NANOscientific

Kylie Cha, Jake Kim, Cathy Lee RATC, Park Systems Corp.

 

Introduction

Lithium-ion batteries (LIBs) are advanced portable energy storage solutions that fuel our daily devices like smartphones, laptops, and cars. To enhance their performance and quality control, we must grasp the nanoscale interaction between morphology and electronic properties. This application note introduces battery basics and how we analyze lithium-ion materials using atomic force microscopy (AFM).

A typical LIB consists of two electrodes, a separator, and an electrolyte solution (Fig. 1 [1]). During operation, electrochemical reactions occur within the electrodes, with ions moving through the electrolyte and electrons passing through an external wire, forming a closed circuit. In discharge, the anode undergoes electrochemical oxidation, while the cathode experiences reduction as chemical energy is converted into electrical energy. A separator prevents electrical shorts between the cathode and anode, and the electrolyte conducts ions but not electrons.

 

 

LIB electrodes comprise active materials, conductive materials, and binders (Fig. 2). The active material generates energy by reacting with the anode material. The cathode’s material is called ‘cathode active material’ and it contains lithium ions. It impacts capacity and power, with materials like lithium cobalt (LCO), lithium manganese oxide (LMO), lithium cobalt aluminum (NCA), and lithium cobalt manganese (NCM) is used. The anode’s material called ‘anode active material’ stores releases lithium ions, primarily using stable graphite. Conductive material, usually carbon black (Super-p), aids electron movement. Binders enhance material adhesion, addressing issues like battery life and charging time due to anode material volume changes during repeated cycles.

 

 

Battery voltage and capacity depend on the active material, while LIB cycling stability relates to electrode integrity. Silicon, a promising anode material, offers ten times greater capacity than graphite [2]. Yet, silicon anodes experience substantial volume changes during charge-discharge in LIBs [3], causing stress, anode fractures, and delamination. Cathode materials also undergo structural changes, impacting functionality [4,5], leading to microcracks, increased porosity, and capacity loss during cycling. To understand LIB failure, we must study local electronic properties and morphology. This paper demonstrates AFM’s ability to provide topography and work function information.

 

In recent years, various analytical methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and AFM have been extensively used in LIB research. While SEM and TEM measurements offer speed, they come with limitations in sample analysis using traditional electron beam imaging. To address this, experiments need to be conducted with diverse conditions and parameters, emphasizing the necessity for a versatile and comprehensive analytical tool.

AFM is increasingly used to study LIB electrode materials due to its micro/nano-meter spatial resolution. It assesses pristine material characteristics, aids electrode fabrication control, and investigates mechanical and electrical degradation. Surface potential measurements detect electrode degradation, including decreased cathode surface potential with more charge/ discharge cycles [8]. Electrical degradation of cathode materials also leads to reduced conductivity and microcrack formation with increased cycles [6].

 

Park Systems introduced PinPoint™ SSRM to comprehensively characterize surface, mechanical, and electrical properties of LIB electrodes [9]. Vacuum measurements suit easily oxidizable or hydrolyzable materials like cycled electrodes.

 

However, for simplicity, pristine samples can be measured under ambient conditions using conventional AFMs. This paper illustrates measurements on a pristine cathode as an initial step in LIB characterization and quality assessment.

 

Materials and Methods

In this study, we investigated the cathode material LiNi_0.8Co_0.15Al_0.05O₂ (NCA) as a promising candidate for lithium-ion batteries (LIBs). The cathode slurry, composed of NCA, a conductive agent, and binder, was applied onto an aluminum (Al) current collector. To facilitate microscopic analysis, a cross-sectional sample was prepared using an argon-ion crosssection polisher. The Al current collector was electrically connected to the AFM’s sample bias line using silver paste. Prior to AFM scanning, scanning electron microscopy (SEM) was employed to evaluate the sample’s structural characteristics and microscopic uniformity.

An automated AFM system (FX40, Park Systems) was utilized, featuring advanced technology for the alignment of the super luminescent diode (SLD) beam and position-sensitive photodiode (PSPD) on the cantilever’s apex. Precise beam focusing within the cantilever minimized optical interference. The incorporation of an automatic tip exchanger and laser aligner reduced labor-intensive tasks and human error. The StepScan™ function within SmartScan from Park Systems facilitated effortless configuration for automated measurements, encompassing the selection of regions of interest, imaging modes, and scan parameters.

Surface potential measurements were conducted using a conductive material-coated tip (PPP-EFM, Nanosensors) in sideband Kelvin probe force microscopy (KPFM) mode. Prior to data acquisition on the sample, we calibrated the work function of the AFM tip by referencing a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface. Subsequently, scanning spreading resistance microscopy (SSRM) measurements were executed to evaluate local conductivity and resistance. A conductive diamond-coated tip (AD-40-AS, Adama Innovations) with a nominal spring constant of 40 N/m was employed to enhance electrical signal stability and minimize tip wear resulting from lateral shear forces. Measurements were conducted at three randomly selected positions on the sample to assess the uniformity of cathode fabrication. Importantly, all AFM experiments were conducted within an environmental control glove box to prevent sample deformation upon exposure to ambient air.

In Fig. 3, SEM images show the cross-sectioned cathode sample with a thickness of approximately 80 ± 10 μm (slurry thickness may vary). The cathode film is securely attached to the Al current collector. The active material particles have a uniform spherical shape and high distribution density, promoting good electrical contact and preventing current collector damage during electrode fabrication. Some minor porosity (indicated by yellow arrows) is observed, likely due to the sample fabrication method, but it’s not expected to significantly affect cathode performance. SEM measurements provide an initial assessment and quality control of the electrode fabrication process by revealing the microscopic structure and uniformity.

 

 

Fig. 4 reveals NCA particles’ work function as 5.0 ± 0.2 eV, in line with the theoretical 4.9 eV. These KPFM results affirm the excellent state of the pristine electrode, ready for battery assembly. Furthermore, Fig. 4 highlights KPFM’s ability to unveil characteristics beyond surface topography inspection. Fig. 5 summarizes surface potential measurements at various locations using KPFM, which provides topography and surface potential simultaneously. Different AFM tips have varying work functions, leading to potential offsets.

 

 

Fig. 6 displays SSRM measurements indicating uniform conductivity across the cathode film, despite a small bias applied to the current collector, suggesting high electrical conductivity in the prepared cathode sample, advantageous for LIB performance. To prevent potential damage to the active material, it is advisable to use PinPoint SSRM mode for battery sample measurements, allowing simultaneous assessment of terrain, electrical, and mechanical properties while minimizing tip wear and sample damage.

 

Conclusion

In summary, this application note demonstrates the preparation and characterization of a pristine cathode for lithium-ion batteries (LIBs). We measured and interpreted the sample's structural, topographical, and electrical properties to enhance our understanding of the relationship between electrode characteristics and battery performance. This approach showcases the effective use of AFM for inspecting and assessing electrode quality in LIBs, paving the way for future battery research and development. In addition to the techniques introduced here, various AFM modes offer comprehensive battery characterization capabilities, making AFM a powerful tool in battery research. These modes include:

- Conductive AFM for current distribution and conductance measurements during battery cycling.

- PinPoint electrical modes for simultaneous mechanical and electrical property measurements of electrodes.

- Electrochemical AFM for in-situ measurements of electrode surface changes during battery electrochemical processes.

- Environment control options, allowing AFM measurements in high vacuum, gases, liquids, and controlled temperature/ humidity conditions.

 

In summary, AFM offers a versatile platform that provides nanoscale correlative information, making it invaluable tool in battery research.

 

 

References

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2. U. Kasavajjula, C. Wang, A.J. Appleby, Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells, J. Power Sources. 163 (2007) 1003–1039.

3. L.Y. Beaulieu, T.D. Hatchard, A. Bonakdarpour, M.D. Fleischauer, J.R. Dahn, Reaction of Li with Alloy Thin Films Studied by In Situ AFM, J. Electrochem. Soc. 150 (2003) A1457.

4. F. Lin, I.M. Markus, D. Nordlund, T.-C. Weng, M.D. Asta, H.L. Xin, M.M. Doeff, Surface reconstruction and chemical evolution of stoichiometric layered cathode materials for lithium-ion batteries, Nat. Commun. 5 (2014) 3529.

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7. W. Zhao, W. Song, L.-Z. Cheong, D. Wang, H. Li, F. Besenbacher, F. Huang, C. Shen, Beyond imaging: Applications of atomic force microscopy for the study of Lithium-ion batteries, Ultramicroscopy. 204 (2019) 34–48.

8. J. Wu, S. Yang, W. Cai, Z. Bi, G. Shang, J. Yao, Multi-characterization of LiCoO₂ cathode films using advanced AFM-based techniques with high resolution, Sci. Rep. 7 (2017) 11164.

9. J.P. Pineda, C. Lee, B. Kim, K. Lee, Electrical and Mechanical Characterization of Li Ion Battery Electrode using PinPoint™ SSRM, Micros. Today. 28 (2020) 48–53.

10. Park Systems Corporation, A New Class of Atomic Force Microscope: FX40, the Automatic AFM