A new biologically inspired battery membrane has enabled a battery with five times the capacity of the industry-standard lithium ion design to run for the thousand-plus cycles needed to power an electric car.
A network of aramid nanofibers, recycled from Kevlar, can enable lithium-sulfur batteries to overcome their Achilles heel of cycle life—the number of times it can be charged and discharged – a University of Michigan team has shown.
The researh was published on Nature : https://www.nature.com/articles/s41467-021-27861-w
The high theoretical specific capacity of 1675 mAh g−1, environmental friendliness, and earth-abundance of elements forming lithium–sulfur (Li–S) batteries make them an attractive platform for energy storage in a variety of technological fields from electric vehicles to robotics and from power grids to aerospace engineering1. However, the diffusion of lithium polysulfides (LPS, Li2SX, 4 ≤ x ≤ 8)2 from cathode to anode drastically reduces their cycle life, overall capacity, and Coulombic efficiency3,4.
Additionally, LPS layers passivate both the electrodes, leading to a significant increase in impedance and thus to energy losses5. The non-uniform surface layer on the anode also promotes the growth of dendrites, which represents another serious issue for Li–S batteries that causes similar issues compounded by short-circuiting and overheating.
The extensive research effort in the past was invested into designing materials for sulfur cathode that would minimize LPS release. It was shown that immobilization of LPS is possible by encapsulating sulfur into microporous carriers made from nanocarbons6,7, conductive polymers8,9, transition metal oxides1, and metal-organic frameworks10,11.
Indeed, nanoporous barriers in the cathode improved the retention of sulfur within the cathode. However, there is still considerable room for improvement in the cycle life and overall performance of Li–S batteries addressing the structural complexity of cathode material and improving electron transport through electrode material12,13.
The problem of the LPS diffusion can also be approached by optimizing the materials design of ion-conducting membranes that can block the LPS transport from S cathode to Li anode. Simultaneously, these membranes must allow the facile transport of Li+ ions5,14.
Significant advances in this area were achieved using coatings from carbonaceous materials15,16,17, polymers18, metal foams19,20,21 metal-oxide layers22,23, and metal oxides with carbon24,25. The great challenge for all of these materials solutions for LPS membranes is to combine at least two contrarian materials properties—efficient ion transport and mechanical robustness in one material or a coating5,15,26.
Among the latter, polymers with high shear modulus are necessary to suppress dendrite growth on lithium anodes27 while high mechanical strength (>98 MPa) and thermal stability (1 h @ 90C with <5% shrinkage) are essential for the longevity of the batteries in real-world conditions28, for instance, in electric vehicles. The prior experimental and computational data show this materials engineering task is difficult5,15,26,29,18 and requires a new approach in materials design.
Here, we show that the ion-selective membranes engineered using sequential deposition of nanofibers enable nearly complete prevention of the LPS diffusion from cathode to anode. The structural design for this membrane was informed by the prior studies of ion channels in cell membranes known for efficient ion transport combined with high ion selectivity30, as well as cartilage known for unique mechanical properties22.
The design of cartilage and several other biological tissues is based on highly interconnected nanofiber networks engendering their unique mechanical, adhesive, and transport properties23,31. Their topology reflected indistinct interconnectivity, and percolation enables effective stress transfer providing the toughness and flexibility, which can be replicated in composites made from aramid nanofibers (ANFs).
ANFs are the nanoscale version of Kevlar fibers32, and multifunctional ANF-based composites have been fabricated inspired by cartilage33. The direct analogy between the organization of nanofibers in ANF membranes and cartilage was recently demonstrated by evaluating their connectivity using Graph Theory34.
ANF-based composites can also be engineered into stratified membranes with nanoscale porosity (np-ANF) and charge sieving capabilities due to the spontaneous adsorption of LPS layer on np-ANF surface. Numerical simulations confirm that negatively charged single-nanometer pores of np-ANF strongly inhibit LPS shuttling while affording rapid transport of Li+ ions.
ANF-based composites have been utilized as electrode materials35,36 and separators37,38,39 in various energy storage systems40,41 including lithium sulfur batteries42 due to their high mechanical and thermal properties, similarly, np-ANF membranes also display a high Young’s modulus of E = 9.2 ± 0.5 GPa (55 times higher than CelgardTM 2400) and high thermal resistance (600 °C).
These properties make possible simultaneous suppression of lithium dendrites extending the life cycle of the Li–S batteries to 3500+ cycles at 3C. The combination of properties found in np-ANF resulted in high efficiency of LPS blocking and remarkable stability over long-term cycling, even for high-temperature environments.