|Title:||Anion hosting graphitic carbon cathodes for dual ion battery|
|Advisors:||Yao, Haimin (ME)|
Zhou, Limin (ME)
|Subject:||Electric batteries -- Design and construction|
Electric batteries -- Materials
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Mechanical Engineering|
|Pages:||xx, 129 pages : color illustrations|
|Abstract:||As an alternative energy storage system, dual ion battery (DIB) with graphite cathode is attractive due to its high operating voltage > 4.0 V versus Li/Li+, environmental benignity, and abundant raw materials. Graphite is capable of different anions (de)intercalation in various systems. However, graphite seldom delivers a reversible capacity higher than 100 mAh g-1 even when voltage reaches 5.0 V (vs. Li/Li+), impeding its further development and commercialization. Other found cathodes, like hard carbon, metal-organic frameworks, polycyclic aromatic hydrocarbon, and aromatic molecules inclusion organic crystalline solids, are capable of anion hosting but deliver even lower capacity compared to graphite. Therefore, exploration of potential high-capacity anion hosting cathodes and capacity optimization of the graphite cathode are performed, contributing to develop and diversify green and sustainable energy storage systems.|
Acknowledging reduced graphene oxide anode's high capacity gifted by increased active exposed edges and surface area, we reckon this law should also be applicable for it as cathode. Because reduced graphene oxide still preserves graphite's bipolarity, the ability to host polarity distinct cation and anion. Hence, hydrothermally reduced graphene oxide (HrGO) is chosen to investigate its hexafluorophosphate (PF6¯) storage capability as a high-performance cathode. Particularly, 3D porous structure of HrGO offers a reversible capacity of 186 mAh g-1 and self-induced electrochemical activation drives the capacity grows to 320 mAh g-1 after 300 cycles, which is the highest DIB cathode capacity ever reported. The electrochemical activation comes from surface area increment, originated from multilayer reduced graphene oxide rolls formation, accumulation and structural order increase in cycling. The formation of rolls is plausibly a result of strain release of reduced graphene layers after electrochemical interaction with PF6¯. Additionally, charge storage mechanism of HrGO is unveiled. At active surface sites of HrGO, PF6¯ is consistently stored in a pseudocapacitive manner. In contrast, at well-crystallized domains, pseudocapacitive PF6¯ uptake occurs at low voltage region while PF6¯ intercalation dominates at higher potentials. Supportive lithium storage also contributes to total capacity. This work offers valuable insights of electrochemical interaction between PF6¯ and reduced graphene oxide and provides guidance for high-capacity cathode construction.
The disclosure of the HrGO's remarkable reversible capacity and long-term cyclability encourages us further an in-depth drilling of it. To verify whether the used concentrated electrolyte contributes to the observed current density independent electrochemical activation of the HrGO in previous study, characterization of the HrGO in three electrolytes with different concentrations is conducted. Since concentration variation is known to affect capacity and working potential of anion hosting cathode. And the operable voltage range for the HrGO is another important parameter. The upper cut-off voltage variation tests are carried out. Additionally, the HrGO can also serve as additive for lithium ion battery (LIB) cathode, for example lithium iron phosphate (LiFePO4, LFP), improving overall capacity. Such hybrid cathode, HrLFP, is a parallel cathode with PF6¯ storage at the HrGO and Li+ delithiation from the LFP simultaneously during cell charging. The prepared parallel hybrid cathode exhibits capacity higher than the theoretical capacity of the LFP. The study confirms that the HrGO is active in the hybrid cathode and demonstrates that the concept is practical.
In this thesis, another hybrid cathode functions in serial manner is also proposed and studied. The cathode incorporates a physical mixture of high working potential graphite matrix interacting with anion and nontoxic LFP as cation host. Since toxic transition-metal free cathode alternatives are highly desirable for sustainable energy storage system advancement. And this hybrid cathode presents a simple and effective approach. Then the proposed hybrid cathode, capable of both cation and anion storage, is examined. Characterization results reveal that the hybrid cathode, during charging, starts with the LFP delithiation in lower voltage region and finishes with graphite intercalation with anion in high voltage region. During discharging, processes are reversed with stepwise anion deintercalation and cation insertion, without interfering individual constituent's proper functioning. The transfer of two polarity distinct ions inside the hybrid cathode is independent, without any added kinetic difficulties. Such hybrid cathode can work stably with the LFP and the graphite in varied proportions. The addition of 40 wt% LFP to graphite cathode shows doubled capacity and excellent long-term cyclability. And the hybrid cathode is also demonstrated cyclable in a full cell configuration.
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