Morphology-Directed Synthesis of LiFePO4 and LiCoPO4 from Nanostructured Li1+2 xPO3+ x.

A LiPO3-type nanostructure has been developed using a simple microwave approach at temperatures as low as 200 °C. This phase presents an ideal architecture for the morphology-directed synthesis of the olivine-type phases LiFePO4 and LiCoPO4, through a simple and scalable solution-based technique. Pure and carbon-composited olivine phases of interconnected nanoparticulate morphologies display excellent performance at high rates (up to 20 C) over 500 cycles in Li-ion battery cells.

Selective and Facile Synthesis of Sodium Sulfide and Sodium Disulfide Polymorphs.

Na2S and Na2S2 were selectively synthesized using a microwave-assisted thermal treatment of a Na+/S solution in tetraglyme between 100 and 200 °C, considerably lower than that of current routes. This novel synthetic pathway yields the Na2S phase in high purity and allows for good selectivity between the polymorphs of Na2S2 (α and ß phases). These materials show promising electrochemical properties and are particularly interesting for the continued development of Na-S batteries.

A new nanostructured γ-Li3PO4/GeO2 composite for all-solid-state Li-ion battery applications.

High-temperature sintering is crucial to achieve good crystallinity and fast-ion conduction in oxide-type solid-electrolytes such as lithium garnets, NASICONs and LISICONs, leading to stiff ceramics which are difficult to integrate in all-solid-state batteries. Developing conventional oxide-based solid-electrolytes in deformable forms that maintain good ion transport properties and allow facile formulation of bulk-type solid-state batteries, hence, remains a challenge. Here, a new γ-Li3PO4/GeO2 composite, that adopts a novel nanostructured architecture and retains deformability after calcination at 500 °C, is successfully synthesized and densified by cold-pressing. Cold-pressed pellets of the new composite showed an ion conductivity that is four orders of magnitude higher than that of the parent γ-Li3PO4 and comparable to those of high-temperature stiff Li3+xP1-xGexO4 ceramics. The γ-Li3PO4/GeO2 composite is stable against high voltages (up to 5 V vs Li+/Li), which suggests a safe use in contact with high-voltage cathodes. The new composite can also be modified to serve as an active anode layer in solid-state cells due to the electrochemical activity of GeO2 at low voltages (<1 V vs. Li+/Li). This study emphasizes the potential of using low-temperature synthesis to develop novel oxide-based nanoarchitectures for all-solid-state battery applications.

Morphology-controlled synthesis of novel nanostructured Li4P2O7 with enhanced Li-ion conductivity for all-solid-state battery applications.

Mechanical stiffness of oxide-type solid-electrolytes is a major drawback which has hindered their practical application in all-solid-state Li-ion batteries to date. Despite their enhanced structural and electrochemical stabilities, lack of deformability of fast-ion conducting oxides impedes the integration of these materials in bulk-type solid-state cells. Deformable solid-electrolytes such as sulfides, on the other hand, lack sufficient electrochemical stability in contact with conventional cathodes. This has recently triggered a search for new materials that combine high ion-conductivity, deformability and sufficient electrochemical stability. Here, we report the synthesis of a novel form of Li4P2O7 that can be densified by cold-pressing and possesses an ion conductivity that is two orders of magnitude higher than conventional Li4P2O7 phases. The material is synthesized by a combination of microwave synthesis and chemical lithiation and adopts a nanostructured morphology with a small amorphous component. The material is electrochemically stable at voltages >5 V vs. Li+/Li, which suggests safe use with high-voltage cathodes. The newly-synthesized material is therefore a bulk, deformable analogue of LiPON, with comparable ion conductivity and phase stability. This research highlights the potential of using novel low-temperature synthetic routes to control the morphology and enhance the electrochemical performance of conventional functional materials.

Stabilization of the cubic, fast-ion conducting phase of Li7La3Sn2O12 garnet by gallium doping.

All-solid-state batteries present promising high-energy-density alternatives to conventional Li-ion chemistries, and Li-stuffed garnets based on Li7La3Zr2O12 (LLZO) remain a forerunner for candidate solid-electrolytes. One route to access fast-ion conduction in LLZO phases is to stabilize the cubic LLZO phase by doping on the Li sites with aliovalent ions such as Al3+ or Ga3+. Despite prior attempts, the stabilization of the cubic phase of isostructural Li7La3Sn2O12 (LLSO) by doping on the Li sites has up to now not been realised. Here, we report a novel cubic fast-ion conducting Li7La3Sn2O12-type phase stabilized by doping Ga3+ in place of Li. 0.3 mole of gallium per formula unit of LLSO were needed to fully stabilize the cubic garnet, allowing structural and electrochemical characterizations of the new material. A modified sol-gel synthesis approach is introduced in this study to realise Ga-doping in LLSO, which offers a viable route to preparing new Sn-based candidate solid-electrolytes for all-solid-state battery applications.

Liquid-Phase Approach to Glass-Microfiber-Reinforced Sulfide Solid Electrolytes for All-Solid-State Batteries.

Deformable, fast-ion conducting sulfides enable the construction of bulk-type solid-state batteries that can compete with current Li-ion batteries in terms of energy density and scalability. One approach to optimizing the energy density of these cells is to minimize the size of the electrolyte layer by integrating the solid electrolyte in thin membranes. However, additive-free thin membranes, as well as many membranes based on preprepared scaffolds, are difficult to prepare or integrate in solid cells on a large scale. Here, we propose a scalable solution-based approach to produce bulk-type glass-microfiber-reinforced composites that restore the deformability of sulfide electrolytes and can easily be shaped into thin membranes by cold pressing. This approach supports both the ease of preparation and enhancement of the energy density of sulfide-based solid-state batteries.

In Situ Diffusion Measurements of a NASICON-Structured All-Solid-State Battery Using Muon Spin Relaxation.

In situ muon spin relaxation is demonstrated as an emerging technique that can provide a volume-averaged local probe of the ionic diffusion processes occurring within electrochemical energy storage devices as a function of state of charge. Herein, we present work on the conceptually interesting NASICON-type all-solid-state battery LiM2(PO4)3, using M = Ti in the cathode, M = Zr in the electrolyte, and a Li metal anode. The pristine materials are studied individually and found to possess low ionic hopping activation energies of ∼50-60 meV and competitive Li+ self-diffusion coefficients of ∼10-10-10-9 cm2 s-1 at 336 K. Lattice matching of the cathode and electrolyte crystal structures is employed for the all-solid-state battery to enhance Li+ diffusion between the components in an attempt to minimize interfacial resistance. The cell is examined by in situ muon spin relaxation, providing the first example of such ionic diffusion measurements. This technique presents an opportunity to the materials community to observe intrinsic ionic dynamics and electrochemical behavior simultaneously in a nondestructive manner.

Li1.5La1.5MO6 (M = W6+, Te6+) as a new series of lithium-rich double perovskites for all-solid-state lithium-ion batteries.

Solid-state batteries are a proposed route to safely achieving high energy densities, yet this architecture faces challenges arising from interfacial issues between the electrode and solid electrolyte. Here we develop a novel family of double perovskites, Li1.5La1.5MO6 (M = W6+, Te6+), where an uncommon lithium-ion distribution enables macroscopic ion diffusion and tailored design of the composition allows us to switch functionality to either a negative electrode or a solid electrolyte. Introduction of tungsten allows reversible lithium-ion intercalation below 1 V, enabling application as an anode (initial specific capacity >200 mAh g-1 with remarkably low volume change of ∼0.2%). By contrast, substitution of tungsten with tellurium induces redox stability, directing the functionality of the perovskite towards a solid-state electrolyte with electrochemical stability up to 5 V and a low activation energy barrier (<0.2 eV) for microscopic lithium-ion diffusion. Characterisation across multiple length- and time-scales allows interrogation of the structure-property relationships in these materials and preliminary examination of a solid-state cell employing both compositions suggests lattice-matching avenues show promise for all-solid-state batteries.

A facile synthetic approach to nanostructured Li2S cathodes for rechargeable solid-state Li-S batteries.

Li-S solid state batteries, employing Li2S as a pre-lithiated cathode, present a promising low cost, high capacity and safer alternative to their liquid electrolyte counterparts, where dissolution of intermediate polysulfide species can result in loss of active material and a subsequent decrease in ionic conductivity. A nanostructured Li2S material would afford greater flexibility in optimising the cathode composite for more harmonious electrode-electrolyte interactions, yet facile routes to such nanoscale materials are limited. Here, we report a facile and scalable microwave approach to directly synthesize nanostructured Li2S from a glyme solution containing lithium polysulfides. As-synthesized Li2S presents an ideal architecture for the construction of free-standing cathodes for all-solid-state Li-S batteries.

Enhancement of the lithium ion conductivity of Ta-doped Li7La3Zr2O12 by incorporation of calcium.

Fast ion conducting garnet materials have been identified as promising electrolytes for all solid-state batteries. However, reliable synthetic routes to materials with fully elucidated cation site occupancies where an enhancement in lithium conductivity is observed remains a challenge. Ca-Incorporation is developed here as a promising approach to enhance the ionic conductivity of garnet-type Li7-xLa3Zr2-xTaxO12 phases. Here we present a new sol-gel synthetic strategy as a facile route to the preparation of materials of a desired stoichiometry optimized for Li+ conductivity. We have found that the ionic conductivity of Li6.4La3Zr1.4Ta0.6O12 is increased by a factor of four by the addition of 0.2 mol of Ca per formula unit. Ca is incorporated in the garnet lattice where it has no effect on the sinterability of the material and is predominately located at the La sites. We anticipate that the ease of our synthetic route and the phases presented here represents a starting point for the further realization of solid state electrolyte compositions with similarly high Li+ conductivities using this methodology.