A Robust AlF3 ALD Protective Coating on LiMn1.5Ni0.5O4 Particles

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A Robust AlF ALD Protective Coating on LiMn Ni O Particles - An Advanced Li-ion Battery Cathode Material Powder Alon Shapira, Ortal Tiurin, Nickolay Solomatin, Mahmud Auinat, Arieh Meitav, and Yair Ein-Eli ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01048 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Energy Materials

A Robust AlF3 ALD Protective Coating on LiMn1.5Ni0.5O4 Particles - An Advanced Li-ion Battery Cathode Material Powder A. Shapira1, Ortal Tiurin1, N. Solomatin1, M. Auinat1, A. Meitav2 and Y. Ein-Eli1, 3 *

Department of Materials Science and Engineering, 3 The Grand Technion Energy Program, Technion- Israel Institute of Technology, Haifa 3200003, Israel. 2 ETV Energy, Ramat-Gan, 5290002 Israel. 1

Key words: Particle by particle, AlF3, ALD coating, LiMn1.5Ni0.5O4 powder, Li-ion batteries.

ABSTRACT The most promising LiMn1.5Ni0.5O4 (LMNO) ultra-high voltage cathode material is not yet commercialized because it is suffering from a capacity fading during cycling, especially at elevated temperatures. Manganese ions dissolution from the cathode and their precipitation on the graphite anode are the main cause of failure of Li-ion batteries (LIB) utilizing LMNO cathode material. In order to mitigate this issue, AlF3 layer was coated directly on LMNO powder particles via Atomic Layer Deposition (ALD). Few nanometers thick coating was individually formed on each particle. The coating protected the particles from corrosion-like phenomenon, when immersed in LIB electrolyte at room temperature (RT) and at 45⁰C. Half-cell electrochemical measurements showed superior performance for the ALD coated AlF3 material over the uncoated material. In the full-cell configuration enhanced capacity retention was observed for cells comprised from cathode materials coated by different AlF3 ALD coatings. Complete Li-ion cells utilizing ALD coated cathode powder in the cathode and a graphite anode exhibited lower initial capacity, which was recovered continuously during cycling at RT and dramatically at 45⁰C during the first ~30 cycles. A different and modified formation process and cycling method, significantly

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improved the lower initial capacity of the Li-ion cells on the expense of a rather shorter cycle life. Even with the new formation cycling, Li-ion cells utilizing ALD coated materials exhibited better cycling performance than cells utilizing pristine material. Fluorination of oxygen impurities in the coating layer or its lithiation are suggested as mechanisms for the recovered capacity. Li-ion cells utilizing ALD AlF3 coated cathode powder were cycled up to 180 cycles, when 150 of them were at 45⁰C.

INTRODUCTION Li ion battery (LIB) is the best rechargeable battery in the market for the past three decades, supplying energy to a variety of devices such as cell phones, lap tops, electric bicycles and a variety of electric vehicles (EVs), whether hybrid of full EVs. With a constant growth of consumer’s demand, the research aiming for better LIBs continues. Even though new LIB’s materials have been introduced during the years,1,2 most of the performance improvements to this day were not in the chemistry of the battery but rather in the materials engineering section.3 Today, almost 30 years after the commercialization of the first LIB, research is still focused on all of the battery's components; namely, switching the carbonaceous anode to other materials with higher capacity4–7 where a silicon based materials are at the top of the list,8,9 using different electrolyte salts and solvents and adding different stabilizing additives to the electrolyte,8,10–13 choosing the best separator,14,15 using lighter current collectors16 and replacing the cathode material, while 5 V cathode materials are drawing a special attention and much interest.2,17–19 In recent years, LiMn1.5Ni0.5O4 (LMNO) cathode material is in the focus of many researchers,13,18,20–27 because of its attractive operation voltage of 4.7 V, a leap of ~1 V from


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today’s commercial LIBs.1 LIB utilizing LMNO cathode material will result in a reduction in the number of cells in the EV's battery pack, whilst supplying the same energy, thus reducing cost and volume needed in the next generation of EV’s power pack. In addition, manganese and nickel are less expensive and more environmentally friendly than cobalt, which is a major element in today’s LIB. Unfortunately, LMNO suffers from capacity fading, especially at elevated temperatures,28,29 a phenomenon which is very severe in the full-cell configuration.30–32 There are several reasons for the capacity fading, where Mn+2 ions dissolving from the cathode via the following disproportionation reaction:33 2Mn 3 (solid)  Mn 4 (solid)  Mn 2 (solution)

[Eq. 1]

and their precipitation as elemental Mn on the anode is considered to be the major factor limiting the cell performances.24,31,34,35 In order to prevent manganese ions dissolution, several approaches have been proposed. These include the replacement of Mn with other metals, whilst keeping the high operation voltage,36 introduction of additives to the electrolyte (also, to cells composed of LiMn2O4 (LMO), which suffers from the same capacity fading10),13,37,38 and surface coating.25,34,39,40 Surface coatings have been researched on other cathode materials in order to prolong the battery's life; Al2O3 on LiCoO241 and other constituent coatings, such as ZnO,42 SiO243 and AlF344 on LMO. Those coatings demonstrated an improvement in the performance of the coated cathode materials. Hydrofluoric acid (HF) is present in the ppm level as an impurity in most common electrolytes based on LiPF6, as the lithium salt, and upon cell operation its concentration increases, due to electrolyte decomposition via the following reactions:



LiPF6  LiF+PF5

[Eq. 2]

PF5 +H 2 O  POF3 +2HF

It was found that traces of HF in the electrolyte accelerates Mn dissolution from the cathode,45 thus accelerating cell’s capacity fading. Cathode coatings serve as a physical barrier between the cathode material and the electrolyte, especially against HF attacks. It appears that metal oxide coating layer serves as a scavenger for HF, as it is chemically reacting, according to the following reaction scheme:46

MO0.5x +xHF  MFx +0.5xH 2 O

[Eq. 3]

The transformation of the metal oxide to the corresponding stable metal fluorides can be directly or through a metal-oxy-fluoride M-Ox-Fy phase. Both cases are accompanied by microstructural changes and volume changes, being calculated to be 64% for the transformation of Al2O3 to AlF3,47 for example. Such volume changes can lead to cracks and even peeling of the new formed metal fluoride layer. In addition, the second product of the above reaction [Eq. 3] is water, which its presence is undesired in the electrolyte, since water lead to an accelerated decomposition of the electrolyte and eventually to a higher production and formation of HF [Eq. 2]. As for the consideration of applying metal fluorides as the coating layers, most of the research so far was performed via a wet chemistry approach. Examples, such as AlF3 on LiMn2O4,48 LiF on LiNi1/3Co1/3Mn1/3O2 (NMC 111)49 and MgF240 or AlF3 on LMNO50 indeed showed electrochemical improvement performances. Wet chemistry coating is simple and inexpensive; however, it lacks a control of the coating thickness or its uniformity. As a result, some particles will be “heavily” coated and some will be barely coated or uncoated at all. In recent years, atomic layer deposition


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(ALD) is being used as a coating technique for LIBs, when the need for thin and uniform layers deposition exists. Not only that, ALD enables an accurate control of the deposited layer thickness by changing parameters like deposition temperature or number of deposition cycles.51 In addition, it is possible to coat the anode or the cathode either in their powder form or the composite electrode, just before cell assembly. So far, most of the ALD research in the field of LIBs was of metal oxide coatings, Al2O3 in particular, as they are the most researched layers in ALD. Examples of metal oxide coatings are ZnO on LMO particles and on its composite electrodes,52 Al2O3 on LiCoO2 particles and electrodes,53 or directly on natural graphite (NG) and its composite electrode,54 Al2O3 on LMNO and graphite electrodes34 and directly on LMNO particles where fluorination of the layer was reported.47 ALD coatings of metal fluorides layer for LIBs are in their early stages, since metal fluorides are less researched than metal oxides in the ALD method. We previously reported on individual particle coating of LMNO powder with MgF2 layer, as 12 ALD cycles at the temperature of 275⁰C resulted in ~6 nm of coating layer.25 Since then, NMC 532 powder was coated with AlF355 and NMC 811 electrodes were coated with LiF, AlF3 and LiAlF4.56 Herein, we present for the first time, AlF3 coating on LMNO powder via ALD using trimethylaluminum (TMA) and hexafluoroacetylacetone as the precursors for aluminum and fluorine, respectively. The layer was examined using electron microscopy and the effect of the coating was analyzed electrochemically in half- and full-cells configuration.

EXPERIMENTAL SECTION Atomic Layer Deposition (ALD): Atomic Layer Deposition (ALD) on LiMn1.5Ni0.5O4 (LMNO) powder was performed in a TFS-200 ALD unit (Beneq Oy, EsPOO, Finland), which was equipped with a



fluidized bed reactor (ALD-FBR). The powder was obtained commercially. Each ALD process was loaded with 20-25 gr of powder. The coating parameters were 2 ALD cycles at 150⁰C (2x150). Each ALD cycle was comprised from two major steps, which introduced the precursors, and two intermediate steps in which ozone (O3) was inserted into the system. After each of these steps; the system was purged with nitrogen. The first major step was insertion of trimethylaluminium (TMA) (Aalfa Aesar) which served as the aluminum precursor, and the second major step was flowing hexafluoroacetylacetone (Sigma Aldrich) into the reactor. There was a total of 8 steps: 2 major, 2 intermediate and 4 system purging). This coating process is based on ALD coating of metal fluorides via metal oxides57 and was previously used by us to deposit MgF2 on LMNO powder.25 Powder Morphology Analysis: HighResolution Scanning Electron Microscopy (HRSEM), Zeiss Ultra-Plus FEG-SEM, was used to analyze the morphology of the powder. Images were taken in a high vacuum mode at 4 kV using secondary electrons (SE) and InLens detectors. Transmission electron microscopy (TEM) images were taken by FEI Titan 80-300 KeV S/TEM with an EDS (EDAX) detector for chemical analysis, as well as FEI T-20 200 KeV. Samples for the TEM were prepared by the dual-beam focused ion beam (FIB) FEI Helios NanoLab DualBeam G3 UC. LMNO Powder Durability and Corrosion Analysis: Four samples of LMNO powder, pristine and 2x150, were stored in LP30 electrolyte 1 M LiPF6 in EC:DMC 1:1 w/w for the following time: 1 week at room temperature (RT)/45⁰C, 1 month at RT and 1 week at 45⁰C followed by 3 weeks at RT. At the end of the storing time, each powder was separated from the electrolyte and was washed twice with acetonitrile followed by drying in a vacuum oven. The powders were than analyzed by HRSEM.


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In addition, Inductively coupled plasma (ICP) analysis was carried with electrolytes that stored LMNO powders (pristine and 2x150). The cathode materials were immersed in the electrolyte for 1 week in 45oC to measure the Mn/Ni dissolution rate in the electrolyte for the pristine and coated powder. Electrochemical Analysis: Pristine and coated powders were analyzed electrochemically in a halfcell and full-cell configuration. Half-cell configuration analysis: electrodes were comprised of 80 wt.% of active material, pristine or coated, 10 wt.% of conductive additive, C-Nergy super C45 (TIM) and 10 wt.% of KYNAR 301F (PVDF). Electrodes with cathode active material loadings of 913 mg, with the area of 1.23 cm2 were pressed for 1 min at 5 ton cm-2 after drying in a vacuum oven at 40⁰C, and then they were inserted back to the vacuum oven for several hours at 120⁰C. PTFE T-cells were assembled in an argon glovebox. Lithium metal was used as counter electrode, two Whatman glass fiber filter papers were used as separators and LP30 electrolyte (Gotion) was used as the electrolyte. Cells were cycled at a constant current density of 0.1 mA cm-2 at RT between 3.5-4.85 V vs. Li/Li+ using Arbin BT2000 battery test system. Full-cell configuration analysis: Full-cell configuration analysis was performed in 100-110 mAh pouch cells that were assembled with a central double-side anode and a one-side cathode from both sides of the anode. The electrode cross section was 23 cm2 with cathode loading of 20 mg cm-2, where the active material was 86%. Each cell was cycled in the following manner: two formation cycles at C/10 followed by charging at C/5 to 4.85V and discharging at C/3 to 4V. The first 30 cycles were cycled at 25⁰C followed by cycling at 45⁰C for the rest of the test.



RESULTS AND DISCUSSION HRSEM images at different magnifications of LMNO powder are presented in Figure 1. The powder contains particles with a wide distribution of sizes, ranging from hundreds of nanometers to several microns. Axmann et al has researched the effect of LMNO's particle size on the electrochemical performance. They have concluded that there is no significant difference (during electrochemical testing) between a powder with a large distribution of particle size, as oppose to powder with the smallest particle size. It appears that when there is a wide distribution of particle size, small particles might fill voids between larger particles, which in turn leads to better contact between the active material particles and the conductive additive in the electrode. The overall result of that is better electrode kinetics.18 With no previous data available on ALD of AlF3 from the proposed precursors in this study and with the recognition that: a. A thin coating layer is needed to protect the cathode material, preventing Mn dissolution; b. At the same time, such coating should be as thin as possible, allowing Li ions intercalation; c. The cell’s impedance must be kept as low as possible, and d. Realizing that the aluminum precursor is much less bulkier than the magnesium precursor used previously, when MgF2 was deposited; 25 The process of depositing 2 ALD cycles with a deposition temperature of 150oC was chosen. Although 1 ALD cycle will theoretically give the lowest impedance, 2 cycles were employed to ensure that ligand hindrance didn't control the growth of the ALD layer58.


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Figure 1: HRSEM images of LMNO powder at different magnifications: (a) 4 K; (b) 20 K.

The TEM image in Figure 2a shows a single LMNO particle coated with AlF3 layer with the thickness of ~2.4 nm. Layers of carbon and platinum were deposited by FIB while preparing the sample. In order to determine the composition of the layer, a STEM/EDS line scan (red/orange line) was applied and its results are presented in Figure 2b-c (same measurements were performed in a different area of the particle, displaying very similar results). The line scan shows elemental measurements results from both the particle and the coating layer. Clear peaks of aluminum and fluorine can be seen in the coating layer. In depth analysis of Figure 2c reveals that all of the element’s values (except of carbon) drop to almost zero after 8 nm. A closer look on each element shows high levels of carbon through most of the scan, originating either from impurities in the layer or from the carbon coating. In addition, high oxygen levels were detected (over 50 at.%), in the beginning of the scan, dropping to approx25 at.% from the middle point of the scan. A steady drop in the amount of nickel and manganese, and small amounts of aluminum and fluorine can also be seen.



Figure 2: TEM images of LMNO powder coated with AlF3 ALD: 2 cycles at 150⁰C (2x150). (a) HAADF image of FIB cross section; (b) STEM image of the selected EDS line; (c) EDS spectrum of b, scan progress from left to right.

Trying to determine the exact composition of the coating layer is not possible from this EDS scan; for example, the high oxygen levels can originate from the LMNO particle or from oxygen impurities resulting from the use of ozone during the ALD process. If the amount of oxygen impurities is high, the coating layer is some sort of Aluminum-Oxy-Fluoride (Al-Ox-Fy) layer. In this case, during the ALD cycle, ozone reacted with TMA to form Al2O3 as was reported before to be possible by Mousa et al,59 and in the following ALD step, some of the fluorine did not actually substitute the oxygen, but actually bonded to the oxygen in the Al-O bond, thus forming a layer of Al-Ox-Fy. At the same time, it is possible to interpret the results as if the oxygen signal derives


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from the LMNO particle solely, since its values are dropping by over a half, around the 5 nm mark. Not only that, in a similar process of depositing metal fluorides over a Si wafer, Putkonen et al reported that oxygen impurities were under the detection levels of the measuring instruments,57 meaning that indeed AlF3 coating layer was deposited. The only similar process of depositing metal fluorides (MgF2) on LMNO particles in an ALD process with ozone as an intermediate step was previously reported by us.25 The STEM/EDS analysis from the MgF2 layer indicates MgF2 layer with carbon and oxygen impurities, strengthening the conclusion that indeed the coating layer in Figure 2a is AlF3. Pristine and 2x150 ALD AlF3 coated LMNO powders were soaked in the electrolyte in order to observe the effect of the electrolyte medium on the material's particles. All of the experiments were performed without applying a potential, thus capturing the interaction between the immersed cathode material (pristine and AlF3 coated) and the electrolyte. Any corrosion effects that were observed may possibly be enhanced once voltage will be applied during cell cycling. On the other hand, the results from these experiments do not take into consideration the effects of changes that can occur to the coating layer while cycling, or the effects of the solid electrolyte interphase (SEI) formation on the cathode electrode.26 It is known that at high voltages, of 4.7 V and up, electrolyte oxidation (such as polymerization of EC to poly(ethylenecarbonate) (PEC)) can occur on the surface of the electrode. Not only that, the surface chemistry of the cathode changes at high voltages,60 and it was also reported that a highly stable surface film is developed on LMNO electrodes, inhibiting pronounced oxidation of alkyl carbonate solvents. Such a phenomena is accelerated at elevated temperatures.61 The SEI is a physical barrier between the cathode and the electrolyte and even if it only serves as a protective film, a deposited coating layer may as



well slow down processes, such as corrosion. Therefore, the stability experiments may indicate whether the AlF3 coating layer protects the LMNO particles or has no effect, at all.

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Figure 3: HRSEM images of LMNO powder immersed in the electrolyte. a-c pristine LMNO: (a) before soaking; (b) after 1 week at RT; (c) after 1 week at 45⁰C. d-f 2x150 AlF3 coated LMNO powder; (d) before soaking; (e) after 1 week at RT; (f) after 1 week at 45⁰C.


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Figure 3a shows pristine LMNO powder before it was soaked in the electrolyte and Figure 3b-c show LMNO powder after it was soaked in the electrolyte for 1 week at RT and at 45⁰C respectively. Generally, raising the temperatures increase the kinetics of all reactions, as well as electrochemical processes. The reaction rate approximately doubles for every 10⁰C increase, meaning that the corrosion rate approximately quadrupled between RT and 45⁰C. After 1 week of soaking at RT, the pristine particle's edges were etched, but no severe damage was observed. However, corrosion effects such as pitting were observed when the temperature was increased to 45⁰C. Figure 3d shows LMNO powder with 2x150 of ALD AlF3 coating before it was soaked in the electrolyte. Only minor and insignificant changes to the powder can be seen after it was soaked in the electrolyte either for 1 week at RT (Figure 3e) or at 45⁰C (Figure 3f). It is very clear that the AlF3 layer is protecting the powder from corrosion caused by the electrolyte. The same trend was kept when the soaking time was increased to 1 month. The changes in the pristine powder can be seen in Figure 4 a-c; the slow corrosion continues at RT and once the powder was soaked for 1 week at 45⁰C, extra 3 weeks at RT made no difference, since the corrosion damage has been already done. On the other hand, the protection of the AlF3 coating continued even when the soaking time was increased to 1 month. Figure 4 d-f indicates the small difference between coated powder before and after soaking. Only minor changes can be observed on the particles facets. The results demonstrate the concern of LMNO stability at elevated temperatures in the electrolyte and how AlF3 coating layer can solve this issue. The stability tests indicated the protection capabilities of the AlF3 coating layer, serving as a physical barrier between the cathode material and the electrolyte.



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Figure 4: HRSEM images of LMNO powder immersed in the electrolyte. a-c pristine LMNO: (a) before soaking; (b) after 1 month at RT; (c) after 1 week at 45⁰C followed by 3 week at RT. d-f 2x150 AlF3 ALD coated LMNO powder; (d) before soaking; (e) after 1 month at RT; (f) after 1 week at 45⁰C followed by 3 weeks at RT.


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Considering the most significant changes on the powders were observed after immersion during 1 week at 45oC, Inductively Coupled Plasma (ICP) measurements were carried out on the electrolyte. This was done to examine the extent of the coating's protective effect by assessing the Mn/Ni dissolution on both pristine and coated powders. The results are shown in Figure 5. After 1 week, Both Mn/Ni amounts in the electrolyte are slightly higher for the pristine powder, as can be seen in fig. 5(a). Fig. 5(b) shows the extent of the protective effect even after 2 weeks under the same conditions. The Mn concentration in the electrolyte of the uncoated powder is x3 higher. Ni dissolution also mitigated by the protective coating.

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Figure 5: Mn/Ni content in the electrolyte after the submersion of pristine/coated powder for (a) 1 and (b) 2 weeks at 45⁰C.

The next issue to be concern of is whether the coating thickness hinders Li ions diffusion and migration. Charge/discharge profiles obtained from half-cells (LMNO vs. Li metal) utilizing pristine and 2x150 AlF3 ALD coated LMNO are displayed in Figure 6. A small plateau was observed



for both cells utilizing the pristine and coated materials at ~4 V, corresponding to the redox reaction of Mn3+/4+ in the nonstoichiometric LiMn1.5Ni0.5O4-δ. The two distinctive plateaus at ~4.7 V attributed to the redox reaction of Ni2+/4+ were also observed.62,63 a

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Figure 6c displays the discharge capacity of half-cells utilizing the two cathode materials (pristine and 2x150 AlF3 coated LMNO). Increased stability is demonstrated in cells utilizing the coated material, albeit with the same slope of deterioration in the capacity, as recorded for cells utilizing the pristine cathode material. In addition, slightly smaller initial capacity was observed for cells utilizing the coated material which was recovered during the first 10 cycles. The major difference between the two materials was observed after 60 cycles, when cells utilizing the pristine material collapsed and cells utilizing the coated material maintained the same deterioration slope. The improved stability and improved coulombic efficiency are attributed to the AlF3 coating, improving the stability of the cathode in the electrolyte. Different cathode coatings have shown improvement in performance of LMNO in the half-cell configuration.25,47,50 Most of this research did not mature into a full-cell analysis. There is data in the literature which shows improvement in the LMNO/graphite system with cathode coating64 and with other solutions such as doping36 or electrolyte additives13. Herein, different AlF3 coated LMNO materials were further studied vs. graphite in the full-cell configuration. In order to accelerate the electrochemical measurements and substantially reduce the measuring time, the experiment’s temperature was raised to 45⁰C after 30 cycles. In this manner, the data was received almost four times faster. In addition, since the batteries based on LMNO are designated for the electric vehicle market, it’s reasonable that the working temperature will be above RT, and thus it is most advantageous to observe and study the behavior of the cell at such elevated temperatures.



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Figure 7: (a) Discharge capacity vs. cycle number of half-cell measurements utilizing pristine and 2x150 ALD AlF3 coated LMNO powder at variety of C-rates; (b) discharge profile of pristine LMNO powder at variety of C-rate; (c) discharge profile of 2x150 coated LMNO powder at variety of Crate.

Pristine LMNO is known for its good rate capabilities. The rate capability is affected by the size of the particles and their morphology65, the electrolyte13 and other factors. Figure 7a compares between the rate capability performance of pristine and 2x150 half-cells. As can be seen the pristine material showed higher capacities than the 2x150 coated material at slower rates (0.1C


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and 0.2C). The coated material had slightly lower initial capacity than the pristine material at the beginning of cycling at 0.1C as was seen in the constant current half-cells. As the rate of the test increased the trend changed and the coated material showed higher capacities. At 1.6C the pristine material is practically not charging and not discharging. As the cycling continued and the rates of the test were reversed, the capacity of the coated material remained higher than the pristine material at the higher rates. At 0.2C the pristine material demonstrates higher capacity for a short while and only decreasing the rate to 0.1C kept the capacity of the pristine material higher for several more cycles. The faster deterioration of pristine LMNO compared to coated material, especially at higher rates was observed before for LMNO coated with Al2O3 via ALD.47 In addition, the discharge profiles in Figure 7b-c show higher polarization for the pristine LMNO which increased as the rate of the test increased. The lower polarization of the 2x150 coated material is another indication of the protection of the ALD coating which slows down capacity fading processes. The C-rate test showed better rate capabilities for the coated material over the pristine material and rapid fading for pristine LMNO at high C-rates. The cycled cells were dismantled and both pristine and coated (2x150) electrodes were taken to the SEM for postmortem analysis. Figure 8 shows no difference between the cycled electrodes. The full-cell configuration charge/discharge curves were divided into two parts, the RT cycles (shown in Figure 9a-b) and the 45⁰C cycles (shown in Figure 9c-d); one can immediately observe the capacity increase for all cells utilizing both materials, after the temperature was raised to 45⁰C. The room temperature first cycle for cells utilizing both materials started from a lower voltage than the rest of the following cycles, which were discharge only to 4 V, and therefore only in this cycle the oxidation reaction of Mn3+/4+ can be seen in the charge curve. In the first cycle



charge curve obtained from the cell utilizing the pristine powder, a steady increase in the capacity is observed between 4.6-4.8 V, corresponding to the Ni2+/4+ reaction, and in the discharge curve of the same cycle, the opposite reaction is manifested in a plateau starting at ~4.75 V.

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Figure 8: Electrodes before cycling for (a) pristine and (b) coated powder; electrodes after C-rate cycling for (c) pristine (d) coated powder.

In cells utilizing the ALD coated powder material, a similar behavior is observed with a substantially lower capacity, manifested in shorter plateaus. In the 5th cycle, the Ni redox reaction is seen as two distinguishable plateaus, positioned at 4.7 and 4.75 V in the charge curve, whereas


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in the discharge curve it is difficult to distinguish between those plateaus. This split in plateaus was recorded for both of the materials. a

b

c

d

Figure 9: Charge/discharge curves of full-cell LMNO vs. graphite. (a) pristine powder at RT; (b) 2x150 AlF3 ALD coated powder at RT; (c) pristine powder at 45⁰C; (d) 2x150 AlF3 ALD coated powder at 45⁰C. First cycle was performed at C/10 and the following cycles at C/5 for charging and at C/3 for discharging.

At the 30th cycle, an additional, a third plateau, was introduced to the charge profile. The third plateau, appearing at ~4.75 V, was previously observed in the LMNO/graphite system for cells



cycled at 55⁰C. It was assumed that changes in the cathode-electrolyte interface were the cause for the appearance of this plateau.66 The third plateau was observed previously only at higher temperatures; however, in the current study, it appeared since the 3rd cycle and deviation of the original second plateau into two “new” plateaus is more visible as the cycle number increases. At 45⁰C, both cells exhibit similar charge/discharge curves. With increased cycling, the charge voltage increases and the discharge voltage decreases. This behavior was attributed to the increase in the cell's impedance.66 The charge/discharge curves in Figure 9 clearly present a better stability for the coated material over the pristine material. Full-cell cycling performances of pristine and 2x150 AlF3 coated vs. graphite are presented in Figure 10a. The AlF3 coated material showed much lower initial capacity than the pristine material. This phenomenon was not observed in the half-cell configuration in that scale. Moreover, both cells exhibited a continuous increase in their capacity during cycling, where the pristine cell showed an increase of only a few mAh g-1, and the coated cell demonstrated a striking capacity increase, of a few dozen mAh g-1. 70% of the capacity recovery was gained during the RT cycling (30 cycles) and 30% during the first 2 cycles at 45⁰C. The uncoated material maintained its capacity for almost 100 cycles and then, the capacity slowly decreases, whereas the 2x150 AlF3 coated cell's capacity retained its capacity for about 30 more cycles at 45⁰C. The coulombic efficiency of both cells was high and stable during cycling (Figure 10a). The IR results presented in Figure 10b explain the discharge capacity behavior of both cells utilizing the cathode materials. After 90 cycles, the IR of the pristine cell is almost 1.5 times higher than that of the cell utilizing the 2x150 powder, and the deviation in capacity retention between the two types of cells starts. After 120 cycles the IR of the cell holding the pristine cathode


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material is almost twice as much than the cells utilizing the 2x150 coated material. Not only that, the IR of the cells utilizing the coated material is climbing slowly throughout the cycling process, which can well explains its rather slow and sluggish capacity fading when compared to the rapid degradation observed for the cell utilizing the pristine material. b

a

Figure 10: (a) Discharge capacity and coulombic efficiency and (b) IR measurements for full-cells utilizing pristine and 2x150 coated powder of LMNO vs. graphite. All cells were cycled for 30 cycles at RT and the rest at 45⁰C.

The lower initial capacity recorded during full-cell cycling is not yet fully understood, but may be attributed to the difference in configuration of the cell. For half-cell electrochemical analysis, PTFE T-cells were used with 'flooded' amount of electrolyte. During full-cell electrochemical, analysis, however, pouch cells were used. They comprised of double-side anode and a one-side cathode (pristine or coated). The amount of electrolyte was much lower than during half-cell



analysis. It is proposed that the wettability of the pristine/coated electrodes is different and is more pronounced in lower volumes of electrolyte, leading to the lower initial capacity. In addition, the pouch cell was comprised from electrodes with much higher mass loading and higher surface area than the T-cells. The improvement shown when using 2x150 ALD AlF3 coated cathode material both in the halfcell and in the full-cell configurations, motivated and led us to further study and improve the ALD coating conditions and thus two more ALD film coatings of AlF3 were investigated. In addition, several parameters of the ALD process were altered. Originally, in the ALD deposition of MgF2, the second intermediate ozone step purpose was to activate the surface (subsequent to the introduction of the fluorine precursor25). TMA is substantially more reactive than the magnesium precursor. Therefore, to reduce oxygen contaminations without compromising the activation of the surface, the second ozone intermediate step was removed. In addition, as water can have a disastrous effect on the cycling of the cells by introducing HF, the carrier gas was switched to Ar, which had less water impurities. The 2 most significant changes were done in the temperature and the number of cycles during deposition. In the first coating, the temperature was increased to 240⁰C, while maintaining the number of ALD cycles at 2 (2x240). In the second modification, the number of ALD cycles was increased to 4, while maintaining the same temperature deposition (4x150). There are not many publications of AlF3 coating via ALD and in most of them, flat surface coating was investigated and not powder coating. Not only that, there are no publications of AlF3 coating using the precursors used in this research. However, a comparison to other AlF3 ALD coatings was performed in order to better understand the impact of the parameter's changes on the coating


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layer. Lee et al deposited AlF3 on Si wafer and Al2O3 using TMA and HF-pyridine and reported a decrease in the growth per cycle (GPC) when raising the temperature over 100⁰C. The GPC was calculated to be 1.02Å at 150⁰C and 0.5Å at 200⁰C.67 Mantynaki et al deposited AlF3 on Si wafer using AlCl3 and TiF4, and a decrease in the GPC from 3Å at 160°C to 0.75Å at 240°C was reported. In addition, they reported an amorphous film formation below 260°C.68 Xie et al deposited AlF3 on Si wafer and on NMC 811 powder using AlCl3 and TiF4 at 250°C. 1Å of GPC was reported at that temperature.56 Jackson et al deposited AlF3 on Si wafer and on NMC 532 powder at 125°C using TMA and TaF5. A GPC of 2Å was calculated for that temperature, and an increase of the GPC was reported over 200°C with a deviation from the ALD process, meaning a non-self-limiting growth caused by the formation of TaC, which causes a multilayer formation.55 All of the GPCs mentioned above were calculated via depositions on Si wafers. The GPC was calculated usingseveral thickness measurements of the deposited layer at different number of cycles; 15 cycles were the least amount of cycles used for calculations. No GPC calculations were made on powder depositions with just several ALD cycles. . However, there is trouble in comparing the GPC for powders vs thin films. not only that it is reasonable to assume that the actual thickness of the coating layer from several ALD cycles will be different than the one calculated from hundreds of cycles preformed on a flat surface, it is also probable that the GPC in the first several cycles is not necessary linear. TEM analysis of the additional coatings showed thicker coating for 4x150 (Figure 11), yet a clear thickness for 2x240 could not be determined due to Pt contamination of the coating. As was explained above, ALD process of coating particles vs films have several clear distinctions, one of which is the number of deposition cycles: on particles only few deposition cycles are needed,



while for films usually at least few dozens of ALD cycles are employed. Therefore, it was important to determine the thickness of the coating directly from the ALD deposition process on the particle, and not from extrapolation of any given data stated in the aforementioned publications. Since most of the particles are in the size of hundreds of nanometers and even larger, it was essential to prepare a FIB sample for the TEM analysis. The thickness of the 4x150 AlF3 coating was estimated to be ~4.6nm, as could be seen in Figure 11a. This is expected, as we know that a higher number of ALD cycles would create a thicker layer. The EDS scan (Figure 11b-c) show presence of Aluminum (red) and Fluorine (green) throughout the spectrum. Unfortunately, the exact thickness of the 2x240 coating was impossible to determine as the TEM and EDS analysis showed a mixed signal of Aluminum which came from the ALD coating and Platinum which originated from the FIB protective coating.

a

b

c

Figure 11: (a) TEM image of LMNO powder coated with AlF3 ALD: 4 cycles at 150⁰C (4x150); (b) zoom-in on the particle; (c) EDS spectrum of the corresponding line in (b).


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Electrochemical cycling of pouch full-cell configuration utilizing the additional modified coated cathode materials is presented in Figure 12. Raising the deposition temperature to 240°C without changing the number of cycles, decreased the initial cell capacity by ~25 mAh g-1. Increasing the number of cycles from 2 to 4 cycles at 150°C increased the cells initial capacity by ~10 mAh g-1, compared to the cells utilizing the 2x150 coated material. Figure 12a shows that a thicker coating (4x150 vs 2x150) produces higher initial capacity: 93 mAh g-1 vs 85 mAh g-1, seemingly contrary to expectations. The difference in capacity can be due to the removal of the intermediate ozone step or the change of the carrier gas. The effect of the change on the deposition properties is still investigated. Importantly, all materials holding the modified coatings procedures being applied showed better performance than the pristine material. Once again, cell utilizing the coated ALD materials exhibited a lower initial capacity that was increased dramatically after the temperature was raised. An attempt to mitigate the lower initial capacity was made by implementing a new formation cycling. So far in our study, each cell was cycled for two formation cycles at a rather slow rate of C/10, in order to build a stable SEI layer 69.

The new formation cycling implanted here, actually proposes to add two formation cycles at

the same slow rate at 45⁰C after the two formation cycles at RT, and only then to continue cycling at RT, as previously was implemented in the study. Changing the formation cycling shortened the number of cycles in which the "capacity recovery" phenomenon took place. This is a most desired result; however, it also shortened the cell’s cycle life. The difference in the initial capacity and capacity retention between the standard formation (S) cycling and modified formation



cycling (M) is presented in Figure 12b-c. The new formation cycling (M) boosted the capacity obtained from Li-ion cells utilizing the coated materials within 4 cycles comparing to 30 cycles for the cells experiencing a standard formation cycle. An increase of 50 mAh g-1 was observed for the cells utilizing 2x240 coated material when the new formation was implemented. The capacity retention of cells cycled with the new formation (M) cycling was better than the pristine cells, which were cycled only with the standard formation, but inferior to the cells utilizing the coated material which were cycled with the standard formation. It is clear from the results that the Liion cells utilizing coated materials are greatly affected by the kinetics, which “kicks in” when the temperature is raised to 45⁰C, as was demonstrated with the modified formation cycling (Figure 12b-c). In general, all AlF3 coatings showed superior or equal results to the uncoated cells in the full-cell configuration. It is not yet known if the cathode material coating experiences some transformation, such as fluorination of oxygen impurities being accelerated at 45°C, as this may enable more lithium intercalation. It is also possible that a different mechanism may take place which involves the insertion of Li from the electrolyte into the coating layer, producing additional pathways for Li ions to migrate. This would be manifested in an improvement in lithium ion conductivity, resulting in a capacity increase that may be also accelerated at higher temperatures. Both possibilities involve volume expansion of the coating layer.70 Capacity recovery is a phenomenon that was reported in the literature, as Kim et al reported on a capacity recovery for electrodes composed form LMNO particles coated with Al2O3 via ALD; the recovery was explained by fluorination of the layer.47 Since there are probably oxygen impurities in the AlF3 layer, it is possible that some fluorination of the layer occurs, which is accelerated at 45⁰C. In this case, the


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shorter cycle life of the cells experiencing a modified formation (M) cycling can be explained by the fast volume changes the layer undergoes. 80% of the volume changes happened fast during the two formation cycles at 45⁰C and only 20% of the volume changes happened during the rest of the cycling at RT. Whereas for the standard formation cycling, 80% of the capacity changes occur in a slow and continuous manner during the 30 cycles conducted at RT, while the remnant 20% took place quite rapidly, during the transition of the temperature increase to 45⁰C at the 31st cycle (Figure 12b-c). At this stage, it is unknown why this phenomenon was not observed in the half-cell measurements. Another mechanism that can explain the capacity recovery is lithiation of the coating layer, implicating that the coating layer changes from AlF3 to a LixAlyFz layer, which has a better Li ions conductivity. These changes are also accompanied by volume changes. Computational reports made by Jung et al showed that Al2O3 can undergo lithiation and a Li3.4Al2O3 layer form, which is the most thermodynamic stable phase. The volume of the layer was expanded 2.1 times, as a result of the lithiation.70 In situ TEM lithiation of naturally oxidizes Al nanowires (4-5 nanometers in thickness) showed an expansion of the Al2O3 layer, equals to a 2.25 times volume expansion,71 well corresponds with the theoretical reports. The calculations shows changes in the Al-Al bonds in the Al2O3 layer and that lithium passes through the coating layer even before the new phase is formed. As the phase is formed, Li ion conductivity increases substantially.70 Structural changes were also observed in Al2O3 ALD layer deposited on Si. The originally insulating layer became ionically conductive, as it changed during lithiation. LiAlO2 which is an excellent Li ion conductor was found on its surface.72 In another computational report by Xu et al, a model for the lithiation of Al2O3 ALD was proposed where Li+ ions exchange trapped



H+ protons in the coated layer thus a new structure, Li0.81Al2O3, is formed. In this case no volume changes were calculated. The new layer had better conductivity than the original insulating layer. Here, it was suggested that the same phenomenon may take place for the amorphous AlF3 ALD layer, since fluorination of amorphous Al2O3 was reported. In addition, it was reported that in thin amorphous coatings, fluctuations in the structure of the layer can lead to significant variation in lithium diffusivity that can either be enhanced or impeded in different regions of the layer.73 Stable phases of LixAlyFz, such as Li3AlF6 or LiAlF4 can form by conversion of AlF3 according to Xie et al, which may be porous due to volume changes, and these phases have better ionic conductivity than the original AlF3 layer. However, the calculations showed that AlF3 cannot experience lithiation and it will remain intact in the working voltages of the cathode in LIBs.56 The theoretical studies indicate that the AlF3 will remain intact as a coating layer, thus lithiation of the coating layer does not explain the improved capacity during cycling. However, the calculations do not take into account the level of purity of the coating layer which is unknown and the exact ratio of Al:F in the layer or the level of porosity, which is also unknown. That means that the layer can experience some lithiation and its conductivity may also increase. Higher temperature will increase the rate of the lithiation and delithiation processes in the material. These processes can be manifested via leaching of lithium ions from the cathode material to the AlF3 coating layer, as was demonstrated by Sun et al. The leaching was shown using TEM and XRD analysis on a wet chemistry coating of AlF3 on layered Li[Li0.19Ni0.16Co0.08Mn0.57]2. In that case the leaching induced the transformation of the initial Li2MnO3 layer phase to the spinel phase.74 The improvement in performance shown by the temperature increase can be explained by better Li ions kinetics and lower charge transfer


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resistance, as was demonstrated before on LiCoO2.75 Previously, the compounds LixPFyOz and LixPFy were found on the surface of LMNO, as part of a film formation on the cathode, analogues to the anode SEI;60,76 those compound were formed when the lithium electrolyte was oxidized at the cathode. a

b

c

Figure 12: Discharge capacity vs. cycle number of full-cell measurements utilizing pristine and different ALD AlF3 coated LMNO powder: (a); pristine, 2x240, 4x150, 2x240. Inset - first 40 cycles. (b) 2x240; (c) 4x150. S – standard formation; M – modified formation. All cells were cycled for 30 cycles at RT and the rest at 45⁰C.



Regarding the effects of the ALD parameters modifications on the full-cell electrochemical behavior: it was expected that raising the temperature (2x240) would result in a thinner coating layer and thus, higher initial capacity; however opposite results were received in this regard. This result might be caused by a denser coating layer which impedes Li ions migration. In addition, increasing the number of ALD cycles (4x150) resulted in an increase in the initial capacity compared to the coating of 2x150, possibly due to the thicker coating layer. This effect, however, could be accounted also due to additional changes and modifications done during the ALD procedure itself.

CONCLUSIONS A new AlF3 coating for LMNO powder was introduced via ALD. LMNO powder material was particularly coated with AlF3 via ALD technique. Three different coatings were studied and examined; each coating differs in the ALD deposition conditions. Overall, electrode composed of the ALD coated materials exhibited improved and enhanced electrochemical behavior, once compared with electrodes utilizing the pristine uncoated material, The cells utilizing the ALD coated cathode materials also exhibited different behavior between themselves. All of the cells utilizing the ALD AlF3 coated powder materials showed improvements in the capacity retention, comparing with cells utilizing the pristine uncoated material. Lower initial capacity was observed in the Li-ion cells utilizing the ALD AlF3 coated materials when cycled against graphite, which was not observed when cycled against lithium. New formation cycling was implemented and this resulted in a recovery and enhancement of the lower initial capacity observed earlier. The disadvantage of the modified formation cycling is manifested in a shorter cycle life. However,


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even with the shorter cycle life, the cells utilizing the ALD AlF3 coated material exhibit better capacity retention than the cell holding the pristine LMNO. Fluorination of the coating layer and lithiation of the layer were suggested as explanations for the capacity recovery phenomenon being observed for all ALD AlF3 coating in the full-cell configuration. The developed ALD AlF3 coating method can be well adopted to other Li-ion cathode materials as well as to be applied on different anode materials. By adjusting the process conditions (temperature, number of cycles etc.), an optimize coating layer will be received, thus improving the performance of LMNO electrode. In addition, ALD is a very generic technique that can be implemented easily by the LIBs industries and it can be modified to other metal fluorides coatings by changing or adding precursors to the process.

Acknowledgements This work was supported by the Transportation Electric Power Solutions TEPS MAGNET - an Israeli Ministry of Economy consortia, and by the Grand Technion Energy Program (GTEP).The authors would like also to acknowledge the support of Planning & Building Committee / ISRAEL Council for Higher Education (CHE) and Fuel Choice Initiative (Prime Minister Office of ISRAEL), within the framework of "Israel National Research Center for Electrochemical Propulsion (INREP)".

AUTHOR INFORMATION Corresponding Author *E-mail [email protected]



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200

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95

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Pristine 2x150 ALD coated Pristine C.E 2x150 ALD coated C.E

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30

Coulombic efficiency (%)

Table of Contents Graphic

Discharge specific capacity (mAh g-1)

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A Robust AlF3 ALD Protective Coating on LiMn1.5Ni0.5O4 Particles

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