Study of Immersion of LiNi0.5Mn0.3Co0.2O2 Material in Water for

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Study of the immersion of LiNi Mn Co O material in water for aqueous processing of positive electrode for Li-ion batteries Marie Bichon, Dane Sotta, Nicolas Dupre, Eric De Vito, Adrien Boulineau, Willy Porcher, and Bernard Lestriez ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00999 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Materials & Interfaces

Study of the immersion of LiNi0.5Mn0.3Co0.2O2 material in water for aqueous processing of positive electrode for Li-ion batteries Marie Bichon*,a, Dane Sottaa, Nicolas Dupréb, Eric De Vitoa, Adrien Boulineaua, Willy Porchera, Bernard Lestriezb aUniversité

bInstitut

Grenoble Alpes, CEA-Liten, 17 Avenue des Martyrs, F-38054 Grenoble, France

des Matériaux Jean Rouxel, UMR CNRS 6502, Université de Nantes, 2 rue de la Houssiniere, BP32229, F-44322 Nantes, France

ABSTRACT

The understanding of the phenomena occurring during water immersion of LiNi0.5Mn0.3Co0.2O2 (NMC) is helpful to devise new strategies towards the implementation of aqueous processing of this high capacity cathode material. Immersion of NMC powder in water leads to both structural modification of the particles surface as observed by HRSTEM, and formation of lithium based compounds over the surface (LiOH, Li2CO3) in greater amount than after long time exposition to ambient air, as confirmed by pH titration and 7Li MAS NMR. The surface compounds adversely affect the electrochemical performance and are notably responsible for the alkaline pH of the aqueous slurry, which causes corrosion of the aluminum collector during coating of the electrode.

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The corrosion is avoided by adding phosphoric acid to the slurry as it lowers the pH, but also enhances the cycling stability of the water-based electrodes due to the phosphate compounds formed at the particles surface, as evidenced by XPS analysis.

KEYWORDS Li-ion battery, LiNi0.5Mn0.3Co0.2O2, aqueous electrode processing, surface modification, phosphoric acid. INTRODUCTION Lithium-ion batteries, which are widely used in portable electronics, are also developed to power electric vehicles. In this regard, recent studies have focused on increasing their energy density, especially by developing new cathode materials1. Layered NMC oxides LiNixMnyCozO2 (x+y+z=1) are attractive cathode material since their reversible capacity is higher than the widely commercialized LiCoO2 cathodes while having a similar operating voltage. Since the reversible capacity of NMC cathodes mainly originates from the oxidation and reduction of the nickel ions, effort has been aimed at synthesizing NMC oxides with high nickel content2. However, Ni-rich materials suffer from surface reactivity upon exposure to air3. Several authors have reported the formation of LiOH and Li2CO3 at the surface of Ni-rich powders after storage in air4,5,6,7. However, there is no general agreement in the literature regarding the mechanisms leading to the formation of these components as the aging process appears to depend on the nature of the material studied (nickel and lithium content, synthesis) or on the storage conditions (temperature and atmosphere). Liu et al.8 studied the degradation of LiNiO2 powder at room temperature in air. They argued that the spontaneous reduction of the Ni3+ ions is responsible for the instability of this material, as it triggers the formation of active oxygen species


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on the surface of the particles, which combine with H2O and CO2 to form adsorbed hydroxyls, bicarbonates and carbonates. These adsorbed species are prone to react with lithium ions and form LiOH, LiHCO3 and Li2CO3. Delithiation and loss of oxygen are believed to prompt the transformation of the outer layers into a cubic phase, as observed by XRD. According to some reports, lithium-based species which are present at the surface of the particles also originate from unreacted lithium-precursor added in excess for the synthesis9,10. Shizuka et al.11 investigated the effect of CO2 on Li1+zNi1-x-yMnxCoyO2 and Li1+zNi1-x-yCoxAlyO2 by measuring the carbonate content for different exposure durations. They concluded that carbon dioxide first reacted with excess lithium from unreacted Li precursor, and then with lithium from the bulk. Although some articles that report the formation of Li2CO3 upon storage in air do not mention the presence of LiOH4,12,13, Faenza et al.14 suggested that carbonate compounds were mainly created through intermediary species formed by reaction with water (LiOH, LiHCO3) and not by direct reaction with CO2 at room temperature. Zhang et al.15 argued that the rapid reaction of LiOH with CO2 to form Li2CO3 accounted for the fact that only Li2CO3 was observed after long period of storage in air4. In many studies, the surface carbonate detected by XPS are assumed to be Li2CO34,8,11,14. Yet, surface compounds mainly composed of nickel carbonate mixed with minor amount of hydroxide and water were found at the surface of LiNi0.8Mn0.1Co0.1O2 after 1-year aging in ambient air, using XPS and Raman spectroscopy16. Though layered oxides with lower Ni content than NCA or NMC811 are still sensitive towards moisture and carbon dioxide, the degradation is less severe. In the case of LiNi1/3Mn1/3Co1/3O2 the aging mechanism might not be exactly the same as proposed for LiNiO2 since in the NMC compound all nickel ions are already in the +II state. Zhang et al.15 detected the presence of LiOH



and Li2CO3 by Raman spectroscopy on the surface of Li1+x[Ni1/3Mn1/3Co1/3]1-xO2 after aging in water. They attributed the modification of the magnetic properties after exposure to humidity to the delithiation of the surface, compensated by the oxidation of nickel ions. Zhao et al.13 also reported a delithiation layer on Li[Li0.2Ni0.2Mn0.6]O2 after storage in air and the presence of Li2CO3 at the surface of the particles, but no change in the oxidation state of Mn and Ni according to XPS results. Shkrob et al.17 investigated the storage behavior of LiNi0.5Mn0.3Co0.2O2 (NMC532) electrodes in humid air. This material is an intermediate between Ni-rich NMC811 and more stable NMC111. The authors observed features akin to aged Ni-rich powders such as amorphous layer, delithiation and formation of a rock-salt phase near the surface. However, they suggested that the delithiation process was paired with a cation exchange between H+ and Li+ besides the well documented redox process. The reactivity of cathode materials towards moisture is a major concern not only for the storage of Ni-rich powders and electrodes, but also for aqueous processing of electrodes. Indeed, replacing toxic N-methyl-2-pyrrolidone (NMP) solvent with water has been seriously considered in order to reduce the cost and environmental impact of the electrode manufacturing process18,19. However, the strong alkalinity of aqueous slurries causes dissolution of the passivation layer on the aluminum collector and pitting corrosion is triggered locally at the Al-Fe-Si intermetallic particles present in the aluminum, resulting in H2 release and formation of Al(OH)321. The corrosion products increase the electrical resistance at the electrode/collector interface which is detrimental to the battery performance. Addition of acid to the slurry to reduce the pH is a straightforward method to avoid corrosion22. In this study, we investigated the effect of aqueous processing on the performance of LiNi0.5Mn0.3Co0.2O2-based electrodes. The previous works on the effect of water on NMC material


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or electrode were mainly considering storage in air rather than aqueous processing. This work aims at clarifying the mechanism occurring to the active material during aqueous electrode preparation. Qualitative and quantitative analysis were first carried out on NMC532 powder after immersion in pure water and acidified water, or long-term exposure in air. Ion leaching was evaluated by ICPAES measurements. TEM analysis revealed chemical and structural evolution at the surface. The amount of surface compounds formed during the aqueous process, such as hydroxides and carbonates, was also determined using 7Li MAS NMR and pH titration. Additionally, XPS measurements were carried out to clarify the nature of the amorphous layer formed at the surface of the particles. Finally, electrodes prepared using NMP, water or acidified water as solvent were cycled in half and full cells to assess the effect of surface evolution on the electrochemical performance. EXPERIMENTAL SECTION Sample preparation Aged powders were prepared by stirring 5 g of LiNi0.5Mn0.3Co0.2O2 powder (NMC532, Umicore, average particle size D-50 = 12 µm) for one hour in 5 mL of distilled water — or 5 mL of distilled water acidified with 118 mg of a solution of H3PO4 at a concentration of 42.5 wt% (SigmaAldrich). The amount of phosphoric acid in the acidified solution was thus 1 wt% of NMC532. The powder was recovered after water evaporation at 50°C, or by filtration at room temperature. In the latter case, the liquid filtrate was also recovered for further characterizations. It will be specified in the text whether the characterizations were carried out on filtered or dried samples. In order to evaporate the water at 50°C, the aqueous mixture was deposited in a large crystallizer so as to get a thin layer of the mixture spread on the glass. If we take into account the time to evaporate the water, the immersion time was actually longer than 1 hour, and we estimate it to be close to 3



hours instead. NMC532 powder was also exposed to ambient air for 1 year to compare the aging behavior in air to water. The description of the different samples is given in Table 1. Table 1: Preparation of the powder samples Ref

Immersion solution

Immersion duration

Storage condition

P0 (pristine) -

-

Ar

P1

H2O

1h

Ar (before and after immersion)

P2

H2O + H3PO4

1h

Ar (before and after immersion)

P3

-

-

Air – 1 year

Characterizations Li, Ni, Mn and Co content in the aqueous filtrate recovered from the immersion of NMC powders P1 and P2 were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Créalins). STEM images and STEM-EELS mappings were recorded using a Cs-corrected FEI Titan Themis microscope operated at an accelerating voltage of 200 kV. Elemental mappings were obtained exploiting EELS spectra recorded in spectrum imaging mode with a Gatan GIF Quantum electron spectrometer. XPS analyses have been achieved on a Physical Electronics Versaprobe II spectrometer. A monochromated Al anode was used as the incident X-ray source (h = 1486.7 eV). An efficient control of charge effects due to the resistive properties of the electrodes’ surface was achieved by using combined use of low-energy ion and electron guns. The internal post-calibration of XPS spectra was carried out by using the C 1s adventitious carbon peak at 284.8 eV.


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The amount of hydroxides and carbonates present at the surface of the NMC powders P0, P1 and P3 was determined using Warder’s titration method23. 5 g of powder was stirred in 100 mL of distilled water in order to dissolve the hydroxide and carbonate compounds. The powders P0 and P3 were stirred in water for only 2 minutes instead of 1 hour for P1. During the 2 minutes immersion time, the pH of the suspension rises due to dissolution of the hydroxide and carbonate compounds. Some LiOH may also be formed in the first stage of immersion which is why we kept the immersion time as short as 2 minutes for the samples P0 and P3. The NMC particles were separated from the solution by vacuum filtration, and the filtrate was titrated with 0.1M HCl solution. 7Li

MAS NMR experiments were carried out at room temperature on a Bruker Avance-500

spectrometer (B0 = 11.8T, Larmor frequencies ν0 (7Li) = 194 MHz). MAS spectra were obtained by using a Bruker MAS probe with a cylindrical 2.5 mm o.d. zirconia rotor. Spinning frequencies of 25 kHz were utilized. 7Li NMR spectra were acquired by making use of a single pulse sequence coupled with a pre-acquisition time of 4.5µs allowing the separation of the surface lithium signal from the bulk signal. By this mean, only diamagnetic species on the surface of the paramagnetic active material were observed24. All spectra displayed in this work were normalized taking into account the number of scans, the received gain, and the mass of sample. 7Li integrated intensities were determined by using spectral simulation (Dmfit software25). The absolute quantification of detected species was performed using i) numerical fit and integration of each set of peaks corresponding to a chemical compound, ii) calibration of this integral using various Li2CO3– electrode mixtures26. Electrode preparation



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Water-based NMC cathodes were prepared by mixing 92wt% NMC532, 4wt% carbon black (Super C65, Timcal), 3wt% latex binder (Zeon) and 1wt% carboxymethyl cellulose (Ashland) in distilled water. In the case of the acidified formulation, phosphoric acid (1wt% of NMC) is added before adding the NMC powder. The slurry pH is measured before coating, and is around 11.8 and 6.5 for the non-acidified and acidified formulation, respectively. In order to compare the electrochemical properties of the NMC powders P0, P1 and P2, NMP-based electrodes were prepared by dispersing 92wt% active material (pristine powder P0 or soaked powders P1 and P2 recovered after water evaporation), 4wt% Super C65 and 4wt% polyvinylidene fluoride (PVDF 5130, Solvay) in N-methylpyrrolidone (NMP, Sigma-Aldrich). The aqueous or NMP-based slurries were coated onto 20 µm thick aluminum foil and pre-dried overnight in an atmospheric oven at 50°C. The active material loading was set at 11.5 mg/cm². The electrodes were then pressed down to 65 µm using a rolling press machine, in order to reach a porosity of 35%. Compositions of the different cathodes formulated are summarized in Table 2. Table 2: Description of the different electrodes Ref

Electrode composition

Solvent

P0NMP

P0 / CB / PVDF (92:4:4)

NMP

P1NMP

P1 / CB / PVDF (92:4:4)

NMP

P2NMP

P2 / CB / PVDF (92:4:4)

NMP

P0H2O

P0 / CB / CMC / latex (92:4:1:3)

H2O

P0H3PO4

P0 / CB / CMC / latex / H3PO4 (91.1:4:1:3:0.9) H2O + H3PO4

Graphite anodes were prepared with graphite (Hitachi), CMC (Ashland), and styrene-butadiene rubber (BASF) at a weight ratio of 97.4:1.3:1.3. The slurry was coated onto copper foil. The electrode loading was 7.3 mg/cm². The electrodes were then pressed to obtain a porosity of 30%.


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Electrochemical measurements Electrode discs (Ø14mm) were cut out of the cathode tapes and dried under vacuum for 48h at 80°C in order to remove residual water. Coin cells were assembled in a glove box, using lithium foil as anode and Celgard 2400 as separator. The electrolyte was composed of 1M LiPF6 in a mixture of ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC), 1:1:3 by weight. Full cells were also assembled with the graphite anode, Celgard 2400 separator and LP10 electrolyte (1M LiPF6 in EC/PC/DMC 1:1:3 by weight + 2% VC). The half and fullcells were galvanostatically cycled at room temperature, in the potential range of 3.0 to 4.3V, and 2.5 to 4.2V vs Li/Li+ respectively. RESULTS AND DISCUSSION Ion leaching and structural evolution The metal content detected in the aqueous filtrates obtained from P1 (immersion in H2O) and P2 (immersion in H2O + H3PO4) was determined by ICP-AES. The relative dissolution for each element is given in Table 3. About 1% of lithium is dissolved in water after soaking NMC for one hour, while the amount of dissolved transition metals is negligible. In acidified water, the metal content is higher than in pure water. Transition metal dissolution is increased by almost a factor 100, but it is still an order of magnitude below the 2.5% lithium dissolution. In both water and acidified water, leaching of nickel ions is higher than that of cobalt, and manganese has the lowest dissolution rate. The same dissolution trend (Li >> Ni> Co> Mn) has been observed with NMC11122,27. By combining DFT and thermodynamics approach, Bennett et al.28 computed the free energy of dissolution of transition metals from NMC to interpret this uneven dissolution trend. In the pH range from 3 to 7, they found that the free energy of dissolution ∆G is lower for Ni < Co



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< Mn. The preferred dissolution of nickel over cobalt and manganese can also be explained by the fact that in the NMC532 material, Mn4+ and Co3+ have to be reduced to divalent Mn2+ and Co2+ to be dissolved, while the nickel ions already in the +II valence state can be more easily solvated. Table 3: Dissolved metal ions content measured by ICP-AES in aqueous filtrate after soaking NMC532 particles for 1h. The values are calculated for each element 𝑥 as the weight ratio 𝑚𝑥𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒 𝑚𝑥𝑝𝑜𝑤𝑑𝑒𝑟.where 𝑚𝑥𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒 is the amount of 𝑥 found in the filtrate and 𝑚𝑥𝑝𝑜𝑤𝑑𝑒𝑟 is the amount of 𝑥 in the powder prior to immersion, also measured by ICP-AES. Filtrate H2O (P1)

H2O + H3PO4 (P2)

% dissolved Li

1.1 %

2.3 %

% dissolved Ni

2.4 10-3 %

1.7 10-1 %

% dissolved Mn

1.3 10-3 %

0.4 10-1 %

% dissolved Co

1.7 10-3 %

0.9 10-1 %

In order to evaluate the impact of ion leaching on the NMC structure, the surface of P1 and P2 powders were examined by transmission electron microscopy and compared with that of pristine P0 powder (Figure 1). Contrary to ICP-AES experiments, the powders P1 and P2 were not filtrated but recovered after evaporation at 50°C of all the water used for their immersion. In these high resolution STEM dark-field images, the bright contrast reveals the transition metals layers while lithium ions are not visible, since much lighter. The structure of the pristine powder P0 consists mainly of a well ordered 𝑅3𝑚 phase with a reduced cation mixing in the extreme surface (~2 nm), where transition metals are observed in the lithium slabs. In contrast, the near-surface region of P2 powder is very different from the P0 pristine powder, indicating significant structural changes. In Figure 1b, no 𝑅3𝑚 phase is observed. This is a result of lithium leaching in aqueous medium


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leading to vacancies at the edge of the particles that were filled with transition metals. Besides, the extreme surface of P2 sample is not as smooth as the pristine one; this could be related to a partial dissolution process. Contrast heterogeneities reveal thickness variations that could result from local deficiencies of transition metals caused by their leaching in acidified water.

Figure 1: HRSTEM-HAADF image of NMC532 powder (a) pristine (P0) and (b) recovered after 1h immersion in acidified aqueous solution followed by water evaporation at 50°C (P2). Figure S1 shows a particle from sample P2 with even higher structural damage, as the nearsurface region is transformed into a polycrystalline phase. Such structural transformation at the edge of NMC particles exposed to humidity have been reported in the literature, along with the formation of amorphous surface films12,17,7,29. We also observed a thin and non-uniform amorphous layer at the surface of P1 and P2, as shown in Figure S2. Such amorphous area is often ascribed to lithium carbonate, either because the presence of this compound is confirmed by other analysis17,30 (notably XPS), or because EDX analysis revealed that this amorphous layer was primarily composed of carbon and oxygen12,29. However in our case, elemental analysis by EELS showed that this amorphous layer is rather composed of TMs, Li and O (



Figure 2). It is interesting to note that the nickel content in this amorphous layer is slightly higher than in the bulk, with a composition of roughly 60% Ni, 20% Mn, 20% Co at the surface compared to 50% Ni, 30% Mn, 20% Co in the bulk. This amorphous layer could be formed by precipitation of dissolved metal ions as it would account for the elemental composition of this amorphous phase considering the dissolution trend Ni>Co>Mn revealed by the ICP-AES measurements. In the case of sample P1, the precipitate might be composed of LiOH, Ni(OH)2, Co(OH)2 and Mn(OH)2 regarding the elevated pH of the aqueous solution, while for sample P2, the layer might rather be mainly composed of phosphate compounds. These assumptions on the nature of the amorphous layer are also supported by XPS analysis, as it will be discussed below.

Figure 2: Elemental mappings on top and associated profiles extracted from EELS analysis of particle P1 shown in Figure S2c. Besides the amorphous layer, we also observed locally at the surface of the particles P1 and P2 the presence of carbonate (Figure S3) and lithium hydroxide (Figure S4) by EELS analysis.


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Quantification of LiOH and Li2CO3 formation

Figure 3 : (a) Content of dissolved hydroxide and carbonate compounds in aqueous filtrate measured by pH titration. The values are standardized by weight of NMC powder. (b) Normalized 7Li

MAS NMR spectra of different NMC532 samples: pristine (P0), stored 1 year in air (P3),

soaked 1h in water or acidified water and recovered after water evaporation (P1 and P2).and (c) Calibration curve obtained by 7Li MAS NMR from mixtures of pristine NMC powder with known amounts of Li2CO3. The points in color correspond to the samples P0, P1, P2 and P3 and were placed on the curve based on the intensities measured in Figure 3b. Formation of surface compounds such as LiOH and Li2CO3 upon exposure of NMC particles to moisture is a phenomenon well described in the literature15,17,31,32. Carbonate and hydroxide content at the surface of NMC pristine (P0), exposed one year to ambient air (P3) and immersed one hour in water (P1), was measured by pH titration (Figure 3a). An initial amount of hydroxide and carbonate compounds is found at the surface of the pristine material P0 (38 µmol/g). After exposition in ambient air for one year, the total amount of these compounds reaches 46 µmol/g and is mainly composed of carbonates as most of the hydroxides must have reacted with the carbon dioxide in air. For the sample immersed 1h in water P1, the total amount of these compounds is even higher (71 µmol/g), notably owing to the formation of hydroxides.



7Li

MAS NMR spectra of NMC powders P0, P1, P2 and P3 are presented in Figure 3b. Note

that, as detailed in the experimental section, the samples P1 and P2 characterized by NMR were recovered after water evaporation at 50°C of the immersed powders. The signal intensity in Figure 3b is proportional to the amount of 7Li in diamagnetic phase in each sample24. As the lithium inserted in the bulk of the paramagnetic NMC particles is not detected in this experiment, this technique provides a quantitative analysis of lithium impurities such as LiOH and Li2CO3. The relative intensity of each sample is reported on the calibration curve obtained from mixtures of pristine NMC powder with known amounts of Li2CO3 (Figure 3c). The sample aged one year in air (P3) contains more lithium impurities than the pristine sample (P0). The immersion in water, followed by solvent evaporation, causes the formation of even more lithium surface compounds (P1), and the reactions are exacerbated in the presence of acid (P2). This trend is consistent with the results obtained by pH titration. The results obtained by pH titration, ICP and NMR analysis are compared in Table 4. In order to compare the values obtained by pH titration and NMR, it is assumed that the species titrated in the aqueous filtrate are solely LiOH and Li2CO3, so that 1 mole of carbonate and 1 mole of hydroxide titrated would correspond to 2 moles or 1 mole of lithium respectively, as reported in Table 4. This assumption is not in accordance with a recent study by Jung et al16 who showed that nickel was involved in the formation of carbonate compounds at the surface of NMC811 particles aged in air. Nevertheless, the amount of surface lithium herewith calculated from the pH titration measurements is consistent with the amount measured by NMR for the samples P0 and P3, suggesting that the carbonate and hydroxide compounds found on the pristine sample and the sample aged in air are mainly LiOH and Li2CO3. In the case of the immersed sample P1, the amount of surface lithium species detected by NMR is higher than the equivalent amount of


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dissolved hydroxide and carbonate measured by the titration method or the amount of dissolved lithium measured by ICP. This discrepancy can be accounted for by the fact that there is a significant difference in the preparation of the samples labeled “P1” and “P2” whether they are characterized by ICP and pH titration or NMR. Indeed, for the latter technique, the powders P1 and P2 were recovered after water evaporation at 50°C, whereas the ICP and titration were conducted on the aqueous solution separated by filtration. The NMR measurements were not conducted on a powder recovered by filtration as LiOH and Li2CO3 species are soluble in water. Furthermore, evaporation of the solvent is what happens during the aqueous processing of NMC electrodes, and the aim of this study is to understand the phenomena taking place in the preparation of water-based electrodes. Henceforth, the sample P1 characterized by NMR was still exposed to water after the 1-hour immersion time, for as long as it took the water to evaporate. Besides, the drying temperature affects the delithiation process and formation of impurities, as illustrated in the supporting information (Figure S5). In this light, it is not surprising to find more lithium compounds at the surface of the powder P1 characterized by NMR rather than in the filtrate analyzed by pH titration or ICP. However, in the case of the sample P2, the amount of 7Li in diamagnetic phase measured by NMR is lower than the amount of dissolved lithium measured by ICP on the acidified aqueous filtrate. Here, it is possible that the NMR signal is attenuated due to the presence of transition metals at the surface of the particles. Indeed, as it will be shown in the next paragraph, transition metal phosphates are formed at the surface of sample P2, and the signal of surface lithium close to these paramagnetic transition metals may be lessened due to the conditions of acquisition used. Despite some small discrepancies between the three quantitative methods due to different sensitivity and accuracy, they all emphasize the same trend, that is a higher amount of surface compounds for P2 > P1 > P3 > P0.



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Table 4: Quantitative results from ICP-AES, pH titration and 7Li MAS NMR analysis. P0

P1

P2

ICP-AES

50 µmol 126 Li/gNMC* Li/gNMC†

pH titration

54 µmol 90 µmol Li/gNMC‡ ‡ Li/gNMC

7Li

NMR

µmol 143 MAS 55 Li/gNMC§ Li/gNMC§

µmol 266 Li/gNMC†

µmol 180 Li/gNMC§

P3 µmol

83 µmol Li/gNMC‡

µmol 85 µmol Li/gNMC§

*Excess

lithium (in µmol/gNMC) present in the pristine powder. This value x is calculated from the stoichiometric ratio Li:M = 1+x obtained by ICP-AES. †Amount

of dissolved lithium measured by ICP-AES on the aqueous filtrate.

‡Amount

of lithium in the form of LiOH and Li2CO3, assuming 1 mol hydroxide = 1 mol Li and 1 mol carbonate = 2 mol Li. §Amount

of 7Li in diamagnetic phase.

Another comment that can be made on the quantification of alkaline species by pH titration is that the calculation of carbonate and hydroxide compounds is based on the hypothesis that these are the sole compounds titrated. However, Li2CO3 is believed to be formed through the intermediate bicarbonate LiHCO35,14,17, which might be present at the surface of NMC particles. Thus, the presence of bicarbonate not taken into account in the calculation of carbonate and hydroxide content might lead to an overestimation of carbonate content over hydroxide content. Analysis of the surface species by XPS The surface of NMC powders P0, P1, P2 and P3 were analyzed by XPS in order to probe the chemical nature of the compounds formed upon aging. As for NMR and TEM studies, samples P1 and P2 were recovered after water evaporation so that impurities formed upon immersion remained at the surface of the particles. Figure 4 shows the O 1s spectra of all samples. The O 1s spectra have been deconvoluted based on information extracted from the C 1s spectra of each samples


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(Figure S6). The C 1s spectra were decomposed with 4 contributions33: C-C/C-H, -CO, -C=O and -CO3. In the O 1s spectra, the area of the contributions positioned between 532-535 eV (-CO3 in green, -C=O in purple and –CO in crimson) were determined from the area of the corresponding contributions in the C 1s spectra, corrected with the appropriate relative sensitivity factor. The peak centered at 529.1 eV corresponds to lattice oxygen and the signal at higher binding energies is assigned to surface oxygen species, mainly carbonate (~532.0 eV) and hydroxide (~531.3 eV). Compared to pristine sample P0, the contribution from carbonate species is higher in air-exposed sample P3 while in soaked sample P1 it is the contribution from hydroxide species that is increased, which is in accordance with pH titration. The lattice oxygen signal in sample P2 is lessened, indicating a thicker surface layer. The presence of phosphates at the surface of the NMC powder immersed in water acidified with H3PO4 was evidenced by a peak centered at 133.5 eV in the P 2p spectrum (see Figure S7). This prompted us to ascribe the main contribution in P2 oxygen spectrum to phosphate compounds. The large area of this peak was determined from the area of the P 2p peak, corrected with the relative sensitivity factor. EDX mapping presented in Figure S8 supports the presence of phosphorus at the surface of the particles of sample P2.



Figure 4: O 1s XPS spectra of different NMC532 samples: (a) pristine (P0), (b) soaked 1h in water and recovered after water evaporation (P1), (c) soaked 1h in acidified water and recovered after water evaporation (P2), and (d) stored 1 year in air (P3). The Ni 2p3/2, Mn 2p and Co 2p3/2 spectra of pristine sample P0 and immersed then dried samples P1 and P2 are presented in Figure 5. The XPS spectra of the transition metals in samples P0 and P1 are similar. The main peak at 855.0 eV corresponds to Ni 2p3/2 in an oxidized form, and its shake-up satellite is positioned at 861.5 eV. The position of the Ni 2p3/2 peak is shifted to lower binding energy when the ratio Ni3+/Ni2+ decreases34. Here, there is no significant reduction of the nickel ions at the surface of the NMC532 particles immersed in water, in contrast to what has been observed in layered oxides with higher nickel content5,8,11. The intensive Co 2p3/2 peak at 780.0 eV and less intense shake-up satellite at 790.0 eV are characteristic of cobalt ions in the +III oxidation state35. The Mn 2p3/2 peak is centered at 642.5 eV which corresponds to Mn4+ 36. The Ni, Mn and Co spectra of sample P2 present features significantly different from P0 and P1, indicating that transition metals are involved in the surface layer formed upon immersion of NMC532 in acidified water. The Ni 2p3/2 peak in sample P2 is positioned at 855.8 eV, which corresponds to the position of the main contribution for Ni(OH)2 and NiO37, where the nickel is in the oxidation state +II. Moreover, the shape and position of the shake-up satellite of the Ni 2p3/2 peak are similar to spectra of nickel phosphate compounds38,39. In the Co 2p3/2 spectrum of sample P2, the shake-up satellite is shifted towards lower binding energy compared to the pristine sample, indicating a reduction of Co (+III) ions into Co (+II). The position and shape of the Co 2p3/2 peak and its shake-up satellite also suggest the formation of cobalt phosphates39,40. In the case of manganese, the position of the peaks Mn 2p3/2 and Mn 2p1/2 are not shifted compared to the pristine sample. The only remarkable difference is a shoulder starting at 645.0 eV. This shoulder is believed


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to be caused by Al Kα X-ray induced Ni LMM Auger peak41, and the intensity of this feature compared to sample P0 and P1 could reflect the enrichment in nickel at the surface of sample P2. However, we cannot rule out the presence of manganese phosphates that would be reflected in the Mn 2p3/2 shake-up satellite peak located around 647 eV42,43 and overlapped by Ni LMM Auger contribution.

Figure 5: XPS spectra of different NMC532 samples: pristine (P0), in black; immersed 1h in water and recovered after water evaporation (P1), in blue; and immersed 1h in acidified water and recovered after water evaporation (P2), in green. Surface modification of NMC532 upon aqueous processing In this section we will further discuss the impact of the aqueous process on the surface modification of NMC532 powder. The pristine powder P0 is initially covered with a certain amount of lithium carbonate and hydroxide. Paulsen et al.23 suggested that there is an equilibrium coverage of hydroxide and carbonate necessary to stabilize the surface of NMC particles at the end of the synthesis. They argue that any attempt to remove these compounds referred to as equilibrium soluble base content (SBCe) — by washing the NMC powder for instance — will deteriorate the structure at the surface and cause poor electrochemical performance29. On the other hand, if at the end of the synthesis some unreacted lithium precursor remains at the surface of the particles



causing high amount of soluble base content, the electrochemical performance of the material will also be lessened10. As the SBCe depends on the particles morphology and composition, Paulsen et al.23 proposed typical values of equilibrium SBC for various types of NMC powders. In the case of large NMC532 particles (BET surface area = 0.27 m²/g) with Li:M ratio close to 1.00 that are used in our study, the SBCe given in their patent is 45 µmol/gNMC. The quantification of LiOH and Li2CO3 by pH titration gives 23.2 µmol/gNMC of hydroxide and 15.4 µmol/gNMC of carbonate, corresponding to 54 µmol/gNMC of lithium, which is also in accordance to the NMR measurement for sample P0. Thus, it appears that the lithium compounds found at the surface of the pristine powder correspond to the equilibrium soluble base content as introduced by Paulsen et al. When NMC532 powder is immersed in water, the basic surface compounds are dissolved, which account for the alkaline pH of the slurry. Then lithium ions close to the surface of the particles react with adsorbed water molecules to form LiOH, leaving the outer layers delithiated. The mechanism related to the charge compensation in the near surface region following the loss of lithium is not well defined. More precisely, it is not clear whether the nickel ions change their oxidation state. One scenario proposed in the literature considers an electrochemical delithiation analogous to the charging process, where nickel ions are oxidized15,14. However, the oxidation of nickel ions is not thermodynamically favored9, and most authors consider instead that Ni3+ ions are spontaneously reduced to the more stable Ni2+ causing delithiation and loss of oxygen8,14,44. Another explanation for the charge compensation associated with the loss of lithium is a cation exchange process, where protons are intercalated into the layered oxide17,14,28. It seems that the redox scenario is predominant in the aging process of layered oxides with high amount of nickel (NMC811, NCA, LiNiO2) that are exposed to ambient air, as Liu et al.45 demonstrated a clear correlation between the amount of impurities (carbonates, hydroxides) and nickel reduction at the


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surface of the particles. However, when the nickel content is reduced in the material, the ratio Ni3+/Ni2+ decreases and the material is less prone to be reduced at the surface through reactions with H2O and CO2. In some reports on Ni-rich layered oxides46,47, the reduction of nickel ions to Ni2+ is hinted by the shift of the Ni 2p3/2 main peak towards lower binding energy, due to the apparition of a contribution near 854,0 eV corresponding to NiO. We did not observe such shift after exposing NMC532 powder to water for one hour. More generally, the Ni 2p3/2, Co 2p3/2 and Mn 2p3/2 XPS spectra of the NMC powder exposed to water were similar to the spectra of the pristine sample, indicating no major reduction of the transition metals at the surface of sample P1. However, the XPS data do not preclude the presence of some intercalated protons at the surface of the layered oxide as an exchange H+/Li+ would not modify the oxidation state of the transition metals. Moreover, the 1H NMR spectrum of sample P1 (see Figure S9) displays a contribution near -10 ppm that does not appear on the pristine powder P0, and which could correspond to protons inserted into the bulk, according to Ménétrier et al48. Besides, Shkrob et al.17 suggested that the proton exchange mechanism is predominant over the redox process during the initial stage of exposure to humid air, which could account for the fact that we did not notice a significant reduction of the nickel ions after 1h-immersion in water. Likewise, Faenza et al.14 analyzed the surface of NCA powder exposed to ambient air for 2 weeks by XPS. Although the Ni 2p XPS spectrum of the exposed sample was poorly resolved because of the thick layer of adsorbed species, the spectra of the same sample annealed at different temperature to remove the impurities were similar to the pristine sample, supporting the idea that nickel reduction occurs only after several months of storage in air5,8. The loss of lithium near the surface of the NMC particles leads to structural reorganization of the outer layers (Figure 1) whereupon lithium sites are occupied by



transition metals. This cation mixing is often attributed to divalent nickel ions migrating from the octahedral 3b TM sites to the octahedral 3a lithium sites49, due to similar ionic radius8. The chemical nature of the surface layer formed on the NMC532 powders was further investigated by XPS. The presence of LiOH and Li2CO3 at the surface of the NMC powders is confirmed by the lines at 531.3 eV and 532.0 eV in the O 1s spectrum (Figure 4), and the line at 55.0 eV in the Li 1s spectrum (Figure S10). However, the contributions around 531-532eV are not exclusively due to LiOH and Li2CO3. The surface of the NMC532 powder also includes hydroxide groups bonded to transition metals, even in the pristine powder P0. Indeed simulations suggested that covalent bonding between adsorbed water molecules and transition metals happened at the surfaces of the crystal having TM-O broken bonds50. Considering the relative intensities of the Ni 2p, Co 2p and Mn 2p photoemission peaks as well as the intensity of the lattice oxygen peak at 529.1 eV in the O 1s spectrum, the ratio Olattice / TM should be close to 2 in theory. However, this ratio is 1.4 for the pristine sample P0, and 0.9 for the sample P1 immersed in water, suggesting that transition metal hydroxides might have precipitated at the surface of the NMC after immersion in water followed by solvent evaporation. Regarding the sample P2 immersed in acidified water, XPS analysis showed that the surface species were mainly composed of phosphate that have reacted with transition metals and lithium. The large presence of paramagnetic transition metals in the surface layer might decrease the signal intensity of 7Li in diamagnetic phase, and the amount of surface lithium on sample P2 measured by NMR might be underestimated. As a matter of fact, the amount of surface lithium measured by NMR for the samples P1 and P2 is respectively 0.14 mmol/gNMC and 0.18 mmol/gNMC, which corresponds to 1.2% and 1.6% of total lithium. By comparison to the dissolved lithium amount measured by ICP, the 1.6% value measured by NMR is indeed lower than the 2.3% dissolution measured by ICP for the sample P2.


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Electrochemical performance of water-based electrodes In order to assess the effect of surface modifications on electrochemical performance, electrodes P0H2O and P0H3PO4 were prepared by processing the pristine powder P0 in aqueous slurry, acidified with H3PO4 in the case of electrode P0H3PO4. NMP-based electrode P0NMP was also prepared for comparison. These electrodes were assembled in half and full cells, using lithium and graphite as anode, respectively. The cells were galvanostatically cycled at 0.1 C-rate for the first two cycles. The specific capacity at these cycles is given in Table S1 and the corresponding voltage profiles are shown in Figure 6. In the case of the half-cells (Figure 6a,c), the initial charge capacity is higher for P0NMP (195 mAh/g) than P0H2O (188 mAh/g) and P0H3PO4 (185 mAh/g), yet the difference between the three electrodes in the following discharge capacity is less striking. This is consistent with the fact that the active material in the water-based electrodes P0H2O and P0H3PO4 contains less lithium than in P0NMP since some of the lithium has been leached out during the aqueous process. Yet, it seems that the active material in P0H2O and P0H3PO4 can be relithiated to some extent at slow discharge rate since these electrodes can deliver a discharge capacity similar to P0NMP. We correlated the gap between the first charge capacity of P0H2O, P0H3PO4 and P0NMP with the delithiation of the active material during the aqueous slurry preparation. Indeed, the amount of dissolved lithium measured by ICP corresponds to a capacity loss of 3 mAh/g and 7 mAh/g for P0H2O and P0H3PO4 respectively, which is in accordance with the 1st charge capacities reported in Table S1. The voltage profile of the electrodes P0H2O, P0H3PO4 and P0NMP assembled in full-cells is different from the half cells (Figure 6b,d). Nevertheless, the electrodes deliver roughly the same capacity upon delithiation as in the half-cell configuration. The ~15-16% capacity loss in the first cycle of the full-cells is explained by the irreversible consumption of lithium at the surface of the graphite



anode to form the SEI. In the following discharge of these full-cells, the water-based electrodes are not relithiated in the same way as in the half-cells since the graphite anode cannot provide additional lithium, so there is a larger gap between the discharge capacities of the water-based and NMP-based cathodes.

Figure 6: Voltage profile at 0.1 C of electrodes P0NMP (black dotted line), P0H2O (blue solid line) and P0H3PO4 (green dashed line) in half cell configuration (a) first cycle, (c) second cycle, and in full cell configuration (b) first cycle, and (d) second cycle. Regardless of the anode, the cells containing water-based electrodes are charged at higher voltage than the cells containing P0NMP, reflecting higher polarization (Figure 6a,b). This polarization during the first charge is caused by the surface modification of the active material upon exposure to water or acidified water, as it will be further discussed later. Cationic disordering and formation of surface compounds evidenced by XPS impede lithium conductivity. This polarization is decreased in the second cycle, indicating that the resistive surface layer is partially dissolved in the electrolyte or decomposed. Grenier et al.52 monitored the evolution of CO2 during


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the charge of NCA electrode covered with Li2CO3 layer, and they suggested that the small spikes in the CV curves were due to gas bubbles forming and leaving the surface of the electrode. Such gas evolution probably also happens during the first charge of the water-based electrodes P0H2O and P0H3PO4 as the differential capacity plot presented similar spikes (see Figure S11). As a result of this high polarization, the water-based cathodes deliver on average a slightly lower discharge capacity than the NMP-based cathodes during the first cycles. Moreover, the voltage profiles of the water-based electrodes assembled in half-cells (Figure 6a) show a voltage peak at the very beginning of the first charge. This peak is more intense for P0H3PO4 than P0H2O, and is absent in the voltage profile of P0NMP. However, this feature completely disappears from all voltage profiles in the subsequent cycle (Figure 6c). Similar observations have been reported for NMC electrodes aged in air16,17,45,30. This initial voltage peak is generally attributed to insulating species formed at the surface of the NMC particles upon aging. However, this peak does not appear in the first charge of the full-cells (Figure 6b). Thus, the initial voltage peak is not caused by a reaction at the cathode, but most likely by the interaction of surface compounds formed on the water-based cathode with lithium foil. With graphite electrode, reactive surface area is larger. The compounds formed on the water-based cathode can be diluted at the graphite electrode surface.



Figure 7: Specific capacity and coulombic efficiency of a) electrodes P0NMP, P0H2O and P0H3PO4 cycling at 0.5 C-rate against a lithium anode and (b) electrodes P0NMP, P1NMP and P2NMP cycling at 0.5 C-rate against a graphite anode. The plots are not always smooth due to temperature variation during cycling. The cycling performance of the water-based electrodes P0H2O and P0H3PO4 and the NMP-based electrode P0NMP in half-cell configuration at 0.5 C discharge rate is displayed in Figure 7a. The initial capacities at 0.5 C are lower than at 0.1 C (cf. Table S1), especially for the electrode P0H3PO4 which showed the highest polarization. However, this electrode exhibits the best capacity retention so that it outperforms the electrode P0NMP after 200 cycles. The cathode processed in water without acidification (P0H2O) has the lowest capacity retention. The cells containing P0H2O and P0H3PO4 exhibit significantly different long-term cycling behavior, although the only difference in the cathode preparation relies on the addition of phosphoric acid in the slurry for P0H3PO4. The voltage profiles and capacity plots in Figure 6 and Figure 7 display only the cycling behavior of one cell of each type, but they are statistically representative of the general trend observed on tens of cells. The most obvious reason that would explain the different cycling behavior between P0H2O and P0H3PO4 is that the water-based electrode prepared without phosphoric acid suffers from corrosion of the aluminum collector. Indeed, NMC aqueous slurries have an alkaline pH due to the presence of alkaline species such as LiOH and Li2CO3. This high pH causes the degradation of the oxide passive film on the aluminum surface when the slurry is coated onto the collector, and corrosion pits appear around the intermetallic particles present in the aluminum alloy20. The corrosion of the aluminum collector is easily detected with the observation of pits in the electrode that are created upon release of H2 that is a product of this corrosion reaction (see Figure S12). Other corrosion products — namely aluminum hydroxides — can hinder the electrical contact between the


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collector and the NMC electrode, and also interact with the active material itself, causing poor performance51. Here, the sharp drop of capacity responsible for the end of life of the cell containing P0H2O is due to the SEI on the lithium anode that has become so thick that it impedes the removal of lithium ions during the discharge. Indeed, after opening the cell we observed that the lithium foil was altered, and it was unable to deliver any capacity when cycled against a fresh cathode. On the other hand, the cathode P0H2O could still deliver 115 mAh/g when cycled at 0.5 C against a fresh lithium anode (see Figure S13). In order to get rid of the detrimental impact of the aluminum corrosion and only see the effect that the aqueous process has on the intrinsic NMC electrochemical performance, NMP-based electrodes were prepared from the powders P1 and P2, soaked 1 hour in water and acidified water, respectively, and recovered after water evaporation. These electrodes, named P1NMP and P2NMP, are cycled at 0.1 C for the first cycle (Figure S14) then at 0.5 C against a graphite anode (Figure 7b). Their electrochemical performances are compared with P0NMP. The electrodes P1NMP and P2NMP in Figure S14a,b show a polarization during the first delithiation similar to P0H2O and P0H3PO4 in Figure 6a,b compared to P0NMP. This result confirms that the polarization of the water-based electrodes in Figure 6 is mainly due to the modification of the NMC surface upon exposure to water and acidified water rather than the electrode composition since a polarization is observed for the electrodes P1NMP and P2NMP that have the same composition as P0NMP. Besides, as observed with the water-based electrodes in Figure 7a, the electrode P2NMP has the best capacity retention, and P1NMP displays the higher capacity loss after 250 cycles. In those conditions, it is clear that the capacity loss of the electrode P1NMP is not due to corrosion of the collector or interaction of impurities with the lithium anode. It rather reveals the effect of the



modification of the NMC532 particles upon exposition to water. Interestingly, the evolution of the NMC532 particles in water acidified with H3PO4 (powder P2) has a distinct impact on the electrochemical performance of electrode P2NMP than P1 has on the electrode P1NMP. These results indicate that the addition of phosphoric acid in a water-based slurry does not only prevent corrosion of the aluminum collector. H3PO4 also interacts with the active material in a way that has a beneficial impact on the capacity retention. Indeed, phosphoric acid reacts with the hydroxide and carbonate compounds, reducing the amount of these impurities, and it seems that the layer of lithium and transition metal phosphates that is formed instead at the surface of the active material during the aqueous process in acidic conditions stabilizes the electrode/electrolyte interface, thus enhancing the cycling stability. This observation is in accordance with other reports in which NMC powder was immersed in H3PO4 solution in order to improve the electrochemical performance through the formation of Li3PO4 surface compounds53,54. In the latter study54, the authors showed that the H3PO4 treatment drastically enhanced the specific capacity of the NMC532, which is not in agreement with our findings. More generally, we can find in the literature examples of waterbased or NMP-based NMC532 electrodes with slightly higher capacity55,56 than shown in Figure 7 as the cycling conditions chosen affect greatly the performance. Yet the results presented here demonstrate that the addition of phosphoric acid in the slurry improves the cycling stability of the water-based electrode P0H3PO4 so that it compensates for the initial capacity loss after some cycles compared to a standard electrode prepared with PVDF (P0NMP). CONCLUSION To better understand the phenomena occurring upon aqueous processing of LiNi0.5Mn0.3Co0.2O2 cathode material, the NMC532 powder was examined before and after immersion in water, acidified or not with H3PO4 (1wt% NMC). This study shows that the aqueous processing modifies


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the surface of the NMC532 particles. As lithium ions are leached in the aqueous solution, transition metals move to occupy the lithium sites and a layer of cation mixing is observed at the surface. LiOH and Li2CO3 are found at the surface of NMC particles that have been exposed to ambient air. Here we show that 1h immersion of the NMC532 powder in water leads to the formation of larger amount of LiOH and Li2CO3 than 1 year exposition to ambient air. In the presence of H3PO4, even more lithium (~2.3%) is extracted from the bulk but the amount of LiOH and Li2CO3 is reduced owing to the formation of lithium and transition metal phosphates instead. The addition of small amount of phosphoric acid is effective in inhibiting the corrosion of the aluminum current collector that occurs during water-based processing of NMC cathodes. The water-based cathode containing 1% H3PO4 shows very good cycling stability compared to conventional NMP-based electrode, although it has a lower initial capacity due to the loss of lithium and higher polarization resulting from the surface transformation in the aqueous process. This good capacity retention is accounted for by the phosphate compounds that are formed at the surface of the active material following the addition of phosphoric acid into the slurry, and that seem to stabilize the electrode/electrolyte interphase. Mitigating the surface reactivity of the active material with water as well as the corrosion of the aluminum collector is crucial for the development of water-based NMC cathodes. In order to design new strategies towards the implementation of aqueous processing of high capacity cathode material, a better understanding of the phenomena occurring during water immersion is helpful. The use of phosphoric acid as an additive addresses some of the issues arising from the aqueous processing of NMC532 electrodes and it should be considered for further developments of water-based formulations. SUPPORTING INFORMATION



HRSTEM-HAADF image of sample P2; STEM-BF images of samples P1 and P2; STEM-EELS carbon mapping of sample P2; STEM-EELS analysis of sample P1; Hydroxide and carbonate content measured by pH titration on aqueous filtrate after immersion of NMC powder for different time and at different temperatures; C 1s XPS spectra of samples P0, P1, P2 and P3; P 2p XPS spectrum of sample P2; TEM-HAADF image and EDX mapping of sample P2; 1H NMR spectra of samples P0, P1, P2 and P3; Li 1s spectra of samples P0, P1 and P2; Charge and discharge capacities of the two first cycles at 0.1C of electrodes P0NMP, P0H2O and P0H3PO4, in half and full cell configuration; Differential capacity plot at 0.1C of electrode P0H2O in half-cell configuration and corresponding voltage profile; Image of the surface of electrodes P0H3PO4 and P0H2O; Capacity retention of electrode P0H2O cycling against a fresh lithium anode; Voltage profile of the first cycle at 0.1 C of electrodes P0NMP, P1NMP and P2NMP in half and full cell configuration. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful to F. Le Cras for helpful discussion on the article content. REFERENCES


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