Improvement of High Voltage Cyclic Performance By Novel Binder for High Energy Lithium Ion Battery Application

Introduction

Lithium-ion batteries (LIB) have been used as high density storage devices for several kinds of applications. Higher energy density and higher durability are required for use of high-performance energy devices. Among various attempts to improve energy density of LIB, technologies which increase battery voltage have emerged as a viable solution.

Binder affects electrochemical performance reaction directly because it exists near the active materials. In a previous report, Co-ion dissolution and Co₃O₄ formation were observed in the electrode with PVdF (Polyvinylidene fluoride) [1].

In this study, we clarified the degradation mechanism of cathode electrode that contain PVdF binder.

Experimental

The cathode electrodes consisted of LiCoO₂, acetylene black, and PVdF. The performance tests were investigated in 40mAh laminate type lithium ion cells. The cycle life test was performed at 4.2V or 4.4V. Solid F-NMR of electrode was used for the analysis of binder degradation after cycle test.

Thin-film electrode of LiCoO₂ was fabricated on Au substrate by rf-sputtering method [2].  X-ray diffraction pattern indicated that high crystalline LiCoO₂ thin-film electrode was prepared.  Binders were spin-coated on the thin-film electrode. Three-electrode cell was used for electrochemical measurements of LiCoO₂ thin-film electrode.  The electrochemical properties of LiCoO₂ thin films were investigated by cyclic voltammetry (CV)

 

Results

Figure 1 shows cycle performance at each cut-off voltage.  Cycle performance was dramatically decreased at high cut-off voltage.  Figure 2 shows the cyclic voltammograms of LiCoO₂ thin-film electrode coated with PVdF in LiClO₄/EC/DEC at 60°C. Around 3.9 V, peaks of lithium-ion de-intercalation /intercalation were observed. With cycling, redox currentsgradually decreased and redox voltage shifted. This result suggested the degradation of LiCoO₂.

We evaluated the structure change of LiCoO₂ after cycle test by TEM spectroscopy. From the TEM spectroscopy, degradation of surface structure was observed at the electrode after cycle test. From the F-NMR, structure of PVdF was slightly changed after the cycle test.  It is suggested that PVdF increased the reactivity of active material.  From ICP measurements, the amount of metal precipitation on anode was also increased after cycle test. And adhesion strengths wassignificantly decreased after cycle test.

In our presentation, we will show a new type of water-based binder developed by Zeon which can contribute to improvement of cell performance, such as improved cycling performance.