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...

This paper reports the results of an initial investigation into the phenomenon of hysteresis in the charge−discharge profile of high-capacity, lithiumand manganese-rich "layered−layered" xLi 2 MnO 3 •(1−x)LiMO 2 composite cathode structures (M = Mn, Ni, Co) and "layered−layered-spinel" derivatives that are of interest for Li-ion battery applications. In this study, electrochemical measurements, combined with in situ and ex situ X-ray characterization, are used to examine and compare electrochemical and structural processes that occur during charge (lithium extraction) and discharge (lithium insertion) of preconditioned cathodes. Electrochemical measurements of the open-circuit voltage versus lithium content demonstrate a ∼1 V hysteresis in site energy for approximately 12% of the total lithium content during the early cycles, which is markedly different from the hysteresis commonly observed in other intercalation materials. X-ray absorption data indicate structural differences in the cathode at the same state of charge (i.e., the same lithium content) during lithium insertion and extraction reactions. The data support an intercalation mechanism whereby the total number of lithium ions extracted at the top of charge is not reaccommodated in the structure until low states of charge are reached. The hysteresis in this class of materials is attributed predominantly to an inherent structural reorganization after an electrochemical activation of the Li 2 MnO 3 component that alters the crystallographic site energies.

laboratory, is operated under contract no. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. We especially thank Danilo Santini of Argonne's Transportation R&D Center for his support and suggestions in carrying out this study. Ralph Brodd reviewed our baseline plant and made several suggestions which we have incorporated in the present design. Fritz Kalhammer and Haresh Kamath of the Electric Power Research Institute have reviewed our work over several years and made suggestions that resulted in improvements. The work was done under the direction of Dennis Dees and Gary Henriksen of Electrochemical Energy Storage who provided guidance in carrying out the work and preparing this manuscript.

Magnesium-substituted Li 4Ϫx Mg x Ti 5 O 12 spinel electrodes (0 < x Յ 1) have been investigated as insertion electrodes for lithium batteries. The substitution of divalent Mg ions for monovalent Li ions in the structure necessitates that the difference in charge must be compensated by a reduction of an equivalent number of Ti cations from Ti 4ϩ to Ti 3ϩ . The substitution increases the conductivity of the [Ti 5/3 Li 1/3 ]O 4 spinel framework by many orders of magnitude, from < 10 Ϫ13 S cm Ϫ1 for insulating Li 4 Ti 5 O 12 (x ϭ 0), in which all the titanium ions are tetravalent, to ϭ 10 Ϫ2 S cm Ϫ1 for Li 3 MgTi 5 O 12 (x ϭ 1.0), in which the average titanium oxidation state is 3.8. The improved conductivity decreases the area specific impedance of Li/Li 4Ϫx Mg x Ti 5 O 12 cells and increases the rate capability of electrodes for small x, typically x ϭ 0.25. The rechargeable capacity of Li 4Ϫx Mg x Ti 5 O 12 electrodes, particularly those with x close to 1 (130 mAh/g), is inferior to that of unsubstituted Li 4 Ti 5 O 12 electrodes (x ϭ 0, 150 mAh/g); the smaller capacity is attributed to the partial occupation of tetrahedral (8a) sites by Mg ions in the spinel structure.

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