Surface Engineering of a LiMn2O4 Electrode Using Nanoscale

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Surface Engineering of LiMn2O4 Electrode using Nanoscale Polymer Thin Films via Chemical Vapor Deposition Polymerization Laisuo Su, Phil M Smith, Priyanka Anand, and B. Reeja Jayan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08711 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Applied Materials & Interfaces

Surface Engineering of LiMn2O4 Electrode using Nanoscale Polymer Thin Films via Chemical Vapor Deposition Polymerization Laisuo Su1, Phil M Smith1, Priyanka Anand2, B. Reeja Jayan1* 1 Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, USA 2 Department of Material Science & Engineering, Carnegie Mellon University, Pittsburgh, PA, USA *Corresponding author: [email protected]

ABSTRACT Surface engineering is a critical technique to improve the performance of lithium ion batteries (LIBs). Here, we introduce a novel vapor-based technique, namely chemical vapor deposition (CVD) polymerization, that can engineer nanoscale polymer thin films with controllable thickness and composition on the surface of battery electrodes. This technique enables us to, for the first time, systematically compare the effects of a conducting poly(3,4-ethylenedioxythiophene (PEDOT) polymer and an insulating poly(divinylbenzene) (PDVB) polymer on the performance of LiMn2O4 electrode in LIBs. Our results show that conducting PEDOT coatings improve both rate and cycling performance of LiMn2O4 electrodes, while insulating PDVB coatings have little effect on these performances. The PEDOT coating increases 10C rate capacity by 83% at 25 °C (from 23 to 42 mA h/g) and by 30% at 50 °C (from 64 to 83 mA h/g). Furthermore, the PEDOT coating extends LiMn2O4 high temperature (50 °C) cycling life by over 60%. A model is developed that can precisely describe capacity degradation exhibited by the different types of cells; based on the aging mechanisms of Mn dissolution and SEI growth. Results from X-ray photoelectron spectroscopy suggest that chemical or coordination bonds form between Mn in LiMn2O4 and O and S in PEDOT film. These bonds stabilize the surface of LiMn2O4, and thus improve the cycling performance. In contrast, no bonds form between Mn and elements in PDVB film. We further demonstrate that this 1

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vapor-based technique can be extended to other cathodes for advanced LIBs.

KEYWORDS: Surface

engineering,

Chemical

vapor

deposition

polymerization, LiMn2O4,

poly(3,4-

ethylenedioxythiophene) (PEDOT), poly(divinylbenzene) (PDVB), Lithium ion battery

INTRODUCTION Research on rechargeable lithium-ion batteries (LIBs) is aimed at extending lifespan, increasing energy and power density, and improving safety.1-4 Compared with anode materials, cathode materials have significantly lower specific capacities, thus limiting the overall performance of LIBs. Additionally, the poor stability of cathode materials in the presence of electrolytes is a major contribution to fast capacity degradation of LIBs during cycling, especially at high cut-off voltages and/or high temperatures.5,6 Modifying the surface of cathode electrodes by coating with an artificial solid-electrolyte interphase (SEI) layer is a widely pursued technique to enhance overall battery performance. The coating has been proved to improve the stability of electrodes,7 suppress dissolution of transition metal elements from cathode electrodes,8,9 and increase electronic and ionic conductivity of electrodes.10 Many types of coatings have been investigated, including oxides, 11 fluorides,9 phosphates,12 and polymers.13 Compared with extensive investigations of inorganic coatings for cathode electrodes, organic coatings do not receive comparable attention, in spite of reports that the actual SEI has a hybrid inorganic-organic composition.14 There are indeed some studies that show the cycling performance of cathode electrodes can be enhanced using organic polymer coatings.13,15,16 However, these studies have an important limitation because the polymer coatings were realized by solution processing, which offers poor control over the film composition, thickness, and functionality. This limits the repeatability, 2


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reliability, and optimization of the coating processes. Moreover, solution processing methods need a large amount of solvent and precursor, and a long time post-heat treatment in order to obtain the desired coating, which largely increases the complexity of making a battery.17 In order to fully explore the potential of organic (e.g., polymeric) surface engineering of LIB electrodes, new coating techniques need to be developed for next-generation LIBs. Chemical vapor deposition (CVD) polymerization is a novel vapor phase process that can deposit polymer thin films ranging in thickness from few nm to tens of μm.18-21 CVD directly converts gas phase monomers into solid films through polymerization of reactive bonds (vinyl or acetylene) present on the monomer, combining the polymerization and coating processes into an efficient single step. The non-line-of-sight arrival of precursors results in coatings of uniform thickness and composition, as well as complete “conformal” coverage of planar and complex surfaces, while retaining the underlying morphology of these structures. Polymeric coatings further provide the flexibility to easily tune functionality of the coatings by selecting monomers with the desired functional moieties. Surface engineering of battery electrodes using organic polymer coatings does not obtain enough attentions compared to that using inorganic coatings.14 More importantly, no systematic comparison between the effects of different functional (such as insulating and conducting) polymer coatings on battery performance has been reported before. In this study, we utilize the CVD polymerization technique to synthesize a conducting polymer (poly(3,4-ethylenedioxythiophene) (PEDOT)) and an insulating/dielectric polymer (poly(divinylbenzene) (PDVB)) thin film on the surface of a LiMn2O4 electrode and compare their performance as artificial SEI layers. Spinel LiMn2O4 is chosen because it is non-toxic and environmentally friendly.22 LiMn2O4 also has high output voltages (3.5-4.3 V) with acceptable specific capacity (120 mAh/g).23 To the best knowledge of the authors, this is the first time that the all dry, solvent-free, low temperature CVD technique is used to grow multifunctional polymer films on the electrodes in LIBs. Our results show that by 3



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tailoring the composition of these polymers, CVD can change their functionality (insulating, conducting), thereby altering the rate and cycling performance of the LiMn2O4 electrode. Such fundamental insights cannot be gained from conventional solution based methods which suffer from poor control over film composition, thickness, and functionality.

EXPERIMENTAL SECTION Chemical vapor deposition Polymerization. Two types of CVD polymerization techniques were utilized for synthesizing polymers, oxidative chemical vapor deposition (oCVD) and initiated chemical vapor deposition (iCVD). The oCVD was applied for growing a conducting PEDOT film and the iCVD for an insulating PDVB film. Figure 1 shows the schematics of the two types of CVD chambers and the corresponding polymerization mechanisms. The procedure for depositing the oCVD PEDOT films was similar to the one reported previously.21 Briefly, an oxidant (e.g. FeCl3) is sublimed by heating and spontaneously reacts with the heated monomer vapors that ﬂow into the oCVD reactor (Figure 1 (a)). Polymerization and thin film growth happens simultaneously on the

surface

of

a

temperature-controlled

substrate

placed.

The

monomer,

3,4-

ethylenedioxythiophene (EDOT), and the oxidant, iron chloride (FeCl3) were purchased from Sigma Aldrich and used as received. The monomer jar was heated to 130 ˚C and the vapor was introduced into the reactor via feed lines. The solid oxidant was placed in a crucible within the reactor and sublimed at 200 °C. The chamber pressure was held constant at 50 mTorr while the stage was controlled at temperatures ranging from 70 °C to 130 °C. Argon was also introduced into the reactor as a carrier gas for the reactants. The deposition was continued until the desired thickness was achieved. The films were then rinsed in methanol for 5 mins to remove any residual monomer and oxidant. As for the iCVD experiment, divinylbenzene (DVB) monomer and t-butylperoxide (TBPO) 4


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initiator were purchased from Sigma-Aldrich and used without further purification. Details about the iCVD polymerization of PDVB have been reported previously, and the deposition conditions are summarized in Table S1 in the Supporting Information (SI).24 Briefly, the monomer DVB was heated in a stainless-steel jar at 65 °C, and its vapors along with vapors of TBPO (at room temperature) were delivered into the reactor at flow rates of 2.0 sccm and 1.3 sccm, respectively. Argon gas at 8.5 sccm was additionally used to control the film deposition rate. The labile peroxide bond of the initiator was thermally cleaved by resistively heated nichrome filaments inside the CVD reactor to produce free radicals that attack vinyl bonds on DVB and initiate free-radical polymerization. iCVD is a substrate-independent process as the substrate temperature remains close to room temperature. Here, the reactor pressure, substrate temperature, and filament temperature were maintained at 0.5 Torr, 25 °C, 230 °C, respectively. A thermocouple was used to calibrate the temperature of the filament, as shown in Fig. S1 (SI), and the calibration results are listed in Table S2 (SI).

Figure 1: Schematics and polymerization mechanisms of (a) oxidative chemical vapor deposition (oCVD) and (b) initiated chemical vapor deposition (iCVD) processes. Precursors are vaporized by heating or reducing pressure, and then introduced into the vacuum chamber where polymerization happens, resulting in a thin film on the surface of samples (e.g., LIB cathode). Insets show the chemical structures of the various monomers (EDOT, DVB) and initiator (TBPO) used in this study to synthesize nanoscale polymer films of PEDOT and PDVB. 5



Material Characterization. Multiple techniques were applied to characterize the synthesized polymers, including scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). SEM and EDS were conducted on Philips XL-30 FEG using 5.0 kV accelerating voltage and the spot size was 5.0 nm (diameter). Silicon trenches with and without a polymer coating were imaged to demonstrate conformality. The trench was 6 µm deep and 1 µm wide with 8 µm spacing between the trenches. 2 nm thick platinum was coated on the surface of samples to reduce surface charging during SEM imaging. TEM was carried out on a Titan G2 80-300 electron microscope operating at 200 kV in bright field. Details of sample preparation steps are shown in Fig. S2 (SI), and samples with and without polymer thin film coating were tested to study the effect of polymer coating on LiMn2O4 particles. FTIR measurements were conducted using a PerkinElmer Frontier spectrometer equipped with an attenuated total reflection (ATR) attachment and a germanium crystal. Baseline-corrected spectra were collected over 700−4000 cm−1 at 1 cm−1 resolution and averaged over 4 scans. Spectra were processed using the Spectrum software package (PerkinElmer). In comparison, FTIR spectra of DVB monomer were also measured with the same settings. Raman spectroscopies of a LiMn2O4 electrode, a PEDOT film, and a PEDOT coated LiMn2O4 electrode were measured using NT-MDT Spectra AFM/Raman system equipped with a visible Raman microscope and CCD detector. The excitation wavelength was 532 nm and spectra were obtained over 60 s at 1.0 cm-1 resolution. XPS measurements were carried out using monochromatized Al Kα radiation (1486.7 eV) as Xray source with a base pressure of 10−8 Pa. The spot diameter was 600 μm during all the measurements. XPS was performed with a pass energy of 50.0 eV and high resolution scans with a step size of 0.1 eV were collected after a survey scan with a step size of 1.0 eV, for carbon 1s, oxygen 1s, sulfur 2p, and manganese 2p. All the binding energies were calibrated from the C 1s hydrocarbon peak (284.8 eV). The obtained XPS spectra were analyzed by AVENTAGE software 6


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with the following parameters: full-width at half-maximum (FWHM) (eV) = 0.5:3.5 and Lorentzian/Gaussian =30%. Electrodes Preparation and Coin Cells Assembly. CR2016 type coin cells with lithium metal as the anode were made to study the effect of polymer coating on the performance of the LiMn 2O4 cathode electrode. Celgard separators, electrolytes, and lithium foils were purchased from MTI cooperation. The electrolyte was ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 v/v) containing 1 M LiPF6. The entire assembly process was carried out in a glove box with an O2 and H2O level maintained below 0.5 ppm. To make LiMn2O4 cathode electrodes, an electrode slurry was made of a 70:20:10 wt% mixture of LiMn2O4, carbon black, and polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone (NMP). The slurry was then spread on an aluminum foil which was used as the current collector. This foil was vacuum dried at 110 °C overnight. These foils were placed in the CVD reactor as mentioned before to obtain surface engineered LiMn2O4 electrodes. Cathode discs of 14 mm diameter with and without polymer coating were punched and collected in glass bottles, with typical mass loading of 2 - 3 mg. The PDVB coated electrode discs were utilized directly, while the PEDOT coated discs were washed by immersing them in methanol for 5 minutes to remove residual monomers and oxidants. These discs were vacuum dried overnight at 110 °C again to get rid of trace water in electrodes. Then, the mass of each disc was measured before being transferred to an argon-filled glove box for coin cell fabrication. Electrochemical characterization. Electrochemical performances of cells were tested using a Biologic VMP3 (Bio-Logic Science Instruments) and LAND battery cyclers (LAND electronics Co., Ltd.). Cells were first cycled 3 times at 0.1C at room temperature (25 °C) before conducting other tests. The voltage range was 3.5-4.3 V and the protocol was constant-current (CC) charge and constant-current (CC) discharge. These settings were kept the same unless otherwise stated. Rate capabilities of cells were then tested via C/3 charging followed by different discharging rates, including C/10, C/3, 1C, 2C, 5C, and 10C. To study the effect of temperature on the conductivity 7



of PEDOT thin films, rate capacity of coin cells with PEDOT coated LiMn2O4 electrodes was tested at 0 °C and 50 °C in TestEquity Model 106 temperature chamber (TestEquity LLC). Then, electrochemical impedance spectroscopy (EIS) was performed at room temperature for all cells. Cells were charged to 4.1 V before measuring the EIS using constant-current constant-voltage (CCCV) charging with C/100 as the cut-off current. EIS was potentiostatically measured by applying an AC voltage of 10 mV amplitude over the frequency range 100 kHz-50 mHz. Finally, cells were cycled by 1C at 50 °C for 150 times, during which the temperature was controlled by Lindberg Blue M furnace (Thermo Scientific). The actual temperature of coin cells was calibrated using a thermistor from U.S. SENSOR corp. and the calibration details can be found in Fig. S3 (SI).

RESULTS AND DISCUSSIONS Material Characterizations. CVD polymerization can uniformly deposit polymer thin films onto complex structures ranging in thickness from few nm to tens of μm.18-20 Figure 2 displays microscope images of polymer thin films coated onto a silicon trench and a LiMn2O4 particle. Figure 2 (a) and (b) compare SEM images of a silicon trench before and after being coated with a 100 nm PDVB film. The film coating was very uniform on the overall surface, including the curved parts, as shown in the enlarged figures of the top and bottom sections. Figure 2 (c), (d) and (e) display TEM images of a LiMn2O4 particle before and after being coated with a 10 nm thick PDVB film and a 20 nm thick PEDOT film. Both PDVB and PEDOT films were conformally coated on the surface of the particle, as shown in Figure 2 (d) and (e). Figure 2 (f), (g) show the cross-sectional image of a LiMn2O4 electrode and the EDS mapping of S element within the selected area. Since S only exists in PEDOT thin films, the uniform distribution of S indicates that PEDOT is uniformly coated on the LiMn2O4 particles all over the electrode. These results suggest that the CVD polymerization technique can engineer the surface of a cathode electrode with uniform polymer thin films. The thickness of the coating can be easily tuned by adjusting deposition parameters like chamber pressure and coating time. Furthermore, the morphology of the cathode electrode 8


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remained the same before and after the coating, which ensures enough surface area for lithium ion intersection during charging/discharging process. All these advantages indicate the superiority of our vapor-based deposition technique compared with solution processing that has little control over coating thickness and conformality.25

Figure 2: Polymer thin films were uniformly coated on the surface of complex structures. SEM images of (a) an uncoated silicon trench and (b) a 100 nm thick PDVB coated silicon trench. TEM image of (c) a pristine LiMn2O4 particle, (d) 10 nm thick PDVB coated on a LiMn2O4 particle, and (e) 20 nm thick PEDOT coated on a LiMn2O4 particle. (f) Cross-sectional image of LiMn2O4 electrode and (g) S elemental mapping in the selected rectangular area in (f), the blue points represent the detected S element. 9



FTIR and Raman spectroscopy were applied to verify the successful polymerization of PDVB and PEDOT thin films. The comparison between these two techniques is discussed in Section S4 (SI). Successful polymerization of PDVB is shown by comparing the FTIR spectra of DVB monomer and PDVB polymer, as in Figure 3. The peak at 903 cm-1 comes from CH2 out-of-plan deformation in unreacted vinyl groups. The reduction of this peak indicates the loss of vinyl groups after polymerization. Additionally, strong peaks at 2870, 2930, and 2960 cm-1 are observed in the PDVB spectrum, allocated to symmetric sp3 CH3, asymmetric sp3 CH2, and asymmetric sp3 CH stretching in the backbone of the newly formed polymer chain.24,26 The relationship between each peak in the spectra and its corresponding functional group can be found in Fig. S4 and Table S3 (SI). Excellent agreement is observed between spectra in this work and prior publications, indicating the PDVB polymer has been successfully synthesized via the iCVD technique.24

Figure 3: FTIR spectra of the DVB monomer and iCVD PDVB. Characteristic peaks in both spectra and the corresponding positions in their molecular structures are numbered.

Raman spectroscopy indicates successful PEDOT thin film deposition on the LiMn2O4 electrode. Figure 4 (a) shows the Raman spectra of a PEDOT film on a silicon wafer with a thermally grown oxide layer. The molecular structure of PEDOT is shown in the top-left section of the Figure. All 10


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Raman peaks are identified and labelled according to published data.27,28 The four main peaks with high wavenumbers are from Cα-Cα inter-ring stretching (1250 and 1270 cm-1), Cβ-Cβ stretching (1371 cm-1), Cα-Cβ symmetrical stretching (1440 cm-1), and Cα-Cβ asymmetrical stretching (1514 and 1567 cm-1). Figure 4 (b) compares Raman spectra of a PEDOT polymer film (on a Si/SiO2 substrate), a LiMn2O4 electrode, and a PEDOT coated LiMn2O4 electrode. The inserted picture shows the position of Raman laser projected onto a LiMn2O4 particle. The result shows that the Raman spectra of a PEDOT coated LiMn2O4 electrode includes peaks from spectra of both LiMn2O4 and PEDOT, indicating that the PEDOT film has been successfully coated on the LiMn2O4 electrode. The peak at 522 cm-1 is from Si-O-Si of SiO2 substrate rather than PEDOT polymer according to the Raman band correlation table.29 The sharp peak at 1640 cm-1 in the spectra of PEDOT coated LiMn2O4 sample is likely from cosmic ray, which appears randomly during Raman spectroscopy measurement.

11



Figure 4: (a) Raman spectra of a PEDOT thin film, the source of each peak is labelled for clarity, (b) comparison of Raman spectra of a LiMn2O4 electrode, a PEDOT thin film, and a PEDOT coated LiMn2O4 electrode, an optical microscopy image is inserted to show the measurement position for a LiMn2O4 electrode. The sources of Raman peaks in PEDOT coated LiMn2O4 are indicated by dash lines.

The thickness of the transparent PDVB polymer thin films can be measured by ellipsometry, but profilometry was used to measure the thickness of PEDOT thin films (which are not optically transparent). Details of the measurement can be found in Fig. S5 and Table S4 (SI). The optimization of polymer coatings will be discussed in our follow-up studies, and the optimization 12


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parameters will include but not be limited to substrate temperature during polymer growth, film thickness, with and without post-deposition annealing, and annealing temperature. All these parameters can significantly affect polymer thin films and thereby affect the electrochemical performance of cathode electrodes. The electrochemical stability of PDVB films was studied using cyclic voltammetry (CV) test, as shown in Fig. S6 (SI). The CV test was conducted within voltage range of 3.5 – 4.3 V with the scan rate of 1 mV/s. Compared to the cell using pristine Al foil as cathode, cell with PDVB coated Al foil does not show new peaks during the scanning process, suggesting the PDVB film is electrochemical stable within the tested voltage range (3.5 – 4.3 V). The PEDOT film has been applied and proved as an effective coating material for LIB cathodes.15 Thus, the electrochemical stability of PEDOT is not repeatedly tested in this research.

Rate capability of the LiMn2O4 Electrode. The conducting PEDOT thin film improves the kinetic performance of the LiMn2O4 electrode, while the insulating PDVB thin film has little effect. Figure 5 (a), (b), and (c) display discharge curves of cells using a pristine LiMn2O4, PDVB coated LiMn2O4 and PEDOT coated LiMn2O4 electrode as the cathode electrode, respectively. The overall shape of the discharge curve remains similar for all the three types of cells, while the voltage drop is affected by polymer coatings. Compared to the pristine cell, the conducting PEDOT thin film coating reduces the voltage drop, especially at high C-rates, while the insulating PDVB coating does not show significant improvement in the voltage drops. This suggests the PEDOT thin film coating decreases the overall resistance of coin cells, and thereby increases the discharge capacities at high rates like 5C and 10C.

13



Figure 5: The effect of polymer coatings on the rate capability of the LiMn2O4 electrode. (a) Discharge curves of coin cells using a pristine LiMn2O4 electrode, (b) a PDVB coated LiMn2O4 electrode, and (c) a PEDOT coated LiMn2O4 electrode. (d) Statistical data summary of cell capacities with respect to C-Rates at room temperature, (e) at low temperature (0 °C), and (f) at high temperature (50 °C). The statistical results are calculated from three coin cells in each case. These results indicate that PEDOT coating improves the rate capability in a wide temperature range from 0 °C to 50 °C, while PDVB coating has little effect. 14


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Figure 5 (d), (e), and (f) compare cell capacities at different C-rates, and each data point is averaged over three samples. All cells were charged at room temperature at C/3, which ensures the same starting point for discharging. Hence, the different discharge capacities are attributed to the polymer coatings only. At room temperature, the insulating PDVB coating does not improve the rate performance, while the conducting PEDOT thin film significantly increases the cell capacity, especially at high C-rates like 10C. The cell capacity increases by 35% at 5C (from 49 mAh/g to 66 mAh/g) and by 83% at 10C (from 23 mAh/g to 42 mAh/g) by coating the LiMn2O4 electrode with a PEDOT thin film. To consider the effect of temperature on the conductivity of PEDOT thin films30, cells using LiMn2O4 electrodes with and without PEDOT coating were also tested at low temperature (0 °C) and high temperature (50 °C), and the results are shown in Figure 5 (e) and Figure 5 (f). The PEDOT coating improves the rate capability of LiMn2O4 electrode regardless of testing temperature. The 10C capacity at 50 °C reaches 83 mAh/g, which is around 80% of the maximum capacity (105 mAh/g) tested at low C-rate (C/10) and 25°C. The 5C capacity at 0 °C is almost tripled (from 6 mAh/g to 17 mAh/g), although the value is still quite low for battery applications. Electrochemical impedances of the three types of cells were measured to further study the kinetic effects of polymer coatings. Figure 6 (a) compares the EIS of the three types of cells, which suggests the conducting PEDOT polymer coating significantly reduces the overall impedance of the cell, while the insulating PDVB coating does not appear to have an effect on the overall impedance. A three-order equivalent circuit model was applied to understand the improvement, as shown in Figure 6 (b).31 The semicircle at higher frequency is ascribed to the charge-transfer process on Li anode (Rct1), while the semicircle at lower frequency is ascribed to charge-transfer process on LiMn2O4 cathode (Rct2).32 The simulation results for all the three types of cells are listed in Table S5 (SI). 15



Figure 6: (a) Electrochemical impedance spectroscopy of three types of cells tested at room temperature with a 4.1 V open circuit voltage, (b) a three-order equivalent circuit model to simulate the EIS data and a schematic that shows different circuit components derived from different partitions on impedance data.

The charge transfer resistances at the interfaces of anode/electrolyte (Rct1) and cathode/electrolyte (Rct2) are largely reduced by the PEDOT coating. Since PEDOT is highly conductive,30 electron transport from the current collector to cathode particles is facilitated, leading to the decrease of Rct2. The reduction of the Rct1 can be ascribed to cross-talk between anode and cathode.33 Such crosstalk has been reported to change the compositions of SEI formed on the surface of both electrodes, and thereby reduces the Rct1. In comparison, the PDVB coating has little effect on the overall charge transfer resistance. It slightly reduces the resistance on anode (Rct1), while increases the resistance on cathode (Rct2). The two effects cancel off and the overall impedance remains similar as the pristine cell. The increased Rct2 is from the insulating PDVB coating on cathode that inhibits the charge transfer process involving Li+, while the reduced Rct1 can also be ascribed to the cross-talk behavior between cathode and anode.33 Although there were some studies that have addressed the cross-talk effect in batteries, more experiments need to be done to confirm that polymer coatings on the LiMn2O4 cathode electrode could affect the SEI formed on the surface of a lithium anode.14,33 Cycling Life Extension at High Temperature. Cells were cycled at 50 °C to study the effect of polymer coatings on their cycling life. At such a high temperature, these cells seriously suffered 16


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from both calendar aging and cycling aging. Herein, an index called equivalent cycle number (Neq) is applied to ensure all cells experienced the same amount of storage time when they go through the same number of cycle.34 (𝑁𝑒𝑞 )𝑖 =

(𝐴ℎ−𝑡ℎ𝑟𝑜𝑢𝑔ℎ𝑝𝑢𝑡)𝑖 2×𝐶𝑎𝑝𝑛𝑜𝑟𝑚𝑎𝑙

(1)

where (Ah-throughput)i is the capacity that a cell has gone through at the ith cycle, Capnormal is the normal capacity of a cell. Applying Neq as the index ensures that the different aging performance among these cells is only from the cycling test. Figure 7 indicates that the PEDOT coating extends cells cycling life, while the PDVB coating does not show an obvious improvement. Figure 7 (a) compares specific capacities of the three types of cells during the cycling test. The PEDOT coating not only increases the initial specific capacity from 105 mAh/g to 115 mAh/g, it also slows down the capacity degradation from 0.21 mAh/g to 0.17 mAh/g per cycle on average. In contrast, the PDVB coating does not enhance the cycling performance. The cycling life improvement from the PEDOT polymer coating is more evident in Figure 7 (b), where capacity remaining is compared among the three types of cells. Generally, cycling life of a cell is defined as the number of cycles that it can undergo before its capacity degrades to 80% of its initial capacity. According to this definition, cycling life of the LiMn2O4 cell is extended from 87 cycles to 122 cycles by the PEDOT coating, suggesting an improvement over 40%. However, the PDVB thin film coating does not extend cycling life. In fact, it reduces the cycle number to some degree.

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Figure 7: Cell cycling life is extended by PEDOT coating, while it is reduced by PDVB coating. (a) Specific capacity with respect to cycle number and (b) capacity remaining with respect to cycle number. The tests were conducted at 50 °C using 1C as the charging and discharge rates.

Manganese dissolution and SEI growth are the main aging mechanisms in Li/LiMn2O4 cells during high temperature cycling tests, as illustrated in Figure 8 (a).35 Mn dissolution is caused by a disproportionation reaction with the help of acids (2 LiMn2O4 + 4 H+ → 2 Li+ + Mn2+ + 2 H2O + 3 λ-MnO2)36. The amount of Mn dissolution is proportional to cycle number during cycling tests.37,38 Hence, the contribution of Mn dissolution to the overall capacity degradation can be described using a linear equation (Eq. (2)). SEI forms on the surfaces of both electrodes and increases cells resistance, which leads to gradually increased overpotential and reduces cells capacity. The SEI growth follows the square root of cycle number or storage time.39 Thus, the effect of SEI growth on cell capacity can be described by Eq. (3). Considering these two effects, the capacity remaining can be related to cycle number by Eq. (4). 𝐶𝑎𝑝𝑙𝑜𝑠𝑠,𝑀𝑛 = 𝑘1 ∗ 𝑁

(2)

𝐶𝑎𝑝𝑙𝑜𝑠𝑠,𝑆𝐸𝐼 = 𝑘2 ∗ √𝑁

(3)

𝐶𝑎𝑝𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 = 1 − 𝑘1 ∗ 𝑁 − 𝑘2 ∗ √𝑁 18


(4)

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Figure 8. (a) Schematic of main aging mechanisms in Li/LiMn2O4 coin cells, including SEI growth and Mn dissolution. (b) Capacity remaining fit from Eq. (4) based on the proposed aging mechanisms. Experiment data is shown every five points for clarity.

Figure 8 (b) shows the fitted curves agree well with experiment data for all the three types of cells, indicating Eq. (4) can accurately describe the capacity remaining with respect to cycle number. This suggests the aging mechanisms illustrated in Figure 8 (a) are the main reasons for capacity degradation during the cycling test in Li/LiMn2O4 coin cells. Parameters k1 and k2 in Eq. (4) are fitted based on average capacity values of each type of cell using the least square method in MATLAB. Fitted values of k1 and k2 with 95% confidence interval are listed in Table 1. PEDOT coated cells have significantly smaller k1 and k2 values compared to pristine cells, suggesting PEDOT coating inhibits both Mn dissolution and SEI growth during the high temperature cycling test. In contrast, the PDVB coating slightly promotes Mn dissolution and SEI growth during the cycling test. This conclusion can be further validated by comparing cell performance after the cycling test, as shown in Fig. S7 (SI). After the cycling test, the PEDOT coated cell has larger capacity and smaller overpotential during discharging compared to the pristine cell, indicating more LiMn2O4 active material remains and less SEI forms during the cycling test.

19



Table 1. Values of parameters k1 and k2 in Eq. (4) fitted by the least square method in MATLAB. The numbers in square brackets beside each value stand for the 95% confidence interval of that value.

k1 (*10-3) 0.97 [0.87, 1.07] 1.18 [1.07, 1.29] 0.73 [0.68, 0.78]

Pristine PDVB coated PEDOT coated

k2 (*10-2) 1.34 [1.26, 1.41] 1.41 [1.34, 1.49] 1.04 [0.99, 1.08]

The Effect of Film Properties on Cell Performance. Rinsing PEDOT films in methanol is a crucial step to improve the electrochemical performance of LiMn2O4. Fig. S8 (SI) compares the effect of PEDOT coating on the performance of coin cells with and without methanol rinsing for PEDOT coated LiMn2O4 electrode after the oCVD experiment. The results suggest that the rate performance of LiMn2O4 reduced without the rinsing step (perhaps due to FeCl3 impurity), compared with the samples with rinsing. However, the rinsing step seemed to have little effect on cell cycling performance. Therefore, rinsing PEDOT coated electrode with methanol is necessary to get the best performance of LiMn2O4. A higher sample stage temperature during oCVD deposition can improve the conductivity of PEDOT films.30 Two stage temperatures, 70 °C and 130 °C, are compared here. Additionally, two thicknesses of the PEDOT polymer coatings, 21 nm and 41 nm, are evaluated at the same stage temperature (70 °C). The electrochemical impedance of the LiMn2O4 electrode is largely reduced by PEDOT coatings at all examined conditions, as shown in Figure 9 (a). However, increasing the conductivity of PEDOT film by using 130 °C as the stage temperature does not further bring down the impedance. In addition, the impedance of the LiMn2O4 electrode does not change a lot when varying the film thickness. It has been shown in literature that the charge transfer kinetics is dominated by the de-solvation process when the SEI is sufficiently conductive.40 The conducting 20


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PEDOT film on the cathode electrode largely increases the conductivity of SEI. As a result, further increasing the conductivity of PEDOT film may not help in continuously reducing the overall impedance of LiMn2O4 electrode; because electron transfer is not the limiting step any longer. Figure 9 (b) compares the capacity degradation of different cells using different PEDOT polymer coated LiMn2O4 electrodes. All the PEDOT coatings show significantly improvement in cells capacity remaining during the cycling test at 50 °C compared to the pristine cells. For the PEDOT films synthesized at the stage temperature of 70 °C, increasing the coating thickness from 21 nm to 41 nm improves the capacity remaining rate. The cycling life is also extended by around 15% (from 122 to 141). Im et al. has reported that the molecular weight, length of conjugation, and the degree of deprotonation of the PEDOT polymer chain can be affected by the stage temperature during the oCVD experiment.30 Interestingly, our results also show that increasing the stage temperature during oCVD polymerization from 70 °C to 130 °C improves the cycling performance and cycling life. The latter is extended by around 20% (from 122 to 147). Therefore, the PEDOT film from the stage temperature of 130 °C has better protection for the LiMn2O4 electrode compared to that from 70 °C.

Figure 9: All PEDOT coatings decrease cell impedance and extend cell cycling life. (a) Electrochemical impedance spectroscopy of four types of cells tested at room temperature with a 4.1 V open circuit voltage; (b) capacity degradation of these cells during cycling at 50 °C and 1C.

21



The coating thickness of PDVB film does not show a significant effect on the improvement of the LiMn2O4 electrode. Fig. S9 (SI) compares the capacity degradation of pristine cells and cells using LiMn2O4 cathodes with different thicknesses of PDVB film coatings. Five thicknesses of coatings were studied, including 23, 43, 56, 74 and 100 nm. The improvement of cycling life is not obvious even when the coating reaches a thickness of 100 nm. To clearly compare the capacity remaining of different cells, Fig. S9 (c) shows the capacity remaining of different cells when they go through 60 times of cycling at 50°C, which further suggests that the PDVB coatings does not help life extension of the LiMn2O4 electrode regardless of the coating thickness. It is believed in the literature that polymer coatings can reduce electrolyte oxidization on the surface of cathode electrodes by introducing a physical barrier between the oxidizing spinel and electrolyte.41,42 However, our study indicates that a physical barrier alone is not enough to improve electrochemical performance of the LiMn2O4 electrode. A physical barrier is formed between electrode and electrolyte by coating an insulating PDVB thin film on the surface of LiMn 2O4. However, the barrier seems to have little effect on the electrochemical performance. Here, we propose that chemical factors, like the formation of chemical or coordinate bonds between polymer thin film and the electrode, are necessary to obtain desired performance improvement. We hypothesize that these chemical factors can help stabilize manganese element by increasing its chemical valence and therefore inhibiting the disproportionation reaction of Mn3+ during the cycling process.43 X-Ray Photoelectron Spectroscopy. The mechanism behind the performance improvement of cathode electrodes from different coatings have not been not fully understood. For inorganic coatings, it is believed that metal oxide coatings scavenge trace hydrogen fluoride acids (HF) in the electrolyte and thus slow down dissolution of metal elements in cathodes; resulting in better performance. However, this understanding was challenged by Bai et al., who reported that the coating of YPO4 on LiCoO2 induces more acidity from the electrolyte.44 Recent studies on 22


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LiNixMnyCozO2 degradation mechanism further disprove the proposed HF based mechanism.45 For organic coatings, Hu et al. proposed a mechanism that elements with strong electronegativity, like O and S, can form bonds with Mn on the spinel LiMn2O4 interface, and therefore stabilize the electrode during its operation in LIBs.41,42 XPS measurements were conducted on LiMn2O4 electrodes with and without the two types of polymer coatings to understand the effect of polymer coating on the cycling performance of LiMn2O4. The effect of PEDOT coating on S 2p, O 1s and Mn 2p binding energies are compared in Figure 10 and the binding energies are listed in Table 2. The effect of PDVB coating on Mn 2p binding energy is displayed in Fig. S10 (SI).

Figure 10: XPS spectra of (a) S 2p (b) O 1s and (c) Mn 2p in (A) a pristine LiMn2O4, (B) a PEDOT thin film, and (C) a PEDOT coated LiMn2O4. Measurement data (dots) are fitted by several individual spectra (colored regions). The combined spectra from these color shaded regions is shown as an envelope that matches well with experimental data (dots). The schematics of these materials are shown in (d).

23



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Table 2: Binding energy of S 2p, O 1s, and Mn 2p from different samples. The Mn-Y represent new bond formation for Mn element.

S (eV) Sample LiMn2O4 PEDOT PEDOT coated LiMn2O4

O 1s (eV)

/ 163.07

/ 164.23

/ 165.2

529.7 531.6 533.07

Mn 2p3/2 641.93 /

163.07

164.34

168.3

531.26

642.23

S 2p3/2 S 2p1/2

S-Cl

Mn (eV) Mn 2p1/2 653.57 / 653.82

Mn-Y / / 646.6

Coating PEDOT thin films on the surface of LiMn2O4 changes the binding energies of S 2p and O 1s in PEDOT and Mn 2p in LiMn2O4, respectively. This suggests the formation of new bonds. S 2p in PEDOT generally has doublet peaks at around 163 eV and 164.3 eV, as shown in Figure 10 (a). Another peak shows up at ~ 165.2 eV, which is attributed to the oxidization of S by Cl during the doping process in CVD PEDOT synthesis.30 Coating PEDOT film onto a LiMn2O4 surface shifts the S-Cl peak from 165.2 eV to 168.3 eV, due to the oxidation of S by O present in the LiMn2O4.46,47 The area ratio under the 168.3 eV peak significantly increases after the coating, indicating some of the undoped S may also be oxidized by O present in LiMn2O4. Figure 10 (b) compares O 1s peaks in the three studied materials, whose binding energy is in the range of 528 eV to 535 eV.48 The peak at 529.7 eV is from the O in LiMn2O4, while the peak at 531.6 eV is from O in Li2CO3, which is formed during LiMn2O4 exposure to air. The evidence of Li2CO3 formation can also be found in C 1s spectra, as shown in Fig. S11. In contrast, only one peak is found in the PEDOT and the PEDOT coated LiMn2O4 samples since XPS can only detect composition near the surface (~10 nm). The O 1s peak in PEDOT reduces from 533.1 eV to 531.3 eV after being coated onto LiMn2O4, indicating the reduction of the O element in the PEDOT film. Figure 10 (c) shows the evolution of Mn 2p peaks after polymer coating. The Mn 2p doublet peaks shift from 641.9 eV to 642.2 eV and from 653.6 eV to 653.8 eV. These suggest that Mn in LiMn2O4 is further oxidized by the PEDOT coating.41 Also, a MnO satellite feature shows up at 646.6 eV after the coating, indicating the surface reconstruction of LiMn2O4 electrode after the coating.49 However, the details 24


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of MnO formation is not fully understood and is not the main focus of this study. Nevertheless, integrating the above results, we can conclude that there are bonds formation between Mn in LiMn2O4 and O and S in PEDOT at the interface of the coating. These bonds further oxidize Mn on the surface of LiMn2O4. Therefore, the stability of the LiMn2O4 is increased since extra energy is needed to destroy the spinel structure. In comparison, Fig. S10 (SI) suggests that PDVB coating does not have significant effect on the binding energy of Mn 2p, indicating no bonds are formed between Mn and elements in PDVB film after coating. This observation may help explain why PDVB coating does not improve high temperature cycling performance of LiMn2O4 electrode, as shown in Figure 7. This study opens a novel research direction in the field of surface and interface engineering for battery materials. The CVD polymerization technique has potential to improve the performance of many other cathodes in LIBs, including high rate cathode LiCoO2, high voltage cathode LiNi0.5Mn1.5O4, and high capacity Ni-rich cathode LiNixCoyMnzO2 (x>0.8). For example, Fig. S12 (SI) shows early test data on LiCoO2. The results show that PEDOT coating significantly improves high voltage (3.0 – 4.5 V) cycling stability of LiCoO2. The cycle number is almost tripled by the PEDOT coating when pristine LiCoO2 decreases to 50% of its initial capacity. In contrast, PDVB coating has little effect on the cycling performance. More studies are underway to uncover the mechanism behind the significant improvement of the LiCoO2 stability after PEDOT coating. The effect of PEDOT coating on other cathode and anode materials will be studied in our future work.

CONCLUSION In this work, the novel versatile CVD polymerization technique is introduced to systematically modify the surface of a LiMn2O4 cathode electrode in LIBs using multifunctional polymer films. For the first time, the effect of PEDOT and PDVB polymers on LiMn2O4 performance is 25



systematically compared. Electrochemical test results show that the conducting PEDOT coating significantly improves the kinetic performance of the LiMn2O4 electrode in the temperature range from 0 °C to 50 °C. In contrast, the insulating PDVB coating does not show much effect on cell rate performance. EIS results further indicate that the PEDOT coating largely reduces charge transfer resistance of Li+ at the interface of electrolyte and both electrodes, while the PDVB coating has little effect. Furthermore, the high temperature (50 °C) cycling life of a LiMn2O4/Li cell is extended by over 60% using the PEDOT thin film coating but declined with the PDVB coating. A model is developed to describe cell capacity degradation based on the main aging mechanisms in Li/LiMn2O4 cells. The simulation results of the model suggest PEDOT coating inhibits both Mn dissolution and SEI growth during the cycling test. The reason of cycling ability improvement from the PEDOT polymer coating is further explained by XPS analysis that there are bonds formed between Mn in LiMn2O4 and O/S in PEDOT, which stabilize the LiMn2O4 cathode material during the cycling test. In comparison, no bonds are formed between Mn and elements in PDVB film. Finally, we prove that this technique has potential to be applied to other cathode materials and LiCoO2 is chosen as an example to illustrate the potential of the technique to enable advanced LIBs. This study opens a novel research direction in the field of surface coating and interface engineering for LIB materials. The CVD polymerization technique has the potential to improve the overall performance of many advanced cathode and anode materials by systematic study of processingstructure-property relationships.

Supporting Information iCVD parameter settings, TEM sample preparation steps, temperature calibration of furnace for high temperature cycling test, FTIR details for DVB and PDVB, ellipsometry measurement for film thickness, electrochemical stability of PDVB films, EIS simulation results, discharge curves 26


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of cells after cycling tests, the effect of FeCl3 on cell performance, the performance of cells with different thicknesses of PDVB coatings, XPS spectroscopy of C 1s for LiMn2O4 electrode and Mn 2p for LiMn2O4 with and without PDVB coating, and preliminary cycling results of LiCoO2 with and without PDVB and PEDOT polymer coatings.

ORCID Laisuo Su: 0000-0002-9307-9357 Phil. M Smith: 0000-0002-7811-5479

Funding The authors acknowledge funding provided by INCUBATE seed funding from Carnegie Mellon University.

ACKNOWLEDGEMENT The authors acknowledge Prof. Jay F Whitacre and Prof. Shawn Litster in Carnegie Mellon University (CMU) for allowing us to utilize their lab facilities. The authors thank Dr. Alex Mohamed (Material Science and Engineering of CMU), Ms. Sneha Shanbhag (Civil and Environmental Engineering of CMU) and Prof. Jun Huang (College of Chemistry and Chemical Engineering of Central South University) for helpful discussion of experiment results. The authors acknowledge Prof. Stefanie A. Sydlik and Mr. Daniel Siroky (Chemistry Department of CMU) for the help of FTIR experiment. The authors acknowledge use of the Materials Characterization Facility at CMU supported by grant MCF-677785 and facilities at Nanofabrication Laboratory at CMU.

27



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Table of Contents

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Surface Engineering of a LiMn2O4 Electrode Using Nanoscale

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