SUPPLEMENTARY INFORMATIONARTICLE NUMBER: 16097 | DOI: 10.1038/NENERGY.2016.97
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Magnetically aligned graphite electrodes for high-rate performance Li-ion batteries
Supplementary information for
Magnetically aligned graphite electrodes for high
rate performance Li-ion batteries
Juliette Billaud1+, Florian Bouville2+, Tommaso Magrini2, Claire Villevieille1*, André R.
Studart2*
+ Authors have contributed equally to this work
* Corresponding authors: [email protected], [email protected]
1- Paul Scherrer Institut, Electrochemical Laboratory, 5232 Villigen PSI, Switzerland
2- Complex Materials, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland
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SUPPLEMENTARY INFORMATION DOI: 10.1038/NENERGY.2016.97
Supplementary Figures
Supplementary Figure 1 | Comparison of the aligned and non-aligned anodes of this study (in
red and blue) with literature values for the areal capacity of anodes versus C-rate.
References: commercial graphite anodes1, Graphite SFG6 and SFG442,3, 3D Li4Ti5O124
Supplementary Figure 2 | Micrograph of graphite flakes (Alfa Aesar, Graphite flake, Natural, -
325 Mesh, 99.8%, metals basis) after alignment in water under a rotating magnetic field
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Supplementary Figure 3 | Setup used for casting of graphite suspensions followed by
magnetic alignment of flakes
Supplementary Figure 4 | Schematic drawing illustrating the effect of static and rotating
magnetic fields on the orientation of flakes.generated by a 400 mT Neodymium magnet. The
white arrow in the bottom left corner indicates the rotation plane of the magnet.
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SUPPLEMENTARY INFORMATION DOI: 10.1038/NENERGY.2016.97
Supplementary Figure 5 | Effect of thickness reduction by calendering on the crystallographic
texture of the initially aligned sample. a. Comparison of the diffractogram obtained at different
thickness reductions on an electrode with aligned graphite platelets. The blue curve
corresponds to the pristine aligned electrode, whereas the red indicates the reference
sample. Peaks indexed with a * correspond to the copper current collector. b. Intensity of the
(002) peak in the aligned electrode after calendering divided by the intensity of the peak for a
reference electrode as a function of the relative decrease in thickness imposed by
calendering.
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Supplementary Figure 6 | SEM micrograph of an aligned electrode after 50
lithiation/delithiation cycles. The current collector has been removed after cycling.
Supplementary Figure 7 | Galvanostatic cycle for high loading (9.1 mg/cm2) electrodes at
C/30 rate for the two first cycles and C rate for the remainder of the test.
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Supplementary Figure 8 | Zeta potential of the graphite powder in water as a function of
the pH (Alfa Aesar, Graphite flake, Natural, -325 Mesh, 99.8%, metals basis).
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Supplementary Figure 9 | Structural characterisation of electrodes. a. Pictures of the aligned
and reference electrodes generated after processing data obtained from the Focused Ion
Beam (FIB) / Scanning Electron Microscopy (SEM) images (structures are displayed at the
same scale). b. Example of diffusivity calculation indicating the boundary conditions assumed.
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Supplementary Figure 10 | Streamline of the diffusive flux for two different concentration
gradient directions for the reference electrode and the aligned ones
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Supplementary Table
Supplementary Table 1| Data used to calculate the tortuosity factor tensor.
Direction
Flux
(mol.m2/s)
C0
(mol/m3) L1 (m) ε Deff (m2/s) D0 (m2/s) τ
Aligned
x 1.59 10-5 10 1.93 10-5 0.32 9.55 10-11 3.00 10-10 3.15
y 2.07 10-6 10 3.67 10-5 0.32 2.38 10-11 3.00 10-10 12.66
z 1.21 10-5 10 2.07 10-5 0.32 7.84 10-11 3.00 10-10 3.84
Not
aligned
x 2.54 10-5 10 1.71 10-5 0.45 9.66 10-11 3.00 10-10 3.08
y 1.96 10-5 10 2.93 10-5 0.45 1.2710-10 3.00 10-10 2.34
z 6.86 10-6 10 1.40 10-5 0.45 2.13 10-11 3.00 10-10 13.95
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Supplementary Notes
1. Comparison of the capacity of various anodes at different charging rates
We calculated the areal capacities of our electrodes and compared them in Supplementary
Fig. 1 with data reported in the literature. Several important aspects are illustrated in this
graph. First and most importantly, it shows that the alignment of flakes alone leads to a
remarkable increase in areal capacity, even if the graphite source used is not optimised for
batteries. Second, graphite flakes have been previously shown to result in high areal
capacities at high rate if the particles size is optimised for lithium insertion (see SFG6
compared to SFG44 grades). This implies that flake geometries (in addition to spherical
mesocarbon microbeads, MCMB-type) can also provide higher capacities that can potentially
rival commercial electrodes if the formulation is further optimised with current industrial know-
how. Finally, commercial graphite anodes exhibit the highest areal capacities, which reflects
the continuous incremental optimisation of these materials in industry. However, these high
capacities are only possible at low rate, as the highly-loaded electrodes (i.e. with an areal
capacities superior to 2 mAh/cm2) could not be cycled above C/33. The comparison between
the performance of experimental anodes with battery-grade graphite flakes (SFG6 and
SFG44) with commercial electrodes indicates that formulations that have been continuously
optimised in industry for decades can increase the area capacity by a factor of 3 as compared
to values reported for standard recipes in the open, academic literature. Such comparison
further emphasises the major breakthrough achieved in our study, since a similar 3-fold
increase in performance was achieved by solely aligning flakes in an orientation that
facilitates mass transport of Li ions through the electrode thickness. Although further work is
still needed, our approach clearly shows the great potential of controlling the architecture of
the electrode alone as a simple and effective means to significantly change the areal capacity
of industrially-relevant graphite anodes.
2. Effect of calendering on platelet alignment
Using X-ray diffraction, we followed the alignment of the platelets after calendering an aligned
electrode at various thicknesses. Changes in alignment of the flakes can be monitored
accurately by measuring the intensity of the (002) peak, as this peak intensity is
representative of the fraction of horizontally aligned platelets.X-ray diffraction measurements
were performed at room temperature with a PANalytical Empyrean diffractometer using Cu
Kα-radiation. The results are summarised in Supplementary Fig. 5. The intensity of the peak
does not increase until the thickness of the electrode has been decreased by 30%, with an
electrode thickness reduction of 45 µm in this case. This means that the aligned structure can
be compressed and densified by 30 % without any major misalignment of the platelets. The
increase in intensity observed if the thickness is reduced further indicates a progressive
reorganisation of the structure, while still preserving a preferred orientation. This is clearly
noticed in Supplementary Fig. 5 b if the results obtained for the aligned sample are compared
with those of a reference electrode as a function of thickness reduction.
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Those preliminary results show that in our sub-optimised case, calendering can be performed
to a certain extent without strongly affecting the electrode alignment
Supplementary Methods
1. Calculation of the effective diffusivity tensor from FIB-Tomography data
1.1 Mesh production and optimisation
The stacks obtained from FIB-Tomography were segmented using the open source software
Fiji5 as described in the main text. The plug-in statistical region merging6 was used with a
number of merged regions of 25. Then a simple threshold was applied (Supplementary Fig. 9
a). The open source plug-in BoneJ7 was used to extract STL files from the binary stacks with
the isosurface function. The STL files represent an unscaled meshed surface, but usually with
a poorly controlled vertices quality and thus need to be cleaned, scaled and optimised before
use. The open source software Meshlab was employed to first remove the unconnected
regions by applying the function “Remove isolated pieces (wt. diameter)” with a diameter of
100 pixels. Then the function “Select self-intersecting faces” was used to clean the mesh
further. A scaling function was also applied to change the mesh size; in our case one voxel
equals to 60x60x60 nm3. The surface mesh was then imported into the open source software
Gmsh8 to produce a 3D mesh. The software native mesh optimising tools was used before
exporting them as NASTRAN files.
1.2. Effective diffusivity calculation
The procedure described here has been adapted directly from an example available in the
software COMSOL Multi-physics 5.2 library. The 3D meshes were imported directly as
NASTRAN files. An automatic face detection function was used to select the different entry
and exit faces (Supplementary Fig. 9 b). The material properties were selected to represent
lithium hexafluorophosphate (LiPF6) in 1:1 EC:DEC, which is the electrolyte composition we
used in our experimental measurements. A bulk diffusion coefficient 𝐷𝐷! = 3 ∙ 10!!" 𝑚𝑚!/𝑠𝑠 of
lithium in the electrolyte was assumed. A constant concentration of 𝐶𝐶! = 10 𝑚𝑚𝑚𝑚𝑚𝑚/𝑚𝑚! was
applied on one face and an external forced convection was applied on the other side, with a
mass transfer coefficient of 𝑘𝑘 = 1 𝑚𝑚/𝑠𝑠 and an external concentration equal to 𝐶𝐶!"# =
0 𝑚𝑚𝑚𝑚𝑚𝑚/𝑚𝑚!. The steady state is reached when the outward flux and concentration reaches a
constant level, named respectively 𝑗𝑗! and 𝑐𝑐!, as shown in Supplementary Fig. 9 b. Under such
condition, Fick’s first law can be written as:
𝑗𝑗! = −𝐷𝐷!"" ∙ ∇𝑐𝑐 (Eq. S1)
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SUPPLEMENTARY INFORMATION DOI: 10.1038/NENERGY.2016.97
where 𝑗𝑗! is the outward flux (equal to 𝑗𝑗! = 𝑘𝑘 ∙ 𝑐𝑐!), 𝐷𝐷!"" is the effective diffusivity coefficient in
the considered direction and ∇𝑐𝑐 is the concentration gradient. Thus, 𝐷𝐷!"" is expressed as,:
𝐷𝐷!"" = 𝑗𝑗! ∙!!
!!!!!= 𝑘𝑘 ∙ 𝑐𝑐! ∙
!!!!!!!
(Eq. S2)
where 𝐿𝐿! is the thickness of the mesh in the considered direction.
Finally, 𝐷𝐷!"" can now be expressed as a function of the bulk diffusion coefficient 𝐷𝐷!:
𝐷𝐷!"" =!!∙ 𝐷𝐷! (Eq. S3)
with 𝜖𝜖 being the electrode porosity calculated from the mesh volume in COMSOL, and 𝜏𝜏 being
the tortuosity factor, which in this case is an adjustable parameter representing the electrode
morphology in the considered direction9. The data resulting from the analysis of the two
stacks shown in Supplementary Fig. 9 are summarised in Supplementary Table 1 and plotted
in Fig. 3 of the main manuscript.
1.3. Streamline of the diffusive flux
To further illustrate the modification of mass transport in our structure, we used the streamline
option of the Comsol Software to plot a number of lines representing the mass flux direction
for two cases, one where the concentration gradient was along the x direction, and one where
it was along the z direction, for both electrodes (cf. Supplementary Fig. 10). Indeed, the lines
are straight and parallel to each other when the tortuosity factor is low, and randomly oriented
when the tortuosity factor is high.
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