Cold Sintering Process for Materials and Their Integration Enabling All-Solid-State Li-ion Batteries

Open Access
- Author:
- Seo, Joo Hwan
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 08, 2020
- Committee Members:
- Clive A Randall, Dissertation Advisor/Co-Advisor
Clive A Randall, Committee Chair/Co-Chair
Donghai Wang, Committee Member
Enrique Daniel Gomez, Committee Member
Ramakrishnan Rajagopalan, Outside Member
Thomas E. Mallouk, Special Member
John C Mauro, Program Head/Chair - Keywords:
- Cold sintering process
All-solid-state Li-ion battery
Solid-state electrolyte
Densification
Volumetric capacity
Ionic conductivity
Interface - Abstract:
- All-solid-state Li batteries (ASSB) have attracted much attention as a next-generation battery system providing high energy density, improved safety, long cycle-life. However, there are a number of challenges to overcome in regard to the practical performance and fabrication process for large scale manufacturing and commercialization, such as low energy density due to low active materials fraction in the electrode composite, poor microstructures with high porosity, high interfacial resistances, the requirement of high temperature and long-time sintering process, and so forth. In this study, the cold sintering process was explored to densify the composite electrodes, solid-state electrolyte at low temperatures for achieving high volumetric capacity and ionic conductivity, respectively. Moreover, the integration process of the composite electrode and solid-state electrolyte layers for an all-solid-state Li-ion battery was developed to demonstrate its feasibility to co-sinter the multilayer solid-state full cell as a low-temperature fabrication process. Cold sintering process was a successful approach to densify cathode and anode composites, which are based on LiFePO4 (LFP) and Li4Ti5O12 (LTO), respectively, for achieving high volumetric capacity density. The cold sintered composite electrodes were demonstrated to have a high density of over 80 % of relative density with good microstructures and to provide high volumetric capacities relative to conventional electrodes fabricated by calendering process. Moreover, the cold sintering process was applied to fabricate the binder-free thin composite electrode tape with high active material fraction and successfully demonstrated to enable a highly densified and thin electrodes with not only high volumetric capacity, but also high rate capability. With regard to the solid-state electrolytes, the cold sintering process was explored to densify the pure Li1.5Al0.5Ge1.5(PO4)3 (LAGP) ceramics and demonstrated its possibility for the high-density LAGP ceramics with excellent microstructures. However, the ionic conductivity of the cold sintered electrolyte was not so promising as expected. The poor ionic conductivity of the cold sintered LAGP ceramics was attributed to the grain boundary with high resistance due to incongruent dissolution and resulting off-stoichiometry and poor crystallinity phases. To overcome this challenge of cold sintering for LAGP, various kinds of strategies for the cold sintering process of LAGP was considered like composite electrolytes mixed with a liquid electrolyte and/or a polymer and a salt. Additionally, the thin solid-state electrolyte tape was cold sintered for practical applications of ASSB. All the cold sintered composites layer can offer low areal resistance with high conductivity, showing the effectiveness of the cold sintering approach to fabricate solid-state electrolyte separator at low temperatures. Furthermore, the garnet Li7Lr3Zr2O12 (LLZO) electrolyte material was investigated to apply cold sintering to develop high conductivity LLZO electrolyte. The LLZO-based composites were cold sintered using dimethylformamide (DMF, C3H7NO). In this composite electrolyte study, a polymer-salt bridge was employed to form active phases between the LLZO grains for achieving high conductivity. The good microstructures with well-developed grain boundaries were characterized. Also, the high conductivity of the cold sintering LLZO composite in the broad temperature range compared to other ceramic or composite electrolytes were demonstrated. An all-solid-state Li-ion battery with a configuration of LTO/LLZO/LFP was fabricated by applying the cold sintering process to co-sinter and integrate three components. The cold sintering process was successfully demonstrated for co-sintering of the multilayered solid battery cell with limited defects. The cold sintered solid full cell was demonstrated to have dense microstructures and good interfaces between different materials developed by employing the polymer-salt composite. The cold sintered battery cell can offer high electrochemical performances such as low internal cell resistance, high capacity, rate capability, and long cyclability. This cold sintering approach is a promising approach to enable all-solid-state batteries fabricated by co-sinter at low temperatures in large scale manufacturing.