Lithium-rich layered oxides, xLi 2 MnO 3 ·(1-x)LiMO 2 (M=Ni, Mn, Co), have been considered as one of the most promising cathode active materials for rechargeable lithium-ion batteries due to the high capacity over 250 mAh g -1 between 2.0-4.8V.However, the commercialized application of these cathodes has so far been hindered by their severe capacity fading and transition metal dissolution during high voltage cycling (>4.5 V vs. Li/Li + ). To overcome this barrier, a double-shelled architecture consisting of inner conductive polyacene layer and outer mesoporous Al 2 O 3 layer is constructed. A polyacene layer with high electron conductivity is first coated on the surface of 0.5Li 2 MnO 3 ·0.5LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode material, followed by a in-sol treatment combined hydrothermal method to produce highly-ordered mesoporous Al 2 O 3 layer. Compared to previous studies, this double-shelled architecture has substantially improved the electrochemical performance of 0.5Li 2 MnO 3 ·0.5LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode material. Two striking characteristics are obtained for this double-shelled lithium-rich layered oxide cathode material: (1) The electrochemical capacity is greatly improved, reaching to 280 mAh g -1 (2.0V-4.8V at 0.1C), (2) the transition from layered phase to spinel is delayed, leading to the superior capacity retention of 98% after 100 th cycle.
M-O stretching vibration.Fourier Transform Infrared Spectroscopy (FTIR) is performed to identify the PAS coating layer. Figure 5 shows the result of FTIR for PAS, LLMO, PL-5 and APL-2 samples. As can be seen in Figure 5, IR spectra of PAS exhibits the peak at 1465 cm -1 and 1638 cm -1 which are in reference to =CH in-plane deformation vibrations and C=O stretching vibrations, respectively 11, 32 . It is noted that it still retains the C=O and =CH peaks for PL-5 and APL-2 samples, indicating that the dehydration and dehydrocyclization will take place between the molecules of phenolic resin at 700℃ 11 and finally form the PAS layer on the surface of PL-5 particles. Two distinct absorption peaks (500-700 cm -1 ) caused by the M-O (M=Ni, Co, Mn) stretching vibration are observed in all samples. Obviously, the M-O stretching bands of the APL-2 sample shift from 620 and 526 cm -1 to 627 and 532 cm -1 , respectively. The shift is due to more Al 3+ ions diffusing into the LLMO particles at the calcining temperature of 500℃. The diffusing of Al 3+ ions into the LLMO could decrease the average bond length of M-O 31 .Figure 7 HRTEM image of APL-2 sample showing the interface between bulk of LLMO, PAS and Al 2 O 3 layer.
Formation of mesoporous structureAccording to previous report 21, 31, 34 , although the cathode materials are effectively protected from the errosion effect of electrolyte by dense Al 2 O 3 coating layer on the surface, the transportation of Li + ions is partially hindered due to the intrinsic insulation of Al 2 O 3 . The occurrence of mesoporous structure in Al 2 O 3 , which consists of textural mesopores and intrinsic interconnected pore systems ...