Reducing cost and increasing energy density are two barriers for widespread application of lithium-ion batteries in electric vehicles. Although the cost of electric vehicle batteries has been reduced by $70% from 2008 to 2015, the current battery pack cost ($268/kWh in 2015) is still >2 times what the USABC targets ($125/kWh). Even though many advancements in cell chemistry have been realized since the lithium-ion battery was first commercialized in 1991, few major breakthroughs have occurred in the past decade. Therefore, future cost reduction will rely on cell manufacturing and broader market acceptance. This article discusses three major aspects for cost reduction: (1) quality control to minimize scrap rate in cell manufacturing; (2) novel electrode processing and engineering to reduce processing cost and increase energy density and throughputs; and (3) material development and optimization for lithium-ion batteries with high-energy density. Insights on increasing energy and power densities of lithium-ion batteries are also addressed.
The effect of calendering and adding graphites of different particle sizes to composite electrodes comprising Si alloy particles was evaluated. It was found that calendered alloy coatings containing graphite results in increased cycling performance, reduced volume expansion and increased energy density compared to a pure alloy coating. Such high energy density, low volume expansion coatings are expected to be practical for implementation in high energy density Li-ion cells.The development of high energy Li-ion cells is of great technological importance. One method to increase Li-ion cell energy is to use active alloys in the negative electrode. Si-based alloys are promising candidates for high energy density anode materials due to the low average voltage and high volumetric capacity 2194 Ah/L (corresponding to Li 15 Si 4 ) of Si. 1 However, the lithiation of Si is associated with a large volume expansion of up to 280%. 2 This expansion can lead to structural degradation of the electrode and the disconnection of active regions from each other and the current collector. The corresponding capacity loss is a major detriment to the application of pure Si as an anode material in Li-ion cells. One way to circumvent this problem is the use of active/inactive alloys. 3 The design principles of active/inactive alloys have been described in Reference 2. The idea of this approach is to dilute the expansion of the active phase during lithiation with an inactive matrix. In this way alloys with lower volume expansion and with good cycle life have been achieved. 4 The performance of composite coatings containing alloy particles is also highly dependent on binder chemistry and electrode processing conditions. Although alloys generally cycle poorly in poly(vinylidene fluoride) (PVDF) binder, Li et al. reported that composite silicon coatings with PVDF binder have significantly better cycle life after a heat-treatment above the PVDF melting point, such that the PVDF forms a continuous film on the Si surface. 5 Better cycling performance can be achieved when the binder forms strong bonds with the alloy particle surface in addition to forming a continuous coating. Hochgatterer et al. investigated Si composite coatings with sodium carboxymethyl cellulose (CMC) binder and concluded that the carboxylic acid sites formation of a covalent bond between the carboxylic acid sites of CMC and Si surface effectively improved the cycling stability. 6 Cycling can be further improved with binders that contain a higher concentration of carboxylic acid sites than CMC, such as lithium polyacrylate (LiPAA), 7 polyacrilic acid (PAA) 8 or alginate. 9 Komaba et al. showed that in addition to forming strong bonds at the alloy surface, PAA binder continuously coats the alloy surface as well. 10 These studies and others 11,12 indicate that good binders for Si alloys should be capable of forming a continuous coating on alloy particles that binds strongly to the alloy surface.After coating, composite electrodes are highly porous. In order to achieve a high vo...
The objective of this paper is to design and optimize the high temperature metalized thin-film polymer capacitor by a combined computational and experimental method. A finite-element based thermal model is developed to incorporate Joule heating and anisotropic heat conduction arising from anisotropic geometric structures of the capacitor. The anisotropic thermal conductivity and temperature dependent electrical conductivity required by the thermal model are measured from the experiments. The polymer represented by thermally crosslinking benzocyclobutene (BCB) in the presence of boron nitride nanosheets (BNNSs) is selected for high temperature capacitor design based on the results of highest internal temperature (HIT) and the time to achieve thermal equilibrium. The c-BCB/BNNS-based capacitor aiming at the operating temperature of 250 °C is geometrically optimized with respect to its shape and volume. "Safe line" plot is also presented to reveal the influence of the cooling strength on capacitor geometry design.
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