Development of high power hybrid cathodes for lithium ion batteries
(2020)
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- Abstract
- In 2014, the group of Prof. J-F Gohy developed a composite material made of redox polymer (PTMA) and conductive carbon for lithium-ion batteries. Later, in Scientific Reports, they described the hybridization of an intercalation material (LiFePO4) and their redox composite in a lithium ion battery cathode. The synergy between the materials enabled the improvement of the charge power performance as well as an enhancement of the lifetime of the electrode. This discovery raised the interest of the Solvay battery team and a collaborative thesis between UCLouvain and SOLVAY started with the objective to bring this electrochemical hybridization concept from the lab to the industry. SOLVAY market studies showed that the most important growth will happen in the automotive sector. Since this sector requires cathodic materials working at higher potential than LFP, higher voltage hybrid cathodes needed to be developed to evaluate the commercial interest of the concept. In such configuration, the synergy between the high voltage intercalation material and the redox composite is favorable to improve the power density during the discharge of the cathode. The Doctiris program was based on the 3 following objectives: (i) Evaluate the scalability of the composite production process (ii) prove that replacing the LiFePO4 by a higher voltage intercalation material will improve the power density during the discharge (iii) Develop the hybrid electrode concept and demonstrate competitive advantages in comparison with existing electrodes. Regarding the objective 1, we were able to propose a scale up synthesis of 500 g of composite material with electrochemical properties close to the lab scale synthesis. Although the performances could still be improved, the feasibility at large scale synthesis of high performances composite material was demonstrated. Regarding the objective 2, we demonstrated that the replacement of LiFePO4 (LFP) by a higher potential material: (LiMn2O4, LMO) enables synergetic improvements of both the specific power during the discharge and the cathode lifetime. Regarding the objective 3, it was necessary to replace the LMO by nickel-rich materials like Li(NixMnyCoz)O2 (NMC, x+y+z=1) which already dominates the automotive market. In this context, the evaluation of hybrid NMC-PTMA cathode brought to light incompatibilities between the two materials penalizing the performances of the electrode. As a matter of fact, the main issue came from the solvent used in the electrode manufacturing step. In contact with water, the nickel-rich oxide is altered and deleterious surface reaction occurs. Since the stability of NMCs towards water has not been solved today, the water processing is not seen as a competitive option. To address the aqueous processing issues, we studied a production process based on an organic solvent (NMP). However, the composite material swells and shrinks during the drying step inducing mechanical stress to the electrode. The optimization of the process decreased the deformation and hybrid electrodes with higher power performances were obtained. However, the remaining mechanical stress impacted negatively the electrode cycle life. Moreover, we observed a lower benefit for thicker cathodes (80 µm) due to lower ion diffusion. As a last solution, we studied an electrode manufacturing process without solvent as it is the only way to valorize the concept in the battery industry. This process was based on the fibrillation of polytetrafluoroethylene to ensure good mechanical properties. The process was optimized towards the production of self-standing electrode films with a thickness of 100 µm. However, the performances of the hybrid was lower than the single constituent NMC reference. The high thickness of the electrode affects the ionic diffusion impacting the redox process of the composite material. Even though we did not observe any improvement, additional work should be carried out in order to improve this process and confirm our conclusions. Our study highlighted the fact that the additional power performances brought by the presence of the composite material depend on the electrode thickness. As the thickness increases, the power improvement decreases. Therefore, the hybrid concept might not be relevant to increase the power performances of thick cathodes (> 50 µm) used in the automotive sector. Further development with more adequate equipment would be required. Beside that the evaluation of thin cathodes (< 50 μm) produced by dry process should be also performed when a more adequate equipment will be available. As a main conclusion, if after optimization with the adequate equipment, the hybrid composite material could offer higher power performances in the case of thick electrodes, this asset would meet the market expectation and could open the route to a valorization in the automotive market.
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Citations
Dolphijn, G. (2020). Development of high power hybrid cathodes for lithium ion batteries. https://hdl.handle.net/2078.5/117662