DEVELOPMENT OF CARBONATE ELECTROLYTES FOR LITHIUM METAL ELECTRODES

The development of energy storage technology is an important topic for facilitating the employment of renewable energy in society. Therefore, current energy storage research is heavily focused on enabling rechargeable high-energy density lithium-based batteries. In particular, permitting reversible electrochemical plating and stripping of the lithium metal negative electrode (or lithium metal anode) in carbonate electrolytes can achieve this goal. Unfortunately, the performance of the lithium metal anode in carbonate electrolytes is plagued by unsafe dendrite formation and poor Coulombic efficiency upon cycling. This dissertation attempts to reveal the role of the composition and structure of the Solid Electrolyte Interphase (SEI) in relation to the performance of the lithium metal anode. Galvanostatic voltammetry was used to characterize the electrochemistry of the lithium metal anode, with Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and Transmission Electron Microscopy to investigate the surface of the lithium metal anode. In chapter 2, a method to electrochemically synthesize lithium metal such that a reliable SEI is generated is introduced, using Cu||LiFePO4 cells. Using this method, in conjunction with the analytical techniques described above, chapters 3 and 4 investigates electrolyte components that significantly improve the performance of the lithium metal anode, fluoroethylene carbonate (FEC) and lithium difluoro(oxalate) borate (LiDFOB), with an explanation proposed. Finally, chapter 5 shows how FEC and LiDFOB can work together to optimize the SEI composition and structure, hence optimizing the performance of the lithium metal anode in carbonate electrolytes.


MOTIVATION
As the global temperature rises, so does the concern about consuming fossil fuels. 1,2,3 In the United States, 6511 million metric tons of carbon dioxide equivalents of greenhouse gases was emitted in 2016. 2 In particular, US economic sectors of electricity and transportation account for more than 50% of the GHG emissions, plotted in Figure 1-1, with each sector contributing about 28% of emissions. 2 In attempt to reduce this fossil fuel consumption, there has been a surge in the development of energy storage technology to facilitate large-scale grid energy storage and electro mobility. 4 So far, the lithium-ion battery is the leader in energy storage technology, ubiquitous in small-scale mobile devices, now being adopted in electric vehicles, and larger energy storage projects. 5,6 However, more breakthroughs in battery technology are required to make energy storage affordable to all consumers.

LITHIUM-ION BATTERIES
The lithium-ion battery consists of four important components, a negative electrode (anode), a positive electrode (cathode), the electrolyte, and separator material. An image of a dry, disassembled CR2032 coin cell with common lithium-ion battery components is shown in Figure 1-2 where is the total charge extracted upon the discharge process and is the total charge input during the charge process.

SOLID ELECTROLYTE INTERPHASE
The electrolyte can react on the surface of electrode materials to generate a Solid Electrolyte Interphase (SEI), which is important for allowing lithium-ion batteries to be charge/discharged for thousands of cycles with high efficiency. 9 Without the SEI, today's rechargeable lithium-ion batteries could not operate with such impressive efficiency and safety. Specifically, the SEI is an electronically insulating surface film that passivates the electrode, permitting lithium cation mobility and preventing further decomposition of the electrolyte. 9 It is composed of inorganic and organic decomposition products of electrolyte components. 9 Top-performing electrolytes have additives, which are chemicals used in low concentrations to generate an ideal SEI upon initial cycling of the battery. For example, vinylene carbonate is a common commercial additive which polymerizes on the surface of graphite upon reduction, improving the stability of the SEI. 8,10-12

LITHIUM METAL ELECTRODE
Lithium metal is considered to be the anode to enable next-generation batteries.
This is because lithium metal has high theoretical gravimetric capacity of 3861 mAh/g, along with its low electrochemical potential. 13,14 However, especially in carbonate electrolytes, a stable SEI for lithium metal electrodes eludes researchers.
Without a stable SEI, the plating and stripping of lithium metal is plagued by dendrite formation, leading to several safety issues, and poor Coulombic efficiency. 8,13,14 Currently, it is difficult to obtain stable Coulombic efficiencies with lithium metal electrodes, where an efficiency of at least 99.9% is required for considering 5 commercial application. 15 Therefore, researchers are motivated to develop lithium metal electrochemistry to enable next-generation battery technology.

ANALYLTICAL METHODS
The methods used to characterize the lithium metal in this work are summarized below with extreme brevity. Galvanostatic voltammetry is typically employed to investigate the electrochemistry of lithium-ion battery materials, suitable for practical operation of lithium-ion batteries. 7 In this mode, the current between working and counter electrodes is fixed, and the cell voltage is measured. By observing the measured voltage, changes in the chemistry at each electrode can be revealed, as the cell voltage is related to the potential difference ( ) between two electrode materials, where is the number of electrons transferred in the cell reaction, is the Faraday In order to characterize the nature of the SEI, employing several techniques that probe the surface of a material is ideal. The first example used throughout this work is Infrared (IR) absorption spectroscopy. Beer's law is applicable to IR spectroscopy, where is the absorbance, ε is the molar absorptivity of the analyte, is the path length of measurement, and is the concentration of the analyte. 16 where is the kinetic energy of photoelectrons, is binding energy of the electron in the atomic orbital from which it originates, is Planck's constant, and is the X-7 ray frequency. 18 Both inorganic and organic SEI components can be identified, as each atomic core is unique, and the penetration depth of XPS is on the order of tens of Ångstroms. 19 where ϴ is the minimum resolvable angular separation of two Airy disks, λ is the wavelength of light used, and D is the aperture diameter. 21 Further, TEM instruments can be equipped with energy-dispersive X-ray spectroscopy, allowing for compositional analysis of the imaged object of interest. In this work, lithium metal is plated on Cu TEM grids and its SEI morphology is investigated.

SUMMARY
Overall, this dissertation attempts to reveal the role of the composition and structure of the SEI in relation to the performance of the lithium metal anode. In chapter 2, a method to electrochemically synthesize lithium metal such that a reliable SEI is generated, is introduced. Using this method, in conjunction with the analytical techniques described above, chapters 3 and 4 investigates electrolyte components that significantly improve the performance of the lithium metal anode with an explanation proposed. Finally, chapter 5 shows how these electrolyte components can work together to optimize the SEI composition and structure, hence the optimizing the performance of the lithium metal anode in carbonate electrolytes.

INTRODUCTION
The plating and stripping of the lithium metal negative electrode in nonaqueous electrolytes has been investigated for decades. [1][2][3] In particular, carbonate solvents have relatively high voltage stability, making them desirable electrolytes for high-energy density lithium batteries. [3][4][5][6] However, the efficiency of plating/stripping lithium in carbonate electrolytes does not meet requirements for commercial application (> 99.9%). 7,8 It is common to measure the plating/stripping efficiency of lithium by assembling Li||Cu cells. [9][10][11][12][13] In this cell design, a small amount of Li is cycled, with an excess reservoir of lithium present. One limitation of this cell design is the difficulty of controlling the design and construction of the solid electrolyte interphase 14 (SEI) on lithium, as the low reduction potential of the lithium metal electrode present during cell construction will cause immediate reaction with electrolyte upon exposure. Thus, a reaction between the electrolyte and the lithium metal electrode will occur before cycling begins. Further, the excess lithium within the cell can significantly increase the cycle life of the cell making it difficult to compare to commercial cells, with a limited supply of lithium. Contrary to Li||Cu cells, Cu||LiFePO 4 cells have air-stable components, facilitating their processing and assembly. 15,16 Further, the in-situ formation of lithium metal and low reactivity of LiFePO 4 ensures additives under investigation do not react with the electrode surface upon construction and are only reduced upon initial cycling. This affords the possibility for controlled design and construction of the SEI on lithium metal since the reduction of the electrolyte can be 16 controlled by current density, cell potential, and the quantity of lithium plated. Finally, given that there is no excess lithium in Cu||LiFePO 4 cells, any observed improvements in capacity retention, Coulombic efficiency, or impedance should be applicable to other lithium metal based battery systems.
Vinylene carbonate (VC) is a well known electrolyte additive for lithium-ion batteries, demonstrating exceptional performance for graphite and several cathode materials. [17][18][19][20][21][22][23][24] Further, the reaction products of VC with lithium have been investigated in detail, using Li||Ni cells [25][26][27] and Li||Cu cells, 10,28 and found to have beneficial performance, typically attributed to poly(VC) within the SEI. However, the effect of added VC has not been investigated with lithium metal anodes in cells without a large excess of lithium. Herein, Cu||LiFePO 4 cells are utilized to investigate the influence of VC for plating and stripping lithium.

RESULTS AND DISCUSSION
The cycling performance of the carbonate electrolytes investigated is provided in Figure 2 decreases significantly over a short number of cycles as expected. 15 Since VC has been shown to have virtually no reactivity on LiFePO 4 , the improvement in cell performance is likely due to modification of the SEI on the negative electrode. 22,29 In general, the addition of VC improves the capacity retention and the Coulombic efficiency, as observed with the EC:EMC, 1% VC, 5% VC, and VC-S electrolytes.
Electrolytes containing 1% and 5% VC have the highest first cycle Coulombic efficiency ~87%. The 5% VC electrolyte has a longer cycle life and better efficiency (~92%), suggesting that increased concentrations of VC in the electrolyte results in the generation of a more stable SEI for lithium metal anodes. However, when employing VC as the solvent the first cycle Coulombic efficiency is reduced significantly to ~58%. After the first cycle, the efficiency improves to ~95%, comparable to reports in the literature. 10,25 After a significant quantity of lithium is consumed irreversibly on the first cycle, the VC-S electrolyte plates and strips lithium more efficiently than the EC:EMC, 1% VC, or 5% VC electrolytes, leading to improved reversible cycling.
The total quantity of lithium stripped each cycle (or the lithium reversibly cycled), summed over all cycles, for each electrolyte is plotted in Figure 2 This suggests that the diminishing benefit of VC, discussed above, may result from high resistance of the SEI film.
ATR-IR spectra of lithium plated on copper foil were acquired after the first charge to 4.0 V at 0.1 mA/cm 2 for the EC:EMC and 5% VC electrolytes and are provided in Figure 2 XPS spectrum of the lithium plated from the EC:EMC electrolyte also contains a large peak at 685 eV characteristic of LiF. The peak associated with LiF is much smaller for the electrolyte containing VC suggesting that VC inhibits LiPF 6 reduction. The surface of the SEI generated from the 5% VC electrolyte is primarily composed of poly(VC).
A chart of the corresponding relative atomic concentrations is provided in

INTRODUCTION
Fluoroethylene carbonate (FEC) has been investigated as an electrolyte additive for lithium-ion batteries which improves the performance of commercial negative electrode materials, such as graphite and silicon. 1−9 Incorporation of FEC has also been reported to significantly improve the cycling performance of lithium metal electrodes, 10,11 which are proposed to be the next generation anodes for lithium batteries. 12 However, the mechanism of performance improvement for lithium metal anodes cycled with electrolytes containing FEC is not well understood.
Previous investigations provide insight into the composition of the solid electrolyte interphase (SEI) 13 18,19 However, LiF is observed in nearly every SEI generated on the surface of anode materials, including lithium. 11 Therefore, a strong understanding of the source of the improved performance for lithium metal anodes in the presence of LiF and polymeric species is lacking.
The mechanism of performance enhancement for lithium metal electrodes  After drying, the TEM grid was placed in a Cryo-Transfer Holder, shutter closed, assembly placed in a sealable Aldrich AtmosBab, allowing for transfer into the TEM without air exposure. Energy dispersive X-ray analysis (EDX) was used to analyze the elemental composition of the surface films on the plated lithium.

RESULTS AND DISCUSSION
The  The C1s, O1s, and F1s XPS spectra of the lithium electrode plated from the mechanism, as previously reported. 30 As lithium is plated from the FEC electrolyte, both LiF and Li 2 CO 3 are formed during the reductive decomposition of FEC. 22 As LiF particle formation is initiated, a high local concentration Li 2 CO 3 is also present 45 resulting in LiF particle capping by a layer of Li 2 CO 3 , thereby controlling the size of LiF nanoparticles. 33 Upon additional cycling, polymeric species are also observed on the outer surface of SEI on lithium metal for the FEC containing electrolytes, further contributing to good cycling performance. While there have been many investigations of the composition of the SEI on anodes in lithium batteries, the results of this investigation suggest that the morphology and nanostructure of the SEI components is critical for lithium metal anodes. The SEI morphology is also likely responsible for the requirement for slow formation cycling of commercial graphite anodes in lithium-ion batteries. 39,40 Developing a better understanding of the role of the nanostructure of the SEI components is required to develop the next generation of lithium batteries.      mechanism not only provides insight for improving lithium metal anodes for batteries, but also expands upon the understanding of the role of LiF in the SEI on graphite electrodes in commercial lithium ion batteries. A superior understanding of the structure and function of the SEI will facilitate the development of next-generation energy storage systems.

INTRODUCTION
Lithium metal is a promising negative electrode material for future highenergy batteries for consumer electronics and electric vehicles. Lithium metal anodes have a very high theoretical specific capacity of 3860 mAh g -1 , extremely low negative potential (-3.04 V vs. standard hydrogen electrode) and low gravimetric density of 0.534 g cm -3 . Thus, application of lithium metal to secondary lithium batteries has been investigated intensively. 1,2 However several barriers exist in commercializing lithium metal anodes, including the formation of lithium dendrites, safety risks caused by dendritic lithium, and low Coulombic efficiency.
Since lithium metal reacts with most common electrolytes, a solid electrolyte interphase (SEI) 3 is generated from the decomposition of the electrolyte on the lithium metal anode during the plating process. The SEI stabilizes lithium metal and prevents further reaction with the electrolyte. While the SEI on lithiated graphite electrodes used in commercial lithium ion batteries has reasonable stability to afford long term cycling performance, a stable SEI on lithium metal anodes has not been observed. The instability of the SEI on lithium metal leads to poor efficiency and irreversible consumption of lithium. Thus, the generation of a thin and stable SEI for lithium metal anodes is critical. Variation of the electrolyte used with lithium metal anodes has been reported to result in significant changes to cycling efficiency and lithium dendrite growth. These variations in electrolyte include, but not are limited to, solid-state or polymer electrolytes, 4-6 concentrated electrolytes, 7 ionic liquids, 8 20 Therefore, the mechanism of LiF generation from the electrolyte and the structure of the LiF particles must strongly influence the electrochemical performance of lithium metal. In addition, the importance of the morphology or nanostructure of SEI components, including LiF, has been proposed for decades, 21,22 however, direct evidence has not been reported. Herein, a unique mechanism for the generation of nanostructured LiF is proposed along with a mechanistic rationale for the improved electrochemical performance of an SEI on lithium metal containing 3x10 -3 atm). After drying, the grid was transferred to the TEM chamber without air exposure using a Cryo-Transfer holder and a sealable Aldrich AtmosBag. Energydispersive X-ray spectroscopy (EDX, INCAx-act, Oxford Instrument) was also conducted to analyze the element composition using beam diameters between 10-25 nm.

RESULTS AND DISCUSSION
The cycling performance of these cells is depicted with Coulombic efficiency versus cycle number (Figure 4-1a) and the total amount of lithium stripped each cycle ( Figure 4-1b). The stripping capacity versus cycle number is also presented in for the source of the significant performance differences. The morphology was also investigated for the LiPF 6 electrolyte for further comparison.
The morphology of plated lithium is dependent on the electrolyte used.
Specifically, the appearance of lithium plated from the LiPF 6 electrolyte is nonuniform (Figure 4-10a). There is no unique morphology observed and many different shapes of lithium (light and dark gray, It is proposed that during the reductive decomposition of LiDFOB, the decomposition products, likely oxalate or CO 2 act as a capping agent [44][45][46] for LiF nanoparticle generation (Figures 4-12a,b). Similar capping agents have been widely used for the synthesis of nanoparticles. A capping agent enables control over the size or shape of particles without agglomeration by modifying the surface of particles.
Oxalates are one of the typical capping agents used to prepare metal oxide nanomaterials. 47 (Fig. 4c). Interestingly, the LiF/Li 2 CO 3 interface at the nanostructured level has been computationally predicted to have high lithium ion conductivity which could also contribute to the good performance of the LiDFOB electrolyte. 50 However, when lithium is plated with the LiBF 4 + LiBOB electrolyte, the size and distribution of the LiF particles is not 80 controlled well due to the poor capping ability of LiBOB compared to LiDFOB. The LiF particles grow much larger and do not evenly coat the surface. In addition, continuous LiBOB reduction during prolonged cycling generates a more resistive surface film on the lithium electrode which quickly leads to cell failure (Figure 4-12d).
The differences in cycling performance can be related to differences in diffusion field gradients at the nanometer scale. Schematic diagrams of the diffusion field on lithium plated with the LiDFOB and LiBF 4 + LiBOB electrolytes are depicted in Figure 4-12e and f. Since LiF has an electronically insulating nature 51 and its cation diffusivity is lower than other SEI components, 52            reported for silicon anodes. 6,7 It has also been reported that FEC generates LiF deposits which may contribute to the improved cycling performance of lithium metal anodes. 8,9 Recent work suggests that FEC can generate nano-structured LiF, creating a uniform diffusion field on the lithium metal electrode, leading to uniform plating and stripping. 9 Furthermore, it has been demonstrated that employing FEC in co-solvent amounts is optimal for achieving high performance lithium metal anodes. 6 Lithium difluoro(oxalate)borate (LiDFOB) has also been reported to generate nano-structured LiF for lithium metal electrodes, thereby improving the electrochemical performance of the lithium metal anode. 10   with previous work. 6,9 This is also evident in Figure 5-2C, since the quantity of reversibly cycled lithium exceeds the best EC electrolyte by more than 1000 mAh/g. and lithium alkyl carbonates (ROCO 2 Li; 1690 cm -1 ), as previously reported. 9,[12][13][14][15] The peaks associated with ROCO 2 Li and Li 2 CO 3 have comparable intensity, suggesting comparable concentrations of these two SEI components for lithium metal plated with both 1.0 M LiPF 6 EC and FEC electrolytes, consistent with previous work. 9 The similar IR spectra for lithium plated with the 1.0 M LiPF 6 EC and FEC but significant difference in cycling performance have been discussed previously, suggesting that the nanostructure of the SEI products is a major factor in electrochemical performance. 9,10 For