Realizing the Embedded Growth of Large Li2O2 Aggregations by Matching Different Metal Oxides for High‐Capacity and High‐Rate Lithium Oxygen Batteries

Abstract Large Li2O2 aggregations can produce high‐capacity of lithium oxygen (Li‐O2) batteries, but the larger ones usually lead to less‐efficient contact between Li2O2 and electrode materials. Herein, a hierarchical cathode architecture based on different discharge characteristics of α‐MnO2 and Co3O4 is constructed, which can enable the embedded growth of large Li2O2 aggregations to solve this problem. Through experimental observations and first‐principle calculations, it is found that α‐MnO2 nanorod tends to form uniform Li2O2 particles due to its preferential Li+ adsorption and similar LiO2 adsorption energies of different crystal faces, whereas Co3O4 nanosheet tends to simultaneously generate Li2O2 film and Li2O2 nanosheets due to its preferential O2 adsorption and different LiO2 adsorption energies of varied crystal faces. Thus, the composite cathode architecture in which Co3O4 nanosheets are grown on α‐MnO2 nanorods can exhibit extraordinary synergetic effects, i.e., α‐MnO2 nanorods provide the initial nucleation sites for Li2O2 deposition while Co3O4 nanosheets provide dissolved LiO2 to promote the subsequent growth of Li2O2. Consequently, the composite cathode achieves the embedded growth of large Li2O2 aggregations and thus exhibits significantly improved specific capacity, rate capability, and cyclic stability compared with the single metal oxide electrode.


Mass calculation of Co 3 O 4 through coulomb's law
The mass of Co 3 O 4 was calculated by the following equation: Where Q is quantity of electricity, M is the relative molecular mass of Co(OH) 2 , N A is the Avogadro's constant, and e is the elementary charge.

Structural characterization
The morphology of pristine, discharged and charged electrode materials were investigated by a field emission scanning electron microscope (FESEM, JSM 6701F, JEOL, Japan). A transmission electron microscope (TEM, JEOL 2100 FEG) was further applied to characterize the microstructure of CP-MnO 2 , CP-Co 3 O 4 and CP-MnO 2 -Co 3 O 4 . X-ray diffraction (XRD, Rigaku D/Max-2400, Japan) patterns of the three samples were conducted on a powder XRD system using Cu Ka radiation. FTIR spectroscopy was analyzed by an IFS120HR system, and the discharge products were pressed with KBr into pellets in a glove box filled with Ar before testing. The chemical species of CP-MnO 2 -Co 3 O 4 were performed on a X-ray photoelectron spectroscope (XPS, Physical Electronics, PerkinElmer PHI-5702) with 1486.6 eV radiation as the excitation source and the spectra were calibrated by using Au 4f 7/2 at 84.0 eV. Nitrogen adsorption-desorption isotherms were used to investigate the porous structure of the samples using a Micromeritics ASAP 2020 volumetric adsorption analyzer at 77 K. The specific surface area was caclulated based on the Brunauer-Emmett-Teller method. Pore size distribution was determined from the adsorption isotherm based on the density functional theory (DFT). The mass loading of metal oxides on CP substrate was measured by an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, optima 2100 DV).

Electrochemical measurements
All Li-O 2 batteries were tested in a typical coin cell system with holes (1 mm diameter) on the positive side, and they were assembled in an Ar-filled glovebox using a Li foil anode, a celgard 2400 separator, an oxygen cathode, and an electrolyte with 1 M LiCF 3 SO 3 in tetraethylene glycol dimethyl ether (TEGDME). The galvanostatic tests were operated with a voltage window of 2.0-4.5 V (vs. Li/Li + ) at ambient temperature. All experiments were performed in high-purity O 2 .

Computational methods
DFT calculations were carried out to understand the adsorptive mechanisms of the LiO 2 on α-MnO 2 , Co 3 O 4 and Li 2 O 2 surfaces. The system energy calculations were performed by using the Vienna ab initio simulation package (VASP). The projector-augmented waves (PAW) method was used to describe the electron-ion interactions, and the spin-polarized generalized gradient approximation (GGA) with Perdew-Wang (PW91) functional was used to treat the exchange-correlation energy of the electrons. The kinetic energy cutoff on the wave function expansion was set as 450 eV. The 5×5×1 k-point mesh was performed using the Monkhorst-Pack scheme. Ionic relaxation was executed with the conjugated gradient method. Gaussian smearing was used with a smearing parameter of 0.2 eV for these calculations. Throughout the calculations, the dipole correction was applied to compensate for the dipole interactions.
The adsorption energy (ΔE adsorption ) of LiO 2 on the surfaces was defined as: Here, the E total is the total energy for the system; E surface is the energy of the clean surface and E LiO2 is the LiO 2 energy in vacuum, respectively.

Description of the calculation detail
Previous experimental results suggested that the facet perpendicular to the central axis of the Li 2 O 2 is the (0001) surface, and the oxygen-rich (0001)  We constructed slab models of the MnO 2 surface with the (020) and (110)  sites and the corresponding adsorption energy is -3.88 eV. Figure 3h with the corresponding energy of -4.63 eV. In detail, the oxygen ending of LiO 2 molecule adsorbing on the Co 3+ site and the Li + adsorbing on the oxygen site that contacts to three Co ions. Table S1 The loading mass of electrode materials on carbon paper (CP), specific surface area (SSA) and correspondences between applied current density based on SSA (1.40 mA m −2 ) and the mass of loading metal oxides.   Figure S1 (a) SEAD patterns of the α-MnO 2 nanorod in Figure 1c and (b) the Co 3 O 4 nanosheet in Figure 2c.

Figure S8
Schematic illustrations of the discharging mechanism for α-MnO 2 nanorod (a) and Co 3 O 4 nanosheet (b). The equations are the corresponding reaction process of MnO 2 (1-6) and Co 3 O 4 (7-15). The characters of "sol-nucleation/seeds" and "sur-seeds" represent the seeds formed through solution and surface, respectively.

Figure S20
The cyclic stability of the three electrodes under the limited capacity corresponding to ~21% of their full capacity at ~103 mA g −1 .

Figure S22
The cyclic stability of the three electrodes with the limited capacity of 320 mAh g −1 at ~103 mA g −1 .
Though Li + can intercalate into the MnO 2 crystal structure, the capacity provided by lithiation is only about 36 mAh g -1 above 2.5 V based on the total mass of MnO 2 and Co 3 O 4 ( Figure S17). When CP-MnO 2 -Co 3 O 4 electrode is tested in oxygen, the capacity provided by lithiation would be less than that tested in Ar atmosphere due to some MnO 6 octahedron channels being occupied by Li x O y . Thus, the cycle test of CP-MnO 2 -Co 3 O 4 electrode at 320 mAh g −1 can also verify its superior cycle stability.