Reversible Cathode Research Articles

Unraveling the Influence of Hetero Atom Doped Carbon Coating and Morphology Control on Electrochemical Behaviour of Na3V2(PO4)2F3 Cathode Material

The rapid growth of the global battery demand has accelerated the search for efficient low-cost and sustainable batteries. Among them, sodium-ion batteries (SIBs) represent one of the most promising electrochemical energy storage systems for stationary and light vehicle applications. The growing interest in SIBs has been catalyzed by the adoption of this battery technology by several companies worldwide.Despite the great achievements recently announced, several challenges persist. Among them, is the limited energy density of SIBs especially when compared to lithium-ion batteries. To tackle the challenges, research has been focused on the rational design of more powerful sodium-ion battery materials. While hard carbon seems to be the anode of choice, several cathode chemistries have been proposed so far. Out of all, polyanionic compounds have garnered widespread attention as promising reversible cathode materials given their rich chemistry, ability to tolerate volumetric stress upon cycling, and high operating potential. Lattice doping with three fluoride ions onto NASICON-type Na3V2(PO4)3 material yields the formation of a fluorophosphate Na3V2(PO4)2F3 (NVPF) with enhanced theoretical capacity and operating potential of 128 mAh g-1 and 3.95V vs. Na+/Na respectively. Nevertheless, performance-limiting traits such as inferior electronic conductivity and the restricted reversible capacity obstruct the overall practical application for high energy density SIB.To address the listed challenges, we have examined the synergistic role of structural modulation and surface modification strategies through the synthesis of boron-doped carbon-coated hollow microsphere Na3V2(PO4)2F3 (NVPF) materials deploying solvothermal, chemical vapor deposition with a subsequent solution-mixing technique. The prepared materials were analyzed via physical-chemical characterizations to elucidate the phase identification, crystal structure, surface morphology, optimal coating thickness, and prevailing oxidation states. Further, the electrochemical activity of the as-prepared materials was demonstrated in half-cell configuration using rate capability and galvanostatic charge-discharge (GCD) cycling within a potential window of 2.5V- 4.3V.In this work, we discuss the comparison between a pristine version of the NVPF (S1) material with two carbon-coated materials, namely a 10 wt% boron-doped carbon-coated NVPF (S2) and 20 wt% boron-doped carbon-coated NVPF (S3). It is found that the optimal conditions for improved rate performance and cyclability are identified for the composite NVPF (S2) material with capacity retention of 97% after 300 cycles when compared to NVPF (S1) and coated NVPF (S3) which displayed capacity holding of 64% and 94%. Further, an in-depth analysis through impedance spectroscopy and post-mortem characterization has been carried out to shed light on the effect of the boron-doped carbon coating on interfacial and structural stability upon cycling.

LiFe0.3Mn0.7PO4-on-MXene heterostructures as highly reversible cathode materials for Lithium-ion batteries

Two-dimensional (2D) heterostructure materials, incorporating the collective strengths and synergetic properties of individual building blocks, have attracted great interest as a novel paradigm in electrode materials science. The family of 2D transition metal carbides and nitrides (e.g., MXenes) has become an appealing platform for fabricating functional materials with strong application performance. Herein, a 2D LiFe0.3Mn0.7PO4 (LFMP)–on-MXene heterostructure composite is prepared through an electrostatic self-assembly procedure. The functional groups on the surface of MXenes possess highly electronegative properties that facilitate the incorporation of LFMPs into MXenes to construct heterostructure composites. The special heterostructure of nanosized-LiFe0.3Mn0.7PO4 and MXene provides rapid Li+ and electron transport in the cathodes. This LiFe0.3Mn0.7PO4-3.0 wt% MXene composite can exhibit an excellent rate capability of 98.3 mAh g−1 at 50C and a very stable cycling performance with a capacity retention of 94.3 % at 5C after 1000 cycles. Furthermore, NaFe0.3Mn0.7PO4-3.0 wt% MXene with stable cyclability can be obtained by an electrochemical conversion method with LiFe0.3Mn0.7PO4-3.0 wt% MXene. Ex-situ XRD suggests that LiFe0.3Mn0.7PO4-on-MXene achieves a highly reversible structural evolution with a solid solution phase transformation (LFMP→LixFe0.3Mn0.7PO4 (LxFMP), LxFMP→LFMP) and a two-phase reaction (LxFMP←→Fe0.3Mn0.7PO4 (FMP)). This work provides a new direction for the use of MXenes to fabricate 2D heterostructures for lithium-ion batteries.

Elucidating the Effect of Pre-Lithiation of Silicon Dominant Anodes on the Full Cell Performance

Over the last few years, there has been a high interest in increasing the energy density of lithium-ion batteries in order to increase the driving range of battery electric vehicles. On a material level, silicon is a promising anode active material candidate due to its high theoretical capacity of 3579 mAh g-1, which is approximately ten-fold higher than that of the currently used graphite anodes with a theoretical capacity of 372 mAh g-1.[1] However, the main drawbacks of silicon are its intrinsic volume expansion of +300 % upon lithiation.[2] This leads to rupturing of the passivating solid electrolyte interphase (SEI), particle cracking, and loss of electronic contact, limiting the lifetime of batteries with silicon based anodes, thus hindering their transition into application.As a possible solution, Jantke et al. proposed the concept of partial utilization of µm-sized crystalline silicon, where only one-third of the theoretical capacity is used for (de)lithiation. Thereby, a crystalline core remains, and the volume expansion on the particle-level is reduced to approximately + 100%.[3] In addition, it was shown that irreversible losses during the formation and upon cycling can be compensated by pre-lithiation of the anode, which increases the lifetime significantly.[4,5] In this study, we investigated a promising cell chemistry for high-energy applications, containing a Si-dominant anode and a cathode based on a Co-free lithium- and manganese-rich NMC. The here used anode consists of 70 wt% silicon, 20 wt% graphite, 2 wt% conductive C65, and 8 wt% LiPAA-binder. Note that graphite mainly acts as a conductive additive within the operating voltage window, with a negligible contribution to the overall anode capacity. The electrochemical testing was conducted at 25 °C in Swagelok® T-cells containing two glass fiber separators, a 1M LiPF6 in FEC:DEC (2:8, v:v) electrolyte and a lithium metal reference.Without pre-lithiation, these full-cells suffered from rapid capacity fading, reaching the 80 % state-of-health (SOH) criterion after ~100 cycles. As will be discussed, our analysis shows that the primary mode of failure in these cells is the loss of cyclable lithium, evidenced by an increase in the end-of-discharge potential at the anode. It was found that with increasing end-of-discharge potential at the anode, the anode resistance at low state-of-charge rises significantly.Therefore, silicon anodes were lithiated in coin half-cells to 25, 50, and 75 % of the reversible cathode capacity (corresponding to 300, 600, and 900 mAh gSi -1). Then, the anodes were harvested in the lithiated state and assembled in a Swagelok® T-cell with the same cathode, separator, and electrolyte as the non-pre-lithiated reference cells. Pre-lithiation leads to similar reversible charge/discharge capacities, but generates a cyclable lithium reservoir in the anode.Successively increasing the pre-lithiation of the anode increases the lifetime from 100 to 550 cycles. The onset of the above-described aging phenomena is delayed by 150 cycles for every additional 25 % pre-lithiation. Further, the influence of aging and lithium inventory on anode and cathode resistance will be discussed, showing that the increase in anode resistance is mainly a function of (de)lithiation rather than cycle number.Acknowledgments:The authors gratefully acknowledge the funding from the BMBF (Federal Ministry of Education and Research, Germany) in the ExZellTUM III project (grant number 03XP0255). We also kindly thank Wacker Chemie AG and BASF SE for providing the anode and cathode active materials.

Joint cationic and anionic redox chemistry in a vanadium oxide cathode for zinc batteries achieving high energy density

AbstractRechargeable aqueous zinc batteries are promising for large‐scale energy storage due to their low cost and high safety; however, their energy density has reached the ceiling based on conventional cathodes with a single cationic redox reaction mechanism. Herein, a highly reversible cathode of typical layered vanadium oxide is reported, which operates on both the cationic redox couple of V5+/V3+ accompanied by the Zn2+ storage and the anionic O–/O2– redox couple by anion hosting in an aqueous deep eutectic solvent electrolyte. The reversible oxygen redox delivers an additional capacity of ∼100 mAh g–1 at an operating voltage of ∼1.80 V, which increases the energy density of the cathode by ∼36%, endowing the cathode system a record high energy density of ∼506 Wh kg–1. The findings highlight new opportunities for the design of high‐energy zinc batteries with both Zn2+ and anions as charge carriers.

A rechargeable Ca/Cl2 battery

Rechargeable calcium (Ca) metal batteries are promising candidates for sustainable energy storage due to the abundance of Ca in Earth’s crust and the advantageous theoretical capacity and voltage of these batteries. However, the development of practical Ca metal batteries has been severely hampered by the current cathode chemistries, which limit the available energy and power densities, as well as their insufficient capacity retention and low-temperature capability. Here, we describe the rechargeable Ca/Cl2 battery based on a reversible cathode redox reaction between CaCl2 and Cl2, which is enabled by the use of lithium difluoro(oxalate)borate as a key electrolyte mediator to facilitate the dissociation and distribution of Cl-based species and Ca2+. Our rechargeable Ca/Cl2 battery can deliver discharge voltages of 3 V and exhibits remarkable specific capacity (1000 mAh g−1) and rate capability (500 mA g−1). In addition, the excellent capacity retention (96.5% after 30 days) and low-temperature capability (down to 0 °C) allow us to overcome the long-standing bottleneck of rechargeable Ca metal batteries.

Pillared Vanadium Molybdenum Disulfide Nanosheets: Toward High-Performance Cathodes for Magnesium-Ion Batteries.

If magnesium-ion batteries (MIBs) are to be seriously considered for next-generation energy storage, then a number of major obstacles need to be overcome. The lack of reversible cathode materials with sufficient capacity and cycle life is one of these challenges. Here, we report a new MIB cathode constructed of vertically stacked vanadium molybdenum sulfide (VMS) nanosheets toward addressing this challenge. The integration of vanadium within molybdenum sulfide nanostructures acts so as to improve the total conductivity, enhancing charge transfer, and to produce abundant lattice defects, improving both the accommodation and transport of Mg2+. Additionally, electrolyte additive-induced interlayer expansion provides a means to admit Mg2+ cations into the electrode structure and thus enhance their diffusion. The VMS nanosheets are capable of exhibiting capacities of 211.3 and 128.2 mA h g-1 at current densities of 100 and 1000 mA g-1, respectively. The VMS nanosheets also demonstrate long-term cycling stability, retaining 82.7% of the maximum capacity after 500 cycles at a current density of 1000 mA h g-1. These results suggest that VMS nanosheets could be promising candidates for high-performance cathodes in MIBs.

Particle size control of cathode components for high‐performance all‐solid‐state lithium–sulfur batteries

AbstractUnderstanding the size effect of each component on battery performance is essential for designing high‐performance Li2S/S cathode for all‐solid‐state Li–S batteries. However, the size effects of different components are always coupled because ball‐milling, an indispensable process to synthesize reversible cathode, simultaneously and uncontrollably reduces the particle size of all the components. Here, a liquid‐phase method, without ball‐milling, is developed to synthesize the Li2S composite cathode, so that the particle size of the active material Li2S and the solid electrolyte Li3PS4 (LPS) can be independently controlled at nano‐ or microscale. This helps reveal that compositing Li2S and the conductive agent at nanoscale is essential for enhancing the reaction kinetics, whereas the nanoscale particle size and homogenous distribution of LPS is important for accommodating the large volume change of the cathode. By reducing the particle size of Li2S to 9.4 nm and that of LPS to 44 nm, the liquid‐phase‐synthesized composite cathode exhibits reversible capacity and 100% utilization of Li2S under 0.1 C rate.

Inhibiting collective cation migration in Li-rich cathode materials as a strategy to mitigate voltage hysteresis.

Lithium-rich cathodes are promising energy storage materials due to their high energy densities. However, voltage hysteresis, which is generally associated with transition metal migration, limits their energy efficiency and implementation in practical devices. Here we reveal that voltage hysteresis is related to the collective migration of metal ions, and that isolating the migration events from each other by creating partial disorder can create high-capacity reversible cathode materials, even when migrating transition metal ions are present. We demonstrate this on a layered Li-rich chromium manganese oxide that in its fully ordered state displays a substantial voltage hysteresis (>2.5 V) associated with collective transition metal migration into Li layers, but can be made to achieve high capacity (>360 mAh g-1) and energy density (>1,100 Wh kg-1) when the collective migration is perturbed by partial disorder. This study demonstrates that partially cation-disordered cathode materials can accommodate a high level of transition metal migration, which broadens our options for redox couples to those of mobile cations.

FeF3 as Reversible Cathode for All‐Solid‐State Fluoride Batteries

Fluoride batteries are attracting intensive attention because they can provide a higher energy density than conventional lithium‐ion batteries. Among various metal fluorides, FeF3 is a promising candidate for the cathode material of fluoride batteries because of its high theoretical capacity. In this report, the reversibility of an FeF3 cathode is investigated in conjunction with fluorite‐type Ba0.6La0.4F2.4 as the electrolyte and Pb as the counter‐electrode material. For the first time, the discharge–charge performance of a fluoride battery using FeF3 cathode is investigated. The initial discharge capacity is 579 mAh g−1, and a capacity of 461 mAh g−1 is retained at the 10th cycle. The reversible conversion reaction mechanism for FeF3 is clarified by X‐ray diffraction and X‐ray adsorption spectroscopy. The results revealed that FeF3 is reduced to FeF2 at the first‐stage plateau and then to Fe metal at the second‐stage plateau; they also reveal that the reverse process proceeded during charging. Ex situ scanning electron microscopy observations show that the morphology of the cathode changed reversibly and that, when the battery is in the discharged state, voids are present because of shrinkage of the electrode.

Blackberry jam fruit (Randia formosa (Jacq.) K. Schum): An Amazon superfruit with in vitro neuroprotective properties

We report on the bromatological profile, the preliminary phytochemical composition, the antioxidant capacity, and the in vitro neuroprotective effects of Blackberry Jam fruit. Carbohydrates were the major macronutrients (20 g/100 g) in both pulp and seeds (36.5 g/100g), that also possess a low lipid content (0.1 g/100 g and 0.7 g/100, and 86 Kcal/100 g, respectively). Eleven compounds including a diterpenoid dimmer, a lactone, coumarins, sphingosines, and flavonoids have been identified for the first time in the pulp and seeds of R. formosa by mass spectrometry. The electrochemical index of the hydroalcoholic extract of the pulp (BJFp respectively) and an appreciable energy value (89 Kcal/100 g) was around 6.8 μA/V and two reversible cathode peaks were observed up to 0.5 V, thus shedding light to its antioxidant capacity, which was also evidenced by the DPPH• and ABTS+ radical scavenging assays (IC50 of 3.30 ± 0.23 μg/mL and 18.17 ± 1.21 μg/mL, respectively). The BJFp protected against oxidative stress in vitro by significantly decreasing both the lipid peroxidation and contents of carbonyl proteins in a dose-dependent manner in murine brain cells treated with H2O2. Conversely, the BJFp increased the superoxide dismutase (EC50 of 11.99 ± 0.35 μg/mL) and catalase (EC50 of 69.53 ± 0.43 μg/mL) activities. The BJFp markedly decreased both the in vitro acetylcholinesterase (IC50 of 227.9 ± 0.34 μg/mL) and butyrylcholinesterase (IC50 of 8.77 ± 0.40 μg/mL) activities. Therefore, besides being a great source of energy, this Amazon superfruit may be promising for enhancing the treatment outcomes of ailments related to oxidative stress, which require further in vivo investigation.

Coupling Fe3 C Nanoparticles and N-Doping on Wood-Derived Carbon to Construct Reversible Cathode for Zn-Air Batteries.

Electrochemical reduction of oxygen plays a critical role in emerging electrochemical energy technologies. Multiple electron transfer processes, involving adsorption and activation of O2 and generation of protons from water molecules, cause the sluggish kinetics of the oxygen reduction reaction (ORR). Herein, a double-active-site catalyst of Fe3 C nanoparticles coupled to paulownia wood-derived N-doped carbon (Fe3 C@NPW) is fabricated via an active-site-uniting strategy. One site on Fe3 C nanoparticles contributes to activating water molecules, while another site on N-doped carbon is responsible for activating oxygen molecules. Benefiting from the synergistic effect of double active sites, Fe3 C@NPW delivers a remarkable catalytic activity for ORR with a half-wave potential of 0.87V (vs. RHE) in alkaline electrolyte, outperforming commercial Pt/C catalyst. Moreover, zinc-air batteries (ZABs) assembled with Fe3 C@NPW as a catalyst on cathode achieve a large specific capacity of 804.4mA h gZn-1 and a long-term stability of 780 cycles. The model solid-state ZABs also display satisfactory performances with an open-circuit voltage of 1.39V and a high peak power density of 78mW cm-2 . These outstanding performances reach the level of first-rank among the non-noble metal electrode materials. This work offers a promising approach to creating double-active-site catalysts by the active-site-uniting strategy for energy conversion fields.

Surface modification of Na0.44MnO2 via a nonaqueous solution-assisted coating for ultra-Stable and High-Rate sodium-ion batteries

• AlPO 4 -coated Na 0.44 MnO 2 is firstly prepared by a nonaqueous solution-assisted technique. • AlPO 4 coating could facilitate the interfacial charge transfer reactions and accelerate the insertion/extraction process of Na. + • The coated samples show excellent rate performance and thermal ability. • Fast charging ability makes it viable for practical applications. Sodium-ion batteries (SIBs) provide an attractive option for large-scale electrochemical energy storage applications. However, the approach to realize highly reversible cathode materials needs further exploration. Our work adopts a universal nonaqueous solution process in coating functional Al-PO 4 layer on cathode material Na 0.44 MnO 2 (NMO) for SIBs. Surface modification of NMO by AlPO 4 coating layer facilitates the interfacial charge transfer reactions and enhances pseudocapacitance's contribution. The optimized 0.5 wt% AlPO 4 -coated Na 0.44 MnO 2 (0.5 wt% AP-NMO) exhibits an enhanced capacity retention ratio of 80.3% after 500 cycles at 5 C (1 C=120 mA g −1 ) and a higher discharge capacity of 92 mAh g −1 at 10 C (1200 mA g −1 ). In addition, it can release a reversible capacity of 42 mAh g −1 at an ultra-high current density of 50 C (6000 mA g −1 ) and maintain 76% of the initial capacity after 2000 cycles. The coated materials also show extraordinary electrochemical stability at 60 °C. This research paves a way to apply an effective surface modification technique for enhancing transition metal oxide cathodes for SIBs.

PxSy nanoparticles encapsulated in graphene as highly reversible cathode for sodium ion batteries

Herein, phosphorus-mediated sulfur nanoparticles encapsulated in reduced graphene oxide nanosheets (P-SrGO-T) were successfully synthesized as the cathode for sodium ion battery by a ball milling and the following thermal treatment. A series of covalent bonds, such as P–S, C–S–C, C–O–P and C–S–P, are formed in this process, which are in favor of fixing the sulfur and suppressing the parasitic shuttle effect of polysulfide. Benefiting from the graphene sheets and these covalent bonds, a high reversible capacity of 637.4 mAh/g was achieved in P-SrGO-T after 100 cycles at the current density of 0.2 A/g. In addition, P-SrGO-T also delivers a high-rate capacity (330.7 mAh/g at 5 A/g) attributing to low charge transfer resistance and faster ion diffusion kinetic. This work pushes the progress forward in developing phosphosulfide cathode for sodium ion batteries.

Earth-abundant magnetite with carbon coatings as reversible cathodes for stretchable zinc-ion batteries

Earth-abundant magnetite (Fe3O4) as cathode materials in aqueous zinc-ion batteries (ZIBs) is limited by its very low capacity and poor cycling. Here, a combined strategy based on carbon coating and electrolyte optimization is adopted to improve the performance of Fe3O4. The Zn-Fe3O4@C batteries display specific capacities of 93 mAh g−1 and 81% capacity retention after 200 cycles. Such performance is attributed to the enhanced electrical conductivity and structural stability of Fe3O4@C nanocomposites with suppressed iron dissolution. Experimental analysis reveals that the charge storage is contributed by diffusion-limited redox reactions and surface-controlled pseudocapacitance. A stretchable Zn-Fe3O4@C battery is further fabricated, showing stable performance when it is bent or stretched. Fe3O4 is a promising cathode material for cost-effective, safe, sustainable and wearable energy supplies.

Electrochemical behavior of CuF2 as reversible cathode in an organic liquid electrolyte for room-temperature fluoride-shuttle batteries

CuF2 is considered a promising positive electrode material for fluoride-shuttle batteries (FSBs). Herein, the electrochemical compatibility of CuF2 in an organic liquid electrolyte comprising equal concentrations (0.25 M) of lithium bis(oxalato)borate (LiBOB) and triphenylboroxine (TPhBX) in addition to saturated CsF salt in tetraethylene glycol dimethyl ether (G4) solution (LiBOB0.25/TPhBX0.25/sat_CsF/G4) was investigated for potential use in FSBs. CuF2 in the LiBOB0.25/TPhBX0.25/sat_CsF/G4 system exhibited comparable initial discharge and charge capacities of 330 and 240 mAh g−1, respectively. While XRD shows that some CuF2 in the inner region of the electrode remains unreacted, XPS analysis clearly confirms the success of the conversion reaction at the electrode surface. In addition, almost no Cu was found in the LiBOB0.25/TPhBX0.25/sat_CsF/G4 electrolyte system in discharged and charged cells. Therefore, since the dissolution of CuF2 and metallic Cu is alleviated through the use of the LiBOB0.25/TPhBX0.25/sat_CsF/G4 electrolyte, LiBOB0.25/TPhBX0.25/sat_CsF/G4 is a promising organic electrolyte system for FSBs.

A Queue-Ordered Layered Mn-Based Oxides with Al Substitution as High-Rate and High-Stabilized Cathode for Sodium-Ion Batteries.

Development of highly stabilized and reversible cathode materials has become a great challenge for sodium-ion batteries. O'3-type layered Mn-based oxides have deserved much attention as one of largely reversible-capacity cathodes featured by the resource-rich and low-toxic elements. However, the fragile slabs structure of typical layered oxides, low Mn-ion migration barriers, and Jahn-Teller distortion of Mn3+ have easily resulted in the severe degradation of cyclability and rate performances. Herein, a new queue-ordered superstructure is built up in the O'3-NaMn0.6 Al0.4 O2 cathode material. Through the light-metal Al substitution in O'3-NaMnO2 , the MnO6 and AlO6 octahedrons display the queue-ordered arrangements in the transition metal (TM) slabs. Interestingly, the presence of this superstructure can strengthen the layered structure, reduce the influence from Jahn-Teller effect, and suppress the TM-ions migrations during long-terms cycles. These characteristics results in O'3-NaMn0.6 Al0.4 O2 cathode deliver a high capacity of 160 mAh g-1 , an enhanced rate capability and the excellent cycling performance. This research strategy can provide the broaden insight for future electrode materials with high-performance sodium-ions storage.

Weaker Interactions in Zn2+ and Organic Ion‐pre‐intercalated Vanadium Oxide toward Highly Reversible Zinc‐ion Batteries

Driven by safety issues, environmental concerns, and high costs, rechargeable aqueous zinc‐ion batteries (ZIBs) have received increasing attention in recent years owing to their unique advantages. However, the sluggish kinetics of divalent charge Zn2+ in the cathode materials caused by the strong electrostatic interaction and their unsatisfactory cycle life hinder the development of ZIBs. Herein, organic cations and Zn2+ ions co‐pre‐inserted vanadium oxide ([N(CH3)4]0.77,Zn0.23)V8O20·3.8H2O are reported as the cathode for ultra‐stable aqueous ZIBs, in which the weaker electrostatic interactions between Zn2+ and organic ion‐pinned vanadium oxide can induce the high reversibility of Zn2+ insertion and extraction, thereby improving the cycle life. It is demonstrated that ([N(CH3)4]0.77,Zn0.23)V8O20·3.8H2O cathodes deliver a discharge capacity of 181 mA h g−1 at 8 A g−1 and ultra‐long life span (99.5% capacity retention after 2000 cycles). A reversible Zn2+/H+ ions (de)intercalation storage process and pseudocapacitive charge storage are characterized. The weaker interactions between organic ion and Zn2+ open a novel avenue for the design of highly reversible cathode materials with long‐term cycling stability.

(Invited) High Coulombic Efficiency Electrolyte Enables Li||Span Batteries with Superior Cycling Performance

Lithium metal is being extensively studied as an anode to replace graphite due to its high capacity and the low electrochemical potential.Current research focus is to mitigate lithium dendrite growth and to increase coulomb efficiency during electrochemical cycling by using various surface coatings and electrolyte additives.1 , 2 In addition, a 3D host can be beneficial due to the reduced volume change.3 While sulfur or metal oxide intercalation cathodes can both in principle achieve full-cell energy densities of 500 Wh kg-1, sulfur-based cathodes are particularly appealing due to their low cost, abundant supply, and high storage capacity. Among these, the SPAN (sulfurized polyacrylonitrile) cathode is particularly promising due to the physical confinement of the small molecular sulfur in the conductive polymer network which provides a high specific capacity of > 550 mAh g-1 and an average discharge potential of ~ 1.8 V. We have recently focused on designing novel electrolyte solution chemistry that enables Li||SPAN batteries with superior cycling performance.In the first example, we designed a high-concentration, ether-based electrolyte with LiTFSI and LiNO3 as the co-salts. In addition to providing excellent protection for lithium metal anodes by forming the solid electrolyte interface (SEI), the electrolyte promotes the formation of a crystalline cathode electrolyte interface (CEI) on the SPAN surface composed of LiF and LiNO2. The CEI effectively prevents the formation of soluble polysulfide species and enables stable cycling of the Li||SPAN battery. The benefit of having effective CEI and SEI layers is also demonstrated in Li||SPAN cells with limited lithium supply, which exhibit lithium cycling efficiency values consistent with or exceeding Li||Cu test results, reaching 99.5%. The development of electrolyte chemistries to enable the formation of effective CEI layers is a promising approach to long-life lithium metal batteries.In another example, we formulated a highly concentrated salt/ether electrolyte diluted in a fluorinated ether, which realized an average lithium coulombic efficiency of 99.37% at 0.5 mA cm−2 and 1 mAh cm−2 for more than 900 cycles. This electrolyte also maintained a record coulombic efficiency of 98.7% at 10 mA cm−2, indicative of its ability to provide fast-charging with high cathode loadings. Morphological studies reveal dense, dendrite free Li depositions after prolonged cycling, while surface analyses confirmed the formation of a robust LiF-rich SEI layer on the cycled Li surface. Moreover, this ether-based electrolyte is highly compatible with the SPAN cathode, where the constructed Li||SPAN cell exhibited reversible cathode capacity of 579 mAh g-1 and no capacity decay after 1200 cycles. A cell where a high areal loading SPAN electrode (3.5 mAh cm−2) is paired with only one-fold excess Li was constructed and cycled at 1.75 mA cm−2, maintaining a coulombic efficiency of 99.30% for the lithium metal. This study provides a path to enable high energy density Li||SPAN battery with stable cycling. Acknowledgements This work was supported by the Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research Program (Battery 500 Consortium) under Contract DE-EE0007764. Part of the work used the UCSD-MTI Battery Fabrication Facility and the UCSD-Arbin Battery Testing Facility. H. Liu, X. Wang, H. Zhou, H.-D. Lim, X. Xing, Q. Yan, Y. S. Meng and P. Liu, ACS Applied Energy Materials, 2018, DOI: 10.1021/acsaem.8b00348.H. Liu, H. Zhou, B.-S. Lee, X. Xing, M. Gonzalez and P. Liu, Acs Applied Materials & Interfaces, 2017, 9, 30635-30642.H. Liu, X. Yue, X. Xing, Q. Yan, J. Huang, V. Petrova, H. Zhou and P. Liu, Energy Storage Materials, 2018, DOI: https://doi.org/10.1016/j.ensm.2018.09.021.

Ultrahigh coulombic efficiency electrolyte enables Li||SPAN batteries with superior cycling performance

Raising the coulombic efficiency of lithium metal anode cycling is the deciding step in realizing long-life rechargeable lithium batteries. Here, we designed a highly concentrated salt/ether electrolyte diluted in a fluorinated ether: 1.8 M LiFSI in DEE/BTFE (diethyl ether/bis(2,2,2-trifluoroethyl)ether), which realized an average coulombic efficiency of 99.37% at 0.5 mA cm−2 and 1 mAh cm−2 for more than 900 cycles. This electrolyte also maintained a record coulombic efficiency of 98.7% at 10 mA cm−2, indicative of its ability to provide fast-charging with high cathode loadings. Morphological studies reveal dense, dendrite free Li depositions after prolonged cycling, while surface analyses confirmed the formation of a robust LiF-rich SEI layer on the cycled Li surface. Moreover, we discovered that this ether-based electrolyte is highly compatible with the low-cost, high-capacity SPAN (Sulfurized polyacrylonitrile) cathode, where the constructed Li||SPAN cell exhibited reversible cathode capacity of 579 mAh g−1 and no capacity decay after 1200 cycles. A cell where a high areal loading SPAN electrode (>3.5 mAh cm−2) is paired with only onefold excess Li was constructed and cycled at 1.75 mA cm−2, maintaining a coulombic efficiency of 99.30% for the lithium metal. Computational simulations revealed that at saturation, the Li-FSI complex forms contact ion pairs, with a first solvation shell comprising DEE molecules, and a second solvation shell with a mix of DEE/BTFE. This study provides a path to enable high energy density Li||SPAN batteries with stable cycling.

Thermal Conductive 2D Boron Nitride for High-Performance All-Solid-State Lithium-Sulfur Batteries.

Polymer‐based solid‐state electrolytes are shown to be highly promising for realizing low‐cost, high‐capacity, and safe Li batteries. One major challenge for polymer solid‐state batteries is the relatively high operating temperature (60–80 °C), which means operating such batteries will require significant ramp up time due to heating. On the other hand, as polymer electrolytes are poor thermal conductors, thermal variation across the polymer electrolyte can lead to nonuniformity in ionic conductivity. This can be highly detrimental to lithium deposition and may result in dendrite formation. Here, a polyethylene oxide‐based electrolyte with improved thermal responses is developed by incorporating 2D boron nitride (BN) nanoflakes. The results show that the BN additive also enhances ionic and mechanical properties of the electrolyte. More uniform Li stripping/deposition and reversible cathode reactions are achieved, which in turn enable all‐solid‐state lithium–sulfur cells with superior performances.