Analysis

[0052] (1) Electrochemical characterization: According to the test results, the argon-filled battery (Example 1) and the battery with a normal structure (Comparative Example 1) have similar charge-discharge curves. Two discharge plateaus appear at around 2.3 V and 1.7 V, which represent the formation of the positive electrode / electrolyte interface layer and the further reduction of sulfur to lithium sulfide, respectively, indicating that the device has normal electrochemical behavior. Figure 2 ).

[0053] (2) Differential electrochemical mass spectrometry characterization: Based on the test results of Experiment Example 1, such as Figure 3As shown in Figure a, the evolution of electrolyte decomposition fragments, including ethylene glycol (HOCHCHOH, m / z=60), ethylene glycol (HOCH2CH2OH, m / z=62), carbon dioxide (CO2, m / z=44), and methane (CH4, m / z=16), can be observed. Combined with the electrochemical curves, it can be seen that the first discharge plateau (interfacial phase growth stage) is the main source of interfacial reactions. Subsequently, the electrolyte undergoes reduction, which also leads to battery decomposition. During charging, the electrolyte remains stable with no gas production signal. The gas production patterns of ethylene glycol and ethylene glycol are similar, indicating that they may be produced simultaneously by the same reaction. The gas production patterns of methane and carbon dioxide are similar, indicating that they originate from the same decomposition pathway, and their gas production accounts for the highest proportion (>90%), therefore, the electrolyte undergoes this side reaction to the greatest extent. This suggests that the interfacial reactions include two types of side reactions in carbonate electrolytes. One type involves the simultaneous breaking of bonds on both sides of the carbonyl carbon atom in the carbonate molecule, decomposing into long-chain fragments ethylene glycol (HOCHCHOH, m / z=60) and ethylene glycol (HOCH2CH2OH, m / z=62). This reaction is relatively slow, reaching its peak value only after the first voltage plateau has ended. The other reaction mode involves decomposition into short-chain fragments carbon dioxide (CO2, m / z=44) and methane (CH4, m / z=16). This reaction is faster and reaches its gas production peak more quickly in the early stages of discharge, representing the main decomposition mechanism for carbonate electrolytes. Figure 3 (b).

[0054] (3) Battery performance comparison: Using the solvent of Example 1-1 as the FEC standard, the I values ​​of several examples can be obtained. 抑制 The values ​​showed that Example 3 exhibited the best inhibition effect, while Example 5 showed no inhibition and instead accelerated electrolyte decomposition. Finally, long-cycle electrochemical performance testing also demonstrated that it possessed the optimal Ig value. 抑制 Example 3 showed a better capacity retention. Similarly, the analysis results of Examples 2-1 to 2-3 were statistically analyzed, as shown in Table 2. Using the DOL / DME ratio of Example 2-1 as a baseline, the addition of LiNO3 suppressed the decomposition of DOL and DME, thereby improving the capacity retention. However, after excessively increasing the amount of LiNO3, the improvement in capacity retention began to decline.

[0055] In summary, the analytical method disclosed in this invention, through its strategy, can ensure real-time analysis of interface reaction pathways by capturing and analyzing volatile fragments of interface reactions under normal battery operation, providing direct evidence related to battery failure. The above description is merely a preferred embodiment of this invention and should be understood as not intended to limit the implementation of the invention. Those skilled in the art can readily make corresponding modifications or alterations based on the main concept and spirit of this invention; therefore, the scope of protection of this invention should be determined by the scope claimed in the claims.

Claims

1. A method for optimizing lithium battery electrolytes based on the analysis of volatile debris, comprising the following steps: (S1) Reference signal acquisition and critical fragment identification: Under the preset standard battery operating conditions, the reference electrolyte formula is tested, and the volatile fragments generated are continuously monitored using a differential electrochemical mass spectrometer. By analyzing the potential, timing and intensity of the fragments, the "critical harmful fragments" that are strongly correlated with the battery capacity decay are identified. (S2) Test Formulation Testing and Signal Acquisition: Under the same test conditions, the electrolyte formulation to be tested is tested, and the integrated intensity S of the key hazardous fragments is recorded. 待测； (S3) Define the side reaction inhibition index I 抑制 Its calculation formula is: I 抑制 =(1-S 待测 / S 基准 ) × 100%; I 抑制 The higher the value, the better the electrochemical performance of the battery with the changed electrolyte formulation compared to the standard battery.

2. The method according to claim 1, characterized in that, The reference electrolyte formulation includes at least one of a solvent, a lithium salt, and a functional additive.

3. The method according to claim 2, characterized in that, The solvent system includes ethers (such as DOL, DME), sulfones (such as TMS), esters (such as DEC, FEC, VC) and their mixtures.

4. The method according to claim 2, characterized in that, The lithium salts include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium hexafluorophosphate (LiPF6).

5. The method according to claim 2, characterized in that, The functional additives include lithium nitrate (LiNO3) and lithium difluorophosphate (LiDFP).

6. The method according to claim 1, characterized in that, The differential electrochemical mass spectrometer is a differential electrochemical mass spectrometer that acquires the entire m / z sequence.

7. The method according to claim 1, characterized in that, The differential electrochemical mass spectrometer and the battery are connected via a four-way valve. The gas path is connected in the following order: differential electrochemical mass spectrometer - four-way valve - battery inlet - battery outlet - four-way valve inlet - differential electrochemical mass spectrometer.

8. The method according to claim 1, characterized in that, When the solvent is a carbonate, CO2 (m / z=44) and / or CH4 (m / z=16) are identified as key hazardous fragments; when the solvent is an ether solvent, CH3OCH2CH2 (m / z=59) and / or CH3CH2O (m / z=45) are identified as key hazardous fragments; when the solvent is a sulfone solvent, SO2 (m / z=48,64) and / or H2S (m / z=34) sulfur-containing fragment features are identified as key hazardous fragments.

9. The method according to claim 1, characterized in that, When the lithium salt is LiPF6, POF3 (m / z=85,104) and / or HF (m / z=20) are identified as critical hazardous fragments; when the lithium salt is lithium borate, boron fluoride BF3 (m / z=49) is identified as a critical hazardous fragment.

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