Li Concentration Change Around Cu/Lipon Interface Measured By Time of Flight Elastic Recoil Detection Analysis
Authors: Kurihara Kyoshi , Nakamizo Shuri , Yamamoto Satoshi , Yasuda Keisuke , Majima Takuya , Yajima Takeshi , Iriyama Yasutoshi 
Published: December 2024

All-solid-state rechargeable lithium (Li) batteries (ASSBs) utilizing inorganic lithium-ion (Li+) conductive solid electrolytes (SEs) are gathering significant interest as a next-generation battery. To enhance the energy density of ASSBs, employing a Li metal anode is crucial due to its substantially higher capacity (3860 mAh g-1) compared to conventional graphite anodes (372 mAh g-1). Among various SEs compatible with Li metal anodes, lithium phosphorus oxynitride glass electrolyte (LiPON) stands out as an especially promising material for stabilizing the Li/SE interface [1]. This is evidenced by the fact that thin-film-type ASSBs employing LiPON and Li metal anodes demonstrate stable charge-discharge cycles over ten thousand cycles without capacity degradation [2].
Regarding the Li/LiPON interface in thin-film ASSBs, it is observed that Li plating-stripping reactions occur stably. Yet, advanced analyses indicate the formation of a reaction layer at this interface. Cheng et al. utilized Cryo-STEM analysis to investigate the Li/LiPON interface, revealing the generation of by-products like Li2O, Li3N, and Li3P within an 80 nm thickness range from the interface [3]. Similarly, Sicolo et al. conducted XPS measurements on the LiPON surface before and after Li deposition, confirming the formation of a reaction layer immediately following Li layer deposition [4]. Theoretical calculations have suggested that LiPON reacts with Li metal and the thermodynamical stability of LiPON is estimated to be 0.68 V vs. Li/Li+ [5]. On the other hand, Schwietert et al. have reported that LiPON does not undergo reductive decomposition less than 0.68 V by taking a metastable state because an activation barrier for the decomposition can retard reductive decomposition [6]. Clarifying this interphase phenomenon is essential for the development of ASSBs with Li metal anodes.
In this study, focusing on the increase in Li content with the reductive decomposition of LiPON, we investigate Li concentration changes around the Cu/LiPON interface by time-of-flight elastic recoil detection analysis (TOF-ERDA) [7]. TOF-ERDA needs no sample pretreatment for the detection of Li and then it can effectively observe Li concentration changes induced by electrochemical reactions around the interface. We confirmed that Li concentration increased around the interface within 30 nm thickness when maintained at 0.006 V vs. Li/Li+ before the Li plating voltage at 0 V vs. Li/Li+. This means that a decomposed region of LiPON forms around the interface within 30 nm thickness in this situation. Electrochemical analysis suggests one of the decomposed materials to be Li3P [4]. These findings basically align with TEM-EELS data, underscoring the utility of TOF-ERDA in analyzing Li concentration changes around interfaces.
Reference

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Acknowledgement
This work was supported by JSPS KAKENHI “Interface IONICS” (JP19H05813, 20H05293, and 22H04617) and by JST GteX, JPMJGX23S2.