Sony's new lithium battery uses tin-based negative electrode materials to increase capacity by 25%

In July 2011, Sony announced that it would use tin (Sn) negative electrode materials to increase the capacity of lithium-ion rechargeable batteries. The battery unit developed this time is a "18650" size (diameter 18mm × height 65mm) with a capacity of up to 3.5Ah. Compared with the company's original 2.8Ah product put into production in 2010, the capacity has increased by 25% (Figure 1). The volume energy density is 723Wh/L. The weight is 53.5g, and the weight energy density is 226Wh/kg. The charging voltage is 4.3V. It is scheduled to start supplying in 2011.

Figure 1: Capacity increased by 25% by using Sn negative electrode
Sony has developed a lithium-ion rechargeable battery "Nexelion" (a) that uses Sn-based negative electrode materials to increase unit capacity by 25% compared to the original. Previously, the capacity was increased by using graphite negative electrode materials, but the capacity increase rate has slowed down recently. Sn has a much larger theoretical capacity than ordinary negative electrode material graphite (b). (Figure (a) was drawn by Nikkei Electronics based on Sony's data)

In fact, this is not the first time that Sony has commercialized lithium-ion rechargeable batteries with Sn as the negative electrode. The company has been selling "14430" size battery cells with a diameter of 14mm and a height of 43mm for camcorders since 2005. This time, it will mass-produce 18650 size products, which are slightly larger than 14430, for notebook computers.

Achieving amorphization of Sn-based materials

To achieve a high capacity of 3.5Ah for 18650 size battery cells, "the only way is to change the negative electrode material" (Hiroshi Inoue, General Manager of Product Design 1 Department, Sony Energy Device LI 1st Business Division). Sony increased the capacity by changing the negative electrode material from the original low-crystalline carbon (hard carbon) to graphite around 1997.

After that, the capacity was increased to about 2.5 times the original by improving the positive electrode material, but the capacity increase rate has been decreasing recently. Therefore, Sony decided to change the negative electrode material of the 18650 unit again after about 14 years and adopted Sn-based materials.

Sn and silicon (Si) have a theoretical capacity nearly 10 times greater than that of graphite, the current mainstream negative electrode material. However, the expansion and contraction of the negative electrode during charge and discharge destroys the crystal structure, so the charge and discharge cycle life is short. This time, Sony solved this problem by amorphizing Sn-based materials (Figure 2). The company achieved amorphization of multiple elements such as Sn, cobalt (Co) and carbon at the nanoscale, inhibiting the shape change of particles during charge and discharge, thereby improving the charge and discharge cycle life of the battery cell.

Figure 2: Structure of Sn-based amorphous material Co-Sn ultrafine particles
The negative electrode material structure of the new lithium-ion rechargeable battery is a carbon phase with ultrafine particles composed of Co-Sn alloy distributed in it. Co forms carbides with carbon and combines with it. (Figure drawn by Nikkei Electronics based on Sony's data)

High capacity also ensures safety

The lithium-ion rechargeable battery developed this time also has improved positive electrodes and separators. The positive electrode material was changed from a ternary material (Li (Ni-Co-Mn) O2) that can balance capacity and safety to lithium cobalt oxide (LiCoO2). The Co material of LiCoO2 is expensive and has poor thermal stability, so the positive electrode material has been shifting to ternary materials, but Sony went against the trend and adopted LiCoO2 with higher capacity.

LiCoO2 will heat up when an abnormality such as an internal short circuit occurs. In this case, oxygen will be generated, and oxygen may burn after reacting with the organic electrolyte. Therefore, Sony "applied a 0.1-1μm thick coating treatment" on the surface of LiCoO2 particles (Inoue). According to the company, this method can reduce the reaction between oxygen and organic electrolyte.

In the case of the diaphragm, a metal oxide ceramic layer several μm thick is formed on both sides of the polyolefin microporous membrane to prevent internal short circuits (Figure 3). The ceramic layer has a three-dimensional structure, and metal oxide particles with a diameter of less than 1 μm are connected to the resin as a binder in a mesh shape. When an abnormality occurs, the resin mesh with high flexibility and adhesion allows the insulator, the metal oxide, to adhere to the foreign matter, thereby limiting the flow of current.

Figure 3: Improving safety through three-dimensional ceramic layer
A three-dimensional ceramic layer is formed on both sides of the diaphragm. Once a metallic foreign body is mixed in and causes damage to the diaphragm, the ceramic layer will be transferred to the surface of the metallic foreign body to prevent large current from passing through. (Figure drawn by Nikkei Electronics based on Sony's data)

Of course, the new cell also has its challenges. For example, after 300 cycles of charge and discharge, the capacity drops to about 80% of the initial capacity. After 500 cycles of charge and discharge, the capacity drops to about 60%, barely reaching the minimum standard for use in notebook computers. In order to expand sales in the future, the charge and discharge cycle of the battery cell must be improved.