NASICON-type Solid Electrolyte Research Articles

On the dependence of ionic transport on crystal orientation in NaSICON-type solid electrolytes

The dependence of ionic transport on crystal orientations in NaSICON-type solid electrolytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal x-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e. when going from Na+ to Ag+, shifts the phase transition temperature by 140 K towards lower temperature. The Arrhenius behavior remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.

Rhombohedral Li1+xYxZr2-x(PO4)3 Solid Electrolyte Prepared by Hot-Pressing for All-Solid-State Li-Metal Batteries.

NASICON-type solid electrolytes with excellent stability in moisture are promising in all-solid-state batteries and redox flow batteries. However, NASIOCN LiZr2(PO4)3 (LZP), which is more stable with lithium metal than the commercial Li1.3Al0.3Ti1.7(PO4)3, exhibits a low Li-ion conductivity of 10−6 S cm−1 because the fast conducting rhombohedral phase only exists above 50 °C. In this paper, the high-ionic conductive rhombohedral phase is stabilized by Y3+ doping at room temperature, and the hot-pressing technique is employed to further improve the density of the pellet. The dense Li1.1Y0.1Zr1.9(PO4)3 pellet prepared by hot-pressing shows a high Li-ion conductivity of 9 × 10−5 S cm−1, which is two orders of magnitude higher than that of LiZr2(PO4)3. The in-situ formed Li3P layer on the surface of Li1.1Y0.1Zr1.9(PO4)3 after contact with the lithium metal increases the wettability of the pellet by the metallic lithium anode. Moreover, the Li1.1Y0.1Zr1.9(PO4)3 pellet shows a relatively small interfacial resistance in symmetric Li/Li and all-solid-state Li-metal cells, providing these cells a small overpotential and a long cycling life.

Crystal structure refinement, microstructure and ionic conductivity of ATi2(PO4)3 (A=Li, Na, K) solid electrolytes

The sintering temperatures were set according to the TG/DSC analysis. The solid-phase sintering method was used to prepare the NASICON-type solid electrolyte in ATi2(PO4)3 (A = Li/Na/K) system. FT-IR spectroscopy conforms to the different chemical bond strengths of Li+, Na+, and K+ in phosphate compositions. The main crystal phases retrieved from the three compositions are NASICON-type ATi2(PO4)3 (A = Li/Na/K) in rhombohedral structure with the R3‾c space group. The presence of heterogeneous LiTiPO5 phases can reduce the grain boundary resistance and increase the density of LiTi2(PO4)3 electrolyte. However, heterogeneous TiP2O7 in NTP and KTP compositions does not contribute to the ionic conductivity. SEM results reveal that the crystal grains of LTP composition are denser than that of NTP and KTP, which is more beneficial to improve ionic conductivity. The LTP electrolyte sintered at 800 °C for 10 h shows the highest ionic conductivity of 3.1 × 10−5 S cm−1 at 25 °C.

Clarifying the relationship between redox activity and electrochemical stability in solid electrolytes.

All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite-, garnet- and NASICON-type solid electrolytes that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is notably larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.

Bayesian-optimization-guided experimental search of NASICON-type solid electrolytes for all-solid-state Li-ion batteries

Experimental Bayesian optimization for Li ion conductivity in NASICON-type LiZr2(PO4)3.

Enhancing the cycling stability of all-solid-state lithium-ion batteries assembled with Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes prepared from precursor solutions with appropriate pH values

NASICON-type solid electrolyte powders Li1.3Al0.3Ti1.7(PO4)3 (LATP) are synthesized by spray-drying and assisted sintering processes. The effects of different pH values of the spray-dried precursor solution on the crystallinity, morphology, ionic conductivity, compaction density and cycling stability of the LATP solid electrolyte sheets are investigated. The LATP-6.0 solid electrolyte sheets present the most perfect microstructure with a compaction density of 2.968 g cm−1 and the highest ionic conductivity of 1.182 E−4 S cm−1 with an activation energy of 0.273 eV at room temperature when the pH value of the precursor solution is adjusted to 6.0. Additionally, the assembled LiCoO2/LATP-6.0/Li all-solid-state coin cell delivers commendable cycling stability with a high discharge specific capacity of 150.5 mAh g−1 at 0.1 C after five activation cycles and retains a specific capacity retention rate of 94.28% after 300 cycles with various current densities at room temperature. Moreover, the LATP-6.0 solid electrolyte sheets maintain an intact morphology and uniformly-distributed elements composition after cycling. Based on these excellent results, the use of the LATP-6.0 sheets as solid electrolytes in high-performance all-solid-state lithium-ion batteries is a promising strategy.

The Role of Metallic Protection Layers in Extending the Stability of Nasicon Electrolytes for Solid-State Batteries

Solid-state batteries with lithium metal anodes are promising candidates for next-generation energy storage devices with improved safety and higher energy density. However, most solid electrolytes with practical ionic conductivities are thermodynamically unstable against lithium metal. Upon interfacial contact with lithium metal, unstable solid electrolytes undergo phase transformations that can be detrimental to battery performance. Lithium aluminum germanium phosphate (LAGP) is a NASICON-type solid electrolyte with relatively high ionic conductivity (10-4 - 10-3 S/cm) and chemical resistance to air and moisture. However, LAGP reacts with lithium to form an electrically-conducting interphase. This interphase continues to propagate from the interface towards the bulk of the solid electrolyte, ultimately leading to failure by causing the solid electrolyte to fracture [1]. In this work, we find that a thin layer of a metal that does not alloy with Li can extend the cycle life of LAGP in symmetric lithium cells from 30 h to more than 800 h at moderate current densities. Interestingly, this significantly improved electrochemical stability is not due to the prevention of interphase formation. Instead, the metal layer alters the morphology evolution of the interphase, creating a uniform and compact interphase. In addition, we find that the inclusion of an electrically insulating thin film, such as Al2O3, between the metal protection layer and the solid electrolyte favors lithium plating and prevents interphase formation over short periods of time. This dual layer approach also extends the electrochemical stability to more than 1000 h during cycling. However, using only the insulator without a metal is not beneficial for electrochemical stability. These results suggest that metallic protection layers may be necessary for attaining stable cycling even if suitable electrically insulating protection layers are developed to completely prevent interphase formation. Our findings are important for enabling the use of a wider range of solid electrolyte materials in solid-state batteries. [1] J.A. Lewis, et at. Interphase Morphology between a Solid-State Electrolyte and Lithium Controls Cell Failure. ACS Energy Lett. 2019, 4, 2, 591-599

Synthesis and Properties of NaSICON-type LATP and LAGP Solid Electrolytes.

Inorganic solid electrolytes play a critical role in solid-state lithium batteries achieving high safety levels and high energy densities. The synthetic approaches to solid electrolytes are important for both fundamental research and practical applications. Li1+x Alx Ti2-x (PO4 )3 (LATP) and Li1+x Alx Ge2-x (PO4 )3 (LAGP) are two representative solid electrolytes with a sodium superionic conductor (NaSICON) structure. Herein, LATP and LAGP solid electrolytes are reviewed from the synthesis perspective, and correlated with their structure and conductive properties, as well as their electrochemical applications in batteries. First, the solid- and liquid-based synthetic methods to LATP and LAGP solid electrolytes and the key influencing factors are described. Second, the crystal structures and phase purities obtained from different synthetic approaches are introduced. Third, the conductive mechanisms, composition effects, and synthetic effects on the conductivities of LATP and LAGP solid electrolytes are compared. Fourth, the electrochemical applications of these two solid electrolytes in full batteries are discussed, including roles as solid electrolytes, composite components in electrodes, and surface coatings on electrodes. In the last section, a brief outlook is provided on the future development of NaSICON-type solid electrolytes for all-solid-state batteries.

Constituting the NASICON type solid electrolyte coated material forming anti-high voltage system to enhance the high cut-off voltage performance of LiNi0.6Co0.2Mn0.2O2 via charge attracts electrostatic assembly

High cut-off voltage in lithium ion batteries (LIBs), now, is the vital threshold for further development of high energy density LIBs. In this work, the NASICON type solid electrolyte as high working voltage and unique ionic transfer rate material is introduced to facilitate the high cut-off performance of Ni-rich cathode material. Herein, considering the uniformity of coating outer layer, the charge attraction is employed as a coating method to form a homogeneous coating layer. The coating properties of LiTi2(PO4)3 coated LiNi0.6Co0.2Mn0.2O2 probed via X-ray diffraction, scanning electron microscope, transmission electron microscope, X-ray photoelectron spectroscopy. Typically, electrochemical tests show that LiTi2(PO4)3 was effectively promoting the electrochemical characters. The modified material preserves 94% after 100 cycles at 1C in voltage of 2.7–4.3 V. Especially, it maintains 92.3%/83.6% in high cut-off voltage of 2.7–4.5 V/2.7–4.8 V for 100 cycles, and keeps 117.4 mA h g−1/101.2 mA h g−1 after 500 long cycles.

Promising high-conductivity Li1.5Al0.5Ge1.5(PO4)3 solid electrolytes: the effect of crystallization temperature on the microstructure and transport properties

High-conductivity NASICON-type solid electrolytes of Li1.5Al0.5Ge1.5(PO4)3 composition have been obtained through glass crystallization. Thermal behavior of the precursor glass has been investigated; glass transition and crystallization temperatures have been determined by DSC. The influence of glass crystallization temperature (750, 800, 820, and 850 °C) on the phase composition, microstructure, molecular structure, and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 glass-ceramics has been studied. It was found that the optimum crystallization temperature for obtaining an electrolyte with the highest conductivity (0.39 mS cm−1 at 25 °C) is 820 °C.

Enhancing High-Voltage Performance of Ni-Rich Cathode by Surface Modification of Self-Assembled NASICON Fast Ionic Conductor LiZr2(PO4)3.

Coating methodology is commonly employed in the enhancement of Ni-rich cathodes for Li-ion batteries as an efficient approach, while its strategy and effect are still great challenges to achieve success in surface modifications for comprehensive electrochemical properties. In this work, the surface of Ni-rich cathode LiNi0.82Co0.15Al0.03O2 (NCA) is modified by intimately coating NASICON-type solid electrolyte LiZr2(PO4)3 (LZP) via a facile approach involving electrostatic attraction. With well-designed architecture and a uniform NASICON-type LZP nanolayer wrapping over the NCA microsphere, the entire electrode demonstrates exceptional Li+ diffusion and conductivity and suppresses the side reaction between electrolyte and electroactive NCA, stabilizing the phase interface with less Li+/Ni2+ cation mixing. As a result, the NCA@LZP can deliver a high reversible capacity of 182 mAh g-1 at 1C in 2.7-4.3 V, maintaining the capacity retention of 84.6% after 100 cycles. More importantly, the structure stability of NCA is enhanced substantially by surface modification of LZP at high cutoff voltage. It achieves a reversible capacity of 204 mAh g-1 and keeps 100.4 mAh g-1 after 500 cycles at 1C in the potential range of 2.7-4.5 V. This effective strategy of using NASICON fast ionic conductor like LZP as a coating layer may provide a new insight to modify the surface of Ni-rich electrode, improving the rate capability and cyclic performance under high voltage.

The preparation of NASICON-type solid electrolyte lithium-ion Li1+xAlxGe0.2Ti1.8-x(PO4)3 by conventional solid-state method

NASICON-type lithium-ion conducting solid electrolyte for Li1+xAlxGe0.2Ti1.8-x(PO4)3 were prepared with solid-state method at 900 °C for 7 h. In this paper, we have studied the property of Li1+xAlxGe0.2Ti1.8-x(PO4)3 solid electrolyte systems with various numbers of Al replaced Ti. The highest total electrical conductivity of the sintered pellets for Li1.45Al0.45Ge0.2Ti1.35(PO4)3 at 25 °C was 1.0×10-3 S/cm, which has NASICON-type crystalline structure without impurity phase. And the sintered pellet as x=0.45 in Li1+xAlxGe0.2Ti1.8-x(PO4)3 systems also has highest relative density of 95.6% and highest three-point strength of 90 Nmm−2, respectively.

LiSnZr(PO4)3: NASICON-type solid electrolyte with excellent room temperature Li+ conductivity

Development of solid electrolytes with good lithium ion conductivity is one of the key pre-requisites of high performing rechargeable all-solid-state lithium batteries. In this work, NA-Super-Ionic-CONductor (NASICON)-type LiSnZr(PO4)3 ceramics fabricated via a sol-gel route were characterized using Synchrotron x-ray diffraction, Raman Spectroscopy, x-ray photoelectron spectroscopy (XPS) and complex impedance spectroscopy. Stabilization of high Li+ conducting rhombohedral (R3¯c) phase at room temperature was confirmed by the Rietveld refinement of synchrotron x-ray diffraction data. LiSnZr(PO4)3 sample sintered at 1000 °C exhibited an excellent room temperature bulk and total conductivity ∼ 0.1 mScm−1 and 1.45 × 10−5 Scm−1 with associated activation energies of ∼0.36 eV and ∼0.38 eV, respectively. Direct current polarization study confirmed the conductivity of LiSnZr(PO4)3 as predominantly ionic in nature. The role of inductive effect in improving the room temperature ionic conductivity by utilizing the electronegativity of counter-cations in NASICON framework is also discussed.

Lithium-ion conducting solid electrolytes of Li1.4Al0.4Ge0.2Ti1.4(PO4)3 and MOx (M = Al, Ti, and Zr) composites

Water-stable and high lithium-ion conducting solid electrolytes have potential application as the electrolyte of solid state batteries and the separator of high energy density aqueous lithium batteries. The lithium-ion conductivity of the water-stable NASICON-type solid electrolyte of Li1.4Al0.4Ge0.2Ti1.4(PO4)3 (LAGTP) was enhanced by addition of TiO2 and Al2O3. The highest conductivity of 1.53 × 10−3 S cm−1 at 25 °C was observed for LAGTP-3 wt% Al2O3, which is approximately 1.5 times higher than that of LAGTP. The conductivity enhancement could be explained by a reduction of the grain boundary resistance. The three-point bending strength was also increased by addition of Al2O3, TiO2, and ZrO2. The LAGTP-10 wt% TiO2 composite showed a bending strength of 209 N mm−2, which was 3 times higher than that of LAGTP, and the conductivity was comparable to that of LAGTP.

Research Update: Ca doping effect on the Li-ion conductivity in NASICON-type solid electrolyte LiZr2(PO4)3: A first-principles molecular dynamics study

In this work, we used a density functional theory-based molecular dynamics simulation to investigate the Ca content-dependent Li-ion conductivity of NASICON-type Li1+2xCaxZr2-x(PO4)3 (LCZP) solid electrolytes (0.063 ≤ x ≤ 0.375) which exhibit a Li-excess chemical composition. The LCZP systems show a higher room temperature Li-ion conductivity and a lower activation energy than pristine LiZr2(PO4)3 (LZP), and the tendencies of those properties agree with the experimental results. In addition, the Li-ion conduction mechanisms in LCZP were clarified by analyzing the radial distribution functions and site displacement functions obtained from our molecular dynamics simulations. For minimal Ca substitution for LZP, the Li-ion conductivity is enhanced because of the creation of interstitial Li ions by Ca doping in the LCZP systems; the frequency of collisions with Li ions dramatically increases. For substantial Ca substitution for LZP, the Li-ion conductivity gradually worsened because some Li ions were trapped at the M1 (most stable) and M2 (metastable) sites near Ca atoms.

Non-uniform lithium-ion migration on micrometre scale for garnet- and NASICON-type solid electrolytes studied by 7Li PGSE-NMR diffusion spectroscopy

The migration behaviours of Li+ in three garnet- and one NASICON-type solid oxide electrolytes were studied on the micrometre scale by pulsed-gradient spin-echo (PGSE) 7Li NMR diffusion spectroscopy to clarify common and specific characteristics of each electrolyte. In these solid electrolytes, clear evidences of grain boundary effects in the diffusion of Li+ were not observed. The Li+ diffusion constants were dependent on parameters such as observation time (Δ) and pulsed field gradient (PFG) strength (g) for all the studied inorganic solid electrolytes. For low Δ values, Li+ ions underwent collisions and diffractions with diffraction distance Rdiffraction [μm]. The apparent Li+ diffusion constants (Dapparent [m2 s-1]) exhibited distributions in a wide range. In this paper, we introduced the apparent diffusion radius, rradius [μm], and compared it with Rdiffraction and mean square displacement (MSD) [μm]; the lengths of these distances were of the micrometre order (10-6 m). The relations between the values of rradius, Rdiffraction and MSD suggested that the migration behaviours of Li+ on the micrometre scale were complicated. Using high Δ and high g values, we obtained an equilibrated value of DLi. The temperature dependences of the number of carrier ions were estimated from the DLi values and ionic conductivities in the four solid oxide electrolytes. For simple comparison and reference, the data of DLi and ionic conductivity of LiPF6 in 1 M solution of propylene carbonate were added.

Molecular Dynamics Simulations on Li-Ion Conduction in Nasicon-Type Solid Electrolytes

As a promising solid electrolyte material for all-solid-state Li-ion batteries (LIBs), Na super ionic conductor (NASICON)-type material is considered to be comparable to perovskite-type or garnet-type ionic conductors. Since the discovery of the fast Na-ion conducting NASICON-type material of Na1+xZr2SixP3-xO12 in 1976 [1], many kinds of Li-ion conducting NASICON-type solid electrolytes have been investigated. One of the most popular among them is Li1+xAlxTi2-x(PO4)3 (LATP), which can be generated by partially substituting Ti4+ with Al3+ in LiTi2(PO4)3 (LTP). Especially, LATP with x = 0.3 exhibits enhanced bulk Li-ion conductivity in the order of 10-3 S/cm at room temperature [2,3]. However, it is also well known that LTP-based materials react with Li metal at around 2.5 V vs. Li+/Li due to the Ti4+/Ti3+ redox reaction [4,5]. Recently, another NASICON-type solid electrolyte LiZr2(PO4)3 (LZP) has been well studied, then it is known to be stable in contact with Li metal due to no redox reaction up to 5.5 V [6]. Therefore, NASICON-type materials would be alternative solid electrolytes for all-solid-state LIBs. In this study, we estimate Li-ion conductivity and activation energy in NASICON-type solid electrolytes using molecular dynamics simulations. The Li-ion trajectory is also analyzed and discussed. The results and discussions will be shown in more detail in our presentation. Acknowledgements This work was supported by the “Materials research by Information Integration” Initiative (MI2I) project of the Support Program for Starting Up Innovation Hub from Japan Science and Technology Agency (JST).

Sol-gel synthesis and electrochemical properties extracted by phase inflection detection method of NASICON-type solid electrolytes LiZr2(PO4)3 and Li1.2Zr1.9Ca0.1(PO4)3

The use of good ionic conductors is a key point in various battery technologies such as Li-air Lithium-Sulfur (Li-S) and All-Solid-State batteries. The determination of the conduction properties as well as the structure in function of temperature and their electrochemical stability are paramount. At the same time, the manufacturing process of these solid electrolytes must be simplified in order to foster the emergence of these technologies. In this context, NASICON-type solid electrolytes LiZr2(PO4)3 (LZP) and Li1.2Zr1.9Ca0.1(PO4)3 (LCZP) were synthetized by a new sol-gel method to simplify the synthesis process compared to the solid-state reaction and to reduce the synthesis temperature from 1200°C to 1100°C. The influence of Ca-doping on crystal structure and transport properties was studied in function of temperature and for the first time the electrochemical stability determined. A method, the Phase Inflection Detection (PID), was developed to better determine the transport properties extracted from Electrochemical Impedance Spectroscopy. The ionic conductivity of LCZP is greater than that of LZP by about 2 decades at room temperature due to the stabilization of the high temperature phase at room temperature by Ca-doping, an increase in the number of lithium mobile ions and a better compactness compared with LZP. The impact of sintering temperature and grain boundaries on transport properties is clearly demonstrated and must be taken into account in the future studies of solid electrolytes. The LCZP material is not stable below 0.6V vs. Li+/Li. It thus presents one of the best electrochemical stabilities, making it a potential candidate for various battery technologies.

Characterization of Sputter-Deposited LiCoO2 Thin Film Grown on NASICON-type Electrolyte for Application in All-Solid-State Rechargeable Lithium Battery.

All-solid-state Li-rechargeable batteries using a 500 nm-thick LiCoO2 (LCO) film deposited on two NASICON-type solid electrolyte substrates, LICGC (OHARA Inc.) and Li1.3Al0.3Ti1.7(PO4)3 (LATP), are constructed. The postdeposition annealing temperature prior to the cell assembly is critical to produce a stable sharp LCO/electrolyte interface and to develop a strong crystallographic texture in the LCO film, conducive to migration of Li ions. Although the cells deliver a limited discharge capacity, the cells cycled stably for 50 cycles. The analysis of the LCO/electrolyte interfaces after cycling demonstrates that the sharp interface, once formed by proper thermal annealing, will remain stable without any evidence for contamination and with minimal intermixing of the constituent elements during cycling. Hence, although ionic conductivity of the NASICON-type solid electrolyte is lower than that of the sulfide electrolytes, the NACSICON-type electrolytes will maintain a stable interface in contact with a LCO cathode, which should be beneficial to improving the capacity retention as well as the rate capability of the all-solid state cell.

Investigation of ionic mobility in NASICON-type solid electrolytes

Impedance spectroscopy and 7Li NMR have been applied to characterize the lithium conducting glass-ceramics membranes of the Li1.5Al0.5Ge1.5(PO4)3 composition with the NASICON-type structure. The 7Li NMR spectra and T1 relaxation times have been compared for the precursor glass and two glass-ceramics annealed for 2 and 6 hours, and analysed with respect to the ionic conductivity in these materials. The 7Li static NMR spectra reveal two components in the glass-ceramics samples: A quadrupole pattern with CQ of 38.7 kHz and 32.5 kHz, and a narrow signal of the Lorentzian or Gaussian lineshape for the samples annealed for 2 and 6 hours, respectively. Variation of the lineshape and the deconvolution parameters point out to the modification of the NASICON framework in the former, which affects the conductivity channels towards improved movement of lithium ions. The NMR data correlate with the conductivity measurements demonstrating enhanced ionic mobility in the glass-ceramics annealed for 2 hours. The 7Li NMR relaxation data seem to be very sensitive to the species with different mobility and reveal the presence of an additional minor component, which can be responsible for decrease of conductivity at longer thermal treatment.