Attainable Energy Density of Microbatteries
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several miniaturized electronic applications. This is the reason why high-performance, all-solid-state, Li-based microbatteries are still sought in earnest. We will demonstrate, herein, a very pragmatic approach that can be employed to improve the attainable or practical volumetric energy density of all-solidstate Li-based microbatteries. We propose a cell design consisting of state-of-the-art electrode materials for lithium-based batteries, with a total active material thickness of 270 μm and a separator (preferably an inorganic Li-ion conductor) membrane thickness of 30 μm; 30 and 40% porosities for the solid-state electrolyte volume in the anode and cathode, respectively, have also been considered. Among the proposed cell chemistries presented in Figure S1, the LiFePO4/graphite and LiFePO4/Li4Ti5O12 systems were chosen to evaluate the effect of packaging because of their compatibility with most inorganic or polymeric solid-state electrolytes. To date, the packaging of thin-film, planar, allsolid-state microbatteries mainly relies on thick substrates and polymeric coatings (mostly Parylene) that cannot completely prevent the permeation of oxygen and water.1,12,13 We therefore propose a novel substrate-integrated packaging technology based on cost-effective wafer-level packaging and thin-film metal deposition. In this case, the deposition of housing and feedthrough structures will have to be done according to the model presented in Figure 2. The model assumes 50 μm for either the substrate or top packaging thickness (t2), as well as a 100 μm width (w) for the frame around the active area. The current collectors are to be considered as part of the packaging volume. The attainable volumetric energy densities of the LiFePO4/ graphite and LiFePO4/Li4Ti5O12 systems as a result of the proposed novel (thin) packaging technology are presented in Figure 3; as expected, they decrease with decreasing battery size. It can also be noticed that, even with the lowest-energydensity electrode materials (LiFePO4/Li4Ti5O12), much higher volumetric energy densities in comparison to the commercial all-solid-state microbatteries can be obtained for battery sizes extremely smaller than 0.1 cm3. Likewise, the proposed LiFePO4/graphite microbattery (as small as 0.1 cm3) will outperform Varta’s CP 1254 coin cell, which has a volume of 0.62 cm3. Indeed, while hermetic packaging with 100 μm width (w) has already been demonstrated for other applications,14 the thickness of the top or bottom housing (t2) as low as 50 μm is imperatively an objective for further development. Such thin packaging is of profound relevance (almost a prerequisite) for microbatteries considering that they are usually integrated into miniaturized electronic products like System-in-Package (SiP) or MEMS devices that have their own housings. The evolutions of the volumetric energy densities of the LiFePO4/graphite and LiFePO4/Li4Ti5O12 systems with
ecently, there is growing interest in miniaturized batteries (called microbatteries) with a small footprint (≤1 cm2) and uncompromised energy densities for a wide range of microelectronic applications including medical implants, hearing aids, and wireless sensor networks.1−3 Generally, microbatteries are characterized by their volumetric energy density (Ev) because the volume that they occupy in the microelectronic device is more important than their weight. In principle, lithium-based (with or without lithium metal as an anode) microbatteries are preferred to other chemistries because of their good energy density. However, the energy requirements (autonomy) of the existing and emerging microelectronic devices crucially impose the need for Libased microbatteries with unprecedented volumetric energy densities. Unfortunately, the attainable volumetric energy density of a Li-based microbattery is very limited, mainly by the fact that the inactive content outweighs the active volume, as a result of the usual non-optimized fabrication and packaging technologies. As schematically shown in Figure 1, the hermetic packaging
Figure 1. Schematic illustration of the factors that require attention for the realization of high-performance microbatteries.
technology of microbatteries requires extraordinary attention in the search for high performance, which is not the case yet. This Viewpoint aims to showcase that, while active materials selection, cell design optimization, and cost-effective fabrication are greatly beneficial, novel packaging of microbatteries is rather indispensable to fully match miniaturization needs with the required volumetric energy densities. To date, significant research efforts have been dedicated to three-dimensional (3D) electrode architectures for microbatteries; nonetheless, that is definitely not the only factor that can help to achieve the anticipated high performance of microbatteries.4−7 The characteristics of some of today’s rechargeable miniaturized lithium-based batteries are presented in Table 1; these examples were chosen for this study, but other manufacturers may exist. It can be noticed that the coin cells (of Wyon and Varta) with thicker electrodes deliver much higher volumetric energy densities than the thin-film solid-sate cells of ST-microelectronics and Ilika. However, the size of the coin cells and the use of liquid electrolyte exclude them from © XXXX American Chemical Society
Received: March 27, 2018 Accepted: April 17, 2018
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DOI: 10.1021/acsenergylett.8b00500 ACS Energy Lett. 2018, 3, 1172−1175
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Cite This: ACS Energy Lett. 2018, 3, 1172−1175
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ACS Energy Letters Table 1. Examples of State-of-the-Art Rechargeable Lithium-Based Microbatteriesa manufacturer type footprint thickness (μm) capacity (μAh) voltage (V) Qa (mAh/cm2) Ev (mWh/cm3) status reference a
EnFilm EFL700
Stereax M250
medical
CP1254
ST Micro thin-film solid-state 25.7 × 25.7 mm2 200 700 3.9 0.13 20 on the market 8
Ilika thin-film solid-state 10 × 10 mm2 750 250 3.5 0.25 12 only for licensing 9
Wyon coin cell ⌀ 2 mm 2000 160 3.7 5.10 94 special product 10
Varta coin cell with coil ⌀ 12.1 mm 5400 60 000 3.7 52.17 358 on the market 11
Areal capacity is represented by Qa, and volumetric energy density is represented by Ev.
based on the electrode/electrolyte stack and footprint only; and x, t and w correspond to the dimensions as given in Figure 2. The proposed novel packaging is based on fixing the packaging dimensions irrespective of the microbattery volume (1−1 × 10−4 cm3). Thus, the packaging volume fractions are 25.6, 26.9, 30.9, 42.8, and 73% for the 1, 0.1, 0.01, 0.001, and 1 × 10−4 cm3 microbattery sizes, respectively. It is worth pointing out that such microbattery design is really feasible based on the recent advances in solid-state electrolytes.15−17 In addition to the 30 μm solid-state electrolyte membrane, the electrodes are to be mixed17 or infiltrated18 with a solid-state electrolyte. No porosity was assumed for the solid-state separator as it can be infiltrated with a solution of the same material (or another: inorganic or polymeric) and dried. In this case, the separator can be originally non-Li-conducting (for example, Celgard, SiO2, Al 2 O 3 , and CeO 2 ) or Li-conducting (for example, Li6.55Ga0.15La3Zr2O12 (garnet), Li1.3Al0.3Ti1.7(PO4)3 (LATP), and Li0.35La0.55TiO3 (LLTO)). The infiltration is easily achievable with polymer electrolytes17 or sulfide inorganic Liion conductors.18 The sulfide inorganic Li-ion conductors are not only soluble in polar organic solvents but also exhibit very high lithium-ion conductivity at room temperature15,17 and hence are recommended for this study.
Figure 2. Top and cross-sectional views of the proposed microbattery design with novel packaging (pink color) and a square footprint. The green part represents the active electrode/ electrolyte content.
respect to the total (including packaging) volume of the microbattery were determined with the following equation Ev =
Evm ·(x 2 × t1) (x + 2w) × (t1 + 2t 2)
(1)
where Ev is the attainable volumetric energy density of the packaged microbattery; Evm is the volumetric energy density
Figure 3. Attainable volumetric energy density as a function of battery size. Comparison of the total energy density of today’s products (blue dots); evolutions of the volumetric energy densities of LiFePO4/Li4Ti5O12 and LiFePO4/graphite cells with novel microbattery packaging are represented by the red and green curves, respectively; and energy densities, according to Figure S1, for all cathodes against Li4Ti5O12 (green band) or graphite (red band). 1173
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As a consequence, the power properties of the proposed novel (2D) microbattery should not be sacrificed regardless of the thick stack (300 μm). Moreover, LiFePO4 and Li4Ti5O12 are high-power electrode materials, and 3D electrode configurations6,7 can still be explored. Certainly, this microbattery design is a pragmatic one that calls for the validation of cost-effective solution-processing or printing technologies for the electrode and solid-state electrolyte materials.19−21 It cannot be overemphasized that while the use of lithium metal is crucial for big-size batteries, it is rather to be avoided for microbatteries because of the usual reflow-soldering temperature limitation;3 moreover, the issue of dendrites with solidpolymer electrolytes and inorganic solid electrolytes (except LiPON) persists, and several inorganic solid-state electrolytes are unstable against lithium metal. In fact, the only commercially available fully inorganic, all-solid-state microbattery that is lithium-metal-free is the Stereax M250 of ilika PLC, which is technologically remarkable; however, the performance can be significantly improved with our proposed novel cell design and packaging concept. Since early 2000, the 3D paradigm of microbatteries2,4,5,22 has received significant attention to improve the performance of microbatteries; however, to the best of our knowledge, no 3D microbattery concept has been commercialized because of the complexities and cost of the processing steps. On the other hand, the concepts of printed batteries aim at mechanically flexible and low-cost batteries but not at extremely small sizes.23 In conclusion, this Viewpoint highlights that, in addition to the research on materials and fabrication methods, extra efforts must be dedicated to novel hermetic packaging of microbatteries, which is nearly indispensable. Generally, microbatteries have very small cell capacities (≤700 μAh); hence, any capacity loss (few microampere−hours per month) as a result of Li-ions reacting with permeated moisture will be dramatic. Indeed, the cell design and novel packaging concept demonstrated herein can be adopted for the realization of allsolid-state Li-based microbatteries with unprecedented performance characteristics. However, it is fundamentally dependent on the successful implementation of advanced microfabrication protocols as well as effective solution processing of electrode and electrolyte materials. This work will truly guide the research and development of better Li-based microbatteries for the emerging advanced microelectronic applications.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (R.H.). *E-mail:
[email protected] (N.A.K.). ORCID
Neil A. Kyeremateng: 0000-0003-0122-1692 Notes
Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the BMBF hearing-aid battery project (Project #: 10043013).
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REFERENCES
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Neil A. Kyeremateng*,† Robert Hahn*,‡ †
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Micro Energy Storage Group, Research Center for Microperipheric Technologies, Technical University of Berlin, 13355 Berlin, Germany ‡ Micro Energy Storage Group, Fraunhofer Institut für Zuverlässigkeit und Mikrointegration (IZM), 13355 Berlin, Germany
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00500. Further information on the proposed electrode materials and their volumetric energy density (Evm) estimation (PDF) 1174
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ACS Energy Letters (16) Wang, Y.; Zhong, W.-H. Development of Electrolytes Towards Achieving Safe and High-Performance Energy-Storage Devices: A Review. ChemElectroChem 2015, 2, 22−36. (17) Kato, Y.; Hori, S.; Saito, T.; Suzuki, K.; Hirayama, M.; Mitsui, A.; Yonemura, M.; Iba, H.; Kanno, R. High-Power All-Solid-State Batteries Using Sulfide Superionic Conductors. Nature Energy 2016, 1, 16030. (18) Kim, D. H.; Oh, D. Y.; Park, K. H.; Choi, Y. E.; Nam, Y. J.; Lee, H. A.; Lee, S.-M.; Jung, Y. S. Infiltration of Solution-Processable Solid Electrolytes into Conventional Li-Ion-Battery Electrodes for All-SolidState Li-Ion Batteries. Nano Lett. 2017, 17, 3013−3020. (19) Zhakeyev, A.; Wang, P.; Zhang, L.; Shu, W.; Wang, H.; Xuan, J. Additive Manufacturing: Unlocking the Evolution of Energy Materials. Adv. Sci. 2017, 4, 1700187. (20) Kyeremateng, N. A.; Dinh, T. M.; Pech, D. Electrophoretic Deposition of Li4Ti5O12 Nanoparticles with a Novel Additive for LiIon Microbatteries. RSC Adv. 2015, 5, 61502−61507. (21) Sun, K.; Wei, T.-S.; Ahn, B. Y.; Seo, J. Y.; Dillon, S. J.; Lewis, J. A. 3d Printing of Interdigitated Li-Ion Microbattery Architectures. Adv. Mater. 2013, 25, 4539−4543. (22) Priimagi, P.; Brandell, D.; Srivastav, S.; Aabloo, A.; Kasemagi, H.; Zadin, V. Optimizing the Design of 3d-Pillar Microbatteries Using Finite Element Modelling. Electrochim. Acta 2016, 209, 138−148. (23) Lanceros-Méndez, S.; Costa, C. M. Printed Batteries: Materials, Technologies and Applications, 1st ed.; John Wiley & Sons Ltd: West Sussex, U.K.; 2018.
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