Enhancing the Electrocatalysis of LiNi_0.5Co_0.2Mn_0.3O₂by Introducing Lithium Deficiency for Oxygen Evolution Reaction

Abstract

LiNi0.5Co0.2Mn0.3O2 (NCM523), as a cathode material for rechargeable lithium-ion batteries, has attracted considerable attention and been successfully commercialized for decades. NCM is also a promising electrocatalyst for the oxygen evolution reaction (OER), and the catalytic activity is highly correlated to its structure. In this paper, we successfully obtain NCM523 with three different structures: spinel NCM synthesized at low temperature (LT-NCM), disordered NCM (DO-NCM) with lithium deficiency obtained at high temperature, and layered hexagonal NCM at high temperature (HT-NCM). By introducing lithium deficiency to tune the valence state of transition metals in NCM from Ni2+ to Ni3+, DO-NCM exhibits the best catalytic activity with the lowest onset potential (∼1.48 V) and Tafel slope (∼85.6 mV dec–1), whereas HT-NCM exhibits the worst catalytic activity with the highest onset potential (∼1.63 V) and Tafel slope (∼241.8 mV dec–1).

New Insight into the Electrocatalysis of Ni-Rich Trimetallic NCM-Based Hydroxides for Water Oxidation

“…Furthermore, the splitting of the Co 2p 3/2 component can be deconvoluted into two major peaks at 780.4 and 782.3 eV (780.2 and 781.5 eV for NCM-523) that can be assigned for the Co 3+ and Co 2+ oxidation states, respectively. 29,36 The high intensity of the Co 2+ state observed in the NCM-523 composition is well consistent with the XRD data for reflecting the Co(OH) 2 species. The core-level spectrum of the Mn 2p component for the NCM-811 composition shows two major peaks at 641.2 and 653.5 eV (642.4 and 654.2 eV for NCM-523) for the spin−orbit regions of the 2p 3/2 and 2p 1/2 components, respectively (Figure 3c).…”

“…These results support the presence of a characteristic Ni 2+ oxidation state in the hydroxides, while the absence of a signal for the Ni 3+ state. , Similarly, the core-level region of the Co 2p spectrum exhibits one spin–orbit doublet and a small satellite structured peak (785.2 eV) for the NCM-811 composition, which could split into two major peaks found at 780.4 and 796.3 eV (780.2 and 795.9 eV for NCM-523) for the Co 2p 3/2 and Co 2p 1/2 components, respectively (Figure b). Furthermore, the splitting of the Co 2p 3/2 component can be deconvoluted into two major peaks at 780.4 and 782.3 eV (780.2 and 781.5 eV for NCM-523) that can be assigned for the Co 3+ and Co 2+ oxidation states, respectively. , The high intensity of the Co 2+ state observed in the NCM-523 composition is well consistent with the XRD data for reflecting the Co­(OH) 2 species. The core-level spectrum of the Mn 2p component for the NCM-811 composition shows two major peaks at 641.2 and 653.5 eV (642.4 and 654.2 eV for NCM-523) for the spin–orbit regions of the 2p 3/2 and 2p 1/2 components, respectively (Figure c) .…”

“…It could also be observed that the NCM-811 composition possesses a higher peak intensity of the Mn 4+ and Co 3+ oxidation states, and the increased content of the higher oxidation state could be beneficial to deliver more active sites in the transition-metal compounds for improved electrocatalytic OER kinetics. 36 The core-level spectrum of the O 1s component of the NCM-811 composition exhibits three major constituents (Figure 3d). The deconvoluted peaks could be located at 528.8, 530.8, and 531.2 eV (529.4, 530.8, and 531.4 eV for NCM-523), which correspond to the existence of M−O (metal−oxygen), M−OH (metal−hydroxyl), and H−O−H species (physically adsorbed or chemisorbed water molecules), respectively.…”

“…30−35 More recently, an investigation of the electrocatalytic performance of OER on typical layered NCM oxides with different crystal structures reported a superior electrocatalytic OER performance for a highly lithium-deficient (DO-NCM) compound, which is due to the tuning of electronic states caused by the introduction of Li vacancies, leading to higher oxidation states of transitionmetal ions (Ni 2+ to Ni 3 + ) in its crystal lattice (Li 1−x (NCM) n+x O 2 ). 36 Other transition-metal-based multimetallic electrocatalysts that have recently received considerable attention include bimetallic Ni−Co phosphate, 37 Mn− Fe−Ni oxide, 38 FeCoNi alloy, 39 Mo-and Fe-modified Ni(OH) 2 /NiOOH, 40 and FeCo 0.5 Ni 0.5 -LDH 41 for improved OER behavior in alkaline electrolytes. However, there are no such reports available on significant NCM hydroxide compositions for the electrocatalytic water oxidation reaction.…”

“…The distinctive NCM composites have hydroxide structures such as Ni 0.5 Co 0.2 Mn 0.3 (OH) 2 (also designated NCM-523) and Ni 0.8 Co 0.1 Mn 0.1 (OH) 2 (NCM-811) that are significantly considered as well-established cathode precursors for LIBs to substantially develop corresponding state-of-the-art lithium-incorporated transition-metal-oxide structures. The characteristic lithium-incorporated layered NCM oxide compositions of Li­[Ni x Co y Mn z ]­O 2 (where x + y + z = 1) (e.g., LiNi 0.8 Co 0.1 Mn 0.1 O 2 and LiNi 0.5 Co 0.2 Mn 0.3 O 2 ) having a structure analogous to that of layered LiCoO 2 materials are used as one of the most efficient and successful commercial cathode materials in LIBs for their low-cost production, better stability, higher capacity, and low toxicity. − More recently, an investigation of the electrocatalytic performance of OER on typical layered NCM oxides with different crystal structures reported a superior electrocatalytic OER performance for a highly lithium-deficient (DO-NCM) compound, which is due to the tuning of electronic states caused by the introduction of Li vacancies, leading to higher oxidation states of transition-metal ions (Ni 2+ to Ni 3+ ) in its crystal lattice (Li 1– x (NCM) n + x O 2 ) . Other transition-metal-based multimetallic electrocatalysts that have recently received considerable attention include bimetallic Ni–Co phosphate, Mn–Fe–Ni oxide, FeCoNi alloy, Mo- and Fe-modified Ni­(OH) 2 /NiOOH, and FeCo 0.5 Ni 0.5 -LDH for improved OER behavior in alkaline electrolytes.…”