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Historically, lithium cobalt oxide and graphite have been the positive and negative electrode active materials of choice for commercial lithium-ion cells. It has only been over the past ~15 years in which alternate positive electrode materials have been used. As new positive and negative active materials, such as NMC811 and silicon-based electrodes, are being developed, it is crucial to evaluate the potential of these materials at a stack or cell level to fully understand the possible increases in energy density which can be achieved. Comparisons were made between electrode stack volumetric energy densities for designs containing either LCO or NMC811 positive electrode and silicon-graphite negative electrodes, where the weight percentages of silicon were evaluated between zero and ninety percent. Positive electrode areal loadings were evaluated between 2.00 and 5.00 mAh cm−2. NMC811 at 200 mAh g−1 has the ability to increase stack energy density between 11% and 20% over LCO depending on percentage silicon and areal loading. At a stack level, the percentage of silicon added results in large increases in energy density but delivers a diminishing return, with the greatest increase observed as the percentage of silicon is increased from zero percent to approximately 25–30%. [ABSTRACT FROM AUTHOR]

This work aims to review and understand the behavior of the electrochemical lithiation onset of amorphous silicon (a-Si) films as electrochemically active material for new generation lithium-ion batteries. The article includes (i) a review on the lithiation onset of silicon films and (ii) a mechanochemical model with numerical results on the depth-resolved mechanical stress during the lithiation onset of silicon films. Recent experimental studies have revealed that the electrochemical lithiation onset of a-Si films involves the formation of a Li-poor phase (Li0.3Si alloy) and the propagation of a reaction front in the films. The literature review performed reveals peculiarities in the lithiation onset of a-Si films, such as (i) the build-up of the highest mechanical stress (up to 1.2 GPa) during lithiation, (ii) a linear increase in the mechanical stress with lithiation which mimics the characteristics of linear elastic deformation, (iii) only a minute volume increase during Li incorporation, which is lower than expected from the number of Li ions entering the silicon electrode, (iv) the largest heat generation appearing during cycling with only a minor degree of parasitic heat contribution, and (v) an unexpected enhanced brittleness. The literature review points to the important role of mechanical stresses in the formation of the Li-poor phase and the propagation of the reaction front. Consequently, a mechanochemical model consisting of two stages for the lithiation onset of a-Si film is developed. The numerical results calculated from the mechanochemical model are in good accord with the corresponding experimental data for the variations in the volumetric change with state of charge and for the moving speed of the reaction front for the lithiation of an a-Si film of 230 nm thickness under a total C-rate of C/18. An increase in the total C-rate increases the moving speed of the reaction front, and a Li-rich phase is likely formed prior to the end of the growth of the Li-poor phase at a high total C-rate. The stress-induced phase formation of the Li-poor phase likely occurs during the lithiation onset of silicon electrodes in lithium-ion battery. [ABSTRACT FROM AUTHOR]

Historically, lithium cobalt oxide and graphite have been the positive and negative electrode active materials of choice for commercial lithium-ion cells. It has only been over the past ~15 years in which alternate positive electrode materials have been used. As new positive and negative active materials, such as NMC811 and silicon-based electrodes, are being developed, it is crucial to evaluate the potential of these materials at a stack or cell level to fully understand the possible increases in energy density which can be achieved. Comparisons were made between electrode stack volumetric energy densities for designs containing either LCO or NMC811 positive electrode and silicon-graphite negative electrodes, where the weight percentages of silicon were evaluated between zero and ninety percent. Positive electrode areal loadings were evaluated between 2.00 and 5.00 mAh cm−2. NMC811 at 200 mAh g−1 has the ability to increase stack energy density between 11% and 20% over LCO depending on percentage silicon and areal loading. At a stack level, the percentage of silicon added results in large increases in energy density but delivers a diminishing return, with the greatest increase observed as the percentage of silicon is increased from zero percent to approximately 25–30%.

This work aims to review and understand the behavior of the electrochemical lithiation onset of amorphous silicon (a-Si) films as electrochemically active material for new generation lithium-ion batteries. The article includes (i) a review on the lithiation onset of silicon films and (ii) a mechanochemical model with numerical results on the depth-resolved mechanical stress during the lithiation onset of silicon films. Recent experimental studies have revealed that the electrochemical lithiation onset of a-Si films involves the formation of a Li-poor phase (Li0.3Si alloy) and the propagation of a reaction front in the films. The literature review performed reveals peculiarities in the lithiation onset of a-Si films, such as (i) the build-up of the highest mechanical stress (up to 1.2 GPa) during lithiation, (ii) a linear increase in the mechanical stress with lithiation which mimics the characteristics of linear elastic deformation, (iii) only a minute volume increase during Li incorporation, which is lower than expected from the number of Li ions entering the silicon electrode, (iv) the largest heat generation appearing during cycling with only a minor degree of parasitic heat contribution, and (v) an unexpected enhanced brittleness. The literature review points to the important role of mechanical stresses in the formation of the Li-poor phase and the propagation of the reaction front. Consequently, a mechanochemical model consisting of two stages for the lithiation onset of a-Si film is developed. The numerical results calculated from the mechanochemical model are in good accord with the corresponding experimental data for the variations in the volumetric change with state of charge and for the moving speed of the reaction front for the lithiation of an a-Si film of 230 nm thickness under a total C-rate of C/18. An increase in the total C-rate increases the moving speed of the reaction front, and a Li-rich phase is likely formed prior to the end of the growth of the Li-poor phase at a high total C-rate. The stress-induced phase formation of the Li-poor phase likely occurs during the lithiation onset of silicon electrodes in lithium-ion battery.