Mitigating Structural Instability of High-Energy Lithium- and

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Mitigating Structural Instability of High Energy Lithium and Manganese Rich LiNixMnyCoz Oxide by Interfacial Atomic Surface Reduction Rosy Sharma, Hadar Sclar, Eliran Evenstein, Shira Haber, Sandipan Maiti, Tali Sharabani, Michal Leskes, and Malachi Noked Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00875 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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Chemistry of Materials

Mitigating Structural Instability of High Energy Lithium and Manganese Rich LiNixMnyCoz Oxide by Interfacial Atomic Surface Reduction Rosy(1,2)*, Hadar Sclar(1), Eliran Evenstein(1,2), Shira Haber(3), Sandipan Maiti(1), Tali Sharabani(1,2), Michal Leskes(3), Malachi Noked(1,2)* (1) Department of chemistry, Bar-Ilan University, Ramat Gan, 52900, Israel (2) Bar-Ilan Institute of Nanotechnology and Advanced Materials, Ramat Gan, 52900, Israel (3) Department of Materials and Interfaces, WIS Israel [email protected] [email protected]

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Abstract Surface modification of electrode materials using chemical treatments and atomic layer deposition are 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 mAh/g), rate capabilities (~15% enhancement at rates of 1 C and 2 C), 50 – 60 mV narrow voltage hysteresis and twice faster 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 HR-TEM, XPS, and solid state NMR (ssNMR), 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.


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1. Introduction Lithium-ion batteries (LIB) 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 LIB has shifted towards powering high-end electric vehicles (EVs).1,2 To meet the increasing need of 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 towards synthesizing new cathode materials is to integrate Li2MnO3 with spinel or metal oxide structures4–6 that results in 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 the Ni,7,8 Mn,9 and Co in the lattice.10,11 The high energy Li and Mn-rich NMC (HE-NMC) material is of specific interest due to its higher specific capacity (>250 mAhg-1).12 Nevertheless, the realization of HE-NMC is hindered by some fundamental challenges.13,14 Among these challenges, there are several critical problems like 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 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 activation


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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 like 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 binders29 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 organo-metallic compounds (e.g. tri-methyl aluminum) at relatively low temperatures. We present herein a detailed electrochemical and spectroscopy study



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of ASR treated HE-NMC in LIB. We mechanistically investigate, and clearly show the effect of ASR on the electrochemical performance of HE-NMC. 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 organo-metallic compounds as optional surface treatment precursors. The self-limiting and surface directed 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. CVD) or multiple precursors and reaction steps (ALD). 2. Experimental Experimental details of material preparation and surface characterizations are provided in the supporting information (Section SI.1). 3. Results and Discussion Surface modifications of electrode materials using ALD or high temperature treatments are documented as an efficient method to enhance EMs 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 relatively simple procedure (e.g. compared to ALD), we examine herein, its effect on the electrochemical performance of the 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 HR-TEM images of the untreated and treated HE-NMC are presented in Figure 1. A ~ 3


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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 transition metals of HE-NMC due to the surface reduction, 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 2(A)) 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 S.2). Interestingly, changes in the transition metal 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 2p3/2 peak from 642.89 eV to 642.58 eV (figure 2(B)).32 The increase in peak splitting in Mn 3s spectra from ~4.46 to 5.17 eV (figure 2(C)) 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 2(D)). 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 atomic surface reduction, the surface layer is different from the conventional Al2O3 that we receive via 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 supplementary information, Fig



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S.1). 3.2 Electrochemical Investigation Figure 3 shows the voltage profile vs. capacity of the 1st and 50th cycle of the treated and untreated electrodes. Figure 3(A) clearly indicates that ASR-HE-NMC exhibited enhanced 1st 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 3(B). The treated sample exhibited approximately 25 mAhg-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 are presented in figure S-2.3 These results indicate that the surface reduction renders stability to the HE-NMC structure and helps in mitigating capacity fading. One of the most important parameters in battery cycling is their overall efficiency or roundtrip efficiency that is dictated by the capacity and the voltage difference between discharge and charge. Furthermore, the difference between the average charge and discharge voltages provides a better comparison of the changes in the Li intercalation/di-intercalation kinetics and increase in the stress/overpotential attributed to the formation of surface passivation layer resulting from the SEI formation and parasitic reactions during cycling. Consequently, Figure 4, presents the changes in the 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 5 depicts the derivative capacity dQ/dV plot for first, second (figure S-3), and 50th cycles. Following previously 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 8 ACS Paragon Plus Environment

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be involved with Li+ extraction from the TM layer and anionic oxidation of oxygen (O2−↔On-, 1

Mitigating Structural Instability of High-Energy Lithium- and

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