In Situ Probing and Synthetic Control of Cationic Ordering in Ni‐Rich Layered Oxide Cathodes

Abstract

Ni‐rich layered oxides (LiNi1–xMxO2; M = Co, Mn, …) are appealing alternatives to conventional LiCoO2 as cathodes in Li‐ion batteries for automobile and other large‐scale applications due to their high theoretical capacity and low cost. However, preparing stoichiometric LiNi1–xMxO2 with ordered layer structure and high reversible capacity, has proven difficult due to cation mixing in octahedral sites. Herein, in situ studies of synthesis reactions and the associated structural ordering in preparing LiNiO2 and the Co‐substituted variant, LiNi0.8Co0.2O2, are made, to gain insights into synthetic control of the structure and electrochemical properties of Ni‐rich layered oxides. Results from this study indicate a direct transformation of the intermediate from the rock salt structure into hexagonal phase, and during the process, Co substitution facilities the nucleation of a Co‐rich layered phase at low temperatures and subsequent growth and stabilization of solid solution Li(Ni, Co)O2 upon further heat treatment. Optimal conditions are identified from the in situ studies and utilized to obtain stoichiometric LiNi0.8Co0.2O2 that exhibits high capacity (up to 200 mA h g−1 ) with excellent retention. The findings shed light on designing high performance Ni‐rich layered oxide cathodes through synthetic control of the structural ordering in the materials.

Prospect and Reality of Ni‐Rich Cathode for Commercialization

“…Along with this morphological instability, the formation of electrochemically inactive phase hinders the synthesis of well-ordered single crystalline nickel-rich cathode materials. [151] The calcination at the high temperature more than 850 °C gave rise to the lithium and oxygen ion loss in the host structure of nickel-rich cathode materials, which generated the structural ordering. [152][153][154] The c/a ratio, which represent the degree of cation ordering, decreased from 850 °C, indicating that the ordered layered structure was transformed to disordered layered structure with the rock salt like structure (Figure 17b).…”

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.…”