Nanoscale Chemical Characterization of Solid-State Microbattery

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Nanoscale chemical characterization of solid state microbattery stacks by means of Auger Spectroscopy and ion-milling cross-section preparation Arnaud Uhart, Jean-Bernard Ledeuil, Brigitte Pecquenard, Frédéric Le Cras, Marina Proust, and Herve Martinez ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07270 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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ACS Applied Materials & Interfaces

Nanoscale chemical characterization of solid state microbattery stacks by means of Auger Spectroscopy and ion-milling cross-section preparation A.Uhart1, J.B. Ledeuil1*, B. Pecquenard2, F. Le Cras3,4, M. Proust5, H. Martinez1*

1

IPREM ECP − UMR CNRS 5254. Université de Pau et des Pays de l’Adour, Hélioparc Pau-Pyrénées,2 Avenue du Président Angot, 64053 Pau Cedex 9, France 2 CNRS, Université de Bordeaux, ICMCB–UPR 9048 and Bordeaux INP, 87 Avenue du Dr. Schweitzer, F33600 Pessac, France 3 CEA LETI, 17 rue des Martyrs, F-38054 Grenoble, France 4 Université Grenoble Alpes, F-38000 Grenoble, France 5 ST Microelectronics, 10 rue Thalès de Milet CS 97155, 37071, Tours, France Corresponding author: [email protected]

Abstract The current sustained demand for ‘smart’ and connected devices has induced a need for more miniaturized power sources, hence for microbatteries. Lithium-ion or ‘lithium-free’ all-solid-state thin-film batteries are adapted solutions to this issue. The capability to carry out spatially resolved chemical analysis is fundamental for the understanding of the operation in an all-solid-state micro battery. Classically cumbersome and not straightforward techniques as TEM / STEM / EELS and FIB preparation method could be used to address this issue. The challenge in this work is to make the characterization of Li based material possible by coupling ion milling cross section preparation method and AES techniques to characterize the behavior of a LiCoO2 positive electrode in an all solid state microbattery. The surface chemistry of the LiCoO2 has been studied before and after LiPON deposition. Modifications of the chemical environments characteristic of the positive electrode have been reported at different steps of the electrochemical process. An original qualitative and a semi quantitative analysis has been used in this work with the peak deconvolution method based on real, certified reference spectra to better understand the lithiation / delithiation process. This original coupling has demonstrated that a full study of the pristine, cycled and post mortem positive electrode in a microbattery is also possible. The ion milling preparation method allows having access to a large area and the resolution of Auger analysis is highly resolved in energy to separate the lithium and the cobalt signal in an accurate way.

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Keywords : All-solid-state thin-film lithium microbatteries LiCoO2 positive electrode Auger Electron Spectroscopy Cross sectional characterization Ion milling

I)

Introduction

The sustained demand for ‘smart’ and connected devices in the context of the development of the internet-of-things and increased healthcare monitoring has induced a need for more miniaturized power sources, hence for microbatteries. A complete integration of the power source in miniaturized devices comprising generally CMOS (Complementary Metal Oxide Semiconductor) components and MEMS (Microelectromechanical systems) sensors or actuators, although not always compulsory, is an asset to optimize their compactness and security. All-solid-state thin-film lithium batteries are adapted solutions to this issue since they can be manufactured with small footprints, by techniques commonly used in the microelectronics industry, i.e. physical or chemical vapor deposition of thin films and patterning methods. Allsolid-state thin film microbatteries are composed of thin layers of different materials. The latter obviously comprise the positive and negative electrode materials, the solid electrolyte and the metal current collectors, but also ionic and/or electronic insulating barriers deposited on the substrate and encapsulation layers preventing water and air to reach the active part of the cell1.The cells are manufactured through a bottom-up process, by depositing by vacuum deposition techniques each material layer on the top of the previous one, to form at the end a


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monolithic component. Finally, at least ten different thin films are synthesized, hence forming at least as many solid-solid interfaces. When designing new batteries, the selection of the active materials (electrodes and electrolyte) is generally directly driven by their intrinsic properties (capacity, electronic/ionic conductivity, operation voltage and electrochemical stability window), that are generally already known or that can be more or less easily forecast, and by their processing ability and manufacturing costs. Unfortunately, the actual performance of the resulting cells in terms of cycle life, calendar life, voltage

profile

and

impedance

is

often

less

than

expected

due

to

unforeseen

electrode/electrolyte reactions and evolution of the electrode/electrolyte interfaces. In all-solid-state batteries, such interactions can lead to both structural and chemical evolutions of the materials in the vicinity of the contact surface, and possibly to the formation of solid interphases. Reactions between the solid electrolyte and electrode materials can occur either during the manufacturing process2,3 or the operation of the cell4,5. As for microbatteries, their sequential manufacturing process by sputtering is particularly prone to induce chemical and structural modifications of the surface of the underlying material during the deposition of the following one, that may not follow thermodynamics rules (formation of metastable interphases)6. In these all-solid-state thin film stacks, it is generally preferable to limit intermixing of the components and to limit the formation of electrode/electrolyte interphase to preserve the intrinsic properties of each material. Nevertheless, in some particular cases, the formation of a proper interphase can be beneficial, as for example to decrease the charge transfer resistance7. But in any case, the generation of adequate solid-solid interfaces has to be monitored and checked by characterization means. Actually, the study of such interfaces in all-solid-state batteries was only reported in few papers, despite the large variety of material combinations that are envisaged in electrochemical cells, due to the difficulty to probe these buried interfacial regions. Techniques used to characterize 3 ACS Paragon Plus Environment


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them include imaging by Scanning Electron Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM), chemical/structural analyses by Electron Energy Loss Spectroscopy (EELS)4 or Energy dispersive Spectroscopy (EDS)5, Elastic Recoil Detection (ERD), Rutherford Back-scattering Spectroscopy (RBS)8, X-ray Absorption Near Edge Structure (XANES)9,

X-ray Photoelectron Spectroscopy (XPS)10, neutron depth profiling11, and

electrochemical

methods

(Electrochemical

Impedance

Spectroscopy

(EIS)12,13,

cyclic

voltametry14). The specific manufacturing process of planar thin film batteries by vacuum deposition techniques, which allow the deposition of very thin films with controlled thickness and chemical composition, could offer a unique opportunity to study the formation of their solid-solid interfaces by XPS spectroscopy. Indeed, despite the fact that XPS probes only a thickness up to 5 nm, it is possible to carry out depth profile analyses of thicker interfacial zones between two materials deposited successively, by using bi-layered samples which top layer has an increasing thickness. Thus, the interfacial zone can be probed from the surface of the substrate material to the core of the second material. Nevertheless, it is very likely that the interface observed this way during the early stages of its formation (< 50 nm) continues to evolve during subsequent deposition of a higher amount (a few µm) of the second material, and possibly the following ones, as it is the case in actual microbatteries. Another way envisaged to get access to ‘real’ buried interfacial regions for XPS analysis would imply time-consuming erosion of the upper material layers15, including the thick (40 µm) encapsulation layers, by argon sputtering that may leads to a modification of the composition of the material15. A third way to reveal the inner interfaces of the microbatteries is to realize a cross-section of the corresponding stack. But in that case, the limited ultimate spatial resolution of XPS would make the analysis very difficult below 1 µm, which is close or even larger than the thickness of the film cross-sections to be characterized.


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To overcome these various obstacles for the determination of the chemical characteristics of the materials inside the multi-layered microbattery at the nanoscale, we developed an original method that associates the preparation of extended cross-sections by ion-milling (crosspolisher), with highly spatially resolved quantitative Auger Electron Spectroscopy (AES) analyses. Note that the quantitative aspect of AES is challenging considering several aspects which are discussed later in the paper. As a case of study, we report here the comprehensive characterization of the LiCoO2 thin film electrode at different stages of the Li/LiPON/LiCoO2 microbattery life, using this method and complementary X-ray Photoelectron Spectroscopy analyses. At first, the surface chemistry of the LiCoO2 positive electrode prior to the LiPON and lithium deposition was carried out by means of XPS. SEM and AES analyses of the cross section of the positive electrode deposited on the microbattery substrate were achieved in order to get a reference for the pristine stack. Then, the cross section of the complete microbattery stack in the pristine state was characterized by SEM, SAM and AES to determine the modifications induced by the deposition of LiPON and lithium on the top of the LiCoO2 layer. Therefore, cycled microbatteries were analyzed using the same experimental approach.

II)

Experimental section

II.1) Micro battery device This study focuses on the characterization of EnFilmTM microbattery and electrode prototypes manufactured by STMicroelectronics Tours (France) (Figure 1 a). These thin film rechargeable lithium batteries are characterized by a 25.7 x 25.7 mm footprint (Figure 1 b) and a nominal capacity of 700 µAh delivered between 4.2 V and 3.0 V. The active part of the microbattery consists of a LiCoO2 cathode (8 µm thick), a LiPON glassy solid electrolyte (3.5-5 µm thick) and 5 ACS Paragon Plus Environment


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a lithium anode (8 µm thick). The latter are successively deposited in this order by sputtering or thermal evaporation on a mica substrate coated with adhesion layers and metal current collectors.

Figure 1: a) Picture and schematic: LiCoO2/LiPON/Li EnFilmTM microbattery stack, b) Two EnFilm microbatteries

II.2) Preparation of microbattery cross-sections by ion-milling The microbattery cross-sections are prepared with a JEOL Cross-Polisher16,17

(JEOL Ltd,

Tokyo, Japan) working under controlled atmosphere (O2

Nanoscale Chemical Characterization of Solid-State Microbattery

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