Capacity fading mechanism of Li2O loaded NiFe2O4/SiO2 aerogel anode for lithium-ion battery: Ex-situ XPS analysis

Abstract

The aerogel form of 0–5 wt% of Li2O loaded NiFe2O4/SiO2 composite aerogel were successfully synthesized using the sol-gel process followed by the supercritical ethanol drying. The addition of small Li2O wt.% enhances the electrical conductivity of NiFe2O4/SiO2 composite aerogels. Based on the AC impedance analysis, the high conducting 5% Li2O + 95% [NiFe2O4/SiO2] composite aerogel with conductivity 2.1 × 10−11 S cm−1 was taken for the detailed analysis. We observed the maximum initial reversible discharge capacity of 930 mA h g−1, and it gradually fades to 370 mA h g−1 at 50th discharge. To elucidate the capacity-fading mechanism, the ex-situ X-ray Photoelectron Spectroscopy (XPS) technique is used to identify the chemical composition at different states of charge-discharge cycling. The capacity fading arises mainly from irreversible LixSiOy accumulation, which causes the loss of active lithium. Meanwhile, the metal to metal oxide conversion is partially reversible between Ni0↔ Ni2+, Fe0↔Fe3+ during the subsequent cycles. To further support, the SEM images were taken for the respective electrodes, and the cell resistance and lithium diffusion coefficient (DLi+) was calculated from the electrochemical impedance spectroscopy (EIS) at the respective samples taken for ex-situ XPS analysis. The DLi+ gradually decreases from 1.8 × 10−13 cm2 s−1 to 9.6 × 10−14 cm2 s−1, and the cell resistance increases during the 1st to 10th discharge cycle. We conclude that the prolonged accumulation of irreversible LixSiOy phase will hinder the DLi+ in subsequent cycles, which leads to capacity-fading. The detailed results and electrochemical performance will be discussed.