Revealing the Dual Surface Reactions on a HE-NCM Li-Ion Battery

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Revealing the Dual Surface Reaction on a HE-NCM Li-ion Battery Cathode and their Impact on the Counter Electrode Daniela Leanza, Carlos A. F. Vaz, Georgian Melinte, Xiaoke Mu, Petr Novák, and Mario El Kazzi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19511 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Revealing the Dual Surface Reaction on a HE-NCM Li-ion Battery Cathode and their Impact on the Counter Electrode Daniela Leanza§, Carlos A. F. Vaz†, Georgian Melinte#, Xiaoke Mu#, Petr Novák§, Mario El Kazzi§* §

Paul Scherrer Institute, Electrochemistry Laboratory, CH-5232 Villigen PSI, Switzerland

†

Paul Scherrer Institute, Swiss Light Source, CH-5232 Villigen PSI, Switzerland

#

Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Hermann-von-Helmholtz

Platz 1, 76344, Eggenstein-Leopoldshafen, Germany

KEYWORDS: Li-rich cathode (HE-NCM), Li4Ti5O12, XPEEM, soft X-ray absorption spectroscopy, cathode-anode cross-talk.

ACS Paragon Plus Environment

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ABSTRACT

The understanding of surface reactions at the electrode-electrolyte interfaces has been a longstanding challenge in Li-ion batteries. X-ray photoemission electron microscopy (XPEEM) is used to throw light into disputed aspects of the high-energy Li-rich Li1+x(NiaCobMn1-a-b)1-xO2 (HENCM) surface reactivity, cycled in aprotic electrolyte against Li4Ti5O12 (LTO). Despite the highly oxidative potential of 5.1 V vs. Li+/Li, there is no formation of oxidized electrolyte by-products layer on any of the cathode particles; instead, a homogeneous organic-inorganic layer builds-up across the particles of the LTO anode arising from the electrolyte and PVDF binder decomposition on HE-NCM. In addition, such layer incorporates, already from the first charge, micrometer-sized agglomerates of transition metals (TMs). The presence of TMs on the anode is explained with the instability of the detected reduced Mn, Co and Ni at the surface of HE-NCM formed mainly during delithiation. Reduced TMs are unstable and prone to be transported to the LTO, where they get further reduced to metallic-like clusters. These results demonstrate that a dual reaction takes place at the HE-NCM-electrolyte interface if subject to high potential, namely, degradation of the surface structure and decomposition of the electrolyte, affecting directly the anode surface through migration-diffusion processes.


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1. INTRODUCTION Li-ion batteries are a key contender for electric mobility, given their potential for providing long autonomy range, fast charging, stable battery life cycle, low cost per kW and safety, which are among the most important requirements for large-scale commercialization. In particular, the goal of achieving higher driving mileage has driven a large research effort towards the optimization of high-voltage positive electrodes, the target being high-energy density and high-power systems. Specifically, lithium-rich high-energy Li1+x(NiaCobMn1-a-b)1-xO2 layered oxide (hereafter referred as HE-NCM) is a material that has been found to be particularly promising to realize those goals, with its high specific charge (above 250 mAh/g)1-2 and high operating potential (up to 4.8 V vs. Li+/Li). However, HE-NCM is characterized by a complex chemical and electrochemical surface behavior that is still neither fully understood nor, by far, optimized for cycling in standard carbonate-based

electrolyte

solutions.

The

higher

specific

charge

of

HE-NCM is attained on the first charge (~ 300 mAh/g) and arises from an irreversible process occurring when Li ions are extracted concomitantly with the oxygen, giving rise to the characteristic extended voltage plateau above 4.4 V.3-5 Simultaneously, on the same plateau, O2n-/O2- redox couple is reported to be reversibly active in the bulk of the material and contributes, together with cation-based redox couples (i.e. from the transition metals), to the enhancement of the specific charge.6-7 The consequent oxygen loss, which primarily involves the cathode surface, results in a structure with a large number of vacancies, which drives the migration of transition metals (TMs) towards the neighboring Li layers. The resulting effect is an inevitable reorganization of the surface crystal structure of HE-NCM, with the widely accepted transition towards a disordered spinel and/or rock salt structure8-13 and is intensified at higher cut-off potentials, especially above 4.6 V vs. Li+/Li.14 This spontaneous transition continues with further


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cycling and causes a dual drop of both the operation voltage and of the specific charge, thus adversely impacting the overall energy density and undermining the original advantage of the Lirich layered structure.14-18 In contrast to the pristine electrode, the surface structural change of the HE-NCM after oxygen release and Li-ions removal is reported to contain reduced TMs, which are more prone to be dissolved into the electrolyte and migrate-diffuse to the anode counterpart.19-22 Besides surface structural deterioration, the high operating voltage and the release of oxygen lead to the oxidation of the carbonate-based electrolyte, with the consequent gas release (e.g. CO2)23-26 and possible formation of an electrolyte by-products layer. On the other hand, below 3 V, the formation and deposition of a carbonate-rich layer and oxygen-containing species was also reported during discharge of a similar layered cathode (Li1.2Ni0.13Co0.13Mn0.54O2) as a result of the reduction of oxygen molecules.10, 27-28 Moreover, on a comparable oxide family with a different stoichiometry LiNi0.4Mn0.4Co0.18Ti0.02O2, an undefined "complex" organic matrix rich in LiF was found on discharged samples, suggested to be partially responsible for the cathode impedance rise.29 Considering all the instantaneous (electro-) chemical reactions that can occur at the interface between the electrolyte and the different components of the electrode, we find that the full picture of the degradation mechanisms of the cathode and their impact on the anode counterpart is still not fully elucidated. Indeed, the discrepancies and uncertainties found in the literature are mainly related to a lack of studies that combine a simultaneous surface investigation of the electrolyte decomposition and the structural degradation on both cathode and anode. In addition, the challenge of studying the confined nature of the electrode-electrolyte interface with a poorly spatiallyresolved surface technique makes it difficult to disentangle between possible preferential reactions occurring on each electrode component and to determine their chemical surface morphology. In this study, we aim to revisit this topic in order to elucidate at the cathode side (i) the surface


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structural damage of the HE-NCM particles when are cycled at high voltage, by monitoring the oxidation states of the TMs at the surface, (ii) the impact of the high voltage on the electrolyte stability and the evolution of the organic-inorganic species across the HE-NCM electrode particles. On the anode side we intend to examine the possible cross-talk, e.g. migration-diffusion of organic-inorganic species and TMs formed at the cathode, together with their homogeneity, morphology and valence state across the entire anode counterpart. To achieve this fundamental understanding of the electrode-electrolyte interactions we employ X-ray photoemission electron microscopy (XPEEM), an advanced synchrotron-based surfacesensitive technique with a high lateral-resolution (better than 70 nm) to obtain simultaneous information on the electrolyte by-product species and oxidation-reduction states of TMs on individual particles of the cathode and anode. With this approach we are able to show the existence of a dual side reaction at the HE-NCM electrode surface, namely, degradation and decomposition of both (i) the HE-NCM particles surface and (ii) electrolyte, respectively. To clarify such processes, we carried out a systematic study of the chemical and electronic state evolution of HENCM when coupled with Li4Ti5O12 (LTO) as a negative electrode. Investigating the surface of the LTO electrode is crucial in our study to identify the possible migration-diffusion of oxidized electrolyte by-product species caused by the parasitic reactions occurring at high operating voltage at the HE-NCM-electrolyte interface. We recently demonstrated, thanks to the high lateral resolution of the XPEEM technique, that carbonate-based electrolyte reduction can indeed occur on the LTO electrode via a process that solely involves the particles of LTO active material, whereas the conductive carbon remains free from any surface layer.30 Thus the detection of surface evolution on conductive carbon particles is a direct proof of the cross-talk of organic-inorganic side reactions species formed at the HE-NCM surface. The primary goal of this study is to


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complement the mechanistic understanding of surface phenomena of a high-voltage cathode in close relation to the processes on the anode counterpart, together with an innovative experimental and technical approach to identify the detrimental effects in a full-cell configuration.

2. EXPERIMENTAL SECTION 2.1 Electrode Preparation and Cycling Conditions

Composite electrodes are prepared by casting a mixture of active materials of Li-rich Li1.17(Ni0.22Co0.12Mn0.66)0.83O2 (HE-NCM) or Li4Ti5O12 (LTO) (Johnson Matthey) with polyvinylidene fluoride binder (PVDF Kynar Flex, Solvay), and Super-C carbon (Imerys), in a ratio of 90:06:04 and 80:10:10, respectively, all suspended in N-methyl-2-pyrrolidone (Fluka) and then spread onto an aluminum foil serving as current collector. After drying the casted slurry at 80 °C overnight under dynamic vacuum, circular 13 mm electrodes were punched out and pressed at 2 tons in order to reduce the surface roughness and porosity (~50%), which also helped to improve the quality of the X-ray photoemission electron microscopy (XPEEM) images. The average secondary particle size of the HE-NCM is between 2-10 μm, while the primary LTO particles are between 1-2 μm (see SEM images Figure S1a and S1b, Supplementary Information). Based on the Brunauer–Emmett–Teller (BET) a specific surface area of 10 m2/g and 5 m2/g is estimated for HENCM and LTO respectively. Prior to cycling, both positive and negative electrodes were reheated at 120 °C in a vacuum chamber connected to an Ar-filled glovebox to remove any remaining water in the electrode. Titanium-made coin-type cells were then assembled using a glass fiber separator inserted between two Celgard 2400 separators, the latter used to avoid silicate contamination at the surface of the electrodes. The main electrolyte used in this study consists of 1M LiClO4 solution


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in a mixture of 1:1 w/w of ethylene carbonate (EC) and dimethyl carbonate (DMC), called hereafter LC30 (BASF). The choice of LC30, as a fluorine-free electrolyte, allows us to easily examine the binder PVDF stability upon cycling, being the only source of fluorine in our system and excluding the fluorine contributions from the commonly used electrolyte salt, LiPF6. The measurements performed on electrodes cycled with LC30 are also compared in some cases with electrodes cycled in LP30 electrolyte (1M LiPF6 dissolved in a mixture of 1:1 w/w of EC and DMC). Galvanostatic cycling is performed on a computer-controlled VMP system in a 3-electrode configuration cell,31 using metallic lithium as the reference electrode, allowing an accurate recording of the potential of both the positive and the negative electrode. The cutoff voltage of the full-cell (HE-NCM vs. LTO) is set between 0.95 – 3.55 V, the equivalent of 2.5 – 5.1 V versus Li+/Li, as referred to the HE-NCM. The electrode balancing of the full-cells is ~ 1.0:2.2 (HENCM:LTO) in mass, considering a specific charge of 300 mAh/g (1st charge) and 160 mAh/g for HE-NCM and LTO, respectively. The loading is ‫ ׽‬3.6 mg/cm2 for the HE-NCM electrode and ‫׽‬8 mg/cm2 for the LTO electrode, respectively. The measurements are carried out at 25 °C using a C/5 rate, by assuming an average specific charge of 250 mAhg-1 for HE-NCM. After disassembling the cells in the glovebox, the electrodes were gently washed with dimethyl carbonate and transferred to the XPEEM with a dedicated transfer chamber to avoid air exposure and surface modification.32 2.2 X-ray photoemission electron microscopy (XPEEM)

Imaging and X-ray absorption measurements are carried out at the SIM beamline installed at the Swiss Light Source (SLS), Paul Scherrer Institute (PSI), Switzerland. All the experiments are performed in ultra-high vacuum (base pressure 5×10-10 mbar), at room temperature, by collecting


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secondary photoelectrons in partial electron yield, which make this technique very surface sensitive with a probing depth analysis in the order of 3 nm. The 13 mm circular electrodes are mounted on the XPEEM sample holder as described in reference33. The upper side of the electrode is grounded with a metallic lid with a circular slit in the middle of the sample to avoid charging effect during the measurements. X-ray PEEM elemental contrast images acquired from the top surface of the electrodes are obtained by a pixel-wise division of two images recorded at the respective X-ray absorption edge and pre-edge. For example, for the HE-NCM electrodes the C K-edge is taken at 285.6 eV, the Mn L3-edge at 641.6 eV and the F K-edge at 693 eV and their pre-edges energies are taken at 278 eV, 637 eV, and 683 eV, respectively. For LTO electrodes the same energies are used for the C and F K-edges, whereas for the Ti L3-edge 458.3 eV and 454 eV are considered for the edge and pre-edge, respectively. For the lowest field of view used for the contrast images in our experiment (20 μm), a single pixel corresponds to ~ 40 nm. The absorption spectra are obtained by recording a sequence of XPEEM images at the specific energies using linearly polarized light. Excepting for the C K-edge, all spectra are normalized by the incoming flux measured from a gold mesh or from the signal from a focusing mirror. However, for the analysis of the C K-edge, gold microparticles deposited on the surface of pristine electrodes were used as a reference, in order to correct for the strong absorption at the carbon edge energies due to presence of carbon on the X-ray optics. From the contrast images one can choose a zone of interest on single particle (e.g. active material or carbon) from where the correspondent local absorption can be measured a posteriori.

3. RESULTS AND DISCUSSION 3.1 XPEEM performed on pristine HE-NCM and LTO electrodes


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We consider first the case of the HE-NCM and LTO electrodes in the pristine state. A reconstructed elemental compositional image of the pristine HE-NCM electrode obtained after dividing the respective edge and pre-edge values at the C K-edge (285.6 eV - 278 eV), Mn L-edge (641.6 eV – 637 eV) and F K-edge (693 eV - 683 eV) is shown in Figure 1a and b. By following the color legend, one can clearly notice the complementary contrast of the HE-NCM (blue) and the conductive carbon (red) areas, whereas the PVDF binder (green) follows the outline of the active material particles, suggesting a preferential coverage of the former particles. Indeed the HENCM particles appear in a cyan color, rather than in blue, due to the overlap with the binder (green). The original elemental contrast images carried out at the individual absorption edges are shown in Figure 1c, d and e. The high spatial resolution of XPEEM is confirmed by the local Xray absorption spectroscopy (XAS) spectra acquired on individual particles of the electrodes. For example, the carbon particles show a strong peak of the characteristic C=C bond towards π* and σ* unoccupied states at the C K-edge (Figure 1f). Conversely, the same absorption spectrum acquired on HE-NCM particles exhibits only a weak π* and σ* transitions (Figure 1i). Moreover one can notice two additional resonances at 288.7 eV and 290.6 eV associated with Li-alkyl carbonates and Li2CO3, as a resulting surface reaction between the Li originating from the surface of HE-NCM pristine particles with atmospheric CO2 due to adventitious exposure to air. This reaction is also-known as surface ageing process and has been observed on similar positive electrodes.34-35 Aging of the oxide layered particles is suggested in the literature as follow: Li(Ni,Mn,Co)O2 + w/2 CO2 + w/4 O2 → Li1-x(Ni,Mn,Co)O2 + w/2 Li2CO3

Reaction (a)


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We obtained similar results for the LTO pristine negative electrode. Despite the smaller size of the LTO particles of only 1-2 μm (see SEM image in Supplementary Information, Figure S1b), the discrimination between carbon, binder and active material is rather clear. The elemental map in Figure 1b, obtained from the overlap of the C K, Ti L and F K-edges and pre-edges, shows that the particles distribution on the LTO anode differs from the HE-NCM electrode. Here, the PVDF binder is spread rather between the LTO and conductive carbon particles. Also, unlike for HENCM, the absence of carbonate components at the C K-edge confirms the higher surface stability of LTO particles towards air exposure (see Supplementary Information, Figure S2). The local XAS signatures of active material, carbon and binder particles confirm that we can successfully study each component of the composite electrode with a negligible contribution from the surrounding area. Further below, we take the full advantage of the XPEEM capability to discern the (electro-) chemical surface reactivity of the HE-NCM and LTO microparticles together with the nanoparticles of conductive carbon and the PVDF binder, to monitor their surface chemical evolution and their dependency on the applied potential. In particular, XPEEM offers unique insights into both the oxidation state changes, by exploiting the electronic sensitivity of the transition metals (TMs) L-edges and the stability of the electrolyte with regard to the electrode particles, via the analysis of the carbon and oxygen K-edges, respectively. We will also examine the binder stability through the evolution of the F K-edge. The PVDF shows a very broad peak centered at 692.8 eV ascribed to the transition of F1s to C-F σ* (Figure 1k and n).


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((b) (b) b)

(a)

carbon

AM

5μm

5μm

C K-edge C

PVDF

(c)

Mn L-edge (f)

σ*

π*

F K-edge (g)

L3

σ*

(h)

L2

5μm 285

290

295

300

305

310

637

Mn

(d)

647

652

657

(i)

C=O

662

683 688 693 698 703 708 713 718

Photon energy (eV)

(j)

(k)

Li2CO3

5μm 285

290

295

300

305

310

637

Photon energy (eV)

F

642

Photon energy (eV)

Photon energy (eV)

(e)

642

647

652

657

662

Photon energy (eV)

(l)

683 688 693 698 703 708 713 718

Photon energy (eV)

(m)

(n)

5μm 285

290

295

300

305

Photon energy (eV)

310

637

642

647

652

657

Photon energy (eV)

662

683 688 693 698 703 708 713 718

Photon energy (eV)

Figure 1. A compositional colored image carried out on pristine (a) HE-NCM and (b) LTO electrodes. The electrode components are displayed in red for the conductive carbon, green for the PVDF binder and blue for the active materials (AM) for both HE-NCM and LTO. XPEEM elemental contrast images carried out on pristine HE-NCM electrode at the (c) C K, (d) Mn L and (e) F K-edges. Local XAS spectra at C K (red square), Mn L (blue circle) and F K (green triangle) -edges acquired on particles of (f-g-h) carbon, (i-j-k) HE-NCM and (l-m-n) PVDF binder.


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3.2 Cycled HE-NCM versus LTO

The voltage profile of the early stage of cycling (first cycle plus the second charge) of the HENCM versus LTO are shown in Figure 2. While the anode is cycled along its characteristic plateau at 1.55 V vs. Li+/Li at both charge and discharge, the cathode profile is changing irreversibly during the first two cycles. The HE-NCM active materials provide a specific charge of 309 mAh/g and 270 mAh/g for the first and second charge, respectively.

Specific charge (mAh/g) Potential (V)

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HE-NCM vs. Li HE-NCM vs. LTO LTO vs. Li

5 4 3 2 1 0

300 250 200 150 100 50 0 0

309 mAh/g

112 mAh/g

2

4

6

8

10

12

14

16

Time (hours) Figure 2. (a) Galvanostatic curve for the 1st cycle (1st charge and discharge) and the 2nd charge performed in a 3-electrode cell. HE-NCM is cycled vs. LTO (blue curve) with LC30 electrolyte at room temperature and using metallic Li as a reference electrode. The squares in the full-cell curve represent the potentials where XPEEM experiments were performed. On the bottom, the evolution of the HE-NCM specific charge in mAh/g over the same cycling is referred to the full-cell HE-NCM vs. LTO.


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In particular, during the 1st charge the HE-NCM shows the typical two plateaus: the first one up to 4.4 V vs. Li+/Li is commonly associated with the oxidation of Ni2+ and Co3+ to an oxidation state of +4 while the Mn remains in +4 state which accounts for 112 mAhg-1of the total specific charge (reaction b).10, 13 Li[Ni(II)Co(III)Mn(IV)]O2 →Li1-x[Ni(III/IV)Co(III/IV)Mn(IV)]O2 + xLi+ + xe-

Reaction (b)

The second plateau, which in our experiment is stopped at 5.1 V vs. Li+/Li, delivers an extra specific charge of 197 mAhg-1, typical of this Li-rich layered oxide composition and associated with the activation of Li2MnO3 phase (reaction c). Li2Mn(IV)O3 → Mn(IV)O2 + 2Li+ + O + 2e-

Reaction (c)

The profile of the voltage and the specific charge for the prolonged cycling (30 cycles) shows the progressive fading of the full-cell, with a loss of almost 50% of the initial specific charge, referred to first charge (see Supplementary Information, Figure S3). 3.3 HE-NCM surface evolution The local XAS spectra acquired on the pristine electrodes are compared to those of the electrodes cycled at different potentials during the early stages of cycling and after 30 cycles (stopped at discharge), as indicated by the squares in the full-cell voltage curve in Figure 2. The C K-edges acquired on HE-NCM electrode separately for conductive carbon and HE-NCM particles are presented in Figure 3a and b, respectively. The C K-edge allows one to track the evolution of the surface layer originated from the electrolyte oxidation process. Surprisingly, in both areas, the C K-edges do not present any additional components upon cycling. As expected, after charging the cathode to 5.1 V vs. Li+/Li, the two components detected in the pristine HE-NCM electrode at


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288.7 eV and 290.6 eV disappear, confirming the decomposition and disappearance of the Li carbonate and carbon-oxygen species from the surface of the HE-NCM particles. Even upon discharge (HE-NCM lithiation), the surface of the particles remain free of carbon-containing electrolyte by-products, in contrast to the lithium carbonate layer formation reported earlier on similar layered oxides.10, 36 We also do not detect additional components in the range between 286 - 291 eV after 30 (Figure 3a and b) or after 112 cycles (Figure S4, Supplementary Information), indicating that the surfaces of HE-NCM and of the carbon particles are not passivated by organicinorganic electrolyte species, as was also reported in recent works.29, 37 The reconstructed contrast image reported in Figure 3c confirms the absence of a detectable surface layer, as we can still distinguish clearly the HE-NCM (cyan) and carbon (red) particles, despite the prolonged cycling of 30 cycles and stopped at discharge.


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carbon

(a) C K-edge on carbon (b) C K-edge on HE-NCM σ*

π*

HE-NCM

σ*

π*

(c) 30 cycles 2 nd charge 1 st discharge

1st charge pristine C=O 285

290

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Photon energy (eV)

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685

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5μm

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(e) F K-edge on LTO MeFx

LiF

695

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(d) F K-edge on carbon MeFx

Li2CO3

700

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680

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(f) F K-edge on PVDF MeFx

LiF

695

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LiF

695

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Photon energy (eV)

Figure 3. Local C K-edge carried out on (a) carbon and (b) HE-NCM particles performed on pristine and cycled cathode at different stages of charge and discharge. (c) Reconstructed contrast image carried out on a HE-NCM electrode after 30 cycles stopped at discharged combining the C K-edge and pre-edge (285.6-278 eV) in red, Mn L-edge and pre-edge (641.6-637 eV) in cyan. Local F K-edges acquired on (d) carbon particles, (e) LTO particles and (f) PVDF binder.

The PVDF binder stability upon cycling is monitored via the F K-edge (Figure 3d, e and f). We observe that the PVDF distribution, as shown in the pristine HE-NCM, does not vary noticeably upon cycling, as the fluorine signal is always in the noise level on carbon particles and more pronounced on HE-NCM particles, confirming that the PVDF still preferentially covering the active material. The peak intensity and the shape do not evolve significantly during the early cycling, neither on carbon, nor on HE-NCM and PVDF. Only a slight intensity increase at around


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690 eV is noticed after 30 cycles on HE-NCM particles, associated with the metal fluorinated species (MeFx). It is important to mention that LiF formation is tracked from the spectral feature at high photon energy (around 703 eV), whereas the formation of MeFx is observed at low photon energy (around 690 eV).38-39 The high background signal detected at 703 eV on HE-NCM and PVDF is a signature of the presence of a thin layer of LiF already at the pristine electrode. Such observation is similarly confirmed by the F1s X-ray photoelectron spectroscopy (XPS) spectra that show a more pronounced LiF component at 685.8 eV in the pristine compared to that of the cycled electrode (see Supplementary Information, Figure S5). Based on this observation, we can confirm that the traces of LiF detected on the pristine electrode are related to a chemical reaction between the PVDF and the Li from the HE-NCM, since PVDF is preferentially covering the HE-NCM active material. The LiF layer does not evolve significantly with cycling and remains relatively thin, suggesting either the stability of the PVDF or its dissolution in the electrolyte at high potential. Similarly, the traces of MeFx detected after 30 cycles are also associated with a reaction between TMs and binder. We also investigate the impact of the salt by replacing the LiClO4 with the LiPF6 salt. The use of a fluorine-rich salt does not modify significantly the surface layer on the HE-NCM and carbon. Indeed, the local F K-edge remains weak with a negligible formation of LiF and MeFx, as in the case of the LC30, even after 65 cycles (see Supplementary Information, Figure S6). The absence of a thick organic-inorganic layer on the cathode is nonetheless surprising considering the high cycling potential of 5.1 V vs. Li+/Li, where the electrolyte is reported to be thermodynamically unstable.40-43 This result indicates that the applied high potential might rather favors the dissolution of the oxidized electrolyte by-products into the electrolyte, rather than their deposition on the cathode surface.


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(a) Mn L-edge

(b) Co L-edge

L3

L3

Mn2+/3+

Co2+

L2

(c) Ni L-edge L3

Co4+

Ni4+

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L2 30 cycles 2 nd charge 1 st discharge 1 st charge

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pristine

Ni2+ 788

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Photon energy (eV)

Figure 4. Local XAS at the (a) Mn, (b) Co and (c) Ni L-edges acquired on single particles of HE-NCM on pristine and cycled cathode at different stages of charge and discharge.

Additionally, the surface structure of the HE-NCM particles is also affected by the applied high potential, as attested by the evolution of the transition metals (TMs) L-edges. The local XAS carried out on the HE-NCM particles at the Mn, Co and Ni L-edges, shown in Figure 4a, b and c, respectively, are used to monitor the evolution of their oxidation states at the same potential values examined previously for the C K-edges. The oxidation states of the transition metals on the surface of the pristine electrode are determined to be Mn4+, Co3+ and Ni2+, respectively, as discussed above (Reaction b). After the 1st charge at 5.1 V vs. Li+/Li (purple curves), the Mn L-edge shows a modification in the spectral region to the left of the main peaks (642.8 eV and 640.8 eV), which is a signature of the presence of reduced species compared to the pristine sample.13, 44 At the same potential, on the Ni L-edge, we observe a broadening at both the L3 and L2 edges, which we interpret as evidence for a higher oxidation state compared to the pristine Ni2+. At the Co L-edge, we notice surprisingly an evolution of the L3-edge’s shoulder at 779.7 eV, as a feature of reduced cobalt, together with a shift of 0.2 eV and 0.4 eV of the main peak of the L3 and L2-edges, respectively, corresponding to Co4+.45-46 The increase of the oxidation state of the TMs is anticipated based on Reaction (b), since the measurements are performed after full delithiation of


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HE-NCM. However, the presence of reduced Mn, Ni and Co is at odds with the latter scenario and suggests that the HE-NCM surface does not follow the bulk charge compensation, as indicated by XAS measurements carried out in transmission mode.47-48 Indeed, the surface of high voltage cathode materials is expected to undergo structural changes upon cycling, as confirmed by TEM studies reported on similar materials.11-12, 49 The surface degradation is believed to be related to the Li+ extraction and oxygen release, which lead to the formation of new, presumably instable phases containing reduced layers of TMs.29 After the 1st discharge (green curves), Ni and Co Ledges resemble the pristine electrode, as expected, due to the reduction to Ni2+ and Co3+, respectively, upon Li+ intercalation (Reaction b). In this process, also the Mn participate with a reduction from Mn4+ to Mn3+, as already reported from both bulk10,

48

and surface50 X-ray

absorption spectroscopy studies on similar oxides. In our case, since we observe already in the first delithiation the presence of reduced Mn, we expect an increase of the two spectroscopic features at 640.8 eV and 642.8 eV. Instead, those two features are less pronounced, allowing us to conclude that the surface layer with reduced Mn2+/Mn3+ ions get dissolved into the electrolyte,19, 44, 51-52 with a similar process occurring for Co and Ni. On the 2nd charge (blue curves), the Ni L3-edge shows a similar evolution as for the 1st charge. In the same fashion, for the Co L3-edge both shoulders evolve at 779.5 eV and at 782.8 eV, even though the former shows a less evident variation than that after the 1st delithiation. Moreover, the energy shift to higher photon energy of the L3 and L2edges is 0.12 and 0.22 eV, respectively, only half the value at the 1st charge. At the Mn L3-edge, we see a decrease of the 642.8 eV and 640.8 eV shoulders compared to the 1st charge/1st discharge, associated with the expected continuous dissolution of the Mn2+/Mn3+ into the electrolyte. Indeed, the formation of reduced TMs at the surface during HE-NCM delithiation competes with their dissolution in the electrolyte and progresses with the number of cycles. This is confirmed by the


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spectroscopic features after long cycling that are a fingerprint of the presence of reduced Mn2+/3+, Co2+/3+and Ni2+. TEM measurements are carried out on three HE-NCM electrodes presented in Figure S7 (Supplementary Information), (a) pristine, (b) 1st discharge and (c) after 10 cycles stopped at discharge. We can clearly observe that no surface structural degradation (SSD) is present on the pristine particles, however as soon as the electrode is cycled, already after the 1st discharge, SSD is observed with a thickness of 2 nm. Interestingly, after 10 cycles the SSD is more pronounced, where we can clearly observe the TMs occupying the Li sites. The thickness of the SSD remains in the range of 2 nm even after 10 cycles which is in line with the XPEEM measurement showing the instability of such a layer containing reduced TMs and prone to dissolve in the electrolyte. 3.4 LTO surface evolution The extent of the cathode degradation is directly reflected on the LTO anode. As shown in Figure 5a and b, the C K-edges measured separately on carbon and LTO particles at the end of the 1st charge show the same components at 286.9 eV (C-OH), 288.7 eV (carbonyl C=O) and 290.6 eV (carbonate CO3-2)53-55 on both particles, associated with electrolyte decomposition by-products. The thickness of the surface layer increases with the number of cycles, as confirmed by the strong attenuation of the C=C bond peak (π* and σ* unoccupied states) after the 1st discharge. The composition of the surface layer and its thickness increase are also in line with the XPS measurements performed on the same LTO electrodes and are shown in the Supplementary Information Figure S9 and S10. The O K-edge (see Supplementary Information, Figure S8) shows the same trend with a strong attenuation of the O1s line related to the electronic transition between the O1s to the O2p orbitals hybridized with the Ti3d.56 This result is quite surprising if one considers


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that, when LTO is cycled against LiFePO4 (upper cut-off potential 4 V vs. Li+/Li), the electrolyte reduction by-products are detected just on LTO particles, forming a thin layer (~1 nm) of organicinorganic species, whereas the carbon particles remain free from any reduced electrolyte byproducts.30 Therefore, the detection of a thicker and homogeneous layer of electrolyte decomposition by-products across the LTO electrode is the signature that an additional process is taking place and is triggered by the reactions occurring at the cathode side (i.e. cross-talk). In other words, the absence of an electrolyte oxidation by-products layer on the HE-NCM electrode at 5.1 V vs. Li+/Li, as demonstrated above, indicates their dissolution in the electrolyte, followed by their migration and/or diffusion to the anode.


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(a) C K-edge on carbon

(b) C K-edge on LTO CO3-C=O C-OH

CO3-C=O C-OH

30 cycles 2 nd charge 1 st discharge 1 st charge

σ* π* 285

290

295

300

305

Photon energy (eV)

MeFx

685

690

285

290

295

300

305

(d) F K-edge on LTO MeFx

LiF

695

pristine

Photon energy (eV)

(c) F K-edge on carbon

680

σ*

π*

700

Photon energy (eV)

705

680

685

690

(e) F K-edge on PVDF MeFx

LiF

695

700

705

Photon energy (eV)

680

685

690

LiF

695

700

705

Photon energy (eV)

Figure 5. The local C K-edges are taken on (a) carbon and (b) LTO areas on pristine and cycled LTO electrodes. Local F Kedges acquired on (c) carbon particles, (d) LTO particles and (e) PVDF binder.

Similarly, we monitored the formation of LiF on the different particles of the LTO electrode by acquiring the F K-edge (Figure 5c, d and e), which confirms its homogenous presence across the anode. The detection of LiF on LTO also directly confirms the strong decomposition and migration of the fluorine-containing binder (PVDF) at high potential at the HE-NCM-electrolyte interface, since the LC30 is a fluorine-free electrolyte. XPS measurements performed on LTO electrodes when cycled versus LiFePO4 (LFP) and HE-NCM after the 1st charge shown in Figure S9 (Supplementary Information) corroborate the previous observation. Indeed, after cycling LTO


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versus LFP, the F1s core level shows slight increase of the LiF component while no intensity increase is observed for the CF2 component. However, when LTO is cycled versus HE-NCM, the LiF component in the F1s increases significantly as well as the CF2 component. We can also observe the intensity increase of the two components CF2 and CH2 associated with the PVDF in the C1s peak. The distribution of the organic-inorganic layer across the LTO electrode can be monitored also from the reconstructed elemental contrast image in Figure 6a, carried out at the carbonate CO3-2 component at 290.6 eV in the C K-edge (red), LiF component in the F K-edge (green) and at the Ti L-edge (blue). The image shows clearly the predominance of LiF species after the 1st charge with the carbonated species covering entirely the LTO particles that appear in magenta (i.e. it derives from the overlap of the blue from the titanium with the red representing the carbon absorption edges). However, after 30 cycles, Figure 6b, the composition of the surface layer changes and the carbonates become predominant in the outermost layer, as previously reported.5758

(a)

(b) Carbonate LiF LTO

5μm

5μm

Figure 6. Reconstructed contrast images using the carbonate component at 290 eV of the C K-edge (red), LiF component at F Kedge (green) and Ti L-edge from LTO (blue) acquired after (a) 1st charge and (b) 30 cycles.

In the cross-talk process between the cathode and anode also TMs are involved. The survey scans in the energy range of 500 - 900 eV acquired on carbon and LTO particles for the 1st charge and


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1st discharge, are reported in the Supplementary Information, in Figure S11. Together with the O and F K-edges signals arising from the LTO particles and PVDF binder, respectively, we witness traces of Mn, Co and Ni that come from the HE-NCM cathode. The presence of TMs on the anode after the first cycle versus HE-NCM was already reported in our group by performing ICP measurement on the graphite electrode, as well as, their quantification compared to long cycling electrodes.59 From the contrast image taken at the absorption edge and pre-edge of Mn L-edge (Figure 7a), one can clearly distinguish TMs agglomerates (bright spots highlighted with a yellow circle). As for nickel and cobalt, they are distributed homogeneously across the LTO, despite being also agglomerated in the Mn clusters. We correlate the detection of TMs on LTO at the 1st charge to the presence of reduced Mn2+/3+, Co2+/3+, Ni2+ observed on HE-NCM at 5.1 V vs. Li+/Li (Figure 4). This behavior confirms that two simultaneous reactions occur at the surface of the HE-NCM at high voltage: (i) the surface structural change generates reduced TMs, and competes with (ii) the simultaneous dissolution of TMs into the electrolyte, followed by their transport to the anode. Indeed, during the discharge, the surface structural modification of HE-NCM is less favorable and no reduced TMs are formed, as confirmed by the less pronounced Mn2+ and Co2+ (Figure 4a and b). The negligible TMs dissolution during the discharge is also supported by the unchanged TMs intensities between the 1st charge and discharge on LTO (see Supplementary Information,Figure S11). The oxidation state of the TMs present at the LTO is monitored through the Mn, Co and Ni L-edges presented in Figure 7c, d and e. The XAS spectra are compared to those on pristine HE-NCM (black dotted lines), used as a reference. After the 2nd charge, the Mn L-edge shows the main absorption at 643.9 eV together with the two features at low photon energies (642.8 - 640.8 eV), attesting the presence of mixed states of 3+/4+.39, 60 Similarly, the Co L3-edge shows a


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predominance of a 2+ oxidation state, considering the correspondent absorption at 779.2 eV and the broadening at the L2-edge. On the other hand, Ni exhibits at both the L3 and L2-edges the featureless absorption edge of the metallic state, however we cannot exclude the presence of Ni in 2+ state which can overlap with metallic component.46 Such oxidation states are not expected, considering that the insertion potential of LTO (1.55 V vs. Li+/Li) is already sufficiently low to reduce all the TMs to a metallic state.21 Taking into account the presence of organic-inorganic species covering the LTO surface, we can assume that the TMs are in the form of carbonated agglomerates, although is difficult to exclude the formation of oxides or other compounds (e.g. fluorides). Significantly, with further cycling, the intensity of the TMs signals increases and shows different valences. Already from the elemental map in Figure 7b it is evident that both the number and the size of TMs clusters increases up to 1 μm.


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(a)

(b)

30 cycles LTO 2nd charge LTO 5 μm

Pristine HE-NMC

(c)

(d) Mn3+ Mn2+

Mn4+

L2

L3

Ni2+,Ni0

Co3+

L2

L3

Mn0

639

(e) Co0

L2

L3

Co2+

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649

654

Photon energy (eV)

659

778

783

788

793

Photon energy (eV)

798

853

858

863

868

873

Photon energy (eV)

Figure 7. XPEEM contrast images acquired at the Mn L-edge on LTO electrode after (a) the first charge and (b) 30 cycles vs. HENCM. The correspondent local XAS are taken on the transition metal clusters at the (c) Mn, (d) Co and (e) Ni L-edges on LTO electrodes upon the second charge (blue curves) and 30 cycles (red curves) and compared with the spectra acquired on HE-NCM particles from the pristine cathode.

The correspondent XAS of Mn, Co and Ni after 30 cycles (Figure 7c, d and e) reveals that these clusters are characterized by a lower valence state compared to the early stage of cycling. Specifically, the Mn shows a mixed +2/+3 state, whereas Co evolves towards the featureless edges of the metallic state and Ni remains metallic-like with more pronounced broadening at the L3 and L2 edges. Widely varied values for the oxidation state of TMs on LTO or graphite have been reported in the literature19, 22, 60-65 and there are indeed some unavoidable reactions that can alter the TMs oxidation states. We suggest that the agglomeration of the TMs in clusters indicates that,


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soluble TM species precipitate at the anode, possibly as carbonates, in the vicinity of nucleation sites. These species can reduce to a metallic state over time, due to the low potential of the anode (1.55 V vs. Li+/Li). However, when the cell is stopped and disassembled, we cannot exclude a reaction with the local environment (i.e. electrolyte and/or the organic-inorganic species of the surface layer) which can oxidize the TMs again. The behavior of the XAS spectra supports this scenario. During the first cycle, the amount of TMs is low and they expose a larger surface area to the local environment, which explains the presence of Mn in +3/+4, Co in +2/+3 and possible Ni in +2 oxidation state. However, upon long cycling, the formation of detected TMs clusters protects the metallic core from the interaction with the external environment, leading to more reduced oxidation states of Mn, Co and Ni, as observed experimentally. We also notice the same behavior in terms of cluster formation and TMs valence states after changing the electrolyte for LP30 (see Supplementary Information, Figure S12). The only striking difference is related to a more pronounced Mn2+ signature, due to the formation of MnF2, as a consequence of the high Fcontaining surface layer, as confirmed from the elemental contrast map shown in the Supplementary Information, Figure S12

4 CONCLUSION

In this work, we overcome the longstanding challenge of studying the surface of Li-ion battery electrodes at the single particles level, without changing their working environment. Thanks to XPEEM spectro-microscopy the surface chemical and morphological evolution of the active material, conductive carbon and binder are separately investigated upon cycling for both HE-NCM and LTO electrodes, enabling us to obtain unique insights into the complex reaction mechanisms taking place at the surface of the cycled Li-ion battery electrodes. The cross-talk


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between the two electrodes is also examined and confirmed for both TMs and organic-inorganic species. Despite the high operating oxidative potential of the HE-NCM electrode (5.1 V vs. Li+/Li), we find that the surface of both HE-NCM and conductive carbon particles are free of oxidized electrolyte by-products. Simultaneously, we demonstrate that decomposed species from the PVDF degradation and electrolyte oxidation can migrate or diffuse to the LTO counter electrode. Additionally, we detect Mn, Co and Ni in +2/+3 oxidation states at the end of the charge (delithiation) on the surface of HE-NCM. The formation of an unstable reduced TMs layer at high potentials, caused by the surface structural modification, is detected when charging (delithiating) the cathode. Such layer dissolves into the electrolyte and is subsequently transported to the LTO anode. The XPEEM chemical contrast images on LTO show that these TMs form micrometersized clusters already from the first charge. Their oxidation states at 1.55 V vs. Li+/Li are found to be +3/+4 for Mn, +2/+3 for Co and +2 for Ni in the early stage of cycling. However, they are further reduced after long cycling, to +2/+3 for Mn and to metallic-like for Co and Ni. We conclude that the two sets of surface parasitic reactions on HE-NCM need to be carefully investigated in the future to determine their direct impact on the overall fading of the cell.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †, §These authors contributed equally.


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Funding Sources The Swiss National Science Foundation is also thanked for financial support (Project 200021_156597).

ACKNOWLEDGMENT This work was performed at the Surface/Interface: Microscopy (SIM) beamline of the Swiss Light Source (SLS), Paul Scherrer Institute (PSI), Villigen, Switzerland. Transmission electron microscopy tests were performed at Karlsruhe Nano Micro Facility (KNMF), Karlsruhe Institute of Technology (KIT).

ASSOCIATED CONTENT Supporting Information. SEM images of pristine HE-NCM and LTO electrodes; XPEEM images and local absorption spectra on carbon, active material and binder for pristine HE-NCM electrode; XPEEM images and local absorption spectra on carbon, active material and binder for pristine LTO electrode; Voltage profile vs. specific charge of the HE-NCM vs. LTO of the fullcell; Local F K-edges on binder, carbon and HE-NCM particles for pristine and cycled HE-NCM electrodes; XPS F1s core level on pristine and cycled HE-NCM electrode; XPEEM contrast image and local F K-edge on a HE-NCM electrode cycled in LP30; TEM measurements performed on pristine HE-NCM particles and cycled particles after 1st discharge and 10 cycles; O K-edges XAS carried out locally on carbon and LTO particles; Local F K-edges on binder, carbon and LTO particles for pristine and cycled LTO electrodes; XPS measurements performed on LTO pristine electrode and LTO cycled vs. LiFePO4 and HE-NCM electrodes; XPS measurements performed


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on LTO cycled vs. HE-NCM after 30 and 200 cycles; XAS survey spectra acquired on carbon and LTO particles after the 1st charge and 1st discharge; XPEEM contrast image and local XAS carried out at the Mn , Co and Ni L-edges and F K-edge on LTO electrode after 65 cycles in LP30.


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Revealing the Dual Surface Reactions on a HE-NCM Li-Ion Battery

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