Layered oxide cathode material is the key material for achieving high energy density lithium ion batteries above 300Wh/kg. Especially by rock salt structural unit Li 2MnO 3And hexagonal layered structural unit LiTMO 2Lithium-rich manganese-based layered oxide cathode material formed (Li 1+xTM 1-xO2, or can be written as xLi 2MnO 3·(1-x)LiMO 2), because it has twice the positive electrode material LiCoO of the first generation lithium ion battery 2Reversible lithium storage capacity has attracted much attention (up to 300 mAh / g). However, this type of material has a continuous voltage fade during electrochemical cycling, which has become a major bottleneck limiting its practical application. The base material has a complex structure and chemical composition. Complex charge compensation occurs in the electrochemical process accompanied by slow structural changes. The mechanism of voltage decay has been lacking accurate understanding and conclusive experimental evidence.
Institute of Physics, Chinese Academy of Sciences / Associate Research Fellow, E01 Group, Key Laboratory of Clean Energy, National Research Center for Condensed Matter Physics, Beijing, China, and Dr. Enyuan Hu, Ph.D., Brookhaven National Laboratory, USA, Researcher Xiao-Qing Yang, Huolin Xin, Argonne, USA Laboratory researcher Jun Lu and Kahlil Amine collaborated with researchers from the National Institute of Standards and Technology to study the lithium-rich manganese-based layered oxide cathode materials using in-situ X-ray absorption spectroscopy and TEM three-dimensional imaging. The voltage decay mechanism (Fig. 1) clarifies the essential relationship between the instability of lattice oxygen ions involved in redox reactions and voltage decay and the effects of different elements on voltage decay, and proposes corresponding solutions. The results of the study were recently published. In Nature-Energy (2018, 3, 690-698), the article is entitled Evolution of redox couples in Li- and Mn-rich cathode materials and mitigation of voltage fade by reducing oxygen release.
The research team used the synchrotron radiation X-ray absorption spectroscopy technology combined with a specially designed analog battery (Fig. 2) to study the lithium-ion battery-rich lithium-manganese layered oxide cathode material in situ. 1.2Ni 0.15Co 0.1Mn 0.55O2In the redox reaction mechanism of different charge and discharge cycles, it was found that the transition metal cations Ni, Co, Mn and lattice oxygen anions participate in the redox reaction, contribute to the lithium storage capacity and evolve with the electrochemical cycle (Fig. 3). Among them, lattice oxygen ions participate in the reaction to contribute a large amount of lithium storage capacity but are unstable. Mn and Co gradually participate in the electrochemical reaction with the electrochemical cycle (reduction leads to voltage decay) and compensate for the capacity loss caused by oxygen participation in the reaction instability. The above results clearly reveal the essential relationship between the reaction mechanism of lattice oxygen participation in high-capacity reaction and the voltage decay of lithium-rich manganese-based materials. Further, the three-dimensional imaging technique of transmission electron microscopy confirmed that the material gradually lost during the electrochemical cycle. Oxygen, and the discovery that the electrolyte reacts with the electrode material exacerbates the material's oxygen loss and leads to more severe voltage decay (Figure 4). This study shows that suppressing the voltage decay of lithium-rich materials requires increasing the lattice oxygen ions in the material during high voltage charging. Stability, and different elements in the material have different effects on voltage decay. This information is designed to be high-capacity The lithium-rich layered oxide lithium battery cathode material with stable structure and no voltage attenuation provides ideas and experimental basis. In addition, this is the first time in-situ real-time study of the valence state evolution of lithium-ion battery materials during long-circulation process using synchrotron radiation experimental technology. Experimental work on performance attenuation mechanism. This method and experimental design have important reference value for future research on failure mechanism in battery and battery materials in long cycle life cycle.
Synchrotron radiation sources can provide a variety of non-destructive in-situ studies of the electrochemical process reaction mechanism of battery materials. 禹 Xiqian and the Institute of Physical Energy Clean Energy Laboratory E01 research team has been working on battery research for many years. In-situ experimental methods have yielded a series of research results. Recently invited to write a review in journals such as Chemical Reviews (2017, 117, 13123-13186) and Accounts of Chemical Research (2018, 51, 290-298) with international colleagues. This paper introduces the experimental method of synchrotron radiation applied to battery materials research. Relevant work has been supported by the Ministry of Science and Technology Key Research and Development Program (2016YFA0202500), the Fund Committee Innovation Group Fund (51421002), the Chinese Academy of Sciences 100 People Program and the Central Organization Department Youth Thousand Talents Program.
Figure 1 Li 1.2Ni 0.15Co 0.1Mn 0.55O2(a) charge and discharge curves and (b) cyclic voltammetry curves for different charge and discharge cycles.
Figure 2 Li 1.2Ni 0.15Co 0.1Mn 0.55O2X-ray absorption spectra of different elements in different charge and discharge cycles.
Figure 3 Li 1.2Ni 0.15Co 0.1Mn 0.55O2Redox reactions of different charge and discharge cycles (a) Contribution of different elements to capacity at different charge and discharge cycles; (b) Changes in lithium ion storage potential due to changes in electronic structure; (c) Different transition metal redox The energy level difference is related to the voltage attenuation.
Figure 4 (a) and (b) Three-dimensional topographical images of electrode particles before and after electrochemical cycling; (c) and (d) statistics of internal pore pore size distribution in material particles before circulation; (e) and (f) after cycling Internal micropore pore size distribution statistics in material particles.
