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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Sun, Xueliang

Abstract

All-solid-state batteries (ASSBs) with polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) or oxide-based solid-state electrolytes (SSEs) are promising candidates for electric vehicles application. However, ASSBs suffer from critical challenges including (i) low electrochemical oxidation window, (ii) poor interface contact, (iii) incompatibility between the SSE and electrode. This thesis, therefore, focuses on various strategies for addressing these problems and understanding the insight mechanisms.

To address the low oxidation window challenge of SPE, surface engineering method was used. The surface coating on LiCoO2, and/or carbon particles with lithium tantalate was conducted. This study disclosed that carbon particles/SPE interface is detrimental to the electrochemical decomposition of SPE. Further, lithium niobium oxide engineering NMC811/SPE interface was done for improving the stability of NMC811 particles and alleviating the decomposition of SPE.

Moreover, the oxidation window of SPE was increased by engineering the end functional group of PEO. Stable performance ASSBs were obtained with the dimethylamine end group SPE. Besides, the binders’ effect was studied. PEO binder are not practical for 4 V class cathodes because of its low oxidation window, while carboxyl-rich polymer binders have superior performance. Mechanism studies showed that they have higher voltage stability and work as a coating material to protect electrode/SPE interface.

The poor contact between oxide-based SSE and cathode particles was addressed with solution method synthesized Li3InCl6 SSE. The incompatibility between NASICON SSEs and sulfur cathodes is tackled with ultra-thin Al2O3 protection.

The discoveries of this thesis provide important guidance to design high performance, high energy density ASSBs.

Summary for Lay Audience

Developing all-solid-state batteries (ASSBs) with nonflammable solid-state electrolytes (SSEs) is important for electric vehicle (EV) applications. However, the problems including (1) instability of solid polymer electrolytes (SPEs) at high voltage, (2) poor interfacial contact, and (3) side reactions at the electrode/SSE interface significantly restrict the development of ASSBs. Several methods were developed to address these problems, and their insight mechanism were investigated in this thesis.

To address the high voltage instability problem of SPEs, interface protection method was used. The interface between the LiCoO2 particles and SPE, the interface of carbon particles and SPE, are protected, respectively, with lithium tantalate. The results indicated carbon particles/SPE is detrimental to the decomposition of SPE. Ni-rich NMC811 cathode should be used for achieving high energy density ASSBs. However, both NMC811 and SPE are not stable at high voltage. The NMC811 electrode/SPE interface was engineered with lithium niobium oxide (LNO), as a result of this, the instability problem of NMC811 and the decomposition of SPE were alleviated with LNO protection.

Modifying the structure of the polymer chain was done by using dimethylformamide solvent to increase the high voltage stability of polymer. As a result of this, higher voltage stability of SPE and higher electrochemical performance of ASSBs were realized with this modified SPE.

In ASSBs, the most used binder is PEO which is not stable at high voltage, thus it is not suitable for high voltage ASSBs. Mechanism studies showed that carboxyl-rich polymer (CRP) binders are more stable at high voltage, therefore, they present better performance in ASSBs.

The poor contact between SSEs and cathodes was addressed by solution synthesized Li3InCl6. Mechanism studies showed that in-situ synthesized Li3InCl6 realized intimately contacts between SSE and cathode. Side reaction between the oxide-based SSE and electrode was also addressed. The sulfur cathode can react with the Ti-containing NASICON SSE, resulting in SSE decomposition. With ultra-thin Al2O3 protection, the stability of the NASICON SSE dramatically increased and the cycling performances of ASSBs were improved.

The discoveries of this thesis provide important guidance to design high performance, high energy density ASSBs.

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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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