Speaker
Description
$LiNi_{0.5}Mn_{1.5}O_4$ (LNMO) emerges as a highly promising material for positive electrodes in Li-ion batteries (LIBs), owing to its elevated operating voltage (4.8 V vs $Li^+/Li$) attributed to the $Ni^{4+}/Ni^{3+}/Ni^{2+}$ redox couples, crucial for development LIBs$^1$.
LNMO crystallizes with different extents of $Ni/Mn$ ordering; in the fully ordered phase ($P4_332$ space group), Ni and Mn ions occupy $4a$ and $12d$ octahedral sites respectively, whereas in the fully disordered phase ($Fd\bar{3}m$ space group), they share the same $16d$ site. Elevated temperatures (>750°C), required to obtain the disordered phase, induce oxygen loss, forming rock salt-based impurities and reducing $Mn(IV)$ to $Mn(III)$.
The degree of $Ni/Mn$ ordering significantly impacts LNMO's performance; disordered LNMO demonstrates enhanced performance at high C-rates and improved cycling stability compared to ordered LNMO$^{2,3}$. Additionally, disordered Mn-rich LNMO ($Ni/Mn = 22/78$) can be prepared without oxygen deficiency, leading to superior electrochemical performance$^4$. This unexpected finding motivates a systematic exploration of the Ni/Mn ratio's effect on LNMO's phase equilibrium and electrochemical behavior. In this investigation, $ex\:situ$ and $in\:situ$ Neutron and Synchrotron X-ray powder diffraction under air and oxygen atmospheres were conducted to monitor the $Ni/Mn$ ordering process during LNMO synthesis with varying initial $Ni/Mn$ ratios (25/75 for stoichiometric LNMO and 23/77 for Mn-rich LNMO).
Our results 1 demonstrate that oxygen release disrupts the $Ni/Mn$ ordering process for both $Ni/Mn$ ratios, indicating that fully disordered LNMO cannot be achieved without rock salt-based impurities. Moreover, our data suggest that the extent of $Ni/Mn$ ordering in LNMO is governed primarily by the concentration of $Mn(III)$ in LNMO's crystal structure, which can be controlled by adjusting the $Ni/Mn$ ratio and partial oxygen pressure during synthesis. In addition, Mn-rich LNMO samples exhibit superior discharge capacity, C-rate capability, and capacity retention, indicating an important role of Mn excess in enhancing the electrochemical performance of LNMO$^5$.
References
(1) Liang, G.; K. Peterson, V.; Wai See, K.; Guo, Z.; Kong Pang, W. Developing High-Voltage Spinel $LiNi_{0.5}Mn_{1.5}O_4$ Cathodes for High-Energy-Density Lithium-Ion Batteries: Current Achievements and Future Prospects. J. Mater. Chem. A 2020, 8 (31), 15373–15398. https://doi.org/10.1039/D0TA02812F.
(2) Casas-Cabanas, M.; Kim, C.; Rodríguez-Carvajal, J.; Cabana, J. Atomic Defects during Ordering Transitions in $LiNi_{0.5}Mn_{1.5}O_4$ and Their Relationship with Electrochemical Properties. J. Mater. Chem. A 2016, 4 (21), 8255–8262. https://doi.org/10.1039/C6TA00424E.
(3) Kim, J.-H.; Huq, A.; Chi, M.; Pieczonka, N. P. W.; Lee, E.; Bridges, C. A.; Tessema, M. M.; Manthiram, A.; Persson, K. A.; Powell, B. R. Integrated Nano-Domains of Disordered and Ordered Spinel Phases in $LiNi_{0.5}Mn_{1.5}O_4$ for Li-Ion Batteries. Chem. Mater. 2014, 26 (15), 4377–4386. https://doi.org/10.1021/cm501203r.
(4) Aktekin, B.; Valvo, M.; Smith, R. I.; Sørby, M. H.; Lodi Marzano, F.; Zipprich, W.; Brandell, D.; Edström, K.; Brant, W. R. Cation Ordering and Oxygen Release in $LiNi_{0.5-x}Mn_{1.5+x}O_{4-y}$ (LNMO): In Situ Neutron Diffraction and Performance in Li Ion Full Cells. ACS Appl. Energy Mater. 2019, 2 (5), 3323–3335. https://doi.org/10.1021/acsaem.8b02217.
(5) Tertov, I.; Kwak, H.; Cabelguen, P-E.; Kumakura, S.; Suard, E.; Fauth, F.; Hansen, T.; Croguennec L.; Masquelier, C. In preparation.
