Electrolyte design for Li-ion batteries under extreme operating conditions.

The ideal electrolyte for the widely used LiNi0.8Mn0.1Co0.1O2 (NMC811)||graphite lithium-ion batteries is expected to have the capability of supporting higher voltages (≥4.5 volts), fast charging (≤15 minutes), charging/discharging over a wide temperature range (±60 degrees Celsius) without lithium plating, and non-flammability1-4. No existing electrolyte simultaneously meets all these requirements and electrolyte design is hindered by the absence of an effective guiding principle that addresses the relationships between battery performance, solvation structure and solid-electrolyte-interphase chemistry5. Here we report and validate an electrolyte design strategy based on a group of soft solvents that strikes a balance between weak Li+-solvent interactions, sufficient salt dissociation and desired electrochemistry to fulfil all the aforementioned requirements. Remarkably, the 4.5-volt NMC811||graphite coin cells with areal capacities of more than 2.5 milliampere hours per square centimetre retain 75 per cent (54 per cent) of their room-temperature capacity when these cells are charged and discharged at -50 degrees Celsius (-60 degrees Celsius) at a C rate of 0.1C, and the NMC811||graphite pouch cells with lean electrolyte (2.5 grams per ampere hour) achieve stable cycling with an average Coulombic efficiency of more than 99.9 per cent at -30 degrees Celsius. The comprehensive analysis further reveals an impedance matching between the NMC811 cathode and the graphite anode owing to the formation of similar lithium-fluoride-rich interphases, thus effectively avoiding lithium plating at low temperatures. This electrolyte design principle can be extended to other alkali-metal-ion batteries operating under extreme conditions.

Aqueous Li-ion battery enabled by halogen conversion-intercalation chemistry in graphite.

The use of 'water-in-salt' electrolytes has considerably expanded the electrochemical window of aqueous lithium-ion batteries to 3 to 4 volts, making it possible to couple high-voltage cathodes with low-potential graphite anodes1-4. However, the limited lithium intercalation capacities (less than 200 milliampere-hours per gram) of typical transition-metal-oxide cathodes5,6 preclude higher energy densities. Partial7,8 or exclusive9 anionic redox reactions may achieve higher capacity, but at the expense of reversibility. Here we report a halogen conversion-intercalation chemistry in graphite that produces composite electrodes with a capacity of 243 milliampere-hours per gram (for the total weight of the electrode) at an average potential of 4.2 volts versus Li/Li+. Experimental characterization and modelling attribute this high specific capacity to a densely packed stage-I graphite intercalation compound, C3.5[Br0.5Cl0.5], which can form reversibly in water-in-bisalt electrolyte. By coupling this cathode with a passivated graphite anode, we create a 4-volt-class aqueous Li-ion full cell with an energy density of 460 watt-hours per kilogram of total composite electrode and about 100 per cent Coulombic efficiency. This anion conversion-intercalation mechanism combines the high energy densities of the conversion reactions, the excellent reversibility of the intercalation mechanism and the improved safety of aqueous batteries.

Author Correction: Aqueous Li-ion battery enabled by halogen conversion-intercalation chemistry in graphite.

In Fig. 3e of this Letter, the labels "Br-Cl1" and "Br-Cl2" should read "Br-Br1" and "Br-Br2", respectively. In the Methods section 'Preparation of electrodes', the phrase "anhydrous LiBr/LiCl was replaced by LiBr·H2O (99.95%; Sigma-Aldrich) and LiCl (99.95%; Sigma-Aldrich)" should read "anhydrous LiBr/LiCl was replaced by LiBr·H2O (99.95%; Sigma-Aldrich) and LiCl·H2O (99.95%; Sigma-Aldrich)". These errors have been corrected online.

Salt-in-Salt Reinforced Carbonate Electrolyte for Li Metal Batteries.

The instability of carbonate electrolyte with metallic Li greatly limits its application in high-voltage Li metal batteries. Here, a "salt-in-salt" strategy is applied to boost the LiNO3 solubility in the carbonate electrolyte with Mg(TFSI)2 carrier, which enables the inorganic-rich solid electrolyte interphase (SEI) for excellent Li metal anode performance and also maintains the cathode stability. In the designed electrolyte, both NO3 - and PF6 - anions participate in the Li+ -solvent complexes, thus promoting the formation of inorganic-rich SEI. Our designed electrolyte has achieved a superior Li CE of 99.7 %, enabling the high-loading NCM811||Li (4.5 mAh cm-2 ) full cell with N/P ratio of 1.92 to achieve 84.6 % capacity retention after 200 cycles. The enhancement of LiNO3 solubility by divalent salts is universal, which will also inspire the electrolyte design for other metal batteries.

Formation of LiF-rich Cathode-Electrolyte Interphase by Electrolyte Reduction.

The capacity of transition metal oxide cathode for Li-ion batteries can be further enhanced by increasing the charging potential. However, these high voltage cathodes suffer from fast capacity decay because the large volume change of cathode breaks the active materials and cathode-electrolyte interphase (CEI), resulting in electrolyte penetration into broken active materials and continuous side reactions between cathode and electrolytes. Herein, a robust LiF-rich CEI was formed by potentiostatic reduction of fluorinated electrolyte at a low potential of 1.7 V. By taking LiCoO2 as a model cathode, we demonstrate that the LiF-rich CEI maintains the structural integrity and suppresses electrolyte penetration at a high cut-off potential of 4.6 V. The LiCoO2 with LiF-rich CEI exhibited a capacity of 198 mAh g-1 at 0.5C and an enhanced capacity retention of 63.5 % over 400 cycles as compared to the LiF-free LiCoO2 with only 17.4 % of capacity retention.

Controlling Intermolecular Interaction and Interphase Chemistry Enabled Sustainable Water-tolerance LiMn2 O4 ||Li4 Ti5 O12 Batteries.

Solid electrolyte interphase (SEI) formation and H2 O activity reduction in Water-in-Salt electrolytes (WiSE) with an enlarged stability window of 3.0 V have provided the feasibility of the high-energy-density aqueous Li-ion batteries. Here, we extend the cathodic potential of WiSE by rationally controlling intermolecular interaction and interphase chemistry with the introduction of trimethyl phosphate (TMP) into WiSE. The TMP not merely limits the H2 O activity via the strong interaction between TMP and H2 O but also contributes to the formation of reinforced SEI involving phosphate and LiF by manipulating the Li+ solvation structure. Thus, water-tolerance LiMn2 O4 (LMO)||Li4 Ti5 O12 (LTO) full cell with a P/N ratio of 1.14 can be assembled and achieve a long cycling life of 1000 times with high coulombic efficiency of >99.9 %. This work provides a promising insight into the cost-effective practical manufacture of LMO||LTO cells without rigorous moisture-free requirements.

Enhancing Li+ Transport in NMC811||Graphite Lithium-Ion Batteries at Low Temperatures by Using Low-Polarity-Solvent Electrolytes.

LiNix Coy Mnz O2 (x+y+z=1)||graphite lithium-ion battery (LIB) chemistry promises practical applications. However, its low-temperature (≤ -20 °C) performance is poor because the increased resistance encountered by Li+ transport in and across the bulk electrolytes and the electrolyte/electrode interphases induces capacity loss and battery failures. Though tremendous efforts have been made, there is still no effective way to reduce the charge transfer resistance (Rct ) which dominates low-temperature LIBs performance. Herein, we propose a strategy of using low-polarity-solvent electrolytes which have weak interactions between the solvents and the Li+ to reduce Rct , achieving facile Li+ transport at sub-zero temperatures. The exemplary electrolyte enables LiNi0.8 Mn0.1 Co0.1 O2 ||graphite cells to deliver a capacity of ≈113 mAh g-1 (98 % full-cell capacity) at 25 °C and to remain 82 % of their room-temperature capacity at -20 °C without lithium plating at 1/3C. They also retain 84 % of their capacity at -30 °C and 78 % of their capacity at -40 °C and show stable cycling at 50 °C.

Azo compounds as a family of organic electrode materials for alkali-ion batteries.

Organic compounds are desirable for sustainable Li-ion batteries (LIBs), but the poor cycle stability and low power density limit their large-scale application. Here we report a family of organic compounds containing azo group (N=N) for reversible lithiation/delithiation. Azobenzene-4,4'-dicarboxylic acid lithium salt (ADALS) with an azo group in the center of the conjugated structure is used as a model azo compound to investigate the electrochemical behaviors and reaction mechanism of azo compounds. In LIBs, ADALS can provide a capacity of 190 mAh g-1 at 0.5 C (corresponding to current density of 95 mA g-1) and still retain 90%, 71%, and 56% of the capacity when the current density is increased to 2 C, 10 C, and 20 C, respectively. Moreover, ADALS retains 89% of initial capacity after 5,000 cycles at 20 C with a slow capacity decay rate of 0.0023% per cycle, representing one of the best performances in all organic compounds. Superior electrochemical behavior of ADALS is also observed in Na-ion batteries, demonstrating that azo compounds are universal electrode materials for alkali-ion batteries. The highly reversible redox chemistry of azo compounds to alkali ions was confirmed by density-functional theory (DFT) calculations. It provides opportunities for developing sustainable batteries.

A Covalent Organic Framework for Fast-Charge and Durable Rechargeable Mg Storage.

High-safety, low-cost, and high-volumetric-capacity rechargeable magnesium batteries (RMBs) are promising alternatives to lithium ion batteries. However, lack of high-power, high-energy, and stable cathodes for RMBs hinders their commercialization. Herein, an environmentally benign, low-cost, and sustainable covalent organic framework (COF) cathode for Mg storage is reported for the first time. It delivers a high power density of 2.8 kW kg-1, a high specific energy density of 146 Wh kg-1, and an ultralong cycle life of 3000 cycles with a very slow capacity decay rate of 0.0196% per cycle, representing one of the best cathodes to date. The comprehensive electrochemical analysis proves that triazine ring sites in the COF are redox centers for reversible reaction with magnesium ions, and the ultrafast reaction kinetics are mainly attributed to pseudocapacitive behavior. The high-rate Mg storage of the COF offers new opportunities for the development of ultrastable and fast-charge RMBs.

An Inorganic-Rich Solid Electrolyte Interphase for Advanced Lithium-Metal Batteries in Carbonate Electrolytes.

In carbonate electrolytes, the organic-inorganic solid electrolyte interphase (SEI) formed on the Li-metal anode surface is strongly bonded to Li and experiences the same volume change as Li, thus it undergoes continuous cracking/reformation during plating/stripping cycles. Here, an inorganic-rich SEI is designed on a Li-metal surface to reduce its bonding energy with Li metal by dissolving 4m concentrated LiNO3 in dimethyl sulfoxide (DMSO) as an additive for a fluoroethylene-carbonate (FEC)-based electrolyte. Due to the aggregate structure of NO3 - ions and their participation in the primary Li+ solvation sheath, abundant Li2 O, Li3 N, and LiNx Oy grains are formed in the resulting SEI, in addition to the uniform LiF distribution from the reduction of PF6 - ions. The weak bonding of the SEI (high interface energy) to Li can effectively promote Li diffusion along the SEI/Li interface and prevent Li dendrite penetration into the SEI. As a result, our designed carbonate electrolyte enables a Li anode to achieve a high Li plating/stripping Coulombic efficiency of 99.55 % (1 mA cm-2 , 1.0 mAh cm-2 ) and the electrolyte also enables a Li||LiNi0.8 Co0.1 Mn0.1 O2 (NMC811) full cell (2.5 mAh cm-2 ) to retain 75 % of its initial capacity after 200 cycles with an outstanding CE of 99.83 %.

High Interfacial-Energy Interphase Promoting Safe Lithium Metal Batteries.

Engineering a stable solid electrolyte interphase (SEI) is critical for suppression of lithium dendrites. However, the formation of a desired SEI by formulating electrolyte composition is very difficult due to complex electrochemical reduction reactions. Here, instead of trial-and-error of electrolyte composition, we design a Li-11 wt % Sr alloy anode to form a SrF2-rich SEI in fluorinated electrolytes. Density functional theory (DFT) calculation and experimental characterization demonstrate that a SrF2-rich SEI has a large interfacial energy with Li metal and a high mechanical strength, which can effectively suppress the Li dendrite growth by simultaneously promoting the lateral growth of deposited Li metal and the SEI stability. The Li-Sr/Cu cells in 2 M LiFSI-DME show an outstanding Li plating/stripping Coulombic efficiency of 99.42% at 1 mA cm-2 with a capacity of 1 mAh cm-2 and 98.95% at 3 mA cm-2 with a capacity of 2 mAh cm-2, respectively. The symmetric Li-Sr/Li-Sr cells also achieve a stable electrochemical performance of 180 cycles at an extremely high current density of 30 mA cm-2 with a capacity of 1 mAh cm-2. When paired with LiFePO4 (LFP) and LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes, Li-Sr/LFP cells in 2 M LiFSI-DME electrolytes and Li-Sr/NMC811 cells in 1 M LiPF6 in FEC:FEMC:HFE electrolytes also maintain excellent capacity retention. Designing SEIs by regulating Li-metal anode composition opens up a new and rational avenue to suppress Li dendrites.

Water-Pillared Sodium Vanadium Bronze Nanowires for Enhanced Rechargeable Magnesium Ion Storage.

Owing to the advantages of high safety, low cost, high theoretical volumetric capacities, and environmental friendliness, magnesium-ion batteries (MIBs) have more feasibility for large-scale energy storage compared to lithium-ion batteries. However, lack of suitable cathode materials due to sluggish kinetics of magnesium ion is one of the biggest challenges. Herein, water-pillared sodium vanadium bronze nanowires (Na2 V6 O16 ·1.63H2 O) are reported as cathode material for MIBs, which display high performance in magnesium storage. The hydrated sodium ions provide excellent structural stability. The charge shielding effect of lattice water enables fast Mg2+ diffusion. It exhibits high specific capacity of 175 mAh g-1 , long cycle life (450 cycles), and high coulombic efficiency (≈100%). At high current density of 200 mA g-1 , the capacity retention is up to 71% even after 450 cycles (compared to the highest capacity), demonstrating excellent long-term cycling performance. The nature of charge storage kinetics is explored. Furthermore, a highly reversible structure change during the electrochemical process is proved by comprehensive electrochemical analysis. The remarkable electrochemical performance makes Na2 V6 O16 ·1.63H2 O a promising cathode material for low-cost and safe MIBs.

Unique aqueous Li-ion/sulfur chemistry with high energy density and reversibility.

Leveraging the most recent success in expanding the electrochemical stability window of aqueous electrolytes, in this work we create a unique Li-ion/sulfur chemistry of both high energy density and safety. We show that in the superconcentrated aqueous electrolyte, lithiation of sulfur experiences phase change from a high-order polysulfide to low-order polysulfides through solid-liquid two-phase reaction pathway, where the liquid polysulfide phase in the sulfide electrode is thermodynamically phase-separated from the superconcentrated aqueous electrolyte. The sulfur with solid-liquid two-phase exhibits a reversible capacity of 1,327 mAh/(g of S), along with fast reaction kinetics and negligible polysulfide dissolution. By coupling a sulfur anode with different Li-ion cathode materials, the aqueous Li-ion/sulfur full cell delivers record-high energy densities up to 200 Wh/(kg of total electrode mass) for >1,000 cycles at ∼100% coulombic efficiency. These performances already approach that of commercial lithium-ion batteries (LIBs) using a nonaqueous electrolyte, along with intrinsic safety not possessed by the latter. The excellent performance of this aqueous battery chemistry significantly promotes the practical possibility of aqueous LIBs in large-format applications.

High-Energy-Density Rechargeable Mg Battery Enabled by a Displacement Reaction.

Because of its high theoretical volumetric capacity and dendrite-free stripping/plating of Mg, rechargeable magnesium batteries (RMBs) hold great promise for high energy density in consumer electronics. However, the lack of high-energy-density cathodes severely constrains their practical applications. Herein, for the first time, we report that a CuS cathode can fully reversibly work through a displacement reaction in CuS/Mg pouch cells at room temperature and provide a high capacity of ∼400 mA h/g in a MACC electrolyte, corresponding to the gravimetric and volumetric energy density of 608 W h/kg and1042 W h/L, respectively. Even after 80 cycles, CuS/Mg pouch cells can maintain a high capacity of 335 mA h/g. Detailed mechanistic studies reveal that CuS undergoes a displacement reaction route rather than a typical conversion mechanism. This work will provide a guide for more discovery of high-performance cathode candidates for RMBs.

A critical review of cathodes for rechargeable Mg batteries.

Benefiting from a higher volumetric capacity (3833 mA h cm-3 for Mg vs. 2046 mA h cm-3 for Li) and dendrite-free Mg metal anode, reversible Mg batteries (RMBs) are a promising chemistry for applications beyond Li ion batteries. However, RMBs are still severely restricted by the absence of high performance cathodes for any practical application. In this review, we provide a critical and rigorous review of Mg battery cathode materials, mainly reported since 2013, focusing on the impact of structure and composition on magnesiation kinetics. We discuss cathode materials, including intercalation compounds, conversion materials (O2, S, organic compounds), water co-intercalation cathodes (V2O5, MnO2etc.), as well as hybrid systems using Mg metal anode. Among them, intercalation cathodes are further categorized by 3D (Chevrel phase, spinel structure etc.), 2D (layered structure), and 1D materials (polyanion: phosphate and silicate), according to the diffusion pathway of Mg2+ in the framework. Instead of discussing every published work in detail, this review selects the most representative works and highlights the merits and challenges of each class of cathodes. Advances in theoretical analysis are also reviewed and compared with experimental results. This critical review will provide comprehensive knowledge of Mg cathodes and guidelines for exploring new cathodes for rechargeable magnesium batteries.

Self-Templated Formation of P2-type K0.6CoO2 Microspheres for High Reversible Potassium-Ion Batteries.

Layered metal oxides have been widely used as the best cathode materials for commercial lithium-ion batteries and are being intensively explored for sodium-ion batteries. However, their application to potassium-ion batteries (PIBs) is hampered because of the poor cycling stability and low rate capability due to the larger ionic size of K+ than of Li+ or Na+. Herein, a facile self-templated strategy was used to synthesize unique P2-type K0.6CoO2 microspheres that consist of aggregated primary nanoplates as PIB cathodes. The unique K0.6CoO2 microspheres with aggregated structure significantly enhanced the kinetics of the K+ intercalation/deintercation and also minimized the parasitic reactions between the electrolyte and K0.6CoO2. The P2-K0.6CoO2 microspheres demonstrated a high reversible capacity of 82 mAh g-1 at 10 mA g-1, high rate capability of 65 mAh g-1 at 100 mA g-1, and long cycle life (87% capacity retention over 300 cycles). The high reversibility of the P2-K0.6CoO2 full cell paired with a hard carbon anode further demonstrated the feasibility of PIBs. This work not only successfully demonstrates exceptional performance of P2-type K0.6CoO2 cathodes and microspheres K0.6CoO2∥hard carbon full cells, but also provides new insights into the exploration of other layered metal oxides for PIBs.

A Pyrazine-Based Polymer for Fast-Charge Batteries.

The lack of high-power and stable cathodes prohibits the development of rechargeable metal (Na, Mg, Al) batteries. Herein, poly(hexaazatrinaphthalene) (PHATN), an environmentally benign, abundant and sustainable polymer, is employed as a universal cathode material for these batteries. In Na-ion batteries (NIBs), PHATN delivers a reversible capacity of 220 mAh g-1 at 50 mA g-1 , corresponding to the energy density of 440 Wh kg-1 , and still retains 100 mAh g-1 at 10 Ag-1 after 50 000 cycles, which is among the best performances in NIBs. Such an exceptional performance is also observed in more challenging Mg and Al batteries. PHATN retains reversible capacities of 110 mAh g-1 after 200 cycles in Mg batteries and 92 mAh g-1 after 100 cycles in Al batteries. DFT calculations, X-ray photoelectron spectroscopy, Raman, and FTIR show that the electron-deficient pyrazine sites in PHATN are the redox centers to reversibly react with metal ions.

Reversible Redox Chemistry of Azo Compounds for Sodium-Ion Batteries.

Sustainable sodium-ion batteries (SSIBs) using renewable organic electrodes are promising alternatives to lithium-ion batteries for the large-scale renewable energy storage. However, the lack of high-performance anode material impedes the development of SSIBs. Herein, we report a new type of organic anode material based on azo group for SSIBs. Azobenzene-4,4'-dicarboxylic acid sodium salt is used as a model to investigate the electrochemical behaviors and reaction mechanism of azo compound. It exhibits a reversible capacity of 170 mAh g-1 at 0.2C. When current density is increased to 20C, the reversible capacities of 98 mAh g-1 can be retained for 2000 cycles, demonstrating excellent cycling stability and high rate capability. The detailed characterizations reveal that azo group acts as an electrochemical active site to reversibly bond with Na+ . The reversible redox chemistry between azo compound and Na ions offer opportunities for developing long-cycle-life and high-rate SSIBs.

A Universal Organic Cathode for Ultrafast Lithium and Multivalent Metal Batteries.

Low-cost multivalent battery chemistries (Mg2+ , Al3+ ) have been extensively investigated for large-scale energy storage applications. However, their commercialization is plagued by the poor power density and cycle life of cathodes. A universal polyimides@CNT (PI@CNT) cathode is now presented that can reversibly store various cations with different valences (Li+ , Mg2+ , Al3+ ) at an extremely fast rate. The ion-coordination charge storage mechanism of PI@CNT is systemically investigated. Full cells using PI@CNT cathodes and corresponding metal anodes exhibit long cycle life (>10000 cycles), fast kinetics (>20 C), and wide operating temperature range (-40 to 50 °C), making the low-cost industrial polyimides universal cathodes for different multivalent metal batteries. The stable ion-coordinated mechanism opens a new foundation for the development of high-energy and high-power multivalent batteries.

Zn/MnO2 Battery Chemistry With H+ and Zn2+ Coinsertion.

Rechargeable aqueous Zn/MnO2 battery chemistry in a neutral or mildly acidic electrolyte has attracted extensive attention recently because all the components (anode, cathode, and electrolyte) in a Zn/MnO2 battery are safe, abundant, and sustainable. However, the reaction mechanism of the MnO2 cathode remains a topic of discussion. Herein, we design a highly reversible aqueous Zn/MnO2 battery where the binder-free MnO2 cathode was fabricated by in situ electrodeposition of MnO2 on carbon fiber paper in mild acidic ZnSO4+MnSO4 electrolyte. Electrochemical and structural analysis identify that the MnO2 cathode experience a consequent H+ and Zn2+ insertion/extraction process with high reversibility and cycling stability. To our best knowledge, it is the first report on rechargeable aqueous batteries with a consequent ion-insertion reaction mechanism.