The Cathode Composition, A Key Player in the Success of Li-Metal

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Cathode Composition, A Key Player in the Success of Li-Metal Solid-State Batteries Andrea I. Pitillas Martinez,† Fred́ eŕ ic Aguesse,† Laida Otaegui,† Meike Schneider,‡ Andreas Roters,‡ Anna Llordeś ,†,§ and Lucienne Buannic*,† †

CIC Energigune, Parque Tecnológico de Alava, 01510 Miñano, Spain Schott AG, Hattenbergstraße 10, 55122 Mainz, Germany § IKERBASQUE, The Basque Foundation for Science, Maria Diaz de Haro 3, 48013 Bilbao, Spain Downloaded via UNIV OF MASSACHUSETTS AMHERST on February 4, 2019 at 10:27:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Polymer−ceramic composite electrolytes are major contenders for applications in commercial Li-metal solid-state batteries. However, the cycling performance and energy density of such batteries are not only related to the capabilities of the electrolyte but also to that of the cathode. Here, we investigate the influence of the formulation of a LiFePO4 cathode, paying close attention to the fraction of the catholyte, on the electrochemical performance of a Li-metal solid-state battery and its theoretical energy density. The ionic conductivity and stripping/plating ability of the polymer− ceramic (PEO−LiTFSI−Li7La3Zr2O12) composite electrolyte are first evaluated before assessing its C-rate performance and cycle life using a Li-metal anode and a LiFePO4 cathode with a competitive active material loading (≈1 mA h·cm−2). It appears that a fraction of catholyte between 30 and 40 vol % is enough to ensure good interfacial contact and ionic transport throughout the cell. It is estimated that, under certain hypotheses, the cells could provide average energy densities of up to 185 W h·kg−1 and 345 W h·L−1, which are well above what are currently reported in the state of the art, because of the high cathode loading targeted in this study. Finally, recommendations for achieving energy density values close to the ones needed for electric vehicle applications (350 W h·kg−1 and 750 W h·L−1) are proposed.

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cathode materials, making their industrial use complex.4 Ceramic oxides, in turn, provide a higher chemical stability and a competitive ionic conductivity, with up to 10−3 S·cm−1 at RT for the Li7La3Zr2O12 (LLZO) garnet.5,6 In addition, this inorganic material exhibits the largest electrochemical stability window.7,8 Nevertheless, it remains challenging to prepare a thin layer (30 vol % is needed, keeping in mind that no more than 40 vol % may be necessary. Even though the LFP/C65 fraction was kept constant for all cathode compositions, there is a minimum amount of carbon needed to obtain a functioning cathode laminate, in particular, at faster C rates and for LFP, a material known to have poor electronic conductivity.24 In the selected formulations, the amount of carbon increases with a decrease in catholyte vol % with 6, 8, and 9 vol % (i.e., 4.6, 5.8, and 6.1 wt %) for the cathodes with 50, 40, and 30 vol % of catholyte, respectively (Table 1). This is coupled to a decrease in irreversible capacity loss (Figure 5). In turn, the polarization of the cells clearly decreases with an increase in catholyte fraction (Figure 5). This could be linked to a better ionic conductivity at the cathode as well as an improved interfacial contact between the electrodes and the electrolyte, both dependent on the fraction of (soft) catholyte present. Further experiments would be needed to properly assess the contribution of these different parameters. Direct cycling of the cathode containing 40 vol % of catholyte at C/5 shows very promising performances, with an initial discharge capacity of 94 mA h·g−1 gradually increasing to 137 mA h·g−1 over the first 21 cycles (Figure 4c). It appears that, at this C rate, the SSB needs some time to electrochemically activate all of the active material present at the cathode

Figure 2. Morphological and electrochemical characterization of the composite membrane. SEM imaging of the (a) top view (SE) and (b) cross section (BSE). (c) Arrhenius plot obtained between 80 and 10 °C with the corresponding Nyquist plots recorded at 30 and 70 °C in the inset. (d) Evolution of the ASR with Li-metal electrodes at 70 °C. (e) Polarization during stripping/plating under various current densities for 2 h steps. The inset shows the polarization observed at 200 μA·cm−2 for 5 h steps, which corresponds to cycling conditions of C/5 for a loading of 1 mA h·cm−2. (f) Stripping/plating at 200 μA· cm−2 for 5 h steps, which corresponds to cycling conditions of C/5 for a loading of 1 mA h·cm−2. F

DOI: 10.1021/acs.jpcc.8b04626 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 5. Voltage profile obtained with the different cathode laminates at C/20, C/2, and 1 C.

Three-Electrode Cell. The contribution of the cathode− membrane and Li metal−membrane interfaces to the resistance of the Li-metal battery was evaluated in a threeelectrode cell with the cathode containing 40 vol % of catholyte (Figure 6). After 24 h at 70 °C, the two interfaces have a similar resistance (≈70 Ω). In Figure 6a, the EIS response shows a first semicircle due to the cathode-membrane interfacial resistance followed by a tail representative of the ionic diffusion at the cathode and the blocking Al current collector. In Figure 6b, our attention will be focused on the semicircle highlighting the Li metal−membrane interfacial resistance, while the tail at lower frequencies may be due to parasitic reactions arising from the three-electrode cell configuration. After charging at C/20, the resistance of the cathode−membrane interface increases while one of the Li metal−membrane interface remains constant. Following discharge at C/20, the resistance of the cathode−membrane interface increases further while that of the Li metal− membrane interface slightly decreases. Continuous increase in the cathode−membrane interfacial resistance was observed upon further cycling indicating a larger contribution of the cathode−membrane interfacial resistance, and implicitly interfacial resistances within the cathode laminate itself, to

Figure 4. (a) C rate performances and cycle life of SSBs containing cathode laminates with a composite catholyte at 1 mA h·cm−2. (b) Rate capability obtained with various cathode formulations. (c) Cycle life of the SSB containing 40 vol % of catholyte at C/5, with no prior conditioning cycling. The open and close symbols are for charge and discharge, respectively. All galvanostatic cycling was performed at 70 °C.

while, at C/20, the highest discharge capacity was obtained on the first few cycles. Interestingly, the full cell is able to perform for more than 60 cycles, clearly exceeding the performances of the Li-metal symmetric cell obtained during stripping/plating, which failed after 14 cycles. This could imply a limitation in terms of the experimental design as during stripping/plating, one Li metal−electrolyte interface is undergoing plating, as in the first charge of a full cell, while the other solid interface is undergoing stripping. Changing the design of the stripping/ plating experiment replacing the second solid interface by a liquid−solid interface could be more representative of the limitations of the solid electrolyte under the stripping/plating experiment. G


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experiments point toward rapid failure of the composite membrane when gradually increasing the current density or directly cycling at 200 μA·cm−2 for 5 h steps (i.e., under C/5 conditions). However, the full cell performances clearly exceed those limits, irrespective of the cathode formulation, indicating a poor design of the stripping/plating experimental setup which should be addressed in the future. Testing of the cathodes containing different vol % of catholyte reveals that a fraction superior to 30 vol % is needed to ensure good ionic conductivity in the cathode and minimize the cell polarization. However, going beyond 40 vol % of catholyte is not necessary as it does not improve the cell performance but decreases its energy density. In parallel, small increases in the carbon content have a significant impact on the irreversible capacity, C rate performance, and cycle life of the cell. The theoretical energy density values calculated on the basis of the weight and volume of the cell components vary slightly based on the volume fraction of the catholyte present at the cathode, but experimental evidence show that the cell with 40 vol % of catholyte performs better. The best experimental system could provide an energy density of up to 182 W h·kg−1 and 345 W h· L−1, if a fully dense system with a cathode loading of 1 mA h· cm2, a 10 μm thick Al collector, and a 15 μm thick Li-metal disk were used. This study clearly highlights the need to change the active material from LFP to high-voltage cathodes and increase cathode loading to at least 3 mA h·cm−2 in order to achieve electric vehicle battery targets and ensure the future application of Li-metal SSBs.

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Figure 6. Interfacial resistances at 70 °C of a three-electrode Li-metal battery containing a cathode laminate with 40 vol % of catholyte: (a) cathode−membrane and (b) Li metal−membrane interfaces.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b04626.

the total cell resistance. In a SSB, the quality of all interfaces is particularly important to ensure fast ionic transport across the cell, and additional studies should be dedicated to further understand the current cell limitations. Outlook. According to the present experimental results, the cathode with the 40 vol % of catholyte provides a better cycling behavior compared to the other cathode formulations, which implies a higher real cell specific energy density. Even if this experimental system were to provide the same experimental performance under the hypotheses used for the calculation of its theoretical energy density (182 W h·kg−1 and 345 W h·L−1), a significant gap remains in order to achieve the recommendations provided by the United States Advanced Battery Consortium on requirements for electric vehicle application (350 W h·kg−1 and 750 W h·L−1 at cell level).25 Increasing the cathode loading to 3 mA h·cm−2, replacing LFP by a highvoltage cathode such as LiCoO2 or LiNixMnyCozO2, and decreasing the electrolyte thickness to 25 μm would make such target values feasible. Future work in our laboratory will aim toward those goals.

DSC and stripping/plating ability of the composite membrane, coin-cell assembly and cathode preparation details, and experimental composition of the cathode formulations (PDF) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 945 29 71 08. ORCID

Andrea I. Pitillas Martinez: 0000-0002-7010-3836 Frédéric Aguesse: 0000-0002-5675-0711 Anna Llordés: 0000-0003-4169-9156 Lucienne Buannic: 0000-0003-3055-4058 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was carried out at CIC Energigune (Spain) and was funded by Gobierno Vasco, within the project framework ETORTEK CIC ENERGIGUNE 2015. A.L. thanks IKERBASQUE for financial support. M.S. and A.R. acknowledge funding from the German Federal Ministry of Education and Research (GLANZ, project number 03X4623A; FELIZIA: 03XP0026D).

CONCLUSIONS In this study, a polymer−ceramic (PEO−LLZO) composite electrolyte was tested in Li-metal SSBs with different cathode formulations to assess the role of the catholyte on the cell performance. The electrolyte presents a homogenous distribution of LLZO particles in its volume and provides a promising ionic conductivity at 70 °C (1.7 × 10−4 S·cm−1) combined to a low activation energy above 53 °C (0.35 eV). Stripping/plating H


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The Cathode Composition, A Key Player in the Success of Li-Metal

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