The continued search for routes to improve the power and energy density of lithium ion batteries for electric vehicles and consumer electronics has resulted in significant innovation in all cell components, particularly in electrode materials design. In this Review, we highlight an often less noted route to improving energy density: increasing the Li + transference number of the electrolyte. Turning to Newman's original lithium ion battery models, we demonstrate that electrolytes with modestly higher Li + transference numbers compared to traditional carbonatebased liquid electrolytes would allow higher power densities and enable faster charging (e.g., >2C), even if their conductivity was substantially lower than that of conventional electrolytes. Most current research in high transference number electrolytes (HTNEs) focuses on ceramic electrolytes, polymer electrolytes, and ionomer membranes filled with nonaqueous solvents. We highlight a number of the challenges limiting current HTNE systems and suggest additional work on promising new HTNE systems, such as "solvent-in-salt" electrolytes, perfluorinated solvent electrolytes, nonaqueous polyelectrolyte solutions, and solutions containing anion-decorated nanoparticles.

Rapid charging of Li-ion batteries is limited by lithium plating on graphite anodes, whereby Li + ions are reduced to Li metal on the graphite particle surface instead of inserting between graphitic layers. Plated Li metal not only poses a safety risk due to dendrite formation, but also contributes to capacity loss due to the low reversibility of the Li plating/stripping process. Understanding when Li plating occurs and how much Li has plated is therefore vital to remedying these issues. We demonstrate a titration technique with a minimum detection limit of 20 nmol (5×10-4 mAh) Li which is used to quantify inactive Li that remains on the graphite electrode after fast charging. Additionally, the titration is extended to quantify the total amount of solid carbonate species and lithium acetylide (Li2C2) within the solid electrolyte interphase (SEI). Finally, electrochemical modeling is combined with experimental data to determine the Li plating exchange current density (10 A/m 2) and stripping efficiency (65%) of plated Li metal on graphite. These techniques provide a highly accurate measure of Li plating onset and quantitative insight into graphite SEI evolution during fast charge.

A key challenge for energy storage and conversion technologies is finding simple, reliable methods that can identify device failure and prolong lifetime. Lithium plating is a well-known degradation process that prevents Li-ion battery fast charging, which is essential to reduce electric vehicle “range anxiety” and enable emerging technologies such as aerial drones and high-performance portable electronics. The ability to detect the initial onset of lithium plating from easily accessible voltage measurements would greatly improve battery safety and feedback controls modeling. In this work, we highlight the application of a differential open-circuit voltage analysis (dOCV) to detect when Li plating begins during a single charge for room-temperature fast charging. We also show that dOCV can identify the Li plating onset during cycling with sensitivity of 4 mAh plated Li per gram graphite, indicating that this method has commercial promise for online Li detection.

Whether attempting to eliminate parasitic Li metal plating on graphite (and other Li-ion anodes) or enabling stable, uniform Li metal formation in 'anode-free' Li battery configurations, the detection and characterization (morphology, microstructure, chemistry) of Li that cannot be reversibly cycled is essential to understand the behavior and degradation of rechargeable batteries. In this review, various approaches used to detect and characterize the formation of Li in batteries are discussed. Each technique has its unique set of advantages and limitations, and works towards solving only part of the full puzzle of battery degradation. Going forward, multimodal characterization holds the most promise towards addressing two pressing concerns in the implementation of the next generation of batteries in the transportation sector (viz. reducing recharging times and increasing the available capacity per recharge without sacrificing cycle life). Such characterizations involve combining several techniques (experimental-and/or modeling-based) in order to exploit their respective advantages and allow a more comprehensive view of cell degradation and the role of Li metal formation in it. It is also discussed which individual techniques, or combinations thereof, can be implemented in real-world battery management systems on-board electric vehicles for early detection of potential battery degradation that would lead to failure.

Layered lithium transition-metal (TM) oxides with a general formula of LiMO 2 (M = Ni, Mn, Co, Al, etc.) are widely used as positive electrode materials for LIBs. [1] Among them, Ni-rich lithium nickel manganese cobalt oxide (LiNi x Mn y Co 1−x−y O 2 , x ≥ 0.8) are considered the most promising due to their high energy density. [7,8] Currently, most commercially available NMC compounds are polycrystalline (PC) secondary particles consisting of submicron-sized primary grains with random orientations. Particle surface is terminated with a variety of crystalline facets that are not optimized for Li transport. [9][10][11] As both Li + diffusion and volume expansion/contraction upon charge/discharge occur anisotropically in the rhombohedral α-NaFeO 2 -type structure, random orientation of the primary grains causes prolonged Li + diffusion pathways and nonuniform Li concentration inside the secondary particles, leading to stress and strain and the eventual intergranular cracking within the particles. [12,13] In addition, the newly exposed surface area from cracking can lead to further parasitic reactions with the electrolyte. [10,14] These issues are greatly exacerbated with increasing Ni content as well as the fast-charging conditions. [15] To address them, approaches such as elemental doping and surface coating have been extensively explored yet they all have met with limited success. [16] It is evident that conventional NMC cathodes are not optimized for fast charge and particle design and engineering are needed in order to minimize internal cracking and improve charge transport capability.Single-crystal (SC) NMCs have recently been shown to deliver enhanced cycling stability under high-voltage operations, gaining the spotlight as promising next-generation high-energy cathode materials. [17,18] The performance improvement is attributed to the lower surface area as well as better cracking resistance due to the absence of grain boundaries, alleviating side reactivities between NMC and the electrolyte which is one of the dominant failure mechanisms in PC-NMCs upon high-voltage cycling. [19] Recent theory work suggested that surface facets also play an important role in the reactivities between NMCs and the electrolyte. In Ni-rich NMCs, it has been shown that (104) surface has the lowest energy whereas (012) and ( 100) are among the ones with the highest surface energy. [20] Surface effect on Li transport is also believed to exist, however, experimental correlations among Ni-rich layered LiNi x Mn y Co 1−x−y O 2 (NMCs, x ≥ 0.8) are poised to be the dominating cathode materials for lithium-ion batteries for the foreseeable future. Conventional polycrystalline NMCs, however, suffer from severe cracking along the grain boundaries of primary particles and capacity loss under high charge and/or discharge rates, hindering their implementation in fast-charging electric vehicular (EV) batteries. Single-crystal (SC) NMCs are attractive alternatives as they eliminate intergranular cracking and allow for grain-level surface optimi...

Fast charging of most commercial lithium-ion batteries is limited due to fear of lithium plating on the graphite anode, which is difficult to detect and poses significant safety risk. Here we demonstrate the power of simple, accessible, and high-throughput cycling techniques to quantify irreversible Li plating spanning data from over 200 cells. We first observe the effects of energy density, charge rate, temperature, and State-of-Charge (SOC) on lithium plating, use the results to refine mature physicsbased electrochemical models, and provide an interpretable empirical equation for predicting the plating onset SOC. We then explore the reversibility of lithium plating for varied deposition rates, amounts, and electrolyte compositions, applying our understanding towards development of electrolytes that reduce irreversible Li formation. Finally, we provide the first quantitative comparison of lithium plating in the experimentally convenient Graphite|Li cell configuration compared with commercially relevant Graphite|LiNi0.5Mn0.3Co0.2O2 (NMC). The hypotheses and abundant data herein were generated primarily with equipment universal to the battery researcher, encouraging further development of innovative testing methods and data processing that enable rapid battery IntroductionThe urgent need to combat climate change has sparked extreme growth in demand for lithium-ion batteries (LIB). Rapid innovation in battery materials and cell design is critical to meet this demand for diverse applications from electronics to vehicles and utility-scale energy storage. Composite graphite electrodes remain a universal component of the LIB and are expected to dominate anode market share through 2030 despite the introduction of silicon and lithium-based materials 1 . The design space for graphite electrodes is immense, with parameters such as the loading, porosity, particle size, binder composition, and electrolyte being carefully selected to meet requirements for lifetime, operating temperature, charge time, and manufacturing. Regardless of design and application, the lithium plating reaction on graphite is a performance and safety concern due to the formation of noncyclable 'dead' lithium metal and salts. While recent studies have focused on Li plating during fast charging, the phenomenon is also pertinent to other operating extremes such as low temperature 2 , overcharge 3 , or system malfunction 4 . Electrochemical (EChem) modeling is an important tool for understanding design tradeoffs that improve graphite performance while avoiding plating. Over decades, Newman-based models that relate cell current density, voltage, temperature, and material properties to graphite intercalation have been enhanced to also estimate lithium plating. [5][6][7][8][9][10] This has led to initial insight into the effect of charge rate, electrode loading, and temperature on lithium plating onset/amount, but simulations rely on debated parameters such as the plating exchange current density or reversibility and are frequently not verified with direct ex...

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In situ neutron reflectometry and GI-XRD reveal the dynamics of SEI formation and layer composition during Li-NRR with current cycling.