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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,...

Increasing the areal capacity or electrode thickness in lithium ion batteries is one possible means to increase pack level energy density while simultaneously lowering cost. The physics that limit use of high areal capacity as a function of battery power to energy ratio are poorly understood and thus most currently produced automotive lithium ion cells utilize modest loadings to ensure long life over the vehicle battery operation. Here we show electrolyte transport limits the utilization of the positive electrode at critical C-rates during discharge; whereas, a combination of electrolyte transport and polarization lead to lithium plating in the graphite electrode during charge. Experimental measurements are compared with theoretical predictions based on concentrated solution and porous electrode theories. An analytical expression is derived to provide design criteria for long lived operation based on the physical properties of the electrode and electrolyte. Finally, a guideline is proposed that graphite cells should avoid charge current densities near or above 4 mA/cm 2 unless additional precautions have been made to avoid deleterious side reaction. Lithium-ion (Li-ion) batteries are currently being used as the primary energy storage device in hybrid, plug-in, and all electric vehicles. This commercialization has been possible only by leveraging decades of previous scientific and engineering advances on materials, electrodes, and cell development. However, interactions in this complex system are still not fully understood. Automotive grade battery cells are required to fulfill a variety of optimization criteria in order to meet customer expectations and enable highly functional, robust and competitive products. In Fig. 1, key cell level criteria are shown for available technology as well as future development goals. Many of these values are highly influential on each other. In order to optimize one of the criteria it is always necessary to critically evaluate the impact on others. Key goals are to increase vehicle range and decrease cost at the same time. Minimizing the fraction of non-active material is an intuitive path to achieve these goals; however, the cell power and rate capability must simultaneously be maintained. [1][2][3] To derive clear development goals, the high level targets can be broken down to specific component requirements on the different levels of a storage system. 1,4 In Fig. 2, an analysis is shown for a state of the art prismatic hard-case automotive cell format. A battery level specific energy of ∼225 Wh/kg is widely accepted to be a critical value for sustainable implementation of long range electric vehicles. It represents a useful ratio between vehicle weight and range. In order to achieve this high specific energy, all subcomponents of the storage system have to meet demanding requirements as well. On the cell level, large format cells are favorable as they reduce the amount of cell housing needed per cell volume. Prismatic cell formats have a positive influence on the packing densi...

We report systematic studies of the microstructural changes of uncoated and AlF 3 -coated Li-rich Mnrich (LMR) cathode materials (Li 1.2 Ni 0.15 Co 0.10 Mn 0.55 O 2 ) before and after cycling using a combination of aberrationcorrected scanning/transmission electron microscopy (S/ TEM) and electron energy loss spectroscopy (EELS). TEM coupled with EELS provides detailed information about the crystallographic and electronic structure changes that occur after cycling, thus revealing the fundamental improvement mechanism of surface coating. The results demonstrate that the surface coating reduces oxidation of the electrolyte at high voltage, suppressing the accumulation of a thick solid electrolyte interface (SEI) layer on electrode particle surface. Surface coating significantly enhances the stability of the surface structure and protects the electrode from severe etching/corrosion by the acidic species in the electrolyte, reducing the formation of etched surfaces and corrosion pits. Moreover, surface coating alleviates the undesirable voltage fade by mitigating layered to spinel-like phase transformation in the bulk region of the material. These fundamental findings may also be widely applied to explain the functioning mechanisms of other surface coatings used in a broad range of electrode materials.

To improve electric vehicle market acceptance, the charge time of their batteries should be reduced to 10-15 minutes. However, achieving 4C to 6C charge rates with today's batteries is only possible for cells with thin electrodes coming at the expense of low energy density and high battery manufacturing cost. An electrochemical model is validated versus high rate charge data for cells with several loadings. The model elucidates that the main limitations for high energy density cells are poor electrolyte transport resulting in salt depletion within the anode and Li plating at the graphite/separator interface. Next, the model is used to understand what future electrode and electrolyte properties can help enable 4C and 6C charging. Ideally, future electrolytes would be identified with 2X conductivity, 3-4X diffusivity, and transference number of 0.5-0.6. Alternatively charging at elevated temperatures enhances electrolyte transport by 1.5X conductivity and 2-3X diffusivity with a negligible effect on transference number. Another effective strategy to enable 4C and 6C charging is reducing electrode tortuosity. Conversely, increasing electrode porosity and negative/positive ratio are ineffective strategies to improve fast charge capability.