Bryant J. Polzin

The Role of Cesium Cation in Controlling Interphasial Chemistry on Graphite Anode in Propylene Carbonate-Rich Electrolytes

Despite the potential advantages it brings, such as wider liquid range and lower cost, propylene carbonate (PC) is seldom used in lithium-ion batteries because of its sustained cointercalation into the graphene structure and the eventual graphite exfoliation. Here, we report that cesium cation (Cs(+)) directs the formation of solid electrolyte interphase on graphite anode in PC-rich electrolytes through its preferential solvation by ethylene carbonate (EC) and the subsequent higher reduction potential of the complex cation. Effective suppression of PC-decomposition and graphite-exfoliation is achieved by adjusting the EC/PC ratio in electrolytes to allow a reductive decomposition of Cs(+)-(EC)m (1 ≤ m ≤ 2) complex preceding that of Li(+)-(PC)n (3 ≤ n ≤ 5). Such Cs(+)-directed interphase is stable, ultrathin, and compact, leading to significant improvement in battery performances. In a broader context, the accurate tailoring of interphasial chemistry by introducing a new solvation center represents a fundamental breakthrough in manipulating interfacial reactions that once were elusive to control.

Formation of Reversible Solid Electrolyte Interface on Graphite Surface from Concentrated Electrolytes

Li-ion batteries (LIB) have been successfully commercialized after the identification of ethylene-carbonate (EC)-containing electrolyte that can form a stable solid electrolyte interphase (SEI) on carbon anode surface to passivate further side reactions but still enable the transportation of the Li+ cation. These electrolytes are still utilized, with only minor changes, after three decades. However, the long-term cycling of LIB leads to continuous consumption of electrolyte and growth of SEI layer on the electrode surface, which limits the batterys life and performance. Herein, a new anode protection mechanism is reported in which, upon changing of the cell potential, the electrolyte components at the electrode-electrolyte interface reorganize reversibly to form a transient protective surface layers on the anode. This layer will disappear after the applied potential is removed so that no permanent SEI layer is required to protect the carbon anode. This phenomenon minimizes the need for a permanent SEI layer and prevents its continuous growth and therefore may lead to largely improved performance for LIBs.

Effects of propylene carbonate content in CsPF6-containing electrolytes on the enhanced performances of graphite electrode for lithium-ion batteries

The effects of propylene carbonate (PC) content in CsPF6-containing electrolytes on the performances of graphite electrode in lithium half cells and in graphite∥LiNi0.80Co0.15Al0.05O2 (NCA) full cells are investigated. It is found that the performance of graphite electrode is significantly affected by PC content in the CsPF6-containing electrolytes. An optimal PC content of 20% by weight in the solvent mixtures is identified. The enhanced electrochemical performance of graphite electrode can be attributed to the synergistic effects of the PC solvent and the Cs(+) additive. The synergistic effects of Cs(+) additive and appropriate amount of PC enable the formation of a robust, ultrathin, and compact solid electrolyte interphase (SEI) layer on the surface of graphite electrode, which is only permeable for desolvated Li(+) ions and allows fast Li(+) ion transport through it. Therefore, this SEI layer effectively suppresses the PC cointercalation and largely alleviates the Li dendrite formation on graphite electrode during lithiation even at relatively high current densities. The presence of low-melting-point PC solvent improves the sustainable operation of graphite∥NCA full cells under a wide temperature range. The fundamental findings also shed light on the importance of manipulating/maintaining the electrode/electrolyte interphasial stability in various energy-storage devices.

Electrolyte volume effects on electrochemical performance and solid electrolyte interphase in Si-graphite/NMC lithium-ion pouch cells

This study aims to explore the correlations between electrolyte volume, electrochemical performance, and properties of the solid electrolyte interphase in pouch cells with Si-graphite composite anodes. The electrolyte is 1.2 M LiPF6 in ethylene carbonate:ethylmethyl carbonate with 10 wt % fluoroethylene carbonate. Single layer pouch cells (100 mA h) were constructed with 15 wt % Si-graphite/LiNi0.5Mn0.3CO0.2O2 electrodes. It is found that a minimum electrolyte volume factor of 3.1 times to the total pore volume of cell components (cathode, anode, and separator) is needed for better cycling stability. Less electrolyte causes increases in ohmic and charge transfer resistances. Lithium dendrites are observed when the electrolyte volume factor is low. The resistances from the anodes become significant as the cells are discharged. Solid electrolyte interphase thickness grows as the electrolyte volume factor increases and is nonuniform after cycling.

Suppressed oxygen extraction and degradation of LiNi x Mn y Co z O 2 cathodes at high charge cut-off voltages

The capacity degradation mechanism in lithium nickel–manganese–cobalt oxide (NMC) cathodes (LiNi1/3Mn1/3Co1/3O2 (NMC333) and LiNi0.4Mn0.4Co0.2O2 (NMC442)) during high-voltage (cut-off of 4.8 V) operation has been investigated. In contrast to NMC442, NMC333 exhibits rapid structural changes including severe micro-crack formation and phase transformation from a layered to a disordered rock-salt structure, as well as interfacial degradation during high-voltage cycling, leading to a rapid increase of the electrode resistance and fast capacity decline. The fundamental reason behind the poor structural and interfacial stability of NMC333 was found to be correlated to its high Co content and the significant overlap between the Co3+/4+ t2g and O2− 2p bands, resulting in oxygen removal and consequent structural changes at high voltages. In addition, oxidation of the electrolyte solvents by the extracted oxygen species generates acidic species, which then attack the electrode surface and form highly resistive LiF. These findings highlight that both the structural and interfacial stability should be taken into account when tailoring cathode materials for high voltage battery systems.

New developments in lithium sulfur batteries

In this work, efforts were conducted in order to mitigate the issue of polysulfides dissolution and hence to improve the capacity and efficiency of Li-sulfur cells. The first approach was achieved by optimizing the amount of sulfur that can be contained in the sulfur/carbon electrode. Five sulfur/carbon ratios were prepared- (1) 50/50, (2) 60/40, (3) 70/30, (4) 80/20, and (5) 90/10- to study the effect of carbon contents on electrochemical cycling. The second approach was by adding nano-sized TiO2 particles having a large specific surface area as the polysulfide adsorbing agent in the electrodes. The impact of nano-sized TiO2 particles in improving the electrochemical properties of sulfur electrodes was investigated using CV measurements and charge/discharge tests. To further enhance the efficiency and cycling stability of Li-S batteries, a novel polysulfide electrolyte was developed. This new electrolyte mainly consisted of pre-dissolved lithium polysulfides (Li2Sx) as an alternative electrolyte salt to replace the lithium bis(trifluoromethanesulfone)imide (LiTFSI). We also used LiNO3 to mitigate the shuttle mechanism that occurs in Li-S cells during the charge and discharge. By creating a dynamic equilibrium at the interface of the cathode and electrolyte, the dissolution of lithium polysulfides, and thus the loss of active materials from the cathode during the discharge and charge of the cell, was greatly prevented.