Polymer layers enhance the compatibility of LATP and electrodes, leading to the superb cycling stability of all-solid-state lithium batteries.
The rational design and exploration of safe, robust, and inexpensive energy storage systems with high flexibility are greatly desired for integrated wearable electronic devices. Herein, a flexible all-solid-state battery possessing competitive electrochemical performance and mechanical stability has been realized by easy manufacture processes using carbon nanotube enhanced phosphate electrodes of LiTi 2 (PO 4 ) 3 and Li 3 V 2 (PO 4 ) 3 and a highly conductive solid polymer electrolyte made of polyphosphazene/PVDF-HFP/LiBOB [PVDF-HFP, poly(vinylidene fluoride-co-hexafluoropropylene)]. The components were chosen based on their low toxicity, systematic manufacturability, and (electro-)chemical matching in order to ensure ambient atmosphere battery assembly and to reach high flexibility, good safety, effective interfacial contacts, and high chemical and mechanical stability for the battery while in operation. The high energy density of the electrodes was enabled by a novel design of the self-standing anode and cathode in a way that a large amount of active particles are embedded in the carbon nanotube (CNT) bunches and on the surface of CNT fabric, without binder additive, additional carbon, or a large metallic current collector. The electrodes showed outstanding performance individually in half-cells with liquid and polymer electrolyte, respectively. The prepared flexible all-solid-state battery exhibited good rate capability, and more than half of its theoretical capacity can be delivered even at 1C at 30 °C. Moreover, the capacity retentions are higher than 75% after 200 cycles at different current rates, and the battery showed smaller capacity fading after cycling at 50 °C. Furthermore, the promising practical possibilities of the battery concept and fabrication method were demonstrated by a prototype laminated flexible cell. KEYWORDS: flexible all-solid-state Li-ion battery, freestanding electrodes, polymer electrolyte, polyphosphazene, PVDF-HFP, carbon nanotube (CNT), LiTi 2 (PO 4 ) 3 , Li 3 V 2 (PO 4 ) 3
In this study, the possibility to characterize the electrochemical characteristics of the particle-polymer interface in dual-phase electrolytes by measuring the contact potential difference with high local resolution is demonstrated. Two different polymer electrolytes, polyethylene oxide (PEO) and poly[bis-2-(2-methoxyethoxy)-ethoxyphosphazene] (MEEP), were investigated in combination with lithium ion conductive Li7La3Zr2O12 (LLZ) particles and two different mixed ionic-electronic conductive ceramic particles: uncoated and carbon coated LiFePO4 (LFP) as typical cathode material and uncoated Li4Ti5O12 as typical anode material. A distinct Volta potential gradient between the particles and the polymer was observable in all cases, except when no lithium salt was present within the polymer matrix. The measured potential gradients can be explained in terms of a contact potential between the polymer electrolyte and the ceramic electrolyte. A more negatively charged space charge layer around LFP particles in PEO matrix and around LLZ particles in MEEP can be explained by enrichment of salt anions in direct vicinity of the particle. Electrochemical characterization with impedance spectroscopy showed an increased conductivity for addition of LFP for PEO while the addition of various particles in different concentrations showed no effect on the conductivity of MEEP. The lithium transference number was unaffected by particle addition for all samples.
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