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Abstract: Li- and Mn-rich cathode materials have been considered as promising candidates for application of lithium-ion batteries in vehicles due to their high capacity and high energy density (capacity around 250 mAh/g). However, these materials suffer from a serious problem of voltage fade. The average discharge voltage keeps decreasing as the cycling goes on and results in decreased energy output after limited cycles. In order to solve this problem, many efforts have been put in to understand the origin of such phenomenon, mostly from structural and chemical points of view. In this work, the important roles of crystallinity, or the microstructure of the cathode material in voltage fade are studied. The study is carried out on Li2Ru0.5Mn0.5O3 as an example. The material is electrochemically prelithiated to around Li3.2Ru0.5Mn0.5O3 first and then delithiated to Li2Ru0.5Mn0.5O3 . It is shown that even though the crystal structure mostly remains the same, the crystallinity is severely damaged by breaking the sub-micro domains into very fine nano-sized domains. Such loss of crystallinity causes much more significant voltage fade in the following cycling compared with those normally cycled samples. The more severe voltage fade caused by low crystallinity can be attributed to the accelerated oxygen loss due to the increased defects and surface area of the prelithiated sample. These results indicate the great importance of crystallinity of the cathode materials. Acknowledgement The work at Brookhaven National Laboratory was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DE-SC0012704. Use of the National Synchrotron Light Source II (NSLS-II) and Center for Functional Nanomaterials at Brookhaven National Laboratory were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contracts No. DE-SC0012704.

In recent years, rechargeable nonaqueous lithium air batteries have attracted much attention as a result of their high energy density [1, 2]. However, it is very difficult for most batteries to work in the ambient air for the reason that the moisture and carbon dioxide in the air will affect their performance [2]. If the reactive gas contains CO2, there will be Li2CO3 formation in the discharging process. It has been reported that Li2CO3 is very difficult to decompose during charging process. However, recently H. S. Zhou reported that the main reaction in the charge process is the decomposition of Li2CO3 if the lithium air battery works in the ambient air [3]. In addition, CO2 is a type of greenhouse gas and is harmful to the climate. Thus, we hope to utilize it in the energy storage and conversion, such as a Li/CO2 battery. In 2011, K. Takechi et al reported a Li/CO2:O2 (from 0 to 100% volume CO2) battery, which didn’t show a reversible charge capacity with a cut-off voltage of 4.5 V with the carbonate-based electrolyte [4]. B. D. McCloskey et al reported a Li/O2 battery with CO2 as a contamination gas (10% volume). This battery employed LiTFSI-DME as electrolyte and a sloped charging voltage profile up to 4.8 V was reported only in the first cycle [5]. Besides, Archer et al reported a primary Li/CO2 battery which cannot be recharged and only work in the high temperature [6]. Recently, Sun and Scrosati reported an electrolyte based on tetraethylene glycol dimethyl ether (TEGDME), with which a rechargeable Li/O2 battery could operate over many cycles under high current rate and could reach a high specific capacity [7]. In our recent work [8], we found that when lithium triflate-TEGDME was used as the electrolyte, the Li/CO2:O2 (2:1, volume ratio) battery and Li/CO2 battery could operate reversibly. The specific capacities of Li/O2 battery, Li/CO2:O2 (2:1) battery and Li/CO2 battery were 2273 mAh g-1, 1808 mAh g-1 and 1032 mAh g-1, respectively based on the weight of Ketjen Black. In addition, from XRD, FTIR and SEM analysis we observed that the main discharge product is Li2CO3 in both batteries and the products can decompose in the recharging process. In Li/CO2 battery, another main discharge product may be carbon which could be confirmed by EELS and Raman spectroscopy with porous Au as cathode. In addition, electrochemical behaviors of a series of air electrodes, kinetic analysis and in situ XRD, TEM, SEM investigation on the products will be reported in this meeting. Based on current investigations, Li/CO2 battery could be a potential candidate as energy storage device if the polarization could be solved. Acknowledgement Financial support from CAS project "Strategic Priority Research Program" of the Chinese Academy of Sciences (XDA01020304) and National project 973 (2014CB932300,2012CB932900) are appreciated. References [1] G. Girishkumar, et al, J. Phys. Chem. Lett., 14 (2010) 2193 -2203. [2] P. G. Bruce, et al, Nat. Mater., 1 (2012) 19-29. [3] T. Zhang, et al, Nat. Commun., 4 (2013) 1817-1823. [4] K. Takechi, et al, Chem. Commun., 12 (2011) 3463-3465. [5] S. R. Gowda, et al, J. Phys. Chem. Lett., 4 (2013) 276-279. [6] S. Xu, et al, RSC Adv., 3 (2013) 6656-6660. [7] H.-G. Jung, et al, Nat. Chem., 4 (2012) 579-585 [8] Y. L. Liu, et al, Energy Environ. Sci., 2013, DOI: 10.1039/c3ee43318h

Li-rich materials have been widely studied as high-energy cathode materials, with a high specific capacity more than 300 mAh/g when charged beyond a voltage plateau of ∼4.5 V [1]. Excess capacity at the voltage plateau has been suggested to be partly compensated by oxidation of oxide ions, causing release of oxygen from the lattice to electrolyte [2]. Electrode performance is significantly influenced by the oxygen removal process. Besides xLi2MnO3·(1−x)LiMO2 (M=Ni, Co, Mn), Li1.2Cr0.4Mn0.4O2 has also been widely studied [3, 4]. The theoretical specific capacity of Li1.2Cr0.4Mn0.4O2 reaches to 387 mAh/g by utilizing the Cr3+/Cr6+ redox, which possibly suppresses the oxygen loss associated with the voltage plateau at ∼4.5 V. The process of Cr3+/Cr6+ redox is reversible at a wide range from 4.8 to 1.5 V [5]. In this work, Li1.2Cr0.4Mn0.4O2 cathode material has been cycled in the voltage range from 4.8 to 1.0 V. The charge-discharge profiles and open circuit voltage curve of Li1.2Cr0.4Mn0.4O2 have been recorded. The low-voltage electrochemical behaviour is similar with (x)LiMn0.5Ni0.5O2·(1−x)Li2TiO3 [6]. It shows a charge capacity of 297 mAh/g during the first charge. When discharged to 1V, the discharge capacity achieved up to 387 mAh/g. It is worthwhile to note that there is a long plateau at the voltage range from 1.7 to 1.0 V, in which part the capacity is reversible during the subsequent cycles. But the voltage profile is not symmetrical; the capacity in the charging process is much less than that of the discharge process at low voltage. GITT, in situ XRD, XAS and HAADF-STEM measurements have been done to clarify the electrochemical behaviors and structure evolution of Li1.2Cr0.4Mn0.4O2 in such wide voltage range. Acknowledgement: Financial support from CAS project (KJCX2-YW-W26), "Strategic Priority Research Program" of the Chinese Academy of Sciences,Grant No. XDA01020304 and National project 973 (2012CB932900) are appreciated. References: [1] M. M. Thackeray, C. S. Johnson, J. T. Vaughey, N. Li and S. A. Hackney, Journal of Materials Chemistry, 2005, 15, 2257-2267. [2] A. R. Armstrong, M. Holzapfel, P. Novák, C. S. Johnson, S.-H. Kang, M. M. Thackeray and P. G. Bruce, Journal of the American Chemical Society, 2006, 128, 8694-8698. [3] Z. H. Lu and J. R. Dahn, Journal of the Electrochemical Society, 2002, 149, A1454-A1459. [4] M. Balasubramanian, J. McBreen, I. J. Davidson, P. S. Whitfield and I. Kargina, Journal of The Electrochemical Society, 2002, 149, A176-A184. [5] N. Yabuuchi, K. Yamamoto, K. Yoshii, I. Nakai, T. Nishizawa, A. Omaru, T. Toyooka and S. Komaba, Journal of The Electrochemical Society, 2013, 160, A39-A45. [6] C. S. Johnson, J.-S. Kim, A. Jeremy Kropf, A. J. Kahaian, J. T. Vaughey and M. M. Thackeray, Electrochemistry Communications, 2002, 4, 492-498.