McMaster University, Department of Chemistry and Chemical Biology, 1280 Main St. W. Hamilton, ON, Canada L8S 4M1.
Solid State Nucl Magn Reson. 2012 Apr;42:26-32. doi: 10.1016/j.ssnmr.2012.01.002. Epub 2012 Jan 23.
The effectiveness of two different selective inversion methods is investigated to determine timescales of Li ion mobility in paramagnetic Li intercalation materials. The first method is 1D exchange spectroscopy, which employs a 90°-τ(1)-90° sequence for selective inversion of a Li resonance undergoing site exchange. The experiment is most easily applied when the first delay period, τ(1), is set to the frequency difference between two resonances undergoing ion exchange. This enables the determination of ion hopping timescales for single exchange pair systems only. To measure ion dynamics in systems having more than one exchange process, a second selective inversion method was tested on two paramagnetic Li intercalation materials. This second technique, replaces the 90°-τ(1)-90° portion of 1D EXSY with a long, selective shaped pulse (SP). Two paramagnetic solid-state materials, which are both cathode materials for lithion ion batteries, were chosen as model compounds to test the effectiveness of both the selective inversion methods. The first compound, Li(2)VPO(4)F, was chosen as it hosts two Li sites with 1-exchange process. The second model compound is a 3-site, 3-exchange process system, Li(2)VOPO(4). For the 2-site material, Li(2)VPO(4)F, the timescales of the single A-B exchange process were found to be within error of one another regardless of the inversion method. For the 3 Li-site material Li(2)VOPO(4), the three exchange processes, AB, BC, and AC, were found to be on the millisecond timescale as revealed using the SP method. These timescales were determined over a variable temperature range where activation energies extended from 0.6 ± 0.1 eV up to 0.9 ± 0.2 eV.
研究了两种不同的选择性反转方法的有效性，以确定顺磁锂离子嵌入材料中锂离子迁移的时间尺度。第一种方法是一维交换光谱学，它采用 90°-τ(1)-90°序列对经历位交换的 Li 共振进行选择性反转。当第一个延迟时间 τ(1)设置为经历离子交换的两个共振之间的频率差时，实验最容易进行。这仅允许确定单交换对系统的离子跳跃时间尺度。为了在具有多个交换过程的系统中测量离子动力学，在两种顺磁锂离子嵌入材料上测试了第二种选择性反转方法。第二种技术，用长的选择性成形脉冲 (SP)代替 1D EXSY 的 90°-τ(1)-90°部分。选择两种顺磁固态材料作为模型化合物来测试这两种选择性反转方法的有效性，它们都是锂离子电池的正极材料。第一种化合物 Li(2)VPO(4)F 被选择，因为它容纳了具有 1 个交换过程的两个 Li 位。第二种模型化合物是具有 3 个位点和 3 个交换过程的 Li(2)VOPO(4)。对于 2 个 Li 位材料 Li(2)VPO(4)F，无论反转方法如何，单个 A-B 交换过程的时间尺度都彼此接近误差。对于具有 3 个 Li 位的 Li(2)VOPO(4)，使用 SP 方法发现三个交换过程 AB、BC 和 AC 都在毫秒时间尺度上。这些时间尺度是在可变温度范围内确定的，激活能从 0.6±0.1 eV 扩展到 0.9±0.2 eV。
