Innovation and Technology

Dr Zhang Biao
Assistant Professor, Department of Applied Physics
Outstanding Young Researcher, Faculty Awards 2021 (Research and Scholarly Activities)

Our research group is committed to designing novel electrode materials for cost-effective and high-energy rechargeable batteries and boosting their power and lifespan through electrode/ electrolyte interface regulation.

Formidable challenges to sustainable development necessitate a technological revolution in energy harvesting and storage. Substantial progress has been made in gathering energy from renewable resources such as solar and wind, which are distributed unevenly and active only intermittently. The efficient use of these clean energy sources demands high-performance energy storage devices, among which batteries are of particular interest owing to their attractive energy density. While Li-ion batteries have conquered the portable electronic market, rechargeable batteries based on Na⁺, K⁺ and Zn²⁺ have received growing attention because of their potential benefits in terms of cost and sustainability.

The natural abundance of Na and K sources makes Na- and K-ion batteries the most attractive alternatives to complement Li-ion batteries for large-scale energy storage. The development of advanced Na- and K-ion batteries is restricted by the lack of appropriate electrode materials for hosting the reversible insertion/ extraction of Na and K ions. In the past five years, we have prepared a number of electrode materials and optimised their performance through microstructure design, including i) ultra-stable polyanionic cathodes such as K₃V₂(PO₄)₂F₃, which demonstrates a minor volume change upon charge/ discharge due to its 3-dimensional open framework; and ii) highly stable carbon anodes. We have synthesised several hard and soft carbon anodes from low-cost precursors, such as lignin. The surface functionalities and atomic structures are tailored to explore the structure/ property relationship and boost capacity. Recently, we have extended the electrode design to aqueous Zn-ion batteries, which use water as the solvent in the electrolyte in place of the flammable organic solvents used in Li-ion batteries, thus significantly enhancing battery safety. Reversible Zn metal anodes with an ultra-long lifespan of over 2,000 cycles have been achieved through electrode architecture design.

Battery advancement relies on materials innovation in both the electrode and the electrolyte. We build electrochemically and mechanically stable solid electrolyte interphases (SEIs) through novel electrolytes, thus stabilising the high-capacity anodes consisting of microsized particles. Severe issues, such as high fabrication cost and low volumetric energy density, complicate the practical application of the widely adopted nanostructured electrodes. As such, we deviate from these widespread strategies and turn to regulate SEIs. The SEIs are formed and covered on the surface of anodes in the first cycle to prevent continuous electrolyte decomposition. It is generally believed that SEIs are fragile and should be protected through delicate electrode control. We demonstrate that the SEIs could be an essential tool for maintaining structural integrity if well designed. Our group has developed several electrolyte formulations to tailor the microstructure of SEIs, which essentially improves the elasticity to accommodate the large deformation of high-capacity alloy and metal anodes. We also reveal the structure–mechanical stability correlation through cutting-edge characterisation techniques.

Energy usage and storage are among the most critical challenges facing our society. The Hong Kong Special Administrative Region Government has placed particular emphasis on these issues, and we are grateful for the continuous support of the Research Grants Council over the last 5 years. The battery is a complex system that demands collective contributions from different fields for its improvement and refinement. We collaborate closely with interdisciplinary experts, including electrochemists and scientists in microscopy and solid mechanics, to enhance our fundamental understanding and facilitate practical implementation. Through these efforts, we are confident that we will reach new heights in the development of sustainable and high-energy rechargeable batteries.

材料創新推動可持續高能量電池發展

張標博士
應用物理學系助理教授
學院傑出表現/ 成就獎2021（研究及學術活動）傑出青年研究員

我們的研究小組致力於設計新型電極材料，使高能量可充電電池更具成本效益，並通過電極／電解質介面調控提高電池功率、延長電池壽命。

能源採集和儲存技術革新是應對可持續發展挑戰的關鍵。從太陽能和風能等可再生資源中收集能源已取得了重大進展。然而，這些清潔能源分佈不均且只能間歇發電，要實現其有效利用有賴於高性能的能源儲存裝置。電池因為擁有較高能量密度而備受青睞。雖然鋰離子電池已經占領了便攜式電子產品市場，但以鈉、鉀和鋅離子為基礎的可充電電池具有成本低廉和可持續發展的潛在優勢，受到越來越多的關注。

自然界擁有豐富的鈉、鉀資源，所以在大規模能源儲存上，鈉離子和鉀離子電池可作為鋰離子電池最具潛力的補充。 先進鈉離子和鉀離子電池的發展主要受限於缺乏合適的電極材料，來承載鈉、鉀離子的可逆嵌入／脫嵌。我們在過去五年已製備了多種電極材料，並通過微結構設計優化其性能，包括i) 超穩定聚陰離子陰極，例如 K₃V₂(PO₄)₂F₃，其三維開放式框架在充放電時僅有微小的體積變化；ii) 高度穩定的碳陽極。我們利用木質素等低成本前驅體合成了幾種硬碳和軟碳陽極，並通過調控表面功能和原子結構，探索了結構與性能之間的關係並提高容量。最近，我們將電極設計擴展到水系鋅離子電池，這種電池以水作為電解質溶劑，以代替鋰離子電池中所用的易燃有機溶劑，顯著提高了電池安全性。通過電極結構優化，實現了具有2000次循環以上超長充放電壽命的可逆鋅金屬陽極。

電池發展有賴於電極和電解液材料的協同創新。我們採用新型電解液構建電化學和機械性穩定的固態電解質介面（SEI），從而穩定由微米尺寸顆粒組成的高容量陽極。被廣泛採用的納米結構電極由於製造成本高昂、體積能量密度低等問題嚴重阻礙了其實際應用。因此，我們摒棄了這種普遍策略，轉而調控SEI。SEI在第一次循環中形成，並覆蓋在陽極表面，以防止電解液持續分解。人們普遍認為SEI是脆弱的，需要通過精細的電極調控來保護。我們提出並證明通過合理設計，可製備高強度SEI，更可以成為保持電極結構完整性的關鍵部分。本小組已開發了幾種電解液配方來調控SEI的結構，有效提高了SEI的彈性以適應高容量合金和金屬陽極在循環過程中的變形。此外，我們還通過尖端的表徵技術揭示了SEI結構與機械穩定性之間的關聯。

能源利用和儲存是社會所面對的最嚴峻挑戰之一。香港特別行政區政府對此特別重視，我們感謝研資局過去五年的持續支持。電池是複雜的系統，需要匯聚不同領域學者攜手合作。我們與電化學、顯微學和固體力學等跨學科專家緊密協作，從而加強基礎研究，並促進其實際應用。通過各方共同合作，我們深信能在開發可持續和高能量可充電電池上再創高峰。