Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Sun, Xueliang (Andy)

2nd Supervisor

Sham, Tsun-Kong

Co-Supervisor

3rd Supervisor

Sun, Xuhui

Affiliation

Soochow University

Joint Supervisor

Abstract

Sodium ion battery is considered as a potential candidate to replace lithium ion battery. To eliminate the need for containment of the liquid electrolyte, the solid-state electrolytes attract more attention. However, solid state electrolytes are still restricted to be applied for commercialization because of the inherent low ionic conductivity and the poor mechanical properties. Therefore, my Ph.D. research is focused on developing advanced solid-state electrolytes by improving ionic conductivity and disclosing mechanisms of the electrolytes.

Firstly, a series of La substituted NASICON electrolytes are prepared to achieve high ionic conductivity of mS/cm, and the effects of La substitution concentration are disclosed.

Secondly, Sc is successfully doped into Na3Zr2Si2PO12 to substitute Zr atoms to improve the ionic conductivity. The dynamics and mechanism show that a redistribution of the Na ions has effect on ionic conductivity.

In the third part, Bi is firstly discovered as a valid substitution element which can increase the ionic conductivity of NASICON to the scale of 10-3 S/cm.

To reduce costs, a cheaper heteroatom Zn is applied as a valid substitution element which can increase the ionic conductivity of NASICON to 10-3 S/cm. Si/P ratio change is the main cause for conductivity improvement.

In the fifth part, a series of NASICON electrolytes with different Si/P ratio are prepared and compared. 3:1 turns out to be an optimal Si/P ratio by ionic conductivities, structure and mechanism studies.

In the sixth part, full batteries NaCrO2/SSE/Na are assembled for testing with NaCrO2 as the cathodes. All NaCrO2/SSE/Na batteries deliver a relatively good reversible capacity at low current below 3 C.

In summary, substitution of La, Sc, Bi, Zn, as well as altering Si/P in NASICON structure are proven could increase the ionic conductivity to 10-3 S/cm. In addition, the detailed mechanisms are revealed by studying electronic and local structure of elements and Na+ transport in the structure. An increased Si/P ratio and enhanced Na+ occupancy for NASICON SSEs can lead to improved ionic conductivities. All NaCrO2/SSE/Na batteries delivered a relatively good reversible capacity at low current below 3 C. Zn and Bi can react with Na metal, leading to a lower capacity. To apply a hybrid solid-state electrolyte consisted of NASICON and a PVdF-HFP based gel polymer electrolyte can effectively prevent the reaction and maintain the high capacity during charge/discharge process.

Summary for Lay Audience

Given the unreliability of intermittent energy sources such as wind and solar energies, rechargeable batteries are one of the most promising energy storage technologies for such sustainable energies due to their reliability and high energy conversion efficiency. Although Li ion batteries (LIBs) have been widely used in our daily life, there are increasing concerns regarding the sustainability of lithium sources because of the limited availability and increasing price. To address these issues, Na ion batteries (NIBs) have attracted much attention. Due to their non-toxicity, low cost, and elemental abundance, NIBs are considered as the promising candidate power sources in recent years.

In NIBs, conventional liquid electrolytes (LEs) still pose a risk of potential leakage and explosions, so solid-state electrolytes (SSEs) have attracted much attention in recent research. SSEs are regarded as an ultimate component for future NIBs that they are expected to improve the durability and safety as well as simplify the cell design.

Among the sodium ion conducting solid state electrolytes (SSEs), NA Super-Ionic CONductor (NASICON) with a general formula of Na1+nZr2SinP3-nO12 (1.6 ≤ n ≤ 2.4), has attracted the most attention due to its high ionic conductivity and low thermal. Na3Zr2Si2PO12 has an ionic conductivity of 10-4 S/cm. To further improve the ionic conductivity, many efforts have been made in past decades. The most common way is element substitution. In this thesis, substitution of La, Sc, Bi, Zn, as well as altering Si/P in NASICON structure are proven could increase the ionic conductivity to 10-3 S/cm. In addition, the detailed mechanisms are revealed by studying electronic and local structure of elements and Na+ transport in the structure. An increased Si/P ratio and enhanced Na+ occupancy for NASICON SSEs can lead to improved ionic conductivities. All NaCrO2/SSE/Na batteries delivered a relatively good reversible capacity at low current below 3 C. Zn and Bi can react with Na metal, leading to a lower capacity. To apply a hybrid solid-state electrolyte consisted of NASICON and a PVdF-HFP based gel polymer electrolyte can effectively prevent the reaction and maintain the high capacity during charge/discharge process.

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