A New Perspective on the Advanced Microblade Cutting Method for Reliable Adhesion Measurement of Composite Electrodes

In Situ Acoustic Diagnostics of Particle-Binder Interactions in Battery Electrodes

Solid-state batteries (SSBs) have seen a dramatic increase in research in recent years because of their ability to address safety challenges associated with flammable liquid electrolytes, and the potential to enable Li metal anodes. However, SSBs present unique challenges, including high interfacial impedances, accommodation of mechanical stresses due to solid-solid interfacial contact, and (electro)chemical instabilities that can evolve during dynamic cycling conditions. Furthermore, a significant challenge facing the scale-up of SSBs is to improve our understanding of processing science needed to enable manufacturing.While a significant amount of attention has been paid recently to the Li metal anode/electrolyte interface, there has been significantly less work on the cathode side of the SSB. In SSBs, the cathodes are typically composites of the active material and solid electrolyte (SE) phases, which experience rate limitations. Analogous to porous electrodes with liquid electrolytes, the rate capability of these composite solid-state electrodes is strongly dependent on electrode thickness (electrode capacity) and electrode composition/microstructure. These results in tradeoffs between energy and power density, which are exacerbated at high charging rates.To address these challenges, we have recently applied operando optical microscopy to directly observe state-of-charge (SoC) gradients in composite solid-state electrodes, using graphite as a model active material [1]. To describe mass transport in these composite electrodes, continuum-scale modeling was performed, which illustrates the critical role of the electrode microstructure and tortuosity on rate capability. These in situ observations are consistent with ex situ measurements of Li-graphite half-cells, where we quantify the rate capability as a function of areal capacity.To overcome these limitations, we engineered composite SSB electrodes with controlled microstructure and 3-D architectures. These electrodes showed improved homogeneity in the local SOCs throughout the electrode thickness, enabling high-rate cycling capabilities. Overall, the insights presented here will enable new strategies to overcome power/energy tradeoffs in SSBs with composite electrodes.[1] A. L. Davis, V. Goel, D. W. Liao, M. N. Main, E. Kazyak, J. Lee, K. Thornton, N. P. Dasgupta, ACS Energy Lett. 6, 2993 (2021)

Silicon has been the focus of many research studies as the next generation high-capacity anode material for lithium-ion batteries. However, the mechanical stability of the material remains a bottleneck to the commercialization of the material. Much work has been devoted to make nanostructured silicon composites to suppress the volume expansion. Yet, the amount of dimensional change on an electrode has not been directly monitored. Here we used an in-situ electrochemical dilatometer to quantify the thickness change of silicon electrodes during charge and discharge in order to develop techniques to stabilize the electrode.Bulk silicon particles with a size of 10-20mm from Sigma Aldrich was used to make the electrodes. We found that the degree of electrode thickness increase depends on the electrode composition (amount of carbon black and binder in the electrode). An electrode with 10wt% polyvinylidene fluoride binder gives as much as 600% increase in thickness after initial discharge. This means that an electrode that is initially 20mm thick is expanded to as much as 140mm thick after lithiation. The amount of thickness change is less when 20% binder is used. Fig. 1 shows the change in electrode thickness with respect to cumulative capacity with 20% carbon black and 20% carboxymethyl cellulose during the first three cycles. As expected, an increase in electrode thickness is observed during lithiation (discharge), and a decrease in thickness during delithiation (charge). The increase and decrease in electrode thickness are however not linear, and a model is devised to explain the mechanical expansion and contraction behaviors in a composite electrode. In brief, the electrode undergoes three stages of expansion during initial lithiation. At the beginning of discharge, the thickness change is small, as the composite electrode contains space between the particles that can accommodate the volume expansion (Stage I). Beyond a certain point, the particles impinge on each other and the volume expansion of the particles lead to an overall increase in the film thickness (Stage II). The amount of increment during stage II is similar to the theoretical increase in volume (dotted line in Fig. 1) for alloying Li with Si. The binder then losses the ability to hold the particles together, leading to an accelerated increase in thickness at the end of lithiation (Stage III). During delithiation, the contraction behavior is different from that during expansion (Stage IV), as seen from the dilatometer results. This is partly because the particle can contract in all three directions, as opposed to one direction during lithiation. The accuracy of the measurement from the dilatometer is verified by cross-sectional SEM test.After understanding the mechanism during charge and discharge, electrodes with different binders were then tested to compare the role of the binder on cycle stability. Electrodes with polyimide (PI) binder show better cycling stability than those with polyvinylidene fluoride and carboxymethyl cellulose, which is attributed to the ability of the PI binder to hold the particles together after expansion. Our results show that binder breakdown is one of the main causes of electrode degradation for Si electrodes. More experimental details and results will be shown during the presentation. With further optimization, even bulk silicon particle has the potential to be used for commercial applications.

Increasing concerns about environmental and energy problems accelerate the demands for high-energy-density batteries. Lithium-ion batteries (LIB) have large energy densities and have been well applied on not only portable devices such as smart phones but also electronic vehicles (EV). However, the energy densities of present LIBs for EV-use seem not to satisfy for a long cruising distance. Present LIBs for EV-use possess large power densities by the sacrifice of energy densities in order to shorten the charging time. In the practical LIBs for portable devices, dense and thick composite electrodes are used. In this case, the ion transport of electrolyte solution through the pores in the composite electrodes becomes a slow process. In order to enhance the energy densities with remaining high power densities, the ion transport rate should be increased for the dense and thick composite electrodes. Unfortunately, to develop dense and thick electrodes with high power densities, fundamental understanding of the ion transport in composite electrodes has not been clear. In this study, ion transportation behavior was investigated with ac impedance spectroscopy. In a conventional use of ac impedance spectroscopy, a composite electrode is used as a working electrode of the measurement cell. In this case, the result of impedance spectroscopy contains all resistances and capacitances accompanying the whole of the charge and discharge reaction including ion transfer through electrolyte and active materials, ion diffusion within active materials, etc. To focus on the impedance of ion transportation of electrolyte, it must be discriminated from that derived from other reaction steps. In order to obtain impedance spectra derived from only ion transportation, composite electrodes without a current collector were used. As a result, we obtained semi-circles, and this semi-circle was assigned to the ion transport process through the graphite composite electrode. As the potential went down, two semi-circles were observed. The characteristic frequency and the resistance of the semi-circle at higher frequency almost did not change. However, at lower frequency, the characteristic frequency and resistance changed. Further, we used a model electrode to understand the semi-circles. The detail analysis will be presented at the meeting.

Present LIBs for EV-use possess large power densities by the sacrifice of energy densities in order to shorten the charging time. In the practical LIBs for portable devices, dense and thick composite electrodes are used. In this case, the ion transport of electrolyte solution through the pores in the composite electrodes becomes a slow process. In order to enhance the energy densities with remaining high power densities, the ion transport rate should be increased for the dense and thick composite electrodes. Unfortunately, to develop dense and thick electrodes with high power densities, fundamental understanding of the ion transport in composite electrodes has not been clear. In this study, ion transportation behavior was investigated with ac impedance spectroscopy. To focus on the impedance of ion transportation of electrolyte through the pores in composite electrode, we prepared anodic aluminum membranes with homogeneous macro- and meso- pores. In addition, graphite composite electrodes were also used. As a result, we obtained semi-circles, and this semi-circle was assigned to the ion transport process through the graphite composite electrode. The detail analysis will be presented at the meeting.

All-solid-state batteries have received significant attention around the world because of high safety, reliability and energy density [1]. Over the past 20 years, all-solid-state batteries fabricated by using thin film techniques have been thoroughly investigated [2, 3]. These batteries are developed as micro-batteries because typically their total thickness is approximately 15 μm, including the protective packaging. Their specific capacities are only between 5 and 100 μAh cm-2, depending on the thickness of electrodes [4]. The main challenge facing all-solid-state batteries is to enhance the surface capacity influenced by the thickness of cathode electrode. However, many attempts to increase the electrode thickness have not been successful because micro-cracks between the components are formed due to the stress generated at the solid electrode-electrolyte interface. Strong kinetic limitations due to the low mobility of the ions and electrons in the increased electrode is also another problem. To overcome these problems, a composite electrode made of multifunctional materials can be used as cathode electrode. The composite electrode should contain the electrochemically active material and should be able to transport electrons and ions in the electrode. In this study, a 0.44LiBO2∙0.56LiF solid electrolyte was used for Li+ conduction pathway in the composite electrode because it has a low melting point (> 700 °C) [5] and is expected to act as a bonding material at the interface resulted from liquid phase sintering. The composite electrode with a strong mechanically framework was fabricated by a simple one-step spark plasma sintering (SPS) technique. The composite formulation as well as sintering parameters were determined by the electrical properties and sintering behaviors of electrode materials. The composite electrode was very dense with very few voids visible in the TEM images (Fig. 1.). The 0.44LiBO2∙0.56LiF solid electrolyte was in close contact with the active materials due to liquid phase sintering. A Li-ion conduction path was formed along the electrode particles. In order to analyze the effects of liquid phase sintering systematically, the electrochemical performance of composite electrode is discussed in detail.

Silicon has been the focus of many research studies as the next generation high-capacity anode material for lithium-ion batteries. However, the mechanical stability of the electrode remains a bottleneck to the commercialization of the material. Many studies were devoted to nanostructured silicon composites with voids to accommodate the volume expansion [1]. Yet, full capability of silicon cannot be utilized because of the low volumetric energy density of these nanostructures. To increase the volumetric energy density compared to graphite, dense silicon electrodes are needed. Volume expansion within the electrode becomes an important factor affecting its cycle stability. In reality, how much is the volume expansion? What is the mechanism during the charge and discharge? Few studies have addressed these issues. In this work, we employed electrochemical dilatometry to measure the thickness change of Si electrodes during charge and discharge to understand the behavior with time. We demonstrate that the amount of electrode expansion and contraction is non-linear with the amount of lithium, which is explained by a model. We identify binder breakdown as a one of the causes of capacity degradation. Better reversibility in thickness change is achieved by using a more flexible binder such as polyimide, resulting in better cycle stability. Bulk silicon particles with a size of 10-20 μm from Sigma Aldrich was used to make the electrodes. The powder was mixed with acetylene black (Alfa Aesar) and binder to make an electrode. Electrode composition and type of binder were varied to study their effects on electrode volume change and capacity during charge-discharge. Typical electrode thickness is between 20-25 μm with 60wt% active material and a packing density of about 1.2 g cm-3. This corresponds to an electrode loading of about 1.5 mg silicon cm-2. The electrodes were assembled in an electrochemical dilatometer (ECD-1 from EL-Cell) with Li metal as counter electrodes for the thickness measurement. Similar device has been used by others to measure thickness change of various electrodes [2,3]. Fig. 1 shows the change in thickness with respect to cumulative capacity of a Si electrode with 20% carbon black and 20% carboxymethyl cellulose during the first three cycles. As expected, an increase in electrode thickness is observed during lithiation (increase in capacity), and a decrease in thickness during delithiation (decrease in capacity). The increase and decrease in electrode thickness are however not linear. A three-stage expansion model is used to describe the observation. At the beginning of lithiation (stage I), the electrode thickness change is small, as the composite electrode contains space between the particles that can accommodate the volume expansion. Beyond a certain point, the particles impinge on each other and the volume expansion of the particles lead to an overall increase in the film thickness (stage II). The amount of increment during stage II is similar to the theoretical increase in volume (dotted line in Fig. 1) for alloying Li with Si, indicating that it is due to structural change within the particle. Further incorporation of lithium into the electrode leads to an accelerated increase in thickness (stage III). The onset of stage III expansion depends on the type of binder used in the electrode, which suggests that it is affected by the ability of the binder to hold the particles together. During delithiation (stage IV), the contraction behavior is different from that during expansion. This is partly because the particle can contract in all three directions, as opposed to one direction during lithiation. Electrodes with polyimide (PI) binder show better cycling stability than those with polyvinylidene fluoride and carboxymethyl cellulose, which is attributed to the ability of the PI binder to hold the particles together after expansion. The results show that binder breakdown is one of the main causes of electrode degradation for Si electrodes. More experimental details and results will be shown during the presentation. Acknowledgement This research is sponsored by the GRF/ECS Scheme (21202014) managed by the Research Grants Council, the Government of the Hong Kong SAR. References Wu, H. & Cui, Y. Nano Today 7, 414-420 (2012). Hantel, M. M. et al. J. Electrochem. Soc. 159, A1897-A1903 (2012). Kim, J. S. et al. J. Power Sources 244, 521-526 (2013). Figure 1

Possible laser processes in battery manufacturing are quite diverse regarding the control of electrochemical characteristics: LIPSS on current collector surfaces are used to adjust the adhesion of composite electrodes to current collectors, laser surface patterning turns ceramic-coated separator materials into superwicking with regard to electrolyte wetting properties, and laser structuring of composite thick film electrodes is applied to generate 3D electrode architectures with shortened lithium-ion diffusion pathways. In the field of cathode thick film development, secondary particles with nanoscaled primary particles are used and ultrafast laser ablation is applied to pattern the composite electrodes to optimize the lithiumion diffusion kinetics by enlarging the active material surface with a view to reducing cell polarization, which develops at high battery power. This enables high energy batteries to be upgraded for operation at high power. In the field of anode development for electromotive vehicles, efforts are being made to develop silicon anodes in order to significantly increase the energy density. In addition, the issue of fast charging, mainly influenced by the anode architecture, is a major topic in research and industrial development. Silicon nanoparticles are used and combined with graphite particles in a binder matrix. The large volume change as a result of the lithiation of silicon during battery operation requires laser structuring of the composite electrodes in order to counteract mechanical degradation. Analogous to cathode materials, the lithium diffusion kinetics for anodes are also significantly enhanced by the applied 3D battery concept. The impact of laser structuring and modification of battery materials on the electrochemical performance with respect to the nanoscale is of considerable relevance for future applications in battery manufacturing.

Rational design of freestanding and high-performance thick electrode from carbon foam modified with polypyrrole/polydopamine for supercapacitors

Spark Plasma Sintering (SPS) has proven to be an effective, rapid, and energy-efficient tool in the sintering of a variety of materials, such as ultrahigh temperature ceramics, composites, thermoelectric and magnetic compounds. Its main difference from the traditional sintering techniques (hot pressing, pressureless sintering, microwave sintering, etc) consists in the use of the direct pulsed current along with a uniaxial pressure to obtain well-compacted materials. Due to the fast heating/cooling rates and internally generated heat (resistive heating), the obtained compounds experience little to no particle growth while shortened sintering times ensure the absence of the unwanted side-reactions. Recently, SPS has been demonstrated to be used not only for the sintering of materials but also for their synthesis making it an attractive technique in the fabrication of thick high energy density electrodes for Li- or Na-ion battery application as well as in the synthesis of battery’s active material compounds or the fabrication of all-solid-state batteries [1-5].High energy density electrodes could be obtained by altering the electrode architectures. In pursuit of increasing the energy density, some methods suggest fabrication of (ultra)thick electrodes while others concentrate on the production of electrodes with a minimum amount of inactive materials (binder-free, conductive additive-free, integrated current collectors, etc). Owing to a rapid and highly efficient sintering process, SPS offers the luxury of obtaining simultaneously binder-free and (ultra)thick electrodes which suffer no structural degradation.Combined with novel integrated current collector architectures, SPS-fabricated (ultra)thick electrodes could provide high active material accessibility while working at elevated current rates thanks to controlled pore network morphology and improved electronic conductivity. In 2018 Elango et. al reported a successful Spark Plasma Sintering of the LiFePO4 and Li4Ti5O12 electrodes with controlled porosity by hard templating method (NaCl leaching) [1]. Obtained by SPS and templating approach ultrathick electrodes (1 mm thick) showed remarkable electrochemical performance at C/20 against lithium metal delivering areal capacities 4 times higher than those of the conventional tape-casted electrodes. This study coming from our group demonstrated the proof-of-concept in the fabrication of Li-ion battery electrodes with an ultrathick design by means of SPS. A follow-up study on the correlation between the electrode’s architecture (porosity level and pore size) and its electrochemical performance has been conducted [6] showing that a moderate increase of porosity and decrease in pore size improves the rate capability of the (ultra)thick electrodes (1 mm thick) by reducing the pore tortuosity and promoting more full electrolyte impregnation.Spark Plasma Sintering-based electrode fabrication technique is expected to be easily transferrable to other battery types like Na-ion batteries. However, commonly used Na-ion battery materials are not as widely produced as their Li counterparts and still require long and tedious synthesis procedures. In our work, we have recently demonstrated that SPS could be used to synthesize a common Na-ion battery cathode material – Na3V2(PO4)2F3 – in under only 40 min[2] which resembles a phase-pure compound with small particle size showing slightly enhanced electrochemical performance when compared to the solid-state synthesized NVPF. SPS-synthesized NVPF requires no further treatment and could be used directly as an active material component in the production of our thick binder-free electrodes.In this presentation, the detailed study on the correlation between the (ultra)thick electrode architecture (porosity, pore size/shape distribution, tortuosity, etc), synthesis/sintering parameters, nature of precursors, and the electrochemical performance will be discussed (both for Li- and Na-ion batteries). An insight into the fabrication of (ultra)thick electrodes with novel integrated current collector architectures and their impact on the structural integrity as well as the electrochemical performance of the electrode will be reported.[1] R. Elango et al., Advanced Energy Materials, Vol 8 Issue 15 (2018)[2] A. Nadeina et al., Energy Technol., 8: 1901304 (2020)[3] F. Lalère, et al, Journal of Power Sources, 247, 975-980 (2014)[4] G. Delaizir, et al., Adv. Funct. Mater., 22: 2140-2147 (2012)[5] A. Aboulaich, et al., Adv. Energy Mater., 1: 179-183 (2011)[6] R. Elango et al., submitted to Journal of Power Sources

Toward understanding the real mechanical robustness of composite electrode impregnated with a liquid electrolyte

Composite electrodes containing active materials, carbon and binder are widely used in Li-ion batteries. It is well known that their morphology influences the electrochemical performance of batteries and we have tuned the parameters of composite electrodes such as porosity, compounding ratio and thickness.1 , 2 )However, the tuning of composite electrodes has been performed on an empirical basis because decision factors for the improved performance in tuning composite electrodes have not clearly understood. From a practical standpoint, it is important to scientifically develop the design guide for composite electrodes. There are a number of resistances in composite electrodes. Since the electrode reaction proceeds in regions with lower resistance, reaction distribution is happened within the composite electrode. It is required to understand the relationship among battery performance, reaction distribution and electronic/ionic conductivity in composite electrodes. Previously some researchers have reported the distribution model from the theoretical aspect.3 , 4 ) Recently there are reports made to directly observe the reaction distribution of composite electrodes.5-8 ) Unfortunately these advanced studies just observed distribution phenomena without addressing the decision factor for the distribution and electrode performance. This study aims to explain their relationship from the experimental results. To investigate the cross-sectional reaction distribution occurring in composite electrodes, X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) technique was applied. For the measurement of electronic / ionic conductivity in composite electrodes we have developed a method for simultaneous measurement of electronic and ionic conductivities in composite electrodes.9) First, we examined the reaction distribution of LiFePO4 and LiCoO2 electrodes. The reaction distribution in mono-phasic materials is relaxed by the potential gradient inside composite electrodes. On the other hand, the reaction distribution using LiFePO4 composite electrodes is remained under the open circuit condition.10) Because our XAS method detects the static information, we selected LiFePO4as the active materials. Composite electrodes using LiFePO4active material with low porosity cause a large polarization in high rate discharge reaction and decrease their capacity. In these electrodes, the discharge reaction occurs preferentially at the top surface of the electrode near electrolyte side. Further, low porosity results in a low effective ionic conductivity. This study clearly reveals the ionic conduction in the composite electrodes is the governing factor of lithium-ion battery performance. Control of ionic conductivities in composite electrodes is important to further improve the performance of lithium-ion batteries. The knowledge is useful to understand the decision factor for charge-discharge performance in Lithium-ion batteries and design principle of composite electrodes. REFERENCES: 1) W.Q. Lu, A. Jansen, D. Dees, G. Henriksen, J. Mater. Res., 25, 1656-1660 (2010). 2) C.C. Chang, L.J. Her, H.K. Su, S.H. Hsu, Y.T. Yen, J. Electrochem. Soc., 158, A481-A486 (2011). 3) M. Doyle, T.F. Fuller, J. Newman, J. Electrochem. Soc., 140, 1526-1533 (1993). 4) J. Newman, W. Tiedemann, J. Electrochem. Soc., 140, 1961-1968 (1993). 5) S.H. Ng, F. La Mantia, P. Novak, Angew Chem Int Edit, 48, 528-532 (2009). 6) J. Liu, M. Kunz, K. Chen, N. Tamura, T.J. Richardson, J. Phys. Chem. Lett., 1, 2120-2123 (2010). 7) K.C. Hess, W.K. Epting, S. Litster, Anal Chem, 83, 9492-9498 (2011). 8) J. Nanda, J. Remillard, A. O'Neill, D. Bernardi, T. Ro, K.E. Nietering, J.Y. Go, T.J. Miller, Adv Funct Mater, 21, 3282-3290 (2011). 9) Z. Siroma, J. Hagiwara, K. Yasuda, M. Inaba, A. Tasaka, J. Electroanal. Chem., 648, 92-97 (2010). 10) H. Tanida, H. Yamashige, Y. Orikasa, Y. Gogyo, H. Arai, Y. Uchimoto, Z. Ogumi, J. Phys. Chem. C, 120, 4739-4743 (2016).

Composite electrodes containing active materials, carbon and binder are widely used in Li-ion batteries. It is well known that their morphology influences the electrochemical performance of batteries. For the improvement of performance in practically used Li-ion batteries, it is necessary to tune the parameters of composite electrodes such as porosity, compounding ratio and thickness. The morphology of composite electrodes varies the effective electronic and ionic conductivity in composite electrodes. However the tuning of composite electrodes has been mainly based on the intuition and experiences, since it is difficult to distinguish the electronic conductivity and the ionic conductivity in composite electrodes. For composite electrodes, the traditional 4-probe method cannot be applied because charge-discharge currents are flew during applying voltage. Therefore, a measurement method for distinction between electronic and ionic conductivities in composite electrodes is required. In this study, we have developed a method for simultaneous measurement of electronic and ionic conductivities in composite electrodes. This method is applied to the various porosities composite electrodes. Fig. 1 shows the measurement setup and the arrangement of the electrodes and samples. Two Al foils were bonded with polypropylene. 75 wt% carbon-coated LiFePO4 powder, 10 wt% acetylene black and 15 wt% PVDF were mixed in 1-methyl-2-pyrrolidinone anhydrous (NMP, Sigma-Aldrich) solvent. The slurries were coated onto the aluminum foils. Drying was done at 70 °C to remove solvent and additional drying was performed at 80 °C in a vacuum oven to vaporize the residual solvent. These LiFePO4-based composite electrodes were pressed at 0 kgf, 300 kgf, 600 kgf, 900 kgf, 1200 kgf pressures to control their porosity. As shown in Fig.1, the electrochemical cell contains 6 probes connecting with the composite electrode. For the electronic and the ionic conduction electrodes, Al foils and Li foils were used, respectively. Two potentiostats and bias voltage were connected to the cell. After open circuit voltage is measured, the two potentiostats were operated with this voltage as the set point. And then, a bias voltage was applied between the two working electrodes. The ionic current was measured at A1 or A3 current meter, and the electronic current was measured at A2 or A4 current meter. The measurement principle is explained in the literature [1]. Fig. 2 shows the effective electronic and ionic conductivities for various porosity composite electrodes. The electronic conductivity is at least 10 times higher than the ionic conductivity. This indicates the dominant contribution to the electrochemical performance is the ionic conductivity. For the high porosity electrodes, the effective ionic conductivity is almost constant. On the other hand, when the porosities of the electrodes are less than 47%, the ionic conductivity decreased. This indicates that the compressed composite electrode structure narrowed the ionic conduction path, resulting the decrease of the ionic conductivity. From the result of the rate capability test in the LiFePO4 composite electrodes, the decrease of porosity causes the decrease of discharge capacity at 10 C rate. Therefore, the low ionic conductivity from low porosity lowers the rate capability of LiFePO4composite electrodes.

The energy storage performance of Li-ion batteries has continuously improved in recent years, reaching over 250 Wh/kg with an annual growth rate of 5.5 Wh/kg [1, 2]. The improved performance is due to the adoption of new active materials such as nickel rich layered oxides, and the maximization of active material fraction in batteries by: I) reducing the thickness of electrochemically inert cell components such as current collectors, separators, and packaging; II) diminishing the contents of inactive materials (e.g., carbon black and binder) in composite electrodes; and III) reducing electrode porosity which minimizes electrolyte amounts. Approaches I and II are physically limited in order to maintain safety and electrochemical performance; increasing the active material loading level or electrode thickness is a promising option that could achieve higher energy densities in the near future. Increasing the thickness of the electrode, however, generally results in electrochemical performance deterioration. Past modeling studies conclude that the high rate performance of porous electrodes is limited by liquid-phase ion transport due to concentration gradients and ion depletion. Experimentally, however, the effects of ionic conductivity and electronic conductivity of thick electrodes on their rate performance have not been examined simultaneously. In this work, composite electrodes with LiNi0.8Co0.1Mn0.1O2 as the active material with various microstructures are prepared to understand the electrochemical performance limiting factors. The electrode composition (80:10:10 weight ratio of active material: carbon black : binder), loading (~ 25 mg/cm2) and porosity (30%) are maintained as the same. On the other hand, electrode microstructure is tuned by controlling the electrode slurry compositions such as solvent and solids content ratio. Such variation in processing conditions sometimes leads to intentional crack formation in the electrodes which serves as a mechanism to influence electrode tortuosity. The contact resistance at the current collector/electrode interface (Rc) and the effective ionic conductivity (κeff) are estimated by measuring the electrochemical impedance spectroscopy (EIS) of a symmetric coin-cell with two identical composite cathodes and electrolytes composed of a non-intercalating salt (TBAClO4), as shown in Figure 1 [3]. The rate and cycling performances are assessed by using galvanostatic charging/discharging tests. For these thick electrodes, Rc, in addition to ionic transport in the electrode, has been found to have a significant influence on electrode rate performance. Based on these observations, potential approaches to improve rate performance of thick electrodes will be discussed. Figure 1. Schematic diagram of the electrochemical impedance spectroscopy (EIS) of the cathode symmetric coin-cell with non-intercalating salt. Note that the real impedance values of the high frequency intercept, the semi-circle, and the 45-degree slope are corresponded to the electrolyte bulk resistance (Rsol), contact resistance at the current collector/electrode interface (Rc), and the ionic resistance in pores (Rion).

3D printing of fast kinetics reconciled ultra-thick cathodes for high areal energy density aqueous Li–Zn hybrid battery