Surface Ion and electron regulation strategies for rechargeable lithium metal batteries

Abstract

With the growing popularity of portable electronic devices and electric vehicles, conventional lithium ion batteries (LIBs) are facing challenges from increasing demands for higher energy density. Instead, lithium metal (LM) has been historically and widely recognised as the most promising negative electrode (negatrode) material owing to its low density (0.53 g cm-3 ), high theoretical specific capacity (3860 mAh g1 ), and lowest (most negative) redox potential (−3.04 V vs. the standard hydrogen electrode). Thus, the lithium metal negative electrode (LMNE) is widely regarded as a crucial part in next-generation energy storage devices such as lithium metal rechargeable batteries (LMRBs). However, uncontrolled dendrite growth can result in infinite volume expansion, sharp capacity degradation, and dangerous short circuit, restricting the application of LMRBs. The non-uniform lithium ion (Li+ ) flow on the surface of the negatrode and the non-uniform distribution of electrons on the surface of the conventional current collector (e.g., Cu) are two key factors that lead to the nonuniformity of lithium (Li) deposition. Thus, this thesis provides new strategies based on the above two crucial factors to alleviate Li dendrites' formation. The work is summarised below.
(1) The surface of the LM is frequently observed to exhibit a non-uniform distribution of Li+ flux in LMRBs. Although nitrogen-containing (N-containing) functional groups in carbon materials are reported to be effective in homogenizing the Li+ flux, the effective interaction distance between Li+ and N-containing groups is relatively small (down to the nanometre scale) according to the Debye length law. Thus, it is necessary to carefully design the microstructure of N-containing carbon materials to make the most of their roles in regulating the Li+ flux. In Chapter 3, porous carbon nitride microspheres (PCNMs) with abundant nanopores have been synthesised and utilised to fabricate a uniform lithiophilic coating layer having pores of both the nano- and micrometre scales on the Cu/Li foil. Physically, the three-dimensional (3D) porous framework is favourable for accommodating volume expansion and guiding Li growth. Chemically, this coating layer can render a suitable interaction distance to effectively homogenise Li+ flux and contribute to establishing a robust and stable solid electrolyte interphase (SEI) layer with Li-F, Li-N, and Li-O-rich contents based on the Debye length law. Such a physical and chemical synergistic regulation strategy using PCNMs can lead to dendrite-free Li plating, resulting in a low nucleation overpotential and stable Li plating/stripping performance in both Li||Cu and Li||Li symmetric cells. Meanwhile, a full cell using the PCNM-coated Li delivered high capacity retention of ~80% after 200 cycles at 1 C and achieved remarkable rate capability. The high-areal capacity pouch cell retained ~73% of the initial capacity after 150 cycles at 0.2 C.
(2) Although the PCNMs with abundant nanopores can homogenise the Li+ flow effectively, it is still difficult to avoid the direct contact between Li and electrolyte during the cycling. Therefore, Chapter 4 reports a method in which a Li-F-rich layer was built in advance on the surface of the Li foil@PCNM electrode as an artificial SEI to reduce the direct contact with the electrolyte. Dimethylacetamide (DMAC) was used as a solvent to promote the self-driven chemical reaction between polyvinylidene fluoride (PVDF) and Li. This facilitated reaction may be attributed to the distinctive solvation structure formed by DMAC with PVDF and moderate structural stability toward LM. The corrosion of Li by different solvents was tested, and X-ray photoelectron spectroscopy (XPS) was applied on the surface of LM treated with different solvents to analyse the effect of solvents on the surface composition of Li. In addition, the effectiveness of the coating material in achieving dendrite-free Li deposits and suppressing the volume expansion of LMRBs (in-situ swelling testing) was investigated. Finally, excellent electrochemical performances of symmetric cells and full cells using modified Li were achieved.
(3) In Chapter 3 and Chapter 4, LMRBs, where pre-placement of LMNE as the main Li resource delivers reversible electrochemical plating/stripping, are promising electrochemical energy storage devices. However, the pre-placement of LMNE will hurt the specific energy of the battery and scientific evaluation of materials, and result in concerns regarding manufacturing costs and safety. The issues mentioned above can be avoided in lithium metal rechargeable batteries with a lithium-metal-free negatrode (LMFRBs). Nevertheless, uncontrolled formation of polymorphous Li deposits, e.g., whiskers, mosses, or dendrites in LMFRBs may result partly from non-uniform interfacial current distribution and internal stress release in the upward direction above the surface of a conventional current collector (e.g., Cu foil). If it occurs in a lithium metal-free negatrode, rapid performance degradation or serious safety problems may be anticipated. The 3D carbon nanotubes (CNTs) skeleton has been proven to effectively reduce the current density on individual CNTs and eliminate the internal accumulation of stress. However, remarkable electrolyte decomposition, inherent Li source consumption due to repeated SEI formation, and Li+ intercalation in CNTs limit the application of the 3D CNTs skeleton. Thus, it is necessary to avoid the side effects of the 3D CNTs skeleton and retain uniform interfacial current distribution and stress mitigation. In Chapter 5, the CNTs network with a soft functional polymer PVDF is reported to form a relatively dense coating layer on the Cu foil. This is expected to shield the contact between the internal surface of the 3D CNTs and the electrolyte. Simultaneously, the Li-F-rich SEI resulting from the partial reduction of PVDF by the deposited Li and the soft nature of the coating layer release the accumulated internal stress in the parallel direction to the current collector surface. As a result, Li deposition without mosses and whiskers has been achieved, leading to improve reversibility of Li deposition and dissolution and stability of the cycling performance of LMFRBs.
(4) Although CNT/PVDF composite coating layer on Cu foil can achieve dendrite-free Li deposition, the entire composite current collector is still physically heavy. In addition, Cu is still chemically inappropriate for LMFRBs. Physically light carbon-based current collectors (CBCCs) may offer sufficiently high conductivity and a strong resistance toward corrosion by oxygen or electrolyte. They can also be engineered to possess suitable macro-, micro- and nanostructures that can assist the more uniform current distribution and hence replace the Cu foil as a preferable deposition substrate. However, there is potential limitations application of CBCCs in LMFRBs. For example, the large surface-area of CBCC (e.g., CNTs) can induce higher consumption of the limited Li source (e.g., SEI). Also, lithiation or electrolyte penetration may lead to mechanical strength reduction. It is anticipated that fragile SEI may result from the lithiophobicity of CBCCs. Last but not least, welding between carbon and metal can be problematic. In Chapter 6, a novel method is described for the preparation of free-standing graphene/PVDF composite current collectors via the coating and etching process. Highly conductive graphene sheets are tightly stacked to reduce the electrochemically reactive surface area and suppress the lithiation behaviour. PVDF not only facilitates the formation of the sturdy free-standing film but also hinders the Li+ insertion and electrolyte penetration to reinforce the mechanical strength of the CBCCs. Meanwhile, the Li-F-rich SEI derived from the partial reduction of PVDF by the deposited Li can considerably reduce the formation of fragile SEI. Also, the newly-formed SEI can further minimise the electrolyte decomposition owing to PVDF’s low Fermi level suppressing electron transfer to the electrolyte. It is also reported that Cu tabs can be bonded firmly to the CBCCs with acceptable resistance via etching, promising practical application. Therefore, basic requirements of the current collector, such as high conductivity, sufficient mechanical strength, viable tab welding, and improved Li deposition and dissolution behaviour in half and full cells have been satisfactorily achieved using this free-standing CBCC. The assembled pouch cell has achieved a remarkable 80% capacity retention after ~50 cycles at 0.1 C.

Date of Award	Jul 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Di Hu (Supervisor), Zhaoping Liu (Supervisor) & George Chen (Supervisor)