Maximizing Electron Transfer Efficiency
Energy in versus Energy out. Is 100% possible?
A battery is a device that when connected to an electric load creates electrical energy via an electrochemical reaction where ions travel from anode to cathode. Primary batteries have a single use as it is discharged over time to supply the needs of the device. Rechargeable batteries can be charged and discharged multiple times. A popular example of a rechargeable battery is the lithium ion battery.
During discharge, lithium ions move from the anode to the cathode, and vice versa during the charging process. The resulting redox (oxidation-reduction) reaction occurs as electrodes are oxidized and reduced back and forth. The battery active material of the anode is typically graphite, while the active material of the cathode is a lithium metal oxide. It is all suspended in a liquid electrolyte providing a medium for the ions to travel back and forth.
Consumers are actually extremely picky when choosing batteries whether they know it or not. Think about the kinds of laptops, electric vehicles, and phones you consider purchasing. You could have all the memory and processing power in the world, but if you do not have the power to supply and sustain it, the device is useless. Therefore, battery lifetime is one of the most important characteristics when determining between each brand or model.
The lithium ion battery was commercialized by Sony in 1991, and researchers have worked to improve its efficiency for the past 29 years. Scientists have come to realize that coulombic efficiency is one of the most important parameters for determining whether a battery will last. Think back to second grade science class when you learned about potential and kinetic energy. Energy loss is a natural phenomenon when dropping a ball from a given height. The schematic below displays how actuality differs from the theoretical expectation.
Coulombic efficiency refers to the ratio of discharge capacity divided by the charge capacity - in layman’s terms it is the discharge energy divided by the fully charged energy of the battery. In a perfect scenario one would want this to be 1.00 or 100% suggesting that the battery is perfectly efficient and there is absolutely zero energy loss. If a battery of this caliber could be made, then electric vehicles could theoretically last forever. Unfortunately, this is not the case as a battery’s performance decays over time due to cycling and side reactions with the electrolyte.
These parasitic (side) reactions occur over time as lithium species react with the electrolyte to form salts and other gases. Salt build-up on the anode/cathode makes it more difficult for lithium ions to intercalate (insert) the electrode lattice, while gas generation can result in internal pressure. These reactions consume electrolyte potentially thickening the medium and reducing ion mobility. All of these phenomenon result in higher resistance which has a negative impact on battery cyclability.
The graph above displays the coulombic efficiency of a battery versus cycle number. At the beginning, the anode and cathode are fresh. During the initial charging process ions are transferred from the cathode to anode. Coulombic efficiency drops initially as parasitic reactions take place with electrolyte. This will prevent the anode from “fully” charging/discharging, because a small film of reacted species builds up on the anode (2). The SEI (solid electrolyte interphase) layer builds up over time and can limit the available space at the anode and cathode for lithium to intercalate.
Over time the SEI matures as a thin film develops on both the anode and cathode (3). The discharge and charge capacity become more similar, therefore causing a slight increase in coulombic efficiency. Below is a very simple schematic showing how the SEI layer (orange) grows over time.
Improving Coulombic Efficiency
Battery manufacturers seek to maximize coulombic efficiency. Material choice, electrolyte, and cell engineering all play a major role in determining the battery’s performance. Material chemistry - LFP, NMC, NCA or other active materials all have specific use cases and I would encourage you to read more about these. Tesla used NCA in their vehicles for the longest time, but they are recently beginning to make battery cells with LFP due to their stability. Secondly, most use confidential blends of electrolyte to limit parasitic reactions. Additions of 1-2% of additives can have significant impacts on long term cycling. As you can see in the image below, researchers have experimented with TMOBX (trimethoxy boroxine) and VC (vinylene carbonate). These additives help stabilize the electrolyte and limit reactions with lithium ions.
Finally, cell electrical engineering is incredibly important. Cathode materials are known to be less stable at higher voltage, so engineers design battery management systems to limit the upper charging voltage. Unfortunately lowering the charge voltage results in a battery with lower energy.
The efficiency of a battery is quite complex, so engineers and designers need to weigh all aspects when tuning to it’s specific use case.
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