The Joint Center for Energy Storage Research (JCESR) , Pacific Northwest National Laboratory , Richland , Washington 99352 , United States.
ACS Appl Mater Interfaces. 2019 Oct 23;11(42):38689-38696. doi: 10.1021/acsami.9b11888. Epub 2019 Sep 27.
Mg batteries have been proposed as potential alternatives to lithium-ion batteries because of their lower cost, higher safety, and enhanced charge density. However, the Mg metal readily oxidizes when exposed to an oxidizer to form a thin MgO passivation surface layer that blocks the transport of Mg across the solid electrode-electrolyte interface (SEI). In this work, the adsorption and thermal decomposition of diglyme (G2) and electrolytes containing Mg(TFSI) in G2 on 10 nm-sized MgO particles are evaluated by a combination of in situ C single-pulse, surface-sensitive H-C cross-polarization (CP) magic-angle spinning (MAS) nuclear magnetic resonance, and quantum chemistry calculations. At 180 °C, neat G2 decomposes on MgO to form surface-adsorbed -OCH groups that are captured as a distinctive peak located at about 50 ppm in the CP/MAS spectrum. At low Mg(TFSI) salt concentration, the main solvation structure in this electrolyte is solvent-separated ion pairs without extensive Mg-TFSI contact ion pairs. G2, likely including a small amount of G2-solvated Mg, adsorbs onto the MgO surface. At high Mg(TFSI) salt concentrations, contact ion pairs between Mg and TFSI are formed extensively in the solution with the first solvation shell containing one pair of Mg-TFSI and two G2 molecules and the second solvation shell containing up to six G2 molecules, namely, MgTFSI(G2)(G2). In the presence of MgO, MgTFSI(G2)(G2) adsorbs onto the MgO surface. At 180 °C, the MgO surface stimulates a desolvation process converting MgTFSI(G2)(G2) to MgTFSI(G2) and releasing G2 molecules from the second solvation shell of the MgTFSI(G2)(G2) cluster into the solution. MgTFSI(G2) and MgTFSI(G2)(G2) tightly adsorb onto the MgO surface and are observed by H-C CP/MAS experiments. The results contained herein show that electrolyte composition has a directing role in the species present on the electrode surface, which has implications on the structures and constituents of the solid-electrolyte interface on working electrodes and can be used to better understand its formation and the failure modes of batteries.
镁电池因其成本低、安全性高和充电密度高而被认为是锂离子电池的潜在替代品。然而，镁金属在暴露于氧化剂时容易氧化，形成一层薄的 MgO 钝化表面层，阻止 Mg 通过固-固电极-电解质界面（SEI）的传输。在这项工作中，通过原位 C 单脉冲、表面敏感的 H-C 交叉极化（CP）魔角旋转（MAS）核磁共振和量子化学计算相结合，评估了二甘醇（G2）和含 Mg(TFSI)的电解质在 10nm 大小的 MgO 颗粒上的吸附和热分解。在 180°C 时，纯 G2 在 MgO 上分解，形成表面吸附的-OCH 基团，在 CP/MAS 谱中位于约 50 ppm 的特征峰处被捕获。在低 Mg(TFSI)盐浓度下，这种电解质中的主要溶剂化结构是溶剂分离的离子对，而不是广泛的 Mg-TFSI 接触离子对。G2 可能包括少量 G2 溶剂化的 Mg，吸附在 MgO 表面上。在高 Mg(TFSI)盐浓度下，Mg 和 TFSI 之间形成广泛的接触离子对，在溶液中形成第一溶剂化壳层包含一对 Mg-TFSI 和两个 G2 分子，第二溶剂化壳层包含多达六个 G2 分子，即 MgTFSI(G2)(G2)。在 MgO 存在的情况下，MgTFSI(G2)(G2)吸附在 MgO 表面上。在 180°C 时，MgO 表面刺激去溶剂化过程，将 MgTFSI(G2)(G2)转化为 MgTFSI(G2)，并将 G2 分子从 MgTFSI(G2)(G2)簇的第二溶剂化壳层释放到溶液中。MgTFSI(G2)和 MgTFSI(G2)(G2)紧密吸附在 MgO 表面上，并通过 H-C CP/MAS 实验观察到。本文的结果表明，电解质组成在电极表面存在的物种中具有定向作用，这对工作电极上的固体-电解质界面的结构和组成有影响，并可用于更好地理解其形成和电池的失效模式。
