Premanand Ramadass

Development of First Principles Capacity Fade Model for Li-Ion Cells

A first principles-based model has been developed to simulate the capacity fade of Li-ion batteries. Incorporation of a continuous occurrence of the solvent reduction reaction during constant current and constant voltage (CC-CV) charging explains the capacity fade of the battery. The effect of parameters such as end of charge voltage and depth of discharge, the film resistance, the exchange current density, and the over voltage of the parasitic reaction on the capacity fade and battery performance were studied qualitatively. The parameters that were updated for every cycle as a result of the side reaction were state-of-charge of the electrode materials and the film resistance, both estimated at the end of CC-CV charging. The effect of rate of solvent reduction reaction and the conductivity of the film formed were also studied.

Mathematical modeling of the capacity fade of Li-ion cells

A capacity fade prediction model has been developed for Li-ion cells based on a semi-empirical approach. Correlations for variation of capacity fade parameters with cycling were obtained with two different approaches. The first approach takes into account only the active material loss, while the second approach includes rate capability losses too. Both methods use correlations for variation of the film resistance with cycling. The state of charge (SOC) of the limiting electrode accounts for the active material loss. The diffusion coefficient of the limiting electrode was the parameter to account for the rate capability losses during cycling.

Solvent Diffusion Model for Aging of Lithium-Ion Battery Cells

This work presents a rigorous continuum mechanics model of solvent diffusion describing the growth of solid-electrolyte interfaces ~SEIs! in Li-ion cells incorporating carbon anodes. The model assumes that a reactive solvent component diffuses through the SEI and undergoes two-electron reduction at the carbon-SEI interface. Solvent reduction produces an insoluble product, resulting in increasing SEI thickness. The model predicts that the SEI thickness increases linearly with the square root of time. Experimental data from the literature for capacity loss in two types of prototype Li-ion cells validates the solvent diffusion model. We use the model to estimate SEI thickness and extract solvent diffusivity values from the capacity loss data. Solvent diffusivity values have an Arrhenius temperature dependence consistent with solvent diffusion through a solid SEI. The magnitudes of the diffusivities and activation energies are comparable to literature values for hydrocarbon diffusion in carbon molecular sieves and zeolites. These findings, viewed in the context of recent SEI morphology studies, suggest that the SEI may be viewed as a single layer with both micro- and macroporosity that controls the ingress of electrolyte, anode passivation by the SEI, and cell perfor

Capacity Fade of Sony 18650 Cells Cycled at Elevated Temperatures: Part II. Capacity Fade Analysis

A complete capacity fade analysis was carried out for Sony 18650 cells cycled at elevated temperatures. The major causes of capacity loss were identified and a complete capacity fade balance was carried out to account for the total capacity loss of Li-ion battery as a function of cycle number and temperature. The three most significant parameters that cause capacity loss were loss of secondary active material (LiCoO2/carbon) and primary active material (Li+) and the rate capability losses. Intrinsic capacity measurements for both positive and negative electrode has been used to estimate the capacity loss due to secondary active material and a charge balance gives the capacity lost due to primary active material (Li+). Capacity fade has been quantified with secondary active material loss dominating the other losses.

Performance study of commercial LiCoO2 and spinel-based Li-ion cells

Abstract The performance of Cell-Batt ® Li-ion cells and Sony 18650 cells using non-stoichiometric spinel and LiCoO 2 , respectively, as positive electrode material has been studied under several modes of charging. During cycling, the cells were opened at intermittent cycles and extensive material and electrochemical characterization was done on the active material at both electrodes. Capacity fade of spinel-based Li-ion cells was attributed to structural degradation at the cathode and loss of active material at both electrodes due to electrolyte oxidation. For the Sony cells both primary (Li + ) and secondary active material (LiCoO 2 )/C) are lost during cycling.

Comparison of the capacity fade of Sony US 18650 cells charged with different protocols

A new varying current decay (VCD) protocol, which charges the Li-ion battery at a faster rate, was developed. The performance of the battery charged using the VCD protocol was compared with the performance of batteries charged with conventional constant current–constant voltage (CC–CV) and constant voltage (CV) protocols. The destructive physical analysis tests at the end of 150 cycles indicated higher impedance for the cells cycled using the VCD protocol compared to the cell charged using the conventional (CC–CV) mode. The observed increase of the impedance was due to a small increase of the potential above the cut-off value of 4.2 for short times. A complete capacity fade material balance as a function of number of cycles was performed in order to account for the total capacity loss due to different charging protocols used. The loss of primary active material (Li + ), the secondary active material (LiCoO2/carbon) and the rate capability losses were determined for Sony US 18650 Li-ion cells and compared for different charging protocols.

Synthesis, characterization and cycling performance of novel chromium oxide cathode materials for lithium batteries

Chromium oxide (CrOx) cathode material (with chromium oxidation state of +5.3) was synthesized by thermal decomposition of chromium trioxide at high temperature and pressure in oxygen atmosphere. The duration of thermal decomposition had a significant effect on the performance of these materials in terms of lithiation capacity. The detrimental effect of CrO 3 and lower oxidation state chromium oxides have been reduced considerably by reducing their amounts in the material. The operating conditions, namely, temperature, pressure and the reaction time were optimized based on synthesizing novel CrOx cathode material with superior properties, such as low irreversible capacity loss, stable capacity and low capacity fade under continuous cycling. These materials are stable intercalation hosts for lithium and were found to be reversible in the entire intercalation range (2.0–4.2 V versus Li/Li + ). The average voltage of these cells is 3 V versus Li/Li + . CrOx-B cathodes exhibit higher capacity than any of the prominent cathode materials used for lithium batteries with an initial lithiation capacity of capacity of 322 mAh/g. The material shows very low capacity fade during cycling and retained 93% of its reversible capacity after 100 cycles.

Safety of Lithium-Ion Batteries

Abstract Lithium-ion (Li-ion) batteries currently represent the state-of-the-art power source for all modern consumer electronic devices. As several new applications for Li-ion batteries emerge like Electric Drive Vehicles (EDVs) and Energy Storage Systems (ESSs), cell design and performance requirements are constantly evolving and present unique challenges to the traditional battery producers. A strong demand for safe and reliable performance of high-energy and high-power density Li-ion batteries thus becomes inevitable. This chapter focuses on the safety aspects of Li-ion batteries on a system level and on a cell level. Also, most commonly practiced abuse tolerance tests have been explained with actual cell test data. Furthermore, internal short and lithium deposition occurring in Li-ion cells and failure mechanism associated with them are discussed.

Capacity fade studies on spinel based Li-ion cells

The performance of Cell-Batt/sup (R)/ Li-ion cells using nonstoichiometric spinel as the positive electrode material has been studied at different charging rates. The capacity of the cell was optimized based on varying the charging current and the end potential. Subsequent to this, the capacity fade of these batteries was studied at different charge currents. For all charge currents, the resistance of both the electrodes does not vary significantly with cycling. Comparison of cyclic voltammograms of spinel and carbon electrode before and after 800 cycles reveals a decrease in capacity with cycling. Low rate charge-discharge studies confirmed this loss in capacity. The capacity loss was approximately equally distributed between both electrodes. On analyzing the XRD patterns of the spinel electrode that were charged and discharged for several cycles, it can be seen that apart from the nonstoichiometric spinel phase, an additional phase slowly starts accumulating with cycling. This is attributed to the formation of defect spinel product /spl lambda/-MnO/sub 2/ according to a chemical reaction, which also leads to MnO dissolution in the electrolyte. EDAX analysis of the carbon samples shows an increase in Mn content with cycling. These studies indicate that capacity fade of spinel based Li-ion cells can be attributed to: (i) structural degradation at the cathode; and (ii) loss of active materials at both electrodes due to electrolyte oxidation.

Capacity fade of Li-ion cells cycled at different temperatures

Cycling characteristics of Sony 18650 cells with LiCoO/sub 2/ as the positive electrode material have been studied at different temperatures with DC charging protocol. The capacity fade of lithium ion cells was found to increase with increase in temperature. Cells cycled at RT and 45/spl deg/C showed a capacity fade of 30% and 36% respectively after 800 cycles, while the cell cycled at 55/spl deg/C failed to continue beyond 500 cycles. The rate capability of the cells continues to decrease with cycling. This can be attributed to increased resistance at both electrodes. Impedance measurements for both full and half-cells show an overall increase in the cell resistance with cycling and temperature. Charge-discharge studies on individual pellet electrodes show a reduced tendency for lithiation for both LiCoO/sub 2/ and carbon. XRD studies of the positive electrode up to 300 cycles shows a decrease in the lithium stoichiometry with cycling and temperature. Capacity fade in Sony 18650 cells is attributed to oxidation of cathode (LiCoO/sub 2/). Both primary (Li/sup +/) and secondary active material (LiCoO/sub 2//C) is lost during cycling.