High Energy Density of Binder Free Sintered Electrodes Made By Spark Plasma Sintering

During the last decades, most improvements in energy density of Li-ion batteries were done at both materials and electrode architecture levels. However, even today, the best batteries (eg laptop) have a small amount of active material (approximately 51% by weight for cells with high energy density), the rest divided between the current collectors 21%, packaging 13%, electrolyte 6.5%, binders and additives 8.5%. Attempts to increase the thickness of the electrodes and the electrode surface capacity (in mAh/cm²) while keeping architectures of conventional electrodes have encountered many difficulties mainly due to limitations in terms of electronic or ionic conduction or due to the limitations of charge transfer at the interfaces whose result is a strong limitation in terms of capacity even at very low rate of cycling.

A very interesting solution to overcome these limitations was proposed 3 years ago by sintering the active material at high temperature providing the electronic contact while maintaining the initial preformed porosity. Indeed previous works have shown the possibility to prepare thick electrodes (LiCoO₂ or LiFePO₄) but the electrode processes are complicated to implement and the porosity is not necessarily controlled effectively [1-3].

In this work, we propose to form thick composite sintered electrodes (active material, carbon, porosity) using a sintering technique of choice which is the Spark Plasma Sintering (SPS) which limits unwanted side chemical reactions thanks to the rapidity of the process. Our preliminary works carried out at LRCS have already shown the feasibility to manufacture by SPS thick porous composites electrodes. Indeed, by incorporating a salt (NaCl) in the electrodes, it is possible to dissolve it after sintering and therefore to generate porosity within the electrode. The porosity (pore size and volume) can be easily controlled by its percentage in volume and the particle size of NaCl. 

Our first electrochemical tests are very encouraging. On a thick electrode of LiFePO4, with 50% volume porosity, it was possible to obtain a capacity of 30 mAh/cm² for a theoretical capacity of 41 mAh/cm² at a rate of C/20, in liquid electrolyte, at room temperature and without additional carbon. Also, with Li₄Ti₅O₁₂(the electrode having a porosity of 50% after sintering and dissolution of the salt), it was possible to obtain the theoretical capacity of 22 mAh/cm² in the first discharge.

In this presentation, we will show the impact of the nature and the morphology of the porosity agent, the SPS conditions together with the addition of the conductor additives on the electrochemical performances of the individual electrodes (Capacity, Power rate studies) and in full cell configurations.

[1] Wei Lai, Can K. Erdonmez, Thomas F. Marinis, Caroline K. Bjune, Nancy J. Dudney, Fan Xu,    Ryan Wartena, and Yet-Ming Chiang, Adv. Mater. 2010, 22, E139–E144

[2] Xue Qin, Xiaohui Wang, Jie Xieab and Lei Wen, J. Mater. Chem., 2011, 21, 12444

[3] Chang-Jun Bae , Can K. Erdonmez , John W. Halloran , and Yet-Ming Chiang, Adv. Mater. 2013, 25, 1254–1258