Publication date: 10th April 2024
LiNi0.5Mn1.5O4 (LNMO) stands out as a highly promising spinel-type positive electrode material for Li-ion batteries (LIBs). Its high operating voltage (4.8 V vs Li+/Li, associated with the Ni4+/Ni2+ redox couple) provides high energy density, a crucial factor for the development of next-generation LIBs [1].
LNMO crystallizes with different extents of Ni/Mn ordering. In the fully ordered phase (P4332 space group), Ni and Mn cations occupy distinct crystallographic octahedral sites, whereas, in the fully disordered phase (Fd3m space group), Ni and Mn share the same crystallographic octahedral site. High temperatures (T > 750°C), required to obtain the disordered phase, lead at T ≥ 700°C to oxygen loss in LNMO and, as a result, to the formation of rock salt-based impurity and the reduction of Mn(IV) cations to Mn(III) [1,2].
The extent of Ni/Mn ordering significantly influences the performance of this positive electrode material. It is shown that the disordered LNMO exhibits superior performance at high C-rates and better cycling stability than the ordered LNMO [3,4]. Moreover, the disordered Mn-rich LNMO (LiNi0.44Mn1.56O4, Ni/Mn = 22/78) can be prepared without inducing oxygen deficiency, resulting in superior electrochemical performance [5]. This unexpected result encourages this systematic investigation of the impact of the Ni/Mn ratio on the phase equilibrium and electrochemical performance of LNMO. In this study, we performed ex situ and in situ Neutron powder diffraction under air and oxygen atmospheres to follow the Ni/Mn ordering process during the synthesis of LNMO with varying initial Ni/Mn of 25/75 (corresponding to stoichiometric LNMO), and Ni/Mn of 23/77 (Mn-rich LNMO).
Our findings reveal that for both Ni/Mn ratios, the Ni/Mn ordering process is disrupted by the oxygen release. In other words, the fully disordered LNMO phase cannot be prepared without rock salt-based impurities, caused by the oxygen release, for the selected Ni/Mn ratios. Our data also suggest that the extent of Ni/Mn ordering in LNMO is controlled solely by a concentration of Mn(III) in the crystal structure of LNMO, which can be adjusted by both the Ni/Mn ratio and partial oxygen pressure during the synthesis of LNMO.
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