Local electronic structure variation resulting in Li 'filament' formation within solid electrolytes.

Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in potentially catastrophic battery failure. At present, this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system, Li7La3Zr2O12, we discover that Li infiltration in solid oxide electrolytes is strongly associated with local electronic band structure. About half of the Li7La3Zr2O12 grain boundaries were found to have a reduced bandgap, around 1-3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li+ ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design.

Lithium/Sulfide All-Solid-State Batteries using Sulfide Electrolytes.

All-solid-state lithium batteries (ASSLBs) are considered as the next generation electrochemical energy storage devices because of their high safety and energy density, simple packaging, and wide operable temperature range. The critical component in ASSLBs is the solid-state electrolyte. Among all solid-state electrolytes, the sulfide electrolytes have the highest ionic conductivity and favorable interface compatibility with sulfur-based cathodes. The ionic conductivity of sulfide electrolytes is comparable with or even higher than that of the commercial organic liquid electrolytes. However, several critical challenges for sulfide electrolytes still remain to be solved, including their narrow electrochemical stability window, the unstable interface between the electrolyte and the electrodes, as well as lithium dendrite formation in the electrolytes. Herein, the emerging sulfide electrolytes and preparation methods are reviewed. In particular, the required properties of the sulfide electrolytes, such as the electrochemical stabilities of the electrolytes and the compatible electrode/electrolyte interfaces are highlighted. The opportunities for sulfide-based ASSLBs are also discussed.

Rational Designed Mixed-Conductive Sulfur Cathodes for All-Solid-State Lithium Batteries.

All-solid-state lithium-sulfur batteries (ASSLSBs) hold great promise for safe and high-energy-density energy storage. However, developing high-performance sulfur cathodes has been proven difficult due to low electronic and ionic conductivities and large volume change of sulfur during charge and discharge. Here, we reported an approach to synthesize sulfur cathodes with a mixed electronic and ionic conductivity by infiltrating a solution consisting of Li3PS4 (LPS) solid electrolyte and S active material into a mesoporous carbon (CMK-3). This approach leads to a uniform dispersion of amorphous Li3PS7 (L3PS) catholyte in an electronically conductive carbon matrix, enabling high and balanced electronic/ionic conductivities in the cathode composite. The inherent porous structure of CMK-3 also helps to accommodate the strain/stress generated during the expansion and shrinkage of the active material. In sulfide-based all-solid-state batteries with Li metal as the anode, this cathode composite delivered a high capacity of 1025 mAh g-1 after 50 cycles at 60 °C at 1/8C. This work highlights the important role of high and balanced electronic and ionic conductivities in developing high-performance sulfur cathodes for ASSLSBs.

Sulfur-Embedded FeS2 as a High-Performance Cathode for Room Temperature All-Solid-State Lithium-Sulfur Batteries.

All-solid-state lithium-sulfur batteries employing sulfur electrodes and a solid electrolyte at room temperature are still a great challenge owing to the low conductivities of sulfur cathodes. In this work, we report room temperature all-solid-state lithium-sulfur batteries using thin sulfur layer-embedded FeS2 (FeS2@S) microsphere composites as active materials in the FeS2@S-Li10GeP2S12-Super P cathode electrode. Setting the cut-off voltage between 1.5 and 2.8 V, only lithiation-delithiation reactions between L2FeS2 and FeSy and direct reaction between Li2S and S will occur, which avoids large volume change of FeS2 caused by the conversion reaction, leading to the structure integrity of FeS2@S. The resultant batteries exhibit excellent rate and cyclic performances, delivering specific capacities of 1120.9, 937.2, 639.7, 517.2, 361.5, and 307.0 mA h g-1 for the FeS2@S composite cathode, corresponding to the normalized capacities of 1645.5, 1252.9, 782.5, 700.2, 478.4, and 363.6 mA h g-1 for sulfur at 30, 50, 100, 500, 1000, and 5000 mA g-1, respectively. Besides, they can retain the normalized capacity of 430.7 mA h g-1 for sulfur at 1000 mA g-1 after 200 cycles at room temperature.

A Highly Reversible, Dendrite-Free Lithium Metal Anode Enabled by a Lithium-Fluoride-Enriched Interphase.

Metallic lithium is the most competitive anode material for next-generation lithium (Li)-ion batteries. However, one of its major issues is Li dendrite growth and detachment, which not only causes safety issues, but also continuously consumes electrolyte and Li, leading to low coulombic efficiency (CE) and short cycle life for Li metal batteries. Herein, the Li dendrite growth of metallic lithium anode is suppressed by forming a lithium fluoride (LiF)-enriched solid electrolyte interphase (SEI) through the lithiation of surface-fluorinated mesocarbon microbeads (MCMB-F) anodes. The robust LiF-enriched SEI with high interfacial energy to Li metal effectively promotes planar growth of Li metal on the Li surface and meanwhile prevents its vertical penetration into the LiF-enriched SEI from forming Li dendrites. At a discharge capacity of 1.2 mAh cm-2 , a high CE of >99.2% for Li plating/stripping in FEC-based electrolyte is achieved within 25 cycles. Coupling the pre-lithiated MCMB-F (Li@MCMB-F) anode with a commercial LiFePO4 cathode at the positive/negative (P/N) capacity ratio of 1:1, the LiFePO4 //Li@MCMB-F cells can be charged/discharged at a high areal capacity of 2.4 mAh cm-2 for 110 times at a negligible capacity decay of 0.01% per cycle.

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.

Construction of 3D Electronic/Ionic Conduction Networks for All-Solid-State Lithium Batteries.

High and balanced electronic and ionic transportation networks with nanoscale distribution in solid-state cathodes are crucial to realize high-performance all-solid-state lithium batteries. Using Cu2 SnS3 as a model active material, such a kind of solid-state Cu2 SnS3 @graphene-Li7 P3 S11 nanocomposite cathodes are synthesized, where 5-10 nm Cu2 SnS3 nanoparticles homogenously anchor on the graphene nanosheets, while the Li7 P3 S11 electrolytes uniformly coat on the surface of Cu2 SnS3 @graphene composite forming nanoscaled electron/ion transportation networks. The large amount of nanoscaled triple-phase boundary in cathode ensures high power density due to high ionic/electronic conductions and long cycle life due to uniform and reduced volume change of nano-Cu2 SnS3. The Cu2 SnS3 @graphene-Li7 P3 S11 cathode layer with 2.0 mg cm-2 loading in all-solid-state lithium batteries demonstrates a high reversible discharge specific capacity of 813.2 mAh g-1 at 100 mA g-1 and retains 732.0 mAh g-1 after 60 cycles, corresponding to a high energy density of 410.4 Wh kg-1 based on the total mass of Cu2 SnS3 @graphene-Li7 P3 S11 composite based cathode. Moreover, it exhibits excellent rate capability and high-rate cycling stability, showing reversible capacity of 363.5 mAh g-1 at 500 mA g-1 after 200 cycles. The study provides a new insight into constructing both electronic and ionic conduction networks for all-solid-state lithium batteries.

Identifying the components of the solid-electrolyte interphase in Li-ion batteries.

The importance of the solid-electrolyte interphase (SEI) for reversible operation of Li-ion batteries has been well established, but the understanding of its chemistry remains incomplete. The current consensus on the identity of the major organic SEI component is that it consists of lithium ethylene di-carbonate (LEDC), which is thought to have high Li-ion conductivity, but low electronic conductivity (to protect the Li/C electrode). Here, we report on the synthesis and structural and spectroscopic characterizations of authentic LEDC and lithium ethylene mono-carbonate (LEMC). Direct comparisons of the SEI grown on graphite anodes suggest that LEMC, instead of LEDC, is likely to be the major SEI component. Single-crystal X-ray diffraction studies on LEMC and lithium methyl carbonate (LMC) reveal unusual layered structures and Li+ coordination environments. LEMC has Li+ conductivities of >1 × 10-6 S cm-1, while LEDC is almost an ionic insulator. The complex interconversions and equilibria of LMC, LEMC and LEDC in dimethyl sulfoxide solutions are also investigated.

Fluorinated solid electrolyte interphase enables highly reversible solid-state Li metal battery.

Solid-state electrolytes (SSEs) are receiving great interest because their high mechanical strength and transference number could potentially suppress Li dendrites and their high electrochemical stability allows the use of high-voltage cathodes, which enhances the energy density and safety of batteries. However, the much lower critical current density and easier Li dendrite propagation in SSEs than in nonaqueous liquid electrolytes hindered their possible applications. Herein, we successfully suppressed Li dendrite growth in SSEs by in situ forming an LiF-rich solid electrolyte interphase (SEI) between the SSEs and the Li metal. The LiF-rich SEI successfully suppresses the penetration of Li dendrites into SSEs, while the low electronic conductivity and the intrinsic electrochemical stability of LiF block side reactions between the SSEs and Li. The LiF-rich SEI enhances the room temperature critical current density of Li3PS4 to a record-high value of >2 mA cm-2. Moreover, the Li plating/stripping Coulombic efficiency was escalated from 88% of pristine Li3PS4 to more than 98% for LiF-coated Li3PS4. In situ formation of electronic insulating LiF-rich SEI provides an effective way to prevent Li dendrites in the SSEs, constituting a substantial leap toward the practical applications of next-generation high-energy solid-state Li metal batteries.

Long Cycle Life All-Solid-State Sodium Ion Battery.

All-solid-state sodium ion batteries (ASIBs) based on sulfide electrolytes are considered a promising candidate for large-scale energy storage. However, the limited cycle life of ASIBs largely restricts their practical application. Cycling-stable ASIBs can be achieved only if the designed cathode can simultaneously address challenges including insufficient interfacial contact, electrochemical and chemical instability between the electrode and electrolyte, and strain/stress during operation , rather than just addressing one or part of these challenges. Chevrel phase Mo6S8 has inherent high electronic conductivity and small volume change during sodiation/desodiation, and is chemically and electrochemically stable with the sulfide electrolyte, and therefore the only challenge of using Mo6S8 as the cathode for ASIBs is the insufficient contact between Mo6S8 and the solid electrolyte (SE). Herein, a thin layer of SE is coated on Mo6S8 using a solution method to achieve an intimate contact between Mo6S8 and the SE. Such a SE-coated Mo6S8 cathode enabled an ASIB with a high cycling performance (500 cycles), even much better than that of the liquid-electrolyte batteries with the Mo6S8 cathode. This work provides valuable insights for developing long-cycle life ASIBs.

High energy-density and reversibility of iron fluoride cathode enabled via an intercalation-extrusion reaction.

Iron fluoride, an intercalation-conversion cathode for lithium ion batteries, promises a high theoretical energy density of 1922 Wh kg-1. However, poor electrochemical reversibility due to repeated breaking/reformation of metal fluoride bonds poses a grand challenge for its practical application. Here we report that both a high reversibility over 1000 cycles and a high capacity of 420 mAh g-1 can be realized by concerted doping of cobalt and oxygen into iron fluoride. In the doped nanorods, an energy density of ~1000 Wh kg-1 with a decay rate of 0.03% per cycle is achieved. The anion's and cation's co-substitutions thermodynamically reduce conversion reaction potential and shift the reaction from less-reversible intercalation-conversion reaction in iron fluoride to a highly reversible intercalation-extrusion reaction in doped material. The co-substitution strategy to tune the thermodynamic features of the reactions could be extended to other high energy conversion materials for improved performance.

Highly reversible zinc metal anode for aqueous batteries.

Metallic zinc (Zn) has been regarded as an ideal anode material for aqueous batteries because of its high theoretical capacity (820 mA h g-1), low potential (-0.762 V versus the standard hydrogen electrode), high abundance, low toxicity and intrinsic safety. However, aqueous Zn chemistry persistently suffers from irreversibility issues, as exemplified by its low coulombic efficiency (CE) and dendrite growth during plating/ stripping, and sustained water consumption. In this work, we demonstrate that an aqueous electrolyte based on Zn and lithium salts at high concentrations is a very effective way to address these issues. This unique electrolyte not only enables dendrite-free Zn plating/stripping at nearly 100% CE, but also retains water in the open atmosphere, which makes hermetic cell configurations optional. These merits bring unprecedented flexibility and reversibility to Zn batteries using either LiMn2O4 or O2 cathodes-the former deliver 180 W h kg-1 while retaining 80% capacity for >4,000 cycles, and the latter deliver 300 W h kg-1 (1,000 W h kg-1 based on the cathode) for >200 cycles.

Existence of Solid Electrolyte Interphase in Mg Batteries: Mg/S Chemistry as an Example.

Magnesium redox chemistry is a very appealing "beyond Li ion chemistry" for realizing high energy density batteries due to the high capacity, low reduction potential, and most importantly, highly reversible and dendrite-free Mg metal anode. However, the progress of rechargeable Mg batteries has been greatly hindered by shortage of electrolytes with wide stability window, high ionic conductivity, and good compatibility with cathode materials. Unlike solid electrolyte interphase on Li metal anode, surface film formed by electrolyte decomposition in Mg batteries was considered to block Mg ion transport and passivate Mg electrode. For this reason, the attention of the community has been mainly focusing on surface layer free electrolytes, while reductively unstable salts/solvents are barely considered, despite many of them possessing all the necessary properties for good electrolytes. Here, for the first time, we demonstrate that the surface film formed by electrolyte decomposition can function as a solid electrolyte interphase (SEI). Using Mg/S chemistry as a model system, the SEI formation mechanism on Mg metal anode was thoroughly examined using electrochemical methods and surface chemistry characterization techniques such as EDX and XPS. On the basis of these results, a comprehensive view of the Mg/electrolyte interface that unifies both the SEI mechanism and the passivation layer mechanism is proposed. This new picture of surface layer on Mg metal anode in Mg batteries not only revolutionizes current understanding of Mg/electrolyte interface but also opens new avenues for electrolyte development by uncovering the potential of those reductively unstable candidates through interface design.

High-Performance All-Solid-State Na-S Battery Enabled by Casting-Annealing Technology.

Room-temperature all-solid-state Na-S batteries (ASNSBs) using sulfide solid electrolytes are a promising next-generation battery technology due to the high energy, enhanced safety, and earth abundant resources of both sodium and sulfur. Currently, the sulfide electrolyte ASNSBs are fabricated by a simple cold-pressing process leaving with high residential stress. Even worse, the large volume change of S/Na2S during charge/discharge cycles induces additional stress, seriously weakening the less-contacted interfaces among the solid electrolyte, active materials, and the electron conductive agent that are formed in the cold-pressing process. The high and continuous increase of the interface resistance hindered its practical application. Herein, we significantly reduce the interface resistance and eliminate the residential stress in Na2S cathodes by fabricating Na2S-Na3PS4-CMK-3 nanocomposites using melting-casting followed by stress-release annealing-precipitation process. The casting-annealing process guarantees the close contact between the Na3PS4 solid electrolyte and the CMK-3 mesoporous carbon in mixed ionic/electronic conductive matrix, while the in situ precipitated Na2S active species from the solid electrolyte during the annealing process guarantees the interfacial contact among these three subcomponents without residential stress, which greatly reduces the interfacial resistance and enhances the electrochemical performance. The in situ synthesized Na2S-Na3PS4-CMK-3 composite cathode delivers a stable and highly reversible capacity of 810 mAh/g at 50 mA/g for 50 cycles at 60 °C. The present casting-annealing strategy should provide opportunities for the advancement of mechanically robust and high-performance next-generation ASNSBs.

Thermodynamics and Kinetics of Sulfur Cathode during Discharge in MgTFSI2 -DME Electrolyte.

Rechargeable magnesium/sulfur battery is of significant interest because its energy density (1700 Wh kg-1 and 3200 Wh L-1 ) is among the highest of all battery chemistries (lower than Li/O2 and Mg/O2 but comparable to Li/S), and Mg metal allows reversible operation (100% Coulombic efficiency) with no dendrite formation. This great promise is already justified in some early reports. However, lack of mechanistic study of sulfur reaction in the Mg cation environment has severely hindered our understanding and prevents effective measures for performance improvement. In this work, the very first systematic fundamental study on Mg/S system is conducted by combining experimental methods with computational approach. The thermodynamics and reaction pathway of sulfur cathode in MgTFSI2 -DME electrolyte, as well as the associated kinetics are thoroughly investigated. The results here reveal that sulfur undergoes a consecutive staging pathway in which the formation and chain-shortening of polysulfide occur at early stage accompanied by the dissolution of long-chain polysulfide, and solid-state transition from short-chain polysulfide to magnesium sulfide occurs at late stage. The former process is much faster than the latter due to the synergetic effect of the mediating effect of dissolved polysulfide and the fast diffusion of Mg ion in the amorphous intermediate.

High-Voltage Aqueous Magnesium Ion Batteries.

Nonaqueous rechargeable magnesium (Mg) batteries suffer from the complicated and moisture-sensitive electrolyte chemistry. Besides electrolytes, the practicality of a Mg battery is also confined by the absence of high-performance electrode materials due to the intrinsically slow Mg2+ diffusion in the solids. In this work, we demonstrated a rechargeable aqueous magnesium ion battery (AMIB) concept of high energy density, fast kinetics, and reversibility. Using a superconcentration approach we expanded the electrochemical stability window of the aqueous electrolyte to 2.0 V. More importantly, two new Mg ion host materials, Li superconcentration approach we expanded the electrochemical stability window of the aqueous electrolyte to 2.0 V. More importantly, two new Mg ion host materials, Li3V2(PO4)3 and poly pyromellitic dianhydride, were developed and employed as cathode and anode electrodes, respectively. Based on comparisons of the aqueous and nonaqueous systems, the role of water is identified to be critical in the Mg ion mobility in the intercalation host but remaining little detrimental to its non-diffusion controlled process. Compared with the previously reported Mg ion cell delivers an unprecedented high power density of 6400 W kg ion cell delivers an unprecedented high power density of 6400 W kg while retaining 92% of the initial capacity after 6000 cycles, pushing the Mg ion cell to a brand new stage.

Flexible Aqueous Li-Ion Battery with High Energy and Power Densities.

A flexible and wearable aqueous symmetrical lithium-ion battery is developed using a single LiVPO4 F material as both cathode and anode in a "water-in-salt" gel polymer electrolyte. The symmetric lithium-ion chemistry exhibits high energy and power density and long cycle life, due to the formation of a robust solid electrolyte interphase consisting of Li2 CO3 -LiF, which enables fast Li-ion transport. Energy densities of 141 Wh kg-1 , power densities of 20 600 W kg-1 , and output voltage of 2.4 V can be delivered during >4000 cycles, which is far superior to reported aqueous energy storage devices at the same power level. Moreover, the full cell shows unprecedented tolerance to mechanical stress such as bending and cutting, where it not only does not catastrophically fail, as most nonaqueous cells would, but also maintains cell performance and continues to operate in ambient environment, a unique feature apparently derived from the high stability of the "water-in-salt" gel polymer electrolyte.

Reverse Microemulsion Synthesis of Sulfur/Graphene Composite for Lithium/Sulfur Batteries.

Due to its high theoretical capacity, high energy density, and easy availability, the lithium-sulfur (Li-S) system is considered to be the most promising candidate for electric and hybrid electric vehicle applications. Sulfur/carbon cathode in Li-S batteries still suffers, however, from low Coulombic efficiency and poor cycle life when sulfur loading and the ratio of sulfur to carbon are high. Here, we address these challenges by fabricating a sulfur/carboxylated-graphene composite using a reverse (water-in-oil) microemulsion technique. The fabricated sulfur-graphene composite cathode, which contains only 6 wt % graphene, can dramatically improve the cycling stability as well as provide high capacity. The electrochemical performance of the sulfur-graphene composite is further enhanced after loading into a three-dimensional heteroatom-doped (boron and nitrogen) carbon-cloth current collector. Even at high sulfur loading (∼8 mg/cm2) on carbon cloth, this composite showed 1256 mAh/g discharge capacity with more than 99% capacity retention after 200 cycles.

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.

High-Performance All-Inorganic Solid-State Sodium-Sulfur Battery.

All-inorganic solid-state sodium-sulfur batteries (ASSBs) are promising technology for stationary energy storage due to their high safety, high energy, and abundant resources of both sodium and sulfur. However, current ASSB shows poor cycling and rate performances mainly due to the huge electrode/electrolyte interfacial resistance arising from the insufficient triple-phase contact among sulfur active material, ionic conductive solid electrolyte, and electronic conductive carbon. Herein, we report an innovative approach to address the interfacial problem using a Na3PS4-Na2S-C (carbon) nanocomposite as the cathode for ASSBs. Highly ionic conductive Na3PS4 contained in the nanocomposite can function as both solid electrolyte and active material (catholyte) after mixing with electronic conductive carbon, leading to an intrinsic superior electrode/electrolyte interfacial contact because only a two-phase contact is required for the charge transfer reaction. Introducing nanosized Na2S into the nanocomposite cathode can effectively improve the capacity. The homogeneous distribution of nanosized Na2S, Na3PS4, and carbon in the nanocomposite cathode could ensure a high mixed (ionic and electronic) conductivity and a sufficient interfacial contact. The Na3PS4-nanosized Na2S-carbon nanocomposite cathode delivered a high initial discharge capacity of 869.2 mAh g-1 at 50 mA g-1 with great cycling and rate capabilities at 60 °C, representing the best performance of ASSBs reported to date and therefore constituting a significant step toward high-performance ASSBs for practical applications.