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Beyond Graphite Are Better Batteries
Although batteries are everywhere, the pursuit of longer life, faster charging, and more power, among other features, is constant. But can this be achieved with metals other than graphite?
Written by Nicole Imeson
MUCH OF THE MODERN WORLD, from electric vehicle fleets to pocket computers, has run on the lithium-ion battery since Sony first commercialized the technology in 1991. Yet, that technological foundation has hit a critical wall, as decades of refinement maximized the design’s energy density.
The global race for batteries that store more energy in less space required scientists to abandon the established, but limited, graphite anode, the primary component that defined the previous era of energy storage. Addressing this critical pivot, researchers at the University of Houston and colleagues from Singapore, South Korea, and China, recently completed a review study, comparing the opportunities and key challenges of replacing graphite anodes with monovalent or multivalent metals.
“One of the key goals was to improve the energy density of the batteries. We wanted to pack more energy into the same volume, reduce cost, and improve safety,” explained Yan Yao, Hugh Roy and Lillie Cranz Cullen Distinguished professor at the University of Houston’s Department of Electrical and Computer Engineering.
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Multivalent vs monovalent
The research team investigated a variety of monovalent and multivalent materials. Monovalent metal anodes, including lithium, sodium, or potassium, possess a much higher storage capacity than graphite. Lithium, for example, holds a theoretical storage capacity roughly 10 times higher than graphite. However, in certain situations, lithium metal proved highly reactive and prone to dendrite formation, needle-like structures that pierced the battery separator, causing short circuiting and creating severe safety issues like fire.
Alternatively, multivalent metal anodes (such as magnesium) boasted an even higher energy density, were more abundant, and developed more ordered plating structures, potentially making them safer.
“Magnesium, the same size as lithium, holds twice as much charge. The interaction between magnesium cation and the intercalation host is much stronger compared to lithium, therefore its diffusion is very sluggish,” Yao explained.
Multivalent metal anodes also formed a denser solid electrolyte interphase (SEI) that significantly reduced the battery’s lifespan. While a specialized electrolyte offsets these disadvantages, it also costs more and scales less easily than current battery technology.
“Magnesium [is] the same size as lithium [and] holds twice as much charge. The interaction between magnesium cation and the intercalation host is much stronger compared to lithium, therefore its diffusion is very sluggish.”
— Yan Yao, Hugh Roy and Lillie Cranz Cullen Distinguished professor at the University of Houston’s Department of Electrical and Computer Engineering
Battery construction
Thermal is a subset of a larger group of energy storage systems, which also includes chemical, mechanical, electrical, and electrochemical, each categorized by the way it delivers heat.
Batteries contain three main components: an anode (typically graphite), a cathode (typically lithium-containing metal oxides), and an electrolyte. When a battery discharges energy, ions flow from the anode (negative electrode) to the cathode (positive electrode). Charging reverses this flow, moving the ions from the cathode back to the anode. This process has been dubbed the “rocking chair,” as the lithium ions rock back and forth between the anode and cathode during charge and discharge cycles. The electrolyte blocks electrons, forcing them to take the long route through the external circuit, generating electric current.
Graphite contains flat graphene layers, which allows lithium ions to reversibly slip in and out during charging in a process called intercalation. The graphite anode stores up to one lithium ion for every six carbon atoms, for a high theoretical specific capacity. This creates a small, manageable change in volume of less than 10 percent, preventing the anode from cracking or degrading over thousands of cycles. Graphite also developed a robust solid electrolyte interphase (SEI) layer that prevented harmful side reactions and capacity fade.
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Market barriers
While lithium metal anodes offer high theoretical capacities, their widespread commercialization suffered significantly from safety and lifespan issues stemming from uncontrolled dendrite growth and high reactivity. Monovalent metal anodes require complex and costly engineering solutions—like solid-state electrolytes or protective coatings—to achieve the long-term cycle stability demanded by the electric vehicle market. High upfront costs and manufacturing complexity prevented immediate, large-scale adoption.
Multivalent metal anodes showed great intrinsic value for their high volumetric capacity and abundance, theoretically making them cheaper and easier to manufacture. However, the fundamental electrochemistry caused sluggish reaction kinetics and high ion migration energy barriers. Consequently, the technologies for multivalent anodes remained in their infancy.
“Magnesium tends not to form dendrites compared to lithium, and plating appeared more uniform. However, to achieve uniform plating, a special type of electrolyte is required. The liquid electrolyte, an inactive component, contributed in a significant way to the magnesium plating,” Yao explained.
With both monovalent and multivalent metal anodes, limitations with electrolytes, cycle stability, and cycle life created a significant barrier to commercialization. This required significantly more research and funding to overcome these challenges.
This project involved collaboration between the University of Houston and fellow energy storage experts in Singapore, South Korea, and China. The group secured funding from the Department of Energy and other consortiums that searched for the next generation of battery technology.
The review study highlighted a stark trade-off. Monovalent metals (like lithium) offered a massive capacity jump but created safety risks from dendrite growth. Conversely, multivalent metals (like magnesium) promised greater abundance and inherent safety but suffered from crippling limitations in sluggish ion movement and poor cycle life.
Ultimately, the researchers concluded that no single battery chemistry could solve all future battery challenges. Instead, successful commercialization requires heavy, directed funding and sophisticated materials science to overcome the core technical barriers to make these high-energy-density alternatives practical for the mainstream market.
Nicole Imeson is an engineer and writer in Calgary, Alta.
Photo: Getty Images
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