Quality control: Graphitization degree and orientation index in graphite anode materials

Introduction

Modern batteries, like Li-ion, have revolutionized our day-to-day life – from enabling smart mobile devices to pollution-free electric cars and intelligent power-management solutions. Batteries also hold the potential to be economical solutions for mass energy storage, complementing renewable energy resources for power grid applications. Irrespective of the cathode chemistry, which can be either LFP or NCM-based, most commercial batteries use graphite as an anode material. Battery-grade graphite is sourced from one of two sources: natural or synthetic. Natural graphite is mined, whereas synthetic graphite is produced from petroleum coke (leftover carbon from petroleum refining) by heating it to temperatures above 2500°C. Favored by EV battery manufacturers for its consistency and performance, synthetic graphite dominates the anode supply chain.

Degree of graphitization

The quality of the synthetic graphite is usually measured using a parameter commonly known as the degree of graphitization. The degree of graphitization is a measure of how similar the synthetic graphite is to ideal graphite. Petroleum coke is a disordered form of carbon, while graphite is a highly ordered crystalline material. When coke is heated to high temperatures it gradually transforms into crystalline graphite. However, when produced at an industrial scale, its transformation into graphite crystalline micro-structure may not be complete, and the final product may still have some disorder present. 

In graphite, the ordered carbon layers are stacked along the c-axis with a spacing of 0.3354 nm. In crystallographic parlance, this is referred to as the interplanar spacing between 002 planes. In disordered carbon, this interplanar spacing increases to 0.3440 nm. As the disordered carbon graphitizes, the d-spacing gradually decreases and moves closer to the crystalline graphite d-spacing. By measuring the d₀₀₂ interplanar spacing of the synthetic graphite, the degree of graphitization (g) can be estimated using the following Equation 1:

Measuring the degree of graphitization 

X-Ray diffraction (XRD) is the best method for measuring the degree of graphitization, as it can directly measure the interplanar d-spacings. The schematics of an XRD measurement are shown below:

From the measured 2θ position of the XRD peak, the interplanar d-spacing can be derived using the following Equation 2:

where λ = 0.154 nm (for a Cu K_α X-ray source).

As the difference between the carbon and graphite d₀₀₂ interplanar spacing is only 0.0046 nm, a very precise measurement of the peak position is required to estimate the degree of graphitization accurately. Absolute measurement of the peak position is prone to errors caused by several factors, such as changes in sample height during sample preparation, instrument misalignment, and more. An absolute peak position measurement in XRD is therefore unlikely to give reliable results on the degree of graphitization. The effect of these errors can be eliminated by using an internal reference standard with a known XRD peak position value. A reference standard is a well-defined crystalline material with a high-intensity peak that is close to, but well separated from, the graphite peak. Pure silicon powder, with its peak of 111 being close to graphite’s 002 peak, is a good internal reference standard for measuring the graphitization degree in carbon materials. Other crystalline materials, such as quartz, corundum, or rutile, can also be used. Usually, a 20-weight percentage of reference standard mixed into the graphite sample is sufficient for the analysis.

If silicon is used as the reference standard, then a diffractogram measured with Cu K_α radiation between 25-30° 2θ provides both the 002 peak of graphite and the 111 peak of silicon. Data produced by an Aeris compact XRD instrument measuring one such sample is shown below in Figure 4. 

The theoretical position of the silicon 111-peak is at 28.44° 2θ.  The measured diffractogram should first be corrected so that the measured silicon 111-peak matches the theoretical value. The d₀₀₂ spacing for synthetic graphite can be calculated from the measured 002-peak position using Equation 2, and the degree of graphitization can then be evaluated using Equation 1. Table 1 below shows the degree of graphitization measured with an Aeris compact diffractometer in five synthetic graphite samples.

Table 1:  Degree of graphitization measured in five synthetic graphite samples, using an Aeris compact diffractometer and a silicon reference standard.

Sample	Graphite peak position after reference correction (2θ)	d002	g%
Gr_Smp1	26.51	3.3611	91.7
Gr_Smp2	26.55	3.3561	97.5
Gr_Smp3	26.49	3.3636	88.9
Gr_Smp4	26.52	3.3599	93.2
Gr_Smp5	26.50	3.3623	90.3

Orientation index

This parameter is relevant to graphite coatings on current collectors. Depending on the coating process and control parameters, graphite particles may have specific orientations, schematically shown in Figure 5. 

Orientations like 110, 100, etc., in which the c-axis of the graphite is in the plane of the current collector, offer much better electronic and ionic conductivity compared with the 001 orientation, in which the c-axis is out-of-plane from the current collector. Due to their elongated crystallographic shape, (long c-axis) graphite particles tend to be oriented along the 001 direction, resulting in high resistance to ion transport. However, their orientation can be randomized by agglomerating these particles into spherical shapes, leading to improved ionic conductivity. In the presence of strong magnetic fields, these can even be preferentially oriented along the 100 or 110 directions, resulting in further improvements to their ionic and electronic conductivities.

The overall crystallographic orientation of graphite particles in the coating can be measured using a parameter called the orientation index. A random orientation means an equal weight fraction of graphite particles oriented along the 001 and 110 (or any other) crystallographic axes. The orientation index (OI) is defined as the ratio of weight fraction oriented along, say, 110 (or any other direction orthogonal to 001) to that oriented along 001. Thus, OI is the ratio of the weight fraction of particles with their c-axis in the plane of the current collector to the weight fraction of particles with their c-axis out of the plane of the current collector. In the case of a random orientation, the OI value would be 1. If particles are preferably orientated along 001, then the OI would be less than 1.

Measuring orientation index

XRD offers an easy and accurate way of measuring OI in graphite coatings. The simplest way to estimate it is from the ratio of the 110 and 004 (fourth-order reflection of 001 planes) peak intensities (areas). Theoretically, in a randomly oriented graphite material, f = I110/I004 = 0.63. A value less than 0.63 would mean that particles are preferentially oriented along 001. Sometimes, the intensity ratio, R = I004/I110 (=1/f) is measured instead. In a random orientation, R = 1.6 and values larger than 1.6 indicate a preferred orientation along 001.

When measured with Cu K_α radiation, the theoretical peak positions for 004 and 110 reflections are at 54.29° and 77.37° 2θ. An example pattern of a graphite coating on a Cu current collector is shown in Figure 6 below:

The peak labels in this graph show the hkl index, the peak area, and crystal phase. In this case, the areas of the 004 and 110 peaks are 373.84 and 73.40 respectively. So,f = 73.40/373.84 = 0.20, andR = 5.1. This indicates a preferred orientation along the 001 axis.

Parameters f or R do not give an absolute value of the orientation index. However, they can be used to compare preferred orientations between different batches of samples – a lower f value (or higher value of R) would mean a higher orientation degree along 001, and a value of 0.63 for f (or 1.6 for R) would mean a random orientation.

An absolute orientation index value can be obtained by the Rietveld refinement of the full XRD pattern, and by using the March Dollase (MD) function to model the peak intensity variation due to the preferred orientation along a specific orientation. Rietveld refinement can also correct the peak intensity variation caused by the finite thickness of the coating layer. As the graphite layer is just a few tens of a micrometer thick, the X-rays will penetrate through the entire layer thickness beyond a certain 2θ angle. The fact that we see Cu peaks in the XRD pattern indicates that X-rays have penetrated through the graphite layer and diffracted from the current collector. Finite thickness causes a reduction in the intensity of peaks occurring at higher angles. Rietveld refinement can also simulate finite layer thickness, giving accurate results for the orientation index. The March Dollase orientation parameter has a value of 1 for randomly oriented samples, and lower values indicate a preferred orientation along the specific direction.

An example of the orientation index obtained from full-pattern Rietveld refinement of the XRD pattern is shown in Figure 7.

Figure 7: Example XRD pattern measured in the angular range of 25° to 90° 2θ. The pattern was Rietveld-refined, including finite thickness correction and March Dollase preferred orientation along 001 (MD 001). 

Results from five samples of two different types are shown below in Table 2.

Table 2 : Orientation index measured with XRD on five samples, using the intensity ratio and March Dollase methods.

Sample	f	R	MD 001
Random orientation (Theoretical)	0.63	1.6	1
GR_Cu_A1	0.2	5.1	0.7
GR_Cu_A2	0.17	6	0.68
GR_Cu_A3	0.26	3.8	0.77
GR_Cu_B1	0.06	17.5	0.51
GR_Cu_B2	0.09	11.4	0.56

The March Dollase orientation parameter can also be reliably deduced from a shorter-range XRD measurement between 40° to 58° 2θ. This can reduce the measurement time significantly. This range can also be covered by a large detector in static mode, opening up the possibility of the real-time online measurement of the OI.

XRD instrument selection

Either a compact XRD (Aeris) or floor-standing XRD (Empyrean) can be used to analyze both the degree of graphitization and the orientation index in graphite powders and coatings.

Aeris XRD

A compact XRD available in 600W or 300W versions. The typical sample measurement time is 5-10 minutes, but can be significantly faster in this type of application.

Key system highlights include:

• No need for an external chiller or computer;
  
• Easy touch-screen operation;
  
• Up to six-position autosampler;
  
• Compact footprint and low maintenance;
  
• Maximum sample size (coated electrode) of up to 35 mm.
  

Empyrean XRD

A full-power XRD operating at 1800W. The typical sample measurement time is 5 minutes.

Key system highlights include:

• Best-quality data with monochromatic optics and energy-dispersive detector;
  
• Up to 45-position autosampler for high throughput;
  
• Possibility of analyzing large electrode samples. 
  

Online XRD

Malvern Panalytical specializes in online automation for roll-to-roll processes. We can customize the solution based on roll width and other process parameters. Please contact us if you are interested in online XRD solutions.

Figure 8 : Schematics of an online XRD

Key online XRD highlights include:

• Fast static-mode measurements using a large detector;
  
• Online real-time data on electrode coating orientation index;
  
• Complete measurement and analysis automation.