Chemically Induced Delithiation and Phase Change of Layered Lithium Rich Nickel Manganese Oxides

Abstract

Lithium-rich nickel manganese oxides (LLRNMOs) experience a large degree of irreversible charge capacity due to an electrochemically induced phase transformation [1–4]. This transformation has been linked to electrolyte decomposition accompanied by outgassing, which limits the commercial viability of this class of materials. Past studies documented that after treating cathodes of similar structure (such as layered LiNiO2, and NMC333) with acidic solutions, the resultant materials resembled untreated electrochemically cycled materials[5–8]. In this study, LLRNMOs were exposed to the acidic solutions of H3PO4, H2SO4, HCl, and HNO3. To the best of our knowledge, this study marks the first time that a lithium-excess cobalt-free cathode was exposed to acidic solutions. X-ray diffraction (XRD) of samples revealed that all acid treatments resulted in (003) shoulder peaks consistent with charged samples [9]. Relithiation of one of these samples resulted in the loss of this shoulder peak. By contrast, XRD of post-acid treatment samples saw patterns with preserved superlattice peaks between 20-25° 2θ, while electrochemically cycled samples do not[10]. Besides these commonalities, XRD patterns suggested the different acid treatments were inducing different phase changes in the materials. Most noticeably, H3PO4 treated samples’ XRD patterns suggested that treatment induced the formation of other secondary phases. X-ray fluorescence (XRF) of samples was conducted to determine the Mn/Ni ratio of samples, verifying that the varied acidic treatments are inducing equally varied phase transformations. Scanning electron microscopy (SEM) was conducted on the samples, and it was found that only the H3PO4-treated materials had altered morphologies. Our results demonstrate that these chemically induced phase transformations are a poor substitute for the electrochemically induced phase transformation. The possible reasons and consequences of this are discussed.[1] A. R. Armstrong, M. Holzapfel, P. Nová, C. S. Johnson, S.-H. Kang, M. M. Thackeray, P. G. Bruce, J. Am. Chem. Soc. 2006, 2006, 8694.[2] W. C. West, R. J. Staniewicz, C. Ma, J. Robak, J. Soler, M. C. Smart, B. V. Ratnakumar, J. Power Sources 2011, 196, 9696.[3] D. Eum, B. Kim, S. J. Kim, H. Park, J. Wu, S. P. Cho, G. Yoon, M. H. Lee, S. K. Jung, W. Yang, W. M. Seong, K. Ku, O. Tamwattana, S. K. Park, I. Hwang, K. Kang, Nat. Mater. 2020, 19, 419.[4] J. Zheng, M. Gu, A. Genc, J. Xiao, P. Xu, X. Chen, Z. Zhu, W. Zhao, L. Pullan, C. Wang, J. G. Zhang, Nano Lett. 2014, 14, 2628.[5] H. Arai, Y. Sakurai, J. Power Sources 1999, 81–82, 401.[6] J. Choi, E. Alvarez, T. A. Arunkumar, A. Manthiram, Electrochem. Solid-State Lett. 2006, 9, A241.[7] S.-H. Kang, C. S. Johnson, J. T. Vaughey, K. Amine, M. M. Thackeray, J. Electrochem. Soc. 2006, 153, A1186.[8] Y. Paik, C. P. Grey, C. S. Johnson, J. S. Kim, M. M. Thackeray, Chem. Mater. 2002, 14, 5109.[9] M. Jiang, B. Key, Y. S. Meng, C. P. Grey, Chem. Mater. 2009, 21, 2733.[10] S. Burke, J. F. Whitacre, J. Electrochem. Soc. 2020, 167, 160518.