Investigation of Failure and Gassing in Advanced Lithium Ion Battery Systems with Electrolyte Optimization as a Solution

Stronger emphasis on sustainability has become a necessity amongst all industries, and the automotive industry is no exception. The push to move toward hybrid electric vehicles (HEVs) and electric vehicles (EVs) has resulted in a need for lithium ion batteries delivering higher power over a wide temperature range with improved safety over a long lifetime. To accomplish these requirements, advanced electrode materials such as the high nickel cathode material LiNi0.8Co0.1Mn0.1O2 (NCM811) or the anode material Li4Ti5O12 (LTO) have been sought after. The high nickel cathode materials come with the desired high capacity suitable for the power needed for automobile applications but comes with safety and cycle life troubles. Looking at the other electrode LTO comes with long cycle life and improved safety compared to the widely used graphite anode but has gassing and capacity setbacks. The purpose of this work has two focuses, anode and cathode, with the common goal of using electrolyte optimization to resolve these advanced material problems. Electrochemical performance testing, gas chromatography, and electrolyte formulation investigation has been conducted to understand the mechanism of gas production with the LTO anode material. Results from this showed the gas evolution is directly related to the electrolyte interacting with the surface of the LTO. By creating a passivation film to protect the surface of the electrode from the electrolyte reactions through additive optimization and electrolyte formulation, we reduced the amount of gas produced by the material. Electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and ATIR-IR spectroscopy were used to characterize the surface film. Using the same concept of electrolyte optimization, additives such as tris(trimethylsilyl)phosphate (TMSP) and Ethoxy pentafluoro cyclotriphosphazene (PFPN) were shown to provide performance benefits to NCM811 cathode material through electrochemical measurements and EIS. Through the experiments conducted and results gathered, this work shows the ability to make the advanced materials, such as NCM811 and LTO, viable materials for successful commercialization in lithium ion batteries.

replacing pure lithium metal with lithium intercalated carbon as an anode material 3 , and functional electrolytes that formed surface films and utilized non-aqueous, organic solvents 4 . Since its commercial debut, the increasing use in consumer goods has caused a demand in research to continue to deliver lithium ion batteries of higher energy and power. Paired with the political, industrial, and technological pushes toward sustainable means of energy and transportation, lithium ion batteries are the most competitive technology to deliver hybrid electric vehicles and electric vehicles due to their high capacity capabilities and long cycle life. To meet these demands, researchers have focused on developing new high capacity cathode materials, optimized battery management systems, advanced anode materials, and multifunctional electrolyte formulations. 1,[5][6][7] With new materials being explored, come new issues that need to be solved including balancing safety, cost, and performance.

Working Concept of Lithium Ion Batteries
The working components of a lithium ion battery consist of a positive electrode (cathode), a negative electrode (anode), and the conductive, lithium ion transporting electrolyte. The two electrodes are separated from each other by a separator, which is most commonly made from a porous polymer membrane. The separator is inert in the system and serves as a means of preventing internal shorting of the cell. During charging, the positive lithium ions travel from the cathode (oxidation process) to the anode (reduction process). During discharge, the reverse happens, and lithium ions move back to the cathode from the anode. The electrolyte should be compatible with all components of the battery while reversibly shuttling the ions. Other components of the battery such as the battery management system (BMS), battery casing, and other engineering factors are also taken into consideration later in the development process. [8][9] Electrodes: Cathode At the birth of lithium ion batteries, the prominent cathode material was LiCoO2 (LCO), which has a layered crystal structure. While this cathode material provides high theoretical specific capacity (274 mAh/g) 5 and strong cycling performance, the cost of cobalt and its low thermal stability leads to a material that proves to be expensive and unsafe. Other metals such as nickel and manganese were explored as replacement metals. Nickle provided high capacity and a lower cost but had cationic mixing and thermal stability issues on its own. Manganese was investigated for the reduced cost and provides improved safety, but the crystal lattice 3 shifts and the metal leaches out causing harm to the anode. LCO provides a rate performance advantage, nickel provides a capacity advantage, and the manganese provides a safety advantage. With all three metals offering different advantages with their own unique complications, the mixing of metals in the cathode material occurred to get a combination of the properties. More cobalt allows for better cycle and rate performance while more nickel allows for higher capacity. Increasing the manganese allows for thermal stability and overall safety in the material. Both nickel and manganese provide cost benefits. 1,5,10 Electrodes: Anode The carbon anode has been the commercially favored and most widely used anode material for more than 20 years. 5 Carbon anode operates at a low working potential versus lithium, is abundant and low cost, and shows the ability to have good cycle life if protected properly. 2,5,11 While suitable for consumer electronics, the demands of an electric vehicle have made it clear that new anode materials are attractive options. The anode material Li4Ti5O12 (LTO) has been highlighted by many researchers as a viable candidate for these higher power applications. LTO has a theoretical capacity (175 mAh/g) 5,12 lower than the carbon anode (>300 mAh/g) and a lower work voltage window , however, it has no volumetric change in the crystal lattice structure, high rate capabilities, long cycle life, and improved safety over the carbon anode. 5,12 Combining LTO with advanced cathode materials does allow for an improvement on the voltage window. That leads to the true problem with the LTO anode, which is the strong gassing that occurs at the surface. [12][13][14]

Electrolyte
The electrolyte for a battery is a complicated system of salt, solvent, and additive components. The solvents most widely used today are linear and cyclic carbonates such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and propylene carbonate (PC). The most widely used salt is lithium hexafluorophosphate (LiPF6). The use of EC with a carbon anode is nearly mandatory as it is used to form a solid electrolyte interface (SEI) layer on the anode surface for surface protection. However, it has a high viscosity which requires it to be mixed with the linear solvents. A perfect electrolyte formulation will be stable over a wide temperature and electrochemical range, have a low viscosity, and good solubility. Additives are added to the solvent mixture as a means of also protecting the anode and cathode material from interacting negatively with the electrolyte through oxidative side reactions. 1,6

Review of the Problem
The main obstacles that lithium ion batteries must overcome to successfully be adapted into the HEV and EV industry involves improved power, safety, lifetime, and cost over a wide temperature range. 1,6 To meet these standards, advanced cathode, anode, and electrolyte must be created. This thesis work confronts these challenges by looking into the gassing mechanism of the LTO anode, how electrolyte can improve this material through surface protection and solvent optimization, and how electrolyte can improve the performance of a sought after high performance cathode material through additive optimization.

8.
Deng, D., Li- presented. Using the EC free formulation and the gas reducing additive, we were able to successful reduce the amount of gas formed and confirm the gas produced by LTO is the result of electrolyte interactions rather than moisture or HF attacks.

Introduction
Lithium-ion batteries have garnered a lot attention due to their beneficial properties in electric and hybrid electric vehicles as well as in energy storage. While lithium titanate (Li4Ti5O12, LTO) has a lower theoretical capacity of 175 mAh g -1 compared to carbon (372 mAh g -1 ), it is an attractive anode material for these applications due to its long cycle life performance, 1.55V working potential which provides safety benefits, and zero volumetric lattice variation during charging and discharging [1][2][3][4] . Despite being regarded as one of the leading anode materials [5] , it suffers from large gas evolution at elevated temperatures causing premature cell life termination [6][7][8] .
Many investigations into this gas production by LTO has been conducted by different researchers throughout the field. One theory that has been researched in the influence of water contamination from humidity, electrolyte contamination, and/or trapped hydroxyl groups on the surface of the LTO during production [7,9,10] . Other research as investigated the lithium salt influence as the most common salt, lithium hexafluorophosphate (LiPF6) is known to decompose at elevated temperatures to form hydrofluoric acid (HF) [12][13][14] , which would be increased if there is water present in the LTO system. While the influence of water is understood to be detrimental to most lithium ion battery systems, recent work has been focusing on another potential cause of LTO gassing; interfacial reactions between the LTO surface and the electrolyte solvents [7][8][9][15][16] . This recent work has shown evidence that the source of majority of gas formed from LTO is from the solvents interacting with the different transition 11 states of the Ti on the outermost layer of the LTO surface. These surface reactions are reported to lead to decarboxylation, decarboxylation, and dehydrogenation of the electrolyte solvents [15] . While many researchers have investigated these findings either in pouch cells at room temperature or in other non-full cell formats at high temperature, we believe is the next step in LTO testing is to investigate in full cell pouch cells at elevated temperatures to create realistic battery scenarios. Therefore, electrolyte formulations replacing the LiPF6 salt with imide salts, introducing an acid scavenging additive, testing an EC free formulation, and trying a novel LTO SEI additive were utilized to investigate some of the details of the gassing mechanism in LTO full cell, pouch cells at high temperature.

Materials
The cathode active material was commercially available BTBM lithium manganese oxide (LMO) and the anode was commercially available POSCO

Electrolyte HF Storage Testing
To verify that the additives react with the electrolyte in the way that the experiment intended them to do, electrolyte underwent storage testing. Each formulation was made in a large batch and allocated in a nitrogen glove box into small aluminum, air tight bottles. Enough samples from each batch were stored to allow for three duplicates for each week measurement to ensure reproducibility. The first sample was tested after blending and right before the other bottles were added to storage.
The HF was measured using a Metrohm titrator with Tiamo software and a Metrohm double platinum wire 6.0341.100 pH electrode. In a Teflon beaker, about 50 grams of crush ice formed from deionized (DI) water and 50 grams and 60 grams of DI water is added. Cold water and ice is used to stall the formation of excess hydrofluoric acid (HF) formation from LiPF6 reacting with moisture leading to false high readings. The beaker is set on the Metrohm titrator propeller stirrer and base, and the electrode is submerged. After the run is set, 5-10 mL of electrolyte is added to the beaker and the sample is titrated to equilibrium with 0.01N NaOH.

Pouch Cell Preparation
Dry cells were dried at 55°C for 12 hours under vacuum prior to filling. Cells were then filled with 9.1 grams of electrolyte and vacuum sealed in an argon dry box.
To ensure proper wetting, the cells rested for 12 hours in a 25°C chamber, unclamped before starting formation and testing. Volume measurements were taken before formation. 13

Electrochemical Testing
Pouch cells were clamped and cycled with a constant current charge at 0.1C rate up to 2.8V using a MACCOR battery cycler. Upon complete charge, the cells were removed from the clamps and placed into a 45°C chamber for 12 hours for aging. Cells that underwent high temperature storage (HTS) underwent the following before storage procedure with tight clamping: CCCV charge to 2.8V at 0.7C with a cutoff current of 0.02C, CC discharge to 1.7V at 1C, and CCCV charge to 2.8V at 0.7C with a cutoff current to 0.02C. In the 100% state of charge (SOC) the cells will undergo any other measurements that need to be taken and then stored under light clamping in a 60°C chamber for 7 days. At the end of the storage time, the cells will be removed from high temperature and undergo any room temperature measurements that are needed. The final step is the following after storage procedure under tight clamping: CC discharge to 1.7V at 1C, CCCV to 2.8V at 1C with a cutoff current of 0.02C, and CC discharge to 1.7V at 1C.
Cells that underwent cycling followed the second formation step with a rate test procedure that cycled between 2.8V and 1.7V with the following cycles: first and 14 second cycles C/2, D/2, third cycle C/2, D/5, fourth cycle C/2, D/2, fifth cycle C/2, 1D, sixth cycle C/2, 2D, and cycles seven through nine C/2, D/2. After rate test cells underwent a second aging step by undergoing the before storage procedure described previously. Cells are then stored under light clamping at 100% SOC for 24 hours.
Cells then undergo the after-storage program described previously. Cells undergo any before high temperature cycling measurements and are placed into the 60°C chamber under tight clamping. The cells are cycled between 2.8V and 1.7V at 1C/1D with a C/10, D/10 cycle every 50 th cycle. Every 300 cycles the cells are suspended in the discharged state for volume measurement at room temperature. After volume measurement, cells then resume the same cycling procedure for another 300 cycles.
All cells were prepared in minimum of two duplicates to confirm reproducibility for all tests conducted.

Gas Analysis
Gas volume was measured before first formation, before aging after first formation, after aging before degassing, before storage and/or cycling, and after storage and/or cycling according to the procedure first described by Aiken et al. The pouch cells were hung from the bottom of scale and tarred. After reaching a stable zero, the cells were submerged completely to a defined level in 25°C deionized water.
The recorded weight of the cell while submersed was then used along with the Archimedes' principle to calculate the amount of gas evolved over time [17].
To measure the composition of gasses, cells were brought into the argon dry box for extraction. A 0.5 mL Vici precision sampling analytical pressure-lok syringe 15 was used to manually extract the gas sample from the cell under argon atmosphere.
The sample was then manually injected into a Varian 450 gas chromatograph equipped with a 19808 ShinCarbon ST column, thermal conductivity detector (TCD), and an argon carrier gas.

Results and Discussion
Electrolyte HF Storage Testing  TMSB. This additive was selected due to the LTO SEI it forms and the performance benefits seen during formulation screening.
After formation, including the aging step, the cells underwent the gas volume measurement. The acid scavenger showed a 9.47% or 1.44 mL increase in hydrogen gas than the baseline indicating removing the HF during the formation was not the source of initial hydrogen gas. The acid scavenger also showed no ethylene or carbon dioxide present.
The formulation containing TMSB showed the lowest amount of gas formed, and despite the percentage of hydrogen gas being 14.1% greater than the baseline, it had 0.535 mL less hydrogen formed in addition to significantly reduced amounts of the carbon gases. This was the smallest amount of hydrogen gas and net gas produced during formation. Further investigation is needed to characterize the SEI, but the protecting layer shows evidence of electrolyte stabilization and reduced interaction with the surface of the LTO during formation.
After cells were removed from high temperature storage for one week, the volume and gas analysis were analyzed again. The gas measured and analyzed during this step is only from storage as the cells were degassed after the formation phase. formulation that reduced the gas evolved after storage was the 1% TMSB formulation, which was 0.721 mL less than the baseline. This is evidence that protecting the LTO can be an effective means of reducing the gas in formation and for long term, high temperature performance. The formulation with DMAc and LiFSI salt replacement showed the largest amount of gas evolved after storage.
The gas composition after HTS showed new gases formed compared to the formation gas composition as seen in Figure 2

High Temperature Cycling
High temperature cycling is conducted at 45°C compared to the 60°C that high temperature storage takes place. Figure 2 During high temperature cycling, the amount of volume of gas each cell produced was measured after every 300 cycles. The volume measurement was conducted at 25°C after the cells were rested to stabilize temperature, and the gas produced is shown in figure 2.7. After 300 cycles, the TMSB and EC free formulations both reduced the amount of gas formed by 4.26% and 2.23% respectively. Following the same trend as formation and storage gassing, Base 2FSI and base with DMAc showed the largest amount of swelling with the acid scavenger formulation producing 10.66 mL of gas; more than double the baseline. After 600 cycles, the EC free and TMSB had reduced the amount of gas by more than 17.8%.
The DMAc formulation had produced so much gas, that the cells floated during measurement and an accurate value could not be obtained. Composition of the gas formed was conducted after 300 cycles for the baseline, but other composition testing is ongoing. Initial results showed similar composition to the high temperature storage test indicating the same gassing mechanism.

Conclusions
The main theories surrounding the mechanism behind LTO gas formation were

Volume of Gases Formed After Formation and Aging
Electrolyte H2        hours under vacuum prior to assembly. Once dried, cells were transferred to a nitrogen glove box and assembled using PRED 2032 type coin cell parts, Celgard polypropylene separator, and 120 µl of electrolyte. Cells underwent 1 hour of resting at 25°C after assembly to ensure complete wetting before formation and cell testing.
Surface analysis was carried out on electrodes extracted from coin cells.

Electrochemical Testing
Formation and Aging -Pouch cells were clamped and cycled with a constant current (CC) charge at 0.1C with a 2.8 V cutoff voltage using a MACCOR battery cycler.
Once charged, the cells were unclamped and placed in a 45 °C chamber for 12 hours of aging. Cells were then degassed and vacuum-sealed in the argon glove box before undergoing a second formation step in which the cells were cycled with a constant current-constant voltage (CC-CV) charge and CC discharge between 2.8 and 1.7 V vs.
Li4Ti5O12/Li7Ti5O11 with the following procedure: 1 cycle at C/10, 1 cycle at C/5, and 1 cycle at 1C. Coin cells did not undergo a degassing step like the pouch cells but otherwise followed the same formation and aging steps. in a 60 °C chamber for 1 week. Upon completing the storage procedure cells followed the following after storage procedure: discharged with CC to 1.7 V at 1C, charged with CC-CV to 2.8 V and finally discharged to 1,7 V at 1C.

High Temperature Storage (HTS) -
Long Term Cycling -After completing the formation and ageing procedure, cells undergo rate testing between 2.8 and 1.7 V according to the following procedure: 2 cycles with C/2, D/2; 1 cycle with C/2, D/5; 1 cycle with C/2, D/2; 1 cycle with C/2, 1D; 1 cycle with C/2, 2D; and 3 cycles with C/2, D/2 (where C = charge rate and D = discharge rate). Once the rate testing is complete, cells undergo the before storage procedure described in the HTS section, stored in the 100% SOC for 24 hours, and undergo the after-storage procedure described in the HTS section. Cells were transferred to a 45 °C chamber (tightly clamped) and cycled between 2.8 and 1.7 V at 1C with a C/10 cycle every 50 cycles.

42
All cells were prepared in duplicate to confirm reproducibility. Representative data are presented.

Gas Analysis
Gas Volume -Gas volume was measured before first formation, before aging after first formation, after aging before degassing, before storage and/or cycling, and after storage and/or cycling according to the procedure first described by Aiken

Results and Discussion
Although gasses formed during formation are typically removed from cells,  Manganese was only detected on the surface of LTO anodes stored in the absence of the borate additives. This suggests that the borate additives prevented manganese dissolution from the LMO cathodes during 1 wee of storage at 60 °C.
C1s, O1s, and F1s core spectra of LTO anodes extracted from cells after 1 week of storage at 60 °C are displayed in Figure 3          All cells were prepared in duplicates to confirm reproducibility. Representative data is presented.

Gas Analysis
Gas Volume -Gas volume was measured before first formation and after aging before

Impedance
Electrochemical Impedance Spectroscopy (EIS) -EIS was measured on all pouch cells before and after high temperature cycling in the discharged state. All measurements were taken at 25°C with a Solartron Analytical modulab 2100A potentiostat with a 5 mv amplitude with the frequency sweep between 1000 kHZ -25 mHz.

Results and Discussion
The results from cycling for 200 cycles at 60°C can be seen in Looking into explanations as to why this occurred, the AC impedance data presented in Figure 4.3 shows B50 with the lowest impedance increase throughout cycling despite the second highest initial impedance. The formulations containing TMSP, B45 and B50, show similar impedance until about 100 cycles. After 100 cycles, B45 increases in impedance rapidly, while B50 does not increase in this way.
Formulations B66, B44, and B45 all have very similar impedance at the end, indicating TMSP is helping the impedance of the cell while PFPN may be not be as beneficial for the impedance. The EIS data in Figure 4.4 gives another look into the impedance trends seen during cycling. B44 shows the largest total impedance before 68 cycling with B45 and B50 having very similar performance again. After cycling, B50 has significantly lower impedance than the other formulations including the baseline indicating the impedance benefits of TMSP. B44 and B45 show similar performance higher than B50 but lower than B66 which is showing the highest impedance of all formulations.
Due to the performance and impedance data, the differential capacity for the first formation is referenced to determine any possible changes between the additive formulations. As Figure 4.5 shows, the formulations containing TMSP display peak changes around 2.38V, 2.7V and 2.85V respectively. At 2.38V, B45 and B50 show a slight shift in the baseline peak. B44 does not show this shift. This is an early indication that TMSP is protecting the formation of or forming a different SEI. To further confirm this, B50 shows the strongest increase in intensity of the peak at 2.7V.
Formulation B45 shows a similar increase, although not as intense. The baseline and PFPN formulation do not show a difference at this peak and remain similar. TMSP formulations also shows a peak at 2.85V which is not present in the baseline or B44.
TMSP formulations also show a leveling of a small peak seen in B66 and B44 around 2. 19V.
Formation gas analysis shows that TMSP containing formulations had the largest amount of formation gasses present as seen in Figure 4.6. While B44 did show a decrease in the formation gassing, the gas composition does not vary significantly from B66. The formation gas produced by the baseline was primarily ethylene and methane at just under 34% each. Figure 4.7 shows that the largest change in formation gas composition was with B50 reducing the amount of ethylene by 8% and the 69 hydrogen gas by almost 2%. This gas reduction was accompanied by an increased in the carbon monoxide produced. TMSP may have stabilized the solvent interactions through SEI formation but forming that SEI releases increased levels of carbon monoxide.

Conclusions
The electrolyte additive TMSP improves the cycling capacity at 60°C while reducing the impedance of the system. TMSP likely does this through SEI formation that stabilizes the electrolyte based on differential capacity and formation gas analysis data. During the SEI formation process, an increased level of gas is produced as a side product. While the main gasses produced in the NCM811/C system are methane, ethylene, and carbon monoxide, TMSP reduces the ethylene and increases the carbon monoxide. This SEI formation and formation mechanism will be confirmed with Xray photoelectron spectroscopy (XPS) surface analysis. The phosphazene additive, PFPN, showed no added benefits to the system. To the contrary, PFPN hurt the cell capacity performance with little improvement to the impedance and formation gas evolution.