Banday Azeem, Shahid Raza, Gupta Mukul, Murugavel Sevi
Department of Physics & Astrophysics, University of Delhi Delhi-110007 India
Department of Physics, Jamia Millia Islamia New Delhi-110025 India.
RSC Adv. 2023 Jun 16;13(27):18332-18346. doi: 10.1039/d3ra01366a. eCollection 2023 Jun 15.
The development of better electrode materials for lithium-ion batteries has been intensively investigated both due to their fundamental scientific aspects as well as their usefulness in technological applications. The present technological development of rechargeable batteries is hindered by fundamental challenges, such as low energy and power density, short lifespan, and sluggish charge transport kinetics. Among the various anode materials proposed, heterosite FePO (h-FP) has been found to intercalate lithium and sodium ion hosts to obtain novel rechargeable batteries. The h-FP has been obtained the delithiation of triphylite LiFePO (LFP), and its structural and electronic properties have been investigated with different crystallite sizes. The synchrotron XRD measurements followed by Rietveld refinement analysis reveal lattice expansion upon the reduction of crystallite size of h-FP. In addition, the decrease in the crystallite size enhances surface energy contributions, thereby creating more oxygen vacancies up to 2% for 21 nm crystallite size. The expansion in the lattice parameters is reflected in the vibrational properties of the h-FP structure, where the red-shift has been observed in the characteristic modes upon the reduction of crystallite size. The local environment of the transition metal ion and its bonding characteristics have been elucidated through soft X-ray absorption spectroscopy (XAS) with the effect of crystallite size. XAS unequivocally reveals the valence state of iron 3d electrons near the Fermi level, which is susceptible to local lattice distortion and uncovers the detailed information on the evolution of electronic states with crystallite size. The observed local lattice distortion has been considered to be as a result of the decrease in the level of covalency between the Fe-3d and O-2p states. Further, we demonstrate the structural advantages of nanosized h-FP on the transport properties, where an enhancement in the polaronic conductivity with decreasing crystallite size has been observed. The polaronic conduction mechanism has been analyzed and discussed on the basis of the Mott model of polaron conduction along with an insightful analysis on the role of the electronic structure. The present study provides spectroscopic results on the anode material that reveal the evolution of electronic states for fingerprinting, understanding, and optimizing it for advanced rechargeable battery operations.
由于锂离子电池电极材料在基础科学方面的重要性以及在技术应用中的实用性，人们对其进行了深入研究。目前，可充电电池的技术发展受到一些基本挑战的阻碍，如能量和功率密度低、寿命短以及电荷传输动力学缓慢等。在提出的各种负极材料中，磷铁矿FePO（h - FP）已被发现可嵌入锂和钠离子主体中，以获得新型可充电电池。h - FP是通过对锂铁磷矿LiFePO（LFP）进行脱锂得到的，并且已经对其不同微晶尺寸下的结构和电子性质进行了研究。同步辐射XRD测量以及随后的Rietveld精修分析表明，随着h - FP微晶尺寸的减小，晶格发生膨胀。此外，微晶尺寸的减小增强了表面能贡献，从而在21 nm微晶尺寸时产生高达2%的更多氧空位。晶格参数的膨胀反映在h - FP结构的振动特性中，在微晶尺寸减小时，其特征模式出现了红移。通过软X射线吸收光谱（XAS）并结合微晶尺寸的影响，阐明了过渡金属离子的局部环境及其键合特性。XAS明确揭示了费米能级附近铁3d电子的价态，该价态易受局部晶格畸变影响，并揭示了电子态随微晶尺寸演变的详细信息。观察到的局部晶格畸变被认为是Fe - 3d和O - 2p态之间共价性水平降低的结果。此外，我们展示了纳米尺寸h - FP在传输性质方面的结构优势，观察到随着微晶尺寸减小，极化子电导率增强。基于极化子传导的莫特模型以及对电子结构作用的深入分析，对极化子传导机制进行了分析和讨论。本研究提供了关于负极材料的光谱结果，揭示了电子态的演变，以便对其进行指纹识别、理解并为先进的可充电电池操作进行优化。
