Search

Rechargeable lithium (Li) metal batteries have received increasing attentions in recent years because of their high theoretical energy densities from the contribution of Li metal anode that has an ultrahigh theoretical specific capacity (3860 mAh g-1), the lowest redox potential (-3.040 V vs. the standard hydrogen electrode) and a low gravimetric density (0.534 g cm-3). However, significant challenges including Li dendrite growth and low Coulombic efficiency still impede the successful deployment of rechargeable Li metal batteries. A lot of efforts have been made in the past 45 years to address these problems, including the application of polymer electrolytes, solid-state electrolytes, ionic liquids, highly concentrated liquid electrolytes, additives, protecting layers, interlayers between Li and separator, nanoscale design, selective deposition, Li-reduced graphene oxide composites, and so on. Although some progresses have been achieved the challenges still remain. It has been reported that ether-based electrolytes are relatively stable with Li metal anode and can allow Li metal anodes to be cycled at reasonably high charging current densities, but these electrolytes are not stable at voltages above 4 V vs. Li/Li+. The conventional Li-ion battery electrolytes based on LiPF6/carbonate-solvents are stable at least up to 4.3 V but they suffer from the low Coulombic efficiency and poor charging current densities due to the reactivity of Li metal with LiPF6 salt and carbonate solvents. In the talk, we present the development of electrolytes based on LiTFSI-LiBOB dual-salt in carbonate solvent mixtures for rechargeable Li metal batteries with conventional Li-ion battery cathodes. It has been found that the dual-salt electrolytes greatly enhance the performances of Li metal batteries in terms of long-term cycling stability and fast charging capability over the conventional LiPF6/carbonate electrolytes. The dual-salt electrolytes also protect Li metal anodes by suppressing the corrosions of Li metal caused by the reactions of Li metal and electrolytes. Certain additives at a small amount level can even further significantly protect the Li metal anode and improve the battery performances. The possible mechanisms have been proposed based on the comprehensive characterizations. The details of the results will be reported at the meeting. Acknowledgements The research has been supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) Program and the Battery500 Consortium. The microscopy and spectroscopy measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

Introduction Na is in the same element group with Li and its diameter is only ~40% larger than a Li ion.1-2 Therefore, it is believed that it has very similar intercalation chemistry in Na-ion batteries as Li in Li-ion batteries. Hard carbon is a promising anode material for both Li-ion and Na-ion intercalation and has almost similar specific discharge capacity.3 Yet, it has been reported that the Na ions and Li ions are quite different when intercalating into hard carbon and the solid state interphase (SEI) on hard carbon anodes has different composition/morphology for Na and Li ions.4-5In this work, we further elucidate how the solid electrolyte interlayer can induce significant difference in the intercalation of Na-ion and Li-ion into hard carbon and hence lead to significant difference in the discharge capacity. Electrolyte additive effect was also investigated. Results and Discussion The same hard carbon electrode was cycled subsequently v.s. Li metal in Li-ion electrolyte and v.s. Na metal in Na-ion electrolyte. The capacity remains similar of ~200 mAh/g when it was switched back to Li-ion batteries. Yet, when the hard carbon electrode was cycled subsequently v.s. Na metal in Na-ion electrolyte and v.s. Li metal in Li-ion electrolyte, the capacity shows significant drop from ~200 mAh/g to ~40 mAh/g when it was switched back to Na-ion batteries. The electrolyte for Li-ion battery and Na-ion battery is only different for the alkali metal ion. Fig. 1. Hard carbon electrodes cycled subsequently v.s. Li or Na metal in Li-ion or Na-ion electrolyte. Acknowledgement The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE’s) Office of Electricity Delivery & Energy Reliability (OE) (under Contract No. 57558). (A portion of) The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. References MD Slater, DH Kim, EJ Lee, and CS Johnson. Adv. Funct. Mater. 23(2013):947. HL Pan, Y-S Hu, and LQ Chen. Energy Environ. Sci. 6 (2013): 2338. D.A. Stevens, J. R. Dahn. J. Electrochem. Soc. 147 (2000), 1271-1273. S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama, A. Ogata, K. Gotoh, K. Fujiwara. Adv. Funct. Mater. 21 (2011), 3859-3867. S. Komaba, T. Ishikawa, N. Yabuuchi, W. Murata, A. Ito, Y. Ohsawa. ACS Appl. Mater. Interfaces 3 (2011), 4165-4168. Figure 1

Lithium- and manganese-rich (LMR) layered-structure cathode materials deliver a much higher energy density than traditional cathode materials such as LiMn2O4 spinel and LiCoO2. However, significant challenges, including voltage fade and limited cycle life in LMR cathodes still remain to be overcome prior to their large-scale market penetration. Here, we report on the direct correlation between voltage fade and atomic level spatial distribution of chemical species among LMR cathode materials (Li[Li0.2Ni0.2M0.6]O2) prepared by several methods, including co-precipitation (CP), sol-gel (SG) and hydrothermal assisted (HA) methods. Quantitative chemical composition analysis was performed on these materials using large-area X-ray energy dispersive spectroscopy (XEDS) mapping. Meanwhile, we used aberration-corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) to acquire detailed crystallographic data on Li[Li0.2Ni0.2Mn0.6]O2 cathode prepared by different methods. The atomic contrast in high-angle annular dark field (HAADF) STEM imaging identified the details of stacking and ordering in the particles while EELS chemical analysis revealed important chemistry information. We found that the materials prepared by the CP and SG methods exhibit a relatively high propensity for Ni segregation, leading to fast voltage fade and poor cycling stability. However, material prepared by the HA method exhibits very uniform Ni distribution and greatly reduced voltage fade. To the best of our knowledge, this is the first direct atomic-scale evidence to correlate atomic level cation uniformities with the electrochemical performance of cathode materials. More importantly, the voltage fade and energy degradation of LMR cathode materials can be significantly mitigated by improving the uniformity of chemical species in atomic level that are strongly affected by the preparation methods and synthesis conditions. These results also shine the light on the current debate regarding to the average/local structure of LMR cathode materials. LMR cathodes prepared by co-precipitation and sol-gel methods exhibit significant Ni segregation and dominated by LiMO2 R-3m phase, where the preferential segregation of Ni blocks lithium ion diffusion channels, weaken nickel-manganese interactions, lead to easy reduction of the manganese ions and fast voltage/capacity fade. In contrast, LMR cathodes prepared by the hydrothermal assisted method form a solid solution dominated by Li2MO3C2/m monoclinic symmetry with largely suppressed Ni segregation and are more stable against excessive lithium ion removal, enhance Ni-Mn interaction and stabilize crystal structure, leads to greatly reduced voltage fade and excellent cycling stability of LMR cathodes. Therefore, LMR cathodes with uniform distribution of chemical species (minimal Ni segregation) are very promising for use in high-energy Li-ion batteries for large-scale practical applications. The fundamental correlation between the atomic level spatial distribution of the chemical species and the functional stability of the materials found in this work also provides new perspective on the design and development of other functional materials with significantly enhanced stability. Acknowledgements This work is supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 18769, under the Batteries for Advanced Transportation Technologies program. The microscopic study described in this paper is supported by the Laboratory Directed Research and Development Program as part of the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL). The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830. The authors also would like to thank M. M. Thackeray for useful discussions.

Silicon (Si) is one of the most promising anode materials for Li-ion batteries because of its very high specific capacity (4212 mAh/g for Li21Si5).1 However, the high specific capacity of silicon is associated with large volume changes (more than 300 percent) when alloyed with lithium. These extreme volume changes can cause severe cracking and disintegration of the electrode and lead to capacity loss. The degradation of the carbonate solvents or the continuous growth of the Solid Electrolyte Interphase (SEI) film during cycling also results in significant irreversible capacity loss because it decreases electrochemical reactivity of electrodes. Fluoroethylene carbonate (FEC) is a well-known electrolyte additive for silicon anodes which can significantly enhance the stability and flexibility of SEI film and improve the stability and Coulombic efficiency of silicon-based anodes.2, 3 Although FEC has demonstrated significant effect on the electrochemical performance of silicon-based anodes, the fundamental mechanisms of this effect are still controversial in the literature. One possible reduction mechanism of FEC on silicon-based anodes is that the FEC first converts into vinylene carbonate (VC) through defluorination and then the resultant VC polymerizes.2, 4, 5 The main products from this mechanism are LiF and poly(vinylene carbonate). Another possible reduction mechanism is that the FEC is reduced through opening the five-member ring to form a (CH2CHFOCO2Li)2 dimer.6 According to these two mechanisms, the FEC reduction products formed in the SEI film should be LiF and poly(vinylene carbonate) or (CH2CHFOCO2Li)2.2, 6 In both cases, the lithium-to-fluorine ratio of the reaction products is close to 1. The majority of the fluorine should exist in either LiF or organic products,4-7but not in both forms. However, our recent analysis found that the ratio of lithium to fluorine is ≥1.5 for the SEI film formed on a silicon-based anode which was cycled in an electrolyte with FEC as additive or pure FEC as solvent (table 1). Our X-ray Photoelectron Spectroscopy (XPS) study also reveals that fluorine exists in both LiF and organic compounds (Figure 1), which cannot be explained by the previous mechanisms proposed in the literature. Therefore, a new mechanism has been proposed (Figure 2) and verified for the reduction and polymerization of FEC and the formation of a stable SEI layer on anode materials. According to this mechanism, FEC is reduced on the anode surface through the opening of the five-member ring, leading to the formation of lithium poly(vinyl carbonate), LiF, and some dimers. Table 1 The percentage of Li, C, O and F in the SEI films in electrolytes (a) 1M LiPF6 in EC/DMC (1:2 in vol), (b) 1M LiPF6 in EC/DMC (1:2 in vol) with 10% FEC and (c) 1M LiClO4in pure FEC Elements samples %Li %C %O %F Li/F ratio Electrolyte (a), 2 cycles 27.7 23.3 24.1 24.9 1.11 Electrolyte (a), 35 cycles 22.7 31.3 37.6 8.4 2.70 Electrolyte (b), 2 cycles 22.9 32.5 32.1 12.5 1.83 Electrolyte (b), 100 cycles 21.0 33.8 31.9 13.3 1.58 Electrolyte (c), 2 cycles 21.1 34.7 32.2 12.0 1.76 Figure 1 XPS spectra of F1s for the SEI film formed in LiClO4/FEC electrolyte Figure 2Proposed possible reduction reaction of FEC Acknowledgment This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 18769 under the Batteries for Advanced Transportation Technologies (BATT) program. A portion of the research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Reference 1. B. A. Boukamp, G. C. Lesh, and R. A. Huggins, Journal of The Electrochemical Society, 128(4), 725-729 (1981). 2. V. Etacheri, O. Haik, Y. Goffer, G. A. Roberts, I. C. Stefan, R. Fasching, and D. Aurbach, Langmuir : the ACS journal of surfaces and colloids, 28(1), 965-976 (2012). 3. N.-S. Choi, K. H. Yew, K. Y. Lee, M. Sung, H. Kim, and S.-S. Kim, Journal of Power Sources, 161(2), 1254-1259 (2006). 4. D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, and U. Heider, Electrochimica Acta, 47(9), 1423-1439 (2002). 5. S. S. Zhang, Journal of Power Sources, 162(2), 1379-1394 (2006). 6. Z.-C. Wang, J. Xu, W.-H. Yao, Y.-W. Yao, and Y. Yang, ECS Transactions, 41(41), 29-40 (2012). 7. H. Wu, G. Chan, J. W. Choi, I. Ryu, Y. Yao, M. T. McDowell, S. W. Lee, A. Jackson, Y. Yang, L. Hu, and Y. Cui, Nat Nano, 7 (5), 310-315 (2012).

Lithium (Li) dendrite formation during repeated charging-discharging cycles is one of the critical challenges for the development of next-generation rechargeable Li metal batteries.[1] The dendrites will penetrate through the separator, causing internal short circuit and even thermal runaway of the battery, which is not acceptable for commercial applications. To address this issue, a widely used approach is to create a strong interphase layer between the electrolyte and the Li metal anode in the form of a protective ion-permeable film, also known as a solid-electrolyte interphase (SEI) film.[2] A stronger and more uniform SEI film makes ion conduction more homogeneous, which decreases protuberant tip formation and thus suppresses Li dendrite growth. These approaches include using different electrolyte solvents, salts and additives in electrolytes, which have been shown to effectively enhance the Li deposition behavior.[3-6] Here we introduce a new electrolyte additive A that acts as an effectively SEI formation agent to suppress the Li dendrite growth. This additive can be in-situ electrochemically reduced to form a proactive layer on Cu substrate at a high potential prior to Li deposition. This thin SEI layer can allow Li ion to pass while prevent the electrolyte erosion with the fresh deposited Li metal, thereby a dendrite-free Li film can be obtained. Figure 1 gives the voltage-time profiles of Li deposition process in 1M LiPF6/PC electrolyte without and with additive A. In the control electrolyte (black line), the voltage curve drops quickly from OCV to about 1.0 V vs. Li/Li+, followed by two short slopes between 1.0 V and 0 V which can be attributed to the reduction of PC solvent and salt anion on the substrate surface to form SEI film. Below 0 V, a long Li plating plateau occurs, corresponding to Li deposition reaction. In the electrolyte with additive A (red line), a high voltage plateau occurred at about 2.0 V, which can be ascribe to electrochemical reduction of the additive A to form a proactive layer on Cu substrate at this potential. This layer is supposed to favor the Li deposition of dendrite-free behavior. Figure 2 compares the morphologies (obtained by optical microscope) of the deposited Li films on Cu substrates using electrolytes without and with additive A. The Li film deposited in the control electrolyte is loose and dendritic (Figure 2a), and the Cu substrate surface is not completely covered by Li clusters. In contrast, the electrolyte with additive A shows significant improvement in the Li film surface quality with the elimination of dendrite formation. The Li deposition is smooth and dense, even reflecting blue metallic luminescence. Detailed electrochemical characterization and study of the working mechanism of this additive A will be discussed in the presentation. Acknowledgements This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the Basic Energy Sciences, Office of Science of the U.S. DOE. References F. Ding, W. Xu, G. L. Graff, J. Zhang, M. L. Sushko, X. Chen, Y. Shao, M. H. Engelhard, Z. Nie, J. Xiao, J. Am. Chem. Soc., 135, 4450-4456 (2013). D. Aurbach, E. zinigrad, Y. Cohen, and H. Teller, Solid state ionics, 148, 405-416 (2002). F. Ding, W. Xu, X. Chen, J. Zhang, M. H. Engelhard, Y. Zhang, B. R. Johnson, J. V. Crum, T. A. Blake, X. Liu, J.-G. Zhang, J. Electrochem. Soc. 160, A1894-A1901 (2013). H. Ota, X. M. Wang, E. Yasukawa, J. Electrochem. Soc., 151, A427-A436 (2004). R. Mogi, et al., J. Electrochem. Soc., 149, A1578-A1583 (2002). K. Kanamura, H. Tamura, S. Shiraishi, Z.-i. Takeha, J. Electroanal. Chem., 394, 49-62 (1995).

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.