Improving the Stability of High-Voltage Lithium Cobalt Oxide with a Multifunctional Electrolyte Additive: Interfacial Analyses

In recent years, various attempts have been made to meet the increasing demand for high energy density of lithium-ion batteries (LIBs). The increase in voltage can improve the capacity and the voltage platform performance of the electrode materials. However, as the charging voltage increases, the stabilization of the interface between the cathode material and the electrolyte will decrease, causing side reactions on both sides during the charge–discharge cycling, which seriously affects the high-temperature storage and the cycle performance of LIBs. In this study, a sulfate additive, dihydro-1,3,2-dioxathiolo[1,3,2]dioxathiole 2,2,5,5-tetraoxide (DDDT), was used as an efficient multifunctional electrolyte additive for high-voltage lithium cobalt oxide (LiCoO2). Nanoscale protective layers were formed on the surfaces of both the cathode and the anode electrodes by the electrochemical redox reactions, which greatly decreased the side reactions and improved the voltage stability of the electrodes. By adding 2% (wt.%) DDDT into the electrolyte, LiCoO2 exhibited improved Li-storage performance at the relatively high temperature of 60 °C, controlled swelling behavior (less than 10% for 7 days), and excellent cycling performance (capacity retention rate of 76.4% at elevated temperature even after 150 cycles).


Introduction
In the past decades, remarkable progress has been made in lithium-ion batteries (LIBs) in the field of portable devices [1,2]. In addition, LIBs can be used as efficient aerospace batteries for small satellites, and LIBs are becoming one of the most widely used energy storage devices due to their relatively high working potential and high energy density [3][4][5][6]. However, it is well known that the energy density of cathode materials is the main factor affecting the performance of LIBs. To meet the requirements of higher capacity and longer cycle life for LIBs, there has been an increase in studies focusing on the development of novel cathodes with higher working potentials [7][8][9]. However, the overcharge of LiCoO 2 can provoke the serious oxidation of the electrolyte at higher potentials and cause a highresistance cathodic film to form, thus causing the capacity fade of the LIBs in the following cycles [10][11][12]. Some side effects have been observed, including the dissolution of transitionmetal ions in the electrolyte and the reduced cycling stability of the cells [13]. It is well known that the conventional organic carbonate solvents have oxidization potentials of 5 V. In addition, the oxidation reaction is catalyzed in the presence of transition-metal ions, and the decomposition of electrolytes is accelerated at lower potentials, leading to unexpected rapid capacity fading [14]. Ethers such as 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) also have high ionic conductivities and coulombic efficiencies [15,16]. However, ethers at typical salt concentrations of 0.1 or 1 M cannot be practically utilized because of their low oxidative stability (less than 4 V vs. Li/Li + ) [16][17][18]. Sulfone-based high-voltage electrolytes with good oxidation resistance have low lattice energy, but their relatively greater wettability and higher viscosity can greatly affect the performances of LIBs [19]. For cell systems containing graphite negative electrodes, sulfone-based electrolytes are also restricted since the stable solid-electrolyte interface (SEI) at the graphite surface cannot be formed [20]. Compared with organic carbonate solvents, room-temperature ionic liquids (RTILs) have exhibited higher thermal stability, lower flammability and volatility, and wider electrochemical windows [21]. However, the compatibilities of these electrolytes are also unsatisfactory due to the low wettability. In addition, the relatively higher melting points have also been found to degrade their low-temperature performances [22].
Electrolytes with excellent electrochemical properties always play an important role in improving the stability of LIBs. However, researchers have had great difficulty in developing novel electrolytes for high-performance LIBs. It is worth noting that the addition of a small amount of additive into the electrolyte is beneficial in forming a protective layer and preventing the solvent penetration; in this way, the possible damages to the electrode structure could be efficiently avoided. In addition, a film-forming additive for high-voltage cathode material in LIBs undergoes oxidation and decomposition reactions on the surface of the positive electrode, and a stable interface film is formed favoring the solvent system, thus reducing or preventing the further oxidation of the solvent system. Lithium bis(oxalato)borate (LiBOB) is a typical inorganic additive for high-voltage LIBs. Phosphides (such as tris(pentafluorophenyl)phosphine (TPFPP), tris(hexafluoro-isopropyl)phosphate (HFiP) and N-(triphenylphosphoranylidene)aniline (TPPA)), sulfonate esters (such as methylene methanedisulfonate (MMDS)), carboxyl anhydrides (such as glutaric anhydride and succinic anhydride), and fluorides (such as 1,1-difluoro-4-phenylbut-1ene (DF)) exhibited similar surface-film-forming characters and could enhance the performance of high-voltage cathodes [23][24][25][26][27][28][29]. The oxidation of LiBOB on the cathode surface was found to generate a cathode passivation layer that inhibited the further oxidation of the electrolyte [30,31].
The complexing additives can form complexes with free transition-metal elements, purify the electrolyte system, suppress the electrolyte decomposition, and improve the highvoltage performance. For example, adiponitrile can inhibit the side reaction between the electrolyte and the surface of the high-nickel positive electrode, and the strong coordination between the nitrile group and Ni 4+ can effectively reduce the formation of electrochemically inert NiO-type rock-salt structure [32].
Recently, dihydro-1,3,2-dioxathiolo[1,3,2]dioxathiole 2,2,5,5-tetraoxide (DDDT) has emerged as an efficient electrolyte additive for LIBs. DDDT has been utilized as an overallfunctional electrolyte additive for high-voltage NCM523/graphite batteries, and enhanced electrochemical performance could be obtained [33]. In this study, to improve the interface stability of high-voltage lithium cobalt oxide (LiCoO 2 ), DDDT-containing electrolyte was used as a multifunctional electrolyte additive. The physicochemical properties of the cells were analyzed, and the underlying mechanisms were investigated. In particular, the protective layers formed electrodes by the electrochemical redox reactions on the surfaces of both the cathode and the anode could greatly decrease the side reactions and improve the voltage stability of the electrodes. The research indicated that the DDDT-containing electrolyte was beneficial for the high-voltage LiCoO 2 batteries, besides the previous breakthroughs towards LiNi 0.5 Co 0.2 Mn 0.3 O 2 /graphite batteries, which may provide a useful reference for the preparation of more stable LIBs by the formation of high-quality interfacial films in the cells.  Figure 1) were provided by Shanshan New Materials (Quzhou) Co., Ltd. The solvent was fully dried by 4A molecular sieve, activated, and configured in an argon (Ar)-filled glove box with water and oxygen below 1 ppm. The electrolyte model is shown in Tables 1 and 2. The moisture and free acid contents of the electrolyte were tested by Metrohm Coulomb Karl Moisture Meter (below 20 ppm) and confirmed by triethylamine titration (below 50 ppm).

Materials, Electrolyte configuration, and Cell production
Battery-level component LiPF6, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), n-propyl propionate (PP), and dihydro-1,3,2-dioxathiolo[1,3,2]dioxathiole 2,2,5,5-tetraoxide (DDDT, Figure 1) were provided by Shanshan New Materials (Quzhou) Co., Ltd. The solvent was fully dried by 4A molecular sieve, activated, and configured in an argon (Ar)-filled glove box with water and oxygen below 1 ppm. The electrolyte model is shown in Table 1 and Table 2. The moisture and free acid contents of the electrolyte were tested by Metrohm Coulomb Karl Moisture Meter (below 20 ppm) and confirmed by triethylamine titration (below 50 ppm). The cathode formulation consisted of 95 wt.% LiCoO2 (provided by Tianjin Bamo Tech Co. Ltd.; Tianjin, China), 2.5 wt.% carbon black, and 2.5 wt.% PVDF. A slurry with a viscosity of about 6000 mPa•s was prepared by dispersing and mixing LiCoO2 and the conductive agent with NMP. By an aluminum foil current collector with the slurry, a positive electrode was obtained after drying. A conductive agent consisting of 95 wt.% graphite (Jiangxi Zichen Tech Co., Ltd.; Yichun, China), 2.5 wt.% carbon black, 1.5 wt.% SBR, and 1 wt.% CMC was used. By dispersing and mixing active materials and the conductive agent with deionized water to form a slurry with a viscosity of about 3000 mPa•s and then coating the copper foil current collector with the slurry, a negative electrode was obtained after drying. The polypropylene separator was obtained from Shenzhen Senior Technology Material Co., Ltd. (China). The negative electrode had an active mass load of approximately 1.07 mg cm −2 , and the positive electrode had an active mass load of approximately 1.86 mg cm −2 . A three-electrode system with Pt metal as the working electrode, lithium metal as the counter electrode, and a reference electrode was used for linear scan voltammetry (LSV). The potential of the electrode was scanned from open-circuit voltage (OCV) to 7.0 V, and the scanning speed was 0.1 mV/s. Cyclic voltammetry (CV) tests of the Li/graphite half cells were performed, and a scanning voltage range of 3-0.01 V was investigated with a sweep rate of 0.01 mV/s. The cells were measured by an eight-channel Solartron potentiostat (model 1470E; Advanced Measurement Technology Inc.; Oak Ridge, USA).
To evaluate the reaction window of the fabricated electrolyte, we assembled the positive and negative electrodes and a separator in a CR2032 coin cell battery. Lithium metal was used as the reference electrode, and model #1-#4 electrolytes were utilized (Table 1). In order to evaluate the electrical performance of the additive on the full battery, we assembled the positive and negative pole pieces and the separator into a wound type 404,798 battery by using model #5-#8 electrolytes ( Table 2).
The surface morphologies of the electrodes before and after cycle tests were investigated by a scanning electron microscope (SEM; Nova Nano SEM450). The functional groups of the formed cathode-electrolyte interface (CEI) layers before and after cycles, as well as their chemical compositions, were confirmed by an X-ray photoelectron spectroscope (XPS; R3000, VG SCIENTA). The crystal forms of the electrode materials before and after cycle tests were investigated by X-ray powder diffraction (XRD; D8 ADVANCE).  The cathode formulation consisted of 95 wt.% LiCoO 2 (provided by Tianjin Bamo Tech Co. Ltd.; Tianjin, China), 2.5 wt.% carbon black, and 2.5 wt.% PVDF. A slurry with a viscosity of about 6000 mPa·s was prepared by dispersing and mixing LiCoO 2 and the conductive agent with NMP. By an aluminum foil current collector with the slurry, a positive electrode was obtained after drying. A conductive agent consisting of 95 wt.% graphite (Jiangxi Zichen Tech Co., Ltd.; Yichun, China), 2.5 wt.% carbon black, 1.5 wt.% SBR, and 1 wt.% CMC was used. By dispersing and mixing active materials and the conductive agent with deionized water to form a slurry with a viscosity of about 3000 mPa·s and then coating the copper foil current collector with the slurry, a negative electrode was obtained after drying. The polypropylene separator was obtained from Shenzhen Senior Technology Material Co., Ltd. (Shenzhen, China). The negative electrode had an active mass load of approximately 1.07 mg cm −2 , and the positive electrode had an active mass load of approximately 1.86 mg cm −2 . A three-electrode system with Pt metal as the working electrode, lithium metal as the counter electrode, and a reference electrode was used for linear scan voltammetry (LSV). The potential of the electrode was scanned from opencircuit voltage (OCV) to 7.0 V, and the scanning speed was 0.1 mV/s. Cyclic voltammetry (CV) tests of the Li/graphite half cells were performed, and a scanning voltage range of 3-0.01 V was investigated with a sweep rate of 0.01 mV/s. The cells were measured by an eight-channel Solartron potentiostat (model 1470E; Advanced Measurement Technology Inc.; Oak Ridge, TN, USA).
To evaluate the reaction window of the fabricated electrolyte, we assembled the positive and negative electrodes and a separator in a CR2032 coin cell battery. Lithium metal was used as the reference electrode, and model #1-#4 electrolytes were utilized (Table 1). In order to evaluate the electrical performance of the additive on the full battery, we assembled the positive and negative pole pieces and the separator into a wound type 404,798 battery by using model #5-#8 electrolytes ( Table 2).
The surface morphologies of the electrodes before and after cycle tests were investigated by a scanning electron microscope (SEM; Nova Nano SEM450). The functional groups of the formed cathode-electrolyte interface (CEI) layers before and after cycles, as well as their chemical compositions, were confirmed by an X-ray photoelectron spectroscope (XPS; R3000, VG SCIENTA). The crystal forms of the electrode materials before and after cycle tests were investigated by X-ray powder diffraction (XRD; D8 ADVANCE).

Electrochemical Properties
Firstly, the analyses of dQ/dV curves were conducted to quantitatively evaluate the performance of the full battery, and the high-temperature storage and the cycle characteristics were also evaluated. The voltages ranged from 3.0 to 4.5 V. Secondly, the AC impedance of the coin cells, i.e., the LiCoO 2 /Li and graphite/Li systems, which charge and discharge were tested after the assembled cells were charged and discharged at 0.2 C for one cycle, respectively. Impedance data were collected in the frequency range 0.01-100,000 Hz with the amplitude of 5 mV. High-temperature storage tests were carried out at 60 • C for three cells per group, and the hot thickness was tested regularly. The recharging currents of 0.7 C (overall cycle 250 and 150 times) were tested at 25 and 45 • C, and the internal resistance of the battery, or the discharge capacity retention (DCR), was tested at 25 • C. Figure 2 shows the CV results for the Li/graphite button-type half cells. Comparing the cells in #1 and #2 electrolytes, it can be found that the redox peak potential of the cell in #2 electrolyte containing 0.5% DDDT appeared at 1.0 V during the first cycle, while it disappeared during the second and the third cycles. We speculated that the reduction reaction occurred during the first cycle due to the addition of DDDT into the electrolyte, which prevented further reaction due to the formation of SEI on the graphite electrode.  Figure 3 shows the results of the linear sweep voltammetry (LSV) of the Li/graphite button-type half cells. Comparing the cell in #1 electrolyte with that in #2 electrolyte, it is obvious that the former showed a higher current than the latter at above 6.5 V. We speculated that the addition of DDDT could improve the oxidation resistance of the electrolyte.  Figure 3 shows the results of the linear sweep voltammetry (LSV) of the Li/graphite button-type half cells. Comparing the cell in #1 electrolyte with that in #2 electrolyte, it is obvious that the former showed a higher current than the latter at above 6.5 V. We speculated that the addition of DDDT could improve the oxidation resistance of the electrolyte. Figure 3 shows the results of the linear sweep voltammetry (LSV) of the Li/graphite button-type half cells. Comparing the cell in #1 electrolyte with that in #2 electrolyte, it is obvious that the former showed a higher current than the latter at above 6.5 V. We speculated that the addition of DDDT could improve the oxidation resistance of the electrolyte. The dQ/dV curve of the full battery containing 1-2 wt.% DDDT electrolyte shows a characteristic peak at around 2.8 V, which is lower than the reference group, indicating that the addition of DDDT is more beneficial than the addition of EC to the formation of the protective film on the negative electrode ( Figure 4). The dQ/dV curve of the full battery containing 1-2 wt.% DDDT electrolyte shows a characteristic peak at around 2.8 V, which is lower than the reference group, indicating that the addition of DDDT is more beneficial than the addition of EC to the formation of the protective film on the negative electrode ( Figure 4).  Figure 5 shows the AC impedance data and the equivalent circuit model of LiCoO2/Li and graphite/Li, and Table 3 shows the fitting experimental results in different electrolytes (#1-#4 EL). The pattern in the impedance spectra is explained by an equivalent circuit diagram ( Figure 5c). The Re represents bulk resistance, which indicates the ohmic resistance of the electrolyte and electrodes. Rf and Cdl1 are the charge-transfer resistance and the double-layer capacitance between the electrolyte and lithium metal corresponding to the semicircle at high frequencies, respectively. Rct and Cdl2 are the charge-transfer resistance and the double-layer capacitance between the electrolyte and cathode corresponding to the semicircle at medium frequencies, respectively. When the capacitances of the electrical double layer and the film are close to each other, the two semicircles will be merged into one semicircle in impedance spectra. W is the Warburg impedance related to the diffusion of lithium ions in the electrode, which is indicated by a slopping line at low frequencies. The combination of Rct and W is called faradic impedance, which reflects the kinetics of the cell reactions. The charge transfer (Rct) resistance of LiCoO2/Li decreased with an increase in the addition amount of DDDT, while no significant differences in the passivation film (Rf) resistance of graphite/Li could be observed. The decomposition products would be accumulated on the electrodes, result-   Table 3 shows the fitting experimental results in different electrolytes (#1-#4 EL). The pattern in the impedance spectra is explained by an equivalent circuit diagram ( Figure 5c). The R e represents bulk resistance, which indicates the ohmic resistance of the electrolyte and electrodes. R f and C dl1 are the charge-transfer resistance and the double-layer capacitance between the electrolyte and lithium metal corresponding to the semicircle at high frequencies, respectively. R ct and C dl2 are the charge-transfer resistance and the double-layer capacitance between the electrolyte and cathode corresponding to the semicircle at medium frequencies, respectively. When the capacitances of the electrical double layer and the film are close to each other, the two semicircles will be merged into one semicircle in impedance spectra. W is the Warburg impedance related to the diffusion of lithium ions in the electrode, which is indicated by a slopping line at low frequencies. The combination of Rct and W is called faradic impedance, which reflects the kinetics of the cell reactions. The charge transfer (R ct ) resistance of LiCoO 2 /Li decreased with an increase in the addition amount of DDDT, while no significant differences in the passivation film (R f ) resistance of graphite/Li could be observed. The decomposition products would be accumulated on the electrodes, resulting in poor electrical contact and remarkable increase of resistance. Obviously, the full cells with DDDT additive in the electrolytes exhibited relatively lower resistance after storage and cycling tests, and we proposed that a newly generated interfacial film guaranteed the higher stability due to the possible decomposition reactions toward the electrolyte being efficiently inhibited. The degeneration of the electrode material could be minimized during the cycling and storage processes, and the parasitic interfacial reactions would be greatly reduced. The AC impedance tests of the half-cells reflected that the DDDT additive could facilitate the formation of the protective layers on the positive electrodes. The results were consistent with the relatively more excellent cycling stability and storage performances of the cells.  The results of DCR discharge at room temperature indicated that the DC internal resistance decreased with an increase in the addition amount of DDDT in the electrolyte ( Figure 6). It is obvious that the smallest DC resistance could be obtained by the addition of 2 wt.% DDDT into the electrolyte.  The results of DCR discharge at room temperature indicated that the DC internal resistance decreased with an increase in the addition amount of DDDT in the electrolyte ( Figure 6). It is obvious that the smallest DC resistance could be obtained by the addition of 2 wt.% DDDT into the electrolyte.

Effect of DDDT on DC impedance
The results of DCR discharge at room temperature indicated that the DC internal resistance decreased with an increase in the addition amount of DDDT in the electrolyte ( Figure 6). It is obvious that the smallest DC resistance could be obtained by the addition of 2 wt.% DDDT into the electrolyte.

Effect of DDDT Additive on High-Temperature Storage
The additive DDDT could significantly improve the high-temperature storage performance of the cells. The cells were stored at 60 • C for 23 days, and the thickness increased by 19.8%, 27.3%, and 36.2% when the amount of DDDT added into the electrolyte was 2 wt.%, 1 wt.%, and 0.5 wt.%, respectively. The thickness of the cell with the base electrolyte even increased by up to~50%, indicating the addition of the appropriate amount of DDDT into the electrolyte was beneficial to the high-temperature storage performance of the cells (Figure 7). The thickness of the battery in 60 • C storage reflected the differences between positive electrode and electrolyte with and without DDDT, and the thickness of electrolyte with 2% DDDT was the smallest in electrolyte groups, reflecting that 2% DDDT can improve the stability of the positive electrode. The additive DDDT could significantly improve the high-temperature storage performance of the cells. The cells were stored at 60 °C for 23 days, and the thickness increased by 19.8%, 27.3%, and 36.2% when the amount of DDDT added into the electrolyte was 2 wt.%, 1 wt.%, and 0.5 wt.%, respectively. The thickness of the cell with the base electrolyte even increased by up to ~50%, indicating the addition of the appropriate amount of DDDT into the electrolyte was beneficial to the high-temperature storage performance of the cells (Figure 7). The thickness of the battery in 60 °C storage reflected the differences between positive electrode and electrolyte with and without DDDT, and the thickness of electrolyte with 2% DDDT was the smallest in electrolyte groups, reflecting that 2% DDDT can improve the stability of the positive electrode. The surface morphological changes of the electrodes (in #5 EL or in #8 EL) stored at 60 °C for 23 days were further investigated (Figure 8). It can be seen that obvious cracks are presented on the LiCoO2 particles in the base electrolyte without additive DDDT, while the surface of the LiCoO2 particles in the 2% DDDT electrolyte group was uniformly covered with a protective layer without any cracks. The results were consistent with the high-temperature (60 °C) storage performance of the full battery in different electrolytes (Figure 7). Table 4 shows the surface elemental distributions of the electrodes after the full battery was stored at 60 °C for 23 days. Sulfur could be detected on both the positive and the negative electrodes of the battery groups with additive DDDT, confirming the sulfide generated on their surfaces. The difference in the elemental distribution of cobalt (Co) on the negative electrode surface indicated that DDDT could be used as a positive electrode protection additive to prevent cobalt leaching and reduce the possible deposition on the negative electrode. The surface morphological changes of the electrodes (in #5 EL or in #8 EL) stored at 60 • C for 23 days were further investigated (Figure 8). It can be seen that obvious cracks are presented on the LiCoO 2 particles in the base electrolyte without additive DDDT, while the surface of the LiCoO 2 particles in the 2% DDDT electrolyte group was uniformly covered with a protective layer without any cracks. The results were consistent with the high-temperature (60 • C) storage performance of the full battery in different electrolytes (Figure 7). Table 4 shows the surface elemental distributions of the electrodes after the full battery was stored at 60 • C for 23 days. Sulfur could be detected on both the positive and the negative electrodes of the battery groups with additive DDDT, confirming the sulfide generated on their surfaces. The difference in the elemental distribution of cobalt (Co) on the negative electrode surface indicated that DDDT could be used as a positive electrode protection additive to prevent cobalt leaching and reduce the possible deposition on the negative electrode.
with the high-temperature (60 °C) storage performance of the full battery in diff electrolytes (Figure 7). Table 4 shows the surface elemental distributions of the electr after the full battery was stored at 60 °C for 23 days. Sulfur could be detected on bot positive and the negative electrodes of the battery groups with additive DD confirming the sulfide generated on their surfaces. The difference in the elem distribution of cobalt (Co) on the negative electrode surface indicated that DDDT c be used as a positive electrode protection additive to prevent cobalt leaching and re the possible deposition on the negative electrode.   Figure 9 shows the structural changes of the positive electrodes in #5 EL and # groups before and after 60 °C storage for 23 days. No obvious differences coul observed for the (006), (102), and (104) peaks of XRD curves, but the 003 peak of th group (the insert illustrated in Figure 9) without DDDT additive was split, while th group with 2 wt.% DDDT electrolyte after high-temperature storage showed   Figure 9 shows the structural changes of the positive electrodes in #5 EL and #8 EL groups before and after 60 • C storage for 23 days. No obvious differences could be Nanomaterials 2021, 11, 609 9 of 16 observed for the (006), (102), and (104) peaks of XRD curves, but the 003 peak of the cell group (the insert illustrated in Figure 9) without DDDT additive was split, while the cell group with 2 wt.% DDDT electrolyte after high-temperature storage showed no significant changes in the peak shape in comparison with that before high-temperature storage. This is because the crystal structure of the positive electrode tended to be destroyed under high voltage and high temperature, and the addition of DDDT into the electrolyte could prevent the positive electrode from damage during the high-temperature storage.

Effect of DDDT additive on cycle performance
DDDT might have a significant impact on the cycle performance of the battery ( Figure 10). After 250 cycles at room temperature, the capacity retention rate remained about 77.7% for the battery in #6 electrolyte group (containing 0.5 wt.% DDDT), while the capacity retention rate in #8 electrolyte group (containing 2 wt.% DDDT) remained up to 84.5%. In contrast, for the battery with the base electrolyte #5 group without additive DDDT electrolyte, the capacity retention rate remained only about 62.1%. After 150 cycles at 45 °C, the capacity retention rate of the battery with the #8 electrolyte group (containing 2 wt.% DDDT) remained around 76.4%, while the battery with the base electrolyte #5 group had a relatively lower value of 52.5%. In order to further reveal the underlying mechanisms of the improved cycle performance of the battery with DDDT additive, AC impedance analyses were performed. Figure 11 shows the AC impedance spectra before and after cycle tests. Table 5 shows the fitted data of the Rct and Rf resistances. From the fitted data, the Rf and the Rct resistances of the battery with 2 wt.% DDDT additive before cycle tests were slightly lower than those of the reference group without additive DDDT. The Rf and the Rct resistances after 50 cycle tests were lower than previous data due to the improved infiltration of electrolyte in the pole piece, and the SEI film formed on the electrode was more stable after cycling over a period of time. As the number of cycle tests increased, the Rf and the Rct re-

Effect of DDDT Additive on Cycle Performance
DDDT might have a significant impact on the cycle performance of the battery ( Figure 10). After 250 cycles at room temperature, the capacity retention rate remained about 77.7% for the battery in #6 electrolyte group (containing 0.5 wt.% DDDT), while the capacity retention rate in #8 electrolyte group (containing 2 wt.% DDDT) remained up to 84.5%. In contrast, for the battery with the base electrolyte #5 group without additive DDDT electrolyte, the capacity retention rate remained only about 62.1%. After 150 cycles at 45 • C, the capacity retention rate of the battery with the #8 electrolyte group (containing 2 wt.% DDDT) remained around 76.4%, while the battery with the base electrolyte #5 group had a relatively lower value of 52.5%.

Effect of DDDT additive on cycle performance
DDDT might have a significant impact on the cycle performance of the battery ( Figure 10). After 250 cycles at room temperature, the capacity retention rate remained about 77.7% for the battery in #6 electrolyte group (containing 0.5 wt.% DDDT), while the capacity retention rate in #8 electrolyte group (containing 2 wt.% DDDT) remained up to 84.5%. In contrast, for the battery with the base electrolyte #5 group without additive DDDT electrolyte, the capacity retention rate remained only about 62.1%. After 150 cycles at 45 °C, the capacity retention rate of the battery with the #8 electrolyte group (containing 2 wt.% DDDT) remained around 76.4%, while the battery with the base electrolyte #5 group had a relatively lower value of 52.5%. In order to further reveal the underlying mechanisms of the improved cycle performance of the battery with DDDT additive, AC impedance analyses were performed. Figure 11 shows the AC impedance spectra before and after cycle tests. Table 5 shows the fitted data of the Rct and Rf resistances. From the fitted data, the Rf and the Rct resistances of the battery with 2 wt.% DDDT additive before cycle tests were slightly lower than those of the reference group without additive DDDT. The Rf and the Rct resistances after 50 cycle tests were lower than previous data due to the improved infiltration of electrolyte in the pole piece, and the SEI film formed on the electrode was more stable after cycling over a period of time. As the number of cycle tests increased, the Rf and the Rct resistances increased more significantly. After 200 cycle tests, the Rf and the Rct resistances In order to further reveal the underlying mechanisms of the improved cycle performance of the battery with DDDT additive, AC impedance analyses were performed. Figure 11 shows the AC impedance spectra before and after cycle tests. Table 5 shows the fitted data of the R ct and R f resistances. From the fitted data, the R f and the R ct resistances of the battery with 2 wt.% DDDT additive before cycle tests were slightly lower than those of the reference group without additive DDDT. The R f and the R ct resistances after 50 cycle tests were lower than previous data due to the improved infiltration of electrolyte in the pole piece, and the SEI film formed on the electrode was more stable after cycling over a period of time. As the number of cycle tests increased, the R f and the R ct resistances increased more significantly. After 200 cycle tests, the R f and the R ct resistances of the battery with base electrolyte increased to 22.46 and 14.05 mΩ, which were much higher than the original values of 17 and 10.2 mΩ, respectively. In contrast, the R f and the R ct resistances of the battery 2 wt.% DDDT additive only decreased to 11.43 mΩ and 3.49 mΩ compared with the original values of 13.59 and 9.02 mΩ, respectively. The results again confirmed that the protective layer formed by DDDT was beneficial in reducing the possible side reactions, thereby greatly reducing the AC impedance.   12 and 13 show the changes in the surface morphologies of the positive and negative electrodes with different electrolytes before and after 250 cycle tests. After 250 cycles of high-temperature cycling, cracks could be found for the LiCoO2 particles with the base electrolytes without any additive. When 2 wt.% DDDT was added into the electrolyte, the interface seemed more normal, again confirming that the DDDT additive had a significant protective effect on the positive electrode at a high voltage of 4.5 V (Figure  12a-d). The morphological changes of the negative electrode with DDDT additive were also relatively more unobvious than those of the negative electrode with the base electrolyte ( Figure 13). TEM analyses further indicated that nanoscale SEI layers were formed, and the SEI layer formed in #8 EL was thicker than that in #5 EL (Figure 12e,f).   Figures 12 and 13 show the changes in the surface morphologies of the positive and negative electrodes with different electrolytes before and after 250 cycle tests. After 250 cycles of high-temperature cycling, cracks could be found for the LiCoO 2 particles with the base electrolytes without any additive. When 2 wt.% DDDT was added into the electrolyte, the interface seemed more normal, again confirming that the DDDT additive had a significant protective effect on the positive electrode at a high voltage of 4.5 V (Figure 12a-d). The morphological changes of the negative electrode with DDDT additive were also relatively more unobvious than those of the negative electrode with the base electrolyte ( Figure 13). TEM analyses further indicated that nanoscale SEI layers were formed, and the SEI layer formed in #8 EL was thicker than that in #5 EL (Figure 12e,f). The obtained results were consistent with the cycle performance tests of the battery, indicating that the DDDT additive had an improvement effect in both the positive and the negative electrodes.
Nanomaterials 2021, 11, x FOR PEER REVIEW 12 dicating that the DDDT additive had an improvement effect in both the positive and negative electrodes.   Figure 14 shows the structural changes of the positive electrode before and afte cycling at 45 °C for the batteries with #5 and #8 electrolyte groups. No obvious d ences could be observed in the (006), (102), and (104) peaks of XRD curves, but the peak of the positive electrode without DDDT additive was split, and the electrode w wt.% DDDT additive after storage showed no significant changes in the peak shap comparison with that before cycling. This was because the positive electrode cr structure was easily destroyed under high voltage and high temperature, and the a tion of DDDT undoubtedly could protect or stabilize its structure.  Figure 14 shows the structural changes of the positive electrode before and after the cycling at 45 • C for the batteries with #5 and #8 electrolyte groups. No obvious differences could be observed in the (006), (102), and (104) peaks of XRD curves, but the 003 peak of the positive electrode without DDDT additive was split, and the electrode with 2 wt.% DDDT additive after storage showed no significant changes in the peak shape in comparison with that before cycling. This was because the positive electrode crystal structure was easily destroyed under high voltage and high temperature, and the addition of DDDT undoubtedly could protect or stabilize its structure.
To assess the composition changes of the electrode surfaces cycled in the base and DDDT-containing electrolytes, XPS analyses of the cycled LiCoO 2 and graphite electrodes after 150 cycles at 45 • C were performed. The XPS spectra of the cycled LiCoO 2 cathode elucidated the effect of the DDDT additive on the CEI composition ( Figure 15). In the C 1s spectra (Figure 15a), the C-C (284.9 eV) and C-H (285.3 eV) peaks of the cathode from the cells cycled in the electrolyte containing DDDT retained higher intensity than the cell cycled in the electrolyte without DDDT. The severe decomposition of electrolytes without DDDT during the cycling process was also confirmed by the C 1s spectra. For the O 1s spectra (Figure 15b), the obtained results of the C-O and C=O peaks were consistent with the C 1s peak analyses. These results could be compared with the intensity of the C-O (286.6 eV) and C=O (289.0 eV) peaks obtained for the cycled cathode in the electrolyte containing DDDT additive, which belonged to the decomposition products including ROCO 2 Li and polycarbonates [33]. Moreover, the Co-O peak of the cathode with DDDT additive was more obvious, indicating that the addition of DDDT promoted the formation of the thinner interfacial layer which relieved the severe oxidative decomposition of the electrolyte and reduced the coverage of decomposition products on the cathode surface.
The cycled cathode in the electrolyte without DDDT possessed higher C-O and C=O intensities, indicating that the addition of DDDT additive can relieve the severe oxidative decomposition of the electrolyte on the cathode surface during the cycling process.  To assess the composition changes of the electrode surfaces cycled in the base and DDDT-containing electrolytes, XPS analyses of the cycled LiCoO2 and graphite electrodes after 150 cycles at 45 °C were performed. The XPS spectra of the cycled LiCoO2 cathode elucidated the effect of the DDDT additive on the CEI composition ( Figure 15). In the C 1s spectra (Figure 15a), the C-C (284.9 eV) and C-H (285.3 eV) peaks of the cathode from the cells cycled in the electrolyte containing DDDT retained higher intensity than the cell cycled in the electrolyte without DDDT. The severe decomposition of electrolytes without DDDT during the cycling process was also confirmed by the C 1s spectra. For the O 1s spectra (Figure 15b), the obtained results of the C-O and C=O peaks were consistent with the C 1s peak analyses. These results could be compared with the intensity of the C-O (286.6 eV) and C=O (289.0 eV) peaks obtained for the cycled cathode in the electrolyte containing DDDT additive, which belonged to the decomposition products including ROCO2Li and polycarbonates [33]. Moreover, the Co-O peak of the cathode with DDDT additive was more obvious, indicating that the addition of DDDT promoted the formation of the thinner interfacial layer which relieved the severe oxidative decomposition of the electrolyte and reduced the coverage of decomposition products on the cathode surface. The cycled cathode in the electrolyte without DDDT possessed higher C-O and C=O intensities, indicating that the addition of DDDT additive can relieve the severe oxidative decomposition of the electrolyte on the cathode surface during the cycling process.
Due to the interactions between LiCoO2 and PVDF, Li-F (684.9 eV) and C-F (687.2 eV) peaks appeared (Figure 15c). In addition, the intensity of Li-F peak for the cathode in the DDDT-containing electrolyte was much lower than that in the electrolyte without additive, further indicating that the cathode surface in the electrolyte containing DDDT was covered with fewer electrolyte decomposition products, which was consistent with the results of the C-O and C=O peak analyses.
In the S 2p spectra (Figure 15d), no peak could be observed for the cycled cathode in the electrolyte without DDDT, while the peak corresponding to Li2S could be detected in the case in the electrolyte containing DDDT, indicating that the addition of DDDT promoted the formation of sulfide on the cathode [29]. For S 2p spectra of the cathode in #8 EL, the peak at 169.2 eV corresponds to ROSO2Li, and the peaks at 169.2 and 163.3 eV belong to ROSO3Li and Li2S, respectively, indicating the additive DDDT was preferentially oxidized to facilitate the formation of the SEI on the cathode. Furthermore, the changes in the XPS patterns of the graphite electrodes in the electrolytes without DDDT and with DDDT additive were confirmed ( Figure 16). In the C 1s photoelectron spectra, the peak of the C-C can be attributed to the graphite, and the peaks of C-O and C=O are assigned to the carbonate species such as RCH2OCO2Li due to the reductive decomposition of the electrolyte. In the O 1s spectra, the intensities of the C=O and C-O peaks for the graphite surface decreased when DDDT was added into the electrolyte compared with the graphite surface with the base electrolyte, indicating that the reductive decomposition of the electrolyte was alleviated and that the DDDT additive facilitated the formation of a passivation layer on the graphite surface. In addition, the graphite anode cycled in the electrolyte containing DDDT showed the lowest Li2CO3 peak intensity, suggesting less electrolyte was decomposed when the DDDT was added. Due to the interactions between LiCoO 2 and PVDF, Li-F (684.9 eV) and C-F (687.2 eV) peaks appeared (Figure 15c). In addition, the intensity of Li-F peak for the cathode in the DDDT-containing electrolyte was much lower than that in the electrolyte without additive, further indicating that the cathode surface in the electrolyte containing DDDT was covered with fewer electrolyte decomposition products, which was consistent with the results of the C-O and C=O peak analyses.
In the S 2p spectra (Figure 15d), no peak could be observed for the cycled cathode in the electrolyte without DDDT, while the peak corresponding to Li 2 S could be detected in the case in the electrolyte containing DDDT, indicating that the addition of DDDT promoted the formation of sulfide on the cathode [29]. For S 2p spectra of the cathode in #8 EL, the peak at 169.2 eV corresponds to ROSO 2 Li, and the peaks at 169.2 and 163.3 eV belong to ROSO 3 Li and Li 2 S, respectively, indicating the additive DDDT was preferentially oxidized to facilitate the formation of the SEI on the cathode.
Furthermore, the changes in the XPS patterns of the graphite electrodes in the electrolytes without DDDT and with DDDT additive were confirmed ( Figure 16). In the C 1s photoelectron spectra, the peak of the C-C can be attributed to the graphite, and the peaks of C-O and C=O are assigned to the carbonate species such as RCH 2 OCO 2 Li due to the reductive decomposition of the electrolyte. In the O 1s spectra, the intensities of the C=O and C-O peaks for the graphite surface decreased when DDDT was added into the electrolyte compared with the graphite surface with the base electrolyte, indicating that the reductive decomposition of the electrolyte was alleviated and that the DDDT additive facilitated the formation of a passivation layer on the graphite surface. In addition, the graphite anode cycled in the electrolyte containing DDDT showed the lowest Li 2 CO 3 peak intensity, suggesting less electrolyte was decomposed when the DDDT was added. In the F 1s photoelectron spectra, the Li x PO y F z , Li x PF y , and LiF would be generated from the decomposition and hydrolysis of LiPF 6 . The peak intensities of Li x PO y F z and Li x PF y from the graphite surface without DDDT were stronger than those cycled in the DDDT-containing electrolytes, which suggests that the superior SEI layer promoted by the additive can effectively relieve the LiPF 6 decomposition on the anode. Furthermore, in the S 2p photoelectron spectra, sulfur-containing species were formed on the graphite surface with the DDDT additive, further indicating that the DDDT additive might have a reduction reaction on the graphite surface. Sulfur-containing species such as ROSO 2 Li and Li 2 SO 4 could alleviate the reductive decomposition of electrolytes and promote the diffusion of lithium ions at the electrode-electrolyte interface, and the results were consistent with the dQ/dV analyses [34][35][36].

Conclusions
For the LiCoO2 system, the stability of the battery system deviates significantly as the voltage increases. Meanwhile, the impedance increases obviously and the storage and cycle performances deteriorate. To prevent side reactions and improve the high-pressure/high-temperature performance and cycle performance of the system, the

Conclusions
For the LiCoO 2 system, the stability of the battery system deviates significantly as the voltage increases. Meanwhile, the impedance increases obviously and the storage and cycle performances deteriorate. To prevent side reactions and improve the highpressure/high-temperature performance and cycle performance of the system, the addition of DDDT to the conventional carbonate solvent system was investigated. Protective layers could be effectively formed on the LiCoO 2 or the graphite electrode surfaces, and the high-temperature storage performance of the battery was significantly improved. From the dQ/dV plots and AC impedance tests, the most appropriate addition amount of DDDT into the electrolyte was 2 wt.%. SEM, TEM, XRD, and XPS analyses confirmed the formation of stable nanoscale SEIs on both the positive and negative electrodes. LiCoO 2 with 2 wt.% DDDT additive exhibited more excellent high-temperature Li-storage performance, controlled swelling behavior, and cycling performance.

Data Availability Statement:
The data presented in this study are available on request from the first author (X.-Q.L., E-mail: xqliao@highpowertech.com).

Conflicts of Interest:
The authors declare no conflict of interest.