Mapping and Modeling Physicochemical Fields in Solid-State Batteries.

The safety and energy density of solid-state batteries can be, in principle, substantially increased compared with that of conventional lithium-ion batteries. However, the use of solid-state electrolytes instead of liquid electrolytes introduces pronounced complexities to the solid-state system because of the strong coupling between different physicochemical fields. Understanding the evolution of these fields is critical to unlocking the potential of solid-state batteries. This necessitates the development of experimental and theoretical methods to track electrochemical, stress, crack, and thermal fields upon battery cycling. In this Perspective, we survey existing characterization techniques and the current understanding of multiphysics coupling in solid-state batteries. We propose that the development of experimental tools that can map multiple fields concurrently and systematic consideration of material plasticity in theoretical modeling are important for the advancement of this emerging battery technology. This Perspective provides introductory material on solid-state batteries to scientists from a broad physical chemistry community, motivating innovative and interdisciplinary studies in the future.

Overpotential-Regulated Stable Cycling of a Thin Magnesium Metal Anode.

To obtain high energy density for magnesium (Mg)-metal batteries, a promising low-cost energy storage technology, a thin Mg-metal anode of tens of micrometers must be used. However, the Coulombic efficiency (CE) and the anode utilization rate (AUR) of thin Mg metal are far from sufficient to sustain a long cycle life. This drawback is closely related to the morphological instability during galvanostatic cycling. In this work, we observed that the morphological evolution of Mg metal can be controlled with a pre-applied overpotential. With a properly pre-applied overpotential (e.g., -0.5 V), we show that the average AUR and the average CE of thin Mg metal (16 µm, equivalent to 6 mA h cm-2) in a Mg/Mo asymmetric cell can be substantially improved from 29.8 to 74.8% and from 97.7 to 99.5%, respectively, under a practical current density of 2 mA cm-2. These advances can theoretically improve the energy density and cycle life of Mg-S batteries to more than 1000 W h kg-1 and 100 cycles, respectively. This work deepens our understanding of the morphological and compositional evolution of Mg metal during stripping and plating processes and suggests a facile and effective method to substantially improve the cycling stability of thin Mg metal.

Room-Temperature All-Solid-State Sodium Battery Based on Bulk Interfacial Superionic Conductor.

All-solid-state sodium batteries (ASSSBs) are attractive alternatives to lithium-ion batteries for grid-scale energy storage due to their high safety and ubiquitous distribution of Na sources. A critical component for ASSSB is sodium-ion conducting solid-state electrolyte (SSE). Here, we report a high-performance sodium-ion SSE with the recently developed bulk interfacial superionic conductor (BISC) concept. The ionic conductivity and areal conductance of the Na+ BISC at 25 °C reaches 6.5 × 10-4 S cm-1 and 260 mS cm-2, respectively. Using NaxCo0.7Mn0.3O2 (x ≈ 1.0, NaCMO) as the cathode active material, all-solid-state Na||NaCMO batteries exhibiting small overpotential and ∼180 cycle life are demonstrated under room temperature. This approach may also be used to prepare other metal ion, such as Mg2+, Al3+, and K+, based all-solid-state batteries.

Miniature non-contact photoacoustic probe based on fiber-optic photoacoustic remote sensing microscopy.

Photoacoustic (PA) remote sensing (PARS) microscopy, featured by non-contact operation, has shown great potential for PA microscopy (PAM) imaging applications. However, current PARS microscopy systems are mainly based on free-space light, making the imaging head bulky and inconvenient to use. These issues hinder selected applications such as PA endoscopy and handheld PAM. Here, we report a miniature probe capable of non-contact PAM based on PARS microscopy. By utilizing fiber-optic components including a wavelength division multiplexer and an optical circulator, the imaging head can be highly miniaturized with a diameter of ∼3.0mm. Also, since all light is transmitted via fibers, the fiber-optic PARS microscopy system is relatively easy to build and facilitates scanning of the probe. In vivo imaging of a zebrafish larva and imaging of lithium metal batteries are conducted using the probe, showing its good imaging capability.

Accelerated Growth of Electrically Isolated Lithium Metal during Battery Cycling.

Severe capacity loss during cycling of lithium-metal batteries is one of the most concerning obstacles hindering their practical application. As this capacity loss is related to the variety of side reactions occurring to lithium metal, identification and quantification of these lithium-loss processes are extremely important. In this work, we systematically distinguish and quantify the different rates of lithium loss associated with galvanic corrosion, the formation of a solid-electrolyte interphase, and the formation of electrically isolated lithium metal (i.e., "dead" lithium). We show that the formation of "dead" Li is accelerated upon cycling, dominating the total lithium loss, with much slower rates of lithium loss associated with galvanic corrosion and formation of the solid-electrolyte interphase. Furthermore, photoacoustic imaging reveals that the three-dimensional spatial distribution of "dead" Li is distinctly different from that of freshly deposited lithium. This quantification is further extended to a solid-state Li/Cu cell based on a Li10GeP2S12 solid-state electrolyte. The lithium loss in the solid-state cell is much severer than that of a conventional lithium-metal battery based on a liquid electrolyte. Our work highlights the importance of quantitative studies on conventional and solid-state lithium-metal batteries and provides a strong basis for the optimization of lithium-metal electrochemistry.

Classical and Emerging Characterization Techniques for Investigation of Ion Transport Mechanisms in Crystalline Fast Ionic Conductors.

Ion transport in crystalline fast ionic conductors is a complex physical phenomenon. Certain ionic species (e.g., Ag+, Cu+, Li+, F-, O2-, H+) in a solid crystalline framework can move as fast as in liquids. This property, although only observed in a limited number of materials, is a key enabler for a broad range of technologies, including batteries, fuel cells, and sensors. However, the mechanisms of ion transport in the crystal lattice of fast ionic conductors are still not fully understood despite the substantial progress achieved in the last 40 years, partly because of the wide range of length and time scales involved in the complex migration processes of ions in solids. Without a comprehensive understanding of these ion transport mechanisms, the rational design of new fast ionic conductors is not possible. In this review, we cover classical and emerging characterization techniques (both experimental and computational) that can be used to investigate ion transport processes in bulk crystalline inorganic materials which exhibit predominant ion conduction (i.e., negligible electronic conductivity) with a primary focus on literature published after 2000 and critically assess their strengths and limitations. Together with an overview of recent understanding, we highlight the need for a combined experimental and computational approach to study ion transport in solids of desired time and length scales and for precise measurements of physical parameters related to ion transport.

LiCrS2 and LiMnS2 Cathodes with Extraordinary Mixed Electron-Ion Conductivities and Favorable Interfacial Compatibilities with Sulfide Electrolyte.

Sulfide-type solid-state electrolytes for all-solid-state lithium ion batteries are capturing more and more attention. However, the electronegativity difference between the oxygen and the sulfur element makes sulfide-type solid-state electrolytes chemically incompatible with the conventional LiCoO2 cathode. In this work, we proposed a series of chalcopyrite-structured sulfide-type materials and systematically assessed their performances as the cathode materials in all-solid-state lithium ion batteries by first-principle calculations. All the five metallic LiMS2 (M = Cr, Mn, Fe, Co, and Ni) materials are superionic conductors with extremely small lithium ion migration barriers in the range from 43 to 99 meV, much lower than most oxide- and even sulfide-type cathodes. Voltage and volume calculations indicate that only LiCrS2 and LiMnS2 cathodes are structurally stable during cycling with the stable voltage plateaus at ∼3 V, much higher than that of the P3m1-LiTiS2 cathode. For the first time, we studied the interfacial lithium transport resistance from a new perspective of charge transfer and redistribution at the electrode/solid-state electrolyte interface. LiCrS2 and LiMnS2 cathodes exhibit favorable interfacial compatibilities with Li3PS4 electrolyte. Our investigations demonstrate that the metallic LiCrS2 and LiMnS2 superionic conductors would possess excellent rate capability, high energy density, good structural stability during cycling, and favorable interfacial compatibility with Li3PS4 electrolyte in all-solid-state lithium ion batteries.

High magnesium mobility in ternary spinel chalcogenides.

Magnesium batteries appear a viable alternative to overcome the safety and energy density limitations faced by current lithium-ion technology. The development of a competitive magnesium battery is plagued by the existing notion of poor magnesium mobility in solids. Here we demonstrate by using ab initio calculations, nuclear magnetic resonance, and impedance spectroscopy measurements that substantial magnesium ion mobility can indeed be achieved in close-packed frameworks (~ 0.01-0.1 mS cm-1 at 298 K), specifically in the magnesium scandium selenide spinel. Our theoretical predictions also indicate that high magnesium ion mobility is possible in other chalcogenide spinels, opening the door for the realization of other magnesium solid ionic conductors and the eventual development of an all-solid-state magnesium battery.

Investigation of Potassium Storage in Layered P3-Type K0.5 MnO2 Cathode.

Novel and low-cost batteries are of considerable interest for application in large-scale energy storage systems, for which the cost per cycle becomes critical. Here, this study proposes K0.5 MnO2 as a potential cathode material for K-ion batteries as an alternative to Li technology. K0.5 MnO2 has a P3-type layered structure and delivers a reversible specific capacity of ≈100 mAh g-1 with good capacity retention. In situ X-ray diffraction analysis reveals that the material undergoes a reversible phase transition upon K extraction and insertion. In addition, first-principles calculations indicate that this phase transition is driven by the relative phase stability of different oxygen stackings with respect to the K content.

In Situ Neutron Diffraction Studies of the Ion Exchange Synthesis Mechanism of Li2Mg2P3O9N: Evidence for a Hidden Phase Transition.

Motivated by predictions made using a bond valence sum difference map (BVS-DM) analysis, the novel Li-ion conductor Li2Mg2P3O9N was synthesized by ion exchange from a Na2Mg2P3O9N precursor. Impedance spectroscopy measurements indicate that Li2Mg2P3O9N has a room temperature Li-ion conductivity of about 10-6 S/cm (comparable to LiPON), which is 6 orders of magnitude higher than the extrapolated Na-ion conductivity of Na2Mg2P3O9N at this temperature. The structure of Li2Mg2P3O9N was determined from ex situ synchrotron and time-of-flight neutron diffraction data to retain the P213 space group, though with a cubic lattice parameter of a = 9.11176(8) Å that is significantly smaller than the a = 9.2439(1) Å of Na2Mg2P3O9N. The two Li-ion sites are found to be very substantially displaced (∼0.5 Å) relative to the analogous Na sites in the precursor phase. The non-molten salt ion exchange method used to prepare Li2Mg2P3O9N produces a minimal background in powder diffraction experiments, and was therefore exploited for the first time to follow a Li+/Na+ ion exchange reaction using in situ powder neutron diffraction. Lattice parameter changes during ion exchange suggest that the reaction proceeds through a Na2-xLixMg2P3O9N solid solution (stage 1) followed by a two-phase reaction (stage 2) to form Li2Mg2P3O9N. However, full Rietveld refinements of the in situ neutron diffraction data indicate that the actual transformation mechanism is more complex and instead involves two thermodynamically distinct solid solutions in which the Li exclusively occupies the Li1 site at low Li contents (stage 1a) and then migrates to the Li3 site at higher Li contents (stage 1b), a crossover driven by the different signs of the local volume change at these sites. In addition to highlighting the importance of obtaining full structural data in situ throughout the ion exchange process, these results provide insights into the general question of what constitutes a thermodynamic phase.

Structures of delithiated and degraded LiFeBO(3), and their distinct changes upon electrochemical cycling.

Lithium iron borate (LiFeBO3) has a high theoretical specific capacity (220 mAh/g), which is competitive with leading cathode candidates for next-generation lithium-ion batteries. However, a major factor making it difficult to fully access this capacity is a competing oxidative process that leads to degradation of the LiFeBO3 structure. The pristine, delithiated, and degraded phases of LiFeBO3 share a common framework with a cell volume that varies by less than 2%, making it difficult to resolve the nature of the delithiation and degradation mechanisms by conventional X-ray powder diffraction studies. A comprehensive study of the structural evolution of LiFeBO3 during (de)lithiation and degradation was therefore carried out using a wide array of bulk and local structural characterization techniques, both in situ and ex situ, with complementary electrochemical studies. Delithiation of LiFeBO3 starts with the production of LitFeBO3 (t ≈ 0.5) through a two-phase reaction, and the subsequent delithiation of this phase to form Lit-xFeBO3 (x < 0.5). However, the large overpotential needed to drive the initial two-phase delithiation reaction results in the simultaneous observation of further delithiated solid-solution products of Lit-xFeBO3 under normal conditions of electrochemical cycling. The degradation of LiFeBO3 also results in oxidation to produce a Li-deficient phase D-LidFeBO3 (d ≈ 0.5, based on the observed Fe valence of ∼2.5+). However, it is shown through synchrotron X-ray diffraction, neutron diffraction, and high-resolution transmission electron microscopy studies that the degradation process results in an irreversible disordering of Fe onto the Li site, resulting in the formation of a distinct degraded phase, which cannot be electrochemically converted back to LiFeBO3 at room temperature. The Li-containing degraded phase cannot be fully delithiated, but it can reversibly cycle Li (D-Lid+yFeBO3) at a thermodynamic potential of ∼1.8 V that is substantially reduced relative to the pristine phase (∼2.8 V).

Thin-film and bulk investigations of LiCoBO3 as a Li-ion battery cathode.

The compound LiCoBO3 is an appealing candidate for next-generation Li-ion batteries based on its high theoretical specific capacity of 215 mAh/g and high expected discharge voltage (more than 4 V vs Li(+)/Li). However, this level of performance has not yet been realized in experimental cells, even with nanosized particles. Reactive magnetron sputtering was therefore used to prepare thin films of LiCoBO3, allowing the influence of the particle thickness on the electrochemical performance to be explicitly tested. Even when ultrathin films (∼15 nm) were prepared, there was a negligible electrochemical response from LiCoBO3. Impedance spectroscopy measurements suggest that the conductivity of LiCoBO3 is many orders of magnitude worse than that of LiFeBO3 and may severely limit the performance. The unusual blue color of LiCoBO3 was investigated by spectroscopic techniques, which allowed the determination of a charge-transfer optical gap of 4.2 eV and the attribution of the visible light absorption peak at 2.2 eV to spin-allowed d → d transitions (assigned as overlapping (4)A2' to (4)A2″ and (4)E″ final states based on ligand-field modeling).

Structural modulation in the high capacity battery cathode material LiFeBO3.

The crystal structure of the promising Li-ion battery cathode material LiFeBO(3) has been redetermined based on the results of single crystal X-ray diffraction data. A commensurate modulation that doubles the periodicity of the lattice in the a-axis direction is observed. When the structure of LiFeBO(3) is refined in the 4-dimensional superspace group C2/c(α0γ)00, with α = 1/2 and γ = 0 and with lattice parameters of a = 5.1681 Å, b = 8.8687 Å, c = 10.1656 Å, and ß = 91.514°, all of the disorder present in the prior C2/c structural model is eliminated and a long-range ordering of 1D chains of corner-shared LiO(4) is revealed to occur as a result of cooperative displacements of Li and O atoms in the c-axis direction. Solid-state hybrid density functional theory calculations find that the modulation stabilizes the LiFeBO(3) structure by 1.2 kJ/mol (12 meV/f.u.), and that the modulation disappears after delithiation to form a structurally related FeBO(3) phase. The band gaps of LiFeBO(3) and FeBO(3) are calculated to be 3.5 and 3.3 eV, respectively. Bond valence sum maps have been used to identify and characterize the important Li conduction pathways, and suggest that the activation energies for Li diffusion will be higher in the modulated structure of LiFeBO(3) than in its unmodulated analogue.