In Situ SEM Study on Electrochemical Lithiation/Delithiation Behavior of Silicon Anodes with Polyimide Binder

Abstract

Lithium-ion batteries (LIBs) currently outperform other competitive batteries, for instance, lead-acid and nickel–metal hydride, but still need extensive improvement to meet the requirements for automotive and large-scale energy storage applications. Silicon is one of the most promising anode materials owing to the high theoretical capacity (3579 mAh g– 1 vs. 372 mAh g–1 (conventional graphite)). However, the dramatic volume change (~300 %) experienced by Si upon alloying/dealloying with Li induces severe pulverization and subsequent loss of electrical connectivity. It inevitably results in fatal capacity decay and hampers practical applications of the Si anode. To address this issue, great research effort focused on the optimization of the electrode composition and structure. Ishikawa and co-workers have reported that micro-sized Si with a polyimide (PI) binder exhibits a stable cyclability over 300 cycles, while the one using other binder showed a rapid capacity decay.1,2 With the aim to clarify the role of binder, the charge-discharge behavior of the Si anode with a PI binder (Si-PI) was investigated by an in situ scanning electron microscope technique,3which was established by our group. Also we have designed a brand-new coin-cell type for easy access to the comparison of the data obtained by conventional electrochemical measurement and in situ SEM observation. To reproduce the practical battery testing condition during the in SEM observation, a new cell for the in situ observation was designed using a commonly-used CR2032 coin cell. The edge of coin cell was carefully cut to observe the cross-section of cell components. Silicon anode with a PI binder was prepared in accordance with previous reports.1,2 Electrochemical measurements were performed using a lithium cobalt oxide (LiCoO2) cathode, a polyolefin separator, a Si anode, and 1 mol L–1 Li[TFSA]–[C2mim][FSA] (Li[TFSA]: lithium bis(trifluoromethanesulfonyl)amide; [C2mim][FSA]: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide) as the electrolyte. The cell was assembled in an Ar-filled glovebox and then immediately transferred into a SEM chamber. Cut-off voltage range was −3.895 V to −2.500 V vs. LiCoO2. For the initial cycle, the cell was charged/discharged at constant current density of 0.2 A g– 1, while constant current constant voltage mode was performed to complete the lithiation. After the second cycle the cells were charged/discharged with a constant current density of 1.0 A g–1. As shown in Fig. 1a, Si-PI anode assembled in the tailored coin cell showed a comparable capacity to previous report1 when tested under vacuum condition, confirming the feasibility of our cell design. At the initial cycle, a big irreversible capacity was recognized, but the charge–discharge curves became to show a better reversible behavior after several cycles. The discharge capacity exceeded 1,000 mAh g– 1 at the 20th cycle. In this investigation, two different positions of Si anodes, namely, edge and center, were observed by SEM. Fig. 1b shows in situ SEM images of the Si anode at the edge. Before the charge, a Si anode, a Cu current collector, and a separator can be clearly distinguished. But after the charge, the morphology is dramatically varied by the known lithiation reaction. Obvious volume expansion of the whole anode was recognized while contraction during the discharge was less noticeable, suggesting that the small contraction is closely linked to the big irreversible capacity observed in the charge–discharge curve at the initial cycle. This behavior is very similar to our previous result on the Si anode with a polyvinylidene difluoride (PVDF) binder.3Afterwards, reversible expansion and shrinkage of the electrode was observed for several cycles, and then the charge–discharge curves showed a good reversibility. Interestingly the expansion of Si anodes was clearly suppressed as compared to the case of our previous research using a PVDF binder. The in situ SEM observation revealed that the mechanical properties of binder play a critical role in electrochemical properties of Si anodes. Acknowledgement Part of this research was supported by the Grant-in-Aid for Scientific Research, Grant Numbers 15H03591, 15K13287, and 15H2202 from the Japanese Ministry of Education, Culture, Sports, Science and Technology and by the ALCA-SPRING program, Japan Science and Technology Agency.