Electrical and Mechanical Characterization of Li Ion Battery Electrode using PinPoint™ SSRM
John Paul Pineda, Cathy Lee, Byong Kim,* and Keibock Lee Park Systems Inc., 3040 Olcott St., Santa Clara, CA 95054
*
byong@parksystems.com
Abstract: Lithium ion batteries (LIBs) are key components of modern electronics and are regularly utilized as their primary power source. Understanding the electrical and mechanical properties of electrode materials plays a major role in the performance improvement of LIBs. In this article, we provide research using PinPoint™ scanning spreading resistance microscopy (SSRM)
to
effectively measure both electrical and mechanical properties of LIB electrode surfaces at a much higher quality in a high-vacuum environment than in ambient conditions. The data collected in this experiment demonstrate that this technique is an effective means for measuring the quantitative and qualitative topographical, electrical and mechanical data of advanced materials with improved image quality and data accuracy.
Keywords: atomic force microscopy, scanning spreading resistance microscopy, lithium ion batteries, conductance, resistance
Introduction Lithium ion batteries (LIBs) are key components of mod-
ern electronics and are regularly utilized as their primary power source [1–3]. LIBs are ubiquitous in a variety of appli- cations ranging from portable devices to electric vehicles because of their high energy density, flexible and lightweight design, low self-discharge, low cost, and long lifespan when compared to other battery technologies [4,5]. Despite these advantages, the reliability and performance of LIBs still need to be improved to meet the requirements of applications such as large-scale energy storage and hybrid electric vehicles (HEVs) [2,6,7]. Extensive research has focused on the devel- opment of four cell materials to achieve better performance: 1) positive and 2) negative electrode active materials (AM), 3) separators, and 4) electrolytes [2]. Understanding the elec- trical and mechanical properties of electrode materials plays a major role in the performance improvement of LIBs. It has been shown that improved adhesion between electrode par- ticles, electrode films, and current collectors leads to better retention of discharge capacity during cycling, especially when electrode materials exhibit faster and/or larger volume expansion [2]. Moreover, enhanced electronic conductiv- ity and ionic diffusion in electrodes also lead to LIB capac- ity improvement [7]. As devices have become more compact, optimizing electrical and mechanical properties on a nano- meter scale has become more relevant, leading to improved interfaces. Tere are several methods that can measure these local
properties. Te more common methods include impedance spectroscopy and nanoindentation [3,8,9]. However,
even
using both methods, one cannot get the full local information about the aforementioned properties. Impedance spectroscopy needs an exact model and only distinguishes between inter- faces and therefore does not provide local information for each
48 doi:10.1017/S1551929520000863
interface. Nanoindentation is destructive and does not provide any electrical information. One of the more effective tools used to overcome the problems encountered in electrical property measurement is scanning spreading resistance microscopy (SSRM). Tis technique simultaneously measures both electri- cal properties and topography. SSRM is an implementation of a well-established spread-
ing resistance profiling (SRP) method used for micro- and nano-scale analyses (Figure 1). Te operation of conduc- tive atomic force microscopy (AFM) and SSRM is identi- cal except that SSRM scans the cross-sectioned surface of a device, whereas in conductive AFM a generalized surface is scanned. Te applications of SSRM include determina- tion of dopant distribution in semiconductor materials as well as exact pn-junction delineation. However, SSRM has some disadvantages, such as rapid wearing of the tip, deg- radation of image resolution, and low signal-to-noise ratio. Tese disadvantages stem from the high frictional force aris- ing from continuous tip-sample contact during scanning. Recently, a new AFM mode developed by Park Systems called PinPoint™ [10] has been coupled with SSRM in the NX-Hivac AFM (Figure 2). Te NX-Hivac provides an innovative solu- tion to avoid the troublesome frictional forces during a scan. PinPoint™ operates in an approach-retract manner ensuring frictionless operation. Tis eliminates the lateral force from continuous tip-sample contact. In addition, the integration of these methods with AFM enables simultaneous acquisi- tion of topography and electrical/mechanical property data without changing the sample or tip. Here we demonstrate that PinPoint™ SSRM effectively measures both electrical and mechanical properties of LIB electrode surfaces at a much higher quality in a high-vacuum environment than in ambi- ent conditions.
Methods and Materials A LIB electrode was investigated using a Park NX-Hivac
AFM system [11]. Te topographical, electrical (resistance and conductance), and mechanical (stiffness and adhesion) data of the sample were measured under high vacuum (10−5
torr range)
to perform a 20 μm×12 μm scan with a resolution of 256×150 pixels. A conductive diamond-coated probe (NANOSENSORS™ CDT-NCHR) with a nominal force constant of 80 N/m was used in this experiment. With PinPoint™ SSRM the topography,
electrical
properties, and mechanical properties of a sample can be acquired simultaneously. Te conductive tip maps the topog- raphy by monitoring its feedback signal and approaches the sample until it reaches a predefined force threshold point. It then measures the Z scanner height and rapidly retracts.
www.microscopy-today.com • 2020 May
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