In 2008, the U.S. National Academy of Engineering highlighted 14 grand challenges for the 21st century, many of which necessitate cutting-edge materials. Addressing this imperative, the VinFuture Foundation presented its October InnovaTalk Webinar on “Smart Materials for Energy Harvesting and Storage.”

The webinar was moderated by Professor Sir Kostya Novoselov, a 2010 Nobel Prize Laureate in Physics and a Member of the VinFuture Prize Council. Renowned for his contributions in graphene material research, condensed matter physics, mesoscopic physics, and nanotechnology, Professor Novoselov holds esteemed positions as the Tan Chin Tuan Centennial Professor at the National University of Singapore (Singapore) and the part-time Langworthy Professor of Physics and the Royal Society Research Professor at The University of Manchester (United Kingdom).

The distinguished speaker was Professor Antonio Castro Neto from the National University of Singapore. He is the Director of the Center for Advanced 2D Materials and co-leads the Institute for Functional Intelligent Materials. During the webinar, he unveiled insights into the horizon of battery technology, specifically focusing on solid-state batteries and their integral role in fostering a sustainable energy future.

Dr. Hieu Nguyen from the Australian National University, also a recipient of the 2021 Vietnam Golden Globe Award in Science and Technology, attended the webinar as a special guest. Representing the Vietnamese energy community, he introduced a novel perspective on solar photovoltaics.

Understanding ionic mobility in solid-state battery

Prof. Antonio Neto, renowned for his expertise in two-dimensional materials, commenced with a concise overview of the battery’s historical and foundational principles, highlighting the distinctions between liquid-state and solid-state batteries. The global trend, he noted, is shifting towards solid electrolytes, primarily driven by the safety hazards posed by their liquid counterpart. These hazards include overheating, gas production, and, in rare instances, battery explosions.

While both liquid and solid-state batteries comprise a cathode, an anode, and an electrolyte, the electrolyte of solid-state battery is solid, which is able to transport ions without conducting electrons. This unique characteristic positions it as an electronic insulator but an ionic conductor. Another important distinguishing characteristic of solid electrolyte from the liquid electrolyte is the role of non-symmetry. Fluid mediums, whether liquids or gases, exhibit inherent symmetries, both translational and rotational.

However, the rigidity of solids, dictated by their crystalline matrix, disrupts these symmetries and influences ionic mobility in solids. Thus, to truly grasp and innovate within solid-state battery technology, Prof. Neto emphasized that one must transition from traditional electrochemistry concepts, which focus on particle movement in liquids, to the pelicular principles of Solid-state Physics.

Delving deeper into the molecular structure of solid-state batteries, Prof. Neto highlighted the challenge of understanding ion movements, especially Lithium, within solids. It’s perplexing to imagine how these particles, given their size, navigate the restrictive confines of a crystal. One important piece of evidence to elucidate this phenomenon is the exponential correlation between conductivity and temperature observed in many solid electrolyte studies. As Lithium atoms travel within a crystal, they leap from one potential minimum to another by surpassing saddle points.

To perform such a leap, Lithium ions must acquire sufficient thermal energy to navigate these hurdles, resulting in the observed exponential temperature dependence. Beyond temperature, the identification of other determinants of ionic mobility within crystals is still a paramount challenge in the field, and to answer this question, it’s crucial to ascertain whether classical or quantum physics governs their movement and if electrons play a role in their transit.

In his team’s pioneering research, Prof. Neto drew parallels between ionic mobility in solid-state batteries and electron mobility in semiconductors. Their landmark paper, “Microscopic Theory of Ionic Motion in Solids,” seeks to discern the forces influencing ionic mobility. This inquiry birthed the Rodin formula, revealing the pivotal role of the topology and geometry of the crystal. The formula revealed three important determinants of ion mobility, including density – a higher crystal density correlates with increased ion mobility; sound velocities – materials that efficiently transmit sound have better ionic conductivity; and the Hessian Matrix – the topology of the crystal structure is fundamental in determining ionic mobility.

Understanding the crucial role of the Hessian Matrix, Prof. Neto’s team has expedited their Hessian matrix calculations using advanced computational techniques, like density functional theory and machine learning for material and battery applications. While early results from these efforts showed substantial improvement compared to the experimental data, many challenges persist in research for polycrystalline material structures.

Concluding his presentation, Prof. Neto articulated a vision: the evolution of solid-state batteries is intricately linked to ionic mobility. Unlike their liquid counterparts, solid-state battery efficiency is determined by the crystal’s geometry and topological structure. Mirroring past semiconductor breakthroughs, the next leap for solid-state batteries depends on crafting novel, structurally robust, and atomically smooth materials.

Advancing solar cell efficiency: Insights from research at the Australian National University (ANU)

At the heart of a solar panel lies its fundamental unit: the solar cell. These cells are interconnected within a panel to augment voltage and current. While the landscape of solar cell research has seen the emergence of over ten distinctive technologies over three decades, the relentless pursuit of peak efficiency endures. The reason for this drive is straightforward—enhanced efficiency translates to reduced production costs. Dr. Hieu Nguyen, in this webinar, delineated the extensive work spearheaded by his team at ANU, encompassing investigations into material attributes, technical advancements, and material engineering tailored for existing and future technologies.

A pivotal aspect in solar cell fabrication, as Dr. Nguyen elucidated, is the material’s light absorption capacity and the required thickness to capture all incident sunlight. A key metric in this process is the absorption coefficient, indicating a material’s prowess in light absorption. As part of their research effort, Dr. Nguyen’s team measures the absorption coefficient for different materials, helping them to characterize materials’ sunlight absorption potential.

Delving deeper, Dr. Nguyen emphasized the importance of discerning both light absorption and emission which materials are promising for solar cell fabrication. By capturing both the absorbed and re-emitted light from a material, one can ascertain the quantum of electron generation, laying the groundwork for predicting the solar cell’s apex voltage. Noteworthy in this context are perovskites and two-dimensional materials. Owing to their minimal thickness, two-dimensional materials have exhibited promise in generating high voltages in solar cell fabrication.

Yet, the journey is riddled with challenges. Dr. Nguyen spotlighted the defects and impurities within materials, which hinder efficiency. In silicon, for example, impurities are more detrimental than dislocations. To help resolve this challenge, Dr. Nguyen’s team developed the Luminescence imaging technique to visualize and analyze these defects. Contrary to conventional imaging that captures reflected light, this technique captured the light emitted from the material itself. Its power was evident when deployed on silicon wafers, revealing an array of defects and impurities. Similarly, in perovskite solar cells, it can flag the degradation along the cell edges, a phenomenon exacerbated by air exposure.

In addition to the diagnostic technique, Dr. Nguyen’s team at ANU has optimized the “passivating contacts” tailored for silicon solar cells. These serve a dual purpose: shielding the solar cell whilst conducting electricity. An innovation in this subject involves the introduction of atomic hydrogen into these structures to neutralize defects, thereby boosting performance. Moreover, for silicon wafers containing impurities and defects, the team has explored methodologies to remove or neutralize these issues. One such technique was using a turbo diffusion process succeeded by a hydrogenation procedure.

Concluding his presentation, Dr. Hieu Nguyen emphasized that solar cell research is an intricate tapestry. At ANU, the team focuses on the fabrication and delves into profound material property studies, diagnostic techniques, and engineering material for improved efficiency. Through these efforts, their ultimate goal is to augment the solar photovoltaic efficacy for universal adoption.