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Improving performance of solid electrolytes

Source: Aramgutang at English Q52, Public Domain

Scientists have known for quite some time that when resistance to the flow of ions in solid electrolytes is reduced the efficiency of fuel cells and batteries can be improved. The only obstacle to achieving this goal is that so far, science has failed to discover which material properties are responsible for the resistance. It is common knowledge that solid electrolyte materials consist of many small crystalline regions, so-called grains, whose boundaries in the materials inhibit the flow of ions through the electrolyte, but what exactly causes this resistance has remained elusive.

Now (2020), scientists at the Argonne National Laboratory and the Northwestern University have researched grain boundaries in a solid electrolyte material. The study employed two powerful techniques — electron holography and atom probe tomography — so that scientists were able to observe the boundaries at an unprecedentedly small scale.

To explore the grain boundaries, the scientists hit a thin sample of the material with a beam of electrons which experienced a phase shift due to the presence of a local electric field in and around it. An external electric field then deflected a portion of the electrons passing through the sample and created an interference pattern.The large and varied electric fields they observed indicated the existence of previously undetected impurities in the material, which could explain the resistance.To further investigate the trace impurities, the scientists tried to determine the chemical identity of individual atoms at the grain boundaries. The electrolyte material in the study was made of ceria which was thought to be almost completely pure, but the tomography revealed small impurities including silicon and aluminium which had possibly been produced during material synthesis. They found that these impurities were the reason that had caused the electric fields across the boundaries to resist the flow of ions. The footprints the impurities left on the overall resistance of the electrolyte closely resembled what scientists would expect from thermodynamic effects alone.

Scientists have long sought to discover ways of increasing the overall efficiency of electrolyte materials. In 2015, scientists investigated the chemistry and interfacial properties of artificial SEI films created by in-situ reaction of a strong Lewis Acid AlI3 additive, Li metal, and aprotic liquid electrolytes. They found that these SEI films increased the interfacial stability of a Li metal anode. They further showed that the improvements came from at least three processes: (i) in-situ formation of Li-Al alloy, (ii) formation of a LiI salt layer on Li, and (iii) creation of a stable polymer thin film on the lithium metal anode.

In 2017, a research team developed a novel electrolyte capable of considerably increasing the energy efficiency and life of lithium-air batteries. Using this electrolyte reduced the excessive voltage that needed to be applied to the cathode of the battery during charging by more than 50% (from the conventional average of 1.6 V or higher to approximately 0.6 V). Also, the energy efficiency of the battery was greatly increased, from approximately 60% to 77%. The scientists were able to more than double the number of lifetime charge-discharge cycles of the battery from the conventional average of 20 cycles or fewer to over 50 by preventing lithium metal dendrite growth, which is considered to be a battery life reducing factor.

In 2019, a team of researchers synthesized magnesium tetrakis (hexafluoroisopropyloxy) borate Mg[B(hfip)4]2 and demonstrated its superior properties as a practical and efficient electrolyte for potential high-energy Mg batteries. Inspired by the promising results in Mg electrolytes, calcium tetrakis (hexafluoroisopropyloxy) borate Ca[B(hfip)4]2 was suggested as a potential electrolyte for room-temperature rechargeable Ca batteries. Ca[B(hfip)4]2 was synthesized by reacting Ca(BH4)2 with hexafluoroisopropanol in dimethoxy ethane (DME).

There are several advantages to analysing electrolytes with electron holography and atom probe tomography. Using these two techniques enabled scientists to create an image of the systems in 3D and gave a clearer picture of the properties of grain boundaries and how they affect resistance in electrolytes. Also, the findings could help to intentionally insert elements into the material that negate the effects of the impurities and lower the resistance at the grain boundaries.

The new insights into battery physics could help scientists to increase the efficiency of solid electrolytes. This in turn might help to improve the performance of many types of sustainable and renewable energy sources and bring society one step closer to the goal of zero-emission energy production.