Development of Novel Solid-State Electrolytes for Sodium Ion Batteries

Fei Sun, The University of Western Ontario

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.