WANG Youchang^a，b，ZHANG Xiaojing^a，b，ZHU Yuwei^a，b，LI Xiaolu^a，b，LI Yuhang^a，SHEN Zhigang^a，b

a. School of Aeronautic Science and Engineering，b. Beijing Key Laboratory for Powder Technology Research and Development，Beihang University， Beijing 100191， China

Abstract

Objective Traditional metal heating films used for preheating lithium-ion batteries are often inflexible， dense， and susceptible to corrosion. In contrast， graphene-based flexible resistive heaters （FRHs）， produced through screen printing， demonstrate great potential due to their outstanding conductivity， stability， and lightweight properties. However， replacing toxic， high-boiling-point organic solvents with water remains a substantial challenge. To address this， this paper prepares an environmentally friendly graphene-based ink using a ternary mixed solvent composed of water， ethanol， and ethylene glycol.

Methods High-quality graphene was initially prepared using the jet cavitation method， with its morphology and structure characterized by scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. A simple sand-milling method was then employed to produce graphene-based water-based ink with varying ratios of graphene and carbon black. A parallel-plate rheometer and a four-probe resistivity meter were utilized to assess the rheological properties of the ink and the conductivity of the printed patterns. SEM was used to explore the mechanism by which carbon black influenced the conductivity of the printed patterns. Finally， graphene-based FRHs were fabricated through screen printing， and their electrothermal properties were tested. These FRHs were applied to low-temperature thermal management in lithium-ion batteries.

Results and Discussion The size and quality of graphene had a significant impact on the rheological properties of the ink and the conductivity of the printed patterns. Graphene produced through jet cavitation method exhibited large lateral sizes （~1 μm）and a high degree of exfoliation， as observed in TEM images. The symmetrical hexagonal patterns in electron diffraction images confirmed its excellent crystallinity. These characteristics contributed to its superior conductivity due to minimal obstruction to carrier mobility. However， the rigid connections between two-dimensional graphene sheets created numerous line contacts， limiting the full utilization of graphene’s specific surface area. Introducing zero-dimensional carbon black particles filled the voids between graphene sheets， constructing a dense conductive network that significantly enhanced conductivity by approximately seven times， from ~3. 2×10³ S∙m^-1 to ~2. 04×10⁴ S∙m^-1. Additionally， the large specific surface area and structure of carbon black increased the static viscosity of the ink while maintaining its viscosity（1. 1~1. 2 Pa·s） at a high shear rate （100 s^-1 ）， making it highly suitable for screen-printing process. Moreover， carbon black enhanced the post-printing viscosity recovery rate，optimizing leveling properties on the substrate. Results indicated that at 15% carbon black content， the ink exhibited optimal rheological properties， and the printed patterns achieved maximum conductivity of 20，383 S/m. FRHs prepared through screen printing showed rapid voltage-independent response time， reaching 150 ℃ within 30 seconds at a low voltage of 8 V， while maintaining uniform temperature distribution. The prepared FRHs could undergo 120 repeated switching cycles， with only a 2. 54% temperature increase after 72 hours of continuous operation and less than 2. 1% temperature variation after 2 000 bending cycles. These results demonstrated excellent reliability， long-term stability， and mechanical performance of the printed graphene-based FRHs. When using FRHs to preheat lithium-ion batteries at voltages of 6 V，7 V， and 8 V， the battery surface temperature rose from -30 ℃ to 20 ℃ in 800，400， and 240 s， corresponding to heating rates of 3. 75，7. 5， and 12. 5 ℃·min^-1，respectively. Preheated lithium-ion batteries could be charged and discharged at rates of 2 C and 5 C， respectively. At 6 V， the battery achieved 80% state of charge （SOC） within 24. 2 minutes， and at 7 V，64. 8% SOC within 30 minutes. Graphene-based FRHs screen-printed with G-ink-15 outperformed conventional external preheating methods and showed comparable performance to internal preheating methods.

Conclusion In this paper， an environmentally friendly graphene-based water-based ink was successfully prepared using a ternary mixed solvent of water， ethanol， and ethylene glycol. The ink exhibits excellent rheological properties and produces highly conductive printed patterns. The prepared FRHs demonstrate a fast response time and outstanding stability， indicating significant potential for thermal management applications in lithium-ion batteries.

Keywords：graphene； water-based ink； screen printing； flexible resistive heater； thermal management

DOI：10.13732/j.issn.1008-5548.2024.06.002