Rechargeable lithium-metal batteries (LMBs) are regarded as the "holy grail" of energy-storage systems, but the electrolytes that are highly stable with both a lithium-metal anode and high-voltage cathodes still remain a great challenge. Here a novel "localized high-concentration electrolyte" (HCE; 1.2 m lithium bis(fluorosulfonyl)imide in a mixture of dimethyl carbonate/bis(2,2,2-trifluoroethyl) ether (1:2 by mol)) is reported that enables dendrite-free cycling of lithium-metal anodes with high Coulombic efficiency (99.5%) and excellent capacity retention (>80% after 700 cycles) of Li||LiNi Mn Co O batteries. Unlike the HCEs reported before, the electrolyte reported in this work exhibits low concentration, low cost, low viscosity, improved conductivity, and good wettability that make LMBs closer to practical applications. The fundamental concept of "localized HCEs" developed in this work can also be applied to other battery systems, sensors, supercapacitors, and other electrochemical systems.

Pristine Li-rich layered cathodes, such as Li(1.2)Ni(0.2)Mn(0.6)O(2) and Li(1.2)Ni(0.1)Mn(0.525)Co(0.175)O(2), were identified to exist in two different structures: LiMO(2)R3[overline]m and Li(2)MO(3)C2/m phases. Upon 300 cycles of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO(2)R3[overline]m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li(2)MO(3)C2/m to spinel involves removal of Li(+) and O(2-), which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF(3)-coating can partially relieve the spinel formation in the layered structure during cycling, resulting in a slower capacity decay. However, the AlF(3)-coating on the layered structure cannot ultimately stop the spinel formation. The observation of structure transition characteristics discussed in this paper provides direct explanation for the observed gradual capacity loss and poor rate performance of the layered composite. It also provides clues about how to improve the materials structure in order to improve electrochemical performance.

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prevent further reactions between the electrolyte and Li metal which decrease the CE, as well as Li metal dendrite growth at the Li surface. Attempts to tackle one or both of these issues have relied on approaches such as tailoring the electrolyte to modify the SEI, [6] introducing an artificial SEI or surface layer on Li metal, [7] and homogenizing the flux of Li + ions during the deposition process. [8] In the pursuit of a high performance LMB, the accurate measurement of Li CE is the most critical factor to predict the cycle life. As shown in Table 1, when the CE is close to 100%, even a 0.1% increase in CE can lead to a dramatic increase in the cycle life of Li metal batteries. However, the measurement of Li CE is often affected by various factors and the measurement methods reported in literature often give different values, even for the same cell design. For example, an extreme caution needs to be taken when handling Li samples during preparation, transfer, and analysis to avoid atmospheric exposure. With so much effort being put into making the Li metal anode a reality, there is an urgent need for a common method for the accurate determination of CE for LMAs and LMBs. Here, we investigated various factors that affect the measurement of Li CE and proposed a more accurate method to determine the CE of Li metal. In this work, we were able to develop a reliable method for measuring CE which can be used as a standardized technique by other researchers and help avoid discrepancy in reported values of CE by different groups. Very accurate values of CE can be calculated and used to quantify the amount of Li consumed during cycling and estimate the cycle life of LMBs.

High-voltage batteries with Li-metal anodes can offer desirable high energy densities. Despite their excellent oxidative stability, sulfones have various limitations to be useful in Li-metal batteries, in particular their instability with Li metal. Here, we achieved a high Li Coulombic efficiency of nearly 99% in a sulfonebased localized high-concentration electrolyte (LHCE) with the addition of a nonsolvating co-solvent. In addition, this co-solvent is highly beneficial for realizing stable battery cycling up to 4.9 V.

Corresponding authors: Chongmin.wang@pnnl.gov, xsun9@uwo.ca, Jiguang.zhang@pnnl.gov # These authors contribute equally to this work. ABSTRACT:The biggest challenge for the commercialization of layered structured nickel rich lithium transition metal oxide cathode is the capacity and voltage fading. Resolving this problem over the years follows an incremental progress. In this work, we report our finding of totally a new approach to revolutionize the cycle stability of aggregated cathode particles for lithium ion battery at both room and elevated temperatures. We discover that infusion of a solid electrolyte into the grain boundaries of the cathode secondary particles can dramatically enhance the capacity retention and voltage stability of the battery. We find that the solid electrolyte infused in the boundaries not only acts as a fast channel for Li ion transport, but also most importantly prevents penetration of the liquid electrolyte into the boundaries, consequently eliminating the detrimental factors that include solid-liquid interfacial reaction, intergranular cracking, and layer to spinel phase transformation. The present work, for the first time, reveals unprecedented insight as how the cathode behaves in the case of not contacting with the liquid electrolyte, ultimately points toward a general new route, via grain boundary engineering, for designing of better batteries of both solid-liquid and solid state systems.

Anode‐free rechargeable lithium (Li) batteries (AFLBs) are phenomenal energy storage systems due to their significantly increased energy density and reduced cost relative to Li‐ion batteries, as well as ease of assembly because of the absence of an active (reactive) anode material. However, significant challenges, including Li dendrite growth and low cycling Coulombic efficiency (CE), have prevented their practical implementation. Here, an anode‐free rechargeable lithium battery based on a Cu||LiFePO4 cell structure with an extremely high CE (>99.8%) is reported for the first time. This results from the utilization of both an exceptionally stable electrolyte and optimized charge/discharge protocols, which minimize the corrosion of the in situly formed Li metal anode.