Yulong Liu1 Qian Sun1 Yang Zhao1 Biqiong Wang1 Keegan Adair1 Yongfeng Hu2 Jinru Liu3 Rong Yang4 Li Zhang4 Shigang Lu4 Xiping Song3 Xueliang Sun1

1, The University of Western Ontario, London, Ontario, Canada
2, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
3, University of Science and Technology Beijing, Beijing, , China
4, China Automotive Battery Research Institute, Beijing, , China

Solid state batteries (SSBs) have been developed to achieve higher energy density and safety by using inflammable solid electrolyte and lithium anode. Nevertheless, the spread of SSBs are impeded by the interface challenges of physical mismatch, chemical reaction, and space charge effect. It is therefore necessary to engineer an interface to reduce the side reactions.[1]
LATP (Li1.3Al0.3Ti1.7 (PO4)3) solid electrolyte is widely investigated for its high ionic conductivity. However, the chemical instability of LATP against Li metal has hindered its application in solid-state batteries because of the side reaction between LATP with Li. The Ti4+ in LATP is easily reduced by Li metal into Ti3+, forming some interphases at the LATP/Li interface.[2] In a recent study done by Janek et al., a mixed (ionic/electronic) conducting interphase (MCI) was observed at the LAT (Ge)P/Li interface, which functioned similarly to the solid electrolyte interphase (SEI) layer formed in batteries with liquid electrolytes.[3]
To improve the stability of LATP against Li metal, intermediate layers such as polymer electrolytes can be utilized at the LATP/Li interface. The side-reactions can be partially mitigated by the chemical stability of the polymer interlayer, however, this introduces additional interfaces (LATP/polymer, Li/polymer) which may have a negative effect on cell performance.[4] Recently, Hu et al. show that introducing an ultrathin Al2O3 via ALD on garnet electrolyte (Li7La3Zr2O12) can dramatically increase the wetting and stability against Li metal after forming a Li-Al-O intermediate layer.[5] It is therefore assumed that ALD coating on LATP can be an effective method in stabilizing the LATP/Li interface. In order to understand the influence of interlayer ionic conductivity on the stability of LATP/Li interface, both Li-ion conducting Li3PO4 and non-conducting Al2O3 interlayers are studied in our design.
Herein, we applied atomic layer deposition coating on LATP surfaces to stabilize the LATP/Li interface by reducing the side-reactions based on our ALD experience [6,7]. In comparison with bare LATP, the Al2O3 coated LATP by atomic layer deposition exhibits a stable cycling behavior with smaller voltage hysteresis for 600 hours, as well as small resistance. More importantly, based on our advanced characterization by HRTEM-EELS, the lithium penetration into LATP bulk and Ti4+ reduction is significantly limited. The results suggest that atomic layer deposition is very effective in improving interface stability of solid-state electrolyte/ electrode.
[1] Sun, C. W, et al. Nano Energy 2017, 33, 363.
[2] Zhu, Y, et al. ACS Appl Mater Interfaces 2015, 7, 23685.
[3] Hartmann, P, et al. The Journal of Physical Chemistry C 2013, 117, 21064.
[4] Borghini, M. C, et al. Journal of Power Sources 1997, 68, 52.
[5] Han, X, et al. Nat Mater 2017, 16, 572
[6] J. Liu et al. Nanotechnology 2015,26,024001
[7] X. Meng, et al. Adv. Mater. 2012, 24,3589.