Influence of Stabilized Lithium Metal Powder Loadings on Negative Electrode to Cycle Life of Advanced Lithium-Ion Capacitors

The cycling performance of Li-ion capacitors (LICs) with various stabilized lithium metal powder (SLMP) loadings on hard carbon (HC) negative electrode was studied and compared. LICs were made with activated carbon (AC) positive electrode and HC/SLMP negative electrode. It was found that the optimal mass ratio of HC to SLMP should be from 5:1 to 8:1 for the LICs with 4.1 V as the maximum voltage to achieve stable cycle life. X-ray photoelectron spectroscopy (XPS) results showed that lithium fluoride (LiF) was observed on the surface of the HC electrode after cycling test. The generation of LiF was considered to be due to a reaction of hydrofluoric acid (HF) with the extra SLMP, lithium alkyl carbonate (semicarbonate, ROCOOLi) or lithium carbonate (Li2CO3) on the surface of the HC electrodes. The extra Li powders protect the solid electrolyte interphase (SEI) layers from reacting with HF so that the surface of the HC electrode is not continuously exposed to electrolyte directly. It was thus confirmed that enough SLMP loadings on HC negative electrode can realize 4.1 V LICs with excellent cycling performance.