Nano Silicon (SiNP) Based Carbon Composite: Flexible Anode System in Lithium Ion Batteries

Abstract

Silicon based anode systems has attracted considerable attention to replace graphite as a high energy density anode material for lithium ion batteries due to the high theoretical capacity of silicon (~4200 mAh/g) however, with colossal volumetric expansion . Over the past few years different approaches have been developed to address and counter the issue of high volumetric expansion (400%) leading to the mechanical degradation of silicon anodes realized during the formation of various phases of Li-Si system during the lithiation process. These approaches involve the use of silicon nanoparticles [1], composite structures of amorphous silicon [2], active-inactive matrix [3], electrodeposited thin films and VACNT-Si heterostructures [4-6]. Amongst these, silicon nanoparticles and nanotubes have shown to withstand the large internal stresses and strain mismatch generated during the expansion process and hence, serve as stable architecture to be exploited as anode in Li/Li+ battery. Silicon based systems as flexible batteries have not been explored well and the mechanical stability of flexible electrodes is expected to be poor due to the localized stresses developed due to the high volume expansion of silicon. The loading density of the active material in flexible electrodes is also restricted due to the limitations in the thickness of the flexible electrode caused by delamination at the material - substrate interface. In the present work novel nanoparticle Si (SiNP)-polymer derived carbon matrix based composite structures have been developed using a solution coating method, without the use of conductive carbon and binders. These flexible composite electrodes obtained after appropriate heat treatment were assembled in a 2025 coin cell and tested in a half cell configuration against lithium foil between the voltage range 0.01V-1V vs. Li+/Li in 1M LiPF6 (dissolved in EC:DEC:FEC=45:45:10) electrolyte. The flexible electrodes showed a first cycle discharge capacity of ~1745 mAh/g (weight of total composite electrode) and a charge capacity of ~1120 mAh/g at a current rate of 50mA/g with a first cycle irreversible loss of ~33% (Fig 1) during the initial testing with respect to Li+/Li system. The loading density of the electrode (SiNP-C) used in the coin cell was 5.5 mg/cm2 with approximately 50% by weight of SiNP in the flexible composite system, which translates to an areal capacity of ~6.05 mAh/cm2 at the end of 5th cycle (current rate = 50mA/g). The first cycle capacity vs voltage plot (Fig 2) shows a plateau at ~0.2V corresponding to the reaction of crystalline silicon with lithium to form LixSi alloy. The first cycle irreversible loss (~33%) is due to formation of SEI layer and the reaction of lithium with undecomposed functional groups present in the polymer derived carbon (0.3V-0.45V in Fig 2). SEM analysis is conducted on the electrodes before and after electrochemical testing to confirm the stability of the flexible composite nanostructures on long term cycling. SEM analysis is also conducted on the electrode in lithiated state to study the effect of volume expansion of SiNPon the mechanical integrity of these flexible electrodes. Results of these studies will be presented and discussed. Acknowledgement: The authors gratefully acknowledge financial support of the DOE-BATT (DE-AC02-05CHl1231) and NSF (CBET-0933141) programs. The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for support of this research.