Quantifying inactive lithium in lithium metal batteries

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Abstract

Lithium metal anodes offer high theoretical capacities (3,860 milliampere-hours per gram)1, but rechargeable batteries built with such anodes suffer from dendrite growth and low Coulombic efficiency (the ratio of charge output to charge input), preventing their commercial adoption2,3. The formation of inactive (‘dead’) lithium— which consists of both (electro)chemically formed Li+ compounds in the solid electrolyte interphase and electrically isolated unreacted metallic Li0 (refs 4,5)—causes capacity loss and safety hazards. Quantitatively distinguishing between Li+ in components of the solid electrolyte interphase and unreacted metallic Li0 has not been possible, owing to the lack of effective diagnostic tools. Optical microscopy6, in situ environmental transmission electron microscopy7,8, X-ray microtomography9 and magnetic resonance imaging10 provide a morphological perspective with little chemical information. Nuclear magnetic resonance11, X-ray photoelectron spectroscopy12 and cryogenic transmission electron microscopy13,14 can distinguish between Li+ in the solid electrolyte interphase and metallic Li0, but their detection ranges are limited to surfaces or local regions. Here we establish the analytical method of titration gas chromatography to quantify the contribution of unreacted metallic Li0 to the total amount of inactive lithium. We identify the unreacted metallic Li0, not the (electro)chemically formed Li+ in the solid electrolyte interphase, as the dominant source of inactive lithium and capacity loss. By coupling the unreacted metallic Li0 content to observations of its local microstructure and nanostructure by cryogenic electron microscopy (both scanning and transmission), we also establish the formation mechanism of inactive lithium in different types of electrolytes and determine the underlying cause of low Coulombic efficiency in plating and stripping (the charge and discharge processes, respectively, in a full cell) of lithium metal anodes. We propose strategies for making lithium plating and stripping more efficient so that lithium metal anodes can be used for next-generation high-energy batteries.

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Fig. 1: Quantitative differentiation of inactive Li by the TGC method.
Fig. 2: Microstructures of inactive Li generated in HCE and CCE imaged by cryo-FIB–SEM.
Fig. 3: Nanostructures of inactive Li generated in HCE and CCE imaged by cryo-TEM.
Fig. 4: Schematic of inactive Li formation mechanism in different electrolytes, based on TGC quantification, cryo-FIB–SEM and cryo-TEM observation.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

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Acknowledgements

This work was supported by the Office of Vehicle Technologies of the US Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) under contract DE-EE0007764. Cryo-FIB was performed at the San Diego Nanotechnology Infrastructure, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the US National Science Foundation (NSF) (grant ECCS-1542148). We acknowledge the UC Irvine Materials Research Institute for the use of the cryo-electron microscopy and XPS facilities, funded in part by the NSF Major Research Instrumentation Program under grant CHE-1338173. The partial pressure measurements and analysis were done using a unique RGA based high vacuum gas evolution system developed under the guidance of I. K. Schuller’s laboratory at UC San Diego. The development of this system were supported by the US Department of Energy, Office of Science, Basic Energy Science (BES) under grant DE FG02 87ER-45332. C.F. thanks D. M. Davies for his suggestions on the manuscript and Shuang Bai for her assistance with the TEM experiment. J.L. thanks W. Wu for helping on figure design.

Author information

C.F., J.L., X.W. and Y.S.M. conceived the ideas. C.F. designed and implemented the TGC system. C.F. designed and performed the TGC, cryo-FIB–SEM, XPS experiments and data analysis. M.Z. collected the cryo-TEM data. C.F., M.Z. and B.L. interpreted TEM data. Y.Z., C.F. and M.C. prepared samples for characterizations. J.Z.L. and Y.Y. helped to set up cryo-FIB instrumentation. F.Y., N.W. and J.G. helped with GC set up and calibration. C.F. and M.-H.L. performed the RGA experiment. J.A., M.A.S. and K.X. formulated and provided the HCE electrolyte. L.Y. and M.C. formulated and provided the GM electrolyte. J.L. and C.F. wrote the manuscript. All authors discussed the results and commented on the manuscript. All authors have approved the final manuscript.

Correspondence to Ying Shirley Meng.

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Extended data figures and tables

Extended Data Fig. 1 Schematic working principle of the TGC method.

By combining [H2O titration on an inactive Li sample and H2 quantification by GC, the amount of metallic Li0 is calculated based on the chemical reaction 2Li + 2H2O → 2LiOH + H2↑.

Extended Data Fig. 2 Supplementary materials for TGC analysis.

ab, Representative voltage profiles of Li||Cu cells in (a) HCE and CCE, plating at 0.5 mA cm−2 for 1 mAh cm−2, stripping to 1 V at 0.5 mA cm−2, 2.5 mA cm−2 and 5.0 mA cm−2 (voltage profiles below 0 V represents the plating process, while those above 0 V represents the stripping process); (b) 2 M LiFSI–DMC, 0.5 M LiTFSI–DME/DOL, 1 M LiTFSI–DME/DOL, 1 M LiTFSI–DME/DOL + 2% LiNO3, CCE + Cs+ and CCE + FEC, plating at 0.5 mA cm−2 for 1 mAh cm−2, stripping to 1 V at 0.5 mA cm−2. c, The isolated metallic Li0 percentage in total capacity loss (Li0/Li0 + Li+). d, SEI Li+ percentage in total capacity loss (Li+/Li0 + Li+). e, Measured H2 area as a function of Coulombic efficiency under a variety of testing conditions. Every data point is an average of three separate GC measurements. The error bars represent the standard deviation, indicating the accuracy and reproducibility of the GC measurement. f, Unit conversion between milliampere-hours and milligrams of Li.

Extended Data Fig. 3 TGC analysis of inactive Li formed under extended electrochemical conditions.

a, The voltage profiles of CCE with different deposition capacities at 0.5 mA cm−2 for 1 mAh cm−2, 2 mAh cm−2, 3 mAh cm−2 and 5 mAh cm−2. b, The corresponding TGC analysis of inactive Li with associated capacity loss and Coulombic efficiency under different deposition capacities. c, The cycling performance of CCE in Li||Cu half-cells at 0.5 mA cm−2 for 1 mAh cm−2. d, TGC analysis showing Li0 and Li+ contents with associated capacity loss after one, two, five and ten cycles, respectively.

Extended Data Fig. 4 XPS analysis of inactive Li SEI components formed in HCE and CCE for various stripping rates.

a, Inactive Li formed in HCE. b, Inactive Li formed in CCE. The stripping rates show negligible impact on SEI components and contents in both electrolytes.

Extended Data Fig. 5 Supplementary materials for cryo-FIB-SEM and cryo-TEM analysis.

ac, Top view, cryo-FIB cross-section and schematic of deposited Li in HCE, respectively. The Li deposited in HCE forms large particles with several micrometres in size, with reduced porosity. df Top view, cryo-FIB cross-section and schematic of deposited Li in CCE, respectively. The Li shows a whisker-like morphology with high porosity. All deposited at 0.5 mA cm−2 for 0.5 mAh cm−2. g, Statistics of inactive Li SEI components formed in HCE, as detected at 50 different sample positions by cryo-TEM. h, Statistics of inactive Li SEI components formed in CCE, as detected at 50 different sample positions by cryo-TEM.

Extended Data Fig. 6 Strategies that may mitigate inactive Li formation.

a, Cross-sectional morphology of Li deposits generated in an advanced electrolyte developed by General Motors (GM), showing a columnar structure. b, The GM electrolyte delivers a first-cycle Coulombic efficiency of 96.2%, plating at 0.5 mA cm−2 for 1 mAh cm−2, stripping at 0.5 mA cm−2 to 1 V. c–f, 3D current collector. c, SEM image of Li deposits on Cu foil. d, SEM image of Li deposits on Cu foam. Both were deposited at 0.5 mA cm−2 for 1 mAh cm−2 in CCE. e, Representative first-cycle voltage profiles of Cu foil and Cu foam, plating at 0.5 mA cm−2 for 1 mAh cm−2, stripping at 0.5 mA cm−2 to 1 V in CCE. f, TGC quantification of inactive Li for Cu foil and Cu foam samples. g, Schematic of an ideal artificial SEI design. The polymer-based artificial SEI should be chemically stable against Li metal and mechanically elastic enough to accommodate the volume and shape change. Meanwhile, the edges of the artificial SEI should be fixed to the Li metal or the current collector, preventing the electrolyte from diffusing and making contact with fresh Li metal. The flexible polymer SEI thus can accommodate expansion and shrinkage during repeated Li plating and stripping. In this way, no Li will be consumed to form SEI during extended cycles, and we can realize anode-free Li metal batteries. h, Influence of pressure on Li plating/stripping. The results are from the HCE, at 0.5 mA cm−2 for 1 mAh cm−2, using a load cell. At each condition, two load cells were measured. The error bars indicate the standard deviation.

Extended Data Fig. 7 TGC calibration and LOD/LOQ analysis.

a, H2 concentration in ppm calibration curve as a function of detected H2 area and verification with certified GSCO H2 calibration gas. b, Converted metallic Li0 mass calibration curve as a function of detected H2 area. c, Nine pieces of Li metal with known mass were tested using the TGC set-up. The strongly linear relationship with detected H2 area indicates the feasibility of this method. d, Comparison between the balance-measured mass and TGC-quantified mass of the commercial Li metal pieces. e, Numerical comparison between the balance-measured mass and TGC-quantified mass of the commercial Li metal pieces. As the accuracy of the balance is two orders of magnitude lower than the TGC (10−5 g versus 10−7 g), the differentials should mainly come from the balance. f, H2 concentration in the blank samples measured for LOD/LOQ analysis. A total of 10 measurements were taken for the LOD/LOQ calculation.

Extended Data Fig. 8 GC chromatogram and N2 interference analysis.

a, GC chromatogram of the background gas from glovebox. b, GC chromatogram of gases with H2 after H2O titration on metallic Li0. c, Glovebox background gas measurements with various sampling amounts. The N2 amounts remain at the same level with various injection amounts, indicating the N2 does not exist in the reaction container. d, Container gas measurements with various sampling amounts after the H2O titration. The N2 amounts still remain identical with different injection amounts, whereas the H2 amounts increase in proportion to the increment of injection amounts, indicating that the N2 does not originally exist in the reaction container but comes from the gas sampling process, and thus will not have any chemical reactions with the inactive Li samples; the H2 quantification is not influenced by the injection sampling process. e, GC chromatogram of 10 µl of air.

Extended Data Fig. 9 Analysis of possible LiH presence in inactive Li.

a-h, Possible influence from LiH in SEI. a–g, The voltage profiles of SEI formation between 0 V and 1 V at 0.1 mA for ten cycles in 2 M LiFSI–DMC (a), 0.5 M LiTFSI–DME/DOL (b), 1 M LiTFSI–DME/DOL (c), CCE (d), HCE (e), CCE + Cs+ (f) and CCE + FEC (g). After the SEI formation, we performed TGC measurements on the current collectors with SEI. h, TGC results of the seven types of electrolytes. No H2 can be detected from any of them, indicating no LiH presence in the SEI of the systems studied. i–n, Possible influence from LiH in bulk inactive Li. To differentiate the two species, we substitute the titration solution with D2O instead of H2O. The D2O reacts with LiH and metallic Li0 to produce HD and D2, respectively. RGA can effectively distinguish between HD (relative molecular mass 3) and D2 (relative molecular mass 4) by partial pressure analysis. i, The D2 standard from the reaction between commercial pure Li metal and D2O. j, The HD standard from the reaction between commercial LiH powder and D2O. kn, Analysis of gaseous products from reactions between D2O and inactive Li forming in 2 M LiFSI–DMC (k), 0.5 M LiTFSI–DME/DOL (l), 1 M LiTFSI–DME/DOL (m) and CCE (n).

Extended Data Table 1 The solubility or reactivity of known SEI species with H2O

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