Semih Afyon

New High Capacity Cathode Materials for Rechargeable Li-ion Batteries: Vanadate-Borate Glasses

V2O5 based materials are attractive cathode alternatives due to the many oxidation state switches of vanadium bringing about a high theoretical specific capacity. However, significant capacity losses are eminent for crystalline V2O5 phases related to the irreversible phase transformations and/or vanadium dissolution starting from the first discharge cycle. These problems can be circumvented if amorphous or glassy vanadium oxide phases are employed. Here, we demonstrate vanadate-borate glasses as high capacity cathode materials for rechargeable Li-ion batteries for the first time. The composite electrodes of V2O5 – LiBO2 glass with reduced graphite oxide (RGO) deliver specific energies around 1000 Wh/kg and retain high specific capacities in the range of ~ 300 mAh/g for the first 100 cycles. V2O5 – LiBO2 glasses are considered as promising cathode materials for rechargeable Li-ion batteries fabricated through rather simple and cost-efficient methods.

A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries

Ga-doped Li7La3Zr2O12 garnet structures are among the most promising electrolytes for all-solid state Li-ion-batteries. The synthesis and processing of garnet-type fast Li-ion conductors depend on conventional sol–gel and solid state syntheses and sintering that are usually done at temperatures above 1050 °C to reach the high Li-ion conducting cubic phase. This process results in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode–electrolyte assembly. Here, we tackle this problem and report a novel low temperature synthesis-processing route to stabilize the cubic phase of Li7La3Zr2O12, while keeping the nanocrystallites at ∼200–300 nm. Li7La3Zr2O12 phases are obtained at temperatures as low as 600 °C by a modified sol–gel combustion method utilizing mainly nitrate precursors, and the sintering temperature is lowered by ∼200 °C compared to the state-of-art. Through a new model experiment, we also shed light on the conditions influencing the tetragonal to cubic phase transformation via homogeneous Ga-diffusion and incorporation occurring at a surprisingly low temperature of ∼100 °C for a post-annealing step. The sintered pellets of the newly obtained Li6.4Ga0.2La3Zr2O12 deliver high bulk Li-ion conductivities in the range of ∼4.0 × 10−4 S cm−1 at 20 °C, and a wide thermal operation window is accessible through its characteristic activation energy of ∼0.32 eV. We report that there is an optimum in sintering-processing conditions for the cubic c-Li6.4Ga0.2La3Zr2O12 solid state electrolytes and their Li-ionic conductivity and the (Raman) near order characteristics that can be tracked through changes in Li–O vibrational modes. Based on this alternative route, low-temperature synthesized powders can be sintered to relatively dense pellets at around only 950 °C. At higher sintering temperatures (e.g. 1100 °C), Li-losses progress as confirmed by structural studies and a reduction of both ceramic pellet density and ionic conductivity, as well as distortions in the Li-sublattice, are found. Through this work, an alternative low temperature processing route for Ga-doped Li7La3Zr2O12 garnet type electrolytes for all-solid state batteries is suggested. The new synthesis method and the use of c-Li6.4Ga0.2La3Zr2O12 nanoparticles could open pathways in terms of preventing Li-loss during the process and advancing future solid electrolyte–electrode assembly options for all-solid state Li-ion batteries.

A Lithium-Rich Compound Li7Mn(BO3)3 Containing Mn2+ in Tetrahedral Coordination: A Cathode Candidate for Lithium-Ion Batteries

Four-coordinate Mn is a rare species owing to the lower ligand-field stabilization compared to the octahedral environment and—to our knowledge—was hitherto only reported with structure-directing multidentate ligands. Li ions tend to be octahedrally coordinated but may also appear in a tetrahedral environment, for example, in Li2O. The title compound Li7Mn(BO3)3 contains a relatively large fraction of cations where both Li and Mn are in tetrahedral coordination environments with the exception of two Li sites. It can be written as M7M II 1(M III 3)O9 with M:O = 11:9 (where M = Li, Mn, B). In addition, there may be a structure-directing effect of the planar borate groups preventing a more-compact arrangement with higher coordination numbers. The peculiar tetrahedral environment of Mn could allow a wide range of oxidation-number switches without trapping specific states, such as octahedral Mn. Together with its large concentration of Li atoms this compound could potentially become a fully lithiated cathode material for lithium-ion batteries, extending the limited range of electric vehicles. The presently employed Lix(Ni,Mn,Co)O2 (NMC) and LixFePO4 (LFP) are quite moderate in terms of capacity with theoretical specific charges of 140 to 170 mAhg . 5] The poor electronic conductor LFP, which can be activated by wrapping up nanoparticles in a conductive composite, has a high cycling stability which is ascribed to the linking phosphate groups. Borate groups may fulfill a similar function but at a lower specific weight. To exchange more charge per mass unit, several oxidation-state switches have to be realized at the redox-active cations, eventually reaching the high oxidation states of the transition metal center. Manganese is especially suited for this purpose. However, at high oxidation states monomeric units like the permanganate anion are formed, which easily dissolve in liquid electrolytes and thus induce battery failure. Herein, inert linker groups such as BO3 units are used to solve part of the problem. The use of monoborates, such as LiMBO3 (M = Fe, Mn, Co) as cathode materials for Li-ion batteries was first investigated by Legagneur et al. but only 4% and 2% Li were extractable for LiFeBO3 and LiMnBO3, respectively, owing to low ionic and electronic conductivities. Yamada et al. obtained about 190 mAhg 1 for LiFeBO3, which is close to the theoretical specific capacity. Recently, we demonstrated a high capacity of 145 mA hg 1 within 4.7–1.7 V for h-LiMnBO3 by employing nanoparticles and a composite electrode utilizing reduced graphite oxide. Even though the theoretical capacities for LiFeBO3 and LiMnBO3, of approximately 220 and 222 mAhg , respectively, are larger than those of presently employed oxides, capacities are still low compared to what in principle is possible and future needs. Herein, we present the new Li-rich compound Li7Mn(BO3)3 which has been synthesized by thermalization of Li2O, MnO, and B2O3 in exact stoichiometric amounts. The colorless crystals are of triclinic symmetry (space group P 1 (no. 2), see Experimental Section) and establish a new structure type. The crystal structure of Li7Mn(BO3)3 (Figure 1) contains six

A low dimensional composite of hexagonal lithium manganese borate (LiMnBO3), a cathode material for Li-ion batteries

The ultrasonic nebulized spray pyrolysis technique has been applied to synthesize amorphous nanospheres, which are further transformed into nano h-LiMnBO3 with an average crystallite size of ∼14 nm. A composite electrode of nano h-LiMnBO3 with reduced graphite oxide and amorphous carbon delivers a high first discharge capacity of 140 mA h g−1 at C/15 rate within 4.5–2.0 V and retains a discharge capacity of 110 mA h g−1 at the 25th cycle. The dissolution of Mn into the electrolyte and the instability of the highly delithiated phases during cycling are suggested as the reasons, which limit the cycling stability of h-LiMnBO3. An improved cycling stability at higher capacities is expected via the combination of the particle size reduction, conductive network formation and the metal site doping strategies.

Investigating the all-solid-state batteries based on lithium garnets and a high potential cathode – LiMn1.5Ni0.5O4

All-solid-state Li-ion batteries based on lithium garnets give new prospects for safer battery operations avoiding liquids, and could enable the integration of high energy density electrode materials. Herein, we critically investigate the structural and chemical stability of the high voltage cathode material, LiMn1.5Ni0.5O4, based on the solid lithium garnet electrolyte LLZO (c-Li6.4Ga0.2La3Zr2O12) for all-solid Li-ion batteries. We manufacture battery cells based on nano-grained synthesized LLZO and composite cathodes (LiMn1.5Ni0.5O4/LLZO/C) fabricated via direct slurry casting of the cathode material and additives on sintered LLZO pellets against metallic Li anodes. The galvanostatic tests of such all-solid-state batteries up to 4.9 V at 95 °C reveal the incompatibility of the solid electrolyte and the cathode material under given conditions. Post-mortem analyses of the all-solid-state batteries demonstrate the formation of new inactive phases at the LLZO/LiMn1.5Ni0.5O4 interfacial region through an irreversible reaction starting at ∼3.8 V during charging. The discovered limited chemical stability under the investigated conditions raises the question if LLZO could be a promising solid-electrolyte for future all-solid-state Li-ion batteries especially at higher operation potentials and demanding operation conditions.

LiMg0.1Co0.9BO3 as a positive electrode material for Li-ion batteries

LiCoBO3 could be a promising cathode material given the electronic and ionic conductivity problems are addressed. Here, Mg substitution in LiCoBO3 is employed to stabilise the structure and improve the electrochemical performance. LiMg0.1Co0.9BO3 is synthesised for the first time via sol–gel method and Mg substitution in the structure is verified by X-ray powder diffraction and energy dispersive X-ray analyses. The electrochemical properties are investigated by galvanostatic cycling and cyclic voltammetry tests. The composite electrode with conductive carbon (reduced graphite oxide and carbon black) delivers a first discharge capacity of 32 mA h g−1 within a 4.7–1.7 voltage window at a rate of 10 mA g−1. The cycling is relatively stable compared to the unsubstituted LiCoBO3. Mg substitution may enhance the electrochemical performance of borate-based electrode materials when combined with suitable electrode design techniques.