Adam Timmons

Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents

The solubility of lithium salts in dimethyl carbonate (DMC) found in solid electrolyte interface (SEI) films was determined. The salt-DMC solutions evaporated, and the salts were transferred into water for ion conductivity measurements. The salts examined included lithium carbonate (Li 2 CO 3 ), lithium oxalate [(LiCO 2 ) 2 ], lithium fluoride (LiF), lithium hydroxide (LiOH), lithium methyl carbonate (LiOCO 2 ,CH 3 ), and lithium ethyl carbonate (LiOCO 2 C 2 H 5 ). The salt molarity in DMC ranged from 9.6 X 10- 4 mol L -1 (LiOCO 2 CH 3 ) to 9 X 10 -5 mol L -1 (Li 2 CO 3 ) in the order of LiOCO 2 CH 3 > LiOCO 2 C 2 H 5 > LiOH > LiF > (LiCO 2 ) 2 > Li 2 CO 3 - X-ray photoelectron spectroscopy measurements on SEI films on the surface of the negative electrode taken from a commercial battery after soaking in DMC for 1 h suggested that the films can dissolve. Separately, the heat of dissolution of the salts was calculated from computer simulations for the same salts, including lithium oxide (Li 2 O), lithium methoxide (LiOCH 3 ), and dilithium ethylene glycol dicarbonate [(CH 2 OCO 2 Li) 2 :LiEDC] in both DMC and ethylene carbonate (EC). The results from the computer simulations suggested that the order in which the salt was likely to dissolve in both DMC and EC was LiEDC > LiOCO 2 CH 3 > LiOH > LiOCO 2 C 2 H 5 > LiOCH 3 > LiF > (LiCO 2 ) 2 > Li 2 CO 3 > Li 2 O. This order agreed with the experiment in DMC within the experimental error. Both experiment and computer simulations showed that the organic salts are more likely to dissolve in DMC than the inorganic salts. The calculations also predicted that the salts dissolve more likely in EC than in DMC in general. Moreover, the results from the study were used to discuss the capacity fading mechanism during the storage of lithium-ion batteries.

Threefold Increase in the Young’s Modulus of Graphite Negative Electrode during Lithium Intercalation

Density functional theory (DFT) is used to reveal that the polycrystalline Youngs modulus (E) of graphite triples as it is lithiated to LiC 6 . This behavior is captured in a linear relationship between E and lithium concentration suitable for continuum-scale models aimed at predicting diffusion-induced deformation in battery electrode materials. Alternatively, Poissons ratio is concentration-independent. Charge-transfer analyses suggest simultaneous weakening of carbon-carbon bonds within graphite basal planes and strengthening of interlayer bonding during lithiation. The variation in bond strength is shown to be responsible for the differences between elasticity tensor components, C ij , of lithium-graphite intercalation (Li-GIC) phases. Strain accumulation during Li intercalation and deintercalation is examined with a core-shell model of a Li-GIC particle assuming two coexisting phases. The requisite force equilibrium uses different Youngs moduli computed with DFT. Lithium-poor phases develop tensile strains, whereas Li-rich phases develop compressive strains. Results from the core-shell model suggest that elastic strain should be defined relative to the newest phase that forms during lithiation of graphite, and Li concentration-dependent mechanical properties should be considered in continuum level models.

Diffusion Induced Stresses and Strain Energy in a Phase-Transforming Spherical Electrode Particle

Lithium insertion and removal in lithium ion battery electrodes can result in diffusion induced stresses (DISs) which may cause fracture and decrepitation of electrodes. Many lithium ion electrode materials undergo formation of two or more phases during lithium insertion or removal. In this work, we mathematically investigate the DISs in phase transforming electrodes using a coreshell structural model. We examine the concentration jumps at phase boundaries that result in stress discontinuities, which in turn can cause cracking. The influence of the mechanical properties of the two phases on stress evolution, stress discontinuity, and strain energy are clarified. The trends obtained with the model may be used to help tune electrode materials with appropriate interfacial and bulk properties so as to increase the durability of battery electrodes.

In Situ Optical Observations of Particle Motion in Alloy Negative Electrodes for Li-Ion Batteries

Direct observation of the particles of active material as they react in situ is necessary to build an understanding of the interaction mechanisms present when large volume-change active materials are employed in composite electrodes. In situ optical observations of a-Si 0.64 Sn 0.36 composite electrodes show dramatic interparticle movement; particles from below the electrode surface are pushed up, particles above the electrode surface shift downward, particles crash and overlap, particles become inactive, and some particles move so far that they become detached from the electrode.

Isotropic Volume Expansion of Particles of Amorphous Metallic Alloys in Composite Negative Electrodes for Li-Ion Batteries

Direct observation of the particles of active material as they react in situ is necessary to build an understanding of the growth characteristics in large-volume-change active materials. A comparison between in situ optical and atomic force microscopy observations shows that particles of amorphous large-volume-change active material expand isotropically upon lithium insertion. Using the isotropic expansion upon lithium insertion characteristic of amorphous alloys, the volume change of other amorphous active materials upon lithium insertion can be measured with an optical microscope by observing only the change in particle area.

In Situ AFM Measurements of the Expansion and Contraction of Amorphous Sn-Co-C Films Reacting with Lithium

In situ atomic force microscopy measurements of patterned amorphous Sn-Co-C sputtered films reacting with Li in an electrochemical cell have been made. Prismatic-shaped patches of Sn 0.34 Co 0.19 C 0.47 were found to undergo reversible volume expansion of 175 ± 5% [(V final - V initial )/V initial ] without fracture. The a-Sn 0.34 Co 0.19 C 0.47 was found to have a reversible specific capacity of 700 ± 10 mAh/g when cycled vs a Li metal negative electrode.

In Situ AFM Measurements of the Expansion of Nanostructured Sn–Co–C Films Reacting with Lithium

The volume changes vs lithium content of nanostructured Sn 1-x-y Co x C y thin films with different compositions have been measured using in situ atomic force microscopy (AFM) experiments. For the films studied, the Co content, x, varied 0.18 < x < 0.5 and the carbon content, y, varied 0 < y < 0.79. In all cases the volume varied linearly with lithium content, and the maximum volume expansion ranged from 142 to 204%, depending on the initial composition, for compositions with y < 0.5. The experimental values have been compared with theoretical estimates calculated based on 8.9 ± 0.5% cm 3 /mol for the alloyed Li atoms as proposed by Obrovac [J. Electrochem. Soc., 154, A849 (2007)]. The agreement is excellent except for the sample that has y = 0.79, or 79% carbon.