Performance Analysis of Separators and Electrolyte and Effects on Solid Electrolyte Interface

Much of the mobile technology used today (e.g. smartphones, tablets, and laptops) is possible because of the lithium ion battery. Because of the high energy density of lithium ion batteries and the continuous improvements in all facets of the technology, larger and more sophisticated batteries are becoming more common for high energy applications such as energy storage for green energy production, electric vehicles (EVs), and hybrid-electric vehicles (HEVs). Despite early success for larger applications, there are some serious problems that still need to be addressed such as cost and battery life. These issues are the motivation for this research. Utilization of different separators and electrolyte formulations can help achieve sustainable cycling performance and lower cost. The impact on cell performance of electrodes made by a new method to manufacture lithium ion batteries that utilizes ceramic separators is explored, which promises to streamline the manufacture process to lower costs and allow for better performance. Electrodes built using a layer by layer deposition method were cycled and analyzed, including SEM images of the cross section to understand the impact of cycling on the layer interfaces. In addition to knowing how the new manufacturing method can impact the performance of a battery, it is also desired to understand how the separator impacts the performance of the battery. Batteries with ceramic separators are thought to perform better than batteries with polyolefin separators. Cells using ceramic and polyolefin separators were built and cycled extensively at elevated temperature to look for performance differences. In a separate effort to make lithium ion batteries cheaper and perform better, manipulating the electrolyte formulation may be key, especially if ethylene carbonate can be replaced with GBL, and LiPF6 could be replaced with a more thermodynamically stable salt such as lithium difluoro(oxalato)borate (LiDFOB). Various LiDFOB electrolyte solutions were made and tested against a standard electrolyte for cycle performance, taken apart, and the surfaces analyzed. The development of a carbonate free electrolyte can be very beneficial to the lithium ion battery, and with thinner and better performing separators, lithium ion battery technology can be better situated for use in large scale applications.


Chapter 1 -Background Introduction to Lithium Ion Batteries
Lithium ion batteries (LIB) are of great interest due to high energy density and high working potentials, making them useful in essential mobile technologies such as smart phones, laptops, and tablets. The reduction potential of lithium vs the standard hydrogen electrode is about -3 V, and lithium is light weight (0.534 g cm -3 ), giving LIB high energy density [1]. The working potentials of most LIB are in the range of 3.5 to 3.9 V [2], depending on the electrodes used, with the potential working voltages increasing to well over 4 V for newer cathode technologies. The operating potentials for LIB are by far higher than any other secondary battery on the market, where chemistries only allow working potentials of 2 V or fewer [2].
Other than high voltage and great energy density, LIB have other notable advantages.
LIB can be designed for high energy purposes or for high power purposes. LIB have a high range of temperature operation from well below freezing to moderately warm temperatures, but the specific range depends on the cell chemistry. While these advantages work in the favor of LIB, they can sometimes work at a disadvantage to LIB.
For instance, a battery can be designed for high power or high energy, with one coming at the expense of the other. While LIB can operate at wide temperature ranges, they tend to have lower capacity at lower temperatures. This can be partially mitigated with electrolytes designed to work at low temperatures. At the higher temperature range, LIB work very well, but will lose capacity at an accelerated rate with chronic exposure. Cost is also a disadvantage for larger format batteries as discussed below.

How a Lithium Ion Battery Works
LIB are composed of a cathode with an aluminum current collector, an anode with a copper current collector, a separator, and electrolyte. The source of lithium is in the cathode material, while the electrolyte facilitates ion movement between the electrodes.
Energy is stored at the anode when the intercalated lithium effectively traps electrons.
The typical lithium ion battery usually operates between 3.0 to up-to 4.3 V with a working voltage around 3.7 V [2], but new cathode technology can increase the working and cut-off voltages [3].

Cathode, Anode, and Separators
The cathode is made of LMO (lithium metal oxides), where M is cobalt (Co), nickel (Ni), or manganese (Mn), or a combination of these metals. Some commonly used cathode materials are LCO (lithium cobalt oxide), NMC (lithium nickel-manganese-cobalt oxide) and NCA (lithium nickel-cobalt-aluminum oxide, where Al is a minor component).
Other cathodes can be made with metal phosphates, such as LiFePO4 (lithium iron phosphate) [2,3]. Capacities of most cathodes is around 150 to 200 mAh g -1 , while the working potentials vary from 3.5 to 3.9 V depending on the cell chemistry [2].
Generally, only half of the available lithium in the cathode is used. If too much lithium is taken from the LMO, the structure can collapse causing capacity fade. Work is being done to expand both the specific capacities and the operating potentials [3].
The anode is generally made with graphite. Graphite has a theoretical capacity of 372 mAh g -1 , but can vary depending on its form. When lithium is fully intercalated into the anode, six carbons can accommodate one lithium. To expand capacity, there is investigation into silicon, which has a theoretical capacity around 4200 mAh g -1 , but has serious issues with volume expansion upon charging [4].
For both the anode and cathode, the active material is combined with binder material, and The job of the separator is to allow for ion flow from one electrode to the other while preventing any electron flow, essentially separating the anode from the cathode. The typical separator is made up of polyolefins, usually polypropylene and/or polyethylene, other kinds of polymers, ceramics, and ceramic/polymer blends [5]. Currently, polyolefins are utilized in most commercial batteries, but ceramic containing separators are gaining lots of attention because they can absorb and retain the electrolyte better and facilitate easy ion transport. The thickness of the separator is on the micron scale, and is typically around 25 µm [5].

Electrolyte and SEI Formation
The main purpose of the electrolyte is to shuttle the lithium ions between the anode and cathode upon charging and discharging the cell. Carbonates are used because they can remain stable at the cathode-electrolyte interface for most voltage ranges, and remain stable at the anode-electrolyte interface after the formation of a reductive layer at that interface known as the solid electrolyte interface (SEI) [1,[6][7][8][9][10].
For purposes of SEI formation, EC is paramount. It is readily reduced on the anode during the first charge cycle. While the linear carbonates are more stable on the anode interface than EC is, they also do not bind to the lithium ions as well due to their low dielectric constants. EC on the other hand has a very high dielectric constant, lower, but similar to that of water [1]. However, EC comes with a big downside in that it is a solid at room temperature making the electrolyte viscous as a result. It needs other solvents to make it thinner and to allow it to be conductive enough to work at low temperatures. The linear carbonates as well as various esters have been used to accomplish this [11,12].
The SEI is a protective layer on the anode/electrolyte interface. This layer forms during the first few cycles to stabilize the electrolyte against the anode surface, which is unstable. Good performing LIB depend on a stable SEI, which will stabilize the electrolyte/electrode interface while allowing ion flow and disallowing electron flow. thermally stable enough, is relatively safe, and it has very good conductivity [1,12]. As compared to other possible salts such as LiBF4, LiAsF6, LiClO4, and other salts, it is not the best in every category, but it is good enough in all categories, which cannot be said for its competitors, which often have a fatal flaw in one or more category. Despite being the best candidate, it is not the ideal candidate, and much research has gone into finding a replacement [1,10]. The major drawbacks of LiPF6 have to do with thermal stability, and chemical stability in relation to protonated impurities [1,10,[12][13][14][15]. At around 40° C, the LiPF6 breakdown will accelerate, reacting with protonated solvents that exists in even the cleanest and driest electrolytes, especially water [9,[12][13][14]. Impurities will react with PF5, which exists in equilibrium with the LiPF6 anion, to help create HF and fluorophosphates that can further degrade the electrolyte and react with the SEI or the electrodes to cause degradation and cell failure [10,13,14].

LIB Problems and Solutions Presented in Thesis
While lithium ion batteries have many advantages, it is the disadvantages that hold it back for larger scale uses. In order to be viable for the automobile market, LIB have to last more than 15 years, must be safe, and sell at lower costs [16]. Many of the Department of Energy goals for LIB include lowering costs to $125 per kWh [16] of useable energy from somewhere near $300 per kWh today [16], achieve energy densities of 400 Wh L -1 and 250 Wh kg -1 [16], and achieve a power density of 2000 W kg -1 by 2022 [16].
The research presented in this thesis is intended to help improve LIB with the DOE goals in mind. Working with Optodot as a collaborator, studies looking into electrodes made with alternative manufacturing processes, studying alternative electrolyte solutions, and studying the separator effects on cell performance can be a key to reaching DOE goals.
With alternative manufacturing, the goal is to increase energy density by reducing the amount of non-active materials used in the cell such as the separator and current collectors while providing a cell that can perform as well or better than conventionally built cells. The alternative manufacturing can also help to lower the costs of the manufacturing process. In formulating alternative electrolytes, it is possible to reduce the cost of the electrolyte while eliminating problematic solvents such as EC. Lastly, understanding how well different separators work in a cell can be a key to identifying the best separator formulations to use in future LIB. The separator can aid or hinder cell longevity, and understanding why can help make batteries last longer.
the electrode stacks investigated are not optimized, the results support good cycling performance for a stacked cell design.

Introduction
Improvement of the cell design of lithium ion batteries is important to lowering manufacturing costs, which is one of the major hurdles for the widespread implementation of lithium ion batteries into electric vehicles [1]. A novel approach to lowering the manufacturing cost of lithium ion batteries is to build cells via a layer by layer coating deposition process as opposed to the current preparation via slurry coated electrodes and free-standing separators which are then assembled via stacking or winding. The layer by layer method can further benefit cells by providing thinner layers, which have the potential to conserve space, improve the volumetric energy density, and decrease the quantity of inactive components (separator, current collectors, and electrolyte) and thus the cost of the materials within the cell. This process also has the potential for battery preparation via layering electrodes in a single manufacturing process, allowing most of the work to be accomplished through automation reducing the coating and assembly cost. The concept of a cell stack has been investigated but there are few reports in the literature [2][3][4][5]. Most of the research in this area has focused on the development of novel separators rather than developing an electrode stack [6]. These investigations focused on polymer separators with ceramic additives [2][3][4]7,8], or developing separators consisting of ceramic and binder [5].
Since the idea of a stacked cell has not been significantly studied or optimized, the goal of this research was to develop a method to prepare a cell stack with comparable performance to traditional cells.

Preparation of the electrodes
Anode stacks are composed of approximately 13 µm of separator, 43 µm of graphite anode and 11 µm of copper. The cathode stacks are composed of approximately 10 µm of separator, 73 µm of LiNi1/3Mn1/3Co1/3O2 (NMC) cathode and 11 µm of aluminum. Each stack is layered into one bonded and inseparable piece rather than existing as separate pieces of metal/electrode and separator as in the current manufacturing process for cells.
The graphite anodes and NMC cathodes (92% active material along with PVdF binder and conductive carbon) were coated directly onto the ceramic separator layer. This is afforded by the small pores of the ceramic separator (30 nm diameter) which prevent penetration of the electrode materials into the separator layer.
The anode stacks were made by first coating a separator layer of aluminum boehmite and polymer onto a silicone-treated polyester release film. The porosity of the separator layer was approximately 43%. The average pore size diameter of the separator layer was approximately 30 nm with a very narrow pore size distribution. Next, a commercial graphite anode layer was coated onto the separator layer. Then, the copper layer was sputtered onto the anode layer, followed by delamination of the release substrate to provide the anode stack. The cathode stacks were made similarly by coating a commercial NMC cathode onto the separator layer on the release film and then sputtering an aluminum layer onto the cathode layer. Delaminating the release film provided the cathode stack.

Assembly of the coin cells and Cycling
Each sample was evaluated in a coin cell versus lithium metal. The electrode stacks were cut to a diameter of 14.7 mm and assembled in a coin cell using 40 µL of electrolyte (1M LiPF6 in 1:1:1 ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC) [9]) and a protective polyolefin separator ring with an inner diameter of 12 mm and an outer diameter of 19 mm. The separator, which is about 10 µm thick, is only large enough to cover the electrode. A polyolefin ring was used to prevent possible shorting at the edges of the electrode due to the presence of loose particles resulting from electrode cutting and may contact the lithium electrode and result in shorting.
Both the Li/graphite and Li/NMC cells were initially cycled at a C/20 rate for five cycles followed by five C/10 cycles, ten C/5 cycles, ten C/2 cycles, and three C/20 cycles. The Additionally, full cells were prepared and cycled at 55° C to simulate accelerated aging.
The cells were full cells constructed as above and used 30 µL of electrolyte. The formation cycling was one cycle at C/20 followed by two cycles at C/10 and two cycles at C/5 rates.
The cells were then cycled at a C/5 rate for ten cycles at 16° C, ten cycles at 55° C and ten cycles at 16° C.

Preparation for SEM
The electrode stacks were imaged using cross sectional SEM with fresh electrode stacks and electrode stacks cycled in graphite/NMC cells. The fresh samples were prepared by cutting a piece of a small part of the electrode with a razor blade with the separator side facing the blade. Cycled cells were opened in an argon filled glove box, rinsed three times with dimethyl carbonate, and vacuum dried overnight to remove the excess DMC then cut with a razor blade.

Cycling of Li/graphite stack and Li/NMC stack cells
The

Graphite stack/NMC stack cells
Graphite stack/NMC stack cells were prepared and cycled (Figure 2.4). The first cycle efficiency is 85 % and the discharge capacity is 150 mAh g -1 . The irreversible capacity on the first cycle is consistent with the formation of a solid electrolyte interface (SEI) on the graphitic anode [10]. The cells retain high capacity as the cycling rate is increased from C/20 to C/2. The cells still deliver 120 mAh g -1 at the fastest rate (C/2) and the cycling efficiency is high for all cells, >98 %. After cycling at high rate, the cells are able to deliver > 90 % of the initial capacity at C/20 (140 mAh g -1 ) suggesting that the cells have stable cycling performance. The cycling performance is comparable to standard graphite/NMC cells prepared with traditional polyolefin separators [11].
The charge-discharge plot of the eighth cycle is provided in Figure 2.5. The charge and discharge plots are symmetrical with a small hysteresis, and are typical for graphite/NMC cells further supporting comparable performance of the graphite/separator stack and NMC/separator stack electrodes to traditional coated electrodes with polyolefin separators.

Elevated Temperature Cycling
Additional graphite stack/NMC stack cells were cycled at 55° C to simulate accelerated aging and is depicted in Figure 2.6. The cells cycled well, with only an 8 % capacity loss during the ten cycles at 55° C. Upon returning the cells to 16° C after cycling at 55° C, the cells retained 88 % of the initial RT capacity. The good cycling stability at elevated temperature can be attributed to the good thermal stability of the separator and the Lewis basic nature of the aluminum boehmite [12]. The cycling performance of the first generation electrode separator stacks suggests that separated coated electrode stacks may be a viable option for the next generation of lithium ion batteries. To better understand the structure of the electrode stacks before and after cycling at 16° C and 55° C, cross sectional SEM images of the electrodes were obtained.

Conclusion
Layered electrode stacks were prepared with copper/graphite/separator and aluminum/NMC/separator. The layered electrode stacks were cycled with lithium metal anodes. The Li/ graphite stack cells have poor first cycle efficiency, but have high discharge capacity after the first cycle ~250 mAh g -1 . The Li/ NMC stack cells have good first cycle efficiency and reasonable discharge capacity (145 mAh g -1 ) and good rate performance up to C/5. However, the graphite stack/NMC stack cells have the best performance. The cells have high first cycle reversibility (85 %), good capacity (150 mAh g -1 ) and good rate performance up to C/2. These cells also cycle well at 55° C consistent with good calendar life performance. Ex-situ analysis of the electrode stacks before and after cycling suggest that cycling does not induce significant changes to the electrode stack structure and is consistent with good cycling behavior, even at 55° C. The results suggest that the use of layered stack electrodes is a promising alternative for the preparation of lithium ion batteries.

Chapter 3 -Carbonate Free Electrolyte for Lithium Ion Introduction
The widespread implementation of electric vehicles (EVs) requires further improvements in lithium ion batteries [1,2,3]. Some of the biggest challenges for lithium ion batteries in EVs are cost, low temperature performance and battery lifetime [2,3]. Improvements in the electrolyte can assist in the resolution of each of these problems [1,4,5]. Most commercial electrolytes are composed of LiPF6 in a mixture of carbonate solvents [5].
However, the high cost and poor thermal and hydrolytic stability of LiPF6 is problematic for the electrolyte [6,7,8]. In addition, ethylene carbonate (EC) is typically a required component of the electrolyte due to the role of EC in the formation of the solid electrolyte interphase (SEI) on the anode [5,[9][10][11][12][13][14]. Since EC is a solid at room temperature, electrolytes containing EC frequently have poor performance at low temperature [15]. and lithium tetrafluoro(oxalato) phosphate (LiTFOP) as additives to LiPF6 based electrolytes to form a more stable SEI [1,14,23,[26][27][28][29][30]. There have also been reports of the use of oxalate salts enabling the cycling of PC based electrolytes due to better SEI formation [30]. The presence of the oxalate group in the anode thus may enable the use of EC free electrolytes and electrolytes with non-carbonate solvents.
The investigation of LiDFOB has been expanded to include carbonate free electrolyte formulations. Esters and lactones are an interesting alternative to carbonate solvents.
Linear esters have been studied as co-solvents due to the high dielectric constants and low freezing points which have been reported to improve the low temperature performance of lithium ion batteries, [15] while lactones such as γ-butyrolactone (GBL) have high dielectric constants [5] and a very wide liquid temperature range (-43.5 to 204 °C). However, the use of GBL as a primary solvent in lithium ion battery electrolytes has been plagued by problems with the stability of the anode SEI [5]. Despite the issues with GBL as a solvent in carbonate based electrolytes, GBL has been studied with LiBOB based electrolytes due to the limited solubility of LiBOB in carbonates [18,31]. In order to investigate the use of novel electrolyte formulations for lithium ion batteries, a comparative study of three electrolytes has been conducted; a standard LiPF6 electrolyte

Equation 1
Where η, ρ, and t are viscosity in mPa s, density in g mL -1 , and time in seconds. The average time and standard deviation were also calculated. The samples were maintained in an environmental chamber set at 20° C for at least four hours to ensure that the temperature was constant. Density was calculated for the samples in a 10 mL volumetric flask and with the sample at 20° C.
To test the cycling performance of the electrolyte formulations, three replicate cells of each electrolyte formulation underwent formation cycling that consisted of one C/20 cycle, followed by two C/10 cycles, and concluded with two C/5 cycles at 16° C. Each set of cells was then cycled for 50 additional cycles at a C/5 rate. The C rates were based on theoretical capacity. After cycling the cells at room temperature, cells were cycled at a rate of C/5 for five cycles with the charge at 25° C and discharged at -10° C, to investigate the low temperature discharge performance, and cells were cycled for 20 C/5 cycles at 55° C, to study accelerated aging.
Electrochemical impedance spectroscopy (EIS) was conducted on cells after 25 cycles at 16° C. Cells were charged at a C/5 rate to 4.1 V followed by holding at constant voltage for 10 hours. For EIS measurements, the cells were held again at constant voltage of 4.1V for a half hour, and EIS was taken from 300 kHz to 20 mHz. where is the intensity of the relative element, and is the sensitivity number of the element, the values obtained from the Multipak software [29]. The C1s graphite (C-C) peak at 284.3 eV was used to confirm the binding energy scale and the F1s peak for LiF at 685.0 eV was used as a secondary reference [14,21,33].

Conductivity
The conductivities of the three electrolytes are depicted in

Viscosity
The viscosity of the standard electrolyte is slightly higher than the viscosity of LiDFOB in GBL/MB electrolyte as provided in Table 3

Electrochemical Impedance Spectroscopy
The cells containing the different electrolytes are analyzed by electrochemical impedance spectroscopy (EIS) after the first 25 cycles at 16 o C as depicted in Figure 3.4, along with an equivalent circuit [34]. For the equivalent circuit, RB represents the bulk resistance of the cell, RSL represents the resistance of the surface layer including the SEI while CSL represents the capacitance of the surface layer and is the response to high AC frequencies, RCT represents the Faradaic charge transfer resistance with CDL representing a double layer capacitance and are at mid-range frequencies, while W is representative of the Warburg impedance at low frequencies [34]. As related to the EIS plots, the first semicircle at lower resistance can be attributed to the high frequencies, the second semicircle can be attributed to medium range frequencies, and the Warburg impedance is the strait line at a near 45° angle and is at low frequencies. The impedance is smallest for the cell cycled with the standard electrolyte followed by the cell containing the LiDFOB in EC/DEC/DMC electrolyte while the cell containing the LiDFOB in GBL/MB electrolyte has the highest impedance. The changes in impedance are primarily related to changes in the surface layer and charge transfer resistances and may result from changes in the structure of the SEI as discussed below.

Scanning Electron Microscopy
SEM images were acquired for both the anodes and cathodes extracted from cycled cells. However, the SEI is relatively thin since the peaks associated with PVDF are still observed in the C1s and F1s spectra. The XPS spectra of the surface of the anodes cycled with LiDFOB electrolytes are significantly different than the XPS spectra observed for the standard electrolyte. This is expected since it has been reported that LiDFOB is involved in SEI formation [1,4,23,25,26,28]. with LiDFOB or LiBOB [14,28]. The cathode surface film is thinner than the SEI observed on the anodes cycled with the LiDFOB electrolytes, since the peaks associated with the PVDF binder are still observable at 290.5 and 286 eV, but thicker than the surface film observed on the cathode cycled with the standard electrolyte since the peak associated with the metal oxide is no longer observable.

Infra-Red Spectroscopy
The anodes and cathodes extracted for cells cycled with different electrolytes were investigated by IR-ATR spectroscopy (

Introduction
Electric Vehicles (EV) are being pursued by auto makers around the world, and this is drawing interest in better, longer lasting lithium ion batteries [1]. The interest of longevity is rooted in the need for the battery to have a long cycle life and a long calendar life for large format batteries used in EV or other large applications. Both the separator and the electrolyte play a key role in the longevity of the battery.
The role of the separator in the battery is to keep apart the anode and cathode to prevent electrical shorting, but this barrier affects the migration of the lithium ions between the electrodes by adding resistance. The separator may also add moisture or impurities to the electrolyte but may also remove them from the electrolyte; the latter has been difficult to demonstrate experimentally. Many researchers have investigated a variety of separator technologies that include plastics, ceramic coated plastics, and ceramic separators [2][3][4][5][6][7].
While many researchers investigate new separator technologies, they often compare their novel separators to some standard, usually a polyolefin, but they do not analyze cell performance beyond basic cycling of their separator, and there is no published research that looks into why certain separators work better than others [3][4][5][6][7]. Most of these papers point to better performance of ceramic containing separators over polyolefin, but the reasons are not thoroughly investigated. It is important to identify why one type of separator provides better longevity than another to help focus further research with regards to the separator and the electrolyte.
To look for these differences, it is necessary to investigate how the separator effects the solid electrolyte interface (SEI), as the SEI has great influence over the performance of the battery [8][9][10][11][12][13]. The SEI forms both organic reduction products and lithium salts on the anode [9,10,13]. One salt that is seen in particular is lithium fluoride (LiF), and this is in part caused by impurities in the electrolyte, especially water [14,15]