| Author: | Gohar, Ghulam Abbas |
| Title: | Chemical kinetics of electrolyte and cathode materials for enhancing fire safety in lithium-ion batteries |
| Advisors: | An, Liang (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Department: | Department of Mechanical Engineering |
| Pages: | xxiii, 178 pages : color illustrations |
| Language: | English |
| Abstract: | Investigating the heterogeneous reaction between the electrolyte and cathode materials is crucial for understanding the safety limitations associated with cathodes in lithium-ion batteries. This thesis focuses on the development of a flow reactor (FR) coupled with gas chromatography (GC) to study the reaction kinetics of the electrolyte with cathode materials. The FR enables precise control over reaction conditions, while GC facilitates the identification and quantification of the various species generated during the reactions. First, the gas-phase reactivities of diethyl carbonate (DEC) in a controlled temperature FR was investigated at temperature range (300~900K) and constant pressure (1 atm) with three residence time 6s, 1hr and 10 hrs. For equivalence ratio (𝜑=1.5), the DEC oxidation begins at temperature of 620K, 420K, and 370K when it is interacted with oxygen content at residence time of 6s, 1hr, 10hr, respectively. A kinetic model was developed for the oxidation of DEC/O2 using CHEMKIN Pro, and the predictions of the model exhibited a satisfactory level of agreement with the experimental results. Furthermore, the oxidation of DEC begins at temperature of 420K, 380K, 410K when it is interacting with three mixtures (1%DEC/4%O2/95%N2), (1%DEC/4%O2/95%N2/NCM622), and (1%DEC/99%N2/NCM622), respectively at residence time of 1 hr. In mixture 1 (where O2 gas serve as sources of oxygen for DEC oxidation), the concentration of different species such as C2H5OH (18.7%), CH2O (27.62%), CH3CHO (10%), CO2 (11.04%), CO (43.5%), and C2H4 (13.8%) are investigated at 500K and 1 atm pressure. In mixture 2 (where Lithium nickel manganese cobalt oxides (NCM622) and O2 gas serve as sources of oxygen for DEC oxidation), higher concentrations of C2H5OH (44.18%), CH2O (67.41%), CH3CHO (66.92%), CO2 (47.05%), CO (43.54%), and C2H4 (28.66%) are examined. In mixture 3 (where NCM622 serve as sources of oxygen for DEC oxidation), DEC is oxidized when interacting with NCM622, resulting in the generation of species such as C2H5OH (37.1%), CH2O (4.97%), CH3CHO (23.08%), CO2 (41.91%), CO (12.92%), and C2H4 (57.54%) at 500K and pressure (1 atm). In addition, scanning electron microscopy (SEM) confirmed the crystal structure and morphology of NCM622 when it was treated with DEC vapor. The EDS analysis investigated the changes in elemental composition of Ni, Co, Mn, and O when NCM622 was subjected to heat treatment and interacted with DEC vapor. In XRD analysis, the value of intensity ratio (I003/I104) of (003) and (104) planes are 1.42 where the extent of unfavourable mixing between Ni and Li cation in treated materials with 1%DEC/99%N2 that indicate negative impact on electrochemical performance. Some functional group attachment is observed in Raman spectrum of NCM622 that is clear indication of chemical change. The observed chemical shifts of the Ni2p3/2 (-0.9eV), Ni2p1/2 (-1.33eV), Co2p3/2 (2.45eV), Co2p1/2 (3.52eV), Mn2p3/2 (-0.2eV), Mn2p1/2 (-1.84eV) and O1s (-0.78eV) atoms in NCM622 provide confirmation of chemical reactions occurring with mixture 2 at 450K. Similarly, the chemical shifts of the Ni2p3/2 (-1.2eV), Ni2p1/2 (-2.35eV), Co2p3/2 (1.37eV), Co2p1/2 (-0.35eV), Mn2p3/2 (0.06eV), Mn2p1/2 (-4.5eV) and O1s (-0.78eV) atoms are observed when using mixture 3 at 450K. This oxidation and decomposition of the electrolyte on the surface of NCM622 are major causes of lithium-ion battery failure. Second, the decomposition of a mixture of diethyl carbonate (DEC) and ethylene carbonate (EC) was examined to start at a temperature of 650K, 450K and 350K with a residence time of 6s, 1hr and 10 hrs respectively in gas phase reaction when O2 gas is source of oxidation of DEC/EC mixture. Furthermore, the oxidation of DEC and EC begins at 450K when it is treated with three mixtures, Mixture 1: (0.7%DEC+0.3%EC)/3.3%O2/95%N2, Mixture 2: (0.7%DEC+0.3%EC)/3.3%O2/95%N2/NCM622 Mixture 3: (0.7%DEC+0.3%EC)/99%N2/NCM622). In mixture 1, where O2 gas serves as the source of oxygen, the oxidation of DEC and EC leads to the formation of species such as C2H5OH (14.25%), CH2O (19.68%), CO2 (5.01%), and CO (1.64%) at a temperature of 500K and a pressure of 1 atm. In mixture 2, where both O2 gas and NCM622 act as sources of oxygen, higher concentrations of species are observed. These include C2H5OH (50.54%), CH2O (45.34%), CH3CHO (46.56%), CO2 (45.08%), and CO (47.57%). In mixture 3, where NCM622 is the source of oxygen, resulting species consist of C2H5OH (35.21%), CH2O (34.97%), CO2 (49.51%), and CO (50.79%) at a temperature of 500K and a pressure of 1 atm. SEM analysis confirmed the crystal structure and morphology of NCM622 after being treated with DEC/EC vapor. Additionally, EDS analysis was performed to examine the elemental composition of Ni, Co, Mn, and O in NCM622 following heat treatment and interaction with DEC/EC vapor. During XRD examination, the intensity ratio (I003/I104) of the (003) and (104) planes was found to be 1.83 in case of mixture 2 and 1.71 in case of mixture 3. This indicates undesirable mixing between Ni and Li cations in the treated materials that has a negative impact on the electrochemical performance of the material. Furthermore, chemical changes in the Ni2p3/2, Co2p3/2, and Mn2p3/2 atoms in NCM622 validate the processes involving DEC/EC vapor. The oxidation and disintegration of the electrolyte on NCM622 contribute to the failure of the lithium-ion battery. In addition, the mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) initiated oxidation at temperatures of 550K and 400K when reacted with an O2 source, with residence times of 6s and 1hr, respectively. Different product species (CH3OH, CH2O, CO2, and CO) are observed during the oxidation of DMC and EC. Furthermore, NCM622 is treated with different mixtures: Mixture 1: (0.7%DMC+0.3%EC)/1.9%O2/97.1%N2, Mixture 2: (0.7%DMC+0.3%EC)/1.9%O2/97.1%N2/NCM622, Mixture 3: (0.7%DMC+0.3%EC)/99%N2/NCM622, In mixture 1, the oxidation of DMC and EC begins to start 450K when interacting with reactive O2 gas and resulting product species such as CH3OH (19.18%), CH2O (51.42%), CO2 (40.23%), and CO (26.72%). In mixture 2, (O2 gas and NCM622 as source of oxygen), following concentrations of species are observed. These include CH3OH (16.89%), CH2O (33.65%), CO2 (21.93%), and CO (59.62%). In mixture 3, where NCM622 is the source of oxygen, resulting species consist of CH3OH (63.92%), CH2O (14.94%), CO2 (37.84%), and CO (13.66%) at 500K and constant pressure (1 atm). SEM analysis revealed that the crystal structure and morphology of NCM622 were successfully confirmed following treatment with DMC/EC vapor. Additionally, EDS analysis was conducted to investigate the elemental composition of Ni, Co, Mn, and O in NCM622 after undergoing heat treatment and interaction with DMC/EC vapor. During XRD examination, it was observed that the intensity ratio (I003/I104) of the (003) and (104) planes was 1.45 for mixture 2 and 1.42 for mixture 3. These values indicate the undesirable mixing between Ni and Li cations in the treated materials, which has a detrimental effect on the electrochemical performance of the material. Furthermore, the chemical changes observed in the Ni2p3/2, Co2p3/2, Mn2p3/2 and O1s atoms in NCM622 provide further validation of the processes involving DMC/EC vapor. Overall, the design of a flow reactor (FR) is a proper fundamental technique for observing the real scenario of the reaction mechanism between the electrolyte and cathode materials in a controlled environment. The oxidation of the electrolyte on the cathode surface is identified as a major contributor to lithium-ion battery failure. These findings enhance our understanding of the reactivity, kinetics, and degradation processes of DEC, the mixture of DEC/EC, and the mixture of DMC/EC with NCM622. The oxidation of DEC, the mixture of DEC/EC, and the mixture of DMC/EC results in the generation of species that exhibit similarities to thermal runaway. These findings provide valuable insights for enhancing the safety and performance of lithium-ion batteries. |
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| Access: | open access |
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