Design strategies of sulfide based solid electrolyte

Abstract

Greenhouse effect is a concern because of the increase in CO2 gas emission due to an increase in the number of fossil fuel powered vehicles and global industrial development. To reduce the CO2 emission, demand for large-sized lithium ion batteries (LIB) have been increased due to the increased demand for electric vehicles (EV) and energy storage systems (ESS). However, conventional LIBs use a flammable organic liquid electrolyte which have the safety problems of lithium ion batteries such as combustion and explosion. To solve these problems, all-solid-state lithium ion batteries (ASS-LIBs) are attracting attention as the best candidate of next generation batteries that use the nonflammable ceramic electrolytes.

However, some technical challenges remain for realizing the high performance of ASS-LIBs using inorganic solid electrolytes. First, to increase energy density of ASS-LIBs, it is needed to suppress the interfacial resistance between the cathode active material and the solid electrolyte. Furthermore, in order to use Li metal anode which is essential for energy density improvement, research to improve interfacial stability of solid electrolyte is needed. Finally, the contact morphology of the solid electrolyte particles was adjusted to enlarge the interfacial area in the electrode.

In Chapter 2, a study on nitrogen and oxygen substitution to improve the lack of electrochemical stability of sulfide solid electrolytes is presented. To realize all-solid-state batteries, solid electrolytes were required some physical properties such as high ionic conductivity, good chemical, thermal and electrochemical stability. Solid electrolytes are mostly classified into sulfide and oxide based systems. Sulfide electrolytes such as LGPS, Li-argyrodite and Li2S-P2S5 glass-ceramics have a high ionic conductivity but have a poor chemical stability with humidity. Several types of sulfide electrolytes have been shown favorable lithium ion conductivities over 10-3 S cm-1, which are comparable to those of liquid electrolytes. However, sulfide solid electrolytes have the disadvantage of lacking electrochemical stability with electrode active materials. Thus, some researchers have attempted to improve the stability of solid electrolytes by attempting to substitute S with O. Given that the bonding energy of P-O are stronger than those of P-S, oxygen substitution would be expected to enhance electrochemical stability. In this chapter, the sulfide glass-ceramics electrolytes with the compositions (78-x)Li2S-22P2S5-xLi2SO4 were synthesized to observe the oxygen doping effect, and their glass-ceramics structures, ionic conductivities and electrochemical stabilities were analyzed. The all-solid-state battery with Li2SO4 doped glass-ceramics electrolyte was demonstrated enhanced all-solid-state battery performances and cyclability.

In addition, nitrogen doping was performed to improve the electrochemical stability of argyrodite solid electrolyte. Nitrogen-substituted Li-argyrodite sulfide solid electrolytes were fabricated using the mechanical milling process and analyzed their structure. Results showed that substituting a small amount of Li3N in the Li-argyrodite system is effective for enhancing ionic conductivity and electrochemical stability. And the interface resistance was effectively reduced by suppressing the degradation of the solid electrolyte due to contact with the cathode by nitrogen substitution of the electrolyte.

In Chapter 3, a study to improve the electrochemical stability of solid electrolytes for the use of Li metal anode is described. We expect to improve the structural stability and chemical durability of the glass matrix when adding Sn to the glass-ceramics electrolytes by the “mixed-former effect”. The combination of several network formers usually provides the possibility of fine-tuning physical and chemical property combinations to fulfil specific needs, and in certain case new characteristics arise as a result of a specific interaction of the two network former species. Along with these benefits, it is known that solid electrolytes containing Sn have improved moisture stability because they form a chemical structure with high binding tendency between Sn and S explained by the hard and soft acids and bases (HSAB) theory.

We applied the similar approach to suppress the side reactions between Li metal and solid electrolytes under oxidizing-reducing environments during the charging and discharging process, and we indeed demonstrated that the proper addition of SnS2 to Li2S-P2S5 glass-ceramics electrolytes improves the chemical and electrochemical stability to lithium metal anode.

In Chapter 4, a study on the effect of bimodal sized electrolyte to maximize the contact area inside the composite cathode is described. The electrodes of all-solid-state lithium ion batteries have composite structure with the electrode active materials, the inorganic solid electrolytes and the conducting agents. A major problem of composite cathode is the lack of contact between the electrode active materials and the solid electrolytes.

The composite electrode layers were fabricated using the bimodal type solid electrolyte. The pores in the composite cathode layer were filled with the small-size inorganic solid electrolyte to lower the interface resistance and to widen the contact areas between the solid electrolyte and the cathode material. The electrochemical properties of composite electrodes for all-solid-state batteries with different structures were evaluated. The bimodal type solid electrolyte for composite cathode layer provides improved all-solid-state battery performances while maintaining excellent stability and durability.