Mitigating Structural Instability of High-Energy Lithium- and

Article Cite This: Chem. Mater. 2019, 31, 3840−3847

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Mitigating Structural Instability of High-Energy Lithium- and Manganese-Rich LiNixMnyCoz Oxide by Interfacial Atomic Surface Reduction Rosy,*,†,‡ Hadar Sclar,† Eliran Evenstein,†,‡ Shira Haber,§ Sandipan Maiti,† Tali Sharabani,†,‡ Michal Leskes,§ and Malachi Noked*,†,‡ †

Department of Chemistry, Bar-Ilan University, Ramat Gan 5290002, Israel Bar-Ilan Institute of Nanotechnology and Advanced Materials, Ramat Gan 52900, Israel § Department of Materials and Interfaces, WIS, Rehovot 7610001, Israel

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‡

S Supporting Information *

ABSTRACT: Surface modification of electrode materials using chemical treatments and atomic layer deposition is documented as an efficient method to stabilize the lattice structure as well as to reinforce the electrode/electrolyte interface. Nevertheless, expensive instrumentation and intrinsic deterioration of the material under high-temperature conditions and aggressive chemical treatments limit their practical application. Here, we report enhanced electrochemical stability and performances by simple atomic surface reduction (ASR) treatment of Li- and Mn-rich 0.35Li2MnO3· 0.65LiNi0.35Mn0.45Co0.20O2 (HE-NMC). We provide mechanistic indications showing that ASR altered the electronic structure of surface Mn and Ni, leading to higher stability and reduced parasitic reactions. We demonstrate significant improvement in the battery performance with the proposed surface reduction, which is reflected by the enhanced capacity (290 mA h g−1), rate capabilities (∼15% enhancement at rates of 1 and 2 C), 50−60 mV narrow voltage hysteresis, and faster (twice) Li+ diffusion. Utilizing online electrochemical mass spectrometry (OEMS), we show in-operando that the reduced surface layer results in suppressed side reactions. We further characterized the surface coating with high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and solid-state NMR before and after cycling. The results presented herein address all the critical challenges associated with the complex HE-NMC material and thus provide a promising research direction for choosing relevant methodology for surface treatment. capacity (>250 mA h g−1).12 Nevertheless, the realization of HE-NMC is hindered by some fundamental challenges.13,14 Among these challenges, there are several critical problems such as voltage fading,15 large first cycle irreversible charge capacity, phase transformation,9 and poor rate capabilities.16 The capacity and voltage fading during the battery life were attributed to the irreversible phase transformation from the layered to spinel structure during the high voltage steps (>4.6 V) which give rise to lattice instability.9 The gradual proliferation of the spinel phase with every charging step eventually results in voltage fading and increasing voltage hysteresis. The capacity fading can also be ascribed to the dissolution of transition metals (TMs) into the electrolyte and their fragmentation and subsequent diffusion to tetrahedral sites which render them electrochemically unavailable.17 Furthermore, the release of O2 during the high-voltage

1. INTRODUCTION Lithium-ion batteries (LIBs) have revolutionized the portable electronics industry for over two decades and proven to be the most impressive scientific and applicative innovation of modern electrochemistry. However, in recent years, the demand for LIBs has shifted toward powering high-end electric vehicles.1,2 To meet the increasing need of the society, LIBs with high energy and power densities, and longer cycle life are required. Unfortunately, current generation cathode materials, (e.g., LiFePO4 and LiCoO2), fall short in terms of energy density for fulfilling the demands of future applications.3 An interesting approach toward synthesizing new cathode materials is to integrate Li2MnO3 with spinel or metal oxide structures4−6 that results in the LiNixMnyCoxO2 (where x + y + z = 1) compound, referred to as NMC. The NMC material exhibits promising electrochemical properties that can further be controlled by varying the proportion of Ni,7,8 Mn,9 and Co in the lattice.10,11 The high-energy Li- and Mn-rich NMC (HENMC) material is of specific interest due to its higher specific © 2019 American Chemical Society

Received: March 3, 2019 Revised: April 25, 2019 Published: April 29, 2019 3840

DOI: 10.1021/acs.chemmater.9b00875 Chem. Mater. 2019, 31, 3840−3847

Article

Chemistry of Materials

relatively simple procedure (e.g., compared to ALD), we examine herein its effect on the electrochemical performance of HE-NMC. We carefully study the electrochemical response of the treated EMs and mechanistically investigate the role and response of the new artificial cathode electrolyte interphase (CEI) formed by our ASR approach. 3.1. Surface Characterization. High-resolution transmission electron microscopy (HR-TEM) images of the untreated and treated HE-NMC are presented in Figure 1. A

activation step leads to side reactions with the electrolyte and results in parasitic reactions that affect the stability of the electrode and reduces its cycle life.18 Furthermore, the fragile and reactive interface facilitates interaction with the electrolyte and other parasitic side products such as acidic HF, resulting in deposition of a thick solid electrolyte interface (SEI) which retards the charge transfer and thus leads to cell polarization, poor rate capabilities, and inferior electrochemical performance.17 In order to mitigate the problems listed above, there is a dire need to stabilize the lattice structure as well as to reinforce the electrode/electrolyte interface. In the past years, significant efforts have been invested in improving the performance of HE-NMC.12,14,19 Several strategies were opted including surface coatings,19−22 surface reduction,23−25 low-temperature activation,13 ion doping,26,27 application of electrolyte additives,28 use of different binders,29 and many more. Among these strategies, protection of the material via surface treatment or surface coating demonstrated effectiveness in maintaining the stability of the electrode/electrolyte interface and suppressing TM dissolution while preserving the bulk structure.12 However, most of the reported methods of coating, surface reduction, or modification involve exposure to atmospheric conditions, high-temperature treatment (calcination steps > 400 °C), and use of many harmful chemicals in long, tedious procedures that inevitably lead to deterioration of the material structure.19−22 Furthermore, these procedures rarely provide accurate control over uniformity, thickness, and the morphology of the resulting surface layer. Atomic layer deposition was reported as an alternative method for accurate surface modification, yet it requires a multistep reaction with an additional inactive thin film on top of the material.30 Recently, we reported a new approach for atomic surface reduction (ASR) of oxides,31 utilizing the vapor phase of reactive organometallic compounds (e.g., trimethyl aluminum) at relatively low temperatures. We present herein a detailed electrochemical and spectroscopy study of ASR-treated HENMC in LIBs. We mechanistically investigate and clearly show the effect of ASR on the electrochemical performance of HENMC. We found that our unique approach suppresses degradation pathways that enforce chemical and morphological changes of the active material over time. This stabilization facilitates better electrochemical response and significantly improves long-term cycling. Our new approach opens a new horizon for ASR of cathodes, with various organometallic compounds as optional surface treatment precursors. The self-limiting and surfacedirected nature of our approach makes it very promising from the practical point of view, especially when compared to more demanding surface coating procedures that require higher temperatures (e.g., chemical vapor deposition) or multiple precursors and reaction steps (atomic layer deposition).

Figure 1. HR-TEM image for the (A) pristine and (B) atomic surface-reduced HE-NMC.

∼3 nm thick, conformal amorphous surface layer on the treated particle was clearly witnessed. To further study the composition of the surface film and the changes in the electronic structure of the TMs of HE-NMC due to the surface reduction, X-ray photoelectron spectroscopy (XPS) measurements were attempted. Figure 2 presents the XPS data observed for the treated and untreated sample. The presence of Al in the treated sample (Figure 2A) confirms the reaction between the surface and the TMA and ensures that our protocol resulted in surface reconstruction. It is important to mention here that the ASR is limited only to the surface (as desired) and imposed no effect on the bulk composition and crystallinity of the material as concluded from the unaltered XRD pattern (Figure S2). Interestingly, changes in the TM oxidation states were also indicated by the shifts in the binding energies for Mn 2p, Mn 3s, and Ni 2p spectra. Specifically, the reduction of manganese during surface reduction is witnessed by a slight negative shift in the binding energy of the 2p3/2 peak from 642.89 to 642.58 eV (Figure 2B).32 The increase in peak splitting in Mn 3s spectra from ∼4.46 to 5.17 eV (Figure 2C) provided further support for the reduction of Mn4+ to Mn3+.23 Similarly, a significant negative shift in Ni 2p was also observed suggesting its reduction to lower oxidation states (Figure 2D). No significant deviation was observed in the XPS spectra of cobalt. From the XPS measurements and previous reports, it can be inferred that the reduced surface layer is composed of an altered electronic structure of Mn and Ni which might help in maintaining the structural integrity of the electrode/ electrolyte interface and suppressing TM dissolution. To examine this hypothesis, the electrochemical performance of the treated and untreated material was next investigated. Furthermore, in our proposed ASR, the surface layer is different from the conventional Al2O3 that we receive via the TMA + H2O ALD process, and the composition lies in between the functionalized Al oxide to defected alumina depending on the nature and chemical environment of the surface oxygen (refer Supporting Information, Figure S1).

2. EXPERIMENTAL SECTION Experimental details of material preparation and surface characterizations are provided in the Supporting Information (Section S1).

3. RESULTS AND DISCUSSION Surface modifications of electrode materials using ALD or high-temperature treatments are documented as an efficient method to enhance EM stability and facilitate a long battery lifetime. Keeping in mind the advantages of our approach for surface modification by ASR in terms of temperature and a 3841

DOI: 10.1021/acs.chemmater.9b00875 Chem. Mater. 2019, 31, 3840−3847

Article

Chemistry of Materials

3.2. Electrochemical Investigation. Figure 3 shows the voltage profile versus capacity of the 1st and 50th cycle of the treated and untreated electrodes. Figure 3A clearly indicates that ASR-HE-NMC exhibited enhanced first discharge capacity in comparison to untreated HE-NMC. Interestingly, the improved capacity became even more significant after 50 charge−discharge cycles as presented in Figure 3B. The treated sample exhibited approximately 25 mA h g−1 (∼12%) more discharge capacity in comparison to the untreated sample. The decline in the capacity for the treated and pristine samples during 50 cycles is presented in Figure S2.3. These results indicate that the surface reduction renders stability to the HENMC structure and helps in mitigating capacity fading. One of the most important parameters in battery cycling is their overall efficiency or round-trip efficiency that is dictated by the capacity and the voltage difference between the discharge and charge. Furthermore, the difference between the average charge and discharge voltages provides a better comparison of the changes in the Li intercalation/diintercalation kinetics and increase in the stress/overpotential attributed to the formation of the surface passivation layer, resulting from the SEI formation and parasitic reactions during cycling. Consequently, Figure 4 presents the changes in the

Figure 4. Mean voltage variation for untreated (blue) and treated HENMC (red) during cycling in a half-cell. (Averaged over three cells).

mean charge voltage as a function of the cycle number for the ASR-HE-NMC. It can clearly be seen that the surface treatment resulted in lowering the average charge voltage by ∼50−70 mV. This gap was found to be stable upon cycling.

Figure 2. XPS spectra of (A) Al 2p, (B) Mn 2p, (C) Mn 3s, and (D) Ni 2p corresponding to the untreated (in black) and atomic surfacereduced HE-NMC sample (treated; in red).

Figure 3. Comparative voltage profile for the (A) 1st and (B) 50th charge−discharge of the untreated (blue) and treated (red) HE-NMC sample in a half-cell at the rate of C/15 and C/3, respectively. (Data from three cells; refer Figure S2). 3842

DOI: 10.1021/acs.chemmater.9b00875 Chem. Mater. 2019, 31, 3840−3847

Article

Chemistry of Materials

treated cathode showed a more intense peak at ∼3.2 V with a slight shift to a higher voltage. Interestingly, during later cycles (e.g., 50th cycle), the difference in peak intensity became much more pronounced. Higher intensity and clearer shapes of “1”, “2”, and “3” peaks for the treated sample highlight the electrochemical stability of the ASR sample in comparison to the untreated one. In addition, the intense peaks corresponding to the treated samples also point toward the improved kinetics of the lithiation and delithiation of the surface reduced sample. The shift of peak “3” toward higher potential in the treated sample further indicates that lithiation of treated HENMC requires lower overpotential and thus is facilitated in comparison to the untreated material. Therefore, we conclude that the surface treatment helped in stabilizing the material during cycling and resulted in improved kinetics for lithium extraction and insertion in both Li and TM layers. To further support the interpretation of dQ/dV plots, electrochemical impedance spectroscopy (EIS) studies were carried out. Figure 6 depicts a Nyquist plot of EIS data36 that was taken at an equilibrium potential of 4.0 V during charge after the 15th and 100th cycles. The values of the parameters from the fitted data (Table 1) and the corresponding equivalent circuit

Figure 5 depicts the derivative capacity dQ/dV plot for 1st, 2nd (Figure S3), and 50th cycles. Following previously

Table 1. Fitted Data for Impedance Spectra at Different Cycling Stages materials untreated HE-NMC

Figure 5. Comparison of the differential capacity versus voltage (dQ/ dV) curves obtained from voltage profiles of (A) 1st and (B) 50th cycle for the treated (red) and untreated (blue) HE-NMC sample in a half-cell.

reported works,13,23 we assigned the peaks in the dQ/dV plots to the following processes: the irreversible oxidation peak ≥4.5 V labeled as “1a” corresponds to the complex electrochemical activation of the material.33 This complex activation step is believed to be involved with Li+ extraction from the TM layer and anionic oxidation of oxygen (O2− ↔ On−, 1 < n < 2) resulting in O2 evolution from the surface.34,35 The partial migration of TMs to the Li+ layer and formation of surface MnO2 also happen due to this activation step,34,35 whereas peak “1” can be ascribed majorly to delithiation as well as Ni and Co oxidation. Peaks “2” and “3” are related to the insertion of Li+ into the TM and Li layers, respectively. It can be clearly seen from Figure 5 that during the 1st cycle, the

treated HE-NMC

cycle number

RS (Ω)

RCEI (Ω)

RCT (Ω)

RS (Ω)

RCEI (Ω)

RCT (Ω)

15th cycle 100th cycle

1.7 1.7

8.0 8.1

22.6 79.5

2.4 2.4

6.7 4.9

7.8 17

models are also shown in Figure 6. The diameter of both semicircles decreased for the TMA-treated material with respect to the semicircle of the untreated material after the 15th and 100th cycle, respectively, which strongly support a decrease in both the surface film resistance (RCEI) and the charge-transfer resistance (RCT) and is in good agreement with the dQ/dV data. Naturally, over cycling, the total resistance (RS + RCEI + RCT) for the cells with both treated and untreated electrodes increases (lithium anode is used in both cases); however, the untreated sample showed >∼3.5 factor increase in RCT compared to