Cell expansion and contraction are major concerns for battery pack and module developers. This work is an effort to understand the behavior of cell expansion due to formation, lithiation and cycling, with measurements made by a system that uses a linear variable differential transformer sensor. For a NCM622-Silicon cell, impact of factors such as initial compression pressure and calendaring of electrodes on cell expansion were explored. Reversible expansion during charge/discharge in a cycle is a function of cell capacity and reversible expansion can be reduced with increasing initial compression. Irreversible expansion during cycling grows linearly with the number of cycles and can also be reduced with initial compression of the cell. With initial compression of 45 psi, measurements showed 3% cell expansion during formation, 4% reversible expansion in a charge/discharge cycle with C/5 rate, and 12% irreversible expansion over 220 cycles. From the data, a hypothesis was developed suggesting that SEI growth and plastic deformation of the silicon particles could be the main causes for irreversible expansion of the cells with silicon electrodes, with the assumption that cells show less than 20% capacity fade over life.

Herein we present a Si electrode fabrication process that includes a phase-inversion step subsequent to slurry-based electrode casting and discuss its consequences for Si//Ni0.6Co0.2Mn0.2O2 cell performance. The phase inversion consists of extracting 1-methyl-2-pyrrolidinone with water and the concomitant coagulation of the polyacrylonitrile binder. Phase inversion improves capacity retention by 50% during C/5 cycling of Si//Ni0.6Co0.2Mn0.2O2 coin cells between 3.0 and 4.2 V. Phase-inversion Si electrodes have (1) 80% porosity compared to 55% for standard electrodes; and (2) bimodal pore size distribution, consisting of micropores (as in standard electrodes) and macropores with dimensions of 2 to 20 μm. The surface film mass growth rate in phase-inversion electrodes is smaller by 24% than in air-dried Si electrodes. Furthermore, during electrochemical cycling, the overall thickness change rate in phase-inversion electrodes is 5x smaller than in air-dried electrodes. Additionally, the high porosity electrodes display a reduced tendency to deform during electrochemical cycling. The insertion of a phase-inversion step into the electrode fabrication process may thus mitigate the volume expansion of the cell, enabling efficient module and pack design, while also increasing battery durability.

We formulated a model that describes the diffusion, volume change, and mechanical compression, coupled with multi-site-multi-reaction theory of the porous electrodes, and we apply the treatment to battery cells with silicon as anode active material. Irreversible thermodynamics and conservation laws have been used to tie all of the equations together. For cell lithiation (charge), changes in the porosity, cell thickness, and cell electrochemical resistance due to increase in active material volume and mechanical compression are calculated. Experimental data on cell expansion is collected on pouch cells with silicon anode and NMC622 the cathode; the model compares favorably with the data. Model simulations show that during the C/5 charge cycle, particle expands by 10% and porosity of the electrode decreases by approximately 8%. The model can be exercised to evaluate the cell operating regime for meeting targets and design considerations. Simulation studies revealed the importance of compression pressure and the spring constant on cell expansion.

Lithium silicide (Li-Si) compounds were synthesized by a reaction between ball-milled commercially available micron sized precursor powders of lithium hydride and silicon. The reaction occurs at ≤500 °C, with the composition controlled by the LiH-to-Si ratio. The products are crystalline equilibrium phases found in the Li-Si phase diagram. The synthesized Li-Si material has a particle size akin to that in the precursor Si, thus eliminating the need for attrition processing before electrode fabrication. Furthermore, the crack-free Li-Si particles may reduce gassing and improve capacity retention in cells. Synthesized Li-Si was also found to be relatively stable during handling in ambient air.