graphite pouch cells operated at 4.5 V

Journal of Power Sources 300 (2015) 419e429

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Effect of LiPF6 concentration in Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells operated at 4.5 V R. Petibon a, L. Madec b, D.W. Abarbanel b, J.R. Dahn a, b, * a b

Dept. of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H4R2, Canada Dept. of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada

h i g h l i g h t s
The effect of LiPF6 concentration on high voltage cycling has been evaluated.
Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells were used as test vessels.
The impedance of the positive electrode increases during high voltage cycling.
High LiPF6 concentration lowers the impedance growth rate of cells operated at 4.5 V.
The SEI of the electrodes is affected by LiPF6 concentration variations.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 July 2015 Received in revised form 25 August 2015 Accepted 24 September 2015 Available online xxx

The effect of LiPF6 concentration in the 1 Me2.5 M range was studied in Li[Ni0.4Mn0.4Co0.2O]2 (NMC(442))/graphite and LaPO4 coated-NMC(442)/graphite pouch cells cycled to high voltage. Electrochemical impedance spectroscopy on symmetric cells revealed that the dramatic impedance growth observed in NMC(442)/graphite cells cycled to high voltage comes from the interface impedance of the positive electrode. The use of high LiPF6 concentrations in the 2e2.5 M range dramatically slowed down the impedance growth of both coated and uncoated NMC(442)/graphite cells containing certain electrolyte additive blends and cycled to high voltage. However no beneficial effect was observed in control cells containing no electrolyte additive. X-ray photoelectron spectra of cycled electrodes of coatedNMC(442)/graphite cells showed that LiPF6 concentration greatly affected the composition of the solid electrolyte interphase of both the positive and negative electrodes of cells containing additives. © 2015 Elsevier B.V. All rights reserved.

Keywords: Li-ion battery high voltage LiPF6 concentration Li[Ni0.4Mn0.4Co0.4]O2/graphite LaPO4 coating

1. Introduction Li-ion batteries are now widely used in portable electronics and electrified vehicles. However in order to improve the marketability of fully electric and plug-in hybrid vehicles, the volumetric energy density of Li-ion cells should be improved. While Li[NixMnyCo1-xyO]2 positive active materials such as Li[Ni0.4Mn0.4Co0.2O]2 (NMC(442) can be cycled up to 4.7 V vs Li/Liþ without structural bulk damage [1], their upper cut off potential is often limited to 4.3 V vs. Li/Liþ. This potential limitation is a direct consequence of severe electrolyte degradation at the positive electrode surface leading to large cell impedance [2e6] and short lifetime.

* Corresponding author. Dept. of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H4R2, Canada. E-mail address: [email protected] (J.R. Dahn). http://dx.doi.org/10.1016/j.jpowsour.2015.09.090 0378-7753/© 2015 Elsevier B.V. All rights reserved.

In order to eliminate electrolyte degradation at high potential, researchers have been focusing on developing new electrolyte additive combinations [2e9], inorganic coatings on the positive electrode material surfaces [10,11], or new electrolyte systems displaying better anodic stability [12e14]. Finding the right path to enable the use of oxide materials over their entire potential window of structural stability would enable Li-ion cells with higher volumetric energy density [15]. In an earlier publication, Ma et al. [2] showed that the use of ternary electrolyte additive blends containing prop-1-ene-1,3 sultone (PES), methylene methanedisulfonate (MMDS) and tris(trimethylsilyl) phosphite (TTSPi) helped stabilize the impedance of NMC(442)/graphite pouch cells cycled at constant current up to 4.5 V. However, Nelson et al. [3] showed that NMC(442)/graphite cells cycled using a constant current charge to 4.4 V followed by a 20 h constant voltage hold at 4.4 V (CC- 20 h CV), displayed a

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dramatic capacity loss and large impedance increase for equal cycling periods. This indicated that the results of constant current continuous “up-down” cycling may lead to a false impression of good performance. Nonetheless, these studies clearly indicated that identifying the right additive blend can enable higher upper cutoff potentials, compared to cells with less suitable additives, while retaining acceptable performance. While tuning an electrolyte system mainly focuses on optimizing the additive blend, tuning salt content might be equally as important. There are now a few reports indicating that salt concentration greatly affects the reactivity of the electrolyte at the surface of both negative and positive electrodes of Li-ion cells. For instance, Nie et al. [16] showed that the composition of the solid electrolyte interphase (SEI) at the surface of graphite electrodes cycled in PC based electrolyte is highly dependent on salt concentration. They showed that going from a 1 M LiPF6ePC electrolyte to a 3.5 M LiPF6ePC electrolyte, the major reduction product changed from lithium propylene dicarbonate to LiF. They associated this difference to a solution structure change between the low concentration and high concentration electrolytes. Similarly, Yamada et al. [17e20], Yoshida et al. [21], and Petibon et al. [22] showed that large concentrations of Lithium bis(fluorosulfonyl)imide allowed the use of atypical solvents such as dimethyl sulfoxide, acetonitrile, glyme ethers, and ethyl acetate with graphite electrodes. Yoshida et al. [21] also showed that the atypically large salt concentration increased the oxidative stability of glyme solvents. Similarly, Petibon et al. showed that NMC(442)/graphite cells with ester based electrolytes and large LiFSi concentration cycle quite well up to 4.4 V [22] and 4.7 V [23]. This change of reactivity was assigned to a peculiar solution structure [20,19], similarly to the findings of Nie et al. [16]. In 2014, Wang et al. [24] published the results of high precision coulometry measurements of LiCoO2/graphite cells cycled to 4.2 V with various LiPF6 concentrations. They reported that while higher LiPF6 concentration increased the parasitic reaction rates in cells without electrolyte additives, it did not have any detrimental effect in cells containing vinylene carbonate (VC). Moreover they showed that increasing the LiPF6 concentration from 1 M to 2 M significantly lowered the impedance of cells stored at an open circuit voltage of 4.2 V. With regard to the findings enumerated above, it seems worthwhile to explore the impact of the LiPF6 salt concentration on the impedance, capacity retention, and surface composition of the electrodes of NMC(442)/graphite cells cycled to high voltage. This article attempts to identify the individual impedance change of the electrodes of Li[Ni0.4Mn0.4Co0.2O]2/graphite cells cycled to high voltage, as well as determine the impact of salt concentration on the impedance and capacity retention of uncoated and LaPO4 coated-NMC(442)/graphite cells with or without additives. In order to achieve this goal several techniques were employed such as electrochemical impedance spectroscopy (EIS) on symmetric cells, open circuit voltage (OCV) storage, constant current cycling and CCCV cycling to 4.5 V as well as X-ray photoelectron spectroscopy (XPS) of cycled electrodes. 2. Experimental

adjusted so the cells could be cycled to 4.7 V without lithium plating. The graphite electrode of both pouch cell types consisted of 96% of 15e30 mm artificial graphite particles mixed with 2% carbon black as conductive additive and 2% carboxymethylcellulose (CMC)styrene butadiene rubber (SBR) as binding agent. The positive electrode of the NMC(442)/graphite cells consisted of 96% of 5e15 mm Li[Ni0.4Mn0.4Co0.2]O2 particles mixed with 2% of carbon black and 2% of polyvinylidene fluoride (PVDF). The positive electrode of the coated-NMC(442)/graphite cells consisted of 96% of 5e10 mm LaPO4 (3% by mass) coated-Li[Ni0.4Mn0.4Co0.2]O2 particles mixed with 2% of carbon black and 2% of PVDF. The pouch cells were vacuum sealed in China without electrolyte and then shipped to Canada. In Canada, the pouch bag of the pouch cells was cut open and the cells were first vacuum dried at 80  C for 14 h to remove any residual water. Cells were filled with 0.75 mL of formulated electrolyte and sealed under vacuum at a pressure of 90 kPa (relative to atmospheric pressure). After filling, the pouch cells were connected to a Maccor 4000 series charger and held at 1.5 V for 12e24 h in a 40  C box to promote wetting. Cells were then charged to 3.5 V at a current corresponding to C/20 and held at that potential for 2 h. In order to remove the gas formed early on during the formation charge [25] the pouch cells were then moved to an argon-filled glove-box to be opened and re-sealed under vacuum. Cells scheduled to be cycled above 4.5 V, were then connected back to the charger and charged to 4.5 V at C/20 and 40  C and held at that potential for 2 h. Cells were then opened and re-sealed under vacuum to remove the gas formed at high potential [26,27]. Cells were finally discharged to 3.80 V and held at that potential until the current dropped to a value below C/1000 for their impedance spectrum to be measured. 2.2. Chemicals The chemicals used in the electrolytes were obtained from BASF (LiPF6, purity 99.94%, water content 14 ppm, EC:EMC, 3:7 by weight, water content < 20 ppm; VC, purity > 99.8%, water content < 100 ppm), Lianchuang Medicinal Chemistry Co., Ltd., China (Prop-1-ene-1,3-sultone (PES), purity 98.20%), Guangzhou Tinci Co. Ltd (methylene methanedisulfonate or 1,5,2,4dioxadithiane-2,2,4,4-tetraoxide (MMDS), purity 98.70%), Sigma Aldrich (1,3,2-Dioxathiolane-2,2-dioxide (DTD), purity 98%) and TCI America (tris(-trimethyl-silyl)-phosphite (TTSPi), purity > 95%). All base electrolyte formulations, to which the additives were added and the salt concentration was varied, in this study consisted of EC:EMC (3:7 w:w) referred to as “control”. 2.3. Electrochemical impedance spectroscopy The electrochemical impedance spectra shown in this report were measured using a BioLogic VMP3 equipped with 2 EIS boards. All impedance measurements were made using a 20. mV input signal from 100. kHz to 10. mHz. The experimental setup did not allow for reproducible solution resistance measurements due to cable and connector impedance. For this reason, all impedance spectra were shifted so they would start at 5 U-cm2 on the real axis at the highest frequency measured (x-axis intercept shifted to 5 Ucm2).

2.1. Pouch cells and cell formation 2.4. Open circuit voltage (OCV) storage experiment Dry (no electrolyte) 220 mAh NMC(442)/graphite and 180 mAh LaPO4-coated NMC(442)/graphite (coated-NMC(442)/graphite) pouch cells were obtained from Li-Fun Technology (Xinma Industry Zone, Golden Dragon Road, Tianyuan District, Zhuzhou City, Hunan Province, PRC, 412000). The negative to positive active mass ratio of the NMC(442)/graphite and coated-NMC(442)/graphite cells was

Cells were first formed following the procedure described in section 2.1. Cells were then put back on the Maccor 4000 charger at 40. C to be cycled twice (two full charge-discharge cycles) between 2.8 V and 4.5 V at C/20. Cells were then charged to 4.5 V at C/20 and held at that potential for 24 h. The cells were then moved to the

R. Petibon et al. / Journal of Power Sources 300 (2015) 419e429

storage station for their OCV to be monitored at 40. ± 0.1  C for 500 h. The open circuit voltage of each cell was monitored using a Keithley model 2750 scanning voltmeter. Cells were connected to the voltmeter only once every 15 min during the first 4 h of storage and then once every 6 h for the remaining 494 h. The connection between the cells and the voltmeter was made through mechanical relays so that the cells experienced true open circuit condition during storage [28]. Cells were then discharged to 3.8 V for their impedance spectra to be measured at 10  C. Cells were then put back on a charger to be charged to 4.6 V at a current of C/20 and held at that potential for 24 h at 40  C. Cells were then moved again to the 40. C storage station for their OCV to be monitored for a period of 500 h. Cells were then discharged to 3.8 V once more for  their volume at room temperature and impedance spectra at 10 C to be measured. 2.5. Symmetric cells constructed from pouch cells The positive electrode and negative electrode symmetric cell construction from pouch cells followed the procedure described by Petibon et al. [29]. After cycling, the cells were charged to 3.65 V and held at that potential until the current dropped below C/1000. The pouch cells were then carefully opened in an argonfilled glove box. From the long doubled-sided electrodes, 6 coincell size (1.54 cm2) positive electrodes and 6 coin-cell size negative electrodes were cut with a precision punch. From the punched (double sided) electrodes, three negative symmetric coin cells and three positive symmetric coin cells were reassembled using one polypropylene blown microfiber separator (BMF, available from 3 M Co. 0.275 mm thickness, 3.2 mg/cm2) [30]. Carbon coated aluminum foil (3 M Co.) was put between the back of the positive electrode and the can. This allows good electronic contact between the can and the positive electrode. This helps reduce the high frequency feature coming from the electronic contact between the can and active material at the back side of the positive electrode [31,32]. Enough electrolyte was added to the separator to fully wet it. An additive-free electrolyte was used since some of the additives are consumed during the formation cycle and extended cycling as shown by Petibon et al. [33]. A positive symmetric cell was constructed using two positive electrodes, and a negative symmetric cell was constructed using two negative electrodes. All impedance spectra were collected at constant temperature by housing the cells in temperaturecontrolled boxes at 10.0 ± 0.3  C with a 20. mV excitation amplitude between 100. kHz and 10. mHz. 2.6. Constant current-constant voltage cycling (CC-CV) Some cells followed the CC-CV procedure described by Nelson et al. [3]. Cells were put in a 40  C temperature box and connected to a Neware battery testing system. One cycle of cycle-hold protocol consisted of a constant current charge to 4.5 V at C/4.5 followed by a constant voltage step at 4.5 V for 20 h, and a constant current discharge to 2.8 V at C/4.5. 2.7. CC-CV procedure coupled with impedance measurements Some of the cells that were subjected to the 4.5 V CC e 20 h CV procedure had their impedance spectra measured every 4 cycles. In that case, cells were connected to a charging system built in house and described in Ref. [3]. This system consisted of a Neware Battery Testing System connected to a computer equipped with a Gamry frequency response analyzer (FRA) card. During impedance measurements, relays disconnected the cells from the Neware cycler and connected them to the FRA card to have their

421

impedance spectra measured. To accommodate the impedance measurement, the CC-CV protocol described above was slightly altered. Every 4 cycles, cells underwent a slow C/20 constant current cycle. During this slow cycle, the impedance spectra of each cell was measured every 0.1 V from 4.5 V to 3.6 V during charge and discharge.

2.8. X-ray photoelectron spectroscopy (XPS) XPS was performed on a SPECS spectrometer equipped with a Phoibos 150 hemispherical energy analyzer and using Mg Ka radiation (hn ¼ 1253.6 eV). To transfer air sensitive samples from the argon-filled glove box to the spectrometer, a special transfer system was used as described in Ref. [34]. Shortly, samples were mounted onto a molybdenum holder using a copper conductive tape (3 M) and placed into the transfer system under argon. The latter was put under vacuum at approx. 103 mbar for 1 h and then connected to the spectrometer where samples were loaded under a pressure of ~103 mbar. All samples were kept at 108 mbar for one night before analysis to allow a strictly identical vacuum procedure. The analyzed sample area was ~2  3 mm2 which gives results representative of the whole electrode. Core spectra were recorded in the fixed analyzer transmission (FAT) mode with a 20 eV pass energy at an operating pressure < 2  109 mbar. Short acquisition time spectra were first recorded as reference to follow any possible sample degradation during the analysis. Data treatment was performed using CasaXPS software. The binding energy scale was calibrated from the C1s peak at 285 eV (CeC/CeH) and the O1s peak at 529.6 eV (O2 anion from the NMC) for the graphite and NMC electrodes respectively. A nonlinear Shirley-type background [35] was used for core peaks analysis while 70% Gaussian - 30% Lorentzian Voigt peak shapes and full width at half-maximum (fwhm) constraint ranges were selected to optimized areas and peak positions. More details about the fitting procedure can be found in Ref. [34]. In order to allow the readers to capture the chemical composition of the SEI of each sample in a simple fashion, a figure summarizing the XPS spectra is shown in this article. The detailed XPS core spectra, fitting of the peaks along with analysis of the spectra and XPS quantification are presented in the supplementary information (Fig. S1, S2 and S3). The SEI thicknesses were estimated using the relative intensity of the NMC active material feature, i.e. the NMC O 1s feature at 529.5 eV. For the graphite electrode, SEI thicknesses were so great that the graphite feature at about 282.5 eV was never observed. The relative intensity was defined as:

Irel ¼ at:%ðsampleÞ=at:%ðfreshÞ

(1)

where at. % (sample) and at. % (fresh) are the atomic percentages for the active material feature of a given sample and for the fresh electrode, respectively, as determined from XPS quantification. Assuming a simple model where the SEI layer was considered as homogenous in thickness (d) with no porosity and with an average composition, the SEI thickness was then estimated using:

Irel ¼ ed=l

(2)

where the inelastic mean free path (IMFP), l, was calculated by averaging values for polyacetylene, LiF and polyethylene [36,37]. This procedure yields l ¼ 2.7 nm and 2.1 nm for photoelectron kinetic energies of ~1000 eV (C 1s) and ~700 eV (O 1s) used in this study.

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3. Results and discussion 3.1. Impedance of individual electrodes from cells cycled to different upper potential cut-offs Fig. 1 shows the impedance spectra of negative electrode symmetric cells (Fig. 1a) and positive electrode symmetric cells (Fig. 1b) made from electrodes taken from NMC(442)/graphite pouch cells filled with 1 M LiPF6 EC:EMC (3:7) þ 2% VC þ 1% MMDS þ 1% TTSPi electrolyte. Cells were cycled to different upper voltage cut-offs for a period of 500 h at 40  C and C/40. The particular additive blend was chosen based on the findings of Ma et al. [37] concerning ternary and quaternary additive blends. Using microcalorimetry Ma et al. [37] showed that these ternary blends provide lower parasitic reactions when cycled up to 4.4 V compared to cells containing conventional additive blends. Fig. 1a and b show that while the impedance of the graphite electrode changed only slightly with full-cell upper voltage cut-off, the impedance of the positive electrode changed dramatically. The changes observed at the negative electrode may be caused by several factors. All pouch cells had the same negative to positive active material ratio. Therefore, cells with lower upper-voltage cutoff would not utilize the graphite fully. As a result, the average potential (vs. Li/Liþ) of the graphite electrode of cells cycled to lower voltage cut-off would be higher than that of cells cycled to higher upper voltage cut-offs. The average potential difference may slightly affect the SEI composition at the surface of the graphite electrode thus affecting its impedance. The change of the impedance observed may also be attributed to the migration of electrolyte oxidation by-products from the positive electrode to the negative electrode. Cells cycled to higher voltage cut-off would exhibit higher parasitic oxidative currents at the positive electrode. The byproducts of these reactions could, in principle, migrate to the negative electrode and react at its surface, thus altering the SEI [38]. This would also change the graphite electrode impedance with

Fig. 1. Area specific negative imaginary impedance as a function of area specific real impedance of negative symmetric cells (a) and positive symmetric cells (b) reconstructed from NMC(442)/graphite pouch cells filled with a 1 M LiPF6 EC:EMC (3:7) þ 2% VC þ 1% MMDS þ 1% TTSPi electrolyte cycled to different voltage cut-offs at 40  C for 500 h. Impedance spectra were collected at 10  C.

upper cell voltage cut-off. Based on the results presented here, it is not possible to assign the origin of the impedance change of the negative electrode to either of the two proposed possibilities. Fig. 1b clearly shows that the impedance growth of NMC(442)/ graphite pouch cells operated at high voltage (>4.3 V) comes from the positive electrode. Fig. 1b also shows that the positive electrode impedance displays two main features at high and medium frequencies. In earlier publications, Gaberscek et al. [31] and Atebamba et al. [39], showed that the high frequency feature of the impedance spectra of positive electrodes corresponded to the contact resistance between the aluminum current collector and the active material in parallel with the electrochemical double layer capacitance of the bare areas of the aluminum surface. This was later supported by impedance measurements of positive symmetric cells and single electrode cells by Petibon et al. [40] and Kim et al. [32]. The medium frequency feature of the impedance spectra of the positive electrodes is attributed to the charge transfer resistance at the surface of the active material as well as the Liþ transfer through the SEI, in parallel with the electrochemical double layer capacitance at the surface of the active material [29,39,41]. Fig. 1b also shows that both the high frequency and the medium frequency features of the positive electrode are affected by the upper voltage cut-off. This suggests that both the active material surface/electrolyte interface and the aluminum/active material interface of the positive electrode are affected by increasing voltage cut-off. The origin of the impedance growth of the current collector/ active material interface (high frequency feature, Fig. 1b) is still unknown. Several researchers reported on the instability of carbon black (conductive additive) at high potential [42e45]. Younesi et al. [43] showed that the impedance of electrodes consisting of carbon black coated on aluminum foil displayed impedance growth when charged to voltages of 4.8 V vs. Li/Liþ and above. They assigned this impedance change to a possible growth of an AlF3 layer at the surface of the aluminum foil. The growth of the high frequency feature of the impedance spectra of the positive electrode cycled to higher voltage cut-off may perhaps come from an increase of the thickness of the passive film at the surface of the aluminum current collector, as well as from the degradation of the carbon black in contact with it. The growth of the medium frequency feature of the impedance spectrum of the positive electrode (Fig. 1b) can only originate from the formation of a resistive SEI film at the surface of the active material. It cannot be primarily caused by loss of electronic pathway between the conductive carbon black and the active material particles due to carbon black degradation (as suggested recently by Metzger et al. [44]) as this would lead to different features in the impedance spectrum. A “transmission line” model of the electrode as shown in Fig. 2a similar to the one proposed by Ogihara et al. [41] was used to probe the effects of changes to the SEI resistance, Rs, the electronic path resistance, Re, and the ionic path resistance, Ri. The AC impedance spectra of the equivalent circuit in Fig. 2b was calculated analytically and also using LTSpice [46]. The spectra calculated both ways showed perfect agreement. Fig. 2b shows the Nyquist plot of the impedance of the model electrode for Re ¼ Ri ¼ Rs ¼ 1 and C ¼ 1 as well as the spectra when each of Re, Ri and Rs are doubled or quadrupled. Fig. 2b shows that changes in Rs impact the diameter of the medium frequency semicircle significantly while changes in Re or Ri change the diameter of the semi-circle only slightly. Changes in Re and Ri also induce a shift of the semi-circle to higher real impedance value. In the experimental results presented here, it is not possible to gauge the changes in Re or Ri with this semi-circle shift since the impedance response of the experimental setup was not reproducible from one cell to the next (see experimental section). Fig. 2c

R. Petibon et al. / Journal of Power Sources 300 (2015) 419e429

Fig. 2. Transmission line model of the electrode (a) used to probe the effects of changes to the SEI resistance, Rs, the electronic path resistance, Re, and the ionic path resistance, Ri Nyquist plot of the impedance spectra (b) obtained using the transmission line model with Re ¼ Ri ¼ Rs ¼ 1 U and C ¼ 1 F (black line), with Rs ¼ Ri ¼ 1 U and Re ¼ 2 or 4 (red lines), Re ¼ Ri ¼ 1 U and Rs ¼ 2 or 4 U (blue lines); Rct (diameter of the calculated semi-circle) as a function of Re (c) with C ¼ 1 F, Ri ¼ Rs ¼ 1 U (red line) and as a function of Rs with C ¼ 1 F, Re ¼ Rs ¼ 1 U (blue line) as modeled with the transmission line model. Changes in Ri lead to the same results as changes in Re. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

shows the Rct (diameter of the semi-circle) as a function of Re with C ¼ 1 F, Ri ¼ Rs ¼ 1 U (red line) and as a function of Rs with C ¼ 1 F, Re ¼ Rs ¼ 1 U (blue line) as modeled with the transmission line model. Changes in Ri lead to the same results as changes in Re. Fig. 2c clearly shows that only changes in Rs impact the diameter of the semi-circle significantly. Fig. 1b shows that Rct increases by more than a factor of 10 from 4.3 V to 4.7 V, which is only possible if the SEI resistance increases as in Fig. 2c.

3.2. Impedance of individual electrodes from cells containing different LiPF6 molarities Fig. 3 shows the impedance spectra of the negative symmetric cells (a, c), and positive symmetric cells (b, d) prepared from NMC(442)/graphite pouch cells. These cells contained EC:EMC (3:7) þ 2% VC þ 1% MMDS þ 1% TTSPi electrolyte with varying LiPF6 molarity and were cycled between 2.8 V and 4.6 V at 40  C for a period of 1000 h. All symmetric cells were made using a 1 M LiPF6 EC:EMC (3:7) electrolyte. The salt concentration was kept at 1 M for all symmetric cells in order to account for the possible Liþ desolvation activation energy differences that a change in LiPF6

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molarity could cause. Results presented by Xu et al. [47], Abe et al. [48,49] and Yamada et al. [50] suggest that the rate determining step for the transfer of Liþ from the electrolyte to the electrode, migrating through the SEI is actually the desolvation of the Liþ in electrolytes without any additives. Going from a LiPF6 concentration of 1Me2 M, the Liþ:EC ratio changes from 1:4 to 1:2. This change in ratio would modify the primary solvation sheath of Liþ. This would, in turn, change the activation energy of the desolvation of Liþ perhaps resulting in a change in the impedance spectra of the symmetric cells. Since this article focuses on stabilizing the impedance of positive electrodes cycled to high voltage, the symmetric cells were made 1 M LiPF6 electrolyte. Fig. 3a and c show that when the salt concentration in the parent pouch cells increased from 1 M to 1.5 M no substantial impedance change in the graphite electrode was observed. However, increasing the salt concentration to 2 M resulted in an impedance increase. Since all symmetric cells were made with 1 M LiPF6 electrolyte, this change must come from a difference in the SEI composition at the negative electrode caused by the high salt concentration in the parent pouch cells. This is consistent with the findings presented by Nie et al. [16] concerning the effect of salt concentration on the composition of the SEI of graphite electrodes. Fig. 3b and d show that the LiPF6 concentration has an even greater impact on the impedance of the positive electrodes of NMC(442)/graphite cells cycled to 4.6 V (notice the y-axis scale difference between Fig. 3a and b). Fig. 3b and d also show that while higher LiPF6 concentrations lead to lower active materialelectrolyte interphase impedance (medium frequency feature), it does not affect the current collector/active material impedance (high frequency feature) significantly. Fig. 3b and d show that higher salt concentration can lower the impedance of positive electrodes cycled to high voltage. Fig. 3 also shows that the positive electrode SEI is more sensitive to salt concentration than that of the negative electrode. 3.3. Effect of salt molarity on the impedance and discharge capacity of cells cycled to 4.5 V Fig. 4 shows the charge transfer resistance, called “Rct”, as a function of cycle number for coated-NMC(442)/graphite cells cycled using the 4.5 V CC- 20 h CV protocol. Here Rct is defined as the difference in the real impedance separating the first and last negative imaginary minima of the Nyquist plot of the cell impedance. Rct then encompasses the contact resistance, charge transfer resistance as well as the resistance of the transfer of Liþ through the SEI at both the positive and negative electrodes. All cells contained a ternary electrolyte additive blend consisting of 2% PES þ 1% MMDS þ 1% TTSPi added to xM LiPF6 (1  x  2.5) in EC:EMC 3:7. Fig. 4a shows that the impedance of coated-NMC(442)/graphite cells with an electrolyte containing 1 M LiPF6 grows as a function of cycle number. This indicates that the LaPO4-coating at the surface of the NMC(442) particle does not prevent the impedance rise when cycled to high voltage. Fig. 4a also shows that over the same cycles, the impedance is voltage dependent. This voltage dependence is even more apparent at higher cycle number. This is similar to the work presented by Nelson et al. on NMC(442)/graphite pouch cells cycled to 4.4 V and 4.5 V. The voltage dependence of the impedance is likely to come from the dynamic nature of the SEI
et al. [51] and Madec et al. [34]. reported by Dupre Fig. 4b, c and d show that while the use of 1.5 M LiPF6 slightly slows the rise of the impedance of coated-NMC(442)/graphite cells, the use of 2 M and 2.5 M LiPF6 dramatically reduces impedance growth (note that the y-axis scale of Fig. 4c and d is a tenth of the yaxis scale of Fig. 4a and b). As a whole, Fig. 4 shows that the impedance of coated-NMC(442)/graphite cells display the same

R. Petibon et al. / Journal of Power Sources 300 (2015) 419e429

Negative electrode

a)

1M cycled to 4.6 V 1.5M cycled to 4.6 V 2M cycled to 4.6 V

Z(Re) /Ω.cm2

120

Positive electrode 800

b)

600

80

400

40

20 0

Z(Re) /Ω.cm2

424

200

c)

300 0

d)

200

12 8

100

- Z(Im) /Ω.cm2

-Z(Im) /Ω.cm2

16

4 0

0.01

1

100

10000 0.01

Frequency /Hz

1

100

10000

0

Frequency /Hz

Fig. 3. Area specific real part of the impedance as a function of the logarithm of the frequency (a and b) and area specific negative imaginary impedance as a function of the logarithm of the frequency (c and d) of negative symmetric cells (a and b) and positive symmetric cells (b and c) reconstructed from NMC(442)/graphite pouch cells filled with electrolytes containing different LiPF6 molarity, cycled between 2.8 V and 4.6 V at 40  C for 1000 h. All pouch cells initially contained an additive blend consisting of 2% VC þ 1% MMDS þ 1% TTSPi. All impedance spectra were collected at 10  C.

behavior as uncoated NMC(442)/graphite cells. That is: increasing the LiPF6 concentration in cells containing electrolytes with ternary additive blends dramatically slows down the impedance growth when cycled at high potential even when using a very aggressive protocol such as 4.5 V CC e 20 h CV. Fig. 5 shows the discharge capacity vs. cycle number of coatedNMC(442)/graphite cells filled with electrolytes containing 1 M or 2 M LiPF6 without electrolyte additive and with a ternary additive blend (2% PES þ 2% DTD þ 2% TTSPi). Cells were cycled at 40. C using the 4.5 V CC e 20 h CV protocol described in the experimental section. Fig. 5 shows that while the discharge capacity of cells without additives suddenly drops after only 15 cycles, the discharge capacity of cells containing the ternary additive blend slowly decreases with cycle number. This is similar to the findings presented by Ma et al. [2] and Nelson et al. [3] on the superiority of ternary additive blends for high voltage cycling of NMC(442)/graphite pouch cells. Fig. 5 also shows that the salt concentration does not have a pronounced effect on capacity retention. For instance, cells with 2 M LiPF6 without additive show the same apparent failure as cells 1 M LiPF6 without additives and cells containing the ternary blend with 1 M or 2 M LiPF6 have virtually the same fade rate. Fig. 6 shows the impedance spectra measured at 10  C at different cycle numbers during the 40  C 4.5 V CC e 20 h CV cycling procedure (see Fig. 5 for the discharge capacity vs. cycle number). Fig. 6a and c shows that cells with 2 M LiPF6 without additives had larger impedance than cells with 1 M LiPF6 without additives up to the 30th cycle. This is different from cells containing ternary additive blends (2% VC þ 1% MMDS þ 1% TTSPi and 2% PES þ 1% MMDS þ 1% TTSPi) such as the ones presented in Fig. 3 and Fig. 4. The impedance difference suggests that while LiPF6 concentration still greatly affects the interfacial chemistry in cells containing

additives, higher concentrations of LiPF6 seem to be detrimental in cells without additives. This is similar to the findings presented by Wang et al. [24]. In their report, Wang et al. showed that while higher LiPF6 concentration had no detrimental impact on the cycling of LiCoO2/graphite cells containing VC, it increased the parasitic oxidative currents at the positive electrode of cells without additives. Fig. 6b and d show that higher LiPF6 concentration leads to a slower growth of the impedance in cells containing a ternary additive blend. This is similar to the results presented in Figs. 2 and 3. 3.4. Effect of salt molarity on parasitic oxidation currents of cells charged to 4.5 V Fig. 7 shows the open circuit voltage of coated-NMC(442)/ graphite cells filled with electrolytes containing 1 M LiPF6 or 2 M LiPF6 without additive or with a ternary additive blend (2% PES þ 1% MMDS þ 1% TTSPi), stored at 40  C and 4.5 V for 500 h (Fig. 7a), and then stored at 40  C and 4.6 V for 500 h (Fig. 7b). Fig. 7a shows that all cells have very similar OCV profiles as a function of time when stored at 4.5 V. This indicates that salt concentration does not affect the parasitic reaction rate at the positive electrode significantly below 4.5 V. By contrast, Fig. 7b shows that cells with higher LiPF6 concentration have a higher voltage drop when stored at 4.6 V. Fig. 7 indicates that the effect of salt concentration on the parasitic oxidation current at the positive electrode is also potential dependent. This implies that while higher salt concentration has a beneficial effect on the impedance when cells are cycled to high potential, it may also have a detrimental effect on parasitic reaction rates when cells are cycled above a certain voltage for cells with and without additives. Cells

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Fig. 4. Charge transfer resistance (Rct, see definition in the text) as a function of cycle number for cells undergoing the 4.5 V CC-CV procedure (see experimental section) at 40  C between 2.8 V and 4.5 V. The impedance spectra from which the Rct was extracted were collected using the FRA-capable cycler developed at Dalhousie University by Greg d’Eon described in Ref. [3].

containing the control electrolyte have similar storage performance to cells containing the additive blend. It then seems that the additive blend does not affect the parasitic oxidation current as opposed to the findings presented by Ma et al. [37]. This is a direct effect of the LaPO4 coating at the surface of the NMC(442) particles. Based on our tests, the coating improves the performance of cells containing no additive, while its effect on cells containing additives is mitigated.

1M LiPF6 EC:EMC (3:7) 2M LiPF6 EC:EMC (3:7) 1M LiPF6 222-PES-DTD-TTSPi 2M LiPF6 222-PES-DTD-TTSPi 200

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Fig. 8 shows schematic representations of the SEI films highlighting the differences between the graphite and NMC SEI films as observed from the XPS measurements of the electrodes of LaPO4coated NMC(442)/graphite pouch cells after 40 cycles of 4.5 V CC e 20 h CV cycling (see experimental section) without additive (control) or with a ternary additive blend (2% PES þ 2% DTD þ 2% TTSPi) and varying LiPF6 molarity. The fitted XPS core spectra and their analysis are given in the supplementary information. The SEI thickness at the NMC surface in cells containing control electrolyte with 1 M LiPF6 was estimated to be ~2.8 nm while the SEI thickness at the graphite electrode of every cell tested was greater than the ~10 nm XPS sampling depth. The thickness of the SEI of the positive electrode drawn in Fig. 8 is proportional to the estimated thickness and scaled relative to the SEI thickness of the positive electrode of control cells with 1 M LiPF6. In each panel in Fig. 8, the number of times a species appears and the font size are proportional to the relative amount of the associated core level peak in atomic percentage (at. %) as measured from the XPS quantification and presented in Fig. S3.

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Fig. 6. Area specific negative imaginary impedance as a function of area specific real impedance LaPO4-coated NMC(442)/graphite pouch cells filled with 1 M LiPF6 EC:EMC (3:7) (a), 2 M LiPF6 EC:EMC (3:7) (b), 1 M LiPF6 EC:EMC (3:7) þ 2% PES þ 2% DTD þ 2% TTSPi (c) and 2 M LiPF6 EC:EMC (3:7) þ 2% PES þ 2% DTD þ 2% TTSPi (d), at various cycle number during the 4.5 V CC-CV cycling procedure (see experimental section) at 40  C between 2.8 V and 4.5 V.

At the graphite surface, electrodes cycled with 1 M LiPF6 without additive (control) showed a large fraction of inorganic compounds, mostly LiF, and few organic species (ether derivatives, -C-O- and alkyl carbonates, ROCO2Li) formed by the degradation of both LiPF6 salt and solvents (EC and/or EMC). Slightly more organic species were found in control electrolyte with 2 M LiPF6, at the expense of LiF. When the ternary additive blend was used, significantly more organic SEI films were found for both LiPF6 concentrations indicating a different reactivity of the electrolyte solvents and salt. Additional unidentified eCOe/-PxFy containing compounds were found when 2 M LiPF6 was used in cells with and without additives. This indicates that the presence of high LiPF6 content modifies the composition of the SEI at the negative electrode of both cells with and without additives. At the NMC surface, thinner and more organic SEI films were observed compared to those found at the graphite surface. The NMC electrodes from cells filled with a control electrolyte showed slightly thicker SEI films compared to NMC electrodes cycled with the ternary additive blend. When 2 M LiPF6 was used, an increase in the LiF content was found for cells both with and without additives. Cells with the ternary additive electrolyte also showed large amount of silicon species -Si- and a small amount of -SOx species (x ¼ 3 or 4). More importantly, the use of 2 M LiPF6 led to a clearly

thinner SEI film at the NMC surface. The thickness of the SEI at the NMC surface of cells containing 2 M LiPF6 and the ternary blend was even smaller than that of the NMC surface of cells containing 2 M LiPF6 without additive. XPS analysis seems to indicate that the lower impedance of cells containing 2 M LiPF6 and the ternary additive blends comes from a combination of a thinner SEI and the presence of Si and sulfur-containing compounds. It is then apparent that the presence of electrolyte additives and high LiPF6 concentration are equally important for the stabilization of the impedance of NMC(442) electrodes cycled to high potential. It is also important to note that while the SEI at the positive electrode of cells containing high LiPF6 molarity and the additive blends seems more stable, it is not more passivating as shown by the results of the open circuit voltage storage data. While SEI thickness might be a factor explaining the impedance decrease, an effect on the crystallographic structure of the surface of the electrode cannot be excluded. For instance, there are several reports in the literature that strongly suggest that the impedance increase observed in oxide positive materials is due to the alteration of the crystallographic structure of the surface of the electrode. In 2002e2003, Abraham et al. [52,53] analyzed the surface of the positive electrode of aged Li[Ni0.8Co0.2]O2/graphite 18650 cells. In their report they showed that the impedance of the positive

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Fig. 7. Cell voltage vs. time during open circuit storage at 40  C and 4.5 V (a), and subsequent storage at 40  C and 4.6 V for LaPO4-coated NMC(442)/graphite pouch cells filled with electrolyte with different LiPF6 concentration and additive blend.

electrode greatly increased. This impedance increase was associated not only with a thicker SEI but also with the formation of a 35 nm thick LixNi1-xO cubic structure. Watanabe et al. [54] also analyzed the surface of Li[Ni0.76Co0.14Al0.10]O2 electrodes after cycling. They suggested the formation of micro-cracks and a NiOlike cubic layer may be responsible for the increase of the kinetics barrier at the positive electrode. Takamatsu et al. and Yogi et al. [55e57] recently reported on the effect of electrolyte additives on the top crystallographic surface of LiCoO2 electrodes. Using total reflection X-ray absorption spectroscopy they showed that the addition of a certain additive prevents the irreversible reduction of the surface Co. They then attributed the impedance growth rate reduction brought by the use of additives to the formation of a surface layer that prevents the irreversible reduction of the surface Co. It is then very likely that SEI composition modification induced by changes in LiPF6 molarity in electrolytes containing certain additives can help prevent such irreversible transition metal reduction at the positive electrode particle surface. The great impedance reduction seen in cells containing high LiPF6 concentrations may then come from a thinner (low impedance), more stable (slow growth rate) SEI preventing the alteration of the crystallographic structure of the positive electrode material (low impedance and slower growth rate). However, this SEI is not more passivating

Fig. 8. Schematic representations of the SEI films on the lithiated graphite and delithiated NMC electrodes taken from LaPO4-coated NMC(442)/graphite pouch cells after 40 cycles of 4.5 V CC e 20 h CV cycling for the different electrolytes, as deduced from the XPS experiments. The heights of the SEI films at the NMC surface in Fig. 8 are proportional to their calculated heights.

because it does not improve the 4.5 V and 4.6 V storage properties of the cells. 4. Conclusions The effect of LiPF6 concentration on the impedance growth and cycling performance of NMC(442)/graphite pouch cells and LaPO4coated NMC(442)/graphite pouch cells was studied using EIS, EIS on symmetric cells, OCV storage, CC-CV cycling and XPS analysis. EIS on symmetric cells showed that the dramatic impedance growth observed in NMC(442)/graphite cells cycled to high voltage

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(>4.3 V) comes from the positive electrode. A transmission line model of the impedance response of the positive electrode strongly suggested that the impedance growth comes from the formation of a resistive SEI and not from inter-particle electronic conductivity loss. It was shown that increasing LiPF6 concentration from 1 M to 2 M dramatically reduces the impedance growth during cycling of cells containing certain additives. Not only did it slow down impedance growth during constant current cycling, it dramatically slowed it down during an electrochemically aggressive 4.5 V CC e 20 h CV cycling protocol at 40  C. It was also shown that increasing LiPF6 concentration in cells without additive did not yield any beneficial impact on the impedance growth rate. OCV measurements during storage showed that while increased salt concentration did not seem to affect parasitic current rates at the positive electrode of LaPO4-coated NMC(442) cells stored at 4.5 V, it made it worse in cells stored at 4.6 V. This indicated that higher salt concentrations might lead to earlier sudden failure in cells cycled above 4.5 V. XPS measurements of LaPO4-coated NMC(442)/graphite cells showed that LiPF6 concentration had a great impact on the composition of the SEI of both the positive and negative electrodes of cells cycled to high voltage. XPS analysis indicated that the effect of LiPF6 concentration on the reactivity of the additives is to be considered. It also suggested that the impedance stabilization seen in cells with high LiPF6 concentration and ternary additive blends come from the formation of a thinner and more stable SEI. However OCV storage experiments showed that this SEI is not more passivating. The report presented here clearly shows that tuning the additive content and salt concentration of Li-ion cells cycled to high voltage can help stabilize the impedance of the positive electrode SEI. The successful development of electrolytes for high voltage Li-ion cells might then involve changing the salt content typically used. It is important to stress that while high salt content stabilizes the impedance growth of NMC(442)/graphite cells cycled to high voltage, it does not reduce the parasitic reaction rate at the positive electrode. This means that while higher salt content might lead to cells with longer lifetime when cycled at high rate (due to impedance reduction) it will not lead to cells with longer lifetime when cycled at moderate rate. Much more work is needed in order to reduce the parasitic reaction rate in order to obtain NMC/graphite Li-ion cells that can be operated to high voltage and still show a long lifetime. We encourage other researchers to assist us. Acknowledgments The authors would like to thank 3M Canada and NSERC for the partial funding of this work. The authors thank Dr. Jing Li of BASF for providing some of the solvents, salts and additives used in his work. Remi Petibon thanks NSERC and the Walter C. Sumner Foundation for Scholarship support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.090. References [1] J. Li, R. Petibon, S. Glazier, N. Sharma, W.K. Pang, V.K. Peterson, J.R. Dahn, Submitted to 3M for Approval, June 2015 n.d. [2] L. Ma, J. Xia, J.R. Dahn, J. Electrochem. Soc. 161 (2014) A2250. [3] K.J. Nelson, G.L. d' Eon, A.T.B. Wright, L. Ma, J. Xia, J.R. Dahn, J. Electrochem. Soc. 162 (2015) A1046. [4] L. Xia, Y. Xia, Z. Liu, Electrochimica Acta 151 (2015) 429. [5] W. Huang, L. Xing, Y. Wang, M. Xu, W. Li, F. Xie, S. Xia, J. Power Sources 267

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