Insights into the Nature and Evolution upon Electrochemical Cycling of Planar Defects in the β-NaMnO2 Na-Ion Battery Cathode: An NMR and First-Principles Density Functional Theory Approach
Insights into the Nature and Evolution upon Electrochemical Cycling of Planar Defects in the β-NaMnO2 Na-Ion Battery Cathode: An NMR and First-Principles Density Functional Theory Approach
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DOI：
10.1021/acs.chemmater.6b03074
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发表时间：
2016-11-22
影响因子：
8.6
通讯作者：
Grey, Clare P.
中科院分区：
文献类型：
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作者：
Clement, Raphaele J.;Middlemiss, Derek S.;Grey, Clare P.
beta-NaMnO 2，是一种高容量钠离子电池阴极，提供ca.当以C/20的速率循环时，190 mAh/g的可逆容量。然而，在100次循环后仅保留70%的初始可逆容量。我们进行了结合固态Na-23 NMR和第一性原理DFT研究β-NaMnO 2在电化学循环后的结构演变。合成的结构包含平面缺陷，被识别为NaMnO 2的α和β形式的纳米畴之间的双平面。GGA+U计算表明，这两种多晶型的形成能在5毫电子伏每公式单位，和相混合物是可能在任何NaMnO 2样品在室温下。Na-23 NMR表明，在合成的材料中，65.5%的Na在β-NaMnO 2结构域中，2.5%在α-NaMnO 2结构域中，并且32%接近孪晶边界。通过NMR观察到在充电开始时和放电结束时的两相反应，与电化学曲线中2.6-2.7 V处的恒定电压平台一致。GGA+U计算的Na脱嵌电位表明，Na提取发生在α-类域，然后在β-类域，最后接近孪晶边界。Na-23 NMR表明，在α-NaMnO 2型网站的Na的比例增加到11%，五个周期后，这表明发生结构重排，导致孪晶边界分离较大的α-NaMnO 2域的主要β-NaMnO 2相。
beta-NaMnO2, is a high-capacity Na-ion battery cathode, delivering ca. 190 mAh/g of reversible capacity when cycled at a rate of C/20. Yet, only 70% of the initial reversible capacity is retained after 100 cycles. We carry out a combined solid-state Na-23 NMR and first-principles DFT study of the evolution of the structure of beta-NaMnO2 upon electrochemical cycling. The as synthesized structure contains planar defects identified as twin planes between nanodomains of the alpha and beta forms of NaMnO2. GGA+U calculations reveal that the formation energies of the two polymorphs are within 5 meV per formula unit, and a phase mixture is likely in any NaMnO2 sample at room temperature. Na-23 NMR indicates that 65.5% of Na is in beta-NaMnO2 domains, 2.5% is in alpha-NaMnO2 domains, and 32% is close to a twin boundary in the as-synthesized material. A two-phase reaction at the beginning of charge and at the end of discharge is observed by NMR, consistent with the constant voltage plateau at 2.6-2.7 V in the electrochemical profile. GGA+U computations of Na deintercalation potentials reveal that Na extraction occurs first in alpha-like domains, then in beta-like domains, and finally close to twin boundaries. Na-23 NMR indicates that the proportion of Na in alpha-NaMnO2-type sites increases to 11% after five cycles, suggesting that structural rearrangements occur, leading to twin boundaries separating larger alpha-NaMnO2 domains from the major beta-NaMnO2 phase.