Spin-Transfer Pathways in Paramagnetic Lithium Transition-Metal Phosphates from Combined Broadband Isotropic Solid-State MAS NMR Spectroscopy and DFT Calculations

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• 作者：Rapha毛le J. Cl茅ment ; Andrew J. Pell ; Derek S. Middlemiss ; Fiona C. Strobridge ; Joel K. Miller ; M. Stanley Whittingham ; Lyndon Emsley ; Clare P. Grey ; Guido Pintacuda
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：October 17, 2012
• 年：2012
• 卷：134
• 期：41
• 页码：17178-17185
• 全文大小：474K
• 年卷期：v.134,no.41(October 17, 2012)
• ISSN：1520-5126

文摘

Substituted lithium transition-metal (TM) phosphate LiFe_xMn_1鈥?i>xPO₄ materials with olivine-type structures are among the most promising next generation lithium ion battery cathodes. However, a complete atomic-level description of the structure of such phases is not yet available. Here, a combined experimental and theoretical approach to the detailed assignment of the ³¹P NMR spectra of the LiFe_xMn_1鈥?i>xPO₄ (x = 0, 0.25, 0.5, 0.75, 1) pure and mixed TM phosphates is developed and applied. Key to the present work is the development of a new NMR experiment enabling the characterization of complex paramagnetic materials via the complete separation of the individual isotropic chemical shifts, along with solid-state hybrid DFT calculations providing the separate hyperfine contributions of all distinct Mn鈥揙鈥揚 and Fe鈥揙鈥揚 bond pathways. The NMR experiment, referred to as aMAT, makes use of short high-powered adiabatic pulses (SHAPs), which can achieve 100% inversion over a range of isotropic shifts on the order of 1 MHz and with anisotropies greater than 100 kHz. In addition to complete spectral assignments of the mixed phases, the present study provides a detailed insight into the differences in electronic structure driving the variations in hyperfine parameters across the range of materials. A simple model delimiting the effects of distortions due to Mn/Fe substitution is also proposed and applied. The combined approach has clear future applications to TM-bearing battery cathode phases in particular and for the understanding of complex paramagnetic phases in general.