UCLA

The aggravated environmental issues and limited resources call for renewable substitutions for fossil energy. In order to enable the wide use of renewable energy resources such as wind power and solar power, energy storage devices and materials have to be developed accordingly. Among all the energy storage candidates, rechargeable batteries, especially lithium- ion batteries (LIBs) show great potential. The high energy and power density of LIBs benefitted from the light-weight of lithium metal is a great advantage over other energy storage devices such as lead-acid batteries. They are also relatively environmental-friendly as a result. LIBs have long cycling life, with little memory effect. The properties of LIBs including physical features and energy storage characteristics are adjustable and flexible with different designs and use of materials, endowing them with broad applications from portable consumer electronics to electric vehicles to grid-scale energy storage.

Anode as one major component of LIBs, has been a research focus for years. In light of the strong need for LIBs with higher energy density, silicon anode materials and lithium metal anode have been especially popular because of their ultrahigh specific capacity that significantly boosts the energy density of the according cells. Given their favorable advantages, they have major drawbacks that decisively hinder their applications in the market. Silicon materials, decided by its alloying lithiation mechanism, have almost 300% volume expansion upon full lithiation, which can cause serious fractures on the electrode and eventual failure. On the other hand, despite the high capacity and low lithiation potential of Li metal, Li dendrite growth is a severe problem that directly leads to a cell failure and even unwanted safety concerns.

In this dissertation, low-cost and durable silicon anode materials are developed. To overcome the major problems of Si anode materials, a covalently-bonded nanocomposite of silicon and poly(vinyl alcohol) (Si-PVA) by high-energy ball-milling of a mixture of micron- sized Si and PVA is designed. The obtained Si nanoparticles are wrapped by resilient PVA coatings that covalently bonds to the Si particles. In such nanostructure, the soft PVA coatings can accommodate the volume change of the Si particles during repeated lithiation and delithiation. Simultaneously, as formed covalent bonds enhance the mechanical strength of the coatings. Due to the significantly improved structural stability, the Si‒PVA composite delivers a lifespan of 100 cycles with a high capacity of 1526 mAh g‒1. In addition, a high initial Coulombic efficiency over 88% and an average value of 99.2% in subsequent cycles can be achieved. This reactive ball milling strategy provide a low-cost and scalable route to fabricate high performance anode materials.

To take a further step, an electrolyte membrane is designed and developed to enable the use of Li metal anode. . Inspired by ion channels in biology systems, we constructed lithium-ionchannels by chemically modifying the nanoporous channels of metal-organic frameworks (MOFs) with negatively charged sulfonate groups. Analogous to the biological ion channels, such negatively charged moieties repel anions while allowing effective transport of cations through the pore channels. Implementing such MOFs as an electrolyte membrane dramatically improves the lithium-ion transference number, enhances the rate capability and durability of the batteries. With the MOF membrane, Li dendrite growth is much suppressed, leading to an improved Coulombic efficiency and a prolonged cycle life.