Exploration of Additives for Improving Performance of High Voltage LiNi0.5Mn1.5O4/Graphite Cells (Applied)

Lithium ion batteries have been investigated extensively due to their widespread applications in portable electronic devices and electrical/hybrid vehicles. However, significant challenges still exist for an extended calendar life at a wide temperature range. Due to the intrinsic drawbacks of the commonly used LiPF6-carbonate electrolyte systems, such as insufficient thermal stability at elevated temperature and unavoidable HF contaminants, much effort has been paid to exploring novel additives. It is well known that the introduction of additives into electrolyte systems is one of the most effective and economic approaches to improve performance of lithium ion batteries. In this dissertation, a novel class of borate compounds has been successfully synthesized and screened as additives for electrolyte of lithium ion battery. Nuclear magnetic resonance (NMR) spectroscopy was utilized to characterize compounds by dissolving additives into deuterated solvents. The cycling performance of these novel additives and other commercialized additives was compared by adding them into 1M LiPF6 EC/EMC electrolyte, LiNi0.5Mn1.5O4/Graphite cells are cycled under both room temperature and elevated temperature up to 4.8V. Electrochemical impedance spectroscopy (EIS) and Linear sweep voltammetry (LSV) were used to investigate electrochemical activity of additives. The investigation of the interrelationship of cycling performance, additive structure, and electrode surface film structure has been conducted by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and FT-IR instruments. SEM and TEM images showed that novel additives can form uniform solid electrolyte interface (SEI) and cathode solid electrolyte interface (CEI). XPS and FT-IR spectra were acquired to analyze main components of SEI and CEI, and they are beneficial for further understanding how addition of additives changed surface chemistry of electrodes. The surface reactions of both additives and electrolytes with the graphitic anode and lithium nickel manganese oxide cathode of lithium ion batteries have been speculated. New additives can not only form more uniform SEI on surface of anode, but also beneficial for forming uniform CEI on surface of cathode.


Background of Lithium Ion Battery
As we stepped into 21century, technology evolved extremely fast, electronic devices such as cell phone, laptop, digital camera have been used more and more frequently in people's daily life. As power source of all these portable electronic devices, lithium ion batteries have been used widely with its unique advantages such as high energy density, longevity and no memory effects, many researchers are contributing their efforts to the development of lithium ion batteries [1][2][3]. Since "energy crisis" is becoming a very big challenge for human society, nowadays researchers also expect that lithium ion batteries can be developed further followed by extend its applications such as being used as main power supply for vehicles to substitute fuel based on non-renewable fossil energy.
Lithium ion battery research has been conducted for many years after the first release by the Sony company in early 1990s [4]. LIBs are usually composed of a cathode with an aluminum current collector, an anode with a copper current collector, a separator, and electrolyte. Lithium cobalt oxide (LCO) [5] is mostly used cathode material in commercial LiBs, and recent years, many other cathode materials such as lithium iron phosphate (LiFePO 4 ) [6], lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O 4 ) [7] and lithium nickel cobalt manganese oxide (NCM) [8] have drawn more and more attentions due to their own advantages. Most commercialized anode material is graphite because lithium ion can intercalate into/ deintercalate from layered graphite structure to store and release energy, silicon material is also explored to be using as anode material due to its high theoretical capacity (around ten times larger than graphite) in recent years. However, its volume expansion during lithium ion intercalation process still limits its commercial application. The permeable polymeric separator is also a very important component of lithium ion battery, it can allow lithium ion passing through but inhibit the conduction of electrons to prevent short-circuit the battery [9]. Polyolefin film separator is mostly used separator in commercialized LIBs. After stacking cathode and anode materials with a separator, lithium ion battery is filled with electrolyte to make it work, the electrolyte During charge process, lithium ion will be extracted from cathode material into electrolyte, then lithium ions will pass through separator, and intercalate into anode structure and neutralized with electrons from external power source, energy will be stored in lithium ion batteries. During discharge process, lithium ion will be deintercalated from anode and go back to the cathode structure, electrons will be released to external circuit.
In an ideal situation, when lithium ions are fully intercalated into the anode, six carbons can accommodate one lithium. A typical chemical reaction in lithium ion batteries is described as following [10]: It is generally believed that during initial charging process, a solid layer called solid electrolyte interface (SEI) which is usually composed of decomposition products of organic solvents will be formed on surface of anode electrodes [11], to prevent further decomposition of electrolyte after the second charge. In mostly used combination of linear and cyclic carbonates, ethylene carbon (EC) is decomposed at a relatively high voltage, 0.7 V vs. lithium, and forms a dense and stable interface [12]. SEI is a electrically insulating yet provides significant ionic conductivity.

Cathode Electrode Materials
Lithium ion batteries are becoming more and more important as they are more widely applied in electric vehicle field due to their high gravimetric and volumetric energy density [13,14]. A large number of researchers have began to focus on exploring new cathode materials which can satisfy huge increasing demand of vehicle scale lithium ion batteries [15][16][17], because the mostly used cathode material-lithium cobalt oxide (LiCoO 2 )-has many limitations such as low energy density, high cost, rare resource of cobalt [18,19].  [56]. In addition to the lithium oxalato borates [57,58], we have recently reported on the beneficial effect of the incorporation of lithium tetralkylborates as Additives for Designed Surface Modification (ADSM) to function as functional group delivery agents to modify the cathode surface [59,60].

LIB Problems and Solutions Presented in Thesis
Lithium ion batteries do have a lot advantages, to achieve higher energy density,

Introduction
Li-ion batteries (LiBs) are widely used for portable consumer electronics and have recently been incorporated into electric vehicles (EVs) [1,2]. The performance requirements for EV applications are much more stringent than portable electronics applications. These requirements include longer cycle life and long term stability upon exposure to moderately elevated temperature (45 °C ). Many of these performance limitations are due to the electrochemical and thermal instability of the electrolyte [3,4].
One of the primary methods to improve the high temperature stability of lithium ion batteries is the incorporation of electrolyte additives, which generate a more stable solid electrolyte interphase (SEI) on the anode [5]. Vinylene carbonate (VC) is a well known SEI-forming additive that is widely used in lithium ion batteries [6][7][8][9].
Incorporation of VC into lithium ion batteries has been reported to improve cycling stability of graphite cells cycled at elevated temperature [10][11][12][13]. The improved stability is typically attributed to the generation of a stable polymer film on the graphite surface [11,14]. Most current commercial lithium ion batteries contain VC as an additive. Thus, cathode materials which are incompatible with electrolytes containing VC are problematic.
Methylene ethylene carbonate (MEC) has also been reported to be a good additive for lithium ion batteries for similar reasons [15,16]. While many anode film forming additives have been investigated with graphite anodes and LiCoO 2 or related layered metal oxides with operating potentials around 4.

Electrochemical test and characterization
Electrolytes consisting of 1. XPS measurements were carried out using a ThermoFisher K-Alpha spectrometer, nder a foc sed monochromatised Al Kα radiation ( ν=1486.6 eV). An air-free transfer vessel (ThermoFisher) was used to avoid any contact with air/moisture. Peaks were recorded with constant pass energy of 50 eV with energy resolution of 50 meV and charge neutralization. Peak positions and areas were optimized by a weighted least squares fitting method using 70% Gaussian, 30% Lorentzian line shapes using Avantage (ThermoFisher) software.
FTIR spectra were acquired on Bruker Tensor 27 with Attenuated Total Reflectance (ATR) accessory with Germanium crystal, 512 scans with a resolution of 4 cm −1 . The IR spectrometer is inside of a N 2 filled glove box.

Electrochemical stability of the electrolytes
Electrochemical stability of the STD and VC or MEC containing electrolytes is Cathodic linear sweep voltammetry of Super C65/Li cells is presented in Figure. For the three electrolytes, STD, MEC, and VC, EC reduction is observed at 0.65 V vs.
Li/Li+. The intensity of the EC reduction peak varies as a function of the additive.
Addition of VC increases the intensity of the EC reduction peak slightly, while addition of MEC diminishes the intensity of the same peak. Despite the fact that both VC and MEC are reduced at a similar potential (~1.5 V vs. Li/Li + ), the nature of passivation layers that are deposited at low potential are likely different due to the structural differences of the additives.

Cycling performance
Charge and discharge curves of graphite/LiNi 0.5 Mn 1.5 O 4 cells using the STD electrolyte, with 0.5% MEC and 0.5% VC are provided in Figure 3. The initial charge capacity of the cells with the MEC electrolyte are 184.3 mAh.g -1 , which is about 30 mAh.g -1 higher than the cells using the STD electrolyte. The charge/discharge efficiency of the cells with the MEC electrolyte (75.2% for the 1st cycle and 96.2% for the 2nd cycle) are lower than the cell with the STD electrolyte (87.2% for the 1st cycle and 98.1% for the 2nd cycle). Poor efficiency of LiNi 0.5 Mn 1.5 O 4 cathodes with STD electrolyte has been previously reported [17]. The cell with VC electrolyte has a charge capacity of 174.2 mAh.g -1 and initial charge/discharge efficiency of 80.7%, but much lower charge/discharge efficiency of 90.4% for the 2nd cycle. This suggests that the VC electrolyte continues to be consumed after the first cycle. The additional charge capacity and lower initial efficiency of the cell with MEC electrolyte suggest more reaction of MEC than STD electrolyte and VC electrolyte [18]. The lower initial coulombic efficiency of cells with the MEC electrolyte suggests that more electrolyte is consumed during the first cycle. However, despite the differences in efficiencies, the discharge capacities are comparable. The efficiencies and discharge capacities suggest that some of and more conductive surface film ( Figure 6) [8,20]. In addition, the initially generated polymers may be reductively or oxidatively unstable on the electrode surfaces leading to further decomposition reactions. Differences in these subsequent decomposition reactions could contribute to the observed differences in performance [21,22]. Ex-situ analysis of the cycled electrodes has been conducted to better understand the differences in electrochemical performance.

4. FT-IR
FT-IR/ATR spectroscopy has been conducted on LNMO cathodes and graphite anodes extracted from cells cycled at 45 °C with different electrolytes.
The IR spectra of the fresh cathodes are dominated by peaks from the PVDF binder at 1400, 1170, 1070, 877, and 840 cm −1 (Figure 9a) Figure 9b). Since the IR has a greater depth of penetration than XPS, the poly(VC) and poly(MEC) are predominantly in the inner SEI. The results are consistent with previous reports utilizing VC or MEC as additives to improve the stability of the anode SEI via the generation of polymeric species [11,[14][15][16].

Conclusions
The effect of MEC and VC addition to the EC/EMC 1. Although improved performance is observed with MEC compared with VC, neither additive provides significant improvement over the STD electrolyte. Unfortunately, at this time the source of the performance decreases in the presence of VC are also unclear.         There have been several reports where electrolyte additives have improved the performance of cathodes cycled to high potential (13)(14)(15)(16)21 spectroscopy.

Electrochemical test and characterization
Battery grade of carbonate solvents, lithium hexafluorophosphate (LiPF 6 ) and The discharged electrodes were briefly (15 s) exposed to air during transfer to the SEM and TEM vacuum chamber. Surface morphology of the cycled electrodes was characterized by scanning electron microscopy (SEM, JEOL5900). The cycled electrodes were exposed to ultrasound in DMC solvent for 3 h to allow homogenous dispersion of the active materials in the solution, and then the dispersed solution was cast on a copper TEM grid (500 mesh) and dried overnight in a vacuum oven. The TEM grids were quickly transferred into the TEM chamber. Imaging was conducted using a JEOL JEM-2100F TEM (Pebody, MA) at 160 eV. The diameter of the beam was 5 nm, and low-dose imaging was employed to minimize electron-beam-induced changes to organic components in the surface layer.
Samples for NMR spectroscopy were prepared in a glove box filled with high purity Ar followed by flame sealing under reduced pressure. Sealed samples were heated in a silicon oil bath at 85 °C. Samples were weighed before and after storage to confirm seal. NMR analyses were conducted on a Bruker 300 MHz NMR spectrometer. 19 F NMR spectra were referenced to LiPF 6 at 65.00 ppm and 31 P NMR spectra were referenced to LiPF 6 at −145.0 ppm, as described previously (31-33).

Characterization of Lithium catechol dimethyl borate (LiCDMB)
The as-synthesized product is purified via crystallizations, and characterized by

Thermal stability
The 19 F and 31 P NMR spectra of the STD electrolyte and STD with 0.5 % added LiCDMB before and after storage for 8-days at 85 °C are presented in Figure 3.

Electrochemical stability
Electrochemical stability of both the STD and the LiCDMB electrolyte have been evaluated on carbon black electrodes with linear sweep voltammetry at high and low potential (10,34). Figure 4.

Anodic linear sweep voltammetry of Super C65/Li cells is presented in
Additive oxidation is clearly observed above 3.5 V vs. Li Cathodic linear sweep voltammetry of Super C65/Li cells is presented in Figure 5.
For the STD electrolyte, the reduction peak of EC at 0.65 V vs. Li/Li + can be clearly observed (35). For the electrolyte containing LiCDMB, the reduction peak for EC is observed at similar potential and intensity. This suggests that the presence of the LiCDMB additive does not affect EC reduction at low potential. There is no evidence for reduction of additive in the 3.0 V-0.7 V vs. Li/Li + potential range.
Investigation of the electrochemical stability at both high and low potentials of the LiCDMB electrolyte suggests reactivity high potential. Additive oxidation is observed

Cycling performance
Cycling performance at 25 °C and 55 °C of LiNi 0.5 Mn 1.5 O 4 /Graphite cells using the STD and LiCDMB electrolytes is presented in Figure 6. Additive concentration of 0.5 % (wt.) is found to be optimal for improved performance of high voltage cells. As seen from Figure 6a

Electrochemical impedance spectroscopy
Electrochemical impedance spectra of cells at a full state of charge (100 % SOC,  (Figure 11).

XPS element spectra
The O 1s and C 1s core spectra of the LiNi 0.5 Mn 1.5 O 4 electrodes are depicted in The C 1s and O 1s XPS spectra of the graphite electrodes are depicted in Figure   14. The C 1s spectrum of the fresh graphite shows high intensity of the C-C peak at 284.3 eV, along with CO x peaks of the CMC binder (40). The anode cycled with the STD electrolyte contains C 1s peaks characteristic of lithium alkyl carbonates and lithium carbonate from carbonate solvent reduction as expected from SEI formation (40,41). The Li 1s, Mn 3p, and Ni 3p spectra of the graphite anodes cycled in electrolytes with and without added LiCDMB are presented in Figure 15. The Li 1s, Mn 3p, and Ni 3p spectra of the anode cycled with the STD electrolyte contain peaks at 49 eV (Mn +IV ) and 48 eV (Mn +III ), 69 eV (Ni +IV ) (44), and LiF at 56 eV ( Figure 15b). This indicates that transition metal dissolution from the cathode surface is occurring followed by deposition on the anode damaging the SEI ( Figure 13b) (45,46).

Introduction
Lithium ion batteries are widely used for portable electronics and are currently being incorporated into electric vehicles due to the high energy density (1, 2). However, there is significant interest in increasing the energy density of lithium-ion batteries (3).
One method to achieve higher energy density is increasing the operating potential of the   (25) have been reported to improve performance at high potential and elevated temperature. Alternatively there have been several investigations of the incorporation of cathode film forming additives that are sacrificially oxidized and/or reduced on the cathode surface to generate a cathode passivation layer (12,26,27). Among these additives, lithium bis(oxalate)borate (LiBOB) has been extensively investigated and provides multiple beneficial effects in the battery system (28). However, oxidation of LiBOB on the delithiated LiNi 0.5 Mn 1.5 O 4 surface at high potential (> 4.5 V vs. Li/Li + ) is accompanied by CO 2 gas generation, which limits its practical application (29). Thus it is important to develop novel cathode film forming additives to improve the performance of high voltage LiNi 0.5 Mn 1.5 O 4 cathodes.
In the present report, a novel sulfonate additive,

Experimental
The baseline electrolyte is 1.

Electrochemical windows of electrolytes
Electrochemical stability of electrolytes is evaluated at high and low potential using carbon black composite electrodes (13,43). Anodic stability of the baseline electrolyte and the DMF-SO 3 -based compositions is presented in Figure 1. Inset data graph of Figure 3 shows the residual current at 4. Test of electrochemical windows of electrolytes with and without added DMF-   (Figure 8). C 1s core spectra of the same cathodes shows important differences when cycled in the three electrolytes. The fresh cathode shows C 1s peaks of C-C the case of 1.0% DMF-SO 3 because of a higher additive content in the bulk electrolyte.
C 1s and O 1s core spectra of the graphite electrodes are presented in Figure 11. Additive reactivity on the graphite anode is checked through the N 1s and S 2p spectra ( Figure 12). N 1s spectra are of stronger intensities than that of the cathodes ( Figure 10). N 1s spectra of the two DMF-SO 3 anodes evidence reduction of the additive with the apparition of new peaks. Original peak of the additive powder at ca. 401 eV is present for the two electrodes. Nevertheless, a new peak at ca. 400 eV appears for the 0.1% DMF-SO 3 graphite that can be attributed to LiN(CH 3 ) 2 CHO formed after the release of the -SO 3 group. More additive reduction is achieved at higher concentration with higher N 1s signal intensity and the apparition of a second reduction peak at ca 399 eV. Sulfur deposition is also observed on the graphite surface. Nevertheless, S 2p is hardly detected for the 0.1% DMF-SO 3  Surprisingly, reactivity of the -SO 3 group is similar on both the cathode and anode side, as identical S 2p peaks have been measured.

FTIR/ATR
FT-IR spectra of LiNi 0.5 Mn 1.5 O 4 and graphite electrodes cycled in the three electrolytes under study are presented in Figure 13. PVDF characteristic peaks mostly dominate the LiNi 0.5 Mn 1.5 O 4 cathode spectrum (1400 cm -1 , 1180 cm -1 , 890 cm -1 ) ( Figure   13a). After DMF-SO 3 addition, a small polycarbonate peak can be detected at 1800 cm -1 , and much more ester and oligomer species can be observed at 1740 cm -1 . These species contribute to most of the CEI composition.
For the anodes, the STD graphite is quite similar to the 0.1% DMF-SO 3 anode. As the concentration of DMF-SO 3 increased to 1.0 %, the anode spectrum shows less lithium alkyl carbonate (1600 cm -1 ) and lithium carbonate (1430 cm -1 ), suggesting that more complicated reactions occurred on the surface of anodes with excess additive. Strong peaks at 1300 cm -1 and 1150 cm -1 show that more C-N stretches can be detected, resulting from the decomposition products of DMF-SO 3 and consistent with significantly increased N 1s core spectrum intensity ( Figure 12).

Conclusions
Electrochemical test shows more electrolyte decomposition with the presence of