United States. Scientists at Rice University's George R. Brown School of Engineering develop a method that helps mitigate lithium loss and improves battery life cycles by coating silicon anodes with stabilized lithium metal particles (SLMPs).
Silicon anode batteries have the potential to revolutionize energy storage capabilities, which is key to meeting climate goals and unlocking the full potential of electric vehicles.
However, the irreversible depletion of lithium ions in silicon anodes imposes a major constraint on the development of next-generation lithium-ion batteries.
Rice's lab of Sibani chemical and biomolecular engineer Lisa Biswal found that spraying anodes with a mixture of particles and a surfactant improves battery life by 22 percent to 44 percent. Battery cells with a greater amount of coating initially achieved greater stability and cycle life.
However, there was a downside. When cycled at full capacity, a greater amount of particle coating led to greater lithium retention, which caused the battery to fade more quickly in subsequent cycles.
Replacing graphite with silicon in lithium-ion batteries would significantly improve their energy density, the amount of energy stored relative to weight and size. Because graphite, which is made of carbon, can contain fewer lithium ions than silicon. Six carbon atoms are needed for each lithium ion, while a single silicon atom can bond with up to four lithium ions.
"Silicon is one of those materials that has the ability to really improve the energy density of the anode side of lithium-ion batteries," Biswal said. "That's why there's currently this push in battery science to replace graphite anodes with silicon ones."
However, silicon has other properties that present challenges. "One of the main problems with silicon is that it continuously forms what we call a solid electrolyte interface or SEI layer that actually consumes lithium," Biswal continued.
The layer forms when the electrolyte in a battery cell reacts with electrons and lithium ions, resulting in a nanometer-scale layer of salts deposited on the anode.
Once formed, the layer isolates the electrolyte from the anode, preventing the reaction from continuing. However, the SEI can break down over subsequent charge-discharge cycles and, as it reforms, irreversibly further depletes the battery's lithium reserve.
"The volume of a silicon anode will vary as the battery is cycled, which can break the SEI or make it unstable," said Quan Nguyen, a doctoral student in chemical and biomolecular engineering and lead author of the study. "We want this layer to remain stable during subsequent battery charge and discharge cycles."
The prelithation method developed by Biswal and his team improves the stability of the SEI layer, meaning that fewer lithium ions are depleted when it forms.
"Prelithation is a strategy designed to compensate for the loss of lithium that normally occurs with silicon," Biswal said. "You can think of this in terms of priming a surface, like when you're painting a wall and you first need to apply a base coat to make sure the paint sticks. Prelitigation allows us to 'priming' the anodes so that the batteries can have a much more stable and longer life cycle."
While these particles and pre-litigation are not new, Biswal's lab was able to improve the process in a way that is easily incorporated into existing battery manufacturing processes.
"One aspect of the process that is definitely new and that Quan developed was the use of a surfactant to help disperse the particles," Biswal added. "This hasn't been reported before, and it's what allows you to have an even dispersion. So instead of them being grouped together or piling up in different pockets inside the battery, they can be evenly distributed."
Nguyen explained that mixing the particles with a solvent without the surfactant will not result in a uniform coating. In addition, spray coating proved to be better at achieving uniform distribution than other methods of application on anodes.
"The spray coating method is compatible with large-scale manufacturing," Nguyen said. Controlling the cell's cycle capacity is crucial to the process.
"If you don't control the capacity at which the cell cycles, more particles will activate this lithium capture mechanism that we discovered and described in the paper," Nguyen said. "But if you cycle the cell with an even distribution of the coating, then lithium won't be trapped."
"If we find ways to avoid lithium capture by optimizing cycle strategies and the amount of SLMP, that will allow us to better exploit the higher energy density of silicon-based anodes," Nguyen added.
