In a previous post, Li-ion Battery Production, I reported how Sony decided to produce the initial Li-ion battery in the same manner as magnetic audio tape. This was because Sony had a surplus of magnetic tape production equipment and technicians, due to the shift in the market for recorded music from audio cassettes to compact disks. The production method helped deliver a significant initial improvement in performance over the nickel-cadmium battery. But it also has produced serious side effects in performance and safety.
The three active components of a rechargeable (secondary) Li-ion battery are the cathode (positive electrode), anode (negative electrode), and electrolyte. In a conventional Li-ion battery, the cathode and anode electrodes are fabricated by coating the active power materials in chemical slurries onto metal foil current collectors. Sheet like electrodes are then wound together, with a polymer separator between them to prevent shorting. Pores in the separator allow the electrolyte to convey ions between the anode and cathode. The naturally cylindrical “jelly roll” configuration is then packaged in a cylindrical metal can or flattened into a pseudo-prismatic configuration and packaged in either a polymer laminate pouch or metal case (see illustration below).
There are a couple of inherent disadvantages with this structure. The first is spatial inefficiency. Within the resulting battery structure, the only materials that store energy are the active anode and cathode powders; all other materials are inactive. Collectively, the inactive materials typically comprise about 43% of the total battery volume. Although necessary for battery construction, from an energy density perspective inactive materials are simply inert waste.
Spatial inefficiency is also a result of safety concerns. Packing more active material into a conventional Li-ion battery and making the electrodes and separator thinner can increase energy density. However, these changes reduce the margin of safety. Microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit. The complex assembly techniques make the elimination of all metallic dust nearly impossible, and ultra-thin polymer separators are more susceptible to impurities than thicker ones.
If enough microscopic metal particles converge on one spot, a major electrical short can develop, and a sizable current will flow between the positive and negative electrodes. This causes the temperature to rise, leading to a thermal runaway, which can produce an explosion and fire. Conventional Li-ion batteries can also become thermally unstable at high temperatures, which can lead to runaway. Cold temperature charging, below 0°C (32°F), can cause permanent plating of metallic lithium on the anode. Such damage can compromise the safety of the battery, making it more vulnerable to failure if subjected to impact or high-rate charging.
The next couple of posts will describe how a 3D cell architecture improves both spatial efficiency and safety in a lithium-ion battery.