Increasing the energy density of Li-ion batteries (LiB) is a key issue. A promising approach is to replace graphite in LIB anodes with silicon. The main challenge is to deal with the large silicon ...volume expansion induced by its lithiation, which damages the mechanical integrity (electronic network) of the electrode, and produces an unstable solid electrolyte interphase (SEI).
We have successfully improved the performance of silicon based anodes by working on various aspects. First, our ball-milled silicon offers the right nanostructure to limit Si particle cracking in addition to be produced using an industrially viable process 1. Second, the use of a mixture of carboxymethylcellulose (CMC) and citric acid (CA) as binder system buffer favors the formation of strong bonds between the binder and the Si particles 2 and a protective layer on the Si nanoparticles that significantly decreases electrolyte reduction 3. Lastly, the use of carbon nanoplatelets as conductive additive insures better ability of the electrode architecture to reversibly expand/contract upon cycling 4.
In this communication, we would like to present a postprocessing treatment (called maturation) that we have recently developed. Indeed, maturation very significantly improves the mechanical and electrochemical stabilities of silicon electrodes made with the CMC/CA binder 5. This treatment consists of storing the electrode in a humid atmosphere for a few days before drying and cell assembly. This results in a beneficial in situ reactive modification of the interfaces within the electrode. Our investigations suggest that the binder tends to concentrate at the silicon interparticle contacts. As a result, the cohesion of the composite film is strengthened. Moreover, the corrosion of the copper current collector, inducing the formation of copper carboxylate bonds, improves the adhesion of the composite film. This results in an impressive improvement of the electrode cycle life.
The calendering of Si-based electrodes is required to obtain a substantial gain in their volumetric capacity compared to conventional graphite electrode. However, the calendering of silicon/carbon nanoplatelets/CMC/CA electrodes induces a major decrease of their cycling stability. This can be attributed to the rupture of the particle-binder bridges during the calendering, lowering the mechanical strength of the electrode. Interestingly, we found that these cohesive bonds can be restored through the maturation treatment. From in-operando dilatometric experiments, it appears that the volumetric expansion is lower and more reversible than for a standard (not-calendered, not-matured) electrode. As a result, a remarkable improvement of the cycle life is observed 6.
Finally, we found that the maturation process is also efficient for silicon electrodes made with the polyacrylic acid (PAA) binder 7.
Acknowledgements
The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC) (grant RGPIN-2016-04524) and Transition Énergétique Québec (TEQ) (grant Techno-0040-0001) for financial support of this work.
References
1 M. Gauthier et al., “A low-cost and high-performance Si-based negative electrode for Li-ion batteries”, Energy Environ. Sci., 2013, 6, 2145-2155.
2 D. Mazouzi et al., “Silicon Composite Electrode with High Capacity and Long Cycle Life”, Electrochem. Solid-State Lett., 2009, 12, A215-A218.
3 C.C. Nguyen et al., “Improved Cycling Performance of a Si Nanoparticle Anode Utilizing Citric Acid as a Surface-Modifying Agent”, Langmuir, 2017, 33, 9254–9261.
4 Z. Karkar et al., “Threshold-like dependence of silicon-based electrode performance on active mass loading and nature of carbon conductive additive”, Electrochimica Acta, 2016, 215, 276-288.
5 C. Real Hernandez et al., “A Facile and Very Effective Method to Enhance the Mechanical Strength and the Cyclability of Si-Based Electrodes for Li-Ion Batteries”,
Adv. Energy Mater
, 2017, 1701787.
6 Z. Karkar et al., “How silicon electrodes can be calendered without altering their mechanical strength and cycle life”, J. Power Sources, 2017, 371, 136-147.
7 Z. Karkar et al., “A comparative study of polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) binders for Si-based electrodes”, accepted for publication in Electrochimica Acta.
The constant demand for lithium-ion batteries with higher energy density requires finding new electrode materials. Silicon-based electrodes are particularly attractive due to the higher gravimetric ...capacity of Si (3579 mAh g
-1
) compared to conventionally used graphite (372 mAh g
-1
). However, during the process of lithiation/delithiation, the silicon material suffers from a huge volume change, leading to the fracturing of the silicon particles, an unstable solid electrolyte interphase layer (SEI) and the disconnection of inter-particle contacts, which all have a negative repercussion on the electrode cycle life. Another serious challenge for commercializing silicon electrodes is to reach a high areal capacity of more than 6 mAh cm
-2
, in order to achieve an energy density improvement over the use of conventional graphite-based anodes. Silicon electrodes with such high areal capacity require very careful design of their formulation at different scales. In particular, a special attention must be paid to creating durable intimate contacts between the active material particles and the conductive additive network, so that sufficient electron transfer could be achieved throughout the electrode from the copper current collector while good mechanical stability of the electrode coating is still maintained.
1
Our group has recently shown that high performance silicon-based anodes can be achieved by combining
(i)
the use of high-energy ball-milling as a cheap and easy process to produce nanostructured silicon powder,
(ii)
the processing of the electrode with carboxymethylcellulose (CMC) binder at pH 3 condition, which promotes the covalent grafting of the CMC to the Si particles;
(iii)
the use of fluoroethylene and vinylene carbonates (FEC/VC) electrolyte additives resulting in a more stable SEI.
2
In the present work, silicon-based electrodes of various areal capacities were prepared by using either carbon black, vapor grown carbon nanofibers, or graphite nanoplatelets as conductive additive.
3
It was observed that the electrical conductivity, capacity retention, and coulombic efficiency of the silicon electrode are significantly affected by the morphological characteristics of the used carbon additives. Spherical-shaped carbon black particles have a strong tendency to agglomerate, and thus fail in creating a conductive network resilient to the silicon particle volume change. In contrast, vapor grown carbon nanofibers maintain more durable contacts with silicon particles, compared to carbon black, by forming a more resilient conductive network due to their wire-like structure.
4
Graphite nanoplatelets also create a continuous conductive network, which limits the mechanical degradation of the electrode coating, likely by playing the role of electrically conducting lubricant.
5
These results demonstrate that the choice of the conductive additive is of crucial importance for the optimization of silicon negative electrodes with commercially relevant areal capacities.
References
(1) Mazouzi, D.; Karkar, Z.; Reale Hernandez, C.; Jimenez Manero, P.; Guyomard, D.; Roué, L.; Lestriez, B.
J. Power Sources
2015
,
280
, 533–549.
(2) Gauthier, M.; Mazouzi, D.; Reyter, D.; Lestriez, B.; Moreau, P.; Guyomard, D.; Roué, L.
Energy Environ.
Sci.
2013
,
6
(7), 2145.
(3) Karkar, Z.; Mazouzi, D.; Reale Hernandez, C.; Guyomard, D.; Roué, L.; Lestriez, B.,
Submitted
(4) Lestriez, B.; Desaever, S.; Danet, J.; Moreau, P.; Plée, D.; Guyomard, D.
Electrochem.
Solid-State Lett.
2009
,
12
(4), A76.
(5) Nguyen, B. P. N.; Gaubicher, J.; Lestriez, B.
Electrochim.
Acta
2014
,
120
, 319–326.
Figure 1
Silicon-based electrode is a promising candidate in lithium-ion batteries (LIB) due to its significantly higher gravimetric capacity (3579 mAh g
-1
) in comparison to that of graphite (372 mAh g
-1
...). However, during the process of lithiation/delithiation, the silicon material suffers from a huge volume change which has a negative repercussion on the electrode cycle life through the fracturing of the silicon particles and of the solid electrolyte interphase layer (SEI) and the disconnection of inter-particle contacts. Our group has recently shown that high performance silicon-based anodes can be achieved by combining
(i)
the use of high-energy ball-milling as a cheap and easy process to produce nanostructured silicon powder,
(ii)
the processing of the electrode with carboxymethylcellulose (CMC) binder at pH 3 condition, which has been proved to be able to promote the covalent grafting of the CMC to the Si particles;
(iii)
the use of fluoroethylene and vinylene carbonates (FEC/VC) electrolyte additives resulting in a more stable SEI. (1)
One of the biggest challenges of commercializing silicon anodes is to reach a high areal capacity of more than 4 mAh cm
-2
, in order to achieve a volumetric energy density improvement over the use of conventional graphite-based anodes. Electrodes with such high areal capacity require careful design of their formulation at different scales, and in particular a special attention must be paid to the tailoring of durable intimate contacts between the active material particles and the conductive additive network so that sufficient electron transfer could be achieved throughout the electrode from the copper current collector.(2)
Here, silicon-based electrodes of various areal capacities were prepared by using either carbon black (Super P, Timcal), vapor grown carbon nanofibers (VGCFs, Showa Denko), or graphite nanoplatelets (GM15, XGSciences) as conductive additive. These electrodes were examined by using SEM, XRD, Raman, electrical four-probe method and galvanostatic charge/discharge tests. The objective was to establish the relationships between the characteristics of the carbon additive and the electrochemical performance of the electrode.
It was observed that the electrical conductivity, capacity retention, and coulombic efficiency of the silicon electrode are significantly affected by the shape, surface area, particle size and crystallinity of the used carbon additives. Spherical-shaped carbon black particles tend to agglomerate and fail in creating a conductive network resilient to the silicon particles’ volume variation. In contrast, vapor grown carbon nanofibers maintain more durable contacts with silicon particles by forming a more resilient conductive network due to their wire-like structure compared to carbon black.(3) Graphite nanoplatelets also create a continuous conductive network and seem to limit the mechanical degradation of the electrode coating, likely by playing the role of electrically conducting lubricant. (4) These results demonstrate that the choice of the conductive additive is of crucial importance for the optimization of silicon negative electrodes with commercially relevant areal capacities.
References
(1) Gauthier, M.; Mazouzi, D.; Reyter, D.; Lestriez, B.; Moreau, P.; Guyomard, D.; Roué, L.
Energy Environ. Sci.
2013
,
6
(7), 2145.
(2) Mazouzi, D.; Karkar, Z.; Reale Hernandez, C.; Jimenez Manero, P.; Guyomard, D.; Roué, L.; Lestriez, B.
J. Power Sources
2015
,
280
, 533–549.
(3) Lestriez, B.; Desaever, S.; Danet, J.; Moreau, P.; Plée, D.; Guyomard, D.
Electrochem. Solid-State Lett.
2009
,
12
(4), A76.
(4) Nguyen, B. P. N.; Gaubicher, J.; Lestriez, B.
Electrochimica Acta
2014
,
120
, 319–326.
Figure 1
For the last 10 years, a tremendous amount of work has been published to solve the problem of capacity fade of silicon-based electrodes which prevents their utilization in commercial lithium-ion ...batteries. The use of Si nanoparticles/nanowires to better accommodate large strain without cracking has developed and is very popular in the academic community. By playing on the nano-architecturing effect or tailoring the composite electrode formulation, several groups have reached up to 1000 cycles in half-cells versus lithium metal 1,2. However, a careful look at the papers shows that in all studies the active mass loading is very low, typically less than 1 mg per cm², and thus the practical surface capacity of the corresponding electrodes is low, typically less than 1 mAh per cm². This is much lower than that of the state of the art graphite-based negative electrode, which reaches up to 5 mAh per cm² in cellular phones for example. As a consequence, although silicon is much more attractive than graphite due to its very high gravimetric capacity (3572 mAh g
-1
versus
372 mAh g
-1
for graphite) and volumetric capacity (2249
versus
779 mAh cm
-3
for graphite), Si-based composite electrodes show lower practical surface capacity, as a matter of fact.
The point is that the cycle life of Si-based electrodes dramatically decreases as the active mass loading increases, e.g. 1000 cycles at 0.5 mg per cm² vs. 50 cycles at 4 mg per cm² (Figure 1). We demonstrated that using copper foam instead of copper foil as current collector shows a great advantage in the cycle life and power performance. More than 400 cycles at an impressive Si loading of 10 mg cm
-
² could be reached, i.e. with a surface capacity of 10 mAh cm
-2
3. The thinness of the composite coating on the foam walls favors a better preservation of the electronic wiring upon cycling and fast lithium ion diffusion. A higher coulombic efficiency in half cells with lithium metal as the counter electrode is achieved by using carbon nanofibers (CNF) rather than carbon black (CB). The possibility to reach in practice higher surface could allow a significant increase of both the volumetric and gravimetric energy densities by 23% and 19%, respectively, for the Cu foam-Silicon//LiFePO
4
stack compared to the Graphite/LiFePO
4
stack of traditional design.
Acknowledgements
Financial funding from the Agence Nationale de la Recherche (ANR) of France (BASILIC project) and the Natural Science and Engineering Research Council (NSERC) of Canada is acknowledged. The authors thank D. Pilon (Metafoam Inc.) for supplying the Cu foams.
References
1 L. Hu, F. La Mantia, H. Wu, X. Xie, J. McDonough, M. Pasta, Y. Cui, Adv. Energy Mater., 1, 1012 (2011).
2 I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov and G. Yushin, Science, 334, 75 (2011).
3 D. Mazouzi, , D. Reyter, M. Gauthier, P. Moreau, D. Guyomard, L. Roué, B. Lestriez, Adv. Energy Mater., DOI: 10.1002/aenm.201301718.
Figure 1.
(a) Surface SEM images of a Cu foam filled with 5 mg of Si/CNF/CMC/Buffer composite electrode (4.6 mg Si per cm
2
). (b) Cycle life as a function of the active mass loading for Foil-Si/CB/CMC/Buffer and Foam-Si/CNF/CMC/Buffer electrodes (Si//Li half-cell with LP30+2%VC+10%FEC, capacity limitation of 1200mAh per g of Si).
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