Hikima Kazuhiro, Hinuma Yoyo, Shimizu Keisuke, Suzuki Kota, Taminato Sou, Hirayama Masaaki, Masuda Takuya, Tamura Kazuhisa, Kanno Ryoji
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan.
Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi, Aichi 441-8580, Japan.
ACS Appl Mater Interfaces. 2021 Feb 17;13(6):7650-7663. doi: 10.1021/acsami.0c18030. Epub 2021 Feb 3.
We evaluated the structural change of the cathode material LiMnO that was deposited as an epitaxial film with an (001) orientation in an all-solid-state battery. We developed an surface X-ray diffraction (XRD) technique, where X-rays are incident at a very low grazing angle of 0.1°. An X-ray with wavelength of 0.82518 Å penetrated an ∼2 μm-thick amorphous LiPO solid-state electrolyte and ∼1 μm-thick metal Li anode on the LiMnO cathode. Experiments revealed a structural change to a high-capacity (activated) phase that proceeded gradually and continuously with cycling. The activated phase barely showed any capacity fading. First-principles calculations suggested that the activated phase has O1 stacking, which is attained by first delithiating to an intermediate phase with O3 stacking and tetrahedral Li. This intermediate phase has a low Li migration barrier path in the [001] direction, but further delithiation causes an energetically favorable and irreversible transition to the O1 phase. We propose a mechanism of structural change with cycling: charging to a high voltage at a sufficiently low Li concentration typically induces irreversible transition to a phase detrimental to cycling that could, but not necessarily, be accompanied by the dissolution of Mn and/or the release of O into the electrolyte, while a gradual irreversible transition to an activated phase happens at a similar Li concentration under a lower voltage.
我们评估了全固态电池中以（001）取向外延生长的阴极材料LiMnO的结构变化。我们开发了一种表面X射线衍射（XRD）技术，其中X射线以0.1°的极低掠射角入射。波长为0.82518 Å的X射线穿透了LiMnO阴极上约2μm厚的非晶态LiPO固态电解质和约1μm厚的金属Li阳极。实验表明，随着循环，结构逐渐连续地转变为高容量（活化）相。活化相几乎没有容量衰减。第一性原理计算表明，活化相具有O1堆积，这是通过先脱锂至具有O3堆积和四面体Li的中间相来实现的。该中间相在[001]方向上具有较低的Li迁移势垒路径，但进一步脱锂会导致向O1相的能量有利且不可逆的转变。我们提出了一种循环结构变化的机制：在足够低的Li浓度下充电至高电压通常会诱导不可逆地转变为对循环不利的相，这可能但不一定伴随着Mn的溶解和/或O释放到电解质中，而在较低电压下类似的Li浓度下会逐渐不可逆地转变为活化相。
