Kaushik Kalaga

Nanosized silicon materials are being developed for use in the anodes of high-energy lithium-ion batteries. However, the high surface areas of these materials increase the rate of parasitic reactions in the electrode, which consume cyclable Li⁺ and degrade battery performance. Prelithiation offers a realistic strategy to compensate for this reactivity, by injecting additional charge into the cell to counterbalance the Li⁺ loss. Interestingly, the benefits offered by prelithiation extend beyond its more obvious purpose. Here, by using a reference electrode in NMC532//Si–Gr cells, we show how prelithiation alters the cycling potentials experienced by the Si-containing anode and how that translates into gains in cycle life. The rate of consumption of the prelithiated charge is lower than that expected from the behavior of non-prelithiated cells. Curiously, the Si particles become partially unresponsive during the C/3 cycling apparently because of kinetic constraints. Electrochemical studies on harvested electrodes in half-cells show that capacities are intact after the long-term cycling and that most of the lithium reservoir is still present in the anode. We conclude that the high capacity retention displayed by the prelithiated cells mainly results from a higher participation of graphite particles during the extended electrochemical cycling.

Safe, fast, and energy efficient cycling of lithium ion batteries is desired in many practical applications. However, modeling studies predict steep Li⁺ ion gradients in the electrodes during cycling at the higher currents. Such gradients introduce heterogeneities in the electrodes, which make it difficult to predict cell lifetimes as different portions of the cell age at different rates. There is a dearth of experimental methods to probe these concentration gradients across the depth of the electrode. Here we use spatially resolved energy dispersive X-ray diffraction to obtain a “movie” of lithiation and delithiation in different sections of the cell and quantify lithium gradients that develop in a porous graphite electrode during cycling at a 1C rate. Inhomogeneity in the total Li content, and in the individual ordered Li_xC₆ phases formed during lithium insertion into (and extraction from) the graphite, has been observed in an operando fashion. The complex dynamics of lithium-staging in graphite with the distinct front propagation of phase changes have been characterized and new features of these dynamics are highlighted here. As large Li⁺ ion gradients contribute to cell polarization, our results suggest that Li plating conditions can be met near the graphite electrode surface, even when the cell is charged at a moderate (1C) rate.

In Situ Lithiated Reference Electrode Four Electrode Design For In-operando Impedance Spectroscopy. JoVE publishes peer-reviewed scientific video protocols to accelerate biological, medical, chemical and physical research. Watch our scientific video articles. JoVE is the world-leading producer and provider of science videos with the mission to improve scientific research, scientific journals, and education. Millions of scientists, educators and students at thousands of universities, colleges, hospitals and biopharmaceutical companies worldwide use JoVE for their research, teaching and learning.

Nickel-rich Ni-Co-Mn (NCM) or Ni-Co-Al (NCA) layered-oxide positive electrodes (cathodes) allow high-voltage operation of lithium-ion cells with increased energy density, but their long-term cycling causes gradual increase in their impedance that slows down lithiation and delithiation of the material. Here we report that pairing of these cathodes with lithium titanate (Li₄Ti₅O₁₂, LTO) negative electrodes (anodes) accelerates this impedance rise when compared to pairing with graphite (Gr) electrodes, during potentiostatic holds (calendar-aging) at sufficiently high (> 4.2 V vs. Li/Li⁺) layered-oxide potentials. Our results suggest that gases generated in the cell play an important role in this impedance rise. In the graphite cells, these gases are gradually depleted by reactions at the Gr electrode, but such reactions do not occur at the LTO electrode. The generation and buildup of gases degrades the electron conduction network within the cathode, and also leads to modifications at the oxide-electrolyte interfaces, causing the observed impedance rise. The calendar-aging also alters cell capacity, with losses observed in the Gr cells and gains observed in the LTO cells. Possible causes and consequences of these changes in the lithium-ion inventory of the cells are discussed.

The demand for high energy densities in Li-ion batteries (LIBs) necessitates exploring bottlenecks of cell performance. Layered oxide positive electrodes, such as LiNi_xCo_yMn_1-x-yO₂ (NCM), and silicon-graphite composite (Si-Gr) negative electrodes in a LIB undergo a series of complex structural and compositional changes during electrochemical cycling. The induced changes have a direct influence on the energy and power output from a battery and a profound dependence on the cycling or aging conditions. The promise of multifold enhancement in energy densities of LIBs using Si based negative electrodes is accompanied by the limited stability of the electrode-electrolyte interface. In commercial LIBs, the solid electrolyte interphase (SEI) layer on the graphite negative electrode facilitates Li+ ion conduction and minimizes reduction of the electrolyte. In cells with Si- containing negative electrodes, the large volume changes of Si during cycling results in continuous degradation of the SEI layer. The formation of a new SEI on the newly-exposed Si surfaces during subsequent cycling causes further trapping of Li+ ions, contributing to performance loss. In the present study, full cells with NCM 523 cathode, Si-Gr (containing 15 wt% Si) anode, and a baseline electrolyte (1.2 M LiPF₆ in Ethylene Carbonate/Ethyl Methyl Carbonate, 3:7 w/w) were tested under calendar life ageing and cycle life ageing protocols between 3 and 4.1 V. The cycle-life cells showed ~92% capacity loss after 100 cycles, while the calendar-life cells showed ~13% capacity loss; the reasons for this difference will be highlighted during the presentation.¹ In order to further improve capacity retention we studied the effect of various additives to the baseline electrolyte, and also examined EC-free electrolyte systems. Advanced diagnostic techniques, including X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) were used to examine the effect of these additives on the SEI layer.^2,3 Insights from electrochemical response in conjunction with post-mortem characterization studies have fostered deeper understanding of the charge transfer mechanisms and the properties of the interfacial layers in the blended negative electrodes. Fundamental knowledge of the nature of this dynamic SEI layer is essential to design better surface films and achieve stable electrochemical cycling. Support from David Howell, Brian Cunningham, and Peter Faguy of the U.S. Department of Energy’s Office of Vehicle Technologies is gratefully acknowledged. This work was performed under the auspices of the US Department of Energy, Office of Vehicle Technologies, Hybrid and Electric Systems, under Contract No. DE-AC02-06CH11357 K. Kalaga et al., manuscript in preparation (2017). K. Kalaga, I.A. Shkrob, R.T. Haasch, C. Peebles, J. Bareño, D.P. Abraham, “Auger Electrons as Probes for Composite Micro and Nanostructured Materials: Application to Solid Electrolyte Interphases in Graphite and Silicon-Graphite Electrodes”, J. Phys. Chem. C 2017, 121, 23333−23346. J. Bareño, I.A. Shkrob, J.A. Gilbert, M. Klett, D.P. Abraham, “Capacity Fade and Its Mitigation in Li-ion Cells with Silicon-Graphite Electrodes”, J. Phys. Chem. C 121 (2017), pp 20640-20649.

Rechargeable batteries capable of operating at high temperatures have significant use in various targeted applications. Expanding the thermal stability of current lithium ion batteries requires replacing the electrolyte and separators with stable alternatives. Since solid-state electrolytes do not have a good electrode interface, we report here the development of a new class of quasi-solid-state electrolytes, which have the structural stability of a solid and the wettability of a liquid. Microflakes of clay particles drenched in a solution of lithiated room temperature ionic liquid forming a quasi-solid system has been demonstrated to have structural stability until 355 °C. With an ionic conductivity of ∼3.35 mS cm–1, the composite electrolyte has been shown to deliver stable electrochemical performance at 120 °C, and a rechargeable lithium battery with Li4Ti5O12 electrode has been tested to deliver reliable capacity for over several cycles of charge–discharge.