Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design
Energy density requirements for next-generation batteries make the Li metal anode a key candidate to replace graphite in Li-ion batteries due to Li’s improved capacity (3,860 mAh/g vs. 372 mAh/g). However, Li anodes display lower Coulombic efficiency (CE, <99.9%) than graphite (>99.95%), resul...
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| Other Authors: | |
| Format: | Thesis |
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Massachusetts Institute of Technology
2023
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| Online Access: | https://hdl.handle.net/1721.1/152954 |
| _version_ | 1811093419825561600 |
|---|---|
| author | Marques Hobold, Gustavo |
| author2 | Gallant, Betar M. |
| author_facet | Gallant, Betar M. Marques Hobold, Gustavo |
| author_sort | Marques Hobold, Gustavo |
| collection | MIT |
| description | Energy density requirements for next-generation batteries make the Li metal anode a key candidate to replace graphite in Li-ion batteries due to Li’s improved capacity (3,860 mAh/g vs. 372 mAh/g). However, Li anodes display lower Coulombic efficiency (CE, <99.9%) than graphite (>99.95%), resulting in faster loss of reversible Li over cycling. This shortfall derives from parasitic Li-electrolyte reactions that lead to accumulation of inactive Li⁰ and electrolyte-derived byproducts, which result in the formation of a native solid electrolyte interphase (SEI). The SEI is an electrolyte-derived passivating layer that ideally protects Li from continued reactions with the electrolyte, and carries the function of regulating Li⁺ exchange between Li and the electrolyte. Despite the importance of the SEI, its properties and composition are challenging to probe experimentally due to its exceedingly low amount (sub-μmolₗᵢ/cm²/cycle at >99% CE). Given these challenges, the framework on which the existing understanding of the SEI was built is largely based on qualitative models, such that knowledge about the optimal chemical composition and functional properties of the SEI has remained remarkably incomplete. To bridge this gap, this thesis focuses on advancing experimental methodologies to precisely quantify (1) functional properties and (2) chemical composition of native SEIs, and explore their relationships with CE.
First, a key functional property that is regulated by the SEI is studied: the rate of Li⁺ exchange between Li and the electrolyte. Measurements of Li⁺ exchange have historically been challenging to interpret due to inconsistent measurement protocols, leading to unclear understanding of its relationship with CE and its role on passivation of the SEI, an often loosely-defined property. Here, distinct but self-consistent electrochemical techniques are used to precisely define and quantify Li⁺ exchange across SEIs, revealing that this functional property varies across electrolytes, increasing from low to high CE, thus ultimately establishing that fast Li⁺ exchange is needed for high CE. These results further revealed a previously-overlooked evolution of Li⁺ exchange with cycling that is unique to high-CE electrolytes, underscoring the importance of SEI formation.
Crucially, imparting beneficial properties to the SEI requires exacting knowledge of the chemical phases that ought to be promoted vs. suppressed at high-CE. This understanding can be gained by examining high vs. low CE SEIs, but has insofar remained largely qualitative due to the lack of quantitative techniques for SEI characterization. Thus, in order to (2) explore SEI chemical compositions that are favored at high CE, the quantitative power of analytical instruments was leveraged to measure byproducts of SEI formation with unprecedented chemical and quantitative resolution. A custom operando GC experiment was developed to measure gas evolution in situ during Li cycling. With sub-nmol/min resolution, these revealed that SEI formation reactions that release CO or CO₂, leaving behind a decarbonylated/decarboxylated byproduct in the SEI, are associated with higher CE. Chemical quantification was extended to post-mortem cells by advancing titration techniques to enable ~nAh resolution of select SEI phases (ROCO₂Li, Li₂C₂, RLi, LiF, P-containing), in addition to the inactive Li⁰. These enabled an unprecedented look into precise SEI compositional breakdown fingerprints that are unique to each electrolyte. In particular, Li₂C₂, a previously-unmeasured phase on Li, showed a strong anti-correlation with CE. Titration was further expanded to include Li₂O and to perform a rigorous statistical analysis on the role of oxygenation vs. fluorination on CE—the latter strategy has been the major motif guiding electrolyte design for the past decade. It was found that Li₂O displayed the strongest positive correlation with CE, surpassing the fluorinated LiF. The critical role of Li₂O was further exploited to create a set of fluorine-free electrolytes which >99% CE, revealing SEI oxygenation to be an alternative and underexplored design space for electrolyte design discovery. Altogether, these quantitative SEI-based descriptors thus reveal new avenues for electrolyte engineering, informed by a fundamental understanding of the “optimal” Li SEI and their associated capacity loss mechanisms at high CE. |
| first_indexed | 2024-09-23T15:44:46Z |
| format | Thesis |
| id | mit-1721.1/152954 |
| institution | Massachusetts Institute of Technology |
| last_indexed | 2024-09-23T15:44:46Z |
| publishDate | 2023 |
| publisher | Massachusetts Institute of Technology |
| record_format | dspace |
| spelling | mit-1721.1/1529542023-11-14T03:06:28Z Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design Marques Hobold, Gustavo Gallant, Betar M. Massachusetts Institute of Technology. Department of Mechanical Engineering Energy density requirements for next-generation batteries make the Li metal anode a key candidate to replace graphite in Li-ion batteries due to Li’s improved capacity (3,860 mAh/g vs. 372 mAh/g). However, Li anodes display lower Coulombic efficiency (CE, <99.9%) than graphite (>99.95%), resulting in faster loss of reversible Li over cycling. This shortfall derives from parasitic Li-electrolyte reactions that lead to accumulation of inactive Li⁰ and electrolyte-derived byproducts, which result in the formation of a native solid electrolyte interphase (SEI). The SEI is an electrolyte-derived passivating layer that ideally protects Li from continued reactions with the electrolyte, and carries the function of regulating Li⁺ exchange between Li and the electrolyte. Despite the importance of the SEI, its properties and composition are challenging to probe experimentally due to its exceedingly low amount (sub-μmolₗᵢ/cm²/cycle at >99% CE). Given these challenges, the framework on which the existing understanding of the SEI was built is largely based on qualitative models, such that knowledge about the optimal chemical composition and functional properties of the SEI has remained remarkably incomplete. To bridge this gap, this thesis focuses on advancing experimental methodologies to precisely quantify (1) functional properties and (2) chemical composition of native SEIs, and explore their relationships with CE. First, a key functional property that is regulated by the SEI is studied: the rate of Li⁺ exchange between Li and the electrolyte. Measurements of Li⁺ exchange have historically been challenging to interpret due to inconsistent measurement protocols, leading to unclear understanding of its relationship with CE and its role on passivation of the SEI, an often loosely-defined property. Here, distinct but self-consistent electrochemical techniques are used to precisely define and quantify Li⁺ exchange across SEIs, revealing that this functional property varies across electrolytes, increasing from low to high CE, thus ultimately establishing that fast Li⁺ exchange is needed for high CE. These results further revealed a previously-overlooked evolution of Li⁺ exchange with cycling that is unique to high-CE electrolytes, underscoring the importance of SEI formation. Crucially, imparting beneficial properties to the SEI requires exacting knowledge of the chemical phases that ought to be promoted vs. suppressed at high-CE. This understanding can be gained by examining high vs. low CE SEIs, but has insofar remained largely qualitative due to the lack of quantitative techniques for SEI characterization. Thus, in order to (2) explore SEI chemical compositions that are favored at high CE, the quantitative power of analytical instruments was leveraged to measure byproducts of SEI formation with unprecedented chemical and quantitative resolution. A custom operando GC experiment was developed to measure gas evolution in situ during Li cycling. With sub-nmol/min resolution, these revealed that SEI formation reactions that release CO or CO₂, leaving behind a decarbonylated/decarboxylated byproduct in the SEI, are associated with higher CE. Chemical quantification was extended to post-mortem cells by advancing titration techniques to enable ~nAh resolution of select SEI phases (ROCO₂Li, Li₂C₂, RLi, LiF, P-containing), in addition to the inactive Li⁰. These enabled an unprecedented look into precise SEI compositional breakdown fingerprints that are unique to each electrolyte. In particular, Li₂C₂, a previously-unmeasured phase on Li, showed a strong anti-correlation with CE. Titration was further expanded to include Li₂O and to perform a rigorous statistical analysis on the role of oxygenation vs. fluorination on CE—the latter strategy has been the major motif guiding electrolyte design for the past decade. It was found that Li₂O displayed the strongest positive correlation with CE, surpassing the fluorinated LiF. The critical role of Li₂O was further exploited to create a set of fluorine-free electrolytes which >99% CE, revealing SEI oxygenation to be an alternative and underexplored design space for electrolyte design discovery. Altogether, these quantitative SEI-based descriptors thus reveal new avenues for electrolyte engineering, informed by a fundamental understanding of the “optimal” Li SEI and their associated capacity loss mechanisms at high CE. Ph.D. 2023-11-13T19:56:49Z 2023-11-13T19:56:49Z 2023-09 2023-09-28T15:51:22.064Z Thesis https://hdl.handle.net/1721.1/152954 In Copyright - Educational Use Permitted Copyright retained by author(s) https://rightsstatements.org/page/InC-EDU/1.0/ application/pdf Massachusetts Institute of Technology |
| spellingShingle | Marques Hobold, Gustavo Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design |
| title | Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design |
| title_full | Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design |
| title_fullStr | Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design |
| title_full_unstemmed | Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design |
| title_short | Quantitative Solid Electrolyte Interphase-Based Descriptors for Lithium Liquid Electrolyte Design |
| title_sort | quantitative solid electrolyte interphase based descriptors for lithium liquid electrolyte design |
| url | https://hdl.handle.net/1721.1/152954 |
| work_keys_str_mv | AT marqueshoboldgustavo quantitativesolidelectrolyteinterphasebaseddescriptorsforlithiumliquidelectrolytedesign |