In Situ Visualized Cathode Electrolyte Interphase on LiCoO2 in High

May 12, 2017 - Charging lithium ion battery cathode materials such as LiCoO2 to a higher voltage may simultaneously enhance the specific capacity and ...
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In Situ Visualized Cathode Electrolyte Interphase on LiCoO2 in High Voltage Cycling Wei Lu,*,† Jiansheng Zhang,† Jingjing Xu,† Xiaodong Wu,† and Liwei Chen*,†,‡ †

i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, P.R. China ‡ Vacuum Interconnected Nanotech Workstation, SINANO, Chinese Academy of Sciences, Suzhou 215123, P. R. China ABSTRACT: Charging lithium ion battery cathode materials such as LiCoO2 to a higher voltage may simultaneously enhance the specific capacity and average operating voltage and thus improve the energy density. However, battery cycle life is compromised in high voltage cycling due to lattice instability and undesired oxidation of electrolyte. Cathode solid-electrolyte interphase (SEI), or cathode-electrolyte interphase (CEI), in situ formed at the cathode−electrolyte interface under high voltage, is critically important in understanding the cathode degradation process and crucial in improving high voltage cycle stability. Here we present in situ atomic force microscopy (AFM) investigation of CEI on LiCoO2 at high voltage. The formation of CEI is only observed at the LiCoO2 edge plane, not at the basal plane. The thin layer of Al2O3 coating completely suppresses the formation of CEI at the edge planes, and is shown to significantly improve coin cell high voltage cycle stability. KEYWORDS: lithium ion battery, lithium cobalt oxide (LiCoO2), solid-electrolyte interphase (SEI), cathode electrolyte interphase (CEI), in situ atomic force microscopy

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Li+.9−11 The electrolyte decomposition products deposit on the cathode surface and form a cathode solid electrolyte interphase (SEI), or cathode electrolyte interphase (CEI) film.12 Stable and dense CEI is believed to be helpful in preventing interfacial reactions including further oxidation of the electrolyte and HF etching to the cathode material, and thus improve the high voltage cycle stability.12,13 On the other hand, unstable and loosely structured CEI is incapable of limiting interfacial reactions, so the quick increase of interfacial impedance and fast degradation of battery performance become inevitable. Therefore, understanding CEI structure and properties is crucial for high voltage performance of cathode materials such as LiCoO2. Various techniques have been used to study the cathode/ electrolyte interfacial processes at high cutoff voltage. In situ Xray absorption spectroscopy (XAS) analyses reveal that the local distortions of the LiCoO2 lattice at the surface will irreversibly extend to the bulk during overdelithiation, which then leads to the degradation of the cathode.10,11 Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) indicate that overcharging causes the progression of Co3+/Co2+ reduction starting from the surface and the chemical states of the surfaces become different from that in the interior.14 These changes at the cathode surface definitely influence the morphology, composition, and structural stability of CEI films. Raman spectroscopy shows that the thickness of the CEI film on LiCoO2 cathode gradually

INTRODUCTION Layered LiCoO2 is one of the most widely used cathode materials in rechargeable lithium ion batteries (LIBs) for portable electronic devices because of its high specific capacity, high tap density, and long cycle life.1 In practical applications, LiCoO2 is generally charged to a cutoff voltage of 4.2 V (vs Li/ Li+). This process extracts 0.5 Li+ per LiCoO2 from the lattice and delivers a reversible capacity of ∼140 mAh g−1. During the past decade, the state-of-art LiCoO2 LIB manufacturing technique has evolved to such a high level that there is little margin for improvement to satisfy the urgent demand for even higher energy density. Charging LiCoO2 above 4.2 V can extract more lithium ions from the lattice and improve the specific capacity of LiCoO2, and thus could be a feasible method to boost the energy density of LiCoO2 LIBs.2−4 However, the overextraction of Li ions from the LiCoO2 lattice leads to a series of side effects that are detrimental to the cycle stability. For example, upon repeated deep Li+ extraction, phase transition to spinel tetrahedral structure may occur, and defect structures may cause quick fading of the capacity.5 Doping the LiCoO2 lattice with Al has been reported to slow down the degradation of lattice structure.6 Processes occurring at the LiCoO2/electrolyte interface under high voltage also contribute to the capacity fading. It has been reported that Co ions near the surface may dissolve into the electrolyte due to the structural changes or the chemical attack by HF, which inevitably exists in LiPF6-containing electrolytes, and the Co loss can be significant when the voltage is higher than 4.2 V.7,8 Furthermore, the degradation of the electrolytes on LiCoO2 cathode surface has been observed at around 4.2 V vs Li/ © 2017 American Chemical Society

Received: March 10, 2017 Accepted: May 12, 2017 Published: May 12, 2017 19313

DOI: 10.1021/acsami.7b03024 ACS Appl. Mater. Interfaces 2017, 9, 19313−19318

ACS Applied Materials & Interfaces

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RESULTS AND DISCUSSION Figure 1a is the SEM (S4800, Hitachi) image of LiCoO2 microcrystals prepared via the molten salt method with regular

increases with increasing cutoff voltage from 4.2 to 4.5 V, which is accompanied by the formation of Li2O and Co3O4 on the surface.15 X-ray photoelectron spectroscopy (XPS) analyses indicate that the oxidation products at the cathode surface may transfer through the electrolyte and become incorporated within the SEI at the anode surface during electrochemical cycling.16 In situ electrochemical impedance spectroscopy (EIS) manifests that the CEI film on the LiCoO2 electrode varies by the cycling state and its resistance decreases during the charge process and increases during the discharge process.17 However, the electrochemical processes occurring at the cathode/electrolyte interface are still ambiguous due to the lack of real time characterization technique, or the low spatial resolution. Besides in situ scanning or transmission electron microscopy,18 atomic force microscopy (AFM) is another powerful tool to study SEI films at the electrode/electrolyte interface.19−29 In situ AFM has been demonstrated to show nanometer-scaled spatial resolution in investigations of electrochemical processes.24−31 Here we use the in situ AFM (Agilent 550, Agilent Technologies) technique to visualize the morphological evolution of the CEI film at the LiCoO2 cathode/electrolyte interface during high voltage cycling. Since the Li+ intercalation/extraction activity in LiCoO2 has been reported to be highly dependent on crystal facet orientation,32,33 micrometer-sized LiCoO2 crystals are prepared for crystalline surface resolved imaging to understand whether the electrochemical anisotropy is related to CEI formation and evolution. It is revealed that the interfacial reaction and CEI film formation are highly dependent on the crystal plane. The CEI films are only formed at the edge plane of LiCoO2 microcrystals and no interfacial reaction occurs at the basal plane. Furthermore, a thin Al2O3 coating layer is found to prevent the interfacial reaction and the formation of CEI at the edge plane.

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Research Article

Figure 1. (a) SEM image and (b) powder XRD pattern of LiCoO2 microcrystals prepared via the molten salt method, the inset in (a) is the equilibrium particle shape of LiCoO2 microcrystal. AFM images of (c) LiCoO2 microcrystals embedded in Al substrate, (d) basal plane, and (e) edge plane of LiCoO2 microcrystals.

shapes and smooth surfaces, and the inset of Figure 1a is the equilibrium particle shape of LiCoO2 microcrystal. The hexagonal crystal face is identified as (0001) surface according to refs 32, 33, and 35. These hexagonal surfaces of LiCoO2 are constructed by the CoO2 layer, which is practically impermeable for lithium ions, and named the basal plane. Both (01̅12) and (1014̅ ) lattice planes expose the edge of the (0001) plane, which provides a pathway for lithium ions, and are named as edge planes.35 The X-ray diffraction (XRD, D8 Advance, Bruker AXS) pattern in Figure 1b is in good agreement with LiCoO2 and no other impure phases are detected. For AFM imaging, the LiCoO2 microcrystals were embedded in an Al substrate in order to provide good electric contact and mechanical connection between LiCoO2 and the Al current collector (Figure 1c). Figure 1d shows the flat surface with clear lattice steps at the basal plane, and Figure 1e shows a much rougher surface with strip-like structures at the edge plane of LiCoO2. The LiCoO2 microcrystal electrode was fixed into a homemade liquid cell with 1.0 M LiPF6 in 1:1:1 (v/v/v) ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate electrolyte and a Li foil counter electrode. The liquid cell was placed into a homemade glovebox filled with ultrapure Ar gas, in which the charge/discharge process and AFM scan were carried out simultaneously. A syringe pump was used to feed electrolyte through the liquid cell during the measurement process to keep the volume and concentration of the electrolyte within the liquid cell constant. In situ AFM was used to investigate the evolution of CEI on the basal plane and the edge plane of LiCoO2 microcrystals during the high voltage electrochemical cycles (2.50−4.50 V vs Li/Li+). The morphology of LiCoO2 on basal plane when charging from the original state (immersed into the electrolyte) to 4.50 V is shown in Figure 2. No CEI film can be detected at all charge and discharge stages. This phenomenon suggests that the basal plane is inactive during the delithiation/lithiation processes, and no oxidation of electrolyte occurs at these surfaces. These results are also consistent with an earlier report that no CEI

EXPERIMENTAL SECTION

Preparation of LiCoO2 Microcrystals by Two-Step Molten Salt Method. First, 2 g of Co3O4, 2.2 g of Li2CO3, and 20 g of LiCl were mixed and ground. The mixture was put in an alumina crucible, heated to 850 °C at a rate of 3 °C min−1, and kept for 20 h in air. After cooling down to room temperature, the product was washed with deionized water for at least three times to remove excess lithium salts and the LiCoO2 microcrystals were obtained. Second, to obtain largersized LiCoO2 microcrystals, the as-prepared LiCoO2 microcrystals were mixed with Li2CO3 and LiCl. Then, the mixture was repeatedly heated to 850 °C and kept for 20 h in air again. After washing, filtration, and drying at 80 °C for 24 h, the LiCoO2 microcrystals with proper size were obtained. Preparation of LiCoO2 Microcrystals Electrode. LiCoO2 microcrystals were spread on two pieces of Al substrates, followed by pressing face-to-face with a pressure of 10 MPa for 1 min. Some LiCoO2 microcrystals were embedded to the Al metal. After separating the two Al substrates and blowing off the non-embedded LiCoO2 powders, LiCoO2 microcrystal electrodes were obtained for AFM measurements. ALD Coating of Al2O3 on LiCoO2 Microcrystal Cathode and Commercial LiCoO2 Powder. Two ALD cycles of Al2O3 film were grown directly onto the LiCoO2 microcrystal cathode and commercial LiCoO2 powders according to the experimental process in ref 34. The precursors utilized for Al2O3 ALD were trimethylaluminum and water. Cambridge NanoTech ALD system was used. 19314

DOI: 10.1021/acsami.7b03024 ACS Appl. Mater. Interfaces 2017, 9, 19313−19318

Research Article

ACS Applied Materials & Interfaces

a higher voltage of 4.50 V, a thin film composed of fibrillar structures is observed covering the entire edge plane (Figure 3c). Upon discharging, the fibrillar film persists to a voltage of 3.90 V (Figure 3d). When discharging to 3.00 V (Figure 3e), the film completely vanishes. To our knowledge, this is the first direct visualization of CEI on LiCoO2 in high voltage cycling. The unique morphology of CEI observed in this in situ experiment corroborates previous reports: the filamentous structure of the film is not compact, and thus could not completely coat the edge plane and block further oxidation of the electrolyte or protect the LiCoO2 from a trace amount of HF in the electrolyte;8,11,37 the disappearance of CEI when discharged to low voltages also testifies that the CEI is unstable in cycling. After the charging/discharging cycle, the parallel line structure on the edge planes is intact but the particles become greater in size (Figure 3e and f). This may be due to the formation of some inorganic residues such as LiF, Li2O, and Li2CO3 as the CEI decompose at low voltage.15,38 In order to enhance the stability in high voltage cycling, coating the cathode materials with protective layers such as metal-oxide, fluoride, or phosphate has been shown to be an effective approach.3,4,16 The coating layers are reported to suppress the formation of thick CEI films and reduce the interfacial resistance.3 Atomic layer deposition (ALD) technique is capable of producing ultrathin coating layers with high precision control of thickness.34,39 The in situ AFM imaging method was used to study the influence of Al2O3 coating layers on the electrode/electrolyte interface of LiCoO2. The entire LiCoO2 microcrystal embedded in Al foil cathode structure was subjected to two cycles of Al2O3 coating (with the thickness of around 0.2 nm) following a published ALD protocol (Cambridge NanoTech ALD, Savannah S100). Figure 4 shows the morphologies of an edge plane with Al2O3 coating

Figure 2. Morphology of the basal plane: (a) and (c) before charging; (b) and (d) after charged to 4.50 V.

film formed on (001) face of epitaxial LiCoO2 films charged to 4.0 V.32 On the contrary, the edge plane shows significant morphological evolution during the electrochemical cycle. Figure 3a shows the edge plane morphology when just submerged into electrolyte. Parallel lines with different height are observed, which are edges of the (001) layers. Many small particles are seen on top of the layer edges. It has been reported that Li2CO3 is present on the LiCoO2 surface due to the reaction of Li+ ions with CO2 and H2O.30,36 Since the LiCoO2 microcrystals had been exposed to the ambient environment, the particles observed on the LiCoO2 edge planes are likely Li 2 CO 3 . Charging to 4.25 V does not cause much morphological change (Figure 3b). When further charged to

Figure 3. AFM images of in situ monitoring of the CEI film formation and decomposition on the edge plane of LiCoO2 crystal: (a) immersed in electrolyte, (b) charged to 4.25 V, (c) charged to 4.50 V, (d) discharged to 3.90 V, (e) discharged to 3.00 V, and (f) discharged to 2.50 V. 19315

DOI: 10.1021/acsami.7b03024 ACS Appl. Mater. Interfaces 2017, 9, 19313−19318

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Combining the results on crystalline surface dependent CEI formation and the effect of extremely thin ALD coating, it can be inferred that the formation of CEI on LiCoO2 under high voltage is strongly related with the surface chemical composition. The basal plane is composed of negatively charged divalent oxide ions. After ALD deposition, Al2O3 surface is also terminated with oxide ions or hydroxyl groups, which are inert toward the oxidation of electrolyte. On the contrary, the edge plane of LiCoO2 has trivalent/tetravalence cobalt ions directly exposed at the surface at high voltage, which is capable of catalyzing the oxidation of electrolyte. This is probably why the filamentous CEI is only observed at uncoated edge plane. It also agrees with an in situ synchrotron X-ray reflectometry study, which reveals that the CEI only forms on intercalation active planes. 32 An in situ total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS) study indicates that the surface trivalent cobalt can oxidize the electrolyte and be reduced to divalent as soon as in contact with the electrolyte.11 However, the in situ AFM imaging does not discover CEI formation when LiCoO2 is immersed into electrolyte without charging. Considering the structure of the edge planes, the amount of surface cobalt ions is about 0.6 ions per nm2 of exposed edge planes. We reason that the electrolyte oxidation product resulting from such a small amount of surface cobalt ion could be too little to be visualized with AFM. However, when charged to high voltage, the cobalt could be repeatedly oxidized to Co(III)/Co(IV) and reduced by the electrolyte, essentially functioning as an electrocatalyst. Consequently, a significant amount of CEI film is formed and detected by in situ AFM.

Figure 4. AFM images of in situ monitoring of edge plane morphology after coating with two ALD cycles of Al2O3: (a) original state before immersion in electrolyte, (b) immersed in electrolyte, (c) charged to 4.50 V, and (d) discharged to 2.50 V.

cycled between 2.50 and 4.50 V. The result is strikingly different from the uncoated LiCoO2. Even when charged to 4.50 V, no obvious CEI film is detected. The edge plane morphology is nearly identical/unchanged during the entire charge/discharge cycle. This means that a coating as thin as two ALD cycles of Al2O3 is sufficient to inhibit the oxidative decomposition of the organic electrolyte and protect the LiCoO2 edge plane in high voltage cycling. Electrochemical performance of CR2025 coin cells with ALD coated commercial LiCoO2 materials verifies the speculation. As shown in Figure 5, the bare LiCoO2 cathode exhibits an initial

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CONCLUSION In summary, the in situ AFM study has revealed that CEI film with loose fibrillar structures is formed on edge plane of LiCoO2 microcrystals under high voltage, which is unstable and decomposes at low voltage. The CEI is not observed on basal planes, or on edge planes with a coating as thin as two ALD cycles of Al2O3. It is highly plausible that the cobalt ions exposed on edge planes play a critical role in catalyzing the oxidative decomposition of electrolyte. It thus points to the direction of controlling surface composition for the improvement of high voltage cycle stability of LiCoO2 and related layered cathode materials.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jingjing Xu: 0000-0002-1050-7660 Notes

Figure 5. Charge−discharge cycle (between 2.50 and 4.50 V) performances of electrodes fabricated using the bare LiCoO2 powders and LiCoO2 powders with two ALD cycles of Al2O3 coating.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding support from Ministry of Science and Technology (Grant No. 2016YFA0200703), the “Strategic Priority Research Program” of the CAS (Grant No. XDA09010600), the Natural Science Foundation of China (Grant No. 21625304, 21473242, 21273273, 21473241, 21503265), and the Collaborative Innovation Center of Suzhou Nano Science and Technology. W. L. thanks the Funding of Creative Young Scientists, CAS.

discharge capacity of 176.9 mAh g−1 when charged to 4.50 V at the current density of 150 mA g−1, but it quickly fades to 19.8 mAh g−1 at the end of the 200th cycle. The capacity decay rate from the first to the 200th cycle is about 1.09% per cycle. With two ALD cycles of Al2O3 coating, the initial discharge capacity is 174.3 mAh g−1, and the discharge capacity at the 200th cycle is drastically improved to be 123.6 mAh g−1. The average decay rate is only 0.17% per cycle. 19316

DOI: 10.1021/acsami.7b03024 ACS Appl. Mater. Interfaces 2017, 9, 19313−19318

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DOI: 10.1021/acsami.7b03024 ACS Appl. Mater. Interfaces 2017, 9, 19313−19318