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Synthesis of Ni-rich Thin Film Cathode as Model System for Lithium Ion Batteries Nathan D. Phillip, Rose E. Ruther, Xiahan Sang, Yongqiang Wang, Raymond R. Unocic, Andrew S. Westover, Claus Daniel, and Gabriel M. Veith ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01982 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019
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ACS Applied Energy Materials
Synthesis of Ni-rich Thin Film Cathode as Model System for Lithium Ion Batteries Nathan D. Phillip,1,2* Rose E. Ruther,5 Xiahan Sang,3 Yongqiang Wang,4 Raymond R. Unocic,3 Andrew Westover,2 Claus Daniel,1,5 Gabriel M. Veith2*
1 The
Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN, 37996, USA
2Chemical
3Center
Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
4
Ion Beam Materials Laboratory, Los Alamos National Laboratory, New Mexico 87545, USA
5
Energy and Environmental Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
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ABSTRACT We demonstrate a process to prepare model electrodes of the Ni-rich layered compound LiNi0.6Mn0.2Co0.2O2. These thin film cathodes are compared with the composite materials to demonstrate the system is a viable platform for isolating interfacial phenomena between the electrolyte and active material without the influence of binders and conductive additives. The appropriate choice of heterolayers was found to influence the preferential orientation of the (101) and (104) planes relative to the (003) plane of the layered R3m crystal structure, enhancing Li+ diffusion and improving electrochemical performance. The addition of a Co interlayer between the Pt current collecting layer and alumina substrate increased the (101) and (104) texturing of the 500 nm Ni-rich film and allowed cells to deliver greater than 50% of their theoretical capacity. This work provides an architecture for isolating complex mechanisms of active materials which suffer from surface reconstruction and degradation in electrochemical cells.
KEYWORDS Ni-rich electrode, magnetron sputter deposition, solid state lithium ion battery, cathode-electrolyte interface, thin film
INTRODUCTION Despite the adoption of lithium ion batteries for large scale energy storage in automotive and electric grid applications, they are yet to reach their full potential due to limited storage capacity, energy density, and lifetime.1-3 The layered rock salt (R3m crystal structure) class of cathode materials is commonly investigated in different stoichiometries of LiNixMnyCozO2, where x + y + z = 1 (NMCxyz). These materials are of commercial interest due to their demonstrated stable 2 ACS Paragon Plus Environment
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capacity up to ~160 mAh/gNMC when cycled to an upper cutoff voltage of 4.3 V vs. Li/Li+ for LiNi1/3Mn1/3Co1/3O2,3, 4 with capacity up to ~200 mAh/gNMC possible for the Ni-rich variants, such as NMC622 and NMC811, but higher upper cutoff voltages must be applied to access that capacity.5, 6 In this regime, electrolyte stability and surface reactivity are critical considerations due to active material and electrolyte degradation which can be detrimental to cell performance and lifetime.6-10 These degradation products, along with surface phase transitions from layered to spinel and rock salt-type structures,11, 12 comprise the cathode electrolyte interface (CEI), which increases cell impedance.13 Understanding this interface is essential to stabilize these materials for commercial implementation. The CEI on Ni-rich NMC can be considered as the complex layer of decomposition products from electrolyte degradation deposited on the cathode active material in contact with the electrolyte. While common electrolyte solvents such as ethylene carbonate (EC) and ethyl methyl carbonate (EMC) have a high intrinsic stability against oxidation, in practical cells with electrolyte salts such as LiPF6 these solvent molecules have a smaller stability window of 1.3-4.3 V vs. Li/Li+.1 It has been suggested that the solvent molecules coordinate with anions such as PF6- from the electrolyte salt to form complexes which can then donate an electron to the cathode surface and possibly spur nucleophilic attack by the anion.14,
15
In contrast to those claims, dissociative
adsorption of EC has recently been reported to be more energetically favorable.16 While these mechanisms are not agreed upon in literature, it is thought that the CEI on Ni-rich NMC can suppress this electrolyte degradation at higher voltages. X-ray photoelectron spectroscopy studies have detected species such as LiF, Li2CO3, RCO3, and LiPFxOy in this passivation layer,17 but this interface is perhaps disrupted due to structural evolution of Ni-rich materials at high voltages. Over-delithiation of the cathode causes progressive structural rearrangement from the layered 3 ACS Paragon Plus Environment
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structure to spinel (LiM2O4, then M3O4) and finally MO-type rock salts.18 This transition is accompanied by the loss of lattice oxygen, providing opportunities for chemical oxidation of the electrolyte, which has been detected by Gasteiger et al. using on-line electrochemical mass spectrometry (OEMS).19 The complex surface environment of the cathode material is further convoluted by the presence of conductive and binding agents typically found in a composite electrode which make it difficult to study the origin and dynamics of the CEI. The so-called “inactive” components have been investigated for their contributions to electrochemical performance and side reactions with the electrolyte. La Mantia et al. found a significant electrolyte oxidation during the first few cycles of electrodes made of carbon black when charged beyond 4.6 V vs. Li/Li+,
20
which is a region of
interest for accessing maximum capacity of Ni-rich materials such as NMC622.21 One of the challenges in determining which factors influence electrolyte decomposition is deconvoluting contributions from the active material and conductive carbon. For example, Demeaux et al. investigated carbon black/PVDF electrodes in an EC/dimethyl carbonate (DMC) 1 M LiPF6 electrolyte and found significant CEI formation on cycling and during storage at open circuit voltage (OCV).22 This suggests not only electronic but chemical contributions of the carbon black and binder electrodes to the CEI, perhaps catalyzing additional solvent decomposition reactions in composite electrodes. Membreno et. al studied the CEI on α-Li3V2(PO4)3 and found that conductive carbon formed a surface layer both spontaneously in electrolyte and electrochemically, comprised of ethers, esters, alkoxides, carboxylates, and carbonates as well as inorganic species from salt decomposition (LiF, LixPOyFz, and LixPFy). They attributed the majority of the SEI formation to the surface of the carbon due to its nanoscale size providing a significantly higher total surface area relative to that of the micron-sized active material particles. 23 This ratio has been 4 ACS Paragon Plus Environment
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measured by the Brunauer-Emmett-Teller method (BET) to find that 89% of the cathode surface area was comprised of conductive carbon (C65) in an electrode containing NMC622, C65, and PVDF in a weight ratio of 91.5/4.4/4.1.7 These contributions to electrolyte decomposition can obscure reactions between the active material and electrolyte, and so it would be prudent to develop a simplified system for studying the complex interactions between the active material and electrolyte. Thin films (
