A breakthrough in battery design could allow engineers to use lightweight lithium as an anode material

'Holy Grail' of lithium battery anode a step closer

Engineers have achieved ‘the Holy Grail’ of battery design by using carbon nanospheres to create a stable pure lithium anode.

Current state-of-the-art batteries, such as those used in Boeing’s all-composite Dreamliner passenger jet and the Tesla Model S electric car are lithium-ion batteries, with lithium in the electrolyte, but not in the anode.

Designers have tried for decades to create an anode of pure lithium as it has the capacity to boost battery efficiency significantly, but the metal’s chemical reactivity and its propensity to expand during charging have so far restricted its use as an anode due to its rapid degradation.

But by using a protective layer of carbon nanospheres to protect the unstable lithium anode, researchers at Stanford University, have managed to create a battery that approaches the shelf life of current battery technology.

To be commercially viable a battery must have a coulombic efficiency – a ratio of the amount of lithium that can be extracted from the anode when the battery is in use compared to the amount put in during charging – of 99.9 per cent or more, ideally over as many charging cycles as possible.

Previous anodes of unprotected lithium metal achieved approximately 96 per cent efficiency, which dropped to less than 50 per cent in just 100 cycles, but the Stanford team's new lithium metal anode achieves 99 per cent efficiency even at 150 cycles.

"The difference between 99 per cent and 96 per cent, in battery terms, is huge. So, while we're not quite to that 99.9 per cent threshold, where we need to be, we're close and this is a significant improvement over any previous design," said Yi Cui, professor of Material Science and Engineering and leader of the research team who have published their findings in the journal Nature Nanotechnology.

Batteries consists of an anode, the negative pole from which electrons flow out and into a circuit, a cathode, the positive pole where the electrons re-enter the battery after travelling through the circuit, and the electrolyte, a solid or liquid loaded with positively charged ions that move between the terminals to allows current to flow out of the battery.

According to Cui engineers have long sought to use lithium as an anode as the metal’s low weight and high energy density could lead to the design of lighter, smaller batteries with more power. “Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail," he said.

But a key engineering challenge is that the lithium ions expand as they gather on the anode during charging.

While all anode materials, including the graphite or silicon used in current lithium ion batteries, expand to a certain extent, the researchers say lithium's expansion during charging is "virtually infinite" relative to the other materials and is also uneven, causing pits and cracks to form in the outer surface.

The resulting fissures on the surface of the anode allow lithium ions to escape, forming hair-like or mossy growths, called dendrites, which short circuit the battery and shorten its life.

A second engineering challenge is that a lithium anode is highly chemically reactive with the electrolyte, meaning it quickly uses up the electrolyte and again reduces its practical life.

"Lithium has major challenges that have made its use in anodes difficult,” said doctoral candidate Guangyuan Zheng and first author of the paper. “Many engineers had given up the search, but we found a way to protect the lithium from the problems that have plagued it for so long."

The solution devised by the Stanford researchers was to build a protective layer of interconnected carbon domes – dubbed nanospheres – on top of the lithium anode that resemble a honeycomb and create a flexible, uniform and non-reactive film to protect the unstable lithium.

The carbon nanosphere wall is just 20 nanometres thick and made of amorphous carbon, which is chemically stable, yet strong and flexible enabling it to move freely up and down with the lithium as it expands and contracts during the battery's normal charge-discharge cycle.

"The ideal protective layer for a lithium metal anode needs to be chemically stable to protect against the chemical reactions with the electrolyte and mechanically strong to withstand the expansion of the lithium during charge," said Cui.

"With some additional engineering and new electrolytes, we believe we can realise a practical and stable lithium metal anode that could power the next generation of rechargeable batteries."Engineers have achieved ‘the Holy Grail’ of battery design by creating a pure lithium anode using carbon nanospheres.

Current state-of-the-art batteries, such as those used in Boeing’s all-composite Dreamliner passenger jet and the Tesla Model S electric car are lithium-ion batteries, with lithium in the electrolyte, but not in the anode.

Designers have tried for decades to create an anode of pure lithium as it has the capacity to boost battery efficiency significantly, but the metal’s chemical reactivity and its propensity to expand during charging have so far restricted its use as an anode due to its rapid degradation.

But by using a protective layer of carbon nanospheres to protect the unstable lithium anode, researchers at Stanford University, have managed to create a battery that approaches the shelf life of current battery technology.

To be commercially viable a battery must have a coulombic efficiency – a ratio of the amount of lithium that can be extracted from the anode when the battery is in use compared to the amount put in during charging – of 99.9 per cent or more, ideally over as many charging cycles as possible.

Previous anodes of unprotected lithium metal achieved approximately 96 per cent efficiency, which dropped to less than 50 per cent in just 100 cycles, but the Stanford team's new lithium metal anode achieves 99 per cent efficiency even at 150 cycles.

"The difference between 99 per cent and 96 per cent, in battery terms, is huge. So, while we're not quite to that 99.9 per cent threshold, where we need to be, we're close and this is a significant improvement over any previous design," said Yi Cui, professor of Material Science and Engineering and leader of the research team who have published their findings in the journal Nature Nanotechnology.

Batteries consists of an anode, the negative pole from which electrons flow out and into a circuit, a cathode, the positive pole where the electrons re-enter the battery after travelling through the circuit, and the electrolyte, a solid or liquid loaded with positively charged ions that move between the terminals to allows current to flow out of the battery.

According to Cui engineers have long sought to use lithium as an anode as the metal’s low weight and high energy density could lead to the design of lighter, smaller batteries with more power. “Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail," he said.

But a key engineering challenge is that the lithium ions expand as they gather on the anode during charging.

While all anode materials, including the graphite or silicon used in current lithium ion batteries, expand to a certain extent, the researchers say lithium's expansion during charging is "virtually infinite" relative to the other materials and is also uneven, causing pits and cracks to form in the outer surface.

The resulting fissures on the surface of the anode allow lithium ions to escape, forming hair-like or mossy growths, called dendrites, which short circuit the battery and shorten its life.

A second engineering challenge is that a lithium anode is highly chemically reactive with the electrolyte, meaning it quickly uses up the electrolyte and again reduces its practical life.

"Lithium has major challenges that have made its use in anodes difficult,” said doctoral candidate Guangyuan Zheng and first author of the paper. “Many engineers had given up the search, but we found a way to protect the lithium from the problems that have plagued it for so long."

The solution devised by the Stanford researchers was to build a protective layer of interconnected carbon domes – dubbed nanospheres – on top of the lithium anode that resemble a honeycomb and create a flexible, uniform and non-reactive film to protect the unstable lithium.

"The ideal protective layer for a lithium metal anode needs to be chemically stable to protect against the chemical reactions with the electrolyte and mechanically strong to withstand the expansion of the lithium during charge," said Cui.

The team’s carbon nanosphere wall is just 20 nanometres thick and made of amorphous carbon, which is chemically stable, yet strong and flexible enabling it to move freely up and down with the lithium as it expands and contracts during the battery's normal charge-discharge cycle.

"With some additional engineering and new electrolytes, we believe we can realise a practical and stable lithium metal anode that could power the next generation of rechargeable batteries," added Cui.

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