Morphology, microstructure, and phase states in selective laser sintered lithium ion battery cathodes

doi:10.1016/j.jmatprotec.2020.116827

Journal of Materials Processing Technology

Volume 288, February 2021, 116827

https://doi.org/10.1016/j.jmatprotec.2020.116827 Get rights and content

Highlights

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Bulk LiNi_0.80Co_0.15Al_0.05O₂ (NCA) is made by binder-free selective laser sintering.
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Single-track morphology is used to refine processing and produce 3D samples.
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Three-dimensional (3D) samples are porous with micron sized grains.
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3D samples exhibit both layered (R-3 m) and rock salt (Fm-3 m) structures.

Abstract

Fabrication of high-energy cathode materials with complex geometries is necessary to enable high-power density in next-generation lithium ion batteries (LIBs). Ceramic three-dimensional (3D) printing techniques are one possible avenue to achieve high performance. Current ceramic 3D-printing methods often suspend the electrochemically active cathode powder in binders that require substantial rheological development to enable printability and secondary binder removal processes to ensure degradation does not occur during electrochemical cycling. In this study, the need for binder additives is circumvented by employing the laser-based 3D-printing technique, selective laser sintering (SLS), for direct fabrication of 3D lithium nickel cobalt aluminum oxide (NCA) cathodes. Thermal stress and part distortion are mitigated by using in-situ substrate heating while operating the 1.07 μm fiber laser in q-switched continuous mode. A parametric single-track study is performed to refine the process parameters for the layer-by-layer selective laser sintering of bulk 3D NCA samples. These bulk 3D samples are porous with 2−3 μm grains and exhibit a dual phase state, including the layered structure (with symmetry R-3 m) and rock salt structure (Fm-3 m). The retention of the electrochemically active layered structure in these samples is promising for the development of binder-free, 3D-printed cathodes for LIBs with enhanced power density.

Graphical abstract

Introduction

Replacement of traditional combustion engines with renewable lithium ion batteries (LIBs) in electric vehicles (EVs) offers a sustainable approach to combating climate change (Green Car Congress, 2019). However, the performance (e.g., energy and power density), cost, and safety of LIBs currently inhibits widespread adoption of EVs (Goodenough and Kim, 2010). Commercially available EVs employ LIBs with graphite anodes and lithium transition metal oxide cathodes (Julien et al., 2015). LIBs are comprised of a positive and a negative electrode separated by an ionically conductive electrolyte and connected with an external electronic circuit. During discharge, electrons become available for useful work by moving through the external circuit as the lithium cations de-intercalate from the negative electrode (anode), shuttle through the electrolyte, intercalate into the positive electrode (cathode), and recombine with electrons (Julien et al., 2015).

Improvements to electrode design can enhance the energy and power density of LIBs (Pang et al., 2019). However, fabrication of high-energy and high-power density cathodes is challenging due to competing requirements. Producing thicker electrodes increases the mass per unit volume, allowing for higher volumetric energy densities; however, high volumetric power densities require electrodes with large surface areas to enable high rate capability (Pang et al., 2019). The development of three-dimensional (3D) printing methods for electrode fabrication offers a promising approach towards the production of high-energy and high-power electrodes by allowing for the construction of batteries with complex geometries.

3D-printing technologies enable the development of highly customizable, scalable electrodes with enhanced performance over conventional electrode fabrication techniques (e.g., high speed roll-to-roll processing, thin film deposition, etc.). Pang et al. (2019) reviewed slurry-based 3D-printing methods that employ pastes loaded with the electrochemically active material (e.g., lithium transition metal oxide) that are deposited, as droplets or filaments, layer-by-layer until the entire part is built. Kim et al. (2007) employed laser direct writing to fabricate porous, thick film LiCoO₂ cathodes for microbatteries. These microbatteries exhibit discharge capacities an order of magnitude higher than microbatteries that employ thin-film sputter deposited cathodes. Although the improvements in performance are desirable, slurry-based 3D-printing techniques are limited by the slow development of precisely controlled inks and pastes, the limited amount of active material that can be loaded into the slurry, and the time consuming post-processing steps required to evaporate out solvents and sinter the active particles after production of the 3D part, as reviewed by Zhang et al. (2017). The development of rapid, single-step, direct fabrication (i.e., slurry-free) methods to 3D-print electrodes is desired to overcome these limitations and enable the realization of high-energy and high-power density LIB cathodes.

Selective laser sintering (SLS) offers a novel approach for 3D-printing of LIB cathodes. During SLS, a high-energy laser beam selectively consolidates regions of a powder bed layer-by-layer until the 3D part is built. Compared to slurry-based 3D-printing methods, SLS allows for significantly increased active material loading by eliminating the need for binders or solvents during processing. The higher active material loading afforded by SLS enables the production of 3D electrode architectures with higher volumetric energy densities by increasing the amount of electrochemically active material per unit volume. Further, SLS does not require solvent evaporation and sintering post-processing steps, allowing for shorter processing times compared to slurry-based 3D-printing methods while still retaining the ability to produce complex 3D architectures.

Although most often utilized for metal 3D-printing, SLS has been explored for technical ceramics, including alumina and yttria stabilized zirconia. Song et al. (2014) prepared yttria stabilized zirconia samples with up to 88 % relative density. However, these samples suffered from low part resolution and significant crack formation. Alternatively, Yves-Christian et al. (2010) employed a dual laser system, a CO₂ laser for preheating the powder bed and a Nd:YAG laser for selective laser sintering, to prepare near fully dense, Al₂O₃-ZrO₂ samples with minimal crack formation and high part resolution. Thus, SLS can produce high resolution ceramic parts without the need for binders but has yet to be explored for preparation of electrochemically active battery materials.

Lithium nickel cobalt aluminum oxide (NCA) is one of the most widely used high performance LIB cathode materials. The high-energy and high-power densities of NCA have enabled its use in commercially available EVs (Julien et al., 2015). The reduced amount of cobalt in NCA, compared to lithium cobalt oxide, lowers the cost and environmental impact; however, the high nickel content makes NCA susceptible to thermal degradation and impairs the usable lifetime (Nitta et al., 2015). Therefore, any method used for the fabrication of NCA parts must be capable of operating at temperatures low enough to avoid this thermal degradation.

This study evaluates the efficacy of using SLS to prepare 3D NCA cathodes without the use of electrochemically inactive components (i.e., binders or solvents). NCA was selected as a model cathode material system for this study because thermal degradation causes changes to the crystal structure producing different phase states that are easily detected by X-ray diffraction (XRD) (Bak et al., 2013). The change in phase state is known to alter electrochemical performance therefore providing insight into the electrochemical activity through facile XRD evaluation of cathodes produced using SLS. A parametric single-track (1DNCA) study was performed to guide development of three-dimensional NCA (3DNCA) components. The 3DNCA samples exhibit surface porosity, desirable morphology, and partial retention of electrochemically active phase states.

Section snippets

Materials

Commercially available NCA (LiNi_0.80Co_0.15Al_0.05O₂) powder (40–300 μm) was used for this study (NEI Corporation, Somerset, NJ, USA). The as-received NCA (AR-NCA) powder was sieved (Gilson) to refine the particle size distribution for SLS (45–106 μm). Particle size assessment of the AR-NCA powder was performed using scanning electron microscopy (SEM) (FEI Magellan 400 XHR SEM). The SEM micrographs of the AR-NCA powder were analyzed using ImageJ (National Institute of Health (NIH), 2018) to

Results

AR-NCA powder is comprised of fine primary particles (d₅₀: 0.52 ± 0.22 μm) (Fig. 3a) agglomerated into large secondary particles (d₅₀: 125 ± 47 μm) (Fig. 3b). The primary particles are cubic and faceted, while the secondary particles are oblong.

Qualitative visual inspection of optical micrographs is often used to determine general trends with respect to individual SLS process parameters. For example, the optical micrographs in Fig. 4 reveal that both laser scan speed and laser beam diameter

Morphology

Proper selection of SLS processing parameters is necessary to ensure multilayer samples with desirable morphologies. For instance, the extensive balling and the formation of voids during SLS can cause insufficient bonding between layers and inhibit printability of multilayer parts. When using 3D-printed cathodes in LIBs, cracks present in the as-deposited SLS samples may grow during the structural fatigue, caused by cycling Li ions in and out of the cathode during charge/discharge, and

Conclusions

Three-dimensional NCA cathodes were prepared using selective laser sintering. A single-track parametric study revealed that the 1DNCA samples are discontinuous below E_c (i.e., VED ≤ 75 J mm⁻³) and continuous above E_c. Continuous 1DNCA samples undergo more extensive cracking with laser beam diameters below 0.65 mm. As such, VED values below E_c were selected for the fabrication of 3DNCA samples to promote desirable morphologies by minimizing crack formation and discontinuity. The graded

CRediT authorship contribution statement

Katherine A. Acord: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft, Visualization. Alexander D. Dupuy: Writing - review & editing. Umberto Scipioni Bertoli: Methodology. Baolong Zheng: Methodology. William C. West: Conceptualization, Supervision. Qian Nataly Chen: Conceptualization, Supervision. Andrew A. Shapiro: Conceptualization, Supervision. Julie M. Schoenung: Conceptualization, Supervision, Project administration, Funding acquisition, Writing -

Declaration of Competing Interest

The authors declare that there are no conflicts of interest.

Acknowledgements

This work is funded by the National Aeronautics and Space Association (NASA) Jet Propulsion Laboratory (JPL) under contract number 1558329. The SEM, EDS, and XRD experiments were performed at the UC Irvine Materials Research Institute (IMRI). The authors would like to acknowledge Dr. James Haley and Aleksandra Vyatskikh, M.S., for their contributions to the laser beam profiling experiments. The author would like to thank Cesar Ponce, UCI undergraduate research assistant, for his contributions

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Morphology, microstructure, and phase states in selective laser sintered lithium ion battery cathodes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results

Morphology

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Acta Mater.

J. Power Sources

Opt. Lasers Eng.

J. Power Sources

Acta Mater.

Phys. Procedia

Nano Energy

Correlating structural changes and gas evolution during the thermal decomposition of charged LixNi0.8Co0.15Al0.05O2 cathode materials

Chem. Mater.

Physical Ceramics

Energy density analysis on single tracks formed by selective laser melting with CoCrMo powder material

Int. J. Adv. Manuf. Technol.

BNEF Annual Li-Ion Battery Price Survey Finds Prices Fell 13% From 2018 to Average $156/KWh in 2019

Integrated melt pool and microstructure control for Ti–6Al–4V thin wall additive manufacturing

Mater. Sci. Technol.

Correlating structural changes and gas evolution during the thermal decomposition of charged Li_xNi_0.8Co_0.15Al_0.05O₂ cathode materials