Feasibility of Chemically Modified Cellulose Nanofiber Based Membrane as a Lithium Ion Battery Separator
Authors: Kim Hyeyun , Guccini Valentina , Salazar-Álvarez Germán , Lindbergh Göran , Cornell Ann 
Published: February 2020

One key concern in the lithium based energy storage device industry is its large environmental impacts, especially in terms of high energy input and components materials resulting in environmental pollutions during manufacturing and waste treatment. To overcome that of the conventional compartments of lithium ion batteries, there have been numerous attempts to replace their materials to such with lower environmental impact. Cellulose has been highlighted as a candidate because of its naturally abundance, low cost, bio-degradability. Besides, its thermal and chemical stability make it even more attractive as lithium ion battery materials.
Cellulose fibrillated to individual nano-sized fibrils has been suggested as a useful material, which can also form nano-structures with high mechanical strength. Particularly, chemically modified cellulose has functionalized groups which can contribute to its excellent dispersibility in aqueous solutions and solvents. Therefore, chemically modified cellulose materials have advantages to be easily fabricated to light weight porous structures favorable for energy storage applications.
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) mediated oxidized cellulose (TOC) has sodium carboxylated groups converted from the primary hydroxyls at the C6 position. The functional groups give repulsion between fibrils, and facilitate dispersion of fibers. By consecutive mechanical disintegration, the pulp can be fabricated to nano-structured materials with a relatively low amount of mechanical energy input. Furthermore, negatively charged surface groups of the separators contribute to the transport of lithium ions, since a polyanionic surface can improve the transport rate of lithium cations. Furthermore, it is produced in large quantities in Nippon Paper Industries, Japan.
Despite of its promising properties, its application for lithium ion batteries has been limited to a small quantity, such as the binders, coating materials, and the mechanical supports of gel-polymer electrolyte.
We have found that TEMPO-oxidized cellulose based separator can be used for Li-ion batteries. From a water-based paper making process, fibrous cellulose treated by low mechanical/chemical energy input was fabricated to meso-porous structured membranes by simple filtration. The manufacturing process includes filtration and sequential solvent exchange with organic solvents to obtain desired structures of separators in terms of thickness, porosity, wettability of electrolyte and ionic conductivity. A battery cell composed of lithium Ni1/3Mn1/3CO1/3O2 (NMC) /Graphite electrodes and such separators showed excellent electrochemical performance.
TEMPO-oxidized celluloses with different amounts of surface charge densities are produced and fabricated to be heat-resistant lithium ion battery separators. TEMPO-oxidized nanofibers with higher charge densities can facilitate the dispersion of cellulose and it contributes to an increase of mechanical strength. However, when fabricated to be a separator and applied for a battery cell, it showed unexpectedly poor electrochemical performance.
We could assume that water remained in the cellulose structure limits the feasibility of high loading of wood based nanocellulose in the lithium ion batteries. Two types of water in TOC of concern are bound water adsorbed to the counter ions, and water in the cellulose amorphous structure which is difficult to remove via normal drying processes. Negatively charged cellulose can interact with higher amounts of water molecules on the cellulose structure while it goes through the water-based manufacturing process. Strong hydrogen bonds between water and cellulose stabilize the cellulose structures. Meanwhile, water can penetrate into amorphous regions of cellulose, whereas it cannot attack the crystalline regions. Therefore, it is inevitable to completely get rid of water from the wood based nanocellulose with low crystallinity and high surface area, prepared by aqueous based process, via normal drying process. Furthermore, since water remained can accelerate unwanted detrimental side reactions, resulting in the degradation of electrolytes and electrodes, it requires a delicate drying process.
NMP (N-Methyl-2-pyrrolidone) was employed as the drying solvent for TOC nanofibers. The membrane immersed in NMP solvent and dried at 110 °C under vacuum showed enhanced electrochemical performance. NMP is an organic solvent widely used in industry because of its dispersibility. It is known as one of the cellulose-dissolving solvents, which can penetrate the cellulose structure, and interrupt the hydrogen bonding.
We investigate the relationship between the electrochemical stability and the characteristics of separator materials. The results will give an overview for our future studies of the applications of cellulose materials produced by different chemical modification processes.

Figure 1