A Japanese research team has improved the design of the silicon anodes that replace the graphite anodes in lithium-ion batteries to improve their lifespan. With the anode incorporation silicon of a polymeric binder composed of a specific polymer significantly improves its structural stability. This ensures that batteries made with an anode of this material last longer and, therefore, are viable to meet the energy needs of electric vehicles and other devices.
Lithium-ion batteries are currently the most widely used both in portable devices and in high energy density applications, such as electric vehicles. Its properties are those that best integrate the minimum criteria necessary to be technically and economically viable. They meet safety requirements, offer the maximum energy density which is limited by the final weight and volume of the battery and its Lifecycle is high. Lastly, they offer high percentages of reuse and recyclingall this, to lowest possible cost.
Nevertheless, its limitations have become apparent, especially with its use in electric vehicles, which require greater durability, energy density and longevity. This is why many research teams have been looking for alternative materials to use in these batteries. One of them is the silicon that would replace the graphite that forms the structure of the anode. Silicon is a very abundant material and therefore much cheaper and has a higher theoretical discharge capacity than graphite.
Silicon has the potential to significantly increase the energy density of batteries. Its capacity is an order of magnitude greater than that of graphite, which is currently used to create the anode structure. At the cell level, energy density could almost doubleproviding obvious benefits in the autonomy of electric vehicles.
However, silicon anodes also have some drawbacks. The problem that arises when silicon is used as the anode material is its high degradation, resulting in poor battery life. Specifically, repeated charging and discharging causes the silicon particles to expand and break apart, forming a thick solid interface (SEI) between the electrolyte and the anode. This SEI limits the movement of lithium ions between the electrodes, which in turn limits the battery’s performance and ability to charge and discharge over time.
The volume changes that the anode undergoes during the charge and discharge cycles lead to the consumption of the electrolyte and lithium and cause mechanical stresses that ultimately result in the loss of electrical and ionic conductivity. Incorporating porosity, electrolytic additives and conductive networks and binders are just some of the solutions that are being developed.
The Researchers at the Japan Advanced Institute of Science and Technology (JAIST)) have shown that adding a specific polymer compound binder to the silicon anode of a lithium-ion battery can significantly improve its structural stability. This makes devices made with an anode of this material last longer and, therefore, are viable to meet the energy needs of electric vehicles.
To improve the performance of silicon anodes, the JAIST team, led by Professor Noriyoshi Matsumi, developed a technique to improve the stability of silicon particles on an anode using a polymer composite binder. The result is the creation of a thin layer of SEI, which prevents the anode and electrolyte from spontaneously reacting with each other, but does not inhibit the flow of lithium ions.
Technically, the researchers used poly(bisiminoacenaphthenequinone) (P-BIAN) and a carboxylate-containing poly(acrylic acid) (PAA) polymer in the compound binder, linked together by hydrogen bonds. The design, comprising n-type conducting polymers (CPs) and proton donor polymers with hydrogen-bonded networks, represents “a promising future in high-capacity electrode materials,” Matsumi said in the news release.
In fact, the composite polymer structure holds silicon particles together and prevents them from breaking apart, while hydrogen bonds allow the structure to repair itself. This means that the polymers can rejoin if they break or degrade. The binder also improved the conductivity of the anode or by limiting the breakdown of the electrolyte.
To test the binder, the team fabricated an anodic half-cell composed of silicon nanoparticles with graphite (Si/C), the binder (P-BIAN/PAA), and a conductive acetylene black (AB) additive. They then subjected the anode to several repeated cycles of charging and discharging.
During this process, the binder stabilized the silicon anode and maintained a specific discharge capacity of 2,100 mAh/g for more than 600 cycles. This is how the researchers explain it in the article published in the journal ACS Applied Energy Materials. For comparison, the capacity of the bare carbon-silicon anode, without binder, dropped to 600 mAh/g in just 90 cycles.
After testing the anode, the team proceeded to take it apart and examine it to check its status. To do this, they used a spectroscopic and a microscopic, in search of cracks that could have been created due to the rupture of the silicon, an effect that is usually very common in this type of anode. After 400 charge and discharge cycles, the anode structure remained practically intact, showing only a few microcracks. This shows that the binder succeeded in improving the structural integrity of the electrode.
In general, these results can be defined as promising and offer a possible scenario for the future use of silicon anodes in lithium-ion batteries. With them, the use of lithium batteries in electric vehicles and even in other types of devices that need high-performance batteries would be optimized.
