Article pubs.acs.org/JPCC
Understanding the Electrochemical Properties of Li-Rich Cathode Materials from First-Principles Calculations Tingting Cao,† Chunsheng Shi,† Naiqin Zhao,†,‡ Chunnian He,† Jiajun Li,† and Enzuo Liu*,†,‡ †
School of Materials Science and Engineering and Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Weijin Road, No. 92, Tianjin 300072, P. R. China ‡ Collaborative Innovation Centre of Chemical Science and Engineering, Weijin Road, No. 92, Tianjin 300072, P. R. China S Supporting Information *
ABSTRACT: The lithium-rich layered oxide materials (LLOs) have attracted much attention as candidates for the next generation of LIBs because of their high voltage and high capacity, which are still poorly understood. In this study, the origin of high voltage and high capacity of LLOs has been comprehensively investigated through first-principles calculations. It is revealed that due to the asymmetric oxidation behavior of Li2MnO3/LiMO2 interface, the transition-metal−oxygen (TMO) layer of Li2MnO3 phase in Li-rich materials gains more electrons from Li layer than that in pure Li2MnO3, which results in the stronger hybrid between Mn-3d and O-2p states enhancing the activity of Mn in Li2MnO3. Moreover, the deintercalated Li-rich models possess smaller spacing than pure LiMO2, which reflects stronger electrostatic interaction between TMO and Li layers. The two factors are both beneficial to the high voltage of the Li-rich materials. However, the asymmetric interface also results in the increase of electronic states of transition metal atoms near the Femi level, which changes the oxidized sequence of Ni2+/Ni4+ and Co3+/Co4+, and reduces the participation of oxygen in the redox process. As a result, the voltage and reversible capacity of Li-rich materials are significantly enhanced compared with that of pure LiMO2.
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INTRODUCTION
and electrochemical properties of LLOs is required to further improve their battery performance. In general, the LLOs described as xLi2MnO3−(1 − x)LiMO2 are composed of the monoclinic Li2MnO3 and the rhombohedral LiMO2 (M = Ni, Co, Mn, Fe, Cr, etc.). The two compounds both possess layered α-NaFeO2-type structure, and the interlayer spacing of (001)monoclinic and (003)rhombohedral is close to 4.7 Å,12 which make the two compounds in LLOs have good compatibility. Whether the integrated material is a homogeneous solid solution or a two-phase composite has always been debated.13 Although no confidential conclusions have been generated on the structure, many studies found the weak peaks at 2θ = 20−25° in the XRD patterns, which can be indexed to the LiMn6 arrangement within the Li2MnO3 showing that the lithium-rich materials may be identified as composite structure of LiMO2 and Li2MnO3.14−17 Particularly, an atomic-resolution observation of the interface between LiMO2 and Li2MnO3 in the Li1.2Mn0.567Ni0.166Co0.067O2 by selected area electron diffraction (SAED) and high-angle annular dark-field (HAADF) STEM revealed that the heterointerface grows along the [001]rh/[103]mon zone axis direction.18 Moreover, when the charging voltage is higher than 4.5 V, the discharge capacity greatly increases, ascribed to the
With traditional fossil fuel resources drying up and the environmental pollution problem becoming increasingly serious, the world is facing the challenge of transforming the electric power production from the burning of fossil fuels to renewable energy.1 The intermittent renewable energies such as solar and wind power give rise to the demand for energy storage. Thus, lithium-ion batteries (LIBs) have drawn plenty of attentions, due to their high energy and power density, long cycle lifetime and friendliness to environment.2,3 However, it is increasingly difficult to develop LIBs with high energy and power densities and good cycle performance to satisfy the urgent needs for the application in electric vehicles and power grids. Compared with anode materials and electrolyte, the cathode materials play a vital role to improve the performance of lithium ion batteries. Recently, the lithium-rich layered oxide materials (LLOs) have attracted much attention as candidates for the next generation of LIBs because their capacities can be larger than 280 mA h g−1 when they are charged to over 4.6 V at room temperature.4−6 However, those series of materials have two significant disadvantages: a huge irreversible capacity loss of 40−100 mA h g−1 in the first cycle and poor rate capability.7,8 Certainly, many efforts are currently made on the Li-rich materials to improve the electrochemical properties.9−11 The investigation on the relationship between the microstructure © 2015 American Chemical Society
Received: October 11, 2015 Revised: December 8, 2015 Published: December 11, 2015 28749
DOI: 10.1021/acs.jpcc.5b09948 J. Phys. Chem. C 2015, 119, 28749−28756
Article
The Journal of Physical Chemistry C Li2MnO3 activation and the extraction of Li+ from Li2MnO3 with the oxygen release.19,20 All the above observations indicate that LLOs is a two-phase composite. The interface of Li2MnO3/LiMO2 exists in LLOs. However, experiment and first-principles calculation show that surface and grain boundary profoundly affect the electrochemical properties of the electrode materials. At the surface of LiCoO2, Li ions are highly mobile and preferentially form a (1 × 1) hexagonal lattice, whereas the surface CoO2 layer shows metallic and insulating phases in LixCoO2.21 Lower Li-ion diffusion energy barrier and higher Li-ion diffusion rate are revealed at LixCoO2 grain boundary area than in grain interiors.22 In the LiNi1/3Co1/3Mn1/3O2 thin film cathode, using advanced scanning probe microscopy techniques, Zhu et al.23 observed that most of the high Li-ion concentration areas are localized at surface defects and grain-boundary-like features. Using first-principles calculations, Hiroki Moriwake et al.24 found that at the twin boundary in the LiCoO2 the cathode voltage is decreased by 0.2 V in the twin boundary, and the diffusion of Li across the twin boundary has higher activation energy than along the twin boundary. Furthermore, it is revealed that the Li ion storage at interfaces contributes to the extra capacity of metal oxide anodes.25 Thus, it is important to investigate the influence of the interface of Li2MnO3/LiMO2 on the electrochemical properties of Li-rich materials. In this study, the microstructure of the Li2MnO3/LiMO2 interface is comprehensively studied, through the comparison of the microstructure of LiMO2, based on the first-principles calculations. The relation between the microstructure and the electrochemical properties is revealed, and the reasons for high voltage and high capacity of Li-rich materials are explored.
Figure 1. Structure models of (a) LiMO2 (M = Co, Ni, Mn) and (b) Li-rich materials in which d1 and d2 represent two kinds of O−O layer spacing.
Li2MnO3 is more than 0.3.8 The matching degree is 98.65% along a axis direction and 98.87% along b axis direction. The supercell of Li2MnO3 is shown in Figure S1a. For both structures, d1 and d2 represent two kinds of O−O layer spacing, where d1 reflects the length of TM−O bond and d2 is determined by the Coulomb interaction between Li layers and TMO layers. For all the systems, ferromagnetic and antiferromagnetic states with alternative up and down spins along TM chains parallel to the b-axis (Figure S1b), the energetically favorable configurations, are both considered,31,32 and the most stable state is adopted for data analysis (Table S1).
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RESULTS AND DISCUSSION Working potential is an important precondition for the application of cathode materials that affects the energy density especially for the Li-rich materials. Since the contributions of entropy and volume effects to cell voltage are expected to be very small (
