Acrylonitrile-grafted poly(vinyl alcohol) copolymer as effective binder for high-voltage spinel positive electrode

Highlights

• PAN-grafted PVA copolymer for the binder of LiNi_1/2Mn_3/2O₄.

• Highly improved electrode performance of LiNi_1/2Mn_3/2O₄ at elevated temperatures.

• The branched structure leading to good coatability with high adhesive strength.

Abstract

Acrylonitrile-grafted poly(vinyl alcohol) copolymer with a branched structure is synthesized and used as binder for a LiNi_1/2Mn_3/2O₄ composite electrode. Electrode performances of composite electrodes with different binders are compared in Li cells at 50 °C. The branched copolymer has better coatability to active materials in comparison to a simple mixture of linear polymers and conventional PVdF as evidenced by hard X-ray photoelectron spectroscopy. Cyclability is effectively improved by using the branched copolymer at elevated temperatures because of high chemical stability of the coated polymer layer and formation of a protective layer on cycles. Moreover, excellent rate-capability is realized by applying the branched copolymer with high adhesive strength, and the composite electrode delivers 70 mAh g⁻¹ of discharge capacity at a rate of 1280 mA g⁻¹.

Introduction

The development of convenient portable electronic devices and electric vehicles has contributed to innovate modern society through the technological progress of state-of-the-art rechargeable lithium batteries as power sources. Nevertheless, there is an ever-increasing demand on energy density of lithium batteries. Rechargeable lithium batteries consist of two different lithium insertion materials; positive and negative electrodes with electrolyte solution, in which a lithium salt is dissolved in aprotic solvent. Since lithium containing oxides for positive electrode materials are supplied as powder forms, polymer binders are used to make sheets of composite electrodes coated on aluminum current collectors. Recently, the polymer binders are being key materials to increase energy density and to improve cyclability of lithium batteries [1], [2], [3], [4], [5], [6]. For positive electrodes, historically, poly(vinylidene fluoride) (PVdF) was the most widely used binder because of superior chemical and electrochemical stability for fluorine-containing polymers. However, adhesive strength of PVdF is not high enough compared with other polymer binders, e.g., sodium carboxymethyl cellulose (NaCMC) [7] and polyacrylates [8] known as water-soluble binders. Moreover, PVdF is easily defluorinated [9] when alkali residues are found in active materials, leading to the gelation of slurry.

In this study, polyacrylonitrile (PAN) possessing oxidation resistant properties is targeted as a base polymer, and is co-polymerized by graft polymerization with poly(vinyl alcohol) (PVA) possessing high adhesive strength through strong hydrogen bonds of hydroxyl functional groups [10]. Both polymers are known to be used as binder for positive electrode materials [11]. A schematic illustration of the new polymer is shown in Fig. 1.

The PAN-grafted PVA (PAN-g PVA) is a branched copolymer, consisting of a straight chain of PVA (40 mol%) with side chains of PAN (60 mol%). The branched structure is beneficial to form slurry with a uniform composition because high physical/chemical interaction with active materials and conductive carbon materials is anticipated [12]. Viscosity of polymer solutions dissolved in NMP is shown in Fig. 2. The viscosity of the PAN-g PVA solution is much higher than those of homopolymers (PVA and PAN) and its simple mixture, indicating that the formation of the branched structure is likely to succeed. Therefore, good dispersion of active materials/conductive carbons is anticipated for PAN-g PVA. Indeed, viscosity of the slurry with PAN-g PVA is much lower than that of PVdF when the rate of shear stress is slow. Infrared spectra of PAN-g PVA shows that hydroxyl groups remain in the polymer (Supporting Fig. S1), and thus high adhesive strength is also expected.

As a positive electrode active material, a high-voltage spinel oxide, LiNi_1/2Mn_3/2O₄, is selected to test electrochemical stability of the PAN-grafted PVA copolymer against exposure to high voltage. Electrode performance of LiNi_1/2Mn_3/2O₄ is easily deteriorated on electrochemical cycles because of high operating voltage of the electrode material, and deterioration is accelerated at elevated temperatures [13]. Therefore, electrochemical properties of LiNi_1/2Mn_3/2O₄ composite electrodes prepared with the PAN-grafted PVA copolymer are examined at elevated temperatures and compared with those of PVdF used as binder. From these results, possibility of the branched copolymer as a non-fluorine binder is discussed for advanced rechargeable lithium batteries.