How to Increase Lithium-ion Battery Capacity

The capacity of commercial lithium-ion batteries is limited by the theoretical capacity of cathode materials. At present, researchers mainly use the development of new cathode materials and improved electrode engineering techniques to increase the battery's charging and discharging capacity. Few studies have focused on improving the battery's capacity by improving the part of the electrode material in the battery. The diaphragm is an important part of lithium-ion batteries, which directly affects the service life, safety, energy density and power density of lithium-ion batteries. Commercial lithium-ion battery diaphragm is usually prepared from polyolefin, and its electrolyte infiltration and thermal stability are poor.

Renewable, inexpensive cellulose diaphragm has the advantages of good hydrophilicity, adjustable structure and high flexibility, and simple production process of thermal stability Gaohe. Nowadays, the research on cellulose base diaphragm is mainly focused on the development of a safe diaphragm with good electrolytic infiltration. Few studies have been made on the preparation of cellulose base diaphragm that can increase battery capacity while chemically functional. It should be pointed out that the traditional commercial diaphragm and its modified diaphragm account for about 15-20 inches of volume in a single battery. Theoretically, the volume of other active materials can be increased accordingly by reducing the thickness of the diaphragm to increase the battery capacity. However, there is no reliable method to prepare ultra-thin diaphragm and maintain the necessary and important characteristics of battery diaphragm.

[Results Introduction]

Recently, Wangzhaohui, a senior researcher at the University of Uppsala in Sweden, and Leiff Nyholm(co-newsletter author) and others published a study entitled "Redox-ActiveSprematorium-IonBatteries" on Advency Science. In this work, the team prepared a flexible mesoporous Redox active diaphragm composed of nanocellulose fibers(NCFs) and polypyrrole(PPy) composites through a simple papermaking process. The Redox active diaphragm has a double layer structure, one of which is an insulating NCF layer about 3 μm thick, and the other side is composed of a PPy/NCF composite layer with Redox activity with adjustable thickness. Among them, the NCF layer acts as the main insulation between the electrodes. The Redox active PPy/NCF composite layer can provide mechanical support for the NCF layer and provide additional capacity for lithium-ion batteries. The team found that the flexible Redox diaphragm had significant advantages over commercial polyethylene diaphragm(PE) in terms of thermal stability and electrolyte wettability. The Redox active diaphragm did not observe the short-circuit during the conceptual verification of the battery cycle, and due to the presence of PPy layers, the battery capacity has significantly increased. In the concept battery, when LiFePO4(LFP) is a positive electrode, lithium ion batteries using Redox active diaphragm can exhibit a capacity of 67 μAhcm-3 / 81 mAhg-1. The capacity obtained is higher than that obtained by a lithium ion battery using a conventional diaphragm(based on the total volume/weight of the diaphragm and the positive electrode). This shows that using Redox active diaphragm can provide a new method to increase the capacity of traditional lithium-ion batteries by replacing the diaphragm.

[Graphic Guide]

Fig. 1(a) Traditional diaphragm, b) Side diagram of Redox-active diaphragm

Electrochemical active diaphragm increases lithium ion battery capacity

Note: Light green area: insulating material; Light grey area: Redox active component

Design idea: by combining the thin insulation layer with the porous support layer composed of conductive Redox materials, a flexible Redox-active diaphragm with a thickness similar to that of a conventional diaphragm can be obtained, not only to ensure the safe operation of the battery, Can also increase the capacity of the battery.

Fig. 2 Preparation and morphological characteristics of activated diaphragm of CO2 reduction

Electrochemical active diaphragm increases lithium ion battery capacity

A) A schematic diagram of the preparation process of the Redox active diaphragm;

B) Photographs of flexible Redox active diaphragm;

C) SEM diagram of NCF layer;

D) SEM diagram with PPy layer;

E) SEM diagram of torn Redox active diaphragm.

Fig. 3 Porous structures of different membranes

Electrochemical active diaphragm increases lithium ion battery capacity

NCF-based diaphragm, Redox active diaphragm, PPy@NCFs composite membrane: a) aperture distribution; (b) Accumulated bore volume.

Fig. 4 Test of thermal stability and wettability of electrolytes

Electrochemical active diaphragm increases lithium ion battery capacity

(a) Thermal stability test of PE diaphragm(figure above) and Redox active diaphragm(figure below) at elevated temperatures(left: before heat treatment; Figure on the right: after heat treatment);

B) Electrolytic hygroscopic test of PE diaphragm and Redox active diaphragm(left: before titration of electrolyte; Right: After the electrolyte is added).

Fig. 5 Electrochemical properties of cells composed of negative electrode and different diaphragm, with LiFePO4 as the positive electrode and Li as the negative electrode

Electrochemical active diaphragm increases lithium ion battery capacity

A) the charge/discharge curve at 02C rate;

B) a cyclic voltammetric curve with a scan rate of 0.2 M Vs-1;

(c) Multiplicity performance;

D) Cyclic stability of CellI.

Note: 1. The thickness of the Redox active diaphragm, NCF diaphragm, PE diaphragm and GF diaphragm is 10, 10, 25 and 255 μm, respectively; 2, CellI: LFP is a positive pole, Li is a negative pole, and the PPy layer of the Redox active diaphragm is in contact with the LFP positive pole; 3, GF diaphragm: glass fiber diaphragm

Figure VI Comparison of the mechanism of capacity increase and the weight/volume capacity of different diaphragm

Electrochemical active diaphragm increases lithium ion battery capacity

A) A schematic diagram of the LFP/Li battery containing the Redox active diaphragm(the NCFs of the diaphragm are in direct contact with the negative electrode of Li);

B) A comparison of the weight/volume of the diaphragm and the positive electrode with the weight/volume of the LFP/Li battery.

Figure VII Comparison of weight capacity of cells with PE as diaphragm(LFP-PPy) / Li and CellI batteries with Redox active diaphragm

Electrochemical active diaphragm increases lithium ion battery capacity

[Summary]

In this work, a design method to obtain a double-layer cellulosic membrane by introducing a porous Redox active layer is proposed to improve the electrochemical performance of lithium ion batteries. Since the Redox active diaphragm can provide additional capacity, when replacing the conventional commercial diaphragm with an Redox active diaphragm, the capacity of lithium-ion batteries with LiFePO4 as the positive pole and Li as the negative electrode will increase from 0.16 mAh to 0.276 mAh. The authors point out that further work can increase the 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 by Redox active diaphragm provides a new idea for the development of high energy density thin film lithium-ion batteries and other electronic products.

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