Abstract
The ion conductivity of solid-state electrolytes (SSEs) has been greatly enhanced to a level comparable to that of liquid electrolytes, which is critical for fast charging/discharging. However, ion transport has primarily been characterized by macroscopic measurements such as electrochemical impedance spectroscopy (EIS) and galvanostatic profiles of charging/discharging. These measurements essentially reflect an average of microscopic ion-transport phenomena. Ion transport in SSEs can be far more complicated, but detailed microscopic transport characterization studies are currently lacking. Theoretical studies have proposed that ion transport can be governed by highly crystalline-orientation-dependent ionic hopping through the ceramic lattice, and some SSEs can even act as one-dimensional or two-dimensional ion conductors.[1][2] Characterization methods are needed that measure in-situ ion transport with nm-scale spatial resolution to improve our understanding and tune the ion conduction in solid-state batteries (SSBs).
Here, we report on nanometer-resolution in-situ ionic and electronic transport imaging of two advanced polymer-ceramic hybrid electrolytes: a polyimin/LixPySz (Li2S/P2S5:30/70) bulk hybrid SSE[3] and a Li-containing polyacrylonitrile (PAN)-coated Li-ion conductive glass ceramic (LICGC) with components of Li2O-Al2O3-SiO2-P2O5-TiO2-GeO2.[4] Hybrid SSE is a promising approach that combines high ion conductivity of ceramic SSE and high stability of polymer SSE. We achieved the nm-resolution in-situ transport imaging by an atomic force microscopy (AFM)-based half-cell setup consisting of an AFM-probe/SSE/Li-metal structure (see Fig. 1). The AFM probe size ensures the nm-scale spatial resolutions, and we detected the small current in the pA‒fA (10-12‒10-15 A) range flowing through the AFM probe by using a logarithmic-scale current amplifier that detects both the ionic conduction and electronic leaking currents through the SSE.
Concerning the bulk hybrid SSE, we found large ionic current fluctuations—five times to one order of magnitude on the LPS region of the SSE. The electronic leaking current is about 5 orders of magnitude smaller than the ionic current. Both the ionic and electronic transport at the LPS/polymer boundary show sharp transitions with no detectable current flowing through the polymer particles. The average ionic current through the LPS region decreases significantly with decreasing polyimine particle size and with extensive cycling, elucidating the Li-dendrite prevention mechanisms of this bulk hybrid SSE by polymer extension into ceramic interparticles.[3] The large ionic current fluctuation is understood in terms of highly anisotropic transport kinetic barriers along the different crystalline axes due to different grain orientations in the polycrystalline ceramic materials;[1][2] this is a new factor to be mitigated to eliminate Li dendrite formation and growth in the SSBs.
Regarding the polymer-coated LICGC SSE, we found significant ionic current fluctuation of 2‒3× the ion current but that is significantly less than in the LPS SSE. This mitigated current fluctuation is likely due to the more homogeneous ion-transport barriers of the glassy material with smaller grains in its amorphous matrix. The electronic leaking current is slightly larger than LPS, four orders of magnitude smaller than the ionic current. The polymer coating reduced the ionic current by several times compared with the bare LICGC SSE without the PAN coating. The PAN coating clearly prevented SSE degradation due to cycling.[4] Without the coating, the SSE after extensive cycling degraded extensively and nonuniformly, with domain sizes of μm to hundreds of μm. The degradation can be ionic blocking with slight electronic leaking as well as highly electronic shunting, depending on different domains or areas. The polymer coating effectively prevented both ionic and electronic degradation. In fact, ionic conduction improved by about 5 times, but electronic leaking was degraded slightly.
Our AFM-based imaging opens up novel characterization of ionic and electronic transport in SSEs. The nm-scale inhomogeneous transport mechanisms as revealed by this operando imaging are far beyond the averaged results provided by macroscopic characterizations.
References
[1] N.D. Lepley, N.A.W. Holzwarth, J. Electrochem. Soc. 159, A538 (2012).
[2] Y. Yang, Q. Wu, Y. Cui, Y. Chen, S. Shi, R.-Z. Wang, H. Yan, ACS Appl. Mater. Interfaces 8, 25229 (2016).
[3] J.M. Whiteley, S. Hafner, C. Zhu, W. Zhang, S.-H. Lee, J. Electrochem. Soc. 164, A2962 (2017).
[4] Y. Yin, C.-S. Jiang, H. Guthrey, C. Xiao, N. Seitzman, C. Ban, and M.M. Al-Jassim, J. Electrochem. Soc. 167, 020519 (2020).
Figure 1