Failure Mechanism for Fast‐Charged Lithium Metal Batteries with Liquid Electrolytes

Abstract

more than four decades ago with a TiS 2based cathode prototype battery, [ 3 ] which was followed shortly thereafter by Moli Energy's brief commercialization of a Li/ MoS 2 battery. Unfortunately, prodigious battery capacity losses were observed when Li metal was used as the anode, especially for high current density charging, which resulted in rapid cell failure and safety concerns. Li metal was therefore replaced with carbon coke and later graphitic carbon as an anode. Subsequently, intercalation cathode materials, such as LiCoO 2 and LiFePO 4 , were then discovered and these, in concert with graphitic carbon, now form the foundation of today's Li-ion batteries. [ 4,5 ] In general, however, Li metal continues to be used in three different categories for battery systems: 1) as a counter electrode in half-cells to evaluate the properties of cathode or anode materials such as LiNi 1/3 Mn 1/3 Co 1/3 O 2 or Si, respectively; 2) as an anode to study cathode materials such as V 2 O 5 , which have no Li source in the lattice; and 3) as an anode for next-generation, high-energy storage technologies such as Li-S and Li-O 2 batteries, as well as Li-S hybrid redox fl ow batteries. 7-9 For these high-energy systems, the Li metal is indispensable, thus marking the importance of obtaining a fundamental understanding of the Li metal failure mechanism during cell cycling.When compared with the original pristine, dense Li metal, the redeposited Li always displays a signifi cantly different morphology, i.e., mossy Li. In addition, some of the redeposited Li may gradually or suddenly lose electrical contact with the bulk material thus becoming inactive in the cell after repeated cycling. [ 6 ] The morphological transformation from dense to porous Li metal also leads to the uneven distribution of the electric fi eld in the Li anode resulting in an evolution in the electrochemical reactions during subsequent electrode cycling, further accelerating the inhomogeneous Li deposition. The end result is generally reported to be the growth of dendritic Li metal, which protrudes from the anode surface leading to cell shorting when contact is made with the cathode. [ 7,8 ] Much effort has been devoted to preventing this dendrite growth. A few common strategies can be identifi ed, including the 1) formation of Li-Al or Li-Mg alloys, [ 2,9 ] 2) use In recent years, the Li metal anode has regained a position of paramount research interest because of the necessity for employing Li metal in nextgeneration battery technologies such as Li-S and Li-O 2 . Severely limiting this utilization, however, are the rapid capacity degradation and safety issues associated with rechargeable Li metal anodes. A fundamental understanding of the failure mechanism of Li metal at high charge rates has remained elusive due to the complicated interfacial chemistry that occurs between Li metal and liquid electrolytes. Here, it is demonstrated that at high current density the quick formation of a highly resistive solid electrolyte interphase (SEI) entangled with Li metal,...

Study of the Mechanisms of Internal Short Circuit in a Li/Li Cell by Synchrotron X-ray Phase Contrast Tomography

“…Nevertheless, the comparison between these two ratios confirms that most of the LmSs electrochemically formed during the 1 st discharge become electrochemically inactive during the 1 st charge while forming the well-known "dead LmSs". 56 This is the first time that the amount of electrochemically deposited LmSs calculated from morphological information is directly correlated with the amount of external electron transfer measured electrochemically. Concerning the locations of preferential Li dissolution and LmS deposition during cycling it has been suggested that such locations are electrochemically more active 15 or possess a high local ionic conductivity.…”

“…56 As a result, the SEI formed on the surface of LmSs during the 1 st discharge electrically insulates most of the LmSs, thereby deactivating them electrochemically during the 1 st charge. 57 Further confirmation of the electrochemical inertness of newly formed LmSs during ensuing electrochemical reaction is obtained by comparing the volume ratio of LmSs plated onto the positive and LmSs plated onto the negative (both calculated from X-ray tomography data) with the analogous ratio calculated from the external electron transfer during the 1 st discharge and charge.…”

“…The currently observed porous LmS phase agrees well with previous characterizations 42 and the highlighted pore structures are attributed to the high-surface area lithium (HSAL) compound that is composed of decomposed lithium salt precipitates according to Eastwood et al 42 We suspect that this HSAL compound consists of porous LmSs covered by SEI formed during electrochemical plating of Li and numerous voids within. 56 If one compares porous LmS (light yellow) with solid and compact LmS (dark yellow) one can conclude that the SEI and the voids in the porous LmS have completely dissolved or depleted. Richard and Dahn have indeed identified an exothermic peak due to SEI decomposition at ~100 ℃ in accelerating rate calorimetry (ARC) studies of the thermal stability of LIBs.…”

A Graphite-Polysulfide Full Cell with DME-Based Electrolyte

“…The pure lithium metal used in the cell also poses crucial challenges in the development of Li-S systems. Chief among them being the formation of Li dendrites and mossy deposits on the Li anode, 5,6 presence of excess lithium which assists the shuttle effect, 7 and low Coulombic efficiency associated with Li metal deposition and stripping which leads to short cycle life. 5 To overcome the shortcomings on the anode side different nonLi metal anodes have been tested such as graphite, [8][9][10][11] hard carbon, 12 silicon, 13,14 tin, 15 and other alloys.…”

Structural Design of Lithium–Sulfur Batteries: From Fundamental Research to Practical Application

“…Due to the imperfect surface of Li foil, lithium is prone to deposit on the Li substrate at defect sites, resulting in lithium dendrite growth during plating, especially for the batteries operated at high current densities [326]. Deposition of Li inside a 3D skeleton has been demonstrated to be a promising strategy and has been widely used to prevent lithium dendrite growth.…”