Solid-state batteries
IN THE search for room-temperature superconductors, scientists are constantly on the lookout for new materials that show superconductivity at temperatures and pres- sures that are easily achieved for real-life applications. Two independent groups of researchers, one Chinese and the other Chinese-plus-american, have discovered such a class of materials in clathrate metal hydrides. Both teams experimentally found a superconducting phase in clathrate calcium hydride (CAH6) at over 200 degrees kelvin (zero degrees C=273 degrees K) and under very high pressures.
The first group, from Jilin University, China, led by Liang Ma, published its results in the April 20 issue of Physical Review Letters. The other group, led by Zhiwen Li of the Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, reported its findings in the May 23 issue of Nature Communications.
This property in metal hydrides has been demonstrated earlier. These are “superhydrides” of either rare-earth elements or actinides. In 2019, one such compound, lanthanum superhydride (LAH10), was shown to supercon- duct at temperatures of up to 260 K but only when subjected to pressures greater than 170 gigapascal (GPA), about 1.7 million atmospheres.
The elements in superhydrides act as anchor sites that hold the compounds’ many hydrogen atoms in an arrange- ment known as clathrate, a cage-like structure. This led researchers to wonder if hydrides containing main-group elements or other transition metals could also form clath- rate structures. In 2012, it was predicted that this material could have superconducting phases at temperatures of 220K or more at a pressure of about 150GPA.
Earlier attempts to synthesise the compound failed because of high reactivity between calcium and hydrogen, which resulted in hydrides with a low hydrogen content. Both the groups now adopted almost similar techniques by using ammonia borane (BH3NH3) as the hydrogen source, which allowed them to synthesise the compound by direct reaction between calcium and hydrogen at a high temperat- ure and pressure.
SOLID-STATE batteries are soon expected to replace lithium-ion batteries, the staple of smartphones and laptops. But on repeated or excessive use, these next-gen batteries develop thin filaments called dendrites, which can shortcircuit the batteries and render them useless. Researchers at the Indian Institute of Science (IISC), Bengaluru, in collaboration with Carnegie Mellon University, Pittsburg, Pennsylvania, have devised a novel strategy to make them last longer and charge faster. The study was published in the June 2 issue of Nature Materials.
Conventional lithium-ion batteries contain a liquid electrolyte (typically made of a lithium salt dissolved in an organic solvent) sandwiched between a positively charged electrode (cathode) made of a lithium compound of a transition metal (such as iron and cobalt) oxide and a negatively charged electrode (anode) made of graphite. When the battery is being charged or is discharging, lithium ions shuttle between the two electrodes. One major issue is that the liquid electrolyte can catch fire at high temperatures.
Solid-state batteries use a solid ceramic electrolyte instead of liquid and a metallic anode made of lithium instead of graphite. In fact, ceramic electrolytes perform better at high temperatures. Lithium is also lighter and stores more charge than graphite, which can significantly cut down the battery cost.
“Unfortunately, when you add lithium, it forms these filaments that grow into the solid electrolyte, and short out the anode and cathode,” the IISC press release quotes Naga Phani Aetukuri of the Solid State and Structural Chemistry Unit of the IISC, and corresponding author of the study.
To investigate this, dendrite formation was artificially induced by repeatedly charging hundreds of battery cells. Examining thin sections sliced out from the lithium-electrolyte interface under a scanning electron microscope, the team found that long before the dendrites formed, microscopic voids developed in the lithium anode during discharge. The team computed that the currents concentrated at the edges of these microscopic voids were about 10,000 times larger than the average currents across the battery cell, which was likely creating stress on the solid electrolyte and accelerating dendrite formation. To ensure that voids did not form, the researchers introduced an ultra thin layer of a refractory metal—metals that are resistant to heat and wear such as tungsten and molybdenum—between the lithium anode and solid electrolyte. These metals do not alloy with lithium. “The refractory metal layer shields the solid electrolyte from the stress and redistributes the current to an extent,” said Aetukuri.
The computational analysis carried out by the Carnegie Mellon collaborators showed that this indeed delayed the growth of microscopic lithium voids. The findings are a critical step forward in realising practical and commercial solid-state batteries.