TDK Front Line

Vol.9 New development in world leading TDK lithium polymer batteries

The pouch cell (laminated type) lithium polymer batteries have been used as the built-in batteries (rechargeable batteries) for smartphones. The main features of the pouch cell are that it is lighter in weight and more flexible in shape compared to other cells that use metal cans. It also replaced organic solvent electrolytes with polymer gels (polymers) to prevent liquid leak, allowing the battery to be safer and more reliable. It is now replacing the square lithium ion batteries, and in addition to the field of ICT devices such as smartphones, the usage of lithium polymer batteries has been expanding to the field of high-capacity “power cells” for drones, automated guided vehicles (AGVs), robots, and electric motorcycles, as well as “mini-cells” for wearable and IoT devices.

Features of lithium ion batteries

Lithium ion batteries (LIB) are often used in smartphones and notebook computers. The innovative rechargeable battery, which uses a lithium compound (e.g., lithium cobaltite) for cathode material and carbon (e.g., graphite) for anode material, was first commercially released in 1991. Its features are superior, having high density in both weight and volumetric energy, and longer commercial life due to smaller self-discharge.

Types and manufacturing methods of lithium ion batteries

A lithium ion battery consists of cathode materials (cathode active materials and current collector) and anode materials (anode active materials and current collector) with a separator and electrolyte that separate them.

The cells (single cell) of a lithium ion battery can come in various shapes such as cylindrical, prismatic, pouch (laminated type), and so on. To increase capacity, a battery must have a larger electrode surface area; winding and stacking methods are available to actualize the manufacturing of such products.

The winding method is usually applied to cylindrical cells and manufactured by winding layers of cathode and anode sheets with an active material on both surfaces of each sheet, and a separator in between using an automatic winding machine. The stacking method is usually applied to pouch cells, and it layers a cathode sheet, separator, and anode sheets that are cut in predetermined sizes in the order mentioned above. Both cylindrical and prismatic cells are put in metal cans filled with electrolytes before they are sealed.

Instead of using metal cans, pouch cells (laminate type) use laminated metal foils to seal plastic films. It is unique because it comes out thinner, lighter, and more flexible in shape. Pouch cells are manufactured with either the winding or the stacking method.

As it uses organic solvent as the electrolyte, a weakness of lithium ion batteries is the risk of fire due to leakage. This is why we created lithium ion polymer batteries (LPB, LiPo batteries) and increased their reliability by using a polymer gel as the electrolyte.

Expanding lithium polymer batteries in both power cells and mini cells

Since its establishment in 1999, TDK’s group company in Hong Kong, Amperex Technology Limited (ATL), has been focused on the manufacturing of pouch cell lithium polymer batteries as its main product. ATL holds the world’s number one share in the lithium polymer battery market for smartphones.

Conventionally, lithium polymer batteries sold by TDK under the ATL brand were mainly for smartphones and other ICT devices, but in order to meet expanding market needs, we have been expanding our product line-up and have consequently started manufacturing high capacity “power cells” for drones, automated guided vehicles (AGVs), robots, and electric motorcycles, as well as “mini cells” for wearable and IoT devices.

Below are some technological challenges in the manufacturing of pouch cells using the winding method, and an explanation of ATL’s unique “Multiple Tab Winding (MTW)” technology.

High-spec power cell using our unique MTW technology

The illustration below shows a structural sample of a pouch cell lithium polymer battery using the winding method. Unlike the sheets in cylindrical cells, they are flatly spiraled for pouch cells.

Lithium polymer batteries created with the winding method have connecting terminals called “tabs” that input and output electricity on both cathode and anode sheets. In a smaller lithium polymer battery, one tab on each of the cathode and anode sheets is all that is needed, but with the increase in the winding number to gain battery capacity, a tab at just one place concentrates electrical current and results in local overheating, which then leads to performance degradation due to increased internal resistance. In order to prevent this local overheating, multiple tabs need to be attached to the cathode and anode sheets.

However, there is a great technological challenge in automatically winding a sheet with multiple tab attachments. Multiple tabs on a wound sheet need to be aligned to be connected to the external electrode lead. The pitches needed to attach the tabs must be adjusted because they must go along with the sheet that becomes longer as the number of windings increases.

With a unique MTW technique, which has been developed by improving the knowledge of winding technology we have accumulated over the years, multiple tabs can be positioned precisely, allowing the development of highly reliable power cells that suppress the increase of inner resistance caused by local heat generation. The illustration below shows the number of tabs and heat distribution caused by local heat generation. You can see that overall heat decreases with the increase in tab numbers.

Optimal for industrial equipment power cells which require warranties longer than 5 years

Generally, lithium ion batteries degrade faster if the temperature they are running at is consistently high. With power cells that have a larger input and output electric current, temperature increase caused by heat generation is more noticeable, and the inner resistance also increases. This in turn causes a decrease in capacity and life span of the battery.

The warranty period of lithium ion polymer batteries for smartphones is only a few years, but with power cells for industrial equipment, a life span of more than 5 years and higher reliability are expected. ATL’s MTW technology is the key to such demand. It suppresses both the temperature increase caused by local heat generation, and the increase of direct current resistance, resulting in better cycle features and longer battery life.

The cylindrical lithium ion batteries tend to collect heat in the center, which degrades heat dissipation and limits the battery size in diameter. On the other hand, pouch cell lithium ion polymer batteries with a flat spiral winding come out thinner, better in terms of heat dissipation, and optimal as power cells for industrial equipment with a large input and output electrical current.

Also, the combination of the MTW technology with wireless electricity will improve the charging of industrial equipmemt power cells, which will lead to reduced costs. For example, AGVs used in manufacturing plants now come with robotic arms not only to transport goods, but also to assemble, process, and pick items. More operational workload with larger power consumption means such vehicles will need a larger battery, but with the utilization of TDK’s wireless power transfer system for industrial equipment, the vehicle can be charged frequently at rest time and during operation, which allows the battery to be smaller and eliminates the need for manual battery charging and exchange.

Strong in flexible manufacturing system and speedy operation

The uniqueness of the lithium polymer batteries provided by TDK under the ATL brand is that they are light, highly flexible in shape, safe and reliable. Our power cells featuring the unique MTW structure are optimal for industrial equipment such as AGVs and robots, drones and electric motorcycles, and also for energy storage systems (ESS) for household charging systems.

The advanced lithium polymer battery technology specialized for pouch cells, as well as our flexible manufacturing systems and speedy customer operations, are strengths unique to ATL. Together with TDK’s material technology and BMS (battery management system), we aim to make the next leap forward.