Monolithic All-Phosphate Solid-State Lithium-Ion ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 22264−22277

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Monolithic All-Phosphate Solid-State Lithium-Ion Battery with Improved Interfacial Compatibility Shicheng Yu,*,† Andreas Mertens,† Hermann Tempel,† Roland Schierholz,† Hans Kungl,† and Rüdiger-A. Eichel†,‡ †

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Institut für Energie- und Klimaforschung (IEK-9: Grundlagen der Elektrochemie), Forschungszentrum Jülich, D-52425 Jülich, Germany ‡ Institut für Materialien und Prozesse für elektrochemische Energiespeicher- und wandler, RWTH Aachen University, D-52074 Aachen, Germany S Supporting Information *

ABSTRACT: High interfacial resistance between solid electrolyte and electrode of ceramic all-solid-state batteries is a major reason for the reduced performance of these batteries. A solid-state battery using a monolithic all-phosphate concept based on screen printed thick LiTi2(PO4)3 anode and Li3V2(PO4)3 cathode composite layers on a densely sintered Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte has been realized with competitive cycling performance. The choice of materials was primarily based on the (electro-)chemical and mechanical matching of the components instead of solely focusing on high-performance of individual components. Thus, the battery utilized a phosphate backbone in combination with tailored morphology of the electrode materials to ensure good interfacial matching for a durable mechanical stability. Moreover, the operating voltage range of the active materials matches with the intrinsic electrochemical window of the electrolyte which resulted in high electrochemical stability. A highly competitive discharge capacity of 63.5 mAh g−1 at 0.39 C after 500 cycles, corresponding to 84% of the initial discharge capacity, was achieved. The analysis of interfacial charge transfer kinetics confirmed the structural and electrical properties of the electrodes and their interfaces with the electrolyte, as evidenced by the excellent cycling performance of the all-phosphate solid-state battery. These interfaces have been studied via impedance analysis with subsequent distribution of relaxation times analysis. Moreover, the prepared solid-state battery could be processed and operated in air atmosphere owing to the low oxygen sensitivity of the phosphate materials. The analysis of electrolyte/electrode interfaces after cycling demonstrates that the interfaces remained stable during cycling. KEYWORDS: phosphate, solid electrolyte, interfacial kinetics, all-solid-state battery, lithium-ion battery prerequisite for further progress in this field is to overcome interfacial challenges in “bulk-type” ceramic all-solid-state batteries.8−12 The origin for the obtained interfacial issues can be traced back either to atomic-scale reactions, such as element interdiffusion and segregation during battery fabrication and operation,9,10 or to formation of space charge layers and unwanted redox reactions due to electrochemical instabilities,13−18 all of which lead to a variation in structure and composition over the interface. On the other hand, microstructural issues, as the formation of poor mechanical contacts and microcracks may occur.3,15,16,19 The former processes usually lead to an interfacial layer with high resistance; the latter mechanisms interrupt the percolation pathways and thus

1. INTRODUCTION Lithium-ion batteries with solid-state electrolyte are currently under development to overcome issues considered critical with respect to the safety in high power large-scale systems for automobile applications when using conventional liquid electrolytes.1−5 However, electrochemical stability of the solid-state electrolyte imposes restrictions to the materials that can be used for electrodes. In particular, the application of metallic lithium or lithiated graphite as anode is challenging since their low reduction potentials could result in the formation of high-resistive layer as side product on the electrolyte surface.6 An alternative approach is a battery design with ceramic anode materials, in which the electrochemical potentials are increased. The electrochemical compatibility of such anode materials to the electrolyte seems to be favorable with respect to long-term stability and reliability, in case the challenges for a suitable design of the interfaces can be met.7 Therefore, as commented by many researchers, a mandatory © 2018 American Chemical Society

Received: April 12, 2018 Accepted: June 12, 2018 Published: June 12, 2018 22264

DOI: 10.1021/acsami.8b05902 ACS Appl. Mater. Interfaces 2018, 10, 22264−22277

Research Article

ACS Applied Materials & Interfaces

based superionic conductors has been reported, enabling the development of solid-state cells with high rate capability.31 While the general feasibility of sulfide-based ceramic all solidstate batteries was demonstrated, the scale-up for production on an industrial scale tends to be risky owing to the high oxygen and moisture sensitivity of the sulfide-based electrolytes. For this reason, phosphate-based materials are considered as the superior choice, as processing under dryroom conditions or even in air atmosphere is possible. Further advantages of these materials are their robustness, as well as inexpensive and environmental friendly raw materials. If high current densities at a competitive capacity could be achieved with this material class, phosphate-based solid-state batteries would provide the most promising concept for large-scale production. Because the main limiting mechanisms reported so far, occur at the contact interface between electrolyte and electrode materials, the obvious question arises of a rational choice of material combination can overcome this interfacial challenge. To circumvent unwanted interfacial reactions, we describe here a choice of a mutually adapted material combination. The corresponding identification process relies on a consideration of suitable combinations of all relevant electrochemical and electromechanical properties. This materials design concept followed here does not focus on searching single highperformance materials but rather identifies an optimum combination of materials that allows overcoming the abovementioned interface limitations. It differs also from highthroughput computing and screening approaches to uncover the relevant properties of all known inorganic materials, but rather consists of a rational materials combinatorics concept.32,33 In particular, the presented “monolithic” solid-state battery combines a phosphate-based electrode pair of Li3V2(PO4)3 (LVP) and LiTi2(PO4)3 (LTP) with a phosphate-based Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte. In this case, the choice of adapted electrode and electrolyte materials were restricted to phosphate backbone structures in order to avoid possible chemical interdiffusions and chemical side reactions at the interfaces of electrodes and solid electrolyte. The material combination of LiTi2(PO4)3//LATP//Li3V2(PO4)3 is expected to be chemically stable, at the interfaces, profiting from the strong M−O (M = V, Ti, Al, and P) bonds.34−37 If a decomposition or unexpected interaction occurs at high potentials, the composition of the side products will be restricted to compounds of the five elements, this simplifies further fundamental investigations. The corresponding allphosphate solid-state battery with a phosphate backbone enables high power density and superior cycling behavior. Moreover, on top of the satisfactory manufacturability and lowest decomposition energetics against oxygen atmosphere of the phosphate materials, the all-phosphate solid-state battery can even be operated in air atmosphere.

hinder an effective charge transport, both limiting the overall charge-transfer kinetics in all-solid-state cells. Generally, the corresponding interfacial processes can be differentiated as of chemical or mechanical origin. Chemical reactions at interfaces are generally circumvented by a reasonable choice of mutually compatible electrodes and electrolyte materials. High structural similarity of the components leads to a reduced interfacial resistance in allsolid-state batteries due to an improved chemical stability, as was evidenced by an all-solid-state lithium-ion battery made from Li10GeP2S12 single material and an all-solid-state sodiumion battery made of phosphates.19,20 Nevertheless, chemical reactions occurring under the application of an electric field as well as the formation of space-charge layers have to be evaluated carefully. C. Hänsel reported an all-solid-state battery consisting of a garnet type c-Li6.4Ga0.2La3Zr2O12 solid-state electrolyte and electrodes of a metallic lithium anode and a LiMn1.5Ni0.5O4 cathode.21 Even though the electrolyte and electrodes are chemically stable, new inactive phases were generated due to the continuous irreversible reactions at the electrolyte−cathode interface at high potential. Space-charge layers are caused by different chemical potentials of lithium in the electrode and electrolyte, which favor Li+ migration and thus lead to a depletion layer at the electrode−electrolyte interface.22 Such a high resistance space-charge layer impairs the lithium-ion transport kinetics at the interface.23 To avoid the undesired chemical reactions, a rational selection of electrode and electrolyte materials with respect to the (electro-)chemical compatibility is necessary. Mechanical interfacial issues are generally circumvented by imposing external pressure on the ceramic cells or by suitable design methods for the electrode∥electrolyte interface. Spark plasma sintering (SPS) was applied for assembling monolithic all ceramic lithium-ion batteries, as for a combination of Li1.5Al0.5Ge1.5(PO4)3 electrolyte and LiFePO4∥Li3V2(PO4)3 electrode pair and for symmetric sodium solid-state batteries, Na3V2(PO4)3∥Na3Zr2Si2PO12∥Na3V2(PO4)3.20,24 By means of sintering, intense contact between the electrode and the electrolyte particles can be obtained by formation of broad sinter necks, providing good transport properties for lithium ions. Crucial point for this method with respect to the process is, however, to limit the inter diffusion of elements between different materials under the influence of high temperatures. In contrast to the SPS methods, elevated temperature can be avoided in assembly techniques like slurry casting and screen printing.25 However, the inter particle connections between the electrode and electrolyte materials are limited to the contact areas. With respect to the volume fractions of active material and electrolyte in the composite electrodes, the casting/printing processes are, in theory, more limited than the sintering method, because a significant part of the composite electrode volume is required for the binder component. On the other hand, from the viewpoint of large-scale practical application, casting and printing methods could be the conjunction of pure ceramic or ceramic-polymer hybrid solid-state battery assembly.26 A commonly pursued option to overcome limitations in power density is the combination of electrolyte materials with “bulk” superionic conductivity and electrode materials used in liquid lithium-ion cells. In this regard, sulfide-based electrolyte materials have been in the focus of current investigations because of their enhanced polarizability compared to oxides.27−30 Most recently, the implementation of sulfide-

2. EXPERIMENTAL SECTION The carbon-coated spindle-like LiTi2(PO4)3 anode, Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte pellet, and carbon-coated needle-like Li3V2(PO4)3 cathode materials were prepared.38−40 2.1. Preparation of All-Phosphate Solid-State Batteries. The corresponding all-phosphate solid-state cell consists of two electrochemically active LTP and LVP electrodes separated by an inactive dense LATP electrolyte. Prior to identifying suitable materials combination, the solid-state cell design has to be identified as electrolyte-supported cell architecture. Principally, such a design offers 22265


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Figure 1. Powder XRD diffraction patterns of LiTi2(PO4)3/C (a), Li1.3Al0.3Ti1.7(PO4)3 powder precalcined at 850 °C (c) and Li3V2(PO4)3/C (e). The reference XRD patterns are LiTi2(PO4)3 (ICSD No.95979), Li1.2Al0.2Ti1.8(PO4)3 (ICSD No.427261) and Li3V2(PO4)3 (ICSD No.167238), respectively. TEM and high-resolution TEM images of LiTi2(PO4)3 (b), Li1.3Al0.3Ti1.7(PO4)3 electrolyte primary powder (d), and Li3V2(PO4)3 (f). the processing advantage of using the dense ceramic electrolyte layer as supporting structure on which the composite electrode layers with optimized active loading are deposited. For better densification of solid electrolyte during sintering, the Li−Al−Ti−P−O powder precursor was precalcined at a relatively low temperature of 850 °C before shaping to pellets by uniaxial die pressing and cold isostatic pressing at 177 and 504 MPa, respectively. The pellets were subsequently sintered at high temperature of 1100 °C for 8 h in air atmosphere with a low heating rate of 0.2 K min−1. The obtained relative density of LATP pellets is 94.2% (theoretical density is 2.93 g cm−3), and the thickness was controlled to be less than 300 μm by polishing with sand paper (P800) to remove the sintering skin and decrease the total resistance of the solid electrolyte. Efforts have been made to optimize the ratio of powder materials in the composite electrode layers. As a result, a mixture of LiTi2(PO4)3/ C powder, Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte powder, carbon black powder (super P, Alfa-Aesar), and ethylcellulose (Carlroth) binder powder in a weight ratio of 45:25:15:15 was used to prepare the composite anode layer for the all-solid-state battery. First, the powder

materials were mixed and grinded for 30 min in glovebox. 1-methyl-2pyrrolidone (NMP, ≥ 99.5%, Sigma-Aldrich) solvent was then added to the powder mixture and mixed for 30 min under vacuum by using a Thinky vacuum mixer (ARV-310, Japan). The obtained anode slurry was directly printed on the LATP solid electrolyte pellet by a manual precision screen printing machine (Model SPC-1, MTI) with the screen of PO015/200/45°/24−30 which has a circular pattern of diameter of 8 mm. After drying overnight at 110 °C under vacuum, the same printing and drying processes were repeated several times, in order to fill the pores from former screen printing processes and to define the active loading of the anode material. Once the anode is prepared, the anode∥LATP composite pellet is used as the substrate for screen printing of cathode on the reverse side. The process and components are same with the preparation of anode layers but by exchanging the LiTi2(PO4)3 to Li3V2(PO4)3. The actual mass loading for anode and cathode are controlled to be about 4.0 mg and 5.0 mg, respectively, corresponding to a capacity loading ratio of 1:1.2 (anode:cathode). After the preparation of electrodes, the allphosphate solid-state batteries are cold isostatic pressed at 504 MPa 22266


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Figure 2. (a) SEM images of monolithic all-phosphate solid-state full cell which demonstrates the overall and interfacial microstructure of the prepared battery. Schematic sketch (b) and photographs (c) of the all-phosphate solid-state battery case used for the electrochemical investigations. for 30 s and further dried at 120 °C under vacuum. Subsequently, 450 nm gold layers are sputtered on the top of both electrodes for better electrical connection. Each cell is assembled in a laboratory scaled sealed battery case to maintain the pressure inside the cell and keeping the layers in contact. 2.2. Structural Characterization. The crystal structure and phase analysis was carried out using powder X-ray diffraction (XRD) measurements with an EMPYREAN (Panalytical, Netherlands) X-ray diffractometer with Cu−Kα radiation, operating at 40 kV, 40 mA and a scanning rate of 1° min−1. Scanning electron microscopy (SEM) images and EDX mapping were taken by using a Quanta FEG 650 (FEI, USA) environmental scanning electron microscope equipped with an EDAX-TSL detector. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) measurements were carried out by using a Tecnai F20 (FEI, USA) transmission electron microscope at an acceleration voltage of 200 kV. Surface images and roughness data of electrolyte pellet were obtained with 5 X objective by confocal laser scanning microscope OLS4100 (Olympus Corp., Japan). 2.3. Electrochemical Measurement. Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements for all the cells were conducted using a VMP3 potentiostat (Bio-Logic, France) in defined voltage ranges and current densities. The performed electrochemical tests were carried out at controlled temperatures in a climate chamber (Binder KB115, Germany). The impedance analyses were conducted for a full battery (LVP∥LATP∥LTP), electrode symmetric cells (LTP∥LATP∥LTP, LVP∥LATP∥LVP) and the LATP electrolyte separately. Both the symmetric cells and blocking electrode cell for LATP were coated with gold and were mounted in the same laboratory-scale battery case as the all-solid-state battery. The blocking electrode cell was prepared by directly sputtering 450 nm thick gold layers on both sides of solid electrolyte pellet. While the symmetric cells were prepared by screen print the mixture of electrode material (LTP/C for LTP symmetric cell; LVP/ C for LVP symmetric cell), carbon black and ethylcellulose binder on the both sides of electrolyte. The temperature was varied between 10 and 50 °C in a climate chamber. The impedance was recorded with a Bio-Logic VSP-300 potentiostat with built-in frequency response analyzer. To gain a higher resolution in the high frequency range, the impedance data were quantified by a 2D-DRT.41 The LATP electrolyte was analyzed in ref 42. For the symmetric cells the mid frequency range between 10 kHz and 1 kHz was analyzed. Tikhonov regularization in generalized form with a uniform, zero-crossing and

boundary condition penalty was applied to stabilize the solution. The transformation was conducted with an RC kernel in the first and a unity kernel in the second dimension, which was represented by the temperature variation between 10 and 50 °C.

3. RESULTS AND DISCUSSION 3.1. Structural Analyses of Phosphate Materials. Phosphate materials of LATP for solid electrolyte, LVP for the cathode, and LTP for the anode were used for setting up the battery. On synthesis of the materials, particular attention was paid to structural phase purity, narrow distribution of particle size and optimized nanostructured particle morphology. The crystal structures of electrolyte and electrodes are highly similar and the corresponding measurements are displayed in Figure 1. Particularly, LTP anode material and LATP electrolyte powder XRD patterns show rhombohedral structure with space group R3̅c. The structure of LVP cathode material is monoclinic with space group P21/n.38−43 The specimens are further characterized by TEM, from which the lattice fringes of the LTP, LATP, and LVP can be seen, demonstrating the high degree of crystallinity of the electrode and electrolyte particles. The TEM images are thus supporting the information obtained by XRD with respect to the crystallization of the electrochemical active materials. For purpose of improving of the interfacial connection the approach of designing the morphology by solvothermal synthesis is applied to establish highly defined microstructures with comparatively large specific surface areas of the electrode materials (12.7 ± 0.4 g m−2 for LTP and 14.4 ± 0.4 g m−2 for LVP).43 As shown in the TEM images in the figure above and the SEM images in Figure S1a and b, the LTP materials exhibit a regular spindle-shape with lengths at the long-side of about 6 μm. The LVP is nanoneedle-like and exhibits a diameter of a single bunch of needle structures ranging from 300 to 500 nm and lengths between 6 and 8 μm. Dense LATP pellets are prepared by using the primary calcined LATP powders, as shown in its SEM image in Figure S1c. Unlike the phase pure precalcined LATP powder, the prepared pellets contain a tiny amount of AlPO4 as secondary phase.38 The presence of an Al22267


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though the active mass loading in the cathode is larger than that in the anode, a thinner layer is obtained for the cathode, owing to the significantly different morphologies of LTP and LVP. While repeating the screen printing processes of electrodes with the same sieve, the slurry tends to penetrate into the previously deposited layers and to fill the pores of previously printed layers resulting in a more compact layer, as evidenced by the thickness evolution of the electrodes (e.g., first cathode layer = 54 μm; second cathode layer = 34 μm; cathode layer final thickness = 120 μm). The final densities obtained for anode and cathode composite layers are larger than 75% of the theoretical densities of the composite layers, as estimated from geometry and weight measurement of the electrodes, which can provide sufficient ionic conductivity through the electrodes. The thickness of each of the electrodes is much larger than for the conventional thin-film solid-state batteries. Layer thickness and capacities of the electrodes are comparable with the electrode thicknesses in liquid electrolyte Li-ion batteries prepared by tape casting. The large volumes of the layers combined with high conductivities, ensured by suitable fractions of LATP and carbon in the electrodes, guarantee satisfactory output capacity of the battery for applications. 3.3. Electrochemical Compatibility of Phosphate Materials. The electrochemical analysis of the electrode materials in half cells vs Li/Li+ resulted in specific discharge capacities of 129 mAh g−1 for LVP and 135 mAh g−1 for LTP at 0.1 C. A high capacity retention and rate capabilities for both materials was obtained. The characteristic performance parameters are summarized in Table S1 and can be found in our previous works.39,40 Moreover, the electrochemical combination of the phosphate electrode pair in a liquid electrolyte battery shows high cycling stability and rate capability.43 Since LTP and LVP exhibit superior ionic conductivity but poor intrinsic electronic conductivity (∼10−8 S cm−1) the LTP and LVP particles were coated with a 3−5 nm carbon layer with the amount of 4.8 and 5.6 wt %, respectively, shown in Figure S2. Evidenced by the outstanding electrochemical performance of the electrode materials in half-cells and full-cell with liquid electrolyte, the electrochemical compatibility of the electrode pair is expected not to be limiting the overall performance of the all-phosphate solid-state battery.43 Furthermore, carbon coating on the surface of active materials can suppress the space charge effects at the interfaces of the electrode and solid electrolyte owing to the improved charge compensation efficiency. The dense LATP solid electrolyte applied in this work exhibits a relative high ionic conductivity, of about 0.1−1 mS cm−1 at room temperature,38,45−48 which is acceptable for a practical lithium-ion battery.49−53 Ceramic solid-state electrolyte materials are claimed to have limited intrinsic electrochemical stability in comparison with liquid electrolytes.54,55 Matching the LATP electrolyte with phosphate electrodes, with significantly lower/higher reduction/oxidation potentials generally results in material decomposition at the interfaces. This decomposition may lead to a continued structural change of the electrolyte which either results in an electronic conductor or in Li+ blocking layers, and thus inhibits the overall battery performance. Nevertheless, since the chemical compositions of the electrolyte materials have been optimized for the intrinsic Li+ migration, predictably the formed new side product interfaces will increase the interfacial resistances of the solid-state batteries. Therefore, the operating voltage of the

rich secondary phase in the sintered LATP pellets was already characterized and proven to enhance the densification significantly.38,44 Therefore, there is no need for precise crystallographic alignment between LATP particles since the low amount of an amorphous interphase can increase the mechanical stability and reduce the interfacial impedance. 3.2. Assembly of Monolithic All-Phosphate SolidState Battery. A monolithic all-phosphate solid-state cell with a LTP∥LATP composite anode and LVP∥LATP composite cathode, both containing 15% conductive carbon and 15% ethylcellulose binder has been built on a densely sintered LATP ceramic electrolyte pellet. Screen printing is employed to prepare the electrode layers, in such way that a force is applied to ensure better attachment of composite electrodes onto dense LATP electrolyte in comparison with conventional tape casting techniques. Unlike SPS assembly technique, the components of all-phosphate solid-state battery are assembled without treatments at elevated temperatures. Thereby potential risks of element inter diffusion and material decomposition, as was reported for LiMn1.5Ni0.4O4∥Li1.4Al0.4Ge1.6(PO4)3 system, are avoided.8 With combining all the refined parameters, as an example of the material composition of electrode layers, the allphosphate solid-state battery exhibits excellent interfacial matching as proven by the cross-section SEM images (Figure 2). To ensure good mechanical contact between the electrolyte and the electrodes, the surface roughness of the LATP electrolyte needs to exceed the size of the electrode particles. After it was polished by sand paper, the average surface roughness of the LATP solid electrolyte pellet has been determined by laser scanning microscopy (Sa = 8.19 ± 0.01 μm, Figure S1d). As the roughness of the LATP electrolyte surface is markedly larger than the mean particle sizes of the conductive carbon, LTP and LVP electrode materials, the mechanical connectivity between electrodes and electrolyte during the battery fabrication is enhanced. The thicknesses of anode, cathode and pure electrolyte zone are 150, 120, and 280 μm, respectively, as shown in the backscattered electrons microscopy images of the cross-section in Figure 2a. Although with further thinning of the electrolyte the electrochemical performance of the solid-state battery could greatly benefit, the handling and processing would be challenging since the cell is chosen to be based on the electrolyte-supported design strategy. For both electrodes, the screen printing process results in excellent initial interfacial connectivity between electrode and electrolyte layers, as indicated in the high magnification SEM images. Moreover these images show that the electrode and electrolyte materials in the electrode zones are homogeneously mixed. Observations of the microstructure of the solid-state battery, thus, hint at the structural and electrical integrity of the electrodes and their interfaces with the solid electrolyte during cycling. The prepared monolithic all-phosphate solid-state cells are placed in a laboratory-scale battery case for the electrochemical tests, as shown in Figure 2b and c. To maintain the high structural stability during cycling, the tightening torque for the screws on the battery case is set to 25 Nm. The thickness of electrodes is controlled by a multistep screen printing process thus optimizing the capacity loading ratio of the electrodes. The relative amount of active materials in the LTP and LVP electrodes were chosen to result in 1:1.2 for the capacity loading. This provides a good matching of the capabilities and sufficient amount of mobile lithium-ions. Even 22268


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Figure 3. (a) Cyclic voltammetry profiles of the LiTi2(PO4)3 anode in the voltage range of 1.6−3.4 V vs Li/Li+ (solid line in blue) and the Li3V2(PO4)3 cathode in the voltage range of 3.0−4.3 V (solid line in red) and 3.0−4.8 V (dash line in red) vs Li/Li+ at a scan rate of 0.1 mV s−1, respectively. (b) Intrinsic electrochemical stability diagram of the all-phosphate solid-state battery. (c) Normalized charge−discharge profiles of the LiTi2(PO4)3 anode and the Li3V2(PO4)3 cathode in the corresponding voltage ranges.

reappears, and thus the two-phase behavior is reasserted for the reinsertion of the third Li+.39 The related discharge−charge profiles of LVP are displayed in Figure 3c, and the apparent steps correspond to these can be found in the CV. The capacity of LVP obtained at 0.1 C is ∼95% of its theoretical capacity (133 mAh g−1) in the voltage range of 3.0−4.3 V, while ∼97% of the theoretical capacity (198 mAh g−1) in the voltage range of 3.0−4.8 V. Figure 3b shows the schematic of the electrochemical compatibility of the phosphate electrodes and LATP electrolyte. The solid electrolyte has an intrinsic electrochemical window referring to the lowest reduction potential of 2.17 V vs Li/Li+ and the highest oxidation potential of 4.21 V vs Li/ Li+,55 as agreed by the LSV curve (cf. Figure S3), which covers the complete phase transition potentials of LiTi2(PO4)3 ↔ Li3Ti2(PO4)3 (2.32 V vs Li/Li+ ) and LiV2(PO4)3 ↔ Li3V2(PO4)3 (4.11 V vs Li/Li+). In the meanwhile, the complete phase transition voltage of V2(PO4)3 ↔ Li3V2(PO4)3 (4.58 V vs Li/Li+) is above the intrinsic oxidation potential of LATP, this results in a solid electrolyte decomposition at the interface, as discussed in the CV result of the all-phosphate solid-state battery in the following section. In short, superior electrochemical compatibility can be achieved for the allphosphate solid-state Li-ion battery with the LTP and LVP phosphate electrode pair and LATP solid electrolyte once the oxidation/reduction reactions are limited to the exchange of two lithium-ions, theoretically. 3.4. Interfacial Charge Transfer and All-Phosphate Solid-State Battery Analysis. For the quantification of the interfacial charge transfer, a three-step approach was applied. First an LATP solid electrolyte pellet was used to quantify the ionic conductivity in the electrolyte grains and grain boundaries. Second, these results were used to determine the electrode−electrolyte interface charge-transfer kinetics at symmetric cells consisting of LATP coated with either LVP or LTP on both sides. This approach was chosen since similar time constants of the charge-transfer processes at both electrodes and thus strongly overlapping impedance contribu-

solid-state battery is preferable to be limited to the intrinsic electrochemical window of the LATP solid electrolyte. The intrinsic electrochemical window of LATP is comparable large as proven by the linear sweep voltammetry result in Figure S3. At 30 °C, the electrochemical window of the as prepared LATP solid electrolyte is ∼2.3 V, which is in good agreement with the calculated results by Zhu et al.55 The electrochemical potential of phosphate electrode materials were assessed in liquid electrolyte half-cells, as the cyclic voltammetry (CV) curves and normalized charge−discharge curves shown in Figure 3a and c. The CV curve of LTP is obtained in the potential window 1.6−3.4 V, as shown in Figure 3a. It clearly demonstrates that the main oxidation− reduction reactions at 2.54 and 2.32 V are characteristic for the reversible deintercalation/intercalation of two lithium-ions from/into rhombohedral LiTi2(PO4)3, which is accompanied by the oxidation−reduction process of Ti(IV) ↔ Ti(III).40,43 The voltage profile for a discharge−charge cycle of LTP halfcell from 1.6 to 3.4 V at a current density of 0.1 C can be seen in the bottom part of Figure 3c. The charge−discharge curve displays a reversible and distinct plateau of ∼2.45 V, which agrees well with the CV curve. The normalized capacity is given by the ratio between the discharge capacity of LTP and its theoretical capacity (∼97% of 138 mAh g−1). The CV curves of LVP were measured in the voltage ranges of 3.0−4.3 V and 3.0−4.8 V, corresponding to two and three of the lithium-ions migrating from/into Li3V2(PO4)3, respectively, whereas the electrochemical behavior of LVP is slightly different in the two voltage ranges. Lithium free vanadium phosphate, V(1) and V(2) exhibits a fairly close average bond distance and an average vanadium valence of +4.5, indicating that the mixed V(IV) and V(V) state does not display charge ordering in this phase. Such phenomena will result in a disordered lithium reinsertion, as is evidenced by the solid solution behavior characterized by the S-shaped curve during the reduction process, as can be seen in Figure 3c. It persists until sufficient Li+ repopulation and vanadium reduction are obtained. At the composition of Li2V2(PO4)3, charge ordering 22269


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Figure 4. Nyquist plots (a and c) and DRT spectra (b and d) of LiTi2(PO4)3 (top) and Li3V2(PO4)3 (bottom) symmetric cells, respectively. The impedances were obtained with temperature variation from 10 to 50 °C.

Figure 5. (a) Charge transfer resistances of symmetric LiTi2(PO4)3 and Li3V2(PO4)3 cells, respectively. The resistances were determined by numerical integration of the peaks in the DRT spectra (Figure 4b and d). Both follow an Arrhenius behavior. (b) Comparison of the measured and simulated impedance data for the all-solid-state battery LiTi2(PO4)3∥Li1.3Al0.3Ti1.7(PO4)3∥Li3V2(PO4)3. The simulation was done using the model shown in the inset. The RQ elements from left to right represent the grain and grain boundary ion transport in the electrolyte, and the charge transfer kinetics at the LiTi2(PO4)3∥Li1.3Al0.3Ti1.7(PO4)3 and Li3V2(PO4)3∥Li1.3Al0.3Ti1.7(PO4)3 interfaces, respectively.

symmetric cells was also done for temperatures between 10 and 50 °C in the same battery case as for the solid-state batteries in order to maintain the similarity of mechanical environment. The impedance data of the symmetric cells are shown in Figure 4a and c for LTP (top) and LVP (bottom), respectively. In the Nyquist plots, temperature dependent semicircles followed by linearly increasing diffusion branches are visible. As expected, the charge-transfer processes at the electrode− electrolyte interfaces overlap with the grain boundary and solid-state diffusion processes and render distinguishing between the three processes very difficult. Thus, a 2D-DRT analysis of the impedance data was performed to gain a higher resolution.41 On the one hand, previous investigations of the LATP solid electrolyte showed, that the grain and grain boundary ion transport mainly contribute to the impedance spectra at frequencies higher than 10 kHz.39 On the other

tions are expected. Third, the determined kinetic parameters were used to simulate the impedance behavior of the all solidstate battery. Since a conventional impedance analysis with equivalent circuit models did not provide a sufficient high resolution a two-dimensional distribution of relaxation times (2D-DRT) analysis was performed.41 The quantification of the ionic conductivity in grains and grain boundaries of LATP, at temperatures between 10 and 50 °C, by the 2D-DRT has been published recently.42 The first dimension (impedance spectrum, Figure S4) was inverted with an RC kernel while the second dimension (temperature) was used to enhance the signal-to-noise ratio by applying a unity kernel. As is shown in Figure S5, at 30 °C the grain and grain boundary conductivities were 2.5 mS cm−1 and 1.6 μS cm−1, respectively, whereas the total conductivity was about 0.3 mS cm−1. The quantification of the electrode−electrolyte charge transfer kinetics at the 22270


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Figure 6. 1st, 5th, 10th, and 50th charge−discharge curves and cycling stability of the monolithic all-phosphate solid-state lithium-ion battery at 0.078 C in the voltage ranges of 0.5−2.2 (a and b) and 0.2−2.8 V (c and d), respectively. All the tests are performed at 30 °C. The specific capacity is calculated by using the mass of active material in composite anode.

For this purpose, an equivalent circuit model was simulated with one RQ element per process (Figure 5b). Since size and geometry of the electrolyte was the same in all-phosphate solid-state battery and solid-state symmetric cells, the resistances and pseudocapacitances of grain and grain boundaries determined from the electrolyte investigations could be directly used and were set as fixed parameters in the model. In contrast, the symmetric cells each contained two electrode−electrolyte interfaces. Thus, the measured values were halved and set as fixed parameters for the LVP and LTP charge transfer process, respectively. The impedance of the model then was simulated in the frequency range between 1 × 103 Hz and 2 × 106 Hz, as shown in Figure 5b. There the impedance values measured at the solid-state battery are depicted also. The simulation was in good agreement with the experimental values, which indicated a full characterization of the solid-state battery dynamics. Moreover, this also showed the high reproducibility of the symmetric and full cell manufacturing process. Regarding the knowledge-based optimization process of the solid-state battery, two important conclusions could be drawn. First, for improving the electrolyte ionic conductivity the grain boundary width and the lithium content within the grain boundaries are the crucial factors.41 Second, for improving the charge transfer kinetics, the activation barrier at LATP∥LVP interface should be further suppressed, for instance by tuning the microstructure or material combination of the composite cathode layer, or construction of lattice-matching between electrode and electrolyte particles. 3.5. Electrochemical Performance of All-Phosphate Solid-State Battery. To gain insight into the compatibility of electrode−electrolyte, and the electrochemical behavior of allphosphate solid-state batteries, cyclic voltammetry (CV) tests (cf. Figure S6) were performed in the voltage range of 0 to 3 V at a scan rate of 0.03 mV s−1. Comparing the first CV profile of

hand, solid state diffusion in the electrodes and the electrolyte contribute to the impedance spectra below 1 Hz.56,57 By only analyzing frequencies in between 100 and 1000 Hz with 2DDRT the contributions of grain boundaries and solid state diffusion to the charge transfer impedance could be minimized. The relaxation time spectra are shown in Figure 4b and d. For a better comparability to the impedance data, the relaxation time τ is depicted as frequency f = (2πτ)−1. The results show one peak in the relevant frequency range. Drawback of this approach was a low signal-to-noise ratio due to peak cut-offs which lead to a peak broadening. The quantification was done by numerical integration of the peak areas, which correspond to the resistance R. The pseudocapacitance Q was determined from R and τ. The interface resistances of both electrodes charge transfer processes showed an Arrhenius like behavior (cf., Figure 5a). The activation energies were 444 (±11) meV for the LVP, and 275 (±24) meV for the LTP, respectively. Previously, the activation energies of grain and grain boundary ion transport in the LATP were determined to 182 (±11) and 430 (±8) meV, respectively.42 An investigation of these values showed that the charge transfer potential barrier at the LTP∥LATP interface was in between those of the grains and grain boundaries, while the potential barrier at the LVP∥LATP interface was more comparable to that in the grain boundaries and thus could be the limiting factor of overall charge transfer. The strongly overlapping impedance contributions of electrolyte grains and grain boundaries, as well as those of the charge transfer kinetics at the electrode−electrolyte interfaces made a direct analysis of the solid-state battery hardly feasible. Instead, the previously quantified impedances of these processes were used to simulate the impedance behavior of the solid-state battery, which afterwards was compared to the measured impedance. 22271


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Figure 7. Rate capability and discharge curves of the monolithic all-phosphate solid-state lithium-ion battery at 30 °C at different C-rates in the voltage ranges of 0.5−2.2 (a and b) and 0.2−2.8 V (c and d), respectively. The specific capacity is calculated by using the mass of active material in composite anode. (e) Cycling performance of the solid-state battery in the voltage range of 0.5−2.2 V at a current density of 0.39 C at 30 °C. The specific capacity is calculated by using the mass of active material in composite anode.

the solid-state battery and CV results of the half-cells shown in Figure 3a, four oxidation peaks in the voltage range of 0.2 to 2.8 V can be attributed to the complete removal of lithium-ions from LVP to LTP, corresponding to a phase transition of Li3V2(PO4)3 → V2(PO4)3. Moreover, the three characteristic reduction peaks of LVP can be revealed in the CV result of the solid-state battery as two peaks due to the sluggish electrochemical kinetics in the solid structure. In the subsequent CV scans, there are apparently redox peaks merging and one oxidation peak (at about 0.8 V in first cycle) is missing. The redox peaks merging are mainly due to the slower lithium-ion diffusion in the solid-state battery system, which is likely to be caused by the kinetic limitations of solid electrolyte or the effects of electrical double layer in the composite electrodes. Thus, a charge behavior of multiphase transition of electrodes turned to a solid solution behavior. The disappeared oxidation peak could be due to the formation of a SEI and irreversible changes at the high voltage in the solid-state battery. The spontaneous and electrochemical formation of the SEI in lithium transition-metal phosphate electrodes during first cycle

has to be attributed to the carbon additive in the electrode composition but not the phosphate materials.43 Hence, the second, fifth and tenth CV profiles are in good agreement with each other indicating high reversibility of the lithiation and delithiation processes of the all-phosphate battery. Although experimentally the LATP exhibits an electrochemical window of 2.3 V, the window of the solid-state electrolyte itself does not directly limit the operating voltage range of all-solid-state batteries. Therefore, two potential windows of 0.5−2.2 V and 0.2−2.8 V have been chosen to include or exclude the electrochemical decomposition of the LATP during the electrochemical tests of the monolithic all-phosphate solidstate batteries. The monolithic all-phosphate solid-state lithium-ion batteries were cycled for 50 cycles at a current density of 0.078 C at 30 °C in each of the voltage ranges. The charge−discharge profiles and the cycling performances, as presented in Figure 6, indicate a trend of continuously increasing Coulombic efficiency with at the same time a slightly decrease in capacity for the first 10 cycles of the solid-state batteries in both voltage 22272


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the battery operated in different voltage ranges. On the basis of these results, the voltage range has to be carefully chosen to avoid the degradation of electrochemical performance of solidstate batteries. To further explore the cycling stability of the prepared solidstate batteries in 0.5−2.2 V, charge−discharge test was performed at 0.39 C as shown in Figure 7e. The battery is able to be cycled up to 500 cycles with minor capacity fading. The discharge capacity of the battery after 500 cycles is 63.5 mAh g−1, corresponding to 84% initial discharge capacity and 46% of the theoretical capacity (138 mAh g−1) of LiTi2(PO4)3 anode. When comparing the active mass related electrochemical performance of the half-cell and full cell employing liquid electrolyte, as shown in previous work, the rate capability and cycling performance of the all-phosphate solidstate battery cannot represent the full ability of the phosphate electrode materials which is in account of the effects of intrinsic ionic conductivity limitation of solid-state electrolyte and the significantly smaller electrolyte/electrode contact areas in solid-state batteries compared to the batteries with liquid electrolyte.43 The charge−discharge behavior in absolute capacities are 0.42, 0.40, 0.36, 0.28, 0.18, 0.12, and 0.05 mAh at current densities from 0.039 to 3.9 C in voltage range 0.5−2.2 V corresponding to Figure 7b. Considering specific capacity, it depends on the point of view what is the appropriate scale base to which the capacity has to be related. In literatures, specific capacities are mostly related to working electrode mass or limiting electrode mass only. When referring to the capacity per gram LTP, the values of the specific capacity (as shown in Figure 6 and 7) for the solidstate battery almost match those obtained with liquid electrolyte cells.34,40,43 Theses figures seems to be useful for battery types which are designed with negligible amounts of anode materials and electrolyte. Indeed when using lithium as an anode, the anode mass required is less than five percent of the cathode mass. For carbon anodes only a slightly higher mass is needed. For battery types with both electrodes ceramic materials, however, in general the mass of both electrodes will contribute significantly to the total mass. Moreover, for solid state batteries, the mass of the solid electrolyte has to be taken into account. Thus, with respect to practical application, the capacities related to volume of electrodes, total volume and total mass of battery seem to be the most suitable figures. Hence, the information regarding to these are listed in Table S2. Owing to the nonoptimized thickness of the electrolyte in the present all-phosphate solid-state battery, such figure shows quite low values. The objective of the present work in first instance was to demonstrate the functionality of an allphosphate solid-state battery with thick film electrodes with respect to stability and rate performance. With respect to the capacity related to surface area, the battery shows performance comparable to the liquid electrolyte battery with LTP and LVP electrodes.43 Considering these objectives, the results demonstrated the cyclability and stability of the all-solid-state phosphate battery. Since the LTP, LATP, and LVP have an intrinsically high stability in air atmosphere, a preliminary cycling test is performed in the voltage range of 0.2−2.2 V in air at 23 °C by applying a constant uniaxial force (3 kN) on the allphosphate solid-state battery in a setup as demonstrated in Figure S7a. The solid-state battery is cycled at a low current

ranges. Specifically, irreversible capacity in the initial cycle was also found in the liquid electrolyte battery with the same electrode materials (LTP∥LP30∥LVP).43 The amount of lithium losses are not substantially different. In both types of cells in the first cycle, the lithium loss is mainly due to the energy loss for the battery activation, irreversible lithium insertion into carbon and the formation of the SEI layers. For the initial capacity loss in solid-state battery cycled in 0.2−2.8 V, the passivation of LATP at cathode∥electrolyte interface could also play a role since the voltage range is slightly out of the intrinsic electrochemical window of LATP. The discharge capacities corresponding to the first 10 cycles of batteries measured in both voltage ranges are in high agreement, whereas the battery cycled in 0.2−2.8 V shows a slightly higher discharge capacity than the battery tested in narrower voltage range. Nevertheless, significant different cycling performance of the batteries can be observed with continued cycling. The discharge capacity of battery cycled in 0.5−2.2 V tends to stabilize at ∼90 mAh g−1 after the first 20 cycles with a discharge capacity retention of ∼88% after 50 cycles, as demonstrated in Figure 6b, proving the excellent cycling stability of this battery at low current density in the specific potential range. In sharp contrast, as shown in Figure 6d, although the battery exhibits relatively high initial discharge capacities in the large voltage window of 0.2−2.8 V, the features of capacity fading and low Coulombic efficiency of the battery caused by intensified polarization limits its application. It is likely that the aging of battery operated in 0.2 to 2.8 V is caused by the exhaustive (de)lithiation of LVP, as well as the electrochemical decomposition of LATP at this high potential. The products of the high oxidation potential of LATP can be O2, LiTi2(PO4)3, Li4P2O7, and AlPO4.55 Even though the presence of tiny amounts of ion conducting compounds of Li4P2O7 may not restrict the ionic transport, AlPO4 and LiTi2(PO4)3 in the composite cathode and at the interface of the electrolyte and cathode inevitable block the lithium-ion transport toward LVP. The rate capability of the all-phosphate solid-state Li-ion batteries was characterized in different voltage ranges, which are 0.5−2.2 and 0.2−2.8 V. As shown in Figure 7a, the battery operated in the voltage range of 0.5−2.2 V reveals outstanding Coulombic efficiency, and the discharge capacity values can be recovered to be basically consistent to the initial values when the current density was retained to 0.03 C after high rate cycling. However, the battery operated in the voltage range of 0.2−2.8 V shows a linear capacity fading with increasing current density, as shown in Figure 7c. Even though when the current density is decreased to 0.03 C, only ∼65% of the initial discharge capacity can be delivered and subsequently continues to fade due to the occurrence of side reactions during cycling. The correlating discharge curves are displayed in Figure 7b and d, in which, owing to the electrochemical polarization of the battery, the dropping of the voltages with the increasing current density for both operating voltage ranges can be seen. Figure 7b shows the discharge behavior of the battery in the voltage range of 0.5−2.2 V at different current densities in the range of 0.039−3.9 C, with the corresponding reversible discharge capacities of 102, 98, 87, 70, 43, 31, and 15 mAh g−1 respectively. Consistent with the former results, higher capacities can be delivered when the battery is operated in the voltage range of 0.2−2.8 V at 0.039 C, as evidenced by Figure 7d. However, once the current density is higher than 0.39 C, there are only tiny differences between the capacities of 22273


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Figure 8. Post-test measurements (a) cross-section SEM images and (b) EDX maps (Al, V, Ti, P, O and C) of the all-phosphate solid-state Li-ion battery after 500 cycles in the voltage range of 0.5−2.2 V at a current density of 0.39 C. The element maps in blue square are refer to zone 1 whereas the element maps in red square are refer to zone 2 in the SEM image.

density about 0.05 C for approximately 1 week for four cycles. The voltage−time curves in good agreement for all cycles (Figure S7b) and the discharge capacity of the battery is slightly increased with an increasing cycle number (Figure S7c). The relative low columbic efficiency, ∼70%, for the first cycle is related to the extended voltage range. The columbic efficiency increases in the following cycles which agreed with

the former observations. Nevertheless, the results are demonstrating the superior chemical and electrochemical stabilities of the employed phosphate materials in air atmosphere, which offers a great advantage for future applications of the all-phosphate solid-state battery, especially when compared with sulfur-based solid-state batteries that 22274


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considered the structural and (electro-)chemical compatibilities of components instead of a single materials performance to play a major role in eliminating the high interfacial resistance of solid-state batteries and to lead the material adjustment and battery architectures. Particularly, phosphates are employed as anode, cathode and solid electrolyte with the aim to establish stable interfaces of all-phosphate solid-state batteries. With respect to processing, the assembly of the battery was accomplished at mild temperature by screen printing of composite electrode layers with compositions balancing sufficient lithium transport and high capacity on presintered ceramic LATP electrolyte. The success of this approach could be confirmed by usage of data, gained by DRT analyses, from the symmetric cells consisting of two identical electrodes and electrolyte to simulate the spectrum of the full cell battery. The features of high chemical and electrochemical stabilities of the combination, as well as the easy manufacturability of the materials offer the possibilities to further optimize the materials for improving the ionic and electronic conductivities and refining the battery structure. The monolithic all-phosphate solid-state battery was cycled for 500 times at 0.39 C with minor capacity fading and superior Coulombic efficiency at 30 °C in the voltage range of 0.5−2.2 V. Moreover, one-third of the theoretical capacity can still be delivered at a current density close to 1 C. These electrochemical properties of the battery are far beyond the recently reported results of bulk-type all-solid-state batteries. Furthermore, benefited by the harmonious material combination, feasibility of operating the all-phosphate solid-state lithium-ion battery in air atmosphere is proven. Herein, our study expresses a promising concept to (electro-)chemically and structurally circumvent the interfacial limitations of all-solidstate batteries.

have limited intrinsic electrochemical window and high sensitivities toward oxygen and moisture. 3.6. Post-Test Characterization. For demonstrating the integrity and stability of the interface between electrolyte and electrodes, a post-test SEM characterization of the microstructure after 500 cycles in the voltage range of 0.5−2.2 V has been performed. The corresponding cross-sectional SEM images and EDX element maps of V, Al, Ti, P, O, and C are illustrated in Figure 8. The battery structure is maintained after long time electrochemical processes indicating superior mechanical stability of the battery, which benefits from the sufficient interfacial matching. The cracks formed as consequences of mechanical pressure and volume changes of materials during electrochemical processes are mainly perpendicular to the interfaces, which is believed not to be the limitation of battery operation since there is no separation of electrolyte and electrode. Obviously, as shown in the EDX maps in Figure 8b, the elemental characteristics for the LATP electrolyte (Al, Ti, P, O) are distributed over all three component layers, as they are present in the electrolyte and the composite electrodes. On the other hand, no observable amounts of elements exclusively present in the carbon-coated pristine electrode materials (C, V) could be observed as segregated over the electrolyteelectrode interface into the electrolyte region. In particular, P and O are uniformly distributed over the entire solid-state battery volume owing to the phosphate backbone over electrolyte and both electrode materials. However, the concentration of Ti, Al, P, and O is not constant and rather follows concentration step functions at the electrode/electrolyte interfaces owing to the different stoichiometry of the phosphate materials in each layer, as well as the different density of the layers. Moreover, V can be only detected in the cathode layer. Accordingly, the rational phosphate material combination resulted in high mechanical and (electro)chemical stability of the monolithic all-solid-state lithiumion battery. In order to investigate the crystallinity of the LATP after 500 cycles in 0.5−2.2 V, the electrode layers are removed carefully by polishing. XRD tests are performed on the bulk (gray) and surfaces of solid electrolyte which were in contacted with cathode (red) and anode (blue), as the results are shown in Figure S8. The LATP main phase remained in all the measured areas as well as the tiny amounts of an AlPO4 impurity which was formed during sintering,38 indicating the high structural stability of the prepared solid-state battery. The electrochemical decomposition mainly took place at the interface between LATP and LTP anode as evidenced by the presence of the Ti(IV)/Ti(III) mixed valence, for example, Li2Ti2(PO4)3 (space group Pbcn), which may be generated by the reduction of LATP at relatively low discharge potentials.48 Since the generated product is capable for lithium-ion storage (serve as electrode), and the amount and thickness of the side product layer is concededly low,58 it unlikely causes tremendous impact on the battery performance as proven by the cycling performance of the battery.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05902. Morphology of anode, cathode, and solid electrolyte, electrochemical performance of the electrodes in liquid cells, characterization of carbon layer on active materials, electrochemical window, EIS data and ionic conductivity of solid electrolyte, cyclic voltammetry of all-solid-state battery, electrochemical performance of the solid-state battery in air atmosphere and the XRD patterns of solid electrolyte after cycling, performance parameter for LiTi 2 (PO 4 ) 3 and Li3 V 2 (PO 4) 3 half-cells and the LiTi2(PO4)3||Li3V2(PO4)3 cell with liquid electrolyte LP30, and specific capacity of all-phosphate solid-state related to different scales, including surface area and volume of electrode and total mass and total volume of battery. PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. CONCLUSION In summary, a high performance all-solid-state battery was constructed by adjusting of electrode materials, solid electrolyte and processing techniques according to the objective to provide a battery with electrochemical stability and minimized interface resistance. With respect to the materials selection, we

ORCID

Shicheng Yu: 0000-0002-6619-3330 Author Contributions

All authors made contributions to manuscript preparation and have given final approval for publication. 22275


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ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the projects “Ionenleiter für hocheffektive Stromspeicher” of the Ministerium für Innovation, Wissenschaft und Forschung from the Federal State of North Rhine-Westphalia (Germany) and “Materials and Components to Meet High Energy Density Batteries II” of the funding program “Excellent battery” from Bundesministeriums für Bildung und Forschung (BMBF) within the framework of the program “Materials Innovations for Industry and Society”, Förderkennzeichen: 03XP0084C. The SEM Quanta FEG 650 (FEI) was funded by the Bundesministerium für Bildung und Forschung (BMBF) under the Project “SABLE”, Förderkennzeichen: 03EK3543.

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Monolithic All-Phosphate Solid-State Lithium-Ion ... - ACS Publications

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