Increasing energy density of rechargeable batteries is highly desired by many emerging applications. It is necessary to identify possible solutions for achieving both high energy density and other required performances. Based on personal knowledge and understandings, this perspective paper summarizes the main scientific and technological problems of solid lithium battries as well as reported solutions. In view of practical application, the features of four types solid lithium batteries with different solid electrolyte are compared. And a roadmap is drawn accordingly. In addition, the technological targets of the energy density of lithium batteries from USA, Japan and China government are listed. The positions of the solid lithium batteries in the roadmap are marked.

All-solid-state lithium ion batteries utilize solid state electrolytes to overcome the safety issues of liquid electrolytes, becoming the most promoting candidate for electric vehicle and large-scale stationary-type distributed power sources. There is an urgent demand for all solid state lithium ion batteries with high energy and power densities and longevity. Materials hold the key to fundamental and practical advances in all-solid-state lithium ion batteries. Most studies have been focused on exploration and preparation of solid electrolytes with high ambient temperature ion conductivity as well as cathode and anode with high energy density, and optimization of interfacial compatibility between electrode and solid electrolyte. This paper is a comprehensive review of the key materials for all-solid-state lithium ion batteries: Various important advances of new solid electrolyte, cathode and anode made in research and practical application, the modification methods to improve the interfacial behavior, and the further development of materials and interfacial issues, which lay a foundation for the analysis of commercial applications prospect of all-solid-state lithium ion batteries.

The traditional rechargeable lithium batteries commonly used a large amount of non-aqueous liquid electrolytes leading to inherent hazards of leakages and fire. All-solid-state polymer electrolytes (ASPEs) attract intensive interests due to their unique properties, such as high safety characteristics, wide operating temperature range and long cycle life. They are expected to be the next generation of commercialized electrolytes in the field of lithium-ion battery. The dendritic growth of lithium metal electrode can also be well suppressed in the process of charging and discharging by ASPEs, because ASPEs usually have excellent mechanical properties. This review presents a brief overview of recent progress in ASPEs based on polyethylene oxide(PEO), polycarbonate, polysiloxane and single lithium-ion conductor. PEO is the first class of ASPEs that are researched extensively, whose high crystallinity give rise to the difficult migration of Li+ and low ion conductivity. Aimed at the issue of crystallinity, researchers have exploited plenty of modifications to lower polymer chains’ crystallinity and improve the conductivity of PEO. Lithium salts are easily dissolved in polycarbonates and resulted polymer electrolyte has higher ion conductivity than PEO because of its strongly polar carbonate group and amorphous state at room temperature, which may be alternative materials of PEO potentially. Besides the carbon-chain polymers, polysiloxane with low glass transition temperature attracts widespread concerns from researchers because of its high conductivity. In addition, migration of anions will only exacerbate concentration polarization of electrolytes in the charge-discharge process, so single lithium-ion conductors without anions’ migration are also worth to exploiting. Finally, the challenges and future trends towards high energy and all-solid-state polymer electrolytes batteries are also commented. PEO should be developed with the organic-inorganic composite system, polycarbonate should be developed with the blend system, polysiloxane should be enhanced with strong mechanical properties, single lithium-ion conductor should be designed with the new polyanion lithium salt that has higher conductivity.

Organic liquids are mostly used as an electrolyte system in commercial lithium-ion batteries. Currently organic liquids electrolytes have certain safety issues caused by the inflammability, explosiveness and leakiness. All solid state lithium ion battreies used inorganic solid electrolytes offer a fundamental solution to the safety issues of conventional lithium ion battreies containing organic electrolytes. The inorganic solid electrolyte has attracted much people's attention recently because of its great advantages: Inhibition the dentrite formation in lithium anodes, the high mechanical robustness, simplification the preparation technology of battery, reducing the manufacturing cost of the battery. This article presents a brief review of the researches on binary sulfide system, ternary sulfide system, preparation and modifications. The prospects of the solid electrolytes are also stated.

Compared with conventional lithium ion batteries, all solid state lithium batteries based on solid electrolytes are new research hotspots, in view of such potential merits as high safety, long cycle life and high energy density. Furthermore, all solid state lithium batteries could be wildly used on electric vehicles and smart grid in the near future. The solid-solid contact resistance of interfaces between electrodes and solid electrolytes is larger than solid-liquid contact resistance. Meanwhile, interface compatibility and stability have significant influence on cycle performance and rate capability. Moreover, grain boundaries in electrolytes determine overall conductivity. Thus, interface issues have great effect on electrochemical performances of all solid state lithium batteries. This paper presents a brief review on all kinds of interfaces in all solid state lithium batteries, covering interface optimizing mechanism and modification methods. The challenges on interface in all solid state lithium batteries are suggested as well.

The space charge layer (SCL) effects result from the requirement of thermodynamic equilibrium at the interface, which were firstly used to account for the abnormal conductivity enhancement in composite conductors. They were lateron developed to qualitatively and quantitatively explain the interfacial transport behaviors in many other systems. Particularly, the SCL effects could be utilized to control the conductivity and construct artificial conductors in nanometer-scale systems. Rechargeable solid state lithium batteries have attracted much attention, especially the SCL effects at the interfaces between the electrolyte and the electrode or inside the composite electrode. In this paper, the principle of SCL based on defect chemistry near the two-phase boundary from the thermodynamical point of view is firstly presented. The SCL effects in several typical conducting systems as well as the influence on properties are reviewed. On this basis, the SCL effects in rechargeable solid state lithium batteries reported so far are reviewed. Characterization techniques of the SCL effects are introduced. Finally, the possibility of utilizing SCL effects to improve battery performance is addressed.

All-solid-state thin film lithium batteries (TFBs) have better safety as well as greater potential in the energy density, power density and cycle life than lithium ion batteries (LIBs). TFBs have the most mature preparation technology among the exiting type of all-solid-state lithium batteries, which performance were excellent and were also taking the lead to achieve the commercialization. The states of art preparation of TFBs were quite different with LIB, which is mainly depending on physical deposition. Every part of TFBs such as cathode, anode, electrolyte and collector was prepared into high density films followed with in situ stack. This review summarizes the progress and advances for the studies of TFBs in the recent decade. We tried to give an airscape from the view points of the whole preparation of TFBs, the existing problems of scientific and technical. The solid inorganic electrolyte film, cathode film and anode film regard as three main steps for TFBs preparation, which progress and key issues of were described separately. On this basis, the whole preparation of TFBs and the TFBs with special structure were discussed, following with the brief advances of substrate, current collector and encapsulation.

 The theoretical specific energy of lithium-air battery is as high as 3505 W•h/kg. The practical specific energy of this electrochemical energy storage system is estimated to be able to 600 W•h/kg, which is a promising value to sustain a driving range of 500～800 kilometers for electric vehicles. Currently lithium-air batteries are facing various challenges, such as stability associated with decomposition of carbon-based cathodes and electrolytes, low electric energy efficiency and power density, questionable applicability due to the common operation environment just in pure oxygen, and the safety issues related to lithium dendrites and so on, in particular, the ability whether can be operated in ambient air or not. Developing solid-state lithium-air batteries can solve the problem of applicability fundamentally, circumvent the safety issues completely, and which is also an important avenue to improve the stability of the battery system. In this paper, we reviewe the progress on the cell construction, the regulation of the electron/electrolyte interface, the cell assembly, the electrochemical performance and the mechanism for the solid-state lithium-air batteries. In every section, the contributions of the recent research progress on the SEAS challenges and the still remained questions will be commented. Based on these review, we attempt to propose some alternative approaches for the next stage, and suggest a development prospective for the solid-state lithium-air batteries.

Novel solid polymer electrolytes, composed of lithium (fluorosulfonyl)(trifluoromethanesulfonyl) imide {Li[(FSO2)(CF3SO2)N], LiFTFSI} and poly(ethylene oxide) (PEO), are prepared by solution casting method. The physicochemical and electrochemical properties of LiFTFSI/PEO [n(EO)∶n(Li+)=16] electrolyte are investigated by the differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), linear sweep voltammogram (LSV), electrochemical  impedance  spectroscopy (EIS), and potentiostatic direct current (DC) polarization. It is demonstrated that LiFTFSI/PEO electrolyte exhibits a relatively high ionic conductivity (σ≈10−5 S/cm) at 25 ℃, sufficient electrochemical stability (4.63 V vs. Li/Li+), and is thermally stable up to 256 ℃. More importantly, the complex of LiFTFSI/PEO displays a relatively high initial discharge capacity (881 mA•h/g) and effectively inhibits the shuttle effect arising from dissolved polysulfides in lithium-sulfur (Li-S) batteries, and exhibits a good cycling performance of Li-S batteries.

Compared with traditional lithium-ion batteries, lithium-sulfur (Li-S) batteries have many advantages such as high specific capacity, high energy density and environment-friendly characteristic, which makes them a promising system for electric vehicles and energy storage. However, the Li-S batteries have been facing problems, such as the shuttle effect, low efficiency, and volume change during cycling. In this paper, we try to suppress the shuttle effect by introducing a LLZO-based hybrid electrolyte. Firstly, we prepare the dense LLZO solid electrolyte with the ionic conductivity of 6.4×104 S/cm; then we use the LLZO as the separator, Li anode, and S-C composite cathode to assemble Li-S test cells. The electrochemical test results indicate that the cell can obtain a Coulomb efficiency closely to 100%. The post-test examination shows that element S can only be found on the side of LLZO facing the S-C cathode, not on the side facing the Li anode, suggesting that the LLZO can effectively block the transport of the dissolved polysulfides.

As a family of promising inorganic solid electrolyte in csrystallized phase, the antiperovskite superionic conductor with general formula of Li3OX(X=F,Cl,Br) received much attention since the day they were sucssesfully synthesized. However many researches pointed out that Li3OX is thermaldynamically metastable with respect to decomposition to Li2O and LiX. In this paper a comparative study is made between the members of this family by means of first-principle calculations with together the lattice-dynamic simulation under the quasi harmonic approximation. The result shows that they share the same atomic configuration with a cubic unit cell and a wide electrochemical window. However the Gibbs formation energy at different temperature and pressure reveals their distinct theromodynamic stability wich may root in their different radius ratios to the oxygen atom. The simulation results are in qualitive agreement with the experimental condition under which the Li3OCl are synthesized and explain the reason why the thermodynamically meta-stable antiperovskite Li3OX can exist to some extend.

A novel free-standing composite electrolyte membranes with high ionic conductivity and good electrochemistry stability is papered through incorporation of sulfide electrolyte 70Li2S-29P2S5-1P2O5 (LPOS) into polyethylene oxide (PEO) matrix. The LPOS particles, acting as active fillers incorporation into the PEO matrix, have a positive effect on the ionic conductivity, lithium ion transference number and electrochemical stability. The lithium ion conductivities of as-prepared composite membranes are evaluated, and the optimal composite membrane incorporating 1% LPOS exhibits an ionic conductivity of 1.60×105 S/cm at room temperature and a maximum ionic conductivity of 1.08×103 S/cm at 80 ℃ and an electrochemical window of 4.7 V. And the LiFePO4/Li battery fabricated with this new composite electrolyte membrane exhibits fascinating cycle performance with high capacity retention. After 50 cycles, the discharge capacity of cell LiFePO4/PEO18-LiTFSI-1%LPOS/Li is 105 mA•h/g at 1 C rate at 60 ℃. It is demonstrated that this new composite electrolyte membrane should be a promising electrolyte applied in solid state batteries based on lithium metal electrode.

LAGP-PEO(LiX) solid composite electrolyte were prepared with Li1.5Al0.5Ge1.5(PO4)3, LiX as conductive components and poly(ethylene oxide) as the binder using solution casting method. Effect of lithum salt, such as LiClO4, LiTFSI and LiBOB, on the ionic conductivity and electrochemical window of solid composite electrolyte were studied. At the same time, chemical and electrochemical cycling stability of interface between solid composite electrolyte and lithium were also studied. The results show that the decomposition voltage of LAGP-PEO(LiX) was higher than 5 V, the ionic conductivity of LAGP-PEO(LiTSFI) is the highest and it is stable at room temperature, but LAGP-PEO(LiBOB) is more stable at 60 ℃. In addition, the rate capability and the cycling performance of solid state LFP lithium ion battery with LAGP-PEO(LiX) were studied. The best properties was present at room temperature when LAGP-PEO(LiTSFI) composite electrolyte was used in battery, while LAGP-PEO(LiBOB) shows its best performances at 60 ℃.

Solid electrolytes do not have the common shortcomings of liquid electrolytes, such as flammability, leakage and low chemical stability. However, solid electrolytes also have a big disadvantage of relatively low lithium ion conductivity of 105～103 S/cm, as compared with the conductivity of liquid electrolytes (102 S/cm). Therefore, the properties of all-solid-state lithium ion batteries based on solid electrolytes are normally rather poor. To increase the lithium ion conductivity of solid electrolytes is a key issue for improving the performance of all-solid-state lithium ion batteries. As a matter of fact, point defects determine the lithium ion conductivity of oxides. To modulate the defect structure of lithium ion conducting oxides is an effective way to enhance the lithium ion conductivity. In this work, two oxides, i.e. perovskite Li3xLa2/3x□1/3-2xTiO3 (0.04x0.16) and garnet Li7La3Zr2O12 are evaluated from the point of view of defect chemistry, and the contributions of various defects to the lithium ion conductivity, the oxygen ion conductivity and the electronic conductivity are elucidated.

Compared to the commercial rechargeable lithium batteries using liquid electrolytes, the rechargeable solid state lithium batteries have attracted much attention, since their great potential in high energy density and safety. As the key materials for rechargeable solid state lithium batteries, the solid state electrolytes need to have the high ionic conductivity, wide electrochemical window, superior mechanical properties, stability against Li and ability to suppressing lithium dendrite growth. To meet the above requirements, organic-inorganic hybrid solid state electrolytes membranes with Li6.4La3Zr1.4Ta0.6O12 (LLZTO) nanopowders and polyethylene oxides (PEO) are prepared. The conductivity and electrochemical properties of PEO-LLZTO and PEO-LiTFSI-LLZTO membranes are comparatively studied. With the insulating PEO, the conductivities of the PEO-LLZTO electrolytes membranes have been greatly improved owing to the percolation effect at the interface, approaching   2×10−4 S/cm at room temperature. Though the conductivity of the PEO-LLZTO electrolytes membranes is slightly lower than that of the PEO-LiTFSI-LLZTO electrolyte membranes (i.e. 6×10−4 S/cm at room temperature), the PEO-LLZTO electrolyte membranes show better electrochemical stability and improved ability of suppressing the lithium dendrite growth. The pouch cells using PEO-LLZTO electrolytes membranes with Li/LiFePO4 and Li/LiFe0.15Mn0.85PO4 show higher energy density, and the batteries can cycle more than 200 times.

This bimonthly review paper highlights 100 recent published papers on lithium batteries. We searched the Web of Science and found 1880 papers online from Jun. 1, 2016 to Jul. 31, 2016. 100 of them were selected to be highlighted. Layered oxide and high voltage spinel cathode materials are still under extensive investigations for studying Li+ intercalation-deintercalation mechanism and evolution of surface structure, and the influences of doping, coating and interface modifications on their cycling performances. Large efforts were devoted to Si based composite anode materials for analyzing the mechanism for Li storage and SEI formation. In-situ technologies are used to analyze the kinetic process and SEI and theoretical work covers the machnism for Li storage, kinetics, SEI and solid state electrolytes. There are a few papers related to electrolyte additives, solid state lithium batteries, Li/S batteries, Li-air batteries, and modeling.