A team of researchers belonging to Brookhaven National Laboratoryfrom the US Department of Energy (DOE), has discovered an additive for the electrolyte of lithium-ion batteries with cathodes rich in nickel that stabilizes charge and discharge cycles. The result of your work could lead to an improvement in energy density of the batteries, which has a direct impact on the autonomy of electric vehicles.
Whether they are small in size, such as those used in consumer electronics, or large-capacity ones, such as those in electric vehicles, there are three main components of any battery. The electrodes, cathode and anodebetween which lithium ions travel to balance the charge created during charging and discharging, which are immersed in a electrolyte which makes it easy to move. When it is in the process of discharging, the ions are released from the anode or negative electrode and travel to the cathode or positive electrode. When the battery is connected to a charger, the opposite occurs and the battery recharges.
The component that makes the battery chargeable and dischargeable is the electrolyte, which in today’s batteries is generally liquid and is usually made up of heavy metals or rare earth metals. The electrons pass through an external circuit that connects the two electrodes while the ions pass through the electrolyte. They both go back and forth between the electrodes during charge and discharge cycles.
The keys to the electrolyte of lithium batteries
The materials of the nickel-rich cathodes (known as NCM being composed of nickel, cobalt and manganese) are arranged in layers in layers. This chemical composition promises high energy density when combined with lithium metal anodes. However, these materials are prone to capacity loss. One of the main problems is the cracking of the particles during the charge and discharge cycles at high voltage. Its capacity to operate at high voltages (above 4 V) is extreme importance since the total energy stored in a battery increases as the useful operating voltage increases.
Another problem is the dissolution of the transition metal from the cathode and its subsequent deposition on the anode. This is known as “crosstalk” in the battery community. During high-voltage charging, small amounts of transition metals in the cathode crystal lattice dissolve and travel through the electrolyte, depositing on the anode. When this happens, both the cathode and the anode degrade. The result is evidenced by poor battery capacity retention.
Additives in the electrolyte
The Brookhaven National Laboratory team led by Brookhaven chemist Enyuan Hu offers a remedy to the degradation problems experienced by nickel-rich cathode materials when operated at high voltages. His work, done as part of the DOE-funded Battery500 Consortium, has been led by the Pacific Northwest National Laboratory (PNNL) and published in the journal Nature Energy.
Paper co-author Sha Tan was conducting research with the electrochemical energy storage group at Brookhaven Lab originally using an additive, lithium difluorophosphate (LiPOtwoFtwo), to improve the performance of batteries at low temperatures. “I found that if I raised the voltage to 4.8 volts (V), this additive really provided great protection to the cathode, making the battery achieve excellent cycling performance,” explains Tan.
What was really happening is that the introduction of a small amount of this additive in the electrolyte eliminated the crosstalk, that is, it reduced the lithium deposits on the anode. As the additive breaks down, it produces lithium phosphate (Li3PO4) and lithium fluoride (LiF) to form a cathode-electrolyte interface, a thin, solid layer that forms on the battery cathode during cycling, with a high level of protection.
“By forming a very stable interface at the cathode, this protective layer significantly suppresses the loss of the transition metal at the cathode surface,” explains Hu. The reduced loss of transition metals helps to decrease the deposition of those transition metals on the anode. In that sense, the anode is also protected to some extent. “We believe that the suppression of transition metal dissolution is one of the key contributors leading to significantly improved cycle performance.”
According to the researchers, adding this additive to the electrolyte, a nickel-rich cathode cycled at high voltages increases energy density and retains 97% of its initial capacity after 200 charge and discharge cycles.
As Hu explains, the improved performance is not the only interesting result. The most common nickel-rich cathode is in the form of polycrystals, that is, an aggregate of many nanometer-scale crystals known as primary particles, grouped together to form a larger secondary particle. Although this is achieved a relatively easy cathode synthesisthis polycrystalline nature causes cracks in the particles which translates into an eventual fading of the capacity.
Some recent research has indicated that single crystal based cathodes may be advantageous over their polycrystalline counterparts for the suppression of particle cracking. However, “according to the results of our work, polycrystalline materials cannot be abandoned because they are easier to manufacture, which translates into a reduction in battery costs” a lower cost in a lower cost ” Hu said. The study suggests that the use of the additives can also effectively address the problem of cracking in polycrystalline materials.
“Our strategy uses a very small amount of additive to achieve such a large improvement in electrochemical performance. In practical terms, this could be a low-cost and easy-to-adopt solution,” adds Hu. Looking ahead, the researchers want test the additive in more demanding conditions to explore whether cathode materials can withstand even more cycles for practical battery use.