Electrochemically active membranes increase lithium ion battery capacity

【introduction】

The capacity of commercial lithium-ion batteries is limited by the theoretical capacity of the cathode material. At present, researchers mainly improve the charge and discharge capacity of batteries by developing new cathode materials and improved electrode engineering techniques. Few studies have focused on improving the capacity of batteries by improving the parts other than the electrode materials in the batteries. Diaphragm is an important part of lithium-ion batteries, which directly affects the life, safety, energy density and power density of lithium-ion batteries. Commercial lithium-ion battery separators are typically made from polyolefins that have poor electrolyte wetting and poor thermal stability.

Renewable, low-cost cellulose separators have the advantages of good hydrophilicity, adjustable structure and high flexibility, high thermal stability and simple production process. Nowadays, research work on cellulose-based separators has focused on the development of safety membranes with good electrolyte wettability, and there has been little research on cellulose-based separators that can simultaneously improve chemical capacity while improving battery capacity. It should be noted that the conventional commercial membrane and its modified membrane account for about 15-20% of the volume in the single cell. In theory, the volume of other active materials can be increased by reducing the thickness of the separator to increase the battery capacity. . However, there has been no reliable method to prepare ultra-thin membranes while maintaining the necessary important characteristics of the battery separator.

[Introduction]

Recently, senior researcher Wang Zhaohui of Uppsala University and Leif Nyholm (co-author) and others published a research titled "Redox-Active Separators for Lithium-Ion Batteries" on Advanced Science. In this work, the research team prepared a flexible mesoporous redox active membrane composed of nanocellulose fibers (NCFs) and polypyrrole (PPy) composites through a simple papermaking process. The redox active separator has a two-layer structure in which one side is an insulating NCF layer of about 3 μm thick, and the other side is composed of a redox-active PPy/NCF composite layer of adjustable thickness. Among them, the NCF layer acts as the main insulation between the electrodes, and the redox-active PPy/NCF composite layer provides mechanical support for the NCF layer while providing additional capacity for the lithium ion battery. The research team found that flexible redox-active membranes have significant advantages over commercial polyethylene separators (PE) in terms of thermal stability and electrolyte wettability; redox-active membranes are used in the proof-of-concept battery cycle. No short circuit was observed, and there was a significant increase in the capacity of the battery due to the presence of the PPy-containing layer. In the concept battery, when LiFePO4 (LFP) is the positive electrode, the lithium ion battery using the redox active diaphragm can exhibit a capacity of 67 μAh cm-3/81 mA hg-1, which is obtained by a lithium ion battery using a conventional separator. Higher capacity (based on the total volume/weight of the separator and the positive electrode). This suggests that the use of a redox-active membrane to replace the membrane provides a new way to increase the capacity of conventional lithium-ion batteries.

[Graphic introduction]

Figure 1 a) Traditional diaphragm, b) Side view of the redox active diaphragm

Note: Light green area: Insulation material; Light gray area: Redox active component

Design Idea: By combining a thin insulating layer with a porous support layer composed of a conductive redox material, a flexible redox active diaphragm having a thickness similar to that of a conventional separator can be obtained, which not only ensures safe operation of the battery, but also ensures safe operation of the battery. Can increase the capacity of the battery.

Figure 2 Preparation and morphological characteristics of redox active separator

a) schematic diagram of the preparation process of the redox active membrane;

b) a photograph of a flexible redox active membrane;

c) an SEM image of the NCF layer;

d) an SEM image containing the PPy layer;

e) SEM image of a torn redox active membrane.

Figure 3. Pore structure of different membranes

NCF based membrane, redox active membrane, PPy@NCFs composite membrane: a) pore size distribution; b) cumulative pore volume.

Figure 4 Thermal stability and electrolyte wettability test

a) Thermal stability test of PE separator (top) and redox active diaphragm (bottom) under elevated temperature conditions (left: before heat treatment; right: after heat treatment);

b) Electrolyte wettability test of PE separator and redox active separator (left: before dropping electrolyte; right: after dropping electrolyte).

Figure 5 shows the electrochemical performance of a battery composed of LiFePO4 as the positive electrode and Li as the negative electrode and different separators.

a) a charge/discharge curve at a rate of 0.2 C;

b) cyclic voltammetry curve at a scan rate of 0.2 mV s-1;

c) rate performance;

d) Cyclic stability of Cell I.

Notes: 1. The thickness of the redox active diaphragm, NCF diaphragm, PE diaphragm and GF diaphragm are 10, 10, 25 and 255 μm, respectively; 2. Cell I: LFP is the positive electrode, Li is the negative electrode, and the redox active diaphragm contains PPy. The layer is in contact with the LFP positive electrode; 3. GF diaphragm: glass fiber diaphragm

Figure 6. Capacity increase mechanism and weight capacity/volume capacity comparison of different diaphragms

a) schematic diagram of an LFP/Li battery containing a redox active separator (the NCFs of the separator are in direct contact with the Li negative electrode);

b) Comparison of weight/volume with different LFP/Li battery weight capacity/volume capacity for different separators and positive electrodes.

Figure 7. Comparison of weight capacity of PE as separator (LFP-PPy)/Li battery and Cell I battery containing redox active diaphragm

【summary】

This work proposes a design method for obtaining a double-layer cellulose-based separator by introducing a porous redox active layer, which can be used to improve the electrochemical performance of a lithium ion battery. Since the redox-active membrane can provide additional capacity, when a redox-active membrane is used in place of a conventional commercial membrane, the capacity of a lithium-ion battery with LiFePO4 as a positive electrode and Li as a negative electrode will increase from 0.16 mA h to 0. 276 mA h. The authors point out that subsequent work can increase more capacity by improving the thickness of the electroactive layer and the composition of the electroactive material. Increasing the capacity of its electrochemical energy storage system through redox active membranes provides a new way to develop high energy density thin film lithium ion batteries and other electronic products.