Operando Microcalorimetry Study on Irreversible Losses in Silicon Anodes
Authors: Allcorn Eric , Harbour Teal , Langendorf Jill , Johnson Linda 
Published: December 2020

Isothermal microcalorimetry (IMC) is an effective characterization tool for the measurement of irreversible reactions within lithium-ion batteries. When carried out on batteries under an electrical load, the measured isothermal heat flow is composed of three sources: ohmic heat, reversible heat, and parasitic heat. When a highly reversible cell - i.e. with coulombic efficiency greater than 99% - is studied, the equation can be reduced to enable simplified measurement of the total parasitic heat per cycle.1 However, for moderately reversible cells like those studied here, equation 1 is employed in combination with complementary measurements to determine ohmic and reversible contributions, and parasitic heat flow is subsequently isolated.2

(1) dQ/dt = I*(E_load - E_eq) + IT*(dE_eq/dT) + dQ_para/dt
Herein, we apply operando IMC to half-cells composed of high silicon content anodes (80 wt.% silicon nanoparticles) versus lithium foil. Silicon is an attractive next-generation anode material for lithium-ion batteries due to it’s low reaction potential and extremely high storage capacity of ~3500 mAh/g. However, silicon electrodes suffer from limited cycle life and continual capacity decay related to mechanical degradation and silicon surface reactivity with the standard carbonate electrolytes employed in lithium-ion batteries. The mechanical degradation arises from the large volume change of silicon during lithiation of nearly 400%, while the surface reactivity is a result of a non-passivating solid-electrolyte interphase (SEI) layer along with continual exposure of fresh active surfaces as the silicon changes volume during cycling.
Both the degree of volume change and degree of surface reactivity are dependent upon the voltage window over which silicon is cycled. In addition, both avenues of capacity loss have different thermal indicators: mechanical pulverization is associated with increased ohmic heat due to higher cell resistance, while surface reactivity losses are correlative with parasitic heat flow. In this study we strive to separate the two causes of capacity loss as a function of voltage window, C-rate, operational temperature, and chosen electrolyte. Figure 1 below shows the parasitic heat generated per mole of irreversible lithium loss for silicon half cells operated at 30°C with 1.2M LiPF6 in EC/EMC 3/7 electrolyte. Cells with a 50 mV lower cut-off have non-linear relations between parasitic heat and capacity loss, indicating (along with ohmic heat values not included here) that mechanical pulverization plays a significant role in capacity loss. While the cell with a 100 mV lower cut-off has a linear relation between lost capacity and parasitic heat flow. This suggests that for the 100 mV cell, the capacity loss is primarily related to surface reactions, and the slope of the data curve yields an average enthalpy of reaction for this SEI layer formation as 105 kJ/mol-Li. Such data can help to inform cell design and operational ranges for optimum cycle life, while providing data of the surface species formed on active silicon.
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Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. SAND2020-5384 A.

Figure 1