Advances in Materials Design for All-Solid-state Batteries: From Bulk to Thin Films

All-solid-state batteries (SSBs) are one of the most fascinating next-generation energy storage systems that can provide improved energy density and safety for a wide range of applications from portable electronics to electric vehicles. The development of SSBs was accelerated by the discovery of new materials and the design of nanostructures. In particular, advances in the growth of thin-film battery materials facilitated the development of all solid-state thin-film batteries (SSTFBs)—expanding their applications to microelectronics such as flexible devices and implantable medical devices. However, critical challenges still remain, such as low ionic conductivity of solid electrolytes, interfacial instability and difficulty in controlling thin-film growth. In this review, we discuss the evolution of electrode and electrolyte materials for lithium-based batteries and their adoption in SSBs and SSTFBs. We highlight novel design strategies of bulk and thin-film materials to solve the issues in lithium-based batteries. We also focus on the important advances in thin-film electrodes, electrolytes and interfacial layers with the aim of providing insight into the future design of batteries. Furthermore, various thin-film fabrication techniques are also covered in this review.

Nanostructure and its effect on electrochemical properties of polyanionic Li2CoSiO4 for lithium ion batteries

This work reports a facile strategy to synthesize carbon-coated Li2CoSiO4/C particles with rich nanostructures by a two-step scheme starting with a low-temperature hydrothermal method. The size and morphology of particle aggregates can be regulated by the (OH-) concentration and viscosity of the precursor solution, a mixture of ethylene glycol and deionized water, for the hydrothermal synthesis. In addition to the good electrical conductivity from the carbon coating, the size of the primary nanoparticles and the mesopore associated with the aggregations play an important role in improving the electrochemical properties of polyanionic Li2CoSiO4. The low-dimensional belt-like and sheet-like Li2CoSiO4/C nanomaterials present a higher reversible capacity 136.6 mAh g-1 and 147 mAh g-1, respectively, in the first charging-discharging cycle between 2 V and 4.6 V. XRD, SEM/TEM, and EDS are used to characterize the crystalline structure and aggregation patterns. TGA and Raman spectra are employed to analyze the carbon coating on different morphologies. The analysis of electrochemical impedance spectroscopy highlights the critical role of the interface between the electrolyte and particles. This study provides insights into the rational design and synthesis of high-performance polyanionic cathodes including silicates.

Antiferromagnetic Inverse Spinel Oxide LiCoVO4 with Spin-Polarized Channels for Water Oxidation

Exploring highly efficient catalysts for the oxygen evolution reaction (OER) is essential for water electrolysis. Cost-effective transition-metal oxides with reasonable activity are raising attention. Recently, OER reactants' and products' differing spin configurations have been thought to cause slow reaction kinetics. Catalysts with magnetically polarized channels could selectively remove electrons with opposite magnetic moment and conserve overall spin during OER, enhancing triplet state oxygen molecule evolution. Herein, antiferromagnetic inverse spinel oxide LiCoVO₄ is found to contain d⁷ Co²⁺ ions that can be stabilized under active octahedral sites, possessing high spin states S = 3/2 (t_2g ⁵ e_g ² ). With high spin configuration, each Co²⁺ ion has an ideal magnetic moment of 3 µ_B , allowing the edge-shared Co²⁺ octahedra in spinel to be magnetically polarized. Density functional theory simulation results show that the layered antiferromagnetic LiCoVO₄ studied contains magnetically polarized channels. The average magnetic moment (µ_ave ) per transition-metal atom in the spin conduction channel is around 2.66 µ_B . Such channels are able to enhance the selective removal of spin-oriented electrons from the reactants during the OER, which facilitates the accumulation of appropriate magnetic moments for triplet oxygen molecule evolution. In addition, the LiCoVO₄ reported has been identified as an oxide catalyst with excellent OER activity.

Structure, Magnetism, and Electrochemistry of LiMg1-xZnxVO4 Spinels with 0 ≤ x ≤ 1

Negative electrode materials with lower operating voltages are urgently required to increase the energy density of lithium-ion batteries. In this study, LiMgVO₄ with a Na₂CrO₄-type structure, LiZnVO₄ with a phenacite structure, and their mixture were treated under a high pressure of 12 GPa and a high temperature of 1273 K, and their electrochemical reactivities were examined in a nonaqueous lithium cell. Synchrotron X-ray diffraction (XRD) measurements and Raman spectroscopy revealed that the LiMg_1-xZn_xVO₄ samples with 0 ≤ x ≤ 1 are in a single phase of the inverse spinel structure that forms a solid solution compound over the whole x range. All of the samples were brown or light black due to the presence of a small amount of V⁴⁺ ions with S = 1/2 and oxygen deficiencies. Since the majority of the vanadium ions are located at the route of the Li⁺ ion conduction pathway, no rechargeable capacity (Q_recha) would be expected. Nevertheless, all LiMg_1-xZn_xVO₄ samples exhibited a Q_recha value of more than 200 mAh g^-1 with an operating voltage of ∼0.8 V. This operating voltage is ∼1.6 V lower than that of LiV₂O₄ with a normal spinel structure. Furthermore, the x = 0.5 sample demonstrated an extremely stable cycle performance over 1 month. Ex situ XRD measurements clarified that the reversible electrochemical reaction can be attributed to the movement of vanadium ions from the tetrahedral 8a to octahedral 16c sites during the initial discharge reaction. Details regarding the crystal structure, magnetism, and electrochemistry of LiMg_1-xZn_xVO₄ are presented.

Unified View of the Local Cation-Ordered State in Inverse Spinel Oxides

Cation ordering/disordering in spinel oxides plays an essential role in the rich physical and chemical properties which are hallmarks of the structural archetype. A variety of cation-ordering motifs have been reported for spinel oxides with multiple cations residing on the octahedral site (or B-site). This has attracted tremendous attention from both experimental and theoretical communities in the last few decades. However, no unified view has been reached, presumably due to the richness of cation species and corresponding complex arrangements emergent in this large family of compounds. In this report, local cation-ordered ground states of (inverse) spinel oxides with two different cations on the octahedral site have been thoroughly investigated using neutron and X-ray total scattering, and a comprehensive theory has been proposed to explain the commonly observed cation-ordered polymorphs. It is found that a cation-zigzag-ordered structure (space group P4₁22) is the ground state for inverse spinel oxides with a pure or strong ionic lattice, while a cation-linear-ordered arrangement (space group Imma) emerges when one of the B-site cations forms very strong directional covalent bonds with lattice oxygen. The degree and length scale of cation ordering is strongly correlated with the charge and ionic radius difference between the two octahedral site cations. More complicated cation ordering schemes can be formed when there is a concomitant charge and orbital ordering which fall on a similar energy scale. This can lead to the formation of orbital-driven cation clusters or the broad concept of "molecules" in solid- state compounds. It is expected these findings will help to better understand the observed physical properties of spinel oxides and thus facilitate design strategies for improved functional materials.

Degradation Mechanism of Dimethyl Carbonate (DMC) Dissociation on the LiCoO2 Cathode Surface: A First-Principles Study

The degradation mechanism of dimethyl carbonate electrolyte dissociation on the (010) surfaces of LiCoO₂ and delithiated Li_1/3CoO₂ were investigated by periodic density functional theory. The high-throughput Madelung matrix calculation was employed to screen possible Li_1/3CoO₂ supercells for models of the charged state at 4.5 V. The result shows that the Li_1/3CoO₂(010) surface presents much stronger attraction toward dimethyl carbonate molecule with the adsorption energy of -1.98 eV than the LiCoO₂(010) surface does. The C-H bond scission is the most possible dissociation mechanism of dimethyl carbonate on both surfaces, whereas the C-O bond scission of carboxyl is unlikely to occur. The energy barrier for the C-H bond scission is slightly lower on Li_1/3CoO₂(010) surface. The kinetic analysis further shows that the reaction rate of the C-H bond scission is much higher than that of the C-O bond scission of methoxyl by a factor of about 10³ on both surfaces in the temperature range of 283-333 K, indicating that the C-H bond scission is the exclusive dimethyl carbonate dissociation mechanism on the cycled LiCoO₂(010) surface. This study provides the basis to understand and develop novel cathodes or electrolytes for improving the cathode-electrolyte interface.

Correlation of intercalation potential with d-electron configurations for cathode compounds of lithium-ion batteries

The d-electron localization is widely recognized as important to transport properties of transition metal compounds, but its role in the energy conversion of intercalation reactions of cathode compounds is still not fully explored.