The Effect of Particle Size and the Material Loading to the Performance and Stability of the LFP Batteries
Authors: Karaman Berke , Couto Luis Daniel , Kinnaert Michel , Job Nathalie 
Published: September 2023

LiFePO4 (LFP) is one of the most studied materials for Li-ion battery cathodes. It provides excellent safety and long cycle life; however, it displays limited capacity when compared to other cathode materials. The physical properties of LFP strongly affect the performance and stability of the final battery. Among them, particle size is known to be crucial and, in addition, the maximum acceptable electrode loading certainly depends on it. Herein, the effect of particle size as well as the loading of the coating on the performance and stability of the LFP electrode is studied by using both galvanostatic charge-discharge and electrochemical impedance spectroscopy. Two powders displaying average particle sizes of 0.88 µm (LFP-1) and 0.26 µm (LFP-2) have been selected to prepare two different sets of electrodes via water-based spray with Xanthan Gum binder. Different coating loadings were prepared on circular current collectors (Æ = 1.55 mm): 3 mg (1.59 mg/cm2), 6 mg (3.17 mg/cm2) and 12 mg (6.35 mg/cm2). Half-cells were analysed according to their capacity, electrochemical properties and capacity retention after cycling.

First, the effect the loading on the capacity of the cells was determined within both LFP-1 and LFP-2 sample groups (Figure 1). Results show that both groups are indeed affected by increasing the active mass loading, but the LFP-2 series to a lower extent, whatever the cycling rate. In the meantime, the rate capabilities of the electrodes were studied by comparing the electrode capacity as a function of the C-rates. Overall, electrodes with the lowest loading (3 mg) can sustain their capacity on higher C-rates regardless of the sample group. It confirms that lower loadings of the coating are preferable for high C-rate applications. The difference between LFP-1 and LFP-2 sets are much more apparent for 6 mg and 12 mg-loaded electrodes as the relative difference can reach 25%; this result shows that active materials with smaller particle size are more suitable for high C-rate applications.

Capacities and capacity retention results were investigated by using EIS in order to understand the electrochemical property difference between the two powders as well as between the different electrode loadings. Firstly, effect of loading amount has been observed and for LFP-1 cells, when the loading increases (from 3 mg to 12 mg), a resistance (Rs) increase (from 15 to 56 Ω) can be observed. Secondly, effect of particle size change has been observed and it has been seen that the charge transfer resistance (Rct) decreases strongly (from 93 to 53 Ω) between LFP-1 and LFP-2 for 3mg loading cells respectively. This is a direct consequence of higher surface area of LFP-2.

Second, the stability of the electrodes was examined. To that aim, half-cells were submitted to a sequence of varied C-rate cycling similar to the performance measurement. Then, they were cycled for 100 cycles at C and, finally, the varied C-rate sequence was applied again. The first and second sequences were compared with each other to obtain capacity retention between the the first and second sequences, which enables observing the effect of ageing.

Results (Table 1) show electrodes with lower loading get less effected by the degradation, regardless of the LFP used. For both powders, the capacity remains high even at high C-rates. The reverse can be told for the electrodes with the highest loading. Regardless of the LFP chosen, the degradation is quite harsh as it can go as low as 28% capacity retention. This suggests that thin coatings are affected less by the degradation while it is quite dramatic when the loading increases. Comparison between LFP-1 and LFP-2 shows the impact of the powder particle size: electrodes made of smaller particles are less effected by the degradation. The difference can go up to 36% and 69% between LFP-1 and LFP-2 respectively.

Electrochemical properties were investigated for 3 mg-loaded electrodes with the EIS before and after cycling. Increase in all observed values (Rs and Rct) has been observed after cycling (Table 2). The increase in values are much more dramatic for LFP-1, which explains the worse stability performance upon long-term cycling.

Overall, LFP-2 cells display lower Rs and Rct as a direct result of high surface area resulting from their lower particle size. Those properties give them the advantage of being more tolerant to increasing loadings and increasing C-rate. In addition, cells prepared with smaller LFP particles are affected less by the degradation, possibly because the morphology is less affected by the cycling. Further study of morphology or crystallography modifications will be conducted to determine the origin of degradation.

Figure 1