Deconstructing Dual-Layer Coated Graphite Anodes: A Multi-Dimensional Physical Property Analysis

1. Introduction

In lithium-ion battery manufacturing, double-layer coating technology has emerged as a practical route to balance the perennial trade-off between energy density and power density. A typical double-layer anode design employs a top layer (near the separator) of small-particle, rate-capable graphite to facilitate rapid lithium-ion intercalation, while the bottom layer (adjacent to the copper current collector) uses large-particle, high-compaction graphite to deliver stable specific capacity. Although the industry often adopts a wet, single-pass co-extrusion process for simultaneous double-layer coating, a critical question remains unanswered: does the intended bilayer structure survive after drying, calendering, and downstream processing? Currently, no universally accepted layer-resolved characterization method exists to verify the preserved gradient distribution or to detect interlayer intermixing.

Figure 1. Multi‑gradient layered electrode design: High porosity in the top layer, high compaction in the bottom layer

In this study, we mechanically peeled a finished, calendered double-layer coated graphite anode and collected powder samples from the top, middle, and bottom regions for multi-dimensional physical characterization. Our objective was to determine whether the original property differences between the top and bottom layers are retained after industrial coating, drying, and calendering.

2. Experimental Methods

2.1 Sample Information

We analyzed a commercial double-layer coated graphite anode roll supplied by a manufacturer. The actual sample was a double-side coated graphite electrode with a total thickness of approximately 208 μm and a current collector thickness of ~3 μm, giving a single-side coating thickness of about 100 μm. According to the design specifications, the top layer (surface side) used a rate-type graphite, the bottom layer (foil side) a capacity-type graphite, with a designed thickness ratio of 1:1 (approximately 50 μm each). The type and content of conductive additives and binder were identical for both layers.

2.2 Powder Sampling by Layer

Using an automatic electrode powder scraping device (LEPS, IEST Instruments) — we sequentially scraped powder samples from three depth intervals on the single-side coating: 0–30 μm, 30–60 μm, and 60–90 μm, as illustrated in Figure 2.

Figure 2. Schematic diagram of powder sampling at different depths on one side of a double-side coated anode.

2.3 Characterization Methods

• Particle size distribution: Laser diffraction particle size analyzer (Topsizer, OMEC, Zhuhai)

• Powder resistivity and compaction density: Powder resistivity & compaction density tester (IEST PRCD Series)

• X-ray diffraction (XRD): SmartLab SE (Rigaku, Japan)

• Raman spectroscopy: XploRA (HORIBA) and RTS2 (Zolix, Beijing)

3. Results and Discussion

Note: The data presented below reflect only the specific double-layer coated anode sample tested in this work. Repeatability was confirmed through parallel experiments. These results do not represent products from other manufacturers or batches but serve as a realistic physical characterization of this particular sample.

3.1 Particle Size Distribution

We used a wet dispersion mode with water as the dispersant. The laser diffraction results (Table 1) reveal a clear gradient in particle size across the three layers. The top-layer powder (rate-type graphite) exhibits the smallest D50 and a narrower size distribution — a feature that shortens the solid-state lithium diffusion path and enhances rate capability. In contrast, the bottom-layer powder (capacity-type graphite) shows larger particles and a broader distribution, which improves packing density and volumetric energy density. The middle layer displays intermediate values.

Table 1. Particle size distribution of top, middle, and bottom layer powders

3.2 Powder Resistivity and Compaction Density

We evaluated powder resistivity and compaction density using a IEST PRCD Series. Figure 3 shows the resistivity–pressure curves. At low to medium pressures (0–80 MPa), the three layers exhibit distinguishable resistivity values, with the top layer showing the highest resistivity (i.e., poorest electronic conductivity). At pressures above 80 MPa, the differences diminish. The higher resistivity of the top layer is consistent with its rate-optimized graphite, which typically contains more structural defects and higher disorder.

Figure 3. Powder resistivity vs. pressure curves for top, middle, and bottom layer samples.

For compaction density, the industry commonly uses two methods: the direct pressure method (measuring density under applied pressure) and the ten-point pressure-release method (measuring the density of the pressed pellet after unloading). These two approaches capture the “under compression” and “after spring-back” states, respectively. The results for both methods are summarized in Figure 4. Under the direct pressure method, the top-layer powder shows similar compaction density to the middle and bottom layers. However, under the pressure-release method, a clear difference emerges: the top layer exhibits a significantly lower compaction density. This behavior suggests that the top-layer (rate-type) graphite may contain secondary particles or a soft-carbon composite structure, providing internal pores and interfacial buffer spaces that lead to pronounced elastic spring-back after unloading. In contrast, the bottom-layer (capacity-type) graphite is more dense and deforms plastically with minimal spring-back — an advantage for achieving high electrode compaction and structural stability.

Figure 4. Powder compaction density of top, middle, and bottom layers: direct pressure method (left) vs. ten-point pressure-release method (right).

3.3 X-ray Diffraction (XRD) Analysis

XRD patterns collected from 5° to 90° at 2°/min are shown in Figure 5. All three layers exhibit typical graphite (002) and (004) reflections.

Figure 5. XRD patterns of top, middle, and bottom layer powders.

From the (002) peak, we calculated interlayer spacing, Scherrer width (crystallite thickness Lc), and graphitization degree. The top-layer powder exhibits a slightly lower graphitization degree, while the middle and bottom layers have d002 values close to that of ideal graphite (0.3354 nm) and much higher graphitization degrees. As shown in Table 2:

Table 2. XRD parameters of top, middle, and bottom layer powders

The narrower (002) peak FWHM and larger Lc in the middle and bottom layers indicate more perfect crystal integrity along the c-axis. We attribute the lower graphitization and higher disorder of the top layer to its rate-type design — moderate reduction in graphitization or introduction of a surface coating, which creates more channels for lithium intercalation/deintercalation at the expense of perfect stacking order. This trend aligns with the powder resistivity results: the middle and bottom layers, with higher crystal perfection and continuous sp² carbon networks, show superior electronic conductivity.

3.4 Raman Spectroscopy

Raman spectroscopy is a powerful tool to probe defects in carbon materials. A higher ID/IG ratio indicates greater structural disorder or more defects.

Figure 6. Representative Raman spectrum (measured from the bottom layer powder).

We used a 532 nm excitation laser and collected data from five points per sample; Table 3 summarizes the average values. Figure 6 shows a representative Raman spectrum measured from the bottom layer powder. The top-layer powder shows the highest ID/IG and a broader G-band FWHM, indicating more surface defects, abundant edge active sites, or an amorphous carbon coating — all hallmarks of rate-type graphite. Middle and bottom layers show lower ID/IG and higher structural order, consistent with the XRD graphitization results.

Table 3. Raman D and G band parameters of top, middle, and bottom layer powders

*Note: The original manuscript’s Table 3 values are presented as above. The top layer shows higher ID/IG and broader G band FWHM, indicating more surface defects or amorphous carbon coating.*

4. Summary

By precisely scraping a double-layer coated graphite anode at controlled depths, we obtained powder samples from the top, middle, and bottom regions and analyzed their differences in particle size distribution, compression/spring-back behavior, and crystallographic characteristics. Our results confirm that for the analyzed double-layer coated graphite anode, the top (surface) layer is a rate-type graphite featuring surface defects or a coating layer, while the bottom layer consists of larger particles with higher graphitization degree and more perfect crystal structure — the capacity-type graphite. The LEPS automatic electrode scraping device (IEST Instrument) proved essential for enabling this layer-resolved structural analysis, offering a robust sample preparation method for process optimization and performance traceability in double-layer coated anodes.