Synthetic Control of Kinetic Reaction Pathway and Cationic Ordering in High‐Ni Layered Oxide Cathodes

Abstract

Nickel-rich layered transition metal oxides, LiNi (MnCo) O (1-x ≥ 0.5), are appealing candidates for cathodes in next-generation lithium-ion batteries (LIBs) for electric vehicles and other large-scale applications, due to their high capacity and low cost. However, synthetic control of the structural ordering in such a complex quaternary system has been a great challenge, especially in the presence of high Ni content. Herein, synthesis reactions for preparing layered LiNi Mn Co O (NMC71515) by solid-state methods are investigated through a combination of time-resolved in situ high-energy X-ray diffraction and absorption spectroscopy measurements. The real-time observation reveals a strong temperature dependence of the kinetics of cationic ordering in NMC71515 as a result of thermal-driven oxidation of transition metals and lithium/oxygen loss that concomitantly occur during heat treatment. Through synthetic control of the kinetic reaction pathway, a layered NMC71515 with low cationic disordering and a high reversible capacity is prepared in air. The findings may help to pave the way for designing high-Ni layered oxide cathodes for LIBs.

Cooling Induced Surface Reconstruction during Synthesis of High‐Ni Layered Oxides

“…[27] Surface Li 2 CO 3 was recently shown to be the main source of CO 2 and CO generated during the first charge, and it reacts with electrolyte to produce LiF and gases, consequently causing impedance increase, capacity and power fade. [22,30,31] Recent studies demonstrate that stoichiometric high-Ni NMC oxides with high structural ordering may be obtained through rational design of synthesis devised to control the cationic ordering as the materials are synthesized, [9,11] but the surface reconstruction, shown as Ni reduction and off-stoichiometry at the particle surface (i.e., Li-deficiency), was found to be an issue. [22,30,31] Recent studies demonstrate that stoichiometric high-Ni NMC oxides with high structural ordering may be obtained through rational design of synthesis devised to control the cationic ordering as the materials are synthesized, [9,11] but the surface reconstruction, shown as Ni reduction and off-stoichiometry at the particle surface (i.e., Li-deficiency), was found to be an issue.…”

“…[1,2] In particular, high-Ni layered oxides, LiNi x Mn y Co z O 2 (NMC; x ≥ 0.7) are now attracting world-wide interest for their high theoretical capacity (≈280 mA h g −1 ), which, however, has been difficult to realize due to the issues associated with high Ni loading: [3][4][5][6][7][8] in addition to cationic disordering (Li/Ni mixing) and the resulted low electrochemical activity, [9][10][11] Transition metal layered oxides have been the dominant cathodes in lithiumion batteries, and among them, high-Ni ones (LiNi x Mn y Co z O 2 ; x ≥ 0.7) with greatly boosted capacity and reduced cost are of particular interest for largescale applications. Despite much research into candidate cathodes for LIBs, transition metal (TM) layered oxides with a hexagonal structure (space group R3m) have remained dominant over the past three decades.…”

“…Despite much research into candidate cathodes for LIBs, transition metal (TM) layered oxides with a hexagonal structure (space group R3m) have remained dominant over the past three decades. [9,11,16,17] It is until very recently that people started to pay close attention to the surface instability, particularly surface reconstruction, including formation of NiO-type rock salt, [11,18,19] Li 2 CO 3 species, [10,14,[20][21][22] and nickel carbonate (NiCO 3 ·2Ni(OH) 2 ·2H 2 O). The high Ni loading, on the other hand, raises the critical issues of surface instability and poor rate performance.…”

“…The reconstruction process occurs predominantly at high temperatures (above 350 °C) and is highly cooling-rate dependent, implying that surface reconstruction can be suppressed through synthetic control, i.e., quenching to improve the surface stability and rate performance of the synthesized materials. [22,30,31] Recent studies demonstrate that stoichiometric high-Ni NMC oxides with high structural ordering may be obtained through rational design of synthesis devised to control the cationic ordering as the materials are synthesized, [9,11] but the surface reconstruction, shown as Ni reduction and off-stoichiometry at the particle surface (i.e., Li-deficiency), was found to be an issue. [12][13][14][15] Significant efforts in developing high-Ni NMC cathodes have been put on studying electrochemical impact, origin of the Li/ Ni disordering, and on improving Li/Ni ordering through optimizing synthesis conditions.…”

“…Rietveld refinements were performed, with the main results provided in Figure 1b and Figure S1 and Table S1 (Supporting Information). 2019, 9,1901915 Scheme 1. In addition, the Lislab distance in the sample by slow cooling is almost the same as that in the sample by quenching (slightly larger; Table S1, Supporting Information).…”