Dr. Yi Cui of Stanford and founder of battery company Amprius, recently published a paper outlining a new structure for battery anodes. Graphite is the most commonly used anode material, durability constraints have prohibited adoption of silicon, despite the fact that silicon should in theory improve battery performance when incorporated into the anode. Historically, silicon anode material has been too fragile. The thermal cycling of battery charging and energy dissipation has caused the anode materials to crack, shortening the life of any battery that used silicon anode materials.
Let’s break down the components of Dr. Cui’s material using the standard materials science framework used in Callister. Process => Structure => Properties => Performance.
Popular press coverage rarely contains any kind of detail on the process by which new materials are made. We’re fortunate here to have a well-written scientific article and some good detail from Physorg. First, sacrificial polymeric nanofibers are formed. Nanofibers are commonly used in porous materials as they are known to have very high surface area, have a large body of scientific literature and numerous industrial scale production methods (see youtube video below for an example). Polyacrylonitrilie (PAN) is commonly used as a polymeric precursor if the goal is to produce carbon nanofibers. Second, the polymeric nanofibers are heated to the point of oxidation – at which point they have been reduced to carbon. This is a common method of manufacturing carbon nanofibers – they will typically have 70% of the fiber diameter of their polymeric precursor. Determining the correct thermal ramp and cooling process to get the desired material requires significant attention. The remaining carbon nanofibers are able to survive the third step, in which silicon is deposited into the carbon scaffold. In the fourth and final step, the carbon is removed through what is loosely described as a ‘hot air process.’
The resulting anode structure is described in Dr. Cui’s paper as a, “Double-walled silicon nanotube”, or DWSN. Equally important to the structure of the individual nanotube, is the structure formed by the nanotubes. Here, we can look at the nanofibrous antecedent to get a feeling as to how the DWSNs are laid out. Most nanofiber processes are known to produce a nonwoven nanofibrous mat; the fibers are randomly aligned. The fibers can be built up to varying levels of thickness, depending on the end performance target. The heat-based formation of the DWSNs most likely moves the material from feeling like a synthetic mat, or membrane, to behaving more like dust or powder. The DWSNs are likely formed into microscopic flakes with nano-scale surface area. It is these flakes, or powder, that are then used to create the anode structure within the battery.
The primary property which will enable the DWSN to work well as an anode is its electrical performance within the battery. Also important, given that it has historically precluded silicon’s use in anodes, is the thermal performance of the material. The DWSNs must survive normal battery operations without experiencing the cracking and structural degradation historically associated with silicon based materials. From a product development and marketing standpoint, this is an important new criteria; the DWSNs must; (1) perform as well as or better than other anode materials within the cell, (2) must show the promised superior performance of silicon to graphite, and (3) accomplish all of this without showing the historical weaknesses of silicon.
The final component is performance – how does the new material perform? From this standpoint, we should focus on performance in system. Does the battery’s performance improve with the presence of the new material? How does it improve? Dr. Cui’s paper touts that the anode is able to cycle over 6,000 times while maintaining over 85% of their initial capacity. This is a significant performance advantage over current materials.
Scaling and Complexity
The two greatest challenges to commercial realization of the improvements offered by DWSNs are scaling challenges and the inherent complexity of a battery system. Batteries are often initially tested as coin cells – those tiny little batteries used in watches and hearing aids. One of the major challenges in the performance of electric vehicles (“EVs”) has been that the size of batteries needed is much, much larger. Batteries have moved from coin cells to notebooks to now car trunks. The challenges encountered at making DWSNs at manufacturing scale may hinder their performance, make them economically unattractive or, if we’re lucky, it may work perfectly.
The components required to innovate within a new battery are significant. The parts are numerous; anode, cathode, separator, electrolyte, package, control system, end application and socioeconomic system. The increased focus on cleantech and energy technologies has led to innovation throughout this system – this is great in that many new options are available, but it is also difficult to coordinate all of these new materials and the various entities and organizations which have created them. Innovation in materials science takes a long time, much longer than it takes in software. Dramatically increasing the number of options available increases the amount of physical product testing required to confirm performance. To reach their end market DWSNs must be the winning component within the winning battery within a socioeconomic system that enables their success.
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