The Effect of Amorphous Silicon Anode Thickness on the Cycling Behavior of High Energy Density Solid State Li-Ion Batteries

Abstract

Novel electronic devices related to the internet of things (IoTs) are reshaping our world in several ways. The emergence of these devices is creating an increasing demand for disruptive innovations, among which high performance on-chip integrated energy storage solutions are the most frequently cited. In this regard, thin film batteries (TFBs) are recognized as one of the most propitious electrochemical devices to meet applications’ demands, considering attractive advantages such as excellent cycle life and miniaturization.1,2 Nevertheless, the commercial widespread application of TFBs and the demonstration of effective on-chip integration have been hindered by (i) a low areal energy density and (ii) encapsulation issues related to metallic lithium anode integration. Recently, great efforts have been made to study the interrelationship between materials, architectures, electrochemical performance and integration; the prevailing tendency has been towards Li-free TFBs, where Li anode is formed in-situ during charging. Despite achieving promising electrochemical performance at the full TFB device, significant capacity decay with cycling is usually reported, thus limiting further applications. In the present work, we report on the development of all solid inorganic thin film lithium ion batteries (TFBs) with an amorphous silicon anode solution. The TFBs were realized on 8’’ substrate with a structure integrating a LiCoO2/LiPON/Si active stack (Figure 1.a, inset). The fabrication process flow was carried out in a clean room environment using the TINY platform. The LiCoO2 thickness was kept constant at 20µm whereas several submicron thick silicon anodes were considered (0, 10, 50, 100, 200, 350, and 1000 nm). It was observed no effect of Si anode thickness on the first cycle charge and discharge capacities. Upon cycling, TFBs with the thinnest Si anodes exhibited excellent capacity retention, with an initial discharge capacity of 680µAh.cm-2 and an average loss of 0.1% per cycle. When increasing Si thickness, an abrupt capacity decay was observed at the early stages of cycling, and was correlated to the anode thickness (Fig 1.a). This behavior was correlated to the electrochemical reactions occurring at the anode level: (i) during the first charge, Li ion exchange between LiCoO2 and Si took place, resulting from LiCoO2 delithiation into Li1−xCoO2 and Li−Si alloying. At higher potentials, Li plating occurred at the negative electrode side, and the potential thereby corresponded to that of LiCoO2 vs Li. (ii) during discharge, Li stripping took place at the anode, concomitant with Li ion insertion into Li1−xCoO2. Upon further discharging, Li ion extraction from Li−Si alloys occurred (Fig. 1-b, c). Accordingly, it is concluded that thinner Si films acted as a uniform and homogeneous seed layer to the initial Li plating reaction, thus enhanced the subsequent cycling stability in comparison to Li-free configuration, and thicker Si films lead to severe mechanical delamination within the active stack, and induced TFB capacity decrease (Fig.1-bc). Furthermore, cycling within specific voltage ranges allowed stabilizing the discharge capacity for thicker Si anodes. These results were validated by ToF-SIMS and FIB-SEM characterization. References (1) Lee, H. S.; Kim, S.; Kim, K. B.; Choi, J. W. Scalable Fabrication of Flexible Thin-Film Batteries for Smart Lens Applications. Nano Energy 2018, 53 (August), 225–231. (2) Liu, L.; Weng, Q.; Lu, X.; Sun, X.; Zhang, L.; Schmidt, O. G. Advances on Microsized On-Chip Lithium-Ion Batteries. Small 2017, 13 (45), 1–12. Figure 1