Dehydration Rather Than HF Capture Explains Performance

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Dehydration rather than HF Capture Explains Performance Improvements of Li-ion Cells by Ceramic Nanoparticles Marco-Tulio Fonseca Rodrigues, Chen Liao, Kaushik Kalaga, Ilya A. Shkrob, and Daniel P. Abraham ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00976 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 1, 2019

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ACS Applied Energy Materials

Dehydration rather than HF Capture Explains Performance Improvements of Li-ion Cells by Ceramic Nanoparticles Marco-Tulio F. Rodrigues, Chen Liao, Kaushik Kalaga, Ilya A. Shkrob, and Daniel P. Abraham* Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois, 60439, USA *Corresponding author: Daniel. P. Abraham, E-mail: [email protected]; Tel: (630)252-4332. Abstract Instability of nickel-rich layered oxides at high voltages is an impediment to their wider use in Liion batteries. HF generated via LiPF6 hydrolysis accelerates corrosion, further destabilizing the material. Ceramic particles are believed to interfere with this corrosion by scavenging HF. Here we show that the likely mechanism is, in fact, disruption of the hydrolytic cycle due to adsorption of water on ceramic particles, of which MgO performed the best. We suggest that hydrolysis prevention rather than post factum remediation of hydrolytic damage makes a more efficient strategy for protection of the high-voltage cathodes.

Keywords: NCM811, LiPF6 hydrolysis, water, HF, capacity fade, impedance rise

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Increasing the energy density of Li-ion batteries is highly desired, as it would extend the driving range of electric vehicles.1 Although nickel-rich layered oxide cathodes such as LiNi0.8Co0.1Mn0.1O2 (known commercially as NCM811) can potentially supply more energy at high voltages, in their delithiated state they become thermodynamically unstable, undergoing oxygen loss from the surface that contributes to impedance rise.2 High voltage exposure also causes acidic dissolution of transition metal ions (in particular, Mn2+) to the electrolyte;3 these ions migrate to the graphite (Gr) anode, where they deposit in the solid electrolyte interphase (SEI) and catalyze detrimental reactions leading to capacity fade.4 This process is further exacerbated by the formation of hydrofluoric acid in the electrolyte bulk.5-8 This acid is formed by the hydrolysis of LiPF6, the commonly used electrolyte salt, as trace water present in the electrodes and electrolyte stepwise converts the hexafluorophosphate (PF6-) anion to HF molecules and oxofluorophosphate (e.g., PO2F2-) anions.9-11 As destructive as HF can be to the cathode, this acid also passivates the aluminum current collector that in the absence of such protection undergoes anodic dissolution at high voltages.12 This beneficial action is one of the reasons why LiPF6 remains prevalent in the industry despite its low hydrolytic stability. While it may be impossible to completely eliminate moisture (and thus HF) from the cell, mitigation strategies can be pursued to reduce acidic dissolution of metal ions. These approaches include creating physical barriers, such as protective coatings, that limit access to the oxide surface, and HF capture with electrolyte additives and dopants.8, 13 Ceramic materials are well known in this regard. Reports abound in the literature demonstrating benefits conferred by thin ceramic coating on the cathodes,5-7, electrode slurries

14, 17

14

anodes

15

and separators.16 Incorporating these materials into

or using them as electrolyte additives have also been shown to improve


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performance.18 Generally, it appears that having ceramic materials in the cell (regardless of their physical form) is beneficial, and the prevalent view is that such materials react with HF thereby preventing acidolysis of the cathode. However, not all observations fit into this scenario. For example, it was reported that mere exposure of the carbonate electrolyte to ceramic nanoparticles can improve the cycle life of LiCoO2//Li half-cells even after these particles are removed from the electrolyte.19-20 These observations raise questions as to what aspect of chemical behavior plays the most important role in the anti-aging action of these ceramics. In this study, we demonstrate that removing water (thereby suppressing HF generation) is more important than the postulated HF scavenging. Details of the experiments are provided in the Supporting Information (SI); the figures referenced from the SI have designator “S”, as in Figure S1. For this demonstration, ceramic nanoparticles (20-210 nm in diameter) were introduced into the baseline (Gen2) electrolyte containing 1.2 mol/dm3 LiPF6 in a 3:7 wt/wt liquid mixture of ethylene carbonate and ethyl methyl carbonate. After a week, the solutions were settled by gravity and the particles were mostly removed by sedimentation. The top supernatant layers were then used to fill NCM811//Gr cells. Galvanostatic cycling of the cells was conducted in the 3.0-4.4 V range based on the protocol described by Long et al. 21. This protocol has four formation cycles at a ~C/10 rate, followed by aging cycles at a ~C/3 rate that includes a 3h constant voltage hold at 4.4V. The aging cycles are interrupted periodically to obtain capacity and impedance data as described in the SI. At the end of the test, each cell experiences ~120 cycles and ~300 h total exposure to 4.4 V. Electrochemical performance of the cells is briefly summarized in Figure 1 (see Figures S1 and S2 for a more complete data set). For our baseline electrolyte (Figure 1a), the initial specific discharge capacity was 204 mAh/g (the mass pertains to the active material in the cathode). The



capacity gradually decreased over time, resulting in a final capacity of 166 mAh/g. This capacity fade (~19%) includes both the Li+ inventory loss (as the ions become trapped in the SEI) 4 and slowdown of Li+ intercalation due to impedance rise.2 Turning to cells containing electrolytes pre-exposed to the nanoparticles, for some materials (e.g. ZnO and Al2O3 in Figures 1b and 1c, respectively), the capacity fade is not significantly affected by the pre-exposure, while for others (e.g., SiO2, Figure 1d) there is significant capacity loss. When ZrO2 and CeO2 are used (Figures 1e-f), the final capacity is slightly higher than for the baseline electrolyte, while for MgO and TiO2 (Figures 1g-h) it is slightly lower (though with a higher capacity retention; see Figure S2). These trends are graphically summarized in Figure 1i, in which the initial discharge capacities and capacity retention are plotted together. Systems to the right of the vertical line in this chart (ZrO2, Al2O3, MgO, TiO2 and CeO2) have better capacity retention than the baseline electrolyte. Only SiO2 lies to the left of the vertical line, presumably because the SEI formation chemistry is affected. Figure 1j shows differential capacity (dQ/dV) plots during the first charge for cells containing electrolyte exposed to SiO2 or ZnO (the latter is representative of most other systems). For SiO2, the peak associated with electrolyte reduction

22

deviates from ~3 V indicating interference during SEI formation, whereas for ZnO

the peak is similar to that for the baseline electrolyte.


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Figure 1. Influence of oxide nanoparticles on the electrochemical cycling of NCM811//Gr cells. Panels a to h show the discharge capacity (per weight of active material in the cathode) in cells containing the baseline electrolyte (in light gray) and in electrolytes pre-exposed to the nanoparticles (in blue), with the materials indicated in each panel. The initial five cycles and the terminal cycle were at a slow C/10 rate, while the remaining cycles are at a C/3 rate (1C corresponding to full discharge in 1 h). The discontinuities in the plots indicate cycles at which pulsed impedance measurements were carried out. (i) The initial discharge capacity plotted vs. capacity retention for different cells. The cross corresponds to the baseline electrolyte (Gen2). (j) Differential capacity vs. cell voltage for selected systems, showing delayed SEI formation in the SiO2 cells.



Figure 2. Area specific impedance (ASI) plotted vs. cell voltage for cells shown in Figure 1. ASI for baseline electrolyte (Gen2, panel a) is reproduced in gray in panels b-h for comparison. The impedance was measured at the cycles indicated in Figure 1.

Impedance data acquired at various stages of aging (indicated by the blue numbers in Figure 1a) are summarized in Figure 2 (test details in the SI). The pulsed current technique gives the cell area specific impedance (ASI) at different depths of discharge. The ASI of the baseline cells increases continuously as the cell ages (Figure 2a). After 100 cycles, the ASI at ~3.7 V increases by a factor of six. Previous studies using reference microelectrodes showed that this increase is predominantly from the cathode; negligible changes are observed in the graphite ASI.2 For the ceramic-modified electrolytes, the initial impedance was comparable to that observed in the baseline cells. For ZnO and TiO2 (Figures 2b-c), no change was observed in the rate of impedance rise, while for Al2O3 (Figure 2d), the impedance rise accelerated. More interestingly, for ZrO2, CeO2, SiO2 and MgO (Figures 2e-h), the impedance rise slowed down. This beneficial effect is particularly notable for MgO and SiO2 (see Figure S3 which plots the initial ASI vs. ASI increase). The capacity retention and impedance rise for different oxides are graphically summarized in Figure 3. It is seen that most of the nanomaterials have relatively little effect on


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electrochemical performance with two outliers: SiO2 and MgO. Both of these materials halve the rate of impedance rise, and MgO also improves capacity retention. These general trends and the unique behavior of MgO and SiO2, can be explained by their ability to interfere with the hydrolytic cycle of LiPF6 in the cell (cf. Scheme 1).

Figure 3. The impedance rise (defined as the difference between the initial and final ASI) obtained at 3.7 V and capacity retention after 120 cycles for cells shown in Figures 1 and 2. The cross indicates the baseline electrolyte (in gray). With the exception of SiO2 and MgO, electrolyte preexposure to the oxides had little effect on cell performance.

We observe that all oxide ceramics chemisorb and dissociate water by hydroxylation of their surfaces, forming thin hydrogel coatings. Additional water molecules then physisorb onto these hydroxylated surfaces, increasing the net moisture uptake.23 At low-to-moderate fluoride concentration, these modified surfaces undergo anion exchange, capturing and immobilizing the fluoride anion.24-25 SiO2 is the exception to this general rule, as the fluorinated compounds solubilize, exposing fresh surface that can react with more fluoride.10



To estimate how the oxides interfere with the hydrolytic cycle responsible for the release of HF acid, we intentionally added ~8,300 ppm of water to freshly prepared nanoparticlecontaining electrolytes. After 7 days of resting, the electrolyte samples were examined using fluorine-19 and phosphorus-31 nuclear magnetic resonance (NMR). In the presence of water, PF6converts to HF, PO2F2- and PO3F2-, which display resonance lines in the NMR spectra.10 The results of our analyses are summarized in Table 1. Advanced stages of hydrolysis are observed in most samples, the oxide particles generally catalyzing PF6- hydrolysis rather than impeding it. The behavior of silica is particularly illuminating in this respect: though less HF was detected (as can be expected given the high reactivity of SiO2 towards HF), the high yield of oxofluorophosphate anions indicated advanced stage of hydrolysis. Indeed, as we observed elsewhere,26 scavenging of HF actually shifts the equilibrium towards additional hydrolysis, as water is reintroduced into the bulk when HF reacts with the oxide (see Scheme 1a). These analytical results suggest that the oxides benefit the cell primarily by removing water from the hydrolytic cycle (Scheme 1b). This propensity readily explains what distinguishes MgO from other oxides, as it is a well-known drying agent, readily dissociating water on its surface.23, 27

For magnesia, only traces of HF and other hydrolytic products are observed, suggesting that it

completely disrupts the hydrolytic cycle (Scheme 1b). When MgO was added to aged Gen2 doped with water (compare the first and last rows in Table 1), the HF concentration before and after the addition of MgO was nearly the same, suggesting low fluoride uptake by MgO. This simple experiment demonstrates that MgO acts by removing water rather than HF from the hydrolytic cycle. We conclude that active removal of water during aging of the cell significantly improves long-term performance of cells cycled at high-voltage.


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Scheme 1. Representation of the effect of oxide particles on cell performance. For a generic oxide particle (panel a), the hydrolytic cycle converting between HF and water in the electrolyte is not disrupted, and can even be facilitated by the ceramic nanoparticles. This cycle includes stepwise hydrolysis of LiPF6 by water molecules in electrolyte and acidolysis and/or F-/OH- ion exchange in the oxide materials (including the cathode and the ceramics added to the electrolyte) by hydrofluoric acid reacting at the surface. For MgO particles, the water is locked in a stable hydrogel on the particle surface, so LiPF6 hydrolysis is largely suppressed and the hydrolytic cycle is disrupted (panel b).



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Table 1. Oxofluorophosphate and Hydrofluoric Acid yields obtained from NMR data after 1 week of aging in electrolyte containing 8,300 ppm water. The baseline electrolyte contained no HF and trace amounts of hydrolysis products before water addition. % LiPF6

Electrolyte/ ceramic

hydrolyzed

baseline

4.0

9.7

SiO2

10.4

4.1

ZnO

9.9

9.0

ZrO2

7.4

13.5

TiO2

7.1

11.9

Al2O3

6.5

9.4

CeO2

4.8

9.0

MgO

0.1

0.0

MgO c

4.6

7.3

a

% HF yield

b

a) the total fraction of PO3F2- and PO2F2- anions relative to all phosphate anions; b) the atom fraction of HF relative to all fluorine compounds; c) added after 1 week aging of the baseline electrolyte.

At first glance, our conclusion appears to contradict the research literature, which suggests that adding ~200-2000 ppm of water to the carbonate electrolyte has little effect on cell performance.28-29 However, careful examination of the literature indicates that these previous experiments were carried out with relatively low upper cut-off voltages (

Dehydration Rather Than HF Capture Explains Performance

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