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Scientists observe how single crystals of a promising cathode degrade in Li-ion batteries
21-01-2025
High-voltage cathodes are considered the future for Li-ion batteries. Now researchers led by the ESRF and CEA have discovered how single crystals degrade (or not) over time and use. The research is out now in Advanced Energy Materials.
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Lithium-ion batteries are widely used in many everyday appliances for their high energy density and efficiency, but face challenges that impact their performance: limited lifespan, safety concerns such as overheating or overcharging, charging speed, scalability for large applications, cost or recycling possibilities.
Researchers are actively studying all of the elements that play a role in the functioning of the batteries. At a fundamental level, battery operation is dependent on (de)lithiation. (De)lithiation is the process where lithium ions leave the battery's positive electrode (cathode) during charge and travel through the electrolyte to the negative electrode (anode). It’s a reversible process, so when batteries are discharged, lithium ions move back to the cathode, generating an electric current, powering the appliances. Investigating cathode materials and its structural evolution during battery operation is paramount for enabling the development of more performant, efficient and reliable energy storage systems.
Next-generation cathode
Scientists led by the ESRF and the CEA have investigated the high-voltage spinel material lithium nickel manganese oxide. This type of cathode material was discovered three decades ago and it is characterized by a very stable structure and, hence, the possibility of reversing the process of delithiation without too much degradation of the system. Another advantage of this material is that it functions at a high voltage, so it could produce more energy, which could then enable higher density of energy storage for applications like electric vehicles. In addition, it is made of manganese and nickel, which are more abundant and less expensive than cobalt, which is currently an essential part of materials for cathodes in commercial electric vehicle batteries.
High-voltage spinel materials still degrade over time despite their apparent stability, however, scientists are working on understanding the mechanisms behind their degradation and how they can be improved and eventually be commercially used. “These materials have a huge potential and many see them as the future for Li-ion batteries but it is not easy to see the processes taking place at the microscale in a single crystal”, explains Tobias Schulli, scientist in charge of beamline ID01 and co-corresponding author of the publication.
Challenging experiment
The team came to ID01 to carry out operando scanning X-ray diffraction microscopy (SXDM) and multi-crystal X-ray diffraction (MCXD) to investigate the microstrain and lattice tilt inhomogeneities inside Li1-xNi0.5Mn1.5O4 cathode particles during electrochemical cycling and how these influence the degradation of the material.
“The experiment was very challenging as we wanted to follow a few individual crystals and how the different regions inside one nanocrystal interact with each other as the reaction was taking place”, explains Schulli. “In a diffraction experiment, the angle is important and so any minuscule change can make you lose the particle”, he adds.
The results showed that some individual cathode crystals can sustain delithiation while others manifest a noticeable lattice deformation during operation. It was found to be caused by inherent defects present in the crystals regardless of the charging and discharging of the battery. Over prolonged cycling, the scientists discovered that such defective particles exhibit more severe degradation while unaffected particles remain in nearly perfect condition. Such selective degradation of particles could be caused by varying crystal quality across the sample and should be an important consideration for designing next-generation high-voltage cathode materials.
The next step for the team is to expand the toolset of battery material characterisation with ptychography, which is rapidly developing the technique of microscopic imaging. It allows to recover full 3D reconstructions of the strain distribution inside the particle for a more detailed investigation of the structural evolution of cathode materials.
Reference:
Vostrov, N. et al, Advanced Energy Materials, 17 January 2025.
https://doi.org/10.1002/aenm.202404933
Text by Montserrat Capellas Espuny
Top image: Evolution of the Bragg reflection corresponding to a single cathode crystallite during battery charging:(I) fully lithiated state at the onset of charging (4.73 V) (II) splitting of the Bragg reflection due to a heterogeneous deformation of the crystal lattice within the imaged particle (4.74 V): both the lithiated (blue) and delithiated (red) phase are present in the crystallite leading to stress and misorientations of the lattice. (III) Charged battery: fully delithiated homogeneous particle (4.75V). Credits: Nikita Vostrov.