Traditional lithium (Li)-ion batteries with liquid electrolytes suffer the problems of poor safety, limited voltage, unstable solid-electrolyte interphase formation and poor cycling performance, all of which can potentially be solved by solid-state lithium batteries (SSLiBs). The widely studied solid-state electrolytes (SSEs) includes LiSICON [1], NASICON-type Li conductors [2], perovskites [3] and garnets [4]. Garnet electrolytes are especially attractive for SSLiBs [4], since it has high total Li-ion conductivity, chemical stability with high voltage Li cathodes, chemical stability with Li anode. Aside from these good electrochemical property, developing an inexpensive and environmentally friendly thin-film preparation methods is critical for its application in SSLiBs. Such a thin film should have very high relative density, in order to prevent Li dendrite growth as well as to further improve total Li conductivity.

Li garnet materials are known to be difficult to sinter, due to Li evaporation at high temperature. Traditional die-pressing and ambient pressure sintering products garnet pellets with low relative density (<96%) and bad grain-grain connection. Currently there are only several papers reporting fully dense garnet (relative density >99%). These works require either high pressure sintering [5] or specially synthesized garnet nano powders [6]. These requirements hinder the cost-effective mass production of garnet thin film. Here in this work, we use a cost-effective tape-casting and sintering method to fabricate fully dense garnet thin films. The thinness of the thin film can be varied from as thin as 20 um to as thick as 1mm. The size of the thin film can be controlled by trimming before or after sintering. Figure 1(insert) shows the transparency of sintered garnet thin wafer. It high relative density is confirmed by SEM in figure 1.

The mechanism behind the fully dense sintering is studied. It is found that a good powder packing in the green tape and garnet particle surface modification by moisture and carbon dioxide during sintering contributes.

Reference:

[1] Xu, X. et al. Chem. Mater. 23, 3798-3804 (2011).

[2] Feng, J. K. et al. Mater. Technol. 28, 276-279 (2013).

[3] Ihlefeld, J. F. et al. Adv. Mater. 23, 5663-5667 (2011).

[4] Thangadurai, V. et al. Chem. Soc. Rev. 43, 4714-4727 (2014).

[5] Sharafi, A. et al. J. of Power Sources 302, 135-139 (2016).

[6] Yi, E. et al. J. of Power Sources 352, 156-164 (2017).

Figure 1: Fully dense garnet thin film has seamless cross section, indicating a very strong grain-grain connection. The insert shows its transparency.