正文阅读顺序审阅

Lithium-ion batteries (LIBs) have been worldwide applied in 3C products, electric vehicles (EVs), and grid energy storage in past

decades. However, current commercialized LIBs cannot fulfill the demand for high energy density and power density with the rapid development of modern industrial society. [ 1-4] As the critical component determining the energy density of LIBs, advanced cathode materials urgently need to be developed, especially to solve the 'range anxiety' in EVs. [5-7] From this perspective, Lithium and manganese-rich layered oxides (LMRO) with a formula of xLi2MnO3 · (1-x)LiTMO 2 (TM = Ni, Co, Mn, etc.) are considered one of the most promising cathode materials for the next generation advanced LIBs owing to their high capacity ( > 250 mAh g -1 ) and high energy density ( > 1000 Wh kg -1 ), far exceeding the traditional cathode materials like LiCoO2 , LiFePO 4 and Ni-rich cathode. [8-14] Based on extensive research, it is consensus that the extraordinarily high capacity of LMROs originated from the hybrid cationic and anionic redox reaction, and the anionic redox triggered above 4.4 V involves reversible oxygen redox occurring in

the bulk (O 2 -→ O2 2 -) and irreversible oxygen gas release on the surface (O 2 -→ O2). [ 15-18]

Although LMROs have advantages in capacity and cost, several inherent drawbacks still hinder the road to commercialization.

First, irreversible oxygen release substantially damages the lattice crystal and facilitates transition metal migration to neighboring Li slabs, engendering layered-spinel phase transition, resulting in cathode degradation and capacity/voltage fading. [ 19-22] Second, oxygen loss would evoke the continual reduction of transition metal to charge compensation, which activates the cation redox reaction of Co 2 + /Co 3 + and Mn 3 + / 4 + , directly leading to the voltage fade. [ 23] Finally, the interface side reaction deteriorates LMROs as well. The releasing O 2 in the form of oxygen radical and O 2 · singlet will attack organic species, such as ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) and lead to their decomposition and the formation of thick solid cathode-electrolyte interphase (CEI) layer on the surface, resulting in inferior Li + diffusions and poor cyclability. [ 24,25] Moreover, LMROs also suffered from surface structure degradation and transition metal dissolution originating from HF corrosion due to the decomposition of LiPF 6 . [ 26,27] Therefore, suppression of oxygen release and side reaction, and construction of uniform and stable CEI layer are vital to enable the practical application of LMROs material.

Various strategies have been employed to solve the drawbacks of LMROs, including surface modification, [28,29] element doping, [30,31] structure design, [32,33] binder readjustment, [34,35] and electrolyte optimization. [ 25,36] Nonetheless, the mentioned strategies still have shortcomings in the complicated method, ineffective protection, and limited improvement of cyclic stability.

The redox couple strategy has been proven to be a facile and efficacious approach to inhibit oxygen release and achieve outstanding electrochemical stability via chemically reducing the peroxide ion O2 2 -back to stable lattice O 2 -during the discharging process. [37] However, previous research mainly focuses on sulfur elements and lacks discovery of other possible redox couples. Phthalocyanine as one type of porphyrin with an 18 /u1D70B electron aromatic cloud around the macrocycle has a Nredox activity, [ 38,39] and is widely applied in organic transistors, organic photovoltaics, and organic light-emitting diodes. [ 40-43] Inspired by this, herein, we introduce copper phthalocyanine (CuPc, C 32 H16 CuN 8 ) and other metal phthalocyanine derivatives into LMROs and find that CuPc serves as the best redox couple. Detailed structural and chemical characterizations combined with electrochemical testing confirm that the N element in CuPc can serve as a redox activity center to reduce surface O 2 2 -to O 2 -, and a synergistic effect of Cu and N element ensures the reaction reversibility of CuPc, thus suppressing surface oxygen evolution and structural degradation. Moreover, CuPc can induce the formation of a uniform and robust LiF-rich CEI on the electrode surface suppress the transition metal dissolution, and prevent the cathode against side reactions. As a result, CuPc-enhanced electrodes exhibit excellent structural stability and deliver outstanding electrochemical performance with higher capacity and voltage retention.

The X-ray diffraction (XRD) patterns shown in Figure S1 (Supporting Information) show that a new diffraction peak appears when the amount of CuPc added increases ( ≥ 5 wt%), but the LMRO and CuPc-enhanced electrodes display the same diffraction reflection. The Rietveld refinements (Figure S2 and Table S1,

In summary, we first develop CuPc as a redox couple to LMRO cathode by a facile and effective method and realize outstanding electrochemical performance with improved capacity and voltage stability. The LMRO@7CuPc maintains over 95.0% capacity retention and 80.3% voltage retention after 500 cycles at 200 mA g -1 . The underlying mechanism of the CuPc redox couple strategy is comprehensively researched with various advanced characterizations and could be summarized as follows. On the one hand, the N ion in CuPc can reduce the O 2 2 -back to lattice O 2 -in time during the charging process to prevent oxygen release, and the oxidized N ion can be reduced back during the discharge process and hence participate in the subsequent redox cycle, meanwhile, Cu ion also plays an important role in the above process to improve the reaction reversibility of N ion. The synergistic effect of Cu and N achieves significant suppression of oxygen evolution, thus alleviating structure degradation, spinel phase transition, and transition metal valence reduction. On the other hand, the CuPc strategy inhibits electrolyte decomposition and induces the formation of a uniform and robust LiFrich CEI, which suppresses side reactions and transition metal dissolution effectively. Furthermore, the comparison between CuPc and other metal phthalocyanine derivatives reveals that dual element redox couple possessing synergistic effect is more desirable for enhancing LMRO performance, and it's believed that these findings may shed light on subsequent redox couple research.

Synthesis of Samples : The Li 1.2 Ni 0.13 Co 0.13 Mn0.54 O 2 material was synthesized by spray pyrolysis followed by high-temperature calcination. Lithium acetate dihydrate (LiAC · 2H2 O, AR), manganese acetate tetrahydrate (Mn(AC) 2 · 4H2 O, AR), cobalt acetate tetrahydrate (Co(AC) 2 · 4H2 O, AR) and nickel acetate tetrahydrate (Ni(AC) 2 · 4H2 O, AR) used as the starting materials with a molar ratio of Li: Ni: Co: Mn = 1.2:0.13:0.13:0.54 were added into 2 L deionized water in a concentration of 0.3 mol L -1 , and 0.8 mol citric acid was added into solution to inhibit the precipitation of metal salts. The mixed solution was stirred for 1 h, and then the spray solution was sent to the pulverizer with a high-press gas of 200 ° C by a peristaltic pump to obtain precursor powders. The collected precursor powders were then calcined at 900 ° C for 10 h in air to obtain LMRO material.

Preparation of Electrodes : LMRO(1.6 g) and conductive additive Super P (TIMCAL) (0.2 g) were mixed via the ball-milling method in a stainlesssteel ball mill jar (120 mL) adding tungsten carbide balls (108 g) and ethanol (80 mL) with 400 r min -1 for 6 h. The obtained mixed materials were dried at 80 ° C in an air-circulation oven for 12 h. Then the obtained mixed materials and binder sodium carboxymethyl cellulose (CMC), CuPc (Macklin, /u1D6FD -form, 90%) with a weight ratio of (95-x):5:x (x = 0,3, 5, 7, 9) were mixed with magnetic stirring for 6 h and cast onto an aluminum foil and then dried at 120 ° C in a vacuum oven for 20 h. CMC is a better binder for LMRO, which can not only stabilize the electrode structure by preventing the electrode materials from detaching from the current collector but also suppress the voltage fading of the LMRO cathode due to Na + ions doping. Most importantly, the dissolution of metal elements from the cathode materials into the electrolyte is also inhibited. [74] The mass loading of the electrode is 2.20 mg cm -2 . The other enhanced electrodes are prepared in the same way by adding Pc, MnPc, FePc, CoPc, NiPc, and ZnPc, respectively.

Material Structure Characterizations : XRD analysis was performed on a Mini Flex 600 X-ray diffractometer (Rigaku, Japan) with Cu-K /u1D6FC radiation

operating at a voltage of 40 kV and current of 15 mA with 2 /u1D703 ranging from 10 ° to 60 ° at a scan rate of 1 ° min -1 . Rietveld refinements of the XRD data were carried out by a General Structure Analysis System (GSAS) software package. HAADF-STEM images, STEM-EDS, and EELS spectra were obtained in the JEM-ARM200F microscope, Spectra 300 microscope, and FEI Talos F200x. The Operando differential electrochemical mass spectrometry (DEMS) was performed in an i-DEMS 100 instrument (Linglu Instruments Co. Lt, Shanghai) with an EI-70 eV ion source and SEM-1100v detector. The cells were assembled in Swagelok-type cells and tested under an Ar gas flow of 0.9 mL mi -1 n at a current density of 40 mA g -1 at the voltage range of 2.0-4.8 V using LAND CT2001A. XPS measurements were performed using an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha) equipped with an Al K /u1D6FC X-ray radiation source (photon energy 1486.6 eV). The collected data were calibrated against the C 1s peak at 284.8 eV. Raman spectrometry was measured in measured on a Renishaw inVia with a 532 nm excitation wavelength. TOF-SIMS analysis was performed on PHI nanoTOF II Time-of-Flight SIMS with pulsed 30 keV Bi 3 ++ ion beam in a high mass resolution mode. All detected secondary ions of interest have a mass range of 2-1850 aum and possess negative polarity over an area of 200 µ m × 200 µ m. To measure the amount of deposited transition metal, the lithium foil of the cycled cell was dissolved in 10 mL HNO3 and analyzed by ICP-OES (Thermo Fisher iCAP PRO).

Electrochemical Measurements : The electrochemical performance was characterized by 2025 coin-type cells. These cells were assembled in an Arfilled glove box (water and oxygen content < 0.1 ppm) with lithium metal foil as the counter electrode, and Celgard-2400 membrane as a separator. A solution of 1 m LiPF 6 in ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) with a volume ratio of 1:1:1 was used as electrolyte. Galvanostatic charge and discharge tests were performed on a multi-channel battery testing system (NEWARE BST-610, China) at different currents with a potential window of 2.0-4.8 V versus Li/Li + at 30 ° C. The standard sample (LMRO) data used in this paper is consistent with the previous work in order to compare the consistency of data. [75]