Analysis of solid electrolyte interface formation reaction and surface deposit of natural graphite negative electrode employing polyacrylic acid as a binder

Highlights

• The XPS spectra of C1s region for NG-3 electrode after discharging up to 2.0 V is shown.

• The NG-3 electrodes using PAA and PVdF as the binders, respectively are shown.

• The amount of the inorganic components of the SEI was low in the case of the PAA binder.

• The binder types have an influence on the SEI composition.

Abstract

We analyzed the solid electrolyte interface (SEI) formation reaction and surface deposit of a natural graphite (NG-3) electrode employing polyacrylic acid (PAA) as a binder in an ethylene carbonate-based electrolyte because it was reported that the initial charge–discharge characteristics of the NG-3 electrode were improved by employing the PAA binder. Poly(vinylidene fluoride) as a binder was used for comparison. We investigated the influence of the binder types on the coating of the NG-3 particles using the B.E.T. specific surface areas. The difference in the above phenomenon was explained by the relationship between the B.E.T. specific surface area and the irreversible capacity. The surface chemical composition of the NG-3 electrode was investigated by FE-SEM/EDX and XPS and then the difference between the PAA binder and the PVdF binder was discussed. The FE-SEM/EDX and the XPS results showed that the amount of the inorganic components of the SEI was relatively small in the case of the PAA binder NG-3 electrode. The AC impedance results showed that the SEI film resistance of the PAA binder NG-3 electrode was lower at 0.2 V. It was clarified that the binder types affected the coating state, the SEI formation reaction, and the SEI film composition.

Introduction

Various improvements in the electrochemical characteristics of the graphite electrode have been reported. As an example, the initial irreversible capacity of the graphite electrode in an ethylene carbonate (EC)-based electrolyte was reduced by employing gelatin as a binder [1], [2]. In addition, the same research group reported that the initial irreversible capacity of the graphite electrode in the EC-based electrolyte was reduced by employing a water-soluble polymer of sodium carboxymethyl cellulose (CMC-Na) as a binder [3]. In recent years, it has been reported that the contact area between the LiCoO₂ particles and graphite particles increased because both the LiCoO₂ and graphite powders were effectively dispersed by employing ammonium polyacrylic acid (PAA–NH₄) as a dispersant, resulting in the improved charge–discharge characteristics of the LiCoO₂ electrode [4].

Based on these reports, we have focused on a water-soluble polymer of polyacrylic acid (PAA) as the binder and covering agent. Our group has reported that the graphite particle coated with PAA enabled the reversible charge (Li⁺ ion intercalation) and discharge (deintercalation) reaction in a propylene carbonate (PC)-based electrolyte; it could effectively improve the charge–discharge characteristics in an EC-based electrolyte [5]. The PAA as a binder for the natural graphite negative electrode [6], for the Si alloy negative electrode [7], and LiFePO₄ positive electrode [8] was then reported for use in a lithium-ion battery. However, the detailed mechanism, which PAA exerts on the improvement of the charge–discharge characteristics, has not yet been clarified.

In this study, we focused on the coating state of the graphite particles coated with PAA, the surface deposit of the NG-3 electrode, and the solid electrolyte interface (SEI) formation reaction during the initial charging because it was considered that they would have an influence on the initial charge–discharge characteristics. Considering the further design of a novel binder, we analyzed of the SEI formation reaction and the chemical composition of the surface deposit to obtain primary knowledge about the natural graphite electrode coated with PAA. We have discussed the influence of the difference in the binder types on the SEI formation reaction and the chemical composition of the surface deposit by mainly analyzing the change in the natural graphite electrode surface during the initial charge–discharge cycling.