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

Abstract

Li-ion batteries (LIBs) are now increasingly used in electric vehicles (EVs) and other large-scale applications, but a bottleneck for their mass adoption is low Li-storage capacity, which is largely constrained by cathodes, typically with a gravimetric capacity equivalent to ≈1/2 that of the graphite anode. 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. [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. The high Ni loading, on the other hand, raises the critical issues of surface instability and poor rate performance. The rational design of synthesis leading to layered LiNi 0.7 Mn 0.15 Co 0.15 O 2 with greatly enhanced rate capability is demonstrated, by implementing a quenching process alternative to the general slow cooling. In situ synchrotron X-ray diffraction, coupled with surface analysis, is applied to studies of the synthesis process, revealing cooling-induced surface reconstruction involving Li 2 CO 3 accumulation, formation of a Li-deficient layer and Ni reduction at the particle surface. 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. These findings may provide guidance to rational synthesis of high-Ni cathode materials. (2 of 10)cycling stability and safety are often compromised due to the surface-related issues. [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. [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). [23] These surface species are electrochemically inactive and poor in electronic and ionic conductivity, so causing high impedance to Li + (de)intercalation during cycling, [24,25] and even power fade as reported in LiNi 0.8 Co 0.2 O 2 . [26] The formation of NiO and Li 2 CO 3 at the surface of prima...

Native lattice strain induced structural earthquake in sodium layered oxide cathodes

“…Recently, researchers leveraged in situ SXRD and computational modeling to investigate the evolution of non-equilibrium kinetic intermediates and the formation of thermodynamic equilibrium phases during these processes 37 , 38 . Although these studies provide valuable guidance for the predictive synthesis of layered oxides, most of them have focused on examining the impact of structural characteristics during the heating/holding process, such as phase impurity, lattice parameter, crystallite size, Li–TM bond length, and Li + /Ni 2+ mixing 39 – 41 . The effect of microstrain evolution during the cooling process, especially under rapid quenching, has been largely ignored.…”

Unveiling the Stabilities of Nickel‐Based Layered Oxide Cathodes at an Identical Degree of Delithiation in Lithium‐Based Batteries

“…A robust EEI could suppress the unfavorable parasitic reactions that cause EEI thickening and surface structure degradation. [ 30,39–43 ] Since surface residual lithium could affect the formation of cathode/electrolyte interphase, [ 44 ] it is necessary to evaluate the amount of residual lithium on both samples. As Figure S8 (Supporting Information) shows, the residual lithium amount in the fresh LNO is comparable to that of NMC811.…”

Kinetic Limitations in Single‐Crystal High‐Nickel Cathodes

“…0.6) to boost capacity while reducing cost. [4] The high-Ni loading, however, leads to lattice collapse during operation at high voltages (> 4.2 V), [5][6][7][8][9] and in the commonly employed polycrystals, anisotropic volume change and the resulted intergranular cracking has been a roadblock to the practical use of high-Ni cathodes. [10][11][12][13][14] In recent studies, long cycling stability was demonstrated in single-crystal high-Ni NMC cathodes.…”