Research Article www.acsami.org
Addressing the Interface Issues in All-Solid-State Bulk-Type Lithium Ion Battery via an All-Composite Approach Ru-Jun Chen, Yi-Bo Zhang, Ting Liu, Bing-Qing Xu, Yuan-Hua Lin, Ce-Wen Nan,* and Yang Shen* School of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: All-solid-state bulk-type lithium ion batteries (LIBs) are considered ultimate solutions to the safety issues associated with conventional LIBs using flammable liquid electrolyte. The development of bulk-type all-solid-state LIBs has been hindered by the low loading of active cathode materials, hence low specific surface capacity, and by the high interface resistance, which results in low rate and cyclic performance. In this contribution, we propose and demonstrate a synergistic all-composite approach to fabricating flexible all-solid-state LIBs. PEO-based composite cathode layers (filled with LiFePO4 particles) of ∼300 μm in thickness and composite electrolyte layers (filled with Al-LLZTO particles) are stacked layerby-layer with lithium foils as negative layer and hot-pressed into a monolithic allsolid-state LIB. The flexible LIB delivers a high specific discharge capacity of 155 mAh/g, which corresponds to an ultrahigh surface capacity of 10.8 mAh/cm2, exhibits excellent capacity retention up to at least 10 cycles and could work properly under harsh operating conditions such as bending or being sectioned into pieces. The all-composite approach is favorable for improving both mesoscopic and microscopic interfaces inside the all-solid-state LIB and may provide a new toolbox for design and fabrication of all-solid-state LIBs. KEYWORDS: lithium ion battery, bulk-type all-solid-state battery, solid-state electrolyte, composite, Li7La3Zr2O12,, PEO
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of cycles,9 their total capacity is rather low because of the small thickness of the cathode layer which contains very small amount of active cathode material. In current thin-film LIBs, substrate and other inactive components occupy a large portion of space inside the battery, resulting in very low surface capacity (5 μAh/cm2 to 100 μAh/cm2).10 Increasing the thickness of the electrodes in all-solid-state batteries is thought to be helpful for decreasing the relative ratio of inactive cell components such as current collectors, and thus increases the specific energy density.12 This approach, however, could not be adopted due to both technical and intrinsic scientific limitations. Current thinfilm-deposition techniques, such as RF-sputtering, pulsed laser deposition or atomic layer deposition9,13−15 are expensive and time-consuming, which makes it rather difficult and not costeffective, if not impossible, to expand the thin-film structure into bulk-type LIBs by simply increasing the thickness of cathode layer. More importantly, increasing the thickness of the active cathode layers also substantially increase the distance for lithium transportation during charge−discharge cycles, resulting in much increased impedance. The cycling performance of the solid LIBs is also significantly compromised by the cracks, both inside the cathode layers and at the interface between electrode and electrolyte layers, arising from the volumetric change of
INTRODUCTION The long-standing challenge for conventional lithium ion battery (LIBs) has been the safety issues related to the flammable organic liquid electrolyte, which limits the mass application of conventional LIBs in electric vehicles (EVs) or other energy storage applications.1−4 The ultimate solution to the fire hazard of conventional lithium ion battery lies in the implementation of all-solid-state LIBs, where the flammable liquid electrolyte is completely replaced by solid-state electrolyte. In addition to their intrinsic safety and reliability, all-solidstate LIBs also exhibit higher energy density and higher power density.5,6 As a result of their highly integrated monolithic structure, composite cathode with higher density allows for higher loading of active cathode materials. Plus, the solid electrolyte of high structural integrity may prevent the growth of lithium dendrites, making it possible for the use of lithium metal as the anode.7,8 These favorable features give rise to much enhanced energy density of all-solid-state LIBs. Among all the all-solid-state lithium batteries with different structure designs, thin-film solid-state batteries have been intensively studied and widely used in microelectronic industries as power supply for ICs or RF devices, where capacity retention instead of total capacity plays a more critical role.9 These film batteries consist of three layers of thin films serving as positive electrode, solid electrolyte and negative electrode, respectively.10,11 Although thin-film solid-state batteries deliver excellent capacity retention of over thousands © 2017 American Chemical Society
Received: December 19, 2016 Accepted: February 28, 2017 Published: February 28, 2017 9654
DOI: 10.1021/acsami.6b16304 ACS Appl. Mater. Interfaces 2017, 9, 9654−9661
Research Article
ACS Applied Materials & Interfaces
of ∼70 μm in thickness. The cathode layer, electrolyte layer and lithium foil are then stacked layer-by-layer and hot-pressed into an all-solid-state LIB of monolithic structure. The all-solid-state battery cell was cycled at a constant current density of 100 μA/ cm2 at 60 °C and delivered an average specific discharge capacity of 155 mAh/g, which translates into a surface capacity as high as 10.8 mAh/cm2. Excellent capacity retention is obtained and no capacity fading can be observed up to at least ten cycles. In addition to the excellent electrochemical performance, the all-solid-state LIB fabricated via the allcomposite approach is highly flexible and ductile. It still works properly after being subjected to harsh conditions such as repeated bending or even cutting into smaller pieces during discharging. We note that not only could the superior mechanical performances of the all-solid-state LIBs be attributed to the composite structure but the all-composite approach also has significant implications on the electrochemical performance of the LIBs. The hot-pressing process during battery assembly is in favor of forming dense and good interfaces between the cathode and electrolyte layer, which gives rise to decreased interface impedance. Plus, the soft polymer matrix employed in the composite cathode layers could tolerate the substantial volumetric change of the active cathode materials during the charge−discharge cycles and prevent the formation of cracks induced by the stress.
active materials during cycling. For these reasons, all-solid-state LIBs could not be fabricated by simply expanding the layer-bylayer structure from thin-film to bulk-type.16 To address these issues, composite approaches have been adopted by preparing composite electrodes consisted of active materials and electronic and ionic conductor additives to increase the thickness of cathode. For instances, bulk-type allsolid-state LIBs with composite cathodes consisted of active cathode materials, inorganic or polymer solid-state electrolyte and other additives on top of Li7La3Zr2O12 (LLZO) pellets as solid electrolyte have been assembled.17−20 Though the thickness of the cathodes in these batteries increased from several microns in thin-film batteries to tens of microns, these batteries contain very thick ceramic electrolyte pellets. As a result, their specific volumetric capacity and energy density are still low. Bulk-type composite electrode prepared by a colloidal crystal templating method and sol−gel infiltration process can exhibit high capacity and energy density, but the preparation processes are too complex and time-consuming.21,22 Delaizir et al. have successfully fabricated a monolithic solid-state lithium battery by spark plasma sintering method with a composite cathode as thick as 400 μm and achieved a high surface capacity of 2.2 mAh/cm2, demonstrating the potential of bulk-type composite electrodes in the application of all-solid-state LIBs.16 However, because of the all-ceramic structure of the battery, the sintering condition was hard to control and a large portion of the active materials could not be fully utilized. We have also prepared compact three-dimensional composite electrodes by one-step sintering of active materials and 0.44LiBO2·0.56LiF and all-solid-state batteries have been assembled using dry polymer electrolyte as the electrolyte layer.23,24 High surface energy density and improved cycling performance have been achieved. The other critical component for all-solid-state LIBs is the electrolyte. Poly(ethylene oxide) (PEO)-based composite electrolytes containing inorganic nanoparticles, such as SiO2 or Al2O3, have been intensively explored and are still considered as the most promising composite electrolyte.25,26 In these PEO-based composite electrolyte, the inorganic nanoparticles act as cross-linking centers to reduce the crystallinity of the polymer, giving rise to increased chain mobility hence higher ionic conductivity. Plus, the strong Lewis acid−base interaction between electrolyte ion species and the surface chemical groups of inorganic nanoparticles can enhance the dissociation and stabilize the anions. It is worth noting that most of the inorganic nanoparticles used in the composite electrolyte so far are inert, meaning that they are themselves of rather low ionic conductivity. Further enhancement of ionic conductivity of these PEO-based composite electrolytes may lie in the employment of nanoparticles with high intrinsic ionic conductivity. For this reason, LLZO or (Li0.35La0.55TiO3) LLTO ceramic particles may hold the promise of achieving higher ionic conductivity in PEO-based composite electrolyte. For instance, Hu et.al., recently demonstrated that the addition of LLZO in PEO can effectively increase the conductivity of the composite electrolyte and block the growth of dendrites.27 In this contribution, we took a synergistic all-composite approach to fabricating highly flexible all-solid-state LIBs. PEO -based composite cathode layers of ∼300 μm in thickness are prepared via hot-pressing using LiFePO4 as active cathode materials and indium tin oxides (ITO) as electronic conductor additive. Composite electrolytes consisted of Al-doped LLZO and PEO dry electrolyte are then cast from solution into layers
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EXPERIMENTAL SECTION
Preparation of Composite Electrolyte. The Al-doped Li6.75La3Zr1.75Ta0.25O12 (Al-LLZTO) powders were prepared by conventional solid-state reaction as reported in our previous work.22 Stoichiometric amount of LiOH·H2O (Aladdin Reagent), La2O3 (Sinopharm Chemical Reagent), ZrO 2 (Sinopharm Chemical Reagent), Ta2O5 (Aladdin Reagent), and Al2O3 (Sinopharm Chemical Reagent) were thoroughly mixed in 2-propanol for 8 h. The mixed slurries were dried and calcined at 900 °C for 6 h. The as-calcined powders were ball-milled in 2-propanol for 13 h followed by drying process. The composite solid electrolytes (CSEs) were prepared by solution method. PEO (Alfa Aesar) with an average molecular weight of 300 000, LiN(CF3S02)2 (LITFSI, Aladdin reagent) and Al-LLZTO were weighted according to the stoichiometric ratio. The PEO-LITFSI concentration ratio was fixed at n(O)/n(Li) = 12 and the weight percentage of Al-LLZTO was varied from 40 wt % to 80 wt % according to the total PEO + Al-LLZTO weight of the CSEs. The AlLLZTO was ultrasonicated in acetonitrile for 4 h followed by the addition of predetermined amounts of PEO and LITFSI. This solution was then stirred at room temperature for 24 h until a homogeneous electrolyte slurry was obtained. The mixture was cast on Teflon plates in an argon-filled glovebox and dried under vacuum at 60 °C to evaporate the solvent and get the CSE films. For ionic conductivity measurement, the electrolyte film was pressed between two stainless steel blocking electrodes. Fabrication of the All-Solid-State Battery. Thick composite cathode layers were prepared with hot-pressing method. Battery-grade LiFePO4 powder was used as the active material of composite cathodes. In2O5Sn (ITO) instead of other cheaper materials was chosen as the electron conducting additive to increase the compaction density of the composite electrolyte, which is in favor of shorter migration paths for lithium ion and gives rise to higher volume energy density. The weight percentage of ITO was set to 10 wt % with respect to the weight of LiFePO4. The PEO-LITFSI concentration ratio was fixed at n(O)/n(Li) = 12 and the weight percentage of PEO was varied from 30 wt % to 10 wt % according to the total PEO + LiFePO4 + ITO weight in the composite cathodes. To ensure homogeneous dispersion of LiFePO4 and ITO particles in the PEO matrix, stoichiometric amount of PEO, LITFSI, LiFePO4, and ITO were first dissolved and thoroughly mixed by stirring in acetonitrile for 12 h to obtain a homogeneous cathode slurry. The slurry was then 9655
DOI: 10.1021/acsami.6b16304 ACS Appl. Mater. Interfaces 2017, 9, 9654−9661
Research Article
ACS Applied Materials & Interfaces completely dried in vacuum oven at 60 °C. The dried precursor of the composite cathode was then subjected to freeze ball-milling at liquid nitrogen temperature for 40 min. For the hot-pressing process, 50 mg of the resultant composite cathode powders were hot-pressed at 40 °C under vacuum into composite cathode layers of ∼300 μm in thickness. For the assembly of all-solid-state LIBs, Au was sputtered on one side of the composite cathode layer as current collector. A composite electrolyte layer of ∼70 μm in thickness was then screen printed on the other side of the composite cathode layer. To control the uniformity of the electrolyte, a steel plate with a hole was installed in the screen printer and the composite cathode was placed under the hole. After the electrolyte slurry flowed through the hole, a uniform electrolyte layer was obtained. The composite electrolyte layer was dried in vacuum oven at 60 °C. Finally, a lithium foil of ∼100 μm in thickness was attached on top of the composite electrolyte layer as anode layer in an argon-filled glovebox. The schematic illustration for the all-solid-state battery is shown in Figure S1. The whole all-solidstate lithium battery was sealed in a LIR 2032 cell before testing. Characterization of the Electrolyte and Battery. The phase composition of the Al-LLZTO was characterized with X-ray diffraction (XRD, RigakuDmax 2500). The electrochemical impedance measurement was carried out with an impedance analyzer (ZAHNER-elektrik IM6) in the frequency range from 1 Hz to 8 MHz at a voltage amplitude of 50 mV. The microstructure of the pellet was characterized with scanning electron microscopy (SEM, JEOLJSM7001 F). The cells were galvanostatically cycled between 2.3 and 3.8 V vs Li/Li+ at 100 μA/cm2 at 60 °C. The testing temperature was controlled by an environmental chamber (Cincinnati Sub-Zero MCB1.2-AC). Constant current charging was used for investigation of the rate capacity with charging current set to 100 μA/cm2 and discharging current varied from 100 μA/cm2 to 300 μA/cm2.
as the content of Al-LLZTO increases, the viscosity decreases substantially. Figure 2 shows the Arrhenius plots for the composite solid electrolytes with different Al-LLZTO contents. It can be
Figure 2. Arrhenius plots for the composite solid electrolytes with different Al-LLZTO contents.
observed that the addition of Al-LLZTO can significantly increase the conductivity of the electrolyte in comparison with other PEO-based polymer electrolytes containing ionic insulating additives.26 The composite electrolyte loaded with 60 wt % Al-LLZTO shows a highest conductivity of 2.48 × 10−4 S/cm at 30 °C, which is comparable to the best results in polymer electrolyte.27,28 Same composite electrolyte also exhibits a wide electrochemical stability window up to ∼6 V, as shown in Figure S4. Higher loading of Al-LLZTO may introduce pores in the composite electrolyte and deteriorate the ionic conductivity. The activation energies (Ea) are calculated using the equation σ = A exp(Ea/kT), where σ is the ionic conductivity, A is the pre-exponential constant, k is the Boltzmann constant, and Ea is the activation energy. For the composite electrolytes with 40 and 60 wt % Al-LLZTO, an obvious change of slope around 60 °C can be observed, which can be explained by the recrystallization of PEO from the amorphous state when it is cooled to the transition temperature at around 60 °C.29 However, no apparent change of slope can be observed in the electrolyte with 80 wt % Al-LLZTO. The decreased content of PEO and the low crystallinity of the PEO in the low temperature range may contribute to this phenomenon. The conductivity at 30 °C and activation energies of the composite electrolytes with different AlLLZTO contents are listed in Table 1. It can be observed that the activation energies of these composites electrolytes below 60 °C are much lower compared with the ceramic-free PEO electrolyte, while the activation energies above 60 °C are of no substantial difference. In addition, the activation energies
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RESULTS AND DISCUSSIONS Enhanced Ionic Conductivity of the Composite Solid Electrolyte. The Al-LLZTO powders used in this study are of pure cubic phase as indicated by the X-ray diffraction (XRD) patterns shown in Figure S2. SEM images of the Al-LLZTO powders (Figure S3) reveal that the powders exhibit uniform size distribution with an average diameter of
