Understanding the loss of electrochemical activity of nanosized LiMn2O4 particles: a combined experimental and ab initio DFT study
Understanding the loss of electrochemical activity of nanosized LiMn2O4 particles: a combined experimental and ab initio DFT study
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DOI：
10.1039/c8ta02703j
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发表时间：
2018-07
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通讯作者：
É. A. Velásquez;Daiane P. B. Silva;J. Falqueto;J. Mejía‐López;N. Bocchi;R. Río;J. Mazo‐Zuluaga;R. Rocha‐Filho;S. Biaggio
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文献类型：
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作者：
É. A. Velásquez;Daiane P. B. Silva;J. Falqueto;J. Mejía‐López;N. Bocchi;R. Río;J. Mazo‐Zuluaga;R. Rocha‐Filho;S. Biaggio
在锂离子电池（LIB）领域，一个普遍的期望是较小尺寸的电极材料导致更好的电池性能。在这里，我们表明，这种推理失败的纳米粒子的LiMn 2 O 4（LMO）低于一定的临界平均尺寸。为此，使用微波辅助水热方法和热处理，我们获得了具有真正小的平均尺寸（4、9和14 nm）的尖晶石LMO纳米颗粒。当通过伏安法和阻抗谱法评估这些LMO用于Li离子提取/插入过程的性能时，与预期相反，纳米颗粒尺寸的减小导致这些过程的渐进阻碍。使用从头算DFT方法，这种现象可以占与最小的LMO纳米粒子的电化学活性显着降低的理论解释。我们发现，随着纳米颗粒的平均尺寸从14 nm减小到1.3 nm，从3.93 V变化到4.64 V，与锂离子插入过程相关的氧化还原电压变得显著更高，从而阻碍了该过程。我们还可以表明，氧化还原电压的增加是由于在较小的LMO纳米微晶石内Li离子扩散的能垒从0.33 eV到至少0.59 eV的相应显著增加，这是由在这种纳米系统的表面上发生的原子弛豫（主要是氧原子）引起的。显然，当非常小的LMO纳米晶体打算用作LIB中的阴极材料时，平均纳米颗粒尺寸不应小于约15 nm的临界值;否则Li离子提取-插入变得显著受阻或不可行。
In the field of Li-ion batteries (LIBs), one prevalent expectation is that smaller sized electrode materials lead to better battery performances. Here, we show that this reasoning fails for nanoparticles of LiMn2O4 (LMO) below a certain critical average size. For this, using a microwave-assisted hydrothermal methodology and heat treatment, we obtained spinel LMO nanoparticles with genuinely small average sizes (4, 9, and 14 nm). When the performance of these LMOs for the Li-ion extraction/insertion processes was assessed by voltammetry and impedance spectroscopy, contrary to expectations, the decrease of the nanoparticle size caused a progressive hindrance of these processes. Using an ab initio DFT methodology, this phenomenon could be accounted for with a theoretical explanation for the significantly decreased electrochemical activity of the smallest LMO nanoparticles. We found that the redox voltage associated with the Li-ion insertion process becomes significantly higher as the average size of the nanoparticles decreases from 14 to 1.3 nm, changing from 3.93 to 4.64 V, thus hindering the process. We could also show that the redox voltage increase is due to a consequent significant increase of the energy barrier for Li-ion diffusion within the smaller LMO nanocrystallite, from 0.33 to at least 0.59 eV, caused by atomic relaxations (mainly of oxygen atoms) occurring on the surface of such a nanosystem. Clearly, when very small LMO nanocrystallites are intended for use as cathode materials in LIBs, the average nanoparticle size should not be smaller than the critical value of ∼15 nm; otherwise Li-ion extraction–insertion becomes significantly hindered or unfeasible.