Lithium metal batteries (LMBs) have long promised to offer high-energy density more than double of that of lithium-ion batteries (LIBs), but in reality faced significant challenges in terms of safety and lifespan. Now researchers have developed a novel structure in the negative electrode of LMBs that can potentially overcome such barriers.
Their results appeared first online on July 9 in Nano Research by Tsinghua University Press and Springer Nature. The same results have been published in the journal's print version in 2021 issue 10 recently.
LIBs are everywhere these days, from mobile phones to laptops, and are also powering the transition away from the greenhouse-gas-emitting use of fossil fuels in road transport. All of these applications demand energy densities beyond what can be offered by LIBs, placing an increasing pressure on researchers to develop alternative batteries with greater energy densities.
Currently, LMBs are theoretically a leading option, which use lithium metal instead of graphite as the negative electrode (anode) in the battery set-up. However, during repeated charging and discharging, lithium ions have the tendency to deposit themselves on the negative electrode as spiky shards called dendrites. When a dendrite grows long enough to touch the positive electrode (the cathode), a short circuit forms, potentially causing a fire.
The formation of dendrites mainly results from the irregular flux of lithium ions in the cells. Past research has shown that if carbon materials have nitrogen-containing functional groups (basically appendages of molecules that have their own distinctive chemical interactions, regardless of the chemical make-up of the rest of the molecule), then these materials can in principle reduce the irregularity of the lithium ion flux.
For this to happen, the interaction distance between lithium ions and these nitrogen-containing functional groups has to be incredibly small-;down to the nanometre scale.
"This means you need to very carefully design the structure of the nitrogen-containing carbon materials to maximize their ability to regulate the lithium ion flux," said Xufeng Zhou one of the corresponding authors of the study from the Advanced Li-ion Battery Engineering Laboratory at the Ningbo Institute of Materials Technology and Engineering.
The researchers have developed what they call porous carbon nitride microspheres, or PCNMs. The 'nitride' part is where the nitrogen comes in. These microspheres are structured such that they exhibit abundant nanopores, or holes on the scale of nanometres, as well as slightly larger ones on the scale of micrometres. When uniformly applied to the surface of a copper or lithium foil to form the negative electrode and tested in the LMB, formation of dendrites was effectively eliminated.
In tests, the researchers found that the LMB using a PCNM coated foil negative electrode achieved a high capacity retention of roughly 80 percent after more than 200 cycles of recharging.
The cause for these significant improvements, as explained by Prof, Zhaoping Liu, who has co-supervised this research, is that the PCNMs offer both physical and chemical improvements. On the physical side, the 3D porous framework of the PCNMs is favourable for absorbing volume changes and for guiding lithium growth. Meanwhile chemically, the foil coating layer produces an effective interaction distance between the lithium ions and the nitrogen-containing functional groups to eliminate the irregularity of the lithium ion flux.
Prof. George Chen of the University of Nottingham in the UK points out that the findings from this research, particularly the established new mechanism for eliminating dendrite deposition, promise a long awaited and very much demanded stable and high capacity negative electrode that can help improve not only various LMBs, but also the supercapattery that is a hybrid device combining the merits of rechargeable battery and supercapacitor.